THE QUALITY OF RADIATION-
VULCANIZED LATEX
By
Leni Lal
Soumya Raju
Reshmi Shankar
2. INTRODUCTION 3
2.1 Composition of latex 3
2.2 Processing of latex 4
2.3 Marketable forms of natural rubber 5
2.3.1 Preserved field latex 5
2.3.2 Latex concentrates 7
2.3.3 Sheet rubber 12
2.3.4 Crepe rubber 13
2.3.5 Technically Specified rubber 13
2.4 Latex compounding 13
2.4.1 Surface active agents 14
2.4.2 Vulcanizing agents 15
2.4.3 Accelerators 15
2.4.4 Activators 15
2.4.5 Antioxidants 15
2.4.6 Fillers 16
2.4.7 Special additives 16
2.5 Preparation of aqueous dispersions 16
2.5.1 Quality of dispersions 18
2.5.2 Preparation of emulsions 18
2.5.3 Deammoniation of latex 19
2.6 Need of vulcanization 19
2.6.1 Method of vulcanization 20
2.6.2 Drawbacks of sulphur vulcanization 20
2.7 Development of radiation vulcanization 20
2.7.1 Evolution of radiation vulcanization 20
2.7.2 Nature of latex used for radiation vulcanization 21
2.8 Production of soluble protein-free latex by radiation process 22
2.9 Details of radiation vulcanization 23
2.9.1 Development of sensitizers 23
2.9.1.1 Sensitizers used in radiation vulcanization 23
2.9.1.2 Sensitizing action of n-BA 24
2.9.1.3 Mechanism of sensitizers 25
2.9.1.4 Sensitizing mechanism of MFA (n-BA) 26
2.10 Antioxidant 28
2.11 Radiation process of natural rubber latex 29
2.11.1 Dose of irradiation 30
2.11.2 Irradiation time 30
2.12 Advantages of radiation vulcanization 30
2.13 Properties of RVNRL 31
2.14 RVNRL films 31
2.14.1 Cytotoxic behaviour of RVNRL films 31
2.14.2 Extractable protein content in RVNRL films 31
2.15 Advantages of RVNRL over SVNRL 33
2.16 Application of RVNRL 33
2.17 Reasons for the huge potentiality of RVNRL 34
2.18 Features of pilot for the radiation vulcanization of latex 35
3 MATERIALS AND EXPERIMENTAL TECHNIQUES 41
3.1 NR latex 41
3.2 Preparation of latex for RVNRL processing 41
3.3 Irradiation 43
3.4 Preparation of RVNRL films 44
3.5 Test for latex 44
3.6 Testing of the final articles 48
3.6.1 Tensile strength 49
3.6.2 Nitrogen content 49
3.7 Machines 50
4 RESULTS AND DISCUSSION 54
4.1 Effect of green strength of cenex on quality of RVNRL 54
4.1.1 Particle size and green strength 54
4.1.2 Raw rubber properties and green strength 58
4.1.3 Effect of green strength on RVNRL film 60
4.1.4 Effect of immersion in calcium nitrate solution on RVNRL film 61
4.1.5 Effect of unpolymerised n-BA on quality of RVNRL 62
4.1.6 Accelerated ageing of RVNRL films 65
4.1.7 Effect of maturation of cenex on production of RVNRL 65
5 SUMMARY AND CONCLUSION 67
REFERENCES
1. AIM AND SCOPE OF THE WORK
Concentrated latex 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, is the 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 brasiliensis is 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
Concentration Of Latex The 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.
CREAMING Particles 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 content is 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 x dR
6 dt
dR/dt=(p- σ )W2R X2
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 from re-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 prevents the 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)
sulphur 100
Dispersol-F 3
Water 97
Ball mill for 48 hours
2. ZDC dispersion (50%)
ZDC 100
Dispersol F 2
Water 98
Ball mill for 24 hours
3. Zinc oxide dispersion (50%)
Zinc oxide 100
Dispersol F 2
Water 98
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
2.5.2 Preparation 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
Oil 100
Oleic acid 2-4
Pbw
Part B
Ammonia (25%soln) 2-4
Water 9-2
Part A and part B are separately heated to about 500c and B is 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 stirred at 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
2.6.2 Drawbacks of sulphur vulcanization It 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 latex is an industrial 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.
Among PFM’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 NR latex, 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:
Ø Radiolysis of NR
RH ® R٠+H
Ø Radiolysis of water
H2O ® H٠, OH٠, eeq
Ø Hydrogen abstraction
RH + OH٠® R٠+H2O
Ø Homopolymerisation
nm ® P٠
Ø Graft polymerization
R + nm ® P٠
Ø Chain transfer
P٠+ RH ® P + R٠
RP٠+ RH (RP + R٠)
R٠+ P (RP٠)
M٠+ RP (RP٠)
Ø Termination
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 = CH RH + CH2 = C٠
| |
O = C - O - R O = 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.
