water permeation through polymers 3 7 update

Partnership in Technology Forum

Water Permeation Through Polymers

Daniel Logan HowellRussell Hallman, Jr.

Ashley Stowe

2

Outline Permeation Overview

– Mechanism– Terminology– Implications– Consequences of Measurement Inaccuracies

Current Methods

Extrans Method– Method Overview– Studies

Polymer Structural Changes and Implications

Conclusions

Overview Current Methods Extrans Structural Changes

3

Permeation Overview Permeation is a standard measure of the quality of the membrane

Permeation = Solubility x Diffusion

Solubility is measure of the permeate’s ability to dissolve and form a homogenous solution with the polymer

Desorption phase is essentially a second adsorption process – Difference in adsorption levels is concentration gradient, the driving force for diffusion

Combination of Fick’s Law and Henry’s Law gives general permeation equation– Concentration gradient represented by vapor pressure difference, p1-p2

𝑑𝑞𝑑𝑡 = 𝑄𝐴𝑝1 −𝑝2ℎ

h


4

Permeation Overview

Permeation: penetration of a permeate through a solid; 2 step process of adsorption and diffusion

Permeance: degree to which a material transmits another substance; normally rate through unit area of material

Permeability: permeance normalized for thickness of the material

Transmission rate: steady flow in unit time through unit area of a body under specific temperature and concentration gradient

Transfer flux: amount of material that passes through an area in a given period of time; overall flux of material that will traverse the film


5

Factors affecting permeation– Temperature– Pressure (for vapors)– Concentration gradient (driving force)– Material thickness– Surface area– Chemical structure and crystallinity

Generally accepted factors– Moisture diffusion occurs through amorphous region– Permeation is constant property of polymer in absence of

degradation for given permeate

Processing profile plays large role in observed permeation rates

Permeation Overview


Amorphous

Interfacial

Crystalline

6

Implications of Permeation Barrier materials are important in many industries and markets

– Pharmaceutical packaging ($54.8B world)– Food and beverage packaging ($329B world)– Protective Clothing ($250.1M US)– Governmental applications ($792M US)

Polymers are required to provide adequate protection level

Protection times must be accurately calculated– Shelf life guarantees for food and pharmaceuticals– Protection level ratings for protective clothing– Consequences vary for different protected materials

Current measurement methods have accuracy and repeatability issues


7

Polymer Selection

Selection between polymers based on multiple factors– Barrier properties– Tensile strength– Hardness– Processability– Cost

Several polymers exist for different barrier applications– Mylar– Various polyethylenes (PE)– Polypropylene– Polystyrene– Several others

Accurate barrier property measurement helps determine optimal polymer


8

Consequences of Inaccuracies Accurate measurement of barrier properties valuable to polymer

manufacturers and customers

Inaccurate barrier property measurements can lead to several problems for industry customers– Inadvertent false shelf-life claims– Unnecessary increased costs

Suboptimal polymer selection Overuse of material

– Inventory obsolescence– Disposal of non-obsolete inventory

Polymer manufacturers also face consequences– Quality issues– Lost sales– Increased manufacturing cost


9

Current Methods Current measurement techniques and equipment are industry-

specific– Standards set forth by groups such as ASTM, DIN, and ISO– Equipment manufactured to test according to standards

Measurement techniques include “weight-gain” methods and “transient measurement” methods

Weight-Gain MethodTransient Measurement Method


10

Current Methods Weight gain methods utilize a desiccant

– Desiccant covered by polymer in controlled atmosphere (i.e., temperature and humidity)

– High-precision microbalance used to monitor weight as moisture permeates the barrier

Transient methods monitor moisture changes on dry side of polymer– Utilize sweep gas or monitor surrounding dry gas in controlled atmosphere– Wet side of polymer contacted by vapor or liquid

Weight-Gain Transient

Advantages • Cheap, easy set up

• More accurate• Observe dynamic

changes in rate

Disadvantages • Inaccurate• Long test times

• Can be expensive• Long test times

• More accurate test method needed to meet industry requirements


11

Alternative Transient Technique: Extrans Transient method employing sweep gas

and utilizing liquid on wet side of polymer (bottom cell)

Test Cell

Process Schematic


Advantages

Constant temperature operation

Low pressure differential across film

Lack of hydraulic pressure driving force

Constant contact between film and fluid

Highly accurate chilled-mirror hygrometer

Maintains nearly constant concentration gradient

12

Measurement Discrepancies Mylar bag tested using Extrans technique found to have permeation

rate that did not agree with vendor report

Permeation rate (20 C) = 3.6 x 10-8 g H2O/min*in2

Vendor reports much higher permeation = 3 x 10-7 g H2O/min*in2


13

Liquid vs. Vapor Interface Schroeder’s Paradox: uptake of solvent in polymer depends on

interaction with the boundary phase– Well-known principle but not well understood– Permeation assumed to be primarily diffusion-limited, especially for liquid interfaces

Need to determine comparability of liquid and vapor contact permeation values

Concentration gradient (diffusion driving force) more constant for liquid interface vs. vapor interface

Adsorption affects could play larger role in permeation rate for vapor interfaces

Could have end-use application impact for certain polymer systems


14

Liquid vs. Vapor Interface

LLDPE – Quick Cooled sample put in contact with liquid and 100% RH vapor

Similar rates observed

No evidence of Schroeder’s Paradox for this system


15

LLDPE

• Equilibrium permeation rate almost immediately realized

• Quenched LLDPE sample run using Extrans technique for several thermal cycles


16

HDPE

• Equilibrium permeation rate approached over several thermal cycles

• Phenomenon observation possible due to accuracy of Extrans technology

• Slow cooled and annealed HDPE sample (more crystalline version of PE) run using Extrans technique for several thermal cycles


17

Structural Polymer Changes• Permeation rate of a polymer changes as a result of thermal cycling• Greater permeation rate conditioning was observed for more highly

crystalline polyethylene• Cycling causes the crystal lattice to change, altering the diffusion path

tortuosity


18

Structural Polymer Changes

Permeation rate changed as a function of time while exposed to constant temperature and concentration gradient

Possible explanation is structural changes within polymer– Swelling due to interaction between water and polymer– Water clustering within amorphous regions or void spaces

Structural changes can affect crystallinity– Suggests concentration-dependent permeation coefficient– Concentration within polymer can affect diffusion rate– Tortuosity of path changes with crystallinity

= H2O


19

Implications of Structural Changes Polymer permeation rates equilibrate as a function of thermal cycling

Suggests that permeation coefficient is dependent on the concentration within the membrane– Crystallinity changes due to swelling and clustering– Tortuosity of diffusion path changes

Greater permeation rate conditioning was observed for more highly crystalline polyethylenes

Thermal conditioning is reversible– Drying polymer will remove inter-lamella water clusters

Understanding this mechanism can allow the fabrication of membranes with precisely controlled barrier properties


20

Conclusions Permeation depends on many factors

– Temperature, pressure, driving force– Crystallinity, diffusion path tortuosity– Thermal history of polymer– Interactions between polymer lattice structure and permeate

Extrans method is superior method that allows changes in these properties to be dynamically observed through permeation rate measurements

Further investigation into mechanism of thermal conditioning can lead to better prediction of polymer barrier properties– Use of spectroscopy to understand changes in crystallinity

Greater understanding of mechanisms affecting permeation rates can allow for the engineering of polymers with precise barrier properties


21

Acknowledgements

Y-12 Office of Technology Maturation

Y-12 Career Advantage Program


water permeation through polymers 3 7 update

Documents