GB2373793A

GB2373793A - A method of crimping oxidised acrylic fibres

Info

Publication number: GB2373793A
Application number: GB0107697A
Authority: GB
Inventors: John Fagge
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-03-28
Filing date: 2001-03-28
Publication date: 2002-10-02
Also published as: GB0107697D0

Abstract

In a process for crimping oxidised acrylic fibres or tows of fibres, the fibres or tows are pre-treated by wetting, in the absence of applied heat, rather than heating as previously considered necessary. Oxidised acrylic fibres or tows 1 are sprayed 2 with water or other wetting agent 4 incorporating chemicals to enhance desired properties of the fibre material prior to passing through a stuffer box 3 where random crimps are formed in the fibres. Excess moisture may be removed in a heater 5 which may form part of a carbonising furnace producing carbon fibres from the oxidised acrylic precursor.

Description

A METHOD OF CRIMPING OXIDISED ACRYLIC FIBRES Field of the Invention The present invention relates to the treatment of acrylic fibres and especially oxidised acrylic fibres, for example in the preparation of carbon fibres. The invention is particularly concerned with methods of crimping oxidised acrylic fibres or tows of such fibres.

Background to the Invention Certain industrial processes require acrylic fibres to be crimped, either as an end product or as an intermediate step in a process leading to a specific end product. A typical example of the latter is in the production of carbon fibres, where crimping enables the fibres to be handled in various textile processes, due to the significant level of interfilament cohesion which is created, compared with straight filaments.

Traditionally, it has always been assumed that crimping has to be performed on acrylic fibres at elevated temperatures. Such fibres can be pushed into a stuffing box and, under pressure, form random crimps. The crimped fibres are then pulled out from the box and may be carbonised, typically in an inert atmosphere such as in a nitrogen furnace, to form carbon fibres. The carbon fibre process was first developed in the mid-1960s, and one area of development since then, has involved crimping oxidised fibres, in which, although refinements have been made to the process, it remains the conventional wisdom that heating of the fibres is an essential prerequisite for crimping.

In the 1980s, Dow Chemical introduced a "low" temperature process in which the fibres are heated to around 600-800oc. The resultant product exhibited different properties, especially as a flame barrier and/or thermal insulator. Prior to the carbonisation stage of the process, the fibres are"knitted", carbonised in the knitted condition, and then"un-knitted", resulting in a crimped carbonised fibre. The overall process is prohibitively expensive and is not regarded as commercially viable. The principle of crimping oxidised fibres has been know since around 1982.

In a particular process, the subject of European Patent No 0624666, the fibres are softened by heating, fed into a horizontal stuffer box maintained under 10-50Kg pressure, withdrawn without tension and finally heated without tension to create a

permanent, non-symmetrical crimped fibre. The precursor fibre is oxidised polyacrylonitril or a p-aramid.

Summary of the Invention In its broadest aspect, the present invention provides a fibre crimping process in which, instead of pre-heating the fibres prior to crimping, they are merely wetted.

In particular, the invention provides a method or process for crimping acrylic fibres in which a precursor fibre is wetted prior to crimping. The wetting step is carried out in the absence of applied heat. The wetting agent is preferably sprayed onto the precursor fibres.

The wetting agent may be water. Alternatively or in addition, other chemicals may be applied to the fibres during the wetting stage.

The invention also provides apparatus for crimping acrylic fibres or tows of acrylic fibres, comprising pre-treatment means to wet said fibres or tows in the absence of applied heat and crimping means to crimp fibres or tows pre-treated by said pretreatment means.

The pre-treatment means preferably comprises spray means for applying the wetting agent, but other means of applying the wetting agent might be used, for example an immersion bath or a lick roller system.

The crimping means preferably comprises a horizontal stuffer box adapted to apply a predetermined pressure to fibres within the box.

In the process, excess moisture remaining in the fibres after crimping may be removed by heating. This heating may occur in a subsequent carbonising step. Alterna tively, depending on their ultimate usage, the crimped fibres may be delivered still moist.

The crimped fibres may be conveyed to carbonising means or may be further processed for other purposes.

Brief Description of the Drawing The invention will be described with reference to the single Figure of the drawings which shows schematically an exemplary process according to the invention.

