Film stack, film roll and method for preventing overstretching when stretching a film stack

Abstract

The present invention provides a film stack. Specifically, a film stack including a base film layer and a removable surface layer disposed on the base film layer is described. Because the removable surface layer produces haze before irreversibly damaging the base film layer, the film stack can prevent accidental over-stretching when the film stack is stretched together.

Description

Background Art

[0001] Film stacks including an overlay layer are used in graphic film applications to protect the printed or otherwise imaged graphic film, especially inks, and increase its durability.A removable top layer is an outer layer that can be peeled off by the user during or after application. Summary of the invention

[0002] In one aspect, the present invention relates to a film stack. The film stack includes: a base film layer, the base film layer is formed of a first material and has a first main surface and a second main surface; a removable surface layer, the removable surface layer is formed of a second material and is disposed on the first main surface of the base film layer; and an adhesive layer, the adhesive layer is disposed on the second main surface of the base film layer. The base film layer is transparent. The removable surface layer is stiffer than the base film layer. The film stack can be stretched to an elongation of 30% without causing irreversible damage to the base film layer.

[0003] In another aspect, the present invention relates to a film stack. The film stack includes a base film layer and a removable skin layer disposed on a major surface of the base film layer. When the film stack is stretched, the removable skin layer provides a visible indication before irreversible damage to the film layer is achieved.

[0004] In another aspect, the present invention relates to a method for preventing overstretching when stretching a film stack. The method includes providing a film stack including a base film layer and a removable surface layer disposed on the base film layer, wherein when the film is stretched, the removable surface layer becomes hazy before irreversible damage to the base film layer occurs; and stretching the film stack to a degree less than or reaching the degree to which the removable surface layer becomes hazy. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 is a schematic side elevation cross-section of a film stack including a base cover film layer and a removable skin layer.

[0006] Figure 2 is a schematic side elevation cross-section of another film stack including two base cover film layers and two removable skin layers.

[0007] Figure 3A-3C are schematic top plan views of an unstretched film stack, a film stack stretched such that a removable skin layer develops haze, and a film stack in which the hazy removable skin layer is delaminated from the film stack to reveal an underlying base cover film layer.

[0008] Figure 4 is a graph showing load force versus elongation for Example 3 (and its components) and Comparative Example 5. DETAILED DESCRIPTION

[0009] Laminations are used in graphic films to protect or enhance the durability of a printed base graphic film. Laminations can provide physical protection, such as protection from abrasion or environmental conditions. Laminations can help extend the usable or optimal life of a printed graphic film product. Typically, laminations are laminated to a printed base graphic film at the time of printing or after printing.

[0010] The outer surface of the overlay film can determine the overall appearance of the film stack to an observer. For example, a glossy outer surface (with little surface structure) provides a high percentage of specular reflection versus diffuse reflection, and generally provides low haze and high clarity. Other desired external surface structures may be desired; for example, certain overlay films may have a matte structure or other textured surface. For this reason, it is important to retain and protect the surface features of the overlay film. Surface scratches and other defects may affect the final appearance of the film stack and may be annoying to customers or observers. Therefore, a protective surface layer is usually provided on the surface of the film to be removed after lamination or final application, which may require scraping or other physical contact that may damage the exposed surface.

[0011] Additionally, the overlay film may be thin, highly conformable, and therefore difficult to handle. The removable skin layer may impart sufficient rigidity for easier handling. Typically, the removable skin layer may be removed once the overlay film is laminated to the base graphic film, or after the combined film stack is adhered to the desired surface or substrate.

[0012] A paper liner or polymer material formed in a separate step is typically laminated to the base overlay. This typically requires additional adhesive layers and attachment steps, which can add complexity to the manufacturing process. Alternatively, a temporary liner is provided on the front side as protection during the installation step. Paper liners are opaque and do not allow the installer to view the graphics during installation.

[0013] Certain removable skin layers described herein may be formed by blown film extrusion or conventional extrusion processes and may be co-formed with the base film, thereby eliminating the need for a separate adhesive and / or heat lamination process. Such removable skin layers may require a moderate peel force to peel the layer from the base film while remaining securely attached during shipping, handling, and installation.

