Improved polyurea coatings from dimethyl-substituted polyaspartic ester mixtures

Abstract: A method for making a polyurea coating by: (a) coating a substrate with a coating composition containing effective coating-forming amounts of (i) a polyisocyanate component, and (ii) an polyaspartic ester mixture. The invention also relates to the coatings made by the method, the polyaspartic ester mixtures used to make the coatings, and methods to make the mixtures.

Description: The invention relates to the field of polyaspartic ester mixtures, and more particularly to the use of polyaspartic ester mixtures in polyurea coating applications. BACKGROUND OF THE INVENTION Two-component polyurea coating compositions containing a polyisocyanate in combination with a polyaspartic ester component are known. They are suitable for the formation of coatings and can be adjusted to produce coatings that are hard, elastic, abrasion resistant, solvent resistant, and especially weather resistant. Despite their widespread use, however, known coating compositions contain disadvantages which limit their use in important applications. Coating compositions with an appreciable amount of polyaspartic esters with dimethyl groups would be desired because dimethyl groups would add desired properties to coatings made from such compositions. U.S. Pat. No. 5,126,170 discloses a process for making polyurethane coatings in which an isocyanate-reactive component b) includes a polyaspartic ester mixture made from an optionally-substituted maleic or fumaric acid ester and a primary amine. Although the patent teaches that maleic acid or fumaric acid ester can be substituted with dimethyl, diethyl and di-n-butyl esters, it has been observed that during the Michael Addition Reaction of dimethyl maleate and primary amines, dimethyl maleate isomerizes to dimethyl fumarate in the presence of amines, according to the following geometric isomerization reaction: The dimethyl fumarate forms long needle-like crystals which no longer participate in the Michael Addition Reaction and prevent the reaction from completing. Although the resulting reaction produces yields of only about 30 to 40%, the entire composition is useless for commercial purposes. This is because the composition contains a mixture of compounds that preclude the formation of a suitable coating. The crude mixture generally contains (i) dimethyl fumarate crystals, (ii) starting diamine material, (iii) mono-primary-amine-monoaspartate and (iv) diaspartate. The presence of crystals in such a mixture prevents the formation of a coating. Filtering the crystals from the mixture has not been an option because filtration removes an appreciable amount of starting material, thereby adding substantial costs. Also, a filtrated mixture contains unreacted primary amines whose presence undesirably speed the crosslinking reaction. U.S. Pat. No. 5,126,170 teaches preparing its polyaspartic esters in a solvent. The use of a 50% methanol reaction medium, however, is not practical in a production situation for the following reasons. First, the use of a 50% solution means that yields of product are half of what could be achieved if the reaction was run without solvent. Second, the methanol is highly flammable and its presence in manufacturing would be a safety hazard. Finally, in order for the polyaspartic ester to be used with polyisocyanates, the methanol would have to be completely removed. Even small, residual amounts of methanol would react with polyisocyanates to form urethanes, which would decrease the crosslink density of the films and so cause a decrease in properties. For the foregoing reasons, it has been desired to develop a method for making a polyurea coating ingredient that contains an appreciable amount of dimethyl-substituted polyaspartic esters. SUMMARY OF THE INVENTION The invention relates to a method for making an asymmetric polyaspartic ester mixture by sequentially (a) forming an ester mixture containing a dimethyl-substituted first ester component and a second ester component substituted with an alkyl group having at least two carbon atoms and (b) reacting the ester component with a propylene oxide amine component, such that the equivalent number ratio of the first ester component and the second ester component is sufficient to prevent the formation of a reaction-stopping crude mixture containing dimethyl fumarate crystals. The invention also relates to a polyurea coating composition containing a polyisocyanate component, the ester component used to make the polyaspartic ester mixture, and the asymmetric polyaspartic ester mixture, a method for making a coating with the polyaspartic ester mixture, and a coating made with the asymmetric polyaspartic ester mixture. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims. DESCRIPTION OF THE INVENTION The invention is based on the surprising discovery that the crystallization that has been observed during the reaction of dimethyl-substituted maleic acid or dimethyl-substituted fumaric acid and a propylene oxide amine can be substantially reduced or eliminated altogether by reacting the propylene oxide amine with a mixture containing dimethyl maleate and a small amount of at least one dialkyl maleate having two or more carbon atoms, e.g., diethyl maleate, dipropyl maleate. By practicing the invention, polyaspartic esters based on dimethyl maleate can now be made simply and directly, without crystallization and without the need for solvents. The asymmetric polyaspartic ester mixtures produced can then be used as isocyanate-reactive components in coating compositions for making polyurea coatings having a novel structure and improved properties. Maleic acid esters and fumaric acid esters include suitable dialkyl maleates or dialkyl fumarates. Suitable dialkyl maleates include dimethyl maleate, diethyl maleate, dipropyl maleate, dibutyl maleate, methyl propyl maleate, ethyl propyl maleate. Suitable dialkyl fumurates include dimethyl fumurate, diethyl fumurate, dipropyl fumurate, dibutyl fumurate, methyl propyl fumurate, ethyl propyl fumurate. The propylene oxide amine component includes any propylene oxide amine that can accomplish the objects of the invention. Suitable polypropylene oxide amines generally include difunctional and multifunctional amines with polypropylene oxide groups. These amines are well known and can be prepared by methods such as those described in German Offenlegungsschrift No. 1,193,671, U.S. Pat. No. 3,236,895, French. Pat. No. 1,466,708. Suitable examples of such difunctional amines include polypropylene oxide diamine can be obtained from Huntsman Corporation under the marks Jeffamine D-2000. Examples of suitable trifunctional polypropylene oxide amines include polyoxypropylene triamine, (Jeffamine T-403), Jeffamine T-3000 and Jeffamine T-5000, also available from Huntsman. It is believed that multifunctional propylene oxide amines, e.g., tetrafunctional polypropylene oxide amines, can also be used. The equivalent number ratio of (i) the dimethyl maleates (or the dimethyl fumurates) to (ii) the dialkyl maleates (dialkyl fumurates) that have at least 2 carbon atoms is sufficient to prevent the formation of a reaction-stopping crude mixture containing dimethyl fumarate crystals. I Generally, the equivalent number ratio is from less than about 7:3, preferably from about 5:5 to more than 0:10, more preferably from about 4.5:5.5 to 1:9, and even more preferably from about 4:6 to about 2:8. Stated in a number percentage basis, the amount of the dimethyl-substituted maleic acid ester or fumaric acid ester is present from less than about 70% to more than 0%, preferably less than 50% to more than 0%, preferably from about 45% to about 10%, and even more preferably from about 40 to about 20%, based on the total number of esters. It has been discovered that these following ranges are critical to accomplish the objects of the invention. The equivalent number ratio of the propylene oxide amine component to the ester component is generally about 1:1. As such, the ratio of the first ester component and the second ester component must be greater than 0:10. The polyisocyanate component used to react with the polyaspartic ester mixtures includes any polyisocyanate, which, when used in accordance with the invention, meets the object of the invention. Suitable polyisocyanates for use as polyisocyanate component in accordance with the present invention include the known polyisocyanates of polyurethane chemistry. Examples of suitable low molecular weight polyisocyanates having a molecular weight of 168 to 300 include 1,4-diisocyanatobutane, 