WO1993002127A1

WO1993002127A1 - Process for making a preimpregnated material

Info

Publication number: WO1993002127A1
Application number: PCT/NL1992/000131
Authority: WO
Inventors: Lucia Elisabeth Peter Wenmakers; Roelof Marissen
Original assignee: Dsm N.V.
Priority date: 1991-07-16
Filing date: 1992-07-16
Publication date: 1993-02-04
Also published as: BE1005106A3

Abstract

The invention relates to a process for making a prepreg consisting of 90-10 % (wt) fibrous reinforcing material and 10-90 % (wt) thermoplastic matrix material by impregnating the fibrous reinforcing material with a dispersion containing thermoplastic material obtained by emulsion polymerization or suspension polymerization, wherein the emulsion polymerization or suspension polymerization is terminated before the thermoplastic particles have an average particle size exceeding the diameter of filaments of the fibres of the fibrous reinforcing material and the dispersion consists of thermoplastic particles in water. A prepreg obtained by applying a process according to the invention can be used for the production of all kinds of composite moulded parts that can be used in, for instance, automotive, aerospace, home furnishing and construction articles, as well as in sports goods such as bicycles and surfing gear, boats, electrical housings, etc.

Description

Process for making a preImpregnated material

The invention relates to a process for making a preimpregnated material comprising 90-10% (wt) fibrous reinforcing material and 10-90% (wt) thermoplastic matrix material by impregnating the fibrous reinforcing material with a dispersion containing thermoplastic material.

Such a process is known from GB-A-2.168.361. In it a description is given showing that fibres are impregnated with a dispersion obtained by dispersing a fine thermoplastic powder in an amount of water. The particles can be obtained by grinding an amount of thermoplastic polymer. The fibres impregnated with the dispersion are dried. The resulting preimpregnated material, also referred to as a prepreg, can be used as fibre-containing thermoplastic intermediate.

The disadvantage of such a process is that a great many steps are needed, including a grinding step. The grinding of a thermoplastic polymer is very awkward, because the polymer is tough and because the polymer may melt and/or clot during the grinding. To avoid this, strong cooling and careful grinding are needed.

The object of the invention is to provide a process that does not have said disadvantages. This is achieved according to the invention in that the dispersion is obtained by emulsion polymerization or suspension polymerization wherein the emulsion polymerisation or suspension polymerisation is terminated before the thermoplastic particles have an average particle size exceeding the diameter of filaments of the fibers of the fibrous reinforcing material and the dispersion consists of thermoplastic particles in water. Emulsion/suspension polymerizations are known in the art and can be applied for obtaining numerous polymers. In US-A-3,238,275, for instance, an emulsion polymerization process is described. US-A-3,238,275 is hereby included in this application for reference purposes. An example of suspension polymerization is described in DE-A-3307033. Emulsion polymerization/suspension polymerization consists in the polymerization of monomers in a medium, in which process the monomers and the polymer virtually do not dissolve in that medium. The result is an emulsion or a suspension consisting of the medium with particles of thermoplastic polymer dispersed therein. US-A-3.897.533 describes a method of impregnating a reinforcing glass fiber mat with a vinylchloride resin emulsion. A first disadvantage of the said method is that the vinylchloride resin emulsion must contain a water- immiscible organic solvent for partially swelling or dissolving the resin particles, to acquire their adhesive properties. Otherwise, the resin particles do not adhere to one another nor to the fibers. Another disadvantage of the said method consists in that only commercially available emulsions are said to be used to impregnate the glass fiber mat.

In the present invention on the other hand, aqueous dispersions of the resin particles are used, directly obtainable from an industrial production plant, where polymers are produced through emulsion polymerisation, and where the polymerisation is stopped before the resin particles reach a certain size, e.g. 1 μm. Commercial polyvinylchloride emulsions which comprise an organic solvent, are excluded from the present application. It is an additional advantage of the present invention that the resin particles in the said aqueous dispersions show good adhesive properties to one another and to the fibers without the necessity of adding any organic solvent to dissolve or swell the particles. The emulsion described in US'533 on the contrary, cannot be used without adding organic solvent. In the preparation of some polymers, a latex is sometimes referred to instead of an emulsion. This is the case, for instance, with acrylonitrile-butadiene-styrene (ABS) copolymers.

