A hot-melt adhesive having improved heat stability

Abstract

The invention relates to an adhesive composition comprising: a) At least one polymer P, b) A stabilizer S comprising b1) At least one first stabilizer S1 and b2) At least one second stabilizer S2, which is different from the at least one stabilizer S1 wherein the first stabilizer S1 is a non-sulfur containing compound having at least two phenolic groups selected from ethylenebis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m- tolyl)propionate] and pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate)the second stabilizer S2 is a sulfur containing compound selected from thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxy-phenyl)propionate] and dilauryl 3,3'-thiodipropionate; wherein the at least one polymer P is selected from at 25 °C solid non-functionalized and functionalized polyolefins;

Description

[0001] A HOT-MELT ADHESIVE HAVING IMPROVED HEAT STABILITY

[0002] Technical field

[0003] The invention relates to hot-melt adhesives having improved heat stability and to use of such adhesives as an assembly adhesive, a laminating adhesive, or as an adhesive for the building of sandwich elements, particularly in automotive industry.

[0004] Background of the invention

[0005] 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. Hot-melt adhesives also include pressure sensitive adhesives (HM-PSA), which are permanently tacky adhesives that are applied as a melt. Non-reactive hot-melt and hot-melt PSA adhesives typically contain a basic polymer matrix comprising amorphous polyolefins, particularly poly-a-olefins or thermoplastic elastomers (TPE), especially styrene block copolymers, and additives, such as tackifying resins, waxes and / or liquid polyolefin resins.

[0006] 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, for example, polyolefins or polyurethane polymers, which are functionalized with isocyanate or silane groups that enable crosslinking of the polymer chains upon contact with atmospheric moisture. The reactive polyolefin-based hot-melt adhesives are typically one-component moisture curable compositions containing as the main polymer component silane-functionalized poly-a-olefins, such as silane grafted poly-a-olefins. Such silane-functionalized poly-a- olefins can be obtained, for example, in a process comprising grafting an olefinical ly unsaturated alkoxysilanes to poly-a-olefins in the presence of a free-radical initiator, such as peroxide. Silane-functionalized poly-a-olefins are commercially available from various suppliers. The crosslinking of the silane grafted poly-a-olefin is accomplished by hydrolyzing the alkoxy silane groups by water followed by dehydrocondensation of the silanol groups, resulting in release of methanol and water.

[0007] A general disadvantage of both reactive and non-reactive hot-melt adhesives is that noncured adhesive compositions have relatively low thermal stability, which limits their use in high temperature applications. One example of such applications is a roller coating process, where the adhesive composition may remain for several minutes in a molten state before being placed between the coating rolls to be applied on a surface of a substrate. In case of insufficient thermal stability, the viscosity of the adhesive may increase to a level, which eventually prevents the application with the rolls. Furthermore, adhesives exhibiting a low thermal stability may change their color from clear to yellow and / or exhibit “angel hairs” and / or “black stains" when stored at high temperatures. Such behavior typically results from degradation of the polymeric constituents of the adhesive and / or from unwanted chemical reactions induced by the application of thermal energy. Changes in appearance of the non-cured or cured adhesive are typically not acceptable, even though they would not have any effect on the performance related properties of the adhesive.

[0008] It is well known that thermal stability of hot-melt adhesives can be improved by using specific antioxidants, also known as heat stabilizers, such as sterically hindered phenolic antioxidants. These are commercially available from various providers, for example, under the trade name of Irganox®, Tinuvin®, and Irgafos® (all from BASF). However, in many cases the use of common antioxidants results only in minor improvement of thermal stability of the adhesive. In fact, many commercially available moisture-curing hot-melt adhesives used today in industrial applications have been found out exhibit non-optimal thermal stability properties. There is thus a need for a hot-melt adhesive having improved thermal stability. Such adhesives are especially suitable for use in roller coating and other demanding industrial application processes.

[0009] Summary of the invention

[0010] The object of the present invention is to provide an adhesive composition, which overcomes or at least mitigates the disadvantages of the prior art adhesives as discussed above.

[0011] Particularly, it is an object of the present invention to provide an adhesive composition exhibiting an improved thermal stability in its non-cured state.

[0012] It was surprisingly found out that the object can be achieved with the features of claim 1 .

