Process for manufacturing carbon fibers

Abstract

Process for manufacturing carbon fibers, includes a first spinning step of a fiber of PAN precursor and a second oxidation/carbonization step of the fiber and the plant thereof. The spinning and oxidation/carbonization steps are performed directly in line and continuously, and hence without any stocking buffer area of a PAN precursor between the two steps. The spinning step is performed at low speed, so that the output speed from the spinning step, downstream of the stretching operations, is a speed falling within the range of the suitable processing speeds in the subsequent oxidation/carbonization step. Moreover, the spinning step is performed in a modular way on a plurality of spinning modules aligned in one or more rows, each spinning module having a productivity not above 10% of the overall productivity of the spinning step. In any individual spinning module, the fibers downstream of the spinning area follow zig-zag, rectilinear paths.

Claims

1. A process for manufacturing carbon fibres, comprising a first step of spinning a fibre of a PAN precursor, and a second step of oxidation/carbonisation of said fibre, wherein: a. said spinning and oxidation/carbonisation step is carried out directly in line and continuously, therefore without any stocking buffer area of PAN precursor between said first and second steps; b. said spinning step is performed at a low speed, so that the outlet speed from the spinning step, downstream of the stretching operations, is a speed falling within a range of suitable processing speeds in the subsequent oxidation/carbonisation step; c. said spinning step is performed in a modular way on a plurality of spinning modules (M) aligned in one or more rows (A, B), each spinning module (M) having a productivity not above 10% of the overall productivity of the spinning step; d. in each individual spinning module (M), the fibres downstream of the spinning area follow zig-zag, rectilinear paths through deflection and driving rollers (3-5), both in a horizontal direction and in a vertical direction, along which paths the various spinning treatments are carried out; and e. the fibre tows coming out of each spinning module (M) are arranged side by side with the fibre tows coming out of the preceding modules and/or the following modules (M), the fibre tows not undergoing lateral deviations with respect to the progress direction of the line, to form a single feeding belt (N) of the oxidation/carbonisation step.

2. The process of claim 1, wherein individual modules (M) in each of said rows (A, B) of modules are slightly offset with respect to one another in a crosswise direction, by an amount corresponding to the overall final width of the tows produced by each module (M).

3. The process of claim 2, wherein said rows (A, B) of aligned modules (M) are mutually superposed and each upper row (B) is overall offset in a crosswise direction with respect to the lower row (A), by an amount corresponding to the overall final width of the belt of tows (NA) manufactured in said lower row (A).

4. The process of claim 3, further comprising a drawing roller assembly (R) configured to align on a same plane the belts of tows (NA, NB) manufactured in each of said rows (A, B) of spinning modules (M).

5. The process of claim 4, wherein said outlet speed of the tows from the spinning step, downstream of the stretching operations, is a speed ranging between 5 and 20 m/min.

6. The process of claim 4, wherein the productivity of each spinning module (M) is not above 5% of the overall productivity of the spinning step of the process.

7. The process of claim 4, wherein each spinning module (M) comprises: a. a tank (1) arranged in a lower portion of the module comprising a coagulation bath of the PAN fibres, in which 2 to 8 spinnerets (2) aligned side by side are soaked; b. at least six sub-horizontal, rectilinear paths between deflection and driving rollers (3), progressing from the lower portion of the module to an upper portion of the module, along which a post-coagulation treatment, a pre-stretching treatment, three or more washing and wet-stretching treatments, and one or more final surface finishing treatments are respectively performed; and c. two vertical, rectilinear paths between pairs of deflection and driving rollers (4, 5), from the top of the module (M) to the bottom of the module and vice versa, along which a collapsing treatment, a steam stretching treatment, and finally a steam annealing treatment of the tows, are respectively performed.

8. The process of claim 4, wherein the productivity of each spinning module (M) is not above 2.5% of the overall productivity of the spinning step of the process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will in any case be more evident from the following detailed description of a preferred embodiment of the same, given purely as a non-limiting example and illustrated in the attached drawings, wherein:

(2) FIG. 1 is a perspective and schematic overall view of the spinning section of a manufacturing plant for carbon fibres according to the present invention;

(3) FIG. 2 is a perspective detail view of the end portion of the spinning section of FIG. 1;

(4) FIG. 3 is a schematic front view which illustratesin an enlarged scaletwo modules of the spinning plant of FIG. 1; and

(5) FIG. 4 is an axonometric view of the two modules illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(6) The object which the inventor set out to achieve with the present invention is to combine the two separate steps of the traditional manufacturing process of carbon fibres in a single in-line process, to thereby obtain a process in which the PAN precursor fibre produced in the spinning section can be supplied directly to the carbonisation section, hence without any type of stocking buffer of PAN precursor fibre between the spinning step and the oxidation/carbonisation step. As a matter of fact, only by achieving this object would it have been possible to fully achieve the main objects of the invention.

