The present invention relates to new materials consisting of thermoplastic aliphatic polysaccharide carbamic acid derivatives, as well as of mixtures of such derivatives with low-molecular urea derivatives, the preparation of such derivatives and mixtures from polysaccharides or polysaccharide derivatives and aliphatic monoisocyanates in a suitable solvent. The materials according to the invention, after removal of the solvents. are preferably processed as thermoplastic material by injection moulding or extrusion. Mouldings, fibres, films, as well as foams or coatings can be produced.
It is known that thermoplastically processable materials can be produced from polysaccharides, in particular from starches or celluloses, by esterification or etherification of the free hydroxyl groups in a homogeneous reaction process (cf. Encyclopaedia of Polymer Science and Technology Vol. 3, 1985).
Thermoplastic materials are also correspondingly obtained, for example, according to EP 626 392, by esterification of polysaccharide hydroxyalkyl ethers.
JP 142938 and Macromolecules 18, 1985, 1746-1752 describe the reaction of cellulose ethers with acid chlorides or with carboxylic anhydrides.
J. Appl. Polym. Sci. 52. 1994, 755-761. and J. Env. Polym. Degr. 3, 1995, 115-118 describe the preparation of thermoplastic cellulose esters from cellulose and long-chain unsaturated fatty acids in pyridine/DMF.
The thermoplastic processability of polysaccharide derivatives is dependent on the average degree of substitution of the anhydroglucose repeat unit, referred to below as DS, which in the case of thermoplastic aliphatic cellulose ester derivatives, for example, is at least approximately 2.0.
To obtain thermoplastic processability, it is necessary to add further auxiliary substances, especially plasticisers,to these polysaccharide-derivatives (F. Müller, Ch. Leuschke, in Becker/Braun: Kunststoff-Handbuch Vol. 3/1, Hanser Verlag, Munich, 1992).
Whereas the etherification is usually carried out by reacting alkali cellulose with epoxides such as, for example, ethylene oxide or propylene oxide, in an inert suspending medium, esterifications are usually carried out in a solvent process using acids as solvents. Here the swelling process also serves to activate the cellulose (Encyclopaedia of Polymer Science and Technology Vol. 3, 1985).
Comparably, the steady reaction of polysaccharides with substituted isocyanates or substituted isothiocyanates likewise succeeds only in the case of easy accessibility of the cellulose chains in homogeneous solution or in solvents having a very strong swelling effect.
Acta Polymerica 32, 1981, 172-176 specifies that dry cellulose does not react satisfactorily with isocyanates in the absence of solvents and further catalysts. Reactions in solvents which are not capable of at least initiating swelling in the cellulose do not produce satisfactory reactions with the cellulose (Ang. Chem., 59, 1947, 257-288).
B. Polym. J., 18, 1986. 259-262 describes the reaction of wood cellulose with alkyl monoisocyanates. The formation of urethane remains limited to the surface of the polysaccharide fibres.
DE 2358808 describes the reaction with long-chain mono- and diisocyanates, which is limited to the surface of cellulose powders.
L. Disserens, in “Neueste Fortschritte und Verfahren in der chemischen Technologie der Textilfasern”, Birkhäuser Verlag, Stuttgart, 1957, presents a survey of the reactions of textile fibres with mono- and diisocyanates which are limited to the surface. Long-chain alkyl isocyanates are preferably used here for the purpose of hydrophobing the textile material. GB 467 992 describes the heterogeneous reaction with isocyanates of textile fibres which have been previously reacted with alkylene oxides. In the reaction carried out in petroleum spirits, however, no significant swelling of the cellulose results.
In addition the mechanical properties of paper can be influenced by the reaction of isocyanates with the cellulose fibres (Acta Polymerica 32, 1981, 172-176). However, for this preferably multifunctional isocyanates, particularly preferably diisocyanates, are used, which are not the subject matter of the present invention.
