Latex products for Health Care-Influence of Compounding
and Process variables on their Quality

D. Joseph Francis
Emeritus Scientist CSIR, Hindustan Latex Ltd
Akkulam, Trivandrum-695017.

Use of natural rubber latex condom as a contraceptive is widely accepted for family planning and hence for the control of population. In the context of increasing incidence of the incurable disease (AIDS), apart from abstinence, the use of high quality condoms is the only healthy way to prevent or reduce the risk of being infected with HIV the causative virus of the disease. The effect or compounding variables and process parameters which significantly affect the pinhole formation and physical properties of condoms, a typical health care latex product, have been studied in detail (1) and the important results are summarized here below. The general finding of this study are application to quality improvement of other rubber products also.

Stabilizer system:
Addition of small amounts of higher fatty acid soaps have been shown to greatly improve the mechanical stability of latex, with an optimum effect when the alkyl chain length of the HFA soap contains 9 to 11 carbon atoms (2,3). Among the six stabilizer systems studied, casein-potassium laurate system was found to be most suitable for condom manufacture. This system gives lower rejection percentage due to pinholes and are able to improve physical properties of condoms. Tensile strength retention after ageing was maximum for this system. Although elongation at break, modulus at 500% elongation, electrical resistance etc. were comparable for all the systems, burst volume and burst pressure values were higher for casein-potassium laurate system compared to casein-potassium oleate which was the control. The lower rejection rate of compounds containing potassium laurate compared to potassium oleate could be attributed to the lower frothing tendency and improved mechanical stability caused by the laurate soap (4).

Vulcanising system:
Vulcanising system is the most important compounding variable which could affect the physical properties and ageing characteristics of condoms. Influence of sulphur concentration in the formulation was studied keeping all other parameters constant. It was found that tensile strength, elongation at a break, modulus etc increased but ageing resistance was decreased. Burst volume and burst pressure of condoms also increased inspite of the increase in modulus with higher sulphur level. It is well known that in NR a high sulphur more flexible and hence can cause higher elongation and tensile strength. However, these polysulphidic crosslinks are more thermolabile and hence ageing resistance of vulcanisates prepared using higher sulphur levels is poorer. The ageing resistance could be improved by a judicious choice of acceleration system. Setsit-NTP accelerator combination gave low ageing resistance whereas Setsit-ZMBT combination improved the ageing resistance. Setsit-ZMBT combination was found to give a tighter cure than Setsit-NTP and this was observed in the burst pressure values also. The higher retention of tensile properties, burst volume and burst pressure of compounds containing ZMBT may be due to the antioxidant activity of ZMBT. NTP-ZMBT combination also gave higher tensile strength, and modulus but burst volume and burst pressure were lower. Ageing resistance was better than the control system, but was not as good as the Setsit-ZMBT combination. When the concentrtion of ZMBT was higher in NTP-ZMBT combination it gave increased values for burst volume modulus and tensile strength. A combination of 0.5 phr of ZMBT and 0.3 phr of NTP was found to be optimum for this system. The low mechanical strength of condoms when dithiocarbamate type accelerators alone were used could be attributed to the peaky cure exhibited by them which causes reversion of vulcanisates even under slight overcuring conditions. Thiazole type are reported togive flatter cure curves (5) than most other types of accelerators. They maintain properties even when considerably overcured. Hence it is desirable to use combination of thiazole type and a dithiocarbamate type accelerator for condom manufacture.

Antioxidants:
An antioxidant is essential to retain the properties of condoms after ageing six antioxidants were examined Vulkanox HS, the amine type antioxidant used was efficient in retaining the properties when added at 0.5 phr and above this level. But higher concentrations were having a discolouring effect. Pentaflex SP which is a phenolic antioxidant also was efficient at 1 phr level to impart comparable tensile strength and brust volume. But the ageing characteristics were much lower compared to Nocrac NS-5 which was the control.
Vanox MBPC and Nocrac NS-5 both bisphenolic type antioxidants were having similar structure and hence same antioxidant efficiency. Vanox MBPC was effective in imparting maximum values of modules at 0.5 phr level whereas Nocrac NS-5 gave the maximum values at 0.75 phr level only. Tensile strength, burst volume and burst pressure values and their retention after ageing increased gradually with increasing concentration of these antioxidants. Ortholinked bisphenols are reported to be more effective than paralinked analogues. The higher retention properties such as burst volume, burst pressure and tensile properties after ageing of condoms may be attributed to this advantageous ortholinkage in the two antioxidants, Nocrac NS-5 and Vanox MBPC. Colour development also lower for ortholinked bisphenols.
Among the six antioxidants studied Irganox 1010 and Vanox SKT were having maximum antioxidant efficiency. 0.5 phr of these antioxidants were able to improve the burst volume, tensile strength and burst pressure of condoms than all the other antioxidants. Both these antioxidants are esters of phenol substituted acids. Combination of Pentaflex SP and Vanox SKT and that of Vulkanox HS and Nocrac NS-5 were having synergistic effect in imparting better ageing characteristics to condoms. The discolouring tendency of Vulkanox HS also was reduced by replaying a part of it with Nocrac NS-5.

