A STUDY ON THE EFFECT OF ZnO AND AMMONIA CONCENTRATION ON LATEX PROPERTIES, GELLING TIME AND PHYSICAL PROPERTIES OF THE FILM
BY MEERA
І . INTRODUCTION……………………………………….
1. Hindustan Latex Ltd……………………………………… 2. Male Contraceptive Condoms………………………………. 3. Natural Rubber Latex……………………………………… 4. Testing Of Latex…………………………………………... 5. Preservation N R Latex…………………………………… 6. Processing Of Latex………………………………………. 7. Latex Compounding……………………………………… 8. Dipping Process…………………………………………..
І І.SCOPE OF THE EXPERIMENT…………………………..
1. Experimental Details……………………………………… 2. Materials Used……………………………………………. 3. Parameters Tested…………………………………………. 4. Frequency Of Testing……………………………………… 5. Preparation Of Latex Compounding………………………… 6. Natural Ageing Of Latex Compound……………………….. 7. Dilution Of Latex Compound……………………………… 8. Preparation Of Mould Samples……………………………. 9. Leaching Of Mould Samples………………………………. 10. Vulcanization & Stripping………………………………… 11. Cutting Of Ring Samples………………………………….. 12. Ageing…………………………………………………… 13. Test Details……………………………………………….
NATURAL RUBBER LATEX
Natural rubber is the nature’s most versatile product, which was discovered in Brazil, in South America 420 years ago, has an exudates mass caused due to drying of milky liquid oozing out of certain trees. This material has multifarious uses and there is hardly any segments of life does not use of rubber-based materials. Guayule Shrub was probably the first known source of rubber, which was discovered in North America. Charles –de-la-Condamine, a French man first discovered Heavea Brasiliensis tree. Now Heavea Brasiliensis is the most important source of natural rubber and more than 97 % of natural rubber is produced from this tree. It obtained from the bark of the rubber tree by a process called tapping that a process of controlled wounding of the plant – a small portion of the bark is removed. This process opens the latex vessels and the latex flows out of the tree that is channeled into containers fitted to the bark.
Natural rubber latex is stable colloidal dispersion of 1,4 Cis Polyisoprene in an aqueous medium. The rubber content of field latex varies from 30-40%. The size of the rubber particle varies from 0.025 to 0.3 microns. Natural rubber is a colloid with a specific gravity of 0.96 – 0.98 and a pH in the range of 6.5 – 7. The dispersed phase is mainly rubber and the dispersion medium is water. In addition to rubber and water, latex contains small quintiles of proteins, resins fats, fatty acids, other lipids and sterol esters, carbohydrates and mineral matter. Lipids in fresh latex consist of fats , waxes ,sterols , sterol esters & Phospholipids . Lipids associated with the rubber and non – rubber particles on latex play a key role in stability and colloid behavior of latex.
The natural rubber is a Polyisoprene. The structure of Polyisoprene is
CH3 │ ▬( ▬CH2 ▬ C═ CH ▬CH2▬ ) n In nature, Polyisoprene occurs as cis as well as trans form, the cis form is natural rubber and trans form is Gutta purcha.
CH2 H Trans (Gutta Purcha) C ═ C
CH3 CH2
CH2 CH2 Cis form of (Natural C ═ C rubber)
CH3 H
Composition of Heavea Latex The following is the approximate composition of natural rubber latex.
Composition In %
Rubber Hydrocarbon 30 -45%
Lutoid particles 10 – 20%
Protein substance 1 – 1.5 %
Resinous Substance 1 – 2.5 %
Applications
N.R latex is a general-purpose elastomer. Its high resilience, low heat build up& excellent dynamic properties coupled with out standing processability make it an ideal rubber for automatic tyres.
Engineering applications
Most engineering applications of natural rubber involves its use as a spring. The main reason for using in spring are , • Excellent resistance to fatigue. • High resilience. • Low heats build up. • Reasonability good bonding with metal.
The typical engineering applications of natural rubber includes anti vibration mounting , flexible couplings , bridge bearings , lock fenders & rail pads .However the most important aspect of natural rubber is that it is economically friendly . I t is a product of nature & the energy requirement for its production is only a small fraction of that required for synthetic rubbers .While production of synthetic rubber causes , large scale production including releasing of large quantities of carbon into the atmosphere , production of natural rubber starts with fixing of the carbon from the atmosphere ,. There use of natural rubber has a definite positive impact on the environment.
Preservation of NR latex
It has been noted that natural rubber latex is the contents of a specialized type of cell in the Hevea Brasiliensis tree. It is a complex biochemical system. NRL consist of negatively charged rubber particle in an aqueous serum. It also contains non rubber constituents like proteins, carbohydrate etc which make them a suitable media for the growth of micro organism. So chemical changes occur shortly after the latex leaves the tree. The first of these changes is that the latex coagulates within a few hours. This process is known as spontaneous coagulation. Spontaneous coagulation is to be distinguished from the second of the obvious changes. This is putrefaction, which sets in at a later stage, with the development of bad odors. Preservation of latex is necessary to prevent both these processes occurring. Preserved latex may be field latex or concentrate latex.
Attributes of a good preservative
1.It should preserve the latex against spontaneous coagulation and putrification.It should destroy or in-activate the micro organisms. 2.It should increase the colloidal stability of the latex. This can be achieved by increasing the pH and hence the preservative should preferably be an alkali. 3.It should deactivate or remove traces of metal ions present in the latex. 4.It should not be harmful to the people or should not have adverse reaction with rubber or the container of latex.
Latex preservatives
Various chemicals have been used as preservatives among which ammonia is the most commonly used chemical.
1) Ammonia
Ammonia is a bactericide and it prevents the microbial action on latex. Moreover being an alkali it increases the pH of the system. Also ammonia reacts with the fatty acid present in the latex to form an anionic soap which increases the colloidal stability of the system.
2) Sodium Sulphite
Sodium Sulphite is used as an anticoagulant on the cup and bucket, especially when the latex is to be used for the production of a form of dry natural rubber known as pale crepe. As the name of this product implies, it is essential to keep discoloration of this type f natural rubber to a minimum. In this connection, it may be noted that sodium Sulphite is used as an enzyme inhibitor in the manufacture of pale crepe, the objective is again being to minimize discolouration.Excess use pf sodium Sulphite in latex retards drying of sheet rubber and gives rise to tackiness.
3) Potassium Hydroxide
The only practically important alternative to ammonia as a sole preservative of NR latex is KOH. This is used for the preservation of 75% m/m evaporated NR latex concentrate. It is an effective bactericide by virtue of its high alkalinity. For the same reason, it gives latex which is colloidally very stable.
High Ammonia preservation System
This is the most commonly used system in India. Here Ammonia at a concentration of 0.7% by wt is added to the centrifuged latex. This system will maintain its stability for long periods. But for field latex the non-rubber constituents are more and hence Ammonia concentration of 1% is used to preserve field latex.
Low Ammonia preservation System
One limitation of the high ammonia system is its low ZnO stability which is more important in the manufacture of some products. In such cases low ammonia systems are preferred. In low ammonia system ammonia is used at a concentration of 0.2% by Wt .Along with it chemicals like boric acid, sodium pentachlorophenate, ZDC are used. The most commonly used low ammonia system is LATZ.Here 0.025% of TMTD & 0.025% ZnO and 0.2% ammonia ( all by wt) is used. Processing of latex About 3 to 4 hours after tapping, the latex is collected from the tree, treated with a stabilizer to prevent premature coagulation, and brought to a factory or small holder central processing center. About 82 to 85% of latex extruded by the tree is collected in this way as field latex. On arrival at factory the latex is sieved and blended. Field latex is either concentrated by removing part of the water to give latex concentrate, or it is deliberately coagulated processing into dry rubber.
Concentration of NR latex The process of latex concentration involves the removal of a substantial quantity of serum from field latex ,thus making the latex richer in rubber content. Concentration of latex is necessary for the following reason: Economy in transportation Preference for high DRC by the consuming industry Better uniformity in quality Higher degree of purity Latex may be concentrated to 60% DRC usually by creaming,centrifuging,evaporation or electro decantation or alternatively coagulated and dried. Creamed latex and evaporated latex are still in production. Centrifuged latex is the standard latex of commercially available in bulk. Centrifuged Latex Centrifuged latex concentrate accounts for more than 90% of total latex concentrate. The main types are available, one is known as high ammonia latex concentrate where the preservative is solely ammonia at a concentration of about 07%.The other is known as low ammonia latex concentrate, where the ammonia content is limited to 02% and is supplemented by secondary preservatives. In centrifuging, latex is subjected to centrifugal force, several thousand times the gravitational force, in a bowl rotating at high speeds whereby individual rubber particles tend to separate into a layer surrounding the axis of rotation leaving an outer serum layer having a comparatively low rubber content. Each layer is removed through annular spacing around the axis of rotation. By controlling the time to which latex is subjected to such forces and by controlling the conditions of operations, latex having an original DRC of 30-38% can be concentrated to DRC of 60 or more.
Creamed Latex Creamed latex is produced by adding a creaming agent, usually ammonium alginate, to the field latex preserved with a higher concentration of ammonia together with secondary preservatives.By creaming, DRC of about 66 to 68% is obtained. Evaporated Latex Evaporated latex is produced by passing field latex, preserved with KOH, through heated film evaporators at reduced pressure repeatedly until the desired rubber concentration is obtained. Electro decantation process This process is carried out in a special rectangular tank containing 1cm apart many groves in which cellophane sheets are fixed. The two electrodes are fixed at each end of the tank. Natural rubber latex is poured in the tank and electrics current is applied. The particles float to the top as a cream, which is removed from time to time. Latex with a solid content of 60-62% is obtained. Advantages of centrifuged latex in the manufacture of dipped goods There are several important features which must be recognized in natural product, which are: High total solids of 60 to 70 %that is ammonia or fixed –alkali stabilized. The latex is readily available in high ammonia forms, which permit deliberate stability control, necessary in latex dipping. It is excellent wet gel strength. It can be prevulcanized prior to use. It can be straight, coagulant, or heat sensitized dipped.
Rapid drying and curing characteristics is typical, unlike most synthetic lattices.
LATEX COMPOUNDING:- The first step of condom production is the compounding of latex. Compounding is done to make the latex suitable for moulding operations and for making the final product confirming to requirements of the end user. The process of mixing various compounding ingredients with latex is known as compounding. Compounding involves choosing the amounts and type of vulcanizing ingredients, activator, accelerator, antioxidant, fillers and pigments and mixing them together when these are vulcanized under appropriate conditions a rubber article appropriate to the requirements specified is obtained. Some times other special purpose additives like viscosity modifier, plasticizer etc are added. All ingredients added to the latex should be brought into a physical state that is comparable to the latex before they are added. Ingredients like water soluble organic acids, salts of polyvalent metal, acidic materials etc should not be added to the latex as they lead to coagulation. Compounding ingredients added to latex may be water-soluble or water insoluble. Water soluble ingredients can be added o latex as solutions in water. But water insoluble ingredients must be added as dispersions or emulsions. Solid ingredients are converted into dispersion before adding to latex. This can be achieved by using equipments like ball mill/attritor. Dispersing agents, wetting agents, protective colloids are also added during the preparation of dispersion to get stable dispersions. The pH of all additives are adjusted to alkaline pH(8-9) before adding to the latex.
Latex Compounding Ingredients
1) Stabilizers & surface-active agents.
Latex stabilizers like alkali’s (KOH, ammonia), protective colloids like casein and numerous surface active agents such as carboxylates, sulphonates...etc are used. Stabilizers are first added in to the latex during its compounding to prevent coagulations of latex compounds, while stripping, storage or during addition of chemicals
2) Vulcanizing agents. Mainly sulphur is used as vulcanizing agent .Sulphur for the use of latex should be of good quality ,and finely dispersed in an aqueous medium . A Sulphur donor like TMTD is also used. Vulcanization is an irreversible process during which a rubber compound, through a change in its chemical structure becomes less plastic and more resistant to swelling by organic liquids and the elastic properties are improved or extended over a greater range of temperatures, when cross links are inserted between the adjacent polymer chains. Vulcanizing agents used for natural rubber are 1. Sulphur 2. Peroxides 3. Sulphur chloride 4. Sulphides (TMTD)……….etc 3) Accelerators Accelerators enhance the rate of vulcanization. At higher temperature it can cut the vulcanization time from hours to minuets or seconds. Different type of accelerators are used for vulcanization .They are
4) Activators ZnO is commonly used as activator.. It can also help to increase the rate of vulcanization.
5) Antioxidents
They reduce degradation from sunlight, heat etc.antioxidants oppose oxidation and in a number of cases suppress many undesirable reactions, promoted either by oxygen or peroxides.
