Patent Publication Number: US-2023158776-A1

Title: Wallpaper and method for manufacturing same

Description:
TECHNICAL FIELD 
     This disclosure is generally related to wallpapers for buildings and methods for manufacturing the wallpapers, and more specifically to energy efficient wallpapers for buildings and methods for manufacturing the energy efficient wallpapers. 
     BACKGROUND 
     Wallpapers can be applied to the exterior and/or interior of a building for various purposes, such as cooling, heating, protection, decoration, waterproofing, noise reduction, etc. Most of the decorative wallcoverings focus on decorative purpose in the commercial environments such as hotels, hospitals, apartment buildings, retail outlets, and schools. Some materials in use for this type of wallcoverings are vinyl, mica, grass-cloth, linen, paper-weave, silk, and wood veneer. Some specialized wallcoverings offer sound reduction. Examples where these specialized wallcoverings are applied include meeting rooms, offices, auditoriums, restaurants, as well as corridors and elevator lobbies. Materials for sound reduction include polyolefin and olefin fibers, and cork/cork veneer. 
     No existing wallcovering is designed to reduce energy cost and consumption of the building. For example, in general 13% of total energy is consumed by heating and cooling of the building envelopes. It is desirable to develop a wallpaper for buildings that can reduce the energy consumption. 
     SUMMARY 
     Described herein are wallpapers that can be applied to exterior and/or interior of a building to reduce energy consumption of the building structures. 
     In one aspect, a wallpaper includes a support portion having a flat surface, and a functional layer disposed on the support portion. The functional layer includes a metallic film and has an emissivity of greater than zero and equal to or less than 0.6. The wallpaper further includes a particulate layer disposed on the functional layer. The particulate layer includes a polymer body and inorganic particles dispersed in the polymer body. 
     In some embodiments, the support portion includes one of fabric-backed vinyl, paper-backed vinyl, non-woven fabric, polyester, wood, metal, stucco, clad, brick, masonry, stone, steel, cement, or concrete. 
     In some embodiments, the wallpaper further includes a first adhesive layer disposed between the support portion and the functional layer. 
     In some embodiments, the functional layer further includes a polymer layer in contact with the first adhesive layer. In some embodiments, the polymer layer includes polyethylene terephthalate. In some embodiments, the polymer layer includes polyethylene, polypropylene, polylactide, poly(glycolic acid), or polybutylene succinate. 
     In some embodiments, the metallic film includes one of aluminum, silver, titanium, polished copper, brass, tin, gold, or a combination of the foregoing metals. 
     In some embodiments, the wallpaper further includes a second adhesive layer disposed between the functional layer and the particulate layer. The polymer of the second adhesive layer and the polymer body include polyethylene. 
     In some embodiments, a thickness of the second adhesive layer is less than a thickness of the particulate layer. 
     In some embodiments, the inorganic particles includes one or more of ZnO, TiO 2 , iron oxide, Fe 2 O 3 , Prussian blue, or silicon. 
     In some embodiments, the particulate layer further comprises a UV stabilizer. 
     In some embodiments, the inorganic particles include nanoparticles or submicron particles. 
     In some embodiments, the wallpaper further includes a decorative layer and/or a protective layer disposed on the particulate layer. 
     In another aspect, a method for forming a wallpaper is provided. The method includes: mixing inorganic particles with a polymer material to form a composite material, the polymer material including a polymer; extruding the composite material to form a particulate layer; laminating a polymer layer on a first surface of a metallic film to form a functional layer; thermally laminating the particulate layer with the functional layer such that the particulate layer is bonded with the functional layer; and adhering a support portion to the functional layer to form the wallpaper. 
     In some embodiments, before extruding the composite material to form the particulate layer, the method further includes: infusing the composite material with an additional material comprising the first polymer. 
     In some embodiments, the method further includes forming a decorative layer on the particulate layer. In some embodiments, the decorative layer is formed by a printing technique, such as flexographic printing, block printing, flatbed screen printing, gravure printing, rotary screen printing, digital printing, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain features of various embodiments of the present technology are set forth with particularity in the appended claims. A better understanding of the features and advantages of the technology will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the disclosure are utilized, and the accompanying drawings of which: 
         FIG.  1    is a diagram illustrating a wallpaper according to one example embodiment. 
         FIG.  2    is a diagram illustrating another wallpaper according to one example embodiment. 
         FIG.  3    is a diagram illustrating yet another wallpaper according to one example embodiment. 
         FIG.  4    is a diagram depicting how a thickness of a polymer film above a functional layer affects emissivity of a wallpaper, according to one example embodiment. 
         FIG.  5    is a diagram illustrating emissivities of the disclosed wallpaper and various competitive wallpapers. 
         FIG.  6 A  is a schematic diagram depicting a testing environment for testing performance of various wallpapers, according to one example embodiment. 
