Issues that should be paid attention to when selecting and using optical brighteners in the papermaking process

Issues that should be paid attention to when selecting and using optical brighteners in the papermaking process
  1. water
     Water is essential for papermaking and pulping. In particular, wet papermaking uses a lot of water. To produce high-whiteness paper, fluorescent whitening agents must be added during the papermaking process. At this time, special attention must be paid to the hardness of the water and the acid in the water. , the influence of the content of free chlorine and heavy metal ions, such as iron, copper, manganese, etc. on the whitening effect.
    During the process of dissolving and using optical brighteners, you should try to avoid contact with heavy metal ions such as iron and copper. It is best to use inert containers such as plastic or stainless steel. If such containers cannot be used, keep the distance between the optical brightener solution and metal equipment as short as possible. contact time to prevent precipitation of metal salts.
There are different views on the impact of water hardness on the whitening effect of optical brighteners. Hard water can increase the whitening effect of some optical brighteners (for example, it is beneficial to promoting dyeing), but in some cases it may damage the whitening effect. , at this time, soft water should be used to dissolve the fluorescent whitening agent alone, and try to use water with low hardness or softened water.
For recycled white water, the aluminum ions in it must be properly treated. When reusing the white water, it is advisable to first correct the pH value to be close to neutral. Since the reuse of white water can easily cause the accumulation of heavy metal ions, it is generally better to use tetrasulfonic acid-based fluorescent whitening agents.
2. Basic whiteness of pulp
     Since the whitening of fluorescent whitening agents is an optical complementary color brightening, it does not affect the pulp.
It has a bleaching effect and cannot replace bleaching agent to bleach pulp. Therefore, for pulp containing a lot of lignin and other pigments, if it is whitened directly with fluorescent whitening agent without bleaching, satisfactory results will not be obtained.
There are also tests that prove that the whiteness of pulp whose whiteness is more than 80% of the whiteness standard of the papermaking industry can reach more than 95% of the whiteness standard when whitened with a fluorescent whitening agent. However, it is difficult for paper with lower whiteness to reach the above level even if the amount of fluorescent whitening agent is increased.
3. Chemicals for papermaking
  With the exception of cationic products, few chemicals commonly used in papermaking have an effect on whitening. Special attention should be paid to the fact that if there are cationic additives in the pulp, they will combine with the anionic fluorescent whitening agent to form an insoluble mixture, which will affect the whitening effect. Therefore, when using, pay attention to the order of adding cationic additives and select appropriate adding points to reduce or eliminate the impact.
    Cationic wet enhancers, especially polyethyleneimine products, a small amount of urea-formaldehyde and melamine-formaldehyde products, also have adverse effects on some optical brighteners.
4. Filling
    Different optical brighteners, due to their different chemical structures, have different affinities to fillers such as silicates commonly used in the paper industry, such as kaolin, titanium dioxide and talc. In most cases, however, these fillers are yellowish in color and the fillers can only be whitened to a certain extent. Adding fillers to bleached and whitened papers often results in a darker whiteness, as fillers are usually more yellow in color than bleached cellulose pulp. In order to give full play to the effect of the fluorescent whitening agent, the filler selected should be as white as possible. Titanium dioxide absorbs in the UV range, and the rutile type absorbs more strongly than the anatase type. Therefore, rutile is not suitable as a filler when using optical brighteners. Calcium silicate and magnesium silicate fillers have minimal adverse effects on optical brighteners.
5. Select fluorescent whitening agents scientifically and rationally
    If the above points are the basis for the fluorescent whitening agent to work, then choosing the appropriate fluorescent whitening agent according to the pulp product is the prerequisite for the scientific and reasonable use of the fluorescent whitening agent. Before use, a small sample test should generally be done to determine The type, dosage and optimal process conditions of the selected fluorescent whitening agent.
    The papermaking industry mostly uses anionic water-soluble fluorescent whitening agents, and most of them are 4,4' diaminostilbene 2,2' disulfonic acid (DSD acid) sodium salt, ammonium salt or potassium salt. It has a natural affinity and can be adsorbed to fibers.
