Toughening of Materials
Day 3
Today, we are introduced to the various methods of toughening materials. As metioned previously, our results from day 2 is out today.
Epoxy with 40% K1
Pure Epoxy
Scoring points for presentation skills
By visual inspection, it seems that our synthetic foam (Epoxy with 40% K1) experienced some shrinkage during curing unlike our pure Epoxy sample which turned out fine. We found out that we can rectify this problem through kneading the hardening sample in the mould every hour instead of leaving it to cure overnight. How serious is the shrinkage? Calculations show that compared to the 7.44% by mass of uncontrolled air pockets in the pure Epoxy sample, the shrinkage caused the percentage to shoot up till a 29.3% high for the 40% K1 sample. This is undesirable as such air pockets would render mechanical stress tests inaccurate due to the differing dimensions of the two Epoxy samples. Nonetheless, it is a vaulable lesson learnt and the samples are still valid for thermogravimetric analysis to compare their thermal resistivity - which will be done in Day 4.
After withdrawing our samples from their moulds, we proceed with the post-curing treatment to maximise the material's mechanical potential. Enter material toughening. For polymers such as our samples, we could apply some heat treatment to our samples to strengthen them (also known as promoting cross-linkaging).
Thermoset polymers, like the ones we made, have a glass transition temperature and a melting temperature - which is the temperature in which the polymer enters a fluid-like phase and the temperature in which the thermoset polymer decomposes. When such thermoset polymers are heated within the range of their glass transition temperature and their melting temperature, polymer strands (which are more mobile during the fluid-like phase) are able to entangle more which makes them harder to seperate. This bounds to improve the sample's mechanical properties as more energy is required to undo this process.
 |
| Cross-Linkage Entanglement of polymer fiber/strands Or your first time in NTU, perhaps? |
Hence this concludes our time spent in Day 3.
Day 4
As mentioned earlier, our samples are ready for some testing after the post-curing treatment. We were introduced to Thermogravimetric analysis - which is essentially a measure of a material's rate of change in weight of the material as a function of temperature or time in a controlled atmosphere. In our case, we used high purity (inert) Nitrogen gas as our atmosphere as we want to prevent undesirable oxidation of our samples. We also cross-referred our sample's performance with other materials such as low performing plastic(PE) from file covers and high performing prototype polymer used in jet turbines(D4). Results are as shown:
As seen above, 40% K1 Epoxy fared better than oridnary plastic (PE) and pure Epoxy. This can be seen from the graph as our sample started loosing weight (dip in y-axis) at a slightly higher temperature. This is because the Epoxy structure contains glass microspheres of air which improves its thermal resistivity as air is an insulator of heat. Not only does it improve the thermal resistivity, the microspheres we added will reinforce its mechanical strength. This is as the microspheres will cause the cracks in the structure to dissipate (linear to branched) when a shear force is applied. As more energy is required to push the integrity of our sample past the threshold (longer distance travelled by cracks), it is harder to fail under stress. This is yet another toughening method which also reduces material density and ultimately lowers weight.
To make our tests more conclusive, we will be embarking on differential scanning calorimetry. Using this technique, it is possible to observe crystallisation and fusion events as well as the glass transition temperature of the different samples. Differential Scanning Calorimetry can also be used to study oxidation as well as other chemical reactions
.
.
TGA Q500 machine we used today for the test
TGA 2950 with typical graphs of the results of other thermal analysis methods
As such, we look forward to yet another day in Pulau NTU.
0 comments :
Post a Comment