- Email: [email protected]
PANI composite yarns
Accepted Manuscript Title: A large-strain weft-knitted sensor fabricated by conductive UHMWPE/PANI composite yarns Author: Jianhan Hong Zhijuan Pan ZheWang Mu Yao Jianguang Chen Yexing Zhang PII: DOI: Reference:
S0924-4247(15)30250-8 http://dx.doi.org/doi:10.1016/j.sna.2015.12.028 SNA 9454
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
3-10-2015 28-11-2015 2-12-2015
Please cite this article as: Jianhan Hong, Zhijuan Pan, ZheWang, Mu Yao, Jianguang Chen, Yexing Zhang, A large-strain weft-knitted sensor fabricated by conductive UHMWPE/PANI composite yarns, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.12.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
A large-strain weft-knitted sensor fabricated by conductive
2
UHMWPE/PANI composite yarns
3
Jianhan Honga,b,c, Zhijuan Panc,d,*,Zhe Wangc,d, Mu Yaoe, Jianguang Chen f, Yexing
4
Zhangg
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
a
College of Textile and Garment, Shaoxing University, Shaoxing 312000, China School of Textile and Clothing, Suzhou Institute of Trade & Commerce, Suzhou 215009, China c College of Textile & Clothing Engineering, Soochow University, Suzhou 215123, China d National Engineering Laboratory for Modern Silk (Suzhou), Suzhou 215123, China e Faculty of Textile & Material, Xi’an Polytechnic University, Xi’an 710048, China f Yancheng Silide Silk Co., Ltd., Yancheng 224311, China g Jiangsu Shenghong Polytron Technologies Inc., Suzhou 215228, China b
Highlights
Weft-knitted strain-resistance sensors from conductive UHMWPE/PANI composite yarns were obtained for the first time.
The resistance of the fabric sensors increased with increasing strain and then decreased when they were further stretched. The fabric sensors exhibited good linearity, sensitivity, and repeatability in the small-strain condition.
Abstract：In this paper, UHMWPE filament yarns were modified to improve their surface energy using atmospheric pressure plasma pretreatment. Then, a novel method based on in-situ polymerization was used for the continuous fabrication of conductive UHMWPE/PANI composite yarn with a conductivity of 0.87±0.1 S/cm. Cylindrical plain weft-knitted fabrics with different densities were fabricated using conductive UHMWPE/PANI composite yarns, and their strain-resistance performances were studied in detail. The results showed that the resistance of the fabrics increased with increasing strain at a relatively low strain range, and it also exhibited good linearity, sensitivity, and repeatability, which were enhanced by increasing density. When the strain was relatively high, the resistance of the fabrics increased with increasing strain and then decreased when the fabrics were further stretched. The repeatability of the fabrics in the large-strain condition was worse than that in the small-strain condition. The repeatability of the fabrics was very different when comparing the first stretching and the second stretching, and the difference then declined and showed good repeatability after repeated stretching three times. Key words：Conductive UHMWPE/PANI composite yarn; Weft-knitted fabric; Strain-resistance; Sensing character In recent decades, sensing technology has drawn increasing attention with the development of measuring techniques, control techniques, and automatic techniques. Sensors have played a more important role in many fields because of environmental requirements and the development of economy, science and technology. Various sensors have been widely used in production automation, energy, transportation, disaster prediction, environmental protection, medical health, and other fields for every aspect of human life [1-6]. With the optimal Corresponding author at: Soochow University, No. 199 Renai Road, Suzhou, PR China. Tel.: +86 13625273222. E-mail address: [email protected] (Z. Pan).
