EP4702068A1

EP4702068A1 - Moisture curable hot-melt adhesive having improved heat stability and low content of residual diisocyanate monomers

Info

Publication number: EP4702068A1
Application number: EP24715842.1A
Authority: EP
Inventors: Martin Schmider
Original assignee: Sika Technology AG
Current assignee: Sika Technology AG
Priority date: 2023-04-24
Filing date: 2024-04-10
Publication date: 2026-03-04
Also published as: WO2024223292A1; CN120882778A

Abstract

The invention relates to a method for producing a moisture-curing hot-melt adhesive composition comprising: A) Reacting at least one first polyol PO1 with at least one polyisocyanate PI, wherein the stoichiometric ratio of isocyanate groups to hydroxyl groups is at least 1.1, preferably 1.2 – 2.5 and B) Adding at least one second polyol PO2 to the to the reaction mixture obtained from step A) and carrying out the reaction, wherein the stoichiometric ratio of isocyanate groups to hydroxyl groups at least 2, preferably at least 2.5, wherein the method further comprises adding at least one poly(meth)acrylate AC in or after step A) and/or in or after step B), wherein the at least one poly(meth)acrylate AC contains no free hydroxyl groups.

Description

MOISTURE CURABLE HOT-MELT ADHESIVE HAVING IMPROVED HEAT STABILITY AND LOW CONTENT OF RESIDUAL DIISOCYANATE MONOMERS

Technical field

The invention relates to a moisture-curing hot-melt adhesive having improved heat resistance and low content of residual diisocyanate monomers and to use of the adhesive as an assembly adhesive, a laminating adhesive, or as an adhesive for the building of sandwich elements, particularly in automotive industry.

Background of the invention

Hot-melt adhesives are solvent free adhesives, which are solid at room temperature and which are applied to the substrate to be bonded in form of a melt. After cooling the adhesive solidifies and forms an adhesive bond with the substrate through physically occurring bonding. Conventional hot-melt adhesives are non-reactive adhesives, which soften again upon heating and are, therefore, not suitable to be used at elevated temperatures. Reactive hot-melt adhesives contain polymers with reactive groups that enable chemical curing of the adhesive, for example, by crosslinking of the polymer chains. Due to the chemically cured polymer matrix reactive hot-melt adhesives do not soften upon heating and these adhesives are, therefore, suitable for use also at elevated temperatures. The chemical curing of the polymers can be initiated, for example, by heating or exposing the adhesive composition to water, such as atmospheric moisture. Moisture curing hot-melt adhesives typically contain polymers functionalized with isocyanate or silane groups, which enables crosslinking of the polymer chains upon contact with atmospheric moisture.

Moisture curing polyurethane hot-melt (PUR-RHM) adhesives consist mainly of isocyanate-functional polyurethane polymers, which have been obtained by reacting suitable polyols, typically polyester and/or polyether polyols, with polyisocyanates, where the reaction is conducted at a molar excess of isocyanate (NCO) groups over hydroxyl (OH) groups. The adhesive composition is cured by reaction of the residual isocyanate groups with water, which results in various chain extension and/or crosslinking reactions of the polymers. A fully cured polyurethane hot-melt adhesive comprises urea and/or urethane bonds and, depending on the starting materials used for providing the isocyanate-functional polymer, ester and/or ether bonds. A crosslinked hot-melt adhesive does not remelt when subjected to heating. However, compared to adhesives with high crosslinking density, such as epoxy or silicone adhesives, the moisture curing polyurethane hot-melt adhesives typically have lower heat resistance properties. This is a significant disadvantage that limits the use of PUR-RHM adhesives in many applications, particularly in bonding of exterior automotive parts, for example headlamps.

Furthermore, the PUR-RHM adhesives typically contain significant quantities of unreacted monomeric diisocyanates (“residual monomers”) since the isocyanate-functional prepolymers are obtained by reacting polyols with a significant stoichiometric excess of monomeric diisocyanates. The application temperature of hot-melt adhesives, such as from 85 to 200°C, particularly from 120 to 160°C, raises the vapor pressure of the residual monomers and causes them to volatilize. Due to the toxicity of certain diisocyanates, additional steps are often taken to reduce their atmospheric concentration in areas to which personnel are exposed. In some cases, ventilated structures such as "down draft booths" are provided to minimize personnel exposure to the volatilized monomers. However, providing such ventilated structures can be costly and difficult. Furthermore, chemicals legislation and classification standards require that polyurethane-based adhesives having a concentration of monomeric MDI of > 1 % must be labeled with an additional hazard statement (H351 ).

There is thus a need for a novel type of moisture curable hot-melt adhesive having improved heat resistance and low content of residual monomeric diisocyanates. Such adhesives are especially suitable for use in bonding of substrates in exterior automotive applications, particularly in headlamp bonding.

Summary of the invention

The object of the present invention is to provide an adhesive composition, which overcomes or at least mitigates the disadvantages of the prior art moisture curable polyurethane hot-melt adhesives as discussed above. Particularly, it is an object of the present invention to provide a moisture curable polyurethane hot-melt adhesive composition having improved heat resistance and low content of residual monomeric diisocyanates.

It was surprisingly found out that the object can be achieved with the features of claim 1 .

Especially, it has been found out that by using a two-step production process comprising adding of at least one poly(meth)acrylate containing no free hydroxyl groups a moisturecuring hot-melt adhesive having improved heat-stability and low content of residual monomeric diisocyanates can be produced.

