U.S. patent application number 10/754753 was filed with the patent office on 2005-07-14 for pet with stress cracking resistance, preform and container made therewith and method.
Invention is credited to Shi, Yu.
Application Number | 20050153084 10/754753 |
Document ID | / |
Family ID | 34739438 |
Filed Date | 2005-07-14 |
United States Patent
Application |
20050153084 |
Kind Code |
A1 |
Shi, Yu |
July 14, 2005 |
PET with stress cracking resistance, preform and container made
therewith and method
Abstract
A preform comprising an open-ended mouth forming portion, an
intermediate body forming portion, and a closed base forming
portion comprising a stress cracking resistant polyester
composition comprising poly(ethylene terephthalate) based resin and
an impact modifier. The preform is blow molded to form a
corresponding container. Polyester compositions and methods are
also disclosed. The impact modifier improves the stress cracking
resistance of low IV PET such that the containers perform similar
to those made of high IV PET.
Inventors: |
Shi, Yu; (Alpharetta,
GA) |
Correspondence
Address: |
SUTHERLAND ASBILL & BRENNAN LLP
999 PEACHTREE STREET, N.E.
ATLANTA
GA
30309
US
|
Family ID: |
34739438 |
Appl. No.: |
10/754753 |
Filed: |
January 9, 2004 |
Current U.S.
Class: |
428/35.7 |
Current CPC
Class: |
C08L 23/0869 20130101;
C08L 9/00 20130101; Y10T 428/1352 20150115; C08L 67/02 20130101;
C08L 51/04 20130101; C08L 53/025 20130101; C08L 67/02 20130101;
C08L 23/0853 20130101; C08L 2666/04 20130101; C08L 2666/24
20130101; C08L 55/02 20130101; C08L 53/02 20130101; C08L 67/02
20130101 |
Class at
Publication: |
428/035.7 |
International
Class: |
B65D 001/00 |
Claims
I claim:
1. A preform comprising an open ended mouth forming portion, an
intermediate body forming portion, and a closed base forming
portion and comprising a stress cracking resistant polyester
composition comprising poly(ethylene terephthalate) based resin and
an impact modifier.
2. A preform as in claim 1 wherein the impact modifier is an
elastomer.
3. A preform as in claim 1 wherein the impact modifier is selected
from the group consisting of polyethylene-based elastomers,
butadiene-based elastomers, and isoprene-based elastomers.
4. A preform as in claim 1 wherein the impact modifier is a
polyethylene-based elastomer.
5. A preform as in claim 1 wherein the impact modifier is a
core-shell impact modifier.
6. A preform as in claim 1 wherein the impact modifier is
ethylene-acrylate copolymer.
7. A preform as in claim 1 wherein the impact modifier is modified
to increase compatibility of the impact modifier with poly(ethylene
terephthalate) based resin.
8. A preform as in claim 1 wherein the impact modifier is modified
to more closely match the reflective index of the poly(ethylene
terephthalate) based resin.
9. A preform as in claim 1 wherein the impact modifier is present
in an amount effective to enhance the stress cracking resistance of
the stress cracking resistant polyester composition relative to a
polyester composition not including the impact modifier.
10. A preform as in claim 1 wherein the impact modifier is present
in an amount from about 1 to about 15% by weight of the stress
cracking resistant polyester composition.
11. A preform as in claim 1 wherein the impact modifier is present
in an amount from about 3 to about 10% by weight of the stress
cracking resistant polyester composition.
12. A preform as in claim 1 wherein the impact modifier is present
in an amount from about 3 to about 6% by weight of the stress
cracking resistant polyester composition.
13. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin has an IV from about 0.65 to about 0.90
dL/g.
14. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin has an IV from about 0.65 to about 0.86
dL/g.
15. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin has an IV from about 0.65 to about 0.80
dL/g.
16. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin has an IV from about 0.68 to about 0.76
dL/g.
17. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin has an IV below about 0.76 dL/g.
18. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin comprises a comonomer for reducing the
thermal crystallization rate.
19. A preform as in claim 18 wherein the comonomer is selected from
the group consisting of naphthalene dicarboxylic acid, diethylene
glycol, isophthalic acid, and 1,4-cyclohexanedimethanol.
