Effect Of Different Hardwood Species And Lay-Up Schemes On The Mechanical Properties Of Plywood

In Estonia, hardwoods form approximately 50 percent of all forest area, where the main species are birch, grey alder, aspen and black alder. Birch has been extensively used by the veneer-based industry, but species like black alder, grey alder and aspen have not been commonly used by the veneer-based products industry due to the lower quality of this resource. It was found that with proper lay-up schemes, these wood species could be successfully used by the veneer-based products industry. By Heikko Kallakas, Anti Rohumaa, Harti Vahermets and Jaan Kers, Tallinn University of Technology

 

In northern Europe, the veneer-based industry plays an important role in the sustainable usage of wood from hardwood species. With the latest concerns about climate change, the focus also falls onto underused and low-quality wood species in Europe. 

In Estonia, hardwoods form approximately 50 percent of all forest area, where the main species are birch (30%), grey alder (9%), aspen (6%) and black alder (4%). The total amount of hardwood harvested in Estonia by wood species is the following: birch, 2,693,000 cubic metres (22.9%); grey alder, 675,000 cubic metres (5.8%); black alder, 712,000 cubic metres (6.1%); and aspen, 660,000 cubic metres (5.6%). 

In the case of birch, this opportunity has been noticed, and the effect of its usage on the climate, by the forest-based industry sector, has been evaluated. 

Birch has also been extensively used by the veneer-based products industry, but wood species like black alder, grey alder and aspen have not been commonly used in plywood products. The reason is not only the availability, but also the lower quality of this resource. 

On the other hand, the price of birch logs is rather expensive compared to these alternative species, which commonly have comparable mechanical properties. However, these hardwood species also contain tension wood, false heartwood, etc., which decreases the economical usability of this wood material, and affects the quality of obtained products.

In the veneer-based product industry, raw material will go through different processing stages, e.g., log soaking, veneer peeling and drying, which all have effects on the obtained veneer’s properties and final product quality. It is shown that birch wood processing for veneer-based products affects birch wood’s physical and chemical properties, as well as on lathe checks development, the integrity of veneer, and finally on the bonding quality and mechanical properties of veneer-based products.

However, similar comprehensive knowledge regarding the effect of processing on alder spp.’s and aspen’s veneer quality for the veneer-based industry is not available. Moreover, there are only a few studies on the different lay-up schemes, mainly for birch and softwood plywood, which could be an optimal way to enhance plywood panels’ mechanical properties for special applications. However, combinations of different hardwood species in one plywood panel are rare. 

The above-mentioned optimisation could be achieved without any significant changes in the current technology or the processing parameters in plywood factory.

The aim of this research is to determine the effect of different lay-up schemes and usages of grey alder, black alder and aspen on the plywood’s mechanical properties, by replacing birch veneer in the plywood core with alternative wood species. 

The main veneer and plywood characteristics will be evaluated according to current standards, e.g., veneer strength perpendicular to grain, plywood bonding and bending strength, and modulus of elasticity. 

All processing parameters will be kept similar to those generally used by birch plywood manufacturers. Hence, this could allow veneer-based industries to use these results without significant changes in current technology or processing parameters.

 

Wood Species & Plywood Lay-Up Schemes

In this study, four different wood species were used to produce the plywood: birch (Betula pendula Roth, density of 640 kg/cubic metres), grey alder (Alnus incana L, density of 467 kg/cubic metres), black alder (Alnus glutinosa L., density of 495 kg/cubic metres) and aspen (Populus tremula L., density of 466 kg/cubic metres). 

All logs were freshly felled in March 2019, at Piirsalu, Lääne County, Estonia, by State Forest Management Centre. Grey alder and black alder trees were cut into logs with nominal length of 2.7 cm, and for birch and aspen a length of 3.2 m. Logs with quality classes B and C were randomly selected for the research. 

The weighted (by area) average stand age of the birch trees was 81 years, grey alder 55 years, black alder 70 years and aspen 74 years. The average diameters of logs peeled in this study were measured from both ends of the peeler logs, average diameter was calculated and results were as follows: birch 25 cm, grey and black alder 20 cm and for aspen 40 cm. The average width of the annual growth rings for birch was 1.7 mm, grey alder was 3.2 mm, black alder was 2.7 mm and aspen was 4.7 mm. 

