3.1 Coupling agent
Eugenol-based silane coupling agent (EBSCA) was synthesized via hydrosilylation. This silane coupling agent enhanced the connection between CNCs and epoxy matrix to achieve sustainable and environment friendly products. Eugenol-based epoxy silane coupling agent with high purity was prepared and used for the surface modification of nano-cellulose crystals. The eugenol epoxy silane-coupling agent, bearing a long chain structure of benzene ring in molecular structure, which could improve the compatibility of CNCs with different bio-based epoxy, contributing to the dispersion state in the matrix, enhancing the overall performance of epoxy-cured products.
3.2 1H-NMR of eugenol base silane coupling agent (EBSCA) coupling agent
Figure-4 shows the 1H-NMR spectrum of eugenol base silane coupling agent (EBSCA) in CDCl3. The signals from the allylic group in coupling agent disappeared owing to the complete hydrosilylation reaction, the new signals (H11, 12) shown in EBSCA assigned at 1 H NMR spectrum confirm the synthesis of EBSCA. 1H NMR (CDCl3) δ ppm: H7 (6.80–6.90); H5,6 (6.65–6.72); H3 (4.13–4.23, 3.92–4.02); H4,11 (3.75–3.89); H2 (3.28–3.39); H8 (2.80–2.88); H1 (2.68–2.73, 2.53–2.63); H9 (1.67–1.82); H12 (1.16–1.29); H10 (0.60–0.73).
3.3 Chemical modification of cellulose nanocrystals with silane coupling agent
Cellulose nanocrystals modified with eugenol-based silane coupling agent (EBSCA) are shown in the Figure-5. The broad peak appeared at 1540 cm− 1 is due to C = C stretching vibration. The peak appeared at 780 cm− 1 attributed to ~ CH bending vibration. This confirmed the successful modification of cellulose nanocrystals, modified by eugenol-based silane coupling agent (EBSCA) [47].
3.4 Scanning electron microscope of native and modified CNCs
The Scanning electron microscope images were obtained on field emission scanning electron microscope (SEM, SU-3500). After sputter coating of gold on the specimen’s surface. The morphologies of the surface of native and modified cellulose nanocrystals images were obtained and clearly showed the surfaces morphologies, reacted with aforementioned epoxies. The native CNCs surface is much rougher than the modified CNCs. In general, their combination has a favorable effect on the performance of composites, which results an increase in the adhesive properties [48]. In Figure-6 (A) shows surface of native CNCs. Figure-6 (B). SEM of cellulose nanocrystals matrix appeared with bio-based epoxy resin, but their diameter decreased after spreading to varying degrees in the epoxy.
However, as additional CNCs aggregation occurred, which prevented the formation of a homogeneous mixture. In general, their combination has a favorable effect on the performance of composites. However, better dispersion was obtained using a modified epoxy resin with low CNCs content. Low concentrations of CNCs are more appropriate than higher, and aggregation resulted with increasing concentrations. Another factor contributing to the uneven distribution is residual epoxy evaporation. Dispersion of CNCs in liquid feed is affected by liquid phase ratio. The presence of multiple break lines in the bio-based epoxy surface indicates the hardness of the material. The modified bio-based epoxy resin particles exhibited bumps and collapsed morphology.
3.5 Surface analysis of modified CNCs
Surface roughness measurements were used as criteria to evaluate the dispersion quality of modified CNCs. Images obtained by atomic force microscopy (AFM) are presented in Figure-7. Qualitatively comparing the micrographs revealed that the addition of modified CNCs have increased the surface roughness, irrespective of the surface modification method. Moreover, the higher loading level, associated with rougher surface. The modified CNCs showed high roughness on surface. Quantitative measurements of roughness had small values, showing that modified CNCs had a smooth surface showed Rq 0.14 µm.
Therefore, it was confirmed that the roughness increased proportionally with modified CNCs loading level. Strong van der Waals forces between modified CNCs could explain the need for higher shear rate. The observed increase in roughness and especially, if the viscosity of the epoxy coating was considered. These roughness measurements demonstrated that modified CNCs were used as a reinforcing agent had relatively good compatibility.
3.6 Adhesive properties of native and modified CNCs
Here, we have also evaluated the adhesives strength of native and modified CNCs, because they have an assured strength and toughness. The average adhesive strength is mentioned in Table-1 and Table-2, using eugenol-based silane coupling agent with the CNCs content, while triethylenetetramine used as a curing agent. The bio-based epoxy resins containing native and modified CNCs at 1, 3 and 5 weight percent (wt%) to evaluate the effect of enhancement potential. With the introduction of tensile modulus and strength at maximum load using steel plates. Modified CNCs has an obvious enhancement effect with 1 wt% and 3 wt% on tensile modulus, but after adding 5 wt% caused agglomeration in the epoxy medium [49, 50]. The maximum value observed for nanocomposites reinforced with 1 wt%. [51]. The tensile data shows that all the modified CNCs samples had a higher tensile strength than the standard one. The results of modified CNCs binding themselves through -OH, creating a high strength linkage, with potentially interaction among them, increasing the adhesive properties. The increasing of modified CNCs loading result in agglomeration between particles, which produce weakness in the material, explaining the comparative decrease in adhesive with individual loading.
