Characteristics

 

 

 

Factors that have an important role in the determination of the advanced composite characteristics are:

 

  • the nature of the matrix (polarity, a surface tension,......)
  • the nature of the reinforcement (its purity, defects)
  • concentration of the reinforcement in the composition, its geometrical form (form, dimension, dimensions distribution)
  • processing conditions
  • the dispersion of the reinforcement in the matrix
  • orientation of the reinforcement in the matrix
  • adhesion between the matrix and the reinforcement 
  • the created micro structure between the reinforcement and matrix.

How the reinforcements matrix with or without reinforcements,,affect some characteristics of the advanced composites (mechanical, dielectrical, rheological, thermic ...) has been shown as follows:

- Dielectrical characteristics- The advanced composites that contain conductive reinforcements - particles in the polymer matrix, for instance, CNT, show an increased conductivity of a few rows of grandeurs, even in a case of very small concentration of the reinforcement - particle (even under 1 wt.%), while the other characteristics of the polymer matrix remain unchanged or insignificantly  changed. There's one critical concentration (tight region of concentration), known as percolation threshold, at which the polymer from the isolator becomes a conductor. It's usual that nano-conductive reinforcements, (especially those with higher length/diameter ratio - CNTs) to have a lower percolation threshold, than the other types of conductive reinforcements with a spherical  form (carbon black), or carbon fibers that have smaller L/D ratio in comparison to CNT. Also, especially important is the dispersion and orientation of the reinforcement in the matrix. If the better dispersion of the reinforcement contributes to smaller percolation threshold (and consequently to higher conductivity of the composite), the CNT’s higher orientation degree in the matrix doesn't mean lower percolation threshold (because the contacts between the nano-tubes has been reduced to minimum and that means smaller conductivity). The orientation of the particle in the polymer matrix is important from the aspect of achieving unique characteristics in certain directions inside the composite, in other words, in achieving anisotropy inside the composite.

- Mechanical characteristics - generally, the mechanical characteristics of the polymer composite (for example, the strenght and the tensile module), depend on the concentration of the reinforcement and its orientation in the matrix. Sometimes, there's a gap between the theoretical predicted characteristics and the characteristics that can really be achieved for the polymer advanced composites. The reason for that (was mentioned above in the text) is the bad dispersion of the reinforcement in the polymer and dispersion inequality in the reinforcements’ characteristics, but also a contribution to that could have the bad adhesion (communication) between the reinforcement / matrix that results with bad matrix stress transfer of the composite towards the reinforcement.

- Rheological characteristics - The high-tensile characteristics of the composites are important from the aspect of their processing, so as for understanding the microstructure of the same composite. Alike the dielectrical characteristics, there's a certain " rheological percolation threshold” here, at which better transfer of the stress between the polymer and reinforcement is achieved. Again, for improving these characteristics and also for better expansion, a functionalization of reinforcement  is needed.

- Thermic conductivity -Generally, the advanced composites have improved thermic characteristics compared to the pure polymers. That improvement of the thermic characteristics of advanced composites isn't with rows of grandeurs as in the case of dielectrical characteristics, but only a modest enhancement. A condition for that enhancement is functionalization of the reinforcement. In other words, reduction of the thermic resistance in the interface between the reinforcement and the polymer.

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