WO2023105120A1

WO2023105120A1 - Linerless adhesive label

Info

Publication number: WO2023105120A1
Application number: PCT/FI2022/050816
Authority: WO
Inventors: Johanna KARHU; Bartosz JANICKI; Stefan Antonius Franciscus BON; Emily May BROGDEN
Original assignee: Upm Raflatac Oy
Priority date: 2021-12-10
Filing date: 2022-12-08
Publication date: 2023-06-15

Abstract

This specification relates to a linerless adhesive label (200, 300) comprising activatable pressure sensitive adhesive which is not tacky at conventional storage and transport conditions, but is activatable into tacky form by external energy. The linerless adhesive label (200, 300) comprises a face (201) and an adhesive layer (202) adjoined to the face. The adhesive layer (202) comprises an activatable pressure sensitive adhesive. The activatable pressure sensitive adhesive comprises a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature Tg1 and a second latex comprising a second polymer having a glass transition temperature Tg2. The Tg1 is lower than the Tg2, difference between Tg2 and Tg1 is at least 60 degrees C, and the Tg1 is at least -60 degrees C. The specification also relates to a method of manufacturing a linerless adhesive label web, a labelled item (320) as well as to an activatable pressure sensitive adhesive composition for linerless adhesive labels.

Description

Linerless adhesive label

Technical field

This specification relates to a linerless adhesive label. Particularly, it relates to a linerless adhesive label comprising activatable pressure sensitive adhesive. Further, the specification relates to a labelled item, a method of manufacturing a linerless adhesive label web as well as to an activatable pressure sensitive adhesive composition suitable for linerless adhesive labels.

Background

Linerless labels are of interest when aiming for sustainable labelling solutions, as the elimination of a release liner reduces the waste produced during the labelling process. Traditionally, face of the linerless label is required to be provided with a release coating in order to enable the label roll to be rolled on itself such that the superposed tacky labels do not permanently stick to each other. However, the release coating typically employs silicone, which often complicates the recycling process and may in the near future fall under the class of forever chemicals. Therefore, more sustainable products entirely eliminating the release coating are desired.

Summary

This specification provides a linerless adhesive label comprising activatable pressure sensitive adhesive which is not tacky at conventional storage and transport conditions, but is activatable into tacky form by external energy. Thus the need for a release coating and liner in the overall label design is eliminated. By eliminating the release liners the waste produced during the labelling process is reduced. Additionally, removal of the release coating also removes the use of silicones therein. Therefore, an overall more sustainable product is provided. Further, as the adhesive of the linerless adhesive label disclosed herein is not responsible for causing blocking when the linerless adhesive label web is rolled on itself, the linerless adhesive label web and the labels thereof disclosed herein are suitable for good quality printing. Further, the activatable pressure sensitive adhesive is suitable for both plastic and natural fibre based material as label face material.

According to an embodiment, a linerless adhesive label is provided. The linerless adhesive label comprises a face and an adhesive layer adjoined to the face. The adhesive layer comprises an activatable pressure sensitive adhesive. The activatable pressure sensitive adhesive comprises a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature T_gi and a second latex comprising a second polymer having a glass transition temperature T_g2. The T_gi is lower than the T_g2, difference between T_g2 and T_gi is at least 60 degrees C, and the T_gi is at least -60 degrees C.

According to another embodiment, a method of manufacturing a linerless adhesive label web is provided. The method comprises arranging a face as a substrate and coating the face with an activatable pressure sensitive adhesive. The activatable pressure sensitive adhesive comprises a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature T_gi and a second latex comprising a second polymer having a glass transition temperature T_g2. The T_gi is lower than the T_g2, difference between T_g2 and T_gi is at least 60 degrees C, and the T_gi is at least -60 degrees C. The method further comprises heating the activatable pressure sensitive adhesive coated on the substrate first at a temperature above the T_gi (first heat treatment) and subsequently at a temperature above the T_g2 (second heat treatment).

According to a further embodiment, a labelled item comprising an item and a linerless adhesive label as described above is provided. The linerless adhesive label is attached to a surface of the item via the adhesive layer of the linerless adhesive label.

