GB2221186A

GB2221186A - Polyamide monofilament with high tenacity and high fatigue resistance

Info

Publication number: GB2221186A
Application number: GB8916180A
Authority: GB
Inventors: Gregory N Henning
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1988-07-15
Filing date: 1989-07-14
Publication date: 1990-01-31
Also published as: BR8903439A; GB8916180D0; JPH0299610A

Abstract

A monofilament from substantially linear polyamide polymer having an RV of at least about 80 and branching of less than about 3 equivalents per 10<6> grams of monofilament, denier of 500-10000, and tenacity greater than 9.5 gms/denier.

Description

TITLE Polyamide Monofilament With High Tenacity And High Fatigue Resistance Background of the Invention The present invention relates to heavy denier, polyamide monofilaments and more particularly relates to heavy denier, polyamide monofilaments having high tenacity and high fatigue resistance.

Heavy denier polyamide monofilaments are useful for embedment in rubber such as in the manufacture of tires and other reinforced rubber goods. Typically, polyamide monofilaments are produced commercially by melt polymerization to produce a polyamide having a formic acid relative viscosity (RV) of between 50 and 70, subsequently spinning the melted polymer and then quenching and drawing the resulting monofilament. While monofilaments having a relative viscosity of up to 70 RV can provide adequate strength for many applications, a monofilament with increased tenacity and fatigue resistance is desired for use in tires in order to enable the reduction of the number of plies and cord ends in the tire.

Summary of the Invention In accordance with the invention, there is provided a monofilament from substantially linear polyamide polymer having an RV of at least about 80, a denier of between 500 and 10,000, a tenacity greater than about 9.5 grams per denier and branching of less than about 3 equivalents per 106 grams of monofilament.

Preferably, the polyamide is poly-(hexamethylene adipamide) and the monofilament of the invention has an RV of at least about 100 and branching of less than about 2 equivalents per 106 grams of monofilament. In another preferred form of the invention, the polyamide is poly- ( -caproamide).

Brief Description of the Drawings Figure 1 is a plot of the log shear stress versus log shear rate for poly(hexamethylene adipamide) which contains essentially no branching; and Figure 2 is a plot of slopes log shear stress versus log shear rate as shown in Figure 1 for poly-(hexamethylene adipamide) with amounts of branching expressed as moles of branching per million grams of polymer.

Detailed Description of the invention The high denier polyamide monofilament of the invention is spun from polyamide polymer. Suitable polyamides include poly-(hexamethylene adipamide) poly-(caproamide), poly-(tetramethylene adipamide) etc., and their copolymers. Preferably, the polyamide is homopolymer poly-(hexamethylene adipamide). Another preferred polyamide is poly-(-caproamide).

The polyamide employed in the monofilament of the invention is a substantially linear polyamide having a formic acid relative viscosity (RV) of at least about 80 and has branching of less than 3 equivalents per million grams of monofilament. Preferably, the linear polyamide has branching of less than 2 equivalents per million grams of monofilament. Relative viscpsity is intended to refer to the ratio of solution and solvent viscosities measured in capillary viscometer at 250C. The solvent is formic acid containing 10% by weight of water. The solution is 8.4% by weight polyamide polymer dissolved in the solvent.

As will become apparent hereinafter, the linear polyamide with the high RV imparts high tenacity and high fatigue resistance to the monofilament provided that the level of branching is less than 3 equivalents per million grams of polymer, preferably less than 2 equivalents per million grams of polymer. Further advantage is obtained when the RV is at least about 100.

The equivalents of branching of the polyamide is determined by observing the rheological behavior of the polymer. Polyamide melts generally show Newtonian flow behavior only under conditions of very low shear rate and shear stress. At shear stress greater than 1 x 105 dyne s/cm2, melted polyamide polymers are non-Newtonian and the degree of deviation from Newtonian behavior is increased by branching. The degree of branching can be determined from rheological data by measuring the extent of deviation from Newtonian behavior such as by plotting log shear stress vs. log shear rate. The plot is essentially linear over a shear rate range of about two decades and the slope of the line can thus be used as a measure of degree of branching.

It is necessary to adjust the water content of the fiber (or polymer) prior to the rheometry measurements in order to minimize molecular weight changes and yield satisfactory reproducibility. The procedure involves cutting the fiber (or polymer) into about 0.5 inch segments (1/8" flake in the case of polymer) and drying a sample 16 hours at 800C under vacuum to reduce the water content to below the equilibrium water content for the,RV of the polymer being tested. The sample is cooled in a desiccator then placed on a balance and allowed to regain sufficient water to adjust the water content of the final sample to the equilibrium water content for the RV of the polymer being tested.

Rheology measurements are made on fiber (or polymer) samples prepared as above at 2850C with an Instron Rheometer equipped with a 0.375 inch diameter cylinder and a 0.02 x 0.750 inch capillary. Measurements are made at 5 crosshead speeds usually over the range of 0.05 to 1.0 inches per minute. The cylinder contains sufficient polymer to permit repeating the measurements three times with each loading. The slope of the log shear stress vs. log shear rate line, calculated by linear regression analysis, is used as a measure of polymer branch content. Poly-(hexamethylene adipamide) polymer, prepared in an autoclave and having an RV of 47.4, is used as a control containing essentially no branching. This polymer gives a slope of the log shear stress vs. log shear rate plot of 0.81 as shown in Fig. 1.A calibration curve, Fig. 2, of branching, b, expressed in equivalents or moles (m) of branching per million grams of fiber or polymer was set up by plotting the slopes of log shear stress vs. log shear rate of poly-(hexamethylene adipamide) polymer samples modified with known amounts of a known branching agent, bis-hexamethylene triamine, opposite the amount of the branching agent in the polymer.

