Polypropylene vs. HDPE: Material Differences and Comparisons

Polypropylene (PP) and HDPE (High-Density Polyethylene) are both commodity thermoplastics. Polyethylene, the basic building block of HDPE, is the highest production volume of plastic in the world, and polypropylene has the second-highest production volume. Polypropylene has excellent fatigue resistance, but it is also used for many single-use plastic applications, such as food packaging. Polypropylene can be manufactured using blow molding, injection molding, and extrusion. 

HDPE has an excellent strength-to-weight ratio as well as good chemical resistance. HDPE is widely used for water tanks, food packaging, and piping. HDPE can be manufactured using extrusion, injection molding, and rotomolding. Both HDPE and PP are considered food-safe. This article will further compare polypropylene vs HDPE in terms of its applications, uses, physical properties, sustainability, and costs.

What is Polypropylene?

Polypropylene (PP) is a thermoplastic with a semi-crystalline structure. It is produced from petroleum products like natural gas through a polymerization process. Propylene can be polymerized into polypropylene by making use of chain growth polymerization. Once polymerized, the polypropylene is extruded into long filaments which are then processed via a pelletizer, which cuts the filaments into pellets to be processed using standard thermal processing techniques like injection molding.  Polypropylene is typically produced in two different forms: a homopolymer and a copolymer. Homopolymeric polypropylene is the most common grade, whereas copolymeric polypropylene is less common. 

Polypropylene is processed using injection molding, blow molding, extrusion, and CNC machining. Polypropylene is widely used for food containers, beverage bottles, fabrics, and chemical process tanks. Polypropylene has excellent chemical resistance, is inexpensive,  and has excellent fatigue strength. For more information, see our guide on All About Polypropylene.

Polypropylene can be identified by the triangular recycling symbol with the number 5 enclosed as indicated in Figure 1 below:

Have you ever wondered what makes everyday items so durable and cost-effective? Enter polypropylene (PP), the star among all commodity plastics. This lightweight thermoplastic is tough, rigid, crystalline, and comes in many forms.But why has PP become a go-to material when you crave superior strength in engineering applications? And, when it comes to the cost game, how does it stack up against the competition in blow molding bottles? Let’s unravel the secrets of polypropylene – the powerhouse of possibilities.