Influence of pressure on bond-line shear strength in laminated veneer products

ABSTRACT

In this study, the bond-line strength of different bonding pressure of veneers in dry (9% moisture content) and wet (immersed in water) conditions was investigated. The results show that peeled European beech veneers need approximately 0.5– 1.2 MPa pressure to achieve a proper bond-line when a urea-formaldehyde adhesive was used. The pressure variation within this range did not significantly affect the strength of the dry veneer, but the strength of the wet veneers decreased as the bonding pressure increased. The type of failure varied, especially for the wet veneers. At high pressure, the wood were stronger than the cohesive strength of the adhesive and the strength of the adhesion between the adhesive and the wood. High pressure, however, caused a compression of the bond-line and resulted in a lack of adhesion.

KEYWORDS:

• LVP
• urea formaldehyde adhesive
• beech

Introduction

A bond-line between veneers that is too thick may result in poor strength. This could occur when the production process does not permit the use of a sufficiently high pressure or as a result of varying pressure distribution due to the three-dimensional shape of laminated veneer products (LVPs). Conversely, pressure that is too high may reduce the amount of adhesive in the bond-line to such a degree that the strength thereof is considerable reduced, and the cell structure may also be densified. It is therefore essential that moulding is performed with a high level of accuracy related to thickness tolerances and that the necessary conditions are provided when preparing the veneers to ensure even, correct pressure for bonding (Stevens and Turner 1970). The aim of this study was to gain a better understanding of how pressure affects the bond-line shear strength of LVPs.

Materials and methods

A shear-strength test was conducted on plane-laminated peeled beech (Fagus silvatica L.) veneers (230 × 230 × 1 mm) bonded with a urea-formaldehyde (UF) adhesive at different pressures in accordance with EN 314-1 (CEN 2005) and at different moisture conditions (dry/wet). Only in-plane straight-grained veneers were used. A total of 12 laminates composed of three veneers were bonded cross-wise at four different pressure levels, each laminate was then divided into seven samples. One of the samples from each laminate was examined with a scanning electron microscope (SEM) to study the micro-structure of the bond-line zone. The remaining samples were prepared for the shear-strength test with half of them in a dry state and the other half in a wet state after soaking in water (Table 1). Bond-line thickness was defined as the thickness of adhesive between veneers.

Table 1. Test conditions in tensile shear-strength test.

Parameter	Quantity	Comment
Veneer conditioning climate	20°C at 50% RH	Before test
Adhesive quantity in bond-line (g/m^2)	175	UF adhesive system (Akzo Nobel Adhesives AB, Sweden, resin A201 and hardener H201) Mix 100:20
Press-plate temperature (°C)	90	–
Pressure (MPa)	0.1/0.5/1.2/3	–
Pressing time (s)	300	–
Dry-state testing conditions	–	20°C at 50% RH
Wet-state testing conditions	–	immersed in water for 24 h

Failure causes were assessed with a loupe and in some cases, with a light microscope. The detected causes of failure were classified as wood (W) failure, adhesive (A) failure or a combination of wood and adhesive (W/A) failure.

Results and discussion

Table 2 summarises the test results.

Table 2. Test conditions and results of shear-strength test.

Parameter	Groups
Pressure (MPa)	∼0.1		0.5		1.2		3.0
Bond-line thickness (µm)	100		60–70		40–50		10–40
Conditioning	dry	wet	dry	wet	dry	wet	dry	wet
Shear strength (MPa)	3.08	3.87	3.23	3.77	3.30	3.39	3.35	3.13
STD (MPa)	0.34	0.18	0.39	0.29	0.28	0.43	0.43	0.41
W failure	7	8	8	7	2		5	2
A failure								3
W/A failure (%)^a	2 (95/5) (60/40)	1 (85/15)	1 (90/10)	2 (40/60) (30/70)	7 (95/5) (90/10) (80/20) 3 x (60/40) (50/50)	9 (90/10) (80/20) (75/25) (70/30) (25/75) (15/85) 2 x (10/90) (5/95)	4 (80/20) (60/40) (50/50) (10/90)	4 (85/15) (20/80) (5/95) (3/97)

^a Percentage between wood/adhesive failure in the respective samples.

SEM studies

With 0.1 MPa pressure, most of the adhesive remained in the bond-line, i.e. did not penetrate the wood cells. With high pressure (3.0 MPa), the amount of the remaining adhesive in the bond-line was low, i.e. the adhesive penetrates the wood cells in the bond-line vicinity to a greater extent with high pressure used than with lower pressures. Cracks (lathe checks) in the veneers were found in the samples at both low and high pressures.

Shear strength of the bond-line

As shown in Table 2, the wet samples were stronger than the dry samples at low pressure. This was probably because wet wood is more ductile than dry wood, and the moistened adhesive may also be more ductile than dry adhesive.

Failure causes

Mean-value of pure wood failure decreases in favour of adhesive failure with higher pressures (Table 2). This was even more obvious with wetted samples, which confirms that the result of high pressure weaken the adhesive bond-line.

Conclusions

Within the range 0.1–3.0 MPa pressure does not have a significant effect on the shear strength of dry samples, the shear strength of wet samples decreases with increasing pressure. The type of failure, varied with pressure level; this was more pronounced for wet samples.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Financial support from Produktion2030, a strategic innovation program supported by VINNOVA [reference number: 2019-00780], Swedish Energy Agency, Formas and the industry is acknowledged with gratitude.