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5.4 Core materials for sandwich construction 5.4.1 Foam Cores Foams are one of the most common forms of core mate- rial. They can be manufactured from a variety of synthetic polymers including polyvinyl chloride (PVC), polystyrene (PS), polyurethane (PU), polymethyl methacrylamide (acrylic), polyetherimide (PEI) and styreneacrylonitrile (SAN). They can be supplied in densities ranging from less than 30kg/m3 to more than 300kg/m3, although the most used densities for composite structures range from 40 to 200 kg/m3. They are also available in a variety of thicknesses, typically from 5mm to 50mm. 5.4.1.1 PVC Foam Closed-cell polyvinyl chloride (PVC) foams are one of the most commonly used core materials for the construction of high performance sandwich structures. Although strictly they are a chemical hybrid of PVC and polyurethane, they tend to be referred to simply as ‘PVC foams’. PVC foams offer a balanced combination of static and dynamic properties and good resistance to water absorption. They also have a large operating temperature range of typically -240°C to +80°C (-400°F to +180°F), and are resistant to many chemicals. Although PVC foams are generally flammable, there are fire-retardant grades that can be used in many fire-critical applications, such as train components. When used as a core for sandwich construction with FRP skins, its reasonable resistance to styrene means that it can be used safely with polyester resins and it is therefore popular in many industries. It is normally supplied in sheet form, either plain, or grid-scored to allow easy forming to shape. There are two main types of PVC foam: crosslinked and uncrosslinked with the un- crosslinked foams sometimes being referred to as ‘linear’. The uncrosslinked foams (such as Airex R63.80) are tougher and more flexible, and are easier to heat-form around curves. However, they have some lower mechanical properties than an equivalent density of cross-linked PVC, and a lower resistance to elevated temperatures and styrene. Their cross-linked counterparts are harder but more brittle and will produce a stiffer panel, less susceptible to softening or creeping in hot climates. Typical cross- linked PVC products include the Herex C-series of foams, Divinycell H and HP grades and Polimex Klegecell and Termanto products. A new generation of toughened PVC foams is now also becoming available which trade some of the basic mechanical properties of the cross-linked PVC foams for some of the improved toughness of the linear foams. Owing to the nature of the PVC/polyurethane chemistry in cross-linked PVC foams, these materials need to be thoroughly sealed with a resin coating before they can be safely used with low-temperature curing prepregs. Although special heat stabilisation treatments are available for these foams, these treatments are primarily designed to improve the dimensional stability of the foam, and reduce the amount of gassing that is given off during elevated temperature processing. 40 - GTC-6-0417PDF Image | GUIDE TO COMPOSITES
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