[0014] Figure 1 is a schematic side elevation cross-section of a film stack including a base cover film layer and a removable skin layer. Figure 1 Included is a film stack 100 that includes a base cover film layer 110 , a removable skin layer 120 , a liner 130 with an optional release layer 132 , a pressure sensitive adhesive 140 , and an optional tie layer 150 .

[0015] The substrate coating layer 110 can be any suitable material and can be formed by any suitable process. In some embodiments, the substrate coating layer includes polyvinyl chloride or is made of polyvinyl chloride. In some embodiments, the substrate coating layer includes polyurethane and cellulose acetate propionate or cellulose acetate butyrate. In some embodiments, the substrate coating layer includes polyester or copolyester, or is made of it. In some embodiments, the substrate coating includes an ionomer resin of a copolymer of ethylene and methacrylic acid, or is made of it. In some embodiments, the substrate coating layer includes ethylene-vinyl acetate (EVA), or is made of it. In some embodiments, the substrate coating layer includes polycarbonate, polypropylene or polystyrene, or is made of it. In some embodiments, the substrate coating layer includes or contains an elastomeric component. Block copolymers and polyamides including polyamides may be suitable. A specific material or a combination of materials may be selected according to its physical, optical and / or rheological properties. In some embodiments, the substrate coating layer may include more than one layer, and the more than one layer includes the same or different materials.

[0016] The substrate coating layer 110 may have any suitable thickness and may be formed by extrusion (such as melt extrusion or blown film extrusion) processes. In some embodiments, the thickness of the substrate coating layer 110 is 25 microns to 100 microns. The substrate coating layer 110 may have a smooth surface structure, or may have a textured surface structure.

[0017] Removable skin 120 can be any suitable material, and can be formed by any suitable material.In some embodiments, removable skin is larger than the rigidity of substrate coating layer.In some embodiments, removable skin 120 can be or comprise polypropylene.In some embodiments, removable skin 120 can be or comprise any other suitable polymer, copolymer or their blend.Removable skin 120 can have any suitable thickness, including the thickness of 10 microns-150 microns.Can select thickness and material to give film stack enough rigidity and keep appropriate peeling force to carry out delamination.In some embodiments, the 90 degree peeling force of delamination can be between 50 grams / inch to 100 grams / inch (about 20 grams / cm to 40 grams / cm).Removable skin can be extruded (melt extrusion or blown film extrusion), solvent casting or coating.

[0018] The removable surface layer 120 has a combination of selected materials and thickness so that when stretched, the surface layer quickly produces haze. Although many materials can produce stretch-induced haze, some materials will show a rapid increase in haze under a certain elongation threshold. For example, the surface layer can increase haze by 50 percentage points or more from 0% to 30%. In some embodiments, the surface layer can increase haze by 60 percentage points or more from 0% to 30%. In some embodiments, the surface layer can increase haze by 70 percentage points or more from 0% to 30%. In some embodiments, the surface layer can increase haze by 80 percentage points or more from 0% to 30%. In some embodiments, the surface layer can increase haze by 90 percentage points or more from 0% to 30%.

[0019] The liner 130 can be provided as a part of the film stack 100 to protect the pressure sensitive adhesive 140 from exposure to the elements, degradation and unintentional adhesion. The liner 130 also allows the film stack 100 to be configured, stored and transported in the form of a roll without self-adhesion. The liner 130 can be any suitable material, generally including paper or polymers (such as polyethylene). In some embodiments, the liner 130 is coated or covered with an optional peeling layer 132, which can enhance the smoothness and / or easy separation between the liner and the pressure sensitive adhesive. The optional peeling layer 132 can be silicone, wax, block copolymer or any other suitable material. In some embodiments, the optional peeling layer 132 can be omitted, or it is not necessary to provide the desired peeling characteristics.

[0020] The pressure sensitive adhesive 140 is disposed between the liner 130 and the substrate lamination layer 120. The pressure sensitive adhesive 140 can be any suitable adhesive, including an acrylic adhesive, an epoxy, or an optically clear adhesive. In particular, for transparent or translucent substrate laminations, the optical characteristics of the adhesive (typically high transmittance, low haze, and high clarity) may be important for maintaining the overall appearance of the film in use. The pressure sensitive adhesive can be extruded (melt extrusion or blown film extrusion), solvent cast, or coated.

