Hot melt Adhesives are most widely used in today’ bonding application. Compared with other types of adhesive, they have low cost, non-toxic, easy to operate. Among most hotmelt glue material types, EVA hot melt adhesive, or eva based hot melt adhesive, (ethylene vinyl acetate adhesive) is mostly known and applied.
EVA hot melt adhesive is a solvent-free, moisture-free, 100% solid meltable polymer. It is a solid at room temperature. Heated and melted to a certain extent, EVA become a fluid and viscous liquid adhesive. After melting, it will be light brown, translucent, or natural white in color. The main components and basic resin is ethylene and vinyl acetate copolymerized under high pressure, and then mixed with tackifiers, viscosity modifiers, antioxidants.
General Introduction of EVA Hot Melt Adhesives
EVA is a plastic material composed of ethylene (E) and vinyl acetate (VA). The ratio of these two chemicals can be adjusted to meet the needs of different applications. The higher the content of VA content, the higher the transparency, softness and toughness.
Ethylene-vinyl acetate copolymer (also known as ethylene-vinyl acetate copolymer) is made by copolymerizing ethylene (E) and vinyl acetate (VA). The polymerization method uses high-pressure bulk polymerization (for plastics) and solution polymerization (PVC processing aids). Emulsion polymerization (adhesive), suspension polymerization is used. Emulsion polymerization is used for vinyl acetate (VA) content higher than 30%, and high-pressure bulk polymerization is used for low vinyl acetate content.
EVA has a wide range of uses. Under normal circumstances, the main products of EVA with vinyl acetate content of less than 5% are for producing films, wire and cable LDPE modifiers, adhesives, etc.; EVA products with vinyl acetate content of 5%-10% are elastic films, etc.; vinyl acetate EVA with a content of 20%-28% is mainly used for hot melt adhesives and coating products; vinyl acetate is 5%-45%, and the main products are films (including agricultural films) and sheets, injection molding and molding products, foam products, hot melt adhesives, etc.
Generally, the content of vinyl acetate (VA) used for EVA hot melt adhesive, is 5%-40%. Compared with polyethylene (PE), EVA introduces vinyl acetate monomer in the molecular chain, thereby reducing high crystallinity, improving toughness and impact resistance. Filler compatibility and heat sealing properties, are widely used in foam shoe materials, functional shed films, packaging molds, hot melt adhesives, wires and cables, toys and other fields.
General History of EVA hotmelt adhesive
Hot melt adhesives are made of thermoplastic resin or thermoplastic elastomer as the main component, adding plasticizers, tackifying resins, antioxidants, flame retardants and fillers, and then melted and mixed to make a solvent-free solid adhesive mixture. Because of its non-toxic, no environmental pollution, convenient preparation and other advantages, it has become the future of the adhesive market development. The world’s annual output has been on an upward trend. Its growth rate is the highest among all types of adhesives, with more and more diversified varieties and more and more applications.
Ethylene and vinyl acetate (EVA) hot melt adhesives are easy to prepare and are widely used in mechanized packaging, furniture making, shoemaking, wireless binding, electronic components and daily necessities bonding. And it quickly become the most widely used and used hot melt adhesives. In 1960, DuPont of the United States first realized industrial production and named the product as Elvax, after that, UCC, USI. Companies such as Bayer, ICI, and Monsanto have successively introduced such products.
EVA hot melt adhesive has large cohesive force, low melting surface tension, and has thermal adhesive force to almost all substances. It has excellent chemical resistance, thermal stability, weather resistance and electrical properties, fast bonding, and wide application. Non-toxic, non-polluting and other characteristics, making it “green glue”, which has attracted more and more attention.
