WO1989003759A1

WO1989003759A1 - Composite material

Info

Publication number: WO1989003759A1
Application number: PCT/GB1988/000931
Authority: WO
Inventors: Dennis John Williams; Gerald Francis Day; Frank Robinson
Original assignee: Courtaulds PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1987-10-30
Filing date: 1988-10-28
Publication date: 1989-05-05
Anticipated expiration: 1990-04-30
Also published as: GB8725470D0

Abstract

A composite material having improved strength comprises a matrix material (5) reinforced by a fabric of knitted yarn (12) embedded within the matrix material (5). The yarn (12) comprises surface fibers (13) which project laterally from the main body (11) into regions (5, 10) of the matrix material located within knitted loops (6) of yarn so as to reinforce those regions.

Description

Composite Material Technical Field

This invention relates to a composite material reinforced with knitted fabric. Background Art

Good strength is normally achieved in composite materials if a high proportion of the reinforcing fibre or yarn is oriented so that its longitudinal direction lies in the direction along which strength is required. This has led to a preference in many applications of composite materials for woven fabric reinforcement, rather than knitted fabric reinforcement because the convoluted nature of the stitch structure in a knitted fabric often means that only a minor proportion of yarn will be oriented in a direction in which good tensile properties are required.

On the other hand, the compliance of knitted fabric gives it advantages in conforming to mould profiles when the fabric is incorporated in a matrix material such as a ther osetting resin. It is therefore desirable to produce knitted fabrics which give improved performance in the reinforcement of composite materials. Disclosure of Invention

According to this invention, a composite material having improved strength comprises a matrix material reinforced by a fabric of knitted yarn embedded within the matrix material characterised in that the yarn has a main body and surface fibres which project laterally from the main body which fibres project into the regions of matrix material located within the knitted loops of yarn so as to reinforce those regions.

The surface fibres of the yarn may be in the form of fibres having free ends at the extremity of projection or in the form of filament loops. The terms "fibres" and "filaments" are used interchangeably except where the context requires otherwise. The yarn is preferably of the core/effect type but may also be a spun yarn having a hairy surface. Suitable core/effect yarns include texturised continuous filament yarns, particularly air-texturised yarns having an effect portion in the form of projecting filamentary loops, boucle yarns, knop yarns and other yarns of the so-called fancy effect type.

The proportion by weight of the projecting surface fibres (the effect portion in a core/effect yarn) is preferably at least 10 per cent and more preferably at least 30 per cent of the total weight of the yarn. A preferred range, particularly for core/effect yarns, is for the projecting surface fibres to be 40 to 60 per cent by weight of the yarn with the main body (or core in a core/ effect yarn) making up the difference.

The improvement in strength possessed by the composite material of the invention compared with a composite knitted from a flat (i.e. untexturised) continuous filament yarn is believed to derive from the reinforcement of the resin-rich regions enclosed by the yarn loops. This reinforcement is provided by the surface fibres projecting into those regions from the main body of the yarn constituting the knitted loops. Crack propagation which can proceed unchecked across resin-rich regions can be stopped or minimised because any path which a crack might follow without meeting a fibre can be made tortuous rather than straight by the presence of the fibres. Improved results in this regard are obtained if the surface fibres are of sufficient length that when they project into the region within the knitted loops there is overlapping of fibres projecting from adjacent or opposite parts of the knitted loops.

The yarn preferably comprises high strength fibres or filaments made from one or more of the materials comprising carbon, aromatic polyamides, silicon carbide, ceramics and glass. Core/effect constructions in which the effect portion is combined with the core portion during yarn manufacture or processing allow the use of different filaments or fibres in the respective portions. For example, the core portion may be of high strength filaments and the effect portion of high strain filaments to give a balance of properties in the composite material.

The matrix material may be of any suitable material including a thermosetting resin, a thermoplastics resin, carbon or a ceramic material. Suitable thermosetting resins include epoxy, polyester and phenolic resins.

