June_AMP_Digital

ccurate multi-component thickness and moisture

data is critical to the productivity and efficiency of

producers of plastic films, extrusion coatings,

blown films, and nonwoven fabrics. Collecting this data on

production lines in real-time ensures that raw materials

are not wasted and that all products meet relevant specifi-

cations. This becomes even more important as polymer

material costs rise. Several different noncontact online

analysis techniques exist, which primarily measure thick-

ness by passing either nuclear or electromagnetic radiation

through the web and measuring its absorption patterns via

a detector on the other side of the material.

Nuclear web gauging instruments are some of the ear-

liest online measurement technologies. Using sources like

Krypton 85 and Promethium 147 to emit a directed beam

of particles at test materials, these instruments can pro-

vide valuable and accurate thickness data but are limited in

measurement capabilities and cost. In addition to obtain-

ing a regulatory license to operate and incurring mainte-

nance costs associated with replacing the source, users of

nuclear web gauging instruments often need to hire a ra-

diation safety officer and submit to annual inspections. De-

pending on the country where the instrument is operated,

users may be subject to additional regulatory and licens-

ing requirements.

Infrared (IR) instruments avoid these burdens by using

infrared light to measure layer thickness. Because every

component in a multilayer web has a specific pattern of in-

frared absorption unique to its chemical structure, web

composition, multilayer thickness data, moisture, and

other parameters can be found by measuring the absorp-

tion and reflection of a light source through a multilayer

web. Two primary types of IR web gauging sensors exist:

Spinning filter wheel and solid-state full spectrum.

Spinning filter wheel sensors

The spinning filter wheel offers the simplest design,

which determines moving web thickness using a ratiomet-

ric measurement technique that compares two IR absorp-

tion readings. These readings are taken through two

different light filters, attached to a spinning wheel posi-

tioned near the web. The first filter is called the reference

filter (R) and it selects a wavelength with very low absorp-

tion for the analyzed material. The second filter is known

as the measurement filter (A) and uses a high absorption

wavelength. After readings are taken, the A/R ratio pro-

vides a result proportional to web thickness.

Spinning filter wheel sensors present several challenges.

Because each material in a multilayer web has its own ab-

sorption pattern, these sensors cannot measure multiple

materials simultaneously without additional A and R filters.

These filters increase upfront instrument costs, require

more maintenance, and compound the risk of failure. Addi-

tionally, any changes to the composition of the web being

measured require new filters. Spinning filter wheels require

a finite amount of time to rotate, so measurements suffer

from spatial displacements between readings. Further, each

filter measures a slightly different spot on the web, making

precise measurements difficult. While design improvements

have incrementally increased filter wheel speed over time,

they are unable to overcome this limitation.

Spatial displacement and the limited number of wave-

lengths measured by filter wheel sensors also make them

prone to error in certain circumstances. Moving webs that

include printed substrates, pigmentation, or nonuniform

base substrates have different absorption patterns than

those that do not, especially when variations are not uni-

form across the entire web (i.e., printed text and paper

board). Sheet flutter can also cause errors, as it can alter

absorption for one or both of the A and R readings. The

same is true for varying light conditions in the area sur-

rounding the instrument.

Solid state full spectrum sensors

Full spectrum IR web gauging instruments were devel-

oped to overcome the regulatory and technical issues of

nuclear and spinning filter wheel IR sensors. Unlike spin-

ning filter wheel sensors, which use only one wavelength of

IR light per filter at a time to take measurements, full spec-

trum IR instruments emit the entire spectrum of near in-

frared (NIR) light and take absorption measurements for

all wavelengths simultaneously. These instruments are able

to continuously measure the complete absorption pattern

of a given material rather than just a single wavelength.

Full Spectrum IR sampling eliminates many of the

problems that come with filter wheel IR web gauging sen-

sors. Because the entire NIR spectrum is sampled simulta-

neously, inaccuracies caused by spatial displacement are

Full Spectrum Infrared Light Analysis

Enhances Web Gauging Applications

ADVANCED MATERIALS & PROCESSES •

JUNE 2014

TECHNICAL SPOTLIGHT

Fig. 1

—

Full spectrum absorption patterns for common web materials,

taken using the Thermo Scientific PROSIS sensor.

2.5

2.0

1.5

1.0

0.5

0.0

IR Absorbance

EVOH

Cellulose

PET

Surlyn

Wavelength