Polypropylene Resins Information
Figure 1: Plastic pellets. Source: Gentlemanrook/CC BY 2.0
Polypropylene plastic plays a role in many aspects of modern life. Everything from toothbrushes to shopping bags to car parts contains polypropylene parts. The low cost and abundance of polypropylene, coupled with its ideal mechanical characteristics, have made it one of the most popular forms of plastic on the market.
Theory of Operation
Polypropylene resin is a type of plastic that's widely used in a variety of applications due to its versatility, durability, and resistance to chemicals. The way polypropylene resin works can be best explained in terms of its production process and the properties that make it suitable for its applications.
Polypropylene is produced through a process called the polymerization of propylene monomers. In this process, propylene gas is exposed to heat and pressure in the presence of a catalyst to form long chains of polypropylene. This forms the basic structure of the polypropylene resin.
Figure 2: The long chains of polypropylene form a white powder. Source: Public domain
For polymerization to occur, the monomers must be carefully prepared. Propylene is a by-product of the oil refining and natural gas processing industries. It's a gas at room temperature, but can be liquefied under pressure. Once the propylene monomers are liquefied, polymerization can begin.
The liquefied propylene is then fed into a reactor along with a catalyst. The catalyst is typically a compound of titanium and aluminum. Under heat and pressure, the catalyst triggers a chemical reaction that causes the propylene molecules to join together, forming long chains of polypropylene. This process is known as polymerization.
The long chains of polypropylene form a white powder. This powder is then melted and extruded into the desired shape, such as pellets, which can be used in a variety of applications. The pellets can be reheated and molded into a variety of shapes, from car parts to food containers.
Specifications
When specifying polypropylene resins, there are several characteristics and properties to consider. These include:
Process Type
This refers to the intended use of the resin, including extrusion, film production, molding, and others. Some resins are better suited for certain processes.
Figure 3: Injection molding machine. Source: 384/CC BY-SA 4.0
Filler Material
Certain polypropylene resins may contain fillers such as fiberglass or minerals to enhance specific properties. Talc or other substances may be added in various ratios to minimize characteristics like shrink rate or to improve other properties.
Features
These can include properties such as being anti-static, biocompatible, electrically conductive, flame retardant, and optical grade.
Industry
The specific industry where the resin will be used can influence the specification. For example, the requirements for resins used in the medical industry may differ from those used in the automotive or construction industries.
Physical Properties
These include properties such as melt flow index (MFI), viscosity, water absorption, use temperature, deflection temperature, thermal conductivity, coefficient of thermal expansion (CTE), tensile strength, tensile modulus, and elongation.
Electrical Properties
These can include dielectric constant, dielectric strength, and electrical resistivity.
Optical Properties
These can include the index of refraction and light transmission. If light transmission or transparency are important characteristics, the polypropylene resin must be specially prepared.
Types
Polypropylene resins can be classified into different types based on the process of polymerization and the structure of the polymer chain:
Polypropylene Homopolymer
This is the most widely used general-purpose grade. It contains only propylene monomer in a semi-crystalline solid form. The homopolymer term refers to the repeated pattern of the polymer’s structure. Main applications include packaging, textiles, healthcare, pipes, automotive, and electrical applications.
Polypropylene Copolymer
This family is produced by polymerizing propylene and ethane. It is further divided into random copolymers and block copolymers. The most common variants include:
- Polypropylene random copolymer
- Polypropylene block copolymer
- Polypropylene impact copolymer
- Expanded polypropylene
- Polypropylene terpolymer
Polypropylene random copolymer is produced by polymerizing ethene and propene together. It usually features ethene units up to 6% by mass. These units are randomly incorporated into the polypropylene chains. These polymers are flexible and optically clear, which makes them suitable for applications requiring transparency and excellent appearance.
Polypropylene block copolymer, in contrast, has a larger ethene content, ranging between 5% and 15%. It has co-monomer units arranged in regular patterns or blocks. The regular pattern makes this thermoplastic tougher and less brittle than the random copolymer. These polymers are suitable for applications requiring high strength, such as industrial usage.
Polypropylene impact copolymer contains a co-mixed polypropylene random copolymer phase and has an ethylene content of 45% to 65%. It is useful in parts that require good impact resistance. Impact copolymers are mainly used in packaging, housewares, film, pipe applications, and also in the automotive and electrical segments.
Expanded polypropylene (EPP) is a closed-cell bead foam with ultra-low density. It is used in automobiles, packaging, construction products, consumer goods, and more due to its higher strength-to-weight ratio, excellent impact resistance, thermal insulation, and chemical and water resistance.
Polypropylene terpolymer is composed of propylene segments joined by monomers ethylene and butane (co-monomer). Polypropylene terpolymer has better transparency than polypropylene homopolymer. The incorporation of co-monomers reduces crystalline uniformity in the polymer, making it suitable for sealing film applications
Features
Polypropylene resins possess a number of key features that make them suitable for various applications:
Versatility
Polypropylene can be formulated to be hard or soft, opaque or transparent, light or heavy, insulating or conductive, neat or reinforced with cheap mineral fillers, short or long glass fibers, natural fibers, or even self-reinforced??.
