An increase in temperature willincrease the permeation rate because the
solubility of the permeant in the polymer will increase, and as the temperature
rises, the polymer chain movement is stimulated, permitting more permeants
to diffuse among the chain more easily.For manygases, the permeant rates
increase linearly with the partial pressureg radient, and the same effect is
experienced with concentration gradients of liquids. If the permeant is highly
solubleinthe polymer, the permeability increase may not be linear.
The thickness of the polymer affects the permeation. An increase in
thickness will generally decrease permeation by the square of the thickness.
However, there are disadvantages to this approach. First, as the lining
thickness is increased, thermal stresses on the bond are increased, resulting
in bond failure. Temperature changes and large differences in coefficients
of thermal expansion aret he most common causes of bond failure. The
thickness and modulus of elasticity of the liningm aterial are two of the
factors that influence these stresses. In addition, as the thickness of the sheet
lining material increases, it becomes more difficult to form, and heat may
have to be supplied. Also, the thickersheets are much more difficult to weld.
Athird factor is cost. As the thickness of the material increases,not only does
the material cost more but the labor costs also increase because of the greater
difficulty of workingw ith the material. If polymers sucha sfl uorinated
ethylene propylene (FEP), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride( PVDF) are being used, the cost may become prohibitive.
The density of the polymer in addition to its thickness willa lso have an
effect on the permeationrate. The higher the specificgravity of the sheet, the
fewer the voids that will be present through which permeation can take
place. Acomparison of the specificgravity between two different polymers
will notg ive an indication of the relative permeationr ates. However,
ac omparison between two linerso ft he same polymer will provide the
difference in the relative permeation rates. The liner havingt he greater
densityw ill have the lower permeation rate.
Other chemical and physiochemical properties affecting permeation are
1. Ease of condensation of the permeant.C hemicals that readily
condense willp ermeate at higher rates.
2. The higher the intermolecular chain forces (e.g., van der Waal’s
hydrogen bonding) of the polymer,the lower the permeationrate.
3. The higher the level of crystallinity in the polymer,t he lower the
permeationrate.
4. The greater the degrees of crosslinking within the polymer, the
lower the permeationrate.
5. Chemical similarity betweenthe polymer and the permeant.When
the polymer and permeant both have similar functional groups,
the permeant rate will increase.
6. Thes maller the moleculeo ft he permeant,t he greatert he
permeationrate.
Sheet Linings
291
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402
solubility of the permeant in the polymer will increase, and as the temperature
rises, the polymer chain movement is stimulated, permitting more permeants
to diffuse among the chain more easily.For manygases, the permeant rates
increase linearly with the partial pressureg radient, and the same effect is
experienced with concentration gradients of liquids. If the permeant is highly
solubleinthe polymer, the permeability increase may not be linear.
The thickness of the polymer affects the permeation. An increase in
thickness will generally decrease permeation by the square of the thickness.
However, there are disadvantages to this approach. First, as the lining
thickness is increased, thermal stresses on the bond are increased, resulting
in bond failure. Temperature changes and large differences in coefficients
of thermal expansion aret he most common causes of bond failure. The
thickness and modulus of elasticity of the liningm aterial are two of the
factors that influence these stresses. In addition, as the thickness of the sheet
lining material increases, it becomes more difficult to form, and heat may
have to be supplied. Also, the thickersheets are much more difficult to weld.
Athird factor is cost. As the thickness of the material increases,not only does
the material cost more but the labor costs also increase because of the greater
difficulty of workingw ith the material. If polymers sucha sfl uorinated
ethylene propylene (FEP), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride( PVDF) are being used, the cost may become prohibitive.
The density of the polymer in addition to its thickness willa lso have an
effect on the permeationrate. The higher the specificgravity of the sheet, the
fewer the voids that will be present through which permeation can take
place. Acomparison of the specificgravity between two different polymers
will notg ive an indication of the relative permeationr ates. However,
ac omparison between two linerso ft he same polymer will provide the
difference in the relative permeation rates. The liner havingt he greater
densityw ill have the lower permeation rate.
Other chemical and physiochemical properties affecting permeation are
1. Ease of condensation of the permeant.C hemicals that readily
condense willp ermeate at higher rates.
2. The higher the intermolecular chain forces (e.g., van der Waal’s
hydrogen bonding) of the polymer,the lower the permeationrate.
3. The higher the level of crystallinity in the polymer,t he lower the
permeationrate.
4. The greater the degrees of crosslinking within the polymer, the
lower the permeationrate.
5. Chemical similarity betweenthe polymer and the permeant.When
the polymer and permeant both have similar functional groups,
the permeant rate will increase.
6. Thes maller the moleculeo ft he permeant,t he greatert he
permeationrate.
Sheet Linings
291
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402