NITROCELLULOSE-URETHANE TRAFFIC PAINT

Abstract: Disclosed is a paint composition suitable for marking a roadway surface, containing the reaction product of nitrocellulose, a modifier, and an isocyanate resin as the binder, a solvent, and a suitable pigment.

Description: This invention relates to nirocellulose-urethane paint compositions and their use. Urethane coatings have long been used for exterior appli­ cations where high durability is required, such as for bridge paints, and aircraft finishes, and they have been proposed for road markings. Urethane coatings are formed by the reaction of isocya­ nate and polyol components. Commercially available nitrocel­ lulose contains fre hydroxyl groups that can react with res­ ins containing isocyanate groups to form urethane linkages. Nitrocellulose/urethane coatings are well-known for furniture finishes and have been proposed for use in pigmented aircraft finishes. Highly durable nitrocellulose/ urethane lacqueers as two-­ part compositions are described in Hercules Incorporated Technical Information Bulletin CSL-202E, issued in May of l984. They contain, as an additional polyol component, any reactive modifying resin that has free hydroxyl functionality, and are described as suitable for finishing wood or metal sur­ faces, with a typical drying time of 20 to 30 minutes for a 50 micrometer unpigmented film. A plasticizer may not be re­ quired, but if used it must have free hydroxyl functionality to react with the isocyanate content. The modifying resins may be modified polyols such as hydroxy acrylic polyols. However, nitrocellulose/urethane coatings have not been used as traffic paints because the known formulations normal­ ly used would remain wet too long (because of slow solvent re­ lease) a problem that has also caused difficulty with both conventional polyester and epoxy liquid compositions and urethane coatings, as described for instance, in U.S. Patent 3,474,057, which attempts to overcome the problem by pre­ heating urethane coatings to a temperature of from 60 to l26°C. Traffic marking paints are classified by "auto-no-track-­ time", defined as the period of time after which a passenger car can pass over a freshly applied pigmented marking layer l50 to 230 micrometers thick without "tracking", i.e., without pick-up of material by a tire and redeposition of the material on the road surface with subsequent revolutions. Paints with a 90 second or less auto-no-track-time are preferred because they do not require placing barriers (such as so-called "traffic cones", the use of which is called "coning") on the road to prevent motorists from driving over the wet paint. The placing and retrieval of such barriers is costly and hazardous to the personnel handling the barriers. Paints which do not require "coning" are usually solvent-­ based and comprised of alkyd or chlorinated rubber vehicles. Under severe service conditions, however, such materials normally have a useful life of six months or less. More dura­ ble marking materials usually are solventless systems and are based on polyester or epoxy resins, or hydrocarbon or rosin ester thermoplastic materials. Their service life when pro­ perly applied can approach three or more years. However, polyester and epoxy liquid compositions have auto-no-track-times of 8-l0 minutes and l0-l5 minutes, respec­ tively, and therefore require "coning". Thermoplastic hydro­ carbon or rosin ester coatings dry quickly by cooling but be­ come soft in hot weather and require expensive application equipment. A heavy application of glass beads (20-25 pounds versus a normal 6-8 pounds per gallon of paint applied) may be used with epoxy resins to eliminate the use of barriers, but that significantly increases the cost. It would be desirable to make available a paint composi­ tion that is solvent-based and has the durability provided by formulation with a nitrocellulose-urethane vehicle, while hav­ ing a drying time short enough to permit its use as a traffic paint for which "coning" would not be required. According to the invention, a solvent-based nitrocel­ lulose-urethane paint composition that comprises the combina­ tion of a pigment, a solvent, a binder, and optionally a cat­ alyst, that is the reaction product of nitrocellulose, an iso­ cyanate resin and a modifier containing -OH functional groups, is characterized in that the modifier is a polymeric hydroxy­ alkyl acrylate, and optionally also contains a plasticizer containing -OH functional groups. The solvent-based paint compositions of this invention, in which the nitrocellulose is used as a part of the polyol component of the urethane vehicle, and the hydroxyacrylic polyol modifier provides additional reactive -OH functional groups, have a 90 second or less auto-no-track-time