3.1.4. Dynamic Properties
As it has been previously shown, these formulations have similar or even enhanced thermal and mechanical properties compared with the reference formulation (ECO-REF). Additionally, a distinguishing property of the formulations prepared with 4-AFD is that the resulting materials show a dynamic behavior. This is attributed to the aforementioned reversible disulfide bridges that, under determined operational conditions, can rearrange giving the materials the ability of being reprocessable, recyclable and repairable (3R).
Relaxation time is defined as the time required for relaxing 63% of the applied stress. Time and temperature dependent relaxation modulus was measured by DMA (data available in
Supplementary Figure S9), to characterize the heat induced malleability and confirm the dynamic behavior of our material prior the reprocessability, recyclability and repairability tests. The stress-relaxation study shows that at temperatures above the T
g, the developed biobased epoxy networks are able to completely relax stress and flow.
Table 3 displays the relaxation times of the prepared materials at different temperatures.
Figure 2 shows the relaxation behavior of all the prepared materials at 200 °C. The results show that the relaxation time clearly depends on the T
g of the material and its crosslink density. Samples with the lowest T
g values require lower temperatures to relax or have shorter relaxation times at a given temperature (ECO-1 and ECO-REF are completely relaxed at 200 °C).
Considering the dependence of the relaxation time and temperature on the T
g of the material, TGA isothermal studies were done at 200 and 230 °C to analyze the thermal stability of networks as a function of time. Results (given in
Figure 3 and
Table 4) show that when the time required to relax the material at a given temperature (above 200 °C) is too long, the degradation of the thermoset may occur due to the thermal degradation of 4-AFD above this temperature.
Figure 3 shows that the weight-loss of the materials after 20 min is similar for all the formulations at each temperature (4–5% at 200 °C and 13–16% at 230 °C).
The thermal degradation, monitored as weight-loss, leads to a decrease of the T
g values. In
Table 4, the results of the DSC studies performed on the residues of the TGA are provided (data available in
Supplementary Figure S10).
Based on the results shown in
Table 4, 200 °C was set as the maximum temperature for the recycling/repairing/reprocessing processes, in order to avoid product degradation. Temperature was set at 40–50 °C above the T
g of each formulation, at which the relaxation time was around 1–2 min (a little bit higher for the ECO-REF,
Table 3). In the case of ECO-4, due to its high T
g, this material would require working at too high temperatures for at least 30 min to be repaired, recycled, or reprocessed. At these conditions, the material would degrade and, therefore, the study of the 3R properties of this formulation was not possible.
Thus, once the relaxation temperatures were set at 40–50 °C above the Tg of the materials, the next step was to test their 3R properties:
The developed materials should be easily reprocessable or reshapable above their Tg, based on their dynamic behavior conferred by the aromatic disulfide bonds in their structure. A hot press and a zig-zag shaped mold were used to change the shape of the materials and demonstrate this fact. After 10 min at the temperature specified for each formulation (40–50 °C above their Tg) a self-standing zig-zag film was obtained.
The same procedure (shown in
Figure 4) was followed for all the samples: a zig-zag shaped mold and the hot-press were pre-heated to the required temperature (150 °C for ECO-1 and ECO-REF; 180 °C for ECO-2 and 200 °C for ECO-3). Once the entire system was tempered, a cured film of the corresponding formulation was placed in the mold in the hot press. Heat and pressure (100 bar) were applied for 10 min so the material has enough time to adapt to the new shape. The mold was let to cool down to room temperature, closed but unloaded.
This process led in all cases to new 3D parts of the previously prepared and cured biobased epoxy thermosets, as shown in
Figure 5.
During service, thermosets may suffer different kinds of damage. Current repair approaches require special techniques and skilled workers, making this possibility slow and expensive. The developed dynamic biobased thermosets can be repaired just applying heat and pressure for a short time (
Figure 6). Here again, the temperature will depend on the relaxation times given by each formulation (set at a temperature 40–50 °C over their corresponding T
g; 150 °C for ECO-REF and ECO-1, 180 °C for ECO-2, and 200 °C for ECO-3).
To show that these materials are repairable, damage was caused on purpose on the surface of a cured film (a cross was made on them with a scalpel). This film was placed then inside the hot-press (in between two Teflon films), which had been pre-heated to the corresponding temperature. The films were left inside the closed hot-press (with no external pressure) for 10 min and the disappearance of the cross could be observed when taking them out of the press, as shown in
Figure 6. The films are flattened due to the pressure put on them by the hot-press.
Thermoset materials are not easily recyclable. In contrast to this, the biobased vitrimers developed in this work can be recycled by a simple process of mechanical recycling and subsequent thermoforming. To show this, the cured sheets were grinded to powder and this powder was then hot pressed for 10 min at a temperature 40–50 °C over its T
g in order to obtain a new sheet (
Figure 7).
The thermomechanical properties of these sheets were characterized in order to check that they did not change compared to the pristine sheets. Results are described below.