rndian Journal of Fibre & Textile Research Vol. 29, March 2004, pp. 19-24

Effect of twist on the performance of tire cord yarns

A S Hockenberger' & S Koral

Department of Textile Engineering, University of Uludag, Bursa, Turkey

Received 23 October 2002; revised received 21 January 2003; accepted 5 March 2003

The effect of twi st on the performance of polyethylene naphthal ate, dimensionally stable polyester and high-tenacity polyester cord yarns has been studied. h is observed that the twi st has a very important effect on the tensile properties, creep properties and fracture mechani sm. An increase in the twi st leve l increases breaking elongation and creep rate but decreases break ing strength. Breaking mechanisms are also affected by the twist level. However, it is found that the material has the most important role on the cord performance.

Keywords: Creep property, Fracture mechanism, Polyester cord, Polyethylene naphthalate cord. Tensil e property

IPC Code: Int. CI. 7 D02G 3/48

1 Introduction Polyester (PET) fibres of high modulus and tenac­

ity are used in industrial applications, such as rein­forcements for rubber articles like tires and hoses. Dimensionally stable polyester (DSP) yarns show high modulus, low shrinkage, excellent dimensional, thermal and chemical stability, no moisture absorp­tion, high strength and very good abrasion resistance'.

Polyethylene naphthalate (PEN) is a high perform­ance and hi gh priced polymer with opportunities in several areas. Characteristic features of the molecular structure of PEN are the rigidity of polymer chains and the occurrence of two trans conformations of naphthal ate ring2

.

Both the polyesters (PET and PEN) are semicrys­talline with closely similar melting temperatures. Dif­ferences in the physical properties of PEN and PET have generally been attributed to the assumed in­creased rigidity conferred on PEN by its constituent naphthyl rings, which are clearly larger than the phenyl rings in PET3

.

The requirement for a tire cord may vary with the basic type of fibre reinforcement. For a tire carcass, the required fibre properties are high tensile strength, good fatigue, durability, dimensional stability, and low heat generation , growth and creep4.

In the present work, three very important and mostly used tire cord yarns, namely PEN, DSP and

aTo whom all the correspondence should be addressed. Phone: 4428176; Fax: +90-224-442802 1; E-ll1ai I :sengonul @uludag.edu.tJ

high-tenacity polyester, have been studied in terms of mechanical performance and an understanding of how does cord configuration affect final cord properties established. Effect of twist on tensile properties, creep behaviour, dimensional stability and fracture mecha­nism has also been studied.

2 Materials and Methods

2.1 Materials

Three different tire cords, namely PEN, DSP and high-tenacity polyester, were used for the study. Fig. 1 shows optical microscopic picture of 2-ply tire cord and Table I shows cord properties.

Tire cords were prepared by twisting the yarns into 2-ply construction with 350, 445, 470 and 530

Fig. I- Cross-section of a cord yarn

20 INDIAN 1. FIBRE TEXT. RES. , MARCH 2004

Table I-Cord spec ifica tions"

Parameter

dtex Filament count Tenacity, g/den Elongation-at-break, %

"Producter's spec ifica tions

Cord type High-tenacity DSP

PET 1100 192 8.1 14

1100 300 8.0 11

PEN

1100 140 10.2 6

Cord type

High-tenac ity PET

Table 2-Tensi Ie results

Twist level Breaking Breaking TPM strength e longa tion

kN %

350 0.162 17.363 445 0.165 20.747 470 0.157 20.030 530 0.153 21.400

Single yarn 0.089 14.6 16

turns/m twist levels by direct cabling equipment. Just DSP 350 0.150 14.431

60 turns/m was applied to the single yarns to keep the filaments together. No thermal process was applied to the cords throughout thi s study. The tire cords and the yarns were in greige form.

2.2 Methods

2.2.1 Tel/sile Tests

Tensile tests were performed on Instron tester 4502 using crosshead speed of 300 mm/min and gauge length of 254 mm. Table 2 shows the breaking elon­gation and breaking strength values.

2.2.2 Creep Tests

Creep tests were performed by means of dead loading conditions. The cords were attached with a pin from one end and the dead load was applied to the other end. Twenty per cent breaking strength of cords was appli ed as creep load. After every 5 min, the elongation in the length was measured by a graded ruler hanged nex t to the cord. To prevent turning and changing the position of loaded cord, a special mechani sm was attached to the specimen through the load . The results are presented as elongation against time under constant load.

2.2.3 Optical Microscopic Studies

Optical microscopic analysis automatic Tri nocliler Stereo (O lympus SZ6045 Model ).

3 Resu,lts and Discussion

3.1 Tensile Properties

was carried out by Zoom Microscope

The stress-strai n curves for the 3 different cord yarns with 4 different twi st levels are shown in Figs 2-4. The tensile tes ts results are g iven in Table 2 .

