WO2019113781A1

WO2019113781A1 - Removable pressure-sensitive adhesives with high peel strength and removability

Info

Publication number: WO2019113781A1
Application number: PCT/CN2017/115637
Authority: WO
Inventors: Biao SHEN; Yurun Yang; Hanpu LI
Original assignee: Avery Dennison Corporation
Priority date: 2017-12-12
Filing date: 2017-12-12
Publication date: 2019-06-20
Also published as: CN109593503B; TW201927967A; CN109593503A

Abstract

Provided herein is a pressure sensitive adhesive (PSA) composition having high levels of removability as well as high peel strength on a variety of substrates. This PSA composition comprises a unique mixture of a polyacrylate base polymer, an epoxy crosslinker, an optional isocyanate crosslinker, and a tackifier. The PSA is solvent-based and can be prepared in any of a number of solvents, including toluene and ethyl acetate.

Description

REMOVABLE PRESSURE-SENSITIVE ADHESIVES WITH HIGH PEEL STRENGTH AND REMOVABILITY

FIELD OF THE INVENTION

The present invention relates generally to pressure-sensitive adhesives (PSAs) , in particular, to improved PSAs that both are removable and have high peel strength. This application also relates to labels containing the improved PSAs as well as to methods of producing same.

BACKGROUND OF THE INVENTION

PSAs are compositions known to provide adhesiveness or tack to various substrates when applied at room temperature. This adhesiveness can provide for instantaneous adhesion to the substrate when subjected to pressure. PSAs are generally easy to handle in solid form and have a long shelf-life, so they are widely used for the manufacture of, for example, self-adhesive labels.

PSAs can generally be divided into two categories: permanent PSAs and removable PSAs. Typically, permanent PSAs are designed to adhere to a substrate without edge lifting These PSAs cannot be removed without damaging either the label or the substrate. Removable PSAs can stick to a substrate without edge lifting and can be removed without damage to either the label or the substrate. Removable PSAs are especially useful when it is desirable to remove and/or reposition the PSAs after being adhered to items. Examples include labelled packages and/or containers that may be subjected to cleaning or recycling to reuse or recover of the actual package/container material after removal of the contents. Such treatments often require the label to be completely separated from the package or container to which it is fastened, without leaving adhesive residues on the surface of the article, so as to simplify the cleaning or recycling process. Having a PSA that can be recyclable and compostable also improves the environmental friendliness of the PSA and facilitates its easy disposal.

On the other hand, in some applications, it is desirable that a PSA has high peel strength so that the label can be stably affixed to the article for desired period of time. A PSA with high peel strength requires a relatively high amount of force to peel the PSA from the substrate or article to which it is affixed. In other applications, it is desirable to have a PSA that has both high removability and high peel strength. For example, for wet tissue packages, it is desirable to adhere the package covers with an adhesive that has high removability, so that when the packages are being opened and closed repeatedly adhesive residue is not left on the package surface. In such applications, high peel strength is also important to prevent the contents from leaking out of the labeled cover.

Conventional PSAs have failed to address the competing features of high peel strength and high degrees removability. Even more so, it have been especially challenging to produce removable PSAs that demonstrate high peel adhesion on low surface energy (LSE) substrates, e.g., polypropylene (PP) , polyethylene (PE) , polyolefin, as well as good removability on high surface energy (HSE) substrates, e.g., polyethylene terephthalate (PET) and various metal substrates. Thus, the need exists for PSAs that provide balanced performance characteristics, which include a combination of high removability and high peel strength on both high and low energy substrates) .

SUMMARY OF THE INVENTION

In some embodiments, this disclosure provides an adhesive composition comprising: a polyacrylate base polymer having an acid value ranging from 2 to 90 mg KOH/g and having a Tg above -41 ℃, greater than 0.15 wt. %of a first crosslinker comprising an epoxy resin, based on the total dry weight of the adhesive composition, and a tackifier having a softening point above 96 ℃.

In some embodiments, when the adhesive is removed from a surface of a substrate that has been in contact with the adhesive for at least three days, the adhesive leaves residues on less than 3%of the surface area. In some embodiments, the polyacrylate base polymer has an acid value ranging from 4 to 75 mg KOH/mg. In some embodiments, wherein the polyacrylate base polymer has a glass transition temperature (Tg) ranging from -40℃ to -5℃. In some embodiments, the polyacrylate base polymer is not a block copolymer. In some embodiments, the polyacrylate base polymer contains alkyl groups that contains between 2 to 14 carbon atoms.

In some embodiments, the average molecular weight of the polyacrylate base polymer ranges from 50000 to 1500000 g/mol. In some embodiments, the molecular weight distribution index of the polyacrylate polymer ranges from 2 to 8. In some embodiments, the polyacrylate base polymer does not comprise unsaturated groups. In some embodiments, the polyacrylate base polymer comprises hydroxyl groups and has a hydroxyl value ranging from 0 to 100 mg KOH/g.

In some embodiments, the adhesive composition comprises 0.16 to 1.0 wt. %of the first crosslinker. In some embodiments, the epoxy resin of the first crosslinker has an epoxide equivalent weight ranging from 60 to 300 g/eq.

In some embodiments, the epoxy resin of the first crosslinker comprises one or more tertiary amine groups in the chemical structure. In some embodiments, the epoxy resin of the first crosslinker comprises a one or more tertiary amine group has the chemical structure of selected from the group consisting of

and mixtures thereof.

In some embodiments, the adhesive composition further comprises a second crosslinker comprising an isocyanate resin. In some embodiments, the adhesive composition comprises 0.01 to 1 wt. %of the second crosslinker. In some embodiments, the isocyanate resin has an NCO content ranging from 10 to 30 wt. %. In some embodiments, the weight ratio of the first crosslinker to the second crosslinker ranges from 0.6 to 20. In some embodiments, the adhesive composition comprises 0.2 to 30 wt. %of the tackifier. In some embodiments, the tackifier comprises a resin of rosin ester, a polyterpene resin, a phenolic resin, or combinations thereof. In some embodiments, the weight ratio of the polyacrylate base polymer to the first crosslinker ranges from 100: 0.9 to 100: 0.01 (100: 0.6 to 100: 0.03 is preferred) . In some embodiments, the adhesive composition has a peel strength from 2 to 12 N/inch on polypropylene according to the FINAT-1 method.

In some embodiments, the adhesive composition has a peel strength from 5 to 13 N/inch on stainless steel according to the FINAT-1 method. In some embodiments, the adhesive composition has a gel content that ranges from 40 to 95 wt. %. In some embodiments, the adhesive composition has a pot life above greater than 4 hours. In some embodiments, the adhesive composition has a curing time ranging from 0.3 to 5 minutes, when cured at 120℃. In some embodiments, the adhesive is substantially free of natural rubber.

In some embodiments, this disclosure provides a label comprising an adhesive layer comprising the adhesive composition disclosed herein. In some embodiments, the label further comprises a facestock layer in contact with the adhesive layer.

In some embodiments, this disclosure provides a method of producing an adhesive formulation, comprising:

dissolving in a solvent at a temperature of less than 50℃

a) a polyacrylate base polymer,

b) a first crosslinker comprising an epoxy resin, and

c) a tackifier having a softening point above 96℃.

In some embodiments, the first crosslinker comprises one or more catalytic units. In some embodiments, the one or more catalytic units are one or more tertiary amines. In some embodiments, the first crosslinker ranges from 0.01 to 0.9 wt. %based on the dry weight of the adhesive composition. In some embodiments, the epoxy resin of the first crosslinker has an epoxide equivalent weight ranging from 60 to 300 g/eq. In some embodiments, the solvent is selected from the group consisting of toluene, ethyl acetate, Isopropanol, xylene, n-hexane, n-heptane, methyl cyclohexane, butyl acetate, acetone, butanone, and 2-Acetoxy-1-methoxypropane.

In some embodiments, the method further comprises coating a facestock with the adhesive formulation, and drying the adhesive formulation. In some embodiments, the method further comprise applying a label comprising the adhesive to a substrate. In some embodiments, the substrate is a low surface energy (LSE) substrate. In some embodiments, the substrate is a high surface energy (HSE) substrate. In some embodiments, the LSE is a surface of a wet tissue package.

In some embodiments, this disclosure provides an adhesive system comprising: a first component comprising i) a polyacrylate base polymer having acid groups and having an acid value ranging from 2 to 90 mg KOH/g, ii) a tackifier having a softening point above 96℃, iii) a solvent; and a second component comprising a first crosslinker comprising an epoxy resin in a solvent; wherein the first component is separate from the second component.

In some embodiments, this disclosure provides a labeled container comprising an adhesive composition disclosed herein.

In some embodiments, the container is a wet tissue package or a book.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to pressure sensitive adhesives (PSAs) that provide advantageous performance characteristics, specifically high degrees of removability as well as high peel strength. These characteristics together can be particularly important when the PSAs are utilized for applications requiring the adhesive to hold a label firmly in place, without leaving adhesive residue upon label removal.

As noted herein, a variety of conventional PSAs are known in the art. Many of these adhesives, however, have been developed for applications where either high removability or high peel strength is preferred. Generally, these PSAs do not demonstrate both high removability and high peel strength. In fact, many components used to contribute to improved adhesion (and therefore to increased peel strength) often have detrimental effects on the PSA’s removability and vice versa.

For example, U.S. Patent No. 8,299,156 describes a water-based emulsion PSA, in which the polymers are emulsified in water in the presence of a surfactant, which is generally required for stabilizing the emulsion. The surfactant negatively affects the crosslinking of the desired polymer and therefore decreases the removability of the PSA.

