WO2004073948A2

WO2004073948A2 - Fibre-reinforced thermoplastics

Info

Publication number: WO2004073948A2
Application number: PCT/CA2004/000233
Authority: WO
Inventors: Douglas W. Lindstrom
Original assignee: Alberta Research Council
Priority date: 2003-02-20
Filing date: 2004-02-19
Publication date: 2004-09-02
Also published as: WO2004073948A3; CA2419345A1

Abstract

A fibre-reinforced prepreg is made of a plurality of fibres impregnated with a thermoset polymer, about which is disposed a thermoplastic coating. A plurality of prepregs may be consolidated into a composite, for example by pultrusion or by forming laminates. The composites formed thereby may be shaped and reshaped when the thermoplastic polymer is in a molten state.

Description

Patent Application

FIBRE-REINFORCED THERMOPLASTICS

FIELD OF THE INVENTION

The present invention relates to the field of thermoplastic pultrusion.

BACKGROUND OF THE INVENTION

Polymers can be reinforced by fibrous materials, such as fiberglass, to provide them with additional strength. One common method of making fibre-reinforced polymer is pultrusion, which is a process of producing continuous lengths of constant cross-section components using long continuous fibres held together with polymeric resin. In the pultrusion process a fibre reinforcing material is pulled through a resin impregnation bath and into a shaping die where the resin is subsequently cured. Traditionally the resin has been a thermosetting polymer, because these polymers have a low viscosity and are able to better penetrate between the fibres than a higher-viscosity thermoplastic polymer. However, these pultrusions are straight, and of constant cross section, and because pultrusions made with thermosetting polymers cannot be reformed once shaped, the attachment of such pultrusions to one another is limited to bolting, gluing, and the like. In fibre-reinforced laminates, sheets or lamina of polymeric material and fibrous material are stacked together, usually in a mold, and pressed at an elevated temperature under pressure to consolidate the material. As thermosetting polymers are often used to penetrate the fibrous material, these laminates suffer many of the same limitations as do pultrusions.

The substitution of a thermoplastic binder would allow reshaping of a pultrusion or laminate after it is formed, greatly increasing its utility in product development. It would also significantly increase the impact strength of the composite material.

The major problem with thermoplastic pultrusion and lamination is the impregnation of the tightly bundled fibres with the viscous thermoplastic resin. Results to date have generated composites that have a high void content and are poorly consolidated. In an attempt to make the thermoplastic pultrusion and laminating commercially viable, several types of processes have been developed, including: co-mingling, pre-impregnation, resin injection, powder coating and solvent baths. To achieve consolidation with existing

D SLegal\030319\00330\1725575vl technology, very slow pulling speeds, or long pressing times, are required. This not only defeats the potential economic advantage of the process, but introduces potential severe material degradation problems resulting from keeping the thermoplastic polymer above its melting point for extended periods of time. At this point, only thin cross-sections can be produced in a satisfactory manner.

SUMMARY OF THE INVENTION

Disclosed herein is a prepreg formed by impregnating a small bundle of reinforcing fibres with a thermosetting polymer to generate a thermoset fibre bundle, and coating the thermoset fibre bundle with a thermoplastic polymer. The diameter of the thermoset fibre bundle may be determined by bending considerations. The prepregs may then be consolidated into a composite, as by pultrusion through a consolidation die, or by lamination. The composite may be reshaped, by heating it to a temperature above the melting point of the thermoplastic polymer.

Accordingly, in one aspect, this invention provides a fibre-reinforced prepreg comprising a plurality of fibres impregnated with a thermoset polymer, and a thermoplastic coating disposed about said fibres and thermoset polymer.

In another aspect, this invention is a fibre-reinforced composite formed by consolidation of the prepregs of this invention. In one embodiment, the composite may be formed by assembling the prepregs in a consolidation die, for example by pultrusion, while the thermoplastic polymer is in a molten state. In yet another embodiment, the composite is formed by weaving the prepregs together into a woven layer, stacking the woven layers together and consolidating the layers.

In another aspect, this invention provides a laminate formed by consolidation of the prepregs of this invention.