The stress-strain data of cord yarns with 4 different twist levels show an increase in breaking ex tension and a decrease in breaking strength . In tex tile yarns , twi st is appli ed to a yarn to increase tensile strength

PEN

0.15

z 0.10 .:£

'0 ro .3

0.05

o

Twis t Level 1- 350 TPM 2 - 445 TPM 3 - 470TPM 4 - 530 TPM

445 470 530

Single yarn

350 445 470 530

Single yarn '

0.141 0.1 45 0. 138 0.076

0.169 0. 149 0. 144 0.125

0.0928

2 3 4 567 Strain, %

15.384 16.960 18.746 11.703

8.374 9.647 9.84 1 10.594 6.258

8 9 10 11

Fig. 2-Load-strain curves of PEN cord yarns wi th different twist levels

0.16

0.14

0. 12

z 0.10 x -0 1l 0.08 -'

0.06

0.04

0.02

Twist Level 1- 350 rPM 2 - 445 rPM 3 - 470 rPM 4 - 530 rPM

10

Slrain, 70

15

4

20

Fig. 3-Load-strai n cu rves of DSP cord yarns wi th different twist levels

HOCKENBERGER & KORAL: PERFORMANCE OF TIRE CORD YARNS 21

Twist Level 0.15 1- 350 TPM

2-445TPM 3 - 470 TPM 4 - 530 TPM

z 0.10 ..><

" ro o -"

0.05

o 5 10 Strain, %

15 20

Fig. 4-Load-strain curves of hi gh-tenac ity polyester cord yarns wi th different twist levels

and for a maximum tensi le strength always an opti­mum twist level, where strength decreases, exists for each type of material. An increase in twi st level puts extra stress on the yarn and increases fibre-to-fibre friction. This can be explained by considering the ef­fect of stress on the cord as the twist increases. As the twist increases, the helix angle (angle between the filament ax is and cord ax is) increases. Thus , tension stresses normal to the cord axis results in greater force, thereby parting the filaments. As the cord twi st increases, the force in the direction of yarn axis in­creases, causing a lower overall breaking strength . A hi gh ly twisted cord behaves like a coil spring. It gives relatively low cord strength but high fatigue resis­tance. In contrast, a sli ghtly twi sted cord acts more like a rod. It gives higher strength but poorer fatigue res istance. Thi s also explains the increase in elonga­ti on-at-b reak with increas ing twi st. Therefore, in cords to have high performance properties and better fa tigue behaviour, twi st levels are preferred beyond the optimum twi st level (Fig. 5) .

Stress-strain curves (Fig. 2) of PEN cords with 4 different twi st levels show two di stinct regions. First region is the elas tic region including yielding and the second region shows the strain hardening, leading to rupture. First region shows no considerable change in shape due to the increased twist level. However, the yield point moves towards the lower position and the modulus decreases. The second region changes with the increase in twist level. The length of the strain hardening region increases. PEN cord yarn with the highest twi st leve l shows rather brittle material be"­haviour with no distinct yield region. The decrease in strength with the increase in twi st is more pronounced compared to the changes in breaking ex tension.

Stress-strain curves of DSP cord yarn with 4 differ­ent twist levels show typical ductile material beha-

Fig. 5-Relationship between strength and twi st for a conventional textile yarn

viour (Fig. 3). Therefore, a short Hooke region with pronounced yielding followed by strain hardening was observed. Compared to PEN cords, here more distinct yielding was observed. Also, the same pattern of decreasing tensile strength with the increase in twist level was recorded. It is important to note that with the twist levels of 350 and 445 TPM, the changes in stress-strain curves and the breaking elongations are not as obvious as those in PEN cords with the same twist levels. When twi st is increased to 470 TPM from 350 TPM, the shape of the tensile curve and the break ing elongation values do not change. However, a distinct loss in strength is obvious. The increase in twi st shows more effect on the breaking elongati on than that on the breaking strength .

On analysing stress-strain curves of hi gh tenac ity polyester tyre cords (Fig. 4) , it is observed that the increase in twi st level does not show a big effect on the shape of the curves. However, it is not easy to es­tablish a pattern o f stress-strain data with increasing twist level. Typical ductile material behaviour was also predominan t for hi gh tenacity polyes ter tyre cords. An increase in twi st from 350 TPM to 470 TPM also increases the strength that is contrary to other materials. A huge increase in breaking elonga­tion was also recorded.

3.2 Creep Properties

Polymeric materials including fibres show creep under static loading conditions. In thermopl astic polymers, creep can be ex tcnsive. Therefore, creep analysis plays a very important ro le on performance expectations. Fig .6 shows a typi cal creep curve of a polymer; the e longation of a fibre plotted as a func­tion of time under constant load .

