Note: Descriptions are shown in the official language in which they were submitted.
S~19L
Back~r_und of the Invention
This invention relates to fusible sheetform articles,
particularly to fusible plastic binding strap that can be joined
by friction fusion, hot knife techniques, or the like manner.
Plastic strap is a convenient and relatively inexpen-
; sive strapping material ~hat has been used ~or a wide variety of
~ tying and packaying operations. For many applications plastic
; strap is uniquely suited by virtue of the inherent elasticity
thereof, e.g., or tying packages subject to dimensional change, ~ -
or to handling situations whereby shock conditions may be imposed
upon the strap loop that surrounds the package. Tying usually is
accomplished by forming a strap loop about the package, shrinking
or reducing the formed loop to a snug fit a~out the pack~ye, and
thereafter joining o~erlapping ends of the strap loop by means of
a wrap-around seal or a fused joint.
Wrap-around seals for plastic strap are generally formed
in a manner analogous to steel strap, e.g~ by crimping a deform-
able metal band around overlapping strap ends so as to form a
mechanical interlock. Such wrap-around seals are not completely
effective, however, because plastic strap has inhexen~ly low shear
strength which xestricts the crimping and interlocking techniques
normally utilized with wrap-around seals.
As an a}ternate strap sealing approach, strap joints
have been formed by melting and fusing overlapping portions of
thermoplastic strap so as to form a joint. For this purpose
heated pressure jaws, high frequency dielectric heating means, ~
ultrasonic welders, and friction fusion devices have been used. ~;
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None of the foregoing joint-forming means are capable of produc-
ing routinely and consistently, and in an economic manner, a seal
that exhibits a joint strength that is greater than about 40 to
50 percent of the plastic strap tensile strength. It is very
desirable, however, to have joint strengths that approach the
tensile strength of the strap much more closely.
Summary of the Invention
; It has now been found that the usibility of plastic
sheetform articles, such as binding strap and the like, can be
improved by forming the articles from a crystalline synthetic
thermoplastic polymer as a laminar composite in which the lamina
or layers are constituted of the same polymer, i.eO, having the
same repeating unit or units in the structural chain, but of a
different average molecular weight. In the laminar composite,
the polymer on at least one fusible face of the produced article
has a relatively higher average molecular we~ght than the same
polymer in the body of the article so that the ultimately formed
joint is in a fused region which contains the relatively higher
average molecular weight polymer. Stated in another way, the
intrinsic viscosity and relative viscosity of the polymer consti-
tuting the fusible face, or faces, is higher than the intrinsic
viscosity and relative viscosity of the polymer in the body of
the article. If melt index is used as the primary measurement
of the molecular weight, then the melt index of the polymer con-
stituting the fusible face is lower than the melt index of the
polymer in the body of the article.
Accordingly, the present invention contemplates a
sheetform, crystalline synthetic ~hermoplastic polymer article
of substantially uniform cross-section and comprising a laminar - -
composite which has a major thickness portion made up of the
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polymer having a relatively lower average molecular weight and
at least one minor thic~ness portion which is made up of the
same polymer but having a relatively higher average molecular
weight. Each minor thickness portion o~ the article has a
thickness that is less than the thickness of the major thick-
ness portion; however, the sum of the thicknesses of the indi-
vidual minor thickness portions on opposite sides of the
sheetform article may be greater than the thickness of the
major thickness portion. The terms "sheetform" and "sheet"
as used herein and in the appended claims designate an article
of manufacture having a thickness greater than about 10 mils.
The minor thickness portion of the sheetform article
defines a fusible face of the article. Both the major thickness
portion and the minor thickness portion of the article are con-
stituted of the same polymer type, and both portions have sub-
stantially similar planar crystalline orientation.
A fusible binding strap which embodies the present
invention likewise is formed as a ribbon of an oriented crystal~
line synthetic thermoplastic polymer having a thickness greater
than about 10 mils. The binding strap has a substantially rec-
tangular and uniform cross-section which is defined by a pair of
opposed major faces and a pair of opposed minor faces or sides.
The binding strap comprises a base layer of the polymer having
a relativaly lower average molecular weight and a generally
planar surface layer contiguous with the base layer, de~ining
at least one major face of the strap, which is made up of a `
polymer of the same general type but having a relatively higher
average molecular weight than the polymer in the base layer.
The axial crystalline orientation aIong the longitudinal dimen-
sion of the strap is substantially similar throughout the strap
cross-section.
