Thermoplastics and Thermosets
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Thermosetting plastics and thermoplastics are both polymers, but they behave differently
when exposed to heat.
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General Definition
Properties
Bio-based Polymers
Advancements
Thermoplastics and Thermosets
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Thermoplastics
Thermosoftening plastics
Thermoplastics and Thermosets
Thermosets
Thermosetting plastics
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— A polymer network crosslinked by
covalent chemical bonds
— Set, cured, or hardened into
permanent shape
— Some thermoplastic  thermosetting
by cross-linking with other materials.
Thermosets
Thermosetting plastics
Thermoplastics and Thermosets
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Thermosetting resins are OFTEN LIQUID AT SOME STAGE in their manufacture or processing.
The liquid-solid state transition that occurs after a thermosetting polymer is heated above its
melting temperature and then cooled down is an irreversible solidification process.
substantially infusible and insoluble. The conversion process involves a chemical reaction
typically triggered by heat, oxygen, UV light, a reagent material, or a catalyst.
During curing, small molecules are chemically linked and create complex interconnected
networks, resulting in a permanent and rigid product. Further heating of thermosets will
result in chemical decomposition and severe structure alteration.
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— A polymer network crosslinked by
covalent chemical bonds
— Set, cured, or hardened into
permanent shape
— Some thermoplastic  thermosetting
by cross-linking with other materials.
Thermosets
Thermosetting plastics
Thermoplastics and Thermosets
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Generally stronger than thermoplastics.
In comparison to thermoplastics, the mechanical properties — tensile strength, compressive
strength, and hardness — of thermosets are not temperature dependent.
Hard, plastic thermosets may undergo permanent or plastic deformation under load. The
higher the crosslink density and aromatic content of a thermoset polymer, the higher the
resistance to heat degradation and chemical attack.
Better suited to high-temperature applications up to the decomposition temperature
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Crosslinking Density
The number of effective cross-links per
unit volume of the thermoset material, in
inverse relation to the molecular weight
between cross-links (Mc).
Functionality — the number of reactive functional sites on the
reactants (monomers).
Chain length — the length between one crosslink and another.
Actual number of functional sites that react (depends on the
process).
Chain mobility between cross-links (depends on the chain
structure).
Thermoplastics and Thermosets
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Cross-link density is a critical parameter that determines the properties of the cured resin,
particularly the mechanical properties.
Generally, a thermoset resin with high cross-link density is harder but more brittle, while low
density leads to increased flexibility, better impact strength and higher elongation.
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Increasing crosslinking density will
Increase:
Decrease:
Modulus
Impact strength
Tensile Strength
Elongation
Hardness
Fatigue Properties
Chemical Resistance
Peel strength
Heat Resistance
Thermal shock resistance
Glass transition
temperature
Coefficient of thermal
expansion
Internal Stress
Brittleness
Thermoplastics and Thermosets
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This rule-of-thumb allows one to tailor the molecular weight between cross-links to optimize
final material performance. Cross-link density, however, is not the only factor in determining
resin properties of the resin (e.g., molecular structure contributes significantly to polymer
morphology as well).
In summary, once the reactants (monomers) are chosen, cross-link density is dependent on
the number of reactive sites that actually reacted during cure. For example, heating the curing
mass increases chain mobility and causes more spatial interaction to end with a cross-link (see
In contrast, to generate higher molecular weight between cross-links, one can: (1) alter the
main reaction process by using reactants (monomers) with a large length to functionality ratio,
(2) add a small percentage of nonreactive diluting agent(s) to improve molecular mobility and
separation of crosslink sites, and/or (3) use mono-functional reactive molecules to react with
and “cap” the open ends of the polymers to stop the reaction.
There are a number of methods for measuring the cross-link density of a thermoset resin:
chemical analysis, infra-red/near-infrared spectroscopy (IR), swelling, Differential Scanning
Calorimetry (DSC), and parallel plate rheometry. These methods measure the extent to which
the active resin sites are consumed. Other methods can measure the properties that relate
directly to the cross-link density: heat distortion temperature, glass transition temperature,
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hardness, electrical resistivity, dynamic and mechanical properties, thermal expansion, and
refractive index.
The most common way is the use of swelling experiments because they are generally easier
and lower cost to perform compared to other options. The degree of swelling is an inverse
dependency on the cross-link density of thermoset networks. The greater the cross-link
density, the lower the degree of swelling.
