CA2164467A1

CA2164467A1 - Lignin-containing isocyanate prepolymer mixtures, their preparation and their use for producing polyurethanes, and also production of the polyurethanes

Info

Publication number: CA2164467A1
Application number: CA002164467A
Authority: CA
Inventors: Werner Hinz
Original assignee: Individual
Current assignee: BASF SE
Priority date: 1994-12-06
Filing date: 1995-12-05
Publication date: 1996-06-07
Also published as: DE19545527A1

Abstract

Isocyanate prepolymer mixtures are obtainable by reacting diphenylmethane 4,4'-, 2,4'- or 2,2'-diisocyan-ate, an isomer mixture of diphenylmethane 4,4'- and 2,4'-or 4,4'-, 2,4'- and 2,2'-diisocyanates or a mixture of diphenylmethane diisocyanates and polyphenyl-poly-methylene polyisocyanates with a solution comprising at least one polyoxyethylene glycol having a molecular weight of from 400 to 4000 and lignin.

Description

Lignin-cont~;n;ng isocyanate ~ e~olymer mixtures, their preparation a~d their use for producing polyure~h~ne~, and al~o production of the polyurethanes The present invention relates to lignin-contain-ing i60cyanate prepolymer mixtures. The invention also relates to a process for preparing ~uch isocyanate prepolymer mixtures. Finally, the invention also provides for the use of the isocyanate prepolymer mixtures of the invention for producing polyurethanes (PU), particularly foamed shaped bodies based on polyurethane and also a process for this purpose.
Polyoxyalkylene polyols prepared using lignin and tannin as initiator molecule6 are known. According to US-A-3,546,199 and US-A-3,654,194, lignin or tannin can be alkoxylated in the presence or absence of solvents u~ing alkylene oxides, for example 1,2-propylene oxide, at from 20 to 250C, at atmospheric or superatmospheric pressure.
The polyoxyalkylene polyol~ prepared have hydroxyl numbers in the range from 50 to 1000, preferably from 200 to 800, and are suitable for producing flexible to rigid PU foams by reaction with organic polyisocyanates.
EP-A-0 342 781 describes the use of lignin in PU
production. Solutions of lignin in tetrahydrofuran (THF) or polyoxyethylene glycol (PEG) are reacted with diphenylmethane diisocyanate (MDI) at 60C or at room temperature. According to the publication, the films obtained therefrom have a good mechanical strength and foams have a good elasticity. No comparative examples without lignin are given. The lignin forms the rigid phase, the PEG the soft phase.
~ ignin can also be dissolved in polyoxyethylene glycols (PEG) and from this solut on be reacted with isocyanates to give polyurethane parts, as described in US-A-3,519,581. For this purpose, the lignin is dissolved 2 1 6446~

