investigation of twisted monofilament cord properties made of nylon 6.6 and polyester
TRANSCRIPT
Fibers and Polymers 2011, Vol.12, No.8, 1091-1098
1091
Investigation of Twisted Monofilament Cord Properties Made of
Nylon 6.6 and Polyester
Berrin Yilmaz
Kordsa Global Endustriyel Iplik ve Kord Bezi San. ve Tic. A.S, Izmit, Kocaeli-41310, Turkey
(Received January 11, 2011; Revised May 13, 2011; Accepted June 22, 2011)
Abstract: A tire is a composite of rubber and reinforcing materials. Polymeric materials used as reinforcing components areusually synthetic yarns. These synthetic yarns have high tenacity, they are made of continuous multifilaments. The yarns areconverted in the cord form to provide desired mechanical, thermal and adhesion properties by a series of conversionprocesses. Besides of multifilament synthetic cords, there are some specific areas in which single ply monofilament cordshave been utilized as a reinforcing element. In this study, new cord structures have been developed by using monofilamentyarns and by imitating multifilament cords. New cord structures exhibited some very interesting cord properties compared toboth single ply monofilament cord and multifilament cords. Monofilament yarns having diameter between 0.23 and 0.50 mmhave been twisted together from 3 to 6 plies based on mixed Taguchi model to form cords. Nylon 6.6 and Polyestermonofilament yarns have been selected because of common application of their multifilament counterpart yarns in tirecarcass and capply. The twisted monofilament cords have been adhesive treated to produce cords ready to adhere rubber. Themechanical properties, thermal stability, adhesion with rubber, fatigue properties under dynamic conditions, retention ofproperties after curing and microscopic analysis of the cords have been analyzed. The cords have been found to possess somebenefits for tire carcass, breaker, belt and belt protective layer applications with their superior fatigue performance, cutresistance, stiffness, ligth weigth etc.
Keywords: Monofilament, Nylon 6.6, Polyester, Tire cord
Introduction
High tenacity Nylon 6.6 and Polyester synthetic continuous
multifilament yarns are well known reinforcing materials for
tire applications. Tire cords made of Nylon 6.6 synthetic
yarns are usually utilized in the cap ply zone of tire. The cap
ply cord structure, such as linear densities, ply numbers,
twist levels, mechanical and thermal property are adjusted
according to the type of tire and design of cap ply in the tire
etc. Polyester synthetic yarn is one of the basic materials for
radial passenger car tires as a carcass member. In recent
years, it seems that developments, particularly in adhesive
systems, make Polyester tire cords a candidate as a cap ply
of some specific tires. The application of steel in radial tires
in Europe goes back to the 1940s and then now steel has
became one of the basic reinforcing materials in radial tires
as a carcass and belt member over many years.
The monofilament cords have very limited and special tire
applications. The literature related to monofilament cords
and its tire applications are limited to patent applications.
The applications of monofilament cords in heavy duty off-
the-road (OTR) tires have been patented [1]. Since OTR
tires run on very harsh road conditions, sharp rocky objects
and uneven terrain cause the chipping and the chunking of
the tire. In order to improve the chip resistance of OTR tires
monofilament cords were applied over the belt and under the
tread of the tire. These monofilament cords have at least
2000 denier, 3,5 g/denier tenacity and adhesive treated with
an epoxy sub-coat and Resorcinol Formaldehyde Latex (RFL)
top-coat.
The obround shaped monofilament cord application in
radial medium truck tires has also been patented [2]. The
puncture of tire by any object from the road surface may be
deep enough causing the exposure of the steel belt package
to water and air. The presence of moisture causes the
corrosion of steel in the belt package and if extensive
corrosion happens, the tire is then scrapped. Therefore, the
replacement of at least one layer of steel belt with a non-
corrosive polymeric monofilament material has been
investigated. The results and observations of tires with
monofilament cords in belt package have been summarized
and were compared to reference tires. The analyses have
shown that there were no worn difference and no evidence to
reduce tire protection. Also, damage stayed localized when
reached to monofilament layer and monofilament layer
could be easily removed by rasping of tire.
Providing a thin and light weight reinforcing belt for
pneumatic tires by using non-metallic monofilament cords
having elongated cross-sections has been patented [3]. The
elongated cross-sectional cords have been expected to
improve the handling performance of tire by resisting lateral
movement and developing higher cornering force. The
useful life of the tire has been extended tire by providing
lighter reinforcing belt and less heat build-up.
Tires carcass reinforced with polymeric monofilament
cords have been investigated and patented [4]. Monofilament
cords with flat cross-section have comprised a plurality of
sub-monofilaments. Each sub-monofilament have circular*Corresponding author: [email protected]
DOI 10.1007/s12221-011-1091-3
1092 Fibers and Polymers 2011, Vol.12, No.8 Berrin Yilmaz
cross-section and interconnected radially to form a rectangular
shape. Those types of monofilament cords have exhibited
better adherence to rubber and have shown better resistance
to the separation from the rubber.
