Indian Journal of Fibre & Textile Research Vol. 22, December [997, pp. 259-263

Developments intyre cords- Some observations

S N Chakravarty

Indian Rubber Institute, 305 South Delhi House, 12 Zamrudpur Community Centre, Kailash Colony, New Delhi 110 048

The importance and role of pneumatic tyre cords and the recent developments in this field have been discussed. Effect of macrostructure and heat treatment on nylon 6 tyre cord's mechanical & dynamic properties, spin-draw and conventional nylon 6 tyre cords, higher denier cord's usage in heavy duty tyre construction and the relationship between tyre performance and tyre cords have been briefly discussed.

Keywords: Hysteresis" Macrostructure, Spin draw, Thermal treatment, Tyre carcass,Tyre cords

1 Introduction Tyre technology has evolved considerably since

the first pneumatic tyre was developed over one hundred years ago. Pneumatic tyre has undergone a continuous evolution in perfonnance capabilities from the beginning of this century. These developments have resulted from the demands of consumer of modem automobiles and advances in automobile as well as tyre technologies. Tyre is a composite material consisting of cords embedded in the rubber matrix. The overall strength and ability to bear load by the tyre depends strongly upon the nature of cord.

2 Pneumatic Tyres Reinforcement makes it possible to employ

inflatable rubber tyres, guaranteeing good spring (shock absorption) action and transmission of frictional force for excellent road holding and high tractive and braking force over a long period of time, even at high speeds and heavy loads.

Heat generation in the tyre places restrictions on load and speed, with the temperature of the rubber constituting the limiting factor. The problem of heat generation can be controlled by:

Increasing the internal pressure (if possible from the dynamic point of view). This would relate to the carcass strength/safety. Factor/load on individual cord. Reducing material volume (by such steps as minimisation of the number of plies, usmg higher denier cord for example). Using materials with low hysteresis. Ensuring good dissipation of heat.

Although the tyre for the most part consists of rubber, the reinforcing materials play a decisive role when it comes to the issues mentioned above.

Strength, breaking energy and dimensional stability, including vulcanization shrinkage, elongation under internal pressure, creep, flexibility, and hysteresis are the parameters of great importance in carcass construction.

Today's tyre reinforcement requirements are best understood by examining the trends in the tyre industry that are taking place in the various parts of the world. In Western Europe, polyester continues to replace rayon in passenger tyres due to the better economics of polyester and the environmental problems associated with the manufacture of rayon. In North America, lighter weight materials and improvements in rolling resistance and unifonnity are needed for O.E. (Original Equipment) tyres. The developing countries, which include China and Southeast Asia, will continue to experience a rapid radialization of passenger and truck tyres, leading to the replacement of nylon with polyester. In India, till now, the total tyre market IS predominated with truck tyre segment' .

The required strength of the carcass is dependent on the tyre's pressure and radius. Rayon, polyamide and polyester all confer higher breaking energies than steel, but this is achieved at the cost of far greater material consumption due to the required multi-ply construction. Because of its high elongation and more particularly becClUse of its insenSItiveness to fatigue and impact, polyamide is suitable for use in mUlti-ply

260 INDIAN J. FIBRE TEXT. RES., DECEMBER 1997

carcasses. Mono-ply carcasses of rayon, steelcord or aramid are dimensionally stable, but those of rayon are not strong enough for truck tyres.

For the bias construction, tyre casmg reinforcement fabric is virtually 100% nylon. Nylon's domination in this application, including both body plies and breakerlbelts, is attributable to its unique combination of strength and durability, particularly with respect to fatigue resistance. This latter characteristic is benefitted by the flexibility of its polymer chain, which unlike rayon, polyester or aramid, contains no restrictive ring structures2 . .

Depending upon the availability and the decision of design engineers, nylon-reinforced truck tyres may be built with either nylon 6 or nylon 66. These two polymer versions differ slightly. The criterion for selection of nylon 6 vs nylon 66 is largely limited to user ' s cost since the physical property differences have negligible impact upon tyre performance3•

In recent years, there is a trend towards spindraw4 nylon technology, because by eliminating multi-stage operation of the conventional production method, a better oriented and heat-set yam of high tenacity and consistency is obtained. Also, there is an advantage due to higher productivity and gain at processing stages.

Future tyre development trends will be driven by both the automotive companies and consumers. The differences between the OE and replacement markets will continue to narrow as the consumers demand the same performance characteristics in both markets. The trend for lighter weight tyres using fewer components and less material for a given component has been driven by the need to reduce costs and address environmental concerns (lower fuel usage).

Evaluation of nylon 6 tyre cords (from different manufacturing sources) with respect to mechanical , morphological, macrostructural and dynamic characteristics at normal and elevated temperatures with heat treatment have shown5 that not only the average molecular weight, but its distribution pattern may also have an effect on performance of tyre cords.

The behaviour of nylon 6 tyre cords at elevated temperature has been evaluated by different workers6,7, The heat treatment of cords at 200°C for 16 h was found to reduce their tensile strength

considerably. This was attributed to the decrease in average molecular weight and change in molecular weight distribution, as ascertained by fraction studies,

3 Relationship between Tyre Performance and Tyre Cords Tyre performance characteristics are affected by

the physical properties of the tyre cords '.