Chinese Patent Application CN102245727 describes a PSA comprising one natural rubber component and polyacrylate component. These rubber-based adhesives are more prone to cause sticky edges and the adhesive strength tends to increase throughout the life of the label, which contributes to a gradual loss of removability. In addition, the PSAs do not employ components having proper acid value and hydroxyl values and do not simultaneously demonstrate well-balanced peel adhesion and removability.

In general, adding tackifiers to a PSA results in an increase peel strength but a decrease in removability.

The PSA of the invention, by including an epoxy resin as a crosslinker, in combination with the tackifier (s) as described in this disclosure, can surprisingly have balanced properties of high peel strength and good removability. In particular, the inventors have now discovered that particular combinations of a polyacrylate base polymer, a crosslinker comprising epoxy resin, and a tackifier, optionally utilized in specific amounts, surprisingly provide for high performance PSAs that demonstrate both high peel strength and high levels of removability. For example, when a polyacrylate base polymer, having acid groups, e.g., carboxyl groups, and an acid value ranging from 2 to 90 mg KOH/g, a crosslinker comprising an epoxy resin, and a tackifier having a softening point above 96℃ are utilized, the resultant PSA surprisingly demonstrates a peel strength on stainless steel ranging from 5 N/inch to 13 N/inch, as measured by the FINAT Test Method 1 (2017) . Advantageously, the PSA also demonstrates superior removability, e.g., using the methods described below.

Polyacryate Base Polymer

The polymer of the solvent-based PSA comprises a polyacrylate base polymer. Various acrylate polymers are known in the art and these polymers generally contain monomers based on the acrylic acid structure, e.g., monomers that consist of a vinyl group and a carboxylic acid terminus, or derivatives thereof. In some cases, the polymer may comprise a single type of acrylate monomer, while in other cases, the polymer may comprise a combination of different types of acrylate monomers. In some embodiments, the acrylate monomers used in the polyacrylate base polymer include butyl acrylate.

In some embodiments, the polyacrylate base polymer contains acid groups and/or hydroxyl groups. In some embodiments, the monomers that form the polyacrylate base polymer include acrylic acid monomers and/or acrylate ester monomers. In some embodiments, the monomers that form the polyacrylate base polymer further include monomer that contain a hydroxyl group ( “hydroxyl group donor” ) and the amount of which present in the polymer directly correlated with the hydroxyl value of the polyacrylate base polymer. Non-exemplary hydroxyl group donors include hydroxyethylacrylate (HEA) , hydroxyl propyl alginate (HPA) and hydroxybenzoic acid (HBA) . The amount of the hydroxyl group donor may be in the range of 0.3 wt. %to 3 wt. %based on the total weight of the monomers that form the polyacrylate base polymer, e.g., 0.4-2 wt. %, or 0.5-1wt. %. In terms of upper limits, the amount of the hydroxyl group donor may be less than 3 wt%, less than 2 wt. %, or less than 1 wt. %. In terms of lower limits, the amount of the hydroxyl group donor may be present in an amount that is greater than 0.3 wt. %, greater than 0.4 wt. %or greater than 0.5 wt. %.

Importantly, the amount of acrylic acid monomers incorporated into the polyacrylate may be directly correlated with the amount of acid groups, e.g. carboxylic acids, in the polyacrylate base polymer; and the amount of acrylic ester monomers incorporated into the polyacrylate base polymer may be directly correlated to the amount of hydroxyl groups in the polyacrylate base polymer. The inventors have found that maintaining specific ratios of the monomers to acid or hydroxyl groups leads to the production of PSAs having the aforementioned combination of desired performance characteristics, e.g., high peel strength and high removability. In some embodiments, the PSA does not comprise any acrylic ester monomer. In some embodiments, the ratio between the acrylic ester monomers and acrylic acid monomers may range from 5.6: 1 to 499: 1, e.g., from 10: 1 to 400: 1, from 20: 1 to 300: 1, from 30: 1 to 200: 1, from 50: 1 to 100: 1, or from 70: 1 to 90: 1, e.g., about 82: 1. In terms of upper limits, the ratio between the acrylic ester monomers and acrylic acid monomers is less than 499: 1, less than 200: 1, less than 150: 1, less than 100: 1, or less than 90: 1. In terms of lower limits, the ratio between the acrylic ester monomers and acrylic acid monomers is greater than 5.6: 1, 8: 1, 10: 1 or 20: 1, e.g., 9: 1.

In some cases, the amount of the acid groups in the polyacrylate base polymer is expressed by an acid value. The acid value of the polyacrylate base polymer is the mass of potassium hydroxide (KOH) , in milligrams, that is required to neutralize one gram of the polyacrylate base polymer. In some embodiments, the acid value of the polyacrylate base polymer ranges from 2 mg KOH/g to 90 mg KOH/g, e.g., from 4 mg KOH/g to 75 mg KOH/g, from 5 mg KOH/g to 35 mg KOH/g, 4 mg KOH to 36 mg KOH, or from 8 mg KOH/g to 75 mg KOH/g. In terms of upper limits, the acid value of the polyacrylate base polymer is less than 90 mg KOH/g, less than 80 mg KOH/g, less than 75 mg KOH/g, less than 50 mg KOH/g, or less than 40 mg KOH/g. In terms of lower limits, the acid value of the polyacrylate base polymer is greater than 2 mg KOH/g, greater than 4 mg KOH/g, greater than 10 mg KOH/g, greater than 20 mg KOH/g, or greater than 25 mg KOH/g.

In some embodiments, the amount of the hydroxyl groups present in the polyacrylate base polymer is expressed by a hydroxyl value. The hydroxyl value of the polyacrylate base polymer is the mass of potassium hydroxide (KOH) , in milligrams, required to neutralize the acetic acid taken up on the acylation of one gram of the polyacrylate base polymer. In some embodiments, the hydroxyl value of the polyacrylate base polymer ranges from 0 mg KOH/g to 100 mg KOH/g, e.g., from 0 to 90 mg KOH/g, from 1 to 50 mg KOH/g, from 5 to 50 mg KOH/g, from 10 to 50 mg KOH/g, form 20 to 80 mg KOH/g, e.g., from 30 to 55 mg KOH/g. Notably, some embodiments of the PSA include a polyacrylate base polymer with no free hydroxyl groups. Those embodiments thus include a polyacrylate base polymer with a hydroxyl value equal to 0 mg KOH/g. In terms of upper limits, the hydroxyl value of the polyacrylate base polymer is less than 100 mg KOH/g, less than 80 mg KOH/g, less than 50 mg KOH/g, less than 20 mg KOH/g, less than 10 mg KOH/g, less than 5 mg KOH/g. In terms of lower limits, the acid value of the polyacrylate base polymer is greater than 0 mg KOH/g, greater than 1 mg KOH/g, greater than 5 mg KOH/g, greater than 10 mg KOH/g, greater than 20 mg KOH/g, or greater than 25 mg KOH/g.

In preferred embodiments, the polyacrylate base polymer is not a block copolymer. A block copolymer is a copolymer formed with two or more monomers that cluster together and form blocks of repeating units. For example, a polymer made up of X and Y monomers joined together like: Y-Y-Y-Y-Y-X-X-X-X-X-Y-Y-Y-Y-Y-X-X-X-X-X-is a block copolymer where -Y-Y-Y-Y-Y-and -X-X-X-X-X-groups are the blocks. Although the polyacrylate base polymer of the invention may comprise more than one types monomers, these monomers are distributed evenly in the polymer chain and do not form blocks of repeating units.

In some embodiments, the polyacrylate base polymer may comprise acrylate monomers that also include alkyl chains. These alkyl chains may vary widely, e.g., linear, branched, cyclical, aliphatic, aromatic, saturated, or unsaturated. The number of carbon atoms in the alkyl chain (s) of the acrylate monomer may vary, ranging from 1 to 20 carbon atoms, e.g., from 2 to 15, from 2 to 13, from 4 to 10, from 4 to 8 carbons. In preferred embodiments, these alkyl chains comprise no more than 20 carbon atoms, e.g., no more than 15 carbon atoms, no more than 12 carbon atoms, no more than 8 carbon atoms, no more than 6 carbon atoms, no more than 5 carbon atoms, or no more than 4 carbon atoms. In preferred embodiments these alkyl chains comprise greater than 1 carbon atom, e.g., greater than 1, greater than 3, greater than 4, or greater than 5 carbon atoms.

The polyacrylate base polymer may contain unsaturated carbon groups, but preferably, the acrylate polymer does not contain any unsaturated carbon groups. In some cases, the acrylate monomer of the polymer may contain an aromatic ring; but preferably, the acrylate monomer of the polymer does not contain an aromatic ring. Monomers have aromatic rings often cannot form high degree of polymerization, and thus may have decreased cohesion and adhesion properties as compared to monomers that do not.

The polyacrylate base polymer, in some embodiments, has a glass transition temperature (Tg) within appropriate ranges and/or limits. The glass transition temperature defines the region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It has surprisingly been found that when a polyacrylate base polymer having a Tg ranging from -40℃ to -15℃ (including the endpoints) , e.g., from -40 ℃ to -15 ℃, from -35℃ to -15℃, from -35℃ to -20℃, from -31 ℃ to -25 ℃, or, about -26.8 ℃, a superior combination of performance features may be achieved, e.g., optimal peel adhesion and removability (see Example II, Table 1) .

