Field of the Disclosure

The current disclosure relates to anti-reflection foam sheets for use in systems with radio wave generating and receiving units.

Background

Radar units are used in a growing number of applications, including in automobiles for collision avoidance, situational awareness for autonomous and semi-autonomous driving, and other general sensing applications. Standing waves can be formed between two reflective surfaces at a fixed distance, including between the radar unit or a surface it is attached to and a protective covering, radome, or vehicle body portion.

Summary

The current disclosure relates to anti-reflection foam sheets for use in systems with radio wave generating and receiving units. In some embodiments, the anti-reflection unitary foam sheet comprising a first foam layer with a first major surface and a second major surface, where the first foam layer comprises a (meth)acrylate matrix with 2% by weight or less acidic content and a Tg of -25°C or greater, and expanded microspheres. The foam layer has a thickness of 0.05 - 3.00 millimeters, a density of 0.10 - 0.80 g/cm ³ , and is hydrophobic. The anti-reflection foam sheet may further comprise additional layers.

Also disclosed are methods for preparing foam sheets. In some embodiments, the method of preparing a foam sheet comprises providing a reaction mixture, partially polymerizing the reaction mixture to provide a curable, coatable syrup, providing expandable microspheres, dispersing the expandable microspheres and additional initiator in the curable, coatable syrup to form a filled curable, coatable syrup, coating the filled curable, coatable syrup on a first release liner to form a curable layer covering the curable layer with a second release liner, polymerizing the curable layer to form a cured layer, and exposing the cured layer to elevated temperatures to expand the expandable microspheres to form a first foam layer. The reaction mixture comprises at least two (meth)acrylate monomers, optionally 2% or less by weight of acidic (meth)acrylate monomer, and at least one initiator. The first foam layer comprises a (meth)acrylate matrix with a Tg of -25°C or greater, and expanded microspheres, where the first foam layer has a thickness of from 0.05 - 3.00 millimeters, a density of 0.10 - 0.80 g/cm ³ , and is hydrophobic.

Also disclosed are anti-reflection unitary foam sheets configured to be disposed between a radar unit and a body portion of a vehicle. The foam sheet layer has a density of about 0.10-0.80 g/cm ³ and comprises a plurality of expanded microspheres, such that when the unitary foam sheet is disposed between the radar unit and the body portion of the vehicle, and bonded to the body portion, the unitary foam sheet a strength of radio waves reflected by the unitary foam sheet when the unitary foam sheet is irradiated with 77 GHz radio waves in a direction perpendicular to the first primary surface from the first primary surface side is at least 10 dB lower than a strength of radio waves reflected by a flat metal surface when the metal surface is irradiated with 77 GHz radio waves in a direction perpendicular to the metal surface.

Brief Description of the Drawings

The present application may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings.

Figure 1 shows a cross sectional view of an embodiment of a foam sheet of this disclosure.

Figure 2 shows a cross-sectional view of an embodiment of another foam sheet of this disclosure.

Figure 3 shows an embodiment of a radar unit incorporating the foam sheet of this disclosure.

In the following description of the illustrated embodiments, reference is made to the accompanying drawings, in which is shown by way of illustration, various embodiments in which the disclosure may be practiced. It is to be understood that the embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure. The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.

Detailed Description

Radio wave generating and receiving units, such as radar (radio detection and ranging) units, may be useful in a diverse and growing application space. For example, as automobiles incorporate more and more sensors in order to enhance driver safety, sense and warn about vehicle surroundings and ambient conditions, and to enable partial or full autonomous driving functions, one or more radar units may be incorporated. For automotive radar applications, microwave generation and receiving units may be used, and so for purposes of this application “radar” and “radio waves” shall include microwave range frequencies as well. For power consumption, safety, and regulatory reasons, these radar units may be relatively low power when compared to those used for, as an example, air traffic monitoring applications. Accordingly, the signal to noise ratios of these lower power units may be more sensitive to interference or attenuation.

In order to protect these radar units from dirt buildup or weather elements such as snow and rain, or, in the case of rotating or moving components, to protect people from injury or accidental damage, the unit is typically protected with a cover. In some cases, this protective cover is referred to as a radome. These units are sometimes embedded within the body of the vehicle. In some embodiments, these units are placed behind or within the emblem or another vehicle fascia, which serves as the protective cover. Depending on the direction of interest, these radar units can be placed at any location on the vehicle. Typically, they are arranged so that the least amount of material is disposed between the radar unit and its potential — or intended — targets for detection.

However, because a protective cover is typically necessary or desirable to use in conjunction with these radar units, the radio waves generated by a radio wave generating unit and received by a radio wave receiving unit must pass through an interface including a sudden increase in electrical permittivity. Relative permittivity for a given frequency, which, as used herein is the ratio of a material’s permittivity to the permittivity of a vacuum, measures the resistance of a material to forming an electric field within itself. Sharp changes in this value — as would be encountered by a radio wave travelling in air at an interface with a non-air material, such as a plastic vehicle fascia, will cause at least some of the radio wave to be reflected at this boundary. The transmitted signal is reflected and detected, giving a false signal. Further, this reflection, combined with potential reflection off of the radar unit or its mount, can lead to the formation of standing waves which can further degrade the signal to noise ratio of the receiving unit by falsely appearing to be a signal.

