Factors affecting whiteness in papermaking process of fluorescent whitening agent

1 The principle of whitening fluorescent brightener

Due to the residual lignin in the fiber and the use of chemical agents in the pulping process, the paper always has yellowish shades. On the principle of optics, the paper absorbs the light energy of shorter wavelengths of natural light, reflected light The blue-violet visible light is a little lacking, and the paper we saw was yellowish. The use of fluorescent whitening agent is to release 420-450nl Il wavelength fluorescence under the stimulation of ultraviolet light (Uv) to supplement the insufficient blue-violet visible light segment wavelength and increase the apparent light sensitivity of total reflected light. Can increase the self-inductance of the paper. Fluorescent brighteners are dyes that have an optically white-feeling effect. Therefore, we also refer to optical brighteners as optical brighteners. Chemical bleaching alone cannot achieve satisfactory whiteness, and excessive bleaching can damage fibers. Thus, fluorescent whitening agents are considered to be the most economical agents for making white paper.

2 Types and utility of optical brighteners for papermaking Most of the optical brighteners used in papermaking are anionic direct dyes. General papermaking fluorescent brighteners can be classified into three categories based on their degree of sulfonation:

General type: It has enough fiber affinity, good pH stability, and good compatibility with starch. It belongs to the tetrasulfonic acid type fluorescent whitening agent, such as NT-3, NT-3B. Direct type: It has good fiber affinity, high fluorescence intensity, small amount, very economical, but poor stability of the pH, belongs to the disulfonic acid type of fluorescent whitening agent, such as NT-8.

Coating type: It has poor affinity with fibers and is not easily adsorbed on fibers, but it can be dispersed in coatings under any conditions, especially for various adhesives and cationic materials. The cost is higher and it belongs to hexasulfonic acid type fluorescent whitening agent. , such as fluorescent brightener RSB.

Special type: It has high affinity with fiber, excellent weathering resistance, high whiteness of finished paper, better acid and alkali resistance, but more expensive, such as fluorescent whitening agent FNW.

3 Factors and countermeasures affecting whitening effect of brighteners

3.1 Pure whiteness

The higher the whiteness of the pulp, the better the whitening effect of the brightener. It is only economically reasonable to use brighteners in high brightness pulps. For example, when the whiteness of the puree is below 70%/SO, the lower the whiteness, the worse the whitening effect. When the amount of whitening agent is the same, the value of whiteness is small. When the whiteness of the original pulp is 80% ISO, it is easier to whiten. Using 4 to 6 kg/t paper liquid, the brightener NT-3 or NT-3B can reach 90% to 95% ISO. If the conditions of the pulping and bleaching process equipment are limited, the whiteness of the pulp can only be less than 75% ISO. It is recommended to use NT-8, but the pH must be above 7 in order to have a better whitening effect. If it is deinking recycled paper, taking into account that some of the waste paper has been used in brighteners, by adding ordinary brighteners to significantly improve the whiteness of the finished paper is difficult, you can choose to try a special brightener F1Nw, due to the price Expensive, it is necessary to choose carefully, NT-3 a B type is also an optional product.

3.2 increase from the amount of agent

Under certain conditions of the process, the whiteness of the original pulp, brighteners are very important for the amount of dry pulp. The first is that as the amount of brightener increases, the whiteness rapidly increases and reaches a certain peak. After the peak, as the amount of whitening agent increases, the peak value of whiteness changes little or even decreases.

When the amount of brightener was increased from 0 to 0.4%, the whiteness increased linearly; however, when the amount was increased from 0.4% to 0.8%, the increase in whiteness value was small. At this time, the cost of brighteners has increased by a factor of two! The amount of brighteners used in production has reached 12 to 16 kg/tonne, while the whiteness still remains unchanged. This is the reason. Exceeding the amount of peak production, adding more brightener dosage, there is no point in improving the whiteness. Of course, at a certain machine, a specific type of paper, and under certain process conditions, what is the amount of whitening agent that achieves the peak value of whiteness, or what is the economical and reasonable amount of whitening agent? Each plant is different. Do some research work. Our recommended economic usage, NT-3, NT-3B is generally 4-8kg/t paper, NT-8 is generally 3-6kg/t paper.

