Basic rubber compounding

BASIC RUBBER COMPOUNDING

Luis TormentoFebruary/2016

Introduction

• The objective of this presentation is to give an overview of rubber compounding. We will briefly focus on:– Elastomer System– Filler System– Protection system– Process Aids– Cure System

What is Rubber?

• The unique property of rubber is that it is elastic. When rubber is stretched, the molecular bonds can be extended out. When released, the molecules coil back to their original shape

• In its raw state rubber consists of long randomly kinked hydrocarbon chains which can slide past each other. Raw rubber is therefore plastic, weak and permanently deformable. The purpose of vulcanization (also called curing) is to chemically link the rubber chains together by "crosslinks" to form a three-dimensional network.

What is Rubber?• The process of vulcanization can improve the quality of

rubber. Raw rubber is heated during vulcanization. This makes the rubber hard, more elastic and strong.

• During the process of vulcanization, some atoms attach themselves to extra loose bonds in the rubber molecule and also cross-link the molecules. The cross-linking locks the molecules in place and prevents slipping. Thus making the vulcanized rubber more strong. Vulcanized rubber is non-sticky and has higher elasticity. It does not loose its properties easily and can be used in a temperature range of - 40 °C to 100 °C.

What is Rubber?

• Vulcanized rubber will have the following characteristics : – the vulcanizate undergoes deformation upon

stretching and when released can recover almost completely its original dimension over time

– the vulcanizate does not dissolve in a good solvent for uncrosslinked rubber but shows swelling

– the properties of a vulcanizate are less sensitive to temperature.

Is only vulcanization sufficient?

• The answer for the question is not, vulcanization will improve some properties but other deficiencies could by improved by a process called Rubber Compounding.

Rubber Compounding

• In any molded rubber product, we have three major influences on the quality of the part, that we can control:– Mold– Process– Rubber formulation.

• The last has the major influence on rubber properties.

Rubber Compounding• In plastics, designers can use material suppliers’ specification

data to determine what material will work best in their application, but in rubber, that information is not readily available.

• The reason is the plastic molder purchases their materials ready to process and the resulting physical and environmental resistance properties are controlled by that supplier.

• In contrast, the rubber molder purchases ingredients from many suppliers and mixes them to form the material that is processed in the mold, therefore, each molder controls the material's and resulting end product's properties.

Rubber Compounding

• Often designers have little experience with rubber and have no idea how to select the rubber that is best for their application.

• If for example he/she will search on general literature for a hydrocarbon/petrol resistant product, the choice naturally will be an NBR. He/she will believe that specifying a NBR and hardness will be sufficient, but it is wrong!

Rubber Compounding• What the designer doesn't realize is that rubber compounders

have over 180 different commercial grades of nitrile from which to select! – These vary by acrylonitrile (ACN) content, viscosity, chemistry, and

supplier ("equivalent" grades usually are not) among other parameters. – On top of that, we will combine the base polymer (often two or more

grades) with approximately 10 - 20 other ingredients.– These other ingredients enable us to achieve the desired physical,

environmental resistance and processing properties required to mold the end product.

– In addition, we have literally thousands of chemicals to choose from when selecting those other ingredients. As you can see the possible combinations and resulting material variations are infinite.

Rubber Compounding

• In the majority of materials, just adding vulcanizing chemicals will produce a product so weak, in its mechanical and environmental resistance characteristics that it is almost unusable.

• Not only can we improve an elastomer’s physical strengths by compounding other ingredients into the mixture of elastomer and sulfur, but also we can improve other characteristics and even create some that do not naturally occur.

Rubber Compounding

• For to make a compound we could divide it on five main system, as follows:– Elastomer system– Filler system– Protection system– Process aids– Cure system

Rubber Compounding• An important detail to note is that nearly all compounders us the

unit of measure of "parts per hundred" (PHR) for their formula. This is a unit of weight for the relationship between the elastomer system and the other systems. If we always utilize 100 parts of elastomer for all formulas then it is much simpler to change the other systems to create changes and different formulas.

• The reason this is so important is that the cure system reacts only with the elastomer system. Thus, as we change all the other systems, the relationship between the elastomer system and the cure system remain constant with a few exceptions that we won’t address here.

