https://orcid.org/0000-0001-8096-4376
Abstract
In practice, for the manufacture of plywood, the veneer is usually dried to a moisture level suitable for bonding. However, anatomical differences within and between veneer sheets are a significant factor and some of them may contribute to significant differences in moisture content (MC). The novelty of this work was to study the possibility of using wood veneers of different MC in adjacent layers in one structure of plywood panels and how this affects the physical and mechanical properties of the panels. The Norway spruce (Picea abies L. Karst) wood veneers with mean MC approximately of 2%, 5% and 7% and phenol-urea-formaldehyde (PUF) resin were used in the experiments. The five-layers plywood panels were manufactured at the two pressing pressures of 0.8 MPa and 1.2 MPa. There was no significant difference in the bending and bonding properties, but only wood failure as measured on bonding specimens which showed higher average values on specimens manufactured with high pressure during hot pressing. Higher differences in MC of each veneer with constant MC of the veneer stack didn’t worse bending and bonding properties of plywood.
Introduction
Plywood is a composite material made from cross-laminated veneers. Softwood plywood is usually used for building construction and adhesives used in the manufacturing process must be resistant to weather, water, etc. when used for exterior purposes.
The manufacturing process of plywood is influenced by three key groups of factors: the type of resin and its chemical properties (pH value, solid content, viscosity, etc.) and the pressing parameters (temperature, pressure, time) in conjunction with various parameters of veneers (density, moisture content, surface roughness, etc.). These factors influence the final physical and mechanical properties of the plywood.
Nowadays, phenol-urea-formaldehyde (PUF) resins have started to become more popular in the manufacturing of exterior plywood. PUF resins obtained under either alkaline or acidic conditions have enhanced application properties in comparison to the properties of phenol-formaldehyde (PF) resins [Citation1,Citation2]. Urea has been added into PF resins in order to improve their curing rate, to lower the content of free formaldehyde, and to reduce the cost of the resin [Citation2–4].
Based on the resin used in the manufacturing process there are three main parameters involved in the mechanisms of hot pressing: temperature, pressure, and moisture content (MC).
The pressing temperature depends on the adhesive and should provide its curing and the creation of chemical bonds. The pressure applied to the press depends on the type of adhesive, its viscosity, the pressing temperature, surface characteristics, and the physical properties of the wood [Citation5]. First of all, the applied pressure onto the veneers brings the wood surfaces together. The pressure influences the penetration of adhesives because it is the driving force for hydrodynamic flow [Citation6]. The pressure applied to the adhesive will force it to spread and penetrate into porous, fibrous materials and into the rough surfaces [Citation7]. The bond quality is affected by the amount of adhesive penetration into the wood substrate during the manufacture of wood composites, i.e. LVLs [Citation8]. An optimum adhesive penetration is needed to repair processing damage of the wood surface, allow better internal surface contact for chemical bonding or interlocking, to improve stress transfer between laminates (plies) [Citation8], promote more efficient use of the adhesive [Citation9] and provide a reliable bondline thickness.
Pressure must be applied uniformly and correctly because resin-based wood adhesives are not able to form strong bonds when applied in thick and variable bondlines due to their low viscosity [Citation10]. In low-density woods, high-pressure forces squeeze the adhesive so deeply into the wood that there is insufficient adhesive to fill the bondline, and it may cause overpenetration and inferior bond strength [Citation11]. On the other hand, low-pressure causes a decrease in shear strength, and does not provide close contact between the surfaces, thus the bondline remains partly poor [Citation12].
Pressure must be consistently applied until the strength of the cured adhesive is higher than the forces of internal vapor. The degree of cure is dependent on the pressing time and temperature and also on the composition of the adhesive. Pressure parameters are influenced by several factors such as the thickness of the composite, thermal conductivity, or MC of the material [Citation13].