Gradient permittivity films — analogous to antireflection films or coatings for optical interfaces, provide a smooth or stepped change in permittivity (versus a smooth or stepped change in refractive index for antireflection films) — from a first medium to a second medium. Typically, the gradient permittivity film’s permittivity varies from being closest to the permittivity of the first medium to being closest to the permittivity of the second medium. For example, the gradient permittivity film could have a varying permittivity that starts close to the permittivity of air on one side and transitions to the permittivity of a plastic vehicle fascia on the other side (which would be attached to the plastic vehicle fascia). This smooth or stepped transition can significantly reduce the dielectric boundary reflection that otherwise occurs at these sharp transitions.

Previous gradient permittivity films typically use varying bulk three-dimensional shapes, such as cones or pyramids. However, in a typical use environment where these films may be exposed to dirt accumulation and weather conditions, these films may become contaminated and ineffective, because they rely on the presence of air in order to provide the gradient in permittivity. Films described herein may be less susceptible to debris and contaminant ingress because a limited portion of the air or gas fraction is exposed to external elements, or in some embodiments the gas volume fraction is completely sealed within the film. In other words, both major surfaces of the gradient permittivity film are primarily a continuous matrix component.

Foam sheets, including in some embodiments multi-layer foam sheets, have been found to function as anti-reflection sheets because they contain entrapped air. However, foams can have drawbacks such as susceptibility to water ingress which can adversely affect the permittivity properties. Also, foams also tend to have rough surfaces that can adversely affect surface adhesion.

Disclosed herein are foam sheets that have the desirable property of hydrophobicity. The foam sheets are prepared from a partially polymerized curable methacrylate-based reaction mixture with expandable microspheres that are coated, cured and expanded. In some embodiments, the foam sheets comprise multiple foam layers. In some embodiments, the foam sheets also have low surface roughness. The foam sheets are suitable for being disposed between a radar unit the body portion of a vehicle.

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singular forms "a", "an", and "the" encompass embodiments having plural referents, unless the content clearly dictates otherwise. For example, reference to "a layer" encompasses embodiments having one, two or more layers. As used in this specification and the appended claims, the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

As used herein, the term “adjacent” refers to two layers that are proximate to another layer. Layers that are adjacent may be in direct contact with each other, or there may be an intervening layer. There is no empty space between layers that are adjacent.

The terms “Tg” and “glass transition temperature” are used interchangeably. If measured, Tg values are determined by Differential Scanning Calorimetry (DSC) at a scan rate of 10°C/minute, unless otherwise indicated. Typically, Tg values for (meth)acrylate copolymers are not measured but are calculated using the well-known Fox Equation, using the homopolymer Tg values provided by the monomer supplier, as is understood by one of skill in the art. In the present copolymers, since the Tg values are often measured as they do not just contain commercially available monomers.

The terms “room temperature” and “ambient temperature” are used interchangeably and have their conventional meaning, referring to temperatures of from 20-25°C.

The term “organic” as used herein to refer to a cured layer, means that the layer is prepared from organic materials and is free of inorganic materials other than the polysiloxane segments of the polyisobutylene-polysiloxane block copolymers. The organic layers are free of the inorganic additives, such as inorganic particles, that are frequently added to modify the properties of organic layers.

The term “(meth)acrylate” refers to monomeric acrylic or methacrylic esters of alcohols. Acrylate and methacrylate monomers or oligomers are referred to collectively herein as "(meth)acrylates”. The term “(meth)acrylate-based” as used herein refers to a polymeric composition that comprises at least one (meth)acrylate monomer and may contain additional (meth)acrylate or non- (meth)acrylate co-polymerizable ethylenically unsaturated monomers. Polymers that are (meth)acrylate- based comprise a majority (that is to say greater than 50% by weight) of (meth)acrylate monomers. The terms “free radically polymerizable” and “ethylenically unsaturated” are used interchangeably and refer to a reactive group which contains a carbon-carbon double bond which is able to be polymerized via a free radical polymerization mechanism.

The terms “polymer” and “oligomer” are used herein consistent with their common usage in chemistry. In chemistry, an oligomer is a molecular complex that consists of a few monomer units, in contrast to a polymer, where the number of monomers repeat units is, in theory, not limited. Dimers, trimers, and tetramers are, for instance, oligomers composed of two, three and four monomer repeat units, respectively. Polymers on the other hand are macromolecules composed of many monomer repeated units.

