METHOD OF IMPROVING THE ADHESION PROPERTIES OF POLYAMIDE FIBERS AND PRODUCTS DERIVED THEREFROM, AND A METHOD OF PREPARING COMPOSITES

Abstract: The adhesion properties of polyamide fibers are improved by treating the fibers with an acid halide, such as oxalyl chloride, in a polar aprotic solvent, and then optionally with a functional compound such as water or glycidol. Fiber reinforced materials having high resistance towards pull-out and other mechanical stresses are made using the so treated polyamide fibers.

Description: Method of improving the adhesion properties of polyamide fibers and products derived therefrom, and a method of preparing composites. The present invention relates to a method of improving the adhesion properties of polyamide fibers and products derived therefrom, by means of a chemical treatment. Polyamide fibers possess numerous favourable properties such as a high elasticity and high tensile strength. In particular, they are suitable for producing fibers because of their high tenacity, rigidity, and wear and abrasion resistance. Fibers of aromatic polyamides excel in particular in having a high modulus, a high strength and a high thermal stability. Therefore, they are pre-eminently suitable for use as a reinforcement in composite materials (composites). However, the adhesion between polyamide fibers and matrix materials is sometimes insufficient as a result of the relative inertness and smoothness of the fiber surface, causing both the chemical and the mechanical bonding to be weak. This is particularly true with aromatic polyamides. Several solutions to this problem have been proposed, which solutions can be divided into mechanical treatments, such as roughening the fibers and applying a coating, and chemical treatments, such as introducing functional groups onto the polymer. Of course, such treatments should not harm the mechanical properties of the fibers to any appreciable extent, but usually this requirement is not met by such treatments. Takayanagi et al. (Polymer J., _1 , 467-474 (1987)) have described a process for treating the surface of poly-p-phenylene terephthalamide (PPTA)-fiber used as a reinforcement for epoxy resin. According to this process PPTA is treated with sodium hydroxide in dimethylsulfoxide (DMSO), whereafter the deprotonated ( etalated) PPTA is reacted with an alkyl halide or an epoxide resulting into a PPTA fiber having functional groups. Reactions of the deprotonated PPTA with other compounds such as acrylonitrile and propylene oxide were also described by Takayanagi et al. (J. Polym. Sci. Polym. Chem. Ed., J21, 31-39 (1983) and J. Appl. Polym. Sci. 27_, 3903-3917 (1982)). Kashani et al. (Polym. Lett. Ed. ^, 363-367 (1977)) have described a similar treatment with sodium in ammonia and ethylene oxide. The adhesion properties of the PPTA were improved after some of such treatments, but the surface of the fiber turned out to be considerably damaged. Penn et al. (J. Adhesion 23, 163-185 (1987)) have described the introduction of side groups on aramide fibers by means of a plasma treatment with methylamine followed by side chain lengthening; however, no improved adhesion of the treated fibers with an epoxy matrix was found. A method has now been found according to which it is possible to considerably improve the adhesion strength of polyamide fibers to matrix materials while maintaining the original mechanical properties of the fibers. The method according to the invention is characterized in that the polyamide fibers or the products derived therefrom are treated with at least one acid halide in an inert, polar, aprotic solvent. In the treatment according to the invention, the acid halide will react with the amide function of the polyamide, substituting a hydrogen atom by an acyl group. This side group may as such or after a further treatment, depending on the nature of the acid halide used, bring about the improved adhesion to matrix materials. In principle any acid halide is suitable for the present method. The acid halide will be chosen depending on the reactivity of the poly¬ amide, on the desired degree of adhesion improvement and on the nature of the material with which the polyamide fibers are combined. If the polyamide has a low-reactive amide function, such as in aromatic polyamides, an acid halide activated by a second functional group, preferably a diacid halide, is advantageously used as the acid halide. For example, an oxalyl, alonyl or phthaloyl halide, or a mono- ester derivative thereof, is used. The halide is advantageously a chloride or bromide. Preferably, oxalyl chloride is used. The treatment according to the invention is carried out in a