Patent Application
20240084147
This disclosure relates to coatings that absorb in the visible band and transmit in the near infrared (NIR) and short-wave infrared (SWIR) bands.
Spectral coatings provide specified transmittance, reflectance or absorption in different bands of the electro-magnetic spectrum. One particular class of coatings provides high absorption in the visible band and high transmissivity in the NIR and SWIR bands. The Visible band spans approximately 400-700 nm. As used herein, the NIR band spans 800-1300 nm and the SWIR band spans 1300-2200 nm. The coatings may be formed from plastics or paint that include SWIR transmissive pigments or dyes or structural layers such as nanofeatures and interference layers or multi-layer films. The coatings may be used, for example, to cover or obscure in an aesthetic manner some type of IR body or device such as an IR Transmit/Receive sensor (Lidar in a car or IR communication on portable devices), solar cells on buildings, or a reflective surface as part of a Cool Coating on a building. The coating provides a color, typically black, that hides what is behind the coating.
US Publication 2021/0096288 entitled “Infrared Transmissive Product” published Apr. 1, 2021 discloses a black layer including a coating film layer that is formed by combining a transparent plastic and at least two types of dyes/pigments that have an infrared transmissivity and produce a black color when mixed together. The black layer has a thickness in a range from 5 microns to 50 microns (0.2 to 2 mil). The black layer contains 50 to 150 parts by mass of dyes/pigments as a whole in relation to 100 parts by mass of transparent plastic.
U.S. Pat. No. 7,727,418 entitled “Infrared Transmissive Thermoplastic Composition and Articles Formed Therefrom” issued Jun. 1, 2010 discloses a thermoplastic composition comprising a thermoplastic polymer, and a dye combination comprising a black dye, and a fluorescent dye, wherein a molded article having a thickness of 2.0 millimeters and consisting of the thermoplastic polymer, the black dye, and the fluorescent dye, has a percent transmission of infrared light according to ASTM D1003-00 of greater than or equal to 50%, when measured at a wavelength of 800 to 1,100 nm, and a percent transmission of visible light, according to ASTM D1003-00 of less than or equal to 15%, when measured at a wavelength of 400 to 650 nm.
Roof coatings are used, in part, to reflect electromagnetic energy to keep a building cool. However, the coolest white roof coatings desired to reflect in the Visible and NIR/SWIR bands are often not accepted by homeowners with visible roofs for aesthetic reasons. Black coatings, which are more aesthetically acceptable, must absorb practically all of the visible band to provide a black appearance. In addition, standard black coatings absorb in the NIR and SWIR bands, greatly raising the roof surface temperature. One approach is to provide a black spectral coating that is reflective in the NIR and SWIR bands. Another approach is to provide a black spectral coating that is transmissive in the NIR and SWIR bands that is positioned over a white coating or an aluminum alloy that reflect in the NIR and SWIR bands. Incident IR radiation passes through the coating and is reflected off of the white coating/aluminum alloy back through the coating instead of being absorbed.
Tore Kolas et. al., “Cool coatings with high near infrared transmittance for coil coated aluminum”, Solar Energy Materials and Solar Cell 196 (2019) 94-104. As described at section 2.3, “Each formulation included the major four prime components in a coating: pigment (NIR-transparent), organic polymer (high durable commercially available polyester), organic solvents and additives (defoameter and dispersion additive). Catalyst and cross link additives were already included in the commerically produced polymer. In one formulation, the pigment was BASF Paliogen Black S0084 at a weight percent of 5%. The formulations were applied to pre-treated Al sheets with an average dry film thickness (DFT) of about 20 μm (0.8 mil) and cured in an oven with heated air at 232 degrees Celsius to cross-link the polymer bonds.
Jie Qin et. al., “The Optical Properties of Black Coatings and Their Estimated Cooling Effect and Cooling Energy Savings”, Journal of Power and Energy Engineering, 2014, 2, 68-75 disclose a formulation for a cool black coating including a pure acrylic emulsion, a black pigment, an extender pigment such as Talcum that is transparent and non-reflective throughout the visible and NIR/SWIR bands and other appropriate paint additives such as a wetting agent, a dispersant, an anti-foaming agent, a leveling agent and a coalescent. To prepare the cool black coatings, the acrylic emulsion and talcum were first added into the mixing setup, followed by the addition of the wetting agent, dispersant and leveling agent. The mixture was stirred and then a prefabricated black pigment dispersion was pumped into the paint mixing setup and stirred. Water was added to adjust the viscosity of the coatings. The cool black coatings were applied with a thickness of 100 to 150 microns (4-6 mil) to a bare aluminum alloy substrate. As shown in Table 1, the black pigment, talcum and dispersant have weight % of 5.0, 25.0 and 0.5 respectively. Based only on the black pigment, the dispersant-over-pigment (DOP) ratio is 0.1. This ratio produces a relatively high concentration of pigment in the coating.
