Field of the invention

This invention relates to sizing compositions for fibres, particularly inorganic fibres such as carbon fibres, glass fibres and silicon carbide fibres. The high strength and modulus of such fibres is used to good effect in the reinforcement of synthetic resins to make composite structures.

Background of the invention Inorganic fibres are usually treated with a sizing composition, commonly as part of the production process, to reduce their susceptibility to abrasion and other mechani¬ cal damage during processing operations such as filament winding and weaving. Inorganic fibres which are to be used as reinforcement in a resin matrix are usually surface- treated with a composition which promotes adhesion between the fibres and the matrix. Both types of composition are commonly referred to as sizes or as sizing compositions.

Certain sizes which are used to protect the fibre against mechanical damage, for example poly(vinyl alcohol), reduce adhesion between the fibre and the resin matrix. They must therefore be removed and replaced by an adhesion- promoting composition.

The development of sizes which will both protect the fibre against mechanical damage and promote adhesion to a resin matrix has been the subject of research.

Sizes have been applied to inorganic fibres both from organic solvent systems and from water-based systems, in the latter case often in the form of an emulsion. Water- based systems are desirable on grounds of ease of use, safety, and lower risk of environmental pollution.

Sizing systems have been developed which are based on

epoxy resins. Such sizes can be applied from water-based systems and serve to protect fibres from mechanical damage. They are highly compatible with and promote good adhesion to the most widely used matrix resin systems, particularly where the latter are epoxy resin systems. Other systems have been developed which are based on condensation pro¬ ducts of ethylene oxide. These generally appear to be more effective in reducing susceptibility to damage than the epoxy type but to be less effective in promoting adhesion.

US Patent 4,420,512 describes a sizing system for carbon fibres which comprises an epoxy resin, an alkoxy- lated glycidyl ether, and an alkoxylated and styrenated derivative of toluene.

Japanese Unexamined Patent Publication 61-28074 describes a sizing system for carbon fibres which comprises an epoxy resin and an alkoxylated bisphenol diglycidyl ether.

European Patent Application 102,705 describes a sizing system for carbon fibres which comprises an epoxy resin and a polyalkylene glycol diglycidyl ether.

Each of these prior art sizing systems is claimed to fulfil the dual requirement of protecting the carbon fibre against mechanical damage and of promoting adhesion to resin matrix systems.

Summary of the invention

According to one aspect of the present invention, a sizing composition for fibres comprises:-

(a) an epoxy resin or a mixture of epoxy resins; and

(b) a thermoplastic polyhydroxyether which is a substan¬ tially linear polymer having the general formula:-

_f_ 0 - D - 0 - E -ϊ-

where D is the radical residuum of a dihydric phenol, E is a hydroxyl-containing radical residuum of an epox¬ ide, and n (the degree of polymerisation) is at least 30 and is preferably 80 or more.

In one embodiment of the present invention the said composition additionally comprises:

(c) a vehicle for components (a) and (b) which is a solvent therefor or an aqueous medium incorporating an emulsifying agent therefor.

In a further embodiment of the present invention, the composition comprising (a) and (b) and an aqueous vehicle

(c) additionally comprises:

(d) a watei—insoluble high-boiling-point solvent for the epoxy resin component (a).

According to another aspect of the present invention, a process for the sizing of fibre comprises treating the fibre with a sizing composition as hereinbefore defined.

According to a further aspect of the present inven- tion, a sized fibre is characterised in that the size on the fibre comprises components (a) and (b) as hereinbefore defined.

Detailed disclosure

We have thus found that sizing compositions which include as essential components epoxy resins (a) and spec¬ ific hydroxylated polymers (b) offer advantages in ease of use and in the properties of the sized fibre, notably in its resistance to abrasion.