In this study, the properties of monofilament cords made
of nylon 6.6 and polyester have been investigated. The effect
of monofilament yarn diameter and linear density, number of
yarn plies to form a cord has been investigated based on a
type of Taguchi design. The properties of the cords have
been analyzed. The idea behind this study is to improve new
cord structures to fulfill some deficient properties of both
single ply monofilament cord and multifilament cords.
Patent literatures summarized above are related to the tire
application of either round shaped or obround shaped single
ply monofilament cords. On the other hand, the reinforcement
of tire rubber with multifilament cords is well known. The
twisted monofilament cords were developed as a novel cord
structures to fill the gap between single ply monofilament
cords and multifilament cords. The new cord structures have
a resemblance to single ply monofilament cords because
they are made of monofilament yarns. At the same time, the
new cord structures have a resemblance to multifilament
cords because of being composed of more than one ply of
monofilament yarns. On the other hand, although similarities
are available they are neither single ply monofilament cords
nor multifilament cords; they have different geometries and
properties. One of the weaknesses of single ply monofilament
cord is the fretting through the bulk of polymer matrix in the
presence of any deformation. Even though good adhesion
can be obtained between RFL coated cord surface and
rubber, the fretting from bulk of polymer of monofilament
creates an open and non-RFL coated cord section. As a
result, defect points created on the surface may cause further
degradation of single ply monofilament cord. Another weak
point is the possibility of crack propagation in a single ply
monofilament cords may proceed up to the cord breakage if
damaged by sharp objects. Thus, cord loses its reinforcing
properties. However, in case of twisted monofilament cords
the crack propagation is limited to only one of the plies and
even if after ply breakage the other plies retain reinforcing
property to a certain extent. Compared to the multifilament
cords, multiply monofilament cords have less frictional
surfaces because of limited amount of plies, this may bring
some fatigue advantages to the tire by positively effecting
the longevity of it. The replacement of one layer of belt ply
by a layer of monofilament cord for special tires decreases
the weight of the tire, thus reducing the rolling resistance
and contributing to less oil consumption.
Experimental
Material
Twisted monofilament cords were prepared by using
nylon 6.6 type G183 and polyester type L183 and K183
(Nexis Fibers, Switzerland). The thickness of G183 type
monofilaments were 0.23- 0.30- 0.40- 0.50 mm and the
corresponding dtex levels were 475, 810, 1420, 2240
respectively. Type L183 with 0.25 and 0.30 mm thickness
levels and type K183 with 0.40 and 0.45 mm thickness
levels were utilized in cord preparation. The dtex levels of
polyester monofilaments were 680, 970, 1730, and 2200.
Machine and Method
The twisting of monofilament cords were made by using
laboratory scale twisting machine (Oerlikon Saurer, Germany).
Monofilament yarns were twisted by combining from 3 to
6 plies with a 50 twist per meter (tpm) level to obtain desired
cord structure determined by the experimental design. The
twisted cords were heat treated by using laboratory scale
single end cord dipping machine (C. A. Litzler, Cleveland,
Ohio). During heat treatment water based RFL [5] solution
was used for nylon 6.6 cords. The dipping of polyester cords
was made first by using epoxy and isocyanate solution with
a solid content of 5 %, followed by RFL based adhesive
solution. The curing temperature applied during dipping
were 225oC for nylon 6.6, 235 oC and 225 oC for polyester
cord dipping.
Mechanical properties of samples were analyzed by using
Instron Mechanical Testing Instrument (Instron, Norwood,
MA). Mechanical tests were carried out in accordance with
ASTM D885, the grip distance was 254 mm and the grip
speed was 300 mm/min.
Thermal shrinkage of the yarns and cords were analyzed
by using Testrite instrument (Testrite Ltd., W. Yorkshire,
England). The cords were hold in the testrite instrument at
177oC for 2 min in the presence of 0.045 g/dtex pretension.
The dimensional change along the yarn axis was recorded as
thermal shrinkage of the yarns and cords. The shrinkage
force of the samples was also measured under the same
temperature, time, pretension conditions; 177 oC, 2 min,
0.045 g/dtex, respectively.
The bending stiffness of cords was measured by using
Instron equipment. A 3 cm cord in length was placed on a
platform where there was a hole in the middle. A hook was
placed on the cord through the hole. While the hook was
pulling down the cord, a force was exerted on the cord to
bend it. The force at a point where the cord began to bend
was recorded as bending stiffness force.
The pull adhesion test was carried out to measure the
adhesion strength of the adhesive treated cords. Each sample
was embedded into the rubber by 0.60-1.00 cm and cured at
143 oC, for 20 min at the pressure of 10.5 kg/cm2. After
curing was completed, the samples were cooled for 30 min
and then the adhesion of the cords was measured by pulling
out the cords from the rubber.