According to yet another embodiment, an activatable pressure sensitive adhesive composition for linerless adhesive labels is provided. The activatable pressure sensitive adhesive comprises a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature T_gi and a second latex comprising a second polymer having a glass transition temperature T_g2. The T_gi is lower than the T_g2, difference between T_g2 and T_gi is at least 60 degrees C, and the T_gi is at least -60 degrees C.

Brief description of the drawings

Fig. 1 illustrates, by way of an example, an exemplary process for adhesive film formation,

Fig. 2 illustrates, by way of an example, a linerless adhesive label according to an embodiment, and

Fig. 3 illustrates, by way of an example, a labelled item according to an embodiment.

The figures are schematic. The figures are not in any particular scale.

Detailed description

The solution is described in the following in more detail with reference to some embodiments, which shall not be regarded as limiting.

In this description and claims, the percentage values relating to an amount of a material are percentages by weight (wt.%) unless otherwise indicated. Unit of thickness expressed as microns corresponds to pm. Unit of temperature expressed as degrees C corresponds to °C. The following reference numbers and denotations are used in this specification:

Sx, Sy, Sz 3D coordinates

200, 300 linerless adhesive label

201 face

202 adhesive layer

310 item

320 labelled item

A label is a piece of material to be applied onto articles or items of different shapes and materials. An article or an item may be a package. A label com- prises at least a face material also referred to as a face stock or a face. A typical way to adhere the label onto an article or an item is by use of adhesive. The label comprising an adhesive layer is referred to as an adhesive label. The adhesive may comprise pressure sensitive adhesive (PSA). A label comprising pressure sensitive adhesive may be referred to as a pressure sensitive adhesive label. Pressure sensitive adhesive labels may also be referred to as self-adhesive labels.

The labels comprising PSA can be adhered to most surfaces through an adhesive layer without the use of a secondary agent, such as a solvent, or heat to strengthen the bond. In that case the adhesive is pressure sensitive as such. Alternatively, the adhesive may be activatable in order to be pressure sensitive. The PSA forms a bond when pressure is applied onto the label at ambient temperature (e.g. between 15 and 35 degrees C) or for cold applications even under freezing temperatures below 0 degrees C or for hot applications in temperatures above 35 degrees C, adhering the label to the item/article to be labelled. Examples of pressure sensitive adhesives include water-based (water-borne) PSAs, solvent based PSAs and hot-melt PSAs. A label may further comprise other adhesive(s).

Term “face” refers to a top substrate of the label, also called as a face stock, a face material or in case of plastic, i.e. filmic, material a face film. The face may have a monolayer structure or a multilayer structure comprising at least two layers. In case of a plastic material the multilayer filmic structure may be coextruded or it may comprise several layers laminated together. The face is the layer that is adhered to the surface of an article/item during labelling through an adhesive layer. The face comprises an adhesive side and a print side. A combination comprising a face and adhesive may be referred to as an adhesive label. The face may comprise e.g. print in order to provide information and/or visual effect. Printable face is suitable for printing by any known printing methods, such as with gravure, flexographic process, offset, screen or letterpress. The print may exist on a top surface, reverse side or both top and reverse side of the face. Further, the label may contain additional layers, for example top coatings or overlaminates to protect the top surface and/or print of the label against rubbing or other external stress. Coating or additional layers, such as a primer, may enable enhancing compatibility of adjacent layers or parts of the label, for example adhesion between the layers. A label comprising a face, a print layer and an adhesive may be referred to as a printed label.

Within context of this specification the labels are so-called linerless labels. The linerless label comprises a mono- or multilayer face and an adhesive on the face. The linerless labels disclosed herein are free of release coating.

Term “web” refers to a continuous sheet of material. The web is generally processed by moving over rollers. Between processing stages, webs may be stored and transported as rolls or reels.