In addition to the knitted fabric reinforcement, the matrix material of the composite material according to the invention may incorporate particulate or short fibre fillers including talcum powder, silica powder and rubber or glass microspheres

The composite material of the invention may be further improved by knitting the fabric in a structure which produces lengths of unlooped float yarn in the fabric. In a double jersey, weft knitted fabric such floats can be produced in the course direction, for example, as in a cross float jersey tuck fabric. It is also possible to lay in additional yarns along the course direction with weft knitted fabrics or along the wale direction in the case of warp knitted fabrics. Both of these techniques can give considerable improvements in strength in the direction of the float or of the laid-in yarn. However, this can be at the expense of a balance of strength properties in the wale and course directions of the fabric which balance is desirable when the composite material is required to have isotropic properties. Another technique which can be employed to vary the strength of the composite material in the wale and course directions of the fabric is to vary the shape -of the knitted loops. In a single jersey fabric knitted from flat, continuous filament yarn, the usual elongated loop structure produces greater wale strength then course strength. Course strength can be improved by changing the shape of the loops so that their width is increased at the expense of their height. This change in shape tends to occur when using the bulky, surface effect yarns employed according to the invention. These yarns also make it possible to control loop shape to suit the desired balance of properties.

According to another aspect of the invention, a process for making a composite material by knitting a fabric from yarn and embedding that fabric in a matrix material as reinforcement therefor is characterised by knitting the fabric from a yarn having a main body and surface fibres which project laterally from the main body, tensioning the fabric during the knitting process and/or subsequently to produce knitted loops of the desired shape for optimum reinforcement and maintaining that desired shape during subsequent processing into the composite material by the inter-engagement of the surface fibres projecting from adjacent portions of yarn.

Tensioning during the knitting process may be varied by the tensioning effect of the fabric draw-down rollers. When a flat bed weft knitting machine having a presser foot facility is used then the pressure of the presser foot can also be controlled. After knitting, if the loop shape is not as desired, then tensioning of the fabric longitudinally will increase the length of the loops to give greater wale direction strength to the composite material. Lateral tensioning, for example by stretching the fabric out on a stenter, can be employed to widen the loops and improve course direction strength.

In a knitted construction having, for example, laid-in yarn in the course direction, this may change the balance too far in favour of the course direction for reasonable isotropy. This balance may be improved in favour of the wale direction by pulling the fabric to elongate the loops.

The use of the yarn as specified according to the invention allows the fabric to be maintained in the desired loop configuration whilst it is being processed into a composite material. This is made possible because the fibres projecting from the main body of the yarn inter- engage and thereby hold adjacent portions of yarn in the set position. This effect promotes general stability of the fabric and facilitates handling in assembly operations involved in the manufacture of composite structures. The effect is especially marked when the projecting surface fibres are in the form of loops.

Weft knitting, particularly employing a machine with a presser foot facility, can be used to produce three- dimensionally shaped structures which can constitute a pre-for of a composite structure. In addition to providing the benefits of increased strength outlined above, the use of the specified yarn to knit such structures facilitates handling of the shaped structure and helps to maintain it in the desired shape during incorporation in the matrix material.

Brief Description of the Drawing

The invention will be further described by way of example, with reference to the accompanying drawing, in which: Figure 1 shows diagrammatically the configuration of two knitted loops in a composite material,

Figure 2 is an enlarged section on the line A-A of Figure 1 extended to show links of a loop of yarn adjacent to that shown in Figure ___. ,

Figure 3 is a diagrammatic representation of two interconnected loops of a fabric knitted from yarn having surface fibres projecting in the form of loops, and

Figure 4 is a diagram including a graph, illustrating the variation in fibre concentration in a region such as that of the section A-A in Figure 1, plotted against displacement across the region. Modes for carrying out the Invention