in a polyoxyethylene glycol (PEG) or a mixture of PEG and polyoxypropylene glycol (PPG) and treated, if appropriate at above 100C, to esterify the carboxyl groups of the lignin. The lignin/polyoxyalkylene glycol solutions obtained are advantageously allowed to cool to below 100C, before being reacted with the polyisocyanates to form polyurethanes. The reaction always takes place in the presence of a surface-active compound.
US-A-3,577,358 describes dissolving the lignin either in PEG or in dioxane for the purpose of the reaction. Curing proceeds over a nll~her of hours at room temperature or alternatively at elevated temperature (~
80C). The polyurethane is isolated by removing the solvent. Lignin and isocyanate react when they are mixed at 120C. The IR spectrum shows that all OH and -N=C=O
groups have reacted.
However, apart from insufficient reactivity of solid or even dissolved lignin, e.g. lignin dissolved in tetrahydrofuran or dioxane, towards isocyanates under the conditions of polyurethane production, a series of other disadvantages stand in the way of the direct use of lignin in polyurethane systems. Their high solvent content very strongly influences the sensitive catalysis of the PU systems, particularly if the lignins are used as solution and not as solid. Industrially, lignins are predom;n~ntly used as "thickeners", and in relatively high concentrations they also have a corresponding viscosity-increasing action in water-containing poly-etherol components. Incompatibility of the lignin with other PU polyol components is frequently also to be observed, which results in the lignin particles, which themselves are very fine, coalescing after making up the polyol mixture, 80 that it is no longer processable.
Some of the lignin OH groups are phenolic in nature, 80 that the polyurethane ~onds obtained therefrom are thermolabile. For the above reasons, lignin polyurethanes were not satisfactory in their processing and even product properties. In general, incorporation of the lignin impairs the mechanical properties even in poly-urethane foams. To obtain PU parts having good properties at all, use is often made of specially fractionated lignins or lignins which have been obtained by a special process (eg. organosolv lignins).
Usual disadvantages of lignin in PU production are insufficient reactivity and insufficient incorpora-tion of the lignin into the PU matrix. Lignin solutions are usually highly viscous and not readily miscible with organic polyisocyanate; in addition, the foams have poor mechanical properties. Removing the high molecular weight fractions of lignin and carrying out the reaction in solution gives PU parts for which a series of advantages have been reported. For example, a lower index is re-quired, cf. CA-A-2,052,487. With kraft lignin itself, the PU polyaddition reaction cannot be carried out in a PEG
solution. The molecular weight of the lignin and the viscosity of the lignin solution in PEG are too high and the miscibility with the isocyanate component is too poor. Special modified lignin, eg. that having a low molecular weight of from 300 to 2000 and better solubility, gives more homogeneous foams having good mechanical properties. A one-shot or a prepolymer procedure can be used. In the latter case, a prepolymer is prepared from lignin/polyols/isocyanates and this i~
then cast into films or can also be foamed by mixing with water/catalysts/stabilizers.
To circumvent the above difficulties associated with the direct processing of lignin, alkoxylation of the lignin has also been proposed. However, this is complicated. In general, owing to the above (processing) difficulties, lignins or lignin derivatives are not currently used on an industrial scale for producing polyurethanes. US-A-3,546,199 and US-A-3,654,194 describe how lignin in solid, pulverulent form or dissolved in reactive cr unreactive solvents can be reacted to give lignin polyetherols, both in the absence of catalysts and with ROH/aniline catalysis. The OH numbers of the polyols obtained are used to back-calculate the 0~ numbers of the lignins. They are from about 600 to 1300. Tannin can also be used like lignin. The OH numbers of the lignin poly-ether polyols are from 50 to 1000.
It is an object of the present invention to provide new readily procesRable isocyanate prepolymer mixtures. A further object of the invention is to indi-cate those isocyanate prepolymer mixtures which, in their further processing into polyurethane products, particu-larly into polyurethane foams, give products which possess improved physical properties, particularly afi regards elongation at break, tensile strength and/or tear propogation resistance. A further object of the present invention is the provision of processes for preparing such isocyanate prepolymer mixtures and for producing polyurethanes having the improved mechanical properties.
We have found that this object is achieved by means of isocyanate prepolymer mixtures as defined in the claims. The process of the invention for preparing such polyisocyanate prepolymer mixtures and also their use ~or producing polyurethanes and polyurethane products and a process for this purpose are likewise defined in the claimR. Preferred embodiments of the invention are given in the following description and the subclaims.
According to the invention, a natural material, ie. a regenerable polyhydroxyl compound, is advantageously used.
According to the invention, the synthetically prepared polyhydroxyl compounds are advantageously replaced co~pletely or at least partially by lignin as a hydroxyl-containing natural material. The use of this regenerable hydroxyl-containing natural material requires no complicated technical syntheses. A further advantage is that lignin obtained in other areas as a waste product can be industrially utilized, if appropriate after slight technical treatment and/or purification. The u~e of novel starting material6 enables, according to the invention, the production of polyisocyanate polyaddition products having different mechanical properties, which in turn open up new possible applicationR.
The isocyanate prepolymer mixtures provided by the invention, which contain urethane groups and reactive isocyanate groups in bonded form, are formed by reacting bl) at least one organic polyisocyanate based on diphenylmethane diisocyanate with b2) at least one polyhydroxyl component.
According to the invention, the polyhydroxyl component consi6ts at least partially of a ~olution of lignin in polyoxyethylene glycol. The isocyanate prepolymer mixture of the invention has an NC0 content of from 2.5 to 30% by weight, based on the total weight of the isocyanate prepolymer mixture, and is obtainable by reacting bl) 4,4'-, 2,4'- and 2,2'-diphenylmethane diisocyanate, an isomer mixture of 4,4'- and 2,4'- or 4,4'-, 2,4' and 2,2'-diphenylmethane diisocyanates or a mixture of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates with b2) a solution comprising b2i) at least one polyoxyethylene glycol hav-ing a molecular weight of from 400 to 4000, and b2ii) lignin.
The ratio of lignin to hydroxyl-containing compound is here preferably such that from 1 to 50% by weight of lignin and from 99 to 50% by weight of liquid comprising polyoxyethylene glycol together form 100% by weight of the solution. The lignin Rolution preferahly consists of from 99 to 80% by weight, in particular from 99 to 90% by weight, of liquid comprising polyoxyethylene glycol, in particular a polyoxyethylene glycol, while the amount of lignin makes up from 1 to 20% by weight, in particular from 1 to 10% by weight, of the solution. In particular, the weight ratio of lignin to liquid compris-ing polyoxyethylene glycol is in the range from 1:5 to 1:20, preferably from 1:6 to 1:14.
The reaction of the lignin solution with the isocyanate is preferably carried out in the absence of surface-active agents.
The lignins used are preferably those which have not been ~ubjected to any special chemical treatment for their further processing. Rraft li~nins are particularly preferred. Such lignins advantageously have an acid number of less than 10.
The preferred solvents for the lignin are poly-oxyethylene glycols having a molecular weight of from 550 to 1800.
The isocyanate prepolymer mixture i8 preferably a liquid mixture having an -N=C=0 content of from 5 to 25% by weight, based on the isocyanate prepolymer mixture. The particularly preferred isocyanate prepolymer mixture has an -N=C-0 content of from 9 to 15% by weight.
In the process of the invention for preparing the isocyanate prepolymer mixtures, the abovementioned components are reacted with one another to form the isocyanate prepolymer mixture. The lignin can be esteri-fied at least partially with the liquid containing polyoxyethylene glycol and thus be chemically bound to tl~e latter. This measure can reduce the acid number to values of less than 1, preferably of less than 0.1. By means of the esterification, the COOH groups of the lignin are transformed and prevented from reacting with the -N=C=0 group~ with carbon dioxide formation. The chemical incorporation of the lignin into the isocyanate prepolymer mixture occurs via the hydroxyl groups present on the lignin, indeed out of the solution with reaction with the isocyanate groups of the polyisocyanate com-ponent.
The isocyanate prepolymers are advantageously prepared by reacting organic polyisocyanates initially _h charged in a heatable stirred vessel-with a defyciency of polyhydroxyl compounds, preferably in the absence of catalysts.
The viscosity of the -N=C=O prepolymer is strongly dependent on the polyoxyethylene glycol and its molecular weights in the range, for example, from 600 to lSOO. In PEG having a molecular weight of 600, the solubility of lignin, in particular unmodified kraft lignin, is still 40~ by weight. Furthermore, in contrast to other polyethylene glycols, PEG 600 is available in PU
quality. In the case of polyoxyethylene glycols of higher molecular weight, the solubility decrea~es (PEG 1500:30%
by weight, PEG 4000:20~ by weight), and in addition the solutions of lignin in PEG 1500 and PEG 4000 are solid at room temperature, which necessitates more complicated handling. Furthermore, the desired -N=C=O content of the isocyanate prepolymer mixture and the desired lignin content of the isocyanate prepolymer mixture influence its viscosity. ~he -N=C=O contents obtained in the prepolymers correspond to the theoretical values when the OH groups of the lignin are included. Dark, but clear and fluid prepolymers are usually obtained. The method of the invention and the isocyanate prepolymer mixtures of the invention have numerous advantages in comparison with the prior art. Lignin dissolved in the polyoxyethylene glycol can be readily handled and proces~ed.
Incorporation of the lignin into the isocyanate prepolymer mixture ena~les any other relatively high molecular weight compounds containing at least two reactive hydrogen atoms to be used for polyurethane production.
In the process of the invention for producing compact or cellular polyurethanes, preferably PU foams, a) relatively high molecular weight compounds con-taining at least two reactive hydrogen atoms, preferably polyhydroxyl compounds, are reacted with b) li~uid polyisocyanate compositions containing urethane groups in bonded form.

