U.S. patent number 3,716,606 [Application Number 05/055,001] was granted by the patent office on 1973-02-13 for method of stabilizing thermo-plastic containers.
This patent grant is currently assigned to Kemp Products Limited. Invention is credited to Patrick Seymour Bazett.
United States Patent |
3,716,606 |
Bazett |
February 13, 1973 |
METHOD OF STABILIZING THERMO-PLASTIC CONTAINERS
Abstract
Thermoplastic containers, particularly polyethylene, can be
stabilized with desired or required control limit on volume and
shape, such that they are capable of being used at temperature
ranges in excess of 110.degree.F. without appreciable or
significant loss of shape or capacity. This is achieved by placing
a small amount of liquid such as water within the bottle and
subjecting the bottle to a microwave radiant energy field such that
the liquid (water) absorbs the radiant energy and boils off into
steam. The vapor generated would appear to be converted (after all
water has evaporated) into superheated vapor which scours the
interior of the bottle causing it to shrink and to stabilize such
that under subsequent elevated heat environments (service
temperatures) it is essentially stable in volume and shape.
Inventors: |
Bazett; Patrick Seymour
(London, Ontario, CA) |
Assignee: |
Kemp Products Limited (Ontario,
CA)
|
Family
ID: |
10388527 |
Appl.
No.: |
05/055,001 |
Filed: |
July 15, 1970 |
Current U.S.
Class: |
264/489; 264/346;
264/900; 264/235; 215/12.2 |
Current CPC
Class: |
C08J
7/08 (20130101); B29C 71/04 (20130101); B29K
2023/06 (20130101); B29K 2995/0049 (20130101); B29L
2031/7158 (20130101); Y10S 264/90 (20130101) |
Current International
Class: |
B29C
71/00 (20060101); B29C 71/04 (20060101); B29d
023/00 () |
Field of
Search: |
;264/22,25,26,235,346 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: White; Robert F.
Assistant Examiner: Kucia; Richard R.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of dimensionally stabilizing a thermoplastic polymeric
container as to capacity and shape by heating the container wherein
the improvement comprises:
providing a container having an internal cavity and composed of a
thermoplastic polymeric material which is generally translucent to
at least one frequency of microwave radiant energy, said container
having a capacity greater than the capacity to which it is to be
stabilized;
introducing into said cavity a material consisting essentially of a
liquid which is generally absorbent to such microwave energy to
which said container is translucent, said liquid having a
vaporization temperature greater than the primary memory
temperature of the thermoplastic material and less than the plastic
deformation temperature of the material,
selecting a microwave radiant energy field which has a frequency
which is generally absorbed by said liquid and to which the
thermoplastic material is generally translucent,
subjecting said container and said liquid to said field to elevate
said liquid to its vaporization temperature and to vaporize said
liquid, so as to heat said container to a temperature in close
proximity to the vaporization temperature to establish a second
memory temperature for the container above that of the first memory
temperature.
2. The method of claim 1 wherein the liquid is water.
3. The method of claim 1 wherein the thermoplastic is
polyethylene.
4. The method of claim 3 wherein the liquid is water.
5. The method of claim 3 wherein the microwave radiant energy field
is selected from the group of microwave energy frequencies
consisting of 890 megahertz.; 940 megahertz.; 2400 megahertz; and
2500 megahertz.
6. The method of claim 4 wherein the microwave radiant energy field
is selected from the group of microwave energy frequencies
consisting of 890 M. H.sub.z.; 940 M. H.sub.z.;, 2400 M. H.sub.z.;
and 2500 M. H.sub.z.
7. A method of dimensionally stabilizing a polyethylene container
as to capacity and shape by heating the container wherein the
improvement comprises:
providing a container having an internal cavity and composed of
polyethylene which is generally translucent to at least one
frequency of microwave radiant energy, said container having a
capacity greater than the capacity to which it is to be
stabilized;
introducing into said cavity a material consisting essentially of
water which is generally absorbent to such microwave energy to
which said container is translucent, said water having a
vaporization temperature greater than the primary memory
temperature of the polyethylene and less than the plastic
deformation temperature of the polyethylene;
said polyethylene having a thermal deformation temperature in
excess of 180.degree.F,;
selecting a microwave radiant energy field which has a frequency
which is generally absorbed by said water and to which the
polyethylene is generally translucent;
subjecting said container and said water to said field to elevate
said water to its vaporization temperature and to vaporize said
water so as to heat said container in close proximity to the
vaporization temperature to establish a second memory temperature
of approximately 160.degree.F. for the container above that of the
first memory temperature.
