Polvnrollvlene
`
`The Definitive User’s Guide and Databook
`
`Clive Maier
`
`Teresa Calafut
`
`Page 1 of 35
`
`BOREALIS EXHIBIT 1040 I
`
`Page 1 of 35
`
`BOREALIS EXHIBIT 1040
`
`

`
`I’0IVllI'0IWIe|lB
`
`The Definitive User’s Guide and Databook
`
`Clive qaaier
`
`Te resa C a I afu 1'.
`
`(007)
`
`Plastics Design Library
`
`Page 2 of 35
`
`

`
`IEPR 23 a Will
`
`‘\*\C_i_P_....-/’
`
`Copyright © 1998, Plastics Design Library. All rights reserved.
`ISBN 1-884207-58-8
`
`Library of Congress Card Number 97-076233
` ..
`
`Published in the United States of America, Norwich, NY by Plastics Design Library a division of
`William Andrew Inc.
`
`Information in this document is subject to change without notice and does not represent a
`commitment on the part of Plastics Design Library. No part of this document may be reproduced or
`transmitted in any form or by any means, electronic or mechanical, includingiphotocopying,
`recording, or any information retrieval and storage system, for any purpose without the written
`permission of Plastics Design Library.
`
`Comments, criticisms and suggestions are invited, and should be forwarded to
`Plastics Design Library.
`
`Plastics Design Library and its logo are trademarks of William Andrew Inc.
`
`Please Note: Although the information in this volume has been obtained from sources believed to
`be reliable, no warranty, expressed or implied, can be made as to its completeness or accuracy.
`Design processing methods and equipment, environment and others variables effect actual part and
`mechanical performance. Inasmuch as the manufacturers, suppliers and Plastics Design Library
`have no control over those variables or the use to which others may put the material and, therefore,
`cannot assume responsibility for loss or damages suffered through reliance on any information
`contained in this volume. No warranty is given or implied as to application and to whether there is
`an infringement of patents is the sole responsibility of the user. The information provided should
`assist in material selection and not serve as a substitute for careful testing of prototype parts in
`typical operating environments before beginning commercial production.
`
`Manufactured in the United States of America.
`
`Plastics Design Library, 13 Eaton Avenue, Norwich, NY 13815 Tel: 607/337-5080 Fax: 607/337-5090
`
`Page 3 of 35
`
`

`
`14
`
`2.3 Effect of morphology on
`characteristics of polypropylene
`
`Due to the ordered crystal structure, seinicrystal-
`line polymers generally have high strength and are
`more chemically resistant than amorphous poly-
`mers. Semicrystalline materials are more opaque
`and can be used at higher temperatures, while
`amorphous rnaterials are generally more transpar-
`ent and have greater toughness and ductility. [772]
`
`2.3.1 Melting point
`The crystalline structure of a solid semicrystalline
`polymer disappears at the melting point, Tm. when
`the material undergoes a phase change from solid
`to liquid. At the melting point. physical properties
`of the material, such as density, refractive index,
`heat capacity, and transparency, change abruptly
`as the material becomes a viscous liquid. Melting
`points are commonly measured using differential
`scanning calorimetry (DSC). [769, 772]
`The melting point of a polymer varies with the
`amount of c1'ystallinity. A perfectly isotactic poly-
`propylene resin has a theoretical melting point of
`about
`l7l°C (34(}°F'); melting points of commer-
`cial
`isotactic resins can range from l60—l66°C
`
`
`
`heatflowAH[W/g]
`
`—— norrnul
`‘ " I oil-reinforced
`mat-nul: PP-homonolyrnnrlu to
`
`0
`
`20
`
`40
`
`60
`
`140
`I20
`100
`60'
`temperature ['62]
`
`I60
`
`180
`
`200
`
`Figure 2.4 A differential scanning calorimetry (DSC)
`melting scan of injection molded polypropylene. in
`DSC. a polypropylene sample is heated. along with a ref-
`erence material, and the energy required to maintain both
`materials at the same temperature is monitored as the
`surrounding temperature is increased. Heat is absorbed
`by polypropylene as it melts, resulting in an endothermic
`peak; the melting temperature given in the literature is the
`highest point of the peak. DSC scans of polypropylene
`frequently show more than one peak, due to polymorph-
`ism or crystalline changes that occur during the heating
`cycle. Bottom scan is normally processed polypropylene.
