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Comparison between HB and HDF made from waste leather.


Due to the ever-increasing scarcity of resources and raw materials in the wood panels industry, it is imperative to look for suitable alternatives to the established resources. Therefore a combination of the traditionally used and newly explored sources may reveal highly innovative ways.

The goal of this study is to provide an insight into the behaviour a wood-leather-composite material and its possible new applications.

Wet White (WW) and Wet Blue (WB) leather particles were used for this analysis and mixed with the wood fibres of spruce and beech. Wood--leather panels' combinations with 0%, 15%, 25%, 35%, and 50%> leather amount were produced in two different production processes (wet and dry discontinuous process). To evaluate how the differences of the two production processes influence the mechanical properties of the panels, investigations concerning the internal bond (IB), bending strength (MOR) and modulus of elasticity (MOE) were conducted.

The leather content of the panels and the production process affected the results of the material properties. In the case of urea-formaldehyde (UF) bonded high-density fibreboards (4,5 mm, 900 kg/[m.sup.3]) the results show a non-linear improvement of the IBs with increasing leather content, whereas MOE and MOR decrease. The hardboards, (3,5 mm, 900 kg/[m.sup.3]) which were produced in the wet process with 1 % phenol formaldehyde resin, show a similar behaviour in MOE and MOR to the HDF panels, but differ in tensile strength. The results give an overview how the leather amount influences the board properties of two different panel types. Furthermore the here published results enable a convincing comparison between the mechanical properties of the two wood-leather composite board types.

Key words: Wood--leather composites, wet process, Wet White, Wet Blue


An analysis of the European wood market shows, that raising the netto imports of round wood can only increase the production of wood products (Barbu et al., 2014). However, untapped timber reserves such as bark need to be mobilized from the Austrian Forest (Schwarzbauer, 2005, Kain, 2012). Based on the current market situation, both options seem to be quite unlikely to reach. The only remaining possibility is to develop new sources of raw materials or to increase the efficiency of the existing industrial use of wood (Barbu et al., 2014).

According to the current market situation and the growing interests in Europe to obtain energy by burning biomass (Mantau et al., 2010), investigations in nearly every section of new material resources have been made through the passed years. Many of these studies set their focus on the upor recycling of by-products, to find possible supplements to wood fibers or particles for wood based panel production (Deppe et al., 1996). Therefore, some research have been made to incorporate lingo-cellulosic materials such as wheat and reed (Han et al., 2001; Halvarsson et al., 2009), industrial hemp fibres (Crowley, 2001), bamboo and rice straw (Hiziroglu et al., 2008) or more exotic materials as coconut fibres (Van Dam et al., 2004) and stream exploded banana bunch fibres (Quintana et al., 2009) in fibre- and particleboards. A forward-looking way of reusing by-products seems to be an up-cycling of waste materials, which occur in the meat production, as chicken feathers and leather shavings (Parkinson, 1998, LMC International, 2007). Most of these by-products are currently recovered thermally or getting landfilled in depositories in Central Europe (Kangaraj et al., 2006, Schroter, 1995). Investigations by Ostrowski (2012) and Grunewald (2012) describe the mechanical and physical influence of leather shavings in medium density fibreboards (MDF) and in insulation mats. Furthermore, Grunewald (2012) presented results according to the distribution of wood fibres and leather particles in panels and determine the structure with a Raman spectroscopy. Wieland et al. (2012) and Stockl (2013) determined an increasing fire resistance with increasing leather-amount in MDF panles. Rindler et al. (2014) described the mechanical properties of high-density fibreboards (HDF) made from waste leather and wood, which is issued by Gerald Lackinger (2009). The corpus of published research, which dealed with fibrematerial, concerned about the usage of new materials produced under laboratory conditions using a dry discontinous process. As the raw material leather incurres with a moisture content > 45% out of the leather production process, a different way of fibreboard production could be more suitable for this material.

This work deals with the comparison between the influence of the leather amount to the mechanical properties of hard-density fibreboards made from wood fibres and leathershavings in a wet process (HB) and high-density fibreboards produced in a dry process (HDF). Investigations concerning the MOE, MOR and IB properties of both boardtypes have been made to determine the behaviour of leather in those processtypes.



The leather material for the presented tests was specified and provided by Gerald Lackinger Consulting (Salzburg, Austria). The leather particles accrued during the thickness shaving process of cattlehides preparation.