Labels may be used in wide variety of labelling applications and end-use areas, such as labelling of food, home and personal care products, industrial products, pharmaceutical and health care products, beverage and wine bottles, other consumables etc. Labels enable providing information, like product specification, on the labelled product(s). Information, e.g. print of a label, may comprise human-readable information, like image(s), logo(s), text, and/or machine-readable information, like bar code(s), QR (Quick Response) code(s). The surface of the labelled article/item may be for example plastics, glass, metal, or paper based. The labelled article/item may be for example a container, such as a bottle, jar, canister, can, tin or the like. The label may also be applied to semi-rigid or flexible packages used for e.g. packaging of food.

There are three main properties which are important to consider in formulation of pressure sensitive adhesives: tack, peel and shear strength. Tack is a dynamic value measured as the force required to remove a substrate after a defined contact time and pressure by that substrate. There are multiple ways to measure tack, including the rolling ball, probe tack and loop tack tests. Peel results from the strength of the bond between an adhesive and the substrate. It is expressed as the force required to separate the substrate from the adhesive, per unit width of the sample. There are multiple ways to measure peel: 90° peel, 180° peel, T peel and floating roller peel. Shear strength is referred to as the holding power of the adhesive based on the adhesive’s cohesive strength. This refers to the ability of the adhesive to resist breaking when two plates, stuck together with the adhesive, are pulled in opposite directions.

A high tack and peel imply that a polymer is able to dissipate large amounts of deformation energy during debonding. A greater shear strength often results in reduced tack and peel performance. Thus each PSA has a delicate balance of each of these properties which has been tailored to its application.

An important property related to the adhesives disclosed herein is their blocking resistance. Blocking resistance refers to the ability of the adhesive film to resist bonding when placed on top of a substrate, such as a face material. Good blocking resistance refers to an adhesive which resists bonding and poor blocking resistance refers to an adhesive which cannot resist bonding. As the linerless adhesive labels disclosed herein lack the release coating, it is important that the adhesive shows good enough blocking resistance.

This specification aims to provide a linerless adhesive label comprising activatable pressure sensitive adhesive. As already mentioned, the linerless adhesive label according to this disclosure is free of release coating. The activatable pressure sensitive adhesive disclosed herein is not tacky at conventional storage and transport conditions, but is activatable into tacky form by external energy. Thus, the linerless adhesive label disclosed herein is a representative of so-called “stick on demand” labels. The adhesive of the label disclosed herein may be called a delayed activatable adhesive. Prior to the activation, the adhesive is not responsible for causing blocking when the linerless label web is rolled on itself, even though the linerless label web lacks the release coating. Conventional pressure sensitive adhesives are tacky at conventional storage and transport conditions, thus requiring either a release coating (linerless labels) or a release liner (label laminates), i.e. a liner substrate material coated with a release coating, for preventing the blocking.

The activatable pressure sensitive adhesive disclosed herein has a two phase structure or a 3D macrostructure comprising polymers with low and high glass transition temperatures, which enable forming soft and hard phases. Due to the specific composition and proportion of soft and hard phases a continuous 3D skeleton is formed. The hard phase forms a 3D supportive structure (interlocked mesh) whereas the soft phase is as isles in between the hard phase. The 3D macrostructure inhibits blocking in reel in storage, but on the other hand allows the soft phase to squeeze and flow out and create adhesion to an article when external energy (for example heat) and pressure is used.

The soft phase forms film in common drying temperatures, whereas the hard phase does not. Thus, after a common drying process a second heat treatment is needed wherein the hard phase is film forming and structured as coherent 3D mesh structure.

The linerless adhesive label disclosed herein provides the effect that the need for a release coating and liner in the overall label design is eliminated. Release liners are not part of the product which reaches the final consumer. By eliminating the release liners the waste produced during the labelling process is reduced. Additionally, removal of the release coating also removes the use of silicones therein. Silicones are a substance to be avoided, since they may be responsible for complicating the recycling process and may in the near future fall under the class of “forever chemicals”, i.e. persistent organic pollutants. Thus, an overall more sustainable product is provided.

Moreover, the adhesive disclosed herein is water-based, and water-based adhesives when compared to solvent-based ones provide better sustainability with less fossil-based raw materials and less volatiles involved both during the manufacturing and end use.

