MXPA06004833A - Adhesive composition - Google Patents

Abstract

There is described a process for preparing a pressure sensitive adhesive having enhanced resistance to water --whitening comprising the steps of

Description

ADHESIVE COMPOSITION DESCRIPTION OF THE INVENTION The present invention relates to the field of adhesives, for example pressure sensitive adhesives (PSAs) and to processes for making such adhesives. A conventional PSA comprises a polymer formulation that is often applied as a transparent layer between the substrate to be bonded (such as a label) and a release layer (such as a silicone-coated film). For health, safety and environmental reasons, it is desired that the process for making and applying PSAs reduce the production of, for example, volatile organic compounds (VOCs) or organic waste solvents. For these reasons, floating PSAs are increasingly preferred. It is also desirable that the PSA coating remains transparent for many uses where aesthetic appearance is important, such as the application of a transparent label to an article to create a non-visible label. (for example in a bottle for drinks). However, articles labeled with PSA labels (for example, beverage bottles) can be subjected to extremes of temperature and humidity. For example, bottles that are ref.172308 pasteurized after filling using water or steam at high temperatures. During use, beverage bottles can be cooled by immersion in cold water or ice. Under such conditions a transparent PSA coating may lose its transparency and whiteness. This can spoil the illusion of a non-visible label appearance and is a particular problem for floating PSAs. Many pressure sensitive adhesives are formed by emulsion polymerization. Without wishing to be limited by any mechanism, the Applicant believes that during the initiation of polymerization by conventional redox initiating agents, some undesirable inorganic components may be formed or remain within the polymer (such as initiators, buffer salts and inorganic hydrophilic salts). . It is believed that these components gradually precipitate when the polymer dries and form hydrophilic islands within the adhesive coating. These islands can absorb huge amounts of water that can grow to a size that is large enough to disperse incidental light. This otherwise increases the opacity of a transparent PSA coating. The PSA formulations of the prior art are described in the following references. Document O03 / 006S17 (Solutia) describes a method of preparing PSA, by adding a preemulsion of monomers to a mixture consisting of water, water soluble and / or polymerizable surfactant which is dispersed in water, and polymerization initiator. WO 01/85867 (Ashland) discloses a PSA made from aqueous latex emulsions, having an average particle size of below 100 nanometers and comprises specific cationic polymerizable alkyl phenol sulfate emulsifiers, such as Hitenol BC- 10 commercially available from Montello. Such emulsions are prepared using redox initiation systems having the aforementioned disadvantages. It is an object of the present invention to provide PSAs that have improved resistance to water whitening. Therefore, broadly in accordance with the present invention there is provided a process for preparing a pressure sensitive adhesive having an improved resistance to water bleaching, comprising the steps of: (a) forming a water mixture of (i) ) an effective initial amount of a polymerization initiator "which produces radicals by thermal decomposition to form a mixture and another optional surfactant; (ii) an effective amount of a polymerizable surfactant that is dispersed in water with a terminal allylamine moiety; salt (es) of. polyoxyalkylene-1- (allyloxymethyl) alkyl ether sulphate and / or mixtures thereof(b) forming a polymerizable aqueous preemulsion comprising (i) a hydrophobic monomer mixture comprising at least one alkyl (meth) acrylate ester of a C? -C alcohol and up to about 30% by weight of the mixture of at least one styrenic monomer, (ii) at least about 1% of the total weight of (i) to (iii) of one or more hydrophilic monomers, (iii) at least about 5% of the total weight of (i) to (iii) of at least one partially hydrophilic monomer (s) selected from N-vinyl pyrrolidone; alkyl (meta) acrylate esters of methanol or ethanol; and / or mixtures thereof, the additional preemulsion comprises effective amounts of surfactant, (c) contacting the pre-emulsion with the water mixture; (d) continuously adding the pre-emulsion to the mixture to polymerize the pre-emulsion to form a latex emulsion, and optionally add another polymerization initiator during the polymerization of the pre-emulsion; and (e) optionally adjusting the pH of the latex emulsion with a suitable base at a pH of about 6.5 to about 9. The applicant has found that the best resistance to water bleaching is achieved with a formulation where the monomer concentration hydrophilic (by the total weight of components (b) (i) to (b) (iii)) comprises about 1% and 10%, preferably from-about 3 to 6%, more preferably about 4% to approximately 5%. Preferred hydrophilic monomers comprise acid functional monomers. more preferably selected from acrylic acid, methacrylic acid, oligomeric acrylic acid (beta-CEA) and mixtures thereof. Other similar acids can also be used. In a further aspect of the present invention there is provided a pressure sensitive adhesive having an improved resistance to water whitening that is obtained or is obtainable by the process of the invention described herein. The polymerization initiator preferably comprises a thermal initiator, more preferably it comprises potassium persulfate. The water-dispersible polymerizable surfactant preferably comprises polymerizable ethoxylated alkyl phenol sulfate surfactants (such as those commercially available from Hitachi Chemical Company under the trade name Hitanol BC-2020. An adhesive composition of the invention preferably comprises prior to polymerization: a) an effective amount of a radical polymerization initiator, (b) polymerizable ethoxylated alkyl phenol sulfate surfactants, and (c) (i) a hydrophobic monomer mixture comprising at least one alkyl (meth) acrylate ester of a 1.-C4 alcohol and up to about 30% by weight of the mixture of at least one styrenic monomer, (ii) at least about 1% of total weight of (i) to (iii) of one or more hydrophilic monomers (iii) at least about 5% total weight of (i) to (iii) of at least one partially hydrophilic monomer (s) selected from N-vinyl pyrrolidone; alkyl (metha) acrylate methanol or ethanol; and / or mixtures thereof. The Applicant has found that bleaching of PSA due to water absorption can be reduced by minimizing the amount of inorganic matter present in the dried floating PSA coating. Without wishing to be limited by any mechanism, the applicant believes that this is achieved by replacing conventional initiators based on a combination of redox components with initiators that produce radicals by thermal decomposition. Thus the persulfate initiators can be used with polymerizable surfactants for the synthesis of an acrylic emulsion which can be dried as a clear coating on a substrate to make an improved PSA with improved resistance to water whitening. Transparent labels attached to a. substrate with such transparent PSAs, remain substantially transparent after immersion in water at 87 ° C for 60 minutes. - The average particle size of the. polymer is preferably less than or equal to 200 nm. The water / immersion whitening tests used to test the adhesives of the invention where they were made are as follows. The release liner was removed by exposing the polymer surfaces to water. UV / visible spectroscopy (UV / VIS) using a Cintra 40 UV-Visible spectrophotometer was used to determine absorbency. The coated Mylar film (self-adhesive) was placed in a bucket that was filled with water at zero time. The light absorbance was measured and recorded for 24 hours at 400 nm (room temperature). The final absorbency increase of the direct coated film was recorded.

