Abstract:
The pot life of a polyurethane composition is increased by providing a composition containing a polyisocyanate, polyol, highly dispersed silica, and a filler wherein at least a portion of the filler is coated.

Description:
This invention relates to retarded uncatalyzed and catalyzed polyurethane compositions having stable reactivity and to their use for adhesives, sealants and casting compounds. 
     BACKGROUND OF THE INVENTION 
     One-component and multiple-component polyurethane compositions have long been used as adhesives, sealants or casting compounds. These applications for polyurethane compositions are reviewed, for example, in Chapters 8 and 11 of Kunststoff-Handbuch, 3rd Edition, Vol. 7, Hanser-Verlag, 1993. In the case of two-component systems, one component generally consists of liquid polyhydroxy compounds optionally containing fillers, catalysts and other auxiliaries while the second component essentially contains a liquid diisocyanate or polyisocyanate either in pure form or as a so-called quasi-prepolymer or in the form of isomer mixtures. By virtue of their good adhesion properties on various materials and their high tensile, tensile shear and peel strengths, coupled with very high breaking elongation, these adhesives, sealants and casting compounds are used in a number of industrial and manual applications, including for example car manufacture, general utility vehicle manufacture, ship building and boat building, general machine construction, the electrical industry and civil engineering. 
     In many applications, the steps involved require very long pot lives which, in addition, are expected to remain constant over the period for which the polyurethane compositions are stored before application. 
     The pot life of a polyurethane composition is understood to be the time left for proper application after complete mixing of a two-component of multiple-component system before the viscosity of the mixture becomes so high through incipient reaction that the mixture can no longer be properly applied. 
     Pot life can be controlled within certain limits through the type and quantity of catalyst used. However, it has not yet been possible for a given formulation to prolong the reaction and hence the pot life any further without a catalyst. In cases such as these, less reactive isocyanates had to be used. It has often been proposed in the general literature and in the patent literature to retard the uncatalyzed reaction by addition of an acid, cf. for example J. H. Saunders, K. C. Frisch, Polyurethanes, Chemistry and Technology, Vol. 1, Chemistry, page 213, Interscience Publ., 1962. For example, the addition of hydrochloric acid or acid chlorides is proposed. However, where this approach is adopted, the reaction is not genuinely retarded, instead all the catalytically active alkaline impurities in the composition are neutralized. However, the addition of acids to adhesives, sealants or casting compounds can lead to corrosion on the surfaces of the substrates to be bonded or encapsulated so that, in many cases, this method cannot be applied. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     Not applicable. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Accordingly, the problem addressed by the present invention was to reduce the reactivity of one-component and, preferably, two-component or multiple-component polyurethane compositions below the so-called &#34;basic reactivity&#34; of the composition, i.e. below the reactivity which the composition has without a catalyst. In addition, the pot life directly related to this reactivity was to remain substantially constant throughout the storage of the unmixed components. 
     According to the invention, the solution to this problem is characterized in that the one-component or multiple-component polyurethane compositions contain 
     one or more diisocyanates or polyisocyanates, 
     one or more diols and/or polyols, 
     highly disperse silicas, 
     fillers and 
     optionally other auxiliaries and additives, the fillers being at least partly coated. 
     In the context of the invention, a &#34;highly disperse silica&#34; is understood to be a fine-particle, substantially amorphous pyrogenic silica with an SiO 2  content of more than 99.8% by weight which is obtained, for example, by hydrolysis of silicon tetrachloride in an oxyhydrogen flame. These silicas are commercially available, for example, under the name of &#34;Aerosil®&#34;. 
     The addition of highly disperse silicas to adhesives, sealants or casting compounds has long been known and is regarded as standard practice. Hitherto, silicas have been added with the object of reducing the sedimentation of fillers in the compositions or to provide the compositions with thixotropic rheological behavior. In the case of paste-like products, however, the addition of Aerosil in accordance with the prior art is often unnecessary because the rheological properties of these products are achieved through the fine-particle fillers. 
     The use of highly disperse silicas for reducing the reactivity of polyurethane systems below their basic reactivity is new and has never been described before. However, the effect of the silicas as a reaction retarder and stabilizer can only be utilized in filler-containing polyurethane systems if the fillers are at least partly coated. 
     The highly disperse silicas used are preferably the non-aftertreated so-called &#34;hydrophilic&#34; silicas which are commercially available with various specific surfaces (BET surfaces). The following commercial products are mentioned by way of example: Aerosil 380, Aerosil 300, Aerosil 200, Aerosil 150, Aerosil 130. 
     In addition to hydrophilic silicas, aftertreated so-called &#34;hydrophobic&#34; silicas may also be used for the compositions according to the invention. Any aftertreated hydrophobic silicas known per se may be used, including for example the silicas aftertreated with dimethyl chlorosilane, dimethyl silazane, long-chain chlorosilanes, for example octyl trichlorosilane, or even polydimethyl siloxanes. These silicas are commercially available, for example, under the names of Aerosil R 202, R 805, R 812, R 972, R 974 or CABOSIL N 70-TS. 
     Suitable fillers are any of the fillers known per se which are widely used in polyurethane chemistry providing they are at least partly coated. Examples of suitable fillers are limestone flour, natural ground chalks (calcium carbonates or calcium magnesium carbonates), heavy spar, talcum, mica, clays or even carbon black. The filler must be at least partly coated, i.e. generally at least 20% by weight and preferably at least 30% by weight of the filler should be coated. The coating of fillers is known per se, the use of coated fillers being recommended in the literature to facilitate their dispersion in the polymer matrix. There is no known reference to the effect of the surface coating on the pot life of polyurethane systems. 
     The coating material may be selected from a large number of compounds. Surface coatings of long-chain saturated or unsaturated fatty acids, particularly stearic acid, and alkali or alkaline earth metal salts thereof, carboxylated polybutadienes, carboxylated poly-α-olefins, resin acids (abietic acid or derivatives thereof) and/or metal salts and/or esters thereof are mentioned by way of example. 
