WO2014078496A2

WO2014078496A2 - Expanding foam core prepreg

Info

Publication number: WO2014078496A2
Application number: PCT/US2013/070033
Authority: WO
Inventors: Scott T. Burr; Jay M. Tudor; Siddharth Ram ATHREYA; Thomas J. Parsons
Original assignee: Dow Global Technologies Llc
Priority date: 2012-11-19
Filing date: 2013-11-14
Publication date: 2014-05-22
Also published as: WO2014078496A3

Abstract

A composite prepreg kit is disclosed that comprises two layers of prepreg with an expandable pre-foam core therebetween, wherein the core optionally further comprises a fibrous material. Also disclosed are methods of manufacturing composite materials from the prepreg kits, and articles made from the prepreg kits.

Description

EXPANDING FOAM CORE PREPREG

FIELD OF THE INVENTION

The present invention relates to composite materials comprising two or more layers of prepreg with a layer of foam between, to prepreg kits for manufacturing composites, and to methods of manufacturing the kits and composites.

INTRODUCTION

When molding articles based on prepreg material systems, there are two basic molding methods commonly used in the industry, autoclave molding, and compression molding.

Autoclave molding involves a single-sided mold placed in a vacuum bag, all placed in an autoclave to provide heat and pressure. The mold half provides a shape and structure on which to build up the prepreg layers and form the kit. Once the required layers have been placed on the mold half in the specified locations, the mold half on which the kit has been built is placed in a vacuum bag, and full vacuum is pulled on the mold half. This step, called the de- bulking step, removes trapped air. In some cases where the buildup is complex and requires many layers, several de-bulking steps may be required.

Once the kit is completed on the mold half and has been de-bulked to remove all entrapped air, the mold half is again placed inside a vacuum bag and placed in an autoclave. The autoclave applies heat and extreme pressure to the outer layer of the kit and forms it onto the mold half. During this heated and pressurized process, the prepreg materials cure and the final composite part is formed.

The autoclave molding process is very labor intensive, usually has a cure time on the order of hours, and is not amenable to high volume production of composite parts.

The second process is termed "compression molding". In this process, the kit may or may not be formed on one side of the molding tool. In the simplest form, the kit is formed on a flat table, placed into the compression mold, and the force of the compression molding machine forces the prepreg kit to conform to the mold. Application of heat results in a cured part. This process is used for either thermoset or thermoplastic materials. The key characteristic of compression molding is that the mold has faces that are "matched," meaning that when the mold is closed, each half of the mold forms the required shape and also the required thickness.

One of the difficulties in compression molding of composite prepreg systems is the accuracy needed in the thickness of the prepreg kit, which results in getting the proper amount of compression of the kit in the molding process. When, for a given application, a composite article needs to have different thicknesses in different areas, this results in a significantly increased accuracy requirement for tooling and resulting kit formation.

U.S. Patent publication 2008/0241576 discloses light weight composites with high flexural strength comprise epoxy foam sandwiched between two layers of facing material have high strength and low weight and can be used to replace steel structures. The facing layer may be fibrous material especially glass or carbon fibres, the facing material is preferably embedded into the epoxy matrix. Alternatively they may be matching box structures or concentric metal tubes. The sandwich structures may be prepared by laying up the fibre; coating and/or impregnating the layer with epoxy resin, laying a layer of heat activatable foamable epoxy material, providing a further layer of the fibrous material optionally coated and/or impregnated with epoxy resin on the foamable material and heating to foam and cure the epoxy materials. Alternatively they may be formed by extrustion of the foamable material between the surface layers.

