GB1567366A - Fibre reinforced composites - Google Patents

Description

(54) FIBRE-REINFORCED COMPOSITES (71) We, HUGHES AIRCRAFT COM PANY, a company organised and existing under the laws of the State of Delaware, United States of America, of Centinela and Teale Street, Culver City, State of California, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to fibrereinforced composite structural materials.

Fibre reinforced composites commonly contain chopped reinforcing fibres in curable resms. Such fibre-reinforced composites have high moduli, percent elongation and strength, but these properties deteriorate on repeated load cycling.

According to the present invention there is provided a composite which comprises a coherent isotropic structurally continuous (as defined herein) three-dimensional network of branched fibrils, said network being embedded in an elastomeric matrix.

By saying that the network is "structually continuous", we mean herein that the fibrils merge into one another at branches that do not contain discontinuities (that is, the fibrils are integrally connected to one another).

The three-dimensional network of polymeric fibres may be prepared from a solution of a fibre-forming polymer by a method as described and claimed in our copending Patent Applications Nos. 21220/77 (Serial No. 1,567,364) and 21221/77 (Serial No. 1,567,365). Such a method involves the precipitation of a polymer from solution when the solution is simultaneously cooled and agitated. The solvent in this method is either polymerizable or extractable.

The fibres are preferably formed of a linear polyolefin, for example, polyethylene, isotactic polypropylene, isotactic poly(butene-l) or isotactic poly(4methylpentene- 1).

The elastomeric matrix (which is of an extensible resin having a pliable, resilient nature) should have the following properties: (a) a sufficiently low viscosity to flow through and impregnate the fibrous network (with or without the application of pressure); (b) a cure temperature below the melting point of the polymer fibres; and (c) a pot-life sufficient to permit complete impregnation before curing.

Suitable elastomers include, for example, curable polyurethanes, silicones, unsaturated carbon chain rubbers, plastisols (particularly plastisols having low fusion temperature) and flexibilized epoxy resins.

A preferred silicone comprises a vinyl terminated dimethyl siloxane, a silane curing agent and a platinum catalyst. Such a silicone is available under the Trade Mark Sylgard 182 from Dow Corning Corporation.

A preferred flexibilized epoxy resin comprises a cocondensate of glycerol and bisphenol A with an epoxide (such as epichlorohydrin), together with an anhydride curing agent. Such an epoxy resin is available under the Trade Mark Scotchcast 280 from the 3M Company.

The elastomer may be introduced into the network either by using a monomer as the solvent in forming the network and then polymerizing the monomer to form the elastomer, or by impregnating the preformed network with a prepolymer which is then cured.

Composites according to the invention are strong and extensible and they retain matrix flexibility during small deformations (they are pliable and resilient). They increase in strength with repeated deformation or stretching and they increase in toughness with repeated loading (a work-hardening or inverse hysteresis effect). Finally, they have good retention in ultimate stretch since the percent elongation at ultimate strength is not decreased by fibre loading.

The behaviour described above is apparently due to the facts that the network may endure large deflections and distortions -of its original shape and return to that original shape without injury, and the individual fibrils of the network develop additional strength with deformation or stretching within the extensible matrix. This may be analogous to the known increase in the strength of fibres caused by drawing.

Another unusual aspect of the composites according to the invention is that, unlike conventional structural fibre-reinforced composites, compatibility between the fibres and matrix (good wetting and adhesion) is not necessary. For example, although polypropylene fibres are somewhat wettable by the above-mentioned Scotchcast, they are not wettable by the above-mentioned Sylgard. The network structure maintains its geometrical integrity upon repeated stretching and relaxation and assures load transfer to the matrix.

Composites according to the invention are useful in structural applications such as the fabrication of flexible electrical components, belts for mechanical power transmission, seals, O-rings, tyres, gaskets and in any application requiring high strength with elastomeric characteristics.

In order that the invention may be more fully understood the following examples are given by way of illustration only.

Example 1.

Polymer solutions were prepared by dissolving isotactic polypropylene in xylene at 125 C. The resulting solutions were vibration ally shaken while cooling until homogeneous fibrous masses were formed.

Subsequently, the xylene was removed from the fibrous masses by Soxhlet extraction with acetone. The masses were then dried in vacuum for 24 hours.

A curable Scotchcast 280 composition was made by mixing 40 parts by weight of resin with 60 parts by weight of hardener.

The mixture was stirred and then heated to 1500F for 15 minutes to effect thorough mixing. The mixture was then outgassed in vacuo until bubble formation ceased and a pressure as low as 10-3 mm Hg was attained.

The polypropylene fibrous mass prepared as described above was impregnated in an evacuated flask. The outgassed resin was introduced slowly via a separatory funnel.

Each impregnation was conducted in a manner which allowed the resin to permeate the fibrous mass from the bottom to an extent which left an excess resin cover. The composites were cured under a positive pressure of 80 psi for 48 hours at 160"F.

Example 2.

Fibrous masses were prepared in similar fashion to Example 1. A curable Sylgard 182 composition was made by mixing 100 parts by weight of resin with 10 parts by weight hardener. The mixture was thoroughly mixed and outgassed.

