US 3104937 A
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p 1963 H, w. WYCKOFF ETAL 3,104,937
METHOD OF MAKING A BINDING STRAP Original Filed Dec. 8, 1960 3,104,937 r/mrnon OF MAKING A BINDING STRAP Harold W. Wychofi and Glen (lneal, .lr., both of Middletown Township, Delaware County, and Samuel S. Starr, Rose Valley, Pa, assignors to American Viscose Corporation, Philadelphia, Pa, a corporation of Delaware Original application Dec. 8, 1960, Ser. No. 74,569, now Patent No. 3,066,366, dated Dec. 4, 1962. Divided and this application Jan. 4, 1962, Ser. No. 171,547
2 Claims. (Cl. 18- 3%) This invention relates to means for strapping, banding or bundling boxes, packages and the like and unitizing the same, and in its more particular aspects the invention deals with an improved method of making a strap. More especially the invention provides an improved strap formed of selectively molecularly oriented polymeric material which is useful in the manner of the well-known steel band strapping. The strap per se is claimed in U.S. Patent No. 3,066,366, which issued December 4, 1962, on application Ser. No. 74,569, filed December 8, 1960, of which the present application is a division.
Nhile steel band strapping is widely used, it is recognized that for many applications such strapping is not entirely satisfactory. Among the undesirable features of steel band strapping is the fact that it rusts, is seriously corroded by certain chemicals, is dilficult to remove from the package without a special tool, generally has sharp edges which make it somewhat dangerous to apply and remove, is difficult to dispose of when it has served its purpose and has a relatively high weight to strength ratio. In order to overcome some or all of these disadvantages, a number of non-metallic strapping materials have been proposed but these substitute straps have generally been formed of materials which must be assembled in a special way or of multiple elements which are associated together to form a fiat strap having a width considerably greater than its thickness. The flat band or strap-like formation is highly advantageous in that it prevents the binding, which is generally under considerable tension, from cutting into the package and greatly facilitates the fastening of the overlapping ends by means of a simple and quickly applied clamp or seal. These non-metallic straps have been relatively expensive to manufacture and have been considerably larger dimensionally than a steel strap of comparable strength, although in some instances the weight to strength ratio has been lower and therefore superior to steel.
It is an object of the present invention to provide a method of making a non-metallic binding device in the form of a strap having an exceptionally low weight to strength ratio, which is completely waterproof, has high impact strength, is not seriously affected by changes in temperature, is easily removable and disposable, safe to use and economical to manufacture.
A more general object of the invention is to provide a method of making a strap formed of polymeric filmor fiber-forming material which has high tensile strength in the longitudinal direction and is tough and non-brittle in the transverse direction, especially with respect to bending.
A still further object is to provide a method of making a molecularly oriented polymeric binding strap so constructed that a longitudinal split will not affect the overall longitudinal strength.
Another object is to provide a method of making a molecularly oriented polymeric binding strap so constructed that a nick or cut in an edge thereof will not seriously weaken it.
Other and further objects, features and advantages of the invention will become apparent as the description of a preferred embodiment thereof proceeds.
Zildfid? Patented Sept. 24-, 1963 It has been mentioned that the strap of the present invention is formed of a polymeric film-and-fiber-forming material and before describing the invention in detail it is believed desirable to mention certain specific materials and some of the known characteristics of films and fibers formed thereof. For reasons of economic feasibility, the preferred material is polypropylene. However, other polyolefins such as polyethylene may be employed, also polycarbonatcs, polyesters, polyamides such as polyhexamethylene adipamide, polyhexamethylene sebacamide and polycaproamides, acrylic resins, polystyrene and any of a great number of other film-andfiber-forming polymeric materials or copolymers. Such materials are long chain linear polymers and are formed into films and fibers by the melt extrusion process; that is, the molten polymer is extruded through an orifice which is a narrow slit if a film is to be made or a more or less circular opening when making fibers. The techniques of treatment immediately after extrusion, particularly the manner and rapidity of cooling vary considerably depending upon whether a film or a fiber is to be formed and also upon the particular polymer. However, these techniques are fairly well developed and need not be gone into here. Sufiice to say that a newly formed film or fiber is generally referred to as unoriented because the molecules thereof are substantially heterogeneously arranged, this being so even though a certain negligible amount of orientation actually does occur be tween extrusion and solidification. Molecularly unoriented films and fibers are characterized by low tenacity and either a high elongation or brittleness. In the case of fibers, these drawbacks are overcome by stretching or drawing the molecularly unoriented fiber to bring about a uniaxial orientation of the molecules longitudinally of the fiber. With films, the stretching is often done both longitudinally and transversely to cause biaxial orientation. The temperature, speed and extent of the draw of course vary with the polymer and the diameter of the fiber or thickness of the film but again these are techniques generally within the skill of the art and are only mentioned here to point out the fact that molecular orientation is necessary to produce a substantial degree of tensile strength in polymeric films and fibers. With these general observations in mind, the present invention will now be described in detail.