1,6-hexamethylene diisocyanate, 2,2,4- and/or 2,4,4-trimethyl-1,6-hexamethylene diisocyanate, dodecamethylene diisocyanate, 1,4-diisocyanatocyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), 2,4'- and/or 4,4'-diisocyanato-dicyclohexyl methane, 2,4- and/or 4,4'-diisocyanatodiphenyl methane and mixtures of these isomers with their higher homologues which are obtained in a known manner by the phosgenation of aniline/formaldehyde condenses, 2,4- and/or 2,6-diisocyanatotoluene and any mixtures of these compounds. Preferred cyclic isocyanates include diphenylmethane 4,4'-diisocyanate (MDI), diphenylmethane 2,4'-diisocyanate, 2,4- and/or 2,6-diisocyanatotoluene. Preferred aliphatic isocyanates include hexamethylene diisocyanate, isophorone diisocyanate, 2,4'- and/or 4,4'-diisocyanato-dicyclohexyl methane. Additional suitable polyisocyanate components include derivatives of the above-mentioned monomeric polyisocyanates, as is conventional in coatings technology. These derivatives include polyisocyanates containing biuret groups as described, for example, in U.S. Pat. Nos. 3,124,605 and 3,201,372 and DE-OS 1,101,394; polyisocyanates containing isocyanurate groups as described in U.S. Pat. No. 3,001,973, DE-PS 1,022,789, 1,222,067 and 1,027,394 and DE-OS 1,929,034 and 2,004,048; polyisocyanates containing urethane groups as described, for instance, in DE-OS 953,012, BE-PS 752,261 and U.S. Pat. Nos. 3,394,164 and 3,644,457; polyisocyanates containing carbodiimide groups as described in DE-PS 1,092,007, U.S. Pat. No. 3,152,162 and DE-OS 2,504,400, 2,537,685 and 2,552,350; and polyisocyanates containing allophanate groups as described, for example, in GB-PS 994,890, BE-PS 761,626 and NL-OS 7,102,524. Suitable polyisocyanates also include polyisocyanates that contain uretdione groups. In one embodiment, asymmetric trimers such as those in U.S. Pat. No. 5,717,091 can be used. Isocyanate group-containing prepolymers and semi-prepolymers based on polyisocyanates can also be used as the polyisocyanate component. These prepolymers and semi-prepolymers generally have an isocyanate content ranging from about 0.5 to 30% by weight, preferably about 1 to 20% by weight, and are prepared in a known manner by the reaction of starting materials, e.g., isocyanate-reactive compounds such as polyols, at an NCO/OH equivalent number ratio of about 1.05:1 to 10:1, preferably about 1.1:1 to 3:1. The asymmetric polyaspartic ester mixtures of the invention are made by combining a suitable polyamine component with an ester component containing maleic acid/fumaric acid mixture containing a dimethyl-substituted maleic acid and a maleic acid/fumaric acid ester substituted with an alkyl group containing at least two carbon atoms in suitable amounts under conditions that favor the reaction of the reactants. The duration of the reaction varies. Reactions involving aliphatic diamines such as hexane-diamine and 2-methyl-1,5-pentanediamine can be fully completed within two weeks. Reactions involving cyclic diamines such as H 12 MDI and 4'-dimethyl H 12 MDI ordinarily take a few months, e.g., 2-3 months, depending on the polypropylene oxide amine used when the reaction has reached 97 or 98% completion. Since it can take as long as 52 to 100 weeks for the reaction to reach 100% completion, it is ordinarily not practical to wait for full completion. Specific duration times can be obtained by routine experimentation. The yields at which the polyaspartic esters are produced are generally at least at about 70, 80 and preferably about 100%. The method is ordinarily practiced without any appreciable amount of an organic solvent, e.g., generally less than about 10% preferably less than 5%, based on the total weight of the solution, and even more preferably no solvents. Generally, when difunctional amines are used, the asymmetric polyaspartic ester mixture includes, in addition to pure compounds, a polyaspartic ester having the formula: in which X is a polypropylene oxide group obtained by the removal of amino groups from a amine corresponding to the formula, X-(NH 2 ) n in which R 1 and R 2 each are the same or different and each is an alkyl group having at least two carbon atoms and n is two. When trifunctional amines are used, the composition includes, in addition to pure compounds, one or both