Polypropylene is a combination of propylene monomer and thermoplastic, popularly used in the packaging and plastic industries. You can recycle these thermoplastics by heating them again and again without much quality compromise. It is also called steel of the plastic industry because it can be copolymerized to enhance its chemical and physical properties.
History of Polypropylene
Polypropylene was discovered in 1951 by Paul Hogan and Robert Banks. However, it was Professor Giulio Natta who polymerized it. Polypropylene became commercially successful and an alternative to other plastic forms. Polypropylene resin has a crystallization property and durability, making it one of the most popular forms of plastic.
This thermoplastic was first used as a catalyst for the polythene industry. Gradually it expanded commercially for different applications. Since 1957, the usage and production of Polypropylene have seen a steep growth worldwide and have emerged as an alternative to various plastics.
Chemical Properties of Polypropylene
It is a linear hydrocarbon polymer that has a methyl group intertwined with the carbon atom. Traditional thermoplastics are different than Polypropylene because thermoplastics cannot be heated more than once, unlike Polypropylene. The constant heating reduces molecular bonding and changes the chemical properties as well.
Moreover, traditional thermoplastics will burn during high temperatures. Polypropylene’s chemical properties are much better than conventional thermoplastics because they do not burn and can be recycled. It has excellent resistance features against acids, bases as well as alcohols. It can also resist the reactions with esters, ketones, aldehydes, and aliphatic hydrocarbons. However, it can partially react with halogenated hydrocarbons. It is highly flammable material also.
Physical Properties of Polypropylene
Polypropylene is a poor conductor of electricity and chemical reactions at high temperatures. Unlike polythene, it has a lower melting density. By adding certain additives to pure Polypropylene, you can enhance its physical properties like rigidity and strength.
Another important physical property is its flexibility. You can bend it again and again without breaking it or deforming the structure. Polypropylene is a very light polymer than the available plastic polymers, making it ideal for lightweight applications.
Mechanical Properties of Polypropylene
Though polypropylene pellets are semicrystalline, you can easily use them for the injection molding process. The lower melting viscosity is responsible for its fluidity. It is a relatively easy and faster way to manufacture polypropylene applications with a better adaptation of technology. The melting point lies between 200-250 degrees celsius, but it can also rise to 300 degrees celsius. Also, take care of the ventilation to avoid burning the thermoplastic.
It is better to know about the melting flow index available for the processing of Polypropylene by manufacturing companies. This will help minimize wastage while processing and a better way to understand the material’s flow behavior. Polypropylene shrinks due to molding patterns and pressures.
Different Types of Polypropylene
Two types of Polypropylene are frequently used in different industries for making applications. Both have their own unique features and purposes.
Homopolymer polypropylene is the basic form of thermoplastic used for the regular stuff manufacturing process. This thermoplastic is produced by copolymerization of propylene and Ziegler-Natta catalysts.
It is easy to use during manufacturing processes like molding, extrusion, or thermoforming. Homopolymer polypropylene has melt and molding temperatures at 210 – 290°C and 20 – 60°C, respectively. Homopolymer polypropylene has a higher strength to weight ratio, processability, impact-resistant, chemical resistance, stiffness, and rigidity.
Copolymer polypropylene is softer but better than homopolymers in terms of strength and durability. They are divided into Block copolymer and Random copolymer. The former has molecular arrangements in blocks and contains ethylene which improves resistance capabilities. The latter has an irregular structure of the co-monomer units.
Block copolymer contains 5-15% ethylene, whereas random copolymer has ethylene between 1-7%. Copolymer polypropylene also has melt and molding temperatures at 210 – 290°C and 20 – 60°C, respectively.
The impact copolymer is a hybrid of Homopolymer and Random Copolymer. It contains 45-65 % ethylene which is responsible for its resistance property. Impact copolymer is popularly used for making automobiles and electrical applications as well as packaging and drainage pipes.
Expanded Polypropylene is a foam-based material that is used to make 3d polymer products. It has a lesser weight ratio than the strength ratio, a bad insulation conductor, and higher resistance to chemical reactions. Thus, it is used to produce packaging products, automotive parts as well as infrastructural materials.
High Melt Strength Polypropylene
High Melt Strength Polypropylene or PP HMS is well known for its impressive mechanical and physical properties. As the name suggests, it is a hybrid form of the normal Polypropylene that has a high melting point and melting durability. Due to the high melting point and heat resistant capability, it is widely used in different applications.
Polypropylene fabric is also rising gradually in the textile industry and can now be seen in various segments of textiles. It is derived from Polypropylene and is a part of the polyolefin group. Earlier, Polypropylene was used as plastic only, but the physical and chemical properties made it possible to use for textile. You do not need to weave Polypropylene to make fabric; instead, you can print it directly.
