EP3433292A1

EP3433292A1 - Melt spinnable copolymers from polyacrylonitrile, method for producing fibers or fiber precursors by means of melt spinning, and fibers produced accordingly

Info

Publication number: EP3433292A1
Application number: EP16714305.6A
Authority: EP
Inventors: Mats Timothy Knoop; Antje Lieske
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2016-03-22
Filing date: 2016-03-22
Publication date: 2019-01-30
Also published as: JP2019513167A; US20190100856A1; WO2017162268A1; KR20180125482A; US11180869B2

Abstract

The invention relates to a method for producing thermally stabilised melt spun fibres in which polyacrylonitrile (PAN) fibres or PAN fibre precursors produced by melt spinning are treated in an aqueous alkaline solution containing additionally a solvent for PAN. The invention also relates to fibres which can be produced according to said method.

Description

Polymacrylonitrile melt-spinnable copolymers, process for making fibers or fiber precursors by melt spinning and correspondingly made fibers

The invention relates to a process for producing thermally stabilized melt-spun fibers in which melt-spun polyacrylonitrile (PAN) fibers or PAN fiber precursors are treated in an aqueous alkaline solution which additionally contains a solvent for PAN. Likewise, the invention relates to fibers that can be produced according to this method.

Carbon fibers are produced according to the prior art by thermal conversion of separately prepared precursor fibers. Materials for the precursor fibers are mainly PAN (co) polymers (acrylic precursor) and pitch. Acrylic Precursorfasern be commercially produced so far only by wet or dry spinning. For this purpose, a solution of the polymers with concentrations <20% either in a coagulation bath or spun a hot vapor atmosphere with the solvent diffusing out of the fiber. In this way, high-quality precursors are produced, but the costs of the methods are comparatively high. This results on the one hand from the required solvents and their handling, on the other hand from the relatively low throughput of solution spinning processes.

Due to the strong inter- and intramolecular interactions of the nitrile groups, the melting point of PAN at 320 ° C is above the decomposition temperature of the polymer. This means that a melt spinning of pure PAN is not possible, the polymer behaves not like a thermoplastic but as a thermoset. At the same time, however, the possibility of producing precursor fibers by means of melt spinning would mean a significant cost savings in the production of precursors, since the throughput in melt spinning is considerably higher and, in addition, no solvents are incurred, which cause costs for procurement and recycling / disposal.

For several decades, therefore, efforts have been made to make PAN susceptible to processing by melt spinning. In principle, it is necessary to distinguish approaches via an external plasticization (mixture of the polymer with additives) and via internal plasticization (copolymerization). In both cases, the interaction of the nitrile groups is disturbed, so that the melting takes place below the decomposition temperature of the polymer. An essential prerequisite for further processing into carbon fibers is the possibility of oxidatively stabilizing the fibers afterwards. This process is carried out at temperatures above 200 ° C and results in the formation of cyclic structures that allow the subsequent carbonization. Of course, this can only be achieved if the fibers do not melt at the stabilization temperatures - which, since the stabilization temperatures are generally higher than the processing temperatures for melt spinning, represents an additional problem to be solved.

Inner softening:

Internal plasticization is achieved by copolymerization with suitable comonomers. As early as 1970 GB 1,270,504 described the preparation of thermoplastic acrylonitrile copolymers for fiber production by melt spinning. 8-50% by weight of aliphatic / aliphatic cyclic alkenes and / or acrylates and 0.2-10 wt .-% sulfonic acid-containing monomers called comonomers. As application area textile fibers are addressed. In the application examples, methyl, ethyl, butyl acrylate, isobutene, vinyl acetate and propene are listed as comonomers; the spinning temperature at the nozzle was 200 to 240 ° C. However, such acidic group-containing copolymers are not suitable for a continuous melt spinning process because the melt viscosity is not stable but over time due to acid catalyzed by the acid group

