CA1080416A

CA1080416A - Oriented containers

Info

Publication number: CA1080416A
Application number: CA237,294A
Authority: CA
Inventors: Robert P. Noonan
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1974-10-31
Filing date: 1975-10-08
Publication date: 1980-07-01
Also published as: DD123066A5; DE2547995A1; JPS5167362A; FR2289320A1; IT1048026B; BE835170A; GB1514277A; FR2289320B1; NL7511933A

Abstract

ABSTRACT
Production of oriented polyethylene terephthalate containers by injecting a polyethylene terephthalate composition into a first mold chamber defining a parison having an axis substantially defined by a core rod and perimeter defined by the wall of the mold, said parison having a tip, a body and a neck; cooling the exterior wall of the polyethylene terephthalate parison to render the outer wall of the parison dimensionally stable; after the outer wall of the parison is dimensionally stable, transferring the parison, while disposed on the core rod, to a second mold chamber, whereby the distance between the outer wall of the transferred parison and the inner wall of the second mold chamber is at any given point between 2.5 and 150%
of the distance across the body of said parison at said point, and wherein the wall of the second chamber is at a higher temperature than the wall of the first chamber which was in contact with the parison tip; when the core rod is at no more than 265°F., injecting a gaseous fluid into the parison at a pressure of at least 10 psig to separate the body of the parison from the core rod and force the exterior wall of the body of the parison to assume the shape of the wall of the second chamber; transferring the parison, while the neck of the parison is disposed on the core rod, to a third chamber, whereby the distance between the outer wall of the trans-ferred parison and the inner wall of the third chamber is at any given point at least 50% of the distance across the body of said parison at said point; and at the orientation temperature of the polyethylene terephthalate parison, injecting a gaseous fluid into the parison to force the exterior walls of the body of the parison to assume the shape of the wall of the third chamber and to orient the walls of the body of the parison.

Description

-This invention relates to the production of oriented polyethylene terephthalate plastic bottles.
Over the last 20 years, plastic containers have replaced glass . 1 :

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~ ~ ' ~ - , :` ' ' ' :' ; i ' ., . ' . . ' ' ' ' ! ' containers in many uses. More recently, there has been con-siderable interest in the production of oriented plastic con-tainers, such as those based on polyethylene terephthalate, since the oriented containers have much better physical pro- -~
perties and/or barrier properties than unoriented containersO
As is well known, oriented plastic containers can be produced by stretching the walls of a polymeric parison only at or below orientation temperature of the particular plastic.
Stretching or blowing above the orientation temperature results in no orientation and accordingly, no increase in physical properties, etc. These processes are complicated by the fact that the orientation temperatures of polymer ~-materials can be substantially below the melting points of the polymers used to produce the polymeric parison, the fact that the walls of the parison must be at a relatively uniform temperature during orientation and the need for rapid pro-duction of bottles on an assembly line basis.
Two general techniques have been developed for producing oriented containers. The first type comprises forming a pre-formed tubular parison, cooling the parison to room temperature, possibly storing the parison for a period of time, and subsequently heating and blowing the pre-formed parison, while disposed in a blow mold, into the desired shape. The second type, called injection blow molding, comprises injection molding or extruding a hot parison, cooling the hot parison to a suitable orientation temperature and then blowing the partially cooled parison, while disposed in a blow mold. The former type of processing -permits rapid production of parisons but results in wasting a substantial portion of polymeric material and is also subject to various other disadvantages. The second type, is subject to the time and temperature problems in cooling the pre-formed parison to the orientation temperature in a short period of time. Polymers such as polyethylene terephthalate, which must be oriented at a temperature more than 200F. less than their melting points, present severe time problems.
The general object of this invention is to provide an improved method of producing oriented polyethylene tere-phthalate containers. A more specific object of this inven-tion is to provide a relatively rapid method of injection blow molding oriented polyethylene terephthalate bottles. ;
Other objects appear hereinafter.
For the purpose of this invention, a parison can be viewed as having three distinct zones, namely the "tip" portion, which corresponds to the bottom of the final container; the "neck" portion, which is substantially the same size in the parison and final container and, the "body" portion, which corresponds to the hoop portion of the container.
I have now found that the objectives of this in- !
vention can be attained by a multi-step process which com-prises:
1. injecting polyethylene terephthalate into a first mold chamber or zone defining a parison having an axis substantially defined by a core rod and peri-meter defined by the wall of the mold;