and high durability, and therefore are particularly suitable for use as traffic marking paints. It is assumed that the non-polar character of the poly­ meric hydroxyalkyl acrylate facilitates the rapid release of the polar solvents (such as esters and ketones) used for nitrocellulose-urethane paint compositions, which tend to be retained by polar polymers. Suitable hydroxyacrylic polyol modifiers include those commercially identified as follows: Acryloid AU-568 AU-608, and QR946 (sold by Rohm and Haas Company); Degalan VP-LS 50, VP-LS 73, and VP-LS l02 (available from Degussa Corporation); and Lumitol H66, H80F, and Hl35, sold by BASF AG. Preferably, the hydroxyfunctional modifiers are copoly­ mers of hydroxyethyl acrylate or methacrylate with an alkyl acrylate or methacrylate such as the methyl, ethyl or butyl derivative. In general the higher the molecular weight of the hydroxyacrylic resins (normally having molecular weights in the order of about l000) provide harder, more brittle films. It may be advantageous to use a mixture of two or more hydroxyacrylic polyol modifiers. Lower molecular weight hydroxyacrylic resins act as plasticizers and give softer, more flexible films, and higher coatings solids. Other plasticizers that may be used are conventionally used in lacquer formulations to achieve a de­ sirable degree of film flexibility, but only those that have reactive -OH functional groups, such as castor oil-azelaic acid alkyd and polyethers derived from propylene glycol. Suitable polymeric plasticizers, identified as Paraplex RG-2 and RGA-2, are available from Rohm and Haas. Although nitrocellulose is compatible with both aromatic and aliphatic isocyanate resins, the aliphatic resins are usu­ ally preferred over aromatic isocyanate resins for exterior applications because the aromatic resins yellow upon exposure to ultraviolet light. However, requirements for color stabil­ ity in traffic paints are not as stringent as in other exteri­ or applications, so combinations of aliphatic isocyanate res ins with the aromatic resins (which are cheaper and cure fast­ er than the aliphatic resins) may be used. Suitable aliphatic isocyanate resins include hexamethy­ lene diisocyanate biurets, trimers of hexamethylene diisocya­ nate, and isophorone diisocyanate trimers. Suitable aromatic isocyanate resins include trimers of toluene diisocyanate, adducts of toluene diisocyanate with polyols such as trimethylol propane, and diphenyl methane diisocyanate resins. Nitrocellulose useful in the paint compositions of this invention has an average nitrogen content of about l0.7 to about l2.2%. Preferred viscosity grades of nitrocellulose, as measured by the falling-ball method at 25°C, are those identified as "l/4 second", "l/2 second" and "5-6 second". The higher viscosity grades, such as l/2 sec or 5-6 sec, give coatings with lower solids contents and thinner dry film thicknesses for the same wet film thickness when applied. The lower viscosiity grades, which for instance may have vis­ cosity values of 30-35 cps and l8-25 cps and supply more sol­ ids in the dry film, give longer no-track-times. An excep­ tionally low viscosity grade (5 cps) gives a much longer no-track-time. Urethane grade nitrocellulose (toluene-wet nitrocellulose with a maximum water content of 3%) as well as toluene-wet nitrocellulose with a maximum water content of 4%, or alcohol-­ wet nitrocellulose, can be used in the paint compositions of this invention. Suitable and preferred ranges for the components of the nitrocellulose-urethane vehicle of the paint composition of this invention are given below. The paint compositions of this invention are typically formulated to maintain molar equivalents of isocyanate and hydroxyl group functionality or a slight excess of hydroxyl to ensure complete reaction of the isocyanate groups. Ratios with an excess of isocyanate exhibit slower drying rates. A catalyst is preferably used to promote the reaction between the isocyanate and hydroxyl groups, particularly when the paint is applied to cold road surfaces. Any catalyst con­ ventionally used in urethane coating systems, such as bismuth 2-ethyl hexanoate or dibutyl tin dilaurate, may be used. Bismuth 2-ethyl hexanoate is preferred. Zinc, tin and cobalt driers are also effective. The catalyst does not appear to affect the no-pick-up-time. Any conventional pigment, or combination of pigments, suitable for traffic marking paints may be used. A combina­ tion of calcium carbonate, talc and titanium dioxide is pre­ ferred for the white paints of the present invention. If yellow paints are desired, yellow pigments such as diarylide yellow toner or lead chromate may be used. The paint compositions of this invention may be formu­ lated at a pigment volume concentration (PVC) between about 15 and what is conventionally known as "the critical PVC" of the composition. The critical PVC is defined as that level of pigmentation in the dry paint where just sufficient binder is present to fill the voids between the pigment particles. Above the critical PVC there is a rapid fall-off in durabili­ ty. A PVC of about 40 is preferred. For a given vehicle system the higher the PVC, the faster the dry and the higher the solids. Polar solvents from which moisture, acids, alcohols, amines and all materials containing reactive hydrogen have been eliminated are used in the paint vehicle. Esters such as ethyl or butyl acetate and ketones may be used. Methyl ethyl ketone is the preferred solvent. The presence of the solvent in solvent-based nitrocellu­ lose-urethane paint compositions provides better wetting of the road surface with varying types and degrees of contamina­ tion (such as dirt, oil, ethylene glycol, rubber). This im­ proved wetting of the road surface eliminates the necessity for the application of a primer or precleaning of the road surface, steps that are required when using some conventional marking materials. The paint compositions of this invention can be formu­ lated for use in either single-stream or two-stream equipment and for both airless and air-assisted application. The com­ positions can be applied at ambient temperature or may be heated before application to a temperature above ambient tem­ perature and below the temperature at which the paint decom­ poses, before application to the roadway surface. Two-stream equipment is preferred. Heating before application will re­ duce the auto-no-track time. If single-stream equipment is used, the polyol component containing the pigment, the isocyanate resin component and the catalyst component are stored separately and are not mixed until shortly before application of the composition to the roadway. If two-stream equipment is used, the polyol component, which also contains the pigment and catalyst, and the isocya­ nate resin component are stored in separate containers until use. The two components do not mix until they reach the ap­ plication nozzle. In either case it may be advantageous to add an anti-­ settling agent to the polyol component. The antisettling agent keeps the pigment in suspension when the polyol compo­ nent is stored for an extended period of time before use. After the paint compositions of this invention are sprayed onto the road surface, glass beads or spheres can be immediately dropped onto the paint while it is still tacky so that the beads become partially embedded in the paint. The resulting line is immediately retroreflective. Conventional application equipment includes a spray head for the paint composition having a glass bead dispensing de­ vice associated therewith and positioned just behind the paint spray head. The glass bead dispensing device is co­ ordinated with the paint spray head to supply glass beads to the freshly applied, undried paint line passing under the glass bead dispenser, or to intermix the paint and glass just before or as the paint hits the road surface. Durability Comparison This series of tests illustrates the durability of the nitrocellulose-urethane traffic paints of the invention as measured by maintenance of "retroreflectance". All parts and percentages are by weight unless otherwise specified. In these tests, the nitrocellulose contributes 0.35 equivalent OH, the hydroxyfunctional resin 0.39 equivalent OH, the hydroxyfunctional plasticizer 0.26 equivalent OH and the isocyanate resin l.ll equivalents NCO to the paint vehicle. The pigment volume concentration is 40. The first three ingredients are combined, and the nitro­ cellulose is added slowly with slow mixing. The mixing is continued until the nitrocellulose is dissolved. The calcium carbonate and talc are dispersed in the nitrocellulose solu­ tion using a high speed disperser. After the remaining methyl ethyl ketone is added, the titanium dioxide is also dispersed at high speed. Components A (polyol plus pigment), B (isocyanate resin), and C (catalyst) are stored separately until use. Immediately before application the following mix is made: The paint is applied at ambient temperature to a Portland cement concrete highway at l5 mils wet film thickness using conventional one stream spray equipment for traffic paint. This application was part of a road test conducted by a state highway department in accordance with ASTM D-7l3, Standard Practice for Conducting Road Service Tests on Traffic Paint. The results of this road test are shown in Table l in which A is a solvent-based nitrocellulose-urethane paint com­ position that is formulated according to the invention, except that the modifier is a conventional modifier for