The term “alkyl” refers to a monovalent group that is a radical of an alkane, which is a saturated hydrocarbon. The alkyl can be linear, branched, cyclic, or combinations thereof and typically has 1 to 20 carbon atoms. In some embodiments, the alkyl group contains 1 to 18, 1 to 12, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, cyclohexyl, n-heptyl, n-octyl, and ethylhexyl.

The term “aryl” refers to a monovalent group that is aromatic and carbocyclic. The aryl can have one to five rings that are connected to or fused to the aromatic ring. The other ring structures can be aromatic, non-aromatic, or combinations thereof. Examples of aryl groups include, but are not limited to, phenyl, biphenyl, terphenyl, anthryl, naphthyl, acenaphthyl, anthraquinonyl, phenanthryl, anthracenyl, pyrenyl, perylenyl, and fluorenyl.

The terms “water absorption” and “water uptake” are used interchangeably and refers to the amount of water that is absorbed by an article as measured by the test method described in the Examples section.

Disclosed herein are anti -reflection unitary foam sheets. In some embodiments, the anti reflection unitary foam sheet comprises a first foam layer with a first major surface and a second major surface. The first foam layer comprises a (meth)acrylate matrix with 2% by weight or less acidic content and a Tg of -25°C or greater; and expanded microspheres. The foam layer has a thickness of 0.05 - 3.00 millimeters, has a density of 0.10 - 0.80 g/cm ³ , and is hydrophobic. In some embodiments, the foam layer has at least one surface roughness of 0.05 - 20.0 micrometers.

The (meth)acrylate matrix is prepared from a reaction mixture comprising at least two (meth)acrylate monomers and optionally 2% by weight or less of an acid-functional (meth)acrylate monomer as is described in greater detail below.

The hydrophobicity of the foam sheet can be measured in a variety of ways, typically it is measured by exposure of the foam to controlled conditions of heat and humidity for certain periods of time and determining the amount of water absorption. In some embodiments, the foam sheets of this disclosure have a water absorption of less than 7400 ppm after 24 hours at 85°C and 85% Relative Humidity. In other embodiments, the foam sheet has a water absorption of less than 5000 ppm after 24 hours at 85°C and 85% Relative Humidity. In some embodiments, the first foam layer has a relative permittivity of 1.1 - 4.0 at 76-77 GHz. Relative permittivity is a well understood scientific term that is its permittivity expressed as a ratio relative to the vacuum permittivity.

In some embodiments, the foam sheet further comprises an adhesive layer on the second major surface of the first foam layer. This adhesive layer can be utilized to attach the foam sheet to a substrate surface, such as the body portion of a vehicle. Any suitable adhesive may be used, such as a pressure sensitive adhesive, or a curable adhesive. Particularly suitable pressure sensitive adhesives include (meth)acrylate-based pressure sensitive adhesives, as these have high compatibility with the (meth)acrylate-based foam layer. Other examples of suitable pressure sensitive adhesives include silicone pressure sensitives. Examples of curable adhesives include both UV-curable adhesives and heat curable adhesives.

Figure 1 shows a cross sectional view of an embodiment of an article with a first foam layer. In Figure 1, foam sheet 100 include foam layer 110, with first major surface 111 and second major surface 112. Adhesive layer 140 is in contact with second major surface 112 of foam layer 110.

In some embodiments, the foam sheet comprises a multi-layer foam article. In these embodiments, the foam sheet described above further comprises a first substrate with a first major surface and a second major surface, where the first major surface of the first substrate is adjacent to the second major surface of the first foam layer. The first substrate can be a variety of substrate layers. In some embodiments, the first substrate comprises a second foam layer. In these embodiments, the second foam layer typically has a thickness of 0.05 - 3.00 millimeters and a density of 0.20 - 0.90 g/cm ³ . Generally, the second foam layer has a relative permittivity that is higher than the first foam layer. In some embodiments, the second foam layer has a relative permittivity of 1.2 - 4.0 at 76-77 GHz.

The foam sheet may comprise additional layers in addition to the first substrate. In some embodiments, the foam sheet further comprises a second substrate with a first major surface and a second major surface, where the first major surface of the second substrate is adjacent to the second major surface of the first substrate. Generally, the second substrate comprises a third foam layer with a thickness of 0.05 - 3.00 millimeters, and a density of 0.21 - 0.95 g/cm ³ . Generally, the third foam layer has a relative permittivity that is the same as or higher than the first foam layer and the second foam layer. In some embodiments, the third foam layer has a relative permittivity of 1.2 - 4.0 at 76-77 GHz.