Eg; Antioxidant SP, Wigstay L (reaction product of p-cresol & di-cyclopentadiene)
DIPPING PROCESS
The dipping process consist essentially in the immersion of a former into suitably compounded latex, followed by slow withdrawal in such a way as to leave a uniform deposit of latex on the former. The thickness of the deposit may be reinforced with subsequent coatings. The process is completed by drying, leaching & vulcanizing the deposit. It is usually desirable to form a rolled bead at the neck of the article, inorder to reinforce the thin rubber to deposit against tearing. There are different dipping methods are used ; 1. Simple dipping 2. Coacervant dipping 3. Heat sensitized dipping 4. Electro deposition
Simple dipping By simple dipping is meant dipping without the assistance of any coagulants. A deposit forms by virtue of the viscosity of latex and of its tendency to wet out of the former. Single dip process gives very thin deposits. Single dipping is usually practiced as a multi-dip process, allowing partial or complete drying between the successive dips .The thickness of the composite deposit is approximately proportional to the number of dips.
Coacervant dipping In this method a fluid coacervant like acetic acid is used to assist the build up of a deposit. It can be worked into two ways, according as the former is dip first into the coacervant or first into the latex compound .The thickness of the deposit obtained is determined by the dwell time and by the stability of the latex towards the particular coacervant, which is used.
Heat sensitized dipping The principle is to employ a heated former and to compound the latex in such a way that it is heat sensitive. The deposit builds up around the former as heat is conducted A way into the surrounding latex .The thickness of the deposit depends on many factors like the degree of heat sensitivity of the latex, the temperature of the former and the heat capacity of the former.
Electro deposition Since the latex particle usually carry a negative charge , it is possible to assist deposition by causing the particles to migrate towards the (positive polarity ) under the influence of a potential gradient.
SCOPE AND OBJECTIVE OF THE EXPERIMENT
SCOPE AND OBJECTIVE
Though dipped goods can be manufactured from post vulcanisable lattices (not pre vulcanized),a certain extend of prevulcanisation is generally given to increase the productivity and to get the required processing characteristics.ZnO is commonly used as the activator for NR latex vulcanized with Sulphur.ZnO along with organic accelerator increases the rate of vulcanization to levels that are required by various industrial applications.Moreover ZnO gives protection against thermal degradation and increases the modulus of the product. Ammonia is used in natural rubber latex as a preservative because it acts as a stabilizer and bacteriacide. During compounding and post compounding operation some amount of ammonia is lost due to evaporation. To compensate this loss additional ammonia and other stabilizers are incorporated into the latex during compounding .Moreover ammonia is added to various compounding ingredients (eg:Sulphur dispersion,accelerator solution etc..) to adjust pH to the required level.The concentration of ammonia can effect the stability of the latex and hence the processing behavior. Also ammonia plays an important role in ZnO thickening of latex compound,which influences the compound stability and flow characteristics.Thus concentration of ammonia and ZnO are very critical as far as processing of latex compound is considered. The main objective of my project work was to study the effect of ZnO and ammonia concentration on the properties of compounded latex,gelling time and physical properties of the film. ZnO is added at different phr and the quantity of ammonia is also varied and the properties of the compounded latex with different formulations were studied.When ZnO is added during compounding,it combines with the ammonia present in the latex to form Zinc Amine complex.The formation of Zinc Amine complex may lead to the changes in viscosity of latex. A brief outline of my project work is as follows:Nine formulations of combinations of ZnO and ammonia were tried.For each compounding, natural ageing, dilution etc….was given All tests (HST,MST,TS,VISCOSITY,PH,ALKALINITY) were carried out.Hand dipped samples were then made.Ring samples were taken from these samples and Tensile strength,Elongation at break and modulus tested.The Gelling Time of the lattices of different formulations were observed.The physical properties of the film(Before Ageing and After Ageing) were also studied.
EXPERIMENTAL DETAILS
Materials Used
Concentrated latex manufactured by supplier namely Thiruvambady Estate (TE) latex was used in the study. • Raw latex: TE • Sulphur • ZnO • Antioxidant • Accelerator-1 • Accelerator-2
SPECIFICATION OF RAW LATEX
Sl.No Test Items Specification
1. Appearance Clear milky white colour without grey/yellow. 2. Odour No putrefactive odour after the neutralisation of ammonia with Boric Acid.
3. DRC(%) 60.0 min by wt.
4. TS - DRC (%) 1.0 Max. (NRC)
5. Total Alkalinity (%) 1.8-2.5 (Titration Method)
6. Viscosity(B-type 50-85 viscometer at 25 deg.C and 60% TS) (CP).
7. VFA No. 0.05max. (KOH g/100g solid )
8. MST (Second) 800-1600 9. pH value at 25deg .C 10-11.5
SPECIFICATION OF COMPOUNDING INGRADIENTS
SULPHUR
Test item Specification
Appearance Light yellow powder
Moisture 1% max.
Residue on 100 mesh sieve (%) 0.05 max.
Residue on 200 mesh sieve (%) 3.5 max.
Ash content 1% max.
Sulphur content (purity) 99% min.
ZnO
Test item Specification
Appearance White powder
Zinc oxide content 99% min. by wt.
Ignition loss 1% max. by wt.
Insoluble matter against HCl 0.1% max.
ANTIOXIDANT
Test Item Specification
Appearance Free flowing, cream coloured powder
Ash 3 % max
Melting point 95 °Cmin.
Moisture 0.5 % max
Residue on sieve (100 mesh) 0.5 % max
Residue on sieve (200 mesh) 1.0 % max
ACCELERATOR - 1
Test item Specification
Physical form liquid
Colour reddish brown
Density, Mg/m3 0.99 +/- 0.02
Solubility* One part dissolves in two parts. Further dilutions turn cloudy.
ACCELERATOR- 2
Test item Specification
Appearance Yellow or reddish brown Translucent liquid.
Content 50.0 % min.
Specific gravity 1.09 - 1.14
Solubility Soluble in water.
FORMULATION DETAILS
Table 1&2 show the details of the Phr & the quantity of the latex during compounding.
TENSILE STRENGTH ELONGATION AT BREAK MODULUS
Frequency of testing
• For twelwe days alternatively( for compounded latex) • Before Ageing & After Ageing (for TS, EB% Modulus)
DETTMINATION OF ALL TEST:
1. Total alkalinity As the alkalinity of the system depends upon the NH3 concentration the ammonia concentration used in this study.
Procedure: About 1.3 gm of latex is weighed accurately and is transffered to a beak containing 200 mi of distilled water. It is stirred and a few drops of Methyl orange indicator added and is titrated against standard 0.1 NHCL in the burette . At the end point a permanent yellow colour appears.
Calculations:
% of NH3=Normality of HCL х 0.017 хVolume of HCL consumed Weight of latex
2. pH
Caliberate the Ph meter: Take any convenient size of sample and adjust the temperature to arrange from 20ºC by midly agitating the sample the Ph and record both the temperature and Ph of the latex sample .
3. Viscosity .
Viscosity of latex is determined by means of a viscometer which measures the torque produced on a specified spindle ritating at constant rotational frequency and at a low shear rate while immersed to a specific depth in latex.
Procedure:
Pour the sample into the beaker the water bath maintained at 25 ºC or 27ºC and stir the sample gently until the temprature is 25 ±2ºC or 27±2ºC record the precise tmperatue. Immediately attach the spindle securely to the motor shaft and attach the guard into the sample in such a way to avoid air being trapped until the surface of the sample is at the mid point of the groove on the spindle shaft . The spindle shall be placed vertically in the sample and in the centre of the beaker .Select the rotational frequency of the instrument.
Switch on yhe viscometer motor and take the equilibrium reading to the nearest unit scale division .20 – 30 sec may ellapse before equilibrium reading is attained . Calculate the viscosity of latex expressd In Cp.
3. Total solids:
Procedure : A suitable mass of sample is taken the dish a little distilled water is added and distributed over the bottom of the dish . With the dish uncovered the specimen is placed in a vented clear oven for 16 hrs at 70 degree centigrade. Then it is taken out cool in a desicator to room temperature and weighed . Repeat drying and weighing until the mass is constant.
Calculation: % of total solids = C – A х 100 B – A
Where A – Mass of weighing dish B – Mass of dish + original sample mass C – Mass of dish + dry sample.
5. Heat stability time:
HST is the minimum time required for the complete coagulation of 50 ml of the completed latex at 90 ± 2ºC.
50 cc of latex is taken in a water beaker . It is then kept in the thermo set 90 ± 2ºC and is stirred continuously with a glass rod . Time for complete coagulation is noted using a stop watch . This gives the HST.
6. MST (MECHANICAL STABILITY TIME)
PRINCIPLE:
A test portion of latex concentrate is diluted to 55% (m/m) total solids content and stirred at high speed. The time required to initiate visible flocculation is recorded, this being regarded as a measure of the mechanical stability
TEST PROCEDURE:
Carry out the determination in duplicate and within 24 h of first opening the sample bottle. If the total solids content and alkalinity of the latex are not known, determine them in accordance with ISO 124 and ISO 125, respectively.
Dilute 100g of latex, in a glass beaker, to 55.0% (m/m) +/- 0.2% (m/m) total solids content with the appropriate ammonia solution. Without delay, warm the diluted latex with gentle stirring to 36 to 37 oC over a water bath. Immediately filter the diluted and warmed latex through the wire cloth and weigh 80.0g +/- 0.5 g of the filtered latex in to the container Check that the temperature of the latex is 35oC +/- 1 oC.
Place the container in position and stir the latex, ensuring that the rotational frequency of the stirrer is 14000 +/- 200 rev/min throughout the test, until the end point is passed.
The arrival of the end-point is preceded by a marked decrease in the depth of the vortex around the stirring shaft.
Determine the end-point by removing a drop of the latex at intervals of 15 s and spreading the sample gently on a suitable surface, for example the palm of the hand, a glass microscope slide, the surface of water or the stainless steel wire cloth. Take the end-point as the first appearance of flocculum. Confirm the end-point by the presence of an increased amount of flocculum in a sample taken after stirring the latex for an additional 15 s.
Procedure for Gelling Time
A test drop of latex is allowed to fall through a glass plate. The glass plate should be kept at an angle of 45º from the horizontal plane and the temperature of the glass plate is maintained at 45º C.A drop of latex is allowed to fall through the glass plate, with the help of a syringe .The time required by the drop to stop the flow is recorded as the gelling time
PROCEDURE FOR TENSILE STRENGTH
• The stripped condoms are cut to get samples in the form of a ring (ring samples ). They are cut using a die. • Hand dipped products are cut to get ring samples . Altogether 60 ring samples are cut from condoms. • Then ring samples are cut from condoms are made from each formulation . Half of them are kept for ageing. • The other half of the ring samples are used to contact tests on tensile strength , EB( Elongation at Break) • The ring samples are clamped and stretched in an UTM (Universal Testing Machine).
AGEING
The ring samples are kept for ageing in an ageing oven at 70C for seven days. They are taken out after the required period and conditioning is given to the samples.
PREPARATION OF LATEX COMPOUND
Latex compounding is done in Test Tank. Test Tank is a jacketed vessel, the capacity of which is about 25 kg. It contains a stirrer in it, which has three blades. The speed of the stirrer can be varied from 0 to 50 rpm.Usually during compounding and pre-vulcanization the rpm is maintained at 20 rpm. It also contain two valves , one valve is used as steam inlet and the other valve is for passing chilled water for the purpose of cooling. The test tank is cleaned thoroughly with water and is dried prior to compounding. Before operating it, the stirrer and the valves are checked . Latex after weighing is sieved through a voil cloth and is loaded to the test tank. The chemicals are added in the required order to the tank.ie first stabilizers, accelerators, antioxidant, activator and vulcanizing agent. These chemicals are added into the latex at regular intervals. After the addition of each chemical the latex is mixed for 5 minutes before adding the next chemical so as to get thorough mixing. The chemicals are added after sieving through the voil cloth. After addition of all chemicals it is stirred continuously for 2-3 hours. Then heating is given by passing steam through the steam line .The temperature of the latex is maintained in the range of 55 2C by a temperature controller. Heating is given until the latex attains the required cure 55C .Usually heating is given for 8-11hours when latex is cured to the required level .Cure is tested by the chloroform method. Then the steam is cut. After giving cooling the compounded latex is unloaded and is kept in a room for maturation for one day.
NATURAL AGEING OF LATEX COMPOUND
Compounded latex was kept for natural ageing. It was kept covered for few days until it gets cured. Continuous stirring was maintained during this period, from this day own words the cure of the latex is tested using chloroform.
CHLOROFORM TEST 5 ml of compounded latex is taken in a beaker and to it ; 5 ml of chloroform is also added .It is then immediately stirred using a glass rod the coagulation gets completed . T he coagulam is allowed to stand 1 to 2 minutes and numerically rated as follows: • If the coagulam forms a tacky lump and breaks stringy, it is judged as under cure. • If the coagulam forms an under lump and breaks shortly, it is judged as normal cure. • If the coagulam forms non-tacky large agglomerates, it is judged as optimum cure. • I F the coagulam forms small dry crumbs, it is judged as over cure.
After maturation the compound is diluted to the required Ts as mentioned below & tested for ammonia, pH viscosity ,TS , HST, MST (all test) which are explained in detail , later in laboratory test methods.
DLUTION OF LATEX Before making condoms, the cured latex should be diluted to the required TS, (521) .After diluting the latex , all tests are carried out (HST,MST,Viscosity,TS) .