The adhesive of the linerless label disclosed herein is not responsible for causing blocking when the linerless adhesive label web is rolled on itself. This means that the label web roll or reel consisting of the superposed label web layers is easily opened for example for printing and subsequent labelling. When opened, fine release is achieved and no adhesive residues or only minor adhesive residues are present on the counter surface, i.e. print side of the face that had been on the roll or reel in contact with the adhesive layer. Therefore, the linerless adhesive label web disclosed herein is suitable for good quality printing. Adhesive composition

The activatable pressure sensitive adhesive disclosed herein is based on two distinct polymer phases. The phase separation is managed with differences in glass transition temperatures, chemistry, morphology and ratio of the polymers.

The activatable pressure sensitive adhesive comprises a binary latex mixture. The activatable pressure sensitive adhesive may consist of the binary latex mixture. However, it may be possible to include a tackifier, especially a tackifier with high softening point, in order to tailor tackiness of the activatable pressure sensitive adhesive. Binary latex mixture refers to a mixture of two latex components. Latex is a stable dispersion (emulsion) of polymer particles in water. Latex solidifies by coalescence of the polymer particles as the water evaporates.

The binary latex mixture disclosed herein includes a first latex comprising a first polymer and a second latex comprising a second polymer. The binary latex mixture may consist of the first latex and the second latex. However, when mixing the first latex and the second latex at least one of a neutralizing agent, a wetting agent, a biocide, a defoamer, a rheology modifier may be added.

The first and second polymers may be acrylate polymers or non-acrylate based polymers. Acrylate polymers are polymerization products, i.e. polymers prepared from monomers including acrylate monomers. Acrylate polymers may consist of acrylate monomer(s) or comprise also non-acrylic comonomer(s). Acrylate polymers are also known as acrylics or polyacrylates. Non-acrylate based polymer may be copolymerized with acrylics. The first and second polymers have incompatible chemistries. Thus, the first and second polymers are non-miscible or have low or limited miscibility.

The first latex comprises the first polymer having a glass transition temperature T_gi. The second latex comprises the second polymer having a glass transition temperature T_g2, and T_gi is lower than T_g2. Glass transition temperature (T_g) refers to a temperature wherein a glass transition for the material occurs. Glass transition refers to a gradual and reversible transition in amorphous materials or in amorphous regions within semicrystalline materials from a hard and relatively brittle ‘glassy’ state into a viscous or rubbery state as the temperature is increased. Typically the glass transition temperature is measured using differential scanning calorimetry (DSC). The glass transition temperature may be measured according to ISO 11357-2 standard.

Within context of this specification the definitions “latex with the polymer of higher/lower T_g” and “latex with higher/lower T_g” are used interchangeably.

The (second) latex with the polymer of higher T_g (T_g2) is so-called ‘hard’ latex. Polymer particles of said latex are responsible for forming hard colloidal particles upon film formation. It is believed that the hard colloidal particles form a 3D interlocked mesh or particle clusters or pillars, thus providing rigidity and support for the adhesive. The polymer with higher T_g comprises monomers with higher T_g. Higher T_g monomers include for example styrene, alpha methyl styrene, Veova 9 (vinyl versatate; vinyl ester of highly branched C9 monocarboxylic acid), isobornyl (meth)acrylate, acrylonitrile, methyl methacrylate, tert-butyl methacrylate, itaconic acid and (meth)acrylic acid. Higher T_g monomers may be responsible for increasing shear strength of the material. The latex with higher T_g may comprise or consist of polystyrene. The hard latex preferably is not pressure sensitive at room temperature (20-25 degrees C). T_g2 is higher than conventional adhesive drying process temperature. For example, T_g2 may be higher than 60 degrees C.

