Fibre reinforced thermoplastics material structure and method of making same

Abstract: A fibre reinforced thermoplastics material structure comprising a first layer which is fully consolidated with uniformly dispersed fibres and a second layer which is formed as an absorbent matrix.

Description: This invention relates to a fibre reinforced thermoplastics material structure and to a method of making such a structure, the resulting article having flow moulded detail on one side and an open, porous structure on the other. This open structure can be subsequently impregnated with a thermosetting resin to impart desirable features. As described in European Patent Application 85300034.7 (European Publication No. 0 148 763), the subject matter disclosed in that application being incorporated by reference herein, consolidated thermoplastic material reinforced with long, stiff fibres will expand when heated to a temperature such that the viscosity of the thermoplastic material is sufficiently reduced to allow movement of the fibres; which occurs due to release of stresses in the fibre network. This phenomenon occurs to a varying extent depending on the type/grade of thermoplastic, the proportion of fibre, and the type/dimensions of the fibre. When such a material is heated and expanded it can be moulded to shape in two ways :- 1) Conventionally, whereby a charge of material is placed in the mould, which when closed, forces the material to flow and fill completely the mould cavity. An article produced in this way is fully consolidated (densified) and can be made to contain intricate detail fully reinforced with glass fibre. Depending on the intended application such an article is either ready for use or it can be coated/painted with suitable materials. However, it cannot be impregnated due to its fully densified state. 2) A sheet of hot, expanded material is inserted to cover the lower tool of the mould. The mass of this sheet is insufficient to fill the mould in a fully densified state so that when the mould is closed to the limit of its travel a semi-consolidated form is produced having a residual porosity. This porous nature can be used as a means whereby the produce can be impregnated with a liquid (resin) if so desired. However, a disadvantage of this forming technique is that flow moulded details (such as deep ribs) cannot be formed without losing porosity in the region of flow. Thus if uniform impregnation is required on one surface of the article some sacrifice in the extent of fibre reinforced moulded detail on the reverse side may be necessary. The invention disclosed herein describes a means whereby it is possible to produce an article having the attributes of both the above methods; i.e. an article having a flow moulded features, such as fibre reinforced ribs, on one side whilst retaining the ability to uniformly absorb liquid resin on the other. It has also been discovered that articles produced by this invention are free of sink marks, which is a problem feature of articles produced by the method 1 above. According to the present invention a method of making a fibre reinforced plastics structure includes laminating a first sheet of fibre reinforced thermoplastics material to a second sheet of fibre reinforced thermoplastics material is characterised in that said first sheet has a fibre content of not more than 30% by weight and will consolidate when cooled after subjection to pressure at a temperature above the melt temperature of the thermoplastics material and said second sheet has a fibre content of more than 60% by weight and will remain substantially unconsolidated and porous when cooled after subjection to said temperature and pressure, and shaping the laminate in a mould at said temperature and pressure so that the first sheet flows to the shape of the mould and is consolidated and bonded to the second sheet which is shaped by the mould and at least a portion of which remains substantially unconsolidated and porous. Preferably a substantial proportion of the fibres are between 7 and 50 millimetres long and 13 microns or less in diameter. Conveniently, the fibres are also in the form of single discrete glass fibres. Such fibres are usually provided bonded together into chopped strand bundles and these must be broken down into single fibres before the sheet is formed. Where the fibres are required to confer structural strength in the laminate, the discrete glass fibres should not be shorter than about 7 millimetres or of a diameter greater than 13 microns, since such fibres which are longer do not adequately reinforce the plastics matrix and fibres which are of greater diameter do not so efficiently reinforce the matrix. Single fibres of other materials having a reinforcement