Published 2/20/2020

Core materials: The basics

Core materials available for composite sandwich structures range from paper, aluminum and thermoplastic honeycomb cores to foams and more.

Honeycomb cores

Honeycomb cores range from paper and card for low strength and stiffness, low load applications (such as domestic internal doors) to high strength and stiffness, extremely lightweight components for aircraft structures. Honeycombs can be processed into both flat and curved composite structures, and can be made to conform to compound curves without excessive mechanical force or heating.

Thermoplastic honeycombs are usually produced by extrusion, followed by slicing to thickness. Other honeycombs (such as those made of paper and aluminum) are made by a multi-stage process. In these cases large thin sheets of the material (usually 1.2 × 2.4 meters) are printed with alternating, parallel, thin stripes of adhesive and the sheets are then stacked in a heated press while the adhesive cures. In the case of aluminum honeycomb the stack of sheets is then sliced through its thickness. The slices (known as ‘block form’) are later gently stretched and expanded to form the sheet of continuous hexagonal cell shapes.

In the case of paper honeycombs, the stack of bonded paper sheets is gently expanded to form a large block of honeycomb, several feet thick. Held in its expanded form, this fragile paper honeycomb block is then dipped in a tank of resin, drained and cured in an oven. Once this dipping resin has cured, the block has sufficient strength to be sliced into the final thicknesses required.

In both cases, by varying the degree of pull in the expansion process, regular hexagon-shaped cells or over-expanded (elongated) cells can be produced, each with different mechanical and handling/drape properties. Due to this bonded method of construction, a honeycomb will have different mechanical properties in the 0-degree and 90-degree directions of the sheet.

While skins are usually of FRP, they may be almost any sheet material with the appropriate properties, including wood, thermoplastics (eg melamine) and sheet metals, such as aluminum or steel. The cells of the honeycomb structure can also be filled with a rigid foam. This provides a greater bond area for the skins, increases the mechanical properties of the core by stabilizing the cell walls and increases thermal and acoustic insulation properties.

Properties of honeycomb materials depend on the size (and therefore frequency) of the cells and the thickness and strength of the web material. Sheets can range from typically 3-50 millimeters in thickness and panel dimensions are typically 1,200 × 2.400 millimeters, although it is possible to produce sheets up to 3 × 3 meters.

Honeycomb cores can give stiff and very light laminates but due to their very small bonding area they are almost exclusively used with high-performance resin systems such as epoxies so that the necessary adhesion to the laminate skins can be achieved.

Aluminum honeycomb

Aluminum honeycomb produces one of the highest strength/weight ratios of any structural material. There are various configurations of the adhesive bonding of the aluminum foil which can lead to a variety of geometric cell shapes (usually hexagonal). Properties can also be controlled by varying the foil thickness and cell size. The honeycomb is usually supplied in the unexpanded block form and is stretched out into a sheet on-site.

Despite its good mechanical properties and relatively low price, aluminum honeycomb has to be used with caution in some applications, such as large marine structures, because of the potential corrosion problems in a salt-water environment. In this situation care also has to be exercised to ensure that the honeycomb does not come into direct contact with carbon skins since the conductivity can aggravate galvanic corrosion. Aluminum honeycomb also has the problem that it has no “mechanical memory.” On impact of a cored laminate, the honeycomb will deform irreversibly whereas the FRP skins, being resilient, will move back to their original position. This can result in an area with an unbonded skin with much reduced mechanical properties.

Nomex honeycomb

Nomex honeycomb is made from Nomex paper — a form of paper based on Kevlar, rather than cellulose fibers. The initial paper honeycomb is usually dipped in a phenolic resin to produce a honeycomb core with high strength and very good fire resistance. It is widely used for lightweight interior panels for aircraft in conjunction with phenolic resins in the skins. Special grades for use in fire retardant applications (e.g. public transport interiors) can also be made which have the honeycomb cells filled with phenolic foam for added bond area and insulation.

Nomex honeycomb is becoming increasingly used in high-performance non-aerospace components due to its high mechanical properties, low density and good long-term stability. However, it is considerably more expensive than other core materials.

Thermoplastic honeycomb

Core materials made of other thermoplastics are light in weight, offering some useful properties and possibly also making for easier recycling. Their main disadvantage is the difficulty of achieving a good interfacial bond between the honeycomb and the skin.