For the emulsion polymerization and suspension polymerization of ABS and of SAN (styrene-acrylonitrile copolymers) reference can be made also to, for instance, Stanford Research Institute (SRI), Process Economics Program, Report no. 20, 1966, the whole document, and particularly to pp. 42-50 and pp. 97-133 and pp. 135-196. In, for instance, the emulsion polymerization of butadiene to form polybutadiene, the polymerization of the emulsion is continued according to the state of the art until the thermoplastic particles have reached a size of 0.05 to 5 μm. In the emulsion polymerization of ABS copolymers the particles may reach a diameter larger than 250 μm. In a suspension polymerization process particles are obtained having a particle size ranging from 10 μm to a few millimetres. The particle size can be influenced by the degree to which the reaction medium in the reactor is stirred. This is a technique known to the person skilled in the art. In an emulsion polymerization process according to the state of the art the medium is removed by coagulation of the particles and the polymeric particles are collected and processed further.

The emulsion polymerization/suspension polymerization according to the invention is preferably terminated when the particles of thermoplastic (co)polymer have reached an average particle size smaller than the diameter of the filaments of the fibres that are to be impregnated by the particles. More preference is given to terminating the emulsion polymerization/suspension polymerization when the particles have reached an average size of 1 μm.

Now, by applying the process according to the invention, the effects are successively that the medium and the thermoplastic particles need not be separated from each other, that the thermoplastic polymer need not be ground, that no water immiscible organic solvent has to be added to swell or partly dissolve the polymeric particles and that the polymeric particles need not be dispersed in a medium.

It is possible also to apply particles of thermosetting (co)polymers, or mixtures of particles of thermoplastic polymer and particles of thermosetting polymer. Preferably, however, the particles substantially consist of thermoplastic polymer, hereinafter referred to as thermoplastic for short.

The thermoplastic can be chosen from the group of thermoplastics that can be prepared in an emulsion polymerization/suspension polymerization. Examples of these are polymers and copolymers of styrene, α-methylstyrene, acrylonitrile, conjugated dienes (rubbers), such as butadiene and isoprene, vinyl acetate, vinyl chloride, acrylate ester, ethylene, vinylidene chloride, acrylamide, maleic anhydride, acrylates, methacrylates, etc.

Examples of such (co)polymers are acrylonitrile- butadiene-styrene (ABS), obtained by grafting styrene and/or α-methylstyrene and acrylonitrile on a rubber, such as polybutadiene rubber (PB) and/or butadiene styrene rubber (BS) and/or butadiene-acrylonitrile rubber (BAN) and/or ethylene-propylene (EPM) rubber and/or ethylene-propylene diene (EPDM) rubber. In addition to the rubbers themselves, polystyrene (PS), high impact polystyrene (HIPS), styrene- acrylonitrile (SAN), polyacrylonitrile (PAN), polyvinyl chloride (PVC) , ethylene vinyl acetate (EVA), polyalkyl(meth)acrylates such as polymethylmethacrylate (PMMA), polyethylene (PE), etc. can be used also. Another option is to use blends of these (co)polymers. The invention makes it possible for polymer blends to be obtained in a simple manner as matrices in the intermediate products. In a subsequent processing step, it will be possible to easily blend the finely distributed polymer particles with each other.

Preference is given to the use of graft copolymers of styrene and/or α-methylstyrene with acrylonitrile on PB, copolymers of styrene and/or α-methylstyrene with acrylonitrile, vinyl acetate copolymers or acrylates.

The medium in which the polymerization takes place may be any water-based dispersion medium as described in the art for emulsion polymerizations/suspension polymerizations. It is possible, after the polymerization, for this aqueous medium to be partly removed, or to be replenished with other liquids in order to optimize the emulsion for the impregnation of the fibrous reinforcing material. It is possible to remove from the emulsion or suspension residual monomers or remainders of catalysts, inhibitors and the like, but this is not necessary.

In order to make the polymers in the emulsion or suspension suited for use in a process according to the invention, it may be necessary for the polymers to be stabilized already in the emulsion or suspension in order to give the polymeric material in the end product a longer life. This stabilization is effected by adding stabilizers, such as antioxidants, antiozonants, UV stabilizers and the like. At room temperature, the stabilizers are usually solids or liquids and must therefore be emulsified or dissolved before blending. The stabilizers are preferably emulsified, because it will then, afterwards, no longer be necessary to remove solvent. Emulsification can be effected, for instance, by adding an emulsifier such as, for instance, soap, optionally at slightly elevated temperature. This soap can be added in such a small amount that it does not affect the properties of the end product.

The fibrous reinforcing material may consist of any kind of fibre. Examples are glass fibres, carbon fibres, aramide fibres, metal fibres, ceramic fibres, thermoplastic fibres and, of course, fibres like jute and cotton. The fibrous reinforcing material preferably consists of a glass, carbon, aramide or polyethylene fibre, or of combinations thereof.

