Permeability

Definition:

  1. To spread or flow throughout...
  2. To pass through opening or small gaps of...
  3. Charge, saturate, suffuse, impregnate, or transfuse
  4. Permeation is the product of two functions - the diffusion between molecular chains and the solubility of the permanent in polymer. Diffusion is driven by a concentration gradient for liquids and a partial pressure gradient for gasses. Solubility is related to the affinity of the PERMANENT for the polymer.
To get a true test of permeation, the test should be run on the final component, due to the structure of polymers and the many variables that can effect is PERMANENT. For example, increasing the temperature raises the permeation rate.

Water Absorption

PTFE <0.01
FEP <0.01
PFA 0.03
ETFE <0.03
PVDF <0.04
PEEK <0.05
LDPE <0.01
HDPE <0.01

Relationship for gaseous permeation is:

P=D S
P is the permeability (cm3(STP)/sec-cm-cm Hg)
D is the diffusion coefficient (sm2/sec)
S is the solubility coefficient (cm3(STP)/cm3-cm Hg)

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