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General Purpose
Phenolics, aminos, polyesters
Engineering
Epoxy, polyurethane
Specialty
Silicones, Polyimides,
Polybenzimidazoles, Allyls,
cross-linked thermoplastics
Thermoplastics and Thermosets
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General purpose thermosets are characterized by average (for thermosets) mechanical properties,
lower resistance to temperature, higher coefficients of expansion, and low cost/commodity-like
production and sales (tons/year).
Engineering thermosets have higher mechanical properties and temperature resistance and they are
perceived to be more durable. They are more expensive and have a moderate production volume
(pounds/year). High-temperature thermosets can resist temperatures.400°F, often for long periods of
time, yet maintain their strength, adhesive, thermal, and electrical resistance properties. Costs are
often higher and manufacturing methods can be quite complex.
Specialty thermosets are useful because of one or more highly specific and unusual property that
offsets any lack of other “good” properties. They are usually very expensive and are produced in
relatively small quantities (pounds/batch).
Overlapping between the three categories often occurs; a general purpose phenolic is often
competitive with an engineering polyimide.
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Biobased
Thermosets
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— Do not set or cure under heat
— Soften into a mobile, flowable state
— Hardens and holds its shape upon
cooling
— May be reheated and reshaped
Thermoplastics
Thermosoftening plastics
Thermoplastics and Thermosets
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At termperatures above Tg, the polymer chains show increased changeableness which allows
the bulk material to flow. The polymer chains associate by intermolecular forces, which
weaken rapidly with increased temperature, yielding a viscous liquid. This is followed by their
subsequent transformation into glassy or semicrystalline hard solids after the cooling
process.
Reversible ang thermal melting – meaning that a restricted number of heating and cooling
cycles can be performed without any structure or functional effects such as color and shape
modification, microstructural alteration, and mechanical dysfunction.
If above Tm is applied, the entire crystalline structure is modified, the linear macromolecular
backbone chain becomes randomly scattered, and the specific physicochemical properties
can be altered.
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Thermoplastics
Thermosoftening plastics
Thermoplastics and Thermosets
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Thermoplastics can be classified as amorphous or semicrystalline plastics.
Amorphous – hard and rigid below Tg. Crystalline have Tm above Tg and degree of
crystallinity affects its mechanical properties.
Above its glass transition temperature and below its melting point, the physical properties of
a thermoplastic change drastically without an associated phase change. Some thermoplastics
do not fully crystallize below the glass transition temperature, retaining some or all of their
amorphous characteristics
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Thermoplastics
Thermosoftening plastics
Has higher impact strength, easier processability, and
superior adaptability to mold complex designs
— superior mechanical
behavior
— chemical stability
— optical transparency
— durability
Thermoplastics and Thermosets
— thermal and electric
behavior
—self-lubrication ability
— hydrophobicity or
waterproofing
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. Amorphous and semi-amorphous plastics are used when high optical clarity is
necessary, as light is scattered strongly by crystallites larger than its wavelength.
Amorphous and semi-amorphous plastics are less resistant to chemical attack and
environmental stress cracking because they lack a crystalline structure. May have
lower solvent resistance than thermosets.
At ambient temperatures, semicrystalline plastics have greater rigidity, hardness,
density, lubricity, creep resistance, and solvent resistance than amorphous
counterparts.
Brittleness can be decreased with the addition of plasticizers, which increases the
mobility of amorphous chain segments to effectively lower the glass transition
temperature. Modification of the polymer through copolymerization or through the
addition of non-reactive side chains to monomers before polymerization can also
lower it.
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Commodity
Acrylates, PE, PS, PVC, TPU
Engineering
ABS, PA, PC
Specialty
Fluoropolymers, LCP, PPS
Thermoplastics and Thermosets
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Commodity
The term commodity plastics is used to describe a category of thermoplastic materials that are
widely used and readily available around the world—hence the term commodity. While they
can be used for structural purposes, they are typically used in cost-sensitive, high-volume
applications such as packaging, clothing, and personal items intended for short term
use.
These materials typically have better mechanical properties than commodity plastics, and as
such are often used for structural purposes. The term engineering is derived from the fact that
their performance can be predicted using traditional engineering calculations.
In addition, there is substantial property data available for these materials, accounting for
both short- and long-term use under a variety of end-use conditions.
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Biobased Thermoplastics
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