Description
This invention relates to an improved stabilized thermoplastic
container and method of making the same.
All thermoplastics have a (thermal) memory point and when they are
caused to extend beyond such (thermal) memory point they remain
dimensionally stable only for an unknown and indeterminate length
of time, dependent upon the temperature to which the material is
subsequently subjected. This is particularly true when service
temperatures (temperatures to which the bottle is subjected to
during its useful life) exceed 110.degree.F. or thereabouts.
It is known that stress patterns which are inherently set up during
the process of manufacture, and which create thermally unstable
thermoplastic products must be substantially removed, if service
conditions demand subjection of a moulded thermoplastic container
to temperatures at or above 110.degree.F. In such processes,
commonly referred to as annealing process, there is an alleviation
of internal stress to a level which creates a new point of
stability above 110.degree.F., but substantially less than the
environmental temperature to which the thermoplastic is subjected
during annealing. It is for this particular reason that annealing
processes are used, and why polyethylene bottles and other
thermoplastic containers are subjected to heat treatment by
conduction or convection over specific time bands; namely, to
attempt to stabilize the container by alleviating the internal
stresses created during manufacture. Hot air ovens are often used
in annealing as well as water-boiling emersion and the like,
controlled under a combination of temperature and related time
during emersion.
It is known that thermoplastic containers or bottles, when
manufactured by the process of blow moulding, vacuum forming or
other present-day techniques, substantially reduce in capacity as
the result of annealing. It is also known that stability of such
annealed thermoplastic containers, is maintained at environmental
conditions substantially less than the temperature employed during
the annealing period, i.e. a bottle annealed by the convention
method as taught by Bailey in his Canadian Pat. No. 787,822 issued
18 June, 1968, entitled "Volume Stabilization of Moulded Plastic
Containers" when subject to an annealing temperature of up to
200.degree.F. for a time period of between 20 to 30 minutes has an
empirical thermal stability level at or about 150.degree.F. When
such bottle is subject after annealing to temperatures at some
point above the stability level of about 150.degree.F. the bottle
again is further fugitive in either or both dimension and capacity,
i.e. retained stresses are further relieved within the bottle,
causing a fugitive reduction in its capacity as well, in some
instances, distortion of the container.
It has now been found that when thermoplastic bottles and the like
are subject to washing for the purposes of re-use or which are to
be filled with a product at about 150.degree.F., the same must be
stabilized to retain the desired capacity limits and requirements
at the requisite temperatures in excess of 150.degree.F.
temperature. It has been found that thermoplastic containers,
particularly of polyethylene, can be stabilized with the desired or
required control limits to a controlled and determined thermal
stability service level, that is to a thermal stability which will
sustain the high environment temperature provided the service
temperature is less than the moulding temperature of the
thermoplastic. This is accomplished by subjecting a thermoplastic
container, preferably a thermoplastic which is not opaque to
microwave radiation selected, to the action of microwave treatment
for a substantially shorter period of time than required by any
other process taught and currently practiced.
It has been further found that an expeditious method, to achieve
the purpose of my invention, namely stability or integrity of
thermoplastic containers, comprises the placing of a controlled
amount of liquid such as water in the bottom of a thermoplastic
container, such as a polyethylene bottle (the polyethylene acting
essentially as a "window" to the microwave radiation, i.e. absorbs
negligible microwave radiation) and the placing of the partially
filled bottle within a microwave radiation field, for example, that
generated by microwave ovens having output in the microwave
broadcast range. The contented liquid (water) rapidly reaches its
vaporization temperature, and is converted into steam due to the
absorption of radiant energy by the liquid, while the polyethylene
acts substantially as a "window" permitting the radiation to pass
through the container wall to the liquid. As a result the liquid
contained is elevated in temperature until it reaches beyond its
boiling point and then vaporizes off. The elevated temperature of
the liquid, and, in its gaseous phase, the vapor phase, scavenges
the inner surface of the bottle heating the same to or above the
boiling point of the liquid, in the instance of water above
212.degree.F. when measured at sea level. The gaseous phase of the
liquid also absorbs radiant energy and is further elevated in
temperature (superheated) to further enhance the thermal scavenging
of the inner surface of the bottle.
The invention therefore contemplates a method of stabilizing
thermoplastic products beyond its initial or primary memory point
(thermal deformation point) but below the plastic deformation point
comprising the steps of:
a. selecting a thermoplastic product which is to be stabilized to a
temperature beyond its primary memory point;
b. placing a liquid absorbant to microwave radiant energy in
intimate proximity to the surface of the thermoplastic product;
c. subjecting the liquid to microwave radiant energy such that the
liquid absorbs radiant energy and is elevated to a temperature in
excess of the (first) primary thermal deformation point of the
thermoplastic such that the thermo-plastic is also heated to a
temperature above that which created the first memory point whereby
the thermoplastic is re-established into a new primary (second)
memory point above the first.
The invention will now be described by way of example reference
being had to the accompanying drawings in which:
FIG. 1 is a schematic of a production method of utilizing the
embodiments of the invention.
FIG. 2 is a three-part graph illustrating the net capacity in cubic
centimeters of thermoplastic (polyethylene) containers prior to
annealing and subsequent to annealing wherein the annealing
processes used are two of the processes of the prior art, a 3 hour
boil -- A, and a 30 minute conduction heat shrink method -- C, and
a third (B) the process embodying the invention.
FIG. 3A is a three part comparison graph illustrating the net
capacity in grams of the containers of FIG. 2A after shrinking --
post boil -- and after one, five and 10 washings for five minutes
at 168.degree.F to 172.degree.F.
FIG. 3B is a three-part comparison graph illustrating the net
capacity in grams of the containers of FIG. 2B after shrinking --
post microwave -- and after one, five and 10 washings for five
minutes at 168.degree.F. to 172.degree.F.
FIG. 3C is a three-part comparison graph illustrating the net
capacity in grams of the containers of FIG. 2C after shrinking --
post oven conduction -- and after one, five and 10 washings for 5
minutes at 168.degree.F to 172.degree.F.
FIG. 4 is a comparative chart showing the total capacity lost in
grams (averaged) for the containers of FIGS. 2A, 2B and 2C
respectively during the 10 washings (after the initial annealing
process).
FIG. 5 is a high-low total variation chart of the capacity losses
in grams of the various containers.
It should be noted that in the above graphs, and the below
described invention, a datum was selected as 3785 grams weight
being equivalent to one United States of America gallon.
Particularly, in order to demonstrate the efficacy and the utility
of the invention, it must be appreciated that thermoplastic bottles
or containers which are commonly in existence in the dairy industry
in North America are subject during their useful life span to
temperatures in excess of 150.degree.F. as a result of either dairy
or consumer washing practices. It might be mentioned in passing
that the North American dairy industry claims as a standard for
dairy washing temperatures at between 140.degree. to 150.degree.F.
It has been found that such practices are tempered by the economies
of higher wash temperature and the demands of good sterile
practices as may from time to time be prescribed by the appropriate
governing bodies and agencies. As a result washing temperatures in
dairies, generally exceed the 150.degree.F. maximum, and usually
range in the vicinity of 165.degree. to 170.degree.F.
As a result, if the memory point of thermoplastic diary bottles are
retained in the neighborhood of 150.degree.F., as they commonly are
under the present state of the art, and such bottles are subjected
during any washing cycle to temperatures in excess of 150.degree.F.
this initiates uncontrollable movement of the body material of the
container construction causing an alteration in the volumetric
capacity of the said bottle. Such alteration of the structural and
capacity integrity continues after each washing or hot rinsing
cycle, causing an unpredictable variance reflected in a new or
re-established volumetric capacity after each wash.
Referring to FIG. 2 36 thermoplastic bottles having internal
capacity slightly in excess of one United States of America gallon,
were taken off the production line immediately after manufacture
thereof. Such bottles were designed to contain an annealing
allowance of approximately four ounces over the desired final
capacity of one U.S. gallon (3785 grams), all bottles were
nevertheless measured for true capacity of contents by weight. The
bottles were marked, and the true capacity of each bottle was
plotted respectively on the graphs shown in FIG. 2 (solid line).
The bottles were then grouped into three equal groups (A, B, C) and
the first group, group A, was subjected to a normal heat treatment
in a boiling water bath, as is often practiced in the art, for 3
hours at 205.degree.F. The capacity of the contents, by weight, of
these containers were once again taken and respectively plotted in
FIG. 2A (dotted lines).
