HK1102438A - Method for producing polyether allophanates using zinc compounds as catalysts - Google Patents

Description

Process for preparing polyether allophanates using zinc compound catalysts

The present invention relates to a process for preparing polyisocyanate prepolymers containing allophanate structural units by using zinc compounds as catalysts, and to the use of these prepolymers for preparing polyurethanes and polyureas.

Polyisocyanate prepolymers containing allophanate structural units are of particular interest because of their high NCO content at lower viscosities. They constitute useful crosslinkers for two-component polyurethane systems and, after blocking of the NCO groups, also for one-component polyurethane systems. Polyurethane systems of this type are generally used for producing coatings.

Polyisocyanate prepolymers containing allophanate structural units are known in principle.

For example, EP-A303150 describes a process for preparing aliphatic allophanates which is carried out at elevated temperatures (> 200 ℃ C.) and without the use of catalysts. However, since rapid heating and cooling are necessary, practical implementation (i.e., in the case of mass production) is almost impossible.

EP-A712840 describes the use of zinc compounds such as zinc stearate, zinc octoate, zinc naphthenate and zinc acetylacetonate as allophanation catalysts. However, in this process, NCO and OH-free urethanes (urethanes) are used for allophanatization. Moreover, the polyisocyanates used for the preparation of the urethanes must always be different from those used for (subsequent) allophanatization. By this process, it is not possible to prepare allophanates based on polyhydroxyl compounds, such as polyether polyols, as the only organic hydroxyl compounds.

The specification of EP-A0682012 includes prepolymers based on diisocyanates and polyethers containing 1 to 4 hydroxyl groups, which prepolymers can be reacted with an excess of diisocyanate using tin (II) compounds to give the corresponding allophanates. However, the tin (II) compound cannot be sufficiently deactivated, and therefore the resultant product undergoes an increase in viscosity during storage and a decrease in NCO content.

It was therefore an object of the present invention to provide a process for preparing (cyclo) aliphatic polyisocyanate prepolymers containing allophanate structural units, by means of which products having a markedly improved storage stability, in particular an improved viscosity stability, can be obtained.

Surprisingly, it has now been found that (cyclo) aliphatic polyisocyanate prepolymers containing allophanate structural units can be prepared by allophanatization using a zinc (II) compound, preferably an zinc alkanoate, as catalyst.

The present invention accordingly provides a process for preparing polyisocyanate prepolymers containing allophanate structural units, in which a) are reacted with b) to give NCO-functional polyurethane prepolymers, the urethane groups of the prepolymers obtained being fully or partially allophanatized by further reaction with c) and d):

a) one or more aliphatic and/or cycloaliphatic polyisocyanates

b) One or more of a plurality of polyhydroxy compounds,

c) a polyisocyanate which may be different from the polyisocyanate described under a),

d) a zinc (II) compound as catalyst.

Examples of suitable aliphatic or cycloaliphatic polyisocyanates are diisocyanates or triisocyanates, such as tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate (1, 6-hexamethylene diisocyanate, HDI), 4-isocyanatomethyl-1, 8-octamethylene isocyanate (triisocyanatononane, TIN), or cyclic systems, such as 4, 4 '-methylenebis (cyclohexyl isocyanate), 3, 5, 5-trimethyl-1-isocyanato-3-isocyanatomethyl-cyclohexane (isophorone diisocyanate, IPDI), and also omega, omega' -diisocyanato-1, 3-dimethylcyclohexane (H, I)₆XDI)。

In components a) and c), hexamethylene diisocyanate (1, 6-hexamethylene diisocyanate, HDI), 4' -methylenebis (cyclohexyl isocyanate) and/or 3, 5, 5-trimethyl-1-isocyanato-3-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI) are preferably used as polyisocyanate. A particularly preferred polyisocyanate is HDI.

Preferably, the same polyisocyanates are used in a) and c).

Any polyol known to the person skilled in the art can be used as polyol of component b), preferably having an average OH functionality > 1.5.

These polyols may be, for example, low molecular weight diols (e.g., 1, 2-ethanediol, 1, 3-and/or 1, 2-propanediol, 1, 4-butanediol), triols (e.g., glycerol, trimethylolpropane) and tetraols (e.g., pentaerythritol), polyether polyols, polyester polyols, polycarbonate polyols and polythioether polyols. Preferred polyols are polyether-based materials of the kind described above.

Preferably, these polyether polyols have a number average molecular weight M_nFrom 300 to 20000 g/mol, more preferably from 1000 to 12000 g/mol, and particularly preferably from 2000 to 6000 g/mol.

It is also preferred that they have an OH functionality of 1.9 or more, more preferably 1.95 or more.

