Abstract:
The invention relates to a process for the deposition of a material on a surface. The process utilizes the steps of providing a material to be deposited on the surface; providing a transport medium comprising liquid, subcritical carbon dioxide; combining the material and the transport medium to obtain a combination of the material and the transport medium; and delivering the combination to the surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from International Patent Application No. PCT/EP2006/004128, filed 3 May 2006, published as WO 2006/1117203 A1 on 9 Nov. 2006, which claims priority from German Patent Application No. 10 2005 020 941.6, filed 4 May 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to a process for the deposition of a material on a surface. 
         [0003]    It is well known that the principal method of applying coatings to a surface has been to use organic solvents as a diluent and carrier. After the coating (diluted and/or carried by the organic solvent) has been applied in liquid form to a surface, the organic solvent vaporizes to leave the coating material on the surface. Unfortunately, it is also known that many organic solvents are hazardous, not only due to their flammability, but also due to their potential to cause environmental and health problems. Some are believed to be carcinogenic. These drawbacks, along with state and federal regulations restricting the use of organic solvents, have resulted in an intense search for alternatives. 
         [0004]    Some of these alternatives have been all-powder spray coatings and water-based coatings. Each alternative has its limitations. There has also been promising results with the use of supercritical fluids as diluents and transport carriers, for example as described in U.S. Pat. No. 4,923,720, entitled “Supercritical Fluids as Diluents in Liquid Spray Application of Coatings”. Generally, as will be known to those of skill in the art, supercritical fluids are substances at a temperature and pressure above their thermodynamic critical point. They have the ability to diffuse through solids like a gas and dissolve materials like a liquid. They can readily change in density upon minor changes in temperature or pressure. These properties make them suitable as a substitute for organic solvents in many applications. Carbon dioxide is traditionally the most commonly used supercritical fluid. 
         [0005]    To the present inventors&#39; knowledge, however, no one has previously discovered that a particular range of subcritical fluids can be used quite effectively as a transport carrier for creating spray coatings. As will be explained more fully hereinafter, this range or envelope of subcritical temperatures and pressures has numerous advantages. 
       SUMMARY OF THE INVENTION 
       [0006]    In its broadest aspect, the present invention provides a process for the deposition of a material on a surface, said process comprising the steps of providing a material to be deposited on said surface; providing a transport medium comprising liquid, subcritical carbon dioxide; combining said material and said transport medium to obtain a combination of said material and said transport medium; and delivering said combination to said surface. 
         [0007]    In another aspect, the present invention provides a process for the deposition of a release agent on a surface of a mold, said process comprising the steps of providing a release agent to be deposited on said surface of said mold to allow an article formed in said mold to release without substantially sticking to said mold surface, providing a supply of liquid subcritical carbon dioxide, combining said release agent and said liquid subcritical carbon dioxide to obtain a combination of said release agent and said liquid carbon dioxide, and delivering said combination to said surface. 
         [0008]    As used herein all terms shall have their ordinary and customary meanings as used in the art (unless otherwise specified), and the described preferred embodiments are not intended to limit the full scope of the invention as recited in the claims. 
         [0009]    The invention uses liquid carbon dioxide as a transport medium for the material to be deposited. Preferably the transport medium consists of liquid carbon dioxide. The liquid carbon dioxide immediately converts into the gaseous phase when being expanded and delivered to the surface. Thereby, the carbon dioxide breaks up the material into extraordinary fine particles and drops. Thus a nearly perfect atomization of the material is achieved. This results in a very homogeneous distribution of the material particles on the surface producing a coating layer of homogeneous thickness. By varying the ratio of carbon dioxide to material to be deposited the thickness of the material layer on the surface can be controlled. The carbon dioxide after having transported and distributed the material to the surface evaporates and disappears without any residue. This is in particular advantageous when food products shall be coated. 
         [0010]    The inventive method has several advantages compared to the prior art. In particular, by using carbon dioxide it is not necessary to use hazardous solvents which might cause environmental or health problems and which induce a flammability risk. 
         [0011]    The invention is preferably used to apply materials such as coatings with a thickness of below 3 mm, more preferably below 1 mm, preferably on a mold surface. In an especially preferred embodiment, the coatings are mold release agents having a layer thickness in the range of about 50 to about 500 micrometers. 
         [0012]    Without wishing to be bound by any theory it is believed that the liquid carbon dioxide and the material to be delivered are homogeneously mixed and thus a fine distribution of the material is achieved when being sprayed. Using carbon dioxide that is in a liquid and subcritical state shows significant advantages. In particular, it is believed that the use of carbon dioxide in a liquid and subcritical form provides homogeneous mixing of a material to be delivered—such as a mold release agent—and the carbon dioxide, even if the material and the carbon dioxide are at different pressures, different temperatures and/or different flow rates. The solubility of the material in the carbon dioxide is considered to be of minor importance. 