3.3 pH and alum dosage

Generally speaking, brighteners can achieve better whitening effect at pH 7.0-9.0, especially NT-8. It is recommended to use above pH 6.5, such as pH below 6.5. NT-3 is recommended because NT-8 is a disulfonic acid type. Although the strength of whitening is higher than that of NT-3, its acid resistance is poor. The addition of alum in the papermaking system has a certain effect on the whitening effect. First, the pH of the system decreases with the addition of alum; the use of alum increases, the positive electropositive property increases, and the interference with brighteners increases; the third is alum. There are more iron ions, and brighteners are less tolerant to iron and copper ions.

3.4 Effect of fillers on brighteners

As the amount of filler increases, the whitening effect of the paper is also affected. When the amount of brightener W is 0.4% and the amount of filler is increased from W = 10% to W = 20%, the whiteness increases by 2 units. Different fillers have different effects on brighteners, such as Ti0, which has a greater effect than kaolin or chalk. Another important point is that different grades of fillers should be selected for paper grades with different whiteness. For example, talc with 83% ISO whiteness should not be used for producing 9o% ISO offset paper.

3.5 The effect of paste and broke on the whitening effect of brightener

It is very important to choose the proper ratio of pulp for papers with different whiteness requirements. The production of high brightness white paper should not use mechanical pulp containing lignin, such as SWG, TMP, C3"MP, etc. Because the effect of lignin on the whitening effect is very obvious. It is difficult to achieve high whiteness for papers containing this type of pulp. Fibers with different bleaching degrees have different reactivity to whitening agents, and from the viewpoint of economic benefits, the amount of recycled paper is indispensable, and the recycling of damaged paper has a negative effect on the whitening effect, which is due to the loss of paper. The presence of cationic chemicals in the product interferes with the whitening effect of the brightener, and therefore the production process should ensure that the ratio of the slurry is stable, and the loss paper must be stably reproduced, so as to ensure the stability of whiteness and hue.

3.6 Effect of Cationic Chemicals on Whitening Effect of Brighteners

As the speed of paper machines increases and the quality requirements increase, cationic chemicals are used in papermaking applications such as cationic polyacrylamides. All cationic chemicals have a negative effect on the whitening effect of brighteners. If handled improperly, even a small amount of cationic chemicals will severely reduce the whiteness of the paper. The stronger the positivity of the chemicals, the greater the effect. For example, the higher the degree of substitution of the cationic starch or the higher the charge density of the polyacrylamide, the greater the interference with the whitening effect.

In high-white high-grade cultural paper, sizing agent, retention agent and brightener are indispensable additives. How to control their negative effects and control the related interference between them is very important. The general measure is to add brighteners and cationic chemicals at different locations, or maintain their distance. It is especially important that no cationic additives be present in the furnish prior to the addition of the brightener. Therefore, it is very important to carefully select the additive points of various materials in the application of papermaking chemicals.

3.7 Application of Brightener in Surface Sizing

Brighteners can be added either in the wet or in the surface sizing. When high brightness papers are produced, they can be added in two places at the same time, but in general it should be avoided to add dyes to the surface sizing agent. , because it will make some paper diseases more prominent. When brighteners are used in surface sizing, full consideration should be given to the mutual interference between brighteners and surface sizing agents or functional chemicals added in surface sizing. At present, some surface sizing agents are positively charged and should not be mixed with brighteners. From the perspective of cost, the quantification of the paper is less than 80g/m2. It is not economically advantageous to add the whitening agent to the surface sizing. Above 80g/m2, there is an economic advantage. The reason is: usually, the surface sizing agent is generally of starch type. When the sheet reaches 80g/m2, the starch does not penetrate into the interior of the paper but remains on the surface of the paper sheet. The brightening agent also remains on the surface of the paper, and the whiteness is obtained. improve.

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