Field of the Invention

The present invention relates to a pressure-sensitive laminate construction, especially to a backing material used in laminate constructions for pressure- sensitive labels. Background of the Invention

One possible and well-known construction of a pressure-sensitive laminate and pressure-sensitive labels or other products made thereof, consists of layers, such as a release liner, also referred as a backing material, and a face material which are laminated together with a pressure-sensitive adhesive layer in between. This kind of construction is also referred to as a labelstock or release-lined laminate hereinafter. Pressure sensitive label is also known as self-adhesive label or self-stick label. The release liner is used to protect the adhesive and allow for efficient handling up to the point where the label face material is dispensed and adhered to a substrate surface. A face material is used as a top layer of the label, also referred to a facestock, and is the layer that is attached to another surface with an adhesive. There exists also release-linerless self-adhesive labelling systems without a protective backing material, but these are special purpose systems not suited for all applications.

The release liner may be, for example, a plastic film or a glassine paper or kraft paper. Release liner is usually coated with a thin layer of a release agent such as silicone. Due to the release properties of the backing material, the release liner can be easily removed to expose the pressure sensitive adhesive for adhering the label to a substrate. The release liner is also referred to simply as liner hereinafter. A general advantage of the plastic film liners is the smoothness of the surface compared to traditional paper liners. Smooth surface of a plastic liner will further generate smoothness of the adhesive coating transferred to the face material. Smooth adhesive layer is benefit, for example, when clarity of transparent labels is preferred. Plastic liners have also better mechanical properties which is an advantage in automatic labelling lines where machinery must dispense and apply labels at high speed. Plastic liners have, for example, reduced occurrence of liner breakages and thus reduced productivity losses on the dispensing and bottling lines.

According to prior art, most commonly used plastic liners are silikonized biaxially oriented polyester films. Typically these liners, even in thickness of 20 microns, exhibit high tensile strength, low elongation and good heat resistance. However polyester liners are higher priced than corresponding paper liners and their recycling is energy consuming process. Silicone will also interfere with the visual appearance causing milkiness of the polyester articles produced from recycled material.

Biaxially oriented polypropylene (BOPP) liners are also used. In comparison to polyester liners, BOPP liners generally exhibit lower tensile strength and higher elongation in the machine direction, particularly at elevated temperatures. Thus, for example, biaxially oriented PP liners tend to shrink excessively during the adhesive drying process.

Summary of the Invention

It is an object of the present invention to provide more economical plastic film used as a release liner for pressure-sensitive labels. It is an object to achieve liner with reduced material costs while at the same time maintaining the adequate properties like mechanical stability of the liner. Another object of the invention is to form release liner which is easily recyclable. These objects can be achieved with release liner described in the present invention.

According to a first aspect of the invention a release liner comprising a machine direction oriented (MDO) plastic film is provided. The release liner is useful for pressure-sensitive laminates and labels made thereof. MDO plastic film may comprise at least one polypropylene homopolymer as a main component and hydrocarbon resin as a minor component. According to a second aspect of the invention a method to produce a release liner for pressure-sensitive label is provided. The method may comprise steps of selecting at least one polypropylene homopolymer as one component and at least one hydrocarbon resin as another component, selecting a relative amount of the at least one hydrocarbon resin in a mixture of the at least one polypropylene and at least one hydrocarbon resin, mixing the at least one polypropylene homopolymer component and the at least one hydrocarbon resin component in said relative amount by melt processing to form a plastic film, orienting said melt processed plastic film in a machine direction, and coating the machine direction oriented plastic film with a releasing agent so as to form a release liner.

According to a third aspect of the invention a method to produce a label is provided. The method may comprise steps of forming a label laminate by combining a release liner comprising a machine direction oriented plastic film including polypropylene homopolymer and hydrocarbon resin with an adhesive layer and a face material, cutting said label laminate so as to form labels. Further embodiments of the invention are presented in the dependent claims.

Minor component may comprise hydrocarbon resin, such as hydrogenated acyclic hydrocarbon resin, saturated aliphatic acyclic hydrocarbon resin, saturated cyclo-aliphatic hydrocarbon resin, or a mixture of two or more thereof.

The release liner may include hydrocarbon resin from 3 to 20 weight-%, preferably from 8 to 15 weight-%. It may further comprise a mineral filler. An amount of mineral filler may be from 2 to 50 weight-%, or from 10 to 30 weight-%.

