WO2014065652A1

WO2014065652A1 - Coated reinforcement fiber, process for the manufacturing of such fiber and polymer composite comprising such fiber

Info

Publication number: WO2014065652A1
Application number: PCT/NL2013/000048
Authority: WO
Inventors: Seyes Morteza Sadat SHIRAZI; Auke Talma; Erik Van De Ven; Jacobus Wilhelmus Maria Noordermeer; Luc Louis Théophile VERTOMMEN; Pieter Jan De Lange
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2012-10-22
Filing date: 2013-10-17
Publication date: 2014-05-01
Also published as: NL1039856C2

Abstract

The invention relates to a reinforcement fiber comprising a coating of epoxy resin having an epoxy backbone, wherein the epoxy resin comprises specific grafts attached to the backbone. The invention further relates to a polymer composite comprising a polymer and a reinforcement fiber of the invention. Reinforcement of polymers with fibers of the invention is enhanced by interaction of the grafts with the polymer.

Description

Coated reinforcement fiber, process for the manufacturing of such fiber and polymer composite comprising such fiber

The invention relates to a reinforcement fiber comprising a coating, to a method for manufacturing such fiber, to a fiber obtainable by such method, to a polymer composite comprising such a fiber, to a method for

manufacturing such polymer composite and to a composite obtainable by such method.

It is known that the properties of a polymeric material can be improved by the incorporation of reinforcement fibers. An example of such polymer composite is rubber reinforced with aramid or polyester fibers. In the manufacturing of such rubber composites, it is important to achieve a strong adhesion of the fibers to the rubber, because untreated aramid or polyester fibers have a specific crystalline surface that normally does not provide sufficient adhesion to the matrix rubber. A fiber surface with sufficient adhesion can be obtained by providing the fibers with a coating that acts as an adhesion promoter, e.g. via a dipping process.

A frequently used coating to improve the adhesion to a rubber comprises a sub-layer of epoxy and a top-layer of Resorcinol Formaldehyde Latex (RFL). The latex in RFL may in particular comprise residues derived from styrene monomer (about 15%), butadiene monomer (about 70%) and vinyl pyridine monomer (about 15%). The standard method for such surface treatment is a an epoxy dipping followed by an RFL dipping. Usually, the epoxy dipping is followed by a high temperature treatment to achieve improved bonding between the fiber and the epoxy.

EP0892007 B1 discloses a process for applying a coating to an unprocessed fiber and its bonding to a rubber composition. The coated fiber is prepared by treating an unprocessed fiber with 1 ) an isocyanate and/or an epoxy compound; and 2) an RFL that comprises at least one rubber latex selected from acrylonitrile-butadiene rubber latex and hydrogenated nitrile rubber latex. In a third step, a fiber-reinforced rubber composition is then prepared by treating the coated fiber with a rubber paste including

acrylonitrile-butadiene rubber composition. The two treatments mentioned occur by dipping cords of the fibers into the different processing liquids. In the art, a cord is usually referred to as a so-called "continuous bundle of fibers". This means that the cord is

(seemingly) endless. A cord is usually composed of twisted bundles (yarns) of fibers, while a bundle is composed of multiple filaments. RFL that is applied to such a cord - as in EP0892007 B1 - is mainly present on the outer surface of the cord, while the fibers and filaments present in the inner side remain largely uncoated. When a polymer matrix is fiber-reinforced, it is usually a continuous fiber that is incorporated.

As opposed to the continuous fiber used for reinforcement as described in e.g. EP0892007 B1 , it is also possible to use so-called "short fibers" for reinforcement. Such short fibers are usually obtained when a continuous bundle of fibers is cut into a plurality of shorter segments. The average length of such short fibers is usually less than 25 mm, less than 15 mm or less than 10 mm, with 0.5 mm as a usual minimum. Preferably, their average length is in the range of 1-6 mm. When such a cutting of a-bundle of fibers is performed, however, the resulting segments lose cohesion and finally disintegrate into their smallest constituents: their filaments. The original structure wherein the RFL coating resides on the outer surface is lost, and a fibrous mixture forms wherein a part of the filaments is (at least partly) coated with RFL, and wherein another part of the fibers may be less coated.

In applications wherein short fibers are required, it is

disadvantageous when the (separated) filaments are poorly coated with RFL because this leads to insufficient adhesion of the fibers to the rubber and to insufficient reinforcement. Alternatively, it is also difficult to provide the RFL coating to the filaments after the cutting into short fibers by the usual dipping process, since the RFL-dipping can hardly be performed on the short and thin filaments.

Another disadvantage of the cutting of RFL-coated bundles of fibers is, that it is difficult to perform the cutting in a rotary cutter. This is due to the rubbery properties of the RFL-coating. The best method of cutting RFL- coated cords is by guillotine cutting, but this method is more expensive than rotary cutting. In conclusion, there is a need to provide short fibers with a coating that improves the adhesion to polymer matrices. There is also a need for a process resulting in a product wherein a large part of the surface of the filaments is coated so as to have the desired adhesion promoting properties, and wherein a large part of the treated filaments have such coating.

Further, a disadvantageous property of an RFL-coating is that it is sensitive to light and air as long as it is not incorporated in a polymer matrix. Accordingly, an RFL-coated fiber needs to be shielded from light and air during its preparation, handling, shipping and storage. The measures to prevent exposure to light and/or air are usually not convenient and are usually costly.

Accordingly, it is an object of the invention to provide a continuous reinforcement fiber having a coating capable of adhering to polymer matrices, wherein the adherence is at least as strong as that of the conventional RFL- coated continuous cord of fibers. It is in particular an object to provide a short reinforcement fiber having a coating with adherence properties that are better than those of known short reinforcement fibers. Since the separate filaments in short fibers have a diameter in the micron scale, it is contemplated that an adherence mechanism on nanoscale could be effective. It is therefore also an object to design an adherence mechanism wherein control can be exerted at the molecular level.

