Radiation dose required for the vulcanization of natural rubber latex

Radiat. Phys. Chem. Vol. 48, No. 4, pp. 505-510, 1996

Pergamon

80969-806X(96)00002-3

Copyright© 1996 ElsevierScienceLtd Printed in Great Britain.All rights reserved 0969-806X/96 $15.00+ 0.00

R A D I A T I O N DOSE REQUIRED FOR THE VULCANIZATION OF NATURAL RUBBER LATEX M. E. HAQUE, l N. C. D A F A D E R , 1 F. A K H T A R l and M. U. A H M A D 2 ~Nuclear Chemistry Division, Institute of Nuclear Science & Technology, Bangladesh Atomic Energy Commission, G.P.O. Box 3787, Dhaka and 2Department of Chemistry, Jahangirnagar University, Savar, Dhaka, Bangladesh (Received 22 May 1995; accepted 26 December 1995)

Al~araet--The radiation dose required for the vulcanization of natural rubber latex was optimized. To enhance the erosslinking, several sensitizers were used. Among the sensitizers, n-butyl acrylate (n-BA) alone was found to be the best one. The effects of concentration of n-BA, mixing and standing time of latex with n-BA on the tensile properties of latex film were investigated. 12 kGy radiation dose, 5 phr n-BA and 30-40 min of mixing time were found to be the optimum conditions for irradiation. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION

EXPERIMENTAL

Radiation vulcanization of natural rubber latex (RVNRL) has been carried out for several decades. At the early stages, radiation was applied on latex for vulcanization (crosslinking) without sensitizing agent. So, higher radiation dose was needed for sufficient crosslinking between the rubber molecules (Minoura and Asao, 1961a). Later on, several kinds of sensitizers were used to reduce the radiation dose required for vulcanization. Halogenated hydrocarbons such as carbon tetrachloride and ehlorofrom were used as sensitizers for radiation crosslinking (Ambroz, 1973; Kartowardoyo and Sundardi, 1977; Lamm and Lamm, 1964; Minoura and Asao, 1961b). Monofunctional and polyfunctionai monomers were also used by some investigators (Makuuchi and Hagiwara, 1984; Makuuchi et al., 1984). Usually some stabilizers are used to prevent the latex from coagulation due to the addition of sensitizer. Zhonghai and Makuuchi (1989) obtained the optimum vulcanization dose of 15 kGy using 5 phr nbutyl acrylate with potassium hydroxide as a stabilizer. Soebianto and Sundardi (1989) claimed to reduce the vulcanization dose to 10 kGy by using hydrogen peroxide together with the sensitizer system n-BA and carbon tetrachloride. In this paper the required radiation doses for vulcanization of natural rubber latex with different sensitizing systems are determined. The other conditions such as effect of mixing time of sensitizer with latex, concentration of sensitizer, standing time of latex after mixing with sensitizer on the properties of irradiated latex film are also investigated.

The latex was collected from Satgaon Rubber Estate of Sylhet zone, Bangladesh and was preserved by aqueous ammonia only. The latex was centrifuged just after bringing to the laboratory using a laboratory scale latex centrifuge machine, model SPL-100, Saito Separator Limited, Japan. Double and triple centrifuged latices were made by repeating the centrifuge process after diluting the concentrated latex by water to 30% total solid content. Ammonia was added to the latex concentrate to make it a high ammonia latex. The total solids content of the latex was adjusted to 50% by diluting with water/aqueous ammonia solution. n-Butyl acrylate (n-BA) from Kanto Chemical Co., Inc., Japan, Carbon tetrachloride (CC14), and Chloroform (CHC13) from BDH, England, n-BA + CC14, n-BA + CHC13 and n-BA + H202 were used as sensitizers. The mixed sensitizers were taken at the ratio of 1:1. Emulsions of the sensitizers were prepared by mixing the sensitizer with water (1:1) and 0.1% soap (commercially available powder soap) for 1 h. Then the emulsion was mixed with latex by stirring for a definite time period. The viscosity was measured by a cone type rotary viscometer model Visconic ELDR, Tokimec Inc., Japan, connected with viscometer controller E-200, Toki Sangyo Co., Ltd, Japan. The latex was then irradiated by T-rays from a source of Co-60 at a dose rate of 8 kGy/h. After irradiation the latex was cast on raised rimmed glass plates and air dried till transparent. The films were leached in water for 24 h and dried in air again until 505

506

M. E. Haque

et al.

