Sustainable Reactive Polyurethane Hot Melt Adhesives Based on Vegetable Polyols for Footwear Industry

The aim of this work is to develop sustainable reactive polyurethane hot melt adhesives (HMPUR) for footwear applications based on biobased polyols as renewable resources, where ma-croglycol mixtures of polyadipate of 1,4-butanediol, polypropylene and different biobased polyols were employed and further reacted with 4-4′-diphenylmethane diisocyanate. The different reactive polyurethane hot melt adhesives obtained were characterized with different experimental techniques, such as Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), softening temperature and melting viscosity. Finally, their adhesion properties were measured from T-peel tests on leather/HMPUR adhesives/SBR rubber joints in order to establish the viability of the used biobased polyols and the amount of these polyols that could be added to reactive polyurethane hot melt adhesives satisfactorily to meet the quality requirements of footwear joints. All biobased polyols and percentages added to the polyurethane adhesive formulations successfully met the quality requirements of footwear, being comparable to traditional adhesives currently used in footwear joints in terms of final strength. Therefore, these new sustainable polyurethane adhesives can be considered as suitable and sustainable alternatives to the adhesives commonly used in footwear joints.

Finger-jointing of green Eucalyptus globulus L. wood with one-component polyurethane adhesives

The use of glued finger joint in green wood, directly from the sawing process, would open the possibility to obtain glued timber from small-sized wood, achieving an efficient use of the original raw material. The gluing of finger-jointed green wood, with moisture content above the fibre saturation point, may improve the efficiency and the manufacturing process of glulam or joinery. This may be especially beneficial for a hardwood such as Eucalyptus globulus L., which is a globally important forest resource, but is a challenging wood to dry. This article presents a study on the possibility to develop finger joints with wood in green state. To compare them, conventional finger joints on dry wood and solid boards without end joints were also manufactured. Cold-setting and fast-curing commercial one-component polyurethane adhesive systems were used. Finger-jointed samples were tested to determine mean and characteristic values (5th percentile) of density, bending strength and modulus of elasticity, and the results were analysed and discussed. Green-glued joints showed no statistically significant differences compared to the solid boards and improved strength properties with respect to dry-glued joints.

Lignosulfonate-Based Polyurethane Adhesives

The feasibility of using lignosulfonate (LS) from acid sulphite pulping of eucalyptus wood as an unmodified polyol in the formulation of polyurethane (PU) adhesives was evaluated. Purified LS was dissolved in water to simulate its concentration in sulphite spent liquor and then reacted with 4,4′-diphenylmethane diisocyanate (pMDI) in the presence or absence of poly(ethylene glycol) with Mw 200 (PEG200) as soft crosslinking segment. The ensuing LS-based PU adhesives were characterized by infrared spectroscopy and thermal analysis techniques. The adhesion strength of new adhesives was assessed using Automated Bonding Evaluation System (ABES) employing wood strips as a testing material. The results showed that the addition of PEG200 contributed positively both to the homogenization of the reaction mixture and better crosslinking of the polymeric network, as well as to the interface interactions and adhesive strength. The latter was comparable to the adhesive strength recorded for a commercial white glue with shear stress values of almost 3 MPa. The optimized LS-based PU adhesive formulation was examined for the curing kinetics following the Kissinger and the Ozawa methods by non-isothermal differential scanning calorimetry, which revealed the curing activation energy of about 70 kJ·mol−1.

Influence of biobased polyol type on the properties of polyurethane hotmelt adhesives for footwear joints

Polyurethanes, one of the most used polymers worldwide, are strongly dependent of non-renewable fossil resources. Thus, boosting the production of new polyurethanes based on more sustainable raw materials is crucial to move towards the footwear industry decarbonisation. The aim of this study is to synthesise and characterise reactive hotmelt polyurethanes from biomass and CO2-based polyols as bioadhesives for the footwear industry. The influence of biobased polyols on the polyurethane structure, and therefore, on their final properties was analysed by different experimental techniques such us Fourier transform infrared spectroscopy (FTIR), Differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), Melting viscosity, Softening temperature and T-peel strength test, in order to assess their viability for the upper to sole bonding process. The results obtained indicated that the incorporation of different amounts of the biobased polyols produces changes in the structure and final performance of the polyurethanes. Therefore, adhesion test carried out by the T-peel test 72 h after the upper -to- sole bonding of the sustainable adhesives show high final adhesion values. These sustainable raw materials provide polyurethane adhesives with additional beneficial non-toxicity and sustainable characteristics, without harming their properties during their useful life.