This occurs in the presence or absence of c) chain extenders and/or crosslinkers, d) blowing agents, e) catalysts and f) auxiliaries.

According to the invention, the polyisocyanate composition b consists at least partially of an isocyan-ate prepolymer mixture as defined above.
In the process of the invention for producing polyurethanes, it is preferred that the relatively high molecular weight compoundq have a functionality of from 2 to 8 and an amine or hydroxyl number of from 25 to 500 and are advantageously fielected from the group of poly-oxyalkylene polyamines and/or polyhydroxyl compounds, in particular polyhydroxyl compounds having a functionality of from 2 to 8 and a hydroxyl numher of from 25 to 500, which are in turn preferably selected from the group of polythioether polyols, polyesteramides, hydroxyl-contain-ing polyacetals, hydroxyl-containing aliphatic poly-carbonates, polyester polyols, polymer-modified polyether polyols, preferably polyether polyols and mixtures of at least two of the specified polyhydroxyl compounds.
Suitable relatively high molecular weight poly-hydroxyl compounds, as are used in (a), advantageously possess, as already indicated, a functionality of from 2 to 8 and a hydroxyl number of from 25 to 500, with preference being given to using polyhydroxyl compounds having a functionality of preferably from 2 to 3 and a hydroxyl nll~her of preferably from 30 to 80 for producing flexible PU foams and polyhydroxyl compounds having a g functionality of preferably from 3 to 8 and in particular of from 3 to 6 and a hydroxyl number of preferably from 100 to 500 for producing rigid PU foams. The polyhydroxyl compounds used are preferably linear and/or branched polyester polyols and, in particular, linear and/or branched polyoxyalkylene polyols, with polyhydroxyl compounds from regenerable natural materials and/or chemically modified regenerable natural materials being particularly preferred. Suitable lignin-free polyhydroxyl compounds (a) also include polymer-modified polyoxy-alkylene polyols, polyoxyalkylene polyol disper~ions and other hydroxyl-containing polymers and polycondensates having the abovementioned functionalities and hydroxyl number~, for example polyesteramides, polyacetals and/or polycarbonates, in particular those which are prepared from diphenyl carbonate and l,6-hexanediol by transester-ification, or mixtures of at least two of the specified relatively high molecular weight polyhydroxyl compounds (a).
Suitable polye~ter polyols can, for example, be prepared from organic dicarboxylic acids having from 2 to 12 carbon atoms, preferably aliphatic dicarboxylic acids having from 4 to 6 carbon atoms and polyhydric alcohols, preferably alkanediols having from 2 to 12 carbon atoms, preferably from 2 to 6 carbon atoms, dialkylene glycols and/or alkanetriols having from 3 to 6 carbon atoms.
Suitable dicarboxylic acids are, for example: succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, decanedicarboxylic acid, maleic acid, fumaric acid, phthalic acid, isophthalic acid and tere-phthalic acid. The dicarboxylic acids can be used either individually or in admixture. In place of the dicarboxyl-ic acids, it is also possible to use the corresponding carboxylic acid derivatives, for example dicarboxylic esters of alcohols having from 1 to 4 carbon atoms or dicarboxylic anhydrides. Preference i~ given to using dicarboxylic acid mixtures of succinic, glutaric and adipic acid in weight ratios of, for example, 20 to 35 :
35 to 50 : 20 to 32, and in particular adipic acid.
Examples of dihydric and polyhydric alcohols, in particu-lar alkanediols and dialkylene glycols, are: ethanediol, diethylene glycol, 1,2- or 1,3-propanediol, dipropylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, l,10-decanediol, glycerol and trimethylolpropane. Prefer-ence is given to using ethanediol, diethylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, glycerol or mixtures of at least two of the Rpecified alkanepoly-ols, in particular, for example, mixtures of 1,4-butane-diol, 1,5-pentanediol and 1,6-hexanediol. It is also possible to use polyester polyols from lactones, eg.
~-caprolactone, or hydroxycarboxylic acids, eg.
~-hydroxycaproic acid.
To prepare the polyester polyols, the organic, for example aromatic and preferably aliphatic, dicarboxylic acids and/or their derivatives and the polyhydric alcohols and/or alkylene glycols can be polycondensed in the absence of catalyst or prefera~ly in the presence of esterification catalysts, advantageously in an atmosphere of inert gases such as nitrogen, helium, argon, etc., in the melt at from 150 to 250C, preferably from 180 to 220C, at atmospheric pres6ure or under reduced pre~sure to the desired acid number which is advantageously less than 10, preferably less than 2.
A-cording to a preferred embodiment, the esterification mixture is polycondensed at the abovementioned tempera-tures to an acid number of from 80 to 30, preferably from 40 to 30, under atmospheric pressure and subsequently under a pressure of less than 500 mbar, preferably from 50 to 150 mbar. Suitable e6terification catalysts are, for example, iron, cadmium, cobalt, lead, zinc, antimony, magnesium, titanium and tin cataly~ts in the form of metals, metal oxides or metal salts. However, the poly-condensation can also be carried out in the liquid phase in the presence of diluents and/or entrainers, for ~ 1 64467 example benzene, toluene, xylene or chlorobenzene, for azeotropically distilling off the water of condensation.
The polyester polyols are prepared by polyconden-sing the organic dicarboxylic acids and/or their deriva-tives with the polyhydric alcohols, advantageou61y in a molar ratio of 1:1 to 1.8, preferably 1;1.05 to 1.2.