In particular, it is aimed to provide a reinforcement fiber having improved adherence properties with respect to the adherence to rubbers.

It is a further object of the invention to provide a coated reinforcement fiber that is less sensitive to environmental influences such as light and air, than RFL-coated reinforcement fibers known in the art. It is also aimed to provide fibers that require less protection from the environment than known coated reinforcement fibers, and to achieve an easier process of preparation and/or a more convenient handling, storage and shipping.

It is also an object of the invention to provide a process for the production of a reinforcement fiber having such improved adherence properties. It is in particular aimed to provide such a process for the

production of a short reinforcement fiber. More in particular, it is aimed that such process provides a coated fiber of which a large part of the surface has the desired adhesion promoting properties, e.g. more than 50%, preferably more than 70%, and wherein a large part of the treated fibers have such coating, e.g. more than 50%, preferably more than 70%.

It is also an object of the invention to provide a fiber-reinforced polymer composition that has improved properties, such as a higher tensile strength than polymer compositions known in the art. It is in particular an object of the invention to provide a reinforced polymer comprising short reinforced fibers that has a tensile strength that is comparable to or higher than that of a known reinforced polymer comprising short fibers.

Therefore, the present invention relates to a reinforcement fiber comprising a coating of epoxy resin having an epoxy backbone, wherein the epoxy resin comprises grafts attached to the backbone, wherein the grafts on the backbone are selected from the group of

-X-C(=0)-X'-R, where - X is attached to a carbon atom of the backbone of the epoxy resin, and where X is O or NR', where R' is a hydrogen atom or a carbon atom of the backbone of the epoxy resin;

- X' is O or NR", where R" is a hydrogen atom or a functional group

comprising at least one carbon atom;

- R is a functional group comprising at least one carbon atom.

In particular, the part of the epoxy resin that is present at the outer surface of the coating comprises grafts attached to the backbone.

A reinforcement fiber is a fiber suitable for reinforcement of a material in which the fiber is incorporated. In this respect, with reinforcement is meant the state or action of being strengthened.

A reinforcement fiber of the invention may in principle be any fiber that is suitable for reinforcement of a material. The reinforcement fiber may be a natural fiber or a synthetic fiber. Natural fibers include those produced by plants, animals, and geological processes. Examples of natural fibers are vegetable fibers such as cotton, hemp, jute, flax, ramie, sisal and bagasse; wood fibers; animal fibers such as silkworm silk, spider silk, sinew, catgut, wool, sea silk and hair; and mineral fibers such as asbestos.

Synthetic fibers are for example metallic fibers, carbon fibers, silicon carbide fibers, glass fibers, mineral fibers and polymer fibers.

A reinforcement fiber of the invention may in particular be a polymer fiber selected from the group of polyamides such as nylon; polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT);

polyvinyl alcohol (PVA); polyvinyl chloride (PVC); polyolefins such as polyethylene (PE), high-performance polyethylene (HPPE) and polypropylene (PP). For the reinforcement of rubbers, usually a polyamide or a polyester fiber is used.

More in particular a reinforcement fiber of the invention is an aromatic polyamide fiber, also known under the shorter name "aramid fiber", such as poly(p-phenyleneterephthalamide). Suitable aramid fibers are those known under trade names such as Kevlar™, Twaron™, Nomex™, New Star™, Teijinconex™ and Technora™.

The reinforcement fibers may be continuous reinforcement fibers or short reinforcement fibers. As explained above, continuous fibers are

(seemingly) endless. Their continuous nature allows their handling in a roll-to-roll manner. Short fibers usually have an average length of less than 25 mm. Preferably, their length is in the range of 0.5-10 mm, more preferably it is in the range of 1-6 mm.

Continuous fibers usually provide more reinforcement to a material than short fibers. Short fibers, on the other hand, are usually advantageous in the manufacture of the reinforced polymers, since their incorporation into the polymer matrix is more straightforward (easier process). Rubber composites comprising short reinforcement fibers are especially suitable for use in the production of V-belts, hoses, tire treads and complex-shaped mechanical goods. An epoxy resin is usually a copolymer that is formed by the reaction of at least a first comonomer, which is an epoxide compound having two or more epoxide groups, and a second comonomer, which in the art is usually referred to as "curing agent". The curing agent is usually a polyamine or polyalcohol. It is, however, not essential that a curing agent is present in the polymerization reaction. In case it is absent, the epoxide molecules react among themselves. A catalyst may be present that initiates the polymerization of the epoxide molecules (a homopolymerisation), but there is no curing agent that forms part of the final structure. In case the epoxide monomer contains a hydroxyl group, the homopolymerization may also be initiated by such hydroxyl group.

The backbone of an epoxy resin is the branched or unbranched chain that is formed by the reaction of a plurality of first comonomers with a plurality of second comonomers. In case a curing agent is not present, the backbone is the branched or unbranched chain that is formed by the

homopolymerisation of epoxide monomers. The backbone of an epoxy resin typically includes hydroxyl and amine groups, which are the result of the copolymerization reaction of first comonomers with second comonomers. In case the epoxy resin is formed by homopolymerizating epoxide molecules, the backbone also contains hydroxyl groups, but in principle does not contain amine groups. The hydroxyl groups that are present on a backbone obtained by homopolymerization may be formed by the hydrolysis of epoxide groups. They, however, may also occur as end-groups on a polymeric chain, which chain is then for example formed by a homopolymerization initiated by a hydroxyl group present in the epoxide monomer.

Depending on the number of epoxide and amine/alcohol

functionalities of each of the comonomers, the backbone of the epoxy resin may be a linear chain (an unbranched backbone) or a network (a branched backbone).

