Polyurethane composition with low diisocyanate monomer content

Abstract

The present invention relates to adhesive polyurethane compositions having a low content of the monomer MDI, to a process for preparing the compositions of the invention, and to the use of the compositions of the invention for hot melt adhesives.

Description

Technical Field

[0001] The present invention relates to an adhesive polyurethane composition having a low content of monomeric MDI, a method for producing the composition of the present invention, and the use of the composition of the present invention for hot melt adhesives.

Background Art

[0002] Polyurethanes are established materials used in the production of a variety of products, from high-performance adhesives, surface coatings and sealants to rigid plastic parts and foams. Polyurethanes are generally obtained from the reaction of isocyanates and polyols, and the properties of polyurethanes can be designed and adapted by selecting appropriate raw materials. Although fully reacted polyurethanes are chemically inert, there remain safety and health concerns, particularly with regard to the isocyanates used. One of the most commonly used isocyanates in the production of polyurethanes is methylene diphenyl diisocyanate, an aromatic diisocyanate with three isomers, abbreviated most often as MDI, that differ by the position of the isocyanate groups around the aromatic ring: 2,2'-MDI, 2,4'-MDI and 4,4'-MDI, with 4,4'-MDI being the most widely used. MDI is known as an allergen and sensitizer, even though it is one of the least dangerous of the commonly available isocyanates. Persons who develop hypersensitivity to isocyanates can experience dangerous systemic reactions with very small exposures, including respiratory failure. Strict engineering controls and personal protective equipment are required for the handling of MDI because of its potential for violent reactions with water and other nucleophilic reagents. MDI has a relatively low human toxicity compared to other organic cyanates, but there remains concern to provide polyurethanes with low levels of monomeric residues of MDI.

[0003] EP1404733 discloses a method for obtaining reactive polyurethanes containing free isocyanate groups but low levels of monomeric asymmetric diisocyanate, such as diphenylmethane-2,4'-diisocyanate (2,4'-MDI), which contains less than 5% 4,4'-MDI and 2,2'-MDI, with a 2,2'-MDI content of less than 0.4%, by reacting it with a polyhydric alcohol such as a diol with a molecular weight of 60 g / mol to 2000 g / mol. The ratio of isocyanate groups to hydroxyl groups can be between 1.05:1 and 2.0:1. The reactive polyurethanes can be used in the production of one-component and two-component adhesives and sealant products.

[0004] In an article titled "A New Monomer-Reduced Polyurethane Hot Melt," pp. 15-18 of Adhesive ADHESIVES & SEALANTS 2004 / 2005, M. Krebs describes so-called "super MDI" as a diisocyanate-forming block consisting of two MDI groups linked by a chain extender that forms a tandem, resulting in a polyurethane with a low monomer MDI content after most of the MDI has been removed by extraction.

[0005] US2004 / 0071977 discloses a moisture-reactive hot-melt composition useful as an adhesive, which is formed by mixing components comprising at least one polyol, at least one polyisocyanate, and at least one silane-functionalized polyolefin. Suitable admixtures according to US2004 / 0071977 have a free NCO content of 0.1% or more, preferably 0.2% or more, more preferably 0.5% or more, and even more preferably 1% or more.

[0006] US6,579,406 refers to a moisture-reactive hot-melt adhesive composition formed by mixing a polyisocyanate, an amorphous polyol, and a component comprising a styrene / allyl alcohol copolymer having a hydroxyl number of 100-300 and a Mn of 1,000-4,000, in an amount of 0.1 to 10% by weight based on the weight of the adhesive composition, wherein the NCO / OH group ratio of the component on an equivalent basis is 1.05 to 2.5, the component does not contain crystalline polyesters or polyethers, and the component contains less than 1% by weight of water based on the total weight of the component. From the examples disclosed in US6,579,406, it will be apparent to those skilled in the art that the monomer isocyanate content in the composition exceeds 0.1% by weight.