Preparation of Polyurethane Adhesives from Crude and Purified Liquefied Wood Sawdust

Polyurethane (PU) adhesives were prepared with bio-polyols obtained via acid-catalyzed polyhydric alcohol liquefaction of wood sawdust and polymeric diphenylmethane diisocyanate (pMDI). Two polyols, i.e., crude and purified liquefied wood (CLW and PLW), were obtained from the liquefaction process with a high yield of 99.7%. PU adhesives, namely CLWPU and PLWPU, were then prepared by reaction of CLW or PLW with pMDI at various isocyanate to hydroxyl group (NCO:OH) molar ratios of 0.5:1, 1:1, 1.5:1, and 2:1. The chemical structure and thermal behavior of the bio-polyols and the cured PU adhesives were analyzed by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Performance of the adhesives was evaluated by single-lap joint shear tests according to EN 302-1:2003, and by adhesive penetration. The highest shear strength was found at the NCO:OH molar ratio of 1.5:1 as 4.82 ± 1.01 N/mm2 and 4.80 ± 0.49 N/mm2 for CLWPU and PLWPU, respectively. The chemical structure and thermal properties of the cured CLWPLW and PLWPU adhesives were considerably influenced by the NCO:OH molar ratio.

Influence of adhesive and layer composition on compressive strength of mixed cross-laminated timber

Different types of wood can be used for making cross-laminated timber (CLT), which is useful as a structural material. Therefore, to assess the viability of mixed cross-laminated timbers prepared with different adhesives, their compressive strength performances were evaluated. Laminae of Japanese larch, red pine, and yellow poplar were used to manufacture eight types of mixed CLTs, which were then tested in a universal testing machine for obtaining the compressive strength. The results were then compared to those obtained from the finite element (FEM) simulation of the CLTs at proportional limit load. The compressive strength of CLTs consisting of Japanese larch laminae, with a high modulus of elasticity, tended to increase. Mixed CLT with polyurethane adhesives showed an average compressive strength that was 14% lower than that of larch CLT, while mixed CLT consisting of red pine and yellow poplar showed an average compressive strength that was 18% lower than that of the larch CLT. The CLT prepared with phenol-resorcinol-formaldehyde adhesive yielded the highest compressive strength among the three adhesives. The FEM-predicted strengths were found to be close to the actual values in all specimens. The obtained results will be useful for selecting material and adhesive for future endeavors.

Influence of Biobased Polyol Type On The Properties of Polyurethane Hotmelt Adhesives For Footwear Joints

The implementation of a Circular Economy model, promoted by the increasingly stricter European policies, demands a comprehensive approach to resource efficiency. In this sense, polyurethanes, one of the most used polymers worldwide, are strongly dependent of non-renewable fossil resources. Thus, boosting the production of new polyurethanes / a new polyurethane based on more sustainable raw materials is crucial to move towards the footwear industry decarbonisation. INESCOP, aware of the footwear industry’s environmental impact, focuses on reducing or removing fossil-based raw materials and opts for eco-friendly ones. These sustainable raw materials provide polyurethane adhesives with additional beneficial non-toxicity and sustainable characteristics, without harming their properties during their useful life. Therefore, the aim of this study is to synthesise and characterise reactive hotmelt polyurethanes from biomass and CO2-based polyols as bioadhesives for the footwear industry. The influence of biobased polyols on the polyurethane structure, and therefore, on their final properties was analysed by different experimental techniques in order to assess their viability for the upper to sole bonding process.