         [0013]    The relative amounts of carbon dioxide and the material which is to be delivered, preferably sprayed, onto the surface can be changed. Thus it is possible to vary the parameters, especially the thickness, of the layer of material to be deposited. Preferably said material and said transport medium are combined in a predetermined ratio. Preferred ratios of material to transport medium range from 1:5 to 1:10. 
         [0014]    In order to avoid over-spray an electrical potential in the range of about 40 to about 85 kV is applied between the nozzle delivering that composition of carbon dioxide and material and the surface. Since carbon dioxide is a polar molecule it is possible to influence the spray characteristics by applying an electrical voltage. 
         [0015]    Suitable coating materials for use with the present invention are numerous and may, for example, be organic or inorganic or mixtures thereof. Both thermosetting and thermoplastic polymers are useful in the present invention. Natural and synthetic polymers, as reactive pre-polymers, reactive monomers or as fully polymerized, may be used. As will be appreciated by those skilled in the art in light of this disclosure, suitable materials for use as coatings in the present invention will preferably not substantially degrade at the temperatures or pressures involved with their admixture with the subcritical fluid of the present invention. Preferred coating materials include vinyl polymers, acrylic polymers, styrenic polyesters, alkyds, polyurethanes, cellulosic esters such as acetate butyrate and nitrocellulose, amino resins such as urea formaldehyde, melamine formaldehyde and other aminoplast polymers and resins, and natural gums. 
         [0016]    Other particularly preferred materials for use as coating materials in the present invention are waxes, polymers, resins, oils, silicon oils, and/or tensides. The coating material may preferably be mixed with water or a solvent which function as a film forming agent. 
         [0017]    Tests have shown that, for an optimum transport medium, CO 2  should be used in its subcritical liquid state. The term “subcritical” means that at least one of temperature or pressure of the CO 2  is below the critical value, that is the temperature is below 31° C. and/or the pressure is below 73.8 bar. 
         [0018]    According to a preferred embodiment, the CO 2  pressure is in the range of about 20 bar to about 90 bar, more preferably between about 60 and about 80 bar. The CO 2  temperature is preferably below its critical value, that is below 31° C., and is more preferably in the range of from about −20° C. to about 25° C. It is to be understood that in a particular application values slightly outside these ranges may also be effective. In addition to acting as a transport medium, the CO 2  can also be used as a cooling agent to cool the surface which shall be coated. In that case the temperature of the CO 2  may be below −20° C. Such temperatures may be achieved by using a cooling unit or a heat exchanger to cool down the CO 2  before it is applied to the surface. 
         [0019]    A cooling unit is in general advantageous since it can be used to control the temperature of the carbon dioxide and to assure that the temperature is below the critical temperature of CO 2 . More preferable, the cooling unit is used to subcool the liquid carbon dioxide in order to make sure that only carbon dioxide in its liquid phase is present. 
         [0020]    The invention can be used to deposit a material on numerous objects, such as metal, casting metal, glass, plastics, textiles, leather, food, drugs, pills, wood, furniture or any composite material. A preferred application of the invention is the coating of molds, for example molds made of metal, wood, plastic, gypsum or stone. 
         [0021]    According to a preferred embodiment the transport medium stream further contains one or more additives selected from the group consisting of a solvent or a reactive compound, especially butane, propane, or dimethyl ether, and combinations thereof. 
         [0022]    After the carbon dioxide and the material to be deposited, for example a release agent, have been brought together it is preferred to feed this combined stream through a static mixer in order to evenly distribute the material within the carbon dioxide. 
         [0023]    While any number of surfaces can be coated in accordance with the teaching of the present invention, most preferred are interior surfaces of molds. The invention in this regard can be used effectively with resin transfer molding (RTM), vacuum molding, compression molding, and open molding techniques. 
         [0024]    It will be appreciated by those of skill in the art that in one preferred embodiment of the present invention, the subcritical transport medium is used to apply coatings to the inside of mold surfaces wherein the coating combines with the molded article to form an outer surface of the molded article when ejected from the mold. This method is sometimes also referred to as in-mold coating. 
         [0025]    The present inventors have found particular and unique advantages of the present invention in forming mold release coatings on the inside of mold cavities. Accordingly, it is preferably used to apply a release agent to the surface of a mold. Thereby, an article which is formed in the mold does not stick to the mold. 
         [0026]    In one preferred embodiment, the present invention is first used with a mold release composition to line the mold cavity and then, utilizing a different coating composition in the inventive process, forming a coating on the mold release coating which adheres to the outer surface of the molded article. 
