BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composition for use in tire innerliners, particularly in tires for motor vehicles.

2. Description of Information Disclosures

The use of butyl rubber and halogenated butyl rubber, to produce innerliners for rubber tires is known. See, for example, U.S. patent 2,943,664, the teachings of which are hereby incorporated by reference.

Although there are many commercially available tire innerliners which are impermeable to air, there is still a need to improve the properties of innerliners, such as heat aging performance, while maintaining adhesive strength.

It has no ^* w been found that a tire innerliner that is made from a composition comprising certain halogen- containing copolymers of a C4 to Cη isomonoolefin and a para-alkylstyrene has improved properties, such as, for example, improved heat aging resistance, particularly for aged flex resistance, tensile strength, and elongation at break, while maintaining low air permeability properties.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a tire innerliner composition comprising: (a) a halogen- containing copolymer of a C4 to C7 isomonoolefin and a para- alkylstyrene, (b) carbon black; (c) a plasticizer oil; and (d) a curing agent.

In accordance with the invention, there is also provided a vulcanized tire comprising an innerliner, at least a portion of said innerliner being made of a composition comprising: (a) a halogen-containing copolymer

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of a C4 to Cη isomonoolefin and a para-alkylstyrene, (b) carbon black; and (c) a plasticizer oil. DETAILED DESCRIPTION OF THE INVENTION

The tire innerliner composition of the present invention comprises a halogen-containing copolymer of a C ₄ to C7 isomonoolefin and a para-alkylstyrene, carbon black, a plasticizer oil, and a curing agent with or without curing agent accelerators. Optionally, the composition may comprise fillers other than the carbon black and rubber compounding additives.

Suitable halogen-containing copolymers of a C ₄ to C ₇ isomonoolefin and a para-alkylstyrene for use as a component of the present tire innerliner composition comprise at least 0.5 weight percent of the para- alkylstyrene moiety. For elastomeric copolymer products, the para-alkylstyrene moiety may range from about 0.5 weight percent to about 20 weight percent, preferably from about 5 to about 20 weight percent, more preferably from about 2 to about 20 weight percent of the copolymer. The halogen content of the copolymers may range from above zero to about 7.5 weight percent, preferably from about 0.1 to about 7.5 weight percent. The halogen may be bromine, chlorine, and mixtures thereof. Preferably, the halogen is bromine. The major portion of the halogen is chemically bound to the para-alkyl group, that is, the halogen-containing copolymer comprises para-halo alkyl groups.

The copolymers of the isomonoolefin and para- alkylstyrene useful to prepare the halogen-containing copolymers suitable as component of the tire innerliner composition of the present invention include copolymers of isomonoolefin having from 4 to 7 carbon atoms and a para- alkylstyrene, such as those described in European patent application 89305395.9 filed May 26, 1989, (Publication No. 0344021 published November 29, 1989). The preferred isomonoolefin comprises isobutylene. The preferred para- alkylstyrene comprises para-methylstyrene. Suitable copolymers of an isomonoolefin and a para-alkylstyrene include copolymers having a number average molecular weight

(Hn) of at least about 25,000, preferably at least about 30,000, more preferably at least about 100,000. The copolymers also, preferably, have a ratio of weight average molecular weight (fiw) to number average molecular weight (fin), i.e., Mw/Mn of less than about 6, preferably less than about 4, more preferably less than about 2.5, most preferably less than about 2. The bro inated copolymer of the isoolefin and para-alkylstyrene obtained by the polymerization of these particular monomers under certain specific polymerization conditions now permit one to produce copolymers which comprise the direct reaction product (that is, in their as-polymerized form), and which have unexpectedly homogeneous uniform compositional distribu¬ tions. Thus, by utilizing the polymerization and bromination procedures set forth herein, the copolymers suitable for the practice of the present invention can be produced. These copolymers, as determined by gel permeation chromatography (GPC) demonstrate narrow molecular weight distributions and substantially homogeneous compositional distributions, or compositional uniformity over the entire range of compositions thereof. At least about 95 weight percent of the copolymer product has a para-alkylstyrene content within about 10 wt. percent, and preferably within about 7 wt. percent, of the average para-alkylstyrene content for the overall composition, and preferably at least about 97 wt. percent of the copolymer product has a para- alkylstyrene content within about 10 wt. percent and preferably within about 7 wt. percent, of the average para- alkylstyrene content for the overall composition. This substantially homogeneous compositional uniformity thus particularly relates to the intercompositional distribution. That is, with the specified copolymers, as between any selected molecular weight fraction, the percentage of para- alkylstyrene therein, or the ratio of para-alkylstyrene to isoolefin, will be substantially the same, in the manner set forth above.

