Development and evaluation of a new formaldehyde-free wood adhesive from renewable materials for making interior plywood

Graduation date: 2012Synthetic adhesives, such as phenol-formaldehyde (PF) resins and urea-formaldehyde (UF) resins, have dominated the wood adhesive market since they were introduced in the wood products industry. Formaldehyde, as a key component in the manufacturing of PF and UF resins, has been classified as a known human carcinogen by the International Agency for Research on Cancer (IARC). The UF and PF resins are derived from petroleum and natural gas. However, petroleum resources are not infinite and not renewable. In recent years, with concerns over fluctuating prices and future-exhaustion of oil, as well environmental issues of the UF resins, soy-based adhesives have again drawn people’s attention as one of the most promising alternatives to petroleum-based adhesives. A soy-based adhesive, consisting of soy flour (SF) and a curing agent (CA) (polyamidoamine-epichlorohydrin (PAE) resin) has been commercialized for production of plywood since 2004. The PAE resin is derived from petrochemicals and is the most expensive component of the soy-based adhesive. In this study, a new formaldehyde-free adhesive, consisting of soy flour (SF) and a new curing agent (CA), was developed and evaluated for making interior plywood. The CA was developed from the reaction of epichlorohydrin (ECH) and ammonium hydroxide in water. ECH can be derived from renewable glycerol.\ud The weight ratio of SF/CA was 7/1 with the total solids content of 36 wt% for the resulting adhesive. The reaction time, reaction temperature, and addition order of reactants in the preparation of the CA; the heat-treatment and storage time of the CA; and the NaOH usage in the SF-CA adhesive were investigated in terms of their effects on the water resistance of the resulting plywood panels. All adhesives containing the CAs (prepared by adding ECH and ammonium hydroxide all at once in water at the temperature of 45 ºC to 60 ºC) enabled all types of plywood panels to meet the water-resistance requirements for interior applications. The heat-treatment of all CAs, except the CA prepared at 50 ºC for five-ply aspen panels, had no effects on improving the water resistance. The storage time of both heat-treated and untreated CAs within at least two months had no significant effects on their viscosities, and the water resistance of the resulting plywood panels