`Top scan is polypropylene with an oriented crystallization.
`produced with a high injection rate and pressure. A small
`high temperature shoulder indicates the presence of more
`than one morphological form. Heating rate was 10 Kfmin
`(t0”C/min; 1B°Frmin). [789]
`
`Morphology and Commercial Forms
`
`(320—331°F) due to atactic material and noncrys-
`talline regions. Melting points decrease dramati-
`cally with lower crystallinity; a syndiotactic poly-
`propylene resin with a crystallinity of 30% melts
`at approximately 130°C (266°F). [693]
`Polymers generally melt over a narrow tem-
`perature range rather than at a distinct point. In a
`resin with a particular crystallinity, variations in
`chain lengths produce lamellae of varying thick-
`nesses, which melt at slightly different tempera-
`tures. The melting point given for a polymer is
`generally the temperature at the highest point of
`the peak on a DSC scan (Figure 2.4) [770, 730]
`The high melting point of polypropylene pro-
`vides resistance to softening at elevated tempera-
`tures. Standard grades of polypropylene can with-
`stand continuous
`service temperatures of over
`107°C (225°F) and over 121°C (250°F) for short
`periods of time. The superior heat resistance of
`polypropylene makes it suitable for sterilization by
`autoclaving at temperatures of over 121°C (250°F)
`and for hot-fill applications. Heat resistance is
`lowered for resins with lower melting tempera-
`tures. [780, 696, 642, 693]
`
`2.3.2 Glass transition
`Amorphous regions of the polypropylene resin un-
`dergo a glass transition (TE) at a temperature be-
`tween -35 and 26°C (-3 i. and 79°F), depending on
`the measurement method, heating rate,
`thermal
`history, and microstructure. The glass transition
`temperature is related to the amount of free vol-
`ume in a polymer. Molecules and segments of
`polymer chains above the glass transition tem-
`perature vibrate and move in noncrystalline poly-
`mer regions. Motions include diffusion, rotation
`about bond axes, and translation under mechanical
`stress. At
`the glass transition temperature, free
`Volume is restricted, and only low amplitude vi-
`brations can occur. This movement continues
`down to absolute zero, at which point all move-
`ment ceases. Resins with lower molecular weight
`generally have lower glass transition temperatures
`due to increased free volume at the ends of the
`polymer chain and lower degrees of chain entan-
`glement. [772, 693]
`Normal use temperatures of polypropylene are
`generally between the glass transition and melting
`temperatures, so that strength and stiffness from
`the crystalline region are combined with toughness
`of the amorphous “tie points”. The low tempera-
`ture (S 5°C; 40°F) brittleness of polypropylene is
`
`© Plastics Design Library
`
`Page 4 of 35
`
`

`
`due to its relatively high glass transition tempera-
`ture; as the temperature drops, approaching the
`glass transition temperature, the resin becomes in-
`creasingly brittle, and impact resistance becomes
`negligible. [693]
`
`2.3.3 Mechanical properties
`
`are
`polypropylene
`of
`properties
`Mechanical
`strongly dependent on its crystallinity. Increasing
`crystallinity increases stiffness, yield stress, and
`flexural strength but decreases toughness and im-
`pact strength. The secant flexural modulus at 1 %
`displacement can range from 2067-2412 MPa
`(300,000—350,000 psi) for polypropylene with ul-
`tra—high crystallinity but decreases to 1378-1654
`MPa (200,000—240,000 psi) for general purpose
`polypropylene of lower crystallinity. In experiments
`with 0L-form,
`isotactic polypropylene,
`increasing
`crystallinity increased the growth of cracks and de-
`creased fracture toughness. [693, 785]
`Spherulite size affects the strength and ductil-
`ity of the resin. In studies of fatigue strength, dif-
`ferent crystallization temperatures were used to