Various types of thermoplastics produce different properties:

ABS: for rigidity, impact strength, toughness, surface hardness and dimensional stability
Polycarbonate: for UV-stability, excellent light transmission, good heat resistance and self-extinguishing properties
Polypropylene:for good chemical resistance
Polyethylene: a general-purpose low-cost core material.

Foam cores

Foams are one of the most common forms of core material. They can be manufactured from a variety of synthetic polymers including polyvinyl chloride (PVC), polystyrene (PS), polyurethane (PU), polymethacrylamide, polyetherimide (PEI) and styreneacrylonitrile (SAN). They can be supplied in densities ranging from less than 30 kg/m³ to more than 300 kg/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 5 to 50 millimeters.

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 came into use as core materials in the 70s. As sandwich structures in marine applications began to be optimized, a need for a low-density, consistent, moisture resistant core material was realized. Formulations were refined over the years and the characteristics of PVC foams fit the needs of the marine industry well. PVC foams are closed-cell, moisture resistant, and have good physical properties when compared to other foams of similar density.

PVC foam is technically an Interpenetrating Polymer Network (IPN) of PVC and polyurea. The interaction of these polymers gives the foam its unique characteristics. Although, the foam is a thermoset, it can still be thermoformed. In addition, it is resistant to many solvents including styrene and most fuels, yet is compatible with most adhesives and laminating resins. PVC foam is closed-cell and has very low moisture absorption. It is self-extinguishing and will not rot. Other inherent properties are excellent fatigue life and good bond strength with common adhesives and resins.

PVC foams are available in a variety of densities from 3pcf to 25 pcf (45 kg/m³to 400 kg/m³). Unlike polyurethane foam, PVC foam is not a two-component pour in place type of foam. The manufacturing process is much more involved. The raw ingredients are mixed together under controlled conditions and dispensed into a mold. The filled mold is then sealed, placed in a large press and heated while remaining clamped shut. After this process is completed, a slab of solid material emerges from the mold. The material is then expanded in a hot water bath to final density and cured. The cured blocks are then cut into sheets of a varying thickness, depending on customer requirements. The process and formulation combine to give the foams their closed-cell nature and excellent physical properties.

PVC foams can be formulated in rigid or ductile versions. Rigid PVC foams, sometimes referred to as crosslinked, have higher heat and solvent resistance than ductile foams. In addition, the physical properties are generally 20-40% higher. Shear strain to failure for rigid PVC foams varies with density between 12-30%. Ductile PVC foams, sometimes referred to as linear, have higher shear elongation to break, typically in excess of 40%. However, ductile foams have lower physical properties, heat and solvent resistance for a given density.

Marine grade rigid PVC foams can be processed up to 180°F and used continuously up to 160°F. They do not get brittle at cold temperatures and can even be used in cryogenic applications. Specialized grades of PVC foams can be processed up to 250°F and used continuously up to 190°F. Ductile PVC foams can be processed up to 150°F and used continuously at 120°F.

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 uncrosslinked 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 HT 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. Typical products include Divincell HD grade.

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 stabilization 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.

Polystyrene foams

Although polystyrene foams are used extensively in sail and surf board manufacture, where their light weight (40 kg/m³), low cost and easy to sand characteristics are of prime importance, they are rarely employed in high performance component construction because of their low mechanical properties. They cannot be used in conjunction with polyester resin systems because they will be dissolved by the styrene present in the resin.

PMI (Polymethacrylimide) foams

For a given density, PMI (polymethacrylimide) foams such as ROHACELL offer some of the highest overall strengths and stiffnesses of foam cores. Their characteristics also include high dimensional stability, a closed cell structure and high fatigue life, and they can be cured and used at elevated temperatures. Their overall cost and performance characteristics mean that, to date, their use has mainly been in higher performance composite parts such as helicopter rotor blades, ailerons and stringer profiles in pressure bulkheads.

Styrene acrylonitrile (SAN) co-polymer foams

SAN foams behave in a similar way to toughened cross-linked PVC foams. They have most of the static properties of cross-linked PVC cores, yet have much higher elongations and toughness. They are therefore able to absorb impact levels that would fracture b

SAN foams are replacing linear PVC foams in many applications since they have much of the linear PVC’s toughness and elongation, yet have a higher temperature performance and better static properties. However, they are still thermoformable, which helps in the manufacture of curved parts. Heat-stabilized grades of SAN foams can also be more simply used with low-temperature curing prepregs, since they do not have the interfering chemistry inherent in the PVCs. Typical SAN products include ATC Core-Cell’s A-series foams.