Specifically, according to the invention, a method for producing a moisture-curing hot-melt adhesive composition is proposed, the method comprising:

A) Reacting at least one first polyol PO1 with at least one polyisocyanate PI, wherein the stoichiometric ratio of isocyanate groups to hydroxyl groups is at least 1.1 , preferably 1 .2 - 2.5 and

B) Adding at least one second polyol PO2 to the to the reaction mixture obtained from step A) and carrying out the reaction, wherein the stoichiometric ratio of isocyanate groups to hydroxyl groups at least 2, preferably at least 2.5, wherein the method further comprises adding at least one poly(meth)acrylate AC in or after step A) and/or in or after step B), wherein the at least one poly(meth)acrylate AC contains no free hydroxyl groups.

As it turned out, the moisture-curing hot-melt adhesive obtained by using the method and containing a catalyst that catalyzes the reactions of isocyanate groups with water also exhibits reduced tendency of expulsion behavior. This is a significant advantage in headlamp bonding applications, where the adhesive is cured at relatively high temperatures and the carbon dioxide released due to the curing reactions tends to build bubbles. If the skin of the adhesive layer is not thick enough or the wetting of the surface not adequate, the released CO2 can press the uncured material out (expulsion behavior) resulting in insufficient adhesion performance. Additional aspects of the present invention are defined in further independent claims. Particularly preferred embodiments are outlined throughout the description and the dependent claims.

Detailed description of the invention

The subject of the present invention is a method for producing a moisture-curing hot-melt adhesive composition comprising:

The prefix “poly” in substance designations such as “polyol” or “polyisocyanate” refers to substances which in formal terms contain two or more per molecule of the functional group that occurs in their designation. A polyol, for example, is a compound having two or more hydroxyl groups, and a polyisocyanate is a compound having two or more isocyanate groups.

The term “polymer” designates a collective of chemically uniform macromolecules produced by a polyreaction (polymerization, polyaddition, polycondensation) where the macromolecules differ with respect to their degree of polymerization, molecular weight, and chain length. The term also comprises derivatives of said collective of macromolecules resulting from polyreactions, that is, compounds which are obtained by reactions such as, for example, additions or substitutions, of functional groups in predetermined macromolecules and which may be chemically uniform or chemically non- uniform. The term “polyurethane polymer” designates polymers prepared by the so called diisocyanate polyaddition process. These also include those polymers which are virtually or entirely free from urethane groups. Examples of polyurethane polymers are polyetherpolyurethanes, polyester-polyurethanes, polyether-polyureas, polyureas, polyesterpolyureas, polyisocyanurates and polycarbodiimides.

The term “isocyanate-functional polyurethane polymer” designates polyurethane polymers comprising one or more unreacted isocyanate groups. The polyurethane prepolymers can be obtained by reacting excess of polyisocyanates with polyols and they are polyisocyanates themselves. The terms “isocyanate-functional polyurethane polymer” and “polyurethane prepolymer” are used interchangeably.

The term “molecular weight” refers to the molar mass (g/mol) of a molecule or a part of a molecule, also referred to as “moiety”. The term “average molecular weight” refers to number average molecular weight (Mn) or to weight average molecular weight (M_w) of an oligomeric or polymeric mixture of molecules or moieties. The molecular weight may be determined by gel permeation chromatography (GPC) using polystyrene as standard, styrene-divinylbenzene gel with porosity of 100 Angstrom, 1000 Angstrom and 10000 Angstrom as the column and, depending on the molecule, tetrahydrofurane as a solvent, at 35°C, or 1 ,2,4-trichlorobenzene as a solvent, at 160 °C.

The term “average OH-functionality” designates the average number of hydroxyl (OH) groups per molecule. The average OH-functionality of a compound can be calculated based on the number average molecular weight (Mn) and the hydroxyl number of the compound.

The term “open time” designates the length of a time period during which an adhesive applied to a surface of a substrate is still able to form an adhesive bond after being contacted with another substrate.

The “amount of at least one component X” in a composition, for example “the amount of the at least one polyol” refers in the present document to the sum of the individual amounts of all polyols contained in the composition. For example, in case the at least one polyol is a polyester polyol and the composition comprises 20 wt.-% of at least one polyol, the sum of the amounts of all polyester polyols contained in the composition equals 20 wt.- %. The term “room temperature" refers to a temperature of ca. 23 °C.

In the first step A) of the method, a first starting mixture comprising at least one first polyol PO1 and at least one polyisocyanate PI is provided and the reaction is conducted to obtain a reaction mixture containing an isocyanate-functional polyurethane prepolymer and unconverted polyisocyanate. The stoichiometric ratio of isocyanate groups to hydroxyl groups in the first starting mixture is at least 1.1 , preferably 1 .2 - 2.5. In the second step B), at least one second polyol PO2 is added to the reaction mixture obtained from step A) to provide a second starting mixture and the reaction conducted to obtain a further reaction mixture. The stoichiometric ratio of isocyanate groups to hydroxyl groups in the second starting mixture is at least 2, preferably at least 2.5, more preferably at least 3.

The reaction conducted in step B) will convert substantially all the hydroxyl groups of the at least one second polyol PO2, particularly at least 95 %, preferably at least 99 %, of the hydroxyl groups of the at least one second polyol PO2.

The reactions in steps A) and B) can be carried out according to conventional methods used for preparation of isocyanate-functional polyurethane polymers. The reactions may, for example, be carried out at temperatures in the range of 50 - 180 °C, preferably 60 - 160 °C, optionally in the presence of a catalyst. The reaction time depends on the temperature employed, but may, for example, be in the range of from 30 minutes to 6 hours, particularly from 30 minutes to 3 hours, preferably from 30 minutes to 1 .5 hours. Suitable catalysts used in the reactions of step A) and B) include, for example, metal catalysts, such as Coscat®83 (from Vertellus Performance Materials Inc.), and tin catalysts.

