The wood-based composites sector plays an important role in national economies in many countries. Plywood is widely used for different applications, such as construction, furniture manufacturing, means of transportation, packaging, decorative purposes, and many others.
In comparison with conventional solid wood products, plywood has various advantages: increased dimensional stability, uniformity and higher mechanical strength, reduced processing cost, availability in larger sizes, better appearance, and biological benefits.
On the other hand, one of the main disadvantages of plywood products is using a large amount of adhesive during its manufacture, which can be up to 20 percent of its total mass.
This disadvantage decreases the plywood product’s ecological balance and makes it less favourable than solid wood, especially when considering resins derived from petrochemical resources. Global production of plywood reached 157 million cubic metres in 2017.
To produce such an amount of plywood, approximately 15 million tonnes of resin are used. Synthetic thermosetting resins based on phenol, urea, formaldehyde, and isocyanates are usually used.
Urea–formaldehyde (UF) resins are incombustible, provide good bonding strength, resistance to fluctuations of temperature, light, and corrosion, have a small curing time, simple manufacturing technology, and low production costs.
However, they also have significant disadvantages, such as fragility, low water resistance, and significant emissions of formaldehyde. Phenol–formaldehyde (PF) resins can improve the bonding strength and water resistance, but they require a longer curing time, higher curing temperatures, higher production costs, and also emit phenol and formaldehyde.
The formaldehyde can irritate the eyes, respiratory and nervous systems, and possibly lead to cancer and leukaemia. Therefore, formaldehyde was reclassified in 2004 by the International Agency for Research on Cancer (IARC) as ‘carcinogenic to humans (Group 1)’, compelling companies to reduce formaldehyde emission to lower levels.
Significant efforts have been made to reduce formaldehyde emissions from wood-based panels by the addition of various additives to the thermosetting resins or by the protection of the product with veneer, varnish, paint, and other coatings.
One of the possible directions is the creation of wood composites based on environmentally-friendly products, where thermoplastics (polyethylene, polypropylene, poly(vinyl chloride), and their copolymers) are used as adhesives. Already, there is a positive experience in the creation and use of wood composites based on thermoplastics.
Waste polyethylene can be used in the manufacture of oriented strand board (OSB) panels, resulting in the enhancement of thickness swelling, humidity, dimensional stability, water absorption, and screw withdrawal resistance.
Laminated veneer lumber (LVL) was manufactured using high-density polyethylene (HDPE) as a binding agent.
The properties of the composite boards were quite similar to or even better than those found in LVL made using thermosetting resin. The thermoplastic polymers were successfully used for coating of birch plywood.
Formaldehyde-free wood–plastic plywood has been successfully produced using thermoplastic polymers as wood adhesive.
Materials for Experiment
Rotary-cut alder wood veneer (Alnus glutinosa Goertn.) with dimensions of 300 mm × 300 mm × 1.6 mm and an average moisture content of 6 percent was used to make plywood panels.
To minimise the influence of wood structure defects on the results of the experiment, the veneer sheets were selected and evaluated for the production of plywood panels.
The veneer sheets were visually checked, and sheets without shocks, cracks, curling, and colours of more or less uniform thickness were selected. Observation on the wood appearance did not show any visible defects.
HDPE film with a thickness of 0.14 mm, density of 0.93 g/cm3 and melting point of 135 deg C was used for the bonding of plywood samples. The plastic film was cut into the same dimensions as the veneers. UF and PF resins were also used for the comparison.
UF and PF adhesives were prepared according to the manufacturer’s instructions. For the preparation of UF adhesive, five percent hardener (ammonium nitrate) and 15 percent filler (wheat flour) were used.
Manufacturing of Plywood Samples
Three-layer plywood samples were prepared. Instead of traditional UF and PF adhesives, a HDPE film was used as an adhesive for manufacturing the plywood samples.
One sheet of HDPE film was incorporated between two veneer sheets, which were laid with the directions of the fiber perpendicular to each other. The laying of the dry HDPE film was very simple.
The influence of hot-pressing pressure (0.8, 1.2, and 1.6 MPa), hot-pressing temperature (140, 160, and 180 °C), and hot-pressing time (1, 2, 3, and 5 min) on the properties of plywood were evaluated.
The pressing temperature depends on the adhesive. The melting temperature of the HDPE film was 135 deg C, implying the value of the lower limit of hot-pressing temperature. This temperature must be over 135 deg C, thus making HDPE flow and penetrate the alder veneers.