(7) The reasons for which this direct combination of the two steps of the traditional process into a single in-line process was neither possible nor conceivable, according to the known art, have already been described in the preliminary portion of this description.

(8) The inventor of the present invention has hence decided to distance himself fully from the traditional approach and has devised a new carbon fibre manufacturing process, characterised, in the spinning step of the PAN precursor fibre, by these fundamental innovative elements: a low output-speed in the final stretching step, i.e. a speed which falls within the range of suitable processing speeds in the subsequent oxidation/carbonisation step (currently 5-20 m/min); yarn-processing path which develops in a highly compact area, using both horizontal and vertical zig-zag fibre paths; modular spinning plant wherein each individual module, which can be joined in series, has a very low productivity (2-8 tows) with respect to the overall process productivity.

(9) An exemplifying diagram of a spinning plant wherein the innovative elements reported above are embodied, and by which the process of the invention can hence be carried out, is illustrated in FIGS. 1 and 2, while the detail of the individual spinning modules is illustrated in FIGS. 3 and 4.

(10) As can be seen in the attached drawings, the illustrated spinning plant, which is an exemplifying, non-limiting embodiment of the present invention, comprises two series of spinning modules, A and B, respectively, arranged one on top of the other and each one consisting of 22 adjacent spinning modules M. Each one of the spinning modules M is for example capable of producing 8 12K tows of PAN precursor.

(11) The overall number of the plant modules M is calculated considering the productivity of each individual module and the requested feeding flow rate of the carbonisation section of the plant. The productivity of each individual module M is preferably below 10% of the overall productivity of the spinning section, more preferably below 5% of such overall productivity and even more preferably below 2.5% of such overall productivity.

(12) According to a particularly interesting feature of the present invention, the individual modules M which make up each one of the series of modules A and B are slightly offset one with respect to the other in a crosswise direction, by an extent corresponding exactly to the overall final width of the tows produced by each module M which, in the example illustrated, is of about 41 mm. Thereby the tows produced by a module can be arranged exactly side by side to the ones produced by subsequent modules Mwithout imposing any lateral deviation to the sameso as to obtain, at the end of each one of the series of modules A and B, a continuous belt N.sub.A, N.sub.B formed by 822=176 tows and hence having an overall width of about 900 mm.

(13) The two series of modules A and B are furthermore mutually offset in a crosswise direction precisely by such distance, so that the belt of tows N.sub.B, coming out from the series of modules B above, can be arranged side by side to the belt N.sub.A, coming out from the series of modules A below, through a suitably arranged drawing roller assembly Rin this case, too, without imposing any crosswise deviation to belts N.sub.A and N.sub.Bso as to form a continuous belt of tows having a width of 1800 mm which is a typical belt size used for feeding the subsequent oxidation oven F of the carbonisation section, which section hence remains fully identical to the one of traditional processes. It is important to stress that the complete absence of crosswise deviations imposed on the PAN precursor fibres during the spinning process and hence during the transport process up to the oxidation/carbonisation oven F, allows to avoid any unevenness of the same, which unevenness would inevitably translate into an irregular crystal structure of the carbon fibres derived from said PAN precursor fibres and hence, in the last analysis, into non-optimal mechanical features of the same.

(14) As stated above, the spinning process occurs at a much lower speed than that of traditional plants and, in particular, at such a speed that the belt of tows N.sub.A N.sub.B coming out from the spinning section, i.e. after the stretching operations, has the inlet speed of oxidation section F of traditional plants, i.e. a speed typically ranging between 5 and 20 m/min.

(15) The structure of each individual spinning module M is immediately understandable from FIGS. 3 and 4 which show a preferred embodiment thereof.

(16) In the lower portion of each module M a spinning tank 1 is arranged containing the coagulation bath of the PAN fibre, wherein between 2 and 8 spinnerets 2 are soaked, arranged side by side. The tows formed by the filaments coming out from spinnerets 2 are collected from spinning tank 1 and are hence led into a path whichunlike what occurs in traditional spinning plantsdevelops both in a horizontal direction and in a vertical direction with a zig-zag path on a series of independently motor-driven rollers 3, 4 and 5. In the illustrated embodiment, 8 rectilinear, sub-horizontal paths are formed between pairs of opposite rollers 3 and along the same paths all the necessary operations, i.e. washing, stretching, drying, stabilising and finishing of the PAN precursor fibres, are performed through a series of devicesknown per se by a person skilled in the field and for this reason not described here in detailthrough which the fibres being formed are caused to pass, simultaneously subjecting them to the action of different aqueous solutions.