In some embodiments, the foam sheet further comprises an adhesive layer on the second major surface of the second substrate. This adhesive layer can be utilized to attach the foam sheet to a substrate surface, such as the body portion of a vehicle. Any suitable adhesive may be used, such as a pressure sensitive adhesive, or a curable adhesive. Particularly suitable pressure sensitive adhesives include (meth)acrylate-based pressure sensitive adhesives, as these have high compatibility with the (meth)acrylate-based foam layer. Other examples of suitable pressure sensitive adhesives include silicone pressure sensitives. Examples of curable adhesives include both UV-curable adhesives and heat curable adhesives. Figure 2 shows a cross sectional view of an embodiment of a multi-layer article with multiple foam layers. In Figure 2, foam sheet 200 includes first foam layer 210, with first major surface 211 and second major surface 217. The foam sheet further includes first substrate 220 which is typically a second foam layer and has first major surface 221 and second major surface 222. The foam sheet includes optional second substrate 230 which is typically a third foam layer and has first major surface 231 and second major surface 232. Adhesive layer 240 is in contact with second major surface 232 of third foam layer 230. It should be understood that since third foam layer 230 is optional, in foam sheets without this layer, adhesive layer 240 would be in contact with second major surface 222 of second foam layer 220.

This disclosure further includes methods for preparing foam sheets such as the foam sheets described above. As mentioned above, the method permits the formation of foam sheets that have the desirable feature of hydrophobicity. In some embodiments, the foam sheets also have low surface roughness.

In some embodiments, the method of preparing a foam sheet comprises preparing a curable, coatable syrup filled with expandable microspheres, coating this syrup on a first release liner to form a curable layer, covering the curable layer with a second release liner, polymerizing the curable layer to form a cured layer, and expanding the expandable microspheres to from a foam layer. The foam layer has a thickness of from 0.05 - 3.00 millimeters, has a density of 0.10 - 0.80 g/cm ³ , and is hydrophobic. In some embodiments, the foam layer has a surface roughness of 0.05 - 20.0 micrometers.

In the above method, preparing a curable coatable syrup filled with expandable microspheres comprises providing a reaction mixture comprising at least two (meth)acrylate monomers, optionally 2% or less by weight of acidic (meth)acrylate monomer, and at least one initiator. The (meth)acrylate monomers are selected to give a polymer with a Tg that is -25°C or greater.

A wide range of (meth)acrylate monomers are suitable for use in the reaction mixture. Typically, the (meth)acrylate monomers are alkyl or aryl (meth)acrylate monomers where the alkyl or aryl group of the (meth)acrylate has an average of 1 to 20 carbon atoms. Since the reaction mixture comprises at least two (meth)acrylate monomers, a wide variety of combinations are suitable. Typically, the reaction mixture comprises a first (meth)acrylate monomer with a relatively low homopolymer Tg and a second (meth)acrylate monomer with a relative higher homopolymer Tg. Generally, the first (meth)acrylate monomer has an alkyl group with about 4 to about 14 carbon atoms and have a homopolymer Tg that is 0°C or lower. Examples include, but are not limited to, butyl acrylate, isooctyl acrylate, lauryl acrylate, iso-stearyl acrylate, 4-methyl-2-pentyl acrylate, isoamyl acrylate, sec-butyl acrylate, n-hexyl acrylate, 2- ethylhexyl acrylate, n-octyl acrylate, n-decyl acrylate, isodecyl acrylate, isodecyl methacrylate, and isononyl acrylate. Generally, the second (meth)acrylate monomer is an alkyl or aryl (meth)acrylate that has a homopolymer Tg that is above 0°C, often above 10°C. Examples include, but are not limited to methyl acrylate, methyl methacrylate, isobomyl acrylate, biphenylyl acrylate, t-butylphenyl acrylate, cyclohexyl acrylate, 4-tert-butylcyclohexyl acrylate, cyclic trimethylolpropane formal acrylate, dicyclopentanyl acrylate, dicyclopentenyl acrylate, dicyclopentenyl oxyethyl acrylate, dimethyladamantyl acrylate, 2-naphthyl acrylate, phenyl acrylate, N.N-dimethyl acrylamide, N,N-diethyl acrylamide, acryloyl morpholine, N-hydroxyethyl acrylamide, N-isopropyl acrylamide, N,N-dimethylamino propylacrylamide, N-vinyl pyrrolidone, N-vinyl caprolactam. Typically, if a hydrophilic monomer is used as the second (meth)acrylate monomer, the amount of such monomer is be less than 10%, more typically less than 5% so that the copolymer retains its hydrophobic nature.

Optionally, the reaction mixture may also include up to 2% by weight of an acid-functional monomer. Examples of acid-functional monomers include acrylic acid, methacrylic acid, and itaconic acid.

The reaction mixture also includes at least one initiator. Typically, the initiator or initiators comprise photoinitiators, meaning that the initiator is activated by light, typically ultraviolet (UV) light. Examples of suitable free radical photoinitiators include DAROCURE 4265, IRGACURE 184, IRGACURE 651, IRGACURE 1173, IRGACURE 819, LUCIRIN TPO, LUCIRIN TPO-L, commercially available from BASF, Charlotte, NC. Generally, the photoinitiator is used in amounts of 0.01 to 1 part by weight relative to 100 parts by weight of total reactive components.

