Hygrothermal behaviour

The following considerations apply for composites that will be exposed to high humidity or immersed in a liquid:

For most polymeric materials Fick's Law applies and the polymeric matrix will take up moisture. For composite materials, while the quantity of liquid taken up may be small, it may have a significant effect on performance. Composite materials exposed to a liquid for an extended period will eventually become saturated. The absorbed moisture plasticises and swells the matrix. This reduces the mechanical properties and the glass transition temperature of the composite and hence the maximum use temperature. After time with moisture, the physico-chemical and thermomechanical properties of composites are further affected by hygrothermal ageing.

• Water penetrates the composite via cracks and voids in the matrix and diffuses through the matrix resulting in reversible plasticisation.
• Moisture will diffuse into the matrix causing swelling and plasticisation. These may result in chemical degradation, or swelling stresses which can result in matrix cracking and/or debonding of the fibre/matrix.
• Loss of properties of the glass fibre: i.e. water molecules penetrate the glass fibre via microcracks on the surface so reducing its mechanical properties.
  

Thermoplastic polymers tend to absorb significantly less water than thermosetting materials, and as such the hygrothermal effect on thermoplastics is negligible.