ES2379506T3 - Heat treatment to feed the compressive strength of PPTA filaments - Google Patents

Abstract

The invention pertains to a method for obtaining a PPTA yarn with increased compressive strength by the steps of air gap spinning a PPTA spin dope to PPTA filaments, bundling the filaments to the yarn, washing and optionally neutralizing and drying the yarn, characterized in that these steps are followed by applying a heat treatment to the PPTA yarn comprising heating the yarn at a temperature of 340 to 510 ºC for 5 sec to 5 min under exclusion of oxygen. The invention further relates to a PPTA yarn thus obtained having filaments with an average compressive strength à c of at least 0.8 + 2.5*10 -3 *(E-100) GPa, wherein E is the modulus in GPa.

Description

Heat treatment to increase the compressive strength of PPTA filaments

The invention relates to a method for obtaining a thread of PPTA [poly) para-phenylene terephthalamide)] with increased resistance to compression and to the PPTA wire thus obtained.

It is an object of the present invention to provide PPTA threads that have filaments with better compression properties. It was found that this objective could be achieved if the yarn receives a further treatment in which the yarn is subjected to particular heating conditions.

Heating a PPTA thread is a usual process to remove water that adheres to the thread after neutralization and washing during the spinning process. Such heating processes are typically performed at about 150 to 250 ° C for usually no longer than a few seconds.

Spinning threads from an anisotropic solution of high molecular weight aromatic polyamides are known in the literature. The process of preparing fully aromatic polyamides is described by Kwolek et al., In US Pat. UU. 3,063,966. The processing for spinning threads of fully aromatic polyamides from anisotropic solutions is described in US Pat. 3,154,610 and in US Pat. 3,414,645. The preparation of anisotropic solutions of aromatic polyamides is described in US Pat. RE 30,352.

Several attempts to improve the compression properties of polymer threads in general and PPTA threads in particular are known. Sweeny described the thermal removal of halogens from aryl moieties followed by cross-linking of halogen-free aryl moieties, in order to create an intercatenary covalent bond between aryl moieties and increase compressive strength (Sweeny, W. , Improvements in compressive properties of high modulus yarns by crosslinking. J. Polym. Sci., Part A: Polym. Chem., 1992, 30 (6): pp. 1111-1122).

DJ. Sweeney at al., In High Perf. Polym., 14, 133-143 (2002) performed a heat treatment of an unmodified PPTA yarn (Kevlar-29 fiber) to improve the compressive strength of the yarn. The wires were slowly heated to 400, 440 or 470 ° C and kept at that temperature for 5 to 10 minutes. The wires were slowly brought to these temperatures using a heating rate (ramp) of 2.5 to 7.5 ° C / min. Therefore, the minimum period in which the threads were maintained between 340 and 400 ° C was 13 minutes and the period between 360 and 400 ° C was at least 10 minutes. Therefore, the threads were heated for relatively long periods and only moderate improvements to compressive strengths between 0.31 and 0.63 GPa were achieved.

The usual heating conditions have been described in US Pat. 3,869,430. The wires are heated within a zone maintained at a temperature of at least 150 ° C under a tension of at least 0.45 cN / dtex. The use of 150 ° C for a period of 60 seconds under a tension of 9 cN / dtex has been found satisfactory for a 171 dtex thread. Higher temperatures can be used, but using very short heating times. Thus, the use of an area at 650 ° C has only given good results with a 360 dtex thread and for periods of 0.6 to 1.0 second at 5.5 cN / dtex. The prolongation of time at this high temperature leads to a higher module, but also to a significant loss of tensile strength. Therefore, zone temperatures as high as 800 ° C or more could only be used with sufficiently short times and low tensions. The use of high temperatures and long times in these conditions leads to excessive degradation of the wire resulting in losses of tensile strength and / or inherent viscosity.

EP 2096199 describes a method for producing aromatic polyamide fibers containing heterocycles. This document does not mention the PPTA. EP 0247889 describes a method for producing PPTA fibers, in which a nitrogen / water vapor atmosphere is created in a heat treatment stage. EP 0759454 describes a method for producing a shaped para-aramid article, in which a heated gas heating stage is used. U.S. patent application 2004/0022981 describes a method for producing a composite material comprising single-walled carbon nanotubes and polyamide. In the US patent application 2004/0022981 a special atmosphere is not described during a heating stage.