Table 1. Tensilepropertiesof the radiationvuleaniz.~natural rubber latex (once centrifuged) film with variousscnsitizers(concentrationof sensitizer= 5 phr; applieddose= 12kGy) Tensile strength

Tear strength

Modulus 300%

Modulus 500%

Sensitizers

(MPa)

(N/mm)

(MPa)

(MPa)

(%)

Nil n -BA CCI 4 CHCI 3 n-BA + CC14 n-BA + CHCI3 n -BA + H2Q 2

14,38 30.66 21.36 24,85 25.59 23,78 15.32

8.25 23.23 18.44 20.17 22.02 24.47 16.53

1.47 2.63 2.33 2.62 2.22 1.72 0.83

1,47 3.18 2.48 2,86 2.82 2.32 1.63

1400 I 100 1200 1200 950 940 1160

transparent. Then the films were heated in an oven at 100°C for l h. The tensile properties were measured by INSTRON, model 1011, England, connected to a personal computer. For the measurement of tensile strength and tear strength ISO methods [(ISO 371977(E) and ISO 34-1975(E), respectively] were followed. The temperature and relative humidity of the testing environment were 25-30°C and 45-50%, respectively. Swelling ratio and gel fraction were measured according to the procedure of BS (BS 1673: Part 4, 1953). RESULTS AND DISCUSSION

Table 1 shows the tentile properties of the films prepared from the latices irradiated with various sensitizer systems. The applied radiation dose is 12 kGy. It is seen that the tensile properties of the

50I

• Reid Intex

Elongation at break

films prepared from the latices irradiated with sensitizer are higher than those of the films prepared from the latices irradiated without sensitizer. Among the sensitizer systems used, n-BA alone is the best one to enhance the radiation crosslinking which improves the tensile properties of the irradiated latex film. When n-BA was used with other co-sensitizers the effect is found to be negative for all. So n-BA was selected as the sensitizer for radiation vulcanization and the conditions for irradiation using this sensitizer were found out and optimized. Figures 1-3 show the tensile properties of the latex film prepared from field latex and concentrated latices of varying number of centrifugation, at various radiation doses using 5 phr n-BA as sensitizer. From Fig. 1 it is seen that with the increase of radiation dose the tensile strength increases and reaches maximum at the dose of 12 kGy. After this dose, there is little change of tensile strength. The

10

x Once centrifuged

• Twioe cen'~ifuged * Thrice centrifuged • Field latex 8

4O

-~-so

•

11

z Once oentrifuged

• Twice oentdfuged * Thrice oentfl~ged

I[

@

z

4

2

10

t

I o

I 12

I 18

0 24

eel., [kQyl Fig. 1. Tensile strength of RVNRL films at various radiation doses,

0

I 5

I 10

I 15

I 20

25

[kGy] Fig. 2. Modulus at 500% elongation at various radiation doses.

Vulcanization dose for NRL 2,600

• Reid I m x

z Once cen~tfuged

• Twice oenldfuged

* Thric, I oenbtfuged

2,000

i

l ,50(3

f¢

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,000

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50g

0 0

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D o s e [kGy]

Fig. 3. Elongation at break of RVNRL films at various radiation doses. same trend is obtained for all the four types of latices, namely, field latex, once, twice and thrice centrifuged latices but the values of tensile strength differ at a particular dose. The once centrifuged latex film possesses maximum tensile strength and the field latex the least. The influence of non-rubber may be attributed to the cause of varying tensile strength of the latex because the field latex and concentrated latex contain varying quantities of non-rubber (Mohid et al., 1989). Figure 2 shows the modulus at 500% elongation of the rubber film at various radiation doses. Modulus increases with the increase of dose. The change of moduli after 12 kGy is not so prominent. The same trend is observed for all types of latices. The elongation at break (Fig. 3) decreases with the increase of radiation dose. It is evident that with the increase of cross-link between the rubber molecule the elongation also decreases. Upon irradiating the latex by y-rays, crosslinking between the c/s-polyisoprene molecules occurs. The addition of n-BA to the latex before irradiation enhances the crosslinking. By the swelling ratio measurement, the crosslinking densities were calculated at various radiation doses. Figure 4 shows the crosslinking densities of the latex film at various radiation doses. The gel content of the film increases with the increase of crosslinking between the molecules. From this figure it is seen that with the increase of radiation dose, the crosslinking density increases. It is remarkable that although the crosslinking density increases with the increase of dose without RI~ 48/4----E

