FIBRE-CONTAINING REINFORCING MATERIAL AND A METHOD OF MAKING SAID MATERIAL

Abstract: A fibre-containing reinforcing material and a method of making said material, and also the use of the reinforcing material for the production of composite materials are disclosed. The reinforcing material comprises a homogeneous distribution of carbon fibres and glass fibres in a weight ratio of from 1:99 to 99:1, a hydrophobic, anionic, colloidal silica in an amount of about 5-20 % by weight of the total fibre weight having been added to the fibres to coat them. In addition, the reinforcing material contains about 5-20 % by weight talc, preferably soapstone, based on the total fibre weight. The reinforcing material is produced by dry dispersion of carbon fibre bundles under shear action for about 10-30 min., silica being added. Glass fibres are added to the mixture of carbon fibres and silica and dry-dispersed together therewith to form a substantially homogeneous mixture, whereupon cationic talc, preferably soapstone, is distributed in the mixture. The finished reinforcing material can be used for the production of fibre-reinforced composite materials.

Description: FIBRE-CONTAINING REINFORCING MATERIAL AND A METHOD OF MAKING SAID MATERIAL The present invention relates to a fibre-containing reinforcing material and to a method of making such a material. The fibre-reinforcing of materials having low tensile strength and low resistance is previously known. For a long time past, different materials have been reinforced with fibres of different types, such as glass, steel, mineral wool and plastic fibres. Fibre-reinforcement mainly aims at increasing the toughness of the material , which, in conjunction with a higher tensile strength, also affects other characteristics and improves the impact strength, wear resistance and dimensional stability of the material. Fibre-reinforced materials are used in many types of products, from sports articles to jet aircraft com¬ ponents. However, technology is still requiring new composite materials having high strength, resistance to fracture and oxidation stability at elevated tempera¬ tures. The present invention aims at obviating or reducing the difficulties ensuing from insufficient strength, resistance to fracture and oxidation stability in con¬ nection with prior art composite materials. This object is achieved in that the invention provides a fibre-con- taining reinforcing material which, when used in the production of fibre-reinforced composite materials, imparts to these materials high strength values also at elevated temperatures of about 100-120°C. It is known that mechanical mixtures of so-called glass filaments of the type roving and carbon fibres when used as reinforcing materials for the production of composite materials significantly improve the strength values and elasticity modulus of the composite material at normal temperatures. At elevated temperatures, however, such as 120°C, the strength values deteriorate markedly because, so far, it has not been possible to produce a homogeneous mixture of the fibres, nor has it been possible to establish chemical and physical bonds between the fibres. Also, it is very difficult to split carbon fibre bundles cut into very short lengths (about 0.5-6.0 mm) into separate carbon fibre filaments. The same difficulty exists with glass filaments of the type roving cut into relatively short lengths and bonded together in parallel filament bundles. Of supreme importance to the effect of the rein¬ forcing material are, inter alia, the length and diameter of the fibres and their chemical and physical surface structure. For a good reinforcing effect, the length/ diameter ratio should be as high as possible. When this ratio, the L/D number, i.e. the fineness of the filament, is increased, the possibilities of intensely working the mixture of fibres in the matrix of the composite material are decreased. Glass filaments of the type roving normally have a diameter of between 10 and 20 μm, while the filament length may vary between a few millimetres up to several centimetres. The limitations of roving type filaments lie in the diameter which cannot be reduced below about 8-10 μm. Consequently, if it is desired to increase the L/D number, there remains, in principle, only the possibility of increasing the filament length. However, if the length is increased, it becomes more and more difficult to achieve a uniform distribution of the filaments admixed to the matrix, and the percentage of roving admixture must therefore be kept low. If a higher roving percentage in the com¬ posite material is desired, the filament length