An earlier post on injection moulding of Dr.S.N.Chakravarty, (President – Elastomer Technology Development Society and Ex-Chairman, Indian Rubber Institute (IRI)), on this site was refreshing to many of the readers who wrote back to me because rubber machinery and rubber processing go hand in hand. One cannot be alienated from the other nor viewed in isolation.

On this note, here is another informative paper sent to me by Dr. Chakravarty – MIXING AND MIX DESIGN: ADVANCES IN MIXING TECHNOLOGY.

Here is Part 1 of this two-part series.

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INTRODUCTION

Rubber compounding is one of, if not the most difficult and complex subjects to master in the field of Rubber Technology. Compounding is not really a science. It is art, part science. In compounding one must cope with literally hundreds of variables in material and machine. There is no simple mathematical formulation to help the compounder. That is why compounding is so difficult a task

OBJECTIVES OF COMPOUNDING

1. to secure properties in the finished product to satisfy service.
2. to attain processing characteristics for efficient utilisation of available equipment.
3. to achieve the desirable properties and processibility at the lowest possible cost.

REQUIREMENTS FOR THE SUCCESS IN COMPOUNDING:

1. The properties and functions of hundreds of elastomers and rubber chemicals are to be understood.
2. Knowledge of the equipment used for mixing, extrusion, calendaring, moulding and vulcanisation are required.

THE INGREDIENTS AND FORMULATION OF A MIX:

Today, 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

 

In addition to the above, reclaimed rubber or vulcanised rubber crumb may be included and according to the manner of their use, function under groups 1, 7 or 8.

SELECTION OF POLYMER AND COMPOUNDING INGREDIENTS:

POLYMER

Rubbers are viscoelastic materials of low rigidity exhibiting large strain elasticity. The deformation imposed on rubber components are far larger than those encountered for most other materials, and the stress-strain relationships are correspondingly more complex.

The ability of rubber to store elastic energy depends largely on the type of polymer used. In general, polymers having relatively high glass transition temperatures exhibit the highest energy losses during deformation. These energy losses are exploited in components  intended to damp motion, but generally the higher the damping obtained the more sensitive are the modules and damping to frequency and temperature.

The tensile strength of a rubber is low compared with other materials but the energy storage capacity at break can be greater than that of an equivalent mass of steel. Failure of rubber components rarely occurs by simple tensile failure: tearing or fatigue crack growth is more likely.

A major factor determining the strength of rubber is an ability to crystallise under the influence of an applied strain. Rubbers possessing this ability (e.g., NR and CR) are intrinsically strong while those that do not crystallise rely on the incorporation of reinforcing fillers to impart adequate strength.

A limitation on the use of rubbers in some applications is the effect of certain fluids. The extent of swelling or property change in a given fluid is critically dependent on both the rubber and the fluid. Selection of a rubber for a given application should take into account its resistance to any fluid it is likely to be in contact with in service. A similar consideration applies to the effects of temperature and the climate in which a product is to be used.

Relative ratings of different polymer vulcanisates are as shown below:

TEMPERATURE RANGE OF MOST COMMON ELASTOMERS

Elastomer Base      Temp Range (°C)
CR                         40 – 100
IIR                         40 – 120
NBR                       40 – 100
NR                         55 – 90
SBR                        50 – 100
CSM                        20 – 120
EPDM                      50 – 150
FPM                        20 – 200
VMQ                       60 – 200

Next on the list would be the vulcanising agent. This would be between sulphur, sulphur donors, metallic oxides, urethane crosslinkers, or resin cures. This decision is somewhat easier since it depends largely on the type of polymer.

The third to choose is for the most appropriate filler and the amount. This of course does not occur with purge gum compounds. The colour desired, the hardness if specified, the service environment will be some of the factors in that decision.

HARDNESS IMPARTING RATIO OF CARBON BLACKS	FOR EVERY 1 POINT RISE IN HARDNESS ADD
SAF                 0.80	SAF                 1.6
ISAF               0.90	ISAF                1.8
HAF                1.00	HAF                 2.0
FEF                 1.05	FEF                  2.1
GPF                 1.10	GPF                  2.2
SRF                 1.20	SRF                   2.4

• For every 1 point rise in hardness add 2.0 phr of carbon black (HAF)
• For every 2.0 phr of process oil added, hardness drops by 1 point.
• For every 2.0 phr of oil added, hardness drops by 1 point.

