Preparation and characterization of a bio-based polymeric wood adhesive derived from linseed oil

Abstract

Increasing concerns over environmental health and safety, as well as diminishing fossil fuel reserves, motivate investigation into bio-based alternatives to petrochemically-derived adhesives. Adhesives derived from plant oils show promise as sustainable alternatives due to their abundance and low cost. Such adhesives are especially important in the growing domain of engineered lumber in which the material properties of low-value timber are enhanced using glued layers and joints. Linseed oil, which is derived from flax seeds, is a relatively unexplored feedstock for the preparation of eco-friendly wood adhesives. Herein, we report the preparation of a linseed oil-based adhesive and assess its strength on Liriodendron tulipifera (yellow poplar), Fraxinus americana (white ash) and Pseudotsuga menziesii (Douglas-fir). The results of a systematic study evaluating the influence of crosslinker choice, open-time, press time and adhesive spread rate on the shear strength of the adhesive-wood assembly are reported, as are results from water-soaking-and-drying tests that indicate excellent resistance of the linseed oil adhesive to water. Ultimately, these results show promise for use of the adhesive in timber applications.

Introduction

Adhesives are critical to industries that shape our lives with applications ranging from aerospace and automobile to medicine and electronics [1,2]. In particular, the wood products industry is heavily reliant on adhesives for building construction, furniture manufacturing and use in paper products [3]. Building with timber is a popular design choice in part because it is aesthetically pleasing and more environmentally friendly than alternatives like steel and concrete [4,5]. Notably, the building and construction industry has seen an increase in large timber structures, such as high-rises and office buildings [6,7]. This construction is enabled by structural composite lumber products such as cross-laminated timber, parallel strand lumber, and glulam in which small cross-sections of timber are glued together to form composite beams [8]. Over two-thirds of wood products are joined together using adhesives [9], and therefore their environmental impact and effect on human health are an area of increasing concern.

Wood bonding is a complex process due to the natural structure of wood [3]. Wood is a porous material primarily composed of cellulose, hemicellulose and lignin. Its porosity leads to the penetration of the adhesive through the wood surface [10]. As a result, the adhesive may fill the cellular lumina, thereby enhancing the surface area of the adhesive bond line, or may permeate through the cell wall, thus creating interpenetrating networks or directly reacting with the polymers composing the cell wall [3,11]. Moreover, an effective adhesive must be able to bond to the dissimilar chemical and structural interfaces that comprise wood, which vary between different wood types. In addition to their compositional complexity, wood substrates are also highly sensitive to both temperature and moisture, which can lead to swelling or shrinking that can significantly alter the strength of the adhesive bond [3,12]. Thus, establishing appropriate conditions for adhesive application and assessing adhesive efficacy across various types of wood, while controlling for temperature and moisture content, is critical in characterizing adhesive-wood bonding.

Currently, wood adhesives are derived from petrochemical precursors. This is problematic due to the diminishing supply of petroleum reserves, rising costs and increasing concern for environmental health and safety. Often, adhesive formulations contain volatile organic compounds and other toxic substances [13]. Specifically, formaldehyde-based adhesives are commonly used in timber engineering due to their high reactivity, high bond strength and low cost, but formaldehyde has been shown to exhibit dose-dependent toxicity [14,15]. Increased attention to and regulation of the emissions from formaldehydes, for example, motivate looking at alternatives [16]. Recently, there has been a strong interest in the substitution of petrochemical content with renewable components. Accordingly, the number of patents on bio-based adhesives has boomed in the last two decades, increasing approximately 400% from 2002 to 2012 [17]. Bio-based alternatives hold substantial promise as they lessen the demand for non-renewable petrochemicals and reduce carbon dioxide emissions [18,19].

Various biomass feedstocks have been explored for use in bio-based wood adhesives. These renewable sources include lignin [[20], [21], [22], [23]], tannin [[24], [25], [26], [27]], starch [[28], [29], [30]] and plant oils [[31], [32], [33], [34], [35], [36]]. Plant oils are of particular interest as they are inexpensive, sustainable and abundant [18]. Moreover, they contain naturally occurring chemical functional groups, including internal double bonds, alcohols or epoxides, which enable chemical modification and polymerization [13,37]. One commonly used subset of reactions, known as click reactions [38], have been extensively studied and successfully used to functionalize and polymerize natural oils [37,[39], [40], [41], [42], [43], [44]] owing to simple reaction conditions, quantitative yields, and short reaction times.

Linseed oil remains a relatively unexplored feedstock for the preparation of sustainable wood adhesives. Derived from flax seeds, linseed oil primarily consists of triglyceride molecules, which are composed of three fatty acid chains connected by ester bonds to a central glycerol moiety. Linseed oil has found use in the wood products industry. It is one of the best natural drying oils available, enabling its use in preservation and finishing [45,46]. While linseed oil-based building blocks have been investigated for applications including coatings [[47], [48], [49], [50]], thermosets [[51], [52], [53]] and composites [54,55], its use as a wood adhesive has remained largely unexplored.

Herein, we test the viability of a wood adhesive derived from linseed oil. Specifically, the adhesive strength is evaluated on three different wood types: Liriodendron tulipifera (yellow poplar), Fraxinus americana (white ash) and Pseudotsuga menziesii (Douglas-fir). The effects of various experimental parameters, including crosslinker choice, open-time, press time and adhesive spread rate were systematically assessed. Finally, water resistance testing revealed excellent resistance of the adhesive to water, a result atypical for most plant-based adhesives. Overall, this study describes a promising pathway towards the application of bio-based adhesives for timber engineering from renewable resources.