Polypropylene is a semi-rigid, relatively inexpensive, and tough thermoplastic linear hydrocarbon polymer resin. Polypropylene has excellent chemical, electrical, and fatigue resistance at high temperature.
It is considered to be a plastic without hazards, thus it is used in the manufacture of products ranging from plastic furniture and machinery to pill containers and syringes.
Polypropylene has many great features, versatility, customizable, and strengths, but it also has some negatives, such as susceptibility to oxidation, flammable, and sensitivity to chlorinated solvents and aromatics.
This article will address many questions about this great material, including how it is made, how it is used, and its advantages and disadvantages.
What is Polypropylene?
Polypropylene (PP) is a thermoplastic with a low-density and stress-cracking resistance created from a monomer known as propene (or propylene).
This linear hydrocarbon resin is represented through the chemical formula (C3H6)n. Its origins can be traced back to the mid-1950s, and it was discovered by Italian scientists. Polypropylene is the second most manufactured plastic, following polyethylene
As a thermoplastic, polypropylene can be heated to its melting temperature, cooled, reheated, and subject to no noticeable deterioration.
This property, in addition to its ability to become a liquid instead of burning when introduced to high temperatures, are also two of the key features that make polypropylene useful in manufacturing applications such as injection molding. Some other properties of this material are:
- Chemical Resistance: Polypropylene is resistant to diluted bases and acids. It is perfect for any container that holds liquid, like cleaning fluids.
- Toughness: Polypropylene is elastic only within a limited range of deflection, and while it is tough because it will experience plastic distortion early in the deformation process. Toughness is the ability of a material to absorb energy and flex without breaking.
- Insulation: high electrical resistance makes polypropylene good for electronic components.
Polypropylene is Made
Polypropylene is synthesized by the polymerization of propylene gas. Propylene gas is polymerized using a catalyst system, typically a Ziegler-Natta catalyst or metallocene catalyst.
The conditions for the polymerization; temperature, pressure, and concentration of reactants, are selected to produce specific grades of polymer.
The polymerization process can occur in several different manufacturing processes, which can be in either gas-phase or liquid-phase:
- Gas-phase polymerization: the reaction occurs in a fluidized bed reactor with propene and hydrogen passing through the catalytic bed in the reactor between 70 °C – 100 °C with 1 – 20 atm of pressure. The reaction of propene is turned into polypropylene as a fine powder, separated from the unreacted propene and hydrogen with cyclone separators. The fine powder is then pelletized and is ready to use. In general, gas-phase polymerization can be made more efficient by using high-activity catalysts to minimize gaseous and liquid waste.
- Spheripol – this polymerization process uses propene in combination with ethylene and hydrogen to produce polypropylene. The reaction occurs with heterogeneous mixing of the catalyst and monomer in liquid in the form of an inert solvent, typically either light hydrocarbons. The slurry of catalyst and monomer is pumped into one or two vertical stirred gas phase reactors where the polymerization occurs. The reaction is complete when the mixture of monomers and polymer powder is removed from the reactors through dip tubes, with the unreacted monomer recycled back to the stirred gas reactor. The polymer is stored for later use.
The Primary Uses of Polypropylene
Polypropylene is utilized in a variety of applications, including but not limited to:
- Material handling
- Packaging
- Medical technologies
- Apparels
- Vehicle parts
- Household products
- Toys
The packaging sector is making pretty substantial use of polypropylene due to its inexpensive price and pliability of application as a replacement for paper and cellophane.
Natural and synthetic fiber pallets, bottles, jars, containers for yogurt, cups for hot beverages, and packaging for food all are produced from polypropylene.
Polypropylene is also used in the automotive industry very often for batteries, bumpers, instrument panels, interior parts and trim, and door trim.
Relative to their safety, polypropylene is used in medical applications since it is resistant to chemical attacks, bacterial attacks, and steam sterilization.
Polypropylene is employed in the manufacture of disposable syringes, diagnostic supplies, petri dishes and primary containers for intravenous use to name a few.
While there are a diversity of processing options available for polypropylene and plastics in general, there is one method that is the most dominant in terms of manufacture, being injection molding.
Polypropylene reacts fast to injection speed and pressure, and it forms quickly in the injection mold. The net result is that moulders can achieve high production rates because it is efficient to mould and solidifies almost immediately after injection.
Because of the combination of performance characteristics of this polymer, it is in a unique position in the injection molding world.
Advantages and Disadvantages of Using Polypropylene
Polypropylene’s advantages are its strength and low cost. Other advantages include:
Advantages of Polypropylene
- Chemical Resistance: Polypropylene is highly resistant to a broad variety of chemicals, including acids, bases, and solvents, making it an exceptional material choice for containers and pipes which are exposed to corrosive materials.
