Low protein latex is produced by enzyme deproteinisation of natural rubber latex. The properties of LOPROL (Table1) was described in a previous paper¹.

It was reported then that with low accelerator levels, post-vulcanised LOPROL gave very low tensile properties. These properties have since been improved by modifications of the formulation and compounding technique and are now above international regulatory requirements.

In the case of prevulcanised LOPROL, very little problems were, however, encountered. Satisfactory tensile properties were easily achieved even with low accelerator sulphur systems.

This paper described a factory trial production of examination gloves from LOPROL undertaken at Sime Latex Products Sdn.Bhd. in Seremban.

High ammonia LOPROL concentrate was produced at the Latex processing factory of the Rubber Research Institute of Malaysia (Experiment Station), Sungai Buloh. It was stored in 205 kg drums which were transported to the glove factory.

The LOPROL compound for the factory trial was prepared using Formulation B of Table 2.The other two Formulations (A and C) were experimented only in the laboratory.

The compound to be used in the trial was diluted to 45% total solids content before being matured for three days. The maturation step is necessary as it improves the tensile properties of LOPROL films. The longer the maturation time, the better are the tensile properties (see Table 8).

As it is anticipated that the MST of LOPROL will be lower and its development slower than in the case of normal centrifuged latex concentrate, a laboratory experiment that subjected the latices to continuous stirring of not more than 40 r.p.m. at room temperature was carried out to compare their stability against mechanical shear forces.

Results obtained (Table 4) show that the coagulum contents of LOPROL were comparable to those of the control HA latex concentrate, albeit slightly higher. Nonetheless, every precaution was taken during the trial to prevent destabilisation of LOPROL caused by mechanical agitation. The maximum time that the latex was stirred continuously was restricted to 30 min for every 24 h of maturation.

Dry coacervant dipping based on freshly prepared calcium nitrate in water was used. The effect of varying the concentration of the coagulant on the thickness of LOPROL films is shown in Table 3. As expected, increasing the coagulant concentration from 7 to 12% resulted in >60% increase in film thickness.

As popularly practised, the wet powder slurry (freshly prepared) was made of bioabsorbable corn starch in water.

Production of examination gloves was carried out on a typical line as shown in Figure 1 using large sized unglazed formers.

LOPROL compound in the maturation tank was agitated for several minutes, filtered and placed in the dipping tank. It was continuously stirred once dipping was in progress.

Several factors have been found vital for successful dipping. They are namely:

• Keeping the temperature of LOPROL compound in the latex dip tank below 28°C. An efficient chiller is therefore essential
• Ensure thorough drying of the coagulant coated formers lest contamination of the latex tank occurs
• Optimise temperatures and duration of ‘drying before vulcanisation’ and ‘vulcanisation’
• Optimise maturation time.

If the last two factors are taken care of, optimal vulcanisate properties would be obtained and very little powder would be required for detackifying.

On line wet-gel leaching of the LOPROL gloves were done in tow stages totalling about 1 min and 8 s. The leaching bath was kept at a constant temperature of 70° ± 5°C.

The process was extended to include tumbling for removal of excess powder and to even out powder contents on gloves.

Several factors were investigated in an effort to improve the tensile properties of the post-vulcanised LOPROL films, namely:

• Temperature of vulcanisation
• Amounts of accelerator
• Types of accelerator
• Activation of acceleration by zinc oxide
• Maturation time.

Figures 2 and 3 show that the higher the temperature of vulcanisation, the lower are the tensile strength values of the films. The values were lowest when LOPROL and HA latex films obtained from post-vulcanisable compounds containing 0.27 p.h.r. ZBUD, 0.7 p.h.r. S and 0.5 p.h.r. ZnO were vulcanised at 120°C.

Similar observations as shown in Figures 4 and 5, were obtained when sulphur systems containing 0.4 p.h.r. of ZDEC accelerator were used.

Lowering the temperature of vulcanisation did bring about an improvement in tensile strength of LOPROL films. However, the strength values remained in the region of 11 MPa – 13 MPa when the temperatures were lowered to 80° - 100°C, suggesting that lowering of temperature alone is not sufficient to achieve the required strength values.

Figures 6,7 and 8 show that increasing the level of ZBUD from 0.27 p.h.r. to 0.6 p.h.r. gave higher tensile strength values for both LOPROL and HA latex films.

However, in the case of ZDEC, increasing its content from 0.4 p.h.r. to 0.7 p.h.r. did not affect the tensile strength values of both LOPROL and HA latex films (Figures 9 and 10).

The use of ZBUD is preferred to ZDEC for LOPROL formulaions. Alternatively, a combination of acclerators such as ZDEC/ZMBT/ZBUD in the order of 0.4 p.h.r., 0.2 p.h.r. and 0.1 p.h.r. has also been found to be suitable (Figure 11).

Table 7 shows that increasing the amount of zinc oxide from 0.5 p.h.r. to 0.7 p.h.r did not appear to increase the tensile strength appreciably.

The state of precure of a latex compound is affected by maturation time and temperature, aqueous phase and the presence of non-rubbers ^3,4. It has been shown that the degree of vulcanisation is lower in highly purified NR latex³, probably due to the absence of non-rubbers, and the necessary presence of added surface-active substances that could cause a slower rate of water removal during drying of the purified latex film. As a corollary, it is reasonable to assume that the state and rate of precure of LOPROL would be lower than those of normal HA latex concentrate regardless of the formulation used. This is confirmed by the results in Table 8 which showed a significant increase in tensile properties when the LORPOL compound was matured for a longer period.

Table 5 shows that the tensile properties of the gloves produced in this trial using a suitably formulated LORPOL compound were well above the minimum requirements of ASTM specifications for examination gloves.

As shown in Table 6, the EP contents of LOPROL Gloves are less than 0.1 mg/g of rubber film. These figures could be further lowered to 0.06 mg/g by a short dry film wash of 30 s.

The use of a low protein latex concentrate has been shown to be a viable process for large-scale production of gloves with reduced amounts of extractable proteins.

The author wished to thank the Director of the Rubber Research Institute of Malaysia for permission to present this paper. Thanks are also extended to Dr Lai Pin Fah, Head of Latex Technology Division for this valuable comments and encouragement. Thanks are also extended to Miss Har Mun Lin and to Sime Latex Rubber Products for permission to run this trial production at their factory.

1. HAFSAH MOHD GHAZALY (1991) Properties of Natural Rubber Low Protein Latex, Paper No.7, Latex Protein Workshop.
2. RUBBER RESEARCH INSTITUTE OF MALAYSIA Manual of Laboratory Methods for Chemical Analysis of Hevea Brasiliensis Rubber. Analytical Chemistry Division.
3. HAFSAH MOHD GHAZALY (1988) Prevulcanisation of NR Latex using Purified Reactants, M. Phil. Thesis, CNAA, UK.
4. LOH,A,A.C.P.(1982)Ph.D.Thesis, CNAA, UK.