GB2190680A - Flame retardant polymers and copolymers of acrylonitrile - Google Patents

Abstract

The flammability of fibrous, fibres or fabric comprising acrylonitrile polymers or copolymers is reduced by incorporating into the polymer preferably before spinning or casting an additive such as a tin halide which promotes charing of the polymer when the polymer is heated or ignited.

Description

SPECIFICATION Flame retardant polymers and copolymers of acrylonitrile This invention relates to the production offlame-retardant polymers and copolymers of acrylonitrile.

In recent years, emphasis on flame-retardant textiles has moved from apparel fabrics towards furnishings, upholstery and bedding, particularly for institutional and other contract outlets. This trend is largelythe consequence of a continuing pattern of calamitous fires, in which textiles were deeply implicated. The most notorious of these fires are those involving furniture with foam upholstery. Many fires in bedding and furniture start with slow, smouldering combustion and can thus cause casualties through carbon monoxide poisoning before they are detected; they may then erupt into conflagrations within so short a timethat escape from a building is impossible because of the heat and smoke generated.Legislation now being enacted to ensure higher standards of safety for domestic furniture imposes a clear and pressing burden on the industry to provide fabrics and upholstery better able than existing materials to satisfythese requirements.

Normal acrylicfibres which are currentiy favoured for upholstery, curtains, carpets and blankets are highly inflammable and generate toxic gases on combustion. The existing routeto flame-retardancy in acrylicfibres is through copolymerisation of acrylonitrile with a halogenated monomer. The modacrylic fibres so produced have, however, inferior textile properties and the problem of toxic gas evolution remains. There is still a great need, therefore, for a fresh route to flame retardancy in acrylicfibres.

According to the present invention, there are provided polymers and copolymers of acrylonitrile, in film, fibrous or other form, which include an additive, said additive being one that will promote charring instead of combustion and melting,when the polymer is heated orignited.

The invention also provides a process for imparting flame retardancyto a polymeric material derived wholly or partly from acrylonitrile comprising incorporating in the polymer an additive which acts to promote charformation when said polymer is heated or ignited.

In a preferred embodiment of the invention the acrylonitrile content of the polymer is not less than 85%.

The invention starts from the knowledgethat polyacrylonitrile (PAN) shares with cellulosics the property of yielding significant amounts of carbonaceous material ('char') when pyrolysed, even in air, under controlled conditions. Under uncontrolled conditions, of course, both polymers burn readily and completely. It iswell known that a primary function of the most common flame-resistant agentsfor cellulose isto promote formation ofthis char, which is relatively difficultto burn, at the expense of the volatile, highlycombustible products normally produced.In seeking means whereby the yield of char from acrylic polymers might be similarly enhanced, the approach differs from most earlier, conventional attempts to reduce flammability, which have relied on copolymerisation, or spinning dope modification with organohalogen or organophosphorus compounds. The poor swelling characteristics of acrylic polymers in water and other acceptable solvents mitigates against extensive use of after treatments.

Additives proposed for use with the present invention include metal salts of both organic and inorganic acids.

The amount of additive included in the polymerwill normally be determined having regard to the nature of the additive itself and the nature of the film fibre or material that is to be made. For example an open weave material has a very much greater surface area than a film ofthe sameweightand therefore requires less additiveto inhibitflammabilitythan the equivalent weight film. Generally, however, incorporation of additives in an amount from 5to 30%w/w preferably lotto 20% w/w will give good results.

When screened by thermogravimetric analysis (TGA) at 9901mien. temperature rise up to 580"C in dry air, many additives were found to enhance the yield of char from solvent-cast films of PAN (Table 1) even at a concentration of only 5% w/w. If corrected for residual solvent (dimethylformamide), these yields are increased by a factor of 1.1. Above 580"C, the carbonaceous residue was slowly oxidised to gaseous products.

