Developing a polymeric human finger sensor to study the frictional properties of textiles: Part II: Experimental results1

Developing a polymeric human finger sensor to study the frictional properties of textiles: Part II: Experimental results1

Ramkumar, S S

Background

Part I of this series elaborated our rationale and methodology behind the development of a polymeric artificial human finger friction sledge to evaluate the frictional characteristics of textile fabrics [6]. In this part of the series, we use the polymeric artificial finger sledge to evaluate the frictional properties of a set of 1×1 rib knitted cotton fabrics varying in loop length and yarn linear density. We fabricate a reciprocating friction tester to provide the to-and-fro motion to the polymeric human finger on fabrics. We then obtain frictional force values from the stick-slip friction trace, which we use to calculate the frictional parameter C, to characterize the frictional properties of the knitted fabrics.

Reciprocating Friction Apparatus

Figure 1 shows the to-and-fro reciprocating sliding friction apparatus for measuring frictional forces at different normal loads in both directions. Zurek et al. used a to-and-fro reciprocating friction tester to characterize the frictional properties of synthetic fabrics [7]. In this study, fabrics are rubbed against each other and their frictional interaction is characterized with the frictional parameter C. In order to better simulate the to-and-fro reciprocating mechanism of a finger on fabric, the standard friction sledge that rubs against the fabrics should have the shape and contour of a human finger. The reciprocating motion of the polymeric artificial finger sledge on fabric is a simple solution to the complex issue of evaluating the frictional feel of textiles. The artificial finger sledge, described in Part I of this series, is used as the standard friction sledge [6]. This polymeric finger sledge is 10 mm wide and 25 mm long. Frictional experiments involve different normal loads on the polymeric finger sledge; the minimum and maximum loads are 5.7 and 35.7 g, respectively, and the normal load is incremented in steps of 10 g. The maximum load that can be used on the sledge is 35.7 g, and any increase in load beyond this value tilts the sledge, preventing it from reciprocating smoothly and steadily. Due to this experimental limitation, loading was undertaken at four levels only.

FRICTIONAL FORCE CALCULATION IN TWO DIFFERENT DIRECTIONS

As described before, the reciprocating sliding friction apparatus is used to measure the frictional forces in two directions. Dead weights are loaded on the friction sledge so that the sledge rubs the fabric in the reverse direction as the cross head of the tensile tester moves downward. This section is concerned with the calculations of frictional forces in the forward and backward directions. The cumulative static and kinetic friction force values are used to obtain the frictional parameter C.

The friction force F^sub 1^ is the resistance force measured as the cross head of the Instron tensile tester moves upward. It has two components, static and kinetic friction forces. The friction force F^sub 2^ is that measured as the cross head moves in the downward direction. Likewise it has two components, the static and the kinetic friction forces. Figure 2 shows the frictional trace as recorded on the chart of the tensile tester in two directions.

The friction forces can be calculated in two ways. Crosshead moving upward: The static friction as recorded in the tensile tester’s friction chart is given by

F^sub s1^ = G + F^sub sr1^ , (1)

where F^sub s1^ is the static friction resistance recorded on the tensile tester’s chart, G is the dead weight, and F^sub sr1^ is the actual frictional resistance. The kinetic friction as recorded on the tensile tester’s friction chart is given by

F^sub k1^ = G + F^sub kr1^ , (2)

where F^sub k1^ is the kinetic friction force recorded by the tensile tester’s chart recorder, G is the dead weight, and F^sub kr1^ is the actual kinetic resistance.

Crosshead moving downward: In this case the dead weight acts in the direction of the motion of the sledge, and the frictional resistance is in the opposite direction. The static friction force as recorded on the tensile tester’s friction chart is given by

F^sub s2^ = G – F^sub sr2^ , (3)

where F^sub s2^ is the static force recorded by the tensile tester’s chart recorder, G is the dead weight, and F^sub sr2^ is the actual static resistance. The kinetic friction force as recorded by the tensile tester’s friction chart is given by

F^sub k2^ = G – F^sub kr2^ , (4)

where F^sub k2^ is the kinetic force recorded by the tensile tester’s chart recorder, G is the dead weight, and F^sub kr2^ is the actual kinetic force. The cumulative static friction force is given by

F^sub s^ = 1/2(F^sub s1^ – F^sub s2^) , (5) and the cumulative kinetic friction force is given by

F^sub k^ = 1/2(F^sub k1^ – F^sub k2^) . (6)

The friction forces in two directions can be conveniently measured with the reciprocating friction apparatus, then used to calculate the cumulative friction forces F^sub s^ and F^sub k^. The cumulative static and kinetic frictional forces are employed to calculate the frictional parameter C.

Experimental Materials and Results

We used a set of 1 X 1 rib knitted cotton fabrics varying in their loop length and yarn linear densities, as given in Table I.

We used the modified friction apparatus to calculate the friction forces in two directions. The polymeric human finger sledge was the standard friction sledge. The fabric whose frictional properties were to be measured was attached to the friction tester’s platform by double-sided adhesive tape. The artificial human finger sledge was able to alter normal loads, and the experimental results were recorded on the tensile tester’s friction chart. The procedure as described in the previous section was used to calculate the cumulative friction forces. The details of the fabrics in this investigation are given in Table I. The experiment was repeated three times on each fabric at each normal load, and the average value was then employed to calculate the cumulative friction forces. The experimental results are given in Tables II and III.

