Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~O 95/14724 PCTlCA94100655
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ppr.yFSTERURETHANE CONDOM
This invention relates to a method for making a non-
rigid, flexible, thin walled, closed-ended, tubular
article of thermoplastic elastomer, to an apparatus '
therefor and to the article produced. More particularly,
this invention relates to a method for drawing, with a
plug assist, very thin walled, non-rigid, flexible,
closed-ended, tubular articles of. thermoplastic elastomer,
1o to a plug assisted drawing apparatus therefor and to
liners, condoms and finger cots produced thereby.
$ACKGROUND OF THE INVENTION
Thin walled tubular articles of thermoplastic
elastomer are known in the art to provide strong, thin,
flexible protective barriers. Protective barriers
having these characteristics are particularly useful to
protect skin surfaces from unwanted contact or
contamination and, yet, to preserve sensitivity of
touch, as with condoms or finger cots used in medical
examination.
U.S.Pat.No. 4,576,156, Dyck, et al., describes a
condom manufactured from a variety of polyurethane
thermoplasticelastomers. In the process for
manufacture, a shaped mandrel is urged into the face of
pre-heated, extruded elastomeric film and the film
assumes the shape of the mandrel with the application of
-vacuum.
U.S.Pat.No. 4,684,490, Taller, et al., describes
a condom manufactured from certain polyurethane
thermoplastic elastomers. In the process for
W0 95I1d72.1 PCT/CA94/00655
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manufacture, a mandrel is coated with an emulsion of
elastomeric material prepolymer and the coating is cured
at elevated temperature.
Additional-methods to manufacture condoms from
thermoplastic elastomers are generally known. In one
process for manufacture, the thermoplastic elastomer is
blown into very thin film, the film is cut into
appropriate preforms and the segments are heat sealed to
form a condom.
Despite the fact that thermoplastic elastomers are
taught to be suitable materials for the manufacture of
the articles described above; despite the fact that
these elastomers may be employed to produce a stronger,
thinner and a more reliably defect-free product as
compared to natural rubber, the present material of
choice; thin walled tubular articles are not
commercially produced from thermoplastic elastomers.
Each of the above described processes is unsuitable for
high volume production for one or more of several
reasons. For instance, the wall thickness of the
resultant product may not be controllable to the desired
tolerance. Also, though the product is thinner and
stronger, the elastomeric modulus may be too high, i.e.,
the elastomer may be too stiff. Further, considering
cure or annealing times required, the through-put of the
machinery required may be too low.
Thus, it is an objectof the present invention to
rapidly produce aubular articles of thermoplastic
elastomer which are soft, flexible and have a uniformly
thin wall thickness.
CA 02177474 2006-03-27
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It is also an object of the present invention to
provide a method for melt forming tubular articles of
thermoplastic elastomer which have a low modulus and a
uniformly thin wall thickness.
It is a further object of the present invention to
provide a method and hardware to draw with plug assist,
tubular articles of thermoplastic elastomer which have a
low modulus and a uniformly thin wall thickness.
It is another object of the present invention to
provide a thermoplastic elastomer which may be drawn into
tubular articles having thin walls and a low modulus.
A final aspect of the invention provides a condom
formed from a polyesterurethane thermoplastic elastomer,
said elastomer characterized by a melt index, measured at
210°C under a 3800 g test load, in the range of from 24-36
g/10 min; 10-25% by weight MDI; 0.1-5% by weight 1,4-
butanediol; 70-89.9% by weight polybutylene/hexylene
adipate having an average molecular weight of 1000 - 3000
Daltons; and 0-5% by weight lubricant; said condom having a
wall thickness of from about 0.005mm to about 0.25mm and a
length to diameter ratio of 2/1 to 20/1.
SUMMARY OF THE INVENTION
Briefly, according to the present invention, there is
provided a method for making a thin walled, closed-ended,
tubular article of a thermoplastic elastomer, said method
comprising:
CA 02177474 2006-03-27
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a) providing a tubular mold having a longitudinal
axis, a terminal end, and an opposite open end, said
open end having a rim, and providing a preform of
thermoplastic elastomer, said preform having two
opposing, substantially coplanar faces and said
thermoplastic elastomer heated to have a viscosity and
elasticity within a range that the preform can be
drawn;
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b) placing a face of said preform of
thermoplastic elastomer on said rim;
c) applying air pressure or vacuum to one face of
said preform to impose an air pressure
differential across the faces of the preform
and axially directing a plug against the
outward face of the preform, thereby drawing
and urging the preform to flow into the mold,
the plug directed at a rate upon contact with
the preform that the preform is not punctured
by the plug;
d) further applying air pressure or-vacuum and
further axially directing the plug against the
preform, the air-pressure or vacuum applied at
a rate. and the plug directed at a rate whereby
the resultant air pressure differential across
the faces of the preform maintains a portion
of the flowing preform away from the wall of
the tubular mold; and
e) at a point where the plug nears the terminal
end, evacuating remaining air from between the
flowing preform and the walls of the mold
whereby the preform is drawn into contact with
the cooling walls of-the mold to form a shaped
article.
Also provided by the present invention is a method
to prepare a thermoplastic elastomeric preform for
forming or drawing into a thin walled, closed-ended,
tubular article having a wall thickness of between about
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0.005 and o.25 mm, said method comprising:
i) providing a tubular mold having a longitudinal
axis, a terminal end, and an opposite open
end, said open end having a rim;
a
ii) heating a thermoplastic elastomer to a
sufficient temperature to substantially
eliminate the crystalline regions, whereby the
viscosity and elasticity of said thermoplastic
elastomer are substantially reduced, and
shaping said thermoplastic elastomer to form a
preform having two substantially coplanar
opposing faces;
iii) cooling said thermoplastic elastomer whereby
the viscosity and elasticity are recovered
within a range that the preform can be drawn;
and
iv) placing a face of said preform of
thermoplastic elastomer on said rim.
Further provided by the present invention is a plug
assembly for use in plug assisted drawing of thin
walled, closed-ended, tubular articles of thermoplastic-
elastomer comprising an axially centered rod, said rod
attached to the base of an axially centered,
frustoconically shaped plug, said plug having a crown
face opposite said base and extending from said crown
face, an axially centered contact projection.
Further provided by the present invention is a plug
assembly for use in plug assisted drawing of thin
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walled, closed-ended, tubular articles of thermoplastic
elastomer comprising an axially centered rod, said rod
attached to a first face of an axially centered, disc
shaped plug, said plug having a second face opposite
said first face and extending from said second face, an
axially centered contact projection.
