In the 1930s, when rubber became one of the essential commodities, selection was never a problem because we had only Natural Rubber (NR) that time. Today, beyond 2010, there are number of elastomers are being used in the industry and the choice is typically important with respect to the competitive advantage of both, durability in the service and cost.

 

NR was called rubber because it could have rubbed out pencil mark. When other synthetic rubbers were produced, they had also similar property of rubbing out pencil mark, but were called elastomers because NR was then typically identified as Rubber. However, both NR and other synthetic rubber (SR) together are called elastomers, because they had typical elastic properties and interestingly, all rubber and elastomers are high polymers. From the time 1930 , industries have increased many folds of time. Engineering requirement in the manufacturing industries, with respect to temperature, pressure and durability have also simultaneously increased and our demand on the applications have also been increased.

CAPTION Fig.1: Asia Pacific Total Elastomers (54%), NR+SR

 

With very competitive demand in the market, all rubber properties cannot be achieved only by NR. Balancing critical demand for rubber applications, that we require in our day to day life, use of SR or blending with SR has become very common practice in the industry today.

 

For example, other than pneumatic tyre, there is hardly any uses of NR these days in automotive industries. Uses of various grades of EPDM, Silicone rubber (Q), Nitrile rubber(NBR), Fluoro Elastomers (FKM) , Perfluoro Elastomers (FFKM) , Hydrogeneted Nitrile rubber (HNBR), Chlorosulphonated Polyethylen (CSM), Polychloroprene(CR) , Polyurethane Rubber (AU/EU), Fluorosilicone Silicone Rubber (FQ) etc. have been increased due to typical automotive parts requirement. Since automobile spares are now mostly manufactured in Asia Pacific countries, they are the largest consumer of total elastomers (Fig.1).

 

CAPTION Fig.2: Only SBR is the highest (47%) synthetic rubber

 

After NR, the next high consuming elastomer is SBR (Fig.2) because of its higher filler and oil loading capability and higher abrasion resistant quality. After SBR, the next high quantity rubber used is BR, followed by IIR (BIIR,CIIR) and EPDM. Recently silicone rubber uses have increased many fold times in Western countries, China, Japan, Korea and in India. However, the total SR uses remains highest in Asia Pacific(Fig.3).

 

CAPTION Fig.3: Asia Pacific Highest Consumer of SR (48%)

 

In critical applications, it is therefore, advisable to give considerable thought, or take advice, on the formulation of the compound. As the potential for 'tailoring' compound to specific applications is essentially limitless, it is often advisable to carry out preliminary qualification tests to ensure that the compound chosen will perform as intended by customer need.

A considerable thought in critical applications, for the formulation of the specific compound need considerable experience with selecting raw materials and art of processing. Very common mistakes by rubber compounder is mostly related to incorrect selection of (1) ingredients, (2) their doses, (3) rubber blends and (4) correct machines. Rubber compounding is an art of developing rubber mixtures with suitable raw material and their doses, that will perform in desired services but with minimum cost possible such that product can be competitive in the market and can be processed well in machines without any difficulties faced by man and machines.

 

There are broadly two classes of Rubbers or elastomers, they are Natural Rubber (NR) and Synthetic Rubber (SR). NR occurs naturally in the plant and hence the name but all synthetic rubbers are man made rubbers and are produced by chemical synthesis. Among the Synthetic elastomers, there is again two category; one is general purpose rubbers (GPR),which can be used as equivalent to NR, e.g., Butadiene Rubber (PBR) and Styrene Butadiene Rubber (SBR) and the other category is specialty elastomers. Specialty elastomers are generally costlier than GPR and are only used in special purpose. Following are the list of specialty elastomers ,which are widely being used in rubber industry beyond 2000:

 

Butyl Rubber (IIR), Chlorobutyl Rubber (CIIR), Bromobutyl Rubber (BIIR), Chlorinated Polyethylene(CM), Chlorosulphonated Polyethylen (CSM), Ethylene Acrylic(EEA) , Ethylene Propylene Rubber(EPM) , Ethylene Propylene Diene Rubber(EPDM), Fluoro elastomers (FKM), Hydrogenated Nitrile Rubber (HNBR), Isoprene Rubber (IR), Nitrile Rubber(NBR) ,  Polyacrylic Rubber (ACM), Perfluoro Elastomers (FFKM), Polychloroprene (CR) , Polysulphide Rubber (TR) , Polyolefin Elastomer (POE),   Polyurethane Rubber (AU/EU) , Silicone Rubber(Q), Fluorosilicone   Silicone Rubber (FQ) etc.