Logs were brought to the TalTech Laboratory of Wood Technology and kept in the same log yard before processing. Thanks to the prevailing climatic conditions (temperature around 0deg C, high relative humidity of the air was between 70% and 90%) it was possible to keep the wood in a wet condition, free of end checks.

Plywood lay-up schemes were made using either only birch veneers, or placing between birch face veneers other hardwood species in the core, for the same lay-up scheme. Six different layups were used: standard, combi, combi mirror, twin, direction face and direction core. 

Plywood products were chosen so that it would be possible to compare birch face veneer lay-ups with different species in the core to birch plywood. Plywood lay-up schemes standard, combi, combi mirror and twin all have the same construction, varying only in wood species. Plywood lay-up schemes’ direction face and direction core have the same number of veneers in the same direction, which only changes when it is in the core or on the face layers.

 

Plywood Processing

Birch, grey alder and black alder logs from the log-yard were cut into peeler blocks nominally 1.4 m in length, and then completely immersed into water tanks at 40 deg C for 24 h. 

As aspen wood is very soft, it does not need soaking, therefore the aspen logs were peeled without soaking and directly brought from the log-yard. 

After soaking, the logs were debarked with the debarking knives to remove any foreign objects like sand and soil, which get stuck on the log surface in the bark and may dull the peeling knife. Subsequently, the logs were rotary peeled on an industrial scale lathe, manufactured by the Raute Corporation into veneer with nominal thickness of 1.5 mm. 

Peeling speed was 60 m/min, knife angle 21° and compression rate 10 percent. After peeling, the veneers were visually graded. The whole veneer ribbon was used for plywood making and the leftover from the peeling was 70 mm from the centre of the wood trunk. 

Veneers were characterised by the presence of the lathe checks, which appeared on the side where the peeler knife had cut. The depth of the lathe checks directly affected the quality of veneers. The veneer side where the lathe checks appear is called loose side, and the side without lathe checks is called the tight side. Then the veneer mat was cut into sheets with dimensions of 900 mm by 400 mm using the guillotine. 

Veneer sheets where dried to target moisture content of 4.5 percent ± 1.5 percent in a laboratory scale veneer dryer at 180 deg C, where humidity inside the dryer was 500–600 g/kg. 

After drying, the veneers where conditioned at relative humidity of 20 percent and temperature of 20 deg C in order to maintain the target moisture content.

The plywood was bonded with commercially available phenolic (PF) adhesive consisting of liquid phenol-formaldehyde resin with solids content of 49 percent. The adhesive was spread (average glue spread of 160 g/sqm) on the veneers with the roller glue spreader. 

For each plywood sample, the veneers were selected randomly. After lay-up, the seven-ply plywood panels with nominal thickness of nine mm were produced with laboratory hot press at temperature of 130 deg C for seven min. 

Pressure was controlled automatically and the pressing cycle was as follows: 1 min—1.8 MPa, then 5 min—1.4 MPa, and then 1 min—0.4 MPa. The panels where then conditioned at 20 deg C and 50 percent relative humidity for one week before machining the specimens. Altogether four panels were made from each sample.

 

Veneer Crosswise Tensile Strength

The veneer crosswise tensile strengths results are in correlation with the wood species’ densities and strength properties. 

Aspen had the lowest test results, followed by grey alder. The lowermost tensile strength for aspen was 1.68 N/sq mm and highest tensile strength was 2.52 N/sq mm. The results for grey alder were 2.78 and 4.41 N/sq mm, correspondingly. The highest variation of test results was found in black alder, which had a lowest result of 1.73 N/sq mm and a topmost result of 5.09 N/sq mm. Birch had the tensile strength results of 2.93 N/sq mm and 5.60 N/sq mm, respectively. However, according to ANOVA, only aspen had a statistically significantly different tensile strength compared to other wood species.

 

Density

The thickness of the plywood panel decreases when using lower density wood species in the plywood core. The results showed that the highest density products have the lowest thickness, and vice versa. 

In this research, the lowest thickness, on sample 18, is placed in seventh position with regards to highest densities. All the veneer sheets of the different species underwent the same conditions concerning the gluing process. Birch has the lowest glue consumption, with 152 g/sqm, alder veneer had the glue consumption around 160 g/sqm, and aspen had 18 percent higher glue consumption, with 179 g/sqm. 

