Botp 115 - Chapter 2 PDF

Department of Chemical Engineering Jubail Industrial College Industrial Chemical Processes - (BOTP 115)  CHAPTER 2 – POLYMERS Polyolefins is family of polymers derived from a particular group of base materials known as olefins, are the world’s fastest growing polymer family. Polyolefins...

Department of Chemical Engineering Jubail Industrial College Industrial Chemical Processes - (BOTP 115)  CHAPTER 2 – POLYMERS Polyolefins is family of polymers derived from a particular group of base materials known as olefins, are the world’s fastest growing polymer family. Polyolefins such as polyethylene (PE) and polypropylene (PP) are commodity plastics found in applications varying from house hold items such as grocery bags, containers, carpets, toys and appliances, to high tech products such as engineering plastics, industrial pipes, automotive parts, medial appliances and even prosthetic implants. Ethylene and propylene are monomers for polyethylene and polypropylene respectively. 2 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polymerization A reaction in which polymer chain is formed by combining large number of small molecules called “Monomers”. Polymerization reaction steps: 1) Initiation - The trick to get the reaction started is to use a catalyst, initiator or promoter. 2) Propagation/Growth - The new radical formed in the first step reacts with another monomer molecule to give a new larger radical. This chain growth continue until propagation is terminated. 3) Termination - Mechanism to stop the propagation are Dis-propagation, Recombination and Chain transfer. 3 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polyethylene Polyethylene (PE) has evolved as a major plastic and is obtained by polymerizing ethylene. Three main types of PE plastics are obtained depending on the type of polymerising process are: 1) Low density polyethylene (LDPE). 2) Linear low density polyethylene (LLDPE). 3) High density polyethylene (HDPE). PE is the polymer (poly + monomer) of ethylene molecules. This product is used to make a variety of plastics. Polymerization of ethylene molecules into heavy molecular weight PE is a reaction in which a chain of macromolecule, is produced by the combination of ethylene molecules. Ethylene is a highly reactive monomer that starts combining with other molecules of ethylene in the presence of a catalyst (Ziegler–Nutta catalyst) under a certain pressure and temperature. 4 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS The reaction steps are in three stages, namely, initiation, propagation, and termination. A radical molecule is formed in the presence of the catalyst in the initiation step. The radical then starts combining with the monomers repeatedly in the propagation stage, which continues indefinitely as long as the monomer molecules are available during reaction until quenched at the termination stage. The properties of a polymer vary with the operating pressure, temperature, and time of reaction. The reaction is exothermic and hence it is essential to control temperature by a proper heat removal system. Three classes of PE, namely, LDPE, HDPE, and LLDPE, are produced in different processes. LDPE is produced in a very high-pressure process, and HDPE and LLDPE are obtained in moderately low pressure processes. 5 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polyethylene usage Polyethylene can be processed by general thermoplastics molding methods. They are mainly used to manufacture thin films, containers, pipes, monofilament, wire and cable, daily necessities, etc. and is applicable in television, radar and other high-frequency insulation materials. With the development of petrochemical industry, polyethylene production has been developing rapidly. In recent years, application of polyethylene as a diffusing agent in the field of nuclear physics, astrophysics, reactor operation to measure the amount of neutrons on the nuclear physics was evident. 6 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS PE is a promising synthetic material with great physical and chemical properties. It has high degree of mechanical properties and excellent combination of good dielectric properties. In addition, the molding process is good and the price is low. They are particularly important in following aspects: 1) Electrical insulation: Due to its high stability, moisture resistance and high dielectric properties, it is an excellent material in the making of insulation material in electrical, non-electrical engineering and many other relevant aspects. 2) Anti-corrosive agents: Can be used for anti-corrosive materials such as pipes, lining and so on. 3) Packaging: PE sheet has properties of low-density, soft, water impermeable, high tear strength and chemical resistance. These characteristics are necessary for packaging materials hence PE film has a high market value in the packaging industry and is gradually replacing celluloid. 