of the following polyaspartic esters: in which X is a polypropylene oxide group obtained by the removal of amino groups from an amine corresponding to the formula X-(NH 2 ) n in which R 1 and R 2 each are the same or different and each is an alkyl group having at least two carbon atoms and n is three. The method provides previously unavailable advantages. Since the method avoids the crystallization that ordinarily forms when dimethyl-substituted esters or maleic or fumaric acid reacts with amines, the method avoids the formation of a commercially useless reaction-stopping crude mixture containing dimethyl fumarate crystals (and other compounds) typically formed by known methods. Also, since the method of the invention does not require the use of solvents, e.g., methanol, it produces polyaspartic ester mixtures in greater yields than solvent-based systems, and avoids the fire hazards typically associated with flammable solvents. Also, since the method does not use solvents such as methanol, the crosslink density and properties of the films are not adversely affected by the polyurethanes which form by the reaction of residual amounts of methanol would react with polyisocyanates to form urethanes. A polyurea coating composition can readily be formed by combining suitable amounts of (a) a polyisocyanate component and (b) an effective coating-forming amount of the asymmetric polyaspartic ester mixture. A coating is made from such a coating composition by (a) coating a substrate with a coating composition including effective coating-forming amounts of (i) a polyisocyanate component, and (ii) the asymmetric polyaspartic ester mixture. The polyisocyanate component and the asymmetric polyaspartic ester component are mixed in a ratio that is generally at least 0.9:1.1, preferably about 1:1 eq:eq, preferably from about 0.9:1.0 eq:eq to 1.5:1.0 and more preferably from about 0.9:1.0 eq:eq to 1.1:1 eq:eq. After the coating compositions have been applied to a suitable substrate, the compositions are hardened by curing at a suitable temperature, e.g., from about 30°C to 150°C. In one embodiment, the polyisocyanate component and the effective coating-forming amounts of the asymmetric polyaspartic ester mixture also reacts with a polyol. Suitable polyols include an suitable polyol, e.g., polyethers such as those in U.S. Pat. No. 5,126,170. The coating can be made on substrates such as cement, asphalt, metal, glass, and wood. The coatings are particularly useful in applications such as spray elastomer, heavy duty maintenance, product finishing, automotive, and flooring applications. The invention will now be described in the following illustrative examples. All references to percentages are by weight unless otherwise indicated. EXAMPLES EXAMPLE 1 A three-neck, round-bottom flask is fitted with stirrer, thermometer, nitrogen inlet, addition funnel and heater. 1000 g (0.5 eq.) polypropylene oxide diamine (Jeffamine D-2000 available from Huntsman Corporation) are added to the reactor at 40 °C. A 0.5 eq. of an 1:1 eq:eq mixture of dimethyl maleate and diethyl maleate is added through the addition funnel. The reaction is exothermic, so that cooling is applied to maintain a temperature below 80°C. After addition is complete, the reaction is heated at 60 to 80°C for an additional 10 hours. The resin is then stored under ambient laboratory conditions until the reaction is complete. No crystals form. EXAMPLE 2 The procedure of Example 1 is repeated except that a 0.65:35 equivalent mixture of dimethyl maleate and diethyl maleate is used. No crystals form. COMPARATIVE EXAMPLE A The procedure of Example 1 is repeated except that 1000 g (0.5 eq.) polypropylenoxide diamine (Jeffamine D-2000 available from Huntsman Corporation) is placed in the reactor at 40°C to melt the amine. 72.1 g (0.5 eq.) dimethyl maleate is added over a one hour period. The reaction is heated at 60°C for nine hours when needle-like crystals begin to grow. COMPARATIVE EXAMPLE B The procedure of Example 1 is repeated except that a 0.8:0.2 equivalent mixture of dimethyl maleate and diethyl maleate is used. Crystals form. Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for illustrative purposes and that variations can be made therein by those skilled in the art without departing from the scope of the invention.

