INTRODUCTION TO POLYMER FILMS
Dr.Stefano Galderisi
SUMMARY
1) Which they are and how they are produced
2) Other operations
3) Differences between films
4) Film properties
5) Applications
6) Conclusions
Most of the films are obtained from petroleum derivatives (natural and renewable sources such as cellulose and starch are another way of obtaining polymers). The main films used in the paper lamination, paper processing and packaging are: polypropylene (PP)*, polyethylene terephthalate (or more simply polyester, PET) and polyethylene (PE). Their production starts from granules which, added to other slippery, anti-oxidising, stabilising ingredients and various additives, are introduced into the extruder, a heated hollow cylinder housing a worm screw.
The extruder homogenises and blends all the components. The compound obtained is pushed through a die supplying a "slab" or semi-molten polymer continuous ”tube”. To give the film the usual appearance, the sheet is laminated in order to reduce the thickness or compressed air is pushed inside the extruded to obtain a polymer bubble of the desired thickness. In addition to reducing its thickness, the film usually requires further processing. The most frequent are: orientation, to give the film the right mechanical an optical properties and treatment to make the film printable and suitable for placing adhesives, inks, varnishes or lacquers. Let's see these two operations in details:
Orientation
It is a mechanical operation performed on the film that allows the long polymer molecules of which the film is made to be spread and sorted. In case of flat head die extrusion, the "slab" is cast onto a cooled rotary cylinder and, after further lamination, the film is transversally (using specific grippers gripping the side of the film) and longitudinally (by adjusting the winding speed of the film downstream)
stretched. In case of "tube" extrusion, a blower introduces pressurized air to obtain a bubble that extends upwards even for many meters. Steel cylinders close the bubble at the top in order to retain the air inside, thus maintaining constant internal pressure values. The pressure value inside the bubble affects the final characteristics of the film.
Treatment
This operation increases the wettability level of the most superficial layer of the film. This improves the interaction with the liquids that deposit on it (e.g. it gives printability to polypropylene). The treatment can be performed by flame (with a burner), by means of a strong potential difference ("corona" treatment), by spreading a polar acrylic layer (typical for PET) or by co-extruding a specific polymer on the film itself.
OTHER OPERATIONS
To the previously mentioned operations, others can be performed in order to integrate new features to the film. Among these:
Spreading the adhesive on the base film (pre-adhesive)
To obtain a thermal film, a layer of adhesive is applied on the film which, upon lamination, must be revived by means of heated coated calender. Compared to non pre-adhesive films (wet film), thermal films have several advantages from different points of view: less expensive laminating machines, faster start-ups, no glue unit, couplings ready for further processing in a shorter time compared to wet, fewer problems with anti-set, lower environmental impact.
Metallization
Metallization is a process in which a very thin layer of aluminium is deposited. This is for aesthetic and functional purposes (oxygen, light and aroma barrier properties). The operation is carried out in a tight chamber in which a high vacuum is performed. The aluminium used for metalising is molten inside a range of pots. An aluminium “steam” is created that deposits on the surface of the film sliding from an unwinding roll to a winding roll. The amount of aluminium deposited on the film can be adjusted by
adjusting the speed of the film.
Colouring
Process used to transform a transparent film to a colored one. This happens by adding mass pigments to the film or by printing the surface.
Embossing
In this case it is a question of impressing on the film surface a design (tipically a repeating texture) by means of a male and female cylinder.
The result is a relief that gives the film a particular tactile effect.
Brushing
The operation requires a cylinder equipped with hundreds of small points to "line" the surface of the metallized film, creating the so-called “brushed” effect. It gives the support a particularly elegant look.
Special
Finally, there are operations used to give particular functions or characteristics to the base film. For example, with the addition/coating of specific additives or copolymers, the film can become more or less slippery, heat resistant (applications such as chicken bag), heat sealable (blister applications), resistant to scratches or fingerprints (applications such as the creation of books, catalogs of greater value), bacteriostatic (to coat surfaces where low bacterial proliferation is required). The films can also be engraved and subsequently metallized to create holograms.
DIFFERENCES BETWEEN 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 ideal for food packaging because, for example, it can withstand sterilization heat treatments or storage at freezing temperatures.
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.
AC
Cellulose acetate is made starting from cellulose monomers (polysaccharide) obtained mainly from wood pulp. It is very transparent, therefore it is particularly appreciated from an optical point of view for high value applications such as cosmetics and perfumery. It is 100% biodegradable and compostable, therefore ecological. Compared to the films described above, cellulose acetate requires greater care by adopting specific precautions. It must be stored well protected from high relative humidity values.
FILM PROPERTIES
The chemical industry has selected a wide range of monomers since each of them gives to the film different characteristics. However, the film properties are also determined by the production process, by the additives and copolymers that are added and by their thickness. Looking at a typical Mag Data data sheet we find the following properties:
Physical:
- Thickness: measured in micron, 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. It provides an indication of the behaviour of the film in the machine. A particularly transparent film is often less slippery.
- Wettability: “it is the ability of a liquid to fully spread onto a flat and horizontal surface of a solid”. It is measured in dyne/cm. In practice, special liquids are used which, once brushed on the film, show the level of treatment based on their retraction/droplets formation speed. The higher the dyne value is (for example 40 rather than 38), the more the film is able to retain adhesives, inks, etc. The lamination tends to decrease this value, as well as the subsequent processing. This must be considered when thinking about the entire phase of realization of the finished product (a book).
Mechanical:
the film undergoes different mechanical stresses. Therefore, it is useful to compare the films to understand which one bears the stresses better. Load at break and elongation at break tests are used to quantify film responses to stresses.
Thermal:
- Dimensional stability (heat shrinkage): during lamination the film absorbs heat and might “shrink”.
- Weldability: for heat-sealable films it indicates the tenacity with which the welding can be carried out. For example to make blister packs.
Optical:
- Opacity (HAZE): ability of the film to let the light pass through it. The lower this value is the more transparent the film. We can find a value of 2,0 for a BOPP up to 0,5 for a PET particularly transparent or 0,7 for a glossy cellulose acetate.
- Brightness: ability of the film to reflect incident light. The value is 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:
- To water steam, oxygen or aromas. It is measured in the quantity of gas that passes in m2 per day through the film at a given temperature. The lower the value, the more the film works as a barrier.
APPLICATIONS
In the paper processing and packaging sector, there are many applications possible. The main ones are:
CONCLUSIONS
As we have seen, there are several films available for lamination and paper converting. Each gives different features and performances. The most suitable film for the specific purpose is determined by the laminator and the final customers requirements. A good final result not only depends on the film, but also on the choice of the paper, inks and correct drying. From the machine settings (coupling temperature, pressure exerted on the film, lamination speed) and compliance of technical times (coupling requires stabilising before proceeding with further processing such as embossing and/or creasing). In fact, although plastic films may seem like an inert material, they react giving significantly different performance depending on their work conditions.
* We would like to mention in this regard Giulio Natta, who was an italian chemist and academic. He won a Nobel prize in Chemistry in 1963 for having discovered and developed the catalysts that are still used today to produce polypropylene on an industrial scale. He can be considered the father of polypropylene as we know it today.