         FIG.  6 B  is a diagram illustrating testing results employing the testing environment as shown in  FIG.  6 A . 
         FIG.  7    is a diagram showing a summary of testing results employing the testing environment as shown in  FIG.  6 A . 
         FIG.  8    is a flow chart depicting a method for forming a wallpaper, according to some example embodiments. 
         FIG.  9    is a flow chart depicting another method for forming a wallpaper, according to some example embodiments. 
         FIG.  10    is a flow chart depicting yet another method for forming a wallpaper, according to some example embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the disclosure. However, one skilled in the art will understand that the disclosure may be practiced without these details. Moreover, while various embodiments of the disclosure are disclosed herein, many adaptations and modifications may be made within the scope of the disclosure in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the disclosure in order to achieve the same result in substantially the same way. 
     Unless the context requires otherwise, throughout the present specification and claims, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” Recitation of numeric ranges of values throughout the specification is intended to serve as a shorthand notation of referring individually to each separate value falling within the range inclusive of the values defining the range, and each separate value is incorporated in the specification as it were individually recited herein. Additionally, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may be in some instances. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Various embodiments described herein are directed to wallpapers that can be applied to exterior and/or interior of a building to reduce energy consumption of the building structures. In one embodiment, a wallpaper includes a support portion having a flat surface, and a functional layer disposed on the flat surface of the support portion. The functional layer includes a metallic film and has an emissivity of greater than zero and equal to or less than 0.6. The wallpaper further includes a particulate layer disposed on the functional layer. The particulate layer includes a polymer body and inorganic particles dispersed in the polymer body. 
     In another embodiment, a wallpaper includes a support portion having a flat surface, a first adhesive layer disposed on the flat surface of the support portion, and a functional layer disposed on the first adhesive layer. The functional layer includes a metallic film. The wallpaper further includes a second adhesive layer disposed on the functional layer, where the second adhesive layer includes a polymer. The wallpaper further includes a particulate layer disposed on the second adhesive layer. The particulate layer includes a polymer body and inorganic particles dispersed in the polymer body. The polymer body includes the same polymer as that of the second adhesive layer. The disclosed wallpapers, when installed inside a building, can reflect heat back into the interior of the building during the winter to reduce energy absorbed by the walls of the building. Moreover, the disclosed wallpaper, when installed outside a building, can reflect sunlight illuminated on the building to reduce heat absorbed by the walls of the building, thus lowering the cost used to cool down the building. 
     Embodiments will now be explained with accompanying figures. Reference is first made to  FIG.  1   .  FIG.  1    is a diagram illustrating a wallpaper  100  according to one example embodiment. The wallpaper  100  includes a support portion  102 , a functional layer  104 , and a particulate layer  106 . The support portion  102  is configured to provide support for other layers of the wallpaper  100 . The support portion  102  is also used to maintain the overall structure integrity of the wallpaper  100  and provides the mechanical strength for the wallpaper  100 . In some embodiments, the support portion  102  may include one of fabric-backed vinyl, paper-backed vinyl, non-woven fabric, polyester, wood, metal, stucco, clad, brick, masonry, stone, steel, cement, concrete, or any other suitable material that can be applied to a building and provide support for the layer structures of the wallpaper  100 . In some embodiments, the support portion is a layer having a thickness of about 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1 mm, or between any two of the above numbers, or more than 1 mm. In some embodiments, the support portion/layer  102  includes at least a flat surface that is compatible with the functional layer  104 . In the illustrate embodiment of  FIG.  1   , the support portion/layer  102  has an upper flat surface in contact with the functional layer  104 . 
     The functional layer  104  includes a metallic film  104   a . The metallic film  104   a  is configured to reflect heat to reduce the radiative heat penetrating into a structure to which the wallpaper is attached. The metallic film  104   a  includes a metallic material that has a low emissivity. For example, the metallic film  104   a  may include one of aluminum, silver, titanium, polished copper, brass, tin, gold, or a combination of the foregoing metals. The metallic film  104   a  is designed to be thin to reduce material cost. For example, a thickness of the metallic film  104   a  may be less than 10 μm, less than 5 μm, less than 3 μm, less than 1 μm, or between any two of the above numbers. 