         [0027]    The processes of the present invention will be described with reference to forming mold release coatings, it being understood that the nature of the coating material selected is the most important variable which differentiates between forming an outer coating on a molded article and forming a release coating which facilitates easy removal of a molded article from a mold (i.e., to allow an article formed in said mold to release without substantially sticking to said mold surface). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The invention as well as preferred details of the invention are described in the following with reference to the accompanying drawings. Herein schematically illustrates 
           [0029]      FIG. 1  a set up for coating a mold with a material in accordance with a preferred embodiment of the invention and; 
           [0030]      FIG. 2  a set up for coating a mold with a material in accordance with another preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The following descriptions of various embodiments of the invention are meant to be exemplary only and are not intended to limit the scope of the invention, which is defined solely by the claims appended hereto. 
         [0032]      FIG. 1  illustrates schematically a device for depositing a coating formulation onto surface  1  in accordance with one preferred embodiment of the invention. A release agent is contained in reservoir tank  2  and can be withdrawn from it using a pump  3 . Optionally reservoir tank  2  is pressurized. A wide variety of release agents are suitable for use in the present invention, preferably selected from the group consisting of resins, waxes and fats, oils, silicon oils, soaps and tensides. For example, and not to be deemed limiting, suitable release agents are high-molecular-weight fatty acid amides, glycerol esters, synthetic polyethylene-based low-molecular-weight polymers, waxes such as carnauba waxes, montan waxes, beeswax, and compositions formulated with wax, silicone and surfactant parting agents. Preferably the release agent in reservoir tank  2  includes a film forming agent, for example water or a solvent. 
         [0033]    As will be appreciated in light of the present disclosure, the selection of a particular release agent will depend on a number of factors such as molding temperatures, pressures, mold residence time, and the materials used to form the molded article. Those skilled in the art will also recognize that the properties of waxes can be modified through various processes. 
         [0034]    The release agent is most preferably brought through an optional reduction valve  5  and check valve  6  to precision control valve  7  that permits a metered introduction of the release agent into a pre-mixing chamber  8 . In this illustrative embodiment, located upstream from precision control valve  7  is return conduit  9  with overflow valve  10  and bypass line  11 . Accordingly, in one embodiment it is preferred that a portion of the release agent is continuously being pumped back into reservoir tank  2  through return conduit  9  so that, preferably, there is release agent present at precision control valve  7  at all times, thereby reducing the deadtime and lagtime involved in supplying release agent to mixer  8  and surface  1 . 
         [0035]    The transport of the transport medium or carrier fluid carbon dioxide is drawn from fluid tank  12  in which the carbon dioxide is preferably kept at a pressure of from about 15 bar to about 20 bar and at a temperature from about −35° C. to about −20° C. and has a purity ranging from technical grade to bulk grade. An alternative to fluid tank  12  for the supply of carbon dioxide can be a bundle of gas cylinders with an ascending pipe which allows to withdraw liquid CO 2 . 
         [0036]    Preferably, fluid carbon dioxide is drawn out of fluid tank  12  via extraction conduit  13  and filtered by filter  14 , after which if flows to fluid pump  15 . A centrifugal pump and a piston pump are examples of two suitable pumps for fluid pump  15 , although other pumps may be suitable. The fluid carbon dioxide is thereby compressed to within the preferred range of subcritical pressure states, for example to 60 to 90 bar, and, for example, has a temperature of less than 25° C. Via fluid pump  15  it is brought to dosing valve  18  through an optional reduction valve  16  and check valve  17 . With the aid of dosing valve  18 , the precise amount of carbon dioxide desired can be fed into pre-mixing chamber  8 . 
         [0037]    Heating coils or the like may be used in connection with extraction line  13  (or a heat controllable filter  14 ) to control temperature of the fluid carbon dioxide if necessary or desired. Preferably the CO 2  after being compressed is cooled down in a heat exchanger  34  by indirect heat exchange with a cooling medium. By cooling the carbon dioxide after compression it is guaranteed that the carbon dioxide is in its liquid state when entering the pre-mixing chamber  8 . Preferably the carbon dioxide is subcooled in cooler  34 . 
         [0038]    Fluid pump  15  is preferably supplied with fluid at all times. For this reason, the compressed carbon dioxide is preferably recirculated into fluid tank  12  through ring line  19 , which is equipped with overflow valve  32  and bypass  33 . In one embodiment, ring line  19  thus preferably carries such a large stream of carbon dioxide that even at maximum capacity, i.e., when flow regulator  18  is feeding the maximum quantity of carbon dioxide into pre-mixing chamber  8 , fluid carbon dioxide is returned to fluid tank  12 . In another embodiment, fluid pump  15  is stopped when no CO 2  is needed and only started to maintain a particular CO 2  pressure, for example 70 bar. 
