Dimensionality change

Solventless Extrusion Process. The solvendess process for making double-base propellants has been used ia the United States primarily for the manufacture of rocket propellant grains having web thickness from ca 1.35 to 15 cm and for thin-sheet mortar (M8) propellant. The process offers such advantages as minimal dimensional changes after extmsion, the elimination of the drying process, and better long-term baUistic uniformity because there is no loss of volatile solvent. The composition and properties of typical double-base solvent extmded rocket and mortar propellant are Hsted ia Table... 

Deterioration. Paintings are composite objects that have high vulnerabiUty. The various materials are adhered to each other, especially in a laminated stmcture, to form a source of potential trouble. Any dimensional change in one of the components or between the components as a consequence of changes in environmental conditions results in a strain on the adhesion of the various parts. Strains can lead to failure of the adhesion. This is one of the principal causes of losses in panel paintings, where the dimensional changes in the wooden support cause losses in adhesion between the paint layer and the support. 

Asbestos-Based Grade. Grade MM is more resistant to heat than Grade C but not recommended for primary insulation at any voltage. It exhibits small dimensional changes when exposed to moisture. 

Fig. 6. Effects on the pressed compact of (a) speed, where A is low and B, high speed compacting (b) powders, where A is soft and B, hard powders (c) dimensional change after sintering and (d) sintering temperatures, where A is high, B, medium, and C room temperature.

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). 

Eig. 7. GTE of a 40% glass-filled PPS as functions of both temperature and direction. GTE values are measured in micrometers of dimensional change per... 

Eig. 2. Radiation-induced dimensional changes in isotropic graphite at various temperatures, nvt = neutron(density)-velocity-time. 

Acrylic Resins. The first synthetic polymer denture material, used throughout much of the 20th century, was based on the discovery of vulcanised mbber in 1839. Other polymers explored for denture and other dental uses have included ceUuloid, phenolformaldehyde resins, and vinyl chloride copolymers. Polystyrene, polycarbonates, polyurethanes, and acryHc resins have also been used for dental polymers. Because of the unique combination of properties, eg, aesthetics and ease of fabrication, acryHc resins based on methyl methacrylate and its polymer and/or copolymers have received the most attention since their introduction in 1937. However, deficiencies include excessive polymerization shrinkage and poor abrasion resistance. Polymers used in dental appHcation should have minimal dimensional changes during and subsequent to polymerization exceUent chemical, physical, and color stabiHty processabiHty and biocompatibiHty and the abiHty to blend with contiguous tissues. 

Special-Purpose Resins, Repair Resins. Fractured acryflc dentures can be repaired with materials similar in composition to cold-cured denture resins. These materials generally cure more rapidly because of the relative simple manipulations involved. The process is quick and there is fltde dimensional change, but the strength of the repaired denture may be only half that of the original appliance (213). Test methods and requirements of these materials are given in ANSI/ADA specification no. 13 for denture cold-curing repair resins. 

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