Radiation grafting of styrene and acrylonitrile to cellulose and polyethylene

Radiation grafting of styrene and acrylonitrile to cellulose and polyethylene

Radiation Physics and Chemistry 55 (1999) 41±45 Radiation grafting of styrene and acrylonitrile to cellulose and polyethylene S. Hassanpour Gamma Irr...

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Radiation Physics and Chemistry 55 (1999) 41±45

Radiation grafting of styrene and acrylonitrile to cellulose and polyethylene S. Hassanpour Gamma Irradiation Center, Atomic Energy Organisation, P.O.B. 11365-8486, Tehran, Iran Received 6 February 1998; accepted 26 October 1998

Abstract Radiation induced graft polymerization is one of the best methods for obtaining material with new properties. In this work, radiation grafting of styrene, mixture of styrene and acrylonitrile to cellulose and polyethylene in the presence of methanol as a solvent by mutual method is discussed. At a low dose rate, high grafting yields were obtained from the two systems used, due to lesser termination of free radicals with the polymer growing radicals and recombination of primary radicals, resulting in a longer chain length of the grafted copolymer. In the system of styrene and acrylonitrile, comonomer technique was used and the styrene controlled the homopolymer formation during graft polymerization. Water uptake of cellulose decreased by increasing the grafting yields. Grafted cellulose can be molded to some extent and in a high percent of grafting, a new transparent material was obtained. By radiation grafting of styrene± acrylonitrile to low density polyethylene a high degree of crosslinking was observed. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Radiation grafting; Cellulose; Polyethylene; Styrene; Acrylonitrile

Introduction Cellulose is a raw material, that forms the basis of an important segment of the textile±paper and packaging industry. Although cellulose has good properties, it also has some undesirable ones such as low tensile strength high moisture transmission and low strength against microbial attachment. By grafting of a monomer to the cellulose backbone, some of the drawbacks of cellulose can be eliminated. Radiation grafting of cellulose and polyethylene has been studied by several authors (Chapiro, 1962; Charlesby, 1960; Chappas and Silverman, 1979; Hebish and Guthrie, 1981; Ang et al., 1983, 1977; Dworjanyn and Garnett, 1989; Garnett et al., 1990). The radiation grafting of polyethylene improves tensile strength and the chemical and heat resistance of

the polymer. The major advantages of radiation grafting are: (a) the reaction is carried out at lower temperature than in chemical operation; (b) the grafting is carried out from gaseous, vapour and liquid phase of the monomer; (c) the modi®ed material is free from residuals of initiator or catalyst. Experimental Styrene and acrylonitrile were supplied by Merck Co. The monomer were puri®ed before irradiation. Polyethylene was a low density ®lm (10100.05 mm) while cellulose was Whatman No. 4 ®lter paper (1010 mm) which were weighed before and after treatment. Polymeric substrates were immersed in methanolic solution of styrene/acrylonitrile (AS). Table 1 lists the composition of the formulations.

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S. Hassanpour / Radiation Physics and Chemistry 55 (1999) 41±45

% gel ˆ …W2=W1† 100

Table 1 The composition of solution for grafting Code

% methanol

% styrene

% acrylonitrile

AS2 AS3 AS4 AS5 AS6 AS7 AS8

20 30 40 50 60 70 80

16 21 24 25 24 21 16

64 49 36 25 16 9 4

Irradiation at higher dose rates was carried out by 60-cobalt in Gamma Cell 220, the dose rate was 7.2 kGy/h and after inserting the lead attenuator, the measured dose rate became 2.4 kGy/h. A Gamma beam 150 at dose rate of 0.036 kGy/h was used for low dose rate. The dose rates were measured by Fricke dosimeter. At the completion of grafting the ®lms were removed from the solution, washed in a soxhelt extraction for 48 h and dried in vacuum oven to constant weight. The solvent used for styrene grafted to polymer substrates was chloroform and for AS grafted samples was acetone. The percent of grafting was determined by: % graft ˆ …W1 ÿ W0†=W0 100 where W0 and W1 represent the weights of ungrafted and grafted samples. The gel content of polyethylene after extraction with xylene for 48 h and drying to constant weight was determined by

W2 is the weight of grafted sample after xylene extraction. Water uptake of the cellulose was determined by immersing the samples in distilled water at room temperature for 24 h. FTIR spectra were recorded by Bruker IFS-45 spectrophotometer. Results and discussion Infrared spectra of grafted cellulose (Fig. 1) and polyethylene (Fig. 2) after solvent extraction show absorption at 2240 cmÿ1 which is attributed to C2N. Absorption at 700 cmÿ1 and combination bands in the 2000± 1600 cmÿ1 region indicated the presence of styrene. E€ect of radiation dose rate The results in Table 2 show that in the presence of AS monomer system the grafting yields at a dose rate of 0.036 kGy/h are higher when compared with the grafting yields at high dose rates. The increase in grafting yield is observed over the whole monomer concentration range studied and particular in (AS4±AS5) the maximum degree of grafting is obtained. A possible explanation is that when the dose rate is high the concentration of free radicals is increased. Accordingly recombination of primary radicals into inert species in the bulk medium is signi®cant and therefore there are only a few radicals available to start copolymerization. Most radicals undergo recombination or initiate homopolymerization. The lower dose rate initiates mainly graft copolymerization due to lesser termination of free radicals with the polymer

Fig. 1. The infrared spectra of (a) ungrafted cellulose, (b) grafted cellulose with AS.

