Manufacture of Cold Retrading Material

S. N. Chakravarty* 

KPS Consultants & Impex Pvt. Ltd.,
 812, Devika Tower, 6 Nehru Place, New Delhi -110019, (India),  
 E-mail : kpspltd@gmail.com

Cold Retrading Material    

Commercial tire retreading provides an economical means of extending the asset utilization of worn-out tires. The worn-out tire, referred to as the casing, is a valuable resource that is often not utilized to the fullest potential. By replacing the worn tread with a new one, the retreaded tire provides performance similar to that of the new tire at a fraction of the cost.

Retreading often offers a less expensive alternative to the purchase of a new tire. To the truck tire customer, however, retreading is an integral part of a tire maintenance and purchasing programme.  A truck tire casing must be able to be retreaded two to three times.

Considering that only about one-fifth of a tire is worn out in service, it makes economic sense to retread the tires for multiple use. Although both cross ply and radial tires can be retreaded, 4 to 8 ply bias tires can be the most easily retreaded. However, the structural performance may fall short of the new tires and service conditions may have to be more closely controlled.

*President – Elastomer Technology Development Society, India

Two main methods, currently in use, are (a) conventional “hot” capping and (b) pre-cured-tread rubber process.

Tire Retreading can be done by the following two processes:

There are two primary methods currently used to retread tires, including: mould cure and precure processes. The preparation of the casing is essentially the same in both cases. In the mould cure process, the tread rubber is applied in the uncured state to the tire casing, and the product is cured in much the same manner as new tires. In the pre-cured technology, the tread is supplied to the retreading factory cured, with the tread pattern already in place. The treads are supplied in rolls of various lengths (typically 3.5 to 10 meters in length), or as rings that can be stretched onto the casing. In the application of both pre-cured methods, a bonding material, the cushion, is applied to the tread or the casing prior to the application of the tread. The majority of the tires retread today consists of truck tires, produced using the pre-cured technologies. A description of this method is given below.

a)         Conventional Process (also known as 'mould cure' or 'hot cure' process) - In this process an un-vulcanized rubber strip is applied on the buffed casing of the tire. This strip takes the pattern of the mould during the process of vulcanization.

b)         Precure Process (also known as 'cold cure') in this process a tread strip, where the pattern is already pressed and precured is applied to the casing.  It is bonded to the casing by means of a thin layer of specially compounded uncured rubber (known as cushion or bonding gum) which is vulcanized by the application of heat, pressure and time.

In the pre-cured rubber (cold process) retreading, better mileage is obtained than with the hot capping process. The tire is inspected for cuts, ply separations, etc. and after inspection, it is repaired with repair compound or patches, and then buffed under inflated conditions to facilitate bonding. Thereafter, the tire is again inflated on an expandable hub and coated with vulcanizing cement on its buffed surface. A layer of bonding / cushion gum is applied around the surface of the tire. The trapped air between the layers is removed and the vulcanized tread strip is applied, its ends spliced and stitched. After this operation, the expandable hubs are collapsed and the tire is deflated and removed for vulcanization. The tire is fitted on suitable rims and inflated with the use of a tube. The inflated tire is then slipped into an envelop and vulcanized in a curing chamber or autoclave at lower temperatures than is normally used for new tire vulcanization.                                    

Tire Preparation

The buffing process provides a contaminant-free surface of uniform texture to allow adhesive of the new tread. During the buffing process, the casing is also brought to a uniform circumference, with the correct toroidal radius and width, to accept the proper tread size for the casing.

Buffing Cum Lug Cutting Machine

After buffing and skiving the casing, a thin layer of contact adhesive is applied to the casing. This material usually referred to as cement, aids in the retread tire fabrication process and helps provide increased adhesion between the casing and the cushion. The cementing of the casings is optional, but widely used in retreading. Cements are solvent-or water based materials.  

It is at this point that the majority of the repair to the casing is performed. The main purpose of repairing the casing is to restore the ability of the casing to maintain air pressure and to return mechanical properties of the casing to a level high enough to endure at least the next use life.

After the completion of the repairing of the casing, the tread can be applied. The application of the tread is referred to, as building the tire, and there are a number of variations to the building process. The main components used in the tire building are the tread and cushion. The tread is supplied to the retread factory fully cured with the desired design and tread width. The back side of the tread is prepared at the manufacturer by buffing with a wire brush drum and the application of a contact adhesive. A polyethylene film is placed on the cemented side of the tread to prevent contamination.

