REUSABLE NON-ADHESIVE TAPE MADE FROM POLYURETHANE

The present invention relates to a reusable tape, particularly a self-binding polyurethane tape.

BACKGROUND TO THE INVENTION

Adhesive tapes are commonly used to fasten things together. Adhesive tapes typically comprise a strip of flexible material, for example cellophane, with an adhesive coating on one side. The coating adheres to surfaces on contact, bonding the strip of material to the surface.

However, such adhesive tapes suffer from numerous disadvantages. It is common for the adhesive coating to leave a residue behind on the surface after removal. This may be undesirable if the fastening is intended to be temporary as the fastened surface will then require cleaning after removal.

These fasteners are also not reusable, as the coating progressively loses its adhesive properties as dirt accumulates on the coating and the coating is removed or may perish.

They are also unsuitable for applications requiring tension, as they are not typically elastic. An example of such an application may be in fastening together a group of pipes in a bundle. If a pipe is removed from the centre of the bundle or the pipes are not efficiently packed and change their arrangement, then the fastener will lose tension and pipes in the centre of the bundle may fall out.

Adhesive fastening tapes are often unsuitable for use in moist environments, as water may dissolve or otherwise attack the adhesive coating, causing the fastening to fall away.

It is an object of the present invention to reduce or substantially obviate these problems by providing a waterproof reusable tape capable of larger elastic deformation than known fasteners. STATEMENT OF INVENTION

According to a first aspect of the present invention, there is provided a self-binding non- adhesive elastic tape made from solid cast polyurethane.

In use, the tape is wound around a structure to be fixed, and adjacent windings of the tape bond to one another, preventing relative slipping and unwinding of the tape. The elasticity of the tape allows it to be stretched while winding so that tension is present in the tape. This provides a large contact force between adjacent windings of the tape, increasing the factional force present so that slipping does not occur despite the increased tension in the tape. If pressure is applied to the tape, the tape binds to itself more strongly so that the free end of the tape is also retained. This allows the tape to fasten or tension a structure without the need for adhesive, which prevents deposition of residue on the structure and allows the tape to be repeatedly used. The free end of the tape may be tucked underneath one of the windings to secure it. Solid cast polyurethane is a material that provides these properties. It is also waterproof and abrasion resistant.

The tape may be elastically deformable to over 500% of its original length. The more the tape is stretched when winding, the more it can reduce its length while still providing tension. This allows a structure of variable size or shape to be tensioned or fastened, as the tape will continue to conform to the structure and provide tension even if the structure changes size or shape in such a way as to allow the tape to shrink in length. The thickness of the tape may be uniform to within 0.15 mm. This improves the longevity and performance of the tape by causing strain to be evenly distributed in the tape, so that all parts of the tape experience the same wear, other factors excluded.

The tape may have an as-cast surface finish on both sides. This allows the tape to be self-bonding. The as-cast finish is smooth and glossy compared to, for example, a cut or skived finish. This allows the tape to be self-binding as the coefficient of friction between two surfaces of the tape is high.

The polyurethane may be made from methylene diphenyl diisocyanate and butane diol. The tape may have a thickness of between 0.25 mm and 5 mm. Within this range of thicknesses, the tape is strong enough not to snap but can be stretched significantly by hand.

The tape may have a Shore A hardness of between 40 and 80.

According to a second aspect of the invention, there is provided a method of manufacturing a polyurethane tape comprising the steps of

a. introducing liquid polyurethane into the interior of a substantially smooth cylindrical rotating heated drum;

b. allowing the polyurethane to cure into a solid sheet; and

c. cutting the polyurethane to form at least one strip.

The drum may rotate at a first speed while the polyurethane is being introduced and at a second speed after the polyurethane has been introduced, the second speed being higher than the first speed. The first speed is slow so that the polyurethane may be introduced without splashing. The higher second speed provides greater centrifugation of the polyurethane, causing it to form a flat and smooth sheet of substantially uniform thickness.

The drum may rotate at a third speed after the polyurethane has partially cured, the third speed being lower than the second speed. By this point the polyurethane has partially cured in a flat and smooth sheet of substantially uniform thickness, so it is not necessary for the drum to rotate at high speed. However, it is still necessary for the drum to rotate so that the heating elements do not cause localised hot spots and so that enough centrifugal force is applied to the sheet for it to retain its shape. The speed is therefore reduced to save energy and minimise wear to the spinning mechanism. The third speed may be the same as the first speed but this is not necessary. Preferably the third speed is as slow as possible without causing localised hot spots or allowing the sheet to deform. Step (a) may be performed by pumping the liquid polyurethane through a hose attached to a linear actuator, the linear actuator being aligned parallel to the axis of the cylindrical drum and adapted to move the hose over the length of the drum. The linear actuator allows the polyurethane to be introduced into the drum with an even distribution along its length, or the distribution to be otherwise controlled to produce a flat sheet. The actuator is more accurate and precise at achieving this than a human holding the hose would be.