The dynamic performance of rubberized cords was carried
out by using dynamic flex fatigue tester (Wallace Instruments,
Redhill Surrey, England). It was a tire simulation test to
Nylon 6.6 and Polyester Twisted Monofilament Cords Fibers and Polymers 2011, Vol.12, No.8 1093
measure the fatigue performance of the cord under dynamic
conditions at which the rubberized cord sample blocks were
flexed around a pulley for a predetermined cycle [6]. The
fatigue testing was carried out at 70 oC for 150000 cycles. At
the end of the test, the adhesion and breaking strength of un-
flexed and flexed samples were measured by using Instron
equipment. The test results of the flexed samples were
compared with the test results of un-flexed samples. The
ratio of flexed sample test result to the un-flexed sample test
result was reported as % retained property.
The mechanical retention properties of cords after
rubberizing were analyzed by curing the cords in rubber test
in which cords were embedded in a rubber and cured at
177oC for 20 min at the pressure of 21 kg/cm2. The cords
were then pulled out from the rubber matrix. The retained
mechanical properties of the rubberized cords were tested by
using Instron instrument. The test results were compared
with the results of un-rubberized cord samples. The ratio of
un-rubberized to the rubberized samples was recorded as %
retained property.
The microscopic cross sectional analysis of the cords were
carried out by using Leica MZFIII microscope (Leica
Microsystems GmbH, Germany).
Experiments were designed by using Minitab statistical
program. Mixed Taguchi method was used as a design of
experiment technique. The orthogonal array used in this
study was L16 (24 12) involving 3 factors, 16 experiment.
The factors were chosen as material thickness (or dtex),
material ply number and material itself. First two factors
were designed with 4 levels whereas the last factor was
designed with 2 levels. Experimental layout of this design
was given in Table 1. Each array referred to an experiment
with predetermined factors and their levels. Factor A was
denoted as thickness of single monofilament yarn where the
thickness levels were explained above in material section.
Factor B was the number of monofilament plies to make
cords, the levels of design was 3, 4, 5 and 6. Factor C is the
monofilament yarn materials, the levels were nylon 6.6 and
Polyester. The test results of experimental design set were
analyzed by using the same statistical program. The main
effect factors and their levels on the cord properties were
determined based on the delta values of statistical analysis.
Delta values corresponded to the difference between the
mean values of two different levels. As delta value increased
the effect of the factor on the desired property also increased [7].
Results and Discussion
The tensile and thermal properties of monofilament yarns
were given in Table 2. The load-elongation (%) curve of
0.30 mm diameter Nylon 6.6 and Polyester yarns were given
in Figure 1. All yarns had similar load-elongation (%)
behavior therefore only one of them, 0.30 mm diameter, was
selected as an example. The breaking strength of the Nylon
6.6 monofilament yarns were between 3.3 and 13.7 kg
depending on the linear density (dtex) of the material. The
Table 1. Experimental layout of Mixed Taguchi L16 type design
Cord
number
Experiment
no
Factor A
(thickness)
Factor B
(ply number)
Factor C
(material)
Cord 1 1 1 1 1
Cord 2 2 1 2 1
Cord 3 3 1 3 2
Cord 4 4 1 4 2
Cord 5 5 2 1 1
Cord 6 6 2 2 1
Cord 7 7 2 3 2
Cord 8 8 2 4 2
Cord 9 9 3 1 2
Cord 10 10 3 2 2
Cord 11 11 3 3 1
Cord 12 12 3 4 1
Cord 13 13 4 1 2
Cord 14 14 4 2 2
Cord 15 15 4 3 1
Cord 16 16 4 4 1
Table 2. Tensile, thermal properties of Nylon 6.6 and Polyester monofilament yarns
MaterialThickness
(mm)Dtex
Breaking strength
(kgf)
Elongation at
break (%)
Lase 3 %
(kgf)
Lase 5 %
(kgf)
Breaking energy
(kgf-mm)
Thermal
shrinkage (%)
Avg/std Avg/std Avg/std Avg/std Avg/std Avg/std
NY 0.23 475 3.34 / 0.038 27.9 / 0.78 0.38 / 0.01 0.51 /0.01 119 / 6.0 2.53 / 0.08
NY 0.30 810 5.10 / 0.06 27.0 / 0.58 0.58 / 0.01 0.80 / 0.01 176 / 7.6 2.60 / 0.06
NY 0.40 1420 8.37 / 0.14 23.0 / 0.44 1.04 / 0.01 1.45 / 0.02 236 / 13.9 2.80 / 0.06
NY 0.50 2240 13.72 / 0.15 22.9 / 0.48 1.78 / 0.02 2.59 / 0.02 423 / 17.1 3.35 / 0.08
PET 0.25 680 2.61 / 0.03 35.8 / 1.05 0.71 / 0.02 0.81 / 0.01 152 / 7.8 -
PET 0.30 970 3.75 / 0.05 37.7 / 1.59 1.0 / 0.05 1.15 / 0.01 233 / 13.5 -
PET 0.40 1730 6.62 / 0.08 39.9 / 0.96 1.70 / 0.02 1.84 / 0.01 380 / 16.1 -
PET 0.45 2200 8.23 / 0.06 37.5 / 0.89 2.31 / 0.02 2.49 / 0.02 512 / 19.3 -
NY: Nylon 6.6, PET: Polyester, and Lase 3 % and Lase 5 %: Load at specific elongation (3 and 5 %).
