The conventional adhesives for production of wood composites are made mainly from fossil derived polymers, based on phenol, urea, melamine, formaldehyde, isocyanate, etc. These fossil-derived adhesives are cost-effective and perform very well regarding bonding performance, mechanical properties, thermal stability and water resistance. Most of the industrially used wood adhesives comprise formaldehyde which is a highly reactive compound, mazking it well suited for its intended use. However, formaldehyde has been identified as a hazardous and toxic compound and in 2004 was re-classified from “probable human carcinogen” to “known human carcinogen“ by the International Agency for Research on Cancer (IARC 2004, EPA 2017). There is sufficient evidence that longterm formaldehyde exposure can cause cancer of the nasopharynx and leukaemia.
Thus, the concern about free formaldehyde emissions from wood composites, especially in indoor applications, along with the increased environmental consciousness related to the sustainability of raw materials and final products, as well as the new stricter environmental legislation, are the main driving factors for shifting the scientific and industrial interest from the traditional formaldehyde-based synthetic resins to the new bio-based adhesives for production of eco-friendly wood composites.
There has been a resurgence of interest and research on using bio-based materials as wood adhesives; however, they have achieved only limited market acceptance. To better understand this low level of replacement, it is important to understand why adhesives work or fail in moisture durability tests. A holistic model for wood adhesives has been developed that clarifies many issues of wood bond durability. This model addresses performance challenges that bio-based adhesives must overcome to compete with synthetic adhesives. Additionally, bio-based adhesives face challenges on economic and process requirements in order to fit into current wood products manufacturing environments. Despite these challenges, bio-based adhesives continue to show great potential for wider acceptance in wood bonding.
Lignin : Bio-based adhesives Lignin: After cellulose, lignin is the second most important component of plant biomass with an estimated 300 billion total tons in the biosphere and an annually resynthesis of about 20 billion tons. It is present in lignocellulosics including wood, grass, agricultural residues, and other plants. Worldwide, more than 50 million tons of lignin accumulate annually as by-product of pulp production (lignosulfonate from sulfite pulping processes and kraft lignin from sulfate pulping processes). About 10% of the technical lignins are exploited industrially whilst the rest is combusted or not utilized at all. The possibilities for use of lignin in adhesive applications have been extensively studied by many authors.
The main interest in lignin is due to its phenolic structure with several favourable properties for formulation of wood adhesives such as high hydrophobicity and low polydispersity. However, the chemical structure of lignin lowers the reactivity of the resin, which is a disadvantage in applications where fast curing times are needed. Lignin-based adhesives can be classified into two groups - lignin-based phenol-formaldehyde adhesives, where lignin is used as a partial replacement of phenol, and formaldehyde-free lignin based adhesives. In different studies lignin is often combined with synthetic resins such as phenol-formaldehyde or urea-formaldehyde resins (to decrease the cost or free formaldehyde emissions.
The most often considered approach is to use lignin as a partial replacement for phenol in phenol-formaldehyde (PF) adhesives. Although PF adhesives are a very large market, the high viscosity of the lignin precludes any significant use of it in many applications, such as with oriented strand board, where the adhesive needs to have a low viscosity for spray or spinning disc atomizer application methods,
This still leaves the exterior plywood industry and some other laminating applications for lignin-phenolformaldehyde (LPF) adhesives. There has been a lot of research touting that lignins can be used up to a 40% replacement of phenol while making products that meet the performance specifications. Problems are that on an industrial scale a fast cure rate and a low cure temperature are economically important. Unfortunately, addition of lignin diminishes the PF adhesive performance in both of these properties.
