One-component Polyurethane Adhesives Research Articles

Application of microscale methods to study the heat-induced delamination in engineered wood products bonded with one-component polyurethane adhesives

One-component polyurethanes (1-c-PUR) are commonly used adhesives for the manufacture of cross-laminated timber (CLT). Typically, these adhesives do not govern the mechanical response of CLT under normal service temperatures. However, when subjected to heating from a fire, failures may transition from within the timber to the bond line interphase zones. CLT bonded with 1-c-PUR has been shown to be prone to heat induced delamination (HID), which may compromise its structural performance at elevated temperatures. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic thermo-mechanical analysis (DTMA) were used to study the thermal and thermo-mechanical responses of engineered wood products and components (i.e. adhesives and timber) and their interphase (i.e. bond line). Two commercially available 1-c-PUR adhesive films from the same manufacturer, two timber species (Norway Spruce and Radiata Pine, as sawdust or as veneers), and four different veneer shear lap combinations, were studied. Shear lap specimens bonded with a conventional 1-c-PUR adhesive consistently experienced bond line mechanical failure in DTMA at about 220-240 °C. The same adhesive films tested via DSC and TGA softened at 240-260 °C. Conversely, a different 1-c-PUR adhesive, formulated specifically for enhanced performance at elevated temperature, displayed no detectable softening in DSC and TGA, and shear lap specimens in DTMA consistently failed within the timber; even at elevated temperatures. The presented thermo-mechanical methods allow identification of failure modes and temperatures, whilst the thermal microscale methods (i.e. TGA, DSC, DTMA) assist in understanding the various factors contributing to failures.

A discussion on the various aspects of the minimum pressing time for polyurethane adhesives

ABSTRACT For many purposes related to the bonding of wood substrates, one-component polyurethane adhesives (1C-PUR) are of great significance. 1C-PUR adhesives are widely used, especially in the cross laminated timber (CLT) production. One major benefit is their ability to be pressed at room temperature. However, not only pressing temperature but also the pressing time is an important economic aspect for manufacturers. Knowing the minimum possible press time offers an economic advantage, however, there have been only few publications on this topic so far. The aim of this work was to investigate and evaluate various aspects of a 1C-PUR adhesive with regard to the minimum pressing time comparing newly developed knowledge-based approaches. The analysis included the strength development of the advanced curing of the adhesive, the development of the final internal bond strength and the minimum required press time for bonded joints to withstand imposed stress. The influence of the wood moisture content and closed assembly time were also investigated. The present study found that closed assembly time had a favourable influence on the required press time period. With immediate testing of the glued joint it also revealed a quicker strength development with lower strength values at a higher wood moisture content.

Durability assessment of one-component polyurethane adhesives for bonding of preservative treated wood subject to artificial ageing

Advancements in the use and applications of engineered wood products is receiving much interest in the construction industry. This is because wood has highly credible characteristics in relation to sustainability. It acts as a carbon store and is a natural renewable material. However, achieving adequate durability requirements and ensuring satisfactory adhesive bondline integrity is fundamental for such engineered products particularly when subjected to an exposed exterior service class environment. Little information exists on the durability performance of preservative treated wood bonded using one-component polyurethanes (1C-PURs). This study therefore involved the accelerated ageing of two 1C-PUR adhesives for the bonding of untreated, copper chromate arsenic and micronised copper azole preservative treated wood of fast growing Radiata pine. The ageing involved both vacuum-pressure soak drying cycling and exposure to a constant long-term high temperature and humidity environment. Resorcinol formaldehyde bonded specimens and solid wood specimens were used as controls. The testing showed that the standard requirements for delamination were satisfied when bonding preservative treated wood with 1C-PUR adhesive. The shear strength of specimens bonded with 1C-PUR adhesive subjected to vacuum-pressure soak drying compared well to the performance of the controls. The predominant failure behaviour was wood failure. The shear strength of all bonded preservative treated wood specimens with both 1C-PUR and resorcinol formaldehyde which were exposed to a constant long-term high temperature and humidity environment demonstrated a general and comparable reduction in strength over time which was statistically significant. High wood failure percentages were recorded at the bond interface which indicated weakening of the wood adherend when subject to the ageing rather than weakening of the adhesive bond. Chemical analysis of bonded specimens after ageing demonstrated that no significant variations were experienced.

