GB2040792A - Thin web with resiliently deformed edges - Google Patents

Abstract

A web of thermoplastic film material having a thickness of 1 to 50 micrometers has resilient deformations along at least one edge such that, when the web is wound into a roll, the deformations provide the web with improved winding and roll properties. <IMAGE>

Description

SPECIFICATION Thin web with resiliently deformed edges Technical Field - This invention relates to webs of thermoplastic film material having a thinness of about 50 micrometers and less, and an indefinite length with edges along the length resiliently deformed to improve the web winding and handling characteristics, during winding, and to improve the web roll quality and appearance, once wound.

Background Art - U.S. 1,651,744, issued December 6, 1927 on the application of H. E. van Derhoef discloses an international and permanent knurling deformation in the edges of film webs. The knurling is provided in a depth to maintain a permanent separation between adjacent convolutions of rolled film material. It is disclosed that the adjacent layers of film do not 'intercontact' anywhere throughout the roll. A representative degree of permanent film thickening is disclosed to amount to a doubling in the thickness of the film web.

U.S. 3,502,765, issued March 24, 1970 on the application of A. W. Spencer discloses substantially the same edge knurling invention as was disclosed in the previous patent; but, additionally, recognizes that such knurling often results in a fluted or cupped web edge. To overcome that additional and secondary web deformation, it is disclosed to preheat the web edge, thus causing a minute shrinkage, immediately prior to application of knurling wheels. This patent specifically discloses polyethylene terephthalate as a preferred thermoplastic web material and discloses that an exemplary increase in edge thickness by knurling is 50%.

While the disclosed film thicknesses range from about 25 to about 250 micrometers, the knurling accomplished by this process is permanent and is performed with the intent of separating adjacent convolutions of wound film web.

U.S. 4,021,179, issued May 3, 1977 on the application of F. J. Pira et al. discloses knurling the edges of a web of thermoplastic material (particularly polyethylene terephthalate) wherein the knurling wheels are heated to provide an increased depth as compared with the knurling depth using unheated knurling rolls.

This patent is concerned with knurling films generally more than 150 micrometers thick and is concerned with providing a deep and permanent knurled pattern to assure separation of adjacent web convolutions in rolled form. While there is some consideration of the loss of deformation in successive layers of knurled film, the concern is that the knurling depth must be maintained to insure permanent separation of layers and roll stability. The patent teaches that knurling which is not permanent and deep is inadequate and undesirable.

Summary of the Invention - Thin web material is difficult to wind into stable rolls. Rolls of such material tend to telescope and, also, develop dimples and ridges and wrinkles radially around the circumference of the roll. As has been previously stated, stable rolls ofthickweb materials, such as, for instance, on the order of 150 micrometers and upward, have been made by deeply knurling, and thereby permanently thickening, the edges of the web material. However, thin web material subjected to such knurling results in rolls which are ridged and wrinked even more than rolls made from that same thin material without knurling. This invention relates to a treatment for the edge of thin web material to permit manufacture of wound rolls having reduced dimples, ridges, and wrinkles and reduced tendency to telescope at high winding speeds.

According to this invention, there is provided a thin web of thermoplastic film material having an indefinite length and a pattern of resilient deformation parallel to the length. The web is about 50 micrometers thick and less, and is more than 1 micrometer and generally more than about 10 micrometers thick. A wound roll of the web is also provided. The resilient deformation of this invention is impressed into the web only to such a degree that the deformation is flattened and substantially completely removed by the forces of adjacent film layers when the web is wound into a roll.

Description of the Invention - Figure lisa figurative representation of the web of this invention as it is wound into a roll of this invention.

Figure 2a is a figurative cross-sectional representation of the web of this invention without the forces of adjacent film layers.

Figure 2b is a figurative cross-sectional representation of several successive wound film layers of the web of this invention.

Figure 3 is a figurative cutaway representation of a portion of the web of this invention.

Figure 4 is a figurative cutaway representation of a portion of another web of this invention.

None of the Figures is drawn to scale and the magnitude of the resilient deformations is exaggerated to make them more visible.

In Figure 1, an indefinite length of web 10 is wound into wound roll 11 about roll core 12 toyield a layer of wound web for each revolution of the roll. A continuous pattern of resilient deformations 13 is impressed into and parallel with at least one edge of web 10; and, in wound roll 11, the pattern of deformation 13 from successive film layers is superimposed on itself.