From Table 1 it can be seen that tin halides are particularly preferred additives. These halides are very soluble in certain preferred solvents for PAN, viz. dimethylformamide or dimethylacetamide, so that high loadings of the additive can be uniformly, or even molecularly, dispersed in the polymer matrix, in whichstate they are at their most effective Table 1 TGA Char Yields ofModifiedPANFilms Additive (5%w/w) Moles/Kg PAN Char yield (%) Stannousbromide 0.18 74 Stannouschloride 0.26 72 Stannouschloride,dihydrate 0.22 72 Stannoudiodide 0.13 68 Stannicchloride,pentahydrate 0.14 66 SnCl4:: 1,1 0-phenanthroline 0.11 65 Zinc chloride 0.37 65 n-Butyltintrichloride 0.18 65 SnCl4:Pyridine(1:2) 0.12 63 (Et4N)2SnBr6 0.06 63 Lead (Il) chloride 0.18 62 3,4,5,6-Tetrabromophthalimide 0.11 61 Ferricchloride 0.31 61 SnCI4:2DMF 0.12 60 Decabromodiphenyl ether 0.05 59 Sandoflam5060* 0.14 57 Dithiodiglycollicacid 0.27 57 Hexabromocyclododecane 0.08 57 Bromocresolgreen 0.07 56 Zinctetrafluoroborate 0.21 56 Thiomalicacid 0.33 56 Ammonium molybdate 0.04(0.28 Mo) 56 None ~ 37 * A proprietaryflame-retardantfor viscose rayon (CH3)2C(CH2O)2P(:S).O.P(::S)(OCH2)2C(CH3)2 For practical purposes, it is importantthatadditive or additives included in PAN fibres remain in the fibres during spinning and subsequent mechanical, thermal, and chemical processes so that the fibres retain char-promoting properties. Stannous chloride, with its high solu bility in water, and susceptibilityto simultaneous hydrolysis and oxidation, could be expected to be unsuitable because of possible losses in a wet-spinning process. Experiments conducted to assess the response of PAN films containing stannous chloride to aqueous extraction (Table 2) showthatthe char promoting ability of stannous chloride additive persisted even after 14 days leaching, although reduced in magnitude.

Table2 TGA Char Yields and COI of Leached PAN + SnCl2(lO%) FILM Leaching Tin TGA Char Yield COI period content at 580"C (% 2) (Days) (g.atom/kg) (%) 0 0.27* 78 22.2 1 - 82 22.2 3 0.30 79.5 22.2 14 0.27 63 22.2 * This value represents only 62% ofthe calculated amount; the remainderwas presumably lost on drying the film.

This discovery indicates that the active char promoting agent may not be SnC12 but possibly a product of its oxidation and/or hydro[ysis, a view supported by the results of Mossbauer spectroscopy, which showed that even unleached modified film contained tin predominantly in the stannic (+4) state. These studies further showed a progressive hydrolysis of Sn-Cl bonds to give Sn-O bonded species, so that after 14 days' leaching, total conversion to a form of SnO2 (possibly alpha-stannic acid) had taken place. However, films modified by addition of powdered SnO2 hydrate had a low char yield (42%). It can also be shown that substantially all ofthe combined chlorine is removed by leaching.

A preferred embodiment of the invention, therefore, comprises the production of flame-retardantfilms and fibres from polyacrylonitrile dispersions and solutions containing tin halides (or other dissolved salts), by treatment of said dispersions and solutions with a controlled amount of water, to convert the tin compound into its hydrolysis products before these dispersions or solutions are cast or spun.

For example, a film was prepared from a solution of PAN containing 10% w/w of SnC12 which had been pre-aged by the addition of sufficientwaterto hydrolysethe stannous chloride in situ and then left for 2 days priorto casting. This film was analysed by AAS before and after leaching; thetin content after7 days was still 95% of the added quantity and char yield remained high at 74%. The pre-ageing process had the desired beneficial effect of preventing loss oftin compound during drying ofthe film, by ensuring hydrolysis ofthe volatile halide complex to a non-volatile hydrated oxide, a factor which of vital importance in the dry-spinning of PAN fibres from solutions containing the tin halide. Equally, pre-formation in the spinning dope of a water-insoluble (i.e. unleachable) derivative permits use of a wet-spinning process. The tin in the cast film was again shown by Mossbauer spectroscopyto be largely in the Sn (IV) state.

Measurements ofthe relative flammability of modified and unmodified PAN films (Table 2) showthat addition of stannous chloride enhanced the Critical Oxygen Index (COl) to a value of 22.2% from 17.5% for an unmodified film. The COl of the film prepared from a PAN SnCl2 (10%) pretreated with water was 21%.

A COl of 22 may not be sufficientforthe most demanding uses of acrylicfibres, but may be adequate for furnishings, upholstery and floor coverings. An important feature of the modified films is that they burned without melting to produce a skeletal char residue of reasonable mechanical strength, whereas PAN controls gave droplets oftarry residue, which dripped away from the descending flame front. The char-promoting effect observed in thermogravimetric studies of SnC12 - modified PAN was thus borne out in the very much more severe conditions of controlled combustion.