The average molecular weight of the polyacrylate base polymer may vary widely. In some cases, the average molecular weight may range from 50,000 to 1,500,000 g/mol, e.g., from 70,000-1,200,000 g/mole, from 100,000-600,000 g/mole, from 200,000-800,000 g/mole, from 300,000-600,000 g/mole or about 450,000. In terms of upper limits, the polyacrylate base polymer can have an average molecular weight of less than 1,500,000 g/mole, e.g., less than 800,000 g/mole, less than 600,000 g/mole. In terms of lower limits, the polyacrylate base polymer can have an average molecular weight of greater than 50,000 g/mole, e.g., greater than 100,000 g/mole, or less than 300,000 g/mole. In some embodiments, the polyacrylate base polymer contains from 2 to 40 wt. %, e.g., from 6 to 30 wt. %, from 10 to 20 wt. %of polymers having a molecular weight of 10,000 g/mole or less based on the total dry weight of the polyacrylate base polymer.

The polyacrylate base polymer may have a molecular weight distribution index ranging from 2 to 8, e.g., from 2 to 5, from 1 to 4, from 2 to 3, from 3 to 4, or from 3 to 4. Molecular weight distribution index refers to the broadness of a molecular weight distribution of a polymer, the larger the index, the broader the molecular weight distribution. Unless expressly noted otherwise, all percentages in this disclosure are weight percentages based on the total dry weight of the PSA.

Suitable commercially available polyacrylate base polymers include Y-1510, Y-1310, Y-1500W, Y-1210, Y-2100 from YASUSA Chemical, BPS5375 from Toyo ink, Etrac7017B, Etarc77307, Etarac7709, Etarac7055 from Eternal, PS8249 and PS8245 from Sumei, Ulrta-Reclo 236A, Ulrta-Reclo 247A from Henkel, NC-310 from Soken.

Crosslinker

The PSA of the present invention comprises one or more crosslinkers. This crosslinker functionally links one polymer chain to another. In preferred embodiments, one crosslinker, e.g., a first crosslinker, comprises an epoxy resin, which may react with the carboxyl groups in the polyacrylate base polymer. The epoxy resin opens the ring and generate one hydroxyl group. The PSA may comprise from 0.16 wt. %to 1.0 wt. %of the first crosslinker, e.g., from 0.16 wt. %to 1.0 wt. %, from 0.18 wt. %to 0.9 wt. %, from 0.25 wt. %to 1 wt. %, from 0.20 wt. %to 0.75 wt. %, from 0.3 wt. %to 0.70 wt. %, and from 0.35 wt. %to 0.5 wt. %, e.g., about 0.25 wt. %based on the total dry weight of the PSA. In terms of upper limits, the PSA comprises epoxy in an amount of less than 1 wt. %, less than 0.9 wt. %, and less than 0.7 wt. %, based on the total dry weight of the PSA. In terms of lower limits, the PSA comprises epoxy in an amount of greater than 0.15 wt. %, e.g., greater than 0.20 wt. %, greater than 0.25 wt. %based on the total dry weight of the PSA.

In preferred embodiments, the epoxy resin may comprise one, two, three, or more tertiary amines. Advantageously, epoxy resins having such properties, when used at appropriate amounts as described above, have been found to be able to react with the polyacrylate base polymer, especially the ones having acid value as disclosed above, with higher rates and at relatively moderate temperatures. As a result, the time required for the crosslinking, i.e., the curing time, is beneficially dramatically shortened. Stated another way, the use of the epoxy crosslinkers having the specific properties, e.g., having one or more tertiary amines, surprisingly has been found to advantageously increase crosslinking efficiency, e.g., shortened curing time and/or lower temperatures required for curing. Unlike conventional PSAs that usually take 30 minutes to 1 hour to cure at a temperature range of 120 –130 ℃, the PSA of the present invention can be cured within a few minutes at similar temperatures, and in some cases at even relatively moderate, lower temperatures, e.g., at 30 ℃ -100 ℃, or 50 ℃ -90 ℃, 45 ℃ -80 ℃, or 50 ℃ -70 ℃. In terms of upper limits, the curing temperature for the PSAs can be less than 100 ℃, less than 90 ℃, less than 80 ℃, or less than 70 ℃. In terms of lower limits, the curing temperature for the PSAs can be greater than 30 ℃, greater than 40 ℃, or greater than 45 ℃.

Curing time disclosed herein refers to the amount of time necessary for a polymer composition to toughen or harden by the crosslinking of polymer chains. In general, if a higher cure temperature is utilized, a shorter curing time can be expected.

Epoxy resins suitable for use in the invention may include but are not limited to the following:

(I) Glycidyl ether of m-xylenediamine

(II) Glycidyl ether of m-cyclohexyldiamine

(III) N, N-Bis (2, 3-epoxypropyl) isopropylamine

(IV) N, N-Bis (2, 3-epoxypropyl) cyclohexylamine

Suitable commercially available epoxy resin crosslinkers include, but are not limited to, ERISYS GA-240 from CVC (having the structure I) , BXX5983 from Toyoink (having structure II) , and Y-202 from YASUSA Chemical (having the structure III) .

It is believed that high epoxy equivalent weight of the epoxy resin used is associated with increased reaction rates and/or degree of crosslinking of the base polyacrylate polymer. Epoxide equivalent weight (EEW) is a parameter that defines the weight of the resin in grams that contains one gram -equivalent of epoxy. In preferred embodiments, the epoxy resin of the PSA has a EEW ranging from 60 to 300 g/eq, e.g., from 60 to 100 g/eq, from 60 to 150 g/eq, from 60 to 250 g/eq, from 75 to 125 g/eq, from 75 to 200 g/eq, from 90 to 120 g/eq, and from 100 to 200 g/eq, or about 100 g/eq. In terms of upper limits, the epoxy resin of the PSA has a EEW that is less than 300 g/eq, less than 250 g/eq, or less than 200 g/eq. In terms of lower limits, the epoxy resin of the PSA has a EEW that is greater than 60 g/eq, greater than 75 g/eq, or greater than 80 g/eq.

In some embodiments, the weight ratio of the polyacrylate base polymer to the first crosslinker in the PSA ranges from 111 to 10,000, e.g., from 166 to 3, 333, from 1,000 to 5,000, or from 2,000 to 3,000. In terms of upper limits, the weight ratio of the polyacrylate base polymer to the first crosslinker is less than 10,000, less than 5,000, or less than 4,000. In terms of lower limits, the weight ratio of the polyacrylate base polymer to the first crosslinker is greater than 100, e.g., greater than 200, greater than 1,000, greater than 2,000, or greater than 3,000.

In some embodiments, the PSA may also include a second crosslinker, which may comprisean isocyanate resin. The isocyanate resin may react with the hydroxyl group of the polyacrylate base polymer, as shown in the reaction scheme below.

Reaction Scheme (I) :

In some embodiments, the PSA comprises from 0.05 wt. %to 0.8 wt. %of the second crosslinker based on the total dry weight of the PSA, e.g., from 0.005 wt. %to 0.4 wt. %, from 0.01 wt. %to 0.4 wt. %, from 0.03 wt. %to 0.3 wt. %, from 0.05 wt. %to 0.2 wt. %, and from 0.08 wt. %to 0.15 wt. %. In terms of upper limits, the PSA comprises the second crosslinker in an amount of less than 0.4 wt. %, less than 0.3 wt. %, less than 0.2 wt. %, or less than 0.15 wt. %. In terms of lower limits, the PSA comprises the second crosslinker in an amount of greater than 0.005 wt. %, greater than 0.01 wt. %, greater than 0.03 wt. %, or greater than 0.05 wt. %.

It is believed that the NCO content of any isocyanate resin, which refers to the weight percent of the N=C=O functional group relative to the total resin, affects the rates and efficiency of the crosslinking reaction. In preferred embodiments, the second crosslinker is an isocyanate resin that has an NCO content ranging from 10 to 30 wt. %based on the total dry weight of the isocyanate resin, e.g., from 10 to 15 wt. %, from 10 to 20 wt. %, from 12 to 15 wt. %, from 15 to 25 wt. %, or from 15 to 30 wt. %. Isocyanate resins having NCO content higher this range typically have low molecular weight and high volatility, and thus may be harmful to the environment or user. In terms of upper limits, the second crosslinker of the PSA has an NCO content that is less than 30 wt. %, less than 25 wt. %, or less than 20 wt. %. In terms of lower limits, the second crosslinker of the PSA has an NCO content that greater than 10 wt. %, greater than 12 wt. %, greater than 15 wt. %based on the total dry weight of the isocyanate resin.

Suitable, commercially available isocyanate resin crosslinkers include, but are not limited to, Desmodur L75, Desmodur N100, and Desmodur N3390 from Covestro.

In some embodiments the PSA comprises both a first and a second crosslinkers. On one hand, it has been discoveredthat epoxy resin crosslinks polyacrylate base polymer and generates a higher crosslinking density than isocyanate resin does. Thus, PSAs having too high an amount of epoxy resin may have an excessively high crosslinking density, which may lead to poor anchorage when applied to substrate by a transfer coating. On the other hand, it has been discovered that increasing the amount of isocyanate resin can increase pot life but will decrease removability. Thus PSAs having too high an amount of isocyanate resin may exhibit poor removability. Accordingly, the inventors have found that specific ratios of epoxy resin to isocyanate resin advantageously provide unexpected combinations of performance features. For example, as disclosed above, the weight ratio of the epoxy crosslinker to the isocyanate crosslinker may range from 0.6 to 20, e.g., from 0.7 to 15, from 1 to 10, from 1 to 8, from 1 to 4, e.g., about 1.25, 1.33 or about 3.5. In term of upper limits, the weight ratio of the first crosslinker to the second crosslinker is less than 20, less than 15, less than 10, less than 8, less than 4. In terms of lower limits, the weight ratio of the first crosslinker to the second crosslinker is greater than 0.5, greater than 0.7, or greater than 1. Maintaining the proper ratio between the first and second crosslinker has been found to be important for controlling the curing reaction speed such that it the PSA can be efficiently cured and also has desired pot life and workability, as shown below. The proper ratio between the first and second crosslinkers is also beneficial for improving the peel strength of the PSA.

