What is polypropylene (PP) and what is its use?
Polypropylene (PP) is a thermoplastic addition polymer made from a combination of propylene monomers. It has a wide range of applications, including consumer product packaging, plastic parts for the automotive industry, and textiles. Philip Oil Company scientists Paul Hogan and Robert Banks first made polypropylene in 1951, and later Italian and German scientists Natta and Rehn also made polypropylene. Natta perfected and synthesized the first polypropylene product in Spain in 1954, and its crystallization ability aroused great interest. By 1957, the popularity of polypropylene had soared, and extensive commercial production had begun throughout Europe. Today, it has become one of the most commonly used plastics in the world.


A medicine box made of PP with a hinged lid

According to reports, the current global demand for PP materials is about 45 million tons per year, and it is estimated that the demand will increase to about 62 million tons by the end of 2020. The main application of PP is the packaging industry, which accounts for about 30% of the total consumption. The second is electrical and equipment manufacturing, which consumes about 26%. The household appliances and automobile industries each consume 10%. The construction industry consumes 5%.

PP has a relatively smooth surface and can replace some other plastic products, such as gears and furniture pads made of POM. The smooth surface also makes it difficult for PP to adhere to other surfaces, that is, PP cannot be firmly bonded with industrial glue, and sometimes must be bonded by welding. Compared with other plastics, PP also has the characteristics of low density, which can reduce weight for users. PP has excellent resistance to organic solvents such as grease at room temperature. But PP is easy to oxidize at high temperature.

One of the main advantages of PP is its excellent processing performance, which can be formed by injection molding or CNC processing. For example, in the PP medicine box, the lid is connected to the bottle body by a living hinge. The pill box can be directly processed by injection molding or CNC. The living hinge that connects the lid is a very thin plastic sheet that can be bent repeatedly (moving in an extreme range close to 360 degrees) without breaking. Although the living hinge made of PP cannot bear the load, it is very suitable for the bottle cap of daily necessities.

Another advantage of PP is that it can be easily copolymerized with other polymers (such as PE) to form composite plastics. The copolymer significantly changes the properties of the material, and can achieve stronger engineering applications compared with pure PP.

Another immeasurable application is that PP can act as both a plastic material and a fiber material.

The above characteristics mean that PP can be used in many applications: plates, trays, cups, handbags, opaque plastic containers and many toys.

What are the characteristics of PP?
The most important characteristics of PP are as follows:

Chemical resistance: diluted alkali and acid do not react with PP, which makes it an ideal container for such liquids (such as detergents, first aid products, etc.).
Elasticity and toughness: PP has elasticity within a certain range of deflection, and will undergo plastic deformation without cracking in the early stage of deformation, so it is usually regarded as a "tough" material. Toughness is an engineering term defined as the ability of a material to deform (plastic deformation rather than elastic deformation) without breaking.
Fatigue resistance: PP retains its shape after a lot of twisting and bending. This feature is particularly valuable for making living hinges.
Insulation: PP material has high resistance and is an insulating material.
Transmittance: It can be made into a transparent color, but it is usually made into a natural opaque color with a certain color transmittance. If high transmittance is required, acrylic or PC should be selected.
PP is a thermoplastic with a melting point of about 130 degrees Celsius, and becomes a liquid when it reaches the melting point. Like other thermoplastics, PP can be heated and cooled repeatedly without significant degradation. Therefore, PP can be recycled and easily recovered.

What are the different types of PP?
There are two main types: homopolymers and copolymers. Copolymers are further divided into block copolymers and random copolymers. Each category has unique applications. PP is often referred to as the "steel" material of the plastics industry, because it can be made by adding additives to PP, or manufactured in a unique way, so that PP can be modified and customized to meet unique application requirements.

PP for general industrial use is a homopolymer. Block copolymer PP is added with ethylene to improve impact resistance. Random copolymer PP is used to make more ductile and transparent products.

How is PP made?
Like other plastics, it starts from the "fractions" (lighter groups) formed by the distillation of hydrocarbon fuels and combines with other catalysts to form plastics through polymerization or polycondensation reactions.

CNC, 3D printing and injection molding features
PP 3D printing

PP cannot be used for 3D printing in filament form.

PP CNC processing

PP is used for CNC processing in sheet form. When making prototypes of a small number of PP parts, we usually perform CNC machining on them. PP has a low annealing temperature, which means it is easily deformed by heat, so it requires a high level of skill to cut accurately.

PP injection

Although PP has semi-crystalline properties, it has very good fluidity due to its low melt viscosity, so it is easy to shape. This feature greatly improves the speed at which the material fills the mold. The shrinkage rate of PP is about 1-2%, but it will vary due to many factors, including holding pressure, holding time, melting temperature, mold wall thickness, mold temperature, and the type and percentage of additives.

Other uses
In addition to conventional plastic applications, PP is also very suitable for making fibers. Such products include ropes, carpets, upholstery, clothes, etc.


What are the advantages of PP?
PP is easily available and relatively cheap.
PP has high flexural strength.
PP has a relatively smooth surface.
PP is moisture-proof and has low water absorption.
PP has good chemical resistance in various acids and alkalis.
PP has good fatigue resistance.
PP has good impact strength.
PP is a good electrical insulator.
What are the disadvantages of PP?
PP has a high coefficient of thermal expansion, which limits its high temperature applications.
PP is susceptible to degradation by ultraviolet rays.
PP has poor resistance to chlorinated solvents and aromatic hydrocarbons.
PP is difficult to spray on the surface because of its poor adhesion properties.
PP is highly flammable.
PP is easy to oxidize.
Despite its shortcomings, PP is generally a good material. It has unique mixing characteristics that other materials can't compare, that is, it can be copolymerized with other polymers to form composite materials, and various additives can be added, which makes it an ideal choice for many projects.

What are PP attributes?
Under standard conditions, that is, an ambient temperature of 25°C and 1 atmosphere of pressure.

Technology Name: Polypropylene (PP)

Chemical formula: (C3H6)n


Resin identification code (for recycling):


Melting temperature: 130°C

Typical injection temperature: 32-66 °C

Heat distortion temperature: 100°C (under 0.46 MPa pressure)

Tensile strength: 32 MPa

Flexural strength: 41 MPa

Specific gravity: 0.91

Shrinkage rate: 1.5-2.0%