efficiency at least as high as glass fibres may, alternatively, be used. A high modulus of elasticity is to be taken as meaning a modulus of elasticity substantially higher than that of the sheet. Fibres falling into this category include glass, carbon and ceramic fibres and fibres such as the aramid fibres sold under the trade names Kevlar and Nomex and will generally include any fibre having a modulus higher than 10,000 Mega Pascals. In order to achieve the result set forth above the method may include using a second sheet of material which has been expanded or in which the proportion of fibres is above that which is possible to achieve full consolidation. This therefore makes us of the fact that in any rigid fibre (for example glass)/polymer combination there is, due to the packing density of the fibres, a critical fibre concentration above which full consolidation or densification of the structure is impossible under normal conditions of pressing and moulding. The method may include investing the porous side portion with a thermosetting or thermoplastic plastics material as proposed in European Patent Application No. 85300035.4 (European Publication No. 0 152 994) the subject matter disclosed in that Application being incorporated by reference herein. If desired the thermosetting or thermoplastic plastics material can be invested in the mould. Thus the thermosetting plastics material can be placed in the mould in a liquid state before investing the porous side. If it is to be invested with a thermoplastics material this can alternatively be provided in the form of a third sheet prior to investment. The thermoplastic materials may for example be of polyethylene, polypropylene, polystyrene, acrilonitrylstyrenebutadiene, polyethyleneterephthalate polybutyleneterephthalate or polyvinylchloride, both plasticised or unplasticised, or an alloy or blends of these materials with each other or other polyamide materials. Other suitable thermoplastics include polyphenylene ether or polycarbonates or polyestercarbonates or thermoplastic polyesters or polyetherimides or acrylonitrile - butylacrylate - styrene polymers or amorphous nylon or polyarylene ether ketone or alloys or blends of these materials with each other or other polymeric materials. With a glass fibre content of more than 60%, that is a material having a critical fibre concentration above which full consolidation and densification of the structure is usually impossible under normal conditions of pressure and moulding, it is difficult to flow mould into intricate shapes but by combining such material with an easily flow mouldable material such as that set forth above, the desired effect can be achieved. The invention can be performed in various ways and various method for producing a glass fibre reinforced thermoplastics article and articles made by the methods will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a diagrammatic view of a mould with a laminate material in it ready for mould; and Figure 2 is a diagrammatic cross-section showing an article produced by the mould shown in Figure 1. As shown in Figure 1 the fibre reinforced thermoplastics material to be moulded comprises an upper sheet 1 of a fibre reinforced thermoplastics material which has glass fibres of a length of 13 mm and a diameter of 11 µm in a polypropylene matrix. The density of glass fibres is approximately 25%. This sheet is laminated on top of a second sheet 2 which is of similar content to sheet 1 but which has a much higher glass fibre content, of approximately 80%. Due to the packing density of the fibres it will be impossible, under normal conditions of pressing and moulding, to achieve a consolidated structure with this second sheet. The mould in which the sheets are to be shaped into an article has an upper tool 3 provided with shaped indentations 4 and a lower tool 5 having a cavity 6. The laminated sheets, which may have been preheated to a predetermined temperature are placed in the mould which is closed to a fixed stop. The high glass content material in sheet 2 conforms to the cavity 6 of the lower tool 5 whilst the low glass content material in sheet 1 is forced to flow into the indentations 4 and take up the shape of the upper tool 3. There is some intermingling of the materials at the boundary layer which ensures adequate bonding. After cooling and deflashing, an article as shown in Figure 2 is produced. The article, indicated by reference numeral 7, has an upper portion 8, formed from the sheet 1, of polypropylene which has become fully consolidated in which the uniformly dispersed glass fibre is indicated by reference numeral 9. The lower portion of the article, which is formed from the sheet 2 is indicated by reference numeral 10, this