PROCESS FOR THE PREPARATION OF THERMOPLASTIC FIBRES

Abstract: A process for the preparation of thermoplastic fibres which comprises: a) spinning a solution of an alternating copolymer of an olefinically unsaturated compound and carbon monoxide, having a number average molecular weight of at least 5000, to fibres through a spinning aperture, said solution having a polymer content in the range of from 0.1 - 50 %m, b) submitting the thus obtained fibres to a solvent-removal treatment to remove substantially all the solvent, and c) stretching the thus obtained substantially solvent free fibres in an environment having a temperature between the glass-transition temperature of said polymer and (T+20)°C, wherein T is the crystalline melting point of the polymer.

Description: The present invention relates to a process for the preparation of thermoplastic polymer fibres. It also relates to composites prepared therewith. Thermoplastic polymer fibres are focusing a lot of attention in view of a very interesting combination of a potentially high strength and a low weight. For this reason they are being widely used as reinforcement in many applications such as car tyres and structural elements. It is generally accepted that for a given polymer the performance properties of the fibres prepared therefrom are amongst others related to the degree of stretching, hereinafter referred to as the draw-ratio, which can be achieved when stretching the spun fibres, which ratio may be enhanced by a high degree of polymer molecule disentanglement. The process generally used for the preparation of thermoplastic polymer fibres, having performance properties as described hereinbefore, is solution- or gel-spinning. These processes differ from the melt-spinning process in that the polymers are dissolved in a suitable solvent or solvent blend prior to being spun, which dissolving step promotes disentanglement of the polymer molecules. A further difference with the melt-spinning process is that said processes permit the processing of considerably higher molecular weight polymers, which is considered to be an additional advantage for the fibre production, as it is known that a high molecular weight can have a beneficial effect on the mechanical properties of products based thereon. It has been reported by J. Smook et al, in a Poster presentation at the Rolduc Abbey Polymer Meeting, April 26-30, 1987, in The Netherlands, that contrary to the excellent results obtained with gel-spun ultra high molecular weight polyethylene, e.g. a maximum draw ratio = 80, similar experiments with polar polymers, such as polyamides and polyesters, only resulted in draw ratios which were very similar to those obtained with the corresponding melt-spun fibres, i.e. draw ratio = 6-7. These results indicate that there is still further need for improvement in the preparation of thermoplastic polymer fibres based on polar polymers , especially as fibres based on polar polymers generally demonstrate superior adhesion characteristics compared to fibres based on non-polar polymers, such as polyolefin-based fibres. As a result of continuing and extensive research and experimentation, the Applicants surprisingly found a type of polar thermoplastic polymer which can very suitably be processed via the solution-spinning process and which results in fibres having a much higher draw-ratio than could be obtained with the fibres based on the polar thermoplastic polymers mentioned hereinbefore, i.e. draw-ratios in the range of 10 to 30. The invention provides therefore a process for the preparation of thermoplastic polymer fibres which comprises: a) spinning a solution of an alternating copolymer of an olefinically unsaturated compound and carbon monoxide, having a number average molecular weight of at least 5000, to fibres through a spinning aperture, said solution having a polymer content in the range of from 0.1-50 %m, b) submitting the thus obtained fibres to a solvent-removal treatment to remove substantially all the solvent, and c) stretching the thus obtained substantially solvent-free fibres in an environment having a temperature between the glass-transition temperature of said polymer and (T+20)°C, wherein T is the crystalline melting point of the polymer. In the context of the present invention the term solution refers to polymer solutions wherein interaction between polymer molecules may occur, which phenomenon may further be concentration related. The term "%m" refers to percentage mass. The term "fibre" wherever used in this specification includes mono- and multifilament fibres. The term "alternating" copolymer refers to those copolymers in which the CO-units in the macromolecules are in alternating arrangement in respect of the units derived from the olefin. Thus, in the macromolecular chains, each CO-unit is positioned next to a single unit of olefin, e.g. ethylene. The copolymer can be a true copolymer of carbon monoxide and one particular olefin, preferably ethylene or they can be copolymers of carbon monoxide and more than one olefin, e.g. ethylene and