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For the purposes of the present invention, suitable
crystalline synthetic thermoplastic polymers are polyamides,
polyesters, polyolefins, and the like. Preferred polymers for
strapping are polyethylene terephthalate, polypropylene, poly-
hexamethylene adipamide (Nylon 66), and polycaprolactam (Nylon 6).
Brief Description of the Drawing
In the drawing,
FIGURE 1 is a perspective view of a binding strap seg-
ment embodying the present invention;
FIGURE 2 is a schematic representation of an extrusion
assemhly suitable for fabricating the strap illustrated in FIGURE
1 and showing an enlarged portion of the extruded strap;
FIGURE 3 is a perspective view of another binding strap
segment made in accordance with this invention;
FIGURE 4 is a schematic representation of an extrusion
assembly suitable for fabricating the strap illustrated in FIGrJ~,
3 and showing an enlarged portion of the extruded strap; and
FIGURES 5A and 5B are sectional elevations of fused
strap joints formed utilizing a composite strap of the present
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invention.
Description of Preferred Embodiments
- When sheetform thermoplastic polymer articles are joined
to one another, overlapping face portions of the articles are
fused together to define a joint. In the case of thermoplastic
polymer binding strap, a strap segment forms a loop which encir-
cles a package to be bound, and the end portions of the strap
segment are overlapped and fused together at an interface region
therebetween. As a result, a closure joint unitary with the
'~ strap is produced having a relatively thin central or interface
region or layer of fused, i.e., merged and resolidified, strap
surface portions. The average overall thickness of the prod~ced
central fused region generally is about 0.001 inch (0O025 mm) to
about 0.004 inch (0.1 mm) using friction fusion techniques. The
thickness of the fused region is somewhat greater if a hot knife
technique is used.
It has now been discovered that the tensile stxength
of the formed joint (joint strength) can be substantially in-
creased, and in an economically advantageous manner, by intro-
ducing into the central fused region, as a unitary part of the
sheetform article, a polymer having a relatively higher average
molecular weight while surrounding or adjacent unfused strap
regions comprise a polymer having a relatively lower average
molecular weight. This condition can be readily accomplished
by providing the article, e.g., binding strap, on at least one
face thereof, with a unitary facing layer of a polymer having a
relatively higher average molecular weight than that of the
polymer which constitutes the major portion of the article
itself. In this manner the strap, or any other sheetform arti-
cle that has to be joined by means of a fused joint, e.g., using
20 friction fusion, hot knife, or similar techniques, can be fabri- ;
cated primarily of a relatively lower cost, relatively lower
molecular weight polymeric material and still provide improved
joint strength by virtue of the presence of a relatively higher
molecular weight polymeric material which provides a relatively ~;
high-strength joint interface.
To produce sheetform articles embodying the present
invention, any crystallizable thermoplastic polymer the crystals
of which can be oriented by mechanical working can be used, in-
cluding polymers that are amorphous as extruded but which can be
converted to a crystalline form by mechanical working, e.g.,
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drawing. Crystalline polymers, of course, are those which exhibit
cxystallographic reflections when examined with X-rays in a
known manner. The polymers may or may not contain plasticizers
that enhance the processability thereof into sheetform articles.
However, the thermoplastic polymers that constitute the sheet-
form article embodying the present invention should have substan-
tially the same crystallizability, i.e., nature and degree o~
crystallinity that is achieved upon mechanical working after
extrusion should be substantially the same in the major and the
minor thickness portions of the produced article. Thus, it is
preferred that the composition of the extruded polymer mass
forming the major and minoX thickness portions of the sheet-
~orm article be substantially the same except that the molecular
weight of the thermoplastic polymer itself is different in these
portions as stated hereinabove.
Some thermoplastic polymers, such as polyesters, if
solidified in a crystalline state immediately after extrusion,
tend to be brittle and are more difficult to orient by subsequent
mechanical working. Accordingly, in such instances it is
preferable to select the extrusion conditions so that the extruded
composite sheetform article initially solidifies in a substantially
; amorphous state from which it is then subsequently converted to
a crystalline state and oriented during mechanical working~
Illustrative of the types of crystalline or crystal-
lizable thermoplas~ic polymers that can be used in the practice
of this invention are the polyesters such as polyethylene tere-
phthalate, copolyesters of terephthalic acid and isophthalic
acid with cyclohexanedimethanol, and the like, the polyolefins
such as polyethylene, polypropylene, and the like, and the poly-
amides such as polycaprolactam, polyhexamethylene adipamide,polyhexamethylene sebacamide, and the like.