The cushion is supplied to the factory in either calendared sheets or as strip stock to be used in an extruder. The calendared cushion is supplied in different widths and thickness. As the first step of the building processes, the casing is placed on a builder machine equipped with an expandable hub, like that on the buffer. The hub is expanded and the leading edge of the tread is cut to provide a uniform uncontaminated surface.

Application of Pre-cured tread on the prepared casing

Enveloping & Curing

The next step in the process is the placing on the “built” tire in a rubber containment device called an envelope.

  

The enveloped tires are placed in a curing chamber that is essentially a large autoclave, steam or electrically heated. The enveloped tires are suspended from a rail system inside the chamber and are connected to exhaust lines inside the chamber (via the valves in the envelope). The tire is then moved down the rail to the rearmost portion of the chamber. Once the desired number of tires is in the camber, the chamber door is closed and the heating and pressurization is started. During the pressurization, the air is allowed to evacuate from inside the envelope.

The operating pressure of the chamber is typically at least 0.3 MPa, with curing temperature between 100°C and 141°C. The cure time is dependent on the thickness of the treads and the composition of the cushion.      

Earlier it was stated that cold retrading of tire using pre – cured tread gives better performance – higher mileage. Why ? Because abraison ( wear ) loss of the tread is much lower giving rise to higher mileage. This is because cold tread material is more compact due to much higher pressure ( hydraulic ) applied during curing of the tread in a hydraulic press compared to a new tire curing in tire mould where pressure is limited because of the steam /  pressure relation ship.

Cold Cure Process – Pre-requisites

It has been well established that heat is the most damaging cause of tire deterioration / ageing, and therefore, its eventual failure.  The critical temperature of rubber is 115ºC, beyond which ageing and deterioration of the tire casing is accelerated, resulting in premature reduction in body strength and leading to failure.  So, technically any retreading system with curing temperature lower than the critical temperature can only be genuinely called a ‘cold process”.  However, another view is that it is the precured tread which is the difference, not lower temperature per se.

In the cold cure process,  factory-cured treads are dense, tough and are of uniform consistency and resilience, as they receive heat and pressure uniformly while moulding, unlike in the conventional retreading process.  The toughened, cured tread is bonded to the tires at considerably lower temperatures, compared to cure mould retreading.

Comparative Features - Conventional vs. Pre-cured Retreading

Feature	Conventional	Pre-cured /cold
Mileage	Lower	Higher
Investment	Comparatively lower.  However, for comparable levels (as that of cold cure process), higher investment is required	Higher
Shelf Life	Limited shelf life for uncured tread strips	Long shelf life
Range	Besides truck and bus, larger tires like OTR tires can also be retreaded	Generally only truck and bus, LCV and  Passenger Car tires
Curing Temperature	Higher temperature 140ºC - 160ºC	Comparatively lower 100ºC-125ºC
Tread Composition- Process-Patterns	-    Natural rubber (NR) extruded unvulcanized strips used usage of different tread pattern is restricted	- Generally synthetic rubber (SR) or a blend or NR/SR with high quality carbon black -extruded and vulcanised (moulded with various designs) strips are used - flexibility in having different tread patterns
Range	Not suitable for radial tires	Ideal for retreading radial tires
Distortion	Tire undergoes distortion while curing in the mould due to variations in tire dimensions	No distortion in tires as no moulds are used
Cost	Lower cost	Marginally higher cost

All the operations such as buffing the tire, building the tread and curing while retreading are carried out in the inflated 'road running condition', without causing any distortion to the original casing unlike the case of mould retreading.  However, this is not in the case of smaller pre cured retreaders not having proper equipment.

Advantages of ‘Pre Cured’ Process:

a)         Precured rubber generally gives more mileage than the conventional rubber due to richer compound, denser tread and flatter profile;

b)         For radial tires, pre-cure retreading is the most ideal process. Radial tire, in its construction, has an inextensible belt.  In the conventional retreading process the matrix (rigid mould) should have exact dimensions matching the dimensions of a built radial tire.  Even a small difference in the matrix dimension affects the ultimate performance of the retreaded tire.  But in the case of pre-cure retreading, radial tires are cured in a chamber and not confined to a rigid matrix or mould, hence there is no distortion.

c)         Longer casing life because tire is retreaded in inflated normal road running condition.  Thus the casing is not put under tension and over stressed as it happens inside a rigid mould.  (Due to distortion of casing, the number of times a tire can be retreaded with conventional retreading is always lesser than with pre-cure retreading).

d)         Better balancing, due to uniform thickness of tread and better buffing and building techniques.

e)         Better traction due to flatter profile of buffing.