Detailed Description of the Illustrated Embodiment The present invention is a significant departure from current processes and methods for crimping acrylic fibres. It has been the long-held belief in the industry that pre-heating of the precursor fibres is an essential step in the crimping process. However, the present invention is based on the realisation that equally good results can be obtained if the fibres are merely wetted, rather than heated, prior to crimping. Wetting appears to have the same effect on the fibres as softening by pre-heating in the prior art. Moreover, the wetting process need be no more elaborate than a simple spraying operation.

It is acknowledged that in some processes, the heating stage may be performed by such techniques as the use of electrical hot-plates, infra-red heating, the application of steam or by a combination of such processes. However, in each case the fibres or tows are intentionally heated to a sufficient temperature to soften the fibres. In stark contrast, the present invention specifically avoids such heating.

Referring to the Figure, precursor fibres or tows 1 are delivered to a production line. The fibres are oxidised acrylic fibres, formed from acrylonitril and possibly incorporating other co-monomers. The fibres are pre-treated 2 prior to entry into a crimping station 3. The pre-treatment stage 2 in the prior art would comprise heating to between 60 C and 900, sufficient to soften the fibres prior to crimping. However, in accordance with the present invention, the pre-treatment stage 2 comprises a wetting stage.

As shown schematically, the fibres pass through a spray chamber or other suitable device 2 where they are sprayed with a wetting agent 4. This agent may simply be water. Water is more than adequate for the purpose of pre-treating prior to crimping. In fact, if the fibres are being crimped as part of an overall carbonising process, it is preferable that only water is used. However, if the crimped fibres have some other end use, it may be desirable for them to be sprayed with other chemical (s) to change the physical properties or the inter-filament cohesion of the product, or change the chemical composition thereof. Such techniques would enable a change in the crimp character or would enhance certain desired properties of the fibres. Under these circumstances,

the chemical (s) may be provided in the wetting agent or they may constitute the wetting agent. Examples of appropriate chemicals could include solutions of dimethyl acet amide, di-methyl formamide, di-methyl sulphoxide, nitric acid, zinc chloride, ethylene carbonate, or sodium thiocyanate.

After wetting, the fibres are conveyed to the crimping station 3. This may be a conventional horizontal stuffer box entered by the wetted fibres. Once inside, the fibres are compressed under a load of around 15-50kg on an area of approximately 0. 02m2 to cause random crimping of the wetted fibres. The process is continuous, with fresh fibres entering the box as crimped fibres exit. The crimped fibres are preferably withdrawn under light tension to conserve their structure or may alternatively be withdrawn under substantially zero tension so as not to impose stresses on the crimped fibres.

The crimped fibres may then pass to a drying station 5, where excess moisture remaining in the fibres after crimping can be removed. In the case where the crimped fibres or tows are to be carbonised, drying can be carried out as an integral part of the carbonising process. Alternatively, the fibres can simply be shipped whilst still moist.

Claims

CLAIMS 1. A process or method for crimping oxidised acrylic fibres or tows of oxi dised acrylic fibres comprising pre-treating the fibres or tows prior to crimping, the pretreatment step comprising wetting the fibres or tows in the absence of applied heat.
2. A process or method according to Claim 1, wherein the wetting step comprises spraying the fibres or tows with a wetting agent.
3. A process or method according to Claim 2, wherein the wetting agent is water.
4. A process or method according to Claim 2, wherein the wetting agent comprises one or more chemicals to enhance predetermined characteristic (s) of the material of the fibres.
5. A process or method according to any of the preceding claims, wherein the crimping step is performed by a stuffer crimper box.
6. A process or method according to Claim 1, further comprising a drying step to remove excess moisture in the crimped fibres or tows.
7. A process or method according to Claim 6, wherein the drying step is performed as part of a carbonising step.
8. Apparatus for crimping oxidised acrylic fibres or tows of oxidised acrylic fibres, comprising pre-treatment means to wet said fibres or tows in the absence of applied heat and crimping means to crimp fibres or tows pre-treated by said pretreatment means.
9. Apparatus according to Claim 8, wherein said pre-treatment means comprises spraying means to spray said fibres or tows with a wetting agent.
10. Apparatus according to Claim 8 or 9, wherein said crimping means comprises a stuffer crimper box.
11. Apparatus according to any of Claims 8 to 10, further comprising drying means to remove excess moisture from fibres or tows crimped by said crimping means.
12. Apparatus according to Claim 11, wherein said drying means comprises a furnace forming part of a carbonising apparatus.
13. A process, method or apparatus for crimping acrylic fibres or tows of acrylic fibres, substantially as herein described with reference to the drawing.