Pellets of fibre-reinforced compositions and methods for producing such pellets

Abstract: Fibre-reinforced pellets comprising a thermoplastics polymer and containing at least 30% by volume of reinforc­ ing filaments extending longitudinally of the pellets which have been produced by cutting a structure made in a continuous process under conditions which give an excep­ tionally high degree of wetting of the filaments by the thermoplastics polymer. The thorough wetting gives rise to a product which can be further processed even in vigorous mixing processes such as injection moulding to give moulded articles with surprisingly high retention of the fibre length in the fabricated article and in which the reinforce­ ment is provided by individually wetted filaments.

Description: This invention relates to pellets of fibre-reinforced compositions containing thermoplastics resins and to methods of producing such pellets. Processes are known in which a fibre-reinforced structure is produced by pulling a tow or roving of glass fibres through a bath of low viscosity thermosettable resin to impregnate the fibres. The structure is subsequently cured by heating. Such processes are known as pultrusion processes. Although such processes have been known for at least 10 years they have not been used commercially to any extent for the production of thermoplastics resin impregnated structures. This is because of difficulties experienced in wetting the fibres when they are pulled through the viscous molten resin. The resulting products have unacceptable properties as a result of this poor wetting. The extent of wetting the fibres achieved by a particular process and thereby providing the basis for making maximum use of the very high levels of physical properties inherent in continuous fibres, such as glass fibres, may be assessed by measuring the extent to which the process provides a product having a flexural modulus approaching the theoretically attainable flexural modulus. The theoretically attainable flexural modulus is calculated using the simple rule of mixtures: E L = V f E f + V m E m where E L is the longitudinal modulus of the composition, V f is the volume fraction of the fibre, E f is the flexural modulus of the fibre, V m is the volume fraction of the matrix polymer, E m is the flexural modulus of the matrix polymer. The use of melts of conventional high molecular weight thermoplastics for impregnating continuous rovings does not permit a high level of flexural modulus to be obtained. For example, United States Patent No. 3 993 726 discloses an improved process of impregnating continuous rovings under high pressure in a cross-head extruder, pulling the rovings through a die and cooling and shaping the rovings into a void-free shaped article. Products obtained using polypropylene are shown in Example 1 to have a flexural modulus of only about 6GN/m 2 , for a glass fibre content of 73% by weight, that is, less than 20% of the theoretically attainable value. Swiss Patent 500,060 describes glass fibre reinforced compositions of high fibre content which have been prepared by processes which include a melt impregnation process. The purpose of the invention is to provide masterbatch granules which are diluted with further non-reinforced resin for injection molding purposes. No information is given on the properties of continuous product produced but it can be deduced that the process does little more than coat strands of fibres with the resin because it is stated that unless the product is chopped at an elevated temperature the product is a mass of shattered fibreglass resin. This is indicative of the fact that bundles of unwetted filaments are still present in the continuous product. It has now been found possible to produce materials in a continuous process having flexural modulus levels which approach the theoretically attainable levels. It has been found that pellets chopped from this continuous product have outstanding advantages for a number of processes. Accordingly, there is provided moulding pellets being cut to a length between 2 and 100mm from a thermoformable fibre-reinforced structure comprising a thermoplastic polymer and at least 30% by volume of aligned reinforcing filaments which have been wetted by a melt of the thermoplastic in a melt pultrusion process, characterised in that the filaments have been wetted by a melt which has a viscosity at low shear rates of less than 100 Ns/m 2 so as to give pellets in which the extent of wetting is at least 70%. The extent of wetting of at least 70% may be assessed by measuring the extent to which the continuous structure has a longitudinal flexural modulus determined by ASTM D790-80 of at least 70% of the theoretically attainable flexural modulus, and preferably at least 90% of the theoretically attainable flexural modulus. The interlaminar shear strength of the structure is in excess of 10 MN/m 2 and preferably in excess of 20 MN/m 2 . The preferred thermoplastic polymers for use in the invention are crystalline polymers having a melting point of at least 150°C and amorphous polymers having a glass transition temperature of at least 25°C. For optimum stiffness the thermoplastic polymer should have a flexural modulus of at least 1 GN/m 2 and preferably at least 1.5 GN/m 2 . Fibre reinforced pellets according to the invention can be produced by a variety of processes which enable good wetting of continuous, aligned, fibres to be achieved. In one of these processes there is provided a process of producing a fibre-reinforced composition comprising drawing a plurality of continuous filaments through a melt of a thermoplastics polymer having a melt viscosity of less than 30 Ns/m 2 , preferably between 1 and 10 Ns/m 2 , to wet the filaments with molten polymer, the filaments being aligned along the direction of draw. Optionally, the impregnated filaments may be consolidated into a fibre-reinforced polymer structure. The viscosity of thermoplastics varies with shear rate decreasing from a near constant value at low shear. In this application we refer to the viscosity at low shear rates (usually referred to as the Newtonian viscosity). This is conveniently measured in a capillary viscometer using a die 1 mm in diameter and 8 mm long, the melt viscosity being determined at a shear stress in the range 10 3 -10 4 N/m 2 . Surprisingly, despite the fact that such a polymer is of lower molecular weight than normally considered suitable in the thermoplastics polymer field for achieving satisfactory physical properties the reinforced compositions have exceptionally good physical properties. Thus when reinforced thermosetting polymer compositions are produced by a pultrusion process the viscosity of the thermosetting