Bis-DMTD as ETU replacement in polychloroprene
1,3-ethylene thiourea (ETU), an efficient curing agent for polychloroprene rubber (CR) and widely used accelerator for EPDM has been classified as a substance of very high concern (SVHC) after REACH evaluation. The French chemicals producer MLPC International has launched a complete study to identify the most appropriate safe replacement solution. A solution based on Bis-dimercapto-thiadiazole (Bis-DMTD) has proved most suitable.
1,3-ethylene thiourea (ETU) is well-known as a very efficient curing agent for polychloroprene rubber (CR). It is also an efficient accelerator for EPDM. However, ETU has been classified on the SVHC list (substance of very high concern) after REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) evaluation. So due to the HSE (health safety and environment) risks of ETU, it is now compulsory for producers of rubber compounds containing ETU or vendors of final rubber parts containing ETU in Europe, to find a replacement solution. The main issue is that all other thioureas are also classified as SVHC and cannot be safer candidates to replace ETU. MLPC International, French chemicals producer and 100 % subsidiary of Arkema, has launched a complete study to identify the most appropriate replacement solution. The MLPC Application Laboratory has conducted an experimental plan comparing alternative solutions to ETU.
This REACH registered solution based on bis-dimercapto-thiadiazole (Bis-DMTD named also SD for Sulfur Donor) is classified non-CMR (carcinogenic, mutagenic, reprotoxic). As a consequence, no complementary compliance test on Bis-DMTD is required by ECHA (European Chemicals Agency).
Two vulcanization formulations based on combination of bis-DMTD (SD) and CBS were identified as suitable: the first with addition of MBTS and the second with sulfur and CTPI. The combination with MBTS gives longer bridges, improving elongation at break and tear strength. The combination with S+CTPI gives a higher crosslinking density, improving heat resistance, crystallization and oil swelling. The study indicates that association of bis-DMTD (SD) with CBS improves the scorch safety of the compound thanks to a delayed reaction without impacting the vulcanization characteristics, the process behavior, the evolution (appearance…) and the mechanical properties of cured rubber.
I. Yarzabal, RFP Rubber Fibres Plastics, 03 2020, 140-144.