To ensure that all adhesives spread and adhere to the substrate, they need to have a & quot; soak & quot; In most adhesives used in packaging, the fluidity is accelerated by dissolving the adhesive in a readily evaporating material (water, solvent, etc.) such as water.
When the blast furnace evaporates it should leave an adhesive which can be easily joined behind. On the other hand, such a solvent is not used in hotmelts. Instead they can be melted before application. Heating causes the hotmelt to solidify, resulting in a bonding property. There are many advantages in using heat as a solvent in partitioned adhesives.
Heat dissipates more quickly in volatile liquids. Even for waterproof layers, this is the case. Rapid heat distribution results in rapid bond formation. For this reason, hotmelts are ideal for applications at high speeds, especially where fast adhesion is important. The absence of a liquid solvent makes it possible for hotmelt adhesives to be transported and stored in small, comfortable forms without fear of freezing, decomposition, decay.
The biggest disadvantage of hotmelt adhesives is; Limited resistance to heat. Thermoplastic materials lose a significant portion of their adhesives even at slightly elevated temperatures, eventually melting again. The strength of most hotmeltes used in packaging starts to decrease at 60 ° C and fall at 70 ° C. The most common uses for packaging are hotmelt, low molecular weight resin and ethylene vinyl acetate copolymer (EVA) combined with a waxy substance.
The additives of EVA are integrative strength, product durability, hot stitching and resistance to heat. Resin is; It is useful in hot stitching, accelerates adhesion, determines large scale color and smell. As the most crystallized material, the wax adjusts the softening point, heat resistance and time. Another hotmelt system is based on several low molecular weight PE and resin.
Here, the PE takes the place of EVA and waxy material in the EVA system. Controls the softening point and application properties while imparting crystallinity to adhesion strength, product durability and integrity. PE-based system shows less elongation, separation and peeling at elevated temperatures than EVA systems.
Its colors are clearer, applications are cleaner, and they are more resistant to cutting at high temperatures. PE based hotmelts are not suitable for EVA types but are oil barrier. If PP is applied instead of crystallized PE, a softer adhesive and an amorphous adhesive will emerge. The amorphous nature of PP systems leads to good adhesion to the films and foil, but also to a reduction in the durability of the application time to prolongation and separation.
As a result, the use of these systems in packaging is limited by film, foil and paper lamination. Hotmelt made from thermoplastic block copolymers are used in packaging applications where rubber structures can be used. In combination with resin and often an oil, it can be converted into pressure sensitive adhesives. In press-sensitive situations, things like time are identified with concepts such as shelf life and application speed. The advantages of press-sensitive compartments often arise during application because solvent-drying systems are not required for their use.
Other PSA systems have different characteristics. It dissolves again like other hotmelts and has limited use at temperatures above 100 ° C. Polyamide, polyester and similar based hotmelts are sometimes used in packaging applications where more heat and oil barrier are required. These are more costly and applications are more difficult than holmeltten. The comparison of physical properties of hotmeltler is customized in solidified, molten and solid state. Fluidity densities; Depending on the temperature and the measured values. Since hotmelts are not completely crystallized materials, there are no definite melting points.
Perhaps the most important feature of hotmeltler is the application period. These properties depend on the physical changes that occur during cooling and bonding in the adhesive. Open time is the maximum time for the lower layers to stick together at a given temperature. If the layers do not adhere during this time, the adhesive will become cold and dry. The freezing time is the minimum time that the layers must adhere after bonding. If the control is done before the end of freezing, the layers are separated from each other.
Open time and freezing time depend on the application heat, the heat of the layers, the heat conductivity of the layers, the shape and the cooling characteristics of the adhesive. The resistance to heat of a hotmelt is a continuous flux measure which combines at high temperatures. Heat barrier tests are carried out at constant temperature or at elevated temperatures in a given area.
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