Dimensionally-stable lightweight core for composite structures

A composition for a pressure molded core of a composite structure, including a thermal plastic resin. The composition also includes a component to reduce the coefficient of thermal expansion, a lightweight high compressive strength filler, and a blowing agent. The thermoplastic resin may be polyetheretherketone, the component may be carbon fibers, and the filler may be hollow glass microspheres.

FIELD OF THE INVENTION 
This invention generally relates to composite structures and, particularly, 
to a composition for a pressure molded core of a composite stucture. 
BACKGROUND OF THE INVENTION 
Many composite structures are fabricated with a core, such as a foam core, 
sandwiched between a pair of skins which may comprise laminated layers. 
The cores often are molded by thermoset or thermoplastic compounds which 
often are not dimensionally controlled or controllable. This is because 
the coefficient of thermal expansion can be different in three mutually 
perpendicular axes, and can be large, when compared to isotropic materials 
of construction, such as metals. A typical epoxy resin used in composite 
materials may have as much as four times the coefficient of expansion of 
metal. Fillers and reinforcements can be added to the resin to add 
dimensional stability. For example, long continuous fibers may be added, 
such as carbon or glass fibers, to lessen the thermal expansion. Such 
fibers can be very effective to control the expansion along the length of 
the fiber but are not very effective in controlling expansion 
perpendicular to their length. 
Attempts have been made to achieve isotropic thermal expansion control, 
while at the same time reinforcing and strengthening the material, by 
adding chopped fiber or fillers. This has become somewhat common and 
examples of such materials are carbon or glass fibers, clay, mica, Teflon, 
wollastonite, molybdenum disulfide, and a variety of other filler 
materials. However, such fibers often become oriented in a common plane, 
yielding low coefficients of thermal expansion in two directions but not 
the third. Although some degree of random orientation of the reinforcement 
has been achieved in thermoplastic resins, the result has not been totally 
satisfactory when forming relatively thin structures. Obviously, the 
reduction in the coefficient of thermal expansion corresponds to an 
increased amount of fiber disorientation. Another disadvantage in the use 
of fibers is that they are relatively heavy when lightweight core 
materials are desired. 
Other attempts to achieve the desired properties in a core material have 
evolved around the addition of a filler material comprising microspheres. 
This material reduces the weight of the core but does not possess the 
dimensional control provided by fibrous material, such as carbon fibers. 
The use of microspheres, sometimes called microballoons, can present an 
additional problem in that the microballoons are prone to be crushed under 
high pressures. This problem is magnified when thermoplastic resins are 
desired in the composition, whereby the thermoplastic resins are quite 
amenable to injection molding, compression molding or extrusion, which 
involves the application of high pressures. Combining microspheres with 
fibrous materials has been proposed but balancing the parameters involved 
has not achieved satisfactory results. To achieve the required density, 
and use fibrous materials, blowing agents may have to be added. The 
conclusion is that the use of thermoplastic resins has become quite 
desirable in core compositions, but with the high pressures involved, the 
dimensional stability desired, and other related parameters and problems 
encountered, a new and improved composition is needed. 
This invention is directed to satisfying the above need and solving the 
above problems. 
SUMMARY OF THE INVENTION 
An object, therefore, of the invention is to provide a new and improved 
composition for a pressure molded core of a composite structure. 
In the exemplary embodiment of the invention, the composition includes a 
thermoplastic resin, a component to reduce the coefficient of thermal 
expansion, a lightweight, high compressive strength filler and a blowing 
agent. 
In a preferred embodiment of the invention, the filler is made up of hollow 
microspheres, such as glass, selected from a group to have on the order of 
10,000 psi compressive strength to withstand the high compression forces 
of injection molding, compression molding or extrusion. Carbon fibers may 
be used as the component to reduce the coefficient of thermal expansion. 
The thermoplastic resin may be polyetheretherketone. The blowing agent may 
be a known agent, such as boric acid. 
A distinct advantage of glass or ceramic microspheres not only resides in a 
reduction of the density of the composition, but provides an unexpected 
beneficial increase in stiffness for the overall structure, as long as the 
integrity of the microspheres is maintained, i.e. preventing crushing or 
collapsing of the microspheres during high pressure of compressive 
fabrication processes.

