Abstract
This article deals with the effects of various parameters on the shear bond strength (SBS) of glued wood. A four-factor analysis showed that the combination of only non-densified wood pieces achieves higher shear bond strength values than densified ones. In this case, only the piece combination was a significant factor. The other factors (glue type, wood species, and number of loading cycles) had no significant effect. Although the differences were not large, a higher SBS was achieved in beech wood glued with polyvinyl acetate (PVAc) glue. Glued wood consisting of the combination of densified and non-densified pieces had slightly lower SBS values. In this case, all the factors were statistically significant. Beech wood had a more significant impact on the SBS than aspen wood. The effect of the type of glue showed an opposite trend than that in the previous variant, i.e., a higher SBS was achieved with polyurethane (PUR) glue. Wood subjected to cyclic loading had slightly higher SBS values than non-cyclically loaded wood. The degree of densification had no significant effect. Glued wood composed entirely of densified pieces showed greater SBS variation between versions.
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Shear Bond Strength of Two-Layered Hardwood Strips Bonded with Polyvinyl Acetate and Polyurethane Adhesives
Miroslav Gašparík,a,* Milan Gaff,a Daniel Ruman,a Vladimír Záborský,a Václava Kašičková,a Adam Sikora,a and Václav Štícha b
This article deals with the effects of various parameters on the shear bond strength (SBS) of glued wood. A four-factor analysis showed that the combination of only non-densified wood pieces achieves higher shear bond strength values than densified ones. In this case, only the piece combination was a significant factor. The other factors (glue type, wood species, and number of loading cycles) had no significant effect. Although the differences were not large, a higher SBS was achieved in beech wood glued with polyvinyl acetate (PVAc) glue. Glued wood consisting of the combination of densified and non-densified pieces had slightly lower SBS values. In this case, all the factors were statistically significant. Beech wood had a more significant impact on the SBS than aspen wood. The effect of the type of glue showed an opposite trend than that in the previous variant, i.e., a higher SBS was achieved with polyurethane (PUR) glue. Wood subjected to cyclic loading had slightly higher SBS values than non-cyclically loaded wood. The degree of densification had no significant effect. Glued wood composed entirely of densified pieces showed greater SBS variation between versions.
Keywords: Shear bond strength; Beech; Aspen; Densification; PVAc; PUR; Cyclic loading
Contact information: a: Department of Wood Processing, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences Prague, Kamýcká 1176, 16521 Prague 6 – Suchdol, Czech Republic; b: Department of Forest Technologies and Construction, Czech University of Life Sciences in Prague, Kamýcká 1176, Praha 6 – Suchdol, 16521 Czech Republic; * Corresponding author: gathiss@gmail.com
INTRODUCTION
Although wood is an exceptional material, people are still trying to improve its desirable properties and eliminate its shortcomings. This is why new materials are created either by modifying the properties of the wood, or by combining wood materials of different properties. The mentioned combination of materials creates widespread wood-based composites that are applied in the most common spheres of the timber industry, such as in the production of furniture and wooden buildings (Stoeckel et al. 2013).
Wood-based composites are one of the core materials in the woodworking industry. In general, wood-based composites are formed by layering wood materials (plies, strips, veneers, strands, pieces, fibres, etc.) (Cai and Ross 2010) and then pressing the material to a final shape with the desired properties. In the construction industry, where these types of composites are most commonly applied, they are divided into two subgroups: structural composite lumber materials (LVL, PSL, LSL, OSL, glulam), and other materials (OSB, fibreboard, plywood, particleboard). Structural composite lumber materials are used for load-bearing elements in buildings and structures, such as rafters, headers, beams, joists, studs, and columns (Stark et al. 2010). Other materials are used for non-load bearing applications, such as tiling, floors, formwork, and thermal and sound insulating layers. In the furniture industry, wood-based composites are used a little differently. Structural composite lumber is not used at all, but some principles of its formation are used in other elements (e.g., LVL and bed piece component). Most materials that are intended for non-load bearing use in construction are used to manufacture load-bearing elements in the furniture industry, with the exception of OSB (not used at all) and fiberboards.