The polyester polyols obtained preferably have a functionality of from 2 to 4, in particular from 2 to 3, and a hydroxyl number of from 240 to 30, preferably from 180 to 40.
However, the polyhydroxyl compounds particularly preferably used are polyoxyalkylene polyols which are prepared by known methods, for example by anionic poly-merisation using alkali metal hydroxides, 6uch as sodium or potas6ium hydroxide, or using alkali metal alkoxides, such as sodium methoxide, sodium or potassium ethoxide or potassium isopropoxide, as cataly6ts and with addition of at least one initiator molecule containing from 2 to 8, preferably 2 or 3, reactive hydrogen atoms in bonded form for preparing polyoxyalkylene polyols for flexible PU
foams and preferably containing from 3 to 8 reactive hydrogen atoms in bonded form for preparing polyoxyalkyl-ene polyols for semi-rigid and rigid PU foams, or by cationic polymerisation using Lewis acids, such as antimony pentachloride, boron fluoride etherate, etc., or bleaching earth as catalysts, from one or more alkylene oxides having from 2 to 4 carbon atoms in the alkylene radical.
Suitable alkylene oxides are, for example, tetrahydrofuran, 1,3-propylene oxide, 1,2- or 2,3-butyl-ene oxide and preferably ethylene oxide and 1,2-propylene oxide. The alkylene oxides can be used individually, in succe6sion or as mixtures. Suitable initiator molecules are, for example: water, organic dicarboxylic acids such as succinic acid, adipic acid, phthalic acid and tere-phthalic acid, aliphal~c and aromatic, unsubstituted or N-monoalkylated, N,N- or N,N'-diaikylated diamines having from 1 to 4 carbon atoms in the alkyl radical, such as unsubstituted, monoalkylated or dialkylated ethylene-diamine, diethylenetriamine, triethylenetetramine, 1,3-propylenediamine, 1,3- or 1,4-butylenediamine, 1,2-, 1,3-, 1,4-, l,S- and 1,6-hexamethylenediamine, phenylene-diamine, 2,3-, 3,4-, 2,4- and 2,6-tolylenediamine and 4,4'-, 2,4'- and 2,2'-diaminodiphenylmethane.
Other suitable initiator molecules are: alkanol-amines such as ethanolamine, N-methylethanolamine and N-ethylethanolamine, dialkanolamines such as diethanol-amine, N-methyldiethanolamine and N-ethyldiethanolamine and trialkanolamines such as triethanolamine, and ~mmQ~ia. Preference is given to using polyhydric, in particular dihydric to octahydric, alcohols and/or alkyl-ene glycols such as ethanediol, 1,2- and 1,3-propanediol, diethylene glycol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, glycerol, trimethylolpropane, penta-erythritol, sorbitol and sucrose and also mixtures of at least two polyhydric alcohols.
The polyoxyalkylene polyols, preferably polyoxy-propylene and polyoxypropylene-polyoxyethylene polyols, have a functionality of from 2 to 8 and hydroxyl numbers of from 25 to 500, with preference being given to u6ing polyoxyalkylene polyols having a functionality of from 2 to 3 and a hydroxyl number of from 30 to 80 for flexible PU foams and polyoxyalkylene polyols having a functional-ity of from 3 to 8 and a hydroxyl number of from 100 to 500 for semi-rigid and rigid PU foams, and suitable polyoxytetramethylene glycols having a hydroxyl number of from 30 to about 280.
Other suitable polyoxyalkylene polyols are polymer-modified polyoxyalkylene polyols, preferably graft polyoxyalkylene polyols, in particular those based on styrene and/or acrylonitrile and prepared by in-situ polymerisation of acrylonitrile, styrene or preferably mixtures of styrene and acrylonitrile, for example in a weight ratio of from 90:10 to 10:90, preferably from 70:30 to 30:70, advantageously in the abovementioned polyoxyalkylene polyols, by a method similar to that given in the German patents 11 11 394, 12 22 669 (US 3 304 273, 3 383 351, 3 523 093), 11 52 536 (G~ 10 40452) and 11 52 537 (GB 987618), and also poly-oxyalkylene polyol dispersions which comprise as dis-persed phase, usually in an amount of from 1 to 50% by weight, preferably from 2 to 25% by weight: for example polyureas, polyhydrazides, polyurethanes containing bonded tert-amino groups, and/or melamine, and which are described, for example, in EP-B-011 752 (US-A-4,304,708), US-A-4,374,209 and DE-A-32 31 497.
Like the polyester polyols, the polyoxyalkylene polyols can be used individually or in the form of mixtures. Furthermore, they can be mixed with the graft polyoxyalkylene polyols or polyester polyols and also with the hydroxyl-containing polyesteramides, polyacetals and/or polycarbonates.
Suitable hydroxyl-containing polyacetals are, for example, the compounds which can be prepared from glycols such as diethylene glycol, triethylene glycol, 4,4'-dihydroxyethoxydiphenyldimethylmethane, hexanediol and formaldehyde. Suitable polyacetals can also be prepared by polymerizing cyclic acetals.
Suitable hydroxyl-containing polycarbonates are those of the type known per se which can be prepared, for example, by reacting diols such as 1,3-propanediol, 1,4-butanediol and/or 1,6-hexanediol, diethylene glycol, triethylene glycol or tetraethylene glycol with diaryl 0 carbonates, for example diphenyl carbonate, or phosgene.
The polyesteramides include, for example, the preA~;n~ntly linear condensates obtained from polybasic, saturated and/or unsaturated carboxylic acids or their anhydrides and polyhydric saturated and/or unsaturated aminoalcohols or mixtures of polyhydric alcohols and aminoalcohols and/or polyamines.
