This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/US98/24404 which has an International filing date of Nov. 17, 1998, which designated the United States of America.
1. Field of the Invention
A method and composition of matter for use as polymeric topcoats for articles and vehicles, such as, aircrafts, naval vessels, clothing and other industrial applications. With regard to an aircraft, xe2x80x9ccold-soakxe2x80x9d of the aircraft wing fuel tank leads to localized wing ice formation under certain environmental conditions. Also, ice forms on the xe2x80x9cpleading edgesxe2x80x9d of the aircraft which detach and enter the jet engines or otherwise influence aerodynamic performance of aircraft wings. Conventional polymer paints and coatings contain a volatile organic content (VOC) that is under increasing regulation by EPA.
2. Background of the Invention
Since 1986, the limit for volatile organic content (VOC) of aerospace topcoatings as set by California Rule 1124 has dropped from around 700 g/l to its present limit of 420 g/l or even lower values. Increasing concern over the impact of organic compounds on the quality of life and environment can be expected to lead to further reduction in permissible VOC in coming years. Achieving durable, functional coatings that comply with the VOC regulations and satisfy functional coating requirements is becoming challenging for the aircraft industry and coating suppliers.
The southern California environmental control agencies require a maximum of 420 grams/liter of volatile organic compounds (VOC) from a coating material. The cyclic prepolymer coatings will reduce the VOC emissions during coating operation to less than 1 gram/liter of coating material. These new coating processes will provide a coatings technology that is environmentally compliant for the future, whereas existing solvent-borne technologies are compliant on a year-to-basis with a questionable future.
Conventional aircraft coatings used on commercial and military aircraft can be either water based or solvent based. Solvent based coatings are the most widely used. Typical solvents such as xylene, toluene and chlorinated aliphatic hydrocarbons, are required in order to control drying times, pigment distribution and surface smoothness of these coatings. These compounds all have unacceptably high VOC. Furthermore, xylene is a carcinogenic compound and the others are suspected hazardous materials both of which present serious employer liability issues. The EPA is strongly advocating a reduction of all solvents with the exception of water to reduce VOC and eliminate potential carcinogens. A water based coating is a natural alternative and has been developed for primer coatings but has yet to produce satisfactory performance as a topcoating. They contain small but significant amounts of VOC.
Historically, coating formulations meet the requirements by using xe2x80x9cexemptxe2x80x9d solvents, or by reclassifying coatings into other categories. Newer approaches for formulations and applications of coatings represented by the approach of the present invention can achieve a reduction of VOC well below 420 g/l, perhaps approaching as low as 0 g/l. This is achieved by using a new polymeric coating technology that will meet the most severe restrictions that are anticipated in the year 2010 less than 100 g/l.
High solids deposition processes are based on water reducible, flame and plasma spray coating processes to implement low VOC coatings and deposition processes through a highly focussed research and development program.
Plasma spray and flame spray processes and flourinated polymer coatings have advantages because current solvent systems have definite limits for reduction of VOC. Although water-reducible systems have potential for further VOC reduction, they have a significant VOC content and may also exhibit adherence problems. A xe2x80x9csuper-critical fluid spray coating systemxe2x80x9d is capable of reducing VOC by 30-70 percent depending on the type of resins and polymers in the parent coating system. However, the equipment is expensive, complex and bulky, and the pigmentation of coatings using this process is limited.
Plasma spraying consists of depositing a coating by flowing a powder coating-inert gas mixture through an electric arc plasma. The thermoplastic powder liquefies and flows on the surface. The advantage of this coating process over air spraying of solvent-borne and water-borne coatings is that no solvent or VOC is produced. Also, many materials can be applied with low surface energies, such as chloro- and fluoropolymers which cannot be air sprayed. The disadvantage is that the process produces an ignition source which is hazardous around aircraft and flammable vapors and liquids. The actual cost of the plasma spray coating process is higher than conventional coating processes, but the service life of the plasma sprayed coating is longer and the coating can be thicker to compensate for wear. Lifecycle costs may be lower than conventional coatings. Typical foot-wear on the surface will not damage these coatings.
The plasma spray process is a mature technology and equipment is available for use. These coatings can be applied directly to aircraft aluminum surfaces to provide a non-icing surface. Limited colors are available in stock powders, but can be formulated for any color. In order to achieve an optimal coating it also will be necessary to formulate binders and pigments with specific properties.
For a thixotropic powder, particles need to coalesce quickly, (tc needs to be short) since xcex7 is time dependent after deformation. The instantaneous viscosity, xcex7, and particle radius, rp, should be small and the surface tension, xcex3, large. For flattening, xcex7, and particularly rp should be small; and xcex3 and particularly h should be large. (Additives may be able to reduce the surface tension, but viscosity seems the primary driver.) Low xcex7 necessitates low molecular weight and higher temperatures, or slower catalysis rate.
Specific flatteners, pigments and other additives are necessary to make an effective topcoat from a resin (binder) promoting coating adhesion, providing ultra-violet (UV) radiation protection and color. These additives must be balanced against the requirements for coalescence and flattening, as increasing content of particulate in the coating increases viscosity. Several specific texts on paint chemistry for the production of a topcoat are available to guide coating formulation.