Also provided by the present invention is a condom
formed from a polyesterurethane thermoplastic elastomer,
l0 said elastomer characterized by a melt index, measured
at 210°C under a 3800 g load, in the range of from 24-36
g/10 min; 10 - 25% by weight MDI; 0.1 - 5% by weight
1,4-butanediol; 70 - 89.9% by weight
polybutylene/hexylene adipate having an average
molecular weight of 1000 - 3000 Daltons; and 0 - 5% by
weight lubricant. -
Further provided by the present invention is a
condom of thermoplastic elastomer, said condom having an
axially centered tubular body, an open end and an
opposite closed end, said tubular body having a maximum
diameter at a point along said axis adjacent to said
closed end and said tubular body having a minimum
diameter at a point along said axis between said point
of maximum diameter to and including said open end.
Additionally, provided by the present invention is
a method to lower the modulus of a thin walled, closed-
ended, tubular article of thermoplastic elastomer, said
method comprising the steps of:
1) stretching said tubular article onto a forming
mandrel; and
~O 95114724 PCTICA94I00655
2) heating said tubular article and mandrel to a
temperature of between 100 and 125°C for a time
sufficient to lower the modulus of said
thermoplastic elastomer.
RT~I~'F DESCRIPTION OF THE DRAWINGS,
FIG. la is an isometric drawing of a rimmed preform
having a flat profile.
FIG. lb is a side centerline cross section of a
rimmed preform having a flat profile.
FIG. 2a is an isometric drawing of a rimmed preform
having a profile.
FIG. 2b is a side centerline cross section of a
rimmed preform having a profile.
FIG. 3 is a schematic of a preheating and plug
assisted vacuum drawing apparatus as taught herein.
FIG. 4a is a isometric drawing of a plug assembly
as taught herein.
FIG. 4b is a side centerline cross section of a
plug assembly as taught herein.
FIG. 4c is a isometric drawing of a plug assembly
as taught herein.
FIG. 4d is a side centerline cross section of a
plug assembly as taught herein.
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FIG. 5 is a tracing of mold cavity pressure as a
function of time in the plug assisted drawing step where
mold vacuum is employed.
FIG. 6 is a tapered condom.
FIG. 7 is a plot of the thickness of a
polyesterurethane condom wall made by the process herein
as a function of distance from the tip end, where the
reported thickness is the maximum, minimum and average
for the circumference at each distance.
FIG. 8 is-a plot of the thickness of a natural
rubber condom wall made by a dipping process as a
function of distance from the tip end, where the
reported thickness is the maximum, minimum and average
for the circumference at each distance.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to thin walled,
closed-ended, tubular articles of- thermoplastic
elastomers, to a process for making the same and to
machinery therefor. Of course, the thinner the walls of
the tubular article, the more critical are the instant
materials and processes. Broadly, the instant invention
is intended to produce a uniformly thin walled tubular
article having a wall thickness of from about 0.005 mm
to about 0.25 mm,of thermoplastic elastomer and a length
to diameter ratio of 2/1 to20/1. The invention is most
beneficially employed where the desired wall thickness
is between about 0.01 mm and about 0.10 mm and the
length to diameter ratio is 3/1 to 10/1. Prior to the
present invention, thermoplastic elastomers could only
09511472.1 ~ ~ ~ ~ ~ ~ ~ - PCTfCA94100655
_ g _
be cast from solvents to uniformly produce wall
thicknesses in this range.
Thermoplastic elastomers are block copolymers
having hard and soft blocks, or domains, in the polymer p
molecule, or compound. The soft domains provide the
rubber like elastomeric properties while the hard or
crystalline domains act as mechanical crosslinks, tying
down the rubbery domains. At processing temperatures,
the hard domains of the thermoplastic elastomer become
amorphous or soften to yield a melt that can be easily
processed by injection molding, extrusion, vacuum
molding, etc.
Suitable thermoplastic elastomers herein include
polyurethanes, polyetherurethaneureas,
polyetherurethanes, polyesterurethanes,
polyester/polyether block copolymers,
styrene/diene/styrene block copolymers, etc. The
present invention may be applied to a broad range of
thermoplastic elastomers, as the suitability of any
given thermoplastic elastomer is more appropriately
judged based on its physical properties than on its
particular type. Controlling the physical properties of
any thermoplastic elastomer is an art specific to that
thermoplastic elastomer. Broadly, it can be stated
herein that the physical properties of any thermoplastic
elastomer are dependent upon hard block type, soft block
type, block arrangement in the polymer, average polymer
. 30 molecular weight, average hard block content with
average number of repeating units, average soft block
content with average number of repeating units and the
use of additives, particularly, waxes, to improve
processability, and, possibly, impact modifiers, to
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improve tear strength. For use herein, a thermoplastic
elastomer should have a Shore A hardness between about
60 and 80, and preferably, between about 60 and 75. The
tensile stress of the thermoplastic elastomer at 100%
elongation, commonly referred to as the 100% modulus,
F
should-be between about 50 and 600 psi, and preferably,
between about 100 and 500 psi. The equivalent 300$
modulus should be between about 450 and 1100 psi, and
preferably, between about 50-0 and 900 psi. The ultimate
elongation should, vary in the range of from about 400
to 800% and the compression set, 24 hrs at 23°C, should
be no more than 25x. Importantly, the thermoplastic
elastomer should have excellent resistance to organic
solvents. All of the above ranges, unless otherwise
stated, are measured at 23°C.
It is another aspect of the thermoplastic elastomer
herein that it should not have a distinct melting point.
In the art of draiaing or extruding thermoplastic
elastomers, an indistinct melting point is one factor in
having a melt which is formable, yet will retain a shape
that it has been given. Within the range of an
indistinct melting point, a melt will exhibit a "green
strength". An indistinct melting point is a
characteristic of.a melt containing a range of molecular
weight polymers. -
A preferred thermoplastic elastomer is a block
copolymer of polyurethane hard blocks with polyester
soft blocks. These thermoplastic elastomers have been
found to have excellent physical strength, superior
abrasion and tear resistance, and excellent tensile
strength. In addition these thermoplastic elastomers
show excellent resistance. to organic solvents.