`produce spherulites of different sizes (37, 54, and
`61 um); higher crystallization temperatures pro-
`duced larger diameter spherulites. In fatigue frac-
`ture tests investigating the effects of a sharp initial
`crack, the resin with smaller spherulites exhibited
`more ductile behavior, with more damage ahead of
`the crack tip and crazing in the surrounding re-
`gion. Larger spherulite sizes resulted in more brit-
`tle behavior, with less material damage. Slow
`crack growth changed to unstable crack growth at
`a critical crack length;
`this length decreased as
`spherulite size increased. [778]
`In investigations of the fracture behavior of nu-
`cleated and non—nucleated isotactic polypropylene
`using the J-integral method, fracture toughness de-
`creased with increasing crystallinity, for both nucle-
`ated and non—nucleated resins. An increase in
`
`sperulite size from 25 to 100 um decreased fracture
`toughness and increased brittleness in non—nucle-
`ated polypropylene. Nucleated polypropylene, with
`a spherulite diameter of 2 pm, exhibited increased
`crystallinity and brittleness, and cracks grew faster
`after initiation, leading to early failure. It was pro-
`posed that this brittleness was due to the higher
`crystallization temperature of nucleated polypropyl-
`ene. Higher crystallization temperatures increase the
`lamellar thicknesses and decrease tie-molecule con-
`
`centrations of amorphous regions; since tie mol-
`ecules provide mechanical continuity between sphe-
`
`15
`
`lower tie molecule concentrations would
`rulites,
`make the amorphous regions of the polymer more
`susceptible to crack propagation. [785]
`
`2.3.4 Haze
`
`Transparency in semigystalline polymers is di-
`rectly related to the crystallinity. Spherulites are
`much larger than the wavelength of visible light
`(0.4—0.7 um), and the refractive index of crystal-
`line regions is higher than that of amorphous re-
`gions. As light rays pass from amorphous to crys-
`talline regions,
`they encounter the large spheru-
`lites, resulting in light scattering; as a result, trans-
`parency is lower, and haze is produced. Due to
`their noncrystalline structure, amorphous materials
`have lower levels of haze than semicrystalline
`materials, and a decrease in crystallinity in a serni—
`crystalline polymer enhances the clarity. Excessive
`reductions in crystallinity can result in unaccept-
`able reductions in strength, stiffness, and resis-
`tance to softening,
`so a compromise must be
`reached that
`is appropriate for the application.
`[693, 780, 786, 774]
`Nucleating agents such as dibenzylidene sor-
`bitols reduce spherulite size to below a level that
`scatters visible light, resulting in a dramatic re-
`duction in haze. Spherulite size distribution is also
`reduced. A higher concentration of nucleating
`agent can result in greater clarity. Clarity can be
`optimized by using a resin with low crystallinity
`with a nucleating agent added and by rapidly
`cooling the molten polymer
`to accelerate the
`crystallization rate. [693, 786]
`
`2.3.5
`
`Sterilization
`
`Polypropylene resins with lower crystallinity exhibit
`greater resistance to embrittlement resulting from
`gamma radiation sterilization. Exposure to gamma
`radiation increases the susceptibility of the material
`to oxidation. Free radicals and ions, highly reactive
`chemical species containing free (nonbonded) elec-
`trons, are created and are then trapped in the crys-
`talline structure of the polymer. These free radicals
`and ions promote covalent bond disruptions in the
`main polymer chain or in pendant methyl groups.
`The resulting chain scission and/or crosslinking of
`adjacent polymer chains increases the brittleness of
`the material. [55, 799, 798]
`associated With the
`Radiation resistance is
`amount of free volume in the polymer structure,
`which allows greater mobility of the polymer chains.
