Titanium Dioxide as a U.V. Screen in Fire Retardant Vinyl Polymers

This invention relates to retarding discoloration of vinyl polymers containing halogenated organic compounds.

Vinyl polymers can be made fire retardant by the incorporation of halogenated organic compounds. Unfortunately, many halogen compounds are sensitive to ultraviolet (U.V.) radiation in that styrene polymers containing halogen compounds tend to discolor rapidly upon exposure to U.V. radiation. Conventional U.V. stabilizers such as benzotriazole and nickel chelate complexes are ineffective for halogenated organics in vinyl aromatic polymers.

Our invention is a composition comprising a vinyl polymer containing a halogenated organic compound and an effective amount of titanium dioxide such that discoloration of the composition is retarded.

We have found that the incorporation of about 0.5 to 10 wt.% and preferably about 1 to 2 wt.% of titanium dioxide retards discoloring of vinyl polymers containing halogenated organics exposed to U.V. radiation, while generally maintaining any flame retardant properties. Although titanium dioxide is well known as a white pigment, the effect observed in styrene or propylene polymers is not a masking of discoloration but is a screening of U.V. radiation which prevents discoloration associated with halogen dissociation.

The ultraviolet radiation which causes discoloration of vinyl polymers containing halogenated organics includes that short wave-length radiation present in sunlight. Therefore, exposure of such polymeric compositions to sunlight will cause a discoloration, although the effect can be intensified in laboratory tests by the use of U.V. lamps.

Numerous halogenated organic compounds produce flame retardance when incorporated into a vinyl polymer in effective amounts usually from about 2 to 50 wt.%. In many cases, a flame retardant promotor or additive such as antimony trioxide (Sb.sub.2 O.sub.3) or zinc borate is used in amounts ranging from about 1 to 10 wt.% with the organic halogen compound to promote fire retardance. Suitable halogenated organics include mono- and poly-chlorinated and brominated aliphatic, cycloaliphatic and aromatic hydrocarbons containing up to about 20 carbon atoms. Suitable halogenated compounds can contain one or more other substituents such as (but not limited to) nitro, ester, acid, hydroxyl, ketone, amine, phosphoric acid esters and the like. Examples of various types of halogenated organic compounds are described in U.S. Pat. Nos. 3,892,667 and 3,892,710 both incorporated herein by reference.

The specific halogenated organic compounds which can be used with the invention herein are characterized as those which (1) contain carbon-halogen bonds, and (2) form a discolored product when incorporated into a vinyl polymer and exposed to U.V. radiation.

The vinyl polymers which can be used in this invention include propylene, ethylene, styrene, alpha-methylstyrene, rubber-modified styrene, styrene-acrylonitrile and acrylonitrile-butadiene-styrene polymers and copolymers. Preferably our invention is useful in propylene polymers and styrene polymers including those rubber-modified styrene polymers containing from about 1 to 25% polybutadiene or styrene-butadiene rubbers.

Preferably, U.V. stable compositions of our invention should contain from about 0.5 to 5 wt.% of an antioxidant such as hindered phenols thio and phosphate esters and other compounds recognized as antioxidants. Incorporation of metallic stearates into these compositions is beneficial.

In order to test the effectiveness of titanium dioxide as a U.V. screen in styrene polymers, test specimens were prepared by dry blending additives with polystyrene pellets. This mixture was extruded in a one-inch Killion extruder at 400.degree.-450.degree. F. and then mini-injected into 100-mil plagues at 400.degree.-450.degree. F. at 900-1000 psi. These test plaques were exposed in a Carbon Arc Fade-Ometer for various times and the color change (Yellowness Index) was measured by a Gardner AC 10 color meter. The results are presented in Table I. The physical and flame retardance properties of these materials are shown in Table II.

A series of polystyrene plaques were prepared and exposed to a Carbon Arc Fade-Ometer. The yellowness was measured after various exposure times and the results are shown in Table III.

Generally, the examples in which titanium dioxide is incorporated show a resistance to coloration on exposure to U.V. radiation.