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Low modulus and low set are achieved in the
. preferred polyesterurethane by selecting, as the soft
block, a long chain diol or combination of diols, which
tend to crystallize little at ambient temperature; and
hard block content of no more than 25%. The rigid hard
block is achieved by reaction of a polyisocyanate and a
short chain diol.
to 25 weight percent of the preferred
10 polyesterurethane is comprised of one or more
polyisocyanates. Preferably the polyisocyanate is a
diisocyanate. Useful diisocyanates-include aromatic and
aliphatic diisocyanates. Suitable diisocyanates include
non-hindered aromatic diisocyanates such as: 4,4'-
methylenebis-(phenyl isocyanate) (MDI); isophorone
diisocyanate (IPDI), m-xylylene diisocyanate (XDI), as
well as non-hindered cyclic aliphatic diisocyanates such
as 1,4-cyclohexyl-diisocyanate, naphthylene-1,5-
diisocyanate, diphenylmethane-3,3'-dimethoxy-4,4'-
diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, and
cyclohexyl-1,4-diisocyanate, as well as combinations
thereof. The most preferred non-hindered diisocyanate
is 4,4'-methylenebis-(phenyl isocyanate) i.e., MDI.
0.1 to 5 weight percent of the preferred
polyesterurethane is comprised of chain extenders.
Suitable chain extenders are lower aliphatic or short
chain glycols having from about 2 to about 6 carbon
atoms. Examples of suitable chain extenders include,
- 30 for instance ,-diethylene glycol, propylene glycol,
dipropylene glycol, 1,4-butane diol, 1,6-hexane dioL,
1,3-butane diol, 1,5-pentane diol, 1,4-cyclohexane-
dimethanol-, hydroquinone di(hydroxyethyl)ether, and the
like, as well as combinations thereof, with 1,4-butane
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diol being preferred.-
70 to about 89.9% by weight of the-preferred
polyesterurethane is comprised of a hydroxyl terminated
polyester. A preferred class of hydroxyl terminated
,
polyester intermediates is generally a linear polyester
having a molecular weight of from about 500 to about
5,000 Daltons, and most preferably from about 1,000 to
about 3,000 Daltons, and an acid number generally less
than 0.8 and preferably less than 0.5. The molecular
weight is determined by assay of the hydroxyl groups.
The polyester intermediates are produced by (1) an
esterification reaction of =one or more glycols with one
or more dicarboxylic acidsor anhydrides, or (2) by
transesterification reaction, i.e., the reaction of one
or more glycols with esters ofdicarboxylic acids. Mole
ratios generally in excess of-more than one mole of
glycol to acid are preferred so as to-obtain linear
chains having a preponderance of terminal hydroxyl
groups.
The dicarboxylic acids can be aliphatic,
cycloaliphatic, aromatic, or combinations thereof.
Suitable dicarboxylic acids which may be used alone or
in mixtures usually have a total of from 4 to 15 carbon
atoms and include: succinic, glutaric, adipic, pimelic,
suberic, azelaic, sebacic,-dodecanoic, isophthalic,
terephthalic cyclohexane dicarboxylic, and the like.
Anhydrides flf the above dicarboxylic acids, such as
phthalic anhydride, or the like, can also be utilized,
with adipic acid being preferred.
The ester-forming glycols can be aliphatic,
aromatic, or combinations thereof; have a total of from
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2 to 12 carbon atoms; and include; ethylene glycol, 1,2-
propylene glycol, 1,3-propanediol, 1,3-butylene glycol,
1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-
dimethylpropane-1,3-diol, 1,4-cyclohexanedimethanol,
decamethylene glycol, dodecamethylene glycol, and
combinations thereof. The combination of 1,4-butanediol
with 1,6-hexanediol being the preferred glycol.
The preferred polyesters of the preferred
polyesterurethane are mixed esters such as
polybutylene/hexylene adipate, polybutylene
adipate/azelate. The most preferred polyester is a
polybutylene hexylene adipate.
In addition to the above polyester intermediates,
numercus other types of polyester intermediates known to
the art and to the literature can be utilized, including
those having different molecular weights and/or contain
branch polyesters therein. For example,
polycaprolactone diols can be used. These are known
polyester reaction products of lactones and bifunctional
compounds having two reactive sites capable of opening
the lactonering. These bifunctional materials may be
represented by the formula HX-R-XH wherein R is an
organic radical which can be aliphatic, cycloaliphatic,
aromatic or hetercyclic and X is O, NH and NR where R is
a hydrocarbon radical which can be alkyl, aryl, aralkyl
and cycloalkyl. Such materials include diols, diamines
and aminoalcohols preferably. Useful diols include
alkylene glycols wherein the alkylene groups contain 2
to 10 carbon atoms for examples, ethylene glycol, 1,2-
- propane diol, 1,4-butanediol, 1,6-hexamethylene diol and
the like.
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The intermediate, such as hydroxyl terminated
polyester, is further reacted with one or more
polyisocyanates and preferably a diisocyanate along with
0
a chain extender, desirably in a "one-shot" process,
that is, a simultaneous co-reaction of the intermediate,
diisocyanate, and the chain extender, to produce a
moderate molecular weight linear polyurethane having a
melt index of from about 1 to about.150 and preferably
from about 1 to about 75 at 2,160 grams test load. The
equivalent amount of diisocyanates to the total amount
of hydroxyl containing components, that is, the hydroxyl
terminated polyester, and the chain extender, is from
about 0.95 to about 1.13~ and desirably from about 0.98
to about 1.06.
~ _
Alternative3y, the urethane can be made in a
conventional two-step process wherein initially a
prepolymer is made from the polyisocyanate and the
intermediate, with the prepolymer subsequently being
reacted With the chain extender. The equivalent ratio
of the one or more polyisocyanates to the hydroxyl
terminated intermediate is generally a sufficient amount
such that, upon subsequent chain extension with a
suitable chain extender, the overall equivalent ratio of
the hydroxyl terminated compound to one or more
polyisocyanates is approximately 0.95 to about 1.065 and
the like.
Examples of the above, as well as other suitable
thermoplastic polyurethanes which can be utilized, are
set forth in Vol. 13 of the Encyclopedia of Polymer
. Science and Engineering, John Wiley & Sons, Inc., New
York, NY, 1988, pages 243-303:.
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In addition, the polyurethane can be blended with
various additives such as, for example, antioxidants,
stabilizers, lubricants, process aids,. Most preferably,
the polyurethane is blended with 0 to about 5 weight
percent of a lubricant composition. Any compatible or
appropriate lubricant composition giving the desired
characteristics can be used. Suitable examples include
fatty bisamide or fatty esters.