The reactive composition components can be mixed by traditional methods known to those skilled in the art. Such methods include mixing, mechanical rolling, hot melt blending, etc. Such techniques are well known in the art.

The reactive mixture is then partially polymerized by exposure to UV radiation to give a curable, coatable syrup as described in, for example, US Patent No. 6,339,111 (Moon, et ah). Because the reaction mixture is only partially polymerized, the resultant coatable syrup remains a curable composition.

Expandable microspheres and additional initiator are dispersed into the curable, coatable syrup to form a fdled curable, coatable syrup. The additional initiator may be same initiator as was used to achieve partial polymerization or it may be a different initiator as described above. Generally, the photoinitiator is used in amounts of 0.01 to 1 parts by weight, more typically 0.1 to 0.5, parts by weight relative to 100 parts by weight of total reactive components.

A wide range of expandable microspheres are suitable. Suitable expandable microspheres are one with low density, high elasticity and low moisture absorption such as those commercially available from Nouryon under the trade name EXPANCEL such as EXPANCEL 920DU20. The amount of expandable microspheres added to the coatable syrup can vary broadly depending upon the nature of the expandable microspheres added and the desired properties of the formed foam layer. In some embodiments, the expandable microspheres have a maximum expansion ratio of 6.4, and the quantity of amount of expandable microspheres range from 0.05% by weight to 25.0 % by weight.

The fdled curable, coatable syrup is coated on a first release liner to form a curable layer using any suitable coating technique. The thickness of the curable layer can vary greatly, typically it is 0.02-2.5 millimeters. Release liners are well known in the adhesive arts and are fdm articles that have at least one surface that exhibits low adhesion to an adhesive, such as a pressure sensitive-adhesive (PSA), so that separation can occur substantially between the adhesive and release liner interface. Release liners typically have a release coating such as a silicone coating or a fluorocarbon coating. A wide range of release liners are commercially available.

The fdled curable, coatable layer is covered with a second release liner to form a curable layer between two release liners. The second release liner may be the same as or different from the first release liner. The second release liner may be added after the curable, coatable layer is formed or it may be added as the coating is formed. In some embodiments, the syrup is coated onto the first release liner and the second release liner is simultaneously contacted to the top surface of the coating.

The curable layer between two release liners is then polymerized to form a filled, cured (meth)acrylate-based matrix. Typically, the curable layer is cured by exposure to UV radiation. The cured layer is then exposed to elevated temperatures to expand the expandable microspheres to form a first foam layer. Typically, the cured layer is placed in an oven to expand the expandable microspheres. The temperature and time to which the cured layer is exposed depends upon the nature of the expandable microspheres used and the level of microspheres present.

The first foam layer thus formed is described in detail above, having a (meth)acrylate matrix with a Tg of -25°C or greater, and expanded microspheres, where the first foam layer has a thickness of from 0.05 - 3.00 millimeters, has a density of 0.10 - 0.80 g/cm ³ , and is hydrophobic. As mentioned above, the first foam layer has a water absorption of less than 7400 ppm after 24 hours at 85°C and 85% Relative Humidity, or even less than 5000 ppm after 24 hours at 85°C and 85% Relative Humidity. The first foam layer has a relative permittivity of 1.1 - 4.0 at 76-77 GHz. In some embodiments, the first foam layer has a surface roughness of 0.05 - 20 micrometers.

The formed first foam layer can be used to form multi-layer foam sheets by removing the second release liner and laminating the exposed second surface of the first foam layer to the first major surface of a first substrate comprising a second foam layer, wherein the second foam layer has a thickness of 0.05 - 3.00 millimeters and a density of 0.20 - 0.90 g/cm ³ . The second foam layers are described above in detail and have a relative permittivity of 1.2 - 4.0 at 76-77 GHz. The second foam layer may be laminated directly to the second surface of the first foam layer or an adhesive layer can be used to adhere the first major surface of the second foam layer to the second major surface of the first foam layer.

The second foam layer may be a foam layer prepared using the same method as used to form the first foam layer, or it may be a foam layer prepared by different methods. The second foam layer may be an acrylic foam tape such as Acrylic Foam Tape Series PX5000 available from 3M Company, St. Paul, MN. In some embodiments, the second foam layer may be a commercially available foam construction such as a polypropylene foam.

In some embodiments, the anti -reflection foam sheet may comprise additional layers. The foam sheet may comprise a second substrate, where the second substrate has a first major surface and a second major surface. In some embodiments, the second substrate comprises a third foam layer. The first major surface of the second substrate may be laminated to the second major surface of the first substrate (typically a second foam layer). The third foam layer has a thickness of 0.05 - 3.00 millimeters and a density of 0.30 - 0.90 g/cm ³ . The third foam layers are described above in detail and have a relative permittivity of 1.2 - 4.0 at 76-77 GHz. The third foam layer may be laminated directly to the second surface of the second foam layer or an adhesive layer can be used to adhere the first major surface of the third foam layer to the second major surface of the second foam layer.

