Lamination processes for composite packaging bags: dry lamination, wet lamination, extrusion lamination

Flexible packaging refers to packaging whose shape can change after filling or removing the contents.

Various bags, boxes, and seals made of paper, aluminum foil, fiber, plastic film, and their composites are all flexible packaging. Single-layer films have different advantages and disadvantages.

For example, polyester film has high strength and excellent oil resistance, gas barrier properties, aroma retention, and moisture resistance, but its heat sealability is poor; while polyethylene film has good moisture resistance and heat sealability, but its gas barrier properties, oil resistance, etc. are poor.

They have performance defects when used alone as plastic packaging, but combining polyester film with polyethylene film can maintain the high strength and good oil resistance, gas barrier properties, aroma retention, and moisture resistance of polyester film, while the film also has good heat sealability.

The role of composite film is to combine multiple layers of film together, overcoming the shortcomings of single-layer films and combining the advantages of each layer of film to become a more ideal packaging material.

In addition, after printing and laminating the film, because the ink is sandwiched between the film layers, the ink layer is protected from direct friction, scratches, and various corrosive substances, which not only solves the problems of color bleeding and color fading in plastic printing, but also avoids the safety and hygiene problems caused by ink directly contacting food and medicine.

The "lamination" in the industry actually means "lamination", which is to stick films of different properties together in a certain way, and then seal them to protect the contents.

The main lamination methods for flexible packaging include dry lamination, wet lamination, extrusion lamination, and co-extrusion lamination.

1、 Dry Lamination

Among various processing technologies for composite films, dry lamination is the most traditional and widely used composite technology in China, widely used in packaging for food, medicine, cosmetics, daily necessities, light industrial products, chemicals, and electronic products.

Dry lamination uses a coating device (generally using gravure intaglio roller coating) to apply a layer of solvent-based adhesive to the plastic film, removes the solvent and dries it in a laminating machine, and then laminates it with other substrates under hot pressing. For example, films and aluminum foil are bonded into composite films.

The process flow is shown in the figure below. Because it is laminated in a "dry" state (solvent-free state), it is called dry lamination.

Dry lamination is suitable for various composite film substrates and the lamination between films and aluminum foil and paper. It has a wide range of applications, excellent resistance to chemical media corrosion, such as alkali, acid, spicy, and oil components in food, water, fragrances, and emulsifiers in cosmetics, and solvents and pesticides in chemicals. It is widely used in packaging with harsh content conditions.

It has high lamination strength, good stability, and high product transparency. It can produce high-end and mid-range composite films, as well as frozen, fresh-keeping, or high-temperature sterilization composite films; it is convenient and flexible to use, simple to operate, and suitable for multi-variety and small-batch production.

However, dry lamination also has defects such as poor safety and hygiene, environmental pollution, and high cost. At the same time, the development of alcohol-soluble and water-soluble adhesives has alleviated the pressure on solvent-based adhesives in terms of safety and hygiene, environmental pollution, and cost to some extent.

Dry lamination still accounts for a large proportion in composite material processing methods, and at present, it is still an irreplaceable composite processing method for extrusion lamination, wet lamination, and solvent-free lamination.

2、 Wet Lamination

Wet lamination is to apply a layer of adhesive to the surface of the composite substrate (plastic film, aluminum foil), and under the condition that the adhesive is not dry, it is laminated with other materials (paper, cellophane) through a pressure roller, and then dried through a hot air duct to become a composite film.

The characteristics of wet lamination are simple process operation, low adhesive consumption, low cost, and fast lamination speed. The adhesives used in the wet lamination method mainly include polyvinyl alcohol, sodium silicate, starch, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyacrylate, and natural resins.

The working principle of the wet lamination machine is basically similar to that of dry lamination. The difference is that dry lamination is to heat the film coated with adhesive through a drying channel, and after the organic solvent in the adhesive volatilizes, it is then hot-pressed and bonded with the composite material; while the wet lamination method is to directly laminate the film coated with adhesive with the composite material, and then enter the drying channel for drying.

When using wet lamination, at least one of the two substrates is a porous material (such as paper or cardboard) so that the diluent contained in the adhesive can volatilize during drying.

Therefore, it is widely used in paper/fiber, paper / paper, paper / cardboard, paper / and aluminum foil composite materials.

3、 Extrusion Lamination

Extrusion lamination is to melt polyethylene and other thermoplastic materials in an extruder, extrude them into a flat die to form a sheet-like film, and then immediately laminate it with another or two films through a cooling roller and a laminating pressure roller.

Compared with other lamination methods, extrusion lamination has the advantages of low equipment cost, low investment, clean production environment, no residual solvent in the composite film, high production efficiency, and simple operation. Extrusion lamination occupies a very important position in the composite processing of plastics.

Extrusion laminated films can be produced by extrusion coating and extrusion lamination.

Extrusion coated film:

Extrusion coating is a method of producing composite film by melting thermoplastic plastics such as polyethylene, flowing them out from a flat die, pressing them onto another substrate between two tightly contacting rollers, and then cooling them.

The unwinding substrate is generally pre-printed PET, BOPP paper, and other materials. For example, instant noodle packaging bags are a typical example produced by extrusion coating, with the following structure: BOPP Printed film / LDPE (or PP ).

Extrusion Laminated Film:

Extrusion lamination involves sandwiching the extruded resin between two substrates, where it acts as an adhesive to bond the two substrates together, and also forms a composite layer.

The first substrate is often PET, BOPP 、 PA paper, and other materials, while the second substrate is generally LDPE 、 CPP aluminum foil, metallized film, and other materials.

Extruded resins are generally PE, PP 、 EVA 、 EAA and other resins. General common washing powder packaging film can be laminated by this method, with the structure as follows: BOPP Printed film / Extruded resin / PE film.

Solvent-free lamination:

The adhesives used are gaining attention because they do not contain organic solvents. In developed countries such as Europe and America, solvent-free lamination has become the main method for producing flexible packaging composite materials.

It is a lamination method that involves coating a substrate with a solvent-free adhesive and directly bonding it with a second substrate. Although it uses adhesives like dry lamination, its adhesives do not contain organic solvents, eliminating the need for drying equipment. Due to its superior environmental friendliness and product performance comparable to dry lamination, it is the future direction of development.

Co-extrusion lamination:

It is a molding method for producing composite films by feeding two or more different plastics, after they have been melted and plasticized by two or more extruders respectively, into a single die, or by merging the plastics supplied by various extruders through a distributor before feeding them into the die.

The different plastics referred to here can be different types of plastics, or the same type but different grades, or the same grade but different formulations.

Co-extrusion lamination has low costs, which can be 20%-30% lower than dry lamination. Furthermore, the co-extrusion lamination process does not use adhesives or anchor coatings ( AC agent), thus ensuring good hygiene and no environmental pollution issues.

However, it is significantly limited by materials; composite films produced using the co-extrusion lamination process are limited to various thermoplastic plastics. If composite materials containing aluminum foil, paper, etc., are required, they cannot be produced using the co-extrusion lamination method.

Additionally, printing cannot be done between composite films. When printing is required on the film, patterns, text, etc., can only be printed on the surface of the composite film.

In summary, each of the aforementioned composite processing methods has its own advantages and disadvantages.

From the current development situation in our country, dry lamination has the widest application range and the largest proportion. Meanwhile, extrusion lamination is developing rapidly, with its proportion second only to dry lamination. Wet lamination and co-extrusion lamination processes are largely limited by materials and cannot become mainstream. Although solvent-free lamination technology is mature in developed countries, it is still basically in its nascent stage in our country and represents the future direction for composite processing methods.

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