must not be set at too high a value, and this of course gives a lower reinforcing effect. To achieve the desired re¬ inforcing effect, one must instead combine several fila- ments to form still larger bundles, so-called flakes, and this in turn adversely affects the mixing with carbon fibres because only a mechanical cooperation in the matrix is obtained, and no chemical or physical fibre-to- fibre adhesion. As a consequence, no essential improvement of the strength values is obtained. The present invention aims at providing a solution to the problem of defibrating carbon fibre bundles into individiual carbon fibres and, furthermore, at providing a homogeneous mixture consisting of carbon fibres and glass fibres and serviceable as a reinforcing material in the production of composite materials. To this end, the present invention provides a fibre- containing reinforcing material which is characterised in that it comprises an essentially homogeneous distribu¬ tion of carbon fibres, glass fibres, hydrophobic, anionic, colloidal silica and cationic talc, the weight ratio of carbon fibres to glass fibres being from 1:99 to 99:1, the silica forming about 5-20% by weight of the total fibre weight, and the talc forming about 5-20% by weight of the total fibre weight. The invention also provides a method of making a fibre-containing reinforcing material, a method which is characterised by a) adding, to carbon fibre bundles, hydrophobic, anionic, colloidal silica in an amount of about 5-20% by weight, based on the total fibre weight of the final reinforcing material, the carbon fibre bundles being divided into individual fibres by mixing under shear action in dry con¬ dition; b) adding, to said mixture of carbon fibres and silica, glass fibres in an amount to give a weight ratio of carbon fibres to glass fibres of from 1:99 to 99:1; and c) distributing, in said mixture of carbon fibres, silica and glass fibres, about 5-20% by weight, based on the total fibre weight, of cationic talc. Finally, the invention also comprises the use of the above-mentioned fibre-containing reinforcing material for the production of fibre-reinforced composite mate¬ rials. These and other characteristic features will appear from the following description and the appended claims. The invention will now be explained in more detail. The fibre-containing reinforcing material according to the invention contains four principal components, i.e. carbon fibres, glass fibres, hydrophobic, anionic, colloidal silica, and cationic talc. These components and the restrictions placed thereon in the context of the invention will be explained below. The carbon fibres used in the context of the in¬ vention are of conventional type and are initially in the form of carbon fibre bundles. This raw material and the production thereof are well known and need not be described in detail. The dimensions of both the carbon fibre bundles and the individual carbon fibres may vary within wide limits. Generally, however, the carbon fibre bundles have a mean diameter of about 10-15 μm, and each bundle comprises about 10-20 individual fibres. The mean diameter of each individual carbon fibre pre¬ ferably is at most about 1 μm. Generally, the carbon fibre bundles are produced in relatively large lengths that are cut into smaller lengths of about 1 cm or less. For the invention, carbon fibre bundles (and thus carbon fibres) having a mean length of about 0.5-6.0 mm are preferred. The reason for this is that the reinforcing effect of the carbon fibre deteriorates at values below about 0.5 mm, while it is difficult to homogeneously admix and distribute carbon fibres longer than about 6.0 mm in the matrix of a composite material. Within the specified preferred limits (at most about 1 μm mean diameter and about 0.5-6.0 mm mean length) the L/D ratio of the fibre, i.e. the fineness number, should be selected such that optimal characteristics are obtained in respect of reinforcing effect and homogeneous distribution within the matrix. Besides carbon fibres, the reinforcing material according to the invention also comprises glass fibres. Also glass fibres and their production method are so well known that no detailed description is necessary. The invention uses glass fibres which from the very beginning are in the form of individual fibres, in con¬ trast to, for example, roving where the filaments are firmly bound together and therefore not readily separable. The glass fibres used in the context of the invention should be relatively thin and preferably have a mean diameter of about 0.5-6.0 μm, most preferred about 0.8-3.0 μm. Furthermore, a mean fibre length