`Amorphous or smectic phases have lower densities
`
`© Plastics Design Library
`
`Morphology and Commercial Forms
`
`Page 5 of 35
`
`

`
`16
`
`and greater free volumes than crystalline regions;
`resins with increased amounts of these less ordered
`phases exhibit
`increased radiation resistance. Poly-
`mers with low isotacticity combined with careful se-
`lection of additives are effective in producing radia-
`tion-resistant materials. [780, 67, 699]
`
`2.4 Orientation
`
`When a polymer crystallizes in the absence of ex-
`ternal forces, the polymer chains are arranged ran-
`domly, in no preferred direction. If the polymer is
`subjected to an external stress (such as flexing) im-
`mediately after crystallization (while it still retains
`heat), the polymer chains align in the direction of
`the external stress. [769, 420] Orientation is used in
`the production of films, fibers, blow molded bottles,
`and living hinges, and some orientation occurs dur-
`ing the injection molding process.
`
`2.4.1
`
`Fibers and films
`
`Orientation can be uniaxial, in which the material is
`stretched or drawn in one direction (machine direc-
`tion), or biaxial, in which the material is stretched in
`two directions (machine direction and cross or
`transverse direction). Uniaxial orientation is used
`for films and fibers, and balanced biaxial orienta-
`tion, in which the machine and transverse direction
`draw ratios are equal,
`is used in films and blow
`molding. Stretching is usually performed at a tem-
`perature slightly below the melting temperature
`(120—l60°C; 248—320°F for polypropylene), when
`crystals are partially melted, and the desired shape
`is maintained during cooling. [642, 774, 772]
`When unoriented fibers
`are
`stretched or
`drawn, the fiber does not become gradually thin-
`ner; instead,
`it suddenly becomes thinner at one
`point, a process called “necking down". In neck-
`ing, the original material becomes oriented locally
`and is separated from unoriented material by a
`relatively sharp boundary. Continued stretching re-
`sults in increased length of the drawn portion at
`the expense of the undrawn portion of the fiber.
`Diameters of both drawn and undrawn portions do
`not change significantly during the stretching pro-
`cess.
`[769, 889] During the orientation process,
`spherulites break apart as-blocks of lamellae slide
`out and rotate, and molecular chains slide past
`each other to become oriented in the same direc-
`tion. Microcracks can form between crystallites,
`and small crystallites stack into long, thin fibrils.
`Oriented lamellae are connected by tie molecules
`
`in amorphous regions. The stereoregular structure
`of polypropylene allows
`the oriented polymer
`chains to fit together well, so that the alignment is
`maintained only by van der Waals forces between
`different polymer molecules. [768, 769, 774]
`Shrinkage of the oriented material can occur
`over time at room temperature, and if heated to a
`temperature approaching the orientation tempera-
`ture, shrinkage becomes significant. To reduce
`shrinkage at room temperature and to raise the
`temperature at which significant shrinkage occurs,
`the material is annealed by heating to a tempera-
`ture just below the melting temperature; the rumor-
`phous tie regions relax, allowing movement of
`some polymer chains in this region into crystalline
`structures. [772, 774]
`
`2.4.2
`
`Effect of orientation on characteristics
`of fibers and films
`
`The alignment of polymer chains in oriented mate-
`rials makes formation of a crystalline structure eas-
`ier, resulting in increased crystallinity. Stiffness and
`strength increases when the applied stress is in the
`orientation direction and decreases compared to
`unoriented material when the applied stress is per-
`pendicular to the orientation direction. Strength re-
`sults from the covalent bonds between carbon atoms
`of the polypropylene chain; orientation increases the
`number of polypropylene chains aligned in the ori-
`entation direction, increasing the number of cova-
`lent bonds. As a result, a smaller amount of material
`can carry the same applied load. [774, 772]
`Permeability in biaxially oriented polypropyl-
`ene film is reduced dramatically due to the in-
`creased crystallinity; molecules cannot easily dif-
`fuse through the crystalline matrix. Permeability to
`moisture is decreased, and oil and grease resis-
`tance is increased. The increase in crystallinity
`also increases its dielectric strength; biaxially ori-
`ented films can withstand three times the Voltage
`of unoriented film before dielectric breakdown oc-
`curs. The nonwettability and stain resistance of fi-
`bers makes them useful
`in applications such as
`marine rope, upholstery, and carpeting. The wick-
`ing action of polypropylene makes it useful
`in
`sportswear; moisture is not absorbed but passes
`through to the other side by capillary action of fi-
`hers with large surface areas. and the material re-
`mains dry. 1642. 783, 773, 705]
`Optical properties are improved in biaxially
`oriented film. In unoriented films, interfaces be-
`tween crystalline and amorphous regions, which
`
`Morphology and Commercial Forms
`
`© Plastics Design Library
`
`Page 6 of 35
`
`

`
`Table 2.1 Effect of increasing Biaxial Orientation on
`Properties of Polypropylene
`
`With Increasing
`Orientation
`
`Tensile Strength at Yield
`
`Modulus
`
`Drop Impact Strength
`
`Cold Temperature Impact
`Strength
`
`Permeability
`Haze
`
`Gloss
`
`Shrinkage
`
`Increases
`
`Increases
`
`Increases
`
`Increases
`
`Decreases
`Decreases
`
`Increases
`
`Increases
`
`refract light to produce haze, are distributed in all
`directions. In oriented film, the alignment of la-
`mellae results in fewer interfaces in the plane of
`the Film. Less light is refracted, reducing haze and
`increasing the clarity.