The third foam layer may be a foam layer prepared using the same method as used to form the first foam layer, or it may be a foam layer prepared by different methods. The third foam layer may be an acrylic foam tape such as Acrylic Foam Tape Series PX5000 available from 3M Company, St. Paul, MN. In some embodiments, the third foam layer may be a commercially available foam construction such as a polypropylene foam.

Since the foam sheets are designed to be attached to the body portion of a vehicle, in some embodiments an adhesive layer can be disposed on the second major surface of the second substrate (typically a third foam layer). The adhesive layer is typically a pressure sensitive adhesive and may optionally be covered by a release liner to protect the adhesive surface until used.

Also disclosed are anti-reflection foam sheets. In some embodiments, the anti -re flection foam sheet comprises a laminate having a first primary surface and a second primary surface, where the laminate comprises a first foam layer having a thickness from 0.05 to 3.00 mm and a density from 0.10 to 0.85 g/cm ³ and comprising a plurality of expanded microspheres, and a second foam layer having a thickness from 0.05 to 3.00 mm and a density from 0.20 to 0.90 g/cm ³ and comprising a plurality of expanded microspheres, where the density of the second foam layer is greater than the density of the first foam layer and the first foam layer and the second foam layer are disposed from the first primary surface side in this order. When the second primary surface of the anti-reflection foam sheet is affixed to an adherend, the strength of radio waves reflected by the anti-reflection foam sheet and the adherend is PI when irradiated with 77 GHz radio waves in a direction perpendicular to the first primary surface from the first primary surface side. The strength of radio waves reflected by a flat metal surface is P0 when the metal surface is irradiated with 77 GHz radio waves in a direction perpendicular to the metal surface, and the adherend has a relative permittivity of from 2.5 to 2.9 at 76 GHz and has a thickness of from 4 to 10 mm in a direction perpendicular to the first primary surface. In the current disclosure, P1-P0 is -10 dB or less.

Figure 3 shows a cross sectional view of an anti-reflection foam sheet of this disclosure is disposed between a radar unit and the body portion of a vehicle. In Figure 3, sheet 300 is an anti-reflection foam sheet that may be a single foam layer sheet as described in Figure 1 or a multi-layer foam sheet as described in Figure 2. Sheet 300 includes foam layer or layers 301 and adhesive layer 340. Sheet 300 is located between the body potion of a vehicle 360 and radar unit 370. Radar unit 370 includes case 371, antenna 372, printed circuit board 373, and radome 374. In Figure 3, sheet 300 is attached to body portion 360 by adhesive layer

Examples These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. All parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight, unless noted otherwise. Solvents and other reagents used were obtained from Sigma-Aldrich Chemical Company; Milwaukee, Wisconsin unless otherwise noted. The following abbreviations are used: cm = centimeters; mm = millimeters; nm = nanometers; pm = micrometers; RPM = revolutions per minute; g = grams; kg = kilograms; mL = milliliters; mJ = milliJoules; RH = Relative

Humidity.

Table of Abbreviations Test Methods

Property measurements

The thickness of the sheet before and after the foaming was measured by a thickness gauge (Mitutoyo 543-500B). The foam sample was cut into 5 cm X 5 cm and then weighed. The density was calculated with the thickness and the weight of the samples.

Tg Calculation

Tg of the acrylate polymers was calculated by the Fox equation based on the Tg of the homopolymers ahown in Table 1.

Fox equation:

Where T _gmix and T _gi are the glass transition temperature of the copolymer and the homopolymer Tg of each component respectively, and OJ, is the mass fraction of component i.

Table 1

Relative Permittivity

Permittivity and tan d were measured by RF impedance analyzer HP4291B + 16453 A at lGHz and Material analyzer system DPS 10-02 (KEYCOM) + Network analyzer PNA N5227A 10 MHz-67 GHz (Keysight) + milli wave module WR10+ 67 GHz-115 GHz (Virginia Diodes Inc.) at 76GHz.

Water uptake

Samples were stored in 85°C, 85% RH chamber more than 24 hours, and water uptake was measured by Karl Fischer titration method, (MCCAT, KF-31 & VA-200), and the vaporing condition was 120°C.

Surface smoothness (Ra)

Surface smoothness Ra was measured by Laser microscope (Olympus OLS4100) using 405 nm.

Average cell diameter and observation of cross section Average cell diameter was obtained by microscope (X200) (Keyence VHZ20) from measuring 5 beads. Cross section of sheet was obtained by the same microscope (X50).

