PP stands for Polypropylene, a type of flexible plastic widely used in various applications. Polypropylene is a polymer belonging to the polyolefin family.

The first plastic product was manufactured in the 1950s and is one of the most versatile plastics. PP can be found in almost every application, from common household items to technical textiles and industrial products.

Plastic products often have plastic identification codes marked on their surfaces. The plastic code for PP is typically “5” within a black triangle with small letters underneath. It usually comes in white, opaque, or translucent colors. However, it can also be formulated with different colors using additives. The raw material is inexpensive and easy to manufacture in forms such as molds, films, and fibers.

Compared to PE materials, PP is stable at higher temperatures—it won’t melt below 160°C (320°F)—making it suitable for a wider range of applications. This is particularly crucial for food-related items such as packaging and equipment with operating temperatures up to 100°C (212°F).

PP has one of the highest recycling rates among plastics. Partly, this is because it’s widely used in packaging, making it easily identifiable (identified by plastic identification code number 5), and partly because it’s used in applications where it’s often the sole material. This makes separation quite efficient and cost-effective during processing. It floats on water, making it easy to separate from mixed waste. For example, packaging containing a mix of polyethylene terephthalate (PET, polyester), PP, and PE; PP and PE will float while PET sinks. Materials containing additives and fillers, such as glass fibers, will not float, making separation more challenging.

100% recycled PP can be used, for example, for injection molding or extrusion, and is estimated to reduce energy consumption by 3/4 or more. Pre-color sorting before recycling increases the value of the recycled material. However, polymers degrade from heat and exposure to ultraviolet (UV) light, so this process cannot continue indefinitely in a closed loop. Mixing recycled material with virgin PP helps offset energy consumption.

PP in Products, Interior, and Lighting

With various applications ranging from solid chairs to elegant lampshades and translucent storage cabinets, there are evidently plenty of design opportunities associated with PP. With excellent injection molding properties and low melt viscosity, PP flows easily in molds, facilitating efficient manufacturing and accurate structure reproduction, even producing large parts in single molds.

PP is considered an ideal choice for kitchen appliances such as rice cookers, ovens, and electric kettles due to its relatively high melting point and chemical resistance.

PP retains static electricity after processing and usage, leading to surfaces prone to dust accumulation. The use of additives can minimize this issue, but it encounters some limitations such as making surface printing nearly impossible and, in some cases, causing white discoloration (blooming) over time, exacerbating this condition.

Woven materials from PP fibers are widely used in industries due to their durability, lightweight, and low cost. Self-reinforced PP (SrPP), a type of synthetic material made from 100% PP, was developed in the 1990s and is commonly known under trade names such as Curv. The production process of SrPP begins with creating a woven fabric. By applying precise heat and pressure, the outer layer of fibers melts and bonds together to form a solid sheet material. SrPP has higher strength, impact resistance, and stiffness compared to compression-molded sheets. It has been successfully used in manufacturing luggage, bulletproof panels, sports equipment, and automotive parts. SrPP is shaped using hot pressing, similar to conventional sheet forming, and is molded to cover cut edges and combine accessories and features. Unlike conventional synthetic materials like glass fiber-reinforced plastics (GFRP), SrPP does not rely on the combination of different materials, offering some environmental benefits.

A similar self-reinforced composite material is produced using high-strength PET plastic. This alternative solution provides higher temperature resistance and impact strength but at a higher cost.

PP in Packaging

PP offers many advantageous properties for packaging. Although its matte image quality is suitable for common foldable sheet packaging, PP fibers with high strength are preferred in industrial packaging. PP can be converted into trays, boxes, lids, and caps with excellent injection molding properties, providing cost efficiency. The success of PP has made it a symbol of image quality and unique feel, associated with global brands like Muji and Tupperware. PP naturally has transparency and can be made even brighter with clarifying additives, making PP containers almost transparent in water. This makes PP directly competitive with other packaging plastics such as PET, polystyrene (PS), and polyvinyl chloride (PVC).

PP is commonly used as sheets for bags, boxes, and foldable items cut from molded grids. Compared to materials like PE, PVC, and ethylene vinyl acetate (EVA) used for similar applications, PP has higher rigidity (and lower elasticity). When compression molded with a glossy finish or textured, similar to injection-molded PP, it has a particularly smooth feel. This is similar to PE and polyamide (PA) in this regard due to its low friction coefficient.

PP in Automotive and Transportation

The automotive industry has been at the forefront of utilizing various types and injection molding techniques to leverage the lightweight and resilient properties of PP.

Short Glass Fiber (SGF) molding enhances PP’s strength-to-weight ratio and stiffness. Typically, the plastic is pre-mixed with glass fiber content ranging from about 10 to 40% by weight. Higher glass fiber content, up to about 60%, is possible but may reduce impact strength (toughness) as increased fiber content decreases flexibility. Applications include fans, casings, engine covers, seat frames, dashboard frames, and other thin-walled structural components.

Glass fibers (GF) affect surface finish and may be noticeable, especially in dark-colored plastics. When image quality is crucial, various finishing techniques are used to conceal them, ranging from thicker structures (effectiveness depends on the fiber content ratio) to coatings. Additionally, parts can be molded with a higher-quality PP outer layer to conceal a high-strength core inside.

Long Glass Fiber (LGF), with lengths ranging from 12 to 25 mm (0.5-1 inch), provides significant mechanical benefits, especially at high temperatures, including minimizing warpage, maximizing strength and stiffness, as well as improving energy absorption capability. Impact resistance is improved fivefold. Similar strength can be achieved as with SGF by using lower LGF concentrations, resulting in lighter parts. However, LGF is more expensive due to the complex raw material production process (compression molding), and it’s also more challenging to injection mold (compared to SGF, offering greater design freedom), hence typically used for applications requiring the highest technical requirements. LGF is often used in structural components, such as front-end modules and underhood applications, replacing more expensive materials like engineering thermoplastics or steel previously.