Tackifier

The PSA of the present invention comprises one or more tackifiers to increase peel adhesion. In preferred embodiments, the PSA comprises 0.2 wt. %to 30 wt. %of the tackifier, e.g., from 0.5 wt. %to 1 wt. %, from 1 wt. %to 5 wt. %, from 1 wt. %to 10 wt. %, from 4 wt. %to 8 wt. %, from 5 wt. %to 15 wt. %, from 10 wt. %to 20 wt. %, and from 10 wt. %to 30 wt. %. In terms of upper limits, the PSA comprises one or more tackifiers in an amount of less than 30 wt. %, less than 20 wt %, or less than 10 wt. %. In terms of lower limits, the PSA comprises the one or more tackifiers in an amount of greater than 1 wt. %, greater than 4 wt. %, greater than 5 wt. %, or greater than 10 wt. %. In terms of upper limits, the PSA comprises the one or more tackifiers in an amount of less than 30 wt. %, less than 20 wt. %, less than 15 wt. %.

In some cases, the tackifiers used in forming the PSA may include rosin resins, polyterpene resins, hydrocarbon resin (e.g., C₉ tackifier) , phenolic resins, or combinations thereof. In a preferred embodiment of the PSA, a polymerized rosin ester is used as the tackifier. Rosin resins are the thermoplastic ester resins produced by reacting rosin acid with alcohol. They are typically derived from either aged tree stumps (wood rosin) , sap (gum rosin) , or by-products of the paper making process (tall oil rosin) and they impart excellent, aggressive adhesion to all polymer types. Rosin resins may often have higher polarity than other types of resins, which can lead to higher adhesion in the PSA. The presence of rosin resin in the PSA can also improve the stability and longevity properties of the PSA. Non-limiting examples of rosin include glycerol ester and pentaerythritol esters.

Although hydrocarbon resin can be used as a tackifier in the PSA, in preferred embodiments, the tackifier of the PSA comprises less than 5 wt. %, e.g., less than 3 wt. %, or less than 1 wt. %, less than 0.5 wt. %, less than 0.1 wt. %of hydrocarbon resin.

The tackifier may have a specific softening point, and the use of specific tackifiers has been found to be beneficial for improving peel strength while minimizing the adverse effects on the removability of the PSA, as shown in the examples. Methods for determining softening point are well known in the art, for example, the Ring and Ball Softening Point (RBSP) , and the Drop Point method (e.g., ISO 4625 or ASTM E28-67/E28-99 or ASTM D36 or ASTM D6493-11 (2017) ) . In one embodiment, the tackifier has a softening point above 96 ℃, e.g., greater than 100 ℃, greater than 110 ℃, or greater than 115 ℃. The inventors have discovered surprisingly that higher softening point contribute to higher peel strength as well as higher removability (see Table 3, Exs. 4 and 5) . It is believed that if molecular composition is the same, the tackifier that has the higher softening point typically has greater molecular weight, the greater molecular weight contributes to the improved peel strength and removability. (See Example III, Table 2, which shows that the higher softening point is correlated with higher removability. ) In terms of ranges the tackifier may have a softening point ranging from 97 to 150 ℃, e.g., from 100-140 ℃, from 110 ℃ to 130 ℃, or from 120-140 ℃. In terms of lower limits, the tackifier has a softening point above 96 ℃, e.g., above 100 ℃, above 110 ℃, above 120 ℃, or above 130 ℃. In terms of upper limits, the softening point of the tackifier is lower than 150 ℃, lower than 140 ℃, or lower than 130 ℃.

Suitable commercially available tackifiers include Pensel D-125, D-160, GA-100 from Arakawa Chemical; RE100, TP2040, TP7042, or TRB115 from Arizona Chemical Company.

Labels

In some cases, the solvent-based PSA can be used to produce a label or other commercial product that has good removability while still having high peel strength. Accordingly, the disclosure also provides a label that comprises the PSA disclosed above. The labels may comprise a facestock layer and an adhesive layer comprising the PSA. In some cases, the label further comprises one or more primer layers and/or a liner, as further described below.

In some embodiments, the label comprises a PSA layer that may comprise a thickness ranging from 10 to 40 microns, e.g., from 15 to 35 microns, from 18 to 30 microns, from 20 to 40 microns, or other ranges in the foregoing amounts. In terms of lower limits, the facestock layer may have a thickness of at least 10 micron, e.g., at least 15 microns, at least 18 microns. In terms of upper limits, the polyolefin films may have a thickness less than 40 microns, e.g., less than 35 microns, or less than 30 microns.

Facestock Layer

The label may have one or more facestock layers. In one embodiment, from the perspective of looking downward to the substrate, the facestock layer is on the top surface of the label, exposed to the environment and is configured to receive printable information, such as barcode or alphanumeric characters.

The facestock layer can include, for example, glassine, kraft, and polyesters, such as polyethylene terephthalate (PET) , polyamides (PA) , polyethylene naphthalate (PEN) , cotton, tissue, paper, fiberglass, synthetic textiles, and polyolefins, such as polypropylene (PP) , ethylene-propylene copolymers, polyethylene (PE) , and combinations thereof. Other polymeric film materials include urethane based polymers such as polyether urethane and polyester urethane; amide based polymers including polyether polyamide copolymers; acrylic based polymers including a polyacrylate, and ethylene/vinyl acetate copolymer; polyester based polymers including a polyether polyester; a vinyl chloride; a vinylidene chloride; a polystyrene; a polyacrylonitrile; a polycarbonate; a polyimide; ABS; polyacrylate; polycarbonate (PC) ; polyamide; polyimide (PI) ; polyamidoimide; polyacetal; polyphenylene oxide (PPO) ; polysulfone, polyethersulfone (PES) ; polyphenylene sulfide; polyether ether ketone (PEEK) ; polyetherimide (PEl) ; metallized polyethylene terephthalate (PET) ; polyvinyl fluoride (PVF) ; polyethylene ether (PEE) ; fluorinated ethylene propylene (FEP) ; polyurethane (PUR) ; liquid crystal polymers (LCPs, class of aromatic polyester) ; polyvinylidene fluoride (PVDF) ; aramid fibers; DIALAMY, (polymer alloys) ; polyethylene naphthalate (PEN) ; ethylene/tetrafluoroethylene; (E/TFE) ; polyphenyl sulfone (PPSU) ; and polymers or polymer alloys containing one or more of these materials.

The thickness or coating weight of the facestock layer may vary depending on the stiffness of the label desired for particular applications. The facestock layer according to certain embodiments of the present invention may comprise a thickness ranging from 100 to 1,000 microns, e.g., from 200 to 800 microns, from 150 to 500 microns, from 300-600 microns, or from 450 to 900 microns, or other ranges in the foregoing amounts. In terms of lower limits, the facestock layer may have a thickness of at least 100 micron, e.g., at least 150 microns, at least 200 microns, or at least 300 micros. In terms of upper limits, the polyolefin films may have a thickness less than 1000 microns, e.g., less than 800 microns, less than 500 microns, less than 400 microns, or less than 300 microns. In some embodiments, the facestock layer is 125 microns.

The facestock layer may optionally be configured to be receptive to printing. For example, the facestock layer may contain one or more printable layers containing an ink-receptive composition that is utilized to form the printable information. A variety of such compositions are known in the art, and these compositions generally include a binder and a pigment, such as silica or talc, dispersed in the binder. Optionally, the printable layer comprises a crosslinker CX-100 (DSM’s polyfunctional aziridine liquid crosslinker) . A number of such ink-receptive compositions are described in US Patent No. 6,153,288, the disclosure of which is hereby incorporated by reference. Printable information can be deposited on the facestock layer using various printing techniques, such as screen printing, dot-matrix, ink jet, laser printing, laser marking, thermal transfer, and so on. In some cases, the facestock layer is receptive to thermal transfer printing.

The inks used for printing on the facestock layer may vary widely and may include commercially available water-based, solvent-based or radiation-curable inks. Examples of these inks include Sun Sheen (aproduct of Sun Chemical identified as an alcohol dilutable polyamide ink) ,

MP (aproduct of Sun Chemical identified as a solvent-based ink formulated for surface printing acrylic coated substrates, PVDC coated substrates and polyolefin films) , X-Cel (aproduct of Water Ink Technologies identified as a water-based film ink for printing film substrates) , Uvilith AR-109 Rubine Red (aproduct of Daw Ink identified as a UV ink) and CLA91598F (aproduct of Sun Chemical identified as a multibond black solvent-based ink) .

The facestock layer may contain one or more topcoats, which enhances printing performance, durability and/or chemical resistance. In one embodiment, the topcoat layer is configured as having its top surface in contact with the bottom surface of the printable layer in the facestock layer. In one embodiment, the topcoat layer of the label typically comprises a resin. Non-limiting examples of the resins that are suitable for use as topcoat include polyester-amino resin and a phenoxy resin, polyester-isocyanate, polyurethane, and polyacrylate.

In some cases, the facestock layer may be a facestock that utilizes activatable inks, e.g., stimulus-activatable inks, such as (for example) laser-activated, pressure-activated, or temperature-activated inks.

The topcoat layer, in accordance with certain embodiments of the present invention, may be applied onto the facestock portion of the facestock layer by any known techniques in the art, such as spray, roll, brush, or other techniques. The printable layer can be formed by depositing, by gravure printing or the like, on the topcoat layer, with the bottom surface in contact with the top surface of the topcoat layer.

