samples of epoxidized sorbitol‐cardanol at a 1 : 1 weight ratio are higher than that of the same ratio of epoxidized isosorbide (60 °C and 93 as compared to 48 °C and 91). Material based on epoxidized isosorbide‐cardanol (3 : 1) exhibits Tg of 83 °C (isosorbide 100, with EEW = 155 g/eq.) and 62 °C (isosorbide 102 with EEW = 158 g/eq.), respectively.
Moreover, cardanol derivative obtained throughout the reaction of cardanol with 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) (Figure 1.38) might be used as the flame‐retardant agent [119].
A triscardanyl phosphate (PTCP) containing phosphaphenanthrene groups is synthesized via dehydrochlorination, epoxidation, and ring opening reaction of cardanol (Figure 1.39).
Figure 1.36 Chemical structure of cardanol NC‐514.
Figure 1.37 Epoxy reactants: epoxidized isosorbide and polyglycidyl ether of sorbitol.
Figure 1.38 Chemical structure of DOPO.
Based on the performed studies (Table 1.5), it was observed that the incorporation of PTCP into DGEBA epoxy resin accelerates the thermal degradation process and improves the char yield.
The same higher char yield is valuable toward improving the flame‐retardant properties of epoxy resins. The LOI value of EP/PTCP‐30% increases from 23.0% (neat epoxy material) to 30.5%. Moreover, compared with neat epoxy resin, the impact strength of EP/PTCP‐30% increases by 29%.
Cardanol is an interesting nonedible by‐product of CNSL industry. Because of the presence of phenolic hydroxyl group, olefinic linkages in the alkyl chain, and aromatic ring, it is a promising nonharmful renewable substituent to BPA. The presence of long aliphatic chain of cardanol in bio‐based epoxy resins results in lower Tg values of obtained materials than those with DGEBA. At the same time, these materials are characterized by very interesting thermal stabilities. Moreover, the mechanical properties of cardanol‐based epoxies are lower than DGEBA epoxies. However, numerous, described in the literature, studies tested coating applications of epoxies obtained with the cardanol derivatives.
Figure 1.39 Synthesis of PTCP.
Table 1.5 Thermal and mechanical properties of PTCP, neat epoxy, and EP/PTCP samples.
| Sample | Thermal properties | Mechanical properties | |||||||
|---|---|---|---|---|---|---|---|---|---|
| T10% (°C) | Tmax1 (°C) | Tmax2 (°C) | Char residue (%) | LOI (%) | Impact strength (kJ/m2) | Tensile modulus (GPa) | Tensile strength (MPa) | Elongation at break (%) | |
| PTCP | 286 | 322 | 500 | 2.8 | — | — | — | — | — |
| EP | 361 | 367 | 554 | 1.4 | 23.0 | 14.9 ± 1.1 | 1.56 ± 0.10 | 40.6 ± 2.5 | 2.2 ± 0.8 |
| EP/PTCP‐10% | 336 | 344 | 554 | 4.2 | 26.5 | 16.8 ± 1.0 | 1.35 ± 0.00 | 46.3 ± 3.4 | 5.5 ± 01 |
| EP/PTCP‐20% | 328 | 344 | 575 | 5.7 | 28.0 | 18.1 ± 1.4 | 1.46 ± 0.03 | 60.8 ± 4.4 | 7.7 ± 1.8 |
| EP/PTCP‐30% | 311 | 335 | 577 | 8.3 | 30.5 | 19.1 ± 0.5 | 1.09 ± 0.09 | 46.7 ± 1.1 | 8.2 ± 0.4 |
Figure 1.40 Chemical structures of isosorbide.
Figure 1.41 Schematic reaction pathway for the production of bio‐based isosorbide.
1.3.4 Isosorbide
Isosorbide (1,4:3,6‐dianhydro‐D‐glucitol) is an organic oxygen‐containing heterocyclic compound composed of two fused furan rings (Figure 1.40).
Isosorbide does not occur naturally. It can be obtained from various raw materials by organic synthesis through different reaction pathways. However, nowadays, isosorbide is produced from diverse polysaccharides through the multistep process that includes several intermediates [120] (Figure 1.41).
The synthesis