20. A preform as in claim 1 wherein the poly(ethylene
terephthalate) based resin comprises a diacid component having
repeat units from terephthalic acid with less than about 5 mole
percent modification and a diol component having repeat units from
ethylene glycol with less than about 5 mole percent diol
modification based on 100 mole percent diacid component and 100
mole percent diol component.
21. A container made by blow molding the preform of claim 1,
wherein the container comprises an open ended mouth forming
portion, an intermediate body forming portion, and a closed base
forming portion.
22. A container as in claim 21 wherein the container is a
carbonated soft drink container.
23. A container comprising an open ended mouth forming portion, an
intermediate body forming portion, and a closed base forming
portion and comprising a stress cracking resistant polyester
composition comprising poly(ethylene terephthalate) based resin and
an impact modifier.
24. A stress cracking resistant polyester composition comprising
poly(ethylene terephthalate) based resin having an IV of from about
0.65 dL/g to less than about 0.86 dL/g and an impact modifier.
25. A stress cracking resistant polyester composition comprising
poly(ethylene terephthalate) based resin having an IV of about 0.68
to about 0.76 dL/g and an impact modifier.
26. A stress cracking resistant polyester composition comprising
poly(ethylene terephthalate) based resin and an impact modifier in
an amount from about 1 to about 10% by weight of the stress
cracking resistant polyester composition.
27. A stress cracking resistant polyester composition as in claim
26 wherein the impact modifier is present in an amount from about 3
to about 10% by weight of the stress cracking resistant polyester
composition.
28. A stress cracking resistant polyester composition as in claim
26 wherein the impact modifier is present in an amount from about 3
to about 6% by weight of the stress cracking resistant polyester
composition.
29. A method for making a stress cracking resistant container
comprising blow molding a preform comprising an open ended mouth
forming portion, an intermediate body forming portion, and a closed
base forming portion and comprising a stress cracking resistant
polyester composition comprising poly(ethylene terephthalate) based
resin and an impact modifier.
30. A method for making a stress cracking resistant preform
comprising an open ended mouth forming portion, an intermediate
body forming portion, and a closed base forming portion comprising
the step of molding a stress cracking resistant polyester
composition comprising poly(ethylene terephthalate) based resin and
an impact modifier.
31. A method for enhancing the stress cracking resistance of a
polyester composition comprising blending an impact modifier into
poly(ethylene terephthalate) based resin, the impact modifier
present in an amount from about 1 to about 15% by weight of the
polyester composition.
32. A method for enhancing the stress cracking resistance of a
polyester composition comprising blending an impact modifier into
poly(ethylene terephthalate) based resin having an IV of from about
0.65 dL/g to about 0.80 dL/g.
33. A method of claim 31 wherein the poly(ethylene terephthalate)
based resin has an IV of from about 0.68 to about 0.76 dL/g.
Description
TECHNICAL FIELD
[0001] This application relates to poly(ethylene
terephthalate)--based compositions and more particularly to stress
cracking resistance of articles such as containers made with
poly(ethylene terephthalate)--based compositions.
BACKGROUND OF THE INVENTION
[0002] Poly(ethylene terephthalate)-based resins, which are
commonly referred to in the industry simply as "PET" even though
they may and often do contain minor amounts of additional
components, have widely been used to make containers for carbonated
soft drink, juice, water and the like due to their excellent
combination of mechanical and gas barrier properties.
Unfortunately, a phenomenon known as environmental stress cracking
(ESC) has been a persistent annoyance in PET containers used for
carbonated beverages since the advent of the one-piece container
design. The ultimate failure caused by ESC is a rupture in the base
of the container resulting in the loss of the container's contents
and subsequent contamination of the surroundings. Clean-up is
laborious. Product and liability losses can be expensive. Usually
10 to 20 stress crack failure containers per million is considered
excessive.
[0003] Direct causes for failures are often difficult to determine.
The factors affecting ESC of PET containers include, but are not
limited to, container production conditions, the stress due to the
pressurization of the carbonated beverage, and the conditions in
container handling and storage.
