MXPA99006031A - Monomer-poor polyurethane prepolymer - Google Patents

Abstract

The invention relates to a monomer-poor polyurethane prepolymer, comprising free NCO groups. Said prepolymer is produced from polyvalent alcohols and diisocyanates having different reactivity and is characterized by the proportion of the NCO groups of the partially less reactive diisocyanate with respect to the NCO groups of the more reactive diisocyanate, the proportion being greater than 6 to 1. However, the inventive prepolymer hardens faster than a known polymer containing primarily more reactive NCO groups. Preferably, TDI is used as partially less reactive diisocyanate and MDI as more reactive diisocyanate. The production of the prepolymers occurs in two stages. The inventive prepolymer is suitable for gluing plastic objects, metals and paper, specially foils, whereby the accelerated migration freedom is quite noticeable.

Description

PREPOLIMERO PU WITH UNDER CONTENT OF MONOMERO This invention relates to a PU prepolymer with low monomer content, which contains free NCO groups and is obtained from polyhydric alcohols and diisocyanates that differ in their reactivity, their production and their use. PU prepolymers of the type in question are described in EP 0 150 444. These are produced by a process in which: - in a first reaction step toluene-2,4-diisocyanate is reacted with polyhydric alcohols in an OH: CO ratio of 4 to 0.55: 1 in the absence of other diisocyanates and, after almost all NCO groups of reactivity relatively high react with some of the OH groups present, - a symmetrical dicyclic diisocyanate, which is no more reactive than the slowly reacting NCO groups of toluene-2,4-diisocyanate from reaction step 1, is added in a second step of reaction in an equimolar or excess amount, based on the free OH groups, or in an amount of 5 to 80% by weight, based on the total amount of the diisocyanates of steps 1 and 2, optionally at elevated temperature and / or in the presence of the common catalysts. Although this known PU prepolymer has a low monomer content, ie only from 1 to 2.5% according to the examples, it is not cured fast enough in many cases. Accordingly, the problem solved by the present invention was to provide a PU prepolymer of low content in monomers containing free NCO groups that allow a faster but sufficiently safe processing. The solution provided by the present invention is defined in the claims and consists of a PU prepolymer of low monomer content containing free NCO groups, which is obtained from polyhydric alcohols and at least two different diisocyanates in their reactivity and which it is characterized in that the ratio of the NCO groups of the partially slower reaction diisocyanate to the NCO groups of the diisocyanate that reacts faster is greater than 6: 1. The ratio between the isocyanate groups is preferably greater than 10: 1 and, above all, greater than 15: 1. It is possible to use various polyhydric alcohols. Aliphatic alcohols containing 2 to 4 hydroxyl groups per molecule are suitable. The primary and secondary alcohols can be used, with secondary alcohols being preferred. In particular, the reaction products of low molecular weight polyhydric alcohols with alkylene oxides containing up to 4 carbon atoms can be used. For example, the reaction products of ethylene glycol, propylene glycol, the butanediols or isomeric ethanediols with ethylene oxide, propylene oxide and / or butylene oxide are suitable. In addition, the reaction products of trihydric alcohols, such as glycerol, trimethylol ethane and / or trimethylolpropane, or higher alcohols, for example pentaerythritol or sugar alcohols, are the alkylene oxides mentioned can be used. Polyether polyols with a molecular weight of 100 to 2000 and preferably of 1000 to 5000, more especially propylene glycol, are particularly suitable. In this way, the addition products of only a few moles of ethylene oxide and / or propylene oxide per mole or the addition products of more than 100 moles of ethylene oxide and / or propylene oxide with polyhydric alcohols of molecular weight Low can be used according to the required molecular weight. Other polyether polyols can be obtained by condensation of, for example, glycerol or pentaerythritol with removal of water. In addition, polyols that are widely used in polyurethane chemistry are obtained by the polymerization of tetrahydrofuran.

