FIBRE REINFORCED COMPOSITE PLASTICS MATERIAL

Abstract: A fibre reinforced plastics structure comprising a porous matrix of thermoplastics material and reinforcing fibres, the pores of said matrix being wholly or partly invested with a thermosetting plastics material.

Description: This invention relates to fibre reinforced plastics materials for use in producing plastics mouldings. In particular, it relates to such materials which embody both thermosetting and thermoplastic components in such a manner that the advantages of both are maximised. Thermosetting materials are usually available to the moulder in the form of liquids, or of powders which soften upon heating. They may be used to impregnate a reinforcing fabric or may be mixed with short reinforcing fibres, or with fillers. When formed into a desired shape and heated to a specific temperature, thermosetting materials undergo an irreversible chemical change and solidify into the formed shape. The resulting article exhibits, in general, good stiffness and strength, and, especially good resistance to heat and prolonged mechanical stress. However, because the thermosetting process is irreversible, reject articles cannot be scrapped and the material reused. They also tend to be relatively brittle. More importantly, the forming process is slow, being of the order of several minutes, because the mould must be held closed until the curing process has been completed. For this reason especially, thermosetting materials do not lend themselves to modern high speed production techniques. As a result, and despite the advantages which articles moulded from thermosetting materials afford, they are being displaced by thermoplastics materials. Examples of thermosetting materials are phenol-formaldehyde resin, urea and melamine formaldehyde resins, epoxy resins, unsaturated polyesters, and polyurethanes. The thermoplastics are usually available to the fabricator in granular or particulate form or as a sheet. They soften upon heating and can be pressed, moulded, extruded or cast into a desired shape into which they solidify upon cooling. The softening and hardening processes are relatively fast so that the article can be moulded, hardened by cooling and ejected from the mould in a matter of seconds rather than minutes. Fillers and reinforcing materials may be included. When the thermoplastics material is in the form of a sheet, the reinforcement will be in the form of fabrics or fibre mats and in the case of granular or particulate materials, in the form of short fibres. Articles moulded from thermoplastic materials tend to be lighter and often tougher than in the case of thermosetting materials. But their stiffness is less, especially under conditions of prolonged stress. Their resistance to heat is, of course, poor. Examples of thermoplastics are the polyolefins, polystyrenes, polyamides, polyvinyl chloride, and the saturated polyesters, together with blends of these materials. Attempts have been made to combine thermoplastics and thermosetting materials in a single article so as to achieve at least some of the technical advantages of each. Thus for example in the manufacture of an article such as a bath tub, an initial moulding is produced rapidly from a thermoplastics sheet by heating and vacuum forming. After coating in and removal from the mould, the outside of the cooled moulding is then covered with liquid thermosetting polyester resin, reinforced with short glass fibres, which is then allowed to cure slowly whilst maintaining its shape. This has the advantage of avoiding long dwell times in the mould, but defects arise in the end product when in use, due to insufficient adhesion between the two materials. This results from differential thermal expansion which produces warping and delamination. US-A- 3 865 661 discloses the concept of combining glass fibre mat with a layer or layers of thermoplastic material and consolidating it under heat and pressure, releasing the pressure to allow the structure to expand and then investing it with a thermosetting resin. It is among the objects of the present invention to provide a method of combining thermoplastic and thermosetting materials in such a manner that articles formed from the resulting composite avoid at least some of the disadvantages of the known techniques. According to the present invention a method of making a composite fibre reinforced plastics structure comprises the steps of forming a foamed aqueous dispersion of from 20% to 60% by weight of single reinforcing fibres having a modulus of elasticity higher than about 1000 mega-pascals and being between about 7 and 50 millimetres long from 40% to 80% by weight of unconsolidated particulate thermoplastic material, draining the dispersion on a foraminous support to form a substantially homogeneous web of uniformly distributed reinforcing fibres with thermoplastic particles retained in the interstices thereof, subjecting the web to heat and pressure to form a molten matrix of thermoplastic with stressed reinforcing fibres dispersed therein, either releasing the pressure from the hot matrix or consolidating the matrix into a sheet by cooling and then reheating the sheet to a molten state so as to permit the consequent relief of the stresses in the fibres to expand the matrix or sheet and render it porous, investing at least the surface of the web with thermosetting resin, and curing the thermosetting resin. The invention also extends to sheets or articles formed from such a plastics structure in which the thermosetting plastics