BACKGROUND OF THE INVENTION
[0001] This invention is related to polyester yarn which has high strength and excellent
durability and is particularly suitable as a rubber-reinforcing material.
[0002] The fiber made from polyethylene terephthalate, or from a polyester in which it is
the main component, has excellent mechanical properties and thermal properties and
it is widely used in the manufacture of tire cords, V belts, conveyor belts and hoses.
In particular it takes the largest ratio in the application as the rubber-reinforcing
material where polyester fibers of high strength and excellent durability with balanced
heat shrinkage and modulus are required.
[0003] As a method of obtaining a polyester fiber of high strength, for example as seen
in US Patent No. 3216187, a method of spinning a polymer having a high degree of polymerization
under a low stress to suppress the molecular orientation and drawing the resulting
undrawn yarn to a maximum possible draw ratio has been previously proposed. However,
although fiber of high strength is obtained by this method, the fiber has a large
heat shrinkage because the amorphous section is highly oriented and one can obtain
only the fiber with inadequate durability.
[0004] For this reason, a method of spinning a polymer having relatively high degree of
polymerization under a high stress and drawing to a low draw ratio to lower the degree
of orientation in the amorphous section and reduce the heat shrinkage to obtain the
fiber of good durability has also been proposed. (U.S. Patent No. 4195052). With this
method, however, it is difficult to obtain fiber of high strength which is adequate
for use as a rubber-reinforcing material.
[0005] Also, when the polyester fiber is used as the rubber-reinforcing material, it is
necessary to subject it to a twisting process and a dip process (process of applying
adhesive and heat-treating). Thus, even if the original yarn has good performance,
the resultant dipped cord frequently cannot satisfy the required performance.
[0006] This invention seeks to provide a polyester fiber which has a high strength and excellent
durability with balanced heat shrinkage and modulus and which exhibits excellent performance
even after being made into a dipped cord.
SUMMARY OF THE INVENTION.
[0007] The embodiment of this invention is as follows:-
[0008] Polyester yarn which is made from polyethylene terephthalate or from a polyester
in which polyethylene terephthalate is the main component, the polyester fiber satisfying
the following property characteristics (a) - (f) simultaneously.
(a) Intrinsic viscosity of at least 0.91, preferably at least 1.0,
(b) Tenacity of at least 8 g/d, preferably at least 8.4 g/d,
(c) E2.25 of 4.5% or less, preferably 3.5% or less,
(d) ΔHmf of at least 11.5 cal/g, preferably at least 12.0 cal/g,
(e) (Tmf - TmF) of at least 20°C, preferably at least 22°C, and
(f) Amorphous orientation (fa) of 0.75 or less, wherein E2.25 is the elongation under a load of 2.25 g/d,
ΔH
mf is the amount of heat at the melting peak in differential scanning calorimeter (DSC),
Tmf is the melting point measured by DSC under a tension of 0.05 g/d, and
TmF is the melting point measured by DSC under no tension.
[0009] The polyester in this invention is polyethylene terephthalate or a polyester in which
polyethylene terephthalate is the main component (at least 90 mol percent) and various
types of dicarboxylic acid and glycol component can be copolymerized therewith in
an amount of up to about 10 mol percent. In order to improve the heat resistance,
it is preferred to reduce the end carboxyl group content of the polyester by reacting
with epoxy compound, carbonate compound, carbodiimide compound, iminoether compound.
[0010] In the following, the property characteristics of the polyester fiber of this invention
are explained.
[0011] First, the polyester fiber of this invention must have a high molecular weight having
an intrinsic viscosity of at least 0.91 as measured by 20°C by use of an equal weight
mixed solvent of phenol and tetrachloroethane. In particular, an intrinsic viscosity
of at least 1.0 is preferred. This is the required condition for the fiber to exhibit
high strength and for increasing the formation of tie molecules which participate
in the improvement of durability.
[0012] The strength is the value obtained using the method of JIS L-1017, i.e., the value
obtained by dividing the load at breaking in a load-elongation test with the measured
denier determined prior to the test. It needs to be at leat 8.0 g/d, preferably at
least 8.4 g/d.
[0013] Also, E
2.25 indicates the elongation at a load of 2.25 g/d in the load-elongation curve at the
strength measurement. The value is obtained by computer treatment. It needs to be
4.5 percent or less, preferably 3.5 percent or less. A low value of this property
means a high intermediate modulus, and it is a necessary condition for reducing the
dip elongation phenomenon in the dip treatment process and for achieving high strength
and high Young's modulus.
[0014] ΔH
mf is measured by use of Perkin Elmers DSC-2C differential scanning calorimeter with
3 mg of sample (original yarn) at a rate of temperature increase of 20°C/minute in
a nitrogen atmosphere. It needs to be at least 11.5 cal/g, preferably at least 12.0
cal/g. This value depends on the degree of crystallization of the fibers and it indicates
the degree of perfection of the microstructure of fibers mainly in the crystalline
section. A large value of this indicates that the fiber has high strength, high modulus
and high durability.