507

having constant value or downward trend within the experimental observation (Fig. 4), the tensile strength shows constant or little downward trend corresponding to same doses at a certain point >/12 kGy (Fig. 1). It can be assumed that for maximum tensile strength a certain number of crosslinking is sufficient which corresponds to a particular dose. In this experiment 12kGy seems to be the optimum dose for this purpose. It is also evident that although the optimum dose is 12kGy for maximum tensile strength (30.93 MPa), the ISO required value of tensile strength (24 MPa) of rubber film needed for the preparation of hand glove can easily be attained by 10 kGy radiation dose for crosslinking between the rubber molecules with 5 phr n-BA if once centrifuged latex is used. The stabifity of the latex upon addition of sensitizer before irradiation is important for the purpose of industrial use of the latex. The stability of latex against monomer loading was determined by adding varying quantities (up to 8 phr) of n-BA to the latex and measuring the change of viscosity before and after irradiation. In this experiment no remarkable increase of viscosity was observed up to 5 phr n-BA. After adding 5 phr n-BA, the viscosity of the latex was 45 mPa.s (at 20 rpm rotor speed of the viscometer). After irradiation the viscosity remains almost constant. The measured viscosity is not too high to coagulate the latex. But coagulation started when the addition of n-BA reaches to 8 phr. The stability of the latex was also tested by allowing the latex to stand for

•- -

• Field latex

x Onoe oentrlfuged

• Twioe oentdfuged

*, Thrioe centrifuged

Z

.

6

I

I

I

10

15

20

26

Fig. 4. Cross link density of RVNRL film at various radiation doses.

508

M . E . Haque et al.

6O • Tindle strength • Tea sll'ength

60

2'°0° /

5O

_T

[0

•

SO

,

~

e &

=o ~

lC

10

I 2

I I 4 6 Conoentration [phr]

0 O

01 0

Fig. 5. Tensile strength and tear strength at various concentrations of sensitizer,

I I I 2 4 6 Conoentmtion [phr]

8

Fig. 7. Elongation at break of latex film at various concentrations of sensitizer.

10

A Modulus SO0% z Modulus 500%

8

40

B

.~-so •

J

4

z

z

,ik

,~o 2o

•

I0

0

I

I

I

2

4

8

8

Conoentratlon [phr] Fig. 6. Modnli at 300 and 500% elongations at various concentrations of sensitizer.

20

I SO

I I ! 40 50 80 Time [minute]

I 70

80

Fig. 8. Tensile strength of latex film at various lengths of mixing time.

Vulcanization dose for NRL

509

Table 2. Tensilepropertiesat variousstanding times of latex after mixingwith sensitizer before irridation (concentration of sensitizer=5phr, applied dose = 12 kGy) Standing Tensile M o d u l u s M o d u l u s Elongation time strength 300% 500% at break (h) (MPa) (MPa) (MPa) (%) 0 24 48 72 96

29.17 28.12 28.71 28.43 29.82

2.24 2.93 2.81 2.92 3.05

various time periods after adding 5 phr n-BA. The viscosity of the latex remains almost unchanged up to 96 h of storage. The physical properties of the film prepared from the latex irradiated with various concentrations of n-BA are shown in Figs 5-7. In Fig. 5 it is seen that the tensile strength increases with the increased concentration of n-BA for a particular radiation dose (12 kGy). The maximum tensile strength is obtained at 5 phr n-BA. On further increase of concentration of n-BA the tensile strength was not increased rather a little downward trend is noticeable. The tear strength was also increased with the increased concentration of n-BA. The moduli at 300 and 500% (Fig. 6) increase with the increased concentration and these attain maximum values at the 5 phr concentration of n-BA. At higher concentrations the modulus at 300% shows down trend and modulus at 500% remains almost unchanged. The elongation at break (Fig. 7) decreases with the