The plastic film may be a monolayer or multilayer film. The film may be oriented from 5 to 8 times in machine direction. The plastic film may have thickness from 15 to 35 microns, preferably around 20 microns. The plastic film may be coated with a release agent Description of the Drawings

In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 a shows, in a cross-sectional view, a release-lined laminate construction,

Fig. 1 b shows, in a cross sectional view, a pressure sensitive label structure,

Fig. 2a shows, in a cross-sectional view, a pressure sensitive label comprising a multilayer release liner, Fig. 2b shows, in a cross-sectional view, a multilayer release liner comprising a MDO multilayer plastic film.

Detailed Description of the Invention It has been observed that prior art liners have some disadvantages. For example monoaxially oriented polypropylene liners tend to be relatively thick compared to biaxially oriented polyester or polypropylene liners, which increases material costs. They also have relatively poor dimensional stability, even in higher thickness values, such as thickness of greater than 50 microns. Additionally, manufacturing equipment for a biaxial orienting (BO) line is higher in cost than a machine direction orienting (MDO) line.

Hereinafter, preferable embodiments according to the present invention will be described. The embodiments are only exemplary embodiments of the invention and a person skilled in the art recognized readily that they may be combined in various ways to generate further embodiments without deviating from the invention. The drawings are schematic. The particular embodiments described below with reference to the drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims. According to the invention, a machine direction oriented (MDO) plastic film is provided. The MDO plastic film is useful as a release liner for pressure- sensitive laminates and labels made thereof. The MDO plastic film may be a monolayer film, also called as a single layer film. Alternatively plastic film may be a multilayer film comprising two or more plastic film layers. Prior to orientation process a monolayer film may be blown or cast film. The multilayer film may be, for example, co-extruded. During co- extrusion the different layers are formed simultaneously and firmly adhered to each other to provide a unitary coextrudate of multiple layers. It is also possible that at least two monolayer films are laminated together. The monolayers may have different composition and/or different degrees of orientation.. Alternatively MDO film may be top coated to form a MDO multilayer film.

Referring to the Fig. 1 a, pressure sensitive label laminate 1 consists of a release liner 6 and a face material 2 which are laminated together with an adhesive layer 4 in between. The release liner may be made repellent towards the adhesive layer by coating the liner with a release agent, such as a thin layer of silicone polymer. The release liner may be a single layer release liner comprising only one MDO plastic film layer i.e. a monolayer.

Referring to the Fig. 1 b, individual labels 3 may be die-cut from the laminate structure 1 . After cutting, the labels may be attached to a liner 6, which may remain uncut. Thus, plurality of labels may remain attached to a liner 6. Alternatively, the labels 3 may be completely separate, i.e. also the liner 6 may be cut.

Referring to the Fig. 2a, a pressure sensitive label laminate construction 5 and label made thereof may have a multilayer release liner 7. The multilayer release liner 7 may comprise a multilayer plastic film structure comprising two or more layers. The multilayer plastic film is also coated from the surface adjacent to the adhesive layer with a releasing agent in order to achieve release properties for the liner.

Referring to the Fig. 2b, the multilayer release liner 7 may comprise a plastic film having three layers. The multilayer plastic film may comprise layers, such as a core layer 7 ^" , first T top layer and second 7 ^{" '} top layer. The top layers may be placed on opposite sides of the core layer. Top layer may also be only on one side of the core layer. Top layers may also be referred to skin layers. The multilayer plastic film may be coextruded and further oriented so that all the coextrudate layers, core and top layer(s), are oriented thus forming a totally machine direction oriented multilayer plastic film structure. The multilayer plastic film may also have a laminated or top coated structure. In this kind of laminated or top-coated multilayer structure at least one of the plastic film layers is machine direction oriented but it may also comprise non- oriented plastic film layer(s) and the formed multilayer plastic film structure is thus partially oriented. However laminated plastic film multilayer structure may also be a totally machine direction oriented if all the separate plastic film layers laminated together are MDO plastic films. A partially or totally machine direction oriented multilayer plastic film construction may also have five or more layers and may thus include an additional intermediate layer or several layers. Thickness of the core layer 7 ^" may be, for example, approximately 80% or more of the total thickness L and/or weight of the multilayer film. The total thickness L of the liner comprising a multilayer or monolayer MDO plastic film may be approximately from 15 to 35 microns, or preferably around 20 microns.