     In some embodiments, the functional layer  104  may be provided with a polymer layer  104   b  to facilitate adhesion with the support portion  102 . In the illustrated embodiment, the polymer layer  104   b  is disposed between the metallic layer  104   a  and the support portion  102 . For example, the polymer layer  104   b  may include polyethylene terephthalate (PET). In some embodiments, the polymer layer  104   b  may include polyethylene (PE) polypropylene (PP), polylactide (PLA), poly(glycolic acid) (PGA), polybutylene succinate (PBS), or other biodegradable plastics. In some embodiments, the polymer layer  104   b  may include flame-resisted PP, PE, PET, PLA, PGA, or PBS. This disclosure, however, is not limited to these examples. Other polymer materials that can provide a flame-resist function and/or improve adhesion between the support portion  102  and the metallic film  104   a  are contemplated for the polymer layer  104   b . In some embodiments, the polymer layer  104   b  may be formed on the metallic film  104   a  by spraying or printing a layer of suitable polymer onto the metallic layer  104   a . In some embodiments, the polymer layer  104   b  may be a pre-prepared thin film and is laminated onto the metallic film  104   a . A thickness of the polymer layer  104   b  is designed to promote strong bonding between the support portion  102  and the metallic film  104   a  to form a durable wallpaper. For this purpose, the thickness of the polymer layer  104   b  may not be less than 500 nm, not less than 1 μm, not less than 2 μm, not less than 3 μm, or not less than 4 μm. On the other hand, the thickness of the polymer layer  104   b  may be designed to save the cost for the polymer layer  104   b . For example, an optimized thickness of the polymer layer  104   b  is about 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm, or between any two of the above numbers. In some embodiments, the polymer layer  104   b  may be omitted from the functional layer  104 . In this embodiment, the metallic film  104   a  is in contact with the support portion  102 . 
     The particulate layer  106  includes a polymer body  106   a  and inorganic particles  106   b  dispersed in the polymer body  106   a . The inorganic particles  106   b  can provide visual effects to the appearance of the wallpaper. For example, the inorganic particles  106   b  allow a user to see color and/or texture of the wallpaper  100 . The inorganic particles  106   b  may be reflective and transparent at visible and near-mid infrared regions, respectively. In some embodiments, the inorganic particles  106   b  may include one or more of ZnO, TiO 2 , iron oxide, Fe 2 O 3 , Prussian blue, or silicon, or other particles that can provide color and visual effects to human eyes. In some embodiments, the inorganic particles  106   b  are uniformly dispersed in the polymer body  106   a . In some embodiments, to create a textured visual outlook, the inorganic particles  106   b  may be dispersed at a first portion of the polymer body  106   a  at a density different from that at a second portion of the polymer body  106   a.    
     A shape of the inorganic particles  106   b  may be varied to obtain desired visual effects. For example, the inorganic particles  106   b  may be round, needle-like, rugby-shaped, or in other shapes. A size of the inorganic particles  106   b  is designed to provide desired visual effects. To be effective for their purpose, an average size of the inorganic particles  106   b  may be, for example, about 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm,  400 , nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, or between any two of the above numbers. 
     The polymer body  106   a  of the particulate layer  106  provides support for the inorganic particles  106   b . The material and the thickness of the polymer body  106   a  are selected to reduce emissivity of the wallpaper  100 . The low emissivity of the particulate layer  106  allows low absorption of radiation energy such that the radiation energy can be reflected by the functional layer  104 . In some embodiments, the polymer body  106   a  includes polyethylene (PE) that may include linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), or high-density polyethylene (HDPE). In some embodiments, a thickness of the polymer body  106   a  is about 10 μm, 15, μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or between any two of the above numbers. The conventional low emissivity building films commonly show solitary metalized silver color, suffering from severe aesthetical limitations, which tremendously hinder their extensive practical applications. The particulate layer  106  of the disclosed wallpapers includes inorganic particles  106   b  that can provide the wallpapers with various colors and appearance. 
     The surface of the particulate layer  106  may look glossy, semi-glossy, or non-glossy to provide choices to users. To reduce glossiness, the inorganic particles  106   b  may be sized to nanoscales, or matt PE materials may be used for the polymer body  106   a , or the surface of the particulate layer  106  may be patterned. 
     In some embodiments, the particulate layer  106  further includes a light stabilizer  106   c  to reduce polymer decay over long-term sun exposure. For example, the light stabilizer  106   c  may be a UV light stabilizer (e.g., UV  783 ). In some embodiments, the light stabilizer  106   c  may be a hindered amine light stabilizers (HALS). 
     In some embodiments, the particulate layer  106  includes 5-20 wt % of the inorganic particles  106   b,  0.5-1 wt % of the light stabilizer  106   c , balanced with the polymer body  106   a.    
     In some embodiments, the particulate layer  106  may further include environmental friendly flame retardants against potential fire in actual usage cases. For example, the environmental friendly flame retardants may be part of the inorganic particles  106   b . In some embodiments, the environmental friendly flame retardants may include one or more of Mg(OH) 2  or Al(OH) 3 . 
     In some embodiments, the wallpaper  100  may alternatively or additionally include a protective layer  110  coated on the particulate layer  106 . The protective layer  110  may be a low-emissivity ultraviolet (UV) coating that can protect the underlying layers. The UV coating can be formed by spraying a UV-curable material on the topmost layer of the wallpaper and curing the UV-curable material with UV radiation. The protective layer  110  may include a durable material that is abrasion resistance and can protect the wallpaper  100  against scratches, tears, and fingerprints. The protective layer  110  may be thin to remain low emissivity. For example, a thickness of the protective layer  110  may be 1-10 μm. 