Radio wave reflection Measurement method : Sparameter Condition 75-11 OGHz sweep

Equipment: Network analyzer PNA-X N5242A (Agilent),

Spectrum Analyzer N9030A (Keysight),

HarmonicMixer 11970W (Agilent), 11970W (Agilent),

AntennaHom MSGH 10-25 (Microwavefactory)

Millimeter-Wave Source Module 83558(HP)

W-BAND amp MPA0208-34 power amp 8349B-1 E8257DS12(HP) power supply amp Source module E8257DS12(HP)

Change in Foam Thickness After Heating (Swell Thickness)

The obtained foam sample was put on a glass plate whose surface was covered with a PTFE Glass Cloth Tape No. 5453 (available from 3M Company), then put in an oven whose temperature was set at 105°C for 24 hours. After the heat aging, the foam was cooled down at room temperature, and the foam thickness was measured again by the thickness gauge (Mitutoyo 543-500B). The thickness change after the heat aging was recorded as % change.

Example 1

0.750 gram of Dis-1, 0.750 gram of Dis-2, 0.225 gram of PI, 90.0 grams of ISTA, and 60.0 grams of IBOA were weighed and put in a 225 mL glass jar, then mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. Oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 225 mL glass jar for 1 minute. Polymer-containing viscous syrup was obtained.

51.6 grams of the syrup prepared above and 9.0 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes.

The pre-adhesive syrup was then poured between two liners (Liner- 1 and Liner-2) on a coating line where the coater head was set so that the gap between the two liners was 0.28 mm. The composition was then exposed to a UV light source of black light fluorescent lamps having a peak emission wavelength of 350 nanometers to provide an approximate total energy of 7790 mJ/cm ² until copolymerization was complete. The acrylate monomer mixture between the two liners was converted into an acryl polymer sheet. The acryl sheet was put in an oven at 150°C without removing the PET liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 1.11 mm and a density of 0.26 g/cm ³ was obtained. Example 2

48.6 grams of the syrup prepared in example 1, and 12.0 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way as described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.22 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 1.04 mm and a density of 0.21 g/cm ³ was obtained.

Example 3

45.5 grams of the syrup prepared in example 1, and 15.0 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.17 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 1.01 mm and a density of 0.18 g/cm ³ was obtained.

Example 4

0.50 gram of Dis-1, 0.50 gram of Dis-2, 0.20 gram of PI, 90.0 grams of 2EHA, and 60.0 grams of IBOA were weighed and put in a 225 mL glass jar, then mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. Oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 225 mL glass jar for 1 minute. Polymer-containing viscous syrup was obtained.

48.2 grams of the syrup prepared above and 12.0 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.22 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 1.06 mm and a density of 0.20 g/cm ³ was obtained.

Example 5

0.30 gram of Dis-1, 0.30 gram of Dis-2, 0.09 gram of PI, 56.4 grams of ISTA, and 43.6 grams of IBOA were weighed and put in a 225 mL glass jar, then mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. Oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 225 mL glass jar for 1 minute. Polymer-containing viscous syrup was obtained.

8.56 grams of the syrup prepared above and 1.5 grams of EMS-2 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.4 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 1.84 mm and a density of 0.22 g/cm ³ was obtained.

Example 6

8.06 grams of the syrup prepared in example 5, and 2.0 grams of EMS-2 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.4 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 2.51 mm and a density of 0.16 g/cm ³ was obtained.

Example 7

7.55 grams of the syrup prepared in example 5, and 2.5 grams of EMS-2 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.4 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 2.95 mm and a density of 0.14 g/cm ³ was obtained.

Example 8

9.39 grams of the syrup prepared in example 1, and 0.67 gram of EMS-2 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.25 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.64 mm and a density of 0.41 g/cm ³ was obtained.

Example 9 8.86 grams of the syrup prepared in example 1, and 0.19 gram of EMS-2 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.32 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.44 mm and a density of 0.75 g/cm ³ was obtained.

Example 10

1.50 grams of Dis-1, 0.45 gram of PI, 90.0 grams of 2EHA, and 270.0 grams of IBOA were weighed and put in a 450 mL glass jar, then oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 450mL glass jar for 1 minute. A polymer-containing viscous syrup was obtained.

37.1 grams of the syrup prepared above and 2.412 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.23 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.51 mm and a density of 0.45 g/cm ³ was obtained.

Example 11

37.0 grams of the syrup prepared in example 10, and 2.948 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.23 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.56 mm and a density of 0.42 g/cm ³ was obtained.

Example 12

0.90 gram of Dis-1, 0.27 gram of PI, 90.0 grams of 2EHA, and 90.0 grams of IBOA were weighed and put in a 450 mL glass jar, then oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 450mL glass jar for 1 minute. A polymer-containing viscous syrup was obtained.

37.1 grams of the syrup prepared above and 0.30 gram of EMS-3 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.46 mm. An acrylic foam having a thickness of 0.46 mm and a density of 0.76 g/cm ³ was obtained.

Example 13

37.0 grams of the syrup prepared in example 12, and 0.60 gram of EMS-3 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.46 mm. An acrylic foam having a thickness of 0.48 mm and a density of 0.59 g/cm ³ was obtained.

