There has been a renewed interest in development of soy-based wood adhesives in recent years. Various new methods have been investigated for improving the strength and water-resistance of wood composite panels bonded with soy-based adhesives. It was demonstrated that the treatment of soybean protein with alkali and protease enzymes significantly improved the strength and water-resistance of plywood samples bonded with the modified soy proteins.
In the twentieth century, soy-based adhesives were the most used glue for making plywood. Soybean-based adhesives, commonly called soy-based adhesives were widely used in the production of wood composites. Soybased adhesives have many advantages such as low cost, easy handling and low press temperature. However, wood composite panels bonded with the soy-based adhesives had relatively low strength and low waterresistance, which caused the adhesives to be replaced by formaldehyde-based adhesives.
There is renewed interest in soy-based adhesives in recent years because soybean is abundant, inexpensive, and readily available. Various chemical or enzymatic modifications of soy protein were investigated for improving the strength and water-resistance of wood composite panels bonded with soy-based adhesives.
Several studies suggested that unfolding the soy protein structure significantly improved the strength and the water-resistance of wood composite panels bonded with modified soy protein. The mechanisms for this improvement were proposed as follows: when the compact protein structure is unfolded, the protein chains can spread well onto wood surfaces and can easily penetrate wood. All those functional groups such as amino groups, carboxylic acid groups and hydroxyl groups in the unfolded protein can well interact with wood components, thus forming strong bonding with wood. Alkali, organic solvents, surfactants, urea, guanidine, and protease enzymes were used to modify soy protein. The modified protein was demonstrated to improve the strength and the water-resistance of resulting wood composite panels when compared with an unmodified soy protein.
The following mechanisms were proposed to explain the improved water-resistance: those modifications turned some hydrophobic amino acids inside out. The hydrophobic amino acids slowed down water penetration into the modified soy adhesives, thus enhancing the waterresistance. It was demonstrated that the esterification of carboxylic acid groups in SPI with ethanol using hydrochloric acid as a catalyst significantly increased the hydrophobicity of the modified SPI, thus improving the water-resistance of plywood bonded the esterified SPI.
The use of curing agents for cross linking soy protein is another effective method for improving the strength and water-resistance of the soy-based adhesives. Conventional curing agents for soy protein include sulfur-containing compounds, epoxy compounds, aldehydes. Sulfur-containing compounds include carbon disulfide, ethylene thiocarbonate, thiourea and they are very good cross linkers for soy protein. Because of their versatility, sulfur-based curing agents are widely used in many industrial applications such as coatings. However, sulfur linkages are susceptible to biological or other degradations.
The amino groups on the protein react with carbon disulfide to produce sulfurized proteins. Then sulfurized proteins can be oxidized to form disulfide linkages. Aldehydes are important protein modifiers. They crosslink and denature soy protein, thus resulting in enhanced water-resistance, increased pot life, improved assembly time tolerance, and increased water-holding capacity. Formaldehyde and paraformaldehyde are very active crosslinkers for soy protein, and they can cause premature gelation. Thus they are used in the modification of soy protein with very small quantities such as 0.1- 1% based on dry adhesive weight. Linkages from the reactions between formaldehyde and soy protein are reversible under hydrothermal conditions.
Soy flour adhesives using a polyamidoamineepichlorohydrin (PAE) resin as a crosslinking agent are used increasingly as wood adhesives for interior products.
Proposed reactions of soy protein-PAE adhesives
The key properties of most often utilised commercial PAE (CPAE) are low solid content and/or high viscosity. Curing agents with a high solid content and low viscosity are preferred for improving the solid content of soybased adhesives. As a result, a lot of research has gone into developing curing agents for soy-based adhesives with a high solid content and low viscosity. To make soybased adhesives, the researchers found that a polyamine solution with a solid concentration of 40–60% and a viscosity of less than 600 cP was optimal.
While soy offers many environmental advantages over other wood adhesives, the benefits don’t stop there. New soy adhesives promise both improved performance and economics to the wood products industry. Formaldehyde pricing depends on the price of methanol, which has fluctuated greatly in the past few years due to worldwide shortages. Urea pricing rose as a result of increased ammonia costs before dropping significantly last year.
The costs of phenol are attributed to the cost of the base stock petroleum, which has followed the same trends as urea. Soy meal/flour costs remained consistent for many years before increasing recently in response to a greater demand. In spite of recent price increases, however, soy meal/flour remains an inexpensive raw material for wood adhesives. The future looks bright for soy-based adhesives as well. New markets are emerging for soy in heat-resistant adhesives, bio based composites and enzymatic processing for new soy hydrolysates.
AUTHOR: Dr. S. C. Sahoo, Scientist-E, Indian Plywood Industries Research &Training Institute, (IPIRTI)