Hot melt adhesives should pay attention to what issues (on)

The use of corrugated boxes in the packaging of goods has been very extensive, and the quality of paperboard and printing has also increased. However, the traditional method of using the adhesive tape or even the strapping sealing method has apparently lags behind. It not only affects the appearance of the overall package, but also has great defects in terms of durability, security, and the like. At present, as a new type of carton and carton packaging sealing, hot melt adhesive sealing plastic box has begun to partially replace the traditional strapping packaging and tape packaging, cartons for various food packaging and wine, Dairy products and other products on corrugated cardboard packaging. Because the hot melt adhesive sealing has the advantages of good package quality, beautiful appearance, low cost, and good anti-theft effect, the application of the hot melt adhesive sealing box is becoming increasingly popular. In view of the fact that the hot melt adhesive sealing is a new technical method, it will inevitably cause some problems in use. Therefore, it is necessary to pay attention to the characteristics and use technology of the hot melt adhesive in order to ensure the quality of the plastic box.

First, the main characteristics of hot melt adhesive and the advantages of the corrugated carton tank

Hot melt adhesives mainly include EVA hot melt adhesives and Pt~`R hot melt adhesives. The EVA hot melt adhesive is an early-use product that has the advantages of fast bonding, low price, easy operation (with reversibility), and easy storage. However, due to the influence of solvents, low temperature, and high temperature conditions on the EVA hot melt adhesive, there are disadvantages such as poor durability, large amount of coating, low bonding strength, and less adaptable materials. For this reason, the moisture reaction was later developed. The curable polyurethane reactive hot melt adhesive (PUR) promotes hardening by the moisture in the air, effectively improving the deficiency of the EVA-based hot melt. The latest PUR hot melt adhesive can be adapted to different paper thicknesses, coating conditions, and printing inks. Its adhesive strength is 40% to 60% higher than that of conventional EVA adhesives. PUR film thickness is lower than that of standard EVA hot melt adhesives. It can be reduced by 50% or less, and PUR hot melt adhesive is unique in that it can firmly bond coated paper and a variety of other materials, including varnish coating, UV curing layer, plastic film and so on.

PUR hot melt adhesives are mass-produced PUR toppers with active terminal groups. Cross-linking reactions occur when the active terminal groups are exposed to moist air or the moisture of the paper reaches a certain level. After the top polymer is wetted, it penetrates well into the paper, achieving a higher viscosity and a higher paper tension. After cross-linking, the PUR film exhibits a higher intrinsic strength (cohesion) so that a very durable adhesive film can be formed. PUR film is not only insensitive to printing inks and solvents, but also has extremely strong durability. PUR is a macromolecular compound with a urethane bond (NH—CO—O). The end of the molecular chain is in the form of an isocyanate group. The basic composition of PUR is a polyol with an active hydrogen at the end. Ester, polyether, etc.) and low molecular weight diols (chain extenders) and isocyanate-based addition polymers. Based on the formation of rigid molecular segments (cohesion between molecules) from the reaction of glycols and isocyanate groups, and the formation of flexible molecular segments (van der Waals attraction) from the reaction of polyols and isocyanate groups, equilibrium is obtained. , to obtain a rigid and flexible PUR adhesive, in addition, when the isocyanate group meets the moisture in the air, an irreversible cross-linking reaction occurs (urea bond, ureidoformate ester chain, titration bond), This forms a strong adhesive layer. Active PUR (Polyurethane Reactive) Hot Melt Adhesive is a wet reactive hot melt adhesive that begins to melt at 90 degrees Celsius and cures when cooled. In addition, since the cross-linking reaction occurs between moisture and moisture in the atmosphere and paper, once the reaction is completed, adhesive strength, heat resistance, cold resistance, solvent resistance, recyclability, and other advantages are exhibited. It promotes hardening by the moisture in the air, which is the main characteristic of PUR hot melt adhesives. The most important point of PUR as a standard hot melt adhesive is its high viscosity. However, compared with EVA, PUR also has disadvantages such as poor workability, long hardening time, and mechanical speed restriction. It takes a long time for the early adhesive effect to occur, and contact with outdoor air may cause cross-linking. React, so we need to prepare a special glue applicator that can block air.