4) After radiation treatment: PE (i) is hard to deform; (ii) will not produce environmental stress cracking; (iii) has strong elasticity; (iv) has excellent electrical insulation and solvent resistance; (v) has high temperature resistance; (vi) has low power factor. Hence the greater performance after radiation puts it into a wider range of uses. For examples, insulating materials for capacitors and transformers and higher temperature parts in aircraft. However, the cross-linking reaction of PE during radiation results in difficulty to undergo subsequent processing. In addition to the above-mentioned purposes, there are other uses such as various medical equipment, spraying metal, wood, fabric and other materials. HDPE can be used as rubber reinforcing agent. 7 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Properties of Polyethylene PE is a thermoplastic polymer, which can be melted to a liquid and remolded as it returns to a solid state. PE is chemically synthesized from molecules that contain long chains of ethylene monomer. Polyethylene is very sensitive to environmental stress (chemical and mechanical) and has poor heat-aging resistance. The properties of polyethylene vary depending on the molecular structure and density. The products of different densities (0.91 to 0.96 g/cm3) can be obtained by different production methods. 8 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polyethylene physical properties Ethylene is transparent in the film state but is opaque in the presence of massive blocks due to a large number of crystals inside and strong light scattering. The degree of PE crystallization is affected by the number of its branches. The more branches, the more difficult to crystallize. The melting temperature of PE crystal is also affected by the number of branches, distributed in the range from 90 to 130 ℃. PE is a white waxy translucent material which is soft, tough, lighter than water, non- toxic and has excellent dielectric properties. It is flammable and continued to burn after the fire. PE is odorless, non-toxic, low-temperature resistant (minimum temperature down to -100 ℃), chemically stable and resistant to most of the acid-base erosions. 9 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS At room temperature it can be slowly dissolved in some organic solvents due to its linear molecular structures. In addition, PE does not swell and has great electrical insulation performance. However, PE is very sensitive to environmental stress (chemical and mechanical role) and has poor heat resistance. The properties of PE vary depending on its molecular structure and density. 10 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polyethylene chemical properties PE has excellent chemical stability. It is resistant to acidic and basic solution such as hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid, amines, sodium hydroxide and potassium hydroxide at room temperature. However, nitric acid and sulfuric acid have a stronger destructive effect to PE. PE is susceptible to photo-oxidation, thermal oxidation and ozone decomposition. Under the action of ultraviolet light, it is prone to degradation. Carbon on PE has excellent light shielding effect. After irradiation there will be cross- linking, broken chain and formation of unsaturated groups and other reflection. Ethylene is produced by polymerization of a thermoplastic resin. Copolymers of ethylene with a small amount of alphaolefins are also included in the industry. 11 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Features of polyethylene Its corrosion resistance and electrical insulation properties (especially high-frequency insulation) are excellent, but can be easily chlorinated, chemical cross-linked, irradiation cross-linking modified. Glass fiber reinforced with low-pressure polyethylene has lower melting point, great rigidity, hardness and strength, high water absorption, good electrical properties and resistance to radiation. Under higher pressure, it has good flexibility, elongation, impact strength and better permeability. High molecular weight polyethylene impacts strength and fatigue resistance. Ethylene is suitable for making corrosion-resistant parts and insulating parts while high-pressure polyethylene is suitable for making films. Ultra-high molecular weight polyethylene on the other hand is suitable for making shock absorption, wear and transmission parts. 12 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Molding characteristics 1) Crystalline material: small moisture absorption, no need to be fully dried, excellent mobility, pressure sensitive, the use of high-pressure in injection molding, temperature uniformity and great filling speed. 2) Great shrinkage values, prone to deformation warping. Cooling should be slow, a cooling system required. 3) Heating time should not be too long otherwise it will break down. 4) Soft plastic parts have a shallow side of the groove, can be of leakage when forced. 