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
combination of sensing principles and material science, sensor technology is used for information monitoring based on a wide variety of functional materials. The sensing principle is based on the physical effect, chemical reaction, and biological reaction, and the functional material is the substantive basis of the sensor technology. In some sense, a sensor can be considered the functional material that can perceive many types of measured signals. Conductive fiber, as a type of functional material, plays a very important role in the sensor field. The resistance of the conductive fiber varies due to its structure change when the fiber was deformed. J. P. Wang[7] deposited conductive PPY on the surface of polyurethane fiber and tested its strain-resistance performance. J. Li [8] manufactured polyamide conductive nanofibers containing carbon nanotubes by using electrospinning and studied the variation of its resistance at different tensile conditions. X. J. Wang [9,10] used carbon fibers as strain sensors in a fiber reinforced cement composite to monitor its internal fracture. The conductive fiber has its own disadvantages of slight deformation, poor resilience, and low sensitivity when used as a strain sensor, owing to the limitation of its structure and properties. The deformation of the structures for the conductive fiber assemblies (woven fabric and knitted fabric) would first occur when they were stretched, for example, when extending the buckling of the yarn and the movement of the loops. Finally, the deformation of fiber occurred. So, the conductive fabric possessed of greater deformability. Furthermore, the resistance of fabric depends on not only on the resistance of fiber but also on its structure. Therefore, a greater variation space of resistance exists in the fabric sensor versus the fiber sensor. There are two main ways to prepare fabric sensors at present. First, they are manufactured using intrinsic conducting materials, such as metal fibers. W. Qureshi [11] prepared a knitted fabric strain sensor using conductive yarns containing metal fibers to monitor respiratory signals. L.W. Li [12] prepared a rib fabric sensor by using stainless steel fibers and developed a respiratory monitoring system. The second way is to provide conductivity to the fabric using conductive treatment methods such as conductive polymer coating and chemical vapor deposition. J. Wu [13] studied the relationship between the strain and resistance of a polyurethane fabric coating with polypyrrole (PPy), and the fabric exhibited high sensitivity (the strain gauge factor was approximately 25). J. Tsang [14,15] prepared a conductive fabric coating with PPy using chemical vapor deposition based on a polyamide/polyurethane plain-knitted fabric, and the fabric possessed high sensitivity (the gauge factor was over 400 when the strain was less than 50%). Y. Li [16], J. W. Ryu [17] and K. Kalleto [18] also performed similar research, and the results showed that the strain sensor made of PPy conductive fabric possessed high sensitivity. From the aspect of fabric sensor applications, E. C. Toni [19] prepared a sensor using a polyamide/polyurethane fabric coating with PPy to monitor the vertical movement of the chest of the participants. The fabric sensor has many obvious advantages, such as low cost, lightweight, breathable, soft, wearable, large deformability, and easy recovery of deformation, versus many other types of sensors. So, the fabric sensor has been widely used in human detection, smart clothing, and other fields. The sensor prepared using metal fibers, which was mentioned above, possessed many shortcomings, such as difficult spinning, poor weaving properties, hard hand feel, and low wearing comfort. The sensor prepared based on the conductive post-treatment of common fabric also had a defect of lacking flexibility on the structure design, for example, it was difficult to implement the intertexture of conductive yarn and common yarn and to design a sensor that met different requirements. The purpose of this study was to solve the main problems of the two types of fabric sensors mentioned above. A fabric sensor with excellent conductivity was prepared using soft organic conductive yarn, which improved the processability for the post-processing of the yarn, the comfortable hand feel and the flexible structure design of the fabric. In previous studies, we developed a method of preparing conductive yarn continually based on in-situ polymerization, and the conductive UHMWPE/PANI composite yarn was also successfully prepared [20,21]. The results showed that the conductive UHMWPE/PANI composite yarn possessed good conductivity and durability. In this paper, four types of flexible weft-knitted fabric sensors with large strain capacities were prepared using the conductive UHMWPE/PANI composite yarn, and their strain-resistance properties were studied. Furthermore, the
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