In yet another aspect, this invention provides a method of making a fibre-reinforced prepreg comprising impregnating a plurality of fibres with a thermoset polymer, to generate a thermoset fibre bundle, and coating the thermoset fibre bundle with a layer of thermoplastic polymer.

DMSLegal\030319\00330\1725575vl In one embodiment of this method, pultrusion is used to impregnate the plurality of fibres with a thermoset polymer.

In yet another aspect, this invention provides a method of making a fibre reinforced composite, comprising pulling a plurality of prepregs through a consolidation die, and consolidating the plurality of prepregs.

In yet another aspect, this invention provides a method of making a fibre reinforced composite, comprising weaving a plurality of prepregs together to make a woven layer of prepregs, stacking a plurality of woven layers together; and consolidating the plurality of woven layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross section of an embodiment of a prepreg of this invention.

Figure 2A and B show cross sections of two different embodiments of a consolidation of prepregs.

Figure 3A shows one embodiment of a woven layer of prepregs and Figure 3B shows one embodiment of a laminate formed by consolidating several layers.

DETAILED DESCRIPTION

As shown in Figure 1, a prepreg 10 includes a plurality of fibres 12 impregnated with a thermosetting polymer 14, the fibres 12 and polymer 14 forming a thermoset fibre bundle 18 that has disposed thereabout a thermoplastic polymer coating 16. A plurality of prepregs 10 may be consolidated into a variety of different composites, such as for example those shown in Figures 2 and 3.

"Consolidation" as used herein means a process that compresses, or otherwise combines, fibre and thermosetting polymer together, or a process that compresses, or otherwise combines, prepregs, or prepregs and thermoplastic polymer together, all processes designed to reduce voids, to achieve a desired density, and to achieve a desired shape. The process may involve the application of pressure, and/or the application of sufficient heat to keep the thermosetting or thermoplastic polymer in a molten state until the desired density and/or shape may be achieved.

D SLegal\030319\00330\1725575vl "Composite" as used herein means a construction or manufacture that may be formed by consolidation of prepregs alone, or in combination with other polymeric materials.

"Polymer" as used herein includes a matrix or a resin.

"Prepreg" as used herein means a thermoset fibre bundle coated with a thermoplastic coating.

"Wet-out" as used herein refers to the impregnation of fibres with molten thermosetting polymer, such that voids between the fibres may be filled with polymer, and to the impregnation of the thermoset fibre bundles with molten thermoplastic polymer, such that voids between the thermoset fibre bundles may be filled with polymer.

The diameter of thermoset fibre bundle 18 may be determined by bending considerations. If one considers a thin rod or strip of material, the bend radius, meaning the radius of curvature that such a rod or strip can bend before material failure, is dependent upon the strip thickness, as the strain imposed by bending is roughly equal to the strip diameter divided by twice the radius of curvature of the bend. Typical glass fibres used in composite materials are 0.013 mm in diameter. If a layer or strip 0.013 mm thick is subjected to 1% strain without damage, the minimum bend radius that may be obtained is 0.65 mm. If several layers are stacked together and bent, the inside bend radius of such a stack could conceivably be as little at 0.65 mm, irrespective of the thickness of the stack. If larger bend radii can be tolerated, thicker layers may be used.

In normal composite design and fabrication, a bend radius of 3 mm may be sufficient for most purposes. If the material of construction is, for example, epoxy coated glass fibers that are able to withstand a strain of about 2.5%, then the rod or strip thickness required to prevent material failure may be about 0.15 mm, or about ten times the thickness of a single glass fibre.

A very small diameter bundle of individual fibres 12 is impregnated with thermosetting polymer 14 to form thermoset fibre bundle 18. Impregnation of the fibres with thermosetting resin may be accomplished in a variety of ways known to those skilled in the art, including for example pultrusion. The thermoset fibre bundle is then covered with thermoplastic polymer coating 16 to generate prepreg 10, a plurality of which may be

DMS egal\030319\00330\1725575vl consolidated into larger bundles or laminates, as described below. Although the thermoset fibre bundles are formed with a thermoset polymer, they are capable of bending, within the limits disclosed above and they may be used to form composites of a variety of shapes other than, but including, straight shapes.