U.S. Patent No. 4,798,763 discloses a method of molding and forming a laminated foamable sheet which contains a solid catalyst that is activated at the molding temperature of the laminate. A laminated foamable sheet is first provided by saturating a glass fiber mat with a urethane foam composition containing a solid catalyst of stearate salt and then sandwich it between two layers of cover sheet. The solid catalyst contained in the urethane composition cannot be activated until it is heated to the molding temperature of the laminated sheet. The laminated foamable sheet is then positioned into a mold having two heated platens and a cavity defined therein with the mold temperature set at a temperature of at least that of the melting point of the solid catalyst contained in the urethane composition. The solid catalyst in the composition melts and causes the urethane precursors to start the foaming reaction such that the laminate expands and fills the cavity of the mold. After a sufficient length of time during which the foamed sheet is cured under heat and pressure, a completed part in the shape of the cavity is removed from the mold.

U.S. Patent Publication 2001/0007269 discloses a process for producing a composite structural element, which includes the steps of providing a thin-section wall part, placing the thin-section wall part into a mold, applying reinforcing elements to the thin-section wall part, placing a counter-mold onto the mold for forming a mold cavity, and introducing a binder having a foaming agent into said mold cavity via one of injection cannulas and nozzles, after a set time delay a foaming of the binder occurring for encapsulating the reinforcing elements on all sides.

There remains a need in the industry for strong prepreg systems amenable to high volume production.

There remains a need for more manufacturing prepreg kits for variable-thickness components that with simplified designing and manufacturing of molds.

There remains a need for lightweight composite materials that are lower cost and suitable for high- volume production.

There remains a need for composite materials that are lighter and lower cost as a result of a foam core.

SUMMARY OF THE INVENTION

It has been surprisingly found that an article comprising two or more layers of prepreg with a heat activatable expandable foam therebetween can be used to economically

manufacture articles having remarkable strength properties.

Without being bound by theory, it is believed that the present invention removes the higher cost and higher density carbon fiber from the less efficient center portion and replaces it with lower cost, low density expandable foam, such as epoxy foam. It is believed that the expandable epoxy moves the high modulus carbon fiber layers away from the neutral axis to increase bending stiffness (I-beam approach to increasing the stiffness).

The present invention provides a laminar prepreg kit comprising a first and a second prepreg layer and an expandable pre-foam layer therebetween, the first and second prepreg layers respectively comprising a first and second fibrous material impregnated with a first and second heat-curable pre-polymer material; the expandable pre-foam layer comprising a heat- activatable pre-foam material that is capable of expanding into a polymeric foam; wherein, when exposed to a temperature capable of activating the heat-activatable pre-foam material, the heat-activatable pre-foam material is capable expanding before the first and second heat- curable epoxy materials set.

The present invention also provides a method of manufacturing a laminar composite article comprising: obtaining a laminar prepreg kit comprising a first and a second prepreg layer and an expandable pre-foam layer therebetween; the first and second prepreg layers respectively comprising a first and second fibrous material impregnated with a first and second heat-curable pre-polymer material; the expandable pre-foam layer comprising a heat-activatable pre-foam material that is capable of expanding into a polymeric foam; enclosing the laminar prepreg kit in a mold having a first and a second side; applying sufficient heat to activate the heat-activatable pre-foam material, wherein the heat-activatable pre-foam material expands, the first prepreg layer is pressed toward the first side of the mold, and the second prepreg layer is pressed toward the second side of the mold, to obtain an intermediate article; applying heat sufficient to cure the first and second heat-curable epoxy materials in the intermediate article; allowing the first and second heat-curable epoxy materials in the intermediate article to cure to form a cured article; and removing the cured article from the mold.

Moreover, the present invention provides a method of manufacturing a laminar prepreg kit comprising: providing a first and a second prepreg layer respectively comprising a first and second fibrous material impregnated with a first and second heat-curable pre-polymer material; providing a core composition comprising a heat-activatable pre-foam material that is capable of expanding into a polymeric foam; forming the core composition into a pre-foam layer that is between, and in contact with, the first and second prepreg layers; wherein when exposed to a temperature capable of curing the first and second heat-curable epoxy materials, the heat- activatable pre-foam material is capable expanding before the first and second heat-curable epoxy materials set. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically illustrates a cross section of a laminar prepreg kit according to the present invention.