According to the invention, the method comprises a further step of adding at least one poly(meth)acrylate AC in or after step A) and/or in or after step B), wherein at least one poly(meth)acrylate AC does not contain any free hydroxyl groups.

Particularly, the at least one poly(meth)acrylate AC is a non-functionalized polymer. The term “non-functionalized” refers to polymers that have not been chemically modified to contain functional groups, such as hydroxyl, carboxyl, epoxy, silane, sulfonate, amide, anhydride group on the polymer backbone. The term “(meth)acrylate” designates in the present disclosure methacrylate or acrylate. The term “poly(meth)acrylate” refers to homopolymers, copolymers, and higher interpolymers of an (meth)acrylate monomer with one or more further (meth)acrylate monomers and/or with one or more further monomers.

Suitable (meth)acrylate monomers include, for example, alkyl(meth)acrylates, such as methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, n-butyl methacrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n-octyl methacrylate, n-nonyl acrylate, lauryl acrylate, stearyl acrylate, behenyl acrylate, and their branched isomers, as for example isobutyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, isooctyl acrylate, isooctyl methacrylate, and also cyclohexyl methacrylate, isobornyl acrylate, isobornyl methacrylate or 3,5-dimethyladamantyl acrylate.

Further suitable comonomers for the synthesis of the at least one poly(meth)acrylate AC include vinyl compounds, such as ethylenically unsaturated hydrocarbons with functional groups, vinyl esters, vinyl halides, vinylidene halides, nitriles of ethylenically unsaturated hydrocarbons, phosphoric acid esters, and zinc salts of (meth)acrylic acid. Examples of further suitable comonomers include, for example, maleic anhydride, styrene, styrenic compounds, acrylonitriles, vinyl acetate, vinyl propionate, vinyl chloride, (meth)acrylic acid, beta-acryloyloxypropionic acid, vinylacetic acid, fumaric acid, crotonic acid, aconitic acid, trichloroacrylic acid, itaconic acid, and maleic acid, and amides thereof.

Especially suitable poly(meth)acrylates include, for example, homopolymers and copolymers obtained by free radical polymerization of one or more (meth)acrylate monomers optionally in combination with at least one further comonomer.

In one or more preferred embodiments, the at least one poly(meth)acrylate AC has

- an acid number determined according to EN ISO 2114:2002-06 standard of not more than 5 mg KOH/g, preferably not more than 2.5 mg KOH/g, more preferably not more than 1 mg KOH/g and/or

- a weight average molecular weight (M_w) determined by gel permeation chromatography (GPC) using polystyrene as standard of 25000 - 100000 g/mol, preferably 35000 - 85000 g/mol and/or

- a glass transition temperature (T_g) determined according to ISO 11357-1 :2016 standard of at or above 5 °C, preferably at or above 35 °C. In one or more embodiments, the reaction between the at least one first polyol PO1 and at least one polyisocyanate PI in step A) of the method is conducted in the presence of the at least poly(meth)acrylate AC.

Particularly, in step A) of the method, the at least one poly(meth)acrylate AC is dissolved in the at least one first polyol PO1 followed by adding of the at least one polyisocyanate PI. It may be preferred that the mixture of the at least one poly(meth)acrylate AC and the at least one first polyol PO1 is dehydrated under vacuum at a temperature of at or above 120 °C before adding the at least one polyisocyanate PI.

Especially, the method can comprise a further step of adding at least one inorganic filler F in or after step A) and/or in or after step B.

Suitable inorganic fillers for use in the moisture curable hot-melt adhesive composition include, for example, natural, ground or precipitated calcium carbonates, optionally coated with fatty acids or fatty acid esters, especially stearic acid, baryte (heavy spar), talc, quartz flours, quartz sand, dolomite, wollastonite, kaolin, calcined kaolin, mica (potassium aluminum silicate), molecular sieves, aluminum oxides, boehmite, cristobalite, aluminum hydroxides, magnesium hydroxide, silicas including finely divided silicas from pyrolysis processes, industrially produced carbon black, graphite, metal powders such as aluminum, copper, iron, silver or steel, and hollow spheres.

The inorganic filler F is preferably present in the moisture curable hot-melt adhesive composition in the form of finely divided particles, preferably having a median particle size dso of not more than 75 pm, more preferably not more than 50 pm. The term “median particle size dso" refers in the present disclosure to a particle size below which 50 % of all particles by volume are smaller than the dso value. The particle size distribution can be determined by sieve analysis according to the method as described in ASTM C136/C136M -2014 standard (“Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates).

In one or more embodiments, the at least one inorganic filler F has a median particle size dso in the range of 0.1 - 35 pm, preferably 0.15 - 25 pm, more preferably 0.35 - 15 pm, even more preferably 0.5 - 10 pm, still more preferably 1 - 10 pm. In one or more embodiments, the at least one filler F is selected from calcium carbonate, kaolin, baryte, talc, quartz flour, dolomite, wollastonite, kaolin, calcined kaolin, and mica, preferably calcium carbonate or dolomite, more preferably dolomite.

Examples of suitable dolomites include natural, ground, or precipitated dolomites, with an optional coating of fatty acids or silanes, particularly stearic acid. The dolomite may for example be dolomite rocks or the mineral. Dolomite as mineral is CaMg[(CO3)]2. Dolomite rocks such as dolomite marble may include other constituents, such as lime, in addition to CaMg[(CO₃)]₂.

In one or more embodiments, the at least one first polyol PO1 is a polyether polyol, preferably a at 25 °C liquid polyether polyol.

