The role of additives in rubber processing

1 What are additives?

In rubber processing, the material is not moulded by mechanical processing alone, but only adjusted to an application-specific compound with defined material properties through the targeted addition of other substances. While the base polymer forms the basis, additives ensure that a material is particularly abrasion-resistant, cold-resistant or chemically resistant, for example.

A seal in a hydraulic system has different requirements to a membrane in a medical device or a valve in aviation. Manufacturers of moulded rubber parts select suitable fillers, plasticisers and other additives depending on the application. This creates the desired elastomer compound from the raw material rubber. The term additives therefore includes fillers, plasticisers and other additives such as antioxidants, flame retardants, UV stabilisers, vulcanisation accelerators, etc.

The potential performance of an elastomer therefore depends not only on its basic chemical structure, but also to a large extent on the balanced interaction of the right additive system.

2 What are fillers in rubber production?

Fillers influence the mechanical, dynamic and processing properties of an elastomer compound to varying degrees. The type, particle size, structure and interaction with the polymer determine whether primarily a reinforcement, a property modification or a cost and process-related adjustment is achieved. A distinction is often made between reinforcing and non-reinforcing fillers depending on the particle size of the filler.

Reinforcing (active) fillers

Active fillers consist of very small particles of ~ 10-100 nm and therefore have a very large surface area. They bind directly to the polymer chain and increase the strength, abrasion resistance and tear resistance of the compound. On the other hand, the higher the filler content, the lower the elasticity and rebound resilience. At the same time, active fillers also change the thermal and electrical properties.

• Carbon black is the most widely used active filler and gives the compound its typical black colour. Carbon black has thermally conductive properties, which also make the rubber compound increasingly thermally conductive at higher filling levels. In addition, the electrical conductivity of carbon black can be used to produce conductive or antistatic components. Depending on the carbon black, conductive ESD rubber components are also possible.
• Silicic acid (silica) enables light-coloured or coloured compounds and improves resistance to ageing and weathering. Due to its electrically insulating effect, components with an insulating effect can be produced by adding silica. Due to its polarity, however, this filler should only be used with coupling agents (e.g. silane) in non-polar rubbers, as it can otherwise lead to poor resistance and loss of properties.
• Nanofillers (e.g. carbon nanotubes, nano-clays, graphene) with even smaller particle sizes open up additional possibilities for high-end applications, as they can influence properties such as gas permeability, thermal conductivity or mechanical performance. 

Non-reinforcing (inactive) fillers

These fillers are often used to reduce costs by increasing volume. Their particle size is usually in the range of a few micrometres. Particle sizes of 100 - 1,000 nm are referred to as semi-reinforcing fillers. Inactive fillers reduce the mechanical properties of the compound if they are used in high proportions. They are therefore only used to a limited extent in high-performance applications.

The most common mineral fillers are kaolin, chalk and talc. They are usually used to optimise costs and specifically influence properties such as hardness, dimensional stability or workability. They are often used in standard applications where the cost-benefit ratio is paramount.

3. how do plasticisers affect the rubber compound?

Plasticisers influence both the processability during production and the performance properties of the resulting compounds. They increase the mobility of the polymer chains and lower the glass transition temperature, making elastomers softer, more elastic and more flexible at low temperatures. At the same time, plasticisers reduce both the tensile strength and the compression set. They also have a cost-cutting effect on the compound by reducing the rubber content.

As with fillers, polar elastomers are only combined with polar plasticisers and, conversely, non-polar plasticisers are only combined with non-polar elastomers. Plasticisers act as an internal lubricant between the polymer chains by increasing the distance between the chains and making the material more flexible and stable at low temperatures. At higher temperatures, incorrect selection or excessive dosage, plasticisers can migrate out of the material or react with chemicals, which can lead to loss of stability or undesirable interactions during use - the rubber „ages“.

Aromatic, paraffinic and naphthenic oils are among the classic plasticisers and facilitate elastomer processing and the adjustment of elasticity. Alternative plasticisers, such as bio-based oils or specially developed blends, are becoming increasingly important as they meet regulatory requirements and reduce the risk of migration or interactions with application media.

4 What other additives are there for rubber compounds?

In addition to fillers and plasticisers, there are a number of other additives that play a decisive role in controlling rubber processing. They have a significant influence on both the properties of the finished rubber product and the processing procedure.

Processing aids 

Processing aids are additives that specifically support the mixing, moulding and vulcanisation process of rubber compounds without primarily determining the performance properties of the finished component. In particular, they influence viscosity, flow behaviour, dispersion of fillers as well as mould filling and demoulding behaviour. Depending on the mechanism of action, a distinction is made between superplasticisers, internal lubricants and blowing agents.

• Superplasticiser cause a targeted, usually short-term degradation of very high molecular polymer chains during the mixing process. This temporarily reduces the viscosity of the rubber, which facilitates the incorporation of fillers and other additives and reduces the soft kneading time required in the mixer. After the mixing process, this effect largely disappears so that the mechanical properties of the vulcanised component are not significantly impaired.

• Internal lubricant act within the compound by reducing the friction between the polymer matrix and fillers. As a result, they improve dispersion during the kneading process and promote uniform flow behaviour during moulding. In addition, they can reduce the adhesion of the rubber to mould surfaces and thus facilitate demoulding. However, too high a proportion can reduce the surface energy of the component and make subsequent processes such as bonding or painting more difficult.

• Blowing agent are used to specifically create porous structures, for example in sponge or sponge rubber. Under the thermal conditions of vulcanisation, they decompose and release gaseous decomposition products - often nitrogen - which lead to the formation of cells in the material. In very small doses, blowing agents can also be used to support mould filling, but are generally associated with a reduction in mechanical strength.