prepolymer resin in the impregnation bath is typically less than 1 Ns/m 2 for good wetting of the fibre. This low value of viscosity may be used because the prepolymer is subsequently converted to a solid form by a heat curing process. By contrast thermoplastics polymers are normally fully polymerised solid materials and are only obtained in liquid form by heating the thermoplastics polymer to melt it. However, the melt viscosity of conventional high molecular weight polymers having acceptable physical properties is usually in excess of 100 Ns/m 2 . Adequate wetting of the fibres in a pultrusion process with a melt of such high viscosity is not possible. Whilst it is possible to reduce melt viscosity to some extent by increasing the melt temperature it is normally the case that an insufficient reduction in viscosity is obtainable below the decomposition temperature of the thermoplastics polymer. The use of a thermoplastics polymer of low enough molecular weight to give a sufficiently low melt viscosity to result in adequate fibre wetting in a pultrusion process surprisingly gives a product of high strength. Thus a fibre-reinforced thermoplastics pellet can be obtained by drawing a plurality of continuous filaments through a melt of the thermoplastics polymer having a melt viscosity of less than 30 Ns/m 2 and preferably between 1 and 10 Ns/m 2 to wet the filaments with molten polymer, the filaments being aligned in the direction of draw. By the term "continuous fibres" or "plurality of continuous filaments" is meant any fibrous product in which the fibres are sufficiently long to give a roving or tow of sufficient strength, under the processing conditions used, to be hauled through the molten polymer without the frequency of breakage which would render the process unworkable. Suitable materials are glass fibre, carbon fibre, jute and high modulus synthetic polymer fibres. In the latter case it is important that the polymer fibres conform to the proviso of having sufficient strength to be capable of being hauled through the polymer melt without breakage disrupting the process. In order to have sufficient strength to be hauled through the impregnation system without breakage the majority of the continuous fibres of the fibrous product should lie in one direction so that the fibrous product can be drawn through molten polymer with the majority of the continuous fibres aligned. Fibrous products such as mats made up of randomly disposed continuous fibrous are not suitable for use in the invention unless they form part of a fibre structure in which at least 50% by volume of the fibres are aligned in the direction of draw. The continuous fibres may be in any form having sufficient integrity to be pulled through the molten polymer but conveniently consist of bundles of individual fibres or filaments, hereinafter termed "rovings" in which substantially all the fibres are aligned along the length of the bundles. Any number of such rovings may be employed. In the case of commercially available glass rovings each roving may consist of up to 8000 or more continuous glass filaments. Carbon fibre tapes containing up to 6000 or more carbon fibres may be used. Cloths woven from rovings are also suitable for use in the present invention. The continuous fibres may be provided with any of the conventional surface sizes, particularly those designed to maximise bonding between the fibre and the matrix polymer. The thermoplastics polymer employed in the process hereinbefore described may be any polymer which will melt to form a coherent mass providing that the melt has a viscosity of less than 30 Ns/m 2 and preferably less than 10 Ns/m 2 . In order to achieve acceptable physical properties in the reinforced composition it is preferred that the melt viscosity should be in excess of 1 Ns/m 2 . As indicated the choice of a polymer in the required melt viscosity range is primarily dictated by the molecular weight of the polymer. Suitable polymers include thermoplastics polyesters, polyamides, polysulphones, polyoxymethylenes, polypropylene, polyarylene sulphides, polyphenylene oxide/polystyrene blends, polyetheretherketones and polyetherketones. A variety of other thermoplastics polymers can be used in the process of the invention although polymers such as polyethylene will not give compositions of such high strength. In the process of impregnating the fibres of the roving, in addition to using a polymer of suitable melt viscosity to bring about adequate wetting it is necessary to maximise penetration of the melt into the roving. This may be done by separating, as far as possible, the rovings into the individual constituent fibres, for example, by applying an electrostatic charge to the roving prior to its entry into the molten polymer or preferably by spreading the roving whilst it is in the molten polymer to separate the constituent filaments. This is conveniently achieved by passing the roving under tension over at least one, and preferably several, spreader surfaces. Further enhancement of wetting occurs if further work is applied to the separated, polymer impregnated, fibres, for example, by consolidating the separated fibres by pulling the impregnated roving from the melt through a die. This die may have the profile desired of the impregnated roving or the impregnated roving may be passed through a further sizing die whilst the polymer is still flowable. Surprisingly, it is advantageous if this die is cooled in order to achieve satisfactory sizing and a smooth passage through the die. When the impregnated rovings emerge from the bath in the form of a flat sheet further work may be applied by passing the sheet between a pair of rollers. The rate at which the roving can be hauled through the impregnation bath is dependent on the requirement that the individual fibres should be adequately wetted. This will depend, to a large extent on the length of the path through the molten polymer bath and in particular the extent of the mechanical spreading action to which the roving is subjected in the bath. The rates achievable in the present process are at least comparable with the rates achievable in thermoset pultrusion processes because the latter process is restricted by the time required to complete necessary chemical reactions after the impregnation stage. In a preferred embodiment the spreader surfaces over which the rovings are pulled to provide separation of the rovings are provided with an external heat input to heat the spreader surface a temperature above the melting point of the particular polymer to be used for impregnating the roving. By this means the melt viscosity of the polymer in the local region of the spreader surface may be maintained at a