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Article

Physical-Mechanical Properties of Light Bark Boards Bound with Casein Adhesives

by
Johannes Urstöger
1,
Günther Kain
1,2,*,
Felix Prändl
1,
Marius Catalin Barbu
1,3 and
Lubos Kristak
4
1
Department of Green Engineering and Circular Design, Salzburg University of Applied Sciences, Markt 136 a, 5431 Kuchl, Austria
2
Higher Technical College Hallstatt, Lahnstraße 69, 4830 Hallstatt, Austria
3
Faculty of Furniture Design and Wood Engineering, Transylvania University of Brasov, B-dul. Eroilor nr. 29, 500036 Brasov, Romania
4
Faculty of Wood Sciences and Technology, Technical University in Zvolen, 96001 Zvolen, Slovakia
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(18), 13530; https://doi.org/10.3390/su151813530
Submission received: 28 July 2023 / Revised: 1 September 2023 / Accepted: 5 September 2023 / Published: 10 September 2023
(This article belongs to the Special Issue The Engineered Wood Products Industry: Sustainable Production, Management, Simulation and Analysis)
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Abstract

:
Based on the background of the limited availability of raw materials in the forestry and timber industry, increased attention applies to sawmill by-products and their potential for future applications. Within the present research, the suitability of a natural binder based on different casein sources, superficially lean curd with different lime ratios, for the production of bark insulation panels from larch bark (Larix decidua) in fraction 12.5–4.0 mm with densities below 500 kg/m³ were discussed and physical and mechanical properties evaluated. In order to obtain a benchmark, control boards bound with powdery casein and urea-formaldehyde resin were fabricated. The evaluation of physical-mechanical characteristics indicated the lean curd panels to be competitive with the references and commercially available insulation materials, whereby higher values could be achieved consistently with a lower lime content of 20% compared to 31%. The best moisture resistance and water absorption were observed with a lime ratio of 31%, whilst thickness swelling and mechanical characteristics were best with a lime content of 20%. Particularly with regard to mechanical properties, panels containing a low proportion of lime in the lean curd blends delivered convincing results, e.g., with an average IB of 0.19 N/mm2, MOR of 1.43 N/mm2 and C of 1.70 N/mm2. In terms of thickness swelling, lean curd adhesives generated unsurpassed values of approximately 11% compared to the casein and urea-formaldehyde references. Additionally, as described in the relevant literature, a considerable influence of density on the mechanical behavior of composite materials was observed. Partly, the lime content significantly influenced the panel properties. The study proved that natural binders based on lean curd and lime are suitable for the production of bark insulation boards and represent a serious alternative to synthetic, oil-based adhesive systems. The results are promising with regard to the market situation due to the availability and pricing of raw materials and technical requirements and demonstrated great potential for further research efforts.

