low temperature prevulcanisation of nr latex...

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

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

101

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

102

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,

DRC, Non rubber solids, Ammonia content, VFA number, KOH number, MST,

103

Brookfield viscosity (cps), Coagulum content and pH. The test methods used are

given in Chapter 2.

TSC and DRC values were determined using conventional method of

heating in hot air oven. VFA number of prevulcanised latex was periodically

determined using Markham-Type Still apparatus. MST was determined using a

mechanical stability apparatus. The viscosity measurements of the prevulcanised

latex were carried out with a Brookfield LVT Viscometer. The experiments

were performed by varying the size and speed of the spindle in order to achieve

a shear stress close to 100%. The pH of the latex was determined using a pH

meter according to ISO standard 976-1986.

NRL was also prevulcanised by heating the compounded latex

containing Zn(bxt)2/ZDC accelerator system at 400C for one hour. The films

were casted at 900C giving different cure times. The casted films were aged at

700C in hot air oven for 24 hours. The variation of swelling parameters of 400C

prevulcanised NRL films with cure time before and after thermal ageing were

reported in Figures 4.29 -4.31. The variation of tensile properties of 400C

prevulcanised NRL films with cure time before and after thermal ageing were

reported in Figures 4.32 -4.37.

RESULTS AND DISCUSSION

4.2 Prevulcanisation of NRL at Room Temperature

4.2.1 Optimisation of Accelerator Concentration and

Prevulcanisation Time

Figure 4.1 shows the variation of chloroform number of latex

compounds A to D with prevulcanisation time.

104

Figure 4.1: Variation of chloroform number with prevulcanisation time of

different latex mixes

The level of prevulcanisation is assessed by coagulation of latex with

chloroform.106 From the Figure 4.1 it is clear that as prevulcanisation time

increases, chloroform number increases and reaches a steady value. The increase

in chloroform number with prevulcanisation time is due to the increase in

crosslinking of rubber molecules inside discrete rubber particles dispersed in the

aqueous phase of the latex. In the case of latex compound A, chloroform number

reaches a steady value of four after two days. For latex compound B to D

chloroform number reaches a steady value after five days. Classical high

temperature procedure (latex compound A) requires only two days for complete

prevulcanisation. The two important parameters which affect the rate of

crosslinking are the temperature of vulcanization and the type of vulcanizing

system used. As temperature is lowered crosslinking occurs very slowly.42 Thus

in room temperature procedure (latex compound B to D) crosslinking occurs

slowly compared to latex compound A. It is evident from the Figure 4.1 that

Zn(bxt)2/ZDC accelerator system can be used for the prevulcanisation of NRL at

room temperature (compound B to D). The positive synergistic effect of

xanthate/ZDC accelerator combination on the cure rate and mechanical

0

1

2

3

4

5

6

1 2 3 4 5 6 7

Chl

orof

orm

Num

ber

Prevulcanisation Time (Days)

A B C D

105

properties of dry natural rubber, carbon balck filled NR and NBR compounds

have been reported earlier.277-283 In the case of latex compound C and D,

chloroform number reaches a steady value of four after five days indicating the

fully vulcanized state of latex particles. Chloroform number of latex compound

B is only three even after five days and it attains a steady state. This indicates a

moderately vulcanized state of latex particles. This clearly shows that the

concentration of accelerator in latex compound B is not enough for complete

prevulcanisation.

Figure 4.2 shows the variation of swell index of latex films prepared

using latex compounds A to D with prevulcanisation time.

Figure 4.2: Variation of swell index of casted films with prevulcanisation

time for different latex mixes

The value of swell index decreases with prevulcanisation time

and reaches a steady value. The decrease in swell index value is due to the

increase in crosslinking between rubber molecules with prevulcanisation time.

Thus swell index can be used as an indicator for the formation of crosslinks. The

extent to which the sample swells is an inverse measure of the degree of

vulcanization. In the case of films casted from latex compound A, swell index

0

2

4

6

8

10

12

1 2 3 4 5 6 7

Swel

l ind

ex


A B C D

106

reaches a steady value of 4.800 after two days. The swell index value of films

casted using latex compound B to D decreases with prevulcanisation time and

reaches a steady value after 5 days. After each day the value of swell index of

latex films prepared from latex compound B is higher than that of latex

compound C and D. The swell index of films casted from compound B attains a

steady value of 6.5 after five days. This again shows that 1 phr Zn(bxt)2 /1 phr

ZDC accelerator concentration is not enough for complete prevulcanisation and

only a moderately vulcanized state can be reached. In the case of films prepared

using latex compound C and D, swell index attains a steady value of 4.684 and

4.733 respectively after five days. This indicates that 1.5 phr Zn(bxt)2/ 1.5phr

ZDC accelerator concentration (compound C) is enough for room temperature

prevulcanisation of NRL. Further increase in the amount of total accelerator

concentration to 4 phr (compound D) does not affect the rate of prevulcanisation

much. Thus swell index measurements shows that the optimum amount of

accelerator for room temperature prevulcanisation of NRL is 1.5 phr Zn(bxt)2

/1.5phr ZDC.

