Global Challenges in Rubber Industries – Importance of In-house Testing

By Kaushik Mishra
Ceat Research Centre, Ceat Limited, Mumbai

INTRODUCTION

“proof of Pudding is in the eating” is an old saying. Thus the criterion for product acceptance is its performance in service. Customer expectations of the Product are ever increasing. Product liability and competitiveness force manufacturers to produce articles of consistent quality at right price. Testing is a tool of utmost importance in achieving these goals, may it be for material, process or final product.

TECHNICAL REASONS FOR TESTING

1. Rubber is a unusual and complex material. When compounded in a particular way it exhibits a unique combination of physical properties.

2. It is highly deformable with possibility of complete recovery. It’s shear and Young’s modulii are far higher than the bulk modulus.

3. Even if rubber compounds are made in identical ways, the properties may not be identical, unlike the behaviour of metal.

4. Conditions and test procedures are of paramount important in rubber testing. For example, variation in testing speed in a tensile machine may result in a significant difference in modulus.

5. Test results may differ depending upon the stress history of the test Specimen.

The above points illustrate the fact that this unusual class of material requires carefully worked out testing procedure. Methods or philosophy used for other materials cannot be simply adopted for rubber if meaningful results are to be obtained.

COMMERCIAL ASPECTS OF TESTING

In India there are a few thousands of small and large scale manufacturers of rubber articles. Over the years local suppliers of rubber articles have become a global players with the development of Internet, e-mail, e-commerce and e-business.

It is hard to believe, but true, that there are many small companies testing the final product by smelling or chewing by skilled operator or supervisor. A product manufactured and tested in this manner can never meet customer expectations. Customer satisfaction is a must to survive in the global market. Therefore proper testing assumes position of great importance.

CURRENT REQUIREMENTS FOR RUBBER PRODUCTS

1. Better material which means

(a) Longer life
(b) High temperature capability (160ΊC+)
(c) Low temperature flexibility (-40ΊC)
(d) Low permeation rate
(e) Odorless
(f) Reliable long term sealing.

2. More relevant test and better data

(a) Long term (>200 hrs) ageing data
(b) Data for CAE/FEA modeling

3. Global availability
4. Design Support
5. Green Materials should

(a) Be Recyclable
(b) Use Recyclates
(c) Eliminate Toxic/Carcinogenic/Hazardous ingredients

6. Low Price, (for example, China could be a big threat not only for rubber industries but also for all others).

Let us take specific example of a critical component of a vehicle, such as a tyre, and understand its global requirements.

 

Features	Present Situation	New Demand
Rolling Resistance Tread Wear Ride (Comfort) Handing Dry Traction Wet Traction	100 100 100 100 100 100	112 110 102 120 120 110

There are 500-600 rubber components used in a vehicle and tyre is just one of them. Therefore it is obvious that each of them component manufacturers has to take into account a set of features pertaining to the specific article. These features need to be tested and ensured both while designing and manufacturing.

VARIETY IN TESTING FOR BETTER UNDER-STANDING AND VALUE ENGINEERING

Tests carried out a few years ago are no more adequate because requirement have changed and additional testing is needed, for example some countries specify the level of ozone resistance required in the tyres used by them depending on the environment. Obviously test becomes customer specific, involving more number of tests. Right product at right cost is the wathword. Naturally selection of materials, optimization of cost and product performance call of variety of tests.

TESTING REQUIRED FOR NEW MATREIALS

New raw materials have been developed and are marketed to enhance process or product performance. A few examples are:

• High vinyl content SBR which provides improved wet traction while maintaining low rolling resistance.
• Solution SBR and chemically modified SBR are used for lower rolling resistance and better wear resistance.
• New antioxidants and antiozonants have been developed to improve aged properties for extended service.
• The carbon black and other filler producers have been constantly trying to develop improved reinforcement materials for better wear and low heat buildup.

To assess the effect of these materials or the product additional tests become a necessity.