[0007] US2014 / 0199540 describes a composite element comprising (a) a thermoplastic and a product of a reaction mixture attached thereto, (b) a polyester polyol having a hydroxy value of 60 to 150 mg KOH / g based on adipic acid or the condensation of adipic acid with isophthalic acid and / or phthalic acid with ethylene glycol, and at least one diol from the group consisting of 1,2-propanediol, 1,4-butanediol and 1,6-hexanediol and trimethylolpropane, and (iii) one or more polyether polyols. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] European Patent No. 1404733 [Patent Document 2] U.S. Patent Application Publication No. 2004 / 0071977 Specification [Patent Document 3] U.S. Patent No. 6,579,406 [Patent Document 4] U.S. Patent Application Publication No. 2014 / 0199540 [Non-patent literature]

[0009] [Non-Patent Document 1] M. Krebs, Adhesive ADHESIVES & SEALANTS 2004 / 2005, pp. 15-18, "New Monomer-Reduced Polyurethane Hot Melt" [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] To date, MDI-based adhesive polyurethane compositions that do not contain monomer residues can only be obtained by using one of the expensive and hard-to-obtain isomers of MDI, namely pure 2,4'-MDI, or by using costly evaporation processes. Since there is still a need to provide polyurethane compositions based on the inexpensive 4,4'-isomer of MDI with a low monomer content of MDI, the object of the present invention is to provide a safe and harmless polyurethane composition.

[0011] In the course of this invention, we surprisingly discovered that by using a mixture of MDI isomers, 2,4'-MDI and 4,4'-MDI, it is possible to obtain a polyurethane composition with an extremely low monomer MDI content. [Means for solving the problem]

[0012] Therefore, the first object of the present invention is an adhesive polyurethane composition obtained from a reaction mixture comprising the following: i) at least one polyester polyol; ii) at least one polyolefin; and iii) A mixture of 2,4' / 4,4'-methylenediphenyl diisocyanate (MDI), Here, the NCO:OH ratio is less than 1.5, and the total content of monomer MDI in the composition is less than 0.1% by weight based on the total weight of the composition.

[0013] Although not bound by theory, the difference in the selectivity of the NCO groups in the 2,4'-MDI isomer and the 4,4'-MDI isomer allows for the acquisition of polyurethanes with a low ratio of NCO groups to OH groups, while simultaneously reducing the monomer MDI content. Furthermore, surprisingly, the adhesive polyurethane composition of the present invention was observed not to exhibit the high viscosity typically seen in polyurethanes with a low NCO / OH ratio. [Modes for carrying out the invention]

[0014] The best results were obtained when the ratio of the 2,4' isomer to the 4,4' isomer of MDI was approximately equal. Therefore, in a preferred embodiment, the mixture of 2,4' / 4,4'-methylenediphenyl diisocyanate used in the reaction mixture to obtain the adhesive polyurethane composition of the present invention contains 30 to 55% by weight of 2,4'-MDI, preferably 40 to 50% by weight of 2,4'-MDI, based on the total weight of the isomer mixture. Preferably, the content of 4,4'-MDI in the isomer mixture is 30 to 55% by weight, preferably 40 to 50% by weight, based on the total weight of the isomer mixture. In a particularly preferred embodiment, the amount of 2,4'-MDI in the isomer mixture is equal to the amount of 4,4'-MDI in the mixture. Therefore, it is preferable that the weight ratio of 2,4'-MDI to 4,4'-MDI is about 1:1. In preferred embodiments, the isomer mixture may further contain 2,2'-MDI, the amount of which is preferably less than 5% by weight, more preferably less than 3% by weight, based on the total weight of the mixture.

[0015] In preferred embodiments, the monomer 4,4'-MDI content is less than 0.05% by weight, preferably less than 0.02% by weight, based on the total weight of the adhesive polyurethane composition.

[0016] In a preferred embodiment, the ratio of NCO to OH in the composition of the present invention is less than 1.4, preferably less than 1.33, and even more preferably less than 1.25. The ratio of NCO to OH refers to the ratio of free NCO groups to free OH groups in the reaction mixture and serves as a measure of the ratio of polyester polyol to diisocyanate.

[0017] The properties of the polyurethane are, in most cases, adjusted by adjusting the polyol component (molecular weight, acid value, hydroxyl value). In a preferred embodiment of the present invention, the polyester polyol used in the reaction mixture preferably has an acid value of 2.0 mg KOH / g or less as measured according to DIN EN ISO 2114.

