GB2147910A - Cross-linked polyurethane emulsion and process therefor - Google Patents

Abstract

Stable crosslinked polyurethane aqueous emulsions are prepared by (1) forming in an inert solvent a branched prepolymer derived from (a) an amine containing alcohol having two or three -OH groups, (b) a hydrophobic polymeric diol or triol, (c) a mixture of an aliphatic and an aromatic polyisocyanate and (d) optionally a urethane-forming catalyst; (2) admixing the reaction product from (1) supra with an amine-containing surfactant; (3) adding the admixture from (2) to water containing a fixed acid with vigorous stirring to form a crosslinked polyurethane emulsion; and (4) removing the inert solvent and a portion of the water so that the emulsion contains about 50 to 75% solids. These emulsions give, on drying, corrosion inhibiting coatings having abrasion resistance.

Description

SPECIFICATION Crosslinked polyurethane emulsion and process therefor 1. Field of the Invention This invention relates to crosslinked, polyurethane emulsions and their production and use.

More particularly, this invention relates to such emulsions of high solids content which can be formulated into products usable in the coatings field.

2. Description of the Prior Art U.S. 3,948,837 teaches making an all aliphatic-NCO capped prepolymer, partially chainextending same with small molecules like ethylene glycol or ethylene diamine, chemically bonding an anionic emulsifier to the partially extended prepolymer in an aqueous bath which completes the chain-extension step using the H20/-NCO reaction.

U.S. 4,183,836 teaches making an anionic urethane dispersion from an aliphatic isocyanate wherein the emulsifier is chemically bound to the polymer.

U.S. 3,388,087 teaches an aqueous dispersion of a quarternized polyurethane composition prepared from a linear prepolymer.

U.S. 4,192,937 teaches a process for the preparation of substantially linear isocyanate polyaddition products having side-chain hydroxyl groups by the reaction of reactive systems which contain free isocyanate groups and oxazolidine groups with water.

U.S. 3,988,278 teaches a latex formed from a prepolymer formed by reacting an aromatic polyisocyanate with a polyol and, thereafter, modifying the thus formed prepolymer with hydrophilic, monofunctional reactants, e.g., diethylethanolamine on chains pendant from the polymer backbone. The prepolymers are then chain-extended in water to create latices of selfemulsified solid water insoluble polyurethane particles.

U.S. 3,479,310 teaches a polyurethane ester which is prepared by dispersing in water a polyurethane containing from 0.02 to about 1 % by weight salt groups. The polyurethane can be dispersed without the aid of additional emulsifying agent.

U.S. 4,331,717 teaches making a linear isocyanate end capped polyurethane oligomer containing internal tertiary amine groups, chain extending with hydrazine or diamines to high molecular weight and then dispersing this linear, -NCO free polyurethane-urea polymer in water containing organic acids which lose their acid character on heating. Apparently the linear structure and resultant hydrophilic characteristics of these polymers (some of which are soluble in water rather than dispersible) obviate their usefulness in the presence of fixed acids such as phosphoric acid, thus requiring the use of special acidic structures that decompose on heating.

This invention provides a hydrophobic, crosslinked, polyurethane aqueous emulsion, having a high (50-75%) solids content. This emulsion gives on drying a corrosion inhibiting coating having abrasion resistance.

Briefly, this invention involves forming a branched prepolymer with amine groups in the backbone thereof and having terminal isocyanate groups. These prepolymers are then admixed with an amine-containing surfactant which on neutralization becomes a cationic emuslifier and then emulsified, chain-extended and crosslinked by adding the prepolymers to water containing a fixed acid.

In more detail the process of the invention for forming a cross-linked polyurethane aqueous emulsion comprises (1) forming in an inert solvent a branched prepolymer derived from (a) an amine containing alcohol having two or three -OH groups, (b) a hydrophobic polymeric diol or triol, (c) a mixture of an aliphatic and an aromatic polyisocyanate and (d) optionally a urethaneforming catalyst; (2) admixing the reaction product from (1) supra with an amine-containing surfactant; (3) adding the admixture from (2) to water containing a fixed acid with vigorous stirring to form a crosslinked polyurethane emulsion; and (4) removing the inert solvent and a portion of the water so that the emulsion contains about 50 to 75% solids.

The average molecular weight of the prepolymer used can be from about 1,500 to about 8,000. These prepolymers and thus the resulting polymers have been provided with at least one internal salt-forming or other hydrophilic groups. The salt-forming or hydrophilic internal group is derived from di- or tri-functional reactants, preferably from tertiary amine-containing compounds having two or three active hydrogen-bearing groups, e.g. triethanolamine, N-methyldiethanolamine and the like. The use of fixed acids such as phosphoric acid or thiosalicylic acid or mixtures thereof to produce a counter ion of the amine-containing prepolymer results in a corrosion inhibiting coating from the emulsion.As used herein, the term "fixed" acid refers to acids that are stable to the conditions of emulsification, compound and curing of the polymer and coatings obtained therefrom without losing their acid character in the range 20-170"C, and which impart corrosion protection to substrates on which the coatings are applied. Mineral acids are the preferred fixed acids, but other acids such as thiosalicylic acid are also operable.

In practising the preferred embodiment of the invention the prepolymer is prepared by charging three moles of an aromatic isocyanate, e.g., toluene diisocynate (TDI), three moles of an aliphatic isocyanate e.g., isophorone diisocyanate (lPDl), three moles of a hydrophobic polyol, e.g., polypropylene glycol (PPG), and one mole of an amine containing alcohol having 3 OH groups, e.g., triethanolamine (TEA), to a stirred container and let the reaction continue for about 1 to 3 hours. During the reaction an exotherm up to about 45"C occurs. To force the reaction to completion a urethane forming catalyst, e.g. dibutyltindilaurate (DBTDL) is usually added to the system. Due to the reaction rates, the following reactions are believed to occur sequentially: (1) Triethanolamine reacts with the aromatic isocyanate.

