One-component moisture-curable adhesive composition

Abstract

The invention relates to a one-component moisture-curing polyurethane adhesive composition comprising an isocyanate-functional prepolymer obtainable by reacting (a) one or more polyisocyanates comprising diphenylmethane diisocyanate; and (b) a polyol mixture comprising (b1) a linear oxybutylene-based diol having 2 primary hydroxyl groups and (b2) a polyether triol having 3 secondary hydroxyl groups, wherein the hydroxyl groups in the polyol mixture (b) in the range of 55-75% are hydroxyl groups from the oxybutylene-based diol (b1), wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 3.5-7.0, wherein the content of free isocyanate groups in the adhesive composition is in the range of 14-17 wt.%. The invention further relates to a method of adhering a first piece of material to a second piece of material with the adhesive composition and to an article, in particular a structural wood product, comprising a first piece of material and a second piece of material adhered with the adhesive composition.

Description

Field of the invention

[0001] The present invention relates to a one-component moisture-curable adhesive composition comprising an isocyanate-functional prepolymer, a method of bonding a first piece of material to a second piece of material using the adhesive composition and an article comprising a first piece of material and a second piece of material bonded with the adhesive composition, in particular a structural wood product. BACKGROUND

[0003] Wood-based panels, also known as laminated wood beams, glued laminated beams or glulam, are structural wood products composed of several layers of dimensioned wood glued together. By laminating several smaller pieces of wood, it is possible to manufacture a single strong large structural component from smaller pieces of wood, which is used, for example, as a ridge beam, a garage lintel, a floor beam, a post or a beam, often in a curved arch shape. Glued laminated beams are used in a wide range of applications in construction. Another glued laminated wood product is cross-laminated timber (CLT), which is a panel product made by gluing together solid sawn wood layers. The plies are oriented perpendicular to the adjacent plies and are glued in a generally symmetrical fashion on the broad faces of the plies, so that the outer plies have the same orientation. Ordinary wood is an anisotropic material, which means that the mechanical properties vary depending on the direction of the applied force. By gluing the wood layers at a perpendicular angle, the panel is able to achieve better structural stiffness in both directions. CLT is different from wood-based panels, which are products where all the plies are oriented in the same way.

[0004] Adhesive compositions used in construction materials such as glued laminated wood products and other structural wood products, also known as engineered wood products, need to meet demanding requirements in terms of adhesion strength under indoor and outdoor conditions. Exposure to moisture and thermal stresses generates tension in the wood due to swelling and shrinking and requires that the adhesive can provide excellent bond strength even under such conditions.

[0005] One-component moisture-cured polyurethane adhesives are known for use in bonding pieces of wood to form structural wood products such as glued laminated wood products. Such adhesive compositions comprise an isocyanate-functional prepolymer which cures upon reaction with moisture. The isocyanate groups in the prepolymer react with water to form amine groups, which react with other isocyanate groups to form urea bonds. When used in the manufacture of glued structural wood products, the adhesive composition is typically applied on the surface of the pieces of wood, such as wood or wood-based material, and then the different pieces of wood are pressed together to cure the adhesive composition and form the structural wood product.

[0006] Such polyurethane adhesives are described, for example, in WO 99 / 19141, US 2005 / 032972 and WO 2021 / 022470.

[0007] A polyisocyanate adhesive for use in the manufacture of wood fiber-based structural laminates (plywood manufacture) is described in WO 99 / 19141. The adhesive comprises an isocyanate-containing prepolymer having an isocyanate content of 5 to 25 %, which can be obtained by reacting a polyol mixture having an average nominal functionality of 2 to 4 and an average equivalent weight of 500 to 3000 with a stoichiometric excess of diphenylmethane diisocyanate. As particularly preferred polyol mixture, a polyoxypropylene-polyoxyethylene polyol having up to 5 % of another polyol is mentioned.

[0008] A fast-curing one-component polyurethane adhesive is described in US 2005 / 032972, which contains a polyisocyanate prepolymer and at least one amino ether polyol having a molar ratio of ether groups to amino nitrogen of 7 to 30. The polyurethane prepolymer is produced by reacting a polyisocyanate with a compound containing isocyanate-reactive functional groups.

[0009] A one-component polyurethane prepolymer composition for waterproof coating applications is described in WO 2021 / 022470. It comprises the reaction product of at least one polyisocyanate and a polyol blend comprising a difunctional polyether polyol having a weight average molecular weight of 3,000 to 9,000 g / mol and a trifunctional polyether polyol capped with 10 to 28 wt.-% of ethylene oxide and having a weight average molecular weight of 5,000 to 8,000 g / mol.

