Thermoplastic Elastomers
Professor Joe Greene
CSU, CHICO
1
Copyright Joseph Greene 2001
Thermoplastic Elastomers
•
•
•
•
•
•
•
•
Introduction
Synthesis
Morphology
Properties and Effect of Structure
Thermodynamics of Phase Separation
Thermoplastic Elastomers at Surfaces
Rheology and Processing
Applications
2
Copyright Joseph Greene 2001
Introduction
• Introduction
– Increase in synthetic thermoset and Natural rubber production over the last 10
years is about the same growth of 2-3% of GNP.
– Growth of thermoplastic elastomers has grown 5-10% per year
• TPEs have replaced themoset rubbers and metals and inorganic glasses
• TPE consumption is to reach 923,000 metric tons (1996) or 6% of world rubber
market and 10% of the US rubber market.
– Current TPEs are not suitable for tire manufacture (50% of world wide rubber
market)
– Advantages of TPEs over conventional rubber
• Ease and low cost of processing
• Variety of properties available
• Possibility of recycling and reuse.
– Disadvantages
•
•
•
•
High cost of raw materials
Inability to have large % of fillers, e.g. carbon balck and silica
Poor chemical and temperature resistance
High mechanical hysteresis (thus can’t be used in tires)
Copyright Joseph Greene 2001
3
Introduction
• Qualifications of TPE
– Ability to be elastically stretched to moderate elongations
– Processability as a melt at elevated temperatures
– Absence of significant creep
• Modulus of TPE blends is affected by temperature, Fig 1
– The higher the temperature, the lower the modulus
– The modulus is an average of the modulus of the two materials
• Average is dependent upon relative amounts of TPEA and TPEB blend
– Random A-B copolymer has range of Modulus over temperature span
– Block Copolymer has more distinct modulus over temperature span
– Segmented block copolymer with imperfect phase segmentation has a plateau
modulus
– Segmented block copolymer with perfect phase segmentation has a plateau
modulus range
4
Copyright Joseph Greene 2001
Introduction
• Six General Classes of Polymers
Thermosetting
Rigid
Epoxy
Urea-formaldehyde
Hard rubber
Highly loaded or highly
Flexible vulcanized rubbers
Rubbery Vulcanized rubbers
Thermoplastic
PS
PVC
PP
PE, Acetate copolymers,
PVC
TPE
• TPEs have similar structural characteristics
– Presence of dissimilar components on a single polymer chain
• Copolymers or terpolymers in random, block, or graft formation
• Macrophase separation is not possible; 2 components separate on smalllength scales (nano-meters)
• Microphase separation caused by crystallinity, hydrogen bonding, ionic,etc.
• Two phases have different properties: One- soft phase and one- hard phase
• Relative concentration of each yield characteristic properties
• Structures of common TPEs are in Fig 2.
– The First is Krayton (Shell) 1960s: SBS, SIS, and SEBS
5
Copyright Joseph Greene 2001
Introduction
• Ethylene-PP copolymers are a major category of TPEs
– This accounts for 30% of the TPE market
– Often these are blended with another material, i.e., homopolymer
• Improves properties of both blend and homopolymer
• Santoprene is a blend of a polyolefin and EPDM (ethylene-propylene-diene)
• EPDM is a small component and exists as the isolated phase. It is crosslinked during
melt processing. Sticks to screw on our extruder.
• EPDM blends can be divided into two classes
– EP-low unsaturation elastomers
• Blended with SBR, NMR, BR, and EPM and vulcanized with sulfur, peroxide and
phenolic compounds
• For peroxide cures of blends, crosslink density, modulus, tensile strength, and
hardness were decreased with liquid SBR and NBR, but increased by BR and CR.
– EP-high unsaturation elastomers
• Blended with EPM improves ozone resistance. Can use ultrahigh EPDM-CR and BR
blends for ozone protection.
• Properties:
• Ref: http://www.polyone.com/
• Ref: http://www.exxon.mobil.com/
Copyright Joseph Greene 2001
6
Introduction
Ref: http://www.polyone.com/
7
Copyright Joseph Greene 2001
Introduction
• Polyurethane elastomers are segmented copolymers with an aromatic
ring containing the hard segment (isocyanate) and a polyether or
polyester oligomer soft segment (polyol).
