1. Field of the Invention
The present invention relates to compositions comprising blends of polycarbonate and polyester. More particularly, the present invention relates to blends of polycarbonate and polyester to which catalyst quenchers are added to control ester-carbonate interchange. The polyester in this invention has epoxy functionality which improves the hydrolytic properties of these catalyst quencher-containing compositions.
2. Related Art
Compositions comprising blends of polyester and polycarbonate have been widely used by industry. For instance, these compositions are widely used in the transportation industry, particularly the automotive industry. These compositions also have been used in the electrical, construction, appliance and houseware industries.
Blends comprising polyester and polycarbonate, however, tend to undergo ester-carbonate interchange, wherein ester linkages in both the polycarbonate and the polyester are believed to be broken and replaced by alkylene carbonate and arylene carboxylate bonds. The result is degradation of the physical properties of the polymers due to hybridization of the molecular linkages therein. Without being held to any particular theory, it is believed that metal catalyst residues from the polymerization reaction forming the polyester promote the ester-carbonate interchange in these compositions. This interchange leads to variability in the articles finally fabricated from the blends.
To deactivate the metal catalyst residues, "catalyst quenchers" have been added to polyester compositions. For example, phosphite quenchers comprise one class of quenchers which has been suggested for deactivating metal catalyst residues in polyester compositions. See J. Devaux, P. Godard, J. P. Mercier, Polymer and Engineering Science, Vol. 22, p. 229 (1982). Phosphorous acid also has been used to quench such metal catalysts.
U.S. Pat. No. 4,532,290 to Jaquiss et al. discloses adding monosodium phosphate and/or monopotassium phosphate quenchers to polyester/polycarbonate blends. Commonly assigned and copending application having Ser. No. 994,794, filed Dec. 22, 1992, and commonly assigned, copending application having Ser. No. 995,243, filed Dec. 22, 1992, disclose quenching catalysts by using metal acid pyrophosphates or polyacid pyrophosphates having the general formula EQU M.sup.z.sub.x H.sub.y P.sub.n O.sub.3n+1
wherein M is a metal, "x" is a number ranging from 1 to 12, and "y" is a number ranging from 1 to 12, "n" is a number in the range of 2 to 10, "z" is a number in the range of 1 to 5, and the sum of xz+y is equal to n+2.
It has been discovered that some catalyst quenchers, e.g., the phosphorus acid or alkyl phosphites, reduce the hydrolytic stability of the polyester/polycarbonate compositions. In certain instances, the stability of compositions comprising these quenchers under hydrolyzing conditions is decreased to a point that the compositions' stability is unacceptable. Hydrolytic stability is usually reflected in the composition's ability to maintain its melt viscosity over time when exposed to moisture under various conditions. Failure to maintain melt viscosity leads to loss of mechanical properties.
U.S. Pat. No. 5,087,665 to Chung et al. discloses a method of improving the hydrolytic stability of blends of polycarbonate and polyester, i.e., polyethylene terephthalate, by adding polyethylene to the blends. The blends disclosed in this patent, however, do not contain catalyst quenchers and the patent does not address hydrolytic stability problems resulting from these additives. As disclosed above, blends of polyester and polycarbonate lacking these quenchers typically exhibit transesterification and change in melt viscosity to a level that the blends result in articles having variable properties. Under extreme conditions, crystallinity will be lost and gaseous by-products will be produced.
Commonly assigned and copending application Ser. No. 987,588, filed Dec. 8, 1992, discloses stabilizing polyesters against hydrolysis by combining the polyester with epoxy compounds in the presence of a catalyst. The resulting polyester composition is shown to have improved hydrolytic stability and melt viscosity stability. Blends of polyester and polycarbonate comprising catalyst quenchers, however, are not disclosed in this application and the hydrolytic stability problems resulting from catalyst quenchers therefore are not expressly addressed.
Commonly assigned and copending application having Ser. No. 952,219, filed on Sep. 28, 1992, discloses using an epoxy compound to stabilize mineral-filled polyesters against hydrolysis. Barium sulfate, zinc oxide, zirconium sulfate, etc., fillers are disclosed. However, this application also does not disclose blends of polyester and polycarbonate comprising catalyst quenchers, nor does it address the hydrolytic stability problems resulting therefrom.
Epoxy compounds also have been added to polyester compositions to improve their thermal stability. See, for example, U.S. Pat. Nos. 3,978,020; 3,886,104; and 4,020,122. The '020 patent and the '104 patent also mention phosphite or phosphonite additives for improved thermal oxidative stabilization. Improving the hydrolytic stability of these compounds is not disclosed, however, nor is there disclosure of improving the hydrolytic stability of compositions comprising blends of polyester and polycarbonate.
There thus is still a need for blends of polyester and polycarbonate which not only possess improved stabilization against ester-carbonate interchanges, but also show improved hydrolytic stability.