The epoxy resin coating of the invention may in principle be any epoxy resin. It can be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic, monomeric or polymeric in nature. Preferred, however, is a poly(glycidylether) of a polyhydric aliphatic alcohol, e.g. an alcohol selected from the group of butanediol, propanediol, ethylene glycol, diethylene glycol, glycerol, polyglycerol, hexanetriol, pentaerythritol, trimethylol ethane and trimethylol propane, hydroxy-containing esters (e.g. castor oil), and mixtures thereof. Particularly preferred is an epoxide resin wherein the epoxide compound is selected from the group of a diglycidyl ether of glycerol, a triglycidyl ether of glycerol, a polyglycidyl ether of polyglycerol and mixtures thereof. The diglycidyl ether of glycerol contains a hydroxyl group that may initiate the homopolymerization of that diglycidyl ether. The polyglycidyl ether of polyglycerol may also contain hydroxyl groups capable of initiating the homopolymerization, since not all hydroxyl groups of the parent polyglycerol may be etherified by glycidyl groups.

Also preferred are phenol-based epoxy resins. Examples of suitable phenol-based epoxy resins are the diglycidyl ethers of bisphenol A, bisphenol F, bisphenol S, resorcinol, hydroquinone, 4,4'-dihydroxydiphenylethane, 4,4'-dihydroxybenzophenone, 1 ,5-dihydroxynaphthalene, and

4,4'-dihydroxybiphenyl, condensed or extended glycidyl ethers of a bisphenol, and glycidyl ethers of polyhydric phenols, for example an epoxy novolac resin.

Other glycidyl ethers of polyhydric phenols are polymers prepared by reacting 1.1 up to about 2 mols of epichlorohydrin with 1 mol of dihydric phenol or by reacting diepoxides with added dihydric phenol. Additional epoxides are glycidyl ethers of polyhydric alcohols, usually made by reacting a polyhydric alcohol and epichlorohydrin with an acidic catalyst such as boron trifluoride and subsequently treating the resulting product with an alkaline dehydrohalogenating agent.

Still other epoxides are glycidyl esters of polycarboxylic acids, such as acids selected from the group of azelaic acid, adipic acid, isophthalic acid, terephthalic acid, dimerized and trimerized unsaturated fatty acids. Useful epoxides also include epoxidized hydrocarbons, such as vinyl cyclohexene dioxide, butadiene dioxide, dicyclopentadiene dioxide, epoxidized

polybutadiene and limonene dioxide. Other epoxides are epoxidized esters, for example, epoxidized soybean oil, epoxidized glycerol trilinoleate, and 3,4-epoxycyclohexylmethyl- 3,4-epoxycyclohexane carboxylate. Still other epoxides are polymers and copolymers of vinyl polymerizable monoepoxides, such monoepoxides being allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate.

Usually, an epoxide compound used in the invention has an average of 2-7 epoxy groups per molecule, preferably it has 2-5 epoxy groups per molecule.

Suitable amine curing agents for the epoxy resin include primary amines (including aliphatic, aromatic and modified amines), polyamides, tertiary and secondary amines, and imidazoles. Preferred curing agents are amines and imidazoles. Preferred amine curing agents are polyamines. More preferred are diamines.

Suitable diamines may be selected from the group of isopropyl diamine, diaminomethane, ,2-diaminoethane, 1 ,3-diaminopropane,

1 ,2-diaminobutane, 1 ,2-diaminopropane, 1 ,4-diaminobutane,

1 ,5-diaminopentane, 1 ,3-diaminopentane, 2,2-dimethyl-1 ,3-diaminopropane, 1 ,5-diamino(2 methyl)pentane, ,6-diaminohexane, 1 ,7-diaminoheptane, 1 ,8-diamino octane, 1 ,9-diaminononane, ,10-diaminodecane,

1 , 12-diaminododecane, 1 ,6-diamino-(2,2,3-trimethyl)hexane, 1 ,6-diamino- (2,2,4-trimethyl)hexane, 1 -amino-3-aminomethyl-3,5,5-trimethylcyclohexane, 1 ,3-bis(aminomethyl)cyclohexane, isophorone diamine, tricyclododecane diamine, dianiline methane, dianiline ether, dianiline sulphone,

2,2',6,6'-tetraethyl(4,4^,-methylenedianiline),

methylenedianiline), ,8-diamino-3,6-dioxaoctane, 1 ,5-diamino-3-oxapentane, alpha, omega-poly(tetrahydrofuryl)diamines, alpha, omega-polyglycol diamines (Jeffamines™), alpha, omega-polypropoxydiamines (Jeffamines™), alpha, omega-poly(ethoxy-propoxy)diamines, 3,5-diamino benzoic acid, 3,4- diaminobenzophenone, 1 ,2-diaminocyclohexane, diaminonaphthalenes, diamino toluenes, m-xylylene diamine, and (ortho-, meta- and

pa a-)diaminobenzene.

Examples of other suitable polyamines are selected from the group of diethylene triamine, triethylene tetramine, tetraethylene pentamine,

polyamide-amine, and adducts of any of these amines. Suitable alcohol curing agents for the epoxy resin include aliphatic and aromatic diols, diols having one or two primary hydroxyl groups, polyols having three or more hydroxyl groups, polyols having one or more secondary hydroxyl groups, polyols having one or more tertiary hydroxyl groups.

Preferred alcohol curing agents are polyols, more preferred are diols.

Suitable acyclic aliphatic polyols may be selected from the group of ethyleneglycol, 1 ,2-propandiol, 1 ,3-propanediol, trimethylolpropane, 1 ,2- butanediol, 1 ,3-butanediol, 1 ,4-butanediol, 2,3-butanediol, 1 ,5-pentanediol, 1 ,4-pentanediol, 2,4-pentanediol 1 ,6-hexanediol, ,5-hexanediol, 1 ,4- hexanediol, 2,5-hexanediol and 3,4-hexanediol. It may also be selected from the group of higher aliphatic diols such as decanediols, dodecanediols, hexadecane diols, octadecanediols, and positional isomers thereof (i.e. isomers differing in the positions of the two hydroxy groups).