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The difference in the average molecular weights between
the major and the minor thickness portion (or portions) varies
depending on the type of polymer that is used and also on the in-
crease in the joint strength that is desirea. Preferably, the
average molecular weight of the polymer in the fusible minor
thickness portion exceeds the average molecularweight in the core
portion by at least about 20 percent, and more preferably by at
least about 50 percent.
Inasmuch as commercially available polymer supplies are
polydisperse, i.e., the polymer is present in a range of molecu-
lar weights, the selection of the polymer for practicing the pre-
sent invention is based on the average molecular weight for that
; polymer. The term "average molecular weight" as used herein re-
fers to the weight average molecular weight of the crystallizable
polymer in the supply used for practicing the present invention ~; ;
and can be determined according to various techniques known in
the art, e.g., light scattering, ultracentrifugation, and the
like. It is not necessary to make an absolute determination,
rather reliance can be had on other well known expedients such as
a determination of intrinsic viscosity, relative viscosity, or
melt index of the polymer.
The intrinsic viscosity of a polymer is directly re-
lated to the molecular weight of the polymer and is usually
obtained from experimentally determined specific relative vis-
cosity values for a polymer solution ~flow time of the polymer
solution through a capillary viscometer divided by the flow
time of the solvent) at several concentrations of the polymer.
The obtained values are plotted and the resulting curve is
extrapolated to infinite dilution ~ero concentration) to obtain
the value for the intrinsic viscosity. Inasmuch as the slopes
of the viscosity-concentration curve for the commercially avail-
able extrudable polymers in the usual solvents therefor are
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known in the art, it is possible to ascertain ~he intrinsic vis-
cosity of a polymer from a single value of relative viscosity.
Accordingly, it is the customary practice to measure only a
single value of relative viscosity and from the measured value
to ascertain the intrinsic viscosity by referring ~o the stan-
dard plots thereof.
The melt index of a thermoplastic polymer is also
related ~o its molecular weight and viscosity and is an indi-
cation of the amount of the thermoplastic polymer that can be
forced through a given orifice at a specific temperature and
in a given time period using a constant force of known value.
The melt indices reported herein are determined according to
ASTM Standard D1238-73 at 230C. and using a 2160-gram force.
In the case of polyesters, e.g., polyethylene tere-
phthalate, for manufacturing composite binding strap embodying
the present invention the intrinsic viscosity of the polyester
forming the fusible, minor thickness portion of the strap pre-
ferably is greater than about 0.7 and exceeds the intrinsic
viscosity of the polymer forming the core portion of the strap
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preferably by at least about 20 percent, and more preferably
by at least about 50 percent.
Binder strap or a similar sheetform article of manu-
facture providing the foregoing advantages is illustrated in
FIGURE 1. Binder strap segment 10 comprises major thickness
portion 11 which is made up of a crystallizable thermoplastic
polymer, e.g., polyethylene terephthalate, having a relatively
lower molecular weight and a unitary minor thickness portion
12 wh1ch is made up of the same polymer but having a relativel~
higher molecular weight. Portions 11 and 12 are of substan-
tially the s~me composition but for the molecular weight ofthe polymer. Minor thickness portion 12 provides a generally
planar surface layer contiguous with and intimately bonded to
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major thickness portion 11, which forms the base layer of the
strap, and defines a fusible face. Minor thickness portion 12
should be at least about one mil (0~001 inch; 0.025 mm) thick,
and usually comprises about 1 up to about 25 percent of the ;
strap thickness, preferably about 3 to about 20 percent of the
strap thickness.
Binder strap of the type illustrated in FIGURE 1 can
be fabricated using the coextrusion assembly schema~ically
depicted in FIGURE 2. Extrusion assembly 15 includes die 16,
single-side feed block 17 and extruder adapter 18. The poly-
meric material which ultimately forms major thickness portion
11 of the extruded strap is fed to die 16 from a first extruder
~not shown) via feed conduit 19, and the polymeric material
which ultimately forms minor chickness portion 12 is fed to die
16 from a second extruder (not shown) via feed conduit 20.
The3e two melt layers o~ the same polymer but of different aver-
age molecular weight merge within die cavity 21 and exit from
the die orifice, without commingling, as a single melt stream
constituted by distinct melt layers. The melt stream is then
solidified, intimately bonding the coextruded layers to one
another. Preferably the polymer in each thickness portion is
; maintained in an amorphous state upon solidification. There-
ater the produced laminar sheet of predetermined configuration
can be hot drawn or otherwise worked to impart the desired
crystallinity, crystalline orientation, and physical charac- ~- ?
teristics to the finally produced product.
