Low free polyurethane prepolymer composition

Abstract

The present invention relates to a polyurethane prepolymer composition, a curable composition comprising these polyurethane prepolymer compositions, and the use of these polyurethane prepolymer compositions as adhesives, the polyurethane prepolymer composition comprising more than 0 wt.% and less than 1.0 wt.% of free diisocyanate monomer, wherein the polyurethane prepolymer comprises less than 80 wt.% of a 2:1 stoichiometric adduct (complete prepolymer) of a diisocyanate and at least one polyol.

Description

Technical Field

[0001] The present invention relates to a polyurethane prepolymer composition, a curable composition comprising these polyurethane prepolymer compositions, and the use of these polyurethane prepolymer compositions as an adhesive, wherein the polyurethane prepolymer composition comprises more than 0 wt.% and less than 1.0 wt.% of free diisocyanate monomer, and wherein the polyurethane prepolymer comprises less than 75 wt.% of a 2:1 stoichiometric adduct (perfect prepolymer) of diisocyanate and at least one polyol. Background Art

[0002] Isocyanate-terminated polyurethane prepolymers are commonly used in the production of polyurethane products such as elastomers, foams, coatings, adhesives, sealants, and binders. However, polyurethane prepolymer manufacturing processes typically result in high residual concentrations of the polyisocyanate monomer used in prepolymer synthesis. Residual polyisocyanate can cause potential health and safety issues and can also be detrimental to the performance and properties of end-use products. For example, due to the migration of these molecules to the interface, residual polyisocyanate can cause an undesired loss of open time, product instability, increased moisture sensitivity, and reduced adhesion. Polyurethane prepolymers containing low levels of residual diisocyanate, less than 1.0 wt.%, preferably less than 0.1 wt.%, based on the total weight of the polyurethane prepolymer, can reduce health and safety risks and improve end-product performance.

[0003] Since residual polyisocyanate can pose significant health and safety risks as well as reduce product performance, many products and methods have been introduced that provide reduced levels of residual polyisocyanate.

[0004] JP08176252 discloses reacting MDI with a straight-chain molecule having Mw = 250 - 4,000 and two active hydrogens in an equivalent ratio (NCO:OH) of 2.5 - 10:1. The free MDI is vacuum distilled to 1 wt.% or less. The examples show polytetramethylene glycol (PTMEG) and ethylene adipate.

[0005] US-B-4,786,703 discloses a method for producing a reaction product comprising a TDI prepolymer, wherein at least about 90% of this prepolymer consists of a prepolymer of two moles of TDI / mole of long-chain diol and the level of unreacted TDI is less than about 0.15%. This document teaches the benefits of a high content of perfect prepolymer of more than 90 wt.%.

[0006] US-A-4,888,442 discloses a method for reducing the free monomer content of polyisocyanate adducts by treating the adduct with 2 - 30 wt.% of an inert solvent in a stirred thin-film evaporator under conditions sufficient to reduce the free monomer content of the polyisocyanate adduct mixture to a level lower than that obtainable in the absence of solvent. No examples show the use of MDI as a suitable diisocyanate for preparing polyisocyanate adducts.

[0007] US-A-4,892,920 discloses a method for producing a prepolymer based on cyclohexane diisocyanate (CHDI) that is free of unreacted CHDI and substantially free of oligomeric CHDI by-products.

[0008] US-A-5,202,001 discloses the preparation of polyurethane prepolymers with low levels of residual organic diisocyanates. The examples show prepolymers prepared from toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and methylene-bis[(4-cyclohexyl)-diisocyanate] (CHDI).

[0009] US-A-5,703,193 discloses a method for reducing the amount of residual organic diisocyanate monomer in polyurethane prepolymer reaction products by distillation in the presence of a blend of inert solvents, one having a boiling point higher and the other having a boiling point lower than the monomer. The comparative examples show the removal of MDI monomer from a PTMEG1000 / MDI prepolymer reaction product.

[0010] US-A-6,133,415 discloses a countercurrent extraction process for preparing polyurethane prepolymers. The examples show the production of an MDI / PTMEG prepolymer with low free MDI.

[0011] US-B-6,174,984 discloses prepolymers of at least one diisocyanate and at least one polyether polyol, where the at least one polyether polyol is selected from ethylene oxide homopolymers, propylene oxide homopolymers, and copolymers of ethylene oxide and propylene oxide, and where the free diisocyanate has been reduced to less than 1% of the prepolymer.

[0012] EP-A-0827995 discloses hot melt adhesives comprising polyisocyanate prepolymers prepared by reacting polyisocyanates having a functionality of at least 2 with polyols having a functionality of at least 2, the reaction product comprising at least 90 wt.% of "complete" prepolymer and less than 2 wt.% of unreacted isocyanate monomer, and the prepolymer having a free NCO functionality in the range of 0.2 to 8 wt.%. This document teaches the benefits of a high content of more than 90 wt.% of complete prepolymer.

