Bonding performance of melamine-urea–formaldehyde and phenol-resorcinol–formaldehyde adhesive glulams at elevated temperatures

Journal Pre-proof Bonding performance of melamine-urea–formaldehyde and phenol-resorcinol– formaldehyde adhesive glulams at elevated temperatures Jian Liu, Kong Yue, Liqin Xu, Jinhao Wu, Zhangjing Chen, Lu Wang, Weiqing Liu, Weidong Lu PII:

S0143-7496(19)30250-7

DOI:

https://doi.org/10.1016/j.ijadhadh.2019.102500

Reference:

JAAD 102500

To appear in:

International Journal of Adhesion and Adhesives

Please cite this article as: Liu J, Yue K, Xu L, Wu J, Chen Z, Wang L, Liu W, Lu W, Bonding performance of melamine-urea–formaldehyde and phenol-resorcinol–formaldehyde adhesive glulams at elevated temperatures, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/ j.ijadhadh.2019.102500. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

1

Bonding performance of melamine-urea–formaldehyde and

2

phenol-resorcinol–formaldehyde adhesive glulams at

3

elevated temperatures

4

Jian Liu1, Kong Yue1,2*, Liqin Xu1, Jinhao Wu1, Zhangjing Chen3, Lu Wang1, Weiqing Liu1,

5

Weidong Lu1

6

1.

7

China.

8

2.

9

Shanghai 200433, P. R. China.

College of Civil Engineering, Nanjing Tech University, Nanjing 211800, P. R.

State Key Laboratory of Molecular Engineering of Polymers (Fudan University),

10

3.

11

University, VA 24060, Blacksburg, USA.

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E-mail address:

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[email protected] (J. Liu)

14

[email protected] (K. Yue)

15

[email protected] (L.Q. Xu)

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[email protected] (J.H. Wu)

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[email protected] (Z.J. Chen)

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[email protected] (L. Wang)

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[email protected] (W.Q. Liu)

20

[email protected] (W.D. Lu)

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*

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Nanjing 211800, China. E-mail address: [email protected]

23

Tel and Fax number: 86-025-58139862

24

Declaration of interest：

25

No conflict of interest exits in the submission of this manuscript, and manuscript is

26

approved by all authors for publication. I would like to declare on behalf of my

27

co-authors that the work described was original research that has not been published

28

previously, and not under consideration for publication elsewhere, in whole or in part.

29

All the authors listed have approved the manuscript that is enclosed.

Department of Sustainable Biomaterials, Virginia Polytechnic Institute and State

Corresponding author: College of Civil Engineering, Nanjing Tech University,

1

1

Abstract

2

To better understand the bonding performance of glued laminated timbers (i.e.,

3

glulams) during fires, phenol–resorcinol–formaldehyde (PRF) and melamine–urea–

4

formaldehyde (MUF) adhesives were used to investigate the effects of elevated

5

temperatures on the glueline shear strength. The wood failure mode was observed to

6

study the heat resistance of the two adhesives. Fourier transform infrared (FTIR)

7

spectroscopy and scanning electron microscopy (SEM) were used to analyze chemical

8

and microscopic changes at various temperatures. The parallel-to-grain shear strength

9

of solid larch wood decreased linearly with increasing temperature. Bonding strength

10

of the wood–PRF glueline exposed to elevated temperatures was similar to that of the

11

solid wood. The wood–MUF glueline exhibited good bonding performance at room

12

temperature but showed poor thermal resistance. Shear strength of wood–MUF

13

glueline was 0 MPa at 280 ºC. Bonding performances of PRF and MUF deteriorated

14

linearly with increasing temperature. FTIR analysis showed that PRF could maintain a

15

relatively intact chemical structure when the temperature was higher than 150ºC, and

16

the structure of the MUF degraded significantly when the temperature was higher than

17

200 ºC.