The (first) latex with the polymer of lower T_g (T_gi) is so-called ‘soft’ latex. Polymer particles of said latex are responsible for forming into a tacky fluid, imbibed into the 3D mesh formed by the hard latex particles. The polymer with lower T_g comprises monomers with lower T_g. Monomers with low T_g include for example butyl acrylate (BA) and 2-ethylhexyl acrylate (EHA). Monomers with low T_g typically have high tack. The soft latex preferably is an acrylic emulsion. Preferably the soft latex shows pressure sensitivity at room temperature (20- 25 degrees C). Within context of this specification, T_gi is at least -60 degrees C. For example, T_gi is from -60 to 0 degrees C.

The difference between the glass transition temperatures of the hard and soft latexes, i.e. the difference between T_g2 and T_gi is at least 60 degrees C. The activatable pressure sensitive adhesive composition disclosed herein thus comprises two latex components, one of which has a lower T_g (first latex) and the other having a higher T_g (second latex). Ratio of the first and the second latexes used is of particular importance in providing an adhesive with desired properties. Too much of the second (‘hard’) latex results in a cracked film, whereas too much of the first (‘soft’) latex results in an adhesive having tack already at room temperature. It has been shown that when using at least 20 wt.% (dry) of the latex with higher T_g an adhesive with reasonably good blocking resistance at temperatures up to 50 degrees C and high tack on activation can be reached. Thus, the amount of the latex with higher T_g of at least 20 wt.% (dry) enables the anti-blocking and non-tackiness during storage and transportation of the linerless adhesive labels. On the other hand, the amount of the latex with lower T_g should be at least 40 wt.% (dry) in order to avoid cracking and enable film formation. In other words, the amount of the latex with lower T_g of at least 40 wt.% (dry) is responsible for providing good enough adhesion properties. The amount of the second latex with higher T_g (T_g2) may be from 20 to 60 wt.% (dry). The amount of the first latex with lower Tg (T_gi) may be from 40 to 80 wt.% (dry). Preferably the amount of the second latex with higher T_g is from 30 to 40 wt.%, more preferably from 30 to 34 wt.%, and the amount of the first latex with lower T_g from 60 to 70 wt.%, more preferably from 66 to 70 wt.% (when dry).

The binary latex mixture disclosed herein may have a particle size distribution ranging from 30 to 500 nm, preferably from 50 to 300 nm, more preferably from 100 to 200 nm. It is preferable that particle size distributions of the both latex components of the binary latex mixture are rather comparable. In case the two latex components have extreme particle size distributions the adhesive film formed from such mixture may be less homogeneous and more unpredictable. The above presented particle distribution range is in general suitable for adhesive film formation and enables producing as high solids latex as possible at manageable viscosity. High solid content adhesives are preferred since lower portion of water has to be removed when drying the adhesive, which relates into lower energy consumption. Development and use of high solid dispersion has positive impact on environment since less material has to be transported and as a result carbon footprint is reduced. Use of a high solid content adhesive in label manufacturing enables a more energy efficient, and thus more sustainable overall process.

The activatable adhesive film is formed by heating the adhesive composition first at a temperature above the lower T_g (T_gi) and subsequently at a temperature above the higher T_g (T_g2). For example, the film formation may be achieved by first heating at 70-80 degrees C (first heat treatment) and subsequently heating at a temperature above the T_g2 (second heat treatment). The first heat treatment corresponds to a conventional drying process of the adhesive.

An exemplary process for the adhesive film formation is schematically presented in Fig. 1 . First, the binary latex mixture comprising soft particles and hard particles is formed. The soft particles are from the first latex having a polymer with lower glass transition temperature T_gi. The hard particles are from the second latex having a polymer with higher glass transition temperature T_g2. The binary latex mixture is cast onto a surface and heated at a temperature above T_gi. In an example, the heating is performed at 70-80 degrees C. As illustrated by Fig. 1 , the hard particles then form a 3D interlocked mesh and the soft particles form into a tacky fluid, imbibed into the 3D mesh. Subsequently, the binary latex mixture is heated at a temperature above the hard latex T_g, i.e. above the T_g2. The second heat treatment causes the hard polymer phase to form a film. In an example, the second heat treatment is performed by heating at 130 degrees C.

