the mechanism of action of uv absorbers
ultraviolet absorbers can strongly and selectively absorb high-energy ultraviolet rays, and use energy conversion to release and consume the absorbed energy as heat or harmless low-energy radiation, thereby avoiding damage to the skin and preventing polymer the polymer is excited due to absorption of ultraviolet energy and then undergoes photophysical and photochemical decomposition.
the light energy absorption and conversion mechanisms of ultraviolet absorbers vary with different types, which are described below:
1. benzophenones
benzophenone uv absorbers are the most widely used type of uv absorbers. this type of ultraviolet absorber has a slow absorption effect on uv-a, uv-b, and uv-c. the ketone group and hydroxyl group in the molecule can generate internal hydrogen bonds to form a chelate ring. after absorbing duv light energy, it undergoes thermal vibration of the molecule, destroys the internal hydrogen bonds, and opens the chelate ring. it converts the energy of uv light into thermal energy and releases it. in addition, the carbonyl group in the molecule will be absorbed by the absorbed uv light energy. excitation produces tautomerism and generates an enol structure. this also consumes part of the energy. in this type of uv absorber, the strength of the hydrogen bond within the molecule is related to its photostability effect. the stronger the oxygen bond, the greater the energy required to destroy it, and the more uv energy consumed in absorption, the better the effect; conversely, likewise. the stabilizing effect is also related to the length of the alkoxy chain on the benzene ring. if it is long and has good compatibility with the polymer, the stabilizing effect will be just right. in benzophenone uv absorbers, there must be a hydroxyl group at the ortho position of the carbonyl group, otherwise the internal hydrogen bond cannot be formed, and it cannot be used as a uv absorber. uv absorbers with an ortho hydroxyl group can absorb 290~380~ m of ultraviolet rays, almost no visible light absorption, no coloring, and good compatibility with polymers. if there are two hydroxyl groups at the ortho position of the carbonyl group, it can absorb ultraviolet light of 300 ~ fzm and also absorb part of the visible light. due to the absorption of visible light, the complementary light will be unbalanced, making the items added with this ultraviolet absorber appear yellow. with high molecular polymers also have poor compatibility, so their uses are limited. although benzophenone without o-hydroxyl groups also has the ability to absorb ultraviolet rays, it will cause its own decomposition when exposed to light, so it is not suitable to be used as an ultraviolet absorber.
2. salicylates
salicylate uv absorbers are the earliest used ones. salicylate also has internal hydrogen bonds in the molecule. the ability of this type of ultraviolet absorber to absorb ultraviolet light is very low at the beginning, and the absorption range is extremely narrow (less than 340vm). however, after being exposed to ultraviolet light for a certain period of time, its absorption gradually increases until it reaches the maximum absorption. this is due to its under ultraviolet irradiation, molecular rearrangement occurs, forming a benzophenone structure with strong ultraviolet absorption ability, thus strengthening its ultraviolet absorption effect. therefore, people call it a pioneer ultraviolet absorber. the bishydroxybenzophenone and its derivatives generated after molecular rearrangement can absorb part of the visible light and appear yellow, causing the substances added with this ultraviolet absorber to turn yellow.