A third group of bottles was taken, group C, and the volumetric
capacity, by weight, of these bottles was measured and plotted on
FIG. 2C. These bottles were then subjected to heat stabilization by
the conduction method as taught by Bailey in his Canadian Pat. No.
787,822 issued on 18 June, 1968, entitled Volume Stabilization of
Moulded Plastic Containers. Such bottles were subjected for a
period of thirty minutes to temperatures in the range of
198.degree. to 205.degree.F. After stabilization by the Bailey
method the volumetric capacity, by weight, of the respective
containers was again taken and the respective values plotted as
shown in FIG. 2C by the dotted lines.
The second group B of bottles was likewise measured, for true
capacity of contents, by weight, and the same plotted on FIG. 2B
(solid lines) thereafter these bottles were stabilized according to
the embodiments of the invention now to be disclosed, in the
following manner. That is to say, 4 ounces of water were placed in
each bottle and the charged bottle was then placed in a radio
frequency microwave field, microwave radiant energy field, as
generated by microwave ovens having output in the microwave
broadcast range of 890; 940; 2,400; 2,500 M. H.sub.z. or other
approved microwave broadcast bands. After about 30 seconds the
water in each bottle started to boil and steam could be seen coming
out of the top. After about ten more seconds steam could not be
seen emanating from the bottle but the level of the water in the
bottle could be seen diminishing in volume. After approximately one
minute more or less all the water had evaporated from within the
bottle. After three minutes the radiation was turned off and the
bottle removed from the oven. After cooling, the volume, by weight,
of the bottle was again taken and respectively plotted on the graph
of FIG. 2B (dotted lines).
Now referring to FIG. 2 it can be seen that bottles subjected to
the three hour boiling, FIG. 2A, all save one, lost sufficient
capacity on boiling that their net capacity was below datum (3785
grams) and in six instances less than 14 grams below datum
(fourteen grams representing one-half fluid ounce). Referring to
bottles subjected by shrinkage according to the embodiments of the
invention it can be seen that only three bottles shrunk in capacity
below datum and that the total deviation of upper limit (12 grams
above datum) and lower limit (7 grams below datum) was less than
the deviation with respect to those bottles subjected to the 3 hour
boil, FIG. 2A. Bottles subjected to the Bailey heat conduction
method of stabilization showed that four bottles stabilized to a
volumetric capacity below datum, one in excess of the fourteen
grams below datum. Further, one bottle shrunk to a capacity of 29
grams above datum, 28 grams representing one fluid ounce. It is to
be noted at this time that in some countries, such as Canada, the
governing authorities do not permit certain containers to be on the
market which have volumetric capacities less than one-half ounce
below their respective datum, or in excess of 1 ounce of their
respective datum.
It therefore can be seen, comparing the three techniques, that the
prior art, of the 3 hour boil, FIG. 2A, and the heat conduction
method of Bailey, FIG. 2C, generate bottles some of which are
beyond acceptable limits. On the other hand bottles stabilized
according to the invention all showed sufficient stabilization to
be within the acceptable limits of datum.
After stabilization all bottles were subjected to ten washing
cycles in duration of five minutes each at a controlled temperature
of 170.degree.F. (168.degree.-172.degree.F.). This wash did not
contain any detergent or soap and might be considered as a hot
rinse. Nevertheless, it was designed to simulate actual dairy
washing cycle conditions as found in a dairy. After each wash the
capacity of each bottle was measured, and respectively tabulated in
FIG. 3 according to the method in which initial stabilization had
occurred namely bottles subjected to the three hour boil method of
stabilization were tabulated in FIG. 3A, bottles stabilized
according to the embodiments of the invention were tabulated in
FIG. 3B, and bottles stabilized according to the Bailey heat
conduction process were tabulated in FIG. 3C. The tables only
indicate the resultant capacity after one wash, five washes and 10
washes respectively. In any event careful examination of the
results indicates that the capacity of bottles, irrespective of the
process of stabilization, migrate to some extent during each wash.
Comparing specifically bottles washed according to the embodiments
of the invention, FIG. 3B with the bottles washed according to the
prior art, FIGS. 3A and 3C, it can be seen that the migration of
bottles washed according to the invention is less than migration
according to the prior art. Specifically, after ten washings,
bottles stabilized according to the 3 hour boil, FIG. 3A, all, save
one, reached or exceeded the lower limit, 14 grams below datum, and
as a result were not serviceable. Moreover after the first wash all
but two bottles were beyond the lower limit of datum.