The scope of this Paper is to highlight the importance and critical aspects of in-house testing. Therefore various test methods will not be touched upon. All the same, lately Dynamic properties have been useful in developing rubber compounds and improving performance of components. Here it will be discussed. Apart from this the different standards, classification of tests etc. will also be covered.

LIST OF COMMONLY USED
TESTS IN RUBBER INDUSTRY:

Tests on unvulcanised rubber

(a) Visco-elastic flow
(b) Cure Characteristics
(c) Tack
(d) Green Strength
(e) Shinkage

TESTS ON VULCANISED RUBBER

1. Specific gravity
2. Shot term stress-strain properties

(i) Hardness
(ii) Stress-strain]

1. Tensile
2. Compression
3. Shear
4. Flexural/Bending

(iii) Tear

1. Trouser
2. Angle
3. Crescent

(iv) Abrasion Resistance

3. Longterm stress-Strain Properties

(i) Creep
(ii) Relaxation
(iii) Set

4. Dynamic Stress-Strain Properties
5. Fatigue

(i) Flex-Cracking
(ii) Cut growth

6. Effect to Temperature

(i) Heat Ageing
(ii) Thermal Expansion
(iii) Transition Temperature
(iv) Low Temperature Test
1. Recovery test
2. Change in Stiffness
3. Brittleness Temperature
4. Crystallization

7. Electrical Test
(i) Resistance and Restivity
(ii) Surface Changes
(iii) Electrical Strength
(iv) Tracking Resistance
(v) Permitivity and Power factor

8. Thermal Properties
(i) Thermal Analysis
(ii) Specific Heat
(iii) Thermal Conductivity
(iv) Thermal Diffusivity
(v) Surface Heat Transfer Coefficient

9. Environmental Resistance
(i) Thermal Analysis
(ii) Specific Heat
(iii) Thermal Conductivity
(iv) Thermal Diffusivity
(v) Surface Heat Transfer Coefficient

10. Permeability
(i) Gap
(ii) Vapour

11. Adhesion/Corrosion/Staining
(i) To Metal
(ii) To Fabric
(iii) To Cord

12. Safety and Health Regulations.

The above tests can be grouped into three classes depending on the purpose for which they are used.

(a) Purely for the quality control
(b) For Performance requirement
(c) For development purposes.

For quality control purposes the tests adopted should be relatively simple and rapid. It is important that the same set of procedures is followed under a set of defined and precise conditions.

When the focus shifts to performance related tests, unlike tests for quality control, the conditions are arbitrarily chosen. These conditions should have relevance to environment of product end use.

In purely research and development area the test conditions will be decided by the scientists or designers to suit individual requirements.

Irrespective of the the purpose of the tests the individual concerned should be fully aware of the details and conditions of the tests and it will be his responsibility to make an intelligent choice.

STANDARDS FOR TEST METHODS AND SPECIFICATIONS.

Generally the source of the standards can be placed into three groups.
(a). International Organization (I.S.O.) – The Principle body and the most important organization in the standards field.

(b). Generally each country has one principle standard organization with membership of ISO. There may be other organizations also issuing standards at national level.

Some will known standards are:

C. Individual Companies: There may be many in-house company standards in existence. These standards appear in commercial contracts. Some of these standards are also used internationally. For example.
1. MG General Motor Standard
2. Ford Standard
3. Chrysler Standard

The important precaution to be excerised while using Standards is that the user must refer to the latest edition and must be aware of all the minutes details.

SOME IMPORTANT ASPECTS OF TESTING

Current efforts in the test method development have concentrated on non-destructive testing. Holographic ultrasonic, infrared radiation, and X-ray testing as well as nuclear, capacitive, Optical and magnetic guage have been developed to give an indication of product quality without destroying the specimen. However, the conventional testing is still very significant and is being continued in rubber industry.

Let us discuss some important aspects of rubber testing which sometimes we tend to forget being a routine activity. Before going into details of these aspects, to understand their importance let us take some practical examples, which occasionally happen in industry.



