1. Introduction
Plywood is one of the most important value-added panel products in the wood industry. In 2018, industrial plywood production in the world reached a record output of 163 mln·m
3. This was the fastest growth among all wood-based panels (particleboard, oriented strand board OSB, and fiberboard), recording 2%/179% and 1%/105% respectively from 2017/2000 to 2018. The global consumption of plywood grew by 2.5% in comparison to 2017 to reach a record consumption of 161 mln·m
3 [
1]. Currently, different kinds of plywood are manufactured using various types of adhesives, mainly formaldehyde-based adhesives such as phenol–formaldehyde (PF) and urea–formaldehyde (UF). These resins have the advantages of low-cost, high reactivity, excellent strength properties, and ease of use for a wide range of conditions for curing [
2]. However, it is well known fact that formaldehyde-based adhesives are not environmentally friendly. One of the main disadvantages of these adhesives is that the production and products from them can release free formaldehyde. Since a significant proportion of plywood products are for indoor applications, the release of harmful volatile compounds, including formaldehyde, is very detrimental to human health [
3]. It is well known that formaldehyde is a human carcinogen [
4], and this fact compels companies to reduce formaldehyde emission to lower levels.
Many studies have been done to reduce the release of formaldehyde or to replace formaldehyde-based adhesives with more environmentally friendly adhesives [
5,
6,
7,
8,
9,
10,
11,
12]. However, very often, such adhesives are not widely used in the manufacture of plywood. They are either too expensive, do not provide proper bonding quality, have insufficient durability against different biological agents, or are not water-resistant.
In addition, at present, plywood factories mainly apply glue on veneers using glue drums. The glue is applied in a liquid state. This process requires a high fluidity of adhesives, and it can cause uneven spreading, uneven thickness of the adhesive line, and as a consequence a deterioration of the bonding strength. Moreover, the operations of mixing and applying the glue are untidy and unpleasant. The ideal plywood adhesive should be of uniform quality and form an adhesive line of uniform thickness [
13].
From the other side, it is well known that environmental pollution problems and a shortage of natural resources are becoming important issues. A huge amount of waste plastics (≈6.3 billion tons) are generated every year all around the world, while only ≈9% of them are recycled and most (≈79%) of them are accumulated in the natural environment [
14]. Plastic is widely used in many applications, especially in the form of disposable products, such as plastic bags, agricultural plastic film, and greenhouse film. These waste products mainly consist of polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC) [
15]. Plastic films are materials that are not able to biodegrade rapidly in the environment. Today, recycling waste products by utilizing them in manufacturing processes is very attractive, because it can prevent environmental pollution and lower production costs.
Due to the lack of suitable adhesives to meet wood industry requirements of being environmentally friendly, low cost, and easy to use, the application of thermoplastics (PE, PP, PS, PVC), and their copolymers, is promising [
16,
17]. The great advantage of such polymers is that the formaldehyde emission of the plywood made by recycled plastics is very low compared to that of ordinary plywood made with urea–formaldehyde resin: the amount of emission is almost zero [
18]. Formaldehyde-free wood–plastic plywood has been successfully produced using thermoplastic polymers as wood adhesive [
18,
19,
20,
21,
22,
23,
24]. The various thermoplastic polymers were used for veneer bonding: low-density polyethylene (LDPE) [
25], high-density polyethylene (HDPE) [
17,
19,
20,
24,
26,
27], PS [
21,
28], PP [
16,
23], or PVC [
29,
30]. The thermoplastic polymers were used for veneer bonding in various forms, such as textile fiber waste (polyurethane, polyamide-6) [
16], recycled plastic shopping bags [
18,
26], or film [
17,
19,
20,
22,
23,
24,
27,
29,
30].
Oh [
25] found that low-density polyethylene can be used as an additive in phenol–formaldehyde resin adhesive for bonding radiate pine plywood. Modified HDPE powder through in situ chlorinating graft copolymerization has been successfully used to manufacture exterior plywood [
31,
32]. LDPE film has also been chosen to develop non-formaldehyde plywood, and the resulting plywood meets the requirement for interior plywood application [
33]. Wood plastic plywood composed of veneer and styrofoam was manufactured, and its vibrational properties were investigated by Hu et al. [
34].