Process variables:
Ball milling time of dispersion was found to affect the pinhole formulation of condoms. As the ball milling time increased, the particle size of solid ingredients decreased gradually and at 72 hours of ball milling optimum was obtained. Above this period, a slight increase in the size of particles was observed and this may be due to the reagglomeration of dispersed particles. Physical properties of condoms also was higher when lower particle size dispersions were used for compounding it was reported (6) that size of sulphur particles is of great importance for the crosslinking reaction. The high tensile strength, modulus and burst volume obtained at 72h ball milling could be attributed to the optimum level of crosslinking obtained by the fine particle size dispersion of sulphur and other ingredients.
Temperature and concentration of mould cleaning soap solution also influences the quality of condoms. A higher than optimum level of soap may lead to frothing and pinhole formation. Optimum temperature of the mould washing solution may be helpful in removing grease, oil etc from the surface of the mould. This if these parameters were not optimum, presence of oily substances on the mould surfaces may lead to the formation of weak spots or pinholes in condoms.
Period and temperature of prevulcanisation and final vulcanisation also significantly influences the physical properties and rejection rate of condoms due to pinholes. Brust volume and burst pressure were maximum at 6 hours of prevulcanisation. Then it decreases with increasing period of prevulcanisation. Pinhole rejection rate was found to increase with increasing temperature of prevulcanisation. 500C was found optimum for obtaining higher modulus at 500% elongation, burst volume and burst pressure. Ageing resistance also decreased when the temperature of prevulcanistion was increased. The adverse effect at higher period and temperature of prevulcanisation may be due to more air entrapment, at higher viscosity, obtained during prevulcanisation. Breaking of polysulphidic in linkages also might have started during ageing, which were formed as a result of prolonged heating at low temperatures of prevulcanisation.
Final vulcanization temperature was found optimum at 700C when duration was kept at 30 minutes. Still higher temperature of vulcanization was found to increase the pinhole rejection rate. It is possiblet that the microlevel air bubbles expand and burst at the higher temperatures leading to large number of pinholes. Adverse effect on properties such as tensile strength and burst volume of condoms at higher temperatures and longer periods of vulcanisation may be due to reversion taking in presence of dithiocarbamate type accelerators. Hence the temperature and period of prevulcanisation as well as final vulcanisation are interrelated in determining the final quality of NR latex condoms.

