Hydroxymethylated Resorcinol Research Articles

Hydroxymethylated resorcinol (HMR) primer to improve the performance of wood-adhesive bonds – A review

A hydroxymethylated resorcinol (HMR) primer was developed in the 1990s to improve the performance of epoxy-bonded wood lamellas for use in moist conditions. Its chemical composition resembles a strongly diluted resorcinol-formaldehyde adhesive, which is applied before the actual adhesive. The effectiveness of HMR priming is proven for different wood species and adhesive types based on delamination tests and shear strength measurements. However, the primer is not used in the industrial mass production of glued laminated timber. The reason can be found in additional process steps, therefore costs, and the fact that certain combinations of currently broadly available wood species and adhesives yield proper bonds without the additional use of a primer. Nevertheless, due to ongoing changes in the availability of wood species, it becomes increasingly important to consider the use of currently less-used wood species, including the corresponding adaptations in the production process. Since it is known that the HMR primer improves the bond performance, but is not used due to necessary process changes, it would be interesting to integrate the primer's functionality into existing adhesive systems. This review summarizes and structures the available information on the HMR primer from previous research. Furthermore, that information is critically discussed and a model of functionality is introduced. Finally, it is evaluated if the existing knowledge about the HMR primer is sufficient to transfer its mode of action into existing adhesive systems or which questions need to be addressed to do so.

Effects of surface treatment and adhesives on bond performance and mechanical properties of cross-laminated timber (CLT) made from small diameter Eucalyptus timber

The feasibility of manufacturing cross-laminated timber (CLT) using fast-grown small diameter Eucalyptus timber was explored in preliminary studies. However, eucalyptus bonded with commercial one-component polyurethane adhesive usually provides poor resistance to delamination and shear force. The effects of priming surface of eucalyptus CLT lamella intended for a bondline and different adhesives on block shear strength (BSS), wood failure percentage (WFP), and rate of delamination (RD) of Eucalyptus CLT made from small diameter lumber were studied via block shear and cyclic delamination tests at both dry and wet states. Four different types adhesives: epoxy resin (EP), emulsion polymer isocyanate (EPI), phenol resorcinol formaldehyde (PRF), and polyurethane (PUR) adhesives were used to manufacture three-layer Eucalyptus CLT panels following adhesive manufacturers’ instructions. Hydroxymethylated resorcinol (HMR) and hygroscopic organic solvent N,N-dimethylformamide (DMF) primers were adopted to priming surfaces of Eucalyptus CLT lamella intended for a bondline. The results indicated that eucalyptus CLT made of small diameter lumber with the four adhesives were comparable to commercial softwood CLT. The maximum wood failure percentage (WFP) and block shear strength (BSS) values of specimens bonded with PUR adhesive at dry state were 85.2% and 3.51 MPa whereas those values of specimens bonded with EPI adhesives at wet state were 58.2% and 1.62 MPa respectively. The minimum rate of delamination (RD) value was 7.6%, which was obtained from specimens bonded with PUR adhesive. The enhancing effects of HMR and DMF priming treatment on bond performance and mechanical properties of Eucalyptus CLT bonded with the above four adhesives did not show a clear trend because their effects depended on the coupling effects of the adhesive and primer. Generally, HMR priming treatment was effective to enhance bond performance and mechanical properties of eucalyptus CLT. After HMR priming, the WFP values of eucalyptus CLT at wet state bonded with PRF and PUR adhesives increased by 27.8%, 12.4% meanwhile the BSS values increased by 31.5%, 4.9%. The RD values of specimens bonded with PRF and PUR adhesives at wet state were close to 0.0% after HMR priming.

Solution‐state NMR analysis of hydroxymethylated resorcinol cured in the presence of crude milled‐wood lignin from Acer saccharum

ABSTRACTResorcinol‐formaldehyde adhesives can reinforce stress fractures that appear from wood surface preparation. Researchers have found that applying the resorcinol‐formaldehyde prepolymer, hydroxymethylated resorcinol, to the surface of wood improves the bond strength of epoxy and polyurethane adhesives to wood. Hydroxymethylated resorcinol is thought to plasticize lignin components and stabilize stress fractures through reactions with lignin subunits and hemicelluloses in wood. In this study, a dilute solution of hydroxymethylated resorcinol (HMR) is cured in the presence of a crude milled‐wood lignin (cMWL) from Acer saccharum and subsequently dissolved in dimethylsulfoxide‐d6 to delineate reactivity with lignin and O‐acetyl‐(4‐O‐methylglucurono)xylan using solution‐state NMR spectroscopy. 1H–13C single‐bond correlation NMR experiments revealed that the HMR only formed 4,4′‐diarylmethane structures with itself in the presence of the cMWL; the 2‐methylols that formed remained free and did not crosslink with resorcinol. Cured HMR resin formed both 4,4′‐ and 2,4‐diarylmethane structures, confirming that the presence of lignin and O‐acetyl‐(4‐O‐methylglucurono)xylan hinders crosslinking at the C‐2 position. No evidence of reactivity between HMR and lignin subunits was found. New peaks consistent with ester linkages were observed in 13C‐NMR spectra of the cMWL sample treated with HMR that may be attributable to HMR moieties condensing with glucuronic acid substituents. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45398.