propylene. In the latter case ethylene is preferably employed as the major olefin. The relevant alternating copolymers are known per se, for example from EP-A 121965, EP-A 213671, EP-A 229408 and US-A 3914391, and likewise, their methods of preparation by catalytic copolymerization are known from these references. Suitable polymerization catalysts are based upon palladium/phosphine systems. Particularly suitable thermoplastic polymers to be employed in the process of this invention are copolymers of ethylene and carbon monoxide, terpolymers of ethylene, propylene and carbon monoxide, preferably those in which the ethylene to propylene molar ratio in the polymer chains is at least 3:1. Other suitable terpolymers are terpolymers of ethylene and carbon monoxide with butene, pentene, hexene, heptene, octene, nonene, decene, dodecene, styrene., methyl acrylate, methyl methacrylate, vinyl acetate, undecenoic acid, undecenol, 6-chlorohexene, N-vinylpyrrolidone and the diethylester of vinyl-phosphonic acid, provided the molar ratio of ethylene to other unsaturated monomer in the polymer macromolecules is at least 3:1, preferably at least 8:1. Preferred such alternating copolymers have a number average molecular weight of at least 10⁴. An especially preferred class of the hereinbefore described alternating copolymers are those based on carbon monoxide and ethylene having a number average molecular weight of more than 10⁶. The preparation of these high molecular weight polymers is the subject of EP-A-0 337 521. Use of other known ethylene/CO copolymers which do not display an alternating structure and which are produced using e.g. peroxide free radical catalysts, is not contemplated in this invention. With solution- or gel-spinning the fibres emerging from the spinning aperture will generally have insufficient strength to allow easy handling, in view of their high solvent content. Sometimes the handleability of the solvent-containing fibres can be improved by cooling. This method may be applied e.g. when the spinning is conducted at elevated temperature. Subsequent cooling of the fibre will cause the polymer solution to gelate, provided an appropriate solvent system had been used in the preparation of the polymer solution, thereby improving the handleability of the fibres. Such gelled fibres will at some stage require a solvent removal treatment, prior to being stretched, in order to convert the fibres into their ultimate form. Frequently 'the handleability improvement is achieved via a solvent removal treatment. In one such a method the solvent removal is achieved by leading the solvent-containing fibres through a shaft through which e.g. hot air can be blown, as a result of which all or part of the solvent may be evaporated. With another and in the process of the present invention preferred method for solvent removal, the solvent-containing fibres are passed through a bath containing a liquid which has the ability to extract the solvent component from the solvent containing fibres, without simultaneously dissolving the polymer. Although in principle any compound may be used for the solvent component of the polymer solution, provided the combination of said compound and the polymer as hereinbefore described results in a polymer solution which can be processed via solution- or gel-spinning, it will be understood by those skilled in the art that when selecting the solvent component for the polymer solution, the conditions under which spinning process is to be conducted, will also be taken into account. In addition to the use of polymer solution based on a single solvent, it is also possible to employ solvent blends, wherein for example the boiling point and/or the dissolution power of the individual solvents may be the same or different. Solvents which may advantageously be employed in the preparation of polymer solution for use in the process of the present invention include hexafluoroisopropanol, m-cresol and mixtures thereof. A number of solvents which as such may be considered to be non solvents for the alternating copolymers as described hereinbefore, may occasionally be beneficially employed, in a minor amount, in combination with the solvents mentioned hereinbefore. Such compounds include aromatic hydrocarbons such as toluene; lower aliphatic alcohols such as methanol, ethanol and isopropanol; aliphatic hydrocarbons such as n-hexane and isooctane; ketones such as acetone and methyl ethyl ketone; and acids such as formic acid and acetic acid. Ethanol is a preferred non-solvent. Depending on the nature of the polymer composition employed for the fibre preparation, most of the non-solvents listed hereinbefore, may conveniently be employed as extracting agent for the solvent removal treatment of the fibres via the extracting procedure. Acetone is a preferred extracting agent for use in the solvent removal treatment. The polymer content of the solutions will preferably be in the range of from 0.25 - 20 %m, and more preferably in the range of from 0.5 - 10 %m. Although in principle the substantially solvent-free