The chloroform numbers and the swell index values given in Figure

4.1 and Figure 4.2 further confirms that the time needed for optimum

prevulcanisation at room temperature is five days.

The extent of prevulcanisation has been analysed by crosslink density

measurements.41 Figures 4.3 and 4.4 show the variation of apparent crosslink

density and chemical crosslink density of latex films prepared using latex

compounds A to D with prevulcanisation time.

107

Figure 4.3: Variation of apparent crosslink density of casted films with

prevulcanisation time for different latex mixes

Figure 4.4: Variation of chemical crosslink density of casted films with


0

0.05

0.1

0.15

0.2

0.25

0.3

1 2 3 4 5 6 7

App

aren

t cro

sslin

k de

nsity


A B C D

0

1

2

3

4

5

1 2 3 4 5 6 7

Che

mic

al c

ross

link

dens

ity(x

10-5

g m

ol-1

cm-3

)


A B C D

108

The crosslink density increases with prevulcanisation time and attains

a steady value. The increase in crosslink density with prevulcanisation time is

due to the increased number of crosslinks formed between rubber molecules

during prevulcanisation. In the case of latex films casted using latex compound

A crosslink density attains a steady value after two days. For films prepared

from latex compound B to D crosslink density gradually increases and reaches a

steady value after five days. After five days, films prepared from latex

compound C has maximum crosslink density compared to that of films prepared

from latex compound B and D. This again confirm that the optimum amount of

accelerator needed for room temperature prevulcanisation of NRL is 1.5 phr

Zn(bxt)2/1.5phr ZDC. Again, the crosslink density of room temperature

prevulcanised latex film (compound C) is higher than that of high temperature

prevulcanised latex film (compound A). This is because of the higher stability of

the vulcanizing agents and lesser degradation of rubber chains due to the non-

application of temperature.

4.2.2 Mechanical Properties of Prevulcanised NRL Films

Figure 4.5 shows the variation of tensile strength of latex films prepared

using latex compounds A to D, with prevulcanisation time.

109

Figure 4.5: Variation of tensile strength of casted films with prevulcanisation


The tensile strength increases with prevulcanisation time and reaches a

steady value. This is due to the introduction of more crosslinks between rubber

molecules during prevulcanisation and also due to the increased ability of latex

particles to coalesce and integrate among themselves when the film dries. In the

case of films prepared from latex compound A, tensile strength reaches a steady

value after two days. It takes five days for attaining steady tensile strength value

for films prepared from latex compounds B to D. After five days, film prepared

from latex compound C attains a maximum tensile strength value of 29.62 MPa.

The time taken for attaining maximum tensile strength can be taken as the

optimum cure time. Films prepared from latex compound A attains maximum

tensile strength after two days indicating complete prevulcanisation. So the

tensile strength studies also suggest that the optimum time for room temperature

prevulcanisation of NRL using Zn(bxt)2/ZDC accelerator system is five days.

The optimum accelerator concentration is 1.5 phr Zn(bxt)2 /1.5phr ZDC.

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7

Tens

ile s

tren

gth

(MPa

)


A B C D

110

Figures 4.6 and 4.7 show the effect of prevulcanisation time on 100%

and 300% tensile modulus values of latex films prepared from latex compound

A to D.

Figure 4.6: Variation of 100% tensile modulus of casted films with


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1 2 3 4 5 6 7

100%

Ten

sile

Mod

ulus

(MPa

)


A B C D

111

Figure 4.7: Variation of 300% tensile modulus of casted films with


Both 100% and 300% tensile modulus values increases with

prevulcanisation time and attains a steady value. This again is due to the

corresponding increase in crosslinking between rubber molecules. For films

prepared using latex compound A, tensile modulus attains a maximum steady

value after two days. It takes five days for attaining maximum steady tensile

modulus value for films casted from latex compound B to D. After five days,

films prepared from latex compound C attain maximum tensile modulus value.

