Hot melt adhesive characteristics
Hot-melt adhesives, referred to as hot-melt adhesives, are solvent-free, fast-curing adhesives. Their main features are:
(1) It is a solid at room temperature, and when it is heated to a certain temperature, it melts into a liquid. Once it cools below the melting point, it quickly becomes a solid.
(2) Fast curing, low pollution, strong adhesion, and the glue layer not only has a certain hardness, but also has a certain toughness.
(3) Has a certain re-adhesion. After being coated on the adherend to be cooled and solidified, it may be reheated and melted, and adhered to the adherend again.
(4) easy to use. In use, as long as the hot melt adhesive is melted and melted into a liquid, it is applied on the surface of the adherend, and after being pressed and cooled, bonding is completed in a matter of seconds, and it can be completely cured in a few minutes.
These characteristics of hot-melt adhesives, especially the characteristics of curing in a short time, are difficult for any water-soluble adhesives. Bookbinding is the use of hot-melt adhesives for fast curing, strong adhesion, easy to use, etc. In the production of glueless books.
Hot melt adhesive properties and composition
There are many kinds of hot-melt adhesives, such as polyamide, polyester, and EVA. Bookbinding hot melt adhesives are EVA type. EVA hot melt adhesive is made by copolymerizing ethylene and vinyl acetate under high pressure. With different proportions of ethylene and vinyl acetate, different hot melts with different molecular weights and properties can be produced to suit different needs.
The main raw material of hot melt adhesive is EVA resin. This kind of resin has high flexibility, does not need to be vulcanized, does not need to add plasticizers, and is rich in rubber-like elasticity, and at the low temperature, the elasticity is reduced, the mechanical properties are basically unchanged, and the temperature resistance is stable and is not easy. Aging, strong bonding force, good curing and so on. However, in use, in order to adjust the fluidity, permeability, softening point, and oxidation resistance of the EVA hot melt adhesive, and to reduce the cost, some additive materials may be added in addition to the polyvinyl acetate vinyl resin. Therefore, commonly used hot-melt adhesives are made of resins, tackifiers, viscosity modifiers, plasticizers, antioxidants, fillers, and other raw materials, and are composed of appropriate proportions.
1.EVA resin
EVA resin is the main body of hot melt adhesive, which determines the basic characteristics of the glue, giving the glue a certain degree of adhesion and cohesion. The higher the content of vinyl acetate in the EVA resin, the greater the viscosity of the hot melt adhesive, but the lower the heat resistance and tensile strength. Conversely, the viscosity decreases, and the heat resistance and tensile strength increase. In general, the resin component accounts for about 50% of the hot melt component.
2. Tackifier
One of the components of the hot melt adhesive is a tackifier. Its main function is to reduce the melt viscosity of the hot melt adhesive, improve the processability, make it easy to wet the adherend surface after the glue is applied, and improve the adhesive strength. In addition, you can also use this to adjust the heat-resistant temperature and open time. Therefore, the tackifier and the resin must have good compatibility. Tackifiers are often made from rosin-modified resins, such as glycerol rosin esters, pentaerythritol rosin esters, and terpene resins. Among them, hydrogenated rosin, terpene, glycerol rosin ester, and rosin hydrocarbon can improve the flowability of the adhesive and have good stability to light, heat, ozone, and low temperature. The softening point of the tackifier is selected between 80 and 100°C, and the amount thereof is about 20% to 40% of the total composition.
3. Viscosity modifier
In hot-melt adhesives, viscosity modifiers can reduce melt viscosity, improve fluidity, shorten the opening time of the glue, reduce the phenomenon of spinning and prevent the glue from sticking. At the same time, it can also reduce costs. Viscosity modifiers generally use waxes, such as paraffin wax, microcrystalline wax, and synthetic wax, in amounts less than 25% of the total composition. If the proportion is too large, the fluidity and permeability of the rubber compound will increase, but the bond strength will decrease and the shrinkage rate of the glue will increase. In severe cases, the back of the book will appear obvious “oil seepage”.
4. Antioxidants
Since hot melt adhesives are often heated to a temperature of 140-170° C. or even higher, they are susceptible to thermal oxidation or aging and decomposition. In order to prevent the rubber from oxidative deterioration by heat for a long time, antioxidants or heat stabilizers, such as di-tert-butyl-p-cresol (BHT), di-tert-butyl-phenyl phenylpropionic acid pentaerythritol, dimethyl sulfide, should be added to the ingredients. Phenol (QD) and other materials. Its amount is generally controlled at 0.1% to 1%.
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The use of EVA hot melt adhesive
1.EVA hot melt adhesive preheating
EVA hot melt adhesive milky white, block, strip and granular. When used, the solid hot melt adhesive must first be preheated. The preheat can be preheated by the oil bath jacket, and can also be preheated directly in the premelting pan by the hot plate device (wireless adhesive bonding motive is adopted). When preheating, the solid hot melt adhesive is poured into a preheated pan and heated at a temperature of 160-200°C. Preheat time is controlled in 1 to 2 hours. The hot melt adhesive is preheated to the set temperature and has good fluidity before it can be used. The hot melt glue that reaches the preheating temperature is released into the rubber bucket of 160-200 DEG C through the thermostatic tube, and can be used for the glue-binding production line.
2. Adhesion and adaptability of EVA hot melt adhesive
In the production process, the adhesive strength of the hot melt adhesive changes with the heating temperature, the quality of the book block paper, the width and depth of the back-back milling, and the operating speed of the glue-binding equipment. Grasping these changing factors is extremely important for improving the adhesive strength and binding adaptability of hot melt adhesives.