nitrocellu­ lose-urethane lacquers (to extend the drying time for compari­ son, emphasizing the durability of nitrocellulose-urethane, with the conventional traffic paint compositions); B is a chlorinated rubber paint conforming to New Jersey Department of Transportation specifications for Type IV traffic paint (this paint is recognized throughout he industry as the most durable traffic paint currently available); C and D meet Commonwealth of Pennsylvania specifications, including a 90-­ second auto-no-track-time; and E meets the specifications of the State of South Carolina, including a 60-second auto-no-­ track-time. *Values given in the table represent the percent of the stan­ dard as measured with a "Michigan" retroreflectometer. The following examples illustrate the invention and are not intended to limit its scope in any way. All parts and percentages are by weight unless otherwise specified. Example l (lA and lC (with comparisons lB and lD) This example illustrates the efficacy of nitrocellulose in reducing the drying time of the traffic paint compositions of this invention. In the following formulations labeled lA and lC, the nitrocellulose provides 0.5 equivalent OH, the hydroxyacrylic resin provides 0.3 equivalent OH, the hydroxyfunctional plas­ ticizer 0.2 equivalent OH, and the isocyanate resin l.0 equiv­ alent NCO. In formulations 2B and 2D, the nitrocellulose in formulations lA and lC has been replaced by additional hy­ droxyacrylic resin on an equivalent basis. Hence, lB and lD each contain 0.8 equivalent OH of hydroxyacrylic resin, 0.2 equivalent OH of hydroxyfunctional plasticizer and l.0 equiva­ lent NCO of isocyanate resin. All of the formulations have a pigment volume concentration of 40. (l) Hydroxyacrylic resin "X" has an equivalent weight of l032 OH, a viscosity of 6000 cps @ 25°C, 58% solids in xylene. (2) Hydroxyacrylic resin "Y" has an equivalent weight of 833 OH, 5000 mPa's dynamic viscosity @ 20°C, 60% solids in a 4/l mixture of xylene/butyl acetate as the sol­ vent. (3) The hydroxyfunctional plasticizer is a castor oil-­ azelaic acid alkyd with an equivalent weight of 484 OH, a viscosity of approximately 70,000 cps @ 23°C, l00% solids. (4) The catalyst solution is bismuth 2-ethyl hexoate, 8.2% metal as supplied, l0% solution in methyl ethyl ketone. (5) The isocyanate resin is a hexamethylene diisocyanate biuret with an average equivalent weight of l9l NCO, a viscosity of ll,500 ± 3,500 mPa's at 20°C, l00% solids. Each mix is made in a container with glass beads as a grinding medium. The polyol portion of each is tumbled over­ night. The pigments and catalyst are added and the mixes tumbled again overnight to disperse the pigments. The isocya­ nate resin is added and the containers are agitated for l0 minutes. The resulting paints are immediately separated from the grinding medium and films are cast on a glass plate to give l5 mils wet film thickness. No-pick-up-times are de­ termined for each according to ASTM D-7ll, Method for No-­ Pick-Up Time of Traffic Paint. This method is a laboratory test for comparing the length of time after application after which would be no-pick-up of traffic marking paint by the tire of an automobile. The results are as follows: Example 2 In this example the nitrocellulose provides 0.5 equiva­ lent OH, the hydroxyacrylic resin 0.3 equivalent OH, the hy­ droxyfunctional plasticizer 0.2 equivalent OH, and the isocya­ nate resin l.0 equivalent NCO. The pigment volume concentra­ tion is 40. In preparing Component A, the nitrocellulose is dis­ solved in methyl ethyl ketone to form a base solution of about 33% solids. The pigments are dispersed in a blend of the nitrocellulose solution, the hydroxyacrylic resin, the plasticizer, and the catalyst on a high speed disperser. The remaining methyl ethyl ketone is then added. The paint is applied at ambient temperature at l5 mils wet film thickness on a Portland cement concrete highway us­ ing conventional spray equipment modified to preblend six parts by volume of Component A (polyol plus pigment and catal­ yst) from one stream with one part by volume of Component B (isocyanate resin) from a second stream. A test conducted in accordance with ASTM D-7l3, Standard Practice for Conducting Road Service Tests on Traffic Paint, shows this paint to have an auto-no-track-time of less than 90 seconds. Example 3 In this example the pigment concentration and the OH and NCO equivalents of the various components of the composition are the same as in Example 2. An anti-settling agent is added to Component A and a polyether derived from propylene glycol is used as the hydroxyfunctional plasticizer. A trimer of hexamethylene diisocyanate is used in Component B. In preparing Component A, the nitrocellulose is dissolved in methyl