The epoxy resin or mixture of epoxy resins comprising component (a) of the sizing composition of the invention may be any which is suitable for use in sizing fibres. Di-, tri-, and tetra-functional epoxy resins and mixtures thereof are preferred. These include epoxy resins of the glycidyl type obtained by the reaction of a bisphenol compound with epichlorohydrin, suitable examples of which are those produced by the Shell Chemical Company under the trade marks Epikote 828 and Epikote 1001. Other suitable epoxy resins of the glycidyl type include:- phenolic epoxy resins derived from the reaction of novolak-type phenol resins and epichlorohydrin; glycidylamine-type epoxy resins; vinyl-ester type epoxy resins derived, for ex¬ ample, from the reaction of a vinyl compound such as acrylcnitrile, vinyl chloride, vinyl acetate or styrene with glycidyl methacrylate; and ether-type epoxy resins obtained, for example, by reacting epichlorohydrin with polyols, polyether polyols or polyhydric phenols to produce the glycidyl ethers thereof. Non-glycidyl-type epoxy resins may also be used, for example cycloaliphatic epoxy resins and epoxidised polybutadiene.

Thermoplastic polyhydroxyethers suitable for use as component (b) and processes for their manufacture are described in GB Patents- 1,006,776 and 1,043,851 and US Patents 3,277,051, 3,297,784, 3,321,549, 3,356,646 and 3,480,695, and the contents of these specifications are incorporated herein by reference thereto.

The thermoplastic polyhydroxyethers forming component

(b) have the general formula given above. The dihydric phenol contributing the radical residuum D can be either a dihydric mononuclear phenol or a polynuclear phenol such as those having the general formula:-

OH

where Ar is an aromatic divalent hydrocarbon radical such as naphthylene or, preferably, phenylene; Y and Y , which can be the same or different, are alkyl radicals, preferab¬ ly having from 1 to 4 carbon atoms, halogen atoms (i.e. fluorine, chlorine, bromine or iodine), or alkoxy radi¬ cals, preferably having from 1 to 4 carbon atoms; r and z are integers having a value from 0 to a maximum value corresponding to the number of hydrogen atoms on the aromatic radical (Ar) which can be replaced by substit- uents; and R is a bond between adjacent carbon atoms (as in dihydroxydiphenyl ) or is a divalent radical such as, for example, -C-, -O-, -S-, -SO-, -SO.-, or -S-S-, or is a

0 divalent hydrocarbon radical such as alkylene, alkylidene or a cycloaliphatic radical (e.g. cycloalkylene or cyclo- alkylidene), a halogenated, alkoxy-subst tuted or aryloxy- substituted alkylene, alkyl idene or cycloal iphatic radical, an alkarylene or aromatic radical (e.g. a halogenated, alkyl-substituted, alkoxy-substituted or aryloxy-substi- tuted aromatic radical) or a ring fused to an Ar group; or R can be polyalkoxy, or polysiloxy, or two or more alkylidene radicals separated by an aromatic ring, a tertiary amino group, an ether linkage, a carbonyl group or a sulphur-containing group such as sulphoxide, and the like.

Particularly desirable dihydric polynuclear phenols have the formula:-

where Y and Y are as previously defined; r and z have values from 0 to 4 inclusive; and R is a divalent satui— ated aliphatic hydrocarbon radical, particularly an alkylene or alkylidene radical having from 1 to 3 carbon

atoms or a cycloalkylene radical having up to 10 carbon atoms. As a specific example may be mentioned 2,2-di(4- hydroxyphenyl )propane, commonly known as Bisphenol A.

The epoxide contributing the hydroxyl-containing radical residuum E can be a monoepoxide or diepoxide. By an "epoxide" is meant a compound containing an oxirane group, i.e. oxygen bonded to two vicinal aliphatic carbon atoms, thus:-