Suitable alicyclic aliphatic polyols may be selected from the group of cyclobutane-1 ,2-dimethanol, cyclobutane-1 ,3-dimethanol, cyclopentane-1 ,2- dimethanol, cyclopentane-1 ,3-dimethanol, cyclohexane-1 ,2-dimethanol, cyclohexane-1 ,3-dimethanol, cyclohexane-1 ,4-dimethanol. In particular it is selected from the group of bis(hydroxycyclohexyl)alkanes, e.g.

2,2-bis(4-hydroxycyclohexyl)propane (also known as "hydrogenated

Bisphenol A").

Suitable aromatic polyols may be selected from the group of catechol, resorcinol, hydroquinone, 1 ,2-benzenedimethanol, 1 ,3- benzenedimethanol and 1 ,4 benzenedimethanol. In particular it is selected from the group of bis(hydroxyphenyl)alkanes, e.g.

2,2-bis(4-hydroxyphenyl)propane (also known as "Bisphenol A").

In a reinforcement fiber of the invention, the grafts attached to the backbone of the epoxy resin may be grafts of different types. It is understood that in a reinforcement fiber of the invention, at least one type of graft is a graft as defined in claim . Grafts of one or more other types may also be present. In case two different grafts are present, the two different grafts may in particular differ in the nature of X. In such case, X = O in the first type of graft and X = NR' in the second type of graft. In particular, X = O in the first and X = NH in the second type of graft. These situations in usually occur when there are nucleophilic oxygen atoms as well as nucleophilic nitrogen atoms in the epoxy backbone that can react with the grafting compound.

The group X of a graft defined as -X-C(=0)-X'-R is bonded to a carbon atom of the backbone of the epoxy resin. In case group X is an oxygen atom or an NH-group, group X is connected to one carbon atom of the backbone. Group X may, however, also represent a nitrogen atom that is bonded to a second carbon atom of the backbone. In that case, the nitrogen atom is bonded to two carbon atoms of the backbone (and to one carbon atom of the graft).

A graft on an epoxy-coated reinforcement fiber of the invention contains a functional group R, which group comprises at least one carbon atom. Usually, the at least one carbon atom of group R is directly connected to group X'. This functional group is designed to interact with the particular polymer matrix wherein the reinforcement fiber is incorporated so as to reinforce that polymer matrix. Preferably, the interaction is a chemical interaction, i.e. a chemical bond.

The functional group R may be any functional group capable of provide bonding with a polymer matrix, such as a hydrophobic or hydrophilic group, a charged or uncharged group, or an organic or inorganic group. In particular, the functional group comprises a heteroatom selected from the group of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, sulfur, chlorine, bromine and iodine. A functional group containing heteroatoms may for example be selected from the group of an alcohol, a ketone, an aldehyde, an acyl halide, a carbonate, a carboxylic acid, a carboxylate, an ester, a hydroperoxide, an ether, in particular an epoxide, a hemiacetal, a hemiketal, an acetal, an amine, an amide, an imine, an imide, a hydroxylamine, an azide, a cyanate, an isocyanate, a nitrate, a nitrile, a thiol, a thioether, a disulfide, a sulfoxide, a sulfone, a sulfinic acid, a sulfonic acid, a thiocyanate, a siloxide, silyl halide, a silene, a silanol, a phosphine, a phosphonic acid, a phosphate, a phosphodiester, a boronic acid, a boronic ester, borinic acid, a borinic ester and a haloalkane such as fluoroalkane, chloroalkane,

bromoalkane and iodoalkane. The functional group may also consist of carbon atoms. It may for example be an alkylene group and an alkynyl group. For example, when the polymer is a rubber, the functional group R in particular comprises an unsaturated carbon-carbon bond, such as an alkylene or an alkynyl group. More in particular, the R group is selected from the group of terminal alkylenes of the general formula -(CH2)n-CH=CH2, internal alkylenes such as -(CH2)m-CH=CH-(CH2)n-CH3, conjugated diene systems such as -(CH₂)n-CH=CH-CH=CH2 and -(CH₂)m-CH=CH-CH=CH-(CH₂)n- Chb, and non-conjugated diene systems such as -(CH2)m-CH=CH-(CH2)n- CH=CH2. In these groups, m and n are chosen independently from each other and are usually in the range of 1-24, in the range of 1-18, in the range of 1- 12, or in the range of 1-6. For example, n = 1 , 2, 3, 4, 5 or 6. In the case of the internal alkylenes such as

and/or conjugated diene systems such as -(CH₂)m-CH=CH-CH=CH-(CH₂)n-CH3, n may also be zero.

For example, when the polymer is a rubber that is to be cured with sulfur, the functional group R preferably comprises an internal carbon-carbon double bond, i.e. a double bond that is not at the end of a carbon chain. It is contemplated that the unsaturated carbon-carbon bond is capable of forming a chemical bond with the elastomer chains of the rubber during the curing.

In another case, when the polymer is a rubber that is to be cured with a peroxide, the functional group R preferably comprises a hydrocarbon moiety that is capable of losing a hydrogen atom in the form of a radical under the action of the peroxide curing agent. More preferably, R comprises a terminal carbon-carbon double bond, i.e. a double bond that is at the end of a carbon chain. It is contemplated that after the abstraction of a hydrogen radical, a chemical bond between the graft and the rubber is formed in a radical reaction.

As stated in claim 1 , the functional group X' may be O or NR". In case it is NR", the functional group R" may be a group that is designed to be capable of providing bonding with a polymer matrix. Thus, both R and R" may be capable of providing bonding with a polymer matrix. Groups R and R" may be identical, but they may also be different. Usually, the at least one carbon atom of group R" is directly connected to the group X'. A free reinforcement fiber of the invention (i.e. one that is not incorporated in a polymer) has a low sensitivity to light and air. This is advantageous, since this requires only moderate exclusion of light and/or air in its manufacturing process, handling, storage and shipping.