[0013] Especially, it was found out that the thermal stability of an adhesive composition can be significantly improved by using a specific combination of a first stabilizer being a non-sulfur containing compound having at least two phenolic groups and a second sulfur containing compound.

[0014] Specifically, according to the invention, an adhesive composition is proposed, the composition comprising: a) At least one polymer P, b) A stabilizer S comprising b1) At least one first stabilizer S1 and b2) At least one second stabilizer S2, wherein the first stabilizer S1 is a non-sulfur containing compound having at least two phenolic groups selected from ethylenebis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m- tolyljpropionate] and pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4- hydroxyphenyljpropionate) and the second stabilizer S2 is a sulfur containing compound selected from thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxy-phenyl)propionate] and dilauryl 3,3'-thiodipropionate and wherein the at least one polymer P is selected from at 25 °C solid non-functionalized and functionalized polyolefins;

[0015] As it turned out, the proposed adhesive compositions are especially suitable for use in roller coating and other demanding industrial application processes, where the adhesive is subjected to high temperatures for prolonged period of time before being applied on a surface of a substrate.

[0016] Additional aspects of the present invention are defined in further independent claims. Particularly preferred embodiments are outlined throughout the description and the dependent claims.

[0017] Detailed description of the invention

[0018] The subject of the present invention is an adhesive composition comprising: a) At least one polymer P, b) A stabilizer S comprising b1) At least one first stabilizer S1 and b2) At least one second stabilizer S2, wherein the first stabilizer S1 is a non-sulfur containing compound having at least two phenolic groups selected from ethylenebis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m- tolyljpropionate] and pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4- hydroxyphenyljpropionate) and the second stabilizer S2 is a sulfur containing compound selected from thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxy-phenyl)propionate] and dilauryl 3,3'-thiodipropionate and wherein the at least one polymer P is selected from at 25 °C solid non-functionalized and functionalized polyolefins;

[0019] 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.

[0020] 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 (Mw) 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.

[0021] 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.

[0022] 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.

[0023] The “amount of at least one component X” in a composition, for example “the amount of the at least one polymer P” refers in the present document to the sum of the individual amounts of all polymers P contained in the composition. For example, in case the at least one polymer P is a poly-a-olefin and the composition comprises 20 wt.-% of at least one polymer P, the sum of the amounts of all poly-a-olefins contained in the composition equals 20 wt.-%. According to the invention, the adhesive composition comprises a stabilizer component S comprising at least one first stabilizer S1 and at least one second stabilizer S2.

[0024] The at least one first stabilizer S1 is a non-sulfur containing compound having at least two phenolic groups.

[0025] The expression “non-sulfur containing" is understood to mean that the molecular structure of the first stabilizer S1 does not contain any sulfur atoms. Furthermore, the term “phenolic group” refers a hydroxylated aromatic ring, whereby the hydroxy group is attached directly to the phenyl, substituted phenyl, or other aryl group.

[0026] The at least one second stabilizer S2 is a sulfur containing compound, which is different from the at least one first stabilizer S1.

[0027] Such first and second stabilizers have been turned out to be especially suitable for improving the thermal stability of the adhesive composition. These types of compounds are commercially available, for example, from BASF under the trade name of Irganox®.

[0028] In preferred embodiments, the stabilizer S comprises ethylenebis(oxyethylene) bis[3-(5- tert-butyl-4-hydroxy-m-tolyl)propionate] as the at least one first stabilizer S1 and dilauryl 3,3'-thiodipropionate as the at least one second stabilizer S2.

[0029] In embodiments, the weight ratio of the at least one first stabilizers S1, the sum of weights of all first stabilizers S1, to the at least one second stabilizers S2, i.e. the sum of weights of all second stabilizer S1 , in the adhesive composition is in the range of from 10:1 to 10: 1 , preferably from 5:1 to 1 :5, more preferably from 3:1 to 1 :3., still more preferably from 2:1 to 1 :2.

[0030] In exemplary embodiments, the weight ratio of the at least one first stabilizers S1 to the at least one second stabilizers S2 is in the range of 1 .5:1 to 1 :1.5, preferably from 1 .3:1 to 1 :1.3, more preferably from 1 .2:1 to 1 :1 .2.