The polypropylene fiber can resist moisture, making it an ideal choice for water-resistant fabrics. It dries faster than other fabrics. Generally, it is used as undergarments. But it absorbs body odors and sweat. Sometimes, they can also cause rashes and skin burns. As Polypropylene cannot endure high temperatures, you need to be careful while washing such fabrics. Do not use hot water for washing them; otherwise, they will shrink or deform.
Polypropylene fabric is very light-weighted and non-reactive to acids and bases. This synthetic fiber is better for winter times due to its thermal conductivity. Polypropylene fabric is also safe from pest attacks. Moreover, you can melt the fabric to make new garments because it is a non-woven material.
One of the limitations of PP fiber is that it is difficult to dye after processing. Even it has limitations on texture variations once you print it. It cannot withstand ultraviolet rays or other heat sources. It can also not blend with some regularly-used gums or epoxies. From the environmental perspective, this synthetic is not good and safe.
Process To Make Polypropylene Fabric
The making of polypropylene fabric requires the following steps-
- Propylene monomer is extracted from crude oil in gaseous form.
- The next step involves the polymerization process of the polypropylene monomer.
- Propylene monomer is then combined together to form a plastic form.
- Add polypropylene additives like plasticizers, fillers, and stabilizers to make propylene fiber.
- Leave the mixed materials to cool off. You can cool it in the form of pellets and bricks.
- Now, these polypropylene pellets or bricks can be reheated to melt for making textile sheets.
- And lastly, you can use these sheets for making the desired textiles by cutting and stitching.
Polypropylene fiber is generally used to make diapers, undergarments, winter apparel, sportswear in the clothing industry.
Why is Polypropylene Popular
Polypropylene has emerged as a consumer-friendly and affordable alternative to other plastics, making it popular. Its popularity lies in the versatility of this thermoplastic. It has excellent chemical resistance properties and is unaffected by organic solvents, acids, and bases.
Polypropylene material is impermeable and absorbs less than 0.01% of its weight in water. Moreover, its tensile strength allows it to hold heavy loads. And the impact resistance is better than polyethylene. So, making it an ideal choice for full immersion applications.
Different Processes For Making Polypropylene
The process for making Polypropylene involves polymerization, where propylene monomer goes through high temperatures and pressure. The propylene monomer turns into a slurry solution under the catalyst. It will generate a translucent polymerized polypropylene which can be further processed. For making Polypropylene, the processing should be at a lower temperature with low pressure.
After polymerization, it can transform into three kinds of chain structures based upon the methyl positions. If there is an irregular arrangement of the methyl group, it will become Atactic Polypropylene. If the methyl group forms a chain on one side of the carbon, it will become Isotactic Polypropylene. And lastly, alternate methyl arrangement will result in Syndiotactic Polypropylene.
Polypropylene is a byproduct of ethylene which can be separated by fractional distillation in low temperatures. By using different catalysts, you can even alter its physical properties for better plastic formation. The versatile properties have boosted its scope in various industrial applications.
Characteristics of Polypropylene
Propylene is popular for many reasons and distinct advantages. Some of the important characteristics are-
Resistance To Electricity
Propylene is a very good insulator of electricity and is capable of resisting electric currents. Due to this feature, this thermoplastic is popular for electrical components.
Non-Reactionary To Acid And Base
Another important characteristic of this thermoplastic is its non-reaction to diluted forms of acids and bases. So it is quite better to make polypropylene-based containers for reactive liquid.
Though Polypropylene has a certain flexible range if you blend it with other filaments, it remains hard and sturdy. This monomer does not deform after bending or twisting. So, Polypropylene is flexible, hard, and retains shape.
Transparency And Appearance
Polypropylene is an opaque thermoplastic, but you can also make it transparent through processing. This makes the plastic suitable for aesthetic applications as well. Light can easily pass through polypropylene objects but for more clear transparency plastics you can rely on better filaments like Polycarbonates.
Polypropylene becomes liquid at their heating temperature (130 degrees Celsius) instead of burning completely. So, you can recycle the existing propylene materials by heating, melting, and cooling. It becomes easy to fill liquid Polypropylene during injection molding.
Production Of Polypropylene Fiber
Polypropylene traces its origin from the United States Of America, but China has emerged as the biggest producer and supplier of this thermoplastic. Since 2016, China has been growing rapidly in polypropylene production. Other countries like Germany, Italy, France, Mexico, and Belgium also produce Polypropylene of net worth.
LyondellBasell is the top company in this field. It is situated in the Netherlands and has franchises in Houston and London. Beijing-based companies Sinopec Group and PetroChina Group are leading producers of Polypropylene and a significant supplier of it worldwide.