 

Elastomers having carbon-carbon double bond on the elastomeric backbone could be cross-linked with sulphur and accelerators. Many of these elastomers are also could be cured with organic peroxides, examples are NR,SBR,BR, AU/EU, CM, CR,CSM,EPM,EPDM,FPM,NBR,HNBR,IR,POE,Q,FQ. Elastomers that cannot be cured with organic peroxides are; ACM,IIR,CIIR,BIIR,ECO.

 

Rubber compounding

 

Rubber compounding is an art of developing rubber mixtures with suitable raw material and their doses, that will perform in desired services but with minimum cost possible such that product can be competitive in the market and can be processed well in machines without any difficulties faced by man and machines. In all rubber industry today, the biggest challenge is cost reduction of a good quality product. During selecting raw materials, therefore, the cost of these will also play a vital role in compound designing.

 

A rubber product might require desired physical properties and ageing properties. For this one need to add particular reinforcing filler or a suitable combination of reinforcing fillers to have desired physical properties. The typical ageing resistant property may be achieved with only NR by adding suitable anti-degradants or, NR could also be blended with synthetic elastomers with better ageing resistant property. NR being cheaper and easily available it is the first choice having good strength, abrasion , tear strength and low heat development in dynamic condition. A synthetic rubber product might require good green strength , in that case either NR or blend of rubber is the choice. For example, for better green strength of CIIR, it is often blended with NR.

 

CAPTION Fig.4: Turn-up Bladders

 

A rubber product may require a specific need , say air retention property or oil resistance property. For the former case the choice is essentially butyl rubber (or, halobutyl rubber , CIIR,BIIR) and for the later it is usually, NBR/HNBR and for both oil resistance and air impermeability, the usual choice is NBR / HNBR rubber (Turn-up bladder for tyre building operation, Fig.4). For a typical product, if the property demands oil resistance at 200 ⁰C, then the choice is FKM (Fluoroelastomers) or Q. For resistance upto 328 ⁰C , it is FFKM.

CAPTION Fig.5: Typical Industrial Gaskets

 

Heat resistance property is typically related to product durability and sustainability at desired temperature and is very important for various industrial gaskets (Fig.5). For temperature resistant rubber compounding and following temperature resistance of the polymer is important, NR ~ 65 °C, SBR ~ 75 °C, NBR ~ 110 °C, HNBR ~ 180 °C, Q ~ 200 °C+, FKM ~ 240 °C, FFKM ~ 328 °C. The temperature ranges quoted are only a rough guide, because the temperature resistant property also depend on the typical compound design as well, depends upon the particular application, and may depend on detailed differences between alternative versions of the same rubber.

 

Rubber compound is always developed as per customer need. For any rubber article, the first choice is the selection of right rubber. Rubber is selected mostly on the basis of :

 

• Cost
• Heat and/or Oil Resistance
• Temperature Requirements
• Energy Absorption
• Seal Ability
• Flex Resistance
• Water Resistance
• Gas Impermeability
• Electrical Properties
• Abrasion Resistances
• Dynamic Properties
• Flame Resistance

 

Rubber compound related definitions

 

• Elastomer, a polymeric material that recovers substantially to its original shape after significant deformation at room temperature.
• Compound, a mixture of elastomer and other materials that is intended to process (mold) satisfactorily and meet end-use specifications.
• Filler, a particulate material added to an elastomer that modifies both the workability and the end-use behavior of the resulting composition.
• Plasticizer, a material added to an elastomer to improve its workability.
• Resins are added to improve rubber tack.
• Waxes also used as plasticizer , are also added for smooth finish of rubber articles.
• Antioxidant, a chemical added to a compound to slow or prevent oxygen attack on the compound.
• Antiozonant, a chemical added to a compound to prevent ozone attack.
• Cross linking agent, a chemical added to a compound to link the long molecules in a polymer together, or to assist in the cross-linking process.
• Accelerator, a chemical added to a compound to increase the rate of cross-linking in the compound.
• For example, sulfur links the long molecules, while an accelerator increases the cross-linking rate.
• Retarder, a material added to an elastomer compound to delay the onset of cross linking (scorch).
• Vulcanization is same as cross-linking but with sulphur.
• Peroxide also helps in cross-linking process.