1. One Institution in India, which was engaged in evaluating aged properties, was using an ageing oven without replenishment of air. Obviously, the results obtained were erroneous and far superior to actual values obtained in other laboratories with the standard multi-cell ageing oven with right type of air replenishment provision. One could imagine the problem associated with such testing and establishing purchase specifications.

2. The second example is concerning a laboratory, which was involved in testing of ozone resistance when tested in that laboratory, results were more than satisfactory but in service, ozone cracking was observed with in a short period. It was finally diagnosed that in the absence of proper calibration of ozone concentration in the test chamber, 30 pphm was present against 50 pphm (Standard) required.

Any testing should have the following intrinsic features:

(a) Variability
(b) Accuracy and precision
© Repeatability
(d) Reproducibility

Two examples cited earlier illustrate some of the above features. If the above aspects are fulfilled the test will be predictable and simulative.

A test is valid if the results are actually a measure of the desired property.

Even though an apparatus and method may be valid, however, the accuracy can be impaired by either instrumental or procedural errors.

A test is precise if the results are closely reproducible.

Precision of the results within the given laboratory is repeatability and between laboratories is reproducibility.

Problems of validity, accuracy and precision can cause a wide scatter of data.

Poor resistibility and reproducibility is ultimately the results of variability in sample history, specimen preparation, inadequate control to test condition, calibration of test and conditioning equipment, etc.

Let us discuss the above aspects in detail, which matter a lot in testing, however, these aspects are ignored and neglected often.

1. SAMPME HISTORY:

• It is necessary that the rubber samples should not be subjected to high temperature or other conditions likely to cause deterioration (ozone and other chemicals) during the storage period.
• The limit of storage temperature and humidity should ideally be 10 to 30ΊC and below 80% R.H.
• Different specimens must be sparated to avoid migration of constituents. Special attention needs to be given when the surface condition of the test piece is important, for example, ozone or paint staining test.
• Acceptable specified Minimum time period between vulcanization and testing is 16 hrs. The maximum of the non-product testing is 4 weeks and for product shall be 3 months.

2. PREPARATION OF TEST PIECES: In which the following processes are involved:

Mixing of compounds, moulding of Sheet
• In mixing of compounds in a two-roll mill of 150-mm dia x 300-mm length temperature control is required within ±5ΊC.
• Tolerance allowed on the weighment of ingredients is 025% or 10 mgs whichever is greater. The limits on the difference between the sum of the mass of the ingredients and final mass of the mixed batch is 0.3% for a gum mix and 0.6% for a filled mix.
• The condition and the time of storage between mixing and vulcanization can affect the properties of the vulcanisate. Hence after mixing, the batch should be stored in a dark and a dry atmosphere. The minimum time allowed between mixing and vulcanization is 2 hrs.. Preferred maximum is 24 hrs. and absolute max is 72 hrs.
• Apart from molding temperature (tolerance of which is ±5ΊC), the loading and unloading of the mould into the press should be done within 45 seconds.

Cutting from the sheet:

• To a cut a dumbbell from a cured sheet only a die and a press are required. There is a tendency to treat this operation as very simple. Without taking much care. The fact is that, accuracy of the final test results depends considerably on the accuracy with which the test place was prepared.
• It is essential that the cutter is very sharp and free from nicks or uneven cutting edges, which would produce flaws in the test piece.
• The effect of blunt cuts was found to lower the tensile strength by 8%
• Poor reproducibility is caused some time by blunt or chipped cutting dies. These small issues need hours of attention, though generally they are taken for granted.

Test Pieces From Finished Products:

• Some times it is desirable to make the test on actual products for which cutting or buffing operation is required.
• In practice cutting from a large product is often carried out in arbitrary fashion using variety of knives and hammers. It is suggested both ISO and BS that cutting should be by a BANDKNIFE LEATHER/RUBBER SLITTING machine. These machines are precise, extremely efficient.
• Buffing is used either for removal of sample irreguarilities or moderate reduction of thickness. It should not be used for removal of large quantity of material while preparing the specimen. High heat generation cause sufficient degradation or rubber surface and properties. A study shows tensile strength is lowered by 15% on soft rubber and for a tire tread the drop was about 5% when a smooth surface was obtained by careful buffing.

