WO1998036111A1

WO1998036111A1 - Manufacture of acrylic fibres

Info

Publication number: WO1998036111A1
Application number: PCT/GB1998/000416
Authority: WO
Inventors: Nigel Philip Briggs
Original assignee: Courtaulds PLC; Akzo Nobel UK PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1997-02-12
Filing date: 1998-02-10
Publication date: 1998-08-20
Anticipated expiration: 1999-08-12
Also published as: AU6000798A; GB9702831D0; ZA981138B

Abstract

A method for the manufacture of an acrylic fibre includes the steps of (1) providing an acrylic polymer dope which comprises an inorganic solvent, a first acrylic polymer in solution in said solvent, a solid organic additive in dispersion, and a dispersing agent, and (2) extruding the dope through a die into an aqueous coagulating bath, thereby forming the acrylic fibre, the dispersing agent being a second acrylic polymer which contains at least one unit of a comonomer which is more hydrophobic than ethyl acrylate. The dope may be made by (1) dissolving the dispersing agent in the inorganic solvent to form a solution, (2) dispersing the organic additive in the solution to form a premix, and (3) mixing the premix with a solution of the first acrylic polymer in the inorganic solvent.

Description

MANUFACTURE OF ACRYLIC FIBRES

Field of the invention

This invention relates to the manufacture of acrylic fibres (that is to say, fibres based on an acrylic polymer, being a polymer which comprises at least 85 percent by weight of acrylonitrile units) which incorporate a solid organic additive such as an organic pigment or a biocidal, biostatic or flame-retardant agent.

Background art Acrylic fibres are known and are described for example in an article entitled "Acrylic Fibres" in Encyclopaedia of Polymer Science and Engineering, Volume 1 (Wiley- Interscience, 1985), pages 334-388. Such fibres are commonly manufactured by wet- or dry-spinning of a solution of an acrylic polymer in a suitable solvent. Such polymer solutions may also be referred to as dopes . Several organic and inorganic solvent systems for acrylic polymers are known, as described in the aforementioned article. Organic solvents can be used in both dry- and wet-spinning processes, but inorganic solvents are generally used only in wet-spinning processes. The industrially-useful inorganic solvent systems are water-based and of strongly ionic character.

It is known that spinning -solutions or dopes which contain in dispersion one or more solid additives such as pigments or biocidal, biostatic or flame-retardant agents can be extruded to produce fibres and films incorporating these additives, thereby conferring valuable properties on these fibres and films. Such solid additives are present in the fibre in particulate form, rather than being dispersed on the molecular scale. It is also known that it is often desirable to utilise a dispersing agent when manufacturing dispersions of such solid additives, both to provide good dispersion of the additive and to hinder its subsequent aggregation. Dispersing agents and their use are described for example in an article entitled "Dispersants" in Kirk-Othmer Encyclopaedia of Chemical Technology, 4th edition, Volume 8 (Wiley-Interscience, 1993), pages 293-311.

It has been found that dopes which contain an acrylic polymer in solution and a solid inorganic additive in dispersion, for example a matt pigment such as titanium dioxide, can generally be prepared and wet- spun into fibres from inorganic solvent systems without excessive difficulty, often without the need for any dispersing agent. In contrast, it has proved difficult satisfactorily to prepare and wet-spin acrylic dopes based on inorganic solvent systems which also contain a solid organic additive in dispersion. Such organic additives are commonly electrically-neutral uncharged materials which are often highly hydrophobic, and they are accordingly poorly compatible with strongly ionic water-based solvents. Use of conventional dispersing agents has failed to supply a satisfactory answer to this problem.