Average degrees of conversion of cellulose with isocyanates are obtained using solvents and catalysts especially when the cellulose is in the swollen condition. This can be achieved through the use of a suitable solvent or else through the use of a derivative of the cellulose or of a polysaccharide together with a suitable solvent. The formation of the urethane bond can also be brought about here by an additional back process (Cell. Chem. Technol. 1, 1967, 23-32).
The swelling of the cellulose can be achieved by initial swelling in water and subsequent exchange for a polar aprotic solvent.
Alternatively, solvents having a strongly swelling or solvent effect on cellulose, such as DMAc, DMSO, DMAc/LiCl or DMSO/formaldehyde mixtures, can also be used directly (U.S. Pat. Nos. 4,129,451, 4,129,640, 4,097,666). Surveys of solvent systems are published in Nevell and Zeronian: Cellulose chemistry and its applications, Wiley, N.Y., 1985, Acta Polym. 36 (1985), 697-698 and Polymer News 15, (1990), 170-175.
Also described as being good swelling or dissolving systems are, for example, morpholine/piperidine mixtures (J. Appl. Polym. Sci., 22, (1978), 1243-1253, as well as amine-N-oxides (Acta Polymerica, 39, (1988), 710-714, U.S. Pat. No. 3,508,941) and metal cation/solvent systems such as Cu/NH3 or Cd/ethylenediamine (Polymer 31, (1990), 348-352).
The reaction of phenyl isocyanate with cellulose in DMSO/paraformaldehyde mixtures is described in J. AppI. Pol. Sci. 27, (1982), 673-685, J. Appl. Pol. Sci. 42, (1991), 821-827, and in Nevell and Zeronian: Cellulose chemistry and its applications, Wiley, N.Y., 1985.
High degrees of conversion are favoured especially by the addition of catalysts, by a reaction period of an appropriately chosen length or by elevated temperatures. On the other hand, the formation of isocyanurates is catalysed by means of amines in DMAc, DMSO or DMF as swelling agents or solvents (Eur. Polym. J., 26 (11), (1990), 1217-1220).
The patents and publications listed below describe the reaction of soluble cellulose acetate having degrees of substitution DS<3.0 with isocyanates. isocyanate prepolymers or isocyanate-functionalised polymers or oligomers in homogeneous solution. After the formation of the urethane, the acetate groups can be saponified and the urethane derivatives of the unsubstituted cellulose can be isolated.
J. Macromol. Sci. Chem. A 16 (1981) 473. Polym. Prepr. Am. Chem. Soc. Div. Poly. Chem. 20 (1979) 574, J. Poly. Sci. Polym. Lett. ed. 111 (12) (1973) 731-735, Macromol. Synth. 7, 101-105, Polymer 21 (1980) 648-650, Polym. Prepr. Am. Chem. Soc. Div. Poly. Chem. 3 (1990) 642, U.S. Pat. No. 3,950,28.
Thermoplastically processable aliphatic carbamates starting from polysaccharides or polysaccharide hvdroxyalkyl ethers are not known.
The object of this invention is to identify reaction conditions for the reaction of polysaccharide derivatives. preferably those from renewable raw materials and derivatives of these, with isocyanates, such that average to high degrees of conversion are obtained. For this was required in particular a reaction under homogeneous conditions, that is. a solution of the polysaccharide derivative in the solvent, and at least a solvent having a very strong swelling effect.
Surprisingly, it has now been found that cellulose derivatives, and particularly preferably cellulose alkyl ethers which are not soluble or highly swellable in the solvents used according to the invention, pass into solution or begin to swell strongly during the reaction with alkyl monoisocyanates and consequently lead to average to high degrees of conversion.
Examples of polysaccharide components which may be mentioned in particular are hydroxypropyl cellulose and hydroxyethyl cellulose. Aprotic polar and nonpolar solvents, preferably cyclic ethers or substituted aromatics, particularly preferably dioxane and toluene, can be used as solvents in the process according to the invention.