2. cooling the exterior wall of the polyethylene tere-phthalate parison to render the outer wall of the parison dimensionally stable;

3. after the outer wall of the parison is dimensionally stable, -transferring the parison, while disposed on the core rod, to a second mold chamber, whereby the distance between the outer wall of the transferred parison and the inner wall of the second mold chamber is at any . ~

given point between 2.5 and 15~d of the distance across the body of said parison at said point, and wherein the wall of the second chamber is preferably at a higher temperature than the wall of the first chamber which was in contact with the parison tip;

4. when the core rod is at no more than 265F., injecting a gaseous fluid into the parison at a pressure af at least 10 psig to separate the body of the parison ~rom the core rod and force the exterior wall of the body of the parison to assume the shape of the wall of the second chamber;

5. transerring the parison, while the neck of the parison is disposed on the core rod, to a third chamber, whereby the distance between the outer wall of the transferred parison and the inner wall of the third chamber is at any given -point at least 50% of the distance across the body of said parison at said point;

6. at the orientation temperature of the polyethylene ~ :
terephthalate parison, injecting a gaseous fluid into the parison to force the exterior walls of the body of the parison to assume the shape of the wall of the third chamber and to oxient the walls of the body of the parison.

For c~nvenience, it is desirable to consider the importance of the various steps in this invention with reference to the production of a polye~hylene terephthalate container having a cylindrical body suitable for use in bottling carbonated beverages, such as pop or beer. In somewhat greater detail the process entails injecting polye~hylene terephthalate at about 529 to 590F. into a test tube shaped first mold chamber or zone 1iL6 defined by a core rod and the chamber walls. Above about 590F., the polyethylene terephatalate degrades. The principle func-tion of the core rod is to define the inner wall of the parison and to serve as a support for transferring the parison from station to station. Preferably the core rod has cooling means to aid in the rapid cooliny of the parison in the ~irst mold and usually contains a conduit for injecting gaseous fluid to -expand the parison in the second the third chambers.
The walls of the first chamber define the outside dimensions or perimeter of the parison with the gap between the core rod and first chamber walls controlling the thickness of the parison. Generally, the walls of the parison can range from about 15 to 400 mils on an average. In order to produce a test tube shaped or closed end parison, there is also a gap between the tip or bottom of the core rod and the chamber bot-tom. The first chamber contains cooling means to obtain rapid cooling of the parison outer walls. Typically the walls of the chamber in close proximity to the closed end or tip of the parison are maintained at a substantially lower temperature than the walls of the chamber in contact with the body or top of the parison since there is usually a greater mass of polymer to be cooled at the tip or closed end.
The parison is maintained in the first chamber until the exterior walls of the polyethylene terephthalate parison are dimensionally stable and the parison can be transferred to a second chamber without changing the parison configuration.
Normally, the parison being transferred has a substantial tem-perature gradient throughout its mass with the dimensionally stable outer wall of the parison being substantially cooler than the rest of the polyethylene terephthalate parison. In effect the parison may be viewed as having a relatively cool ~8~
skin or shell containing the hotter polyethylene terephthalate.
Of course, the parison cannot be oriented effectively while there is this large temperature gradient. However, forming this relatively cool skin or shell while the parison is part-ially supported by the core rod permits a substantial saving in the time necessary to maintain the polymeric material in the injection mold or first chamber and accordingly a reduc-tion in production time.
The second chamber which must be larger in the hoop dimension than the first chamber (except in the neck portion), may be viewed as an equilibration chamber where the temperature gradient throughout the parison is reduced. This chamber is extremely important since it permits relatively rapid equilib-ration of polyethylene terephatalate parison which must be oriented at a temperature more than 200F. less than poly-ethylene terephthalate melting point. This chamber reduces the cycle time by about 25~ and increases productivity by about 33~. Temperature equilibration is facilitated by separating the body and tip of the parison from the core rod by injecting a gaseous fluid into the parison at a pressure of at least 10 psig.
As pointed out by Ninneman in U.S. Patent 3,244,778, separation of the parison from contact with a metal surface by injecting air removes the parison from heat transfer relation-ship with the metal surface. However, unlike Ninneman, this invention requires the injection of gaseous fluid at 10 psig or higher, preferably 40 to 120 psig. If the gaseous fluid is at less than 10 psig or the core rod is above 265F., poly~
ethylene terephthalate does not separate properly from the core rod. Generally, best results with polyethylene tere-phthalate are attained with a core rod temperature of 100 to 200F. Further, the separation of the parison body and tip from the core rod has to be carried out in a chamber where the chamber walls in contact with the parison tip is above the temperature of the tip portion of the walls of the first chamber. This higher temperature is necessary to raise the skin or shell temperature of the parison to a temperature at which air pressure can expand the parison. If the wall tem-; perature of the parison is too low, the tip portion of the -~
parison will not assume the shape of the second chamber. -- ;
The separation or tolerance in the hoop dimension between the parison and the second chamber should on an aver-age range from about 2.5% to 150% of the distance across the body of the parison. In the case of a tubular or test tube shaped parison, the separation between the parison wall and the chamber should be equal to at least 2.5% times the out-side diameter of the parison or 5% times the outside radius of the parison. Stated a different way the chamber inside diameter should be 105 to 300% of the outside diameter of the parison. If the tolerance is too small, it is difficult to ~20 obtain the desired rapid temperature equilbration of the parison. On the other hand if the tolerance is too large, it is difficult to obtain the desired orientation in the third chamber. The walls of the second chamber can be maintained above or below the orientation temperature of the polyethylene terephthalate parison.
The parison, whose neck portion is still disposed on the core rod, is then transferred to the third chamber to produce an oriented bottle. The separation or tolerance in the hoop dimension between the parison and the third chamber should be on an average at least 50% of the distance across the parison or distance across the second chamber. In the case