3, benzotriazoles
the mechanism of action of benzotriazole uv absorbers is similar to that of benzophenones. benzotriazoles have a wide absorption range of ultraviolet rays and can absorb light with a wavelength of 300~~m. it hardly absorbs visible light above ~m, so the product will not be discolored. in addition, the mechanism of action of substituted acrylonitrile and triazine ultraviolet absorbers is presumed to be cis-trans isomerization, converting light energy or releasing harmless energy. acrylonitrile-substituted ultraviolet absorbers can absorb ultraviolet light of 290 to 320 fm, and also do not absorb visible light, and will not cause the applied object to become discolored. three-class ultraviolet absorbers can absorb ultraviolet light of 300~~m. ultraviolet absorbers can absorb auxiliary color genes (such as -nh, -oh, -soh, -cooh, etc.) and chromophoric groups (such as c:n, n:n, n:o, c:) with wavelengths below ~m. o etc.). they are all connected to the aromatic core. organic nickel can also be used as an ultraviolet absorber, but it is generally classified as a quencher (also known as a deactivator or matting agent, or a laser state quencher, an energy quencher), and its ability to absorb ultraviolet rays is higher than that of a quencher. the connection is low, but it can prevent the dissociation caused by the absorption of ultraviolet rays by the polymer. organic nickel complexes are molecules that are excited into excited states through the interaction with polymers during ultraviolet light irradiation, returning the excited state to the ground state and converting ultraviolet energy into low-energy spectral emission without causing damage. , thereby protecting the polymer from damage. so it is called quencher because its mechanism of action is different from that of general uv absorbers. uv absorbers change the structure of the molecules themselves, while quenchers dissipate energy just through energy transfer between molecules. in addition, light shielding agents such as carbon black, titanium dioxide, zinc oxide, zinc barium, etc. are also a type of materials that can absorb or reflect ultraviolet rays. a barrier is set up between the polymer and the light source to absorb or reflect ultraviolet rays before they reach the polymer surface. when it is absorbed and reflected, it prevents ultraviolet rays from penetrating deep into the polymer. among them, carbon black is the most efficient. free radical scavengers are also a type of substance that can produce photostability. they are piperidine derivatives with steric hindrance and are called hindered amine photostabilizers
uv absorber�products. if you use a benzophenone uv absorber, such as ‘s uvinul dp-uv, it contains two hydroxyl groups. it can form hydrogen bonds with cellulose, wool, silk, and polyamide fibers and has affinity. it can dye fibers and can be used for exhaust dyeing treatment. the dosage is dp-uv 10～20 (o.w.f). treat at 8o℃ for 30m[a]. pad dyeing method dp-uv 10o～z00g/l can also be used. add 30g/l of softener and pad dye at 6o℃ and dry at 100℃ for 3 minutes. it can also be used in combination with waterproofing agent to make a uv-proof tent, with waterproofing agent 60g/l.dp uv 8o～1 50g/l, catalyst (such as mgci 2·6h2o) 5g/l. padding, drying and baking bake (1 50℃ .2min). this application method achieves a washable effect. the shielding rate for 280~vm ultraviolet rays can be selected above 85.
3 the method of making microcapsules and then applying them to fabrics is to use microencapsulation technology to make ultraviolet absorbers into microcapsules. the capsules are preferably made of high molecular polymers, such as styrene and acrylate, and are polymerized while microcapsules are made by microencapsulation method. wrap the ultraviolet absorber in the core of the capsule, and then use adhesives and cross-linking agents to fix the microcapsules on the fabric. textiles with an ultraviolet shielding rate of more than 85% can be made. the microcapsules can slowly release the ultraviolet absorber, thereby achieving long-term protection. effective uv protection purpose.
4. use the coating application method to add the ultraviolet absorber to the coating agent. carry out coating, drying, and baking. other functional finishing agents can also be added to the coating liquid. tents and hats made by this method. the method is simple, but the weather resistance and wash resistance still need to be improved.
(2) application in polymers
to test the effect of ultraviolet absorbers, the transmittance of ultraviolet light with a wavelength of 280~4o0fm is generally tested on textiles. use an ultraviolet spectrophotometer to measure the transmittance (or light transmittance), and use the area integral value of the transmittance to obtain the shielding rate, which is calculated according to the following formula. (1-)×10. (1-t~)x100%x is the ultraviolet shielding rate of the sample treated with ultraviolet absorber. is the area integrated value of the transmittance curve without trial mixing. l is the area integrated value of the transmittance curve with treated trial mixing. l is the area integrated value of the transmittance curve with untreated trial mixing. for specific operation methods, please refer to the lin hong: article. according to the regulations on ultraviolet shielding processing, the class a standard is a shielding rate of 90%. the b-level standard is a shielding rate of 80 or more, and the c-level standard is a shielding rate of 50 or more. the ultraviolet transmittance reduction rate is generally 4.5 to 74.5. in polymer products, the effect of ultraviolet absorbers is expressed by measuring the aging effect. to measure its weather resistance, two methods are generally used: one is outdoor atmospheric exposure. the other is artificial accelerated aging. regularly test samples and observe aging condition. of these two methods, the outdoor exposure method is the more reliable. the results of the outdoor exposure method are related to the exposure location, exposure time, and the mode and angle of the exposure rack. our country often chooses guangzhou. the united states often chooses florida and arizona. the key to the artificial aging method is the energy distribution of the aging machine. choose a light source that is closer to the energy distribution when the sun reaches the ground and has better simulation. the advantage is that it is easy to control and manage.