Primer layers

Optionally, the label disclosed herein comprises one or more primer layers and the one or more primer layers may be situated between the facestock layer and the adhesive layer.

Optional Components

In some cases, the PSAs, facestock layer, topcoat layer, or primer layer may optionally include one or more fillers, antioxidants, UV-absorbers, photo-stabilizers, and/or fillers. These additives may be incorporated into the adhesive in conventional quantities using conventional equipment and techniques. For example, representative fillers can include tale, calcium carbonate, organo-clay, glass fibers, marble dust, cement dust, feldspar, silica or glass, fumed silica, silicates, alumina, various phosphorus compounds, ammonium bromide, titanium dioxide, antimony trioxide, antimony trioxide, zinc oxide, zinc borate, barium sulfate, silicones, aluminum silicate, calcium silicate, glass microspheres, chalk, mica, clays, wollastonite, ammonium octamolybdate, intumescent compounds and mixtures of two or more of these materials. The fillers may also carry or contain various surface coatings or treatments, such as silanes, fatty acids, and the like. Still other fillers can include flame-retardant agents, such as the halogenated organic compounds. In certain embodiments, the topcoat layer may include one or more thermoplastic elastomers that are compatible with the other constituents of the layer, such as etherified melamine, hydroxylated polyester, polyester-melamine, and other suitable elastomers.

Liner

In some embodiments, the label further includes a liner deposited on the opposite side of the surface of the reactive adhesive layer that contacts the facestock layer. A releasable liner can be positioned adjacent to the reactive adhesive layer such that the reactive adhesive layer is disposed, or sandwiched, directly or indirectly between the bottom surface of facestock layer and the releasable liner. The releasable liner may function as a protective cover such that the release liner remains in place until the label is ready for attachment to an object. If a liner or release liner is included in the label, a wide array of materials and configurations can be used for the liner. In many embodiments, the liner is a paper or paper-based material. In many other embodiments, the liner is a polymeric film of one or more polymeric materials. Typically, at least one face of the liner is coated with a release material such as a silicone or silicone-based material. As will be appreciated, the release coated face of the liner is placed in contact with the otherwise exposed face of the adhesive layer. Prior to application of the label to a surface of interest, the liner is removed to thereby expose the adhesive face of the label. The liner can be in the form of a single sheet. Alternatively, the liner can be in the form of multiple sections or panels.

The liner used in the label may have a thickness ranging from 20 to 150 micron, e.g., from 30 to 120 micron, from 60 to 100 micron, or from 50 to 90 micron. In terms of upper limits, the thickness of the label is less than 150 micron, e.g., less than 130 micron, or less than 100 micron. In terms of lower limits, the thickness of the label is greater than 20 micron, e.g., greater than 30 micron, or greater than 40 micron.

Various additives can also be added to one or more of the facestock layer, the primer layer, the adhesive layer, or liner layers to obtain a certain desired characteristic. These additives can include, for example, one or more waxes, surfactants, talc, powdered silicates, filler agents, defoamers, colorants, antioxidants, UV stabilizers, luminescents, crosslinkers, buffer agents, anti-blocking agents, wetting agents, matting agents, antistatic agents, acid scavengers, flame retardants, processing aids, extrusion aids, and others.

Peel strength

The PSA of the present invention demonstrates high peel strength. This high peel strength is believed to be attributed, at least in part, to the presence of the aforementioned tackifiers. Peel strength is the average force required to remove an adhesive laminated under specified conditions on a substrate, from the substrate at constant speed and at a specified angle. Peel strength evaluations can be performed by testing 180° stainless steel peel for 20 minutes and for 24 hours according to the FINAT Test Method 1 (2017) ( “FINAT-1” ) . When applied on polypropylene substrate, the PSA may demonstrate a peel strength from 2 N/inch to 12 N/inch on according to the FINAT-1 method, e.g., from 2 N/inch to 11 N/inch, from 3 N/inch to 10 N/inch, and from 4 N/inch to 9 N/inch. In terms of upper limits, the PSA demonstrated a peel strength of less than 12 N/inch, e.g., less than 11 N/inch, or less than 10 N/inch on polypropylene substrate. In terms of lower limits, the PSA demonstrated peel strength of greater than 5 N/inch, greater than 6 N/inch, greater than 7 N/inch on polypropylene substrate.

When applied on stainless steel, the PSA may demonstrate a peel strength from 5 N/inch to 13 N/inch according to the FINAT-1 method, e.g., from 5 N/inch to 7 N/inch, from 5 N/inch to 8 N/inch, from 8 N/inch to 12 N/inch, and from 9 N/inch to 13 N/inch. In terms of upper limits, the PSA demonstrated a peel strength of less than 13 N/inch, e.g., less than 12 N/inch, or less than 10 N/inch on stainless steel. In terms of lower limits, the PSA demonstrated peel strength of greater than 5 N/inch, greater than 6 N/inch, greater than 7 N/inch on stainless steel.

Removability

As discussed above, the PSA of the present invention demonstrates a high degree of removability. This high degree of removability is believed to be attributed, at least in part, to the presence of the crosslinkers as disclosed above. The inclusion of crosslinkers increases the cohesion properties of the PSA such that the adhesive can be removed from surface of the substrate without leaving residues. The removability properties of the PSA can be typically determined as follows:

· adhere the PSA to the surface of a substrate (e.g., stainless steel) for a period of time, e.g., at least three, at least five, at least ten days to age the PSA at a high temperature and an environment having humidity above 80%. Typically, the temperature ranges from 40 ℃ to 90 ℃; then

· peel the PSA from the surface; then

· visually inspect the surface for any adhesive residue remaining --less residue remaining on the substrate surface indicates a higher removability; then

· assign a numerical value based on the amount of remaining adhesive.

The numerical value may be based on a scale of 1 to 5, with 5 being leaving no residue on the substrate after the label is peeled (e.g., leaving residue on less than 3%of the surface area that has been in contact with the PSA) , 3 leaving a little residue (e.g., leaving residue on less than 20%of the surface area that has been in contact with the PSA) , and 1 leaving a significant amount (e.g., more than 40%) of residue on the substrate. In general, the removability of 4 or 5 is considered acceptable for most applications.

Curing Time

The PSA of the present invention surprisingly and unexpectedly demonstrates high crosslinking efficiency. It is believed that this high crosslinking efficiency is a result of the appropriate amounts and the properties of the one or more crosslinkers used in the PSA. For example, epoxy crosslinker having one or more tertiary amines can significantly improve the curing efficiency. This is often reflected in that, as compared to conventional PSAs, the PSA disclosed herein has a relatively shorter curing time (see above) . In preferred embodiments, when cured at 120 ℃, the curing time of the PSA may range from 0.5 to 5 minutes, e.g., from 0.5 minute to 1 minute, from 1 minute to 2 minutes, from 1 minute to 3 minutes, from 2 minutes to 4 minutes, or from 4 minutes to 5 minutes. In terms of upper ranges, the curing time is less than 7 minutes, less than 6 minutes, less than 5 minutes, or less than 3 minutes. In terms of lower ranges, the curing time is greater than 0.5 minute, greater than 1 minute, greater than 2 minutes, or greater than 3 minutes.

Gel Content

The PSA of the invention also has high gel content as a result of sufficient crosslinking of the polyacrylate base polymer by the crosslinker. Gel content is typically determined by dissolving the adhesive in a suitable solvent for at least one day and separating the solution containing the soluble fraction of the adhesive from the insoluble fraction of the adhesive. The insoluble fraction, i.e., the gel, is dried and weighed against the total dry weight of the adhesive. Gel content in general is positively correlated with the degree of crosslinking, the cohesion strength of the PSA, and the removability of the PSA. In some embodiments, the gel content of the PSA ranges from 40 to 95 wt. %based on the total dry weight of the PSA, e.g., from 45 to 95 wt. %, from 50-90 wt. %, or from 60 to 85 wt. %.

Pot life

The PSA of the invention has improved pot life. Pot life refers to the amount of time it takes for the viscosity of an initially mixed composition to double. Pot life is a good indication how fast the PSA cures and also the workability of the PSA. Typically, timing begins from the time the product is mixed, and is measured at working temperature, e.g., room temperature. For commercial production of a PSA, it is important that the pot life be long enough to permit handling of the composition in a manufacturing line but not so long as to slow the manufacturing process. The PSA disclosed herein comprises crosslinkers, e.g., epoxy, at appropriate amounts so that optimal pot life can be obtained. The PSA of the present invention generally has a pot life ranging from 4 hours to 32 hours, e.g., from 4 hours to 8 hours, from 4 hours to 16 hours, from 8 hours to 10 hours, from 8 hours to 16 hours, and from 16 hours to 32 hours. In terms of lower limits, the PSA has a pot life that is greater than 4 hours, e.g., greater than 8 hours, greater than 10 hours, or greater than 15 hours. In terms of upper limits, the PSA has a pot life that is less than 32 hours, e.g., less than 25 hours, less than 20 hours, or greater than 16 hours.

Adhesive formulation

This disclosure also provides an adhesive formulation which comprises a solvent, a polyacrylate base polymer, a tackifier, a first crosslinker comprising epoxy, and optionally a second crosslinker comprising isocyanate resin, as disclosed above.