     In some embodiments, the wallpaper  100  has a low emissivity of greater than zero but lower than 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55 or 0.6, or between any two of the above numbers. Emissivity is defined as the ratio of the energy radiated from a material&#39;s surface to that radiated from a perfect emitter. In some embodiments, to provide the wallpaper  100  with a low emissivity, a thickness of the particulate layer  106  is about 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or between any two of the above numbers. 
     Reference is now made to  FIG.  2   .  FIG.  2    is a diagram illustrating a wallpaper  200  according to one example embodiment. The wallpaper  200  includes a support portion  202 , a functional layer  204 , a particulate layer  206 , and a decorative layer  208 . The support portion  202 , the functional layer  204 , and the particulate layer  206  are similar to the support portion  102 , the functional layer  104 , and the particulate layer  106  of  FIG.  1   , and their descriptions can be referred to those in connection with  FIG.  1    and will be omitted. 
     The decorative layer  208  may provide color and aesthetic looking for the wallpaper  200 . In some embodiments, the decorative layer  208  may include inks of various colors and textures. The decorative layer  208  may provide patterns or graphics on the particulate layer  206 . The decorative layer  208  may be a pattern/graphic layer. In some embodiments, the decorative layer  208  may have a rough surface facing outward. The decorative layer  208  is designed to have a low emissivity to reduce absorption of the radiative heat. For this purpose, the decorative layer  208  has a thickness of less than 5 μm, 4 μm, 3 μm, 2 μm, or 1 μm, or between any two of the above numbers. The decorative layer  208  may be formed by a printing technique, such as flexographic printing, block printing, flatbed screen printing, gravure printing, rotary screen printing, digital printing, etc. 
       FIG.  3    is a diagram illustrating a wallpaper  300  according to one example embodiment. The wallpaper  300  includes a support portion  302 , a first adhesive layer  304 , a functional layer  306 , a second adhesive layer  308 , and a particulate layer  310 . The support portion  302  is configured to provide support for other layers of the wallpaper  300 . The support portion  302  is also used to maintain the overall structure integrity of the wallpaper  300  and provides the mechanical strength for the wallpaper  300 . In some embodiments, the support portion  302  may include one of fabric-backed vinyl, paper-backed vinyl, non-woven fabric, polyester, wood, metal, stucco, clad, brick, masonry, stone, steel, cement, concrete, or any other suitable material that can be applied to a building and provide support for layer structures of the wallpaper  300 . In some embodiments, the support portion is a layer having a thickness of about 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1 mm, or between any number of the above, or more than 1 mm. In some embodiments, the support portion/layer  302  includes at least a flat surface that is compatible with the first adhesive layer  304 . In the illustrate embodiment of  FIG.  3   , the support portion/layer  302  has an upper flat surface in contact with the first adhesive layer  304 . 
     The functional layer  306  includes a metallic film  306   a . In some embodiments, functional layer  306  may optionally include a polymer layer  306   b . The metallic film  306   a  is configured to reflect heat to reduce the radiative heat penetrating into a structure to which the wallpaper is attached. The metallic film  306   a  include a metallic material that has a low emissivity. For example, the metallic film  306   a  may include one of aluminum, silver, titanium, polished copper, brass, tin, gold, or a combination of the foregoing metals. The metallic film  306   a  is designed to be thin to reduce material cost. For example, a thickness of the metallic film  306   a  may be less than 10 μm, less than 5 μm, less than 3 μm, less than 1 μm, or between any two of the above numbers. 
     In some embodiments, the functional layer  306  is provided with a polymer layer  306   b  to facilitate adhesion with the first adhesive layer  304 . In the illustrated embodiment, the polymer layer  306   b  is disposed between the metallic layer  306   a  and the first adhesive layer  304 . For example, the polymer layer  306   b  may include PP, PE, PET, PLA, PGA, or PBS. In some embodiments, the polymer layer  306   b  may include flame-resisted PP, PE, PET, PLA, PGA, or PBS. This disclosure, however, is not limited to these examples. Other polymer materials that can provide a flame-resist function and/or improve adhesion between the first adhesive layer  304  and the metallic film  306   a  are contemplated for the polymer layer  306   b . In some embodiments, the polymer layer  306   b  may be formed on the metallic film  306   a  by spraying or printing a layer of suitable polymer onto the metallic layer  306   a . In some embodiments, the polymer layer  306   b  may be a pre-prepared thin film and is laminated onto the metallic film  306   a . A thickness of the polymer layer  306   b  is designed to promote strong bonding between the support portion  302  and the metallic film  306   a  to form a durable wallpaper. For this purpose, the thickness of the polymer layer  306   b  may not be less than 500 nm, not less than 1 μm, not less than 2 μm, not less than 3 μm, or not less than 4 μm. On the other hand, the thickness of the polymer layer  306   b  may be designed to save the cost for the polymer layer  306   b . For example, an optimized thickness of the polymer layer  306   b  is about 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm, or between any two of the above numbers. 