1094 Fibers and Polymers 2011, Vol.12, No.8 Berrin Yilmaz
tenacity (breaking strength (g)/dtex) values of the Nylon 6.6
yarns were in the range of 6.0-7.0 g/dtex. The breaking
strength of polyester monofilament yarns were between 2.6-
8.2 kg within 680-2200 dtex range. The tenacity values of
polyester monofilament yarns were around 3.8 g/dtex.
Although the linear densities of both type of monofilament
yarns were close to each other at equivalent thickness levels,
the tenacity of polyester monofilaments were low. Elongation at
break values of Nylon 6.6 monofilament yarns was within
23-28 % range, whereas that of polyester monofilament
yarns had higher breaking elongations such as 36-39 %. The
initial elongations of Polyester monofilament were lower
than Nylon 6.6 monofilament yarn. For instance, 0.30 mm
Polyester needed 1 kg load to elongate by 3 %, whereas
Nylon 6.6 needed 0.58 kg load for the same percent
elongation. Up to 5-8 % elongation Polyester required more
load to elongate at the same percentage. After this point the
elongation behavior of the monofilaments was reversed. The
breaking energy of Nylon 6.6 and Polyester monofilament
yarns were between 119-423 kg-mm and 152-512 kg-mm,
respectively. The toughness of the material (breaking energy
divided by linear density) 0.17-0.25 kg-mm/dtex for Nylon
6.6 and 0.22-0.24 kg-mm/dtex for Polyester monofilament
yarns. Nylon 6.6 exhibited a decrease in toughness with
increased thicknesses; on the other hand Polyester possessed
more consistent values within a thickness range. Thermal
shrinkage of Nylon 6.6 monofilaments were within 2.5-3.3 %.
Polyester monofilaments did not have any retraction in the
fiber direction, instead elongated under thermal shrinkage
testing conditions.
As mentioned in experimental sections the cords were
prepared based on Table 1 arrays and the test results of these
cords were given in Table 3(a). The load elongation curve of
5 plies 0.25 mm twisted monofilament Nylon 6.6 and 4 plies
0.30 mm Polyester cords were given in Figure 2 as a typical
example. These two cord structures were selected particularly
out of the 16 cord samples because of having linear densities
close to each other. All Polyester and Nylon 6.6 twisted
monofilament load-elongation curves were similar to that of
given in Figure 2. Taguchi main effect analysis (Figure 3)
showed that, the breaking strength of the cords increased
Figure 1. Load/elongation curves of nylon 6.6 and polyester
monofilament yarns having 0.30 mm thickness.
Table 3. (a) Properties of adhesive treated twisted Nylon 6.6 and Polyester monofilament cords prepared in accordance with mixed Taguchi
design
Cord
number
Cord
definition*
Breaking strentgh
(kg)
Elongation at
break (%)
EASL 44N
(%)
LASE 3 %
(kg)
LASE 5 %
(kg)
Avg / Std Avg / Std Avg / Std Avg / Std Avg / Std
Cord 1 0.23×3, ny 8.98 / 0.10 23.2 / 0.64 11.6 / 0.04 0.9 / 0.00 1.5 / 0.01
Cord 2 0.23×4, ny 12.36 / 0.05 22.7 / 0.42 9.2 / 0.07 1.4 / 0.02 2.1 / 0.03
Cord 3 0.25×5, pet 12.41 / 0.07 12.6 / 0.47 2.6 / 0.09 1.9 / 0.12 2.56 / 0.17
Cord 4 0.25×6, pet 14.70 / 0.33 10.9 / 0.31 2.1 / 0.09 2.7 / 0.12 2.62 / 0.16
Cord 5 0.30×3, ny 14.46 / 0.17 20.4 / 0.35 7.4 / 0.05 1.8 / 0.01 2.8 / 0.02
Cord 6 0.30×4, ny 19.46 / 0.20 21.5 / 0.40 5.9 / 0.03 2.4 / 0.01 3.7 / 0.02
Cord 7 0.30×5, pet 17.59 / 0.09 24.1 / 1.2 1.7 / 0.05 6.9 / 0.08 9.5 / 0.05
Cord 8 0.30×6, pet 21.10 / 0.17 24.2 / 0.56 1.4 / 0.08 8.3 / 0.16 11.4 / 0.12
Cord 9 0.40×3, pet 20.49 / 0.12 24.0 / 0.66 1.6 / 0.11 7.3 / 0.16 10.0 / 0.1
Cord 10 0.40×4, pet 27.13 / 0.16 24.0 / 0.69 1.3 / 0.10 9.3 / 0.25 13.0 / 0.18