of about 0.5-15 mm is preferred, about 0.8-3 mm being most preferred. Within these diameter and length ranges, the desired high fineness numbers are obtainable which permit optimal mutual adjustment of a good reinforcing effect and sim¬ plicity of admixture to and distribution in the matrix material. The invention is not restricted to the use of glass fibres of any special type or glass composition and, in principle, all types of glass fibres suitable as reinforcing fibres may be used. An especially preferred glass fibre type are glass fibres of so-called C glass. Both the amount of carbon fibres and the amount of glass fibres in the fibre-containing reinforcing material according to the invention may be varied within wide limits. Generally, the weight ratio of carbon fibres to glass fibres may vary from about 1:99 to about 99:1, preferably from about 10:90 to about 40:60. It is prefer¬ red to use a lower proportion of carbon fibres which are more expensive than glass fibres. The third component of the fibre-containing re¬ inforcing material according to the invention is hydro- phobic, anionic, colloidal silica. The principal object of the silica in the context of the invention is to serve as a dispersing agent for dividing the carbon fibre bundles into indiviual fibres. Without being bound to any specific theory, it is assumed that the silica penetrates into the fibre bundles and is distributed on the surface of the individiual carbon fibres, thus imparting to the fibres a negative charge which has a repelling effect conducive to fibre separation. Colloidal silica is obtainable in several different forms, among them 1 ) silica aerogel which is a colloidal silica which is produced by replacing the water of a silica hydrogel by a low-boiling water-miscible liquid, heating in an autoclave above the critical tempe¬ rature of the liquid, and then venting the auto¬ clave; 2) so-called fumed or pyrogenic silica which is a colloidal silica obtained by burning silicon tetrachloride and collecting the resulting silica fumes; 3) precipitated silica produced by destabilising water-soluble silica under conditions that do not permit the formation of a gel structure, but instead cause a flocculation of silica par¬ ticles to form agglomerates, for example by the addition of sodium ions to a solution of sodium silicate, aluminium silicate, copper silicate, magnesium silicate and zinc silicate. Colloidal silica is generally in the form of agglom¬ erates built up of extremely small primary particles of a size within the range of about 5-20 nm. The extremely small size of the primary particles imparts to colloidal 2 silica a large surface area of about 50-400 m /g or more. Colloidal silica usually is hydrophilic, but the inventor has found that it should be hydrophobic for a successful division and separation of the carbon fibre bundles. To the extent that the silica is not initially hydrophobic, it can be made hydrophobic by a special surface treatment which, per se, is previously known. The invention prefers colloidal silica of the so-called fumed type, and an especially preferred silica is the one marketed by Degussa under the tradename Aerosil R 972. This is a hydrophobic, anionic, colloidal silica having the following characteristics Surface area according to BET ( /g) 120+30 The primary particle mean size (nm) 16 pH (according to DIN 53200) in 4% aqueous dispersion 3.5-4.1 Si0 2 >98.3 A1 2 0 3 <0.05 Fe 2 0 3 <0.01 Ti0 2 <0.03 HC1 <0.05 Among other types of silica, mention may be made of the one which is marketed by Cabot Corporation under the trade name Cab-0-Sil. In the present invention, the amount of silica is about 5-20% by weight, preferably about 5-15% by weight, based on the total fibre weight. Generally, it may be said that the amount of silica should be suffi- cient to coat also the surface of the glass fibres, thereby to provide a more uniform charge distribution in the fibres. The fourth component of the fibre-containing re¬ inforcing material according to the invention is cationic talc, preferably the variant known as soapstone. Soapstone is greenish-grey, while talc is practically white. The general term "talc" usually includes (a) the mineral talc, (b) steatite which is a compact variant of talc, and (c) the rock type soapstone. The mineral talc is a hydrated magnesium silicate of the ideal composition Mg^i.O. Q (OH) „ . The talc content of commercial talc is high and usually lies at about 97% by weight. In the production of commercial talc, talc mineral is crushed and comminuted and then purified by flotation to give a talc product having a high talc content and whiteness. Soapstone