Gluing wood is one of the basic methods of bonding wood, known since ancient times. In terms of gluing wood, both the bonding strength and the properties of the glue are important. These characteristics are affected by the gluing conditions (open time, curing method, moisture content of wood, and necessary ambient temperature), as well as the length of the entire production process and the strength of the final product. Today, the most widely used adhesives in the furniture and construction industries are polyvinyl acetate (PVAc) and polyurethane (PUR), which are both defined as elastic polymers (Stoeckel et al. 2013). PVAc glues (most often dispersed in the form of an emulsion) are thermoplastics characterized by the need for an absorbent surface, and their glass-transition temperature (Tg) is close to room temperature (Motohashi et al. 1984; Minelga et al. 2010). Their mechanical properties are closely linked to changes in temperature, which is why even the slightest change in their Tg affects their mechanical properties under specified usage conditions (Motohashi et al. 1984). The advantage of PVAc glue is its water solubility, low curing temperature, non-toxic composition, transparent adhesive joints, and its ease of use. PUR glues are thermoplastics most often cured using the wood’s moisture content, which reacts with the glue’s isocyanate groups (Ren and Frazier 2012). PUR glue for wood creates strong joints with good flexibility, cures at a normal ambient temperature, and is more water resistant than PVAc glue, but it requires a higher moisture content of the glued material or air (Vick and Okkonen 1998).
The bonding quality of glued wood-based composites is affected by various factors. First of all, there are the basic physical and structural properties of the adherent wood, such as moisture content, density, porosity, properties of the cell wall, lumen, and extracellular spaces, which can sometimes change when the wood is modified (impregnation, grinding, and surface treatment). Another group of factors are the properties of the glue in its application (e.g., penetration behavior and surface free energy) and curing (e.g., creep and stiffness) states (Follrich et al. 2007; Li et al. 2015). Finally, the gluing parameters (adhesive spread, pressing time and temperature, and applied pressure) determine the final properties of the glue and therefore the entire glued wood-based composite (Follrich et al. 2007).
The suitability of an adhesive for bonding wood under certain conditions can be determined by various characteristics. The most commonly used reference parameter for the bond strength of the glued wood is its shear bond strength (Serrano 2004; Burdurlu et al. 2006; Konnerth et al. 2006; Raftery et al. 2009; Derikvand et al. 2014). The bond strength is the ability of the bonded wood to hold the wood-based composite elements together, ensuring its overall strength. If the bond has a low strength, then the wood-adhesive interface fails; the elements stop sharing the load and the entire composite will have poor characteristics (Le and Nairn 2014).
This research focuses on examining the shear bond strength of beech and aspen wood consisting of pieces bonded with PVAc and PUR adhesives.
The main goal was to determine the effects of wood densification, combination of pieces, wood species, and number of cycles on the shear bond strength values.
EXPERIMENTAL
Materials
European beech (Fagus sylvatica L.) and European aspen (Populus tremula L.) woods were used for testing. Thin wood pieces of three thicknesses (3, 5, and 9 mm), 25 mm in width and 600 mm in length were produced. These pieces were conditioned to an equilibrium moisture content (EMC) of 8% (ɸ = 40 ± 3% and t = 20 ± 2 °C). The EMC represented the final moisture content of furniture and wooden joinery (flooring and cladding) for interior use, according to EN 942 (2007) and ČSN 91 0001 (2007). The pieces of both wood species were divided into two groups: the first group consisted of non-densified pieces, and the second group consisted of pieces designed for densification.
Densification
Pieces intended for densifying were cold-pressed in a UPS 1000 hydraulic press (RK MFL Prüfsysteme GmbH, Germany) without previous plasticizing. The press was closed for 5 min, and gradual densification of the pieces began. The pieces were kept densified for 2 min. In the last stage, which lasted 3 min, the press was opened and the pressure was released. Subsequently, the pieces relaxed for 5 min.
Table 1 shows all the individual densities of the non-densified and densified pieces for each combination.
Table 1. Average Density Values of Pieces at Moisture Content of 12%
N – non-densified, D – densified pieces
Sample preparation
The one-component dispersion water-proof polyvinyl acetate (PVAc) adhesive AG-Coll 8761/L D3 (Agglu SK Ltd., Slovakia) and moisture-curing one-component polyurethane (PUR) adhesive Neopur 2238R (Neoflex S.L., Spain) were used for the gluing of pieces. According to EN 204 (2001) these PVAc and PUR adhesives belong to durability classes D3 and D4, respectively. The adhesive properties are shown in Table 2.
