Composites usually absorb moisture until saturation, but saturation depends on the relative humidity. If the latter decreases, then the composite will make a new equilibrium with a lower amount of absorbed moisture. The process is reversible, but slow. If after a given time, the material released water, it is because the surrounding medium became drier than before. I don't expect this phenomenon to occur at constant relative humidity. If you measured a weight loss, however, this is probably related to another phenomenon (for example, diffusion of additives outside the polymer, favoured by the plasticizing effect of absorbed water, or hydrolytic degradation releasing small molecules).
Alain
How do you mean by reversed? Do you by any means mean water is rather lost after a certain time?
Dear Alain Sir,
in my experiment,The absorption of composite laminate increases up to170 days after that absorption reversed.Then once again the absorption increases after 210 days.I can understand the first increase and decrease.I could not able to make a comment on second increase.Can you suggest me regarding this second increase of absorption?
Did you control the relative humidity to which your composite was submitted ? Was it strictly constant ? If not, you may obtain the changes that you have seen. And how did you measure the water uptake ? Are you sure that you only absorbed water ? As your results are not logical "a priori", we have to look for uncontrolled phenomena which may have occurred.
Alain
Dear sir,
Laminates in sea water.No control over relative humidity.Simply room atmospheric conditions.moisture absorption is calculated from measuring of weight periodically.
I have taken marine applications.For marine applications,There is no control over atmospheric conditions.
So the composites were directly soaked into sea water ? I agree that they first started to absorb water, but then water acted as a plasticizer and produced the extraction of something initially inside the composite. If the weight further increased, this should correspond to a succession of other phenomena, maybe the colonisation of the composite by microorganisms ?
Alain
In my informed opinion two phenomena are at work here, namely sorption and osmosis. The increase in weight in the first 170 days was solely due to the absorption of water by sorptive capacity (hygroscopicity) of the laminate which must have probably died to equilibrium moisture content way below fibre saturation point (25-30% MC) at the time of immersion. When laminate has absorbed water close to green conditions (100%MC), and sorptive capacity has been fulfilled, the two distinct regions of two different solute concentrations, one within wood and the other outside wood in sea water are now at play. At this point sorptive capacity is fully offset (zero) and osmotic pressure in favour of the region of seawater is fully created and exerted. From this point (between 170-210 days) on, water rather moves from the laminate (region of lower concentration) to the surrounding seawater (region of higher concentration) till osmotic equilibrium is reached. When osmotic equilibrium has been reached and osmotic pressure if fully offset, a sorptive capacity is re-established to move water into the laminate (as occurs after the 210th day).
All other conditions being constant, the duo could alternate on. Shalom!
FRP is a polymer based composites and in frp the polymer part is little weak than fiber part in terms of bond strength. So, when frp is exposed to any favor liquid then the first step occur the void filling and then develop the osmotic pressure to cause swelling of polymer. If it is continuously expose to same favoring media the swelling stabilized for some time at first phase of absorption and again erupted by second phase when those polymer jelly dissociated from the bond surface. To understand better it is required to know activation period of reaction.
For the above mentioned problem,
For water,
water absorption increase and decrease because of absorption equilibrium and osmotic equilibrium respectively.It is ok
But for ultimate load,
The ultimate load plot shows that initially load decreases with water uptake.Then load increases with reverse osmotic condition.This down and up nature continues .....
My question is
once the material is weakened by absorption ,How does it regain its strength ?
I have attached the plot for reference.
As I said, water is a plasticizer. If the phenomenon is reversible, as you indeed observed, it means that plasticization is the major phenomenon. If it was not, it would indicate that in addition to that, irreversible (chemical) ageing would occur as well.
Alain
The more water cellulosic materials absorbs the more pliable it is ( or less strong it gets). Thus, fluctuations in moisture content would influence material strength accordingly in seawater. Shalom!
What are the material components of your composite? If water absoprtion is a problem it may be you are using the wrong adhesive between the layers. Some adhesives are water activated.
Do you have a brand name or chemical composition for the epoxy? What is glass polyester? Is it a fiber glass and polyester blend?
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