is a natural product consisting minera- logically of talc mixed with a high content of chlorite and minor amounts of carbonate and amphibole. For example, soapstone from Handδl in Sweden has the following mineral composition talc about 67% by weight chlorite about 18% by weight carbonate about 8% by weight amphibole about 3% by weight ore material about 4% by weight One example of mineral talc is the one obtainable under the trade name Finntalk P40 from Outokumpu Oy, Finland. It has a mean diameter of about 10-20 μm, a talc content of about 97% by weight, a loss on ignition of 7% by weight, and an oil absorption value of 32 g oil/100 g talc. The melting point is 1375°C. As an example of a soapstone suitable in the context of this invention, mention may be made of the soapstone type H340 from Handδl, Sweden. This is a micronised soapstone having a mean diameter of 5-10 μm. The talc content is about 67% by weight, and the loss on ignition about 8% by weight. The oil absorption value is 55 g oil/100 g soapstone, and the melting point is 1500°C. It is also possible to use, in the present invention, mixtures of talc minerals and soapstone. The proportions between talc and soapstone may vary within wide limits, preferably from a weight ratio of about 30:70 to about 70:30, more preferred from about 40:60 to about 60:40, and most preferred about 50:50. The particle size of the talc is not critical in the invention, but usually lies within the range of about 5-10 μm. As mentioned above, the talc is cationic, and by incorporating talc in the fibre-containing reinforcing material of the invention it is intended to make the talc act as a cationic additive to the anionic carbon fibres and the glass fibres, but also to the matrix material which is preferably selected among anionic polymer materials, such as polyolefins (polyethylene, polypropylene), polyvinyl chloride, polyesters, poly- acrylates and polymethacrylates, polyurethanes, bituminous products, etc. The amount of talc in the fibre-containing rein¬ forcing material according to the invention may vary within wide limits and is generally about 5-20% by weight, based on the total fibre weight. After this description of the components of the fibre-containing reinforcing material, the production of this material in accordance with the invention will now be described. In a first step in the method according to the in¬ vention of making fibre-containing reinforcing materials, carbon fibre bundles are slushed to divide the bundles into individual carbon fibres which are then separated from one another. The slushing and separating step is carried out by mixing the carbon fibre bundles under shear action in the dry state. For the sake of simplicity, this treatment will be called dry dispersion hereinafter. It was found that, for dry dispersion, it is not enough that the carbon fibre bundles are mixed under the action of shear forces in order to obtain a satisfactory carbon fibre separation. Surprisingly, however, it was found that a highly satisfactory dry dispersion with efficient carbon separation is obtained if hydrophobic, anionic, colloidal silica is supplied during the mixing step under shear action, as defined above. The function of the silica during the dry dispersion has not been fully explained, but it is assumed, without binding the in¬ vention to any specific theory, that the extremely small primary particles of the negatively charged silica pene- trate into the carbon fibre bundles and are distributed across the surfaces thereof, whereby a unitary, repellent, negative surface charge is obtained on the fibre surface. The repellent surface charge on the carbon fibre surfaces is assumed to be highly conducive to the efficient carbon fibre separation. It is understood that the dry dispersion of the carbon fibre bundles can be carried out in any machine mixing the carbon fibre bundles under the action of shear forces. Suitable machines for dry dispersion in accordance with the invention are high-speed mixers of the type Lδdige, Drais or Cowles-Dissolver. The mixer is adjusted to a suitable operating speed, preferably 1000-3000 rp , and dry dispersion is carried out for a time sufficient to achieve satisfactory separation, which is easily seen from a routine check for which the dry-dispersed fibre mixture is combined with matrix material and spread on a glass plate. If the spread-out fibres show an isotropic distribution in the matrix, the dry dispersion is satisfactory. The dry dispersion time usually is about 10-30 min. In a second step in the method according to the invention, glass fibres are supplied to the carbon fibre