Impregnation resins are slightly viscous, organic liquids that are used in the forest products industry for wood modification. They typically contain formaldehyde and are composed of dimers and trimers of the main molecule. These can become polymer solutions upon curing inside of a wood substrate, imparting stabilizing properties. Impregnation of these resins involves a vacuum chamber procedure that completely disperses the resin into the wood. Once inside of the wood, the resin can diffuse into the cell wall and enhance the physical strength of the wood even further.
Applications
Impregnation resins were designed to be used in the forest products industry to modify wood and enhance its natural properties. Wood impregnation resins typically are designed to end up inside wood cell walls, where they generally increase strength, hardness, dimensional stability, and resistance to decay. In recent years, interest in wood, or lumber, has increased because less energy is needed to convert trees into wood than what is required by other building materials. Although wood does have advantages over other building materials, it also has major disadvantages. Wood swells in high humidity and shrinks when the air is dry. It can also decay if allowed to stay moist for extended periods, and has variable properties from board to board. Resin impregnation can improve all these properties. The impregnation of resins into wood substrate involves a vacuum treatment process that disperses a monomer solution throughout the finer structures inside of the wood. Polymerization of the resin is accomplished by curing with heat, setting the impregnation resin into a solid substance state. Impregnating the wood with resin followed by curing bulks the cell wall of the wood and imparts enhanced properties. Demand for impregnating resins is proliferating across the globe due to increased efficiency in the overall electric insulation process of automotive and electronic parts.
While there are no new commercial developments with impregnation resins for wood modification, there are constant innovations being discovered in industry and research institutes.
Types of impregnation resins
Phenol-formaldehyde resins
Phenol-formaldehyde resins (PF) were the first commercially relevant impregnation resins, made by reacting phenol and formaldehyde, creating a polymer network inside of the wood upon curing. Phenol can react with formaldehyde at the ortho and para positions, generating mono, di, and trimethylolphenol as the reaction products. Besides being the first synthetic polymers to be commercialized, studies show that PF resins have superior performance over other resins in terms of imparting strength, stability, and decay resistance to the wood substrate. The dimensional stability displayed by the wood samples impregnated with PF resins is mostly a function of the mass of resin inside the wood cell wall. PF impregnated wood is sometimes called impreg. Before curing, PF resins act as cell wall plasticizer, allowing wood to be compressed more easily. After cure, the PF prevents wood from springing back. This has been used to make high density, very hard wood products, known as “compreg” or “staybwood”. The increase in both strength and stability of the wood is due to the cross-linking of the resin components and the resin's ability to bulk the cell wall. PF is black, obscuring any natural color of the wood.
Urea-formaldehyde resins
Urea-formaldehyde resins (UF) are a class of impregnation resins for wood modification made by reacting urea with formaldehyde. This resin can be polymerized after impregnation into the wood substrate by oven-curing. UF resins are beginning to be used less in the wood modification industry due to the fact that they are less durable than other impregnation resins and do not stand up well to harsh weather conditions. When exposed to water, UF resins can hydrolyze to release formaldehyde out of the wood substrate. For this reason, they are mostly used as impregnation resins when the wood substrate will be used for indoor purposes. Wood is modified for outdoor use to improve stability against weather conditions, therefore PF resins are more suitable for this purpose.