Disclosed herein is a means for using the benefits of both thermosetting and thermoplastic polymers, in pultrusion and other technologies. A low viscosity thermosetting polymer may be used initially to achieve penetration of polymer into the fibre bundle, thereby avoiding the main problem associated with using thermoplastic polymers in pultrusion - poor penetration in to fibres. However, the diameter of the fibre bundle may be kept small enough to permit the thermoset fibre bundles to be capable of bending, and therefore the thermoset fibre bundles may be bent to some extent.

A thermoplastic polymer may be used to consolidate prepregs 10 into a composite, thereby adding to the workability and impact strength of the composite as compared to a fibre- reinforced composite made with a thermosetting polymer. Composites may be formed by consolidation of a plurality of prepregs. Consolidation may be achieved by pultrusion or by forming a laminate, however it is not limited to these processes. With regard to pultrusion, the prepregs may be drawn through a die and the thermoset fibre bundles 18 may then become embedded in the thermoplastic as the coating becomes molten and melds together. As the thermoset fibre bundles are larger in diameter than the fibres themselves, the penetration of the thermoplastic polymer between the thermoset fibre bundles may be improved over straight application of thermoplastic polymer directly onto a fibre bundle. Accordingly, provided herein is a fibre-reinforced composite made in one embodiment by pultrusion with a thermoplastic polymer and yet penetrated by the polymer. The composite may be shaped and reshaped unlike composites of the same size made with a thermosetting polymer alone. Likewise, also provided herein is a fibre- reinforced laminate penetrated by a thermoplastic polymer, and which may be shaped and reshaped.

The fibres 12 may be made of any of a number of suitable materials, including graphite, carbon, polymerics and glass. The fibres 12 may be assembled loosely together, as in a roving, or they may be woven, braided, or twisted together, as in a yarn. In one

DMSLegal\030319\00330\1725575vl embodiment, fibres 12 that are continuous along the length of the thermoset fibre bundles 18 may be used, however there may be no limitation as to fibre length.

Thermosetting polymers useful herein include thermosetting urethane and vinyl esters, polyesters, phenolics and epoxies. However, other thermosetting polymers may be used to make the thermoset fibre bundle 18 disclosed herein, provided that the polymer may be suitably bonded, as outlined below, to an appropriate thermoplastic polymer. Included herein are all such thermosetting polymers that are now known or which hereafter become known and which meet this requirement. The thermosetting polymer may include an amount of thermoplastic polymer or other additives.

As a non-limiting example, one method of manufacturing the thermoset fibre bundles 18 may be to pull strands of continuous fibres arranged in bundles of diameter Vτ mm to 2 mm, but not restricted to these dimensions, through a bath containing the thermosetting resin of choice. These wetted bundles may then be pulled through a shaping die that may or may not be heated. This removes excess resin and entrapped air from the bundle. Pulling speed may not be important and it has been found that 1 m/min through several 10's of m/min works adequately. Care may be taken to prevent undue bending of the fibers during this process, so that they do not break.

The thermoset fibre bundles 18 may be any shape in cross section and are coated with a layer of thermoplastic polymer, to make prepreg 10. The prepreg may, or may not be, circular in cross-section, and may vary in shape from the shape of the thermoset fibre bundle used to produce it.

Thermoplastic polymers useful herein include thermoplastic urethane, polystyrene and acetonitrile butadiene/styrene (ABS). Other thermoplastic polymers may be used to make the prepreg 10 disclosed herein, provided that the polymer selected may be bonded, as outlined below, to an appropriate thermosetting polymer. Included herein are all such thermoplastic polymers that are now known or which hereafter become known and which meet this requirement. The thermoplastic polymer may include an amount of thermosetting polymer or other additives.

The specific thermoplastic and thermosetting polymer combinations may be chosen such that the two types of polymers will bond integrally to one another to provide a composite

DMSLegal\030319\00330\1725575vl with adequate structural integrity for the purposes for which the composite will be used.

Additionally, the melt temperature of the thermoplastic polymer should not result in significant degradation of the thermosetting polymer. As non-limiting examples, the inventors have achieved suitable bonding between the following pairs of polymers: thermosetting urethane/thermoplastic urethane, vinyl ester/polystyrene and vinyl ester/ABS.