The OH functionality of these polyethers is preferably < 6, more preferably < 4.

Such polyether polyols can be prepared in a conventional manner by alkoxylating suitable starter molecules with base catalysts or using double metal cyanide compounds (DMC compounds).

Particularly suitable polyether polyols of component b) are those of the above-mentioned kind which contain less than or equal to 0.02 milliequivalents of unsaturated end groups per gram of polyol (meq/g), preferably less than or equal to 0.015 meq/g, more preferably less than or equal to 0.01 meq/g (measuring method: ASTM D2849-69).

Such polyether polyols have a particularly narrow molecular weight distribution, i.e.polydispersity (PD ═ M)_w/M_n) From 1.0 to 1.5 and/or an OH functionality of > 1.9. The polyether polyol preferably has a polydispersity of from 1.0 to 1.5 and an OH functionality greater than 1.9, more preferably greater than or equal to 1.95.

Such polyether polyols are prepared conventionally, in particular by alkoxylating suitable starter molecules using double metal cyanide catalysts (DMC catalysts). This method is described, for example, in US-a5158922 (e.g. example 30) and EP-a 0654302 (page 5, line 26 to page 6, line 32).

Examples of suitable starter molecules for the preparation of the polyether polyols include simple low molecular weight polyols, water, organic polyamines having at least two N-H bonds, or any desired mixtures of these starter molecules. Alkylene oxides which are particularly suitable for the alkoxylation reaction are ethylene oxide and/or propylene oxide, which can be used in any order or as a mixture in the alkoxylation reaction.

Preferred starter molecules for the preparation of polyether polyols by alkoxylation, in particular by the DMC process, are simple polyols, such as ethylene glycol, 1, 3-propylene glycol and 1, 4-butanediol, 1, 6-hexanediol, neopentyl glycol, 2-ethyl-1, 3-hexanediol, glycerol, trimethylolpropane, pentaerythritol, and hydroxyl-containing low molecular weight esters of such polyols with dicarboxylic acids, or low molecular weight ethoxylated or propoxylated products of such simple polyols, or any desired mixtures of these polyols.

The isocyanate group-containing polyurethane prepolymers are prepared by reacting the polyhydroxyl compounds of component b) with an excess of the polyisocyanates of a). The reaction is generally carried out at temperatures of from 20 to 140 c, preferably from 40 to 100 c, with or without catalysts known to those skilled in the art of polyurethane chemistry, such as tin soaps, for example dibutyltin dilaurate, or tertiary amines, for example triethylamine, or diazabicyclooctane.

The allophanatization reaction is then carried out by reacting the polyurethane prepolymers containing isocyanate groups with polyisocyanates c) with the addition of suitable catalysts d) for the allophanatization reaction, where the polyisocyanates of component c) and the polyisocyanates of component a) may be identical or different. The acid additive may then be added for stabilization purposes before removing excess polyisocyanate from the product by methods such as thin film distillation or extraction.

The molar ratio of the OH groups of component b) to the NCO groups of the polyisocyanates of components a) and c) is preferably from 1: 1.5 to 1: 20, more preferably from 1: 2 to 1: 15, very preferably from 1: 5 to 1: 15.

Preference is given to using zinc alkanoates (II) as catalysts for component d). Preferred zinc alkanoates (II) are based on 2-ethylhexanoic acid and on linear aliphatic C₄To C₃₀Zinc alkanoate of carboxylic acid (II). Particularly preferred compounds of component d) are zinc (II) bis (2-ethylhexanoate), zinc (II) bis (n-octanoate), zinc (II) bis (stearate) or mixtures thereof.

The allophanatization catalyst is generally used in an amount of up to 5% by weight, based on the total reaction mixture. Preferably 5 to 500ppm of catalyst is used, more preferably 20 to 200 ppm.

The acid additives optionally used are Lewis acids (electron-deficient compounds) or Bronsted acids (protic acids) or compounds which are capable of liberating such acids by reaction with water.

These acid additives may be, for example, organic or inorganic acids or other neutral compounds, such as acid chlorides or esters, which react with water to form the corresponding acid. Particular mention may be made of hydrochloric acid, phosphoric acid esters, benzoyl chloride, isophthaloyl dichloride, p-toluenesulfonic acid, formic acid, acetic acid, dichloroacetic acid and 2-chloropropionic acid.

If acid additives are used, preference is given to using organic acids, such as carboxylic acids, or acid chlorides, such as benzoyl chloride or isophthaloyl chloride.

The above acid catalysts may also be used to deactivate the allophanatization catalyst. Furthermore, they can improve the stability of the allophanates prepared according to the invention in the event of thermal stress during thin-film distillation or other storage of the products after preparation.

