Foam Core Materials in the Marine Industry
by Trevor Gundberg, Composite Materials Engineer with DIAB Inc. Continued from Page 1
Foam Core Properties and Applications
While core shear stiffness and strength are paramount in sandwich construction, other properties are also important. Compression strength is needed to withstand localized loads such as dropping an anchor on a deck or resting a craft on trailer mounts. Graphs in Figures 4 and 5 compare different foam core materials in shear and compressive strength.
CL PVC Foam: This foam is based on a thermoplastic PVC (Poly Vinyl Chloride) and crosslinked (or highly bonded) thermoset polyurea, or for short, a crosslinked PVC (ex. Divinycell, Klegecell, Airlite). This foam provides higher strength/stiffness in both static and dynamic situations, good temperature stability up to 180 0 F, good fatigue resistance, and a closed cell structure for low resin/water absorption. One problem encountered when using crosslinked PVC's is some grades are not compatible with some epoxy prepregs, and if the foam is not specifically treated, some prepregs will not sufficiently bond. Crosslinked PVC's are typically used in decks, superstructures, hull bottoms and sides, bulkheads, and transoms.
SAN and L PVC Foams: These foams are based on the thermoplastic Styrene Acrylo Nitrile (ex. Corecell) and a non crosslinked or "linear" PVC (ex. Airex R63), respectively. Typical reasons for using these materials are: high toughness, good impact resistance/energy absorption, good fatigue resistance, and a closed cell structure. Some drawbacks to using these foams are: comparatively lower strength/stiffness, high temperature problems, and is susceptible to styrene attack (the styrene in the resin may seep through the foam, leaving the resin uncured and the foam degraded). These foams are typically used in areas where high impact is prevalent, such as hull bottoms and sides.
PUR and PIR Foams: The polyurethane (PUR) and polyisocyanurate (PIR) foams have good compression strength and moderate physical properties at higher densities, but have a tendency to be friable, or deteriorate with time. Due to this, these foams are typically used in non-stressed acoustical and insulation panels. Higher density less friable versions of these foams are used extensively in transoms (due to their high compressive strength), while the lower density materials are used as formers or in stringers.
There are also other versions of these foams tailored for specific purposes, such as high temperature CL PVC's and SAN's for use with prepregs. Other foams such as PMI (ex. Rohacell) and PEI (Airex R82) are generally used in aircraft due to their relatively high cost and marine environment issues (e.g. water absorption)..
Foam vs. Balsa Wood and Honeycomb
Balsa wood and more commonly plywood have been used extensively in boat construction for many years. While these materials provide excellent compression and stiffness properties for a relatively low cost, they can be heavy, susceptible to water absorption, and will eventually rot. Foam cores can be much lighter, fungi resistant, and do not absorb water or any other fluids encountered in a marine environment. There is also evidence that foam cores have better fatigue resistance than balsa wood. Laminates made with foam cores can last longer and weigh less than wood cored laminates, while producing adequate physical properties. Wood is still a viable material to be used in areas where highly localized compression loads or through fittings are present (such as engine mounts and around cleats), where the appropriate high-density foam core may be too expensive.
Honeycomb materials such as Nomex (aramid paper and phenolic resin) and aluminum honeycomb are staples in the aerospace field due to their high strengths, high temperature stability, and low weight. While these properties work well for aerospace applications, honeycomb does have some drawbacks in a marine environment. Honeycombs have a relatively small area for the skins to bond to, producing weak core/skin bonds and poor fatigue resistance. The open cell structure of honeycombs is susceptible to water infiltration and bond degradation. Processing honeycomb materials also requires much higher end processing equipment and materials, such as autoclaves and high temperature epoxy prepregs.
The following figures compare the shear and compressive strengths of some foam cores with balsa wood and various honeycombs. NOTE: The 1/8" and 3/16'" designations for the honeycombs represent the cell size.
Foam Core Materials in the Marine Industry
by Trevor Gundberg, DIAB Inc.