1 Overview
As the main binding process today, the wireless glue binding has many advantages. First, the hot melt adhesive used in glue binding is non-toxic, solvent-free, non-polluting, almost odorless, and has a fast curing speed. It is suitable for use on high-speed binding linkage motives. The binding speed meets the need for fast, efficient, and high-quality printing. Second, it can bind thick books, breaking the limitation on the thickness of books made of iron wire, and the formed rubber film has a certain degree of elasticity, which alleviates the difficulty of opening the books with iron wire books. . Due to various advantages, hot melt adhesives have become the mainstream of bookbinding processes. Therefore, the quality of hot melt adhesives becomes the key to the binding of books.
2 EVA hot melt adhesive composition
Hot melt adhesive for bookbinding is a kind of solventless thermoplastic compound. It is solid at room temperature, white or yellowish, and it becomes liquid with good fluidity after being heated and melted. It is smeared on the adherend and cooled and solidified. Strength. EVA hot melt adhesive consists of basic resin, tackifier, viscosity modifier, filler, antioxidant and other components.
1) Basic resin
The basic composition of EVA hot melt adhesive is ethylene-vinyl acetate polymer resin EVA which is a copolymer of ethylene and vinyl acetate under high pressure. Its melting temperature, melt index (MI) and molecular weight and vinyl acetate VA content. Adjusting both results in a variety of copolymers. Practice has proved that vinyl acetate VA content of 20% to 30% by weight of the resin has good strength and toughness, suitable for the adhesive of the basic resin. The chain is more flexible, and the non-polar polyethylene part gives the melted glue better fluidity and the formed film has a certain elasticity. The low-molecular-weight EVA resin has desirable fluidity required for hot-melt adhesives, so the base resin generally has a low molecular weight resin having a vinyl acetate (VA) content of 20% to 30%.
2) Tackifiers
Due to the poor thermal stability and wettability of the base resin polymer, the adhesive force is lost after the temperature is lowered, and therefore, some eutectic substances are added to adjust the adhesiveness. When selecting the hot melt adhesive viscosifier, the key considerations are the chemical composition, softening point, price, color, thermal stability, and meltability with other components of the hot melt adhesive. Among them, softening point, thermal stability, and meltability are the most important. Commonly used tackifiers include gum rosin, wood rosin, modified rosin, terpene resin, petroleum resin and the like.
3) Viscosity modifier
The main function of the viscosity modifier is to reduce the melt viscosity, increase the fluidity, wettability, regulate the solidification rate, and make the operation more convenient. Paraffin waxes are commonly used as viscosity modifiers. Paraffin wax is the most effective modifier for hot melt adhesives. The choice of the type and amount of wax has a great influence on the success or failure of the formula. If the amount of film is too large, it is easy to produce shrinkage. If it is serious, it will leak oil traces. If it is too small, it will not achieve the purpose of dilution. Paraffin waxes commonly used in hot melt adhesives include paraffin wax, microcrystalline wax, and polyethylene wax.
4) Packing
To reduce costs, adjust viscosity, and reduce shrinkage, some fillers need to be added. Titanium dioxide and other materials are usually used as fillers in bookbinding hot melt adhesives. At the same time, they can adjust the color of the glue to match the white paper. But do not use too much, otherwise it will reduce the adhesive strength of the glue.
5) Antioxidants
The use of hot-melt adhesives must be heated at high temperatures for a long time, in order to ensure that it does not decompose in this situation, do not cross-link, to add some antioxidants. Antioxidants commonly used in hot melt adhesives are: 1010, BHT, etc., and the amount used is generally not more than 1%.
3 performance of hot melt adhesive
1) Viscosity and fluidity
Viscosity and flowability are one of the more important properties of hot melt adhesives and are closely related to the sizing properties. The viscosity of the hot melt can be reduced by selecting EVA with a melt index of MI 或 or a tackifier with a low melt viscosity. The paraffin wax is the one that has the greatest impact on this property of hot melt adhesives. Because wax is the least viscous component in hot melt adhesives, increasing the amount of wax can significantly reduce the viscosity of hot melt adhesives and increase their fluidity. Therefore, the viscosity of the hot melt adhesive is mainly regulated by the type and amount of the wax and the melt index of the EVA resin.
2) Elongation and flexibility
Flexibility is an indicator that bookbinding glue must control. If the flexibility of the glue is not good, the book after binding will not be easy to open, or it will be brittle, and the turning resistance will be reduced. The molecular weight of EVA resin directly affects the flexibility of the glue. The smaller the melt index, the less the flexibility of the glue. Paraffin wax also affects the flexibility of the rubber. In formulas, using microcrystalline wax or synthetic paraffin instead of ordinary wax can increase the flexibility of the rubber. The amount of isomerized and cyclized hydrocarbons in the wax molecules is high, and the resulting rubber has a high elongation. Therefore, two or more waxes can be mixed and used in the formulation, and the flexibility of the rubber can be adjusted by adjusting the proportion thereof.
3) Opening hours
The open time of the hot melt adhesive refers to the time from when the hot melt adhesive is applied to the adherend to the adhesion of the book's back or side glue to another adherend. The two adherends must be completed at the time specified by the hot melt adhesive so as to ensure the bonding strength. For the composition of the hot melt adhesive, the addition of the wax always shortens the opening time, and the degree of influence varies with the nature and amount of the wax. In general, the greater the amount of wax, the higher the melting point and the greater the degree of crystallinity, the shorter the opening time and the curing time of the hot melt adhesive.
4) Low temperature resistance
The low temperature resistance of hot melt adhesives is related to the reliability of the quality of the books. To ensure that the book does not loose or fall due to brittleness in the winter, the hot melt adhesive used must have good low temperature resistance, and the low temperature resistance of the hot melt adhesive depends on the flexibility of the glue, so the control glue The flexibility is very important. According to the plastic area to adjust the flexibility of the size of the rubber.
5) Aging
Rubber books and magazines are stored for a long time in the natural environment, and aging and brittleness occur, which means that the resin, rosin, and paraffin in the colloids undergo oxidative decomposition during storage. Another cause of aging is that the material used in colloids has a high content of low-melting substances, resulting in a reduction in the anti-aging properties of the glue. Therefore, when selecting materials, good quality materials should be used to ensure the quality of the glue.
4 Use of hot melt adhesive
The choice of hot melt adhesive should be based on different paper and different models to choose different types of hot melt adhesive. That is, general offset paper can be used with low viscosity and good fluidity after melting. If thick offset paper and coated paper are to be used, glue with high viscosity and strong adhesive strength should be selected. When the amount of sizing is reached, the use temperature should be reduced as much as possible. Before use, it should be preheated. Generally, it should be preheated for 1 to 2 hours. After reaching the heating temperature and having good fluidity, it is added to the plastic tank of the machine. The use temperature should be kept constant and generally controlled at 160°C to 170°C.
When the hot melt adhesive is heated, the temperature must be strictly controlled because the heating temperature of the hot melt adhesive is closely related to the adhesive fluidity and the adhesive strength. At 170°C, the hot melt adhesive has the best adhesive force. Above and below this temperature, the adhesive strength is reduced.
Hot-melt adhesives used in the plastic pot for a long time, there will be some aging deposits in the plastic pot, in order to ensure the quality of glue, it is necessary to regularly clean the plastic pot, remove some of the aging sediment, and then add all the new heat Melt glue.
Source: "Printing World"
