Effect of surface treatment and knit structure on comfort properties of wool fabrics

Maryam Naebea, Volker Lutzb, Bruce A McGregora, Xungai Wanga,c and David Testerd

a Institute for Frontier Materials, Deakin University, Victoria 3216, Australia bInstitut für Textiltechnik der RWTH Aachen, Germany c School of Textile Science and Engineering, Wuhan Textile University, Wuhan, China d CRC for Sheep Industry Innovation Ltd., CJ Hawkins Homestead, University of New England, Armidale NSW 2351, Australia.

Corresponding author email: maryam.naebe@deakin.edu.au

ABSTRACT

The comfort properties of the pique and single jersey knitted wool fabrics were investigated

using the Wool ComfortMeter. The fabrics were knitted in three cover factors and treated

with either plasma or a silicone softening agent and were compared with untreated fabrics.

Plasma treatment did not show significant effects on the comfort value. However, silicone

polymer significantly reduced Wool ComfortMeter values suggesting that the silicone coating

reduced the number of protruding fibres on the fabric surface. Regardless of treatment used,

pique fabrics showed a lower Wool ComfortMeter value, and therefore were perceived to be

more comfortable than the single jersey structure. While the effect of cover factor was not

significant, in fitted model to predict the Wool ComfortMeter value of fabrics, mass/unit area

and fabric thickness were significant predictors along with fabric structure and finishing

treatment.

Keywords: Surface modification; plasma treatment; silicone polymer; next-to-skin comfort;

pique and single jersey

Introduction

The surface of the wool fibre is complex and its morphology and chemistry play important

roles in the functional properties of wool fabrics. Many chemical and/or physical treatments

are used to modify the fibre surface and improve the performance properties of wool fabrics

(Alexander and Hudson, 1963).

Plasma treatment is one of special interest as it can replace less environmentally friendly

finishing processes and create new surface characteristics. It has been considered as an

effective surface modification technique which induces chemical and morphological changes

on the surface of wool fibre (Naebe et al., 2010a; Naebe et al., 2010b; Naebe et al., 2011a;

Ratnapandian et al., 2011). The plasma treatment parameters (pressure, treatment time,

discharge power and the nature of the gas) control the effectiveness of plasma treatment.

Previous studies (Kim and Kang, 2002; Naebe et al., 2011b) have shown that plasma

treatment increased the stiffness of the fabric with increasing treatment time and plasma

power. Handle evaluation of treated fabrics has shown that plasma treatment made the fabric

harsher and longer plasma treatment times resulted in a serious impairment of fabric handle.

It has been shown that plasma treatment modifies the surface lipid layer of wool fibres,

particularly by the removal the fatty acid layer from the surface and then increases the

coefficient of friction of wool fabric, hence resulting in the harsher handle (Naebe, 2009).

Silicone based softening agents are normally associated with enhancements in fabric softness.

Their surface energies are lower than the wool fibre. Since they have a high tendency to

spread on the fibre during drying, there is a strong tendency for softener to form a continuous

layer between adjacent fibres (Rippon, 2008). When silicone polymer is applied to the fabric,

the formation of an elastic network on the surface of the fibre makes the fabric easier to bend

and twist (Naebe, 2009). Therefore, the net effect of softening agent on wool fibres is an

initial decrease in fabric stiffness and improvement in the handle of the treated fabrics. The

results of a subjective evaluation of the effects of fabric softness and harshness were

consistent with the bending stiffness evaluation (Naebe et al., 2011b). The mechanism for the

reduction in prickliness of silicon treated fabrics appears to be reduced fibre/fibre friction

rather than a modification in fibre modulus (Naylor et al., 1992).

The Wool ComfortMeter device has been developed to objectively assess the comfort

properties of knitted fabric. There is a good correlation with the results of a subjective

assessment in wearer trials of fabric prickle responses and this objective technique (Tester,

2010; McGregor & Naebe, 2012). Since the instrument has been designed to detect changes

in wool fibre bending stiffness (Ramsay et al., 2012), it is expected to be able to detect the

changes in comfort on both softener and plasma treated fabrics. The aim of this study is to

compare the effect of plasma treatment and softener on the comfort properties of wool fabrics

assessed by the Wool ComfortMeter.