 

 

Elastomer blends

 

Elastomer blends often creates problem when two different types of unsaturated rubbers are mixed and vulcanized together. For example, NR and IIR have two different unsaturation level and hence both sulphur , ZnO and black flows more towards polar rubber, on NR phase, and results undercure in IIR phase and the resultant blend vulcanizate becomes spongy and cannot be used.

 

GPR (NR,SBR,BR) rubber could be blended to any proportion. For higher synthetic rubber level (BR,SBR) , accelerators dose is often adjusted to higher side and sulpur level is adjusted to lower side, because for equivalent curing, BR, SBR requires more accelerators as compared to NR. Stearic acid is added 2-3 phr with only synthetic elastomer and for NR, stearic acid dose of 0.5 phr is enough.

 

CAPTION Fig.6 : Micro Dispersion of Rubber Blends

 

Practically most of the polymers are not miscible to 100%, polymer blends usually consist of micro-dispersion of one rubber into the other rubber and this results after intensive mixing of these two different polymers. These micro dispersed rubber often has dimensions around 0.1-1.5 nm(Fig.6). When fillers are also mixed into such blends, a situation may develop in which the filler unevenly distributed between two phases. Such uneven distribution of fillers, naturally effects the uniformity of compound physical properties. In most blends the effect on the properties of blended elastomers depend on:

 

1. The polymer compatibility
2.  Distribution of fillers in different phases and
3. The degree of cross-links between rubber phases

 

Though NR,SBR,BR could be blended to any proportion , yet the blended phases are not compatible to hundred percent and there is also phase separation, where, on proper identification one can witness that there is phase separation with NR & SBR, NR & BR, BR & SBR. However, upon proper mixing these phase differences could be minimized (Fig.7) such that the resultant blend gets cured almost homogeneously . That is why very highly dispersed NR (5 to 10 parts) could also be co-cured with IIR.

 

 

CAPTION Fig.7 : Well Dispersed Rubber Blends

 

 IIR cannot be blended with GPR but can be blended with EPDM (having ENB diene content between 2-3 mole%) to any proportion. Higher diene content EPDM rubber (ENB, >9.0% mole) could be well blended with GPR. If high diene content EPDM is blended with IIR, filler, sulphur, accelerator and zinc oxide flows more towards EPDM than IIR. IIR could be blended with CIIR and BIIR to any proportion. Such blend is often used in making tyre inner-tubes and hose jacket compounds. When CIIR and BIIR doses are on the higher side with IIR (>60phr) it is worthwhile that zinc oxide is added in the final batch since zinc oxide is curative for CIIR & BIIR.

 

Besides zinc oxides, CIIR and BIIR can also be cured with sulphur/accelerator system as well. However, for very good heat resistant property, they are often cured with ZnO. Highly dispersed plastic (LDPE) could also be blended with CIIR/BIIR with no detrimental effect but with improvement on air permeability.

 

CIIR and BIIR could be blended to any proportion with GPR. Such blend is often used in tyre inner liner. When CIIR and BIIR doses are on the higher side (>60phr) both zinc oxide and amine type anioxidant/antioxonates are added in final batches as these are curatives in CIIR and BIIR.CIIR blend with GPR and EPDM is used in PC sidewall for glossy finish sidewall and addition of CIIR also help to reduce the curing time of PC tyre. Blend of EPDM/NR/SBR and EPDM/NR/SBR/CIIR are often used in tyre side wall compound for better look.

 

CR rubber is not normally blended in the industry as it is mostly used in adhesive industry. However, they can be blended to any proportion with GPR. In adhesive industry crystallinity is important and CR gives the highest degree of crystallinity among all general-purpose rubber. CR could be blended with IIR , close to 5-15 phr, for bladder making and in general, only 5.0 phr is added in the beginning of the mixing cycle.

 

In bladder mixing, Zinc oxide could be mixed with CR in master batch. CR is premasticated in mixing mill for making bladder compound, before adding in Banbury.CR/BR blend is used in hose covers.CR could also be blended with GPR at any proportion like CIIR. Both zinc oxide and amine type antioxidant / antioxonates are added in final batches as these are curatives in CR and CIIR.

 

In general Silicone rubber (MQ,PMQ,VMQ) cannot be blended with any other rubber because of phase difference problem but highly dispersed EPDM could be blended with it upto 10 -15 phr. EPDM/Q blend is used in heat resistant cover roll compound.