18

Keywords

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Glulam; Glueline; Shear strength; Mechanical degradation models

20

1. Introduction

21

Glued laminated timber (glulam) is widely used in construction. However, its use

22

entails fire risk because of the inherent flammability of wood materials and

23

adhesives[1]. Thus, research on the fire resistance of glulams used in construction is

24

essential. Wood burns and carbonizes, which causes its structural failure[2]. The

25

mechanical properties of wood structures at elevated temperatures are different from

26

those at room temperature[3].

27

Compared with that of log-based timber structures, the fire resistance of glulam

28

in modern timber structures is different due to the presence of adhesives[4]. The

29

bonding performance of such adhesives is affected by the temperature. However, BS

30

EN 1995-1-2 does not take this potential adverse influence into consideration. A large

31

number of experiments have shown the wood bonding performance deteriorates with

32

increasing temperature[5,6]. However, the influence of elevated temperatures on wood

33

and the adhesive interface remains to be elucidated. 2

1

The thermal performance of the adhesives affects the bonding strength and

2

failure models of glulams. Frangi[ 7 ] studied the bonding strength of phenol–

3

resorcinol–formaldehyde (PRF) and polyurethane (PUR) adhesives at elevated

4

temperatures and found that the shear strength decreased with increasing temperature.

5

The PUR adhesive retained 40% of original bonding strength when the temperature

6

rose from 20 ºC to 70 ºC, and the bonding strength of PRF adhesive began to decrease

7

at 180–190 ºC. When the temperature was higher than 150 ºC, PUR exhibited

8

significant loss in the bonding capability. PRF showed a better bonding performance

9

than PUR at high temperatures. George[8] found that the modulus of PRF was 2250

10

MPa at room temperature and began to decrease after 175 ºC. Da Silva[9] determined

11

the mechanical properties of wood composites of various adhesives (Redux 326 paste,

12

Redux 326 film, Hysol EA 9359.3, and Supreme 10HT.) at high temperatures (–

13

55—to 200 ºC). The results of tensile and shear tests showed that the stiffness and

14

strength exhibited a linear relationship with the temperature. Clauβ[10] found that the

15

shear strength of bonded wood joints as well as the stiffness of prepolymer films

16

increased significantly for a higher content of urea hard segments independent of the

17

temperature. Yang[11] studied the fire resistance of resorcinol resin adhesive (RF)

18

glulam and stated that the modulus of elasticity (MOE) and modulus of rupture (MOR)

19

decreased for a prolonged burning time. The decreases in the MOE and MOR were

20

related to the residual area (section modulus and moment of inertia). Some adhesives

21

can be ductile and exhibit creep at high temperatures. Na[12] studied high-temperature

22

creep properties of polyurethane glulam components. Thermo mechanical analysis

23

(TMA) showed that the MOE decreased with increasing temperature. The MOEs at 50

24

and 100 ºC were 16% and 22% lower, respectively, than that at 40ºC.

25

When the wood structure is subjected to bending loads, the glueline near the

26

neutral axis can withstand the maximum shear stress. However, the wood tissue next

27

to the glueline decomposes at a high temperature, resulting in poorer bonding

28

performance. There have been many studies on the shear strength of glueline in

29

small-scale specimens exposed to high temperatures, but degradation of wood with

30

glueline at high temperatures was not taken into consideration. Urea–formaldehyde

31

adhesive has a low cost and is thus used widely. However, it is not suitable for

32

outdoor use and suffers from low durability. Usually, melamine is added to it to

33

improve the durability. PRF and melamine–urea–formaldehyde (MUF) are common

34

resins used in making glulam, and it would be interesting to study their performance 3

1

in practical applications.

2

The purpose of the study was to investigate the shear strength of glulams using

3

PRF and MUF as adhesives at elevated temperatures. Fourier transform infrared

4

(FTIR) spectroscopy and scanning electron microscopy (SEM) were employed to

5

reveal the influence of elevated temperatures on the strength of wood and glueline in

6

glulams. The results can provide a reference for the fire-resistant design of wood

7

structures.

