With the continuous development of our country’s social economy and the continuous improvement of people’s living standards, people’s demands for packaging materials are also getting higher and higher, not only to protect goods and facilitate transportation, but also to be able to greatly increase the added value of goods. Intensity, printability, cost, etc. impose higher requirements on corrugated paper manufacturers, and the future corrugated box industry will inevitably move toward high strength, low basis weight, and high printing fineness.
In the actual production of corrugated paper, there are mainly two ways to improve the quality of corrugated paper. One is to improve the performance of raw materials, and the other is to use chemicals rationally. As we all know, the raw materials for the production of corrugated paper are recycled paper, and the performance of raw materials is poor. If we start from the perspective of improving the structure of raw materials, optimizing waste paper materials or adding high-quality pulp, there will undoubtedly be more pressure on the cost. By using some chemicals reasonably, not only the cost is lower, but also the performance of the products can be significantly improved to meet the market demand. At present, the chemicals used in the production of corrugated paper mainly include reinforcing agents and sizing agents.
1. Reinforcing agents used in the production of corrugated paper
The reason why corrugated paper can meet the needs of commodity transportation and packaging is mainly due to its special structure and good strength properties. Among them, the ring pressure strength and stiffness are the main strength and performance indicators of corrugated medium. At present, based on the requirements of environmental protection and cost, reducing the quantification of corrugated paper is a trend in the development of corrugated cartons. At the same time, in order to maintain or even further increase the strength of products, it is only possible to use some enhancers with lower costs and significant reinforcement effects. Although the use of the reinforcing agent will cause the thickness of the paper sheet to decrease, it does not affect the strength of the single fiber, but can increase the bonding strength and the Z-direction strength of the fiber, thereby improving the ring pressure strength and reinforcing [1]. .
Some enhancers currently used have hydroxyl, amino, and other groups in their molecules and can form hydrogen bonds with the hydroxyl groups of the fibers to increase the binding strength of the fibers, thereby significantly improving the stiffness and ring pressure of the corrugated base paper. strength. According to the synthetic monomer, the currently used enhancers mainly include starch chemical aids, polyacrylamide enhancers, and copolymer emulsion type enhancers [7, 8].
1.1 Starch enhancer
The varieties of modified starches for paper use include anionic starch dry strength agent, oxidized starch, phosphate starch, carboxymethyl starch, starch xanthate, nonionic starch, cationic starch, zwitterionic starch, and starch and vinyl monomers. Graft copolymers account for a large proportion of papermaking enhancers. Among them, the starch used for reinforcing corrugated paper mainly includes anionic starch, cationic starch, and sprayed starch [1, 2].
Anionic starch can effectively increase the stiffness of the sheet, thereby improving the crushing strength of the corrugated base paper. The use of inter-layer spray starch can not only increase the bond strength between layers, but also improve the stiffness of the cardboard, thereby increasing the paperboard's ring crush strength, using spray starch in the corrugating medium of the entire waste paper, and the amount of paper sheet 135g/ M2, with a dosage of 3%, can increase the ring pressure strength by about 50%; with an amount of 6%, it can increase the ring pressure strength by about 67%. Applying cationic starch to corrugated base paper can increase both tensile strength and crushing strength, but it is more effective for increasing the split length of the paper sheet than the crush strength. The cold-dissolved cationic starch is used for 70% of the cardboard and 30 The corrugated pulp made up of % of paper and paper was added in an amount of 2.0 and the ring pressure index was increased by 23.92%~26.97%. Its use effect is equivalent to that of hot-soluble cationic starch (28.3%), but the former can be pasted at room temperature and is easy to operate. Lower cost.
1.2 Polyacrylamide (PAM) type enhancer
Polyacrylamide is a multifunctional water-soluble linear polymer. In the industrial production, acrylamide monomers are mainly prepared by catalyzing hydration with acrylonitrile as a raw material, and then acrylamide is radically polymerized into various polyacrylamides with relative molecular masses. PAM and its derivatives can be used as flocculants, thickeners, paper enhancers and liquid drag reducers [3,4].
When polyacrylamide is used as a papermaking reinforcing agent, because the fiber itself has a negative charge, and the large amount of secondary fibers is used at present, the environment of the wet part of papermaking is becoming more and more severe, and contains a variety of hetero ions instead of ionic polycondensation. Acrylamide itself has no charge and cannot be combined directly with the fiber, and its reinforcing effect is very small. Therefore, it must be modified first to produce an anionic, cationic or amphoteric polyacrylamide before use. In recent years, the use of polyacrylamide enhancers has gradually increased and there is a tendency to gradually replace modified starches.
Anionic polyacrylamide (APAM) is used as a dry reinforcement and must be complexed with Al3+ in the presence of high concentrations of aluminum sulfate. Electrostatic attraction causes the negative charge (R-COO-) carried by the polyacrylamide molecule to bind with the fiber. The negative charges are bridged to form a coordination bond, forming a spatial network structure, to enhance the role [1]. Because it must be paper-made under strong acidic conditions, and its charge is single, it is susceptible to the interference of the hetero-ion in the pulp, and the enhancement effect is poor. Therefore, it is not widely used at present.
