Fluorescent bleach is a fluorescent dye or white dye, which is also a complex organic compound. Its characteristic is that it can stimulate incident light to produce fluorescence, so that colored substance can obtain a sparkling effect similar to fluorite and make substance visible to naked eye very white, achieving a whitening effect.
1. How fluorescent bleach works
1.1. Reflection of light based on bleaching principle of fluorescent bleaches
When light hits an object, a certain range of wavelengths is absorbed and rest are reflected. It can be said that color of objects perceived by our naked eyes is color of reflected light, and this is optical perception of reflected light stimulation. The concept of falling, absorbing and reflecting light as shown in picture:
Our naked eyes usually see only light with a wavelength of 400-700 nm. This wavelength is called visible range, and this light is called visible light. When light in visible wavelength range is split, light can be seen changing from blue-violet (short wavelength) to red (long wavelength), which is called a spectrum. If all these lights are mixed in a certain proportion, it will look white, and if luminous flux is reduced in same proportion, it will change from gray to black and, finally, to a state of pitch darkness. This white-gray-black series is called achromatic. Given above picture, incident natural light is reflected in same proportion in visible range, and object is achromatic. On other hand, in visible spectrum, if specific light is relatively high, its color will be very noticeable, going from achromatic to colored, we think it is object's own color. Therefore, so-called whiteness means that all spectral segments of visible part reflected by object are in same proportion, that is, achromatic total and diffuse reflection.
1.2 Complementary color of fluorescent whitening agent Whitening principle
Red, blue and yellow are called three primary colors, and almost all colors can be made by mixing them. Moreover, when two colors of light are mixed and result is white, these two colors are called complementary colors to each other, that is, colors located diagonally on color wheel in figure below are complementary colors to each other. From this image, we can learn that yellowing of object is due to lack of blue light and loss of balance.
After understanding light and color, whitening principle of fluorescent whitening agent is well understood. Fluorescent whitening agent can absorb invisible ultraviolet light (wavelength range is about 360-380nm) and convert it to long blue or violet wavelength. visible light can compensate for unwanted yellowness in matrix, and at same time reflect more visible light than original incident light wavelength in range of 400-600nm, so that product looks whiter, brighter and brighter. Its function is to convert invisible ultraviolet light absorbed by product into purple-blue fluorescent light, which complements original yellow light to become white light and enhances whiteness of product under sunlight.
Second, classification of fluorescent whitening agents
The molecular structure of a fluorescent bleach mainly includes two parts: a continuous conjugated system containing aromatic rings and one or more substituent groups. The conjugated system contains certain π-electrons. After absorbing ultraviolet rays, x-electrons are excited from ground state to active state, and then return to ground state in a very short time. At this time, blue fluorescence with a wavelength of 420-450nm is emitted. According to its structure, it can be divided into following nine categories:
Carbocyclic fluorescent whitening agent refers to a type of fluorescent whitening agent that does not contain a heterocycle in parent moiety, and a substituent in the parent moiety does not contain a heterocycle. There are three main types of parent molecules that make up carbocyclic fluorescent whitening agents: 1,4-stilbenbenzene, 4,4'-stilbenbiphenyl, and 4,4'-divinylstilbene, which are typical varieties: ER fluorescent whitening agent, CBS-X fluorescent whitening agent , LeukophorEHB (foreign varieties).
Triazinylaminostilbene fluorescent whitening agents with this type of structure are most diverse among existing fluorescent whitening agent products, and more than 80% of fluorescent whitening agents belong to this type of structure. It is widely used for bleaching cellulosic fabrics. , paper, recycled fibers and detergents Typical varieties are DMS fluorescent bleach.
This variety is also called in our country: Jinjin 33# fluorescent bleach, which is often used in solid detergents.