Fig. 2 schematically illustrates one method of manufacturing a laminar prepreg kit according to the present invention.

Fig. 3 schematically illustrates a method for manufacturing a laminar composite article according to the present invention.

Fig. 4 shows structures of comparative and inventive composites of the Examples.

Fig. 5 shows the 3-point bend finite element model of the Examples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to cured laminar composite articles and to processes for their manufacture from an uncured or partially cured laminar prepreg kit.

The invention provides a laminar prepreg kit comprising an expandable pre-foam core within the outer layers of prepreg. When activated, e.g., when heat from a compression mold is transferred into the kit, the foam is designed to rapidly and forcefully expand well before the prepreg material has cured, thereby pressing against each prepreg layer against a side of the matched mold. This enables the use of variable thickness prepreg kits without having to accurately design and manufacture a matched mold.

The expanding system could be tailored to meet the needs for any particular application by delivering the right amount of force at the right time and temperature during the molding process, while also providing the needed structural requirements in the final cured laminar composite article.

As seen in Fig. 1, laminar prepreg kit 1 comprises three layers: prepreg layers 2 and 3, and an expandable pre-foam layer (or core) 4 therebetween.

Prepreg as used herein refers to a fibrous material that has impregnated with a curable material (e.g., an epoxy) and then is preferably partially cured.

Any fibrous material can be used for the prepreg, and many are well-known to those of skill in the art. Some fibrous materials include para-aramids (e.g., Kevlar or Twaron), fiberglass, aramids, boron, alumina, silicon carbide, or quartz fibers. Fibrous materials may be woven, non- woven, or unidirectional, woven being preferred. Combinations of these are also included, such as two or more layers of woven or unidirectional sheets, preferably with different orientation.

Uncured prepregs can be tacky, which can make them more difficult to handle.

Accordingly, prepregs that are non-tacky or have reduced tackiness are preferred. In this regard, the use of dispersion based prepregs may be preferred in the present invention over a traditional liquid resin infused prepregs. A dispersion based prepreg may provide a tack-free surface which may provide benefits in the manufacturing process or provide improved handling characteristics. Suitable dispersion-based resins (e.g., epoxy, polyurethane, and/or thermoset) include those known in the art. Some preferred dispersion-based resins include those described in U.S. Patent Application Nos. 61/599,062 and 61/599,068 (both filed February 15, 2012), 61/603,455 and 61/603,463 (both filed February 27, 2012). and in U.S. Patents 5,539,021, 5,688,842, and 6,156,806, all of which are incorporated by reference in their entireties.

Alternatively, a prepreg can be partially cured, preferably prior to manufacturing the laminar prepreg kit. That is, the prepreg can be B-staged, e.g., to hold the shape.

The expandable pre-foam layer preferably comprises a polymerizable pre-polymer and a foaming agent. Various chemical and/or physical blowing agents may also be used.

The pre-polymer can comprise any suitable material, and may comprise, without limitation, epoxy, polyurethanes, expandable thermoplastic systems, and many others.

In a preferred embodiment, the prepreg prepolymer and the pre-foam pre-polymer are selected so that the two polymers bond securely when cured. In a more preferred embodiment, the same type of prepolymer is used in the pre-foam as in the prepreg. For example, an epoxy- based prepreg is preferably used with an epoxy-based prefoam, a polyurethane-based prepreg is preferably used with a polyurethane-based prefoam, a thermoplastic-based prepreg is preferably used with a thermoplastic-based prefoam, etc.

In a preferred embodiment, a toughened epoxy resin is used, which may be in the foam/pre-foam, in the prepreg, or both. By toughened epoxy resin is meant an epoxy resin with a toughening agent such as an elastomeric toughener. A preferred toughened epoxy resin comprises an epoxy resin, an elastomeric toughener, a rubber (e.g., a core-shell rubber), a curing agent, and (when in the pre-foam) a foaming agent. Some suitable toughened epoxy resins are described in U.S. Patent Application No. 12/510,323, filed on July 28, 2009, which is incorporated herein in its entirety.

