At present, due to good industry prospects and considerable return on investment, businesses and manufacturers of all sizes have entered this huge market. However, the domestic automotive glazing film market lacks national standards and industry norms. According to statistics, there are more than 2,000 brands currently sold in China. In order to make products more distinctive and selling points, many businesses constantly introduce new product concepts. According to different application fields, the current glass window films can be roughly divided into three categories: building glass window films, automobile glass window films and safety glass window films.
Building glass window film: the main purpose is energy saving and heat insulation, with anti-ultraviolet and safety functions. This kind of film can also be divided into thermal reflection film and low emissivity film. Thermal reflective film, attached to the glass surface, allows visible light and near infrared light to penetrate the room, but far infrared light cannot. Therefore, there is enough light to enter the room, and most of the heat of solar energy is reflected back, so that the indoor temperature will not rise too much in hot summer, thus reducing the indoor air conditioning load and achieving the effect of saving air conditioning costs and energy. The low radiation film can transmit a certain amount of short-wave solar radiation energy, so that the solar radiation heat enters the room and is absorbed by indoor objects; At the same time, it can keep more than 90% of the long-wave infrared reflection radiated by indoor objects indoors. Low-emissivity film can make full use of solar radiation and long-wave radiation energy of indoor objects. Therefore, it can be used in cold areas and heating buildings to achieve the effect of heat preservation and energy saving.
Adhesive film for automobile glass window: it is attached to the inner surface of automobile glass and has the functions of energy saving, heat insulation, ultraviolet ray isolation, security and theft prevention. At the same time, the requirements for visible light transmittance and visible light reflectivity of window films are also very high, especially for front windshield films. According to "Technical Conditions for Motor Vehicle Operation Safety", "The visible light transmittance of the front windshield is not allowed to be less than 70%, and all the windows are not allowed to be attached with a mirror reflective sunshade film". Because it is used for automobile windshield, the visible light transmittance after film pasting must be greater than 70% to meet the requirements of national standard GB7258-2004.
Safety glass window film: there is a layer of glue between the film and the glass, and the film itself has good tear resistance and penetration resistance. Even if the glass is broken, it can maintain the original overall state and prevent the secondary injury to the human body caused by accidental glass breaking and splashing. Its main function is safety and explosion-proof, and it is generally used in some important places or exhibition halls of financial institutions such as banks, jewelry store windows, museums, hotel halls and so on. At the same time, this film has certain heat insulation and ultraviolet shielding ability, and its transparency is also high.
As far as production technology is concerned, glass window films can be divided into three categories: dyed films, primary color films and vacuum coating. Dyed films can be divided into ordinary films, surface dyed films and interlayer dyed films. Vacuum coating can be divided into vacuum evaporation coating and magnetron sputtering coating; According to the different coating materials, it can be divided into metal film and ceramic film.
Ordinary film: commonly known as tea paper, it is made by mixing dyes in glue and coating it on polyethylene substrate. It is the oldest production method, and it is rarely used now because it is not heat-insulated and ultraviolet-proof, and its shelf life is short.
Surface dyeing film: The dye coated on the surface of PET substrate has soft hand feeling, lack of sufficient toughness and explosion resistance, ultraviolet radiation resistance, easy fading and short shelf life.
Interlayer dyed film: the dye is coated between two layers of PET substrates, which is soft to the touch and has certain anti-ultraviolet aging ability. Because of its low cost, this dyed film is still available in the market.
Dyed film features: low visible light reflectivity, lack of good thermal control performance, easy to fade (generally purple), easy to bubble after long-term use.
Primary color film: refers to the glass window film without adding organic dyes. Up to now, there are only two kinds of optical grade PET films, gray and colorless, and other colors of PET films can only be achieved by dyeing. The primary color film is characterized by good hand feeling, difficult fading and long shelf life.