The MC of veneers and the water content in the applied adhesive affect the heat transfer through the veneers [Citation14]. Moreover, the MC in the veneer combined with the water in the adhesives will also significantly affect the wetting, spreading, penetration and even curing of the adhesives [Citation15]. The MC of all veneers should be around 7% before bonding [Citation16]. For such MC the conventional thermosetting adhesives provide high quality bonding of plywood with satisfactory physical and mechanical properties. However, differences in initial MC, differences in drying conditions, anatomical differences within and between veneer sheets are significant factors and some of them may contribute to significant differences in MC. Therefore, in practice, to ensure uniform sheets MC as well as uniform distribution of MC within the sheet, an excessively wet or dry veneers are rerouted for re-drying or re-conditioning, respectively [Citation17]. However, this requires additional costs. Vick [Citation15] determined that an MC of veneer of 3–5% at the time of hot pressing was satisfactory for softwood plywood intended for construction and industrial uses. Gumowska et al. [Citation14] showed that when using veneers with 5% MC in comparison with 10% MC, it is possible to produce veneer-based composites significantly reducing the pressing time up to a half of the conventional time when using cyclic pressing.
Several authors [Citation18] also found that the best bonding results were obtained in plywood panels with veneers having 4–6% MC whereas panels from veneers of 16–18% MC showed the lowest mechanical properties. Bekhta et al. [Citation19] to bond the high MC (15%) birch veneers used PF resin modified by wheat starch, rye flour, resorcinol and phenol-resorcinol-formaldehyde resin. They found that based on the shear strength values the properties of specimens met the European standard EN 314-2 for bonding quality of class 3 and such plywood panels can be used in exterior conditions. In another study, a series of pressing temperatures (140, 150 or 160 °C) and veneer MCs (5, 10, 15, 20 25 or 30%) were selected for manufacturing the surface-densified (SD) plywood [Citation20]. The high and low MCs (5, 10, 15, 20 25 or 30%) veneers were placed in the top- or bottom-surface and 5% MC – in the core of the seven-plies plywood panels. The authors showed that mechanical properties, including modulus of rupture (MOR), modulus of elasticity (MOE) and hardness of the SD plywood were significantly higher than the control plywood made from the veneers with the same 5% MC. The greatest increment of MOR, MOE and hardness was 54, 104 and 144%, respectively, which could be attributed to the high density of surface veneers.
The MC of veneers and the water in the adhesive influence the curing process during hot pressing and the final physical and mechanical properties of the plywood. More studies were performed on possibilities of bonding high moisture veneers using different approaches [Citation19, Citation21–23]. However, no information was found in the literature regarding the impact of lower (<8%) MC fluctuations within and between veneer sheets on the properties of veneer-based materials. Such MC fluctuations can change the physical and mechanical properties of veneer-based materials. Therefore, the novelty of this work was to study the possibility of using wood veneers of different lower (<8%) MC in adjacent layers in one structure of plywood panels and how this affects the physical and mechanical properties of the panels manufactured at high and low pressing pressures. This work should provide better understanding of the bonding qualities of softwood plywood using PUF resin in conditions of veneers MC fluctuations.
Materials and methods
Materials
Rotary cut veneer was manufactured in the plywood company, located in the Czech Republic, from one log of Norway spruce (Picea abies L. Karst). Industrial size veneers without visible defects were cut into sheets of 600 × 600 × 2 mm and dried after manufacturing. Before drying the veneer, sheets were split into three groups: I group – veneers were dried to the average 2% MC, II group – veneers were dried to the average 5% MC and III group – veneers were dried to the average 7% MC.
A commercial PUF resin purchased from the manufacturer (Prefere Resins Finland Oy) was used for this experiment. The PUF adhesive was prepared by mixing the 400 grams of PUF resin with 138 grams of hardener and 220 grams of water. The average conventional and constant mass solid content of the PUF resin/adhesive were 54.31%/43.68% and 52.53%/43.87%, respectively, as measured according to EN 827 (2005). The flow time of PUF resin, as measured by the Ford cup (100 ml with a diameter of hole 6 mm and the length 4 mm) according to EN 12092 (2001), was on average 96 s. The average value of flow time immediately after mixing the resin with the hardener and water was 47 s. After 20, 40 and 60 min the flow time of PUF adhesive were 46 s, 32 s and 26 s, respectively. The average values of the pH for PUF resin/adhesive were 12.06 and 11.62, respectively, as measured according to EN 1245 (2011).
