01 Sep 2014 How do I calculate barrier properties?
It is always good to discover that it’s not just you who gets confused by something. A delightful paper1 by Prof Kay Cooksey’s team at Clemson university certainly cheered me up. It showed that in terms of barrier calculations there is a lot of confusion, even among expert book authors,. The problem is that sometimes we get data in terms of Transmission Rate (TR) which is the actual amount of (say) oxygen or water that gets through a layer, and sometimes get the Permeability (P) which is a property of that material. To convert from one to the other just needs a thickness. But what happens when you have some TR values for some materials at thicknesses different from the ones you are using, or have some TR values and some P values? It all becomes a mess for just about everyone. The paper went on to give a reliable way to do the calculations, with some worked examples. I’ve simply implemented their method allowing the use of any combination of 6 layers. The app (which includes all the relevant formulae) is at Barriers.
The layer thicknesses for those layers being used are entered and then you can choose to enter a set of known P values, see the individual TR values and get the overall P and TR values, or you can enter a set of known TR values to get the overall TR along with the P values for future reference.
You will notice that no units are specified. This is deliberate – the intention is that you do all the calculations in units that make sense to you and as long as they are consistent it doesn’t matter what they are. In other AbbottApps there is a choice of “sensible” units, but that isn’t possible for permeation. In the app I’ve added the capability for you convert between 2688 (!) different versions of such units. For example, if you ever need to convert between kg.μm/(m2.s.kPa) to g.in/(cm2.day.mm/Hg) that’s no problem.
The problem with writing apps is that once you start there’s no obvious place to stop. Some key formulae on oxygen and water vapour product lifetime from Prof Cooksey’s Keynote talk at the AIMCAL Charlotte conference in 2013 were too good not to add, then Dr Charles Bishop asked me to add an app on the “three model” approach to multi-layer barriers: Electrical, Coverage and Pinhole. This last model demonstrated the frightening (to me) effects of pinholes on high barrier performance. This in turn led to a page on adhesion and cleanliness for those who need low pinhole counts. By the time you read this column there may be other barrier-related apps included. This is definitely not my area of expertise so I urge all readers to let me know of problems/issues and suggestions for other calculations that can help all of us do a better job.
This blog first appeared in the Converting Quarterly Magazine from AIMCAL and I am grateful to the Editor, Mark Spaulding, for permission to re-use the text.
1Kay Cooksey et al, Predicting Permeability & Transmission Rate for Multilayer Materials, Food Technology, 53, 60-63, 1999