Among the polyether polyols mentioned, the reaction products of low molecular weight polyhydric alcohols with propylene oxide under conditions where at least the secondary hydroxyl groups are partially formed are particularly suitable. Other suitable polyether polyols are described, for example, in DE-OS 2 559 759. Polyester polyols having a mollecular weight of 200 to 10,000 are also suitable for the process according to the invention. A first embodiment is characterized by the use of polyester polyols formed by the reaction of low molecular weight alcohols, more specifically ethylene glycol, propylene glycol, glycerol or trimethylol propane, with 1 to 50 moles of caprolactone. Other suitable polyester polyols can be obtained by polycondensation. In this way, the dihydric and / or trihydric alcohols can be condensed with less than the equivalent amount of dicarboxylic acids and / or tricarboxylic acids or reactive derivatives thereof to form polyester polyols. Suitable dicarboxylic acids for this condensation reaction are succinic acid and higher homologues thereof containing up to 12 carbon atoms, unsaturated dicarboxylic acids, such as maleic acid or fumaric acid, and aromatic dicarboxylic acids, more particularly isomeric italic acids . Suitable dicarboxylic acids include citric acid and trimellitic acid. The polyester polyols of the aforementioned dicarboxylic acids and glycerol, which have a residual content of secondary OH groups, are particularly suitable for the purpose of the invention. The polyols based on polycarbonates can also be used. The polycarbonates can be obtained, for example, by reaction of diols, such as propylene glycol, butan-1,4-diol or hexan-1,6-diol, diethylene glycol, triethylene glycol to tetrylene glycol, or mixtures of two or more of these diols with diaryl carbonates, for example diphenyl carbonate or phosgene. Other suitable polyol components are polyacrylates containing OH. These polyacrylates can be obtained, for example, by the polymerization of ethylenically unsaturated monomers containing an OH group. Monomers such as these may be obtained, for example, by the esterification of ethylenically unsaturated carboxylic acids and dihydric alcohols, the alcohol generally being present in a small excess. The ethylenically unsaturated carboxylic acids suitable for the esterification are, for example, acrylic acid, methacrylic acid, crotonic acid or maleic acid. The corresponding OH-containing esters are, for example, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate or 3-hydroxypropyl methacrylate or mixtures of two. or more of them. Finally, the polyols can also be based on polybutadiene, styrene / butadiene rubber or nitrile rubber. In the context of the invention, the expression "two diisocyanates that differ in their reactivity" is applied to two different diisocyanates that differ in their reactivity to alcohols.In the selection of these different diisocyanates, the following observations must be taken into account: in the reaction of the diisocyanates with alcohols, the reaction rate of the first diisocyanate group is considerably higher than that of the second diisocyanate group.This applies in particular to monocyclic diisocyanates and also to other diisocyanates containing groups in different chemical environments, In general, it is found that the di-cyclic diisocyanates or, more generally, the symmetrical diisocyanates have a higher reaction rate than the second isocyanate group of the non-symmetrical or monocyclic diisocyanates.Therefore, the first diisocyanate is a non-symmetrical diisocyanate that contains d The NCO groups that differ in their reactivity. Specific examples are toluene-2,4-diisocyanate (TDI), isophorone diisocyanate and 2,4-diphenyl methane diisocyanate. The second diisocyanate is a symmetrical isocyanate, preferably a dicyclic diisocyanate. It is important that the reactivity of their isocyanate groups to the hydroxyl groups be greater than that of the terminal isocyanate groups of the non-symmetrical diisocyanate which reacts on one side (reactive diluent). Therefore, diaryl diisocyanates are especially suitable, with, for example, 4,4 '-diphenyl methane diisocyanate (MDl) and / or substituted 4,4-diphenylmethane diisocyanates being preferred. The concentration of the NCO groups that differ in their reactivity in the prepolymer according to the invention is determined by nuclear resonance spectroscopy (NMR). By "low monomer content" is meant a low concentration of initial diisocyanates.Their concentration is below 2, preferably less than, and more preferably less than 0.5% by weight, based on the prepolymer.The concentration is determined by gas chromatography or HPLC The PU prepolymer according to the invention is produced by a process in which: - in a first reaction step, the diisocyanates containing NCO groups differing in their reactivity (non-symmetrical diisocyanates) are reacted with polyhydric alcohols in an OH: CO ratio of 4 to 0.55: 1 and, after almost all the fast NCO groups have reacted with some of the OH groups present, - a diisocyanate (symmetrical diisocyanate) more reactive than the NCO groups that react The isocyanate of reaction step 1 is slowly added in a second reaction step in less than the equivalent amount, based on the free OH groups, optionally at elevated emperatures and / or in the presence of the common catalysts. Therefore, according to the invention, a polyhydric alcohol is reacted with non-symmetrical diisocyanate in an OH: NCO ratio of 4 to 0.55: 1 in a first reaction step until most of the reactive NCO groups of the diisocyanate have Reacted almost completely with some of the OH groups available, but slowly reacting groups have not yet reacted to a significant degree, if they have reacted somewhat. This point of the reaction can be determined analytically by following the course of the reaction. The course of the reaction can be followed by spectroscopy (IR) or by titration. In the process according to the invention, the molar ratios used are selected so that the free OH groups are still present after most of the reactive isocyanate groups have reacted. In the second reaction step, a symmetrical diisocyanate is introduced into the low viscosity reaction mixture obtained in the first reaction step, this symmetrical diisocyanate being selected for its higher reactivity as compared to the NCO groups now attached to the diisocyanate polymer. symmetrical. In simple terms, it can be said that, in accordance with the invention, in a low viscosity reaction medium containing functional OH and functional NCO constituents, a reaction is carried out between the functional OH constituents and a symmetrical diisocyanate to form a polyurethane prepolymer. In the process according to the invention, the product of the first reaction step is used with a reactive diluent without any reaction taking place between the OH groups and the NCO groups bonded to the slowly reacting polymer of the non-symmetrical diisocyanate. To obtain the reaction products of a non-symmetrical diisocyanate, more specifically toluene-2,4-diisocyanate, with polyhydric alcohols which, according to the invention can be used as a solvent or 'reactive diluent' in the second reaction step, is it is important to maintain a certain proportion between hydroxyl groups and isocyanate groups, in this way, the suitable products that still contain OH groups after most of the reactive NCO groups have reacted, are formed when the number of OH groups divided by the number of isocyanate groups is between 4 and 0.55 and, preferably, between 1 and 0.6 To carry out the second step of the process according to the invention, the symmetrical diisocyanates, more particularly dicyclic, are reacted with the remaining OH groups in the functional OH and functional NCO reaction products of the first stage as reactive diluents For the reaction of the second stage, the molar ratio of the OH groups, expressed as the quotient of the OH groups divided between the isocyanate groups, is greater than 1.0 and, preferably, 1.1 to 12, based on the remaining groups. In order to carry out the process according to the invention, the diisocyanates are preferably reacted with polyhydric alcohols at elevated temperature. Temperatures of 40 to 120 ° C are suitable, with temperatures of 80 to 95 ° C being preferred. For laboratory batches (ca. 1 kg), a reaction time of about 1 hour for the first stage and 2 to 20 hours for the second stage may be advantageous. The temperature in both stages is of the order of 80 ° C. In all cases, the term of the reaction is reached when there is no longer any reduction in the number of isocyanate groups. This can be determined analytically by titrating the isocyanate groups and is the case, for example, after 2 hours to 5 days at room temperature. The PU prepolymer thus produced is used, preferably together with conventional hardeners and / or humidity, if desired in the presence of common organic solvents and accelerators, for bonding plastic articles and metals, particularly films, preferably at temperatures of 20 to 120. ° C. In the context of the invention, it is understood that hardeners are polyfunctional oligomeric compounds of low molecular weight that are selected according to the type and number of functional groups so that they react with the isocyanate groups of the prepolymer and crosslink them. The hardeners preferably contain amino groups, carboxylic acid and epoxide. The coupling agents can also be used, in particular to improve resistance to chemicals and resistance to filler or content in the case of metals. Specific examples of the coupling agents are titanates and Si compounds, more particularly siloxanes.