Referring to FIG. 3C and bottles stabilized according to the Bailey
heat conduction method one bottle showed migration below the lower
limit of datum after the first wash and it remained below that
limit during all washes. All other bottles seemed to be
satisfactory within the acceptable limits about datum. Referring to
FIG. 3B after the first wash one bottle reached a capacity of the
lower acceptable limit and it showed continual reduction in
capacity beyond the lower acceptable limit after the first washing.
All other bottles remained within acceptable limits during
successive washes.
Now, referring to FIGS. 4 and 5 and particularly FIG. 5 is was seen
that the hi-low total variation which bottles migrate during the
washing cycle is less with bottles stabilized according to the
embodiments of the invention than with either the two prior art
techniques. In fact careful analysis shows that bottles shrunk by
the Bailey heat conduction method shrunk 80 c.c. during the 30
minutes of stabilizing. After ten washes they shrunk an additional
7 c.c. approximately. On the other hand bottles shrunk according to
the embodiments of the invention shrunk 106 c.c. during the 3
minutes of stabilization and only approximately 3 c.c. during the
10 washings. Nevertheless it must be admitted that the total
aggregate capacity lost after 10 washings, with bottles stabilized
according to the embodiments of the invention is somewhat higher
than with bottles stabilized according to Bailey, see FIG. 4, it is
nevertheless less than bottles stabilized according to the 3 hour
boil method.
From the above results it is clear that thermoplastic bottles or
containers stabilized according to the invention are significantly
more stable and have a higher integrity than those of the prior
art. They are thus capable of being used in environment
temperatures in the range of 170.degree.F. Such containers are
therefore particularly useful for food and other products which are
elevated in temperature above 150.degree.F, in order to decrease
their viscosity and to make then pourable, for example, jellies,
jams and the like, but below the thermal distortion point of the
thermoplastic. Furthermore, dairy bottles which are subjected to
the stabilization processes embodying the invention will have a
greater inherent stability and integrity to maintain volumetric
capacity than bottles presently existing in the dairy industry.
Now referring to FIG. 1, thermoplastic bottles 10 which have just
been moulded to an interior capacity slightly larger than the final
capacity designed may be placed as by a hand 11, on a continuous
conveyor belt 12. The bottle 10, is then carried to beneath a
nozzle 13, which injects into the bottle 10, a microwave absorbing
fluid such as water 14. The bottle 10, then progressively moves
into a microwave oven 16, which emits radiant microwave energy 17.
The energy 17, passes through the bottle 10, essentially unabsorbed
but is absorbed by the water 14, elevating it in temperature to its
boiling point. The steam (vapor) 18, thereby generated is then
superheated, scavenges the inside of the bottle 10, and shrinks or
stabilizes the bottle. At the same time the excess steam 18'
(vapor) emanates from the top of the bottle pervades the inner part
of the oven. As a result this steam might have some scavenging
effect on the outside surface of the bottle 10, (but this is
difficult to ascertain at the present). The bottle 10, then is
carried out of the oven 16, all or some of the water having been
evaporated, and the same is then removed from the conveyor belt 12,
as by hand 19 or automatically.
It will be appreciated that though the invention has been described
in one embodiment utilizing a bottle of polyethylene thermoplastic,
other thermoplastics can be used for the bottle. In such cases, the
thermoplastic must have properties which permit it to act
essentially as a "window" to the microwave radiant energy used.
This allows the liquid to absorb the radiation and thereby be
elevated in temperature.
If, on the other hand, thermoplastics which are opaque to microwave
radiation are used, i.e. thermoplastics which do not act as
"windows," the use of a liquid absorbent to microwave radiation
need not be as critical.
It has been found that for the preferred results that the
polyethylene bottles, above described, are satisfactorily
stabilized using water as the liquid. The amount of water to be
used may vary with the size of the bottle to be stabilized;
nevertheless, it would appear that for bottles having a volume of a
pint, a minimum amount of water to be used is about 1 ounce.
It might also be mentioned that a different selection of liquids
with higher vaporization temperatures will be necessary as more
sophisticated thermoplastics with higher plastic deformation points
are used, if it is desired to increase the range of environmental
and service temperatures for thermoplastic products.
* * * * *