Claims: A method for making a polyaspartic ester mixture comprising the sequential steps of: (a) forming an ester component including (i) a first ester component selected from the group consisting of esters of dimethyl-substituted maleic acids, esters of dimethyl-substituted fumaric acids, and (ii) a second ester component selected from the group consisting of esters of maleic acid esters substituted with alkyl groups having at least two carbon atoms and esters of fumaric esters substituted with alkyl groups having at least two carbon atoms; and (b) reacting the ester component with a propylene oxide amine component including an amine selected from the group consisting of propylene oxide difunctional amines and propylene oxide trifunctional amines; wherein the first ester component and the second ester component are used in amounts that are sufficient to prevent the formation of a reaction-stopping crude mixture containing dimethyl fumarate crystals. The method of Claim 1, wherein the first ester component and the second ester component are used in an equivalent number ratio that is from less than 7:3 to more than 0:10. The method of Claim 1, wherein the method is practiced without any appreciable amount of an organic solvent. The method of Claim 1, wherein difunctional amines are used and the asymmetric polyaspartic ester mixture includes a polyaspartic ester having the formula: wherein X is a polypropylene oxide group obtained by the removal of amino groups from a amine corresponding to the formula, X-(NH 2 ) n in which R 1 and R 2 each are the same or different and each is an alkyl group having at least two carbon atoms and n is two. The method of Claim 1, wherein trifunctional polypropylene oxide amines are used, and the polyaspartic ester mixture includes a polyaspartic ester component selected from the group consisting of esters having the following formulae: wherein X is a polypropylene oxide group obtained by the removal of amino groups from an amine corresponding to the formula X-(NH 2 ) n in which R 1 and R 2 each are the same or different and each is an alkyl group having at least two carbon atoms and n is three. An ester component used in a method of Claim 1. An asymmetric polyaspartic ester mixture comprising the reaction product of: (a) a pre-formed ester mixture including (i) a first ester component selected from the group consisting of esters of dimethyl-substituted maleic acids and esters of dimethyl-substituted fumaric acids, and (ii) a second ester component selected from the group consisting of esters of maleic acid esters substituted with alkyl groups having at least two carbon atoms and esters of fumaric esters substituted with alkyl groups having at least two carbon atoms; (b) a propylene oxide amine component including an amine selected from the group consisting of difunctional polypropylene oxide amines, trifunctional polypropylene oxide amines, and tetrafunctional polypropylene oxideamines; wherein the first ester component and the second ester component are used in amounts that are sufficient to prevent the formation of a reaction-stopping crude mixture containing dimethyl fumarate crystals. The asymmetric polyaspartic ester mixture of Claim 7, wherein the first ester component and the second ester component are used in an equivalent number ratio is from less than 7:3 to more than 0:10. The asymmetric polyaspartic ester mixture of Claim 7, wherein the propylene oxide amine component further includes tetrafunctional polypropylene oxide amines. The asymmetric polyaspartic ester mixture of Claim 7, wherein the method is practiced without any appreciable amount of an organic solvent The asymmetric polyaspartic ester mixture of Claim 7, wherein difunctional polypropylene oxide amines are used and the asymmetric polyaspartic ester mixture includes a polyaspartic ester having the formula: wherein X is a polypropylene oxide group obtained by the removal of amino groups from a amine corresponding to the formula, X-(NH 2 ) n in which R 1 and R 2 each are the same or different and each is an alkyl group having at least two carbon atoms and n is two. The asymmetric polyaspartic ester mixture of Claim 7, wherein polypropylene oxide trifunctional amines are used, and the polyaspartic ester mixture includes a polyaspartic ester component selected from the group consisting of esters having the following formulae: wherein X is a polypropylene oxide group obtained by the removal of amino groups from an amine corresponding to the formula X-(NH 2 ) n , wherein R 1 and R 2 each are the same or different and each is an alkyl group having at least two carbon atoms and n is three. A polyurea coating formed by the asymmetric polyaspartic ester mixture of Claim 7. A method for making a polyurea coating comprising: (A) coating a coating composition to a substrate, the coating composition comprising: (a) a polyisocyanate component; (b) an asymmetric polyaspartic ester mixture comprising the reaction product of: (1) a pre-formed ester mixture including (i) a first ester component selected from the group consisting of esters of dimethyl-substituted maleic acids and esters of dimethyl-substituted fumaric acids, and (ii) a second ester component selected from the group consisting of esters of maleic acid esters substituted with alkyl groups having at least two carbon atoms and esters of fumaric esters substituted with alkyl groups having at least two carbon atoms; (2) a propylene oxide amine component selected from the group consisting of difunctional amines, trifunctional amines, and tetrafunctional amines; wherein the first ester component and the second ester component are used in amounts that are sufficient to prevent the formation of a reaction-stopping crude mixture containing dimethyl fumarate crystals. B) curing the composition.

Inventors: ROESLER RICHARD R, SQUILLER EDWARD P, ROESLER, RICHARD R., SQUILLER, EDWARD P.
Applicants: BAYER AG, BAYER CORPORATION
Application year: 2000
Main classification: C08G18
IPC codes: C09D175, C08G18