[0004] Production conditions are critical contributors to the ESC
phenomenon. Manufacturing variations can play a significant role in
determining the ESC resistance. Minor changes in processing
conditions are reported to have significant effect on stress crack
resistance. During line conveying and container cleaning stress
crack agents in the form of chemicals contact PET container. The
origin of a crack in a PET container is normally located at the
areas where stress concentration exists. The stress cracking agent
can attack the base by many mechanisms, but normally starts from
craze development as a result of the cracking agent attack. Crazes
can then develop into cracks and failure may occur in forms of a
rupture or leakage. Optimum processing conditions, which are
difficult to manage consistently, are required to prevent ESC.
[0005] ESC can be also caused by excessive stress seen in the
carbonation of beverages. Beverages are often over pressurized
during the filling process to increase shelf life. However, over
pressurization creates added stress to the container and thus a
higher possibility for rupture.
[0006] Physical aging has been found to be an important
contributing factor for ESC of PET containers. Studies have shown
that the container ESC resistance decreases exponentially with
hours of aging. The physical aging that containers experience
before filling may be a primary factor responsible for the
increased rate of failures often observed during the hot summer
months.
[0007] Several methods have been reported to improve ESC resistance
of one-piece PET containers for carbonated soft drinks (CSDs).
Proper and robust base design of the PET container is one method
used to reduce ESC. A well-designed base is required to tolerate
manufacturing variations and can protect containers from excess
stress. Base design is a critical component in today's container
manufacture. However, base design can not protect containers from
contact with stress-inducing chemical agents.
[0008] Another method for reducing ESC is a modification to the
blow molding process. The modification achieves optimal base
weight, minimum residual stress, and/or maximum crystallinity in
the base. This method, however, has the same problem inherent with
all the methods above: the nature of the material is not changed.
Therefore, the reduction of stress cracking is just a matter of
degree, depending on the nature of the chemical agent, the contact
time and other factors.
[0009] A third method is the use of special lubricants on container
line conveyors. These lubricants have fewer tendencies to attack
PET containers as stress cracking agents and thus reduce the stress
cracking of PET containers. This method, however, can not solve
problems associated with other chemicals such as cleansers,
contaminants from water, and the like that can cause stress
cracking. So ESC still occurs, just to a lesser degree during
filling.
[0010] Still another method is to coat the PET container, at least
around the base. The coating itself is very inert to chemical
agents which are normally strong enough to attack PET. This method,
however, requires additional steps in the container making process
and thus adds more cost to the container. The method is not
applicable in all container making or filling sites either due to
space limitations.
[0011] Additionally, increasing the molecular weight or intrinsic
viscosity (IV) of PET has been found helpful in reducing the
occurrence of ESC in PET containers used for carbonated beverage. A
minimum IV of 0.80 dL/g is desirable in making a CSD container,
with preferred IV's being reported to be 0.82 dL/g or higher.
Higher IV or higher molecular weight PET has fewer chain ends and
is believed to have less interaction with chemical attacking
agents, which is believed to result in less stress cracking. In
addition, higher IV PET (above 0.80 dL/g) is believed to have more
chain entanglement, which can dissipate more stress than
traditionally lower IV PET (below 0.76 dL/g). Normally, when a
craze forms in PET during the attack of the chemical agent, stress
is localized at the point of craze formation and is transferred
through the expansion of crazes to form cracks. A higher IV resin
is believed to be able to reduce the effect by transferring the
stress partially to the other chains without further expansion of
the crazes to form cracks. However, higher IV PET requires longer
solid state polymerization (SSP) time, and higher injection molding
temperature due to higher melt viscosity. Unfortunately, this means
higher IV PET is more expensive to make than lower IV PET.
Nevertheless, the general trend in recent years is to use higher IV
PET of about 0.84 dL/g to avoid the annoyance of the ESC problem
even though higher IV PET costs more to produce and convert from
resin to container.
[0012] Lower IV PETs, such as 0.72 to 0.76 dL/g, however, have been
used for a few container applications, such as still water
containers, where stress cracking is seldom an issue. Application
of such lower IV PETs in one-piece-CSD containers has been limited
solely because of its bad performance in stress cracking
resistance. Modifications to PET compositions have been reported in
an effort to counter ESC effects so that lower IV PET can be used
for CSDs. One example is the addition of a comonomer, such as
naphthalene dicarboxylate to enhance the stress cracking
resistance. In any case, a higher IV PET has always been used for
carbonated beverage applications. Having different resins for
packaging different products, however, creates more inventories in
the converting facilities. Some facilities do not have enough
capability to have so many inventories. Thus, having a lower IV PET
that resists stress cracking problems would be greatly desired by
the industry so that a lower IV resin can be used for both CSD and
water application reducing resin cost as well as inventory
cost.
