Composite core materials are used to provide thickness to a laminate, this dramatically increases the shear strength (stiffness) without adding very much weight. Below are some of the most common composite core materials used along with their various advantages and disadvantages.
End-Grain Balsa Wood
Balsa is one of the most common core materials used in composites. It is extremely stiff yet very lightweight. Balsa is commonly used in boats, wind turbine blades, and structural composite panels. Do to the natural properties of the balsa wood, thermoset resins bond extremely well to this core material. The resins partially absorb into the first layer of the balsa, creating an extremely strong mechanical bond.
Balsa is a fast growing tree, and most all of the balsa in the world is derived from Ecuador in South America. It is naturally resistant to rot and mold, although like all wood products, it is susceptible to absorbing moisture. It is also considered a "green" composite core material as it is derived from a renewable source (balsa trees).
In recent years, balsa has come in extremely high demand due to the increased production in wind turbine blades. The two major companies providing balsa are:
- Baltek, a division of the company 3A Composites out of Switzerland.
- I-Core Composites, a new but growing composite core material company.
Polyurethane foam is perhaps the most common core materials used in composites because it is readily available and low cost. Used in a myriad of composite applications, this core material is "closed-cell," meaning, the foam can never absorb moisture. The following are reasons why composite manufacturers select polyurethane foam as a core material:
- Low cost
- Hi rigidity
- Easy to machine
- Good insulation R-value
- Bonds well to composite laminates
Polyurethane foam is commonly used in composite structures requiring insulation, such as structural insulated panels, known as SIPs. Common densities of this composite core material range from 2 - 12 pounds per cubic foot. Some of the major manufacturers of polyurethane foam include:
Another type of composite core material commonly used is honeycomb. This is perhaps the lightest weight option, as the majority of the core is air. Honeycomb cores are most often manufactured out of the following materials:
- Paper (such as fire-resistant Nomex)
- Plastic (most often polypropylene)
In honeycomb cores, the cells are open, and because of this they are not suitable for vacuum infusion and other processes using liquid thermoset resins. However, these cores are commonly used in aerospace applications in conjunction with pre-preg skins.
Manufactures honeycomb core include:
PET Foam Core
PET is a common plastic, best know for its use in soda bottles, carpet, and fleece clothing. Manufactures of PET foam extrude this plastic creating a composite core material that is similar to polyurethane. It is light weight, closed cell, and structural.
Perhaps the biggest advantage of PET foam is it's ability to thermoform. Meaning, under pressure and temperature this core material can take on the shape of the mold. Additionally, in theory, this foam can be recycled at it's end-of-life, similar to the recycling of soda bottles. PET foam manufacturers include:
EPS Foam Core
EPS foam core (expanded polystyrene) can be used in composites, but is not as common as the other mentioned core materials. Similar to a foam coffee cup, EPS is good at insulation and is lightweight, but it is also not very structural. EPS is extremely inexpensive and is used in building panels and in sporting equipment such as epoxy surfboards.
However, EPS cannot be used as a composite core material with thermosetting resins that use styrene. Solvents such as styrene will dissolve the EPS.
PVC foam is another structural composite core material made popular by the boat building industry. This type of foam is extremely consistent with little to no air bubbles, and has good mechanical and structural properties. It also bonds well to composite resins of all types. However, PVC foam is often more expensive then other types of composite core materials.
The two most common types of PVC foam are: