Protein industrially used in the sector of wood-based panels is primarily derived from soy. During the thirties, soybean was incorporated in phenolic resins mainly as a filler or extender to decrease the cost of petroleum based plastics  while soy-based adhesives have been used to manufacture common wood products such as plywood for over 70 years. Currently few soy protein based adhesive systems have been developed to produce lower cost or more environmentally friendly systems. For example, one of the primary successes of United Soybean Board funded research is the development of two different soy-based adehesive systems suitable for use as glues in the production of interior hardwood plywood, oriented strand board and softwood plywood. The first system is soy phenol formaldehyde resin, where soy flour is converted to a soy hydrolyzate which in turn co-reacts with a phenolic resin. Low-cost soy meal/soy flour soy flour is converted to a soy hydrolyzate that can be substituted for up to 40 percent of the more expensive phenol component. The second resin involves the use of soy flour in foamed glue extruded systems for laminating plywood veneers. In this system, soy flour is substituted for animal blood for use as a foaming agent for phenol formaldehyde resins. The main benefit for foamed glues over other application systems is the savings in resin usage. Other benefits are reduced glue spread, reduced glue waste and minimal clean-up time. Reduced resin content also results in reduced formaldehyde emissions .
A new group of adhesives have been developed by researchers from the College of Forestry at Oregon State University. The discovery resulted from the curiosity of Kaichang Li, an assistant professor of wood chemistry, who was harvesting mussels one day at the ocean's edge. Li observed mussels being pounded by ocean waves, and wondered how they could cling so tenaciously to rocks by their thread-like tentacles. A study of the chemistry of this attachment ultimately led to creation of a new group of adhesives based on renewable natural materials, such as soy bean protein or wood lignin. The new adhesives may replace some of the formaldehyde-based wood adhesives currently used to make wood composite products, such as plywood, oriented strand board, particle board, and laminated veneer lumber. A key advantage of this new adhesive is its superior strength and water resistance. The discovery has been protected with patents while the exploitation rights have been transferred to the Columbia Forest Products that market the resin, under the trade name PureBond,® for application on hardwood plywood panels and other composite wood products.
The soy flour component is a renewable resource and, while not studied extensively as a building material component, it is not expected to have health impacts as significant as the formaldehyde and MDI-based binders.
Water-blown rigid polyurethane foams have been prepared by introducing a small portion of soy flour [56, 57]. Soy flour contains about 50% proteins and 30% carbohydrates, which are polyfunctional molecules bearing many hydroxyl groups. These moieties react with isocyanate to form different polyurethane formulations. Compared to petroleum-based polyols, soy flour can be applied to modify or to improve the physical and chemical properties of polyurethane foams, while reducing the overall cost.
2.7 Natural Oils
The first bio-based adhesive patent was issued on Nov 11, 2003 as US Patent No. 6,646,033, entitled “Pressure Sensitive Adhesives from Plant oils”. The patent was disclosing the use of soy oil and Dow Chemical expressed interest in developing this high volume bio-based material.
Epoxidized plant oils like soybean oil, epoxidized castor oil and fatty acids have been used for the production of sustainable epoxy resins . These epoxidized oils are converted directly, either in the presence of thermally latent catalysts to initiate polymerization or in the presence of anhydrides as curing agent .
Trials have been made to mechanically reinforce epoxy resins derived from plant oil with different inorganic fillers such as glass, carbon and mineral fibers .
Plant oils are also used in the synthesis of polyurethane resins. These resins may be either thermosetting or thermoplastic because their properties may be tailored by reactions with various polyols and isocyanates.
Figure 10: Synthesis of polyurethane where R and R’ are of aliphatic or aromatic nature.
In sustainable thermosetting materials, polyurethane is currently prepared starting from renewable polyols, while the isocyanate component is made from petroleum resources . To obtain vegetable oil polyols, the most used methods are based on the epoxidation of plant oils, or air oxidation followed by their soft-hydrolysis or maleinization carried out in one or two step precedures. This leads to the formation of hydroxyl functions in the middle of the fatty acid chains . Nevertheless, more recent advancements in biotechnology promise the synthesis of isocyanate compounds from renewable resources. Tamani et al.  developed soy-based polyurethane networks without the aid of diisocyanates. The authors converted epoxidized soybean oil into carbonated soybean oil by reaction with CO2 and tetrabutylammonium bromide as catalyst. The carbonated soybean oil was reacted with amine functionalized monomers to produce polyurethane networks.
The hydrolysis or transesterification of fatty acids triacyl esters (oils and fats) gives glycerol [64, 65]. In the traditional manufacturing process bio diesel is produced by a transesterification reaction between vegetable oil and methanol, catalyzed by KOH.
Currently, industry, government and academy are increasing their efforts to develop new and improve existing glycerol chemistry, and figure 11 depicts some of the platform chemicals that can be derived from glycerol .
Figure 11: Platform chemicals derived from glycerol. (Source: J. A. Kenar, Lipid Technology, November 2007, Vol. 19, No. 11.)
Hyperbranched polyether (HBP) obtained from glycerol blended with urea-formaldehyde (Figure 12) improves the hardness (16%) and the compressive shear strength (17%) of the cured urea - formaldehyde polymer, whereas water absorption remains unaffected. It was also shown that blending UF resins with hyperbranched polyethers can be an effective tool for controlling mechanical properties and dimensional stability of the polymeric systems.
Figure 12: Anionic polymerization of glycerol carbonate to hyperbranched polyether (HBP), using partially deprotonated trimethylolpropane (TMP) as an initiator
Some interesting glycerol derivatives that are already used in the wood-based panel industry are:
- Glycerol triacetate (triacetin) [67, 68] that can be used as a cure accelerator in various binder formulation such phenol – formaldehyde and tannin or lignin binder systems.
- Poly-glycerol and poly-glycerol ethers and esters [69, 70], are also very interesting compound due to their high content of hydroxyl groups that can be either react with formaldehyde (scavenger) or with urea or melamine to improve overall resin properties.
- Diglycidyl ether (DGE) and polyglycidyl ether (PGE) type compounds could also react with condensed tannins and/or lignin. Due to the demonstrated reactivity of diglycidyl and polyglycidyl ether compounds with hydroxylated reactants for different applications in general and with lignin, with potential application in the wood adhesives field it is important to consider the relative chemical similarity of lignin with tannin structures, both being polyphenolic in nature.
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