Anti-ageing agent

Protect the material from the damaging effects of oxygen (antioxidants), ozone (antiozonants) and UV radiation. During ageing, radicals are formed which, together with oxygen, form peroxide radicals. These attack other chains and thus lead to further ageing. Fine surface cracks form, especially when stretched. Anti-ageing agents react with the radicals on the surface and bind them. This creates oxidation products that additionally seal the surface. Depending on the agent used, these are visible as brownish discolourations on the surface and are therefore rarely used in light-coloured or coloured components. Due to the low proportion of rubber, they usually do not change the properties apart from resistance to ageing and weathering, but can lead to changes in the vulcanisation process.

Vulcanising agent

Comprises all chemicals that contribute to the cross-linking of the polymer chains during vulcanisation. They are responsible for turning the plastic rubber compound into an elastic rubber component. Depending on the chemical composition of the compound and the subsequent application of the component, sulphur, peroxides, metal oxides or special crosslinkers are used.

Sulphur vulcanisation

Sulphur vulcanisation is used when unsaturated rubbers (double bonds in the polymer chain) such as NR, SBR, BR, NBR and EPDM (with dienes) are used. The heat in the vulcanisation process causes sulphur bridges to form between the polymer chains consisting of one or more sulphur atoms (C-S_x-C). These crosslink the chains and ensure the elasticity typical of rubber. As sulphur alone is relatively inert, vulcanisation accelerators, which form free sulphur radicals, are used to speed up the cross-linking process. The ratio of sulphur to accelerator determines how long the sulphur chains become. The higher the proportion of accelerators and the lower the proportion of sulphur, the shorter the sulphur chains will be. Long sulphur chains promote dynamic properties and high tensile strength, but have a lower heat resistance. Conversely, short sulphur bridges have better heat and ageing resistance, but lower tensile strength.

Peroxide crosslinking

Peroxide crosslinking is mainly used when particularly temperature-stable or media-resistant properties are required. Peroxides decompose at temperatures between 140 and 180 °C to form free radicals, which release hydrogen atoms from the polymer chains. These free sites can then be cross-linked and form carbon bridges (C-C). Carbon-based cross-links are thermally more stable and also more resistant to ageing than sulphur bridges. However, they have a lower tensile strength and tear resistance and are prone to brittle fractures with low tear propagation resistance. Peroxide cross-linking can be used with EPDM, NBR, HNBR and FKM to achieve increased heat resistance. Silicone (VMQ) is almost exclusively crosslinked with peroxides, as sulphur, for example, cannot form bridges to Si-O bonds.

Metal oxide cross-linking

Metal oxide cross-linking is used in particular for halogen-containing rubbers such as CR, CSM and halogen-containing IIR derivatives (CIIR and BIIR). Zinc oxide or magnesium oxide bind the resulting HCl molecules and crosslink the chains with a small amount of added sulphur.  

Functional additives and special additives

These additives are not used for processing or cross-linking, but instead give the vulcanised component additional functional or regulatory properties, for example in terms of appearance, fire behaviour or testability. They are used specifically to fulfil material-specific requirements of individual applications or standards without significantly changing the basic structure of the compound.

• Pigments influence the visual appearance and enable individual colour designs. In black rubber, this effect is usually caused by the filler carbon black. Pigments can also take on functional tasks, for example through reflective or light-stabilising properties. Pigments are often used in combination with anti-ageing agents to prevent discolouration and migration of the particles.
• Flame retardant, which are based on halogens, produce a protective slag in the event of fire which protects the component. However, many of these substances are banned for environmental reasons. For this reason, halogen-free flame retardants based on aluminium oxide or magnesium oxide are increasingly being used today, which release water when they decompose and cool the component. The addition of flame retardants is often necessary in order to achieve UL 94 certification. Halogenated elastomers such as CR and FKM are inherently more resistant to flames, while unsaturated polymer chains (NBR, NR, SBR) are often highly flammable.
• X-ray opaque additives are added to a compound to make the component visible under an X-ray machine. These are used, for example, for silicone hoses in medical technology or FKM seals for the oil & gas industry.

5 What role do additives play in rubber processing?

The true strength of elastomer compounds lies not in the individual additives alone, but in the interaction between them. This is particularly evident in the interaction between fillers and plasticisers. The two pursue different objectives, but are strongly interlinked and together determine how a compound ultimately behaves.

• Balance between strength and flexibilityA higher proportion of fillers generally increases strength and abrasion resistance, but can reduce elasticity. Plasticisers counteract this effect by increasing the mobility of the polymer chains and thus restoring the desired flexibility.
• Influence on processingFillers often increase the viscosity of the compound, making it more difficult to process. This effect can be compensated for by the targeted use of plasticisers so that the compound remains easy to mould.
• Long-term behaviourWhile fillers can improve ageing and media resistance, plasticisers harbour the risk of migration or interaction with chemicals. The trick is to harmonise both additive groups in such a way that mechanical stability and chemical resistance are maintained over long periods of time.

These examples make it clear that the properties of a component cannot be controlled by the base polymer alone, but result from the coordinated combination of fillers, plasticisers and other additives. The decisive factor here is not the maximisation of individual effects, but the deliberate balancing of strength, flexibility, processability and long-term stability. Therefore, the exact conditions of use of a component are always required for special applications in order to specifically optimise the rubber properties. Speciality compounds can therefore exceed the properties of a base polymer many times over. Conversely, compromises in other areas must be accepted.