Effect of Fibre Orientation on the Bond Properties of Softwood and Hardwood Interfaces

Increasing concerns regarding carbon emissions and climate change are prompting a shift toward the use of sustainable materials in the construction industry. Engineered timber products are gaining attention in the construction industry due to advancements in lamination techniques and adhesives as well as the renewable characteristics of wood. Bond properties play a significant role in engineered timber products. In Australia, Radiata Pine (RP, softwood) and Shining Gum (SG, hardwood) share a large proportion of local and native plantation forest resources. The present paper investigates the bond behaviours of Australian softwoods (RP–RP), hardwoods (SG–SG) and hybrid-wood (RP–SG) combinations in both parallel (PAL) and perpendicular (PER) bonding directions using one-component polyurethane adhesives. The results indicate that most of the softwood samples were subjected to wood-side (timber) failure, whereas hardwood samples failed due to delamination but exhibited higher strength and stiffness regardless of bond direction. In contrast, bond direction had a significant effect on the bond characteristics of hybrid configurations. Improved bond properties were observed when bonded in PAL directions; however, negative effects were seen when bonded in PER directions. Obtained characteristic (5th percentile) shear bond strengths for RP–RP–PAL, RP–SG–PAL and SG–SG–PAL samples were 3.88 MPa, 6.19 MPa and 8.34 MPa, whilst those for RP–RP–PER, RP–SG–PER and SG–SG–PER samples were 3.45 MPa, 2.96 MPa and 7.83 MPa, respectively.

Photodegradation risk evaluation of polyurethane gluelines in wood products by infrared spectroscopy and mechanical tests

During outdoor use, glued wood products are exposed to rather harsh conditions, which involve moisture and temperature fluctuations, attacks by microorganism, UV radiation etc. For some risks, the effect on glued wood products are already known, however less investigated are the risks concerning UV radiation. It has been established that weathering affects only the surface of the wood without influencing the mechanical properties. However, polyurethanes are known to undergo more significant degradation with structural changes, strength reduction and surface erosion. The objective of the study was to investigate whether photodegradation caused by UV irradiation can impair the bond quality of glued wood products bonded with one-component polyurethane adhesives. In the study, considerable attention was attributed to the investigation of the chemical changes by FTIR methods focusing on the differences as well as normalisation possibilities of the spectra. The results showed that UV significantly degraded the chemical structure and mechanical properties of the one-component polyurethane films as well as introduced a number of microcracks in the gluelines. Despite that, glued wood products after UV irradiation showed only minor or none at all reduction in the bond quality characterised by wood failure percentage. The only identified aspect that had some effect was the thickness of glueline, which should be kept at minimum. The chemical analysis revealed two stable bands, which could be used for normalisation of polyurethanes FTIR spectra during weathering investigations. The corresponding bands are attributed to the aromatic and isocyanurate ring structures.

Finger-jointing of green Eucalyptus globulus L. wood with one-component polyurethane adhesives

The use of glued finger joint in green wood, directly from the sawing process, would open the possibility to obtain glued timber from small-sized wood, achieving an efficient use of the original raw material. The gluing of finger-jointed green wood, with moisture content above the fibre saturation point, may improve the efficiency and the manufacturing process of glulam or joinery. This may be especially beneficial for a hardwood such as Eucalyptus globulus L., which is a globally important forest resource, but is a challenging wood to dry. This article presents a study on the possibility to develop finger joints with wood in green state. To compare them, conventional finger joints on dry wood and solid boards without end joints were also manufactured. Cold-setting and fast-curing commercial one-component polyurethane adhesive systems were used. Finger-jointed samples were tested to determine mean and characteristic values (5th percentile) of density, bending strength and modulus of elasticity, and the results were analysed and discussed. Green-glued joints showed no statistically significant differences compared to the solid boards and improved strength properties with respect to dry-glued joints.

Effect of cellulosic fillers addition on curing and adhesion strength of moisture-curing one-component polyurethane adhesives

Abstract In this work, microcrystalline cellulose (MCC) and sawdust (SD) green fillers have been demonstrated as reactive fillers and moisture reservoirs for enhanced curing and adhesion strength of one-component moisture-curing polyurethane (PU) adhesives. The chemical reaction between the MCC or SD fillers and isocyanates present in the PU adhesives was tracked using Fourier-transform infrared spectroscopy (FTIR), while the joint strength was determined using lap-shear tests. The results showed that the addition of MCC or SD leads to an accelerated curing rate through the early consumption of isocyanates. The lap-shear strength of unreinforced PU adhesive joints was achieved in 12 hours. In turn, the curing time in MCC and SD reinforced joints was achieved in 5 hours. The addition of MCC and SD fillers also enhanced the lap-shear strength of the joints under full cure conditions. An autocatalytic growth model was used to describe the lap-shear strength variation with respect to curing time.

