Wet end chemicals added before the pulp is put online, if not used properly, not only cannot produce the expected benefits, but also can have adverse effects on paper production or paper product quality. This article introduces the most common problems in the use of wet end chemicals.

From a technical perspective, the most common problem encountered in the application of wet end chemicals is the inability to achieve the expected results, resulting in paper machine testing failure or product replacement.

There are three main reasons for this situation,

Firstly, the modern paper industry has great diversity, from raw materials to equipment, from technology to management, different paper machines have significant differences, and one or several products are difficult to meet the different needs of most paper machines;

Secondly, due to market competition, paper mills are constantly adopting various new types of paper machines and paper making processes that are suitable for this paper machine, which makes many paper making processes highly specialized and often unsuitable for general application and problem-solving experiences and methods;

Thirdly, in the process of paper machine testing or application of products, it is inevitable to encounter problems of one kind or another.

Due to the continuous production characteristics of paper machines, once problems arise, they often need to be solved immediately. In such cases, there is often not enough time to adjust to achieve the best application effect of the product.

Retention aid

Retention aids are the most problematic wet end chemicals, mainly because they affect the fundamental factor of paper, namely the interweaving of the paper. Retention aids can affect the entire papermaking process through retention.

The most common problem in the application of retention aids is the deterioration of the uniformity of the paper. At present, all retention aids rely on flocculation mechanisms (flocculation and coagulation), and the result of flocculation is the formation of flocs. When the flocculation is too strong, the uniformity of the paper will deteriorate.

Therefore, when applying retention aids, it is necessary to control the molecular structure, dosage, and addition method of retention aids. Retention aids mostly have drainage functions, and improving the drainage performance of pulp often offsets the influence of flocculation on uniformity to a certain extent.

In order to produce effective retention effects, at least one product in the retention aid system should be a high molecular weight organic flocculant. When using solid flocculants, the solubility of the product is very high

Important. Flocculants have a wide range of uses, and different uses have different requirements for the solubility of products. For example, flocculants used for paper making must have a very low insoluble content, while flocculants used for wastewater treatment have low requirements. Modern large paper locomotives are becoming faster and faster, so high molecular weight flocculants are needed to resist the high shear forces experienced in the pulp flow and forming process.

When using high molecular weight flocculants, the solubility of the product is more important. Excessive insoluble substances in the product can cause difficulties in paper making.

dyestuff

Pigments, similar to paper fillers, cannot form effective chemical binding with fibers themselves. Therefore, when used in the wet end, they need to be retained in the paper by the action of retention aids.

When the pigment is not well retained, the unreserved pigment will accumulate in the white water at the wet end, eventually forming color spots on the paper surface.

Some dye products may experience uneven concentration during long-term storage. If the product is not stirred evenly during use, the lower concentration dye in the upper layer will enter the papermaking system, which will significantly increase the amount of dye used.

The vast majority of dyes used in the wet end are negatively charged, and some even have strong negativity.

If the dye dosage is high or the addition point is too close to the cationic wet end chemical, it may affect each other, causing color spots or reducing the product's effectiveness. Usually, a certain distance needs to be maintained between the addition points of dyes and cationic chemicals.

Fluorescent whitening agent

Fluorescent brighteners are substances with strong negative charges that can affect the electrical properties of pulp. High dosage or improper addition may reduce the efficiency of other cationic wet end chemicals.

When producing high whiteness cultural paper, in addition to adding fluorescent brighteners to the wet end, a high amount of fluorescent brightener needs to be used in surface sizing.

With the reuse of damaged paper, the demand for cationic charges in pulp will significantly increase, resulting in an increase in the amount of cationic wet end chemicals such as retention aids.

To reduce the impact of fluorescent whitening agents on other wet end chemicals, they are usually added before all cationic chemicals.

starch

In terms of dosage, starch is the third largest category of substances after fibers and fillers when used for wet end addition. Starch is an ideal nutrient source for microorganisms, and its circulation in white water can promote microbial reproduction and have adverse effects on the paper making process.

Starch usually needs to be gelatinized before it can be added to the pulp. When gelatinization is incomplete, starch particles that are not fully swollen can form transparent holes on the paper - "fish eyes", or cause sticking to the cylinder and the mesh.

Sizing agent

AKD and ASA synthetic sizing agents are widely used in neutral and alkaline papermaking. These two sizing agents will undergo hydrolysis during the white water cycle, causing deposits in the wet, pressing, and dry front of the paper machine, causing sticking and even paper breakage.

ASA hydrolysates initially exist in the form of free acids in pulp. When ASA hydrolysates encounter calcium carbonate in the pulp, the free acids will transform into calcium salts, reducing their solubility and depositing them.

Therefore, when analyzing ASA hydrolysates, the presence of calcium is often found.

Fungicide

Modern paper making processes widely use fungicides to inhibit the growth and reproduction of microorganisms in paper making systems. Fungicides are usually added to the paper machine sizing system, white water system, and storage tanks for some wet end chemicals.

There are various types of fungicides used in the paper making process, among which brominated nitrostyrene, benzothiazole, benzylammonium, and other types of products may affect the sizing effect of AKD or ASA if the addition point is not appropriate.

These fungicides have strong negative electricity and will react with the sizing agent, resulting in a decrease in the sizing effect of the sizing agent. To avoid mutual reactions between fungicides and other papermaking chemicals, compatibility experiments are usually required before use.

Defoamer

Defoamers achieve the purpose of defoaming by changing the surface chemical condition of bubbles, which is similar to the principle that sizing agents achieve water resistance by changing the surface properties of fibers. Therefore, improper use of defoamers can affect the sizing effect of sizing agents.

filler

The filler itself has chemical inertness, and its impact on paper making mainly has two aspects. On the one hand, using fillers with smaller particle sizes, such as precipitated calcium carbonate, can increase the difficulty of retention, especially when the retention method using retention aids is incorrect.

On the other hand, many paper mills use self-made or purchased pulpy calcium carbonate. Due to the need to use filler dispersants when producing pulpy calcium carbonate, most of the filler dispersants are anionic organic compounds. As the filler enters the pulp, it also increases the difficulty of retention.

Filler is also one of the important factors that cause paper to lose powder and hair during printing, which is particularly important for newsprint. Newspaper often uses deinked pulp as the raw material, and the ash content of the finished paper mainly comes from the ash content in magazine paper and the added fillers during paper making.

These two types of ash substances perform differently in terms of retention at the wet end and the shedding of powder and hair on the paper. The addition of fillers is more prone to the problem of powder and hair shedding. It is necessary to pay attention to increasing the amount of additional fillers to increase the ash content of the paper.

Paper making production cannot do without wet end chemicals. Due to the complexity and uniqueness of the paper making process, as well as the diversity of wet end chemicals, it is common to encounter situations where the results of certain products are not ideal, and even cause problems in paper making production.

Proper use of wet end chemicals not only requires a deep understanding of their principles of action and methods of use, but also requires rapid and effective adjustment based on experience when problems arise.