1. Introduction

Tree bark is considered the second most important tissue of a tree [1] and accrues in the sawmill and timber industry as a by-product in large quantities. Despite existing utilization options, there is justified interest in alternative application opportunities with a higher degree of added value [2]. Research on the natural functions of bark indicated the opportunity as green thermal insulation material [3]. Bark and the associated use for various wood-based panels have already been addressed in manifold scientific work, e.g., Volz [4], Xing et al. [5], and Kraft [6], finding a negative influence of increased bark content on mechanical properties. Kain et al. [2,7] developed insulation boards with urea-formaldehyde, exhibiting remarkable results in thermal conductivity compared to conventional insulation materials. Urstöger et al. [8] investigated the suitability of different bark types in combination with cement. In order not to combine an ecological and sustainable raw material such as bark with mineral oil-based resin systems, adhesives based on renewable resources are desirable. To ensure the production of environmentally friendly insulation materials entirely from ecological sources, synthetic adhesives must be replaced by a biological, natural binder in the spirit of cradle-to-cradle, a concept in which all products are recyclable, i.e., a sustainable strategy to produce products without the addition of harmful chemicals that enable a value chain in a natural cycle [9].
In the development of the forestry and timber industry, adhesive for bonding of solid wood and wood particles occupy an important role in the efficient use of wood as a resource and in the manufacture of modern wood products [10]. Until the middle of the 20th century, in addition to phenolic resins, protein-based glues, for example, from blood, soy, bone, and casein, considered to be a preferable source in the production of wood-based materials, e.g., production of plywood for the construction of wooden aircraft, but have been replaced by synthetic, resin-based binders, as these offered many advantages, e.g., cost reduction, better moisture resistance, etc. and thus often fallen into oblivion [11]. Several of these mineral oil-based adhesives, such as urea-formaldehyde (UF), melamine formaldehyde (MF) or phenol formaldehyde (PF), etc., are problematic due to formaldehyde release and limited biodegradability but have been used extensively in wood-based material production for decades [12,13]. Nowadays, natural biogenic adhesive systems are largely made from polyphenols (tannin, lignin), polysaccharides (starch), proteins (casein, blood, etc.), natural resins, and waxes [10,11]. These original, largely formaldehyde-free, organic binders from renewable resources are currently experiencing a renaissance, although despite intensive research, for example by Pizzi et al. [14], Kain et al. [7,12], Guo and Wang [15], Solt et al. [16], Herzog et al. [17], Hussin et al. [18], etc., still lead a niche existence. Extensive research efforts concerning tannin and lignin adhesives can be registered. Cesprini et al. [19], for example, used renewable tannin-based adhesives from quebracho extract and furfural for particleboards. Jorda et al. [20] analyzed quebracho tannin bio-based adhesives for the production of plywood, and Chen et al. [21] utilized modified lignin for particleboard application. Vitola et al. [22] additionally evaluated by-products of plant origin as lightweight aggregates for bio-composites with starch binders, producing panels with low thermal conductivity and high strengths. Besides the ecological advantages, decreased water resistance and durability appeared with these adhesives [12,23].
Casein, a phosphorus protein from skimmed milk obtained using precipitation, is one of the oldest natural binding agents [24,25]. The use of casein-based binders for gluing wood dates back to ancient Egypt, the Roman Empire, and the Middle Ages. Even in these early times, craftsmen worked with casein and bone glues, as the required raw materials were already available [26,27]. Traditionally, casein in the form of lean curd was mixed with alkaline substances, for example, burnt or slaked lime, to prepare glues, but nowadays, it is available in pure powder form. Today, the main use of casein adhesives is in very special sustainable applications, such as environmentally friendly bottle labeling [15,28]. With about 3–4%, it is considered the most important protein component of milk and accounts for approximately 80% (about 25 g/L) of the total protein quantity [15]. The structure is characterized by a predominantly random spiral pattern, able to bind calcium due to phosphoseryl groups, and occurs in large casein micelles (porous, spherical aggregates) [29], forming a complex entity of colloidal calcium phosphate (CCP) and proteins (α-, β- and κ-casein) [30]. This consists of several molecular units organized and held together by non-covalent, intermolecular binding interactions [31].
The present research focuses on the possibility of producing insulation panels from larch bark by using an alternative to synthetic, resin-based binders in the spirit of cradle-to-cradle for an entirely ecological origin. The aim of this study is to investigate and evaluate whether casein adhesives from different sources, such as powder and in the form of lean curd, are suitable for the production of bark insulation boards based on larch bark (Larix decidua), focusing on physical and mechanical properties, whereby casein powder and UF serve as reference materials.