In a composite material reinforced with knitted fabric made from a flat continuous filament yarn (i.e. uncrimped and untexturised) , the eye in the head of the loops, for example that region 5 of the loop 6 in Figure 1, will be rich in matrix^' material (e.g. resin) and will contain relatively little yarn. The effect is to some extent exaggerated in Figure 1, because only two knitted loops are shown, but along a section line such as A-A in Figure 1, yarn from other loops will often be largely in a different plane from the yarn of loop 6. In fact, micrographs of composite materials reinforced with knitted fabric have been obtained which correspond closely with Figure 2 and show that crack propagation in the composite material tends to follow a line such as the broken line 7, passing round the fibre reinforcement 8, in this case a yarn, and extended straight across resin-rich regions 5 and 10. By having surface fibres projecting into the resin-rich areas 5 and 10, the strength of the composite material can be considerably improved. Crack propagation in pure resin regions requires less energy than where fibres are present to act as reinforcement. Thus, if a yarn 12 as specified according to the invention is used to knit the reinforcing fabric, as shown in Figure 3, the strength of the resulting composite material will be increased. In this case, the projecting fibres, projecting from the main body 11 of the yarn, are in the form of loops 13 of filaments such as would be produced in a core/effect, air-texturised continuous filament yarn or a boucle yarn.

The redistribution of the fibre reinforcement within the matrix of the composite material which is achieved by using a yarn as specified to produce the knitted reinforcing fabric, is illustrated in Figure 4. This shows a graph of fibre concentration (as defined above) along the y-axis against the distance moved along the x-axis by the element of volume used to determine the fibre concentration. The variation in fibre concentration is illustrated in relation to the two limbs 14 and 15 of the head of a knitted loop along the section shown in the upper part of Figure 4 (i.e. a section such as the section A-A in Figure 1). Flat, untexturised continuous filament yarn will produce a fibre concentration curve such as that indicated by reference numeral 16. Use of a yarn as specified according to the invention will result in a curve such as 17 and, if a sufficient proportion of surface fibres project from the main body of the yarn, the curve 17 may be modified by the introduction of the portion 18 shown in dotted line. This may be produced by the effect of the overlapping of the surface fibres projecting from the two limbs 14 and 15 of the knitted loop into the region between them.

By using a yarn in which filaments in the main body of the yarn (for example the core portion or a proportion of it in a core/effect yarn) are made of a high tensile strength fibre such as high strength polyamide, higher strength composite materials can be achieved. The laterally projecting surface fibres in the yarn may advantageously be a high strain fibre, preferably having a greater strain to break than the matrix material of the composite material. High tensile strength filaments in the body of the yarn may, of course, advantageously be combined with high strain filaments projecting laterally from the main body, for example in loop form.

The invention is illustrated by the following Example.

Example

Two fabrics A and B, were knitted in the same construction on a 5 gauge flat bed knitting machine having a presser foot facility. Both fabrics were knitted using continuous filament glass yarns of 1660 overall decitex comprising individual filaments of 6 microns diameter.

However, the yarn used to make fabric A, which was the control fabric, was a flat, untexturised yarn whereas the yarn used to make fabric B was an air-texturised yarn made using a "Taslan" (Registered Trade Mark) type of air-texturising jet. The latter yarn comprised a core portion of essentially straight filaments from which projected an effect portion comprising loops of filaments projecting laterally from the core portion. The effect or loop portion of this yarn constituted 57 per cent by weight of the total yarn weight.

The fabric construction of both fabrics was a weft knitted double jersey construction in which the two layers of jersey were joined by a tuck stitch. Each jersey layer was knitted using three yarn feeds with the yarns knitting on successive needles so that each yarn knitted on every third needle and floated across the two intermediate needles in a so-called 2 1 cross float jersey structure. Fabric A had a weight of 1,800 gms/sq.metre and fabric B had a weight of 1,413 gms/sq. metre. Composite materials were made using each of fabrics A and B by pouring a thermosetting resin onto the respective fabrics and working the resin into the fabrics manually.

The resin used was an epoxy resin made by Ciba-Geigy Limited coded LY 1927 GB with hardener HY 960.

For each fabric, A and B respectively, two plies of resin-impregnated fabric were placed with their wales in alignment in a compression press pre-heated . to a temperature of 60°C. and compressed to squeeze out excess resin. The resin was then allowed to cure over 9 minutes to produce a 3 mm thick plate of composite material having a volume fraction of glass fibres of 50 per cent in the case of fabric A and 45 per cent in the case of fabric B. Rectangular test samples were cut from each of the plates some with the fabric wales in alignment with the long edges of the samples and some with the fabric courses in such alignment. The samples were then tested using an Instron apparatus for tensile strength and tensile modulus according to the test method ASTM/D/3039 and for flexural strength and flexural modulus according to the test method ASTM/D/790.

