Aliphatic and Cycloaliphatic Amines as Hardeners
Rosso et al. investigated the property improvements of piperidine-cured DGEBA by modification with 5 wt % nanosilica. Although the tensile strength remained unchanged, the tensile modulus was increased by more than 20%. The fracture toughness (K1c) was improved by 70%, and G1c was improved by more than 140%.
In a continuation of this work, Wetzel et al. explored the fracture and toughening mechanisms using Al2O3 and TiO2 nanoparticles of similar sizes (ca. 20 nm) in 4,4′-methylene bis(2-methylcyclohexyl-amine) cured DGEBA. They identified crack-deflection processes, crack pinning, and energy dissipation rather than debonding at the particle–resin interface as reasons for the toughness improvements. A very detailed description is given in Wetzel's Ph.D. thesis.
The fracture behavior of piperidine-cured DGEBA with various nanosilica concentrations at high and low temperatures was reported by Deng et al. They found that the toughness increased significantly at RT and 50°C with a maximum at approximately 5 wt % nanosilica. At 70°C, they found no increase in the modulus or toughness. At 0 and −50°C, the improvements were much smaller; this indicated different mechanisms at different temperatures.
The curing kinetics of a modified DGEBA cured with piperidine were studied by Rosso and Ye. The addition of 1–5 vol % nanosilica led to a higher reactivity in curing and an alteration in crosslinking. They suggested the formation of an amino-rich interphase region around the silica nanoparticles, which could have been responsible for the property improvements.
Haupert et al. looked into the tribological properties of DGEBA cured with an aliphatic amine. They found improvements at nanosilica addition levels above 2 vol % and a maximum at 5.5 vol %. The wear resistance was improved by 30%.
DGEBA and DGEBF cured with an accelerated aminoethyl piperazine were the subject of studies by Dittanet. She reported an increase in the modulus of 108% for DGEBA and 90% for DGEBF at a 30 wt % addition level of nanosilica. A significant reduction in coefficient of thermal expansion (CTE) was reported as well.
The cyclic fatigue properties of a piperidine-cured DGEBA was studied by Mai et al. They found a fatigue life improvement of 145% with 2 wt % nanosilica, an even slightly higher improvement at 6 wt % nanosilica, but only a 56% improvement at 10 wt % nanosilica. Apparently, there is no linear relationship between the silica content and increased fatigue performance, but a maximum seems to exist at a certain addition level.
Liang and Pearson investigated the toughening mechanisms of piperidine-cured DGEBA. In addition to 20-nm silica nanoparticles, they used 80-nm particles from a small-scale manufacturer, which might have had a different surface modification. Neither particle sizes influenced Tg (≤24.6 wt % of silica). The modulus increased by approximately 20% for both particle sizes. The compressive property and toughness increases were nearly identical as well. The authors concluded that the influence of particle size was negligible in the range of 20–80 nm.
The interactions between silica nanoparticles and diethylene triamine cured DGEBA before and during network formation was the subject of research by Baller et al. In the first stage of isothermal curing, there was no difference between epoxy resins with different nanosilica contents, whereas later in curing, the reaction rate was reduced, probably due to the reduced mobility of the matrix with increased nanoparticle content.
This study was continued by Philipp et al., and the generalized Cauchy relation was investigated. It seems that the cured epoxy resins with different amounts of nanosilica incorporated behaved similarly to porous silica glasses, and this indicated a perfect distribution of monodisperse silica nanoparticles.
Tsai et al. investigated nanosilica-containing DGEBA cured with a modified isophorone diamine. The modulus was increased up to 19% and K1c was increased by 81% with 40 wt % nanosilica. Again, the improvements increased with increasing nanoparticle addition.
Furthermore, Tsai and Chang explored the damping properties of isophorone-diamine cured DGEBA and reported slightly improved damping properties (+3,24%) at 10 wt % nanosilica addition.