`in addition.
`the smoother
`spherulitic surfaces of oriented film scatter less re-
`flected light, so that gloss is increased. 1881, 774]
`Shrinkage is higher in oriented rnate-rials, clue
`to relaxation of the aligned structure back to a
`more random morphology. if not annealed. films
`exposed to temperatures close to the orientation
`tcrriperaturc will shrink to a structure similar to
`that of the unoriented material, a property that is
`exploited in heat shrinkable films. [642, 881]
`General effects of biaxial orientation on poly-
`propylene are listed in Table 2.1. [695]
`
`Injection molding
`2.4.3
`Orientation in injection molding arises from shear
`stress during flow and cooling of the molten
`polymer as the mold is filled. Orientation is pro—
`duced in the direction of polymer flow. During
`mold filling, polymer chains are stretched and un-
`coiled due to shear and elongational forces; chains
`then relax slightly during the cooling period before
`crystallization occurs. After urystalli:»'.ation,
`the
`remaining orientation is frozen in. The time and
`extent of relaxation is dependent on properties of
`the particular‘ resin, such as molecular weight and
`molecular weight distribtition,
`the difference in
`temperature between the polymer melt and the
`mold walls, and processing conditions. [770]
`The Flow and cooling process produces a skin-
`core morphology in iitjection molded parts. De-
`pending on the flow pattern of the melt. different
`morphologies can be observed between the skin, at
`the surface of the mold where cooling is rapid. and
`
`17
`
`the core, in the center of the mold where cooling is
`slower. In some cases, a continuous decrease in
`orientation has been observed from the skin to the
`core layers. The skin layer is highly oriented due
`to shear forces during flow and rapid cooling.