Then the selection of accelerators and activators are made. Their choice depends primarily on the vulcanising agents chosen, next the polymer and then 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.

The last fundamental question to be resolved is the age resistor package. Two conditions have to be satisfied here, providing suitable protection against the environment and not choosing agents inimical to the softeners or curative system.

Antioxidant Class	Natural ageing	Heat ageing	Flexing	Resistance to Staining	Copper & manganese
Acetone/Diphenylamine  condensates	2-4	4-6	1-5	1-2	2
Acetone/Aniline condensates	2-4	4	1-2	1-2	1
Phenyl-betanapthyl amines	4	4	4	1	2
Para-Phenylene diamines	4	4	4-6	1	4-5
Substituted Phonols	2-3	1-3	1-2	5-6	1

Obviously for some compounds further choices have to be made: fire retardance, electrical conductance, particular colour etc.

COMPOUND FOR VULCANISATE PROPERTIES

Hardness : Hardness, as measured by an indentation test is a semi empirical measure of modulus .

Tensile Strength : Tensile strength is one of the most widely determined properties of rubber. Tensile strength varies widely with rubber type and is sensitive to compounding, passing through a maximum as cross-link density and hardness are raised. Elongation at break usually decreases with increasing stiffness.

Tear resistance : Tear is a localised strength failure in a material whose bulk strain is below its breaking point. Resistance can be improved by reinforcing fillers.

Fatigue resistance : Like other materials rubber can undergo fatigue failure as a result of crack growth and , as in the case of tear, this can occur at tensile deformations well below the breaking strain.

Fatigue life increases rapidly as the maximum strain imposed is reduced. Atmospheric oxygen can reduce the fatigue limit and increase the rate of crack growth. At strains below the mechanico-oxidative fatigue limit as slow crack growth many occur as a result of ozone attack in unsaturated rubbers. IN strain crystallising rubbers, fatigue life is enhanced in components that do not return to zero strain during cycling.

Compression set : The extent of resultant deformation when rubber is subjected to a distorting load after release of load is known as compression set.

1. Optimum resistance to compression set is developed as cure continues beyond the      level normally considered adequate to obtain a good general level of properties.
2. Thiurmas in conventional systems give lower compression set than do thiazoles and sulphonamides alone.
3. Partial replacement of sulphonamide with thiuram gives compression set resistance approaching that of thiuram used alone. These systems give good compromise between cost and technical performance.
4. Semi EV system are superior to conventional system.

 

Tension Set : The tensile analogue of compression set is termed tension set and is defined a residual tensile strain in a rubber after it has been stretched either to a given tensile strain and released.

Resistance to liquids : By proper selection of the rubber type and other compounding ingredients, products can be designed for satisfactory use with wide  range of liquids. Contact with an aggressive liquid can have two effects on rubber. The more obvious is change of dimension due to swelling which may be positive because of absorption of liquid or negative because of extraction of soluble compounding ingredients, e.g. Ester plasticizers by fuels. Swelling is diffusion controlled process. The rate of penetration depends more on the viscosity of the fluid rather than its exact chemical nature.

Ageing resistance : It is known that many factors such as oxygen, ozone, sunlight, metal irons, heat and mechanical conditions may markedly contribute to the deterioration of rubber with the passage  of time. Although complete prevention of degradation is impossible inhibitions can be done by use of antioxidants to minimise oxidation and carbon black to reduce sunlight effects.

Heat resistance : In general, resistance to high temperature is a function of polymer structure and Crosslinking systems.

Low temperature resistance : As the temperature falls there is an increase in stiffness the rubber passing through an intermediate transition  state until it becomes brittle solid  (glass hardening). The use of plasticizer may improve the low temperature flexibility. An additional factor in low temperature behaviour is crystallisation which may occur with certain rubbers particularly with NR and CR.

In Part 2 of this article, you will read more deeply into PRINCIPLES OF MIXING.

Dr. Chakravarty can be reached on kpspltd@gmail.com

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