The preferred polyesterurethane thermoplastic
elastomer may be produced in a two step process. In a
first polymerization, polyester blocks are produced by
reacting the diol and the diacid to produce diol
polyester blocks. In a second polymerization, the diol
polyester blocks are reacted with a mix of diisocyanate
and at least one diol monomer, the latter two in
appropriate ratio to the polyester block and to each
other to produce the desired molecular weight for the
polymer and the hard blocks. However, the most
2o preferred process for making the thermoplastic
polyesterurethane is a "one shot" polymerization
process. The diols, diisocyanate and chain extenders
are reacted at a temperature above 100 °C and desirably
above 120 °C. The reaction is exothermic and results in
the preferred polyesterurethane.
By way of example, a specific polyesterurethane
thermoplastic elastomer, has a molecular weight in the
range of from 145,000 to 190,000 Daltons, corresponding
to a melt index, measured at 210°C under a 3800 g test
load, in the range of from 24-36 g/l0 min; 10 - 25% by
weight MDI; 0.1 - 5% by weight 1,4-butanediol; 70 -
89.9% by weight polybutylene/hexylene adipate having an
average molecular weight of 1000 - 3000 Daltons; and 0 -
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5% by weight lubricant. This specific polyesterurethane
has the properties of Table I.
TABLE I
,
ProoertY ASTM METHOD
Value
Hardness, Shore D2240 74 A
Specific Gravity . D792 1.17
DSC Thermal D3418
Transition Temperature, C
Ti -4 0
T~ -10
T~ 27
Tm(max) 165
Stress-strain, psi D412
100% modulus 500
300% modulus 905
ultimate tensile
strength 5100
ultimate elongation (%j 550
Tear Strength, pli D1938 244
Compression Set, % D395
22h/23C 23
22h/70C 74
The articles of the present invention are formed
from the thermoplastic elastomer, firstly, in a preform
operation and, secondly, -in a plug assisted drawing
operation. In the preform operation, the t hermoplastic
elastomer is made into an appropriate prefo rm and heated
to a suitable temperature for plug assisted drawing. In
the plug assisted drawing operation, the pr eform is
~O 95114724 PCTICA94100655
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drawn with a plug assist into the thin walled tubular
articles herein. Finally, the articles may be annealed
to remove residual stress.
At a minimum, it is required of the preform
operation that a thermoplastic elastomer is heated to
eliminate the crystalline phase and thereby,
substantially, the elasticity; that the thermoplastic
elastomer is shaped into a preform having essentially
two opposing substantially coplanar faces; that the
preform is cooled, recovering viscosity and elasticity
within a range that the preform can be drawn; and that
the preform is positioned over the mold cavity for
drawing with plug assist. Each of these steps, without
more, is well within the capabilities of the skilled
artisan. However, disclosed as follows are preferred
and contemplated methods for accomplishing each of these
steps.
Physically, the preform may take a variety of
shapes with the functional requirement being that the
preform is useful in a plug assisted drawing apparatus
of the type disclosed herein. As the necessary
characteristic, the preform should have essentially two
opposing substantially coplanar faces which, when
restated, is intended to mean that the preform is a thin
flat piece of thermoplastic elastomer. In its specific
embodiments, the preform might be a square panel or a
round disc which is cut and/or stamped from a larger
extruded sheet or the preform might be a ribbon or sheet
from which the article is cut at the time of plug
assisted drawing. Of course, in either case, the
extrusion, molding or stamping of the preform might be
performed at the site of the plug assisted drawing
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operation and it may even be advantageous to integrate
the two operations. The preform may simply have planar
faces or the face of the preform may be profiled to
obtain some advantage in thermoplastic elastomer melt
distribution in the mold during the plug assisted
drawing operation. Referring to figures la and lb and
figures 2a and 2b, there are shown possible disc-shaped
preforms having a profiled surface which might be
advantageously employed herein. Although it is well
within the ability of a person skilled in the art to
choose an appropriate shape and size for a preform given
a particular product, it is recommended that preforms
herein have a thickness no greater than about 5 mm and
no less than about 0.5 mm, and preferably between 1 and
2.5 mm.
The preform should be heated to a forming
temperature at which the rate of standing deformation is
insufficient tosubstantially deform the preform during
the time period of the preform operation, and at which
the elasticity and viscosity of the melt is within the
range that the preform can be drawn into a thin walled,
closed-ended, tubular article. This range of elasticity
and viscosity at which the melt can be drawn may be
characterized in two ways. The first characteristic of
this range is that, to as great a degree as possible,
resistance to flow in thethermoplastic elastomer melt
should be viscous and not elastic. The second
characteristic of this range is that under the combined
effect of viscosity and elasticity, the thermoplastic
elastomer should not stick, tear or warp when drawn to
the desired thickness. In the case of the
polyesterurethane described above, the heating was
carried out to a temperature between 190 and 210°C and
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the melt was cooled for drawing to a temperature between
145 and 185°C and preferably between 150 and 170°C. This
temperature range for drawing corresponds roughly to a
melt viscosity (i.e. complex viscosity), as measured
with a Rheometrics Mechanical Spectrometer at the =
forming temperature, in the range of 75,000 to 150,000
Poise and an elastic modulus, at the forming
temperature, in the range of from 600,000 to 1,200,000
dyne/ cm2 .
As stated above, it is necessary to the process
described herein that the elastomeric character of the
thermoplastic elastomer be minimized for the plug
assisted drawing operation. That is, the principal
resistance to the initial deformation of the preform in
the plug assi-sted drawing operation should be viscous
resistance. This can be accomplished by substantially
eliminating the crystalline domains through a controlled
heat history of the thermoplastic elastomer. Simply,
the thermoplastic elastomer should be first heated to a
sufficient temperature to substantially eliminate the
crystalline domains and, subsequently, cooled to the
temperature of drawing. As crystalline domains do not
immediately reform, there is some time period in which
the thermoplastic elastomer may be deformed without
substantial elastic resistance to deformation. It is
preferred that the thermoplastic elastomer be
immediately cooled and drawn with plug assist, since the
reformation of the crystalline domains begins with
cooling and proceeds until a crystalline domain content
consistent with temperature is regained. For any given
thermoplastic elastomer, and using the technique of
first heating to eliminate the crystalline domains, it
is a trivial matter-to optimize the temperature to which
W 0 95114724 PCTICA94100655
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the thermoplastic elastomer should-be cooled for
drawing. In one preferred method, a preform at room -
temperature is heated to a sufficient temperature to
substantially eliminate the crystalline domains. In the ,
short time period for which the preform is held at this
temperature, the standing deformation of the preform is
insignificant. Immediately, the preform is allowed to
cool to the temperature of plug assisted drawing and
drawn. In another preferred method, thermoplastic
elastomer is heated to a sufficient temperature to
substantially eliminate the crystalline domains and
extruded into a ribbon preform. The preform is cooled
to the temperature of plug assisted drawing and
successive segments drawn.