portion being porous and unconsolidated. The article can be used in various ways, for example, it can be bonded to another article by utilizing the porous structure as a key for an adhesive or molten thermoplastic which unifies the two articles. Alternatively it can be made into a unitary article itself by filling the porous structure. In order to do this a thermosetting resin can be poured/injected into the lower tool 5 (in this particular configuration) and the mould closed again so that the resin is forced into the porous, absorbent layer 10. After the curing the article is removed and possesses the attributes of a fully reinforced moulded thermoplastic on the upper side and a reinforced thermoset resin on the other. Additionally, sink marks are obscured, even before the impregnation of the porous layer with a liquid resin. This construction enables fast forming of flow moulded detail on one side of the article combined with a smooth thermoset layer on the other side. Thus, the article can have a good finish and have one surface capable of withstanding high temperature and with sufficient detail on the other surface for stiffening or other requirements. Table 1 specifies the theoretical and measured void contents of unconsolidated material having a glass content above the critical level at which consolidation can be achieved. The void content of the material, which subsequently renders it capable of impregnation was both calculated theoretically and determined by an oil absorption test. It will be seen that there was good agreement between the two modes of evaluation. Table 2 sets out 8 examples of laminates formed from high and low glass content starting materials as specified in Note 1 of the Table. It will be seen from the oil absorption test carried out on the side of each laminate formed from the high glass content component that the oil absorption (and therefore the void content) values were substantially consistent with the purpose for which the laminate is to be used. Example 9 Unconsolidated samples of 70% glass fibre 12 millimetres long, 11 microns diameter/30% polypropylene powder material having a substance of 1000 g/m² and 25% glass fibre 12 millimetres long, 11 microns diamter/75% polypropylene material having a substance of 2000 g/m² were cut to 22 cms diameter, which was the effective diameter of the mould. The samples were oven heated at 200°C for 7 minutes and placed in a press mould at a temperature of 100°C with the 25% glass content material uppermost. On closing the press to a 3 mm final gap, a dish was formed having an absorbent lower surface and moulded ribs and bosses in the upper surface. The press was raised and cooled to a temperature of 50°C , 40 g of a thermosetting resin (sold under the Trade name Modar 824 LT by ICI Ltd.) were poured into the mould and the press reclosed, thus forcing the resin into the absorbent layer, with excess resin being forced out of the tool. After curing, the dish was weighed and calculated to have picked up 18 g of resin, which imparted a smooth, glossy finish to the lower surface. Example 10 Example 9 was repeated with unconsolidated sheet material samples of 80% glass fibre 12 millimetres long 11 microns diameter/20% polypropylene powder having a substance of 1000 g/m² and 25% glass fibre 12 millimetres long 11 microns diamter/75% polypropylene material having a substance of 2000 g/m². A 24 g resin pick up resulted, with the lower surface having a finish similar to that of Example 9. Example 11 The following samples were prepared. Two discs 23 centimetres in diameter and having a substance of 2000 grams per square metre of unconsolidated permeable sheet like material comprising 25% glass fibres 12 millimetres long and 11 microns in diameter and 75% polypropylene powder, bonded together. One disc 21 centimetres in diameter and having a substance of 500 grams per square metre of unconsolidated permeable sheet like material comprising 80% glass fibres 12 millimetres long and 11 microns in diameter, and 20% polypropylene powder, bonded together. One disc 17 centimetres in diameter of polycarbonate film 1 millimetre thick, sold under the trade name LEXAN by General Electric Co., which acts as a third sheet. The samples were heated in an oven at 205°C for seven and a half minutes and then placed together, in the order listed, in a press mould at a temperature of 100°C. The press was then closed and a pressure of 2000 lbs per square inch applied for one minute. The resulting laminated moulding proved to be substantially bonded. Example 12 The procedure of Example 11 was repeated, but with a polycarbonate film sample 21 centimetres in diameter which replaced the disc of 17 centimetres in diameter in Example 11. The resulting laminated