“Green” vulcanization: research opens up new eco-friendly possibilities

“Vulcanization” a simple word that hides production processes that have evolved following the birth of the modern rubber industry. The vulcanization process takes place under strictly controlled temperature (at least 130°C) and pressure conditions in special ovens. This process significantly improves the mechanical and physical properties of the product thanks to chemical reactions that create sulphur "bridges" between the polymer chains of the raw material. Elastomers have the shape of long intertwined polymer chains and the bonds created by the sulphur bind these chains in something that resembles a network: it is no coincidence that this process is called cross-linking, which constitute bonds that bind the elastomers: even if they move thanks to their elasticity, they are then "forced" to return to their original position, featuring the behaviour of a material that is elastic and tough at the same time, with significant resistance to oxidation and weather conditions. For a long time, not only sulphur has been used in the vulcanization process: this is in fact supported by additives of many types such as plasticizers, antioxidants, accelerators and activators. These are sometimes chemically aggressive substances, but the results of a study, conducted by Ali Ansarifar, Kornkanok Noulta, George Weaver and Upul Wijayantha of Loughborough University (United Kingdom), aim to reduce the use of chemical products and their cost.
A not so harmless mix
These researchers give the example of components for the polymerization of the facade sealant compound (Cws), which contains 5 phr (parts per hundred rubber) of zinc oxide ZnO, 1 phr of stearic acid, 2.75 phr of accelerators and 1 phr of sulphur. These chemicals are added at various stages of mixing and must disperse evenly to produce effective, consistent cross-links in the rubber. The excessive use of these chemicals is not only expensive and harmful to the environment but also makes homogeneous mixing difficult. If this does not happen completely, these additives can re-agglomerate inside the product and migrate towards the surface, which lowers the quality of the final product. The object of the study was zinc oxide, treated with a sulphenamide accelerator in an organic solvent to produce a powder that was used together with sulphur to vulcanize an ethylene-propylene-diene monomer (EPDM) rubber. The rubber compounds were tested at high temperature to determine the scorch time ts2 (the time in which the vulcanization of the compound begins), the optimal hardening time t95, the hardening speed and the minimum and maximum torques. These last 2 quantities indicate the torque applied in the mixer and are an index of the workability of the compound: ML is the one detected at the beginning of the process while MH refers to complete vulcanization.
Meticulous preparation
The components used in the experiment are available from various suppliers: the raw EPDM rubber used was Keltan 6951C from Lanxess, the reinforcing filler Mercap 100, based on kaolin, was supplied by Imerys Ceramics and is pre-treated to reduce its polarity (which could lead to agglomeration) and prevent it from absorbing moisture; the very fine granules lead to a surface area of 25m²/g. The sulphur was produced by Solvay while an accelerator, Santocure Tbbs from Sovereign Chemicals, was used to treat zinc oxide from Harcros Durham Chemicals in order to have a single component to use as an additive. The amount of Tbbs required to provide monomolecular coverage of zinc oxide was determined to be 35 mg/g based on the surface areas of the Tbbs molecule and ZnO. This starting value, which led to a very slow polymerization, was then progressively increased up to 350 mg of Tbbs to coat each gram of zinc oxide: this value led to a good polymerization, comparable to what is normally obtained with much higher loads by Tbbs. A subsequent evaporation test demonstrated that most of the Tbbs was adsorbed on the ZnO surface. The compounds were prepared by placing first the raw rubber into the mixer, mixed for 30 seconds, and then the sulphur and the powder mix of Tbbs and ZnO, mixing for another 8 minutes. The powder was progressively increased from 0.75 phr to 7 phr and the sulphur from 1 phr to 4 phr to determine the effects of the different quantities: in total 36 rubber compounds were prepared, and their polymerization properties were measured, such as the aforementioned scorch time ts2, the optimal hardening time t95, the hardening speed, the minimum and maximum torques ML and MH and the Cure Rate Index - CRI - which measures the vulcanization rate: prolonged process over time can lead to a worsening of the properties of the vulcanized compound.
For a more sustainable vulcanization
The results of the numerous tests conducted on the various formulas agree on the fact that treating ZnO with a sulphenamide-based accelerator in an organic solvent (dichloromethane) is an efficient method for obtaining the desired polymerization and reducing the use of chemical additives. Early estimates suggest that chemicals are reduced by approximately 63% by weight for the same sulphur content in a CWS formula. Another obvious advantage is the ability of the user of the compounds to precisely know the effect of the "powder" on the properties of the compound for different dosages of Sulphur. Since the essential parameters of vulcanization - scorch time, polymerization times and CRI - are not influenced by changes in the dosage of the powder, it is possible to have optimal polymerization by changing the quantity of Sulphur to obtain the best curing of the compound in question. The results also highlighted that if the necessary sulphur load is higher than 4 phr, then the polymerization properties depend on the dosage of the powder: this simplifies the selection of additives for vulcanization and shortens the mixing cycle to produce the compounds. To cure a 1 to 4 phr of sulphur vulcanized EPDM rubber, 3.2 to 5.63 phr of powder will be needed for the desired polymerization, which will occur without the addition of stearic acid. The right dosage of powder depends on the quantity of Sulphur and therefore the correlation between the two can be used to determine the exact quantity of powder (which can replace the two commonly used accelerators and activators) for a given Sulphur load. Other benefits include the 63% fewer chemicals mentioned earlier, 53% less ZnO, a simpler compound design, a safer working environment and shorter mixing cycles due to fewer chemicals used in the process.
Lower costs and fewer chemicals
We also need to consider the important cost implications when using this powder in rubber vulcanization. The market price of TBBS is around 2 US dollars/kg, zinc oxide is quoted at 2.46 US dollars/kg and stearic acid at 0.77 to 0.94 dollars/kg. The CWS curing system has 5phr of ZnO, 1phr of stearic acid and 2.75 of accelerators, for a total cost of approximately $18.57. As an example, if the rubber is cured with the lowest powder dosage, which is 3.2 phr (Tbbs/ZnO: 0.83/2.37), the chemicals used will cost $7.46, or 60 % less without considering the 63% reduction in weight of chemical substances and 53% less ZnO, much to the environment’s benefit. This "powder" is therefore very convenient and can replace the classic polymerization system also in other industrial rubber products, with a substantial reduction in the use of unsafe chemical. Also focused on the reduction of zinc oxide is the European Safe-Vulca project, financed by Eit RawMaterials and led by the University of Milano-Bicocca. The consortium's research partners include the Fraunhofer Institute, Radboud University, the French Commission for Alternative Energy and Atomic Energy and Monolithos Ltd while the task partner is Pirelli. The activator under study is known as ZnO-NP@SiO2-NP and involves ZnO nanoparticles anchored to silica particles: it is therefore a filler that behaves at the same time as a hardening agent and promises to reduce the quantity of zinc oxide necessary for vulcanization.