Flame-retardant metal-clad dielectric sheeting comprising a non-woven fibrous layer provided with dimensional stability under etching and soldering conditions by a polyester-diepoxide adhesive

DETAILED DESCRIPTION In general, non-woven webs used in metal-clad dielectric sheeting of the invention are prepared by first blending the desired discontinuous or staple fibers; then forming the blend into a fluffy nonwoven web, preferably by conventional air-layering processes such as Rando-webbing or garnetting; then preferably preshrinking by means of exposure to elevated temperatures via circulating air oven or infrared heating; then impregnating the web with a binder material, as by passing the web between two squeeze rolls, one of which is partially immersed in a dispersion or solution of the binder material; and finally drying and compacting the web as by passing the impregnated preshrunk web through an oven and then between heated pressure rollers (which, in the preferred embodiment, also crosslinks the binder material).
The staple fibers used in the fiber blend should have lengths that provide good tear strength and ease of web formation ("staple" fibers means discontinuous fibers).
Rando-webbing, garnetting or equivalent air-laying processes are generally convenient to use with staple fibers longer than about 0.
3 centimeter and preferably longer than 1.
5 centimeters.
Fibers longer than about 8 or 10 centimeters are generally not used, even on a garnett machine.
It is preferred that the discontinuous or staple fibers of the invention be fibers having filament diameters corresponding to 0.
5 to 20 denier, preferably less than 10 denier.
The fibers should be included in an amount of at least 10 pounds per ream, and preferably in an amount of 20 pounds per ream to provide adequate tear strength to the backing, although the weight will vary somewhat depending on the amount and type of high-strength fiber used.
High-temperature-resistant fibers are important to the dimensional stability of dielectric sheeting of the invention.
Temperature resistance can be measured by observing fibers laid on the surface of a molten solder bath heated to 500.
degree.
F.
for 10 seconds.
Fibers that show a significant distortion (such as polyethylene terephthalate fibers, which curl into a ball in this test) do not have sufficient heat-resistance, and dielectric sheeting made completely from such fibers will not have the needed dimensional stability