solvent which dissolves the acid halide, but which does not react with it to any noticable extent, and which, if necessary, makes the polyamide accessible for chemical reaction. Making the polyamide accessible can be effected by allowing the surface layer of the polyamide fibers to swell somewhat in the solvent; however, the polyamide should not dissolve in the solvent. In general, the above requirements are met by polar, aprotic organic solvents. Solvents are understood to comprise also mixtures, at least one component of which is polar. Suitable solvents are halogenated hydrocarbons, ethers, ketones, esters, organic nitro compounds, sulfoxides, sulfones, amides and similar solvents. Useful examples of these are chlorobenzene, o-dichlorobenzene, nitromethane, nitrobenzene, tetramethylene sulfone (sulfolane), N-methylpyrrolidone and acetone. In case of aliphatic polyamides, the solvent may be weakly polar or even apolar, like carbon tetrachloride. In case of aromatic polyamides, the solvent should be more polar; preferably sulfolane, optionally combined with another solvent, is used. If desired, the solvent may contain an acid binding agent, but this is not necessary. The ratio between acid halide and solvent is not critical and can be, for example, between 2 and 50 vol.% of acid halide in the mixture, but also outside this range if desired. The amount of acid halide to be used with respect to the polyamide depends on the desired degree of adhesion improvement, i.e. on the number of amide groups that should be substituted; in general, an excess of acid halide is used. The reaction time and the reaction temperature are also chosen according to the desired degree of adhesion improvement. Depending on the polyamide fiber and the reaction temperature respectively, the treatment can take a few seconds up to several hours. The reaction can be performed at ambient temperature or at elevated temperature, provided that the properties of the polyamide do not decrease. The upper limit is usually between 200 and 300°C. If required, the reaction Is performed under pressure. The polyamide fibers treated with the acid halide can be used as such for reinforcing the matrix materials, especially if the acid halide has a second functional group which can form bonds with groups present in the matrix material. Such a second functional group can also be used, however, to introduce another, desired, function into the side chain thereby increasing the adhesion properties even further. If the treated polyamide contains a second acid halide function, it is preferably reacted with a compound containing hydroxy, amino and/or mercapto groups used as a coupling agent, whether or not in an organic solvent. This agent may contain other functional groups ensuring a coupling with the matrix material. This second functional group can be chosen according to the nature of the matrix material. For example, for bonding to epoxy resins, polyesters and other ther osetting matrix materials, a hydroxy group or epoxy group, or possibly an amino group or thiol group can be selected; for polycarbonates, an amino group is suitable and for other thermoplasts such as poly(vinylchloride), polyolefines, polyacrylates and the like, a vinyl group and a long alkyl group are also suitable; for polyimides, polyamide-imides and polyether-imides, an amino group or a vinyl group is suitable as a bonding improving group; for poly- sulfones and polyether-sulfones, a thiol group or an amino- or vinyl group combined with a phenylene group is suitable; for poly(ρhenylene oxide), a thiol group and for poly(alkylene terephthalates) a metha- crylate group, a thiol group or an epoxy group; for elastomers, such as styrene-butadiene rubber, polyurethanes, polysulphides and the like, an epoxy group, a halogen atom or a thiol group is useful. In the second treatment, the acid halide can thus be reacted with water, alcohols, carboxylic acids, amines, thiols and such compounds and in particular with bifunctional compounds such as water, diols, hydroxycarboxylic acids, aminoalcohols, diamines, mercaptoalcohols, dithiols, unsaturated alcohols, haloalcohols and epoxyalcohols, whereby the two functions are preferably substituted at the ends of the molecule. The bifunctional compounds can also contain a phenylene group or an ester group in their chain. Examples are 3-aminopropanol, ethylenediamine, trimethylene- diamine, 4-aminobenzyl alcohol, 1,2-ethanedithiol, 1,3-propanedithiol, allyl alcohol, 4-vinylphenol, glycidol, 2-hydroxyethyl methacrylate, 3-chloropropanol etc. Upon reaction with such a bifunctional compound, a possible excess of the acid halide is preferably removed first, in