Suitable polyether polyols, also known as polyoxyalkylene polyols include especially polymerization products of ethylene oxide, 1 ,2-propylene oxide, 1 ,2- or 2,3-butylene oxide, tetrahydrofuran or mixtures thereof, optionally polymerized by means of a starter molecule having two or more active hydrogen atoms, such as, for example, water, ammonia or compounds having two or more OH- or NH- groups such as 1 ,2-ethanediol, 1 ,2- and 1 ,3- propanediol, neopentyl glycol, diethylene glycol, triethylene glycol, isomeric dipropylene glycols and tripropylene glycols, isomeric butanediols, pentanediols, hexanediols, heptanediols, octanediols, nonanediols, decanediols, undecanediols, 1 ,3- and 1 ,4- cyclohexanedimethanol, bisphenol A, hydrogenated bisphenol A, 1 ,1 ,1 -trimethylolethane, 1 ,1 ,1 -trimethylolpropane, glycerol, aniline, and mixtures of the aforesaid compounds.

Use can be made both of polyoxyalkylene polyols which have a low degree of unsaturation (measured according to ASTM D-2849-69 and expressed as milliequivalents of unsaturation per gram of polyol (meq/g)), produced for example by means of double metal cyanide complex catalysts (DMC catalysts), and of polyoxyalkylene polyols having a relatively high degree of unsaturation, produced for example by means of anionic catalysts such as NaOH, KOH or alkali metal alkoxides.

Particularly suitable polyether polyols include polyoxyalkylene diols or poly-oxyalkylene triols, especially polyoxyethylene diols or polyoxyethylene triols. Especially suitable are polyoxyalkylene diols or polyoxyalkylene triols, more particularly polyoxypropylene diols and triols, having a number average molecular weight (Mn) in the range of 1000 - 30000 g/mol, and also polyoxypropylene diols and triols having a number average molecular weight (Mn) of 400 - 8000 g/mol. Suitable polyether polyols are commercially available, for example, under the trade name of Acclaim®, Desmophene®, and Arcol® (all from Covestro).

In one or more embodiments, the at least one first polyol PO1 has a hydroxyl number determined according to ISO 4629-2:2016 standard of 15 - 100 mg KOH/g, preferably 25 - 85 mg KOH/g, more preferably 35 - 75 mg KOH/g, even more preferably 45 - 65 mg KOH/g.

In one or more embodiments, the at least one second polyol PO2 is polyester polyol, preferably a at 25 °C solid polyester polyol, more preferably a at 25 °C solid crystalline polyester polyol.

Suitable polyester polyols can be obtained by reacting dihydric and trihydric, preferably dihydric, alcohols, for example, 1 ,2-ethanediol, diethylene glycol, triethylene glycol, 1 ,2- propanediol, 1 ,3-propanediol, dipropylene glycol, 1 ,4-butanediol, 1 ,5-pentanediol, 1 ,6- hexanediol, 1 ,8-octanediol, 1 ,10-decanediol, 1 ,12-dodecanediol, dimer fatty alcohol, neopentyl glycol, glycerol, 1 ,1 ,1 -trimethylolpropane or mixtures of the aforesaid alcohols, with organic dicarboxylic acids or tricarboxylic acids, preferably dicarboxylic acids, or their anhydrides or esters, such as succinic acid, glutaric acid, 3,3-dimethylglutaric acid, adipic acid, suberic acid, sebacic acid, undecanedioic acid, dodecanedicarboxylic acid, azelaic acid, maleic acid, fumaric acid, phthalic acid, dimer fatty acid, isophthalic acid, terephthalic acid, and hexahydrophthalic acid, or mixtures of the aforesaid acids. Polyester polyols made from lactones such as from s-caprolactone, also known as polycaprolactones, are also suitable.

Preferred polyester polyols include those obtained by reacting adipic acid, sebacic acid or dodecanedicarboxylic acid as dicarboxylic acid and hexanediol or neopentyl glycol as dihydric alcohol. Further examples of suitable polyester polyols include polyester polyols of oleochemical origin. Polyester polyols of this type may be prepared, for example, by complete ring opening of epoxidized triglycerides of a fat mixture comprising at least partially olefinically unsaturated fatty acids, with one or more alcohols having 1-12 carbon atoms, and by subsequent partial transesterification of the triglyceride derivatives to give alkyl ester polyols having 1-12 carbon atoms in the alkyl radical. Particularly suitable crystalline and partially crystalline polyester polyols include adipic acid/hexanediol polyester and dodecanedicarboxylic acid/hexanediol polyesters. In one or more embodiments, the at least one polyester polyol PO1 has a softening point determined by Ring and Ball method according to ISO 4625-1 :2020 standard of at least 85 °C, preferably at least 95 °C, more preferably at least 105 °C, even more preferably at least 110 °C.

In one or more embodiments, the at least one second polyol PO2 has:

- a softening point determined by Ring and Ball method according to ISO 4625-1 :2020 standard of 35 - 100 °C, preferably 40 - 90 °C, more preferably 45 - 85 °C, even more preferably 50 - 80 °C and/or

- a hydroxyl number determined according to ISO 4629-2:2016 standard of 5 - 100 mg KOH/g, preferably 10 - 85 mg KOH/g, more preferably 15 - 75 mg KOH/g, even more preferably 20 - 65 mg KOH/g and/or

- an acid number determined according to EN ISO 2114:2002-06 standard of not more than 15 mg KOH/g, preferably not more than 10 mg KOH/g, more preferably not more than 5 mg KOH/g.

Suitable at 25 °C solid partially crystalline and crystalline polyester polyols are commercially available, for example, under the trade name Dynacoll® 7300-series (from Evonik Industries).