Vacuum evaporation coating: By applying thermal evaporation (for pure metal with low melting point) process, various metal, alloy or oxide coatings can be coated on the film. Vacuum evaporation technology usually adopts pure aluminum coating, which can make films with different visible light transmittance and reflectivity, so that the films have the best solar control performance.
Vacuum sputtering coating: usually in a very large vacuum chamber, in a low-pressure inert gas environment, under the action of electric energy, charged ions are used to impact various metals or metal synthetic targets, so as to make multi-layer dense metal films with low reflection and high thermal insulation, which are orderly and evenly coated on the surface of polyester films, and compounded into various unique and stable colors and high-level transmittance selectivity, so as to ensure that the products have sufficient toughness, explosion-proof, thermal insulation, UV resistance of 99% and long shelf life.
Ceramic membrane: glass window membrane containing ultra-thin ceramic coatings such as SiO2 _ 2, TiO2 _ 2, etc., without metal layer, which does not interfere with GPS satellite navigation system, radio communication and mobile phone reception in the car at all; At the same time, it does not oxidize and fade, and it is easier to maintain and durable than metal film. It has low reflectivity and moderate light transmittance. Gu Wei's amber ceramic membrane, 3M's Jing Rui automobile heat insulation membrane and Johnson & Johnson's basalt series automobile membrane all belong to this kind of products.
All the above glass window film products must have one-way protection function, that is, they can prevent the damage from outside to inside in one direction, but there is no obstacle to the damage from inside to outside. When fire, earthquake, car accident and other sudden disasters occur, indoor personnel can easily break through the glass to escape. The structural schematic diagram of the glass window film is as follows:
1. Wear-resistant layer: composed of wear-resistant polyurethane, the hardness can reach 4h;
2. Color PET safety base layer: It is made of high strength and high transparency PET polyester and pigment through melt extrusion and biaxial stretching. Because the pigment is sandwiched in the PET film, it can prevent oxidation and discoloration, and its service life is as long as 8 years.
3. Metal heat insulation layer: a nano-level metal layer with high infrared reflectivity, such as aluminum, silver and nickel, is deposited on the PET film by vacuum evaporation or vacuum magnetron sputtering;
4. Composite adhesive: composed of polyurethane adhesive with good weather resistance and high transparency;
5. Ultraviolet absorption layer: composed of special ultraviolet absorber, which can block 99% ultraviolet rays;
6. Transparent PET safety base layer: it is composed of high-strength and high-transparency PET polyester film, with the purpose of clamping the metal layer, preventing metal oxidation and prolonging the life of the metal film;
7. Installation glue: composed of acrylate glue, with good weather resistance and high transparency;
8. High transparent PET release film.
2.3 working principle
2.3. 1 Working principle of glass window membrane thermal insulation
There are three forms of heat conduction: radiation, conduction and convection. Thermal insulation film mainly uses radiation and convection to insulate heat, and mainly protects the radiant heat of the sun.
Sunlight is composed of light waves (electromagnetic waves) of various bands, and solar energy is the energy of these electromagnetic waves. After being filtered by the atmosphere, these energies reach the surface of the earth in the form of light waves of different bands, and these energies are converted into heat energy in the form of radiation. The higher the energy, the greater the converted heat.
The solar spectrogram can be divided into three bands:
Ultraviolet band: The wavelength is between 280-400nm, which has strong penetrating power and can make melanin deposit on human skin and deepen the color. Excessive ultraviolet radiation can lead to skin cancer, which can lead to the fading of carpets, curtains, fabrics and furniture paints.
Visible spectral band: the wavelength is between 380 and 780 nm, and it is characterized by a unique spectrum visible to the naked eye. The visible spectrum band can be further divided into different colors (red, orange, yellow, green, indigo and purple), which has no direct harm to human body.
Infrared band: The wavelength is between 700-2400nm (nanometer), which is characterized in that we can directly feel the "invisible" heat of sunlight, which contains the most energy, so the heat is also high.