Table 2. Adhesives Properties
A toothed spatula was used for the glue application, laying a thin film of glue at the recommended level of 160 g/m2. The samples were pressed in an industrial press GS 6/90 (SCM GROUP S.p.A., Italy) under 0.6 MPa at a temperature 20 °C during 20 min. Subsequently, the samples were conditioned in the humidity chamber HCP 108 (Memmert, Germany) to a moisture content of 8% (ɸ = 40 ± 3% and t = 20 ± 2 °C). Lastly, the 2-layered samples were cut to final dimensions of 600 × 35 × h mm3 (thickness was dependent on the combination of pieces).
Table 3 shows samples with various combinations of pieces. One group consisted of 120 glued samples made of non-densified pieces and the second group consisted of 480 glued samples created by various combinations of densified and non-densified pieces. One half of the samples were intended for cyclic loading testing.
Table 3. Marking and Parameters of Samples
3, 5, and 9 are the original thicknesses of the pieces in millimeters
N – non-densified pieces, D – densified pieces
Cyclic loading
At least 24 h after bonding, cyclic loading was carried out. Cyclic loading testing, based on uniaxial bending stress, was carried out using a cycling machine (designed by Milan Gaff at the Faculty of Forestry and Wood Sciences, CULS Prague) with a distance between supporting pins of 490 mm. Cyclic loading was based on bending loading with repetitive loading and releasing. The cyclic loading rate was 60 cycles/min. Maximum load was set to 90% of the limit of proportionality. The limit of proportionality was found during the preliminary tests. Its value is important for avoiding the sample overloading during the cyclic loading. Therefore, the loading value was set up to 90% of the proportionality limit. Subsequently, the samples underwent 10,000 cycles and were compared with non-cyclic loaded samples (0 cycles).
Many of glued furniture parts are loaded cyclically during their use (e.g. lamellar grid of chairs and bed furniture). Therefore, the impact of cyclic loading should be examined for glued wood designed for such furniture components.
Shear bond strength
After cyclic loading, the clear samples were cut to 110 × 25 × h mm3 (sample thickness according to Table 3) and were conditioned to an equilibrium moisture content (EMC) of 12% (ɸ = 65 ± 3% and t = 20 ± 2 °C). The shear bond strength (SBS) was represented by tensile-shear strength using a lap joint test according to EN 205 (2003) (Fig. 1). Tensile-shear strength was determined using the universal testing machine UTS 50 (TIRA, Germany). The constant loading speed was set to 5 ± 0.5 mm/min such that the time required to reach failure was between 30 s and 50 s. Maximum loading force was directly recorded into the computer software.
Fig. 1. Testing sample dimensions
Note: l1 – sample length (mm), l2 – shear area length (mm), b – sample width/shear area width (mm), h – piece thickness (mm).
Evaluation and Calculation
The shear bond strength values were evaluated using MANOVA, specifically utilizing Fisher’s F-test in STATISTICA 13 software (Statsoft Inc., Tulsa, Oklahoma, USA). The results were evaluated using a 95% confidence interval, which reflects a probability of 0.05 (P < 0.05).
The tensile-shear strength was calculated according to EN 205 (2003) and Eq. 1:
(1)
where τ is the tensile-shear strength parallel to the fibers (MPa), Fmax is the maximum loading force recorded at the breaking point (N), l2 is the length of the shear area (mm), and b is the width of the shear area/sample (mm).
The wood density was determined according to ISO 13061-2 (2014) and Eq. 2,
(2)
where ρw is the density of the sample at moisture content w (kg/m3), mw is the weight of the sample at moisture content w (kg), and Vw is the volume of the sample at moisture content w (m3).
The moisture content of the samples was determined according to ISO 13061-1 (2014) and Eq. 3,
(3)
where w is the moisture content of the sample (%), mw is the weight of the sample at moisture content w (kg), and m0is the weight of the oven-dry sample (kg). Oven-drying was carried out according to ISO 13061-1 (2014).
