Films description
Traditionally the films we know come from oil processing. However, today we can mention other raw materials such as cellulose, starch and sugars. These are classified as renewable sources and are typically used to make biodegradable and compostable films.
Le pellicole più comunemente utilizzate sono: il polipropilene (PP), il polietilentereftalato (PET) e il polietilene (PE). Examples of compostable films are cellulose acetate and PLA.
The production of traditional films is similar to each other. It starts with granules that are introduced into extruders or heated hollow cylinders that house a worm screw inside. The extruder has the task of homogenizing and melting all the components. The compound obtained is pushed through a die which provides a continuous “plate” or “tube” of semi-melted polymer.
Thanks to the operation of “orientation” or stretching of the sheet both longitudinally and transversally, the appearance, thickness and properties we know are given to the film. In the image below an example of sequential orientation. From left to right, it is possible to appreciate in order: the granule loading hopper (resin), the actual extruder (Twin-screw extruder), the die (T-die), first orientation station in the machine direction ( Machine direction stretcher) and the second station of orientation in the transverse direction (Transverse direction stretcher). The operation ends with the wrapping of the film (Winder)
The film can receive further processing such as corona treatment, application on one side of acoating,metallization.
Corona treatment
The goal is to increase the wettability level of the most superficial layer of the film in order to allowoptimal adhesion of paints, inks, adhesives. The process involves exposing the film to a highly oxidative environment which promotes the formation of radical groups on the surface of the film. This increases the surface energy of the film or the polar affinity with inks, paints, etc. (increase in printability).
Application of a coating
If we want to give the film a particular function such as a better printability by chemical affinity, a heat-sealing side or make afilm dry (pre-adhesive) we can spread a special polymer layer on one side of the film. The application will take place through coextrusion processes or feeding the die with two or more extruders (online application) or with spreading techniques on the base layer of the ready-made film (off-line application).
Metallization
That is the deposition of a very thin layer of aluminum (a few hundred Angstroms thick). This process gives the film a silvery appearance and addsgas and light barrier characteristics. The function can therefore be simply aestheticor functionalin the case of packaging applications. The process is carried out in a high vacuum chamber in which very pure aluminum filaments (see photo) are melted in special crucibles forming an aluminum vapor which sublimates directly on the surface of the film. It is possible to adjust the amount of aluminum deposited on the film by adjusting the speed of the film.
Characteristics of the main conventional films
BOPP
Polypropylene is the most widely used film in lamination. Excellent optical characteristics, resistant, good transparency. Permeable to oxygen.
PET
Polyester is a more resistant film than BOPP, it is more transparent and has a low oxygen permeability. It is also more thermally stable and therefore suitable for food packages that must undergo a passage in the microwave.
PE
Polyethylene is a softer and more flexible film than both polypropylene and polyester. It is less transparent than BOPP, permeable to oxygen and is particularly suitable for contact with food. Frequently used to make packages heat sealable.
Property
The chemical industry has selected a wide range of monomers since each of them gives the film different characteristics. However, the properties of the films are also determined by theproduction process, by theadditives and copolymers that are added and by their thickness. Looking at a typical data sheet we find the following properties:
Physics
Thickness: measured in microns, it affects the transparency, consistency and barrier effect of the film.
Specific weight: measured in g / cm3 it is the ratio between weight and volume.
COF: coefficient of friction, aptitude of the film to slip. Provides an indication of the behavior of the film in the car. Often a particularly transparent and even less slippery film.
Wettability: “is the ability of a liquid to spread completely on a flat and horizontal surface of a solid”. It is measured in dynes / cm.
Mechanics
The film undergoes different mechanical stresses, it is therefore useful to be able to compare the films to understand which one best bears these stresses. The tensile strength at break and elongation at break tests are used to quantify the response of the film to stresses.
Thermal
Dimensional stability (heat shrinkage): during lamination the film absorbs heat and can “shrink”.
Weldability: for heat-sealable films it indicates the tenacity with which the welding can be carried out. For example to make blister packs.
Optics
Opacity (HAZE): ability of the film to let light through it. The lower this value, the more transparent the film. We can find a value of 2.0 for a BOPP up to 0.5 for a particularly transparent PET or 0.7 for a glossy cellulose acetate.
Gloss: ability of the film to reflect incident light. The value will be low for transparent films and high for metallized films.
Optical density: in metallized films it quantifies the quantity of metal deposited on the surface.
Barrier
With water vapor, oxygen or aromas. It is measured in the quantity of aeriform which passes in m2 per day through the film at a given temperature. Thelower the value, the more the film is a barrier to their passage.
In conclusion
Numerous types of films are available for paper converting and packaging applications. Each of them provides different characteristics and performances that must be evaluated with respect to the specific application required.