A precursor
yarn or thread is made by passing a
central element, e.g., a filament, a tow, or a flat member, through a bath of a binder, such as a low-temperature hot-melt
adhesive, and aqueous urethane, or an acrylic material, with which is mixed a quantity of hard-shelled microspheres which expand when heated to a higher temperature. This is then covered by a sheath, e.g, of PVC,
polyurethane,
polyester,
acrylic resin,
polycarbonate,
polypropylene, or
polyethylene in a second bath. When this product is heated to a
transition temperature which is characteristic of the microspheres chosen, the microspheres expand, swelling the sheath. Such a precursor could be woven into a fabric and then heated, so that as the
yarn expands the fabric mesh becomes tighter, reducing its
porosity. This would be useful as a
yarn in making papermaker's felts. Heat-expansible threads made similarly would be useful in manufacture of shoes, sails, tents, clothing and other items where
porosity is undesirable; that is, the product could be sewn together using the thread of the invention and then heat-treated, causing the thread to expand, sealing the holes made in sewing. Such threads would also be useful for decorative purposes, e.g., as embroidery yarns, and likely for other purposes. The yarns of the invention can also be disposed in a desired position and infused with a hardening resin while heat is applied to cause the microspheres to expand, forming a lightweight and stiff
composite structure.