Example 14

37.0 grams of the syrup prepared in example 12, and 0.90 gram of EMS-3 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.46 mm. Through this post-foaming process, an acrylic foam having a thickness of 0.50 mm and a density of 0.50 g/cm ³ was obtained.

Example 15

3.60 grams of Dis-2, 0.54 gram of PI, 180.0 grams of 2EHA, and 180.0 grams of IBOA were weighed and put in a 450 mL glass jar, then oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 450 mL glass jar for 1 minute. A polymer-containing viscous syrup was obtained.

50.6 grams of the syrup prepared above and 2.177 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.30 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.60 mm and a density of 0.53 g/cm ³ was obtained.

Example 16

3.60 grams of Dis-2, 0.54 gram of PI, 144.0 grams of 2EHA, and 216.0 grams of IBOA were weighed and put in a 450 mL glass jar, then oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 450 mL glass jar for 1 minute. A polymer-containing viscous syrup was obtained.

50.6 grams of the syrup prepared above and 2.177 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.30 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.60 mm and a density of 0.54 g/cm ³ was obtained.

Example 17

3.60 grams of Dis-2, 0.54 gram of PI, 108.0 grams of 2EHA, and 252.0 grams of IBOA were weighed and put in a 450 mL glass jar, then oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 450 mL glass jar for 1 minuet. A polymer-containing viscous syrup was obtained.

50.6 grams of the syrup prepared above and 2.177 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.30 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.59 mm and a density of 0.54 g/cm ³ was obtained.

Example 18

3.60 grams of Dis-2, 0.54 gram of PI, 90.0 grams of 2EHA, and 270.0 grams of IBOA were weighed and put in a 450 mL glass jar, then oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 450 mL glass jar for 1 minuet. A polymer-containing viscous syrup was obtained.

50.6 grams of the syrup prepared above and 2.177 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.30 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.59 mm and a density of 0.54 g/cm ³ was obtained.

Example 19

50.6 grams of the syrup prepared in example 18, and 1.651 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.34 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.63 mm and a density of 0.60 g/cm ³ was obtained.

Example 20

27.8 grams of the syrup prepared in example 1, and 2.010 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.21 mm. The acryl sheet was put in an oven at 150°C without removing the liners on both side of the sheet for 10 minutes to expand the microspheres. Through this post-foaming process, an acrylic foam having a thickness of 0.54 mm and a density of 0.41 g/cm ³ was obtained.

Example 21

The acrylic foam obtained in example 20 was laminated with an Acrylic Foam Tape PX5005 whose thickness is 0.50 mm. The anti-reflection performance of this laminated foam tape was evaluated by measuring the reflection level described below.

A metal plate (aluminum plate, 3 mm thick) having a flat metal surface was prepared. The intensity P0 (dB) of the reflected radio wave when the metal plate was irradiated with a radio wave of 75 GHz to 110 GHz in a direction perpendicular to the main surface was measured by the S-parameter method at 77GHz and 80GHz. A radio wave reflection reduction sheet was attached to the surface of a polycarbonate molded body (relative permittivity: 2.7 / 76 GHz, thickness: 7.2 mm) prepared as an adherend in a direction in which the second main surface was in contact with the adherend. Subsequently, the intensity P 1 [dB] of the reflected radio wave when the radio wave reflection reducing sheet was irradiated with a radio wave of 77 GHz or 80 GHz in a direction perpendicular to the main surface of the radio wave reflection reducing sheet was measured by the S-parameter method. Reflection Level was calculated by the following formulas.

Reflection Level (dB) = PI - P0

The reflection level of the laminated acrylic foam tape was -16.1dB(77GHz), -13.2dB(80GHz). Comparative example 1

0.50 gram of Dis-1, 0.15 gram of PI, 90.0 grams of 2EHA, and 10.0 grams of AA were weighed and put in a 225 mL glass jar, then mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. Oxygen gas was purged out by bubbling nitrogen gas in the monomer mixture for 10 minutes, then 350 nm black light lamp was turned on to irradiate UV light to the 225 mL glass jar for 30 seconds. A polymer-containing viscous syrup was obtained.

8.1 grams of the syrup prepared above and 2.0 grams of EMS-1 were weighed into a plastic cup and mixed by the Planetary Centrifugal Mixer at 2000 rpm rotation for 2 minutes. The composition was polymerized and then foamed in the same way described in example 1, except the gap of the coater head was set so that the gap between the two liners was 0.23 mm.

Table 2: Foam Properties * Relative permittivity at 76 GHz of Example 5-14 & 20, Comparative example 1 in Table 2 were calculated values based on the measured value of relative permittivity at 1 GHz. The correlation between the values were confirmed to be linear by the relative permittivity values of Example 1, 2, 3 and the equation of the trend line was used for the value conversion. y=0.8052x+0.101 where y is the relative permittivity value at 76GHz and x is the relative permittivity value at 1GHz.

Table 3: Foam Properties