RESULTS AND DISCUSSION
Combination of Non-Densified Pieces
Table 4 presents a statistical evaluation of the influence of factors on the shear bond strength of the samples with non-densified piece combination. Only the piece combination was statistically significant (P < 0.04). The wood species, glue type, number of cycles, as well as the interaction of all factors, did not significantly influence the shear bond strength.
Table 4. Statistical Evaluation of the Shear Bond Strength for Combinations of Non-densified Pieces
The mean shear bond strength value for beech wood (12 MPa) was approximately 8.1% higher than that of aspen (11.1 MPa; Fig. 2). The differences in shear bond strength values between the different wood species were probably caused by the different anatomical structures (length of wood fibers, porosity, water absorption, and surface roughness), which affect the adhesion of the glue to the surface of the wood pieces.
Fig. 2. Influence of wood species on tensile-shear strength
Shear bond strength also depends on the type of adhesive used. It is generally believed that PUR adhesives achieve slightly higher strength values. In this research, no significant differences between PUR and PVAc were observed. However, the shear bond strength found using PVAc glue was 1.7% higher than that with PUR glue (Fig. 3), which is the opposite of the general assumption.
Fig. 3. Influence of glue type on tensile-shear strength
The effect of piece combinations proved to be the only statistically significant factor. Although the area for finding the shear bond strength was the same for all combinations (10 × 25 mm), the thickness of individual pieces changed, which also changed the overall thickness of the samples. The shear bond strength values increased with the increase in sample thickness (Fig. 4).
While the samples composed of 3 mm pieces achieved shear bond strength of 9.9 MPA, the samples composed of 5 mm pieces achieved shear bond strength of 11.9 MPa. The highest value of shear bond strength, 12.6 MPa, was found in the samples composed of 9 mm pieces.
Fig. 4. Influence of pieces combination on tensile-shear strength
Cyclic loading had almost no effect on the shear bond strength. The shear bond strength in the samples without cyclic loading were slightly higher, but only by 1.1%, in comparison to the cyclically-loaded samples (Fig. 5). Both adhesives created a sufficiently flexible bond that could withstand cyclic loading, and the bonded sample therefore remained intact.
Fig. 5. Influence of number of loading cycles on tensile-shear strength
Figure 6 shows the combined effect of piece combination, glue type, and wood species on the samples without cyclic loading. The shear bond strength is only clear in one case: in beech pieces bonded by PVAc glue. In this case, the shear bond strength increased in proportion to the increase in the thickness of the pieces, or samples. Other cases did not confirm this relationship. It can be concluded that slightly higher shear bond strength values were achieved with PVAc glue.
Fig. 6. Influence of piece combination, glue type, and wood species on the tensile-shear strength without cyclic loading
The combined effect of piece combination, glue type, wood species, and cyclic loading had a different character than that in the case of wood without cyclic loading (Fig. 7). This case is characterized by a distinctive trend in the shear bond strength dependency on the piece combination.
Fig. 7. Influence of piece combination, glue type, and wood species on the tensile-shear strength after cyclic loading by 10,000 cycles
Large differences between different wood species were also found, where higher shear bond strength was measured in beech wood samples. The influence of the glue type exhibited smaller differences than in the previous case.
Comparing these results with other studies, it is clear that the SBS values are similar in most cases (Table 5).
For example, Konnerth et al. (2006) reported an SBS value in laminated beech wood of 10.8 MPa using PVAc glue, and 9.7 MPa using PUR glue.
On the other hand, Derikvand and Pangh (2016) found higher SBS values using PUR glue (13.4 MPa) than using PVAc glue (12.3 MPa) to bond Oriental beech (Fagus orientalis Lipsky).
Král et al. (2015), who investigated plasma surface treatment and subsequent bonding of beech wood with polyvinyl acetate glue, reported a relatively lower SBS value of 8.2 MPa.
Tiryaki et al. (2015) reported an even lower SBS value, 6.9 MPa, in the bonding of Oriental beech wood (Fagus orientalis Lipsky) using PVAc glue.
Özçifçi and Yapici (2008) reported an even lower SBS value for aspen wood bonded with PUR glue – a value of 4.9 MPa.