[0013] US-B-6,866,743 discloses a prepolymer composition based on MDI or TDI suitable for use in non-structural polyurethane adhesive compositions, which consists essentially of at least 80 wt.% of fully prepolymer and less than 2 wt.% of free MDI monomer.

[0014] US-B-6,884,904 discloses an MDI / polypropylene polyether prepolymer composition suitable for use in polyurethane adhesive compositions, which consists essentially of at least 80 wt.% of fully prepolymer and less than 2 wt.% of free MDI monomer. A polyurethane prepolymer composition containing less than 75 wt.% of fully prepolymer and less than 1.0 wt.% of residual diisocyanate monomer is not disclosed.

[0015] US-B-6,943,202 discloses a polyurethane prepolymer with an NCO content of at least 70% of the theoretical NCO content of the pure ABA structure and preferably at least 80% of the theoretical NCO content of the pure ABA structure.

[0016] WO-A-01 / 040340 discloses that polyurethane compositions with low levels of monomeric diisocyanate can be prepared in a two-step process, in which a glycol component with a molecular weight less than 2000 is reacted with a monomeric diisocyanate with a molecular weight less than 500 in the first step. The molar ratio of MDI:polyol is preferably 5:1 to 10:1 because it is beneficial for the formation of the final prepolymer (after removal of solvent and free MDI monomer), which has an NCO content of at least about 80% of the theoretical NCO content of the pure ABA structure. In the second step, the resulting low-monomer macromolecular diisocyanate is reacted with a polyol to form a reactive prepolymer with isocyanate end groups. Such polyurethane compositions are mentioned in this document as useful adhesives for reactive one-component or two-component adhesive / sealant materials, which adhesives can contain solvents and can also provide for the preparation of reactive hot melts as long as the polyol is appropriately selected.

[0017] US-A-2004 / 259968 discloses a composition comprising at least one reaction product of a polyol with a mixture of a stoichiometric excess of an asymmetric polyisocyanate and a high molecular weight polyisocyanate, the asymmetric polyisocyanate having a molecular weight below 500 and an NCO functionality of 1.75 to 2.5. In a non-inventive comparative example, a prepolymer based on 4,4'-MDI and PPG-750 in a ratio of 5:1 is disclosed, with a residual amount of monomeric MDI < 0.1%.

[0018] US-A-2005 / 154172 discloses a polyurethane prepolymer which is the reaction product of IPDI with polypropylene glycol (PPG) having Mw = 4200 g / mol and PPG having Mw = 6300 g / mol. The prepolymer contains 0.05 wt.% of residual IPDI, the amount of oligomeric adduct is 44% and the NCO content is 1.5 wt.%. This document does not mention using MDI as the diisocyanate and the adhesive properties of the disclosed prepolymer.

[0019] EP-A-1746117 discloses a polyurethane prepolymer which is the reaction product of 4,4'-methylenediphenyl diisocyanate (4,4'-MDI) with a trifunctional polyether polyol (PPO; 2095; prepared by the addition reaction of polypropylene oxide and ethylene oxide using glycerol as the initiator molecule), and the prepolymer contains 0.083 wt.% of unreacted monomer (residual MDI), the amount of the complete ABA structure is at most 80 wt.% and the NCO content is 2 wt.%.

[0020] US-A-2007 / 060731 discloses that when constructing the ABA structure of defined isocyanates and polyols, it is not desirable to form oligomeric polyurethanes because such a defined structure has a positive impact on the performance characteristics of, for example, compact elastomers such as thermoplastic polyurethanes or castable elastomers. Reacting an asymmetric diisocyanate such as 2,4'-MDI with PPG-450 provides a prepolymer in which the bis(carbamate) is more than 80 area-% (determined by gel permeation chromatography (GPC)). Such an asymmetric diisocyanate has a lower reactivity. 2,4'-MDI is also not easily obtainable on an industrial scale.

[0021] Considering the prior art, it is obvious that a polyurethane prepolymer having a high content of "complete" prepolymer and a low content of oligomers is preferred.

[0022] There has long been a desire to provide a polyurethane prepolymer having a low content of free diisocyanate monomer which still provides a cured polyurethane having comparable or even better physical properties such as tear strength or adhesive properties such as high shear strength compared to those polyurethane prepolymers having a high NCO functionality.