Experimental

Design and material

The fabric design was: two knitting structures, three cover factors (CF) and three finishing

treatments. The fabric structures were single jersey and pique. The fabrics were knitted with

pure wool yarn of mean fibre diameter 19.4 μm and yarn count of 1/40 Nm. All fabrics were

knitted on a circular knitting machine with positive feed in 24 machine gauge in CF: loose

(L), medium (M) and tight (T) with corresponding values of 1.37, 1.45, 1.54 for single jersey

and 1.45, 1.54, 1.62 for pique fabrics, respectively. The finishing treatments were: control

fabrics, untreated (UT), plasma treated (PT), and treated with softening agent (ST).

Plasma treatment

An atmospheric pressure, ambient air plasma machine, manufactured by CSIRO Materials

Science and Engineering (Finn et al., 2006) was used. This device is a form of dielectric

barrier discharge plasma with one flat stainless steel mesh electrode and a series of glass

covered metal rods as the dielectric-barrier electrode. The fabric was placed in the 3 mm gap

between the glass electrode array and the mesh. Ambient air was passed through the rods, the

fabric and then the mesh with a face velocity of approximately 1 m/s. The operating

frequency and voltage were 2.6 kHz and 19 kV, respectively. The power input was ~10

kW/m2. Fabrics were treated by passing them through the device very slowly providing an

exposure time of approximately 10 seconds and a very high level of treatment.

Softening treatment

A silicone based softener, Megasoft Jet (Ciba), was applied to fabrics by a pad dry method.

Fabric samples were dipped in a freshly made aqueous solution containing 500 g/l water and

500 g/l softener emulsion for 30 sec prior to padding. The samples were padded to a wet

pick-up of 50 % and dried on a pin frame in a Benz laboratory stenter for a 3 min dwell at

150 °C.

Comfort evaluation

The testing procedure using the Wool ComfortMeter (WCM) followed the established test

method (Naylor et al., 2010). Briefly, five (30 cm × 30 cm) samples were prepared for each

fabric. As 18 fabrics were used (2 structures × 3 cover factors × 3 treatments), a total of 90

samples were prepared. The back of the samples was lightly steamed and fabrics were

conditioned at the standard conditions for over 20 hours prior to testing on the WCM. Fabric

comfort evaluation was carried out on the back of the sample in a random order each time.

Previous research on the influence of temperature and humidity on the WCM results (Naebe

et al. 2012) has confirmed that the WCM operates best at standard conditions. Then the

average of five values along their standard deviation (s.d) were determined for each fabric

(Table 1).

Mass per unit area and fabric thickness Measurements

Mass per unit area and fabric thickness were determined at standard conditions. Fabrics were

cut into 100 mm2 round samples using a ZWEIGLE circle cutter. Three samples were

prepared. Care was taken that the samples were cut from the middle part of the fabric roll to

avoid uneven edges which might have influenced the results. A precision scale (± 0.1 mg),

Metler Toledo Ltd., was used to determine the mass of each sample. The average of the mass

per unit area (g/m2) was calculated.

Fabric thickness was measured using Mesdan Lab thickness gauge with an applied pressure

of 0.5 kPa. The average of five measurements (± 0.01 mm) in different area of each sample

was reported as the average thickness.

Scanning electron microscopy

Surface morphology of the fabric samples was evaluated using a Field Emission Scanning

Electron Microscope (Hitachi S4300 SE/N). The images were taken at a working distance of

15 mm and an accelerating voltage of 1.2 kV.

Statistical analysis

To study the influence of knitting structure, CF and finishing treatment on the WCM value,

a randomized block ANOVA was applied to the data using SPSS Statistics 17 (SPSS

Manuals, Harvard University) . There were no outliers. Differences between treatments were

determined using a P-value = 0.05. Further analysis using the Post Hoc Multiple

Comparison using Least significant difference (LSD) was carried out to compare all

possible pairs of factor levels. Linear regression modelling was used to determine the effects

of significant physical attributes, including fabric thickness and fabric mass/unit area, upon

the WCM value.

Results and discussion

Table 1 gives the mean and s.d. of WCM values, mass per unit area and thickness for each

knit structure, CF and fabric treatment.