u -

A monoepoxide contains one such oxirane group and provides a radical residuum E containing a single hydroxyl group, a diepoxide contains two such oxirane groups and provides a radical residuum E containing two hydroxyl groups. Satur- ated epoxides, by which term is meant epoxides free from ethylenic unsaturation (i.e >C = C<) and acetylenic un- saturation (i.e. -C = C-), are preferred. If the epoxide is a monoepoxide it must contain another functional group capable of linking with a dihydric phenol to form a sub- stantially linear polymer with that phenol, for example a halogen atom. Particularly preferred are halogen-sub¬ stituted saturated monoepoxides (for example, the epihalohydrins) , and saturated diepoxides which contain solely carbon, hydrogen and oxygen, especially those wherein the vicinal or adjacent carbon atoms form a part of an aliphatic hydrocarbon chain. Oxygen present in such diepoxides in addition to oxirane oxygen can be present as ether oxygen -0-, oxacarbonyl oxygen

0 It - C - 0 - carbonyl oxygen

0

II

- C - and the like,

Specific examples of onoepoxides include epihalo- hydrins such as epichlorohydrin, epibromohydrin, 1,2-epoxy- -1-methyl-3-chloropropane, 1 ,2-epoxy-1-butyl-3-chloropro- pane, 1 ,2-epoxy-2-methyl-3-fluoropropane, and the like.

The hydroxyl-containing radical residuum E is prefer¬ ably of the formula:-

R^ R° H I I I

- C - C - c -

I ( (

H OH H where R^ and R are hydrogen or methyl and are the same or different.

The degree of polymerisation n of the thermoplastic polyhydroxyether (b) is at least 30, preferably at least 80. It is generally no more than 200, preferably no more than 150.

Suitable thermoplastic polyhydroxyethers for use as component (b) are manufactured by Union Carbide Corporation as UCAR (Trade Mark) phenoxy resins with the designations PKHC, PKHH and PKHJ. These resins have the formula

where n iε approximately 82 to 123, and have molecular weights in the range 25,000 to 35,000.

Another suitable UCAR phenoxy resin (which is a modification of those described above) is UCAR PKHW-35 which is a carboxylated resin of the above type which has been amine-neutralised.

The epoxy resin or mixture of epoxy resins (a) preferably comprises the major proportion of the content

of(a) plus (b) of the size composition. The thermoplas¬ tic polyhydroxyether (b) may comprise at least 2 per cent by weight of the total content of (a) and (b), preferably 5 to 30 per cent by weight.

The vehicle for the components (a) and (b), i.e. component (c) of the size composition, is preferably an aqueous medium incorporating an emulsifying agent. The emulsifying agent may be a nonionic or, preferably, an anionic surfactant. Examples of suitable nonionic surfac- tants are nonyl phenyl poly(ethylene glycol) alkyl al¬ cohols containing around 10 to 20 ethylene glycol units, for example those sold by the GAF Corporation under the trade mark Antarox. Preferred examples of anionic surfac¬ tants are nonyl phenyl poly(ethylene glycol) alkyl sul- phonates containing around 10 to 20 ethylene glycol units, for example those sold by the GAF Corporation under the trade marks Fenopon EP-100 and EP-120. Anionic surfac¬ tants in the form of a salt with a non-metallic cation, for example ammonium, are preferred. The proportion of emulsifier employed is usually within the range of 5 to 20 per cent by weight based on the combined weight (a) and (b).

The combined concentration of (a) and (b) in the aqueous emulsion is usually in the range 0.5 to 10 per cent by weight, preferably 0.5 to 2 per cent by weight, chosen to provide the required viscosity for application to the fibres and the required level of add-on to the fibres. Emulsion viscosities in the range 7 to 20 mPa.s may be used, with a preference for viscosities in the range 10 to 15 mPa.s.

The resins may be incorporated into an aqueous emul¬ sion by melting them and then adding them slowly into a mixture of the emulsifying agent and warm water which is being stirred under high shear.

An alternative and preferred method for preparation of an aqueous emulsion is as follows. The resins are blended in the melt under high shear and an emulsifying agent is added. Water is added while high-shear mixing is continued, until an emulsion is formed.