The pre-foam can comprise any suitable foaming agent, that is, any material that expands into a foam, or allows the core to expand into a foam, when exposed to suitable conditions. A heat activatable pre-foam material is preferred. The pre-foam material is preferably activated by the same type of condition (e.g., heat) as the prepreg. The pre-foam material preferably rapidly expands to an expanded foam, that is, it preferably expands and reaches final volume on a time scale that is short relative to the curing time of the prepreg.

The expandability of the foam should depend on the particular application for which the composite is intended. The foam preferably expands sufficient that its density is less than the density of the prepreg, thereby reducing the weight of a finished composite article compared to one that does not have a foam core, e.g., compared to one that comprises one or more layers of prepreg and no foam core. The foam is preferably capable of expanding by at least 50% of its original volume, more preferably by at least 100% of its original volume, more preferably by at least 150% of its original volume, or 200% of its original volume.

If the foam expands too much, this might result in a loss of strength and/or might reduce the ability of the laminar composite article to absorb impact energy. The foam is preferably capable of expanding up to 400% of its original volume, more preferably up to 350% of its original volume, more preferably up to 300% or 250% of its original volume.

In general, it is preferred to use a foam that is capable of expanding by a greater amount than the free space provided in the mold. For example, one can use a foam that expands 200% maximum (triples in volume), where the mold only provide 100% of free space compared to the amount of material put into the mold. This helps to ensure that the foam fully fills the mold cavity. This also helps to provide enough expansion pressure to get good adhesion. The ratio of foam expansion capability (percentage increase) to mold free space (percentage relative to material in the mold) is preferably greater than 1, more preferably greater than 1.25, 1.5 or 1.75. The ratio is preferably less than 4, more preferably less than 3.5, 3, or 2.5. Some preferred ratios include 1.9, 2 and 2.1.

The density of the expanded foam need not be uniform, but can be different in different parts of the article. For example, in an area where a mold physically confines the foam, the foam might be limited in its capacity to expand. Therefore, expanded foam in such an area could be more dense than in an area where the foam is permitted more space to expand.

The core layer may also comprise one or more additional materials that improve or alter properties of the laminar prepreg kit and/or the laminar composite articles made therefrom.

The core layer may comprise a material to physically increase strength, e.g., stiffness or crash durability. The core layer may comprise, for example, a fibrous material, such as carbon fiber or carbon fiber/resin, either of which may be new or recycled. Use of recycled material in this manner can help to provide valuable reinforcement to the expanded foam and/or expandable pre-foam; decrease overall cost of the laminar composite articles; and/or provide a useful application for recycled materials. Other fibrous materials may be used in the core layer, such as woven or non-woven textiles, e.g., non-woven carbon fiber, para-aramids (e.g., Kevlar or Twaron), fiberglass, aramids, boron, alumina, silicon carbide, or quartz fibers. Moreover, one or more geometric reinforcement layers may be included in the expanding foam layer. The geometric reinforcement layer may be in addition to, or instead of, any fibrous materials. Some preferred geometric reinforcement layers include honeycomb, metal lattice, or screens.

If the prepreg has holes or pores, it is preferred that the expanding foam be prevented from, or not be capable of, flowing into the holes or pores in the prepreg. Accordingly, in a preferred embodiment, the expanding foam is not capable of flowing into the prepreg, e.g., through holes in the prepreg. This can be accomplished by, for example, encapsulating the pre- foam, by using an intermediate layer between the pre-foam and the prepreg, or by using a sufficiently non-porous prepreg. A preferred method is encapsulation, in which the pre-foam material may be encapsulated in a polymeric envelope such that the polymeric envelope expands as the foam expands, but does not allow the expanding foam to flow through small holes or openings in the outer layers. A preferred encapsulation method is shown in U.S.