considerably lower value than the polymer in the bulk of the impregnation bath. This process has the advantage that a very small proportion of the polymer can be raised to a relatively high temperature so that a low impregnation viscosity can be obtained with minimum risk of degradation to the major proportion of the polymer in the bath. This in turn greatly reduces the problem arising from the fact that some polymer may remain in the bath for an almost unlimited period during a given processing period because the polymer supply in the bath is continually replenished. Thus some of the polymer present at the start of a processing period may still be present in the bath at the end of the processing period. Despite this long dwell time in the bath such polymer will have been subjected to a less severe thermal history than if the whole of the polymer in the bath had been subjected to a high temperature to obtain a low viscosity throughout the bath. A further advantage of the local heating process is that polymers of poorer thermal stability may be used. Furthermore, polymers of higher molecular weight may be used because the lower degradation arising from the lower overall thermal history enables a higher temperature to be used locally to produce lower viscosity melt. The supply of polymer to the impregnation bath may be in the form of polymer powder which is melted in the bath by external heating elements or by the internally disposed heated spreader surfaces or alternatively the bath may be fed with molten polymer using, for example, a conventional screw extruder. If the bath is provided with heated spreader surfaces the polymer melt delivered from the extruder should be at as low a temperature as possible to minimise thermal degradation. The use of a melt feed has the advantages of easier start-up, better temperature control and the avoidance of unmelted polymer lumps which result in various processing problems particularly when very thin structures are produced. The impregnated fibre product may be pulled through a means for consolidating the product such as a sizing die. The temperature of the die has been found to have a significant effect on the process. Although it would be predicted that a hot die should be used to minimise friction in the die and to aid consolidation it has been found that a die which is held at a temperature at or above the melting point of the polymer used gives rise to an erratic stick-slip behaviour as the product is pulled through the die. It has been found that it is preferable to use a cooled die and to ensure that the surface temperature of the pultruded section entering the die is at a temperature of not more than 20°C above the softening temperature of the polymer. By "softening temperature" we mean the lowest temperature at which the polymer can be sintered. This can be achieved by blowing air on the lace during its path between the impregnation bath and the die and/or spacing the die from the impregnation bath. If the pultruded section is too hot polymer is squeezed out as the product enters the die. This leaves a deposit at the entrance to the die which builds up and can score the pultruded section as it passes through the die. The pultruded section should not be cooled to a temperature below the softening point of the polymer because it will be too difficult to shape the product in the sizing die. In a further process for producing fibre-reinforced pellets according to the invention it has been found possible to achieve satisfactory wetting even though the thermoplastics polymer used has a melt viscosity significantly in excess of 30 Ns/m 2 . This is achieved in a process wherein after tensioning and aligning a plurality of continuous filaments to provide a band of contiguous filaments, the band is passed over a heated spreader surface so as to form a nip between the band and the spreader surface, a feed of a thermoplastics polymer is maintained at the nip, the temperature of the spreader surface being sufficiently high to give a polymer melt of viscosity capable of wetting the continuous filaments as they are drawn over the spreader surface and the fibre reinforced composition produced is chopped into pellets. Whilst it is preferred that the polymer melt in the cusp of the nip has a viscosity of less than 30 Ns/m 2 , a high back tension on the filaments fed to the spreader surface will ensure that polymer impregnation in the nip area is favoured, so that it is possible to produce a well impregnated band at a significantly higher viscosity than 30 Ns/m 2 . Thus this process provides a means of maximising the molecular weight of the polymer that may be used in a thermoplastics polymer pultrusion process and in pellets chopped from the resulting product. In one embodiment of this process of producing the pellets the continuous filaments are most suitably tensioned and aligned by pulling them from rolls or reels over a series of spreader surfaces, such as the surfaces of rods. This enables bundles of filaments to be spread out as far as possible into individual filaments which are under considerable tension. These filaments are guided to provide a band of contiguous filaments as they pass over a heated spreader surface. The shape of the spreader surface and the angle of contact of the filament band with the surface should be such as to provide a nip between the band and the heated spreader surface. A thermoplastics polymer powder is fed to the nip and the heated spreader surface is maintained at a temperature sufficient to melt the thermoplastics polymer. The melt impregnates and wets the fibres of the band as the band passes over the heated spreader surface. This process may be further modified by providing at least one further heated spreader surface with which the at least partially polymer impregnated fibre band forms a second nip by means of which a further supply of polymer melt may be impregnated into the fibre band. Either surface of the partially impregnated band can be used to form the working surface of the nip. The amount of polymer in the reinforced structure is controlled to a large extent by the tension in the band and the length of path over which the band contacts the heated spreader surface. Thus where the band is under high tension and contacts the spreader surface over a substantial area, so that the band is strongly urged against the spreader surface, the polymer content of the reinforced structure will be lower than under low tension/short contact path conditions. The heated spreader surfaces and any subsequent heated or cooled surfaces used to improve impregnation or to improve surface finish are preferably in the form of cylindrical bars or rollers. These may be stationary or capable of