Effect of Heat and Humidity on the Properties of High-Pressure Laminates

The objective of this study was to predict the properties of composite structures from the properties of individual components. Physical and mechanical properties were obtained on four 1/16-inch general-purpose decorative laminates, one flake-type particleboard, and two commercial backing sheets. A statistical analysis was made of the data and the results reported. Properties evaluated were: 1) free dimensional movement due to moisture change, parallel and perpendicular to the machine direction; 2) coefficient of thermal expansion in the two directions; 3) modulus of elasticity in tension in the two directions; 4) vapor transmission rate of the decorative laminates and the backing sheets; and 5) equilibrium moisture content of the various materials. In general, decorative laminates, backing sheets, and particleboard expand in width and length with an increase in their moisture content. Due to the orientation of the fibers in the plastic laminates, and possibly to the restraint placed upon the paper web as it dries during manufacture, movement across the machine direction of the sheets is about twice that in the machine direction. The rate at which strips of plastic laminates increase in dimension when exposed at a higher relative humidity appears to be related, primarily, to the thickness of the laminates. Sheet size of the plastic laminates specimens has little or no influence on the amount of dimensional movement of the laminate. The coefficients of thermal expansion of the decorative laminates were greater than those for the backing sheets, but this may have been due to the influence of the melamine-impregnated top sheet on the decorative laminate. For both decorative laminates and backing sheets, the values for the across-the machine direction were about twice those for the machine direction. The water vapor transmission rate appears to be linear. with change in the thickness of plastic laminates. The tensile properties of plastic laminates and particleboard increase as their moisture contents decrease. Backing sheets and decorative laminates have nearly the same level of equilibrium moisture contents. The values found in this study were one-fourth to two-thirds those of particleboard in the 30 to 80 percent relative humidity range. Although the brands of decorative laminate and backing sheets tested were similar in behavior, the differences in physical properties among the brands indicated that they should not be used interchangeably.

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