The acid additive is generally added in an amount such that the molar ratio of acid sites of the acid additive to deactivated sites of the catalyst is at least 1: 1. However, it is preferred to add an excess of the acid additive.

Thin-film distillation is the preferred method for separating off excess diisocyanate and is generally carried out at temperatures of from 100 to 160 ℃ and pressures of from 0.01 to 3 mbar. The residual monomer content after this operation is preferably less than 1% by weight, more preferably less than 0.5% by weight (diisocyanate).

All steps of the process according to the invention can, if appropriate, be carried out in the presence of an inert solvent. The inert solvent as used herein refers to a solvent that does not react with the reactants under the given reaction conditions. Examples are ethyl acetate, butyl acetate, methoxypropyl acetate, methyl ethyl ketone, methyl isobutyl ketone, toluene, xylene, aromatic or (cyclo) aliphatic hydrocarbon mixtures or any desired mixtures of these solvents. However, the reaction according to the invention is preferably carried out in the absence of solvents.

The components involved in the preparation of the isocyanate group-containing prepolymer and the allophanatization can be added in any order. However, preference is given to adding the polyether polyol b) to the polyisocyanate of the first added components a) and c) and finally adding the allophanatization catalyst d).

In a preferred embodiment of the present invention, the polyisocyanates of components a) and c) are initially charged into a suitable reaction vessel and then heated with stirring (if appropriate) at from 40 to 100 ℃. Then, after the desired temperature has been reached, the polyhydroxyl compounds of component b) are added with stirring and stirring is continued until the NCO content is equal to or slightly below the theoretical NCO content of the polyurethane prepolymer, estimated according to the chosen stoichiometry. The allophanatization catalyst d) is then added and the reaction mixture is heated at 50 ℃ and 100 ℃ until the NCO content is equal to or slightly below the desired NCO content. After addition of the acid additive as stabilizer, the reaction mixture is cooled or directly subjected to a thin-film distillation. In the course of the distillation, excess polyisocyanate is separated off at a temperature of from 100 to 160 ℃ and a pressure of from 0.01 to 3 mbar, with a residual monomer content of less than 1%, preferably less than 0.5%. After the thin film distillation, other stabilizers may be added, if desired.

The allophanates formed in this process generally correspond to the general formula (I),

in the formula:

Q¹and Q²Each independently of the others, is a radical of a linear and/or cycloaliphatic diisocyanate of the kind described above, preferably- (CH)₂)₆-，

R¹And R²Each independently is hydrogen or C₁-C₄Alkyl radical, R¹And R²Preferably hydrogen and/or a methyl group,

y is a radical of a starter molecule of the above-mentioned kind having a functionality of from 2 to 6, and is thus

n is a value from 2 to 6, depending on the different starter molecules used, and is not necessarily an integer,

m preferably corresponds to the number of monomer units, so that the polyether on which the structure is based has a number-average molecular weight of from 300 to 20000 g/mol.

The allophanates obtained are preferably compounds corresponding to the general formula (II),

in the formula:

q is a radical of a linear and/or cycloaliphatic diisocyanate of the above-mentioned kind, preferably- (CH)₂)₆-，

R¹And R²Each independently is hydrogen or C₁-C₄Alkyl radical, R¹And R²Preferably hydrogen and/or a methyl group,

y is a radical of a difunctional starter molecule of the above-mentioned kind,

m corresponds to the number of monomer units, so that the polyether on which the structure is based has a number-average molecular weight of from 300 to 20000 g/mol.

The number average molecular weight of the allophanates prepared according to the invention is generally from 700 to 50000 g/mol, preferably from 1500 to 15000 g/mol, more preferably from 1500 to 8000 g/mol.

The allophanates prepared according to the invention generally have a viscosity at 23 ℃ of from 500 to 100000mPas, preferably from 500 to 50000mPas, more preferably from 1000 to 7500mPas, and still more preferably from 1000 to 3500 mPas.

The products obtained by the process of the invention are distinguished in particular by viscosity stability. The viscosity increase after 7 days of storage at 50 ℃ is preferably less than 10%.

For example, the allophanates of the invention are used to prepare polyurethanes, polyureas or polyurethane-ureas by reaction with suitable polyols or polyamines, respectively, or with mixtures of the two. The process is carried out at or below room temperature, or at elevated temperature (baking). The polyurethanes and/or polyureas thus obtained are particularly suitable as coatings.

Accordingly, the present invention also provides a coating composition comprising:

A) one or more allophanates of the invention and

B) at least one diol or polyol and/or

C) At least one linear and/or cyclic aliphatic, araliphatic and/or aromatic diamine or polyamine.