either free or driven rotation. For example, the first impregnation surface may be a freely rotating roller which will be caused to rotate by the band at the speed of the band so that attrition of the fibre prior to impregnation or sizing by the melt is reduced to a minimum. It has been observed that if the first roll is rotated (either freely or driven) in the direction of the movement of the fibre at up to the speed of the fibre any accumulation of loose fibre on the band is carried through the system. This self-cleaning action is particularly useful in preventing an accumulation of fibre at the first roll which could cause splitting of the band. After the band has picked up some molten polymer, preferably after being provided with further molten polymer on the other side of the band by means of a second freely rotatable heated surface, the fibre is much less susceptible to attrition and may be subjected to treatments to improve wetting of the fibres. Thus the polymer-containing band may be passed over at least one roller driven in a direction opposite to that of the travel of the band to increase the local work input on the band and maximise wetting. In general, the degree of wetting and the speed of the process may be increased by increasing the number of surfaces at which there is a work input. A further advantage of the process employing a band of fibre to form a nip, over the process which requires the use of a bath of molten polymer, is that of reducing the risk of degradation. Thus the relatively small amount of polymer present in the nip between the fibre band and the spreader surface ensures that large quantities of polymer are not held at an elevated temperature for prolonged periods. Provision can also be made to include a scraper blade at positions at which polymer is fed to the nip to remove any excess polymer which might accumulate during processing and which might be subject to thermal degradation. The impregnated products of the processes hereinbefore described are chopped into lengths for subsequent fabrication. These may be used in conventional moulding or extrusion processes. When glass fibre is used the fibre content of the product of the invention should be at least 50% by weight of the product to maximise the physical properties of the product. The upper limit of fibre content is determined by the amount of polymer required to wet out the individual fibres of the roving. In general it is difficult to achieve good wetting with less than 20% by weight of polymer although excellent results are obtainable using the process of the invention to incorporate 30% by weight of polymer in the fibre-reinforced composition. The pultruded products of the invention are also suitable, when chopped to appropriate dimensions for providing selective reinforcement in shaped articles moulded from polymeric materials in a process in which at least one preformed element consisting of a product according to the present invention is located in a mould to provide reinforcement in a selected portion of the finished moulding and polymeric material is moulded around the in situ reinforcement to provide a shaped article. The invention not only permits the fibrous reinforcement to be located in the shaped article with maximum effect with respect to the stresses the shaped article will be subjected to in use but overcomes the processing problems involved in producing such high strength articles by alternative processes. In particular the process can be used to produce such reinforced articles using the high productivity injection moulding process using conventional thermoplastic polymers with melt viscosities of 100 Ns/m 2 or more. It may be advantageous in some applications to use the preformed element at a temperature at which it is readily pliable so that it can be more readily located in the mould, for example by winding the heat softened preformed element on a mould insert. The moulding process used may be any process in which a shaped article is formed from a polymeric material in a mould. The polymeric material may be a thermoplastics material which is introduced into the mould as a melt, such as in injection moulding or as a powder, as in compression moulding. Included in the term "compression moulding" is the process of compressing a polymer powder without melting and subsequently sintering the "green" moulding outside the mould. The thermoplastics polymeric material moulded in the mould may also be derived by introducing into the mould a monomer or monomers, or a partially polymerised medium which is held in the mould until fully polymerised, for example, under the influence of heat or chemical activators or initiators. It is preferred that the polymer which is moulded around the preformed insert is the same as or is at least compatible with the polymer used to impregnate the preformed insert. The impregnated products obtained from the processes hereinbefore described find particular utility when chopped into pellets or granules in which the reinforcing fibres have a length of at least 3 mm and preferably at least 10 mm. These products may be used in the conventional fabrication process such as injection moulding and show advantages over prior art products in pellet form because the fibre length in the pellet is retained to a much greater extent in articles fabricated from the pellets of the invention than when using the prior art products. This greater retention of fibre length is believed to be a result of the greater protection afforded to the individual reinforcing filaments in the product of the invention by virtue of the good wetting by polymer which arises from use of the processes hereinbefore described. This aspect of the invention is particularly important because it enables reinforced articles to be formed in versatile operations, such as injection moulding, which employ screw extrusion processes to melt and homogenise the feed material, with a surprisingly high retention of fibre length and consequent enhancement of physical properties. Thus the product of the invention enables moulded articles to be obtained from fabrication processes which employ screw extrusion which articles contain at least 50% and preferably at least 70% by weight of the fibres in the article of a length at least 3 mm long. This is considerably longer than currently obtainable from the commercially available reinforced products. An alternative process of forming moulded articles by melting and homogenising the products of the invention, that is lengths between 2 and 100 mm, is by calendering. For example, a sheet product can be made in this manner. The products suitable for injection moulding may be used directly or may be blended with pellets of other