Rubber Compounding

• Elastomer system– One or more elastomers blended, to enhance or

achieve desired properties.– Elastomer selection for improved processing

Rubber Compounding• NATURAL RUBBER

– The raw material to make natural rubber actually does come from trees– Produces compounds with high tensile strength, tear strength, tear and abrasion resistance– Can be used at lower temperatures, low compression set, and high resilience– Not recommended for severe applications with oil and solvent exposure; subject to aging by sun,

ozone, and heat– Also not good for applications in contact with concentrated acids or alkalis– Maximum continuous operating temperature is about 225°F

• NEOPRENE (CHLOROPRENE)– Good general purpose rubber with properties close to natural rubber, but is synthetically produced– Better resistance to oils and solvents compared to natural rubber but similar low compression set– Can be compounded for flame resistance– Good weathering resistance– Poorer low temperature performance compared to natural rubber– Not good in applications with concentrated acids or alkalis– Maximum continuous operating temperature is about 275°F

Rubber Compounding• NITRILE (BUNA)

– Much better oil and solvent resistance compared to either natural rubber or Neoprene– Recommended for most oil field applications– Can be formulated for use at low temperatures– Good compression set and abrasion resistance, but poor weathering resistance– Can be used with concentrated acids and alkalis but there are better alternatives– Maximum continuous operating temperature is about 275°F

• HNBR (HYDROGENATED NITRILE)– Similar to Nitrile but with improvements in heat and ozone resistance– Can be formulated for low temperature applications– Excellent for oil field service– Usually not recommended in applications with concentrated acids or alkalis– Very high cost– Maximum continuous operating temperature is about 350°F

Rubber Compounding• STYRENE BUTADIENE (SBR)

– Originally developed as a low cost substitute for natural rubber– Good water resistance and abrasion resistance– Poor weathering resistance, but can overcome with specific raw materials– Not recommended for contact with oils and solvents– Not used with concentrated acids or alkalis– Maximum continuous operating temperature is about 225°F

• BUTYL– Very good resistance to most gases including air– highly resistant to ozone and weathering– Abrasion resistance close to natural rubber and good for concentrated acids and

alkalis– Not recommended for petroleum product exposure– Maximum continuous operating temperature is about 300°F

Rubber Compounding• EPDM

– Exceptional resistance to weathering and ozone– Excellent resistance to water, most gases, steam, and heat aging– Good for exposure to concentrated acids and alkalies, but not recommended

for exposure to oils and solvents– Maximum continuous operating temperature is about 350°F

• FKM (VITON®)– High cost, but high performance material– Outstanding resistance to most chemicals, oils and solvents– Good oxidation and ozone resistance– Maximum continuous operating temperature is about 650°F– "Viton" is a trademark of DuPont and signifies material produced by DuPont

Rubber Compounding

• Filler System– They are composed of:

• Reinforcing filler• Semi and non-reinforcing filers• Plasticizers

Rubber Compounding

• Filler System• Reinforcing filler – typically are:

– Carbon black– Precipitated silica– Fumed silica

Rubber Compounding

• Filler System• Semi and non-reinforcing filers

– Typically are used to:» Reduce cost» Improve processing» Increase hardness» Tensile strength» Tear resistance» Abrasion resistance

Rubber Compounding

• Filler System• Plasticizers

– Plasticizers (UK: plasticisers) or dispersants are additives that increase the plasticity or fluidity of a material. The plasticizers also control the hardness. The dominant applications are for plastics, especially PVC, and rubbers. The properties of other materials are also improved when blended with plasticizers including silica, carbon black, clays, and related products

Rubber Compounding

• Protection system– All polymers & products based on them are subject to

degradation on exposure to the degradative environments such as: - Storage aging - Oxygen - Heat - UV Light & Weathering - Catalytic degradation due to the presence of heavy metal Ions (Cu, Mn, Fe etc.) - Dynamic Flex - Fatigue - Ozone (Static / Dynamic / Intermittent exposure) These factors degrade rubbers / rubber products causing substantial changes in their technical properties and ultimately lead to their failure during service or shorten the expected service life in the absence of Antioxidants.

Rubber Compounding

• Protection system– Antioxidants– Antiozonants– Initiators/promoters

Rubber CompoundingType of Degradation Initiating /

Accelerating FactorsDegradation Causes

Type of Failure

Storage Aging Surrounding conditions Oxygen, Light, Heat, Humidity

Loss of elasticity and tensile strength

Aging due to Heat Heat Oxygen Loss of elasticity and tensile strength

Aging due to Light & Weathering Light, UV light, heat, humidity, surrounding conditions

Oxygen Formation of crazed surface, loss of elasticity and tensile strength

Soluble Metal ion (Cu,Mn,Fe) catalyzed oxidation

Cu, Mn, Fe, Co & Ni Ions as their rubber soluble fatty acid salts

Oxygen Rapid loss in elasticity and tensile strength.

Flex-Fatigue cracking Intermittent mechanical strains

Oxygen, Ozone, Flaws

Appearance of cracks (cracking patterns usually complex)

Ozone Cracking Continuous / Intermittent strains

Ozone Extensive cracking at right angles to the force causing strain

Rubber Compounding

• Process Aids– High productivity is very important to the rubber

industry because it is usual to encounter large variations of raw materials.

– Processing aids are used mostly to improve the production process, i.e. flow rate, homogeneity, tackiness etc.; This product range includes lubricants, tackifiers, homogenizers, chemicals dispersing agents, peptizers, plasticizers, flow promoter and oil.