material has cured. The porous matrix may be formed in various ways. Thus for example the fibres and plastics components can be bonded into an air permeable structure in the manner described in co-pending European Patent Application No. 85300031.3 filed 3 January 1985 or it may be formed by a fibre reinforced thermoplastics material which has been consolidated by heating and cooling under pressure and then reheated to cause the resilience of the fibres to re-expand it and render the sheet porous as described in co-pending European Patent Application No. 85300034.7 filed 3 January 1985. The fibre reinforced plastics structure of the invention affords the fast forming capability of the thermoplastics material with the superior mechanical and heat resistance properties of the thermosetting material. Because the two materials are combined in an intimate manner, separation is virtually impossible. Furthermore, there is little risk of distortion and separation due to differential thermal expansion. The matrix may be completely impregnated or invested with thermosetting liquid plastics material, in which case articles formed from the matrix will be relatively dense. Or the surface layers only of the matrix may be impregnated, for example, by coating. This will result in a lighter article but may provide a sufficient increase in stiffness as compared with the unimpregnated matrix, together with a sealed surface which will prevent the ingress of other fluids such as water or oil, into the expanded central zone. An excess of liquid thermosetting materials may also be applied to the surface where the article to be formed is required to have a very smooth, glossy appearance. This is particularly desirable where the moulded article is to be used as a substitute for an element previously formed from sheet metal. Such surfaces are difficult to achieve with conventional fibre reinforced materials. A low temperature curing liquid resin may be used to impregnate the matrix, which is then stored to enable curing of the resin to proceed slowly at room temperature, or at least at a temperature considerably below the softening point of the thermoplastics material. Alternatively, the porous matrix is initially impregnated with a relatively slow curing thermosetting resin. It is then heated rapidly to the moulding temperature of the thermoplastics component, rapidly transferred to a moulding press and pressed into the desired shape before the curing of the thermosetting component is complete. In a further variation, a thin, porous, fibre reinforced thermoplastics matrix is fully impregnated with the thermosetting resin, heated to the moulding temperature of the thermoplastics material and laminated with one or more hot unimpregnated sheets and then formed in a hot press. In a variation of the last mentioned procedure, a further impregnated sheet is added to the other side of the unimpregnated sheets which thus form an unimpregnated core in the article when formed. The invention will now be further described with reference to the accompanying drawings in which :- Figure 1 a is a cross-section through a first form of fibre reinforced plastics structure according to the invention, Figure 1 b is a cross-section through a plastics moulding formed from a structure of Figure 1 a , Figure 2 a is a cross-section through a consolidated fibre reinforced thermoplastics sheet, Figure 2 b is a cross-section through an intermediate structure formed from the sheet of Figure 2 a , Figure 2 c is a fibre reinforced plastics structure according to the invention formed from the structure of Figure 2 b and, Figure 2 d is a plastics moulding formed from the structure of Figure 2 c . Referring first to Figure 1 a , this shows a porous sheet like structure 1 consisting of a porous matrix of glass reinforcing fibres 2 and particulate thermoplastics material 3, the matrix being impregnated throughout with a thermosetting resin 4. Optionally, the thermosetting resin 4 may be applied only to the surfaces of the matrix so that the core regions remain free of thermoset material when cured. The matrix comprised of the fibres 2 and particulate thermoplastics material 3 may for example be made in accordance with the process set out in copending European Patent Application No. 85 300 031.3 Filed 3rd January 1985. Figure 1 b shows a moulding 5 formed from the impregnated sheet material of Figure 1 a . In order to form a moulding, the sheet 1 is initially heated until the thermoplastics content 3 has become plastic. The sheet is then quickly transferred to the mould and moulding effected before the thermosetting resin has cured. After the mould temperature has been reduced sufficiently to solidify the thermoplastic component, the moulding can be removed from the mould so that the thermosetting material can cure slowly at a controlled temperature. Turning now to Figure 2, Figure 2 a shows a consolidated sheet 10 comprising glass reinforcing fibres 11 dispersed through a solid thermoplastics matrix 12. Since the sheet 10 has been formed by the application of heat and pressure and then cooled whilst under pressure, the fibre reinforcement remains trapped in a stressed condition within the solid thermoplastics matrix. Turning now to Figure 2 b , this shows the sheet 10 after heating. As the thermoplastics content 12 becomes plastic, it adheres to the fibres 11 which, being released from their trapped condition, relax from their stressed condition and cause the whole structure to expand and form an open porous matrix 13. Figure 2 c shows the