         [0039]    The release agent (having a preferred viscosity of from about 50 centipoise (0.05 Pa s) to about 20000 centipoise (20 Pa s)) is preferably fed to the liquid subcritical carbon dioxide flowing into pre-mixing chamber  8 . For release agents having a higher viscosity heating elements may be used to decrease the viscosity of the release agent. To accomplish this, the release agent is sprayed into pre-mixing chamber  8  and dispersed in the carbon dioxide stream. If desired, an additional substance in container  22 , for example butane, propane or dimethyl ether, can also be sprayed into the carbon dioxide stream, via conduit  21 . 
         [0040]    The mixing of the release agent, or in general of the material, and the carbon dioxide can be carried out by continuously spraying the material into the carbon dioxide flow or by discontinuously mixing a predefined amount of material with a predefined amount of carbon dioxide. 
         [0041]    The mixture of carbon dioxide, release agent and additional substance, if any, is most preferably mixed a second time in a static mixer  23  and subsequently sprayed onto surface  1  using spray nozzle  24  (for example, a nozzle, available from the Graco company). The pressure of the mixture of carbon dioxide and release agent at the nozzle  24  is about 80 bar. When the liquid carbon dioxide expands in spray nozzle  24 , the release agent breaks up into small droplets that are evenly dispersed or entrained in the spray. As it is discharged from nozzle  24 , the carbon dioxide then evaporates, leaving behind substantially no residue on surface  1 . 
         [0042]    The feed line for the release agent as well as the feed line for carbon dioxide are provided with flow meters  35 ,  36 . Flow meters  35 ,  36  measure the amount of release agent and the amount of carbon dioxide and send corresponding signals to a control unit  37 . Control unit  37  preferably controls control valve  7  and dosing valve  18  to provide a pre-determined ratio of carbon dioxide to release agent to the pre-mixing chamber  8 . 
         [0043]    Control valve  7  and/or dosing valve  18  are preferably designed a kind of as piston-pump which with each stroke feed a defined volume into pre-mixing chamber  8 . By changing the speed of the piston-pump the average amount of carbon dioxide or release agent fed to the pre-mixing chamber  8  can be changed. The piston-pumps are for example driven by a hydrodynamic or hydrostatic drive which allow to feed a defined amount of carbon dioxide or material independent of their pressure or temperature. 
         [0044]    To reduce over-spray an electrical voltage  38  of 40 to 85 kV is impressed between the nozzle  24  and the surface  1 . Thereby, corners and holes of the surface can be easier reached. The applied voltage depends on the flow rate of carbon dioxide and release agent. 
         [0045]      FIG. 2  illustrates another preferred arrangement for performing the process according to the present invention. Identical reference numbers refer to identical components in both figures. 
         [0046]    The process shown in  FIG. 2  differs from that in  FIG. 1  in the way the subcritical carbon dioxide is supplied. Here too, in a preferred embodiment the carbon dioxide is preferably stored in fluid form in a fluid tank  12 . However, this embodiment provides for a gas compressor  30  instead of fluid pump  15  for increasing the pressure of the transport medium. For reasons of redundancy, to enable the continuous supply of carbon dioxide in the event of a failure of gas compressor  30 , most preferably it is useful to install a second gas compressor  31  in parallel with gas compressor  30 . 
         [0047]    Gas compressor  30  preferably runs only when spray nozzle  24  is in operation and fluid carbon dioxide is to be sprayed out. A return loop such as ring line  19  in the embodiment in  FIG. 1  may not be necessary in this alternative preferred embodiment. In order to achieve a subcritical CO 2  temperature a cooler  34  is provided after compressor  30 . The carbon dioxide transported with gas compressor  30  is fed into pre-mixing chamber  8 , the release agent is admixed with the carbon dioxide, and the resulting release agent formulation is sprayed onto the mold via spray nozzle  24 . 
       EXAMPLE 
       [0048]    The inventive process is utilized to form a mold release coating on the inside of a mold cavity having a temperature of about 60° C. 7 parts of liquid, sub-critical carbon dioxide at a temperature of about 20° C. are mixed with 1 part of an active component consisting of 20% wax and 80% solvent as a film forming agent. That combination is sprayed onto the mold surface via a nozzle. The pressure of the combination of carbon dioxide and active component at the nozzle is about 80 bar. Thus a homogeneous layer of the active component is formed on the mold surface. Then a MDI-based cold cure foam formulation is introduced into the coated mold cavity and hardened. The resulting article can easily be released from the mold cavity without sticking to the mold surface. 
         [0049]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It should be understood that while preferred embodiments have been shown and described, all changes and modifications that come within the spirit of the invention are desired to be protected.