3. CONDITIONING: Apart from the actual test condition (temperature and humidity) which are specified for each test in the standard, the properties of the rubber compounds depend on conditioning of specimen before actual testing. All the test methods specify the conditioning period, prior to test, in the standard atmosphere. The objective of conditioning is to brig the test pieces in equillbrium with the test atmosphere.

Requirement for Conditioning:

• Team & Relative Humidity:

a) 23°C & 50% R.H. &
b) 27°C & 65% R.H-> useful for tropical countries.

1. Test where only temp is specified, it should be 23 or 27°C
2. For textiles or composite o textile and rubber the condition should be 20°C and 65% RH
3. List for testing at sub-ambient temperature is given in the standard.

For all the above standard Tolerance:
±2ΊC on temperature
±5ΊC on RH

• Time:
When both temperature and RH are controlled the standard conditioning time is a minimum of 16 hrs. if only temperature is to be controlled the period is min of 3 hrs. At the subnormal and elevated temperate, it is simply specified that conditioning time should be sufficient for the test piece to reach equilibrium.


• Special cases like

1. After accelerate aging
2. Preparations other than molding like buffing or cutting
3. Mechanical Conditioning.
ISO & BS Specify different conditions periods, which will be available in the standard.

Apparatus Involved in Conditioning

• Air Conditioned room: Rubber conditioning and testing calls for temperature and humidity control. Therefore, the test room requires air conditioning. Skipping this necessity for economy reasons will not be advisable because repeatability and reproducibility of results will be adversely affected.
• Hygrometers and thermometers: For humidity control the standard hygrometer (Wet & dry bulb type) should be kept in a place where air is circulating around the hygrometer at a velocity of not let less than 3 m/sec. Dry and wet bulb hygrometer is the most reliable. Some people because of their size and relatively lower price choose simple AIR OR PAPER hygrometer. They are very often inaccurate and required calibration regularly.

The standard mercury thermometer is taken for granted as the most accurate, however, they too need careful inspection for separation of mercury in the bulb. While measuring temperature, immersion of the bulb to the right depth is to be ensured.

4. CALIBRATION: is defined as “the set of operations which establish, under specified conditions, the relationship between values indicated by a masuring instrument or measuring system, or values represented and the corresponding known values of the measurement.”

While all aspects of a laboratory’s operations require systematic controls, it is calibration of the test equipment which gives rise to most of the problems. It is expensive too. A few years ago all pieces of equipment were not formally calibrated in most polymer laboratories. It is still not universally accepted that all the test equipment and every parameter of each instrument require formal calibration. For example, apart from calibrating the force scale of tensile machine, there are also requirements for speed of traverse, associated cutting dies and dial gauges.

Dynamic Testing: let us take an example of Dynamic Testing, which is very much sensitive to test conditions. All the static tests treat rubber as an elastic material, where as in fact it is Viscoelastic material. Hence its response to Dynamic Stressing is a combination of an elastic & viscous response.

• Elastic response is required to

Maintain the shape of the rubber article after cyclic deformation. In this process energy is retained and stored. It reduces heat build up and improves flex fatigue. In the case of tyres, rolling resistance reduces resulting in fuel economy.

• Viscous response is important to
Dissipate noise and shock vibration in bushings, seals & tyres. Contribute to the frictional forces required to prevent tyre slippage while cornering. In viscous behavior energy applied to the body causes deformation and the deformed body does not return to its original shape. Therefore, the part of the applied energy gets dissipated and converted in to heat.

• Information obtained from dynamic testing:
• Resilience: The ration of energy released on recovery, to the energy required to produce deformation
• Hysteresis: Percent energy loss per cycle.
• Modulus: ratio of stress and strain under vibratory condition.
• Damping: is a result of hysteresis.

• Depending upon the Dynamic stressing, tests are divided into three main categories:

• Free Vibration: In which the test piece is set into oscillate and the amplitude allowed to decay due to damping in the system.