matrix 13 of Figure 2 b after impregnation or surface coating with the thermosetting resin 14 as described with reference to Figure 1. The impregnated or coated matrix 13 is then heated and moulded into a moulding 15, as Shown in figure 2 d in the manner described above with reference to Figures 1 a and 1 b . Preferably, the fibrous content of the sheet is between about 7 and about 50 millimetres long since this will permit free flow of the fibres with the thermoplastc content of the composite sheet during the moulding process. Example No. 1 Impregnation with liquid thermosetting resins Together with a balancing weight percentage to 100 percent of glass reinforcing fibre 12 millimetres long and 11 µm diameter, (Grade R18D supplied by OCF Ltd.), each of the following thermoplastic powders was dispersed in an aqueous foam, from which a web was formed and then dried. Matrix Sample 1 polypropylene powder (Grade PXC 81604) supplied by ICT Ltd. - 52 percent by weight Sample 2 polyvinyl chloride powder, (Grade Corvic S57/116) supplied by ICI Ltd. - 72 percent by weight. Sample 3 acrylic resin powder, (Grade Diakon LG 156) supplied by ICI Ltd. - 70 percent by weight. The three examples thus obtained were consolidated into flat dense sheets in a hydraulic press, heated to 190°C (polypropylene), 210°C (polyvinyl chloride) or 200°C (acrylic) and a pressure of 13,8-20,7 bar (200-300 psi) was applied at the end of the heating cycle of approximately 5 minutes. The press was then opened to varying extends for the different samples, which were thus allowed to re-expand to various thicknesses and densities, and then to cool. Control portions of samples 1, 2 and 3 were then tested for Flexural Modulus and Ultimate Tensile Strength, whilst the remainders were divided into two impregnated by quick immersion in different liquid thermosetting resin solutions: either : phenol-formaldehyde resin (Grade MS 7814) supplied by Ciba-Geigy or : melamine-formaldehyde resin (Grade BL434) supplied by B.I.P. Chemicals, dissolved in water to a concentration of 50 percent by weight. The impregnated samples were then dried in an air-circulation oven at 100 o C for a period of 1 hour approximately, cooled and weighed in order to determine the pickup of resin solids, and then cured in a flat press at 170-200 o C, either at contact pressure (so as to produce only partly-impregnated materials) or under a pressure of 17,2 bar (250 psi), when cure and consolidation were achieved in approximately 5 minutes. They were then cooled and tested as above. The results are summarised in the attached Table No. 1. They show that the impregnation of the expanded samples with thermosetting resins produced an increase in strength and stiffness; also the samples remained sufficiently thermoplastic at the end of the drying cycle to be capable of consolidation before curing. Visual examination showed the samples to be uniformly and finely impregnated, with no visible boundaries between the thermoplastic and thermosetting regions. Samples No. 1A and 4 in the table were also tested for hardness at different temperatures by the following procedure. A piece of the sample resting on a metal support was placed in an air circulating oven, and the temperature raised to 150 o C. After 5 minutes the support and the sample were quickly taken out of the oven, and the hardness of the sample was measured using a Shore D instrument supplied by Shore Instrument & Manufacturing Co. Another piece of the sample was then placed on the support in the oven, and the procedure repeated at several higher temperatures. The results of the tests are shown in Table 2 below : Example No. 1 Description Temperature °C Hardness (SHORE) Control = no impregnant 10 80 15 75-76 80 62-67 120 57-65 160 31-33 180 6 Impregnated with phenol formaldehyde resin 10 53-65 15 53-59 80 52-55 120 51-53 160 40-48 180 37-40 The results show that the material impregnated with liquid thermosetting resin retains its hardness over a greater range of temperatures. Also, the material no longer expanded in thickness on reaching the higher temperatures, demonstrating the restraining action of the thermosetting additive. Exmample No. 2 Impregnation with liquid thermosetting resins (B) Formability A sample was prepared as for samples 3 and 4 in Table 1 of Example No. 1. with the following difference. After impregnation and drying, the sample was pre-heated briefly in an air circulation oven at 180 o C, and placed quickly in a shaped tool in a toggle-type press heated at 180 o C. The press was closed fast to a pressure of some 250 psi for a period of 5 minutes. Upon opening the press, a well-formed article of a moderate degree of draw, was released. Example No. 3 Impregnation with liquid thermosetting resins by transfer from a thermoplastic film (A) Physical Properties When dealing with resins which cure too fast to be pre-heated to the moulding temperatures of the thermoplastic matrix; or which produce copious irritant or flammable fumes during such pre-heating, the following modification of the procedure in Example No.1 was adopted. Samples prepared as Nos. 1,2 and 3 in Table No. 1 of Example No.1. were pre-heated in an oven for 5 minutes at 200 o C, or until they visibly expanded in thickness, and became soft and flexible to touch. In the meantime, two pieces of polycarbonate film (Grade Lexan) of a weight of 300g/m² each, and a thickness of 0.26 mm, supplied by General Electric Co. (Europe) Ltd. were cut to the approximate size of the sample, and spread out flat on a table. A quantity of unsaturated