Fusible composite binding strap

A sheetform article such as a binding strap and the like which exhibits improved fusibility is a laminar composite made of a crystalline synthetic thermoplastic polymer. The composite has a major thickness portion or base layer of the polymer having a relatively lower average molecular weight and a minor thickness portion of the same polymer but having a relatively higher average molecular weight. The minor thickness portion defines a fusible face of the sheetform article, and all thickness portions of the article have substantially similar planar crystalline orientation.

BACKGROUND 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 inexpensive strapping material 
that has been used for a wide variety of tying and packaging operations. 
For many applications plastic strap is uniquely suited by virtue of the 
inherent elasticity thereof, e.g., for 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 about the package, and thereafter 
joining overlapping 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 deformable 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 inherently low shear strength which restricts the crimping and 
interlocking techniques normally utilized with wrap-around seals. 
As an alternate 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. None of the foregoing joint-forming means are capable of 
producing 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 fusibility 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.e., 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 weight 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 constituting 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 
constituting 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 thermoplastic polymer article of substantially uniform 
cross-section and comprising a laminar composite which has a major 
thickness portion made up of the polymer having a relatively lower average 
molecular weight and at least one minor thickness portion which is made up 
of the same polymer but having a relatively higher average molecular 
weight. Each minor thickness portion of the article has a thickness that 
is less than the thickness of the major thickness portion; however, the 
sum of the thicknesses of the individual 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 constituted of the same polymer type, and both 
portions have substantially similar planar crystalline orientation. 
A fusible binding strap which embodies the present invention likewise is 
formed as a ribbon of an oriented crystalline synthetic thermoplastic 
polymer having a thickness greater than about 10 mils. The binding strap 
has a substantially rectangular 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 
relatively lower average molecular weight and a generally planar surface 
layer contiguous with the base layer, defining 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 along the longitudinal 
dimension of the strap is substantially similar throughout the strap 
cross-section. 
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, polyhexamethylene adipamide (Nylon 66), and polycaprolactam 
(Nylon 6).

