Polyurethane adhesive based on polyol monomers BHET and BHETA depolymerised from PET waste

Abstract

During the development of the polyurethane (PU) industry, one of its biggest challenges has been synthesis and formulation-related due to a lack of renewable resources and thus an over-reliant dependence on fossil reserves which has led to considerable research effort focused on alternative renewable sources of supply. In this study, PET bottle waste and castor oil have been considered as alternative sources for the synthesis of a PU adhesive. A cheap and environmentally friendly synthesis pathway for polyols is provided using depolymerization reactions of recyclable materials. The structures of the polyols have been demonstrated using Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. Different NCO/OH ratios with calculated amounts of MDI were used to prepare polyols which were then employed during PU preparation and their effects on the resultant PUs were investigated. Solvent-free polyurethane adhesives were synthesized. Changes in the properties of PU synthesized by the addition of castor oil and boron species to the structure were also determined. The thermal behaviour of PU samples were characterized by DSC-TG; and functional group detection and covalent bonding formation were investigated via FTIR analysis.

Introduction

Polyethylene terephthalate (PET) is a semi-crystalline thermoplastic polyester with high strength, transparency and safety characteristics [1]. Since PET is a durable and strong material, it is commonly used in storage products like plastic bottles. However, the durability of this material brings problems because great quantities of household polymeric waste can cause serious environmental pollution. PET is a non-degradable plastic under normal conditions as there is no known organism that can consume it [2]. Current methods of disposing of plastic waste, has been shown to be harmful to humans and marine wildlife. For example, according to a study published in 2017, 21 different types of table salt were examined in Spain and plastics were found in all of them [3]. Sixteen different table salt brands were considered in a similar study in 2018 in Turkey [4]. In 2018, 93% of 259 bottles of water supplied from 19 different regions of nine countries were found to contain micro plastic contamination [5]. In this case, it is very important to reduce the amount of plastic waste and to produce valuable materials from low cost sources in the recycling of PET.

Polyurethane (PU) is a polymeric material which can be produced from a reaction between polyol and isocyanate groups [6]. It is the only class of polymer that can display thermoplastic, elastomeric and thermoset behavior depending on its chemical and morphological makeup [7]. This degree of versatility has resulted in its use in a widespread range of application areas. The application areas and properties depend on the chemical nature of the reactives, reaction parameters, different NCO:OH stoichiometric ratios, together with the use of chain extenders and catalysts [8]. One of the well-known fields of PU synthesis is in adhesive technology where several different types of adhesives have been available in the market. Although urea-formaldehyde and phenol-formaldehyde type adhesives are among the most common types of adhesives, particularly in wood bonding applications, they have recognized drawbacks such as sensitivity to hydrolysis, high energy consumption (curing temperature at 130–160 °C) and toxicity caused by the release of formaldehyde [9]. Polyvinyl acetate is another adhesive type which has been employed in bonded wooden structures which has been shown to suffer from factors such as low resistance to heat and moisture, susceptibility to creep under long-term stress and a low gap filling ability [9]. PU adhesives have been recognized as potential alternatives to other adhesive types since they have been shown to demonstrate excellent adhesion and wetting properties on wood substrates, chemical versatilities, resistance to weathering, flexibility in formulation and reasonable curing time [10]. A critical problem in the synthesis of PUs is that the current polyol industry is heavily dependent on oil-based resources [11]. Because natural sources are limited, searching for different ways for polyol synthesis is an important issue.

Several studies have considered the synthesis of PU from PET waste. In 2015, Taheri et al. published an article concerning a glycolized PET based PU [12] where a depolymerization reaction was performed with propylene glycol and the resulting product was used to obtain a PU which was investigated to assess properties such as compressive strength, modulus, thermal stability and the thermal conductivity coefficient. In 2014, a PU was synthesized from bis(2-hydroxyethyl) terephthalate (BHET), polyethylene glycol as a chain extender and hexamethylene diisocyanate [13]. The resulting PU and alginate were then blended and their swelling degree was investigated. Sinha et al. used BHET based phosphorous containing polyols and diisocynates to generate a flame retardant PU coating [14]. Here, post consumed polyester clothes were subjected to glycolysis to obtain BHET. Beside these publications, several studies have considered the synthesis of PU from the aminolysis of PET. Bis-(3-hydroxy propyl) terephthalamide (BHPTA) was synthesized and reacted with ε-caprolactone, then assessed for pencil hardness, scratch resistance, impact resistance, anticorrosive properties and chemical resistance [15]. In 2019, a bioresorbable poly(ester amide) was developed using soybean oil, bis(2-hydroxyethyl) terephthalamide (BHETA) and other low cost renewable resources such as citric acid, sebacic acid and mannitol [16]. It was concluded that the synthesized polymer might be an alternative low cost biopolymer for bone tissue engineering application. Gupta et al. prepared a polymer modified bitumen (PMB) using in situ polymerization of BHETA and MDI in the bitumen [17]. They compared it with styrene butadiene styrene (SBS) based PMB in terms of conventional binder tests. For another BHETA based PU, Abdouss et al. synthesized it with hexamethylene diisocyanate, polyethylene glycol and BHETA in DMF and then thermal stability, strength and stiffness of PUs were studied [18].

PET based polyurethanes also have been used in adhesive applications. For example, in one study PET was depolymerised with glycol glycoside which was obtained from potato starch [19]. With changes in glycoside content, three PET depolymerised oligomers were generated and used to obtain adhesives for styrene butadiene rubber strips. According to Zawadzki and coworkers, an alcolysis reaction of PET with diethylene glycol (DEG) was carried out under nine different conditions by changing time, temperature, amount of DEG and catalyst [20]. Reactions between PET polyol, TDI and DEG were conducted in methylethylketone as solvent. The resulting adhesive was used to bond Pinus taeda wood which resulted in a shear bond strength of 7.5 MPa.

In the light of this information, it can be seen that although there has been a considerable number of PET depolymerization studies, most have not related with adhesive technology. On the other hand, studies which include adhesive applications have generally focused on obtaining oligomers from PET waste. In this study, PET waste was used to obtain two different monomers (BHET, BHETA) and they have been used to generate a PU adhesive. In addition, the effect of castor oil on the production of a PU adhesive has also been examined. In order to examine the effect of boron on the shear strength of a PU adhesive, boron compounds were also introduced to the reaction medium.