The film material of this invention is, as stated, thin; and, also, it is of a nature to accept the resilient deformations of material. The film is generally thermoplastic organic polymeric material and is generally from about 1 to 50 micrometers thick. The film is most often at least 10 micrometers thick, however. While this invention can be practiced on a variety of film materials, including polyolefins, halogenated or not, polyamides, and polyesters, generally, coated or not by other materials, the invention presently finds special utility on films of polyethylene terephthalate. Film webs of polyethylene terephthalate about 50 micrometers thick and less, have, until now, been extremely difficult to wind at high speeds into stable rolls of acceptable appearance.

The thin polyethylene terephthalate web material with resilient deformations of this invention can be wound into rolls at as much as twice the winding rate as can be used for such web material having smooth edges and no edge treatment. Air is entrapped between layers of thin web material during high speed winding and the resilient deformations along web edges cause successive web layers to be held slightly spaced apart for a time adequate to permit the entrapped air to escape. On the other hand, web material with smooth edges is formed into rolls having so-called "slip dimples" caused by entrapped air which cannot escape through sealed-together smooth film web edges. "Slip dimples" have the appearance of blisters formed under the surface layers of wound rolls. Individual slip dimples ordinarily have an initial size of about 2-4 millimeters in diameter and may have a final diameter of 10-15 millimeters at the surface of a finished roll.

The web material of this invention can be wound at high rates into rolls which exhibit smooth, wrinkle-free, surfaces and a greatly reduced risk of telescoping. It is believed that the resilient deformations serve as minute tension springs each of which positively but gently maintains the position of one web edge with respect to adjacent upper and lower web edges in a roll. As a roll is more completely wound, the deformations or dents are more completely removed or flattened until they are substantially completely flattened and the web material is in contact with adjacent upper and lower layers across the complete width of the roll. Web material having smooth edges and wound at a lower rate so as to avoid slip dimples nevertheless has a tendency toward either telescoping, should the winding tension be slightly low, or forming "MD" (machine direction) ridges, should the winding tension be slightly high. As stated above, telescoping is prevented, in the web material of this invention, by the multitude of gentle, resilient, pressure points which each exert a minute radial force on the roll and, thereby, maintain a slight lateral tension on the wound web.

The web material of this invention can be coated and rewound faster than web material with smooth edges and with less blocking between adjacent layers; and the web material of this invention can be coated from edge-to-edge resulting in less waste than in coating web material having knurled edges which cannot be coated.

The reason for occurrence of MD ridges as is mentioned above, is not entirely understood. MD ridges tend to form in wound rolls of thin web material sometime after the rolls are made. For example, MD ridges tend to appear on the outer surface of rolls of thin polyethylene terephthalate film about one to five, usually two, days after the rolls are made, if the film web has smooth edges. Because the reason for occurrence of MD ridges in rolls of smooth-edged film is not understood, the reason for the absence of such ridges in rolls of the web material of this invention is, likewise, not understood. It is in the case, however, that a smooth roll of the thin polyethylene terephthalate web of this invention does not later generate and exhibit MD ridges.

The resilient deformations in thin webs of this invention differ greatly from knurling which is often applied to the edges of webs to maintain a separation of successive web material layers. Knurling, such as is described in some of the prior art discussed previously, is a permanent deformation of a film web generally accompanied by some substantial displacement of material. Knurling, intentionally and actually, results in a permanent thickening of material over the knurled area and causes a substantial and permanent separation of at least the edges of adjacent web layers in a roll. Knurling forces, applied to film webs of only about 1 to 50 micrometers thickness, cause permanent deformations which result in rolls of web material having hard edges at the areas of knurling and severe wrinkling transversely across the roll in the unsupported film material at the center of the roll between the areas of knurling. On this point, the benefit of the present invention is particularly realized when the web width-to-thickness ratio is at least 104.

As stated, the resilient deformations in the web of this invention are substantially completely removed by being flattened by the cumulation of forces encountered in the winding of several successive layers of web over earlier-wound layers. Being resilient, the deformations return when the web is unwound. The returned deformations are, of course, useful in rewinding the web material in the same way as they were useful in the initial winding.