Referring now to the drawing:
FIGURE 1 is a view partially in side elevation and partially in section of the apparatus for making the strap;
FIGURE 2 is an elevational view showing a pair of cooperating rolls which are used to groove the strap;
FIGURE 3 is a perspective view of the strap;
FIGURE 4 is a diagrammatic view showing the effect of stretching upon the strap.
The molten polymer, e.g., polypropylene, is extruded from a hopper 10 through a substantially rectangular orifice into a brine tbath 11 contained in a tank 12. The brine is maintained at a low temperature and in the bath the polymer becomes solidified to form a sheet or band as indicated at 13. The band is led about a roller 14 rotatably mounted in the lower portion of the tank and thence between a pair of driven shaping rolls 15 and 16, the structure and purpose of which will later the eX- plained. From the rolls 15 and 16 the band passes around a roller 17 located in the lower portion of the tank and thence upwardly about a transfer roller 18. The shaping rolls 15 and 16 may be adjustably mounted for vertical movement as a unit so as to provide more or less travel of the band through the bath before it reaches these rolls and likewise the roll 17 may be adjustable vertically to regulate the distance traveled through the bath after the band leaves the shaping rolls. By regulating the temperature of the bath and the time the band is in the bath before reaching the shaping rolls, the band can be caused to reach these rolls in a completely solidified condition or in a condition wherein only the surface areas are solidified and the interior is still molten. At any rate, by the time the band reaches the transfer roller 18 it is completely solidified and is in the form of a relatively wide band of molecularly unoriented polymeric material. As previously mentioned, such a molecularly unoriented band has a low tenacity.
While uniaxial longitudinal orientation of the molecules can be produced by cold drawing, this can best be accomplished at commercially suitable speeds by heating the band to a point somewhere between the softening range and the melting range of the polymer. In order to raise the temperature of the band to the desired stretching \or drawing temperature, it is led from the transfer roller 18 into a warm water bath 19 contained in a tank 20, guide rollers 21 and 22 serving to direct the band through the warm water bath. From guide roller 22 the band passes between a low speed draw roll 23 and nip roll 24 and then between a high speed draw roll 25 and nip roll 26. The low and high speed draw rolls are preferably located quite close together for a reason presently to be explained and may be coated with a material such as rubber.
If the shaping rolls 15 and 16 were omitted, the band as it left the draw roll 25 would have a substantially rectangular cross-section and, due to the stretching which takes place between the draw rolls 23 and 25, the molecules would be uniaxially oriented longitudinally of the band. Such a band would have, because of the uniaxial orientation of the molecules, high tensile strength in the longitudinal direction but would be brittle in the transverse direction so that if folded sharply about the longitudinal axis it would split. It should be mentioned that the band 13, as extruded and as it leaves the stretch rolls 23 and 25, will normally be considerably wider than the final product and that after stretching it is slit by suitable means, not shown, into a plurality of relatively narrow straps. It is in this narrower form that the transverse brittleness is particularly objectionable. Furthermore, when a split develops as the result of an impact or from bending, the material fibrillates at the injured portion and the split does not necessarily extend exactly longitudinally of the strap but may run oil to one side, thus very considerably reducing the overall tensile strength of the strap.
According to the present invention, transverse brittleness and its subsequent deleterious clients are avoided by the use of the shaping rolls 15 and 16. Referring now to FIGURE 2, the roll 15 is provided with a series of cylindrical ribs 27 and the roll 16 is provided with a similar series of ribs 28. Rollers 15 and 16 extend the width of the band 13 and as the band passes therebetween the ribs 27 and 2S indent the opposite faces of the band so as to provide a longitudinally extending series of grooves in the opposite faces of the band. The ribs 27 and 28 are directly opposite one another so that the grooves cut into the opposite faces form the band into alternating thick and thin portions which extend longitudinally therealong. Preferably, the edges of ribs 27 and 28 are slightly rounded as are the portions between adjacent ribs.
When the grooved band is stretched between the draw rolls 23 and 25 the molecules of the thick portions become substantially entirely uniaxially oriented longitudinally of the band and for a reason which :can best be explained in connection with FIGURE 6, the molecules of the thin portions become biaxially oriented.