(2) The polypropylene glycol reacts with a substantial portion of the aromatic isocyanate.

(3) The aliphatic isocyanate reacts with the remaining -OH groups of the polypropylene glycol.

The reactions are carried out in an inert solvent such as methylethyl ketone, acetone or dioxane.

As a result of the reaction rates a prepolymer having the following idealized structure is obtained:

For certain applications where somewhat different properties are required, an alternate route for preparing the amine-containing prepolymers of this invention may be used. That is, the prepolymer can be prepared from a tertiary amine-containing alcohol having only two hydroxyl groups, or from blends (mixtures) of such dihydroxy tertiary amines with various trihydroxy compounds including trihydroxy tertiary amines, e.g., triethanolamine as described above.We will exemplify this alternate route as follows: To the reactor is charged two moles of an aromatic isocyanate, e.g., a polymethylene polyphenylisocyanate having an average -NCO functionality of 2.3 and commonly referred to as polymeric MDI (PM Dl), 2.6 moles of an aliphatic isocyanate, e.g., isophorone diisocyanate (IPDI), 2.6 moles of a hydrophobic polyol, e.g., polypropylene glycol (PPG), and one mole of an amine-containing alcohol having two -OH groups, e.g.. N-methyldiethanoamine (NMDEA), to a stirred container and let the reaction continue for 1 to 3 hours. During the reaction an exotherm up to about 45"C occurs. To force the reaction to completion, a urethane-forming catalyst, e.g., dibutyltindilaurate (DBTDL) is usually added to the system.Due to reaction rates. the sequence of reactions outlined above will take place, and inert solvents similar to those described above are used to facilitate the preparation of the prepolymer. As a result of the differential reaction rates, a prepolymer having the following idealized structure is obtained:

For attainment of best physical properties (tensile strength, resiliency, abrasion resistance, solvent resistance, heat distortion resistance, outdoor weathering resistance and the like), it is critical that the prepolymers of the present invention be branched rather than linear in structure.

Branched prepolymers, on subsequent chain extension and crosslinking during emulsification (and subsequnt drying or "curing" of applied coatings derived therefrom) will lead to the formation of crosslinked, elastomeric polyurethane products having a balance of the desirable physical properties outlined above. In the first example, above, the branching units were furnished by the trihydroxy tertiary amine (TEA), whereas in the alternate route prepolymer the branching units were furnished by the polyfunctional polymeric MDI (PMDI). When NMDEA is used together with TDI or pure (non-polymeric) MDI, branching units may be supplied in various ways such as by incorporating into the starting composition small amounts of other polyfunctional reactive comonomers such as TEA, glycerol, trimethylolpropane, pentaerythritol, toluene2,4,6-triisocyanate, polymeric MDI and the like.

The thus formed prepolymers are then admixed with an amine-containing surfactant, e.g., N,N-dimethyloctadecylamine under atmospheric conditions. The amine-containing surfactants used herein on neutralization become cationic emulsifiers. The thus emulsifier-containing prepolymers are then added with stirring to an equal weight of water containing a fixed acid, e.g., phosphoric acid in an amount ranging from stoichiometric up to a 10% excess to produce a counter ion of the amine-containing prepolymer and the tertiary nitrogen in the emulsifier. The reaction is continued for 1 to 1 2 hours with vigorous foaming (CO2 evolution) taking place. The formed hydrophobic, crosslinked, polyurethane aqueous emulsions contain roughly about 25% solids.The emulsions are concentrated to approximately 50-57% solids by passing them through a thin film evaporator at temperatures ranging from about 20 to 50"C or a centrifuge or an ultrafiltration membrane apparatus.

Stoichiometry of Prepolymer Step Reactants are chosen by first selecting a central unit comprising an amine containing alcohol having two or three -OH groups.

Stoichiometry calls for an average of one amine containing alcohol residue per molecule of prepolymer. This is the key ingredient and is central in the molecular structure as shown in the schematics set out hereinbefore. This amine reactant can have three -OH groups or only two.

As stated supra, it is also possible and sometimes expedient for achieving somewhat different properties to use mixtures (blends) of these and other dihydroxy and trihydroxy compounds.

From the central amine containing alcohol moiety one builds the backbone of the prepolymer structure through selection of an appropriate polymeric hydrophobic polyol. This hydrophobic polyol is tied to the central amino moiety through bis-urethane linkages by use of diisocyanate reactants and is further end-capped with a diisocyanate through a monurethane linkage leaving a free -NCO group on each terminus of the prepolymer molecule. It is important to use a mixture of an aliphatic and an aromatic isocyanate and, due to the difference in reactivity ratios of these reactants, the aliphatic moieties will predominate at the terminus locations because they react more slowly than aromatic isocyanates with -OH groups.The choice and positioning and ratio of hydrophobic polyol moieties in the prepolymer backbone is critical only in that it leads to a final average prepolymer molecular weight from about 1 500 to about 8000. The choice, positioning and ratio of diisocyanate moities is critical only in that the object is achieved to prepare stable, crosslinked polyurethane aqueous emulsions having a high (50-75%) solids content when the prepolymer is dispersed in water under the emulsification conditions specified in the invention.

Further clarification of the stoichiometry of the prepolymer formation step may be possible by paraphrasing the idealized schematics supra with the showing of the terminus -NCO functions as follows:

Obviously mixtures of TEA and NMDEA in the same prepolymer would lead to an even more complex structure than the ones exemplified herein.