Two generic types of coatings are relevant, aircraft topcoatings and industrial maintenance (IM) coatings. Requirements for aircraft topcoatings are stringent. Typically, aircraft topcoat requirements are specified by the military specifications MIL-C-83286B, xe2x80x9cAliphatic Isocyanate Urethane Coating for Aerospace Applicationsxe2x80x9d, MIL-C-85285, xe2x80x9cHigh Solids Polyurethanesxe2x80x9d, or Boeing Military Specifications such as BMS 10-60, xe2x80x9cProtective Enamel.xe2x80x9d
An EPA reports summarizes the competitive low VOC coating processes and chemistries available in 1991; the principal ones being powder, waterborne, radiation curable and high solids coatings. The summary of this older reference still appears to represent a good economic and technical assessment of coating possibilities. This report also emphasizes that VOC from coating stripping operation is also considered one of the VOC consequences of the selection of method of coating. Table 1 summarizes coating/application methods and issues in this report.
Besides the genre of application for coatings, the actual deposition method is an important element in controlling VOC. High solids coatings, for example, achieve low VOC by eliminating the solvent classically used for coalescence, flow and flattening. They rely instead on mechanisms such as thermal or kinetic energy to achieve these ends water-based coatings replace organic solvents with water.
The EPA sets forth VOC requirements for industrial maintenance coatings, namely, primers, sealers, topcoats, etc., used in outdoor aggressive environments on structures such as bridges, ships, and hydraulic structures. The proposed VOC limit was 350 g/l; and the 2004 limit (proposed) was 300 g/l.
This reports describes in detail the VOC measurement methodology (EPA Reference Method 24, a distillation of ASTM standard test methods) and describes the calculation of VOC emissions. Manufacturers claim that the ASTM D-2369 can produce inordinately high VOC levels, particularly in marine and architectural coatings, as it requires the coating to be heated to 110 C. (230 F.) where excessive loss of volatile components by coating decomposition may occur.
Camouflage topcoatings must meet low VOC requirements and also very stringent chemical agent resistance requirements. These requirements consist of resistance to chemical decontamination/wash as well as other severe requirements.
The baseline coating is a two components solvent-borne polyester/polyisocyanate binder system that is lead, chromium, 1,1,1 trichlorethane free. This candidate new xe2x80x9clow VOCxe2x80x9d coating is a waterborne/dispersible/reducible coating using polyisocyanates and polyesters from Miles, Inc. including Bayhydrol XP-7044 WD polyester, Bayhydur XP-7007 WD polyisocyanate and de-ionized water reducer as needed.
Typical fillers are cobalt green spinet, chromium oxide, magnesium ferrite and carbazole violet pigments with diatomaceous silica, magnesium silicate and amorphous silica extender pigments. These pigments are added to the polyester component and polyisocyanate is diluted with suitable solvent to meet viscosity of both components and meeting stoichiometry.
This coating met all requirements of specification except CAR (chemical agent resistance). VOC is estimated to be xcx9c300 g/l. A fundamental problem of these WB/WD/WR coatings is film porosity that allows chemical agents to penetrate the coating. The CPVC (critical pore volume content) appears to determine gloss. A high CPVC value in the candidate coating is a problem for CAR. The author of the study cites the strategy for improving the CPVC is use of additives to improve wetting/flow/dispersion in this WB/WD/WR system.
It would appear that a well designed, low VOC aircraft topcoat may also met CARC requirements.
Table 2 outlines the basic performance characteristics of aircraft coatings.
Regulatory bodies tend to restrict the use of deposition systems that have low transfer efficiency. The California AQMD regulations require minimum transfer efficiencies of 60-85% and maximum gun tip gas pressure of 10 psi. Currently, only HVLP and electrostatic spray processes can meet these requirements.
Existing levels of corrosion protection should be maintained with new low VOC topcoatings. Corrosion (aqueous) requires the presence of water, cations and oxygen. strontium chromate is an important additive to inhibit corrosion. Coating strategy has been to achieve a physical barrier between the substrate and the external environment to prevent moisture and radiation induced coating degradation.
Since moisture egress is a virtual certainty, coating adhesion becomes a very important coating characteristic. The mechanism of adhesion is either chemical or physical. Although chemical pretreatment of the surface substrate can enhance secondary chemical bonding, and in some cases even achieve primary bonding, the major adhesion mechanism is the mechanical interlocking of the coating with the microscopic surface roughness created in anodizing.
The practical lifetime of a military or commercial coating is 4-8 years. This lifetime requirement imposes significant demand for resistance to environmental degradation.
Traditionally an epoxy primer and polyurethane topcoat are used for aircraft applications. Epoxy primer/polyurethane topcoatings are highly refined to meet the military requirements. Since epoxides are brittle and have very low UV stability, they are used as a primer and the external coating provides the UV protection. The epoxide coatings provide superior resistance to moisture penetration and subsequent corrosion. The combination of the two also has very low water absorption, vapor transmission rate and UV resistance.