Cord 11 0.40×5, ny 43.10 / 0.23 20.7 / 0.36 1.9 / 0.07 5.7 / 0.08 8.61 / 0.11
Cord 12 0.40×6, ny 51.49 / 0.43 21.2 / 0.51 1.6 / 0.04 6.8 / 0.05 10.2 / 0.10
Cord 13 0.50×3, pet 25.81 / 0.19 23.4 / 0.63 1.1 / 0.11 9.6 / 0.27 13.1 / 0.19
Cord 14 0.50×4, pet 33.71 / 0.16 23.1 / 0.68 1.0 / 0.14 12.3 / 0.44 17.1 / 0.30
Cord 15 0.50×5, ny 67.37 / 0.32 21.1 / 0.45 1.4 / 0.16 8.8 / 0.22 13.3 / 0.33
Cord 16 0.50×6, ny 80.66 / 0.37 20.1 / 0.28 1.2 / 0.10 11.0 / 0.22 16.8 / 0.35*Monofilament yarn thickness×ply number, material.
Nylon 6.6 and Polyester Twisted Monofilament Cords Fibers and Polymers 2011, Vol.12, No.8 1095
with increasing thickness and ply number. The thickness had
highest influence (delta 7.90), then followed by ply number
(delta 3.94) and finally material type (delta 0.82). Different
breaking strength values could be interchangeable obtained
either by changing the thickness of monofilament yarn or the
ply number. For instance, 4 ply 0.23 mm Nylon 6.6 twisted
monofilament cord (cord 2) gave 12,4 kg breaking strength,
the same breaking strength was achieved by using 5 ply
0.25 mm Polyester monofilament cord (cord 3). The flexibility
of obtaining the same breaking strength with different
combinations seemed to be advantageous for a cord design.
The tenacity of the twisted monofilament cords were
calculated by dividing the total breaking strength of the cord
to the total linear density (dtex) of the same cord. Thickness
and ply number did not have any influence on the tenacity.
The tenacity values of the cords were driven by material
type, i.e. the values of Nylon 6.6 cords were between 6.6-
7.2 g/dtex, whereas that of Polyester cords were between
3.6-3.9 g/dtex (Table 3(a)).
Elongation at break values of both Nylon 6.6 and
Polyester twisted monofilament cords were very close to
each other. The breaking elongations of materials as a yarn
form were above 35 and 23 %, for Polyester and Nylon 6.6
respectively. After heat and adhesive treatment the breaking
Table 3. (b) Properties of adhesive treated twisted Nylon 6.6 and Polyester monofilament cords prepared in accordance with mixed Taguchi
design
Cord
number
Cord
definition*
Breaking energy
(kgf mm)
Shrinkage
(%)
Bending stiffness
(g)
Thickness
(mm)
Adhesion strength
(kg)
Avg / Std Avg / Std Avg / Std Avg / Std Avg / Std
Cord 1 0.23×3, ny 280 / 13.7 3.4 / 0.00 39.0 / 2.0 0.43 / 0.01 12.8 / 0.61
Cord 2 0.23×4, ny 364 / 12.1 3.4 / 0.01 53.0 / 2.0 0.48 / 0.01 17.4 / 1.06
Cord 3 0.25×5, pet 562 / 30.6 1.9 / 0.06 130 / 23 0.53 / 0.01 14.6 / 1.53
Cord 4 0.25×6, pet 569 / 82.9 2.0 / 0.06 243 / 62 0.67 / 0.01 18.2 / 0.99
Cord 5 0.30×3, ny 385 / 6.16 3.4 / 0.06 89.0 / 2.0 0.58 / 0.01 16.8 / 1.47
Cord 6 0.30×4, ny 585 / 23.2 3.5 / 0.00 117 / 2.0 0.63 / 0.01 17.6 / 1.78
Cord 7 0.30×5, pet 789 / 52.7 2.0 / 0.06 206 / 15.0 0.72 / 0.01 15.0 / 0.68
Cord 8 0.30×6, pet 953 / 29.6 2.0 / 0.06 292 / 50.0 0.83 / 0.06 15.2 / 0.71
Cord 9 0.40×3, pet 872 / 31.3 2.3 / 0.06 335 / 28.0 0.74 / 0.01 14.7 / 0.93
Cord 10 0.40×4, pet 1137 / 41.3 2.4 / 0.00 387 / 18.0 0.82 / 0.01 17.1 / 2.15
Cord 11 0.40×5, ny 1223 / 90.0 3.6 / 0.07 334 / 8.0 0.97 / 0.01 19.4 / 0.86
Cord 12 0.40×6, ny 1598 / 66.6 3.6 / 0.01 419 / 10.0 1.15 / 0.01 20.2 / 1.03
Cord 13 0.50×3, pet 1084 / 42.1 2.4 / 0.06 428 / 16.0 0.85 / 0.30 10.8 / 1.19
Cord 14 0.50×4, pet 1399 / 50.5 2.4 / 0.00 548 / 26.0 0.94 / 0.20 27.9 / 6.25
Cord 15 0.50×5, ny 2124 / 71.6 3.3 / 0.12 830 / 40.0 1.48 / 0.01 43.8 / -
Cord 16 0.50×6, ny 2327 / 115 2.8 / 0.15 1050 / 27.0 1.49 / 0.01 41.5 / -*Monofilament yarn thickness ×ply number, material.