[0023] It has now unexpectedly been found that, contrary to the long-standing teachings in the prior art, compared to polyurethane prepolymers having a low amount of residual diisocyanate monomer and an adduct of 75 wt.% or more of diisocyanate with at least one polyol in a 2:1 stoichiometry, polyurethane prepolymers (complete prepolymers) having more than 0 wt.% and less than 1.0 wt.% of a low amount of residual free diisocyanate monomer and less than 75 wt.% of an adduct of diisocyanate with at least one polyol in a 2:1 stoichiometry exhibit the desired performance advantages, based on the total weight of the polyurethane prepolymer. SUMMARY OF THE INVENTION

[0024] The present invention relates to an isocyanate-terminated polyurethane prepolymer composition comprising a polyurethane prepolymer (the polyurethane prepolymer being the reaction product of an excess of diisocyanate with at least one polyol) and free diisocyanate monomer in an amount of more than 0 wt.% and less than 1.0 wt.%, preferably less than 0.5 wt.% and most preferably less than 0.1 wt.%, based on the total weight of the polyurethane prepolymer, wherein the polyurethane prepolymer comprises less than 75 wt.%, preferably less than 73 wt.%, more preferably less than 70 wt.% of an adduct of diisocyanate with at least one polyol in a 2:1 stoichiometry, based on the total weight of the polyurethane prepolymer. DETAILED DESCRIPTION

[0025] The present invention relates to a polyurethane prepolymer composition based on the reaction of an excess of diisocyanate with at least one polyol, which has a low level of residual diisocyanate monomer and a high oligomer content.

[0026] The polyurethane prepolymer composition is the product of the reaction of at least one polyol containing "n" (at least 2) OH groups with an excess of diisocyanate. The polyurethane prepolymer reaction product comprises oligomers and so-called "complete" prepolymers. The desired high oligomer content of the prepolymer composition is > 20 wt.%, based on the total weight of the prepolymer composition, or conversely, it can be expressed in terms of the content of the adduct of diisocyanate with polyol in a 2:1 stoichiometry being < 75 wt.%.

[0027] Adducts (complete prepolymers) of diisocyanates with at least one polyol in a 2:1 stoichiometry and methods for their preparation are well known and described in the art, for example in EP-A-0288823, EP-A-0370408, EP-A-0370392, EP-A-0827995, EP-A-1237967, EP-A-1237971, EP-A-1249460, EP-A-1253159, EP-A-1499653 and EP-A-1553118.

[0028] The adducts of the diisocyanates of the present invention with at least one polyol in a 2:1 stoichiometry are stoichiometrically blocked products of one polyol molecule (B) with two diisocyanate molecules (A). In the case of a diol (bifunctional polyol (B)), the stoichiometric ratio of diisocyanate to polyol in the reaction product is 2:1. The complete prepolymer is substantially an adduct containing only one molecule of polyol (B) in each prepolymer molecule A:B:A (or A2B).

[0029] For a bifunctional polyol (B) (n = 2), the oligomers of the present invention are any species having a composition greater than the complete 2:1 molecular ratio (A:B:A; di(carbamate)) such as 3:2 (A:B:A:B:A; tri(carbamate)) or 4:3 (A:B:A:B:A:B:A).

[0030] The present invention claims that a polyurethane prepolymer composition (1) contains, based on the total weight of the polyurethane prepolymer, less than 75 wt.%, preferably less than 73 wt.%, more preferably less than 70 wt.% of an adduct of diisocyanate with polyol in a 2:1 stoichiometry, and (2) contains more than 0 wt.% and less than 1.0 wt.% of unreacted and thus free diisocyanate monomer. In a preferred embodiment, the present invention claims that such a diisocyanate prepolymer reaction product (1) contains more than 30 wt.% of an adduct of diisocyanate with at least one polyol in a 2:1 stoichiometry, and less than 75 wt.%, preferably less than 73 wt.%, more preferably less than 70 wt.% of an adduct of diisocyanate with at least one polyol in a 2:1 stoichiometry, and (2) contains more than 0 wt.% and less than 0.1 wt.% of unreacted diisocyanate monomer.

[0031] The polyurethane prepolymer reaction product contains a free prepolymer NCO functionality in the range of 0.2 to 15 wt.%, preferably 0.5 to 8 wt.% and more preferably 5 to 7 wt.%. The free NCO content is usually determined in weight % according to ASTM D1638-70.

[0032] In one embodiment, the polyurethane prepolymer composition of the present invention contains free prepolymer NCO groups in the range of 0.2 to 15 wt.%, preferably 0.5 to 8 wt.%, more preferably 5 to 7 wt.%, and unreacted diisocyanate monomer in an amount greater than 0 wt.% and less than 1.0 wt.%, preferably less than 0.5 wt.% and more preferably less than 0.1 wt.%.

[0033] In a preferred embodiment of the present invention, based on the total weight of the prepolymer, the polyurethane prepolymer contains at least 30 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol and less than 75 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol, preferably less than 75 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol, more preferably less than 70 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol, even more preferably less than 65 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol, or conversely at least 20 wt.% of oligomers, preferably at least 25 wt.% of oligomers, more preferably at least 30 wt.% of oligomers and even more preferably at least 35 wt.% of oligomers.

[0034] Diisocyanate

[0035] The diisocyanates of the present invention are not particularly limited. Suitable diisocyanates of the present invention include aliphatic diisocyanates, alicyclic diisocyanates, polycyclic diisocyanates, aromatic diisocyanates and aliphatic-aromatic diisocyanates.