5) May melt and hence not to use with organic solvents to prevent cracking 13 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Low Density Polyethylene (LDPE) High pressure processing using tubular or stirred autoclave reactors yields low density polyethylene. In a tubular reactor, pure liquid ethylene (99.99 %) is mixed with hydrogen peroxide and forced through the tubular reactor at very high pressure (3500 atm), surrounded by a cooling medium to extract heat of polymerization reaction. Oxygen is used with hydrogen peroxide as the initiator.The reaction takes place in solution and the heat of the reaction is given as 3650 kj/kg The temperature in the reactor is controlled above 200°C to avoid crystallization of LDPE, which will otherwise damage the reactor tube. Conversion per pass is about 20 %. 14 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Effluent from the reactor, consisting of the product and the unconverted monomer, is separated by high and low pressure separator vessels. Ethylene is recycled to the reactor. The overall conversion achieved by recycling is about 95%−97%. Finally, the molten LDPE is withdrawn from the low pressure separator and extruded, followed by cooling, drying, and pilling. As tubular reactors are prone to plugging and poor heat transfer problems, a thick- walled, stirred tank vessel reactor is used. High-speed agitation helps in good heat transfer. LDPE is used for making films, sheets, tubes, blocks, insulation, hoses etc. But it cannot be used at temperatures above 80°C because deformation will occur. It has excellent dielectric properties, good elasticity up to −60°C, and it is anticorrosive. It is soft and waxy and is used to make films. 15 Prepared by: Abdulrahman Ali Al Malawi CHAPTER 2 – POLYMERS 16 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS High Density Polyethylene (HDPE) HDPE is manufactured by the suspension polymerization method. In this method, high purity ethylene is introduced into the reactor vessel in which a catalyst (Ziegler-Nutta catalyst, Titanium tetrachloride (TiCl4) in alkyl aluminium) is suspended in benzene at a pressure of 20−35 atm and at a temperature of 60°C−80°C. The ratio of alkyl aluminium and titanium chloride determines the size of the polymer. The greater the ratio, the greater the molecular weight of the polymer. After the reaction, the polymer mixture is separated from ethylene and inerts in a flash drum. The polymer is water washed and filtered to recover the catalyst (water soluble) and reused. However, recovery of the catalyst from the polymer is not complete. Catalyst consumption is about 1 g of titanium (Ti) per 1500 kg of polymer. This is due to the high residual presence of the catalyst within the polymer. The presence of this impurity limits the HDPE’s applications. A modern catalyst has been developed that can yield 6000 kg polymer per gram of Ti, where the contamination of Ti metal in the polymer is a few parts per million. 17 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS HDPE has a rigid and translucent property and is suitable for making electrical goods, bottles, ropes, etc. The main economic advantage of HDPE is that it can be manufactured at much lower pressure as compared to LDPE. HDPE has a higher melting temperature than LDPE and has almost the same glass transition temperature. ❑Process parameters: ✓ Temperature: 80⁰C ✓ Pressure: 2-8 kg/cm2 ✓ Residence Time: 1h ✓ Conversion : 20% ✓ Comonomers: 1-BUTENE OR PROPYLENE ✓ Catalyst: Propriety catalyst ✓ Chain terminator: Hydrogen ✓ Polymerization mode: Gas phase or in liquid phase where hydrocarbon is used as diluent. ✓ Reactor: Autoclave, Fluidized bed ✓ Commercial process: BP Chemicals, Union Carbide, DuPont, Himant (Italy), CX Process, Mitsubishi, CdF Chemie Tech. 18 Prepared by: Abdulrahman Ali Al Malawi CHAPTER 2 – POLYMERS 19 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS UNIPOL Process The process produces low density polyethylene and high density polyethylene using low pressure in gas phase. Wide range of polyethylene is produced using proprietary solid and slurry catalyst. Fluidized bed reactor using proprietary solid and slurry catalyst. Gaseous ethylene, comonomer and catalyst are fed to fluidized bed reactor containing a fluidized bed of growing polymer particles operating at 25 kg/cm2 and 100 ⁰C. Polymer density is easily controlled from 0.915 to 0.97 g/cm. 20 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Linear Low Density Polyethylene (LLDPE) LLDPE is a copolymer of ethylene and 1- butene having linear structure. It is produced by a low pressure fluidized bed process where the temperature and pressure are 100°C and 7−20 atm (0.7−2 MPa), respectively. Gaseous monomers are used to fluidize the polymer granules previously prepared. During polymerization, additional polymers are produced of specific sizes depending on the seed polymers present. Unreacted monomers are separated from the effluent and recycled to the reactor. A long residence time in the range of 3−5 h is required for a reaction. 