[0018] In a more preferred embodiment, the polyester polyol used in the present invention is characterized by a hydroxyl value of 20 to 45 mg KOH / g, preferably 25 to 40 mg KOH / g, as measured according to DIN EN ISO 4628-2. In an even more preferred embodiment of the present invention, the polyester polyol has an average molecular weight of 1000 to 4500, preferably 1500 to 4000, as measured from the hydroxyl value.

[0019] In a preferred embodiment, a mixture of polyester polyols is used. The mixture may contain two or more, preferably three or more, polyester polyols.

[0020] The reaction mixture yielding the polyurethane composition of the present invention contains at least one polyolefin. Surprisingly, the presence of a polyolefin in the reaction mixture has been found to improve the Green strength of the composition. The polyolefin used in the reaction mixture to obtain the polyurethane composition of the present invention is preferably an amorphous poly-α-olefin selected from a mixture of polymers and copolymers obtained from the copolymerization of monomers selected from the group consisting of ethylene, propene, 1-butene, and 1-hexene. The copolymerization is preferably carried out in the presence of a Ziegler-Natta type catalyst. Preferably, the molar mass of the polyolefin used is in the range of 7000 to 24000, preferably 7300 to 23800. The polyolefin used preferably has a glass transition temperature Tg of -15 to -50°C, preferably -20 to -40°C, as measured according to ASTM D 3418. In particularly preferred embodiments, a polyolefin having a ring softening point of 60 to 200°C, preferably 75 to 180°C, as determined according to DIN 52011, is used. Suitable polyolefins are commercially available, including, for example, Vestoplast sold by Evonik, Eastoflex sold by Eastman Chemical, and Rextac sold by Rextac LLC. In a particularly preferred embodiment, the reaction mixture yielding the adhesive polyurethane composition of the present invention does not contain silane-functionalized polyolefins, and the content of silane-functionalized polyolefins in the reaction mixture is preferably less than 0.5% by weight, preferably less than 0.1% by weight, and particularly less than 0.01% by weight, based on the total weight of the reaction mixture.

[0021] The polyurethane composition of the present invention is characterized by a low ratio of NCO to OH, yet it exhibits a low viscosity desirable for most applications. In a preferred embodiment, the polyurethane composition of the present invention has a drain viscosity η of 5000 to 35000 mPas, preferably 6000 to 30000 mPas, measured at 130°C to 150°C using a Brookfield thermocell with spindle 27.5 rpm in accordance with DIN EN ISO 2555 / 2000 - 01.

[0022] Another object of the present invention is a method for producing an adhesive polyurethane composition according to the present invention. This method is characterized by the order in which the components are mixed and reacted and includes the following steps. a) Providing a solution comprising at least one polyester polyol and at least one polyolefin; b) Drying the solution of at least one polyester polyol and at least one polyolefin; c) Adding a mixture of 2,4'- and 4,4'-methylenediphenyl diisocyanate (MDI) to the dried product of step b) to obtain the desired polyurethane composition, The ratio of NCO to OH in step c) is characterized by being less than 1.5.

[0023] Surprisingly, it has been found that the polyurethane composition obtained by the method of the present invention exhibits a very low monomer MDI residue content of less than 0.1% by weight based on the total weight of the composition.

[0024] The adhesive polyurethane composition of the present invention can be prepared under mild conditions. In preferred embodiments, the polyurethane composition according to the present invention is prepared at a reaction temperature of less than 150°C, preferably less than 130°C. In particularly preferred embodiments, the reaction temperature is 80°C to 150°C, preferably 90°C to 130°C, and more preferably 100°C to 115°C. Surprisingly, it has been found that the selectivity of different NCO groups in the reaction mixture can be improved if the reaction temperature is kept within the scope of the claims and the monomer MDI content in the polyurethane composition is particularly low. The reaction time can be adjusted to improve the reaction yield. In even more preferred embodiments, the reaction time is greater than 40 minutes, preferably greater than 60 minutes.

[0025] In a preferred embodiment, the drying of the solution of at least one polyester polyol and at least one polyolefin is carried out under reduced pressure, preferably at a pressure of 1000 to 5000 Pa (10 to 50 mbar), and more preferably at a pressure of 2000 to 4000 Pa (20 to 40 mbar).

[0026] Furthermore, it was found that the drying process is best carried out at a temperature in the range of approximately 100 to 150°C to avoid thermal decomposition of the polymer. Therefore, the drying process of the method of the present invention is preferably carried out at a temperature of 100 to 130°C, preferably 110 to 125°C.

[0027] To accelerate the reaction, a catalyst may also be used in the method of the present invention. The catalyst is preferably added to the mixture of polyester polyol and polyolefin in step a) of the method of the present invention.

[0028] As catalysts, all known compounds capable of catalyzing isocyanate reactions can be used. Examples include titanates such as tetrabutyl titanate and tetrapropyl titanate; tin carboxylates such as dibutyltin dilaurate (DBTL), dibutyltin diacetate, and tin octoate; tin oxides such as dibutyltin oxide and dioctyltin oxide; organoaluminum compounds such as aluminum trisacetylacetonate and aluminum trisethylacetoacetate; chelate compounds such as titanium tetraacetylacetonate; triethylenediamine, guanidine, diphenylguanidine, 2,4,6-tris(dimethylaminomethyl)phenol, morpholine, N-methylmorpholine, 2-ethyl-4-methylimidazole, 1,8-diazabicyclo-(5,4, The catalyst is 0)-undecene-7 (DBU), 1,4-diazabicyclo[2,2,2]octane, N,N-dimethylpiperazine, 1,8-diazabicyclo[5.4.0]undec-7-ene, dimorpholinodimethyl ether, dimorpholinodiethyl ether (DMDEE), or a mixture thereof. The catalyst is preferably used in an amount of 0.01 to 5% by weight based on the total weight of the reaction mixture.

[0029] To further adjust the properties of the adhesive polyurethane composition, additional polyester polyols may be added after the addition of the 2,4'- / 4,4'-MDI mixture. Therefore, in a preferred embodiment, the method of the present invention includes a further step d) of adding additional polyester polyols after the addition of the 2,4'- / 4,4'-MDI mixture.

[0030] Adhesive polyurethane compositions with a low ratio of NCO to OH tend to increase in viscosity when stored for extended periods. Surprisingly, however, it has been found that the undesirable increase in viscosity can be avoided by treating the resulting polyurethane composition with an additional isocyanate source or monofunctional alcohol. Therefore, in preferred embodiments, the method of the present invention includes step e), further treating the polyurethane composition obtained in step c) or d) with an isocyanate source or monofunctional alcohol. Preferably, the amount of isocyanate source or monofunctional alcohol is in the range of 1 to 5% by weight based on the total weight of the composition. Preferably, the isocyanate source is an NCO-terminated prepolymer with a low content of residual free 4,4'-MDI monomer, preferably less than 0.1% by weight based on the total weight of the prepolymer. Even more preferred is an NCO prepolymer with a total NCO content of 5 to 10%, preferably 6 to 9%, as measured according to the Spiegelberger method. The NCO-prepolymer preferably has a viscosity of 35,000 to 200,000 mPas, preferably 50,000 to 100,000 mPas, at 50°C according to DIN EN ISO 2555 / 2000-01, Brookfield Thermosel, spindle 27.5 rpm.

[0031] The adhesive polyurethane composition of the present invention can be used in a variety of adhesive applications. A further application of the present invention is the use of the adhesive polyurethane composition of the present invention for hot melt adhesives.

[0032] The present invention will be described in more detail with respect to the following embodiments, which should not be understood as limiting the spirit of the invention. [Examples]

[0033] The monomer content of the obtained polyurethane composition was measured by HPLC. OR4088, available from Henkel AG & Co., KGaA, was used as the NCO-terminated prepolymer.

[0034] Example 1: 345 g of the polyester polyol mixture was placed in a reactor and heated to 142°C. Once the temperature was reached, 44.3 g of polyolefin and 0.03 g of phosphorous acid were added, and the mixture was stirred at 140°C for 45 minutes. After the components were completely dissolved, 76.13 g of calcium carbonate was added, and the mixture was dried at 30 mbar and 120°C for 1 hour. Then, 30.7 g of a mixture of 2,4'-MDI and 4,4'-MDI (NCO:OH = 1.31) was added, and the reaction was continued for 45 minutes. Subsequently, 15.4 g of NCO-terminated prepolymer was added.

[0035] Monomer content: 2,4'-MDI: 0.07% by weight 4,4'-MDI: 0.01% by weight Drainage viscosity: η(150°C) = 26400 mPas; after 2 hours, the viscosity was measured at 31500 mPas at 150°C using a viscometer.

[0036] Example 2: 300 g of PPG2000, 105 g of polyester polyol, and 0.03-phosphorous acid were placed in a reactor and heated to 136°C. After reaching the temperature, 44 g of polyolefin was added, and the mixture was vacuum-dried at 30 mbar and 120°C for 1 hour. Next, 54.9 g of 2,4'- / 4,4'-MDI (NCO:OH = 1.25) was added, and the reaction was continued for 45 minutes. After that, an additional 75 g of polyester polyol was added. After 30 minutes, 9.5 g of NCO prepolymer was added, and the reaction was continued for a further 15 minutes.

[0037] Monomer content: 2,4'-MDI: 0.03% by weight 4,4'-MDI: 0.01% by weight Wastewater viscosity: η(130°C) = 6100 mPas; after 2 hours, the viscosity was measured to be 8300 mPas at 130°C using a viscometer.

[0038] Example 3: 278.9 g of a polyester polyol mixture, 24 g of PPG1000, and 0.03 g of phosphorous acid were placed in a reactor. After heating the mixture to 145°C, 48 g of additional polyolefin was added, and the reaction was continued at 140°C for 70 minutes. After the components had dissolved, 86.5 g of calcium carbonate was added, and the mixture was dried at 30 mbar and 123°C for 1 hour. Then, 66.8 g of 2,4'- / 4,4'-MDI (NCO:OH = 1.42) was added, and the reaction was continued for 30 minutes. Next, 6 g of hexadecane-1-ol was added, resulting in an NCO:OH ratio of 1.33, and the reaction was continued for 15 minutes.

[0039] Monomer content: 2,4'-MDI: 0.08% by weight 4,4'-MDI: 0.01% by weight Wastewater viscosity: η(150°C) = 13200 mPas; after 2 hours, the viscosity was measured to be 19900 mPas at 150°C using a viscometer. Preferred embodiments of the present invention include the following: [1] i) at least one polyester polyol; ii) at least one polyolefin; and iii) A mixture of 2,4'- / 4,4'-methylenediphenyl diisocyanates (MDIs) An adhesive polyurethane composition obtained from a reaction mixture containing, Here, the NCO:OH ratio is less than 1.5. An adhesive polyurethane composition characterized in that the monomer MDI content in the composition is less than 0.1% by weight, based on the total weight of the composition. [2] The adhesive polyurethane composition according to [1], characterized in that the NCO:OH ratio is less than 1.4, preferably less than 1.33, and more preferably less than 1.25. [3] The adhesive polyurethane composition according to any one of [1] to [2], characterized in that the content of 4,4'-MDI is less than 0.05% by weight, preferably less than 0.02% by weight, based on the total weight of the composition. [4] The adhesive polyurethane composition according to any one of [1] to [3], characterized in that the at least one polyester polyol has an acid value of 2.0 mg KOH / g or less as measured according to DIN EN ISO 2114. [5] The adhesive polyurethane composition according to any one of [1] to [4], characterized in that the at least one polyester polyol has a hydroxy value of 20 to 45 mg KOH / g, preferably 25 to 40 mg KOH / g, as measured according to DIN EN ISO 4628-2. [6] The adhesive polyurethane composition according to any one of [1] to [5], characterized in that the at least one polyester polyol has an average molecular weight of 1000 to 4500, preferably 1500 to 4000, as measured from its hydroxyl value. [7] The adhesive polyurethane composition according to any one of [1] to [6], characterized in that the polyolefin used in the reaction mixture is an amorphous poly-α-olefin selected from polymers obtained from copolymerization of monomers selected from the group consisting of ethylene, propene, 1-butene, and 1-hexene, and a mixture of copolymers obtained therefrom. [8] The adhesive polyurethane composition according to any one of [1] to [7], characterized in that the polyolefin has a glass transition temperature Tg determined according to ASTM D 3418 of -15 to -50°C, preferably -20 to -40°C. [9] The adhesive polyurethane composition according to any one of [1] to [8], characterized in that the drain viscosity η, measured at 130°C to 150°C using a Brookfield thermocell with a spindle of 27.5 rpm in accordance with DIN EN ISO 2555 / 2000-01, is 5,000 to 35,000 mPas, preferably 6,000 to 30,000 mPas.

[10] The following steps a) A step of providing a solution comprising at least one polyester polyol and at least one polyolefin; b) A step of drying a solution of at least one polyester polyol and at least one polyolefin; c) Adding 2,4'- / 4,4'-methylenediphenyl diisocyanate to obtain the desired polyurethane composition. Includes, A method for producing an adhesive polyurethane composition according to any one of [1] to [9], characterized in that the ratio of NCO to OH in step c) is less than 1.5.

[11] The method according to

[10] , characterized in that the reaction is carried out at a reaction temperature of less than 130°C, preferably less than 115°C.

[12] The method according to either

[10] or

[11] , characterized in that the reaction time is more than 40 minutes, preferably more than 60 minutes.

[13] The method according to any one of

[10] to

[12] , further comprising step d) adding an additional polyester polyol after adding a mixture of 2,4'- / 4,4'-MDI.

[14] The method according to any one of

[10] to

[13] , further comprising step e) of further treating the polyurethane composition obtained in step c) or d) with an additional isocyanate source or monofunctional alcohol.

[15] Use of any of the adhesive polyurethane compositions described in [1] to [8] for hot melt adhesives.

Claims

1. The following steps a) A step of providing a solution comprising at least one polyester polyol and at least one polyolefin; b) A step of drying a solution of at least one polyester polyol and at least one polyolefin; c) Adding a mixture of 2,4'- / 4,4'-methylenediphenyl diisocyanate (MDI) to obtain a desired polyurethane composition. Includes, A method for producing an adhesive polyurethane composition, characterized in that the ratio of NCO to OH in step c) is less than 1.5, Polyolefins are amorphous poly-α-olefins selected from polymers obtained by copolymerization of monomers selected from the group consisting of ethylene, propene, 1-butene, and 1-hexene, and mixtures of copolymers obtained therefrom. The monomer MDI content in the composition is less than 0.1% by weight, based on the total weight of the composition. The content of 4,4'-MDI in the composition is 0.01% by weight or more and less than 0.05% by weight, based on the total weight of the composition. The aforementioned mixture of 2,4'- / 4,4'-methylenediphenyl diisocyanate (MDI) contains 30 to 55% by weight of 2,4'-MDI. A method further comprising step e) further treating the obtained polyurethane composition with an additional isocyanate source or monofunctional alcohol.

2. The method according to claim 1, further comprising step d) adding an additional polyester polyol after adding a mixture of 2,4'- / 4,4'-MDI.

3. The method according to any one of claims 1 to 2, characterized in that the NCO:OH ratio is less than 1.

4.

4. The method according to any one of claims 1 to 3, characterized in that the content of 4,4'-MDI is less than 0.02% by weight based on the total weight of the composition.

5. The method according to any one of claims 1 to 4, characterized in that the at least one polyester polyol has an acid value of 2.0 mg KOH / g or less, as measured according to DIN EN ISO 2114.

6. The method according to any one of claims 1 to 5, characterized in that the at least one polyester polyol has a hydroxy value of 20 to 45 mg KOH / g as measured according to DIN EN ISO 4628-2.

7. The method according to any one of claims 1 to 6, characterized in that the at least one polyester polyol has an average molecular weight of 1,000 to 4,500, as measured from its hydroxyl value.

8. The method according to any one of claims 1 to 7, characterized in that the polyolefin has a glass transition temperature Tg determined according to ASTM D 3418 of -15 to -50°C.

9. The method according to any one of claims 1 to 8, characterized in that the composition has a drain viscosity η of 5,000 to 35,000 mPas, measured at 130°C to 150°C using a Brookfield thermocell and a spindle of 27.5 rpm in accordance with DIN EN ISO 2555 / 2000-01.

10. Use of an adhesive polyurethane composition obtained by any one of claims 1 to 9 for use as a hot melt adhesive.