Optimized synthesis and characterization of polystyrene graft copolymers and preliminary assessment of their biodegradability and application in water pollution alleviation technologies

https://doi.org/10.1016/j.polymdegradstab.2007.01.019Get rights and content

Abstract

With more and more plastics being employed in human lives and increasing pressure being placed on capacities available for plastic waste disposal, the need for biodegradable plastics and biodegradation of plastic wastes has assumed increasing importance in the last few years. Keeping in view the environmental pollution caused by the waste polystyrene and to make the waste polystyrene technologically important, we have modified/functionalized the polystyrene with natural polymers and hydrophilic monomer through graft copolymerization. The present paper discusses the optimum conditions for the synthesis of graft copolymers and characterization of these polymers with SEMs and FTIR and thereafter biodegradation studies of these polymers by soil burial method. The present paper also discusses the effect of crosslinker concentration on the swelling and metal ion sorption (As+5 uptake) through the functionalized polystyrene, with the intention to make use of these polymeric networks in water pollution alleviation technology. It has been observed that percent As5+ uptake decreases from 80% to 60% as the crosslinker concentration increases from 0.032 mM to 0.162 mM in the polymeric networks. It has also been observed from the degradation studies that the grafting of starch onto polystyrene has induced 37% degradation after 160 days soil burial treatment and no degradation has been observed in case of grafting of acrylic acid onto polystyrene.

Introduction

Polystyrene (PSty) is one of the widely used thermoplastic than any other polymer, for the reason that of its excellent physical properties, low cost and ease of fabrication [1]. Nevertheless, its hardness, limited water absorption and chemical composition make it inert and resistant to microbial attack and it remain in the nature without any deformation for very long time, and cause environmental pollution [2], [3]. Modification/functionalization of polystyrene is one of the alternatives to overcome such problem of disposal and make it technologically important. Addition of functionalities in the plastic waste makes it labile to microbial attack. Modification with natural polymers, such as gelatin [4], wool [5], natural rubber [6], starch [7], [8] and lignin [9], [10] is the desirable approach to degrade it in the natural environmental conditions with in a reasonable time frame which make plastic susceptible to biodegradation [11].

Functionalization of polystyrene can be carried out through chemical method [12], photo-irradiation method [13] and graft copolymerization with vinyl monomers [14], [15]. Incorporation of polar functionalities onto synthetic polymer will induce hydrophilicity in these polymers [16], [17] that is the step towards induction of biodegradation and for its use in enrichment separation technology. Functionalized polystyrene finds applications in metal ions sorption for use in water alleviation technologies. Different resin based membranes have been reported for the removal of heavy metal ion toxicity from the soil [18], [19]. Polystyrene–polypropylene fibrous exchangers with amino polyaceticacids as fibrous sorbent are used for the separation, removal, and recovery of heavy metal ions in the fields of waste-fluid treatment, the food industry and in analytical industry [20].

Copolymerization of polystyrene and starch was carried out in twin-screw extruder achieving up to 60% grafting thus forming partially biodegradable plastic [21], [22], through blend formation [23], [24], [25]. Galgali and coworkers had adopted various strategies of anchoring mono- and disaccharides on to functionalized polystyrene (polystyrene–co–maleic anhydride 14% by weight) to induces biodegradability in plastic [26], [27]. Biodegradation of polyolefins and modified polystyrene has been studied through various methods but soil burial method is one of the promising method [2]. Kathiresan has studied the biodegradation of polythene bags and plastic cups after 2, 4, 6, and 9 months of incubation in the mangrove soil. Among the bacteria, Pseudomonas species degraded 20.54% of polythene and 8.16% of plastics in one month. Among the fungal species, Aspergillus glaucus degraded 28.80% of polythene and 7.26% of plastics in one month and found that mangrove soil is a better source for plastic biodegradation [28]. Kawabata and coworkers have studied that poly(styrene–co–N-benzyl-4-vinylpyridinium chloride) could be made degradable by activated sludge in soil when the oligo-styrene portion was sufficiently small [29].

Keeping in view the environmental pollution caused by the waste polystyrene and to make the waste polystyrene technologically important, we have modified/functionalized the polystyrene with natural polymers and hydrophilic monomer through graft copolymerization. The present paper discusses the optimum conditions for the synthesis of graft copolymers, characterization of these polymers with SEM and FTIR and thereafter biodegradation studies of grafted product by soil burial method. The present paper also discusses the effect of crosslinker concentration on the swelling and metal ion sorption (As+5 uptake) to make use of these polymeric networks in water pollution alleviation technology.

Section snippets

Materials and method

Polystyrene (packaging material waste) (PSty), acrylic acid (AAc) and Arsenic Test (sensitive) kit (Merck-Schuchardt, Germany), ammonium persulfate (APS), N,N′-methylenebisacrylamide (N,N-MBAAm), starch and tetrabutylammoniumbromide (TBAB) from S.D. Fine Mumbai, India were used as-received.

Graft copolymerization

One gram of polystyrene dissolved in minimum amount of toluene (3 mL) was taken in 100 mL round bottom flask placed on a water bath maintained at a definite temperature. A known amount of ammonium persulfate

Results and discussion

Ammonium persulfate (APS) is a well-known and has been extensively used as initiator for vinyl polymerization. Hydroxyl radicals arising from the reaction between SO4radical dot− and water are responsible for graft copolymerization.

Mechanism of the polymerization in the presence of the initiator is presented as:

  • Initiation

−O3S–O–O–SO3− → 2SO4−radical dotSO4−radical dot + H2O → HSO4− + radical dotOHRH + SO4−radical dot → Rradical dot + HSO4−RH + radical dotOH → Rradical dot + H2OM + radical dotOH → M–OHM + SO4−radical dot → radical dotM–SO4−2

  • Propagation

R–H + radical dotM–OH → Hradical dot + radical dotR–M–OHradical dotR–M–OH + nM → R–(M)n–Mradical dot–OHRradical dot + nM → R–(M)n−1Mradical dotradical dotM–OH + nM → HO–(M)n–Mradical dot

  • Termination

R–(M)n–Mradical dot–OH +

Conclusion

From the above discussion it has been concluded that waste polystyrene when modified via graft copolymerization of natural polymer starch, has induced the hydrophilicity and degradability in the plastic waste. From metal ion sorption study it is evident that these polymeric networks developed from the waste polystyrene can be used for the removal, separation, and enrichment of hazardous metal ions in aqueous solutions and can play an important role for environmental remediation of municipal and

Acknowledgement

The authors wish to thank the Ministry of Environment and Forest, Government of India for providing the financial assistance to this work.

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