PSAs according to the present invention exhibit an improved resistance to water bleaching. The formulation has utility in for example clear label applications (beverage / beer bottle label) where the floating PSA is subjected to extremes of temperature, water and / or humidity such as during pasteurization or in an ice tray. Using the combination of potassium persulfate and Hitanol BC 2020 the Applicant has found that both the water-whitening resistance and the adhesive function of the PSA can be improved. Other features of the invention are described in the claims. The invention will be illustrated by some non-limiting examples: Example 1 A copolymer latex was prepared from acrylate and alpha methyl styrene as follows. A preemulsion of monomer was prepared by mixing 2-ethyl hexyl acrylate (307 g), alpha-methyl styrene (45 g), methyl acrylate (65.86 g), methacrylic acid (9.56 g), acrylic acid (13.4 g), t- dodecyl mercn (0.6 g), water (135.8 g) and Hitanol BC-2020 (25.3 g). A two-liter glass reactor (equipped with a reflux condenser, thermal coupler and a two-blade agitator) was charged with water (310.11 g), potassium persulfate (1.18 g) and Hitanol BC-2020 (3.45 g) . Separately, a stock solution (2.86% by weight) of potassium persulfate (72.12 g) was prepared. The reactor was heated with water and when the temperature reached 78 ° C then a stock solution of 6 g of potassium persulfate was added. Using an automatic feeder, the monomer preemulsion was added at a rate of 3.03 g per minute for the next 190 minutes. The temperature was maintained at 83 ° C and the polymerization charge was continuously stirred at 240 rpm. Every 20 minutes, another 6 g of potassium persulfate stock solution was added to the reactor for the next 220 minutes. After adding the final potassium persulfate charge, the reactor temperature was raised to 88 ° C for an additional 60 minutes. The reactor was held at 88 ° C for the next 60 minutes and then cooled to room temperature and the latex was collected by filtration. The latex had a solid content of 51.52%. This was determined by placing the known amount of latex in a tin weighing heavy aluminum, which was dried at 150 ° C for 60 minutes. The tin was weighed again to calculate the solids. The average latex particle size was 83 nm as measured by a Horiba model LA-910 laser light scattering particle size analyzer. The Brookfield viscosity of the latex was 4700 cp, measured using a Brookfield LV 11+ viscometer with a # 3 spindle at 30 rpm.