Cyclization reaction of nitrile groups inevitably increases. In US

3,499,073 is the production of thermoplastic PAN copolymers with

Help an organometallic catalyst described. The polymers - including homopolymers of the PAN - could be processed to monofilaments at temperatures of 250 ° C to 295 ° C. GB 1,294,044

Acrylonitrile copolymers containing 60-70% of acrylonitrile, 25-30% of methacrylonitrile and 5-10% of acrylates or methacrylates, and softening points between

125 ° C and 175 ° C have. At the described softening point, however, the polymer is not present as a melt but as a flexibilized film. In US 4,107,252 copolymers of acrylonitrile with 12-18wt% styrene and 13- 18wt% isobutene are presented which have melting temperatures between 175 ° C and 260 ° C. However, the comonomer contents are far too high to be able to produce carbon fibers from such copolymers. Generally, it is believed that a PAN copolymer suitable for carbon fibers has an average chain length of> 9 consecutive

Acrylonitrile must contain. Realistically, only about 10 mol% comonomer content can be tolerated. EP 0 030 666 describes thermoplastic acrylonitrile copolymers with up to 96% acrylonitrile content for hoses and films prepared by grafting acrylonitrile onto a

Elastomer phase arise. Such polymers have branched structures due to grafting and are not suitable for fiber production. GB 2,356,830 describes the thermoplastic deformation of

Acrylonitrile polymers with 96-100% acrylonitrile content by a particular pressure and temperature regime, however, the author excludes the possibility of use for melt spinning due to the necessary high extruder pressures. The largest internal plasticization patents are held by Standard Oil. No. 5,618,901 describes thermoplastic acrylonitrile copolymers consisting of 50-95% of acrylonitrile and 5-50% of a comonomer, wherein first of all all conceivable comonomer classes (acrylates, methacrylates, acrylamides, methacrylamides, acrylamide derivatives, methacrylamide derivatives, nyl esters, vinyl ethers, vinylamides, vinyl ketones, styrene, halogen-containing monomers, ionic monomers, acid group-containing monomers,

amino group-containing monomers, olefins and combinations). The examples give methylstyrene, styrene, vinyl acetate, methyl acrylate and methyl methacrylate with at least 15wt% share (corresponds to about 10 mol% for methyl acrylate) on the copolymer. The copolymers could be extruded at 200 ° C. US Pat. No. 6,114,034 describes the preparation of fibers from precisely these copolymers, with only methyl acrylate and vinyl acetate being used as comonomers in the exemplary embodiments, in a proportion of 15-25% by weight. It was possible to produce fibers with diameters of 3-8 dtex and strengths up to 29 cN / tex (15% methyl acrylate) or 55 cN / tex (25% methyl acrylate); the spinning temperatures were between 210 ° C. (55,000 g / mol) and 240 ° C (90,000g / mol). In WO 00/50675 the use of the above-described copolymers for the production of carbon fiber

Precursors disclosed. In terms of content, however, the patent does not go beyond US 6,114,034. Preferred comonomers are methyl acrylate, ethyl acrylate, methyl methacrylate and vinyl acetate. However, our own work shows that such copolymers are not stable

Have melt viscosity. A copolymer with 10 mol% of methyl acrylate and a molecular weight of 126,000 g / mol showed after 20 min at 200 ° C, an increase in complex viscosity by 35%, a copolymer with 20 mol%

Methyl acrylate and a molecular weight of 68,000 g / mol still an increase in the complex viscosity by 10%. This is also confirmed by the scientific literature (Bhanu et al., Polymer 43 (2002) 4841-4850).

External Softening: US 2012/0027944 A1 describes the use of ionic liquids in mass ratios of 1: 1 to 1: 0.25 for the external plasticization of PAN homopolymers. US 2007/0270529 Al describes the blending of PAN with aromatic engineering thermoplastics with the addition of

Compatibilizers and thermal stabilizers. In either case, after melt spinning, the polymer still contains the outer plasticizers, so that a lower quality precursor fiber and a reduction in carbon yield in the carbonization are inevitable.