The allophanates prepared by the process of the invention are distinguished by their high compatibility with the abovementioned components B) and C). In particular the combination of A) and C) can form a homogeneous (polyurea) coating.

The coating compositions concerned may be applied to a surface by conventional techniques such as spraying, dipping, flow coating or pouring. After flash evaporation to remove any solvent present, the coating is cured at ambient conditions or at elevated temperatures of 40 to 200 ℃.

The coating composition can be applied to, for example, metals, plastics, ceramics, glass and natural substances, and the substrate can be subjected to any pretreatment beforehand if necessary.

Examples

In the absence of any contrary indication, all percentages are to be understood as weight percentages.

The NCO content was determined by back-titration of the excess di-n-butylamine added with hydrochloric acid.

The viscosity was measured at 23 ℃ using a rotational viscometer from Haake.

The color number was measured in accordance with DIN EN 1557 (Hazen).

Comparative example 1

275.5 g of 1, 6-hexamethylene diisocyanate were first mixed with 120 mg of a 10% solution of isophthaloyl chloride in n-butyl acetate, and the mixture was heated to 100 ℃ with stirring. Then, over the course of about 3 hours, 324.3 g of polypropylene glycol prepared by the DMC catalyst process (without base) (unsaturated group content < 0.01 meq/g, molar weight 2000 g/mol, OH number 56 mg KOH/g, theoretical functionality 2) were added. The reaction mixture was then heated at 100 ℃ until the NCO content had reached 20.7%. The temperature was then lowered to 90 ℃ and 50 mg of tin (II) bis (2-ethylhexanoate) were then added and the allophanatization reaction was completed by stirring the reaction mixture, i.e. the NCO content had dropped to 18.4%.

However, when this catalyst was used, the allophanatization reaction was still incomplete even after continuous stirring (about 8 hours), and the NCO content reached only 19.7%.

Comparative example 2

275.5 g of 1, 6-hexamethylene diisocyanate were initially heated to 100 ℃ with stirring. 324.4 g of a polypropylene glycol prepared by the DMC catalyst process (without base) (unsaturated group content < 0.01 meq/g, molar weight 2000 g/mol, OH number 56 mg KOH/g, theoretical functionality 2) were then added in the course of about 3 hours. The reaction mixture was then heated at 100 ℃ until the NCO content had reached 20.7%. The temperature was then lowered to 90 ℃ after which 50 mg of tin (II) bis (2-ethylhexanoate) were added and the reaction mixture was stirred until the NCO content was 18.4% (about 6 hours). After the addition of 50 mg of isophthaloyl chloride, the excess 1, 6-hexamethylene diisocyanate was removed by thin-film distillation at about 0.5 mbar and 140 ℃.

The allophanatization reaction proceeded almost completely to give a clear, colorless product having an NCO content of 5.47% and a viscosity of 3725mPas (23 ℃ C.).

Example 1

275.5 g of 1, 6-hexamethylene diisocyanate were first mixed with 120 mg of a 10% solution of isophthaloyl chloride in n-butyl acetate, and the mixture was heated to 100 ℃ with stirring. Then, over the course of about 3 hours, 324.3 g of polypropylene glycol prepared by the DMC catalyst process (without base) (unsaturated group content < 0.01 meq/g, molar weight 2000 g/mol, OH number 56 mg KOH/g, theoretical functionality 2) were added. The reaction mixture was then heated at 100 ℃ until the NCO content had reached 20.7%. The temperature was then lowered to 90 ℃ after which 50 mg of zinc (II) bis (2-ethylhexanoate) were added and the reaction mixture was stirred until the NCO content was 18.4% (about 6 hours). 50 mg of isophthaloyl chloride were added and the excess 1, 6-hexamethylene diisocyanate was removed by thin-film distillation at about 0.5 mbar and 140 ℃.

The allophanatization reaction proceeded almost completely to give a clear, colorless product having an NCO content of 5.75% and a viscosity of 3360mPas (23 ℃ C.).

Example 2

275.5 g of hexamethylene 1, 6-diisocyanate were reacted with 324.3 g of polypropylene glycol in the presence of 50 mg of zinc (II) bis (2-ethylhexanoate) in the same manner as in example 1, and were stabilized with 50 mg of isophthaloyl chloride before thin-film distillation, except that isophthaloyl chloride was not added to hexamethylene 1, 6-diisocyanate.

This gives a clear, colorless product having an NCO content of 5.75% and a viscosity of 4230mPas (23 ℃).