The invention further relates to a process for the manufacturing of a reinforcement fiber, comprising

1) providing a reinforcement fiber with a coating of epoxy resin;

2) contacting the reinforcement fiber comprising a coating of an epoxy resin with a compound selected from the group of 0=C=N-R and Z-C(=0)-O-R, where R is a functional group comprising at least one carbon atom and Z is a leaving group. The reinforcement fiber with a coating of epoxy resin is usually grafted on filament level, but the grafting reaction may also be performed on a yarn that contains epoxy-coated filaments.

Providing an uncoated fiber with an epoxy coating is known to the person skilled in the art. It can for example be performed according to the methods described in "The application of textiles in rubber"; D. B. Wooton; Ch. 5; Rapra Publ.; Exeter; UK; 2001.

It is usually performed by dipping the uncoated fiber in a curable epoxy coating composition, followed by curing the composition present on the fiber to an epoxy coating. The curing is usually performed at a temperature of at least 200 °C.

The curable epoxy coating composition is in particular applied as an aqueous solution or dispersion or as an organic solution or dispersion which preferably contains 0.3-10% by weight of the epoxide compound. The amount of the epoxide compound in the solution or dispersion is preferably so chosen that it has the desired viscosity and so that the desired amount of the epoxide compound is taken up by the fiber. After having been applied to the fiber, the epoxy compound is usually cured, so that a practically water- insoluble coating is formed having the desired properties. The amount of cured epoxide compound present on the yarn is usually in the range is 0.01- 5% by weight, preferably in the range of 0.03-1.0% by weight. The grafting occurs with a grafting compound of the formula O=C=N- R (an isocyanate) or of the formula Z-C(=0)-OR (a formate ester), wherein R is a functional group comprising at least one carbon atom and Z is a leaving group.

Generally, a leaving group is an atom or group (charged or uncharged) that becomes detached from an atom in what is considered to be the residual or main part of the substrate in a specified reaction. In the process of the invention, Z usually is a halogenide such as chloride, bromide or iodide. Preferably, the halogenide is chloride. Thus, the formate ester grafting compound in that case is a chloroformate ester, for example allyl chloroformate. The formate ester grafting compound may also be an anhydride of the formate ester, in particular a mixed anhydride. A mixed anhydride is preferably an anhydride of the formate ester with a carboxylic acid, i.e. Z is Rc-C(=0)-O- where R_c is a group on the carbon atom of the carboxygroup such as an alkyl group or an aryl group. For example, the mixed anhydride is the anhydride of acetic acid with allyl formate, i.e. C2H5- C(=0)-O-C(=O)-O-C₃H₅.

The functional group R of the grafting compound is in principle the same as the R-group described hereinabove for the grafted polymer coating. This means that when a graft with a particular R group is desired on the backbone of the epoxy, that particular R group is in principle the same as the R group of the grafting compound used to introduce a graft with that R group.

The grafting is usually performed by reacting the epoxy-coated fibers with the grafting compound in an inert solvent, e.g. a solvent selected from the group of chloromethylene, chloroform, diethyl ether, tetrahydrofuran (THF) and mixtures thereof. The grafting reaction may be perfomed at temperatures above room temperature. It may then be performed at the reflux temperature of the solvent, e.g. in refluxing THF.

When the formate esters (i.e. Z-C(=O)-O-R) are used for grafting, a base is usually present in the reaction mixture to scavenge the protonated leaving group HZ.

A reinforcement fiber of the invention where X' = NR" is usually prepared by post-treating the corresponding coated fiber product where X = NH that is obtainable by the process of the invention. Such post-treatment may for example comprise reacting the product with a strong base to deprotonate the NH-group, followed by reacting it with a haloalkyl or haloaryl.

An advantage of the process of the invention and of the fibers of the invention is that with the introduction of well-defined grafts, control can exerted over the coating (in particular over its surface) at the molecular level (nanometer level). The design at the molecular level allows an accurate tuning of the grafts, so that the grafts can be designed to be compatible with a desired polymer matrix, thereby achieving an excellent adherence of the reinforcement fiber to the polymer matrix.

A consequence of the control at nanometer level is that the

introduction of the grafts on the backbone of the epoxy coatings results in an almost negligible increase of the thickness of the coating as compared to the filament diameter of the fiber. In contrast, in coatings known in the art, such as an RFL-coating, the thickness may increase with 2 μιτι or more upon coating. In case of a 10-12 μιτι thickness of the separate filaments, this is a significant increase in the thickness of the coated fiber.

A process of the invention is in particular suitable for bundles of fibers that are separated at the filament level. The conventional RFL-coated short fibers are prepared by coating bundles of fibers with RFL, followed by cutting the bundles of fibers into short fibers. However, many fibers in a bundle of fibers (in particular the inner fibers) may remain insufficiently coated with RFL in this way. Cutting of the bundle of fibers into smaller segments releases these insufficiently coated filaments, which provide less adhesion to a polymer when incorporated therein. This results in less reinforcement of a polymer composite than when all filaments would be sufficiently coated.

Short fibers of the invention, however, do not have this disadvantage. When short fibers are grafted with a process of the invention, a cord is first cut into short fibers, after which the grafting occurs. Accordingly, the process of the invention in principle allows the treatment of all separate filaments, so that grafts can in principle be introduced at essentially the entire surface of essentially all fibers present. Thus obtained fibers provide more adhesion to a polymer than fibrous mixtures containing uncoated and/or insufficiently coated fibers, which results in more reinforcement of a polymer composite.

Cutting the fibers may be done by any way known to the person skilled in the art. For example, cutting can be performed by rotary or guillotine cutter.

The invention further relates to a polymer composite comprising a polymer and a reinforcement fiber of the invention. This means that the fiber is incorporated in the polymer matrix, i.e. that the fiber and the polymer are combined into one substance.