DETAILED DESCRIPTION OF THE INVENTION 
Generally, the invention contemplates incorporating hollow microspheres, 
such as glass or ceramic microballoons, randomly oriented fibers such as 
carbon or glass, and a chemical blowing agent to the thermoplastic resin 
to achieve the desired end results in a core for a composite structure, 
wherein the core has low specific gravity, low and relatively isotropic 
coefficient of thermal expansion, and improved stiffness. Such materials 
are also useful in and of themselves as a lightweight structural material. 
For example, the thermoplastic resin may comprise polyetheretherketone, 
polyetherketone, poly arylene sulfide, poly arylene ketone, poly aryl 
sulfone, or poly aryl ether sulfone. Glass microballoons and short carbon 
fibers are added to the thermoplastic resin. This product is extruded 
through a face plate and then cut into short lengths, typically 1/8 inch 
long and 1/12 inch wide, in a cylinder. This injection molding pellet is 
treated with the same care as conventional polymer pellets in terms of 
protection from moisture, etc. 
The pellets then are mixed with a suitable amount of chemical blowing 
agent, such as boric acid, in the hopper of an injection molding machine. 
Alternatively, the blowing agent may be incorporated in the molding 
pellets themselves. The blowing agent could also be sodium tetra hydrido 
borate or potassium tetra hydrido borate. Normal precautions against 
contamination are followed by the use of shielded enclosures or dryers. 
The resin then can be injected molded into a suitable tool, with allowance 
for subsequent action of the blowing agent and the expansion of the 
material in the tool. Although generally flat or planar thicknesses of 
core material are common for subseqeunt sandwiching between outer skins or 
layers, molding the core composition of this invention can be done to 
fabricate irregular structures, such as housings for dynamoelectric 
machines, or the like. Other irregular shaped composite structures are 
contemplated. Copending applications Ser. No. 191,250, filed May 6, 1988, 
and Ser. No. 139,007, filed Dec. 29, 1987, which are assigned to the 
assignee of this invention are incorporated herein by reference. 
Since pressure molding techniques often employ pressures as much as 30,000 
psi, the microspheres should be selected from a group having proportionate 
size and wall dimensions to achieve a compressive strength on the order of 
10,000 psi. This will enable the microspheres to withstand the overall 
molding forces without crushing or collapsing. 
The composition of the invention may have up to 40% of a component to 
reduce the coefficient of thermal expansion, up to 40% of the lightweight 
high compressive strength filler, and between 0.01% and 0.25% blowing 
agent, with the thermoplastic resin making up the balance. 
The properties achieved by the above-described composition have been 
unexpected. The hollow microspheres yield lower density and also greater 
flexural and tensile stiffness. The short carbon fibers yield better 
control over other properties, such as providing a low coefficient of 
thermal expansion or better thermal or electrical conductivity. Lower 
fiction coefficients and higher impact strengths have been encountered 
than when using known core compositions which may be readily available. 
The added blowing agent reduces density, and parts have been fabricated as 
low as 0.5b/cc. The combination of the invention has achieved low density 
as well as the additional properties gained. 
The microspheres not only yield lower weight but obtain higher stiffness. 
The increase in tensile modules of such a lightened product has been 
unexpected. Expectations would appear to be a degraded value of stiffness 
with less density. Instead, the incorporation of the hollow microspheres 
has added greater stiffness in an unexpected fashion within the overall 
combination. Since many structural parts are stiffness-critical, rather 
than strength-critical, the invention has a wide range of applications. 
It will be understood that the invention may be embodied in other specific 
forms without departing from the spirit or central characteristics 
thereof. The present examples and embodiments, therefore, are to be 
considered in all respects as illustrative and not restrictive, and the 
invention is not to be limited to the details given herein.