Лако-красочные материалы — производство

Poly(vinyl acetate) (PVAc) adhesives are another important type of thermoplastic adhe­sive, especially in furniture manufacturing and carpentry. They form the bondline in a physical process by losing their water content to the two wooden adherends. PVAc adhe­sives are ready to use, have a short setting time, and give flexible and invisible joints. They are easy to clean and show long storage life. Limitations are their thermoplasticity and their creep behavior. Due to the manifold variations available (homo — or copolymerization products, unmodified or modified, with or without plasticizers) PVAc adhesives show a great variety of processing and bonding properties. The various formulations differ in viscosity, drying speed, color of the bondline, flexibility or brittleness, hardness or smooth­ness, and other characteristics.

The bonding priniciple of PVAc adhesives is based on the removal of the water by penetration into the wood substrate or by evaporation to the surrounding air. The forming

of the bondline also requires the application of proper pressure. The final bond strength is reached after migration of the residual water away from the bondline. The minimum temperature of film formation (or white point) is 4-18°C, depending on the type of the adhesive and the addition of plasticizers. This temperature is determined mainly by the glass transition temperature Tg of the polymer used which for PVAc is approximately 28°C. Parameters influencing the drying time are the type of the adhesive, the type of wood surface, the wood substrate absorption behavior, the wood moisture content, relative humidity and temperature of the surrounding air, the amount of adhesive applied, and the temperature of the adhesive and the wood surfaces.

Depending on the formulations, various grades of water resistance can be achieved. For the two-component PVAc adhesives, crosslinking and hence a thermosetting behavior is obtained by addition of hardening resins (e. g., based on formaldehyde), complex form­ing salts [based on chromium (Cr III), e. g., chromium nitrate, or aluminum (Al III), e. g., aluminum nitrate] or isocyanate. The bondlines are then resistant against high tempera­tures and the influence of water.

The addition of comonomers during polymerization enables a higher flexibility to be obtained compared to PVAc homopolymers. This causes also a lower glass transition temperature and a lower minimum film formation temperature. Possible comonomers are acrylic acid esters (butylacrylate, 2-ethylhexylacrylate), dialkylfumarates, ethylene, and others.

Plasticizers soften the film and increase both adhesion and setting rate. The most common are phthalates, adipates, and benzoates. The amount added can be in a broad range of 10-50% by weight. They affect swelling and softening of the PVAc particles and hence ensure the film-forming capabilities at room temperature, the tack of the still wet and of the dried bondline, and a better water and moisture resistance of the bondline. Disadvantages are the lower resistance of the bondline against heat, possible migration of the plasticizers, and an enhanced cold flow.

Fillers (calcium carbonate, calcium sulfate, aluminum oxide, bentonites, wood flour) increase the solid content of the dispersion, and they are added up to 50%, based on PVAc. The purpose of their addition is the reduction of the penetration depth, a thixo­tropic behavior of the adhesive, gap filling properties, and the reduction of the adhesive costs. Disadvantages can be the increase of the white point and possibly the more marked tool wear rate due to greater hardness of the adhesive. Other components in PVAc for­mulations are defoamers, stabilizers, filler dispersants, preservatives, thickeners (hydro- xyethylcellulose, carboxymethylcellulose), poly(vinyl alcohols), starch, wetting agents, tackifiers, solvents (alcohols, ketones, esters), flame retardants, and others.

The PVAc bond strength decreases at higher temperatures due to the thermoplastic behavior of the adhesive itself. The higher the average molar mass of the polymer, the smaller this temperature-dependent loss of strength. Under long term load, PVAc bon­dlines are susceptible to cold flow, especially when plasticizers are included in the formula­tion. Both effects limit the heat resistance of a PVAc bondline and generally the long term strength under load at higher temperatures (>40°C) as well.