– Materials microphase separate because incompatibility between aromatic rings
and soft segment. Fig 2
– Dow Chemical Pellethane: Commercial MDI (Methyl diisocyanate)
– 15% of elastomer market is for polyurethanes
– Copolyesters can be the hard segment PTMT and PTMO. Hytrel from Dupont
• Other TPEs in Fig 2:
– Copolyamide elastomers (similar to copolyesters)
• Crystallization provides driving force for phase separation.
• Low chemical permeability and good low temperature properties.
– Ionomers have small mole fraction of monomers (<10%) containing ionic
functionality.
• Are not segmented into phases, rather the ionic groups are distributed
randomly along polymer backbone. Incompatibility between ionic and
nonpolar polymer backbone leads to formation of ionic rich domains.
8
• Surly from DuPont is shown in Fig 2.
Copyright Joseph Greene 2001
Synthesis
• Condensation polymerization
– For copolyesters, polyurethanes, and copolyamides.
– For TPEs a molecule containing two reactive functional groups of
one type (diisocyanate) reacts with another molecule containing
two reactive functional groups of another type (diol) to form a
polymer.
• Requires high conversion (>99%) to produce high molecular weight product.
• TPE properties are weakly dependent on overall molecular weight, thus
distribution is not very important.
– Polyurethanes can be polymerized in solution or bulk
• Solution PUs have less crosslinking and more uniform hard and soft
segments MW.
• Bulk PUs (most common) have higher MW, partially caused by crosslinking
• Reacted at 80 to 120°C and are highly exothermic and thus heat must be
removed from reacting polyurethane, otherwise degradation can occur
9
Copyright Joseph Greene 2001
Synthesis
• Condensation polymerization
– Copolyesters- similar to polyurethanes except the hard segments
are diesters.
– Produced with melt phase transesterification
• Oligomeric diol, chain extender, and diester are mixed and react at T >
200°C in presence of titanate catalyst. Yields prepolymer
• Temp is raised to 250°C to produce polymers from second
transesterification. Caution: Temp < 260°C to prevent degradation.
– Co-polyamides are produced from ractions of carboxylic acids and
diamenes, acid chlorides and diamines, and carboxylic acids and
isocyanates.
– Most common is with ester or amid linkages
– High temperatures are needed to produce high molecular weight
material
10
Copyright Joseph Greene 2001
Synthesis
• Anionic polymerization
– Produces a polymer with an extremely narrow MW distribution in
contrast with condensation polymerization.
– SBS and SIS (triblock polymers) are produced using high vacuum
process which eliminates oxygen and water.
– Important to produce a polydiene black with a high 1,4 structure so
that the Tg of soft phase is low.
• Accomplished with nonpolar solvent, toluene or cyclohexane
– SEBS is formed through hydrogeneration of SBS
– Three polymerization methods produce styrene copolymers
• 3-stage process with monofunctional initiator
• 2-stage process with monofunctional initiator and difunctional linking agent
• 2-stage process with a difuntional iniator.
– Important is the control of stoichiometry exactly so that no
diblocks are formed
11
Copyright Joseph Greene 2001
Synthesis
• Copolymerization
– Ethylene-propylene copolymer TPE and other PP-olefin
coploymers are produced from Ziegler-Natta processes.
• Ziegler-Natta polymers are reactions catalyzed by a mixture of alkyl metal
halides and transition metal salts.
– Often produce crystallizable stereospecific products.
» Example, PP-olefin copolymers, PP crystallites provide hard phase for the
TPE.
» Long blocks, necessary for phase separation result from the inherent
reactivity ratios of the components.
» Kinetics (steps in reactions) are not well understood.
• Blends of EP copolymers with isotactic PP are often made in 2-step reaction
process
– Reactor contains only PP monomer; second reactor contains PP and ethylene.;
Other blends are made by mixing them in a melt; Blending condition have large
effect on resulting properties.
• Free radical polymerization is used to produce many ionomers, e.g., PE and
methacrylic acid.