8

2. Materials and methods

9

Clear larch (Larix gmelinii) specimens (density of 604 kg/m3, growth ring

10

thickness of approx. 3 mm) were used to make glulams in this study. The wood

11

specimens were conditioned in a 20 ºC and 65% relative humidity environment for

12

more than two weeks to achieve an equilibrium moisture content.

13

PRF was provided by Dynea (Shanghai, China). MUF was synthetized in a

14

laboratory according to a method reported by Zhou[13].

15

2.1 Preparation of specimens

16

The glulams were made following EN 301 guidelines. The dimension of the test

17

specimens for the wood–adhesives tensile shear test was 20mm×10mm×150mm (see

18

Fig. 1). A spread of PRF and MUF was prepared according to Zhou[13], where the

19

solid contents of PRF and MUF were 35.7% and 50%, respectively. The adhesive

20

hardeners of PRF and MUF were 100/20 and 100/100, respectively. The hardeners

21

were added into the adhesives and then they were homogenized by a mixer running at

22

a pumping capacity of 200 L/min for at least 3 min. A spread of 350g/m2 PRF and

23

250g/m2 MUF were applied on a single face, and the double lap shear specimens were

24

pressed at 1.0 N/mm2 for 8 h at room temperature. To determine the effect of

25

adhesives on the shear strength of glueline exposed to elevated temperatures clearly,

26

tangential shear strength tests of solid wood were conducted. The glulams were placed

27

in a 20 ºC and 65% relative humidity environment for one week before they were

28

tested. Nine temperatures (20, 50, 70, 110, 150, 200, 220, 250, and 280ºC) and eight

29

replicates were used for the tensile shear strength.

30

4

1 Fig. 1. Dimension of double lap shear test specimen

2 3

2.2 FTIR spectra

4

FTIR spectroscopy analysis is a powerful tool for determining the composition

5

and structure of polymers[14]. It was used in this study to investigate changes in the

6

adhesives at elevated temperatures (20, 100, 150, 200, and 220 ºC) of glueline. The

7

cured adhesives were collected and ground into powders with 40 mesh (see Fig. 2).

8

After 1 h of heating at the elevated temperatures, the PRF and MUF adhesives were

9

analyzed with an IL7EH163CD FTIR spectrometer (PerkinElmer Company, China).

10

The spectral range measured was between 2500 cm-1 and 400 cm-1 and the spectral

11

resolution was 4 cm-1. Each spectrum presented here is the average of 32 successive

12

measurements in order to minimize the measurement error. Five measurements were

13

performed for each sample.

14 (a) PRF

15

Fig. 2. Cured adhesives powders of PRF and MUF.

16 17 18 19

(b) MUF

2.3 Shear strength tests The tensile shear strength of glueline was tested, and the test setup is shown in Fig. 3.

5

Nitrogen supply Testing machine

Heating cabinet 1 2

(a) Mechanical properties test setup

(b) Setup for tensile shear test

Fig. 3. Setup for shear strength of glue line

3 4

Nine temperatures (20, 50, 70, 110, 150, 200, 220, 250, and 280 ºC) were

5

investigated. Some specimens were used to test the core temperature of wood

6

specimens in the heating cabinet, and some specimens were used to test the strength.

7

After the core temperature in a specimen reached the target temperature, the

8

specimens for strength tests were kept for another 1 h in a GDX 300 atmosphere

9

chamber (MTS Systems Co., Ltd., China) with internal dimensions of 300×300×600

10

mm3 (Fig. 3). A combination of K-type thermocouples (Shanghai Xinghui Automation

11

Instrument Factory, China) and a DX1012 temperature patrol measure meter

12

(Yokogawa, Japan) was used to monitor the target temperature. The temperature was

13

recorded per min. The chamber was filled with nitrogen to simulate a hypoxia

14

environment.

15 16

Tensile load was applied at a rate of 1 mm/min. The shear strength was determined from the maximum tensile force (Qv) using Equation (1)

τT =

QV b × tv

(1)

17

where τT is the shear strength at a temperature of T ºC, expressed in Nmm-2; Qv is

18

maximum tensile force applied to the specimens during the test, expressed in N; b and

19

tv are the width and depth of cross-section, respectively, in mm.

