Molding compositions based on acetal polymers having a high degree of crystallinity have been in use for many years. They have been used in many applications, as for example, automobile bumper extensions and instrument panels; plumbing supplies, such as valves, shower assemblies, flush tank components, faucets and pipe fittings; tool components; and household and personal products.
These crystalline acetal polymers have excellent physical properties. However, for certain applications, improved impact resistance is highly desirable. Typically, impact strength of a crystalline polymer is improved by blending the crystalline polymer with an elastomer to form shock-absorbing rubbery domains in the crystalline polymer. This approach to impact resistance is most successful when there is a strong interaction between the surface of the rubbery domains and the crystalline polymer. Elastomeric polymers which have strong interactions with a crystalline polymer also are useful for bonding two surfaces of the crystalline polymer and for bonding the crystalline polymer to other materials.
Acetal copolymers have recently been synthesized wherein the compositions have been chosen to yield non-crystalline acetals whose chemical structure is similar enough to the chemical structure of the crystalline acetals to interact well with them.
For example, commonly assigned U.S. Pat. No. 4,788,258 discloses acetal copolymers derived from trioxane and 1,3-dioxolane, with the 1,3-dioxolane content being between about 65 and 75 mol percent of the polymer based on the total monomer composition. These polymers have a glass transition temperature that is less than about -60.degree. C. Blends of these non-crystalline copolymers with crystalline acetal polymers show improved impact resistance over that of the unblended crystalline acetals.
Commonly assigned U.S. Pat. No. 4,758,608 teaches non-crystalline acetal terpolymers which are curable in ultraviolet light. These terpolymers are synthesized from trioxane, 1,3-dioxolane, and formals of monoethylenically unsaturated aliphatic diols. When they are cured with multifunctional crosslinking monomers, insoluble, rubbery, non-tacky polymers are obtained which are useful for blending with crystalline acetals to produce resins with improved impact properties.
Copending and commonly assigned U.S. Ser. No. 406,641 discloses acetal copolymers made from 1,3-dioxolane and 1,3-dioxepane. These copolymers are elastomeric and have glass transition temperatures which are as -80.degree. C. Blends of these copolymers with crystalline acetal polymers exhibit improved impact resistance.
There are currently a limited number of elastomeric acetal polymer compositions, and only one elastomeric acetal polymer that can be crosslinked has been reported, as cited above. Additional compositions are needed so that workers of skill in the art will have a broader range of elastomers that can be blended with crystalline acetal resins to achieve the desired combination of properties for specific end-uses. Such properties include, for example, impact resistance, compatibility with other additives and fillers, tensile strength, and stable morphology.