In addition, since the relative reactivity of para- alkylstyrene with isoolefin such as isobutylene is close to

preferably within about 7 wt. percent, of the average para- alkylstyrene content for the overall composition. This substantially homogeneous compositional uniformity thus particularly relates to the intercompositional distribution.. That is, with the specified copolymers, as between any selected molecular weight fraction, the percentage of para- alkylstyrene therein, or the ratio of para-alkylstyrene to isoolefin, will be substantially the same, in the manner set forth above.

In addition, since the relative reactivity of para- alkylstyrene with isoolefin such as isobutylene is close to one, the intercompositional distribution of these copolymers will also be substantially homogeneous. That is, these copolymers are essentially random copolymers, and in any particular polymer chain the para-alkylstyrene and isoolefin units will be essentially ranpodomly distributed throughout that chain.

The halogen-containing copolymers useful in the practice of the present invention have a substantially homogeneous compositional distribution and include the para- alkylstyrene moiety represented by the formula:

in which R and R ¹ are independently selected from the group consisting of hydrogen, alkyl preferably having from 1 to 5 carbon atoms, primary haloalkyl, secondary haloalkyl

the heat of polymerization, inlet pipes for monomers, catalysts and diluents, temperature sensing means and an effluent overflow to a holding drum or quench tank. The reactor is purged of air and moisture and charged with dry, purified solvent or a mixture of solvent prior to introducing monomers and catalysts.

Reactors which are typically used in butyl rubber polymerization are generally suitable for use in a polymerization reaction to produce the desired para-alkyl¬ styrene copolymers suitable for use in the process of the present invention. The polymerization temperature may range from about minus 35*C to about minus 100*C, preferably from about minus 40 to about minus 80*C.

The processes for producing the copolymers can be carried out in the form of a slurry of polymer formed in the diluents employed, or as a homogeneous solution process. The use of a slurry process is, however, preferred, since in that case, lower viscosity mixtures are produced in the reactor and slurry concentration of up to 40 wt. percent of polymer are possible.

The copolymers of isomonoolefins and para-alkyl¬ styrene may be produced by admixing the isomonoolefin and the para-alkylstyrene in a copolymerization reactor under copolymerization conditions in the presence of a diluent and a Lewis acid catalyst.

Typical examples of the diluents which may be used alone or in a mixture include propane, butane, pentane, cyclopentane, hexane, toluene, heptane, isooctane, etc., and various halohydrocarbon solvents which are particularly advantageous herein, including methylene, chloride, chloroform, carbon tetrachloride, methyl chloride, with methyl chloride being particularly preferred.

An important element in producing the copolymer is the exclusion of impurities from the polymerization reactor, namely, impurities which, if present, will result in complexing with the catalyst or copolymerization with the isomonoolefins or the para-alkylstyrene, which in turn will prevent one from producing the para-alkylstyrene copolymer

product useful in the practice of the present invention. Most particularly, these impurities include the catalyst poisoning material, moisture and other copolymerizable monomers, such as, for example, metal-alkylstyrenes and the like. These impurities should be kept out of the system.

In producing the suitable copolymers, it is preferred that the para-alkylstyrene be at least 95.0 wt. percent pure, preferably 97.5 wt. percent pure, most preferably 99.5 wt. percent pure and that the isomonoolefin be at least 99.5 wt. percent pure, preferably at least 99.8 wt. percent pure and that the diluents employed be at least 99 wt. percent pure, and preferably at least 99.8 wt. percent pure.

The most preferred Lewis acid catalysts are ethyl aluminum dichloride and preferably mixtures of ethyl aluminum dichloride with diethyl aluminum chloride. The amount of such catalysts employed will depend on the desired molecular weight and the desired molecular weight distribution of the copolymer being produced, but will generally range from about 20 pp to 1 wt. percent and preferably from about 0.001 to 0.2 wt. percent, based upon the total amount of monomer to be polymerized.