Advantages of Conventional retreading process

a)         More economical (compared to cold cure retreading).

b)         Comparative advantage (vs. pre cure retreading due to difficulties in  setting up franchisees everywhere in case of latter - high investment).

c)         Better aesthetics (compared to pre cure retreading since better finish given to the tire sidewall also).

Tire Retreading – Salient Features

i)    Since labour is one of the main components of tire retreading in the organized sector, it is more profitable to undertake high value addition business in the organized sector (i.e. retreading of truck and bus, light truck, jeep etc.).  Hence, pre cure retreading of other categories of tyres (especially farm, two wheelers etc.) is not as popular.

ii)   Retreading, as per international experience, finds greater and ready acceptance in the commercial segment since the main objective is 'savings' whereas in the passenger segment the focus is 'safety' followed by 'aesthetics'.  Moreover, savings are also very low in passenger segment.

iii)  In India, trucks carrying loads above 16 ton and plying on long distance routes do not find operational economy in using retreaded tires. However, trucks and buses plying short distance routes (and loading pattern upto 12 ton) find retreading a more economical option.

iv)  The current trend is going in favour of pre-cured primarily due to the following reasons:

• Better road conditions (with resultant less damage to tire casing)
• Better driving habits (getting more mileage even from retreaded tires)
• Overloading is comparatively less

·         Presence of large number of 'job shops' for retreading with each specializing in a special part of the process - repairing, buffing, curing etc. being handled by different persons/processors who, over the years, have gained sufficient experience and expertise in conventional retreading.

Pre-cured Tread Manufacturing Process

Compound Mixing

Typical Compound Formulation of Pre-cured Tread, Solution and Cushion Gum

Ingredients	Pre-Cured  Tread Compound phr	Solution Compound phr	Cushion Gum Compound phr
Natural Rubber ( RMA 4 )	70	100	100
Polybutadine Rubber (High cis type)	30	0	0
Rubber crumb ( 40 mesh )	5	0	0
WT Reclaim Rubber	5	0	10
Peptizer	0.15	0.2	0.3
Zinc Oxide (White Seal)	4	4	5
Stearic Acid	3	1.5	1.5
Antioxidant TDQ	1	1	1.5
Antioxidant 4020	1	0	0
MC Wax	0.8	0	0
Carbon Black N 339 / 220	65	0	0
Carbon Black N 550	0	35	30
Rubber Process Oil 710	12	12	0
Pine Tar	0	0	12
Wood Rosin	0	6	5
PF Resin	0	2	4
MF Resin	0	2.8	0
Insoluble Sulfur	0	0	3
Sulfur	2.3	3	0
Accelerator  NOBS	0.8	0	1
Accelerator TMTD	0	0	0.25
Retarder PVI	0.1	0	0.15
	200.15	167.50	173.70

Rubber compound is prepared by mixing rubber with different ingredients like fillers, process oil, activators, accelerators, curing agents, antioxidants etc. In order to achieve desired level of properties of the product. It is necessary to reinforce rubber with different fillers and vulcanize with sulfur with the help of accelerator etc.

 Mixing and mastication are carried out in an Internal Mixer ( Banbury ) or Kneader or Two Roll mixing mill. 

Rubber compounding is one of the most difficult and complex subjects to master in the field of Rubber Technology. There is no simple mathematical formulation to help the compounder. That is why compounding is so difficult a task.

Objectives of Compounding

a)               to secure properties in the finished product to satisfy service.

b)                         to attain processing characteristics for efficient utilisation of available equipment.

c)                          to achieve the desirable properties and possibility at the lowest possible cost.

Requirements for the success in compounding :

1.         The properties and functions of large number of elastomers and    

            rubber chemicals are to be understood.

            2.       Knowledge of the equipment used for mixing , extrusion , 

                      calendaring , moulding and vulcanisation are required.

A technical vulcanisate  is made up  of the following constituents :

1.                Base polymer or blend of polymers

2.                Crosslinking agents

3.                Accelerators

4.                Accelerator modifiers (Activators / retarders )

5.                Antidegradents

6.                Reinforcing fillers

7.                Processing aids

8.                Diluents

9.                Colouring materials

10.           Special Additives

After this, the selection of accelerators and activators are made. Their choice depends primarily on the vulcanising agents chosen , the polymer and  curing and service conditions. Plasticisers and / or softeners have to be compatible with the elastomer, effective with the type of filler, and should not cause problems of their own.

Finally, the question to be resolved is the age resistor package. Two factors are to be kept in mind while selecting antidegredants, providing suitable protection against the heat & environment (oxygen / ozone etc.) and those are not inimical to other additives or curative system.