3.1 Burst Strength

The tensile strength of cords is related to burst strength of the carcass of a tyre by the following relationship:

Burst strength = Nt JlK = 3.14 x Pp (r} - r~ax ) / sin a

where N is the total number of cords in a tyre; tJl,

the average tensile strength of cords; Pp, the burst pressure; r c, the radius from the centre of rotation to the crown of tyre; r max> the radius from the centre of rotation to the maximum section width of

a tyre; a, the crown angle; and K, the efficiency factor.

3.2 Tyre Endurance

Different types of separations related to tyre cords can occur. Most separations are due to poor adhesion between the cord and the carcass rubber. Initial adhesion between cord and rubber as well as uniformity and adhesion retention are important. Adhesion uniformity is more important than a high average adhesion since unevenness causes stress separations which tend to promote separations. Surface activity of the cord (like amount and type of spin finish) plays an important role in adhesion.

3.3 High Speed Endurance During operation, a tyre continually stores and

releases mechanical energy. At very high test speeds (e.g. 193 km/h) , a tyre sometimes generates enough heat to cause a tread separation or chunk-outs, often starting at a small internal flaw in the hottest and thickest part of the tyre, usually the tread shoulder. The heat generated by the tyre cords in a bias tyre is sufficient to significantly affect the maxImum operating speed of the tyre. In Indian context, for truck tyres the same phenomenon is observed at low speed/higher load/rough road condition.

CHAKRA VARTY: DEVELOPMENTS IN TYRE CORDS 261

Extensive research in different laboratories showed8 the significance of the mechanical loss of tyre cords in tyre temperature rise during service (Fig. 1). With polymeric fibres such as PET, nylon 6 and nylon 66, the mechanical loss is controlled by the viscoelastic responses of the fibre, and the contribution of ;nter-filament friction is minimal. With steel cords, on the other hand, the individual filaments are elastic and the loss is almost entirely due to inter-filament friction . Regardless of these fundamental differences, both the metallic and polymeric tyre reinforcing media exhibit substantial mechanical loss that greatly affects the performance of tyres. A major difference between the two types of reinforcement is, however, the temperature dependence of the mechanical loss. With metallic cords, the loss is independent of temperature in the temperature range encountered in tyres. With polymeric fibres, on the other hand, the loss exhibits a maximum at the glass transItion temperature of the fibre.

3.4 Power Loss In a bias tyre, 30-40% of the total power loss, a

significant portion of the heat generated, is because of the carcass cords. Nylon 6 bias truck tyres operates 10-2SoC cooler than those made of nylon 66. These differences apparently correspond mainly to the relative rates of heat generation of the cords.

Viscoelastic properties of nylon tyre cords vary from greige cords and tensilized cords to cords pulled from tyres after being in service for some time. However, after curing of tyres and testing or driving of tyres under severe driving conditions, these differences are greatly diminished and well-conditioned nylon 6 and nylon 66 tyre cords exhibit the mechanicalloss8, shown in Fig. 2.

Analytical and experimental determinations of · tyre temperature profiles were conducted8 on a pair of bias truck tyres which were built with identical design using the same rubber stock but with different cords (nylon 6 and nylon 66). The purpose of this study was to determine whether the difference in the heat generation characteristics of the cord results in a detectable difference in the tyre temperature. Fig. 3 shows the resulting temperature profile at the crown-shoulder region for 104 kmIh. Comparing the data of Fig. 2, nylon

3 (c)

~ a I 0

X -g-~ N c:I: a: ~ 2 c-o 111 .- -c E I... U ~-c 0'11 ~ I... t!)~ -0 ell :I: 0

3

2

-0- ~ Nylon 6 Cord

• •

(0 )

, , , ,

• Nylon 66 Cord

o-~

,,' "tJ, 0'

\

'0 ...

... ... '0

' ... '0

50 100 150 Tempe-roturt!':C

Fig. 1-Mechanical loss (heat generation rate) vs temperature curves for nylon 6 and nylon 66 cords: (a) from experimental measurements, (b) with a vertical shifting to account for differences in degree of crystallinity, and (c) with a vertical and horizontal shifting to compare the slopes of curves before and after the temperature at th'O peak.of loss [Source: Rubber Chern Technol, 60(4)(\987)668]

66 cords in the temperature range above 110°C is reflected in higher temperature at the shoulder region.

'The result clearly shows that the viscoelastic properties of the tyre components directly affect

262 INDIAN J. FIBRE TEXT. RES., DECEMBER 1997

e

o

= .~ e ... ~1 ... '" ... =

-Nylon6

----- Nylon 66

o 150 Temperature ~C

Fig. 2-Heat generation rate of tyre cords under various strain amplitude and temperature [Source: Rubber Chern Technol. 60(4) (1987) 660]

Nylon 6 Reinforced

Nylon 66 Reinforced

\

\

\

\

\

Fig. 3---Effect of cord heat generatiqn on tyre temperature (0C) profile of bias truck tyres at 104 km/h [Source:j ~lbbe, Chern Technol. 60(4) (1987) 662]

the temperature rise in a rolling tyre . The fact that the shoulder temperature of a bias truck tyre rolling at 120 kmIh reaches nearly 160°C shows that how easily high speed driving can push the tyre temperatures to the levels where the tyre wall starts failing because of combined temperature and stress effects.

