Difference Between PP and PET

The key difference between PP and PET is that PP is a saturated polymer, whereas PET is an unsaturated polymer.

The term PP stands for polypropylene while PET stands for polyethylene terephthalate. These are polymer materials made of many monomers. The monomer used to produce the polymer material is shown as the repeating unit of the polymer. For PP, the monomer is propylene. The repeating unit of PET shows ethylene terephthalate.

1. Overview and Key Difference
2. What is PP
3. What is PET
4. Side by Side Comparison – PP vs PET in Tabular Form
5. Summary

What is PP?

In polymer chemistry, the term PP stands for polypropylene. It is a polymer material containing propylene repeating units. Another name for this material is “polypropene”. The general formula for this polymer is [CH(CH₃)CH₂]_n. Polypropylene falls under the category of thermoplastic polymers, and it has applications as both fibers and plastics. This material becomes soft upon heating and can be remoulded into different shapes, which is a characteristic property of thermoplastic polymers. Furthermore, this polymer material is made via addition polymerization. The major application of PP is its use as a packaging material.

Figure 01: Repeating Unit of PP

PP is cheap because it is one of the most recyclable plastics available. Unlike in its monomer, this material has no double bonds in its polymer structure; thus, it is a saturated polymer material.

When considering the tacticity of PP, there are three types of tacticities that can be observed in polypropylene: isotactic, atactic and syndiotactic. Isotactic polymer structure contains polymer chains containing the pendant group on the same side. Atactic polymer structure contains polymer chains containing a methyl group in a random manner. In the syndiotactic structure, the methyl groups are located in an alternating pattern.

The most important properties of PP include low density, good transparency, recyclability, and stretchability. Furthermore, some common applications of PP include the production of films for food packaging, textile industry (for the production of carpets, etc.), production of consumer goods, etc.

What is PET?

In polymer chemistry, the term PET stands for polyethylene terephthalate. It is a polymer material containing repeating units of ethylene terephthalate which has the chemical formula (C₁₀H₈O₄)_n. This material falls under the category of thermoplastic resin. PET is a type of polyester.

Figure 02: Repeating Unit of PET

Usually, PET is produced from ethylene glycol and dimethyl terephthalate (or terephthalic acid). It has two types of chemical reactions in the production process. They are the transesterification reaction and the esterification reaction.

PET is colorless, and usually, it is in the semi-crystalline state. The rigidity of this material depends on the production process. PET is strong and impact resistant. It is considerably good as a barrier for moisture and gas. Also, it acts as a good barrier for alcohol and some other chemicals. However, it turns into white color when exposed to chloroform and toluene.

Majority of PET in the world are synthetic polymers. There are many important applications of this polymer including the production of soft drink bottles, production of fibers for clothing in the textile industry, production of containers for food and beverage storage, as a substrate in thin film solar cells, as a waterproofing barrier in undersea cables, etc.

What is the Difference Between PP and PET?

PP stands for polypropylene while PET stands for polyethylene terephthalate. The key difference between PP and PET is that PP is a saturated polymer, whereas PET is an unsaturated polymer. Also, PP is produced via addition polymerization of propylene while PET is produced via condensation polymerization of ethylene glycol and dimethyl terephthalate.

Moreover, applications of PP include the production of films for food packaging, textile industry (for the production of carpets, etc.), production consumer goods, etc. Meanwhile, PET is important for the production of soft drink bottles, production of fibers for clothing in the textile industry, production of containers for food and beverage storage, as a substrate in thin-film solar cells, as a waterproofing barrier in undersea cables, etc.

Below infographic shows more comparisons related to the difference between PP and PET.

Summary – PP vs PET

The terms PP and PET stand for polypropylene and polyethylene terephthalate, respectively. The key difference between PP and PET is that PP is a saturated polymer, whereas PET is an unsaturated polymer.

Reference:

1. “Polyethylene Terephthalate (PET): A Comprehensive Review .” PET Plastic (Polyethylene Terephthalate): Uses, Properties & Structure, Available here.

Image Courtesy:

1. “Polypropylen” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
2. “Polyethyleneterephthalate” By Schippmeister – Own work (CC BY-SA 4.0) via Commons Wikimedia