20

3. Results and discussion

21

3.1 physical properties of solid wood

22

The wood density and its moisture content (MC) affect its mechanical

23

properties[15]. Generally, high density is always associated with high strength. As

24

shown in Fig. 4, MC decreased linearly with increasing temperature in the range of

25

20–150 ºC. At 150 °C, the wood moisture evaporated completely and the wood MC 6

1

was 0%. This observation is consistent with that made by Clauβ[16]. The density

2

reduction can be attributed to the moisture evaporation from the wood at temperatures

3

below 150 ºC and wood pyrolysis and volatilization above 200 ºC [15]. 30 12

20

8 15

6

10

4

5

2 0

0

50

100

150

200

250

0 300

Temperature/°C

4 5 6

Density loss/%

Moisture content/%

25

Moisture content Density loss

10

Fig. 4. Wood moisture content and density loss as a function of temperature

3.2 Shear strength of solid wood

7

The relationship between the shear strength of solid wood and temperature is

8

shown in Fig. 5. The shear strength of solid wood decreased with increasing

9

temperature. At room temperature, the shear strength of larch was 9.65 MPa. The

10

shear strength of solid wood decreased rapidly at 20–110 ºC and at a slow rate at

11

110–150 ºC. The decrease in the strength was highly correlated with that in the

12

wood density (see Fig. 4). At 150 ºC, the shear strength of larch was 61% of the

13

initial strength. From 150 to 300 ºC, the shear strength decreased linearly with

14

increasing temperature. The shear strength decrease may be attributed to

15

depolymerization[17]. Hemicellulose and lignin started to pyrolyze at 200–225

16

ºC[18]. 10

Shear strength/MPa

8

6

4

2

0

17 18 19 20

0

50

100

150

200

250

300

Temperature/°C

Fig. 5. Shear strength of solid wood as a function of temperature

3.3 FTIR spectroscopy analysis Fig. 6(a) shows the FTIR spectra of PRF adhesives at elevated temperatures. The 7

1

intensities of the absorbance bands related to benzene ring of PRF adhesive at 1630

2

cm-1 (2) and 1530 cm-1 (3) decreased significantly after thermal treatment, as well as

3

the absorbance bands related to methylene at 1470 cm-1 (4) and C–O–C ether bond at

4

1100 cm-1 (5). When the temperature was between 20 and 150 ºC, the chemical

5

structure of the PRF adhesive remained relatively intact. However, when the

6

temperature reached 150 ºC, the chemical structure of PRF began to change. The C–

7

O–C ether bond broke at 150 ºC when subjected to thermal shock. The methylene

8

bridge was destroyed, and the damage was due to the pyrolysis of the ether bond.

9

From 150 to 200 ºC, the PRF began to pyrolyze with the ether bond cleavage. The

10

characteristic peak of C=O bond at 1740 cm-1 (1) could be seen more clearly. The

11

chemical structure of PRF adhesive did not change much when the temperature was

12

higher than 200 ºC.

13

Fig. 6(b) shows the FTIR spectra of MUF adhesives at elevated temperatures.

14

The intensities of most absorbance bands remained almost unchanged after thermal

15

treatment at temperatures below 150ºC. These absorbance bands are related to the

16

C=O vibration of MUF adhesive at 1680 cm-1 (2), an amide II band caused by

17

coupling between NH deformation vibration and C–N stretching vibration at 1580

18

cm-1 (3), C–H bending and stretching vibration at 1390 cm-1 (4), C–O–C ether bond at

19

1210 cm-1 (5), and hydroxymethyl C–O at 1060 cm-1 (6). These results indicate that

20

there was no significant change in the chemical structure when the temperature was

21

below 150 ºC. At 200 ºC, the amount of -OH in the methyl group decreased

22

significantly, and a distinct characteristic peak was produced at 2150 cm-1 (1). This