To produce binder strap of the type illustrated in
FIGURE 3, i.e., having base layer or core 22 flanked on each
side by generally planar, contiguous surface or facing layers
30 23 and 24, an extrusion assembly 25 shown in ~IGURE 4 can be ;
utilized. More specifically, die 26 is provided with double-
sided feed block 27 and extruder adapter 28 which together form
a ~nitary assembly. Feed conduit 29 is defined by apertures in
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adapter 28, feed block 27 and die 26, and serves to convey to
die cavity 31 the molten polymeric material which, upon extrusion
and solidification, forms the aforesaid base layer or core 22 of
the extruded strap segment. Feed conduits 30 and 32 are provided
in feed block 27 or supplying the relatively higher molecular
weight polymeric material which ultimately forms surface layers
23 and 24. Streams of molten, relatively higher molecular weight
polymeric material exiting into die cavity 31 from feed conduits
30 and 32 merge without commingling, with the molten polymeric
material exiting from feed conduit 29 so as to produce a single,
three-layer melt stream which is extruded from die cavity 31 and
solidified. The coextruded, multi-layer ribbon of polymeric
material can be hot-drawn, rolled, or otherwise worked to impart
thereto the desired degree of crystallinity and crystalline
orientation.
For binding strap having the polymer of relative~y
higher molecular weight on both major faces thereof, the thick-
ness of the facing layers can be relatively small because when
the strap portions to be sealed are overlapped, the total thick-
ness of the desired polymer of relatively higher molecular weightthat is available for fusion is doubled.
Plso, binding strap can be coextruded as a ribbon which
is reduced to the desired thickness and width dimensions of the
ultimate strap product upon mechanical working; however, it is
usually more expeditious to coextrude a sheet of substantial
width that is mechanically worked to achieve the desired thick-
ness and subsequently cut to produce binding strap having the
desired width.
A weld or joint produced in a loop formed ~y thermo-
plastic strap similar to the strap produced in FIGURE 4 isshown in FIGURES 5A and 5B. The strap is provided on both faces
thereof with respective minor thickness portions 23 and 24 of a
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polymer having a molecular weight at least about20 percent higher
than the molecular weight of the polymer which constitutes major
thickness portion 22. The strap loop is formed so that for over-
lapping strap ends 34 and 35 the minor thickness portions 23 and
24 are contiguous with one another. Vpon joining o~ strap ends
34 and 35 by insertion of a hot sealing blade between the con-
tiguous minor thickness portions 23 and 24, or by rubbing the
thickness portions against one another as in friction fusion
joint-forming techniques, the contiguous regions thereof in the
joint area are softened or molten and, upon cooling while under
pressure, fuse together to form c~ntral, fused interfacial region
36 which is primarily, and in some cases exclusively, constituted
by the polymer of relatively higher molecular weight and which
region is substantially surrounded by the polymer of relatively
lower molecular weight in unfused major thickness portions 22.
In FIGURE 5A the inLG-facial region includes also some of the
po-ymer of relatively lower molecular weight and in FIGURE 5B
the interfacial region is made up only of the polymer having rela-
tively higher molecular weight. The thickness of the central
fused region can be about 1 to about 20 percent of the thickness
of the overlapping strap ends 34 and 35.
For optimum joint strength it is desirable that the
binder strap welding conditions, as well as the thicknesses of
the contiguous minor thickness portions are selected so that
the central fused region is maintained solely within the minor
thickness portions.
During joint formation r the original crystalline
orientation of the polymers present in what ultima~ely becomes
the central fused region of the joint is modified or obviated,
thus the crystalline orientation of thecentral fused region is
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usually different than the crystalline orientation of the strap
portions adjacent thereto.
The present invention i5 further illustrated by the
following examples. -
ExAMæLE I: Composite Polyethylene Terephthalate Strap
One half-inch wide and 0.020 inch thick polyethylene
terephthalate strap is produced by coextrusion and subsequent
crystallization and orientation of polyethylene terephthalate
having intrinsic viscosity of about 0.6 with the same polymer
having intrinsic viscosity of about l.1. Coextrusion is carried
out so that the polymer ha~ing the relatively higher intrinsic
viscosity forms a surface layer about 0.0015 inch thick on one
major face of the extruded strap. A11 layers of the extruded
strap are crystalline and have substantially similar planar cry-
stalline orientation.
Segments of the pl-~d~ced strap are joined utilizing
conventional hot knife techniques to produce joint strengths
in excess of about 80 percent of strap strength. Consistently
high joint strengths are obtained by fusing the layer of rela-
tively higher intrinsic viscosity, i.e., molecular weight, tothe layer of relatively lower intrinsic viscosity, i.e.,
molecular weight, as well as by fusing together both layers of
relatively higher intrinsic viscosit~.