Suitable polyisocyanates to be used as the at least one polyisocyanate PI include, for example, aliphatic, cyclo-aliphatic, and aromatic polyisocyanates, especially diisocyanates, particularly monomeric diisocyanates. Non-monomeric diisocyanates such as oligomeric and polymeric products of monomeric diisocyanates, for example adducts of monomeric diisocyanates are also suitable but the use of monomeric diisocyanates is preferred.

The term “monomer” designates a molecule having at least one polymerizable group. A monomeric di- or polyisocyanate contains particularly no urethane groups. In the context of the present invention, oligomers, or polymer products of diisocyanate monomers such as adducts of monomeric diisocyanates are not monomeric diisocyanates.

An isocyanate is called “aliphatic” when its isocyanate group is directly bound to an aliphatic, cycloaliphatic or arylaliphatic moiety. The corresponding functional group is therefore called an aliphatic isocyanate group. An isocyanate is called “aromatic” when its isocyanate group is directly bound to an aromatic moiety. The corresponding functional group is therefore called an aromatic isocyanate group.

According to one or more embodiments, the at least one polyisocyanate PI is a diisocyanate, preferably a monomeric diisocyanate, more preferably a monomeric diisocyanate having a number average molecular weight (Mn) determined by gel permeation chromatography (GPC) using polystyrene as standard of not more than 1000 g/mol, preferably not more than 500 g/mol, more preferably not more than 400 g/mol.

Examples of suitable monomeric diisocyanates include, for example, 1 ,6-hexamethylene diisocyanate (HDI), 2-methylpentamethylene 1 ,5-diisocyanate, 2,2,4- and 2,4,4-trimethyl- 1 ,6-hexamethylene diisocyanate (TMDI) and mixtures of these isomers, 1 ,10 decamethylene diisocyanate, 1 ,12-dodecamethylene diisocyanate, lysine diisocyanate, lysine ester diisocyanate, cyclohexane 1 , 3-d i isocyanate and cyclohexane 1 ,4-diisocyanate and mixtures of these isomers, 1-methyl-2,4- and -2,6-diisocyanatocyclohexane and mixtures of these isomers (HTDI or H6TDI), 1-isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane (= isophoronediisocyanate or IPDI), perhydro-2,4'- and - 4,4'-diphenylmethane diisocyanate (HMDI or H12MDI) and mixtures of these isomers, 1 ,4- diisocyanato-2,2,6-trimethylcyclohexane (TMCDI), 1 ,3- and 1 ,4-bis(isocyanato- methyl)cyclohexane, m- and p-xylylene diisocyanate (m- and p-XDI) and mixtures of these isomers, m- and p-tetramethyl-1 ,3- and 1 ,4-xylylene diisocyanate (m- and p-TMXDI) and mixtures of these isomers, bis(1-isocyanato-1-methylethyl)naphthalene, 2,4- and 2,6- tolylene diisocyanate and mixtures of these isomers (TDI), 4,4'-, 2,4'- and 2,2'- diphenylmethane diisocyanate and mixtures of these isomers (MDI), 1 ,3- and 1 ,4- phenylene diisocyanate and mixtures of these isomers, 2,3,5,6-tetramethyl-1 ,4- diisocyanatobenzene, naphthalene 1 ,5-di isocyanate (NDI), 3,3'-dimethyl-4,4'- diisocyanatobiphenyl (TODI), and dianisidine diisocyanate (DADI).

According to one or more embodiments, the monomeric diisocyanate is selected from the group consisting of 4,4'-, 2,4'-, and 2,2'-diphenylmethane diisocyanate and mixtures of these isomers (MDI), 2,4- and 2,6-tolylene diisocyanate and mixtures of these isomers (TDI), 1 ,6-hexamethylene diisocyanate (HDI), and 1-isocyanato-3,3,5-trimethyl-5- isocyanatomethylcyclohexane (IPDI). Furthermore, a person skilled in the art knows that the technical grade products of diisocyanates may frequently contain isomer mixtures or other isomers as impurities. According to one or more embodiments, the monomeric diisocyanate is selected from the group consisting of MDI and IPDI. Suitable monomeric diisocyanates are commercially available, for example, under the trade name of Lupranat® (from BASF) and Desmodur (from Covestro).

In one or more embodiments, the method for producing a moisture-curing hot-melt adhesive composition further comprises adding at least one catalyst that catalyzes the reactions of isocyanate groups with water and/or at least one further polyisocyanate to the reaction mixture obtained from step B).

Examples of suitable catalysts include metal-based catalysts such as dialkyltin complexes, particularly dibutyltin(IV) or dioctyltin(IV) carboxylates or acetoacetonates, such as dibutyltindilaurate (DBTDL), dibutyltindiacetylacetonate, dioctyltindilaurate (DOTDL), further bismuth(lll) complexes such as bismuthoctoate or bismuthneodecanoate, zinc(ll) complexes, such as zincoctoate or zincneodecanoate, and zirconium(IV) com-plexes, such as zirconiumoctoate or zirconiumneodecanoate.

Further examples of suitable catalysts include compounds containing amine groups such as, dimorpholinodialkylethers and/or dimorpholino substituted polyalkylene glycols, for example 2, 2'-dimorpholinodiethyl ether and 1 ,4-diazabicyclo[2.2.2]-octane. Combinations of two or more catalysts may also be used, preferred combinations including of one or more metal-catalysts with one or more morpholine amine compounds.