`Shish—kebab structures (Figure 2.5) are produced,
`with lamellae oriented in" the flow direction, and
`some B—form crystallites form due to the high melt
`strain in this region. [734] In the core region, there
`is less melt strain, in addition to slower cooling, so
`that any orientation that occurs has time to relax
`back to an unoriented morphology. As a result, a
`spherulitic structure is produced, with little or no
`orientation. A continuous increase in unoriented
`spherulitic morphology is observed from the skin
`to the core. [734, 770]
`2
`Another type of morphology often observed
`exhibits three layers: a skin layer, a core layer, and a
`layer of maximum orientation between the skin and
`core. The skin layer has been attributed to elonga-
`tional forces arising from fountain flow at the front
`and displays either no orientation or a limited
`amount of orientation. In fountain flow,
`the melt
`front stretches and balloons out to cover the mold
`wall. Cooling is rapid, and the melt freezes when it
`meets the mold walls. The frozen layer along the
`walls produces a non—uniform velocity gradient in
`the transverse direction of the mold; melt
`in the
`center, with the highest velocity, then flows out to
`the mold walls and freezes. [526, 787, 774]
`Maximum orientation and some B—form spheru-
`lites occur in the shear zone. The polymer melt in
`this region is attached to the frozen skin layer but is
`still moving with the melt front, producing high
`shear strain. In the center core region,
`low shear
`strain results in low orientation. [787, 734, 772]
`
`u,=2.5x r,‘
`
`v, : strength sell-reinloreed
`In = strength normal
`
`Figure 2.5 Drawing of a shlsh-kebab structure in
`polypropylene. Shear iorces during melt flow into the
`mold cavity produce deformation of spherulitic structures,
`and lamellae become aligned in the flow direction. [739]
`
`© Plastics Design Library
`
`Morphology and Commercial Forms
`
`Page 7 of 35
`
`

`
`Crystallinity increases from the skin layer to
`the core, due to a higher cooling rate at the sur-
`face, and formation of
`[3-form crystallites de-
`creases. Spherulite size increases from the skin to
`the core region — the higher cooling rate at the
`mold walls produces faster nucleation and smaller
`spherulites. [770, 734]
`Processing parameters and resin properties can
`affect morphology. In experimental studies, melt
`temperature was the most important parameter at-
`fecting spherulite size and skin layer thickness. In-
`creasing melt
`temperatures resulted in decreased
`skin layer thicknesses due to chain relaxation from
`increased cooling times, and spherulite diameter de-
`creased due to slower nucleation rates. Skin layer
`thickness also decreased with increasing mold tem-
`perature, due to decreased cooling rates, but
`in-
`creased slightly with increases in holding time. Skin
`layer
`thickness
`also increased with increasing
`polymer molecular weight, due to a longer relaxa-
`tion time For longer polymer chains. Splierulite di-
`ameter increased with increasing melt temperatures
`due to slower nucleation rates. [734, 770]
`
`2.4.4 Effect of orientation on characteristics
`of injection molded parts
`
`Orientation imparts a directionality or anisotropy
`on the properties of injection molded parts. Me-
`chanical properties depend on the direction of the
`applied stress; oriented materials are generally
`stronger than uriorientetl materials when stress is
`applied in the flow direction, but weaker with
`stress applied in the transverse direction.
`Tensile strength is high in the flow direction,
`due to to the strength of covalent bonds between
`carbon atoms along the polymer chain, but low in
`the normal or transverse direction, due to the weak
`intermolecular‘ forces between polymer chains. With
`a completely oriented,
`extended polypropylene
`chain, ultimate strength in the flow direction could
`be as high as 16 GPa (2,300,000), with a modulus
`of 50 GPa (7,200,000 psi); this is not achieved in
`practice due to the spherttlitic crystal structure. Ini-
`pacl strength follows the same pattern as tensile
`strength.
`1772.
`789]. Tensile modulus.
`yield
`strength, and flexural modulus all increase in the
`flow direction as orientation increases. Softening
`temperatures are higher in the skin layers relative to
`the core layers. [770]
`Orientation induces residual stress in injection
`molded parts, which can result in decreased impact
`resistance and warpage. Stress relaxation can re-
`
`sult in shrinkage; when exposed to elevated tem-
`peratures after demolding, an oriented material
`will shrink more in the direction of orientation
`than in the transverse direction. Shrinkage has
`been correlated with the area fraction of the skin
`layer. [7'l‘0. 772]
`Orientation influences thermal, chemical and
`electrical properties of injection molded parts. The
`thermal cond activity increases in the [low direction,
`while the coefficient of linear thermal expansion in-
`creases in the transverse direction and decreases in
`the flow direction. The electrical dissipation factor
`decreases in the orientation direction, and penne-
`ability increases in the transverse direction. [772]
`Orientation affects the radiation resistance of
`polypropylene. In experiments that determined the
`break-angles of plaques after ga1'nma irradiation.