order to avoid possible formation of other polymers. When the polyamide has been treated for example with oxalyl chloride, a polyamide having recurring groups of formula 1 in the chain is obtained. Reaction of the latter with methanol, ethylene- diamine, ethane dithiol, allyl alcohol, 3-chloropropanol or glycidol provides a polyamide having the groups with formulae 2-7 respectively. Reaction with water yields groups having a carboxylic function of formula 8 and/or, after decarboxylation, groups having an aldehyde function of formula 9. Preferably, water or glycidol is used for epoxy resins and poly¬ esters. The polyamide fibers and the products derived therefrom may comprise any kind of polyamide, i.e. both homopoly ers and copolymers. The polyamides can be derived from aminocarboxylic acids as well as from diamines and dicarboxylic acids. The method according to the invention is of particular advantage for fibers of aromatic polyamides, i.e. poly¬ amides containing p- or m-phenylene groups and/or other aromatic groups such as thiazolylene and benzimidazolylene groups in the chain. Well-known examples are the aramides poly-m-phenyleneiεophthalamide (MPIA), poly—p—phenyleneterephthtalamide (PPTA) and poly-p—benzamide, as well as copolymers of MPIA and PPTA. The preferred polyamide Is poly-p-phenyleneterephthtalamide, known under the trade names Kevlar (DuPont) and Twaron (AK.Z0). The process of improving the adhesion properties of polyamide fibers according to the invention may, if desired, be performed continuously. The fibers are then passed through a solution of the acid halide and subsequently, if necessary, through a bath containing the coupling compound. Prior to the treatment with acid halide, the fibers can be passed through a bath, optionally heated, containing a solvent only, such as sulfolane, in order to improve the accessibility of the fibers for the acid halide. The present invention also relates to the polyamide fibers and products derived therefrom which are thus obtained, having improved adhesion properties. The present invention also relates to a method of producing composite materials (composites) wherein polyamide fibers or products derived therefrom having improved adhesion properties are incorporated into a matrix material such as an epoxy resin, a polyester, a poly- urethane, a thermoplast, an elastomer and the like, thus transferring the mechanical properties of the polyamide fibers to the composite. The present invention therefore also relates to the composites thus produced. The process of improving the adhesion of polyamide fibers according to the present invention with respect to matrix materials does not adversely affect the mechanical properties of the polyamide fibers; this is shown by the high tensile strength of the polyamides treated according to the method of the invention which is unchanged with respect to that of the corresponding untreated polyamide fibers. Upon analysis of the fiber surface with the aid of scanning electron microscopy (SEM) it appears that the surface after treatment is as smooth as before treatment. Thus, the adhesion increase obtained is the result of the chemical modification and not of a physical alteration of the surface. The nature of the binding with the matrix material ensuring the increased adhesion can vary: full chemical bonding such as by reaction of, for example, an amino or hydroxyl group with an acid chloride group or an epoxy group or of an epoxy group with a carboxylic acid group in the matrix; hydrogen bridging, for example, between a hydroxyl group and an ester or amide group; and lipophllic and similar interactions. Generally, full chemical bonding provides the greatest improvement in adhesion properties. The adhesion or bonding strength can be measured by means of mechanical tests such as pull out tests, providing the maximum force which can be applied on the fiber without the fiber being pulled out of the matrix. This force divided by the embedding depth of the fiber is a measure of the adhesion. The bonding strength can also be determined by means of bending tests such as the ILSS (interlaminar shear strength) or on the basis of the fatigue behaviour. In the matrix materials prepared according to the invention, the bonding strength appears to be 20-60% higher than in corresponding materials comprising the polyamide fibers which have not been treated. High temperature curing of the materials (for example 140°C) does not adversely affect the adhesion of the poly¬ amide fibers previously treated according to the invention. The method according to the invention can also be used for coupling other groups than adhesion promoting groups to