The heating step of the preform operation might be
carried out in an oven and/or extruder barrel.
Referring to Figure 3, in the instance where an oven is
employed, it is suggested that the preform 1 is
positioned in the oven 2 horizontally and heated by
contact with hot air and exposure to an IR source. A
suggested IR source might-be nothing more sophisticated
than the glowing elements of an electric resistance
heater 3 or the IR souce may be a lamp designed to emit
IR radiation. With the relatively thin preforms
employed in the process herein and relying primarily on
an IR heat source, the temperature of the preform can be
rapidly changed to minimize standing deformation and to
increase through-put. Where it.is attempted to heat the
preform at an incline or positioned vertically, the
weight of the preform will aggravate standing
deformation.
Additional and possibly more advantageous methods
~O 95114724 PCTICA94100655
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of heating the preform are contemplated. One possible
method of heating the preform might utilize microwave
radiation. In this case, an appropriately designed oven
would be required, as well as dopants in the
thermoplastin elastomer to convert the radiative energy
to heat. In a second method of heating, the plug
assisted drawing might be performed on a preform off an
extruder where the preform has cooled to an appropriate
temperature for drawing.- In this case, there would be
no need for a heat source other than the extruder.
The heating of the preform, as well as the
controlled heat history are well within the skill of the
art.- Applicants have critically applied these
techniques to the plug assisted drawing of tubular
articles of thermoplastic elastomer.
As stated above, it is the purpose of the plug
assisted drawing operation to mold the preform into the
thin walled tubular articles herein. A first feature of
the plug assisted drawing operation is the use of a plug
assist, or mandrel, complementing the application of an
air pressure differential across the faces of the
preform to both urge and draw the heated preform into
the mold. A second feature of the plug assisted
drawing operation is the dynamic relationship of air
pressure differential across the faces of the preform to
plug extension into the mold.
3o In regard to imposing and controlling an air
pressure differential across the faces of the preform,
two techniques might be employed. In one technique, the
pressure differential is the result of positive and
negative guage pressure applied to the outward face of
W0 95114724 PCTICA94100655
- 22 -
the preform with the mold cavity maintained at a
constant pressure, preferably atmospheric. In a second
technique, the pressure differential is the result of a
positive and negative gauge pressure applied within the
mold cavity, with the preform sealably mounted on the f
rim of the mold. Whether the preform is drawn into the
mold with an air pressure differential created by an
external positive pressure or internal negative
pressure, it is considered herein to be "drawn". The
two techniques are considered equivalent. The preferred
technique for the description herein is the second
technique where air pressure differential is controlled
by applying vacuum to the mold. However, this is mainly
due to the fact that the prototype machinery where air
pressure diffential is controlled
in this manner is simpler to build and operate.
Herein, the pressure differential across the faces
of the preform will be stated as a positive value where
the pressure is into the mold and as a negative value
where the pressure is out of the mold. Thus, where a
positive differential pressure is applied to the
preform, it may be created by mold vacuum or by an
external pressure. Similarly, where a negative
differential pressure is applied, it may be the result
of a pressurized mold or an external vacuum.
Figure 3 depicts an embodiment of the present
invention where air pressure differential is controlled
from within the mold. Referring to Figure 3, heated
preform 1 is positioned on rim 4 of mold 5 with plug
assembly 6 fully retracted, whereby the mold is sealed.
With the mold-sealed; air is evacuated from the mold by
means of vacuum source 7 through vacuum port 8.
O 95114724 PCTICA94100655
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Simultaneously or subsequently to the time that the air
is drawn from mold 5, plug assembly 6 is extended into
the mold to make contact with the preform and urge it
into the mold. Through the combined effect of air
evacuation and plug extension, the deformation of E
preform 1 into the mold is substantially accelerated.
Plug extension rate and air evacuation rate combine to
produce a dynamic mold pressure which, as described
below, may alternatively assist or resist the action of
the plug on the deforming thermoplastic elastomer. With
the plug assembly fully extended, the remaining air in
the mold is evacuated by means of vacuum source 7 and
the molded article is removed. Figure 3 may be modified
to impose the air pressure differential with apparatus
on the outward face of the preform. In this technique,
a pressurizeable cavity may be brought into sealed
contact with the outward face of preform 1, the contact
opposite the rim 4. This pressurizeable cavity would
have a sealed opening through which plug assembly 6
would extend and a pressure port to which the equivalent
of vacuum source 7 would be attached. Vacuum port 8
could be open to the atmosphere.
As stated above, the process herein is most
beneficially applied to tubular articles of
thermoplastic elastomer having thin walls and a length
to diameter ratio within the boundaries recited.
Providing an appropriately sized mold in which to
produce such an article is clearly within the skill of
the art. Since the mold need not be heated or
excessively pressurized, a variety of materials are
available.
In regard to the first feature of the plug assisted
WO 95114724 PCTICA94/00655
- 24 -
drawing operation, the purpose of the plug assembly is
to urge the deforming preform into the mold cavity and,
in doing so, to bias the flow of the deforming
thermoplastic elastomer preform axially along the mold
cavity, avoiding contact with the mold walls until the
point at which it is nearly fully drawn. The plug
assembly comprises the plug itself attached at its base
to a rod. The purpose of the plug is to provide a
surface to contact the deforming preform. The rod
provides a support on which to mount the plug and by
which the plug may be extended into the mold cavity.