Although their chemical mechanism of action is unknown, it seems that char-promoting additives such as stannous chloride function predominantly in the solid phase, presumably by promoting the frequently postulated 'cyclisation' reaction of PAN leading to a "ladder" polymer. Inevitably, however, there are other competing reactions, probably proceeding via a free-radicai routes, whereby fission of the polymer backbone leads to production ofvolatile, flammable fragments. To suppress combustion ofthesefragments, and therebyfurtherenhance flame-retardancy, a different agent is needed, which for example may act in the vapour phase, by scavenging the highly reactive radicals responsibleforflame-propagation, a quite distinct mode of action from that of most char-promoters.

In a further embodiment of the invention, therefore, a combination of a tin halide and a radical inhibitor is incorporated into the PAN film orfibre. Many radical phase inhibitors are known; ammonium bromide, organohalogens such as organobromine compounds and organophosphorus compounds are particularly effective, particularly when used in conjunction with an oxide of antimony. They are illustrated by, but not limited to, the examples in Table 3.

In each case, further enhancement ofthe COI resulted. The PAN in these examples was a copolymer containing approximately 85-90% acrylonitrile. Modification of PAN homopolymer, i.e. 100% acrylonitrile, such as is used in the production of industrial quality acrylic fibres, with SnC12 (10%) and NH4Br(10%) produced a film with a COl of 25.2%.

Table3 Additives COI (%O2) None 17.6 Stannouschloride (10%) 22.2 SnCl2(10%) + Ammonium Bromide (10%) 26.4 " " + 3-Bromopropylamine hydrobromide (10%) 25.8 " " + Chlorendicanhydride(10%) 24.6 t n + Pentabromodiphenyl ether(10%) 24.0 If desired one or more additives can be included in the polymer which havethe effect of suppressing destabilisation.

The following examples further illustrate the invention.

Example 1 To a solution of a fibre-grade acrylic copolymer containing at least 85% acrylonitrile, (10 g) in N,N-dimethyl formamide (DMF. 40 g),was added anhydrous stannouschloride (1 g) dissolved in the minimum quantity of DMF.Following thorough mixing, the solution was de-aerated under reduced pressure, cast onto release paperata knife gap of 500 am,and dried in a fan-assisted oven at 1000Cfor20 minutes to give afilm 75-90 Fmthick (95 g/m2). When subjected to thermogravimetric analysis (TGA), at a heating rate of 99"C pew minute under a flow of dry air (25 cm3/min), a char yield of 79%, measured at 580"C, resulted. Measurement of flammability carried out according to ASTM D2863 on the said film, gave a Critical Oxygen Index (COI) of 22.2%andacontinuousfilm ofcharmaterial.

Example2 Example 1 was repeated with the exception that stannous chloride was replaced with lead (II) chloride (0.5 g). TGAofthe dried film gave a cha r yield, at 580"C, of 62% Example 3 Example 1 was repeated with the exception that stannous chloride was replaced with ammonium molybdate (0.5 g), which, being insoluble in DMF, was ground to a fine powder and passed through a 53 Fm test sieve prior to mixing with the polymer solution. TGA ofthe dried film gave a char yield, at 580"C, of 56%.

Example 4 Afilm prepared as in Example 1 was allowed to stand in deionised waterfor7 days at room temperature.

This leached film gave a char yield, at 580"C, of 66% by TGA, and a COl of 22.2% with production of a continuous film of char material.

Example 5 The acrylic polymer used in Example 1 g) was dissolved in DMF (35 g) and to this solution was added stannous chloride (1 g) in DMF (4.5 g).Afterthorough mixing, water (0.5 g) in DMF (0.5 g)was added with stirring and the solution leftto age for at least4 days. The aged solution was cast and dried as in Example 1; the resulting film gave a charyield, byTGA, of 76% and a COl of 21.0% (continuous film of char material).

Example 6 Example 1 was repeated with the exception that stannoud chloride was replaced with stannous bromide (0.5 g). TGA of the dried film gave a char yield, at 580"C of 74%.

Example 7 ExampleS was repeated with the exception that a second additivetribromoaniline (1 g), dissolved in the minimum quantity of DMF, was stirred into the aged solution, which was then de-aerated, cast and dried as in Example 1. The char yield, by TGA at 5800C, was 77% and the COI was 23.4% (continuous film of char material).

