Properties of plywood manufactured from compressed veneer as building material

Materials and Design 30 (2009) 947–953

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Properties of plywood manufactured from compressed veneer as building material Pavlo Bekhta a, Salim Hiziroglu b,*, Oleg Shepelyuk a a b

National University of Forestry and Wood Technology of Ukraine, Department of Wood-Based Composites, Lviv 79057, Ukraine Oklahoma State University, Department of Natural Resource Ecology and Management, Stillwater, OK 74078-6013, USA

a r t i c l e

i n f o

Article history: Received 24 April 2008 Accepted 1 July 2008 Available online 5 July 2008 Keywords: Veneer compression Cold rolling Strength properties Roughness

a b s t r a c t The objective of this study is to evaluate some of the physical and mechanical properties of plywood manufactured from compressed veneer of birch (Betula pubescens) and alder (Alnus glutinosa) using a cold rolling process. Surface roughness, tensile strength parallel and perpendicular-to-grain orientation of veneer sheets before and after compression process were tested. Shear strength, bending strength, and compression strength of the plywood samples made from both compressed and non-compressed veneers were also determined. Based on the results of the study, overall mechanical properties of veneer and plywood improved as compression degree veneer increased from 5% to 15%. This also resulted in less adhesive consumption up to 20% as well as enhanced surface characteristics of veneer samples by 40%. Plywood samples required lower pressure ranging from 25% to 30% for those manufactured from regular veneer. It appears that veneer compression process can be considered as an alternative method to improve both physical and mechanical properties of experimental plywood panels which can be used for building applications. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction In general adhesive bonding of plywood consists of three steps, namely application of adhesive to the surface of veneer, assembly of veneer into panels, and pressing. Application of adhesive to the veneer sheets during the plywood manufacturing is one of the most important parameters influencing properties of the panel. Interaction between liquid adhesive and veneer surface depends on specifications of binder and surface quality of veneer [16]. Surface roughness of veneer also plays an important role on depth of penetration of adhesive into the veneer, uniform distribution of adhesive, and strength of glue line between veneers. Improving surface roughness of veneer enhances glue bonding during the press [17,18,21]. Acceptable strength properties of plywood cannot be achieved unless veneer is sanded to have smooth surface. However sanding not only increases overall production cost but also creates dust problem during the manufacture. Improvement of the quality of adhesive bonding is possible by the activation of the wood surface by using various chemicals or mechanical processes such as densification of veneer [3–5,8]. The concept of wood densification dates back to the early 1900s [16]. The motivation of compression of wood is to provide an increase of strength properties of products so that they can be used for structural application * Corresponding author. Tel.: +1 405 744 5445; fax: +1 405 744 3530. E-mail addresses: [email protected] (P. Bekhta), [email protected] (S. Hiziroglu), [email protected] (O. Shepelyuk). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.001

where high strength characteristics are required. Wood compression techniques have been applied to wood for various applications. One of the previous studies related to the compression of high moisture content of veneer using a roller press aimed to remove water from the samples mechanically [1]. It has also been reported that thermal conductivity of compressed wood of Japanese cedar has increased as a result of radial compression [2]. There is a limited research carried out in the area of bonding of compressed wood. Jennings investigated shear strength of compressed yellow poplar as function of different adhesive type [6,15]. Densified samples bonded using urea formaldehyde and phenol formaldehyde exhibited similar strength properties to each other and control specimen [15]. However polyvinyl acetate (PVAc) bonded compressed veneer from the same species showed higher mechanical properties than those of control samples. Plywood manufactured compressed birch and alder veneer sheets would have a potential for various constructional applications as building material with a better properties and lower production cost than those of traditionally produced plywood panels. Currently there is no information on both physical and mechanical properties of plywood made compressed veneer of above species. Therefore the objective of this study is to have initial data on some of basic properties veneer of birch and alder before and after compression is applied to their surface. Also experimental plywood panels from both types of veneer of two species were produced to evaluate tensile strength characteristics if they can be used for structural purposes.