mixture. The parameters of the glass fibres are those mentioned above, and the glass fibres are added to the dry dispersion machine. Although the glass fibres may be supplied after the dry dispersion of the carbon fibre bundles has been terminated, they are usually added at the final stage of the dry dispersion of the carbon fibre bundles (for example after about 10-12 min. of dry dispersion). In both cases, operation of the mixer is continued to achieve an essentially homogeneous distri bution of the glass fibres among the carbon fibres. Unlike the carbon fibres, the glass fibres are from the beginning in the form of individual, extremely thin fibres having a negative surface charge. Since the surfac charge level may vary between the carbon fibres and the glass fibres, it is preferred to add the silica in a slightly excessive amount so that there will be enough silica also for partly coating the glass fibre surfaces. In this manner, there is achieved a uniform distribution of the surface charge between carbon fibres and glass fibres, and when the glass fibres are added to the system of carbon fibres and colloidal silica, the glass fibres are highly uniformly distributed during the continued dispersion and mixed to form an entirely homogeneous system with the carbon fibres and the silica. The mixing time after the glass fibres have been added preferably amounts to about 5-15 min. In a third step in the method according to the invention, cationic talc, preferably soapstone as defined above, is added to the dry-dispersed mixture of carbon fibres, glass fibres and silica. Thus, it was found in the present invention that talc, in particular soap- stone, acts as an adhesion agent between the fibres and the matrix of the composite material. The reason why talc is an efficient adhesion agent has not been fully explained, but is presumably due to the fact that talc is a material having a cationic, hydrophobic surface, and by its positive surface charge the talc gives an excellent adhesion to the negatively charged fibres and, simultaneously, good adhesion to the matrix material which preferably is anionic, such as anionic polymer materials of the type mentioned above. By the interaction of the talc, the carbon fibres and the glass fibres, there is obtained, in accordance with the invention, a unique fibre-containing reinforcing material for the production of entirely novel types of composite materials. In the reinforcing material according to the invention, the carbon fibres primarily give an improved impact strength to the composite material, while the other constituents also provide substantially higher rigidity and dimensional stability at higher temperatures. In the method according to the invention, the talc is added by dry dispersion, and this applies also to the remaining constituents, the dry dispersion being conducted until the talc has been substantially homogeneously distributed in the mixture, which is usually achieved within about 5 min. It has so far not been possible, with existing reinforcing materials, to maintain the strength and oxidation stability of composite materials at higher temperatures. By the novel fibre-containing reinforcing material according to the invention, which is hydrophobic in character and in which a chemical and physical inter¬ action occurs between carbon fibres and glass fibres, and also with the matrix material, excellent adhesion between the components of the composite material is obtained, and it has surprisingly been found that the strength values and the stability of the composite ma¬ terial can largely be maintained at more than 90% also at elevated temperatures, for example a temperature of 120°C for 200 hours. The corresponding values after a similar heat treatment of rovings and glass fibres alone or carbon fibres alone are about 60-70%. Another important aspect is the orientation of the fibre materials in the matrix of the composite mate¬ rial. To obtain a uniform strength in all directions, the fibres should be isotropically oriented, i.e. uni- formly distributed in all directions. In conventional technique, however, the fibres are oriented more or less perpendicular to the direction of flow during form¬ ing, which results in an anisotropic distribution and strength. The fibre-containing reinforcing material according to the invention has surprisingly been found to give an isotropic fibre orientation, which means a uniformly distributed strength of the material. It is assumed that this isotropy is the result of the charges of the components included and the interaction established between carbon fibres and glass fibres and also between