Aliphatic isocyanate and polyester are highly developed UV resistant topcoatings and their literature is well documented. Typical aircraft topcoats have a dry film thickness of 0.002+/xe2x88x920.0003 inch (50.8+/xe2x88x927.8 micrometers). Set and hard dry time is typically 2 and 6 hours, respectively. Fully developed properties may not be attained until about 7 days aging.
Fillers such as talc and mica are used to provide an oriented distribution to serve as secondary radiation and physical barriers, silica and metal silicates, carbonates and sulfates are added as physical fillers that reduce gloss and increase opacity.
Water-borne coatings are one approach to compliant coatings. The primary strategy is to achieve a solution of emulsion of polymer powders whose surfaces are modified with hydrophilic groups. The major difficulty with water-borne coatings is that the use of water as a solvent leads to more porous coatings and adhesion problems related to organic surface contamination.
High solids or powder coatings are also one route to achieving low VOC levels. One approach is to reduce the solvent content of the coating, but this shortens pot life and greatly increases viscosity. These factors increase surface roughness. By moving to lower molecular weight resins, one can achieve improved viscosity and flatter coatings. However, polyisocyanate cured powders have shorter pot life and reduced flexibility due to the more rapid and extensive cross linking due to the lower molecular weight. One strategy to improve this is to use polymers with very narrow molecular weight distribution.
An EPA study and a follow-up publication evaluated six (6) coatings. The six types are solvent-borne polyurethane, waterborne epoxy primer w/latex topcoat, solvent alkyd primer/waterborne acrylic, 2 component polysiloxane topcoat, water reducible alkyd primer/acrylic topcoat and solvent alkyd primer/solvent alkyd enamel (standard baseline). The study compared impact, adhesion, pencil hardness and solvent tests and outdoor exposure tests of these coatings. The VOC data from candidate coatings in this study is useful for aerospace topcoats. Of particular interest in this assessment of IM coatings is the determination that a two component polysiloxane coating with a low VOC of about 84 g/l performed extremely well. These two studies showed the polysiloxane coatings exhibited the best VOC levels and performance in environmental testing.
A second study is also grouped in the IM coatings discussion. Although the study intended to coat F-15 aircraft, only ground vehicles were coated. It consisted of an evaluation of supercritical spray coating and a high pressure-low volume (HPLV) process called ULV (ultra low volume) spraying. The polyurethane coatings had a baseline VOC of about 420 g/l which is too high for current requirements. The study found the supercritical coating process to be unacceptable for field use, and found that the ULV process reduced emissions by about 50%, primarily by reducing the total paint sprayed in the coating process. This result shows that both the coating process itself as well as the formulation of the coating can have significant impact on total VOC emission. This study attempted to spray high solids coatings unsuccessfully. The major problem encountered was very slow drying. This was a result of an improper level of catalysts in the coating.
IM coatings for bridges using principally an epoxy mastics and silicone rubbers, that are not particularly relevant to aircraft topcoats, were evaluated. Cyclic salt-fog/freeze provided a relatively rapid method to differentiate coating performance in a short time period. Specific VOC content was not stated, but all the evaluated coatings were at or below 340 g/l. A xe2x80x9clow-VOCxe2x80x9d acrylic aliphatic polyurethane topcoat exhibited the best gloss retention.
The basic objectives of this invention are to produce a polymeric binder or matrix for a coating with extremely low, or zero VOC content that can be used as a top coat for many applications and in aircraft applications to achieve specific coating characteristics to prevent icing of aircraft wings. Icing on critical aircraft surfaces may create a condition which might impair the stability of the aircraft. The specific areas are referred to as xe2x80x9ccold-soakxe2x80x9d areas and some other areas on the xe2x80x9cleading edgesxe2x80x9d of the wings and engine nacelles. The present invention eliminates the adhesion of ice on these surfaces. Environmental icing due to weather is a related problem, but is not the direct problem concerning the present invention.
Ice will not adhere to the surface of certain polymer coatings with low surface energy such as Teflon. This is a consequence of the high contact angle between the water droplet and the surface that establishes a non-wetting surface. One objective of the present invention is to implement such coatings and a deposition process. Effective implementation will also result in a coating formulation and deposition process, with a very low VOC emission.
Coatings formulations for prevention of icing problems includes the following properties:
Low surface energies to prevent icing.
Adhesive to aircraft surfaces.
Protection of substrates from corrosion.
Resistance to jet fuel and hydraulic fluids.
Other properties for coatings specifications.
Coating materials are selected for low surface energy properties and general coating properties. The coating materials are polymerized fluoropolymers that possess good low temperature properties, e.g., do not embrittle at xe2x88x9245xc2x0 C., and do not soften at elevated temperatures of 90xc2x0 C.
Two parallel benefits of this approach may be achieved. The coating process is potentially adaptable for coating an entire aircraft or other commercial item. The combination of the coating process and the coating formulation reduces volatile organic compounds (VOC) well below the current Environmental Protection Agency (and California) limits for the forseeable future.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.