Based on the application for cool coatings, the critical parameter is not transmittance of the coating in the NIR/SWIR band but is the total reflectance off of the cool black coating and the aluminum alloy/reflective base coat. This coating may achieve the total reflectance through a combination of direct reflectance off of the cool black coating and transmittance through the coating and reflectance off of the alloy or base coat. This may have the beneficial effect of reducing the transmittance specifications of the coating in the NIR and SWIR bands.
US Publication 2021/0096288 mentions at para [0005] formation of the black layer by applying a paint made of a single black pigment such as carbon black or Ketjen black. However, the light transmissivity rises in a section of the wavelength region of visible light rays that is slightly lower than the boundary with the wavelength region of infrared rays. This suggests that a paint containing a single black pigment does not provide adequate absorption in the visible band for a NIR/SWIR transparent coating.
The present specification discloses a water-based acrylic latex paint having a dispersant-over-pigment (DOP) ratio between 0.5 and 1.5 to form a coating in which the pigment has a low concentration and is highly dispersed throughout to absorb in the visible band to produce a color (e.g., black) and is transmissive in the NIR and SWIR bands.
In an embodiment, the variables that determine absorption in the visible band and transmission in the NIR and SWIR bands include the DOP ratio between 0.5 and 1.5, a weight percentage of the pigment between 1-2% wt. in the paint formulation and a thickness of the coating between 2 and 4 mil. More typically, the DOP ratio is between 0.8 and 1.5.
In an embodiment in which the visible band spans 300-700 nm, the NIR band spans 800-1300 nm and the SWIR band spans 1300-2200 nm, the coating has an average transmittance of less than 20% over the visible band and average transmittances greater than 60% transmissive over the NIR band and greater than 80% transmissive over the SWIR band. Preferably, the transmittance specification is satisfied at each wavelength within the bands. An exemplary formulation that meets these specifications includes a binder of RHOPLEX™ AC-261F, a pigment of Paliogen® Black L0086 and a dispersing agent is EFKA® PX4310 in which the DOP ratio is approximately 1 and that coating dries with a thickness of 2.5 mil with a black color.
In an embodiment, the pigment is selected to provide a cut-off wavelength between the visible and NIR bands. A single pigment may be used to provide a “black” color with the requisite spectral properties. Additional pigments may be included to provide non-black colors although this may reduce the transmissivity in the NTR and SWIR bands.
In an embodiment, to control the viscosity of the pre-mixture, the dispersing agent is a high molecular weight compound of over 2,000 grams per mole. The dispersing agent suitably includes an acrylic-based block co-polymer including an amine-functional block to anchor onto the pigment.
In an embodiment, the rheology modifier is selected to provide “associative thickening” to the paint. The rheology modifier suitably includes a hydrophobically modified alkali-soluble rheology modifier.
In an embodiment, the pigment is mixed with the dispersing agent to exhibit a DOP ratio between 0.5 and 1.5. Mechanical crush energy is applied to the pre-mixture to de-aggregate pigment particles. The pre-mixture is then mixed with the solvent, binder and rheology modifier to form the paint.
In different embodiments that paint may be used to provide a coating to cover or obscure in an aesthetic manner some type of IR body or device. For example, the paint may be applied to a base having a transmissivity that spans the NIR and SWIR bands to cover transmitting and receiving units for an IR sensor. Other applications of the paint formulation in which good absorption in the visible band and high transmission in the NIR and SWIR bands are contemplated by this disclosure.
These and other features and advantages of this disclosure will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
The present disclosure provides a water-based acrylic latex paint that when applied dries to form a coating that absorbs in the visible band to produce a visible color (e.g., black) and is transmissive in the NIR and SWIR bands.