Sometimes it may be useful to treat the polymers or thermoset fibre bundles, to facilitate bonding between the thermoset and thermoplastic polymer. Bonding between the layers of polymer may also be achieved, as non-limiting examples, by applying an adhesive layer to the thermoset fibre bundle, or by roughening the surface of the thermoset fibre bundle to aid in mechanical keying of the thermoplastic polymer. If styrene-based thermoplastic/thermosetting polymer pairs are used, bonding between the two polymer types may be strengthened by dissolving some of the thermoplastic polymer in the thermosetting polymer, before using the thermosetting polymer in the pultrusion (Balazek and Griffiths, In situ Integration of Thermoplastic Extrusions into the Thermoset Pultrusion Process, 47^th Annual Conference, Composites Institute, The Society of the Plastics Industry Inc., Feb., 1992, s.lO-C; ρp.1-3; Balazek, et al. Pultrusion/Extrusion Method, U.S. Patent #4,938,823)

After wet-out of the fibres 12 with the thermosetting polymer 14, the thermoplastic coating 16 may be applied immediately, such as by using a coating die. The thermoplastic polymer coating 16 may be applied before or after curing of the thermosetting polymer. In one embodiment, coating 16 may be applied after the thermosetting polymer has gelled, and before curing, as a better bonding between the two polymer types may be achieved if this approach is used.

After application of the thermoplastic polymer coating 16 to thermoset fibre bundle 18, the coating may be allowed to cool to below its melt point. If this is done, the prepregs 10 may be spooled and coiled for storage and possibly transport to another location.

Alternatively, after coating the thermoset fibre bundle with thermoplastic polymer coating 16, the resultant prepreg 10 may immediately be consolidated with other prepregs to form composites.

DMSLegal\030319\00330\1725575vl One way of consolidating the prepregs may be to pass the prepregs 10 through a consolidation die by pultrusion. The prepregs 10 may be assembled loosely together, as in a roving, or they may be woven, braided, or twisted together, as in a yarn. If the thermoplastic coating is still in it's molten state, the prepregs may immediately be consolidated by the application of pressure and optionally, additional heat. If the thermoplastic coating has been permitted to cool to a temperature below its molten state, the thermoplastic coating may be heated until it melts, before consolidation of the prepregs

10 by the application of pressure and optionally, additional heat. As is apparent, additional thermoplastic polymer may be used during this process. The thermoset fibre bundles 18 may be bent, having reference to the bending limitations outlined above, in order to mold the final composite into a selected shape. The whole structure may then be cooled to below the thermoplastic melt temperature, to form a consolidated part.

Various embodiments of a composite made by pultrusion of thermoset fibre bundles 18, are shown Figure 2. Figure 2A shows a plurality of prepregs 10 that are consolidated, as indicated by arrow (a), into a composite 20 that is approximately circular in cross-section, and comprised of a plurality of thermoset fibre bundles 18 that are arranged randomly in the composite and that are of varying diameters. Figure 2B shows a composite 200 that is approximately rectangular is cross section and comprised of a plurality of thermoset fibre bundles 18 that are arranged in a regular pattern and that are of two consistent diameters.

Another means of consolidating the prepregs 10 is shown in Figure 3. Figure 3 A shows a plurality of prepregs 10 that are woven into a layer 30. A plurality of layers 30 may then be stacked together as shown in Figure 3B and consolidated into a laminate 40, as represented by arrow (a) in Figure 3B. The prepregs in layer 30 may additionally be consolidated before stacking the layers 30 together. The thermoset fibre bundles 18 may be bent, according to the bending limitations outlined above, in order to mold the laminate into a selected shape.

Alternatively, or in addition, layers 30 may be stacked together with layers of another material, such as additional thermoplastic polymer capable of bonding to layers 30. The entire stack of layers may then be consolidated, as by the application of pressure and optionally, heat, to form a laminate.

DMSLegal\030319\00330\1725575vl After the composite has been formed, such as by pultrusion or by making a laminate, the composite may be reshaped by heating the composite to a temperature above the melting temperature of the thermoplastic, molding it and cooling it to below the melting temperature of the thermoplastic.