According to the invention, a release liner comprises a machine direction oriented plastic film including at least one polypropylene (PP) homopolymer as a main component and at least one hydrocarbon (HC) resin as a minor component. Mixing of these components, in order to form non-oriented plastic film, may be done by conventional melt processing techniques, such as blown-film or cast-film extrusion. PP homopolymer consists only propylene as repeating monomer unit. After extrusion, the non-oriented plastic film is stretched in machine-direction orientation process. Thus the film is monoaxially oriented. This oriented plastic film will also be designated as a MDO film. Also other stretching methods may be used. Plastic films may be oriented or stretched approximately from 5 to 8 times in the machine direction. Orientation temperature may be approximately 20-40 °C below the melting temperature of the polypropylene homopolymer. The plastic film may comprise from 3 to 20 weight-% of hydrocarbon resin. Alternatively, the amount of HC may be from 8 to 15 weight-%. Hydrocarbon resin may be amorphous and it may have low molecular weight. For example, weight average molecular weight (M _w ) may be around 1500 Da and M _w /M _n may be around 1 .7. HC may be derived from aromatic petrochemical raw materials. Hydrocarbon resin may be a hydrogenated acyclic hydrocarbon resin, fully hydrogenated to a saturated aliphatic acyclic hydrocarbon resin, saturated aliphatic cyclic (cyclo-aliphatic) hydrocarbon resin compound, or a mixture of two or more thereof.

Polypropylene and hydrocarbon raw materials may be, for example, commercial materials, such as presented in the following Table 1 .

Table 1 . Commercial raw materials

Material/Brand name Company

Polypropylene homopolymer

HC101 BF Borealis

HC1 10BF Borealis

Moplen 1073 Lyondell Basell

PP4352F1 Exxon Mobil

Hydrocarbon resin

Masterbatch MDPPH6040 PolyOne

Masterbatch MDPPH6025 PolyOne

Oppera Exxon Mobil

In a liner comprising a MDO multilayer plastic film at least one layer includes a mixture of polypropylene (PP) homopolymer as a main component and hydrocarbon (HC) resin as a minor component. For example, only the core layer of the multilayer film may comprise such mixture. Other layers, like first and second top layer or intermediate layer(s), may have different film compositions. Other layers may comprise, for example, different polymers and/or one or more additives as a minor component. Alternatively, at least one top layer or intermediate layer may include HC resin. The MDO plastic film may also contain at least one filler material, such as a mineral filler, for example, silica in the form of particles. Other filler materials known in the art are also possible. Filler is preferably used in a core layer of a multilayer film. An amount of filler may be from 2 to 50 weight-% or from 10 to 30 weight-% based on the total weight of the filled polymer Addition of filler component may increase heat stability and/or mechanical stability of the plastic film. A certain degree of heat stability is required, for example, during the printing and die-cutting operation when the label laminate may reach a temperature of 50-60 °C. At this elevated temperature also the elongation of the MDO film should be kept in a minimum. MDO plastic film which is loaded with the filler material will not be perfectly transparent but rather have a milky-white appearance. This kind of appearance will be beneficial especially when said MDO plastic film is used as a release liner for pressure sensitive labels. The milky-white appearance will be an advantage for the optical detection of the labels during the dispensing process.

The advantages of the MDO plastic films according to the invention are to be illustrated in the following examples.

Reference examples 1 and 2 (Ref 1 and Ref 2) are commercially available biaxially oriented polypropylene films (BOPP) having thickness value around 30 μιτι.

Example 3 (Ex 3) is a machine direction oriented multilayer film comprising three layers. The film has been stretched in machine direction and has orientation ratio of 6.5 and a thickness of around 35 μιτι. Core layer comprises 85 weight-% of polypropylene (PP) homopolymer and 15 wt-% hydrocarbon resin (HC). Skin layers comprise 98 wt-% of polypropylene (PP) homopolymer and minor amount of additives. The thickness of the core layer is about 80% from the total thickness of the film.