Figure 4.8 shows the variation of tear strength of films

prepared using latex compound A to D with prevulcanisation time.

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6 7

300%

Ten

sile

Mod

ulus

(MPa

)


A B C D

112

Figure 4.8: Variation of tear strength of casted films with prevulcanisation


The tear strength increases with prevulcanisation time and attains a

steady value. Films prepared using latex compound A attains maximum tear

strength after two days. Films prepared from latex compound B to D takes five

days for attaining maximum steady tear strength value. After five days films

prepared from latex compound C attains higher tear strength value (67.56N/mm)

compared to that of films prepared using latex compound B and D. This value is

higher than that of high temperature prevulcanised latex compound A. This

might be attributed to the lower concentration of polysulphidic type of

crosslinks at high vulcanization temperature than that of low vulcanization

temperature.285

Figure 4.9 shows the variation of elongation at break values of films

prepared using latex compound A to D with prevulcanisation time.

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7

Tear

str

engt

h (N

/mm

)


A B C D

113

Figure 4.9: Variation of elongation at break value of casted films with

revulcanisation time for different latex mixes

Elongation at break values decreases with prevulcanisation time and

reaches a steady minimum value. For films prepared from latex compound A, it

takes two days for attaining minimum steady value. In the case of films prepared

using latex compound B to D, elongation at break attains a steady minimum

value after five days. After five days films prepared from latex compound C

shows lower elongation at break value compared to that of films prepared from

latex compound B and D.

4.2.3 Thermal Ageing Study

Figure 4.10-4.15 shows the effect of thermal ageing at 700C for 24

hours on crosslink density, tensile strength, tear strength, 100% tensile modulus,

300% tensile modulus, and elongation at break (%) values of films casted using

both high temperature prevulcanised latex (compound A) and room temperature

prevulcanised latex (compound B to D).

0

200

400

600

800

1000

1200

1 2 3 4 5 6 7

Elo

ngat

ion

at B

reak

(%)


A B C D

114

Figure 4.10: Effect of thermal ageing at 700C for 24 hours on crosslink density

of latex films prepared from different latex mixes

Figure 4.11: Effect of thermal ageing at 700C for 24 hours on tensile strength

of latex films prepared from different latex mixes

0

1

2

3

4

5

6

A B C D

Che

mic

al c

ross

link

dens

ity(x

10-5

g m

ol-1

cm-3

)

Latex compound

Before Ageing After Ageing

0

5

10

15

20

25

30

35

A B C D

Tens

ile S

tren

gth

(MPa

)

Latex compound


115

Figure 4.12: Effect of thermal ageing at 700C for 24 hours on tear strength of

latex films prepared from different latex mixes

Figure 4.13: Effect of thermal ageing at 700C for 24 hours on 100% tensile

modulus of latex films prepared from different latex mixes

0

10

20

30

40

50

60

70

80

A B C D

Tear

Str

engt

h (N

/mm

)

Latex compound


0

0.2

0.4

0.6

0.8

1

1.2

1.4

A B C D

100%

Ten

sile

Mod

ulus

(MPa

)

Latex compound


116

Figure 4.14: Effect of thermal ageing at 700C for 24 hours on 300% tensile

modulus of latex films prepared from different latex mixes

Figure 4.15: Effect of thermal ageing at 700C for 24 hours on elongation at

break of latex films prepared from different latex mixes

0

0.5

1

1.5

2

2.5

3

A B C D

300%

Ten

sile

Mod

ulus

(MPa

)

Latex compound


0

200

400

600

800

1000

1200

A B C D

Elo

ngat

ion

at B

reak

(%)

Latex compound


117

From the figures it is evident that for films casted using latex

compound B to D, the values of crosslink density, tensile strength, tear strength

and tensile modulus (100% and 300%) increases after thermal ageing. For films

casted using latex compound A, these properties undergo a slight decrease after

thermal ageing. The elongation at break value decreases after thermal ageing for

latex compound B to D and this value increases for latex compound A.