Principles of Mixing

Vulcanizable polymers cannot be used without compounding. Various additives like curative system, protective system, reinforcing agents, cheapeners and other process aids have to be mixed to the polymer or polymer blend “to make a coherent homogenous mass of all these ingredients, which will process satisfactory and on Vulcanisation will give the product capable of giving the desired performance, all with the minimum expenditure of machine time and energy.”

Due to the partly elastic nature and very high viscosity of rubber, power intensive study machinery like mixing mills or internal mixers is necessary to achieve the mixing of additives into the polymer. The ingredients are in form of liquids, solid powders or solid agglomerates.

The mixing of solid ingredients into the solid polymer occurs in phases (Fig.1) during subdivision large lumps or agglomerates are broken down into smaller aggregates suitable for incorporation into the rubber. For instance carbon black pellets which have dimension of the order of 250-2000 µm get broken down into aggregates with dimensions of the order of 100 µm. Then these aggregates are absorbed or incorporated into the rubber to form a coherent mass. During mixing, shearing of the rubber generates shearing stress in rubber mass which imposes in turn shear stress on these aggregates and breaks these into their ultimate fine size which in case of carbon blacks is of the order of about 1µm. in size. This phase is also known as intensive mixing or homogenization in micromolecular level. Distribution or homogenization in micromolecular level or extensive mixing is “the moving of the agglomerates / particles from one point to another, without changing the shape of the particle to increase the randomness of the mixture”.

Figure – 1

Even after all ingredient is incorporated, dispersion / distribution of the ingredient is not complete Good distribution is comparatively easy to achieve by paying proper attention to  cutting and folding operations  on a mixing mill or by just prolonging the mixing cycle in an internal mixer . Dispersion however is dependent on the shear stresses generated within the polymer and hence good dispersion may not be achieved by prolonged mixing. Careful consideration is necessary not only as regards the time of the mixing cycle but also for the order of addition of ingredients to the rubber. Viscosity break down occurs during mixing and is essential for smooth processing of the stock.

Degree of dispersion of carbon black has profound influence on the physical properties of the vulcanisate. Undispersed carbon black (normally taken as carbon black agglomerates bigger in size than 9µm) act as gritty particles. Under tension, cracks develop at these spots. Failure properties like tensile strength, tear strength and consequently abrasion resistance come down as the degree of dispersion comes down.

Conditions for good dispersion in Internal Mixer

Narrow Clearance between Rotor

Tip and Chamber wall

(High Rate of Shear)

Correct Volume Loading

Adequate Ram Pressure

High Viscosity of Polymer

Low Polymer Temperature

(High Viscosity and More Prominent

Elastic Characteristics of Raw polymer)

For higher shear stress generation inside the polymer mass, polymer should have high viscosity. The temperature should be low so that thermoplasticity does not lead to lowering of polymer viscosity.

Any sweeping of carbon black at the end of mixing cycle is to be avoided in regular production. The MB is cooled / aged and then added to the cracker mill. Mechanical working of the cooled MB improves the degree of dispersion further.  MB is worked on cracker mill warming mills, feed mill and then to the extruder.

It is possible to follow the mixing process in the internal mixer with the help of power / time curve (or amperage of drive motor / time curve). When carbon black is added the torque does not rise immediately. The carbon black added as palletized black is about 30% higher than the total chamber volume. As the carbon black is slowly absorbed into the rubber the torque increases. As more and more carbon black gets absorbed, stock volume becomes lower and the power curve comes down.

Based on the power curve data on experimental batches, criteria like constant time or constant temperature are selected as dumping criteria. With constant time or constant temperature as the dump criteria, there will be variation in quality of the compound produced.

The better criterion is the constant energy criterion. This is very versatile, and will automatically take care of any minor variation in operating conditions as well as of even major ones to give a consistent quality output It can also be kept constant even when rotor rpm is changed or ram pressure is increased, while the time or temperature criteria will have to be re-established after a series of experiments.

 Two Roll Mixing Mill

  

Internal Mixer

Extruder and Extrusion Process

Extruders are machines, which shape rubber to a profiled strip by forcing it through a die. In the simplest form an extruder consists of four basic components viz. (a) a devise system (b) a barrel ( c )  a ram or screw for forcing the rubber through the barrel and (d) a head holding the die which ultimately gives the desired shape.

Extruder drive system comprises of an AC or DC motor along with a reduction gear unit using V-Belt drive for power transmission or connected directly using suitable couplings. The extruder drive has to turn the extruder screw at the desired speed. It should be able to maintain a constant screw speed because fluctuation in screw speed will result in throughput fluctuation, which in turn will cause fluctuations in the dimensions of the extrudate.

