Polypropylene Totally Explained

Everything about Polypropylene totally explained

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Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles (for example, ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. An addition polymer made from the monomer propylene, it's rugged and unusually resistant to many chemical solvents, bases and acids. Its resin identification code is . Melt processing of polypropylene can be achieved via extrusion and molding.
   Common extrusion methods include production of melt blown and spun bond fibers to form long rolls for future conversion into a wide range of useful products such as face masks, filters, diapers and wipes.
   The most common shaping technique is injection molding, which is used for parts such as cups, cutlery, vials, caps, containers, housewares and automotive parts such as batteries. The related techniques of blow molding and injection-stretch blow molding are also used, which involve both extrusion and molding.
   The large number of end use applications for PP are often possible because of the ability to tailor grades with specific molecular properties and additives during its manufacture. For example, antistatic additives can be added to help PP surfaces resist dust and dirt. Many physical finishing techniques can also be used on PP, such as machining. Surface treatments can be applied to PP parts in order to promote adhesion of printing ink and paints.

Chemical and physical properties

Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low density polyethylene (LDPE) and high density polyethylene (HDPE); its Young's modulus is also intermediate. Through the incorporation of rubber particles, PP can be made both tough and flexible, even at low temperatures. This allows polypropylene to be used as a replacement for engineering plastics, such as ABS. Polypropylene is rugged, often somewhat stiffer than some other plastics, reasonably economical, and can be made translucent when uncolored but isn't as readily made transparent as polystyrene, acrylic or certain other plastics. It can also be made opaque and/or have many kinds of colors through the use of pigments. Polypropylene has very good resistance to fatigue, so that most plastic living hinges, such as those on flip-top bottles, are made from this material. Very thin sheets of polypropylene are used as a dielectric within certain high performance pulse and low loss RF capacitors.
   Polypropylene has a typical peak melting point of ~160°C (320°F), as determined by Differential Scanning Calorimetry (DSC). Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an autoclave. Food containers made from it won't melt in the dishwasher, and don't melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminium foil (both heat-resistant materials). After the product has cooled, the tubs are often given lids made of a less heat-resistant material, such as LDPE or polystyrene. Such containers provide a good hands-on example of the difference in modulus, since the rubbery (softer, more flexible) feeling of LDPE with respect to PP of the same thickness is readily apparent. Rugged, translucent, reusable plastic containers made in a wide variety of shapes and sizes for consumers from various companies such as Rubbermaid and Sterilite are commonly made of polypropylene, although the lids are often made of somewhat more flexible LDPE so they can snap on to the container to close it. Polypropylene can also be made into disposable bottles to contain liquid, powdered or similar consumer products, although HDPE and polyethylene terephthalate are commonly also used to make bottles. Plastic pails, car batteries, wastebaskets, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature.
   The MFR (Melt Flow Rate) or MFI (Melt Flow Index) is an indication of PP's molecular weight. This helps to determine how easily the melted raw material will flow during processing. Higher MFR PPs fill the plastic mold more easily during the injection or blow molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease.
   There are three general types of PP: homopolymer, random copolymer and impact or block copolymer. The comonomer used is typically ethylene. Ethylene-propylene rubber added to PP homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to PP homopolymer decreases the polymer crystallinity and makes the polymer more transparent.

Degradation

The polymer is however, liable to chain degradation from exposure to UV radiation such as that present in sunlight. For external applications, UV-absorbing additives must be used. Carbon black also provides some protection from UV attack. The polymer can also be oxidised at high temperatures, a common problem during moulding operations. Anti-oxidants are normally added to prevent polymer degradation.

Synthesis

An important concept in understanding the link between the structure of polypropylene and its properties is tacticity. The relative orientation of each methyl group (CH₃ in the figure at left) relative to the methyl groups on neighboring monomers has a strong effect on the finished polymer's ability to form crystals, because each methyl group takes up space and constrains backbone bending.
   Like most other vinyl polymers, useful polypropylene can't be made by radical polymerization due to the higher reactivity of the allylic hydrogen (leading to dimerization) during polymerization. Moreover, the material that would result from such a process would have methyl groups arranged randomly, so called atactic PP. The lack of long-range order prevents any crystallinity in such a material, giving an amorphous material with very little strength and only specialized qualities suitable for niche end uses.
   A Ziegler-Natta catalyst is able to limit incoming monomers to a specific orientation, only adding them to the polymer chain if they face the right direction. Most commercially available polypropylene is made with such Ziegler-Natta catalysts, which produce mostly isotactic polypropylene (the upper chain in the figure above). With the methyl group consistently on one side, such molecules tend to coil into a helical shape; these helices then line up next to one another to form the crystals that give commercial polypropylene many of its desirable properties.
   More precisely engineered Kaminsky catalysts have been made, which offer a much greater level of control. Based on metallocene molecules, these catalysts use organic groups to control the monomers being added, so that a proper choice of catalyst can produce isotactic, syndiotactic, or atactic polypropylene, or even a combination of these. Aside from this qualitative control, they allow better quantitative control, with a much greater ratio of the desired tacticity than previous Ziegler-Natta techniques. They also produce narrower molecular weight distributions than traditional Ziegler-Natta catalysts, which can further improve properties.
   To produce a rubbery polypropylene, a catalyst can be made which yields isotactic polypropylene, but with the organic groups that influence tacticity held in place by a relatively weak bond. After the catalyst has produced a short length of polymer which is capable of crystallization, light of the proper frequency is used to break this weak bond, and remove the selectivity of the catalyst so that the remaining length of the chain is atactic. The result is a mostly amorphous material with small crystals embedded in it. Since each chain has one end in a crystal but most of its length in the soft, amorphous bulk, the crystalline regions serve the same purpose as vulcanization.