     The first adhesive layer  304  is configured to bond the support portion  302  and the functional layer  306 . In some embodiments, the first adhesive layer  304  may be a glue. For example, the first adhesive layer  304  may include a water-based or an oil-based glue, or polymer glue, or other glues that can provide sufficient bonding strength between the support portion  302  and the functional layer  306 . In some embodiments, before applying the first adhesive layer  304  to the support portion  302 , a surface of the support portion  302  to receive the first adhesive layer  304  may be surface-treated to increase the bonding strength. For example, the surface of the support portion  302  may be plasma treated. 
     The particulate layer  310  include a polymer body  310   a  and inorganic particles  310   b  dispersed in the polymer body  310   a . The inorganic particles  310   b  can provide visual effects to the appearance of the wallpaper. For example, the inorganic particles  310   b  allow a user to see color and/or texture of the wallpaper  300 . The inorganic particles may be reflective and transparent at visible and near-mid infrared regions, respectively. In some embodiments, the inorganic particles  310   b  may include one or more of ZnO, TiO 2 , iron oxide, Fe 2 O 3 , Prussian blue, or silicon, or other particles that can provide color and visual effects to human eyes. In some embodiments, the inorganic particles  310   b  are uniformly dispersed in the polymer body  310   a . In some embodiments, to create a textured visual outlook, the inorganic particles  310   b  may be dispersed at a first portion of the polymer body  310   a  at a density different from that at a second portion of the polymer body  310   a.    
     A shape of the inorganic particles  310   b  may be varied to obtain desired visual effects. For example, the inorganic particles  310   b  may be round, needle-like, rugby-shaped, or in other shapes. A size of the inorganic particles  310   b  is designed to provide desired visual effects. For example, an average size of the inorganic particles  310   b  may be about 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400, nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, 1000 nm, 1100 nm, 1200 nm, 1300 nm, 1400 nm, 1500 nm, 1600 nm, 1700 nm, 1800 nm, 1900 nm, 2000 nm, or between any two of the above numbers. 
     The polymer body  310   a  of the particulate layer  310  provides support for the inorganic particles  310   b . The material and the thickness of the polymer body  310   a  are selected to reduce emissivity of the wallpaper  300 . In some embodiments, the polymer body  310   a  includes PE materials, e.g., LLDPE, LDPE, or HDPE. In some embodiments, a thickness of the polymer body  310   a  is about 10 μm, 15, μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, or between any two of the above numbers. 
     The surface of the particulate layer  310  may look glossy, semi-glossy, or non-glossy to provide choices to users. To reduce glossiness, the inorganic particles  310   b  may be sized to nanoscales, or matt PE materials may be used for the polymer body  310   a , or the surface of the particulate layer  310  may be patterned. 
     In some embodiments, the particulate layer  310  further includes a light stabilizer  310   c  to reduce polymer decay over long-term sun exposure. For example, the light stabilizer  310   c  may be a UV light stabilizer (e.g., UV  783 ). In some embodiments, the light stabilizer  310   c  may be a hindered amine light stabilizers (HALS). 
     In some embodiments, the particulate layer  310  includes 5-20 wt % of the inorganic particles  310   b,  0.5-1 wt % of the light stabilizer  310   c , balanced with the polymer body  310   a.    
     In some embodiments, the particulate layer  106  may further include environmental friendly flame retardants. 
     The second adhesive layer  308  is configured to bond the functional layer  306  and the particulate layer  310 . Because the second adhesive layer  308  is disposed on the functional layer  306  and may increase the emissivity of the wallpaper  300 , the material and thickness of the second adhesive layer  308  are selected to reduce its impact on emissivity. In some embodiments, the second adhesive layer  308  may include a material that is the same as or similar to that employed in the polymer body  310   a  of the particulate layer  310  to increase the bonding strength and reduce its impact on increased emissivity. For example, the second adhesive layer  308  may include or consist of one of PE materials, e.g., LLDPE, LDPE, or HDPE. The thickness of the second adhesive layer  308  is selected to be less than that of the particulate layer  310  reduce its impact on emissivity. For example, a thickness of the second adhesive layer  308  may be about 3 μm, 5 μm, 6 μm, 8 μm, or 10 μm, or between any two of the above numbers, to reduce emissivity. 
     In some embodiments, the wallpaper  300  has a low emissivity of greater than zero but lower than 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, or between any two of the above numbers. In some embodiments, to provide the wallpaper  300  with a low emissivity, a total thickness of the particulate layer  310  and the second adhesive layer  308  is about 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or between any two of the above numbers. 