moulding was found to have bonded well with the polycarbonate film partially enveloping the side edges of the moulding without creases having been formed. Example 13 Samples were prepared as in Example 11, and the same procedure followed except that the samples comprised of polycarbonate were heated separately at 250°C for four minutes and the bottom platten of the press mould (in contact with the polycarbonate film) was maintained at 140°C during moulding. The procedure was then repeated twice with unconsolidated permeable samples comprising polycarbonate had glass fibre contents of 70% and 60% respectively. The resulting laminated mouldings in all three cases were found to have bonded together with, and exhibited a greater resistance to forced delamination, than the mouldings of Examples 11 and 12. Example 14 38 centimetre square samples were prepared from the following materials. Unconsolidated permeable sheet like material having a substance of 2000 grams per square metre and comprising 25% glass fibres 12 millimetres long and 11 microns in diameter and 75% polypropylene powder, bonded together. Unconsolidated permeable sheet like material having a substance of 500 grams per square metre and comprising 80% glass fibres 12 millimetres long and 11 microns in diameter and 75% polypropylene powder, bonded together. Polycarbonate film sold under the trade name LEXAN by General Electric Co. in thicknesses of 1 millimetre, 0.5 millimetre and 0.25 millimetre . The two unconsolidated samples were heated to 205°C for seven and a half minutes in an oven, and the 1 millimetre thick polycarbonate film to 250°C for four minutes. The samples were then placed in a platen press at a temperature of 100°C in the sequence listed and a pressure of 13790 Kilo Pascals (2000 lbs per square inch) applied for 1 minute. The foregoing procedure was then repeated on a second and a third occasion in which the 0.5 millimetre and 0.25 millimetre polycarbonate films respectively were substituted for the 1 millimetre film. A good bond between the three components of each of the resulting laminates was found to have been formed. Example 15 The three laminates produced in Example 14 were cut to a diameter of 22 centimetres. A 15 centimetre diameter piece of unconsolidated material of the kind from which the first sample was cut in Example 14 was then laid on each of the circular laminates. The resulting assemblies were in sequence heated to 205°C for seven and a half minutes and subjected to press moulding in a mould heated to 120°C at 13790 Kilo Pascals (2000 pounds per square inch) for 1 minute. The three resulting mouldings were found to be well formed and well bonded. Example 16 Samples were first prepared for moulding as an in Example 9. The first sample, comprising 75% glass fibre 12 millimetres long 11 microns diameter and 30% polypropylene was then oven heated to 200°C for seven minutes and then placed in the same press mould as had been used in Example 9. On closure of the mould, the structure of the sample was compacted so that the molten polypropylene wetted out the surfaces of the glass fibres. As the press was raised, the resilience of the glass fibres caused the wetted out fibrous structure substantially to reassume its porous configuration prior to pressing. After the first sample had sufficiently cooled to be handled, it was removed from the mould and allowed to cool fully. 15 g of thermosetting resin sold under the trade name Modar 824 LT by ICI Ltd. were then poured into the mould and the first sample then returned to the mould. The press was then closed so that the thermosetting resin invested the pores in the lower surface of the sample. After curing, this produced a dish-like structure having a smooth and glossy lower surface, and a tangled open fibrous upper surface. The structure thus produced can be removed from the mould for storage and integral moulding, at a later date, with a second fibre reinforced sheet comprising a substantially higher proportion of thermoplastic, or it may be integrally moulded with such a sheet immediately, as described below. Whilst the thermosetting resin was curing in the mould, the second sample comprising 25% glass fibre 12 millimetres long 11 microns diameter and 75% polypropylene was heated to 200°C for seven minutes and placed in the mould on top of the previously moulded first sample. The press was then closed for a second time so as to cause the hot material at the lower face of the second sample to integrate with the tangled fibrous upper surface of the first sample already in the mould. Because of the relatively high thermoplastic content of the second sample, it also flow moulded without difficulty to conform to the profile of the upper portion of the mould.

