GB1561771A - Process for the manufacture of heat-resistant polyurethane elastomers - Google Patents

Description

(54) PROCESS FOR THE MANUFACTURE OF HEAT RESISTANT POLYURETHANE ELASTOMERS (71) We, BASF AKTIENGESELL SCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to a process for the manufacture of a heat-resistant polyurethane elastomer, from a predominantly linear polyhydroxy-compound, a polyisocyanate and a chain lengthener.

The prior art shows that polyurethane elastomers based on polyhydroxy - compounds of fairly high molecular weight, diisocyanates and low molecular weight chain lengtheners have well-balanced properties; these materials have therefore proved of excellent value for a large number of applications.

In spite of their advantages in respect of mechanical strength and wear resistance, the polyurethane elastomers of the prior art suffer from some serious shortcomings which substantially restrict their uses.

The outstanding shortcoming is inadequate heat resistance, leading to premature softening and hence to the failure of parts exposed to severe mechanical conditions. This particularly proves troublesome if these parts are exposed to dynamic stresses.

Hitherto, polyurethane elastomers having improved heat resistance were only obtainable by using starting materials which are very expensive and not readily accessible eg.

naphthylene - 1,5 - diisocyanate, or by using physiologically dangerous substances such as methylene - bis - (2 - chloroaniline).

The present invention provides a process for the manufacture of a heat-resistant polyurethane elastomer from at least one predominantly linear polyhydroxy compound having a molecular weight of 800 to 10,000 polyisocyanate, a chain lengthener and, if desired, a catalyst and one or more other additives, wherein the at least one polyhydroxy compound is reacted with an amount of polyisocyanate such that the ratio of OH groups to NCO groups is from 1:0.66 to 1:0.85, and the resulting adduct possessing terminal hydroxy groups is subsequently reacted with a diol as chain lengthener and an excess of symmetrical aromatic diisocyanate which forms a crystalline or crystallizable diurethane and/or polyurethane with the diol, under conditions such that in the final product the ratio of the number of isocyanate groups to the number of all hydrogen atoms capable of reacting with isocyanate is from 1.0:1 to 1.2:1, the process being carried out such that the final product has a softening point or softening range at or above 1500C.

The advantage of the polyurethane elastomers manufactured by a process within the invention is that in addition to their known high mechanical strength they have softening points or softening ranges at or above 150or, preferably from 150 to 200 C (measured by thermo-mechanical analysis using the TMA attachment for DuPont Thermal Analyzer 990), the starting materials, molar ratio OH: NCO groups and process conditions being chosen within the defined limits so as to give such a final product. The reaction of the adduct obtained from a polyhydroxycompound having a molecular weight of 80010,000 and a polyisocyanate and possessing terminal hydroxyl groups, to give a polyurethane elastomer can be carried out by conventional methods, i.e. in a single stage, in accordance with the one-shot process, by stirring in the diol chain lengthener and then adding the symmetrical aromatic diisocyanate, or in two stages, in accordance with the prepolymer process, by reacting the adduct possessing terminal hydroxyl groups, with the symmetrical aromatic diisocyanate to give a prepolymer containing isocyanate groups and then carrying out chain lengthening with the diol chain lengthener. Preferably, the economical one-shot process is used to manufacture the polyurethane elastomers of the invention.

All predominantly linear polyhydroxy-comw pounds having a molecular weight of 80e 10,000, polyisocyanates and low molecular weight diol chain lengtheners known from the prior art can be used to manufacture the polyurethane elastomers in accordance with the process of the invention. However, it is essential, and must be borne in mind, that only those symmetrical aromatic diisocyanates which form crystalline or crystallizable products with the added diol chain lengthener can be used for the reaction of the adduct possessing terminal hydroxyl groups with excess of the diisocyanate, whilst the adduct containing hydroxyl groups can itself be manufactured from a predominantly linear polyhydroxy - compound having a molecular weight of 800--10,000 and any desired polyisocyanate.