Characterization of wood-adhesive bonds in wet conditions by meansof nanoindentation and tensile shear strength

The performance of adhesive-hardwood bonds can often be sensitive tohumidity and temperature variation. Therefore, it is frequently challenging to achievestandard requirements for structural applications. To gain a better understanding of thewood-adhesive bond, the properties of the individual constituents as well as the localinterface of European beech (Fagus sylvatica L.) woodcell walls in contact with structural adhesives were analyzed by means ofnanoindentation. These results are compared to classical lap-shear strength. Asadhesives two different one-component polyurethane adhesives (1C PUR) and a phenolresorcinol formaldehyde adhesive (PRF) were used. In one case, the beech wood wasadditionally pre-treated with an adhesion-promoting agent (primer) prior to bonding with1C PUR. Beech wood joints were analyzed subsequent to several treatments, namelystandard climate, after wet storage and in re-dried conditions. In addition, theinfluence of the primer on the hydroxyl accessibility of beech wood was investigatedwith dynamic vapor sorption (DVS). The lap-shear strength revealed good performance indry and re-dried conditions for all adhesives on beech. Both polyurethane adhesivesobtained deficits when tested in wet conditions. The use of a primer significantlyimproved the PUR performance in wet condition. DVS experiment demonstrated a decrease inhydroxyl group accessibility when using a high primer concentration. As novelty,nanoindentation was used for the first time to characterize the localwood–adhesive-interface properties in wet conditions. Nanoindentation showed that alltested 1C PUR perform quite similar in room climate, while PRF achieves considerablehigher values for reduced E-modulus and hardness. Wet storage led to a considerablereduction in mechanical properties for all adhesives, while the highest relative changewas observed for PRF. After re-drying, the adhesives re-gained a large part of theiroriginal mechanical properties in room climate. No distinct effect of the primer on thelocal micromechanical properties could be detected with nanoindentation in terms ofspecific work of indentation.

Improvement of ash (Fraxinus excelsior L.) bonding quality with one-component polyurethane adhesive and hydrophilic primer for load-bearing application

The increasing quantity of hardwood growing in the Swiss forest represents an opportunity for the future of the wood industry. Glued laminated timbers made of hardwood offer higher strength and stiffness properties than the softwood equivalents, and they are often used in combination with softwood for local reinforcement. Additionally, their visual aspect can be an attractive selling argument. Until now, phenol resorcinol adhesive (PRF, Aerodux 185) has preferably been used for the bonding of ash wood. Despite its high bonding performance, this adhesive has several disadvantages, including a dark brown glue line and a strong emission of formaldehyde both during the gluing process and when in use. One-component polyurethane (1C-PUR) adhesives, widely used in load-bearing structures made of softwood, represent an interesting alternative for hardwood gluing. These formaldehyde-free adhesives require no mixing and present an invisible glue line without any emissions after full curing. Preliminary results have shown that the combination of a slow-reacting 1C-PUR adhesive with a water-based primer system obtained similar results as Aerodux 185 in laboratory conditions with respect to shear strength and delamination resistance. This paper focuses on the optimization of the gluing process of two different 1C-PUR adhesives and their combination with a special primer on ash wood to develop an economically attractive industrial application process. The influence of several factors, such as primer-water concentration, primer activation time and primer quantity, were investigated using a delamination test according to EN 302-2. It was shown that it is possible with 1C-PUR to reach the normative standard requirements according to EN 15425. The best results were obtained with a primer-water concentration of 10% and a primer-water applied quantity of 20 g/m2. The influence of the primer activation time was shown to have only a small effect. 1C-PUR with a long closed assembly time was shown to perform even better. Additionally, the reduction of the lamellar thickness led to a reduction of delamination. By considering these parameters, it was possible to develop an industrial process for producing high-performance glued laminated timber out of ash wood and glued with a certified and commercially available 1C-PUR adhesive.

About the influence of a water-based priming system on the interactions between wood and one-component polyurethane adhesive studied by atomic force microscopy and confocal Raman spectroscopy imaging

In recent years one-component polyurethane (1C-PUR) adhesives have been increasingly applied in loadbearing timber constructions. However, a low wood fracture percentage at wet stage and a comparatively low performance in delamination tests need to be overcome to fulfill certain technical standards and to achieve an even higher acceptance in the global market of wood adhesives. Via the use of suitable primers, in this case a water-soluble primer (WSP) system, it is possible to significantly improve these parameters. Nevertheless, the exact mode of action of the primer remains so far unclear. In this study, primer treated wood surfaces were characterized in detail by atomic force microscopy (AFM) and the influence of the primer on the penetration of the 1C-PUR adhesive into wood cell walls was further analyzed by confocal Raman spectroscopy imaging. AFM investigations of primer treated surfaces reveal a significant increase in adhesion compared to non-primer treated surfaces and indicate a penetration of the primer into the cell wall. However, Raman spectroscopy shows that the primer application does not lead to a substantial and deep penetration of the 1C-PUR adhesive into wood cell walls. Slight changes in the spectral properties of the primer treated samples at the interface between the bond line and the adjacent cell wall, point towards changed interactions between wood and polyurethane.