In such an elongation-time curve, followin g four regimes can be characteri zed by differen t behaviour of the creep rate:

(i) Regimc la-instantaneous initial elongation

22 INDIAN J. FIBRE TEXT. RES., MARCH 2004

c o ro OJ c 52 ill

III

Time

Fig . 6-Elongation-time curve of typical thermoplast ic fibre

(ii) Regime Ib-creep rate decreases with the in­crease in elongation (primary creep)

(iii) Regime If-creep rate is approximately con­stant (secondary creep)

(iv) Regime III-creep rate increases again, signal­ling imminent failure

Initial elongation and the elongation caused by primary and secondary creep are important for the application of fibre, and therefore this study was fo­cused on these regimes.

Fig. 7 shows elongation-time creep curves of the cords with the increasing twist level. Initial elongation was effected by twist level. The highest the twist \evel, the highest is the initial elongation . This is true for all types cords. For the lowest twist level for all cords, the creep rate decreases with the increase in elongation after initial elongation. With the increase in time, secondary creep shows a very low creep rate, almost a plateau. This is an irreversible elongation . With the increase in twist level, primary creep rate increases, but the initial recovery rate is not affected by the twist level. The cord shows the same recovery behaviour regardless of its initial elongation, primary creep rate and the twist level. This is different for PEN cord with the highest twist level (Fig. 7b).

3.3 Fractu re Mechanism

3.3.1 DSP Cord Fig. 8 shows the broken ends of the DSP cord yarn

with 4 different twist levels. On analysing broken ends of cord with 350 TPM twist (Fig. 8a), it is ob­served that the both pli es ac t together on loading and then break, giving clear ends. The twi ~ t along the cord until the breaking region does not distort. On analys­ing the cord with 470 TPM twist (Fig. 8c), the twist is

§ ,;f 0 ..... ..... t':S OJ) 0 0 @

3 r-------------------------~

2

0 0

3

2

1

0

0

3

2

(a)

.•.•.•....•. ... •....•.... -·- .. ··35 0 TPM

-0-- 445 TPM

.•• & ••• 470TPM

~530TPM

20

(b)

20

(c)

40 60

40 60

o ~------~------~------~~

o 20 40 60

Time, min

Fig. 7-Creep curves of high-tenacity polyester cord (a), PEN cord (b), and DSP cord (c) with diffe rent twist leve ls

slightly distorted and one of the two plies wraps around the other at the broken end . With the increase in twist level, the twist distortion along the cord was more pronou ced. Broken ends also change their ap­pearance with the increase in twist level. When more twist is applied to the cord during loading, one of the two plies caITies more load and therefore the other wraps around it. The one, carrying the more load, probably breaks earlier. This is very clear with 530 TPM cord as two plies can be separately observed.

3.3.2 fligh-Tel/aci/y Polyester Cord

Fig. 9 shows the frac ture mechanisms of high­tenacity polyester cord yarns with varying twist level. Here, with 350 and 445 TPM, similar breaking mechanism was observed. Both the plies act together

HOCKENBERGER & KORAL: PERFORMANCE OF TIRE CORD YARNS 23

Fig.8-DSP cord [a-350 TPM, b-445 TPM, c-470 TPM, and d-530 TPM]

Fig. 9-High-tenacity polyester cord [a-350 TPM, b-445 TPM, c-470 TPM, and d-530 TPM]

Fig. IO-PEN cord [a-350 TPM, b-445 TPM, c-470 TPM, and d-530 TPM]

24 INDIAN J. FIBRE TEXT. RES., MARCH 2004

throughout loading and carry the same amount of load. Therefore, broken ends show same length tips. However, the fibres show irregular orientation at the tip. Twist along the cord remains even. But with in­creasing twist level , ply separation was again ob­served and with the highest twist level an uneven twi st distribution along the cord recorded. One of the plies was twisted around the other one.

3.3.3 PEN Cord

Fig. 10 shows the broken ends of PEN cord yarns. With PEN cords, even the lowest twist shows two different ply breaking. Almost with all twist levels, twi st along the cord is distorted. One of the plies was wrapped around the other one and broken ends for all twist levels showed this predominantly, which is dif­ferent than other cords.

4 Conclusion The amount of twist applied to the cord affects the

mechanical behaviour of cord. It increases breaking elongation and decreases breaking strength. Creep behav iour is also affected by applied twist level. With increasing twist level, the creep rate increases. There­fore, for a cord, an optimum twist level should be de-

cided. Breaking mechanisms arc also affected by the twist level. On the other hand, cord material plays more important role on the performance properties as all the materials behave differently with increasing twist levels. PEN cords show the most decrease in strength with increasing twist level. In terms of creep behaviour, no important change was observed be­tween 350 TPM and 470 TPM twists. For all cords, with 530 TPM twist an important fall in strength was recorded. Applying extra twist also puts an extra cost. Therefore, 470 TPM twist level can be considered suitable for sufficient performance requi rements.

Acknowledgement The authors are thankful to the KORD-SA, Turkey,

for their support and valuable information.

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1813 .

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