The polyisocyanates used to form the emulsions of the instant invention are mixtures of aromatic and aliphatic isocyanates. Operable aromatic isocyanates include, but are not limited to, tolylene diisocyanate, triphenylmethane-4,4',4"-triisocyanate, benzene-1,3, 5-triisocyanate, toluene-2,4,6-triisocyanate, diphenyl-2,4,4'-tri-isocyanate, xylene diisocyanate, chlorophenylene diisocyanate, diphenylmethane-4,4'-diisocyanate, naphathalene-1 , 5-diisocyanate, xylene-alpha, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, 3,3'-dimethoxy-4,4'-bi-phenylene diisocyanate, 2,2', 5, 5'-tetramethyl-4,4'-bi-phenylene diisocyanate, diisocyanate, 4,4'-methylene bis(phenyl-isocyanate), 4,4'-sulfonyl bis(phenylisocyanate) and 4,4'-methylene diorthotolylisocyanate.

As used herein, the term aliphatic polyisocyanate encompasses all polyisocyanates except those wherein the -NCO groups are attached directly to an aromatic ring. Thus the term includes cycloaliphatic diisocyanates such as isophrone diisocyanate (IPDI) and it also includes structures such as

and the like. Operable aliphatic isocyanates include, but are not limited to, ethylene diisocyanate, trimethylene diisocyanate, diicyclohexyl methane-4,4'-diisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate and 2,2,4-trimethyl-1 6-hexane diisocyanate.

The aromatic and aliphatic diisocyanates are added in approximately equal molar amounts in the preferred embodiment, but may be varied from this ratio to improve properties or achieve lower costs, taking precautions to maintain a sufficient concentration of aliphatic isocyanate groups required to permit the formation of stable emulsions.

Tertiary amine alcohols operable herein to form the prepolymer include, but are not limited to, N-alkylated diethanolamines, N-arylated diethanolamines and N-alkarylated diethanolamines of the formula:

N-alkylated dipropanolamines, N-arylated dipropanolamines and N-alkarylated dipropanolamines of the formula:

wherein said formulas R = alkyl having 1-18 carbon atoms or alkyl substituted or unsubstituted phenyl or benzyl; tripfopanolamine, i.e., N-(CH2CH2CH2-OH)3; N,N-bis (2-hydroxyethyl)-piperazine, i.e.,

1 -[N, N-bis(2-hydroxyethyl)amino]-2-propanol, i.e.,

and the like.

The products of this invention are intended for use in making corrosion inhibiting coatings having abrasion resistance. Thus it is critical in achieving these objectives that the coatings be hydrophobic and relatively impermeable to moisture. Because the major backbone elements of the prepolymer structure are comprised of high molecular weight polymeric polyols, these polyols are chosen to be hydrophobic. By virtue of ready availability and low cost the polyalkylene either diols and triols are the preferred polymeric polyols of this invention. It is well known in the art, however, that polyethyelene glycols are water soluble and hydrophilic, and are therefore to be avoided for the purposes of this invention, as are copolymers of ethylene oxide with higher alkylene oxides wherein the ethylene oxide content is greater than about 40 mole percent.Thus, it is important that the admixtures of polymeric polyols used to make the prepolymer be sufficient hydrophobic as to be immiscible, i.e., forms two layers, when mixed with an equal volume of water. Hydrophobic polyols operable herein to form the prepolymer include, but are not limited to, polyalkylene-ether diols and triols, especially polypropylene glycols, polybutylene glycols and polytetramethylene ether glycols of MW. 400-4,000; poly(caprolactone) diols and triols of M.W. 500-3,000; hydroxy-terminated poly(ethylene adipate) of M.W. 500-3,000; hydroxy-terminated poly(1 ,4-butylene adipate) of MW.

500-3,000; poly(butadiene diol of M.W. 500-3,000 and the like.

Aside from the amine containing surfactant N,N-dimethyl-octadecyl amine there are various ethoxylated amines, commercially available under the tradename "Ethomeen" from Armak Co.

These ethoxylated amines are prepared by addition of ethylene oxide to a primary amine, thus:

These ethoxylated amines contain a percent minimum of tertiary amine of 96%.

The following examples will explain, but expressly not limit, the instant invention. Unless otherwise noted, all parts and percentages are by weight.

The tensile and elongation properties of films cast from the emulsion were measured in accord with the procedures set out in ASTM-D638.

Example I Prepolymer Preparation To a stirred, 3-necked, round bottom, 50-liter flash are charged 11 , 1 00 g methylethyl ketone, 1,448.5 g toluene diisocyanate (80-20 by weight mixture of 2,4-/2,6-isomers), 1,848.1 g of isophorone diisocyanate, 413.1 g of triethanolamine and 8,533.1 g of polypropylene glycol having a molecular weight of about 1,025. The reactants are stirred for about 3 hours at which time the exotherm (up to 40"C) begins to subside. 11.1 g of dibutyltin-dilaurate are added to the reaction to force the reaction to completion. The final NCO content was 0.24 meq/g (theoretical NCO is 0.36 meq/g).

Example 2 Emulsion Preparation Into a 30-gallon (1141) open drum equipped with 4 equally spaced vertical baffles are charged 23,292 g demineralized water containing 321.4 g of phosphoric acid (85%).

Into another container are combined 23,292 g of the reaction mixture from Example 1 along with 132.7 g of a emulsifier, i.e, N,N-dimethyloctadecylamine, commercially available from Armak under the Tradename "Armeen DM-1 8D". The admixture was stirred for about 5 minutes to obtain a homogeneous admixture. The thus emulsifier-containing prepolymer solution was then charged to the phosphoric acid containing water with high speed stirring and the reaction was continued for about 1 hour. The emulsifier on neutralization by the opposing counter ion (e.g., phosphoric acid) becomes a cationic surfactant. During the reaction copious foaming (CO2 evolution) took place which subsided considerably after about one hour. The resultant emulsion was about 25% solids.