Evaluation of Adhesion Promotion Techniques for Structural Bonded Timber Joints

Long-term durability of a structural adhesive joint is an important requirement, because it has to be able to support the required design loads, under service conditions, for the planned lifetime of the structure. Epoxy adhesives, whilst not ideal, are currently the best family of adhesives for in situ repair operations. As long as the bonded joint remains dry and unexposed to high service temperatures, epoxy adhesives produce strong bonds to timber. However, once they are exposed to severe stresses as a result of repeated water soaking and drying cycles, the bonded joint delaminates and does not fulfill the requirements for structural timber adhesives intended for exterior exposure. One way of improving bond durability is through the use of surface treatments prior to bonding. In this study, the effects of corona discharge surface treatment, hydroxymethylated resorcinol (HMR) and γ-glycidoxypropyltrimethoxysilane (GPMS) adhesion promoters on the durability enhancement of pine, iroko, and oak bonded joints were evaluated. The results proved that surface modification methods for adhesion promotion can be adapted to cellulosic substrates with significant improvements in bonded joint durability.

Influence of chemical treatments on moisture-induced dimensional change and elastic modulus of earlywood and latewood

Abstract To better understand the performance of bonded, coated, and modified wood, knowledge of how these processes alter the dimensional change and mechanical properties of wood at a given moisture content (MC) are important. These localized influences on earlywood (EW) and latewood (LW) properties are not well understood. In the present study, the influence of chemical treatments by hydroxymethylated resorcinol (HMR) and acetylation on moisture-induced dimensional change and longitudinal modulus of elasticity (MOE) of isolated EW and LW specimens of Loblolly pine (Pinus taeda L.) was evaluated. The dimensional change was not altered by the HMR treatment, whereas acetylation lowered it by ∼50% in EW and LW in both radial and tangential directions. The MOE was not influenced by the two chemical treatments tested. Based on results of swelling, shrinkage, and MOE it can be concluded that chemical treatment does not modify EW selectively compared with LW neither in radial nor in tangential orientation.

Influence of wood properties and bonding parameters on bond durability of European Beech (Fagus sylvatica L.) glulams

• Considerable progress has been made recently to promote glue-laminated beams of European beech (Fagus sylvatica L.) for load-bearing structures in engineering. However, further investigations into the bond durability are required. The objective of the present study was to analyse material and manufacturing factors and their combination to improve the resistance in delamination tests as required for the use in building service class 1. • In the experimental work, the influence of selected wood properties, manufacturing conditions, and the effect of a hydroxy-methylated resorcinol primer on the delamination resistance of beech glulams bonded under laboratory and industrial conditions with a melamine-urea-formaldehyde adhesive were investigated. • The highest delamination resistance of the laboratory samples was achieved with the smallest lamellae thicknesses of 29 mm. A closed assembly time of more than 30 min and the use of the primer further enhanced the bond durability. Higher delamination rates observed in glulams manufactured industrially were related to the relatively thick (38 mm) and often twisted beech lamellae used. • The results suggested that bond durability required for service class 1 can be attained by restricting lamella thickness in beech glulam to a maximum of 30 mm and by using a minimum closed assembly time of 40 min.

Hydroxymethylated Resorcinol (HMR) and Novolak-Based HMR (n-HMR) Primers to Enhance Bond Durability of Eucalyptus globulus Glulams

The effectiveness of the hydroxymethylated resorcinol (HMR) and the novolak-based HMR (n-HMR) primers to enhance bond durability of Eucalyptus globulus glulams was evaluated according to EN 391 (Delamination) and EN 392 (Shear strength) for outdoor load-bearing timber structures (Service Class 3). All glulams prepared under increasingly more stringent conditions with both primers, surpassed the requirements for delamination and shear strength. Specifically, when used with a one-component polyurethane adhesive: lamellas did not need to be planed before bonding; the primer amount could be reduced to 50%; and the adhesive could be applied to only one of the bonding surfaces. Additionally, lamellas primed with the n-HMR were effective for 2 weeks prior to bonding. The novolak-based primer overcomes the main limitations in using the original HMR primer, offering more flexibility for the same efficiency, and consequently, it is a more attractive option for manufacturing Eucalyptus globulus glulams.