[0021] In some embodiments, pressure sensitive adhesive 140 may have enough structure to maintain its shape (that is, non-flowing) within the exposure temperature and humidity range. In some embodiments, adhesive may be completely or partially cross-linked. Adhesive may include pigment, dye or other coloring agents. In some embodiments, the thickness of adhesive may be between 10 microns to 100 microns. In some embodiments, adhesive may include partly embedded microbeads made of materials such as glass, ceramic or polymer resin or their agglomerates kept together with suitable binder materials. In some embodiments, microbeads may be matched with the structured adhesive refractive index and / or be transparent.

[0022] Because the substrate coating layer may not be inherently firmly bonded to the pressure-sensitive adhesive, some embodiments may include an optional bonding layer 150. Bonding layer (also commonly referred to as primer or primer layer) can be any suitable material or composition with any suitable thickness. The selection of bonding layer is to ensure that there is enough adhesion (to prevent interlayer bonding failure) between the substrate coating layer and the pressure-sensitive adhesive. In some embodiments, the bonding layer may include polyamide or copolyamide. Some materials may be used as a barrier layer alternatively or in addition to this, to prevent plasticizer, water, solvent or other contaminants from migrating to the adhesive from the front side of the film stack. The bonding layer can be extremely thin: less than 10 microns thick, less than 6 microns thick, less than 5 microns thick, less than 4 microns thick, less than 3 microns thick, less than 2 microns thick or even less than 1 micron thick. The bonding layer can be solvent cast, coated or even extruded or (with one or more of other layers) co-extruded.

[0023] The film stack 100 can be stretched to a certain extent together as a whole without causing irreversible damage to the substrate coating layer. In some embodiments, the film stack 100 can be stretched to an elongation of 30% without causing irreversible damage to the substrate coating layer. In some embodiments, the film stack 100 can be stretched to an elongation of 50% without causing irreversible damage to the substrate coating layer. In some embodiments, the film stack 100 can be stretched to an elongation of 100%, an elongation of 200% or more without causing irreversible damage to the substrate coating layer. Stretchable helps to apply such films to complex, curved or channeled surfaces with minimal lifting. However, each stretchable film has a limit value at which irreversible plastic deformation occurs and eventually breaks or tears. In some cases, the stress of the overstretched film will cause the film to lift from the surface to which it is adhered. In some cases, the film can produce optical artifacts, such as turbidity.

[0024] It is important to avoid overstretching during use and installation. In the embodiments described herein, when the film stack is stretched, the removable surface layer produces haze when or before the substrate cover film layer is irreversibly damaged optically or physically. In some embodiments, the removable surface layer produces haze when or before any of these layers is irreversibly damaged.

[0025] Thus, the constructions and stacks described herein can be used because they provide a visual indication to the installer or user that the base cover film is approaching the physical limit of its recommended elongation. Because the removable skin layer can be peeled off after application, the user can stop stretching at or around the time haze is observed, proceed with film installation after being warned of approaching irreversible damage, and remove the skin layer with the confidence that the underlying base cover film layer has retained its optical and physical qualities.

[0026] In some embodiments, the film stack described herein is laminated to a substrate graphic film (i.e., a colored or digitally printed decorative film), and the entire stack can still be stretched together. As described elsewhere, the overall film stack may have a recommended or safe elongation limit. In some embodiments, this limit may be 30%. In some embodiments, this limit may be 150%. In some embodiments, this limit may be 100%, 200% or greater. Similarly, the removable surface layer produces haze when or before the substrate overlay layer and / or the substrate graphic film is overstretched to failure.

[0027] Figure 2 is a schematic side elevation cross-section of another film stack including two base film layers and two removable skin layers. Film stack 200 is similar to Figure 1 The film stack 100 in FIG. 1 ; however, Figure 2 An alternative configuration is shown in which two base cover layers are formed and stored on a single liner layer. Film stack 200 includes two removable skin layers 220, two base cover layers 210, two optional tie layers 250, two pressure sensitive adhesive layers 240, and a single liner 230, which may or may not include a release coating on one or both surfaces.

[0028] Figure 2 Each component in Figure 1 As described above, the difference is that the manufacture of a particular film stack can be advantageously adapted to an extrusion process. The formation of two usable film sides with a single liner can provide more economical manufacturing, storage and shipping, and less waste. In addition, when the film stack 200 is stored or configured in the form of a roll, the removable skin layer 220 acts as a protective liner to minimize surface indentations or scratches on the substrate cover film.

