Process for preparing polyethylene terephthalate useful for beverage containers

A process for reducing the acetaldehyde content and the generation rate of acetaldehyde of polyethylene terephthalate chip which has an intrinsic viscosity of from about 0.55 to about 0.75 deciliter per gram as calculated from measurements made on an 8 percent by weight solution in o-chlorophenol at 25.degree. C. in order to render the polyethylene terephthalate suitable for making containers, which process comprises

• (a) crystallizing said polyethylene terephthalate chip by agitating and simultaneously heating it to a temperature of from about 110.degree. to about 240.degree. C. for at least about 3 minutes in order to impart a crystallinity of at least about 30% to said polyethylene terephthalate and to reduce the acetaldehyde content of said polyethylene terephthalate to less than about 20 parts per million, and
• (b) stabilizing said crystallized polyethylene terephthalate chip by moving it downward and heating it in a stabilization zone at a temperature of from about 180.degree. to about 220.degree. C. for from about 2 to about 20 hours in a counter-current stream of air and maintaining an air to chip ratio at a predetermined value of at least about 0.8 standard cubic foot of air per minute/pound of polyethylene terephthalate per hour and at a vapor velocity of at least about 0.5 foot per second, said air having a dew point of less than about -30.degree. C., in order to reduce the acetaldehyde level of said crystallized polyethylene terephthalate to less than about 2.5 parts per million, to reduce the generation rate of acethaldehyde to less than about 3.0 parts per million per minute, and to increase the intrinsic viscosity to a value in the range of from about 0.60 to about 0.95 deciliters per gram.

A continuous process for reducing the acetaldehyde content and the generation rate of acetaldehyde of polyethylene terephthalate chip which has an intrinsic viscosity of from about 0.62 to about 0.68 deciliters per gram as calculated from measurements made on an 8% by weight solution in o-chlorophenol at 25.degree. C. in order to render the polyethylene terephthalate suitable for making containers, which process comprises

• (a) crystallizing said polyethylene terephthalate chip by agitating and simultaneously heating it to a temperature of from about 150.degree. to about 210.degree. C. for from about 5 to about 20 minutes in order to impart a crystallinity of from about 35 to about 45% to said polyethylene terephthalate and to reduce the acetaldehyde content of said polyethylene terephthalate to less than about 10 parts per million and the moisture content to from about 0.02 to about 0.03%, and
• (b) stabilizing said crystallized polyethylene terephthalate chip by moving it downward and heating it in a stabilization zone at a temperature of from about 190.degree. to about 210.degree. C. for from about 6 to about 12 hours in a counter-current stream of air and maintaining an air to chip ratio at a predetermined value of from about 0.95 to about 2.5 cubic feet of air per minute/pound of polyethylene terephthalate chip per hour and at a vapor velocity of from about 1 to about 2 feet per second, said air having a dew point of from about -40.degree. to about -80.degree. C., in order to reduce the acetaldehyde level of said crystallized polyethylene terephthalate to less than about 1.5 parts per million, to reduce the generation rate of acetaldehyde to a value of from about 2.2 to about 2.6 parts per million per minute, and to increase the intrinsic viscosity to a value in the range of from about 0.70 to about 0.80 deciliters per gram.