GB1567367A - Fibre reinforces composites - Google Patents

Description

(54) FIBRE-REINFORCED COMPOSITES (71) We, HUGHES AIRCRAFT COMPANY, 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 fibre-reinforced composites.

Fibre-reinforced composites are conventionally prepared by the addition of natural or synthetic fibres to a liquid resin, and then solidifying the resin by freezing or curing to form a matrix around the fibres. The fibres may be, for example, in the form of loose strands, woven cloth or continuous lengths. However, the fibres as added to the matrix resin are in their final form and are not changed or improved in their inherent properties by becoming an intinìate part of the composite.

It is known to mix two immiscible polymers followed by extrusion of the mixture in a completely molten state, whereby some degree of orientation and fibre formation of one polymer within the other takes place. Fibrous materials have been extracted from such extruded mixtures by solvent extraction or by mechanical filtration of the matrix. Such an extrudate does not form a useful composite however.

Generally reinforced composites are prepared by impregnating strips of glass, carbon or other fibres into large sheets, cutting and laying-up the sheets into the desired configuration, and curing with heat and pressure, for example, in a press, autoclave or vacuum-bagged mould.

Composite structural materials having increased strength to weight ratios and low cost, are continually being sought.

According to the invention there is provided a method of forming a fibrereinforced composite, which comprises deforming a composite comprising a coherent isotropic structurally continuous (as defined herein) three-dimensional network of branched fibrils embedded in a thermoplastic matrix, with the composite at an elevated temperature which is above the softening point of the matrix and below the melting point of the fibres, the deformation being performed so as to orient the fibres, and cooling the composite while maintaining it in the deformed state so as to obtain a formed product.

By saying that the network is "structurally 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).

In the products obtained, the fibres have a degree of orientation, that is relative to the random orientation of the fibres in the initial composite, so that the stiffness of the composite and its strength in the direction of orientation of the fibres are enhanced.

Such composites may be in the form of rods, bars or flat panels. Multilayer composites may be made by laminating together a plurality of such panels with the directions of orientation of the fibres in the panels at various orientations to one another.

The composite to be deformed is preferably prepared by the method disclosed and claimed in our co-pending Patent Application No. 21220/77 (Serial No.

1,567,364) in which a thermoplastic matrix is used (our said application also describes the use of non-thermoplastic matrices).

The deformation may be effected by a tensile or compressive force, for example by pressing the composite at an elevated temperature in a mould which permits flow of the thermoplastic matrix in the desired direction(s) of orientation.

Fibres within the composite are thereby oriented and stretched by the viscous flow of the matrix.

In the accompanying drawings: Figures la and lb schematically illustrate typical apparatus for causing a composite to flow in a linear manner; Figure 2 represents a composite before and after a treatment according to the invention; Figure 3 is a photograph of a composite prepared in accordance with the invention wherein the fibrous reinforcement is radially oriented; Figure 4 is a photomicrograph of a broken end of a stretched composite formed according to the invention; and Figure 5 is a photograph of fibres which have been extracted from a composite prepared according to the invention and subsequently dried.

Figure la shows the deformation of a circular-sectioned composite 11 between flat platens 12 (one of which may be stationary). The composite is softened by heat and pressure is applied by means of the platen or platens to cause it to flow. If flow axially of the cylindrical composite is prevented by mould walls, flow will take place substantially only in the direction of the arrows shown in the Figure, that is it will be uniaxial.

This flow imposes a differential shearing action between the matrix and the fibres, thus causing alignment, orientation and stretching of the fibrous mass. The fibres are aligned or orientated substantially parallel to the direction of flow of the flowing polymer by the viscous drag forces. This phenomenon is enhanced by the fineness of the fibrous reinforcement.

The composite is also compressed or flattened by the application of pressure which causes the flow. The pressure is maintained and the composite is allowed to cool below the solidification temperature of the polymer matrix. When cooled, the fibres within the composite remain aligned or orientated. It is believed to be this alignment or orientation of the three-dimensional fibres within the cured matrix which causes the increases in thermo-mechanical and tensile strength of the pressed composite.

According to the invention composites may be prepared by hot press moulding (compression moulding) or injection moulding, whereby the resulting composite has enhanced mechanical and thermal properties.

In order that the invention may be more fully understood the following Examples and Tests, in which all percentages and ratios are by weight, unless otherwise indicated, are given by way of illustration only.

EXAMPLE 1 To 30 ml of xylene in a large test tube was added enough polyethylene and isotactic polypropylene in a ratio of 3:1 to yield a 4.5% concentration of polymers, which were dissolved at about 125"C and the test tube was placed on a shaker head.