[0010] An adhesive composition is described in EP 1072620 comprising an isocyanate mixture of 2,4'- and 4,4'-diphenylmethane diisocyanate and a polyol mixture comprising propylene glycol and a polyether triol, wherein the polyol mixture has an OH number of 73 mg KOH / g.

[0011] A two-component polyurethane adhesive composition is described in US 2021 / 024795 comprising diphenylmethane diisocyanate and a polyol mixture comprising a glycerol-initiated propoxylated and ethoxylated triol and a hydrophobic polyol based on a hydrophobically modified (polyoxybutylene) diol.

[0012] The balance between the longest assembly time (AT) and the shortest press time (PT) is very important for one-component moisture-curing polyurethane adhesive compositions. The longest assembly time is the time during which the pieces of material on which the adhesive composition has been applied can be assembled before pressing. The applied adhesive composition must still be curable when these pieces are pressed together. The shortest press time is the time in the press needed to give sufficient adhesion of the pressed pieces of material to allow further processing of the structural product. A ratio of AT / PT of 1 : 1 to 1 :2.5 is usually acceptable. The AT / PT ratio is ideally close to 1.1, preferably between 1 : 1 and 1 :2.5, more preferably between 1 : 1 and 1 :2. Adhesive compositions with a very good AT / PT ratio are usually worse in mechanical properties such as wet and dry adhesion strength compared to products with a less favourable AT / PT ratio. Therefore, fibres are usually added to one-component moisture-curing polyurethane adhesive compositions to improve mechanical properties such as tensile strength in the machine direction.

[0013] There is a need in the art for one-component moisture-curing polyurethane adhesive compositions that combine a favourable AT / PT ratio and good mechanical properties even without fibres. SUMMARY

[0015] It has now been found that a one-component moisture-curable adhesive composition comprising an isocyanate-functional prepolymer provides good adhesion and has a favourable AT / PT ratio if the prepolymer is prepared by reacting an excess of a polyisocyanate with a specific mixture of polyols and if the content of free isocyanate groups in the adhesive composition is in the range of 14 to 17 wt.%.

[0016] The present invention thus provides in a first aspect a one-component moisture-curing polyurethane adhesive composition comprising an isocyanate-functional prepolymer obtainable by reacting:

[0017] (a) one or more polyisocyanates comprising diphenylmethane diisocyanate; and

[0018] (b) a mixture of polyols comprising:

[0019] (b1) a linear oxybutylene-based diol having 2 primary hydroxyl groups; and

[0020] (b2) a polyether triol having 3 secondary hydroxyl groups,

[0021] wherein in the range of 55 to 75% of the hydroxyl groups in the mixture of polyols (b) are hydroxyl groups from the oxybutylene-based diol (b1),

[0022] wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 3.5 to 7.0, wherein the content of free isocyanate groups in the adhesive composition is in the range of 14 to 17 wt.-%.

[0023] Due to its excellent adhesion properties and its advantageous AT / PT ratio, the one-component moisture-curable polyurethane adhesive composition according to the first aspect is particularly suitable for bonding pieces of material, in particular pieces of wood such as timber or lumber, to form structural wood products.

[0024] The present invention thus provides in a second aspect a method of bonding a piece of a first material to a piece of a second material, comprising contacting a surface of the first material with a surface of the second material, wherein at least one of the contacting surfaces has the adhesive composition according to the first aspect of the present invention applied thereto prior to the contacting.

[0025] The present invention provides in a last aspect an article comprising a piece of a first material and a piece of a second material bonded with the adhesive composition according to the second aspect of the present invention, in particular a structural wood product. DETAILED DESCRIPTION

[0027] The adhesive composition of the present invention is a one-component moisture-curable composition. It comprises an isocyanate-functional prepolymer, preferably in an amount of at least 80 wt.-%, more preferably at least 85 wt.-%, even more preferably at least 90 wt.-%. The adhesive composition can comprise the isocyanate-functional prepolymer in an amount of at most 100 wt.-%, preferably at most 99.8 wt.-%.

[0028] The isocyanate-functional prepolymer can be obtained by reacting:

[0029] (a) one or more polyisocyanates comprising diphenylmethane diisocyanate; and

[0030] (b) a polyol mixture comprising:

[0031] (b1) a linear oxybutylene-based diol having 2 primary hydroxyl groups; and

[0032] (b2) a polyether triol having 3 secondary hydroxyl groups,

[0033] wherein in the range of 55 to 75% of the hydroxyl groups in the polyol mixture (b) are hydroxyl groups from the oxybutylene-based diol (b1),

[0034] wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 3.5 to 7.0.

