In fact, what the industry calls "compounding" means "laminating", which means that films with different properties are glued together in a certain way and then sealed to protect articles.The compound processing methods of flexible packaging mainly include dry compound, wet compound, extrusion compound, coextrusion compound, etc.
Among the various processing technologies of composite films, dry compounding is the most traditional and widely used compounding technology in our country. It is widely used in the packaging of food, medicine, cosmetics, daily necessities, light industrial products, chemicals, electronic products, etc.Dry compounding is to coat a layer of solvent-based adhesive on a plastic film by a coating device (usually gravure screen roll coating), remove the solvent by a laminating machine and dry it, and then compound it with other substrates in a hot pressing state, such as films, aluminum foils, etc. to form a composite film. since it is compounded in the "dry" state (solvent-free state) of the adhesive, it is called dry compounding.
Dry compounding is suitable for compounding various composite film substrates and films with aluminum foil and paper. It has a wide application range and excellent corrosion resistance to chemical media. For example, food contains alkali, acid, spicy, grease and other components, cosmetics contains water, essence, emulsifier and other components, chemicals contain solvents, pesticides and other components, and are widely used in packaging with harsh content conditions.Its composite strength is high, its stability is good, and its transparency is high. It can not only produce high-grade and middle-grade composite films, but also produce frozen, fresh-keeping or high-temperature sterilization composite films.The utility model has the advantages of convenient and flexible use and simple operation, and is suitable for production with multiple varieties and less batch.However, dry compounding itself also has the defects of poor safety and sanitation, environmental pollution, higher cost, etc.The development of alcohol-soluble and water-soluble adhesives has eased the pressure of solvent-based adhesives on safety and hygiene, environmental pollution and cost to some extent.Dry compounding still occupies a large proportion in the processing methods of composite materials. At present, extrusion compounding, wet compounding and solvent-free compounding cannot be replaced.
Wet compounding is to coat a layer of adhesive on the surface of the composite substrate (plastic film, aluminum foil), compound with other materials (paper, cellophane) by a pressing roller under the condition that the adhesive is not dry, and then dry through a hot drying tunnel to form the composite film.Wet compounding is characterized by simple process operation, less adhesive consumption, low cost and fast compounding speed.Adhesives used in wet compounding mainly include polyvinyl alcohol, sodium silicate, starch, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyacrylate, natural resin, etc.The working principle of the wet compound machine is basically similar to that of the dry compound machine, except that the dry compound machine heats the film coated with the adhesive through a drying tunnel, and after the organic solvent in the adhesive volatilizes, the film is thermally pressed and bonded with the composite material;In the wet compounding method, the film coated with adhesive is directly compounded with the composite material and then enters a drying tunnel for drying.
When wet compounding is used, at least one of the two substrates is a porous material (such as paper or cardboard) so that the diluent contained in the adhesive volatilizes when drying.Therefore, wet compounding is widely used for compounding materials such as paper/fiber, paper/paper, paper/paperboard, paper/aluminum foil, etc.
Extrusion compounding is to extrude thermoplastic materials such as polyethylene into a flat die after melting in an extruder, and form a sheet-like film which is compounded with another film or two films through a cooling roller and a composite pressing roller immediately after flowing out.Compared with other compounding methods, extrusion compounding has the advantages of low equipment cost, less investment, clean production environment, no residual solvent in the composite film, high production efficiency, simple operation, etc. Extrusion compounding occupies a very important position in the compounding processing of plastics.Extrusion composite films can be prepared by extrusion coating and extrusion compounding.
Extrusion coated film
Extrusion coating is a method of making composite films by melting thermoplastic plastics such as polyethylene and flowing out of a flat machine head, pressing the thermoplastic plastics towards another substrate between two closely contacted rollers, and cooling the thermoplastic plastics.The unwinding substrate is generally printed PET, BOPP, paper and other materials, such as instant noodle packaging bags, which are typical examples of extrusion coating production. Its structure is BOPP printing film/LDPE (or PP).
Extruded composite film
Extrusion compounding is to sandwich the extruded resin between two kinds of base materials. It acts as an adhesive to bind the two kinds of base materials together and is also a composite layer.The first substrate is usually PET, BOPP, PA, paper and other materials, the second substrate is usually LDPE, CPP, aluminum foil, aluminized film and other materials, and the extruded resin is usually PE, PP, EVA, EAA and other resins.General ordinary washing powder packaging film can be compounded by this method, and its structure is as follows: BOPP printing film/extrusion resin/PE film.
Adhesives used in solvent-free compounding have attracted much attention because they do not contain organic solvents. In developed countries such as Europe and the United States, solvent-free compounding has become the main method for the production of flexible packaging composites.It is a compounding method that uses solvent-free adhesive to coat the substrate and directly adhere it to the second substrate. Although the adhesive is used like dry compounding, the adhesive does not contain organic solvent and does not need a drying device.Due to its superior environmental friendliness, the product performance can be the same as that of dry compounding, which is the future development direction.
Coextrusion compounding is a molding method for preparing composite films by melting and plasticizing two or more different plastics through two or more extruders and then feeding the plastics into a pair of die or feeding the plastics supplied by various extruders into an inlet die after merging through a distributor.The different plastics mentioned here can be different kinds of plastics, the same kind but different brands of plastics, or the same brand but different formulations of plastics.The cost of coextrusion compounding is low, which is 20%-30% lower than that of dry compounding, and no adhesive or anchor coating agent (AC agent) is used in the coextrusion compounding process, so that the coextrusion compounding process has good sanitation and no environmental pollution problem.However, due to the obvious limitation of materials, the composite films produced by coextrusion process are limited to various thermoplastics. If composite materials containing aluminum foil, paper, etc. are needed, they cannot be produced by coextrusion process.In addition, it is not allowed to print between the composite films, and only patterns, characters and the like can be printed on the surface of the composite film when the film needs to be printed.
To sum up, all the above composite processing methods have their own advantages and disadvantages. Judging from the current development situation, dry composite has the widest application range and the largest proportion. At the same time, extrusion composite has developed rapidly, and its proportion is second only to dry composite. However, wet composite and coextrusion composite processes are restricted by materials and cannot become the mainstream. Solvent-free composite technology has matured in developed countries and is the development direction of composite processing methods in the future.