2. Materials and Methods

The larch bark used could be obtained almost pure and dried (approx. 13–16% moisture content) in the fraction 12.5–4.0 mm via the company Barkinsulation GmbH (Hallein, Austria) who themselves buy the raw bark in the sawmill Graggaber Peter GmbH in Unternberg, Salzburg, Austria (Figure 1). Larch bark was used because it was shown in previous work [32] that it has the lowest thermal conductivity of industrially used species in central Europe.
Lean curd from pasteurized milk from Gmundner Molkerei GmbH (Gmunden, Austria) and pit lime (hydrated lime soaked in water for at least three months) from Baumit GmbH (Waldegg, Austria) applied to prepare the adhesive mixtures, casein powder from Kremer Pigment GmbH and Co. KG (Aichstetten, Germany) (Figure 2), sodium silicate (Na2SiO3) from Hedinger GmbH and Co. KG (Stuttgart, Germany) and urea-formaldehyde (Primere 10F102) from Metadynea (Krems, Austria) with a solid content of 62.90% and a viscosity of 500 mPa·s for the references.
Based on the tree bark material mentioned, 20 × 500 × 500 mm boards with a target density in climate 20 °C/65% (temperature/relative humidity) of 450 kg/m3, corresponding to a dry density of approx. 393 kg/m3 were produced in combination with the following adhesive formulations visualized in Table 1.
The recipe for “Casein reference” was adopted from Schwarzenbrunner [33]. The following solid contents could be determined using kiln-drying (dry by wet weight; EN 827:2006 [34]) for the binders: Lean Curd 1 = 31.08%, Lean Curd 2 = 27.52%, casein reference = 27.21% and UF reference = 62.90%. The adhesives were prepared according to the above-mentioned recipes and mixed using a disperser from VMA-Getzmann GmbH (Reichshof, Germany). Subsequently, the bark material and the respective adhesive were mixed in a plowshare mixer. After about 3 min, the resin-coated bark particles were formed into a single-layer mat in a mold frame (500 × 500 mm; Figure 3) and pressed with a Höfer laboratory press (Taiskirchen, Austria) at temperatures of 150 and 180 °C and total cycle times of 6 (pressing time factor = 18 s/mm) and 8 min (pressing time factor = 24 s/mm) resulted for pressing of boards with lean curd and casein (binder content = 15%), and 7 min (pressing time factor = 21 s/mm) with UF (binder content = 10%). Pressing parameters and binder content of lean curd and casein were determined using a preliminary series of tests, while the parameters for UF data were supplied by Barkinsulation GmbH (Hallein, Austria). Finally, the manufactured boards were conditioned at 20 °C and 65% relative humidity (RH) for at least three days.
In order to ensure scattering, several samples were taken from different areas of the panels (EN 326-1:2005 [35]). Density determination followed EN 323:2005 [36]. By using X-rays (Dense-lab X from EWS, Uhingen, Germany), density profiles were generated. The physical properties measured included moisture content (MC) according to EN 322:2005 [37], thickness swelling (TS), and water absorption (WA) after 24 h water immersion by EN 317:2005 [38]. The mechanical characteristics were measured using a Zwick Roell GmbH Co. KG Z 250 universal testing machine (Ulm, Germany). Internal bond (IB) was determined according to EN 319:2005 [39], modulus of elasticity (MOE) and modulus of rupture (MOR) following EN 310:2005 [40], and compressive strength (C) as per EN 29469:2023 [41].
To determine the influence of the factors of lime content, press temperature, and the covariate density, specifically for lean curd blends, Pearson correlation coefficients and a two-factor analysis of covariance (ANCOVA) were performed using the software package “IBM SPSS Statistics 27”.
To demonstrate the explanatory content of the factor influence, partial eta-squared values were applied according to Backhaus et al. [42]. Factor influence of lime content and press temperature, including the covariate density, was studied according to the experimental design (Table 2):

3. Results

Examples of the insulation panels with lean curd, casein, and UF are displayed in Figure 4.

3.1. Panel Density

According to a defined initial target density of 450 kg/m³, the panels deviated on average by 1.7–4.7% due to the inhomogeneous mat strewing caused by manual filling of the press form (Table 3). The similarity of the density distribution of the different binders was reflected in the density profiles over the entire cross-section of the panels. Boards with lean curd blend revealed, on average, a 3.5% higher density over the cross-section than the references of casein and UF. Edge effects due to the high pressing pressure on the surfaces occurred for the lean curd blends (Figure 5).

