99 Dibqufs . 4 LOW TEMPERATURE PREVULCANISATION OF NR LATEX USING Zn(bxt)2/ZDC ACCELERATOR SYSTEM 4.1 INTRODUCTION Prevulcanised NRL is a very convenient raw material used for the production of various dipped goods. 41-43 Prevulcanisation is the process of partially crosslinking rubber particles in the latex stage without affecting the colloidal stability of the latex. Thus prevulcanised latex, in effect, is a latex of vulcanized rubber. The appearance of prevulcanised latex is very similar to vulcanized latex and the original fluidity of latex is retained during prevulcanisation. During prevulcanisation crosslinking of the rubber molecules takes place inside discrete rubber particles dispersed in the aqueous phase of the latex without affecting their state of dispersion appreciably. 43 Using prevulcanised latex effective control of the physical properties can be exercised before the articles are manufactured from it. Prevulcanised latex is used nowadays for the development of products, since initial crosslinking of the rubber particle is possible during prevulcanisation, and complete vulcanization is achieved by simply drying the product. This enables the manufacturer to decrease the time required for an optimum cure in the circulating hot air oven. An additional benefit is that less time at elevated temperatures means less opportunity for oxidation degradation. Prevulcanised latex is widely used for the manufacture of various dipped goods such as gloves, toy balloons, condoms, catheters, adhesives, latex foam, latex thread, textile combining, latex composites and blends. 41 Different techniques were used for the prevulcanisation of NRL. They are reaction with sulphur and peroxide, irradiation using UV and γ-rays. 45,55,78
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Dibqufs!.!4!LOW TEMPERATURE PREVULCANISATION OF NR LATEX USING Zn(bxt)2/ZDC ACCELERATOR SYSTEM
4.1 INTRODUCTION
Prevulcanised NRL is a very convenient raw material used for the
production of various dipped goods.41-43 Prevulcanisation is the process of
partially crosslinking rubber particles in the latex stage without affecting the
colloidal stability of the latex. Thus prevulcanised latex, in effect, is a latex of
vulcanized rubber. The appearance of prevulcanised latex is very similar to
vulcanized latex and the original fluidity of latex is retained during
prevulcanisation. During prevulcanisation crosslinking of the rubber molecules
takes place inside discrete rubber particles dispersed in the aqueous phase of the
latex without affecting their state of dispersion appreciably.43 Using
prevulcanised latex effective control of the physical properties can be exercised
before the articles are manufactured from it. Prevulcanised latex is used
nowadays for the development of products, since initial crosslinking of the
rubber particle is possible during prevulcanisation, and complete vulcanization
is achieved by simply drying the product. This enables the manufacturer to
decrease the time required for an optimum cure in the circulating hot air oven.
An additional benefit is that less time at elevated temperatures means less
opportunity for oxidation degradation. Prevulcanised latex is widely used for the
manufacture of various dipped goods such as gloves, toy balloons, condoms,
Different techniques were used for the prevulcanisation of NRL. They
are reaction with sulphur and peroxide, irradiation using UV and γ-rays.45,55,78
100
Among the various techniques, sulphur prevulcanisation is the most commonly
used process in latex industry. Production of sulphur prevulcanised latex
involves heating of raw latex with dispersions of various compounding
ingredients such as sulphur, accelerator and activator until the required degree of
crosslinking is obtained.153 The rate of prevulcanisation reaction varies with
different vulcanizing systems and the extent of prevulcanisation has a profound
effect on the final vulcanisate properties.41 The effect of various accelerators and
other compounding ingredients and various formulation for the crosslinking of
NRL been reported.111,112
One of the major factors determining the quality of dipped rubber
products is the temperature of prevulcanisation. Optimum properties are
obtained when crosslinking is done at the lowest possible temperature. Low
temperature prevulcanization results in products of good quality and appearance.
The effect of temperature on the rate of prevulcanisation and its effect on the
final vulcanisate properties have been reported.42 The time and temperature
needed for prevulcanisation depends on the vulcanizing system used.
