Adhesives for Achieving Durable Bonds with Acetylated Wood

Abstract

Acetylation of wood imparts moisture durability, decay resistance, and dimensional stability to wood; however, making durable adhesive bonds with acetylated wood can be more difficult than with unmodified wood. The usual explanation is that the acetylated surface has fewer hydroxyl groups, resulting in a harder-to-wet surface and in fewer hydrogen bonds between wood and adhesive. This concept was evaluated using four different adhesives (resorcinol-formaldehyde, emulsion polymer isocyanate, epoxy, and melamine-formaldehyde) with unmodified wood, acetylated wood, and acetylated wood that had been planed. Strikingly, acetylation did not hinder adhesive bonds with a waterborne resorcinol-formaldehyde adhesive that bonded equally well to both unmodified and acetylated yellow poplar. An epoxy adhesive bonded better to the acetylated wood than to the unmodified wood, in contrast to an emulsion polymer isocyanate, which gave less durable bonds to acetylated than to unmodified wood. Planing of the acetylated wood surface prior to bonding reduced bond durability for the epoxy adhesive and increased the amount of surface hydroxyl groups, as measured using X-ray photoelectron spectroscopic analysis of the trifluoroacetic anhydride-treated wood. These experiments showed that wood modification is useful in understanding wood-adhesive interactions, in addition to determining how to develop adhesives for acetylated woods.

Selected previous findings on the factors influencing the gluing quality of solid wood products in timber construction and possible developments: A review

“…Bongers et al (2016) stated that better results can be achieved by gluing acetylated wood using various PUR adhesives compared to gluing untreated wood. The good adhesive strength of acetylated wood when using resorcinol formaldehyde adhesives is shown in Figure 5, as reported by Frihart et al (2017). Similar results were obtained by Olaniran et al (2021) in tests performed on rubberwood.…”

Quasi-static and fatigue performance of bonded acetylated rubberwood (Hevea brasiliensis, Müll. Arg.)

“…The increase in TSS of the acetylated samples with all the adhesives in wet condition could be an indication of a reduced fibre saturation point as acetylation limits moisture uptake by reducing the availability of hydroxyl groups for moisture sorption (Frihart et al 2017). The TSS of wood decreases as a function of increasing moisture content.…”

“…All three adhesives performed well with acetylated rubberwood in dry condition. However, there may be a few exceptions as shown by Vick and Rowell (1990) and Frihart et al (2017), who reported a decrease in TSS of some adhesives such as emulsion polymer isocyanates (EPI), urea formaldehyde (UF), and epoxy-polyamide adhesive even in dry conditions. Compared to the values obtained in dry conditions, the TSS values of unmodified rubberwood in wet condition are generally reduced yielding values in the range of 6 ± 0.44 MPa for PUR, 6.6 ± 0.59 MPa for MUF and 6.9 ± 0.65 MPa for PRF (Fig.…”

“…However, the WFP of unmodified and acetylated wood bonded with PUR has been reported to be low in wet condition (Bongers et al 2016;Vick and Rowell 1990). Bonded acetylated wood also performed well with the use of resorcinol-formaldehyde adhesive for both planed and un-planed surfaces, while epoxy adhesives performed better with unplaned acetylated wood surfaces in both dry and wet conditions (Frihart et al 2017).…”

Comparative Adhesive Bonding of Wood Chemically Modified with Either Acetic Anhydride or Butylene Oxide

“…Research in this area is greatly aided by a paper that studied the bonding of 18 different adhesives to yellow poplar acetylated at different modification levels [19]. Additional studies have been done on adhesive bonding of acetylated wood [20][21][22]. We are aware of only of two citations regarding bonding to alkylene oxide-modified wood, which are preliminary discussions of our results [23,24].…”