The air- side surface of the polyurethane may be heated after step (c) to eliminate surface bubbles. This improves the quality of the air-side surface finish and improves the strength of the tape, as bubbles would weaken the tape. A gas heat gun may be used for this purpose.

The polyurethane may be formulated to have a gel time of between 3 and 9 minutes. The gel time is preferably 6 minutes. Within this range, the tape may be manufactured efficiently without requiring excessive curing time and there is enough time to transfer the liquid polyurethane from its source to the drum, and for the polyurethane to attain a flat and smooth distribution within the drum, before it gels and is rendered difficult to work.

The polyurethane may be formulated to have a curing time of between 12 and 16 hours. Within this range, optimal curing characteristics are achieved and the tape may be manufactured efficiently without requiring excessive curing time. The polyurethane may be made from methylene diphenyl diisocyanate and butane diol.

A release agent may be introduced into the drum before step (a). This improves the drum- side surface finish of the polyurethane and expedites removal of the polyurethane from the drum.

The release agent may be a silicone release agent.

The drum may be polished. This improves the drum-side surface finish of the polyurethane and expedites removal of the polyurethane from the drum. The drum may be polished by introducing abrasive particles into the rotating drum.

The drum may be provided with a cutting strip, the cutting strip being a linear channel in the surface of the drum extending along the entire length of the drum and filled with polyurethane. This allows the polyurethane sheet to be cut by a blade within the drum to expedite removal of the polyurethane from the drum without unnecessarily blunting the blade on the interior surface of the drum which is likely to be harder than polyurethane.

The drum may be provided with a weight to balance the effect of the cutting strip on the inertia tensor of the drum. The weight may be positioned near the cutting strip. If the drum is not so-balanced, its rotation may become eccentric, resulting in non- uniformity in the polyurethane sheet. Specifically, a thickening of the sheet will occur opposite the cutting strip.

According to a third aspect of the invention, there is provided a method of manufacturing a polyurethane sheet comprising the steps of a. introducing liquid polyurethane into the interior of a substantially smooth cylindrical heated drum rotating at a first speed; b. after the polyurethane has been introduced, changing the speed of rotation of the drum to a second speed, the second speed being higher than the first speed;

c. after the polyurethane has partially cured, changing the speed of rotation of the drum to a third speed, the third speed being lower than the second speed;

d. allowing the polyurethane to fully cure into a solid sheet.

This method allows polyurethane sheets to be manufactured more economically. The first speed is slow so that the polyurethane may be introduced without splashing. The higher second speed provides greater centrifugation of the polyurethane, causing it to form a flat and smooth sheet of substantially uniform thickness. After the polyurethane has partially cured in a flat and smooth sheet of substantially uniform thickness, it is not necessary for the drum to rotate at high speed. The speed is therefore reduced to save energy and minimise wear to the spinning mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which:

Figure 1 shows a section of polyurethane tape wound around an object in perspective view;

Figure 2 shows a polyurethane tape wound around a bundle of pipes in cross-sectional view;

Figure 3 shows a drum for manufacturing polyurethane tape in perspective view; and Figure 4 shows the drum of Figure 3 in cross-sectional view.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to Figure 1 , an embodiment of a self-binding non-adhesive elastic tape is indicated generally at 10. The tape 10 is in the form of a strip of natural width 5 cm and natural thickness 1 mm. In other example embodiments, the natural width may be for example 9mm, 18mm or 42mm, or any similar width. The natural thickness may be around 0.8mm, 1mm, or 1.2mm, but preferably less than 2mm. The tape may be provided in strips of any length. It is found that casting the polyurethane material in thin sheets, less than 2mm thick, preferably around 0.8mm thick, produces a tape with different material properties than if the same precursor materials are used to cast a thicker sheet which is then sliced into thinner sheets. The tape is formed of polyurethane.

The polyurethane has a Shore A hardness of between 50 and 60.

The polyurethane has a 100% Modulus of between 1 MN/m ² and 1.5 MN/m ² . The 100% Modulus is defined as the force needed to stretch the material to twice its original length. The polyurethane has a 300% Modulus of between 1.6 MN/m ² and 3 MN/m ² . The 300% Modulus is defined as the force needed to stretch the material to four times its original length.