Polypropylene is manufactured and supplied all over the world by China and some European nations, but the final processing or stitching is done in Asian countries like India, Pakistan, Indonesia, and others. Here, the labor is cheap and easy to find than rest countries.
Costing Of Polypropylene Fiber
The cost of polypropylene fibers is relatively cheap if bought in bulk. But it is growing expensive due to a shortage in demand. There is huge competition among leading countries to expand their market and supplies, making the prices fair and low.
With the advancement of polyester fabric, it is losing hold of the market. Due to these constraints, it is becoming expensive than other synthetic fibers, which are ultimately paid for by the customers. The other non-apparel polypropylene fabrics are cheaper than available fibers.
Fibre And Their Types
Just like different kinds of Polypropylene, it has two types of polypropylene fibers.
Homopolymer Polypropylene Fibre
The homopolymer polypropylene fiber is a polypro plastic. It does not contain any kind of additives. It is pure form and carries all properties of Polypropylene like softness, prone to heat damage, and others. So, it is not used so much for such kinds of fabrics.
Copolymer Polypropylene Fibre
Unlike homopolymer polypropylene fiber, copolymer polypropylene fiber contains additives to enhance the physical and chemical properties of the thread. It is further divided into block copolymer polypropylene and random copolymer polypropylene.
The former has co-monomer units in the form of blocks or square patterns, and the latter has a random arrangement of the co-monomers. Both kinds are popularly used to make fabrics.
How do additives improve PP properties?
Pure Polypropylene is difficult to use and process but by adding suitable additives, you can surely enhance its physical and chemical properties. Some frequently used polymer additives like conductive fillers like clays, talc, calcium carbonate, flame retardants, pigments, glass fibers, or lubricants improve their features and properties during processing.
Thus, adding different additives and polymerization is cost-effective and increases its efficiency, making it an alternative to traditional plastics.
Polypropylene Processing Method
Polypropylene can be processed through different manufacturing processes. Some of the popular methods are CNC, Injection Molding, Thermoforming, and Crimping. In CNC processing, you need to be careful because polypropylene sheets can melt at high temperatures. It is a slippery and soft material that makes it difficult to cut smoothly, but it is possible with better CNC machines.
Injection molding of Polypropylene is another manufacturing method. For this purpose, you need to use polypropylene pellets. It is easier to mold because of its low melting viscosity. Different factors like melting temperature, injecting pressure, the proportion of additives, and pellet thickness can interfere with efficient processing.
Uses of Polypropylene
Polypropylene is popular for manufacturing a range of applications. It ranges from home decor, living hinges, plastic kitchenware, packaging, fiber to industrial applications. Polypropylene is cheaper, flexible, and available material for packaging, toiletries, robes, carpets, and many more.
Polypropylene with additives is easier to process for 3d printing process. It is perfect for low-strength products. It is prone to warping and lower melting point issues, but blended PP can now cope with these issues. You can 3d print complex models, prototypes, functional models, and small components easily in Polypropylene.
Different Types Of Polypropylene Films
Polypropylene is popular for film extrusion due to its versatile properties. It has emerged as an alternative to cellophane or paper films. It has lower prices and higher resistance to breakdown, making it an ideal choice for this purpose.
They are available as cast film, and bi-axially orientated PP (BOPP). The packaging industry has its largest share in polypropylene film usage, especially in confectionery stores and food packaging.
Cast Polypropylene Film
Cast polypropylene or CPP is known for its versatility. It has excellent resistance to higher temperatures, is not prone to puncture or deformities, and has greater transparency. It also has a high permeability to water vapor and acts as a barrier to moisture.
Biaxially Oriented Polypropylene Film
Biaxially oriented polypropylene film or BOPP has a two-directional molecular chain responsible for its increased tensile strength and stiffness. It can also resist oxidation, moisture, and higher temperatures. Moreover, it comes in transparent, glossy, opaque as well as a matte finish.
Advantages of Polypropylene
You have the advantage of using this thermoplastic through different kinds of technology like CNC machining, molding, crimping, and others. This enables you to print them in different shapes and sizes that fit your requirements. It is popularly used for manufacturing living hinges or flexible segments attached to the parent material.
Polypropylene hinges are very flexible and can easily bend up to 360 degrees without breaking. However, they cannot hold heavy weights or bear high pressure but are ideal for lightweight applications. The flexibility of Polypropylene allows it to bend repeatedly without any damage.
Moreover, it is cheaper than other available options. You can easily process it for use in prototype models. You can easily combine Polypropylene with other polymers to boost its mechanism. The copolymerization of Polypropylene enhances the strength and durability as well as the physical properties of thermoplastic.