Characterization of aluminium one-component polyurethane adhesive joints as a function of bond thickness for the automotive industry: Fracture analysis and behavior

The automotive industry is increasing, nowadays, its use of high performance structural adhesives in order to reduce vehicle weight and increase the crash resistance of automotive structures. One-component polyurethane adhesives are attracting rising interest as an alternative to conventional rigid structural adhesives. This is due to several benefits such as the excellent impact resistance they provide. This paper investigates experimentally the mechanical behavior of one-component polyurethane adhesive joints and specifically the effect of the adhesive layer to be used in several parts for buses structure. Furthermore, it characterizes the fracture behavior of the adhesive layer with various bond’s thicknesses using mode-I and mode-II fracture testing. The fracture toughness under both modes is determined using Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests, respectively. Assessment of the various fracture tests indicated that DCB and ENF provide the same shape of evolution of the fracture energy versus adhesive thickness. Moreover, the test results were able to fully mechanically characterize the adhesive and demonstrate that the adhesive has not only high mechanical strength but there is an optimal adhesive thickness, which allows a high toughness for bonded joints. This result proves that it is worthy to use the one-component polyurethane adhesive in the automotive industry.

The effect of free isocyanate content on properties of one component urethane adhesive

One-component polyurethane adhesives were prepared from difunctional linear polyether polyol and two types of isocyanates in order to achieve excellent adhesion to various substrates. The structure of the prepolymer was varied by using different stoichiometric excesses of isocyanate in the range between 5 and 24wt%. The simulation of synthesis between polyol and two isocyanates with 15wt% of the NCO groups was performed on a rheometer in order to determine the optimal reaction time. The average molecular weight, determined by GPC, and Brookfield viscosity of the prepolymers decreased with increasing amounts of free isocyanate (NCO) concentration. The structure of the prepolymers as well as the type and amount of side reactions as a function of the free NCO was determined by NMR analysis. The cross-linking, arising from allophanate linkages, was more pronounced at higher amounts of free NCO. Thermal properties of synthesized urethane prepolymers were investigated by DSC and TGA techniques. The glass transition temperatures (Tg) of the prepolymers depend on the type of isocyanate used for the synthesis. Adhesion strength was also tested. Maximum tensile strength was observed at 15wt% of NCO for both series of prepolymers.

Improving the thermal stability of one-component polyurethane adhesives by adding filler material

The aim of the current study is to improve the thermal stability of one-component moisture-curing polyurethane adhesives. The approach here tends to add suitable filler materials to the adhesive and to study the resulting effects. The investigation covers mechanical tests to determine the shear strength of the glued wood joints according to EN 302-1 (2004). Furthermore, the distribution of the filler material within the adhesive is shown by means of environmental scanning electron microscopy combined with energy-dispersive X-ray spectroscopy analysis. The thermal stability of the glued wood joints could be significantly improved by adding chalk with a volume fraction of 30% to the adhesive.

Thermal stability of glued wood joints measured by shear tests

The thermal stability of glued wood joints is an important criterion to determine the suitability of adhesives in the field of engineered wood. During their product life, glued wood joints can be exposed to high temperatures in various ways (direct exposure to the sun, fire, etc.). Thereby the cohesiveness of the adhesive must not degrade. This raises the question of how the strength of bonding changes under thermal load. The current investigation covers the influence of temperature (T=20 to 220°C) on the shear strength of glued wood joints. Different adhesive systems were investigated. With increasing temperature, the shear strength of solid wood and also of glued wood joints decreased. There were big differences in thermal stability and failure behaviour between the adhesive systems as well as within the polyurethane group. The thermal stability of one-component polyurethane systems can be greatly varied by modifying their chemical structure. Well adapted one-component polyurethane adhesives reach a strength similar to that of phenol resorcinol resin.

Factors influencing timber gluability with one-part polyurethanes – studied with nine Australian timber species

Abstract The bulk and surface properties of blocks of nine Australian wood species of commercial importance were investigated to elucidate the factors affecting timber gluability with structural one-component polyurethane adhesives. Cross-lap joints were prepared from freshly sanded blocks and the joints were subjected to creep loading in a condensing humidity environment. The median tensile strength (MTS) of the joints was found to improve with decreasing phenolic extractives content, lower timber density and decreasing lipophilic surface extractives content. The latter was assessed from O/C atomic ratios of the timber surfaces determined by X-ray photoelectron spectroscopy (XPS). The content of bulk lipophilic extractives and lignins and wettability of the surface as determined by the sessile drop method did not reveal significant correlations with the adhesion properties. The adhesion tests indicated significant gluability differences between the species investigated.