‘Cure’ your way to environmentally friendly sports shoe sole manufacturing

By March 26, 2021 No Comments

If one does a quick survey with people asking how the manufacture of shoes impacts the environment, it would not be surprising to know that a majority believe that shoes affect the environment mostly AFTER they are thrown out. Hardly anyone thinks about the life cycle of manufacture of shoes that impacts the environment.

In the various stages of manufacture, machinery and chemicals are used to produce the shoes. Fossil fuels are used to power the equipment that produces greenhouse gases when burned. On an average the production of one shoe produces 30 pounds of carbon dioxide. With 15 billion shoes produced each year, simple multiplication will tell you the damage to our environment.

Add to this transporting the finished goods to places where the consumer resides, and environmental damage adds up. Most footwear manufacturing companies worldwide have chosen to build factories in third world countries like ours (though we no longer can use that tag), with cheap labour.

Then with chemicals used in the manufacture of soles and in preservation, when it comes to leather, the tally goes up with environmental damage. Unseen costs of health of the workers in these industries is not even calculated, when using all these detrimental components for manufacture.

The last step in the shoe life cycle, which is disposal, also contributes greatly to the environmental impacts of the shoe industry with most of the shoes landing up in landfills.

Sustainability in the Shoe Industry

Big shoe brands have very lofty goals for sustainable production processes and materials. They have gone on record stating that their goal is to increase their productivity while also cutting their environmental impact in half. One negative social impact that is a sad reality for nearly every shoe company is outsourcing the manufacturing process to other countries where there is little, or no child labour laws. Many companies have seen what they are doing and have begun to right the ship. They have taken initiatives to make their factories more eco-efficient, extract their raw materials more sustainably, and improve the working conditions in factories across the globe.

One way they are doing this is with their leather extraction process, where they refuse to obtain leather from the Amazon biome. Also, all the tanneries they obtain their leather from are certified by the Leather Working Group for sustainability, and 80% of their tanneries have the “gold standard” for leather.

Yet, little is known or spoken off, about the chemicals that go into making the sole of the footwear.

Monitoring restricted substances

It is important to monitor and eliminate the use of restricted substances in all stages of the product supply chain. This aspect of sustainability is one that is governed by legislation so, regardless of a company’s stance on environmental issues, it is something that must be carefully managed. 

This would include ensuring that all rubber chemicals suppliers and contractors which supply the manufacturer stick to guidelines on the use of certain chemicals and carry out routine checks testing to confirm that those suppliers are following the guidelines. Also included in this would be measuring solvent usage (either the total volume used in a factory or facility, or the amount used in a certain footwear style) and introducing measures to reduce it.

In the rubber industry and more especially in the manufacture of the rubber sole used for sports shoes, there is a need to use ultra-accelerators with improved cure performance. It has been noted that N-nitrosamines compounds in general and N-nitrosamines in particular have severe carcinogenic effects on various life forms including primates. This has been documented by various international agencies for research on cancer. That is the reason we have come up with nitrosamine free rubber accelerators

Thiazoles used in the rubber industry causes cancer and allergies.  Rubber companies have come under pressure to seek to reduce and eliminate thiazoles in the manufacturing and end products.  Thus, this has an impact on the curing systems on natural and synthetic rubbers.  Legislation then, is the driving force to find better products.  Chemicals used have to provide solutions for heightened demand for health and safety and environmental matters.

Background to legislation

The State of California Proposition 65 lists chemicals known to cause cancer or to interfere with the reproductive toxicity – 2 -MBT is on this list.  Countries like Germany have strict limits set for inhalable dust for 2 MBT.  MBT is generated by many delayed action accelerators, including MBTS which is commonly used in rubber shoe sole formulations.

Likewise, various N-nitrosamines are listed as causing cancer and appear on the State of California Proposition 65 list and Footwear Manufacturers ‘Restricted Substances List (RSL).

There is now a new footwear standard ISO/CS 19577 and also TRGS 552 that has set limits for airborne dust for the manufacturing and storage areas for rubber. 

What has happened in the Specialty chemicals industry is significant research has been done to find a solution that meets with legislative requirements and more importantly products that puts health, safety and environmental standards in the forefront.  

Although MBTS has been used considerably, environmentally friendly chemicals that offer substitution of MBTS and also offer a scorch time delay to the ultra-accelerators without generating regulated N-nitrosamines or leaving any thiazole residues in the cured rubber, are now commercially available. 

Robinson Brothers, UK is a specialty chemical manufacturer and supplier that has done considerable work in this area. They have developed a technology called Robac Technology that focuses specifically on chemicals used in dry and wet rubber applications, plus additional polymer additives.  They offer a range of nitrosamine safer accelerators.  

Robac Technology products Robac AS100 and Robac Arbestab Z are used as accelerators to produce rubber or latex goods, which comply with the BS EN 71:2016 Safety of Toys Standard and BS EN 12868:2017 Child Use and Care Articles Standard for N-Nitrosamines and N-Nitrosatable substances. Robac AS 100 serves industries like sport, adhesives, coatings, and medical devices and has no detrimental effect on the physical properties of rubber. This is approved and contains no materials that are on the Restricted Substances List. RSL.

Robac AS 100 for sports shoe soles

More especially, when manufacturing sports shoe soles and those with multicolours, as is the fashion these days, each multicomponent article requires a ‘fast’ and ‘slow’ cure.  Many thick rubber articles require a ‘slow’ cure, requiring longer processing safety, while ‘fast’ cures require relatively shorter processing safety. Both ‘Slow’ and ‘Fast’ cures are required to produce the final article in the same cure time at a typical temperature, for example 150ºC.

Also, for optimization at a production scale homogeneous mixing is needed for Robac SC to work effectively. In order to achieve this both dispersive and distributive mixing have to occur in the correct order. Dispersive mixing reduces particle size, requires high shear forces and high viscosity. Whereas, distributive mixing redistributes ingredients evenly, requires lower shear forces and constant movement of material around the mixing chamber.

In conclusion

Many brands and retailers now recognize that specific groups of consumers will make choices based on their perception of products which have limited environmental impact. The challenge facing the industry is how to measure these impacts and convert them into a meaningful message, which ultimately offers the consumer real choice, but in the short term is both credible and effective at reducing the impact of footwear throughout its entire lifecycle.

Once a product’s environmental qualifications and credentials can be measured and quantified by suitable metrics, it can be compared to other products. This will give greater, more informed choice to consumers and retailers alike. Perhaps more importantly, it will provide the drive and incentive for other manufacturers to create better, more sustainable products that compare favorably with the leading environmental products.

I.R.Tubes Private Limited are distributors for environmentally safe chemicals and is a distributor for Robinson Brothers UK for Robac AS 100 and other products of Robac Technology

Raju Jethmalani
IRTubes Pvt. Ltd., Pune

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