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2. Materials and methods Rotary cut veneer sheets of birch (Betula pubescens) and alder (Alnus glutinosa) were used for the experiments. A total of 100 defect free samples with dimensions of 300 mm by 300 mm and with two thicknesses of 1.3 mm and 1.5 mm were conditioned to a moisture content of 8% prior to any tests carried out. A laboratory rolling device was employed for densification of the samples. A schematic of rolling device is illustrated in Fig. 1. The surface of the veneer samples was passed through between the rolls of cold press while pressure was applied in parallel and perpendicular grain orientations. Force of the veneer compression was identified as compression degree ranging from 5% to 25% with an interval of 5%. Compression degree (Dv) in percent can be defined based on below equation

Dv ¼ ½ðSv1  Sv2 Þ=Sv1   100; where Sv1 and Sv2 are veneer thickness before and after rolling, respectively. Thickness of each sample was measured using a calliper at nine different points at accuracy of 0.01 mm prior and following the rolling process. The gap between drums of rolling device was determined based on veneer thickness and the necessary compression degree. Surface roughness of veneer samples was also measured using a stylus type equipment. A portable profilometer unit consisting of main unit and pick-up which has a skid type stylus with 5 lm tip radius. The stylus traverses the surface at a constant speed of 1.0 mm/s over 12.5 mm. Although there are numerous roughness parameters which can be calculated from the vertical movement of the stylus during its tracing the surface mean peak-to-valley height (Rz) was used to have a general idea about the roughness characteristics of the sample. Detail definition of Rz roughness parameter is described in previous works [13,14,19,20]. A total of 15 random measurements were taken from the surface of each samples for roughness measurements. Tensile strength of the samples in parallel and perpendicular grain orientations was determined on 20 mm by 200 mm and 120 mm by 240 mm specimens, respectively. Seventy samples were tested on a Universal Testing Machine equipped with a load cell having 5000 kg capacity. Properties of the test specimens were evaluated based on GOST standards [10–12]. Five-ply plywood samples were manufactured from both compressed and non-compressed veneers using a commercially produced phenol formaldehyde adhesive with a rate of 135 g/m2. Panels were pressed at a Raute Laboratory press using a pressure of 1.8 MPa, a temperature of 125 °C for 8 min. Thereafter, the influence of the various technological factors of pressing on the properties of plywood made from densified veneer was studied. Levels and intervals of the factors variation are shown

in Table 1. The plywood samples were made from the birch veneer obtained under the parallel-to-grain rolling at Dv = 15% and n = 2. Eight panels for each type of process were manufactured as shown in Table 1. Shear strength of the samples boiled in water for one hour was determined. The properties of plywood were also measured after conditioning of the specimens at the temperature 20 °C and 65% relative humidity according to GOST standards [9,11]. 3. Results and discussion 3.1. Influence of rolling direction on the veneer strength The comparative data of tensile strength of both non-densified and densified rotary-cut veneer and solid wood in parallel- and perpendicular-to-grain directions are shown in Table 2. Peeling process resulted in approximately 23.7% and 86.9% reduction of tensile strength of the samples parallel and rain perpendicularto-grain orientations, respectively. This can be related to development of the internal strain due to cracks in the veneer during its peeling. However, the densification of veneer enhanced strength properties of samples even comparable to that of solid wood. In particular, birch veneer of the 1.5 mm thickness reached to the strength of solid wood at Dv = 25% and n = 1 (Fig. 4). The influence of the direction of rolling on strength of veneer samples is illustrated in Figs. 2–4. Depending on the frequency rate of rolling and compression degrees tensile strength values of the birch samples parallel-to-grain, perpendicular-to-grain for 1.3 and 1.5 mm thicknesses ranged from 114.5 to 158 MPa, from 130 to 164 MPa, from 1.15 to 1.68 MPa and from 0.76 to 1.98 MPa, respectively. Findings revealed that the direction of the rolling does not essentially influence tensile strength parallel-to-grain of veneer samples. It seems that 20% compression degree is threshold where strength values were decreasing beyond that point. This may suggest that deformation of veneer has occurred at the perpendicular-to-grain rolling with a high compression degree. However a different mechanism was observed for tensile strength perpendicular-to-grain. Tensile strength slightly increased at 5% and 10% compression degree followed a decrease beyond 10% point which can be due to destructive influence of the densification at Dv = 20% and 25% in the samples. 3.2. Influence of species on strength of compressed veneer The effect of wood species on tensile strength parallel-to-grain of compressed veneer is shown in Fig. 5. The strength of both alder and birch samples increased with increasing of the compression degree. In particular, tensile strength of birch samples with thickness of 1.5 mm increased by 18.8%, while the samples made from alder with the same thickness had an average increase of 34.7% when the compression degree increased from 5% to 25%. Higher strength increase of alder samples can be related to greater porosity of alder veneer which may result in higher densification so that it had greater strength properties. However opposite behaviour,