carbon fibres, glass fibres and matrix material. To illustrate the invention and its advantages. the following nonrestrictive Examples are given. Examples 1-6 Material for the production of the fibre-containing reinforcing material according to the invention, in the percentages indicated in the Table below, were batched into a high-speed Lδdige type mixer of high shear action, which was operated at about 2000 rpm. The carbon fibre bundles had a length of about 3 mm and a diameter of about 10-15 μm. Each bundle contained about 10-20 in- dividual carbon fibres having a diameter of about 0.7 μm. The L/D number of the carbon fibres was 4280. The glass fibres consisted of C glass and had a mean length of about 2.7 mm and a mean diameter of about 0.8 μm. The L/D number of the glass fibres was about 3375. The hydro- phobic, anionic, colloidal silica was Aerosil ® R 972 from Degussa. The talc was soapstone H-340 from Handδl. For testing the properties of the fibre-containing reinforcing material, it was admixed to a matrix con¬ sisting of Crastine S 600 which is a polyester plastic from Ciba-Geigy. In all cases, the amount of admixed reinforcing material was 20% by weight. For the production of the reinforcing material, the carbon fibre bundles were first dry-dispersed for about 10 min. together with the silica, whereupon the glass fibres were added, and dry dispersion was continued for about 15 min. Then the soapstone was added, and dry dispersion was continued for about 5 min. more. Table I Constituent/Example 1 Unit=% Glass fibre 100 Carbon fibre Silica Soapstone Bending of sample to rupture, N 560 Tensile strength N/mm 2 , 1350 1980 1670 1585 1420 1610 Compressive strength N/mm 2 , 600 1410 1180 900 630 950 * The carbon fibre bundles were not dispersed. Anisotropic distribution. It appears from Table I that glass fibres alone or carbon fibres alone (Examples 1 and 4, respectively) give unsatisf ctory results, and that also glass fibres alone or carbon fibres alone in combination with silica and soapstone (Examples 5 and 6, respectively) insigni¬ ficantly improve the result. On the other hand, a combina¬ tion of glass fibres and carbon fibres together with silica and soapstone, in accordance with the invention (Examples 2 and 3) give excellent results. The reinforcing material according to the invention can be used for a variety of applications and a variety of products, such as reinforcing material in safety helmets, straps for ear muffs, car components and many other articles for which a preferably isotropic fibre reinforcement is required.

Claims: CLAIMS 1. Fibre-containing reinforcing material, c h a r ¬ a c t e r i s e d in that it comprises an essentially homogeneous distribution of carbon fibres, glass fibres, hydrophobic, anionic, colloidal silica and cationic talc, the weight ratio of carbon fibres to glass fibres being from 1:99 to 99:1, the silica forming about 5-20% by weight of the total fibre weight, and the talc forming about 5-20% by weight of the total fibre weight. 2. Fibre-containing reinforcing material as claimed in claim 1, c h a r a c t e r i s e d in that the carbon fibres have a mean length of about 0.5-6.0 mm. 3. Fibre-containing reinforcing material as claimed in claim 1 or 2, c h a r a c t e r i s e d in that the individual carbon fibres have a mean diameter of at most about 1 μm. 4. Fibre-containing reinforcing material as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the glass fibres have a mean length of about 0.5- 15 mm. 5. Fibre-containing reinforcing material as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the glass fibres have a mean diameter of about 0.5-6.0 μm. 6. A method of making a fibre-containing reinforcing material , c h a r a c t e r i s e d by a) adding, to carbon fibre bundles, hydrophobic, anionic, colloidal silica in an amount of about 5-20% by weight, based on the total fibre weight of the final rein¬ forcing material, the carbon fibre bundles being divided into individual fibres by mixing under shear action in dry condition; b) adding, to said mixture of carbon fibres and silica, glass fibres in an amount to give a weight ratio of carbon fibres to glass fibres of from 1:99 to 99:1; and c) distributing, in said mixture of carbon fibres, silica and glass fibres, about 5-20% by weight, based on the total fibre weight, of cationic talc. 7. Use of the fibre-containing reinforcing material as claimed in any one of claims 1-5 for the production of fibre-reinforced composite materials.

Inventors: HJORTH INGEGERD, HJORTH, INGEGERD
Applicants: BILSOM AB
Application year: 1989
Main classification: C08K13
IPC codes: C08K3, C08K7