**6. Modified polyurethane/epoxy IPNs**

**5. Polyurethane/epoxy interpenetrating polymer network**

At the beginning of cure reaction, *T*<sup>g</sup>

of IPNs. The increase in *T*<sup>g</sup>

in *T*<sup>g</sup>

Covalent bond

6 Aspects of Polyurethanes

Polymer 1

**Figure 4.** Interaction in IPNs.

remain unreacted and act as plasticizer leading to lower *T*<sup>g</sup>

Due to high modulus, strength, and mechanical properties, epoxy resins have been used in high‐performance structural composites. However, engineering applications of epoxy resins have been limited in several cases such as damping materials [35]. In contrast, polyurethane is a flexible and elastic polymer with low mechanical strength [36]. PU prepolymers have been prepared and incorporated into epoxy resin to form IPNs. The mechanical properties of PU/epoxy IPNs largely depend on the amount of polyurethane in the blend network. Jin et al. [6] investigated tensile properties of soybean oil‐based PU/epoxy IPNs. The tensile strength and tensile modulus of PU/EP IPNs with 5–20 mass% PU were lower than pure epoxy. This means that the addition of PU turned epoxy to the rubbery elastomer increased the elon‐ gation at break by 13‐fold as compared to pure epoxy. However, increasing the amount of epoxy increased tensile strength and tensile modulus drastically. IPNs basically integrate the structure and properties of these versatile polymers (epoxy and polyurethane). Integrated per‐ formance of epoxy/polyurethane networks have been improved by the structural modification. The glass transition temperature of PU/epoxy IPNs provides important information about the miscibility of the blend components and blend compatibility. Moreover, the glass transition temperature of PU/epoxy IPNs also provides information about the cure rate of the reaction.

Semi-IPN Fully formed IPN

Polymer 2

epoxy. The cure rate of epoxy is usually slower than that of PU. Some of the epoxide groups

the cure reaction. When reaction proceeds and an IPN is formed, this may result in an increase

ture through the reaction of hydroxyl groups of epoxy with isocyanate [37].

of PU/epoxy IPNs is usually lower than that of neat

can be attributed to the miscibility/formation of graft struc‐

values of IPNs at the beginning of

Polyurethane/epoxy IPNs have been prepared by adopting several modifications. The reinforced PU and diglycidyl ether of bisphenol A (DGEBA) IPN composites have been prepared with aramid fibers. The mechanical properties have been found to improve [38]. The PU/epoxy IPNs exhibited higher tensile and Izod impact strength. Montmorillonite‐filled polyurethane/epoxy IPN nanocomposites were prepared, and the influence of hydrogen bonding on free volume and miscibility of clay was studied [39]. Polyethylene glycol‐based polyurethane and epoxy IPN composite filled with fly ash have also been studied [40]. Semiconductive polyaniline and polyurethane/epoxy nanocomposite have been prepared for electromagnetic interference (EMI) shielding and charge dissipation applications. The damping properties of the modified PU/epoxy IPN composites have also been studied [41]. *T*g , contact angle, interfacial, and mechanical properties have been investigated [42]. Recently, Kausar and Rahman Ur [43, 44] reported modified epoxy/polyurethane interpenetrating networks and their composites. Damping of vibration is a critical problem in the design of structural materials because excessive vibration may cause damage to the surroundings or the material components. To solve this problem, polymeric and composite IPNs with high damping properties around glass transition temperatures have been focused [45]. Chen et al. [46] prepared a series of castor‐oil‐based polyurethane/epoxy resin graft IPNs modified by hydroxy‐terminated liquid nitrile rubber (HTLN). **Figure 5** and **Table 1** show the damping properties of HTLN‐modified PU/epoxy IPN composites at 10 Hz. The glass transition tem‐ perature (corresponding to the peak of tan *δ*) was shifted to higher values with HTLN addition compared with that of pure IPNs. *T*<sup>g</sup> of 5% loaded composite was increased to 71°C relative to neat IPNs (68.2°C). The 5% HTLN‐modified PU/epoxy also showed good damping properties.

**Figure 5.** DMA curves of PU/EP IPN composites as a function of the HTLN content at 10 Hz [46].

The tensile strength of polyurethane/epoxy IPN composites filled with montmorillonite (MMT) has been studied [8]. According to **Figure 6**, 1% MMT resulted in the maximum tensile strength of the composites. The tensile strength of all the filled composites increased by about 40% relative to pure PU/epoxy IPNs. The results were attributed to the large interface area and strong interfacial adhesion between the IPN matrix and MMT. **Figure 7** shows the impact strength of MMT‐filled PU/epoxy IPN composites. The impact strength primarily increased and then decreased with the increase in the MMT content. The impact strength reached higher values for 1 and 3% MMT contents. The results showed strong mutual interactions between the filler and matrix, and thus the impact strength of MMT‐modified composites was improved.


**Table 1.** DMA data of PU/EP IPN and HTLN‐modified PU/EP IPN composites at 10 Hz [46].

**Figure 6.** Tensile strength of MMT‐filled PU/EP IPN composites as a function of the MMT content [8].

**Figure 7.** Impact strength of MMT‐filled PU/EP IPN composites as a function of the MMT content [8].

The tensile strength of polyurethane/epoxy IPN composites filled with montmorillonite (MMT) has been studied [8]. According to **Figure 6**, 1% MMT resulted in the maximum tensile strength of the composites. The tensile strength of all the filled composites increased by about 40% relative to pure PU/epoxy IPNs. The results were attributed to the large interface area and strong interfacial adhesion between the IPN matrix and MMT. **Figure 7** shows the impact strength of MMT‐filled PU/epoxy IPN composites. The impact strength primarily increased and then decreased with the increase in the MMT content. The impact strength reached higher values for 1 and 3% MMT contents. The results showed strong mutual interactions between the filler and matrix, and thus the impact strength of MMT‐modified composites was improved.

 **(°C)**

**PU/epoxy/HTLN (%) tan δ Tg**

8 Aspects of Polyurethanes

50/50/0 1.063 68.2 50/50/5 1.105 71.0 50/50/10 1.031 70.9 50/50/15 1.081 70.0 50/50/20 1.041 71.9

**Table 1.** DMA data of PU/EP IPN and HTLN‐modified PU/EP IPN composites at 10 Hz [46].

**Figure 6.** Tensile strength of MMT‐filled PU/EP IPN composites as a function of the MMT content [8].