[0035] The one or more polyisocyanates (a) comprise diphenylmethane diisocyanate (MDI). The diphenylmethane diisocyanate can be 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate or a mixture of two or more thereof. For the purposes of the present specification, the term diphenylmethane diisocyanate refers to diphenylmethane diisocyanate as such and does not include polymeric, oligomeric or modified diphenylmethane diisocyanates such as carbodiimide-, uretonimine- or isocyanurate-modified diphenylmethane diisocyanates.

[0036] The one or more polyisocyanates can comprise polyisocyanates other than diphenylmethane diisocyanate such as isophorone diisocyanate (IPDI), hydrogenated MDI, uretonimine-modified diphenylmethane diisocyanate, polymeric MDI or hexamethylene diisocyanate. Preferably the one or more polyisocyanates have an average nominal isocyanate functionality in the range of 2.0 to 2.3, more preferably 2.0 to 2.1, even more preferably 2.0 to 2.05, still more preferably 2.0 to 2.04. Reference herein to average nominal isocyanate functionality relates to the number average per molecule of isocyanate groups.

[0037] If polyisocyanates other than diphenylmethane diisocyanate are comprised within the one or more polyisocyanates (a), it is preferred that the amount thereof is at most 25 wt.-%, more preferably at most 10 wt.-%, based on the total weight of the one or more polyisocyanates (a). It is particularly preferred that the one or more polyisocyanates consist of diphenylmethane diisocyanate, even more preferably of 4,4'-diphenylmethane diisocyanate or a mixture of 4,4'-diphenylmethane diisocyanate and 2,4'-diphenylmethane diisocyanate and / or 2,2'-diphenylmethane diisocyanate.

[0038] The 4,4'-diphenylmethane diisocyanate content of the one or more polyisocyanates (a) is in the range of 50 to 100 wt.-%. Such mixtures are commercially available, for example, as Suprasec 2004, Suprasec 6004 (from Huntsman), Lupranate MX 119 / 1 (from BASF), Cosmonate JG-50K, Cosmonate PI (from Kumho Mitsui Chemicals) or Ongronat CO4050 (from Wanhua).

[0039] If the one or more polyisocyanates consist of diphenylmethane diisocyanate, the average nominal isocyanate functionality of the one or more polyisocyanates (a) is 2.0.

[0040] The polyol mixture (b) comprises a linear oxybutylene-based diol having 2 primary hydroxyl groups (b1 ) and a polyether triol having 3 secondary hydroxyl groups (b2).

[0041] The oxybutylene-based diol (b1 ) is linear and has 2 primary hydroxyl groups. The 2 primary hydroxyl groups are terminal hydroxyl groups. Such diols can be obtained by polymerization of oxybutylene which is also known as tetrahydrofuran and are also known as polytetrahydrofuran, poly(tetramethylene ether) diol, poly(tetramethylene oxide) or poly(1,4-butanediol). Preferably the diol (b1 ) has a number average molecular weight in the range of 500 to 2,500 g / mol, more preferably 500 to 1,500 g / mol. Such diols (b1 ) are commercially available for example as PolyTHF 650, PolyTHF 1000, PolyTHF 1400 (from BASF).

[0042] The polyether triol (b2) has three secondary hydroxyl groups. The polyether triol (b2) is preferably a polyethylene ether, a polypropylene ether, a polybutylene ether or a hybrid thereof, more preferably a polyethylene ether, a polypropylene ether or a hybrid thereof. Preferably the polyether triol (b2) has a number average molecular weight in the range of 500 to 2,500 g / mol, more preferably 500 to 1,500 g / mol. An example of a particularly suitable polyether triol (b2) is Voranol CP1050 (from Dow) which is a polypropylene ether triol having a number average molecular weight of 1,000 g / mol.

[0043] The amount of oxybutylene-based diol (b1 ) in the polyol mixture (b) should be such that in the range of 55 to 75 %, preferably 60 to 70 % of the hydroxyl groups in the polyol mixture (b) are hydroxyl groups from the oxybutylene-based diol (b1 ). It has been found that a hydroxyl content from the oxybutylene-based diol (b1 ) in the range of 55 to 75 % in the polyol mixture (b) improves the AT / PT ratio compared to lower contents.

