A useful method for thorough dehydrochlorination of Poly(vinylidene chloride-co-vinyl chloride) using Zinc(II) oxide

Highlights

• PVDC-PVC copolymer dehydrochlorinated at a low temperature (around 423 K) in the presence of ZnO.

• Only a small percentage of chlorine atoms remained with a low amount of HCl.

• ZnO particles dispersed into the polymer matrix were effective for thorough dehydrochlorination.

• The efficiency of dehydrochlorination was a result of increasing the contact area with ZnO particles.

• Scission or isomerization to polyene also proceeded over a short period, forming a microporous carbonaceous solid.

Abstract

A poly(vinylidene chloride)-based polymer is a good material for food packaging as a barrier for oxygen and steam; however, there are problems related to chemical recycling due to low temperature degradation. To aid in developing a chemical recycling method without the evolution of harmful hydrochloric acid (HCl) and organochlorines, we have proposed a dehydrochlorination process at a low temperature (around 423 K) using zinc(II) oxide (ZnO). ZnO particles dispersed into the polymer matrix were found to be effective for thorough dehydrochlorination because the reaction promoters (zinc chloride [ZnCl₂] and water) spontaneously penetrated the polymer via exothermic reactions. As a result, only a small percentage of chlorine atoms remained with a low amount of HCl. The efficiency of dehydrochlorination was a result of increasing the contact area with ZnO particles using a heat-kneading apparatus. The catalyst ZnCl₂ was involved in the cross-linking reaction to form C–C single bond. Due to exothermic reaction, the scission or isomerization to polyene and aromatics also proceeded over a short period, leading to a microporous carbonaceous solid with a surface area of 5 × 10² m²/g.

Introduction

Chlorinated plastics are useful materials because of their excellent mechanical, electrical, and chemical properties. A copolymer consisting of poly(vinyl chloride) (PVC) and poly(vinylidene chloride) (PVDC) is a raw material used in food packaging as an oxygen and steam barrier. The PVDC copolymer (PVDC–PVC) brings difficulties in chemical recycling, compared to pure PVC [1], because of its low degradation temperature. Solid PVDC begins to degrade around 393 K, with hydrochloride gas evolution beginning at 463 K [[2], [3], [4]]. The evolved HCl gas from the thermal degradation often causes corrosion of recycling equipment. Additional organic chlorine gas is generated as trichlorobenzene at 503 K [5]. Strict control of the dehydrochlorination temperature has been reported to produce Cl-free hydrocarbons from PVDC-containing mixed plastics [6]. Therefore, a dehydrochlorination process operated at a low temperature for PVDC-containing polymers is of high importance in an environmentally-benign recycling process for waste plastics.

Pyrolysis of PVDC–PVC has provided microporous carbon [7] with a specific surface area of 700–1300 m²/g [[8], [9], [10], [11]]. Although they have been investigated in electrical and molecular sieve applications, a higher temperature (973–1273 K) has been required (for homogeneous PVDC polymer, at least 673 K is necessary [12]). Chemical functionalization of PVDC–PVC by dehydrochlorination, including substitution and carbonization, is also a valuable solution for an alternative recycling process operated in a low temperature region. A wet treatment dechlorination of PVDC–PVC has been proposed by using ethylene glycol/aqueous sodium hydroxide (NaOH), where Cl is substituted with OH groups at 463 K [13,14]. The wet processes also enable substitution with nucleophilic functionalities [15]. In a dry process, metallic zinc particles have been used as an absorber of HCl through a mechano-chemical reaction [16].

We have investigated dry process dehydrochlorination of chlorine-containing polymers. Dehydrochlorination of PVC proceeds in the presence of zinc oxide (ZnO) at lower temperature (at around 473 K) than that of pure PVC, accompanied by an evolution of water and a slight amount of HCl (within 1%) without organic gases [17]. At the initial stage, PVC (particle size of 50–150 μm) is reacted with partial ZnO particles, forming an acidic aqueous solution comprised of water and zinc chloride (ZnCl₂). The acidic solution penetrates into the PVC matrix as a reaction promoter accompanied by the dissolution of other ZnO particles. In addition, we have deduced that ZnCl₂ in the acidic solution promotes the dehydrochlorination through the cross-linking C–C single bond formation in/among polymer chain(s). As a result, 70% of the particulate PVC was dehydrochlorinated. It has also been reported that ZnCl₂ and Zn₂OCl₂ act as catalysts for cross-linking reactions of chlorinated poly(isoprene-o-isobutylene) to form a new C–C single bond [18,19]. Therefore, using ZnO, instead of ZnCl₂ and zinc oxide chloride (Zn₂OCl₂), plays an important role in suppressing HCl gas evolution during the dehydrochlorination process as well as acting as a precursor of the reaction promoter. We have translated the learnings of other chlorinated polymers to PVDC–PVC in this study using ZnO. Since the penetration of the liquid reaction promoter is of importance in enhancing dehydrochlorination, we have devoted efforts to increase the contact area between ZnO and PVDC–PVC using organic solvents (tetrahydrofuran) and without organic solvents (heat kneading) via mixing.