The relatively high molecular weight polyhydroxyl compounds (a) can, depending on the application of the isocyanate prepolymer mixture~ (b), be completely or preferably partially replaced by low molecular weight chain extenders and/or crosslinkers. In the production of flexible PU foams, the addition of chain extenders, crosslinkers or, if desired, mixtures thereof can be advantageous for modifying the mechanical properties of the PU foams, eg. the hardness. In the production of rigid PU foams, the use of chain extenders and/or cross-linkers can usually be omitted. Suitable chain extendersare difunctional compounds, suitable crosslinker6 are trifunctional and higher-functional compounds, each having molecular weights of less than 400, preferably from 62 to about 300. Examples of chain extenders are alkanediols, for example those having from 2 to 6 carbon atom~ in the alkylene radical, such as methanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol, and dialkylene glycols such as diethylene, dipropylene and dibutylene glycol, and examples of crosslinkers are alkanolamines, eg. ethanol-amine, dialkanolamines, eg.diethanolamine, and tri-alkanolamines, eg. triethanolamine and triisopropanol-amine, and trihydric and/or higher-hydric alcohols such as glycerol, trimethylolpropane and pentaerythritol.
Useful chain extenders or crosslinkers are also the low molecular weight ethoxylation and/or propoxylation products, eg. those having molecular weights up to about 400, of the abovementioned polyhydric alcohols, alkylene glycols, alkanolamines and also of aliphatic and/or aromatic diamine6.
Chain extenders which are preferably used are alkanediols, in particular l,4-butanediol and/or 1,6-hexanediol, alkylene glycols, in particular ethylene glycol and propylene glycol, and preferred crosslinkers are trihydric alcohols, in particular glycerol and trlmethylolpropane, dialkanolamines, in particular diethanolamine, and trialkanolamines, in particular triethanolamine.
The chain extenders and/or crosslinkers (c) which are preferably used in producing flexible PU foams can be used, for example, in amounts of from 2 to 60% by weight, preferably from 10 to 40% by weight, based on the weight of the relatively high molecular weight compounds (a).
Further polyisocyanates can be used in admixture with the isocyanate prepolymer mixtures (b) of the invention for producing the PU. Specific examples are:
alkylene diisocyanates having from 4 to 12 carbon atoms in the alkylene radical, for example l,12-dodecane diisocyanate, 2-ethyltetramethylene 1,4-diisocyanate, 2-methylpentamethylenel,5-diisocyanate,2-ethyl-2-butyl-pentamethylene 1,5-diisocyanate, tetramethylene 1,4-diisocyanate and preferably hexamethylene 1,6-diiso-cyanate; cycloaliphatic diisocyanates such as cyclohexane 1,3- and 1,4-diisocyanate and also any mixtures of these isomers, l-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (isophorone diisocyanate), hexahydrotolylene 2,4- and 2,6-diisocyanate and also the corresponding isomer mixtures, dicyclohexylmethane 4,4'-, 2,2'- and 2,4'-diisocyanate and also the corresponding isomer mixtures, and preferably aromatic diisocyanates and polyisocyanates, for example tolylene 2,4- and 2,6-diisocyanate and the corresponding isomer mixtures, diphenylmethane 4,4'-, 2,4'- and 2,2'-diisocyanate and the corresponding isomer mixtures, mixtures of diphenyl-methane 4,4'- and 2,4'-diisocyanates, polyphenyl-poly-methylene polyisocyanates, mixtures of diphenylmethane 4,4'-, 2,4'- and 2,2'-diisocyanates and polyphenyl-polymethylene polyisocyanates (raw MDI) and mixtures of raw MDI and tolylene diisocyanate. The organic diisocyan-ates and polyisocyanates can be used individually or in the form of their mixtures.
Organic polyisocyanates which have been found to be very useful are mixtures of diphenylmethane diisocyan-ates and polyphenyl-polymethylene polyisocyanates, preferably those having a diphenylmethane diisocyanate content of at least 35% by weight, eg. from 45 to 95% by weight and in particular from 48 to 60% by weight, 80 that such raw MDI compositions are particularly prefer-ably used.
Suitable blowing agents (d) for producing the cellular polyurethanes are water and/or liquids and gases which are liquid at room temperature, which are inert towards the liquid isocyanate prepolymer mixtures and have boiling points below 50C, in particular from -50C
to 30C, at atmospheric pressure, and also mixtures of gaseous and liquid blowing agents. Examples of such preferred gases and liquids are alkanes such as propane, n- and iso-butane, n- and iso-pentane, preferably indus-trial mixtures of n- and iso-pentane, and cycloalkanes such as cyclopentane, alkyl ethers such as dimethyl ether, diethyl ether and methyl isobutyl ether, alkyl carboxylates such as methyl formate, and halogenated hydrocarbons such as dichlorofluoromethane, trifluoro-methane, 1,1-dichloro-1-fluoroethane, monochlorotri-fluoroethane, monochlorodifluoroethane, difluoroethane, dichlorotrifluoroethane, monochlorotetrafluoroethane, pentafluoroethane, tetrafluoroethane and dichloromono-fluoroethane. The cellular polyurethanes, preferably PU
foams, are produced using, in particular, water, linear and cyclic alkanes having from 5 to 7 carbon atoms and m~xtures thereof. The blowing agents mentioned by way of example can be used individually or as mixtures. Blowing agent~ which are not used are chlorofluorocarbons, which damage the ozone layer.
The liquids having boiling points below 50C can alAo be mixed with (cyclo)alkanes, eg. hexane and cyclo-hexane, and alkyl carboxylateR, eg. ethyl formate, ha~ing boiling points above 50C, as long as the blowing agent mixture ha~ a boiling point advantageously below 38C.
The amount of blowing agent or mixture required can be experimentally determined in a simple manner as a function of the type of blowing agent or blowing agent mixture and also of the mixing ratios. The blowing agents are usually used in an amount of ~rom 0.1 to 30 parts by weight, preferably from 1 to 25 parts by weight, based on 100 parts by weight of the components a - c.