EXAMPLE II: Composite Polyethylene Terephthalate Strap
Polyethylene terephthalate having an intrinsic viscos-
ity of about 0.8 is coextruded with polyethylene terephthalate
having an intrinsic viscosity of about 1.2 to produce, after
drawing, strap about 5/8-inch wide and about 0.020 inch thick
and so that the polyethylene terephthalate having the relatively
higher intrinsic viscosity forms a surface layer about 0.001
3 5
inch thick on one major face of the extruded strap. After coex-
trusion, the extruded article is crystallized and oriented to
provide substantially similar planar crystalline orientation in
Rll layers thereof.
Segments of the produced strap are formed into loops, ~
and the ends thereof are overlapped and joined by friction fusion.
Joint strengths in excess of about 85 percent of strap strength
are obtained.
EXAMPLE III: Composite Polypropylene Bindin~LStrap
Oriented polypropylene binding strap having a thickness
of about 0.030 inch is produced by coextrusion and subsequent
drawing of polypropylene having an average melt index of about
0.2 and polypropylene having an average melt index of about 6
into a sheetform article that is subsequently cut into ribbons
about one half-inch wide and suitable as binding strap. Coextru-
sion is effected so that the polypropylene having the relatively
~oweX melt index forms a surface layer about 0.003 inch thick
on each side of the sheetform article produced after drawing.
All la~ers of the produced strap are crystalline and have substan-
~ially similar planar crystalline orientation.
EXAMPLE IV: Composite Polyethylene Terephthalate Binding Strap
Polyethylene terephthalate having intrinsic viscosities
of about 0.6 and about 1 is coextruded and subsequently crystal-
lized and oriented by drawing under tension so as to produce one
half-inch wide strap having a thickness of about 0.020 inch, and
having minor thickness portion which is a layer of the polymer
having intrinsic viscosity of about 1 on each major face of the
strap. Each of the minor thickness portions in the produced
strap is about 0.092 inch thick, and all strap portions have
30 substantially the same planar crystalline orientation. `~
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Control strap of substantially the same overall dimen-
sions is produced in a similar manner and with similar planar
crystalline orientation, but using only polyethylene terephthal-
ate having an intrinsic viscosity of about 0.6.
Strap segments of each type of produced strap are
superposed so that a face of one segment is contiguous with a
face of the other segment, and are then welded together using a
torsion bar type laboratory friction fusion welder at a welding
time of about 0.004 second and welding pressure of about 10,000
1~ to about 13,000 pounds per square inch (p.s.i.). The produced
welds are contained within the layers of the relatively higher
intrinsic viscosity material.
Upon testing for joint strength, the fol'~owing is
observed:
Control Strap Composite Strap
joint strength, % 55 80
EXAMPLE V: Composite Nylon Binding Strap
Polyhexamethylene adipamide (Nylon 66) binding strap
having a thickness of about 0.020 inch is produced by coextrusion
and subsequent crystallization and orientation of Nylon 66 having
a relative viscosity of about 225 and Nylon 66 having a relative
viscosity of about 50. The coexlrusion is performed so that a
surface layer about 0.004 inch thick and constituted by the Nylon
66 of the relatively higher relative viscosity is provided on
each major face of the produced strap. All layers of the pro-
duced strap are crystalline and have substantially similar planar
crystalline orientation.
Segments of the produced strap are formed into loops
and joined by ~riction fusion so as to produce a weld within
contiguous layers of the Nylon 66 having the relatively higher
relative viscosity. The welds, when tested for joint strength
exhibit a joint strength of about 60 percent of strap strength.
This compares favorably with a joint strength of only about
40~ that is attained under same conditions using Nylon 66 strap
having a relative viscosity of about 50.
EXAMPLE VI: Composite Polyethylene Terephthalate Binding Strap
In a manner similar to Example IV, oriented crystalline
binder strap is produced with each face of the strap defined by
a 0.0036 inch thick layer of the polyethylene terephthalate hav-
ing the relatively higher intrinsic viscosity.
Control strap having substantially the same overall
dimensions and crystalline orientation is produced from poly-
ethylene terephthalate having the relatively lower intrinsic
viscosity (i.e., I.V. = 0.6).
Upon testing for joint strength, welds produced in the
same manner and on the same equipment as in Example IV, the fol-
lowing is observed.
Control Strap Composite Strap
joint strength, % 57 92
The foregoing specification is intended as illustrative
and is not to be taken as limiting. Other variations within the
spirit and scope of this invention are possible and will readily
present themselves to one skilled in the art.
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