Various patents (for example EP 0 406 709 A2; US 4,238,442; US Pat 4,163,770) describe the use of water as external plasticizer, either alone or in combination with other solvents such as

Nitroalkanes, glycols, etc. To process such plasticized PANs by melt spinning, a special melt spinning apparatus is required in which the fiber is spun into a print zone to allow first fiber forming; during the subsequent relaxation, the water evaporates in a controlled manner. For these processes, PAN copolymers were used which correspond to the classical PAN precursors used for wet spinning processes, which in addition to about 99-96% acrylonitrile units generally contain about 3-6% of a (meth) acrylate and about 0.5- Contain 1% of an acid group-containing monomer. Analogous to the internally plasticized systems operated above, problems which are attributable to instable melt viscosities always occur for the described systems with external plasticizers.

The oxidative stabilization of the fibers is for low-boiling plasticizers such as water and / or the o. G. said organic solvents thermoplastic fibers made no challenge; after evaporation of the outer plasticizer, the fiber is again infusible and can be stabilized analogous to solution-spun PAN fibers.

Overall, regardless of the type of plasticization, the thermal stability of the melt-spinnable copolymers is the decisive criterion for a technically feasible melt spinning process. Any thermal instability prevents a long-term stable spinning process. It leads to cracks in hotspots in the spinning extruder, the spinning pump and the spinneret and defects in the spun fiber. This problem arises with all the solutions of the inner and outer plasticizing described in the prior art. Based on this, it was an object of the present invention to provide precursors based on polyacrylonitrile, which show an improved suitability for melt spinning processes.

This object is achieved by the copolymer having the features of claim 1 and the method for producing fibers by melt spinning with the

Characteristics of claim 12 and solved by the fiber with the features of claim 17. The other dependent claims show advantageous developments.

Preferably, the copolymer, preferably with the addition of at least one outer plasticizer, in the temperature range of 160 to 220 ° Cverspinnbar.

The copolymer according to the invention preferably has, with the addition of at least one outer plasticizer, a constant or decreasing melt viscosity with increasing temperature up to 240 ° C., in particular up to 260 ° C. This proves that the copolymer of the invention has a particularly high stability of the melt viscosity.

Preferably, the copolymerization is carried out by a precipitation polymerization, an emulsion polymerization and / or a polymerization in a solvent. The solvent is preferably selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium thiocyanate solution and mixtures thereof.

The proportion of the at least one alkoxyalkyl acrylate is preferably 3.0 to 4.9 mol%, particularly preferably 3.5 to 4.8 mol%.

A further preferred embodiment provides that a mixture of comonomers of at least one alkoxyalkyl acrylate and at least one

Alkyl acrylate is present. The proportion of the at least one alkyl acrylate may preferably be in the range from 1 to 5 mol%.

A further preferred embodiment provides that a mixture of at least one alkoxyalkyl acrylate and at least one vinyl ester is present. The proportion of the at least one vinyl ester is preferably in the range from 1 to 5 mol%. It is further preferred that the copolymer has a weight average molar mass (Mw) in the range of 15,000 to 80,000 g / mol.

Likewise, it is also possible that the comonomers used are alkoxyalkyl acrylates in combination with alkyl acrylates and vinyl esters.