We used these normalized mean friction force values to calculate the friction parameter C. We found that the friction force-normal load relationship can be represented by an equation of the form F/A = C(N/A)^sup n^. The correlation values between the experimental and calculated friction values are given in Table IV.

As is evident from Table IV, there is a good correlation between the experimental and calculated friction force vales. This suggests that the relationship F/A = C(N/A)^sup n^ is valid for representing the frictional relationship between the polymeric finger sledge and the fabrics investigated. Frictional parameter C values of knitted fabrics calculated with the artificial human finger sledge are given in Table V. The frictional parameter C is used to compare and characterize the frictional properties of cotton knitted fabrics. Because C is the measure of the surface characteristics, we decided to use C alone to compare the frictional properties of cotton knitted fabrics. Furthermore, the material/friction index n depends on the material properties of the sliding surfaces [4]. In this study, the sliding surfaces are cotton and polyvinylsiloxane, and they remain the same in all the experiments. This feature enables frictional comparisons of cotton knitted fabrics with the friction factor C.

TYPICAL FRICTION TRACE OF ARTIFICIAL FINGER SLEDGE ON KNITTED FABRIC

As shown in Figure 3, the stick-slip trace of the artificial finger sledge on knitted fabric does not have pronounced peaks and troughs, which are common when a fabric rubs against another fabric. It is clear that the contour and surface profile of the artificial finger sledge influences the pattern of the stick-slip trace. Since the plateau of the artificial finger sledge rubs against the fabric surface, the pattern is smooth rather than showing peaks and troughs. In addition, the fabrics investigated in this study are knits, which are generally softer than woven fabrics, resulting in smooth frictional traces. Both the profile of the standard friction sledge and the fabric structures play significant roles in determining the stick-slip pattern.

Discussion

INFLUENCE OF KNITTED STRUCTURAL VARIABLES ON THE FRICTION PARAMETER C

Figures 4a and b show the effect of loop length and yarn linear density on the frictional properties of knitted fabrics. It is evident from Figure 4a that as the loop length of the fabric increases, the C value decreases, showing that the frictional resistance of fabrics decreases with increased loop length. Loose fabrics offer less resistance to the smooth motion of the polymeric friction sledge than tight fabrics. As the loop length increases, the tightness of the fabrics decreases, and this reduced tightness helps reduce the plowing component of the friction of the polymeric finger on the knitted fabrics. Similarly, for fabrics made from finer yarns, the frictional resistance is less, and this is due to the reduced contact area and hence the adhesion component of friction. This trend is clearly shown in Figure 4b. We can quantify the change in the friction parameter values for different knitted fabrics using percent differences in the frictional parameter values. The fabric with the lowest friction factor parameter is considered to be the base fabric in each set of fabrics, against which other fabrics are compared. The base fabric against which comparisons] are made for set 1 is fabric D and for set II fabric O. The percent change in the friction factor is calculated as follows:

The percent changes in friction factor values for both static and dynamic friction for different fabrics are given in Table VI. Most recently, Ramkumar used a similar approach to quantify the changes in the friction values after enzyme finishing [5]. The friction parameter C is calculated from the intercept of the regression equation relating the friction force and normal load values. The change in the intercept values is represented as a percentage change. This method of quantifying change is logical, so we have made no attempt to test the significance by other methods. This is because the test for equality of intercepts in the pooled multiple regression model relies on the assumption of common normal errors, which may not be true in this particular case. Therefore, a percentage analysis is a logical and intuitive solution to the differences in the friction factor values [2].

The methodology we have adopted in this study is different from that adopted by Kawabata. Kawabata’s friction testing involves a steel sensor at a normal load of 50 gf, and the frictional property of fabrics is characterized by the coefficient of friction [mu]. Therefore, comparing the results from our study with those from the Kawabata tester is not logical. In other studies, we used subjective evaluations to determine the smoothness of the same set of knitted fabrics [1, 3], employing a paired comparison technique. Five subjects who were knowledgeable about fabric quality participated in the study, ranking the fabrics for their smoothness. The higher the rank of the fabric, the lower its friction. Results from these studies revealed that tighter fabrics were perceived to be rougher and scored lower ranks [1, 3]. Our friction evaluation of knitted fabrics with the polymeric finger sledge resulted in higher friction parameters for tighter fabrics than for lighter fabrics. Based on our results, it seems that there is agreement between the results from the frictional evaluation with the polymeric finger sledge and the subjective evaluation.

Conclusions

We have adopted a novel approach to characterize the frictional properties of textiles. We have developed an artificial polymeric human finger to evaluate the frictional properties of a set of 1X1 rib cotton knits. Our polymeric finger mimics the shape and surface features of a human finger. In order to simulate the frictional interaction of human fingers with fabrics, we have developed a friction apparatus capable of reciprocating motion and used it to calculate the cumulative friction forces in the two directions with the artificial finger sledge. The frictional properties of knitted fabrics can be characterized with the friction parameter C. Both loop length and yarn linear density influence the frictional interaction of the artificial finger sledge on knitted fabrics.

The work reported in this paper has enabled us to measure fabrics in a way similar to the tribological interaction of human fingers with fabrics. Our apparatus is a simple and novel way of characterizing the frictional feel and hand of fabrics.

ACKNOWLEDGMENTS

S. S. Ramkumar acknowledges the CVCP of the United Kingdom for the provision of an Overseas Research Student award and the University of Leeds for Tetley and Lupton scholarships. Dr. Ramkumar received the Lee Educational Trust Bursary, UK, for this work on the study of knitted fabrics.

Copyright Textile Research Institute Jul 2003

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