Producing polyglycerol polyester polyol for thermoplastic polyurethane application: A novel valorization of glycerol, a by-product of biodiesel production

Abstract

The production of biodiesel generates glycerol as a by-product that needs valorization. Glycerol, when converted to polyglycerol, is a potential polyol for bio-based thermoplastic polyurethane (TPU) production. In this study, a novel polyglycerol polyester polyol (PPP) was developed from refined glycerol and coconut oil-based polyester polyol. Glycerol was first converted to glycerol acetate and then polymerized with coconut oil-based polyester polyol (CPP) as secondary polyol and phthalic anhydride. The resulting PPP polymerized at 220 °C and OH:COOH molar ratio of 2.5 exhibited an OH number of <100 mg KOH·g sample^-1, an acid number of <10 mg KOH·g sample^-1, and a molecular weight (MW) of 3697 g mol^-1 meeting the polyol requirement properties for TPU (Handlin et al., 2001; Parcheta et al., 2020) [1-2]. Fourier-transform infrared (FTIR) spectroscopic characterization determined that higher reaction temperatures increase the polymerization rate and decrease the OH and acid numbers. Further, higher OH:COOH molar ratios decrease the polymerization rate and acid number, and increase the OH number. Gel permeation chromatography determined the molecular weight of PPP and suggested two distinct molecular structures which differ only in the number of moles of CPP in the structure. A differential scanning calorimetric (DSC) experiment on a sample of PPP-based polyurethane revealed that it was able to melt and remelt after 3 heating cycles which demonstrates its thermoplastic ability. The novel PPP derived from the glycerol by-product of biodiesel industries can potentially replace petroleum-derived polyols for TPU production.

Keywords: Bio-based; Coconut oil-based polyol; Glycerol; Polyglycerol; Thermoplastic polyurethane.

Fig. 1

General reaction equation of polyurethane synthesis.

Fig. 2

Effect of OH:COOH molar ratio on viscosity (a), OH number (b), and acid number (c). All reactions were performed at 220 °C for 4 h.

Fig. 3

Full Fourier transform infrared (FTIR) spectra (a), partial FTIR spectra showing the OH absorption (b) of the remaining alcohol groups and partial FTIR spectra showing CO absorption (c) of the ester groups formed by the reaction of alcohol and carboxylic acid groups of the polyglycerol polyester polyol synthesized using different reaction temperatures (160, 180, 200 and 220 °C) and an OH:COOH molar ratio of 1.0 for 4 h.

Fig. 4

Full Fourier transform infrared (FTIR) spectra (a), partial FTIR spectra showing the OH absorption and aromatic CH absorption (b) of the remaining alcohol groups and the CH bonds of the phenyl group of phthalic anhydride, and partial FTIR spectra showing CO absorption (c) of the ester groups formed by the reaction of alcohol and carboxylic acid groups of the polyglycerol polyester polyol synthesized using different OH:COOH molar ratios (1.0, 1.5, 2.0, and 2.5) and a reaction temperature of 220 °C for 4 h.

Fig. 5

Gel permeation chromatogram of polyglycerol polyester polyol (black, left y-axis) synthesized using 220 °C reaction temperature and 2.5 OH:COOH molar ratio. Molecular weights were calculated based on polystyrene standards (blue, right y-axis).

Fig. 6

Synthesis of polyglycerol polyester polyol from coconut oil-based polyester polyol, phthalic anhydride, and glycerol monoacetate from the acetylation of glycerol to glycerol acetate. The two proposed structures of polyglycerol polyester polyol were based on the two molecular weights observed on its gel permeation chromatographic experiment where the difference of the observed molecular weight corresponds to the molecular weight of coconut oil-based polyester polyol. Structure (a) has two molecules of coconut oil-based polyester polyol while structure (b) has only one.

Fig. 7

Proposed additional structures based on the presence of glycerol and 1,3 glycerol diacetate in glycerol acetate; structure (a) has one and (b) has two coconut oil-based polyester polyol similar to Fig. 6b and a respectively but with a random arrangement of glycerol, glycerol monoacetate and glycerol diacetate on both of its alcohol groups, while structure (c) is a random arrangement of glycerol, glycerol monoacetate, glycerol diacetate and phthalic anhydride without a coconut oil-based polyester polyol in the structure.

Fig. 8

Differential scanning calorimetry thermogram showing the endothermic curves of the first, second and third heating of a thermoplastic polyurethane synthesized from 100% polyglycerol polyester polyol and MDI from −60 °C to 240 °C at 20 °C·min⁻¹..

Fig. 9

Full FTIR spectra of thermoplastic polyurethane synthesized from 100% bio-based polyglycerol polyester polyol and MDI.