Characteristics of protective film and its application in the aluminum industry

Structure and material characteristics of protective film

The protective film is generally a polyacrylate type protective film, and the basic structure of the polyacrylate type protective film (Figure 1) is as follows from top to bottom: isolation layer; Printing layer; film; Adhesive layer. [2] (1. Isolation layer; 2. Printing layer; 3. Film; 4. Adhesive layer) Figure 1 Structure of polyacrylate protective film

1.1 Film

Thin films are generally made from low-density polyethylene (PE) and polyvinyl chloride (PVC) as raw materials. It can be obtained through extrusion molding, injection molding, and blow molding. Due to its relatively low price and better environmental friendliness, 90% of the film is made of polyethylene, mainly through blow molding process. There are many types of polyethylene with different melting points and densities.

1.2 Colloids

The quality of the protective film is determined by the characteristics of the colloid. There are two types of pressure-sensitive adhesives used for protective films: solvent based polyacrylate adhesive and water-soluble polyacrylate adhesive, which have different characteristics.

1.2.1 Solvent based polyacrylate adhesive

Solvent based polyacrylate adhesive uses organic solvents as a medium to dissolve acrylic monomers. The colloid is very transparent, with low initial viscosity and excellent aging resistance, lasting up to 10 years. When exposed to ultraviolet light, the colloid also slowly solidifies. After corona treatment, the film can be directly coated with polyacrylate adhesive without the need for a primer. Polyacrylate adhesive is relatively hard and has poor fluidity, so the adhesive force of the protective film is relatively slow to exert. Even after pressure, the colloid and the surface to be pasted cannot fully contact each other. After being left for 30 to 60 days, it will fully contact the surface to achieve the final adhesion. Moreover, the final adhesion is often 2 to 3 times greater than the adhesion when pasted. If the adhesive of the protective film is suitable for cutting in the board factory, the end user may have difficulty tearing off the film, or even cannot tear it off.

1.2.2 Water soluble polyacrylate adhesive

Water soluble polyacrylate adhesive is a type of adhesive that uses water as a medium to dissolve acrylic monomers, and basically has the characteristics of solvent based polyacrylate adhesive. However, the adhesive should avoid and reduce contact with water vapor to avoid residue. Due to its environmental friendliness and the absence of solvent recovery equipment, water-soluble polyacrylate adhesive is often used in developing countries to produce protective films.

1.3 Characteristics of Colloids

1.3.1 Adhesive force

The force required to peel off the protective film from the surface after a period of time. The adhesion force is related to the material being applied, pressure, film application time, angle at which the film is peeled off, and temperature. Generally, with the increase of time and pressure, the adhesion force will also increase. The viscosity of the protective film should not increase too much to ensure that there is no residue when tearing off the film. Generally, a 180 degree peel test is used to measure adhesion.

1.3.2 Cohesive force

Refers to the strength inside the colloid. As a protective film, the cohesive force of the colloid must be very high, otherwise when tearing off the protective film, the colloid will crack inside and cause residual glue. The measurement method of cohesion: Attach a protective film to the surface of stainless steel, hang a certain weight of weight on the protective film to measure how long it takes for the protective film to be pulled off by the weight. If the adhesive force is greater than the cohesive force, tearing off the protective film will break the connecting bonds between the adhesive molecules, resulting in residual adhesive.

1.3.3 Adhesion

Refers to the binding force between colloids and films. If the adhesion force is greater than the adhesion force, tearing off the protective film will break the connecting bonds between the adhesive molecules and the film, resulting in residual adhesive.

1.3.4 UV resistance linearity

Polyacrylate adhesive is UV resistant, and after adding UV stabilizers to the transparent polyacrylate adhesive protective film, its UV resistance can reach 3 to 6 months. Generally, climate simulation equipment is used to test the UV resistance of protective films. Temperature and radiation intensity are adjusted and condensed water is used to simulate climate change. A cycle of 3 hours of high humidity environment and 7 hours of UV irradiation is used. A 50 hour cycle experiment is equivalent to approximately one month of outdoor storage.

Application of protective film in aluminum industry

The Chinese aluminum industry started in the 1950s and has undergone nearly 60 years of development, becoming a global producer and consumer of aluminum. In the huge aluminum market in China, domestic brands are stacked together, and their quality can compete with international brands. They have an absolute majority in the market and have established professional associations to form a unified national standard to ensure the quality of aluminum. The protective film is a layer of plastic film attached to aluminum materials, which aims to protect the produced aluminum materials from damage during a series of processes such as handling, inventory, transportation, processing, and installation. After the installation of the aluminum materials is completed, the installation team tears them off to make the surface of the aluminum materials smooth and new, and to achieve the desired decorative effect.

There are various types of aluminum products in the Chinese market, and the surface treatment technology for aluminum is constantly improving. Various surface treatment technologies for aluminum materials in China include mechanical polishing, chemical polishing, anodizing coloring, electrophoretic coating, chemical coloring, fluorocarbon spraying, electrostatic powder spraying, surface pattern treatment, etc. Different adhesive protective films need to be selected for different aluminum surfaces.

Protective film products generally come in four series: low viscosity, medium viscosity, high viscosity, and ultra-high viscosity; Colors include transparent, milky white, blue, black and white, etc; The thickness is 30-200 μ m, so different protective films need to be selected according to the different surfaces of the aluminum material. Depending on the surface and film pressure, the rate of viscosity increase varies. Generally, the protective film should be peeled off after sufficient time to determine whether the viscosity is appropriate.

It is extremely important to choose the appropriate colloid and adhesive protective film for a given surface, and the subsequent processing steps are the key to determining the adhesion. The subsequent processing generally includes cutting, bending, drilling, stamping, etc. Generally, low adhesion protective films are used for smooth surfaces, such as mechanical polishing and chemical polishing of aluminum materials; Select medium adhesive protective film for moderately rough surfaces, such as anodizing coloring, electrophoretic coating, chemical coloring, fluorocarbon spraying, and smooth electrostatic powder coating aluminum; Choose high adhesion protective film for extremely rough surfaces, such as electrostatic powder sandblasted aluminum; If laser cutting is required, due to the thermal shrinkage of the protective film, a highly adhesive protective film should be selected.

In addition to viscosity, users also need to test the tensile strength and elongation of the protective film to ensure that its flexibility and strength meet the requirements for use.