Iran’s petrochemical industry was established in late 1950s. Availability of enormous hydrocarbon reserves and also having an exemplary domestic market, the industry rapidly developed in the ensuing years. Since its inception, the industry has travelled a long and challenging road from its humble origins to become a significant source in the global petrochemical market. The genesis of petrochemical industry in Iran dates back 1964, when the National Petrochemical Company (NPC) was set up to plan for the development of this high-potential industry.
JET Company is a prominent manufacturer, supplier and exporter of several petrochemical products in the Middle East. Having had an extended existence in international markets, JET Company is able to present its services in all aspects of this business, from consultation to exporting. Polypropylene is one of the JET’s specialties which makes us distinguished from our rivals, as we are able to find the most proper product for our customers among plethora of products promptly.
What is Polypropylene?
Polypropylene is a semi-crystalline polymer which is used vastly due to its unique mixture of attributes, cost and ease of manufacturing. All grades consist of polymer, a neutralizer and antioxidants. Other additives like clarifiers, nucleators, slip additives, UV stabilizers, silica, talc, and calcium carbonate are added to impart specific functionality. The polymer may be a pure homopolymer made by polymerizing propylene, a random copolymer made from propylene and another monomer (like ethylene), or an impact copolymer made by dispersing rubber in polypropylene matrix.
Polypropylene can be processed by a diversity of fabrication methods like film/sheet extrusion, multifilament, non-wovens, injection molding, blow molding and profile extrusion.
Different Grades of Polypropylene
Homopolymer PP is a polypropylene material which is used the most in the HPP, RCP, and ICP family of products. It is produced in several different reactor design using catalysts that link the monomers together in a stereospecific manner, resulting in polymer chains that are crystallizable. Whether they crystallize and to what extent depends on the conditions under which the entangled mass of polymer chains transitions from the melt to the solid state or how a heat-softened solid PP material is strained during a further fabrication procedure like fiber drawing.
Homopolymer PP is a two-phase system since it contains both crystalline and noncrystalline regions. The noncrystalline, or amorphous, regions are composed of both isotactic PP and atactic PP. The isotactic PP in the amorphous regions is crystallizable, and it will crystallize slowly over time up to the limit that entanglement will allow. The extent of crystallization after the initial fabrication step of converting PP pellets or powder to a molded article will slowly increase over time, as will the stiffness. A widely accepted model of HPP morphology likens the solid structure to a system consisting of pieces of stiff cardboard linked together by strands of softer material. In the areas represented by flat pieces of cardboard, PP polymer chains weave up and down into close-packed arrays called crystallites (‘‘little crystals’’), which are called lamella by morphologists. The soft strands linking the pieces of stiff cardboard are polymer chains that exit one crystallite, enter another, and then begin weaving up and down in another crystallite. The crystallizability of the chains is one factor that determines how thick the crystallites will be, and the thickness of the crystallites determines how much heat energy is required to melt them (the melting temperature). A typical HPP has an array of crystallites from thick ones to very thin ones, and these manifest themselves as an array of melting points. Homopolymer PP is marketed mainly by melt flow rate (MFR) and additive formulation into fiber, film, sheet, and injection molding applications. Melt flow rate is an indicator of the weight-average molecular weight as measured by the ASTM or ISO MFR test method.
Random copolymers are ethylene=propylene copolymers that are produced in a single reactor by copolymerizing propylene and small amounts of ethylene (usually 7% and lower). The copolymerized ethylene changes the properties of the polymer chains significantly and results in thermoplastic products that are sold into markets in which slightly better impact properties, improved clarity, decreased haze, decreased melting point, or enhanced flexibility are required. The ethylene monomer in the PP chain manifests itself as a defect in the chain regularity, thus inhibiting the chain’s crystallizability. As the ethylene content increases, the crystallite thickness gradually decreases, and this manifests itself in a lower melting point. The amount of ethylene incorporated into the chain is usually dictated by the balance between thermal, optical, and mechanical properties.
