This article provides a brief overview of the salient aspects of polycarbonate films, from the peculiarities of the polymer used to the differentiations of the various films on the market. The most common processing methods and sectors of use will also be briefly summarized.
Peculiar characteristics of polycarbonate
Polycarbonate (PC) is a thermoplastic polymer with chemical/physical characteristics that make it highly sought after in various industrial fields.
- Thermal properties: glass transition point of 147°C.
- Mechanical properties: polycarbonate offers exceptional impact resistance, transforming impact energy into thermal energy and deforming locally instead of shattering like pure pmma.
- Optical properties: high transparency to visible light.
- Fireproof characteristics: extruded polycarbonate in sheets or films is classified V2 if thickness <1.5mm, HB if 1.5mm ≤ thickness < 10mm, V0 if thickness ≥ 10mm according to UL 94.
- Chemical stability characteristics: resistance to mineral acids, aliphatic hydrocarbons, petrol, greases, oils, alcohols (except methyl alcohol) and water below 70°C.
The differences between polycarbonate sheet and film
Polycarbonate extrusion is a process that becomes more complicated as the thickness is reduced. Specifically, among the industrial polymers used, polycarbonate film requires careful processing, due to the poor malleability of the polycarbonate itself, and for this reason, during the extrusion of polycarbonate films, particular attention is paid to the temperature and speed parameters, in addition to specific measures on the geometry of the extrusion cylinder, the extrusion mouth itself and the transport calenders used.
In general, machines calibrated for the extrusion of polycarbonate films, which vary as standard in thickness between 100 µm and 750 µm, are set to speeds that are considerably lower than machines used for the extrusion of compact polycarbonate sheets. Film machines can also extrude polycarbonate films of a few dozen µm in thickness as a lower limit and go above a millimetre, if required.
The machines used for the extrusion of compact polycarbonate sheets are instead calibrated as standard for thicknesses that exceed one millimeter up to a maximum of approximately 20 mm. These latter machines for the production of polycarbonate panels can also be used in the lower range up to a minimum of 500 µm in thickness. Given the difference in the extrusion process, the optical quality of the polycarbonate film is greater than that of the polycarbonate sheets in the range in which sheets and films overlap in thickness. In the films there are fewer optical distortions resulting from variations in the refractive index at various points and a lower tolerance is guaranteed with respect to black spots resulting from the carbonization of particles in extrusion. The films offer a more stringent tolerance to variations in thickness, eliminating problems due to vibrations during production, which are reflected on the surface of the semi-finished product and on the optical quality.
Pure, blend and coextruded polycarbonate film
There are films of polycarbonate mixed in mass with other polymers, such as the PBT of Bayfol® films, which increases the resistance of the film to chemical agents compared to pure polycarbonate.
In addition, there are films of polycarbonate coextruded with a thin film of PMMA on one of the two surfaces of the film, such as Makrofol® SR 906. These films make the surface harder mechanically, as well as more resistant to chemical agents, while maintaining the basic physical properties of polycarbonate.
Unlike films with an anti-scratch treatment, coextruded films in PC/PMMA can be thermoformed without problems and above all without having the shelf-life restrictions typical of anti-scratch polycarbonate films suitable for thermoforming. The latter in fact have a coating that is not completely cross-linked, which is hardened during or after thermoforming, and therefore very unstable, with a short-term expiry and the need for storage in thermally controlled environments.
Surface roughness
Polycarbonate films are available with various surface finishes. The surface matt finish is simply linked to the roughness impressed on the polycarbonate sheet. Polycarbonate films on the market range from glossy to incremental matt finish in more than 5 different levels. Various combinations on both sides ensure a varied choice of combinations.
Matt finish has both an optical and tactile effect.
From an optical point of view, depending on the increase in surface roughness, distorting the wave front, the material first becomes anti-reflective until it presents diffusive characteristics, which can be used for lighting systems.
From a tactile point of view, the material gives a warmer and softer sensation to the touch, which is used for example in input systems, such as touch keypads.
Specific treatments to functionalize films
- UV resistance: polycarbonate naturally interacts with UV rays, yellowing and reducing mechanical performance. Special additives in the mass make polycarbonate suitable for outdoor use.
- Anti-scratch/hard-coated for increased resistance to both scratches and chemicals. Anti-scratch films can also be treated to have UV resistance, which allows for outdoor use.