[0031] Particularly, the sum of the weights of the at least one first stabilizer S1 and the at least one second stabilizer S2 makes up at least 0.1 wt.-%, preferably at least 0.15 wt.-% of the total weight of the adhesive composition. In embodiments, the sum of weights of the at least one first stabilizer S1 and the at least one second stabilizer S2 makes up 0.05 - 2 wt.-%, preferably 0.1 - 1.5 wt.-%, more preferably 0.25 - 1 .25 wt.-%, still more preferably 0.35 - 1 of the total weight of the adhesive composition.

[0032] The adhesive composition is a hot-melt adhesive. The term “hot-melt adhesive” refers to solvent free adhesives, which are solid at room temperature and which are applied to a substrate in form of a melt. In the present disclosure, the term “hot-melt adhesive” is considered to encompass also pressure sensitive hot-melt adhesives (HM-PSA).

[0033] In embodiments, the adhesive composition has a softening point determined by the Ring & Ball method as defined in ISO 4625-1 :2020 standard of at or above 85 °C, preferably at or above 95 °C, more preferably at or above 105 °C.

[0034] Hot-melt adhesives can generally be provided as non-reactive or reactive compositions, particularly moisture curing compositions. Typically, moisture curing hot-melt adhesives contain one or more different types of polymers having functional groups, especially isocyanate and / or silane groups.

[0035] The term “silane group” refers to hydrolyzable groups bonded directly to a silicon atom via Si-O- bonds, particularly alkoxy groups or acyloxy groups, such as of formula (II) wherein R1represents an alkyl group with 1 to 8 carbon atoms, especially with 1 to 5 carbon atoms,

[0036] R2represents an alkyl group with 1 to 8 carbon atoms, especially between 1 to 5 carbon atoms, and index a has a value of 0, 1 or 2, especially 0 or 1 .

[0037] Silane groups have the property of undergoing hydrolysis on contact with moisture. In so doing, they form organosilanols, these being organosilicon compounds containing one or more silanol groups (Si-OH groups) and, by subsequent condensation reactions, organosiloxanes, these being organosilicon compounds containing one or more siloxane groups (Si-O-Si groups).

[0038] Particularly, the adhesive composition may be a non-reactive polyolefin-based hot-melt adhesive, a reactive polyolefin-based hot-melt adhesive, especially a moisture curing polyolefin-based hot-melt adhesive

[0039] In first exemplary embodiments, the adhesive composition is a polyolefin-based hot-melt adhesive, wherein the at least one polymer P is selected from at 25 °C solid nonfunctionalized and functionalized polyolefins, particularly from non-functionalized and silane-functionalized polyolefins, preferably form at 25 °C solid non-functionalized and functionalized poly-a-olefins, especially from non-functionalized and silane-functionalized poly-a-olefins, particularly from at 25 °C solid non-functionalized and silane-functionalized amorphous poly-a-olefin (APAO).

[0040] The term "functionalized polymer" designates in the present document polymers which are chemically modified so as to contain one or more functional groups. In contrast, the term "non-functionalized polymer" designates polymers which are not chemically modified so as to contain functional groups, for example, epoxy, silane, sulfonate, amine, amide, or anhydride groups. Furthermore, the terms “non-functionalized and “unmodified” can be used interchangeably.

[0041] Suitable at 25 °C solid non-functionalized or functionalized polyolefins, particularly poly-a- olefins, for use as the polymer P include, for example, homopolymers, copolymers, and terpolymers of monomers selected from ethylene, propylene, 1 -butene and higher a- olefins. Especially suitable at 25 °C solid polyolefins, particularly poly-a-olefins, include homopolymers of propylene, copolymers of propylene and ethylene, copolymers of propylene and 1 -butene or other higher a-olefins, homopolymers of ethylene, copolymers of ethylene and propylene, copolymers of ethylene and 1 -butene or other higher a-olefins, and terpolymers of ethylene, propylene, and 1 -butene.

[0042] In embodiments, the at least polymer P is selected from at 25 °C solid non-functionalized and functionalized amorphous poly-a-olefin (APAO), particularly from at 25 °C solid non- functionalized and silane-functionalized amorphous poly-a-olefin (APAO). 2023-0092WC

[0043] 9 / 25

[0044] The term "amorphous poly-a-olefin" designates in the present disclosure poly-a-olefins having a low crystallinity degree, such as in the range of 0.001 - 10 wt.-%, particularly 0.001 - 5 wt.-%. A crystallinity degree of a polymer can be determined by using differential scanning calorimetry (DSC) measurement to determine the heat of fusion of the polymer, from which the degree of crystallinity is calculated. In particular, the term “amorphous poly- a-olefin” designates poly-a-olefins lacking a crystalline melting temperature (Tm) as determined by DSC or equivalent technique.