fibres prepared according to the process of the present invention may already be stretched in an environment having temperature just above the glass transition temperature of the polymer, it is very difficult to visualise stretching under these conditions as part of an economically viable process. Hence in practice stretching will be conducted at considerably higher temperatures. Preferably the temperature of the environment will be in the range of from (T-100)°C to (T+10)°C and more preferably in the range of from (T-50)°C - T°C. The environment wherein the fibres are to be stretched may be a liquid environment, such as an organic or inorganic oil or oily substance, or a gaseous environment such as oxygen, nitrogen, air, carbon dioxide or one of the inert gases. Preferably the environment is inert with respect to the polymer fibre. Nitrogen is a preferred inert gaseous environment for conducting the stretching operation. Stretching may be effected in a single step or in more than one step. With the latter mode of operation the temperature of a preceding step will generally be lower than that of the consecutive step. When required the fibres based on the alternating copolymers as described hereinbefore may contain appropriate amounts of one or more auxiliary compounds such as antioxidants, stabilisers, processing aids, dyes and the like. Should the presence of one or more of said auxiliary compounds be required, it or they may advantageously be introduced during or after the preparation of the polymer solution. The fibres prepared according to the present process find utility as reinforcing or structural material in many applications, e.g. in sheets, optionally (pre)shaped, made of relatively non-stiff or relatively inexpensive material. Advantageous is especially the fact that by this process higher melting, more viscous polymers can be processed to fibres better than by alternative, known methods of fibre production such as melt spinning. Very suitably the base material to be reinforced is another polymer, which should be compatible with the fibres and adhering well thereto. According to another aspect of the present invention, there are provided composites of thermoplastic matrix material reinforced with fibres made from thermoplastic material, characterized in that the fibres have been prepared using the process according to the invention and the thermoplastic matrix material comprises an alternating copolymer of carbon monoxide and an olefinically unsaturated compound having a number average molecular weight below that of the copolymer from which the fibre is made. Preferably the fibres have been prepared from an ethylene/carbon monoxide copolymer and the matrix material comprises an ethylene/propylene/carbon monoxide copolymer which has a crystalline melting point of at least 15 °C below that of the fibre copolymer. Another suitable matrix material is polypropylene, having a melting point of about 220 °C. Such composites are suitably prepared by compression moulding, having a good temperature control and no critical fibre dimensions. The products show a good wetting between reinforcement fibre and matrix, and have a higher tensile strength and modulus than the neat polymer. Similar composites could be made using fibres prepared by another spinning technique, for instance by melt spinning. However, this would not allow to use fibres made of material of high melting point/viscosity/molecular weight, which is needed for good reinforcing properties. The fibre content of the composite is suitably at least 5 %m, preferably from 10-20 %m, based on the mass of the matrix material. The invention is further illustrated with the following examples for which the following information is provided. Solution preparation: The polymer solutions having a viscosity in the range from 10⁴-10⁵ m.Pa.s were prepared by dissolving the appropriate amount of polymer in the desired solvent, in a jar or bottle, on a roller table at approximately 20 °C. Fibre spinning: The various polymer solutions were spun with the aid of a motorized syringe injector in combination with a 20 ml syringe, which was connected to a 300 mm Polytetrafluoroethylene capillary having an internal diameter 1.75 mm. The free end of the capillary, having a diameter of 1.3 or 1.75 mm and which acted as the spinning aperture, was positioned above the extraction bath with the opening parallel to the surface of the bath. The speed at which the polymer solution was ejected from the capillary into the extraction bath, hereinafter referred to as linear spinning speed, could be varied by altering the setting of the ejector. The distance over which the fibre travelled through the extraction bath, containing acetone as extracting solvent, was approximately 1.20 m, while the distance between bath and reel on which the substantially solvent-free fibre was collected, was 1.50 m. Fibre stretching : The fibres were stretched in a stretching chamber or oven, having a length of 20 cm and containing nitrogen having a temperature as indicated hereinafter. Stretching was