Screw type extruders  

A screw extruder consists of five components (a) drive system (b) a feed hopper ( c ) a screw rotating within (d) a barrel (e) a head and die. The basic principle is that the screw carries material from the feed hopper by acting as a conveyor or a hump providing pressure to extrude or force the material along the machine barrel through the head and the die.

Feed hopper:  the purpose of which is to receive material and pass it down to the flights of the screw. It is mostly supplied in the form of strip.

Barrel : within which the screw rotates, the usual clearance is approximately 0.40 mm. Normally, the barrel is fitted with a detachable liner  in the form of a sleeve which is highly wear and corrosion resistant made of hardened steel . The barrel is made double walled for steam or water circulation so that a constant temperature is maintained in the extruder head.

 Screw: A conventional extruder screw has three geometrically different sections.  

Feed section (closest to the feed opening) generally has deep flights and consists of     approximately 1/5^th of the length of the screw. The material in this section will be mostly in solid state.

Metering section (closest to the die) usually has shallow flights and consists of approximately 2/5^th of the length of the screw. The material in this section will     

 mostly be in molten state.

The third section, which connects the feed section and the metering section, is called as compression section. In going from feed section towards the metering section, a             compression of the material in the screw channel takes place, which is essential for the proper functioning of extruders. The compression favours streamline flow and helps to eliminate air and also ensures a constant pressure in the head.

Extruder designation often used is the length to diameter (L/D) ratio. This is an important factor in the selection of extruders to match process requirements. The length of a rubber extruder depends on whether it is a hot feed or cold feed extruder. Hot feed extruders are usually very short about 3D to 5D while cold feed ones range from 12D to 20 D).

Die: The purpose of which is to give the compound the desired shape.

The extrudates shrink along their length and increase in thickness and width, the behaviour being termed  “ die swell “ which depends upon  (i) rheological characteristics compound  (ii) shape of head and extrudate (iii) pressure in the head (iv) the head and compound temperatures .

The extruder is operated in such a manner so that temperature is gradually more from feed to discharge, the die being the hottest part.

Cold Feed Extruder

In recent year there has been an upward trend in production of cold feed extruder. In general sense an extruder which is fed with the compound at a temperature above ambient may be said to be a hot feed extruder. On the other hand any extruder where compound is fed at ambient temperature or below may be called a cold feed extruder.  Some of the differences may be classified as under.

Parameter	Hot Feed	Cold Feed
	Prewarmed hot feed above room temperature	No need to feed hot compound. It is fed at room temperature.
L : D Ratio	Generally hot feed extruders are having 5: 1 or 4: 1	Generally vary between 12: 1 to 16 : 1
Mills	A set of mill is required	No mills are required
Floor space requirement	More	Less - Because of no mills.
Power & water consumption	More	Less
Temperature control	Compound homogenised on mills & is a manual job	Compound homogenised with better control .
Feed system	Through a feed mill	Must-as metal pieces may harm the pins.

Pin Type Cold Feed Extruder

Most of the Cold Feed Extruders are now equipped with metallic pins fixed along the circumference in full length of screw. These pins help in homogenizing the compound. These number may be optimised by process & if required pins from last zone i.e. towards head may be blocked. These pins are generally 8 to 10 in circumference in 10 rows will have 8 X 10 = 80 pins.

The rubber compound is passed through a hot feed or cold feed extruder to produce rubber blanks of suitable size (width and thickness) for use in the next manufacturing step i.e. curing in a hydraulic press.

The extruded and cut rubber blanks are placed in tread dies and cured in a steam heated hydraulic press at suitable curing temperature and pressure. After the curing cycle is completed cured treads with desired tread pattern are taken out and cooled.

Cushion Gum

A three roll calandering machine is used to prepare “cushion gum” i.e. uncured rubber sheet that acts as an adhesive layer between the pre-cured tread and tyre casing during the pre-cured retreading process.

Cushion Gum Roll

The term “to calendar” is defined as “to press between rollers or plates in order to make smooth & glossy sheet”. Calandering of elastomer fit into later part of the defenition.

Calendar can be of two bowl or three bowl or multiple bowl machines which is used for Calendar rolls are not perfectly cylindrical but have different shapeRoll  a    -convex, Roll  b-concave, Roll  c   -parallel , Roll  d-convex.

Solution

A solution churner vessel is used for the preparation of cement solution (contact adhesive) applied to the casing and helps provide increased adhesion between the casing and the cushion.

Figure 2: Tire Tread

Multidaylight Hydraulic Press & Die

Multidaylight Hydraulic Press & Die