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Next-generation BOPP

With the help of Conenor's innovative new TRIAXCELL® process technology, the microcellular structure of biaxially oriented cast films and sheets can be tailored to the specific properties required by individual applications – helping ensure that these materials can go on meeting changing packaging needs.

The success story of biaxially oriented polypropylene film, or BOPP, continues apace, with a steady stream of new innovative packaging solutions being developed for a wide variety of end-products.

Originally launched in the early 1970s, some 400,000 t/a of BOPP cast film is produced annually today, and BOPP has completely replaced cellophane. Food and bakery packaging is the main current application area for BOPP, but good potential exists in replacing paper and aluminium foil.

Today's production lines are typically 10 metres wide and run at over 500 metres a minute. The downside of this development, however, is that new lines call for high initial capital investments, they cannot be used efficiently in new product development, and they are not particularly flexible in terms of logistics.

This leaves very little space for customising products, which has been one of the main contributors to the success of BOPP, and is a capability that must be retained if the success story of these film materials is to continue. And that is where Conenor's new TRIAXCELL® technology comes in.

The gas diffusion chamber in Conenor's TRIAXCELL® pilot line.

The process

The innovation behind Conenor's new process begins by MD-orienting a nucleated and extruded sheet of BOPP to form microscopically small needle-like cavities. The film is then run through a gas diffusion chamber to pressurise the cavities, after which the film is rapidly oriented in the line transverse direction in a TD stretcher. This gives the cavities a lens-like form, and they form microcells with extended cavity height with the aid of the internal overpressure in the cavities.

A SEM image of TRIAXCELL-produced film.

Films with different cell structures can be produced, ranging from thicknesses as thin as 20 µm. Controlling the cell structure with the help of MD/TD/gas chamber parameters also enables more rigid films and sheets to be produced.

A major benefit of Conenor's technology is that it requires only a small amount of relatively inexpensive equipment, keeping the overall length of a production line down to no more than 15 metres, and investment costs at a reasonable level. It also uses widely available polypropylene grades or other general polymers, and does not require expensive or toxic additives.

In addition, the technology can be retro- fitted to existing lines at minimal cost.

New properties

Using the TRIAXCELL process, a converter can produce a microcellular BOPP film with a density of 0.3 kg/dm3 or less, half that possible with standard cavitated film using established technologies – resulting in considerable weight and material savings.

High opacity and rigid cellular structure translate into improved printing properties, which are becoming critical in a market where downgauging solid BOPP film for packaging needs is increasingly popular.

The benefits of microcellular BOPP film do not stop with reduced weight. The TRIAXELL process also gives film other new properties, such as a soft and warm touch and improved barrier properties and electrical behaviour, all of which has the potential to open the door to its use in new application areas.

Opportunities in electrical applications

One such area is electromechanical applications, where multi-directional elasticity and constantly charged cavity walls are a definite plus.

A film with built-in electrode layers generates electricity when compressed, and its high sensitivity (200 pC/N) makes it ideal for sensor applications.

If a voltage is applied to the electrodes, film thickness begins to vary as a result of the electrostatic forces generated, and the film can be used as an actuator to produce sound, for example. The lower the film density, the higher the sensitivity. Special cavity shapes, for applications where film needs to be compressed or elongated, for example, can be optimised in the TRIAXCELL process.

In the case of circuit board materials, a uniform highly cavitated cell structure is ideal to achieve the lowest possible dielectric constant (?r) and minimum dielectric loss factor (tan?). In electrical applications, cyclic polyolefins (COC/COP) perform better than polypropylene, due to their enhanced stability at temperatures up to 150°C.

Pilot line

As a technology development company, Conenor does not manufacture BOPP film or sheet itself commercially, and focuses instead on licensing its patented TRIAXCELL know-how worldwide.

A pilot line in Finland is available for material and product trials and sampling. This can produce both cavitated and solid material at up to a maximum finished film width of 1600 mm, depending on the transverse direction orientation ratio, which can be freely adjusted online to any value up to x9.

Microcellular and solid BOPP film at widths up to 1600 mm can be produced using Conenor's pilot line.
> Markku Vilkki
(Published in High Technology Finland )