The differences in SBS values among various studies are caused by the different physical and mechanical properties of the wood (surface roughness, porosity, density, etc.), the variability of the adhesives used (viscosity, open time, fillers and hardeners, water resistance D1 – D4, etc.), as well as the gluing conditions (temperature, pressure, and pressing time).
Table 5. Mean Values of the Tensile-shear Strength for Combination of Non-densified Pieces
*Values in parentheses are standard deviations
Combination of Non-Densified and Densified Pieces
Table 6 contains a statistical evaluation of factors that affect the shear bond strength. All individual factors and their combined effect were statistically significant (P <0.05; Table 6).
Table 6. Statistical Evaluation of the Shear Bond Strength for Combinations of Densified and Non-Densified Pieces
As in the previous case of the combination of non-densified pieces, in the combination of densified and non-densified pieces the shear bond strength was higher in beech wood than in aspen wood (Fig. 8). The average value for the glued wood consisting of beech pieces was 26.8% higher than in aspen pieces.
Fig. 8. Influence of wood species on tensile-shear strength
The type of adhesive used had an opposite effect on the shear bond strength (Fig. 9) than in the combination consisting only of non-densified pieces. The shear bond strength of the wood glued with PUR glue was 9.1% higher than that with PVAc glue. The smoother surface of the pieces, as a result of densification, had a positive effect on the adhesion of the glue. The works of Özçifçi (2006), Özçifçi and Yapici (2008), as well as Frihart and Hunt (2010) confirm that higher surface smoothness increases the shear bond strength of wood.
Figure 10 shows the shear bond strength for different combinations of non-densified and densified pieces. Based on the course of the curves in the graph, it can be concluded that the combination of only densified pieces exhibits the greatest differences in shear bond strength values.
Fig. 9. Influence of glue type on tensile-shear strength
Paradoxically, the glued wood consisting of pieces densified by 20% achieved lower shear bond strength values in all cases, whereas the thickness of the individual pieces had no significance. However, glued wood consisting of densified and non-densified pieces showed smaller differences, and as the thickness of the pieces increased, there was a gradual increase in the shear bond strength.
Fig. 10. Influence of piece combination on tensile-shear strength
Although cyclic loading was a statistically significant factor, its probability P <0.04 was close to P <0.05. Based on this result, it can be concluded that the number of cycles had the smallest effect on the shear bond strength of all the factors. The shear bond strength of the glued wood subjected to cyclic loading was 7.4% higher than that of wood without cyclic loading (Fig. 11).
Fig. 11. Influence of number of cycles on tensile-shear strength
The assessment of the effect of all the factors on the SBS of wood not subjected to cyclic loading is shown in Fig. 12. The glued wood consisting of both wood species did not achieve a clear trend in SBS values for any combination of densified and non-densified pieces. A clear dependence on the piece thickness or their combination in terms of densification was not confirmed. The use of PUR glue resulted in greater fluctuations in SBS values in beech wood. These results confirmed the assumption that the glued beech wood achieved higher SBS values than the aspen wood with the use of both types of adhesives.
Fig. 12. Influence of piece combination, glue type, and wood species on the tensile-shear strength without cyclic loading
The effect of the combination of all the factors (wood species, glue type, combination of pieces, and cyclic loading) on the SBS is shown in Fig. 13. In this case, there were greater differences between the combinations of densified and non-densified pieces in glued beech wood. Glued wood consisting of both densified and non-densified pieces achieved significantly higher SBS values but a dependence on the piece thickness was not confirmed in compariosn with the combination of densified pieces. Glued beech wood without cyclic loading also achieved higher SBS values than those of aspen wood. The SBS of aspen wood did not have a definite dependence on the combination of pieces or their thickness in any of the cases.