PREPARATION OF MOULD SAMPLES
Borosilicate glass moulds are used for condom sample preparation .These moulds are cleaned by washing and kept in an air oven for drying (70C) .Then it was kept in room temperature to bring down the temperature. Diluted compounds after removing the surface skin was used for dipping. • First dip was given with very slow withdrawal speed and the last drop dripping from the nipple portion was allowed to full down. • Then the mould is inverted quickly and rotated so that the film formed on the mould will have uniform thickness through out. • It was rotated over a hot plate to speed up the drying process .About 5 pieces were given the first dip and then started the second dipping process. • For the second dip , the level of latex should be slighter lower than that of the first dip so that all the beading portion, the second dip level will be at a lower level . • Initial drying was done after inverting the mould and rotating over a heated hot plate .For getting a hand made samples this is done manually. • Five hand dip condoms were made from each latex compound. So altogether 30 mould samples were made .Beading is done by hand. LEACHING OF MOULD SAMPLE After drying, the product will firmly stick on to the moulds . In order to soften and to facilitate easy removal from the moulds , the product should be immersed in an alkali bath (1 % ammonia) at 70C . This can also remove unwanted and soluble substances present in the dipped samples. VULCANIZATION AND STRIPPING
Vulcanization of the mould samples is done before stripping the products .All the moulds were kept on a metal stand and placed in an air oven kept at 80C for 60 mts .After vulcanization time is over , the moulds were taken out , cooled to room temperature and stripping is carried out with the help of silica powder. CUTTING OF RING SAMPLES The stripped condoms are cut to get samples in the form of a ring (ring samples ). They are cut using a die. • Hand dipped products are cut to get ring samples . Altogether 60 ring samples are cut from condoms. • Then ring samples are cut from condoms are made from each formulation . Half of them are kept for ageing. • The other half of the ring samples are used to contact tests on tensile strength , EB( Elongation at Break) AGEING The ring samples are kept for ageing in an ageing oven at 70C for seven days. They are taken out after the required period and conditioning is given to the samples. Conditioning is given for three days at 25 deg C and RH of 50% & then tensile properties are measured..
DATA TABULATIONS
FORMULATION 1
TEST PARAMETERS
2nd DAY
4th DAY
6th DAY
8th DAY
10th DAY
AMMONIA 0.5 0.5
0.5
0.5
0.5
Ph 10.6 10.6 10.6 10.55 10.5
VISCOSITY 14.5 14.5 14.6 14.6 14.6
TS 51.0 51.0
51.0
51.0
51.0
HST 940 900
920
900
900
MST 435 435 435 430 430
FORMULATION 2
TEST PARAMETERS
2nd DAY
4th DAY
6th DAY
8th DAY
10th DAY
AMMONIA 0.75 0.75 0.75 0.75 0.75
Ph 10.85 10.85 10.8 10.8 10.8
VISCOSITY 15.5 15.5 15.6 15.6 15.6
TS 51.0 51.0 51.0 51.0 51.0
HST 970 970 970 962 960
MST 445 445 440 440 440
FORMULATION 3
TEST PARAMETERS
2nd DAY
4th DAY
6th DAY
8th DAY
10th DAY
AMMONIA 0.9 0.9 0.9 0.9 0.9
Ph 11 11 10.95 10.95 10.9
VISCOSITY 14.4 14.4 14.5 14.5 14.5
TS 51.0 51.0 51.0 51.0 51.0
HST 1050 1035 1030 1030 1028
MST 435 435 435 430 430
FORMULATION 4
TEST PARAMETERS
2nd DAY
4th DAY
6th DAY
8th DAY
10th DAY
AMMONIA 0.5 0.5 0.5 0.5 0.5
Ph 10.6 10.6 10.55 10.5 10.5
VISCOSITY 18.5 18.5 18.6 18.6 18.6
TS 51.0 51.0 51.0 51.0 51.0
HST 660. 660 650 650 650
MST 230 228 230 230 230
PARAMETERS 2nd day 4th day 6th day
8th day 10th day
AMMONIA 0.75 0.75 0.75 0.75 0.75
PH 10.8 10.8 10.8 10.8 10.75
VISCOSITY
18.4
18.4
18.5
18.5
18.5
TS 51 51 51 51 51
HST 740 740 740 730 738
MST 237 235 235 230 230 FORMULATION 5
FORMULATION 6
PARAMETERS 2nd day 4th day 6th day 8th day 9th day
STUDIES ON FACTORS AFFECTING THE QUALITY OF RADIATION- VULCANIZED LATEX
By
Leni Lal
Soumya Raju
Reshmi Shankar
2.INTRODUCTION3
2.1Composition of latex 3
2.2Processing of latex 4
2.3Marketable forms of natural rubber 5
2.3.1Preserved field latex5
2.3.2Latex concentrates 7
2.3.3Sheet rubber 12
2.3.4Crepe rubber 13
2.3.5Technically Specified rubber 13
2.4Latex compounding13
2.4.1Surface active agents 14
2.4.2Vulcanizing agents15
2.4.3Accelerators 15
2.4.4Activators 15
2.4.5Antioxidants15
2.4.6Fillers 16
2.4.7Special additives 16
2.5 Preparation of aqueous dispersions 16
2.5.1Quality of dispersions 18
2.5.2Preparation of emulsions 18
2.5.3Deammoniation of latex19
2.6Need of vulcanization 19
2.6.1Method of vulcanization 20
2.6.2Drawbacks of sulphur vulcanization20
2.7Development of radiation vulcanization20
2.7.1Evolution of radiation vulcanization 20
2.7.2Nature of latex used for radiation vulcanization 21
2.8Production of soluble protein-free latex by radiation process 22
2.9Details of radiation vulcanization 23
2.9.1Development of sensitizers 23
2.9.1.1Sensitizers used in radiation vulcanization 23
2.9.1.2Sensitizing action of n-BA 24
2.9.1.3Mechanism of sensitizers25
2.9.1.4Sensitizing mechanism of MFA (n-BA) 26
2.10Antioxidant 28
2.11Radiation process of natural rubber latex29
2.11.1Dose of irradiation30
2.11.2Irradiation time 30
2.12Advantages of radiation vulcanization 30
2.13Properties of RVNRL 31
2.14RVNRL films 31
2.14.1Cytotoxic behaviour of RVNRL films 31
2.14.2Extractable protein content in RVNRL films 31
2.15Advantages of RVNRL over SVNRL 33
2.16Application of RVNRL 33
2.17Reasons for the huge potentiality of RVNRL 34
2.18Features of pilot for the radiation vulcanization of latex 35
3 MATERIALS AND EXPERIMENTAL TECHNIQUES41
3.1NR latex 41
3.2Preparation of latex for RVNRL processing 41
3.3Irradiation 43
3.4Preparation of RVNRL films 44
3.5Test for latex 44
3.6Testing of the final articles 48
3.6.1Tensile strength 49
3.6.2Nitrogen content 49
3.7Machines 50
4RESULTS AND DISCUSSION54
4.1Effect of green strength of cenex on quality of RVNRL 54
4.1.1Particle size and green strength 54
4.1.2Raw rubber properties and green strength58
4.1.3Effect of green strength on RVNRL film 60
4.1.4Effect of immersion in calcium nitrate solution on RVNRL film61
4.1.5Effect of unpolymerised n-BA on quality of RVNRL 62
4.1.6Accelerated ageing of RVNRL films65
4.1.7Effect of maturation of cenex on production of RVNRL 65
5SUMMARY AND CONCLUSION 67
REFERENCES
1. AIM AND SCOPE OF THE WORK
Concentratedlatex is used in the production of large number of products. The latex concentrate is compounded with necessary ingredients mainly for vulcanization. Vulcanization can be done either before or after the shaping process. The former is called pre vulcanization and the latter post vulcanization. Pre vulcanized latex is a convenient material for the latex goods manufacturing industry because it can be used directly for the manufacture of goods without the need for compounding and vulcanization. Generally latex is pre-vulcanized using sulphur and accelerator. Such sulphur vulcanized latex produce nitrosamines which is a toxic material and hence hazards to people working with or using latex based products. Some of the chemicals used for pre-vulcanization can also cause allergic problems. Moreover pre-vulcanization is carried out by heating latex at 55-60oC for durations varying from 4-5 hours and hence it is an energy consuming process.
Hence it is required that pre-vulcanized latex used for production of latex products should be non-toxic and available by an easy and less energy intensive process.
Radiation vulcanization of latex dose not requires sulphur or the conventional nitrosamines liberating accelerators. It is produced by exposing latex treated with suitable monomers like n-butyl acrylate to gamma radiations. In the conventionally prepared Radiation Vulcanized Natural Rubber Latex (RVNRL) there can be some un-polymerized n-butyl acrylate due to which the latex has a bad smell. However if latex films prepared from RVNRL is subjected to heating for small durations at about 1000c, the n-butyl acrylate can be completely converted in to hydrolyzed or polymerized forms with no residual n-BA.
Hence an attempt is made to produce radiation vulcanized non-toxic natural rubber latex of high quality with no residual toxic chemicals that can be used for the manufacture of rubber products. 2. Introduction to Natural Rubber
Hevea brasiliensis, isthe only major commercial source of natural rubber and more than 97% of natural rubber is produced from this tree. This tree is popularly called rubber tree.
The main crop from Hevea brasiliensisis a white or a slightly yellowish opaque liquid known as latex. It is obtained from the bark of the tree by process of tapping. The dry rubber content of the latex varies depending on the age of the tree, tapping system, climatic condition and colonel variation. All forms of coagulated rubber obtained from the field are collectively known as field coagulum.
The fundamental changes in the properties of NR through vulcanization removed most of its susceptibility to climate condition and its limitation as a raw material for a large number of rubber products like tires, foot wear, hoses, belting, foam, mattresses etc. This is because of the important properties like high resilience; high shock absorbing quality, excellent dynamic mechanical properties. A disadvantage as compared to synthetic rubber is that it has poor ageing resistence towards oils, fuels, oxygen, ozone and high temperature.
2.1 Composition of Latex
Latex that comes out of the tree is a white or slightly yellowish opaque liquid. Fresh Latex is slightly alkaline or neutral. Upon storage it becomes acidic due to bacterial action.
Latex consists of a suspension of extremely fine rubber particles in an aqueous liquid or serum. The main constituent of natural rubber latex is of rubber hydrocarbon. This substance has seldom been obtained perfectly pure. It is closely associated with resinous matter and protein. Freshly tapped latex of Hevea brasiliensis contain in addition to rubber hydrocarbon, a large number of non-rubber materils suspended in latex. The non-rubber constituents occurring are the greatest quantity in field latex and proteins, lipids, quadraketone and inorganic salts. The last two components occur entirely in the aqueous phase or serum. The lipids are nearly all on the surface or in the interior of the rubber particles, and the proteins are distributed between the serum and rubber serum interface.
TYPICAL COMPOSITION OF FRESH NATURAL RUBBERS LATEX.
a. Rubber hydrocarbon-30-40 %
b. Water-55-65 %
c. Proteinaceous substance -2-2.5 %
d. Fat and related components-1-2 %
e. Ash-<1>
f. Sugar-1-1.5 %
2.2 Processing of Latex
About 3-4 hours after tapping, the latex is collected from the tree, treated with an anticoagulant (if necessary) to prevent premature coagulation and brought to factory. Ammonia is the most common anticoagulant used though others such as sodium sulphite and formaldehyde are also still in use. About 80-85 % of the crop is collected as latex.
Ca (NO3)2 Latex continues to flow slowly for several hours after the initial collection. This latex is not collected but coagulate spontaneously in the collection cup. This is known as cup lump. A small amount of latex coagulates as thin film on the tapping cut to form tree lace. Some latex also drips to the ground to form earth snap. The coagulated materials known as field or natural coagulum constitute about 15- 20% of total crop and are collected on next tapping day.
2.3 Marketable Forms of Natural Rubber.
Marketable forms of natural rubber include
2.3.1 Preserved field latex
2.3.2 Latex concentrates
2.3.3 Sheet rubber
2.3.4 Crepe rubber
2.3.5 Technically specified rubber
2.3.1 Preserved field latex
To keep latex for longer periods bacterial activity should be suppressed so as to prevent coagulation. This can been accomplished by addition of preservatives. Such latex is called preserved field latex.
Preservation of latex
Shortly after latex is obtained from rubber tree, bacterial action begins and in order to preserve latex in an uncoagulated condition for more than a few hours after tapping it is necessary that some preservatives be added. Ammonia is the most popular latex preservatives. Preservative should be added as soon after tapping as possible.
Why Preservation is necessary?
Natural rubber latex is a negatively charged colloidal dispersion of rubber particles in an aqueous serum. The presences of non-rubber constituents like proteins, carbohydrates etc in latex make it a suitable medium for growth of microorganisms. Because of the proliferation of microorganisms, organic acids are produced and these decrease the stability of latex and eventually coagulate it. This is called spontaneous coagulation that takes places within a period of 6-12 hours. Hence if latex is to be kept for longer periods, bacterial activity should be suppressed. This is accomplished by the addition of preservatives.