Table 1
Adhesive properties of bio-based epoxy with native and modified CNCs
Formulation SIEEP4 bio-based epoxy | Tensile strength | Tensile modulus | Elongation at Break |
MPa | MPa | dl% |
1% modified | 2190 | 13.50 | 0.6 |
3% modified | 2100 | 13.20 | 0.5 |
5% modified | 2150 | 15.70 | 0.6 |
1% unmodified | 2160 | 10.50 | 0.7 |
3% unmodified | 2040 | 10.50 | 0.7 |
5% unmodified | 2090 | 11.00 | 1.1 |
Table 2
Adhesive properties of bio-based epoxy with native and modified CNCs
Formulation SIEEP2 bio-based epoxy | Tensile strength | Tensile modulus | Elongation at Break |
MPa | MPa | dl% |
1% modified | 1940 | 14.70 | 1.3 |
3% modified | 2000 | 16.00 | 1.6 |
5% modified | 2410 | 15.30 | 1.9 |
1% unmodified | 1840 | 14.16 | 1.3 |
3% unmodified | 1870 | 15.60 | 1.2 |
5% unmodified | 2000 | 16.00 | 0.9 |
3.7 Bio-based epoxy with native and modified CNCs mechanical properties
The main challenges encountered to use bio-based epoxy resins and their composites; including the design of high processability (low-viscosity) epoxy resins and cured epoxy resins with high stress-strength and toughness that can be recycled and reused. Here, we have also tried our best to study different types of bio-based epoxy for mechanical properties. We have found good mechanical properties in our synthesized bio-based epoxy. The results are much better, because the remaining bio-based epoxy (SIEEP4 and SIEEP2) are hyper-branched epoxy resins. This is an important epoxy resin that belongs to one sub-class of dendrimers and contain highly branched topological structures with a high content of functional groups. The deformable topological structure, good solubility with other matrixes, high chemical stability and low viscosity makes them more important. Triethylenetetramine (TETA) is used as curing and toughening agent for thermoset materials for bio-based epoxy resins. In recent years, scientists have made significant progress in synthetic methods of low viscosity. The synthetic approaches of hyper-branched that includes esterification, etherification, polymerization and oxidation of double bonds, and hydrosilylatin reaction. The low viscosity of curing agent can improve the mechanical properties of SIEEP4 and SIEEP2 (bio-based epoxy codes) by separating the entangled molecular chains of bio-based epoxy. Among the wide applications of TETA, one of the important use in the industrial field is their simultaneous reinforcing and toughening function on epoxy. Existing methods of simultaneous reinforcing and toughening of bio-based epoxy includes the use of nanomaterials, block polymers and hyper-branched epoxy resins. The homogeneous dispersion and size of nanoparticles, as well as good adhesion between these bio-based epoxies and nanoparticles are critical factors, influencing the degree of improvement of strength and toughness.
To cope with aforementioned challenges, which includes highly efficient recycling, improving our understanding of the homogeneous reinforcing and toughening mechanism. We have prepared degradable hyper-branched bio-based epoxy resins by esterification and thiol-ene reaction for high-performance. The cured bio-based epoxy composites demonstrated good mechanical properties and degradability, high tensile strength and toughness.
Evaluation of mechanical properties of the bio-based epoxy was investigated by performing normal stress tests, as shown in Figure-8 and Figure-9. The inelastic deformation was observed with increase in applied stress as shown in the curvature portion of the curves. Eventually, fracture strength resulting in breakage. Modified CNCs exhibited a yielding behavior. Such a yielding phenomenon was not observed by composite samples having native CNCs matrix. This reinforcing effect was attributed to the high rigidity and the increased crystallinity of modified CNCs. It should be pointed out that the loss of the ductility was accompanied. Furthermore, the observed reinforcing effect maintained at high temperature condition. The storage modulus was 1622 MPa for 1wt% modified CNCs composites, which was considerably higher than 1429 MPa value of the native CNCs. In a sharp contrast, the composites exhibited higher storage modulus since the existence of CNCs within polymeric matrix, which restricted the molecular motion. For semi-crystalline polymer like modified CNCs, its strength is usually proportional to its crystallinity. Thus, the increased crystallinity played a key role in reinforcement of composite’s properties. On the other hand, the high rigidity nature of CNCs also contributed to reinforcement.
3.8 Contact angle of native CNCs with bio-based epoxy
The contact angle is an angle, which is conventionally measured through the liquid, where a liquid vapor interface meets a solid surface. The contact angle of water on native and modified CNCs with bio-based epoxies were carried out to demonstrate the wettability of native and modified CNCs as shown in the Figure-10.
The abundant –OH groups of native CNCs are responsible for hydrogen bonding with the water molecules (CA = 55 ± 2°). The native CNCs lost their hydrophilicity after modification with bio-based epoxy. The modified CNCs showed hydrophobic behavior (CA = 102 ± 2°) due to the presence of eugenol based-silane coupling agent (EBSCA).