- With flame retardant added in the mass, to guarantee a V-0 classification according to the UL 94 standard.
- Laserable: special additives in the mass make the film suitable for security laser printing on identity documents.
- Optically diffusing, with different degrees of transparency to light in the visible field, depending on the use.
Protective liner
PC films can be protected by a liner on glossy surfaces, to protect them from scratches until processing. Matt surfaces cannot be protected by a liner, primarily for reasons of adhesion, and also because small scratches are not visible on a surface that is not completely glossy.
Liners are specific to the process to be performed, and can be transparent or colored to indicate the sides of the film. Usually the colored liner is initially removed on the printing side, while the transparent liner remains applied to be able to check the quality of the print itself, keeping the opposite side protected. Liners can also be specifically designed for processes such as thermoforming or in-mould decoration.
Typical processes on polycarbonate films
Printing
Polycarbonate films, as well as polycarbonate sheets, show good printability without the need for a primer for the adhesion of inks or paints. Printing can be done by screen printing, digital printing, UV digital, offset printing, pad printing, etc.
Polycarbonate naturally has a good surface tension, which allows good wettability and therefore adhesion of the inks. However, for safety, manufacturers of compact polycarbonate films and sheets always indicate to heat treat the semi-finished product before printing. This process, which is indicated in temperature and minutes necessary to release the water molecules absorbed by the polycarbonate on the surface, is usually specified as a function of the thickness of the semi-finished product.
If necessary, to further increase the wettability of the surface, it is possible to carry out a Plasma treatment or a Corona treatment to increase the surface energy, typically expressed in dyn, thus increasing the hydrophilicity of the surface.
Hot thermoforming
Polycarbonate films can be thermoformed, since polycarbonate is a thermoplastic polymer. After printing, brought to the softening temperature, the film is made to adhere to a mold thanks to a vacuum system. To protect the film from contact with the mold, the transparent protective liner is left on the film which must resist thinning due to elongation and at the same time be removable without difficulty at the end of the process.
Embossing
Embossing is used for low-thickness details, such as the raised keys on a keyboard.
Polycarbonate is a material that lends itself well to this type of processing, allowing polycarbonate films to be an optimal choice compared to other plastic films.
High pressure forming – HPF
The HPF technique is used when you want to maintain high positional precision for applications that require it, such as car speedometers. In this case, the film is heated to a lower temperature than in thermoforming, and then the film is made to adhere to the mold thanks to the pressure generated by a jet of hot air that pushes it onto the mold. With HPF, you can produce components with less depth than with thermoforming.
Again, both the inks and the protective film must be suitable for withstanding the temperatures and stresses generated by the process.
Film Insert Molding (FIM)/Injection Molding Decoration (IMD)
FIM/IMD is a technique that allows the surface of a plastic component to be functionalized and decorated using a plastic film (for example polycarbonate films), through the back injection or overmolding of a thermoplastic polymer.
The film, previously printed and thermoformed, is placed in a mold having the same shape taken by the film during thermoforming, and from the inside of the mold a compatible polymer is injected to adhere to the film itself or to the printed areas. Usually special inks for polycarbonate are used, suitable both to resist the elongations in thermoforming and to adhere the back-injected polymer. Often a transparent resin is used between the last layer of ink and the overmolded polymer to protect the ink and ensure good adhesion with permitted polymers.
The specific configuration of the mold, and in particular the configuration of the entry points of the injected polymer, must be studied following the special guidelines of the manufacturers (of films, machinery and inks) to ensure an optimal flow of the polymer during the injection phase. The protective film of the polycarbonate film, similarly to the thermoforming process, must be able to withstand the temperatures involved during the FIM process, which varies depending on the polymer injected back.
Gluing
Polycarbonate films can be glued, even after printing, ensuring the ink adhesion level necessary to prevent the color layer from delaminating on the film. For this application, optimized adhesives and inks are obviously necessary, which are often recommended by the manufacturers themselves on the technical data sheets.
Areas of use of polycarbonate film
Polycarbonate in film and compact sheets is used in various sectors that require both the intrinsic characteristics of polycarbonate itself and the specific technical characteristics offered by the functionalized material.
From the transport sector (railway, maritime, aeronautical, automotive) to the screen printing, graphic, industrial, electronics (input systems, keyboards, screens), lighting (diffusion and reflection of light), helmet production, identity cards, white goods industry, polycarbonate film represents an important piece for many industrial products that we all use daily.
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