**4. Interpenetrating polymer network (IPN)**

Cl O

4 Aspects of Polyurethanes

Epichlorohydrin

**Figure 2.** Preparation of bisphenol‐A epoxy.

C

O

O

O

<sup>O</sup> <sup>O</sup>

**Figure 3.** Different types of epoxy resin.

O

O

O

O

O

Diglycidyl ether of bisphenol-A

O O <sup>O</sup> <sup>O</sup>

Tetrafunctional epoxy resin Trimethylol propane

O

Cycloaliphatic epoxy resin Novolac epoxy resin

O

HO OH Bisphenol A

> H2 C

> > n

N N O

O O

O

O O O

O

<sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup>

Epoxy Resin n = 1-15

OH

H2 C C H

> Interpenetrating polymer network (IPN) is a unique type of polyblend. IPNs can be sim‐ ply defined as a mixture of two or more crosslinked polymeric networks [30]. The interpenetrating polymer network can be generated using physical or chemical interlock‐ ing between the polymer chains [31, 32]. IPNs can be considered as the crosslinking of one polymer component in the presence of another polymer component to form crosslinked polymer network. In this regard, fully formed interpenetrating polymer network and semi‐ interpenetrating polymer network have been identified [33]. Fully formed interpenetrat‐ ing polymer network are acknowledged by the presence of crosslinks in both the network polymers, and the whole polymer components are crosslinked. Semi‐IPN exists when one of the components is crosslinked and other is linear or non‐crosslinked [34]. Semi‐IPN is also referred to as pseudo‐IPN. In *in situ* crosslinking, results in polymer chains are well inter‐ locked. The full IPN and semi‐IPN are shown in **Figure 4**. IPNs are sometimes confused with the polymer blends, block, graft, or crosslinked copolymers. However, there exist differences. An IPN can be distinguished from block copolymers as IPNs swell in the presence of solvents but does not dissolve. Moreover, IPNs exhibit characteristic morphologies. Polyurethane has been applied in elastomer, leather, foam, coatings, and fibers. Epoxy resin has been intro‐ duced in polyurethane systems to form epoxy/polyurethane interpenetrating polymer net‐ work structure. Various methods have been used to prepare and modify the properties of epoxy/polyurethane IPNs.

**Figure 4.** Interaction in IPNs.
