Note: Descriptions are shown in the official language in which they were submitted.
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1 BACKGROUND OF THE INVENTION
This invention relates to glass containers and particularly
to a method of producing glass containers of greatly increased
effective strength and shock resistance by a combination of ion
exchange treatment of the outer surface of the container to
provide a compressive stress layer and a subsequent jacketing
of the container with a syn-tehtic resin coating of such -thick-
ness as to form an enveloping shock resistant sheath.
Ion exchange strengthening of glass surfaces to strengthen
*he same by producing a compression layer at the glass surface
is known in the art. For instance, Poole et al United States
Patent 3,607,172, dated September 21, 1971, discloses a method
of producing such a compres.sive layer in soda lime glass by
spraying hot glass containers with a water solution of a potas-
sium salt to substitute potassium ions for sodium ions a-t -the
glass surface, the potassium ions having a larger atomic
diameter than the sodium ions.
Also, application of a synthetic resin coating to glass
bottles is known in the art. Representative patents showing
such coatings are Smith et al United States Patent 3,362,843,
dated January 9, 1968 and Clock United States Patent 3,415,673,
dated December 10,1968. In the latter patent glass containers
are dipped in polyethylene solutions to provide an adherent
substrata and are then provided with a coating of e-thylene-
acrylic acid copolymer to a thickness of about.010" by
spraying the coating material on the glass article with an
electrostatic powder spray gun, then heating the article to
fuse the power to -the glass article. ~
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10368Z3
1 In a Poole et al United States Patent 3,743,491, dated
July 3, 1973, there is disclosed a method of strengthening glass
articles by ion exchange treatment and also rendering the articles
abrasion-resistant by applying polyethyene or similar synthetic
resin materials to the glass surface after firstlreating the
la-tter with metal oxides such as tin oxide or various titanates
to provide a surface to which -the final polyethylene coa-ting
will adhere. In this method the polyethylene coating is rela-
tively thin and con-tributes nothing to the strength or shock
resistance of the container, being provided solely to make the
container surface lubricious and thus render the same resistant
to abrasion.
The treatment of glass containers to render them abrasion
resis-tant by applying a metal oxide coating to the glass con-
tainer while the same is in a heated condition and then treating
the same with a polyethylene or other polymer coating after the
glass has cooled to approximately 200 F. is widely practiced
and well known. Eowever, the so-called "cold end" coating of
polyethylene or other polymer is not in any sense comparable to
the resin of -the present invention. In conventional abrasion
resistant treatment the final polyethylene coating is molecular
in thickness and is physically unmeasurable, being of the order
of a small fraction of a micron. This coating is applied merely
to give the article surface lubricity and does not in any sense
form a physical film or envelope of any strength characteristics
whatever.
SUMMARY OF TEE INVENTION
In practising the method of the present invention -the glass
surface is first subjected to ion exchange -treatment to produce
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1 a compressive surface layer by substituting ions of larger atomic
diameter for ions of smaller atomic diameter. This may be effected
as fully described in the aforementioned U. S. Patent 3,607,~72
wherein a water solution of tripotassium phosphate is sprayed on
the hot glass surface a-t a temperature a-t or slightly below the
s-train point of the glass so that the solution pyrolyzes to deposit
the po-tassium salt on the glass surface. The glass is then cooled
gradually so -that its temperature remains sufficiently high for a
period of time which is long enough to permit effective ion ex-
change.
Other potassium salts may be employed, for instance, di-
potassium hydrogen phosphate or potassium carbonate, although -tri-
potassium phosphate is believed to be of superior effectiveness.
Subsequen-t to the ion exchange strengthening step a laye~ of
plas-tic material i9 applied of sufficient -thickness to form a
protective plastic envelope a~out -the article, such envelope
having a minimum thickness of approximately .003" and preferably
from .004" to .010". The only upper limitation on thickness of
the plastic shea-th is one of economy of material and cost of appli-
cation.
Glass con-tainers being treated in accordance with the method
of -the present invention do not require the abrasion resistance
treatment described above and disclosed in Patent 3,743,491
unless there is a substantial delay between the ion exchange
strengthening step and the final provision of the plastic
sheath, in which case the abrasion resistance treatment will
prevent scratching of the strengthened glass surface prior -to
the final plastic sheath application.
