One of the fundamental reasons for the rapid growth of EVA (ethylene-vinyl acetate) adhesives is that they have a fast solidification rate. Generally, the curing time is about 10 seconds, and the fastest is even 1 to 2 seconds. Non-toxic and non-flammable, it is an environmentally friendly product. EVA hot-melt adhesives have a wide range of applicability, such as PP (polypropylene), PE (polyethylene) and other non-polar materials, can also achieve better bonding results. The adhesive layer has good low temperature resistance, water resistance and moisture resistance. Due to the poor rigidity of EVA, compared with polyester and polyamide hot-melt adhesives, the softening point is low, and its softening point around the world is around 100°C, thus limiting its application range.
The basic composition of EVA hot melt adhesive is EVA resin, tackifying resin, wax and stabilizer. EVA resin is the introduction of a short chain branch composed of polar acetic acid genes in the ethylene branch, disrupting the original crystalline state, resulting in EVA tends to "plasticizing effect", reducing the crystallinity of ethylene on the branch, Increases the distance between polymer chains, making EVA resin more flexible and elastic. Tests abroad have proved that EVA resin with VA (Vinyl Acetate) of 12% does not break at -76°C. Therefore, EVA resin does not need to be added with any plasticizer and other additives. It is safe and can be used as food packaging. , pharmaceutical packaging and medical instrument packaging.
The pure EVA resin has a high adhesive force, which is due to the introduction of the polar group VA on the ethylene branch, which makes the EVA hot melt adhesive have excellent adhesion and printability. The VA content in EVA resin can range from low to high percentages. Therefore, the percentage of VA in the resin and MI (melt index, reflecting the molecular weight size) are the two most important factors that directly determine and influence the properties of EVA. Generally, the VA content is high, the adhesive force is increased, the wettability to the adhered material is good, the curing time is slow, and the heat resistance is reduced. The high melt index means that the molecular weight is low, the viscosity of the colloid decreases, the wettability is good, the heat resistance is reduced, and the adhesive force is not changed.
In addition, the electrical properties of EVA resin are linearly related to its VA content. The VA content is high, the volume resistance decreases, the breakdown voltage decreases, and the dielectric loss and dielectric constant increase. So when we use EVA adhesive as a heat-seal coating, we can use high-frequency current energy to heat seal, but polyethylene can not.
EVA has higher tensile strength at break in comparison with EEA (ethylene-ethyl acrylate), EAA (ethylene-propylene ester), and EMA (ethyl methacrylate) in ethylene-based copolymers. With the VA content varies. If the MVA of the EVA is different, the fracture tensile strength changes more.
The tensile strength at the EVA yield point decreases as the VA content increases. If the VA content exceeds 25%, it becomes very small. The flexural rigidity of EVA is equal to that of soft PVC, which decreases with the increase of VA, while MI increases and the bending rigidity becomes smaller.
EVA resin selected EVA hot melt adhesive, the VA percentage is generally 20 to 40, MI is 6 to 400. The preparation and use temperature of hot-melt adhesives is between 180°C and 200°C. If this temperature is exceeded, EVA will undergo thermal decomposition. Due to the presence of VA groups on the side chains, at high temperatures, deacetic acid reacts first to produce free acetic acid, which has a strong pungent odor and pink spots.
EVA resin has a good compatibility, it can be miscible and blended with other various resins to improve the physical properties. Such as EVA resin and nitrocellulose blend, so that nitrocellulose has excellent flex resistance, friction resistance and heat sealing. Hard PVC is blended with 5% to 10% EVA to improve fluidity and impact resistance. EVA was blended into a polyamide resin to develop good resistance to bending and coloration at low temperatures.
Tackifying resins include rosin, rosin esters, polyfluorene resins, low molecular weight polystyrene, C5, C9 olefin resins, and the like. It has good compatibility with EVA resin, can increase the adhesive force of the hot melt adhesive, improve the thermal adhesiveness, fit effective time, and improve the wettability of the hot melt adhesive to the adhesive surface. The choice of tackifying resin should pay attention to its thermal stability. General olefin resins are better than rosin derivatives. But rosin derivatives provide better adhesion.
Commonly used waxes are paraffins, microcrystalline waxes, synthetic waxes, low molecular weight polyethylenes, and the like. In EVA hot melt adhesives, the role of the wax is to reduce viscosity, improve wetting, reduce cure time, and reduce costs. However, it can cause brittleness and reduce adhesion. The amount of the formula should be used with caution. The microcrystalline wax has the least influence on the adhesion of the glue to the glue. Synthetic wax is superior to microcrystalline wax and paraffin in improving the heat resistance of the rubber and reducing the cost of the glue.
In the printing industry, EVA hot melt adhesives were first used in book-end-motivation as early as the end of the 1970s. They are suitable for machines with a speed of 2500 to 5,000 vehicles per hour. By the mid-1980s, with the introduction of domestic high-speed packaged-motivation, improved hot-melt adhesives met the speeds of 10,000 to 13,000 vehicles per hour, basically solving the problem of hindering the binding of books.
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