thermoplastics products. These other products may be of the same polymer but having higher molecular weight or may be of a different polymer providing that the presence of the different polymer does not adversely affect the overall balance of properties of the composition. The other products may be an unfilled polymer or may contain a particulate or fibrous filler. Blends with materials containing the conventionally produced reinforced moulding powders, that is moulding powders with reinforcing fibres up to about 0.25 mm long are particularly suitable because the overall reinforcing fibre content of the blend can be kept high to produce maximum strength even though the shorter reinforcing fibres do not contribute so effectively as the long fibres present from the product of the present invention. The chopped form of the continuous pultrusion is also very useful as a feedstock for a method in which a fibre-reinforced shaped article is produced by extruding a composition comprising a settable fluid as a carrier for fibres at least 5 mm in length through a die so that relaxation of the fibres causes the extrudate to expand to form an open fibrous structure containing randomly dispersed fibre as the extrudate leaves the die and compressing the porous structure produced whilst the carrier is in a fluid condition into a shaped article. By "settable" is meant that the fluid may be "set" into such a form that it holds the fibre in the random orientation, which occurs on extrusion. Thus, for example, the settable fluid may be a molten thermoplastics material which is extruded in its molten state and then set by cooling until it freezes. Preferably the expanded extrudate is extruded directly into a mould chamber provided with means for compressing the porous extrudate into a shaped article and the extrudate is compressed into a shaped article before the extrudate is caused or allowed to set. The extrudate formed in the process contains randomly dispersed fibres so that the only orientation of fibres in the shaped article is that which might arise as a result of the compression process. The process can be used at high fibre loadings, that is in excess of 30% by volume of fibre. Little fibre breakage occurs in the process so that shaped articles of exceptionally high strength measured in all directions in the article can be obtained. Pellets obtained by chopping the pultruded product of the present invention into lengths of at least 5 mm and preferably at least 10 mm are preferred. The upper limit is determined by the extent of the problems encountered in feeding material to the extruder which will melt the product. Lengths at least up to 50 mm can be employed although with long lengths the amount of fibre breakage increases so that the benefit of long fibre length is partially eroded. When the relatively low molecular weight polymers, for example polymers having a melt viscosity below 30 Ns/m 2 and preferably below 10 Ns/m 2 are used to achieve adequate wetting of the roving, although it is surprising that such a product has such high levels of physical properties, the invention does not exclude the subsequent processing step of increasing the molecular weight of the polymer in the composition by known techniques. Such techniques include solid phase polymerisation in the case of condensation polymers, the use of cross-linking agents or irradiation techniques. In the case of increasing the molecular weight using cross-linking agents it is necessary to intimately mix these in the composition. This may only be practicable if they are already present ouring the impregnation process but in such cases care must be taken to ensure that they are not activated before the wetting process is complete. The invention is further illustrated with reference to the following examples. EXAMPLE 1 Copolymers of polyethylene terephthalate in which 20% by weight of the terephthalic acid had been replaced by isophthalic acid and having the intrinsic viscosity values listed in Table 1 were used to prepare polymer melts in a bath at a temperature of approximately 290°C. A glass roving containing 16000 individual filaments was pulled through the molten polymer over one spreader bar situated in the bath at a rate of 30 cm/minute giving a dwell time in the bath of 30 seconds. The impregnated roving was pulled through a 3 mm diameter die in the wall of the bath and then cooled. The viscosity of the melt and the intrinsic viscosity of the polymer feedstock and the polymer in the reinforced composition were measured. The extent of the wetting of the fibres and the void content were assessed by comparing the weight of a completely wetted length of impregnated product with the same length of product with an unknown extent of wetting. The completely wetted control material is obtained by operating the pultrusion process at a very slow rate with a low viscosity melt so that a completely transparent product is obtained. Thus the completely wetted standard is taken to be a sample which is transparent and which has been prepared under conditions which optimise the parameters favourable to wetting. The extent of wetting values given in the table are derived from the relationship: Degree of wetting = M 2 - M 1 M 0 - M 1 x 100% where the mass per unit length of the transparent sample is M 0 , the mass per unit length of the glass is M 1 and the mass per unit length of the sample to be assessed is M 2 . The void content is given by subtracting the percentage degree of wetting from 100%. The strength of the product was assessed by measuring the force required to break a specimen of the 3 mm rod in flexure placed across a 64 mm span. The results obtained are given in Table 1. Intrinsic viscosity (dl/g) Melt viscosity during pultrusion (Ns/m 2 ) Extent of fibre wetting (%) Force at break (N) Feedstock After pultrusion 0.18 0.15 0.2 100 129 0.4 0.35 1.8 100 158 0.43 0.38 3.0 100 151 0.45 0.4 6 96 143 0.49 0.44 15 86 154 0.60 0.5 30 70 122 EXAMPLE 2 The polymer used in Example 1 having an intrinsic viscosity of 0.45 dl/g was evaluated over a range of melt temperatures and haul through rates. The results obtained are recorded below in Table 2. Melt temperature (°C) Melt viscosity (Ns/m 2 ) Extent of fibre wetting (%) Force at break (N) Haul through rate of (cm/min) 21 36 60 21 36 60 255 18 98 84 62 102 70 40 275 10 100 100 90 121 114 70 290 6 100 100 92 139 127 94 310 3 98 98 92 155 140 94 EXAMPLE 3 A PET homopolymer having a melt viscosity at 280°C of 6 Ns/m 2 was pultruded as described in Example 1 using a glass fibre made up of filaments of diameter 17 µm at 280°C using a single spreader bar and a line speed of 30 cm/minute to give a pultruded rod approximately 3 mm diameter. The glass content