Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1993-03-23
2001-07-10
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S432000, C524S439000, C524S593000, C524S366000, C525S327300
Reexamination Certificate
active
06258884
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of polymers, and more specifically to polymers that resist mineral deposition in aqueous environments.
BACKGROUND OF THE INVENTION
Polymeric acetal resins (polyacetals) are resistant to degradation in aqueous environments and are therefore useful in applications in which the polymer is in contact with water for extended periods of time. Polyacetals also have excellent physical and mechanical properties. This combination of properties makes polyacetals particularly useful in plumbing application, such as water faucets and showerheads.
A problem that is encountered with essentially all materials when used in plumbing fixtures is the accumulation of mineral deposits. This problem occurs with metals as well as plastics. The problem is especially noticeable in showerheads, where large volumes of water pass through small openings at elevated temperatures. The mineral build-up is generally more noticeable when “hard” water is used (i.e. water with a high mineral content) and is often a problem in rural areas which rely on wells for water. This mineral accumulation results in unsightly deposits, and in some instances, such as showerheads, the accumulated minerals can eventually interfere with the operation of the plumbing fixture. If a material can be found which resists the accumulation and adhesion of such mineral deposits, certain plumbing fixtures, such as showerheads, could be designed with a longer useful life.
Inorganic fillers are often included in polymeric acetals and other polymers to modify the physical properties according to the particular end use. However, it does not appear that practitioners in the art have recognized that certain inorganic fillers improve the resistance of specific polymers to the build-up and adhesion of mineral deposits.
SUMMARY OF THE INVENTION
Compositions have now been discovered which resist the build-up and adhesion of mineral deposits in aqueous environments containing dissolved minerals. These compositions comprise a polymeric acetal and a sufficient quantity of a zinc-containing inorganic filler to give the composition resistance to the build-up and adhesion of mineral deposits. The zinc-containing inorganic filler can be included at a level of about 1% to about 20% by weight, more preferably at a level of about 5% to about 10% by weight, and most preferably at a level of about 7.5% by weight.
Zinc oxide and zinc sulfide are the preferred zinc-containing inorganic fillers, with zinc oxide being most preferred. The composition can also include other additives such as mold lubricants, plasticizers, fillers, glass fibers, nucleating agents, antioxidants, formaldehyde scavengers, chain scission inhibitors, ultraviolet light inhibitors, impact modifiers, acid scavengers, and colorants. A method is also disclosed for making shaped articles which resist the deposition of minerals. The method consists of the steps of first making a blend of polyacetal, zinc-containing inorganic filler, and other optional additives such as those listed above; and second, shaping the blend to form the desired article.
DETAILED DESCRIPTION OF THE INVENTION
The composition of the current invention is a blend comprising a polymeric acetal and a zinc-containing inorganic filler. Polymeric acetals are characterized in general as having recurring oxymethylene repeat units of the following formula:
—O—CH
2
—
Polymeric acetals that are useful in making composition of the current invention generally have a fairly high content of oxymethylene units (generally greater that about 85%). These materials are commercially available from a number of manufacturers as homopolymers, copolymers, terpolymers, and the like. These highly crystalline acetals, described briefly hereinbelow, are well known in the art and have been reviewed extensively. For example, a review of polymeric acetals entitled “Acetal Resins,” by T. J. Dolce and J. A. Grates, can be found in the Second Edition of
Encyclopedia of Polymer Science a Engineering,
John Wiley and Sons, New York, 1985, Vol. 1, pp.42-61. Additional information on acetal copolymers can be found as part of the Detailed Description in commonly assigned U.S. Pat. No. 4,788,258.
Typically, acetal homopolymers, or poly(oxymethylenes), are prepared by polymerizing anhydrous formaldehyde or trioxane. Oxymethylene homopolymers and usually stabilized against thermal degradation by end-capping with, for example, ester or ether groups, such as those derived from alkanoic anhydrides (e.g. acetic anhydride) or dialkyl ethers, (e.g. dimethyl ether), or by incorporating stabilizer compounds into the homopolymer. Commercially available acetal homopolymer is made by polymerizing anhydrous formaldehyde in the presence of an initiator, after which the polymer is end-capped by acetylation of the hemiacetal end groups with acetic anhydride in the presence of sodium acetate catalyst. Methods for making end-capping acetal homopolymers are taught in U.S. Pat. Nos. 2,786,994 and 2,998,409. Acetal homopolymer is well know in the art and is commercially available under the trademarks DELRIN® and TENAC®.
Polymeric acetals which have been found to be especially suitable for use in the composition of the present invention are crystalline oxymethylene copolymers having repeat units which consist essentially of oxymethylene groups interspersed with oxy(higher alkylene) groups represented by the general formula:
wherein each R
1
and R
2
is hydrogen, a lower alkyl group, or a halogen substituted lower alkyl group, each R
3
is a methylene, oxymethylene, lower alkyl or haloalkyl substituted methylene or lower alkyl or haloalkyl substituted oxymethylene group, and n is zero or an integer from one to three, inclusive. Each lower alkyl group preferably contains one or two carbon atoms. Oxymethylene groups generally will constitute from about 85 to 99.9 percent of the recurring units in such copolymers and are generally incorporated by ring-opening polymerization of trioxane in the presence of an acidic catalyst. The oxy(higher alkylene) groups are incorporated into the polymer by copolymerizing a cyclic ether or cyclic formal having at least two adjacent carbon atoms in the ring in addition to trioxane. The cyclic ether or formal is incorporated by the breaking of an oxygen-to-carbon linkage. The preferred oxy(higher alkylene) group is oxyethylene, having the formula:
—O—CH
2
—CH
2
—
Oxyethylene is incorporated into the polymer by copolymerization of ethylene oxide or 1,3-dioxolane with trioxane.
The preferred crystalline acetal copolymers as described above, which have a structure consisting essentially of oxymethylene and oxyethylene groups, are thermoplastic materials having a melting point of at least 150° C. They normally are millable or processable at temperatures ranging from about 175° C. to about 200° C. They are normally highly crystalline, having a polymer crystallinity from about 60% to about 90% or greater.
These oxymethylene copolymers normally are stabilized after manufacture by degradation of unstable molecular ends of the polymer chains to a point where a relatively stable carbon-to-carbon linkage prevents further degradation of each end of the polymer chain. Such degradation of unstable molecular ends is generally effected by hydrolysis, as disclosed, for example, in U.S. Pat. No. 3,219,623 to Berardinelli. The oxymethylene copolymer may also be stabilized by end-capping, again using techniques well known to those skilled in the art, as for example by acetylation with acetic anhydride in the present of sodium acetate catalyst.
A particularly preferred class of oxymethylene copolymers is commercially available under the name CELCON® acetal copolymer. CELCON acetal copolymers typically are copolymers of about 98% (by weight) trioxane and about 2% ethylene oxide. CELCON is a registered trademark of Hoechst Celanese Corporation, the assignee of the present invention. CELCON polymers are widely available and are well known. The compositions of the current invention may be made using any
Asinovsky Olga
Hoechst Celanese Corporation
Jubinsky James A.
Seidleck James J.
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