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With China Plastic Machinery
With China Plastic Machinery
Plastic bonding is a process that uses a variety of means and methods to extend, thicken or form a specific shape of plastic profiles or parts. This process belongs to the category of secondary processing of plastics. Plastic connection mainly includes three forms: bonding, welding and mechanical connection. Each method has its own unique advantages and disadvantages. In practical applications, it should be reasonably selected according to specific needs.
Plastics are solid materials with synthetic or natural polymer compounds as the main components, which may or may not contain compounding agents. They are molded under certain temperature and pressure conditions and can maintain the shape of the product. Plastics can be roughly divided into two categories: thermoplastics and thermosetting plastics. Thermosetting plastics are soluble before molding. Once solidified, they become solid and cannot be softened or dissolved by heating. They may decompose when exposed to strong heat. Therefore, they do not have the ability to be repeatedly molded, but have higher strength and resistance. In contrast, thermoplastics soften when heated, and solvents can dissolve them. They can be repeatedly molded, but due to their lower strength, they have poor heat resistance and solvent resistance.
Plastic bonding includes bonding between plastic itself and between plastic and other materials (such as metal or non-metal). When performing plastic bonding, it is first necessary to clarify the specific type of plastic to be bonded, and then select the appropriate adhesive and implement the appropriate bonding process. According to the thermal properties of plastics, plastics can be divided into thermoplastics and thermosetting plastics. Thermoplastics can be distinguished by heating or solvent dissolution, while thermosetting plastics cannot. There are various bonding methods for thermoplastics, including hot melt bonding, solvent bonding, solution bonding and adhesive bonding, while thermosetting plastics can only be bonded with adhesives. Some low-surface-energy, difficult-to-bond plastics, such as polytetrafluoroethylene, polyethylene, and polypropylene, require special surface treatment to achieve the ideal bonding effect. These treatments usually involve chemical methods, otherwise the bonding effect may not be good. For plastics that are easy to bond, solvents can be used directly for bonding, such as chloroform bonding of plexiglass and acetone bonding of eyeglass frames, which can all achieve good bonding effects.
The correct bonding process is very important because different types of plastics have different surface treatment methods. Some plastics only need solvent degreasing, while others need roughening or chemical treatment. Plastic bonding usually cures at room temperature, but for plastic to metal bonding, heating and curing are sometimes required. In this case, attention should be paid to the technology and conditions of heating and curing, and whether the bonded plastic can withstand the heating temperature should also be considered. In addition, cooling after heating and curing also needs to follow relevant technical requirements. There are four main methods for plastic bonding: hot melt bonding, solvent bonding, solution bonding and adhesive bonding. For thermoplastics, all four methods can be used; for thermosetting plastics, only adhesive bonding can be used.
Hot melt bonding is a bonding method that utilizes the characteristics of thermoplastics being heated and melted, placing them in a bonding flow state at a high temperature (about 95 to 260 degrees Celsius), then closely contacting and applying pressure, and obtaining a certain strength after cooling. It is suitable for bonding and heat sealing of tube sheets and films such as polyethylene, polyvinyl chloride and polypropylene.
Solvent bonding is to use the solubility characteristics of plastics to apply appropriate solvents to the bonded parts, wait for the solvent to wet the surface and soften it, then overlap and apply pressure, and the solvent will harden after evaporation. This method is often used for temporary repairs or occasions with low requirements due to the large stress and insufficient bonding, and is only suitable for bonding between the same materials.
Solution bonding is a method of mixing solvents with the powder of the plastic to be bonded, adding appropriate additives, and preparing a solution of a certain concentration for plastic bonding. Although this method is difficult to control accurately, the bonding strength is higher than the solvent method. If a binary or ternary mixed solvent is used, the effect is better.
Adhesive bonding can be used for self-bonding of various plastics, and can also bond plastics with non-metallic or metal materials. Since different plastics require different adhesives, caution should be exercised when selecting adhesives.
Surface treatment of plastics is a key step to ensure good bonding. Usually, water is used to clean and remove impurities such as dirt, dust, grease, etc. For small-sized plastic parts, ultrasonic cleaning can also be used. For different types of plastics, special degreasing solvents are needed for wiping, dipping, spraying or steam degreasing to remove mineral oil, vegetable oil and animal oil on the surface. Next, use tools such as sandpaper, files, and grinders to roughen the plastic surface, but it should not be over-roughened to avoid affecting the bonding strength. After roughening, the surface residue should be removed with a brush, dry cloth or compressed air, and wiped again with a degreasing solvent to thoroughly remove the oil and then dry for use. Flame treatment refers to the use of a flame generated by the combustion of a combustible gas to instantly burn the surface of the plastic to be bonded at high temperature, causing an oxidation reaction to occur, thereby improving the bonding effect. For example, after using an oxy-acetylene flame (gas welding gun) to treat difficult-to-bond plastics (such as polyethylene and polypropylene), conventional adhesives can be used for bonding.
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