[0044] The polyol mixture (b) can comprise polyols other than the oxybutylene-based diol (b1 ) and the polyether triol (b2), preferably in an amount of less than 50 wt.-%, more preferably less than 30 wt.-%, even more preferably less than 10 wt.-%, based on the polyol mixture (b). In a particularly preferred embodiment, the polyol mixture (b) is essentially free of any polyol other than (b1 ) and (b2). Reference herein to a polyol mixture which is essentially free of any polyol other than (b1 ) and (b2) relates to a polyol mixture which comprises at most 1 wt.-%, preferably at most 0.5 wt.-%, of any polyol other than (b1 ) and (b2), even more preferably to a polyol mixture which is free of any polyol other than (b1 ) and (b2).

[0045] The isocyanate-functional prepolymer can be obtained by reacting an excess of a polyisocyanate with a polyol. The equivalent ratio between isocyanate groups in the one or more polyisocyanates (a) and OH groups in the polyol mixture (b) is in the range of 3.5 to 7.0, preferably 4.0 to 6.5, more preferably 4.5 to 6.0. A larger excess of polyisocyanate can lead to a lower molecular weight of the resulting prepolymer, which can adversely affect the adhesion strength and the AT / PT ratio.

[0046] Preferably, the isocyanate-functional prepolymer has a weight average molecular weight of at least 1,500 g / mol, more preferably at least 2,000 g / mol. Particularly preferred is a weight average molecular weight in the range of 2,000 to 5,000 g / mol.

[0047] Reaction conditions for the preparation of isocyanate-functional prepolymers from polyisocyanates and polyols are known in the art. Any suitable reaction conditions can be used.

[0048] Reference herein to number average molecular weight or weight average molecular weight relates to the number average molecular weight or weight average molecular weight determined by size exclusion chromatography using polystyrene standards. A suitable determination method is ISO 16014-1.

[0049] The adhesive composition can comprise components other than the isocyanate-functional prepolymer, such as organic solvents, reactive diluents, curing catalysts, pigments, fillers and one or more additives, such as defoamers and thickening agents.

[0050] The adhesive composition can comprise an organic solvent, preferably in an amount of at most 10 wt.-%, more preferably at most 5 wt.-%, even more preferably at most 2 wt.-%, based on the total weight of the adhesive composition. If an organic solvent is present, the organic solvent is preferably a polar organic solvent such as a ketone, an ester or an ether.

[0051] The adhesive composition can comprise a curing catalyst for curing the moisture cure reaction, typically in an amount in the range of 0 to 0.4 wt.-%. Suitable curing catalysts are known in the art. Tertiary amines, organometallic compounds and strong bases are known to be suitable as curing catalysts. A particularly preferred curing catalyst is 2,2'-dimorpholinodiethylether (DMDEE).

[0052] The adhesive composition can comprise any suitable additives commonly used in adhesive compositions, for example defoamers, thickening agents and / or wetting agents. Preferably, the adhesive composition comprises a defoamer and / or a thickening agent.

[0053] The adhesive composition has a free isocyanate group content in the range of 14 to 17 wt.-%, based on the total weight of the adhesive composition. Free isocyanate groups are free isocyanate groups in the prepolymer and any unreacted polyisocyanate monomer. Further polyisocyanate, preferably diisocyanate, more preferably diphenylmethane diisocyanate, can be added to the prepolymer to achieve a free isocyanate group content in the range of 14 to 17 wt.-% in the adhesive composition.

[0054] More preferably the adhesive composition has a free isocyanate group content in the range of 15.0 to 16.5 wt.-%.

[0055] The free isocyanate group content is determined according to ASTM D2572-19.

[0056] The adhesive composition is preferably free of water or contains only trace amounts of water. Preferably the adhesive composition contains less than 1.0 wt.-%, more preferably less than 0.2 wt.-%, even more preferably less than 0.01 wt.-% of water, based on the total weight of the adhesive composition. Water is explicitly not added to e.g. adjust the viscosity of the coating composition, but water is only present in trace amounts, if at all, as part of typical adhesive additives.

[0057] The adhesive composition is preferably free of fibers.

[0058] In a preferred embodiment the adhesive composition is free of any compounds other than the isocyanate-functional prepolymer, the additional polyisocyanate, the curing catalyst and the additives.

[0059] The one-component moisture-curing adhesive composition is particularly suitable for bonding pieces of wood or other material to form a structural product.

[0060] The present application thus provides a method of bonding a piece of a first material to a piece of a second material, comprising contacting a surface of the first material with a surface of the second material, wherein at least one of the contacting surfaces has the adhesive composition according to the first aspect of the present application applied thereto prior to the contacting.

