The advancement of bis(2-hydroxyethyl)terephthalate recovered from post-consumer poly(ethylene terephthalate) bottles compared to commercial polyol for preparation of high performance polyurethane
Graphical abstract
Introduction
Polyurethane (PU) is one of the most versatile polymeric materials and is used in a broad spectrum of industrial applications. The applications of PU include foam and non-foam materials. PU foam (PUf) is formed by the reaction of a polyol with a suitable diisocyanate/polyisocyanate in the presence of a catalyst and blowing agents. PUf can be made flexible, semi-rigid, or rigid by tailoring and selecting the relevant types and amounts of polyol, diisocyanate or polyisocyanates, blowing agents, surfactants, and catalysts to meet specific demands [1]. In the PU foam industry, rigid PUf is widely used as thermal and electrical insulation materials in refrigerators, construction buildings, furniture, and chemical pipelines for transportation due to its exclusive combination of excellent properties, such as lightweightness, low thermal conductivity, low water absorption, high compressive strength, and high energy-absorbing ability [2], [3].
Commonly, only a few diisocyanates/polyisocyanates are generally used, whereas the different polyols typically determine the eventual properties of the obtained polyurethane [4]. To date, PUf is mainly derived from petrochemical-based polyether and polyester polyols, such as polyethylene glycol (PEG), as the starting material.
Regarding plastic waste treatment, scientists have recently focused on the rapid progress of innovative and eco-friendly recycling technologies. Furthermore, the replacement of virgin petrochemical-based resins by recycled or environmentally friendly resources is currently a ubiquitous trend worldwide, leading to a reduction in the utilization of materials derived from petroleum feedstocks [5], [6], [7]. Accordingly, recycling polymers is also an effective way to reduce the accumulation of plastic waste in the environment.
Poly(ethylene terephthalate) (PET) is an important commercial thermoplastic that is widely used in specific applications, ranging from textiles, thin films, and packaging materials to the production of single-use soft drink and water bottles. Unfortunately, the irresponsible utilization of this non-biodegradable and chemical-resistant plastic has led to its massive accumulation in the environment, and PET has become the most recycled material of all thermoplastics [8]. Chemical recycling is the primary process of converting PET waste into monomers/oligomers and other chemical substances by total or partial depolymerization. Of the feasible chemical recycling approaches, the glycolysis of PET is one of the most sustainable methods that meets the requirements for renewable material development [9], [10], [11], [12], [13], [14]. Glycolysis is a transesterification reaction that converts PET into oligomers with terminal hydroxyls in the presence of catalysts. The glycolysis of PET can produce monomers/oligomers with different chemical structures and types by varying the type of glycol, such as ethylene glycol (EG), diethylene glycol, propylene glycol, poly(ethylene glycol) or glycerol. When excess EG is used, the main glycolyzed product of PET is bis(2-hydroxyethyl) terephthalate (BHET), which is the raw feedstock for synthesizing virgin PET, unsaturated polyester resin [15], [16], [17], [18], and polyurethane foam (rPUf) [9]. This process could effectively and usefully reduce abundant amount of waste PET and produce very high quality technical polymers.
rPUf (PUf based on BHET) and cPUf (conventional PUf based on a commercial polyol) are considered to be highly combustible materials, that is, susceptible to fire when exposed to an ignition source, and they easily and continuously release a large amount of heat and toxic smoke. Therefore, anti-fire safety is an essential prerequisite for most PUf applications [19], [20], [21]. To enhance the flame retardancy for special applications, the incorporation of flame retardant (FR) additives is a necessary objective. These flame retardants are commonly composed of halogens, phosphorus, or nitrogen elements [22], [23], [24]. Some flame retardant additives mainly act in the gas phase by forming free radicals, which could capture active radicals in the combustion zone to stop free radical chain reactions. Another type of flame retardant primarily promotes the formation of condensed and thermally stable residual char, which prevents the exposure of inner layer materials to oxygen and heat sources, thereby protecting them [25]. Interestingly, the combination of both gas- and condensed-phase flame retardant mechanisms is applied in rPUf, and as a consequence, a better fire performance can be obtained.
In previous work, we investigated the fabrication of an excellent flame-retardant rigid polyurethane foam based on a novel oligo-ester-ether-diol, a product of the glycolysis of recycled PET with diethylene glycol [26]. To the best of our knowledge, only one paper has been published on the use of BHET prepared from recycled PET waste from textile sources as the diol and dimethyl methylphosphonate (DMMP) as the FR to produce flame-retardant rigid polyurethane foams [9]. To date, no literature reports comparing the flame-retardant behaviors, thermal stabilities, and physicomechanical properties of rPUf and cPUf have been found.
Therefore, this is the first investigation detailing the comprehensive comparisons and characterizations of the flammability, thermal stability, and physical properties by various methods, which enabled the behaviors of rPUf (polyurethane foam derived from BHET obtained from waste PET bottles) and cPUf (polyurethane foam based on a commercial polyol) with non-halogen FRs to be evaluated and compared in detail.
Based on the outcomes of this study, the advantage of BHET prepared from waste PET bottle glycolysis is its application in PUfs to obtain a more durable foam with more stable thermal properties and excellent fire resistance at rather low loadings of FRs, especially in a high moisture environment. The outcomes of this study also confirmed that rPUf/FRs prepared from BHET not only satisfied the thermal stability and fire safety requirements of polymer applications, but were also composed of a high percentage of waste PET bottles from related industries, which can help reduce the amount of recycled polymer materials and improve waste management.
Section snippets
Materials
Post-consumer PET soft drink bottles were collected and cut into small pieces of 5 × 5 mm2 after removing the caps and labels. The PET flakes were washed with distilled water to remove remaining impurities and then oven-dried at 60 °C until their weight remained unchanged. Ethylene glycol (EG) and zinc acetate dihydrate (Zn(CH3COO)2·2H2O) were purchased from Xilong, China. Polyol (Voracor CR 765, hydroxyl value: 360 mg KOH/g, viscosity and density of 800 mPa·s and 1.12 g/cm3, respectively, at 20 °C)
BHET preparation and characterization
In the PET production, BHET was prepared as an intermediate by a bottom-up process, by a condensation of terephthalic acid with ethylene glycol or by transesterification of dimethyl terephthalate with ethylene glycol (Scheme 1). If PET waste was used as a starting material for glycolysis by EG, the same BHET was created. This is the top-down process for BHET preparation.
The structure of BHET was confirmed by 1H NMR characterization (Fig. 1). The signals in the 1H NMR spectrum (DMSO, ppm) were
Conclusions
The difficulty of using solid BHET was overcome by heating a BHET mixture until it melted, quickly cooling it and then keeping it above 70 °C to avoid solidification during mixing with a diisocyanate.
In summary, the results of this study showed that rPUf and cPUf with non-halogen flame retardants, such as TPP, OP, and ATH, exhibited notably efficient flame retardancy and high thermal stability. rPUf and the rPUf/FRs exhibited much higher fire resistance performances than cPUf and the cPUf/FRs,
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Notes
The authors declare no competing financial interest.
Funding sources
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.02–2017.317.
Ms Chi Thi Pham was supported by the Domestic Master/PhD Scholarship Programme of Vingroup Innovation Foundation.
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