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Polymers and composites

Which is a characteristic of thermosetting resins?

  1. AThey soften and melt when heated and set again on cooling, so they can be remolded any number of times
  2. BTheir molecular chains are crosslinked into a three-dimensional network, and once cured they do not melt when heated
  3. CTheir molecular chains are linear without crosslinks, so even after curing they dissolve readily in solvents
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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.

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More Polymers and composites questions

  1. Which statement about the glass transition temperature Tg and melting point Tm of a crystalline (semicrystalline) polymer is correct?
  2. A polymer sample consists of equal masses of molecules with a molecular weight of 1.0×10⁴ g/mol and molecules of 1.0×10⁵ g/mol. Which correctly gives the number-average molecular weight Mn and the weight-average molecular weight Mw?
  3. Take the density of perfectly crystalline polyethylene as 1.000 g/cm³ and that of fully amorphous polyethylene as 0.850 g/cm³. Which value is closest to the crystallinity (mass fraction) of a sample with a density of 0.925 g/cm³?
  4. For the same type of polymer, what generally happens as the crystallinity increases?
  5. When a plasticizer is added to poly(vinyl chloride) (PVC), what happens to the glass transition temperature Tg and the properties of the material?