Examples of polyhydroxy-compounds having a molecular weight of 800-10,000 are hydroxyl - containing polyesters, polyester amides, polyethers and/or polyacetals, and preferably those having molecular weights of from 1,000 to 5,000. The polyhydroxy compounds must be at least predominantly linear, ie. they must be essentially difunctional for the purpose of the isocyanate reaction. The polyhydroxy-compounds mentioned may be used as single components or as mixtures.

Suitable hydroxyl - containing polyesters and polyester amides can be manufactured, for example, from dicarboxylic acids of 2 to 12 carbon atoms and di-hydric alcohols, if appropriate with addition of amino alcohols or di amines. Examples of suitable dicarboxylic acids are aliphatic dicarboxylic acids, such as succinic acid, glutaric acid, adipic add, suberic acid, azelaic acid and sebacic add, and aromatic dicarboxylic acids, such as phthalic acid, isophthalic acid and tere phthalic acid. The dicarboxylic acids can be used individually or as mixtures. To manu facture the polyesterols it can at times be advantageous to use, in place of the carboxylic acids, the corresponding carboxylic acid deri vatives, such as carboxylic acid esters of 1 to 4 carbon atoms in the alcohol radicals, carboxylic acid anhydrides or carboxylic acid chlorides. Examples of di-hydric alcohols are glycols of 2 to 16 carbon atoms, prefer ably of 2 to 6 carbon atoms, such as ethylene glycol, diethylene glycol, butane - 1,4 - diol, pentane - 1,5 - diol, hexane - 1,6 - diol, decane - 1,10 - diole, 2,2 - dimethyl - pro pane - 1,3 - diol, 2,2,4 - trimethyl - pentane 1,3 - diol, propane - 1,3 - diol, propane - 1,2 diol, dipropylene glycol and tripropylene glycol. Depending on the desired properties, the di-hydric alcohols can be used individually or as mixtures with one another or with small amounts of diamines or amino alcohols, such as ethylene diamine, 1,4 - diaminobutane, piperazine, ethanolamine or N - methyldiethanolamine. Esters of carbonic acid with the above diols, especially with diols of 4 to 6 carbon atoms, eg. butane - 1,4 - diol and/or hexane - 1,6 - diol, condensation products of w - hydroxycarboxylic acids,-eg. of Rw hydroxycaproic acid, and, preferably, poly merization products of cyclic lactones, eg. of substituted or unsubstituted e - caprolactones, can also be used. The hydroxyl - containing polyesters and polyester - amides preferably have molecular weights of from 800 to 5,000.

Suitable polyether - ols can be manufactured by reacting one or more alkylene oxides, where alkylene is of 2 to 4 carbon atoms, with a starter molecule which contains two active hydrogen atoms in the molecule. Examples of suitable alkylene oxides are ethylene oxide, 1,2 - propylene oxide, epichlorohydrin, 1,2 butylene oxide and 2,3 - butylene oxide. The alkylene oxides may be used individually, in alternate succession or as mixtures. Examples of suitable starter molecules are water, amino alcohols such as N - alkyldiethanolamines, eg.

N - methyldiethanolamine, and diols, eg.

ethylene glycol, propylene glycol, butane 1,4 - diol and hexane - 1,6 - diol. Further suitable polyetherols are hydroxyl - contains ing polymerization products of tetrahydrofuran. The hydroxyl - containing polyethers which, like the above polyesterols, are of predominantly linear, ie. difunctional, structure, have molecular weights of from 800 to 10,000, preferably from 1,000 to 5,000. Suitable polyacetals are, particularly waterinsoluble formals, eg. polybutanediol formal and polyhexanediol formal.