[0036] In a preferred embodiment, the diisocyanate of the present invention is methylene diphenyl diisocyanate (MDI), p-phenylene diisocyanate (PPDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate (HDI), cyclohexyl diisocyanate (CHDI), isophorone diisocyanate (IPDI) or toluene diisocyanate (TDI).

[0037] In a more preferred embodiment, a symmetric diisocyanate is used to prepare the polyurethane prepolymer.

[0038] In a more preferred embodiment, 4,4'-methylene diphenyl diisocyanate (4,4'-methylene-bis(phenyl isocyanate); 4,4'-diphenylmethane diisocyanate; 4,4'-MDI)) is used to prepare the polyurethane prepolymer.

[0039] Polyol

[0040] The present invention is not limited to the use of a specific polyol and more than one may be used. The polyols suitable for the present invention may be selected from any polyols known in the art.

[0041] Polyols include compounds having more than one hydroxyl group. The formation of such polyols is well known in the art.

[0042] In many embodiments, diols (having a hydroxyl functionality of 2.0) are preferred over triols or polyols having a higher hydroxyl functionality to avoid an increase in viscosity.

[0043] In some embodiments of the present invention, the polyol includes at least one polyester polyol, at least one polyether polyol, at least one polycaprolactone polyol, at least one polycarbonate polyol, or a combination thereof.

[0044] Preferred polyols are polyether polyols based on polypropylene oxide, also known as polypropylene glycol (PPG), which include, but are not limited to, polypropylene polyether polyols having a functionality of 2 or greater and an average equivalent weight between 100 and 8,000. Also included are ethylene oxide-capped PPG and PPG containing low amounts of monohydric alcohol.

[0045] Other polyols that may be used include alkylene glycols such as diethylene glycol (DEG), other difunctional or polyfunctional alkylene ether polyols such as poly(tetramethylene ether) glycol (PTMEG) and polyethylene oxide, polyester polyols, polyester polyols from polycaprolactone, and hydroxyl-terminated polybutadiene.

[0046] The above polyether and polyester polyols are commonly used in the production of polyurethane prepolymers. Polyols may be blended such that at least one polyol (single polyol or blend) used in the preparation of the prepolymer has a weight average molecular weight (Mw) in the range of about 50 to 16,000 g / mol, preferably 250 to 4,000 g / mol and more preferably 500 to 1,200 g / mol. In another preferred embodiment, at least one polyol includes two polypropylene glycols having different molecular weights. The average molecular weight may be determined by gel permeation chromatography (GPC).

[0047] In a preferred example, at least one polyol of the polyurethane prepolymer composition of the present invention includes a first polypropylene glycol having a weight average molecular weight Mw of 200 to 800 g / mol, preferably 200 to 600 g / mol, more preferably 400 to 600 g / mol, and a second polypropylene glycol having an average molecular weight of 800 to 12,000 g / mol, preferably 800 to 4,000 g / mol and more preferably 800 to 2,000 g / mol.

[0048] Method for preparing polyurethane prepolymer

[0049] The polyurethane prepolymer according to the present invention is prepared by reacting an excess of diisocyanate with at least one polyol.

[0050] In a preferred embodiment, the polyurethane prepolymer of the present invention is prepared by reacting at least one polyol with an excess of MDI (preferably 4,4'-MDI).

[0051] In some embodiments, the polyurethane prepolymer is prepared by reacting an excess of diisocyanate (preferably MDI, more preferably 4,4'-MDI) with at least one polypropylene glycol.

[0052] The polyurethane prepolymer of the present invention can be prepared under the following conditions: heating the reaction mixture of polyol and diisocyanate at 50°C to 150°C for 10 min to 24 h, preferably heating at 60°C to 100°C for 2 h to 6 h.

[0053] The method for synthesizing the polyurethane prepolymer is well known in the art. Generally, the polyurethane prepolymer of the present invention is prepared using standard industrial reaction processes and conditions known in the art for the production of polyurethane prepolymers.

[0054] In a preferred embodiment, the addition of at least one polyol to the diisocyanate is carried out in batches rather than dropwise.

[0055] The polyurethane prepolymer of the present invention is generally prepared using an excess of diisocyanate monomer, which results in a polyurethane prepolymer composition containing unreacted monomers (e.g., unreacted diisocyanate or "free" diisocyanate). Levels of free diisocyanate monomer of 20 wt.% or more may be encountered based on the polyurethane prepolymer composition.

[0056] In a preferred embodiment, the prepolymer reaction is not catalyzed. The use of a catalyst may result in trace residues of the catalyst in the polyurethane prepolymer reaction product, which may affect the curing process.

[0057] The polyurethane prepolymer of the present invention is a "low free monomer" polyurethane prepolymer (also referred to as "low free" or "LF" or "low isocyanate" = "LNCO").

[0058] One of ordinary skill in the art can understand that the "free" monomer isocyanate groups are present in a lower amount compared to conventional polyurethane prepolymers, i.e., the polyurethane prepolymer composition of the present invention has less than 1.0 wt.% free diisocyanate monomer based on the total weight of the polyurethane prepolymer.

[0059] Based on the total weight of the polyurethane prepolymer, the unreacted diisocyanate monomer in the prepolymer reaction product is removed to a concentration of more than 0 wt.% and less than 1 wt.%, preferably less than 0.5 wt.%, and most preferably less than 0.1 wt.%. A polyurethane prepolymer without any residual free diisocyanate monomer would have an unfavorably high viscosity.

[0060] The amount of free diisocyanate monomer in the prepolymer composition can be determined according to common measurement methods such as HPLC (high performance liquid chromatography) measurement.

[0061] The amount of adducts and oligomers can be determined by MALDI-MS.

[0062] Any method suitable for reducing the amount of free diisocyanate monomer in the polyurethane prepolymer composition to the low level of the present invention can be employed. A variety of methods are known for minimizing the residual isocyanate content of polyisocyanate monomers, such as wiped film evaporation, solvent-assisted distillation / codistillation, molecular sieves, and solvent extraction. Preference is given to vacuum distillation, especially thin film or stirred film evaporation under vacuum.

[0063] Curable polyurethane prepolymer composition

[0064] The present invention further relates to a curable polyurethane prepolymer composition comprising the polyurethane prepolymer composition of the present invention and at least one curative.

[0065] Curing agent

[0066] Suitable curatives for the curable polyurethane prepolymer composition of the present invention include diamines, polyols, or blends thereof.

[0067] Examples of diamines include aromatic diamines and aliphatic diamines, primary and secondary amine-terminated polyether polyols, and bifunctional, trifunctional, and polymeric amines.

[0068] Examples of polyols include polyester polyols or polyether polyols, which can be diols, triols, and tetraols, having primary, secondary, and / or tertiary alcohol groups. Preferred polyol curatives are 1,4-butanediol (BDO) or hydroquinone bis(2-hydroxyethyl) ether (HQEE). These polyols can be mixed with diamines.

[0069] The ratio of prepolymer to curative is generally in the range of 0.5:1 to 1.5:1, preferably 0.7:1 to 1.2:1, and more preferably 1.1:1 to 0.90:1.

[0070] Additive

[0071] The curable polyurethane prepolymer composition of the present invention optionally further comprises additives such as catalysts; thickeners; tackifying resins such as abietic acid, abietic acid esters, terpene resins, terpene phenolic resins or hydrocarbonaceous resins; fillers such as silicates, talc, calcium carbonate, clay or carbon black; plasticizers such as phthalates; thixotropic agents such as bentonite, fumed silica, urea derivatives, fibrillated or pulped short fibers; colorants such as color pastes and pigments or desiccants.

[0072] Optional catalysts include tertiary amine catalysts and suitable organometallic catalysts such as those derived from tin, zirconium and bismuth.

[0073] Adhesive

[0074] The present invention also relates to a method for joining or sealing two substrates, comprising the steps of:

[0075] (1) applying the curable polyurethane prepolymer composition of the present invention to a substrate, and

[0076] (2) bringing the curable polyurethane prepolymer composition applied to the substrate into contact with a second substrate such that a bond is formed.

[0077] In one embodiment, the polyurethane adhesive composition used in the method of the present invention comprises the above-mentioned polyurethane prepolymer reaction product, which can be prepared by reacting a diisocyanate (preferably 4,4'-MDI) with a polypropylene polyether polyol having an average molecular weight of 500 to 1,100 g / mol.

[0078] Substrates that can be adhesively bonded include cold-rolled steel, aluminum, glass fiber reinforced polyester (FRP), sheet molding compound (SMC), plastics, wood and glass.

[0079] Accordingly, the present invention further relates to the use of the curable polyurethane prepolymer composition of the present invention as an adhesive. In a preferred embodiment, the polyurethane prepolymer composition comprises a polyurethane prepolymer obtained by reacting an excess of 4,4'-MDI with at least one polypropylene glycol and contains more than 0 wt.% and less than 1.0 wt.%, preferably less than 0.5 wt.%, and more preferably less than 0.1 wt.% of free 4,4'-MDI monomer based on the total weight of the polyurethane prepolymer, wherein based on the total weight of the polyurethane prepolymer, the polyurethane prepolymer contains less than 75 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol, preferably less than 73 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol, and more preferably less than 70 wt.% of a 2:1 stoichiometric adduct of a diisocyanate and at least one polyol.

[0080] In a preferred embodiment, the amount of the ABABA oligomer is 25 wt.% or more based on the total weight of the polyurethane prepolymer.

[0081] The present invention also encompasses the use of the polyurethane prepolymer composition of the present invention for preparing a curable polyurethane prepolymer composition.

[0082] The present invention also encompasses the use of the polyurethane prepolymer composition of the present invention as a one-component foam (OFC).

[0083] The present invention also encompasses the use of the curable polyurethane prepolymer composition of the present invention as an adhesive at room temperature or as a hot melt adhesive.

[0084] Compared with an adhesive composition of a non-inventive polyurethane prepolymer based on a low amount of oligomer or a non-inventive conventional polyurethane prepolymer with a high amount of residual diisocyanate monomer, using the polyurethane prepolymer composition of the present invention in an adhesive composition provides an adhesive that exhibits the following: improved shear strength after curing at room temperature for 1 or 7 days and health and safety benefits associated with lower levels of volatile diisocyanate monomers of less than 1.0 wt.% based on the total weight of the polyurethane prepolymer.

[0085] Examples

[0086] The following materials were used in the examples:

[0087] M 4,4'-Methylene diphenyl diisocyanate; MDI; CAS No.: 101-68-8; (commercially available from Covestro)

[0088] 1200 Polypropylene glycol; PPG-500; Mw = 500 g / mol; CAS No.: 25322-69-4; (commercially available from BASF)

[0089] 1,100 / 1 Polypropylene glycol; PPG - 1100; Mw = 1100 g / mol; CAS No.: 25322 - 69 - 4; (Commercially available from BASF)

[0090] Tri(propylene) glycol Tripropylene glycol; TPG; CAS No.: 24800 - 44 - 0; (Commercially available from Sigma Aldrich)

[0091] Adhesion experiment

[0092] Polyol (Ethylene dicyanide) tetra - 2 - propanol; CAS No.: 102 - 60 - 3; Commercially available from Sigma Aldrich

[0093] 325F Talc; Commercially available from Barretts Minerals

[0094] Polyol PPG - 2000 Polypropylene glycol; PPG - 2000; Mw = 2,000 g / mol; Commercially available from Covestro

[0095] TS 720 Fumed silica; Commercially available from Cabot

[0096] Aluminum powder, 325 mesh, grade 101 Commercially available from Toyal America

[0097] Microbeads (10 mm) Commercially available from Cataphote

[0098] Aluminum coupon Commercially available from AlKemix

[0099] Methyl ethyl ketone (MEK) Commercially available from Sigma Aldrich

[0100] 218 Single - coat adhesive (Commercially available from Lord)

[0101] 1,4 - Butanediol (BDO) Commercially available from Sigma Aldrich

[0102] Hydroquinone bis(2 - hydroxyethyl) ether (HQEE) Commercially available from Sigma Aldrich

[0103] Method:

[0104] Free NCO content (NCO%)

[0105] The free NCO content was determined in wt% according to ASTM D1638 - 70.

[0106] Residual monomer content:

[0107] The amount of free 4,4'-MDI monomer in the polyurethane prepolymer reaction product was determined by HPLC (High Performance Liquid Chromatography).

[0108] Oligomer content:

[0109] The oligomer content of the polyurethane prepolymer composition was measured using MALDI - MS. The intensities of each structural type were added together, and Table 2a shows the relative amounts. Samples were prepared with a mixture of 20 mg / mL DCTB (trans - 2 - [3 - (4 - tert - butylphenyl)-2 - methyl - 2 - propenylidene] malononitrile) in THF, 10 mg / mL sample in THF, and 20 mg / mL NaTFA in THF at a ratio of 10:10:1. Acquisition was performed in linear mode (positive ion, mass range 300–30000). Data analysis was performed by baseline subtraction (smoothing SG7 points, Centroid 0.3 width S / N 1).

[0110] Example 1 (Inventive)

[0111] 6046 g of 4,4'-methylenediphenyl diisocyanate (4,4'-MDI) was added to a reactor and heated to 50 °C. Then, 2896 g of PPG - 500 and 1646 g of PPG - 1100 were added. The reaction temperature was maintained at 80 °C for 4 hours. Excess residual 4,4'-MDI monomer was removed from the reaction product by thin - film distillation under reduced pressure to a level of at least 0.1 wt.% residual 4,4'-MDI, and the total NCO content % was 6.05 wt.%.

[0112] Example 2 (Inventive)

[0113] 2880 g of 4,4'-MDI was added to a reactor and heated to 50 °C. Then, 603 g of PPG - 500 and 1017 g of PPG - 1100 were added. The reaction temperature was maintained at 80 °C for 4 hours. Excess residual 4,4'-MDI monomer was removed from the reaction product by thin - film distillation under reduced pressure to a level of at least 0.1 wt.% residual 4,4'-MDI, and the total NCO content % was 6.10 wt.%.

