The hot melt adhesive is a solid at room temperature, gradually softens when heated, and turns into a viscous liquid having a certain fluidity and penetrating power, and can produce a certain wetting affinity on the surface of the substrate, and solidifies the condensed substance after cooling. China's printing market supplies books glued hot melt adhesives, mainly EVA hot melt adhesives. The production and promotion of EVA hot-melt adhesives in China is not a long time. Because the promotion of glue-binding technology has made the supply and demand of hot-melt adhesives for printing companies continue to increase, most domestic manufacturers are lagging behind due to weak technical capabilities of their factories. , The lack of necessary inspection and control measures, the low technological content of products produced, the lack of stable and reliable quality support, and the uneven quality of products, which seriously affect the quality of post-press processing, resulting in bookbinding accidents. Therefore, the use of industry standards to regulate EVA heat Melt glue production and use is necessary, this article for EVA hot melt adhesive product characteristics, talk about its technical standards cover several aspects.
1. 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 of the hot melt adhesive is slowed down, which may cause the book to be deformed or cannot be cut during cutting, thereby affecting the production efficiency of the linkage line, thereby increasing the Production cost; When the softening degree is too high, the curing speed will be accelerated, but it will affect the effective bonding time of the hot melt adhesive (ie, open time), and the opening time is too short, which will cause the book block and the cover to be separated from the cover. Generally, the hot melt adhesive is required. Softness is between 80～90°C.
2. Adhesion, flow properties
Due to the special use of hot melt adhesives, it is required that the hot melt adhesives have a certain melt viscosity, so as to ensure that the hot melt adhesives have good fluidity and wettability at the melting temperature, and have sufficient penetration ability and stickiness to paper fibers. 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; and if the melt viscosity is too high, the fluidity will be poor, which will affect the binding effect of the back of the book, and it may cause loose leaves or cracks on the back of the book when the book is turned.
3. Adaptability to temperature
The sensitivity of hot melt adhesive to temperature is relatively high. Generally, a good hot melt adhesive should have properties such as low temperature resistance, high temperature, and thermal stability to temperature. The low temperature resistance of hot melt adhesive is directly related to the cold binding of rubber seals. The safety of use, while the high temperature performance is directly linked to the aging properties of the hot melt adhesive.
In general, the better the toughness of the hot melt, the better the low temperature performance and vice versa. The low-temperature performance test of hot-melt adhesives is generally in a low-temperature environment, because the colloid is not easy to be brittle at this time; the high temperature performance of hot-melt adhesives mainly refers to the hot melt adhesive at a certain temperature (usually slightly higher than the melting temperature). , Will not produce colloidal discoloration, viscosity drop and other phenomena; the thermal stability of hot melt adhesive refers to its anti-aging properties; good thermal stability means that the hot melt adhesive will not produce discoloration of glue during long-term heating process. 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 the color and viscosity of the hot melt at a certain constant temperature for a period of time. One of the simpler methods is to thermostat the sample at a higher temperature, and the change in adhesive with poor anti-aging properties will be very obvious.
4. Tensile strength, hardness
In order to make the packaging of books and magazines well-formed, there is no phenomenon such as loose-leafing or page-flooding during frequent indexing, folding and flattening. Hot-melt adhesives must have appropriate hardness (usually HS 78° to 90°) and suitable toughness. . It is necessary to have high tensile strength and appropriate elongation at break under the premise of full penetration of the paper fiber. Generally, the performance of the hot melt adhesive is evaluated by two indicators of hardness and tensile strength. of.
The hardness of the hot melt adhesive is directly related to the flexural strength of the glue book. High hardness, book pages can withstand a certain gap between the folding force, the back is strong and beautiful. In addition, the high hardness can also be appropriately considered to reduce the thickness of the adhesive layer and reduce the consumption of the hot melt adhesive. Conversely, if the hardness is low, it is necessary to increase the thickness of the adhesive layer to increase the strength, especially a thicker book with a large number of grams of double-glued pages or multiple pages of thick sheets.
In the premise that the hot melt adhesive is sufficiently compatible with the paper, that is, the hot melt adhesive has sufficient penetration of the paper fiber, the hot melt adhesive should also have a high tensile strength to ensure the adhesive strength and the use of the hot melt adhesive to the book. Strength, so that the book does not occur in the use of loose pages, page breaks and cracking phenomenon. When the tensile strength of the hot melt adhesive is greater than 4 MPa, it is not only beneficial to the quality control of the book set by glue, but also can reduce the overall cost of the production. Toughness, hardness, and strength are interrelated. When the hot-melt adhesive has an elongation at break of less than 100%, the back of the book will be brittle and break due to the non-impact of external force, and conversely, it will be elastic due to its high elongation at break, resulting in exceeding the multi-directional affinity of colloid and paper fibers. The summed and degummed pages or deformed due to excessive flattening.
5. Color and odor
Hot melt adhesive appearance, in addition to color printing paper needs to add the appropriate color material, the general high-quality rubber or film surface smooth, shiny, no rough particles of impurities. The color of general high-quality hot-melt adhesive products is milky white or light yellow, soft and shiny, and the density is generally about 0.93g/cm3. Glossy glues indicate that the raw materials have poor miscibility. If the raw materials cannot be fully melted, they will lose the proper function of the raw materials. This type of glue has poor permeability to paper and poor adhesion. The quality of glue binding is not guaranteed. The gas emitted by the melt during the melting process should also be within the evaluation index. Generally, the operators prefer non-toxic and odorless hot melt adhesives. However, in the process of selecting hot melt adhesives and evaluating hot melt adhesives, they are non-toxic. The hot melt adhesive is the first to be guaranteed, so the color and odor of the hot melt adhesive should also be in its evaluation system.
6. Specific gravity
As long as the glue is placed in the water, the specific gravity of the glue can be detected. The high-quality glue should not sink in the long-term floating water surface. The glue with a specific gravity greater than 1 will sink into the bottom of the water, and the glue with a specific gravity less than 1 will float on the water. For a rubber with a specific gravity greater than 1, the overall performance index is lower than the hot melt adhesive with a specific gravity of less than 1. In addition, the greater the specific gravity of the hot melt adhesive, the greater the weight of the single-use adhesive when the adhesive layer is the same, and this will also significantly increase production. cost.
Compared with foreign markets, China's hot-melt adhesives market has a certain gap in its internal quality, process means, and raw materials and production equipment performance. In view of the domestic hot-melt adhesives market, the development of a set of hot melt adhesive quality assessment standards is It is very necessary to analyze the performance of the hot melt adhesive based on the properties of the hot melt adhesive itself. It is expected that it will help users understand the hot melt adhesive better. It has a certain reference role in selecting the hot melt adhesive. .