Preferably, symmetrical and/or asymmetrical cycloaliphatic and, in particular, aliphatic and/or aromatic diisocyanates are used to manufacture rhe adducts possessing terminal hydroxyl groups from polyhydroxycompounds having a molecular weight of 800-10,000. Specific examples are cycloaliphatic diisocyanates, eg. 1,4 - cyclohexanediisocyanate, 4,4' - diisocyanatodicyclohexylmethane and 3 - isocyanatomethyl - 3,5,5 trimethylcyclohexyl isocyanate (isophorone diisocyanate), aliphatic ciiisocyanates, eg.

ethylene diisocyanare butane diisocyanate, decane diisocyanate, 2,2,4 - (2,4,4) - trimethylhexamethylene diisocyanate and, preferably, 1,6 - hexamethylene diisocyanate, and aromatic diisocyanates, eg. diphenyl diisocyanates, m- or pphenylene diisocyanate, 1,5 - naphthalene - diisocyanate, 2,4 - and 2,6 - toluylene diisocyanates and their isomer mixtures, 2,2'-, 4,4'- and 2,4' - diphenylmethane diisocyanates and their isomer mixtures and, preferably, 4,4' - diphenylmethane diisocyanate.

As has already been mentioned, it is essential to the invention that the adduct containing terminal hydroxyl groups should be reacted with a symmetrical aromatic diiso- cyanate which forms a crystalline or crystallizable urethane with the diol chain lengthener.

Examples of suitable symmetrical aromatic diisocyanates are 1,4 - phenylene diisocyanate, 1,5 - naphthylene diisocyanate and 4,4' diisocyanato diphenylmethane.

The said symmetrical aromatic diiso cyanates,, especially 4,4' - diisocyanatodiphenylmethane, give polyurethane elastomers having particularly advantageous mechanical properties and high heat resistance, so that they are used preferentially for the reaction according to the process of the invention.

Low molecular weight diol chain lengtheners which may be used are usuallv diols of 2 to 6 carbon atoms, especially those having an even number of carbon atoms, or esters of terephthalic acid with glycols of 2 to 4 carbon atoms. Examples are di - (ethylene glycol) terephthalate and di -(burane - 1,4 diol) terephthalate. Ethylene glycol, butane 1,4 - diol and hexane - 1,6 - diol are used preferentially, butane - 1,4 - diol in particular having proved excellent. If the process according to the invention is used to manufacture cellular polyurethane elastomers, water can also be used as an additional chain lengthener.

To manufacture the adduct containing terminal hydroxyl groups the polyhydroxy compound having a molecular weight of 800 10,000 is reacted with any symmetrical or asymmetrical polyisocyanate in an amount such that the ratio of OH groups to NCO groups is from 1:0.66 to 1:0.85, preferably from 1:0.71 to 1:0.85. The hydroxyl-containing adduct obtained is then reacted with an excess of a symmetrical aromatic diisocyanate and a diol chain lengthener. In this reaction it is essential-rn order to obtain polyurethane elastomers having the excellent properties referred to above-that the ratio of the number of isocyanate groups to the total number of all hydrogen atoms capable of reacting with isocyanate in the final product is from 1:1 to 1.2: 1, the best results being obtained with values from about 1:1 to 1.1:1.

To manufacture the polyurethane elastomers of the invention, the starting components are generally reacted at from 800 to 2000C, preferably from 1000 to 1500C. In detail, an advantageous method of manufacture is the following: the anhydrous polyhydroxy compound is reacted, under anhydrous conditions, with an appropriate amount of any desired polyisocyanate to give an adduct having terminal hydroxyl groups. The further conversion to the polyurethane elastomer can then be carried out either by the two-stage prepolymer process or by the more economical one-shot process. In the one-shot process, the diol chain lengthener is first stirred into the melt of the adduct containing terminal hydroxyl groups, the symmetrical aromatic diisocyanate is then stirred in and after the melt has been homogenized dissolved gases are removed by application of reduced pressure. The bubble-free, pourable melt is then poured into moulds and cured. If further processing is carried out by the prepolymer process, the adduct containing terminal hydroxyl groups is first reacted with the symmetrical aromatic diisocyanate to give a prepolymer with terminal isocyanate groups, the diol chain lengthener is then stirred in and after removing dissolved gases the mixture is poured into moulds and cured.

The curing of the mouldings to give the end product is generally carried out by heating at from 1000C to 2000C, preferably from 1000C to 150 C.

Of course, any known conventional assistant or additive can be added to the reaction mixture, eg. catalysts, plasticizers, dyes, fillers, antioxidants and agents to protect against hydrolysis.

