Recycled Thermoplastic Polymer Hot Melts Utilization for Birch Wood Veneer Bonding

Abstract

Today, thermosetting polymer resins containing formaldehyde are mainly used in the production of plywood composites. These glues are toxic due to the generation of gaseous products, chiefly formaldehyde, during the production and exploitation of laminated materials, which pollute the environment. In order to decrease the emission of toxic products from these resins, natural product additives (lignins, tannins etc.) or fully biodegradable polymers such as starch, polylactides etc., are used. These glues are not toxic, but they have poor adhesion to wood, low water resistance and are more expensive. Our previous investigations have shown the possibility of using non-toxic virgin polyolefins as hot melts for plywood production. The aim of this work was to investigate the use of cheaper waste products, such as recycled thermoplastic polymers (polyolefins, polyamides), as glue hot melts for wood veneer bonding instead of traditional thermosetting resins, which are based on formaldehyde. Different recycled thermoplastic polymers produced from tetra package waste, domestic film waste, recycled polypropylene and recycled polyamide-6 were used as hot melts for birch wood veneer bonding. The content of the recycled adhesives was determined using differential scanning calorimetry (DSC) and the tetra package waste content of different types of polyethylenes, with additives of polypropylene and polyethylene terephthalate (PET), was fixed. Virgin high-density polyethylene and polypropylene were used as references. The fluidity of all the recycled materials, using melt flow index (MFI) measurements, was 1.96 g/10 min for polyethylenes and 1.94 g/10 min for polypropylene; this adhesive fluidity is sufficient to form full contact with the surface of the wood veneer sheets. Adhesive activity of the recycled hot melts was evaluated by preparing single overlap joints and specimens were tested for shear strength. The optimal technological parameters for producing samples were noted: pressure 2 MPa, contact time 1–2 min and a temperature for polyethylenes of 130°C, for polypropylene of 180°C, for PA-6 at 220°C. The highest values of shear strength were observed for specimens glued with recycled polypropylene (10 MPa), in the case of polyethylenes, the maximum was 5–6 MPa. The mixed fracture mechanism of adhesive joints was fixed after visually examining the surface of the broken samples. It was shown that to reach high adhesive strength it is also necessary to improve the cohesive strength of the adhesive layer. Polyethylene terephthalate fabric waste was used as reinforcement for strengthening the adhesive layer, which improved the bending strength and modulus of the two layers laminate bonded with recycled polypropylene. Direct correlation between high adhesive properties and high bending strength was not observed on all occasions. Water exposure experiments (23°C, 72 h) showed a decrease of adhesive strength to constant values of 2–2.2 MPa, for all investigated adhesives, which are similar to observations using 4 mm thick industrial plywood, corresponding to the 3^rd class of water resistance. These investigations highlight the excellent prospects for the use of recycled polyolefins as hot melts for wood bonding.