Features of EVA hotmelt adhesive
It is usually a solid at room temperature, and melts into a liquid when heated to a certain extent, and once it is cooled below the melting point, it quickly becomes a solid (that is, it solidifies);
It has fast curing, low pollution, strong adhesion, and the adhesive layer has a certain degree of flexibility, hardness and toughness;
The glue is applied to the glue layer after cooling and solidification on the adherend, and it can also be heated and melted again to become a glue body and then bond with the adherend, with a certain degree of re-stickiness;
When using, as long as the hot melt adhesive is heated and melted into the required liquid state, and applied to the adhered object, the bonding and solidification can be completed within a few seconds after being pressed. And the hardening, cooling and drying can be achieved within a few minutes.
Usage of EVA hot melt adhesive in bookbinding, as an example
For EVA hot-melt adhesives, there are generally two types of binding: high-speed glue and low-speed glue. High-speed glue has a slightly faster curing speed, and low-speed glue has a slightly slower curing speed. According to different materials, books are divided into offset paper and coated paper glue; and there are back glue and side glue. Different EVA hot melt glue should be chosen to meet the strength and needs of the adherend, so as to achieve a good bonding effect. To use hot melt adhesive, you must master the preparation procedures before use, such as pre-gluing, temperature, etc.
1. Preheating of EVA hot melt adhesive
Before using EVA hot melt adhesive, the solid glue must be preheated and melted. There are two preheating methods: one is preheating with an oil bath. That is, the jacketed melting pot is preheated; the other is preheating with an electric plate, that is, directly preheating in the preheating melting pot with an electric heating plate device. The preheating time is generally 2 hours. When the gel reaches the required heating temperature and has good fluidity, the glue is released into the working glue pot with a temperature of 160～200℃ through a constant temperature pipeline, for use in gluing binding books, etc.
2. Adhesion and adaptability of EVA hot melt adhesive
In the actual production process, the adhesive force of the hot melt adhesive will vary with the heating temperature of the hot melt adhesive, the difference of the adherend material, the width and depth of the milling back, the height of the glue, and the operation of the binding machine speeds, etc. This will get different bonding effects.
(1) Heating temperature of hot melt adhesive
The softening point of hot melt adhesive would generally be above 80°C. That is, when heated to 80°C, the colloid would begin to soften and melt. This temperature is only the melting temperature of the hot melt adhesive. To make it melt to the extent that it can bond books, the heating temperature has to rise to 130-180°C. At this temperature, the viscosity, fluid and viscosity of the colloid can be all suitable for the binding of books and books.
(2) The relationship between book paper and gluing temperature
The paper textures of making books are different, so the temperature of gluing should also be different. This is not only because of the different fibers of the paper, but more importantly, the different thermal conductivity of the colloid due to the type and texture of the paper, which changes the cooling rate. Take the thermal conductivity of coated paper (also known as coated paper) and relief paper (non-coated paper) as an example, the cooling rate of the glue of the former is faster than that of the latter. Because the inorganic matter contained in coated paper is about 10 times higher than that of non-coated paper such as letterpress, news, offset paper, etc., and inorganic matter has good thermal conductivity, which can make the hot melt adhesive cool down faster.
For example, when the gluing temperature is both 170°C, the tensile test value of the hot melt adhesive of non-coated paper can meet the expected requirements, while that of the coated paper cannot. Therefore, when gluing the coated paper, it is necessary to increase the temperature of the glue solution and increase its strength.
(3) The relationship between the open time of EVA hot melt adhesive and the operating speed of production equipment.
In the production of perfect binding processing, there are three time stages that must be strictly controlled when using hot melt adhesives, namely opening time, curing time and drying time for cooling and hardening. The opening time refers to the time for applying glue on the back of the book. The curing time is the time for the cover and the back of the book to be glued together. The cooling, hardening and drying time is the time for the book with the cover to be cooled and finalized after curing. Only after these three time stages can the book be finalized and achieve the desired shapes.
