On the composition, classification and use of hot melt adhesives

1 The basic composition and function of hot melt adhesive

EVA hot melt adhesive is a solid, fusible polymer that does not require solvents and does not contain moisture. EVA hot melt adhesive is solid at room temperature, heated to a certain temperature into a liquid that can flow and have a certain viscosity. EVA hot melt adhesive generally consists 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, and temperature and deformation resistance.

2) Tackifiers are the main additives. In the case of only EVA resin, the adhesive after melting at a certain temperature has a certain adhesive force, but after the temperature is lowered, the adhesive force and the permeability to the paper are reduced, and the book block and the cover cannot be firmly fixed. Bonding. Therefore, it is necessary to add a tackifier that can increase the bond strength. Tackifiers are generally polymeric rosins, terpene resins, and the like.

3) viscosity modifier Its role is to reduce the melting temperature, increase fluidity and wettability, adjust the solidification rate, in order to achieve the purpose of bonding firmly. Viscosity modifiers are generally selected from microcrystalline wax or paraffin wax.

4) Antioxidant Its role is to prevent the EVA hot melt adhesive from producing high-temperature aging during melting, ensuring that the adhesive properties do not change under high temperature conditions. Antioxidants generally use di-tert-butyl p-methylphenols.

2 Hot melt adhesive performance classification

1) Classification by the speed of glue binding In order to ensure the binding speed of books, different types of hot melt adhesives should be used at different speeds. Hot-melt adhesives are divided into three types according to the speed of glue binding: low-speed glue hot melt adhesive (about 3600 / h), medium-speed glue hot melt adhesive (about 8000 / h) and high-speed glue hot glue (10000 / Hours or so).
Hot melt adhesive opening time and curing time are closely related to the speed of glue binding. The opening time refers to the time when the hot melt adhesive is applied to an adherend and adhered to another adherend. The open time means that the two adherends must be bonded within a specified time so as to ensure the bond strength. The opening time for low-speed machines is 15 to 20 seconds, medium-speed machines are 5 to 10 seconds, and high-speed machines are 2 to 7 seconds. The curing time of the hot melt adhesive refers to that after the cover and the book block are adhered within a specified time, cooling is required after a certain period of time in order to turn the pressurization. This time is called the curing time. If the hot-melt adhesive has not been cured, turned or cut after shaping, the back of the book will be deformed, and the hot-melt adhesive will stick on the blade when cutting, which will affect the product quality. When used, it should be reasonably selected according to the length of the conveyor belt, the conveying speed, and seasonal changes. Hot melt curing time is generally 3 to 5 minutes.

2) According to paper classification Can be divided into coated paper and general paper hot melt adhesive. Use different types of paper, use the appropriate hot melt adhesive, because the paper surface finish, permeability, thermal conductivity is different.

3) Classification according to the purpose of the use of glue is divided into side glue and back glue. The hot melt adhesive used for side adhesive has good fluidity, low viscosity, long opening time, long curing time, and high adhesive strength. Book side glue requirements to achieve thin and uniform, strong bonding, in order to ensure the quality of books and cutting requirements. Two kinds of glue can not be used wrong.

4) According to the color classification of plastic In addition, if it is a product of the same grade, yellow plastic is better than white plastic. Because of its formula and process, only titanium dioxide is added to the white glue. Due to the poor fusibility of titanium dioxide, the hardness and viscosity of the white rubber are reduced, and precipitation is liable to occur, which makes it difficult to clean the heating device such as a preheated barrel. Once deposited on the heating site, the heating effect is reduced.

3 Use of hot melt adhesive

1) Heating conditions Warm up before use. General preheat 1 ~ 2 hours, to reach the heating temperature has fluidity, and then into the 160 °C ~ 180 °C plastic tank, in order to normal boot use. To strictly control the use of temperature. If the temperature is too high, the adhesion force will decrease, the fluidity and permeability will increase, the viscosity and strength will decrease, and the bond strength of the publications will be affected. When the temperature of the glue is too low, the viscosity increases, fluidity and permeability decrease, the glue thickens and becomes brittle, and the adhesive effect becomes worse. In order to prevent degumming and loosening, the sizing temperature should be controlled within the range of 170°C to 180°C. To ensure the quality of the glue, the glue pan should be cleaned regularly to remove the aged sediment. In cold and cold seasons, do not forcefully press the back of the book to avoid freezing and breaking. Use it at room temperature to thaw.

2) The stability of the hot melt adhesive. It refers to the degree of stability that occurs after repeated use or long-term heating. 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) Simple identification of hot melt adhesive quality. Hot melt adhesives have many quality indicators and require special instruments and appropriate testing environments. In the absence of equipment, firstly, the appearance of the adhesive is the same as whether or not the bubbles are present. The adhesive was then melted to make a tape about 1 cm wide and 0.2 cm thick. After cooling, the tape is stretched by hand, and it is observed whether the stretching rate can reach the standard, whether the strength is sufficient, and whether the fracture port can maintain the color unchanged. If there is a powdery substance, it indicates that the filler is too much or the mixing is not uniform, or the filler particle size is too large, which will cause a major quality accident. Next, the micelles were stored in an environment at 0° C., and they were removed after 24 hours, and they were struck with a hard object. If brittle cracking occurred, the low-temperature resistance was poor. When 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 Operational process requirements

1) The adhesive thickness is generally between 0.8 and 1.2 mm, and the adhesive layer should be uniform. Adhesives that are too thin will affect the bond strength, and too thick will increase the cost. The glue length should be slightly shorter than the cover size 1~2mm. If it is too long, it will cause the remaining glue to accumulate on the machine, which will easily cause wild glue on the back of the book or cause the cover to be uneven and damaged, which will easily cause the transmission components to malfunction; if the length is too short, it will cause The back of the book and the empty air of the book affect the quality of the book.

2) The upper side glue width is generally 3~7mm, and the thinner and more uniform the better. In order to ensure its fluidity, the sizing temperature should be controlled at about 180°C. Do not use gum as the side adhesive, otherwise it will start to sever.

3) The amount of milling on the back of the book should be determined according to the thickness of the signature, paying particular attention to the folding style of the book page to prevent short-stemming. To make the last page of each signature into a single page, the milling must be flush and the back of the book cannot be milled into bevels, otherwise the thickness of the adhesive layer is not uniform.

4) The back of the book must be slotted and the depth and spacing of the slots must be the same. Milling quality is directly related to the book's adhesive strength, to avoid generating loose pages, page breaks. It requires that the groove is smooth, the paper is clean, the spacing is 5~10mm, and the groove depth is 0.5~1.2mm, so as to facilitate glue filling and increase the tensile strength.

Source: "Printing World" Author: Cheng Jiamin

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