The PU prepolymer according to the invention is distinguished by the following positive or advantageous properties: - only 7 days, contrary to 14 days, are required for curing at room temperature and normal atmospheric humidity. This is surprising because the PU prepolymer according to the invention actually contains fewer reactive NCO groups than the known PU prepolymer according to EP 150 444. - early adhesive strength at 1.6 N / 15 mm - is distinctly greater than in the case of the known PU prepolymer where it is less than 0.3 to 0.6 N / 15 mm. The early strength was measured under the following conditions: a strip of 15 mm wide sheet is subjected to a T delamination test immediately after joining (delamination angle 2 x 90 °, delamination speed 100 nm / minute). the monomer content of unreacted diisocyanates is much less than 15 by weight [sic] - the non-volatile diisocyanate content is also lower, as manifested in the accelerated absence of migration. In the system according to the invention this is achieved only in 4 days while, in the known system, it takes 10 to 14 days to achieve it. The absence of migration is determined as follows (see Deutsche Lebensmittel-Rundschau 87 (1991), pages 280 and 281). A welded plant bag is filled with 3% acetic acid and stored at 70 ° C. After storage for 2 hours, the contents of the bag are diazoated, subjected to azo coupling and concentrated in a C? 8 column. The concentration is then determined photometrically. The migration of non-volatile diisocyanates and other compounds can cause problems, for example in the sealing of laminates of bonded films, particularly CPA / EVA laminates. The increase in viscosity at 70 ° C as a function of time is much flatter in the case of the PU prepolymer according to the invention, than in the case of the known PU prepolymer. This flatter life curve makes processing easier by ensuring a longer processing time. The prepolymers according to the invention are suitable as such or in the form of solutions in organic solvents for bonding plastics, metals and paper, but especially for laminating textiles, aluminum and plastic films and films covered with metal or oxide and papers. Conventional hardeners, for example, polyhydric alcohols of relatively high molecular weight can be added (two-component system) or surfaces of defined moisture content can be directly linked using the products according to the invention. The laminates of films produced with the products according to the invention are characterized by their high processing safety during heat sealing. This can possibly be attributed to the greatly reduced content of the low molecular weight products that can migrate in the prepolymers. In addition, the prepolymers according to the invention can also be used as extrusion, printing and metallizing primers and for heat sealing. The invention is illustrated by the following methods: Comparative Example In a three-necked flask, equipped with a stirrer, thermometer and drying tube, 411.7 g of a polypropylene glycol (OH number 109 mg KOH / g) were mixed with 104.4 g of toluene-2,4-diisocyanate and The resulting mixture was heated with stirring. Half hours after reaching a melting temperature of 90 ° C, the NCO value of 4.56% was measured by titration. This was only below the theoretical value of 4.88%. After the addition of 25 g of 4,4'-diphenylmethane diisocyanate, the reaction mixture was stirred for 2 hours at 90 ° C, after which an NCO content of 4.61% (theoretical 4.65%) was measured. % of monomeric TDI 0.03% NCO = 4.7 (4.77 theoretical)% of monomeric TDI 2.5 OH: CO (1) = 1.1 viscosity: 1980 mPas / 60 ° C OH.-NCO (2) = 1: 1.6, based on the OH remaining from stage 1.