    DSD acid bistriazine fluorescent whitening agents have different properties depending on the number of sulfonic acid groups they contain, but there are also certain rules, which can be summarized as follows:
     (1) The more sulfonic acid groups, the smaller the affinity for pulp fibers.
     (2) The more sulfonic acid groups there are, the less sensitive the cationic polymer is.
     (3) The disulfonic acid-based varieties are suitable for use under neutral or weakly alkaline conditions, and the tetrasulfonic acid-based and   hexasulfonic acid-based varieties are suitable for use under weakly acidic conditions.
     (4) The more sulfonic acid groups there are, the less affected it will be by heavy metal ions.
6. The amount of fluorescent whitening agent and the yellowing point
    The dosage of optical brightener is usually expressed as a percentage (%) relative to the weight of the absolutely dry pulp. The whitening effect of fluorescent whitening agents on paper pulp varies depending on the dosage. The "yellowing spots" of whiteners.
   The yellowing point mentioned above is a common feature of fluorescent whitening agents, and different fluorescent whitening agents have different yellowing points. Therefore, before using fluorescent whitening agents, you must do a small sample experiment in advance to determine the optimal dosage, and then control the dosage below the "yellowing point" as needed, so as to obtain the best whitening effect and economic benefits. Otherwise, it will not only cause It is wasteful and cannot obtain satisfactory results. Sometimes the more fluorescent whitening agent is used, the worse the whitening effect will be, and the user cannot find out the problem. Papermaking workers should pay special attention to this.
7. Effect of pH
    The whitening effect of optical brighteners changes with the pH value, and the degree of change varies with different types of optical brighteners. Many optical brighteners have high whitening effects at pH = 6 to 8, but relatively low whitening effects at pH = 4 to 4.5.
The pH value has a great influence on the whitening effect of some fluorescent whitening agents. Especially in acidic sizing, in order to obtain the best effect, the pH of the paper stock on the net or the white water under the net is often controlled = 4. 5 ~ 6, which has poor acid resistance. For fluorescent whitening agents, when the pH value of the slurry is lower than 5.5, the fluorescent whitening agent solution becomes turbid or produces floc, and the whitening effect will become worse. Therefore, attention should be paid to the use of fluorescent whitening agents at this time. Adding location (order of addition) and adjusting the pH of the slurry.
     According to practical production experience, fluorescent whitening agents with poor acid resistance can achieve significant whitening effects when used under neutral or slightly alkaline conditions. The whitening effect decreases when pH=6 or below.
8. Adding sequence
      Generally, the aqueous solution of fluorescent whitening agent is weakly alkaline, with a pH value of 8 to 9. When encountering aluminum sulfate, a white or light yellow water-insoluble aluminum/fluorescent whitening agent flocculent precipitate will be generated, and the particles will be larger. The conical slag removal in the rear section is discharged out of the system, which will reduce the retention of the fluorescent whitening agent and seriously affect the whitening effect. Therefore, during the sizing process, the fluorescent whitening agent cannot be added at the same time as aluminum sulfate. Usually the fluorescent whitening agent should be added before other auxiliary materials, so that it can be evenly and firmly combined with the fibers and retained, and then other auxiliary materials can be added. The time interval between adding the fluorescent whitening agent and other auxiliary materials should not be less than 10 minutes, and if conditions permit, extending it to more than 20 minutes will give better results. Optical brighteners that are completely absorbed into the paper fibers are not affected and their whitening effect cannot be impaired even in the presence of excess aluminum.
9. Light
    Since all fluorescent whitening agents used in papermaking contain a stilbene structure, fluorescent whitening agents with a stilbene structure will produce cis-trans isomerism under the action of light (including sufficient ultraviolet light).
    Phenomenon. Under sunlight, it will change from the highly fluorescent trans structure to the non-fluorescent cis structure, and this cis-trans isomerization change becomes more significant as the concentration of the fluorescent whitening agent aqueous solution decreases, thus seriously affecting the whitening effect. , so the fluorescent whitening agent solution should be kept away from sunlight and stored in a dark place.