The fiber may incorporated in the polymer matrix in any form, e.g. as cords, woven fabrics, knitted fabrics or reed screens.

In a preferred embodiment, the fiber is chemically bonded to the polymer in which it is incorporated.

Depending on the application, a polymer composite according to the invention may comprise 100 wt parts of the polymer and 2-30 wt parts of short fibers. In a particular application, a composition according to the present invention may comprise 100 wt parts of the polymer and 3-6 wt parts of the fibers. At least 3 wt parts of fibers may be required for sufficient reinforcement properties, while the presence of at most 6 wt parts may ensure good dispersion of the fibers.

The polymer may be selected from the group of synthetic organic polymers, biopolymers, semi-synthetic polymers and inorganic polymers. Semi-synthetic polymers are polymeric reaction products of chemical reactions applied to biopolymers, e.g. esters and ethers of cellulose or amylose and vulcanized natural rubber. The polymer may in particular be selected from the group of polyethylenes (HDPE or LDPE), polypropylenes, polyvinyl chloride, polystyrene, polyamides, poly(tetrafluoroethylene), polyurethanes, poly(ethylene terephthalate), rubbers and polyesters. The polymer usually is a rubber, preferably a vulcanizable rubber.

If present, the vulcanizable rubber component may be a natural rubber or a synthetic rubber, or mixtures thereof. Examples of a vulcanizable rubber are butyl rubber, styrene-butadiene rubber, chloroprene rubber, ethylene propylene rubber, alkylated chlorosulfonated polyethylene, hydride- nitrile rubber, a mixed polymer of hydride-nitrile rubber and a metal salt of an unsaturated carboxylic acid, and an ethylene- alpha -olefin elastomer. An ethylene-alpha-olefin elastomer may comprise ethylene propylene rubber (EPR) and/or ethylene propylene diene monomer (EPDM) rubber. Examples of the diene monomer are dicyclopentadiene, methylenenorbornene, ethylidenenorbornene, 1 ,4-hexadiene, cyclooctadiene and the like. EPDM is desirable for its excellent high and low temperature resistance.

If the vulcanizable rubber component comprises EPDM, an EPDM iodine value of 3 to 40 is preferred. If the iodine value is less than 3, vulcanization of the rubber may not be adequate. As a result, abrasion or adhesion problems may occur. If the iodine value is over 40, the scorch time for the rubber composition may become too short. It thus may be difficult to handle, which may result in a decreased heat resistance of the rubber. The iodine value (or iodine adsorption value) is a measure for the unsaturation of a chemical substance and is defined as the mass of iodine in grams that is consumed by 100 grams of a chemical substance.

The polymer composite may further comprise customary additives. Preferably, at least 80 wt% of the polymer composite is formed by the polymer and the fibers. More preferably, at least 90 wt% or at least 95 wt% of the polymer composite is formed by the polymer and the fibers.

A polymer composition according to the invention may for example comprise known rubber compound additives appropriate for the particular use of the polymer composition. Such known additives include stearic acid, zinc oxide, antidegradants, coupling agents, lubricants, process oils and curing additives.

A composition of the present invention may also include

antidegradants, for example in the range of 4.5-10 phr. Such antidegradants may be a combination of antiozonants and antioxidants for rubber

compositions as conventionally used for cured rubber compositions intended to be exposed to atmospheric conditions and dynamic applications.

Representative examples of such antidegradants are polymerized 2,2,4- trimethyl 1 ,2-dihydroquinoline and A/-1 ,3-dimethylbutyl-A/'-phenol para- phenylenediamine. The first can be obtained as Flectol(R) TMQ from the Flexsys America LP. Company; the latter can be obtained as Flexzone(R) 7F and mixed with aryl-p-phenylene diamines, such as Wingstay(R) 100 from The Goodyear Tire and Rubber Company.

Rubber processing oils may be present in a range of 0-10 phr, for example 1-10 phr. Such oils may be used to enhance processability of the unvulcanized rubber composition and/or to enhance the balance of a combination of tensile strength and elongation of a sulfur vulcanized rubber composition. Rubber processing oils may include, for example, aromatic hydrocarbon oils, naphthenic oils, paraffinic oils or ester oils. Aromatic rubber processing oils as well as mixtures of alkylated naphthenic and aromatic hydrocarbon rubber process oils, and their use as processing aids, are well known to those having skill in the preparation of rubber compositions.

Tensile tests on fiber-reinforced polymer composites of the invention (containing reinforcement fibers of the invention) revealed that such composites have a comparable or even higher stiffness than conventional RFL-coated fibers.

The invention further relates to a product obtainable by a process of the invention.

The invention further relates to a process for the manufacturing of a polymer composite, comprising:

- providing a mixture comprising a reinforcement fiber according to the invention and a polymer;

- curing the mixture.

The curing step increases the adhesion between the fiber of the invention and the polymer. For example, when the polymer as well as the grafts on the epoxy comprise unsaturated carbon-carbon bonds, curing with sulfur has the effect that the unsaturated carbon-carbon bonds participate in the formation of a chemical bond between the polymer and the grafts.

The curing step may also harden the polymer itself, e.g. by providing cross-links in the polymer. This may result in a higher mechanical interaction between the fiber and the polymer.

When the polymer is a vulcanizable rubber, sulfur or an organic peroxide may be used to cross-link that rubber. The mixture of a reinforcement fiber and a polymer as prepared in the first step of the process of the invention may therefore further comprise sulfur or an organic peroxide. The organic peroxide may be selected from di-f-butyl peroxide, dicumyl peroxide, f-butyl cumyl peroxide, 1 ,1 -di(f-butyl peroxy)-3,3,5- trimethylcyclohexane, 2,5-dimethyl-2,5-di(f-butyl peroxy)hexane, 2,5-dimethyl- 2,5-di(benzoyl peroxy) hexane, 1 ,3-bis(tert-butyldioxyisopropyl)benzene, t- butyl peroxy benzoate, f-butylperoxy-2-ethylhexyl carbonate and mixtures thereof. The organic peroxide or the mixture of organic peroxides may be used in the range 0.005 to 0.02 mol per 100 g of an ethylene-alpha-olefin elastomer. The presence of the sulfur and/or an organic peroxide may also induce the formation of a chemical bond between the rubber and the reinforcement fiber.