Table 1 Levels and intervals of the factor variation Factor

Pressure of pressing (P), MPa Temperature of pressing (T), °C Time of pressing (s), min Glue spread (q), g/m2 Fig. 1. Schematic of veneer rolling set up.

Level of the factor variation Lower (1)

Basic (0)

Upper (+1)

1.2 110 5 110

1.8 125 8 135

2.4 140 11 160

Interval of factor variation

0.6 15 3 25

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P. Bekhta et al. / Materials and Design 30 (2009) 947–953 Table 2 Mechanical properties of veneer (Sv = 1.5 mm) and wood Parameter

Species Birch Densified veneer (experimental data)

Solid wood (data from Borovikov and Ugoliev [7])

Non-densified veneer (experimental data)

Densified veneer (experimental data)

Solid wood (data from Borovikov and Ugoliev [7])

128.2

130–164

168

63.5

64–113.6

101

1.42–1.5

0.76–1.98

11.1

–

–

7.2

5.4% strength reduction of birch samples was observed when the frequency rate of the rolling was increased as can be seen in Fig. 5. Such reduction was more pronounced in the case of alder samples with an average value of strength 25.2% which can be related to higher mechanical properties of birch than that of alder [7]. Calculations also testified that the parallel-to-grain tensile strength (TSparallel) of veneer dependence on the compression degree of veneer which can be described by the following linear functions (for Sv = 1.5 mm under the parallel-to-grain rolling at n = 2): for alder veneer TSparallel = 1.0714  Dv + 61.757 for birch veneer TSparallel = 1.1583  Dv + 127.9.

Parallel-to-grain tensile strength (MPa)

Parallel-to-grain tensile strength (MPa) Perpendicular-to- grain tensile strength (MPa)

Alder

Non-densified veneer (experimental data)

160

140

120

100 0

5

10

15

20

25

30

Parallel-to-grain tensile strength (MPa)

120

100

80

60

40 0

5

10

15

20

25

30

Perpendicular-to-grain tensile strength (MPa)

Compression degree (%) at the parallel-to-grain rolling 2

1.8

1.6

1.4

1.2

1 0

Compression degree (%) at the parallel-to-grain rolling

110 100 90 80 70 60 50 40 5

10

15

20

25

30

Compression degree (%) at the perpendicular-to-grain rolling Fig. 2. Effect of compression degree and direction of rolling on the parallel-to-grain tensile strength of alder veneer with thickness of Sv = 1.5 mm at different frequency rate of rolling. (N: n = 1; d: n = 2; s: n = 3).

Perpendicular-to-grain tensile strength (MPa)

Parallel-to-grain tensile stength (MPa)

120

0

5

10

15

20

25

30

Compression degree (%) at the parallel-to-grain rolling 2 1.75 1.5 1.25 1

0.75 0.5 0

5

10

15

20

25

30

Compression degree (%) at the perpendicular-to-grain rolling Fig. 3. Effect of compression degree on the tensile strength of birch veneer with thickness Sv = 1.3 mm at different frequency rate of rolling. (N: n = 1; d: n = 2; s: n = 3).