(3) Stilbene triazoles
This type of fluorescent bleach came before. The disadvantage is that fluorescent tone is greenish, and bleaching whiteness of fiber is not high enough, and now it is gradually leaving market. So far, two types of fluorescent whitening agents have been used: symmetrical and asymmetric. Typical varieties with asymmetric structure are: TinopalPBS, mainly used for bleaching cotton fiber and some synthetic fibers in papermaking, soap making and other industries, with blue fluorescence,
A typical species with a symmetrical structure is: Blankophor BHC, which is mainly used for bleaching and washing cotton fiber.
The production of benzoxazoles is second only to triazinylaminostilbene fluorescent bleaches. Most varieties are high-performance fluorescent bleaches with high prices. Benzoxazole groups are very easy to introduce into molecule, and they participate in conjugation of electrons in molecule to lengthen conjugated chain of molecule. CI Fluorescent Bleach 393, whose trade name in my country is OB-1 Fluorescent Bleach, is widely used for bleaching polyester fiber resin mother liquor.
There is another type of asymmetric structure. The structure of a typical variety is as follows: it is not usually used as a separate component, but is often used together with other fluorescent whitening agents with a similar structure to form a mixed fluorescent whitening. agent.
(5) Furans, benzofurans and benzimidazoles
Furan, benzofuran, and benzimidazole are not precursors to fluorescent whitening agents, but are building blocks of fluorescent whitening agents. They can be combined with other building blocks (such as biphenyl) to form a good fluorescent whitening agent. The combination of furan and biphenyl is structurally similar to combination of styrene and biphenyl. Such combinations containing sulfonic acid groups are highly soluble in water and are particularly suitable for bleaching nylon and cellulose fibers. The structure of a typical connection is as follows:
1,3-diphenylpyrazoline compounds have a strong blue fluorescence. A typical option is DCB Blancofor (CI 121 Fluorescent Bleach): The trade name in my country is DCB Fluorescent Bleach, which is widely used to bleach acrylic fibres.
Coumarin itself has a very strong fluorescence, and various substituents can be introduced into positions 4 and 7 to make it a fluorescent whitening agent of practical importance. However, fastness properties developed at an early stage are not good, instead, more complex substituent groups, especially heterocyclic groups, are introduced into positions 4 and 7, and resulting grades have higher whiteness, higher light fastness. Typical varieties are UvitexWGS (fluorescent whitening agent 52 C.I.):
With basic structure of coumarone, it is used in celluloid, PVC plastics, etc. and has a strong blue fluorescence.
1,8-Naphthalimide and its N-derivatives have a strong green-yellow fluorescence and have been used as fluorescent dyes such as: C.I. Solvent Yellow 44, amino acylation, then maximum fluorescence wavelength of these compounds is shifted to blue, which is suitable for use as fluorescent whitening agents. The naphthalimide fluorescent whitening agents currently in use are mostly derivatives substituted with one or two alkoxy groups in positions 4 and 5. Typical varieties are: MikawhiteAT (C.I. fluorescent whitening agent 162), no domestic production has been observed. It is used for fibers such as polyester, rayon and nylon and has a blue fluorescence.
Pyrene-based XMF fluorescent bleach is mainly used for bleaching polyester, nylon and acetate fibers, as well as for bleaching polystyrene and polyvinyl chloride. In addition to being used to bleach fibers, it is also widely used in stationery products such as fluorescent markers.
Third, main properties and applications of optical brighteners commonly used in plastic products
3.1 Fluorescent Bleach OB
CAS number: 7128-64-5, chemical name 2,2'-(2,5-diphenylthio)bis[5-(1,1-methylethyl)]benzoxazole. Molecular formula: C26H26N2O2S, molecular weight 430. The appearance of product is light yellow or milky white powder, and light transmission is 296% at 450nm and 297% at 500nm. According to structural type, this is a benzoxazole compound, sparingly soluble in water, but soluble in alkanes, mineral oil, and common organic solvents.
OB fluorescent bleach can be used to bleach thermoplastics, polyethylene, polystyrene, polyethylene, polypropylene, ABS, acetate fiber, paint, coating, printing ink, etc. OB fluorescent bleach is used to bleach polymers at various stages. processing gives processed products a bright white-blue sheen.