3.2. Physical and Mechanical Properties of the Bark Panels

Results of thickness swelling (TS), water absorption (WA), internal bond (IB), modulus of elasticity in bending (MOE), modulus of rupture (MOR), and compressive strength (C) are summarized in Table 3.
The density, except for TS (p = 0.054), exhibited a highly significant influence on the board properties (MC, WA, IB, MOE, MOR, C) with a high explanatory content of 0.312–0.735, consistent with the literature on bark panels of Kain et al. [2,7]. Lime content exerted a highly significant influence on MC, IB, and MOR (p < 0.001), with a significant effect on C (p = 0.023). The press temperature exhibited no significant impact on the characteristics, but C was highly significantly (p < 0.001) and IB significantly (p = 0.003) influenced by the interaction between lime content and press temperature. Density displayed over three times higher explanatory content than lime content and the interaction between lime content and press temperature, with regard to partial Eta2 (Table 4).
The relation between density to MOR and C was revealed in the statistical value range, according to Kuckartz et al. [43], a very high positive (R > 0.7) to MC, IB, and MOE and a high positive correlation (R > 0.5). Between density and WA, a medium negative (R < −0.5) and to TS, a low negative correlation (R < −0.3) occurred. In addition, with the exception of TS, all physical and mechanical properties exhibited a statistically high significant influence of the panel density, whereby the influence of the density on TS could not be clearly rejected at a significance level of 5% (p = 0.052; Table 5).
The lean curd blends averaged a difference of approx. 0.8% (LC1: 16.03%, SD = 0.56%; LC2: 16.83%, SD = 0.65%) in the values for MC after conditioning at 20 °C/65% RH and remained about 3% below the casein reference. “UF” exhibited the lowest values, with an average of about 12.43% (SD = 0.14%). The considerably higher scatter of moisture content of casein-bound panels compared to the other binders was noticeable and could be related to the significantly shorter pot life compared to lean curd, the associated poorer mixing, and insufficient blending of the particles. The density revealed twice as much influence on MC as the lime content. On average, TS appeared to be lowest for “LC2” with about 11.22% (SD = 1.53%), but was about 1% below “CA” and 3.5% lower than “UF”. Even in terms of WA, the adhesives from lean curd demonstrated positive properties; the values exceeded the casein reference by only 5%, which achieved the best results with an average of 45.73% (SD = 6.46%), whereby lean curd ranged clearly below the synthetic, oil-based urea-formaldehyde by about 9% beneath (Figure 6). Due to the rubbery nature of casein, comparable to the consistency of chewing gum, a clear swelling behavior with low water absorption appeared.
In terms of IB, “LC2” with an average of 0.19 N/mm2 (SD = 0.04 N/mm2) exceeded “LC1” by 25%, “CA” by about 8%, and “UF” by approx. 18%. The MOE of “LC2” reached about 9% higher results than the second lean curd mixture and surpassed the references by 14 and 21% (Figure 7). Compared to “LC1”, the MOR of “LC2” with 1.43 N/mm2 (SD = 0.27 N/mm2) on average was 19%, and for “UF” around 22% higher, but in contrast, only 1% above “CA”. In the case of C, the lean curd adhesives clearly outperformed the reference values by approx. 14 and 11% (Figure 8).