Rubber Compounding

• Process Aids– Mixing Aids– Molding Aids

Rubber Compounding

• Cure system– Vulcanizing Agents– Activators– Accelerators– Scorch Retarders

Rubber Compounding• Cure system

– To vulcanize, you will have to bring the following conditions : • Energy : mostly thermal energy • Vulcanization agents:

– Sulfur – Organic peroxides – Metallic oxides – Diamines...

• Active sites on the polymer – Halogen atom (Br, Cl...) – Insaturation ( -C=C- ) – Chemical function ( -COOH or epoxy groups )

• Accelerators are chemicals which accelerate the cross-linking reactions.

Rubber Compounding

• Cure systemDepending on the elastomer, you will have the following possibilities :

Vulcanization system Concerned elastomers Examples

Sulfur (and accelerators) All the elastomers with an insaturation on their structure NR, IR, SBR, BR, EPDM, IIR, NBR, HNBR

Organic peroxides All the saturated elastomers, but also insaturated elastomers to improve the thermal properties CPE, CSM, MQ, HNBR, EVM, FKM

Metallic oxides Halogenated polymers CSM, CR

Amines Miscellaneous ACM, EAM, FKM, ECO

Resins (mostly formo-phenolic resins) Butyl rubber (IIR) and some EPDM IIR


– Two factors are very important in the vulcanization of rubber: • The density of crosslinking (the frequency with which a rubber

chain is linked to others) • The nature of the crosslink.

– There is a clear maximum in strength properties at a certain level of crosslinking but it is often advantageous to exceed this level to increase resilience and resistance to set. These latter properties improve with increasing hardness (or modulus), brought about by increasing crosslinking. However, increasing the hardness by means of raising the filler loading has a harmful effect on set and resilience.


– Vulcanization with sulfur• Elemental sulfur is the predominant vulcanizing agent for general-

purpose rubbers. It is used in combination with one or more accelerators and an activator system comprising zinc oxide and a fatty acid (normally stearic acid). The most popular accelerators are delayed-action sulphenamides, thiazoles, thiuram sulphides, dithocarbamates and guanidines. Part or all of the sulfur may be replaced by a sulfur donor such as a thiuram disulphide.

• The accelerator determines the rate of vulcanization, whereas the accelerator to sulfur ratio dictates the efficiency of vulcanization and, in turn, the thermal stability of the resulting vulcanizate.


– Vulcanization with sulfur– In rubber an accelerator to sulfur ratio typically of 1 : 5 is called a conventional vulcanizing

system and it gives a network in which about 20 sulfur atoms are combined with the rubber for each inserted chemical crosslink. The term 'chemical crosslink' is used because chain entanglements can behave as physical crosslinks once 'locked in' by chemically inserted crosslinks. Most of the crosslinks are polysulphidic (ie with a bridge of not less than three sulfur atoms) and a high proportion of the sulfur is in the form of cyclic sulphide main chain modifications. This combination provides good mechanical properties and excellent low temperature resistance, but polysulphidic crosslinks are thermally unstable and reversion can occur at high vulcanizing temperatures and high service temperatures.

– An accelerator to sulfur ratio of 5 : 1 is typical of an efficient vulcanizing (EV) system where no more than 4 - 5 sulfur atoms are combined with the rubber for each chemical crosslink. Most of the crosslinks at optimum cure are monosulphidic or disulphidic and only a relatively small proportion of the sulfur is wasted in main chain modifications. This combination provides very much enhanced thermal stability, both under aerobic and anaerobic conditions, but some mechanical properties may be impaired.

Rubber Compounding

• Cure system– Vulcanization with sulfur– An intermediate accelerator to sulfur ratio of 1 : 1

is typical of a semi-efficient vulcanizing (semi-EV) system and provides properties between those of conventional and EV systems.

– The same principles apply to synthetic rubbers, although the optimum accelerator to sulfur ratio may not be the same as in natural rubber.


– Vulcanization with sulfur• The most common vulcanizing agent is sulfur which is used with an

accelerator and other auxiliary chemicals. In a conventional vulcanization system employing 2 to 3 parts of sulfur per 100 parts of rubber each crosslink formed consists of a chain of several sulfur atoms. However, a sulfur vulcanizate may have only one atom of sulfur in each crosslink in rubber vulcanized by an efficient vulcanization (EV) system which may use as little as 0.25 parts of sulfur. The differences in the physical properties conferred on natural rubber by these different systems should be noted.

• Vulcanization is activated by zinc oxide and stearic acid and the process is "accelerated" by the addition of small quantities of complex sulfur-based chemicals, typically sulphenamides which not only speed up the process, but also influence the properties of the vulcanizate, especially its resistance to ageing.