Examples

In order to form an adhesive with a 3D interlocked mesh structure, two latexes for the binary latex mixture was produced: a hard latex with higher T_g and a soft latex with lower T_g.

General polymerization method for the hard latex is as described below. Reactions were carried out in a 250 ml double walled glass reactor, equipped with an external circulating heating bath, a Teflon anchor type stirrer fitted around 2 cm from the bottom of the reaction vessel, a condenser and a temperature probe. Samples were taken throughout each reaction to analyze conversion and particle size. Lakeland PAE 136 (0.32 g) in water (100 g) was added to the reactor. This was degassed, via nitrogen bubbling, for 30 minutes, along with each of the following in separate round bottom flasks: monomer mixture (95 g), brij L23 (9.2 g) in water (5.6 g) and APS (0.2 g) in water (8 g). Once degassed, the monomer charge (8 g) was added to the reaction vessel. The APS charge (8 ml) was added at time zero. The feeds (monomer and brij solution) began at 20 minutes and fed for 200 minutes. The reaction mixture was stirred throughout at 220 rpm and heated via inbuilt water jacket. The total reaction time was 4 hours.

General polymerization method for the soft latex is as described below. Reactions were carried out in a 250 ml double walled glass reactor, equipped with an external circulating heating bath, a Teflon anchor type stirrer fitted around 2 cm from the bottom of the reaction vessel, a condenser and a temperature probe. Samples were taken throughout each reaction to analyze conversion and particle size.

Lakeland PAE 136 (0.32 g) in water (102 g) was added to the reactor. This was degassed, via nitrogen bubbling, for 30 minutes, along with each of the following in separate round bottom flasks: monomer mixture (90 g), brij (9.2 g) in water (5.6 g) and APS (0.2 g) in water (7.33 g). Once degassed, the monomer charge (8 g) was added to the reaction vessel. The APS charge (8 ml) was added at time zero. The feeds (monomer and brij solution) began at 30 minutes and fed for 90 minutes. The reaction mixture was stirred throughout at 220 rpm and heated via inbuilt water jacket. The total reaction time was 3 hours.

Higher T_g (hard) latex was produced using styrene (polymer T_g of 99.85 degrees C) and methacrylic acid (polymer T_g of 227.85 degrees C) to give a predicted T_g of 102.7 degrees C. The experimental T_g of the hard latex was found to be 98.02 degrees C.

The predicted T_g is calculated by using the Equation 1 (Fox equation) ion 1 )

where T_g, mix and Tg are the glass transition temperature of the mixture and of the components, respectively, and Wi is the mass fraction of component i.

Butyl acrylate based dispersion stabilized by mixed (anionic and non-ionic) emulsifier system was utilized as the lower T_g (soft) latex. The experimental T_g of the soft latex was -40 degrees C. Solids content of the soft latex was 64%.

Solids content of the hard latexes varied from 39 to 45%. The monomer content of the hard latexes was from 38 to 44%. The monomer:water ratio was from 0.6 to 0.8 (g/g).

The binary mixtures of the higher T_g latex and lower T_g latex described were film formed using various component ratios. Each binary latex mixture was film formed at 70 degrees C (10 minutes) and cured at 130 degrees C on a PET (polyethylene terephthalate) surface. Wet film height of 100 pm was used, the predicted dry film height being from 54 to 55 pm. Casting speed of 70 mm/s was used.

When testing for tack and blocking, it was found that a compromise between too much hard latex, resulting in a cracked film, and too much soft latex, resulting in tack at room temperature had to be established. For the studied components it was found that the best compromise was between 30 to 34 wt.% hard particles (higher T_g latex) and 66 to 70 wt.% soft particles (lower T_g latex). The adhesive films fulfilling these conditions showed reasonably low blocking (i.e. good blocking resistance) at 50 degrees C and high tack on activation.

Effect of the curing time on tack and blocking of the adhesive films was studied by preparing adhesive films with hard-soft ratio (higher T_g latex:lower T_g latex) of 30:70 and varying the curing time (10 minutes, an hour and 6 hours). The longer curing time resulted in reduced tack and lower blocking.