4. Discussion

According to “EN 10456/2010 [44]”, MC of conventional insulation materials ranges below 10% for wood wool cement bonded lightweight boards with a similar density of 250–450 kg/m3 and under 15% for polyurethane foam (PUR foam) with a considerably lower density. Data obtained in the present study for bark boards with lean curd amounted to 16.38% (SD = 0.72%), i.e., slightly above the specified values and approx. 4% higher than the UF reference (around 12%; consistent with Kain et al. [2] insulation panels), associated with high material moisture of the bark of 13–16%, density fluctuations, and considerably lower solid content. Counteracting effects would be feasible by pre-drying the bark material to 7–9% and using a spreading device to lay the mat.
A different situation is given for TS after 24 h of immersion in water, where the benchmark implemented by Schwemmer et al. [45] for insulation materials made from cattail of below 15%, and the values achieved by Kain et al. [2] for bark insulation boards (approx. 18%) were clearly undercut with around 11% for lean curd blends. Additionally, the results remained about 1% under the casein and with 3.5% considerably below the UF reference. Further, the values generated for particleboards with tannin bonding by Cesprini et al. [19] could be considerably surpassed by up to 60% on average.
In terms of WA, lean curd mixtures achieved about 9% better values compared to urea-formaldehyde, whereby the lowest rates resulted from the casein reference. In comparison to other sustainable bark insulation materials, e.g., Kain et al. [7] with tannin adhesives, lean curd generated, on average, about 26% lower WA results.
In the case of TS and WA, the situation differed from MC, as better properties could be obtained due to a lower lime content. With the exception of MC, the physical properties demonstrated the clear potential of natural binders compared to the oil-based UF reference, representing the range of values of conventional synthetic resins.
Insulation materials suffer high stresses in the panel plane, for example, due to wind. The IB characterizes the individual particle adhesion. Especially for “LC2” with a reduced lime content, considerably higher values of up to 0.27 N/mm2 occurred by comparison with wood wool lightweight boards with a similar density or the UF reference. At lower densities, the panels produced ranged within the values reported by Chen et al. [21], producing particleboards bound with lignin adhesives (density = 650 kg/m3). At higher densities, the results ranged within the value spectrum of expanded and extruded polystyrene. Particularly, the values at 20% lime content for both pressing temperatures coincide with data from Kain et al. [2] for bark panels with urea-formaldehyde (Figure 9).
Focusing on MOE (material parameter defining the relationship between tension and strain in the deformation of solid material with linear elastic behavior), the lean curd mixture with reduced lime content amounted to 313.26 N/mm2 (SD = 54.27 N/mm2), surpassing “CA” by 14 and “UF” by 21%, with large scatter. In comparison to Kain et al. [7], within similar density ranges, values for bark panels with tannin remained clearly below by approx. 42%.
According to “EN 622-4/2019”, a standardized wood fiber insulation board must possess a MOR of 0.8–0.9 N/mm2, depending on the application in dry, wet, or outdoor areas and a thickness starting at 19 mm. For “LC2”, rated only about 1% below “CA”, the mean MOR amounted to approx. 1.43 N/mm2 (SD = 0.27 N/mm2; lower limit 95% confidence interval of mean values = 1.30 N/mm2, upper limit = 1.57 N/mm2) and consequently considerably exceeds the specified range. In addition, the results remained clearly above “UF” (about 22%) and the average MOR of 0.76 N/mm² generated by Kain et al. [2].
The C of insulation materials is generally strongly determined using the density, as demonstrated not only in the relevant literature (e.g., Xing et al. [5] and Kain et al. [2]) but also in the present study using a correlation analysis with a coefficient of determination of R2 = 0.59. The C of bark boards bonded using lean curd exceeded comparable insulation materials, as wood wool lightweight boards and bark panels by Kain et al. [2], along with the references of casein (over 13%) and UF (over 11%; Figure 10).
The panel density considerably influences the mechanical characteristics of composite materials according to literature, e.g., Xing et al. [5] and Kain et al. [2]. This became apparent in the specific evaluation selected by the lean curd blends “LC1” and “LC2” in the present investigation. The relations between density to MC, IB, MOR, MOE, and C revealed a high, positive correlation (R > 0.5 to 0.7), to WA and TS a medium to low, negative correlation (R < −0.3 to −0.5), corresponding to relevant literature, e.g., Gößwald et al. [47].

5. Conclusions

As a continuation of Kain et al. [2], the present research focused on the possibility of producing insulation panels from larch bark by using an alternative to synthetic, resin-based binders in the spirit of “cradle-to-cradle” for entirely ecological origin. The processing of lean curd in combination with lime successfully enabled the bonding of particles, allowing the production of a synthetic resin-free bark insulation board. The results clearly indicated that the physical and mechanical properties (TS, WA, IB, MOE, MOR, C) of bark panels bonded using lean curd resulted in high-quality properties and are competitive with commercially available insulation materials.
Except for MC, better values could be achieved with a lower lime content (20% compared to 31%). Focusing on the equilibrium moisture content (MC), the assumption remains that with a higher lime content (31% compared to 20%), less water is absorbed due to less hydrophilic functional groups in the calcium carbonate contained in lime. In contrast, this effect could not be investigated for TS and WA, requiring further research. The lime content proved to be an influential factor and apparently generates different advantages between physical and mechanical properties. Additional studies investigating even lower lime ratios and different proportions should be considered.
As expected, density and partly lime content significantly influence the panel properties, whereby board density exhibited a considerably higher explanatory power. With regard to pressing temperature, an ambiguous trend emerged due to the fluctuation of the findings, as there was no statistical significance, requiring further investigation. An interaction between lime content and pressing temperature resulted in a significant difference in IB and C. Depending on the lime proportion, for example, the best values for TS were found at 180 °C with 20% lime and for C at 150 °C with 31%.
Future considerations should focus on thermal properties and on upscaling to industrial equipment. Problems could arise due to the homogeneity of the boards, the delivery of lean curd to production companies, the pressing time due to the increased moisture after pressing, the high binder content of 15% compared to the industrial scale (approx. 10%), etc. Primarily, future tests regarding the sprayability of the binder and the thickness of the panels are required.