The pH of the latex was initially 2.35 as measured by an Orion model pH meter. The latex was neutralized with ammonium hydroxide to a pH of 6.29. Water bleaching test of PSA The neutralized latex of Example 1 was coated in a Mylar film of a thickness of 1 mil (= 0.254 mm). The film was air dried for 10 minutes and heat dried at 90 ° C for 5 minutes. The Mylar coating was laminated with a release liner. To test the resistance to water whitening, the release liner was removed, thus exposing the surface of the polymer to the water. The Mylar coated film was immersed in a water bath at a constant temperature of 88 °. C for 60 minutes. After 60 minutes the color change of the film was visually evaluated and classified on a scale of 0 to 5. Level 5 bleaching indicates that the film turned matt white while level 0 means that the film remained clear, transparent without any visible discoloration. The color change of the Mylar film coated with the PSA of Example 1 was rated 0, ie no whitening or discoloration of any kind was detected. The following comparative example (Comp A) is intended to demonstrate the effect of using a redox initiation system of t-butyl hydroperoxide used with zinc formaldehyde sulfoxylate (such as that commercy available under the tradename Formop.on). This is based on an example published in WO 01/85867 Al. COMP A. The equipment used was the same as that described in example 1. The preemulsion of monomer was prepared by mixing water (129.6 g), sodium bicarbonate (0.8 g), 2-ethyl hexyl acrylate (259.8 g), _ _e.sJüi.re or ___ (_ 6_1.3 g) _, methacrylate-methyl (30.66 g), methacrylic acid (4 g), alpha-CEA (oligomeric acrylic acid, 20 g), hydroperoxide of t-butyl (1.2 g), and Hitanol BC 1025 (25% solids, 14.85 g). The reactor was charged with water (236.2 g) and Hitanol BC-1025 (7.42 g). Two Formopon solutions were prepared separately. The start charge of Formopon consists of 1.4 g of Formopon dissolved in 10 g of water and the stock solution of Formopon is 1.4 g dissolved in 48 g of water used for the incremental addition of Formopon to the reactor charge. Thus, the reactor was heated with water. When the temperature reached 60 ° C, 4% (approximately 20.3 g) of pre-emulsion was added to the reactor charge. The reactor charge was continuously stirred at 240 rpm. At 70 ° C the Formopon start charge was added and the heating continued. When the temperature reached 80 ° C, 1.4 g of Formopon solution were added and the monomer preemulsion was added simultaneously at a rate of 2.7 g per minute for 180 minutes with 1.4 g of Formopon stock solution added every 10 minutes also by 180 minutes The temperature was maintained at 80 ° C throughout the reaction. After ten minutes of adding the preemulsion de-monomer- and. The Formopon mother-solution-solution was added with an additional 2.5 g of Formopon stock and the reactor was maintained at 80 ° C for an additional 60 minutes. After 60 minutes, the reactor was cooled to below 50 ° C, water solution (2.5 g) and t-butyl hydroperoxide (0.15 g) were added and the load was mixed for 10 minutes. After 10 minutes, 2.5 g of Formopon stock solution was added and the reactor was maintained at 50 ° C for 30 minutes. After 50 minutes, the charge was cooled to below room temperature and the latex was collected by filtration. The latex had 48.7% solids and the particle size was 79 nm. The latex was neutralized with ammonium hydroxide at a pH of 7.02 and was coated directly on a 1 mil Mylar film. The film was prepared as described in Example 1 and tested for water whitening. After 60 minutes in water at 88 ° C, the film became matt white and a level 5 whitening was observed. It is noted that in relation to this date, the best method known to the applicant to carry out the cited invention, is the one that is transparent of the present description of the. invention.