Manufacturing of plywood
Five-layer plywood panels of 600 × 600 mm dimensions and a thickness of 10 mm were made in the laboratory press. A different MC (2, 5 or 7%) of veneers were selected for manufacturing the plywood (see ). The plywood specimens were made from veneers of mixed MC (2% and 7%) (M) and the control specimens (C) – from veneers of the same 5% MC. The schematic of veneers in the layers in plywood are illustrated in . All the plywood panels were hot-pressed at 0.8 MPa and 1.2 MPa, and at a pressing temperature of 135 °C and a pressing time of 600 s. The adhesive spread was 150 g/m2 based on wet mass. PUF adhesive was applied onto one side of every uneven ply with a hand roller spreader. The plies were assembled perpendicularly to each other.
Figure 1. Scheme of the formation of the plywood.
Table 1. Technological parameters for manufacturing the plywood specimens.
Moisture content of veneer and plywood specimens
Moisture content (MC) was determined according to the methodology EN 322 (1993) on pieces of veneer with dimensions 50 × 50 mm and on the plywood 50 × 50 mm. The MC of the veneers was determined before pressing. The plywood specimens were conditioned at a temperature of 20 °C and relative humidity (RH) of 65% and then measured.
Density and density profile of plywood specimens
Density was determined according to the methodology EN 323 (1993) on 10 specimens from each panel. Further, the density profile was measured on one specimen from each panel. The specimens with dimensions of 50 × 50 × 10 mm (conditioned at 20 °C and 65% RH) were measured at interval 0.01 mm through the specimen thickness (10 mm) using an X-ray densitometer (X-RAY, Dense-Lab).
Bending and bonding properties
Mechanical testing of plywood specimens was carried out on a Zwick®Z050 universal testing machine with testXpert v11.02 software and a 50 kN load cell (Zwick GmbH & Co. kg, Ulm, Germany). Specimens were tested after conditioning at a temperature of 20 °C and relative humidity of 65%.
Three-point bending tests according to EN 310 (1993) were measured on the specimens with a surface layer in a parallel or perpendicular direction on ten specimens for each direction with dimensions 250 × 50 mm. The determination of the modulus of elasticity (MOE) and modulus of rupture (MOR) were then calculated.
Bonding quality was tested according to EN 314-1 (2004) on specimens with dimensions of tested area 25 × 25 mm. There were three exposures of testing: Exposure 1 (dry test) – after conditioning at a temperature of 20 °C and relative humidity of 65%; Exposure 2 – soaking in water 20 °C for 24 h; Exposure 3 – boiling in water for 6 h and cooling in water 20 °C at least 1 h.
The wood failure (WF) percentage was made on all specimens from testing bonding quality. The wood failure percentage was compared with pictures in EN 314-1 (2004).
Statistical analysis
The data were processed in STATISTICA 10 software (StatSoft Inc., USA), evaluated using a one-factor analysis of variance (ANOVA), and completed with Tukey’s honest significance test (HSD test).
Results and discussion
Moisture content and density
The MC of plywood specimens made from veneers with different MC showed a significant increase compared to the control specimens (). Also, there was an increase in the density of plywood with higher pressure 1.2 MPa used for manufacturing both type of specimens, although the difference between them was insignificant. Specimens 1.2 C and 1.2 M showed the highest average density, caused by the densification of the wood veneer. The veneers used in this experiment were made from the same sheet from the same log to avoid variability in density.
Table 2. The physical properties of the veneer and plywood.
Density profile
The density profile for plywood made with higher pressure (1.2 MPa) showed steeper peaks in the bondline than plywood made with lower pressure (). These peaks showed better bonding because of the presence of the thinner bondline with higher density; the density of resin is about 1220 kg/m3. It is known [Citation24], that with decreasing bondline thickness the bonding strength increases.
Figure 2. the density profile of tested specimens.
Bending and bonding properties
Even though the values of MOR and MOE of longitudinal and transverse 0.8 C, 1.2 C, 0.8 M and 1.2 M specimens are different from each other, but as we can see (), the difference between these values is insignificant. The lower values of MOR and MOE for control specimens and specimens with different MC obtained at higher pressing pressure than MOR and MOE values obtained for the same specimens at lower pressure can be explained by the lower density of surface layers in specimens 1.2 C/1.2 M, than in the 0.8 C/0.8 M specimens (). First of all, the density of the surface layers determines the MOR and MOE indicators. It is known that there is a direct relationship between the strength and density of the plywood [Citation25, Citation30]; the higher density – the higher strength.