The density measuring results showed that the combi mirror lay-up scheme (samples 5, 10 and 15) gave the highest density for the plywood combined with different wood species. The lowest density was achieved with the twin lay-up scheme (samples 8, 13), where higher amounts of lower density wood species were available. 

Statistical analysis of the results showed that there are three density groups, in which the means are not significantly different from one another. 

The first group is formed from plywood made solely from birch veneers (standard layup scheme), and has the highest density. The second group is formed from all the other lay-up schemes, and the third group is formed from the twin lay-up scheme, which has the lowest density.

 

Bonding Quality

The best shear strength (2.39 N/sq mm) was achieved by the combi mirror samples combining the birch and black alder wood veneers (sample 10). Almost all of the lowest results in the groups of species came from the lay-up with direction core. 

As all the bonding quality tests were performed on the middle layer, the direction core was the only lay-up scheme wherein the glue line was pulled between layers that were parallel to each other. 

For all the specimens, wood fibre failure was determined to an accuracy of 10 percent. Starting from the lowest, the wood fibre failure outcomes were: 43.3 percent for black alder, 46.3 percent for birch, 61.8 percent for aspen and 79.8 percent for grey alder. 

According to these results, grey alder showed very good bonding with PF glue, followed by aspen. Black alder and birch were on the same level. 

While aspen had higher glue consumption and shear strength, its apparent cohesive wood failure was also higher. The results showed that the best bonding quality was achieved with the combi, combi mirror and twin lay-up schemes with all the wood species, which, according to ANOVA analysis, are not significantly different from each other. 

However, the lay-up scheme using the direction core showed the lowest values, and it was statistically significantly different from the other lay-up schemes used in this study.

The bending test results showed that the lowest bending strength (fm) values were obtained in the crosswise direction, and they belong to the direction face lay-up samples (31.1 N/sq mm for birch, 19.5 N/sq mm for grey alder, 22.5 N/sq mm for black alder and 26.4 N/sq mm for aspen). 

In the parallel direction, birch sample 3 had the highest bending strength with the direction face lay-up (121 N/sq mm). The highest compressive extension came from the same sample in a crosswise direction: 29.4 mm. 

Despite high compressive extension in the crosswise direction, sample 3 is placed in 15th position out of 18 according to its bending strength in this direction. In the crosswise direction, the combi plywood lay-up sample 9, with birch and black alder, had the highest bending strength (64.6 N/sq mm). 

In the parallel direction, the average bending strength results are: 99.3 N/sq mm for grey alder, 98.0 N/sq mm for black alder, 107.1 N/sq mm for aspen and 114.3 N/sq mm for birch. In the crosswise direction, the average bending strength results are: 42.2 N/sq mm for grey alder, 46.3 N/sq mm for black alder, 51.4 N/sq mm for aspen and 48.5 N/sq mm for birch.

The same as with the bending strength results, the four lowest results for modulus of elasticity (MOE) belong to the crosswise direction face lay-up (1614 N/sq mm for birch, 1361 N/sq mm for grey alder, 1206 N/sq mm for black alder and 1543 N/sq mm for aspen samples). 

In the parallel direction, the birch and grey alder plywood sample 7 had the highest bending MOE, with the direction face lay-up (12,239 N/sq mm), while in the crosswise direction, the combi mirror plywood lay-up sample 15 and the twin lay-up sample 18, both with birch and aspen, had the highest bending MOE (5019 N/sq mm and 4937 N/sq mm, respectively). 

In the parallel direction, the average bending MOE results are: 10,440.4 N/sq mm for grey alder, 9377.4 N/sq mm for black alder, 10,864.8 N/sq mm for aspen and 10,460.0 N/sq mm for birch. 

In the crosswise direction, the average bending MOE results are: 3010.8 N/sq mm for grey alder, 3201.8 N/sq mm for black alder, 4011.8 N/sq mm for aspen and 3157.7 N/sq mm for birch. 

The analysis of results based on the standard Pearson’s method shows that there is strong correlation between bending strength and MOE. The correlation coefficient in the parallel (fm, 0) wood grain direction was r = 0.77, and for the crosswise wood grain direction (fm, 90) it was r = 0.93.

The veneer crosswise tensile strengths were according to expectations, as birch had the highest tensile strength of the species and aspen the lowest. In addition, it was shown that grey alders tensile strength results were weaker than black alder’s. 