Example 3 Emulsion Concentration The resultant emulsion from Example 2 was passed 3 times through a Kontro thin film evaporator, Model No. R-55-3H-1, under the following operational conditions: Impeller speed-650 rpm; Clearance-middle setting; Throughout-first pass: 120-140 g/min.; second pass: 160-180 g/min.; third pass: 200-300 g/min.; Water jacket temperature-45-50"C; Condenser coolant-5 C; Vacuum-less than 20 in. Hg. (98 kPa).

The resultant emulsion was concentrated to a 60% non-volatiles content.

The following example shows the stepwise preparation of the prepolymer: Example 4 To a 5-liter round bottom flask equipped with stirrer and thermometer was charged 1 850 g of methylethyl ketone, 241.4 g of touene diisocyanate (80-20 by weight mixture of 2,4-/2,6isomers) and 68.9 g of triethanolamine. The reaction was continued for 3 hours with stirring.

The final product was a light tan color and had an NCO content of 0.659 meq/g (theoretically, NCO equals 0.655 meq/g). The exotherm reached 33.2"C.

1,422 g of polypropylene glycol having a molecular weight of 1 ,025 were added to the reaction and the reaction was continued with stirring for 1 hour. 1.85 g of dibutyltindilaurate was added to the reaction and the reaction was continued for an additional 1 + hours. 308 g of isophorone diisocyanate were then added to the reaction and the reaction was continued for an additional 1 + hours with stirring. The final NCO content was 0.26 meq/g.

1 ,000 g of the thus formed prepolymer solution were mixed with 3.8 9 of a cationic emulsifier, i.e., N,N-dimethyloctadecyl amine and then charged to a 1 gallon (3.81) blender containing 1 ,000 g of deionized water and 13.05 g of 85% phosphoric acid. The admixture was blended for 7 minutes with high speed agitation. The thus formed emulsion was degassed and concentrated in a thin film evaporator to a solids content of 62.8%. The emulsion was cast into rectangular molds and dried overnight resulting in coherent self-supporting films of 15-20 mil-thickness (0.38-0.51 mm). The thus formed films had a tensile strength of 386 psi. (2.7 MPa) and a percent elongation of 798%.

Various emulsions were made up in accord with the procedure and reactants of Example 4 except that the emulsifier was varied. The results of the cast film from the emulsions formed are shown in TABLE l: TABLE I PREPARATION CONDITIONS AND PHYSICAL PROPERTIES OF CATIONIC POLYURETHANE Emulsification Physical Properties Example Emulsifier Wt.% Emulsifier No. Based on Solids Tensile Prepolymer Conc. Strength Elong.

psi ~~~~~~~~~~~ ~~~~~~~~~~~~~~~ ~~~~~~ psi (MPa) ~~~~~~ 5 (a) 1.5 61.8 341 (2.35) 822 6 (b) 1.5 62.1 317 (2.19) 804 7 (c) 1.5 65.4 277 (1.91) 820 8 (d) 1.5 64.2 33 (2.32) 795 9 (e) 1.5 63.8 2n (1.99) 799 (a) Ethomeen S/12 = bis(2-hydroxyethyl)soyamine, sp.gr.

0.911 at 250C; M.Wt.=352 containing 2 moles ethylene oxide, commercially available from Armak (b) Ethomeen S/15 = polyoxyethylene (5) soyamine, sp.gr.

0.951 at 250C; M.Wt.=484 containing 5 moles ethylene oxide, commercially available from Armak (c) Ethomeen 5/20 = polyoxyethylene (10) soyamine, sp.gr.

1.020 at 250C; M.Wt.=704 containing 10 moles ethylene oxide, commercially available from Armak (d) Ethomeen S/25 = polyoxyethylene (15) soyamine, sp.gr. 1.040 at 25"C; M.Wt. = 924 containing 1 5 moles ethylene oxide, commercially available from Armak (e) Ethomeen 18/60 = polyoxyethylene (50) soyamine, sp.gr. 1.115 at 60"C; M.Wt. = 2469 containing 50 moles ethylene oxide, commercially available from Armak The hydrophobic polyurethane emulsions of the instant invention can be used per se or may be formulated with conventional additives to form abrasion-resistant, anticorrosive coatings.

Such coatings can be used on various substrates including metals, wood, glass, concrete, plastic, fabric, fiber, paper and the like including surfaces previously coated or painted with other classes of coating materials. One field in which the coatings are especially useful is in the automotive industry. Therein the formulated aqueous emulsion can be used as underbody coatings, chip-resistant coatings, rocker panel coatings, wheel weil coatings and for washresistant replacements for cavity conservation waxes. The products of the present invention are durable, wear-resistant, corrosion protective and do not contain massive amounts of solvent that may contribute to air pollution during application in the factory environment.

In formulating the coatings, conventional additives such as thinners, thixotropic agents, antioxidants, antiblistering agents, pigments or dyes, anti-UV agents, anticorrosion additives and possibly extending "fillers" (inorganic powders, oils, resins), reinforcing agents (fibers, platelets, crosslinkers, latexes), thickeners, plasticizers and the like can be added to the emulsion.

The additives are each added in amounts ranging from about 0.01 to 25% by weight of the aqueous emulsion. The additives can be added prior to emulsification but, preferably, are added after the emulsion is formed by adding each additive subsequently in an appropriate order with stirring for about 1 to 10 minutes. In many instances it is preferred to admix the additives with small amounts of water to form a paste or master batch prior to adding it to the system.