(1) Heating temperature of hot melt adhesive
The softening point of the hot melt is generally within the range of 70 to 100°C. This temperature range is only the melting temperature of the glue. To make the hot melt glue capable of adhering to the book, the heating temperature must reach 150-180°C. According to foreign data, the glue on the ordinary book paper (press paper and newsprint paper) is glue-bonded. When the glue temperature is 170°C, the hot-melt adhesive's pull test value is 8.83±0.44N/cm. When the adhesive temperature rises to 180°C, the hot-melt adhesive's tensile test value will drop to 7.33±0.33N/cm. On the other hand, when the temperature of the glue drops to 155°C, the pull strength of the hot melt is only 5.22±0.25N/cm.
Thus, the heating temperature of the hot melt adhesive should not be too high or too low. If the temperature is too high, although the fluidity, wettability, and permeability of the glue are strengthened, the bond strength will decrease, which will affect the tensile strength of the glue. Conversely, if the temperature is too low, the viscosity will increase, but it will affect the fluidity of the glue, as well as the wettability and permeability of the book block and the tensile force of the glue, and may even fail to bond. Therefore, the heating temperature and the gluing temperature of the hot melt adhesive must be controlled within an optimal range.
(2) The relationship between the temperature of the book block paper and glue
The core of the book is different from the paper, and the gluing temperature of the hot melt adhesive should also be different. This is not only because the fiber properties of the paper are different, but more importantly, the thermal conductivity is different, that is, the cooling rate is different. Taking the thermal conductivity of coated paper (coating paper) and non-coated paper (topography paper) as an example, the former cools the glue faster than the latter. Because coated paper contains nearly 10 times more inorganic than non-coated paper, and inorganic materials have good thermal conductivity, the coated paper allows the hot melt to cool faster. The hot-melt adhesive pull test shows that if the sizing temperature is also 170°C, the uncoated paper can reach a pull value of 8.83±0.44N/cm, while the pull value of the coated paper can only reach 1.86±0.11N/cm (65g). /m2 coated paper) and 1.46±0.11 N/cm (100 g/m2 coated paper). Therefore, to improve the pull value of the hot melt adhesive on the coated paper, the gluing temperature must be increased. Practice has proved that the gluing temperature is generally about 195 °C. At this temperature, the pull value can reach 4.1 to 3.91 N/cm (The above data is extracted from foreign sources for reference only).
(3) Opening time and production operating speed of hot melt adhesive
In the use of hot-melt adhesives, there are two time variables that must be strictly controlled, one is the open time of the hot melt adhesive, and the other is the curing time.
The opening time means that the process of applying the hot melt adhesive to the adherend must be completed within the bonding time specified by the hot melt adhesive. Otherwise, the adhesive layer surface will be conjunctival beyond the specified time, so that the two adhesives cannot be adhered firmly. According to the type of hot melt adhesive, the opening time is also different, it can be 7 to 15 seconds, it can also be 15 to 20 seconds. After the two adherends are adhered within the specified time, they must be cooled for a certain period of time before they can be pressurized or turned. Otherwise, the adhesive force of the hot melt adhesive may be affected, and the product may be deformed or the pages may fall off. This cooling time is called the curing time. The curing time varies according to the model of the hot melt adhesive, and it has a length or a short time. The general curing time is 3 to 5 minutes.
The open time and curing time of the hot melt adhesive are closely related to the speed of the machine, especially the opening time. For example, if the opening time of the hot melt adhesive is 7 to 15 seconds, then the book block must be at the 15th after the milling and backing of the book block, starting from the application of the glue and until the cover is glued with the back of the book (without gauze and paper jam on the back of the book). Within seconds, the hot melt adhesive can achieve a good adhesive effect. If the machine runs slowly, the cover cannot be bonded within 15 seconds, and the bond between the cover and the back of the book is not strong and the bond strength is poor. Therefore, to determine the open time of hot melt adhesives, it is necessary to consider the normal operating speed of the glue-binding equipment.
In addition, it is necessary to grasp the curing time of the hot melt adhesive, because the glue is brushed to the back of the book, it will take 3 to 5 minutes before the basic curing. Before it has reached or nearly complete cure, the book cannot be cut and the page cannot be flipped. Otherwise, it will cause the back of the book cover bow wrinkles, book back deformation and book off and other ills. Therefore, if the interval between the book and the three-sided cutting process is shorter after the book is glued, you should try to extend the time so that the glue can be fully cured before cutting.
Hot melt adhesive precautions
(1) The hot melt adhesive can only be heated with the oil bath spacer or with a sealed hot plate. Never use open flame heating.
(2) After the hot melt adhesive is preheated with a preheating pot, it must be released into the rubberized glue bucket before it can be used. Therefore, the amount of glue in the preheated pan should be suitable. If too much is added, the adhesive can be repeatedly melted, and the colloid can easily be aged, which affects the adhesive force. Therefore, it is necessary to add less.
(3) In the hot melt adhesive opening time, complete the binding work of the book, if you encounter downtime or other failures, the books in the book folder brushing the end of plastic and not on the side of the plastic book, to remove timely to prevent the appearance of books The phenomenon of the core stalling on the rubber wheel.
(4) The temperature of the molten colloid must be strictly controlled within the use temperature range. It is necessary to prevent the temperature of the glue liquid from being too low, affect the flowability, wettability and permeability of the glue, and prevent the temperature from being too high, which will affect the viscosity of the glue. Causes problems such as aging of glue.
(5) The glue pan that melts the glue should be cleaned periodically. It is best to clean it twice a year to ensure that the glue pan is clean. Care should be taken when cleaning, do not use paraffin as a cleaning agent, because paraffin wax is flammable, there is a safety hazard, it is recommended to use a dedicated cleaning agent. In addition, the sensitivity of the temperature display must be ensured to ensure that the display temperature is reliable and
