LOCTITE NS 5540 is a thermally reactive, heat-cured, liquid sealant for threads and flanges used in high temperature and high pressure applications.LOCTITE NS 5540 is a brown, high strength, thermally reactive, heat-cured, liquid sealant for threads and flanges that is used in high temperature and high pressure applications. It will not cement flanges together thus allowing future repairs of metal-to-metal joints and is unaffected by thermal cycling. It has been successfully used to seal flanges carrying up to 2900 psi steam.