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of a tubular or test tube shaped parison the separation between the parison wall and the third chamber should be equal to at least 50~ times the outside diameter of the parison formed in the second chamber or 100% times the outslde radius of the parison. Stated a different way the third chamber should have an inside diameter of at least 200% up to about 600% of the outside diameter of the parison. Orientation is accomplished by injecting a gaseous fluid at 40 to 500 psig, preferably 80 to 250 psig at a suitable orientation temperature. As in-dicated below, the lower orientation temperature employed, the higher the pressure of gaseous fluid.
The polyethylene terephthalates useful in this in-vention contain at least 75 mole percent terephatalate units and at least 75 mole percent ethylene glycol units.
While the aforesaid description is directed primarily to the production of monoaxially oriented containers, biaxially oriented containers can be produced advantageously by using a third chamber or mold which is approximately 40~ to 600% longer in the axial direction than the second mold chamber. Orienta-tion in the axial direction is also facilitated by using an extendible core rod which can be used to stretch the parison in the third chamber.
The following examples are merely illustrative~ In the following examples, the conditions recited in each chamber are repeated for each composition or structure placed in the chamber.
Example 1 Ninety-five hundredths I.V. (inherent viscosity) homopolymeric polyethylene terephatate at 560F. was injected into the first stage of a Rainville 30 ton 4-station modular injection blow molding machine modified to contain two blow :. , ., . :