The fibrous reinforcement can be applied in the form of short fibres or in the form of long fibres. Long fibres may be endless monofilaments or bundles of short filaments twisted together. The fibres can be used at random or in the form of a woven, knitted or plaited fabric, a fleece or mat or otherwise.

The process according to the invention further consists of the impregnation of an amount of fibrous material by immersing it in an emulsion bath or in a suspension bath. It is possible also for the emulsion or suspension to be applied by spraying or for the emulsion or suspension to be applied by rolling or by hand, for instance by pouring. The mixing of emulsion or suspension and fibres in a mixing device is another option.

It is further possible to have the impregnation carried out by mixing an amount of fibrous reinforcing material with an emulsion or suspension and subsequently to suck the medium off through filters or otherwise, in which process a large amount of thermoplastic is left behind with the fibres.

It is further possible to have the emulsion polymerization/suspension polymerization carried out in the presence of at least a part of the fibrous reinforcing material.

The dispersion preferably contains 20 to 80% (wt) solid material, particularly 30-50% (wt) . If the thermoplastic material content is too high, the impregnation proceeds with greater difficulty. If the percentage is lower, the number of particles that will stick to the fibrous material will be too small, depending on the impregnation process.

The amount of fibrous reinforcing material is generally as high as possible. If the reinforcement is in the form of woven fabrics, the percentage by volume of fibres is between 30 and 70 and preferably between 50 and 60 (calculated on fibres + thermoplastic). If the reinforcement is in the form of short fibres,.the percentage by volume is generally about 25. The impregnated fibrous material is dried at a temperature ranging from 50 to 140°C. Higher and lower temperatures are possible, but preferably the temperature is above the film forming temperature of the dispersion. The drying time of the prepreg may, at 100°C, amount to a few minutes.

The film forming temperature of a dispersion is the temperature at which the particles of the dispersion form a film spontaneously. This temperature is determined by a number of parameters, such as the material which the particles consist of, the diameter of the particles and the medium. If the film forming temperature is higher than

100°C, while the medium is water, preference is given to raising the pressure during the drying.

Particularly, at least an extra film former is added to the dispersion, as described in EP-A-368.412. The advantage of a special film former is that it reduces the temperature at which a film is formed by the dispersion in combination with the film former.

The prepreg has a fairly long storage life. It can be processed in the ways in which thermoplastic prepregs can usually be processed, for instance by heating the prepreg in a mould until the thermoplastic melts, moulding and cooling till the article consolidates, or by heating the prepreg outside the mould and subsequently consolidating it in a cold mould, or by autoclaving. Further options include pultrusion and filament winding. If short fibres are used, the prepreg may also be injection moulded or extruded. Then the temperatures and pressures to be set depend on the polymeric materials used in the impregnation of the prepreg. A prepreg obtained by a process according to the invention can be used for the production of all kinds of composite moulded articles that can be used in, for instance, automotive, aerospace, home furnishing and construction articles, as well as in sports goods, such as bicycles and surfing gear, boats, electrical housings, etc. The invention will be elucidated by means of the following examples without being limited thereto.

Example I

Preparation of the emulsion

An amount of α-methyl-styrene-acrylonitrile copolymer emulsion (α-methyl-SAN emulsion) was mixed in a 50/50 weight ratio with acrylonitrile-butadiene-styrene emulsion (ABS emulsion) with 16% rubber, both acquired according to an emulsion polymerization process as described in US-A-3,238,275 and stabilized by the addition of an Irganox^R emulsion a stabilizer. The Irganox^R emulsion was prepared by melting 100 g Irganox* 245 of the firm of Ciba- Geigy in the presence of 10 g stearic acid at 100°C and subsequently cooling the melt to 80°C. To it, 2 g KOH (dissolved in 50 ml water and heated to 80°C during firm stirring) was added slowly. After that, 150 ml water with a temperature of 80°C was added to the mixture during firm stirring.

Example II

Impregnation of the woven fabric Using a laboratory impregnating machine of the firm of Babcock a fabric was impregnated with the emulsion of example I by passing it through two successive baths at a speed of 1 m/min. The woven fabric is a glass fabric with the same amount of fibres in the twill and in the weft direction (a so-called 50/50 fabric) and a weight of 280 g/m². After the impregnation baths, the fabric is passed through two ovens, one behind the other, with temperatures of 140 and 100°C respectively, in which the woven fabric dries. After drying the prepreg formed contained 55% (vol.) glass.