[0029] Figure 3A-3C is an unstretched film stack (such as a combination of Figure 1 Schematic top plan views of film stacks described and shown, a film stack stretched so that a removable skin layer develops haze, and a film stack in which the hazy removable skin layer is delaminated from the film stack to reveal an underlying base cover film layer.

[0030] In the first step, Figure 3A A top view of the film stack is shown with the removable skin layer 320 visible. In some embodiments, the removable skin layer 320 is substantially transparent or clear, and the surface of the underlying substrate cover film is visible through the top surface.

[0031] In the second step, Figure 3BA top view of a film stack in which a removable surface layer 320 and any underlying layer (such as a substrate coating 310) are stretched is shown. In some embodiments, this can be an elongation of 130% or even 150%. The removable surface layer produces haze due to stretching. In some embodiments, producing haze means that the haze changes by more than 50 percentage points from unstretched to stretched. In some embodiments, producing haze means that the haze changes by more than 60 percentage points from unstretched to stretched. In some embodiments, producing haze means that the haze changes by more than 70 percentage points from unstretched to stretched. In some embodiments, producing haze means that the haze changes by more than 80 percentage points from unstretched to stretched. In some embodiments, producing haze means that the haze changes by more than 90 percentage points from unstretched to stretched. The substrate coating below does not produce haze, and remains within its recommended stretching range for physical and optical integrity.

[0032] In the third step, Figure 3C A top view of a film stack in which a removable skin layer is layered with a base cover film layer is shown. The removable skin layer 320 retains its haze, but can be easily and cleanly removed to reveal the protected base cover film 310 underneath. The skin layer can be removed before, during, or after final installation on a desired substrate.

[0033] Example

[0034] Material

[0035]

[0036] Preparation of Examples

[0037] Examples 1 to 3 and Comparative Examples 1 to 5 were prepared on a 3-layer blown film line. The line had a custom set 3 layer die with a 2 inch opening connected to 3 independent 3 / 4" Brabender single screw extruders (Brabender GmbH & Co. KG, Kulturstraβe 49-55 47051 Duisburg, Germany). Examples 4, 5 and 6 and Comparative Example 6 were prepared on a seven layer pancake stack die (LF-400 Coex 7 layer model from Labtech Engineering). The airflow to the die was manually controlled to achieve a blow up ratio of approximately 2:1. The bubble was then collapsed approximately ten feet above the die, traversed a roller, and then wound onto a 3 inch core and rolled up. The feed was supplied by 7 independent 20 mm diameter extruders (Labtech Scientific LE20-30 / C HA model single screw extruders).

[0038] The compositions of the Examples and Comparative Examples are listed in the following table.

[0039] 3-layer blown film

[0040]

[0041] 7-layer blown film

[0042]

[0043] Test Method

[0044] 90° peel force measurement

[0045] 90 degree peel test was performed on a peel tester (80-91LAB MASTER peel and adhesion tester) manufactured by Testing Machines, Inc., New Castle, DE 19720 USA, New Castle, Delaware, USA. The sample was conditioned in a CTH (constant temperature 73F and humidity 50%) room for at least 24 hours. The conditioned sample was cut into 2.5 cm (TD, transverse) by approximately 30.5 cm (longitudinal) strips. The sample was laminated to a metal plate using 3M optically clear adhesive 8171, with the ionomer film side facing the adhesive, and the peeling force for peeling the peelable epidermis was measured. The 90 degree peel force (in grams) was measured at a speed of 120 inches / minute.

[0046] Table 3: 90° peel force

[0047] sample Peel force, g Example 4 86 Example 5 92

[0048] Optical haze measurement

[0049] The optical haze of the samples was measured with a Haze-Gard Plus haze meter, commercially available from BYK Instruments, Columbia, Maryland 21046-2729, USA. The samples were cut into 38 mm x 127 mm (1.5" x 5.0"). The optical haze of the unstretched samples was measured and the samples were stretched to 30%. Prior to measurement, these samples were laminated to a clear 3 mm thick 60 mm x 240 mm general purpose acrylic panel using 3M Optically Clear Adhesive 8171. The skin side was on the air side. The haze results are listed in the table below.