Halogenation of the polymer can be carried out in the bulk phase (e.g., melt phase) or either in solution or in a finely dispersed slurry. Bulk halogenation can be effected in an extruder, or other internal mixer, suitably modified to provide adequate mixing and for handling the halogen and corrosive by-products of the reaction. The details of such bulk haiogena ion processes are set forth in U.S. Patent No. 4,548,995, which is hereby incorporated by reference.

Suitable solvents for solution halogenation include the low boiling hydrocarbons (C4 to C7) and halogenated hydrocarbons. Since the high boiling point para- methylstyrene makes its removal by conventional distillation impractical, and since it is difficult to avoid completely solvent halogenation, it is very important where solution or slurry halogenation is to be used that the diluent and

halogenation conditions be chosen to avoid diluent halogenation, and that residual para-methylstyrene has been reduced to an acceptable level.

With halogenation of para-methylstyrene/ isobutylene copolymers, it is possible to halogenate the ring carbons, but the products are rather inert and of little interest. However, it is possible to introduce halogen desired functionality into the para- methylstyrene/isobutylene copolymers hereof in high yields and under practical conditions without obtaining excessive polymer breakdown, cross-linking or other undesirable side reactions.

It should be noted that radical bromination of the enchained para-methyl styryl moiety in the useful copolymers for the practice of this invention can be made highly specific with almost exclusive substitution occurring on the para-methyl group, to yield the desired benzylic bromine functionality. The high specificity of the bromination reaction can thus be maintained over a broad range of reaction conditions, provided, however, that factors which would promote the -ionic reaction route are avoided (i.e., polar diluents, Friedel-Crafts catalysts, etc.).

Thus, solutions of the suitable para- methylstyrene/isobutylene copolymers in hydrocarbon solvents such as pentane, hexane or heptane can be selectively bro inated using light, heat, or selected radical initiators (according to conditions, i.e., a particular radical initiator must be selected which has an appropriate half- life for the particular temperature conditions being utilized, with generally longer half-lives preferred at warmer hydrogenation temperatures) as promoters of radical halogenation, to yield almost exclusively the desired benzylic bromine functionality, via substitution on the para-methyl group, and without appreciable chain scission and/or cross-linking.

This reaction can be initiated by formation of a bromine atom, either photochemically or thermally (with or without the use of sensitizers) , or the radical initiator

used can be one which preferentially reacts with a bromine molecule rather than one which reacts indiscriminately with bromine atoms, or with the solvent or polymer (i.e., via hydrogen abstraction) . The sensitizers referred to are those photochemical sensitizers which will themselves absorb lower energy photons and disassociate, thus causing, in turn, disassociation of the bromine, including materials such as iodine. It is, thus, preferred to utilize an initiator which has a half life of between about 0.5 and 2500 minutes under the desired reaction conditions, more preferably about 10 to 300 minutes. The amount of initiator employed will usually vary between 0.02 and 1 percent by weight on the copolymer, preferably between about 0.02 and 0.3 percent. The preferred initiators are bis azo compounds, such as azo bis isobutyronitrile (AIBN) , azo bis (2,4 dimethyl valero) nitrile, azo bis (2 methyl butyro) nitrile, and the like. Other radical initiators can also be used, but it is preferred to use a radical initiator which is relatively poor at hydrogen abstraction, so that it reacts preferentially with the bromine molecules to form bromine atoms rather than with the copolymer or solvent to form alkyl radicals. In those cases, there would then tend to be resultant copolymer molecular weight loss, and promotion of undesirable side reactions, such as cross- linking. The radical bromination reaction of the copolymers of para-methylstyrene and isobutylene is highly selective, and almost exclusively produces the desired benzylic bromine functionality. Indeed, the oniy major side reaction which appears to occur is disubstitution at the para-methyl group, to yield the dibromo derivative, but even this does not occur until more than about 60 percent of the enchained para-methylstyryl moieties have been monosubstituted. Hence, any desired amount of benzylic bromine functionality in the monobromo form can be introduced into the above stated copolymers, up to about 60 mole percent of the para- methylstyrene content.

