Wood adhesives from waste-free recycling depolymerisation of flexible polyurethane foams

Highlights

• Wastes from flexible polyurethane foam are taken to landfills and incinerated, causing environmental problems.

• The traditional recycling approach was related to the generation of by-products requiring a significant energy input.

• The new recycling approach is energy favorable and all products recovered can be processed and re-used.

• The research proposes the use of recovered polyol in adhesive formulations for wood specimens.

Abstract

Flexible polyurethane foams (FPUF) are common plastic materials used in industry or consumer goods (heat and sound insulation, washing sponges, car seats). FPUF wastes are frequently taken to landfills and incineration, which causes environmental problems. The current method of physical recycling for rebound foams seems to be efficient, however, with limited usage in the structure of mattresses or construction insulations. Disadvantages of the current chemical recycling methods of FPUF consist of the slow reaction process, high energy consumption, and production of secondary waste by-products. This article deals with the special technique of the controlled depolymerisation of the studied FPUF. FPUF was depolymerised in an ambience of non-reactive liquid at a temperature of 130 °C, only giving liquid products. The depolymerisation process and the final product was characterised by titration methods, Fourier Transform-Infrared Spectroscopy (FT-IR), Gas Chromatography/Mass Spectrometry (GC/MS), and Gel Permeable Chromatography (GPC). The recycled polyol (product of depolymerisation) was tested in model polyurethane adhesive formulations and evaluated for shear tensile strength on spruce wood test pieces.

Introduction

Flexible polyurethane foams (FPUFs) are frequently used for upholstery and mattress manufacturing, for kitchen and bath sponges, and of course, for heat and/or sound insulation in buildings and industrial use (Ashida, 2006; Defonseka, 2019). Significant production volumes are dedicated to the automotive industry, mainly to the production of car seats (Beneš et al., 2007; Mark, 2013). Polyurethanes (PURs) and FPUFs are both polymers produced by polyaddition polymerisation of isocyanates and polyols (Kumar and Gupta, 2018). They are crosslinked polymers completed with specific additives according to the final use of the polymer, i.e. pigments, fillers (calcium carbonate, aluminium hydroxide, silica, kaolin, etc.), flame retardants, and foam stabilisers (Delfosse and Bosshard, 1976; Ferrigno, 1962; Haddick et al., 1972; May and Rose, 1992; Nunes et al., 2000; Weil and Levchik, 2004).

The crosslinked character of FPUFs is the dominant reason for difficult waste disposal (Popov et al., 2010). Landfilling is problematic due to the voluminous character of polymer foam of low density, lack of degradability, and also very slow biodegradation (Howard, 2002, 2011). Similarly, incineration in combustion units is not desirable because nitrogen-containing polymer generates harmful gases such as isocyanate fumes, hydrocyanic acid, or nitrogen oxides (Günther et al., 2018; McKenna and Hull, 2016). It concludes that the recycling of FPUF is an important field in the polymer industry.

Mechanical recycling means that the material is reused in its polymer form. The standard way of mechanical recycling is making rebond flexible foam from pieces of chopped PUR foam waste (less than 10 mm in diameter) and binder (Stone et al., 2000). The rebonded foams are produced in standard densities ranging from 60 to 300 kg m⁻³ (Rebonded Flexible Foams, Isopa 08-97-REC.-0025-FACT SHEET, n.d.). They are successfully used for specific mattress manufacturing and even more frequently for building insulation. Another type of mechanical recycling is foam regrinding or powdering, in which the resulting powder is mixed with virgin materials, and new polyurethane foam is created. Micro-milling of FPUFs is difficult because of low glass transition temperature T_g which is about −50 °C (Piszczyk et al., 2014); the foam is very elastic and resistant to foam cell breaking at ambient temperature. It contrasts with rigid insulation foams, which can be used in milled form and as a very fine powder, applied as an active filler in polyurethane binders and adhesives (Beran et al., 2020).