Further experiments were performed in order to study the adhesion and blocking of the linerless labels. Coated paper or PET film was used as the label face material. The adhesive composition included 65 wt.% (dry) soft latex and 35 wt.% (dry) hard latex. Adhesion tests were performed by activating the adhesive on PET or PP (polypropylene) substrate by heating with a thermoelement from the label face side for 2-3 seconds. Higher activation temperature (135 degrees C vs. 130 degrees C) was shown to have positive effect on the adhesion. The samples with coated paper face showed good or very good adherence on both the PET and PP substrates. For the samples with PET face, the adherence on PET was shown to be somewhat lower, whereas on PP good or very good adherence was achieved.

Ability of the adhesive to resist blocking was studied for the samples described above by exposing the samples for varying conditions. The samples were prepared by placing two linerless adhesive labels on top of one another, i.e. superposed, such that the adhesive layer of the upper label touches the print side of the face of the lower label. This corresponds to the structure on which the linerless labels are when being on a roll or reel. Test conditions included 1 ) room temperature (23 degrees C), 50% RH (relative humidity), 24 h, under press; 2) 50 degrees C, 24 h, under 4.5 kg; and 3) 50 degrees C, 75% RH, 24 h, under 8 kg. Blocking was shown to be minimal or tolerable for both samples (with paper or PET film faces) under all conditions studied. Tolerable blocking refers to a situation wherein the linerless adhesive label shows fine release when opened, i.e. when separating the upper and lower labels, and there are only some adhesive residues on the counter surface.

Further blocking tests were performed in order to study the effect of the second heat treatment on the blocking resistance of the samples. Besides the samples described above, reference samples were used that were prepared by not exposing the linerless label to the second heat treatment (at a temperature above the T_g2) in the manufacturing process. Blocking was tested under the following conditions: 50 degrees C, 75% RH, 24 h, under 16 kg. Both reference samples (with paper face or PET face) showed full blocking under the tested conditions. The sample could not be separated from the test surface. This emphasizes the importance of the second heat treatment in the manufacturing process. The samples prepared according to the manufacturing process disclosed in this specification in principle showed minimal or tolerable blocking. After blocking tests the counter surfaces on which the blocking had been studied were printed and the printing quality was investigated. Good printing quality was achieved in all samples wherein the printing was possible, i.e. the linerless label was detachable from the counter surface (no blocking).

Linerless adhesive label

Fig. 2 illustrates a side view in the S_x,S_z-plane of a linerless adhesive label 200 according to this disclosure. The linerless adhesive label 200 comprises a face 201 and an adhesive layer 202 adjoined to the face. The face 201 may comprise or consist of plastic material or natural fibre based material, such as paper.

The adhesive layer 202 comprises or consists of activatable pressure sensitive adhesive as described above. Thus, the activatable pressure sensitive adhesive comprises a binary latex mixture including a first latex comprising a first polymer having a T_gi and a second latex comprising a second polymer having a T_g2.

The linerless adhesive label disclosed herein may be used for providing a labelled item 320, as is illustrated in Fig. 3. The labelled item 320 comprises an item 310 and a linerless adhesive label 300 disclosed above. The linerless adhesive label 300 is attached to a surface of the item via the activatable pressure sensitive adhesive of the linerless adhesive label. When attaching the linerless adhesive label 300 to the surface of the item, the label is exposed to external energy in order to activate the adhesive. The item 310 may be for example a beverage bottle.

Linerless adhesive label web comprises multiple linerless adhesive labels disclosed herein. The linerless adhesive label web is manufactured by arranging a face as a substrate and coating the substrate with activatable pressure sensitive adhesive. The manufacturing method includes two heat treatments: heating first at a temperature above the lower T_g (T_gi) (first heat treatment) and subsequently at a temperature above the higher T_g (T_g2) (second heat treatment). The first heat treatment is performed at a temperature that is lower than T_g2. For example, the first heat treatment may be performed at 70-80 degrees C. In an example the second heat treatment may be performed at 130 degrees C.