Patent Application No. 12/510,323. In some embodiments, it may be preferred to permit the expanding foam to penetrate the prepreg. However, if the expanding foam is permitted to expand into the surface layer this could disrupt fiber alignment of the surface layer. This would be more likely to occur if the surface layer is not a prepreg, e.g., if it is a woven textile not pre-impregnated with a pre- polymer. Accordingly, in some embodiments this feature is not preferred.

Laminar prepreg kits can be made in many different ways designed by persons of ordinary skill in the art. One exemplary method is schematically shown in Fig. 2. As shown, rolls 21 and 22 hold the prepregs (e.g., resin impregnated woven carbon fiber). Pump/reservoir 25 applies (laminates) a layer of the expandable pre-foam material 23 between prepreg layers 26 and 27. The resulting three-layer laminar prepreg kit 20 may be rolled up onto take-up roll 28, which may optionally include a barrier sheet or release film (not shown) to prevent adjacent layers of the prepreg from contacting and/or adhering to each other. Alternatively, laminar prepreg kit 20 may be cut and stacked or otherwise processed and/or transported prior to forming into a cured laminar composite article.

While Fig. 2 shows only one prepreg on each surface, the present invention includes use of two or more prepreg layers on one or both sides. When a multiple prepreg layer is used, adjacent prepregs may or may not have foamable material therebetween, and may or may not have a non-expanding material (e.g., an epoxy or polyurethane layer) therebetween. The present invention includes multiple layers of prepreg and pre-foam. Some possible

arrangements can include, for example, prepreg-pre-foam-prepreg; prepreg-pre-foam- prepreg-pre-foam-prepreg; prepreg-pre-polymer-prepreg-pre-foam-prepreg; etc. Other arrangements are also possible. Use of two or more prepreg with non-expanding material therebetween can maintain or increase stiffness in the outer fiber layer.

An exemplary method of making a cured laminar composite article is schematically shown in Fig. 3. Referring to Fig. 3a, laminar prepreg kit 30 is placed into mold 33, which may comprise first face 31 and second face 32. Referring to Fig. 3b, the mold is closed, which may leave gaps 34 and/or 35 between prepreg 30 and faces 31 and/or 32. Depending on the shape of the mold and the thickness of the laminar prepreg kit, gaps 34 and 35 may be continuous, discontinuous, or absent. Referring to Fig. 3c, heat is applied causing the pre-foam core to expand to form foam 36, causing prepregs 37 and 38 to be pressed against mold faces 31 and 32 (which would be in addition to any pressure exerted by the mold itself prior to foam expansion). Continued application of heat causes pressed prepregs 37 and 38 to cure. As shown in Fig. 3d, after curing is complete, the mold is opened and cured laminar composite article 39 is removed.

The inventive expandable foam core compensates for variable prepreg thicknesses in a prepreg parts without making changes in the mold. The core is self adjusting to changes in thickness. The expandable foam core smoothens the transitions between multiple prepreg layers. The expandable foam core provides internal mold pressure to possibly help overall part quality. The expandable core allows for using thinner prepreg sheets which will help in draping in the mold.

The inventive prepreg kit can be used immediately or soon after manufacture, or it can be stored , e.g., for transportation and/or later use. If stored, depending on the properties of the prepreg and/or pre-foam, it may be preferred to store the inventive prepreg kit under controlled conditions, e.g., under refrigeration, in order to prevent or slow the curing process. It is preferred to use prepreg and/or pre-foam that do not require refrigeration both for cost of handling, and for extended shelf life.