Production of the PSA

The present invention also relates to methods of producing a PSA. The methods include providing a solvent, a tackifier, a first crosslinker comprising an epoxy resin, optionally a second crosslinker, and polyacrylate base polymer having an acid value ranging from 2 to 90 mg KOH/g, and a tackifier having a softening point above 96 ℃; and mixing the solvent, the first crosslinker, and the polymer, and the tackifier to form an adhesive formulation. Any of the aforementioned embodiments of the polyacrylate base polymer, the tackifier, the first cross-linker, and/or the second crosslinker can be used to produce an adhesive formulation. An exemplary PSA formulation is shown in Table 1. This process is commonly referred to as compounding. The compounding can occur under a temperature less than 50 ℃, e.g., between 20 ℃ and 40 ℃, or between 20 ℃ and 30 ℃, or under any temperature below the triggering temperature for curing.

A variety of solvents can be used to dissolve the components of the PSA. Suitable solvents include those that demonstrate proper evaporation rate and in which the various components show good solubility. In preferred embodiments, the solvent is a petroleum-based solvent. Suitable solvents include but are not limited to, aromatic solvents, aliphatic solvents, ester solvents, xylene, ethyl benzene, isopropyl alcohol, and combinations thereof. Examples of aromatic solvents include aromatic rings with alkyl substitution (e.g. toluene) . Examples of ester solvents include esters of 3 or more carbon atoms (e.g. methyl acetate, or ethyl acetate) . In some embodiments, two or more solvents can be used to dissolve various components above to produce the adhesive formulation.

The amount of solvent (s) used for producing the adhesive formulation may vary depending on the desired viscosity that is suitable for coating on the substrate or other layers. Typically, the solvent is present in the adhesive formulation in an amount ranging from 5 to 50 wt. %, e.g., e.g., from 8 to 45 wt. %, from 10 to 40 wt. %, from 15 to 25 wt. %, e.g., about 17 wt. %. In terms of lower limits, the solvent is present in an amount of greater than 5 wt. %, e.g., greater than 8 wt. %, greater than 10 wt. %, or greater than 12 wt. %, greater than 15 wt. %, or about 17 wt. %, based on the total weight of the adhesive formulation. In terms of upper limits, the solvent is present in an amount of less than 50 wt. %, e.g., less than 40 wt. %, less than 30 wt. %, or less than 20 wt. %, based on the total weight of the adhesive formulation.

Typically, when in storage, the solution containing the polyacrylate base polymer, optionally also containing the tackifier, is kept separate from the crosslinkers to prevent undesired crosslinking. The crosslinkers can be added to the solution containing the polyacrylate base polymer immediately prior to the producing of the adhesive and/or the labels. Thus, in some embodiments, the disclosure provides an adhesive system comprising: a polyacrylate base polymer having acid groups and having an acid value ranging from 2 to 90 mg KOH/g, a first crosslinker comprising an epoxy resin, and a tackifier having a softening point above 96 ℃; wherein the crosslinker is separate from the polyacrylate base polymer.

The adhesive formulation, as prepared above, has good coatability with a typical viscosity of 100-5,000 cps, e.g., 200-4,000 cps, 300-3,000 cps, 400-2,000 cps, 300-600 cps, or about 500 cps. In terms of lower limits, the viscosity is greater than 100, e.g., greater than 200 cps, greater than 300 cps, or greater than 400 cps. In terms of upper limits, the viscosity is less than 5,000, less than 4,000 cps, less than 2,000 cps, less than 1,000 cps. Methods for measuring viscosity are well known, for example using the Brookfield Viscometer method, testing the flow resistance of the fluid by low and medium rate rotation.

In some embodiments, the adhesive formulation as produced above can then be coated onto a facestock using a solvent coater by knife over roll, slot die, or comma coating. The formulation may be coated to form an adhesive layer having a coat weight of at least 5 grams per square meter (gsm) , e.g., at least 10 gsm or at least 15 gsm. In terms of upper limits, the formulation may be coated to form an adhesive layer having a coat weight of 60 gsm or less, e.g., 50 gsm or less, or 40 gsm or less. In terms of ranges, the formulation may be coated to form an adhesive layer having a coat weight from 5 to 60 gsm, e.g., from 10 to 50 gsm or from 15 to 40 gsm, depending on the end use of the adhesive layer. The facestock/liner coated with the formulation above then can be dried as further described below and processed into labels. In some cases, it is used as a transfer adhesive without being associated with a facestock.

The coating process is typically performed in an oven having multiple temperature zones, e.g., at least 2 zones, at least 3 zones, at least four zones, at least five zones, or at least six zones. The temperature zones may range from 30 to 200 ℃, e.g., from 40 to 150 ℃ or from 60 to 130 ℃. The temperature may increase from the first to last zone, though multiple zones may be at the same temperature.

Once coated, the adhesive may be dried in an oven, for a predetermined drying time. The drying oven can have a temperature of greater than 100 ℃. The rate of solvent evaporation increases with temperature. The drying time can be at least 2 minutes, at least 4 minutes, at least 6 minutes, at least 8 minutes, at least 10 minutes, at least 15 minutes, at least 20 minutes, at least 25 minutes, at least 30 minutes, at least 40 minutes, at least 50 minutes, or at least 1 hour.

In some embodiments, the adhesive can is laminated onto a liner. Suitable liners are described above.

Labeling articles

The present invention also relates to methods of applying a label comprising the PSA to an article and labeled articles. The methods include providing an article defining an outer surface, and a label in accordance with an embodiment. The methods further include affixing the label to the outer surface of the article, thereby applying the label to the article. In some embodiments, the article is a container, a book, or a package that has a cover that the reclosure and repositioning of which is frequently required. The adhesive can be affixed to the cover.

The surface of the article whereto the PSA is applied may have low surface energy (LSE) or high surface energy (HSE) . Non-limiting examples of LSE include polyolinfins, polypropylene, and polyethylene. Non-limiting examples of HSE include metal and Acrylonitrile Butadiene Styrene (ABS) plastics. Traditionally, it is challenging for an adhesive to be able to bind to low surface energy substrate with sufficient peel strength. It is also challenging for an adhesive to be able to be removed without leaving adhesive residues on HSE substrate. The unique compositions disclosed herein, e.g., by including the appropriate tackifiers thus increase peel strength in LSE substrates. The inclusion of the unique epoxy resin as disclosed herein also increase the cohesion of the PSA thus improve removability, especially on HSE substrates.

The labels can be affixed to the articles in a batch, continuous, or semi-continuous fashion. Prior to application, one or more liners can be removed from the labels to thereby expose the adhesive face of the labels. The adhesive face and label is then contacted with the container (s) or article (s) and the labels applied thereto. Adhering may also include one or more operations of pressing or otherwise applying a pressing force against the label to promote contact and/or adhesion with the container; activating and/or curing of the adhesive such as by heating and/or exposure to UV light; and/or drying operations.

Adhesive system

Also provided is an adhesive system comprising: a first component comprising a polyacrylate base polymer having acid groups and having an acid value ranging from 2 to 90 mg KOH/g, a tackifier having a softening point above 96 ℃, and a solvent. The first component can be prepared by dissolving the polyacrylate base polymer and tackifier in the solvent. The system further comprises a second component comprising a first crosslinker comprising an epoxy resin in a solvent. Typically, the first component and the second component are stored separately and only mixed to produce the PSA disclosed above shortly before applying to substrate, e.g., less than 16 hours, less than 8 hours, or less than 4 hours before the application. This prevents premature crosslinking of the polyacrylate polymer and increases workability. The solvent used in the first component may be the same as or different from the solvent used in the second component.

In some embodiments, the second component may further comprise a second crosslinker comprising isocyanate, as described above.

The materials in the first and/or second components of the adhesive system may be present in the amounts such that the PSAs produced have the properties described in this disclosure.

Embodiments

Embodiment 1: An adhesive composition comprising: a polyacrylate base polymer having an acid value ranging from 2 to 90 mg KOH/g and having a Tg above -41 ℃, greater than 0.15 wt. %of a first crosslinker comprising an epoxy resin, based on the total dry weight of the adhesive composition, and a tackifier having a softening point above 96 ℃.

Embodiment 2: The adhesive composition of Embodiment 1, wherein when the adhesive is removed from a surface of a substrate that has been in contact with the adhesive for at least three days, the adhesive leaves residues on less than 3%of the surface area.

Embodiment 3: The adhesive composition of Embodiment 1, wherein the polyacrylate base polymer has an acid value ranging from 4 to 75 mg KOH/mg.

Embodiment 4: The adhesive composition of any of the preceding Embodiments, wherein the polyacrylate base polymer has a glass transition temperature (Tg) ranging from -40℃ to -5℃.

Embodiment 5: The adhesive composition of any of the preceding Embodiments, wherein the polyacrylate base polymer is not a block copolymer.

Embodiment 6: The adhesive composition of any of the preceding Embodiments, wherein the polyacrylate base polymer contains alkyl groups that contains between 2 to 14 carbon atoms.

Embodiment 7: The adhesive composition of any of the preceding Embodiments, wherein the average molecular weight of the polyacrylate base polymer ranges from 50000 to 1500000 g/mol.

Embodiment 8: The adhesive composition of any of the preceding Embodiments, wherein the molecular weight distribution index of the polyacrylate polymer ranges from 2 to 8.

Embodiment 9: The adhesive composition of any of the preceding Embodiments, wherein the polyacrylate base polymer does not comprise unsaturated groups.

Embodiment 10: The adhesive composition of any of the preceding Embodiments, wherein the polyacrylate base polymer comprises hydroxyl groups and has a hydroxyl value ranging from 0 to 100 mg KOH/g.

Embodiment 11: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition comprises 0.16 to 1.0 wt. %of the first crosslinker.

Embodiment 12: The adhesive composition of any of the preceding Embodiments, wherein the epoxy resin of the first crosslinker has an epoxide equivalent weight ranging from 60 to 300 g/eq.

Embodiment 13: The adhesive composition of any of the preceding Embodiments, wherein the epoxy resin of the first crosslinker comprises one or more tertiary amine groups in the chemical structure.