polyamide fibers. Thus, polyamide fibers and products derived therefrom can be made hydrophobic by reaction of the polyamide treated with acid halide, with a long chain alcohol, optionally in combination with phenyl groups. In a similar way, dyes can also be coupled to polyamides, thus providing fibers having a desired colour. The method can also be used to make fibers fire resistant ' by coupling the fibers with halogen and/or phosphorous containing groups. With tne method according to the invention it is aLso possible tυ in> rea&e t e bonding power of aliphatic polyamide hot melts by cross,- linking. To that purpose, (meth)acryloyloxyalkyl groups can for e5.ar.iple De coupled to the polyamides, whereby the shear stren fa th of the aohesive layer is increased as a resuLt of the cross-linking. Exampl I PPTA-fibers (Twaron D1000, extracted, 1000 filaments, dtex=l6t:0) re loosely wound around a cage-shaped glass support and kept in a bath of sulfolane/ xalyl chloride (9:1 vol/ vol) at 50-60°C for 1 hour. The fiDers were removed from the bath, and after draining, they were kept in distilled water for 1 minute. The thus treated fibers were dried under reduced pressure and stored in a desiccator. The carbon, nitrogen and oxygen content of the dried fiber was determined by means of photo- electron spectroscopy. The results are summarized in Table A. The tensile strength was determined by means of closed loop experiments with yarns/f ibers/mul t f ila ents twisted by 1 turn/cm (1 strand). The stretching rate used was 10% min- . The results are summarized in Table A. Two strands of treated PPTA-fibers were twisted by 1 turn/cm and embedded in a disc of epoxy resin. The epoxy system was that of Ciba-Geigy: epoxy: LY556; curing agent: HT972; cure treatment: (1) from room temperature to 80°C by 2°C/min.; (2) 2 hours at 8U°C; (3) from 80°C to 120°C by °C/rain.; (4) 2 hours at 120°C; (5) from 120°C to room temperature by 6 °C/min. Determination of bonding strength Tne bonding strength of the multif ilament was determined by means of a bundle pull-out test. Thereby, the end of the fiber is fixed on a hook, which is connected to the force recorder of the tensile strength tester. After the multif ilament has been twisted by 1 turn/cm, the epoxy disc is fixed on a specially engineered clamp by applying a slight (pre)strain. The final testing takes place at a rate of 10 mm/min. At the same test configuration and test performance, the pull-out force divided by the embedded length is a measure of the adhesion between fiber/ir.ult if i lament and matrix. For a comparison of the bonding strength of the f ibers with different denier/ tex values , it is necess ary to determine the shear strength defined by K 2K Ti = = __^ a embedded outer surface of multifilament 71 ^.dl wherein K = maximum force (N) measured d = diameter (mm) twisted fiber/ ultifilament = diameter (aim) of the hole in the epoxy disc present after pull-out 1 = embedded length (mm) and to compare these with each other. ILSS values are measured using test rods, made by immersing a fibre bundle in a resin mixture without inclusion of air. The number of yarns in the fibre bundle is such that the fibre content of the rods is 50 % by volume. The resin is degassed and curing conditions are as described above. The bundle pull-out force, the shear strength and the ILSS values are summarized in Table A. Comparative example The chemical composition and the tensile strength of the same PPTA-fibers as used in Example 1, however, not treated, were measured. After twisting and embedding in epoxy resin as in example 1, the pull-out force was determined. The results are summarized in Table A. Example 2 The procedure of Example 1 was followed, with the exception that the fibers treated with sulfolane/oxalyl chloride were not held in water, but instead in methanol. The corresponding results are summarized in Table A. Example 3 The procedure of Example 1 was followed, with the exception that the sulfolane/oxalyl chloride treated fibers were not held in water but were first rinsed with dry dichloromethane in order to remove any remaining oxalyl chloride, then held in ethylenediamine, and finally rinsed with dichloromethane again. The corresponding results are summarized in Table A. Example 4 The procedure of Example 1 was followed, with the exception that the sulfolane/oxalyl chloride treated fibers were not held in water but were first rinsed with dry diethyl ether in order to remove any remaining oxalyl chloride, then immersed in a mixture of glycidol and diethyl ether (1/10 vol/vol) and then again rinsed with diethyl ether. The corresponding results are summarized in Table A. Examples 5 and 6 The procedure of Example 4 was followed, with the exception that the fibers were held in the sulfolane/oxalyl chloride bath for only 5 min. (Example 5) and 1 min. (Example 6) respectively, instead of 1 hour. The pull-out force according to these examples is given in Table B, along with that of Example 4. Examples 7 and 8 The procedure of Example 1 was followed, with the exception that the vol/vol-ratio of sulfolane/oxalyl chloride in the bath was 5:1 (Example 7) and 12:1 (Example 8) respectively. The pull-out strength of the material, prepared for the remainder according to Example 1, was 86 N/mm (Example 7) and 83 N/mm (Example 8) respectively. 