It will be understood that the temperature that activates expansion of the pre-foam (the foaming temperature) need not be the same as the temperature that activates curing of the prepregs (the curing temperature), and that each of the prepregs may also cure at different temperatures depending, e.g., on their compositions. Regardless the temperatures required for foaming and curing, it is preferred that the pre-foam expands and exerts pressure on the prepreg before the curing process causes the prepreg to set. That is, it is preferred that the pre-foam expand while the prepregs are still sufficiently flexible in order to allow the expanding foam to press the prepregs against the mold, and to take the shape of the mold where appropriate, without compromising (e.g., weakening or breaking) the prepregs. It is preferred that the foaming temperature be about the same as, or lower than, the curing temperature. If expansion of the foam is fast enough compared to the rate at which the prepreg sets and/or cures, then it would be acceptable for the foaming temperature to be higher than the curing temperature(s). Therefore, after the foam expands, the temperature can be increased, decreased, or maintained at the same level, until curing is complete.

Moreover, the expanding foam typically generates heat (exothermic reaction) which can help cure the prepreg layers, especially if two or more separate prepreg layers are used on one or both surfaces.

Any type of mold can be used, preferably a heated mold. The mold preferably contains space to allow expansion of the laminar prepreg kit to the desired final thickness. It will be understood that depending on the complexity of the shape of the desired laminar composite article, the mold may comprise two or more sections. Thus, for example, the "first face" of the mold that contacts that first prepreg may comprise two or more pieces of a mold. It is understood that this might cause a seam or line to appear in the laminar composite article where the sections of the mold come together. If desired, the seam or line can be removed or hidden by further processing (e.g., sanding and painting), or can be positioned where it would not be noticeable (e.g., the interior surface of a car panel).

EXAMPLE

Some embodiments of the invention will now be described in the following Examples, wherein all parts and percentages are by weight unless otherwise specified. The following describes an example composite created from an expandable epoxy foam in comparison to a standard prepreg composite. As schematically shown in Fig. 5a, the comparative carbon fiber and epoxy resin prepreg composite for comparison purposes uses 4 layers total (0/90 weave, -45/45 weave, -45/45 weave, and 0/90 weave) and has no pre-foam core. The inventive composite schematically shown in Fig. 4b comprises a layer of expandable epoxy between two layers of 0/90 weave impregnated woven carbon fiber. The core comprises expandable epoxy with blowing agent according to Example 3 of U.S. Application 12/510,323 (incorporated by reference), capable of 210% expansion, and is baked for 30 minutes at 177° C. Expansion room in the mold is 100% based on volume of the prepreg kit.

The two composite parts are evaluated using finite element analysis (FEA). To evaluate, a 2"x8" beam of each composite is subject to a three point bend load. The beam is simply supported on each end, and is loaded in the center (20 loads of IN each, uniform across the width of the beam) to bend. Fig. 5 shows the FEA model used. The deflection of the beam at the center is monitored and used to compute the stiffness.

Table 1 shows the properties used to define the comparative and inventive composite beams. E is Young's modulus (GPa), G is shear modulus (GPa), and υ is Poisson's ratio.

Table 1

The finite element analysis is completed using several iterations on each beam. For each iteration the thickness of the beam is changed, in order to generate a weight versus stiffness curve for each of the composites. The stiffness is calculated using the maximum deflection at the loading point divided by the applied load (20 N). Tables 2 and 3, respectively, show the attributes, weight and stiffness results, for the comparative composite, and the inventive expandable core composites.

Table 2

Layer Total

Max Deflection Stiffness Total Weight Thickness Thickness Load (N)

(mm) (N/mm)

(mm) (mm) (g)

0.4 1.6 2.209 20 9.05 25.27

0.6 2.4 0.655 20 30.53 37.90

0.8 3.2 0.276 20 72.46 50.54

1.0 4.0 0.141 20 141.84 63.17 The layers of the comparative composite (Table 2) are applied symmetrically, with layer 1=0/90 weavel; layer 2=-45/+45; layer 3=-45/+45 weave; and layer 4=0/90 weave.

Table 3

The layers of the inventive composite (Table 3) are applied symmetrically as shown in

Fig. 4(b) (0/90 weave; prefoam; 0/90 weave).

