Resins skin-core structures

Results are presented of experiments undertaken by Gaiker in the manufacture of sandwich panels containing foam cores based on PETP recycled by a solid state polyaddition process developed by M G Ricerche. Panels were produced with glass fibre-reinforced unsaturated polyester and epoxy resin skins, and allthermoplastic panels with PE, PP, PS and glass fibre-reinforced PETP skins were also produced. EVA hot melt adhesives and thermoset adhesives were evaluated in bonding glass fibre-reinforced PETP skins to the foam cores. Data are presented for the mechanical properties of the structures studied. 

Entries 1-6 in Table I reveal an approximate constancy of volumetric CTE through the XYDAR 300, 400, and SRT-300 series resins. This result is not surprising, since the molecular structures of these materials are similar. However, the CTE values for a molded part reflect macroscopic structure (skin/core) as well as domain or molecular level morphology. The LCP molded part must therefore be regarded as a composite structure. These considerations as well as differences in basic molecular composition may explain the lower volumetric CTE for Vectra A-950 relative to the neat XYDAR resins, at least over the measured temperature range of 0-150 C. These factors may also be responsible for the lower anisotropy of the neat XYDAR 300 series resin. 

A second opportunity with coextruded structures is also related to the use of virgin resin skin layers. Recent approval for food contact of coextruded films with PCR core layers has been granted based on the precedent of food contact approval for virgin resins. There is some concern about migration of compounds from the core to the skins, and this has led to tight controls over the source of the PCR for the core layers. 

In a sandwich structure, the foam is used as the core with two skins of reinforced resin sheets firmly stuck on the foam to obtain high rigidity. The sandwich composite behaves as an I-beam see Figure 6.11. 

Thermoplastic structural foams with bulk densities not less than 50% of the solid resin densities are considered. Cellular morphology, uniform-density cell behaviour, the I-beam concept in designing, core-density profile and the role of the skin, mechanical properties, and ductile-brittle transitions are discussed. 63 refs. 

Structural foam is the term usually used for foam produced in an injection molding press and made of almost many thermoplastic resin. Structural foam is always produced with a hard integral skin on the outer surfaces and a cellular core in the interior, and is used almost exclusively for production of molded parts. The process is thus ideally suited for fabrication of parts such as business machine housings (commonly for ABS), and similar parts or components in which lightweight and stiffness are required. 

Several processes are employed for the manufacture of structural foams. In the high-pressure process, the first step is to fill a mould solidly with the resin under pressure. While it is still soft, the mould is expanded or a core retracted, which provides an interior space to be foam-filled by expansion and foaming inward of the still-soft resin or by injecting foam into the Interior space. In either case, the result is a dense skin surrounding a cellular core. In the low-pressure process, a mould is partially filled and the molten resin expands to fill the mould, forming a skin upon contact with the walls of the mould. 

A mixture of Phenolic Microballoons and resin binder has a putty-like consistency. It can be molded to shape, trowelled onto surfaces, or pressed into a core. Curing gives a high strength, low density (0.144 g/cm ) foam free of voids and dense areas, and without a brittle skin. Syntactic foams are used in widely diverse applications, including boat flotation aids structural parts in aircraft, submarines, and missiles structural cores for wall panels and ablative heat shields for reentry vehicles and rocket test engines. 

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