US-A-3_.630, 985 discloses a process for preparing a stable spinning composition comprising an acrylonitrile polymer dissolved in a concentrated inorganic solvent solution therefor and a surface-active agent (e.g. laurylamine acetate) insoluble in said solution. This process involves dispersing the surface-active agent in the solution at a particle size less than 50 micrometres, maintaining the dispersion under agitation, and defoaming the dispersion as a film under reduced pressure at a temperature which is at least equal to the boiling point of the composition at the reduced pressure employed until the foam content is below 0.5% by volume based on the total volume of the composition. The surface-active agent may be present in an amount of about 0.1 to 10% by weight based on the weight of the polymer. Problems of agglomeration of the surface-active agent and of foaming of the spinning composition are said to be thereby avoided. Disclosure of the invention

According to the invention there is provided a method for the manufacture of an acrylic fibre which includes the steps of:

(1) providing an acrylic polymer dope which comprises an inorganic solvent, a first acrylic polymer in solution in said solvent, and a solid organic additive in dispersion,- and

(2) extruding the dope through a die into an aqueous coagulating bath, thereby forming said acrylic fibre,

characterised in that the dope comprises a dispersing agent which is a second acrylic polymer which contains one or more units of a comonomer which is more hydrophobic than ethyl acrylate.

A solid organic additive is one which is essentially insoluble in both the inorganic solvent and the coagulating bath, so that it becomes incorporated into the fibre in particulate form with a high degree of efficiency. Examples of such additives include, but are not limited to, organic pigments (including carbon black) and organic biocidal, biostatic and flame-retardant agents. Other examples include polymer- containing particles, for example polymeric matting agents such as polystyrene particles or microencapsulated phase- change materials of the kind disclosed in US-A-4, 756, 958. Mixtures of additives may be used. The amount of each such additive will often be within the range from 0.01 to 15 percent by weight based on the weight of the fibre.

The first acrylic polymer may be any fibre-forming polymer of the conventional kind known in the manufacture of acrylic fibres. Such polymers typically comprise by weight from 89 to 95 percent of acrylonitrile units, from 4 to 10 percent of non-ionic comonomer units and from 0.5 to 1 percent of ionic comonomer units. Examples of non-ionic comonomer units include units of methyl (meth) acrylate, vinyl acetate and acrylamide. Examples of ionic comonomer units include units of methallyl sulphonic acid, styrene sulphonic acid, 2 -methyl -2 - sulphopropylacrylamide , itaconic acid, dialkylaminoalkyl (meth) acrylates and the vinyl pyridines.

The inorganic solvent may be any of those known in the art as spinning solvents for acrylic polymers. Suitable solvents of this kind include aqueous sodium thiocyanate, aqueous zinc chloride and aqueous nitric acid. The coagulating bath is commonly aqueous and can be considered to be diluted spinning solvent .

The second acrylic polymer should be soluble in the inorganic solvent and essentially insoluble in the coagulating bath. It thus becomes incorporated in the acrylic fibre, and accordingly it poses little or no problem as regards recovery of spinning solvent or cross-contamination with other product lines. This is an advantage of the invention.

Examples of the comonomer which is more hydrophobic than ethyl acrylate (hereinafter referred to as the hydrophobic comonomer) include the C₃ to C₁₀, preferably the C to C₈, further preferably the C₄ to C_6ι alkyl esters of (meth) acrylic acid, particularly the primary alkyl esters. Further examples include the vinyl esters of the C₄ to C_X1, preferably the C₅ to C₉, alkanoic acids, particularly the straight-chain alkanoic acids. Further examples include derivatives of esters of these kinds which contain partially or fully fluorinated alkyl groups. Silylated compounds may also be used. It will be appreciated that the desirable proportion of hydrophobic comonomer in the second acrylic polymer will depend to some extent on the nature of the organic additive and of the inorganic solvent in which it is to be dispersed. The proportion should be sufficiently high so that the organic additive is maintained in dispersion, but it should not be so high that formation of the dispersion is attended by excessive foaming. Polymers can readily be assessed for their suitability in any particular case by trial and error. A proportion of the hydrophobic comonomer in the second acrylic polymer in the range from 0.5 to 10, often from 0.5 to 5, more particularly from 1 to 3 , or from 2 to 7, percent by weight based on the weight of the second acrylic polymer may be found suitable.