Impact copolymers are physical mixtures of HPP and RCP, with the overall mixture having ethylene contents on the order of (6–15% wt%. These are sold into markets where enhanced impact resistance is needed at low temperatures, especially freezer temperature and below. The RCP part of the mixture is designed to have ethylene contents on the order of 40–65% ethylene and is termed the rubber phase. The rubber phase can be mechanically blended into the ICP by mixing rubber and HPP in an extruder, or it can be polymerized in situ in a two-reactor system. The HPP is made in the first reactor and the HPP with active catalyst still in it is conveyed to a second reactor where a mixture of ethylene and propylene monomer is polymerized in the voids and interstices of the HPP polymer powder particle. The amount of rubber phase that is blended into the HPP by mechanical or reactor methods is determined by the level of impact resistance needed. The impact resistance of the ICP product is determined not only by its rubber content but also by the size, shape, and distribution of the rubber particles throughout the ICP product.
Reactor products usually give better impact resistance at a given rubber level for this reason. As the rubber content of the ICP product is increased, so is the impact resistance, but this is at the expense of the stiffness (flexural modulus) of the product. Consequently, polymer scientists often describe a product as having a certain impact–stiffness balance. The stiffness of the ICP product is dictated by the stiffness of the HPP phase and the volume of rubber at a given rubber size distribution in the product. The impact resistance is dictated by the amount and distribution of the rubber phase in the ICP product.
The process technology for PP manufacture has kept pace with catalyst advances and the development of new product applications and markets. In particular, the relationship between process and catalyst technology was vividly symbiotic and that of a partnership. Advances in one technology had always exerted a strong push–pull effect on the other to improve its performance. The progress in process technology has resulted in process simplification, reduction in investment and manufacturing cost, improvement in plant operability, constructability, and broader process capabilities to produce a wider product mix.
Solvent polymerization process
Since the PP particles are dispersed in the form of a slurry in the solvent with the solvent polymerization process, this is also called the slurry polymerization process, and it was a representative manufacturing process that was the main current in the first generation.
Fig 1. Schematic flow diagram of Sumitomo’s solvent polymerization process
Fig. 1 shows the first-generation solvent polymerization process developed by Sumitomo Chemical. The Sumitomo Chemical solvent polymerization process was one that initially introduced technology from the Italian company Montecatini, which was the first in the world to industrialize PP, but subsequently Sumitomo Chemical made a large number of technical improvements of its own and licensed them to a number of companies. Solvent polymerization used an autoclave provided with an agitator for the reactor, and the conditions are a temperature of 50 to 80°C and pressure of approximately 1 MPa. It is carried out in the presence of hexane, heptane or another inert hydrocarbon solvent where polymerization inhibitors have been eliminated. In the first generation, PP particles were obtained after going through separation and recovery of unreacted propylene, deashing (decomposition and elimination of the catalyst using alcohol), washing in water, centrifugal separation and drying for the aftertreatment processes. In addition, a process for separating the AP, which was produced as a secondary product at 10% of the amount polymerized was necessary at one time, and therefore, the AP was separated using its solubility in the polymerization solvent. Not only was this process complicated, but also the cost burden was large because of the separation and purification of the particularly large amount of alcohol and water used in deashing from the solvent that was recovered. Subsequently, in the second generation, the deashing process was omitted because of improvements in catalyst activity, and the large amounts of alcohol and water became unnecessary.
While the process was simplified in this manner, the omission of the process for eliminating secondary AP had to wait for the advent of superior catalysts that gave a high level of stereoregularity that made possible a reduction in the proportion of secondary AP generated in addition to increasing the polymerization activity.
Bulk polymerization process
The bulk polymerization process is also called the mass polymerization process, and solvents such as hexane and heptane are not used. It carries out polymerization in liquefied propylene. It aims at simplifying the process by also using the propylene monomer, which is the raw material, as the solvent.
