OPTIMIZATION OF CURE CYCLE TIME OF A BIAS TRUCK LUG TYRE BY STUDYING THE CURE EQUIVAL
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OPTIMIZATION OF CURE CYCLE TIME OF A BIAS TRUCK LUG TYRE BY STUDYING THE CURE EQUIVALENTS IN ISOTHERMAL AND NON-ISOTHERMAL CONDITIONS


INTRODUCTION
Curing is a process in which the viscoelastic rubber is converted into three-dimensional networks by tying up of all independent chains. Uncured rubber is a viscoelastic fluid, which has poor physical properties and undergoes deformation easily. Curing process reduces the flow of rubber material. And also reduces the amount of permanent deformation after the removal of deforming force. Vulcanization also imparts strength, stiffness, modulus as well as the resistance to fatigue and abrasion.
Generally the tyre cure cycle provides a 3T condition, i.e.
 Time
 Temperature
 Tension
These three factors together determine the chemical and physical properties of tyre. This chemical reaction is a function of curatives, time and temperature.
The curing of tyres takes place in curing presses, which hold the tyre moulds.
The heat is transferred to the green tyre in the mould both from inside and out side - Out side through heated platens (indirect heating) or by direct steam heating (Dome) and from inside through bladders containing heating media like steam & Hot water.
By applying heat and high pressure, the tyre is minimum cured to the point where the net work force of the compound is higher than the pressure resulting from the volatile compounding ingredients.







OBJECTIVE
To optimize the cure cycle time of a bias truck lug tyre by studying the cure equivalents in isothermal and non-isothermal conditions.
In order to achieve so, develop and follow a new model of study for determination of blow point which gives due weightage to geometry and gauge of the coldest portion of the tyre, and yet involves minimum development scrap.

Scope of study
The curing stage in tyre manufacturing is of most significance as this stage determines physical and chemical properties of the tyre. Tyre performance and life expectancy are affected if cure time is insufficient and hence compromises on quality. However, rising industry demands constantly require increase in productivity by shorter cycle time. The project is an attempt of optimizing cure cycle time without compromising on cure safety time and so, preserve the quality of the product. Also, the study follows an unconventional experimental method aimed at reducing developmental scraps.
The project also deals with the effect of varying retarder concentration on cure cycle. This data can be of importance when dealing with controlling cure time by varying retarder concentration.

THEORETICAL BACKGROUND

• Vulcanization: Introduction
• Accelerated Sulfur Vulcanization
• Vulcanization Retarders
• Heat engineering principles for Rubber
• Thermocouple and Blow Point Study
• Curing Equipment









VULCANIZATION
Vulcanization is the process by which mainly plastic rubber is converted in to elastic rubber or hard rubber state. The process which is brought about by the linking of macromolecules at their reactive sites, this process is also known as cross linking. Sulfur is combined in the vulcanization network in a number of ways, it may be present as monosulphidic, disulphidic or polysulphidic but it may also be present as pendent sulphidic or cyclic monosulphidic & disulphidic group.






Most of the properties of the vulcanizates are dependent on these cross links in between the macromolecular structure. The system of vulcanization can mainly effect the concentration of these cross links. A conventional vulcanization with high sulfur to accelerator ratio produces more poly-suphidic cross links. Both, efficient vulcanization system, which uses high accelerator to sulphur ratio & semi efficient vulcanization system, which uses compromised concentration of sulphur to accelerator (in between conventional and efficient vulcanization system) can produce more mono & disulphidic cross links.
As the number. of mono & disulphidic cross links are increased, it will increase oxidation resistance, thermal resistance, reversion resistance in natural rubber vulcanization and it also provides reduced compression set. But these systems show some draw backs i.e. inflexible processing safety, outdoor & poor initial vulcanisate properties, high cost & necessity for careful storage etc. So the application of this system is limited to few necessary situations. The modules, hardness etc. are increased by increasing the number of polysuphidic cross links, but this also reduces the elongation at break, fatigue life etc. of the polymer vulcanisate.
Not only the system of vulcanization, but also the process of vulcanization affects the vulcanisate properties. Vulcanization is a dynamic process, not all the cross links survive the vulcanisation process. Instead existing cross page link are broken down and new ones are formed continuously. The modulus rises as more cross links are formed than are broken down. At the plateau stage, newly formed cross links are in balance with those that are broken down. And when reversion takes place more are broken down than are formed. This will cause variation of vulcanisate property of natural rubber vulcanisate.
The degree of cross linking can show a marked influence in tensile stress, elongation at break, rebound resilience at lower temperature, tear resistance, tension set, compression set, fatigue resistance (heat buildup), resistance to swelling etc. The degree of cross linking can also show the less influence on tensile strength, rebound resilience at room temperature, dynamic damping at room temperature, abrasion resistance, gas permeability, low temperature flexibility, electrical resistance etc.
Physical property changes on Vulcanization