Figures 2a and 2b are figurative representations of the film web of this invention. In Figure 2a, one edge of web 10 is shown in cross section through the pattern of resilient deformations 13. The web 10 is shown out of contact with other web layers and the resilient deformations 13 are present to their fullest extent. In Figure 2b, one edge of several successive layers of web 10 are shown in cross section as they would be on a roll.

The resilient deformations 13 are increasingly flattened from outer layer (10') through successive layers of the roll to inner layer (10"). The adjacent deformations do not normally match and fit into one another and they are not herein shown to do so. It should be mentioned that substantially complete collapse of the resilient deformations may be accomplished after winding several layers onto a roll or it may occur after only a few layers have been wound.

The resilient deformations can be present in a pattern of from as few as one to as many as six rows or more; and the deformations can take the form of dents or grooves or any other equivalent discontinuity of minute proportion. In Figure 3, there is a figurative cutaway representation of the surface of a portion of web 10 of this invention with resilient deformations 14. The deformations 14 can be pressed into web 10 in any pattern of spaced-apart rows and can generally have any shape. The pattern of the deformation or dents is preferably established such that the distance between dents is from about 0.5 to 10 millimeters. While the lower spacing limit is not believed to be especially critical, the upper limit is chosen to maintain a sufficient number of pressure points to obtain the desired performance of the web during winding and in roll form.

Figure 4 is a figurative cutaway representation of the surface of a portion of web 10 of this invention with resilient deformations 16. The deformations 16 can be pressed into web 10 as a pattern of parallel grooves, as specifically indicated in Figure 4, or the grooves can be present in any skewed patterns along the edge of the web. Grooves are considered to be elongated dents and, when they do not intersect, adjacent grooves should preferably be spaced not more than 6 millimeters apart. Where high speed winding is important, it is recommended that any pattern of grooves should provide a path across the pattern free from deformation to permit escape of entrapped air during the winding operation.

The resilient deformations can be applied to a thin web of thermoplastic material by any means which will cause a plastic deformation without substantial lateral displacement of material. For example, even a knurling wheel, such as that described in previously cited U.S. 4,021,179, could be used provided that the force of deformation must be well below that force which would cause permanent deformation and lateral displacement of film material. When a knurling wheel is used, it should be used against a smooth resilient roll so that web material drawn between the wheel and the resilient roll will be pressed into the roll and be only dented rather than permanently deformed by lateral displacement of web material as would occur if the knurling wheel were used against a hard roll or against a mated knurling wheel.

The web of this invention is made by drawing a smooth-edged thin web through a pair of wheels or rolls; -- one member having feet for pressing into the edge of the web, and the other member having a smooth, resilient, surface for accepting the pressures of the footed member. The spacing of the rollers is very carefully controlled so as to maintain the desired, light, force of deformation.

Although the particular preferred forces of deformation will vary with varied conditions and film thicknesses and kinds, it is preferred that when the footed wheel is made such that a total area of about 3-19 square mm contacts the film at any given time, it should do so with a force of about 4'35 newtons, thus providing a pressure of deformation of about 1200-7300 kPa. The above forces and pressures are exerted through the film against a wheel of elastomeric material such as rubber or polyurethane having a resilient hardness of 75-95 as determined on a Shore "A" Durometer Scale or 45-55 as determined on a Shore "D" Durometer Scale.

The resilient deformation of this invention is a deformation of such a small degree that it is difficult to measure. In fact, unless lighting conditions are favorable, the deformations are sometimes difficult even to see. The degree of deformation, while not easily described or measured, is easily determined by simple web winding experiments. Thin film webs are drawn through rollers, as previously described, with the forces of deformation adjusted to be in the range previously described -- much below the forces which would be expected by an experimenter familiar with the knurling of plastic films -- and are then wound into rolls for evaluation. Different materials and different thinnesses require different pressures to obtain the resilient deformation of this invention and simple winding experiments will yield those values.