3.51 3.61 3.53 3.87 4.19

increase of sensitizer concentration. Without sensitizer crosslinking between the rubber chains by radiation is very little. So the resulting film of the latex without sensitizer exhibits very high elongation (1400%). As the concentration of sensitizer increases the crosslinking increases and thus the elongation decreases. The stability of the latex after mixing with sensitizer but before irradiation was also tested by allowing the latex to stand for various lengths of time periods. As stated before the viscosity remains unchanged up to 96 h of storage. The tensile properties of the film, made from the latex allowed to stand for certain length of time period, were measured. Table 2 shows the effect of various standing times after mixing sensitizer with the latex before irradiation on the tensile properties of the latex film. The tensile strength, and elongation at break are not changed by standing the latex up to at least 96 h. However, the moduli at 300 and 500% elongations

10

g.ooo

• Modulul 800%

z Modulus 5 0 0 %

8

i

950 900 900 950 1000

1 ,cO0

Z e

i l.g00 ts •I t

&

&

I 80

I 60

I 70

4 X

L 1[

m

X

:

A

f

t a.

2

20

400

I 80

I 4O

, I 8O

I eO

I 7O

80

Time [minute] Fig. 9. Moduli at 300 and 500% elongations at various lengths of mixing time.

0 20

I 30

I 40 Time

00

[minute]

Fig. I0. Elongation at break of latex film at various lengths of mixing time.

510

M. E. Haque et al.

show little upward trend within this range of standing time. Homogeneous mixing of sensitizer with the latex is essential for proper crosslinking of the rubber molecules by radiation. For this purpose the sensitizer is well stirred with latex for various lengths of time period. Figures 8-10 show the effect of mixing time on the tensile properties of irradiated latex film. The tensile strength increases with the increase of mixing time (Fig. 8) reaching maximum (28.42 MPa) at 40 min mixing. On further increase of mixing time it remains almost constant. The moduli at 300 and 500% elongations (Fig. 9) and elongation at break (Fig. 10) have attained maximum values by 30 min mixing. From these results it seems that 30-40 min mixing time is sufficient to attain the ISO required value of tensile strength for the rubber film for preparing hand gloves.

REFERENCES

Ambroz H. (1973) J. Polym. Sci. Syrup. No. 42, 1339. Kartowardoyo S. and Sundardi F. (1977) Studies on the preparation and uses of Co-60 gamma ray irradiated natural latex. J. Appl. Polym. Sci, 21, 3077.

Lamm A. and Lamm G. (1964) Some important factors in the radiation treatment of natural rubber latex. Proc. 1962 Tihany Symp. Radiat. Chem., Akade'miai Kiado', Budapest, p. 245. Makuuchi K. and Hagiwara M. (1984) Radiation vulcanization of natural rubber latex with polyfunctional monomers. J. Appl. Polym. Sci. 29, 965. Makuuchi K., Hagiwara M. and Serizawa T. (1984) Radiation vulcanization of natural rubber latex with polyfunctional monomers--II. Radiat. Phys. Chem. 24, 203. Minoura Y. and Asao M. (1961a) Studies on the yirradiation of natural rubber latex. J. AppL Polym. Sci. 5, 233. Minoura Y. and Asao M. (1961b) Studies on the yirradiation of natural rubber latex. The effects of organic halogen compounds on crosslinking by y-irradiation. J. Appl. Polym. Sci. 5, 401. Mohid N., Makuuchi K., Yoshii F. and Ishigaki I. (1989) Effect of non-rubber components on sensitized RVNRL. Proc. Int. Syrup. Radiat. Vulcanization of Natural Rubber Latex, p. 157, JAERI-M 89-228, Japan. Soebianto Y. S. and Sundardi F. (1989) Effect of hydrogen peroxide on radiation vulcanization of natural rubber latex sensitized with carbon tetrachloride and n-butyl acrylate. Proc. Int. Syrup. Radiat. Vulcanization of Natural Rubber Latex, p. 319. JAERI-M, 89-228, Japan. Zhonghai C. and Makuuchi K. (1989) n-Butyl acrylate as a sensitizer for radiation vulcanization of natural rubber latex. Proc. Int. Syrup. Radiat. Vulcanization of Natural Latex, p. 326, JAERI-M, pp. 89-228, Japan.