These several time stages of using hot melt adhesive are closely related to the operating speed of the equipment. For example, the open time of hot melt adhesive is generally 7-15 second. And this time is the time for the perfect binder to perform milling and transmission. The process of applying glue to the cover and the back of the book (excluding the jammed back of the book) must be completed within 15 seconds, so that the hot melt adhesive can be used to achieve a good bonding effect. If the equipment is running at very slow speed, and the bonding work cannot be completed within 15 seconds, the bonding effect is definitely not ideal. And there will be problems such as weak bonding, failure to stick, books falling apart after being booked, and pages falling. Therefore, the opening time of the hot melt adhesive should be considered in consideration of the normal operating speed of the perfect binder. The curing time of the hot melt adhesive is generally the same as the opening time. Of course, users hope that the curing time will be shorter to facilitate the binding and setting of the book.
In addition to the opening and curing time, the cooling, hardening and drying time are also required to be basically accurate, especially for the glue used in the production line of wireless glue binding. That’s because this kind of production line is a linked assembly line from the collating machine, which collate to the book cutting, all parts of it are connected with each other. After the book is glued with cover of the bag, the time for the book to be transferred to the cutting part must conform to a certain rule, so that the book cutting can meet the ideal requirements. Cooling, hardening and drying can ensure the finished product.
This process should be completed within 3 minutes, otherwise it will cause deformation of the book back, unstable size of the finished product, sticking to the knife, etc., which will affect the quality of the finished product and cause undue losses. Therefore, the length of the book delivery conveyor (conveyor belt) of the linked production line should be selected according to different operating speeds. Generally, the length of the common conveyor belt is between 40 and 55 meters. It can also be longer or shorter according to needs, but it must be guaranteed. Books with a cover should be moved on the conveyor belt to cool for more than 3 minutes. The quality of book binding is determined by many factors such as paper, hot melt adhesive, binding materials and craftsmanship.
Environment of using EVA hot melt adhesive
There are more and more books using EVA hot-melt adhesive for the binding process. Almost all of them are in use in large, medium and small processing plants across the country. There will still be an upward trend in the future. This is due to the fast publication of books using this adhesive, short cycle, good appearance of books, saving several processes. But often because the printing factory is eager to get started, strive for more work, and more processing, but ignore the scientific nature of the environment and conditions for using hot melt adhesives, so many problems have appeared in the processing. Here are a few points for reference by the user:
The influence of temperature and humidity in the production plant
EVA hot melt adhesive is a kind of thermoplastic adhesive. The opening time after application is affected by indoor temperature and humidity. Generally, the indoor temperature (that is, the workshop) should be kept at a constant temperature. The best temperature is between 15～26℃, and the humidity should be kept around 50%. However, it is understood that among the units that use hot melt adhesives across the country, it is very rare that this requirement can be met. Therefore, when hot melt adhesives are used in many areas, there have been too many bubbles in the summer, which is difficult to solidify and cool. In winter, the adhesive solidifies, the cooling time is shortened, the bonding is not strong or sticking, and the book breaks after sticking. In other areas, because of the high humidity, the book is not shaped after the cover is wrapped, and it is impossible to cut and so on. These phenomena are directly related to the temperature and humidity of the workshop. So request:
(1) For companies with good conditions, it is best to install constant temperature equipment and humidity measuring instruments in the workshops that use wireless gluing equipment to ensure the normal temperature and humidity.
(2) Others should also strive to create conditions for scientific and standardized management and production. In the absence of these conditions, the following principles can be followed temporarily: when the temperature is high in summer, the temperature of the hot melt adhesive should be reduced by about 10 ℃, so that the opening and curing time of the adhesive can basically meet the reasonable requirements.
(3) The books processed by wireless adhesion processing should be kept at a temperature of 1～45℃ in the storage warehouse or workshop. And it is strictly forbidden to stack them against walls, heating (in winter), windows and open air for a long time. Avoid damp or excessive heat to deform the paper, change the adhesive, and cause irreparable losses.
Precautions
EVA hot melt adhesive is different from general cold glue. According to its characteristics, the following points must be mastered and paid attention to in use:
1. To understand and master the various models and technical performance (including technical parameters) of each EVA hot melt adhesive type. Different types of hot melt adhesives have different opening time and curing time. In particular, it is necessary to strictly distinguish the usage of summer glue and winter glue. The opening and curing time will also change when the temperature is different.