[0045] Suitable silane-functionalized poly-a-olefins can be obtained, for example, in a process comprising grafting an olefinically unsaturated alkoxysilanes to poly-a-olefins in the presence of a free-radical initiator, such as peroxide.

[0046] Suitable amorphous poly-a-olefins for use as the at least one polymer P include, for example, amorphous atactic polypropylene, amorphous propene rich propylene-a-olefin copolymers, and amorphous propene rich propylene-a-olefin terpolymers. Such amorphous poly-a-olefins are known to a person skilled in the art and they can be obtained, for example, by polymerization of a-olefins in the presence of a polymerization catalyst, such as a Ziegler-Natta catalyst or a metallocene catalyst or any other single-site catalyst.

[0047] Suitable non-functionalized or functionalized amorphous poly-a-olefins, particularly nonfunctionalized and silane-functionalized amorphous poly-a-olefins, are commercially available, for example, under the trade name of Vestoplast® (from Evonik Industries), under the trade name of Eastoflex® (from Eastman Corporation), and under the trade name of REXtac® (from REXtac LLC).

[0048] Particularly suitable at 25 °C solid non-functionalized and functionalized amorphous poly- a-olefin (APAO) for use in the polyolefin-based hot-melt adhesive have:

[0049] - a softening point (Ts) determined by using the Ring and Ball method as defined in ISO 4625-1 :2020 standard of 60 - 200 °C, preferably 75 - 185 °C, more preferably 85 - 175 °C and / or

[0050] - an average molecular weight (Mn) in the range of 2500 - 35000 g / mol, preferably 3000 - 30000 g / mol, more preferably 5000 - 25000 g / mol and / or - a melt viscosity at 190 °C determined according to DIN 53019 standard of not more than 150000 MPa s, preferably not more than 135000 MPa s, more preferably not more than 125000 MPa s and / or

[0051] - xylene cold soluble content (XCS) determined at 25 °C according ISO 16152-2005 standard of at least 80 wt.-%, preferably at least 90 wt.-%, more preferably at least 95 wt.- % and / or

[0052] - a heat of fusion (Hf) as determined by DSC measurements of not more than 35 J / g, preferably not more than 30 J / g, more preferably not more than 25 J / g.

[0053] Particularly, the polyolefin-based hot-melt adhesive may comprise 5 - 85 wt.-%, preferably 15 - 75 wt.-%, more preferably 25 - 65 wt.-%, of the at least one polymer P.

[0054] In embodiments, the polyolefin-based hot-melt adhesive further comprises: c) At least one tackifying resin TR and d) Optionally at least one wax W.

[0055] The term “tackifying resin” designates in the present document resins that in general enhance the adhesion and / or tackiness of an adhesive composition. The term “tackiness” designates in the present document the property of a substance of being sticky or adhesive by simple contact. The tackiness can be measured, for example, as a loop tack. Preferred tackifying resins are tackifying at a temperature of 25°C

[0056] Examples of suitable tackifying resins include natural resins, synthetic resins and chemically modified natural resins.

[0057] Examples of suitable natural resins and chemically modified natural resins include rosins, rosin esters, phenolic modified rosin esters, and terpene resins. The term “rosin” is to be understood to include gum rosin, wood rosin, tall oil rosin, distilled rosin, and modified rosins, for example dimerized, hydrogenated, maleated and / or polymerized versions of any of these rosins.

[0058] Suitable terpene resins include copolymers and terpolymers of natural terpenes, such as styrene / terpene and alpha methyl styrene / terpene resins; polyterpene resins generally resulting from the polymerization of terpene hydrocarbons, such as the bicyclic monoterpene known as pinene, in the presence of Friedel-Crafts catalysts at moderately low temperatures; hydrogenated polyterpene resins; and phenolic modified terpene resins including hydrogenated derivatives thereof.

[0059] The term “synthetic resin” refers to compounds obtained from the controlled chemical reactions such as polyaddition or polycondensation between well-defined reactants that do not themselves have the characteristic of resins.