[0013] Impact modifiers have been used in amorphous polymer as well
as high crystalline polymers to improve the impact resistance of
these polymers. For polyesters, impact modifiers are widely used to
improve the low temperature or room temperature impact resistance
of highly crystallized PET homopolymers (often referred to as
"CPET") or amorphous polyesters made using high levels of glycol
modification (more than 10 mole percent of non-ethylene glycol
modifiers). The impact stress is normally applied in a rapid
movement, such as dropping the article from a high distance or
applying a rapid high stress to the article through an external
force. These modifiers are believed to form a two-phase morphology
in the polymer. The modifiers form a rubber-like phase which can
absorb the impact energy and effectively transfers the stress from
the CPET or amorphous polyester phase. Although impact modifiers
are generally known for the above applications, there has been no
attempt of using the same family of chemicals to improve the stress
cracking resistance of PET containers for carbonated beverage
application, especially those made with lower IV PET.
[0014] Although much effort has been made to understand and solve
the stress cracking problem, the root causes of the stress cracking
are still unknown. Stress cracking can have costly ramifications in
terms of clean-up or liability. As can be seen, no one method
currently available solves the stress cracking problems of PET
containers used for CSDs. Rather, the stress cracking problem of
PET containers has been carefully managed through optimized bottle
design, bottle making, and product filling processes. Therefore,
there remains a need to prevent or decrease environmental stress
cracking in PET containers. Accordingly, it is to the provision of
such that the present invention is primarily directed.
SUMMARY OF THE INVENTION
[0015] This invention addresses the foregoing issues in the prior
art by providing a preform having an open ended mouth forming
portion, an intermediate body forming portion, and a closed base
forming portion and being made of a stress cracking resistant
polyester composition comprising a poly(ethylene terephthalate)
based resin and an impact modifier. This invention also encompasses
a container made by blow molding such preform made of the stress
cracking resistant polyester composition.
[0016] Without wishing to be bound by theory, the impact modifier
is believed to improve the relaxation phenomenon of the
poly(ethylene terephthalate) based resin such that a lower
molecular weight (i.e. lower intrinsic viscosity) poly(ethylene
terephthalate) based resin behaves as a higher molecular weight
resin and resists stress cracking. In preferred embodiments the
poly(ethylene terephthalate) based resin has an IV of from about
0.65 to about 0.90 dL/g, more preferably from about 0.65 to about
0.86 dL/g, even more preferably from about 0.65 to about 0.80 dL/g
and still even more preferably from about 0.68 to about 0.76 dL/g.
Thus, as a result of this invention, less expensive, lower IV
poly(ethylene terephthalate) based resin when combined with the
impact modifier can be used to make such preforms and containers.
Further the same preform and container can be used for a variety of
applications including CSD and bottled water containers.
[0017] Other objects, features, and advantages of this invention
and the preferred embodiments will be appreciated from the
following drawings, detailed description of embodiments, and
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a sectional elevation view of an injection molded
container preform made in accordance with a preferred embodiment of
this invention.
[0019] FIG. 2 is a sectional elevation view of a blow molded
container made from the preform of FIG. 1 in accordance with a
preferred embodiment of this invention.
[0020] FIG. 3 is a perspective view of a packaged beverage made in
accordance with a preferred embodiment of this invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0021] The present invention encompasses a preform and a container
made by blow molding the preform, both having an open ended mouth
forming portion, an intermediate body forming portion, and a closed
base forming portion and comprising a stress cracking resistant
polyester composition comprising a poly(ethylene terephthalate)
(PET) based resin and at least one impact modifier. Preferably, the
PET based resin has a low intrinsic viscosity (IV), that is below
about 0.76 dL/g. The impact modifier surprisingly enhances the
stress cracking resistance of the low IV PET based resin such that
the low IV PET based resin behaves as a higher IV resin. Thus, the
container made utilizing this invention is particularly suited for
use as a carbonated soft drink (CSD) container. The use of the
impact modifier makes it possible to use a cheaper low IV PET based
resin for the CSD container, and use the same container for other
applications such as bottled water, alcoholic beverages, juices,
and the like.
