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Currently, we use a variety of plastic films, such as PE, PET, BOPP, and PVC. There are several ways to preliminarily identify plastic materials:

1. The product’s appearance, shape, and usage.
2. Physical characteristics.
3. Burning properties.
4. Solubility.
5. Relative density.

Identifying Plastic Based on Usage

For instance, single-layer plastic bags used for food packaging are typically made of polyethylene (PE), while plastic casings for sausages are often made of polyvinylidene chloride (PVDC). Many modern plastic packaging products are labeled with their material, so by observing the markings, the material of the product can be easily determined.

1. Identifying Single-Layer Plastic Films

For single-layer plastic films, distinguishing them by appearance and use is usually possible, though experience is required. For example, if you encounter a film, you should first examine its appearance, noting characteristics such as glossiness, transparency, and stiffness. Colorless, transparent films with a glossy, smooth, and stiff surface are often biaxially oriented polypropylene (BOPP) or polyethylene terephthalate (PET).

Films that feel soft might be polyvinyl alcohol (PVA) or soft PVC. Transparent films that turn milky white when rubbed are likely polyethylene (PE) or polypropylene (PP). Films that produce a crisp sound when shaken are generally PET. After making these observations, further analysis of physical properties and burning characteristics can help confirm the material.

Plastic materials dissolve in specific solvents. The solubility of plastics in different solvents can also be used to distinguish various types of plastic.

2. Identifying Composite Packaging Films

Composite plastic films are made from multiple single-layer films, so their identification methods differ slightly. First, consider the performance characteristics of the individual layers and the purpose of the composite. For example, opaque composite films typically include aluminum foil or metalized layers, while high-barrier films are often multi-layered. By analyzing the appearance and mechanical properties of the film, you can roughly determine the base material—such as high-strength PET, puncture-resistant PA (nylon), or easily tearable PP. PET and PA are typically laminated with PE or CPP to solve heat-sealing problems.

Additional Methods for Identifying Composite Films

1. Natural Curling: Composite films usually curl naturally, which becomes more apparent when placed in hot water.
2. Different Textures: If the inner and outer layers of a composite film are made from different materials, the texture of each side will differ, which can be detected by careful touch.
3. Tear Test: By cutting a small notch in the film and tearing it at an angle, layers of the composite can be easily separated based on the differing tear resistance of the base materials.
4. Printed Layers: If a film has a printed layer, and the ink does not rub off, this suggests the printing layer is sandwiched between two layers, indicating a composite film.

These are simple methods to differentiate between single-layer and composite films. More reliable identification can be achieved through physical property testing. Using thermal analysis, the characteristic temperatures on the thermal spectrum can help identify composite films like PP/PE, PET/PB, or PA/PE by their respective melting peaks.

The microscope method is another way to identify composite films. Different plastics have varying refractive indices and birefringence values when stretched. Under polarized or phase contrast microscopy, each layer of the composite will display different colors, making it possible to distinguish between single-layer and composite films.

Separation and Identification of Composite Film Components

Once you identify a composite film, the next step is to separate its components for further analysis.

1. Peeling Method: Films made through extrusion or dry lamination can be peeled apart by stretching or rubbing, due to uneven deformation. Heating the film may soften the adhesive layer, allowing separation. High-crystallinity polymers can also be separated based on their differing melting points.
2. Dissolution or Solvent Method: Different resins have varying solubility or swelling properties. A solvent can penetrate the interface and alter surface tension, helping to separate the layers. Once the layers are separated, the base materials can be analyzed and identified, but care must be taken to avoid interference from adhesives.