[0060] Monomers that may be polymerized to synthesize the synthetic resins may include aliphatic monomer, cycloaliphatic monomer, aromatic monomer, or mixtures thereof. Aliphatic monomers can include C4, Cs, and Ce paraffins, olefins, and conjugated diolefins. Examples of aliphatic monomer or cycloaliphatic monomer include butadiene, isobutylene, 1 ,3-pentadiene, 1 ,4-pentadiene, cyclopentane, 1 -pentene, 2-pentene, 2- methyl-1 - pentene, 2-methyl-2-butene, 2-methyl-2-pentene, isoprene, cyclohexane, 1 - 3-hexadiene, 1 -4-hexadiene, cyclopentadiene, dicyclopentadiene, and terpenes. Aromatic monomer can include Cs, C9, and C10 aromatic monomer. Examples of aromatic monomer include styrene, indene, derivatives of styrene, derivatives of indene, coumarone and combinations thereof.

[0061] Particularly suitable synthetic resins include synthetic resins made by polymerizing mixtures of unsaturated monomers that are obtained as by-products of cracking of natural gas liquids, gas oil, or petroleum naphthas. Such synthetic resins obtained from petroleum -based feedstocks are also characterized as “petroleum resins” or “hydrocarbon resins”. These include also pure monomer aromatic resins, which are made by polymerizing aromatic monomer feedstocks that have been purified to eliminate color causing contaminants and to precisely control the composition of the product. Tackifying hydrocarbon resins typically have a relatively low average molecular weight (Mn), such in the range of 250 - 5000 g / mol and a glass transition temperature of above 0°C, preferably equal to or higher than 15°C, more preferably equal to or higher than 30°C.

[0062] Examples of suitable hydrocarbon resins for use as the tackifying resin TR include, for example, C5 aliphatic hydrocarbon resins, mixed C5 / C9 aliphatic / aromatic hydrocarbon resins, aromatic modified C5 aliphatic hydrocarbon resins, cycloaliphatic hydrocarbon resins, mixed C5 aliphatic / cycloaliphatic hydrocarbon resins, mixed C9 aromatic / cycloaliphatic hydrocarbon resins, mixed C5 aliphatic / cycloaliphatic / C9 aromatic 2023-0092WG

[0063] 12 / 25 hydrocarbon resins, aromatic modified cycloaliphatic hydrocarbon resins, C9 aromatic hydrocarbon resins, polyterpene resins, and copolymers and terpolymers of natural terpenes as well hydrogenated versions of the aforementioned hydrocarbon resins. The notations "C5" and "C9" indicate that the monomers from which the resins are made are predominantly hydrocarbons having 4-6 and 8-10 carbon atoms, respectively. The term “hydrogenated” includes fully, substantially, and at least partially hydrogenated resins. Partially hydrogenated resins may have a hydrogenation level, for example, of 50%, 70%, or 90%.

[0064] Preferably, the at least one tackifying resin TR is a non-functionalized tackifying resin. The term "non-functionalized tackifying resin" designates tackifying resins which are not chemically modified so as to contain functional groups such as epoxy, silane, sulfonate, amide, or anhydride groups.

[0065] In embodiments, the at least one tackifying resin TR has:

[0066] - a softening point measured by a Ring and Ball method according to ISO 4625-1 :2020 standard in the range of 65 - 175 °C, preferably 70 - 165 °C, more preferably 80 - 155 °C, and / or

[0067] - a number average molecular weight (Mn) in the range of 150 - 5000 g / mol, preferably 250 - 3500 g / mol, more preferably 250 - 3000 g / mol, and / or

[0068] - a glass transition temperature (Tg) determined by dynamical mechanical analysis (DMA) as the peak of the measured loss modulus (G”) curve using an applied frequency of 1 Hz and a strain level of 0.1 % of at or above 0 °C, preferably at or above 10 °C, more preferably at or above 15 °C.

[0069] Suitable hydrocarbon resins are commercially available, for example, under the trade name of Wingtack® series, Wingtack® Plus, Wingtack® Extra, and Wingtack® STS (all from Cray Valley); under the trade name of Escorez® 1000 series, Escorez® 2000 series, and Escorez® 5000 series (all from Exxon Mobile Chemical); under the trade name of Novares® T series, Novares® TT series, Novares® TD series, Novares® TL series, Novares® TN series, Novares® TK series, and Novares® TV series (all from RUTGERS Novares GmbH); and under the trade name of Kristalex®, Plastolyn®, Piccotex®, Piccolastic® and Endex® (all from Eastman Chemicals). In exemplary embodiments, the polyolefin-based hot melt adhesive contains 5 - 75 wt.-%, preferably 15 - 65 wt.-%, more preferably 25 - 55 wt.-% of the at least one tackifying resin

[0070] TR

[0071] The term “wax” designates in the present disclosure substances that have a waxy consistency and have a melting temperature or melting temperature range of above normal room temperature, particularly above 25 °C.