The materials of manufacture and the shape of the plug
may vary widely. Of course, any choice of materials and
any plug design should have as objectives, that the
material not stick to the hot preform, that surface area
of contact be minimized at the same time that sufficient
surface is presented to prevent inappropriate stress or
punctures-in the material, that contact with the plug
minimize heat loss to the plug, etc. It is not an
objective of the plug design to provide an appropriate
surface against which to compressively shape the
preform. The plug might be manufactured of non-stick
polyolefins, such as, poly(perfluoro-olefins), e.g.
teflon, or aluminum, optionally having a textured
surface. The shape of the plug might be ellipsoidal,
toroidal, paraboloidal, a segmented paraboloid with
perpendicular bases, barrel shaped, spherical sector
shaped, frustuconical (frustum of a right circular
cone), cylindrical, etc.- Referringto figures 4a and
4b, a preferred plug is a frustoconically shaped plug
having extending from its crown face, an axially
centered contact projection. Referring to figures 4c
and 4d, another preferred plug is a truncated cylinder
or disc having extending from a radial face, an axially
~O 95114724 PCTICA94I00655
- 25 -
centered contact projection. Also, it is preferred that
the plug is made of a textured aluminum.
In the preferred method of using the plug assembly
and mold, the mold is aligned along a vertical axis with s
the rim defining an opening centered about the axis on
one end and the vacuum port located at the opposite,
terminal end at a point where.the terminal end
intersects the vertical axis. The plug assembly is
mounted along the vertical axis and the plug is
extensible into the mold cavity along the vertical axis
for substantially the length of the mold. It should be
made clear that the plug is not a compressive member
and, thus, should clear the rim, the mold walls and the
terminal end of the mold by distances greater than the
thickness of any thermoplastic elastomer with which it
is in contact at that point. In line with this, the
plug should only be capable of extending substantially
the length of the mold clearing the terminal end, at
least, not only by the distance just stated, but also by
a sufficient distance that-the drawn end of the preform
is not prematurely entrained in flowing air as it exits
the mold port in the final drawing step as described
below.
The pressure or vacuum source is not critical
herein. Persons skilled in the art will readily be able
to choose a means for-pressurizing air or- drawing a
vacuum from, for example, a vacuum reservoir maintained
3o by an ejector or rotary compressor; a rotary compressor;
or a single-stage, single-acting piston. The preferred
pressure or vacuum source is the single-stage, single-
acting piston connected to the pressure or~vacuum port
without a reservoir. Where such a piston is sized to a
W0 95/14724 PCTICA94/00655
~1?'~4~~
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volume larger vhan the mold itself, then the mold may be
filled or evacuated in a single draw of the piston and a
discharge of the piston back into the mold at the end of
the drawing operation will assist in clearing the mold
for subsequent plug assisted drawing operations.
As restated from above, the second feature of the
plug assisted drawing operation, is the dynamic
relationship of air pressure differential across the
faces of the preform in the mold to plug extension into
the mold. This relationship of pressure differential as
a function of plug extension can be divided into three
separable stages over the length of plug extension.
The first stage is the relationship of air-pressure
differential to plug extension up to or about the time
of contact between the preform and the extending plug.
In the first stage, thereis either a positive
differential applied to-the preform drawing it into the
mold or a neutral or zero-pressure differential is
maintained and extension of the plug into the mold is
begun. At the latest, it isnecessary to control a
positive pressure differential or actively maintain the
neutral pressure differential at the point at which the
extending plug makes contact with the preform and the
preform is thereby urged into the mold. If controlling
the pressure differential is delayed much after contact
of the extending plug with the preform, a damaging
pressure will build as the preform is urged into the
mold against a-sealed mold or external cavity. It is
preferred in the first stage that a positve pressure
differential is applied to the preform prior to contact
between the extending plug and the preforin.
Specifically, sufficient pressure--differential is
~O 95/14724 PCTICA94100655
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applied and the rate of plug extension is set such that
the preform is deformed by action of the pressure
differential to a substantially hemispherical shape in
the time between the application of the pressure
differential and contact of the preform by the plug. It
is preferred in this first stage that the preform be
drawn by the pressure differential to a shape and given
a momentum of deformation better suited for contact with
the plug. Prior to plug contact with the preform, the
pressure differential should at least be sufficient to
deform the contacting face of the preform into a concave
shape and preferably into a hemispherical shape.
However, past the point at which the deformimg preform
is hemispherical in shape, further deformation by the
pressure differential alone will draw the body of the
deforming preform into the tubular mold where continuous
contact with the mold wall will-be made as it is drawn.
This contact with the mold wall produces thick non-
uniform walls in the molded article for the length of
such contact. Plug contact should therefor be made with
the preform prior to a point at which the shape of the
deforming preform is substantially tubular.
Specifically-as to the rate of plug extension in the
first stage, it must clearly be axially greater than or
equal to the axial draw rate of the deforming preform
resulting from the pressure differential. However, the
rate of plug extension should not be so great that upon
contact with the preform, the preform is punctured.
Suitably, the pressure differential in the first stage
should range from 0 to 500 mm-Hg, and preferably from 5
to 200 mm-Hg. These pressures are not absolute
pressures, but relative-pressures where 0 mm-Hg is no
pressure across the preform.
W0 9511.172-1 PCTlCA94100655
- 28 -
The second stage is the relationship of air
pressure differential across the faces of the preform to
plug extension from at or about the time of contact
between the extending plug and the deforming preform to
about the point at which the plug is fully extended. In a
this second stage, the air flow rate from the pressure
or vacuum source is such that when combined with the
extension of the plug into the mold, there results a
pressure differential at or about 0 mm-Hg. It is
l0 desired in this second stage that the deforming preform
be fully extended into the mold cavity with mold wall
contact only in the area of the mold rim assembly. This
can be accomplished where the plug is employed to urge
the deforming preform into the mold cavity, biasing the
flow of the deforming thermoplastic elastomer preform
axially into the mold cavity, and the pressure
differential is used to prevent contact between the
drawn thermoplastic elastomer and the mold walls until
about the point at which the plug is fully extended. By
reducing the pressure differential to about 0 mm-Hg, the
pressure which would otherwise urge the preform into the
mold and, conmitantly, into the mold wallis equalized,
with the result that thin walls of the drawn article are
not pressed into the wall. By the drawing action of the
plug, the thin,walls of the drawn article are pulled
nearly to a line extending from the trailing point of
contact with the plug to the rim assembly. Further, by
the action of hoop stress, which resists deformation in
the drawn thermoplastic elastomer, the thin walls of the
drawn article are pulledradially inward toward the axis
of plug extension. Thus, a pressure differential of up
to 25 mm-Hg and preferably up to 5 mm-Hg might be
tolerated in the second stage depending on draw rates,
thermoplastic elastomer, preform temperature, and so on.