     In some embodiments, the wallpaper  300  may further include a decorative layer  312  disposed on the particulate layer  310 . The decorative layer  312  may provide color and aesthetic looking for the wallpaper  300 . In some embodiments, the decorative layer  312  may include inks of various colors and textures. The decorative layer  312  may provide patterns or graphics on the particulate layer  310 . The decorative layer  312  may be a pattern/graphic layer. The decorative layer  312  is designed to have a low emissivity to reduce the radiative heat penetration. For this purpose, the decorative layer  312  has a thickness of less than 5 μm, 4 μm, 3 μm, 2 μm, or 1 μm, or between any two of the above numbers. The decorative layer  312  may be formed by a printing technique, such as flexographic printing, block printing, flatbed screen printing, gravure printing, rotary screen printing, digital printing, etc. 
     It is to be understood that the various layers as shown in  FIGS.  1 - 3    may be selected for a wallpaper. For example, the protective layer  110  of wallpaper  100  may be applied to wallpapers  200  and  300 . 
       FIG.  4    is a diagram depicting how a thickness of a polymer film above a functional layer affects emissivity of a wallpaper, according to one example embodiment. In the illustrated embodiment, the functional layer includes an Al metallic film while the polymer film on the metallic film is PE. As shown in  FIG.  4   , the emissivity of the polymer film is increased with increased thickness of the polymer film. To be effective to reflect heat, the thickness of the polymer film of the disclosed wallpaper is selected to be equal to or less than 100 μm. 
       FIG.  5    is a diagram illustrating emissivity of the disclosed wallpaper and various competitive wallpapers. As shown in  FIG.  5   , all competitive wallpapers have an emissivity of about 0.9, while the disclosed wallpaper has a much smaller emissivity below 0.3. More thermal radiation will be reflected when the emissivity of a wallpaper is lower. This demonstrates that the disclosed wallpaper has a better reflection than the competitive wallpapers in the current market. When applied to an outer surface of a building, the disclosed wallpaper can better reflect sunlight back to the external environment and reduce heat absorption by the building. Moreover, when applied to an interior surface of a building, the disclosed wallpaper can better reflect heat generated by a heating system during the winter back into the rooms, thereby saving energy consumed by the heating system. 
       FIG.  6 A  is a schematic diagram depicting a testing environment  600  for testing performances of various wallpapers, according to one example embodiment. The testing environment  600  includes a space enclosed by a wall  602 . The wall  602  is insulted with a polystyrene foam. A wallpaper  604  for testing is attached to one surface of the wall  602 . A temperature sensor  606  is attached to an internal surface of the wall  602  away from where the wallpaper  604  is attached to. During the testing, a heating lamp is turned on in front of the wallpaper  604  while the temperature at the space is measured by the temperature sensor  606 . In the testing environment, the temperature at the space can be continuously monitored. The temperature in the space is expected to increase while the heating lamp is turned on. 
       FIG.  6 B  is a diagram illustrating testing results employing the testing environment  600  as shown in  FIG.  6 A . As shown in  FIG.  6 B , the temperature increases at a higher rate for the competitive wallpaper than for the disclosed wallpaper, indicating a better heat reflection for the disclosed wallpaper. After heating by the heating lamp for 30 minutes, the temperature within the space is 3.3° C. higher when the competitive wallpaper is applied to the wall  602  than when the disclosed wallpaper is applied to the wall  602 . 
       FIG.  7    is a diagram showing a summary of testing results employing the testing environment  600  as shown in  FIG.  6 A . The summary indicates that with the disclosed wallpaper, the temperature at the space is on average 3° C. cooler than the competitive commercial products. This testing confirms that the disclosed wallpaper makes the room cooler, reducing the cooling energy for a comfort living space. 
       FIG.  8    is a flow chart depicting a method  800  for forming a wallpaper, according to some example embodiments. At  802 , to prepare a particulate layer, inorganic particles are mixed with a polymer material to form a composite material/masterbatch. The polymer material includes a polymer. In some embodiments, the inorganic particles may be added up to 50% by weight of the composite material. In some embodiments, a light stabilizer may be added to the composite material up to 5% by weight. In one embodiment, the polymer of the polymer material may include or consist of one of LLDPE, LDPE, or HDPE. The inorganic particles may be reflective and transparent at visible and near-mid infrared regions, respectively. For example, the inorganic particles may include one or more of ZnO, TiO 2 , iron oxide, Fe 2 O 3 , Prussian blue, or silicon, or other particles that can provide color and visual effects to human eyes. In some embodiments, the inorganic particles may include an environmental friendly flame retardant. The inorganic particles may be uniformly or non-uniformly dispersed in the polymer material. A average size of the inorganic particles may be 20-2000 nm. 
     At  804 , the composite material formed at  802  is extruded to form a particulate layer. In some embodiments, before  804  the composite material may be infused with an additional material at  806  to provide more functions to or change the property of the resulting particulate layer. For example, at  806  the composite material may be infused with additional PE pellets to reduce the content of the inorganic particles to 5-20 wt % inclusive in the composite material (and in the resulting particulate layer) and reduce the content of the light stabilizer to 0.5-1 wt % inclusive in the composite material (and in the resulting particulate layer). The particulate layer has a thickness of about 10-100 μm to maintain a low emissivity. 