It is desirable that the termination reactions be minimized during bromination, so that long, rapid radical

chain reactions occur, and so that many benzylic bromines are introduced for each initiation, with a minimum of the side reactions resulting from termination. Hence, system purity is important, and steady-state radical concentra¬ tions must be kept low enough to avoid extensive recom¬ bination and possible cross-linking. The reaction must also be quenched once the bromine is consumed, so that continued radical production with resultant secondary reactions (in the absence of bromine) do not then occur. Quenching may be accomplished by cooling, turning off the light source, adding dilute caustic, the addition of a radical trap, or combinations thereof.

Since one mole of HBr is produced for each mole of bromine reacted with or substituted on the enchained para- methylstyryl moiety, it is also desirable to neutralize or otherwise remove this HBr during the reaction, or at least during polymer recovery in order to prevent it from becoming involved in or catalyzing undesirable side reactions. Such neutralization and removal can be accomplished with a post- reaction caustic wash, generally using a molar excess of caustic on the HBr. Alternatively, neutralization can be accomplished by having a particulate base (which is relatively non-reactive with bromine) such as calcium carbonate powder present in dispersed form during the bromination reaction to absorb the HBr as it is produced. Removal of the HBr can also be accomplished by stripping with an inert gas (e.g., N ₂ ) preferably at elevated temperatures.

The bro inated, quenched, and neutralized para- methylstyrene/isobutylene copolymers can be recovered and finished using conventional means with appropriate stabilizers being added to yield highly desirable and versatile functional saturated copolymers.

In summary, halogenation to produce a copolymer useful in the present invention is preferably accomplished by halogenating an isobutylene-para-methylstyrene copolymer using bromine in a normal alkane (e.g., hexane or heptane) solution utilizing a bis azo initiator, e.g. , AIBN or

VAZO® 52: 2,2 -azobis(2,4-dimethylpentane nitrile), at about 55 to 80*C, for a time period ranging from about 4.5 to about 30 minutes, followed by a caustic quench. The recovered polymer is washed in basic water wash and water/isopropanol washes, recovered, stabilized and dried.

In addition to the halogen-containing copolymer of a C4 to C7 isomonoolefin and a para-alkylstyrene, the tire innerliner composition of the present invention also comprises carbon black, a plasticizer oil, and a curing agent. Optionally, the composition may comprise a rubber component of certain rubbers other than the halogen- containing copolymer of a C4 to C7 isomonoolefin and the para-alkylstyrene.

Suitable optional rubber components for the composition of the present invention, in addition to the halogen-containing coplymer of the isomonoolefin and the para-alkylstyrene, are rubbers selected from the group consisting of natural rubber, styrene-butadiene rubber, polybutadiene rubber, a terpolymer of ethylene, propylene, and a nonconjugated diene (EPDM) , and mixtures thereof. The additional optional rubber component may be an oil extended rubber of these suitable rubbers.

Furthermore, the tire innerliner composition may, optionally, comprise a component selected from the group consisting of a filler other than carbon black, a rubber compounding additive and mixtures thereof. The carbon black may be derived from any source. Suitable carbon black includes channel black, furnace black, thermal black, acetylene black, lamp black and the like. The filler and/or additive may be any conventional filler and/or additive generally used with rubber.

Suitable plasticizer oils include hydrocarbon plasticizer oils such as paraffinic, naphthenic or aromatic petroleum oils. The preferred plasticizer oil is a paraffinic petroleum oil. Suitable hydrocarbon plasticizer oils include oils having the following general characteristics:

Property Preferred Minimum Maximum

API'gravity at 60*F 15-30 10 35

Flash Point, 'F 330-450 300 700 (open cup method)

Pour Point, *F -30 to +30 -35 60

SSU at 100'F 100-7,000 50 20,000

The optional other filler may be a non- reinforcing filler, a reinforcing filler, an organic filler, and an inorganic filler, or mixtures thereof.

Suitable fillers, other than carbon black, include calcium carbonate, clay, silica, talc, titanium dioxide and mixtures thereof. Suitable rubber compounding additives include antioxidants, stabilizers, rubber processing aids, pigments and mixtures thereof. The rubber processing aids may be an oil such as paraffinic, naphthenic, or aromatic processing oils. Suitable antioxidants include hindered phenols, amino phenols, hydroquinones, certain amine condensation products and the like. The preferred additives are fatty acids, low molecular weight polyethylene, waxes and mixtures thereof. A preferred fatty acid is stearic acid. Mixtures of other fatty acids can be used with the stearic acid.