conductive UHMWPE/PANI composite yarn’s applications in sensor technology were also studied. 1. Experimental 1.1 Preparation and characters of conductive UHMWPE/PANI composite yarn The UHMWPE filament yarn (444dtex/406f) used in this paper was provided by Xiangsheng High Strength Fiber Materials Inc.(Hangzhou, China). The detailed preparation process of conductive UHMWPE/PANI composite yarn was as follows: UHMWPE filament yarn was modified to improve its surface energy using an atmospheric pressure plasma pretreatment. Then, the yarn was successively immersed in an aniline monomers solution and the mixed reaction solution of oxidant (150 g/L ammonium persulfate) and doping acid (1.75 mol/L sulfuric acid) to make the yarn possess aniline monomers, oxidant, and doping acid on its surface. The yarn ran continuously (12 m/min) under traction so that it experienced a conductive treatment. In the end, the yarn was loosely placed in a standard environment so that the aniline reacts with the mixed reaction solution for at least 24 h to form PANI. The surface morphologies of the UHMWPE yarns before and after the conductive treatment were obtained using an S-4800 cold field emission scanning microscope (Hitachi Limited, Japan). The resistances of UHMWPE/PANI composite yarns were measured with an Agilent 4339B high resistance meter (Agilent Technologies Co., Ltd., USA) at 20℃and 65% RH, and the electrical conductivities of the UHMWPE/PANI composite yarns were calculated according to equation 1:
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
L ……（1） RS
where σ is the electrical conductivity (S/cm), R is the electrical resistance (Ω), and L and S are the length (cm) and cross section area (cm2) of UHMWPE/PANI composite yarns, respectively. To specify the variation in the resistance of the UHMWPE/PANI composite yarn along the length direction, the resistance test of the yarn began with the head of the yarn and was tested every one meter. The total measurements were 200 m. 1.2 Preparation of fabric sensors Weft-knitted fabric generates large deformation while it is under small stress, whereas it exhibits excellent recovery capability. The deformation of the weft-knitted fabric is mainly due to the transfer of the yarns in the loops for a certain range of deformation, which avoids the deformation of yarns and the fatigue damage of the knitted fabric after they are repeatedly stretched. Thus, in this paper, cylindrical plain weft-knitted fabric was selected to study its strain-resistance property. Fig. 1 shows the structure of the cylindrical plain weft-knitted fabric prepared by an E12 hand-operated flat knitting machine, and there are 60 wales for the length of 10 cm in the weft-knitted fabric when it was off the loom. The fabric was knitted using polyurethane yarns (26.6dtex) and conductive UHMWPE/PANI yarns to improve its recovery ability, and the mass percentage of polyurethane in the fabric was 4.4%. Four types of weft-knitted fabrics with different densities were prepared by adjusting the yarn’s sinking depth for the loop forming triangle in the knitting machine and to study the effect of fabric density on its sensing property.
34 35
Fig. 1 Structure of cylindrical plain weft-knitted fabric
36
Tab. 1 Density of weft-knitted fabrics
Sample
1#
2#
3#
4#
Courses per inch 1 2 3 4 5 6 7 8 9 10 11 12
18.2
19.0
21.1
22.3
1.3 Resistance testing of fabric sensors The resistance changes of the fabric sensors were recorded using a homemade apparatus to investigate their strain sensitivity, which is shown in Fig. 2. The homemade apparatus consisted of high resistance instrument 1 and two clamps (4 and 5). Clamp 4 was fixed on baseplate 2, and clamp 5 could reciprocate along the slideway that was fixed on the baseplate. Conductive fabric 6 could generate different tensile deformations when it was fixed between the two clamps. The fabric was held by the two clamps with a clamping length of 5 cm, and the resistance of the fabric was measured and recorded using the high resistance instrument. Then, clamp 5 moved 1-mm outward, and the resistance value was recorded again. The resistance value was recorded every 1 mm that clamp 5 moved by. To investigate the repeatability of the fabric sensor, the fabric returned to its original length after it was stretched to a certain length, and six repeated tests were performed. An elastic latex film was put in the middle of the fabric to prevent the connection of the fabric insides, which may distort the results.
13 14 15 16
1.4 Modeling of the conductive knitted fabric Figs. 3 a and b show the structure of the weft-knitted fabric and its equivalent electric circuit of a single loop,
17
respectively. RL1 is the length resistance of the needle loop, RL 2 is the length resistance of the limb, RL3 is the
18
length resistance of the sinker loop, and RC1 is the contact resistance of the adjacent loops (up sinker loop and down
19
needle loop). RC 2 is the contact resistance of the adjacent loops on the left and right sides. RC 3 is the contact
20 21 22 23 24 25 26
resistance of limbs. The length resistance of the loop depends on its electrical conductivity and length. The contact resistance of the loops depends on the contact length and the contact force, that is, the longer the contact length is, the greater the contact force is and the smaller the contact resistance is. When the fabric was stretched along the longitudinal direction, each section of the loop would transfer, the limb became longer and the circle loop became shorter; thus, the length resistance would change. Furthermore, the contact resistance would also change with the variation of contact length and contact force. Thus, the fabric could be used as a strain sensor due to its total resistance change with the variation of tensile elongation.
Fig. 2 Homemade apparatus for resistance testing of sensing fabrics
27 28
Fig. 3 Structure of (a) weft-knitted fabric and (b) the equivalent electric circuit of single loop
1 2 3
Weft plain construction is made up of continuous loops that are connected to each other in the transverse direction and are intermeshed together in the longitudinal direction. Thus, the weft plain construction has many sensor units that connect to each other repeatedly. The resistance of every sensor unit could be calculated as per Fig.
