Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-02-20
2001-02-13
Hoke, Veronica P. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S128000, C524S147000, C524S303000, C524S417000, C524S494000, C524S605000
Reexamination Certificate
active
06187848
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to glass fiber filled polyester molding compositions with reduced discoloration on thermal aging.
BACKGROUND OF THE INVENTION
Moldable glass filled crystalline resin blends offer a high degree of surface hardness, solvent resistance, strength, stiffness, heat resistance, good appearance and colorability. However, polyester formulations typically begin to discolor within days on continuous exposure, in air, to temperatures exceeding about 120° C. The presence of flame retardant additives can cause additional issues to color stability as seen in U.S. Pat. No. 5,371,123 where interaction of certain brominated FR agents and certain glass coatings gave rise to undesirable color.
Hindered phenol antioxidants and phosphites have been used to improve the retention of mechanical properties in polyesters as described in the “Plastics Additives Handbook” R, Gachter and H. Muller, Hanser-Gardner 1996, p92-95. Various additives have been used to improve the retention of polyester viscosity and molecular weigh in the melt. U.S. Pat. No. 5,589,530 to Walsh describes blends of low and high molecular weight polybutylene terephthalate which are melt stabilized by the addition of certain phosphorus-containing compounds. Other additives are described as UV stabilizers, pigments, colorants, fillers, plasticizers, processing aids, antioxidants and the like. Addition of zinc phosphate, sodium acid pyrophosphate, hindered phenols, and pentaerythritol tetrastearate are specifically set forth.
Despite all this work light colored polyester formulations tend to discolor over time, especially when subjected to temperatures exceeding about 120 degrees Centigrade. This discoloration is especially noticeable in polyesters that have white or light color pigmentation. The thermal aging results in a color shift which renders the polyester undesirable for applications such as oven handles or trim, and other appliances subject to heat such as toasters, broilers, dryers, hair styling devices and especially clothing irons. Hence, it is desirable to provide improved polyester resins having desirable properties of strength, stiffness, resistance to deformation at high temperature, as well as good color stability over time when subjected to relatively high temperatures.
SUMMARY OF THE INVENTION
The compositions of the present invention provide for enhanced color stability without undesirable affecting the physical and chemical properties of the polyester resin.
According to the present invention, there is provided a lightly colored, reinforced polyester molding composition having color stability at relatively high temperatures, comprising (a) from 40 to 97 weight percent of a polyester resin; (b) from about 3 to about 60 weight percent silica based filler or fiber (c) and less than about 5 percent by weight additional ingredients which include a combination of stabilizers consisting essentially of an acidic phosphorus quencher, a thioester and a phosphonite. The blend may optionally have from 2 to 30 weight percent of a flame retarding composition (d). Preferred flame retardants are polymeric aromatic bromine compounds with an antimony oxide synergist
Surprisingly, the above combination of acidic quencher, thioester and phosphonite and absent other stabilizers such as hindered phenols, gives enhanced retention of original color on thermal aging.
Addition of polyolefins to polyesters has beneficial effects as described in Holub et.al., U.S. Pat No. 4,122,061, which describes polyester reinforced blends of poly(1,4-butylene terephthalate) and poly(ethylene terephthalate) impact modified with the addition of a polyolefin or olefin based copolymer resin, and in Liu, U.S. Pat. No. 4,564,658, which describes PBT with linear low density polyethylene. We have found that the presence of these polyolefin resins in the blends of this invention is detrimental, accelerating color formation even in the presence of phosphorus and sulfur based additives that are usually beneficial to polyolefin stabilization.
Thus polyester compositions of the prior art tend to undergo undesirable discoloration. This is surprising in view of the fact that hindered phenols are used as stabilizers for polyolefins and their synergy with phosphorus and sulfur based antioxidants is a basic tenet of polymer stabilization. The compositions of the present invention are characterized by the absence of a significant amount of a polyolefin and the use of thioester, phosphonite and acidic quencher combinations as stabilizers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The reinforced polyester molding composition includes the following major ingredients which are described as follows: thermoplastic polyester resins, silica based fillers or fibers, especially glass fibers, and mixtures of phosphonite, thioester and acidic quenchers. In the preferred composition flame retardant additives are also employed.
Polyester components include crystalline polyesters such as polyesters derived from aliphatic or cycloaliphatic diols, or mixtures thereof, containing from 2 to about 10 carbon atoms and at least one aromatic dicarboxylic acid. Preferred polyesters are derived from an aliphatic diol and an aromatic dicarboxylic acid having repeating units of the following general formula:
wherein n is an integer of from 2 to 6. Ar is a C
6
-C
20
aryl radical comprising a decarboxylated residue derived from an aromatic dicarboxylic acid.
Examples of aromatic dicarboxylic acids represented by the decarboxylated residue Ar are isophthalic or terephthalic acid, 1,2-di(p-carboxyphenyl)ethane, 4,4′-dicarboxydiphenyl ether, 4,4′ bisbenzoic acid and mixtures thereof. All of these acids contain at least one aromatic nucleus. Acids containing fused rings can also be present, such as in 1,4- 1,5- or 2,6-naphthalene dicarboxylic acids. The preferred dicarboxylic acids are terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid or mixtures thereof.
The most preferred polyesters are poly(ethylene terephthalate) (PET), and poly(butylene terephthalate) (PBT), poly(ethylene naphthanoate) (PEN), poly(butylene naphthanoate) (PBN), poly(propylene terephthalate) (PPT), poly(cyclohexanedimethylene terephthalate) (PCT) or mixture of these resins.
Also contemplated herein are the above polyesters with minor amounts, e.g., from about 0.5 to about 15 percent by weight, of units derived from aliphatic acids and/or aliphatic polyols to form copolyesters. The aliphatic polyols include glycols, such as poly(ethylene glycol) or poly(butylene glycol). Such polyesters can be made following the teachings of, for example, U.S. Pat. Nos. 2,465,319 and 3,047,539.
The preferred poly(1,4-butylene terephthalate) resin used in this invention is one obtained by polymerizing a glycol component at least 70 mol %, preferably at least 80 mol %, of which consists of tetramethylene glycol and an acid component at least 70 mol %, preferably at least 80 mol %, of which consists of terephthalic acid, and polyester-forming derivatives therefore.
The polyesters used herein have an intrinsic viscosity of from about 0.4 to about 2.0 dl/gm. measured in a 60:40 phenol/tetrachloroethane mixture or similar solvent at 23-30° C. VALOX Registered TM 315 polyester is particularly suitable for this invention. Preferably intrinsic viscosity is about 1.1 to about 1.4 dl/gm.
Blends of polyesters may also be employed in the composition. Preferred polyesters blends contain poly(ethylene terephthalate) and poly(1,4-butylene terephthalate). When blends of these preferred components are employed the polyester resin component can comprise from about 5 to about 50 parts by weight poly(ethylene terephthalate) and from about 95 to about 50 part by weight poly(1,4-butylene terephthalate) based on 100 parts by weight of both components combined.
Also, blends of relatively low molecular weight polybutylene terephthalate resin may be used with a relatively high molecular weight polybutylene terephthalate as set forth in U.S. Pat. No. 5,589,530 to Walsh. The low molecular weight polybutylen
Finan James M.
Gallucci Robert R.
Pixton Matthew R.
General Electric Company
Hoke Veronica P.
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