Polyethylene and polypropylene are two of the most common plastics used in the food packaging industry. On the surface they seem very similar, however, they each have certain well-defined characteristics and functions. Do you know the difference between them?
Today we are going to look at the characteristics of each of these polymers to help you understand the 5 differences between polypropylene and polyethylene food packaging.
POLYETHYLENE AND POLYPROPYLENE: GENERAL CHARACTERISTICS
When we talked about plastics identification codes, we explained that polyethylene is classified as number 2 – HDPE: High–density polyethylene — and number 4 – LDPE: Low-density polyethylene. Polyethylene is a polymer made of ethylene, and depending on how it polymerises, the properties of polyethylene change.
Polypropylene, however, is classified as number 5 – PP. It is also a plastic polymer, but its monomer is propylene, which is easily produced and has a high degree of purity.
THE DIFFERENCES BETWEEN POLYETHYLENE AND POLYPROPYLENE FOOD PACKAGING
1. CHEMICAL AND HEAT RESISTANCE
- High temperatures: polypropylene withstands high temperatures better than polyethylene. While the first remains in perfect condition between 140ºC and 170ºC, the highest temperature the latter can withstand is between 105ºC and 115ºC
- Low temperatures: in this case, polyethylene is more resistant at low temperatures, maintaining its characteristics as low as -80ºC. Polypropylene only preserves its properties above 0ºC.
- Chemical resistance: in spite of the fact polyethylene has a better resistance to wear and tear, it is polypropylene that stands out for its high resistance to abrasive acids, this being one of the main differences between the two plastics.
Polyethylene is a very elastic material that stretches easily, making it the perfect material for food packaging. Polypropylene is much less flexible, although it is notably more difficult to break. Depending on the orientation the plastic is given – BOPP or OPP –, it will have a greater or lesser degree of flexibility.
3. LIGHTNESS AND COLOUR
Polypropylene is a much more lightweight material than polyethylene. Regarding colour, in its natural form the first is a translucid white, while polyethylene is colourless. Different industrial extrusion processes are used to develop the best features for the packaging industry, in other words, better transparency, tearability, pierceability, etc.
Polypropylene and polyethylene food packaging is easy to recycle, as both materials are thermoplastics. This means that they melt when exposed to heat processes and can be moulded repeatedly. In the specific case of these two plastics, they can be given a new use after recycling. Polyethylene is the more widely recycled of the two, however.
5. USES IN FOOD PACKAGING
The different features of both plastics mean their uses as food packaging are often very different. Let’s look at some examples:
- Microwaveable packaging: thanks to its resistance to high temperatures, polypropylene is perfect for microwaveable packaging.
- High hot-tack: food packaging with high hot-tack, or resistance to heat sealing, is perfect for packaging nuts, rice, and sauces. It is principally made of biaxially-oriented polypropylene.
- Polypropylene is a good material to use for heat treatments, such as sterilisation and pasteurisation, as it withstands both treatments perfectly. However, polyethylene can only withstand pasteurisation.
OTHER USES OF POLYPROPYLENE AND POLYETHYLENE
Polyethylene and polypropylene can both be used for purposes other than food packaging:
- Insulation: the special polypropylene film PP FLOC is principally used as insulation for car-interior parts, as well as for doors and windows.
- Plastic bags: polyethylene is one of the most commonly used materials in plastic bags. These may be shopping bags, bin bags, etc.
- Adhesive tape: most adhesive tapes are made from polypropylene because it is particularly strong.
- Toys: high-density polyethylene is commonly used to make children’s toys.
- Storage boxes: stackable plastic storage boxes are generally made from polypropylene because it is tough and resistant to abrasion.
- Agricultural use: greenhouses, geothermal blankets (normally hybrids of PE and PP).