It has also been found, surprisingly, that the products A of this process according to the invention are thermoplastically processable and especially the by-products B of the process according to the invention improve the thermoplastic processability of the products according to the invention. This surprising result of the procedure according to the invention could not have been anticipated by the person skilled in the art. The person skilled in the art had to anticipate that the by-products B would decompose during subsequent thermoplastic processing and form interfering gaseous decomposition products.
The molar ratio of the components A and B is given by the formula
The molar ratio B/(A+B) is particularly preferably at least 0.05 and in particular 0.1 to 0.2. If the limiting value of 0.5 in the above formula is exceeded, that is, if the proportion of by-product is increased, the product no longer possesses mechanical properties which render possible its practical use.
The function according to the invention of the product B operates only if the melting temperature of B is lower than the processing temperature.
The new compounds correspond to the general structure:
wherein polysaccharide-O represents a substituted or unsubstituted polysaccharide unit and R is a substituent of a polysaccharide-OH group having either the structure
or the structure
wherein A is a linear polyether chain having the following structure:
wherein D denotes a linear aliphatic branched or unbranched chain having 2 to 12 C atoms, O represents an oxygen atom and n is a number equal to or greater than 1.
B represents a substituted carbamic acid having the structure
wherein E denotes a linear or branched aliphatic chain having 1 to 18 C atoms.
The invention accordingly provides thermoplastically processable urethane derivatives of polysaccharides or polysaccharide ethers which have been reacted with isocyanates or blocked isocyanates at average degrees of substitution (referred to the anhydroglucose unit) of DS equal to 0.3 to 3.0, preferably 0.6 to 2.5.
The invention also provides the mixture of the polysaccharide urethane derivatives according to the invention with low-molecular symmetrical and asymmetrical aliphatic urea derivatives, having chain lengths of 1 to 20 carbon atoms.
The invention further provides a process for the preparation of such thermoplastic polysaccharide urethanes, polysaccharide ether urethanes and mixtures of these with low-molecular urea derivatives, which is characterised in that the polysaccharide or the polysaccharide ether is introduced into a nonsolvent together with a suitable catalyst and is then converted by means of an isocyanate into the corresponding urethane, the polysaccharide urethanes or polysaccharide ether urethanes thus formed and the low-molecular urea derivative being soluble in the solvent or suspending medium used. The product can be isolated in the process according to the invention by a precipitation or by an evaporating extrusion.
For the synthesis, a cellulose derivative is refluxed in a suspending medium and, after addition of a catalyst, the isocyanate compound is added dropwise. Here it is to be regarded as surprising that the isocyanate reacts with the polysaccharide or with the polysaccharide ether in a solid-liquid two-phase reaction in standard suspending media. In suspending media such as dioxane and toluene the reaction products pass into solution during the reaction and can be isolated by distillation or precipitation in nonsolvents, for example, with water, acetone, or alkanes.
The degree of conversion of the polysaccharide derivative can be adjusted within wide limits through the stoichiometry and the progress of the reaction. In this connection a degree of substitution of greater than 0.5 is sufficient to provide a thermoplastically processable material. At normal pressure and room temperature, polysaccharides and polysaccharide derivatives may contain a small per cent by weight of water. The secondary reaction of the isocyanate compounds with water leads to the formation of by-products. among which are preferably disubstituted ureas. The extent of the formation of by-products is here highly dependent on the water content of the reaction solution. These by-products have melting temperatures <200° C. and, unless they are removed during the working-up of the polysaccharide derivatives, lead to a distinct improvement in the thermoplastic processability of the product. This outcome is surprising to the person skilled in the art and was not predictable.
Industrially available polysaccharide derivatives of any molecular weight are suitable for the synthesis.
If polysaccharide ethers and in particular cellulose ethers are used. examples of suitable cellulose ethers are methyl cellulose. ethyl cellulose or benzyl cellulose having average degrees of substitution of less than or equal to 2.5. hydroxyethyl cellulose, hydroxypropyl cellulose, dihydroxypropyl cellulose, hydroxybutyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methyl-hydroxybutyl cellulose, ethylhydroxypropyl cellulose, ethylhydroxyethyl cellulose, carboxyalkyl cellulose, sulphoalkyl cellulose, cyanoethyl cellulose and mixed ethers thereof.
