Both PP and PE resins are thermoplastic materials, derived from olefin monomers - a term for the characteristic carbon double bonds in the molecular structure of these monomers. The obvious common origin has given these two plastics many similarities.
So, what is olefin, and the characteristics of polyolefin family resins?
Olefins are a group of hydrocarbons belonging to the alkene family, with the feature of containing a carbon-carbon double bond in the structure. Alkenes include: Propene (C3H6), Butene (C4H8), Pentene (C5H10), Hexene (C6H12), Heptene (C7H14), Octene (C8H16), Nonene (C9H18), Decene (C10H20), …
Polymers made from olefin monomers are called polyolefins or polyalkenes, which are classified as thermoplastics. They are much simpler and easier to manufacture than other plastics, extremely easy to shape, and can be reshaped over and over again, just by repeating the heating, molding, blowing and cooling processes.
Polyolefins are not much different from crude oil in terms of molecular structure. This is both an advantage but also a major drawback, as they have very similar properties to oil and are not fire resistant. But in return, they are considered as the cleanest plastic with a simple and pure structure. Their production does not require the addition of harmful chemicals such as styrene, chlorine or bisphenol A. In addition, most olefins float on water, making polyolefins very easy to distinguish from other synthetic materials, especially during sorting for recycling.
Commercially dominant, the most produced of the polyolefin group are polyethylene (PE) and polypropylene (PP). There are also a number of specialized but less common polyolefins such as polyisobutylene and polymethylpentene...
Polyolefins can appear as oil-like liquids or colorless or white solids, and their properties are mainly determined by their molecular weight and degree of crystallinity. Polyolefins with a low degree of crystallinity (between 0 and 20%) have elastomer-like properties. A moderate degree of crystallinity (between 20 and 50%) is associated with a plastic resin, and a degree of crystallinity above 50% constitutes a hard resin.
Most polyolefins are made by treating monomers with metal catalysts, and the polymerization of this resin is highly exothermic. Polyolefin surfaces will not be effectively bonded together by solvent welding or adhesives, as most of them have excellent chemical resistance, unaffected by common solvents. While they are extremely chemically inert, their strength degrades at very low or high temperatures. Therefore, for plastic films in this group such as PE or PP, heat sealing is a common technique for bonding between layers. However, with the development of modern science, people have successfully developed a number of super adhesives such as cyanoacrylate and acrylate, that can be used to glue these materials.
Both PP and PE plastics, as well as other polyolefins, can be easily processed using common forming techniques such as injection molding, blow molding, extrusion and heat pressing. Easy handling, low cost combined with good physical and chemical properties make them a popular plastic, a good choice for a multitude of applications in the plastics industry.
PP and PE both belong to the group of Olefin-based thermoplastics
Comparing PP and PE plastic, which one is better?
In addition to the common characteristics of olefin-based resins, PP and PE resins also have their own advantages and disadvantages, compatibility with each type of application. There are really no good and bad materials, only different performance, so the purpose is also different. Therefore, finding the most suitable materials is an important goal for businesses to aim for.
Let's try to compare some properties between PP and PE plastic
Heat resistance
PE plastic is divided into many types according to the density of branching in the molecular structure, and the heat resistance of each type will be very different. The melting point of HDPE is about between 130 and 140 °C, LDPE has a lower melting point with a range of 105 to 115 °C. Meanwhile, polypropylene can withstand maximum temperatures up to 170°C, about 40%-50% higher than polyethylene.
From a heat resistance perspective, polypropylene does not deform at 150°C, so the product has a higher range of applications than polyethylene. For example, for the production of products that need to be disinfected frequently, for use in the microwave or dishwasher. However, PP becomes more brittle at low temperatures, cold resistance is only at -20°C. In contrast, PE has stronger cold resistance, up to -80°C, suitable for frozen food packaging.
Both PP and PE plastics exhibit poor resistance to UV radiation, and are not fire resistant.
Mechanical properties and protection
Polyethylene has low stiffness but is very ductile and resistant to impact. When subjected to strong impact, they will only be stretched or deformed, not broken. While polyethylene is soft, they are available in many forms, low, medium and high density, making them quite versatile and suitable for a wide range of products.
Polypropylene is stiffer, also flexible, and quite durable. The strength of PP allows it to bend multiple times at the same point without cracking or breaking, they will retain their shape despite wear and tear.
