Process for producing a multifilament yarn

Description:

The invention pertains a process for producing a multifilament yarn from a molten thermoplastic polymer comprising melt-spinning of the thermoplastic polymer, extruding the molten thermoplastic polymer through a spinneret to form a plurality of filaments, and passing the filaments through a cooling zone to enable the spun filaments to be solidified to produce an undrawn yarn, applying a primary spin finish to the undrawn yarn and continuously drawing the undrawn yarn in one or more stages to produce a drawn yarn, applying a secondary spin finish during or after drawing and subsequently winding the drawn yarn.

Such a process is known from U.S. Pat. No. 6,077,468, wherein a molten thermoplastic polymer fiber is extruded through a spinneret to form a plurality of filaments. The filaments are cooled, typically by passing through an air quenching apparatus maintained at or slightly below room temperature. The filaments are then bundled and directed across guides or kiss rolls, whereupon they are treated with a primary spin finish. After receiving the primary spin finish treatment, the filaments are generally stretched. Stretching may be accomplished over a number of godets or pull rolls that are at elevated temperatures (e.g., from 85 -115 ⁰ C) sufficient to soften the thermoplastic polymer. By rotating the rolls at different speeds, stretching of the filaments can be obtained.

Spin finishes can be applied to fibers at different stages of the production process, depending upon what balance of performance properties are demanded from the fiber at that particular production stage. A primary spin finish is generally applied to the fibers soon after they are extruded from the spinneret, cooled, and bundled,

but prior to stretching the fiber. The primary spin finish reduces fiber-to-metal or fiber-to-ceramic friction while the fiber travels along the early stage production equipment. Furthermore the static electricity is reduced and the cohesion of the filaments is improved.

While ideally the primary spin finish would have properties, which eliminate the need for any secondary spin finish, this is not always possible. For example, fiber- to-metal or fiber-to-ceramic friction may be different from the required friction of the final article. A primary spin finish must be optimized to allow the initial stages of yarn production to proceed in an efficient manner. If the succeeding stages have different requirements, a secondary finish will have to be applied. A secondary spin finish will also have to be applied if the primary spin finish is removed, or almost removed, during a processing step. Application of a secondary spin finish is often necessary during the later stage production (i.e., after stretching, crimping and texturizing of the fiber).

As primary and secondary spin finishes often aqueous solutions, emulsions or solutions in organic solvents are applied. Aqueous solutions and emulsions have the drawback that they require often troublesome emulsification steps and are sensitive to bacterial growth. By using solutions in volatile organic solvents air pollution will be increased. Additionally and in particular, when being used as secondary spin finish, the hot yarns while travelling at high speed through the emulsions or solutions cause extra evaporation and splashing of the solvents with all the detrimental effects attributed to this effect.

In order to tackle this problem U.S. Pat. No. 6,077,468 proposes to use low melting, high solids spin finish compositions instead. Those compositions exhibit a melting point in a range from 25 to 140 ⁰ C.

While possibly avoiding the problems related to splashing and evaporation of the solvents, other drawbacks of the prior art, as to mention the adhesion of the yarn to rubber and the homogeneity of the finish application to the running yarn still remain to be solved.

According to this invention, the improvement lies in a process as described in the introductory paragraph or in the preamble of claim 1 , wherein the yarn temperature when entering the secondary spin finish is greater than 150 ⁰ C, that the secondary spin finish contains one or more adhesion activating components in an oil based composition and that the oil based composition consists of components with an evaporation rate of less than 10 wt.-% at the given yarn temperature.

The evaporation rate is determined according to the so-called Noack volatility. The Noack volatility of an oil is defined as the weight loss of the oil when it is held under isothermal conditions at a given temperature for a period of 1 hour under a constant flow of air. The Noack volatility can be quantified according to ASTM D6375-05 "Standard Test Method for Evaporation Loss of Lubricating Oils by Thermogravimetric Analyzer (TGA) Noack Method".

Compared to the teaching of the prior art, that discloses e.g. a temperature range between 85 and 140 ⁰ C, the yarn temperature when entering the secondary spin finish in the process of the invention is considerably higher. At first sight, this seems to render the process even more problematic when looking at the splashing and evaporation problems described above.

On the other hand, however, the higher temperature of the running yarn and the presence of the adhesion activating agents in the secondary spin finish is a necessary condition for improved adhesion properties.