P. Bekhta et al. / Materials and Design 30 (2009) 947–953

180 170 160 150 140 130 120 0

5

10

15

20

25

30

Parallel-to-grain tensile strength (MPa)

Parallel-to-grain tensile strength (MPa)

950

150

130

110

90

70

50 0

2.5

2

1.5

1

0.5 0

5

10

15

20

25

0.5

1

1.5

2

2.5

3

3.5

Frequency rate of the parallel-to-grain rolling Parallel-to-grain tensile strength (MPa)

Perpendicular-to-grain tensile strength (MPa)

Compression degree (%) at the parallel-to-grain rolling

30

Compression degree (%) at the parallel-to-grain rolling

180 160 140 120 100 80 60 40 20 0 0

5

10

15

20

25

30

Compression degree (%) at the parallel to grain rolling

Fig. 4. Effect of compression degree on the tensile strength of birch veneer with thickness Sv = 1.5 mm at different frequency rate of rolling. (N: n = 1; d: n = 2; s: n = 3).

Fig. 5. Effect of compression degree and frequency rate of rolling on the parallel-tograin tensile strength of veneer with thickness of Sv = 1.5 mm at different wood species. (N: birch; d: alder).

3.3. Influence of the rolling on veneer strain change

increase of the compression degree and the frequency rate of the rolling. If the shear strength of plywood made from birch veneer of 1.5 mm thickness increases on the average by 5.1% with increase of the frequency rate of the rolling from 1 to 3, therefore the shear strength of plywood made from veneer with 1.3 mm thickness increased 28.6%. Glue penetrates into cracks of veneer and formed a strong glue line resulted in enhanced strength properties of plywood samples. The bending strength of plywood increased 14.1% with increasing of compression degree from 5% to 15% then the subsequent prompt of the reduction took place by 35.5% (Dv = 25% bending strength reduced by 51.2%). The bending strength of plywood increased 18.1% for Dv = 5–15% and reduced 24.2% for Dv = 25% with the increased of n from 1 to 3.

It was established that rolling of the rotary-cut veneer is characterized by the change of its strain size. Plastic strain of veneer is transformed into a residual strain. Residual strain (e) increased on the average in 1.2 times with the increase of the veneer thickness from 1.3 to 1.5 mm, and on average 1.7 times with increasing of the frequency rate of the rolling from 1 to 3, and on average in 2.01–2.61 times with increasing of the compression degree from 5% to 25%. The residual strain dependence of the compression degree of birch veneer (Dv), frequency rate of the rolling (n) and veneer thickness (Sv) can be described by the following function in percent:

e ¼ 0:43  S0:72  D0:64  n0:47 : v v 3.4. Influence of veneer rolling on mechanical properties of plywood

3.5. Influence of the rolling of veneer on the roughness of veneer and plywood

Influence of the rolling of veneer on shear and bending strength properties of plywood samples is shown in Fig. 6. Both properties of the samples initially improved with an increase of the compression degree up to 15% and then declined. It appears that the compression degree more adversely influenced bending strength of the samples as compared to that of shear strength beyond 15% threshold. In particular, the shear strength of plywood increased by 17.6% (Sv = 1.3 mm) and by 6.4% (for Sv = 1.5 mm) with an increase of the compression degree from 5% to 15%. It is known fact that uniform glue line has a significant effect on the strength properties of plywood. Thinner samples under identical conditions are subjected to greater strength destruction with

The experiments showed that surface roughness of veneer improved with increasing with compression degree as can be seen in Fig. 7. However, when roughness of veneer in terms of Rz value reduced from 110 to 65 lm Rz value of plywood samples reduced from 84 to 52 lm. Results of the experiments showed that roughness of veneer and plywood decreased with the increasing n value from 1 to 2 and then surface quality of the samples reduced again which can be related to the quantity of cracks on the veneer surface. It is also important to take into consideration of not only variables of the rolling (Dv and n) but also variables of the pressing of plywood (P and T) which influence overall roughness of plywood.