of the product was varied by altering the number of strands in the rovings fed to the bath. The flexural modulus and force at break were determined as a function of glass content using a 64 mm span. Weight % glass Flexural Modulus (GN/m 2 ) Force at break (N) By calculation By ash content 62 63 42(3) 217(19) 56 37(2) 201(36) 52 38(3) 204(16) 47 27(5) 191(24) 43 17(4) 112(25) 37 36 12(1) 92(25) (Five determinations in each case, figure in parenthesis indicates standard deviation.) These results indicate an approximate plateau in modulus and strength in the region 50 to 65% glass (by weight). EXAMPLE 4 Conventional grades of polypropylene have viscosities at low shear rates in excess of 100 Ns/m 2 and are not conveniently processed by pultrusion. For example the melt viscosity of 'Propathene' HF11, a polypropylene homopolymer, is about 3000 Ns/m 2 at low shear at 280°C or about 10000 Ns/m 2 at 230°C. In order to make a polymer suitable for pultrusion 'Propathene' HF11 was blended with 0.1% calcium stearate, 0.1% 'Irganox' 1010 and 0.5% 'Luperco' 101XL ('Luperco' 101XL is an organic peroxide dispersed with calcium carbonate) so that degradation would occur. This composition was pultruded at 30 cm/minute using a single spreader at temperatures of 230°C and 290°C. At 230°C (melt viscosity 30 Ns/m 2 ) the wetting was poor. At 290°C (melt viscosity 17 Ns/m 2 ) the wetting was moderate. EXAMPLE 5 A sample of 'Victrex' polyethersulphone having an RV of 0.3 was pultruded with the glass fibre used in Example 3 at 405°C at 21 cm/minute using a single spreader bar (having a melt viscosity of 30 Ns/m 2 ) giving a moderately wetted extrudate. At lower temperature, where the viscosity was higher, the sample was poorly wetted. EXAMPLE 6 The wetting of the rovings is clearly influenced by the number of spreader bars, and for the same operating conditions an increase in line speed can be effected by increasing the number of spreaders for any degree of wetting. The glass fibre used in Example 3 was pultruded at 280°C with PET homopolymer using a single spreader and a speed of 20 cm/minute to give a totally wetted product (transparent). The dwell time in the bath under these conditions was about 30 seconds. The use of three spreaders allowed an increase in line speed to 120 cm/minute for a transparent well-wetted pultrusion. The dwell time under these conditions was about 10 seconds. EXAMPLE 7 A number of polymers were used according to the general procedure of Example 1 to produce pultruded sections from a glass roving containing 16000 filaments. The roving was pulled through molten polymer over one spreader bar at a rate of 15 cm/minute to give a product containing about 65% by weight of glass in each case. The polymers used, the melt temperatures employed, the melt viscosities at those temperatures and the properties obtained are detailed in Table 4. Polymer Type Processing Conditions Physical Properties Melt Temperature (°C) Melt Viscosity (Ns/m 2 ) Flexural Modulus (GN/m 2 ) Flexural Strength (MN/m 2 ) Interlaminar Shear Strength (MN/m 2 ) Poly(ethylene terephthalate) 290 6 31 550 42 Nylon 66 320 30 28 600 - Poly(methyl methacrylate) 250 8 30 550 - Polypropylene 260 5 30 350 10 Polyetheretherketone 400 20 30 650 45 Polyethersulphone 350 8 30 500 - Polyphenylene sulphide 320 5 40 750 23 In the case of the polyethylene terephthalate the pull through speed was increased above 15 cm/mm to examine the effect of void content on physical properties. Table 5 below records properties measured on the 3 mm diameter rod produced. These indicate that a void content of less than about 5% gives superior properties. Void content (%) Flexural Modulus (GN/m 2 ) Flexural Strength (MN/m 2 ) Interlaminar Shear Strength (MN/m 2 ) Flexural Modulus as percentage of calculated value 0.2 31 550 42 91 5 31 570 42 91 6 28 550 41 90 10 24 480 32 82 13 23 440 36 74 15 20 330 29 71 EXAMPLE 8 A sample of carbon fibre-reinforced polyetherketone was prepared by pulling a carbon fibre tape containing 6000 individual filaments through a bath of molten polyetherketone at a temperature of 400°C at a speed of 25 cm/minute. A product having a flexural modulus of 80 GN/M 2 , a breaking stress of 1200 MN/m 2 and an interlaminar shear stress of 70 MN/m 2 was obtained. EXAMPLE 9 This example illustrates how the mechanical properties of pultrusions vary with the volume fraction of fibre and with the resin type. The samples are compared at fixed volume concentration. The low flexural strength of the composites based on polypropylene is a reflection of the tendency for less stiff resins to fail in a compressive mode. Polypropylene resin has a modulus of about 1 GN/m 2 while polyethylene terephthalate has a modulus of about 2 GN/m 2 . The pultrusions were produced according to the general procedure of Example 1 with resins of the preferred viscosity level, about 3 Ns/m 2 . Resin Volume fraction glass % Flexural Modulus (GN/m 2 ) Flexural strength (MN/m 2 ) Interlaminar shear strength (MN/m 2 ) PET 40 21 630 26 50 31 690 60 38 800 Polypropylene 40 26 340 10 50 33 340 60 38 310 This Example indicates a clear preference for a resin of high modulus for applications where high compressive strength is required. EXAMPLE 10 A rod of 65% by weight glass fibre in PET was chopped to 1 cm lengths and diluted on a 50/50 basis with normally compounded material containing 30% by weight of short glass fibre in PET. This mixture was injection moulded using normal technology to give ASTM bars having the following properties in comparison with normally compounded material containing 50% by weight of glass fibre in PET. Blend containing pultruded material Normally compounded Flexural Modulus 16.1 14.4 Impact Energy notched Izod test 270 J/m 120 J/m Glass content (wt %) 47% 45% Inspection of ashed sections of the moulding revealed that most of the long fibres had been retained through the moulding operations. This unexpected property is believed to result from the low void content or high degree of fibre wetted by polymer in the chopped pultruded material. EXAMPLE 11 Various Examples of pultruded materials including PET with 60% by weight of glass fibre and PEEK containing 60% by weight of carbon fibre were chopped to 1 cm length and moulded using the method described in European Patent Publication No. 56702 in which an expanded reinforced material is produced by extrusion through a die of short length, preferably of zero length, and is subsequently compression moulded to give three dimensional shaped articles containing 60% by weight of long fibre. Pultruded material is especially suitable for this application because the high level of wetting obtained effectively protects the fibres and reduces the attrition between