Re: Thermosets and thermoplastics

From: Guy Allen Brady (
Date: Tue May 13 1997 - 18:41:14 EEST

Here's my general take on the subject:

Thermoplastics are typically long-chain polymers. Properties are
determined by length of chain, stiffness of chain and how the chains pack
together (can be crystalline e.g. polyethylene). Typically they are not
crosslinked, but can be. Thermoplastics, to most folks, are those
plastics which can be melted and resolidified without degradation - this
means the polymer chains can slide past one another easily, translating to
lower modulus and/or lower viscosities.

I think of thermosets as a "processing name" (where thermoplastics is a
"property name") because it adequately describes how they are made. Mix
together monomers, crosslinking agents and catalysts, pour in mold, heat
to "set" the shape. Most of what we call thermosets are 3D networks made
from crosslinking chains of those monomers together. That's because the
monomers are typically multifunctional and can form bonds in many
directions, leading to the 3D network. You can make a thermoset from
single and di- functional monomers, but (depending on the monomer) you'd
likely form a "thermogoo" - no 3D network for strength. As a result of
the 3D network, you've got a structure that will not flow when heated like
a thermoplastic (primarily 2D chains wrapped together). You end up
breaking the bonds (crosslinks) and fundamentally changing the structure
(thermal degradation) upon heating.

I think that's enough for now.

          G. Allen Brady -- Graduate Research Assistant
Materials Science and Engineering - The University of Michigan
2219 H.H. Dow Bldg. 2300 Hayward Street Ann Arbor, MI 48105
work: 313/936-0177 fax: 313/747-4807 email:

On Tue, 13 May 1997, M. Burns ( wrote:

> Good morning,
> A technical question for the materials (plastics) geniuses out there:
> As I understand it, plastics (i.e. organic polymers) may be
> classified as either thermoplastics or thermosets, the difference being
> whether they respond to heat by (thermoplastics:) reversably softening and
> melting or (thermosets:) irreversably degrading.
> Now it seems that individual types of plastics are classified as one
> or the other of these two. E.g. ABS = thermoplastic; PMMA = thermoset.
> But I've also heard that what makes a thermoset is crosslinking.
> So my question is this: (a) Can any plastic be made a thermoset
> by crosslinking? and (b) Can any plastic be made a thermoplastic by
> preventing crosslinking?
> Thanks for your help with this fairly basic question.
> Best regards,
> Marshall Burns
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