Polymer Process Engineering

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Polymer Process Engineering

• Introduction

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PRIMER

• Fundamental concepts language
• Nomenclature
• Chemical bonding, chemical interactions,
  entanglements
• Molecular weight
• Thermal transitions

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WHAT IS A POLYMER?

• Berzelius (1883) Poly (many) mer (unit)
• Polystyrene polymerized in 1938 polyethylene
  glycol made in 1860s
• Early polymer products were based on cellulose-
  gun cotton nitrated cellulose

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A polymer is

• Long chain molecule, often based on organic
  chemical building blocks (monomers)
• Long molecules (Mw 100,000 Da) have solid-like
  properties
• The chain may be amorphous (no regular
  structure), crystalline (a regular repeating
  structure), crosslinked,
• Dendrimers and oligomers have different
  properties

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HOW DO YOU BUILD A MOLECULE?

• Chemical structure
• Chain morphology constitution, configuration,
  conformation
• Degree of polymerization number of repeating
  units
• Building block sources hydrocarbons, renewable
  materials

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Building blocks

• 5 of petroleum goes into polymers
• Sustainable use is possible
• Energy recovery is possible if solid polymers are
  combusted

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Building methods

• Chain (addition)

• Step (condensation)

• Example polyethylene (PE) from ethylene
• Small number of reacting chains at any one time,
  which can grow into long molecules prior to
  termination
• Long reaction times needed to achieve high
  conversions

• Example poly(ethylene terephthalate) (PET) from
  terephthalic acid and ethylene glycol
• Endgroups react to build the chain long reaction
  times needed to achieve high polymer

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Multiple building blocks

• Copolymers, terpolymers,
• Using multiple building blocks leads to polymers
  with intermediate properties or unique properties
  compared to the homopolymers

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Several copolymer configurations
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Chain configurations

• Linear repeating units are aligned sequentially
• Branched large segments branch off the main
  chain/carbon backbone
• Crosslinked/network chemical crosslinks between
  chains add mechanical strength
• EXAMPLES?

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Multiphase systems

• Composites
• Structural
• Random
• Other
• Nanocomposites
• Blends
• Dispersed lamellae, cylinders, spheres

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HOW DO WE CLASSIFY POLYMERS?

• Structure chemical, configuration
• solid performance (mechanical thermal
  properties)
• other

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Mechanical Thermal

• Thermoplastic solidified by cooling and
  reheated by melting
• Thermosets retain their structure when reheated
  after polymerization (usually crosslinked)
• Elastomers rubbers, deform readily with applied
  force
• Thermoplastic elastomers
• other

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WHAT IS IN A COMMERCIAL PRODUCT?

• Very few commercial products are pure
• MWD molecular weight distribution
• additives

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Polymers vs. metals
Why do we use polymers?
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Polymeric materials

• Compete well on a strength/weight basis
• Easy to form into 3D shapes
• Creep under load is usually poor this behavior
  is usually corrected by adding fillers or fibers
• Low corrosion in the environment compared to
  metals
• Generally good solvent resistance

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Thermoplastics

• Commodities 75 of the polymer volume used is
  with 4 polymer families, polyethylene,
  polystyrene, polypropylene and poly(vinyl
  chloride) low cost
• Intermediate higher heat deflection temperatures
• Engineering plastics can be used in boiling
  water
• Advanced thermoplastics extreme properties

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Thermosets

• High moduli, high flex strengths, high heat
  deflection temperatures
• Shape is retained during thermal cycling
• Often made with step/condensation polymerization
  systems
• Crosslinking is usually used

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HOW DO WE MAKE A PART?

• Polymerization
• Formulation
• Fabrication

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Formulation

• Additives are used to modify properties and/or
  lower costs
• Additives heat stabilizer, light stabilizer,
  lubricant, colorant, flame retardant, foaming
  agent, plasticizer
• Reinforcement particulate minerals, glass
  spheres, activated carbon, fibers
• Blends, alloys, laminates

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Additives can change

• Processing properties
• Performance properties
• Composites polymers with fiber fillers
• Packaging multiple layers often used

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Formulation operations

• Thermoplastics melting or solvent processing
• Thermosets additive addition to monomers or to
  prepregs

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Fabrication

• Varies by industry sector
• Adhesive
• Coating
• Elastomer
• Plastic
• fiber

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Overview of the polymer industry
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Commodity plastics
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Film blowing
High strength films are achieved by orienting the
crystallites. The film is biaxially oriented the
wind-up rolls stretch the film in the machine
direction and the expansion of the film radially
provides a hoop stress force.
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Wire coating
Wire coating speeds can be high, and process
start-up is challenging. Metal wires may need
sizing, or wetting agents in the polymer melt for
good adhesion.
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Calendaring
Thin and thick section calendaring is used to
make wide sheets (8-12 ft).
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Bottle blowing
The parison is inflated, developing biaxially
orientation similar to that of blown film. The
sides of the mold provide cooling, quickly
freezing in the orientation developed during
the blowing process. When this process is used to
make soda bottles of PET, the orientation is
critical to achieving low carbon dioxide
permeation rates (and long bottle shelf life).
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Compression molding
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Thermoset applications
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Elastomers

• The polymers used for elastomers usually have
  very low heat deflection and melt temperatures
• Solids with good mechanical properties are made
  by crosslinking polymer chains together
• The molecular weight of elastomer parts is the
  size of the object
• Vulcanization of rubber uses sulfur to provide
  crosslinks between the CC bonds of natural
  rubber.

