Web uniformity is critical for all filtration applications; however, nonwovens manufacturers often do not prioritize web uniformity in their product development activities as much as fiber size, unit weight, and other physical properties. Web uniformity should be a priority since it is a major controlling factor in critical applications.

Spot-to-spot web uniformity is an individual value analysis in predetermined measurement points in the nonwoven web rather than production lot averages. As nonwoven manufacturers race to be more competitive, they often increase production speeds to reduce cost; in many cases, this action has the negative result of drastically reduced web uniformity. Today some nonwovens vary in unit weight, thickness, pore size and air permeability by 50% or more when one square inch is compared to another square inch in the same web.   

Generally, as unit weight decreases the spot-to-spot web non-uniformity becomes more apparent. To overcome this deficiency, filter manufacturers and end users of nonwovens have gravitated toward higher unit weights, which significantly increase their cost.
Additionally, the degree of spot-to-spot web uniformity in a particular nonwoven limits its capability in many applications.  Today, applications where uniformity is most critical, such as reverse osmosis membrane support substrate, generally use nonwovens such as synthetic wet laid.  Spunbond nonwovens captured some of these critical applications in past years but with the general deterioration in uniformity in the past few years no longer participate in this market.  With increased emphasis on spot-to-spot web uniformity, spunbond nonwoven manufacturers have the potential to recapture some of these critical applications. 
 
Actual trials using some of the newest spunbond technologies available today have shown that when run under specific conditions a very uniform spunbond can be produced approaching the uniformity of wetlaid. 

Measurement of Web Spot-to-Spot Uniformity
The characterization methods used for process control and product release by many manufacturers do not accurately reflect spot-to-spot web uniformity since multiple samples taken from across the width of the web combine to calculate production lot averages.  Additionally, the samples measured are usually too large to reflect accurately spot-to-spot web uniformity.  The smaller the sample size, the better web uniformity can be characterized. 

Contour Mapping for Evaluating Web Spot-to-Spot Uniformity
Contour mapping can also be a valuable tool for detecting localized areas of non- uniformities in nonwoven webs.  This method can show differences in uniformity where conventional statistical methods show little or no difference.  Contour maps can include measurements such as unit weight, air perm, thickness, and pore size. 

Contour Mapping Sensitivity vs. Sampling Frequency and Size of the Test Area
 
The accuracy of contour mapping directly relates to the size of the test area and the frequency of testing in developing the data grid.  For example, a contour plot of air permeability (as measured by ASTM D-737[1]) using a one square inch sampling head gives more accurate data than a three square inch sampling head.  Additionally, a sampling frequency at points 6 inches MD and 6 inches CD (6x6 inch centers) is more accurate than one using 12x12 inch centers.
 
On-Line instrumentation
 
Laboratory analysis for spot-to-spot web uniformity requires extensive testing and time.  The Textest FX3500 Combiscan instrument[2], available from the ATI Corporation, provides an automated method of generating air permeability contour data on-line.
 
Conclusion
Using the techniques described above, manufacturers can and should place more emphasis on spot-to-spot web uniformity.  The benefits will allow growth by penetration into more critical applications and may allow in some cases reduction of web unit weight for given applications.

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Frank Baker has served as principal of Frank Baker Associates since 2013. He has more than 30 years of experience in nonwovens and filtration, working with DuPont and Fiberweb Nonwovens. More info: www.frankbakerassociates.com