By contrast, conventional plywood made from the commercial UF and PF resins is generally hot-pressed at approximately 105 and 145 deg C, respectively. Therefore, a range of 140 to 180 deg C was considered for hot pressing plywood panels using HDPE film as adhesive.
The physical and mechanical properties of HDPE-bonded plywood were compared with UF and PF plywood, and relevant plywood standards.
After hot pressing, the plywood samples were subjected to a cold-press stage that was performed at room temperature for five min, which was used to reduce the distortion and stress of the plywood. Three replicate panels were manufactured for all the conditions and control.
Then after bonding, the plywood panels were air conditioned for five days. After air conditioning in a standard climate (T = 20 ± 2 °C, RH = 65 ± 5 percent), standard samples were taken from each panel to determine the appropriate physical and mechanical properties: 20 samples per shear strength test, 6 samples per MOR/MOE test, and 6 samples per dimensional changes test.
Physical and Mechanical Properties
Thickness, density, bending strength (MOR), and modulus of elasticity (MOE) in bending, shear strength, water absorption, and thickness swelling of plywood samples were determined according to the standards.
For the shear strength test, one half of the samples were tested in dry conditions and the other half in wet conditions after soaking in water at 20 ± 3 °C for 24 h.
Mechanical properties of the samples, MOR and MOE in bending were carried out in parallel and perpendicular directions, depending on the surface layer. Physical properties of the samples, water absorption (WA) and thickness swelling (TS), were conducted in accordance with EN-317.
Before testing, the weight and thickness of each sample were measured. Conditioned samples of each type of plywood panel were fully immersed in distilled water at room temperature for 2, 24, 48, and 72 h.
The samples were removed from the water, patted dry, and then measured again. The samples were weighed to the nearest 0.01 g and measured to the nearest 0.001 mm immediately.
Furthermore, the measurement of the core temperature that can be achieved inside the veneer package under given pressing conditions of plywood samples was undertaken. Temperature changes were measured using thermocouples connected to an PT-0102K digital multichannel device.
In addition, micromorphological properties were evaluated by microscopic imaging.
Thickness and Density of Plywood Panels
The aim of the veneer thickness measurement was to find effects of the different conditions of pressing on the tolerance of a pressed plywood panel.
In the case of using HDPE film as adhesive in the production of plywood, it is very important to choose the pressing parameters so that the thickness of finished plywood is within acceptable limits, to avoid unnecessary losses of wood raw material.
Analysis showed that the temperature, pressure, and time of the hot pressing, as well as the type of adhesive used, significantly affects the thickness and density of the HDPE-bonded plywood panels.
It can be seen that the average thickness of HDPE-bonded plywood panels made at different hot-pressing temperatures, pressures, and times is not smaller but even exceeds the thickness of control plywood using UF and PF adhesives, which is essential for the industrial application of this technology.
The smallest thickness (3.99 mm) and the highest density (619.1 kg/m3) had plywood samples made using PF adhesive. The largest thickness (4.70 mm) and the smallest density (536.8 kg/m3) had plywood samples made using HDPE film.
Primarily, this can be explained by the high hot-pressing pressures of plywood samples and the increased adhesive dosage in the case of using UF and PF adhesives.
As the time of the hot pressing increases from one to three min, the thickness of the plywood samples decreases at temperatures of 140 and 180 deg C, and increases at a temperature of 160 deg C.
This is because at a higher hot-pressing temperature and longer time of pressing, the wood becomes more plastic and more easily compressed.
The hot-pressing pressure also significantly influences the thickness and density of plywood samples. With an increase in pressing pressure from 0.8 to 1.6 MPa, the thickness of the plywood samples decreases by 6.0 percent, and the density of samples increases by 7.6 percent.
Nevertheless, plywood panels containing HDPE film were pressed at a lower pressure than the control panels.
In this case, the average compression ratio of plywood made using HDPE film was smaller—5.4, 3.04, and 8.05 percent, respectively, for 0.8, 1.2, and 1.6 MPa compared with the compression ratio of 8.5 and 13.5 percent for control UF and PF plywood, respectively.
Moreover, plywood containing HDPE film was manufactured with 19 percent less adhesive spread than the adhesive spread used for the control panels. In addition, its compression ratio will be smaller because less moisture was brought with the adhesive into the veneer package, and such package, in turn, is less densified (wood is deformed more heavily).