Catalysts (e) which can be used in PU production are preferably compounds which strongly accelerate the reaction of the hydroxyl-containing component (a) with the isocyanate prepolymer mixtures (b) of the invention or the mixtures of polyisocyanate prepolymer mixtures (b) and further organic polyisocyanates. Suitable catalysts are, for example, organic metal compounds, preferably organic tin compounds such as tin(II) salts of organic carboxylic acids, eg. tin(II) diacetate, tin(II) diocto-ate, tin(II) diethylhexanoate and tin(II) dilaurate, and the dialkyltin(IV) salts of organic carboxylic acids, eg.
dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate, dioctyltin diacetate, and dibutyltin dimer-captide, and strongly basic amines, for example amidines such as 2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, tertiary amines such as triethylamine, tributylamine, dimethylbenzylamine, N-methylmorpholine, N-ethylmorpho-line, N-cyclohexylmorpholine, dimorpholino-diethyl ether, N,N,N',N'-tetramethylethylenediamine, N,N,N',N'-tetra-methylbutanediamine, N,N,N',N'-tetramethylhexane-1,6-diamine, di(4-N,N-dimethylaminocyclohexyl)methane, pentamethyldiethylenetriamine, bis(dimethylaminoethyl) ether, bis(dimethylaminopropyl)urea, dimethylpiperazane, 1,2-dimethylimidazole, 1-azabicyclo[3.3.0]octane, alkan-olamine compounds such as triethanolamine, triisopropan-olamine, N-methyldiethanolamine and N-ethyldiethanolamine and dimethylethanolamine, tris(N~N-dialkanol~miTlo~lkyl)-s-hexahydrotriazine, in particular tris(N,N-dimethyl-aminopropyl)-~-hexahydrotriazine, tetraalkylammonium ~.ydroxides such as tetramethyl~m~ium hydroxide, and preferably 1,4-diazabicyclo[2.2.2]octane. Preference is given to u~ing from 0.001 to 5% by weight, particularly from 0.05 to 2% by weight, of catalyst or catalyst combination, ba6ed on the weight of the component (a).
Auxiliaries (f) can, if desired, additionally be incorporated into the reaction mixture for producing the compact or cellular polyurethanes, preferably PU foams.
Examples which may be mentioned are surface-active substances, foam stabilizers, cell regulators, flame retardants, fillers, dyes, pigments, antistatic agents, hydrolysis inhibitors, fungistatic and bacteriostatic substances.
Suitable surface-active substances are, for example, compounds which serve to aid the homogenisation of the isocyanate prepolymer mixtures and may also be suitable for regulating the cell structure of the PU
foams. Examples which may be mentioned are emulsifiers such as the sodium salts of castor oil sulfates or of fatty acids, and also salts of fatty acids with amines, eg. diethylamine oleate, diethanolamine stearate, diethanolamine ricinoleate, salts of sulfonic acids eg.
alkali metal or Am~o~ium salts of dodecylbenzenesulfonic or dinaphthylmethanedisulfonic acid, and ricinoleic acid;
foam stabilizers such as siloxane-oxyalkylene copolymers and other organopolysiloxanes, ethoxylated alkylphenyls, ethoxylated fatty alcohols, paraffin oils, castor oil or ricinoleic esters, Turkey red oil and peanut oil, and cell regulators such as pyrogenic silica, paraffins, fatty alcohols and dimethylpolysiloxanes.
Furthermore, oligomeric polyacrylates having polyoxy-alkylene and fluoralkane radicals as side groups are suitable for improving the emulsifying action, the cell structure and/or ~tabilization of the foam. The surface-active substances are usually used in amounts of from 0.01 to 5 parts by weight, based on 100 parts by weight of the component (a).
Suitable flame retardants are, for example, diphenyl cresyl phosphate, tricresyl phosphate, tris(2-chloroethyl) phosphate, tris(2-chloropropyl) phosphate, tris(1,3-dichloropropyl) phosphate, tris(2,3-dibromopropyl) phosphate and tetrakis(2-chloroethyl)-ethylene diphosphate.
Apart from the halogen-substituted phosphates mentioned, it is also possible to use inorganic flame retardants such as hydrated aluminum oxide, antimony trioxide, arsenic oxide, ~ ~n;um polyphosphate, expanded graphite and calcium sulfate, or cyanuric acid deriva-tives such as melamine, or mixtures of at least two flame retardants such as Am~onium polyphosphate and melamine and/or expanded graphite, and also, if desired, starch for ~k;ng the PU foams produced from the isocyanate prepolymer mixtures flame resistant. In general, it has been found to be advantageous to use from 5 to 50 parts by weight, preferably from 10 to 40 parts by weight, of the specified flame retardants or mixtures per 100 part~
by weight of the components (a) to (c).
For the purposes of the present invention, filleræ, in particular reinforcing fillers, are the customary organic and inorganic fillers and reinforce-ments known per se. Specific examples are: inorganic fillers such as siliceous minerals, for example sheet silicates such as antigorite, serpentine, hornblendes, amphiboles, chrysotile, zeolites, talc; metal oxides such as kaolin, aluminum oxides, aluminum silicate, titanium oxides and iron oxides, metal salts ~uch as chalk, barite, and inorganic pigments such as cadmium sulfide, zinc sulfide, and also glass particles. Suitable organic fillers are, for example: carbon black, melamine, rosin, cyclopentadienyl resins and graft polymers.
The inorganic and organic fillers can be used individually or as mixtures and are advantageously incorporated into the reaction mixture in amounts of from 0.5 to 50% by weight, preferably from 1 to 10% by weight, based on the weight of the components (a) to (c).
Further details about the abovementioned other customary auxiliaries (f) can be found in the specialist literature, for example the monograph of J.H. Saunders and K.C. Frisch "High Polymers" Volume XVI, Poly-urethanes, Parts 1 and 2, Interscience Publishers 1962 or 1964, or the Kunststoff-Handbuch, Polyurethane, Volume VII, Carl-Hanser-Verlag, Munich, Vienna, 1st and 2nd Edition, 1966 and 1983.
Further preferred features and embodiment~ of the invention are given in the following examples.