The relative hydrophilicity or hydrophobicity of a monomer may be estimated by measurement of the surface tension of a homopolymer of such monomer. Surface tension data on a variety of polymers are set out in Polymer Handbook, 3rd edition (Wiley-Interscience, 1989) at pages VI/411 to VI/434, particularly with reference to "Polymer Interface and Adhesion" by S . Wu (Marcel Dekker, 1982) . The surface tension measured at 20°C of a homopolymer of the hydrophobic comonomer is preferably at least 2 mN/m lower, more preferably in the range from 2 to 25 mN/m lower, further preferably in the range from 3 to 6 mN/m lower, than that of a homopolymer of ethyl acrylate of comparable molecular weight.

The second acrylic polymer may also include groups derived from one or more other comonomers, for example vinyl acetate or the C, or C₂ alkyl esters of (meth) acrylic acid, for example in the range from 1 to 10 percent by weight based on the weight of the polymer. The second acrylic polymer may include groups derived from one -or more ionic monomers, as with the first acrylic polymer. The second acrylic polymer may be the same as the first acrylic pol mer. Alternatively to being a random copolymer, the second acrylic polymer may be a block or graft copolymer.

The second acrylic polymer may conveniently be made by conventional techniques of olefinic polymerisation. Alternatively, an acrylic polymer may be chemically modified so that it complies with the requirements of the second acrylic polymer, for example by transesterification, although such techniques may generally be less preferred. The molecular weight of the second acrylic polymer is not especially critical but is conveniently of similar magnitude to that of the first acrylic polymer, for example a number-average molecular weight in the range from 10,000 to 200,000. Desirably, the molecular weight of the second acrylic polymer is sufficiently high that each polymer chain contains by number-average more than one, preferably two or more, units of the hydrophobic comonomer.

The amount required of the second acrylic polymer depends to some extent on the nature of all the materials in the system, and it can conveniently be determined empirically in any particular case. The considerations which apply are similar to those described in connection with the proportion of the hydrophobic comonomer. In general, an amount in the range from 0.1 to 5 percent by weight based on the weight of the solid organic additive may be found suitable. It will be appreciated that the amount of the second acrylic polymer in the fibre will as a result be very low and will accordingly have little if any observable effect on fibre properties. This is an advantage of the invention.

Care should desirably be taken to avoid bridging flocculation; this is a phenomenon in which a single polymer chain binds two or more particles together. Accordingly, use of a second acrylic polymer having too high a molecular weight or containing too high a proportion of the hydrophobic comonomer is preferably to be avoided.

Polymer spinning dopes are of high viscosity. Accordingly, dispersion of the solid organic additive directly into a polymer dope is generally disfavoured. It is generally preferable to disperse the additive in a low-viscosity system which can then be mixed with a solution of the first acrylic polymer. According to a preferred embodiment of the invention, the acrylic polymer dope may be prepared by a method which includes the steps of: (1) dissolving the dispersing agent in the inorganic solvent to form a solution,-

(2) dispersing the solid organic additive in the solution to form a premix; and

(3) mixing the premix with a solution of the first acrylic polymer in the inorganic solvent, thereby forming said acrylic polymer dope.

If mixtures of additives are to be used, it will be appreciated that it will often be operationally more convenient to form separate premixes for admixture into the dope .

The dispersion step may be carried out in any convenient known manner. Preferably, the dispersion step includes or is followed by a milling step performed on the premix, for example using a bead mill or ball mill to reduce particle size, to break down aggregates of particles and no ensure good dispersion.

The mixing step is conveniently performed by injecting the premix into conventional polymer dope comprising a solution of the first acrylic polymer in the inorganic solvent shortly before extrusion of the dope. The mixing step preferably employs a mixing device such as a barrel mixer or static mixer. If desired, the mixing step may be performed in two or more stages, whereby the premix is first mixed with only a (first) portion of the stream of conventional polymer dope (optionally diluted with solvent) passing to the extrusion die, thereby forming an intermediate mixture which is then mixed with the remainder (second portion) of the conventional polymer dope to yield the acrylic polymer dope ready for extrusion.