When the polymer is a vulcanizable rubber, a vulcanization

accelerator may also be used. The mixture of a reinforcement with a polymer as prepared in the first step of the process of the invention may therefore further comprise a vulcanization accelerator. Examples of vulcanization accelerators are thiazoles, thiurams and sulphenamides. The thiazole vulcanization accelerator may be selected from the group of 2- mercaptobenzothiazole, 2-mercaptothiazoline, dibendothiazyl disulfide, zinc salts of 2-mercaptobenzothiazole, and mixtures thereof. The thiuram vulcanization accelerator may be selected from the group of

tetramethylthiuram monosulfide, tetramethylthiuram disulfide,

tetraethylthiuram disulfide, A/./V-dimethyl-ZS/./V-diphenylthiuram disulfide, and mixtures thereof. The sulphenamide vulcanization accelerator may be selected from the group of A/-cyclohexyl-2-benzothiazyl sulphenamide, Λ/,Λ/'- cyclohexyl-2-benzothiazyl sulphenamide, and mixtures thereof. Other vulcanization accelerators, such as e.g. bismaleimide and ethylenethiourea, may be used either alone or in combination with any of the accelerators mentioned above.

In a process of the invention wherein a vulcanizable rubber is cross- linked with e.g. sulfur or an organic peroxide, the curing step usually comprises heating the vulcanizable rubber to at least 70 °C, preferably to at least 150 °C. The duration of the heating is usually related to the time after which a certain degree of curing has taken place. For example, heating may be stopped when 50%, 75% or 90% of the total number of curable bonds of the rubber has actually been cured. When the heating is stopped, the reaction mixture is usually allowed to cool down to ambient temperatures.

After a mixture comprising rubber and a reinforcement fiber is provided in the first step of the process, the mixture may be extruded to form one or more sections, which may then be inserted into a suitable mold and cured at a temperature of e.g. 150 °C to form a rubber composite such as a pneumatic tire.

The invention also relates to a polymer composite obtainable by a process as described hereinabove.

The invention further relates to an object comprising a polymer composite of the invention, such as V-belts, conveyor belts, transmission belts, hoses (in particular radiator hoses), tires (in particular tire treads). In particular, the polymer composite of such objects comprises a vulcanized rubber composition.

EXAMPLES Preparation of grafted reinforcement fibers of the invention using allyl chloroformate.

Epoxy coated short aramid fibers were provided by Teijin Aramid BV. A round-bottomed flask fitted with a reflux condenser attached was filled with 200 ml THF and 10 grams of the dry epoxy coated aramid fibres. The content was refluxed at 66°C during 1 hour. Subsequently 1.0 ml of allyl

chloroformate and 1.0 ml of triethylamine were added to start the grafting reaction. The mixture was refluxed at 66°C for 2 hours and allowed to cool down to room temperature during another 2 hours. They were washed with 100 ml of ultrapure water and 100 ml of pure ethanol to remove catalyst and other by-products and finally they were dried for 48 hours in the air and stored in a dry and dark place. Preparation of grafted reinforcement fibers of the invention using allyl isocvanate.

Epoxy coated short aramid fibers were provided by Teijin Aramid BV. A round-bottomed flask fitted with a reflux condenser attached was filled with 200 ml THF and 10 grams of the dry epoxy coated aramid fibres. The content was refluxed at 66°C during 1 hour. Subsequently 1.7 mL of allyl isocyanate and 0.24 ml DBTDA (dibutyltin diacetate) were added to start the grafting reaction. The mixture was refluxed at 66°C for 2 hours and allowed to cool down to room temperature during another 2 hours. The fibres were removed from the flask and filtered over a redband paper filter (Fisher) to remove the solvent. They were washed with 100 ml of ultrapure water and 100 ml of pure ethanol to remove catalyst and other by-products and finally they were dried for 48 hours in the air and stored in a dry and dark place.

Preparation of a polymer composite comprising fibers coated with epoxy and RFL (comparative example)

A polymer composite comprising conventional epoxy/RFL-coated fibers was prepared by mixing the fibers with an EPDM type of rubber of the following composition: EPDM type Keltan 8340A (100 phr), carbon black N- 550 (105 phr), oil (8 phr), stearic acid (1 phr), PEG 2000 (2.5 phr), Parkadox 14/40 (7,5 phr), TRIM (4 phr).

For the mixing, an industrial scale internal mixer was used, and the curing agents and the fibres were added on a two-roll mill. The milling direction was the longitudinal direction of fibre orientation. After determining the cure characteristics with a Rubber Process Analyzer (RPA) of Alpha Technologies the compounds were cured at 170°C for their t9o+2 mins.

Preparation of a polymer composite comprising allyl-grafted epoxy coated fibers of the invention A polymer composite comprising fibers of the invention was prepared using the same method as that used to obtain a composite comprising epoxy/ RFL-coated fibers described hereinabove. Testing of polymer composite

The tensile properties are investigated by a method described in RUBBER CHEMISTRY AND TECHNOLOGY, Vol. 84, No. 2, pp. 187-199 (2011). The tensile tests are performed using a Zwick Z1.0/TH S tensile tester, speed 200 mm/min, in the longitudinal direction of fiber orientation on the polymer samples. The fractured surfaces of tensile bars are studied with electron microscopy using a bench-top NeoScope JCM-500 device, after coating the samples with a very thin layer of platinum. Dynamic mechanical analysis (DMA) is done on samples using a Metravib Viscoanalyser

DMA+150, in strain sweep mode, at the frequency of 10 Hz and ambient temperature.