Rubber Compounding

• Cure system– Vulcanization with sulfur

• It is not possible to list all the chemicals used as accelerators, but some of the main groups used in association with natural rubber are: thiazoles; sulphenamides and guanidines.

• Vulcanization using sulfur alone is sluggish, but, when the sulfur level is increased to make the vulcanization faster, a problem of sulfur blooming arises. Organic accelerators were developed to overcome the sluggishness of sulfur vulcanization and to prevent blooming of unreacted sulfur.

• You can choose one (or mostly) a combination of the following accelerators.

Rubber Compounding


• You can choose one (or mostly) a combination of the following accelerators.

• When a compounder sets out to develop a new curing system, he proceeds in two steps.

• First, a base system is selected which will provide a level of performance processing and curing systems close to his requirements.

• Secondly, it will be necessary to refine that system to meet the needs of the available processing equipment.

Rubber Compounding


Classe of accelerators Examples Speed of vulcanisation

Guanidines DPG, DOTG Medium

Thiazoles MBT, MBTS Semi-fast

Sulphenamides CBS, TBBS, MBS Fast, delayed action

Thiurams TMTD, TETD, TMTM Very fast

Dithiocarbamates ZDMC, ZDEC Super fast

Rubber Compounding

• Summary of sulfur crosslinkObjective Problems Approach

To reduce cure time Scorchiness increases (use retarder) Modulus increases hence elongation and tear resistance decrease

Use a faster accelerator Increase accelerator level Increase accelerator level and reduce S level (EV system)

To improve aging resistance

High proportion of monosulfidic links, loss of dynamic flex fatigue properties CONVENTIONAL CURE : S : 2.5 phr CBS : 0.6 phr S1 = 0% (S2 + Sx ) = 100%

Use high (accelerator/S) ratio e.g.EV systemCBS : 1.5 phrDTDM (S-DONOR) : 1.5 phrTMTD : 1.5 phrS1 LINKS = 46%(S2 + Sx ) = 54%

Effect of (S/accelerator) ratio

Higher compression set and reversion on fatigue life possible due to breakdown of Sx to S1

Lower compression set and S1, can crystallize hence higher brittlenesspoint temperature

Conventional cure gives higher polysulfidic links and better fatigue resistance EV gives higher monosulfidic links and poor fatigue resistance Tensile strength decreases as Sx > S1 > C -- C

Curing temperature Higher temperature causes reversion in conventional cure system, i.e. Sx break down to S1

As cure temperature increases, time reach optimum cure decreases

Activators ZnO or MgO is required to develop full potential of acceleratorsStearic acid or salts of fatty acids are required


– Vulcanization with peroxide• Elastomer crosslinking can be carried out by means of peroxides as well.

The procedure can be applied in the case of both diene and saturated (silicone, urethane, ethylene-propylene, etc.) rubbers.

• Dicumyl peroxide performs crosslinking of NR, SBR, nitrile rubber, resulting in vulcanizates with good cold and aging resistance.

• Higher tensile strength of the vulcanizates obtained with peroxides as vulcanizing agents, can be obtained by the addition of small amounts of sulfur, amines or unsaturated compounds.

• By comparison with the vulcanization with sulfur and accelerators, peroxides produce a lower reaction rate and the resulting vulcanizates have a lower tensile strength, scorching tendency and unpleasant smell.

Rubber Compounding• When creating a new compound there are three main criteria compounders

use to guide them. Listed in order of importance they are as follows:– 1. Customer Requirements– 2. "Processability"– 3. Cost

• Nearly all new compounds are modifications of some existing formulations. Nowadays, development of a completely new compound is seldom attempted. Moreover, such an attempt is usually unnecessary. In order to be efficient and effective in rubber compounding, chemist should take full advantage of technical information readily available inside as well as outside of his organization. He must be analytical, resourceful, and innovative. The following is a useful procedure to guide compound development.

Rubber Compounding• 1. Set specific objectives (properties, price, etc.)• 2. Select base elastomer(s).• 3. Study test data of existing compounds.• 4. Survey compound formulations and properties data presented

by material suppliers in their literature.• 5. Choose a starting formulation• 6. Develop compounds in laboratory to meet objectives.• 7. Estimate cost of compound selected for further evaluation.• 8. Evaluate processability of compound in the factory.• 9. Use compound to make a product sample.• 10. Test product sample against performance specification.

Rubber Compounding• 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 part art, part science. In compounding, one must cope with literally hundreds of variables in material and equipment. There is no infallible mathematical formulation to help the compounder. That is why compounding is so difficult a task.

Contact

LT QuimicosAv. Pedro Severino Jr., 366 Cjto 35

04310-060 – São Paulo – SP – BrasilLuis TormentoNPD Director

[email protected]: +55 (11) 5581-0708

mailto:[email protected]

Basic rubber compounding

Engineering

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