Example 1 Preparation of the lignin-containing prepolymers of the invention Preparation of the lignin solution:
The proportion of lignin indicated in the table was dissolved in the corresponding polyoxyethylene glycol (PEG) and the solution was dewatered for 8 hours at 160C. Partial esterification of the lignin with the PEG
occurred during this phase.

Preparation of the isocyanate prepolymer mixture:
To prepare the iRocyanate prepolymer mixture, the isocyanate was initially charged at 80C and the lignin-containing PEG component was allowed to run in slowly while stirring. Subsequently, stirring was continued for a further 2 hours at 80C to complete the formation of the lignin-containing i~ocyanate semiprepolymers.

Table 1 -N=C=O prepolymers from lignin solutions in polyether glycols ~xperi- Lignin ~ Polyol~ Lignin;Polyol Isocyan-ment (Weight ratio) ate~
1 Lig AT PEG60058:760 MI
2 Lig AT PEG600110:680 MI

3 Lig AT Peg60040:360 MI

4 Lig AT Pegl500 40:360 MI

~xperi- Lignin solution: -N=C=O Viscosity Lig~in ment Isocya~ate (Wt.%)mPas (Wt.%) (Weight ratio) 1 82:118 12.913500 3 2 79:120 14.218400 5 3 300:300 9.25121000 5 4 300:300 11.88180 5 Notes:

Lig AT = Kraft lignin Indulin AT from We~tvaco, Charle~ton, SC, USA
PEG 600 = Polyethylene glycol having an average molecular weight of 600 PEG 1500 = Polyethylene glycol having an average molecular weight of 1500 MI = Mixture of diphenylmethane 4,4'~ and 2,4'-diiso-cyanate Exam~le 2 Production of polyurethane parts Open-celled flexible polyurethane foams were produced using the liquid, lignin-containing prepolymers of the invention as described in Example 1. The process-ing properties of the polyols and the properties of the flexible PU foams were e~m;ned. For comparison, use was made of a formulation into which no lignin-containing prepolymers had been incorporated.
The base polyetherol used was a glycerol-initiated polyoxypropylene-polyethylene block polyol having a primary hydroxyl end group content of greater than 80%, a hydroxyl number of 35 mg of ROH/g of-polyol and a viscosity of 850 mPas (measured in accordance with DIN 51562 at 25C, using an Ubbelohde viscometer).
The cell-opening polyol used was a glycerol-initiated polyoxypropylene-polyoxyethylene polyol having an ethylene oxide content of 70% by weight, based on the alkylene oxide content, a hydroxl number of 42 mg of KOH/g of polyol and a viscosity of 980 mPas at 25C
measured using an Ubbelohde viscometer.
As compari~on substance, use was made of a polyisocyanate mixture cont~ining urethane groups and having an -N=C=O content of 24.5% by weight obtained from diphenylmethane diisocyanate (40.7% by weight), polyphenyl-polymethylene polyisocyanates (30.3% by weight), polyoxypropylene glycol having an average molecular weight cf 2000 (10% by weight) and the abovementioned cell-opening polyol (10% by weight).
To produce the flexible PU foams, the polyol and isocyanate components were intensively mixed at an NCO
index = 80 (80 -N=C=O group~ per 100 OH groups) and the reaction mixture was poured into an open beaker (deter-mination of the reaction times and the free-foamed bulk density) and into a heatable mold (mold temperature 50C) having the dimensions 400x400xlOO mm (determination of the mechanical propertieR of the foams) and allowed to foam therein.

Formulation for producing flexible polyurethane foams in parts by weight.

Component (A): Mixture comprising:
Base polyetherol 54.55 Cell-opening polyol 5.0 Water 3.0 Stabilizer ~ 0.20 Diazabicyclo[2.2.2]octane 0.20 33~ by weight in dipropylene glycol N,N-dimethylaminopropylamine 0.30 N,N,N',N~-tetramethyl-4,4'-diamino-dicyclohexylmethane 0.45 Glycerol 1.20 Tri 6 ( chloropropyl) phosphate 5.00 ' Foam stabilizer based on silicone, Tegostab ~ B8680 from Goldschmidt Component (B): Isocyanate mixture as described in Table In the experiments described in Table 2, the polyisocyanate mixture containing urethane groups which was used as comparison substance was replaced in the proportions indicated by the lignin-containing prepoly-mers of the invention. It can be seen that both the elongation at break and also the tensile strength and the tear propogation resistance of the flexible foams are improved.
In experiment 5, only the comparison substance was used as isocyanate component, while in experiments 6 and 7, 75 parts by weight of the comparison substance and 25 parts by weight of the respective lignin-containing prepolymer as described in Example 1 were used in each case. The actual amounts of the polyisocyanate component in the three experiments were always selected in such a way that the indicated -N=C=O/OH ratio of 80:100 resulted.