Embodiment 14: The adhesive composition of any of the preceding Embodiments, wherein the epoxy resin of the first crosslinker comprising comprises a one or more tertiary amine group has the chemical structure of selected from the group consisting of

and mixtures thereof.

Embodiment 15: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition further comprises a second crosslinker comprising an isocyanate resin.

Embodiment 16: The adhesive composition of Embodiment 14, wherein the adhesive composition comprises 0.01 to 1 wt. %of the second crosslinker.

Embodiment 17: The adhesive composition of Embodiment 14 or 15, wherein the isocyanate resin has an NCO content ranging from 10 to 30 wt. %.

Embodiment 18: The adhesive composition of any of Embodiments 15 –17, wherein the weight ratio of the first crosslinker to the second crosslinker ranges from 0.6 to 20.

Embodiment 19: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition comprises 0.2 to 30 wt. %of the tackifier.

Embodiment 20: The adhesive composition of any of the preceding Embodiments, wherein the tackifier comprises a resin of rosin ester, a polyterpene resin, a phenolic resin, or combinations thereof.

Embodiment 21: The adhesive composition of any of the preceding Embodiments, wherein the weight ratio of the polyacrylate base polymer to the first crosslinker ranges from 100: 0.9 to 100: 0.01 (100: 0.6 to 100: 0.03 is preferred) .

Embodiment 22: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition has a peel strength from 2 to 12 N/inch on polypropylene according to the FINAT-1 method.

Embodiment 23: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition has a peel strength from 5 to 13 N/inch on stainless steel according to the FINAT-1 method.

Embodiment 24: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition has a gel content that ranges from 40 to 95 wt. %.

Embodiment 25: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition has a pot life above greater than 4 hours.

Embodiment 26: The adhesive composition of any of the preceding Embodiments, wherein the adhesive composition has a curing time ranging from 0.3 to 5 minutes, when cured at 120℃.

Embodiment 27: The adhesive composition of any of the preceding Embodiments, wherein the adhesive is substantially free of natural rubber.

Embodiment 28: A label comprising an adhesive layer comprising the adhesive composition of any of the Embodiments 1 –27.

Embodiment 29: The label of Embodiment 28, further comprising a facestock layer in contact with the adhesive layer.

Embodiment 30: A method of producing an adhesive formulation, comprising:

dissolving in a solvent at a temperature of less than 50℃

a) a polyacrylate base polymer,

b) a first crosslinker comprising an epoxy resin, and

c) a tackifier having a softening point above 96℃.

Embodiment 31: The method of Embodiment 30, wherein the first crosslinker comprises one or more catalytic units.

Embodiment 32: The method of any of Embodiment 30 to 31, wherein the one or more catalytic units are one or more tertiary amines.

Embodiment 33: The method of Embodiment 30, wherein the first crosslinker ranges from 0.01 to 0.9 wt. %based on the dry weight of the adhesive composition.

Embodiment 34: The method of Embodiment 30 or 31, wherein the epoxy resin of the first crosslinker has an epoxide equivalent weight ranging from 60 to 300 g/eq.

Embodiment 35: The method of any of Embodiments 30 –34, wherein the solvent is selected from the group consisting of toluene, ethyl acetate, Isopropanol, xylene, n-hexane, n-heptane, methyl cyclohexane, butyl acetate, acetone, butanone, and 2-Acetoxy-1-methoxypropane.

Embodiment 36: The method of any of Embodiments 30 –35, further comprising: coating a facestock with the adhesive formulation, and drying the adhesive formulation.

Embodiment 37: The method of any of Embodiments 30 –36, wherein the method further comprise applying a label comprising the adhesive to a substrate.

Embodiment 38: The method of Embodiment 37, wherein the substrate is a low surface energy (LSE) substrate.

Embodiment 39: The method of Embodiment 37, wherein the substrate is a high surface energy (HSE) substrate.

Embodiment 40: The method of Embodiment 38, wherein the LSE is a surface of a wet tissue package.

Embodiment 41: An adhesive system comprising: a first component comprising i) a polyacrylate base polymer having acid groups and having an acid value ranging from 2 to 90 mg KOH/g, ii) a tackifier having a softening point above 96℃, iii) a solvent; and a second component comprising a first crosslinker comprising an epoxy resin in a solvent; wherein the first component is separate from the second component.

Embodiment 42: A labeled container comprising an adhesive composition of any of the Embodiments 1-27.

Embodiment 43: The labeled container of Embodiment 42, wherein the container is a wet tissue package or a book.

EXAMPLES

The following examples are offered to illustrate, but not to limit the claimed invention.

Example I: The Effect of Polymer Tg on Performance

PSA compositions were prepared as described in this disclosure. The adhesive compositions comprised a polyacrylate base polymer, an epoxy resin crosslinker (Y-202 from YASUSA Chemical) , and a rosin resin tackifier (D-125 from Arakawa) .

Polyacrylate base polymers with varying glass transition temperatures (Tg) were utilized to determine the effect of Tg on the PSA’s performance characteristics. PS8245 from Sumei was used in Ex. 1, 1500w from YASUSA Chemical was used in Ex. 2, and BPS5330 from Toyo ink was used in Comparative Example A. The amounts of the tackifier in the PSA compositions were varied according to the Tg of the polymers in order to maintain the Tg of PSAs at about the same temperature. The amount of polyacrylate base polymer was about 90 wt. %based on the total dry weight of the PSA. The polyacrylate base polymer had an acid value of 8 mg KOH and a hydroxyl value of 5 mg KOH/g. The crosslinker Y-202 was an epoxy resin that has an epoxide equivalent weight of about 100 g/eq. All weight percentages in this and other examples of this application were based on the solid weight of the PSA. Labels comprising the adhesive compositions were produced. Performance results are listed in Table 1 below. Removability was rated on a scale from 1 to 5, where 1 is the worst and 5 is the best.

SS stands for stainless steel

PP stands for polypropylene

As shown in Table 1, when the amount of the crosslinker was the maintained at a similar level, a decrease in Tg of the polymer and an increase in the amount of tackifier increased peel strength but decreased removability (see Comparative Example A) . In contrast, Examples 1 and 2 demonstrated a balance of good peel strength and good removability, e.g., a removability value between 4.5 and 5 and a peel strength on polypropylene substrate between 2 N/inch and 7 N/inch or a peel strength on stainless steel between 5 N/inch and 13 N/inch. The results in Table 2 show that an adhesive having a polymer having Tg values of -25 ℃ and -31 ℃ performed better than an adhesive having a polymer having a Tg of -41 ℃, despite the fact that the Tg of all adhesives as a whole (not only the polymer) were about the same. All three examples showed a pot life of at least 16 hours, which is sufficient for most applications the adhesive is intended to be used.

Example II: The Effect of Tackifier Softening Point on Performance

Four additional PSA compositions were prepared as discussed above using the components and weight percentages shown in Table 3. The adhesive compositions comprised a polyacrylate base polymer (1500w from YASUSA Chemical) and an epoxy resin crosslinker (Y-202 from YASUSA Chemical) , and various tackifiers. The epoxy resin crosslinker was present in each PSA composition in an amount of 0.25 wt. %; the amount of tackifier is shown in the table, the rest of the formulation was the polyacrylate base polymer. In this example, the polyacrylate base polymer had an acid value of 8 mg KOH and a hydroxyl value of 5 mg KOH/g. Various tackifiers were utilized to determine the effect of tackifier properties on the PSA’s performance characteristics. The results are shown in Table 2.

The results of Table 2 show that PSAs, Ex. 4 and Ex. 5 having tackifiers that had a softening point as claimed, i.e., above 96 ℃, showed good removability, in contrast to comparative examples B and C, which had a softening points of 96 ℃ and 85 ℃ respectively and had poor removability –2.5 and 2 respectively. Exs. 4 and Ex. 5 also showed excellent peel strength. The results also showed that a higher amount of tackifier corresponded to a higher peel strength and when the amount of the tackifier is the same, the tackifier having higher softening point results in both higher peel strength and higher removability.

Example III: The Effect of Tackifier Amount on Performance

Four additional PSA compositions were prepared as discussed above using the components and weight percentages shown in Table 3. The adhesive compositions comprised a polyacrylate base polymer that was synthesized using 10 wt. %of acrylic, 84.5 wt. %of 2-ethylhexyl acrylate (2-EHA) , and 0.5 wt. %hydroxyethylacrylate (HEA) , an epoxy resin crosslinker (GA-240 from CVC) , and D-125 from Arakawa as a tackifier. In one example, epoxy (GA-240) and isocyanate (M20, from BASF) were used as the first and the second crosslinkers, respectively. GA-240 has an epoxide equivalent weight of 100 g/eq M20 has a NCO content of 31%. The polyacrylate base polymer had an acid value of 75 mg KOH/g and a Tg of -26.8 ℃, a hydroxyl value of 1 mg KOH/g, and had a ratios of acrylic ester monomer to acrylic acid monomer of 9: 1. The average molecular weight of the polyacrylate base polymer was 300,000, and the molecular weight distribution index is 4. The weight percentage of the crosslinkers and the tackifiers are shown in the table and the rest of the PSA composition is made up of the polyacrylate base polymer. The amount of the tackifier was varied to determine the effect of tackifier content on the PSA’s performance characteristics. The results are listed below in Table 3.

