Coating processes – Spray coating utilizing flame or plasma heat – Organic containing coating
Reexamination Certificate
2000-08-15
2002-08-13
Cameron, Erma (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Organic containing coating
C427S385500, C427S421100
Reexamination Certificate
active
06432486
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 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.
BACKGROUND OF THE INVENTION
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, “cold-soak” of the aircraft wing fuel tank leads to localized wing ice formation under certain environmental conditions. Also, ice forms on the “pleading edges” 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 “exempt” 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<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 “super-critical fluid spray coating system” 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, (t
c
needs to be short) since &eegr; is time dependent after deformation. The instantaneous viscosity, &eegr;, and particle radius, r
p
, should be small and the surface tension, &ggr;, large. For flattening, &eegr;, and particularly r
p
should be small; and &ggr; and particularly h should be large. (Additives may be able to reduce the surface tension, but viscosity seems the primary driver.) Low &eegr; 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, “Aliphatic Isocyanate Urethane Coating for Aerospace Applications”, MIL-C-85285, “High Solids Polyurethanes”, or Boeing Military Specifications such as BMS 10-60, “Protective Enamel.”
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.
TABLE 1
Comparison of Coatings/Application Methods
Coating/Process
Type
Applications
Advantages
Disadvantages
Waterborne
metal coating
low VOC,
surface finish
coatings
automotive
water cleanup
humidity control
low fire hazard
Powder
aerospace,
surface finish,
cost, oven curing,
coatings
automotive,
low VOC,
color matching,
metal
durability,
Faraday effect
transfer
efficiency
High-solids
most
low VOC,
short shelf and
coatings
applications
color matching,
pot life,
transfer
maintaining
efficiency
workable
Viscosity
High Volume/
most
surface finish,
uniform coverage
Low Press
Deng Fenghua
Eckert Charles A.
Liotta Charles L.
Liu Zhengui
Paris Henry G.
Birch & Stewart Kolasch & Birch, LLP
Cameron Erma
Delta Airlines, Inc.
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