[0061] The present application provides in a third aspect an article comprising a piece of a first material and a piece of a second material bonded with the adhesive composition according to the first aspect of the present application.

[0062] The materials can be any suitable material such as wood, metal, plastic, paper, glass, concrete, gypsum or other mineral materials such as stone or brick. The pieces of the first and second material can be of different materials. Preferably the pieces of material are pieces of wood.

[0063] The article can be any suitable article. Preferably the material is wood and the article is a structural wood product.

[0064] The application is further illustrated by means of the following non-limiting examples. Examples

[0065] The polyisocyanates and polyols used in the examples are given in Table 1.

[0066] Table 1 Polyisocyanates and polyols

[0067]

[0068] Preparation of isocyanate-functional prepolymer

[0069] The polyisocyanate and polyol were added under nitrogen protection to a three necked reactor equipped with a mechanical stirrer and a cooler. The amounts and types are given in Table 2. The reactor contents were mixed until the exothermic reaction was complete (no further temperature increase) and the reactor was heated to a temperature of 50°C and maintained at this temperature until all polyol was reacted (until the free isocyanate content was equal to the theoretically achievable isocyanate content according to ASTM D2572-19). This was usually after 1.5-2 hours. The reactor was then cooled to room temperature and the resulting prepolymer was collected from the reactor. The molecular weight and the free isocyanate content of the so prepared prepolymer were determined.

[0070] Preparation of adhesive composition

[0071] The adhesive composition was prepared by adding 0.2 wt% of a curing catalyst (Jeffcat DMDEE), 0.05 wt% of an antifoam agent (Foamaster MO 2115) and 0.8 wt% of a thickener (Carbosil TS 720) to the prepolymer. Diphenylmethane diisocyanate (Suprasec 2004) was added until the achieved free isocyanate group content was between 15.5-16.0 wt% based on the total weight of the adhesive composition.

[0072] Test methods

[0073] Longitudinal tensile shear strength

[0074] A beech wood panel assembly was prepared according to EN 302-1 :2013 with a thick bead (0.5 mm gap joint) using each adhesive composition. The adhesive composition was applied in excess (double sided application). The time between glue-up and testing was 13 days (at 65% relative humidity and 20°C).

[0075] The assembly time was approximately 4 minutes, the press time was 16 hours and the applied pressure was 8.0 kg / cm 2 .

[0076] The longitudinal tensile shear strength of the assembly bonded with a thick glue line was determined after treatment A4 as described in EN 302-1 :2013: A4: 6 hours soaking in boiling water followed by 2 hours soaking in cold water (20°C). The samples were tested in the wet state.

[0077] Closure assembly time (AT)

[0078] The closed assembly time of the adhesive composition was determined according to EN 14080:2013.

[0079] Assemblies were prepared by gluing together 5 spruce thin boards of size 285 x 145 x 30 mm using each adhesive composition. The application amount of adhesive composition was 120 g / m 2 The thin boards were assembled into a stack directly after application of the adhesive composition. The assembly was pressed for 16 hours after different time intervals (assembly time). The applied pressure was 8.0 kg / cm 2 .

[0080] The delamination of the glue line was determined according to EN 14080:2013, Annex C using Method B after 5 days at 20°C and 65% relative humidity.

[0081] The closed assembly time is the longest assembly time that can be applied without exceeding the delamination values in Table 9 of EN 14080:2013.

[0082] Shortest press time (PT)

[0083] The shortest pressing time of the adhesive composition was determined according to EN 14080:2013.

[0084] Assemblies were prepared by gluing together 5 spruce thin boards of size 285 x 145 x 30 mm using each adhesive composition. The application amount of adhesive composition was 120 g / m 2 The assemblies were pressed after 2 minutes assembly time with an applied pressure of 8.0 kg / cm 2 The pressing time was variable.

[0085] The delamination of the glue line was determined according to EN 14080:2013, Annex C using Method B after 5 days at 20°C and 65% relative humidity.

[0086] The shortest pressing time is the shortest pressing time required without exceeding the delamination values in Table 9 of EN 14080:2013.

[0087] The longitudinal tensile shear strength, AT and PT are shown in Table 2.