The polyurethane has an ultimate tensile strength of between 12 MN/m ² and 20 MN/m ² . The ultimate tensile strength is defined as the force needed to stretch the material until it breaks.

The polyurethane has an elongation at break of between 750% and 1700%. The elongation at break is preferably at least 750%, more preferably more than 1000%, even more preferably more than 1250%. The elongation at break is defined as how much the material can stretch before it breaks, as a percentage of its original dimensions.

The polyurethane is preferably a methylene diphenyl diisocyanate. The polyurethane can have either a polyester or a polyether backbone. The polyurethane is cross linked with butane diol and an independently injected non-mercury-based catalyst.

A silicone additive is preferably added to the polyurethane. The silicone additive makes up between 0.1 % and 5% of the tape by weight. The tape 10 is shown wound around an object 12. Consecutive windings 14 of the tape 10 overlap and contact one another. The tape 10 has been stretched beyond its natural length. This provides tension in the tape 10. The radial component of the tension in each infinitesimal element of the tape 10 is balanced by a contact force exerted on that element by the object 12. The tangential component of tension in infinitesimal element of the tape 10 is balanced by tension in neighbouring infinitesimal elements of the tape 10.

The tensional forces in the free end of the tape 10 are not balanced in this manner because there is no neighbouring element on one side. Instead, the tangential component of tension in the free end of the tape 10 is balanced by a factional force between the free end of the tape 10 and the winding directly underneath it. The coefficient of friction between the free end of the tape 10 and the winding directly underneath it must be large enough that the maximum factional force is greater than the tension in the tape 10. To increase this coefficient of friction, it may be necessary to tuck the free end of the tape 10 underneath the topmost winding (which would otherwise have been the winding directly underneath the free end of the tape 10). Both surfaces of the free end of the tape 10 are then in contact with adjacent windings, and are subject to contact forces which balance the radial component of tension in the upper adjacent winding.

Alternatively, pressure may be applied to the free end of the tape 10 to bond it more strongly to the winding directly underneath it.

The as-cast finish of the surfaces of the tape 10 provides bonding and a high coefficient of friction between successive windings 12 of the tape 10.

Referring now to figure 2, a bundle of pipes wrapped with a tape is indicated generally at 20. The tape 22 extends around a perimeter of the bundle of pipes 24, 26, contacting the pipes 24 at their outer extremities. If the central pipe 26 is removed, the tension in the tape 22 will cause the other pipes 24 to move into the space left by the removal of pipe 26. The tape 22 will reduce in length and continue to conform to the perimeter of the bundle. The tape will still be tensioned as the new smaller length is still greater than the natural length of the tape 22. Referring now to Figure 3, a drum is indicated generally in perspective view at 30. The drum 30 includes a linear actuator 32. The drum 30 rotates at a variable speed. The interior surface 34 of the drum 30 is heated. Referring now to Figure 4, the drum 30 is shown in cross sectional view. The interior of the drum 30 includes a cutting strip 36. The cutting strip 36 is provided by a linear channel in the interior wall of the drum 30 which is filled with polyurethane. The polyurethane filling is flush with the interior surface of the drum 30. The polyurethane filling is softer than the wall of the drum 30. A coating of polyurethane 38 is shown within the drum 30.

In this illustration, the coating 38 includes a bulge 40, which has resulted from unbalancing of the drum 30 due to the replacement of drum material at 36 with lighter polyurethane. A weight 42 may be attached to the drum 30 near the cutting strip 36 to re-balance the drum 30. If the weight is present and is correctly calibrated, the bulge 40 will not form. The weight 42 and bulge 40 are only illustrated on the same figure in the interest of conciseness. A method of manufacturing polyurethane sheets will now be described. The interior surface of the drum 30 is treated with a release agent, for example a silicone release agent. Liquid polyurethane is introduced into the interior of the drum 30 via the hose. The drum spins at a first speed during this process. This is the casting speed. The casting speed is tailored to the polyurethane material and to the properties of the sheet to be cast. If the casting speed is too high, the polyurethane may splash. If the casting speed is too low, then the resulting sheet may not be smooth and flat.

The first speed is between 140 rpm and 180 rpm. An open end of the hose is mounted on the linear actuator 32 within the drum. Another end of the hose is connected to a source of liquid polyurethane, for example a mixer. The polyurethane is made from methylene diphenyl diisocyanate and butane diol. Liquid polyurethane is dispensed out of the open end of the hose into the interior of the drum 30. The flow rate of the liquid polyurethane is constant.