Preservatives
Various chemicals are used as preservatives among which ammonia is of prime importance. Other chemicals used along with ammonia are known as secondary preservatives.
Attributes of Preservatives
It should destroy or inactivate microorganisms.
It should contribute positively to the colloidal stability of latex by increasing the charge on the particles and the potential at the rubber –serum interface.
It should deactivate or remove traces of metal ions present in latex.
It should not be harmful to people.
It should not have adverse reaction on rubber or containers of latex.
It should be cheap, readily available and convenient to handle.
Importance of Ammonia as a Preservative
Use of ammonia was described in 1853. Now ammonia is the most widely used preservative. Ammonia at a concentration of 0.7-1.0% by weight of latex is added for preservation. This treatment preservers latex and maintains it in a stable colloidal condition almost indefinitely. Also during storage the higher fatty acid esters present in latex get hydrolyzed into ammonium soaps, which improve the mechanical stability of latex (1).
As a bactericide, ammonia is effective at concentration above 0.35%, being an alkali; it imparts an alkaline reaction to latex thereby enhancing the magnitude of negative charge on the particles and the potential at the rubber- serum interface, thus improving the stability.
2.3.2 Latex concentrates
About 12% of the world’s NR is processed in the form of latex concentrates. More then 65% of this comes from Malaysia. Concentration of latex increases the rubber content in the latex to 60% or more from an initial value of about 30-35% in the field latex.
Concentration Of LatexThe process of latex concentration involves the removal of a substantial quantity of serum from field latex and thus making latex richer in rubber content. Concentration of latex is necessary because of four reasons.
a.Economy in transportation.
b.Preference for high DRC by the consuming industry.
c.Better uniformity in quality.
d.Higher degree of purity
Various processes have been proposed for concentrating latex. Out of these, four have emerged as of special importance
CREAMING
CENTRIFUGATION
EVAPORATION
ELECTRODECANTATION
But only creaming and centrifugation are the commonly used method for the concentration of latex.
CREAMINGParticles dispersed in a fluid medium and subjected to gravitational field have a tendency to move relative to the dispersion medium if the density of the particles differs from that of the dispersion medium. In the case of natural rubber latex, the density of the rubber particles is less than that of the density of the dispersion medium. So the rubber particles tend to rise to the surface of the dispersion medium, this process is known as creaming.The process of concentration by creaming is a comparatively simple one. The ammonia preserved latex is placed in a tank, the solution of creaming agent is added and thoroughly stirred and the mixture is allowed to stand for a period of at least two days until a concentrate of the desired dry rubber contentis obtained. The serum is then drawn off.
Creaming agents: most commonly used creaming agents are ammonium alginate, sodium alginate and tamarind seed powder.
CENTRIFUGATION
Concentration by centrifugation was first realised by Biffin in 1898. This is a method currently used for the concentration of NR latex. About 90% of the NR latex concentrate used in industrially is produced by Centrifugation. The remaining 10 % of NR latex is produced either by creaming or by evaporation.
Centrifugation is in effect a type of accelerated creaming process; in which the motion of the particles is effected by centrifugal field rather that gravitational field. By analogy the products obtained from the centrifugal concentrate is known as cream and the latex obtained as a bi-product known as skim.
Theory of Centrifugation
The motion of a Centrifuging latex particle relative to the dispersion medium in which it is suspended is confined to the radial direction. For practical purposes, the so-called corolis-motion, that is the lateral movement of the particle relative to the dispersion medium, can be neglected. The theory of centrifugation is analogous to that of creaming. The acceleration due to gravity ‘g’ being replaced by the centrifugal acceleration associated with the circular path which any given particles is constrained to follow. The theory of centrifugation is more complex than that of creaming because where as gravitational acceleration is somewhat independent of particle location. If the radius of particles path is ‘R’ then the centrifugal acceleration is directly proportional to the distance of the latex particle from the axis of centrifugal.
It is also necessary to consider the rate at which particle separation occurs. The average steady speed at which a particle moves through the dispersion medium depends up on the balance between the centrifugal force which tends to accelerate motion and the viscous drag which tends to retard motion and the viscous drag which tends to retard motion. For a spherical particle of diameter ‘X’ at a distance ‘R’ from the axis of the centrifuge the centrifugal force is ЛX3(p-σ)w2R/6 and the magnitude of the force of viscous drag is 3Лηx/ dR/dt. Where ‘η’ is the viscosity of the dispersion medium. The direction of the viscous drag is opposed to that of the centrifugal force. Thus the steady value of dR/dt is such that
ЛX3(p- σ )W2R=3Л η xdR
6dt
dR/dt=(p- σ )W2RX2
18 η
Centrifugation in practice
Ammoniated field latex is usually transported to the factory and fed by gravitational flow in to field latex tank. From the tanks, the latex is fed to the centrifugal machines.
L-D-Lavel latex centrifuging machine
The machines consist of a rotating bowl in which a set of concentric conical metallic separates discs enclosed. Latex enters the bowl through a centered feed tube and passes to the bottom of the bowl through a distributor. A series of small holes on a separator discs, positioned at a definite distance from the center, allow the later to get distributed and broken up in to a number of conical shells within the bowl, which rotates at a speed of around 6000 rpm. By maintaining a small clearance between successive conical shells, the maximum distance, which a particle has to traverse in order to pass from the skim to the cream, is made very small.
Minimum required quantity of ammonia shall be added to latex before centrifuging. Most of the ammonia added to field latex goes in to the skim usually latex is to be ammoniated and kept overnight before being centrifuged, thus providing the time for the sludge to settle down. During the working of machine, inverts to the axis of rotation and then empties from the bowl through the holes in to a stationary gulley. The cream is separately colleted in bulking tanks, its ammonia content estimated and adjusted to a minimum of 0.6% on latex and packed in drums.
Efficiency of centrifuging process is defined as the proportion of the total rubber recovered as concentrate. The important factor which affects the efficiency are feed rate, angular velocity of machines, length of regulating screws, DRC of the field latex. The shorter screw increases the DRC of the cream but reduces the efficiency, since the proportion by volume of input, which emerges, as skim increases; Non-rubber content in the cream will be less.
EVAPORATION
Evaporation methods yield a concentrate having properties altogether different from those of centrifugally concentrated latex. This is primarily due to the fact that in evaporation the only constituent removed from latex is water; in addition certain non-volatile stabilizers must be used. Since all of the natural constituents of latex are retained, such concentrates are referred to as whole latex and the rubber obtained from the concentrate as whole latex rubber. Latex is evaporated by application of heat, with or without the assistance of a vacuum, hot air currents over the evaporation surface, agitation or other means of hastening the removal of water.
ELECTRODECANTATION
The electro decantation process is based on the phenomenon of stratification resulting from electro dialysis of latex. The latex is placed in a cell having walls constructed of permeable diaphragms such as cellophane. On either side of the cell are electrodes in a conducting aqueous medium, as in conventional electro dialysis cell. When a potential is imposed across the cell the latex particles move toward the diaphragm nearest the anode. If the potential is correctly adjusted, the particle is not coagulated at the diaphragm, but because of its density tends to move upward along the membrane wall. Frequent reversal of current prevents accumulation at the diaphragm and eventual coagulation. For maximum efficiency several factors must be controlled including potential gradient, frequency of current reversal, charge on particles, area of membranes etc.
2.3.3 Sheet Rubber
Sheet rubber is produced from field latex. Operation includes sieving, bulking and standardization of latex addition of chemicals, coagulation, sheeting, dripping and drying.
Raw rubber sheets are of various types depending upon their method of drying. They are
a)Ribbed smoked sheets (RSS)
b)Air dried sheets (ADS) and
c)Sun dried sheets
2.3.4 Crepe Rubber
When coagulum from latex or any form of field coagulum or RSS cuttings after necessary preliminary treatment is passed through a set of a creping machine crinkly lace like rubber is obtained. This when dried, is called crepe rubber. They include
a)Pre coagulated crepe
b)Sole crepe
c)Pale latex crepe
d)Estate brown crepe
e)Smoked blanked crepe
f)Remilled crepe
g)Flat bark crepe
2.3.5 Technically Specified Rubber
The main drawbacks in the marketing of RSS and different types of crepe are;
a)Multiplicity of grades posing problems to the consumer.
b)Non-availability of technical information on quality of rubber.
c)Poor presentation of rubber in large bareback bales prone to contamination.
These difficulties are reduced in TSR and new methods of processing and presentation were developed to market natural rubber in compact medium-sized blocks wrapped in PE and graded adopting technical standards. These are called TS block rubber.
2.4 Latex Compounding
Rubber particles in natural latex are polydisperse and have colloidal size. Because of the presence of alkali, the pH of latex is frequently in the range of 9-12. The conversion of NR latex into a product is accomplished in many ways. In all latex processes a stable colloidal system is maintained until it is desired to be destabilized and converted to a solid product. Ingredients for a latex compound are selected so as to achieve the desired processing behaviours and product properties. The different ingredients used in a latex compound are.
Surface active agents
Vulcanizing agents
Accelerators
Activators
Antioxidants
Fillers and
Special additives
The water-soluble materials are added as solutions, insoluble solids as dispersions and insoluble liquids as emulsions.
2.4.1 SURFACE ACTIVE AGENTS
Surface active agents are substances which bring about marked modification in the surface properties of aqueous medium, even though they are present only in every small amount (of the order of 1% or less). All the chemicals that are used as stabilizes, dispersing agents, emulsifiers, wetting agents, foam promoters, foam stabilizers, viscosity modifiers, protective colloids come under this group. Chemically they can be classified as anionic, cationic, ampotheric and non-ionogenic types. The dispersing agent prevents the dispersed particles fromre-aggregating alkyl sulphonates are used for this. Emulsifying agents are used to make miscible two liquids that are normally immiscible. Soaps usually oleates, formed in situ, function as emulsifying agents. Wetting agents are used to reduce the interfacial tension between two surfaces. Many of the stabilizers and dispersing agents are good wetting agents also. Proteins, alginates, cellulose derivatives and polyvinyl alcohol are used as viscosity modifiers and protective agents in the processing of latex compounds.
2.4.2 VULCANISING AGENTS
The normal vulcanizing agent for natural rubber latex is sulphur. Thiuram polysulphides such as tetramethyl disulphide is used as curative in latex compounds which require specific property such as heart resistance and non-staining of metal parts by sulphur. Butyl xanthogen disulphide in conjuction with a zincdithiocarbomate may be used to vulcanize natural rubber latex films in the absence of additional sulpher. Organic peroxides and hydroperoxides may also be used to vulcanize natural rubber latex films.
2.4.3 ACCELERATORS
These are chemicals added to latex to reduce the vulcanization time and also to increase the technological properties of vulcanized product. The most important class used latex compounding are the metallic and amine dialkyldithiocarbamates.
2.4.4 ACTIVATORS
Zinc oxide is used as the activator for the vulcanization process and its effect include increase in tensile strength and modulus of the vulcanizate.
2.4.5 ANTIOXIDANTS
The common antioxidants used in latex work fall into two classes.
a.Amine derivatives which are powerful antioxidants but which tend to cause discoloration of rubber during ageing.
b.Phenolic derivatives, which are generally less effective antioxidant but have the advantage of not causing discoloration. Phenolic type effective antioxidants, especially styrenated phenol, are widely used.
2.4.6 FILLERS
Fillers are added to latex .in order to modify its properties and reduce cost. No effect analogous to the reinforcement of dryrubber by fillers is observed when the same fillers are incorporated in latex compound. Precipitated silica, china clay, whiting and precipitated calcium carbonate are the important non-black fillers used in latex compounding.
2.4.7 SPECIAL ADDITIVES
Depending on the nature of the process, or on end use, ingredients such as gelling agents, foaming agents, flame proofing agents, tackifiers, colors, anti-tack agents areused12
2.5 Preparation of Aqueous dispersions.
Most of the solids in gradients of the latex is insoluble in water and hence the particles size of the ingredients should be reduced to that of the rubber particles.
The solid material is made to disperse in water by grinding action in presence of a dispersing agent. The dispersing agents preventsthe dispersed particles from reaggregation .For very fine particle size ingredients like zinc oxide the quantity of dispersing agents required is 1% by eight whereas for materials like sulphur , 2-3% by wt is required. Different types grinding equipments are Ball mills, ultrasonic mill, attrition mill, colloidal mills etc.
Out of these most widely used equipment is the ball mill. The efficiency of ball mill depends on the following factors;
a)Speed of rotation of the jar.
b)Size of the balls.
c)Materials of the ball.
d)Viscosity of slurry.
e)Ratio between the volume of the charge and of the balls.
f)Period of ball milling.
g)
1. Sulfur dispersion (50%)
Pbw (parts by weight)
sulphur100
Dispersol-F3
Water97
Ball mill for 48 hours
2.ZDC dispersion (50%)
ZDC100
Dispersol F2
Water98
Ball mill for 24 hours
3.Zinc oxide dispersion (50%)
Zinc oxide100
Dispersol F2
Water98
Ball mill for 24 hour
2.5.1 Quality of dispersions.
Consistency in the quality of dispersion is highly desirable and it is recommended that suitable test should be applied to dispersion immediately before addition to the latex.