     Preferred diols and/or polyols for the binder are liquid polyhydroxy compounds containing two or three hydroxyl groups per molecule, for example difunctional and/or trifunctional polypropylene glycols with molecular weights in the range from 200 to 6,000 and preferably in the range from 400 to 3,000. Statistical and/or block copolymers of ethylene oxide and propylene oxide may also be used. Another group of preferred polyethers are the polytetramethylene glycols which are obtained, for example, by the acidic polymerization of tetrahydrofuran and which have molecular weights in the range from 200 to 6,000 and preferably in the range from 400 to 4,000. 
     Other suitable polyols are the liquid polyesters which can be obtained by condensation of di- or tricarboxylic acids, for example adipic acid, sebacic acid, glutaric acid, with low molecular weight diols or triols such as, for example, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, butane-1,4-diol, hexane-1,6-diol, glycerol or trimethylol propane. 
     Another group of polyols which may be used in accordance with the invention are polyesters based on ε-caprolactone which are also known as &#34;polycaprolactones&#34;. 
     However, polyester polyols of oleochemical origin may also be used. Oleochemical polyester polyols may be obtained, for example, by complete ring opening of epoxidized triglycerides of a fatty acid mixture containing at least partly olefinically unsaturated fatty acids with one or more alcohols containing 1 to 12 carbon atoms and subsequent partial transesterification of the triglyceride derivatives to alkyl ester polyols containing 1 to 12 carbon atoms in the alkyl group (see, for example, DE-A-3 626 223). Other suitable polyols are polycarbonate polyols and dimer diols (Henkel KGaA) and, in particular, castor oil and derivatives thereof. Hydroxyfunctional polybutadienes, for example of the type commercially available under the name of &#34;Poly-bd&#34;, may also be used as polyols for the compositions according to the invention. 
     Preferred diisocyanates or polyisocyanates are aromatic isocyanates, for example diphenyl methane diisocyanate either in the form of the pure isomers, as a mixture of the 2,4&#39;-/4,4&#39;-isomers or even in the form of diphenyl methane diisocyanate (MDI) liquefied with carbodiimide which is commercially available, for example, under the name of Isonate 143 L. So-called &#34;crude MDI&#34;, i.e. the isomer/oligomer mixture of MDI which is commercially available, for example, under the name of PAPI or Desmodur VK, may also be used. So-called &#34;quasi-prepolymers&#34;, i.e. products of the reaction of MDI or toluene diisocyanate (TDI) with low molecular weight diols such as, for example, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol or triethylene glycol, may also be used. Although the isocyanates mentioned above are the particularly preferred isocyanates, aliphatic and cycloaliphatic diisocyanates or polyisocyanates, for example hydrogenated MDI (H 12  MDI), tetramethyl xylylene diisocyanate (TMXDI), 1-isocyanatomethyl-3-isocyanato-1,5,5-trimethyl cyclohexane (IPDI), hexane-1,6-diisocyanate (HDI), the biuretization product of HDI, the isocyanuratization product of HDI or dimer fatty acid diisocyanate, may also be used in special cases. 
     Although a preferred embodiment of the compositions according to the invention is catalyst-free, catalysts may also be used. Suitable catalysts are any of the usual organometallic compounds known in polyurethane chemistry such as, for example, iron compounds and, in particular, tin compounds. Examples of such compounds are 1,3-dicarbonyl compounds of iron, such as iron(III) acetyl acetonate, and in particular the organotin compounds of divalent and tetravalent tin, more especially the Sn(II) carboxylates and the dialkyl Sn(IV) dicarboxylates or the corresponding dialkoxylates, for example dibutyl tin dilaurate, dibutyl tin diacetate, dioctyl tin diacetate, dibutyl tin maleate, tin(II) octoate, tin(II) phenolate or even the acetyl acetonates of divalent and tetravalent tin. Other suitable catalysts are the highly active tertiary amines or amidines which may optionally be used in combination with the tin compounds mentioned above. Suitable amines are both acyclic and, in particular, cyclic compounds, for example tetramethyl butane diamine, bis(dimethylaminoethyl)-ether, 1,4-diazabicyclooctane (DABCO), 1,8-diazabicyclo-(5.4.0)-undecene, 2,2&#39;-dimorphol inodiethyl ether or dimethyl piperazine or even mixtures of the above-mentioned amines. 
     In addition, the adhesive according to the invention may optionally contain stabilizers. Stabilizers in the context of the present invention are antioxidants, UV stabilizers or hydrolysis stabilizers. The choice of these stabilizers is determined on the one hand by the main components of the composition and, on the other hand, by the application conditions and by the loads which the bond, joint or encapsulation is expected to withstand. If the polyurethane prepolymer is made up predominantly of polyether units, antioxidants--optionally in combination with UV stabilizers--are mainly required. Examples of suitable antioxidants are any of the commercially available sterically hindered phenols and/or thioethers and/or substituted benzotriazoles and/or amines of the HALS type (hindered amine light stabilizer). 
     If the polyurethane prepolymer is essentially made up of polyester units, hydrolysis stabilizers, for example of the carbodiimide type, are preferably used. 
     In addition, the compositions according to the invention may contain other auxiliaries and additives known per se, for example plasticizers (such as phthalic acid esters) or additional thixotropicizing agents (for example Bentone, urea derivatives, fibrillated or pulped chopped fibers) or pigment pastes or pigments. 
     The polyols and the polyisocyanates are preferably used as a two-component system, the so-called resin component containing the polyols and also the fillers, the highly disperse silica and, optionally, drying agents. Suitable drying agents are any of the usual water-binding agents such as, for example, monofunctional isocyanates, orthoformic acid esters and--in a particularly preferred embodiment--sodium aluminium silicates in the form of molecular sieves. 
     In general, the so-called hardener component only contains the diisocyanate or polyisocyanate or the &#34;quasi-prepolymer&#34;, although this component, too, may optionally contain small quantities of fillers, thixotropicizing agents or pigments. 
     As mentioned at the beginning, the polyurethane compositions according to the invention may be used as adhesives, sealants and/or casting compounds in a number of applications. They are particularly suitable for assembly bonding and structural bonding, for example in the manufacture of sandwich elements. 
     Particularly preferred embodiments of the invention are described in more detail in the following. The quantities mentioned in the are parts by weight unless otherwise indicated. 
     EXAMPLE 1 (Comparison Example) 
     Non-retarded catalyst-free polyurethane system 
     