Although FPUFs are successfully recycled by the mechanical means described above, there is also the possibility to recycle them by chemical methods. Chemical recycling of PUR foams is a well-studied theme known as PUR foams chemolysis. The chemical recycling includes aminolysis, acidolysis, alcoholysis, hydrolysis, glycolysis, etc., according to the agent used in the recycling process, with glycolysis being the most elaborate method (Aguado et al., 2011; Behrendt and Naber, 2009). Unfortunately, chemolysis is not a very widely used method of FPUF recycling in common practice due to the high energy required for the whole process. The latest review of thermo-chemical recycling techniques was reported by Deng et al. (2021). Of course, foam pyrolysis may be used because FPUFs feature high energy content. During the pyrolysis process, the issue of gaseous emissions must be solved. The thermo-chemical process at a mild temperature of 150–200 °C may be useful for yielding toluene diisocyanate (TDI) or polyols, as reported. Similarly, gasification of FPUFs to syngas (fuel) is described.

The essence of chemolysis is the reversal of a reversible polycondensation reaction to the monomer units from the polymer chains. Generally, chemolysis involves the reaction between polyurethanes and substituents. It results in the formation of simpler compounds like recycled polyol, some secondary solid waste substituted polyurea, and a small amount of diamines (Shinko, 2018; Gama et al., 2020). Thus, the polyols obtained can be used to prepare new PUR products, but not flexible foams. Diamines and polyureas are by-products containing nitrogen from the original PUR polymer. Diamines contain harmful compounds and are considered a potential health hazard (Albrecht and Stephenson, 1988). Substituted polyureas are solid, insoluble polymers that have to be separated and disposed of. The formation of these products poses a disadvantage, and another reason why FPUF polymers are rarely recycled in practice.

This study aims to control the depolymerisation process in the way of maximal elimination of these by-products. Currently, there is no method for the recovery of diisocyanate due to its very high reactivity in the conditions of the recycling process with traces of water, glycols, or degraded polyols being found (Behrendt and Naber, 2009). The typical reaction of diisocyanates with water results in carbon dioxide and diamine (Fig. 1a). The reaction provides the principal source of gas for the foam blowing and the heat power for the expansion of the polyurethane. Additionally, the free isocyanate takes part in other reactions due to its high reactivity. For example, the isocyanate together with a diamine gives substituted urea (Fig. 1b) (Sharmin and Zafar, 2012; Simón et al., 2015).

The present study is focused on a novel method of chemical FPUF recycling consisting of the partial catalysed depolymerisation of PUR at a mild temperature. The product of this type of depolymerisation is liquid, directly applicable in polyurethane chemistry. It has been developed in an effort to avoid any secondary waste. The new method works with non-reactive liquid, which is not used up during the depolymerisation process.

Chemical reactions are characterised by their thermodynamic balance and equilibrium constant. The polyurethane reaction is an exothermic process using a catalyst for shifting the balance to the polyurethane product at room temperature (Van Gheluwe and Leroux, 1983). In conditions of studied liquefaction of FPUF, the balance is moved back to the reagents due to elevated temperature and the usage of a suitable catalyst. It is possible to run the reaction backward from polymer to monomers. No co-reactant is necessary with this method. Organic cyclic carbonate, specifically propylene carbonate (PC), was used as a medium. It is a polar aprotic solvent (Czompa et al., 2019; Schaffner et al., 2010), which has no reactivity with components. The advantage of PC is that it is biodegradable, non-toxic, odourless and has a low vapour pressure. It can be synthesised from cheap and renewable feedstocks on an industrial scale (Gautam et al., 2017). In addition, PC facilitates the proton transfer during catalysed polymer decomposition.

The general reaction mechanism of polymerisation/depolymerisation is shown in Fig. 2.

Reaction 1 is a general polyurethane reaction mechanism with highlighted equilibrium; reaction 2 is the description of the urethane catalysis mechanism (where R³N is the generally used tertiary amine catalyst).

The recycled FPUFs (which are composed of predominantly polyol and resinous PUR-polyurea oligomers) were characterised by Fourier Transform-Infrared Spectroscopy (FT-IR), Gas Chromatography/Mass Spectrometry (GC/MS), Gel Permeation Chromatography (GPC), and the determination of amine and hydroxyl numbers. The recycled product was tested in the model polyurethane adhesives as one-component (1C) and two-component (2C) formulation, where the tensile strength of the wood to wood bond was measured.