**3. Epoxy thermoset**

hard and soft segment structures, molecular weight, polydispersity, and crosslinking ability. Epoxy polymers are tough and flexible with good corrosion and chemical resistance. The epoxy reactions are highly temperature dependent, epoxy oligomer/monomer‐dependent, and energetic [3, 4]. Polymerizations can be typically carried out at or above ambient temper‐ ature (90–130°C). Solvent‐free reactions are also used. Recently, there has been considerable interest in the development of mixtures with a second reactive polymer to generate interpen‐ etrating polymer network (IPN) [5]. Interpenetrating polymer network is often considered as a novel type of polymer alloy, also known as polyblend. IPNs of polyurethane and epoxy have been reported by several investigators [6, 7]. In order to improve mechanical proper‐ ties, thermal resistance, and damping properties of polyurethane, epoxy has been introduced in polyurethane systems to form polyurethane/epoxy IPN structure. Polyblends of linear polymers as well as grafted IPNs due to covalent bonding between the polymers have been established. Semi‐IPN has also been developed from linear and crosslinked polymers [8]. Glass transition behavior and morphology studies have been used to explore the effective IPN. Generally, IPNs show excellent engineering properties due to synergetic effect induced by the compatibility of individual components in PU/epoxy IPNs [9]. Polyurethane/epoxy IPNs have been widely applied to foams, coatings, fibers, leather, and other applications [10]. To broaden the applications of PU/epoxy IPNs, these network structures have been modified using various filler structures. In this chapter, initially brief outlook on polyurethane and epoxy is illustrated. Afterward, the interpenetrating polymer network is discussed with spe‐ cial focus on polyurethane/epoxy IPNs. Applications of these networks produced by using

the two unique polymers (polyurethane and epoxy) have also been conversed.

Polyurethane (PU) is one of the most important classes of thermoplastic polymers having versatile structural relevance. Polyurethane elastomers are segmented copolymers consist‐ ing of hard and soft segment domains [11]. Soft domains are consequential of a macrodiol, while hard segments are derived from diisocyanate (**Figure 1**). When a short chain compound commonly referred as chain extender is used, polyurethanes are considered as segmented. In shape memory polyurethane (an important class of PU), hard segments in polyurethane are also known as fixed phase*,* while soft segments are termed as reversible phase*.* In general, hard and soft segments are incompatible with one another to create microphase separation. This segregation is principally responsible for excellent mechanical and other physical properties. Phase separation between the segments has been found to influence by hard seg‐ ment structure, soft segment structure, molecular weight, weight fraction, polydispersity, and crosslinking. Polyurethane exists in several forms such as rubbery materials, liquid, soft solids, and thermoplastic. Few types of polyurethanes also exist as thermoset materials. A wide range of potential applications of polyurethanes have been achieved due to tailoring the essential features of PU. General applications of PU range from foam mattress to medical implant to engineering components [12, 13]. The advantage of the choice of polyurethane in advance applications is the ease of synthesis, processability, tailorability, chemical nature of hard and soft segments, and phase separation properties. Hydrogen bonding phenomenon

**2. Prolog to polyurethane**

2 Aspects of Polyurethanes

Epoxy is generally illustrated by three‐membered rings known as epoxy or oxirane or ethoxyline group. Epoxy resin is a prepolymer having more than one epoxide group. It is generally a low molecular weight compound. The most common type of epoxy resin is bisphenol‐A epoxy resin (**Figure 2**). It is a type of epoxy resin produced by the reaction of epichlorohydrin with bisphenol‐A in the presence of a basic catalyst. The properties of epoxy resin depend on the number of monomers in the epoxy chain [18]. Epoxy with low molecular weight usually has high viscosity and exists in the liquid state. High molecular weight epoxy occurs in solid state [19]. Depending upon the chemical structure of epoxy, they may have excellent electrical properties, thermal stability, UV stability, and weather ability [20]. In addition to linear epoxy resins, they can be cycloaliphatic, tri‐functional, and tetra‐functional epoxy resins [21, 22]. Various types of epoxy resins are shown in **Figure 3**.

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

O

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

Among these types, tetra‐functional epoxy resin has high crosslinking density and high thermal resistance. Novolac epoxy resin is produced by the reaction of aromatic novolac resin with epichlorohydrin. It has high crosslinking density due to the bulk of epoxide groups. Because of crosslinking property, it has excellent chemical, thermal, and solvent resistance properties [23]. Another other phenomenon is hardening of epoxy resins. The thermosetting resins are hardened using extensive range of hardening agents. The properties of thermosets also depend on the specific combination of hardening agents and epoxy resins constituting a system [24]. Molecular structure of hardening agents affects the final structure and properties particularly the glass transition temperature of epoxy. The hardening agents are also of various types such as amine, anhydride, and catalytic hardening agents [25]. Different types of amine hardening agents have been employed in epoxies [26]. The reactions between amine hardener and the epoxy are generally nucleo‐ philic addition. These hardeners provide excellent chemical, physical, and electrical prop‐ erties to the epoxy systems [27]. Hardening can be performed at room temperature, or in the presence of light, or heat. Room temperature hardeners provide high tensile properties, electrical conductivity, thermal resistance, and low *T*<sup>g</sup> to epoxy systems [28]. Aliphatic poly‐ amines, aromatic amines, and alicyclic polyamines are generally room temperature harden‐ ers. Hardening time has been reduced using photocuring process, relative to heat curing or room temperature process [29].