[0072] Suitable waxes to be used in the acoustic damping material include in particular synthetic waxes, for example, petroleum waxes, such as paraffin wax, petrolatum, and microcrystalline waxes, polyolefin waxes, polyethylene glycol waxes (Carbowax), amide waxes, and chemically modified waxes, such as hardened or hydrogenated waxes, for example, Montan ester waxes.

[0073] Particularly, the at least one wax W may be selected from the group consisting of paraffin waxes, polyolefin waxes, microcrystalline waxes, and amide waxes, especially from paraffin waxes and polyolefin waxes.

[0074] The term “paraffin wax” refers in the present document to hard, crystalline wax composed mainly of saturated paraffin hydrocarbons. The paraffin waxes are typically obtained from petroleum distillates or derived from mineral oils of the mixed-base or paraffin-base type.

[0075] Especially suitable paraffin waxes include Fischer-Tropsch waxes, particularly having a softening point determined by using the Ring and Ball method according to ISO 4625- 1 :2020 standard of 75 - 150 °C, particularly 80 - 140 °C, such as 85 - 130 °C.

[0076] The term “polyolefin wax” refers in the present document low molecular weight polymers of linear or branched a-olefins having from 2 to 30 carbon atoms and an average molecular weight (Mn) in the range of 5000 - 25000 g / mol. They include both homopolymers and copolymers of the above mentioned linear or branched a-olefins. Polyolefin waxes can be obtained by thermal decomposition of polyolefin plastics, particularly polyethylene plastic, or by direct polymerization of olefins.

[0077] Suitable polymerization processes to obtain polyolefin waxes include, for example, free- radical processes, where the olefins, for example, ethylene, are reacted at high pressures 2023-0092WG

[0078] 14 / 25 and temperatures to give more or less branched waxes and processes, where ethylene and / or higher a-olefins, particularly propylene, are polymerized using metalorganic catalysts, for example Ziegler-Natta or metallocene catalysts, to give unbranched or branched waxes. The polyolefin waxes have generally at least partially crystalline structure.

[0079] Especially suitable polyolefin waxes include polar-modified polyolefin waxes. These types of waxes may be used in the polyolefin-based hot-melt adhesive, for example, to improve the adhesion to polar substrates. Suitable polar-modified polyolefin waxes include waxes that are obtained by grafting polar olefin monomers, for example, a-[3-unsaturated carboxylic acids and / or derivatives thereof, for example (meth)acrylic acid or maleic acid anhydride and / or substituted and / or unsubstituted styrene, to polyolefin waxes.

[0080] Especially suitable polar-modified polyolefin waxes include maleic acid anhydride- functionalized polyolefin waxes, particularly selected from ethylene and propylene homo and copolymer waxes grafted with maleic acid anhydride, for example polypropylene and polyethylene waxes grafted with maleic acid anhydride.

[0081] Suitable maleic acid anhydride-functionalized polyolefin waxes may have a maleic anhydride grafting degree of at least 1 wt.-%, for example at least 3 wt.-%, particularly 2 - 15 wt.-%, preferably 4 - 15 % wt.-% and / or a melt viscosity at a temperature of 170 °C determined according to DIN 53019 standard in the range of 10 - 10000 mPa s, for example 100 to 5000 mPa s, particularly 500 - 3500 mPa s.

[0082] In exemplary embodiments, the polyolefin-based hot-melt adhesive contains 0.5 - 20 wt.- %, preferably 1 - 15 wt.-%, more preferably 1 .5 - 10 wt.-% of the at least one wax W.

[0083] Generally, the 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.

[0084] 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.

[0085] 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 adhesive composition.

[0086] The 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.

[0087] A further aspect of the present invention is use of the adhesive composition of the present invention as an automotive assembly adhesive, laminating adhesive, or as an adhesive for the building of sandwich elements.