The only variation from these test methods was in the size of sample tested which was 10 mm wide by 190 mm long in the case of the tensile tests and 10 mm wide by 100 mm long in the case of the flexural tests.

The test results are shown in the following table:-

The test results for the composite material made using fabric A show the usual pattern for knitted fabric reinforcement of higher strength and modulus in the wale direction compared with the course direction. This produces an anisotropic composite material. This pattern is maintained despite the use of a knitting construction which produces float yarns in the course direction.

In comparison, the test results for the composite material made using fabric B, i.e. according to the invention, show a reversal of the usual pattern, with higher strengths in the course direction then in the wale direction. Compared with the control sample incorporating fabric A, the tensile and flexural strengths in the course direction are substantially increased and there is a comparative fall in tensile and flexural strengths in the wale direction which brings the strengths in the two directions more into balance. Tensile and flexural modulus is affected in the same way to produce a very good balance of modulus between the wale and course directions. Industrial Applicability T_his invention finds application in the field of composite materials reinforced by fabrics of knitted yarn.

Claims

1. A composite material having improved strength comprising a matrix material (5) reinforced by a fabric of knitted yarn (12) embedded within the matrix material characterised in that the yarn (12) has a main body and surface fibres (13) which project laterally from the main body (11) which fibres (13) project into the regions (5, 10) of matrix material located within the knitted loops (6) of yarn so as to reinforce those regions.

2. A composite material as claimed in claim 1, characterised in that the surface fibres (13) of the yarn (12) project sufficiently from the main body of the yarn to provide overlapping of fibres projecting from adjacent or opposite parts of the knitted loops of the fabric.

3. A composite material as claimed in claim 1, characterised in that the projecting surface fibres (13) comprise greater than 10 per cent by weight of the yarn (12).

4. A composite material as claimed in claim 3, characterised in that the projecting surface fibres (13) comprise greater than 30 per cent by weight of the yarn (12).

5. A composite material as claimed in claim 4, characterised in that the projecting surface fibres (13) comprise between 40 and 60 per cent by weight of the yarn (12).

6. A composite material as claimed in claim 1, characterised in that the yarn (12) is a core/effect yarn.

7. A composite material as claimed in claim 6, characterised in that the core/effect yarn (12) is an air- texturised continuous filament yarn in which the effect portion is in the form of laterally projecting loops (13) of the filaments.

8. A composite material as claimed in claim 1, characterised in that the yarn (12) comprises high strength fibres or f laments made from one or more of the materials comprising carbon, aromatic polya ideε, silicon carbide, ceramics and glass.

9. A composite material as claimed in claim 1, characterised in that the matrix material (5) comprises a thermosetting or thermoplastic resin or carbon or a ceramic material .

10. A composite material as claimed in claim 1, characterised in that the fabric is a double ersey, weft knitted fabric.

11. A composite material as claimed in claim 1, characterised in that the fabric is knitted in a construc¬ tion producing lengths of unlooped, float yarn in the fabric.

12. A composite material as claimed in claim 1, characterised in that additional yarns are laid into the knitted material.

13. A composite material as claimed in claim 6, characterised in that the core portion of the yarn com- prises fibres or filaments which are different from the fibres or filaments of the effect portion.

14. A composite material as claimed in claim 13, characterised in that the core portion of the yarn com¬ prises high strength filaments and the effect portion comprises high strain filaments.

15. A process for making a composite material by knitting a fabric from yarn (12) and embedding that fabric in a matrix material (5) as reinforcement therefor charac¬ terised by knitting the fabric from a yarn (12) having a main body (11 ) and surface fibres (13) which project laterally from the main body, tensioning the fabric during the knitting process and/or subsequently to produce knitted loops of the desired shape for optimum reinforcement and maintaining that desired shape during subsequent processing into the composite material by the inter-engagement of the surface fibres projecting from adjacent portions of yarn.

16. A process for making a composite material as claimed in claim 15, characterised in that the fabric is weft-knitted into a three-dimensional 1y shaped structure.

17. A process for making a composite material as claimed in claim 15, characterised in that the fabric is knitted on a flat bed knitting machine employing presser foot tensioning of the yarn being knitted.