Polyurethane foams

The polyurethane foam world is very large and diverse — chances are good you are sitting on some kind of flexible polyurethane foam right now — but the useful products for composite-core applications are rigid foams.

The term “rigid polyurethane foam” comprises two polymer types: Polyisocyanurate formulations, and polyurethane formulas. There are distinct differences between the two, both in the manner in which they are produced, and in the performance of the results.

Polyisocyanurate foams. Polyisocyanurate foams (or “trimer foams”) are generally low density, insulation-grade foams, usually made in large blocks via a continuous extrusion process. These blocks are then put through cutting machines to make sheets and other shapes. Polyisocyanurate foams have excellent insulating value, good compressive-strength properties, and temperature resistance up to 300°F. They are made in high volumes at densities between 1.8 and 6 pounds per cubic foot, and are reasonably inexpensive. Their stiff, brittle consistency, and their propensity to shed dust (friability) when abraded can serve to identify these foams.

It is this friability that limits the utility of polyisocyanurate foams in composite panel applications, as this lack of toughness at the foam surface can cause failure of the foam-to-laminate bond under conditions of vibration or flexure. For this reason, structural use of these foams is often limited to internal-mold shapes for stringers and hat-section reinforcements in FRP boat construction. Here the foam has no supportive function but to provide a form for the fiber-and-resin composite laid over it.

Other uses include under-slab insulation in cold-storage buildings, and below-grade insulation for other building structures.

Polyurethane foams. Polyurethane foams, on the other hand, are considerably different, and more useful in composite constructions. These foams are made in large blocks in either a continuous-extrusion process, or in a batch-process. The blocks are then cut to make sheets or other shapes. They are sometimes also individually molded into discrete part-shapes.

Isocyanate foam polymers, while not as heavily cross-linked as polyisocyanurate materials, offer many cost-efficient advantages for users. Foam densities range from approximately 2 pounds per cubic foot, up to 50 pounds per cubic foot. Unlike thermoplastic foams (PVC, SAN), the unit cost of polyurethane foam increases in a more linear fashion with density; e.g., a 20-pound per cubic foot polyurethane foam will be approximately twice the cost of a 10-pound foam.

There can be considerable differences in foam strength, at the same density, depending on the foam production process used. This results from differences in chemical formulation required to make foams via different production methods, and the curing temperature of the foam while in production.

Also, if flammability is a concern, it is useful to know what kind of blowing-agent is employed to create cells in the foam. Many producers use carbon-dioxide (a by-product of the foam-making chemical reaction) to create cells in their foams. Other producers have switched from chlorofluorocarbon (HCFC, HFC) blowing agents to pentane in low-density foam manufacturing processes, which can have a deleterious effect on flame-resistance.

Polyurethane polymer foams can be made considerably tougher and less-friable than the polyisocyanurate foams, mostly at the expense of some modulus and high-temperature strength properties. Nevertheless, these foams can be useful (depending on formulation) to temperatures as high as 275°F, while retaining a substantial portion of their strength and toughness. This allows them also to be used in panel applications along with high-temperature curing prepregs, cured in ovens or autoclaves.

Typical applications include use as an edge close-out for honeycomb aircraft-interior panels, structural shapes (transom cores, bulkhead core, stringers, motor-mounts etc.) in FRP boat building, impact-limiters and crash-pads, RTM cores, mold-patterns and plugs, sports-equipment core material, and composite tooling.

Polyisocyanurate/Polyurethane Foams

There are producers of polyisocyanurate/polyurethane foams, a blending of the two foam types, trying to get the best of both worlds. These foams offer some improvements in strength values (compared to polyurethane foams) and a reduction of friability (compared to polyisocyanurate foams) with a sacrifice in temperature resistance.

Still, the result of this combination is a compromise, and may not present the best properties of both polymers in some applications. These foams are limited to densities of 2-8 pounds per cubic foot.

For the latest on core materials used in composites manufacturing, please see compositesworld.com/zones/core.