The best example of polypropylene's value is through one of its most common uses: thin plastic hinges on bottles of soy sauce, sauces... You can still bend, open and close it repeatedly without breaking, with a range of motion up to 360 degrees.
PP plastic has less expansion or contraction than PE plastic, so PE bags will have the advantage of flexibility, while PP bags will have high formability.
LDPE plastic bags have flexible strength and flexibility
Reaction to chemicals
Polyethylene is water resistant and durable, but offers poor protection against gases, such as carbon dioxide. Chemically, PE resins are not affected by many bases and acids. This material has good resistance to machine oils, especially oil in air compressors. Therefore, they are suitable materials for the manufacture of pneumatic pipes.
Polypropylene has a markedly lower life in the presence of oil. However, it is still the material with the highest chemical resistance rating in the plastics category. At room temperature, polypropylene is resistant to most organic solvents and fats. They can also withstand acids and bases, high electrolytes without cracking or oxidizing. As a result, PP is widely used in packaging for hot and cold liquids, dairy products, laboratory chemicals, detergents and reverse osmosis systems.
PP pipes can also be used as an alternative to metal piping systems. Unlike steel pipes, PP pipes have very impressive corrosion resistance.
Resistance
PP plastic has the ability to be used as a perfect insulator, and is often used as electrical plugs and sockets. The high electrical resistance of polypropylene also opens up a lot of potential applications for electronic components.
PE is also an effective insulator. However, this material is easily electrostatically charged. This property can be compensated by adding an antistatic agent, carbon black or graphite during the manufacturing process.
Optical properties
PP is translucent when uncolored. When comparing plastic bags of the same thickness, it looks like PP bags are clearer than PE bags. But it's not as transparent as acrylic or glass. Meanwhile, polyethylene colors can be almost transparent, milky or opaque. This difference comes from the molecular structure, temperature when extruding, blowing and film thickness. LLDPE is the most optically clear while HDPE is the most opaque.
The transparency of PE plastic is different from that of PP plastic. They still allow light to pass through, but do not facilitate a clear representation of the image.
PP bags look more transparent than PE bags
Recyclability
The advantage of thermoplastic materials, is that they can be easily recycled by shredding, melting and then cooling them into a different shape. This makes thermoplastics a highly valuable and environmentally sustainable material. However, in cases where the melting point of plastics is too great, recycling them is not always the best option. In addition to polypropylene and polyethylene, ABS, PC, PVC and Acrylic are also thermoplastics.
Both polypropylene and polyethylene are very easy to recycle, but in general, polyethylene is recycled more widely, and costs are also lower because it uses less heat.
Cost
There is no fixed price for each type of plastic, because their cost depends on many factors. This type of plastic may cost more or less than another depending on the time. But most of the time, polyethylene tends to be a bit cheaper than polypropylene, due to the lower heat required in the manufacturing process and the abundance of raw materials. However, the market has also recorded many times that the price of PE plastic has increased dramatically more than PP plastic due to high demand, or the price of oil and other products from crude oil has increased, causing petrochemical plants to adjust capacity for each product line. At these times, PP plastic produced from natural gas becomes competitive in price and stable in supply.
The high price of plastic in some times does not change the choice of raw materials of plastic companies. This selection is largely based on performance and suitability for each type of product.
The cost of both PP and PE plastic is generally still very cheap compared to other materials, but this is not a top consideration at all. Importantly, these are materials that have become ubiquitous and are available anywhere, in any quantity. Your plastic materials will always be ready for production, completely without worrying about supply interruptions.
Application of PP and PE plastic in industries
Poly bag
PE is an excellent, low-cost material suitable for many packaging needs. Applications of LDPE in the packaging industry often take advantage of the material's outstanding versatility, such as squeezable sauce bottles, shrink film, poly bags, zipper bags, ... For HDPE, tensile strength high making them suitable for the production of pails, trays, water bottles...
The plasticity and load-carrying capacity also make polyethylene bags outstanding in the industrial packaging industry, making bags for heavy and large items.
Meanwhile, polypropylene is often used to produce straps, adhesive tapes, fashion and jewelry packaging, food containers, ... With its high temperature resistance, PP plastic containers can be used in microwave ovens or sterilizer machines, dishwasher.
Yarn and woven fabric
Due to the excellent tensile strength of HDPE, yarn made from HDPE is the first choice for ropes and sports nets, fishing lines and agricultural uses.
Polypropylene yarn is extremely valuable to the fashion industry, as it is very resilient and resistant to wear and tear, sunlight, mold and bacteria...