– High pressure process and temperature with presence of neutralizing metal
12 salt
to improve properties
Copyright Joseph Greene 2001
Synthesis
• Molecular Weight and Chain Structure
– Determining MW is difficult in some polymers
• Required to dissolve polymer in a solvent so that the polymer chains behave
individually.
• TPEs are composed of two dissimilar polymers, thus a good solvent for one
might not work with the other component.
– Leads to aggregates of polymer and an aggregate MW, not true MW.
• Gel Permeation Chromotography (GPC) is most often used.
– MW is reported based upon reference to a polymer standard (Usually PS) and
for a certain solvent.
• Accurate MW is possible for triblock copolymers, because they are
produced via anionic polymerization, theoretical MW is close to actual MW.
– Block copolymer is applied to macromolecules made up of sequences with
different chemical, physical, or tactic structures represented by A, B, and C
– Triblocks are ABA and ABC as linear (Block) or branched (graft) copolymers.
– Polyurethanes and copolyesters are block copolymers made by condensation
» Length of hard segment is very important and affects phase separation
13
Copyright Joseph Greene 2001
Morphology of TPEs
• Morphology of the TPE are important and defines the shape
of the different polymers in the TPE.
– Seven equilibrium morphologies exist. (A=minor, B=major)
•
•
•
•
•
•
•
Isolated spheres of A in a continuous matrix of continuous B
Hexagonally packed isolated cylinders of A in continuous B
Ordered bicontinuous double diamond of A in continuous B
Alternating lamella of A and B
Ordered bicontinuous double diamond of B in continuous A
Hexagonally packed isolated cylinders of B in continuous A
Isolated spheres of B in continuous A
– Shown in Fig 4 and TEM in Fig 5-8
– Once the general morphology (spheres, cylinders, etc.) has been
specified, then specific questions are asked for:
• Domain spacing, radii, and arrangement in space.
14
Copyright Joseph Greene 2001
Morphology of TPEs
• Morphology will have large effect on properties
– Soft phase is the continuous phase which maintains the elastomeric
behavior.
– Hard segments tend to crystallize into lamellae (overlapping
layers) both phases are continuous.
• Example, polyurethanes and copolyesters
– Rarely, is the hard segment the continuous phase.
– Common morphology is Fig 9 for SBS triblock copolymers.
• Isolated hard phase acts as intermolecular tie points for elastomeric soft
phase. Hard segment domains are 5 to 50nm.
• TPEs with crystallizable hard segments (PU) have crystallites that form
lamella which turns into spherulites, Fig 10
– Chain axis of hard segment radiates outward from a central point.
– Interface between hard and soft segments is often considered a separate phase.
» Adhesion between interface and polymer is important because failure often
occurs in interface
» Systems with narrow interfaces usually show more complete phase
15
separation.
Copyright Joseph Greene 2001
Morphology of TPEs
• Studies of Morphology
– Transmission Electron Microscopy (TEM)
• In TEM, a sample is bombarded with electrons and the number of electrons
which travel through the sample is measured as a function of position.
– The number of electrons that pass through a sample is proportional to the
sample thickness and the electron density.
– Electron density may not be identical for each phase in a TPE; hence the
number of electrons passing through the two phases may be different and will
cause a difference in shading and gray tones.
• Highest quality TEMs are from extremely well-ordered morphologies.
– If morphology is not well ordered, then the average domains of one type that a
single electron passes through does not vary with position.
– Because electron density differences between atoms that constitute polymers are
small, a high electron density additive (OsO4 or RuO4) must be added.
– Styrene-diene block copolymers are optimum materials for TEM
» SBS example in Figs 5-8
– Copolyesters, polyurethanes, and ionmers are also studied with TEM (Fig 11)
16
Copyright Joseph Greene 2001
Morphology of TPEs
• Infrared Spectroscopy
– Fourier Transform Infrared (FTIR) spectroscopy
• One of the most common techniques used for chemical identification of
many plastics, especially polyolefins.
• The FTIR technique provides information on chemical structure of the
sample material.
• Infrared spectroscopy measures the frequencies of infrared light absorbed by
the bonds that make up a chemical compound. Every type of bond has a
natural frequency or energy of vibrations.