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P. Bekhta et al. / Materials and Design 30 (2009) 947–953

2.2

Roughness of veneer (10 -6 m)

120

Shear strength (MPa)

2.1

2

1.9

1.8

1.7 0

5

10

15

20

25

110 100 90 80 70 60 50

30

10

15

20

25

30

Compression degree (%) at the parallel-to-grain rolling

160

80

Roughness of plywood (10-6 m)

150

Bending strength (MPa)

5

0

Compression degree (%) at the parallel-to-grain rolling

140 130 120 110 100 90 80

70

60

50

40

70 0

5

10

15

20

25

30

Compression degree (%) at the parallel-to-grain rolling Fig. 6. Effect of compression degree on the strength properties of plywood from birch veneer with thickness of Sv = 1.5 mm at different frequency rate of rolling. (N: n = 1; d: n = 2; s: n = 3).

3.6. Influence of press parameters on the physical and mechanical properties of plywood made from compressed veneer as a theoretical approach Regression equations of the shear strength (SS), bending strength (BS) and compression of plywood (C) based on experimental data are shown below:

SS ¼ 1:78799P þ 0:494066T þ 1:04587s  0:21q þ 0:92599P2  0:00132T 2  0:0324s2 þ 0:00123q2  0:02646P  T þ 0:03299P  s  0:01404P  q  0:002625T  s  0:000635T  q  0:002042s  q  21:266; BS ¼ 106:19712P þ 5:03049T  5:40182s þ 0:68378q  9:31086P2  0:01023T 2  0:305768s2 þ 0:003357q2  0:389583P  T þ 0:663194P  s  0:109583P  q þ 0:057083T  s  0:009783T  q þ 0:029083s  q  395:486; C ¼ 1:64093P þ 0:525T þ 1:03456s  0:02671q  0:21225P2  0:00254T 2  0:0768s2  0:00055q2 þ 0:00847P  T þ 0:14653P  s þ 0:04483P  q þ 0:00128T  s þ 0:00107T  q þ 0:000617s  q  35:983: The variables P, T, s and q are given in Table 1. The highest value of the shear strength resulted in when P = 1.8 MPa, T = 125 °C, s = 8 min and q = 135 g/m2 press parameters were used. The shear strength increased by 136%, 48.5% and 41.7% with increasing of pressure values from 1.2 to 1.8 MPa at a temperature of 110, 125 and 140 °C, respectively. Similar trend

5

0

10

15

20

25

30

Compression degree (%) at the parallel-to-grain rolling Fig. 7. Effect of compression degree on the roughness of birch veneer with thickness of Sv = 1.3 mm and plywood at different frequency rate of rolling. (N: n = 1; d: n = 2; s: n = 3).

for shear strength was also observed for s and q as illustrated in Fig. 9. The shear strength decreased when P, T, s and q increased over 1.8 MPa, 125 °C, 8 min and 135 g/m2 accordingly. As can be seen from Fig. 8 using of densified veneer allowed to reach a required value (>1.5 MPa) in accordance with GOST standard at lower press values [9]. Satisfactory shear strength of plywood was reached already at the minimum press time of s = 5 min (Figs. 8 and 9). It seems that increasing heat conductivity of compressed veneer result in reduced press time. Also bending strength and compression of plywood increased with increasing of P, T, s and q. However, the increase of P, T, s and q offers not only improvement of bending strength but also leads significant over expenditure of raw material due to substantial increase of temperature and the increase of overall manufacturing cost. It should be emphasized that the shear strength of plywood exceeds the regulated value of shear strength (>1.5 MPa) according to GOST-9624 (1993) at any values of investigated factors. Finally some combinations of modes of pressing densified veneer which allow making plywood at lower pressure and time of pressing and glue spread are presented in the Table 3. The adhesive coaters in which the rolling of veneer and glue application is carried out simultaneously represent a method to compress veneer and increase the strength of plywood [6]. 4. Conclusions Experimental plywood panels from densified veneer by rolling were developed. Based on the findings of this study cold rolling of veneer before adhesive application resulted in improved physical and mechanical properties of plywood and reduced the

