Talking about the technical standard of book EVA hot melt adhesive
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The hot melt adhesive is a solid at normal temperature, gradually softens upon heating and becomes a viscous liquid having a certain fluidity and permeability, and can have a certain wet affinity to the surface of the substrate, and then solidifies and condenses after cooling. China's printing market supplies bookbinding wireless adhesive hot melt adhesives, mainly based on EVA hot melt adhesives. The production and promotion of EVA hot melt adhesives in China is not long. Due to the promotion of the adhesive binding process, the supply and demand of hot melt adhesives by printing companies are rising, but most domestic manufacturers are weak due to weak technical strength of the factory. Lack of necessary testing and control methods, the production of products with low technical content, no stable and reliable quality support, uneven product quality, seriously affecting the quality of post-press processing, resulting in binding accidents, so use industry standards to regulate EVA heat The production and use of melt glue is necessary. This article focuses on several aspects of its technical standards for the characteristics of EVA hot melt adhesive.
Softness
The softening degree of the hot melt adhesive is directly related to the fixing speed. When the softening degree is too low, the curing speed will be slower, which will cause the book to deform or not cut during cutting, which will affect the production efficiency of the linkage line, thereby increasing Production cost; when the softness is too high, the curing speed will increase, but it will affect the effective bonding time (ie, opening time) of the hot melt adhesive. If the opening time is too short, the book block and the cover will be disengaged, and the hot melt adhesive is generally required. The softness is between 80 and 90 °C.
2. Adhesion, flow properties
Due to the special use of hot melt adhesive, the hot melt adhesive is required to have a certain melt viscosity, so as to ensure good flowability and wettability of the hot melt adhesive at the melting temperature, and sufficient penetration and adhesion to the paper fiber. Connection strength. If the melt viscosity of the hot melt adhesive is too low, the amount of glue on the back of the book will be insufficient; while the melt viscosity is too high, the fluidity will be poor, which will affect the bonding effect of the book back, and it is easy to produce a loose leaf or a crack on the back of the book when the book is turned.
3. Adaptability to temperature
Hot melt adhesives have high sensitivity to temperature. Generally, good hot melt adhesives should have low temperature resistance, high temperature and thermal stability to temperature. The low temperature resistance of hot melt adhesives is directly related to the adhesive book in cold environments. The safety used, and the high temperature resistance is directly related to the aging properties of the hot melt adhesive.
In general, the better the toughness of the hot melt adhesive, the better the low temperature performance, and vice versa. The low temperature performance test of hot melt adhesive is generally in a low temperature environment, because the colloid is not easy to be brittle at this time; the high temperature resistance of hot melt adhesive mainly refers to the hot melt adhesive at a certain temperature (generally slightly higher than the melting temperature). It does not cause colloidal discoloration and viscosity decrease; the thermal stability of hot melt adhesive refers to its anti-aging property, and good thermal stability means that hot melt adhesive does not cause discoloration of glue during long heating. At the same time, the surface is not crusted during use. Generally, the thermal stability of the hot melt adhesive should be expressed as a certain range of increase in color and viscosity after heating for a certain period of time under constant temperature conditions. One of the simpler methods is to thermostat the gel at a higher temperature, and the change in the gel with poor anti-aging properties is very easy to check.
4. Tensile strength and hardness
In order to make the book packaging well-formed, there is no loosening or falling pages when frequently flipping and folding flattening. The hot melt adhesive must have appropriate hardness (generally HS78°~90°) and should have suitable toughness. . Under the premise of sufficient penetration of paper fibers, it is necessary to have high tensile strength and appropriate elongation at break. Generally, the properties of hot melt adhesives are evaluated by two indexes of hardness and tensile strength. of.
The hardness of the hot melt adhesive is directly related to the flexural strength of the adhesive book. The hardness is high, and the book page can be subjected to a certain degree of bending resistance, and the back of the book is firm and beautiful. In addition, the high hardness can also be considered to reduce the thickness of the adhesive layer and reduce the unit consumption of the hot melt adhesive. On the other hand, if the hardness is low, it is necessary to increase the thickness of the rubber layer to improve the strength, in particular, a large-gram double-bonded book page or a multi-printed thick book with a good paper quality.
Under the premise that the hot melt adhesive is sufficiently compatible with the paper, that is, the hot melt adhesive has sufficient penetration into the paper fiber, the hot melt adhesive should also have a high tensile strength to ensure the adhesion and use of the hot melt adhesive to the book. Strength, so that the book does not occur during the use of loose pages, page breaks and cracks. When the tensile strength of the hot melt adhesive is more than 4 MPa, it is beneficial to the quality control of the bookbinding book and the overall cost of the production. Toughness, hardness and strength are interrelated. When the elongation at break of hot melt adhesive is less than 100%, the back of the book will be brittle and cracked due to the resistance to external force, and conversely, it will be elastic due to its excessive elongation at break, resulting in more affinities than colloid and paper fibers. Debonded off the page, or deformed due to excessive flattening.
5. Chroma and smell
In terms of appearance, hot melt adhesives need to be added with corresponding color materials for color printing paper. Generally, high-quality rubber compounds or film surfaces are smooth, shiny, and have no coarse foreign particles. Generally, the quality of high-quality hot-melt adhesive products is milky white or light yellow, and the gloss is soft and the density is generally about 0.93g/cm3. Glossy glue indicates that the raw material has poor miscibility. If the raw material is not fully melted, it will lose the proper function of the raw material. The adhesive has poor permeability to paper, and the adhesive strength is not strong. The quality of the adhesive is not guaranteed. The gas emitted by the melt during the melting process should also be within the evaluation index. Generally, the operator tends to be non-toxic and odor-free hot melt adhesive, but it is non-toxic during the selection of hot melt adhesive and evaluation of hot melt adhesive. The hot melt adhesive is the first to be guaranteed, so the color and odor of the hot melt adhesive should also be in the evaluation system.
6. Specific gravity
As long as the glue is placed in the water, the specific gravity of the glue can be detected, and the high-quality glue should not float for a long time. Generally, the rubber with a specific gravity greater than 1 will sink to the bottom of the water, and the rubber with a specific gravity of less than 1 will float on the surface. For cements with a specific gravity greater than 1, the comprehensive performance index is lower than that of hot melt adhesives with a specific gravity less than 1. In addition, the higher the specific gravity of the hot melt adhesive, the greater the weight of the single adhesive when the adhesive layer is the same, and this will significantly increase the production. cost.
Compared with foreign markets, China's hot melt adhesive market has a certain gap in its internal quality, process methods and performance of raw materials and production equipment. For the current hot melt adhesive market in China, a set of hot melt adhesive quality evaluation standards is It is necessary that the above analysis only analyzes the relevant properties of hot melt adhesives from the properties of hot melt adhesives. It is expected to help the users to better understand the hot melt adhesives and have a certain reference role in the process of selecting hot melt adhesives.