In addition to possessing the conventional high mechanical strength of polyurethanes, the polyurethanes manufactured according to the invention are particularly distinguished by softening points higher than those of prior art materials. They are therefore, exceptionally suitable for applications where the material is exposed to heat or to dynamic stress. Types of use which may be mentioned are dynamically stressed rotating rollers, vehicle tyres, dust hoods over motors and gearboxes and seals. They may be used both in the compact form and in a cellular form.

In the Examples which follow, parts and percentages are by weight, unless stated otherwise.

EXAMPLE 1 311.7 parts (0.15 mol) of a hydroxyl-containing polyester of adipic acid and ethylene glycol, having a hydroxyl number of 54, are dehydrated in a stirred flask, equipped with a thermometer and reduced pressure attachment, for one hour at 100cm and 15 mm Hg, and are then reacted with 31.25 parts (0.125 mol) of 4,4' - diphenylmethanediisocyanate at 1000C for 30 minutes. The resulting hydroxyl-containing adduct is reacted with 81.25 parts (0.325 mol) of 4,4' - diphenylmethane - diisocyanate at 1000C for 20 minutes to give a prepolymer containing isocyanate groups. To manufacture the polyurethane elastomer, a mixture of 25.1 parts (0.279 mol) of butane - 1,4 - diol and 0.05 part of acetyl - acetone is worked, at 800 C, into the prepolymer containing isocyanate groups. After the reaction mixture has been homogenized, the air dissolved therein is removed under reduced pressure and the polyurethane melt is poured into moulds and heated for 24 hours at 1200C. The thermal characteristics of the polyurethane elastomer manufactured by the process of the invention were examined by thermal analysis using a TMA attachment for DuPont Thermal Analyzer 990. For this purpose, a 5 mm thick sample was subjected to the load of a test ram of 2.54 mm diameter under a weight of 100 g, and at the same time the sample was heated at a rate of 5 C/min. The softening of the sample was measured from the penetration of the test ram.

Measurement showed that the sample under went linear expansion between the glass tran sition temperature of the soft phase and the melting of the crystalline hard phase. The sample started to soften at from 1850 to 1900C.

The accompanying diagrammatic drawing shows as a solid line the softening curve for this sample, but no absolute units are put on the degree of softening illustrated. As can be seen, the curve follows a straight line from 0 C until 1850--1900C, there being no secondary softening range.

COMPARATIVE EXAMPLE 1A If the procedure followed is as described in Example 1, but the entire amount of 4,4' diphenylmethane - diisocyanate is added all at once to the hydroxyl-containing polyester, a polyurethane elastomer is obtained which, when examined thermo-mechanically, exhibits a secondary softening range of from 500 to 800 C, with final softening starting at about 1400 C. The softening curve for this sample is shown by the dotted line in the accompanying diagrammatic drawing, the secondary softening range being shown.

Accordingly, a polyurethane elastomer with heat resistance improved by more than 1000C was obtainable, from the same starting components, by means of the process of manufacture according to the invention.

EXAMPLE 2 311.7 parts (0.15 mol) of a hydroxyl-conraining polyester of adipic acid and ethylene glycol, having a hydroxyl number of 54, were converted, by the method described in Example 1, first to an adduct possessing terminal hydroxyl groups by means of 31.25 parts (0.125 mol) of 4,4' - diisocyanatodi phenylmethane and then to a polyurethane elastomer by means of 107.5 parts (0.43 mol) of 4,4' - diisocyanatediphenylmethane and 34.11 parts (0.379 mol) of butane - 1,4 diol.

Thermo-mechanical analysis of the product showed only one softening range, which started at 1900C.

COMPARATIVE EXAMPLE 2A A prepolymer containing isocyanate groups was manufactured from 311.7 parts (0.15 mol) of a hydroxyl-containing polyester according to Example 2 and 138.75 parts (0.495 mol) of 4,4' - diisocyanatodiphenylmethane, at 1000C, and was then crosslinked with 28.96 parts (0.321 mol) of butanediol.

Thermo-mechanical examination showed a softening range of from 700 to 90 C with final softening starting at about 170-175 C.