Table 1. Mean Wool ComfortMeter (WCM ± s.d.) value, mass per unit area (GSM) and thickness for single jersey and pique knitted fabric at each cover factor and finishing treatment

Treatment Cover factor

Single jersey Pique

WCM value

GSM (g/m2)

Thickness (mm)

WCM value

GSM (g/m2)

Thickness (mm)

L 713±89 173±0.6 0.68±0.01 509±93 174.1±1.8 1.01±0.02

Untreated M 691±94 195.5±0.1 0.63±0.02 547±32 180.5±1.3 0.95±0.01

T 659±61 207.5±1.6 0.65±0.01 535±57 187.8±0.1 0.95±0.01

L 713±65 182.6±1.9 0.71±0.01 462±80 205.3±0.1 0.98±0.01

Plasma M 687±30 185.5±0.9 0.73±0.01 519±36 201.2±0.8 0.96±0.01

T 682±113 209.6±1.6 0.73±0.02 551±38 211.5±0.1 0.96±0.02

L 545±66 178±0.3 0.59±0.01 457±80 171.1±0.4 1.02±0.01

Softener M 469±77 197.5±0.1 0.63±0.01 488±86 175.9±0.3 0.92±0.01

T 583±78 221±0.1 0.7±0.01 394±85 187.7±1.1 0.91±0.02

The results of analysis of variance of WCM value are summarized in Table 2. Both knitting

structure and finishing treatment had an independent effect on WCM value (P < 0.001).

However cover factor had no effect. None of the interaction terms between factors were

significant.

Table 2. Analysis of variance results for treatment affects on the Wool ComfortMeter evaluation of the knitted fabrics at three cover factors and three finishing treatment

Factor Sum of square Degree of freedom F-value P-value Model 29780000 17 304.91 <0.001

Knitting structure 455609.03 1 83.97 <0.001 Cover factor 12.81 2 0.001 0.999 Finishing treatment 270716.89 2 24.95 <0.001 LSD results for treatments are summarised in Table 3. As there was no difference in WCM

value between the untreated and plasma treated fabrics (P = 0.73) they have similar next-to-

skin comfort. There were significant differences in WCM value between softener treated

fabrics and both the untreated and plasma treated fabrics (P < 0.001).

Table 3. Least significant difference `s post hoc comparisons for the main effects of finishing treatments on Wool ComfortMeter value

Treatment (I) Treatment (J) Mean Difference (I-J) P-value Plasma Treatment Silicone Treatment112.93* <0.001

Untreated -6.56 0.73 Silicone Treatment Plasma Treatment -112.93* <0.001

Untreated -119.48* <0.001

Testing the statistical significance of parameters shows no difference in WCM value with

increasing CF, though our previous study, which had a larger number of fabrics and a wider

range in CF, (McGregor and Naebe, 2012) showed that CF may affect the WCM value.

The CF describes the density of a knitted fabric which in this study is defined as √Tl

in which

T is the yarn count in tex and l is loop length. It has been pointed out (Baird and Foulds,

1968) that some relaxation shrinkage nearly always occurs in the yarn and causes a reduction

in the loop length and a corresponding increase in cover factor when fabrics are relaxed in

water. The need to measure CF after wet relaxation and conditioning has also been

mentioned. In addition, in recent study (Saha, 2012) the effect of plasma treatment on CF was

investigated and changes in the CF as a result of treatment was reported.

These findings might indicate that the silicone treatment and plasma treatments may have

changed the nominal calculated CF and so may explain why the statistical analysis did not

detect any significant change in WCM value with increasing CF. Manipulating CF changes

fabric mass/unit area and fabric thickness, and WCM values may be more sensitive to these

direct measurements of fabric properties than CF. Therefore, further analysis using fabric

mass/unit area was carried out.

The multiple regression modelling of the WCM value rejected CF (P = 0.787) and the final

model was the form of (R2 = 0.990) with no interceptor:

WCM value = β1 Knitting structure + β2 Thickness+ β3 Finishing treatment + β4

mass/unit area

where treatment was coded 0 for untreated fabrics and 1 for plasma treated and 2 for silicone

treated fabrics, and β1, β2, β3 and β4 are the coefficient of the model estimated in the Table 4.