The tire innerliner composition of the present invention also comprises a curing agent.

Suitable curing agents include peroxide cures, elemental sulfur cures, sulfur-containing compound cures, and non-sulfur cures. For example, the curing agent may be zinc oxide. Optionally, curing agent accelerators may be used such as dithiocarbamates, thiurams, thioureas, and mixtures thereof; zinc oxide-free cures may also be used such as, for example, litharge, 2-mercaptoimidazoline, and diphenyl guanidine; 2-mercaptobenzimidazole, and N.N'- phenylene- bismaleimide. Organic peroxide may be used as curing agents, such as, for example, dicu yl peroxide, benzoyl peroxide, α ,α'-Bis(tertiary butyl peroxy)

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A preferred curing system comprises zinc oxide, sulfur and 2,2 ^/ - benzothiazyl disulfide (MBTS) accelerator.

The tire innerliner composition of the present invention may comprise the halogen-containing copolymer of a C4 to C7 isomonoolefin and a para-alkylstyrene in an amount ranging from about 40 to about 70, preferably from about 45 to about 65 weight percent, the carbon black in an amount ranging from about 20 to about 45, preferably from about 25 to about 40 weight percent; the plasticizer oil in an amount ranging from above zero to about 25, preferably from about 5 to about 20 weight percent; the total amount of other fillers and additives in an amount ranging from above zero to about 50, preferably from above zero to about _. 20 weight percent; and the curing agent in an amount ^' ranging from about 1 to 6, preferably from about 1 to 3 weight percent, all said percentages being based on the weight of the total composition.

If other optional rubbers are present, the weight percent of the halogen-containing copolymer is adjusted in such a way that the total rubber content of the composition may fall within the range of 40 to 70 weight percent, preferably 45 to 65 weight percent of the total composition. In the embodiment in which one or more optional additional rubbers are present in the composition, it is perferred that the halogen-containing copolymer of an isomonoolefin and para-alkystyrene be present in an amount of at least about 50 weight percent of the total rubber content, that is, of said halogen-containing copolymer plus the optional additional rubbers.

The tire innerliner composition of the present invention may be vulcanized by subjecting it to heat according to any conventional vulcanization process. Typically, the vulcanization is conducted at a temperature ranging from about 100*C to about 250*C, preferably from about 150'C to about 200*C, for a time period ranging from about 1 minute to several hours.

The tire innerliner composition of the present invention may be used in producing innerliners for tires, for example, motor vehicle tires such as truck tires, bus tires, passenger automobile, motorcycle tires, and the like. The improved heat aging resistance of the present composition makes it particularly suited for use in truck tires to increase the retreading capability of the tire.

Suitable tire innerliner compositions may be prepared by using conventional mixing techniques including, e.g., kneading, roller milling, extruder mixing, internal mixing (such as with a Banbury* mixer) , etc. The sequence of mixing and temperatures employed are well known to the skilled rubber compounder, the objective being the dispersion of fillers, activators and curatives in -the polymer matrix without excessive heat buildup. A useful mixing procedure utilizes a Banbury mixer in which the copolymer rubber, carbon black and plasticizer are added and the composition mixed for the desired time or to a particular temperature to achieve adequate dispersion of the ingredients. Alternatively, the rubber and a portion of the carbon black (e.g.; one-third to two-thirds) is mixed for a short time (e.g., about 1 to 3 minutes) followed by the remainder of the carbon black and oil. Mixing is continued for about 5 to 10 minutes at high rotor speed during which time the mixed components reach a temperature of about 140'C. Following cooling, the components are mixed in a second step on a rubber mill or in a Banbury mixer during which the curing agent and optional accelerators, are thoroughly and uniformly dispersed at relatively low temperature, e.g., about 80 to about 105 ^* C. Variations in mixing will be readily apparent to those skilled in the art and the present invention is not limited to any specific mixing procedure. The mixing is performed to disperse all components of the composition thoroughly and uniformly.

The innerliner composition is formed, typically, into a sheet, such as by calendering. The sheet may then be placed adjacent to an intermediate layer (e.g., carcass layer) of an uncured tire which is being formed on a tire

building drum. Tires are generally built on a drum from at least three layers, namely, an outer layer, an intermediate layer, and an inner layer (e.g., innerliner). After the uncured tire has been built on a building drum, the uncured tire may be placed in a heated mold to shape it and heat it to vulcanization temperatures and, thereby, to produce a cured unitary tire from the multi-layers.

