Issues that should be paid attention to when adding fluorescent whitening agents to plastics
1. Whitening effect of fluorescent whitening agents
The whitening effect is often expressed as whiteness, which is not only related to the amount of fluorescent whitening agent added, but also related to the compatibility of the resin and the light resistance of the plastic. Fluorescent whitening agents with good compatibility and good light resistance have good and long-lasting whitening effects. The most direct and effective way to test the whitening effect of fluorescent whitening agents is to conduct small sample experiments.
2. Dosage of fluorescent whitening agent
The amount of optical brightener is often expressed as a percentage relative to the mass of the plastic. The amount of fluorescent whitening agent used in plastics and its chemical structure, dispersion and plasticization in plastic formulations
agent or other additives. The dosage of optical brightener is generally 0. 05%-0. 1% (weight of plastic), for ultra-white varieties, the dosage can reach 0. 5% (weight of plastic) or more; for transparent plastics, the dosage is 1/100 to 1/10 of the above dosage. The appropriate dosage should be determined through small sample testing, in order to meet the application requirements. It is not that the more the amount of fluorescent whitening agent is used, the better the whitening effect will be under any circumstances, but there is a certain concentration limit, which exceeds a certain limit value. , not only has no whitening effect, but even yellowing occurs. Excessive dosage of fluorescent whitening agent can also cause incompatibility, migration, and reduced anti-aging properties.
The use of two or more different fluorescent whitening agents, if properly selected, will broaden the fluorescence spectrum and enhance the fluorescence intensity, thereby achieving the purpose of synergy and improving its application performance.
3. The influence of pigments in plastic formulations on the whitening effect
Since the function of the optical brightener is to convert ultraviolet light into visible blue light or violet light, the components that have the greatest impact on the optical brightener itself are those that can absorb ultraviolet light, namely white pigments and ultraviolet light stabilizers. Since titanium dioxide in white pigments can absorb the 380nm light wave in ultraviolet light, if it exists in plastic products, it will reduce the whitening effect of the fluorescent whitening agent. Since different crystal forms of titanium dioxide absorb 380nm light waves differently, the anatase type can only absorb 40% of the 380nm light; the rutile type can absorb 90%. If titanium dioxide is combined with a fluorescent whitening agent, anatase should be used Type titanium dioxide and appropriately increase the amount of fluorescent whitening agent.
White pigments vary in how much UV light they absorb. The ultraviolet light in the area that can act on most fluorescent whitening agents is also absorbed by anatase and rutile titanium dioxide. Zinc sulfate has a slightly poorer ability to absorb this part of ultraviolet light. This difference is most obvious in molded articles colored with white pigments containing optical brighteners. When the concentration of fluorescent whitening agents is the same, generally speaking, the white color achieved is the strongest when zinc sulfate is used, and the white color is the weakest when using rutile titanium dioxide. Under certain conditions, the whiteness cannot be significantly improved with rutile pigments. If the whiteness needs to be improved, the concentration of the optical brightener should be increased. For example, in a polyethylene formula, use 2.0% rutile pigment and 0.2% optical brightener, or use 5% anatase Type pigment and 0.05% optical brightener, you can get a white effect immediately.
4. The influence of UV absorbers
UV absorbers can absorb UV light, thereby reducing the whitening effect of fluorescent whitening agents. The effectiveness of optical brighteners depends on the properties of other pigments and UV absorbers in the formulation. Therefore, in products using fluorescent whitening agents, it is best to use histamine-based light stabilizers that do not discolor. If UV absorbers must be used, the dosage of fluorescent whitening agent should be appropriately increased.
The figure on the next page shows two UV absorbers widely used in plastics at 300 UV absorption in the 390nm range. The figure also shows the UV absorption characteristics of the dibenzoxazole structured fluorescent whitening agent. As can be seen from the figure on the next page, the absorption ranges of UV absorbers and optical brighteners overlap. The fluorescent whitening agent on the surface layer loses its whitening ability under the action of ultraviolet light, so the white color is weaker than when the formula does not contain ultraviolet light absorbers. Because of this limitation, it is recommended that optical brighteners be used only in formulations that do not contain UV absorbers.
UV absorption properties of fluorescent whitening agents and UV absorbers
A-Optical brightener 0. 002% B-Benzene type UV absorber 0. 002%
C-Diarylamine oxalate 0. 002% D-Benzotriazole type UV absorber 0. 002%
5. Other influencing factors
1. Form of fluorescent whitening agent (dosage form)
There are already fluorescent whitening agent masterbatches in China, which are much more convenient to use than ordinary powdered fluorescent whitening agent products and have good effects. If there are no special requirements, it is advisable to use optical brightening agent masterbatches for whitening.
2. Mechanical equipment
When using fluorescent whitening agents, the mechanical equipment (such as rollers, grinders, etc.) must be very clean. Trace amounts of red or yellow and other miscellaneous pigments will greatly weaken or even eliminate the whitening effect of the fluorescent whitening agent, so special attention should be paid to .
