Polypropylene films & laminating structures
Polypropylene film according to production method, properties and different uses can divided into three types; cast polypropylene film (CPP), Inflation PP (IPP) and biaxially oriented PP (BoPP) film.
Polypropylene films are one of the most prepotent barrier films, versatile and economic than ever used in various packaging applications. Due to its excellent moisture barrier, clarity, gloss, durability, the ability to print, embossability, aroma strength, dimensional stability, processability and other; markets of PP films are quickly changed and a number of new applications take market share of other plastic films. PP films compete with various other materials such as PVDC films, polyester films, nylon films, polyethylene films, cellulose films, wax paper, aluminium foil, and others on the basis of performance and cost.
Product developers soon have found many qualities within PP films which made it as one of the most versatile film for a wide variety of applications. Polypropylene is a unique material which can be steam-sterilized or autoclaved without damage and resists environmental stress cracking when subjected to most chemical tests and of course, PP offers higher heat resistance and is therefore more suitable for retort packaging or hot filling. It also offers an opportunity for better grease resistance compare to LLDPE/LDPE.
The polypropylene film produced with Cast Polypropylene is offers magnificent transparency and external glossy qualities. It is extraordinary for packaging snacks as well as being used as a sealant film for retorting purpose due to its excellent heat sealing characteristic and stabilizing dimensions of packaged contents.
Surface treatment needed
Generally, plastics have chemically inert and nonporous surfaces with low surface tensions causing them to be non-receptive to bonding with substrates, printing inks, coatings, and adhesives. Polyethylene and polypropylene are the lowest in surface energy of the various plastics and are the two materials most often subjected to surface treatment to improve their bonding characteristics.
To ensure sufficient adhesion of printing inks, lamination adhesive or metal from metallization, the surface energy of CPP must be increased so that it achieves a minimum surface tension of 38 mN/m (dyne/cm).
Corona treatment is the preferred method on cast film lines to achieve an increased surface energy.
Back treatment is moderately usual occurrence with cast film which stands up from wrinkles and flexure on the reverse face. This phenomenon can be avoided by the prevention of air between the film and the surface of the dielectric roll. Back treatment may hinder with subsequent heat sealing and cause blocking, especially over print which results in unpleasant stripes.
Surface tension can be measured using a treatment pen or test solution immediately after Winding. Surface tension will reduce by roll aging about two points or more depending on the polymer additive levels. Excessive treatment may give rise to blocking, unacceptable odours or colouration. The treatment will usually cause some heating of the web. Steps must be undertaken to prevent this from causing creasing.
Coextruded PP in lamination process
The trend in flexible packaging film is moving towards multilayer co-extrusion film structures with enhanced film properties and performance features. When using the coextrusion procedure it is possible to combine different materials/properties in a sandwich construction. Coextrusion in general, and polyolefin coextrusion in particular, have gained a dominant position over the last few years.
In particular the 3-layer coextrusion in a layer combination ABA or ABC is a preferred version of PP for manufacturing multilayer constructions.
In lamination cast PP film, high crystalline homo PP can be applied in either core or skin layer and co-extruded with copolymer resin.
Laminated structures lend themselves naturally to applications in demanding end-uses of the flexible packaging industry, like food packaging with medium to long shelf-life.
Similarly, a combination of excellent mechanical and barrier performance provides new opportunities in FFS packaging.
High crystalline homo PP co-extrusion film allows converters to achieve effective production cost saving as lamination step is not necessary and good printing quality is still ensured. In addition, it also provides down gauging potential and this will also lead to direct material cost savings significantly.
Usually each laminate film will have its own specific properties. To achieve the best result, there are some aspects that need to be considered before deciding what type of lamination will suit a particular application.
In conventional laminating processes, aluminium is considered a major component, usually bonded to a coextruded layer of polyamide (PA), PET or polystyrene film. Since aluminium is a perfect barrier but does not seal and has low puncture resistance, it will often be used in combination with another layer that makes up for its weaknesses.
For example, a structure with good sealing might include a PET film laminated to an aluminium foil and polypropylene. PP will provide the heat sealing capability along with strong puncture resistance.
Example of CPP in some applications
PP – Twist films
For twist wrap film applications, high crystalline homo PP is used as core layer to provide the stiffness or as surface layer to allow metallization.
The film structure provides clear economic advantage versus the traditional packing as it offers not only down gauging potential but also flexibilities in subsequent converting. Good twist retention and excellent dead-fold are guaranteed.
FFS packaging films (BOPP substitution)
This is a substitution of laminates with BOPP structures or BOPP films
Typically produce on PP cast or PE blown film/BOPP with PP homo and Terpolymer or Raco sealing layer
Vacuum metallisable cast polypropylene (VMCPP) film applications
High crystalline homo PP is recommended for the core layer in vacuum metallisable cast polypropylene (VMCPP) film applications.
High film stiffness is a critical requirement to achieve even aluminum deposition onto the running base cast PP film during the metallisation process. On top of that, VMCPP film makers will also enjoy the following benefits such as good heat resistance and excellent barrier properties of VMCPP films using high crystalline home PP. In addition, absence of migratory additive allows it to achieve and maintain high pretreatment level before secondary processing.
Stand up pouch lamination film
This is a substitution of aluminium, white tin and glass bottles, trays, cans and boxes by a polymeric stand up pouch concept.
Typical structure often is SiOx coated PET/PA/PP laminates. PP has mainly the function of a sealing layer. It provides the tenacity and body of the film.
Used for packaging of soups, sauces, solid food, pet food, Detergent and cosmetics packaging.
Peelable structures/films based on PP sterilisable and non-sterilisable.
Typical structure is a Coex film, consisting of PP core (homo, raco or block) and thin peelable layer (10 μm)
Used for all kinds of films for ready to eat meals, ice cream boxes, desserts, soft cheese, dairy products. Peelable flexible structures for food and detergents - peelable against itself. Sterilisable, peelable secondary packaging for medical pouches.
The particular demands for packaging solutions – high impact and shelf presence, resistance to high processing temperatures, and strong distribution challenges has led flexiblepackaging converters to continuously strive for films that offer those properties. Along the path of development, they found PP one of the best substrate to use in lamination. Today in PolyFame Technologies Inc. we have the aptitude to offer high quality machinery that allow manufacturer to produce Cast PP as well as other equipment to Print or laminate them for various packaging solutions.
- K IN0070/GB FF 2007 10 BB – Polypropylene Cast line by Borouge
- Corona treatment: An overview by David A. Markgraf
- Technical Guide - Elevated stiffness with high crystalline homo polypropylene by Borouge
- Polypropylene films Processing- Packaging Films 1-2014, By Matthew Tabassi
- Future direction of lamination in retort packaging- PackagingFilms 4-2013, By Matthew Tabassi