Author Contributions

Conceptualization, J.U. and G.K.; methodology, J.U. and G.K.; software, J.U.; validation, G.K. and F.P.; formal analysis, G.K. and M.C.B.; investigation, J.U. and L.K.; resources, G.K., M.C.B. and L.K.; writing—original draft preparation, J.U., G.K. and L.K.; writing—review and editing, F.P., M.C.B. and L.K.; visualization, J.U.; supervision, G.K. and F.P.; project administration, G.K. and M.C.B.; funding acquisition, G.K. and M.C.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Ministry of Education, Science, Research and Sport of the Slovak Republic, grant Nr. VEGA 1/0714/21.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful for the support of the company Barkinsulation GmbH (Hallein, Austria) and to Thomas Wimmer at Campus Kuchl of Salzburg University of Applied Sciences.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Larch bark, dried, shredded, and fractionated to 12.5–4.0 mm.
Figure 1. Larch bark, dried, shredded, and fractionated to 12.5–4.0 mm.
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Figure 2. Casein as a powder (left) and in lean curd (right).
Figure 2. Casein as a powder (left) and in lean curd (right).
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Figure 3. Coated particles in a mold frame (left) and pre-pressed cake before pressing (right).
Figure 3. Coated particles in a mold frame (left) and pre-pressed cake before pressing (right).
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Figure 4. Larch bark insulation 20 mm panels glued with lean curd (foreground), casein powder (middle), and UF (background).
Figure 4. Larch bark insulation 20 mm panels glued with lean curd (foreground), casein powder (middle), and UF (background).
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Figure 5. Density profiles of bark panels for different binders per factor level, including press time (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
Figure 5. Density profiles of bark panels for different binders per factor level, including press time (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
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Figure 6. Thickness swelling (left) and water absorption (right) after 24 h of water immersion dependent on binder (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
Figure 6. Thickness swelling (left) and water absorption (right) after 24 h of water immersion dependent on binder (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
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Figure 7. Internal bond (left) and modulus of elasticity in bending (right) dependent on binder (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
Figure 7. Internal bond (left) and modulus of elasticity in bending (right) dependent on binder (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
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Figure 8. Modulus of rupture (left) and compressive strength (right) dependent on binder (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
Figure 8. Modulus of rupture (left) and compressive strength (right) dependent on binder (LC1…Lean Curd 1, LC2…Lean Curd 2, CA…Casein reference, UF…UF reference).
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Figure 9. Internal bond of insulation materials by comparison (data with the exception of LC, CA, and UF, according to Pfundstein et al. [46] and Kain et al. [2]).
Figure 9. Internal bond of insulation materials by comparison (data with the exception of LC, CA, and UF, according to Pfundstein et al. [46] and Kain et al. [2]).
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Figure 10. Compressive strength of insulation materials by comparison (data with the exception of LC, CA, and UF, according to Pfundstein et al. [46] and Kain et al. [2]).
Figure 10. Compressive strength of insulation materials by comparison (data with the exception of LC, CA, and UF, according to Pfundstein et al. [46] and Kain et al. [2]).
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Table 1. Casein-based adhesive formulations of tests.