21 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polypropylene (PP) Polypropylene (PP) is one of the most commonly used thermoplastics in the world. Polypropylene uses range from plastic packaging, plastic parts for machinery and equipment and even fibres and textiles. It is a rigid, semi-crystalline thermoplastic that was first polymerised in 1951 and is used widely today in a range of domestic and industrial applications. PP has the lowest density among commodity plastics. PP has an excellent chemical resistance and can be processed through many converting methods such as injection molding and extrusion. Polypropylene is a polymer prepared catalytically from propylene. Polypropylene is a free-color material with excellent mechanical properties and it is better than polyethylene for the previous reasons. Polypropylene is a downstream petrochemical product that is derived from the olefin monomer propylene 22 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS The polymer is produced through a process of monomer connection called addition polymerization. In this process, heat, high-energy radiation and an initiator or a catalyst are added to combine monomers together. Thus, propylene molecules are polymerized into very long polymer molecules or chains. There are four different routes to enhance the polymerization of any polymer: solution polymerization, suspension polymerization, bulk polymerization and gas-phase polymerization. However, polypropylene properties vary according to process conditions, copolymer components, molecular weight and molecular weight distribution. Polypropylene (PP) is a type of polyolefin that is slightly harder than polyethylene. It is a commodity plastic with low density and high heat resistance. Its chemical formula is (C3H6)n 23 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Properties of Polypropylene 1) Melting Point - The melting point of PP occurs 5) Chemical Resistance at a range. ❑Excellent resistance to diluted and concentrated ❑Homopolymer: 160 - 165°C acids, alcohols and bases. ❑Copolymer: 135 - 159°C ❑Good resistance to aldehydes, esters, aliphatic 2) Density - PP is one of the lightest polymers hydrocarbons, ketones. among all commodity plastics. This feature ❑Limited resistance to aromatic and halogenated makes it a suitable option for hydrocarbons and oxidizing agents. lightweight\weight saving applications. 6) Flammability: PP is a highly flammable material. ❑Homopolymer: 0.904 - 0.908 g/cm3 7) PP retains mechanical & electrical properties at ❑Random Copolymer: 0.904 - 0.908 g/cm3 elevated temperatures. This occurs in humid ❑Impact Copolymer: 0.898 - 0.900 g/cm3 conditions and when submerged in water. It is a water-repellent plastic. 3) It exhibits good resistance to steam 8) It is sensitive to microbial attacks, such as sterilization. bacteria and mold. 4) PP has good resistance to environmental stress cracking. 24 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polypropylene Uses PP finds application in packaging, automotive, consumer good, medical, cast films, etc. Depending on how it is produced and formulated, PP can be: 1) Hard or soft, 2) Opaque or transparent, 3) Light or heavy, 4) Insulating or conductive, 5) Neat or reinforced with cheap mineral fillers, short or long glass fibers, and natural fibers or even self-reinforced. Polypropylene has a slippery, tactile surface, making it ideal for plastic furniture and Low friction applications, such as gears in machinery and vehicles. 25 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS It is highly resistant to chemical corrosion, making it an excellent choice for packaging for cleaning products, bleaches and, and first-aid products. Polypropylene has high insulation properties too, making it safe to use for plastic casing in electrical goods and cables. In its fiber form, PP uses are not limited to not tote bags but also encompass a much wider range of other products, including ropes, twine, tape, carpets, clothing and camping equipment. Its waterproof properties makes it effective for the marine sector. In the automotive industry, PP is also used widely, for example in for battery casings, trays and drink holders, bumpers, interior details, instrumental panels and door trims. The medical world appreciates the waterproof properties of polypropylene too, as well as its flexible strength, resistance to mold, bacteria and chemical corrosion. Some medical applications include syringes, medical vials, Petri dishes, pill containers, and specimen bottles 26 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Types of Polypropylene 1) Homo-polymer PP (HPP): containing only propylene monomer in a semi-crystalline solid form. 2) Random Co-polymers: produced by polymerizing ethene and propene together. Polypropylene containing ethylene as a co-monomer in the PP chains at levels in the range of 1-8% and this is referred to as a random copolymer (RCP). 