[0114] Example 3 (Comparative)

[0115] 4182 g of 4,4'-MDI was added to a reactor and heated to 50 °C. Then, 443 g of PPG-500 and 1500 g of PPG-1100 were added. The reaction temperature was maintained at 80 °C for 4 hours. Excess residual 4,4'-MDI monomer was removed from the reaction product by thin-film distillation under reduced pressure to a level of at least less than 0.1 wt.% residual 4,4'-MDI, and the total NCO content % was 5.87 wt.%.

[0116] Example 4 (Comparative)

[0117] 1584 g of 4,4'-MDI was added to a reactor and heated to 50 °C. Then, 900 g of PPG-500 and 1516 g of PPG-1100 were added. The reaction temperature was maintained at 80 °C for 4 hours. The NCO content % of the reaction product was 5.90 wt.%.

[0118] The content of residual 4,4'-MDI monomer > 5 wt.%.

[0119] Table 1 presents the NCO %, residual free 4,4'-MDI, and oligomer content of the MDI / PPG polymers of Examples 1 - 4.

[0120] Table 1a.: Characteristics of the prepolymer compositions

[0121]

[0122] *Inventive example; **Calculated

[0123] Based on the wt.-% of the measured free NCO groups and the molecular weights of the MDI monomer and polyol used, the oligomer content was calculated, and the results are shown in Table 1a. The calculation of the oligomer content was based on the assumption that the higher molecular weight oligomers were mainly ABABA, and the amount of even higher oligomers such as ABABABA was neglected.

[0124] However, the prepolymer composition was additionally analyzed by MALDI MS to determine the exact amounts of the ABA adduct and ABABA and ABABABA oligomers, where A represents 4,4'-MDI and B represents PPG.

[0125] Table 2a.: Composition of the MDI PPG prepolymers determined by MALDI-MS

[0126] Example ABA [wt.%] ABABA [wt.%] ABABABA [wt.%] 1* 58.4 30.4 11.3 2* 64.5 28.8 6.7 3 76.0 20.6 3.4 4 49.4 32.6 18.0

[0127] *Inventive example

[0128] Table 2a shows the amounts of the ABA adduct and oligomers of Examples 1 to 4 measured using MALDI mass spectrometry.

[0129] Inventive Examples 1 and 2 showed a residual 4,4'-MDI content of less than 0.1 wt.% and an oligomer content of more than 25 wt.% based on the total weight of the polyurethane prepolymer. Thus, in Inventive Examples 1 and 2, the amount of the 2:1 stoichiometric adduct of 4,4'-MDI and PPG was less than 75 wt.% based on the total weight of the prepolymer composition.

[0130] Example 5

[0131] The prepolymer compositions of Examples 1 - 4 were evaluated by room temperature curing on aluminum coupons in the following structural adhesive compositions:

[0132] Table 2.: Components of the adhesive composition

[0133]

[0134] The adhesive composition was prepared by mixing Part A and Part B at an NCO:OH ratio of 1:0.9 while adding 1 wt.% microbeads. The substrates were prepared according to the steps of the following substrate preparation method:

[0135] (1) Abrasion

[0136] The aluminum coupons were partially abraded to a partial distance of 6" with a 20 / 40 Crushed glass @ 80 psi Nozzle. The expected profile on the aluminum coupons was 2 to 3 mm.

[0137] (2) Solvent washing

[0138] The surface was washed with acetone and then air dried.

[0139] Then the adhesive composition was applied to one side of a 2.54 x 12.7 cm aluminum coupon to cover at least 3.23 cm 2 area and then mated with a second substrate coupon to obtain a total 3.23 cm 2 lap overlap. The samples were cured at room temperature and 50% humidity. Samples were prepared and tested after 1 day and 7 days according to ASTM D10002 - 10 (Standard Test Method for Apparent Shear Strength of Single - Lap - Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal - to - Metal)). All tests were conducted at room temperature.

[0140] Table 3: Lap Shear Strength Data without Adhesive

[0141]

[0142] *Inventive Example; **50% Humidity

[0143] The results in Table 3 show that, compared with the adhesive compositions based on non-inventive prepolymers with a low amount of oligomers (Example 3) or non-inventive conventional high amounts of free diisocyanate monomers (Example 4), the adhesive compositions based on the polyurethane prepolymers of Examples 1 and 2 of the present invention have higher shear strengths after 1-day and 7-day curing.

[0144] Example 6

[0145] Repeat the adhesive testing procedure as described in Example 5, except that as the third step of the substrate preparation process, apply the adhesive on the sanded and washed surface of the aluminum test piece, and then apply the adhesive composition as follows:

[0146] (3) Adhesive

[0147] Prepare an adhesive composed of a 50:50 blend of methyl ethyl ketone (MEK) & Adhesive 218. Immerse the aluminum test piece in the adhesive and then hang it vertically to dry.

[0148] The shear strength test results are shown in Table 4.

