Strength, lumber compressive

Large knots drastically reduce strength and are a major cause of downgrade in lumber. They are undesirable in fibre products. Knots are very dense, some 2-3 times as dense as stem wood and are frequently above 1000 kg m . This is a consequence of compression wood formation and heavy resinification after mortality or pruning resin can account for 30% or more by weight of knot wood. Knots are hard to penetrate with chemicals and are inadequately pulped, while in mechanical pulping knots are resistant to defibration. 

Species sueh as oak and beech check quite readily, and to avoid this problem the humidity is kept high early in the kiln schedule. Also the temperature is kept low in order to maintain the timber s strength. Only as the lumber dries and becomes stronger can the humidity be lowered and the temperature raised to provide more rapid drying eonditions. Surfaee ehecks forming early in the kiln schedule may close up later when the surfaee fibres go into compression and the core into tension, although the failure plane remains in the tissue. Such checks can be revealed subsequently as hairline streaks if the wood is stained. 

Plastic lumber is recycled plastics processed such as commingled plastic, polyethylene plastic, and polypropylene plastic. To improve their performances different developments have been used such as specialty additives (lubricants, deoxidizers, etc.). An example is by adding as low as 10 wt% of short glass fiber to these recycled plastics can double their strength. Other fibers used include hemp, flax, and sisal. They are principally extruded other processes are used such as injection and compression molding, to produce products competitive to wood lumber on land and in the water. Compression molding allows for a deep-molded grain and a much more dense board. The density also helps the product resist moisture absorption and improves weatherability. 

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