Table 1. Casein-based adhesive formulations of tests.
AdhesiveLean Curd (%)Lime (%)Casein (%)Water (%)Sodium Silicate (%)UF (%)
Lean Curd 169.0031.00 ---
Lean Curd 280.0020.00 ---
Casein reference-16.0223.1158.172.70-
UF reference-----100.00
Table 2. Experimental design with factors of lime content and press temperature.
Table 2. Experimental design with factors of lime content and press temperature.
AdhesiveAbbreviationLime ContentPress Temperature (°C)Press Time (Min)Panel Count
Lean Curd 1LC10.3115082
18061
82
Lean Curd 2LC20.2015082
18082
Casein referenceCA0.1618081
UF referenceUF-18071
Table 3. Physical and mechanical properties of bark panels (standard deviation in parentheses).
Table 3. Physical and mechanical properties of bark panels (standard deviation in parentheses).
Density TS/WA
(kg/m3)		TS
(%)		WA
(%)		Density IB
(kg/m3)		IB
(N/mm2)		Density MOE/MOR
(kg/m3)		MOE
(N/mm2)		MOR
(N/mm2)		Density C
(kg/m3)		C
(N/mm2)
LC1466.8111.5550.87458.950.15464.58284.831.17457.751.67
(27.27)(1.30)(5.45)(29.47)(0.04)(24.39)(63.00)(0.32)(26.86)(0.24)
LC2474.2211.2250.73472.860.19469.04313.261.43470.071.70
(22.05)(1.53)(4.40)(25.37)(0.04)(18.73)(54.27)(0.27)(22.87)(0.15)
CA469.9312.4645.73468.020.18451.78269.461.45458.481.46
(18.40)(1.60)(6.46)(23.86)(0.05)(20.50)(40.14)(0.23)(22.45)(0.20)
UF448.8014.8959.45445.310.16438.85248.681.12438.681.50
(15.78)(1.52)(1.96)(13.31)(0.05)(18.46)(58.24)(0.21)(15.90)(0.18)
TS thickness swelling, WA water absorption, IB internal bond, MOE modulus of elasticity, MOR modulus of rupture, C compressive strength.
Table 4. p-values and partial Eta2 (in parentheses) of the analysis of variance for individual factors of lean curd blends.
Table 4. p-values and partial Eta2 (in parentheses) of the analysis of variance for individual factors of lean curd blends.
MCTSWAIBMOEMORC
Sample size63636363363663
Variance homogeneity0.0050.0280.3910.4480.5040.5600.598
Density0.000 ***0.0540.000 ***0.000 ***0.000 ***0.000 ***0.000 ***
(0.405)(0.062)(0.385)(0.312)(0.416)(0.688)(0.735)
Lime content0.000 ***0.5510.5470.000 ***0.1690.000 ***0.023 *
(0.291)(0.006)(0.006)(0.276)(0.060)(0.339)(0.086)
Press temperature0.0640.3370.5030.4040.1590.0620.296
(0.058)(0.016)(0.008)(0.012)(0.063)(0.108)(0.019)
Lime content x Press temperature0.5650.1400.3390.003 **0.9600.1450.000 ***
(0.006)(0.037)(0.016)(0.140)(0.000)(0.067)(0.181)
Significance levels: *** ≤0.001, ** ≤0.01, * ≤0.05. MC moisture content, TS thickness swelling, WA water absorption, IB internal bond, MOE modulus of elasticity, MOR modulus of rupture, C compressive strength.
Table 5. Correlation coefficients (R) and significances of the lean curd blends between density to physical and mechanical board properties according to Pearson.
Table 5. Correlation coefficients (R) and significances of the lean curd blends between density to physical and mechanical board properties according to Pearson.
MCTSWAIBMOEMORC
Sample size63636363363663
Correlation coefficient (R)0.667−0.246−0.6080.5800.6180.7620.822
p-value0.000 *0.0520.000 *0.000 *0.000 *0.000 *0.000 *
Significance levels: * ≤0.001. MC moisture content, TS thickness swelling, WA water absorption, IB internal bond, MOE modulus of elasticity, MOR modulus of rupture, C compressive strength.
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Urstöger, J.; Kain, G.; Prändl, F.; Barbu, M.C.; Kristak, L. Physical-Mechanical Properties of Light Bark Boards Bound with Casein Adhesives. Sustainability 2023, 15, 13530. https://doi.org/10.3390/su151813530

AMA Style

Urstöger J, Kain G, Prändl F, Barbu MC, Kristak L. Physical-Mechanical Properties of Light Bark Boards Bound with Casein Adhesives. Sustainability. 2023; 15(18):13530. https://doi.org/10.3390/su151813530

Chicago/Turabian Style

Urstöger, Johannes, Günther Kain, Felix Prändl, Marius Catalin Barbu, and Lubos Kristak. 2023. "Physical-Mechanical Properties of Light Bark Boards Bound with Casein Adhesives" Sustainability 15, no. 18: 13530. https://doi.org/10.3390/su151813530

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