Figure 4: The melting point of polypropylene occurs at a range. Source: SomovPA/CC BY-SA 4.0
High Melting Point
The melting point of polypropylene occurs at a range. For polypropylene homopolymer, it's 160° C to 165° C and for copolymers it is often in the 135° C to 159° C range??.
Low Density
Polypropylene is one of the lightest polymers among all commodity plastics. This feature makes it a suitable option for lightweight/weight saving applications.?
Chemical Resistance
Polypropylene has excellent resistance to diluted and concentrated acids, alcohols, and bases. It also has good resistance to aldehydes, esters, aliphatic hydrocarbons, and ketones. However, it has limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents??.
Flammability
Polypropylene is a highly flammable material.?
Mechanical and Electrical Properties at Elevated Temperatures
Polypropylene retains its mechanical and electrical properties at elevated temperatures. This occurs in humid conditions and when submerged in water. It is also a water-repellent plastic?.
Steam Sterilization
Polypropylene exhibits good resistance to steam sterilization? and environmental stress cracking?.
Recycliability
Many forms of polypropylene are able to be recycled. Containers and parts that can be recycled will typically be marked with a recycling symbol for easy visibility.
Figure 5: Recycling symbol. Source: Anton Poliakov/CC BY-SA 4.0
Impact Resistance
Certain types of polypropylene, such as the impact copolymer, are useful in parts that require good impact resistance??.
Strength-to-Weight Ratio and Thermal Insulation
Expanded polypropylene has a higher strength-to-weight ratio and excellent thermal insulation, making it useful in a variety of applications.
Manufacture
Polypropylene is manufactured through a process known as polymerization, specifically through the polymerization of propene monomer. There are two main synthesis methods used to produce polypropylene:
- Ziegler-Natta Polymerization
- Metallocene Catalysis Polymerization
Ziegler-Natta Polymerization is the most common method for the production of polypropylene. This process uses titanium chloride as a catalyst and an organoaluminum compound as a co-catalyst. The polymerization process takes place under high pressure and at high temperatures.
Figure 6: Mechanism of Ziggler Natta catalysis .Source: Polimerek/CC BY-SA 3.0
Metallocene catalysis polymerization is a more recent method that uses metallocene catalysts for the polymerization process. This method provides better control over the polymer's molecular structure, which allows for the creation of polypropylene with specific properties tailored to certain applications.
Applications
Polypropylene resins are versatile and used in a wide array of applications due to their durability, resistance to many chemical solvents, bases, and acids, and their ability to function as both a plastic and a fiber. Some of the key applications of polypropylene resins include:
Packaging
Polypropylene is widely used in packaging applications. This includes film applications for packaging and labeling, food containers, plastic bottles, straws, bags, and caps.
Figure 7: Polypropylene is used in the textiles industry. Source: Pixabay
Automotive
In the automotive industry, polypropylene is used for applications like interior trim, battery cases and trays, bumpers, fender liners, and more. Its use is favored due to its low cost, easy processability, and recyclability.
Textiles
Polypropylene is used in the textiles industry for making indoor-outdoor carpets, carpets for cars, and other textile products due to its durability and resistance to wear and tear.
Healthcare
In healthcare, it is used for applications such as syringes, medical vials, and intravenous bottles. It's also used in the production of lab equipment, diagnostic devices, and surgical sutures.
Pipes
Polypropylene is used to manufacture various types of pipes, including those used for industrial applications, sewage systems, and irrigation.
Figure 8: Polypropylene is used to manufacture various types of pipes. Source: Emilian Robert Vicol/CC BY 2.0
Electrical Applications
In electrical applications, polypropylene is used as an insulating material. It's also used to manufacture electrical components like cables, wires, and light fittings.
Consumer Goods
It is used in a variety of consumer goods such as appliances, toys, furniture, and more. Polypropylene’s low cost and wide availability result in its use in many different applications.
Standards
Polypropylene, along with most other commodity plastics, tends to be regulated fairly well. Many of the standards associated with polypropylene are related to the final application of the polypropylene. This helps ensure that the final propylene products will have the proper characteristics and meet the requirements for the application. Some standards include:
- CFR — Code of Federal Regulations Title 21 Part 177 — Polymers
- ASTM D5857-17
- ISO 19069-1:2015 — Plastics — Polypropylene (PP) molding and extrusion materials
- ASTM D2673-14(2022) — Standard Specification for Oriented Polypropylene Film
- Many others
These standards apply to propylene in various ways and will not be applicable to all polypropylene products. Some of these standards concern how polypropylene is produced and methods for injection and extrusion while other standards apply to items like food products or medical products. For any polypropylene product, it is important to be aware of any applicable standards.