The coatings obtained can be air-dried to evaporate the water but, preferably, are oven-dried at temperatures ranging from about 75-150"C for periods ranging from about 5-60 minutes.

The coatings are tested for abrasion resistance in both the dry and wet state on a Shot Blaster manufactured by Auer, Mannheim, West Germany (Model Strahlanlage-ST800A). The dry or wet coatings to be tested are sometimes cured at 150"C for 30 minutes prior to abrasion testing. The coatings to be tested wet are covered with a +11(6.4 mm) thick absorbent cotton which has been soaked in demineralized water. The cotton covered test panel is put in a polyethylene bag and sealed and then put in a second polyethylene bag and sealed. The panel is then placed in an oven at 70"C for one week. The cotton is then removed from the panel and both bags resealed and the panel is put in a cold chamber maintained at - 20"C for 2 hours.

The sample is removed from the bags and allowed to come to room temperature then conditioned at 23"C, 50% RH for 2 to 4 hours before abrasion testing.

The Shot Blaster abrasion test is the same for both the dry and wet panels. The test consists of shot blasting the urethane polymer coated panel with a crushed spherical cast steel shot type GP-1 4 Wheelabrator-Allevard at an air pressure of 70 psi (1.1 7 MPa) at an angle of 90, until full penetration of the coating to expose bare steel is visibly noted. For dry samples a blasting period in excess of 300 seconds is considered acceptable. For wet samples a blasting period greater than 200 seconds is acceptable.

The following example shows the improvement in physical characteristics of a cured coating by the use of additives such as UV stabilizers and antioxidants after accelerated weathering: Example 10 Using the reactants and procedures set out in Examples 1-3, a concentrated emulsion having a non-volatile content of 52.5% was prepared. To 800 g of said concentrated emulsion was added 1.05 g Tinuvin-328, 1.5 g of Tinuvin-770 (both UV stabilizers), commercially available from Ciba-Geigy, and 0.5 g of Santonox-R (an antioxidant), commercially available from Monsanto Chemical Co., with stirring for 5 minutes.The additive-containing emulsion was then drawn down on a "3 X 6" (7.6cm X 15.2cm) steel panel electrocoated with a composition, commercially available from PPG under the tradename "ED-3076". The coating was dried over night to a thickness in the range 0.007"-0.015" (0.18-0.38 mm). The dried coated panel was cured in a forced air oven for 30 minutes at 150"C and then scribed in an X design (2" (5.1cm) each leg).

The cured panel was then subjected to a simulated weathering test in a QUV accelerated weathering tester manufactured by Q-Panel Co. The operating conditions in the tester are alternating 8-hour exposure to fluorescent UV light at a temperature of 65"C followed by 4 hours of condensing humidity without light at 40"C for a total exposure of about 1,000 hours.

Thus, the panels were subjected to 693 hours of UV exposure and 299 hours of condensing humidity exposure.

Following testing, the following properties were assessed qualitatively: Color-visual observation rated 0-4 with 0 no change and 4 worst; Orange peel-wrinkle effect from weathering rated 0-3 with 0 no effect and 3 most wrinkled; Tackiness--stickness of coating rate 0-2 with 0 none, 1 slight and 2 some: Rust-appearance of rust on the steel under the scribed portion of the electrocoat rated 0-2 with 0 none, 1 slight and 2 some; Adhesion ability to have pull coating from electrocoat rated 0-2 with 0 good adhesion and 2 poor adhesion; Tear-ability to tear the coating till moving same -in the adhesion test rated 0-1 with 0 no tear and 1 some tear.

A control run using the same emulsion as a coating which was subjected to the same weathering test without the addition of any UV stabilizers or antioxidants was also carried out.

The results are shown in TABLE ll: TABLE II Example Stabilized Color Orange Tackiness Rust Adhesion Tear No. Peel (a) Yes O 0 1 0 0 0 (b) No (control) 2 2 0 0 0 0 As can be seen from the Table, the addition of UV stabilizers and antioxidants improves the color and smoothness of the weathered coating.

Example 11 In this example an emulsion using the reactants and prepared by the procedures as set out in Examples 1-3 was employed. The emulsion had a non-volatile content of 60%. 100 g of this emulsion was charged to a Waring blender and 0.61 g of 8-hydroxy quinoline (1% by weight) was added. The mixture was blended for 3 minutes. The thus blended mixture was coated onto a non-coated cold rolled steel panel, and the panel was dried overnight resulting in a coating having a thickness of 0.03155" (0.8mm). The thus coated panel was cured at 150"C for 30 minutes in an air oven. The cured panel was then subjected to a simulated weathering test in a QUV accelerated weathering tester manufactured by Q-Panel Co.The operating conditions in the tester for these panels were 2 hours exposure to fluorescent UV light at a temperature of 65"C followed by 8 hours of condensing humidity without light at 40"C for a total exposure of about 1,000 hours. The resultant tested coating was light brown to tan in color, had no blisters and there was no rust under the coating.

In a controlled run wherein no 8-hydroxy quinoline was added to the composition, the tested coating was tan to light brown in color but had light rust forming under the coating.

Example 11 was repeated using 2% by weight 8-hydroxy quinoline with the resulting coating being light tan with no blisters and no rust under the coating.

One problem encountered when using an aqueous emulsion as a coating is blistering. This is due to the water being trapped beneath the cured skin layer of the composition during curing.

To overcome this problem, anti blistering or antiskinning agents are added to the composition.

These materials slow down the skin cure thereby allowing the water to escape. It has been found that anti blistering agents such as ethylene glycol, propylene glycol, 1,4-butanediol and the like are operable herein. The following examples show the increase in film thickness of the cured coatings obtainable without blistering by use of antiblistering agents.