Mechanism of metallocene catalysts

The reaction of many metallocene catalysts requires a co catalyst for activation. One of the most common co catalysts for this purpose is Methylalmuinoxane (MAO). Other co catalysts include, Al(C₂H₅)₃.There are numerous metallocene catalysts that can be used for propylene polymerization. (Some metallocene catalysts are used for industrial process, while others are not, due to their high cost.) One of the simplest is Cp₂MCl₂ (M = Zr, Hf). Different catalyst can lead to polymers with different molecular weights and properties. Active research is still being conducted on metallocene catalyst.
In the mechanism the metallocene catalyst first reacts with the co catalyst. If MAO is the co catalyst, the first step is to replace one of the Cl atoms on the catalyst with a methyl group from the MAO. The methyl group on the MAO is replaced by the Cl from the catalyst. The MAO then removes another Cl from the catalyst. This makes the catalyst positively charged and susceptible to attack from propylene.
Once the catalyst is activated, the double bond on the propene coordinates with the metal of the catalyst. The methyl group on the catalyst then migrates to the propene, and the double bond is broken. This starts the polymerization. Once the methyl migrates the positively charged catalyst is reformed and another propene can coordinate to the metal. The second propene coordinates and the carbon chain that was formed migrates to the propene. The process of coordination and migration continues and a polymer chain is grown off of the metallocene catalyst.

History

Polypropylene was first polymerized on March 11 1954 by Giulio Natta. At first it was thought that it would be cheaper than polyethylene.

Practical applications

A common application for polypropylene is as Biaxially Oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear bags. When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products.
   In New Zealand and also in the US military, polypropylene, or 'polypro' (New Zealand 'polyprops'), is the material used for the fabrication of cold-weather gear, such as a long-sleeve shirt or long underwear, in addition to warm-weather gear such as Under Armour clothing, which can easily wick away sweat. These polypro clothes are not easily flammable, however, they can melt, which may result in severe burns if the service member is involved in an explosion or fire of any kind. Polypropylene is also used as an alternative to polyvinyl chloride (PVC) as insulation for electrical cables for LSZH cable in low-ventilation environments, primarily tunnels. This is because it emits less smoke and no toxic halogens, which may lead to production of acid in high temperature conditions.
   Polypropylene is also used in particular roofing membranes as the waterproofing top layer of single ply systems as opposed to modified bit systems. Its most common medical use is in the synthetic, nonabsorbable suture Prolene, manufactured by Ethicon Inc. Polypropylene is most commonly used for plastic moldings where it's injected into a mold while molten, forming complex shapes at relatively low cost and high volume, examples include bottle tops, bottles and fittings.
   Recently it has been produced in sheet form and this has been widely used for the production of stationary folders, packaging and storage boxes. The wide colour range, durability and resistance to dirt make it ideal as a protective cover for papers and other materials. It is used in Rubik's cube stickers because of these characteristics.
   The availability of sheet polypropylene has provided an opportunity for the use of the material by designers. The light weight, durable and colourful plastic makes an ideal medium for the creation of light shades and a number of designs have been developed using interlocking sections to create elaborate designs.
   Polypropylene sheets are a popular choice for trading card collectors; these come with pockets (nine for standard size cards) for the cards to be inserted and are used to protect their condition and are meant to be stored in a binder.
   Polypropylene has been used in hernia repair operations to protect the body from new hernias in the same location. A small patch of the material is placed over the spot of the hernia, below the skin, and is painless and is rarely, if ever, rejected by the body.
   The material has recently been introduced into the fashion industry through the work of designers such as Anoush Waddington who have developed specialized techniques to create jewellery and wearable items from polypropylene.
   Expanded Polypropylene: EPP

Future Applications

Recent development of a specialty extruded polypropylene called Innegra S" has produced a high modulus thread that's 66% lighter than comparably deniered fiberglass. Dr. Brian Morin patented a process that creates a high modulus fiber that has potential in specialty applications ranging from marine ropes, marine hulls, kayak paddles, aircraft pallets, ballistic applications and other lightweight structral applications.

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