polyester resin (Grade Crystic 198), catalysed with Catalyst Powder B, both supplied by Scott Bader Co. Ltd. was poured on to the surface of each piece of film, to form two thin puddles. When the sample was sufficiently hot, it was quickly taken out of the oven, and placed down on to the surface of the puddle on the first piece of the film, when the liquid resin became immediately absorbed in the porous expanded surface, to act as a weak adhesive. The other side of the sample was then lowered on to the surface of the puddle on the second piece of film, with the same result. The whole was replaced in the oven for a period of 10-20 seconds, i.e. until the films visibly softened but did not melt and shrink. The sample was then taken out, and placed between the platens of a hydraulic press, pre-heated to 120 o C, and then kept for 15 minutes at contact pressure. It was then removed from the press, cooled and tested. The results are shown in the Table below : Example No. 3 Sample Number Resin Pick-up percent by weight Flexural Modulus MPa Ultimate Tensile Strength MPa 10A 44 2700 33 9A 50 3100 50 8A 54 3100 53 5A 65 3300 48 4A 73 3200 49 7A 76 2900 41 6A 98 3400 48 It will be appreciated that in order to facilitate handling in large scale production, the details of the procedure above may be altered: e.g. a partly gelled, or high-viscosity resin may be coated on to the film, and the film then applied to the hot sample, rather than vice-versa. Example No. 4 Impregnation with liquid thermosetting resins by transfer from a thermoplastic film Samples were prepared as in Example No. 3 using various amounts of liquid resin puddle on the film. After re-heating in the oven, they were placed in the shaped tool of Example No. 2 in a toggle-shaped press, and the press closed for 30 seconds at 130 o c, to a pressure of some 17,2 bar (250psi). They were then removed, held lightly in a clamp, and left to complete the cure in an oven at 160 o C for 10 minutes. Well formed, undistorted articles with a strongly adhering glossy film surface, were obtained. In a variant of this procedure, a relatively small puddle of the liquid resin was spread on one piece of film only, and applied to one side only of the sample. The sample still released well after 30 seconds from the press, and a glossy well-adhering surface film was obtained, as before, but only on one side. In another variant of this procedure, the film used was oriented polypropylene, which had been sprayed with a release agent before the puddle was applied to it. After formation and curing, the film was stripped off, leaving a smooth hard, non-glossy, partly thermoset surface. Example No. 5. In-mould coating of expanded samples with liquid thermosetting resins. A sample of reinforced sheet material having a substance of 3000g/m² was made by the process described in Example No. 2 with the following composition. polypropylene powder (as in Example No. 2.) 60 parts by weight glass fibre (as in Example No. 2) 40 parts by weight The sample was heated in an air circulating oven at 200 o C for approximately 5 minutes, that is until it had visibly expanded in thickness and became soft and flexible to touch. The tool described in Example No. 2 was pre-heated to 130 o C in a toggle-type press. A quantity of unsaturated polyester resin (as in Example No. 2 sufficient to cover the lower part of the tool to a grammage of approximately 500 g/m² was poured into the lower part of the tool. The hot sample was then quickly transferred from the oven into the tool, the press closed fast to a pressure of some 17.2 bar (250psi), and held closed for 10 minutes, i.e. until the resin was sufficiently cured for the sample to release cleanly. The sample was then held lightly in a clamp, and allowed to cure in an oven at 140 o C for sufficient length of time to achieve full cure. The article obtained was seen to be uniformly penetrated by the resin over the lower surface to give a hard and smooth thermoset surface. Some of the resin was also seen to have penetrated to the other side of the article with similar results. In a variant of this procedure, the sample consisted of two pieces each of 1500 g/m² which were heated together, as before, to produce a two-layered sheet, of different compositions in each layer: top layer 25% glass fibre by weight : 75% polypropylene bottom layer 48% glass fibre by weight : 75% polypropylene The bottom layer was seen to be much more expanded in thickness (as expected from the teaching in European co-pending Application No. 85 300 034 Filed 3rd January 1985) and the bottom layer showed only very little expansion. The top part of the tool (in contact with the top layer) was modified to include more complex detail of ribs and bosses, and was mounted in a hydraulic press capable of closing to a pressure of 103 bar (1500psi) over the area of the article. Upon release, the article was seen to be uniformly impregnated over the bottom surface, and only very little resin had penetrated to the top surface. However, the latter side reproduced faithfully the fine detail of the tool. It will be appreciated that the choice of the curing time will depend on the type of resin used for impregnation, and that much faster times may be obtained by this means. Also, the liquid resins may be replaced by powdered resins, known for in-mould coating, if so desired. In either case, the microporous structure of the hot expanded sheet permits even and thorough penetration, ensures good adhesion of the coating, and the imparts thermoset properties to the article.