Films prepared from room temperature prevulcanised latex

(compound B to D) undergoes an exposure to high temperature (700C) for 24

hours during thermal ageing. At this high temperature, further crosslinking

occurs between rubber molecules due to the consumption of excess crosslinking

reagents. Thus the increased crosslink density after thermal ageing can be

explained with the formations of new crosslinks by free curative residues such

as elemental sulphur, cure accelerator residues and zinc complexes remained in

the vulcanisate.287-289 Especially, free sulphur remained in the vulcanisate reacts

well with rubber chains.289 A pendent sulfide group terminated by an accelerator

residue reacts with another pendent group of the neighbouring rubber chains

leading to the formation of new crosslinks.287,289-291 The mobility and mixing of

polymer chains across the particle-particle interface increases by the application

of temperature during thermal ageing. This would promote gradual coalescence

between rubber particles and lead to better film forming properties. This, along

with increased crosslinking between rubber molecules cause an increase in

tensile strength of films prepared using latex compound B to D. The decrease in

crosslink density and tensile properties of films prepared from latex compound

A (which already undergoes an exposure to temperature of 55-600C for 3 hours)

is due to the degradation of rubber chains during accelerated thermal ageing.

Figure 4.16 shows the scanning electron micrograph of tensile

fracture surface of films prepared using high temperature prevulcanised latex

(compound A).

118

Figure 4.16: SEM photograph of tensile fracture surface of high temperature

prevulcanised (ZDC system) NRL film

Figure 4.17 and 4.18 shows the SEM photograph of tensile fracture

surface of films prepared from room temperature prevulcanised latex

(compound C) before and after thermal ageing.

Figure 4.17: SEM photograph of tensile fracture surface of room temperature

prevulcanised (Zn(bxt)2/ZDC accelerator system) NRL film

before thermal ageing

119

Figure 4.18: SEM photograph of tensile fracture surface of room temperature

prevulcanised (Zn(bxt)2/ZDC accelerator system) NRL film after

thermal ageing

Tensile fracture surface of films prepared from compound C before

thermal ageing is homogeneous compared to that of films prepared from latex

compound A. Figure 4.16 shows crystal like structures. This may be due to the

presence of microcoagulums of rubber particles formed in latex compound A

during heating of latex for prevulcanisation at high temperature (55-600C) for 3

hours. This will lead to colloidal destabilization of latex compound. This crystal

like structures may also be due to the separation of one of the vulcanizing

ingredients, which cause a reduction in vulcanization efficiency. In Figure 4.17,

surface is regular due to the interparticle crosslinking between rubber particles.

Here the chance of formation of rubber microcoagulums is less due to the non-

application of temperature. The homogeneous surface may also be due to the

non separation of vulcanizing agents. After thermal ageing tensile fracture

surface of films prepared using compound C become somewhat rough (Figure

4.18). The roughness of tensile fracture surface may be considered as an

indication of the energy consumed during fracture of the materials.292 This

indicates the formation of more crosslinks after thermal ageing due to the

increased mobility and mixing of polymer chains across the particle-particle

interface.

120

4.2.4 Effect of Storage on the Colloidal Properties of Room Temperature

Prevulcanised NRL

NRL particles were covered by some proteins and phospholipids which

give colloidal stability to NRL.15 The colloidal stability of prevulcanised latex

depends on many factors such as the nature of the latex, amount of KOH and

carboxylate soap, dosage of vulcanizing ingredients and prevulcanisation

conditions such as time and temperature.284 There are two opposing factors

which affect the colloidal stability of prevulcanised latex. One is the presence of

residual vulcanizing ingredients such as ZnO which reduces the colloidal

stability by ZnO thickening. The other one is the addition of alkalies and

carboxylate soaps which increase the stability by increasing the negative charge

on the surface of rubber particles and by increasing surface adsorption. The

properties of sulphur prevulcanised latex may undergo changes during storage

because of the presence of surface-active agents and residual vulcanizing

ingredients. During storage, chemical composition of latex changes significantly

due to the action of bacteria, enzymes and preservatives. These changes are

reflected in the properties of latex particularly, MST, VFA number, KOH

number and hence they received most attention.293

From the Figures 4.19 - 4.21, it is clear that TSC, DRC and

non- rubber solids of room temperature prevulcanised latex remains almost

constant during the entire period of storage of 30 days and is comparable to high

temperature prevulcanised latex. Ammonia content of the room temperature

prevulcanised latex decreases slightly on storage (Figure 4.22).