Example 12 Using an emulsion prepared by the procedure and reactant set out in Examples 1-3 (64.4% non-volatiles), varying amounts of antiblistering agents were admixed with the emulsion with high speed agitation for 5 minutes. Each emulsion was separately coated on 1 x 3" (2.5 X 7.5cm) steel panels electrocoated with a composition commercially available from PPG under the tradename "ED-3076". The coatings were dried for 20 minutes at 95"C and then cured in a forced air oven for 30 minutes at 120"C. The results are set forth in TABLE Ill:: Antiblistering Agent Antiblistering Agent Film Thickness Content Content % mils (mm) None (control) 0 20 (0.51) Ethylene glycol 5 41 (1.04) Ethylene glycol 7.5 44 (1.12) Propylene glycol 5 39 (0.99) 1,4-Butanediol 7.5 40 (1.02) As can be seen from TABLE III, the use of antiblistering agents allows for a 100% increase in film thickness before blisters are obtained.

Example 13 In this example an emulsion using the reactants and prepared by the procedures set out in Examples 1-3 was employed. The emulsion had a non-volatile content of 64.4%. To 100 g of this emulsion was added 5 g of 1,2-propylene glycol and the emulsion was agitated for 5 minutes. This emulsion will hereinafter be referred to as Emulsion A.

To another 100 parts of the emulsion was added 10 g of 1,2-propylene glycol with stirring for 5 minutes. This emulsion will hereinafter be referred to as Emulsion B.

Emulsions A and B were each separately coated on 4 X 1 2" (10.2 X 30.5cm) steel panels electrocoated with a composition, commercially available from PPG under the tradename "ED-3076". The coatings were dried overnight to a thickness of approximately 0.015" (0.38mm) and then cured in a forced air oven at 150"C for 30 minutes. The coatings were then tested for abrasion resistance in the wet and dry state in accord with the procedure set out supra for the shot blaster test.The results are set forth in TABLE IV: TABLE IV Coating Dry Blasting Wet Blasting Thickness Period Period Sample inches (mm) seconds seconds Emulsion A 0.0155 (0.39) 1053 382 Emulsion B 0.0146 (0.37) 942 367 Example 14 The emulsion of Example 3 was coated on a 4" x 12" (10.2 X 30.5cm) steel panel electrocoated with a composition, commercially available from PPG under the tradename "ED-3076". Coating was dried overnight to a thickness of 0.015" (0.38mm). The dried, coated panel was cured in a forced air oven for 30 minutes at 150"C.

The cured panel was then subjected to a dry gravelometer abrasion test in accord with SAE J-400 with 5 pints (2.81) of gravel. The thus abraded panel was then placed in a salt fog chamber (a Singleton SCCH Corrosion Test Cabinet, manufactured by the Singleton Corp., Cleveland, Ohio). The Salt Spray (Fog) Test was run in accord with the procedure of ASTM B117-73. The cabinet contained a salt solution of 5 parts by weight NaCI in 95 parts distilled water and was maintained in the exposure zone at a temperature in the range 33.3-36.1 C.

The exposure time in the cabinet was 48 hours. On examination of the panel for rust spots, none were found.

Under the same conditions except that the thickness of the cured coating was 0.012" (0.30 mm), an occasional rust spot was seen indicating that the electrocoat coating had been damaged exposing bare metal. Standard commercial underbody coating compositions based on mineral filled polyvinylchloride plastisols required a coating thickness of 0.040" (10.2 mm) in order to prevent similar damage of the electrocoat coating leading to the onset of corrosion.

Example 15 Preparation of Concentrated Emulsion To a 200-gallon (7.571) jacketed reactor equipped with an agitator were charged 388.5 pounds (176.2 kg) of methyl ethyl ketone, 50.7 pounds (23.0 kg) of toluene diisocyanate (80-20 by weight mixture of 2,4-/2,6-isomers), 64.7 pounds (29.3 kg) of isophorone diisocyanate, 14.5 pounds (6.6 kg) of triethanolamine and 298.7 pounds (135.5 kg) of polypropylene glycol having a molecular weight of about 1025.The reactants were stirred for about 3 hours at which time the exotherm (up to 40"C) begins to subside. 0.4 pounds (0.2 kg) of dibutyltin dilaurate was added to the reaction and the reaction was continued for an additional 5 hours at temperatures ranging up to 110"F (43"C). The final NCO content was below 0.3 meq/g. 4.6 pounds (2.1 kg) of a cationic emulsifier, i.e. N,N-dimethyloctadecylamine, commercially available from Armak under the tradename "Armeen DM-1 8D" was added.

Stirring was continued for 1 5 minutes and then the content of the reactor was discharged into two 55-gallon (2081) drums for temporary storage. The reactor was rinsed with methyl ethyl ketone and the rinse was discarded.

To the reactor was then charged 81 5 pounds (370 kg) of demineralized water and 11.5 pounds (5.2 kg) of phosphoric acid with high speed agitation over a 5-minute period. With the high speed agitator running the contents of the two 55-gallon (2081) drums were recharged to the reactor and stirring was continued for 1 hour. During the reaction foaming (CO2 evolution) took place which subsided considerably after about 1 hour. The resultant emulsion was about 26% solids.

824 pounds (374 kg) of the resultant emulsion were concentrated by removal of all the methyl ethyl ketone and a portion of the water in 2 passes through a Pfaudler heat jacketed Wiped Film Evaporator (WFE). In the first pass at an operating pressure in the range 97-103 mm.Hg., a jacket temperature of 1 55'F (68"C) and a feed rate of about 134.4 Ibs/hr. (61.0 kg/hr), the concentration was raised to about 55.7% solids. The second pass was accomplished with the feed prewarmed to 1 25'F (52"C) and a WFE jacket water temperature of 176"F (80"C). The operating pressure was 87 mm.Hg. at a feed rate of 231 Ibs/hr (105 kg/hr). The concentrate from this second pass weighed 444 pounds (201 kg) and the percent solids was 68%.