Vulcanization of a molded tire, typically, is carried out in heated presses under conditions well known to those skilled in the art.

Curing time will be affected by the thickness of the tire to be molded and the concentration and type of curing agent as well as the halogen content of the halogenated copolymer. However, the vulcanization parameters can readily be established with a few experiments utilizing e.g., a laboratory characterization device well known in the art, the Monsanto Oscillating Disc Cure Rheometer (ODR, described in detail in American Society for Testing and Materials, Standard ASTM D 2084) .

The following examples are presented to illustrate the invention. All parts and percentages herein are by weight unless specifically otherwise indicated.

EXAMPLE

Experiments were performed to compare compositions in accordance with the present invention with a composition comprising a bromobutyl rubber in a tire innerliner type formulation. The results of these experiments are summarized in Tables I, II, III and IV. In these tables, composition A and B were compositions in accordance with the present invention. Composition C was not a composition in accordance with the present invention.

Copolymer Z was a brominated isobutylene-para-methylstyrene copolymer comprising 1.9 wt. % bromine and 5 wt. % par - methylstyrene moieties.

Copymer Y was a brominated isobutylene-para-methylstyrene copolymer comprising 1.9 wt. % bromine and 10 wt. % para-

methylstyrene moieties.

Copolymer X was Exxon bromobutyl rubber grade 2222 comprising 2 wt. % bromine.

Compositions A through C were compounded using a laboratory internal mixer. The mix cycle used is shown in Table I. The test methods used are shown in Table V.

Table I

COMPOSITION

Mixing cycle Conventional Mixing Cycle

0.0':Polymer,Black.

Loading 1.600 L

1.5' :Oil,Stearic Acid.

1 or 2 passes 2 passes

3.0' :Cleaning

Dump temp *C 14C*C/add curative on the mill

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3.5 ^, :Dump at 140*C.

(1) Flexon* 876 is a paraffinic oil, Exxon Chemical Co.

(2) MBTS is 2,2'- benzothiazyl disulfide

Table II

COMPOSITION

Ingredients _B-

Copolymer Z 100 Copolymer Y 100

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Table III

COMPOSITION

Ingredients

Copolymer Z 100 Copolymer Y 100 Copolymer X 100 Carbon Black N-660 60 60 60 FLEXON 876 15 15 15 STEARIC ACID 1 1 1

ZnO 2 2 3

SULPHUR 1 1 5

MBTS 2 2 5

CURED COMPOUND, ORIGINAL PROPERTIES

PRESS CURE 160 ^α c Hardness Shore A Modulus 100% Mpa Modulus 300% Mpa Tensile Strength MPa Elongation at Break %

MONSANTO FLEX RESISTANCE 140% ext, cam 24 in, kcycles max, kcycles avg, kcycles

PRESS CURE 160'C PELL ADHESION, Self kN/m Min. kN/m Max. Type of failure

PRESS CURE 160'C

PEEL ADHESIO l ¹ ' kN/m

Type of failure

PRESS CURE 160'C

PEEL ADHESION ( ² - kN/m

Type of failure

TABLE III . continued

Footnotes:

(1) Adhesion to a chafer compound based on 25/75 natural rubber/butadiene rubber.

(2) Adhesion to a carcass compound based on 50/50 natural rubber/ styrene-butadiene rubber.

(3) Adhesion to a carcass compound based on 100 percent natural rubber.

TABLE IV (continued) AGED PROPERTIES 7 DAYS AT 150* C (Repeated)

C

Table v

Property Test

Mooney Viscosity ASTM D1646

Mooney Scorch ASTM D4818

Oscillating Disc Reheometer ASTM D2084 (ODR - Monsanto)

Press Cure ASTM D2240/D412

Elongation at break ASTM D412

As can be seen from the data in Tables I, II, in, and IV, the compositions in accordance with the present invention, that is, Composition A and B had improved heat aging properties relative to comparative composition C, particularly for aged flex resistance, tensile strength, and elongation at break, and similar good cure properties and cured original properties as comparative Composition C.