A simple test to check the quality of dispersion is to add a drop of the dispersion into water taken in a tall glass jar .A cloudy appearance indicates good dispersion whereas rapid setting of the ingredients to the bottom of the jar shows poor dispersion
2.5.2Preparation of emulsion
Compounding ingredients which are water immiscible liquid should be emulsified in water before addition into latex. Eg for these type of compounding ingredients is antioxidant SP oil etc;-The emulsifying agent generally used is fatty acid soaps. The most stable emulsions are prepared in which the soap is formed in-situ. The following recipes are eg for preparing 50% anti-oxidant sp or oil emulsions.
Pbw
Part A
Oil100
Oleic acid2-4
Pbw
PartB
Ammonia (25%soln) 2-4
Water9-2
Part A and part B are separately heatedto about 500c and Bis then add to A in small quantities under high speed stirring. Emulsions should be prepared as and when it is required
2.5.3 Deammoniation of latex
Ammonia is the common preservative used for natural rubber latex. Hence sufficient quantity for ammonia is added to natural rubber latex before it is marketed.0.7 to 1% of ammonia is added to preserve latex for longer periods. In latex form, high ammonia content leads to uncontrolled gelling of the foam by the reaction between ammonia and ZnO. High NH3 content in dipping compounds leads to the formation of webs between adjacent protruding part, and also leads to frequent surface film formation particularly in compounds containing higher amount of ZnO.
Hence latex is deammoniated to reduce the ammonia content to say 0.2-0.3%.To reduce the ammonia content in latex, a current of moist air is blown over the surface of the latex while it is stirredat about 50 r.p.m.
2.6 Need of vulcanization
To increase the properties of NR, we have to cross-link different molecular chains together. This interlinking of polymer chains with or without bridging constituents is termed as vulcanization.
2.6.1 Method of vulcanization
Vulcanization is mainly done by using Sulphur as the vulcanizing agent. Accelerators increase the rate of Vulcanization. TO get good results, several other chemicals like activators, antidegradents etc. with sulphur or sulphur donors are added. In conventional vulcanization, vulcanization is done by adding excess sulphur and fewer amount of accelerators. In efficient vulcanization system, less amount of sulphur and more amounts of accelerators are added.
2.6.2 Drawbacks of sulphur vulcanizationIt is reported that SVNRL (Sulphur Vulcanized Natural Rubber Latex) products like examination gloves, surgical gloves, catheters etc. May cause health problems in some of the users. This is due to the residual traces of accelerators or chemicals, which causes allergy (Type IV). The residual proteins causes allergy (Type 1).
Moreover, nitrosamines are produced in the latex products by this nitrosation reaction of secondary amines generated from antioxidants or accelerators. These accelerators are carcinogenic. Hence it becomes necessary to develop a new technology to vulcanize NR latex to avoid the above health problems.
2.7 Development of radiation Vulcanization
The radiation vulcanization of natural rubber latex has been investigated for a long time since early 1960’s. The technique was not been used in industries due to the high cost of irradiation, low quality products and ambiguous advantages of the products. But recently, significant progress has been made in cost reduction and quality improvement in support of International Atomic Energy Agency(IAEA) and United Nations development program known as Regional Co- operative Agreement (RCA). In RVNRL, the cross – linking of rubber particles in natural rubber latex is brought about by Gamma radiation coming from a Co-60 source.
2.7.1 Evolution of radiation vulcanization
Rubber latexis anindustrial raw material for producing products like gloves, rubber threads, catheters, balloons, foam rubber, nipple etc. Rubber is to be vulcanized for ensuring proper dimensional stability of products manufactured out of it. Conventionally, vulcanization is carried out using sulphur and accelerators. This is a chemical reaction out of which long polymeric molecules of rubber are interlinked through sulphur bridges. The strength of rubber increases by vulcanization. This reaction is necessary in all rubber product manufacturing operations. Conventional vulcanization requires some chemicals which are allergic and carcinogenic (8). So possibilities of vulcanization without chemicals were explored and the evolution of radiation vulcanization is in this context.
2.7.2 Nature of latex used for radiation vulcanization
Latex collected from plantation will have a rubber content of 30-40%. For industrial uses, this latex is concentrated to 60% rubber content and this concentrated latex is used for RVNRL production.
2.7.3 Source of irradiation
Generally, the source of radiation is Co-60. It is a radioactive isotope of cobalt with an atomic weight of 60. It does not exist in nature. It is produced by bombarding a suitable nucleus of Co-59 with a neutron in nuclear reactor. The reaction is as follows.
27 Co 59+on1®27Co60
This Co-60nucleus being un stable, emits energy in the form of gamma rays and beta rays. But these beta rays are recaptured by the source itself.
27Co60 ® γ-ray +β-ray+ 28Ni60
The following table gives information about Co-60 source
Name
Cobalt
Symbol
Co
Atomic number
27
Atomic weight
58.993
Mass Number
59
Melting Point
1495 deg C
Boiling Point
2900 deg C
Crystal Structure
Hexagonal
Density (g/ml)
8.9
Wavelength of Gamma rays of 1.25 MeV (Co-60)
102-105 nm
Frequency of gamma rays
3x1022Hz
The gamma source is stored and shielded inside lead flask of specific thickness. Gamma radiations are high-energy electromagnetic rays with lower wavelength and penetrating power.
2.8 Production of Soluble Protein- free latex by radiation process.
During irradiation of NR latex for vulcanization, the latex proteins under go disintegration, which lives a high soluble protein content. In order to follow up the effects of radiation on NR protein, field latex was irradiated with gamma rays and the soluble protein concentration in the rubber phase and the scrum phase where analyzed. It was found that the water solubility of proteins in the latex increases with increasing dose. In the soluble protein content in the cream face (rubber) decreased whereas that in the serum phase increased with radiation dose SDS-PAGE analysis revealed that the 27 KD protein together with 14KD appear in the radiation vulcanized latex up to a radiation dose of 160 KGY and at 320 KGY in the disappear due to disintegration by radiation. A new process for the preparation of protein-free latex has been developed (3).
In the new process the radiation pre vulcanized centrifuged latex is subjected to dilution and then centrifuged in the case of field latex, it is irradiated first and then centrifuged after dilution. The new process results in prevulcanized latex almost free from solid protein. Tensile strength of the sample produced from the new process is comparable to that from the conventional radiation process.
2.9 Details of Radiation vulcanization.
Radiation vulcanization of natural rubber latex can be obtained by irradiating NRL without using Sanitizer. But the vulcanization dose (DV-the dose at which the maximum tensile property of irritated latex is found) needed will be about 200-300 KGY. This is too high to be used for industrial application. So it become necessary to degrees the dust to a considerable extent which is active by adding a suitable sanitizer.
2.9.1 Development of Sensitizer.
In order to decreases the doe to a considerable extent, it is necessary to add a suitable sensitizer. Requirements of a sensitizer are
Non-toxic
Easy availability and low cost
Easy removal of uncreated part.
2.9.1.1 Sensitizers used in radiation vulcanization
a)Halogenated hydrocarbon
Halogenated hydro carbon such as CCl4, CHCl3 etc were found to be good sensitizes. Five parts of CCl4 was necessary for 100 parts of dry rubber in latex for getting proper sensitization. CCl4 gave good Tb values for RVNRL film and DV was also reduced to 20 KG. But the toxic effect of CCl4 made it and unacceptable material for the industry. Also there are environmental problems like the destruction of stratospheric ozone layers by using halogenated hydrocarbons in large scale.
b) Polyfunctional Monomers (PFM)
Many PFM’s (containing more than two polymerisable C=C) can be used to enhance cross-linking of polymers. It was postulated that the sensitizing efficiency of PFM’s depends on its solubility in NR as well as on its reactivity with NR to be cross-linked.
AmongPFM’s, at first only divinyl benzene was investigated as sensitizer for RVNRL. But its sensitizing efficiency is not satisfactory. After that, neopentyl glycol dimethacrylate (ANPG) were suggested as sensitizers. But these diacrylate reduce the colloidal stability of latex. Moreover, it produces skin irritation and causes damage to the human body. So PFM’s also turned out to be unacceptable for RVNRL processing (2).
c) Monofunctional Acrylates (MFA)
MFA such as 2-ethyl hexylacrylate (2EHA) and n-butyl acrylate (n-BA) also accelerate RVNRL. The sensitizing efficiency increases with increasing molecular weight of MDA. Thus 2-EA was selected as sensitizer. But smell of the gloves was very bad due to the residual 2-EHA, which also causes skin irritation and other health problems. The attempts at remove residual 2-EHA, by drying failed due to its low vapour pressure. The complete polymerization of 2-EHA by irradiation was not practical because further irradiation results lowering of tensile strength (4).
2.9.1.2 Sensitizing action of n-BA
n-BA is selected as the suitable sensitizer for RVNRL because it can be easily removed by drying owing to its high vapour pressure. But the addition of n-BA increases the viscosity of NRL, sometimes causing coagulation. To stabilize NRL against n-BA, KOH is added as stabilizer. In practice, phr KOH is enough to stabilize the latex containing 5 phr of n-BA.
2.9.1.3 Mechanism of sensitizer
It is usually believed that polyfunctionality of the sensitizer is essential to enhance the radiation induced cross-linking. In PFM’s, the sensitizing mechanism consists of two steps:
-The first step is the graft polymerization of PFM into NRlatex, forming pendant chain containing many polymerisable C=C bonds.
-The second step is the polymerization of the C=C bonds.
But in the case of MFA containing only one polymerisable C=C bond, sensitizing occurs through other mechanism. The mechanism was investigated using a system of liquid polyisoprene (LIP) and 2-EHA is added to LIP to increase the ratio of gel formation and also to enhance the cross-linking of LIP through the polymerization of 2-EHA.
The sensitizing mechanism of MFA including its chain transfer is illustrated in the following equations:
ØRadiolysis of NR
RH®R٠+H
ØRadiolysis of water
H2O®H٠, OH٠, eeq
ØHydrogen abstraction
RH + OH٠® R٠+H2O
ØHomopolymerisation
nm ®P٠
ØGraft polymerization
R + nm ® P٠
ØChain transfer
P٠+ RH ® P + R٠
RP٠+ RH (RP + R٠)
R٠+ P (RP٠)
M٠+ RP (RP٠)
ØTermination
R٠+ R٠(R- R)
RP٠+ R٠ (RP – R)
Here, RH, M and P are MFA respectively.
P and RP are growing chain radical of monomer and rubber respectively.
Chain transfer to monomer may be illustrated as follows:
R٠ + SH2 = CHRH + CH2 = C٠
||
O = C - O - RO = C - O - R
2.9.1.4 Sensitizing mechanism of MFA (n-BA)
The mechanism of sensitizing action of MFA can be represented in the following manner.
CH2 = CH - COOR γrays( C٠H = CH – COOR + H٠
(acrylate radical)
As acrylate radical formed more mobile than the rubber chain; it attacks a double bond in the adjacent chain.
ØC٠H – C = CH - CH2 ---+--- CH2 – C = CH - CH2 ---
||
CH3CH3
CH3
|
( --- CH2 – C – C٠H - CH2 ---
|
--- CH - C = CH - CH2 ---
|
CH3
(Cross linked structure)
2.10 Anti-oxidant
The most effective way of improving the thermal and oxidative degradation of RVNRL is by adding suitable anti-oxidant. Several anti-oxidants are available for protecting RVNRL from thermal and oxidative degradation. The anti-oxidant selected should not cause any allergic reaction and generate nitrosamine, which are carcinogenic.
Hence TNPP may be used as an anti-oxidant. The structure of TNPP is given below.
Preparation of TNPP: 21g of TNPP is accurately weighed in a beaker and 2g of oleic acid is added to it. The mixture is heated until becomes free-flowing liquid. The 12.5ml of ammonia is diluted with 82.5ml of water. To this, well-cooled TNPP mixture is added with continuous stirring. This is further stirred for half an hour on magnetic stirrer until no oily appearance exists when added to water.
Actual effects of TNPP and other anti-oxidants on ageing property of RVNRL films were examined as measured ad measured by the retention of Tb and Eb of the film aged at 100 deg Celsius for 20 hours. The Tb and Eb of the films before ageing were 30 Mpa and 1020% respectively with 2phr TNPP. Also the aged film containing TNPP possessed better transparency and low discoloration than those films containing any other anti-oxidant. Consequently, TNPP was selected as the suitable anti-oxidant for RVNRL.
2.11 Radiation processing of NR latex
The following chart gives a brief picture of radiation vulcanization of NR Latex.
NR latex( Mixing( Irradiation
n-BAMixing (Radiation vulcanized NRL
(Leaching & drying)
Latex productAnti oxidant
Irradiation of latex is generally carried out at about 50% dry rubber content (DRC). But the cenex has a dry rubber content of 60% on adding sufficient ammonia water to cenex. We have to decrease the DRC to 50%. The typical properties of preserved field latex are given in the following table:
Dry rubber content
36.5 %
Ammonia content
1.33 %
Volatile Fatty Acid number
0.03 %
Magnesium content
53 ppm
2.11.1 Dose of irradiation
Dose can be defined as the energy absorbed per unit mass of the irradiated material at the place of intercept. Its unit is ‘rad’ (radiation absorbed dose)
1 rad = 100erg/g
SI unit of dose id J/Kg and is called ‘Gray’. The symbol is Gy.