 

EPDM, being a good elastomer as weather resistant and heat resistant is often blended with number of other elastomers to get the benefit of the vulcanisates.

EPDM/CR blend are very popular in making gaskets. EPDM/IR blend is widely used in car wiper rubber blades. EPDM/SBR blends are used in gaskets, sponges and hose stocks. EPDM/CSM blend is used in transmission belt, conveyor belt and in hose covers. EPDM/LDPE blend is very popular in making cable insulation compound.

 

NBR in general, is not blended with other elastomers as this rubber having higher degree of polarity , is exclusively used for oil resistance property. It may have acrylonitrile content ( ACN) ranging from 18-50%. Incase of higher oil resistance, the elastomeric grade is selected with higher ACN. For better abrasion however, 10-20 phr of BR could be added to NBR with the aid of good dispersing agents , used in shoe sole, high abrasion resistance rolls and in conveyer belts. Higher ACN content will have better abrasion property. NBR could be cured both by sulphur/accelerators or by peroxides. Hydrgenated NBR (HNBR) has emerged into market with better heat resistant property as compared to NBR. For intermediate heat resistant property NBR and HNBR could be blended.

 

NBR/SBR blends used in hydraulic hose tubes, high pressure hose, belt cover, idler roll compounds and in gasket compounds. NBR/PVC blend and NBR/PVC/BR blend are used for roll cover compound, very popular in electric cable insulation and in closed cell sponge applications in shoe industry. XNBR/PVC blend is used for heavy duty cable jackets, roller cover, belt cover, hose cover stocks etc. NBR/IR blend and NBR/TR blend is popular in colored or non-black roll covers. The later is mostly used in printing roll cover compound.

The Rubber Board, which is observing its 77th anniversary this year, organised a press conference to showcase a novel method developed by the Rubber Research Institute of India (RRII) for recovering high-quality rubber from skim latex.

The new technique creates rubber lumps by treating the skim latex with a specific chemical mixture for 24 hours, then allowing the acid to coagulate. After that, these lumps may be dried and turned straight into skim crepe. About 3–4 percent of the rubber used to make Centrifuged Latex (Cenex) is skim latex, a by-product of the process. Cenex is frequently used in the production of goods including balloons, condoms, and gloves. About 10 percent of the natural rubber produced in India is converted into Cenex via centrifugation, which is accomplished at more than 40 centrifuging facilities.

The current method for recovering skim rubber is the acid coagulation of skim latex, which produces a slurry of skim rubber. After being packed into many plastic sacks, this slurry is allowed to solidify and dewater for two weeks. Skim crepe is made by further processing the resultant skim powder. But this traditional approach is labour-intensive, unrefined and results in bad odours, and it only generates rubber of poor grade. It has also caused public outcry and legal conflicts in the vicinity of Cenex enterprises, and it presents difficulties for wastewater treatment facilities. With the innovative method created by RRII, plastic bags are no longer used and processing takes only twenty-four hours. In addition to improving environmental sustainability and guaranteeing full recovery of premium rubber from skim latex, it drastically reduces offensive odours.

The Federation of Latex Processors (FLP), a group of owners of centrifuged latex factories, has been granted access to this technology on a fee basis. Other organizations and Cenex units can also purchase it. The product is marketed as Indian Purified Skim Rubber (IPSR) by the Rubber Board, which is also working on a patent for the concept. Rubber companies nationwide have expressed a strong interest in employing IPSR in product manufacture as it provides a competitive edge because of its improved quality and reduced cost.

Several efforts to solve various problems that exist in the rubber value chain were also addressed at the press conference. At the press conference, it was revealed that a nationwide celebration of the Silver Jubilee of the National Institute for Rubber Training (previously the Rubber Training Institute) would be organised, with participation from all relevant parties. M Vasanthagesan IRS (Executive Director, Rubber Board), Dr T Siju (Rubber Production Commissioner), Dr M D Jessy (Director-in-Charge, Rubber Research Institute of India) and representatives of the Federation of Latex Processors were in attendance.

Kraton Corporation, a leading global sustainable producer of speciality polymers and high-value bio-based products derived from pine wood pulping co-products, has expanded its CirKular+ product line with the launch of CirKular+ Paving Circularity Series C5000 in line with its long-term vision to create innovative solutions for a sustainable tomorrow.

