So, how do we measure bulk (as the inverse of density)? You must first measure thickness, which is the layman’s usage of the word bulk. There we have our first big challenge. That is there is no such ‘thing’ as bulk. By that I mean there is no universal agreement on how to measure it. There are literally dozens of ways to define or measure nonwovens thickness and all will give a different value. Thus, this is very different than say length, or temperature or thickness. No matter what ruler or thermometer we use, everyone will get very close to the same number (once unit conversion is done if needed). This is not at all the case with nonwovens. The value of thickness you get very much depends on the specific measure you use. Most use some variation of a probe that is loaded to a certain force against a flat platen with the nonwoven sandwiched in between, such as shown in Figure 1. The problem is, however, that there is little agreement on how much force is to be used. If the probe pushes harder, the ‘measured’ thickness is smaller. There is also little agreement on how big the probe area should be. If the probe area is larger, the ‘measured’ thickness is larger.
Another key difference is whether you measure single or multi-ply thickness. Due to nesting of surface roughness, you are likely to get a single ply measure thicker than a 10 ply measurement (divided by 10). If you are doing product development, perhaps single ply measurement might be slightly more relevant to end-use. However, multi-ply measurement has better resolution and would be more predictive of packaging such as in a wound roll or box or bag.
However, the problem is even worse when you consider that we may also define in-roll thickness to help us predict wound roll diameters (for a given roll length). Here, the laws of physics require that the in-roll thickness be smallest near the core and largest at the outside of the wound roll and neither will be the same as the test lab measures it. Analogously, we may be interested in how a nonwoven product fills a box, bag or other container. Here, the pressures are different still. In a box the pressures may be low, think facial tissue in a box. In a bag, the pressures might be higher to reduce shipping and warehousing volume.
So how does the industry work in this ‘Tower of Babel’ situation where a key parameter such as bulk is measured in dozens of languages? If you only are concerned with your own plant and nothing else, we might select one measurement method. But what if your customer uses a different measure? What if you want to predict roll diameters or box fill? Well, it is possible to convert any one measure to any other measure using a conversion chart, such as illustrated in Figure 2. However, this requires a bit of experimental time and effort to get. Worse yet, that chart would be different for each and every grade you made or your customer wanted. It would also have to be done for each and every combination of measurement methods you might need to convert. In short, nothing like the simple formula that converts Fahrenheit to Celsius or vice versa. I do not overstate the complexity. One Fortune 100 company I consulted with had no less than 10 totally different measures of thickness in sanctioned official use, and all gave different answers.
As an individual, we must recognize that measurement, or more accurately, the definition of thickness is not a settled matter. We must be careful that when we state a number when speaking to others. It should always be in proper units and that it always be accompanied with the test method name/ID under which the measure was made. As an industry, we might wean out some of the many odd measurements, just to reduce variation. As an industry, we might select methods that use lower probe pressures (less force and larger probe area) because they work on sturdy grades as well as delicate grades. In fact, this is part of the reason we got where we are. The pressures used by TAPPI for a hundred years is about 7 psi and while suitable for paper (and very sturdy nonwovens), is not a good choice for anything soft and fluffy. That led the nonwovens people to creative test development for thickness, which as we have now just seen, might have been too creative because of the proliferation of similar but not quite the same measures. Finally, we might favor the multi-ply measurement over the single ply measurement because it is more accurate (law of large numbers) and because it probably applies over a wider array of applications and interests.
We will continue this discussion in the next column by talking about how to preserve bulk through demanding processes such as winding.