In actual use, OB fluorescent whitening agent should be added at 0.01~0.05% by weight of plastic, and it can be fully mixed with various plastic particles. Similar foreign products include: UvitexOB.
3.2 Fluorescent Bleach HE
Molecular formula: C24H16N2, main component is derivatives of bisstyrene type, molecular weight 332. Appearance - yellow powder, white-blue tint. The fluorescent whitening agent does NOT have good fluorescence intensity and high whiteness. Good thermal stability, high temperature 300*C does not decompose. Excellent dispersibility, easy to mix with plastics. It is suitable for bleaching and brightening PP, PE, PS, ABS and other plastics, especially PVC profiles, sheets, plates, pipes and cables.
In actual use, add fluorescent bleach HE20--60g (% by weight of plastic raw material) for every 100kg of plastic, and in special cases (for example, when proportion of filler is high), this must be determined experimentally. ). The dosage of HE fluorescent whitening agent on PVC cable material is only half that of PF, and whiteness is much higher than that of PF.
3.3 Fluorescent Bleach KCB (FBA367)
CAS No.: 5089-22-5 (63310-10-1) refers to a benzoxazole type fluorescent whitening agent, and its chemical name is 1,4-di(benzoxazolyl-2-yl)naphthalene. Appearance - light yellow or milky white crystalline powder.
KCB fluorescent bleach is widely used in various fields, such as plastics, coatings, paints, textiles, etc. It can prevent product from discoloration, anti-yellowing, prevent aging, and prolong service life. Good whitening effect, better whitening effect on PVC, PP and other plastic products, especially perfect whitening and brightening PU leather products, and not yellowing or fading in long-term storage.
In actual use, amount of KCB Fluorescent Whitening Agent is typically 0.01-0.05% by weight of object to be bleached. The bleaching agent is fully mixed with required bleaching granules and shaping process can be carried out.
3.4, fluorescent bleach OB-1
CAS No: 1533-45-5. Chemical name - 2,2-(4,4-distyryl)bisbenzoxazole, appearance - bright yellow crystalline powder.
This product has a melting point of 351-358*C, withstands high temperatures of 375°C, has best heat resistance of all bleaches, especially suitable for high melting point plastics such as polyester (PET), OB-1 has a strong pole fluorescent whitening power, good stability, compatible with many polymers, is an effective fluorescent whitening agent for plastic products, widely used in ABS.PS.HLPS.PA.PC.PP.EVA and hard PVC and other plastics. OB-1 is also commonly used to correct polyester fiber, polypropylene fiber, nylon fiber, nylon and polyester, after adding OB-1 it also greatly improves whiteness. After adding OB-1 to recycled polyester fiber waste, bottle material and chips, recycled material will look whiter, and color of various recycled materials will become whiter and more uniform. For bleaching high temperature plastics, OB-1 is highly recommended.
OB-1 fluorescent bleach is added at 0.01~0.05% to plastic or polyester granules before forming and processing various plastic products or polyester fiber spinning, and can be fully mixed with plastic.
3.5 Fluorescent Bleach FP-127 (FBA378)
Appearance: light yellow or milky white crystalline powder, maximum absorption peak: 368 nm, maximum fluorescence reflectance: 436 nm, specific gravity: 1.23 g/m3.
Fluorescent bleaching agent FP-127 has a good whitening effect on various plastics and plastic products, especially on products made of polyurethane, polypropylene and other plastics, as well as artificial leather products. yellowing and no fading during long-term storage.
Add bleach FP-127 in an amount of 0.01-0.05% by weight of plastic before molding and processing various plastic products and mix well with plastic particles.
3.6. Fluorescent bleach KSN (FBA368)
CAS number: 5242-49-9, molecular formula: C29H20N2O2. Appearance - bright yellow powder. Insoluble in water, soluble in high boiling organic solvents such as benzene trioxide. The structural type is a stilbenbisbenzimidazole compound.