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of wax-treated wood

The effects of hydroxymethylated resorcinol (HMR) on the tensile shear strength of wood joints treated with montan ester wax were studied with polyvinyl acetate (PVAc) and melamine formaldehyde (MF) adhesives. Untreated specimens which were bonded with MF and PVAc had similar bond strength properties under dry conditions, but MF bonded specimens’ tensile shear strength values were higher than that of PVAc glued specimens under wet conditions. With both adhesives tested, dry tensile shear strengths of wax-treated specimens were in a comparable range to those of untreated wood, while under wet conditions the strength values of wax-treated specimens were significantly lower than those of the controls.

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of silicone modified wood glued with a polyvinyl acetate adhesive

A hydroxymethylated resorcinol (HMR) was used as a coupling agent to improve glueability of silicone (aminofunctional polydimethylsiloxane) modified pine sapwood (Pinus sylvestris) with polyvinyl acetate (PVAc). Priming with HMR did not significantly increase tensile shear strength of unmodified and silicone treated wood under dry conditions. Under wet conditions, tensile shear strength of the control specimens was increased by 37% and of the silicone modified specimens by 13%. Despite of higher shear strength under wet conditions, the primed silicone treated specimens displayed lower degree of wood failure than the unprimed treated specimens.

Tensile strength of softwood butt end joints. Part 2: Improvement of bond strength by a hydroxymethylated resorcinol primer

In a previous study it was shown that the mechanical stability of an end-grain joint bonded with a one-component polyurethane adhesive (PUR) was insufficient compared with melamine–urea–formaldehyde and phenol–resorcinol–formaldehyde bonding. Based on this, the aim of this study was to improve the mechanical stability of the end-grain joint by means of a hydroxymethylated resorcinol (HMR) primer and by increasing the spreading quantity. To study the effect of HMR and the increased spreading quantity on the adhesive bond strength of end-grain to end-grain-bonded wood samples, three-part Norway spruce wood specimens were tested in tension. Before bonding, each end-grain surface was treated with an aqueous solution of HMR. The two axially orientated outer parts of the specimens were jointed with the middle part using a PUR adhesive. Compared with untreated, i.e. non-primed samples, the tensile strength of HMR-treated specimens was more than doubled. Furthermore, a positive effect of increased adhesive spread was shown for untreated PUR-bonded samples. An increase in adhesive spread by a factor of 1.6 led to an improvement in tensile strength by a factor of about 2.6.

X-ray photoelectron spectroscopy of wood treated with hydroxymethylated resorcinol

This paper attempted to use X-ray photoelectron spectroscopy (XPS) to determine the surface chemistry of wood treated with hydroxymethylated resorcinol (HMR), a coupling agent that enhances the durability of wood adhesive bonding. Wood veneers of hard maple were soaked in HMR solution for either 1, 15, or 30 min. XPS analyses showed that the treated surfaces exhibited an increase in the non-oxidized carbons (C1), a decrease in C–O bonds (C2), and an unaltered or decreased C O or O–C–O bonds (C3), when compared to the control (unsoaked) samples. Despite these findings, the oxygen-to-carbon (O/C) ratios increased with HMR treatments, and the attributes of carbonate species ( CO 3 2 - and HCO 3 - ) were identified. Further analyses revealed that these species were of sodium salts, which also formed in sodium hydroxide (0.08 N) treatment of wood. Overall, this study paves the way for relating the surface oxidation states of the treated wood to the adhesive bond strength, with an ultimate attempt to identify the possible roles of surface chemistry in adhesion durability of wood products.

Resorcinol-formaldehyde reactions in dilute solution observed by carbon-13 NMR spectroscopy

A recently discovered coupling agent, hydroxymethylated resorcinol (HMR), based on resorcinol-formaldehyde, can greatly enhance wood-to-epoxy resin bond durability in exterior applications. However, for HMR to be most effective, it needs to be prepared a few hours before it is applied to the wood surface. In this study, carbon-13 nuclear magnetic resonance (NMR) spectroscopy was used to monitor com- position of HMR as a function of time to characterize which chemical groups are present in solution when HMR is applied. A quantitative assessment of formaldehyde-derived groups required the use of 99% 13 C-enriched formaldehyde. Hydroxymethyl groups, primarily attached to the 4-position of resorcinol, and hemiformal groups formed very quickly. Signals from methylene linkages between resorcinol rings began to appear 20 min into the reaction. Formaldehyde was consumed quickly; 95% was bound to resor- cinol rings within 1.7 h. By 3 h, 16% had been converted to methylene linkages, and by 8.3 h, 40% was converted. Another set of NMR experiments was used to monitor the dependency of peak positions of resorcinol solution as a function of pH. These experi- ments showed significant effects, especially between pH 7.7 and 9.1, which explains chemical shift changes observed during the HMR reaction. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1760-1768, 2000