The fibrous mass was impregnated as in Example 1, and the resin cured for 16 hours at 160"F without positive pressure application.

In the accompanying drawings: Figure 1 is a stress-strain curve in which a composite according to Example 1 (referred to in the drawing as "In Situ Fiber Composite") is compared with a commercial fibre-reinforced Scotch cast and with unfilled Scotchcast; Figure 2 is a stress-strain curve in which a composite according to Example 2 is compared with unfilled Sylgard; Figure 3 illustrates the effect of cyclic deformations on the physical properties of unfilled Sylgard; Figure 4 illustrates the effect of cyclic deformations on the physical properties of a composite according to Example 2; Figure 5 illustrates the effect of load cycling on an unfilled resin sample; and Figure 6 illustrates the effect of load cycling on a composite according to the invention.

The stress-strain curves shown in Figure 1 clearly show the increase in toughness (area under the curve) of the composite according to Example 1 compared with the unfilled material (control), while the modulus (slope of the curve) and the ultimate elongation are little changed. This small change in the ultimate elongation and modulus, and the increase in toughness, are in contrast to the properties indicated in Figure 1 (Commercial Fiber Composite) for Scotchcast containing chopped commercial polypropylene fibres (highly drawn, high strength fibres having a tensile strength of about 40,000 psi) in the same amount (about 7 Ó) as fibres are present in the composite of Example 1.

The strain behaviour of the conventional composite is significantly reduced, even at low strains, and it has reduced elongation with stress at all levels.

Figure 2 shows that the toughness of the composite according to Example 2 has increased as a result of incorporation of the "in situ" formed fibres, as has the percent elongation. The ultimate strength decreased a little because the non-wetting silicone matrix begins to separate at the higher .elongations. Nevertheless, the composite is a tougher material than the unfilled material over deformations of practical interest, i.e.

pre-ultimate.

Figures 3 and 4 illustrate cyclic stressstrain curves (between 0 and 0.5 strain) for unfilled Sylgard and the composite of Example 2 respectively. The unfilled material behaves elastically with little dissipation in energy. In contrast, the composite of Example 2 is capable of dissipating energy, yet increases in strength with repeated load cycling.

Figures 5 and 6 illustrate the weakening effect of load cycling on unfilled Sylgard and the strengthening effect of load cycling on the composite according to Example 2 respectively.

WHAT WE CLAIM IS: 1. A composite which comprises a coherent isotropic structurally continuous (as defined herein) three-dimensional network of branched fibrils, said network being embedded in an elastomeric matrix.

2. A composite according to claim 1, in which the polymeric fibres are formed of at least one linear polyolefin.

3. A composite according to claim 2, in which the polyolefin is one or more of polyethylene, isotactic polypropylene, isotactic poly(butene- 1) and isotactic poly(4-methyl pentene-l).

4. A composite according to any of claims 1 to 3 in which the matrix comprises a curable polyurethane, a silicone, an unsaturated carbon chain rubber, a plastisol or a flexibilized epoxy resin.

5. A composite according to claim 4, in which the silicone comprises a vinylterminated dimethyl siloxane, a silane curing agent and a platinum-containing catalyst.

6. A composite according to claim 4, in which the flexibilized epoxy resin is derived from glycerol, bisphenol A and epichlorhydrin, and contains an anhydride curing agent.

7. A composite substantially as described herein in Example 1 or in Example 2.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. over deformations of practical interest, i.e. pre-ultimate. Figures 3 and 4 illustrate cyclic stressstrain curves (between 0 and 0.5 strain) for unfilled Sylgard and the composite of Example 2 respectively. The unfilled material behaves elastically with little dissipation in energy. In contrast, the composite of Example 2 is capable of dissipating energy, yet increases in strength with repeated load cycling. Figures 5 and 6 illustrate the weakening effect of load cycling on unfilled Sylgard and the strengthening effect of load cycling on the composite according to Example 2 respectively. WHAT WE CLAIM IS:

1. A composite which comprises a coherent isotropic structurally continuous (as defined herein) three-dimensional network of branched fibrils, said network being embedded in an elastomeric matrix.

2. A composite according to claim 1, in which the polymeric fibres are formed of at least one linear polyolefin.

3. A composite according to claim 2, in which the polyolefin is one or more of polyethylene, isotactic polypropylene, isotactic poly(butene- 1) and isotactic poly(4-methyl pentene-l).

4. A composite according to any of claims 1 to 3 in which the matrix comprises a curable polyurethane, a silicone, an unsaturated carbon chain rubber, a plastisol or a flexibilized epoxy resin.

5. A composite according to claim 4, in which the silicone comprises a vinylterminated dimethyl siloxane, a silane curing agent and a platinum-containing catalyst.

6. A composite according to claim 4, in which the flexibilized epoxy resin is derived from glycerol, bisphenol A and epichlorhydrin, and contains an anhydride curing agent.

7. A composite substantially as described herein in Example 1 or in Example 2.