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