Black alder showed higher variation in crosswise tensile strength results. These higher variations in results may be due to the log and veneer quality. Previous studies have shown that the quality variation in these wood species is higher, and they might contain some quantity of false heartwood, which has different properties, e.g., density and higher moisture content. 

Another explanation of the result variation could also be the different strength properties of the earlywood and latewood in the veneer yield, which have different strength properties.

 

Mass to Volume Ratio

One of the main reasons why some products have a higher density comes from its nature, as density is the ratio of mass to volume. All the plywood panels were pressed with the same pressing cycle and parameters. 

This means that veneers peeled from lower density wood species were more compressed than higher density wood species. The density results showed that highest density products have the lowest thickness, and vice versa. The analysis of results based on the standard Pearson’s method showed that the thickness and density of veneer panels are negatively correlated (r = −0.41). 

This can be explained via the densification effect of the low-density wood species, where the permanent deformation of wood cells occurs during the exertion of higher pressures. Therefore, in panel number 18, the high density and low thickness were mainly caused by the five layers of low-density aspen veneer in the core of panel. 

Due to the compression of the low density aspen veneers, the overall density of the plywood increased due to the densification effect. Another factor that influences the density of the plywood is the glue consumption of different wood species’ veneers. 

Increasing the glue consumption also increases the density of the plywood panel. The higher glue consumption is attributed to the surface roughness of the different wood species. This surface roughness affects the gluing, where greater surface roughness means the consumption of greater amounts of glue, which could therefore also affect bonding quality.

The bonding quality results showed that all the plywood lay-up combinations passed the required one N/sq mm value, according to standard EN 314-2 (1999), which means there are no requirements for apparent cohesive wood failure. 

This shows that all the combinations with lower density wood species bonded well. The best shear strength of sample 10 (combi mirror of birch and black alder) is because of the similar densities of birch and black alder wood, and the slightly higher glue consumption of the black alder veneers, which all together gives the highest shear strength in this research. The same trend was seen with the combi plywood with grey alder (sample 4), where the higher glue consumption of grey alder veneers increased the plywood shear strength, and led to the second-best shear strength value of 2.28 N/sq mm. 

The higher shear strength values of lower density wood species (black alder, grey alder and aspen) compared to birch can be attributed to the fact that they all achieved higher glue consumption than birch. Previous research has also stated that lower density wood needs more adhesive to glue. 

Higher glue consumption increases the shear strength of the plywood. Therefore, the lowest density wood—aspen—had the highest values of shear strength. In this research, all plywood panels with the combi mirror lay-up scheme had the highest apparent cohesive wood failure, since the veneers in their cores are from lower density wood species. For example, aspen and grey alder showed 100 percent, and black alder 80 percent, apparent cohesive wood failure in combi mirror products.

Bending properties were tested in the parallel and crosswise directions relative to the wood grain. In crosswise bending, the veneers were weak, as in these samples (3, 7, 12, 17) there are two outer layers parallel to the weakest grain direction. 

Therefore, these two parallel veneer layers will break easily under tension. On the other hand, they gave some of the best results in the parallel grain direction with the direction core lay-up. 

The bending strength test results showed that the grey alder plywood had the weakest bending strength properties, which can be explained via the low modulus of rupture of grey alder itself. 

Oddly, aspen showed high bending test results, which could be explained via the high glue consumption and the densification effect of the low-density aspen layers during the plywood making process, which increasing the strength of the aspen plywood samples. 

It is generally known that densification and higher glue consumption increase the density and strength properties of wood. Due to this, the aspen and birch combined plywood panels are comparable with all the birch layered plywood panels with regards to bending strength. 

The same trends were found in the bending MOE values, where the plywood panels with highest MOE in the parallel wood grain direction showed the lowest results in the crosswise direction. Birch, grey alder and black alder have similar MOE values, while aspen showed the highest values for MOE. 

This effect can be explained via the higher glue consumption and greater compression of aspen veneer layers (densification effect) during the plywood production process. Another reason could be that the aspen itself has higher MOE values than grey alder and black alder, but these are still lower than the birch. 

The results showed that the bending strength increases in the parallel direction when there are more parallel layers in the plywood, and at the same time decreases in the crosswise direction for all the wood species. 

The stiffness is affected by glue consumption and the number of lower density wood layers in the plywood, which are compressed more densely together than the birch layers (densification of the lower density wood).

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