- Durability and Toughness: PP has excellent levels of impact resistance and is extraordinarily tough. PP can be subjected to considerable wear and tear, making it ideal for long usage products, such as automotive parts and industrial components.
- Lightweight: One of the critical benefits of polypropylene is very low density and produces lightweight products. This is increasingly advantageous in both packaging and automotive applications where weight savings are extremely important.
- Thermal Resistance: Polypropylene has a relatively high melting point in comparison to a number of plastics and can be used in applications involving high temperatures. PP is often used in microwaveable containers, and automotive engine components.
- Flexibility and Versatility: PP can be formed into many various shapes and forms, ranging from rigid products to flexible films and fibers. This flexibility and versatility of the polymer makes it applicable to a vast array of products such as packaging, textiles, and everyday consumer goods.
- Moisture Resistance: PP is considerable moisture resistant and therefore remains dry because of minimal moisture absorption. This quality makes PP a perfect material of choice for many products, including outdoor furniture and marine ropes where water resistance is of utmost importance.
Disadvantages of Polypropylene
- Vulnerability to UV Deterioration: Polypropylene is susceptible to degradation from UV exposure after prolonged exposure, and limits its use in outdoor applications to when it is stabilized with UV inhibitors.
- Restricted Temperature Range: Although PP has good thermal properties, it will become brittle at very low temperatures and restricts use in extreme cold.
- Flammability: Polypropylene is flammable and may add to fire spread. It requires additives for applications that require flame resistant properties.
- Environmental Issues: Similar to many other plastics, polypropylene is not biodegradable, is made from non-renewable fossil fuels, and there is plenty of concern about the environmental issues it raises, but recycling campaigns can help alleviate the problem.
- Bonding Issues: Polypropylene has a low surface energy, so it is difficult to bond with adhesives and coatings as is. Special surface treatment or primers must be used to improve bonding.
- Electrical Conductivity: Polypropylene is an electrical insulator, which can be a problem in applications where electrical conductivity is necessary.
The Difference Between Polypropylene and Polystyrene
The key difference between polypropylene and polystyrene is that polypropylene is more durable. Polystyrene’s monomer is styrene while polypropylene’s monomer is propylene.
In addition, polystyrene has a phenyl group as its pendant group, and polypropylene has a methyl group. Each polymer’s tacticity is altered by the addition of these groups.
Let us now take a look at a quick overview of the main characteristics of these two materials that differentiate them:
- Production process: While polyproplyene is made using chain-growth polymerization, polystyrene is produced using the method of free radical vinyl polymerization.
- Uses: Polystyrene is used for making single-use drinking cups, home devices and components of computer housings. Polypropylene is used for making containers, packaging and parts of vehicles.
- Chemical resistance: Polystyrene has some acid and base resistance, but that is not as much as polypropylene has between some chemicals.
- Reusability: Obviously, Polystyrene products can be single-use as the products are often used in large quantities and disposed The final use is generally limited to overall temperatures of 100°C, and it can’t be exposed to a second use. Polypropylene products can have multiple uses.
The Difference Between Polypropylene and Polyester
Polyester can take on some water, whilst polypropylene cannot The differences between polyester and polypropylene.
Both polyester and polypropylene are polymer materials, but are used in different situations based on the properties needed in the final product.
Let’s now take a look at a brief synopsis of the respective issues of the key properties that differentiate these two materials:
- Production process: polypropylene is produced by addition polymerization of propylene, whereas polyester is the byproduct of condensation polymerization of a diol combined with a dicarboxylic acid.
- UV and heat resistance: polypropylene can begin degrading after long term UV exposure and extremely high temperatures, while polyester can handle higher temperatures and has UV resistance.
- Moisture absorption: polypropylene is more aqueous resistant to polyester because it distributes moisture rather than absorbs it which is why it’s good for athletic wear.
- Cost: while it is typically cheaper to manufacture polypropylene than polyester, this does not always mean that the end product will be cheaper. The price is greatly affected by brand names and type of products.
- Uses: polypropylene is mainly used in packaging, as opposed to polyester which is used primarily in textiles.
Conclusion
Polypropylene has been used across many industries because of its flexibility, durability, chemical resistance, and many other significant properties, including being the top user of all the packaging.
Polypropylene is a BPA free thermoplastic polymer that is not deemed a health risk for humans, although it may degrade or leach chemicals when exposed to significant amounts of UV radiation or exposed to sustained high temperatures.
There are various methods of molding plastics in manufacturing; however, polypropylene is primarily produced through injection molding processes.