100rads = 1Gy
When 1Kg of matter absorbs 1J of energy, then this material is said to have received a dose of 1Gray. When 1Kg matter absorbs 1000J of energy, it is said to have received a dose of 1Kgy.
2.11.2 Irradiation time
Irradiation time is calculated using the formula
Irradiation time = Dose / Dose rates
The value of dose rate changes from day today, which is obtained from the table. The dose rate decreases day to day due to the disintegration of Co-60.
2.12 Advantages of radiation vulcanization
Latex producers and product manufacturers were compelled to develop a new vulcanization method for overcoming problems of allergy and nitrosamine base toxicity. RVNRL is completely free from toxic chemicals and it is non-allergic. The chemicals used are KOH and n-BA. KOH is removed completely by leaching. N-BA itself polymerizes and the residue will be removed on drying due to high vapour pressure. The antioxidant added is tris nonylated phenyl phosphite (TNPP), which gives a transparent appearance to the RVNRL film (5) .The other advantages are the following:
ØLonger shelf-life period
ØLower modulus
ØBiodegradability
ØLess extractable protein content
ØLow emission of sulphur dioxide
ØLower ash content
ØNo problem associated with zinc contamination in the effluent generated
ØNo problem associated with chemical stability or zinc oxide thickening
2.13 Properties of RVNRL
The property of RVNRL, which is different from that of cenex used for irradiation properties of RVNRL, is given below.
DRC
60.45 %
TSC
62.0 %
NH3
0.72 %
VFA Number (Volatile Fatty Acid number)
0.015
Mechanical Stability Time at % TSC
1320 Sec
Coagulum content
0.009
2.14 RVNRL films
RVNRL films can be prepared by casting or dipping. After casting the film, it has to be leached for sufficient time to extract proteins, which are degraded and changed into soluble proteins by the effect of radiation. Otherwise these proteins will cause cytotoxic problems. Generally, films are leached in cold water for 4hours. After leaching, the film shows excellent physical properties (7). RNRL films containing 2phr TNPP as anti-oxidant show very good ageing behavior. The tensile properties of RVNRL films are shown in the following table.
Property Before ageing
100 % Modulus ,(MPa)
0.74
300 % Modulus ,(MPa)
1.32
500 % Modulus ,(MPa)
2.2
700 % Modulus ,(MPa)
6.35
Tensile strength , (MPa)
25.4
Elongation at break, (%)
1050
2.14.1 Cytotoxic behavior of RVNRL films
Conventional sulphur vulcanizates contain Zinc dithiocarbamate residues which cause cell damage RVNRL films do not contain dithiocarbamates.
Quantitative cell cytotoxicity evaluation by a ‘colony separation assay method’ indicated that the cytotoxicity in RVNRL films is much weaker than those of conventional sulphur vulcanized films.
The cytotoxic potential of a material is expressed as the extract concentration which suppresses the colony formation to 50% of the control (IC50). Sulphur vulcanized natural latex have low IC50 values, i.e., higher cytotoxicity than RVNRL films. Again cytotoxicity of RVNRL films decreases with increasing leaching periods. Thus RVNRL films are much safer than sulphur prevulcanised latex.
2.14.2 Extractable Protein Contents in RVNRL Films
A film of RVNRL which has not been leached shows a high level of extractable protein. This is because irradiation facilitates the dissolution of proteins on NR latex particles by breaking the polypeptide chains.
The allergic reactions due to the soluble proteins were investigated by the Passive Cutaneous Anaphylaxis (PCA) test. The PCA test demonstrates weaker allergenicity (6) for RVNRL films than sulphur cured latex films. It has been reported that more that 90% of the total extractable proteins in RVNRL films could be removed by extraction with 2% ammonia solution for 24 hours. However such long leaching is impossible for industrial purposes.
The residual extractable protein content in RVNRL products can be reduced by prolonging the leaching period and/or increasing the temperature of the leaching bath.
2.15 Advantages of RVNRL over SVNRL
ØAdvantages of RVNRL over NR latex rubbers vulcanized using sulphur are
ØAbsence of toxicity (free from nitrosamines and accelerator induced allergies).
ØAbsence of sulphur and zinc oxide.
ØBetter latex stability (longer shelf life).
ØLower modulus (suit the requirements for specific applications).
ØBetter product clarity (better coloration).
ØEnvironment-friendly process (non-polluting effluent).
ØLower ash remain (reduce environment problem).
ØTransparency.
2.16 Application of RVNRL (9)
a) Examination Gloves
b) Condoms
c) Toy Balloon
d) Catheter
e) Other medical and pharmaceutical products
2.17 Reasons for the huge potentiality of RVNRL
1) Absence of nitrosamines.
RVNRL does not contain any vulcanization accelerators based on dithiocarbamates or thiurams that can liberate volatile nitrosamines. Many of the volatile nitrosamines are proved to be carcinogenic.
2) Very low cytotoxicity.
Leached RVNRL films are almost pure cross-linked rubber containing only small amounts of anti-oxidants. Hence RVNRL films exhibit very low cytotoxicity.
3) Low emission of SO2 and less formation of ashes on burning.
In sulphur vulcanized films, sulphur to the extent of 0.5-3.0phr is added (depending on the cure systems).Hence sulphur vulcanizates on burning liberate substantial quantities of SO2. Due to the presence of other mineral matter, sulphur vulcanizates also leave ashes on burning. However RVNRL films contain no sulphur or chemicals and hence produce very little SO2 and ashes on burning.
4) Transparency and softness (low modulus).
Due to the absence of any added chemicals, thin films of RVNRL are highly transparent. RVNRL films are generally softer due to their lower modulus.
5)Biodegradability on weathering.
Sulphur vulcanizates contain residues of accelerators which have some bactericidal activity and hence sulphur vulcanization offers some resistance to biological degradation. Since RVNRL is free of any residual chemical, they easily undergo biodegradation.
6) Less extractable protein content.
During irradiation, proteins are degraded and hence more water soluble. During leaching, they are removed. Hence leached RVNRL films contain less extractable proteins and thus cause less allergic reactions.
7)No problems associated with chemical stability or ZnO thickening.
ZnO is generally added as an activator to vulcanization in conventional vulcanization systems. However, ZnO can dissolve in the aqueous phase of latex and to cause thickening of latex. In RVNRL, no ZnO used and it is free from problems that generally accompany.
8)No problems associated with Zn contamination in the effluent generated.
Dissolved Zn is considered to be a pollutant in waste water. In a latex product manufacturing unit using RVNRL, the waste water generated is free of dissolved zinc
2.18 Features of Pilot for the Radiation Vulcanization ofLatex
The completion of the latex irradiator at Kottayam has marked the beginning of “Radiation Induced Vulcanization” as an industrial process in India. This pilot plant is the result of the collaborated efforts of BARC, the multi-disciplinary research center and the Rubber Board, the prime organization promoting the rubber industry in India.
The basic requirements of an irradiator are:-
* A source for irradiating the product
* Biological shielding for protecting the plant personnel during irradiation
* A shielding to the source when not in use
The source used for the latex irradiator is Co-60 and the biological shielding is by concrete. The gamma source is stored and shielded under water and is brought up for irradiation. It is a batch process where the latex is irradiated in a vessel with stirrers.
GAMMA CHAMBER 5000
Description:
Gamma chamber 5000 is a compact self shield cobalt-60 gamma irradiator providing an irradiation volume of 5000 cc.Gamma chamber can also be used in many other research applications which require irradiaton of materials with ionizing radiations to varying doses.
Specifications of gamma chamber 5000
vMaximum Co-60 source capacity: 444 TBq(12000 Ci)
vDose rate at maximum capacity: 0.9 Mega Rad/hr at the center of sample chamber
vDose rate uniformity: +25% or better radially
-25% or better radially
vIrradiation volume: 5000 cc
vSize of sample chamber: 17.2 cm(dia) X 20.5 cm(ht)
vShielding material: Lead and stainless steel
vWeight of the unit: 5600 kg
vSize of unit: 125 cm X 106.5 cm X 150 cm
vTimer range: 6 sec onwards
Irradiation cell and concrete shielding:
The irradiation cell is 4m x 4.5m with the storage pool and product movement space. To minimize the streaming of dose near the personnel entry door, the access corridor to the cell is made in the form of a maze (labyrinth) of width enough for regular transport of latex vessel and the source cask when required.
Cement concrete (2.35g/cc) is used in the construction of biological shield. The concrete wall separating the cell and the laboratory is 1800mm thick. The thicknesses of other walls are decided by their number in a direction and their distance from source.
From the safety point of view, the irradiator can be divided into three zones, viz;
a) Controlled areas namely cell, labyrinth and cell roof where entry is prevented during irradiation
b) Restricted areas namely control room, service room, and de-mineralization plant room.
c) Unrestricted areas namely latex handling area.
Source Storage Water Pool:
The requirements of the pool are:-
a) To accommodate the source trolley and guide rails.
b) To accommodate the source cask at the time of source handling.
c) To provide enough shielding during storage and source loading operation.
To meet the above requirements, the ceramic tile lined pool is designed to have the dimensions 2.6m X 2m X 6.3m depth.
Latex Vessel and Transportation:
The NRL is irradiated in a trolley mounted stainless steel vessel of 1000litre capacity. It has a central hollow portion meant for the Co source during irradiation. Rails and turntables are laid for easy, manual movement of latex vessel from the handling area to the cell. The rails are extended over the pool. The final position of the latex vessels in the top of the pool; at the bottom of which the Co source is normally stored. The trolley of the latex vessel is aligned with the source position using mechanical fixtures. Three stirrers driven by a remotely located motor, maintain the homogeneity of the latex.
Radiation Source Assembly:
Radiation source assembly is in the form of a cylindrical cage with 12 positions for the source units. Each unit is about 460mm in length and 27mm in diameter. It can contain 100kCi of Co-60 normally (currently the irradiator at Rubber Board contains only about 23kCi of Co-60. These integrated source units for wet storage are fabricated and tested as per ISO-2919-1980 by BRIT and are used in all the wet storage type irradiators used in India. The sources transported on type B(U) package is taken on rail mounted trolley and taken to the cell where it is lowered into the pool. The source loading operation is done remotely with special tools. The sources are firmly fitted on the source cage. The cylindrical source cage of 200mm diameter is mounted on an underwater trolley.
Source Hoist System:
The underwater trolley, with the source assembly mounted on it, is fitted with 8 wheels to move on four guides. The hoisting is achieved using a set of pulleys and a pair of wire ropes actuated by gravity. The central hollow portion of the latex vessel as well as the source cage is designed to have a safe clearance during source hoist. The hoist system is working with a velocity ratio of four.
Ventilation:
Adequate ventilation is provided for the removal of ozone and oxide of nitrogen from the irradiation cell. The system consists of an axial flow fan (5000cubic meter/hour) with connected ducts. Two inlets are provided in the cell at different elevation while the exhaust outlet is 3m above the building.
Controls and Interlocks:
Control system is based on relay logic and mechanical, hydraulic and electrical end elements. It has displays for knowing the source, door and other important parameters. Distinct audio alarms operate during the source operation, source transit and emergency conditions. The control system incorporates:-
Procedural sequence for source raising
Source lowering conditions
Safety interlocks
Procedural sequence for source raising consists of
1)Monitoring the preconditions and safe levels of pool water, radiation etc.
2)Search operation
3)Source raising
Conditions for source raising consists of
1)Failure of power, ventilation, hydraulic pump etc.
2)Breach of any interlock
3)Actuation of source disable switch, tripwire or emergency push button
Safety interlocks consists of
1)Source door interlock
2)Hydraulic bypass valve-latch bar interlock
3)Radiation level-door interlock
4)Pool water level interlock
5)Ventilation interlock
6)Latex vessel alignment interlock
7)Rope stretch interlock
8)Smoke/fire detector interlock
3 MATERIALS AND EXPERIMENTAL TECHNIQUE
3.1 NR LATEX
The preserved field latex was obtained from Pilot Latex Processing Center of RRII – Chethakkal – Ranni – Kerala.Centrifuged latex was prepared at PLPC using L-D-Lavel latex centrifuging machine, operating at 7000 rpm.
3.2 Preparation of latex for RVNRL processing
(1)Collect field latex from only normally tapped trees to avoid higher non-rubber constituent and higher bacterial population in the field latex.Before collection, the buckets and barrels are to be washed thoroughly with water and then rinsed with formalin.
(2)Filter the field latex using a 40 mesh sieve into clean and rinsed barrels.
(3)Preserve the field latex using ammonia at the rate of 1.2% on the weight of latex.
(4)Transfer the PFL into desludging tanks and after determining the magnesium content and the required amount of DAHP such that the magnesium content in the processed latex should be below 10ppm.Stir the latex and sand for 30 minutes
(5)Add ammonium laurate at 0.02% on the weight of field latex and stir.
(6)Allow to stand for a minimum period of 24 hours for every meter of depth.