The temperature of the running yarn upon entering is preferably between 150 and 220 ⁰ C, even more preferably between 170 and 200 ⁰ C. This can be controlled by varying the distance between the finish applicator and the drawing section of the spin process. The closer the point of application lies to a heated godet, the more equals the yarn temperature upon entering the secondary spin finish that of the respective godet. As a consequence, it is possible to determine the yarn temperature when entering the secondary spin finish via the temperature of the heated godet in the drawing section.

"Drawing" in the context of the invention is to be understood in its broadest sense, both accomplished by a draw ratio greater and smaller than 1. While draw ratios

greater than 1 lead to an increase of the filament length, those smaller than one are commonly known as relaxation.

It is preferred that the yarn speed when entering the secondary spin finish is greater than 3000 m/min, more preferably greater than 5000 m/min, even more preferred between 5500 and 7000 m/min.

Multifilament yarns produced at speeds greater than 5000 m/min are suitable as tire yarns, whereas yarns produced at lower speeds are better suited for so-called technical applications, such as reinforcements for conveyor belts.

Preferably the oil based composition of the secondary spin finish in the claimed process is a neat oil. Using this type of oil based composition the problems related to splashing and evaporation of the solvent in every concentration can be eliminated.

As used herein, the term "neat oil" means a substance or a composition of substances that is liquid at room temperature and in particular does not comprise a solvent or any dispersion means. Further the neat oil preferably exhibits a cinematic viscosity in the range up to 200 mm ² /s.

The neat oil consists preferably of ethoxylated fatty esters. The neat oil may also contain additives typically found in a finish formulation, such as antistatic agents, antioxidants, UV stabilizer etc.

Most commercially available neat oils for the spinning process have the lack of a too high viscosity (friction problems) or a too high evaporation (high emission). An optimum composition of the neat oil should fulfil the requirements for frictional behaviour and environmental requirements.

Application of the secondary spin finish can for example be performed by a highspeed yarn finish applicator as described in WO 2002/024987 A1 , the contents of which is herein incorporated by reference.

Suitable adhesion activating components are known to those skilled in the art and encompass in particular epoxides, such as glycerol polyglycidyl ether, in combination with amine hardeners like fatty amines. A preferred embodiment, in particular when using a neat oil as the oil based

composition is, however, the requirement that the adhesion activating components should be soluble in the neat oil.

In the process according to the invention it is preferred if the drawing of the undrawn yarn is performed at a draw ratio in the range from 1.5 to 4.0. Depending on the spinning speed the wind-up velocity in such continuous process is then in a range from 3000 to 8000 m/min.

Typically, there is also a relaxation step or heat setting step of the drawn yarn before winding up takes place. The application of the secondary spin finish can take place before or after the relaxation step. If a multistage drawing is performed the application of the neat oil as secondary spin finish can be done after the first drawing stage or later. Preferably, the neat oil application takes place after the final drawing step is performed, i.e. before the winding-up is taking place. The person skilled in the art also refers to an "after oiling" step in such cases.

Although the inventive process is not restricted to a particular thermoplastic polymer, it is preferred that the thermoplastic polymer is a polyester or a co- polyester. Even more preferred as thermoplastic polymer is a polyester that contains 95 mol-percent or more ethylene terephthalate as a repeating unit.

In such a way so-called adhesion activated polyethylene terephthalate can be conveniently manufactured.

The process according to the invention is particularly suitable for those yarns that exhibit higher tenacity and for that reason it is preferred if the tenacity of the drawn yarn is 60 cN/tex and more (measured according to ASTM 885).

For economic reasons it is preferred that also as the primary spin finish a neat oil is applied, even more preferably if both the neat oils applied as the secondary spin finish and as the primary spin finish are of the same type. For the application of the neat oil in the primary step, commercially available finish applicator guides can be used.

Thus the process of the invention is a combination of after-oiling, higher temperature of the yarn and the use of an adhesion activating finish composition based on 100 % pure oil.

Preferably, the cooling zone of the process according to the invention comprises two parts, wherein in the first and upper part of the cooling zone a gaseous cooling medium, such as air, is directed in such a way that it flows through the multifilaments transversely and that the cooling medium leaves the multifilaments practically completely on the side opposite the inflow side, and in the second and lower part of the cooling zone the multifilaments being cooled further essentially through self-suction of the gaseous cooling medium surrounding the multifilaments. Such cooling process is described in more detail in WO 2004/005594 A1 , the contents of which hereby is incorporated by reference.

Further it is preferred, if between the spinneret and the beginning of the cooling zone there is tube of a length between 10 and 40 cm, which can be heated or not, in order to achieve a retarded cooling of the multifilaments under the spinneret before entering the cooling zone.