Contrary to most of the prior art attempts where modified PPTA was used, the present invention allows the preparation of standard spinning solutions and wet spinning techniques with dry jet (air chamber) that can be used without modifications of aryl residues unlike described above. The threads were prepared from a PPTA that had a compressive strength as high as 1.3 GPa or even higher.

The standard search for compressive strength of filaments of PPTA has been published in a work of

M.G. Northolt and D.J. Sikkema in Adv. in Pol. Sci., 98, page 336 (1990). These authors describe that the compressive strength of polymer filaments increases when the module does so, but for PPTA filaments the compressive strength Oc does not exceed the maximum value of approximately 0.85 GPa.

without using halogenated or otherwise modified aryl moieties, said treatment being fundamentally different from the usual heat treatments that are performed to dry the thread.

For this purpose the invention relates to a method for obtaining a PPTA thread with the characteristics of claim 1.

Therefore, this method makes the PPTA threads have a better compressive strength by applying a heating process for a relatively long time with relatively low tensions. This heating stage is an additional stage of the stages normally used to prepare PPTA threads, including spinning in an air chamber, which includes coagulating the spun filaments and grouping them into a thread, followed by a

or more process steps well known as washing, neutralization, drying, winding and the like.

The optimum improvement in compressive strength is obtained when the heat treatment of the thread is carried out at a temperature of 360 to 480 ° C for 20 seconds to 2 minutes. It was found that the results improved at a higher temperature at the lower end of the claimed period of time, therefore, the best results are obtained when the heat treatment is performed at a temperature of 410 to 440 ° C for 20 seconds to 1 minute, or even better when performed at a temperature of 425 to 435 ° C for 20 seconds to 1 minute. In general, shorter times are used at higher heating temperatures.

The best results are obtained when in addition to the temperature and the heating time the voltage is also controlled. The heat treatment is performed at a voltage of 0.1 to 0.5 cN / dtex.

The heating stage of this invention is carried out excluding oxygen. The presence of oxygen during this stage of treatment has a negative influence on the tenacity of the thread. Therefore, oxygen must be removed as much as possible from the heating device, such as an oven, for example by passing an inert gas, such as nitrogen, helium and the like. In any case the amount of oxygen should be brought to less than 1% by volume, preferably to less than 0.5% by volume.

PPTA threads were spun from anisotropic solutions of aromatic polyamides in sulfuric acid, but other solvents, such as N-methyl-2-pyrrolidone (NMP) / calcium chloride, can also be used. The aromatic polyamides were prepared using conventional polycondensation reaction techniques. Spinning solutions could be prepared by mixing the polymer with ice-cold sulfuric acid. The threads can be obtained by the known wet spinning process with dry jet.

The present method of additional heat treatment can provide PPTA threads in which the filaments have compressive strengths that had never been obtained before. Therefore, at any module the compressive strength of the filaments (of the PPTA threads) is superior to that of similar threads that have not undergone additional heat treatment. Therefore, the invention also aims at PPTA threads obtained by the method described above. Said thread has an average compressive strength O c of its filaments of at least 0.8 + 2.5 · 10-3 · (E-100) GPa, where E is Young's modulus in GPa. The term "average compressive strength" (O c) means the average of three values of compressive strength measured from 3 strands of the thread that are taken at random.

Therefore, according to M.G. Northolt and D.J. Sikkema common PPTA threads have filaments with a compressive strength of 0.5 GPa to a 50 GPa module, while this PPTA thread after heat treatment of this invention has a compressive strength of at least about 0 , 68 GPa. PPTA threads with a compressive strength of 0.95 GPa or higher are threads that have never been obtained before. Therefore, PPTA threads having filaments with compressive strengths of at least 0.95 GPa, more preferably at least 1 GPa are new. These threads with extremely high compressive strength filaments are obtained by selecting optimal heating conditions with respect to the temperature and the heating time.

The invention is further illustrated by the following non-limiting examples.

General method for heat treatment.

The treatment was carried out in a 3.66 m long oven. The oven contained nitrogen with a residual level of less than 0.3% by volume of oxygen.

Compressive strength measurement

The threads were equilibrated at 21 ° C and 65% relative humidity (RH). The compressive strength was determined by the elastic loop test (abbreviated ELT by the English expression Elastica Loop Test) according to D.J. Sinclair, J. Appl. Phys., 21 (1950) 380. In this test, a filament is curved or folded in a single asymmetric loop where the diameters are a few millimeters, submerged in paraffin, covered with a glass slide, and then gradually contracted . During the contraction, images are taken, followed by the measurement of the c and a axes, where the c axis is the largest diameter and the axis at the smallest diameter of the loop. The compressive strength O  It is calculated from c *, which is determined by taking the intersection of two lines that can be drawn through the c / a graph before and after the formation of the turn curve. The value of c * is obtained by semi-automatic tracing of both lines. Before the formation of the turn curve, the theoretical ratio c / a is 1.34. The compressive strength is calculated according to the equation:

5 Oc is the compressive strength in GPa E is Young's module in GPa from tensile measurements r is the initial radius of filament in mm, determined by optical microscopy c * is the value of the c axis at which the c / a ratio begins to deviate from 1.34.