(7)Then centrifuge the latex and add ammonium laurate at 0.05% on the weight of the cenex.Bring the ammonium content of the cenex to 0.7%.
(8)Adjust the DRC to 60%.
(9)Transfer the cenex into barrels that are property cleaned with water and rinsed with 1% formalin.
(10)Store the barrels away from exposure to sunlight.
(11)The optimum period required for the cenex is 20 days from the date of collection of field latex to the date of processing of RVNRL.
Specifications of HA cenex for RVNRL processing
Properties
Value
Dry rubber content ,% min
60.0
Non-rubber solids, % max
2.0
Alkalinity as NH3 ,% min
0.70
Mechanical stability time, sec
1200 sec
KOH number max
0.6
VFA no, max
0.02
Coagulam content, % max
0.001
Sludge content ,% max
.010
Copper content in ppm
0.8
Manganese content in ppm, max
10
Magnesium content in ppm, max
50
Colour on visual inspection
No pronounced blue or gray
3.3 Irradiation
HA centrifuged latex having a DRC of 64%, 62.9% and 62.4% is mixed with 0.3 phr of 50% KOH as stabilizer and 0.01% ammonium laurate.The DRC is decreased to 55% by adding 1% ammonia water.Then 5 phr normal butyl acrylate (n-BA) as 50% emulsion is added as sensitizer with stirring.
The mixture is stirred for 30 minutes (The total quality in 2 litres).This is then kept for irradiation in the gamma chamber.5000 unit (self shielded cobalt 60 irradiator) by giving a dose of 15 KGy.
Eg: Sample No: 4
Initial DRC = 64
HA cenex = 1687.5 g
Dry weight = wet weight X DRC/100
=1687.5 X 64/100
Dry weight = 1080 g
50% KOH (3 phr) = 6.48 g
Ammonium laurate (0.01%) = 0.54 g
Ammonium water (1%) = 161 g
Stir these for 30 minutes
Then add
50% n-BA emulsion
n-BA emulsion was added in small quantities directly to the latex while latex was being stirred efficiently for 30 minutes.The prepared latex was kept for 24 hours before irradiation.
Irradiation time = Dose / Dose rate
= 15 KGy / 1.265 KGy/hr
= 11 hours 50 minutes 57 seconds.
After irradiation 2 phr antioxidant tri nonylated phenyl phosphate (TNPP) is added and kept for overnight.
3.4 Preparation of RVNRL films.
The films are casted by pouring the irradiated latex on a leveled glass plate of suitable dimension.After spreading the latex, the excess latex is poured out and the glass plate containing latex is kept overnight for drying.The films are taken out from the glass plate by dusting.The film is dusted to prevent adhesion to itself and to other articles.Tale was used for dusting, and then leached in cold water for 4 hours, dried at room temperature till transparent and again dried at 700 c for 4 hours in hot air oven.After that it is taken out and kept in a decicator.The film to be tested is cut into dumbbell strips.The physical properties of casted films were determined in each case using UTM (Universal Testing Machine)
3.5 Test for latex
1)Total solids content
2)Dry rubber content
3)Ammonia content
4)Mechanical stability time
5)Viscosity
1) Total solids content
It is defined as the amount in grams of total solids present in 100 g of latex.
Procedure:
Weight correctly clean petridish (w1) grams.Add about 2 gms of latex into a petridish and again weight correctly (w2) grams.Gently swirl the dish so that the latex is distributed over the bottom of the dish and dry the specimen in an oven at 70 ± 20 C for 16 hours or 2 hours at 100 ± 20 C.Cool the sample to room temperature and weigh again (w3).The total solid content is calculated as
TSC = (w3 – w1) X 100
(w2 – w1)
W1-weight of Petri dish
W2-weight of Petri dish + sample
W3-weight of Petri dish + decide sample
2) Dry rubber content
The quantity of rubber present in latex is calculated from its dry rubber content (DRC).This is defined as the quantity in grams present in 100gms of latex.The DRC of latex falls in the range 30-40.
Procedure:
Standard laboratory method:
About 10 – 15 gms of representative sample is taken in a container from a stopped 50 ml conical flask.The latex is coagulated with sufficient 2% ascetic acid and heated over a steam bath until a clear serum is obtained.The coagulam is thoroughly washed, rolled to a thin film of around 2mm and placed in a thermostatically controlled over about 700 C for 16 hours.The dried rubber obtained is cooled in a desicator and weighed in a chemical balance.
DRC (%) = weight of rubber X 100
weight of latex
= g/w X 100
Non-rubber content:
It is calculated from subtracting dry rubber content from total solid content.
Non-rubber content =TSC – DRC
3) Ammonia content
It is defined as the quantity of grams of ammonia present in 100 g of latex.
Procedure:
About 5 – 10 gms of latex in a 50 ml stopped conical flask is accurately weighed and transferred to a 250 ml beaker containing 150 ml distilled water and correct weight of latex transferred is assessed by the difference method, while transferring latex to the beaker.Add 6 drops of 0.1 % alcoholic solution of methyl red to the latex sample, in the beaker and titrate with 0.2 N hydrochloric acid until the colour changes from yellow to light pink.Ammonia content is calculated as:
Ammonia content (%) = 1.7 X N X V
W
N – Normality of Hcl
V – Volume of Hcl
W – Weight of latex in grams.
4) Mechanical Stability Time (MST)
Latex is stirred at a high speed (14000 rpm) and the time taken to produce visible signs of clotting is recorded as a measure of mechanical stability.
Procedure:
Dilute 100 gm of well mixed sample of concentrated latex to 55.0 ± 0.2 percent total solid content with appropriate quantity ammonia solution.Warm on a water bath to 36 – 370 C.Filter through a viol cloth or stainless steel 80 mesh sieves and weigh 80. 0 + 0.5 gm into the testing container.Check that the temperature of the latex is 35 + 10 C.Place the container in position and start the machine.Make sure that the speed of the machine is 14000 ± 200 rpm.The arrival of the end point is preceded by a marked decrease in level of latex in the container.Determine the end point by dipping the clean glass rod into the latex to about 15 mm and drawing it only once gently over the palm of the hand.
5)Viscosity
The viscosity of latex sample is measured by means of Brookfield viscometer (model LVT) at 25 ± 20 c.
Procedure:
Determine the TSC of the given latex sample after sieving through a voilcloth or 40 mesh sieve.Adjust the TS of latex to 60% by adding distilled water and cooled to 25 ± 20 C.Remove the bubbles on the surface of the latex using a filter paper.Measure the viscosity with spindle no: 2 at rpm 60 using digital or analog type Brookfield viscometer.
3.6 Testing of the final articles
The films are prepared from RVNRL was tested for specification as per IS 4770-1991 (10).
Classification of test
The following shall consist to type tests.
* Tensile Strength
* Nitrogen Content
Tensile Properties
The tensile properties where determined from dumb-bell samples. The samples were continued at room temperature and tested on a universal testing machine. The modeless, elongation at break and tensile strength were recorded. The test was continued on samples before the ageing and after ageing. A dumb-bell specimen is shown below.
r
A= overall length;
B= width of end;
C=Length of narrow parallel portion;
D= Width of narrow parallel portion,
R = Large radius,
r= Small radius.
3.6.1 Tensile Strength
The tensile stress requires to stretch the test pier to the breaking point, the conditions of being such that the stress is substantially uniform over the cross section.
Principle of method: -
In this test the dumb-bells are stretched in a tensile testing machine at a constant rate of transverse of the driven grip. Reading of load and elongation are taken as required during the uninterrupted stretching of test piece and when it breaks.
The experiment was conducted on RVNRL. The required dumb-bell samples were cutout and subjected to test.
vTemperature of the test; the test was carried out at 27+2c
Experiment: - The dumb- bell was inserted into the grip of the tens ting machine, with the samples arranged symmetrically so that the tension will be uniform over the cross section. The machine was started and the distance betweens the centers of the reference lines was measured until the test piece breaks.
3.6.2 Nitrogen content
Deammoniation of nitrogen content by ‘kjeidhal method’.
Reagents :
- 0.1g sample
- Conc:H2SO4
- 600/0 NaOH
- Catalyst mixture (15g K2SO4 +2g Cu (SO4)2) +1g
Selenium)
- 2 0/0 boric acid
Indicator (methyl red –methylene blue)
Apparatus:
Semi – micro – kjieldhal digestion and distillationapparatus.
- 5ml burette calibrates at every 0.02ml.
Procedure:
A small amount of the sample was digested by mixing sample, catalyst mixture, and conc:H2SO4 in a beaker when the mixture is clear, the sample is digested. Transfer digested sample to a distillation flask. Add 10ml of 600/0 NaOH solution and distill. The distillate is collected in a conical flask containing 10ml of 20/0 boric acid and titrate against 0.1 NH2SO4. Indicator used ismethyl red –methylene blue.
3.7 MACHINES
* Universal Testing Machine
The HSK –S Universal Testing Machine is designed to test a wide spectrum of materials such as plastic, rubber, yean, adhesives, finished components, wire, paper, foil, food, textiles, etc, in tension compression, flexure or shear. This machine is accurate and easy to use and its capability is enhanced by direct connection of a pointer through which a comprehensive test report and high resolution group can be quickly obtained. In this designing of this system, extemomus capability is assured. Contracting and non- contracting extensometer can be supplied by this system.
Universal testing machine
This testing machine can be connected to a PC and can be operated by windows bared software. The most complex test can be made simple and access to comprehensive data analysis is possible.
Control Unit
Powerful built in machine function are accessed through the robust control unit with its large, easy to load, back lit crystal display and alpha numeric keypad.
Functions include:-
- Real time test graphic display
- Storage and retrieval of test profiles
- Digital display of force and displacement at any instant
Through out the test
- Manual cross head, up, down stop and jog control
- Dedicated functions keys for fast access
- Input of test data and formatting test report.
Features of machines are:
- Easy to read back lit liquid crystal display
- Graphic or numeric display mode
- Alpha numeric data entry
- High speed return, 1500mm/min
- Cross head job facility
- Robust load frame
-Test report of result with mean, median and standard
deviation
-PC control unit
Graph and test report
Testing machine is connected directly to a printer. A test report and high resolution can be obtained with in 15 seconds by pressing a single key. Results may be measured and calculated from the printed graph.
*Particle Size Analyzer
Particle Size Analyzer
Particle size of slurry and latex was determined based on the dynamic light scattering techniques using a MALVER nanosizer UK. Filler dispersion was studied on vulcanized using dispersion analyzer from tech pro USA. The mechanical properties were determined from relevant ASTM standards (tensile properties ASTM D412-92, Heat built up ASTM D623-G3, Abrasion Resilience ASTM D5963-96, hardness ASTM D2240-95, Compression set ASTM D395-89 resilience. ASTM D2632-92, Tear strength ASTM D624-98). The ageing tests were carried out according to ASTM D 573, after ageing at 1000C for 3 days. SEM study was conducted by using a Hitachi Scanning Electron microscope (model 2400).tensile fracture surface of vulcanizates was coated with gold to carryout SEM study.
4RESULTS AND DISCUSSION
4.1. Effect of green strength of cenex on quality of RVNRL
4.1.1Particle size and green strength
Latex is a colloidal dispersion of rubber particle in an aqueous serum. Each rubber particles contains few to several hundred rubber molecules. Cross linking of these molecules can also occur while latex is in the tree.
The latex contains rubber of various particles size. During centrifugal concentration of ammonia preserved NR latex comparatively bigger molecules separate as cream fraction and smaller molecules as skim fraction. There is variation in the size of rubber molecules and amount of non-rubber ingredients for the two fractions of the NR obtained during centrifugation.
In the centrifuge during the high speed of rotation the rubber particles are subjected to centrifugal force. Like creaming the process of centrifugation is opposed by Brownian motion of dispersed particles. Eventually equilibrium of particle concentration with the distance from the centrifuge axis is established between the opposing tendencies of separation and dispersion.
The average speed at which a particle moves through the dispersion medium during centrifugation depends upon balance between centrifugal force and viscous drag.
dR =(ρ-σ)ω2R.x2
dt18 η
ρ-density of serum
σ-density of rubber particles
ω-angular speed of the particle
R-radius of the particle path
x-diameter of rubber particles
η-viscosity of serum
For a spherical equation (4) shows that speed of movement of rubber particles by a centrifugal force is directly proportional to the density difference between rubber particles and dispersion medium (serum)
Thus it is possible to get centrifuged latex fraction containing comparatively only bigger particles with lower proportion of the smaller particles. Among the more important factors which affects the composition of concentrated latex in relation to preserved field latex (incoming Latex) are
vFeed rate
vRotational frequency of centrifuge
vLength of regulating screw.
The length of regulating screw can be varied by inserting screws of selected length into the skim discharged orifice. In this way fine control can be exercised over the equilibrium difference between the density of discharging skim and that of cream. A shorter screw length encourages a greater difference in density between skim and cream and hence the cream tends to have increased rubber content and the proportion by volume of skim also increases. Latex of varying particles size is obtained by using skim screw of 9 cm, 10.5 cm, and 11 cm. Thus cenex using a short screw of 9 cm has a higher DRC than those prepared using screws of 10.5 cm and 11 cm length.
