My country's wood pulp, especially long-fiber wood pulp, is in short supply. Many paper types need to be equipped with short-fiber broad-leaved wood pulp, straw pulp, and recycled paper pulp. This will inevitably lead to a decrease in the degree of paper, so it is necessary to take measures to increase the strength of the paper. The strength of the paper is controlled by the bonding strength between the fibers. The bonding strength between fibers is directly related to the number and quality of hydrogen bonds formed by the bonding part. The application of paper reinforcement is a good way to strengthen the bonding between fibers, which can avoid the increase of the paper's tightness and temperature deformation caused by the increase of the beating degree. The opacity, absorbency, air permeability and even the tearing degree decrease.
The paper reinforcements currently used can be divided into dry reinforcements and wet reinforcements according to their functions. Dry strengthening agents (hereinafter referred to as dry strength agents) mainly include starch, modified starch, vegetable gum, polyvinyl alcohol and polyacrylamide (PAM), among which the most commonly used are PAM and cationic starch.
PAM can be divided into non-ionic type (NPAM), cationic type (CPAM), anionic type (APAM) and amphoteric type (EPAM) according to its basic structural characteristics. The retention rate of paper enhancer decreases, and the enhancement effect of paper decreases. In the past two to three decades, the research and development of enhancers can be said to be a method of assisting retention. Because there are anionic impurities in the wet end of the paper machine, the retention performance of the cationic reinforcer is relatively good.
NPAM is not easy to be absorbed by the fiber, so the reinforcement effect is not good. APAM bears the same negative charge as fibers and fillers, and aluminum sulfate is needed to increase APAM's role in attaching to fibers and fillers. However, since acid sizing usually uses aluminum sulfate as a precipitating agent, APAM is generally used in papermaking systems for acid sizing. CPAM is suitable for a wide range of pH and can be used in papermaking systems for acid sizing and neutral sizing. Relatively speaking, the stability of CPAM is worse than that of APAM, and the price is also higher. To solve these problems, many methods have been proposed.
PAM paper enhancers have been in practical use for more than 40 years. During this period, the raw materials and papermaking environment have undergone significant changes, and the additives have also been improved in various ways. It is difficult to obtain satisfactory results by using a certain kind of additives alone. It is developing in the direction of mixing APAM and CPAM, cross-linking high-density PAM, PAM-starch. formula.
1. PAM modified CPAM
1. PAM reacts with cationized amino-aldehyde resins, which has been reported in early US patents. Its advantage is that it can obtain cheap products. Amino-aldehyde resin itself has a large number of amino groups, which is very easy to combine with paper fibers. After cationization, it is easier to retain.
CPAM not only has a significant enhancement effect on paper, but also has a significant effect on the retention of fillers and fine fibers. Its papermaking pH range from acidic to neutral can get better results. But this product has a fatal weakness, namely limited storage time.
Moore and others developed this process. He used PAM with a relative molecular mass of (20~29) x 10* and acidified trimethylol melamine at 50~100C for 15s to 45min to get the product. See Table 5-1 for the enhancement of various paper indexes.
Table 5-1 PAM enhancements to paper
2. Mannich reaction modifier
The Mannich reaction is usually modified by dialkylamine, and its general formula is as follows:
This modified product has relatively good performance even if its substitution degree is low. In addition, its price is relatively cheap, so a large number of amphoteric paper enhancers currently on the Japanese market-part of this type. The key to this type of product is how to solve the free formaldehyde, dialkylamine or hydroxymethylated dialkylamine remaining in the product; how to extend the life of the product and the degree of cation modification.
Xu Zhenxing and others studied various influencing factors of amine methylation by PAM. He first mixed formaldehyde with dimethylamine in a ratio of 1:1.2 (amount of substance) to produce dimethylaminomethanol, and then added a human system to react with PAM to generate CPAM. There is very little free formaldehyde in the product (<0.5%), and the stability period can reach more than 3 months. Mannich modified CPAM can also be prepared by reacting glyoxal with PAM. The earlier acetalized PAM was used as a wet strength agent for paper. The relative molecular mass of PAM is not greater than 2.5x10*, and the relative molecular mass through glyoxalation has increased. The functional group formed is CONHCHOHCHO, which reacts in an aqueous solution to form a polymer aqueous solution, which will occur at room temperature for 8 days. gel. Dauplaise et al. prepared glyoxalated PAM by inverse emulsion polymerization, and also added a cationic gene to make it have better stability and retention. But the process is relatively complicated and the cost is relatively high,
See Table 5-2.
Table 5-2 Stability of glyoxalated PAM prepared by inverse emulsion method
This product as a dry strength agent added to the pulp can improve the mutual strength between the fibers, so that the finished paper has higher bursting resistance, folding resistance and paper strength. Without reducing the strength, a higher proportion of waste paper ice, straw pulp, forage wood pulp and ground wood pulp can be used to produce high-grade paper, reducing the paper weight and reducing the beating time. As a wet strength agent, it also has excellent performance, low dosage, low cost, adding 0.2% can get good dry and wet strength, pH range of use is 4.0~8.0. .
3. Hofmann rearrangement reaction product
When PAM and NaOCl strong oxidant can be partially aminated under alkaline conditions, some amide groups can be changed to amine groups, which is the Hofmann rearrangement reaction of amides.
The product shows a significant enhancement effect, if it is low-quality pulp, the effect is greater. The disadvantage is that the primary amine group forms a lactam ring with the ortho-amide group and gradually loses its cationicity. In order to overcome this shortcoming, the Hofmann reaction of PAM is carried out under the coexistence of dimethylpropylamine or choline: the former generates urea type, and the latter generates urethane type. This product has many advantages (such as cationic groups are tertiary and quaternary amine groups) copolymerization type CPAM, such as large enhancement. .
Hofmann rearrangement products have a short CPAM validity. In order to reduce the formation of lactam rings, it has been proposed to react at low temperature: acrylamide and part of acrylonitrile, N, N-dimethylacrylamide copolymerization and rearrangement reaction, but the storage time is still not long. The best way is to make it in the paper mill and use it locally. According to the literature, when the reaction temperature is 50 ~ 110C, the rearrangement reaction time can be reduced to tens of seconds to several minutes, and the reactor can be installed on the papermaking machine to achieve local use.
4. PAM and Polyethyleneimine (PEI) The reaction product of PAM and PEI is relatively effective. There are a series of products in the early days, but due to the high price, the development is not fast.
5. PAM and hydrazine reaction
PAM reacts with hydrazine first, then acidified with nitrous acid, and then heated to obtain the product. Since the product contains isoflurate groups, it can react with hydroxyl groups to form covalent bonds, so the degree of enhancement is greater. However, the price of hydrazine is relatively high, and it must be used in large amounts. However, as a reactive paper enhancer, it has great potential for development.
Compared with the previous Mannich modified CPAM and Hofmann rearrangement CPAM, this type of product has better stability and acceptable price. And the performance is better, it is worthy of vigorous development in the future-class products. With the continuous development of China's economy, the demand for paper has risen rapidly, and the global environmental awareness has continued to increase, prompting people to reduce the amount of long-fiber wood pulp as much as possible. Therefore, the status of paper reinforcing agents must be higher and higher. getting bigger. As a paper enhancer with excellent performance, CPAM has huge development potential. If the stability and price of CPAM can be effectively solved, it is feasible to fully promote CPAM.