23

indicates that isocyanate groups had formed during thermal treatment. At higher

24

temperatures, such as 220 ºC, the characteristic peak of the isocyanate group could be

25

seen more clearly. The characteristic absorption peak of the melamine ring indicates

26

that the chemical structure of MUF changed drastically at higher temperatures. The

27

chemical structure of the cured MUF adhesive remained unchanged between 20 and

28

200 ºC, other than a trace of isocyanate being produced. However, above 200 ºC, the

29

amount of -OH in the methylol group at 2150 cm-1 (1) decreased significantly, and a

30

large amount of isocyanate was produced, and the melamine ring was destroyed.

8

280℃ 220℃

200℃

Absorbance

Absorbance

220℃

150℃ 100℃ 20℃

200℃ 150℃ 100℃

1 2000

1800

2 3 1600

20℃

5

4 1400

1200

1000

800

600

-1

1 2 3 4

1

2

3

4

5

6

2400 2200 2000 1800 1600 1400 1200 1000 800 600

-1

Wave number/cm

Wave number/cm

(a) PRF

(b) MUF

Fig. 6. FTIR spectra of PRF and MUF adhesives at different temperatures

3.4 Bonding performance of glueline

5

Based on observation, the average shear strength of PRF glueline was slightly

6

less than that of solid wood at room temperature. The strength of PRF glueline as a

7

function of temperature is shown in Fig. 7a. The pattern is similar to that of solid

8

wood in temperatures ranging from 20 to 150 ºC. The wood failure percentage of

9

PRF–glueline was higher than 75%. Therefore, the strength of PRF glulam depended

10

on the strength of wood. The reduction in the strength of the larch PRF glulam was

11

due to the density reduction of the wood during thermal treatment. As the temperature

12

increased, the wood underwent pyrolysis. The strength of the PRF adhesive was

13

affected by residual stress in the glueline and shear stress. The failure at wood

14

decreased at 150–200 ºC because of the pyrolysis. In the range of 150–300 ºC, the

15

shear strength of PRF–glueline decreased more than that of the solid wood. Failure

16

occurred more at the glueline, and the failure modes of the PRF adhesive shifted from

17

wood to mixed wood–glue failure.

18

MUF adhesive showed excellent bonding performance at room temperature. The

19

relationship between the bonding strength and temperature is shown in Fig. 7b. The

20

decrease in the MUF bonding strength was slightly faster than that of the solid wood

21

at 20–110 ºC. 95% failure percentage of MUF glulam occurred at wood. The bonding

22

strength depended on the shear strength of solid wood below 110 ºC. Based on FTIR

23

spectroscopy, the isocyanate started to form at 150 ºC. A large amount of isocyanate

24

was generated in the MUF adhesive at temperatures above 200 ºC, and at this

25

temperature the melamine ring broke. The structure of the cured MUF adhesive was

26

destroyed, and the failure percentage at wood was only 30% at the higher temperature.

27

As shown in Fig. 7b, when the temperature was higher than 150 ºC, the shear strength 9

3

function at the elevated temperature. The failure modes of MUF–glueline shifted from

4

wood to mixed wood–glue failure and to glue failure at elevated temperatures. 10

100

8

80

6

60

4

40

0

5

Larix gmelinii-PRF Larix gmelinii Wood failure percentage

2

0

50

100

150

20

200

250

0 300

10

100

8

80

6

60

4

40

0

Temperature/°C

Larix gmelinii-MUF Larix gmelinii Wood failure percentage

2

0

50

100

150

20

200

250

Wood failure percentage/%

MUF–glueline was destroyed without any shear strength. MUF adhesive lost its

Shear strength/MPa

2

Wood failure percentage/%

of the MUF glueline decreased significantly. When the temperature reached 280 ºC,

Shear strength/MPa

1

0 300

Temperature/°C

(a) Shear strength of larch and bonding

(b) Shear strength of larch and bonding

strength of PRF–glueline

strength of MUF–glueline

6

Fig. 7. Shear strength of PRF and MUF adhesive glueline as a function of temperature in

7

comparison with that of solid wood

8

The percentage of wood failure at the elevated temperatures is shown in Fig. 7.

