Effects of Layer Characteristics on the Properties of Three-Layer Particleboards

Layer properties of specific gravity, tensile strength, and stiffness parallel to the surface were determined on three-layer boards. Face material was 0.020- x 0.50- x 2-in. disk cut flakes; core material was 3/4-in. pulp chips flaked to 0.02 in. thick in a ring-type flaker. Experimental variations included board thicknesses of 1/2, 3/4, and 1 in., face weights of 0.167 and 0.333 lb/ft2, random and alined face flakes, board densities of 30 and 40 lb/ft3, and fast, slow, and normal press closures. Average face layer specific gravity was higher in thicker boards, but increased at the same rate with increasing amounts of face material. The ratio of face layer density to board density remained nearly constant regardless of total board density. Tensile stiffness (parallel to the surface) of the alined face flake layers was 2.5 to 3.5 times as great as that of the random face layers and increased in a nonlinear fashion with increasing face layer specific gravity. Steam shock treatments used in conjunction with a fast closure exaggerated the shape of the density gradient curve but were detrimental to the tensile stiffness of the face layers. Graphs show that a three-layer board with 30 pct random face material achieves 40 to 50 pct of the potential stiffness increment of a homogeneous board made with all face material. The same 30 pct addition of alined material accounts for 60 pct of the possible stiffness increase in an all-alined board. The dependence of effective board MOE on face-core weight ratios for various material types is shown. Linear expansion perpendicular to the grain was shown to be within acceptable commercial limits for boards having up to 40 pct alined material. Bending stiffness predictions made with mathematical formulae developed for three-layer and multi-layer analyses methods were compared to results of a standard center-point loading test and to a two-point loading test. Stiffness measurements using the center-point loading method were uniformly lower than those obtained using the two-point loading method. Prediction precision averaged 78 pct of the two-point testing method when the three-layer analysis method was used, but rose to 92 pct when comparisons were made using the multilayer analysis. Stiffness prediction precision varied with the type of board and were closer to actual results in board types showing a “shallow” density gradient.

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