All rubber products start out as a “rubber compound”. Rubber compounds start with a rubber polymer, either natural or one of the many synthetic polymers, fillers, plasticizers, anti-oxidants, process aids, activators, accelerators and curatives. Many of the chemical ingredients are classified as hazardous or toxic chemicals, and some may be listed as carcinogens. Handling and processing of these chemicals create both environmental and safety concerns.
Hazardous Waste
Ventilation systems and dust collectors are necessary for workers handling and weighing the rubber chemicals and for workers mixing and processing the uncured rubber compound. Personal protection equipment may also be necessary for these workers. The material collected in the dust collectors must be tested to determine whether it is a hazardous waste. It would be a hazardous waste if it is reactive, corrosive, flammable or contains chemicals that are listed hazardous as wastes.
Hazardous waste must be listed on a manifest and sent for disposal at a hazardous wastesite. Non-hazardous waste can go to local sanitary landfills or may have to go to an industrial landfill, depending on applicable environmental regulations.
Air Pollution
Some rubber products require a rubber cement application in the manufacturing process. Rubber cements are made by mixing the uncured rubber compound with a solvent. The solvents used in this process are usually classified as volatile organic compounds (VOCs). Processes that use VOCs must have some type of emission-control equipment. This equipment can be a solvent recovery system or a thermal oxidizer. A thermal oxidizer is an incineration system that destroys the VOCs by combustion and usually requires a fuel supplement such as natural gas. Without emission control equipment the VOCs can cause health concerns in the factory and in the community. If the VOCs are photochemically reactive, they will affect the ozone layer.
When rubber parts are cured and the curing vessel is opened, curing fumes rush out of the vessel and from the rubber part. These fumes will be in the form of smoke, steam or both. Curing fumes can carry unreacted chemicals, plasticizers, mould lubes and other materials out into the atmosphere. Emission controls are needed.
Ground and Water Pollution
Storage and handling of VOCs must be done with extreme caution. In past years, VOCs were stored in underground storage tanks, which in some cases resulted in leaks or spills. Leaks and/or spills around underground storage tanks generally result in soil and groundwater contamination, which triggers expensive soil and groundwater remediation. The best storage choice is above-ground tanks with good secondary containment for spill prevention.
Waste Rubber
Every manufacturing process has process and finished goods scrap. Some of the process scrap can be reprocessed in the intended product or other product processes. However, once the rubber is cured or vulcanized, it can no longer be reprocessed. All cured process and finished goods scrap becomes waste material. Disposal of scrap or waste rubber products has become a worldwide problem.
Every household and business in the world uses some type of rubber product. Most rubber products are classified as non-hazardous materials and therefore would be non-hazardous waste. However, rubber products such as tyres, hose and other tubular products create an environmental problem as related to disposal after their useful life.
Tyres and tubular products cannot be buried in a landfill because the void areas trap air, which causes the products to rise to the surface over time. Shredding the rubber products eliminates this problem; however, shredding requires special equipment and is very expensive.
Smoldering tyre fires can generate large amounts of irritating smoke that can contain a wide variety of toxic chemicals and particulates.
Incineration of Scrap Rubber
One of the options for disposing of scrap rubber products and process scrap rubber from the manufacturing processes is incineration. Incineration might initially seem to be the best solution for disposal of the numerous “worn out” rubber products that exist in the world today. Some rubber-manufacturing companies have looked at incineration as a means of disposing of scrap rubber parts as well as cured and uncured rubber-process scrap. In theory, the rubber could be burned to generate steam that could be used back in the factory.
Unfortunately, it is not that simple. The incinerator must be designed so as to handle air emissions and would most likely require scrubbers to remove such contaminants as chlorine. Chlorine emissions generally would come from burning products and scrap that contain chloroprene polymers. The scrubbers generate an acidic discharge that may have to be neutralized prior to discharge.
Almost all rubber compounds contain some type of fillers, either carbon blacks, clays, calcium carbonates or hydrated silica compounds. When these rubber compounds are burned, they generate ash equivalent to the filler loading in the rubber compound. The ash is collected either by wet scrubbers or dry scrubbers. Both methods must be analysed for heavy metals prior to disposal. Wet scrubbers most likely will produce a wastewater that contains 10 to 50 ppm zinc. This much zinc being discharged into a sewage system will create problems at the treatment plant. If this occurs, then a treatment system for the removal of zinc must be installed. This treatment system then generates a zinc-containing sludge that must be shipped out for disposal.
Dry scrubbers generate an ash that must be collected for disposal. Both wet and dry ash is difficult to handle, and disposal can be a problem since most landfills do not accept this type of waste. Both wet and dry ash can be very alkaline if the rubber compounds being burned are heavily loaded with calcium carbonate.
Finally, the amount of steam generated is not enough to supply the full amount necessary to operate a rubber-manufacturing facility. The scrap rubber supply is inconsistent, and efforts are currently underway to reduce scrap, which would reduce the fuel supply. The maintenance cost of an incinerator designed to burn rubber scrap and rubber products is also very high.
When all of these costs are taken into consideration, incineration of scrap rubber may be the least cost-effective method of disposal.
Conclusion
Perhaps the best solution to environmental and health concerns associated with manufacturing rubber products would be good engineering control for producing and compounding powdered chemicals used in rubber compounds, and recycling programmes for all uncured and cured rubber process scrap and products. The powdered chemicals collected in dust-collector systems could be added back to rubber compounds with the appropriate engineering controls, which would eliminate the landfilling of these chemicals.
Controlling the environmental and health issues in the rubber industry can be done, but it will not come easy or be free. The cost associated with controlling environmental and health problems must be added back to the cost of rubber products.