9

Before 110 ºC, 75–80% PRF glulam failure occurred in the wood, while the

10

corresponding amount was about 95% for MUF glulam. MUF showed a better

11

performance than PRF at lower temperatures. At 150–200 ºC, PRF and MUF glulams

12

experienced mixed failure whereby half of the failure occurred in the wood and half

13

occurred in the glueline. At 200–250 ºC, for PRF glulam, the failure was still a mixed

14

one in wood and glueline. However, for MUF glulam, the wood failure percentage

15

was lower than 30%, and the shear strength was determined by the glueline or

16

adhesive-related. The failure occurred at glueline. In summary, PRF glulam showed

17

better performance than MUF glulam at high temperatures.

18

3.5 Scanning electron microscopy (SEM) observation

19

SEM was used to detect microscopic changes in the glueline regions to validate

20

the failure mode of glueline. SEM images of the microscopic changes in the glueline

21

region are presented in Figs. 8 and 9.

22

10

glueline

1 2 3

(a) 20ºC

(b) 220ºC

(c) 280ºC

Fig. 8. SEM images of glueline with PRF adhesive at elevated temperatures glueline

4 5 6

(a) 20ºC

(b) 220ºC

(c) 280ºC

Fig. 9. SEM images of glueline with MUF adhesive at elevated temperatures

7 8

At 20 ºC, the thickness of PRF–glueline was larger than that of MUF–glueline

9

due to the deeper penetration of MUF adhesive into the wood cell[19] (see Figs. 8a and

10

9a); PRF and MUF adhesives both remained intact at 20 ºC. When the temperature

11

reached 220 ºC, as shown in Figs. 8b and 9b, the wood disintegrated partially; cracks

12

around the glueline appeared in the PRF–specimen but an appreciable fraction of the

13

PRF adhesive remained. There were no cracks in the MUF specimen, but the MUF

14

adhesive was not able to hold wood together. This could be due to the heat resistance

15

of PRF being higher than that of wood, which in turn had a higher heat resistance than

16

that of MUF adhesives at 220 ºC. The thickness of glueline increased because of

17

adhesives penetration at high temperatures. Wood failure occurred for a small

18

percentage of MUF–glueline at 220 ºC, while the wood failure percentage of PRF–

19

glueline did not change significantly. When the temperature rose to 280 °C, as shown

20

in Figs. 8c and 9c, the wood charred and the PRF and MUF adhesives were also

21

destroyed by the heat.

22 23 24

3.6 Mechanical degradation models The shear strength of glueline is presented in Figs. 10–12. Fig. 10 compares the 11

1

experimental solid wood degradation model with EN 1995-1-2. The relative shear

2

strengths in the study were 1, 0.66, and 0 at 20, 150, and 300 ºC, respectively. Fig. 11

3

shows the relative shear strength model of PRF and MUF adhesives at elevated

4

temperatures. The PRF reduction coefficients were 0.92, 0.56, and 0 at 20, 150, and

5

300 ºC, respectively, while those for MUF were 0.99, 0.58, and 0 at 20, 150, and 280

6

ºC, respectively. Fig. 12 compares the results obtained by the model with previously

7

reported ones. Clauβ[16] studied the shear strength of beech PRF and beech MUF

8

glueline. Each showed better shear strength than the experimental fitting model in the

9

study, since beech exhibits better strength than larch. Moritzer[ 20 ] and Frangi[7]

10

calculated fitted curves with integrating different types of adhesives. The reduction

11

coefficient, Kθ, was calculated using the following formulae:

Relative shear strength

1.0

Experimental model EN1995-1-2 model

(1.0) 0.8

(0.66) 0.6 0.4

(0.4) 0.2 0.0

(0.0) 0

50

150

200

250

300

Temperature/°C

12 13

100

Fig. 10. Comparison of experimental solid wood degradation model with EN1995-1-2

14

Relative shear strength

1.0 0.8

Fitting model of PRF Fitting model of MUF

(0.92)