4~i molds and temperature control means in the core rods and walls of the first three modular stations. The core rods were thermostatted at 125F. In the first mold chamber or injection mold station, the portion of the chamber walls in contact with the tip of the parison and the bottom half of the parison body were thermostatted at 42-46F. while the portion of the chamber walls in contact with the neck of the parison and upper portion ``~
of the parison walls were thermostatted at 160F. The first chamber had a .8" diameter opening in the hoop direction with `~
from 80 to 140 mil gap between the 4-1/2" long core rod and chamber walls, the largest gap being toward the bottom of the parison body and tip area. After nine seconds in the first station, the injection mold was opened and the core rod bearing the parison was transferred to the second mold station having a 1" diameter in the hoop dimension while a fresh charge of polyethylene terephatalate was injected into the vacant first chamber. A gaseous fluid was injected from the core rod in the second station into the parison at 40 psig while the walls of the second chamber were maintained at 185F. After nine seconds residence time, the first and second mold chambers were opened and the parisons in the first and second stations were advanced to the second and third stations, respectively, while disposed on their core rods and fresh polyethylene terephthalate was injected into the vacant first chamber. A gaseous fluid was injected from the core rod in the third station into the parison at 120 psig to orient the polymer and fill the third chamber, which was 2-1/2" in diameter in the hoop dimension and whose chamber walls were at 42 to 46F. After nine seconds residence time the molds were opened and the parisons were dis-posed on their core rods were advanced. The finished bottle leaving the third chamber was ejected at the fourth station.

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: Eight ounce polyethylene terephthalate bottles pro-duced in the aforesaid manner had the following average pro-perties: -Property Hoop Axial ~ Elongation 3.0 2.5 ASTM D 1708 Tensile yield strength9,340 psi 6,830 psi ASTM D 1708 ~:
Tensile Modulus479,000 psi389,000 psi ASTM D 1708 :, Bottle weight 23.3 grams Midwall thickness0.72 mm Density 1.351 g/cc Average dfflp impact 12.2' ASTM D 2463 ~, ~
~ Example 2 ;. :
A bottle having somewhat less orientation than the bottle produced in Example 1 was prepared in essentially the .
same manner except the first mold walls were maintained at `
: 150F., the second mold walls were maintained at 120F., the ` third mold walls were at 48F., a 6 second cycle time was used and the core rods were at 250F. In this case the bottles broke after dropping approximately 8 feet.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of forming an oriented polyethylene terephthalate container which comprises the steps of:
(1) injecting a polyethylene terephthalate composition into a first mold chamber defining a parison having an axis sub-stantially defined by a core rod and perimeter defined by the wall of the mold, said parison having a tip, a body and a neck;
(2) cooling the exterior wall of the polyethylene terephthalate parison to render the outer wall of the parison dimensionally stable;
(3) after the outer wall of the parison is dimensionally stable, transferring the parison, while disposed on the core rod, to a second mold chamber, whereby the distance between the outer wall of the transferred parison and the inner wall of the second mold chamber is at any given point between 2.5 and 150% of the distance across the body of said parison at said point, and wherein the wall of the second chamber is at a higher temperature than the wall of the first chamber which was in contact with the parison tip;
(4) when the core rod is at no more than 265°F., injecting a gaseous fluid into the parison at a pressure of at least 10 psig to separate the body of the parison from the core rod and force the exterior wall of the body of the parison to assume the shape of the wall of the second chamber;
(5) transferring the parison, while the neck of the parison is disposed on the core rod, to a third chamber, whereby the distance between the outer wall of the trans-ferred parison and the inner wall of the third chamber is at any given point at least 50% of the distance across the body of said parison at said point; and (6) at the orientation temperature of the polyethylene tere-phthalate parison, injecting a gaseous fluid into the parison to force the exterior walls of the body of the parison to assume the shape of the wall of the third chamber and to orient the walls of the body of parison.

2. The method of Claim 1. wherein the gaseous fluid in step 4 is at from about 40 to 120 psig.

3. The method of Claim 1, wherein the parison formed in step 1 is test tube shaped and the parison walls are on an average from about 15 to 400 mils thick.

4. The method of Claim 1, wherein the core rod in step 4 is at 100 to 200°F.

5. A method as in Claim 1, 2 or 4 wherein the parison is tubular, and wherein the second chamber inside diameter is 105 to 300% of the outside diameter of the parison.

6. A method as in Claim 1, 2 or 4 wherein the parison is tubular, and wherein the third chamber has an inside diameter of at least 200% up to about 600% of the outside diameter of the parison.