Disadvantages of Polypropylene
However, Polypropylene has few drawbacks that limit its potential use. One of the major issues is its inability to blend with certain kinds of glues and surface bonding. These glues are compatible with other plastics, but Polypropylene becomes an exception.
So, instead, you need to go for the welding process for joining the parts. Another issue is of higher coefficient of friction which means that Polypropylene has more friction force than the normal force. It has oxidation problems if left at higher temperatures.
It is difficult to process pure Polypropylene because it has a low melting point and is very soft. The prints can goof up the structure due to uneven cutting. You will require additional efforts to smooth it.
Properties of Polypropylene
Polypropylene can easily be fabricated to make different objects. The copolymerization enhances its chemical and physical properties for a better manufacturing process through molding, CNC, or thermoforming. Moreover, this thermoplastic is also used as a fiber to produce handbags.
The technical name of this thermoplastic is Polypropylene pp, and its chemical formula is (C3H6)n. The melting point of Polypropylene pp is around 130°C, whereas, for the injection molding, a temperature up to 66°C is enough. The tensile and flexural strength is 32 MPa and 41 MPa, respectively. Polypropylene PP is a saturated polymer that is used for both plastic and fiber applications.
Polypropylene has its biggest share in packaging applications, where it has replaced traditional packaging materials. The highest consumption of Polypropylene is in the food and confectionery packaging. The tobacco industry is the second-highest consumer of Polypropylene. It has emerged as a cheaper alternative to traditional packaging materials like cellophane.
Polypropylene material is also good for reusable, safe, and non-reactive storage containers. All kinds of stuff are PP-based: containers and bottles for edible oil or water and caps, wrappers, and tetra packets.
It is also heavily used in the automotive sector for interior parts. The mono-material solution is used through molded technology to produce automobile cushions, bumpers, films, and covers. The exterior accessories are also now using polypropylene material with better additives.
Properties like chemical and electrical resistance, ease of processing, durability, rigidity, and colored polypropylene films have contributed to its increasing demand.
Polypropylene is a consumer-friendly and durable polymer. A long list of consumer-centric applications like home decor and furniture, garden utilities, traveling stuff, or animated toys has its presence.
One of the unique features of this thermoplastic is that it can be used as fiber also. Thus it expands the polypropylene scope in the textile industry as well. It is popular for making undergarments, diapers, winter wears, and sportswear. Another application involves ropes, carpets, drinking straws, sunglasses, and tote bags.
Another important sector for the consumption of Polypropylene is industrial products and applications. Be it for big manufacturing factories or infrastructural units; Polypropylene is a highly recommended material. Drainage pipes, transportation gears, sheets, and other constructional aids are made of this thermoplastic.
Polypropylene is also used to make different medical applications and devices like vials, specimen bottles, trays, pill bottles, syringes, and other items. The properties of non-reaction to chemicals and electrical insulation are responsible for its growth in the medical field.
Is Polypropylene Harmful?
Though Polypropylene is from the plastic category, it is generally considered safest than all available plastics for commercial purposes. FDA has approved Polypropylene safe for food containers. It is used for packing milk products. Even polypropylene-packed food can be heated in the microwave too.
Environmental Protection Agency has also approved Polypropylene as safer than the rest. But, research in 2019 has pointed to certain toxicity levels in various proportions in few polypropylene items. This toxicity is affecting androgen hormones, causing stress in the cellular levels of the human body.
Plastic contains harmful chemicals that leech out and mix with the food in longer durations. It also reacts with the food at high temperatures. However, Polypropylene has a lower toxic chemical called BPA, or Bisphenol A. BPA is an organic synthetic compound used in water bottles, food containers, and medical aids. Phthalates chemicals soften the plastic to make it more flexible. But these chemicals are also harmful to the human body.
You can consume phthalates by inhaling their fumes, or children may chew their plastic toys containing the chemicals. It might seep into your food and cosmetic products through plastic containers. So, it is better to avoid plastic use or at least keep the usage minimal. Phthalates and BPA are very harmful because they can cause cancer, hormonal imbalances, asthma, development, and reproductive issues.
Certification Of Polypropylene
International Organization for Standardization (ISO) has certified polypropylene products. It has ISO 9001 certification for general propylene products and ISO 13485 certification for medical polypropylene products. Another certification as ISO 19069-1:2015 for tensility, resistance to moisture, impact, and melting speed is also given.
Recycling Of Polypropylene
Polypropylene is considered 100% recyclable. Unlike other plastics, it is recyclable, and its Plastic Recycling Code is 5. Recycled Polypropylene is used to make battery cases, brooms, brushes, signal lights, and many more items.
To recycle polypropylene plastic, you need to melt the plastic at 250°C. And blend it with pure Polypropylene for reusing it. Generally, it is safe to use because no serious harm or chemical toxicity has been found yet.