Talking about the composition, classification and use of hot melt adhesive
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1. The basic composition and function of hot melt adhesive
EVA hot melt adhesive is a solid, fusible polymer that does not require solvent or moisture. EVA hot melt adhesive is solid at normal temperature, and is heated to a certain temperature to become a liquid which can flow and has a certain viscosity. EVA hot melt adhesives generally consist of the following four basic components.
1) EVA resin
EVA resin is its main component, which determines the properties of hot melt adhesives, such as bond strength, melting temperature, tensile strength, temperature resistance and deformation properties.
2) Tackifier
It is the main additive. In the case of only EVA resin, although the glue has a certain adhesive force after melting at a certain temperature, the adhesion and the permeability to the paper are lowered after the temperature is lowered, and the book block and the cover cannot be firmly fixed. Bonding. Therefore, it is necessary to add a tackifier which can improve the bonding strength. The tackifier is generally a polymerized rosin, a terpene resin or the like.
3) Viscosity regulator
Its function is to lower the melting temperature, increase the fluidity and wettability, and adjust the solidification speed to achieve the purpose of firm bonding. Viscosity modifiers generally use microcrystalline paraffin or paraffin wax.
4) Antioxidants
Its function is to prevent high temperature aging of EVA hot melt adhesive during melting, and to ensure that the bonding performance does not change under high temperature conditions. Antioxidants are generally selected from the group consisting of di-tert-butyl-p-methylphenols.
2, hot melt adhesive performance classification
1) Classification by binding speed
In order to ensure the bookbinding speed, different types of hot melt adhesives should be selected according to different machine speeds. Hot melt adhesives are divided into three types according to the speed of binding: low-speed adhesive hot melt adhesive (3,600 copies/hour), medium-speed adhesive hot melt adhesive (about 8,000 copies/hour) and high-speed adhesive hot melt adhesive (10,000 copies/ About an hour).
The opening time and curing time of hot melt adhesives are closely related to the speed of binding. The opening time refers to the time when the hot melt adhesive is applied to one of the adherends and bonded to the other adherend. The open time means that the two adherends must be bonded within the specified time to ensure the bond strength. The opening time of the low speed machine is 15 to 20 seconds, the medium speed machine is 5 to 10 seconds, and the high speed machine is 2 to 7 seconds. The curing time of the hot melt adhesive means that after the cover and the book block are bonded within a predetermined time, it is necessary to cool after a certain period of time to turn the pressurization. This time is called the curing time. If the hot melt adhesive has not been solidified, the product is flipped or cut, the back of the book will be deformed, and the hot melt adhesive will stick to the blade during cutting, which will affect the quality of the product. It should be used reasonably according to the length of the conveyor belt, the conveying speed, and the seasonal changes. The hot melt adhesive curing time is generally 3 to 5 minutes.
2) Sort by paper
Can be divided into coated paper and general paper hot melt adhesive. For different types of paper, the corresponding hot melt adhesive should be used because the surface finish, permeability and thermal conductivity of the paper are different.
3) Classification by glue use
Divided into side glue and back glue. The hot melt adhesive used for the side glue has good fluidity, low viscosity, long opening time, long curing time and high bonding strength. The book side glue is required to be thin and uniform, and the bond is firm, so as to ensure the quality and cutting requirements of the book. The two glues cannot be used incorrectly.
4) Classified by color of glue
In addition, if it is a product of the same grade, yellow gum is better than white glue.
Because of its formula and process, only titanium dioxide is added to the white glue. Due to the poor meltability of titanium dioxide, the hardness and viscosity of the white rubber are lowered, and it is easy to precipitate, so that the heating device such as the preheating barrel is inconvenient to clean, and once deposited on the heating portion, the heating effect is lowered.
3, the use of hot melt adhesive
1) Heating conditions
Preheat before use. Generally preheating for 1 to 2 hours, after reaching the heating temperature and having fluidity, it can be placed in the plastic tank of 160 ° C ~ 180 ° C to be used normally. The temperature of use should be strictly controlled. When the temperature is too high, the adhesion will decrease, the fluidity and permeability will increase, and the viscosity and strength will decrease, which will affect the bond strength of the book. When the temperature of the glue is too low, the viscosity increases, the fluidity and permeability decrease, the glue becomes thick and brittle, and the bonding effect deteriorates. In order to prevent degumming and loosening of the sheet, the temperature of the coating should be controlled within the range of 170 ° C to 180 ° C. In order to ensure the quality of the sizing, the rubber pot should be cleaned regularly to remove the aged precipitate. In the cold and cold seasons, do not force the back of the book to avoid freezing and brittle fracture. It should be thawed at room temperature during use.
2) Stability of hot melt adhesive. It refers to the degree of stability that is exhibited after recycling or heating for a long time. If the thermal stability is poor, the color of the glue will become darker, the viscosity will increase, the permeability will decrease, and the bond strength will be affected.
3) The quality of hot melt adhesive is easily identified. Hot melt adhesives have many quality indicators, and require special instruments and appropriate testing environment. In the absence of equipment, first of all, the appearance of the gel type is consistent, with or without bubbles. Next, the glue was melted to form an adhesive tape having a width of about 1 cm and a thickness of 0.2 cm. After cooling, the tape is stretched by hand to see if the stretch rate can reach the standard, whether the strength is sufficient, and whether the fracture mouth can maintain the color. If there is a powdery substance, it indicates that the filler is excessive or unevenly mixed, or the filler particles are too large, which may cause a major quality accident. Next, the rubber particles were stored in an environment of 0 ° C, and after 24 hours, they were taken out and beaten with a hard object. If brittle cracking occurred, the low-temperature resistance was poor. After the glue is placed at a temperature of 40 ° C for a certain period of time, if the glue is soft, self-adhesive or sticky, it indicates that the softening point of the glue is low.
4, operating process requirements
1) The thickness of the adhesive is generally between 0.8 and 1.2 mm, and the adhesive layer should be uniform. Too thin a back coat affects the bond strength, and too thick increases the cost. The length of the glue should be slightly shorter than the cover size of 1~2mm. If it is too long, it will cause the glue to stick on the machine. It will cause the wild rubber on the back of the book or cause the cover to be uneven and damaged. It is easy to cause the fault of the transmission component to be too short. The sky and the feet are empty, which affects the quality of books.
2) The width of the upper side rubber is generally 3 to 7 mm, and the thinner the more uniform the better. In order to ensure its fluidity, the temperature of the glue should be controlled at about 180 °C. Do not use the backing glue for the side glue, otherwise it will lift the bar line.
3) The milling amount of the back of the book should be determined according to the thickness of the signature. Pay special attention to the folding method of the book page to prevent the phenomenon of shorts. To make the innermost page of each signature into a single page, the milling should be flush, and the back of the book cannot be milled into a bevel, otherwise the thickness of the glue layer is not uniform.
4) The back of the book must be grooved and the depth and spacing of the groove are the same. The quality of the milling groove is directly related to the adhesive strength of the book, avoiding the occurrence of loose pages and dropped pages. The milling groove is required to be smooth and clean, the paper wool is clean, the spacing is 5~10mm, and the groove depth is 0.5~1.2mm, which is beneficial for filling glue and improving tensile strength.