Mass Transfer Considerations in Hygiene Products
Carol Blaney, Ph.D.Blaney Consulting
OUTLINE
1. Business perspective – infant diapers
2. Problem – customer complaint
3. Background – brief overview of garment function
4. Strategy – mass transfer considerations
5. Execution – modeling and experimental 6. Conclusions - problem solved
1. Business perspective
$26 billion dollar global industry Fiercely competitive Growing (Asia-Pacific)
1. Business Perspective
New technology: New polymers New fillers New processes
Innovations/Improvements Ultra high permeability never before possible
2. Problem – customer complaint
Positive: skin health (dryer skin) Negative: customer says ‘it leaks’
(but it wasn’t actually leaking)
3. Background – overview of garment function
Goal: ‘dry’ skin
Testing has consistently shown: 1. Hydrated skin is abraded more easily, more
susceptible to infection2. Skin hydration is beneficially reduced by
(a) use of SAP, and (b) by reducing barriers to water vapor transfer out
of diaper.
3. Background – overview of garment function
3. Background – overview of garment function
Before Insult (all layers) Insult (coversheet) Transfer fluid away from skin (transfer
layers) Retain fluid away from skin (pulp,
superabsorbent) Continue removing moisture out of garment
4. Strategy
Initial Goal: lower skin hydration
New Goal: Solve perception of ‘leakage’ Lab tests show no holes big enough to cause a
real ‘leak’ of liquid urine
Young-Laplace Equation
Visualizing criteria for leakage across a microporous film:
Simple model:
Maximum pore size allowed
Why does diaper seem to be leaking?
What do you feel when you feel wetness? What is it about water that feels wet? Warm water doesn’t feel as wet as cold water.
Hypothesis: ‘wet’ means hand is losing heat. Outercover feels ‘cold’, not ‘wet’. Cause: Evaporative cooling
Remedy?
Remedy #1. Lessen evaporative cooling
Problem: raises humidity inside diaper
Raises skin hydration, compromises baby’s skin health
Another Remedy?
Remedy #2. Lessen the human perception of evaporative cooling by reducing heat transfer from hand to diaper (while keeping evaporation levels high)
How? Lower heat transfer coefficient of the outercover
add a layer of air.
New goal
Minimize heat transfer into diaper outercover
AND Maximize moisture transfer out of diaper
this includes maximizing water evaporation
and hence maximizing evaporative cooling
New question:
But will the extra layer of ‘heat insulation’ (air) add to the diffusional resistance of moisture vapor leaving the diaper?
Quickest way to answer this: Model
4. Strategy - Modeling
Modeling is fastest way to see relative effects of various layers on overall moisture transfer
Use it to intelligently select the most effective fabric prototypes
AND
Test these fabrics in a product to see if it eliminates customer complaints.
End of Section 4 (Strategy)
We now have a clear strategy based on our hypothesis that ‘leakage’ perception is due to heat loss from hand:
Use a Spacer layer (air) To Reduce heat transfer Hopefully it won’t appreciably reduce moisture flux
Section 5: Execution
Modeling Experimental
Modeling
Steady state, ordinary diffusion of water vapor through air (~ideal gas):
J = - D(C)
Microporous Films
Fickian Diffusion in one direction
JA = - DAB dcA/dZ = - DAB (P/RT) dYA/dZ
Ideal gas mixture of components A and B at constant T, P
Microporous Films
NA = (1 – YA)-1 [ -DP/(RT) dYA/dZ ] Where D = [ __1__ + _1_ ]-1
DF) DK
Boundary conditions for integration
At Z=0, YA=YA1
At Z=Z0, YA=YA2
Result for single layer:
MVTR = NA =
DP/(Z0RT) ln [(1-YA2)/(1-YA1)]
Add boundary layer:
MVTR = NA =
ln [(1-YA3)/(1-YA1)]_
Z0RT + h0RT
DF)P DFP
Moisture flux (g/m2/day)
0
5000
10000
15000
20000
25000
No spacer MB BCW thin SB SMS
Experimental
Recruited and trained a sensory panel to perceive ‘dry’ vs ‘wet’ when they touched the diapers.
Set up laboratory: A dozen torsos that mimic real diaper use
Realistic urine insults Heated skin surface, correct anatomyHumidity sensorsMass balance every hour
Experimental Results
Experimental Results
1. Perception of ‘dry’ correlated with lowered heat transfer coefficient of the outercover composite (hypothesis was correct)
Experimental Results
1. Perception of ‘dry’ correlated with lowered heat transfer coefficient of the outercover composite (hypothesis was correct)
2. Lower humidity levels inside diaper correlated with higher MVTR values (of outercover composite)
Experimental Results
1. Perception of ‘dry’ correlated with lowered heat transfer coefficient of the outercover composite (hypothesis was correct)
2. Lower humidity levels inside diaper correlated with higher MVTR values (of outercover composite)
3. Modeling correlated with experimental results, confirming our understanding.
Summary
A thin, open nonwoven ‘spacer’ layer was selected for commercialization: Lowered thermal conductivity to address ‘complaints’ Didn’t lower MVTR too much strong cost effective machine capacity available
Patent protection obtained (Patents: 6,673,980; 6,663,611; 6,660,086; 6,583,331; 6,177,607)
6. Conclusion
Mission Accomplished
Company remained a leader in diapers, able to advertise skin health benefits
Without customer complaints
Credits (Blaney)
Solved riddle of ‘leakage perception’ Proposed strategy Applied model to gain clarity Set up lab (with electrician) Established testing protocol Wrote up patent draft with preliminary
claims
My technical team
Recruited and trained an in-house sensory panel Constructed ‘diaper’ products Arranged material property testing Ran lab tests Collected data
Commercialization Team
Assessed material availability and cost
Ran trials to produced materials for testing
Commercialized the ‘winner’
Model/Programming
Sigma Software in Roswell, GA
Theory for Model Mass Transfer – Chemical Engineering Series
Sherwood, Pigford, Wilke McGraw Hill © 1975