According to the invention, a method is also provided for producing fibers or fiber precursors by melt spinning, in which i. a copolymerization of 99.9 to 80 mol% of acrylonitrile with at least one comonomer selected from a) from 0.1 to less than 5 mol%, in particular from 0.1 to 4.9 mol%, of at least one alkoxyalkyl acrylate of the general formula I,

With

R = CnH2n + 1 and n = 1-8 and m = 1-8, in particular n = l-4 and m = l-4

10 b) 0 to 10 mol% of at least one alkyl acrylate of the general formula II With

R = CnH2n + 1 and n = 1-18, c) 0 to 10 mol% of at least one vinyl ester of the general

20 Formula III

With

25 R = CnH2n + 1 and n = 1-18, d) 0 to 10 mol% of at least one vinyl ether of the general formula IV

• R IV

30

With

R = C _n H ₂ n ₊ i and n = 1-18,

in which the sum of the comonomers a) to d) is at most 20 mol

35%.

in the presence of at least one initiator, ii. the copolymer is spun into mono- or multi-filaments with a melt spinning unit containing an extruder, a melt pump and at least one nozzle suitable for spinning.

It is possible that immediately before or during the extrusion in (ii.) At least one plasticizer is added, which is in particular selected from the group consisting of water, acetonitrile, nitroalkanes, alkyl alcohols, ionic liquids, glycols, dimethyl sulfoxide, dimethylformamide, dimethylacetamide , N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium thiocyanate solution and mixtures thereof. If a plasticizer is added, the spinneret can be followed by a pressure chamber to prevent explosive evaporation of plasticizers boiling below processing temperatures.

In the case where the fibers are carbon fibers, the following two subsequent steps are preferably carried out: i. a stabilization of the filaments by a temperature treatment at temperatures of 200 to 350 ° C and ii. a carbonization of the filaments at temperatures of 800 to 1200 ° C.

The stabilization of the filaments is preferably carried out at temperatures of 220 to 320 ° C, more preferably in the range of 250 to 300 ° C.

It is further preferred that in the copolymerization in step (i.) The radical polymerization initiator is selected from the group consisting of azo compounds, peroxides, hydroperoxides, alkyl peroxides,

Peroxodicarbonates, peroxyesters, dialkyl peroxides, persulfates,

Perphosphates, redox initiators and mixtures thereof.

According to the invention fibers are also provided, in particular carbon fibers, which can be produced by the method described above.

With reference to the following example, the subject matter according to the invention will be explained in more detail, without this being based on the specific embodiment shown here. restricting forms of example

A PAN copolymer having the following characteristics was prepared in an emulsion polymerization with a final solids content of 20%:

Composition: 4.4 mol% methoxyethyl acrylate, 3.1 mol% methyl acrylate, 0.5 mol% itaconic acid

The polymer dispersion was precipitated in 0.1% aluminum sulfate solution, the precipitated polymer washed extensively with water and ethanol and dried in vacuo at 50 ° C.

The relative viscosity of a 1% DMSO solution at 30 ° C is 1.8.

The polymer powder was mixed in a mixer with acetonitrile and water such that the total mixture contained 14wt% acetonitrile and 14wt% water. Subsequently, the polymer could be melt-spun using a 40-hole die at a die temperature of 190 ° C., the melt spinning unit used being equipped with a pressure chamber downstream of the spinneret in accordance with US 5,168,004. The fibers had a single filament denier of 5 dtex, a round cross-section, and a compact, void-free internal structure.

Claims

claims

Melt spinnable copolymer of polyacrylonitrile (PAN) preparable by a copolymerization of 99.9 to 80 mol% of acrylonitrile with at least one comonomer selected from a) 0.1 to less than 5 mol% of at least one alkoxyalkyl acrylate of all formula I.

With

R = C _n hi and n = 1-8 and m = 1-8, in particular n = l-4 and m = l-4 b) 0 to 10 mol% of at least one alkyl acrylate of the general formula II

R = C _n H ₂ n + i and n = 1-18, c) 0 to 10 mol% of at least one vinyl ester of the general formula

III

R = C _n H _{2n +} i and n = 1-18, d) 0 to 10 mol% of at least one vinyl ether of the general formula IV

With

R = C _n H ₂ n ₊ i and n = 1-18,

wherein the sum of the comonomers a) to d) is at most 20 mol% and the copolymer has a weight-average molar mass (Mw) in the range of 10,000 to 150,000 g / mol.