[0022] Conventionally, impact modifiers have been used in thermal
formed high crystallinity PET (CPET) articles to improve the low
temperature impact resistance. Such articles include amorphous
polyester films or thermal formed high crystallinity PET trays used
in low temperature freezer storage and high temperature oven-able
applications. The high crystallinity is normally achieved through
thermal crystallization. Therefore, a PET homopolymer is normally
used to enhance the thermal crystallization rate. The high
crystalline PET homopolymers used for such application normally
also have high IVs of above 0.80 dL/g and preferably above 0.90
dL/g to achieve the good impact resistance in low temperature
applications. Further, in order to improve the impact resistance,
normally a loading level of 10 weight percent or more of the impact
modifier has to be incorporated into CPET. In this invention, the
same family of chemicals that are used as impact modifiers improves
the stress cracking resistance of low IV PET based resins at much
lower loading, preferably lower than 10 weight percent, such that
the low IV PET based resin can perform similar to that of high IV
PET materials in applications including, but not limited to CSD
containers. The containers are made through injection blow molding
processes, where the thermal crystallization is minimized in the
injection molding to obtain clear preforms and the crystallinity is
achieved through strain induced crystallization during the blow
molding process.
[0023] Suitable impact modifiers for use in this invention include
any impact modifier compatible with PET based resins. Preferred
impact modifiers include elastomers. Still more preferred suitable
impact modifiers include elastomers based on polyethylene,
butadiene or isoprene, such as polybutadiene, polyisoprene, natural
rubber, styrene-butadiene (SBR), acrylonytrile-butadiene,
styrene-butadiene-styre- ne or hydrogenated SBS block copolymers,
or acrylonytrile-butadiene-styren- e polymers containing high
levels of butadiene.
[0024] The impact modifiers that are especially useful are the ones
modified to enhance the compatibility with PET based resins, such
as polyethylene based elastomers. Examples of desirable
polyethylene based elastomers include ethylene-acrylate copolymers
such as ethylene/methylacrylate, ethylene/ethyl acrylate,
ethylene/butyl acrylate and ethylene/methylacrylate/glycidyl
methacrylate, or ethylene/vinyl acetate copolymers, or the
copolymers of butadiene/MMA/styrene. U.S. Pat. Nos. 5,409,967 and
5,652,306 and PCT application WO00/15717 disclose the composition
of such impact modifiers, and the specifications thereof are
expressly incorporated herein by reference.
[0025] Suitable impact modifiers also include those with a
core-shell structure which are described in U.S. Pat. No. 5,409,967
and PCT application WO00/15717. These core-shell modifiers normally
contain a hard shell from methylmethacrylate copolymers and a core
made from either butadiene methcrylate/butadiene-styrene copolymer
or butyl acrylate copolymers. Examples of suitable core-shell
impact modifiers include those available from Rohm & Haas
Company under the trade name Paraloid.
[0026] Impact modifiers can contain added or reacted-to-the-chain
compatibilizers. The compatibilizers are those functional groups
that can enhance the compatibility or the miscibility of the impact
modifiers with PET based resin. These modifiers include those
described in PCT application WO 00/15717.
[0027] Normal impact modifiers have a different reflective index
than that of PET based resins, and the resultant container is white
or hazy. One way to solve this is to use modified impact modifiers.
Impact modifiers can be modified to match the reflective index of
the impact modifiers to that of PET based resins so that the
resultant container is clear. The impact modifiers can also be
modified with core-shell technology such that the compatibility as
well as the reflective index is matched with PET based resins.
Further, the impact modifiers can be modified by manipulating the
particle sizes of the modifiers such that they are below the
visible light wave length, normally below 0.1 microns. All these
modifications can be achieved by those skilled in the art.