Fig. 13. Influence of piece combination, glue type, and wood species on the tensile-shear strength after cyclic loading by 10,000 cycles
The SBS values obtained have some similarities with other studies (Table 7) as stated previously. Similar values were reported by Özkaya et al. (2015), who found the SBS value of 8.1 MPa in beech LVL glued with PVAc glue. Slightly higher values were found by Kläusler et al. (2014), who reported an SBS value of 12.5 MPa for beech wood glued with single-component PUR glue. Shukla and Kamden (2008), who studied LVL consisting of thin aspen veneer and PVAc glue, found SBS values in the range of 2.1 to 3 MPa. In this case, the densification of the pieces played a certain role in the results, which was most clearly demonstrated in the gluing of the pieces densified by 20%. The combination of densified and non-densified pieces balanced the SBS values of the glued wood. Similar studies indicate that densification has no significant effect on SBS values. For example, Kurowska et al. (2010) found that densification of veneers has no significant effect on the SBS value of plywood glued with UF glue.
Wood, as a natural heterogeneous material, is affected by many factors during gluing. The quality of the adhesive bond depends primarily on the properties of the wood, as well as the gluing parameters. One of the most basic wood properties, that affect the gluing process, is density. Generally speaking, the shear bond strength increases along with the density. However, increasing density requires higher compression pressure, depending on the surface flatness. Hardwood species with higher density require a smoother surface for bonding, while softwood species require a rougher surface. Excessive compression pressure has a negative effect, because it densifies the wood and pushes the glue out of the glue line (Seldiačik and Sedliačik 2000). Densification or pressing of wood can eliminate surface roughness to a certain extent, but it also causes a decrease in the wood’s porosity (Schneider and Wagner 1974, Patyakin et al. 2008, Plötze and Niemz 2011), which limits the ability of the adhesive to bind to its surface.
Table 7. Mean Values of the Tensile-shear Strength for Combinations of Densified and Non-Densified Pieces
Values in parentheses are standard deviations
3, 5, and 9 are the original thicknesses of the pieces in millimeters
The outer and inner surface of the wood is considered. The outer surface, which consists of the wood’s anatomical structure (porosity, differences between spring and summer wood, the width of annual rings, etc.), and the type of machining tool, is measured by the surface roughness and waviness. The surface roughness of the wood is strictly dependent on the method of processing. A milled surface does not require high pressure for gluing in order to achieve higher shear bond strength. In a sanded surface, the higher the compression pressure, the higher the shear strength. The inner surface is the set of all capillary cavities from wood fiber lumens, the spaces between the fibrils and submicroscopic areas between microfibrils and micelles (Sedliačik and Sedliačik 2000). The wood surface affects the penetration of the adhesive into its structure, and therefore the adhesion between the wood and the glue.
Based on these facts, it is important to further examine the various effects on the interaction between wood and glue. It is necessary to examine wood as an adherend with certain anatomical properties (porosity, surface roughness, waviness) and changes to these properties through technological operations (densification, machining, modifications) based on the properties of the adhesive and gluing parameters.
CONCLUSIONS
- In general, glued wood consisting only of non-densified pieces achieved higher SBS values in both wood species. As expected, higher SBS values were found in glued beech wood. However, statistical probability was only confirmed for the pieces combination (P > 0.04), where there was a slight increase in SBS values as the piece thickness increased. The other factors had no significant effect on the SBS. The difference in the SBS between PUR and PVAc glue was only 1.7% in the favor of PVAc. The comparison of SBS values of glued wood subjected to cyclic loading and wood not subjected to cyclic loading showed a difference of only 1.1%.
- Glued wood consisting of a combination of densified and non-densified pieces had lower SBS values. All the factors were statistically significant. The SBS of glued beech wood was 26.8% higher than that of aspen wood. The combination of non-densified and densified pieces achieved lower SBS values than those of glued wood consisting only of densified pieces. PUR glue ensured 9.1% higher SBS as compared to PVAc glue, which was the opposite of the trend as in the previous variant (combination of NN pieces). Cyclic loading of glued wood increased the SBS by 7.4% compared to wood not subjected to cyclic loading.
ACKNOWLEDGMENTS
The authors are grateful for support by the University-Wide Internal Grant Agency of the Faculty of Forestry and Wood Science at Czech University of Life Sciences Prague, project CIGA 2016-4311 as well as by the Ministry of Agriculture of the Czech Republic, project NAZV QJ1520042.
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Article submitted: August 30, 2016; Peer review completed: November 5, 2016; Revised version received and accepted: November 14, 2016; Published: November 23, 2016.
DOI: 10.15376/biores.12.1.495-513