A resin that forms a three-dimensional network by a curing reaction and does not melt when heated after curing. Examples include epoxy and phenolic resins.
Japanese name: 熱硬化性樹脂
Thermosetting resin appears in 2 practice questions across 2 topics, including Polymers and composites, Materials selection and design.
Field: Polymers and composites
Thermosetting resins harden as a low-molecular-weight base resin reacts with a curing agent to build a three-dimensional network. Epoxy resins cure with amine or acid anhydride hardeners, unsaturated polyesters by radical polymerization with styrene, and phenolic resins by condensation of phenol and formaldehyde. The point at which the reaction has connected molecules throughout the system is the gel point, after which the resin no longer flows. As the reaction continues and the Tg of the network catches up with the curing temperature, the molecules can no longer move and the reaction stops, which is called vitrification.
In thermoplastics the chains are held together only by intermolecular forces, so they flow when heated and can be reshaped any number of times. In thermosets the chains are crosslinked by covalent bonds, so they do not melt when heated and do not dissolve in solvents, only swell. Thermosets do have a glass transition temperature, however, and above it their modulus drops greatly. The Tg of epoxies ranges from around 100 °C to over 200 °C depending on the hardener and curing conditions, and heat resistance is set by the crosslink density and degree of cure.
With insufficient cure, Tg is low and strength and chemical resistance do not fully develop. The curing temperature and time (the cure schedule) are therefore set by measuring the remaining reaction heat by DSC to find the degree of cure and checking Tg by DMA. Where needed, the resin is post-cured at a higher temperature. The curing reaction shrinks the volume, and cooling shrinks it further, producing residual stress and warpage in parts and in CFRP. Epoxies have relatively small cure shrinkage, which suits applications requiring dimensional accuracy.
Thermosets excel in heat resistance, dimensional stability and electrical insulation, and are used as CFRP matrices, glass–epoxy printed circuit boards (FR-4), heavily silica-filled semiconductor encapsulants, adhesives and paints. The phenolic resin (Bakelite) invented by Baekeland in 1907 was the first true synthetic resin. On the other hand, the inability to melt and remold them is a major recycling problem, and research is under way on recovering fibers by pyrolysis or chemical decomposition and on vitrimers, whose bonds can be rearranged by heating.
Answer: B. Their molecular chains are crosslinked into a three-dimensional network, and once cured they do not melt when heated
In thermosetting resins such as epoxy and phenolic resins, the curing reaction chemically bonds (crosslinks) the molecular chains into a three-dimensional network. Once cured they do not melt when heated; at high temperature they decompose before they flow. They have excellent dimensional stability, heat resistance and solvent resistance, and are used as semiconductor encapsulants, printed circuit boards and the matrix of CFRP. Softening and melting on heating and being remoldable any number of times are characteristics of thermoplastics such as polyethylene and PET. Thermoplastics have linear or branched chains without crosslinks, and many dissolve in solvents. A drawback of thermosets is that they are difficult to recycle.
Not necessarily. The name comes from curing irreversibly on heating, but there are also two-part epoxy adhesives that cure at room temperature and resins that cure with ultraviolet light. What they have in common is that they form a crosslinked network after curing.
Yes. Even in a crosslinked network, partial motion of the chains begins above Tg, and the modulus drops by one to two orders of magnitude to a rubbery state. Because of the crosslinks, however, the resin does not flow, and further heating decomposes it before it melts.
Vulcanized rubber is also a crosslinked network and likewise does not melt on heating. But its crosslink density is low and its Tg is far below room temperature, so it is treated separately from hard thermosets, as an elastomer that shows rubber elasticity at room temperature.
In thermosetting resins such as epoxy and phenolic resins, the curing reaction chemically bonds (crosslinks) the molecular chains into a three-dimensional network.
Which is a characteristic of thermosetting resins?Thermosetting resins have a network structure and do not melt when heated, so unlike thermoplastics they cannot be melted and remolded and are difficult to recycle as materials.
Which is an appropriate materials selection and design poli…CFRP (carbon-fiber-reinforced plastic) Crystallinity Glass transition temperature Rule of mixtures Viscoelasticity