ethyl ketone to form a base solution of about 33% solids. The pigments and antisettling agent are dispersed in a blend of the nitrocellulose solution, the hydroxyacrylic resin, the plasticizer, and the catalyst on a high speed dis­ perser. The remaining methyl ethyl ketone is then added. The paint is applied at ambient temperature at l5 mils wet film thickness on a Portland cement concrete highway using conventional spray equipment modified to preblend six parts by volume Component A (polyol plus pigment and catalyst) from one stream with one part Component B (isocyanate resin) from a second stream. A test conducted in accordance with ASTM D-7l3 shows this paint to have an auto-no-track-time of 90 seconds. ("Aromatic l00" is a complex mixture of liquid hydrocar­ bons sold by Exxon). Example 4 and 4B White traffic paint compositions are prepared as follows. In this example the OH and NCO equivalents contributed by the various components of the composition are the same as in Example l. An alternative pigmentation for a white traffic paint is illustrated in Example 4A. The pigment volume con­ centration is l7 for Example 4A and 40 for Example 4B. In preparing Component A the nitrocellulose solution is dissolved in methyl ethyl ketone to form a base solution of a­ bout 33% solids. The pigment is dispersed in a blend of the nitrocellulose solution, the hydroxyacrylic resin, the plasti­ cizer, and the catalyst on a high speed disperser. The re­ maining methyl ethyl ketone is then added. The paints are applied and compared with the control using the same procedure as in Example 3. Satisfactory auto-­ no-track times are obtained. A control white paint conform­ ing to New Jersey Type IV specifications applied at ambient paint temperature on the same highway on the same day using conventional spray equipment failed the auto-no-track-time requirement at l80 seconds. Examples 5A and 5B Yellow traffic paint compositions are prepared as fol­ lows. In this example the pigment volume concentration and the OH and NCO equivalents contributed by the various compo­ nents of the composition are the same as in Example 2. Example 5A is a lead-free yellow paint composition. The paints are prepared and applied using the same procedure as in Examples 4A and 4B. Satisfactory auto-no-track times are obtained.

Claims: 1. A solvent-based nitrocellulose-urethane paint compo­ sition that comprises the combination of a pigment, a solvent, and a binder that is the reaction product of nitrocellulose, an isocyanate resin and a modifier containing -OH functional groups, characterized in that the modifier is a polymeric hydroxyalkyl acrylate. 2. A paint composition as claimed in claim l, further characterized in that the polymeric hydroxyalkyl acrylate is a copolymer of a hydroxyalkyl acrylate or methacrylate with an alkyl acrylate or methacrylate. 3. A paint composition as claimed in claim l, further characterized in that the modifier also contains a plasticizer containing -OH functional groups. 4. A paint composition as claimed in claim 3, further characterized in that the plasticizer is a castor oil-azelaic acid alkyd resin or a polyether derived from propylene glycol. 5. A paint composition as claimed in any of the pre­ ceeding claims, further characterized in that the isocyanate resin is hexamethylene diisocyanate biuret, a trimer of hexa­ methylene diisocyanate, or a diphenylmethane diisocyanate resin. 6. A paint composition as claimed in any of the pre­ ceeding claims, further characterized in that the composition also contains a catalyst to promote the reaction between the isocyanate and hydroxyl groups, particularly when the paint is applied to cold road surfaces. 7. A paint composition as claimed in any of the preceed­ ing claims, further characterized in that the catalyst is bismuth 2-ethyl hexanoate. 8. A process for preparing a paint composition for ap­ plication onto a roadway surface comprising forming a paint composition as claimed in any of the preceeding claims, by mixing a solution of nitrotrocellulose with the modifier, an isocyanate resin solution and a pigment. 9. A process for forming a paint marking on a roadway surface comprising forming a paint composition as claimed in claim 8 in a closed system, and spraying the paint composi­ tion onto the roadway surface. l0. A process for forming a paint marking on a roadway surface as claimed in claim 7 in which the paint composition is heated to a temperature above ambient temperature and below the temperature at which said paint composition decom­ poses, before application to the roadway surface.

Inventors: WINCHESTER, CHARLES MIRACK
Applicants: HERCULES INCORPORATED
Application year: 1986
Main classification: C08G18
IPC codes: C09D5, B05D7, C08L67, C09D175, C08G18, C08L75, C08L33