P. Bekhta et al. / Materials and Design 30 (2009) 947–953

3.5

3.5

3

3

Shear strength (MPa)

Shear strength (MPa)

952

2.5 2 1.5 1

2.5 2 1.5 1 0.5

0.5

0

0 1

2

1.5

2.5

100

3

110

120

Pressure of pressing (MPa)

Bending strength (MPa)

Bending strength (MPa)

140

150

160

170

150

160

170

170

160 150 140 130 120 110

1

2

1.5

160 150 140 130 120 110 100 100

100

2.5

110

120

3

14

14

12

Compression (%)

16

12 10 8

130

140

Glue spread (g/m 2)

Pressure of pressing (MPa)

Compression (%)

130

Glue spread (g/m 2)

10 8 6

6

4 100

4 1

2

1.5

2.5

3

110

120

130

140

150

160

170

Glue spread (g/m 2)

Pressure of pressing (MPa) Fig. 8. Effect of pressing pressure and temperature on the properties of plywood at s = 8 min and q = 135 g/m2. (N: T = 110 °C; d: T = 125 °C; s: T = 140 °C).

Fig. 9. Effect of glue spread and pressing time on the properties of plywood at P = 1.8 MPa and T = 125 °C. (N: s = 5 min; d: s = 8 min; s: s = 11 min).

Table 3 Some modes of plywood manufactured from densified veneer. Mode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

P (MPa)

1.2 1.2 1.2 1.2 1.8 1.8 2.4 2.4 1.2 1.2 1.2 1.8 1.8 1.8 2.4 2.4

T (°C)

110 140 140 140 125 125 140 110 125 125 125 125 140 140 125 125

s (min) 5 5 5 11 8 5 5 11 5 8 8 5 8 5 8 5

q (g/m2)

160 160 110 110 110 135 110 110 110 110 160 110 110 110 110 110

Shear strength (MPa)

Bending strength (MPa)

Compression of plywood (%)

Experiment

Model

Experiment

Model

Experiment

Model

2.47 3.16 2.80 2.83 2.75 1.73 3.02 3.02 – – – – – – – –

2.65 3.18 2.87 2.72 2.75 1.77 2.92 2.95 2.43 2.76 3.24 2.36 2.84 2.56 3.40 2.95

121.4 148.5 134.9 144.7 132.9 123.8 141.6 124.1 – – – – – – – –

122.8 144.6 132.8 143.0 135.9 135.2 144.1 126.4 116.9 122.2 145.6 129.4 150.9 141.8 142.9 135.2

4.47 6.2 5.19 6.62 9.4 9.09 10.12 11.52 – – – – – – – –

4.29 6.62 5.05 6.42 9.51 9.33 10.38 11.55 5.26 6.58 7.44 7.93 9.43 7.79 12.29 10.44

P. Bekhta et al. / Materials and Design 30 (2009) 947–953

adhesive spread when degree of compression was increased up to 15%. It appears that the direction of the rolling of veneer did not essentially influence on the parallel-to-grain tensile strength of veneer. Besides parallel-to-grain tensile strength of veneer initially increased when compression degree increased from 5% to 20%, and frequency rate of the rolling was increased from 1 to 3, and then certain reduction was observed. Compression degree of veneer during the rolling can be specified that will enable the receiving of plywood with the predetermined physical and mechanical properties. The densification/rolling of veneer influences on all operations of the technological process of the manufacturing of plywood reduces the time of pressing, pressure of pressing, temperature of pressing and glue spread (owing to its smaller penetration into pores of the compressed wood), and there is also an opportunity of the economy of fine wood due to the reduction of grinding. Besides, improving strength properties a number of decorative qualities of the plywood made from densified veneer are improved with the application of the rolling of veneer, in particular, the lustre becomes more intensive and the structure of wood is better shown.

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