Characteristics of Polypropylene and Polyethylene
Two of the most ubiquitous forms of plastic across the globe are polypropylene and polyethylene. For good reason – these plastics have incredible material characteristics that provide incredible conveniences at inconceivably low costs. Well, incredibly low prices – the costs to our environment are externalized, and are rapidly growing.
Polypropylene
Some of the products made with polypropylene and frequently used in the packaging world are adhesive tapes, strapping used to seal pallets and boxes, and boxes and crates used for product storage. Because Polypropylene is so durable, it enables stackability which is an incredibly useful feature in logistics/shipping contexts.
A list of some of the advantages of Polypropylene:
- Inexpensive
- Moisture-resistant
- High flexural strength – resists failing when bending
- Good ph resistance for both bases and acids.
- Good fatigue resistance
- Excellent electrical insulator.
- Easily repaired
- Food safe – one of the only plastics that don’t leach into food
Polypropylene disadvantages:
- Can’t be used in high-temperature applications.
- Photodegradable by ultraviolet light
- Low resistance to chlorinated solvents and aromatics.
- Difficult to paint
- Very flammable
- Susceptible to oxidation.
Polyethylene
Some of the most commonly used products for interior packaging and manufactured with polyethylene are polyethylene plastic bags, which include those used in stores and those used for shipping, and even garbage bags. Also widely used are stretch film, bubble wrap, protective foam sheets, or plastic boxes with grids.
A list of some of the advantages of Polyethylene:
- Can be recycled!
- Durable and water-resistant – performs despite exposure to the elements
- Very malleable with a high impact strength – it stretches
- Can be almost transparent to opaque
- Excellent electrical insulator
- Low density makes it light, and suitable for packaging
- Can be used in both high and low temperatures
Polyethylene disadvantages:
- Degrades very slowly – may remain in landfills for decades
- Mainly derived from petroleum or natural gas
- Manufacture is energy-intensive, and a significant source of carbon emissions
- Recycling process is complex and lengthy
- Can become electrostatically charged
All these beneficial characteristics make polypropylene and polyethylene materials widely used in the packaging world, as well as in other sectors such as toys, DIY, and automotive.
How to identify polypropylene and polyethylene
This video is a fantastic tutorial on how to identify what kind of plastic you’re dealing with.
Polypropylene and Polyethylene chemistry
Both compounds are polymers, i.e. long chains formed by the repetition of a unit molecule or monomer. From a chemical composition point of view, the difference is that polyethylene is a polymer of ethylene (two carbon and four hydrogen atoms) (CH2-CH2)n; while polypropylene is a polymer of propylene (three carbon and six hydrogen atoms) (CH2- CH2-CH2-CH2)n.
Polyethylene is the most widely used plastic in the world and is used to manufacture most of the plastic products that surround us (shopping bags, detergent containers, gel, cleaning products, etc.).
For example, the linear polyethylene of very high density and high molecular weight, HMW-HDPE (high density polyethylene) is used for the manufacture of parts for industrial machinery that require high strength, while branched polyethylene or very low density and low molecular weight, LMW-VLDPE (very low density polyethylene) is used, among other applications, as plastic film.