Since no solvent other than the liquefied propylene is used, the energy costs for the steam, electricity, etc., required for recovering the solvent may be greatly reduced. The bulk polymerization process is a process that is representative of the second generation, but it coexisted with the first generation, and even now when the third generation is the main current, there are times when it is advantageous for the manufacture of propylene homopolymers. It plays a part in a variety of commercial process groups Fig. 2 shows the second generation bulk polymerization process developed by Sumitomo Chemical, but if it is compared with the first generation solvent polymerization process (Fig. 1), we can see that it has been made much simpler. This Sumitomo Chemical process has been licensed to several companies, and has been evaluated highly.
Fig 2. Schematic flow diagram of Sumitomo’s bulk polymerization process
It is characterized by the use of a continuous extraction tower that was developed by Sumitomo Chemical and has a special internal structure.5) Furthermore, in addition to using a high performance catalyst that Sumitomo Chemical developed itself, we were successful in being the first in the world to greatly simplify deashing and the secondary AP elimination process by providing a countercurrent washing system that uses refined liquefied propylene.
The typical operating conditions for the bulk polymerization process are a temperature of 50 to 80°C and a pressure that is roughly the vapor pressure of propylene. It changes according to the temperature, but is in a range of 2 to 4 MPa. Since liquefied propylene, which is a monomer, is used for the solvent, the polymerization reaction is rapid, and the retention time is shortened.
Since the volumetric efficiency is greatly improved, the reactor size for obtaining the same production capacity
can be smaller than it was conventionally. However, even though there is high productivity, the heat elimination
surface area is insufficient for removing the polymerization heat if the size of the device is reduced.
Therefore, in the case of a stirred tank reactor, a special external heat exchanger that implements measures for
preventing adherence of the polymer is used. On the other hand, loop reactors where the surface area for heat elimination can be increased relative to the reaction volume have become practical.
The bulk polymerization process is a process with many advantages like these, but it is not suitable for the manufacture of the polymers known as impact copolymers.
Impact copolymers are a mixture of a propylene homopolymer component with a comparatively low molecular
weight and a rubber component, which is an ethylene- propylene copolymer with a comparatively high molecular weight. This has improved impact strength at low temperatures while at the same time maintaining
the rigidity, which is one of the superior original characteristics of PP, as much as possible. It is mainly used in
injection molding applications starting with automobile components. Industrially, it is obtained by polymerizing the latter following the polymerization of the former, and during continuous production, individual reactors are required for polymerization of each of the components. To polymerize the rubber component, the reaction composition must have a high ethylene concentration, but if ethylene is dissolved in the liquefied propylene to the point of obtaining the required ethylene concentration with bulk polymerization, the overall reaction pressure increases, so there have been almost no practical implementations. In addition, since the rubber component is dissolved in the liquefied propylene, there is a problem with the limitations for polymerization of the rubber component.
Vapor phase polymerization process
The vapor phase polymerization process falls under the category of bulk (mass) polymerization processes carried out only with monomers in the broad sense, but since polymerization is carried out in propylene gas rather than in liquefied propylene, it is handled as a process different from conventional bulk polymerization.
It is positioned as a third-generation process, but the history is longer than expected, and the technology already existed when first generation processes were the main current. The vapor phase polymerization at that time was inferior in terms of quality because there was no process for separating the very many AP secondary
products, and products were limited to special applications.
However, with the subsequent complete elimination of deashing and AP removal operations because of the rapid improvement in catalyst performance, further simplifications were achieved in the process, and it achieved a position as the third generation process capable of manufacturing high performance products with
diverse levels of quality. Fig. 3 shows the initial thirdgeneration vapor phase polymerization process developed
by Sumitomo Chemical for manufacturing impact copolymers.6) Manufacturing impact copolymers requires at least two reactors, and a supply line for ethylene, which is a comonomer, is installed for the second stage reactor so that the rubber component can be polymerized.
