MIXING

MILLING

All of the commercially available HNBRs can be mixed either in a two-roll mill or in an internal mixer. Most HNBRs are mixed by the use of internal mixing equipment. This is done to improve the quality of the finished compound while also significantly reducing the typical mix times incurred when done on a two-roll mill. Typically, HNBRs are two-pass mixed in an internal mixer where the first pass through the mixer withholds the cure chemicals and is typically dropped out of the mixer at approximately 140°C. This masterbatch is then run through the mixer a second time adding the cure chemicals and again dropped from the mixer at approximately 100-110°C.

The milling of an HNBR compound is typically quite easy. HNBR compounds tend to build heat quickly; therefore, the use of full cooling capabilities is typically recommended. Normally one should start by setting the mill gap at approximately ¼”. This ensures that you will get the shear action required to finish the mixing process while being thin enough to dissipate any excess heat generated during this process. One would normally drop the compound on the mill from an internal mixer from above or feed a stored compound one sheet at a time near the ends of the mill. After banding the compound on the mill, cross-cutting the compound 5 – 7 times from each end is usually adequate to complete this process. Batching off of the mill can easily be accomplished via automatic systems or by hand.

HNBR elastomers are typically cured with either peroxide or sulfur/sulfur-donor cure systems. Laboratory comparisons of sulfur/sulfur-donor and peroxide cured HNBR compounds indicate that peroxide curing provides better compression set and heat resistance. Because HNBR has fewer highly reactive allyl position hydrogens versus other diene-based elastomers, such as NBR and SBR, it is necessary to add 50-100% more peroxide in order to produce excellent curing characteristics.

The hydrogenation of various diene-containing elastomers has been well known for many years. Berthelot began the earliest experiments in 1869, with many other workers carrying out numerous experiments under different conditions and with different catalyst systems. Pummerer et al., in the early 1920s and Staudinger about 1930 were notable contributors to the hydrogenation of polymer systems.

Hydrogenated nitrile butadiene rubber or HNBR was first developed in the late ‘70s and early ‘80s. Though commercial production did not begin until 1984, there were numerous companies looking at the feasibility of producing this type of elastomer. Of those who chose to evaluate this polymer, three companies emerged: Bayer Corporation, Zeon Corporation, and Polysar.

Of these three, Zeon Corporation was the first to commercialize HNBR in March of 1984 with Bayer and Polysar very close behind. Zeon Corporation’s initial manufacturing site was in Takaoka, Japan. The other initial manufacturing site was Polysar’s in Orange, Texas. However, Polysar eventually sold that business to Bayer Corporation who now owns and operates that facility.

Today, there are two manufacturers of HNBR worldwide. Zeon Corporation manufactures HNBR in Takaoka, Japan, while their subsidiary, Zeon Chemicals L.P., produces in Pasadena, Texas, all under the trade name Zetpol□. Bayer Corporation produces HNBR in Orange, Texas, and in Leverkusen, Germany under the trade name Therban©.