Figure 1 shows the results of a tensile test wherein samples of rubber and rubber composites having different fiber coatings have been subjected to stress (elongation). Figure 1 displays the tensile curves of a rubber without fibres (graph indicated as "WF") and of the fiber-reinforced rubbers (graphs indicated as "RFL", "IS" and "CH"). The fiber-reinforced rubbers comprise 5 phr of RFL-coated fibers (RFL), 5 phr of epoxy-coated fibers grafted with allyl isocyanate (IS) or 5 phr of epoxy-coated fibers grafted with allyl chloroformate (CH), wherein the fibre orientation is in longitudinal direction. Figure 1 demonstrates that the Young's modulus of the composites comprising fibers coated with grafted epoxy is comparable or even higher than that of composites comprising fibers coated with RFL (first linear part of the curves, i.e. up to approximately 6% elongation). At the same elongation of e.g. 5%, the composites comprising fibers according to the invention show a higher tensile stress than those comprising fibers coated with RFL. Thus,

reinforcement fibers of the invention provide a polymer with a comparable or even higher stiffness than conventional RFL-coated fibers do. For comparison, figure 2 shows the results of tensile tests performed on different conventional samples of rubber and rubber composites. Figure 2 displays the tensile curves of a rubber without fibres (graph indicated as "WF") and of the fiber-reinforced rubbers (graphs indicated as "St", "EpT" and "RFL"). The fiber-reinforced rubbers comprise 5 phr of uncoated fibers (St), 5 phr of epoxy coated fibers (EpT) or 5 phr of RFL-coated fibers (RFL), wherein the fibre orientation is in longitudinal direction. This figure demonstrates the difference in tensile properties of rubbers containing uncoated fibers

(indicated with "St") or standard epoxy coated fibers (indicated with "EpT") on the one hand, and rubbers containing epoxy coated fibers treated with RFL (indicated with "RFL") on the other hand (the graphs indicated with "RFL" and "WF" in figures 1 and 2 being identical). From figures 1 and 2, it is thus also evident that reinforcement fibers of the invention (i.e. epoxy coated fibers having the grafts) provide a polymer with a substantially higher stiffness than epoxy coated fibers that lack the grafts do.

Claims

1. A reinforcement fiber comprising a coating of epoxy resin having an epoxy backbone, wherein the epoxy resin comprises grafts attached to the

. backbone, wherein the grafts on the backbone are selected from the group of

-X-C(=0)-X'-R, where

- X is attached to a carbon atom of the backbone of the epoxy resin, and where X is O or NR', where R' is a hydrogen atom or a carbon atom of the backbone of the epoxy resin;

- X' is O or NR", where R" is a hydrogen atom or a functional group

comprising at least one carbon atom;

- R is a functional group comprising at least one carbon atom.

2. A reinforcement fiber according to claim , wherein the reinforcement fiber is an aramid fiber.

3. A reinforcement fiber according to claim 1 or 2, wherein the reinforcement fiber is a short reinforcement fiber, in particular a short reinforcement fiber having an average length in the range of 1-6 mm.

4. A reinforcement fiber according to any one of claims 1-3, wherein R is

selected from the group of:

-(CH2)n-CH=CH2, where n is in the range of 1-6;

where m is in the range of 1-6 and n is in the range of 0-6;

-(CH2)n-CH=CH-CH=CH2, where n is in the range of 1-6;

-(CH2)m-CH=CH-CH=CH-(CH2)n-CH3, where m is in the range of 1-6 and n is in the range of 0-6; and

-(CH2)m-CH=CH-(CH2)n-CH=CH2, where m is in the range of 1-6 and n is in the range of 1-6.

5. A reinforcement fiber according to any one of claims 1-4, wherein the epoxy resin is a poly(glycidylether) of a polyhydric aliphatic alcohol, in particular an epoxy resin wherein the epoxide compound is selected from the group of a diglycidyl ether of glycerol, a triglycidyl ether of glycerol, a polyglycidyl ether of polyglycerol and mixtures thereof.

6. A reinforcement fiber according to any one of claims 1-5, wherein

R = -CH2-CH=CH2 and X' = O; or wherein

R = -CH₂-CH=CH₂ and X' = NH.

7. Process for the manufacturing of a reinforcement fiber according to any one of claims 1-6, comprising

1) providing a reinforcement fiber with a coating of epoxy resin;

2) contacting the reinforcement fiber comprising a coating of an epoxy resin with a compound selected from the group of O=C=N-R and Z-C(=0)-0- R, where R is a functional group comprising at least one carbon atom and Z is a leaving group.

8. Process according to claim 7, wherein the R group of 0=C=N-R or Z-C(=0)- O-R is selected from the group of:

-(CH2)n-CH=CH2, where n is in the range of 1-6;

-(CH2)m-CH=CH-(CH2)n-CH3, where m is in the range of 1-6 and n is in the range of 0-6;

-(CH₂)n-CH=CH-CH=CH₂, where n is in the range of 1-6;

9. A polymer composite comprising a polymer and a reinforcement fiber

according to any one of claims 1-6.

10. A polymer composite according to claim 9, wherein the polymer is a rubber, in particular a vulcanizable rubber.

11.A polymer composite according to claim 9 or 10, wherein the fiber is

chemically bonded to the rubber.

12. Process for the manufacturing of a polymer composite, comprising

- providing a mixture comprising a reinforcement fiber according to any one of claims 1-6 and a polymer;

- curing the mixture.

13. Process according to claim 12, wherein the polymer is a vulcanizable rubber and wherein the mixture further comprises sulfur or an organic peroxide.

. A polymer composite obtainable by the process of claim 12 or 13.

15. Object comprising a polymer composite according to any one of claims 9-11 and 14, such as a V-belts, conveyor belts, transmission belts, hoses and tires.