[0015] The melting points of fiber (Tmf and TmF) are measured as follows: 3 mg of the monofilament
of original yarn is wound over a copper plate of length 10mm, width 3 mm and thickness
of 0.5 mm, under a load of 0.05 g/d or under no load, to a width of 5mm; the front
end of the sample is tied and the excess copper plate is cut off; then, the measurement
is made under the same conditions as in the measurement of ΔH
mf. In the reference side cell of DSC, a copper plate of the same shape is inserted.
[0016] TmF is the ordinary melting point and Tmf indicates the super-heating property caused
by the tie molecules. With the fibers of this invention, the value of Tmf-TmF (= ΔT)
is at least 20°C, preferably at least 22°C characteristically. The value of ΔT is
an important indicator of the amount of tie molecules formed. The fiber having a high
ΔT has a strong connection of the crystalline section and amorphous section by the
tie molecules, and this contributes effectively to the prevention of breading phenomenon
which occurs at the boundary between crystalline and amorphous sections; as the result,
high strength and high durability are exhibited.
[0017] The amorphous degree of orientation (fa) is obtained by the following equation from
the birefringence Δ n, degree of crustallization X, the intrinsic birefringence of
crystal Δ nc (= 0.220), the intrinsic birefringence of amorphous section Δ na (0.275),
and the crystalline orientation function fc:-

Here, Δ n is determined by the Berek compensator method using an ordinary polarized
microscope. f
c is obtained from the following equation with the average orientation angle ϑ measured
by wide angle X ray diffraction, i.e. the average orientation angle ϑ obtained by
the analysis of average angle width of the diffraction pattern:-
fc = 1/2 (3 cos² ϑ-1)
X is determined from the following equation from the sample density ρ measured
at 25°C by a density gradient method using ligroin and carbon tetrachloride, the crystal
density ρc (=1.455 g/cm³) and amorphous density ρ a (=1.335 g/cm³):-

fa is related to the durability against the repeated elongation-compression fatigue
when the polyester fiber is used as the rubber-reinforcing material. With the polyester
fiber of this invention, f
a must be 0.75 or less.
[0018] As to the f
c of the polyester fiber of this invention, there is no particular restriction but
it generally exhibits a value of about 0.94. The value of X is generally about 0.40
to 0.45.
[0019] The polyester fiber of this invention which satisfies the above-mentioned characteristics
simultaneously has high strength and excellent durability and also has a balanced
heat shrinkage and modulus. Even after being made into the dipped cord, it exhibits
excellent performance.
[0020] In the following, the method of making the polyester fiber of this invention is described.
[0021] First, molten polyester of high intrinsic viscosity (at least 0.91) from a polycondensation
apparatus is directly fed to a spinning machine or is first made into chips which
are melted in an extruder and then is fed to a spinning machine. Spinning is effected
in a manner known per se at a take-up rate of about 700 to 5000 m/minute. In anticipation
of the drop in the viscosity which usually occurs in spinning, there is used a polyester
of somewhat higher viscosity than the desired intrinsic viscosity of the polyester
in the fiber to be obtained.
[0022] In the spinning, to achieve uniform cooling and enhance the uniformity of yarn strands,
it is necessary to have optimal combinations of the following conditions in connection
with the intrinsic viscosity of polyester and spinning speed: number of filaments
in the yarn, denier of filament, diameter and arrangement of the discharge hole of
spinning die, spinning temperature, length of the heated hood, length of the cooling
zone, temperature, speed and mode of blowing (circumferential blowing or side blowing)
of the cooling air.
[0023] In order to obtain the polyester fiber of this invention, it is necessary to use
the high stress spinning method and it is desirable that the stress on the spun yarn
is 0.05 to 1.0 g/d. As for the method of increasing the stress of spun yarn, one can
use a method involving increasing the spinning speed (take-up speed) or a method involving
increasing the rate of cooling of the spun yarn. In this way, the birefringence of
the resultant undrawn yarn is made to over 15 x 10⁻³, preferably 25 x 10⁻³ to 70 x
10⁻³.
[0024] Next, this undrawn yarn is drawn in one stage or multiple stages while heating by
heating roller, heating plate, steam jet. Drawing can be done by a spin draw method
which is conducted as a continuation of the spinning or by a two-process method in
which the undrawn yarn is first taken up and then drawn subsequently. As for the draw
ratio, an optimal condition is selected by combining the spinning stress at spinning
and the drawing temperature and time.
[0025] In the following example, the invention is described in further detail.
EXAMPLE
[0026] To the chips of polyethylene terephthalate having an intrinsic viscosity shown in
Table 1, N-glycidyl phthalimide was added in an amount indicated in Table 1. This
was fed to an extruder type melt spinning machine and was spun at a spinning temperature
of 300 to 310°C (set to the optimal temperature in this range) using a spinning die
having 252 holes of diameter 0.6 mm. Spun yarn was passed through a heated hood of
100 mm length at an atmosphere temperature of 300°C. Then, cooling air at 18°C was
blown from the circumferential direction at 40 m/minute velocity over a length of
300 mm. Spinning oil agent was applied by use of an oiling roller. Then the yarn was
taken up at a speed of 2000 m/minute to a take-up roller which was heated to 85°C.