The CirKular+ Paving Circularity Series addresses the changing demands of the paving industry to use more reclaimed asphalt and lower greenhouse gas (GHG) emissions. It allows for the addition of up to 50 percent or more reclaimed asphalt to the asphalt mix in surface layers while enhancing performance and processability. It increases the modified asphalt's resilience while maintaining resistance to permanent deformation and lowering the pavement's lifetime carbon footprint.  C5000 satisfies requirements for asphalt surface materials and is compatible with Warm Mix Asphalt technology.

Pedro Lopes, Kraton Global VP of Strategic Marketing, Product Management and Supply Chain, said, “Launching the Paving Circularity Series is a testament to our commitment to sustainability and innovation. In Europe, the paving industry is actively working to increase circularity and reduce GHG emissions in alignment with the European Green Deal and the EU’s 2050 climate neutrality goals. The new series helps unlock value-added circularity by enabling polymer-modified bitumen (PMB) producers and contractors to increase the reuse of reclaimed asphalt, thereby reintegrating it back into the road surface layer.”

Perth-based CTS Tyre Recycling is forging ahead with its plans to reshape the options for disposal of end-of-life tyres in Western Australia.

The company, a part of the wider Cometti Group, a family-owned business with more than 40 years of experience in the tyre industry, has invested more than USD 40 million in a state-of-the-art recycling plant at Neerabup, north of Perth, that processes waste tyres into crumb rubber, tyre derived products, reusable high tensile steel wire and reusable textile. The company also made some additions to its management ranks and expanded its links with industry associations as it moves forward with its strategy. These tyres, along with all other sizes, will be remanufactured in the new Neerabup factory into new, high-value goods like load-restraining matting, gym matting, equestrian and farm matting and acoustic underlay.

Leigh Cometti, the company’s Managing Director, identified a potential to diversify into the recycling of end-of-life tyres, concentrating on some of the huge off-the-road tyres utilised in the mining services and agricultural industries. Over 90 percent of the bigger OTR tyres now in use in Western Australia are thought to end up in landfills. Tyres are commonly buried in pits left behind during excavation, which results in the greatest landfill disposals in the Pilbara area. The recycling programmes will lessen the need for more virgin products in addition to decreasing landfill discharge.

Cometti has brought on two seasoned senior level managers to support him as the company grows. Joseph Jeevaraj has joined as Chief Financial Officer, while Darren Rodwell has been named Chief Operating Officer. Both positions work for the Cometti Group, which also owns the Bunbury Trucks Sales and Service dealership, CTS People and Mechanics Recruitment.

Continental and the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH have extended their project to improve sustainability in smallholder natural rubber farming by three years. About 5,000 indigenous smallholders in West Kalimantan, Borneo, Indonesia, have already received training since 2018 on how to grow premium natural rubber in accordance with well-defined sustainability standards. Co-funded by Continental and the German Federal Ministry for Economic Cooperation and Development (BMZ), the initiative aims to engage an additional 1,000 smallholders by 2027.

Lack of knowledge and low farmer income are two major issues in natural rubber farming that are addressed by the Continental and GIZ initiative in the region of Kapuas Hulu, which is home to a UNESCO biosphere reserve. Farmers are taught sustainable agriculture practices and cultivation techniques through the training offered by Continental and GIZ. This enhances the rubber's quality, boosts yield, streamlines the supply chain and raises rubber producers' profits. Additionally, this aids in putting into practice the crucial subject of preserving or reviving biodiversity in the project region. Transparency is also guaranteed across the natural rubber supply chain, from the rubber tree to its usage in Continental's manufacturing, thanks to the implementation of a digital tracking system.

In addition to increasing local capacity, both the companies are actively assisting in enhancing smallholders' quality of life and encouraging environmentally friendly methods of growing natural rubber. The objectives of the EU Regulation on Deforestation-Free Supply Chains (EUDR) are clearly supported by Continental, which has long been actively dedicated to increased supply chain sustainability and transparency. In addition to Continental's tyre facilities, other manufacturers can purchase the sustainably sourced natural rubber produced as part of the project, and more partners are being invited to join.

Dr Michael Radke, Head of Sustainability in Purchasing, Continental, said, “As one of the largest tyre manufacturers in the world, we have a particular responsibility in the natural rubber supply chain. That is why we are committed at all levels and are building capacity locally. Over the past few years, together with GIZ, we have shown that we can make the natural rubber supply chain transparent and at the same time increase farmers’ incomes. Now we want to reach even more smallholders and create the framework conditions for the successes achieved to be sustained.”