Claims: A method of making a composite fibre reinforced plastics structure, comprising the steps of forming a foamed aqueous dispersion of from 20% to 60% by weight of single reinforcing fibres having a modulus of elasticity higher than about 1000 mega-pascals and being between about 7 and 50 millimetres long from 40% to 80% by weight of unconsolidated particulate thermoplastic material, draining the dispersion on a foraminous support to form a substantially homogeneous web of uniformly distributed reinforcing fibres with thermoplastic particles retained in the interstices thereof, subjecting the web to heat and pressure to form a molten matrix of thermoplastic with stressed reinforcing fibres dispersed therein, either releasing the pressure from the hot matrix or consolidating the matrix into a sheet by cooling and then reheating the sheet to a molten state so as to permit the consequent relief of the stresses in the fibres to expand the matrix or sheet and render it porous, investing at least the surface of the web with thermosetting resin, and curing the thermosetting resin. A method of making a fibre reinforced plastics structure as claimed in claim 1 in which the matrix is completely impregnated or invested with thermosetting liquid plastics material. A method of making a fibre reinforced plastics structure as claimed in claim 1 in which only the surface layers of the matrix are impregnated with the thermosetting plastics material. A method of making a fibre reinforced plastics structure as claimed in any one of preceding claims 1 to 3 in which an excess of liquid thermosetting plastics material is applied to the surface of the matrix to provide a smooth glossy appearance. A method of making a fibre reinforced plastics structure as claimed in any one of preceding claims 1 to 4 in which a low temperature curing liquid resin is used to impregnate the matrix which is then stored to enable curing of the resin to proceed slowly at room temperature considerably below the softening point of the thermoplastics material. A method of making a fibre reinforced plastics structure as claimed in any one of preceding claims 1 to 5 in which the porous matrix is initially impregnated with a relatively slow curing thermosetting resin, is then heated rapidly to the moulding temperature of the thermoplastics component, rapidly transferred to a moulding press and pressed into a desired shape before the curing of the thermosetting component is complete. A method of making a fibre reinforced plastics structure as claimed in any one of preceding claims 1 to 6 in which a thin porous fibre reinforced thermoplastics matrix is fully impregnated with the thermosetting resin, heated to a moulding temperature of the thermoplastics material and laminated with one or more hot unimpregnated sheets and then formed in a hot press. A method of making a fibre reinforced plastics structure as claimed in claim 7 in which a further impregnated sheet is added to the other side of the unimpregnated sheet or sheets which form an unimpregnated core in the article when formed. A method of making a fibre reinforced plastics structure as claimed in claim 1 in which the thermosetting plastics material carried on a film or sheet which is applied to the matrix. A method of making a fibre reinforced plastics structure as claimed in claim 9 in which the thermosetting plastics material is located on a film or sheet of thermoplastics material which is then applied to the matrix and then heated and pressed. A method of making a fibre reinforced plastics structure as claimed in claim 9 in which a partly gelled or high viscosity resin is coated on the film or sheet which is then applied to a hot matrix and pressed. A fibre reinforced sheet or article made by the method set forth in any one of the preceding claims.

Inventors: RADVAN, BRONISLAW, TILL, ANDREW
Applicants: THE WIGGINS TEAPE GROUP LIMITED
Application year: 1985
Main classification: B29C67
IPC codes: C08J5, B29C70, B29C35, B29C67, B29B15