Introduction to Polypropylene by gregoria

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                 Introduction to Polypropylene
                      INTRODUCTION                                   A slurry reactor is usually a large diameter
                 Polypropylene (PP) resins are a general class       pipe in the form of a loop that is filled with a
                 of thermoplastics produced from propylene           non-reactive diluent such as isobutane.
                 gas. Propylene gas is derived from the              Propylene and catalyst are fed into the
                 cracking of natural gas feedstocks or               reactor. As the PP is produced, it forms into
                 petroleum by-products. Under broad ranges           small "crumbs" which are insoluble in the
                 of pressures and temperatures, propylene            diluent and circulate as a slurry. A portion of
                 generally polymerizes to form very long             this slurry is continuously removed and the
                 polymer chains. However, to make                    PP crumbs are separated, dried and
                 polypropylene resins with controlled                conveyed to the extrusion area.
                 configurations of molecules (tacticity) at
                 reasonably acceptable commercial rates,             A "bulk" system operates in a similar fashion
                 special catalysts are required.                     except liquefied propylene is used as the
                                                                     diluent for the slurry. The use of propylene
                 By utilizing techniques such as multiple            is advantageous because there is no need to
                 reactor configurations, polymerizing other          separate any unreacted propylene from the
                 gases such as ethylene or butene in                 diluent.
                 conjunction with propylene to form
                 copolymers, using special additives to control      In the gas phase process, no liquid is used
                 crystallinity, etc., different types of PP resins   and the reaction takes place in a fluidized or
                 can be produced. The ability to produce so          mechanically agitated bed of polypropylene
                 many variations of a basic material permits         powder. All liquid separation and treating
                 the manufacturer to tailor PP resins for            steps are essentially eliminated in this
                 diverse applications, such as packaging films;      process.
                 clear, rigid food containers; appliance parts;
                 automotive bumper fascia and batteries;                 POLYPROPYLENE HOMOPOLYMERS
                 medical syringes; etc. Following is a review of     Only propylene is used in the reaction
                 the four basic categories of PP resins, how         producing PP homopolymers and usually
                 they are produced, and their key properties.        they are made on a single reactor system.
                                                                     PP homopolymers exhibit high stiffness,
                     POLYMERIZATION OF PP RESINS                     high temperature resistance and excellent
                 PP resins are generally produced in large,          chemical resistance. End products usually
                 continuous reactor systems operating in a           have a hard surface that resists scratching
                 slurry, “bulk" or gas phase mode. In all cases,     and staining.
                 the resultant PP products are pelletized on
                 compounding extruders where additives such
                 as antioxidants, processing aids, etc. are
                 incorporated.




                                                                                           (Continued on Page 2)



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                 Introduction to Polypropylene                                   (continued)


                PP homopolymers are available in a wide          in the polymer are reduced and clarity, gloss
                range of melt flow rates and are used to         and stiffness are enhanced. Clarified
                produce, for example, oriented film,             random copolymers are commonly used for
                thermoformed deli containers, appliance          food storage containers and bottles.
                parts, caps and closures, monofilaments,         Although not as stiff as homopolymers,
                non-woven cloth and carpet fibers. PP            random copolymers can still be used for
                homopolymers do not have good impact             “hot fill” applications.
                strength at low temperatures, so
                homopolymers are not generally suitable for          IMPACT COPOLYMERS
                applications exposed to temperatures below       PP impact copolymers provide the highest
                normal refrigeration.                            level of impact strength, especially at sub-
                                                                 ambient conditions. These materials are
                     RANDOM COPOLYMERS                           manufactured in a two-reactor system,
                One way to improve the impact strength of        operating in series. In the first reactor, PP
                PP at low temperatures is to add a second        homopolymer is produced. Instead of
                component, ethylene, in the reactor step.        transferring this product to extrusion
                The resulting PP product is called a             compounding, it is conveyed to a second
                “random copolymer.” The addition of the          reactor which also contains a high
                second component reduces the crystallinity       concentration of ethylene. The ethylene, in
                of the PP to some extent and provides two        conjunction with the residual propylene left
                benefits: marginal impact improvement at         over from the first reactor, copolymerizes to
                lower temperatures and clarity.                  form an ethylene-propylene rubber (EPR).
                                                                 The resultant product has two distinct
                The amount of ethylene added is usually          phases: a continuous, rigid homopolymer
                two-to-four percent. In the resulting            matrix and a finely dispersed phase of EPR
                polymer, the ethylene molecules are              particles. This rubber phase absorbs and
                randomly inserted along the polypropylene        disperses the energy of any impact load to
                chain. These polymer chains are more             prevent part failure while overall stiffness is
                flexible so their flexural modulus (stiffness)   maintained by the rigid homopolymer
                is reduced but impact resistance improves        matrix.
                slightly at room temperature and below.
                Such PP random copolymer products are            Although impact modification can be (and
                used in applications such as “see-through”       has been) done by mechanically mixing
                freezer storage containers, syrup and            rubber into a PP homopolymer, it is not
                household chemical bottles, VCR cases,           possible to achieve either the level of
                tackle box components and storage boxes.         dispersion or a small enough rubber particle
                If special additives called “clarifiers" are     size (about five microns) to obtain the
                also added, the sizes of the crystallite areas


                                                                                    (Continued on Page 3)




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            Introduction to Polypropylene                                                              (continued)


               maximum rubber efficiency that can be                                  levels result in products with excellent
               achieved by in-reactor synthesis. By                                   impact strength even at temperatures as
               carefully adjusting the amount and                                     low as -30°C. Although some amount of
               composition of the EPR, a wide range of                                stiffness is lost, these materials can still be
               impact/stiffness combinations can be                                   used in material handling crates, highway
               produced. Impact copolymers are used in                                dividers, etc. However, the largest
               diverse applications including medical waste                           application for TPOs is filled and reinforced
               bags, automotive batteries and heavy duty                              compounds which are replacing many of the
               tool and tackle boxes. Impact copolymers                               more expensive engineering thermoplastics
               are also used as the base materials for                                in automotive and appliance applications,
               producing filled and reinforced compounds                              such as paintable bumper fascia and interior
               for many automotive structural applications.                           panels.
               Because these materials are composed of
               two distinct phases, impact copolymers tend                                 SUMMARY
               to be translucent to opaque.                                           PP resins are one of the more versatile
                                                                                      families of thermoplastics. By tailoring the
                   OLEFINIC THERMOPLASTIC                                             process or the composition, a supplier can
                   ELASTOMERS                                                         produce products ranging from semi-rigid,
               These products, often referred to as TPOs,                             extremely tough, elastomeric types to very
               are a relatively new PP development,                                   rigid grades that can withstand severe
               Historically, the highest rubber content that                          environments, such as autoclave
               could be incorporated into an impact                                   sterilization at high temperatures. In
               copolymer in the second reactor was about                              between are grades suitable for films,
               20-25%. With the advent of new catalyst and                            filaments, automotive battery cases and
               process technologies, these capabilities                               rigid packaging with exceptional optical
               have widened considerably and some                                     properties, all sharing the same
               products have been developed with rubber                               characteristics of easy processing, durability
               content of 50% or more. Such high rubber                               and chemical resistance.


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