The Effects of Rain Water, Temperature on Phenolic Resin Bonded Wood Based Panel Products

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Phenol-formaldehyde (PF) resin curing and bonding in the presence of wood is a complex process. Any changes in wood aspect (such as wood surface chemistry and texture) or resin aspect (such as viscosity and composition) may influence the rate of resin cure and bonding quality. Kinetic study of PF resin will provide a useful and more direct way in understanding the basic interaction between wood and resin. This information will also help the manufacturers of wood composites to develop or formulate a suitable adhesive for a particular wood species.

Phenol-formaldehyde (PF) resin is a widely used thermosetting adhesive for exterior-grade wood composites. During the wood-composite manufacturing process, the resin undergoes polymerization reaction with itself and chemical reaction with wood under various environmental conditions, which comprise temperature, relative humidity (RH), moisture content (MC) etc. These variables may significantly affect the resin curing and bonding behavior and consequently affect the final performance of wood composites. In addition, the resin bonding is also influenced by some wood-related factors, such as density and porosity, shrinking and swelling, surface texture and wet-ability.

Experimental study was carried out to study the bonding strength vs humidity by curing Phenolic Resin at different temperature of 110°C, 120°C, 130°C and 140°C is summarized for the 90% RH Bond strength development as a function of bonding temperature (110°C, 120°C, 130°C and 140°C) at 90% RH.

The data of the study indicates that the extent of bond strength development was highly dependent on the environmental relative humidity. Bond strength of PF resin built up faster and achieved finally higher bonding at 41% RH than it did at 75% RH or 90% RH. Increasing bonding temperatures from 110°C to 140°C caused the bond strength to develop at a much higher rate, while the absolute inhibiting effect of relative humidity on bonding greatly decreased. When wood based panels are exposed to the rainwater during transportation or application, in certain situation core delamination occurs. Normal rain water has a pH of 5.6 (slightly acidic).

This is because it is exposed to the carbon dioxide in atmosphere. The carbon dioxide gets dissolved in the rain water and forms carbonic acid. When this acidic water comes in contact with wood based panels it makes swelling and wick the bonding, hence starts delaminating at the edges first.

At 110°C, 41% environmental relative humidity provides better bonding conditions than relative humidity of 75% or 90%. With increasing temperature, the bond strength developed faster and the effect of relative humidity became less significant.

So to understand the basic interaction between wood and resin under various environmental conditions the information similar to that acquired is helpful to resin manufacturers to optimize resin for specific wood composite manufacturers, and on the other hand, wood composite manufacturers who are seeking to improve their products by optimizing manufacturing conditions.

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