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 encircles 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 produced central fused region generally 
is about 0.001 inch (0.025 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 strength of the formed joint 
(joint strength) can be substantially increased, and in an economically 
advantageous manner, by introducing 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 article that has to be joined by means of a fused joint, e.g., 
using friction fusion, hot knife, or similar techniques, can be fabricated 
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, including polymers that are amorphous 
as extruded but which can be converted to a crystalline form by mechanical 
working, e.g., drawing. Crystalline polymers, of course, are those which 
exhibit crystallographic 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 sheetform article embodying the present 
invention should have substantially the same crystallizability, i.e., 
nature and degree of 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 minor thickness portions of the sheetform 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 crystallizable thermoplastic 
polymers that can be used in the practice of this invention are the 
polyesters such as polyethylene terephthalate, copolyesters of 
terephthalic acid and isophthalic acid with cyclohexanedimethanol, and the 
like, the polyolefins such as polyethylene, polypropylene, and the like, 
and the polyamides such as polycaprolactam, polyhexamethylene adipamide, 
polyhexamethylene sebacamide, and the like. 
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 increase in the joint strength that 
is desired. Preferably, the average molecular weight of the polymer in the 
fusible minor thickness portion exceeds the average molecular weight 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 molecular weights, the selection of 
the polymer for practicing the present invention is based on the average 
molecular weight for that polymer. The term "average molecular weight" as 
used herein refers 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 related to the molecular 
weight of the polymer and is usually obtained from experimentally 
determined specific relative viscosity 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 (zero concentration) to obtain the value for the 
intrinsic viscosity. Inasmuch as the slopes of the viscosity-concentration 
curve for the commercially available extrudable polymers in the usual 
solvents therefor are known in the art, it is possible to ascertain the 
intrinsic viscosity 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 to the standard plots 
thereof. 
The melt index of a thermoplastic polymer is also related to its molecular 
weight and viscosity and is an indication 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 230.degree. C. and using a 2160-gram force. 
In the case of polyesters, e.g., polyethylene terephthalate, for 
manufacturing composite binding strap embodying the present invention the 
intrinsic viscosity of the polyester forming the fusible, minor thickness 
portion of the strap preferably is greater than about 0.7 and exceeds the 
intrinsic viscosity of the polymer forming the core portion of the strap 
preferably by at least about 20 percent, and more preferably by at least 
about 50 percent. 
Binder strap or a similar sheetform article of manufacture providing the 
foregoing advantages is illustrated in FIG. 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 
which is made up of the same polymer but having a relatively higher 
molecular weight. Portions 11 and 12 are of substantially the same 
composition but for the molecular weight of the polymer. Minor thickness 
portion 12 provides a generally planar surface layer contiguous with and 
intimately bonded to 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 FIG. 1 can be fabricated using the 
coextrusion assembly schematically depicted in FIG. 2. Extrusion assembly 
15 includes die 16, single-side feed block 17 and extruder adapter 18. The 
polymeric 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 
thickness portion 12 is fed to die 16 from a second extruder (not shown) 
via feed conduit 20. These two melt layers of the same polymer but of 
different average 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. Thereafter the produced laminar sheet of predetermined 
configuration can be hot drawn or otherwise worked to impart the desired 
crystallinity, crystalline orientation, and physical characteristics to 
the finally produced product. 
To produce binder strap of the type illustrated in FIG. 3, i.e., having 
base layer or core 22 flanked on each side by generally planar, contiguous 
surface or facing layers 23 and 24, an extrusion assembly 25 shown in FIG. 
4 can be utilized. More specifically, die 26 is provided with doublesided 
feed block 27 and extruder adapter 28 which together form a unitary 
assembly. Feed conduit 29 is defined by apertures in 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 for 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 relatively higher molecular weight 
on both major faces thereof, the thickness of the facing layers can be 
relatively small because when the strap portions to be sealed are 
overlapped, the total thickness of the desired polymer of relatively 
higher molecular weight that is available for fusion is doubled. 
Also, 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 thickness and subsequently cut to produce binding strap having the 
desired width. 
A weld or joint produced in a loop formed by thermoplastic strap similar to 
the strap produced in FIG. 4 is shown in FIGS. 5A and 5B. The strap is 
provided on both faces thereof with respective minor thickness portions 23 
and 24 of a polymer having a molecular weight at least 20 percent higher 
than the molecular weight of the polymer which constitutes major thickness 
portion 22. The strap loop is formed so that for overlapping strap ends 34 
and 35 the minor thickness portions 23 and 24 are contiguous with one 
another. Upon joining of strap ends 34 and 35 by insertion of a hot 
sealing blade between the contiguous 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 central, 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 FIG. 5A the interfacial region includes 
also some of the polymer of relatively lower molecular weight and in FIG. 
5B the interfacial region is made up only of the polymer having relatively 
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, the original crystalline orientation of the 
polymers present in what ultimately becomes the central fused region of 
the joint is modified or obviated, thus the crystalline orientation of the 
central fused region is usually different than the crystalline orientation 
of the strap portions adjacent thereto. 
The present invention is further illustrated by the following examples. 
EXAMPLE 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 1.1. Coextrusion is 
carried out so that the polymer having the relatively higher intrinsic 
viscosity forms a surface layer about 0.0015 inch thick on one major face 
of the extruded strap. All layers of the extruded strap are crystalline 
and have substantially similar planar crystalline orientation. 
Segments of the produced 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 relatively higher intrinsic viscosity, i.e., molecular 
weight, to the layer of relatively lower intrinsic viscosity, i.e., 
molecular weight, as well as by fusing together both layers of relatively 
higher intrinsic viscosity. 
EXAMPLE II 
Composite Polyethylene Terephthalate Strap 
Polyethylene terephthalate having an intrinsic viscosity 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 inch thick on one major face of the extruded strap. After 
coextrusion, the extruded article is crystallized and oriented to provide 
substantially similar planar crystalline orientation in all 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 Binding Strap 
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. 
Coextrusion is effected so that the polypropylene having the relatively 
lower melt index forms a surface layer about 0.003 inch thick on each side 
of the sheetform article produced after drawing. All layers of the 
produced strap are crystalline and have substantially 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 crystallized 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 0.002 inch thick, and all strap portions have substantially 
the same planar crystalline orientation. 
Control strap of substantially the same overall dimensions is produced in a 
similar manner and with similar planar crystalline orientation, but using 
only polyethylene terephthalate 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 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 following is observed: 
______________________________________ 
Control Strap 
Composite Strap 
______________________________________ 
joint strength, % 
55 80 
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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 coextrusion 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 produced strap 
are crystalline and have substantially similar planar crystalline 
orientation. 
Segments of the produced strap are formed into loops and joined by friction 
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 having the relatively higher intrinsic 
viscosity. 
Control strap having substantially the same overall dimensions and 
crystalline orientation is produced from polyethylene 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 following is observed. 
______________________________________ 
Control Strap 
Composite Strap 
______________________________________ 
joint strength, % 
57 92 
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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.