S. Hassanpour / Radiation Physics and Chemistry 55 (1999) 41±45

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Fig. 2. The infrared spectra of (a) ungrafted polyethylene, (b) grafted polyethylene with AS.

growing radicals and recombination of primary radicals resulting a longer chain length of the grafted copolymer and elimination of homopolymer (Kiatkamjornwong et al., 1993). E€ect of monomer Fig. 3(a±b) shows that the yield of grafting with AS are higher for the two polymeric substrates when compared with styrene alone. This may be related to Gvalue of the two monomers that for acrylonitrile is higher than styrene (GAn=2.4±5.6, GSt=0.66±0.69). For this reason, in styrene graft polymerization the Table 2 E€ect of dose rate on grafting yield of AS to polyethylene and cellulose at di€erent dose ratesa Code

AS2 AS3 AS4 AS5 AS6 AS7 AS8

% graft ab

Cellulose bc

cd

ab

0.5 18 27 18 21 9 3

2 21 26 27 46 23 21

Ð Ð 2320 2410 2090 1250 Ð

19 36 49 29 25 6 3

Polyethylene bc cd 21 23 25 41 35 10 7

Ð 340 5430 5500 2410 1260 240

a

Total dose 4 kGy. a: dose rate, 7.2 kGy/h. c b: dose rate, 2.4 kGy/h. d c: dose rate, 0.036 kGy/h. b

Fig. 3. Radiation grafting of styrene and AS to (a) Cellulose, (b) Polyethylene at 4 kGy, dose rate = 0.036 kGy/h.

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S. Hassanpour / Radiation Physics and Chemistry 55 (1999) 41±45

Fig. 4. The gel content of PE.

formation of homopolymer is very low. Whereas in acrylonitrile graft polymerization the formation of homopolymer is very high. In the system of styrene and acrylonitrile, the styrene controls homopolymer formation during graft polymerization, Garnett and co-workers used MMA as a model monomer and they reached the same conclusion (Viengkhou et al., 1997). In styrene acrylonitrile graft polymerization, at low dose rate after irradiation to 4 kGy, the solution in

which the samples were immersed disappeared and the samples changed in size and shape. The cellulose samples which were square ®lms, changed in thickness turning into cubic form. The polyethylene ®lm samples, which were also square in form, increased about ®ve times in size but retained the geometric form. At low dose rate, the polyethylene and cellulose swelled during reaction and grafting occurred homogeneously throughout the whole ®lm. This process con-

Fig. 5. The water uptake of cellulose.

S. Hassanpour / Radiation Physics and Chemistry 55 (1999) 41±45

tinued until the two polymers were totally grafted in the depth. Variation in properties In the case of polyethylene, after extraction of AS grafted samples with xylene, a high degree of crosslinking was observed (Fig. 4). In the samples grafted with styrene no crosslinking were obtained. It could be concluded that when the yield of grafting is very high in acrylonitrile, its termination occurs via the combination of two growing chains of grafted branches and leads to a tri-dimensional network. In contrast, cellulose samples after extraction with acetone showed decrease in the size of ®brillar of fragments that occurred at low dose rate and the cellulosic sheets broke into short fragments and particles. This could be explained by, polyethylene being a polymer of crosslinking type and cellulose being a polymer of degrading type. An experimental observation showed that the grafted cellulose can be molded to some extent, and in a high percent of grafting a new transparent material was obtained. Grafting onto cellulose usually reduced the water uptake due to the hydrophobic nature of grafted branches. Fig. 5 shows the water uptake of grafted samples.

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References Ang, C.H., Davis, N.P., Garnett, J.L., Yen, N.T., 1977. The nature of bonding during ®lm formation on polyole®ns and cellulose using UV and ionising radiation initiation. Radiat. Phys. Chem. 9, 831. Ang, C.H., Garnett, J.L., Long, M.A., Levot, R., 1983. Novel additives for accelerating radiation grafting of monomers to polymers in acid media. Radiat. Phys. Chem. 22, 831. Chappas, W.L., Silverman, J., 1979. The e€ect of acid on the radiation-induced grafting of styrene to polyethylene. Radiat. Phys. Chem. 14, 847. Chapiro, A., 1962. Radiation Chemistry of Polymeric System Interscience, London Chapter 12. Charlesby, A., 1960. Atomic Radiation and Polymers Pergamon Press, Oxford Chapter 23. Dworjanyn, P.L., Garnett, J.L., 1989. The role of multifunctional acrylates in radiation grafting and curing reactions. Radiat. Phys. Chem. 33, 429. Garnett, J.L., Jankiewicz, S.V., Sangster, D.F., 1990. Mechanist aspect of the acid and salt e€ect in radiation grafting. Radiat. Phys. Chem. 36, 571. Hebish, A., Guthrie, J.T., 1981. The Chemistry and Technology of Cellulosic Copolymers Springer, Berlin Chapter 3.

Acknowledgements

Kiatkamjornwong, S., Chvajarenpun, J., Nakason, C., 1993. Modi®cation on liquid retention property of cassava starch by radiation grafting with acrylonitrile. 1. E€ect of gamma irradiation on grafting parameters. Radiat. Phys. Chem. 42, 50.

The author would like to thank Dr M. Sohrabpour, Director of the Gamma Irradiation Center, for supporting this work.

Viengkhou, V., Ng, L.T., Garnett, J.L., 1997. The e€ect of additives on the enhancement of methyl methacrylate grafting to cellulose in the presence of UV and ionising radiation. Radiat. Phys. Chem. 49, 597.