[0149] Table 4: Lap Shear Strength Data Using Adhesive

[0150]

[0151] *Inventive Example; **50% Humidity

[0152] The data in Table 4 show that comparable shear strengths can only be achieved between the adhesive compositions based on the inventive prepolymer composition and the adhesive compositions based on non-inventive prepolymer compositions by using an additional and disadvantageous process step of applying the adhesive.

[0153] Example 7

[0154] Cast elastomers were prepared from the prepolymers of Examples 1 to 4 using conventional techniques. The polyurethane prepolymer compositions were cured using 98% stoichiometric 1,4-butanediol (BDO) or hydroquinone bis(2-hydroxyethyl) ether (HQEE), and then post-cured at 115 °C for 16 hours. Physical properties were obtained using the following ASTM test methods: hardness (ASTM D2240); split tear (ASTM D-470); trouser tear (ASTM D-1938); die C tear (ASTM D-624); modulus, tensile strength at break, elongation at break (ASTM D638). Tables 5 and 6 show the data.

[0155] Table 5.: Physical properties of cast elastomers based on polyurethane prepolymer compositions cured with HQEE

[0156]

[0157] *Inventive

[0158] Table 6.: Physical properties of cast elastomers based on polyurethane prepolymer compositions cured with BDO

[0159]

[0160] *Inventive

[0161] Compared with Non-inventive Examples 3 and 4, the low monomer, high oligomer prepolymers of Inventive Examples 1 and 2 showed more excellent split tear, trouser tear and die C tear strengths.

[0162] Table 7.: Physical properties of cast elastomers based on polyurethane prepolymer compositions cured with HQEE

[0163]

[0164] n.d. = not determined

[0165] Compared with Non-inventive Examples 3 and 4, the low monomer, high oligomer prepolymers of Inventive Examples 1 and 2 showed higher moduli.

[0166] Table 8.: Physical properties of cast elastomers based on polyurethane prepolymer compositions cured with (1,4-butanediol) BDO

[0167]

[0168] Compared with Non-inventive Examples 3 and 4, the low monomer, high oligomer prepolymer of Inventive Example 1 showed higher modulus.

Claims

1. A polyurethane prepolymer composition, comprising: (a) a polyurethane prepolymer which is a reaction product of an excess of diisocyanate and at least one polyol, and (b) free diisocyanate monomer in an amount greater than 0 wt.% and less than 1.0 wt.%, based on the total weight of the polyurethane prepolymer, wherein, based on the total weight of the polyurethane prepolymer, the NCO content of the polyurethane prepolymer is 5 to 7 wt.%, and the polyurethane prepolymer contains less than 75 wt.% of the 2:1 stoichiometric adduct of the diisocyanate and the at least one polyol, wherein the diisocyanate is 4,4'-methylenediphenyl diisocyanate, and the at least one polyol is a propylene oxide-based polyether polyol, and wherein the at least one polyol comprises: a first polypropylene glycol having a weight average molecular weight Mw of 200 to 600 g / mol; and a second polypropylene glycol having an average molecular weight of 800 to 12,000 g / mol.

2. The polyurethane prepolymer composition according to claim 1, wherein, based on the total weight of the polyurethane prepolymer, the polyurethane prepolymer composition contains more than 0 wt.% and less than 0.5 wt.% of the free diisocyanate monomer.

3. The polyurethane prepolymer composition according to claim 1 or 2, wherein, based on the total weight of the polyurethane prepolymer, the polyurethane prepolymer composition contains more than 0 wt.% and less than 0.1 wt.% of the free diisocyanate monomer.

4. The polyurethane prepolymer composition according to claim 1 or 2, wherein, the first polypropylene glycol has a weight average molecular weight Mw of 400 to 600 g / mol; and the second polypropylene glycol has an average molecular weight of 800 to 4,000 g / mol.

5. The polyurethane prepolymer composition according to claim 1 or 2, wherein, the prepolymer contains more than 30 wt.% and less than 75 wt.% of the 2:1 stoichiometric adduct of the diisocyanate and the at least one polyol.

6. The polyurethane prepolymer composition according to claim 1 or 2, wherein, the second polypropylene glycol has an average molecular weight of 800 to 2,000 g / mol.

7. A curable polyurethane prepolymer composition, comprising the polyurethane prepolymer composition according to any one of claims 1 to 6 and a curing agent.

8. The curable polyurethane prepolymer composition according to claim 7, wherein, the curing agent is a diamine, a polyol or a blend thereof.

9. The curable polyurethane prepolymer composition according to claim 7, wherein, the curing agent is 1,4-butanediol or hydroquinone bis(2-hydroxyethyl) ether.

10. A method for adhesively bonding or sealing two substrates, comprising the steps of: (1) applying the curable polyurethane prepolymer composition according to any one of claims 7 to 9 to the substrates, and (2) Bring the curable polyurethane prepolymer composition applied to the substrate into contact with a second substrate such that a bond is formed.

11. Use of the curable polyurethane prepolymer composition according to claim 7 as an adhesive.

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