Process for preparation and after-treatment of polyester pellets
AssigneeZimmer, Inc.
InventorsHans Kerpes, Ulrich Thiele
Life of this patent
Estimating the term…
Abstract
Process for simultaneous removal of aldehyde, solid-phase postcondensation and drying of linear polyesters and copolyesters by after-treating the polyester pellets with a diffusion surface of 1.95 to 2.90 m.sup.2 /kg, an individual pellet weight of 5 to 15 mg and a bulk density of 750 to 900 kg/m.sup.3 at 140.degree. to 210.degree. C. with a flow of a dry gas mixture consisting of 60-90 vol % N.sub.2, 5-21% O.sub.2 and 0.1-25% CO.sub.2 (total 100%) at a quantity ratio of 0.5 to 2.5 kg/h gas mixture per kg/h polyester.
Claims6
Process for production and thermal after-treatment of pellets of ethylene terephthalate homopolymers or copolymers starting from polyesters having an intrinsic viscosity within the range of 0.45 to 0.95 dL/g, whereby less than 1 ppm free acetaldehyde and less than 12 ppm bound acetaldehyde are present in the after-treated pellets comprising the steps of
- a) partially crystallizing the polyester by extruding the polyester in a molten state to form strands,
- cooling said strands to a temperature in the range of glass transition temperature to 30.degree. C. below the melting point at at least the surface,
- drawing said strands by a factor of at least 2 at said cooling temperature, and
- immediately chopping into pellets said strands at a temperature in the range of said drawing temperature to 30.degree. C. below the melting point
- said pellets having a specific surface area in the range of 1.95 to 2.90 m.sup.2 /kg,
- an individual pellet weight in the range of 5-15 mg, and
- a bulk density in the range of 750-900 kg/m.sup.3,
- and b) heating said pellets for 2 to 20 hours at a temperature of 140.degree.-185.degree. C. in the presence of a flowing gas mixture consisting of
- 60-90 vol % N.sub.2,
- 5-21 vol % O.sub.2,
- 0.5-25 vol % CO.sub.2 (total 100%),
- and having a dew point of less than -20.degree. C.
- the velocity of flow being 5-40 m/min at a quantity ratio of 0.5 to 2.5 kg/h of gas mixture per 1.0 kg/h polyester.
Dependencies are inferred from the claim text.