I made a comparison study of monolithic epoxy and epoxy-graphene composites exposed to water absorption. Tensile test was performed on dry and immersed samples. After water immersion, the monolithic epoxy show higher tensile strength compared to dry epoxy samples. How does water increase tensile strength of monolithic epoxy? If degradation take place, why the value increase?
Probably, the effect of epoxy plastification which may increase tensile strength because plastification influences on the failure process, namely, excludes untimely brittle failure.
Probably, the effect of epoxy plastification which may increase tensile strength because plastification influences on the failure process, namely, excludes untimely brittle failure.
It is a curious result. I think It is because interaction between graphene, water and epoxy Because If only epoxy, the behaviour will decrease, as article attached.
I imagine that graphene will prevent the hydrogen in water to bond in the epoxy chain. Meanwhile, water molecules will bem included among epoxy chain, as an impurity, helping to reinforce the material.
I hope to help you
I would tend to agree with Serge's answer, although some more aspects may also be important. When unexpected results occur, one should verify that the result is not a result of an uncontrolled parameter.
My first thought would be, 'how significant is the difference and how many samples were tested for each condition?' Without seeing a typical stress strain curve for each case and without knowing the extent of the variation and standard deviations of each cases it is hard to tell if the difference is real.
Also polymers tend to have significant variations in properties between batches and a difference in measured properties may very well come from there if the samples from different batches were not mixed between conditions.
Also, depending on the cure cycle of the epoxy (and mostly for room temperature cured epoxy), samples tested at different time intervals after manufacturing may provide different values of strength as cure may continue in the time between the test runs.
As Serge mentioned, epoxy is usually quite brittle and may therefore be very sensitive to surface finish or the presence of defects such as voids. If the surface finish of the sample were not the same or if one batch had more voids, this may explain the difference.
If moisture uptake is effectively the culprit, I see two possible explanations.
First, adding to Serge's answer, although I have usually seen and measured decrease in epoxy strength after saturation, plastified resin may become more ductile and more tolerant to surface defects, even more so considering that the adsorbed water concentration will often by higher on the outer surface of the sample.
Second, in cases where there are large gradients of moisture within the sample, the differential swelling between the relatively dry core and saturated outer surface may also cause compressive stresses on the outer face.
I hope this helps.
Best regards,
Laurent
Thank you to Dr. Serge, Dr. Gilberto and Dr. Laurent for your good explanation.
You did not present the received values, it is possible they within an experimental error
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