Tyre is a dynamic product; the property of a tyre vulcanizate mainly depends on its degree of cross linking, the system of vulcanization used, the vulcanization process and the ageing conditions, heat buildup etc. on the service affects the properties of tyre vulcanizates.







Accelerated Sulfur Vulcanization
The purpose of the cure systems is to cross-link the polymer matrix of the compound mixture. There are many ways to cross-link the unsaturated double bonds of the elastomers and a few systems, to form C-C cross-link. C-C cross –link can be made with peroxides or resin cure systems. However these are not used in tyre compounds. The predominant types of cross-link feature achieved by both soluble and insoluble sulfur and accelerated systems. In addition, ZnO and fatty acid are important cure activators. Retarders are used to improve process safety or to delay cure during processing stages of tyre manufacture.
By cross-linking the rubber changes from plastic to elastic state. In the plastic state the polymer molecules can move more or less freely, especially at elevated temperatures (Macro-Brownian motion). As more cross-links are formed the vulcanizate becomes tighter and the forces (stress forces) necessary to achieve a given deformation increase.

Fig 2 .1




Degree of vulcanisation (cross-link density)

The number of cross-links formed depends on the amount of vulcanization agents, its activity reaction time. In sulfur vulcanization various types of cross-links are formed depending upon the quantity and activity of the accelerator. These cross-links can be anything between mono-sulphidic to poly-sulphidic.

Fig 2.2
Different cure systems and cross page link type
Earlier S was commonly used at about 8phr, this compound vulcanized in about 4hrs at 140oC. But in modern compounds require only 2 –3phr of S to form an elastic vulcanizate product. Depending upon the S level there are three cure systems – conventional (CV), efficient (EV) & Semi efficient (Semi EV).
Conventional System (CV): - In the case of compounds, which contain high proportion of sulfur (1.5 – 3phr), the network formed will be polysulphidic, which are thermally unstable. These crosslinks will decompose at elevated temperatures leading to loss of cross-links. The final network will be mainly di or poly sulphidic and hence liable to further decomposition.
Efficient System (EV): - The term arises from the efficiency of utilization of sulphur per cross-links. This cure system formulated with small amounts S (0 – 0.5phr) and high proportion of accelerators or sulphur donors (2-6phr) will have the final network cross-linked with mainly mono sulphide bonds and there will be relatively few modifications of cyclic sulphide or conjugated triene type. Use of sulphur donor (TMTD or TETD) alone produces low tensile strength.
Semi efficient system (Semi EV): - These produce network structures having efficient cross links (mono sulphide) percentage lying between those of EV & CV systems (see table). Semi EV cross-links has thermal stability lying between EV and CV system. Sulphur donors are used to replace part of the S used in normal S cured system to produce Semi EV.

Cross page link type CV(%) EV(%) SemiEV(%)
Poly sulphides 65 0 40
Di sulphides 25 25 25
Mono sulphides 10 75 35


Figure 2.3
The above figure is the schematic representation of the effect of curing time on the distribution of crosslinks in an accelerator – sulphur compound. The poly sulphidic cross links initially formed, break down to form more cross links containing smaller units of sulphur. Thus form a new network structure in the compressed state.
The ideal molecular arrangement for heat resistant sulphur cured rubber is predominant in mono sulphidic page link with minimum modification to the main rubber chains. This corresponds, efficient views of sulphur and the term EV system is often used.


Fig 2.4
The heat resistance as measured by retention of tensile strength and elongation improves across the diagram from CV through Semi EV to the EV systems. The Compression set, which is measured at elevated temp. is an indication of the thermal stability of the cross-link.

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