         ______________________________________                   % by weight______________________________________Resin componentCastor oil, OH value around 160                     29.0Polypropylene glycol, OH value around 240, trifunctional                     5.0Na--Al silicate in castor oil (1:1 mixture)                     6.0Limestone flour, uncoated 20.5Limestone flour, coated   39.5Hardener componentDiphenyl methane-4,4&#39;-diisocyanate (crude MDI)                     100Mixing ratio of resin to hardener                     6:1Pot life                  45      minsPot life after 8 weeks&#39; separate storage of the resin and                     37.5    minshardener components______________________________________ 
    
     EXAMPLE 2 (Invention) 
     Retarded catalyst-free polyurethane system 
     
         ______________________________________Resin component______________________________________Castor oil, OH value around 160                     29.0Polypropylene glycol, OH value around 240, trifunctional                     5.0Na--Al silicate in castor oil (1:1 mixture)                     6.0Limestone flour, uncoated 20.5Limestone flour, coated   39.0Aerosil 150               0.5Hardener component and mixing ratio asin Example 1Pot life                  60      minsPot life after storage for 8 weeks                     60      mins______________________________________ 
    
     Comparison of Example 1 (Comparison Example) with Example 2 shows that the addition of Aerosil in accordance with the invention in Example 2 does not lead to any change in pot life, even after 8 weeks&#39; storage of the resin and hardener components. In addition, the pot life as a whole is prolonged as required. 
     However, if only uncoated limestone flour is used as the filler, pot life cannot be influenced by the addition of Aerosil. 
     EXAMPLE 3 (Invention) 
     Accelerated catalyst-containing polyurethane system 
     
         ______________________________________Resin component______________________________________Castor oil                27.0Polypropylene glycol, OH value around 240, trifunctional                     3.0Na--Al silicate in castor oil (1:1 mixture)                     6.2Limestone flour, uncoated 49.475Limestone flour, coated   13.5Aerosil 200               0.81,4-Diazabicyclo[2.2.2]octane (33.3% solution indipropylene glycol)       0.025Hardener component and mixing ratioas in Example 1Pot life                  35      minsPot life after 12 weeks&#39; storage                     35      mins______________________________________ 
    
     This Example shows that, even in the case of catalyst-containing formulations, pot life remains constant after prolonged storage where Aerosil and at least partly coated filler are used in accordance with the invention.