Example 4 (Ex 4) is a machine direction oriented multilayer film comprising three layers. The film has been stretched in machine direction and has orientation ratio of 7 and thickness of around 35 μιτι. Core layer comprises 85 weight-% of polypropylene (PP) homopolymer and 15 wt-% hydrocarbon resin (HC). Skin layers comprise 98 wt-% of polypropylene (PP) homopolymer and minor amount of additives. The thickness of the core layer is about 80% from the total thickness of the film. All example materials were tested in order to get values for stiffness , tensile strength and elongation in machine direction (MD), and elongation and stiffness values in cross direction (CD). At least five parallel samples were tested. These results are presented in the following Table 2. In the table standard deviation values are presented in parenthesis, n/a refers to not available.

Table 2 Mechanical properties of the films

The plastic films comprising HC resin have, for example increased tensile strength and reduced elongation in the machine direction of the MDO film. Reduced elongation of the plastic film used as a liner for labels is beneficial, among other things, in order to avoid stretching of the liner and to obtain optimum register control during the printing operation as well as exact placement of labels during the dispensing operation.

It is also an advantage, that due to the addition of the HC resin, the thickness L of the MDO polypropylene film can be reduced whilst appropriate mechanical stability, such as stiffness, tensile strength, tear resistance and elongation properties of the film may be maintained. Increased strength properties allow the film to be down gauged to a thickness values of about 15-20 microns whilst maintaining excellent mechanical stability. In the economical point of view, reduced film thickness and thin liners are preferred due to the decreased material costs. Thin label structure allow also more labels to be involved per roll and thus reduced number of roll changes and increasing labelling efficiency.

It is an another advantage, that due to the decreased elongation the MDO plastic film has increased mechanical stability in machine direction of the plastic film. It is beneficial, because of the high tension and stress placed on the film in that direction, during further processes such as printing, die-cutting and dispensing of the label. The liner should not elongate under the high tension and stress placed upon it, even under warm conditions experienced, for example when operating a printing press for lengthy periods. The waste material between the labels (matrix), which is stripped away and reeled up after die-cutting operation, should be strong enough not to break, and the liner should not tear at high-speeds during the dispensing operation when the liner is removed from label and the label is attached to the substrate surface.

It is also an advantage, that the manufacturing equipment for providing machine direction oriented films, i.e. a machine direction orienting (MDO) line, is much lower in cost than the cost of a biaxial orienting (BO) line. MDO line may also be preferred in label industry since special formulations and tailor-made products in relatively small quantities could not be economically produced on a BO-line. It is a further advantage, that the laminates and labels made thereof are environmental friendly. The liner material is recyclable and may be reused, for example in products such as in injection moulded polypropylene articles.

The various aspect of the invention are illustrated by the following examples.

1 . A machine direction oriented plastic film used as a release liner for a pressure-sensitive label, wherein the plastic film comprises a polypropylene homopolymer and a hydrocarbon resin. 2. The machine direction oriented plastic film according to example 1 , wherein the hydrocarbon resin is a hydrogenated, saturated aliphatic acyclic or saturated aliphatic cyclic hydrocarbon compound or mixture of two or more thereof.

3. The machine direction oriented plastic film according to example 1 , wherein an amount of hydrocarbon resin is from 3 to 20 weight-% or from 8 to 15 weight-%.

4. The machine direction oriented plastic film according to example 1 , wherein the film comprises mineral filler.

5. The machine direction oriented plastic film according to example 4, wherein the amount of mineral filler is from 2 to 50 weight-% or from 10 to 30 weight-% based on the total weight of the filled polymer. 6. The machine direction oriented plastic film according to example 1 , wherein the plastic film is a monolayer film.

7. The machine direction oriented plastic film according to example 1 , wherein the plastic film has a totally or partially machine-direction oriented multilayer film structure.

8. The machine direction oriented plastic film according to example 1 , wherein the plastic film has a thickness from 15 to 35 microns, preferably around 20 microns.

9. A method for producing a release liner for pressure-sensitive label, the method comprising at least the following steps:

- selecting at least one polypropylene homopolymer as one component,

- selecting at least one hydrocarbon resin as another component, - selecting a relative amount of the at least one hydrocarbon resin in a mixture of the at least one polypropylene and at least one hydrocarbon resin,

- mixing the at least one polypropylene homopolymer component and the at least one hydrocarbon resin component in said relative amount by melt processing to form a monolayer or multilayer plastic film,

- orienting said plastic film in a machine direction; and - coating the machine-direction oriented plastic film with a releasing agent to form a release liner.

10. The method according to example 9, wherein the plastic films are oriented in from 5 to 8 times in machine direction.