     At  808 , a functional layer is formed to include a metallic film. The metallic film includes a metallic material that has a low emissivity. For example, the metallic film may include or consist of one or more of aluminum, silver, titanium, polished copper, brass, tin, gold, or a combination of the foregoing metals. A thickness of the metallic film may be less than 10 μm, less than 5 μm, less than 3 μm, less than 1 μm, or between any two of the above numbers. In some embodiments, a polymer layer may be provided to the functional layer to facilitate adhesion with an adhesive layer between the functional layer and a support portion/layer, discussed below. In some embodiments, the polymer layer may include PET. In some embodiments, the polymer layer may include PE PP, PLA, PGA, or PBS, or other biodegradable plastics. In some embodiments, the polymer layer  104   b  may include flame-resisted PP, PE, PET, PLA, PGA, or PBS. In some embodiments, the polymer layer may be formed on the metallic film by spraying or printing a layer of suitable polymer onto the metallic layer. In some embodiments, the polymer layer may be a pre-prepared thin film and is laminated onto the metallic film. In some embodiments, the polymer layer may be formed on the first surface of the metallic film by depositing a metal thin film on the polymer layer via evaporation techniques. In some embodiments, an optimized thickness of the polymer layer is about 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm, or between any two of the above numbers. In some embodiments, the polymer layer is optional and may be omitted from the functional layer. 
     At  810 , the particulate layer is thermally laminated with the functional layer such that the particulate layer is bonded with the functional layer. For example, the particulate layer and the functional layer are brought together and heated at a temperature (e.g., 310-350° C. or above the meting temperature of the particulate layer) to bond the two layers together. 
     At  812 , a support portion is adhered to the functional layer via an adhesive layer to form a wallpaper. 
     In some embodiments, at  814  a decorative layer may be formed on the particulate layer to provide visual effects, such as colors and/or textures, to the wallpaper. The decorative layer may have a thickness of less than 5 μm, 4 μm, 3 μm, 2 μm, or 1 μm, or between any two of the above numbers. The decorative layer may be formed by a printing technique, such as flexographic printing, block printing, flatbed screen printing, gravure printing, rotary screen printing, digital printing, etc. 
     It should be understood that the sequences of the method  800  may be modified and different from those explained above. In some instances, some of the operations  802 - 814  may be omitted. For example, in some embodiments, operation  814  may be omitted. 
       FIG.  9    is a flow chart depicting a method  900  for forming a wallpaper, according to some example embodiments. At  902 , to prepare a particulate layer, inorganic particles are mixed with a polymer material to form a composite material/masterbatch. 
     At  904 , the composite material is extruded to form a particulate layer. In some embodiments, before  904  the composite material may be infused with an additional material at  906  to provide more functions to or change the property of the resulting particulate layer. For example, at  906  the composite material may be infused with additional PE pellets to reduce the content of the inorganic particles to 5-20 wt % inclusive in the composite material (and in the resulting particulate layer) and reduce the content of the light stabilizer to 0.5-1 wt % inclusive in the composite material (and in the resulting particulate layer). The particulate layer has a thickness of about 10-100 μm to maintain a low emissivity. 
     At  908 , a functional layer is formed to include a metallic film. The metallic film include a metallic material that has a low emissivity. For example, the metallic film may include or consist of one of aluminum, silver, titanium, polished copper, brass, tin, gold, or a combination of the foregoing metals. A thickness of the metallic film may be less than 10 μm, less than 5 μm, less than 3 μm, less than 1 μm, or between any two of the above numbers. In some embodiments, a polymer layer is provided for the functional layer to facilitate adhesion with an adhesive layer between the functional layer and a support layer, discussed below. In some embodiments, the polymer layer may include PET. In some embodiments, the polymer layer may include PE PP, PLA, PGA, or PBS, or other biodegradable plastics. In some embodiments, the polymer layer  104   b  may include flame-resisted PP, PE, PET, PLA, PGA, or PBS. In some embodiments, the polymer layer may be formed on the metallic film by spraying or printing a layer of suitable polymer onto the metallic layer. In some embodiments, the polymer layer may be a pre-prepared thin film and is laminated onto the metallic film. In some embodiments, the polymer layer may be formed on the first surface of the metallic film by depositing a metal thin film on the polymer layer via evaporation techniques. In some embodiments, an optimized thickness of the polymer layer is about 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm, or between any two of the above numbers. In some embodiments, the polymer layer is optional and may be omitted from the functional layer. 