The aqueous emulsion may also comprise a water- insoluble high-boiling-point epoxy resin solvent (d) as a coagulating agent. Examples of suitable solvents are the diethers and ether esters of ethylene glycol and propylene glycol and their oligomers. Specific examples are 2- methoxypropyl acetate, 2-(2-methoxypropyloxy)propyl acetate, and 2-ethoxyethyl acetate. Preferred amounts are in the range 5 to 20 per cent by weight on the combined weight of resin solids (a) and (b).

If the size composition is to be used with the resins in solution, then a suitable organic solvent is used for the vehicle (c). Dichloromethane, methyl ethyl ketone and acetone are examples of suitable solvents. The total concentration of (a) plus (b) in the solvent may be again in the range 0.5 to 10 per cent by weight, preferably 0.5 per cent to 2 per cent by weight, depending upon the desired application viscosity and add-on level to the fibre. The solution viscosities used are usually lower than with emulsion systems, for example in the range 5 to 10 mPa.s.

The invention includes fibres sized with a size composition according to the invention. Preferably the fibres are carbon fibres. Such carbon fibres may be of any commercial type including those derived from the pyrolysis of polyacrylonitrile fibres or cellulose fibres and those derived from pitch. Carbon in the form of graphite is also included.

The sized fibres of the invention may be in any suitable form, for example as staple fibres, as continuous

filaments or tow, or in a fabric form derived from such fibres or filaments. The sizing operation, however, is preferably carried out on continuous filament tow both for convenience and to provide the protection of the size at an early stage in the processing of the fibre. The size composition may be applied by passing a tow of filaments through a dip bath of the size composition followed by a drying stage. Passage of the impregnated tow through a hot air oven at a temperature of 120 to 180°C for the required time, usually less than two minutes, is a suitable method of drying.

The add-on level of size may be in the range 0.1 to 5.0 per cent by weight based on the weight of the fibre. Levels in excess of this are usually unnecessary and may start to affect eventual composite properties adversely. A preferred range is 0.5 to 2.0 per cent by weight based on the weight of the fibre.

The sized fibres of the invention give a much im¬ proved performance in processing whilst retaining the properties which are required in composite structures made by incorporating the sized fibres in a matrix resin. This improved performance is seen in a lower level of fibre damage with a resulting better appearance and processabil- ity of the product. Thus, for example, there is less fibre shedding and less fibre agglomeration and hairiness on the surface of a processed tow. This leads to cleaner equipment, fewer process breakdowns and a better product on completion of processing.

The sized fibres of the invention may be used with any suitable matrix resin for the production of prepregs and composite structures, for example with epoxy resins, unsaturated polyester resins, bismaleimides, polyimides and phenolic resins.

The invention is illustrated by the following Ex-

amples in which the test methods used were as follows:-

Dry Abrasion Test

This test is used to collect debris in the form of broken filaments present within a fibre tow after manufac- ture. In this case, it is used as a measure of the ability of the size composition to protect the filaments against further abrasion and to hold broken filaments in place by cohesive action.

The sized fibre tow is pulled off a package held on a creel against a tension strap applied to the package mount¬ ing with a weight of 200 grams. The tensioned tow is passed at a speed of 15 metres per minute between a pair of foam pads held in a stainless steel clamp which exerts a pressure at the pad surface of 2.5 kPa. The debris col- lected on the foam pads is collected and weighed after a set time, usually 1 to 15 minutes, and expressed as the weight of debris collected in grams per metre of tow passing through the pads in the set time.

Static Roller Test

This test is used to measure the likely damage to a fibre tow by passing it around a series of stationary stainless steel pegs. Damage arises by both bending and abrasive action and the size is tested for its ability to protect the fibres against this damage as well as its ability to withstand the bending forces itself by reason of its flexibility.

The equipment comprises five stainless steel pegs each of 10 mm diameter mounted horizontally by ends fixed into a vertical plate with three pegs on an upper horizon- tal row and two pegs underneath and between on a lower horizontal row. The tow is passed in a zig-zag path, consecutively over and under upper and lower pegs, turning

through an angle of 120° at each peg. The tow is unwound from the package under tension as in the Dry Abrasion Test and is passed through the peg unit at a speed of 15 metres per minute for the time required for 50 grams of tow fibre to pass through the peg unit. The debris falling from the tow as it passes through the peg unit is collected in a box located under the peg unit and weighed. This weight of debris in grams is the expressed result of the test.

Combined Test

The Static Roller Test and the Dry Abrasion Test are run in series in that order. In this case, the Dry Abrasion Test is given the same basis as the Static Roller Test, i.e. passage of 50 grams of tow fibre, so that the combined result is expressed in terms of total weight in grams of debris collected in the box and on the foam pads.

Composite Preparation and Testing

Carbon fibre tow sized with a size composition according to the invention was made first into a unidirec¬ tional prepreg and then into a cured, compression-moulded composite. The composite was tested for Inter!aminar Shear Strength (ILSS) using the ASTM D234 test method and for Ultimate Flexural Strength (UFS) and Ultimate Flexural Modulus (UFM) in the longitudinal direction using the CRAG Test Method 200 as described in the CRAG Technical Report No. 88012.

In each case the tow used was a 12,000 Grafil carbon fibre tow sized as specified in the respective Examples.

Grafil is a Registered Trade Mark of Courtaulds pic. For the ILSS test, the sized tow was passed through a resin bath of the following composition:-

parts by weight

A tetra-functional epoxy resin 100

(MY720 ex Ciba-Geigy Ltd.)

Nadic methyl anhydride 115

Amine curing catalyst (K61B 4 ex Shell Chemical Co. Ltd.)

The resin-impregnated tow was wound on a drum to form a unidirectional prepreg in which the volume rai-.io of fibre:resin was 60:40. The prepreg was cut off the drum and laid up in a compression mould as sixteen unidirection¬ al layers. The mould was then operated at a pressure of 17 mPa (1 tonne per square inch of laminate), starting at a temperature of 15°C, with the temperature being raised at 3°C per minute to a plateau of 120°C, held for 45 minutes, and then raised again at 3°C per minute to a higher plateau of 180°C which was held for 120 minutes. The moulded laminate was cooled, released from the mould and tested for ILSS as specified.

For the UFS and UFM tests, prepreg and composite were made in the same way as described above but in this case the resin composition was as follows:-

parts bv weight

A difunctional epoxy resin (Epikote 100 828 ex Shell Chemical Co. Ltd.)

Nadic methyl anhydride 90

K61B amine curing catalyst (Shell) 4

and the compression moulding sequence was operated with a

first temperature plateau of 105°C for 135 minutes and a second temperature plateau of 175°C for 20 hours.

Example 1

An unsized, 12,000 filament carbon fibre tow (Grafil XAS) was passed through a dip bath containing the follow¬ ing resin size composition as a 1 per cent by weight solution in methylene chloride:-

parts by weight based on total resin content

Epikote 828 epoxy resin 54 (Shell)

Epikote 1001 epoxy resin 36 (Shell)

UCAR PKHH phenoxy resin 10 (Union Carbide)

The sized tow was then passed through an air drying oven at an air temperature of 130°C for a residence time of 40 seconds. The size add-on was 1 per cent by weight on the weight of fibre.

As a control, for comparison purposes, a similar tow was sized in the same way but in this case omitting the phenoxy resin component and with the two epoxy resin com¬ ponents in the same 60:40 weight proportion. Add-on was again 1 per cent by weight on weight of fibre.

Example 2

The procedure of Example 1 was repeated using a resin size composition as follows:-

parts bv weight based on total resin content

Epikote 828 epoxy resin 42 (Shell)

Epikote 1001 epoxy resin 28 (Shell)

UCAR PKHH phenoxy resin 30 (Union Carbide)

The sized tows of Examples 1 and 2 were tested for processabi1 ity using the Combined Static Roller Test and Dry Abrasion Test as described and the results were as follows:-

Example 3

A size composition in aqueous emulsion form was made using the resin composition specified in Example 1. The resins were melted and were poured dropwise into the high- shear zone of a volume of water containing the anionic surfactant sold by GAF Corporation under the trade mark Fenopon EP100 and being agitated by a high shear mixer, to form an emulsion. The proportion of surfactant used was

17 per cent by weight based on the total weight of resin and the total resin content of the resulting emulsion was 1.0 per cent by weight.

