|M.Sc Student||Waisman David|
|Subject||Development of a Characterization Method of Surface Energy|
for Carbon Fibers in Phenolic Polymeric Matrix
|Department||Department of Polymer Engineering||Supervisor||Professor Emeritus Abraham Marmur|
|Full Thesis text - in Hebrew|
During a composite material manufacturing, fibers are soaked in a polymeric matrix. The ultimate goal is obviously for a total wetting of the fibers. This phase is critical and essential, since the success of this stage determines the composite material properties.
It is clear that the extent of fiber wetting is determined by the quality of matrix-fiber interaction, or in other words the surface tension of the fiber.
During this research, an attempt to develop a characterization technique to the extent of fiber wetting (surface energy), of different fibers, was made. After the surface energy characterization is completed, it will be possible to determine, while comparing to a matrix with known surface tension and taking into account errors, the expected wetting of a carbon fiber with a specific matrix.
There are many techniques to characterize the surface tension and contact angle. Still when it comes to fibers, the techniques have their drawbacks. This technique is supposed to be simple and practical (separating a single fiber out of a bundle, in order to characterize these properties, is rather difficult). Also some of the techniques are inadequate for fibers - it's almost impossible to get a drop shape on a fiber (due to size matters) or on a fiber sheet (due to the fiber porosity). Also, when dealing with contact angles there are two main drawbacks. The first is the inability to achieve significant precision of the calculated or seen contact angle. The second is the meaning related to the calculated or seen contact angle, due to contact angle hysteresis.
In the current researched technique the main principle is to find empirically the extent of wetting by the lowest concentration of the less polar component in the solution, for which a total wetting situation is achieved (the contact angle θ is equal zero). This concentration is called CSC, critical spreading concentration. This parameter is much more sensitive than a measured contact angle, for surface wetting. Practically a series of binary solutions is examined and in each solution the extent of particles settling is reviewed. For a solution in which a full settling is observed a contact angle and a solid surface tension calculation is carried out. In this research a number of fibers are examined - PAN, silica, rayon, kevlar, basalt and carbon T700.
This technique is relatively simple, direct and gives results very quickly. It has already been tested successfully in the past on sphere particles and polyethylene fibers. Finally the CSC values, produce by calculations (indirectly), contact angle differences with significant precision.
This research shows that it is possible to notice a solution concentration at which full settling has occurred, and in fact, characterize the surface tension at this point. At this concentration, the fiber surface tension is approximately equal to the solution surface tension. Also it is possible to notice different CSC values for different fibers. Finally it is shown that though the contact angle approaches zero value at settling, it is not exactly zero.