4
4. Suppose RC1 s on the left and right sides of the diagram were equal and so were RL 2 s; thus, RL1 and RC 3 would
5 6
act as the electrical bridges when the voltage was applied according to the figure. Because there was no current flowing through them, they had no influence on the total resistance of the sensor unit. Therefore, the resistance of
7
the sensor unit could be regarded as the two RC1 s that were connected in series and then were connected in parallel
8
with two RL 2 s that were connected in series or as two RC1 s that were connected in parallel and then were connected
9
in series with two RL 2 s that were connected in parallel. Equation 2 was used to measure the resistance of a sensing
10
element.
11 12
Fig. 4 Calculation of resistance
R11
13
RC1 R L 2 ……（2） 2
14
The resistance of the unit that made up one row and two wales could be measured in Fig. 4b. It was assumed
15
that the resistance of the same part (same color in Fig. 4b) of every loop was the same and that the values of RC 2
16
and RL3 had no influence on the total resistance of the unit. So, this unit ( R12 ) could be regarded as two R11 s that
17
were connected in parallel. R12 can be described by equation 3:
R12
18 19 20
The resistance of the unit that made up two rows and one wale could be measured in Fig. 4c. Thus, this unit ( R21 ) could be regarded as two R11 s that were connected in series, and R21 can be described by equation 4:
R 21 2R 11 R C 1 R L 2 ……（4）
21 22 23 24
R11 RC1 R L 2 ……（3） 2 4
All of the equivalent resistances of plain weft-knitted fabric with different sizes could be measured using the same method, as shown in Table 2. Tab. 2 Equivalent electric circuit of fabrics with different loops
course
1
wale 1
R11
RC1 R L 2 2
2
3
……
n
R21 2R11
R31 3R11
……
Rn1 nR11
…
R11 2 R11 R13 3 ……
m
R1m
2 3
R12
R11 m
3R11 2
……
R23
R33 R11
…… ……
R2 m
…… 3R R3 m 11 m
nR11 2 nR11 Rn 3 3 ……
……
Rnm
R22 R11
2 R11 3 …… 2 R11 m
R32
Rn 2
nR11 m
1 2
number of wales.
3 4 5 6 7
Thus, planar weft plain fabric could be considered as a sensor that is made up of many of the same sensor units, which are connected with each other in a transverse direction and a longitudinal direction. However, the current transferred from one loop row to another in the longitudinal direction in the cylindrical plain weft-knitted fabric and also moved along the fiber length because the fabric was knitted using a single yarn. As shown in Fig. 5, the current transferred from one loop row to the next row in a corkscrew pattern.
8 9
Fig. 5 Resistance network of cylindrical plain weft-knitted fabric
Note: in the subscript of the resistance, the previous number represents the number of rows, and the next number represents the
10
Thus, the total resistance of the cylindrical plain weft-knitted fabric ( Rtotal ) could be regarded as the total
11
resistance of the plain weft-knitted fabric ( Rnm ) with the same rows and wales connected in parallel with the total
12
length resistance of the yarn ( RL ), and Rtotal could be calculated using equation 5:
13 14 15 16 17 18 19 20 21 22 23
Rtotal Rnm // RL
Rnm R L ……（5） R nm RL
Based on the above analysis, it is assumed that the resistance of the loops with the same color in Fig. 5 was all of the same, the resistance of the cylindrical plain weft-knitted fabric depended on the total length resistance of the yarn, the length resistance of the limb, and the contact resistance of the adjacent row loops and had nothing to do with the length resistance of the needle loop, the length resistance of the sinker loop, the contact resistance of the longitudinal adjacent loops, and the contact resistance of the right-and-left limbs. Actually, every length resistance and contact resistance in the whole fabric were not identical with those mentioned above because the conductivity of the yarn was not the same in the length direction, and the contact force and contact area of the loops were also not completely consistent with each other due to the difference in size and the morphology of the loops. Thus, the total resistance of the cylindrical plain weft-knitted fabric was affected by the length resistance of the needle loop, the length resistance of the sinker loop, the contact resistance of the longitudinal adjacent loops, and the contact
1 2 3 4 5 6 7 8
resistance of the right-and-left limbs. The greater these resistances were, the higher the total resistance of the fabric was. 1.5 Sensing performance of conductive weft-knitted fabric Many sensing performance specifications could be used to evaluate the property of a sensor. The linearity, sensitivity, and repeatability were proposed in this paper. Linearity was defined by the percentage of the maximum deviation between the calibration curve with the fitting straight-line and the full scale of the sensor under definite conditions, which can be described by the following equation [22]:
L
9
10
Ymax ……（6） YF S
where L is the linearity, Ymax is the maximum deviation between the calibration curve and the fitting
11
straight-line, and YF S is the full scale of the sensor.