Cationic polyacrylamide (CPAM) is an excellent enhancer. The cationic groups in the CPAM molecular structure are easily adsorbed on the negatively charged fibers, acting as bridging, promoting the interaction between the fibers, and at the same time adsorbing Reinforcing agents on the fibers help the amide groups form hydrogen bonds with the hydroxyl groups on the fibers, thereby increasing the strength of the paper. CPAM can not only improve stiffness and crush strength, but also effectively improve dry strength performance [1,2,4]. Compared with starch additives, cationic polyacrylamide can significantly improve the crush strength.
The reinforcement agent for amphoteric polyacrylamide type paper uses hydrogen bond between the amide group (-CONH2) in the polymer chain and the hydroxyl group (-OH) on the fiber (this bond energy is greater than the Van der Waals force between the molecules). Fibers are intertwined and reinforced to achieve the purpose of increasing paper dry strength [1,2,4]. In addition to the hydrogen bonding effect of the amphoteric polyacrylamide copolymer, the cationic functional group on the polymer chain can directly form an ionic bond with the cellulose negative charge, while the anionic functional group can be combined with the Al3+ in the system through coordination complexation. And the fiber forms a coordination bond, so through the role of amphoteric polyacrylamide, it can promote the formation of a cross-linking network between fibers. Including crack length, ring pressure strength, etc.). As amphoteric polyacrylamide has its unique advantages, it has become the focus of research at home and abroad.
Currently used in the production of corrugated paper are mainly cationic polyacrylamide and amphoteric polyacrylamide type. Adding 1% to 1.5% cationic polyacrylamide enhancer to ordinary waste paper pulp can increase the ring crush strength by 23%~30%[2]. Wang Jin et al. synthesized cationic polyacrylamide by free radical copolymerization and used it as a reinforcing agent for corrugated paper roll strength. The results show that the molecular weight of cationic polyacrylamide has a greater influence on the ring crushing strength of corrugated paper, with a molecular weight of 400,000. The left and right cationic polyacrylamides are best for improving the crushing strength and tensile strength of the corrugated base paper, and its reinforcing effect increases with the increase of the amount. Adding 0.6% of the copolymer to the recycling box pulp can make the corrugated paper The ring crush strength increased by 22.1%. For the amphoteric polyacrylamide type, there are also many related studies. Domestic Peng Xiaohong and Shen Jiarui synthesized poly(methacryloxyethyl trimethyl ammonium chloride-CO-acrylamide-CO-acrylic acid) amphoteric paper enhancer. According to experiments, after adding pulp, the dry strength and tear of the paper have been significantly improved. The synthetic modified polyamide resin (BNP-III-2) produced by Sichuan Bangbei Chemical Co., Ltd. is amphoteric complex ionic type. It contains a large number of active groups such as hydroxyl, amide and carboxyl groups, and is used in 90% of ordinary waste pulp and When 10% kraft pulp paperboard production 160g/m2 corrugated paper, adding 1% of BNP-III-2, the ring pressure index can all reach the C-class standard, saving cost [1]. Chen Gang and others compared the effect of cationic starch, amphoteric starch, PAM, and other papermaking aids in the improvement of corrugated papermaking. It was found that cationic PAM was the most effective for improving the strength of paper sheets, and the rest were amphoteric PAM, cationic starch, Amphoteric starch and anionic PAM, but with the increase in the amount of cationic starch, the use of cationic starch is better than the cationic PAM, considering the cost, the use of cationic starch is more favorable.
1.3 Copolymer emulsion type enhancer
Copolymer emulsion type enhancer is a new type of high-efficiency paper sizing agent, it uses a variety of functional monomer polymerization under certain conditions. According to the requirements of use, cationic or anionic monomers can be introduced on the polymer chain, and accordingly, cationic emulsion polymers or anionic emulsion polymers can be prepared. They can be used alone or in combination with other additives. At present, there are many researches on increasing the crushing strength of corrugated base paper by adding such additives [1,2,4].
Cationic emulsion type enhancers for papermaking are generally copolymers of styrene, acrylate, acrylamide, and quaternary ammonium salts. For example, methyl acrylate, styrene, acrylamide and methacryloyloxyethyltrimethylammonium chloride are used as raw materials to prepare a cationic copolymer (PMAA) emulsion by soap-free emulsion polymerization and added to straw pulp in an amount of 0.75% to 1.25% (for absolute dry pulp), the ring pressure strength index increased by 37.1% to 41.6%, and the tensile strength and burst strength increased by 30% and 100%, respectively [1].
The application of polyvinyl acetate in anionic emulsion is better. Polyvinyl acetate has a good reinforcing effect on the crushing strength of corrugated base paper. It is added to the original corrugated medium made from straw pulp or waste pulp and interwoven with each other to form a network structure. Just like adding "rebar" at the molecular level in the paper, the fibers in the paper can be better joined together. The shape is more fixed. It can increase the ring compression strength while increasing the physical index such as puncture strength and burst strength. Relevant data show that: vinyl acetate on the waste paper corrugated paper reinforced effect is more obvious, it can make its ring pressure strength increased by 20% to 130%. The reinforced corrugated base paper can meet or exceed certain physical specifications of the high-strength corrugated base paper currently used for exporting packaging cartons.