[0050] Haze results

[0051] Haze of unstretched sample Haze of 30% stretched sample Increased haze levels Comparative Example 1 9.41 8.93 -0.48 Comparative Example 2 5.95 7.82 1.87 Comparative Example 3 19.3 15.6 -3.7 Comparative Example 4 21.3 21.8 0.5 Example 1 3.52 90.8 87.28 Example 2 3.33 98.4 95.07 Example 3 2.48 98.8 96.32 Comparative Example 5 6.33 5.3 -1.03 Example 4 31.5 93 61.5 Example 5 30.1 98.3 68.2 Comparative Example 6 3.84 3.64 -0.2

[0052] Film tensile properties

[0053] After conditioning the film in a CTH (Constant Temperature and Humidity, 23°C room temperature and 50%) room for 24 hours, the samples were cut into 38 mm x 127 mm (1.0" x 5.0") in the machine direction. Testing was performed using a stretcher (Model: EMSYSU 4242) from Instron, Norwood, MA 02062, at a crosshead speed of 12" / min (304.8 mm / min) with a 2 inch (50.8 mm) gap. Each of the examples was capable of stretching up to 150%.

[0054] Figure 4 Load force (lbs / inch) versus elongation (%) for selected examples and assemblies of examples are shown. In the elongation range used for processing and lamination (low level), the stiffness of the skin layer 420 is greater than the stiffness of the base film layer 410. Similarly, the full construction of Example 3 440 (including the rigid skin layer) is more rigid than Comparative Example 5 430.

Claims

1. A membrane stack comprising: a base coating layer, the base coating layer being formed of a first material and having a first main surface and a second main surface; a removable surface layer formed of a second material and disposed on the first major surface of the base coating layer; and an adhesive layer disposed on the second main surface of the base coating layer; wherein the base coating layer is transparent; wherein the removable surface layer is more rigid than the base coating layer; wherein the film stack can be stretched to an elongation of 30% without causing irreversible damage to the base film layer, wherein the second material is polypropylene, and wherein when the film stack is stretched together, the removable skin layer increases in haze by more than 50 percentage points from unstretched to stretched to 30%.

2. The film stack of claim 1, wherein the base cover film layer remains transparent when the film stack is stretched together.

3. The film stack of claim 1, wherein the removable skin layer has a thickness between 10 microns and 150 microns. 4 . The film stack of claim 1 , wherein the film stack comprises a bonding layer between the adhesive layer and the base cover film layer.

5. The film stack of claim 1, wherein the first material and the second material are extrudable.

6. The membrane stack of claim 1 wherein the first material comprises polyvinyl chloride.

7. The film stack of claim 1, wherein the first material comprises polyurethane.

8. The membrane stack of claim 1, wherein the base cover membrane layer comprises an ionomer.

9. A roll of film comprising the film stack according to claim 1, wherein the film stack is disposed on a liner.

10. A membrane stack comprising: Base coating layer; and a removable skin layer disposed on a major surface of the base film layer, wherein the removable skin layer is formed of polypropylene and when the film stack is stretched together, the haze of the removable skin layer increases by more than 50 percentage points from unstretched to stretched to 30%; Wherein, when the film stack is stretched, the removable surface layer provides a visible indication before irreversible damage is caused to the base cover film layer.

11. The film stack of claim 10, wherein the visible indication is a sudden increase in haze.

12. The film stack of claim 11, wherein the sudden increase in haze is an increase of at least 50 percentage points.

13. A method of preventing overstretching when stretching a film stack, the method comprising: Providing a film stack, the film stack comprising a base film layer and a removable skin layer disposed on the base film layer, wherein when the film stack is stretched, the removable skin layer becomes hazy before irreversible damage to the base film layer occurs, and wherein the removable skin layer is formed of polypropylene, and when the film stack is stretched together, the haze of the removable skin layer increases by more than 50 percentage points from unstretched to stretched to 30%; The film stack is stretched to less than or to a point where the removable skin layer becomes hazy.

14. The method of claim 13, wherein stretching the film stack means stretching to an elongation of 30%.

15. The method of claim 13, wherein stretching the film stack means stretching to an elongation of 50%.

16. The method of claim 13, wherein stretching the film stack comprises stretching the film stack to a point where the haze of the removable skin layer increases by 50 percentage points.

17. The method of claim 13, further comprising the step of removing the removable surface layer.

18. The method of claim 13, comprising the step of further stretching the film stack.

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