Example 3

502.4 g of 1, 6-hexamethylene diisocyanate were first heated to 100 ℃ with stirring. Subsequently, 297.5 g of polypropylene glycol prepared by DMC catalysis (molar weight 1000 g/mol, OH number 112 mg KOH/g, theoretical functionality 2) were added in the course of about 3 hours. The reaction mixture was then heated at 100 ℃ until the NCO content had reached 28.2%. The temperature was then lowered to 90 ℃ and 70 mg of zinc (II) bis (2-ethylhexanoate) were added and the reaction mixture was stirred until the NCO content was 25.1%. 40 mg of dibutyl phosphate were added and the excess of 1, 6-hexamethylene diisocyanate was removed by thin-film distillation at about 0.5 mbar and 140 ℃.

This gives a colourless product having a Hazen colour number of 0, an NCO content of 8.95% and a viscosity of 3500mPas (23 ℃ C.).

Example 4

336.0 g of 1, 6-hexamethylene diisocyanate were first mixed with 120 mg of a 10% solution of isophthaloyl chloride in n-butyl acetate, and the mixture was heated to 100 ℃ with stirring. Then, over the course of about 3 hours, 263.8 g of polypropylene glycol prepared by DMC catalysis (without base) (unsaturated group content < 0.01 meq/g, molar weight 2000 g/mol, OH number 56 mg KOH/g, theoretical functionality 2) were added. The reaction mixture was then heated at 100 ℃ until the NCO content had reached 26.1%. The temperature was lowered to 90 ℃ and then 50 mg of zinc (II) bis (2-ethylhexanoate) were added and the reaction mixture was stirred until the NCO content was 24.3%. 50 mg of isophthaloyl chloride were added and the excess 1, 6-hexamethylene diisocyanate was removed by thin-film distillation at about 0.6 mbar and 140 ℃.

This gives a colorless, transparent product having an NCO content of 6.45% and a viscosity of 2860mPas (23 ℃).

Example 5:

50 grams each of the allophanates prepared according to comparative example 2 and example 1 were stored in tightly sealed glass bottles placed in a 50 ℃ dry box. It can be seen from the following values that the viscosity of the allophanates prepared according to the invention rises only slightly (< 8%), without a significant drop in the NCO content (< 2.1%), whereas the viscosity of the samples prepared using the tin catalyst increases considerably (about 50%):

allophanates of example 1

0 day, 7 days and 14 days

NCO content [% ] 5.755.715.63

Viscosity (23 ℃ C.) [ mPas ] 336034403620

Allophanate of comparative example 2

0 day, 7 days and 14 days

NCO content [% ] 5.475.315.29

Viscosity (23 ℃ C.) [ mPas ] 372544005570

Claims (9)

1. A process for preparing polyisocyanate prepolymers containing allophanate structural units, in which a) are reacted with b) to give NCO-functional polyurethane prepolymers, the urethane groups of the prepolymers obtained being fully or partially allophanatized by further reaction with c) and d):

a) one or more aliphatic and/or cycloaliphatic polyisocyanates

b) One or more of a plurality of polyhydroxy compounds,

c) a polyisocyanate which may be different from the polyisocyanate described under a),

d) a zinc (II) compound as catalyst.

2. The process for preparing polyisocyanate prepolymers containing allophanate structural units according to claim 1, wherein the same type of polyisocyanates is used in components a) and c).

3. The process for preparing stabilized polyisocyanate prepolymers containing allophanate structural units according to claim 1 or 2, wherein 1, 6-hexamethylene diisocyanate is used as polyisocyanate in components a) and c).

4. Process for preparing polyisocyanate prepolymers containing allophanate structural units according to any one of claims 1 to 3, characterized in that polyether polyols are used in component b).

5. The process for preparing polyisocyanate prepolymers containing allophanate structural units according to any one of claims 1 to 4, wherein, in component d), use is made of prepolymers based on 2-ethylhexanoic acid and/or on linear aliphatic C₄To C₃₀The zinc alkanoate (II) of the carboxylic acid serves as a zinc catalyst for the allophanatization reaction.

6. Polyisocyanate prepolymers containing allophanate structural units, preparable by a process as claimed in any one of claims 1 to 5.

7. Use of the polyisocyanate prepolymers containing allophanate structural units according to claim 6 for the production of coatings, adhesives and/or sealants.

8. A coating composition comprising:

A) one or more polyisocyanate prepolymers containing allophanate structural units as claimed in claim 6, and

B) at least one diol or polyol, and/or

C) At least one linear and/or cyclic aliphatic, araliphatic and/or aromatic diamine or polyamine.

9. A substrate coated with a coating prepared from the polyisocyanate prepolymer containing allophanate structural units according to claim 6.