~03~;~23
Since the glass is subjected to the potassium salt solu-
tion spray at a temperature in the general range of 1000 F. and
is then gradually cooled to a temperature of approximately 20Q F.,
and since the plastic application in preferred embodiments des-
cribed later herein requires the glass to be at a temperature of
approximately 600 F., it is desirable to effect the plastic
coating treatment by interruption of the cooling process which
occurs during p~ssage of the articles through the usu~l annealing
lehr, such interruption occurring when the glass reaches the proper
temperature for plastic application.
In one broad aspect the present invention provides the
method of producing a strengthened soda-lime glass container com-
prising replacing alkali-metal ions in an exterior surface thereof
with larger alkali-metal ions with the glass at an elevated tem-
perature below the strain point thereof to produce a compression
layer of glass at such surface, and subsequently forming on said
surface a synthetic resin coating of a thickness of at least
0.003" up to about 0.010" to provide a protective, shock-resisting,
shard-retaining encapsulating sheath for said container, said
synthetic resin coating being applied with said container at an
elevated temperature between approximately 300 and 600 F.
EMBODIMENTS OF THE INVENTION
In practising the present invention bottles are sprayed
with a water solution of tripotassium phosphate while the bottles
are at or slightly below 1000 F., the approximate strain point
of soda lime glass of the constituency commonly used in beverage
bottles, for instance. The bottles may be thus treated shortly
after they leave the bottle forming machines at a point when they
are at the above approximate temperature. The bottles may be
sprayed at lower temperatures but are then preferably raised in
temperature to approximately 1000 F., and held at such tempera-
ture for about five minutes and then coolei in about fif-teen
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03G823
I,inutes. This "soaking" period is required to effect the proper
degree of substitution of potassium ions for sodium ions in the
glass surface. All of the foregoing is set forth more fully in
the above cited Poole et al United States Patent 3,607,172.
Other methods which exchange larger ions for smaller ions
in the glass surface may also be employed to produce the desired
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10368Z3
compressive layer in the glass surface by chemical means.
Dipotassium hydrogen phosphate and potassium carbonate are other
potassium salts which may be employed although strengthening is
believed to be achieved to the fullest extent by the employment
of the aforesaid tripotassium phosphate. Following the "soaking"
period the bottles are washed to remove excess salt from the
glass surface.
Following the foregoing ion exchange strengthening of
the glass article the plastic jacket or sheath is applied. By
way of example, powdered plastic materials which may be employed
are so-called ionomers which are ionic polymers, the term being
generally applied to that class of poly~,ers in which ionized
carboxyl groups create ionic cross links in the intermolecular
structure. A representative ionomer resin is one manufactured
by E.I. DuPont de Nemours & Co. Inc. by whom it is identified as
"Surlyn-AD 5001".
This resin, ground to 100 mesh or finer, can readily
be applied by the use of an electrostatic 5pray gun and in such
application the bottles are heated or held at a surface
temperature of approximately 600 F. The spray guns are operated
with a controlled standard air pressure, a controlled cascade
pressure, and a constant particle charge having a potential of
70,000 volts (70 Kv.). In the tests and demonstrations made in
demonstrating the effectiveness of the present invention the
spray gun tip was maintained at a distance of 12" from the
bottle surface, the bottles being rotated on their vertical axes
and electrostatic spraying was effected for various time periods
from 2 seconds to 7 seconds. The bottles were then permitted
to cool to room temperature.
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~0368Z3
1 Another representative plastic material which has been
employed in tests and demonstrations of the present invention is
cellulose acetate butyrate of 150 mesh par-ticle size. This
material may be purchased from Eastman Chemical Products, Inc.
by whom it is identified as Tenite CAB Powder Formula 7400-W.
This material may be applied with the same electrostatic spray
gun technique described in the previous example and with the
bottles at approximately the same temperature.
It has been found that plastic sheaths having a thickness
of from .004" to .010" give good results although a sheath
thickness as low as .003" may be employed with satisfactory
result.
The foregoing two resin compositions and others may be
applied by employing fluidized bed techniques in place of the
electrostatic spray gun of the foregoing examples. In flui-
dized bed application the powdered plastic material is main-
tained in a fluidized co~dition by an ascending column of dry
air and the heated article is exposed to the fluidized powdered
plastic material for several seconds or until the desired
thicXness of coating is achieved. The plastic particles coming
into contact with the heated surface of the glass bottle or
- other article fuse to form a smooth continuous coating after
being removed from the bed.
A full description of fluidized bed coating techniques
will be found in "Technical Proceedings of the Forty-Ninth
Annual Conven-tion, American Electroplaters Society, June 24-
28, 1962", beginning at page 99 thereof. Asthere shown, the
temperature of the article to be coated varies somewhat with
various plastic powders. T~.lith polyethylene the required
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1036823
- 1 temperature of -the article is from 300 to 500 F. with cellu-
lose acetate butyrate from 500 - 600 F. The required
temperature for applying other common thermoplastic resins is
given at page 102 of the aforesaid ar~icle.
The best results are substantially -the same with either
of the foregoing plastic coatings and whether the same be
applied by electrostatic spray gun application, or by means of
a fluidized bed. It is likewise immaterial from a breaking
strength standpoint whether the bottles, in addition to the
10 ion exchange strengthening, are treated for abrasion resistance
in accordance with the aforesaid U. S. Patent 3,743,491 or not.
The following are average test results of a large
number of 12-oz. lightweight beer bottles dropped to land on
their bottoms:
With untreated bottles or bottles having the standard
h tin oxide hot end trea-tment and polyethylene acid end treat-
ment, filled and capped, bottles broke a* an average drop
height of 2'5".
Encapsulation of the above bottles, whether otherwise
20 untreated or whether they have the above abrasion resistance
treàtment, increased the drop height at which breakage
occurred to 3', the thickness of the plastic sheath or en-
capsulation being .004~l.
Bottle~, either plain or having abrasion resistance
treatment, when subjected to the foregoing tripotassium phos-
phate ion exchange strengthening, broke at an average drop
height of 4'4".
The same ion exchange s-trengthened bottles with a sub-
sequent encapsulating plastic sheath of a thickness of .004
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1 broke at an average drop height of 5'8".
In addition to the increase in effective strength and shock
resistance of the foregoing treatment, the plastic sheath or
envelope renders the containers safer in handling and transpor-
tation since the plastic sheath retains the bottle contents
and bottle fragments or shards in instances where the glass
containers become broken.
As stated above, the foregoing treatments were applied to
a large number of 12-oz. lightweight amber beer bottles. ~he
term "lightweight" is used here to describe conventional non-
returnable bottles which are lighter in weight than convention-
al multiple trip bottles. However, the improvement in breaking
strength produced by the combination of ion exchange strength-
ening and plastic encapsulation of the present invention is of
such degree that beverage bottles and similar glass containers
may be further materially lightened in weight by manufacturing
them with thinner walls;than has heretofore been practicable.
The saving thus effected in material cost offsets to a substan-
tial degree the cost of the plastic employed in encapsulating
the containers.
The increase in strength of bottles treated in accord-
ance with the present invention as reflected in the increased
drop height at which the average bottle drops as se-t forth
above is highly beneficial but does not tell the full story of
the importance of the combined ion exchange strengthening and
plastic encapsulation which are inherent in the present inven-
tion. Bottles which have been strengthened by ion exchange
treatment which provides a compressive stress layer at the
surface of the bottle may initially resist breakage when
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1 .dropped from a given height but are very likely to become
chipped and such chipping removes the compressive stress layer
at the chipped area so that subsequent dropping from much lesser
heights would result ink~eakage.
In addition to the advantages discussed earlier herein
the plastic encapsulation prevents this type of surface chipping
o the bottle and thus preserves the compressive stress layer
and consequently the initial streng-thening resulting from the
above ion exchange treatment. Thus the practical strength
increase and resistance to breakage of the bottles are very
much greater than is indicated by the above test figures.
A further advantage in treating glass containers accord-
ing to the present invention, which has been briefly alluded
to above, resides in the fact that when encapsulated bottles
do break, despite the ion exchange strengthening which forms
part of the present invention, the glass fragments or shards
are retained within -the plastic capsule and generally speaking
the con-tents of broken bottles are likewise retained within
the encapsulating sheath.
Preferred embodiments of the present invention have
been described herein to illustrate the underlying principles
of the invention but it is to be understood -hat numerous modi-
fications may be made without departing from the broad spirit
and scope of the invention.
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