Claims (6)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. Process for preparing a pressure sensitive adhesive having an improved resistance to water bleaching, characterized in that it comprises the steps of: (a) forming a water mixture of (i) an initial effective amount of a polymerization initiator that produces radicals by a thermal-decomposition to form a mixture and an optional additional surfactant; (ii) an effective amount of a polymerizable surfactant that is dispersed in water with a terminal allylamine moiety; salt (s) of polyoxyalkylene-1- (allyloxymethyl) alkyl ether sulfate and / or mixtures thereof, (b) forming a polymerizable aqueous preemulsion comprising (i) a hydrophobic monomer mixture comprising at least one alkyl ester (meth) acrylate of a C! -C4 alcohol and up to about 30% by weight of the mixture of at least one styrenic monomer, (ii) at least about 1% of the total weight of (i) to (iii) of one or more hydrophilic monomers, (iii) at least about 5% of the total weight of (i) to (iii) of per. at least one partially hydrophilic monomer (s) selected from N-vinyl pyrrolidone; alkyl (metha) acrylate esters of methanol or ethanol; and / or mixtures thereof, the pre-emulsion additionally comprises effective amounts of surfactant. (c) contacting the pre-emulsion with the water mixture; (d) continuously adding the pre-emulsion to the mixture to polymerize the pre-emulsion to form a latex emulsion, and optionally add another polymerization initiator during the polymerization of the pre-emulsion; Y (e) optionally adjusting the pH of the latex emulsion with a suitable base at a pH of from about 6.5 to about 9.
2. 2. Adhesive composition obtained and / or obtainable by a process according to claim 1, characterized in that the adhesive comprises, prior to polymerization: (a) an effective amount of a radical polymerization initiator. (b) polymerizable ethoxylated alkyl phenol sulfate surfactants; and (c) (i) a hydrophobic monomer mixture comprising at least one alkyl (meta) acrylate ester of a C? -C4 alcohol and up to about 30% by weight of the mixture of at least one styrenic monomer, (ii) at least about 1% of the total weight of (i) to (iii) of one or more hydrophilic monomers, (iii) at least about 5% of total weight of (i) through (iii) of minus one partially hydrophilic monomer selected from N-vinyl pyrrolidone; alkyl (metha) acrylate esters of methanol or ethanol; and / or mixtures thereof.
3. 3. Self-adhesive label, characterized in that it comprises a pressure-sensitive adhesive according to claim 2.
4. 4. Item characterized in that it is labeled using a self-adhesive label according to claim 3.
5. 5. Method for labeling an article, characterized in that it comprises the steps of: (a) distributing a self-adhesive label in accordance with claim 3, and (b) adhering the label distributed on the article.
6. 6. Labeling article, characterized in that it is obtained and / or obtainable by a method according to claim 5. SUMMARY OF THE INVENTION A process for preparing a pressure sensitive adhesive having improved resistance to water bleaching is described, comprising the steps of: (a) forming a water mixture of (i) an effective initial amount of a polymerization initiator, which produces radicals by thermal decomposition to form a mixture and an optional additional surfactant; (ii) an effective amount of a polymerizable surfactant that is dispersed in water with a terminal allylamine moiety; salt (s) of polyoxyalkylene-1- (allyloxymethyl) alkyl ether sulfate and / or mixtures thereof, (b) forming a polymerizable aqueous preemulsion comprising (i) a hydrophobic monomer mixture comprising at least one alkyl ester ( meta) Cx-d alcohol acrylate and up to about 30% by weight of the mixture of at least one styrenic monomer, (ii) at least about 1% of the total weight of (i) to (iii) of one or more hydrophilic monomers, (iii) at least about 5% of the total weight of (i) a (iii) of at least one partially hydrophilic monomer selected from N-vinyl pyrrolidone; esters of alkyl (meta) crilate of methanol or ethanol; and / or mixtures thereof, the pre-emulsion further comprises effective amounts of surfactant; (c) contacting the pre-emulsion with the water mixture; (d) continuously adding the pre-emulsion to the mixture to polymerize the pre-emulsion to form a latex emulsion, and optionally add another polymerization initiator during the polymerization of the pre-emulsion; and (e) optionally adjusting the pH of the latex emulsion with a suitable base at a pH of about 6.5 to about 9.