It will be appreciated that the premix desirably contains a high proportion of the solid organic additive in order to avoid unnecessary dilution effects but that this proportion should not be so high that the premix is unstable or otherwise difficult to process. Although this proportion depends to some extent on the nature of the components in the system, an amount in the range from 10 to 60 percent by weight may generally be found suitable.

The invention is illustrated by the following Examples, in which parts and proportions are by weight unless otherwise specified: -

Example 1

A monomer mixture consisting of acrylonitrile (2404 g) and n-butyl acrylate (49.1 g) was prepared. Initiator solutions consisting of (1) potassium persulphate (28.66 g) dissolved in water (to 10 1) and (2) sodium metabisulphite (116.66 g) and ferrous ammonium sulphate (0.468 g) dissolved in water (to 10 1) were prepared. The monomer mixture and the two initiator solutions were concurrently fed into a stirred continuous reactor vessel (2.6 1) at rates of 38, 46 and 46 ml/min respectively. The temperature of the reaction mixture was 55 °C and residence time was 20 rain. The polymer was collected by filtration, washed and dried. Monomer conversion was 78.5%. A sample of the polymer was dissolved in dimethylsulphoxide (0.5% w/v) and its viscosity measured in an Ostwald viscometer (Grade B) at 25 °C. The inherent viscosity (IV) calculated from this measurement was 1.2. The polymer (10 parts) was dissolved in 51% aqueous sodium thiocyanate (90 parts) to form a stock solution.

Stock solution (5 kg) was added with stirring to 52% aqueous sodium thiocyanate solution (70 kg) . If desired, a conventional antifoam may be added . 2, 4, 4' -Trichloro-2' -hydroxydiphenyl ether in powder form (a bactericidal agent available from Ciba-Geigy AG under the Trade Mark IRGASAN DP 300) (25 kg) was slowly added to the solution to form a premix in the form of a thin paste. The premix was milled by single passage through a Dyno (Trade Mark) bead mill (25 1 rating) over a period of 3 hr . The milled premix (1 part) was mixed with polymer dope (1 part) and 52% aqueous sodium thiocyanate solution (3 parts) to form an injectible premix (injector dope) . This latter premix was then filtered and injected into a polymer dope stream feeding a fibre spinning machine. The polymer dope in this stream contained acrylic polymer (monomers acrylonitrile 93%, methyl acrylate 6%, 2 -acrylamido-2 -methylpropanesulphonic acid 1%, all to the nearest percent; IV 1.5) (13%) , sodium thiocyanate (45%) and water (42%) .

The rate of injection was selected to yield fibre- containing 0.7% of the additive. Mixing was effected by means of an in-line multi-element static mixer (available from Sulzer AG) . The dope was then extruded through a spinnerette (180,000 holes of 80 micron diameter) into an aqueous coagulating bath (S.G. 1.068,-12°C) to yield acrylic fibre, which was stretched (xl.5 at 60°C; then x5 at 100°C) , washed free of solvent, soft finished, dried and crimped.

In a comparative experiment, the same organic additive

(IRGASAN) was added with stirring to 52% aqueous sodium thiocyanate without the use of any dispersing agent. The additive tended to float on the surface of the mixture. Upon milling, the mixture became viscous, to resemble a very stiff whipped cream. Microscopic examination showed the presence of a multitude of small air bubbles, surrounded by particles of the additive. The experiment was not taken further, because the presence of air bubbles in a polymer spinning dope is most undesirable .

Example 2

Milled premixes were made according to the general procedure of Example 1 but containing tolnaftate as organic additive instead of IRGASAN. With tolnaftate, it was found that a copolymer based on 95 parts acrylonitrile and 5 parts butyl acrylate gave superior results to the copolymer based on 98 parts acrylonitrile and 2 parts butyl acrylate of Example 1; in the sense that the premix contained less entrained air and appeared more mobile (thinner) .