Referring now to FIGURE 4, a band or strap is represented as having alternating thick square pontions 29 and thin rectangular portions 30. 29 are held between a pair of rolls 31 and 32 so that when the band is stretched in a plane perpendicular to the plane of the paper, the thick portions are unable to move toward one another. Now, suppose that the thick The thick portions portions 29 before drawing, are 3 mm. on a sideand the distance between the squares is 3 mm. Now, if the band is stretched say to nine times its original length, the cross-sectional area of the thick squares 29 must reduce to one-ninth their original area which means that each square will now be 1 mm. on each side. The distance between the centers of adjacent squares 29 does not change because they are firmly held by the rollers 31 and 32, which incidentally would be spring urged toward one another so that they could move closer together as the squares are reduced in size. In FIGURE 4 the squares 29 are shown in dotted lines after their area has been reduced and it will be observed that the thin portions 39, also shown in dotted lines after stretching and which are joined to adjacent squares, have now actually increased in width in addition of course to being considerably reduced in thickness. This transverse stretching of the thin portions 30 is in addition to the longitudinal stretching produced by pulling on the larger portions 29 and results in biaxial orientation of the molecules in the thin portions. The transverse orientation in the thin p rtions does not result entirely from the fact that they are stretched but is accomplished partially through the fact that the thin portions naturally tend to become narrower as they are stretched in the plane transverse to the paper and are constrained from doing so by the fact that they are attached to the thick portions 29 which themselves are prevented from moving closer together by the rollers 31 and 32.
After the band 13 has been stretched, it is slit as aforesaid into desired strap widths and a portion of one of the Y straps is shown in FIGURE 3 at 33 and it will be observed that the Wide band has been slit through two of the thin sections. In FIGURE 3, which incidentally is drawnto an entirely different scale than FIGURE 2, the thick portions of the strap are indicated at 34, the thin portions at 35 and the aforementioned grooves at 36. The thin portions 35 are biaxially oriented for precisely the same reasons as the thin portions 30 in the example described in connection with FIGURE 4. In other words, the draw rolls prevent the thick portions 34 from moving toward One another and since the diameters or peripheries of the thick portions are reduced due to the stretching, the thin portions 35 are necessarily strained sideways which results in biaxial orientation. If the draw rolls 23 and 25 and their associated nip rolls 24 and 26 are spaced far apart, the band width will be maintained only at the draw rolls and that portion between the draw rolls will be able to contract sideways as the band is elongated, but by placing the draw rolls close together this sideways contraction is minimized. Thus to obtain the greatest biaxial orientation of the molecules of the thin portions of the band the draw rolls are placed as close together as possible. Because of the biaxial orientation of the molecules in the thin portions the transverse tensile strength of the strap per unit cross-section is greater in the thin portions than in the :thick portions.
When the strap as thus constructed is bent about its longitudinal axis, the bending naturally takes place in the thin sections and since these sections are biaxially oriented the strap can readily bend along those lines without crackmg. Should an unusual impact result in actually splitting the strap, the split will form in one of the thin sections and since the longitudinal tensile strength of the strap is supplied primarily by the uniaxially oriented thick portions, .a spht in one of the thin portions does not appreciably aifect the holding power of the strap. Furthermore, a
any split which should develop will continue along the thin portion rather than run out through an edge of the strap as it might if the strap were the same thickness all the way across.
The improved transverse toughness can of course be obtained by grooving only one side so as to obtain the necessary alternating thick and thin sections. While the alternating thick and thin sections have been described 5 as being formed by the shaping rolls 15 and 16, it is contemplated that the desired strap configuration could be obtained by extruding the molten polymer through an appropriately shaped nozzle.
Having thus described the invention, what is claimed is: 1. The method of making a binding strap comprising forming a molecularly unoriented strip of polymeric fiberforming material having in transverse cross-section alternating relatively thick and relatively thin portions, stretching said strip longitudinally by exerting a pull on the relatively thick portions whereby the molecules of the relatively thick portions become substantially uniaxially oriented longitudinally of the strip and the molecules of the relatively thin portions become biaxi'ally oriented.
2. The method of making a binding strap comprising extruding a molten polymeric fiber-formin-g material into a quenching bath to form a ribbon having a substantially rectangular cross-section and an unoriented molecular structure, forming a series of longitudinally extending grooves in one face of the ribbon whereby the ribbon is provided with longitudinally extending thick and thin portions alternating transversely thereof, stretching the ribbon longitudinally while preventing any substantial widthwise contraction thereof whereby the molecules of the thick portions become substantially uniaxially oriented longitudinally of the ribbon and the molecules of the thin portions become biaxially oriented.
References Cited in the file of this patent UNITED STATES PATENTS 2,079,584 Hazell et al. May 4, 1937 2,582,294 Stober Jan. 15, 1952 2,728,950 Annesser Jan. 3, 1956 2,750,631 Johnson June 19, 1956 3,051,987 Mercer Sept. 4, 1962
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