Coating processes – Base supplied constituent – Resin or rubber base
Reexamination Certificate
1999-02-16
2001-02-06
Beck, Shrive (Department: 1762)
Coating processes
Base supplied constituent
Resin or rubber base
C427S372200, C427S421100
Reexamination Certificate
active
06183815
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
FIELD OF THE INVENTION
The present invention relates to a process for forming a self-assembled layer on a metal surface. Still more particularly, the present invention relates to a method for coating a thiol-containing solution onto a treated metal substrate. The thiol has been designed to not have the stench characteristic of conventional thiols, the methods of application employ benign chemicals and simple techniques, and the treated metal surfaces feature improved corrosion resistance even though the layer is invisible.
BACKGROUND OF THE INVENTION
There are many instances where it is desirable to protect the surface of a metal from exposure to air or water in the environment. In particular, it is often desirable to minimize the exposure of the metal surface to moisture, oxygen, sulfur-containing gases and other reactive molecules that may be present in the air. One simple way to achieve this goal is to encapsulate the metal object in a polymeric container. In the case of rare coins, encapsulation of the coin and a small gap of air surrounding the coin in airtight plastic case is referred to as “slabbing.” Alternately, the metal object itself can be coated with a wax or polymer in solution, such an acrylic polymer lacquer, that contacts and covers the coin with a relatively thick, transparent layer. This layer is obvious to the naked eye, changes the appearance or sheen of the metal, and is subject to yellowing, cracking and peeling as the polymer ages. When the object is large, slabbing or encapsulation of the metal object in an airtight plastic container is not practical. Aesthetic or other considerations may preclude coating the metal with a layer of polymer. Therefore, alternative methods of protection are desired.
The coating of a metal surface by self assembly of organic molecules has been reported in the literature. The process yields a well defined surface that moderates and mediates the chemistry of the underlying metal. By self assembled, it is meant that the molecules in the coating layer each attach one of their ends to the surface, and then “assemble” in a uniform geometric pattern on the metal, thereby aligning themselves consistently and uniformly and forming a layer whose thickness is approximately equal to the length of one molecule.
For example, the self assembly of thiol compounds, i.e. those having the general formula H(CH
2
)
n
SH, on metals like gold, silver or copper is known in the art. In the self assembled layer, the sulfur atoms are bound to the metal surface and the alkyl tails are pointed away from the metal surface. This outermost layer of hydrocarbons tends to make the surface of the layer at least slightly hydrophobic. A severe disadvantage to these alkyl thiol metal surface treatments, however, is that the thiol compounds tend to have prohibitively unpleasant odors. Although alkyl thiols having higher molecular weights are less odiferous, they exhibit reduced solubility in most practical solvents.
Hence, it is desired to provide a self-assembling metal surface treatment that produces a gas-, water-, oil- and corrosion-resistant outer layer without producing an undesirable odor during the application of the layer, and without employing hazardous chemical solvents. It is further desired to provide a metal surface treatment that is a simple and easy to apply and relatively inexpensive.
SUMMARY OF THE INVENTION
The present invention provides a self-assembling metal surface treatment that produces a gas-, water-, oil- and corrosion-resistant outer layer. The present metal surface treatment is simple and easy to apply and relatively inexpensive. According to a preferred embodiment, a solution of a fluoroalkylamide thiol in a light alcohol is applied to the metal surface that is to be protected and maintained in a wet state for a predetermined amount of time, which serves to allow the thiol molecules to self-assemble into a monolayer on the metal surface. One way to accomplish the self-assembly period is to spray a thin layer of the thiol solution onto the metal surface and then allow it to dry undisturbed. An alternative technique consists of dipping the metal into the thiol solution and allowing it to remain in the solution for a predetermined self-assembly period before removing it from the solution and drying it.
In alternative embodiments, the fluoroalkylamide thiol is replaced with alkyl amide thiols to lower the cost of the thiol. Similarly, the light alcohol solvent can be replaced with a hydrofluoroether or a fluoroalkane if a non-flammable solution is desired, although the integrity of the self-assembled layer is decreased. In any case, the thiol monolayer is kept in wet contact with the metal surface as the thiol self-assembles in a monolayer during the evaporation of the solvent. Excess coating material (thiol) is removed by rinsing and/or polishing, and the surface is dried.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
According to the present invention, a coating solution comprising an alkylamide thiol dissolved in a low molecular weight alcohol is applied to the metal surface that is to receive the protective layer. Details of these steps and compositions are as follows.
Thiols
In a preferred embodiment, the thiol compounds used to form a protective layer on the metal surface comprise fluoralkyl amide thiols having the general formula F(CF
2
)
n
(CH
2
)
p
CONH(CH
2
)
m
SH, where n, m and p can each be 0-20. It is preferred that n be 1-20. F(CF
2
)
n
is the fluoroalkyl section of the molecule and is outermost when the protective layer self-assembles. Hence, it provides a fluorinated outer surface that provides the tarnish resistance. The CONH group is the amide section, which provides hydrogen-bonding between the thiols, and SH is the thiol head group that chemisorbs to the metal/tarnished metal surface. A particularly preferred embodiment of the present coating compound is F(CF
2
)
6
CONH(CH
2
)
2
SH. It is believed that the integrity of the layer formed by these compounds is enhanced by the action of van der Waals forces between the coating molecules and by hydrogen bonding between thiols at the amide group.
An alternative to the use of fluoralkyl amide thiols comprises alkyl amide thiols with the general formula H(CH
2
)
n
CONH(CH
2
)
m
SH, where n and m are each 1-20. It is expected that these compounds will be less expensive than the fluoralkyl amide thiols. The alkyl amide thiols may still be free of odor because the hydrogen bonding group lowers the vapor pressure of the neat thiol. Although the protective layer formed by alkyl amide thiols on the surface of the metal will not have a fluorinated characteristic, it comprises hydrocarbons and so is expected to repel water to some extent. Nevertheless the fluorinated compounds are presently preferred, as fluorinated coatings are more water repellant and are also oil repellant.
An outstanding characteristic of the present thiols is their lack of any detectable odor. The unpleasant and dangerous stench associated with conventional alkylthiols is the primary reason that they have not been used more extensively. Although several commercial silver tarnish-inhibiting products contain alkylthiols, their strong stench is readily detected when the products are used.
Solvent
The preferred solvent should be one that is safe, easy to handle, and inexpensive, as well as one that dissolves the self-assembling compound. One suitable class of solvents is the hydrofluoroethers or fluoroalkanes. These are nonflammable, and are effective solvents for the desired compounds. A disadvantage of the hydrofluoroethers and fluoroalkanes is that they are relatively expensive, and the integrity of the self-assembled layer is inferior to that observed in other solvents.
Preferred solvents therefore are light alcohols or alcohol water mixtures. By light alcohols are meant alcohols having six or less carbon atoms. For example, alcohols such as isopropyl alcohol, normal propanol or mixtures of either propanol with small amounts (3
Beckman Eric
Enick Robert M.
Beck Shrive
Calcagni Jennifer
Conley, Rose & Tayon, P.C
University of Pittsburgh
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