Ye et al. reported increases in the modulus from 2.9 to 3.3 GPa (with 10 wt % nanosilica) and 3.6 GPA (with 20 wt % nanosilica) for a DGEBA resin cured with piperidine.G1c was increased from 238 to 458 and 666 J/m2 (improvements of 92 and 180%, respectively).
Liu et al. looked further into K1c of piperidine-cured DGEBA. They found an increase in the modulus and K1c with increasing loading level: 22% increase in the modulus and 304% increase in G1c at 20 wt % nanosilica.
In another study, Liu et al. examined cyclic fatigue crack propagation and reported significant improvements in the fatigue lifetimes for 6 and 12 wt % nanosilica. They discussed extensively the contribution of the different toughening mechanisms identified at high and low loading levels.
In continuation of earlier work, Dittanet and Pearson tried to identify the influence of the nanoparticle size on the toughening of a piperidine-cured DGEBA epoxy resin. They used particles with average sizes of 23, 74, and 170 nm from a small-scale manufacturer and reported improved properties with increasing addition levels of nanosilica. The modulus was increased by approximately 60% at 30 vol % nanosilica addition regardless of the particle size. G1c was improved by 221% for the 23-nm particles, 239% by the 170-nm particles, and 317% by the 74-nm particles. Interesting was the reduction of the CTE size dependence as well; the 23-nm particles performed best.
Mechanisms for the toughening effect of nanosilica were discussed as well, and the model from Kinloch et al. was confirmed; see the next two sections in this article. Matrix shear banding was the dominant mechanism, matrix void growth was secondary, and the debonding of silica nanoparticles had only a minor effect.
Table 1 gives an overview of the increases in the modulus and K1c (at RT) versus addition levels of nanosilica. The same particles were used together with piperidine as a hardener, and identical curing conditions were used.
Table 1. Properties of Piperidine-Cured Epoxy Resins with Various Nanosilica Contents
|SiO2 content (wt %)||Modulus (GPa)||K1c (MPa m1/2)|
|0||2.80 ± 0.03||2.86 ± 0.11||2.86 ± 0.08||0.967 ± 0.07||0.89||0.95 ± 0.03|
|2||2.89 ± 0.07||2.90 ± 0.06||2.88 ± 0.03|| || ||1.01 ± 0.04|
|4||2.98 ± 0.15|| ||2.93 ± 0.03|| || ||1.14 ± 0.06|
|5|| || || ||1.66 ± 0.11|| || |
|6||2.94 ± 0.07||2.98 ± 0.08||2.98 ± 0.10|| || ||1.26 ± 0.04|
|8||3.18 ± 0.12|| ||3.10 ± 0.15|| ||1.43||1.39 ± 0.07|
|10|| ||3.14 ± 0.14||3.14 ± 0.14|| || ||1.57 ± 0.02|
|12|| || ||3.20 ± 0.05|| || ||1.70 ± 0.05|
|20|| || ||3.48 ± 0.14|| || ||2.11 ± 0.01|
As a short summary, I concluded that the tensile strength remains more or less unchanged by the addition of silica nanoparticles. At very high addition levels, there may be a slight increase. The modulus increases with increasing concentrations of silica nanoparticles. However, toughness and fatigue improvements have been either reported to increase steadily or have a maximum at 5–6% loading levels.
Poly(ether amine)s as Hardeners
Ma et al. reported for DGEBA cured with a difunctional short-chain poly(ether amine) an increase in the modulus by 32% at a 10 wt % addition level of nanosilica. At a 20 wt % addition level, the modulus increased by 40%. G1c increased by 110 and 274%, respectively. By extensive microscopic work, the initiation and development of a thin dilatation zone and nanovoid formation were identified as the dominant toughening mechanisms.
Kinloch et al. investigated DGEBA and a DGEBA/DGEBF blend cured with a difunctional short-chain poly(ether amine). They reported only very small increases in the modulus (17 and 10%, respectively) with 20 wt % nanosilica. Toughness by means of G1c was improved in both cases by approximately 280%. A linear increase with increasing addition level was found. The toughening mechanisms were investigated and compared with theoretical predictions. Localized plastic shear bands initiated by the stress concentrations around the periphery of the silica nanoparticles were the main contributor to the increase in toughness. The debonding of the nanoparticles seemed to be less important, as only approximately 15% of the nanoparticles were found to debond. However, the plastic void growth following the debonding contributed to the toughness increase.