Fig 3. Schematic flow diagram of Sumitomo’s vapor phase polymerization process
Moreover, manufacturing is fundamentally possible with one reactor for polymers other than impact copolymers. The typical operating conditions are a temperature of 50 to 80°C and a pressure in the range of 1 to 2 MPa. Various types of reactors, such as stirred tanks and fluidized beds, have been developed by various companies, but while there are small differences in construction costs and variable costs, these are not determiners of the differences in the final product costs. The competition between makers can be said to be mainly in the area of product quality.7) Along with the further improvements to the process, Sumitomo Chemical has made great strides on the quality front by commercializing a variety of polymer designs based on our own high performance catalyst technology.
Fibers and Fabrics
Fibers are produced by various kinds of extrusion processes. Fibers include slit film or slit tape. The advantages offered by PP include low specific gravity which means greater bulk per given weight, strength, chemical resistance and stain resistance. There are different applications for fibers like slit film, staple fibers, nonwoven fabrics and monofilaments. Slit film is a wide web extruded film. The major application of slit film is in carpet backings. Nowadays, carpet backings are produced from PP more than from natural jute fibers. The reason is that jute fibers damage faster than PP fibers in high moisture weather. High humidity weather will allow higher absorption of water that yields to mold attack. Slit film applications are twine, woven fabrics for feed and fertilizer sacks, sand bags and bulk container bags, tarpaulins, mats, screens for erosion prevention and geotextiles to stabilize soil beds. More conventional fibers than slit-film fibers are known as continuous filament fibers and they result from extrusion. Staple fibers are short fibers ranging from less than an inch to a little less than a foot in length depending on the application. Nonwoven fabrics are the most common single fiber application for PP usage. There are three types of nonwoven fabrics: thermo-bonded, spun-bonded and melt-blown. The fabrics of each differ from another in properties and appearance. For example, spun-bonded fabrics are strong whereas melt-blown fabrics are soft. However, this type of fabrics often used by combinations of two types together. The fibers formed in the melt-blowing process are very fine and allow for the production of lightweight uniform fabrics that are soft and not strong. Fabrics from fine melt-blown fibers are utilized in medical applications because they allow the passage of water vapor but prevent the penetration of liquid water and aqueous solutions. Monofilaments are produced by extruding PP through a plate containing many small holes and then monofilaments are quenched into a water bath which cools the fabrics. Twisting bundles of monofilament together gives us applications like rope, twine and fishing nets that are strong and moisture resistant; making them ideal in marine applications, Figure 4.
Figure 4. Monofilaments extrusion process
Strapping is similar to slit film but fabrics here are thicker approximately in the order of 20 mils. Fibers are produced either from a direct extrusion or from slit sheets. Applications include securing large packages, boxes or to hold stacks together. The most important property of strapping fibers is strength where, sometimes, fabrics can replace steel.
An extrusion process of PP produces films. Film is less than 10 mils thick. The film uses embrace food products, tobacco and clothing. There are two broad classes of films: cast films and oriented films. Cast films are manufactured by depositing a layer of liquid plastic onto a surface and stabilizing this form by allowing melt to cool or by evaporation of solvent. Film thickness is usually in the range of 1-4 mils. An important feature of cast films is softness. Both homo-polymers and random copolymers are used in cast films. Cast films are converted to products like bags, pages, sheet protectors, tapes and pressure-sensitive labels. Bi-axially oriented polypropylene film (BOPP) is another film type that is produced by extruding the plastic through a circular die followed by expansion cooling. Two methods are widely used for producing BOPP films: tenter process (film thickness 0.5-2.5 mils) and tubular process (film thickness 0.25-2 mils). BOPP films have excellent clarity and gloss properties. They are printable when using some additional surface treatment technology. The main applications for BOPP films are in flexible packaging where the major use is in snack food packaging. BOPP film provides resistance to moisture vapor to keep snacks crisp and fresh tasting and provides a heat-sealable layer. Also, BOPP films are used in packaging of bakery products and many adhesive tapes. An opaque film is a special kind of BOPP which is used in packaging products such as candy, chocolate bars, soaps and labels on soft drink bottles. Moreover, BOPP films are used in electrical applications to store energy.