(0.58)

0.6

(0.56) 0.4 0.2 0.0

15 16

(0.99)

0

50

100

150

200

250

300

Temperature/°C Fig. 11. Relative bonding strength models of PRF and MUF adhesives at elevated temperatures

12

Relative shear strength

1.0

PRF of Clauβ MUF of Clauβ Experimental PRF fitting model Experimental MUF fitting model Moritzer Frangi

(0.84)

0.8

(0.73) (0.70) (0.72) (0.58)

0.6

(0.56) (0.42)

0.4 0.2

k=2*exp((1.33*(200-T)/200)^2.8)/10.46

0.0

0

50

100

150

200

250

300

Temperature/°C 1 2

Fig. 12. Comparison of results of experimental model with previously reported results

3

1 τT  Kθ = = 0.66 τ0  0

Kθ . PRF =

K θ ,MUF =

τ T ,PRF τ0

τT ,MUF τ0

T = 20o C T = 150o C

(2)

T = 300o C

0.92  = 0.56 0  0.99  = 0.58 0 

T = 20 o C T = 150 o C T = 300 C

T = 20o C T = 150o C T = 280 C o

4

where Kθ, Kθ,PRF, and Kθ,MUF are the reduction coefficients of solid wood, PRF–

5

glueline, and MUF–glueline at T ºC, respectively; τT, τT,PRF, and τT,MUF are the shear

6

strength of solid wood, PRF–glueline adhesive, and MUF–glueline at T ºC,

7

respectively, expressed in Nmm-2; and τ0 is the parallel-to-grain shear strength of solid

8

wood at room temperature, expressed in Nmm-2. The reduction factor Kθ in the

9

corresponding temperature range was obtained by linear interpolation.

10

The reduction coefficient Kθ was observed to be different from that calculated

11

using BS EN1995-1-2, as shown in Fig. 10. BS EN1995-1-2 used 100 °C as the

12

turning point of the wood of 0% MC. However, the reduction factor Kθ at 150 °C as

13

the turning point accurately represents the experimental results. Owing to the nitrogen

14

in the hypoxia environment, no degradation owing to oxidation occurred. The 13

(3)

o

(4)

1

mechanical properties appeared to be better than those given by BS EN1995-1-2.

2

As shown in Fig. 11, since the bonding strengths of PRF–glueline and MUF–

3

glueline at low temperatures mainly depended on the shear strength of the solid wood,

4

the reduction coefficients Kθ,PRF and Kθ,MUF were very similar to the Kθ of the solid

5

wood. The reduction coefficient of the wood–PRF bonding strength Kθ,PRF was 0.92

6

and that of wood–MUF Kθ,MUF was 0.99 at room temperature. The inflection point of

7

PRF (MUF) glulam Kθ,PRF (Kθ,MUF) was 150 ºC. At room temperature, MUF exhibited

8

better bonding performance than PRF. However, PRF performed better than MUF at

9

high temperatures. The reduction coefficient could be used as reference in the design

10

and simulation of fire-resistant wood construction materials.

11

Conclusions

12

The bonding strengths of larch glulam using PRF or MUF adhesives decreased

13

with increasing temperature. Between 20 and 150 ºC, the glueline shear strength of

14

the PRF or MUF glulams strongly depended on the shear strength of the wood.

15

Between 150 and 300 ºC, the bonding strength of the adhesives decreased, which was

16

related to wood pyrolysis and adhesive disintegration. At 220 ºC, PRF maintained its

17

chemical structure mostly intact, while MUF underwent significant chemical

18

breakdown.

19 20

Acknowledgements

21

This work was financially supported by the National Natural Science Foundation of

22

China (Grant No. 51978331) and the State Key Laboratory of Molecular Engineering

23

of Polymers (Fudan University) (Grant No. K2019-22).

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 14

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