Claims: A method of making a fibre reinforced plastics structure (7) which includes laminating a first sheet (1) of fibre reinforced thermoplastics material to a second sheet (2) of fibre reinforced thermoplastics material characterised in that said first sheet (1) has a fibre content of not more than 30% by weight and will consolidate when cooled after subjection to pressure at a temperature above the melt temperature of the thermoplastics material and said second sheet (2) has a fibre content of more than 60% by weight and will remain substantially unconsolidated and porous when cooled after subjection to said temperature and pressure, and shaping the laminate in a mould (3,5) at said temperature and pressure so that the first sheet (1) flows to the shape of the mould and is consolidated (8) and bonded to the second sheet (2) which is shaped by the mould (3,5) and at least a portion (10) of which remains substantially unconsolidated and porous. A method as claimed in claim 1 characterised in that a substantial proportion of the fibres are between 7 and 50 millimetres long and 13 microns or less in diameter. A method as claimed in claim 2 characterised in that the fibres are in the form of single discrete fibres. A method as claimed in claims 1, 2 or 3 characterised in that the reinforcing fibres have a modulus of elasticity higher than 10,000 Mega Pascals. A method as claimed in claims 1 to 4 characterised by using a second sheet (2) of material which has been expanded, or in which the fibre content is above that which is possible to achieve full consolidation. A method as claimed in claims 1 to 5 characterised by investing the said porous portion (1) of said second sheet (2) with a thermosetting or thermoplastic plastics material before or after laminating with said first sheet (1). A method as claimed in claim 6 characterised by investing the porous portion (1) with thermosetting or thermoplastic plastics material in the mould (3,5). A method as claimed in claim 7 characterised in that the thermosetting plastics material is placed in the mould (5) in a liquid state prior to said laminate. A method as claimed in claim 7 or claim 8 characterised in that said porous portion (1) is invested with a thermoplastics material which is provided in sheet form and integrated with said laminate in the mould (3,5). A method as claimed in claim 9 characterised in that the thermoplastics material sheet is or larger overall dimensions than the porous portion (1) of the second sheet (2) whereby part of the thermoplastics material sheet is caused to integrate with at least part of one side edge of the second sheet (10). A method as claimed in claim 8, claim 9 or claim 10 characterised by moulding the second sheet (2) under pressure, investing it with the thermosetting or thermoplastics material and then laminating said second invested sheet (2) with said first sheet (1). A method as claimed in claim 13 characterised by placing the second sheet (2) in the mould (3,5), applying pressure, releasing said pressure to allow said sheet (2) to resume its porous configuration prior to pressing, placing the thermosetting or thermoplastic plastics material in the mould (3,5), applying pressure to invest said second sheet (2) with the thermosetting or thermoplastic plastics material and subsequently laminating said invested second sheet (2) to said first sheet (1). A method as claimed in any one of the preceding claims characterised by pre-heating the first (1) and second (2) sheets before placing them in the mould (3,5). A method as claimed in any one of the preceding claims 1 to 13 characterised in that the fibres are glass fibres. A method as claimed in any one of the preceding claims 1 to 14 characterised in that the thermoplastics materials is polyethylene, polypropylene, polystyrene, acrilonitry-styrenebutadiene, polyethyleneterephthalate, polybutyleneterephthalate or polyvinylchloride, both plasticised or unplasticised, or an alloy and blends of these materials with each other or other polymeric materials. A method as claimed in any one of the preceding claims 1 to 14 characterised in that the thermoplastics materials are polyphenylene ether or polycarbonates of polyestercarbonates or thermoplastic polyesters or polyetherimides or acrylonitrile-butylacrylate-styrene polymers or amorphous nylon or polyarylene ether ketone or alloys or blends of these materials with each other or other polymeric materials.

Inventors: BIGGS STEDMAN IAN, RADVAN BRONISLAW, BIGGS, STEDMAN IAN, RADVAN, BRONISLAW
Applicants: WIGGINS TEAPE GROUP LTD, THE WIGGINS TEAPE GROUP LIMITED
Application year: 1987
Main classification: B29C67
IPC codes: B29K105, B29C43, B29C70, B32B27, B32B5, B29L9