Example 1 (invention): In a three neck flask, equipped with a stirrer, thermometer and drying tube, 575.3 g of a polypropylene glycol (OH number 109 mg KOH / g) and 156.9 g of a polypropylene glycol (OH number) 267 mg KOH / g) were mixed with 238.5 g of toluene-2,4-diisocyanate and the resulting mixture was heated with stirring. Half hours after reaching a melting temperature of 90 ° C, the NCO content was measured by titration and was only below 4.5%. After the addition of 28 g of 4,4'-diphenylmethane diisocyanate, the reaction mixture was stirred for 2 hours at 90 ° C, after which an NCO content of 4.57% (theoretical 4.59%) was measured. % monomeric TDI 0.03% NCO = 4.57 (4.59 theoretical)% monomeric TDI 0.2 OH: CO (1) = 0.68: 1 viscosity: 1000 mPas / 70 ° C OH: NCO (2) = 10: 1, based on the remaining OHs of stage 1.

Example 2 (invention): heavy quantities 524.0 g of polyester (OH 137 index) 106.0 g PPG (OH number 113) 106.0 g PPG (OH number 267) 244.0 g TDI (NCO: 48.0%) 21.0 g MDl (NCO: 33.5%) Method: Three-necked flask apparatus equipped with a contact thermometer, stirrer (+ motor), drying tube and heating fungus. Procedure: The polyester and the two PPG were introduced into the flask and mixed homogeneously. The TDI was then added, followed by heating to 50 ° C. The temperature rose under the effect of the exothermic reaction. The temperature was then maintained at 90 ° C by cooling.