     At  910 , an adhesive layer is coated on a second surface of the metallic film, where the second surface is opposite to the first surface on which the polymer layer is formed. The adhesive layer includes a polymer that is the same as that of the particulate layer formed at  902 - 906  to increase the bonding strength between the particulate layer and the functional layer. For example, the adhesive layer may include or consist of a PE material. A thickness of the adhesive layer is less than a thickness of the particulate layer. For example, the thickness of the adhesive layer may be about 3 μm, 5 μm, 6 μm, 8 μm, or 10 μm, or between any two of the above numbers. The adhesive layer may be coated on the second surface of the metallic film by spray coating, printing, spinning coating, slit coating, and other suitable coating techniques. In some embodiments, no glue is used as the adhesive layer in this step. 
     At  912 , the particulate layer is thermally laminated with the functional layer such that the adhesive layer is in contact with the particulate layer. After the adhesive layer is coated on the second surface of the metallic film, the particulate layer and the functional layer are brought together and heated to a temperature (e.g., 310-350° C. or above the meting temperature of the adhesive layer) such that the adhesive layer binds the functional layer to the particulate layer. Because the adhesive layer and the particulate layer contain a same polymer, a strong bonding can be achieved. 
     At  914 , a support portion is adhered to the functional layer with another adhesive layer (e.g., a glue) to form a wallpaper. 
     In some embodiments, at  916  a decorative layer may be formed on the particulate layer to provide visual effects, such as colors and/or textures, to the wallpaper. The decorative layer may contain inks and be formed by a printing technique, such as flexographic printing, block printing, flatbed screen printing, gravure printing, rotary screen printing, digital printing, etc. 
     It should be understood that the sequences of the method  900  may be modified and different from those explained above. In some instances, some of the operations  902 - 916  may be omitted. For example, in some embodiments, operation  916  and/or operation  906  may be omitted. 
       FIG.  10    is a flow chart depicting a method  1000  for forming a wallpaper, according to some example embodiments. At  1002 , to prepare a particulate layer, inorganic particles are mixed with a polymer material to form a composite material/masterbatch. 
     At  1004 , the composite material is extruded to form a particulate layer. In some embodiments, before  1004  the composite material may be infused with an additional material at  1006  to provide more functions to or change the property of the resulting particulate layer. For example, at  1006  the composite material may be infused with additional PE pellets to reduce the content of the inorganic particles to 5-20 wt % inclusive in the composite material (and in the resulting particulate layer) and reduce the content of the light stabilizer to 0.5-1 wt % inclusive in the composite material (and in the resulting particulate layer). The particulate layer has a thickness of about 10-100 μm to maintain a low emissivity. 
     At  1008 , a metallic film is formed on the particulate layer to form a functional layer. The metallic film may be deposited on the particulate layer by a metallization technique. For example, the metallic film may be deposited on the particulate layer by evaporation, electroplating, etc. In some embodiments, the metallization process may be a physical vapor deposition method. The solid metal (e.g. Al) source is first vaporized in a vacuum and deposited on a surface of the particulate layer, where the deposition temperature typically is lower than the melting point of the particulate layer. A typical thickness of metallic film is greater than about 50 nm but no more than 1000 nm. 
     The metallic film include a metallic material that has a low emissivity. For example, the metallic film may include or consist of one or more of aluminum, silver, titanium, polished copper, brass, tin, gold, or a combination of the foregoing metals. 
     At  1010 , a support portion/layer is adhered to the functional layer such that the functional layer is sandwiched between the particulate layer and the support portion/layer. 
     In some embodiments, at  1012  a decorative layer may be formed on the particulate layer to provide visual effects, such as colors and/or textures, to the wallpaper. The decorative layer may be formed by a printing technique, such as flexographic printing, block printing, flatbed screen printing, gravure printing, rotary screen printing, digital printing, etc. 
     It should be understood that the sequences of the method  1000  may be modified and different from those explained above. In some instances, some of the operations  1002 - 1012  may be omitted. For example, in some embodiments, operation  1012  and/or operation  1006  may be omitted. 
     In summary, the disclosed wallpapers have a low emissivity and can reflect heat. When used inside a building, the disclosed wallpapers can reflect heat back into the interior of the building during the winter to reduce energy absorbed by the walls of the building. When installed outside a building, the disclosed wallpapers can reflect sunlight illuminated on the building to reduce heat absorbed by the walls of the building, thus lowering the cost used to cool down the building. 
     The layer structures of the disclosed wallpapers are strong and durable, and can be employed both inside and outside of a building. The disclosed wallpapers reduce energy cost and consumption of the building due to their low emissivity. In some embodiments, the integrity of the wallpapers is improved over conventional wallpapers thanks to using an adhesive between the functional layer and particulate layer that does not include a traditional glue. In the embodiment, the adhesive between the functional layer and particulate layer contains a polymer used in the particulate layer. 
     The foregoing description of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. Many modifications and variations will be apparent to the practitioner skilled in the art. The modifications and variations include any relevant combination of the disclosed features. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical application, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalence.