Foils
Polyolefin shrink films are materials produced on the basis of polyethylene or polypropylene.
Polyolefins are polymers containing only carbon and hydrogen with long carbon chains -C-C-C-, which form the basic backbone of the chains of the polymers themselves. It can be said that polyolefins are polymeric hydrocarbons.
Polyolefins are a very important industrially important group of polymers. The production of polyethylene, polypropylene accounts for about 80% of the weight of all synthetic polymers produced. These polymers have very good performance characteristics, the monomers for their production are obtained directly from crude oil and their polymerisation poses no major technical problem.
Polyethylene lub polieten – ethene polymer
Industry symbol: (PE).
Polyethylene is flexible, waxy, transparent, thermoplastic. It loses elasticity when exposed to sunlight and moisture. The synthesis of polyethylene is an example of radical polymerisation.
PE films have low permeability to water vapour, are easily permeable to vapours from organic substances, and are not resistant to hydrocarbons and their chlorine derivatives. They are resistant to acid, alkali and salt solutions and low temperatures.
PE-HD (high density PE, PE-HD)
High-density polyethylene. Obtained by low-pressure polymerisation. It is harder in comparison to PE-LD, has a higher mechanical strength, a higher melting point (125°C), higher barrier to gases and higher chemical resistance, shows greater brittleness at lower temperatures, is less transparent (milky white) . The most common commercial grades of PE-HD are PE 80 and PE 100. Density is 0.94-0.96 g/cm³.
LLDPE (linear low density PE)
Low density linear PE (short, unbranched chains are formed by copolymerisation of ethene with longer-chain alkenes). Density – 0.915-0.935 g/cm³.
Poly(vinyl chloride)
Polyvinyl chloride, polyethylene chloride, polychloroethene (PVC, PVC) – plastics obtained by polymerisation of the monomer vinyl chloride.
It has thermoplastic properties, high mechanical strength and is resistant to many solvents.
The abbreviation PVC, commonly used in Poland, is logically and linguistically incorrect. International nomenclature uses only the abbreviation PVC – from poly(vinyl chloride). In Polish, the correct abbreviation is PVC – from Polish poly(vinyl chloride). PVC is not a correct abbreviation of either the international or Polish name.
Polypropylene
A polymer from the polyolefin group that is made up of mers with the formula: -[CH2CH(CH3)]-. It is obtained by the low-pressure polymerisation of propylene. Polypropylene is one of the two most commonly used plastics, along with polyethylene. Items manufactured from this plastic usually bear the symbol PP.
Polypropylene is a hydrocarbon-based thermoplastic polymer, i.e. it can be made liquid when the temperature is increased and solidified again when the temperature is lowered, without any change in chemical properties.
PP is obtained by polymerising propene (commonly known as propylene, CH2=CHCH3), which is derived from crude oil. Today, most polypropylene is produced by the Ziegler-Natta process, in the gas phase using catalysts of organometallic compounds suspended on special substrates.
PP is the material with the lowest density of the widely used polymers. It exhibits high chemical resistance, especially at room temperature, where it is almost completely resistant to acids, bases and salts as well as organic solvents. Only strong oxidants such as fuming sulphuric or nitric acid, bleaching bases and non-polar liquids (benzene, carbon tetrachloride, methyl chloride) attack it at this temperature. Prolonged contact with copper also has a damaging effect on PP, so brass fittings should be used for permanent PP-copper joints.
A competing material to polyolefins for the production of shrink film is polyvinyl chloride. However, due to its vastly inferior mechanical properties and waste treatment problems, sales of PVC film in developed countries are declining significantly.
Shrink foils can be divided into two types:
- single-layer extruded from one or a mixture of granules
- multilayer – formed from several layers of similar or different polymers in order to obtain special properties of the final product
- monoaxial – oriented in one direction (lengthwise or crosswise)
- biaxial – oriented in both directions
To compare the properties of different materials, it has become accepted to use standardised test methods (ASTM):
optical properties:
- clarity [clarity] (percentage distortion of the image viewed through the material, the higher the value the clearer the image)
- gloss (reflection of light from the surface, the higher the value the more glossy the surface of the film)
- haze (the hazier the product viewed through the film, the higher the value the less clear the image)
mechanical properties:
- flexibility at low temperatures informs us about the ability of the film to work (sealing and shrinking) in adverse conditions [cold temperature flex].
- modulus [modulus]- a measure of the film’s stiffness (the higher the modulus, the higher the stiffness; as a rule, a higher modulus means better performance on very fast packaging machines
- slippage [slip] on cold (the higher the value, the lower the possibility of blocking the packaging machine due to the resistance of the film sliding over machine parts) and on hot (the higher the value, the easier it is to place the last package in the collecting box, however some products require a film with low slippage to avoid slipping of packages from pallets)
- coefficient of friction between two layers of film [coefficient of friction COF].
- impact strength (the higher the value, the greater the resistance to puncture caused by a sharp tool or a fall)
- tensile strength (the higher the value, the greater the resistance to tearing caused by a sharp tool or a fall).
- elongation tells us how much the film will stretch before breaking
- seal strength is a measure of the resistance of the weld to tearing
- tear propagation allows us to evaluate the tendency of the material to continue to tear after it has been cut
- shrink tension is a measure of the amount of force exerted by the film during shrinking on the packaged product
- shrinkage [free shrink] is a percentage of the shrinkage of the foil
physico-chemical properties:
- water vapour permeability [M.V.T.R.] – a measure important in food packaging that tells us about the ability of the film to remove water from the package
- oxygen permeability [O.T.R.] – a measure important in food packaging which tells us about the ability of the film to reduce the flow of oxygen into the package
Advantages and disadvantages of PVC foil:
- very easy to seal and shrink (low requirements for packaging machines)
- low shrink temperature
- fast shrinking
- poor seal strength
- need to use thicker films than POF due to low puncture and tear resistance
- very sensitive to temperature differences (at low temperatures the film breaks, at high temperatures (from 30°C) the film shrinks uncontrollably)
- unsuitable for high-speed machines due to both the strength and highly corrosive fumes, which cause corrosion of the machines.
Polyolefin shrink foils can be produced from pure granules or blends of different intermediates. Additives allow specific film properties to be obtained. These include:
- Polyethylene (PE) enables sealability and increases the mechanical resistance of the foil
- Polypropylene (PP) provides high stiffness, heat resistance and excellent optical properties
- Ethylene-vinyl acetate copolymer (EVA) reinforces the foil, improves sealability and shrinkage
- Ethyl vinyl alcohol (EVOH) improves oxygen barrier properties
- Polyamide (PA) improves tear resistance and provides a gas barrier
Production process for POF foils (Exlfilm and PC Film):
- Mixing of selected granules in the right proportion
- Melting of the mixture at high temperature and pressure (extrusion).
- Forming a multilayer tube
- Blowing and stretching the tube to the correct thickness and shrinkage (orientation)
- Crosslinking, i.e. exposing the film to X-rays (only for Exlfilm FP Plus and PC Solid crosslinked films).
- Winding the film onto mother rolls, slitting the film and winding it onto rolls for sale. Depending on the type of machine on which it is to be used, the film is either folded into a half-sleeve or wound in tape form.
In recent years, the use of film in thicknesses of 12.5 or 13mic, 15, 19 and 25mic has become established. Other film thicknesses are also offered, but due to low demand these are niche products (8, 11 or 38 micrometres).
As a standard, paper sleeves with an internal diameter of 3 inches, i.e. 76mm, are also used for film winding (for very fast machines, film is produced on sleeves with double the diameter in order to reduce the number of turns of the roll on the machine, thus avoiding too much electrification of the film during the packaging process).