**solid refers to that the weight percentage was based on the total dry weight of the PSA.

The results of Table 3 show that keeping the amount of tackifier at a lower level, e.g., 4 wt. %or 8 wt. %, provides for a PSA having excellent removability and good peel strength. In addition, the results showed that maintaining the proper ratio between the first and second crosslinker is beneficia l –when other components are the same. PSAs using a mixture of an epoxy resin crosslinker (GA-240) and an isocyanate resin crosslinker (M20) at a ratio of 1.33, as in Ex. 11, showed higher peel strength as compared to adhesives using epoxy resin crosslinker alone. Finally, consistent with the Examples I-III above, increasing the amount of tackifier increased the peel strength of the adhesive.

Example IV: The Effect of the Acid Value of the Polymer on Performance

Two additional adhesive compositions were prepared as discussed above and by employing varying acid value of the polymer as shown in Table 5. GA-240 from CVC was used as the epoxy crosslinker in an amount of 0.30 wt. %based on total solid weight of the adhesive and D-125 from Arakawa was used as the tackifier. Polyacrylate base polymers with varying acid values were utilized to determine the effect of acid value on performance characteristics. The PSAs all had a hydroxyl value of about 5 mg KOH/g. The results are shown in Table 5. The polyacrylate base polymers were synthesized using 2-Ethylhexyl acrylate (EHA) (acrylate monomer) , acrylic (acrylic acid monomer) , and hydroxyethylacrylate (HEA) (hydroxyl group donor) . Ex. 12 was synthesized using 84.27 wt. %2-EHA, 0.53 wt. %acrylic, 0.5 wt. %HEA and 15.7 wt. %methyl acrylate (MA) . Ex. 13 was synthesized using 80 wt. %2-EHA, 4.8 wt. %acrylic, 0.5 wt. %HEA and 14.7 wt. %MA.

The results show that the acid value of the polymer affected both the peel strength and the removability of the adhesive. As shown in Table 4, a higher acid value, e.g., 36 mg KOH/g, provides for higher peel strength and higher removability. Without being bound by theory, it is believed that this is because the polyacrylate base polymers having the higher acid values can react more efficiently with the respective epoxy crosslinker.

Example V: The Effect of Weight Ratios of the First to Second Crosslinkers on Performance

Three additional compositions were prepared as described above using components and weight percentages as shown in Table 6. The adhesive compositions comprised a polyacrylate base polymer (Y2310 from Yasusa) and a tackifier (TP2040) . The base polymer had an average molecular weight of 420,000, an molecular weight distribution index of 3.6. The PSAs uses epoxy resin (Y-202) and isocyanate resin (L75) as the first and second crosslinkers, respectively. Y-202 is an epoxy resin that has an epoxide equivalent weight of 100 g/eq L75 has an NCO content of 13.3 wt. %based on the total dry weight of the isocyanate resin. The polyacrylate polymer had an acid value of 80 mgKOH/g and a hydroxyl value of 5 mgKOH/g. The weight ratios of the first to second crosslinkers were varied to determine the effect of the ratio on the PSA’s performance characteristics. The results are shown in Table 5.

The results of Table 6 show that, as compared to Comparative Example D, in which the weight ratio of the first to second crosslinker was 0.5: 1, Ex. 14 and 15 which had higher weight ratios showed improved removability. Although both Ex. 14 and Ex. 15 had sufficient pot life, Ex. 15, which has a lower ratio, i.e., a higher amount of isocyanate resin, showed even longer pot life. This indicates that keeping the weight ratio of the first to second crosslinkers in the PSA above 0.5: 1 is important to maintain high removability and superior pot life.

While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. In view of the foregoing discussion, relevant knowledge in the art and references discussed above in connection with the Background and Detailed Description are all incorporated herein by reference. In addition, it should be understood that aspects of the invention and portions of various embodiments and various features recited above and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriate combined with other embodiments as will be appreciated by one of skill in the art. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.

Claims

An adhesive composition comprising:

a polyacrylate base polymer having an acid value ranging from 2 to 90 mg KOH/g and having a Tg above -41 ℃,

greater than 0.15 wt.%of a first crosslinker comprising an epoxy resin, based on the total dry weight of the adhesive composition, and

a tackifier having a softening point above 96 ℃.
The adhesive composition of claim 1, wherein when the adhesive is removed from a surface of a substrate that has been in contact with the adhesive for at least three days, the adhesive leaves residues on less than 3%of the surface area.
The adhesive composition of claim 1, wherein the polyacrylate base polymer has an acid value ranging from 4 to 75 mg KOH/mg.
The adhesive composition of any of the preceding claims, wherein the polyacrylate base polymer has a glass transition temperature (Tg) ranging from -40℃ to -5℃.+
The adhesive composition of any of the preceding claims, wherein the polyacrylate base polymer is not a block copolymer.
The adhesive composition of any of the preceding claims, wherein the polyacrylate base polymer contains alkyl groups that contains between 2 to 14 carbon atoms.
The adhesive composition of any of the preceding claims, wherein the average molecular weight of the polyacrylate base polymer ranges from 50000 to 1500000 g/mol.
The adhesive composition of any of the preceding claims, wherein the molecular weight distribution index of the polyacrylate polymer ranges from 2 to 8.
The adhesive composition of any of the preceding claims, wherein the polyacrylate base polymer does not comprise unsaturated groups.
The adhesive composition of any of the preceding claims, wherein the polyacrylate base polymer comprises hydroxyl groups and has a hydroxyl value ranging from 0 to 100 mg KOH/g.
The adhesive composition of any of the preceding claims, wherein the adhesive composition comprises 0.16 to 1.0 wt.%of the first crosslinker.
The adhesive composition of any of the preceding claims, wherein the epoxy resin of the first crosslinker has an epoxide equivalent weight ranging from 60 to 300 g/eq.
The adhesive composition of any of the preceding claims, wherein the epoxy resin of the first crosslinker comprises one or more tertiary amine groups in the chemical structure.
The adhesive composition of any of the preceding claims, wherein the epoxy resin of the first crosslinker comprising comprises a one or more tertiary amine group has the chemical structure of selected from the group consisting of

and mixtures thereof.
The adhesive composition of any of the preceding claims, wherein the adhesive composition further comprises a second crosslinker comprising an isocyanate resin.
The adhesive composition of claim 14, wherein the adhesive composition comprises 0.01 to 1 wt.%of the second crosslinker.
The adhesive composition of claim 14 or 15, wherein the isocyanate resin has an NCO content ranging from 10 to 30 wt.%.
The adhesive composition of any of claims 15–17, wherein the weight ratio of the first crosslinker to the second crosslinker ranges from 0.6 to 20.
The adhesive composition of any of the preceding claims, wherein the adhesive composition comprises 0.2 to 30 wt.%of the tackifier.
The adhesive composition of any of the preceding claims, wherein the tackifier comprises a resin of rosin ester, a polyterpene resin, a phenolic resin, or combinations thereof.
The adhesive composition of any of the preceding claims, wherein the weight ratio of the polyacrylate base polymer to the first crosslinker ranges from 100: 0.9 to 100: 0.01.
The adhesive composition of any of the preceding claims, wherein the adhesive composition has a peel strength from 2 to 12 N/inch on polypropylene according to the FINAT-1 method.
The adhesive composition of any of the preceding claims, wherein the adhesive composition has a peel strength from 5 to 13 N/inch on stainless steel according to the FINAT-1 method.
The adhesive composition of any of the preceding claims, wherein the adhesive composition has a gel content that ranges from 40 to 95 wt.%.
The adhesive composition of any of the preceding claims, wherein the adhesive composition has a pot life above greater than 4 hours.
The adhesive composition of any of the preceding claims, wherein the adhesive composition has a curing time ranging from 0.3 to 5 minutes, when cured at 120℃.
The adhesive composition of any of the preceding claims, wherein the adhesive is substantially free of natural rubber.
A label comprising an adhesive layer comprising the adhesive composition of any of the claims 1–27.
The label of claim 28, further comprising a facestock layer in contact with the adhesive layer.
A method of producing an adhesive formulation, comprising: dissolving in a solvent at a temperature of less than 50℃

a) a polyacrylate base polymer,

b) a first crosslinker comprising an epoxy resin, and

c) a tackifier having a softening point above 96℃.
The method of claim 30, wherein the first crosslinker comprises one or more catalytic units.
The method of any of claim 30 to 31, wherein the one or more catalytic units are one or more tertiary amines.
The method of claim 30, wherein the first crosslinker ranges from 0.01 to 0.9 wt.%based on the dry weight of the adhesive composition.
The method of claim 30 or 31, wherein the epoxy resin of the first crosslinker has an epoxide equivalent weight ranging from 60 to 300 g/eq.
The method of any of claims 30–34, wherein the solvent is selected from the group consisting of toluene, ethyl acetate, Isopropanol, xylene, n-hexane, n-heptane, methyl cyclohexane, butyl acetate, acetone, butanone, and 2-Acetoxy-1-methoxypropane.
The method of any of claims 30–35, further comprising: coating a facestock with the adhesive formulation, and drying the adhesive formulation.
The method of any of claims 30–36, wherein the method further comprise applying a label comprising the adhesive to a substrate.
The method of claim 37, wherein the substrate is a low surface energy (LSE) substrate.
The method of claim 37, wherein the substrate is a high surface energy (HSE) substrate.
The method of claim 38, wherein the LSE is a surface of a wet tissue package.
An adhesive system comprising:

a first component comprising i) a polyacrylate base polymer having acid groups and having an acid value ranging from 2 to 90 mg KOH/g, ii) a tackifier having a softening point above 96℃, iii) a solvent; and

a second component comprising a first crosslinker comprising an epoxy resin in a solvent;

wherein the first component is separate from the second component.
A labeled container comprising an adhesive composition of any of the claims 1-27.
The labeled container of claim 42, wherein the container is a wet tissue package or a book.