The solution was allowed to cool while the frequency of oscillation of the head was decreased from 700 to 70 hertz, the decrease being periodically interrupted by vibration at a constant frequency, whereby a three-dimensional fibrous mass resulted. The fibrous mass was removed from the test tube and washed with hot acetone in a soxhlet extractor for 24 hours and then vacuum dried and impregnated with catalyzed styrene so that the fibrous mass became embedded in the styrene.

The latter was then cured at 500C for 7 days followed by post curing at 1000C for 20 hours.

A first cylindrical sample of the resulting composite was pressed into a sheet 0.050" thick by pressing between flat platens at 1500C (as shown in Figure la) so as to obtain substantially uniaxial orientation of the fibres. The cylindrical sample contained about one inch length of pure polystyrene containing no fibres, which was simultaneously pressed into sheet form, enabling the strength of unfilled polystyrene to be compared directly with the composite.

A second sample of the composite was pressed in a mould as shown in Figure lb (in which reference numeral 13 indicates unfilled polystyrene). The initial sample had a cross-section as indicated at the left hand side of Figure 2 and the resulting composite had a cross-section as indicated at the right hand side thereof.

A photograph, using polarized light, of a plug pressed in this manner is shown in Figure 3, in which radial orientation of the fibres can be clearly seen.

EXAMPLE 2 A composite was formed by polymerizing a 1/1 mixture of styrene and methylmethacrylate in which an isotactic fibrous mass of polypropylene had been grown, the concentration of the polypropylene in the resin prior to fibre formation being approximately 7%.

A bar of this composite of approximately one half inch in diameter was drawn at a temperature of 135"C (approximately 300C above the glass transition of the resin matrix) to approximately four times its original length.

Subsequently tensile specimens were cut from the stretched composite and compared with material which had not been subjected to stretching. The results were as follows: TABLE 1 Tensile Strength % Elongation P.S.I. at break a. Control (1:1 styrene/methyl methacrylate copolymer without reinforcement) 7.3x 103 4.3% b. Composite (with undrawn fibrils) 4.7x103 1.7 c. Drawn composite 13.6x103 16.1% A photomicrograph of a broken end of the stretched composite following the tensile test is shown in Figure 4. The fibrous structure is clearly evident.

EXAMPLE 3 Three composite sheets (prepared as for the first sample of Example 1) were laid one above the other with the prevailing fibre direction as follows: I st layer 0 2nd layer +600 with respect to layer 1 3rd layer -60 with respect to layer 1 The resulting crossplied laminate was tested for tensile strength and was found to have substantially isotropic properties.

Test 1 Pressed sheets with uniaxial fibre orientation were produced by the method employed for the first sample of Example 1. The fibres were subsequently extracted by dissolving the matrix in toluene at 500C for 4 hours followed by extraction with acetone. The extracted and dried fibres are shown in the photograph, Figure 5.

Attempts were made to measure the tensile strengths of these extracted fibre mats. It was impossible to determine the absolute cross-sectional area of material loaded because of the fibrillated nature of the mats. Approximate values of the cross-sectional areas were calculated from the sample weights and inherent bulk density (approximately 1 gm/cm3) in the following fashion.

W ab= Dc where ab=cross-sectional area W=sample weight D=bulk material density c=sample length Tensile strengths of about 13,000 psi were obtained by first measuring ultimate load-to-failure for each sample and using the approximate cross-sectional area values. Unfortunately, the measured load values must be considered approximate also because of the fact that the fibres are reticulated and not uniformly collimated along the loading or c axis.

Fibre tensile strengths were also determined by calculating their contribution to the composite tensile strengths in the following way.

(T.S.)comp7Vr(T.S.)r (T.S.) Vf where (T.S.)=fibre tensile strength (T.S.)r=resin tensile strength (T.S.)cOmp=composite tensile strength Vr=volume fraction of resin Vvolume fraction of fibres.

This method indicated fibre tensile strengths on the order of 80,000 psi. This value approaches the theoretical strength of 3.7x106 psi (H. Mark, "Polymer Science and Materials", Wiley-Interscience, N.Y. 1971, Chapter 11) which would obtain for perfect orientation and alignment of the fibre polymer molecules.

Tests 2 to 13 A number of composite sheets were prepared as for the first sample of Example 1, but with different degrees of deformation. A further number of sheets were prepared by pressing unfilled polystyrene in exactly the same manner. Finally, a number of control samples were prepared with and without reinforcement which samples were not pressed into sheet form. (The reinforced control samples were prepared as described for the first sample of Example 1, but without the pressing step).

The tensile strengths of these samples were measured in the direction of deformation and orientation, and the average for each type of sample is given in the following Table 2.