[0088] Table 2: Prepolymers and adhesive compositions: ingredients and properties

[0089]

[0090]

[0091] Comparative Example

[0092] a NCO or OH equivalent

[0093] AT: closed assembly time

[0094] PT: shortest press time

Claims

1. A one-component moisture-curable adhesive composition comprising 80 to 100 percent by weight of an isocyanate-functional prepolymer obtainable by reacting: (a) one or more polyisocyanates comprising diphenylmethane diisocyanate; and (b) a polyol mixture comprising: (b1) a linear oxybutylene-based diol having 2 primary hydroxyl groups; and (b2) a polyether triol having 3 secondary hydroxyl groups, wherein in the range of 55 to 75 percent of the hydroxyl groups in the polyol mixture (b) are hydroxyl groups from the oxybutylene-based diol (b1), wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 3.5 to 7.0, and wherein the content of free isocyanate groups in the adhesive composition is in the range of 14 to 17 percent by weight as determined according to ASTM D2572-19.

2. The one-component moisture-curable adhesive composition according to claim 1, wherein the oxybutylene-based diol (b1) has a number average molecular weight in the range of 500 to 1,500 g / mol as determined by size exclusion chromatography using polystyrene standards according to ISO 16014-1.

3. The one-component moisture-curable adhesive composition according to claim 1, wherein the polyether triol (b2) has a number average molecular weight in the range of 500 to 1,500 g / mol as determined by size exclusion chromatography using polystyrene standards according to ISO 16014-1.

4. The one-component moisture-curable adhesive composition according to claim 2, wherein the polyether triol (b2) has a number average molecular weight in the range of 500 to 1,500 g / mol as determined by size exclusion chromatography using polystyrene standards according to ISO 16014-1.

5. The one-component moisture-curable adhesive composition according to claim 1, wherein the polyol mixture (b) is essentially free of any polyol other than (b1) and (b2).

6. The one-component moisture-curable adhesive composition according to claim 2, wherein the polyol mixture (b) is essentially free of any polyol other than (b1) and (b2).

7. The one-component moisture-curable adhesive composition according to claim 3, wherein the polyol mixture (b) is essentially free of any polyol other than (b1) and (b2).

8. The one-component moisture-curable adhesive composition according to claim 4, wherein the polyol mixture (b) is essentially free of any polyol other than (b1) and (b2).

9. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, wherein in the range of 60 to 70 percent of the hydroxyl groups in the polyol mixture (b) are hydroxyl groups from the oxybutylene-based diol (b1).

10. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 4.0 to 6.

5.

11. The one-component moisture-curable adhesive composition according to claim 9, wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 4.0 to 6.

5.

12. The one-component moisture-curable adhesive composition according to claim 10, wherein the equivalent ratio between the isocyanate groups in (a) and the hydroxyl groups in (b) is in the range of 4.5 to 6.

0.

13. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, wherein the content of free isocyanate groups in the adhesive composition is in the range of 15.0 to 16.5 wt.-%, determined according to ASTM D2572-19.

14. The one-component moisture-curable adhesive composition according to claim 11, wherein the content of free isocyanate groups in the adhesive composition is in the range of 15.0 to 16.5 wt.-%, determined according to ASTM D2572-19.

15. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, wherein the one or more polyisocyanates (a) consist essentially of diphenylmethane diisocyanate.

16. The one-component moisture-curable adhesive composition according to claim 14, wherein the one or more polyisocyanates (a) consist essentially of diphenylmethane diisocyanate.

17. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, comprising 85 to 99.8 wt.-% of the prepolymer.

18. The one-component moisture-curable adhesive composition according to claim 16, comprising 85 to 99.8 wt.-% of the prepolymer.

19. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, further comprising one or more additives.

20. The one-component moisture-curable adhesive composition according to claim 18, further comprising one or more additives.

21. The one-component moisture-curable adhesive composition according to claim 19, further comprising a defoamer and / or a thickener.

22. The one-component moisture-curable adhesive composition according to any one of claims 1 to 8, further comprising a curing catalyst.

23. The one-component moisture-curable adhesive composition according to claim 20, further comprising a curing catalyst.

24. The one-component moisture-curable adhesive composition according to claim 22, wherein the curing catalyst is 2,2'-dimorpholinodiethylether.

25. A method of adhering a first piece of material to a second piece of material, comprising contacting a surface of the first piece of material with a surface of the second piece of material, wherein at least one of the contacting surfaces has the adhesive composition according to any one of claims 1 to 24 applied thereto prior to the contacting.

26. An article comprising a first piece of material and a second piece of material adhered with the adhesive composition according to any one of claims 1 to 24.

27. The article according to claim 26, wherein the materials are wood and the article is a structural wood product.

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