them which causes fibre breakdown. EXAMPLE 12 Blends were made of conventional glass filled PET (short fibre compounded material prepared by extrusion compounding with PET of IV - 0.75) and chopped 10 mm pultrusions (prepared according to Example 3). These blends were injection moulded to give discs 114 mm diameter and 3 mm thick filled from a rectangular side gate 1.5 mm thick by 10 mm wide. These samples were subjected to impact in an instrumented falling weight impact test and the failure energy noted. Test Wt % short fibres Wt % long fibres Total Wt % fibres Failure energy J Standard deviation 1 30 - 30 4.8 (0.6) 2 45 - 45 5.3 (0.6) 3 22.5 15 37.5 7.3 (0.6) 4 15 30 45 8.7 (0.4) 5 7.5 45 52.5 8.8 (0.4) 6 - 60 60 8.7 (1.0) All samples filled the mould with similar ease. This is because the polymer used to prepared the pultrusion samples was of lower molecular weight than that used to prepare the short fibre compound and this lower molecular polymer offset the increased resistance to flow due to the long fibre. The results clearly indicate increased failure energy for the long fibre filled material despite the lower molecular weight of the polymer which would normally be expected to contribute to brittleness. Note especially the comparison between tests No. 2 and No. 4 and the same total weight percentage of fibres. We further noted that whereas the short fibre mouldings splintered on impact allowing pieces of sharp plastic to fly off when more than half the weight fraction was of long fibres the mouldings failed in a safe manner all broken pieces remaining attached to the main part. Ashing the mouldings after test revealed that much of the long fibre glass had retained most of its original length. Considerably more than 50% by weight of the original long fibres in the mouldings were more than 3 mm long. The samples were also assessed for flexural modulus, anisotropy ratio, Izod impact strength and IV of the polymer in the mouldings. The values in the table below indicate reduced anisotropy and good notched impact relative to short fibre products. Test Bar flexural modulus (GN/m 2 ) Disc flexural modulus (GN/m 2 ) Anisotropy ratio Izod impact IV 0° 90° notched (Jm -1 ) unnotched (Jm -1 ) 1 10.3 9.7 6.3 1.54 70 414 0.53 2 16.5 14.4 8.9 1.62 90 590 0.524 3 11.4 10.7 7.3 1.47 117 432 0.46 4 14.1 10.9 8.2 1.33 172 326 0.4 5 15.1 10.3 7.7 1.34 195 312 0.34 6 15.4 10.8 8.7 1.24 177 219 0.28 EXAMPLE 13 14 tapes of continuous carbon fibres (supplied by Courtaulds Ltd and designated XAS carbon fibres), each containing 6000 individual filaments were drawn at a rate of 25 cm/minute over a series of stationary guide bars to provide a band of width about 50 mm having a tension of about 100 lbs. When the fibres had been guided into contiguous relationship they were pulled over a single fixed heated cylindrical bar of 12.5 mm diameter. The temperature of the bar was maintained at about 380°C. A powder of polyetheretherketone having a melt viscosity of 20 Ns/m 2 at this temperature was fed to the nip formed between the carbon fibre band and the fixed roller. The powder melted rapidly to provide a melt pool in the nip which impregnated the fibre band passing over the roller. The structure was passed over and under five further heated bars without the addition of further polymer. A carbon fibre-reinforced sheet containing 58% by volume of carbon fibre and having a thickness of 0.125 mm was produced. The product was found to have the following properties: Flexural Modulus 130 GN/m 2 Flexural Strength 1400 MN/m 2 Interlaminar Shear Strength 90 MN/m 2 EXAMPLE 14 The procedure of Example 13 was used with a polyethersulphone having a melt viscosity of 3 Ns/m 2 at 360°C to produce a reinforced product containing 40% by volume of carbon fibre. The temperature of the roller was maintained at about 360°C. The product had a flexural modulus of 80 GN/m 2 and a flexural strength of 700 MN/m 2 . EXAMPLE 15 The procedure of Example 29 was used with the commercially available polyethersulphone PES 200P (available from Imperial Chemical Industries PLC) having a viscosity of 800 Ns/m 2 at 360°C. The roller temperature was maintained at about 360°C and a product with 44% by volume of carbon fibre was produced. The product had the following properties: Flexural modulus 60 GN/m 2 Flexural strength 500 MN/m 2 Interlaminar shear strength 25 MN/m 2 EXAMPLE 16 The general procedure of Example 13 was followed to produce an impregnated sheet using 14 tapes of continuous carbon fibres (Courtaulds' XAS, 6K tow) and polyetherether ketone having a melt viscosity of 30 Ns/m 2 at 370°C. In the apparatus five cylindrical bars each of diameter 12.5 mm were heated to 380°C. The 14 tapes were drawn under tension to give a band 50 mm wide passing into an adjustable nip formed by the first two bars with their longitudinal axes in a horizontal plane. The band subsequently passed under and over three further heated bars also having their longitudinal axes in the same horizontal plane. The use of the first two bars to form a nip enabled a polymer feed to be fed on both sides of the band. To avoid spillage of polymer two retaining metal sheets were placed in contact with the first two heated bars, disposed along the length of the bars, to provide a feed trough. Polymer powder was fed to either side of the band passing through the first two heated bars. The powder melted rapidly forming a melt pool in the two nips between either side of the band and each heated bar. The gap between the first two bars was adjusted so that when the haul-off was run at 0.5 m/minute the carbon fibre was coated with polymer and the resulting impregnated tape contained approximately 60% by weight of carbon fibre and 40% by weight of polymer. Adjustments to the fibre content were found to be achievable in several ways: 1 varying nip gap, 2 varying pretensioning, 3 varying the number of filaments fed to the nip, 4 varying the powder feed rate, 5 varying the temperature of the bars at the nip (with the resin used in this example the temperature range preferred was not greater than 400°C because of degradation and not less than 360°C because of the onset of crystallisation), 6 varying the haul-off rate. The tape so formed appeared to be well wetted and was about 0.1 mm thick. EXAMPLE 17 The tape described in Example 16 was cut to lengths of 150 mm and stacked in a matched die compression moulding tool. This tool was heated to 380°C, in a conventional laboratory press, and compressed so that the moulding was subjected to a pressure of between 2 and 5 x 10 6 N/m 2 . The moulding was held at pressure for 10 minutes (approximately half of which time was required for the mould and sample to reach equilibrium temperature) and