12 13 14
Sensitivity could be defined as the ratio of output and input variation in a stable state, which is usually represented by a gauge factor. The sensitivity is the ratio of the change in electrical resistance and the strain in this paper, which can be described by the following equation:
G
15 16 17 18 19 20 21
where G is the gauge factor, R is the difference in the resistance before and after stretching, L is the variation of the sample’s length, and L0 is the original length of the sample. Repeatability could be defined as the inconsistency of the characteristic curves of the input quantity that continuously changes within the full scale range in the same direction under the same condition, which could be represented by the percentage of the maximum value of the standard deviation of two or three times and full scale [22]. The following equation was used to measure the repeatability of a sensor.
k
22
23
R / R 0 ……（7） L / L 0
2 ~ 3 100 ……（8） YF S
where k is the repeatability and is the standard deviation.
24 25
Repeatability is the percentage of the maximum value of the standard deviation of two times during the tensile process and full scale in this paper.
26
2. Results and discussion
27 28 29 30 31 32 33 34 35 36
2.1 UHMWPE/PANI composite yarn Fig. 6 shows the morphology of UHMWPE yarns before and after the atmospheric pressure plasma pretreatment and the conductive treatment. It can be seen from Fig. 6b that the surface roughness of the UHMWPE fiber was improved effectively after the atmospheric pressure plasma pretreatment, which is helpful for more aniline and reaction solution absorption and is uniformly scattered on the surface of fibers during conductive treatment. Furthermore, the improvement of surface roughness for UHMWPE fiber contributes to the uniformity and fastness of PANI on the fiber surface, which is in favor of the enhancement of conductivity and the durability of the composite conductive yarns, and this phenomenon is consistent with the previous study [23]. A complete PANI layer was coated on the surface of the UHMWPE fiber, and the gaps between fibers were also filled with PANI, which contributed to the fibers’ bond in the UHMWPE filament yarn, as shown in Fig. 6c.
1 2 3
Fig. 6 Surface morphology of (a) UHMWPE, (b) plasma pretreated UHMWPE, and (c) conductive UHMWPE/PANI
4 5 6 7 8
Fig. 7 shows the distribution of electrical conductivity for the UHMWPE/PANI conductive yarn within 200 meters in the length direction. It can be seen that the conductive yarn exhibited excellent electrical conductivity, that the average conductivity was 0.87 S/cm and that the deviation was approximately 0.1 S/cm, which further indicated that conductive UHMWPE/PANI composite yarn with high conductivity and uniformity could be prepared using the continuous conductive treatment method based on in-situ polymerization.
9 10 11 12 13 14
15 16 17 18 19 20
composite yarn
Fig. 7 Variation of electrical conductivity of UHMWPE/PANI conductive yarn in length direction
2.2 Effect of tensile strain on the resistance of fabric sensor Fig. 8 shows the variation in the resistance of weft-knitted fabrics with different densities under the condition of small strain. It can be seen that these weft-knitted fabrics have a similar strain-resistance change rule, and the resistance of all of the fabrics increased with the increase in strain.
Fig. 8 Strain-resistance curves of weft-knitted fabric with different densities in small strain
The variation in the resistance of weft-knitted fabrics with different densities under the condition of large strain was shown in Fig. 9. The sensing property of the weft-knitted fabrics under the condition of large strain was significantly different than that under the condition of small strain. The resistance of weft-knitted fabrics increased with the increase in strain at the initial stage of tension, whereas when the strain arrived at a certain point, the
1
resistance would decrease when the strain was further increased.
2 3
Fig. 9 Strain-resistance curves of weft-knitted fabric with different densities in large strain
4 5 6 7 8 9
The resistance of weft-knitted fabric depends mostly on the total length resistance of yarn, the length resistance of limbs, and the contact resistance of the adjacent row loops. The total length resistance of yarn was constant during the deformation. When the cross-sectional area and conductivity of the yarn were fixed, the length resistance was only affected by the length of limbs, which was determined by the applied force in the course of tension. The greater force applied, the greater length resistance due to the longer yarn transferred from the sinker loop and needle loop, as shown in Fig. 10, which were recorded by a 3D optical microscope (Keyence, Japan) in different stretching.