EXAMPLE 3 If the procedure followed is as described in Example 1, but 25.0 parts ,0.1 mol of 4,4' - diisocyanatodiphenylmethane are first added so as to produce an adduct possessing terminal hydroxyl groups, which is reacted, in a second reaction stage, with 98.75 parts (0.395 mol) of 4,4' - diisocyanatodiphenylmethane and 28.96 parts (0.321 mol) of butane - 1,4 - diol, there is obtained a polyurethane elastomer which starts to soften at 1800C, and does not possess a second softening range at lower temperatures.

WHAT WE CLAIM IS: 1. A process for the manufacture of a heatresistant polyurethane elastomer from at least one predominantly linear polyhydroxy compound having a molecular weight of 800 to 10,000, polyisocyanate, a chain lengthener and, if desired, a catalyst and one or more other additives, wherein the at least one polyhydroxy compound is reacted with an amount of polyisocyanate such that the ratio of OH groups to NCO groups is from 1:0.66 to 1:0.85, and the resulting adduct possessing terminal hydroxy groups is subsequently reacted with a diol as chain lengthener and an excess of symmetrical aromatic diisocyanate which forms a crystalline or crystallizable diurethane and/or polyurethane with the diol, under conditions such that in the final product the ratio of the number of isocyanate groups to the number of all hydrogen atoms capable of reacting with isocyanate is from 1.0:1 to 1.2:1, the process being carried out such that the final product has a softening point or softening range at or above 1500C.

2. A process as claimed in claim 1, wherein the polyisocyanate used to manufacture the adduct possessing terminal hydroxyl groups is a symmetrical and/or asymmetrical cycloaliphatic, aliphatic and/or aromatic diisocyanate.

3. A process as claimed in claim 2, wherein the polyisocyanate used is 1,6 - hexamethylene diisocyanate, 4,4' - diisocyanatodiphenylmethane or 1,5 - naphthylene diisocyanate.

4. A process as claimed in any one of the preceding claims, wherein 1,4 - phenylene diisocyanate, 4,4' - diisocyanatodiphenylmethane or 1,5 - naphthylene diisocyanate is used as the symmetrical aromatic diisocyanate.

5. A process as claimed in any of claims 1 to 4, wherein ethylene glycol, butane - 1,4 diol or hexane - 1,6 - diol is used as the chain lengthener.

6. A process as claimed in claim 1 or 2, wherein 4,4' - diisocyanatodiphenylmethane

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. TMA attachment for DuPont Thermal Analyzer 990. For this purpose, a 5 mm thick sample was subjected to the load of a test ram of 2.54 mm diameter under a weight of 100 g, and at the same time the sample was heated at a rate of 5 C/min. The softening of the sample was measured from the penetration of the test ram. Measurement showed that the sample under went linear expansion between the glass tran sition temperature of the soft phase and the melting of the crystalline hard phase. The sample started to soften at from 1850 to 1900C. The accompanying diagrammatic drawing shows as a solid line the softening curve for this sample, but no absolute units are put on the degree of softening illustrated. As can be seen, the curve follows a straight line from 0 C until 1850--1900C, there being no secondary softening range. COMPARATIVE EXAMPLE 1A If the procedure followed is as described in Example 1, but the entire amount of 4,4' diphenylmethane - diisocyanate is added all at once to the hydroxyl-containing polyester, a polyurethane elastomer is obtained which, when examined thermo-mechanically, exhibits a secondary softening range of from 500 to 800 C, with final softening starting at about 1400 C. The softening curve for this sample is shown by the dotted line in the accompanying diagrammatic drawing, the secondary softening range being shown. Accordingly, a polyurethane elastomer with heat resistance improved by more than 1000C was obtainable, from the same starting components, by means of the process of manufacture according to the invention. EXAMPLE 2 311.7 parts (0.15 mol) of a hydroxyl-conraining polyester of adipic acid and ethylene glycol, having a hydroxyl number of 54, were converted, by the method described in Example 1, first to an adduct possessing terminal hydroxyl groups by means of 31.25 parts (0.125 mol) of 4,4' - diisocyanatodi phenylmethane and then to a polyurethane elastomer by means of 107.5 parts (0.43 mol) of 4,4' - diisocyanatediphenylmethane and 34.11 parts (0.379 mol) of butane - 1,4 diol. Thermo-mechanical analysis of the product showed only one softening range, which started at 1900C. COMPARATIVE EXAMPLE 2A A prepolymer containing isocyanate groups was manufactured from 311.7 parts (0.15 mol) of a hydroxyl-containing polyester according to Example 2 and 138.75 parts (0.495 mol) of 4,4' - diisocyanatodiphenylmethane, at 1000C, and was then crosslinked with 28.96 parts (0.321 mol) of butanediol. Thermo-mechanical examination showed a softening range of from 700 to 90 C with final softening starting at about 170-175 C. EXAMPLE 3 If the procedure followed is as described in Example 1, but 25.0 parts ,0.1 mol of 4,4' - diisocyanatodiphenylmethane are first added so as to produce an adduct possessing terminal hydroxyl groups, which is reacted, in a second reaction stage, with 98.75 parts (0.395 mol) of 4,4' - diisocyanatodiphenylmethane and 28.96 parts (0.321 mol) of butane - 1,4 - diol, there is obtained a polyurethane elastomer which starts to soften at 1800C, and does not possess a second softening range at lower temperatures. WHAT WE CLAIM IS:

1. A process for the manufacture of a heatresistant polyurethane elastomer from at least one predominantly linear polyhydroxy compound having a molecular weight of 800 to 10,000, polyisocyanate, a chain lengthener and, if desired, a catalyst and one or more other additives, wherein the at least one polyhydroxy compound is reacted with an amount of polyisocyanate such that the ratio of OH groups to NCO groups is from 1:0.66 to 1:0.85, and the resulting adduct possessing terminal hydroxy groups is subsequently reacted with a diol as chain lengthener and an excess of symmetrical aromatic diisocyanate which forms a crystalline or crystallizable diurethane and/or polyurethane with the diol, under conditions such that in the final product the ratio of the number of isocyanate groups to the number of all hydrogen atoms capable of reacting with isocyanate is from 1.0:1 to 1.2:1, the process being carried out such that the final product has a softening point or softening range at or above 1500C.

2. A process as claimed in claim 1, wherein the polyisocyanate used to manufacture the adduct possessing terminal hydroxyl groups is a symmetrical and/or asymmetrical cycloaliphatic, aliphatic and/or aromatic diisocyanate.

3. A process as claimed in claim 2, wherein the polyisocyanate used is 1,6 - hexamethylene diisocyanate, 4,4' - diisocyanatodiphenylmethane or 1,5 - naphthylene diisocyanate.

4. A process as claimed in any one of the preceding claims, wherein 1,4 - phenylene diisocyanate, 4,4' - diisocyanatodiphenylmethane or 1,5 - naphthylene diisocyanate is used as the symmetrical aromatic diisocyanate.

5. A process as claimed in any of claims 1 to 4, wherein ethylene glycol, butane - 1,4 diol or hexane - 1,6 - diol is used as the chain lengthener.

6. A process as claimed in claim 1 or 2, wherein 4,4' - diisocyanatodiphenylmethane

is used as the polyisocyanate and the syan- metrical aromatic diisocyanate and butane 1,4 - diol is used as the chain lengthener.

7. A process as claimed in any of claims 1 to 6, wherein the at least one polyhydroxy compound is reacted with an amount of the polyisocyanate such that the ratio of OH groups to NCO groups is from 1:0.71 to 1:0.85 to form the adduct possessing terminal hydroxy groups.

8. A process as claimed in any of claims 1 to 7, wherein the predominantly linear polyhydroxy compound having a molecular weight of 800 to 10,000 is a polyether or polyacetal or a polyesteramide.

9. A process for the manufacture of a heat-resistant polyurethane elastomer carried out substantially as described in any of the foregoing Examples 1, 2 and 3.

10. Heat-resistant polyurethane elastomers when obtained by the process of any of claims 1 to 9.