~G' 95/14724 PCTlCA94100655
- 29 -
Also, depending on these same factors, a negative
pressure differential might be advantageous in the
second stage, with the pressure differential running
from 0 to -25 mm-Hg and preferably from 0 to -5 mm-Hg.
A negative pressure differential would tend to hold the
thin walls of the drawn article radially inward and away
from the mold wall. Of course, the negative pressure
differential should not be so great that the thin walls
of the drawn preform contact the rod of the plug
assembly. In order to maintain the pressure
differential within the desired ranges of the second
stage, the volumetric displacement within the mold
resulting from the axial extension of the plug and the
drawn preform into the mold must be roughly balanced by
the airflow rate from the vacuum or pressure source.
The third stage defines the relationship of air
pressure differential across the faces of the preform to
plug extension from at or about the point at which the
plug is fully extended to the point at which the mold is
fully evacuated and the preform is drawn into full
contact with the mold walls. It is the purpose of the
actions in the third stage to finally shape the thin
walled tubular article. The plug is fully extended in
the third stage. Air pressure differential is raised as
necessary to evacuate the mold and bring the walls of
the tubular article into deforming contact with the mold
walls. Preferably, the action of raising the air
pressure differential lifts the expanded preform from
contact with the plug which permits an unentangled
retraction of the plug. Contact of the expanded preform
with the mold walls cools the resultant thin walled
tubular article sufficiently that it may be removed from
the mold.
WO 95/14724 PCTICA94I00655
- 30 -
In combining the three stages of plug extension and
pressure differential into a single continuous drawing ,
operation, there are several options. For a given mold
volume, mold length, thermoplastic elastomer, etc.,
there is likely a constant plug extension rate which
when combined with a constant rate of air flow from the
pressure or vacuum source will result in a pressure
differential and plug extension profile falling within
the boundaries just described. of course, while the
resulting profile might be the most convenient from the
point of view of equipment and operation, a superior
product might be produced by having various plug
extension rates and various rates of air flow to achieve
the stated objectives o.f the three stages.
Thus, in a preferred embodiment, there is provided
a method for making a thin walled tubular article of a
thermoplastic elastomer, said method comprising:
a) aligning an elongated mold cavity along a
substantially vertical axis, said elongated mold
cavity defined by a mold comprising: a terminal
end, having surfaces intersecting said vertical
axis, radially defined sides, substantially
parallel to said vertical axis, ~a working end,
comprising a rim assembly defining an opening
centeredabout said vertical axis, and means to
evacuate.said mold cavity, said means to evacuate
comprising a port attached to said terminal end at
a point where said terminal end and said vertical
axis intersect,
b) aligning a plug along said vertical axis, said
plug extensible along said vertical axis through
~O 95I1d724 ~ ~ ~ ~ ~ ~ ~ PCTICA94I00655
- 31 -
said opening into said mold cavity to a point
substantially the length of said mold cavity, said
plug clearing said rim assembly, said sides and
said terminal end by distances substantially
greater than the thickness of any contacted a
thermoplastic elastomer at that point;
c) positioning, said plug retracted, a
substantially planar preform of thermoplastic
l0 elastomer across said opening, said preform having
opposing top and bottom faces; said preform heated
to a temperature at which the rate of standing
deformation is insufficient to substantially deform
the preform during the time period of said
positioning step and at which the viscosity and
elasticity are within a range thatthe preform can
be drawn; and said preform radially extending from
said vertical axis in a 360° arc to a point beyond
said opening as defined by said rim assembly,
whereby at least one face of said preform is in
contact with said rim assembly, whereby said cavity
is sealed, and whereby said incipient flow is along
said vertical axis with the force of gravity;
d) applying an air pressure differential across
said top and bottom faces of said preform and
directing said plug along said vertical axis to
contact a face of said preform,- whereby the preform
is drawn by said pressure,differential and urged by
said plug to flow along said vertical axis and into
said cavity whereby the air of said mold cavity is
evacuated through said,port;
e) drawing and urging said preform into said mold
WO 95/1-172. PCTICA941D0655
- 32 -
for substantially the length of the mold cavity,
where the rate of plug extension combined with the
rate of air evacuation produces a.pressure
differential whereby the drawn and urged preform is
held away from the mold walls, said plug extended
to a point that the preform is not in contact with
the terminal end of the mold; and
g) evacuating the mold, whereby the preform is
drawn into full contact with the cooling walls of
said mold whereby a shaped article is formed.
Following the plug assisted drawing operation, it
may be desirable to further modify the elastomeric
properties of the drawn article by thermal annealing
process. This type of process for a thermoplastic
elastomer is described by Nallicheri, R. A. and Rubner,
24. F., Macromolecules, 190, 23, 1005-1016. Basically,
to remove residual stress: in the vacuum drawn article
and thereby lower modulus, the article is annealed by
heating it at a moderate temperature for an extended
period of time and, optionally, shaping it during
heating by stretching it over a mandrel. In the case of
polyesterurethane article, the annealing process might
be carried out by simply heating, in a convection oven
at 130°C for 4 hours. Generally, an annealing process
might be carried out at a temperature of from 100°C to
140°C for a time period ranging from 1 to 24 hours.
The process described herein is most--advantageously
employed to produce condoms or finger cots. In the case
of condoms a molded article herein will have a length of
from about 125.to about 225 mm and a diameter of from
about 30 to about 50 mm.- In regard to the shape of
~O 95/1472-i PCTICA94f00655
'!
- 33 -
condoms now marketed, the relatively low elastic modulus
of natural rubber at 30% extension, in the range of from
30 to 50 psi allows a very wide range-of the population
to comfortably utilize a single product of uniform
shape. However, persons skilled in the art of condom f
manufacture know that suitable thermoplastic elastomers
now available have a higher elastic modulus than natural
rubber at 30% extension, i.e., in the range of from 80
to 150 psi. Thus, to enable a broader range of the
population to comfortably employ the condom produced
herein, it has been found advantageous to taper the
condom in order to control the location in which the
pressure caused by the strain of the condom during use
is applied. Referring to Figure 6, there is depicted a
tapered condom in one embodiment contemplated herein.