Prior to labelling, linerless adhesive labels are formed from the linerless adhesive label web by cutting.

The labelling process includes activation of the adhesive by external energy. External energy may be provided for example by utilizing radiation, convection or conduction, and/or by mechanical treatment, such as pressure. External energy by provided by radiation may include heat radiation, UV radiation, IR radiation, visible light, or other suitable radiation in order to activate the adhesive. When heat is employed for activating the adhesive, either by way of radiation, convection or conduction, the linerless adhesive label may be exposed to a temperature of at least 100 degrees C, for example 105 degrees. Naturally, the choice of the external energy to be used in activation depends on the adhesive composition and the face material of the linerless label, as well as on the nature of the item to be labelled. Activation of the adhesive may be achieved using more than one kind of external energy. For example, activation may be achieved using heat and pressure.

Claims

Claims:

1. A linerless adhesive label (200, 300) comprising a face (201 ) and an adhesive layer (202) adjoined to the face, the adhesive layer (202) comprising an activatable pressure sensitive adhesive, wherein the activatable pressure sensitive adhesive comprises a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature T_gi and a second latex comprising a second polymer having a glass transition temperature T_g2, the T_gi being lower than the T_g2, difference between T_g2 and T_gi being at least 60 degrees C, and the T_gi being at least -60 degrees C.

2. The linerless adhesive label (200, 300) according to claim 1 , wherein an amount of the second latex is at least 20 wt.% of the binary latex mixture when dry.

3. The linerless adhesive label (200, 300) according to claim 1 or 2, wherein an amount of the first latex is at least 40 wt.% of the binary mixture when dry.

4. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the activatable pressure sensitive adhesive is non- tacky at temperatures up to 50 degrees C.

5. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the activatable pressure sensitive adhesive is activatable by external energy.

6. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the second polymer includes monomer(s) selected from styrene, alpha methyl styrene, vinyl versatate, isobornyl (meth)acrylate, acrylonitrile, methyl methacrylate, tert-butyl methacrylate, itaconic acid and (meth)acrylic acid.

7. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the second polymer is polystyrene.

8. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the T_g2 is higher than 60 degrees C.

9. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the first latex is an acrylic emulsion.

10. The linerless adhesive label (200, 300) according to any of the preceding claims, wherein the binary latex mixture has a particle size distribution ranging from 30 to 500 nm.

11 . A method of manufacturing a linerless adhesive label web, the method comprising

- arranging a face (201 ) as a substrate,

- coating the face (201 ) with an activatable pressure sensitive adhesive, the activatable pressure sensitive adhesive comprising a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature T_gi and a second latex comprising a second polymer having a glass transition temperature T_g2, the T_gi being lower than the T_g2, difference between T_g2 and T_gi being at least 60 degrees C, and the T_gi being at least -60 degrees C,

- heating the activatable pressure sensitive adhesive coated on the substrate first at a temperature above the T_gi (first heat treatment) and subsequently at a temperature above the T_g2 (second heat treatment).

12. The method according to claim 11 , wherein the first heat treatment is performed at 70-80 degrees C.

13. A labelled item (320) comprising an item (310) and a linerless adhesive label (200, 300) according to any of the claims 1 -10, wherein the linerless adhesive label (200, 300) is attached to a surface of the item via the adhesive layer (202) of the linerless adhesive label.

14. An activatable pressure sensitive adhesive composition for linerless adhesive labels, wherein the activatable pressure sensitive adhesive 19 comprises a binary latex mixture including a first latex comprising a first polymer having a glass transition temperature T_gi and a second latex comprising a second polymer having a glass transition temperature T_g2, the T_gi being lower than the T_g2, difference between T_g2 and T_gi being at least 60 degrees C, and the T_gi being at least -60 degrees C. The activatable pressure sensitive adhesive composition according to claim 14, capable of forming a 3D macrostructure that is non-tacky at temperatures up to 50 degrees C. The activatable pressure sensitive adhesive composition according to claim 14 or 15, wherein the activatable pressure sensitive adhesive composition is activatable by external energy.