TABLE 2 Tensile Work to Number % Defor- Fibre concen- strength Failure Test of samples mation tration x 10-3 psi psi in/in 2 6 4700 7% 7.02 - 3 (comparative) 6 4700 0% 2.70 4 6 1000 7% 3.20 - 5 (comparative) 5 1000 0% 2.70 6 (comparative) 4 4700 0% 3.35 7 3 4700 7% 6.6 8 (comparative) 3 4700 0% 4.6 9 4 4700 7% 7.6 10 7 4700 7% 8.5 15.3 Il (comparative) 7 4700 0% 4.5 37.2 12(control) 6 0 0 3.0 13 (control) 9 0 7% 3.3 The samples of Tests 6 to 9 were all laminated sheets (both unfilled polystyrene and filled polystyrene).

The improvement in the properties of samples according to the invention can be seen by comparing the results of Tests 2 and 3, 4 and 5, 7 and 6, 9 and 8, and 10 and 11, respectively.

WHAT WE CLAIM IS: 1. A method of forming a fibre-reinforced composite, which comprises deforming a composite comprising a coherent isotropic structurally continuous (as defined herein) three-dimensional network of branched fibrils embedded in a thermoplastic matrix, with the composite at an elevated temperature which is above the softening point of the matrix and below the melting point of the fibres, the deformation being performed so as to orient the fibres, and cooling the composite while maintaining it in the deformed state so as to obtain a formed product.

2. A method according to Claim 1, in which the fibres are formed of

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. Fibre tensile strengths were also determined by calculating their contribution to the composite tensile strengths in the following way. (T.S.)comp7Vr(T.S.)r (T.S.) Vf where (T.S.)=fibre tensile strength (T.S.)r=resin tensile strength (T.S.)cOmp=composite tensile strength Vr=volume fraction of resin Vvolume fraction of fibres. This method indicated fibre tensile strengths on the order of 80,000 psi. This value approaches the theoretical strength of 3.7x106 psi (H. Mark, "Polymer Science and Materials", Wiley-Interscience, N.Y. 1971, Chapter 11) which would obtain for perfect orientation and alignment of the fibre polymer molecules. Tests 2 to 13 A number of composite sheets were prepared as for the first sample of Example 1, but with different degrees of deformation. A further number of sheets were prepared by pressing unfilled polystyrene in exactly the same manner. Finally, a number of control samples were prepared with and without reinforcement which samples were not pressed into sheet form. (The reinforced control samples were prepared as described for the first sample of Example 1, but without the pressing step). The tensile strengths of these samples were measured in the direction of deformation and orientation, and the average for each type of sample is given in the following Table 2. TABLE 2 Tensile Work to Number % Defor- Fibre concen- strength Failure Test of samples mation tration x 10-3 psi psi in/in 2 6 4700 7% 7.02 - 3 (comparative) 6 4700 0% 2.70 4 6 1000 7% 3.20 - 5 (comparative) 5 1000 0% 2.70 6 (comparative) 4 4700 0% 3.35 7 3 4700 7% 6.6 8 (comparative) 3 4700 0% 4.6 9 4 4700 7% 7.6 10 7 4700 7% 8.5 15.3 Il (comparative) 7 4700 0% 4.5 37.2 12(control) 6 0 0 3.0 13 (control) 9 0 7% 3.3 The samples of Tests 6 to 9 were all laminated sheets (both unfilled polystyrene and filled polystyrene). The improvement in the properties of samples according to the invention can be seen by comparing the results of Tests 2 and 3, 4 and 5, 7 and 6, 9 and 8, and 10 and 11, respectively. WHAT WE CLAIM IS:

1. A method of forming a fibre-reinforced composite, which comprises deforming a composite comprising a coherent isotropic structurally continuous (as defined herein) three-dimensional network of branched fibrils embedded in a thermoplastic matrix, with the composite at an elevated temperature which is above the softening point of the matrix and below the melting point of the fibres, the deformation being performed so as to orient the fibres, and cooling the composite while maintaining it in the deformed state so as to obtain a formed product.

2. A method according to Claim 1, in which the fibres are formed of

polyethylene, isotactic polypropylene, isotactic poly(butene- 1), polystyrene, polyethylene oxide or nylon.

3. A method according to Claim 1 or 2, in which the matrix is formed of polystyrene or a polystyrene/methyl methacrylate copolymer.

4. A method of forming a fibre-reinforced composite having orientated fibrous reinforcement, substantially as herein described in Example 1 or 2.

5. A fibre-reinforced composite having orientated fibrous reinforcement formed by a method as claimed in any of Claims 1 to 4.