These materials are used to make printed circuit boards, structural composites, and other varieties of special-purpose products. If you examine the composition of these yarns under a magnifying glass, you will find a bulk of chopped or continuous strands of glass in them. This type of fiberglass is used as insulation to protect plastic materials due to their unique properties of thermal and electrical conductivity. http://www.gyfiberglassproduct.com/fiberglass-yarn/