Table 1: Initial latex properties
ParameterLatex samples
Cenex1
Cenex 2
Cenex3
Dry rubber content,%
64.30
63.08
62.38
Total solid content,%
65.56
64.22
63.72
Non-rubber solids,%
1.26
1.14
1.34
Alkalinityas ammonia,%
0.58
0.57
0.56
Mechanical stability time, sec
1053
749
740
Volatile fatty acid number
0.01
0.01
0.01
KOH number
0.49
0.52
0.53
PH
10.25
10.25
10.25
Brookfield viscosity Spindle
No:2, 50 rpm,MPa.s
304
224
200
Magnesium content, ppm
Trace
Trace
Trace
The particle sizes of the different fractions are shown in figures (1) and (2). The particle size varies from 200 nm to 300 nm for concentrated latex and from 60-200 nm for skim latex and is close to the values reported earlier Cenex 1 contains comparatively higher particle size as compared to Cenex 2 and 3. Cenex 2 contains comparatively lower particles compared to cenex 3. It is also known that molecules present in bigger particles are highly branched and that in smaller particles are linear. So rubber molecules in latex concentrate is highly branched and molecules in skim latex comparatively linear with a higher proportion of non rubber ingredients (11, 12, and 13). Hence it is expected that the level of branching follows the order 1>3>2. The variation in particle size between 2 and 3 are less as seen from figure 1.
Cenex 3
Cenex 2
Cenex 1
Figure 1: particle size distribution of cenex
Skim latex 3
Skim latex2
Figure 2: particle size distribution of skim latex
4.1.2 Raw Rubber Properties and Green Strength.
Raw rubber properties of the 3 skim fractions and latex fractions are shown in Table 2.Cenex contain lower nitrogen content, lower Ash content, and lower Acetone extract and lower PRI compared to other fractions. The variation in initial plasticity is very less among the three samples, which is prominent for samples, which is subjected to acetone extraction.
A rough estimate of the molecular/ branching can be obtained from initial plasticity values. Cross linking reaction involving proteins also contribute to enhancement in initial plasticity. This reaction leads to formation of a gel fraction. (Fractions insoluble in solvents )in latex .Cross linkingtake place by polar interactions between the proteinacoeusmaterials and oxygen groups on thepoly isoprene molecule .The higher initial plasticity for cenex2and 3 can be due to cross linking reactions involving protienacousmaterialscenex 2and 3 contain highernitrogen contentand higher acetone extractable(2). As protein also act as aantioxidantslatex containinghigherproportion ofprotein shows higher PRI. Comparatively higherinitialplasticity and PRIis shown by skim rubbers and is attributed to higher proportions of proteins .Hence it is expected that based on particle size cenex 1 has a higher molecular weight/branching compared to cenex 2 and 3. The green strength is also expected to be higher. As the skim fraction has smaller particles it is expected that non rubber ingredients present are also higher.
Table 2: Raw rubber properties
Parameters
Cenex
1
Cenex 2
Cenex 3
Skim 1
Skim 2
Skim 3
Nitrogen content,% w/w
0.35
0.45
0.41
2.36
2.78
3.02
Ash content ,% w/w
0.05
0.07
0.07
0.35
0.38
0.34
P0
34
38
37
42
53
48
P30
7
9
8
10
19
15
PRI
21
24
22
24
36
21
P0 of Acetone extracted sample
8
9
7
31
47
31
Acetone extract,%
1.72
2.02
1.96
4.61
8.56
7.51
The green strength of three cenex samples are shown in Table 3.
Table3. Green strength of three cenex samples
Latex sample
Tensile strength, MPa
300% modulus, MPa
500% modulus, MPa
Elongation at Break, %
Cenex 1
2.29
0.34
0.38
1741
Cenex 2
1.82
0.36
0.39
1599
Cenex 3
1.53
0.42
1.45
1459
As seen comparatively higher tensile strength and elongation is shown by cenex 1. A higher modulus shown by cenex3 can be due to cross linkingreactions involving the proteins. Green strength is reported to increase with nitrogen content and then tend to decrease with further increase in nitrogen content16. Thuslatexcontaining very low or very high contentof protein can have lower green strength .The effect of protein on strength of RVNRL is not fully understood .However there are reports to show that tensile strength of latex films irradiated after leaching was lower than that recorded without leaching(17,18) .
4.1.3 Effect of green strength on RVNRLflims
A comparatively higher tensile strength is obtained for cenex of high initial strength
Tensile strength of RVNRL obtained on cenex stored for 22 days is shown in table 4.Tensile strength of cenex 1 is greater that of 2 and 3.This is because green strength of cenex 1 is higher. There are earlier reports that cenex of high green strength produces RVNRL of high tensile strength (14, 15). On exposure of latex to gamma radiations cross-links occur between molecules by free radical formation which results in enhanced tensile strength. Radiation cross linking of NR chains can be enhanced by addition of n-butyl acrylate (n-BA).
Table 4. Tensile properties of RVNRL prepared from cenex stored for 22 days
Latex sample
Tensile strength, MPa
300% modulus, MPa
500% modulus, MPa
Elongation at break ,%
Cenex 1
23.50
1.15
1.60
1448
Cenex 2
21.01
1.15
1.67
1322
Cenex 3
22.09
1.19
1.88
1282
Schematic representation of cross linking inside a rubber particle by the use of n-BA is shown
Gamma Radiation
Latex particlePresence of n-BA
Vulcanized Latex ParticlesGraphted polymer chain
The properties irradiated latex is shown in Table: 5
Table 5. Latex properties irradiated after 22 days
Parameter
Latex samples
Cenex 1
Cenex 2
Cenex 3
Dry rubber content, % w/w
56.15
56.63
55.65
Total solids content, % w/w
58.29
57.9
57.62
Non-rubber solids, % w/w
2.14
1.27
1.97
Alkalinity as ammonia, % w/w
0.55
0.56
0.47
Mechanical stability time, sec
2485
2511
2592
pH
9.7
9.7
9.7
Brookfield viscosity, Spindle No:2,
50 rpm, MPa.s
132
120
124
As observed from table 5, RVNRL has a higher total solids and DRC suggesting some amount of polymerizations has occurred during irradiation. Due to the presence of fatty acid soaps high MST is recorded by all samples. n-BA undergoes both hydrolysis and polymerization during irradiation.
4.1.4Effect of unpolymarised n-BA on quality of RVNRL
Some n-BA is likely to remain unpolymarised during irradiation and due to this RVNRL generally has an unpleasant odour. This is because of the presence of residual n-BA. Reports show that n-BA get hydrolyzed when RVNRL is heated at 80ºC for 4 hours leaving no residual n-BA in RVNRL. On heating films at 100ºC it is observed that there is an increase in tensile strength, modulus and elongation at break. This suggests that the post vulcanization process also increases the tensile strength as shown in table 6.
Table 6. Tensile properties of post vulcanized RVNRL films
Latex sample
Tensile strength, MPa
300% modulus,MPa
500% modulus,MPa
Elongation at break, %
Cenex 1
25.72
1.05
1.47
1370
Cenex 2.
23.55
1.01
1.80
1450
Cenex 3.
28.62
1.06
1.54
1415
4.1.5Effect of immersion in calcium nitrate solution on RVNRL Films
Effects of leaching in calcium nitrate solution onquality of RVNRL films are shown in Tables 7a, 7b and 7c.On leaching the films in Calcium nitrate solution there is an increase in the tensile strength o n RVNRL films in Table7. As the concentration of calcium nitrate solution increase, the tensile strength does not increase but there is a slight increase in modulus. The way in which RVNRL is converted to films like through drying of cast films, or drying of coagulant dipped films is well known to affect the properties of dry films. The tensile strength of NR films depends on the degree of cross linking include chemical and physical cross links. Inter particle cross linking can improve properties of RVNRL. This cannot be attempted during irradiation as this can lead to coagulation of latex. When films are immersed in salt solution it is possible that calcium ions participate in an inter particle cross-linking process and consequently there is an increase in tensile strength.
Ca++
Coo- Coo-
Coo- Coo-
Ca++schematic representation of inter particle cross-link formation through Ca++
Table7a. Effect of leaching in Ca (NO3)2 on quality of RVNRL – Cenex 1
Leaching condition
Tensile strength,MPa
300% modulus, MPa
500% modulus, MPa
700% modulus, MPa
Elongation at break,%
Un leached
23.39
1.15
1.62
2.7
1450
2.5%, 1 h
25.6
1.23
1.63
2.88
1460
5%, 1 h
23.75
1.19
1.66
2.83
1400
10%, 1 h
22.47
1.68
1.71
2.96
1350
Table 7b. Effect of leaching in Ca (NO3)2on quality of RVNRL – Cenex 2
Leaching condition
Tensile strength,MPa
300% modulus, MPa
500% modulus, MPa
700% modulus, MPa
Elongation at break,%
Un leached
21.01
1.13
1.66
2.8
1320
2.5%, 1 h
21.74
1.15
1.57
2.68
1370
5%, 1 h
20.04
1.0
1.45
2.42
1300
10%, 1 h
20.8
1.2
1.7
3.07
1320
Table 7c. Effect of leaching in Ca (NO3)2 on quality of RVNRL – Cenex 3
Leaching condition
Tensile strength,MPa
300% modulus, MPa
500% modulus, MPa
700% modulus, MPa
Elongation at break,%
Un leached
20.82
1.19
1.88
3.14
1280
2.5%, h
22.88
1.21
1.72
2.89
1400
5% , h
22.00
1.20
1.70
2.89
1335
10%, h
23.57
1.23
1.82
2.90
1411
Post Vulcanization of RVNRL films after leaching in 2.5% Ca(No3)2 solution is given in Table 8.
There is an increase in tensile strength after post vulcanization for all three RVNRL samples.
Table 8. Effect of leaching and post Vulcanization of RVNRL films, (1000C/24 hrs)
Latex Sample
Tensile strength,MPa
300%
modulus, MPa
500% modulus, MPa
Elongation at break,%
Cenex 1
28.66
1.05
1.60
1500
Cenex 2
24.88
1.00
1.40
1540
Cenex 3
27.33
1.20
1.80
1450
4.1.6Accelerated ageing of RVNRL films
Post vulcanized RVNRL films were aged at 100ºC for 24 hrs and results are shown in Table 9. It is observed at that comparatively good retention of properties are obtained for the RVNRL films.
Table 9. Tensile properties of post vulcanized RVNRL films aged at 1000C/24 hrs
Latex Sample
Tensile strength,MPa
300%
modulus, MPa
500% modulus, MPa
Elongation at break,%
Cenex 1
22.63
1.43
1.87
1463
Cenex 2
21.12
1.40
2.13
1493
Cenex 3
22.15
1.72
2.81
1539
4.1.7Effect of maturation of cenex on production of RVNRL
Radiation vulcanized natural rubber latex was prepared using cenex 1 after maturation for 7,22 and 35 days.It is observed that latex matured for about 22 days give better tensile strength as compared to fresh cenex.Several reactions take place during storage of latex. Hydrolysis of phospholipids along with protein are two prominent reactions. Along with this there is micro gel formation also. These lead to increased tensile strength. The tensile strength obtained afterstorage of cenex for different durations is given in Table10 and a lower tensile strength is shown by latex stored for 35 days as compared to others.
Table 10. Effect of maturation of cenex 1 on tensile strength of RVNRL films (Before post vulcanization)
Period of maturation, days
Tensile Strength (Unleached),MPa
Tensile strength (leached), MPa
After 7 days
20.93
23.71
After 22 days
23.50
25.60
After 35 days
22.58
24.00
5 Summary and Conclusion:-
High ammonia preserved field latex was prepared and magnesium content was lowered to a negligible amount by mixed with diammonium hydrogen phosphate with a settling period of 24 hrs.
Concentrated latex samples of different particle sizes were produced by centrifuging using skim screws of different length.
Particle size of cenex and skim latex was determined. Raw latex properties were also determined. RVNRL was prepared from cenex stored for different periods. Properties of the vulcanized latex were determined. Films were prepared by casting in shallow glass dishes. The films were leached in different concentration of calcium nitrate solution and subjected to post vulcanization at 1000C for 24 hrs and properties were determined. The retention in properties after ageing at 1000C for 24 hrs was also determined.
Quality of RVNRL produced depends on the properties of natural rubber latex such as maturation, micro gel structure, particle size, concentration process, magnesium ions, VFA, MST and preservatives.
By using skim screws of shorter length, cenex of high DRC and bigger particle size was obtained. Cenex of high DRC also had high green strength resulting in high tensile strength for the films from radiation vulcanized latex. Leaching in calcium nitrate solution increases tensile strength. Post vulcanization of the films further increases the tensile strength.
RVNRL films prepared from cenex matured for 22 days showed comparatively better tensile properties. Comparatively good retention in properties were obtained after ageing of the films.
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5.Sebastian.M.S. George V, Britto.I.J, Vijayakumar.K.C, and Jacob.J, ‘Preliminary studies on the dipping characteristics of radiation vulcanized NR lated’, Rubber India, 51(6): 7-10.
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