By Kenny Corscadden & Kerin Lester
Accurate measurement and assessment of tissue softness is an
extremely complex problem. Historically, consumer sensory panels
have been used by the industry to determine softness even though
panel ranking is subjective, time-consuming, and ultimately costly.
This subjective perception makes reliable comparison for product
development and quality control purposes difficult.
Tissue softness incorporates consumer perceptions affected by
bulk, ply, smell, surface softness, and personal preferences.
It also includes scientific parameters such as apparent density,
stiffness or flexibility, surface roughness, and compressibility.
Softness will thus be impacted by process conditions such as TAD
and embossing, additives and raw materials.
In several publications1,2,3, methods for the evaluation
of tissue softness have resulted in correlation of panel assessed
tissue softness with various physical properties
of a range of tissue products. Kim et al4 used the
Kawabata Evaluation System (KES), requiring a number of instruments
to measure various physical properties. Using the same approach,
Hoffman5 related the results
of physical measurement through a predicted softness
model to consumer assessed softness results for a wide range of
tissue products. Pan et al6 and Brodeur7 have
investigated the correlation of out-of- plane (ZD) ultrasonic
measurements with tissue properties, although, until recently
such an instrument has not been available commercially. Hollmark
and Pan showed good correlation between bulk and surface softness
and a combination of out-of-plane (ZD) ultrasonic measurements.
The different properties and consumer requirements of facial
tissue, bathroom tissue, and hand towels demand a different combination
of measurement properties to provide good correlation with panel
assessment.
This paper presents a method for the evaluation of bathroom
and facial tissue using ultrasonic parameters from a single instrument.
The problem behind accurate evaluation of tissue softness is
twofold:
- The requirement to measure, analyse and combine measurements
of various physical properties.
- The relationship of the results of such analysis to
consumer perception of softness.
Consider the main physical properties of tissue softness: compressibility
or density, flexibility or stiffness (bulk softness), surface
smoothness and surface chemistry (surface softness). OPUS, an
out-of-plane ultrasonic instrument, measures ZD soft platen thickness
(TAPPI T551), but employs a 20kPa load pressure for the analysis
of tissue.
This pressure provides sufficient compressibility to load the
sample without adversely impacting its integrity.
While under load, an ultrasonic signal is applied
which travels through the sample at the same time the thickness
is measured. Time and distance measurements allow calculations
directly related to ZD tensile stiffness. By knowing the basis
weight in g/m2, density is calculated, thus providing sufficient
information to determine bulk softness. Calculation of ultrasonic
impedance and attenuation, which are impacted by bulk softness
and fibre alignment in the Z direction under compression, provide
information relating to surface softness.
ASSESSMENT OF BATHROOM TISSUE
This methodology employs a three-dimensional
model, published by Waterhouse8, which proposes the
use of three properties: impedance (Z); mass specific attenuation
(A/W); and basis weight (W); along with coefficients, in the following
equation:
Softness = a0Z + a1b (A/W)
+ a2W (Eq 1)
The impedance, attenuation, and basis weight results obtained
using OPUS for a range of bathroom tissue products were compared
to panel ranked softness evaluation for the same samples. The
regression coefficients (a0, a1 and a2), in equation 1 were determined
empirically using OPUS and the panel ranking using least-squares
estimation.
The application of the model was validated using a completely
new data set. Six commercially available bathroom tissue products – Angel,
Charmin, Cottonelle, Scott Extra, Scott Regular and a no name
brand – were obtained. Two sheets from each roll were selected
at random from each of the five rolls. The samples
were then measured using OPUS. The results for Z, A, and A/W were
then applied to equation 1, along with the pre-determined coefficients
(a0, a1 and a2), to produce a
predicted softness value for each product, with a higher value
indicating greater softness. The predicted softness ranked Charmin
the softest and the no name brand the least soft.
The samples were then assessed using panel ranking, and the
results compared to the predicted softness (Figure 1). The results
demonstrate a 96.85% correlation between softness assessed by
panel and the model predicted softness when using a completely
different sample set and predetermined coefficients.
ASSESSMENT OF FACIAL TISSUE
Facial tissue is generally not embossed
and is much thinner than bathroom tissue. Its softness is predominated
by tensile strength and surface properties9. Both these properties
are affected by stiffness, density and fibre orientation. Thus
for facial tissue, research has shown that only ultrasonic impedance
is necessary to assess softness. This has been investigated by
comparing impedance values obtained using OPUS for a range of
12 commercially available facial tissue samples with softness
results obtained by panel assessment. The results show a correlation
of 82% (Figure 2).
CONCLUSION
Z-directional ultrasonic technology can be used to
evaluate different properties of softness for facial tissue, bathroom
tissue, and towel with unsurpassed ease and excellent correlation
to existing methods. By applying the ultrasonic measurements from
OPUS of impedance, attenuation, thickness, and stiffness, sufficient
information was provided to evaluate softness of bathroom tissue
when applied with regression coefficients to equation 1.
The method has been validated using completely separate data
sets for a range of commercially available bathroom
tissue resulting in a 96% correlation with human
sensory panels. Using the same OPUS technology, the measurement
of ultrasonic impedance of facial tissue under load and human
panel evaluation of facial tissue softness correlates 82%. TW