For this investigation two different leather types, which are defined as Wet White (WW) and Wet Blue (WB), were used. The tanning process of Wet White uses a synthetically non-chromatic tanning, whereas in Wet Blue chromium is connected with the proteins of the hides in a stable bond. Both leather types were pre-fractionated (particles < 2 mm) at a moisture content of > 45%.

For the dry process the leather particles got dried to average moisture content of 8 [+ or -] 1%.

The unglued wood fibres (Picea Abies) for the dry process where provided by MDF Hallein GmbH with a moisture content of 8 [+ or -] 1%. For the wet process wood fibres (1/3 bark residuals and 2/3 Fagus Sylvatica fibers) with moisture content of 45% were used.


Fibreboards with dimensions 450 x 450 x 4.5 mm (dry process) and 450 x 450 x 3.5 mm (wet process) were produced under laboratory conditions. Both panel types were manufactured with different leather types Wet White (WW) and Wet Blue (WB) and varying leather-fibre ratios (leater amount in the board: 0%, 15%, 25%, 35%, and 50%). To obtain a significant comparison, both board types were manufactured with the same average density of 900 kg/[m.sup.3].

For the dry process an amount of 10% urea-formaldehyde resin (UF) was used. The adhesion occurred in a plowshare blender with a two-substance nozzle upright section to obtain a homogenious glue application. For this process a nozzle with a hole diameter of 2.3 mm and a pneumatic pressure of 2 bar was used. After the adhesion of the fibres a conventional vacuum cleaner "Bosch GAS 50-M" was used to soak the glued material of the blender and thereby counteract the agglomeration behaviour of the glued fibres. Further the fibres were distributed manually in a frame and finally pressed in a Hoefer HLPO 280 automated hot press at 80[degrees]C with a pressing factor of 1 min/mm.

For the wet processed leather-fibreboards beech wood fibres (u = 45%) and leather particles, Wet Blue (u = 69%) and Wet White (u = 45%), were used. The material got mixed in 10 I white water from the fiberboard production process. During the blending process 1% of phenol formaldehyde resin was added. The fibre-leather liquid got filled in a dewatering press with an operating pressure of 100 bar. Subsequently the pre-compressed fibre mat was pressed for 4 min and 120 bar at a temperature of 200[degrees]C in a Burkle hotpress.

In total 10 panels were produced (Table 1).


After the pressing process the samples from the wet and the dry production process, were conditioned in a standard climate at 20 [+ or -] 1[degrees]C and 65 [+ or -] 1% relative humidity, until the equilibrium moisture was reached.

To eliminate the density variation in the edge region, the panels were trimmed to a dimension of 400 x 400 mm. Subsequently the test pieces were cut out of the boards according to the EN 326-1:2005, to ensure randomised test results.

The testing samples for the modulus of rupture (MOR) and modulus of elasticity (MOE) were prepared by the following equation (1), which is in accordance to the OENORM EN 310:2005.

l = 20t + 50

l ... length, t ... thickness

Based on this formula, the samples for MOE and MOR test were 50 x 140 mm for the dry process panels and 50 x 120 mm for the wet process.

To analyse the internal bond (IB) the test was conducted according to OENORM EN 319:2005 and therefore the testing samples were prepared in the dimensions 50 x 50 mm. To counteract a weak performance of the adviced hot melt glue (EN319) in the glue layer during the IB testing, PUR (Kleiberit 501 PUR-Leim D4) resin was used to fix the HB and HDF samples to the testing crosshead.


The determination of the mechanical properties of the panels was performed on a Zwick Roell Z 250 universal testing machine. For all mechanical tests, a total of five samples (n=5) of each combination and process were tested.

2.4.1 Internal Bond

The IB strength was determined according to the OENORM EN 319:2005. The force was applied with a constant speed to achieve a breaking of the specimen within 60 [+ or -] 30 seconds.

2.4.2 Modulus of Rupture and Modulus of Elasticity

The MOR and MOE were evaluated according to OENORM EN 310:2005. The samples were tested for each of the two similar panels of each combination. The specimens were tested with a continuous crosshead-speed to achieve a breaking within 60 [+ or -] 30 seconds. The MOE was obtained from the linear values between 10 to 40% of the maximal load.


To evaluate the data of the mechanical tests, an Analysis of Variance (ANOVA) with the density as a covariate was used to determine the explanatory effect of each parameter on the model. To determine the statistical influence of the leather amount, the leather type and the process type on the tested data, a one-way Welch-ANOVA was used to compensate the unequal variance. The general level of significance was 0.05.