Hot-pressing time had a significant impact on the shear strength of plywood samples. Too short pressing time is insufficient for good adhesive penetration.
The time used should be enough for heating the inner areas of the plywood samples allowing the melting of the HDPE and the drying of the wood veneer at the same time.
Increasing the pressing time from one to three minutes leads to increased shear strength. The subsequent increase in pressing time to five minutes leads to increasing the time for production and decreasing the shear strength.
This can be explained by the fact that after hot pressing for five min, the HDPE film was completely melted, and the thickness of the film could be reduced if the pressing time was longer than 3 minutes because part of the film was melted and flew out from the plywood, resulting in a lack of polyethylene film and, finally, the shear strength of plywood samples decreases.
Moreover, with long-term hot pressing, the molten plastic film penetrated into the wood and, consequently, caused a decrease in the shear strength of plywood.
Therefore, the hot pressing could not also be so long as to prevent thermal degradation of the wood veneer, decomposition and fracturing of the HDPE film.
At the various hot-pressing pressures and temperatures, the highest shear strength values are observed for the pressing time of three minutes. A similar trend in the impact of pressing time on the properties of plywood is described in the work.
They concluded that the optimal parameters were a plastic use of 100 g/m2, a hot-pressing temperature of 150 deg C, and a hot-pressing time of six minutes.
The pressure also played an important role because it is responsible for providing close contact between both materials and helping the flow of the HDPE into voids and irregularities of the wood veneer.
Therefore, high pressure is recommended for enhancement of adhesion properties with veneer.
This study also found that processes that kept the polypropylene molten at the surface of the wood for a longer time, higher temperature, and higher pressure achieved much better interlocking than the short process cycle.
To confirm the above-described processes of the melting and flowing of HDPE film during the hot-pressing operation, measurements of the temperature inside the veneer package for different types of adhesive used and different pressing conditions were made.
In another study, it was shown that the polypropylene melted during hot pressing and made good contact with veneer surfaces penetrating into the lumina of wood cells, lathe checks, and other spaces open on the veneer surface.
These authors indicated that the anchoring effect of polypropylene, which had penetrated into various wood elements and spaces in the veneer, contributed dominantly to the gluability.
Therefore, because wood is a porous material, a mechanical interlock is the most likely bonding mechanism involved.
Effect of Adhesive Types on the Plywood
Plywood bonded with UF and PF resins were manufactured for comparison with plywood bonded with HDPE film. All plywood contained approximately equal adhesive dosage.
The mechanical properties of HDPE-, UF-, and PF-bonded plywood panels were comparable despite the fact that these three polymers have considerably different mechanical properties, which affect the final properties of the panels.
The lower TS and WA of the plywood could be explained by the fact that HDPE film filled the micropores of the wood veneers and covered a larger surface area of the hygroscopic wood component and thus prevents water penetration into the wood veneers.
HDPE-bonded plywood samples were made using a lower adhesive dosage (130 g/m2) and at a lower hot-pressing pressure than UF and PF plywood samples. However, their properties are not inferior to these traditional plywood.
In addition, HDPE-bonded plywood can be attributed to environmentally-friendly plywood. The formaldehyde emission of the plywood made from recycled plastics is very low; compared with that of ordinary plywood made with urea–formaldehyde resin, the amount of emission is almost zero.
In addition, the use of HDPE film makes plywood more flexible and simplifies the production of different bent constructions from plywood.
The values of the mechanical properties of the plywood panels obtained in this work were significantly higher than those obtained in other works, which mainly used poplar and very rarely eucalyptus, as well as different films.
It can be assumed that wood species may have an effect on the ability to be bonded with a thermoplastic film.
The thermoplastic polymers were successfully used for the bonding of alder plywood.
The findings of this work indicate that HDPE film adhesive gave bending strength, MOE in bending, and shear strength values of alder plywood panels that are comparable to those obtained with traditional UF and PF adhesives.
Moreover, this is despite the fact that the adhesive dosage and pressing pressure were less than when UF and PF adhesives were used. Among the HDPE-bonded alder plywood panels, all the highest mechanical properties values were obtained for panels produced with high pressing temperature and pressing time.
The highest shear strength was achieved at 160 deg C when the pressing time was increased to three minutes. The smallest shear strength was achieved at 140 deg C and a pressing time of one to two minutes.
Environmentally-friendly high-density polyethylene-bonded formaldehyde-free alder plywood panels have been successfully produced using thermoplastic polymers as an adhesive.