The third problem caused by dimensional instability is warpage. Most decorative laminates consist of a two-layered aramid-formaldehyde resin impregnated paper backing with more than 5 layers of phenol-formaldehyde resin impregnated paper. The triethyleneamine-containing formaldehyde resin portion of the laminate is more than the phenol-formaldehyde-containing resin. Unstable. This indicates that the two layers of the factory laminate are subjected to different forces when there is any change in moisture content. The slab responds by deviating from its original planar form, which is called warping. Because most decorative laminate panels are ultimately attached to a flat surface, warpage makes this attachment difficult, if not impossible.
From a practical point of view, the degree of warpage is related to the amount and type of resin used for each layer, the density of the laminate, and whether or not a non-porous laminate is obtained. Controlling the volatile content of each individual prior to pressing is an effective way to adjust dimensional stability to achieve an ideal equilibrium state within the laminate.
Epstein analyzed this phenomenon and he tried to create a digital model. In his model, he assumes that the laminate is completely elastic, ie the stress is proportional to it. This seems to be a good hypothesis, because the stress-strain relationship is completely straight until the stress approaches the rupture state. He also adopted an equilibrium state, that is, the internal stress was used to estimate the warpage, and the internal stress existed after the equilibrium with the surrounding environment. To avoid the renaturation of the anisotropic structure in three directions, Epstein limited his model to one direction. According to the test, this would be equal to a warp of a narrow strip (ie, the length is much larger than the width). The Epstein model was extensively studied by others and they found that there is sometimes a good substantial consistency between the model and the experimental data. And in other respects it is not very good. A good correlation is usually obtained between the prediction at low humidity and the measured warpage, although the predicted warpage is often higher than the measured warpage. There is little correlation at high humidity.
If individual layers from the composite laminate are separately pressed and consolidated, it is shown that the individual layers are subjected to larger dimensional changes than the corresponding layers in the composite laminate. It is apparent that adjacent layers inhibit variations in their neighbors in the composite laminate. The value of the stress involved in this inhibition is completely in the straight line region of the stress-strain curve. This eliminates the possibility of stress suppression exceeding the plastic limit of the material. The conclusion is that the difference between the observed and the calculated warpage is due to the intrinsic drilling elasticity of the laminate, ie the modulus is not a constant but a value that varies with time. This will cause the stress to disappear with time. In practice, laminates are not subject to the level predicted by the elastic model but at a lower level of internal stress. The degree of warpage observed will thus be low and there is no predictive value.
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