PUR hot melt adhesives began with the first generation of the 1980s (available at the drupa show in 1988) and have evolved into the fourth generation. Since the viscosity of the first-generation hot melt adhesive is not strong enough, only a strong viscosity can be obtained after the reaction takes place. Generally, the best adhesive strength is achieved after 2 days to 5 days. Second-generation PUR hot melt adhesives were introduced around the mid- to late-1990s. For the first-generation deficiencies, it increased cure speed, strength, and availability. If the paper contains a certain amount of moisture and the working environment has a suitable air humidity, better adhesive strength can be obtained after 4 hours of gluing. However, if the weather is cold and the relative humidity of the air is less than 20%, cross-linking of the PUR adhesive will be affected. The third-generation PUR has improved the cross-linking firmness, and the chemical reaction of the product has been accelerated. If the environment is suitable, it can be completely cross-linked and fixed in just one hour. The moisture in the paper can complete the moisture-curing cross-linking reaction, thus making PUR possible. The curing speed is greatly improved. However, since the third-generation PUR hot melt adhesive can achieve a preliminary adhesive strength in a relatively short period of time, its fluidity is not easy to control, especially in wet areas, and therefore, improving the flowability of PUR has become a new topic. . Nowadays, the fourth-generation PUR hot melt adhesives have begun to be applied. It not only enhances the curing speed and adhesiveness, but also improves the flowability of PUR, prolongs the residence time of PUR in plastic pots, and can achieve the desired initial viscosity. strength. In view of the superiority of the fourth generation PUR, it has gradually replaced the second and third generation PUR products. After nearly 20 years of development, the use of PUR adhesives in UV-coated papers and high-filled papers has become more and more popular.

Hot-melt adhesives were first used for bookbinding, to replace the traditional use of inconvenient, poorly-preserved wire binding and behind the process, production speed is very slow sewing binding, which greatly promoted the book printing to high-quality high-speed steering The process of binding the books produced a historic revolution. Later, with the increasing demand for carton packaging printing quality, hot melt adhesives began to be used in corrugated carton sealing applications. Hot-melt adhesive sealing is the use of a hot melt machine to melt the hot melt into a liquid, which is then sent to the surface of the carton through the throat of the hot melter and the spray gun. The hot melt adhesive is then cooled and bonded. It can meet the requirements of carton sealing, and overcome the defects of the traditional carton sealing form. Since the transport cartons must be able to withstand temperature and humidity changes and rough handling, it is difficult to meet this requirement with tape. Due to poor adhesive strength, low tensile strength, and poor adhesion to coated or oily paperboard. The hot melt adhesive has good binding force with the wet product, strong penetration force, physical and chemical bonding effect between the corrugated paper and the linerboard, and the integrated strength of the bonded carton is enhanced, and the carton is not easy to be deformed and cracked.

In addition, the hot melt adhesive coating system can apply pressure on key parts of various shapes, such as folding lids in cartons. Secondly, the adhesive tape has low temperature resistance and is easy to open in the cold climate in the north; while the hot melt adhesive has very good performance in high temperature and low temperature, such as PUR hot melt adhesive can resist high temperatures above 100 degrees Celsius or minus 40 degrees Celsius. Low temperature harsh environment. In addition, the hot melt adhesive has a storage period of up to 2 years, and the storage area is small. There is no special storage requirement. The tape storage period is only half a year. It also requires a certain storage condition, or it will easily deteriorate. In addition, the use of hot melt adhesives in corrugated carton sealing provides good anti-theft function. As the adhesive penetrates into the corrugated medium, any attempt to open the carton tears the fibers. For the sealing of the tape, it can be cut with a knife, and the contents are removed and sealed again unconsciously. In today's commodity packaging, merchants require packaging to convey products, but also to convey the product and company image through the packaging design. Therefore, the fewer factors that hinder appearance, the better. Traditional tape often obscures images and interferes with the transfer of packaging information. The hot melt adhesive is glued between the flaps, which can provide more display space and make the packaging product look beautiful. The use of hot melt adhesive sealing speed, the entire bonding process only takes 1 second to 3 seconds to complete, suitable for mass production, low energy consumption, low cost. The standard corrugated carton hot melt adhesive is approximately 32 yuan/1000 cases, which is the middle price of hot melt adhesive. The middle price of the tape seal is about 120 yuan/1000 boxes. If hot-melt adhesives are used, the addition of glue during processing will not cause a shutdown, while the replacement of adhesive tape will require shutdown. Moreover, no partitions may be installed in the box, and the strength of the sealing is greater than the strength of the sealing of the inner partition when the tape is used to seal the box, which can reduce the cost of the partition.

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