– A stiffer bond is normally associated with higher frequency. The region of an
IR spectra between 600 and 1400 cm-1, is known as the fingerprint region
because of the absorption patterns in this region are often unique to a particular
compound.
– IR spectroscopy can indicate the presence of functional groups such as alkanes,
alcohols, nitriles, and amines in a chemical compound.
• The IR spectrum is compared with some known compound.
• Polymers are more complex than for functional groups or simple
compounds.
– The peak position in the IR spectra for polymers is affected by the type of bonds
in the chemical structure or repeating units, but can shift slightly due to17polymer
chains on the backbone.
Copyright Joseph Greene 2001
FTIR Spectroscopy of LDPE
LDPE Neat
256 Scans
12-08-00
18
Copyright Joseph Greene 2001
FTIR Spectroscopy
• In FTIR, a beam of infrared light is passed through a sample
and the amount of absorbed radiation is measured as a
function of frequency.
– Absorption occurs if the frequency of the light is the same as the
frequency of a normal mode of vibration.
• Method is sensitive to molecular bonding between atoms.
• IR spectroscopy helps identify materials as chemical characterization,
interchain interactions, hydrogen bonding, and crystallization
– Beer’s law relates absorbance A of a vibration to its absolute
concentration: A =  t c
• Where,  is the absorption coefficient, t is the path length, and c is the
concentration.
– Higher concentration of the bond in the polymer yields higher absorbance.
19
Copyright Joseph Greene 2001
FTIR Spectroscopy
• Hydrogen bonding
– Secondary bonding of hydrogen atoms to an atom containing
unpaired electrons.
• For TPEs, hydrogen bonding (3kcal/mole) is half order of magnitude less
than covalent bonds (C-C and C-H).
• For PU and polyamides, hydrogen bonding occurs between urethane or
amide hydrogen and the carbonyl oxygen.
– Hydrogen bonding occurs between hard segments and causes phase separation
and hard segment crystallization.
• Causes a shift toward lower wavenumber (lower energy) in the vibration of
the bonds involving the hydrogen donating group and the hydrogen
accepting group which indicated that both primary bond strengths have
diminished because of the secondary hydrogen bonding.
– Typical TPE which can have hydrogen bonding, vibrations of functional groups
participating in hydrogen bonding are split into bands termed bonded and free.
– Table II: wavenumbers which have been assigned to the bonded and free bands
for the C=O and N-H vibrations for a variety of polymers.
– Small but noticeable effect of the polymer type on the exact wavenumber for the
vibration.
20
– Two vibrations listed for some C=O stretches are due to the ordered (crystalline)
and disordered bonded
states.
Copyright
Joseph Greene 2001
FTIR Spectroscopy
• Table II. Band Assignments of Hydrogen Bonded
Functional Groups in TPEs
Material
PU:hexamethylene
diisocyanate-butanediol
PU:toluene (TDI)
diisocyanate-butanediol
PU: MDI diisocyanatebutanediol PTMO
Polyamide: Nylon 11
Bond
Free
Bonded
C=O
N-H
1720 cm-1
3440 cm-1
1685-1700 cm-1
3320 cm-1
C=O
N-H
1740 cm-1
3460 cm-1
1700 cm-1
3300 cm-1
C=O
N-H
C=O
N-H
1733 cm-1
3420 cm-1
1680 cm-1
3450 cm-1
1703 cm-1
3320 cm-1
1636-1645 cm-1
3300 cm-1
• Figure 12 Morphological Model for C=O Stretch
Vibration
21
Copyright Joseph Greene 2001
FTIR Spectroscopy
• Wide-Angle X-Ray Scattering
– Wide-angle X-ray scattering (WAXS) measures electron
density variations with distances on the order of
angstroms, which is the interatomic distances
• WAXS is used to study thermoplastic elastomers with
crystalline hard or soft segments.
• Fig 14. Weight fraction crystallites versus soft segments
• Small-Angle X-ray Scattering
– Measures scattering very close to the primary beam and
measures lengths of scales on nano distances
– Probes two-phase morphology of TPEs
– SAXS measures interphase morphology and WAXS
measures intraphase morphology
22
Copyright Joseph Greene 2001