The slow curing of the LPF compared to the PF is a negative for the industry because when too much lignin is added, the lignin becomes more of a filler than a coreactant in the formulation due to lignin’s slow cure rate. The addition of cure accelerators allows PF adhesives to cure quicker and at lower temperatures, but more powerful accelerators would need to be used with the lignin. Lignin can be used in the adhesive formulation by either adding it from the beginning of the PF reaction, or by blending in methyolated lignin (product of reacting formaldehyde with lignin) (Gardner and Sellers 1986). Furthermore, if the PF adhesive needs oligomers to stabilize the cell wall and repair surface damage (Frihart 2009), then adding lignin, which has too high of a molecular weight to enter the cell wall, will require that the formulation be readjusted to have more low molecular weight PF components and require the lignin to bridge the two surfaces.
Tannin: Tannins are natural polyphenols divided into two classes of chemical compounds of mainly phenolic nature: hydrolysable tannins and condensed tannins. Tannins occur naturally in bark, wood, leaves and fruits of different plant species but only a few plants have high enough concentration to make their extractions worthwhile. Tannins can be extracted from pine, oak, chestnut, wattle, eucalyptus, myrtle, maple, birch, willow, etc. Different extraction methods exist and they have a significant impact on the adhesive properties of tannin extracts. Extraction of the plant material and subsequent purification of the isolates, followed by spray drying, yield powdered tannins. Other components of the extraction include sugars, pectins and other polymeric carbohydrates, amino acids, as well as other substances.
Hydrolysable tannins have been successfully used as partial substitutes (up to 50%) of phenol in the manufacture of phenol-formaldehyde resins (Kulvik 1976, 1977). However, the naturally low macromolecular structure, the low level of phenol substitution they allow, limited worldwide production, and relative high price makes them less interesting compared to the condensed tannins (Pizzi 2003, 2006). Condensed tannins with a yearly production of 200 000 tons make up more than 90% of the world´s commercial production (Pizzi 2003, 2006).
Studies focused on developing resins totally free of formaldehyde by combining tannins with other biobased material, e.g. protein (Li et al. 2004). Santos et al. 2017 have studied the possibility of completely removing formaldehyde from adhesive formulations by developing particleboard adhesive based on tannins, extracted from industrial lignocellulosic wastes.
The application of tannins as adhesives for woodbased panels depends mainly on the content of reactive polyphenols and the reactivity of these components towards formaldehyde. Tannins can be used as adhesives alone (with a formaldehyde component as cross linker) or in combination with amino plastic or phenolic resins. MDF produced with tannins replacing parts of phenol in phenol-urea formaldehyde resins or even with 100% tannin resin can meet interior grade requirements but usually not exterior grade specifications.
Tannins are very different from lignins in supply, cost, and reactivity, although both are aromatics. Of the two types of tannins, most of the research has been done on the one most commercially available, the condensed tannins. The tannin availability is about 200,000 tons per year from a variety of different plant species, including bushes and trees. Tannins are very reactive due the high content of phenolic groups, and can be compared more to resorcinol in reactivity than to phenol (Pizzi 2003b, Pizzi 2013).
Like lignin, condensed tannins are higher in molecular weight so the PF formulation needs to be adjusted to obtain the proper flow and cell wall infiltration characteristics. The high reactivity of tannins has allowed them to be cured without the use of formaldehyde (Pizzi 2013).Another key difference is that commercial lignin is a by-product of other processes (pulping for papermaking and cellulosic ethanol), while tannins are the main product of extraction of plant materials. Thus, the tannin has to bear more of the production costs than does lignin. Consequently tannins are higher in price and more of a localized product compared to PF resins. In the right circumstance, they have been used commercially with good success.
Bio-based adhesives provide an eco-friendly and sustainable alternative to the conventional adhesive systems used in wood-based panel industry. All natural adhesive raw materials, presented above, can significantly reduce the negative environmental impact of harmful formaldehyde and volatile organic compound emissions from wood-based panels. However, there are still substantial challenges for the complete replacement of petroleum based wood adhesives with bio-based adhesives, mainly because of their relatively poor water resistance, low bonding strength and large natural variations due to different growing conditions.