DGEBA modified with various amounts of nanosilica and cured with a difunctional short-chain poly(ether amine) was by Tsai et al. as well. They found exactly the same 17% improvement in the modulus at 20 wt % nanosilica like Kinloch et al. and a 40% improvement at a 40 wt % loading level. The strength was slightly improved at the 40 wt % level. Three-point bending tests showed an improvement in the flexural strength with increasing addition of silica nanoparticles up to 16%. The toughness increase was found to be very small because of the fact that K1c of the unmodified resin was quite high. The improvements reached a plateau at approximately 10 wt % nanosilica. One has to take into account the fact that the curing conditions were different.
The work of Jajam and Tippur focused on a DGEBA blended with 15% n-butyl glycidyl ether cured with a commercial hardener formulation consisting of poly(ether amine), trimethyl hexane diamine, benzene-1,3-dimethane amine, nonyl phenol, and substituted phenol. In addition to nanosilica, they tested micrometer-sized spherical glass particles with a mean diameter of 35 μm. They found a linear increase of K1c with increasing addition level for both particles. At 10 vol %, the nanosilica provided a 78% enhancement relative to the 35-μm glass particles. In dynamic fracture tests, both materials showed improved dynamic K1c values with increasing loading levels. However, the nanosilica showed only a minor improvement of 34% at a 10 vol % addition level. In another study, it was confirmed that the addition of nanosilica did not necessarily improve the toughness when a fast impact occurred. Nevertheless, it was shown that quite significant improvements could be achieved when commercial resin systems were used, with the hardeners typically being complex amine blends. The effects found for nanosilica modification of epoxy resins have not been limited to model systems.
Aromatic Amines as Hardeners
Kinloch et al. also investigated a high-performance, high-Tg epoxy resin system similar to the industrial benchmark RTM6. Nanosilica-filled TGMDA was cured with a blend of 4,4′-methylenebis(2,6-diethylaniline) and 4,4′-methylenebis(2,6-diisopropylaniline). At a 10 wt % nanosilica loading level, the modulus was increased by 26%, and G1c was increased by 146%, although it was still at a very low level of 172 J/m2.
DGEBA cured with 3,3′-diaminodiphenyl sulfone (3,3′-DDS), tested by Rhoney et al., showed a reduction in gel time with increasing silica levels without much change in the cure profile. The Tg, determined by thermomechanical analysis, was lowered from 163 to 146°C at approximately 33 wt % nanosilica. The CTEs at 80° (below Tg) and 200°C (above Tg) were measured, and a reduction of approximately 20% was found for approximately 33 wt % nanosilica.
Ma et al. studied DGEBA cured with 4,4′-DDS. The modulus was increased by 18% at a 10 wt % addition level of nanosilica. Doubling the addition level to 20% increased the modulus by 40% compared to the neat epoxy resin. G1c was improved by 49 and 81%, respectively. Transmission electron microscopy showed some dilatation in the propagated crack-tip area and some nanovoid formation.
The research of Gurung was based on a DGEBA cured with 4,4′-DDS as well. An industrially available nanosilica epoxy masterbatch was compared of silica nanopowder modified with aminopropyl triethoxysilane. Gurung reported an acceleration of curing at the beginning of curing caused by the industrial nanosilica as well. A significant drop in Tg from 173 to 130°C at approximately 33 wt % nanosilica was found by both thermomechanical analysis and differential scanning calorimetry studies. The modulus was improved by 59%, but the stress at break was reduced by 12%. The industrial material performed better than the “homemade” nanosilica, and this was attributed to a better particle dispersion.
It is interesting to see the effects of the different network densities derived from the two different DDS molecules when they were modified with the same nanoparticles with regard to the reduction of Tg.