Far-infrared rays and near-infrared rays of all bands constitute 53% of solar energy, ultraviolet rays account for 3%, and visible light accounts for 44%.
The working principle of glass window film: aluminum, gold, copper, silver and other metals are made into multi-layer to dense high thermal insulation metal films by vacuum spraying or magnetron sputtering technology. In general, shell electrons (free electrons) in metallic materials are not bound by nuclei. When the light wave irradiates, the electric field of the light wave makes the free electrons absorb the energy of the light, produce oscillation with the same frequency as the light, and then release the light with the same frequency as the original light, which is called light reflection. The higher the conductivity of metal, the shallower the penetration depth and the higher the reflectivity. These metal layers will selectively reflect all kinds of heat energy in the sun, including infrared, ultraviolet and visible light heat energy, and then the color on the film will absorb the solar heat radiation and release it to the outside for the second time, and take away some heat with the outdoor airflow. Thereby effectively playing the role of heat insulation.
The metal reflective materials of glass window films are mostly highly conductive materials such as gold, silver, aluminum and copper. The reflectivity of several metal reflective films at different wavelengths is as follows.
Table 1. Reflectivity of several metal reflective films to different wavelengths
Metal species 800 nm reflectivity 650 nm reflectivity 500 nm reflectivity
Aluminium 86.7 90.5 9 1.8
Silver 99.2 98.8 97.9
Gold Gold 98.0 95.5 47.7
Copper 98. 1 96.6 60.0
2.3.2 Explosion-proof working principle of glass window film
Whether it is ordinary glass or tempered glass, its disadvantage is that the splash of glass fragments may cause harm to human body. When the glass window film is attached to its inner surface, the adhesive on the glass window film makes them merge into a whole and become a composite material that can withstand external forces.
The safety substrate in the glass window film is transparent polyester film (PET), which has great tensile strength and impact strength. Especially, when two or even three layers of polyester film are glued together by laminating, its mechanical properties are doubled, which makes it have very strong impact resistance. When impacted by external force, the safety base can play an effective buffering role; Even with the alternating external force, the unique pressure-sensitive adhesive layer can firmly stick the glass fragments on the surface of the window film, and only a small amount of powdered glass fragments are scattered, which can minimize the external damage.
2.4 Main performance indexes of glass window film
Visible light transmittance: the percentage of visible light (380-780) passing through the glass/glazing film composite layer.
Visible light reflectance: the percentage of visible light (380-780) reflected by the glass/glazing film composite layer.
Solar energy transmittance: the percentage of solar energy passing through the glass/glazing film composite layer.
Ultraviolet blocking rate: the percentage of ultraviolet rays (280-400) reflected by the glass/glass window film composite layer.
Solar reflectivity: the percentage of solar energy reflected by the glass/film composite layer.
Solar energy absorption rate: the percentage of solar energy absorbed by the glass/film composite layer. The absorbed solar energy can radiate to both sides of the glass/film composite layer at the same time, and the side with high airflow speed will radiate more energy.
Total solar energy transmittance: the sum of the part of solar energy that passes through the glass/glass window film composite layer and the absorbed inward radiation solar energy. Adiabatic rate is measured by the heat passing per square foot per hour (BTU/ hour/ft2, where 1BTU=262 calories). The thermal insulation rate of transparent glass is generally 20 btu/ hour/square foot.
The total solar reflectivity: that is, the thermal insulation rate is the sum of the solar energy reflected by the glass/glass window film composite layer and the solar energy absorbed and radiated to the outside.
Shielding coefficient: the total solar energy transmittance of glass/glass window film composite layer relative to glass.
U value: hourly, indoor and outdoor temperature difference per degree, and thermal radiation per square foot.
According to different uses, the performance requirements are also different. Generally speaking, its main indicators are visible light transmittance, visible light reflectivity, heat insulation and ultraviolet blocking rate.