MXPA97006790A - Method for preparing an adhesive composition that functions with heat comprising isocyanate prepolimeros with low content of free monomers and oligome - Google Patents

Abstract

A method for adhesively bonding or sealing two substrates, which comprises: (1) applying at about 50øC onto a substrate a moisture-curable, solvent-free urethane heat-melting adhesive composition comprising the reaction product of the polyisocyanate prepolymer of (a) a polyol and (b) polyisocyanate, (2) contacting the adhesive placed on the substrate copper, so that the baking bond is formed, and (3) exposing the adhesive bond to environmental humidity causing a crosslinking reaction within the adhesive, whereby it forms a bond cured by moisture. The improvement is characterized in that the polyisocyanate prepolymer is prepared by reacting a polyisocyanate with a functionality of at least two with a polyol with a functionality of at least 2, the product of the reaction comprises at least 905 by weight of the prepolymer "perfect" and less than 2% by weight of the unreacted isocyanate monomer and the prepolymer having a free NCO functionality in the range of 0.2 to 8% by weight

Description

ADHESIVES THAT FUNDEN WITH HEAT. UNDERSTAND UNDER CONTENT OF FREE MONOMERS. LOW CONTENT OF PREMIUMERS OF OLIGOMER ISOCYANATE DESCRIPTION OF THE INVENTION The present invention relates to urethane adhesives that melt with heat, moisture curable, extrudable comprising a urethane prepolymer. Heat-melting adhesives and urethane adhesives that cure with moisture are well known in the generic adhesive classes. Heat-melting adhesives generally comprise the thermoplastic material, which can be conveniently applied by extruding the adhesive composition at elevated temperatures onto a workpiece to which it is desired to attach another workpiece. The bonded adhesive increases in integrity as the composition cools. Heat-melting adhesives generally have good strength before firing, ie initial strength before completing the hardening of the joint and also have the additional benefits of being easy to handle and free of solvents, which must be evacuated during the Hardening or curing process of the adhesive. However, hot melt adhesives have the disadvantage that after cooling to form a bond line, the thermoplastic composition can be temperature sensitive. In other words, the bonding mass may lose the bond strength as the temperature of the workpiece and the bond line increases. In addition, the hot melt adhesive tends to be physically unstable in the presence of hydrocarbon solvents and some organic compositions. In contrast, many curable urethane adhesives have little resistance to baking. In this way, after the application of a curable urethane adhesive, the structure to be joined must be extremely supported until the urethane can be cured to a highly crosslinked, elastic, strong bonding line. Additionally, solvents can be used in these adhesives, which may need to evaporate during curing. However, once cured, these urethanes are resistant to traction and have little or no sensitivity to temperature. They also offer good resistance to many solvents after curing. Clearly, a single adhesive composition having both hot curing and melting properties is an advantageous goal, since the resulting adhesive, in theory, could possess rapid bonding and strength, high baking strength, cured, crosslinked adhesive bonding. The U.S. No. 4,623,709 discloses polyurethane prepolymers in which the polyhydric alcohols are initially reacted with isocyanate groups that react faster from an asymmetric diisocyanate with the slowly reacting group which remains intact, after which the reaction products are combined with a symmetrical diisocyanate of which the equally reactive isocyanate group reacts more rapidly than the slowly reacting groups of the first polyfunctional isocyanate compound. The products obtained by this process are supposedly distinguished by a low residual monomer content. The U.S. No. 5,441,808 discloses hot melt adhesive compositions combining the characteristics of thermoplastic heat melting adhesives and reactive adhesives and are prepared by mixing a thermoplastic polyester polyether elastomer with a polyisocyanate prepolymer. The U.S. No. 3,931,077 discloses high viscosity heat melting adhesive compositions comprising a specific high viscosity reactive urethane prepolymer, a specific ethylene-vinyl acetate thermoplastic polymer, and a tackifier resin of the phenolic or abietic acid type. The U.S. 4,585,819 describes the combination of an isocyanate prepolymer, a thermoplastic polyurethane or polyester and a synthetic resin, which may be a ketone resin, a hydrogenation product of the condensation resins of acetophenone and their mixtures. The U.S. 4,775,719 discloses a thermally stable, heat-curing, moisture-curing polyurethane adhesive composition comprising an ethylene-vinyl monomer having an ethylene content of 55% by weight or less, a polyisocyanate prepolymer, and a polyisocyanate resin. aromatic stickiness The U.S. No. 4,808,255 discloses a thermally stable, heat-curing, urethane-cure, hot melt adhesive comprising a thermoplastic polymer, a heat curable, compatible urethane polyalkylene polyol prepolymer and a tackifier. The U.S. 4,820,368 discloses a thermally stable, moisture curing, urethane heat-curing adhesive comprising a thermoplastic polymer, a curing, compatible urethane polyester polyol prepolymer and a tackifying agent. The present invention provides an adhesive composition that melts heat of a moisture curable, improved urethane having low melt viscosities and good bake resistance. A method for bonding or sealing two substrates using such an adhesive composition is also provided. The method generally comprises: (1) applying to a substrate a moisture-curable urethane heat-curable, solvent-free adhesive composition comprising the reaction product of the polyisocyanate prepolymer of (a) a compound that contains at least two active hydrogen atoms according to the Zerewittenoff test, preferably a polyol and (b) a polyisocyanate, the adhesive composition being applied at a high enough temperature to melt-expel the adhesive composition; (2) contacting the melt-extruded adhesive composition, placed on the substrate to a second substrate such that a baking bond is formed; and (3) exposing the adhesive bond to moisture causing a crosslinking reaction with the adhesive, thereby forming a bond that cures with the moisture. According to the present invention, the prepolymer used in the moisture-curable, hot-melt adhesive composition is prepared by reacting a polyisocyanate having an NCO functionality of at least 2 with a compound containing X active hydrogen atoms according to the invention. with the Zerewittenoff test (where X is at least two), preferably a polyol having an OH functionality of at least 2, the prepolymer containing free NCO in the range of 0.2 to 8% by weight and less than 2% by weight of the unreacted isocyanate monomer. At least 90% by weight, preferably 95% by weight of the reaction product of the prepolymer obtained by the reaction of the polyisocyanate with the active hydrogen-containing compound, should consist of a prepolymer of X moles of polyisocyanate per mole of such compound containing active hydrogen, ie a "perfect" prepolymer. In the preferred embodiment, at least 90% by weight, preferably 95% by weight, of the product of the reaction of the prepolymer obtained by the reaction of toluene diisocyanate (TDI) with a long-chain polyol having OH groups comprises a "perfect" prepolymer of 3 moles of TDI per mole of long-chain polyol. Advantageously, the adhesive composition comprising such a prepolymer is a liquid of low viscosity at the application temperature and forms a solid by cooling, which serves to adhere the two substrates brought into contact with it. Subsequently, the prepolymer reacts with ambient humidity to form a cross-linked polyurethane-urethane with improved strength and durability. Compared with the conventional urethane prepolymers of the prior art, the present prepolymer has lower viscosity at the application temperature, which results in better wetting and adhesion to the substrate. In addition, the lower viscosity also allows the application to use lower processing temperatures, which expands the number and type of substrates that can adhere. The significantly lower monomer content results in reduced potential for worker exposure. A more regular resin structure in the prepolymer will result in faster crystallization and development of "baking resistance". In addition, the lower oligomer / monomer content by crosslinking results in a more uniform network structure with improved properties relative to conventional materials. The moisture curable urethane heat-melting adhesive compositions used in the present inventive method comprises an NCO-terminated urethane prepolymer (also called a polyisocyanate prepolymer) prepared by the reaction of a polyisocyanate having an NCO functionality of at least 2 with a compound containing at least two active hydrogen atoms according to the Zerewittenoff test, preferably carrying out the reaction in an NCO to active hydrogen at an equivalent ratio of > 4: 1, preferably 6-8: 1. The unreacted polyisocyanate monomer in the reaction product of the prepolymer is removed by distillation or other treatment at a concentration of minus 2%, preferably less than 0.1%. The product of the prepolymer reaction should contain free isocyanate, ie, free NCO functionality, in the range of 0.2 to 8% by weight, preferably, 0.5 to 3% by weight. Such prepolymers useful in the hot melt adhesive composition can be prepared according to the teachings of the U.S. 4,786,703, which description is incorporated for reference. A useful device for washing an unreacted isocyanate monomer is a clean film evaporator. Exemplary of compounds containing at least two active hydrogen atoms according to the Zerewittenoff test are those compounds that contain the functionalities >2 OH, NH and / or SH, especially a polyol. Thus, in the preferred embodiment, a polyol having an OH functionality of at least 3 is reacted with a diisocyanate in an NCO to OH in an equivalent ratio of > 4: 1, preferably 6-8: 1. The products of the prepolymer reaction are prepared using suitable organic polyisocyanates well known in the art including, for example, hexamethylene diisocyanate, phenylene diisocyanate, toluene diisocyanate (TDI) and 4,4'-diphenylmethane diisocyanate (MDI). Particularly suitable are 2,4- and 2,6-TDI individually or together as their commercially available mixture with percentages that are approximately 65-100% of the 2,4- and 0-35% of the 2,6-isomer by weight. Other suitable isocyanate mixtures of isocyanates commercially known as "unpurified MDI", also known as PAPI, which contain about 60% DI together with other isomers and analogs of higher polyisocyanates. Polyether and polyester polyols are commonly used in the preparation of polyurethane prepolymers. The hydroxyl terminated polyethers are typically polyalkylene ether glycols, such as poly (ethylene ether) glycol, poly (propylene ether) glycol and polytetramethylene ether glycol. Other polyethers are prepared by the copolymerization of cyclic ethers, such as ethylene oxide, propylene oxide and trimethylene oxide with various aliphatic diols such as ethylene glycol, butanediols, for example, 1,3- and 1,4-butanediols and the like. The polyester polyols can also be used to produce the polyurethane prepolymers and these would include hydroxyl-terminated polyesters such ethylene adipate, polypropylene adipate, polyhexamethylene adipate and copolyesters prepared by copolymerization of ethylene glycol and propylene glycol with the above polyesters, which include adipate of poly (1,4-butylene-ethylene) and poly (1,4-butylene-propylene) adipate. The polyester and the above polyester polyols are conventionally for producing polyurethane prepolymers and can be blended, such that the polyol composition (single or mixed) used in making the prepolymers typically has an average Mn range of about 1,000 to 60,000. and typically from 4,200 to about 25,000. Preferred prepolymers suitable for use in the hot melt adhesive composition were prepared by reacting toluene diisocyanate (TDI) with a long chain triol in such form to minimize the formation of the byproduct and thereby produce a prepolymer on the which is essentially "perfect" in terms of a stoichiometric ratio of TDI to polyol, ie, 3: 1. In the synthesis of the preferred prepolymer, it is important to maintain a high equivalent ratio of TDI to the polyol, for example, a range of 4 to 20: 1. As the TDI to polyol level approaches 4: 1 and then stoichiometric, higher amounts of high molecular weight oligomers are formed, which detract from the performance characteristics of the moisture curable urethane prepolymer adhesive compositions. . Preferably, the equivalent ratio of TDI to polyol in the eß reaction of 6 to 8: 1 equivalents of TDI to polyol to produce oligomers at less than 10% by weight, preferably less than 2% by weight. The prepolymers should have an isocyanate content substantially reacted to that of the equivalent stoichiometric ratio of TDI to polyol, ie, 2: 1. This objective is achieved by maintaining an equivalent ratio of isocyanate to polyol at moderate temperatures. Maintaining a high equivalent ratio of TDI feed to polyol is extremely important because when the equivalent ratio of TDI to polyol is decreased to slightly above stoichiometric, for example an excess of 10%, which is conventional, the oligomers are formed. The formation of higher oligomer results in lower operating properties and higher viscosity of the prepolymer. A lower level of isocyanate reacted in the prepolymer decreases the performance properties in the hot melt adhesive, because it reduces the amount of moisture that can be reacted with the prepolymer to produce the cured adhesive. Ideally, in the preparation of a prepolymer for use in this invention, the actual isocyanate content should be at least 90% and preferably at least 95% of the stoichiometric level of isocyanate for a "perfect" prepolymer, for example an equivalent 2: 1 ratio of TDI to polyol. The levels of TDI that reacted below about 95% and particularly below 90% of the stoichiometers, are direct evidence of the formation of the TDI byproduct and / or oligomer formation. During the removal of the unreacted TDI from the prepolymer reaction product, the temperature should be kept as low as possible to avoid reaction of the free TDI with the prepolymer. The formation of substantial oligomer and other side reactions may occur during this phase of elimination. For this reason, if distillation techniques are used to eliminate the free TDI, the clean film distillation technique, optionally using an inert gas countercurrent wash, should be considered to avoid oligomerization during the purification of the prepolymer. For the efficiency of the process, it is preferred that the TDI isomers that reacted are within the same ratio by weight as are present within the commercial supply. This level of isomer that reacted is achieved by using a higher concentration of 2,6-TDI in the feed that is normally present, since the 2,6-isomers have a lower reactivity than the 2,4-TDI isomer and recycling the excess back to the reactor as a feed. Since the degree of 2,4- and 2,6-TDI that reacted is a function of the concentration of each isomer in the feed, the feed to the reactor must have a 2,6-TDI content of 60 to 60% to produce the level that reacted of 20-30% by weight of the TDI that reacted. In addition to the polyisocyanate prepolymer, the urethane heat-melting adhesive composition may optionally contain other thermoplastic components, which will not detrimentally affect the adhesive character of the composition. Representative examples of such thermoplastic components include the thermoplastic polymers described in U.S. 5,441,808 and other compatible vinyl polymers such as copolymers of vinyl acetate-ethylene, ethylene-vinyl acetate and .copolymers of vinyl alcohol; ethylene acrylic compounds including copolymers of ethylene acrylate, ethylene methacrylate, ethylene butylacrylate and ethylene hydroxyethylacrylate; polyamides; polyolefins; and polyesters. Preferably, the thermoplastic components comprise a vinyl polymer, a vinyl acetate-ethylene copolymer, or mixtures thereof. To improve the tackiness and to impart pressure sensitive qualities to the adhesive, a tackifier can be incorporated into the adhesive composition. This tackifier can be an ester of rosin, an aromatic resin, or mixtures thereof. Representative examples of the rosin esters, which are useful in the present invention include glycerol rosin ester, rosin pentaerythritol ester and hydrogenated versions of the foregoing. Representative examples of the aromatic resins include resins of -methylstyrene, polystyrene, cumorone, indene and vinyltoluene. Preferably, the tackifier is an aromatic resin and, more preferably, is styrene resin. More preferably, the tackifier is a-methylstyrene resin. The fillers can be incorporated into the adhesive composition in the form of particulate matter, fibers or powders. These fillers can be any of the fillers which will not interfere with the other components in the adhesive composition. Types of fillers include calcium carbonate, ceramics, glass, silica, quartz, mica, treated clay, titanium dioxide, boron nitride, graphite, carbon black, asbestos, metals, barium sulfate, talc or mixtures thereof . Preferred fillers include calcium carbonate, silica, clay, carbon black or mixtures thereof. The plasticizer can also be incorporated into the adhesive composition of the present invention. The plasticizer can be any compound, which does not interfere with the effectiveness of the other components and which facilitates the processing and increases the hardness and flexibility of the adhesive composition. Representative plasticizers include liquid aromatic ester plasticizers, including esters of dioctyl phthalate; solid plasticizers, including dicyclohexyl phthalate, cyclohexane dimethanol dibenzoate; and the plasticizer available under the SANTICIZER trademark of Monsanto. In addition to the optional components indicated above, various other compounds can be added to the adhesive composition. These compounds include catalysts, colorants, inhibitors, antioxidants, UV light absorbers, waxes, adhesion promoters (for example silane) and other conventional additives. The isocyanate prepolymer is present from about 5 to 100% by weight of the adhesive composition. Preferably, it is present from about 25 to 100% by weight and more preferably, from about 50 to 100% by weight. The portion of the thermoplastic polymer of the adhesive composition is present from 0 to about 80% by weight of the adhesive composition, preferably from 0 to 50% by weight.