Postvulcanised, Prevulcanised and Radiation vulcanized NR latex condoms:
Using LATZ latex, condoms were prepared in three different ways. Latex was compounded and postvulcanised condoms were prepared in one case. In another case the latex compound was given a prevulcanization before the dipping process. Radiation vulcanization of latex was carried out in the third case. Rheological characteristics at different temperature and shear rates, effect on pinhole formation, influence of different stabilizer system and affect of the presence of antioxidant in these studied. Thus a comparative evaluation of the three modified lattices were carried out with respect to tensile properties, burst volume, burst pressure and ageing characteristics of condoms.
Viscosity values were higher for radiation vulcanized and prevulcanised lattices compared to post vulcanized latex. But the viscosity difference was higher at low shear rates. But increase in temperature was found to decrease the difference.
Pinhole rejection percentage was higher for post vulcanized latex and lowest for RVNRL condoms. Absence of any compounding dispersion in RVNRL may be contributing to the low pinhole formation rate. Presence of an additional stabilizer, KOH, may also be contributing to the lower rejection due to pinholes. But in the case of prevulcanised latex the froth might have escaped while prolonged heating for 8 hours and this may be the reason for comparatively low rejection rate due to pinholes of prevulcanised latex than postvulcanised.
Presence of a stabilizer system in all three lattices were found to increase the quality of condoms with respect to low pinhole formation and higher physical properties. Among the three stabilizer system studied, casein-potassium laurate combination was most effective in reducing the pinhole formation and improving the tensile and burst volume properties. Polyvintyl alcohol-casein system increased the viscosity of latex compound and pinhole rejection percentage of aged condoms was higher compared to unaged in presence of this stabilizer system. Potassium oleate-casein system was inferior due to high pinhole formation rate and lower physical properties compared to potassium laurate-casein system. Hence casein-potassium laurate system was found suitable for condoms of all the three types of latices.
With respect to tensile strength and burst volume of condoms of the three lattices, postvulcanised condoms were superior over two types. RVNRL condoms were having very low physical properties and this result is in agreement with those of previous workers (6). Presence of nonrubber particles which effect the fusion behaviour of latex may be a reason for their low strength. High heat stability and mechanical stability of RVNRL indicate a low extent of vulcanistion and this also may be contributing to the low physical properties. Breaking of polysulphidic linkages formed during prevulcanisation latex. Low wet gel strength of prevulcanised latex also may be contributing to the comparatively low properties of the prevulcanised latex condoms.
Thermo-oxidative ageing of condoms of three modified lattices and the effect of antioxidant in them were studied. It was noted that presence of an antioxidant is essential in all these lattices to obtain improved ageing characteristics. Nocrac NS-5, the bisphenolic type antioxidant was used for the comparative evaluation. Post vulcanised latex condoms were found better in ageing resistance compared to prevulcanised and RVNRL condoms. Low polysulphidic linkages (7). Prevulcanisation of latex compound at 550C may be the reason for the high rate of formation of polysulphidic linkages and during ageing they may be break easily to reduce the strength properties of condoms. Ageing characteristic of RVNRL is also found very poor as reported by Makuuchi(8). But Nocrac NS-5 was found to improve the ageing characteristics of RVNRL with respect to tensile properties and burst volume. But the retention properties were much below after ageing. Hence further work is required in the field of RVNRL to make use of its advantages properties without sacrificing the physical properties and ageing characteristics of the vulcanisate.

CONCLUSIONS

1. Potassium laurate-casein combination was found to be the most suitable stabilizer system for NR latex condoms.
2. Quantity of sulphur and type of accelerators were found to affect the tensile properties, burst volume and burst pressure of condoms:Setsit-ZMBT and ZMBT-NTP combinations were found to impart high mechanical strength to compared to NTP-Setsit control system.
3. Presence of an antioxidant was found essential to improve the ageing characteristic of NR latex condoms. Among the six different antioxidants studied, Vanox SKT and Irganox 1010 were found most suitable for NR latex condoms to retain their physical properties after ageing. Synergistic effect also was observed when combination of Vulcanox HS and Nocrac NS-5; and Pentaflex SP and Vanox SKT; were used for latex compounding.
4. Ball milling time of dispersions upto 72h could decrease the particle size of ingredients which improved the mechanical properties of condoms and also led to a reduction in pinhole rejection rate.
5. Concentration and temperature of mould cleaning soap solution was also found to affect the quality of condoms. For the alkyl aryl polyether type mould cleanin agent, optimum concentration was 0.5-0.7% and temperature around 600C.
6. Period and temperature of prevulcanisation and final vulcanisation are interrelated in determining the final properties of latex vulcanisation.
7. Post vulcanised latex condoms were found to have better physical Properties and ageing characteristic compared to prevulcanised and radiation vulcanised latex condoms.
8. Pinhole rejection percentage of RVNRL condoms were lower than those of post vulcanised and prevulcanised latex condoms whereas physical properties and ageing characteristic were inferior.

REFERENCES

1. J. Maret, ph.D Thesis, Cochin University (1997)
2. I.S.F. Chen and C.S. Ng, Rubber Chem. Technology 57, 243 (1984)
3. D.C. Blackley, A.A. Nor Aisah and R. Twaits, Plast. Rubber. Mater. Appl. 4, 77 (1979)
4. T.D. Pendle and A.D.T. Gorton, The mechanical stability of natural lattices, Rubb.Chem.Technol.986, 51 (1978)
5. C.C. Davis and J.T. Blake, “The Chemistry and Technology of Rubber, Reinhold Publishing Corp; New York, USA, p.300 (1937)
6. B. Garner, F. Danes, D. Delaunay (Lab, Termocinetique, ISITEM, 44087, Nantes, France) Thermochim Acta, 222 (1), 115-25 (Fr) (1993).
7. A.D.T. Gorton and T.D. Pendle, N.R. Technol 12, 1 (1981)
8. K. Makuuchi and K. Tsushima “Improvement of Ageing Properties of Latex Rubber Vulcanished with Radiaton” J. Soc. Rubber Ind. Japan 61,710 (1988).