An unsized, 12,000-filament carbon fibre tow (Grafil XAS) was passed through a dip bath containing the emulsion size and then dried as in Example 1. Size add-on was 1.0 per cent by weight based on the weight of fibre.

As a control, a similar tow was sized in the same way with an emulsion size which in this case omitted the phenoxy resin component but with the two epoxy resin com¬ ponents in the same 60:40 weight proportion. Add-on was again 1 per cent by weight on weight of fibre.

The sized tows were tested for processabi1ity using the Combined Test as described and the results were as follows:-

Example 4

In this Example, sizing was carried out on a fibre production line in contrast with the off-line procedure used for Examples 1 to 3. A carbon fibre tow was sized on-line using the size formulation and procedure of Example 1. In this case the control tow was sized on-line using the size formulation of Example 3. Both tows were tested for processabi!ity using the Static Roller Test, the Dry Abrasion Test (using the time taken for 50 grams of tow fibre to pass as the set time), and the Combined Test. The results were as follows:-

Exampl e 5

Further samples of the sized tow of Example 4 and the control tow were tested using the Static Roller Test at different tow tensions and at a much higher tow speed. The results (grams of debris) are shown below:-

(a) Standard Static Roller Test Speed - 15 metres/minute

(b) Higher Static Roller Test Speed - 50 metres/minute

Examp l e 6

The procedure of Example 3 was repeated on a 12,000- filament carbon fibre tow of very high strength and modulus (Grafil 43-750). The results of the Combined Test are as follows:-

Example 7

Samples of a 12,000-filament carbon fibre tow (Grafil XAS) were sized by the procedure of Example 3, one accord¬ ing to the invention and the other as described in rela¬ tion to the control. These sized tows were processed into prepregs and then moulded into composites according to the procedures described earlier. The composites were tested for ILSS, UFS and UFM. The results are shown in the following table:-

Example 8

A concentrated sizing formulation was prepared according to the following recipe :

parts by weight

Epikote 828 epoxy resin (Shell) 50 Epikote 1001 epoxy resin (Shell) 32 UCAR PKHH phenoxy resin (Union Carbide) 4 Fenopon EP-120 surfactant (GAF) 14 De ineral ised water 83

The liquid 828 resin was stirred by means of a high- shear mixer together with the solid 1001 resin, the PKHH resin, the surfactant and the demineral ised water to form an emulsion.

An unsized 12,000-fi lament carbon fibre tow (Grafil XAS) was passed through a dip bath of the above sizing formulation containing 1 per cent by weight solids. The sized tow was passed over a roller maintained at 120°C, the residence time on the roller being 30 sec. Size add-on was 0.7 per cent by weight on the weight of fibre.

A control experiment was carried out using a sizing formulation made to the same recipe but with the omission of the PKHH resin. Size add-on was again 0.7 per cent weight on the weight of fibre.

The sized tows of Example 8 were tested for proces¬ sabi1 ity using the Combined Test and the results were as follows:

Exampl e 9

Example 8 was repeated with the difference that 10 parts by weight 2-ethoxyethyl acetate, sold by Union Carbide Corporation under the trade mark UCAR Cellusolve Acetate, were added to the sizing formulation as a coalesc¬ ing agent. The sized tow was tested against the same control as that of Example 8:

Example 10

An unsized continuous multifilament glass fibre tow was passed through a dip bath and dried in the manner of Example 8, but with the difference that the dip bath contained 1.5 per cent by weight solids. Size add-on was

0.9 per cent by weight of the weight of the fibre.