C.H. Smith

Fabrication of Open-Cell Polyethylene Foam

density and cell size. In addition, the foam was to have the highest compressive modulus and the lowest creep obtainable. These requirements precluded the use of the gas expansion method for preparing foamed polyethylene. However, a review of the literature suggested a process for making microporous polyolefins by first preparing an admixture of poreformer and resin followed by a leaching step. This procedure appeared to provide adequate control to

A New Potting Material — Expandable Polystyrene Bead Foam

Many present-day electronic packages are designed using sophisticated and expensive circuitry. A potting material that could be easily removed with common solvents was needed to reduce the cost and time of reworking defective components in these units. Polystyrene bead foam (PSBF) appeared to have several advantages over conventional encapsulants such as rigid polyurethane foam and filled epoxy compounds for this application. The density of the PSBF can be readily adjusted from 0.02 g/cm/sup 3/ to 0.60 g/cm/sup 3/ and easily dissolved in most hydrocarbon solvents with minimal damage to other plastic parts and electrical components in the unit. In addition, it is relatively inexpensive, requires very little clean-up, and is an excellent shock absorbing material.

Structural Application Characteristics of a Rigid Urethane Foam System

various applications in the manufacture of aircraft (2, 3). Other uses of rigid urethane foams as structural components, particularly for the protection of electronic components, have also been described in the literature (4, 5, 6). Although rigid urethane foams have many desirable engineering properties, the use of these foams as structural materials has not been widely adopted. There are some applications where urethane foams are utilized for structural purposes; however, when compared to the extensive use of the material for thermal insulation, this usage is relatively small. Perhaps one reason for the limited structural applications for these materials has been lack of detailed information on the

THERMAL AGING OF CURED ETHYLENE/VINYL ACETATE AND ETHYLENE/VINYL ACETATE/VINYL ALCOHOL ELASTOMERS

Accelerated aging characteristics of ethylene-vinyl acetate (EVA) and ethylene/vinyl acetate/vinyl alcohol (VCE) elastomers were evaluated after exposure to controlled thermal and atmospheric environments to provide information for predicting the long term stability of the materials. Composite test specimens were prepared by placing discs of the materials in contact with discs of open-cell silicone foam which were then clamped between stainless steel plates. The composite samples were thermally aged at 60°, 80°, and 100°C in closed containers back-filled with either dry air or dry nitrogen for periods of from 2 weeks to 8 months. At the appropriate intervals, samples were subjected to a three-part analysis; (1) measurement of the migration of unbound chemicals between materials by weight change, (2) solvent swell determinations to evaluate the degree of crosslinking, and (3) measurements of residual unbound chemicals by solvent extraction. Little change was found in the VCE samples under the most severe conditions, while the EVA materials degraded considerably as indicated by the dramatic increase in crosslink density and the identification of degradation products. A degradation mechanism was proposed based upon the data collected.

Polyether-Polyol- and Polyester- Polyol-Based Rigid Urethane Foam Systems

For the past 10 years polyurethane polymers have been one of the fastest growing segments of the plastics industry. Over 900 million pounds of polyurethanes were used in 1970. Approximately 30 percent of this amount was consumed in the form of rigid urethane foam products (1). Initially, polyester polyols were employed in both rigid and flexible foam formulations. In 1957, however, polyfunctional polyether polyols were introduced for use in urethane foam systems. Even though the development of rigid polyether foams

The Effect of Cure Temperature and Isocyanate Index on the Softening Point of Some Rigid Urethane Foams

The Bendix Corporation, Kansas City Division, produces precision, molded-to-size, rigid, urethane foam parts for structural support applications. Some of these foam supports must function in a temperature range from -55 to 200°C. The maximum service temperature of these parts, however, is controlled to a great extent by the softening point and load-bearing characteristics of the foam. In prior work, the thermal stability of these materials was evaluated as a function of molecular structure and processing characteristics [1, 2] . More recent activity has been directed toward the effects that cure temperature and the isocyanate index have on the softening point of rigid urethane foam, and therefore the maximum service temperature that

Effects of Chemical Bonds on Some Physical Properties of A Rigid Urethane Foam

a series of rather complex molecular rearrangements and countless chemical reactions leading to the formation of a wide variety of chemical bonds. The final polymer contains a great diversity of chemical structures, making available a very wide range of physical properties (2). The two most important reactions common to both carbon dioxide and fluorocarbon-blown foam systems are, however, those that lead to the production of urethane and allophanate linkages. The urethane linkage in a foam system is generated from the reaction of an isocyanate with a hydroxyl-terminated resin (Equation 1). This can be considered the chain-propagating reaction since the reaction product (urethane linkage) can then further react with another isocyanate molecule to form the allophanate linkage (Equation 2). The reaction, as shown in Equation 2, can continue, resulting in additional crosslinking and branching of the polymer. The major chemical reactions common only to foams blown from carbon dioxide are the formation

Effect of Voids on the Compressive Strength of Molded Rigid Urethane Foam

are often employed in the aerospace industry as a means of nesting modular electronic subassemblies within an assembly. These foam structures, because of their high strength-to-weight ratio and good shock mitigating and thermal insulating properties, afford a number of advantages over other types of supports utilized for this application. The principal problem associated with the use of foam supports is the difficulty in manufacturing them to meet precise requirements of dimension and strength. Both internal and external voids constitute yet another major concern in the production of these parts. Such voids may be caused by incomplete filing of the molds, entrapped air, large bubbles of gas in the foam, or the collapse of cells prior to gelation of the polymer. Little, if any, corrective action can be taken in regard to internal voids. However, external voids sometimes occur in non-critical areas of the part, and if these voids can be filled with a suitable

Physical / Chemical Properties of Rigid Urethane Foam On Exposure to Metals

w designing equipment to process w hen rigid urethane foams, factors such as output rate, type of mixing head, reliability of pumps, and temperature controls must be given consideration. The literature contains references covering these topics in detail. However, little attention has been given to the materials used in the construction of the processing equipment. These materials actually constitute another important guide to equipment selection. This work concerns the results of prolonged exposure of various metals used in the construc-