Specifically, the tapered condom herein has an axially
centered tubular body, an open end and an opposite
closed end, with the tubular body having a maximum
diameter at a point along said axis adjacent to said
closed end and with the tubular body having a minimum
diameter at a point along said axis between said point
of maximum diameter to and including the open end. The
ratio of maximum diameter to minimum diameter should
fall within the range of 1.05/1 to 1.4/1 and preferably
between 1.1/1 to 1.25/1. The location of the point of
maximum diameter is preferably immediately adjacent to
the closed end of the condom. The location of the point
of minimum diameter might be anywhere along the tubular
body but preferably is closer-to the open end of the
condom than to the closed end. There may be multiple
points of minimum diameter as where the condom tapers to
a ribbed tubular body or the point of minimum diameter
might comprise a segment of the tubular body with a
constant diameter. - For a condom having an elastic
W095114724 , PCTICA94100655
~1'~'~4'~~
- 34 -
modulus within the range stated immediately above, it
has been found that the most advantageous dimensions for
a condom having utility for-the widest range of the
population are 37 - 40 mm diameter at the minimum
tapered to 40 - 45 mm diameter at the maximum. The
tapered condom may be produced utilizing a tapered mold
in the drawing of the condom and/or by annealing the
condom on a tapered mandrel.
A bead may be incorporated onto the condom at its
open end in order-to insure its retention in use and to
facilitate handling and rolling of the condom. The bead
may be installed by two methods. In the first method,
the condom is placed on a mandrel former, the excess
material trimmed, the film rolled down to form a bead
and the bead fused into a solid ring. In the second
method, a ring bead is prepared in a separate operation
and fastened at the rim of the condom by a thermal
fusion or adhesion process. This second method is
referred to as."importing the bead".
The following example for the manufacture of a
condom is offered by way of illustration and not by way
of limitation.
A~naratus
The equipment herein is divided into two major
components, a preheating oven and a plug assisted vacuum
forming unit. The oven was an insulated metal box with
temperature controlled heating elements and exhaust fan
to provide the desired ambient temperature conditions
within the oven. Also located with the oven, were two
(2) temperature controlled Mid-Infrared (Mid-IR) heaters
~0 95/14724 PCTICA94J00655
'~i ~~~~~
- 35 -
with fused quartz emitter plates and a radiant energy of
40 W/cm~ and each having a radiating surface area of 100
cm2. A track with a servo/computer controlled shuttle
system was provided in the lower part of the oven where
the preforms, mounted in holders, could be horizontally
transported into the oven for heating, and subsequently
horizontally transported through the oven and mounted
onto the vacuum forming unit. The vacuum forming unit
was a female glass mold in the shape of a condom mounted
about a vertical axis with the terminal end of.the mold
downward. The mold had a tapered geometry with the
diameter increasing from the open end towards the
terminal end. The mold had a total length of 260 mm, a
diameter of 38.5 mm at the open end and reach a maximum
diameter of 42 mm. The approximate volume of the mold
was 311 cm'. A single-stage, single-action vacuum pump
was connected to the mold through a vacuum port mounted
on the terminal end at the point where the mold
intersects the vertical axis. The vacuum pump had a
single-action volume of 590 ml, and could thus evacuate
the mold in a single draw of the piston. The position
of the piston was controlled via a computer controlled
servo motor to provide a precise vacuum profile
throughout the forming process. The mold was
encapsulated by a jacket where a liquid was circulated
to control the temperature of the mold at about 15°C. At
the open end of the mold, there were two metal flaps and
ancillary hardware that were capable of combining with
the preform holder and sealably mounting the preform. A
plug assembly comprising a metal rod and a
frustoconically shaped contact element was axially
mounted above the open end of the mold in such a manner
that it was extensible into the mold. The~position of
the contact element was controlled by a computer
W0 95/Id724 PCTICA94/00655
21'~'~ 4 '~ ~
- 36--
controlled servo motor to provide a precise rate of
displacement into the mold and controlled depth in the
mold. The contact element was made of Teflon with a
roughened surface (50 grit) and was axially aligned with
the base upward-and the crown face downward. The
heights of the frustum was 20 mm, base diameter 31.33
mm and crown diameter 35 mm. Axially extending from the
crown face was a contacting projection having a length
of 6 mm.
_ -
Preparation of the Preform ,
A continuous sheet of the polyesterurethane,
Estane~ 58238-032P polyesterurethane (a registered
trademark of The B.F. Goodrich Company), having the
properties of Table I was extruded to a thickness of
from about 1.0 to about 1.9 mm. From this continuous
sheet were stamped circular discs of from about 25 to
about 30 mm in diameter. The discs were conditioned for
at least 24 hours at room temperature and a relative
humidity in the range of from-about 20% to about 30%. A
circular preform was placed onto the metal holder,
mounted horizontally on the guide track and
automatically horizontally transported into the
preheated 110°C oven. The metal holder was such that the
preform disc would sit on a 6.3 mm ledge with a 45 mm
diameter opening such that the bulk of the preform was
exposed on both the top and bottom face to allow rapid
and uniform heating. The preform was then immediately
exposed to the two (2) Mid-IR heaters with a peak wave
length of 28 ~m at a temperature of 750°C. One heater
was located 35;mm above the preform and the other heater
was located 35mm below the preform. The.heaters were
in continuous operation, thus the duration of the IR
~O 95/14724 PCT1CA94100655
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- 37 -
exposure-was controlled by the transport system. The
preform was heated in this manner to a.point where it
began to soften and "sag" under its own weight. This
occurred with a radiant heater exposure time of about 19
seconds. At this point, the temperature of the preform,
as measured by a narrow band (7.92 Vim) infrared
thermometer, was approximately 230°C. After the preform
was exposed to the radiant heaters, the preform was
horizontally transported along the track through the
l0 preheated 110°C oven, in a time of approximately 8
seconds, and into the vacuum drawing apparatus which was
at about room temperature, and sealingly clamped into
position at the mold opening.
~lacuum Drawin~t
The plug assisted vacuum drawing operation was
initiated with initiation of plug extension. The
elapsed time from the moment that the preform left the
oven to the moment that forming began was about 2
seconds. The plug displacement profile was controlled
by computer in the range of from about 30 to about 50
mm/sec. Figure 5 is a representative plot of mold
pressure and plug extension as a function of time during
the vacuum forming. At about, or immediately before,
the moment the plug contacted the hot preform, the
vacuum was applied to act concurrently with the pushing
action of the plug. The vacuum drawn condom was
subsequently removed from the mold and annealed at 130°C
for 4 hours. Figure 7 is a representative plot of
thickness as a function of surface distance from the tip
end fora condom as made above. Figure 8 is a similar
plot for a natural rubber condom produced~from latex by
a dipping process.