Consumer goods
HDPE can be found in many consumer goods such as ice bins, trash cans, and small water tanks. And LDPE is present in children's toy products, especially flexible toys.
PP plastic is common in spoons, forks, dishes and other plastic hand tools and equipment.
Pipes and accessories
One of the most common uses of HDPE is for the production of pipes and fittings. Resistance to chemical decomposition and high durability make HDPE pipes used in many industries such as water supply, wastewater, gas... HDPE pipes are also used to protect electrical wires and telecommunication cables.
Softer and more flexible LDPE material, used for irrigation pipes, simple hoses. LDPE is also used as the sheath material of cables to provide physical protection and insulation.
The insulating ability of PP plastic is even superior to LDPE. They are widely used and work smoothly in electrical sockets, electronic products and applications.
Polypropylene is also a good alternative to traditional metal parts in the automotive industry due to its easy handling in injection molding. Their easy moldability and low thermal expansion make them an excellent choice for interior and exterior parts, fenders, dashboards, and more.
Furniture
Plastic furniture is widely used because of its superior moisture resistance, easy cleaning, compactness and low cost.
The material of plastic furniture is also divided into several types depending on the intended use. Durable PE plastic provides a long shelf life. PP plastic is hard but more susceptible to damage when subjected to strong impact, but is resistant to high temperatures and less prone to stains. In addition to plastic tables and chairs, wardrobes, and kitchen cabinets that have proved their ability to replace traditional wood materials, there are also interior accessories such as wall panels, ceiling tiles, and plastic fonts that are also gradually becoming a trend.
Medican
PP plastic is resistant to microbial growth and resistant to many chemical compounds, resistant to high temperatures when sterilizing. As a result, they are quickly becoming a wonder material for the medical industry. It is present in laboratories and hospitals in many forms, especially in surgical sutures. Po Polypropylene is the ingredient that makes up the synthetic suture marketed as “Prolene”. In addition, PP plastic is also used to produce syringes, bottles, surgical instruments, patient bottles...
During the recent Covid-19 epidemic, Polypropylene also served for essential items such as alcohol spray bottles and medical masks. Their electrostatic charge traps droplets and prevents them from penetrating deeper. Polypropylene is also impermeable to water, so virus and bacteria particles can't use moisture to survive either.
PP fiber is used as an antibacterial layer in medical masks
How to distinguish PP and PE plastic
PP and PE plastic as we have seen, have many similar characteristics, so it is difficult to distinguish. If you are a professional plastic manufacturer or user, you should be able to easily make an immediate guess about the plastic material you see. But there's no guarantee these predictions are completely accurate. The emergence of more and more plastics and their variations, along with new manufacturing techniques, has made it increasingly difficult to distinguish plastics based on experience alone.
There is a small trick that collectors of plastic scrap often use, which is to watch the transformation and scent of plastic when burning them. In the plastic world, each type has its own unique scent and shape when subjected to the impact of a flame. The plastic will melt in drops, or form bubbles, only shrinks and produces dense smoke, or it may not change at all. The flame will also have specific colors, scents or distinctive smokes.
However, burning and smelling scents to distinguish plastics is definitely not recommended, because of the serious effects that can happen to human health. And there is a first rule to remember, is to never burn any material in an enclosed space. The common product of combustion of any material is carbon monoxide (CO gas), which can be deadly easily in an enclosed space.
In the case of PE and PP resins, the combustion reaction occurs quite similarly: a blue flame with a yellow top, molten plastic dripping down like candle wax.
The key difference to be able to distinguish PE and PP plastic through the process of burning, is probably the scent. When burned, PE plastic smells of molten candle wax, while PP has the smell of engine oil. Some types of PP plastic when burned also have a smell of candle wax, but it is more pleasant than the smell of PE plastic. This odor occurs with PP resins that contain both propylene and ethylene monomers in their composition (copolymers), produced by a type of reaction known as copolymerization.
Both PP and PE plastic have a fairly simple structure, consisting of only two elements hydrogen and carbon. Although they are flammable, the combustion process only releases relatively small toxic substances into the environment. If you really want to burn some plastic for some reason, then polyethylene and polypropylene are the safest options. However, this is only a relative theory, because in reality, today's plastic products tend to contain additives with increasingly complex chemical compositions, in order to improve their properties and performance.
Some characteristics when burning to distinguish plastics