3) Impact Co-Polymers: containing a co-mixed RCP phase that has an ethylene content of 45-65%. In general, polymers consisting of identical monomers are called homo-polymers where polymeric compounds with more than one type of monomer in their chains are known as co-polymers. 27 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Polypropylene Production It is made from the polymerization of propene monomer. There are two main syntheses to produce polypropylene: 1) Ziegler-Natta polymerization. 2) Metallocene catalysis polymerization. Upon polymerization, PP can form three basic chain structures. They depend on the position of the methyl groups: 1) Atactic (aPP) – Irregular methyl group (CH3) arrangement 2) Isotactic (iPP) – Methyl groups (CH3) arranged on one side of the carbon chain 3) Syndiotactic (sPP) – Alternating methyl group (CH3) arrangement 28 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Process Technology for PP Polypropylene polymerization process have undergone a number of revolutionary changes since the production of crystalline polypropylene were commercialised in 1957 by Motecatini in Italy and Hercules in U.S. Commercial polypropylene processes are based on low pressure processes using Ziegler-Natta catalyst that produces a product with an isotactic content of 90% or more. PP is manufactured by catalytic reaction in a stirred tank reactor, where Titanium and aluminium halides are used as catalysts at a temperature of 60°C−70°C and a pressure of 1−2 MPa. An unreacted monomer is recycled after it is separated from the catalyst and polymer mixture in a flash chamber under vigorous stirring conditions. The mixture of polymer and catalyst is then passed to a centrifugal separator where a catalyst and polypropylene polymer is recovered. Further processing of the spent catalyst in the presence of alcohol is carried out to recover the active components of the catalyst for its reuse. 29 Prepared by: Abdulrahman Ali Al Malawi CHAPTER 2 – POLYMERS 30 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS UNIPOL Process for PP The process produces homopolymer, random copolymer and impact copolymer polypropylene. Polymerization takes place in a fluidized bed reactor using slurry reactor (TiCl4 supported on MgCl2 in slurry form in mineral oil. Co-catalyst TEAL, purified propylene and ethylene in case of random PP), purified H2 and selectivity control agent is continuously fed to the reactor. Temperature 35 oC and pressure 33 kg /cm2 is maintained in the reactor. Spheripol Process The Spheripol process is a modular technology consisting of three main process steps – catalyst and raw material feeding, polymerization and finishing. The catalyst, liquid propylene and hydrogen for molecular weight control are continuously fed into the loop reactor. The bulk polymerization typically occurs in two tubular loop reactors filled with liquid propylene and optional gas-phase copolymerization reactors. The finishing section consists of highly efficient liquid propylene vaporization operations at very high polypropylene concentrations, separation of the unconverted monomers, and complete recycling of the monomers back to the reactor. 31 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS This process can produce a broad range of polypropylene polymers including homopolymer PP, random copolymers and terpolymers, heterophasic impact, speciality impact as well as high stiffness copolymers. In this process, homopolymer and copolymer polymerization takes place in liquid propylene within the loop reactor. Heterophasic impact copolymerization is achieved by adding a gas phase reactor. 32 Prepared by: Abdulrahman Ali Al Malawi  CHAPTER 2 – POLYMERS Borstar PP Process The Borstar PP process developed by Borealis is based on a double loop reactor and a fluidized-bed reactor in tandem connection for the production of HPP and RPP. The loop reactor is operated under supercritical conditions with increased comonomer and hydrogen concentration, and higher temperature avoiding gas bubbles. For producing the impact PP, additional one or two fluidized-bed reactors are connected in series. Using the HY/HS catalysts of Borealis, which are prepared from emulsion strategy from two-phase liquid/liquid contact, Borstar PP technology produces high performance HPP and RPP with very low ash content and narrow molecular weight distribution in the supercritical loop reactor due to excellent heat removal and high productivity. The rubber phase of the impact copolymers is typically generated through the 3rd and 4th gas-phase reactors. Using the multi-reactor processes described, it is feasible and practical to further broaden PP material properties meeting up the ever-demanding customer needs. 33 Prepared by: Abdulrahman Ali Al Malawi CHAPTER 2 – POLYMERS (a) prepolymerizer; (b) loop reactor; (c) first gas-phase reactor; (d) second gas-phase reactor; (e) third gasphase reactor; (f) coolers; (g) separators; (h) low-pressure degasser; (i) dryer; (j) purge bin 34 Prepared by: Abdulrahman Ali Al Malawi