Protex M fiber contains approximately 2x the amount of Antimony Trioxide as compared to Protex C.
It is designed to be used primarily in blends with non-FR fibers such as cotton or rayon (non-FR). A traditional but still active blend is 55/45 Protex M/cotton (no topical FR treatments required). The M fiber is typically sold in a 2 Denier x 38 or 51 mm cut staple length.
Protex C is designed to be used in multiple fiber blends, including meta and/or para aramids, cellulosic (cotton or rayon).
The blend portion of Protex C can range from 20 to 85%, depending on its blend partners. Most dual hazard fabrics will use Protex C fiber in their blend. The C fiber is typically sold in a 1.5 Denier x 38 mm cut staple length. Yarns blended with Protex C will normally be finer and are typically stronger than yarns spun with Protex M, due to the lower Denier of Protex C.
ArcWear does not recommend one fiber over another but many of our clients have asked this question. When specifying a fabric that does not have a strong brand things like this can make a positive or negative difference.