3.5 Tread Wear Since the cord modulus contributes to the

stiffness and load distribution of the tyre print, a higher cord modulus implies less tread wear and longer tread life .

3.6 Tyre Size and Shape The size and shape of an inflated tyre depends

upon the design of the tyre and the modulus of the cord. Because cord processing affects modulus and creep, careful process control must be maintained to obtain uniform size and shape.

3.7 Tyre Groove Cracking

Cords with less creep produce less groove cracking. Post cure inflation of nylon tyre reduces tyre growth and the tendency of the grooves to crack.

3.8 Flat Spotting and Tyre Non-Uniformity Thermoplastic tyre cords such as nylon and

polyester tend to shrink readily when they are heated above their Tg. Therefore, the higher the Tg of the cord the lesser is the chance of flat spotting.

3.9 Tyre Cornering Force

Generally, the higher the cord modulus, the stronger is the cornering force developed at any particular steering angle.

4 New Nylon Cord Du Pont has recently developed a nylon 66

monofilament tyre cord (rectangular in shape with rounded corners) which is totally anew concept with the elimination of twisting at the cord prepa-ration stage. Besides the advantage at the fibre manufacturer's end, the various positive ~ttributes of the tyre made of this monofilament cord are reduced tyre weight, increased plunger energy, reduced flat spotting, aesthetic, high speed performance, cooler running, etc. Two different denier (2000/1 & 300011) nylon 66 monofilament cords were chosen fof comparing with various

CHAKRA V ARTY: DEVELOPMENTS IN TYRE CORDS 263

multifilament tyre cords with respect to tensile, dynamic mechanical and thermal properties in order to find out their suitability 10 tyre performance.

Considering the stress-strain and dynamic mechanical properties, monofilament cord shows its superiority over the multifilament tyre cords. Unlike multifilament cords, monofilament cords do not show any peak tan 8 value. Above 100°C, monofilament cords show least tan 8 compared to multifilament cords which is of practical importance considering their low heat generation in tyre under dynamic condition.

5 Higher Denier Cord Theoretical investigation and practical

expenence have shown that the increase in the thickness of multiffiament yams positively influences cord properties. Physical and mechanical properties of cord are greatly dependent on their structure-the thickness of multifilament and single yams, the number of twists and their relationship.

The first twisting of cord has a great effect on

size truck tyre and OTRs (off the road tyre) the speed of the vehicle is not high and in practical applications not much effect of fatigue has been observed. In OTRs, 1890 Dx2 cord is preferred as the load bearing factor is primary in this case. Also, it is understood that some of the developed countries have started using 1800 D cords for heavy duty truck tyre in place of 1260 Dx3. Indian tyre industry have initiated commercial trials with 1890 D.

Usage of 1890 Dx2 cords would lead to process time saving (at dip unit, calendar unit and tyre curing stages), higher output from same machines (no additional investment and tyre curing, time shortening due to lower gauge/mass) and energy saving. Usage of 1890 D tyre cord can lead to defect improvements like tread separation, ply separation, crack, etc. 1890 D would reduce casing growth but ply gauge will increase. All these are subject to casing strength control.

References I Mukhopadhayay R, (Personal Communication). 2 Lambillottee B D, J Coated Fabric, 18 January (1989)

162.

all its physical and mechanical properties. During 3 the first twisting the length of single yam shortens 4 while during the second twisting the cord yams are elongated as a result of their twisting in the direction opposite to the first twist. The thickness

Lambillottee B D & Eiber G S, Rubber World, 209(1) (1993) 27. Chakravarty S N, Mustafi S K & Rizhwani H K, Comparative sllIdy ofspill dra w alld cOllvelltiollallly /O/ f 6 ty re cord, paper presented at the International Conference on Rubber & Rubber like Materials, Jamshedpur, 6-8 November 1986. of filaments has a pronounced effect on the 5

properties of cord yams. It has been found that the decrease in the linear density of filaments produces favourable effect on the fatigue resistance of 6 twisted cord yams.

In view of the above observations, 1890 Dx2 7 cords might exhibit lower fatigue resistance 8 compared to 1260 Dx2 cords. However, for large

Chakravarty S N & Mustafi S K, Proc., Int Rubber COllf 'RubberCon 93 ', (Indian Rubber Institute, Delhi), 1993, 149. Mukherjee A K, Gupta B D, Kulkarni S G, Chauhan D S & Chakravarty S N, J Appl Polym Sci, 30 (1985) 4417 . Mukherjee A K & Chakravarty S N, Rubber Chem Rev, 12( II) (1983) 30. Prevorsek D C, Murthy S & Kwon Y D, Rubber ('hem Technol, 60(4) (1987) 665.