Example 3

The following procedure was used for screening purposes. Aqueous sodium thiocyanate solution (52%) was taken, and an acrylic polymer (ca. 1%) was dissolved in it . A portion of solution (ca. 10 ml) was placed in a polystyrene screw-capped tube, and to this a small amount of the additive to be dispersed (ca. 50-100 mg) was added. The tube was mechanically tumbled end-over-end (ca. 1-2 rpm) for about 6 hr, after which it was left to stand overnight. The quality of dispersion was then assessed visually on the scale 1 - heavy settling (worst) , 2 = settling, 3 = dispersion and 4 = good dispersion (best) . The results shown in Table 1 were obtained for a variety of pigments:

Table 1

Key to column headings. 1 = no acrylic polymer (comparative) . 2 = conventional acrylic polymer, as in the polymer dope stream of Example 1 (comparative) . 3 = 98:2 acrylonitrile/butyl acrylate. 4 = 95:5 acrylonitrile/butyl acrylate. 5 = 95:5 acrylonitrile/isopropyl methacrylate . 6 = 95:5 acrylonitrile /2-ethylhexyl methacrylate.

All pigments were supplied by Ciba. A dash in Table 1 means that no test was made.

It can be seen from Table 1 that all the acrylic polymers of the invention (columns 3-6) gave improved results with at least some of the organic additives used.

Claims

1. A method for the manufacture of an acrylic fibre which includes the steps of:

characterised in that the dope comprises a dispersing agent which is a second acrylic polymer which contains at least one comonomer unit which is more hydrophobic than ethyl acrylate .

2. A method according to claim 1, further characterised in that the comonomer unit in the second acrylic polymer is selected from the group consisting of units of C₃ to C₁₀ alkyl esters of (meth) acrylic acid and units of vinyl esters of the C₄ to C_X1 alkanoic acids.

3. A method according to claim 1 or claim 2, further characterised in that the comonomer in the second acrylic polymer is such that the surface tension of a homopolymer of the comonomer, measured at 20°C, is lower by a value in the range from 2 to 25 mN/m than the surface tension of a homopolymer of ethyl acrylate of comparable molecular weight.

4. A method according to claim 3, further characterised in that the surface tension of a homopolymer of the comonomer, measured at 20°C, is lower by a value in the range from 3 to 6 mN/m than the surface tension of a homopolymer of ethyl acrylate of comparable molecular weight.

5. A method according to any one of the preceding claims, further characterised in that the proportion of the comonomer unit in the second acrylic polymer is in the range from 0.5 to 10 percent by weight based on the weight of the second acrylic polymer.

6. A method according to any one of the preceding claims, further characterised in that the amount of the second acrylic polymer in the dope is in the range from 0.1 to 5 percent by weight based on the weight of the organic additive.

7. A method according to any one of the preceding claims, further characterised in that the inorganic solvent is selected from the group consisting of aqueous sodium thiocyanate, aqueous zinc chloride and aqueous nitric acid.

8. A method according to any one of the preceding claims, further characterised in that the solid organic additive is selected from the group consisting of organic pigments, biocidal, biostatic and flame-retardant agents, polymeric matting agents and microencapsulated phase- change materials .

9. A method according to any one of the preceding claims, further characterised in that the acrylic polymer dope is prepared by a method which includes the steps of:

(1) dissolving the dispersing agent in the inorganic solvent to form a solution,-

(2) dispersing the solid organic additive in the solution to form a premix,- and

(3) mixing the premix with a solution of the first acrylic polymer in the inorganic solvent, thereby forming the acrylic polymer dope.

10. A method according to claim 9, further characterised in that the premix contains from 10 to 60 percent by weight of the solid organic additive.

11. A method according to either claim 9 or claim 10, further characterised in that the premix is mixed with a first portion of solution of the first acrylic polymer in the inorganic solvent to form an intermediate mixture which is then mixed with a second portion of solution of the first acrylic polymer in the inorganic solvent, thereby forming the acrylic polymer dope.