As the liquid polyurethane is poured, the linear actuator 32 causes the open end of the hose to move in the drum 30. The open end of the hose moves parallel to the rotation axis of the drum 30 from one end of the drum 30 to the other. This motion may be repeated a number of times. The surface of the drum 30 rotates past the open end of the hose while the linear actuator 32 moves. When sufficient polyurethane has been dispensed to coat the interior of the drum 30 with a layer of polyurethane of the required thickness, liquid polyurethane ceases to be dispensed. In this embodiment, the polyurethane coating is 0.75 mm thick.

The chemistry of the liquid polyurethane is adapted to provide a gel time of between 3 and 9 minutes. If the polyurethane sets too fast, it may set before it has time to spread to a uniform thickness. If the polyurethane does not set fast enough, it may splash on pouring or be perturbed by resonant vibration of the drum, resulting in a non-uniform finish.

The speed of rotation of the drum 30 now increases to a second speed. This happens approximately 2 minutes after dispensing the polyurethane into the drum is complete. The second speed is high enough for centrifugal effects to dominate over gravity at the interior surface of the drum. This causes the polyurethane to spread into a uniform coating on the interior surface of the drum 30.

The second speed is between 180 rpm and 240 rpm. The drum 30 rotates at the second speed until the polyurethane is partially cured. This typically takes between 10 and 20 minutes. In this embodiment it is 15 minutes.

When the polyurethane is partially cured the speed of rotation of the drum is decreased to a third speed. This is the curing speed. The curing speed is between 140 rpm and 180 rpm. Because the polyurethane has partially cured, it is no longer necessary for the centrifugal effect to be as large as it was when the polyurethane was liquid, as the polyurethane is able to retain its shape. However some centrifugal force is still required to prevent the sheet from collapsing, as the sheet is still flexible. While the drum rotates at the curing speed, further curing takes place. The drum is rotated at the curing speed for between 30 and 40 minutes, preferably for 35 minutes.

In an alternative embodiment of the process, the drum could be rotated at significantly higher speeds. For example, above 800rpm. The drum is heated to assist curing. External heaters are disposed outside the drum, proximal to the curved wall of the drum. The surface of the drum is heated to between 80 and 90 degrees Celsius. The air-side surface of the polyurethane is heated to substantially the same temperature.

The air- side surface of the polyurethane is heated to eliminate air bubbles. A heat gun is passed over the surface. The heat gun could be attached to a linear actuator or could be operated by a worker. When the polyurethane is cured enough to permit cutting and handling, it is removed from the interior of the drum 30. This may be between 20 and 60 minutes after the introduction of the polyurethane into the drum, but is preferably 40 minutes after the introduction of the polyurethane into the drum. The polyurethane is cut with a blade along the cutting strip 36. It is then removed from the drum 30 as a rectangular sheet.

The rectangular sheet is then placed in a cure oven for between 12 and 16 hours. The rectangular sheet is preferably in the cure oven for 16 hours. This completes the curing of the polyurethane. This general type of manufacturing process, in which polyurethane is poured into a mould (in this case a rotating drum) and then heated to cause it to cure with surfaces of the polyurethane during the curing process being either exposed to the air or against the mould (drum), is referred to as a solid cast process. This is to distinguish it from, for example, reaction injection moulding or extrusion. The surfaces obtained by the solid cast process are found to be smooth and self-binding. The mould (drum) is smooth and centrifugation is sufficient to ensure uniform thickness. The centrifugation, combined with the process of exposing the air-side surface to heat (for example through use of a heat gun) ensures that any air bubbles are removed from the polyurethane, to create a final as-cast surface with the desired properties.

A method of manufacturing polyurethane tape will now be described. This method consists of the method of manufacturing polyurethane sheets described above, with the additional step of cutting the rectangular polyurethane sheet into elongate strips. The polyurethane sheet is cut with a blade. The blade may be a fixed blade or an oscillating blade.

This method produces an as-cast finish on both sides of the tape, because the surfaces of the resulting tape retain the finish imparted to them by the solid cast manufacturing process. The edges of the tape obtain a cut finish in this process, but the as-cast finish is retained on the larger surfaces. This would not be the case if the polyurethane were cast as a block and then cut or shaved off into thin sheets. Instead, in the solid cast process, the polyurethane is cast into a form which is the same thickness as the finished tape, so cutting is only required at the edges of each strip of tape. The as-cast finish imparts self-binding properties to the tape, allowing a strong bond to be formed between surfaces of the tape that are in contact with one another, particularly after application of pressure, without the use of an adhesive.

These embodiments are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.