conducted in a single or a two-step stretching procedure, with the latter mode of operation, stretching was conducted in two identical ovens with intermediate cooling of the fibre. The degree of stretching was regulated by adjusting the roller speed. The reel-up speed was fixed at 2 m/min for a single-step stretching procedure , while with a two-step stretching process the fibre was introduced into the oven at a speed of 0.16 m/min. Fibre testing : The potential performance properties of the fibres were assessed by measuring the modulus and tensile strength of the fibres with the aid of a table model tensile tester, equipped with a load cell having a max. capacity of 2000 g. The modulus was determined employing a cross head speed of : mm/min, while for the strength a cross head speed of 5 mm/min was used. Prior to testing, the samples were conditioned at 23 °C and 50 % rel. humidity for at least 24 h. Polymers tested: The following polymers were used for fibre preparation. A. Ethylene/CO copolymer melting temp.(T M ) approx 260 °C, M n ≧ 10⁶ B. Ethylene/CO copolymer T M approx 260 °C M n = 1.3x10⁵ C. Ethylene/propylene/CO terpolymer T M approx 218 °C, M n = 4.6x10⁴ Examples I-VII Polymer solutions of the composition as indicated in Table 1 hereinafter, were spun at approximately 20 °C, and subsequently passed through the extraction bath. The stretching of the substantially solvent-free fibres was conducted under the conditions as indicated in Table 1 which table also includes the results of the fibre testing. Examples I-III demonstrate the influence of the solvent composition on the fibre spinning and fibre properties. Examples III and IV demonstrate the influence of the polymer composition albeit that polymer C also has a lower M n than polymer A. Examples II and V demonstrate the possible influence of molecular weight. Example V also demonstrates the positive effect of a two-step stretching procedure over a single-step stretching procedure. Examples V, VI and VII demonstrate the influence of a solvent blend containing a non-solvent. In nearly all the examples the effect of the temperature at which the fibres have been stretched is shown. Comparative Example Fibres were prepared by melt spinning from polymers B and C, but it appeared to be impossible to do so from polymer A. The diameter of the solution spun fibres appeared to be generally smaller, about 30 µm, than the diameter of melt spun fibres, about 100 µm. Example VIII The copolymer C used in the preceding Example IV was dry-blended with 15 %m, based on the mass of copolymer C, of chopped fibres made of the ethylene/CO copolymer identified as B in the preceding Examples. The fibre diameter was 30 µm and their length was around 0.5-1.0 cm. The blend was compression moulded to a sheet, at 233 °C for 1 minute at 3 bar and 2 minutes at 45 bar. The sheet of thickness 3.0 mm, had a tensile strength of 68 MPa and a tensile modulus of 1.9 GPa, when tested according to ASTM 638M-81 (die M1). A similar sheet, moulded and tested under the same conditions, but without fibre reinforcement, gave a tensile strength of 58 MPa and a tensile modulus of 1.6 GPa.

Claims: A process for the preparation of thermoplastic polymer fibres which comprises a) spinning a solution of an alternating copolymer of an olefinically unsaturated compound and carbon monoxide having a number average molecular weight of at least 5000, to fibres through a spinning aperture, said solution having a polymer content in the range of from 0.1 - 50 % by mass. b) submitting the thus obtained fibres to a solvent-removal treatment to remove substantially all the solvent, and c) stretching the thus obtained substantially solvent-free fibres in an environment having a temperature between the glass-transition temperature of said polymer and (T+20)°C, wherein T is the crystalline melting point of the polymer. A process as claimed in claim 1, wherein the polymer is an ethylene/carbon monoxide copolymer. A process as claimed in claim 1 or 2, wherein the copolymer has a number average :olecular weight of at least 10⁴. A process as claimed in claims 2 and 3, wherein the ethylene/carbon monoxide copolymer has a number average molecular weight of more than 10⁶. A process as claimed in any one of claims 1-4, wherein the solvent present in the polymer solution is hexafluoroisopropanol, m-cresol, or a mixture thereof. A process as claimed in any one of claims 1-5, wherein the solvent component of the polymer solution includes a minor amount of a non-solvent. A process as claimed in any one of claims 1-6, wherein the polymer content of the polymer solution is in the range of from 0.25-20 % by mass. A process as claimed in any one of claims 1-7, wherein the solvent-removal is achieved by passing the solvent-containing fibres through a bath containing an extracting liquid for the solvent component of said fibres.

Inventors: VAN BREEN, ADRIAAN WOUTER, VAN DEURSEN, JOSEPHUS HUBERTUS, WETSER, FRANCISCUS IGNATIUS MARIA
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: D01F6
IPC codes: D01F6, C08G67, D01D5