[0088] A still further aspect of the present invention is a method for adhesively bonding a first substrate to a second substrate, the method comprising steps of:

[0089] I) Heating an adhesive composition according to present invention to provide a melted adhesive composition,

[0090] II) Applying the melted adhesive composition to a surface of the first substrate,

[0091] III) Contacting the applied adhesive with a surface of the second substrate, and

[0092] IV) Physically curing the applied adhesive by cooling and / or chemically curing the applied adhesive with water, preferably with atmospheric moisture.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] Another aspect of the present invention is use of a stabilizer SB for improving the heat stability of an adhesive composition, preferably a hot-melt adhesive composition, wherein the stabilizer SB comprises: b1) At least one first stabilizer SB1 and b2) At least one second stabilizer SB2, wherein the first stabilizer SB1 is a non-sulfur containing compound having at least two phenolic groups and the second stabilizer SB2 is a sulfur containing compound. In embodiments, the first stabilizer SB1 is the first stabilizer S1 and / or the second stabilizer SB2 is the second stabilizer S2.

[0098] In embodiments, the adhesive composition is a polyolefin-based hot-melt adhesive composition.

[0099] Particularly, the adhesive composition comprising the stabilizer SB may be selected from the polyolefin-based hot-melt adhesive as discussed above. In preferred embodiments, the first stabilizer SB1 is a non-sulfur containing compound having at least two phenolic groups selected from ethylenebis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m- tolyl)propionate] and pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate) and the second stabilizer SB2 is a sulfur containing compound compound selected from thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxy- phenyl)propionate] and dilauryl 3,3'-thiodipropionate.

[0100] Examples

[0101] The followings compounds and products shown in Table 1 were used in the examples. Table 1

[0102] Preparation of adhesive compositions The reference and inventive adhesive compositions were prepared by mixing of the first and second stabilizer to the corresponding commercially available adhesive material.

[0103] Measurement methods The adhesive compositions were characterized using the following measurement methods. 2023-0092WC

[0104] 19 / 25

[0105] Storage stability of non-reactive polyolefin adhesive

[0106] Storage stability of the commercially available the non-reactive polyolefin-based hot-melt adhesive SikaMelt-172 with inventive stabilizer package (S) and without an inventive stabilizer package was determined by measuring the viscosity at a temperature of 180 °C and a rotating speed of 5 revolutions per minute.

[0107] The inventive stabilizer package S contained the stabilizer S11 and S21 at a weight ratio of 1 :1.

[0108] The inventive adhesive formulations contained 0.6 wt.-% of the stabilizer package S.

[0109] The results of the viscosity measurements are shown in Table 2 below. It is evident that the adhesive formulation without the inventive stabilizer package exhibits a higher change in viscosity compared to the same adhesive with the inventive stabilizer package. Table 2

[0110] Roller coating stability of the reactive polyolefin-based adhesive

[0111] Roller coating stability of a commercially available reactive polyolefin-based hot-melt adhesive SikaMelt-890 with inventive stabilizer package (S’), with comparative stabilizer packaged (S” or S’”) and without an inventive stabilizer package was determined by measuring the increase in viscosity in an open roller over a specific period of time.

[0112] The inventive stabilizer package S’ contained the stabilizer S11 and S21 at a weight ratio of 1 :1.

[0113] The comparative stabilizer package S” contained the stabilizer S12 and S21 at a weight ratio of 1 : 1.

[0114] The comparative stabilizer package S’” contained the stabilizer S12 and S24 at a weight ratio of 1 : 1 .

[0115] The inventive adhesive formulations contained 0.6 wt.-% of the stabilizer package S’ or S” or S’”.

[0116] The stability measurement was conducted by providing the tested hot-melt adhesive in a roller coater and determining the viscosity of the material after every 10 minutes. The stringing behavior of the material was evaluated by laminating a TPO-foil with the tested adhesive. The measurement was ended when the tested adhesive showed significant formation of agglomerates and / or stringing behavior.