[0028] The impact modifier is present in the stress cracking
resistant polyester composition in an amount effective to enhance
the stress cracking resistance thereof relative to a polyester
composition not including the impact modifier. Preferably, the
impact modifier is present in the stress cracking resistant
polyester composition in an amount from about 1 to about 10 weight
percent of the stress cracking resistant polyester composition.
More desirably, the impact modifier is present in an amount from
about 3 to about 10 weight percent and even more preferably in an
amount from about 3 to about 6 weight percent.
[0029] The PET based resin preferably comprises a diacid component
having repeat units from terephthalic acid with less than about 5
mole percent modification and a diol component having repeat units
from ethylene glycol with less than about 5 mole percent diol
modification based on 100 mole percent diacid component and 100
mole percent diol component. The diacid modification can be a
comonomer from any of a number of diacids, including adipic acid,
succinic acid, isophthalic acid, phthalic acid, 4,4'-biphenyl
dicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and the like.
The diol modification can be a comonomer such as
cyclohexanedimethanol, diethylene glycol, 1,2-propanediol,
neopentylene glycol, 1,3-propanediol, and 1,4-butanediol, and the
like. The PET base resin preferably further comprises a comonomer
for reducing the thermal crystallization rate thereby allowing for
the manufacture of relatively clear containers.
[0030] In preferred embodiments, the IV of the PET based resin, as
measured according to ASTM D4603-96, is from about 0.65 to about
0.90 dL/g, more preferably from about 0.65 to about 0.86 dL/g,
still more preferably from 0.65 to 0.80 and further more preferably
from about 0.68 to about 0.76 dL/g. According to ASTM D4603-96, the
IV of PET based resin was measured at 30.degree. C. with 0.5 weight
percent concentration in a 60/40 (by weight fraction)
phenol/1,1,2,2-tetrachloroethane solution.
[0031] As is well known to those skilled in the art, containers can
be made by blow molding an injection molded preform. Examples of
suitable preform and container structures and methods for making
the same are disclosed in U.S. Pat. No. 5,888,598, the disclosure
of which is expressly incorporated herein by reference in its
entirety.
[0032] Turning to FIG. 1, a container preform 10 is illustrated.
This preform 10 is made by injection molding the stress cracking
resistant polyester compositions of this invention and comprises a
threaded neck finish 12 which terminates at its lower end in a
capping flange 14. Below the capping flange 14, there is a
generally cylindrical section 16 which terminates in a section 18
of gradually increasing external diameter so as to provide for an
increasing wall thickness. Below the section 18 there is an
elongated body section 20.
[0033] The preform 10 illustrated in FIG. 1 can be blow molded to
form a container 22 illustrated in FIG. 2. The container 22
comprises a shell 24 comprising a threaded neck finish 26 defining
a mouth 28, a capping flange 30 below the threaded neck finish, a
tapered section 32 extending from the capping flange, a body
section 34 extending below the tapered section, and a base 36 at
the bottom of the container. The container 10 is suitably used to
make a packaged beverage 38, as illustrated in FIG. 3. The packaged
beverage 38 includes a beverage such as a carbonated soda beverage
disposed in the container 22 and a closure 40 sealing the mouth 28
of the container.
[0034] The preform 10, container 22, and packaged beverage 38 are
but examples of applications using the stress cracking resistant
compositions of the present invention. It should be understood that
the compositions of the present invention can be used to make
preforms and containers having a variety of configurations.
[0035] Embodiments of the present invention are further illustrated
below by way of examples, which are not to be construed in any way
as imposing limitations upon the scope of the invention.
EXAMPLE 1
[0036] Sample container preforms were injection molded using two
different PET based resins and different stress crack resistance
modifiers. Sample 1 was made with a first PET resin (R1), a
commercially available CSD grade PET copolymer resin with an IV of
0.83, and Samples 2-6 were made with a second PET resin (R2), a
commercially available water grade PET copolymer resin with an IV
of 0.74. The compositions of the Sample preforms are set forth in
Table 1.
[0037] Each preform was injection molded using 500 mL Contour
design 28g preforms under conditions that were set to produce
molded samples with no crystallinity, haze or other visual
imperfections based on the control resin variable. The PET resins
R1 and R2 were dried overnight at 280.degree. F. to a moisture
level below 50 ppm. The stress crack resistance additives,
poly[ethylene-co-butyl acrylate] (PBA, Aldrich #43,077-3) and
poly[ethylene-co-methyl acrylate] (PMA, Aldrich #43,266-0), were
dried over the weekend in a vacuum oven with no heat applied to
avoid the sticking of additives. An Arburg 320H was used for the
injection molding trial.