Thus, undrawn yarn having a birefringence as shown in Table 1 was obtained. (At this
time, spinning stress was in the range of 0.08 to 0.20 g/d).
[0027] Birefringence of the undrawn yarn was measured by unheating the take-up roller and
winding the undrawn yarn on this and taking a sample from this.
[0028] Also, without winding up the undrawn yarn, the yarn was given a first stage drawing
to a draw ratio of 1.45 between the above-mentioned heated take-up roller and an unheated
No. 1 Nelson roller. Next, between the No. 1 Nelson roller and a No. 2 Nelson roller
which was heated to 240°C, the yarn was passed through 400°C steam jet apparatus to
effect a second stage drawing. Then, between the No. 2 Nelson roller and a conditioning
roller which was heated to 100°C, the yarn was subjected to a relation at a ratio
as shown in Table 1. The yard is then wound up. The draw ratio in second stage drawing
was adjusted to give a total draw ratio as indicated in Table 1 taking the relaxation
ratio into consideration.
[0029] The physical properties of the drawn yarn (original yarn) of 1000 d/252 f are shown
in Table 1.
[0030] The denier change produced by changing the relaxation ratio of drawn yarn was adjusted
by adjusting the extrusion rate. The end carboxyl group content in the drawn yarn
obtained was in the range of 9.5 to 10.8 g/eq/10⁶ polymer in all cases. To this original
yarn, downward twisting of 49 twists/10 cm was applied in the Z direction by use of
a ring twisting machine. Next, two yarn strands were combined and upward twisting
of 49/10 cm in S direction was applied to make a greige cord. This was dip-treated
by use of a single dipping machine made by the Litzler Company to obtain a dipped
cord.
[0031] The conditions of the dipping treatment and the method of evaluation of the performance
of dipped cord were as follows:-
Conditions of the Dipping Treatment.
(1) Dip Solution:-
[0032] A mixed solution consisting of 83 weight parts of resorcinol-formalin-Gentac latex
solution (1 weight part of the reaction product of resorcinol and fomaldehyde in a
mol ratio of 1:1.2, mixed with 4.3 weight parts, in solid content, of latex to prepare
a solution having 20 weight percent concentration; pH adjusted to 9.5 with NaOH) and
17 weight parts of Vulcabond E.
[0033] Gentac latex is a trade name of General Tire Company; it is a butadiene-styrene-vinyl
pyridine latex. Vulcabond E is a trade name of Vulnax Company; it is an ammonia-water
solution of 2,6-bis(2′,4′-dihydroxyl phenyl-4-chlorophenol having a solid content
of 20 weight percent,
(2) Conditions of drying and heat treatment in the dip treatment:-
[0034] Drying zone 80°C x 30 sec
Curing zone 240°C x 80 sec x 2 times
Dip tension 0.5 kg/cord
Performance of the Dipped Cord
(1) Tensile Properties
[0035] Measurement was done by the JIS L 1017 method. However, heat shrinkage was measured
under the condition of 180°C x 30 minutes.
(2) Fatigue Properties
[0037] Measurement was made by the JIS L 1017, Goodyear's Mallory Tube Method with tube
test angles of 60° and 90°at 850 rpm and the rotation was reversed at 30 minutes.
When the angle was 60°, the cord was taken out carefully from rubber at 3 hours of
fatigue and the tensile strength was measured to determine the strength retention
ratio.
[0038] Also, when the angle was 90°, the time (minutes) to the rupture of tube by fatigue
was measured.
[0039] The performance of the dipped cords is shown in Table 1.
[0040] As can be seen from the results of Table 1, polyester fibers which simultaneously
satisfy the properties defined in this invention have excellent total balance of the
strength, heat shrinkage, modulus and fatigue resistance in the dip cord performance
which expresses the performance as rubber-reinforcing material directly. Thus, they
are good as rubber-reinforcing materials; No. 5 - 8 have particularly good performance.

[0041] A. IV of chips; B. Amount of N-glycidyl phthalimide added; C. Birefringence of undrawn
yarn x 10³; D. Relaxation ratio (%); E. Total draw ratio; F-K. Original yarn properties;
F. IV; G. Tenacity (g/d); H. E
2.25 (%); I. ΔH
mf (Cal/g); J. Tmf-TmF(°C); K. Amorphous orientation fa; L-Q. Dip cord performance;
L. Tenacity (g/d); M. Elongation (%); N. Heat shrinkage (%); O. Modulus (g/d); P-Q.
Fatigue properties; P. angle 60° (%); Q. Angle 90°(%).
*It was impossible to take the cord out.
Note:
(1) Unit of the amount of addition of N-glycidyl phthalimide is wt%.
(2) No. 1-8 are examples of application; No. 9-11 are comparative examples; No.
12 is a reference example showing the example of commercial high strength type polyester
fiber.
[0042] This invention provides polyester fiber which has high strength and excellent durability
and balanced heat shrinkage and modulus and is suitable as rubber-reinforcing material
exhibiting excellent performance even after being made into dipped cord.