[0117] The roller coating process was conducted using the following process parameters:

[0118] Roller temperature: 165 °C

[0119] Application weight: ca. 100 g / m2

[0120] Application roller speed: 5 m / min

[0121] Transfer roller speed: 0.5 m / min

[0122] Metering gap = 0.4 mm

[0123] The results of the roller coating stability measurements are shown in Table 3 below. It is evident that the comparative adhesive formulation without the inventive stabilizer package loses its stability after 10 minutes of roller coater processing. The adhesive formulation with comparative stabilizer packages S” and S’” lose their stability after 35 and 20 minutes respectively, whereas the adhesive formulations with the inventive stabilizer package S’ remains stable for a significantly longer time. Table 3

Claims

Claims1 . A hot-melt adhesive composition comprising: a) At least one polymer P, b) A stabilizer S comprising: b1 ) At least one first stabilizer S1 and b2) At least one second stabilizer S2, wherein the first stabilizer S1 is a non-sulfur containing compound having at least two phenolic groups selected from ethylenebis(oxyethylene) bis[3-(5-tert- butyl-4-hydroxy-m-tolyl)propionate] and pentaerythritol tetrakis(3-(3,5-di-tert- butyl-4-hydroxyphenyl)propionate) and the second stabilizer S2 is a sulfur containing compound selected from thiodiethylene bis[3-(3,5-di-tert-butyl-4- hydroxy-phenyl)propionate] and dilauryl 3,3'-thiodipropionate; wherein the at least one polymer P is selected from at 25 °C solid nonfunctionalized and functionalized polyolefins.

2. A hot-melt adhesive composition according to claim 1 , wherein the stabilizer S comprises ethylenebis(oxyethylene) bis[3-(5-tert-butyl-4-hydroxy-m- tolyl)propionate] as the at least one first stabilizer S1 and dilauryl 3,3'- thiodipropionate as the at least one second stabilizer S2.

3. The adhesive composition according to any one of previous claims, wherein weight ratio of the at least one first stabilizer S1 to the at least one second stabilizer S2 is in the range of from 10:1 to 10:1 , preferably from 5:1 to 1 :5.

4. The adhesive composition according to any one of previous claims, wherein the adhesive composition has a softening point determined by the Ring & Ball method as defined in ISO 4625-1 :2020 standard of at or above 85 °C, preferably at or above 105 °C.

5. The adhesive composition according to any one of previous claims, wherein the adhesive composition is a polyolefin-based hot-melt adhesive.

6. The adhesive composition according to claim 5, wherein the composition further comprises: c) at least one tackifying resin TR; d) optionally at least one wax W.

7. The adhesive composition according to claim 6, wherein the at least one tackifying resin TR is a hydrocarbon resin, particularly a hydrogenated hydrocarbon resin.

8. The adhesive composition according to claim 6, wherein the at least one wax W is a polyolefin wax, particularly a polar-modified polyolefin wax.

9. The adhesive composition according to claim 6, wherein the at least one wax W is a polypropylene wax, particularly a polypropylene wax grafted with maleic acid anhydride.

10. The adhesive composition according to any one of previous claims, wherein the at least one polymer P is selected from at 25 °C solid non-functionalized and functionalized poly-a-olefins, especially from at 25 °C solid non-functionalized and functionalized amorphous poly-a-olefins (APAOs).11 . The adhesive composition according to any of previous claims, wherein the at least one polymer P is selected from at 25 °C solid non-functionalized and silane-functionalized poly-a-olefins, particularly from at 25 °C solid non- functionalized and silane-functionalized amorphous poly-a-olefins (APAOs).

12. Use of the adhesive composition according to any one of previous claims as an automotive assembly adhesive, laminating adhesive, or as an adhesive for the building of sandwich elements .

13. 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 1-11 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, andIV) Physically curing the applied adhesive by cooling and / or chemically curing the applied adhesive with water, preferably with atmospheric moisture.

14. An article obtained by the method according to claim 13.

15. Use of a stabilizer SB for improving the heat stability of a hot-melt adhesive composition, preferably a polyolefine-based hot-melt adhesive composition, wherein the stabilizer SB comprises: b1 ) At least one first stabilizer SB1 and b2) At least one second stabilizer SB2, wherein the first stabilizer SB1 is a non-sulfur containing compound having at least two phenolic groups selected from ethylenebis(oxyethylene) bis[3-(5-tert- butyl-4-hydroxy-m-tolyl)propionate] and pentaerythritol tetrakis(3-(3,5-di-tert- butyl-4-hydroxyphenyl)propionate) and the second stabilizer SB2 is a sulfur containing compound selected from thiodiethylene bis[3-(3,5-di-tert-butyl-4- hydroxy-phenyl)propionate] and dilauryl 3,3'-thiodipropionate.

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