A commercially available acrylic paint is a fast-drying paint made of pigment suspended in acrylic polymer emulsion and plasticizers, silicon oils, defoamers, stabilizers, or metal soaps. Most acrylic paints are water-based but become water resistant when dry. Water-based acrylic paints are used as latex house paints, as latex is the technical term for a suspension of polymer microparticles in water. An inorganic or organic pigment is a colorized material that is completely or nearly insoluble in water. Color of pigments arises because they absorb only certain wavelengths of visible light. The bonding properties of the material determine the wavelength and efficiency of light absorption. Absorption over the entire visible band produces a black color. These commercially available paints also absorb light in the NIR and SWIR bands. A dispersing agent is a substance, typically a surfactant, that is added to a suspension of pigment particles in the solvent (water) to improve the separation of the particles and to prevent their settling or clumping. A DOP ratio of approximately 0.2 is typical for acrylic paints. A high concentration of pigment (e.g., at least 20% by weight) is required to achieve maximum coverage and absorption of visible light. A rheology modifier is typically added to the mix to form a non-Newtonian fluid that provides “associative thickening” so that when the paint is applied to form a coating it does not “bead up” but remains as a uniform thin coating. A defoamer is a chemical additive the reduces surface tension to prevent the formation of bubbles.
To provide a water-based acrylic latex paint that is highly absorptive in the visible band and highly transmissive in the NIR and SWIR bands without sacrificing the formability characteristics (e.g., not runny, not sag, clean film, no brush strokes etc.) and ambient drying characteristics requires more than just substitution of a NIR/SWIR transparent pigment for a standard color pigment.
Referring now to
The formulation of the water-based acrylic latex paint that produces coating 10 to meet such transmittance specifications requires a pigment that has a low concentration (e.g., <5.2% wt.) in the dried coating and is highly dispersed (e.g., pigment particle diameter <0.8 microns) throughout the coating. If the concentration is too high, the transmittance in the NIR and SWIR bands will not meet the specification. If the pigment is not adequately dispersed, the absorption in the visible band will suffer and the specification will not be met.
To achieve this, a formulation of the water-based acrylic latex paint includes an acrylic latex binder, a solvent of water, a pigment, a dispersing agent, a rheology modifier and a defoamer. The binder and dispersive agent components being transmissive in the NIR and SWIR bands (e.g., >90% on average). The pigment has a cut-off wavelength that lies between the visible and NIR bands. Below the cut-off the pigment is absorptive and above the cut-off the pigment is transmissive.
The mixture has a dispersant-over-pigment (DOP) ratio between 0.5 and 1.5 and more typically between 0.8 and 1.5. This ensures the low concentration of pigment in the coating. To ensure that the pigment is highly disperse throughout the coating, the dispersing agent preferably has a molecular weight higher than 2,000 grams per mole. Furthermore, dispersing agents that have a block co-polymer including an amine-functional block to anchor onto the pigment have better absorption characteristics in the visible band than lignosulfonate-based dispersing agents.
The rheology modifier is suitably selected to provide non-Newtonian fluid that provides “associative thickening” to the formulation. A hydrophobically modified alkali-soluble rheology modifier is a good choice for sag resistance as it forms networks through inter-connections of, for example, hydrophobic acrylic ester units to provide the associative thickening.
Referring now to
Referring now to
As shown in
Each ingredient in the formulation was identified based on maximizing transparency in the NIR and SWIR bands while minimizing transparency in the visible band. However, upon blending, it was noted that drastically different films were being made, in terms of film formation as well as end-properties, depending on other factors that had to be controlled. Achieving the desired end-properties in the formed film is predicated on 1) the spectral properties of each ingredient in the formulation (i.e., the “component level”; 2) their dispersion in the cast layer (i.e., the “interaction level”); and 3) the structure that all ingredients in aggregate form within the coating (i.e., the “system level”).
Referring now to
As shown in plot 90, the transmissivity 84 for a 2.5 mil coating is on-average less than 20% in the visible band, greater than 60% in the NIR band and greater than 80% in the SWIR band. The transmissivity satisfies the specifications at virtually every wavelength in each of the bands. Note, the drastic reduction in transmissivity at approximately 2300 nm is caused by the acrylic latex binder. Note, the transmissivities 86 and 88 for 7 and 9 mil coatings are highly absorptive in the visible band because of the additional thickness. However, the transmissivity fails to meet spec in the NIR/SWIR bands.
In different embodiments that paint may be used to provide a coating to cover or obscure in an aesthetic manner some type of IR body or device. For example, the paint may be applied to a base having a transmissivity that spans the NIR and SWIR bands to cover transmitting and receiving units for an IR sensor. Other applications of the paint formulation in which good absorption in the visible band and high transmission in the NIR and SWIR bands are contemplated by this disclosure.
Referring now to
While several illustrative embodiments have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