The tackifier is used from about 0- to 60% by weight, preferably from about 0 to 40% by weight. The silane can be incorporated into the composition for certain applications and be present from about 0 to about 2% by weight, preferably from about 0 to about 1% by weight. Additionally, a filler material may be present from about 0 to about 50% by weight, the plasticizer from about 0 to 10% by weight, the wax from about 0 to 40% by weight and a catalyst from about 0 to 1% by weight. weight. There are numerous methods for the preparation of the adhesive compositions of the present invention. Many of these will be obvious to those with skill in the art. One of such variation is that the prepolymer can be prepared separately or simultaneously with the preparation of the adhesive composition. To be used as an adhesive or sealing material, the solid composition can be placed in a cylinder or a bucket of a supply equipment of the plate type can be used to supply the adhesive. The adhesive composition is extruded at a temperature that is at least sufficient to melt the extrudate of the adhesive composition on a substrate, for example from about 50 ° to about 150 ° C. Prior to curing with adhesive moisture, a second substrate is contacted with the adhesive. The adhesive has sufficient resistance to baking, in such a way that the external mechanical support is not necessary while the adhesive cures with moisture. Suitable substrates for use with the adhesive of the present invention include glass, metal, polycarbonate, acrylic, ABS, PVC, vinyl, wood and the like. The urethane hot-melt, reactive adhesive composition is cured at the bond line based on the presence of ambient moisture, such as moisture in the atmosphere or on the substrate, particularly moisture in the wood. Another means of curing the binding line involves the addition of active compounds for the adhesive in the binding line, which has an active hydrogen atom available for reaction with the NCO groups of the prepolymer. A variety of reactive compounds having free active hydrogens are known in the art, including hydrogen sulfide, polyols, amines, ammonia and other active compounds. EXAMPLE 1 A polyisocyanate prepolymer is prepared by the reaction of one equivalent of modified ethylene adipate with functional trimethylolpropane 3.0 (OH # = 14) with 8 equivalent of 2,4 / 2,6-TDI (ratio of 50/50% by weight) at 50 ° C until all the hydroxyl functionality is consumed. The second, processing stage such as the clean film evaporator is used to remove the unreacted TDI monomer at a level of <0.1% by weight and produce a prepolymer resin with the total free isocyanate content of about 1%. When heated to 80 ° C, the resulting material would exhibit good melt flow, absence of gels and good wetting on a variety of surfaces. United assemblies prepared using this urethane prepolymer would exhibit good baking resistance and high final properties as determined by standard adhesive tests. The corresponding conventional materials prepared at < 3 equivalents of isocyanate groups per equivalent hydroxyl groups would be either too viscous or non-homogeneous (due to gel formation) and could not be processed as heat-melting adhesives. These materials would also contain concentrations of monomeric diisocyanate in an excess of 2% by weight. The present invention provides a method for joining two substrates using a moisture-curable urethane, heat-set adhesive composition.

Claims (8)

1. CLAIMS 1. A method for bonding or adhesively sealing two substrates which comprises: (i) applying to a substrate a moisture-curable urethane heat-curable adhesive composition, without solvent, comprising the reaction product of the polyisocyanate prepolymer of (a) a compound containing X active hydrogen atoms according to the Zerewittenoff test, where X is at least 2), and (b) a polyisocyanate having an NCO functionality of at least two, the adhesive composition which is applied at a high temperature sufficient to melt the extruded adhesive composition; (ii) contacting the molten extruded adhesive placed on the substrate to a second substrate such that a bond to the bake is formed; and (iii) exposing the adhesive bond to moisture to cause a crosslinking reaction within the adhesive, whereby a cured bond with moisture is formed, characterized in that the reaction product of the polyisocyanate prepolymer comprises at least 90% by weight. weight of a prepolymer, comprising X moles of polyisocyanate per mole of active hydrogen-containing compound, the product of the reaction of the prepolymer containing less than 2% by weight of unreacted polyisocyanate monomer and the free NCO-containing prepolymer in the range from 0.2 to 8% by weight. The method according to claim 1, characterized in that the polyisocyanate prepolymer is a product of the reaction of (a) a polyol containing groups XOH (where X is 2 or 3), and (b) a diisocyanate and the adhesive composition is applied from 50 ° to 150 ° C. 3. The method according to claims 1 and 2, characterized in that the reaction product of the prepolymer contains less than 0.1% by weight of the unreacted polyisocyanate monomer. 4. The method of compliance with the claim 3, characterized in that the prepolymer contains free NCO in the range of 0.5 to 3% by weight. 5. The method of compliance with the claim 4, characterized in that the polyisocyanate prepolymer is a product of the reaction of (a) a triol and (b) toluene diisocyanate comprising at least 95% by weight of a prepolymer comprising 3 moles of toluene diisocyanate per mole of triol. 6. The method according to claims 1-5, characterized in that the polyisocyanate and the active hydrogen-containing compound are reacted in an equivalent ratio of NCO to active hydrogen of > Four. 7. The method according to claims 1-6, characterized in that the adhesive composition that melts with heat applied to the first substrate, comprises 5-100% by weight of polyisocyanate prepolymer, 0-80% by weight of thermoplastic polymer, of 0 -60% by weight of the tackifier, 0-2% by weight of silane, 0-50% by weight of filler, 0-10% by weight of plasticizer and 0-40% by weight of wax . The method according to claim 7, characterized in that the heat-melting adhesive composition applied to the first substrate comprises 25-100% by weight of polyisocyanate prepolymer, 0-50% by weight of thermoplastic polymer, 0 -40% by weight of the tackifier, 0-1% by weight of silane, 0-50% by weight of filler, 0-10% by weight of plasticizer and 0-40% by weight of wax.