8β TABLE A ©o 2 Example formula % C % N % 0 Tensile strength Pull-out force 2 ) Shear ILSS Z found ) /calculated strength (N/mm 2 ) | (N/mm 2 ) Comparative 77/77.8 8/11.1 15/11.1 2.2 57.4 18.0 56.5 jJ 64.3 7.1 28.6 1 70 5 25 2.2 84.8 27.4 64.5 _9 72.7 9.1 18.2 2 2 67/66.7 5/ 6.6 28/26.7 2.2 76.0 24.4 60.5 3 _3 . 65/64.7 17/17.6 18/17.6 2.1 76.7 ' 24.7 4 I 70/66.7 6/ 5.6 24/27.7 2.1 100.0 32.2 69.5 3 > 1) by means of photoelectron spectroscopy 2) maximum force during pull-out test divided by embedding depth (usually 1.3-1.7 mm) 3) the highest values reported Ln literature (See EP 6275 for example) are 68-70 N/mm 2 ; the highest value posslbLe Is about 70 N/mm 2 being the Internal shear strength of PPTA. co Oι 00 i TABLE B Example Duration Pull-out force (N/mm) Shear strength (glycidol) (min) (vide Table A) (N/mm 2 ) 4 60 100.0 32.2 5 5 91.7 29.4 6 1 89.0 28.6 Example 9 Example 1 was repeated with the exception that the fibers were held in the sulfolane/oxalyl chloride bath for only 1 min. The pull-out force and shear strength values were 78.9 N/mm and 25.3 N/mm 2 respectively. Examples 10 and 11 Examples 1 and 3 were repeated, with the exception that after the hardening of the epoxy resin, a hot curing treatment at 140°C for 4 hours was performed. The pull-out force was unchanged. Example 12 PPTA-fabric ( evlar 49-mats from Ten Cate, Type VD 230, extracted) was treated with oxalyl chloride/sulfolane according to the procedure of Example 1 and subsequently, according to Examples 1, 2, 3 and 4, with water, methanol, ethylenediamine and glycidol, respectively. The measured carbon, nitrogen and oxygen content of the dry, treated fabrics is summarized in Table C. The results of the treated fabrics correspond with those obtained with fibers (compare Table A). TABLE C Treatment formula %C %N %0 None (comparison) 73 21 Oxalyl chloride + water 11 70 5 25 + methanol 2 67 5 28 + ethylenediamine 63 16 21 + glycidol 7 70 5 25

Claims: C L I S 1. A method of improving the adhesion properties of polyamide fibers or products derived therefrom, by means of a chemical treatment, characterized in that the polyamide fibers or the products derived therefrom are treated with at least one acid halide in an inert, polar, aprotic solvent. 2. The method of claim 1 " , characterized in that the acid halide is a diacid halide or a monoeεter thereof. 3. The method of claim 2, characterized In that the acid halide is oxalyl chloride. 4. The method of any of claims 1-3, characterized in that the solvent Is sulfolane. 5. The method of any of claims 1-4, characterized in that the polyamide is aromatic. 6. The method of claim 5, characterized in that the polyamide is p-phenyleneterephthalamide. 7. The method of any of claims 1-6, characterized in that the polyamide fibers or products derived therefrom, treated with acid halide, are subsequently treated with a compound containing one or more hydroxy, amino and/or mercapto groups and which may also contain one or more vinyl groups and/or halogen atoms. 8. The method of claim 7, characterized in that the polyamide fibers or products derived therefrom, treated with the acid halide, are treated with water or glycidol. 9. Polyamide fibers and products derived therefrom having improved adhesion properties, obtained using the method of any of claims 1-8. 10. A method of preparing composite materials, characterized in that polyamide fibers or products derived therefrom, obtained by using the method of any of claims 1-8, are incorporated into a matrix material. 11. The method of claim 10, characterized in that the fibers or products obtained using the method of claim 8 are incorporated into an epoxy resin or a polyester. 12. Composite material, obtained using the method of claim 10 or 11. ***

Inventors: MERCX FRANCISCUS PETRUS MARIA, VAN TURNHOUT JAN, MERCX, FRANCISCUS, PETRUS, MARIA, VAN TURNHOUT, JAN
Applicants: TNO, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST- NATUURWETE
Application year: 1989
Main classification: D06M13
IPC codes: D06M13