[0038] The preforms were blow molded into containers using a Sidel
SBO1/2 machine. The blow molding process conditions that produced
an acceptable container with appropriate section weights was
established for each variable.
[0039] The intrinsic viscosity (IV) of each Sample was measured
according to ASTM D4603-96 and the results are shown in Table
1.
1TABLE 1 Resin/Additive Variables Actual % IV Sample Resin Additive
Additive Added (dL/g) 1 R1 Control-None N/A 0.795 2 R2 Control-None
N/A 0.696 3 R2 PBA 3.0 0.681 4 R2 PBA 5.0 0.670 5 R2 PMA 3.0 0.689
6 R2 PMA 5.0 0.679
[0040] The stress cracking resistance of the containers made from
Samples 1-6 was measured and the results are shown in Table 2
measured by caustic stress cracking method. In the test,
twenty-five containers were randomly selected. Due to the limited
number of containers, the number of containers tested varied from 8
to 25 in different cases. The relative comparison was therefore
used. The samples were carbonated to about 56 psi of CO2 and stored
for 24 hours in an environment of 72.degree. F. and 50% RH. The
base of each sample container was immersed in a 0.1% NaOH solution.
The % pass indicates the % of containers that passed the test and
did not exhibit stress cracking.
2TABLE 2 Caustic Stress Crack Results Average Minimum Maximum
Failure Time Failure Time Failure Sample (min.) .sigma. (min.) Time
(min.) % Passed 1 180 0 180 180 100% 2 123 49 26 180 9% 3 180 0 180
180 100% 4 180 0 180 180 100% 5 175 11 150 180 75% 6 180 0 180 180
100%
[0041] The results in Table 2 clearly showed that the addition of
the stress cracking resistance modifiers dramatically improved the
stress cracking resistance of the containers made from low IV PET
resins. The low IV resins with stress cracking resistance modifiers
performed similarly to that of higher IV resins. In this case, the
IV of the low molecular weight PET container is 0.68 and the IV of
the high molecular weight PET container is 0.80, but their stress
cracking performance is the same. Sample 2, the low IV resin
without the modifier, could not pass the stress cracking test.
EXAMPLE 2
[0042] Sample container preforms were injection molded using two
different PET based resins and different stress crack resistance
modifiers. Sample 1 was made with a first PET resin (R3), a
commercially available CSD grade PET copolyester resin with an IV
of 0.84, and Samples 2-4 were made with a second PET resin (R4), a
commercially available water grade PET copolyester resin with an IV
of 0.74. The composition of the Sample preforms is set forth in
Table 3.
[0043] The stress cracking resistance additive used was a
commercially available Paraloid EXL core-shell impact modifier
supplied by Rohm & Haas. The preforms were injection and blow
molded according to Example 1. The containers were then subjected
to the same tests and the results are shown in Table 4.
3TABLE 3 Resin/Additive Variables Actual % IV Sample Resin Additive
Additive Added (dL/g) 1 R3 Control-None 0 0.782 2 R4 Control-None 0
0.674 3 R4 EXL 3.0 0.651 4 R4 EXL 6.0 0.638
[0044] The stress cracking test was performed the same as in the
Example 2. All variables had 25 containers for testing. The
absolute failure time was thus used for the comparison. The average
failure time for different variables is shown in the table 4. The
longer the failure time, the better the stress cracking resistance.
It is seen that both 3 and 6 wt % of the Paraloid EXL additive
improved the stress cracking resistance of the low IV resin. For 6
wt %, it was very similar to that of the high IV resin.
4TABLE 4 Stress Cracking Test Results Average failure Standard
deviation Sample time (minutes) (minutes) 1 168.0 11.3 2 27.0 34.2
3 112.8 39.5 4 137.4 7.7
[0045] It should be understood that the foregoing relates to
particular embodiments of the present invention, and that numerous
changes may be made therein without departing from the scope of the
invention as defined by the following claims.
* * * * *