Table 4. A list of the regression coefficients and statistical significance of included terms in the final model for Wool ComfortMeter values of treated and untreated knitted fabrics. Values are relative to untreated single jersey fabrics. Adjustment to final model β s.e. t-value P-value Knitting structure -435.1 106.93 -4.07 0.001 Thickness 1109.2 330.23 3.36 0.005 Finishing treatment -45.6 19.16 -2.38 0.032 Fabric mass/unit area 2.1 0.69 3.06 0.009

The softened fabrics were different from and had lower WCM values compared with the

plasma treated and untreated fabrics. The existing knowledge on the effect of silicone

polymer on the surface of the wool fabrics confirms a reduction of both bending modulus

(Kim and Kang, 2002) and friction coefficient (Naylor et al., 1997; Kim and Kang, 2002) of

wool fibres. Earlier work has shown that the plasma treatment resulted in a higher bending

modulus (Naebe at al, 2011b; Kan and Yuen, 2009) and a higher friction coefficient (Kim

and Kang, 2002; Thomas, 2007 ) on wool fabrics compared with untreated fabrics.

Since the operating principles of the WCM means it is sensitive to the changes in wool fibre

bending stiffness (Ramsay et al. 2012), the observed effects are as expected for the softener

treated fabrics. While, for the plasma treated fabrics a similar WCM value compared with

that of untreated fabrics indicates no change in the fibre bending modulus. The increase in

fabric bending modulus is due to fibre friction increases and not increases in the component

fibre bending modulus. .

Therefore, the changes in fibre bending modulus could explain the observed differences in

WCM values. However the WCM device is also sensitive to the density and number of the

protruding fibres from the surface of the fabrics ( Ramsay et al., 2012). It is also possible that

the lower WCM values occurring after silicone treatment are a result of less fibre on the

fabric surface interacting with the recording head of the WCM. It has been known for many

years that the application of silicone polymers can glue protruding fibres together or cause

them to adhere to the surface of the yarn and therefore not protrude (Figure 1, Makinson,

1975; Naebe et al., 2011b).

Figure 1. SEM image of wool fabric treated with Silicon

Regardless of treatment, the pique fabrics showed lower WCM values compared to the

corresponding single jersey fabrics. This appears to confirm the report of pique fabrics

producing lower prickle responses during fabric wearer trials where other aspects of fabric

design and construction and WCM assessment were also accounted for in prediction models

(McGregor et al., 2012). While the same yarn is used for both pique and single jersey fabrics,

the pique fabric is thicker, more stable to treatments and changes in CF and has a more open

surface structure (Kane et al., 2007) (Figure 2). Accordingly, more protruding fibres sit deep

in the surface of the pique fabric compared with single jersey fabrics. Thus the lower WCM

value of pique is probably due to less fibres protruding enough to be available for detection

by the WCM. The results are in good agreement with the objective evaluation of sensory

responses of knitted single jersey and pique fabrics, where pique fabrics were found to be

more comfortable (Kane et al., 2007).

Figure 2. Back face of untreated single jersey (left) and pique (right) fabrics of cover factor 1.45 used in the present study

Conclusion

Although silicone treatments are normally associated with enhancements in softness, plasma

treatments are known for their harsh effects on fabric handle. While the Wool ComfortMeter

was able to assess the changes affected by silicone on treated fabrics, the differences between

plasma treated and untreated fabrics were not significant. It is likely that modification in fibre

modulus is not the only mechanism involved. It is believed that gluing the protruding fibres

together by silicone polymer is the reason for reduction in Wool ComfortMeter value of

silicone treated fabrics compared with untreated and plasma treated fabrics, as less individual

fibres can be detected by Wool ComfortMeter. Though cover factor did not show to be

significant on the comfort evaluated by Wool ComfortMeter device, mass per unit area and

thickness of fabrics were significant in the predicted model. Pique fabrics showed a lower

Wool ComfortMeter value than single jersey fabrics.

ACKNOWLEDGMENT The authors would like to express their appreciation to L. Staynes, F. Dean and Dr. N. Finn

from CSIRO, Materials Science and Engineering, Belmont, Australia. This work was funded

by The Cooperative Research Centre for Sheep Industry Innovation Ltd.

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