Table 3. Bending properties of longitudinal and transverse specimens.
Although the total density of the 1.2 C specimens is greater than the density of the 0.8 C specimens, but as can be seen from , the density of the outer layers is greater in the 0.8 C specimens than in the 1.2 C specimens. Therefore, the 0.8 C specimens also demonstrated higher MOR values and MOE. As for the 0.8 M and 1.2 M specimens, the density of the surface layers in them is practically the same, so the values of MOR in the longitudinal direction are practically identical (53.7 MPa and 54.4 MPa, respectively).
In contrast to palm plywood, bonded with PUF resin with a similar density like plywood made in this research had the average MOR of 30.7 N/mm2 and MOE of 4370 N/mm2 [Citation26]. Several authors [Citation27] also observed the lack of difference between the values of MOR (63 N/mm2 and 69 N/mm2) and MOE (5819 N/mm2 and 6212 N/mm2) for LVL bonded with melamine urea formaldehyde (MUF) adhesive at similar, as in this study, pressures of 0.75 MPa and 1.25 MPa. But authors stated that increasing the pressing pressure to 1.5 MPa has already led to a significant increase in MOR and MOE values.
The effect of pressing pressures and fluctuations in the MC of veneer in one veneer stack on the bonding strength of plywood specimens is shown in , and the average values of the bonding strength and wood failure are given in the . A very significant conclusion emerges from the findings, which is important for practice. In production conditions, during the formation of a veneer stack, fluctuations in the MC of the veneer within the range of 2–7% in one stack will not affect the worsening of the bonding strength. The average bonding quality in Exposure 1 was 1.6 N/mm2 with wood failure 33%, bonding quality was higher compare to the dry bond quality at Palm plywood 1.10 N/mm2 with wood failure 100% [Citation26]. The average bonding quality for plywood made from PF resin with different types of modifying agents was 1.2 N/mm2 [Citation19]. The wood failure in this experiment in all Exposures () is lower than WF in other studies [Citation26, Citation28]. The bonding strength of the plywood made with PUF resin pressed in 130 °C was in dry conditions 1.69 N/mm2 with average WF 35% and after 4 h of boiling 1.27 N/mm2 with WF 50%. Bonding quality and wood failure were higher when the plywood was made at 150 °C [Citation4]. The bonding strength of the plywood made with PUF co-condensed resin pressed in 150 °C with pressure 0,98 MPa was in dry conditions 1.82 N/mm2 with average WF 85% and after 4 h of boiling 1.24 N/mm2 with WF 80% [Citation28]. The bending properties and bonding quality of plywood panels can be improved by combining different wood species as well as the densified and non-densified veneer in one panel [Citation25, Citation29,Citation30].
Figure 3. Bonding quality and wood failure of specimens in three different exposures (Exposure 1 – conditioning 20 °C and 65% RH; Exposure 2 – soaking in water 20 °C for 24 h; Exposure 3 – boiling in water for 6 h and cooling in water 20 °C at least 1 h).
Table 4. The bonding quality and wood failure of plywood specimens.
There was a significant increase in WF in Exposure 1 between the specimens 0.8 M and 1.2 M, 29% and 57%, respectively. There was also significant difference in WF in Exposure 3 between 0.8 M and 1.2 M, 25% and 42%, respectively. There was a trend observed in that higher pressure for manufacturing panels increases percentage of WF, confirmation of the trend will be with a higher difference between the pressure of manufacturing panels.
Conclusions
The most important finding of this study is that fluctuations in the MC in the range of 2–7% within and between veneer sheets in one plywood panel do not change the properties (except MC) of the panels.
Higher pressing pressure provides better bonding quality for plywood panels made by combining veneer sheets with different MC in one panel, which was confirmed by higher average values of the bonding strength and WF in these panels. This is true for all three exposures of testing. Higher and steeper peaks in the bondline for density profiles in the plywood panels also confirm better bonding in panels made at higher pressing pressure.
Findings can be easily implemented in practice. They also slightly simplify the preparation process of veneer prior to adhesive application and pressing, especially its drying and sorting processes, because higher fluctuations in the MC of the veneer sheets can be allowed.
Disclosure statement
No potential conflict of interest was reported by the authors.
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