121

Figure 4.19: Effect of storage on TSC of compound C and its comparison

with compound A

Figure 4.20: Effect of storage on DRC of compound C and its comparison with

compound A

60

60.1

60.2

60.3

60.4

60.5

5 10 30

Tota

l Sol

ids C

onte

nt (m

ass%

)

Storage Time (Days)

57

57.5

58

58.5

59

59.5

60

5 10 30

Dry

Rub

ber

Con

tent

(mas

s%)

Storage Time (Days)

122

Figure 4.21: Effect of storage on non-rubber solids of compound C and its

comparison with compound A

Figure 4.22: Effect of storage on ammonia content of compound C and its


1

1.05

1.1

1.15

1.2

1.25

1.3

5 10 30

Non

-rub

ber

Solid

s (m

ass%

)

Storage Time (Days)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

5 10 30

Am

mon

ia c

onte

nt (m

ass%

)

Storage Time (Days)

123

Figure 4.23: Effect of storage on VFA Number of compound C and its


VFA number of room temperature prevulcanised latex increases on

storage (Figure 4.23). But it does not exceed the acceptable limit of VFA

number for the concentrated latex industry i.e, 0.15. During storage, the bacterial

degradation of latex constituents causes the formation of short chain fatty acids

(mostly of formic, acetic and propionic acids) with the resultant decrease in the

pH value of the latex (Figure 4.28) and a corresponding increase in VFA

number.236 Thus VFA number is used as an important measure of the level of

deterioration and stability of the latex and it is a good index of the state of

preservation of the latex.293

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

5 10 30

VFA

Num

ber

Storage Time (Days)

124

Figure 4. 24: Effect of storage on KOH number of compound C and its


The KOH number of the room temperature prevulcanised latex

increases slightly on storage (Figure 4.24). This indicates an increase in the

concentration of acids which are present as ammonium salts.235 But even after

one month, the value is very much less than the acceptable limit of 1.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

5 10 30

KO

H N

umbe

r

Storage Time (Days)

125

Figure 4.25: Effect of storage on Brookfield viscosity of compound C and its


The increase in Brookfield viscosity during storage (Figure 4.25)

is due to ZnO thickening of latex and also due to the slow liberation of Zn ions

from the accelerator.43 The zinc ions tend to destabilize the latex by neutralizing

some of the fatty acid anions and proteinate anions on the rubber particle

surface.

0

10

20

30

40

50

60

70

80

90

5 10 30

Bro

okfie

ld v

isco

sity

(cps

)

Storage Time (Days)

126

Figure 4.26: Effect of storage on MST of compound C and its comparison

with compound A

MST of room temperature prevulcanised latex increases on storage

(Figure 4.26). On storage ammonia present in the latex, alkali and potassium

soap added during compounding, slowly hydrolyze the proteins and

phospholipids to fatty acid anions and other products. The liberated fatty acid

anions are adsorbed at the particle interfaces and thus enhance the stability of

latex due to a higher surface charge and therefore a higher repulsive energy

between particles.293 This accounts for the increase in MST of the prevulcanised

latex.

High temperature prevulcanised latex has low value of MST

compared to room temperature prevulcanised latex. Thus, one of the advantages

of room temperature prevulcanised latex is its high colloidal stability, which will

be of significant technological importance to the rubber dipped goods

manufacturing industry providing a means for effecting better product quality

control and improvement. Another important advantage is its non-consumption

of energy during prevulcanisation which will make the process highly economic.

0

200

400

600

800

1000

5 10 30

Mec

hani

cal S

tabi

lity

Tim

e (s

)

Storage Time (Days)

127

Figure 4.27: Effect of storage on coagulum content of compound Cand its


Coagulum content of the room temperature prevulcanised latex

remained almost constant during storage (Figure 4.27) and is comparable to that

of high temperature prevulcanised latex.

Figure 4.28: Effect of storage on pH of compound C and its comparison with

compound A

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

5 10 30

Coa

gulu

m c

onte

nt (m

ass%

)

Storage Time (Days)

0

2

4

6

8

10

12

5 10 30

pH

Storage Time (Days)

128

4.3 Prevulcanisation of NRL at 400C

Using Zn(bxt)2/ZDC accelerator system, temperature of sulphur

prevulcanisation could be reduced from 55-600C (conventional high temperature

procedure) to 400C and heating period of latex could be reduced from 2-3 hours

to one hour. The reduction in time and temperature of prevulcanisation

decreases the opportunity for rubber degradation and help for the improvement

of physical properties of thin film products. So Zn(bxt)2/ZDC accelerator system

can be used for the cost effective factory production of rubber dipped goods like

gloves, balloons, condoms, catheters etc and thus contribute to the energy saving

process.

Figure 4.29 shows the variation of swell index of 400C prevulcanised

NRL films with cure time, before and after thermal ageing.