Example 8 Example 7 was repeated with the exception thattribromoanilinewas replaced with alumina hydrate (1 g) which, being insoluble in DMF, was added as a sieved (53 m particle size) powder. The char yield ofthefilm, by TGA at 580"C, was 75% and the COI was 24.9% (continuous film of char material).

Example9 Example 7 was repeated with the exception thattribromoaniline was replaced with triphenyl phosphite (1 g).Thecharyield ofthefilm, byTGA at 580 C, was 71% and the COI was 25.7% (continuous film of char material).

Asimilarfilm containingthesame level oftriphenyl phosphite but withoutstannous chloridewas prepared and this was found to give a charyield byTGA of 60% and had a COl of only 18.3%, illustrating the need to include the tin component for significant COI exaltation.

Example 10 Example 7 was repeated with the exception that tribromoaniline was replaced with zinc diethyldithiocarbamate (1 g). The char yield of the film, by TGA at 580, was 77% and its COI was 24.3% (continuous film of char material).

The films which were tested in the foregoing examples were extremely lightweight and correspond roughly to the woven material ofthe nature of a light voile. However, the problem that the invention seeks to deal with occurs predominantly with heavierweight material such as furnishings fabrics, carpets, overalls and the like. The results obtained in the examples provide a good indication that the invention will be even more effective with heavierweightmaterial.

Claims (22)

1. A polymer or copolymers of acrylonitrile in film, fibrous or other forum which includes an additive that will promote charring ofthe polymer instead of combustion and/or melting when the polymer is heated or ignited.

2. A polymerorcopolymer of acrylonitrile as claimed in claim 1, wherein the additive is dispersed or dissolved in a solution ofthe polymerpriortofilm casting orfibre spinning thereof.

3. A polymerorcopolymerofacrylonitrile as claimed in claim 1 or claim 2, wherein the additive comprises a pluraiity of substances.

4. A polymer or copolymer of acrylonitrile as claimed in any preceding claim wherein the additive comprises a halide or a complex halide of tin (II) ortin (IV) or a mixture thereof.

5. Apolymerorcopolymerofacrylonitrile as claimed in claim 4,wherein the additive isstannous chloride.

6. Apolymerorcopolymerofacrylonitrileas claimed in any preceding claim, wherein the additive is present in an amountoffrom 5to 30%w/w.

7. A polymer or copolymer of acrylonitrile as claimed in any preceding claim wherein the additive, if not already oxidised, is subjected to oxidative hydrolysis prior to film casting or fibre spinning ofthe polymer.

8. A polymerorcopolymerofacrylonitrile as claimed in any preceding claim, and comprising afurther additive which acts as a vapour phase flame retardant.

9. A polymer or copolymer of acrylonitrile as claimed in any preceding claim, and comprising an additional additive which acts to suppress depolymerisation.

10. A process for imparting flame retardancyto a polymeric material derived wholly or partly from acrylonitrile comprising incorporating in the polymer an additive which acts to promote charformation when said polymer is heated or ignited.

11. A process as claimed in claim 10, wherein the additive is dispersed in a solution of the polymer prior to casting orfibre spinning.

12. A process as claimed in claim 10 or claim 11, wherein the additive comprises a plurality of substances.

13. A process as claimed in any of claims 10to 12, wherein the additive comprises a halideora complex halide oftin (II) ortin (IV) or a mixture thereof.

14. A process as claimed in any of claims 1 Oto 13wherein the additive is stannous chloride.

15. A process as claimed in any of claims 10to 14, wherein the additive is present in an amount of from 5 to 30% w/w.

16. A process as claimed in any of claims 10 to 15 wherein the additive, if not already oxidised, is subjected to oxidative hydrolysis priorto film casting orfibre spinning of the polymer.

17. A process as claimed in any of claims 10to 16, wherein a furtheradditive which acts as a vapour-phase flame retardant is included in the polymer.

18. A process as claimed in any of claims 1 to 17 wherein an additional additive which acts to suppress depolymerisation is included in the polymer.

19. A polymer or copolymer of acrylonitrile substantially as described herein with reference to any one of the Examples.

20. A process for imparting flame retardancyto a polymeric material derived wholly or partlyfrom acrylonitrile substantially as described herein with reference to any one of the Examples.

21. Polymeric materials produced by the process as claimed in any of claims 10 to 18 or 20.

22. Article comprising polymeric materials as claimed in any of claims 1 to 9 or 19 or as produced by the process as claimed in anyofclaims lotto 18or20.