Figure 2. Load/elongation curves of nylon 6.6 (0.25 mm×5 plies)
and polyester (0.30 mm×4 plies) twisted monofilament cords. Figure 3. Main effect analysis for the mean values of breaking
strength of the cords prepared in accordance with mixed Taguchi
design.
1096 Fibers and Polymers 2011, Vol.12, No.8 Berrin Yilmaz
elongations were stabilized within 21.3-24.2 % range for
Polyester cords and 20-23.2 % range for Nylon 6.6 cords
(Table 3(a)). Heat treatment conditions influenced Polyester
elongation values more than that of Nylon 6.6. The ranking
of parameters based on main effect analysis was obtained as
material type, ply number and thickness in the decreasing
order of delta values (2.19, 1.13 and 0.91) respectively.
Referring to the values given in Table 3(a) and curves
given in Figure 2, partial load elongation at 4.5 kg of
Polyester and Nylon 6.6 twisted monofilament cords had
significantly different values. Polyester twisted monofilaments
had very high initial modulus, low elongations compared to
Nylon 6.6 under the same load. Polyester twisted cord
retained its lower elongating character up to 10 %
elongation. After that point Nylon 6.6 twisted monofilament
cords exhibited higher modulus values than Polyester
twisted monofilament cords. The main effect analysis of
elongation at 4.5 kg of all cords proved that thickness of
monofilament (delta: 5.17) has the highest influence. Ply
number (delta: 3.88) and material type (delta 3.44) had
almost very similar influence on partial load elongations. As
the thickness of monofilament, ply number increased, partial
load elongation at 4.5 kg decreased. Load at specific elongation
at 3 and 5 % values also supported this difference. As the
thickness increased, the load required to elongate the cord by
3 or 5 % also increased. The ply number also caused the
similar change, as the ply number of cord increased, the load
for 3 and 5 % elongations increased. If the cord was made of
polyester, the cord had higher load for the same elongation
values. The ranking of delta values of main effect analysis
for load at 3 and 5 % elongations pointed out the following
decreasing order; thickness, ply number and material type.
Ductile behavior of twisted monofilaments was calculated
by dividing the elongation at break and elongation at partial
loads to the total linear density of the cords separately. Main
effect analysis denoted that the thickness of the monofilament,
ply number of the cord and Polyester material reduced the
ductile behavior per unit linear density. For instance; the
ranking of delta values driven from the main effect analysis
of elongation at partial load 4.5 kg was in the order of
thickness, ply number and material (0.34, 0.27 and 0.198),
respectively. The mean values of ductility cords for 0,25 mm
and 0.50 mm thickness monofilaments were 0.35 and 0.012
respectively. The mean values of ductility for 3 plies and 6
plies cords were 0.29 and 0.025, respectively. The mean
values of ductility of Nylon 6.6 was higher than Polyester
monofilament cords (0.23 versus 0.3).
The breaking energy of twisted cords (Table 3(b)) was
attributed to the energy absorption ability of the material
upon external disturbances. The graph related to the main
effect analysis for means of breaking strength of the Taguchi
design was given in Figure 4. As the thickness (delta 1289)
and ply number (delta 706) increased the breaking energy of
the cords increased also. Polyester twisted monofilament
cords have less breaking energy values than Nylon 6.6
twisted monofilament cords (delta 190). The breaking toughness
of twisted monofilament cords was calculated by dividing
the breaking energy to the total dtex value of the cords to
calculate the energy absorption capacity per unit linear
density. The toughness values of the cords were between
0.14 and 0.20 kg-mm/dtex. The main effect analysis proved
that the effect of material type had slightly higher influence
on toughness compared to monofilament diameter and ply
number.