End point of the reaction to NCO = 5.4%. The MDl was then added, followed by heating to 95 ° C. The reaction mixture was stirred at this temperature for 1 hour. NCO after 1 hour: 4.3% (theoretical 4.3%). The hot mixture was packaged in containers. MDl: 0.4% (theoretical < 1.0%) TDI: 0.3% (theoretical < 0.1%) NCO: 4.3. Viscosity: «5,700 mPas [Brookfield DVII (Thermocell /) /Sp.27/20 r.p.m. / 70 ° C] Adhesion of the laminate A universal machine for voltage testing, with a synchronous recorder is required for the measurement diagrams. The strength range should be between 10 and 50 N and should be adjusted according to the expected level of adhesion. 15 mm wide strips are prepared using a strip cutter. Before the strips are immobilized, they separate slightly. The delamination rate is 100 mm / min, delamination angle 90 ° and delamination length 5 to 10 cm according to the range of variation. The result is expressed as adhesion of the laminate in N / 15 mm and the separation pattern is visually evaluated (adhesive or cohesive failure, adhesive residue).

Claims (10)

1. A PU prepolymer with low monomer content containing free NCO groups, obtainable from polyhydric alcohols and at least two diisocyanates that differ in their reactivity, is characterized in that the ratio of the NCO groups of the partially reacted diisocyanate to the NCO groups of the diisocyanate that reacts faster is greater than 6: 1.

2. The PU prepolymer, as mentioned in claim 1, is characterized in that the ratio of the NCO groups of the diisocyanate that reacts slower to the NCO groups of the diisocyanate that reacts faster is greater than 10: 1 and, more specifically , greater than 15: 1.

3. The PU prepolymer, as mentioned in claim 1 or 2, is characterized in that the primary or secondary aliphatic alcohols containing from 2 to 6 and, preferably from 2 to 4, hydroxyl groups are used as the polyhydric alcohols.

4. The PU prepolymer, as mentioned in at least one of the preceding claims, is characterized in that the polyhydric alcohols used are selected from diols containing from 2 to 5 carbon atoms, triols containing from 3 to 6 atoms. of carbon, tetraols containing from 4 to 8 carbon atoms, OH functional esters and / or OH functional polyesters with molecular weights of up to 10,000 and, more specifically, in the range of 1000 to 5000.

5. The PU prepolymer, as mentioned in at least one of the preceding claims, characterized in that a symmetrical aromatic diisocyanate, more specifically MDl, is used as the diisocyanate that reacts faster.

6. The PU prepolymer, as mentioned in at least one of the preceding claims, is characterized in that a non-symmetrical isocyanate, more specifically TDI, is used as the diisocyanate that reacts slower.

7. A process for the production of the PU prepolymer, as mentioned in at least one of the preceding claims, is characterized in that: - in a first reaction step, the diisocyanates containing NCO groups that differ in their reactivity (non-symmetrical diisocyanates) ) are reacted with polyhydric alcohols in an OH: CO ratio of 4 to 0.55: 1 in the absence of other diisocyanates and, after almost all the fast NCO groups have reacted with some of the OH groups present, - a diisocyanate (diisocyanates) symmetrical) more reactive than the slower isocyanate NCO groups of reaction step 1, is added in a second reaction step in less than the equivalent amount, based on the free OH groups, optionally at elevated temperature and / or in the presence of the common catalysts.

8. The process, as mentioned in claim 7, is characterized in that the two reaction steps are carried out at temperatures of 40 to 102 ° C and, preferably, at temperatures of 80 to 95 ° C.

9. The use of the PU prepolymer mentioned in at least one of the preceding claims together with conventional hardeners and / or moisture, optionally in the presence of organic solvents and accelerators and common additives, for joining plastic, metal and paper articles, more specifically films .

10. The use, as mentioned in claim 9, is characterized in that the connection is made at temperatures between room temperature and 120 ° C.