The effect of vulcanization time and storage on the stability and
physical properties of high temperature prevulcanised NRL has been reported
earlier.284 At present sulphur prevulcanisation is done by conventional high
temperature procedure. In this procedure NRL is prevulcanised by heating the
latex with dispersions of sulphur and an accelerator such as ZDC to 50-800C for
2-3 hours. This will affect the colloidal stability of the latex. Thus one of the
main drawbacks of high temperature prevulcanised latex is its low colloidal
stability. Thus lowering of prevulcanisation temperature has paramount
importance in latex goods manufacturing industry.
Nature of the accelerators used for crosslinkng has a major effect on the temperature of sulphur prevulcanisation. Xanthates and dithiocarbamates are the two important ultrafast accelerators used in latex industry. In latex technology,
use of mixed accelerators for prevulcanisation has received much attention since such systems usually exhibits synergism. The primary aim of this work is to
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develop a novel accelerator system for low temperature prevulcanisation of NRL
without affecting its colloidal stability of the latex and with improved physical properties of the latex. This chapter describes the use of Zn(bxt)2 in combination
with ZDC for low temperature prevulcanisation of NRL. The chapter reports the results of the studies conducted on (1) sulphur prevulcanisation of NRL under room temperature conditions using Zn(bxt)2/ZDC accelerator combination and
its comparison with the prevulcanisation at (55-60)0C using ZDC alone as accelerator. The mechanical properties of films casted using these prevulcanised
latices and their crosslink efficiency were evaluated during the course of prevulcanisation. (2) Effect of thermal ageing at 700C for 24 hours on the tensile properties of these latex films was studied. (3) Effect of storage on the colloidal
properties of room temperature prevulcanised latex was studied and these properties were compared with that of high temperature prevulcanised latex. (4)
use of Zn(bxt)2/ZDC accelerator system for prevulcanisation of NRL at 400C was also tried.
EXPERIMENTAL
Zn(bxt)2 was prepared in the laboratory as reported earlier.285 NRL
was compounded as per the formulations given in Table 4.1.
Table 4.1: Formulation of latex mixes
Ingredients Parts by weight(g)
A B C D
NR latex 10% KOH 10% Potassium oleate 50% S 50% ZDC 50% ZnO 50% Zn(bxt)2
167 1.5 0.75 2.5 1.5 1.0 -
167 1.5 0.75 2.5 1.0 1.0 1.0
167 1.5 0.75 2.5 1.5 1.0 1.5
167 1.5
0.75 2.5 2.0 1.0 2.0
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After compounding, the latex mixes were kept for maturation at room
temperature for 24 hours. Compound A was prevulcanised by heating to (55-
60)0C for three hours. After the maturation time, chloroform test of the
compounded lattices (A-D) were done at an interval of one day for 7 days and
the chloroform numbers are reported in Figure 4.1. Simultaneously, films were
casted from these latex mixes in glass cells according to ASTM D 1076-88, at
room temperature. Equilibrium swelling test, chemical crosslink density and
tensile property measurements of the latex films were done at an interval of one
day for 7 days to determine the optimum time for prevulcanisation. The
equilibrium swell index was calculated and reported in Figure 4.2. The
reciprocal of swell index (1/Q) was reported as the apparent crosslink density in
Figure 4.3. The total chemical crosslink density was also determined by
equilibrium swelling method using Flory- Rehner equation.109,110 The results are
given in Figure 4.4
Dumb bell and crescent shaped tensile and tear specimens were punched
out of the casted films. Stress-strain measurements were carried out at a
crosshead speed of 500 mm/min on a Zwick Universal Testing Machine. Tensile
and tear strengths were measured according to ASTM D 412-87 (method A) and
ASTM D 624-86 respectively. The results are reported in Figures 4.5-4.9.
Effect of thermal ageing at 700C for 24 hours on crosslink density and
tensile properties of prevulcanised latex films were studied as per ASTM D 865-
88 and are reported in Figures 4.10-4.15. SEM photographs of the tensile
fracture surface of high temperature prevulcanised latex film and that of room
temperature prevulcanised latex film before and after thermal ageing were taken
and their results are given in Figures 4.16, 4.17 and 4.18 respectively.
The colloidal properties of the prevulcanised latex (compound C) were
studied after 5 days, 10 days and 30 days of storage and compared with that of
high temperature prevulcanised latex (compound A) and the results are given in
Figures 4.19-4.29. The important colloidal properties studied here are TSC,