`samples tested in the transverse direction showed
`dramatic losses in ductility. while samples tested
`in the orientation direction generally retained duc-
`tility up to 1000 hours after sterilization. Flexural
`moduli of identical samples not irradiated showed
`only a small orientation effect, with moduli meas-
`ured in the flow direction slightly higher. [700]
`
`2.4.5
`
`Living hinges
`
`The high strength in the direction of orientation is
`used in the production of integral or living hinges
`— molded-in, one-piece hinges that allow part
`opening and closing without use of a mechanical
`hinge. Polypropylene living hinges can be flexed a
`million times before failure and are used in appli-
`cations such as videocassette cases and fishing
`tackle boxes. [420, 773]
`The design of a polypropylene living hinge in-
`cludes a thin section (hinge web) connecting two
`heavier sections (Figure 2.6). A recess is located in
`the upper portion of the thin region, to prevent any
`cracking that could result in hinge breakage. An
`are below the thin section allows for flexing and
`orientation. The polymer chains are initially uncri-
`ented after molding; the hinge is flexed immedi-
`ately after molding, while the part temperature is
`still high, to induce orientation. The hinge web is
`stretched during the initial flexing, and polymer
`chains become aligned perpendicular to the hinge.
`As with any orientation process, strength in the
`orientation direction is greatly increased due to
`carbon-carbon covalent bonds,
`resulting in a
`tough, fatigue—resistant hinge. [420, 690, 773]
`
`Morphology and Commercial Forms
`
`© Plastics Design Library
`
`Page 8 of 35
`
`

`
`Hinge as molded —
`random conliguratiori
`oi molecular ciiains
`
`Hinge closed after
`molding — web
`stretched by bending.
`Molecular chains
`aligned
`
`Oriented hinge -
`molecuiar chains
`aligned perpendicular
`ll} hinge — shaded
`area represents
`neck-down due 10
`ilexing
`
`Figure 2.6 Formation ot a living hinge, shown for a
`fishing tackle box. Top, hinge immediately after mold-
`ing. A thin section [hinge web) connects two larger sec-
`tions (iid and box). A recess at the top of the hinge web
`prevents cracking. and an are below allows for flexing
`and orientation. Polymer chains are unoriented. Middle.
`hinge is closed after molding, while the part is still warm,
`to induce orientation. Bottom, the oriented hinge. Poly-
`mer chains are aligned perpendicular to the hinge, pro-
`viding high fatigue strength. Neck-down occurs in the
`shaded regions due to flexing. [420]
`
`2.5 Commercial Forms of
`
`Polypropylene
`
`Polypropylene is produced commercially in differ-
`ent forms, depending on the properties desired.
`Polypropylene homopolymer contains only propyl-
`ene monomer in the polymer chain. Homopolymer
`provides stiffness and toughness but exhibits low
`impact strength at low temperatures, and clarity is
`too low for some applications. Propylene copoly-
`mers contain one or more different types of mono-
`mers in the polymer chain. Random copolymers are
`used in applications requiring higher clarity or a
`lower melting point, and impact copolymers are
`used in automotive and other applications that re-
`quire high impact resistance at low temperatures.
`Thermoplastic olefins and thermoplastic Vulcani-
`zates provide elastomeric properties for automotive,
`medical, and other applications.
`
`2.5.1 Homopolymers
`
`The chemistry and morphology of polypropylene
`homopolymer have been described in the preced-
`ing sections. The primary application of homo-
`polymers is in the extrusion of fibers and filaments
`for Cordage, webs, carpet backing and face yarns,
`upholstery fabrics, apparel, filters, geotextiles, dis-
`posable diapers, medical fabrics, fabric for auto-
`mobile interiors, bags, and strapping tape. Ori-
`ented and unoriented films are also extruded for
`
`pressure-sensitive tapes, electrical applications,
`shrink film, and packaging for retortable pouches.
