In paper, the direction of arrangement of the fibers is more or less parallel to the plane of the sheet. The radial dimension of the fibers is a major component of the thickness of the sheet, so the dimensional change is greatest. The fibers are more random inside the plane of the sheet. Orientation, such that in-plane expansion or contraction results in a combination of fiber axial and fiber radial dimensional changes. Because in the longitudinal direction, the fiber axis has a slightly higher orientation. The laminate is more dimensionally stable in this direction than in the transverse direction.
This dependence of the anisotropic swellability of the plastic laminate on fiber orientation was fully confirmed by the test of highly oriented substrates. The hardwood flakes were carefully pulped, washed and dried. This produces a paper-like sheet with the fibrous structure originally present in the wood. That is, all of the fibers are substantially parallel in one direction of the sheet. These flakes are saturated with phenolic resin impregnation and pressed and consolidated into a plastic laminate.
A comparison of the dimensional stability test of the resulting sheet plastic laminate with a similar sample (ie, a paper laminate) made by commercial saturation. The figures also include wood and phenolic resins for making the sheets. As expected, the fibers in the sheet are almost completely aligned in a straight line, so that the dimensional change is increased in the fiber transverse direction (sheet width), and the dimensional change is not reduced in the fiber direction (sheet length). In the width direction, the sheet laminate changes almost four times larger than the randomly oriented paper laminate. In the length direction, the opposite was observed, that is, the sheet laminate only changed by 0.03 writing, while the paper laminate changed by 0.30%.
The data also shows how the resin and fibers interact to limit the expansion and contraction of the laminate. The resin reduces the lateral expansion of the sheet laminate fibers, while the fibers inhibit the expansion of the resin in the fiber direction. This similar relationship is evident in laminates made from impregnated saturated paper, but due to the more random orientation of the fibers, the numerical effect is lower. Wood can also be viewed as a natural plastic laminate in a cellulosic fiber-containing lignin (lignin) matrix, and Figure 183 shows that wood is subject to greater dimensional changes than sheet laminates. This may be due to the lower concentration of resin (wood rope) in wood and/or the difference in hydrophobicity between the two resins.
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