The following tables give an overview of the obtained values of the mechanical properties (IB, MOE and MOR) and the measured density. Table 2 displays the calculated averages of the HDF boards and Table 3 shows the means of the HB boards with WW and WB particles. The upper value in the box represents the mean of the tested panel and the data in brace below their standard deviation (sd). The number of specimens of IB, MOE, and MOR was 5 and the number of density measurements 10.

As clearly visible in table 2 all WW and WB blending ratios of HDF panels obtained similar density means. According to results of the mechanical properties it is to say, that the WW HDF panels reached overall higher means compared to the WB HDF panels with the same leather ratio. Only WB 50 achieved higher results in case of IB compared to WW 50%. In case of MOE and MOR the panels with 15% Wet Blue amount show better properties than the WW 15% panel.

In table 3 it can be seen, that all WW and WB blending ratios of HB boards reached similar density means. According to results of the mechanical properties it is to say, that the WB-HB panels achieved overall higher means compared to the WW-HB boards with the same leather ratio (expect WB 50%). In case of MOE and MOR there is no explicit difference between Wet White and Wet Blue noticeable.

3.1.1 Internal Bond

Figure 1 and 2 show the medians (line in the box) and the interquartile range (whole box) of the IB results, which indicate a relation between transverse tensile strength and leather content in the panels. The range of the wiskas shows 95% of the measured values. Figure 1 indicates the test results of the fibreboards, which were produced, in the dry process and figure 2 displays the wet process.

Allover it is to say that the IB values are increasing with rising leather content in the dry production process. Only WB 15%, WB 25% and WW 50% show a lower value than the pure wood panel. As visible in both figures (1,2) it can be seen, that the medians of the boards from the dry process reached IB between 0.7 and 1.7 N/[mm.sup.2] and the wet produced panels only between 0.15 to 0.5 N/[mm.sup.2]. However, the medians of WB 50 dry process and WW 50% wet process represent the strongest tensile strength properties. The analysis of variance (p=0.05) shows that the amount as well as the type of leather and the process type, highly significant, influences the IB.

3.1.2 Modulus of Elasticity

Figure 3 and 4 show the medians and standard deviations of the MOE results. Figure 3 indicates the dry process, while figure 4 displays the wet process.

Interestingly both leather types (WW, WB) show a lower MOE value with a leather amount of 50% compared to the pure wood panels in the dry process. All other blending ratios of the dry process performed with better MOE medians. According to the wet process it is to say, that decrease of elasticity appears in a more continous way than the behaviour of the dry process.

The comparison of the medians show, that no median of the blending ratios could achieve higher results than the pure wood panel. Comparing the influence of the leather amount between dry and wet process, it is to say, that the medians of MOE are in both cases between 1800 and 3500 N/[mm.sup.2]. According to the analysis of variance (p=0.05) it is to say that the amount of leather is highly significant for both fibreboards, but the type of leather shows only a significant influence on HDF and FIB. Further the type of process is highly significant.

3.1.3 Modulus of Rupture

Figure 5 and 6 show the medians and standard deviations of the MOR results for the dry and the wet production process.

The results for the MOR are similar to the MOE values. Also here the same bahaviour of material with increasing leather content is visible. While in the dry process only the 50% leather blendings obtain lower values than the pure wood panel, in the wet process the pure wood board performed the best. The analysis of variance (p=0.05) shows that the amount as well as the type of leather and the process type influences the IB highly significant.


The results for the internal bond (IB) showed interesting insights into the importance of the process. The hardboards showed a weaker performance than the high-density fibreboards. A reason for this could be the, in 2.3 mentioned, problem with the weaker screen side of the hardboards. Although the used PUR adhesive performed well, the sieve surface obviously was too unregular to perform better. The second reason could be the low adhesion amount in the hardboards. It seems that the amount of 1% PF is to low with rising leather content, as pure leatherparticles do not possess the attribute of a self-bonding character.

The values of the modulus of rupture and elasticity investigations were highly satisfying and showed good performance for both pressing types. A significant difference could be noticed between the standard deviations. The results in the dry process showed a significantly higher standard deviation than the wet process. This reveals to the forming of the fibremat before the pressing. The forming occurs much more homogenious in the wet process than in the dry laboratory process.

