PET film itself is transparent - with a light transmittance of over 90%, it is widely used in packaging, optical films, and electronic substrates. But after vacuum aluminum plating, the same PET film becomes opaque like a mirror, with visible light reflectivity reaching 89% to 97%.

The transition from "transparent" to "mirror like" is not due to any chemical changes in PET, but rather because a layer of aluminum metal with a thickness of only tens of nanometers has been deposited on the surface of the film. Aluminum has extremely high reflectivity for visible and infrared light in metals, and when deposited on the surface of PET, it forms a continuous reflective interface, giving the film a metallic reflective property.

2、 Reflection and insulation: dual functions of the same material

The core function of PET aluminum coated reflective film can be decomposed into two layers.

Visible light reflectionIt's the first level. The reflectivity of aluminum coating for visible light is usually between 89% and 97%, depending on the thickness and surface flatness of the aluminum layer. In LED lighting fixtures, this feature is used to maximize light efficiency output - redirecting "off center" light towards the exit surface and reducing light absorption losses inside the fixture. The traditional approach uses aluminum or stainless steel reflectors, which have high reflectivity but are costly, heavy, and difficult to process. The PET aluminum plated reflective film achieved the same level of reflection effect at a cost of about 50%.

Infrared and thermal radiation reflectionIt's the second level. Aluminum has a much higher reflectivity for infrared radiation than most metals. The UV transmittance of PET film after aluminum plating is extremely low, only below 5%, and a large amount of infrared radiation is reflected. In building insulation applications, this feature allows the film to "block out" the heat from solar radiation rather than "absorb it in". In the fields of automobiles and new energy, PET aluminum plated film is used as a thermal insulation layer for battery packs, reducing the working temperature fluctuations of the battery pack by reflecting thermal radiation.

3、 Aluminum layer thickness: the core variable determining performance

The thickness of the aluminum layer is the most critical process parameter for PET aluminum coated reflective film. It's not about 'thicker is better', but about finding a balance between reflectivity, barrier performance, flexibility, and cost.

The thickness of aluminum layer is usually characterized by optical density (OD). The higher the OD value, the thicker the aluminum layer, and the lower the transmittance. In industrial products, the OD value is usually adjustable between 1.62 and 3.6, corresponding to different reflectivity and barrier properties. At the same substrate thickness, the higher the OD value, the better the continuity and density of the aluminum layer, and the stronger its ability to block oxygen and water vapor. The oxygen permeability can be as low as 1.13 cm ³/m ² · d · 0.1MPa, and the water vapor permeability can be as low as 1.86 g/m ² · 24h.

However, the thicker the aluminum layer, the better. When the thickness of the aluminum layer exceeds a certain threshold, the increase in reflectivity tends to saturate - because the reflection on the surface of the aluminum layer is close to complete, and further increase in thickness has limited contribution to optical performance. Meanwhile, an excessively thick aluminum layer may increase the rigidity of the film, reduce its flexibility, and affect subsequent die-cutting and bonding processes. In practical selection, engineers need to choose the appropriate OD value based on the specific reflectivity requirements and barrier level of the application.

4、 Vacuum aluminum plating: growing optical grade aluminum layer on PET surface

The aluminum layer preparation of PET aluminum coated reflective film mainly adopts vacuum evaporation process. In a high vacuum environment, high-purity aluminum wire is heated to the evaporation temperature (about 1200-1400 ° C), and aluminum vapor condenses and deposits on the surface of high-speed PET film, forming a continuous and dense aluminum layer.

The core control points of this process include:

5、 Key Performance: What Engineers Should Pay Attention to

When evaluating PET aluminum coated reflective film, the following indicators are the most critical:

visible light reflectanceIt is the core indicator for measuring optical performance. The reflectivity of the high gloss aluminum plated film can reach ≥ 89%, and the glossiness can reach 135%. In LED lighting applications, reflectivity directly affects the light efficiency output of the lamp.

Infrared blocking rateIt determines the thermal insulation performance. The aluminum plating layer can efficiently reflect ultraviolet and infrared rays, and a large amount of infrared rays are reflected or diffused. In building and automotive insulation applications, infrared barrier ratio is the primary criterion for selection.

barrier performanceIncluding oxygen permeation rate and water vapor permeation rate. The presence of aluminum layer significantly reduces the permeability of PET film to oxygen and water vapor. The oxygen permeability can be as low as 1.13 cm ³/m ² · d · 0.1MPa, and the water vapor permeability can be as low as 1.86 g/m ² · 24h.

mechanical propertiesIn terms of aspect, the tensile strength of PET aluminum plated film is usually ≥ 170 MPa, and the elongation at break is ≤ 200%. Good mechanical strength makes the film less prone to damage during die-cutting, stamping, and bonding processes.

heat resistanceIn terms of long-term use temperature, PET substrate can reach 120 ° C. Some products have a temperature resistance range of -40 ° C to 130 ° C.

Data Source:Advanced Institute of TechnologyAnd industry public information

6、 Application Layout: From LED Lighting to Building Energy Efficiency

7、 Selection considerations: from reflectivity to processing adaptability

• Selection of reflectivity and OD value:High reflectivity applications (such as LED lighting) should choose high brightness products (reflectivity ≥ 89%); For applications that require higher barrier properties (such as packaging), products with higher OD values should be selected. The higher the OD value, the thicker the aluminum layer, and the stronger the barrier, but the flexibility decreases accordingly.
• Selection of substrate thickness:The thickness range of PET aluminum plated film substrate covers 4.5 μ m to 188 μ m. Thinner substrates (6-12 μ m) are suitable for scenarios that require high lightweight and flexibility; Thicker substrates (50-100 μ m) are suitable for building and automotive applications that require high mechanical strength and dimensional stability.
• Single sided and double-sided aluminum plating:Single sided aluminum plating provides the characteristics of reflection on one side and insulation on the other side; Double sided aluminum plating provides double-sided reflection function, suitable for scenarios that require double-sided reflection or double-sided conductivity.
• Temperature resistance and processing technology:The long-term use temperature of PET substrate is 120 ° C. In applications that require higher temperatures, it is necessary to evaluate whether the substrate meets the process requirements. The flexibility of the film makes it easy to cut, fold, and shape, making it suitable for die-cutting processing.

8、 Conclusion

The essence of PET aluminum coated reflective film is an engineering material that combines the mirror reflection characteristics of aluminum with the lightweight and flexible properties of PET film. It is not a trade-off between "reflection" and "flexibility", but a vacuum aluminum plating process that unifies the two in the same material system - the aluminum layer is responsible for managing light and heat, and the PET substrate is responsible for providing structural support and processing convenience.

From the reflective film of LED lamps to the insulation layer of building curtain walls, from the thermal management of new energy vehicle battery packs to the barrier protection of food packaging - PET aluminum plated reflective film is unifying the seemingly independent engineering propositions of "light reflection" and "heat control" into one material solution. Understanding the engineering relationship between aluminum layer thickness, reflectivity, and barrier performance is the key to upgrading PET aluminum coated reflective film from a "roll of reflective film" to an "engineering decision".