Process of making polyester pellets

Details Process of making polyester pellets Process of making polyester pellets

2001-01-22

2003-09-02

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

With severing, removing material from preform mechanically,...

To form particulate product

C525S437000, C525S444000, C528S194000, C528S298000, C528S300000, C528S301000, C528S302000, C528S308000, C528S308100, C528S308300, C528S308600, C528S308800

Reexamination Certificate

active

06613259

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to novel polyester pellets and a process for preparing the polyester pellets. More particularly, the invention relates to polyester pellets having excellent gas barrier properties, transparency and heat resistance and to a process for preparing the polyester pellets.
BACKGROUND OF THE INVENTION
Because of their excellent gas barrier properties, transparency and mechanical strength, saturated polyesters such as polyethylene terephthalate are widely used for containers such as bottles. Particularly, the bottles obtained by biaxial stretching blow molding (draw blow molding) of polyethylene terephthalate are excellent in transparency, mechanical strength, heat resistance and gas barrier properties, so that they have been extensively used as containers (PET bottles) to be filled with drinks such as juice, soft drinks and carbonated beverages.
Such bottles are generally produced by a process comprising molding a saturated polyester into a preform having a neck part and a body part, inserting the preform in a mold of given shape, and subjecting the preform to stretching blow molding to stretch the body part, thereby producing a bottle having a neck part and an oriented body part.
The polyester bottles, particularly polyester bottles for drinks such as juice, are required to have heat resistance high enough for heat sterilization of the contents therein, and therefore the bottles are generally further subjected to heat treatment (heat setting) after the blow molding to improve the heat resistance.
In the polyester bottles obtained by the above process, the neck parts are unstretched and inferior to the stretched body parts in the mechanical strength and the heat resistance. In general, therefore, the neck parts of the preforms are heated to crystallize prior to the blow molding, or the necks of the bottles obtained by blow molding are heated to crystallize, thereby improving the neck parts in the mechanical strength and the heat resistance.
In recent years, the sizes of bottles produced from the polyester resins (particularly polyethylene terephthalate) tend to be made smaller. In case of such small-sized bottles, the contact area between the contents and the bottle body part per unit volume is increased, and thus loss of gas or permeation of oxygen from the outside may have a marked influence on the contents, resulting in decrease of shelf life of the contents. Accordingly, the polyester resins are required to have more excellent gas barrier properties than before.
In order to improve the heat resistance and the gas barrier properties of the polyester resins, an attempt to blend polyethylene terephthalate with polyethylene isophthalate has been proposed (see Japanese Patent Publication No. 22302/1989). The blend of polyethylene terephthalate and polyethylene isophthalate, however, generates acetaldehyde when it is melt kneaded at a high temperature to improve compatibility, and this causes problems such as change of taste of the contents filled in the container and lowering of transparency. Further, the polyethylene isophthalate adheres to the screw to prolong the residence time, and this causes another problem of scorching. Furthermore, when polyethylene isophthalate is amorphous, polyethylene terephthalate is required to be dried by an ordinary drier, then cooled and blended in a dry state with the polyethylene isophthalate, followed by molding the blend. Therefore, the cost of equipment for various steps from drying to molding is great, and much space is necessary for the equipment.
To cope with the above problems, there has been proposed a polyester comprising ethylene glycol and a dicarboxylic acid component which comprises terephthalic acid as a major ingredient and isophthalic acid. This polyester, however, does not always have sufficient heat resistance and gas barrier properties and sometimes generates acetaldehyde. Accordingly, development of polyesters having further improved heat resistance and gas barrier properties and hardly generating acetaldehyde is desired.
OBJECT OF THE INVENTION
The present invention has been made with a view to solying such problems in the prior art as mentioned above, and it is an object of the invention to provide crystallized polyester pellets having excellent gas barrier properties, transparency and heat resistance and hardly generating acetaldehyde and to provide a process for preparing the polyester pellets.
SUMMARY OF THE INVENTION
The novel polyester pellets according to the invention are polyester pellets made of a polyester which comprises dicarboxylic acid constituent units derived from dicarboxylic acids containing terephthalic acid and isophthalic acid and diol constituent units derived from diols containing ethylene glycol and 1,3-bis(2-hydroxyethoxy)benzene, and which has the following properties:
constituent units derived from terephthalic acid are 15 to 99.5% by mol and constituent units derived from isophthalic acid are 0.5 to 85% by mol, both based on the total amount of the dicarboxylic acid constituent units (i),
constituent units derived from ethylene glycol are 25 to 99.5% by mol and constituent units derived from 1,3-bis(2-hydroxyethoxy)benzene are 0.5 to 75% by mol, both based on the total amount of the diol constituent units (ii),
the intrinsic viscosity is in the range of 0.5 to 1.5 dl/g, and
the melting point (Tm (° C.)), as measured by a differential scanning calorimeter, satisfies the following formula (I):
[1/527−0.0017·ln(1−(
m
I
+m
B
)/200)]
−1
−273
<Tm≦
265  (I)
wherein m
I
is a proportion (% by mol) of the constituent units derived from isophthalic acid to all of the dicarboxylic acid constituent units, and m
B
is a proportion (% by mol) of the constituent units derived from 1,3-bis(2-hydroxyethoxy)benzene to all of the diol constituent units;
said polyester pellets having a density of not less than 1,350 kg/m
3
.
The melting point (Tm (° C.) of the polyester desirably satisfies the following formula (I′):
[1/527−0.0017·ln(1−(
m
I
+m
B
)/200)]
−1
−273
<Tm≦
265  (I′).
The acetaldehyde content in the polyester pellets is preferably not more than 20 ppm, particularly preferably not more than 10 ppm.
The process for preparing polyester pellets according to the invention comprises
blending (A) polyethylene terephthalate before solid phase polymerization having an intrinsic viscosity of 0.3 to 0.8 dl/g, in an amount of 99 to 20% by weight, with (B) a polyethylene isophthalate copolymer before solid phase polymerization having an intrinsic viscosity of 0.3 to 0.9 dl/g, in an amount of 1 to 80% by weight, said polyethylene isophthalate copolymer comprising dicarboxylic acid constituent units derived from dicarboxylic acids containing terephthalic acid and isophthalic acid and diol constituent units derived from diols containing ethylene glycol and 1,3-bis(2-hydroxyethoxy)benzene,
pelletizing the blend, and
crystallizing the pellets.
It is preferable that the blend is heated to precrystallize it and then subjected to solid phase polymerization.
The blend preferably has a heat-up crystallizing temperature of not higher than 190° C.
Another process for preparing polyester pellets according to the invention comprises
blending (C) polyethylene terephthalate after solid phase polymerization having an intrinsic viscosity of 0.5 to 1.5 dl/g, in an amount of 99 to 20% by weight, with (B) a polyethylene isophthalate copolymer before solid phase polymerization having an intrinsic viscosity of 0.3 to 0.9 dl/g, in an amount of 1 to 80% by weight, said polyethylene isophthalate copolymer comprising dicarboxylic acid constituent units derived from dicarboxylic acids containing terephthalic acid and isophthalic acid and diol constituent units derived from diols containing ethylene glycol and 1,3-bis(2-hydroxyethoxy)benzene,
pelletizing the blend, and
crystallizing the pellets.
In the present invention, the blend may be subjected to solid phase polymerization

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