`Sheets greater than 250 um (10 mil) in thickness
`are used for counter tops and tank liners; thinner
`sheets are thermoformed into packaging contain-
`ers. Injection molde’d homopolymers are used in
`automobile parts, appliances, housewares, pack-
`aging containers, trigger Sprayers, furniture, and
`toys. [795, 788]
`
`2.5.2 Random copolymers
`
`Random copolymers are produced by adding the
`comonomer, ethylene or, less commonly, 1-butene
`or l—hexene, to the reactor during the polymeriza-
`tion reaction: [691]
`
`CH,=CH, CH,=CHCH2CH3
`
`ethylene
`
`I-butene
`
`CH,=CHCH2CH,CH2CH,
`I -hexene
`
`The comonomer substitutes for propylene in the
`growing polymer chain. Insertions are randomly or
`statistically distributed along the chain and can con-
`sist of single monomers, as shown for ethylene in
`Figure 2.7, or multiple monomers (two or more se-
`quential ethylene molecules along the polymer
`chain). Random copolymers generally contain 1-7
`wt.% ethylene, with 75% single and 25% multiple
`insertions. In practice, depending on the catalyst,
`polymerization Conditions, and the reactivity of the
`comonomer compared to propylene, random co-
`polymers can become somewhat blocky, with some
`regions of the polymer chain containing only poly-
`propylene units and other regions containing only
`comonomer. [693, 795, 691]
`The structure of random copolymers is similar
`to isotactic polypropylene, but the regular, repeat-
`ing arrangement of atoms is randomly disrupted
`by the presence of comonomer units. The effect is
`similar to that of increasing atacticity. Crystallinity
`
`© Plastics Design Library
`
`Morphology and Commercial Forms
`
`Page 9 of 35
`
`

`
`34
`
`bilization in experimental studies and had lower
`volatility and extractibility. It was not absorbed by
`fibers present in the formulation and did not inter-
`act with pigments during fiber spinning. [885]
`
`3.4.5
`
`Screeners
`
`Pigments added to a formulation to provide opac-
`ity or translucence, such as carbon black, titanium
`dioxide, and zinc oxide, act as UV screeners by
`absorbing or reflecting ultraviolet
`light. Carbon
`black absorbs ultraviolet and visible light through-
`out the entire spectium and may also act as a free
`radical scavenger. It can be used at concentrations
`as low as 1 — 2%. The stabilization of resins con-
`taining carbon black can be enhanced by addition
`of antioxidants or HALS. Titanium dioxide (rutile)
`reflects light and is effective at high loadings.
`Some pigments can act as synergists with com-
`pounds such as phosphites and nickel-organic salts
`to improve embrittlement time for polypropylene
`by over 60%. [822, 878]
`
`3.4.6
`
`Evaluation of UV stability
`
`The most accurate test of UV stability is use of the
`material in its intended end-use environment over a
`period of time. Due to the long—term nature of out-
`door weathering tests, accelerated testing using arti-
`ficial light sources (xenon arc lamp. sunshine car-
`bon arc lamp, mercury arc lamp, fluorescent sun
`lamp} is common. Filtered xenon most accurately
`reproduces the spectral energy distribution of sun-
`light, while light sources with significant emission
`below 290 nm can produce different results than
`those obtained with long—term, outdoor weathering.
`Accelerated exposure tests may underestimate the
`effectiveness of HALS due to the very high levels of
`UV radiation produced. [8l9, 821, 843, 885]
`1"
`
`3.4.7 Use of light stabilizers
`
`Use levels of light stabilizers range from 0.05-
`2.0%, depending on the type of stabilizer, part
`thickness. presence of other additives, type of resin,
`and application requirements. Benzophenones and
`hindered amines are widely used in polypropylene.
`A combination of stabilizers is used to obtain the
`desired stabilization; highly stabilized polypropyl-
`ene contains an ultraviolet absorber, a phosphite
`stabilizer, and a nickel quencher or hindered amine.
`[821, 878, 822]
`Compatibility with the resin is more important
`in light stabilizers than in antioxidants, since they
`are used at higher concentrations. Higher concen-
`
`trations of stabilizers can be dissolved at high
`temperatures than at low temperatures, so that sta-
`bilizers dissolved during resin processing may ex-
`hibit blooming, migration of the stabilizer to the
`part surface, when the resin is cooled. Blooming
`and turbidity can occur if the stabilizer is incom-
`patible with the resin; compatibility of the gener-
`ally polar light stabilizers is more difficult
`to
`achieve with nonpolar