Figure 4.29: Variation of swell index of 400C prevulcanised NRL films with

cure time, before and after thermal ageing

From the Figure 4.29 it is clear that as cure time increases swell index

value decreases and reaches a minimum value for 25 minutes cured sample. So

the optimum cure time was found to be 25 minutes at 900C.

0

1

2

3

4

5

6

15 20 25 30

Swel

l Ind

ex

Cure Time (Minutes)


129

Figure 4.30 shows the variation of apparent crosslink density of 400C

prevulcanised NRL films with cure time, before and after thermal ageing.

Figure 4.30: Variation of apparent crosslink density of 400C prevulcanised NRL

films with cure time, before and after thermal ageing

Figure 4.31 shows the variation of chemical crosslink density of 400C


0

0.05

0.1

0.15

0.2

0.25

0.3

15 20 25 30

App

aren

t cro

sslin

k de

nsity

Cure Time (Minutes)


130

Figure 4.31: Variation of chemical crosslink density of 400C prevulcanised

NRL films with cure time, before and after thermal ageing.

Figures 4.32-4.37 shows the variation of tensile properties of 400C


0

1

2

3

4

5

6

15 20 25 30

Che

mic

al c

ross

link

dens

ity( x

10-5

g m

ol-1

cm-3

)

Cure Time (Minutes)


131

Figure 4.32: Variation of tensile strength of 400C prevulcanised NRL films

with cure time, before and after thermal ageing.

Figure 4.33: Variation of elongation at break value of 400C prevulcanised NRL


0

5

10

15

20

25

30

35

15 20 25 30

Tens

ile S

tren

gth

(MPa

)

Cure Time (Minutes)


10701080109011001110112011301140115011601170

15 20 25 30

Elo

ngat

ion

at B

reak

(%

)

Cure Time (Minutes)


132

Figure 4.34: Variation of 100% tensile modulus of 400C prevulcanised NRL

films with cure time, before and after thermal ageing.


films with cure time, before and after thermal ageing.

0

0.2

0.4

0.6

0.8

1

1.2

15 20 25 30

100%

Ten

sile

Mod

ulus

(MPa

)

Cure Time (Minutes)


0

0.5

1

1.5

2

15 20 25 30

300%

Ten

sile

Mod

ulus

(MPa

)

Cure Time (Minutes)


133





0

0.5

1

1.5

2

2.5

3

3.5

4

15 20 25 30

500%

Ten

sile

Mod

ulus

(MPa

)

Cure Time (Minutes)


0

1

2

3

4

5

6

7

8

15 20 25 30

700%

Ten

sile

Mod

ulus

(MPa

)

Cure Time (Minutes)


134

From the figures it is evident that the crosslink densities and tensile

properties increases as cure time increases and optimum properties were

obtained for 25 minutes cured samples. Elongation at break values decreases as

cure time increases and reaches a minimum value for 25 minute cured samples.

Thus both swelling measurements and tensile property measurements suggests

that the optimum cure time was 25 minutes at 900C. Properties of latex

prevulcanised at 400C were improved after thermal ageing at 700C for 24 hours.

4.4 CONCLUSIONS

Zn(bxt)2/ZDC accelerator combination can bring about prevulcanisation

of NRL at room temperature. It takes 5 days for optimum

prevulcanisation at room temperature using Zn(bxt)2/ZDC system. The

amount of Zn(bxt)2 and ZDC required for the prevulcanisation is

optimized as 1.5 phr each.

The tensile properties were found to be better for the room temperature

prevulcanised latex compared to high temperature (55-600C)

prevulcanised latex. Tensile properties of films prepared from room

temperature prevulcanised latex is improved after ageing but that of

prevulcanised latex prepared by heating decreases.

The colloidal properties of room temperature prevulcanised latex,

prepared using Zn(bxt)2/ZDC accelerator combination is found to be

superior to that of conventional high temperature prevulcanised NRL.

MST of room temperature prevulcanised latex increases on storage and

has high value compared to high temperature prevulcanised NRL. Even

after one month of storage, the values of VFA number and KOH

number of room temperature prevulcanised latex are much less than the

acceptable limits of these values for the concentrated latex industry.

The properties like TSC, DRC, non-rubber solids and coagulum content

of room temperature prevulcanised latex have values comparable to

high temperature prevulcanised NRL.

Zn(bxt)2/ZDC system can also be used for the prevulcanisation of NRL

at 400C.

low temperature prevulcanisation of nr latex...

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