The following examples show the criticality of using an emulsion of a high solids content in order to obtain a sprayable coating which is adequately abrasion resistant.

Example 16 Portions of the emulsion from Example 1 5 were diluted with demineralized water to obtain emulsions having the following solids content: EMULSION WEIGHT PERCENT SOLIDS A 64.1 B 60.9 C 35.7 D 22.9 The emulsions were then compounded as indicated followed by additon (with good stirring) of a thixotropic agent. The amount of thixotropic agent used in each case was the minimum amount required to prevent the composition from sagging or dripping when held suspended from the blade of a vertically positioned spatula, i.e., the amount required to achieve true thioxtropic behavior.

To 262.1 g of Emulsion A was added 0.52 g of a thixotropic agent, i.e., hydroxyethyl cellulose, commercially available from Hercules under the tradename "Natrnsol". To 217.2 g of Emulsion B was added 1.3 g of Natrosol. To 423.4 g of Emulsion C were added 8.5 g of Natrosol, and to 588.5 g of Emulsion D were added 1 7.2 g of Natrosol. After said additions all emulsions were thioxtropic and Emulsion A had a viscosity of 77.6 poise, Emulsion B had a viscosity of 33.1 poise, Emulsion C had a viscosity of 5.2 poise and Emulsion D had a viscosity of 4.0 poise. The viscosity measurements were performed on a Contraves rheomat 11 5 at 20"C and 350 rpm using measuring system 114.

An attempt to make the diluted emulsion thixotropic with a different thixotropic agent, i.e., a polyacrylamide commercially available from Dow Chemical under the tradename 'Separan" AP273 failed as the amount of Sepran to make the 35.7% solids emulsion thixotropic caused the emulsion to gel.

Example 17 The thioxtropic Emulsions A-D were coated on 3 inch X 6 inch (7.6 X 1 5.2 cm) steel panels electrocoated with a composition, commerically available from PPG Industries, Inc. Under the tradename "ED-3076". The panels were than all dried over night and in addition half of the panels containing each of the emulsions were additionally dried at 150"C for 30 minutes.

All the panels were then subjected to the shot blaster abrasion test in both the dry and wet state as set out herein before. The results are shown in Table V: TABLE V Abrasion Resistance (a) Dry Wet Panel Coating Ti(b) Coating Ti(b) Coating Curing Thickness (seconds) Thickness (seconds) (in.) (mm) Actual Normalized(c) (in.) (mm) Actual Normalized(c) Emulsion A Air 0.0132 (0.34) 502.9 419.1 0.0110 (0.28) 256.2 256.2 64.1% solids Air+150 C/30 min.0.0127 (0.32) 516.7 447.5 0.0110 (0.28) 199.8 0.2 parts Natrosol Emulsion B Air 0.0153 (0.39) 448.7 322.6 0.0118 (0.30) 257.9 240.4 60.9% solids Air+150 C/30 min.0.0151 (0.38) 742.4 540.8 0.0136 (0.35) 635.7 514.1 0.64 parts Natrosol Emulsion C Air 0.0160 (0.41) 457.1 314.3 0.0147 (0.37) 104.8 78.4 35.7% solids Air+150 C/30 min.0.0150 (0.38) 1056.7 775.1 0.0202 (0.51) 62.5 33.9 2.08 parts Natrosol Emulsion D Air 0.0202 (0.51) 378.7 206.1 0.0142 (0.36) 58.9 45.6 22.9% solids Air+150 C/30 min. 0.0180 (0.46) 779.2 476.1 0.0185 (0.47) 51.7 30.7 Natrosol (a) All data in Table are based on an average of 2 panels.

(b) Ti = Time in seconds for split steel shot to penetrate the coating. 300 or more seconds is acceptable for dry panels and 200 or more seconds is acceptable for wet panels.

(c) Values mathemathically normalized to 0.0110 inches (0.28 mm) coating thickness, the minimum thockness required to meet both wet and dry performance specifications and thereby give maximum economy of coating cost. Values in boxes do not meet specifica standards.

As can be seen from the data in TABLE V, emulsions with low solids content, i.e., below 50 weight percent solids, require the addition of relatively large amounts of a thixotropic agent. At high addition levels the thixotropic agents, although necessary for spraying, especially in the case of vertical panels to preclude run-off, cause the resultant coating invariably to fail the abrasion test under wet exposure conditions and in one occasion under low temperature curing conditions even when tested dry. Emulsions C and D yielded coatings that failed wet abrasion even at uneconomical thicknesses of 0.0150 to 0.0202 inches (0.38 to 0.51 mm).

Example 18 To 100 parts of the emulsion of Example 1 5 having a solids content of 68% was added 0.025 parts of a thixotropic agent, i.e., a polyacrylamide commercially available from Dow Chemical under the tradename "Separan" AP273 in 7 parts of water with stirring. The resultant thixotropic emulsion had a viscosity of 20 poise and a yield-stress of 1 30 dynes/cm2 as measured on a Contraves rheomat 11 5 at 20"C and 350 rpm using measuring system 11 4.

This material will hereinafter be referred to as Emulsion 1.

