Industrial problems and the solutions barrier

>Most people hardly give vapour permeability a second thought, which is a pity as issues with it cost UK manufacturing and processing industries several billion pounds a year. They range from rusty machinery to catastrophic equipment failure and from spoiled foods or pharmaceuticals through to sub-standard performance, computer break-down, poor print quality and leaking hydrocarbons. The list goes on and on.

>So, exactly what is permeability? Technically it's the ability of a gas or vapour to seep, or 'permeate' through a material. But to explain it in more practical terms, if you have a coating, container or seal with water or hydrocarbons on one side, you hope to keep it there. The trouble is that the liquid's vapour can pass (permeate) through the actual material, coating, barrier or seal to reappear as liquid again on the other side. You can't prevent it happening, but you can reduce it to a level where it doesn't cause a problem.

>It isn't always easy, though. As an example, common materials like silicone and cellulose are good barriers against liquid water, but water vapour passes almost straight through them, as if they weren't there at all. And sadly, every coating, film and material acts in a different way: one that is good at keeping hydrocarbons in, may be very poor at keeping water vapour, carbon dioxide, or oxygen out.

>We try to reduce the problem in a number of ways - choosing different materials, using films and coatings, sealing and manufacturing things differently. Each of these present different problems and opportunities, and sometimes you can even make matters worse.

>A poignant but easy to understand example comes from the very different world of trying to preserve medieval buildings. Here, applying a modern concrete rendering to keep rainwater out also prevented the natural moisture inside from getting out. Many buildings that had been sound for centuries rotted, decayed and collapsed. Barriers, then, work both ways and can trap as well as preserve.

>Another unexpected result comes from the pharmaceutical world where packs of drugs are provided in single (or unitary) dose blister packs with moulded plastic on one side and a foil on the other. Although the plastic is usually a good barrier against moisture and contamination, the very act of thermoforming it into shape can allow up to four times more moisture to penetrate through to the finished product.

>Measuring permeability

>The problems that can be caused by vapour permeability range across most equipment and manufacturing processes, as well as most products, and the solutions are just as widespread and, often, even more specialised and ingenious. This raises a crucial point - just because you know the stated value for the permeability of a material, does not mean you can rely on that value for your production. Manufacturing and other conditions can change it radically. The only real-world solution is to measure and quality control the permeability of the finished component or product. Surprisingly, over-tightening the lid on some tubs can actually allow more water vapour to get in.

>Until thirty years ago the most realistic way to measure vapour permeability involved sealing a little cup with the material, weighing it, waiting several weeks or months and weighing it again to see how much vapour had passed through. This was the gravimetric technique, and whilst it yielded a reasonably accurate result, it took both time and money. Fortunately, a series of instrumental techniques have now been developed which can take a measurement both quickly and accurately, sometimes in as little as 30 minutes and virtually always within a day. Depending on the material and the specific vapour (for example water or hydrocarbon vapour, Oxygen, Carbon Dioxide, etc), results are usually accurate in the Parts Per Million (PPM) range, though sometimes Parts Per Billion are achieved.  

>With some equipment you can measure the permeability of either the finished component or a sample of material - even if the sample is purely a thin coating.   Perhaps surprisingly you can even measure the permeability of an edible film, in-situ - a film on a pizza, for example, that's designed to keep the base firm and the topping moist. Often more useful is testing the finished component - from a unitary dose drug pack through to an O-ring seal or enclosure. Naturally the technique can be used under a range of temperatures and mechanical pressures as joined materials or multi-layer laminates may have different thermal expansion coefficients.

>There are two ways to measure the permeability of enclosed components and products. The most reliable is to include, for example, a water source in the container, then seal it as normal on production equipment. This is put in a special chamber in the permeability meter which has a dry gas pumped through it. Any water vapour that escapes from the container walls is easily measured. The alternative involves passing the dry gas through the enclosure itself, but placing the component in a temperature and humidity controlled chamber.

>Any part can be tested for permeability - either by using a specially created jig (as with the pizza) or by sealing off the other parts with non-permeable materials. You can also test material samples, including just about anything from a foil barrier through to a coating (such as an anti-rust coating), or from a small sample up to an entire structure such as the wall of a house.

>Vapour permeability is a surprisingly complex subject, and industrial films, multi-layer laminates and coatings are even more so. Results can be counter-intuitive and some seeming solutions may make the problem worse. However, getting it right can lead to better manufacturing and improved products with longer working lives with better performance. With permeability, what starts out to be a barrier may well end up as a solution.