The thermal shrinkage of twisted monofilament cords at
177oC, for 2 min, with 0.045 g/dtex pretention were obtained
between 1.9-3.8 % (Table 3(b)). Recalling the thermal shrinkage
of the yarn materials, Nylon 6.6 twisted monofilaments
retained their shrinkage after heat treatment. Polyester
twisted monofilament cords developed a certain level of
thermal shrinkage as an average value of approximately
2.0 %. Regarding to the statistical main effect analysis for
mean values, yarn thickness and cord ply number did not
have significant influence on thermal shrinkage. Material
type was dominating parameter, where Nylon 6.6 twisted
monofilament cords had high shrinkage, with an average
value of 3.3 %. The retraction force was measured under the
same thermal conditions. Nylon 6.6 monofilament cords
exhibited a retraction force between 0.3 and 3.2 kg, polyester
monofilament cords exhibited a retraction force between
0.4-1.5 kg depending on the composition of the cord. The
statistical analysis denoted that, as the thickness of the
monofilament yarn increased the retraction force also
increased. As the ply number of the cords increased the
retraction force decreased slowly.
The bending stiffness of twisted monofilament cords was
a distinguishing property of the cords (Table 3(b)) from the
multifilament cords. The thicker monofilament cords (delta
0.59) had higher bending stiffness values. As the ply number
increased (delta 0.28), the stiffness also increased. Twisted
Figure 4. Main effect analysis for the mean values of breaking
energy of the cords prepared in accordance with mixed Taguchi
design.
Nylon 6.6 and Polyester Twisted Monofilament Cords Fibers and Polymers 2011, Vol.12, No.8 1097
monofilament cords had higher stiffness compared to the
equivalent dtex multifilament cord. For instance, multifilament
cord with 1400 dtex×2 ply (total dtex 2800) structure
required 0.040-0.050 kg bending strength, Cord 5 with
0.30 mm×3 ply (total dtex 2430) required 0.09 kg bending
strength. In tire belt package applications stiffer material
may contribute to the generation of handling force, better
traction and tread wear performance. The stiffness of the
monofilament cords was also adjusted by changing either
the diameter of the monofilament or by the ply number of
the cords.
The adhesion of twisted monofilament cords was one of
the critical points compared to the multifilament cords. The
adhesion values of twisted monofilaments were given in
Table 3(b). A typical example of multifilament cords is 1400
dtex which composed of 210 individual filaments. After the
RFL adhesive treatment the adhesive solution penetrated to
at most 3 to 4 filaments, thus, chemical reactions were
developed between several filaments and adhesive. Another
attribute of multifilament cords were the surface roughness
because of filamentary nature, it increased the surface area
of cord and thus increased surface contact between rubber
and cord, and also increased the mechanical interlocking.
However, monofilaments did not have filamentary structures;
their surface was quite smooth and there was no chance of
adhesive solution to be penetrated inside the monofilament,
as a consequence less contact surface area for rubber
adhesion was available. In this study it was observed that the
adhesion of twisted equivalent dtex monofilament cords and
multifilament cords had similar adhesion values. For
instance, Cord 2 had 1900 dtex linear density and 17.4 kg
adhesion strength, the adhesion strength of multifilament
cord having 1880 dtex was expected to be on average 16.0 kg.
If the adhesion behavior of twisted monofilament cord was
compared to the single ply monofilament cord, twisted
monofilament cords exhibited higher adhesion values. For
instance single ply monofilament cord made of 0.35 mm
diameter and 2240 dtex linear density possessed 15.6 kg
adhesion strength. Although the dtex of Cord 2 was smaller
than 2240 dtex, Cord 2 had higher adhesion value. On the
other hand, the circumference of Cord 2 and 0.35 mm single
ply cord was measured as 1.96 and 1.1 mm. The difference
between the adhesion values was most probably originating
from the surface area in favor of twisted monofilament
cords.
The fatigue performance of twisted monofilament cords
were carried out under extreme conditions. The fatigue
behavior of Nylon 6.6 twisted monofilament cords was
analyzed as a function of monofilament yarn thickness and
cord ply number (Figure 5). The adhesion retention after
fatigue test was influenced by ply number and thickness. As
the ply number increased, the adhesion retention decreased,
considering all cord couples as Cords 1-2, Cords 5-6, Cords
11-12, Cords 15-16 the behavior was obvious. As the
number of plies increased, more contact surface was created
and more friction was realized. Therefore, under dynamic
conditions more heat was produced and most probably the
produced heat caused adhesion degradation. The thickness
of monofilament yarns also affected the adhesion retention.