then cooled under pressure to 150°C before removal from the press. The cooling stage took approximately 20 minutes. The mould was allowed to cool to ambient temperature and the moulding was then extracted. Mouldings ranging in thickness from 0.5 mm (4 plies) to 4 mm (38 plies) were formed in this way. During the moulding operation a small amount of polymer was squeezed out of the mould as flash so that the mouldings contained 62% by weight of carbon fibre by comparison with 60% by weight in the original tape. The mouldings were then cut using a diamond wafering saw to give specimens suitable for mechanical testing by flexural techniques. The following results were obtained: Property Specimen span/depth ratio Value Flexural modulus 70:1 130 GN/m 2 Flexural modulus 30:1 115 (6) GN/m 2 Flexural strength 30:1 1191 (55) MN/m 2 Transverse flexural strength 5:1 98 (11) MN/m 2 Interlaminar shear strength 5:1 81 (4) MN/m 2 (figures in parentheses indicate standard deviation). EXAMPLE 18 Using the same equipment as Example 16 some poorly wetted tapes were produced by starving some sections of the tape while flood feeding others. The overall fibre content of the tape was the same as in Example 4 but many loose fibres were apparent on the surface of the tapes while other areas were rich in resin. These tapes were stacked and moulded as described in Example 17 taking care that poorly wetted regions of one tape were placed adjacent to resin rich areas in the next tape. Visual inspection of the mouldings showed that substantial unwetted areas remained and loose fibres could easily be pulled from the surface. The mechanical properties of these mouldings was inferior to those noted in Example 33 and in particular the interlaminar shear strength was variable and low, values of 10 MN/m 2 (by comparison with 81 for the well wetted sample) being common. This Example illustrates that fibre wetting takes place primarily at the impregnation stage and not during the secondary, moulding, stage. It is, however, believed that some wetting could be achieved by this secondary stage if higher pressures and longer dwell times were employed. EXAMPLE 19 Carbon fibre ('Celion' 6K and 3K tows) were impregnated with a thermotropic polyester containing residues of hydroxynaphthoic acid, terephthalic acid and hydroquinone having a melt viscosity at 320°C of 7 Ns/m 2 . The equipment was the same as described in Example 32 except that the bars were maintained at 320°C. A tape 0.1 mm thick containing 62% by weight of carbon fibre was of good appearance. EXAMPLE 20 Various pieces of scrap material produced from working Examples 16 to 18 including some material with excess resin were broken up and fed to a conventional screw extruder and compounded to form granules. The granules contained carbon fibres which were up to 0.25 mm long. These granules were injection moulded using conventional moulding technology under normal operating conditions for filled PEEK. The mouldings had the following properties by comparison with the best available commercial grade of carbon fibre filled PEEK prepared by conventional compounding operations: Compounded scrap scrap Best commercial grade Wt % carbon fibre 30 55 Modulus 32 GN/m 2 13 GN/m 2 Tensile strength 250 MN/m 2 190 MN/m 2 Surface quality Good Good This Example illustrates that the product of this invention is capable of being converted to a product for conventionaa processing which is in some way superior to that available by present technology. Also the scrap from various long fibre operations such as sheet preparation, lamination, filament winding etc can be reclaimed to give a high performance material. The characteristic of reclaimability is of great economic significance when working with expensive raw materials such as carbon fibre. EXAMPLE 21 The optimum tensions in rovings when operating according to the method of Example 31 were determined by measuring the tension before impregnation and at the haul-off stage in an individual roving containing 6000 filaments (14 rovings were used in Example 29 and the operating tension will in practice be 14 times the values given below). The values quoted below were judged to be the minimum operation tension (Case 1) and the maximum operating tension (Case 2) for the particular rovings, polymer type and equipment used. With tension values below those of Case 1 there was a tendency for misalignment of fibres and splits in the tape produced. With tension values above those of Case 2 increased fibre attrition was observed with loose fibres accumulating on the band. For a different set of conditions (rovings, polymer type etc the values obtained wwll be different but can be readily optimised to produce a good quality product. Tension before impregnation Haul-off tension Case 1 0.14 kg 2.4 kg Case 2 0.37 kg 3.8 kg

Claims: Moulding pellets being cut to a length between 2 and 100 mm from a thermoformable fibre-reinforced structure comprising a thermoplastic polymer and at least 30% by volume of aligned reinforcing filaments which have been wetted by a melt of the thermoplastic in a melt pultrusion process, characterised in that the filaments have been wetted by a melt which has a viscosity at low shear rates of less than 100 Ns/m 2 so as to give pellets in which the extent of wetting is at least 70%. Moulding pellets according to claim 1 wherein the structure from which the pellets are cut has a longitudinal flexural modulus determined by ASTM D790-80 of at least 90% of the theoretically attainable flexural modulus. Moulding pellets according to either of claims 1 or 2 wherein the pellets can be injection moulded into an article in which the fibres are present in the form of individually dispersed filaments at least 50% by weight of which retain a length of at least 2 mm in the moulded article. Moulding pellets according to any one of claims 1 to 3 in which the molecular weight of the polymer has been increased after the filaments have been wetted by the thermoplastic polymer. Moulding pellets which have been obtained by melt homogenising the product of any one of claims 1 to 4, extruding the molten product and chopping it into pellets. A composition comprising a blend of moulding pellets according to any one of claims 1 to 4 and pellets of other thermoplastic products. A fibre-reinforced thermoplastic moulded article formed from the pellets of claim 1.

Inventors: COGSWELL FREDERIC N, HEZZELL DAVID J, WILLIAMS PETER J, COGSWELL, FREDERIC NEIL, HEZZELL, DAVID JOHN, WILLIAMS, PETER JOHN
Applicants: KAWASAKI CHEM HOLDING, KAWASAKI CHEMICAL HOLDING CO., INC.
Application year: 1982
Main classification: B29B9
IPC codes: C08J5, C08L33, C08L77, C08L67, B29B15, C08K7, C08L23, B29B9, B29C45, B29C70, C08L81, C08L71