PET system:PET Solid State Polymerizing System

Pellet quality from a used PET bottle is improved by a molecular weight adjustment

The difference in the pellet quality is equalized by solid state polymerizing and a diversion as virgin PET resin.


Molecular Weight Adjustment For Crushed PET Bottle Resin Material

The quality demand and conditions for using a reground PET Bottle.
As the resin for PET Bottle molding by Solid State Polymerization.

One of the reasons, which can not divert virgin PET resin to the solid state polymerization process of the reground PET resin as it is because the quality of the pellet resin, which carries out solid state polymerization, differs.

By the reground PET resin, in order to pelletizing at the melting extrusion process of reground PET Bottle molding, quality deteriorates by heat decomposition.

Degree of intrinsic viscosity of the resin is falling, but also the value of a carboxyl end group and aldehyde increases at the same time, then it has a bad influence on a color tone.

The increase in a carboxyl end group reduces polymerization speed and solid state polymerization conditions differ from the conditions of virgin PET resin.

Moreover, since degradation will progress reground resin further if prolonged solid state polymerization processing is carried put, in order to prevent this, compared with virgin resin, adaptation of quiet conditions is needed.

Since the degree of demand of quality is severe, especially in case of the resin used for PET Bottles, it is necessary to fully consider at the setup of solid state polymerization conditions.

System Features

N2 gas and PET chips do the piston flow, by adopting the hopper system for all process of crystallization, drying and maturing. In the hopper structure of our system, circular flow of the resin chips is generated by equipping a diamond shaped cone at the lower cone part of the hopper, and all resin chips are heaved up (bulk density become small), then interspace is increased. And the resistance of the gas flow become smaller, consequently, the gas flows uniformly against the cross section area. Through the above action, resin chips are not damaged, and can be moved, making N2 gas and resin chips piston flow. (uniformly drying, minimizing the detention time range) Uniform air flow (surface wind velocity) is 0.6 m/s or 0.06Nm3/

Because the revolving speed of agitator for crystallization hopper is as low as 1.5 to 2.5 rpm, generation of dust or micro particles is low. Also, micro particles generated are exhausted with the gas and collected by bag filter.

We adopted WATER SCRUBBER for scrubbing N2 gas.

# eg. N2 supplied = 1.5Nm3/kg.h (99.99vol%)
N2 gas at the scrubber inlet to outlet is
EG = 110 ppm / AA = 32 ppm -> EG = 5 ppm / AA = 1 ppm

Effect of this scrubber is as good as that of EG scrubber, actually better than EG scrubber.

As makeup N2 gas reduction measures, our adsorbing rotor has a unique structure, so N2 gas leak is equal to zero during the action of adsorption, purge, and regeneration. Stable dew point, and no leak N2 gas result in the reduction of the makeup N2 gas. With 1000mmAq of differential pressure between each zone, there is no leakage of the N2 gas.

A Demand of Quality

The high temperature polymerization is easier for the removal and reduction of impurities which are

  • 1. Acetaldehyde
  • 2. Low cyclic oligomer contained
  • 3. High whiteness
  • 4. Transparency
  • 5. Molding properties (for injection molding and orientation blow molding)

as a demand item to quality. # 210 degC is better than 200 degC, and the processing temperature also has 230 degC removal and the reduction effect higher than 210 more degC.)

Therefore, the factors, which participate in low temperature polymerization, will be "2", "3", "4" and "5". A low whiteness is generated so certainly that it because high temperature. Acetaldehyde that is sensitive to heat and the oxygen of very small quantity always exists that it is easy to form a coloring substance. Moreover, the item, which affects the molding properties of PET, which reached the target degree of polymerization, has the little ingredient of polymer (degree of degradation, crystalline nuclear particle). In addition, there are the degree distribution of polymerization and a crystallization state, and generally it is thought that it becomes so disadvantageous that it becomes high temperature. A hot minute particle and a hot modification particle will tend to be in the degree of polymerization and a crystal state sharply different from a normal tip. Moreover, the molding of the polymer containing them spoils transparency. Usually, it is not perfect to form the removal equipment of such a minute particle (dust, fine and cut miss tips) in a solid state polymerization process although it carries out.

Design Conditions

  • 1. The piston flow ability of a tip and nitrogen is secured in the state where adopt a move floor and nitrogen gas circulates under normal pressure.
  • 2. Polymerization temperature makes time 8 hours at 217 degC.
    (High temperature is applied to high degree product of polymerization for non bottles).
  • 3. Nitrogen gas and the flow ratio of PET are set to about 1.2 to 1.8 Nm3/kg.h.
    (increases flux in the case of the high degree of polymerization. eg: about 2.0 Nm3/kg.h)
  • 4. A crystalline, dryness, and maturing temperature (preheat / heat up) equipment process with a hopper dryer type, and use a stirring mill in the stage crystallization.
  • 5. It is carrying out to reproduction (refining) of nitrogen gas by the easy method and save the money.
  • 6. Target concentration of nitrogen gas is carried out in Four Nine.
    (oxygen concentration of about 10 ppm)
  • 7. The flux and the amount of reproduction of nitrogen gas are reduced,and energy saving and save of cost are aimed at.
  • 8. It regards so that the trouble by reduction of dust, its removal equipment,and the oligomer of tip may not occur.