• Impact Vibration: Test piece is subjected to part of deformation only.

• Falling ball rebound type

• Pendulum rebound type

• Forced vibration: Oscillation is maintained by external means.

• In free Vibration test two well known equipment are:

• Yerzley oscillograph – it consists of a horizontal beam pivoted so as to oscillate vertically and in so doing deform the test piece mounted between the beam and a fixed support. A pen attached to one end of the beam records the decaying train of oscillation on a revolving drum chart. Yerzley resilience is calculated from the ratio of two successive decaying amplitudes recorded on the chart.

• Torsion Pendulum- the bottom of a ribbon-shaped specimen is rigidly attached while the top is fastened, either solidly or throught torsion wire, to a structure having as adjustable moment of inertia. When the test is performed, the inertial system is displaced through a small angle and released. The shear modulus is calculated from the period. Shorter the period greater the modulus.

• In impact vibration commonly available equipment are:

• Falling ball rebound- if the drop height of a steel ball is divided in to 100 equal parts, the rebound height is equal to the resilience.

• Pendulum rebound test- the specimen, held at a rest position of the pendulm (zero degree), and is impacted by the center of percussion of the arm. The angle of rebound is followed on a scale and resilience calculated by the formula:


      1-cos (angle of rebound)
R = ------------------------------- x 100
         1-cos (angle of fall)

The correlation between the rebound tests is very poor. Results are influenced by plunger weight, design or impacting head, drop height, penetration and energy-absorbed n the apparatus. In any rebound test the specimen should be preconditioned with about six impacts before a reading is taken.

Rebound tests are very sensitive to bulk temperature but not to the surface temperature. Hence conditioning of the samples before testing is very important.

• Forced Vibration at resonance
• Forced Vibration away from resonance
• Constant stress
• Constant strain
• Another classification could be on the basis of the vibrator used for oscillating the sample:
• Mechanical (force range max 2 Hz)
• RAPRA sinusoldal strain machine
• Electromagnetic (force range up to 104 Hz)
• Closed-loop servo hydraulic (force range up to 100 Hz)
• MTS/DMTS (FROM GABO), etc.
• The international standard for force vibration measurement is ISO 1464. it does not specify any particular machine but calls for apparatus giving forced sinusoidal displacement cycles in shear by means of any of the above vibrator, the displacement being determined to an accuracy of ±2.5%, tan delta be measurable to ±5% when the testing is done at 10±0.5 Hz.


• No standard gives all details of carrying out the actual tests. This is basically dependent on the apparatus used. It may be somewhat difficult to decide which dynamic test machine should be used in a given circumstance this will depend to a large extent on for what the results are needed and how they will be used.
• One should understand the important criteria for dynamic testing i.e. the results are dependent on the test conditions, test piece shape, mode of deformation. Strain amplitude, strain history, frequency and temperature. It is not sufficient to speak about dynamic test results, unless the details of the measurement are specified.

CONCLUSION

In this paper we have tried to consolidate our thought in understanding the importance of some of and how rigidly these aspects are followed in laboratories. It was not possible to describe the minute details of all the testing, however, our efforts was to demonstrate what level of importance should be given to preparatory activities, and test conditions apart from the test per se, to achieve better product through optimal product design.

REFERENCES

1. RAPRA review: 1992 vol. 5 No. 10,
2. physical testing of rubber: R.P. Brown 2nd edition.
3. Rubber technology: Maurice Morton, 2nd edition.
4. ISO, Guide 25, General requirements for the competence of calibration and testing laboratories.
5. ISO, 4661, part 1 & 2 Rubber, Vulcanisate, Preparation of samples and test pieces.
6. ASTM, 1995, Vol. 9.10 & 9.02.
7. Rapra, Polymer testing, 1995.

ACKNOWLEDGEMENT

We express our thanks to CEAT management specifically to Mr. P.K. Mohamed, Mr, P.V. Alva and Mr. N. Ganesh for their help and encouragement during Preparation of this paper. I would also like to thank Mr. Ganesh Kamath for his valuable contribution.