2. When using EVA hot melt adhesive, do not directly heat it with an open flame. Be sure to use spacer oil to heat (the boiling point of water should reach 100°C), or use a sealed electric heating plate.
3. Before using the solid EVA hot melt adhesive, it must be preheated with a preheating device and then released into the working glue box of the glue liquid. The preheating time is generally about 2 hours. When using glue, do not directly mix solid glue blocks (that is, glue that has not been preheated) into the glue box to prevent poor fluidity of the glue or apply the incompletely melted glue to the back of the book, resulting in uneven glue application.
4. The amount of glue in the preheated glue pot should be controlled properly. If the amount is too much, the glue will melt in turn, which will cause deterioration and aging, lower viscosity, and affect adhesion.
5. During the opening time of the hot melt adhesive, the entire bonding process must be completed. If the machine stops due to a failure, the books in the book holder must be taken out in time, and the book block must not stay above the gluing wheel.
6. When repairing wireless glue binding equipment for a long time, the pre-gluing and working glue pot should be turned off to prevent the aging of the glue.
7. The temperature of the molten colloid should be strictly controlled within the range of use.
8. It is necessary to clean the pre-gluing pot and the working glue box regularly to keep the glue pot clean to prevent the accumulation of impurities in the glue and affect the accuracy of temperature control, and even cause the thermostat to malfunction and cause the colloid to burn. Generally, under normal production and operation, the preheated glue pot should be cleaned every 3 months. The working glue box (for glue application) should be cleaned every half a month. A system should be established for cleaning the glue pot to facilitate implementation.
Features of EVA Hot Melt Adhesive
1. Biodegradable: it will not harm the environment when discarded or burned.
2. The price is similar to that of PVC: EVA is more expensive than toxic PVC, but relatively cheaper than phthalate-free PVC.
3. Lighter weight: The density of EVA ranges from 0.91 to 0.93, while that of PVC is 1.32.
4. Odor-free: EVA does not contain ammonia or other organic odors.
5. Does not contain heavy metals: Comply with the relevant international toy regulations (EN-71 Part 3 and ASTM-F963).
6. No phthalate: suitable for children’s toys and will not cause the risk of plasticizer release.
7. High transparency, softness and toughness: the application range is very wide.
8. Super low temperature resistance (-70C): suitable for freezing environment.
9. Resistant to water, salt and other substances. It can maintain stability in large amounts of application.
10. High heat adhesion: It can be firmly attached to nylon, polyester, canvas and other cloths.
11. Low bonding temperature: can speed up the production speed.
12. Can be silk-screened and offset printing: can be used for more fancy products (but must use EVA ink).
Application range:
Household appliances, hardware tools, toys, handicrafts, tourism products, cultural goods, cosmetics; lining materials for sports shoes, sports insoles, luggage back pads, surfboards, kneeling pads; sports protective gear, car interiors, shoe material fitting; base materials for high-end foam tape products; ice storage, cold-proof buildings, Light mats, sun visor materials; shock-proof cushioning packaging for electrical appliances, precision instruments, instruments and other electronic products in a wide range of fields.
EVA Hot Melt Adhesive Formulation
| Element | Mass percentage | Ingredient description |
| EVA | 30-35% | Main body resin |
| Terpene resin | 6-15% | Tackifying resin |
| C9 petroleum resin | 10-20% | Tackifying resin |
| Microcrystalline wax/paraffin wax | 3-8% | Viscosity modifier |
| Dibutyl phthalate | 1-4% | Plasticizer |
| Calcium carbonate | 25-35% | Filler |
| BHT | 0.5-2% | Antioxidant |
| Tert-Butyl Peroxy-2 Ethyl Hexyl Carbonate | 0-1% | Primary crosslinker |
| Fumed silica | 0-1% | Matting agent |