1. Field of the Invention
The invention relates generally to a polyurethane molding compound. More particularly, the invention pertains to a biodegradable and thermoplastically processible polyurethane molding compound, which is capable of being spun, in particular using the melt spinning technique, into elastic filaments.
2. Description of Related Art
Nonwoven [formed] fabrics made of elastic spun filaments constitute the main component of incontinence articles, such as diapers and sanitary napkins, because of their excellent strength properties and their good wear comfort.
Since disposable products are involved, particular attention must be paid to the biodegradability of such filament-formed fabric. However, in spite of the excellent capability of linear polyurethane molding compounds, i.e., polyurethane molding compounds made of purely aliphatic components, to be spun into filaments having excellent elasticity and strength properties for precisely those articles named above, a disadvantage of these molding compounds is that they are not sufficiently biodegradable. Thus, DE 42 03 307 C1 describes a polyurethane molding compound that is capable of being processed thermoplastically in sinter-powder form to manufacture leather-grain sintered sheeting for dashboards. This molding compound can be manufactured exclusively from linear components, namely from 100 parts by weight of a polyol mixture from an aliphatic polycarbonate diol and from a polyester diol based on adipic acid, hexane diol and neopentyl glycol, and from hexamethylene diisocyanate. 1,6 butane diol serves as a chain lengthener. The NCO coefficient, formulated from the quotients of the equivalency ratios of isocyanate groups multiplied by 100 and of the sum of all hydroxyl groups contained in the molding compound, inclusive of those of the chain lengtheners, lies at 97 to 99, which signifies a diisocyanate deficiency.
Nothing is said about the capacity of this molding compound to be spun into continuous filaments. However, tests did, in fact, reveal that the compound can also be processed into filaments because of its low-viscosity melt. These types of polyurethanes are especially resistant to ageing because of their high concentration of polycarbonate in the molecular skeleton.
When no additional protective agents are used, the effect of moisture causes pure polyester polyurethanes having the polyol component based on adipic acid and glycol to be hydrolytically degraded. The polyester component in the soft segment is saponified by water, and the polyurethane chains split into shorter units. This degradation occurs even under mild conditions, i.e., at temperatures and at an atmospheric humidity, which correspond more or less to the climatic conditions of interest herein.
Tests showed a reduction in the stability of polyester polyurethanes by 50% after a storage life of four years in a normal climate (23.degree. C., 50% relative humidity). Apparently, the resulting low-molecular breakdown products, predominantly polyurethane hard segments, are very resistant to any further degradative effect of moisture and oxygen.