Abstract
This study investigated the performance of polyurethane adhesives prepared with various combinations of wheat starch that had been modified by isophorone diisocyanate (MS), two polyol types (1,3-propanediol (PD) and glycerol (Gly)), native wheat starch (NS), and 4,4′-diphenylmethane diisocyanate (pMDI) at a NCO:OH weight ratio of 1:1. Two more adhesives were also synthesized with NS, PD, or Gly and pMDI blends and served as controls. The thermal behavior of the adhesives before and after the curing process, as well as their rheological performance and lap shear strength, were analyzed. Differential scanning calorimetry (DSC) showed a reduction in curing temperature and heat by adding MS. The thermal stability of the cured adhesives was slightly increased by MS addition. The viscosity of the adhesives that contained MS substantially increased at a linear ascendant ramp of shear, while the controls exhibited relatively low viscosity during the whole shear rate spectrum from 0.1 to 100 s−1. The tensile shear strength of wood veneers was also significantly increased by the incorporation of MS under both dry and wet measuring conditions. The maximum dry shear strength was obtained for the adhesive with Gly polyol and a higher content of MS and was comparable to the control adhesive with pMDI.
Highlights
The global wood adhesives and binder market is increasing tremendously and is expected to exceed USD 21 billion by 2024 [1]
The exothermic curing peak, Tmax, of the control adhesive that contained PD (NS-PD-polymeric diphenylmethane diisocyanate (pMDI)) was at 85.7 ◦ C, while the formulation with Gly (NS-Gly-pMDI) exhibited a Tmax of 127.2 ◦ C. This might be related to the different reaction rates between isocyanate groups of pMDI and primary and secondary hydroxyl groups in the polyols [25]
It is known that the reaction of isocyanate groups with a primary hydroxyl group is about 3.3 times faster than with a secondary hydroxyl group [26]
Summary
The global wood adhesives and binder market is increasing tremendously and is expected to exceed USD 21 billion by 2024 [1]. Besides the hydrolyzation issue of formaldehyde-based adhesives under warm and humid conditions, increasing concerns related to formaldehyde emissions, as a potential hazard to the environment and human health, have forced the wood industry to find alternative, formaldehyde-free adhesives [2]. Offers non-formaldehyde emitting solutions, and it has been introduced as an alternative adhesive in the wood panel industry [3]. PUWAs are generally prepared by the reaction between hydroxyl and isocyanate groups to form urethane linkages [4] and demonstrate strong adhesion and good durability. They are not considered a sustainable solution, as most of them are petroleum derived
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