Mechanism and research status of wet strength agent for papermaking
Issuing time:2017-02-07 11:17
With the development of modern science and technology, the use of paper is more and more extensive, some of which require to be under the action of water, or in water application and processing. Such as sea drawings, special map paper, banknote paper, advertising poster paper, cotton paper, tablecloth paper, industrial filter paper, photographic base paper, tea filter paper, etc. It is necessary to give this kind of paper a certain wet strength performance, that is, adding wet strength agent in the paper making process or in the post-processing process, so that the paper has a certain wet strength.
1. Some concepts related to wet strength
1.1 wet strength
When the finished paper is saturated by water, most of the strength will be lost, and the remaining strength is called wet strength. It is difficult to define an absolute value for the wet strength of paper. Generally, the wet strength of paper is expressed as the ratio of wet 2 to dry strength, expressed as a percentage, where the strength usually refers to the tensile strength. The wet strength of paper can be divided into initial wet strength and re wet strength. The initial wet strength refers to the strength of the finished paper from the paper machine after rewetting with water. Generally speaking, the wet strength refers to the re wetting strength. Generally, non wet strength paper can only retain about 2% ~ 10% of its dry strength after being saturated with water. The wet strength of the paper can reach 20% ~ 40% of the dry strength of the base paper after adding the wet strengthening agent. At present, the paper products with wet strength over 50% can be made.
1.2 wet strength agent and wet strength paper
Generally, the strengthening agent that can keep the dry strength of the base paper more than 15% after being fully wetted with water is called wet strength agent . If the wet strength of the paper is more than 15% of its dry strength, it is called wet strength paper. Wet strength paper is usually distinguished by the duration of wet strength. Non wet strength paper will lose its strength within a few seconds after being saturated with water. Some wet strength papers only shorten the time of strength loss. This kind of paper is generally called "temporary" wet strength paper, while some wet strength agents can produce "lasting" wet strength.
2. Mechanism of wet strength
The strength of paper depends on the strength of the fiber itself, the strength of the connection between the fibers, and the arrangement and distribution of the fibers in the paper, that is, the binding force between the fibers and the fibers themselves, among which the most important is the binding force between the fibers. This kind of binding force is mainly the binding force of hydrogen bond. The ability of cellulose fiber to form hydrogen bond is due to the existence of cellulose hydroxyl group. The combination of hydrogen bond between fibers makes the fibers in the paper combine with each other without adhesive to give the paper a certain strength. D. Dunlop Jones believes that when paper is wet, in order to retain some of its initial dry strength, there are generally one or several methods: strengthening the original fiber bonding; protecting the existing fiber bonding; forming a new bond that is not sensitive to water; and creating a network of physical coating on the fiber. Generally speaking, there are two kinds of mechanisms for increasing the wet strength of paper: one is called "protection" mechanism, that is to protect the existing fiber bonding mechanism. The mechanism thinks that the wet strength agent will produce a cross chain network structure around the fiber, which can prevent the fiber from swelling and water absorption, so as to maintain the existing fiber hydrogen bond. This mechanism is considered as concentric cross chain mechanism. The other is the "strengthening" mechanism, that is, the formation of new, water-resistant inter fiber bonds and mechanism. This mechanism considers that there is a cross-linking structure between the fiber and the wet strength agent, which still exists after the other natural bonds are destroyed by water. Through the new water-resistant bond formed between the fiber and the wet strength agent, such as covalent bond and hydrogen bond, the fiber is connected by cross-linking. This mechanism is also known as "strengthening new bond or interlacing connection" mechanism. When the wet strength agent is added in the paper making process, there are not only interlaced fibers, but also added wet strength agent molecules in the mixed pulp contacting the wet strength agent and the fiber. After drying, the chemical changes occur between them and form bonds. Due to the wetting effect of the wetting agent, the water swelling agent prevents the fiber from penetrating into the fiber again.
3. Wet strength agent commonly used in paper industry
The main production method of early wet strength paper was adding water resistant agent into pulp to make the paper have water-resistant protective layer and adapt to its application, such as adding paraffin, polyvinylidene chloride, etc. It does not substantially improve the wet strength of the paper. Later, in the 1930s, it was found that some water-soluble synthetic resins were added to the paper pulp and solidified on the paper machine to give the paper wet strength. Since then, the application of wet strength agent has been developed rapidly. Generally, the wet strength agent must have the following four characteristics: it must be a high polymer and have a certain mechanical strength to protect the bonding between fibers from swelling and damage; it must be cationic and can be adsorbed on the negatively charged fiber to achieve rapid and complete retention; it must be water soluble or water dispersive to ensure uniform distribution in the slurry; it must be able to form a chemical network structure, The reaction is thermosetting, so that the paper has a certain resistance to water swelling. The selection of wet strength agent in paper industry should be based on the end use of paper, the price and characteristics of wet strength agent.