From the mechanical point of view it is definitely worth to produce hardboards with a leather amount up to 50%. The huge advantage of the wet process is economically by leaving a prior drying process of the raw material and using less adhesive.

To the opinion of the author further research concerning the physical properties of leather hardboards should be made to obtain a better understanding of probable future applications.


The authors gratefully acknowledge for the support of the Austrian Research Promotion Agency (FFG) under grant no.836988.


Department of Forest Products Technology and Timber Constructions, Salzburg University of Applied Sciences, Markt 136a, 5431 Kuchl, AUSTRIA


Department of Forest Products Technology and Timber Constructions, Salzburg University of Applied Sciences, Markt 136a, 5431 Kuchl, AUSTRIA

Marius C. BARBU

Department of Forest Products Technology and Timber Constructions, Salzburg University of Applied Sciences, Markt 136a, 5431 Kuchl, AUSTRIA



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Table 1: Total overview of the pressed boards

Leather/Fibre Ratio   Leather Type   Production Process

0/100                 WW             Dry
                      WB             Dry
15/85                 WW             Dry
                      WB             Dry
25/75                 WW             Dry
                      WB             Dry
35/65                 WW             Dry
                      WB             Dry
50/50                 WW             Dry
                      WB             Dry

Table 2: Estimated mean and standard deviation (sd)
of the mechanical properties of WW and WB HDF-boards

Type [%]   Density      IB           MOE              MOR
           [g/[cm.      [N/[mm.      [N/[mm.          [N/[mm.
           sup.3]]      sup.2]]      sup.2]]          sup.2]]

           n=10         n=5          n=5              n=5

Wood HDF   0.80 (.04)   0.92 (.19)   2083.3 (110.1)   29.8 (3.8)
WW15       0.92 (.08)   1.23 (.46)   2697.8 (358.0)   37.1 (5.1)
WW25       0.97 (.07)   1.52 (.19)   3230.2 (806.1)   47.8 (9.9)
WW35       0.97 (.04)   1.39 (.14)   2599.5 (222.5)   35.2 (4.2)
WW50       0.88 (.10)   0.98 (.34)   1902.7 (414.4)   25.3 (6.9)
Wood HDF   0.80 (.04)   0.92 (.19)   2083.3 (110.1)   29.8 (3.8)
WB15       0.95 (.06)   0.74 (.20)   2988.5 (384.9)   38.0 (5.1)
WB25       0.87 (.04)   0.85 (.18)   2508.0 (163.5)   33.4 (3.4)
WB35       0.92 (.05)   1.04 (.11)   2361.2 (558.4)   31.8 (9.1)
WB50       0.92 (.07)   1.65 (.16)   1863.2 (260.3)   28.5 (4.4)

Table 3: Estimated mean and standard deviation (sd) of
the mechanical properties of WW and WB HB-boards

Type [%]  Density      IB           MOE              MOR
          [g/[cm.      [N/[mm.      [N/[mm.          [N/[mm.
          sup.3]]      sup.2]]      sup.2]]          sup.2]]

          n=10         n=5          n=5              n=5

Wood HB   0.97 (.02)   0.27 (.07)   3232.2 (186.4)   37.5 (2.3)
WW 15     0.96 (.03)   0.19 (.17)   2756.6 (223.5)   32.8 (3.4)
WW 25     0.98 (.03)   0.22 (.17)   2737.7 (134.3)   33.1 (2.0)
WW 35     0.98 (.04)   0.26 (.23)   2557.7 (223.4)   33.7 (3.3)
WW 50     0.96 (.02)   0.33 (.16)   1799.8 (268.7)   25.5 (3.6)
Wood HB   0.97 (.02)   0.27 (.07)   3232.2 (186.4)   37.5 (2.3)
WB 15     0.95 (.03)   0.22 (.24)   2772.2 (290.7)   31.1 (5.4)
WB 25     0.95 (.01)   0.33 (.05)   2436.9 (84.8)    31.3 (1.8)
WB 35     0.94 (.01)   0.33 (.06)   2366.2 (100.6)   28.2 (2.4)
WB 50     0.95 (.03)   0.22 (.11)   1919.9 (140.5)   24.1 (2.3)
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Author:Rindler, Axel; Solt, Pia; Barbu, Marius C.
Publication:Forest Products Journal
Article Type:Report
Geographic Code:4EUAU
Date:Jul 1, 2015
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