Best Mode and Industrial Applicability - To demonstrate the invention using polyethylene terephthalate film, the edges of a web 12 micrometers thin and about 2 1/2 meters wide were drawn through pairs of rollers, one at each edge, to press resilient deformations into the web. One of the rollers on each edge was a footed wheel having a diameter of 3 cm and a width of 2.5 cm, and feet of about 0.75 square mm spaced 2 mm apart axially and 2 mm apart radially. Each footed roller had two axial rows of feet which acted against a smooth polyurethane roller having a diameter of 10 cm, a width of 250 cm and a hardness of 90 as determined on the Shore "A" Durometer Scale. The rollers were adjusted such that they exerted about 1250 kPa of total pressure through all of the feet onto the film placed therebetween. That pressure, in the above conditions, on thin polythyleneterephthalatefilm yielded a web of this invention having resilient deformations; and, when wound at a rate in excess of 8 meters per second made stable, unwrinkled rolls. As a comparative test, when that same film, but without resilient deformations, was wound at a rate up to only 6 meters per second, after about 2 days, severe MD ridges appeared on the surface of the roll. When that film was rewound at a rate in excess of 6 meters, it exhibited a tendency to telescope during winding. As an additional comparative test, when that same film, but with edges knurled using a knurling wheel of the same design as that described above except that there were four axial rows of teeth and a knurling force of about 125 newtons (corresponding to a knurling pressure of about 13,500 kPa) and wound at a rate in excess of 6 meters per second, the rolls had extremely hard edges and a loose'and severely wrinkled center and the web wandered from side to side yielding a roll with uneven edges.

Using the same footed roller against the same resilient roller as described above, films of polyethylene terephthalate of this invention having a variety of thinnesses have been made using the following pressures.

Film thinness Pressure (Micrometers) (k Pascal) Minimum Preferred Maximum 12 1048 1247 1446 23 2756 3273 3790 36 4312 4740 5168 50 5636 6435 7234 The preferred pressures indicated above are the pressures which have been found to yield the film exhibiting the most desirable characteristics under the conditions described herein. The maxima represent the pressures beyond which the film product takes on the appearance and handiing characteristics for film having edges herein described as knurled. While the benefits of the resilient deformations of this invention are realized to whatever extent those deformations are present, the minima set out above represent the practical lower limit for pressures usefui to obtain the deformations under the conditions described herein.

Claims (12)

1. A web of thermoplastic film material having a thickness of about 1 to 50 micrometers and an indefinite length with a continuous pattern of resilient deformations along at least one edge parallel to the length.

2. A wound roll of a web of thermoplastic film material having film material with a thickness of about 1 to 50 micrometers and a width-to-thickness ratio of at least 104 wherein the web has a continuous pattern of resilient deformation along at least one edge parallel to the length of the web.

3. The web of Claims 1 or 2 wherein the thermoplastic film material is polyethylene terephthalate.

4. The web of Claims 1 or 2 wherein the film material has a thickness of more than about 10 micrometers.

5. The web of Claims 1 or 2 wherein the pattern of resilient deformation is provided in from one to six rows.

6. The web of Claims 1 or 2 wherein the pattern of resilient deformation is provided by from one to six rows of dents spaced apart such that each dent is from about 0.5 to 10 millimeters from adjacent dents.

7. The web of Claim 1 wherein the resilient deformation is impressed into the web only to such a degree that, when the web is wound into a roll whereby the pattern of resilient deformation is wound onto itself in successive film layers on the roll, the deformation is substantially completely removed by the forces of adjacent film layers.

8. The wound roll of Claim 2 wherein the pattern of resilient deformation is superimposed on itself in successive film layers and wherein the deformation is substantially completely removed by the forces of adjacent film layers.

9. A process for pressing resilient deformations into an edge of a web of thermoplastic film material having a thickness of about 1 to 50 micrometers and an indefinite length comprising, drawing the edge of the web between a pair of rollers, one member of the pair having feet for pressing into the web and the other member of the pair having a smooth, resilient, surface for accepting the pressures of the footed member, the pair of rollers being spaced apart such that the web drawn therethrough is subjected to web deforming forces adequate to create plastic deformation and less than those required for permanent deformation with lateral displacement of thermoplastic material.

10. The process of Claim 9 wherein, after the drawing step, the web is wound into a roll whereby the resilient deformations are wound onto themselves in successive film layers on the roll and are substantially completely removed by forces from adjacent film layers.

11. A process for pressing resilient deformations into an edge of a web of thermoplastic film material having a thickness of about 1 to 50 micrometers substantially as hereinbefore described with reference to the accompanying Drawings.

12. Aweb of thermoplastic film material substantially as hereinbefore described with reference to the accompanying Drawings.