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Fibers

• Fibers are based on highly crystalline polymers
  that can be oriented axially to have great
  strength. Orientation (cold drawing) develops
  crystal structure in the solid.
• Most natural fibers from biomass are based on
  cellulose spider silk has different compositions
  and is based on a set of copolymers

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Elastomer polymers
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Synthetic fibers
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Coatings

• Coatings. Major area for expansion solar cells,
  windows, Supplier base is highly fragmented.
• Paints. Major area for expansion vehicles,
  Materials supplier base is clustered painting
  systems base is clustered user base is
  fragmented

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Adhesives

• Highly fragmented market.

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Foams

• Major area insulation for housing, sound
  control,
• Materials polystyrene, polyurethanes,
• Reaction injection molding example

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Composites

• Thermosets and thermoplastics
• Sheet molding compounds
• Filament winding

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HOW DO WE NAME POLYMERS?

• Polymer nomenclature is widely varied.
• Trademarks and common names may be
  industry-sector specific.
• Nomenclature Polymer Handbook. Chapter 1.

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Source-based names

• Source-based name when the polymer is derived
  from a single (original or hypothetical) monomer
  or random co-/ter-polymers
• Poly(vinyl alcohola)
• Poly(styrene-co-butadiene)
• Polyformaldehyde (not polyoxymethylene)b
• Poly(ethylene oxide) (not poly(ethylene glycol)b
• a when the name is long, parentheses are used
  to separate the name from poly
• b - actually the second name is quite common

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Structure-based names

• Structure-based name when the constitutional
  repeating unit (CRU) has several components
• The CRU is independent of the monomers and
  polymerization methods
• Poly(hexamethylene adipamide)
• Poly(ethylene terephthalate)

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copolymers
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Polymers with other backbones
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WHY ARE LONG CHAIN MOLECULES SOLIDS?

• Bonding along the backbone is not extraordinary.
• With long chains, secondary valence forces,
  integrated over the entire chain, provide
  considerable bonding forces.
• Chain entanglements provide physical linkages.

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Chemical bonding in polymers

• Most primary bonds along the backbone are
  covalent
• Secondary valence bonds
• Much smaller forces than the covalent bonds, but
  become significant when integrated over the
  entire chain
• Consider the forces acting on this macromolecule
  as it is pulled through the tube surrounding
  its structure in three dimensional space
• As each chain segment moves, it must overcome the
  local interactions at the tube surface
• Longer chains will have more resistance to motion

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Secondary valence forces

• Secondary valence forces affect the glass
  transition, the melting temperature,
  crystallinity, melt flow,
• They include nonpolar dispersion, polar dipoles,
  polar induction, and hydrogen bonds

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WHAT ARE TYPICAL CHAIN LENGTH DISTRIBUTIONS?

• Few synthetic polymers are monodisperse, i.e.,
  have one chain length.
• Many biological polymers do have specific
  molecular weights, e.g., proteins, DNA,
• The molecular weight distribution has critical
  effects on polymer properties in the melt and
  solid states.

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Typical effects of molecular weight distributions

• Homopolymers with different molecular weight
  distributions may be insoluble in each other

Linear alkane properties
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MWD - oligomer

• Poly(a-olefin) PAO6
• Synthetic base oil vehicle use
• Trimer, tetramer, pentamer, hexamer, heptamer
• Based on 1-decene
• Ionic polymerization
• Differential distribution by size exclusion
  chromatography
• PeakFit used for curve deconvolution

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Two polyethylenes

• Weight frequency, differential distributions
• Number-average molecular weights are the same
• Weight-average molecular weights are different
• Narrow MWD PD 5.7
• Broad MWD PD 15
• Differences in flow, tensile and appearance
  properties

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HOW DOES CHAIN LENGTH AFFECT PROCESSING?

• In-class exercise

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HOW DOES CHAIN LENGTH AFFECT PERFORMANCE?

• In-class exercise

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WHAT ARE IMPORTANT THERMAL TRANSITIONS?

• Thermal properties are often key criteria used to
  select polymers for specific applications.
• Five regions of viscoelastic behavior (many
  polymers have all five) lt glass transition,
  power law region, rubbery plateau, rubbery flow,
  fluid flow
• Other crystalline solids, crosslinked
  elastomers

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Five regions of viscoelasticity

• Use amorphous polymers below Tg
• Use crystalline polymers below Tm
• Crosslinked elastomers at G
• Melt processing between B and C

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Typical G vs T plots
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regions

• Viscoelasticity most polymers creep(slow flow)
  under long-term stress. Creep may not be
  recoverable, i.e., the sample may not recoil to
  its original dimensions. Over short periods of
  time, polymers are elastic.
• Solid yield and fracture elasticity for e lt
  0.1 PS is brittle and fails at low elongations.
  PE yields, and then undergoes cold drawing to gt
  300 elongation.

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BUILDING A GLOSSARY
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POLYMER SCIENCE DIRECTIONS

• Medical applications are a rich applications area
  for polymers.
• Local variations in surface roughness at the
  nanoscale can induce strains in cell membranes,
  leading to the growth of F-actin stress fibers
  that span the length of the cell.
• W.E. Thomas, D. E. Discher, V. P. Shastri,
  Mechanical regulation of cells by materials and
  tissues, MRS Bulletin, 35 (2010), 578-583.

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Cells feel their environment

• Tissues are hydrated natural polymers with
  controlled elasticity
• Most animals cells require adhesion to a solid to
  be viable
• Tissue elasticity ( kPas) is important for
  regulating cell growth, maturation and
  differentiation. Brain 0.2 lt E lt 1 kPa fat 2
  lt E lt 4 kPa muscle 9 lt E lt 15 kPa cartilage
  20 lt E lt 25 bone 30 lt E lt 40 kPa
• Nanoroughness seems to affect a number of cell
  processes
• 3D scaffolding is important
• Mechanotransduction cells adhere to surfaces via
  adhesive proteins attached to adaptor proteins,
  to the actomyosin cytoskeleton.