Another thixotropic emulsion was prepared by adding 5 parts ethylene glycol and 0.08 parts "Separan" Ap 273 in 8 parts water to 100 parts of the emulsion of Example 15. This material will hereinafter be referred to as Emulsion 2. Emulsion 2 had a viscosity of 4.3 poise and a yield-stress of 65 dynes/cm2 when measured under the same conditions as Emulsion 1. Each emulsion was sprayed using commercial equipment at a spray pressure of 3500-4000 psi (24.1-27.6MPa) on steel panels electrocoated with a composition commercially available from PPG Industries Inc. under the tradename "ED-3076." The thus coated panels dried "tack-free" after one hour at room temperature. The panels were then subjected to the shot blaster abrasion test in both the dry and wet state as set out supra. The results are shown in Table VI.

TABLE VI Dry Wet Panel Coating Ti(sec) (a) Coating Ti(sec.) (a) Coating Thickness Thickness (in.? (mum) (in.) (mm) Emulsion 1 0.0123 (0.31) 350.6 0.013 (0.35)277.8 Emulsion 2 0.0123 (0.31) 379.5 0.0130(0.33)256.7 (a) Ti equals time in seconds for split steel shot to penetrate the coating. 300 or more seconds is acceptable for dry panels and 200 or more seconds is acceptable for wet panels under the Renault-Puegeot abrasion test.

The results in TABLE VI show that emulsions of the instant invention with high solids content, i.e., above 50%, which can be made thixotropic with a minimum amount of thixotropic agent, result in coatings having good abrasion resistance.

Claims (22)

1. Process of forming a crosslinked polyurethane aqueous emulsion which comprises (1) forming in an inert solvent a branched prepolymer derived from (a) an amine containing alcohol having two or three -OH groups, (b) a hydrophobic polymeric diol or triol, (c) a mixture of an aliphatic and an aromatic polyisocyanate and (d) optionally a urethane-forming catalyst; (2) admixing the reaction product from (1) supra with an amine-containing surfactant; (3) adding the admixture from (2) to water containing a fixed acid with vigorous stirring to form a crosslinked polyurethane emulsion; and (4) removing the inert solvent and a portion of the water so that the emulsion contains about 50 to 75% solids.

2. Process according to claim 1 wherein the said prepolymer is formed by reacting in an inert solvent (a) the amine containing alcohol having 2 or 3-OH groups, and (b) the aromatic polyisocyanate; and adding (a) the hydrophobic polymeric diol or triol, (b) the aliphatic polyisocyanate, and, optionally, (c) the urethane-forming catalyst.

3. The process according to Claim 1 or 2 wherein the amine-containing alcohol is triethanolamine, the hydrophobic polymeric diol is polypropylene glycol, the aliphatic polyisocyanate is isophorone diisocyanate, the aromatic isocyanate is toluene diisocyanate and the fixed acid is phosphoric acid.

4. The process according to Claim 1 to 2 wherein the amine-containing alcohol is Nmethyldiethanolamine, the hydrophobic polymeric diol is polypropylene glycol, the aliphatic polyiisocyanate is isophorone diisocyanate, the aromatic polyisocyanate is polymethylene polyphenyl-isocyanate and the fixed acid is phosphoric acid.

5. Process according to claim 1 substantially as described in any one of the foregoing Examples.

6. A branched prepolymer composition which comprises the reaction product in an inert solvent of (a) an amine containing alcohol having two or three -OH groups, (b) a hydrophobic polymeric diol or triol, (c) an aliphatic polyisocyanate, and (d) an aromatic polyisocyanate.

7. A prepolymer according to Claim 6 wherein the mole ratio of aliphatic to aromatic polyisocyanate is about 1:2 to about 2:1.

8. A prepolymer according to Claim 6 wherein the mole ratio of aliphatic to aromatic polyisocyanate is about 1:1.

9. A crosslinked polyurethane aqueous emulsion having about 50-75% solids content comprising the reaction product of (a) an amine containing alcohol having two or three -OH groups, (b) a hydrophobic polymeric diol or triol, (c) a mixture consisting essentially of 25-75 mole percent aliphatic polyisocyanate and 75-25 mole percent aromatic polyisocyanate, and (d) water containing a fixed acid.

10. A crosslinked polyurethane aqueous emulsion composition having a solids content of 50 to 75 weight percent comprising the reaction product of (a) an amine containing alcohol having two or three -OH groups.

(b) a hydrophobic polymer diol or triol, (c) an aliphatic polyisocyanate, (d) an aromatic polyisocyanate, and (e) water containing a fixed acid, said aliphatic and aromatic polyisocyanate being present in a mole ratio in the range of about 1:3 to about 3:1 and the ratio of the total isocyanate equivalents to the total -OH equivalents being in the range of about 1.6:1 to about 2.4:1.

11. A cross-linked polyurethane aqueous emulsion when formed by the process of any of claims 1 to 5.

1 2. The emulsion of any of claims 9 to 11 containing in addition a thixotropic agent.

1 3. The emulsion of any of claims 9 to 1 2 which contains in addition at least one antioxidant.

1 4. The emulsion of any of claims 9 to 1 3 which contains in addition a UV stabilizer.

1 5. The emulsion of any of claims 9 to 1 4 which contains in addition at least one corrosion inhibitor.

1 6. The emulsion of any of claims 9 to 15 which contains in addition at least one antiblistering agent.

1 7. The emulsion of any of claims 9 to 16 which contains in addition at least one colouring agent.

1 8. The process of forming a coating on a substrate which comprises coating said substrate with an emulsion as claimed in any of claims 9 to 17, drying said emulsion to remove the water therefrom and, thereafter, heating the resultant coating to a temperature in the range about 20 to 160"C for a time sufficient to obtain a cured coating.

1 9. The cured coating resulting from the process of Claim 1 8.

20. An automotive underbody coating produced by the process of Claim 1 8.

21. A chip-resistant automotive rocker panel coating produced by the process of Claim 1 8.

22. A wash-resistant automotive cavity conservation coating produced by the process of Claim 18.