10
11 12 13 14 15 16 17
Fig. 10 Changes in structure of sensing fabric after (a) 0%, (b) 6%, (c) 10%, (d) 20%, (e) 30% and (f) 40% stretching
The influencing factors of contact resistance were very complex. The contact resistance increased under low tensile stress due to the decrease in contact area, which was caused by the decrease in the sinker loop, the needle loop, and the length of contact yarn. When the tensile stress reached a certain level, the contact area increased due to more fibers transferring to the contact surface of the yarn as the extrusion deformation of yarn occurred, which led to the decrease in contact resistance [24], as shown in Fig. 11.
1 2
Fig. 11 Movement of fibers in two yarns
3
The contact resistance was also significantly affected by the contact force beside the contact area. The contact
4
resistance can be described by the equation: Rc KFc m , where Rc is the contact resistance, Fc is the contact
5 6 7 8 9 10 11
force, K is the constant related to material, and m is the constant related to contact condition [25]. The contact resistance decreased exponentially with increasing contact force. In conclusion, the resistance of the weft-knitted fabric sensor was highly dependent on the length resistance and contact resistance, which was affected by the length of limbs and the contact area between loops, and the resistance increased with increasing strain under a small strain range. As the strain reached a certain point, the contact resistance decreased due to the increase in contact force and contact area when further increasing the strain, which led to a decrease in the total resistance of the weft-knitted fabric sensor.
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
2.3 The linearity of weft-knitted fabric sensor The resistance of weft-knitted fabrics with different densities increased linearly with the increase in strain under the condition of small strain, as shown in Fig. 8. However, the resistance and the strain were not linearly related under the condition of large strain. Thus, the linearity of the weft-knitted fabric sensor was studied within only 10% strain. By the regression analysis of the experiment data, the fitting equations of resistance and strain and their correlation coefficients were obtained, as shown in Table 3. The correlation coefficient of the regression equations increased with the increase in fabric density, which indicated that the linearity of the weft-knitted fabric sensor increased with the increase in its density. This phenomenon may be explained in that the strain of every loop of fabric with high density was relatively small versus the fabric with low density because the applied strain was the same, and the variation in the resistance for the weft-knitted fabric sensor was highly dependent on the variation in loop length and the contact area between up-and-down loops, which possess relatively high linearity. When the strain of a single loop became larger, the contact force between up-and-down loops would play a more important role in changing the total resistance of the weft-knitted fabric, whereas the variation in resistance caused by contact force was non-linear, which was the reason why the linearity of weft-knitted fabric with low density was relatively low.
28 29 30 31 32 33
Tab. 3 The fitting results of the strain sensing characters of sensing fabrics with different densities
Sample Fitted equation Correlation 1# y=0.0117x+0.1794 0.9716 2# y=0.0189x+0.1278 0.9939 3# y=0.023x+0.1593 0.9971 4# y=0.0252x+0.1513 0.9973 2.4 The sensitivity of weft-knitted fabric sensor The sensitivity was an important indicator of a sensor. The greater the sensitivity is, the more sensitive the sensor is. The sensitivity was measured within only 10% of the strain because high linearity was required. The gauge factor of the sensing fabrics with different densities as measured according to equation 7, and the results were shown in Table 4. The gauge factor increased with the increase in the density of the sensing fabric because the fabric with high density possessed more sensor elements and more obvious changes than that of the fabric with relatively
1 2
low density. Tab. 4 The gauge factor of sensing fabrics with different densities
Sample
1#
2#
3#
4#
Gauge factor
7.38
13.26
13.69
15.47
3 4
Thus, to prepare the weft-knitted fabric with high sensitivity, the sinking depth should be decreased to improve the density of weft-knitted fabric.