Ø C٠H = CH – COOR + --- CH2 – C = CH - CH2 ---
|
CH3 CH = CH - COOR
|
--- CH2 – C٠ – CH - CH2
|
CH3
Ø CH = CH - COOR
|
--- CH2 – C٠ – CH - CH2 --- + --- CH2 – C = CH - CH2 ---
| |
CH3 CH3
CH = CH - COOR
|
® --- CH2 – CH - CH + C٠H – C = CH - CH2---
| |
CH3 CH3
Ø C٠H – C = CH - CH2 --- +--- CH2 – C = CH - CH2 ---
| |
CH3 CH3
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-BA Mixing ( Radiation vulcanized NRL
(Leaching & drying)
Latex product Anti 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:
Ø Longer shelf-life period
Ø Lower modulus
Ø Biodegradability
Ø Less extractable protein content
Ø Low emission of sulphur dioxide
Ø Lower ash content
Ø No problem associated with zinc contamination in the effluent generated
Ø No 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).
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
Ø Advantages of RVNRL over NR latex rubbers vulcanized using sulphur are
Ø Absence of toxicity (free from nitrosamines and accelerator induced allergies).
Ø Absence of sulphur and zinc oxide.
Ø Better latex stability (longer shelf life).
Ø Lower modulus (suit the requirements for specific applications).
Ø Better product clarity (better coloration).
Ø Environment-friendly process (non-polluting effluent).
Ø Lower ash remain (reduce environment problem).
Ø Transparency.
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.
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 of Latex
The completion of the latex irradiator at Kottayam has marked the beginning of “Radiation Induced Vulcanization” as an industrial process in
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
v Maximum Co-60 source capacity : 444 TBq(12000 Ci)
v Dose rate at maximum capacity : 0.9 Mega Rad/hr at the center of sample chamber
v Dose rate uniformity : +25% or better radially
-25% or better radially
v Irradiation volume : 5000 cc
v Size of sample chamber : 17.2 cm(dia) X 20.5 cm(ht)
v Shielding material : Lead and stainless steel
v Weight of the unit : 5600 kg
v Size of unit : 125 cm X 106.5 cm X 150 cm
v Timer 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
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
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
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.
The experiment was conducted on RVNRL. The required dumb-bell samples were cutout and subjected to test.
v Temperature 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 distillation apparatus.
- 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 is methyl 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
4 RESULTS AND DISCUSSION
4.1. Effect of green strength of cenex on quality of RVNRL
4.1.1 Particle 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
dt 18 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
v Feed rate
v Rotational frequency of centrifuge
v Length 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
| Parameter Latex 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 |
| Alkalinity as 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 |
| 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 latex 2
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 linking take place by polar interactions between the proteinacoeus materials and oxygen groups on the poly isoprene molecule .The higher initial plasticity for cenex 2and 3 can be due to cross linking reactions involving protienacous materials cenex 2and 3 contain higher nitrogen content and higher acetone extractable(2). As protein also act as a antioxidants latex containing higher proportion of protein shows higher PRI. Comparatively higher initial plasticity and PRI is 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 cenex 3 can be due to cross linking reactions involving the proteins. Green strength is reported to increase with nitrogen content and then tend to decrease with further increase in nitrogen content16. Thus latex containing very low or very high content of 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 particle Presence of n-BA
Vulcanized Latex Particles Graphted 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 |
| 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.
4.1.4 Effect 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 |
4.1.5 Effect of immersion in calcium nitrate solution on RVNRL Films
Effects of leaching in calcium nitrate solution on quality 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++
Table 7a. 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)2 on 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 |
4.1.6 Accelerated 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 |
4.1.7 Effect 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 after storage 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.
References:
1. Blackley.D.C., ‘ Polymer Latices’ – Vol 1, 2 & 3
2. Makuuchi, K., Haziwara. M and Serizawa. T, ‘Radiation Vulcanization of Natural Rubber Latex with poly-functional monomers’ – Radiation Chemistry, pp 203-207.
3. Varghese.S., Katsumura.Y, Mukuuchi.K, Yoshii.F,., ‘Production of soluble Protein-free Latex by Radiation Process’, Radiation Physics and Chemistry.
4. Sabharwal.S., Das.T.N, Chaudhari.C.V, Bharadwaj.Y.K and Majali.A.B., ‘Mechanism of n-Butyl Acrylate sensitization action in radiation vulcanization of natural rubber latex’ Radiation Physics and Chemistry, Vol. 51, pp 309-315, 1998.
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· IS 3400 (part 17) – 1974
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14. J.R.Puig, atomic energy Rev. 9, 373 (1971)
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16. Sebastian.M.S (1999), ‘How radiation vulcanized latex is better’, Rubber Asia, 13 (1): 76-79.
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