Example III

The prepreg of example II was cut into pieces measuring 12 cm x 20 cm. The pieces of prepreg were piled up in such a manner that the thickness and the fibre orientation were correct for the composite article to be obtained. This pile was dried at 100°C for two hours. After that, the pile was compressed in a Bucher press for 5 minutes at 240°C under a pressure of 40 bar and subsequentl cooled in it to 100° under the same pressure. The press was opened and the laminate formed was taken out. Using a diamond saw, test specimens were sawn from the laminate formed, which test specimens were dried again for two hours at 100°C.

Of these test specimens the mechanical properties were determined. The results are shown in table 1.

Example IV

The process of example I was repeated using α-methyl-SAN only. With the emulsion obtained, a fabric was impregnated by hand. From the prepreg obtained test specimens were made according to the process of example III. Of these test specimens mechanical properties were determined. These, too, are shown in table 1.

Table 1; Results of the measurement of the mechanical properties in examples III and IV.

Example III Example IV

4-point flexural strength

(ASTM D790) 479 MPa 639 MPa 4-point flexural modulus

(ASTM D790) 27 GPa 34 GPa tensile strength (ASTM D 3039-76) 355 MPa tensile modulus (ASTM D 3039-76) 22 GPa

ILSS (ASTM D 2344-76) 45 MPa 53 MPa HDT A (ASTM D 648-82) 143°C

IZOD (ISO 180) 97 kJ/m²

ASTM = American Standard Test Method ILSS = Interlaminar Shear Strength HDT = Heat Deflection Remperature IZOD - Notched Impact Resistance ISO = International Standard Organisation

Table 1 shows that impregnation with α-methyl SAN yields a product with better mechanical properties than impregnation with ABS. The advantage of the prepreg with ABS, however, was that the film formation was better and that it was more flexible and consequently easier to handle in the processing.

Example V A SAN emulsion with and without 10% (wt) methylmethacrylate (MMA) incorporated therein, obtained according to a process described in US-A-3,238,275, in the preparation of which a part of the acrylonitrile is replaced by MMA, was used for impregnating a woven fabric by hand and subsequently making test specimens. The results of the four- point flexural and ILSS and HDT measurements are shown in table 3.

Table 3; Results of the measurements in example VI

SAN SAN with 10% MMA

4-point flexural strength (ASTM D790) (MPa) 504 526

ILSS (ASTM D 2344-76) (MPa) 41.8 52.5

flexural modulus (ASTM D 790) (GPa) 27.0 27.5

HDT A (ASTM D 648-82) (°C) 129.3 134.4

With 10% (wt) MMA the composite formed has a better temperature resistance and better mechanical properties than the composite article obtained from woven fabric impregnated with just SAN.

Claims

C L A M S

1. Process for making a preimpregnated material comprising 90-10% (wt) fibrous reinforcing material and 10-90% (wt) thermoplastic matrix material by impregnating the fibrous reinforcing material with a dispersion containing thermoplastic material obtained by emulsion polymerization or suspension polymerization wherein the emulsion polymerization or suspension polymerization is terminated before the thermoplastic particles have an average particle size exceeding the diameter of filaments of the fibres of the fibrous reinforcing material and the dispersion consists of thermoplastic particles in water.

2. Process according to claim 1, characterized in that the average particle size of the particles in the emulsion or suspension is smaller than 1 μm.

3. Process according to any one of claims 1-2, characterized in that the dispersion contains 20 to 80% (wt) solid material.

4. Process according to claim 3, characterized in that the dispersion contains 30 to 50% (wt) solid material.

5. Process according to any one of claims 1-4, characterized in that the impregnated fibrous reinforcing material is dried at a temperature above the film forming temperature of the dispersion.

6. Process according to claim 5, characterized in that the impregnated fibrous reinforcing material is dried at a temperature of 50 to 140°C.

7. Process according to any one of claims 1-6, characterized in that the impregnated fibrous reinforcing material is dried under elevated pressure.

8. Process according to claim 7, characterized in that at least a film former is added to the dispersion.

9. Process according to any one of claims 1-8, characterized in that the thermoplastic is chosen from the group consisting of graft copolymers of styrene and/or α-methylstyrene with acrylonitrile on poly- butadiene, copolymers of styrenen and/or α-methylstyrene with acrylonitrile, vinyl acetate copolymers and acrylates and mixtures of these.

10. Process according to any one of claims 1-9, characterized in that the fibrous reinforcing material is chosen from the group consisting of glass, carbon, aramide and polyethylene fibres.

11. Preimpregnated material obtained or obtainable by applying a process according to any one of claims 1-10.

12. Composite article obtained by processing a preimpregnated material according to claim 11.

13. Process or prepreg as described in substance in the specification and/or the examples.