Hot melts based on HDPE, polyurethane textile fiber waste, and unmodified and modified sulfate lignin adhesives for wood veneer bonding were used by Grinbergs et al. [
35]. Cui et al. [
18] replaced traditional adhesives with compounds made with recycled plastic shopping bags, mainly composed of PE, PP, PVC, and PS, in order to make hot-melt plywood using various amounts of plastic film, different hot-pressing temperatures, and different hot-pressing times. The results show that the bonding strength of plywood does not increase with increasing amounts of plastic film. The optimum hot-pressing parameters are as follows: 100 g·m
−2 of recycled plastic, a hot-pressing temperature of 150 °C, and a hot-pressing time of 6 min.
The results showed that polystyrene wastes can be used in plywood manufacturing as an alternative bonding material for interior uses [
21,
28]. It was shown that low-pressure pre-heating is a necessary step allowing for increasing the final shear strength of the wood thermoplastic joints and avoiding the indentation of thermoplastic particles to wood [
28].
According to the findings of another work [
16], the use of HDPE, PP, polyurethane, and polyamide-6 textile fibre waste hot melts for wood veneer gluing guarantees the shear strength of the material that considerably exceeds the adhesive strength of industrial plywood glued with phenol–formaldehyde resins. Moreover, their use would eliminate glue toxicity and considerably improve environmental protection.
Fang et al. [
19] demonstrated that it is possible to manufacture environmentally friendly plywood directly bonded with unmodified plastic film. The main component of the plastic film used in the experiments was HDPE with a thickness of 0.05 mm per layer. The results showed that the highest strength was obtained under hot pressing conditions (pressure, 0.7 MPa; temperature, 160 °C; time, 1 min·mm
−1; and film dosage of two layers), which was comparable to that of commercial UF-bonded plywood. The pressing temperature had a notable effect on adhesive penetration and bonded joints formed. In another work [
27], it was found that the HDPE film dosage positively affects the properties when ranging from 61.6 to 246 g/m
2. The performance of these plywood panels was comparable to those of plywood made with commercial UF resins.
The effect of various surface treatments on the adhesion between wood veneers laminated with a thin PVC film has been investigated using XPS, contact angle, and surface tension measurements, and a lap shear-test [
29]. Wood veneers were treated with silanes (amino and chloro), phthalic anhydride, and maleated polypropylene for surface modification. The adhesion between PVC and wood veneer laminates was significantly improved when wood veneers were treated with amino-silane, while no improvement was observed for the other adhesion promoters.
To improve the interfacial adhesion between the wood veneer and HDPE film, silane A-171 was used to treat the surface of poplar veneer by spraying [
36]. The shear strength and water resistance of plywood are greatly improved thanks to the silane surface modification. When one layer of HDPE film was used as an adhesive, it caused a 293.2% increase in shear strength, and a 34.6% and 40.8% reduction in water absorption and thickness swelling, respectively. In addition, the wood failure also increased from 5% to 100% due to the silane modification.
In another work [
37], to improve the interfacial adhesion between wood veneer and HDPE film, wood veneer was thermally modified in an oven or chemically modified by vinyltrimethoxysilane. The results showed that both modifications reduced veneer hydrophilicity and led to an enhancement in shear strength, wood failure, and water resistance of the resulting plastic-bonded wood composite. However, the strength of silane-treated plastic-bonded wood composite was still much lower than thermosetting resin-bonded composites at higher temperatures. Moreover, this chemical-based method was limited in its industrial use because of high modification costs and relatively complicated processing.
The use of thermoplastic film as an adhesive for the bonding of veneer is the most promising. Apart from the fact that the plastic film is formaldehyde-free, its use also has several other advantages compared with using liquid adhesives. Dry adhesive film is simpler to apply than wet adhesives; all of the untidy and unpleasant mixing and spreading operations in wet gluing are wholly removed from the plywood factory by the use of dry adhesive film. The dry adhesive film contains in each square meter of surface precisely the same quantity of adhesive, equal quality, uniform composition, exactly the same bond strength, and the same standard thickness [
13].
Plywood is produced from softwood and hardwood species, and the species used in its manufacture determine the physical and mechanical properties of the plywood [
38]. However, no information was found in the literature regarding the impact of various wood species and different type of thermoplastic polymers, particularly co-polyamide and сo-polyester on the properties of plastic-bonded plywood. Therefore, the purpose of this study was to obtain a better understanding of the bonding process of plastic plywood when using various wood species and different types of thermoplastic polymers.