INTRODUCTION TO POLYMERIC FILM

INTRODUCTION TO POLYMER FILMS

Dr.Stefano Galderisi

SUMMARY

1) Which are they and how are they produced

2) Other operations

3) Differences between films

4) Film property

5) Applications

6) Conclusions

Most of the films we know are obtained from petroleum derivatives (natural and renewable sources like cellulose and starch represent another way of obtaining polymers). The main films used in plasticising, paper processing and packaging are: polypropylene (PP), polyethylene terephthalate (or more simply polyester, PET) and polyethylene (PE).

The main films are produced starting 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 components. The obtained mix is pushed through a die supplying a “slab” or semi-molten polymer continuous “tube”. The sheet is laminated to give the film the usual look in order to reduce thickness or compressed air is pushed inside the extruded to obtain a polymer bubble of the wanted thickness. Film normally requires further processing in addition to thickness reduction. The most frequent are: orientation to give the film the correct mechanical and optical properties and treatment to make the film printable and suitable for placing adhesives, inks, paints or lacquers on it.

Details of these two operations are given below:

Orientation

It is a mechanical operation performed on the film to stretch and tidy the long polymer molecules the film consists of. 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 (adjusting downstream film winding speed) stretched.

A blower pressurises air to obtain a bubble extending upwards, for many metres also, in case of “tube” extrusion. Steel cylinders close the upper part of the bubble so as to contain the air, thus maintaining internal pressure values constant. The value of pressure inside the bubble affects the film’s final features.

Treatment

This operation increases wettability level of the most superficial film layer. This improves interaction with liquids depositing on it (e.g. printability is given to polypropylene). The Treatment can be flame (with a burner), by means of significant 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

Other operations can be added to the above in order to integrate new features to the film. Among these:

Spreading of adhesive on base film (pre-adhesive)

An adhesive layer is applied onto the film that, upon plasticising, must be revived by means of heated coater calender, to obtain a film dry. From different points of views, the film dry have different benefits compared to non-pre adhesive films (film wet): less expensive laminating machines, quicker start-ups, no glue unit, ready couplings for subsequent processing in shorter times compared to wet, less problems with anti-set, lower environmental impact.

Metallization

Metallization is a process where a very thin layer of aluminium is deposited. This for aesthetical or functional purposes (oxygen, light and aroma barrier properties). The operation is carried out in a tight chamber where 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 the unwinding roll to the winding roll. The amount of aluminium deposited on the film can be adjusted by adjusting film speed.

Colouring

Process used to transform a transparent film to coloured. This happens by adding mass pigments to the film or by printing the surface.

Embossing

Using one male and one female cylinder, a drawing is embossed on the film surface (normally, a repeating theme). The result is a relief that gives the film a particular tactile effect.

Brushing

This foresees the “scratching” of the metalised film surface by a cylinder equipped with hundreds of small bits, thus creating the so-called “brushed” effect. It gives a particularly elegant look to the support.

Special

There are operations used to give particular functions or features to the base film. For example, addition/spreading of specific additives or copolymer to give film greater or minor slipperiness, thermoresistance (chicken bag like applications), heat-sealability (blister applications), resistance to scratches or finger prints (realisation of prestigious books or catalogues applications), bacteriostatic (to cover surfaces requiring low bacterial proliferation). Holograms can be created by engraving and subsequently metalising the films.


DIFFERENCES BETWEEN FILMS

BOPP

Polypropylene is the most used film in plasticising. Excellent optical features, resistant, good transparency. Permeable to oxygen.

PET

Polyester is more resistant film than BOPP, more transparent and has low permeability to oxygen. It is good for food packaging as it supports thermal sterilisation treatments or freezing temperature storage.

PE

Polyethylene is a softer and more flexible film compared to polypropylene and polyester. It is less transparent than BOPP, permeable to oxygen and particularly suitable for contact with food.

AC

Cellulose acetate is made starting from cellulose monomers (polysaccharide) mainly obtained from wood pulp. Extremely transparent therefore pleasant to the eye for high value applications like in cosmetics and perfumery. It is 100% biodegradable and compostable, therefore ecological. Cellulose acetate requires greater care in its use, adopting specific measures, compared to previously described films. Also, it must be kept well protected against high relative humidity values.


FILM PROPERTY

The chemical industry has selected a wide range of monomers as each gives different features to the film. The production process, additives and copolymers added and their thickness also determine the film properties.

Note the following properties by looking at a typical Mag data technical sheet:

Physical

Thickness: measured in micron, affects transparency, consistency and barrier effect of film.

Specific weight: measured in g/cm3 it is the ratio between weight and volume.

COF: coefficients of friction, behaviour of film in slipping. Gives indication on film in machine behaviour. 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. Specific liquids are used in practice which, once spread onto the film, show the level of treatment according to their forming of droplets/shrinkage speed. The higher the dyne value (e.g. 40 rather than 38), the more can the film withhold adhesives, inks, etc. Plasticising tends to decrease this value as do subsequent processings. This must be considered when thinking of the overall realisation of the finished product (a book).

Mechanical

Films must be compared to understand which supports stresses best as they are subjected to different mechanical stresses. Load at break and Elongation at break tests are used to quantify film response to stresses.

Thermal

– Dimensional stability (Shrinking with heat): film absorbs heat and can “shrink” during plasticising.

– Weldability: heat-sealable film indicates persistence with which welding can be made. For example, for realising blister packaging.

Optical

– Haze: film ability to let light enter. The lower this value is the more transparent the film. We can find a value equal to 2.0 for BOPP, up to 0.5 for a particularly transparent PET or 0.7 for shiny Cellulose acetate.

– Brightness: film ability to reflect incident light. The value is low for transparent films and high for metalised films.

– Optical density: it quantifies amount of metal deposited on the surface in metalised films.

Barrier

To water steam, oxygen or aroma. It is measured in gas amount, passing m2 per day through the film at a given temperature. The lower the value the more the film works as barrier at their passing.


APPLICATIONS

There are many applications possible in the paper processing and packaging sectors, the main ones are:

• To ennoble a mould, making colours livelier. From a visual and tactile point of view, plasticising also gives a more sumptuous and elegant look to the support.

• To protect paper and ink against wear and foreign agents.

• To barrier paper (food packaging).

• To show the product inside a box (opening).

• To make a paper support heat-sealable.

• To give container suitability upon contact with food.


CONCLUSIONS

We have seen there are different films available for plasticising and paper processing. Each gives different features and performances. The most suitable film for the specific purpose is determined by the plasticiser’s experience and final customer requirements.

A good final result not only depends on the film, being only one of the “cake’s ingredients”, but on the choice of paper, inks and their correct drying. On machine settings (coupling temperature, pressure on film, lamination speed) and respect of technical times (coupling requires stabilising before proceeding with subsequent processings like embossing and/or creasing).

Despite plastic films appearing to be inert material, they react giving significantly different performance depending on their work conditions.


Giulio Natta was an Italian chemist and academic. He won a Nobel Prize in Chemistry in 1963 for having discovered and set-up catalysts still used to date for producing polypropylene on an industrial scale basis. He can be considered the father of polypropylene as we know it today.