Copolymer according to claim 1,

characterized in that the copolymer in the temperature range of 160 to 240 ° C, preferably with the addition of at least one outer plasticizer, is spinnable.

Copolymer according to one of the preceding claims,

characterized in that the copolymer, preferably with the addition of at least one outer plasticizer, having a rising temperature to 240 ° C, in particular to 260 ° C constant or decreasing melt viscosity.

Copolymer according to one of claims 2 or 3,

characterized in that the external plasticizer is selected from the group consisting of water, acetonitrile, nitroalkanes, alkyl alcohols, ionic liquids, glycols, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone,

Ethylene carbonate, propylene carbonate, aqueous sodium thiocyanate solution and mixtures thereof.

5. Copolymer according to one of the preceding claims,

characterized in that the copolymerization is carried out by a precipitation polymerization in an aqueous medium, an emulsion polymerization in an aqueous medium and / or a polymerization in a solvent.

6. Copolymer according to the preceding claim,

characterized in that the solvent is selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate,

Propylene carbonate, aqueous sodium thiocyanate solution and mixtures thereof.

7. Copolymer according to one of the preceding claims,

characterized in that 3.0 to 4.9 mol%, preferably 3.5 to 4.8 mol% of the comonomer in a) are present.

8. Copolymer according to one of the preceding claims,

characterized in that 1 to 5 mol% of the comonomer in b) are present.

9. Copolymer according to one of the preceding claims,

characterized in that 1 to 5 mol% of the comonomer in c) are present.

10. Copolymer according to one of the preceding claims,

characterized in that 1 to 5 mol% of the comonomer in d) are present.

11. A copolymer according to any one of the preceding claims,

characterized in that the copolymer has a weight average molar mass (Mw) in the range of 15,000 to 100,000 g / mol.

Process for producing fibers or fiber precursors by melt spinning, in which i. a copolymerization of 99.9 to 80 mol% of acrylonitrile with at least one comonomer selected from a) 0.1 to less than 5 mol% of at least one

Alkoxyalkylacrylats of the general formula I,

With

R = CnH _{2n +} i and n = 1-8 and m = 1-8, in particular n = l-4 and m = l-4 b) 0 to 10 mol% of at least one alkyl acrylate of the general formula II

With

R = C _n H ₂ n + i and n = 1-18, c) 0 to 10 mol% of at least one vinyl ester of the general formula III

With

IV with

R = C _n H ₂ n ₊ i and n = 1-18,

wherein the sum of the comonomers a) to d) is at most 20 mol% is carried out in the presence of at least one initiator, the copolymer with a melt spinning plant containing an extruder, a melt pump and at least one suitable for the Verspin nen nozzle to mono- or multi Filaments is spun.

Method according to claim 12,

characterized in that immediately before or during the extrusion in ii) at least one external plasticizer is added, which is in particular selected from the group consisting of water, acetonitrile, nitroalkanes, alkyl alcohols, ionic liquids, glycols, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium thiocyanate solution and mixtures thereof.

Method according to one of claims 12 or 13,

characterized in that the fibers are carbon fibers, wherein in further steps iii. a stabilization of the filaments by a temperature treatment at temperatures of 200 to 350 ° C and iv. a carbonization of the filaments takes place at temperatures of 800 to 1200 ° C.

15. The method according to claim 14,

characterized in that the stabilization of the filaments at temperatures of 220 to 320 ° C, in particular from 250 to 300 ° C.

16. The method according to any one of claims 12 to 15,

characterized in that the initiator is selected from the group consisting of azo compounds, peroxides, hydroperoxides, alkyl peroxides, peroxodicarbonates, peroxyesters, dialkyl peroxides, persulfates, perphosphates, redox initiators and mixtures thereof.

17. fiber producible by spinning the copolymer according to any one of claims 1 to 11.

18. Fiber according to the preceding claim,

characterized in that the fiber is a carbon fiber.