10 Module measurement, toughness, EAB and resistance

The modulus, toughness, elongation at break (abbreviated EAB by the English expression elongation at break) and the resistance of the threads were measured according to ASTM D885.

Preparation of the spinning solution

In a mixing vessel, solid sulfuric acid was initially mixed at temperatures below 10 ° C

15 which had a concentration of 99.8% and high molecular weight PPTA. Subsequently, the temperature of the mixture of polyamide and sulfuric acid was allowed to rise to room temperature resulting in a dry sandy mass, for use in the preparation of the spinning solution. Alternatively, the PPTA and concentrated sulfuric acid can be mixed in a twin screw extruder at room temperature to directly obtain the spinning solution.

20 Preparation of the threads

The PPTA spinning solution was spun to obtain a strand of filaments of different linear densities and finally washed and dried in a conventional manner. The thread was heat treated in a tube oven at the temperatures and times indicated in Table 1 in an inert environment (max. 0.25% by volume of O2). The properties of these threads are shown in Table 1. PPTA is poly (para-phenylene terephthalamide).

25 Table 1 Mechanical properties of PPTA threads after heat treatment

CN / dtex voltage

Warm-up time, seconds Temperature, ºC Linear density, dtex Tenacity, mN / tex EAB% Module, GPa Resistance, J / g Compressive strength, GPa

- *

- - 89 2195 3.2 91 35.7 0.56

0.36

28 380 86  2019 2.5  115 24.9 0.94

- *

- - 71 2242 3.05 103 34.6 0.63

0.22

28 360 66  1978 2.3  123 22.7 0.98

(+) 0.75

28 360 66  1988 2.3  128 22.3 0.98

(+) 2.27

28 360 65  2103 2.2  135 22.8 0.86

0.22

28 400 65  2144 2.4  124 24.7 1.05

(+) 0.75

28 400 65  2085 2.3  130 22.8 1.01

(+) 2.27

28 400 65  2111 2.1  138 22.1 1.05

0.22

28 430 65  2052 2.2  128 22.4 1.25

(+) 0.75

28 430 65  2079 2.1  137 22 1.11

(+) 2.27

28 430 65 2032 2 145 19.9 0.99

(+) 2.27

56 400 65  2124 2.2  140 21.8 0.95

0.22

 56 430 64 1903 2 134 18.8 1.22

- *

- 1684 2030  3.09  91.3 30.8 0.5 #

(+) 1.8

28 410 1649 1700 1.99 122.4 16.6 0.87

(+) 1.7

28 430  1642 1670 1.95 123 16.0 0.96

 * Comparative example: without heat treatment. (+) Comparative example: Voltage outside the scope of the invention. # Dear

Claims (4)

1. A method of obtaining a poly (para-phenylene-terephthalamide) (PPTA) yarn with increased compressive strength by means of the air chamber spinning steps of a PPTA spinning solution to obtain PPTA filaments, the filaments to form the thread, wash and optionally neutralization and 5 drying of the wire, where these steps are followed by the application of a heat treatment of the PPTA wire comprising heating the wire at a temperature of 340 to 510 ° C for 5 seconds to 5 minutes, characterized in that the heat treatment is performed at a tension of 0.1 to 0.5 cN / dtex in an amount of oxygen less than 1% by volume. 2. The method according to claim 1, wherein the heat treatment is carried out at a temperature of 10 360 to 480 ° C for 20 seconds to 2 minutes.

3.

The method according to claim 1, wherein the heat treatment is carried out at a temperature of 410 to 440 ° C for 20 seconds to 1 minute.

Four.

The method according to claim 1, wherein the heat treatment is carried out at a temperature of 425 to 435 ° C for 20 seconds to 1 minute.

A poly (para-phenylene terephthalamide) (PPTA) yarn obtained by a method according to claim 1, which has filaments with an average compressive strength Oc of at least 0.8 + 2.5. 10-3 · (E-100) GPa, where E is Young's module in GPa.