While the invention has been described in conjunction with the disclosed embodiments, it will be understood that the invention is not intended to be limited to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

The following examples are intended only to illustrate and describe the invention rather than limit the claims that follow.

EXAMPLE

Profile Pultrusion A standard shape, such as a rod was pultruded using the vinyl ester and glass fibres to make the thermoset fibre bundle. An injection head to fit on a standard die was fabricated. The method of feeding molten thermoplastic polymer to the injection head was a small laboratory extruder. Thermoset fibre bundles were made with a diameter of typically lmm. These were pulled through a cross-feed injection head held at a temperature of about 250°C. Pulling speeds were approximate 1 m/min. The injection rate of the thermoplastic polymer, polystyrene, was the minimum allowed by the extruder. The prepregs were consolidated in a cylindrical die about 5 mm in diameter. After cooling in a water spray, the consolidated prepregs were easily bent over a pipe heated to about 200°C.

Scanning electron micrographs (SEM) of pultruded glass fiber roving-reinforced polystyrene composites (at a resolution is 100 μm) demonstrate that there is a very thin vinyl ester layer in the interface between the glass fiber and the polystyrene matrix. This interface is dim, indicating that that vinyl ester has a good bonding with polystyrene. The SEM's also show that the glass fiber roving is well impregnated with thermoset resin, as no void is found among the glass fibers.

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Claims

CLAIMSWe claim

1. A fibre-reinforced prepreg comprising:

(a) a plurality of fibres impregnated with a thermoset polymer, and

(b) a thermoplastic coating disposed about said fibres and thermoset polymer.

2. A fibre-reinforced composite formed by consolidation of a plurality of prepregs of claim 1.

3. The composite of claim 2 formed by:

(a) assembling the plurality of prepregs in a consolidation die, and

(b) consolidating the plurality of prepregs.

4. The composite of claim 3 wherein the plurality of prepregs are woven together before they are assembled in the consolidation die.

5. The composite of claim 2 formed by:

(a) weaving the plurality of prepregs together to make a woven layer of prepregs,

(b) stacking a plurality of woven layers together; and

(c) consolidating the plurality of woven layers.

6. The composite of claim 5 wherein a layer of another polymeric material that is able to bond with the woven layer upon consolidation, is inserted between one or more pairs of woven layers, before consolidation of the plurality of woven layers.

7. A fibre-reinforced composite formed by reheating the composite of claim 2, 3, 4, 5 or 6, to a temperature at which the thermoplastic polymer is molten, reshaping the composite, and cooling the composite to a temperature at which the thermoplastic polymer is no longer molten.

DMSLegal\030319\00330\1725575vl

8. A fibre-reinforced laminate formed by:

(a) weaving a plurality of prepregs of claim 1 together into a woven layer,

(b) stacking a plurality of woven layers together; and

(c) consolidating the plurality of woven layers.

9. The laminate of claim 8 wherein a layer of another polymeric material that is able to bond with the woven layer upon consolidation, is placed between one or more pairs of the woven layers.

10. A fibre-reinforced laminate formed by reheating the laminate of claim 8 or 9 to a temperature at which the thermoplastic polymer is molten, reshaping the laminate, and cooling the composite to a temperature at which the thermoplastic polymer is no longer molten.

11. A method of making a fibre-reinforced prepreg comprising:

(a) impregnating a plurality of fibres with a thermoset polymer, to generate a thermoset fibre bundle, and

(b) coating the thermoset fibre bundle with a layer of thermoplastic polymer.

12. The method of claim 11, wherein pultrusion is used to impregnate the plurality of fibres with a thermoset polymer.

13. A method of making a fibre reinforced composite, comprising:

(a) assembling a plurality of prepregs of claim 1 in a consolidation die, and

(b) consolidating the plurality of prepregs.

14. The method of claim 13, wherein pultrusion is used to assemble the plurality of fibres in the consolidation die.

15. A method of making a fibre reinforced composite, comprising:

DMSLegal\030319\00330\1725575vl (a) weaving a plurality of prepregs of claim 11 together to make a woven layer of prepregs,

(b) stacking a plurality of woven layers together; and

(c) consolidating the plurality of woven layers.

DMSLegal\030319\00330\1725575vl