Sheet / Thermoforming
Thermoforming process involves heating of a thermoplastic sheet to its softening point followed by forming of the softened sheet into a desired shape by mechanical means and finally solidification into the desired shape. Extrusion process produces a sheet that is greater than 10 mils in thickness and typical thickness is about 40 mils. Sheet width is usually between 2-7 ft. Sheets are used in the production of thermoformed containers for rigid packaging applications. Differences between PP and polystyrene in producing rigid packages perceived by noticing that PP resins resist the stress cracking in the presence of fatty products whereas polystyrene resins do not. PP is tougher and more rigid than polystyrene. Also, PP has lower specific gravity than polystyrene which allows us to manufacture lighter weight containers at the same thickness and shape.
In this process, granules of polymer are heated until melting. Then, the molten material is injected into a closed mould. The mould normally consists of two halves which are held together under pressure to overcome the force of the melt. After that, the injected material is allowed to cool and solidify in the mould. The two halves of the mould are then opened and the molding is become out, Figure 5. Usually, mould shapes have very complex geometries that provide designers with infinite number of design possibilities. Thin-wall injection molding is used for rigid packaging container applications. The thickness does not usually exceed 25 mils and often less than that. Rigid packaging containers are used in consumers’ items such as food storage containers and water bottles. Water bottles have many shapes and can be round, square, flat or tall. Injection molding is the best choice for producing containers with verities of shapes easily. Housewares applications include storage systems, toys, sports equipment, paintbrushes and garden furniture. Screw caps for bottles and jars are some examples of closures applications produced from PP. Further, injection molding includes some appliances and hand tools applications like coffee makers, can openers, blenders and mixers as well as different medical applications such as disposable syringes.
Figure 5. Injection molding process
The basic principle of blow molding process is to produce a hollow object by blowing a thermoplastic with hot air. A heated thermoplastic hollow tube is known as parison is placed inside a closed mould before blowing. The parison takes the shape of the mould, after blowing, and retains the shape upon leaving it. Bottles and jars are the main products of blow molding process. There are three types of blow molding: extrusion blow molding in which produced PP bottles have hot-filling capability and good contact clarity, injection blow molding to produce relatively small bottles and wide-mouth jars and injection stretch blow molding to produce bi-axially oriented jars and bottles with greater clarity, strength and barrier properties. Typical applications are water bottles, shampoo bottles and lubricant/pesticides containers.
Polypropylene has a large presence in vehicles. For example, one of the original uses is in battery cases and AC ducts. Since PP is considered as the lightest thermoplastic due to its low density, 0.9 g/mL, much of the plastics in new cars are PP because car companies tend to reduce the overall weight of their cars to save some gas expenses for customers. Also, interior trim and several exterior components are made completely of PP or PP compounds. Interior trim like doors, pillars, quarter panels, and consoles are all molded of PP. Weight reduction has been an important factor so PP became a major material for automotive exterior parts. A special known material that is produced from PP called thermoplastic olefin (TPO) is used in car bumpers. TPO is also used in air dams, body side claddings, rocker panels and even grills in some vehicles. A study in 2005 showed that the global consumption of polypropylene by end use application is primarily for injection molding and fibers, Figure 6.
Figure 6. Global consumption of polypropylene by end use application
Polypropylene has many other applications associated with plastics in medical or laboratory tools, plastic tubs, plastic containers, wastebaskets, pharmacy prescription bottles, cooler containers, dishes, pitchers, rugs, insulation for electrical cables, stationery folders, storage boxes, light shades, loudspeaker drive units and water filters or air-conditioning-type filters. Furthermore, PP is used to produce clothes or even products related to clothing like diapers or sanitary products where PP is treated to absorb water (hydrophilic) rather than naturally repelling water (hydrophobic). PP is perfect for fabrication of cold-weather base layers and under-armor clothing. Another interesting PP application is called polypropylene sheet foam, Table 1.
Table 1. Applications of Polypropylene Sheet Foam