To determine the amount of branching in a fiber or polymer sample, the slope of log shear stress vs. log shear rate of the sample is determined and the amount of branching, b, in equivalents or moles per million grams of fiber or polymer is read from the calibration curve.

While the above relates to determining branching in poly-(hexamethylene adipamide) fiber or polymer samples, a similar method may be set up for any polyamide.

To prepare the monofilament of this invention, with its novel combination of high RV, high tenacity, and low level of polymer branching, requires a particular high RV polyamide and a special spinning process. Relative to the polyamide, conventional melt polymerization methods are not useful in preparing linear polyamide polymer of greater than 80 RV of the quality needed to prepare the high-tenacity, fatigue-resistant monofilament of this invention. Conventionally melt-polymerized polyamide polymer of greater than 80 RV contains a significant amount of branching and, thus, cannot be spun and efficiently drawn to monofilaments with greater than 9.5 gpd tenacity and high fatigue resistance. The branching is believed to be caused by a combination of a cyclization reaction, catalyst-induced branching and time/temperature above 2800C duringpolymerization. One method to avoid branching and prepare linear polyamide polymer is to use a two-step polymerization process comprised of: (1) Conventional melt-polymerization to prepare solid polymer of moderate levels of RV, up to a maximum of about 70 and preferably not greater than 50, followed by; (2) solid-phase polymerization at comparatively low temperatures of between 180-240tC to an RV of greater than 80, and preferably to an RV of 100 or more. A conventional autoclave or continuous-polymerizer method can be used in step (1). Suitable methods are disclosed in U.S.

Patent Nos. 3,509,107 and 3,357,955. Preferably, the polymer from step (1) should have an RV of 30 - 50. Solid polymer chips (or flake) from step (1) are then further polymerized in a tumbler-type polymerizer unit at a temperature of, for example, 2000C, with a stream of nitrogen flowing over the polymer chips at the rate of 3 L per hour per kg of polymer. The polymerizing polymer is sampled periodically and polymerization is continued until the desired RV of greater than 80, and preferably of about 100 or more, is attained. In this way, linear high RV polyamide polymer is prepared which is suitable for the preparation of the monofilaments of this invention.

The linear polyamide polymer of greater than about 80 RV prepared as above must be spun and drawn by special processes in order to achieve monofilaments having the unique combination of properties of this invention. A preferred spin-draw process involves the coupled steps of spinning, quenching, drawing and winding as disclosed in U.S. Patent No. 4,009,511, which is incorporated herein by reference. Process steps like those of the referenced patent provide sufficient draw for core orientation while providing a surface with reduced orientation are employed to achieve the high tenacity and highly fatigue-resistant monofilament of this invention.

The deniers of the monofilaments of this invention are in the range of 500-10,000. For use in tires, the denier of the monofilament is preferably in the range of 750-10,000.

While the cross-section of the monofilament can be one of a variety of shapes, it is particularly desirable that an obround cross-section be employed. Obround is a generally flat, ribbon-like cross-section with rounded corners. The obround cross-section makes it generally easier to manipulate the filament in end-use applications.

The polymer of the monofilaments of this invention is substantially linear, having less than 3 equivalents, preferably less than 2 equivalents, of branching per 106 grams of polymer. In addition, the RV must be high to achieve the objectives of this invention, greater than 80 RV and, preferably, at least about 100 RV. The combination of high RV and linearity of polymer are both needed to achieve the high tenacity of greater than 9.5 gpd and significantly enhanced fatigue resistance of this invention. For example, monofilaments made with 100 RV linear poly-(hexamethylene adipamide) polymer with less than 3 equivalents per 106 grams of branching exhibit 25% better fatigue resistance than monofilament made with conventional 70 RV polymer.By contrast, monofilaments from substantially linear 70 RV poly-(hexamethylene adipamide) even when spun and drawn by the preferred process of U.S. 4,009,511, have tenacities only in the range of 8.6 to 9.2 gpd. Moreover, that level of tenacity cannot be improved upon by employing non-linear 100 RV polymer containing greater than 3 equivalents of branching per 106 grams of filament, prepared by conventional melt-polymerization processes.

Preferably, the polyamide monofilaments of this invention are those prepared from poly-(hexamethylene adipamide) or poly-(-caproamide) polymers. More preferably, such monofilaments have an optically visible, porous surface layer of 3-15 microns as determined by the method disclosed in U.S. Patent No. 4,009,511, the orientation of which is lower than that of the core of the filament. The surface layer is further characterized by a parallel refractive index, n//, of less than 1.57 as determined by the test method given in U.S. Patent No.

4,009,511.

Claims

Claims:

1. A monofilament from substantially linear polyamide polymer having an RV of at least about 80, a denier between about 500 and about 10,000, a tenacity greater than about 9.5 grams per denier and branching of less than about 3 equivalents per 106 grams of monofilament.

2. The monofilament of claim 1 wherein said polyamide is poly-(hexamethylene adipamide);

3. The monofilament of claim 1 wherein said polyamide is poly-(C-caproamide). 2 or 3'

4. The monofilament of claim ltphaving an RV of at least about 100. of claims 1 to 4

5. The monofilament of any onelhaving an obround cross section. of claims 1 to 5

6. The filament of any oneAhaving a denier of from 750 to 10,000. of claims 1 to 6

7. The monofilament of any onethaving branching of less than about 2 equivalents per 106 grams of monofilament. of claims 1 to 7 of claims 1 to 7

8. The monofilament of any onejhaving an optically visible, porous surface layer about 3-15 microns thick having an orientation lower than the core of the filament, said surface layer having a parallel refractive index, n//, of less than 1.57.

9. A spun/drawn linear polyamide monofilament in accordance with claim 1 substantially as hereinbefore described.