KSN fluorescent bleach has a chemical structure similar to OB-1, but its bleaching effect on polyester fibers and plastic products is better than that of OB-1, and its compatibility with plastics is better than that of OB-1. A very small amount can give a very good whitening effect, which is significantly inferior to OB-1. KSN not only has excellent high temperature resistance, but also excellent sun and weather resistance. KSN fluorescent bleach is also suitable for bleaching polyamide, polyacrylonitrile and other polymer fibers, and can also be used in films, injection molding and extrusion materials. Adding KSN at any stage of processing synthetic polymers has a great effect. Good bleaching effect.
Add KSN fluorescent whitening agent at 0.01-0.05% equivalent to weight of plastic or polyester pellets before various plastic molding or polyester fiber drawing processes, and mix completely evenly with materials.
Four, Precautions when choosing fluorescent brighteners commonly used in plastic products
4.1 The processing and molding temperature of plastics is relatively high, often reaching 200-300°C or even higher, requiring selected fluorescent bleach to be stable at that temperature. Below are melting point figures for each type of bleach, which refer to temperature at which pure bleach changes from solid to liquid when heated. The higher melting point, better resistance to high temperatures.
(1) OB-1: melting point 355-360℃
(2) FP-127: melting point 219-221℃
(3) KCB: melting point 210℃-212℃
(4) KSN: melting point 335-345℃
(5) OB: melting point 198-203℃
The use of some bleaches requires bleach to be resistant to hightemperature, so you need to know melting point of bleach. (If melting temperature is below theoretical value indicated in instructions, bleach may not be a pure product)
4.2、 Most of outdoor advertising and shop signs we see are made of plastic. For this type of plastic, which has been exposed to air for a long time, in order to maintain its bright color, fluorescent plastic bleach must have extremely high weather resistance, that is, it is not easy to change color and turn yellow after long-term use. After many years of research and experimentation, we have come to conclusion about stability:
4.3, Many fluorescent bleaches partially dissolve in plastic at a certain processing temperature, at which time fluorescent bleaches easily migrate into these substrates. Thus, fluorescent whitening agent must have excellent migration stability and cannot migrate or penetrate other materials.
a: OB-1 has advantage of a good initial color and a small amount of additive will have a good whiteness. However, duration of its whiteness is very short, and there is a risk of precipitation if amount of pure product exceeds 2/10,000 of plastic resin. Not suitable for soft foods.
b: OB is most resistant to migration and yellowing. But it has worst whiteness of any bleach when adding same amount of grams.
c: FP is not recommended for use in polyethylene products as there is a risk of precipitation.
4.4. Chemically inert and compatible with other additives. In processing of plastics, many additives are necessarily added, such as titanium dioxide, plasticizers, pigments, fillers, etc. Among them, titanium dioxide can also absorb some of ultraviolet rays, which practically reduces light absorption of fluorescent bleach. agent, thus affecting bleaching agent effect of plastic bleaching agent. If both are to be used at same time, their compatibility must be considered. Some additives, such as hindered amine light stabilizers, since their light stabilization mechanism is an ultraviolet absorption mechanism, their presence will also affect absorption of ultraviolet light by fluorescent whitening agents and affect whitening effect of whitening agents.
4.5, high purity. For fluorescent whitening agents, high purity means high fluorescence efficiency. The presence of a small amount of impurities, sometimes even a small amount of solvent, reduces whitening effect of fluorescent whitening agent due to fluorescence quenching effect.
4.6, non-toxic and harmlessden for environment. In our lives, plastic products play an important role and often come into contact with food or human body. The fluorescent bleach used for this type of plastic must be non-toxic and meet certain hygiene standards such as FDA, MITL, EU sanitation. Regulations, etc. In addition, it is also very important to be environmentally friendly during use or even when it is thrown away.
Source:Lianda Chemical, Hongyi Chemical, Renke Team Combination
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