The preferences given above for the at least one polyisocyanate PI are also applicable to the at least one further polyisocyanate. As it turned out, adding small amounts of the further polyisocyanate to the reaction mixture obtained from step B) can result in further improvement of the heat stability of the moisture-curing hot-melt adhesive composition. However, the amount of the at least one further polyisocyanate is preferably adjusted such that the content of monomeric diisocyanates remains under 1 wt.-%.

The preferences given above for the at least one first polyol PO1, the at least one first polyol PO2, the at least one polyisocyanate PI, the at least one poly(meth)acrylate AC, the at least one inorganic filler F, the at least one catalyst, and to the at least one further polyisocyanate apply equally to all further aspects of the present invention unless stated otherwise.

A further aspect of the present invention is related to a moisture-curing hot-melt adhesive composition obtained by using the method of the present invention. The moisture-curing hot-melt adhesive composition is preferably a one-component adhesive composition. The term “one-component composition” refers in context of the present disclosure to a composition in which all constituents of the composition are stored in a mixture in the same container or compartment.

Particularly, the moisture-curing hot-melt adhesive composition comprises 5 - 45 wt.-%, 10 - 40 wt.-%, more preferably 15 - 35 wt.-%, of the at least one poly(meth)acrylate AC.

In one or more embodiments, the moisture-curing hot-melt adhesive composition may comprise 0.5 - 35 wt.-%, preferably 1 - 30 wt.-%, more preferably 2.5 - 25 wt.-%, of the at least one inorganic filler F.

Due to the two-step production process used for producing the adhesive composition, the moisture-curing hot-melt adhesive composition preferably has a content of monomeric diisocyanates of not more than 1.5 wt.-%, preferably not more than 1 .0 wt.-%, more preferably not more than 0.75 wt.-%, based on the total weight of the adhesive composition.

In one or more embodiments, the moisture-curing hot-melt adhesive composition further comprises the at least one catalyst that catalyzes the reactions of isocyanate groups with water, wherein the at least one catalyst preferably makes up 0.005 - 2.00 wt.-%, preferably 0.05 - 1 .00 wt.-%, of the total weight of the adhesive composition.

In one or more embodiments, the moisture-curing hot-melt adhesive composition further comprises the at least one further polyisocyanate, wherein the at least one further polyisocyanate makes up not more than 1 .5 wt.-%, preferably not more than 1 .0 wt.-%, of the total weight of the adhesive composition.

The moisture-curing hot-melt adhesive composition of the present invention can further comprise auxiliary substances and additives, for example, those selected from the group consisting of plasticizers, adhesion promoters, UV absorption agents, UV and heat stabilizers, optical brighteners, pigments, dyes, and desiccants.

Examples of suitable UV stabilizers that can be added to the adhesive composition include, for example, sterically hindered phenols, and suitable UV-absorbers include, for example, hydroxybenzophenones, hydroxybenzotriazoles, triazines, anilides, benzoates, cyanoacrylates, phenylformamidines, and mixtures thereof.

The total amount of such auxiliary substances and additives is preferably not more than 10 wt.-%, more preferably not more than 5 wt.-%, based on the total weight of the moisturecuring hot-melt adhesive composition.

The moisture-curing hot-melt adhesive composition of the present invention has good workability under typical application conditions of hot-melt adhesives, particularly at temperatures in the range of 85 - 200 °C, meaning that at the application temperature the adhesive has sufficiently low viscosity to enable application to a substrate in a molten state. The adhesive composition also develops a high initial strength immediately after the application to a substrate upon cooling even before the initiation of the crosslinking reaction with water, particularly with atmospheric moisture.

Another aspect of the present invention is related to use of the moisture-curing hot-melt adhesive composition of the present invention as an automotive assembly adhesive, preferably for bonding of exterior automotive parts, more preferably for bonding of headlamps.

Another aspect of the present invention is a method for adhesively bonding a first substrate to a second substrate, the method comprising steps of:

I) Heating a moisture-curing hot-melt adhesive composition according to the present invention to provide a melted adhesive composition,

II) Applying the melted adhesive composition to a surface of the first substrate,

III) Contacting the applied adhesive with a surface of the second substrate, and

IV) Chemically curing the applied adhesive with water, preferably with atmospheric moisture.

The first and second substrates can be sheet-like articles having first and second major surfaces defined by peripheral edges and defining a thickness there between or three- dimensional shaped articles. In the method for adhesively bonding a first substrate to a second substrate, the adhesive composition is heated to a temperature above the softening point of the adhesive composition and applied to the surface of the first substrate in molten state using any conventional technique, for example, by means of a roller, brush, or by pouring out and further distributed by means, for example, of a roller, a scraper, or a notched trowel.

After the applied adhesive has been contacted with the surface of the second substrate, the adhesive composition develops a certain initial adhesive strength by physical curing, i.e. , upon cooling. Depending on the application temperature and on the embodiment of the adhesive composition, particularly on the reactivity of the adhesive, the chemical curing reactions may begin already during the application of the adhesive composition on the surface of the first substrate. Typically, however, majority of the chemical curing occurs after the application of adhesive, particularly, after the applied adhesive has been contacted with the surface of the second substrate.

The first and second substrates can be composed of any conventional material including polymeric material, metal, painted metal, glass, wood, wood derived materials such as natural fiber polypropylene (NFPP), and fiber materials. Suitable polymeric materials include, for example, polyethylene (PE), in particular high density polyethylene (HDPE), polypropylene (PP), glass-fiber reinforced polypropylene (GFPP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polystyrene (PS), polycarbonate (PC), polymethylmethacrylate (PMMA), acrylonitrile butadiene styrene (ABS), polyamide (PA), and combinations thereof. The first and second substrates can be composed of a single layer or of multiple layers of different types of materials. The layer(s) composed of polymeric materials can further contain additives such as fillers, plasticizers, flame retardants, thermal stabilizers, antioxidants, pigments, dyes, and biocides.

Examples

The followings compounds and products shown in Table 1 were used in the examples. Table 1

Preparation of inventive adhesive composition

The liquid polyether polyol (PO1 ) and the polymethacrylate (AC) were charged into a stainless-steel reactor. The mixture was kept under vacuum with stirring at 140 °C for 120 minutes to dewater the components and to obtain a homogeneously mixed mixture. The temperature of the thus obtained mixture was lowered to 120 °C and the polyisocyanate (PI1 ) was added to the mixture under a nitrogen blanket. The thus obtained first starting mixture was reacted with stirring for 45 minutes under vacuum at a temperature of 120 °C to obtain a first reaction mixture containing an isocyanate-functional polyurethane prepolymer and unconverted polyisocyanate. The second polyol (PO2), the filler (F), and the pigment were then added to the first reaction mixture and thus obtained second starting mixture was reacted with stirring for 45 minutes under vacuum at a temperature of 120 °C to obtain a second reaction mixture containing reduced amount of monomeric diisocyanates. The catalyst (CA) and the further polyisocyanate (PI2) were then added to the second reaction mixture under nitrogen blanket. After mixing for 45 minutes under vacuum, the obtained adhesive composition was stored at room temperature under exclusion of moisture.

The proportions of compounds shown in Table 2 have been calculated based on the total weight of the produced adhesive composition. Reference adhesive composition

A polyurethane-based moisture curing hot-melt adhesive composition, which is commercially available under the trade name of SikaMelt®-700, was used as reference adhesive composition.

Measurement methods

The adhesive compositions were characterized using the following measurement methods.

Open time

The sample adhesive composition provided in a sealed tube was first preheated in an oven to at temperature of 110 °C for a time period of 30 minutes. After the heating, a sample of 20 g of the molten adhesive was applied with a doctor blade to surface of a silicone paper strip (B700 white, Lautenberg & Sohn KG) placed on a heating plate. The silicone paper strip had dimensions of 30 cm x 10 cm and the adhesive was applied as a film having a thickness of 500 pm and dimensions of 30 cm x 6 cm. Before applying the adhesive film, the silicone paper strip and the doctor blade were heated to a temperature of 110 °C with the heating plate.

Immediately after application of the adhesive, the silicone paper strip was removed from the heating plate and placed (with the adhesive film facing upwards) on a sheet of plywood at room temperature (23 °C) and the time was recorded as the starting point of the measurement. Every 10 seconds a short strip of silicone coated paper having dimensions of 10 cm x 1 cm and formed in a roll (non-siliconized surface facing outwards) was placed on the adhesive film and then slowly removed to separate the strip from the adhesive film. The procedure was repeated until the paper strip could not be removed from the adhesive film without damaging the paper strip or the adhesive film. The time interval between the starting point of the measurement and the last sampling point was recorded as the open time (in seconds) of the adhesive composition

The values of open time presented in Table 2 have been obtained as an average of three measurements conducted with the same adhesive composition. Softening point was measured by Ring and Ball method according to ISO 4625-1 :2020 standard.

Tensile lap-shear

The adhesive was kept in an oven at 130 °C for more than 30 min to ensure the adhesive was provided in a molten state. After the heating, a sample of the molten adhesive was applied on the surface of a wood substrate having dimensions of 9 cm x 2 cm x 5 mm. The adhesive was applied as a coating film having dimensions of 2.5 cm x 1 cm and a thickness of 1 mm.

Immediately after the application of the adhesive, a second wood substrate having same dimensions as the first wood substrate was positioned over the first wood substrate along the edge of the adhesive film to form a test composite element. The second wood substrate was pressed firmly against the first wood substrate to remove air from adhesive bond. A weigh of 150 g was placed on the top surface of the second wood substrate. Any adhesive squeezed out from the joint was trimmed off with a knife. Lap shear strength (LSS) of the test composite element was measured according to EN 1465 standard using a material testing apparatus (Zwick Z 020) and a test speed 10 mm/min.

The lap shear strengths were measure with test composite elements, which had been stored for 3/6/10/20/30 minutes after the bonding of the first wood substrate with the second substrate to investigate the green (initial) adhesive bond strength obtained with the tested adhesive composition.

The lap shear strength values for each adhesive composition presented in Table 2 have been obtained as an average of three measurements conducted with identical test composite elements prepared by using the same adhesive composition.

Tensile strength and elongation at break

The adhesive composition provided in a sealed tube was preheated in an oven to at temperature of 110 °C for a time period of 30 minutes. After the heating, a sample of 40 g of the molten adhesive was applied with a doctor blade to surface of a silicone paper strip (B700 white, Lautenberg & Sohn KG) placed on a heating plate. The silicone paper had dimensions of 60 cm x 10 cm and the adhesive was applied as a film having a thickness of 500 pm and dimensions of 60 cm x 6 cm. Immediately after the application of the adhesive, the silicone paper strip was removed from the heating plate and stored at standard climatic conditions (23 °C, 55 % relative humidity) for a period of 7 days.

The measurements were conducted using a method based on DIN 53504 standard. Five rectangular test specimens having dimensions of 2.0 cm x 8.0 cm were cut from a cured adhesive film having a thickness of 500 pm (cured for 14 days at 23 °C/50% relative humidity). The test specimens were clamped into the tensile testing machine (Zwick Z 020) and pulled apart with a speed of 100 mm/min (test conditions 23°C, 50% relative humidity). The tensile strength and elongation at break were determined based on the measured maximum tensile stress.

The values of tensile strength and elongation at break presented in Table 2 have been obtained as an average of five measurements conducted with the same adhesive composition. at 130 °C

The sample adhesive composition provided in a sealed tube was preheated in an oven at a temperature of 130 °C for a time period of 30 minutes. After the heating, a sample of 12.3 g of the adhesive composition was weighted and placed in a disposable sleeve to a viscometer. The viscosity was measured at temperature of 110 °C at 5 revolutions per minute using a Brookfield DV-2 viscometer with a spindle No. 27 equipped with a Thermosel system. The values obtained with 20 minutes of tempering at the measurement temperature and five minutes of measurement were recorded as representative viscosities.

Shear adhesion failure temperature (SAFT)

The adhesive composition provided in a sealed tube was preheated in an oven at a temperature of 140°C for a time period of 20 minutes. After the heating, a sample of molten adhesive was applied on the surface of a wood specimen (pine) having dimensions of 9 cm x 2 cm x 5 mm and having a 1 mm copper wire on its surface as a spacer. The adhesive was applied as a film having dimensions of 2 cm x 2 cm and a thickness of 1 mm. Immediately after the application of the adhesive a second wood specimen (pine) having same dimensions as the first wood specimen was positioned on the first wood specimen along the edge of the adhesive film. The second wood specimen was pressed firmly against the first wood specimen to remove air from adhesive bond. A weigh of 150 was placed on the top surface of the second wood specimen. Any adhesive squeezed out from the joint was trimmed off with a knife. The test specimens consisting of bonded wood specimens were then stored for 14 days in controlled standard ambient conditions (23°C, 40-60 % relative humidity).

The test specimens were suspended vertically from one end of the first wood specimen on a metal hook and placed in an oven. A metal weight corresponding to a static load of 1 kg was attached to the lower end of the second wood specimen of each test specimen. Three test specimens at a time were placed in the oven for the heat stability measurement.

In the heat stability measurement, the oven was first heated to a temperature, which is 40°C below the anticipated bond failure temperature. The test specimens were kept at this starting temperature for 60 minutes. In case no bond failure occurred, the temperature of the oven was increased by 10°C and the measurement was continued for another 60 minutes. The temperature of the oven was increased in steps of 10°C following the procedure as described above until a bond failure occurred. The last measured temperature before the bond failure occurred was recorded as the heat stability temperature (SAFT).

The SAFT values presented in Table 2 have been obtained as an average of measurements conducted with three identical test specimens prepared with samples of the same hot-melt adhesive composition.

Residual monomeric diisocyanate content

The amount of unconverted monomeric diisocyanates in the hot-melt adhesive composition was determined by means of HPLC (detection via photodiode array) and the measured values are expressed in % by weight, based on the total weight of the hot-melt adhesive composition. Table 2 ^aSikaMelt-700

Claims

1 . A method for producing a moisture-curing hot-melt adhesive composition comprising:

2. The method according to claim 1 , wherein the at least one poly(meth)acrylate AC has an acid number determined according to EN ISO 2114:2002-06 standard of not more than 5 mg KOH/g, preferably not more than 2.5 mg KOH/g.

3. The method according to claim 1 or 2, wherein the reaction between the at least one first polyol PO1 and at least one polyisocyanate PI in step A) is conducted in the presence of the at least poly(meth)acrylate AC.

4. The method according to any one of previous claims further comprising adding at least one inorganic filler F in or after step A) and/or in or after step B.

5. The method according to claim 4, wherein he at least one filler F is selected from calcium carbonate, kaolin, baryte, talc, quartz flour, dolomite, wollastonite, kaolin, calcined kaolin, and mica, preferably dolomite.

6. The method according to any one of previous claims, wherein in step A) the at least one poly(meth)acrylate AC is dissolved in the at least one first polyol PO1 followed by adding of the at least one polyisocyanate PI.

7. The method according to any one of previous claims, wherein the at least one first polyol PO1 is a polyether polyol, preferably a at 25 °C liquid polyether polyol, preferably having a hydroxyl number determined according to EN ISO 4629-2:2016 standard of 15 - 100 mg KOH/g, preferably 35 - 75 mg KOH/g.

8. The method according to any one of previous claims, wherein the at least one polyisocyanate PI is a diisocyanate, preferably a monomeric diisocyanate, preferably having a number average molecular weight (Mn) of not more than 1000 g/mol, preferably not more than 500 g/mol.

9. The method according to any one of previous claims further comprising adding at least one catalyst that catalyzes the reactions of isocyanate groups with water and/or at least one further polyisocyanate to the reaction mixture obtained from step B).

10. A moisture-curing hot-melt adhesive composition obtained by using the method according to any one of previous claims.

11. The moisture-curing hot-melt adhesive composition according to claim 10 comprising 5 - 45 wt.-%, preferably 15 - 35 wt.-%, of the at least one poly(meth)acrylate AC.

12. The moisture-curing hot-melt adhesive composition according to claim 10 or 11 comprising 0.5 - 35 wt.-%, preferably 2.5 - 25 wt.-%, of the at least one inorganic filler F.

13. The moisture-curing hot-melt adhesive composition according to any one of claims 10-12 having a content of monomeric diisocyanates of not more than 1.5 wt.-%, preferably not more than 1 .0 wt.-%, based on the total weight of the adhesive composition.

14. Use of the adhesive composition according to any one of claims 10-13 as an automotive assembly adhesive, preferably for bonding of exterior automotive parts, more preferably for bonding of headlamps.

15. A method for adhesively bonding a first substrate to a second substrate, the method comprising steps of:

I) Heating an adhesive composition according to any one of claims 10-13 to provide a melted adhesive composition,