Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1999-11-22
2001-04-10
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S081000, C521S182000
Reexamination Certificate
active
06214897
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to polyester polymer compositions and to the closed cell polymer foams made therefrom. In particular, there is provided a semi-semi-crystalline polycondensed branched polyester polymer blended with an aromatic polycarbonate to provide enhanced melt strength to polyesters, and the closed cell polymer foams made therefrom simultaneously having improved impact strength and lower density.
BACKGROUND OF THE INVENTION
Foamed sheets of polyester are often made by extruding a polyester composition out of an annular die and over a “cooling can” after which the somewhat cooled polyester is cut to form a sheet that is taken up on a roll. A common problem encountered in extruding molten polyester onto a moving belt is that the polyester extrudate tears as it is being pulled over the cooling can. This type of failure is often due to the lack of adequate polyester polymer melt strength. A polyester polymer possessing high melt strength resists the tendency of tearing or ripping when subjected to tensile forces. Accordingly, it would be desirable to increase the melt strength of an extrudable polyester polymer to allow a continuous take up of foamed sheet onto a roll and reduce the down time needed to re-align a torn extrudate onto or over a cooling can.
U.S. Pat. No. 4,544,677 is directed to foamable thermoplastic compositions made with a mixture of polycarbonate resins, optionally polyester resins, a foaming agent, and a nucleating agent having a specific aspect ratio. The optional polyester resins disclosed are linear, and additional amounts of acid or glycols could be added in amounts ranging from 0.5 wt. % to 50 wt. % based on the total composition. We have found, however, that linear polyesters are not well suited to extruding foamed sheets because the resin composition has insufficient melt strength. Without sufficient melt strength and die swell, the molten resin both rips and tears as it is pulled over or onto the cooling can, and fails to yield a product with low density. Certain foaming methods, such as injection molding techniques as described in U.S. Pat. No. 4,544,677, do not require the use of a resin of great melt strength because the resin is not placed under tension or taken up during the foaming of the resin. Accordingly, there remains a need to develop a resin which has high melt strength.
U.S. Pat. No. 5,360,829 discloses foamed polyester sheets made from amorphous polyesters, 20 to 100 parts by weight of a polycarbonate resin based on 100 parts by weight of the polyester, and a nucleating agent in order to impart to the foamed sheet a uniform cellular morphology and a high foaming ratio while maintaining its mechanical properties and dimensional stability. The patentees noted that when crystalline polyesters were blended with polyolefins, the cellular morphology of the resulting foamed sheets was not uniform, and the foaming ratio was too low. Accordingly, the patentees recommended using amorphous polyesters in combination with polycarbonates to overcome this deficiency. Amorphous polyesters, however, even in foamed compositions, suffer the drawback in that they are excluded from being used in high temperature applications. It would be desirable to find an alternative polyester composition that possesses high melt strength without this noted deficiency.
The foamed sheets of polyester find a number of consumer uses, many of which subject the polyester foam sheet to high impacts. Therefore, it is also desirable to make a polyester foam sheet which has high impact strength. One way of doing so is to increase the density of the polyester foam, either by increasing the amount of polymer per unit volume, or by adding reinforcing agents. Neither method is an attractive solution. Both increase the cost of the foam. Further, in the latter method, the amount of reinforcing agent that can be added is limited by the increase in viscosity to the polymer melt resulting from the reinforcing agent.
Attempts to decrease the density of the polyester foam have also lead to a corresponding decrease in the impact strength of polyester foams. It would be highly desirable to make a polyester polymer that overcomes this longstanding direct relationship between the foam density and impact strength; that is, there exists a need to make a polyester foam that simultaneously has a low density and an improved impact strength. Accordingly, in addition to making a polyester polymer which processes well by virtue of its high melt strength, as an additional embodiment, it is desirable to make a polyester foam sheet, and a thermoformed article from the sheet, which has a good cellular structure, low density, and high impact strength.
U.S. Pat. No. 5,502,119 suggests stabilizing blends of polyesters and aromatic polycarbonates with a particular organosilicate. The types of polyesters described therein are not polycondensed branched types of polyesters. U.S. Pat. No. 5,504,130 describes filled thermoplastic compositions where about 15-50% of at least one poly(phenylene ether) resin is compatibilized with about 20-80% of at least one polyester resin by using from 3-50%, preferably of from about 8-20%, of an aromatic polycarbonate having a weight average molecular weight of at least about 40,000. These patents, however, do not address the problem of how to improve the melt strength of polyester compositions, or how to additionally make foamed sheets having good cellular uniformity and high impact strength at low density.
SUMMARY OF THE INVENTION
There is now provided a closed cell polymer comprising a mixed polymer composition comprising:
a) a semi-crystalline polyester composition comprising a polycondensed branched polyester polymer, and
b) an aromatic polycarbonate polymer.
This polymer mixture has a high melt strength and can be extruded and pulled across, over, or through a die with a low frequency of ripping or tearing.
In another embodiment of the invention, there is provided a mixed polymer composition comprising:
a) a polyester composition in an amount of at least 90 wt. %, based on the weight of all polymers in the mixed polymer composition, said polyester composition consisting of polyester polymers, said polyester polymers comprising polycondensed branched polyester polymers, and
b) aromatic polycarbonate polymers in an amount ranging from 0.01 wt. % to 10.0 wt. %, preferably from 0.01 wt. % to 2.0 wt. %, based on the weight of the mixed polymer composition.
There is also provided a foamable composition comprising a combination of blowing agent and mixed polymer composition comprising:
a) a semi-crystalline polyester composition comprising a polycondensed branched polyester polymer, and
b) an aromatic polycarbonate polymer.
Processes for extruding the mixed polymer compositions, making a foamed sheet and thermoforming the foamed sheet made with the mixed polymer composition in the presence of a blowing agent are also included.
An unexpected further advantage observed in the closed cell polymers is that they had a more uniform, fine, and closed cell morphology; and were thicker and less dense than the same closed cell polymers made only with linear polyesters as the polyester component or without the aromatic polycarbonate. The thermoformed sheets made from the foamed sheets using this composition also had a higher impact strength compared to the same thermoformed sheets made only with linear polyesters as the polyester component or without the aromatic polycarbonate.
DETAILED DESCRIPTION OF THE INVENTION
The foamable composition comprises the mixed polymer composition, blowing agent, and any other optional additives, such as impact modifiers, fibers, stabilizers, and flame-retardants. The foamable composition for purposes of calculating amounts of each ingredient includes the sum total of all ingredients used in the embodiment.
The mixed polymer composition comprises:
a) a polyester composition comprising a polycondensed branched polyester polymer, and
b) an aromatic polycarbonate polymer;
along with any optional hydrocarbon polymers and oligomers hav
Johnston Walter Francis
Rollick Kevin
Scoarste John Michael
Snell Tung Deborah Ann
Foelak Morton
Shell Oil Company
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