Suitable suspending media and solvents are ketones such as, for example, methyl ethyl ketone, ethers and cyclic ethers such as, for example, dimethoxyethane, dimethoxymethane, dimethyl ether, diethylene glycol dimethyl ether. dioxane and tetrahydrofuran, acetals, hydrocarbons and polar aprotic compounds such as dimethyl sulphoxide, dimethylformamide, dimethylacetamide, N-methylmorpholine, N-methylpyrrolidone. trialkyl phosphate, ethyl acetate as well as nonpolar aprotic solvents such as toluene. Dioxane and toluene are preferred.
Suitable isocyanate compounds are aliphatic linear and branched monoisocyanates having saturated or unsaturated alkyl chains such as, for example, methyl isocyanate, ethyl isocyanate, propyl isocyanate, isopropyl isocyanate, butyl isocyanate. pentyl isocyanate, hexyl isocyanate, heptyl isocyanate, octyl isocyanate, nonyl isocyanate, decyl isocyanate, undecyl isocyanate, dodecyl isocyanate, tetradecyl isocyanate. hexadecyl isocyanate, octadecyl isocyanate, the corresponding isothiocyanates as well as any mixtures of the above-mentioned monoisocyaniates and isothiocyanates.
Suitable catalysts for the reaction are amines, particularly sterically hindered tertiary organic amines such as trimethylamine, triethylamine, tributylamine, tetramethylene-diamine, pyridine, N,N-dimethylcyclohexyldiamine, N,N-dimethylbenzylamine, 4-pyrilidinopyridine, permethyldiethylenetriamine, 1,4-diazabicyclo[2.2.2]octane, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5-diazabicyclo[4.3.0]non-5-ene and any mixtures thereof.
The catalysts conventional in polyurethane chemistry such as, for example, organotin compounds. are equally suitable.
During the catalysis the quantity of amine added affects the degree of derivatisation of the polysaccharide. For the reaction with the polysaccharide derivative, the amine is added in a molar ratio to the polysaccharide of from 0.01 to 3, preferably from 0.1 to 1.
The temperature of the reaction of the polysaccharide derivative with the isocyanate is 20° C. to 150° C., preferably 40° C. to 130° C., particularly preferably 50° C. to 120° C. The reaction times are from 0.5 to 16 hours, preferably 0.5 to 8 hours, particularly preferably 0.5 to 4 hours.
The polysaccharide ether esters obtainable by the process according to the invention are soluble or highly swellable in organic solvents such as dioxane. tetrahydrofuran, toluene, methylene chloride. chloroform.
Moreover, these compounds are thermoplastically processable after removal of the solvents and can be converted by means of the conventional processing techniques such as extrusion into. for example, injection mouldings, films or fibres.
The process according to the invention is also characterised in that by-products, particularly preferably dialkylureas, are formed during the reaction, depending on the water content of the polysaccharide component. The retention of these by-products in the product improves the thermoplastic processability of the cellulose ether urethane derivatives.
The polysaccharide derivatives and mixtures thereof with low-molecular urea derivatives according to the invention are suitable for the production of coatings, fibres. powders, films, foams and mouldings. Their properties can be varied by preparing blends of any composition with other components such as, for example, polysaccharides and polysaccharide derivatives or polyurethanes. Combination with plasticisers, fire-protecting agents, pigments and processing aids such as, for example, long-chain fatty acid amides, is also possible here. Especially the addition of plasticisers such as, for example, triethylene glycol, lactic esters or tartaric esters, improves the thermoplastic processability and renders possible processing even at low degrees of substitution such as, for example, a DS equal to 0.6.
The subject matter of the present invention is explained in more detail by means of the available Examples.