Test, testing dimensional stability

IPC-TM-650 recommends a method of testing dimensional stability in a 9 x 10-in rectangle for both MD and TD. One standard-size test sheet is too small to represent the whole web. 

Dimensional Stability. Plastics, ia general, are subject to dimensional change at elevated temperature. One important change is the expansion of plastics with increa sing temperature, a process that is also reversible. However, the coefficient of thermal expansion (GTE), measured according to ASTM E831, frequendy is not linear with temperature and may vary depending on the direction in which the sample is tested, that is, samples may not be isotropic (Eig. 7). 

Most of over six million dentures produced annually in the USA are made of acrylics (PMMAs) that includes full dentures, partial dentures, teeth, denture reliners, fillings and miscellaneous uses. Plastics have been edging into the dental market for over a half century. Even before the introduction of acrylics to the dental profession in 1937, nitrocellulose, phenol-formaldehyde and vinyl plastics were used as denture base materials. Results, however, were not wholly satisfactory because these plastics did not have the proper requisites of dental plastics. Since then, PMMAs have kept their lead as the most useful dental plastics, although many new plastics have appeared and are still being tested. Predominance of PMMAs is not surprising, for they are reasonably strong, have exceptional optical properties, low water absorption and solubility, and excellent dimensional stability. Most denture base materials, therefore, contain PMMA as the main ingredient. 

The moisture content of a plastic affects such conditions as electrical insulation resistance, dielectric losses, mechanical properties, dimensions, and appearances. The effect on the properties due to moisture content depends largely on the type of exposure (by immersion in water or by exposure to high humidity), the shape of the product, and the inherent behavior properties of the plastic material. The ultimate proof for tolerance of moisture in a product has to be a product test under extreme conditions of usage in which critical dimensions and needed properties are verified. Plastics with very low water-moisture absorption rates tend to have better dimensional stability. 

Creep and stress-relaxation tests measure the dimensional stability of a material, and because the tests can be of long duration, such tests are of great practical importance. Creep measurements, especially, are of interest to engineers in any application where the polymer must sustain loads for long periods. Creep and stress relaxation are also of major importance to anyone interested in the theory of or molecular origins of Viscoelasticity. 

Space structures, which in service represent the ultimate in inaccessibility, are stressed mechanically during launch but the principal cause of subsequent degradation is environmental. Dimensional stability is the principal concern. In general, complete structures rather than individual materials are exhaustively tested in space simulators before they are launched. There is now sufficient information available with different materials for estimates to be based largely on experience. 

The use of a water-soaking, oven-drying series of cycles for the determination of dimensional stability of wood is a severe test (although it may produce useful data) and it does not necessarily reflect the conditions that wood will encounter in service conditions. As a result, some workers determine dimensional stability by subjecting samples to different relative humidities. It can be readily understood that dimensional stability values determined using different methods will not be comparable and it therefore needs to be explicitly stated how these values were obtained. 

The results of both liquid water and water vapor tests show that acetylation of lignocellulosic materials greatly improves dimensional stability of composites made from these materials. 

Inert Simulants for High Explosives. In certain physical tests on weapons explosives, eg the effects of humidity and temp cycling on the dimensional stability of the weapon, it is desirable to simulate the explosive filler by an inert filler. Many such inert simulants have been proposed and tested (Refs 1, 2 3). Simulants for Comp B TNT have been patented (Ref 4). The simulant for Comp B consists of a mixt of 30% of 1,2 hydroxystearic acid, 5% wood rosin 65% dead-burned gypsum. The simulant for TNT is a 40/60 mixt of hydroxystearic acid dead-burned gypsum... 

The most common dimensional measurements relate to the size of test pieces because this information is required for virtually all physical test methods. There is also sometimes need to measure dimensions of components of the apparatus, such as the thickness of spacers in compression set tests. Other aspects of dimensional measurement that are relevant to rubber testing include extensometry, surface roughness, dimensional stability and dispersion. 

Generally, vulcanised rubber is dimensionally very stable (unless it is strained), which probably explains the lack of standard test methods for this property. In this context, thermal expansion and swelling in liquids are properties considered in their own right and not normally thought of as being measures of dimensional stability. This is a different situation to that which exists with plastics where a number of dimensional stability tests are in existence. If a measure of dimensional change is required, the appropriate dimensions of a suitable sized test piece can be measured by any of the methods mentioned in this chapter before and after an ageing treatment. 

Specifications, Standards, Quality Control, and Health and Safety Factors. Formerly, there was an Insulation Board Institute representing the insulation board industry, but the decline in the market and number of producers has led to its demise. Currendy (ca 1997), the industry is represented by the American Hardboard Association (AHA). Specifications and standards are found in American National Standards Institute (ANSI) standard for CellulosicFiberboard (7). The standard includes descriptions of the various types and classes of fiberboard, as well as requirements for physical and dimensional stability properties. Quality control tests are limited to a few basic strength and stability tests, including bending strength, bond strength, and moisture resistance. 

Acetylation of cellulose to the triacetate has been carried out without breaking down of the structure with acetic anhydride containing pyridine to help open up the cell wall structure and to act as a catalyst (71). This led Stamm and Tarkow (72) to test the liquid phase reaction on wood. High dimensional stabilization without break down of the structure was obtained, but excessive amounts of chemical were used. They hence devised a vapor phase method at atmospheric pressure that proved suitable for treating veneer up to thicknesses of 1/8 inch. Acetic anhydride pyridine vapors generated by heating an 80-20% mixture of the liquids were circulated around sheets of veneer suspended in a box lined with sheet stainless steel. Hardwood veneer,... 

Burrows in 1959 contributed a study based on making a floor tile from the cork fraction of Douglas-fir bark (34). Added binder in the dry-process tiles was either 5% butadiene styrene or diethylene glycol. Comparison tests were made against tiles from Mediterranean oak cork. Dimensional stability was better in Douglas-fir cork tiles, and most other properties compared favorably. No known commercial application resulted. 

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