(17) In particular, in the first two rectilinear paths between rollers 3, immediately downstream of spinning tank 1, post-coagulation and pre-stretch treatments are performed, in the four subsequent intermediate paths washing and wet-stretching treatments are performed, while in the two final paths surface finish treatments are performed. At the end of this series of treatments the tows of fibres being formed, which have in the meantime arrived at the top of module M are brought back to the bottom of the same according to a rectilinear vertical path which extends between a first pair of stretching rollers 4 and a second pair of stretching rollers 5; the pair of rollers 4 is heated, so that when passing on the same the fibres are dried and caused to collapse (collapse=fibre density increase, under tension and heat, due to collapsing of the possible alveolar structure of the same generated by solvent removal).

(18) Along the rectilinear path between the pairs of rollers 4 and 5 a steam stretching device 6 is furthermore provided through which the fibres are caused to pass in order to undergo the final stretching determined by the rotation speed difference between the pair of rollers 5 and the pair of rollers 4. From the pair of rollers 5 the tows of PAN fibres are finally brought back to the top portion of module M, in a second, vertical, rectilinear path through a steam annealing device 7, and finally from here they are sent to the oxidation section together with those coming from the preceding or subsequent spinning modules M, of the same series A or B.

(19) Due to the fact that spinning is performed at low speed, the length of the treatment paths can be particularly short, despite maintaining satisfactory permanence times within the individual fibre-processing devices. This allows to limit the overall size of spinning modules M to particularly low values; as an example, in the illustrated embodiment the longitudinal dimension of the modules, or more precisely the pitch between two subsequent modules, is of 1250 mm, while the height of the modules is below 2200 mm.

(20) Since in each one of modules M there is a relatively low production of fibre, the width of rollers 3-5 can be easily dimensioned so as to houseeven in the first spinning steps where the fibre bulk is highesta larger number of lower-denier tows or of tows consisting of filaments having low linear density, so as to be able to keep the overall productivity of each module M constant, regardless of the number of processed tows and of the linear density of the individual filaments making up said tows.

(21) The overall length of a spinning plant according to the present invention is hence about 30 meters, also comprising a drawing roller assembly R which provides to arrange belts N.sub.A and N.sub.B side by side and to feed oxidation section F. Such overall length is not only much shorter than the one of currently used spinning plants, but even comparable to the one of the sole creel feeding traditional carbonisation plants. Using the process and plant according to the present invention it is hence possible to innovate the operation of existing plants at a very low cost and with a dramatic efficiency boost, both in terms of the quality of the finished product and of the cost of the same.

(22) As a matter of fact, it is evident from the detailed description reported above that the carbon fibre manufacturing process according to the present invention fully reaches the set main object, since the step of winding on the bobbin the PAN precursor at the end of the spinning step, is therein fully eliminated. The problems that such winding used to determine are hence cleared, both in terms of tow homogeneityand hence of the quality of the carbon fibre obtained from said PAN precursor fibresand in terms of the plant costs and running costs connected to the winding/transport/unwinding of the bobbins of PAN precursor.

(23) The manufacturing process of carbon fibres according to the present invention furthermore allows to achieve also the other additional objects of the invention and, in particular: a dramatically improved efficiency in case of tow breakage, since in this case it is not necessary to halt the entire production of the spinning section, as occurs in traditional plants, but only that of the individual module M affected, with a minimal loss of productivity which, for example, in the illustrated embodiment, is equal to about 2.3% of the overall productivity; a high process flexibility, i.e. the possibility to produce tows with a low denier or with filaments having low linear density without negative effects on productivity. As a matter of fact, the modularity of the proposed technical solution does not pose a substantial limit to the theoretic overall width of the spinning section, equal to the sum of the widths of the small rollers 3-5 used in each of modules Mwhereon the overall denier of the processed fibres can hence be maintained unchanged even working with low-denier tows or with filaments having low linear densitythereby providing spinning lines which are much more efficient than conventional spinning lines, where the maximum width of the rollers represents a limit for line productivity when working with low-denier tows. Moreover, the production of the above said low-denier tows or of tows with filaments having low linear density can be implemented only in a portion of the spinning plant modules M specifically adapted for this purpose, thereby improving plant flexibility also from this point of view.

(24) However, it is understood that the invention must not be considered limited to the particular embodiment illustrated above, which represents only an exemplifying embodiment thereof, but that a number of variants are possible, all within the reach of a person skilled in the field, without departing from the scope of the invention, as defined by the following claims.