Table 2 ~xperiment 5 (Com- 6 (In- 7 (Inven-parison) vention) tion) Comparison 6ub~tance 100 75 75 Ligprep 4 25 Ligprep 3 25 Viscosity at 25C, mPas ~ 100 420 780 Beaker times:
Start time [6ec] 12 10 12 Setting time [~ec] 67 52 71 Rising time [eec] 75 62 90 Free-foamed den~ity [g/l] 48.37 48.48 52.80 Molded foamsO:
Mold temperature [C] 51.2 50.8 51.1 Cushion weight [g] 824 757 792 Core density [g/l] 50.3 49.7 46.8 Open cell content [%] 3 2.5 2.5 Compressive hardne~s [kPa]
20% 1.8 1.0 1.4 40% 3.0 1.9 2.4 60% 6.0 4.5 4.8 Compressive ~et [%] 7.7 25.8 19.5 Elasticity [cm] 45.9 38.1 40.7 Elongation at break [%]

Ten6ile strength [kPa]

Tear propogation resistance [N/mm]
_ 0.20 0.23 0.27 .~otes on Table 2:

100 g of the components (A) (polyol) plu8 ~B) (poly-isocyanate) corre~ponding to the ratio given were introduced into a beaker having a capacity of 1000 ml.

O 16 l cu6hion mold, demolding time 5 min, open cell content: ~ubjective evaluation after 5 min after demolding. Scale 1 - ~ very open, 5: very closed.

Foam tests were carried out as follows:

Density determination DIN 53420 Elasticity mea~urement: Rebound resilience measured by an internal BASF method Compressive set DIN 53572 Compressive hardness DIN 53577 Tear propogation resi~tance DIN 53515 Tensile strength DIN 53571.

The figures in Table 2 6how that the isocyanate prepolymer mixtures of the invention are suitable for producing polyurethane foam articles having improved properties. In particular, the increased elongation at break, the increased tensile Rtrength and the increase tear propogation resistance are notable and repre~ent a surprising result.

Claims

1. An isocyanate prepolymer mixture containing urethane groups and reactive isocyanate groups in bonded form, which has an NCO content of from 2.5 to 30% by weight, based on the total weight of the isocyanate prepolymer mixture, and is obtainable by reacting b1) 4,4'-, 2,4'- and 2,2'-diphenylmethane diisocyanate, an isomer mixture of 4,4'- and 2,4'- or 4,4'-, 2,4' and 2,2'-diphenylmethane diisocyanates or a mixture of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates with b2) a solution comprising b2i) at least one polyoxyethylene glycol hav-ing a molecular weight of from 400 to 4000, and b2ii) lignin.

2. An isocyanate prepolymer mixture as claimed in claim 1, wherein the solution b2 comprises, based on the total weight of the solution b2:

b2i) from 99 to 50% by weight of at least one polyoxy-ethylene glycol having a molecular weight of from 400 to 4000 and b2ii) from 1 to 50% by weight of lignin, and wherein, in particular, the weight ratio of lignin to solvent is in the range from 1:5 to 1:20, preferably from 1:4 to 1:14.

3. An isocyanate prepolymer mixture as claimed in any of the preceding claims, which has an -N=C=O content of from 5 to 25% by weight, based on the total weight of the isocyanate prepolymer mixture.

4. An isocyanate prepolymer mixture as claimed in any of the preceding claims, wherein the lignin used is one which has not been subjected to any special chemical treatment for this further processing.

5. An isocyanate prepolymer mixture as claimed in any of the preceding claims, wherein the solution b2 consists of the components b2i and b2ii.

6. The use of the isocyanate prepolymer mixture as claimed in any of the claims pertaining to isocyanate prepolymer mixtures in the production of polyurethane.

7. A process for preparing an isocyanate prepolymer mixture as claimed in any of the claims pertaining to isocyanate prepolymer mixtures, which comprises reacting the organic polyisocyanate (b1) defined in any of the claims pertaining to isocyanate prepolymer mixtures with the polyhydroxyl component (b2).

8. A process as claimed in claim 7, wherein the polyhydroxyl component (b2) is, prior to the reaction with the polyisocyanates, treated at a temperature of from 60 to 130°C, preferably under a pressure of as much as 30 mbar, for a period of from 1 to 8 hours, particu-larly preferably in the presence of esterification catalysts.

9. A process for producing compact or cellular polyurethanes, preferably PU foams, by reacting a) relatively high molecular weight compounds con-taining at least two reactive hydrogen atoms, preferably polyhydroxyl compounds, with b) liquid polyisocyanate compositions containing urethane groups in bonded form in the presence or absence of c) chain extenders and/or crosslinkers, d) blowing agents, e) catalysts and f) auxiliaries, wherein the polyisocyanate compositions (b) consist at least partially, preferably to the extent of from 1 to 80%, in particular to the extent of from 10 to 50% by weight, based on the weight of the component b), of an isocyanate prepolymer mixture as claimed in any of the claims pertaining to isocyanate prepolymer mixtures.

10. A process as claimed in claim 9, wherein the relatively high molecular weight compounds (a) have a functionality of from 2 to 8 and a hydroxyl number of from 25 to 500 and are preferably polyhydroxyl compounds which are selected, in particular, from the group of polythioether polyols, polyesteramides, hydroxyl-contain-ing polyacetals, hydroxyl-containing aliphatic poly-carbonates, polyester polyols, polymer-modified polyether polyols, preferably polyether polyols and mixtures of at least two of the specified polyhydroxyl compounds.

11. A process as claimed in claim 9 or 10, wherein the blowing agent (d) used is water, a linear or cyclic alkane having from 3 to 7 carbon atoms, or mixtures thereof.

12. A process as claimed in any of the claims per-taining to the production of polyurethane, wherein the polyurethane is obtained by reacting the high molecular weight compounds (a) and the polyisocyanate compositions (b) in the presence of a blowing agent (d) to form a polyurethane foam article in a mold.