As the thickness increased, the adhesion retention decreased;
i.e. examples of cord couples such as Cords 1-5, Cords 2-6,
Cords 11-15, Cords 12-16. If the adhesion retention of
twisted monofilament cord was compared to the multifilament
cord at equal linear densities, it was observed that twisted
monofilament cords performed better by retaining their
properties more. The Cord 5 having 2430 dtex linear density
possessed approximately 79 % adhesion retention after 150000
cycle of fatigue, a multifilament cord having 1400×2
structure (total 2800 dtex) possessed approximately 43 %
adhesion retention after 130000 cycle of fatigue. The
breaking strength retention of twisted monofilament cords
realized between 80-100 %. Only 0.50 mm 5 and 6 plied cords
had low breaking strength retention, the rest of the cords
retained their breaking strength. If a comparison was made
with multifilament cord having similar linear density;
1400×2 cord retained the breaking strength by approximately
83 % after 130000 cycles, whereas Cord 5 did not show any
breaking strength loss after 150000 cycle fatigue. Twisted
monofilament Nylon 6.6 cords had superior performance
under dynamic conditions compared to multifilament cords.
However, the fatigue performance of Polyester twisted
monofilaments was not as successful as Nylon 6.6 case.
During fatigue testing, fatigue sample piece turned around a
pulley simulating shoe shine motion. Polyester cords
embedded into rubber matrix had brittle failure at the end
points of shoe shine motion. Most probably high bending
stiffness of Polyester prevented flexible movement of the
cord and as a consequence brittle failure occurred. Therefore,
a reliable fatigue data could not be obtained for twisted
monofilament cords.
The breaking strength retention of twisted monofilament
Figure 5. Adhesion strength retention and breaking strength
retention after the fatigue test of nylon 6.6 twisted monofilament
cords prepared in accordance with mixed Taguchi design.
1098 Fibers and Polymers 2011, Vol.12, No.8 Berrin Yilmaz
cords under static conditions was analyzed by cure in rubber
test. Cords 1, 2, 3, 4 four samples were selected out of the
prepared cord samples. Cords 1, 2, 3 and 4 retained the
original braking strength; cord 4 retained approximately
90 % of its original breaking strength.
The alignment of each ply in a cord cross-section was
analyzed by using microscopic analysis. Some examples of
cross-sectional view of 3, 4, 5, 6 ply cords were given in
Figure 6(a)-(d). The appearances of these cross-sections were
very similar to steel cord cross-section, but different than
that of multifilament cord cross-sections. Twisted monofilament
cord made of 3 plies yarn has triangular cross-section, the
cord made of 4 plies yarn has rhomboid structure. The
twisted monofilament cords with 5 plies of yarn have
pentagonal structure which might not be stable. Because the
hole in the middle of plies was big enough and one of the ply
might shift to the hole, thus cord cross-section might change
to rectangular shape. In case of 6 plies twisted monofilament
cord, the cross-section had a circular shape and one of the
plies were located in the middle of the cord without leaving
a hole as in case of 5 plies.
Conclusion
In this study the properties of twisted monofilament cords
made of Nylon 6.6 and Polyester were investigated. The
twisted monofilament cords were made of monofilament
yarns and those monofilament plies were twisted together
and RFL dipped by simulating multifilament cords. Actually
twisted monofilament cords were interpreted as a cord
having properties between monofilament and multifilament
cords. It was observed that, cord properties were adjusted
flexibly by playing with monofilament diameter and the
number of plies in a cord. The load-elongation (%) behavior
of nylon 6.6 and Polyester monofilament cords were
different from each other. The breaking strength of cords
was changed proportionally between 9 and 80 kg with the
diameter of monofilament yarn and with the number of
monofilament ply. The Polyester monofilament cords exhibited
rayon like high initial modulus. The thermal shrinkage of
adhesive treated cords was almost on similar levels. The
adhesion of the designed cords was on acceptable level
compared to multifilament cords and single ply monofilament
cords. Nylon 6.6 twisted monofilament cords exhibited
prevailing fatigue performance under dynamic conditions.
High fatigue resistance was a desired property for carcass
and aircraft tire application and twisted monofilament cords
might contribute to this application. The cross-sectional
analysis showed that cord geometry was shaped by the ply
number of the monofilaments forming the cord. Since
twisted monofilament cords were made of several plies, they
had better cut resistance compared to single ply monofilament
cords for protective layer of belt package of special tires.
The bending stiffness of twisted monofilament cords was
adjustable by diameter and ply number of monofilaments.
High bending stiffness of twisted monofilaments might
contribute to the handling performance of tires. As a
replacement material of one of the steel belt layer in the belt
package of a tire, lighter tires was possible to be produced
and might contribute to less energy consumption of the tire.
Twisted monofilament cords can be a candidate for different
tire applications.
Acknowledgement
The author would like to thank all Kordsa Global R&D
Laboratory technicians for their valuable observations and
contributions.
References
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i
Figure 6. Microscopic cross-sectional views of twisted monofilament
cords (a) cord made of 3 ply monofilament yarn, (b) cord made of
4 ply monofilament yarn, (c) cord made of 5 ply monofilament
yarn, and (d) cord made of 6 ply monofilament yarn.