The inventive expandable core composites have advantages over the comparative prepreg composite. The inventive expandable core composite provides approximately a 100% increase in stiffness at equivalent weight to the comparative composite. Conversely, the inventive expandable core composite can provide approximately a 25% reduction in weight at equal stiffness. The inventive 30% carbon fiber filled expandable core composite increases the stiffness over the unfilled version, though the carbon fiber also adds weight. The present invention provides lighter weight at equal bending stiffness.

A variety of section sizes can be created to customize the composite based on the needs of the application. In addition, various numbers of woven fiber plies can be used based on the application.

Claims

CLAIMS:

1. A laminar prepreg kit comprising a first and a second prepreg layer and an expandable pre-foam layer therebetween,

the first and second prepreg layers respectively comprising a first and second fibrous material impregnated with a first and second heat-curable pre-polymer material;

the expandable pre-foam layer comprising a heat-activatable pre-foam material that is capable of expanding into a polymeric foam;

wherein, when exposed to a temperature capable of activating the heat-activatable pre- foam material, the heat-activatable pre-foam material is capable expanding before the first and second heat-curable epoxy materials set.

2. A method of manufacturing a laminar composite article comprising:

obtaining a laminar prepreg kit comprising a first and a second prepreg layer and an expandable pre-foam layer therebetween; the first and second prepreg layers respectively comprising a first and second fibrous material impregnated with a first and second heat-curable pre-polymer material; the expandable pre-foam layer comprising a heat-activatable pre-foam material that is capable of expanding into a polymeric foam;

enclosing the laminar prepreg kit in a mold having a first and a second side;

applying sufficient heat to activate the heat-activatable pre-foam material, wherein the heat-activatable pre-foam material expands, the first prepreg layer is pressed toward the first side of the mold, and the second prepreg layer is pressed toward the second side of the mold, to obtain an intermediate article;

applying heat sufficient to cure the first and second heat-curable epoxy materials in the intermediate article;

allowing the first and second heat-curable epoxy materials in the intermediate article to cure to form a cured article; and

removing the cured article from the mold.

3. A method of manufacturing a laminar prepreg kit comprising:

providing a first and a second prepreg layer respectively comprising a first and second fibrous material impregnated with a first and second heat-curable pre-polymer material; providing a core composition comprising a heat-activatable pre-foam material that is capable of expanding into a polymeric foam;

forming the core composition into a pre-foam layer that is between, and in contact with, the first and second prepreg layers;

wherein when exposed to a temperature capable of curing the first and second heat- curable epoxy materials, the heat-activatable pre-foam material is capable expanding before the first and second heat-curable epoxy materials set.

4. The prepreg kit of claim 1, or the method of claim 2 or 3, wherein the heat- activatable pre-foam material comprises a foaming agent and a prepolymer selected from an epoxy resin prepolymer, a polyurethane prepolymer, or a thermoplastic prepolymer.

5. The prepreg kit or method of any of the above claims wherein the heat- activatable pre-foam material further comprises a third fibrous material.

6. The prepreg kit or method of any of the above claims wherein the third fibrous material comprises carbon fiber, para-aramid fibers, fiberglass, aramid fibers, boron fibers, alumina fibers, silicon carbide fibers, or quartz fibers.

7. The prepreg kit or method of any of the above claims wherein the first and second fibrous materials comprise a para-aramid, fiberglass, an aramid, boron fibers, alumina fibers, silicon carbide fibers, or quartz fibers.

8. The prepreg kit or method of any of the above claims wherein all three of the first and second heat-curable pre-polymer material and the heat activatable pre-foam material comprise an epoxy resin prepolymer, comprise a polyurethane prepolymer, or comprise a thermoplastic prepolymer.

9. A cured composite article made from the prepreg kit or method of any of the above claims.

10. The prepreg kit, method, or cured article of any of the above claims wherein the pre-foam material is encapsulated.