5 6 7 8 9 10 11 12 13 14
2.5 The repeatability of weft-knitted fabric sensor The strain-resistance performance of the weft-knitted fabric was measured under the condition of small strain, and the fabric reverted to the original length after the first test. Six cycles were performed to measure the repeatability of a weft-knitted fabric sensor. Fig. 12 shows the variation of the resistance in several extension processes and the increase in strain. Within 10% of strain, the weft-knitted fabric sensors with different densities all possessed excellent repeatability, and a slight difference was found between the resistances of the same fabric sensor that were tested under the same strain at different extension and recovery times. This phenomenon could be attributed to the polyurethane filament with excellent resilience that was used to prepare the weft-knitted fabrics, and the 10% strain was far less than the ultimate elongation of the weft-knitted fabrics, which makes a contribution to the full recovery of the loops of the fabrics.
15
16
1
2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23
Fig. 12 The repeatability of the sensing character of sensing fabrics in small strain
The repeatability error of the sensing character of sensing fabrics with different densities in small strain was shown in Table 5. It can be seen that the repeatability errors were smaller than 8% and decreased with the increase in the density of the weft-knitted fabrics. The elongation of the single loop of the weft-knitted fabric with relatively high density was less than that of the weft-knitted fabric with relatively low density when the strain was the same, which contributed to the recovery of the fabric and went against the error of the resistance. So, the fabric with high density possessed higher repeatability. The elongation of the single loop of the weft-knitted fabric with low density was larger, and the fabric had difficulty returning to its original shape and size, which lead to a larger repeatability error. Tab. 5 Repeatability error of the sensing character of sensing fabrics with different densities in small strain
Sample
1#
2#
3#
4#
k /%
7.56
7.23
7.02
6.36
Fig. 13 shows the repeatability of the sensing character of sensing fabrics in large strain. It can be seen that the repeatability of the sensing fabric in large strain was significantly different from that in small strain. In the first testing cycle, the resistance of the fabric is remarkably different between the extension and recovery time. In addition, the resistance of the weft-knitted fabric tested the first time was also very different from the second time. The resistance tested the first time was smaller than that of the second time when the strain was smaller than a certain value, and the opposite results occurred when further increasing the strain. Furthermore, the difference of the resistance changing curve obtained in the second time is very slight with that tested in the third time. The resistance changing curve that was tested the third time agreed with the following three times very well, which indicated that the resistance of the fabric showed good repeatability after the fabric experienced repeated stretching three times. This phenomenon can be attributed to that relatively large irreversible deformation that occurred in the fabric after the first stretch, and the deformation decreased with the increase in the extension processes.
1
2
3
4
1 2 3
Fig. 13 The repeatability of sensing character of sensing fabrics in large strain
3. Conclusion
4 5 6 7 8 9 10 11 12 13 14 15 16
Conductive UHMWPE/PANI composite yarn with a conductivity of 0.87±0.1 S/cm was continuously prepared using in-situ polymerization based on the UHMWPE filament yarns with an atmospheric pressure plasma pretreatment. The conductive UHMWPE/PANI composite yarns were used to prepare the cylindrical plain weftknitted fabric with different densities. The strain-resistance property of the cylindrical plain weft-knitted fabric was also studied. The results show that the resistance of weft-knitted fabric increased with increasing strain and exhibited excellent linearity, sensitivity, and repeatability under the condition of small strain. Furthermore, the linearity, sensitivity, and repeatability increased with the increase in the density of the weft-knitted fabric. Under large strain, the resistance of the weft-knitted fabric increased with the increase in strain in the early stage and then decreased when further increasing the strain after the strain reached a certain value. The weft-knitted fabric sensor possessed relatively poor repeatability due to the irreversible deformation of the UHMWPE/PANI yarn and the damage of the structure of the PANI on the yarn surface. Additionally, the resistance of the weft-knitted fabric tested the first time was significantly different from that of the second time for large strain, whereas the resistance of the fabric showed good repeatability after the fabric experienced repeated stretching three times.
17
Funding
18 19 20 21
This work was supported by the Natural Science Foundation of Jiangsu Province（BK20150360, BK20141267）, the Cooperative Innovation Fund of Jiangsu Province（BY2014059-12）, the Technology Innovation Fund of Science and Technology Enterprises of Jiangsu Province（BC2014166）, and the Science and Technology Support Program Funds of Suzhou（SG201444）.
22
References
23 24 25 26 27 28 29 30 31 32 33
[1] S. S. Varghese, S. Lonkar, K.K. Singh, et al. Recent advances in graphene based gas sensors. Sensors and Actuators B: Chemical, 2015, 218:160-183. [2] M. Aliofkhazraei, N. Ali. Recent Developments in miniaturization of sensor technologies and their applications. Comprehensive Materials Processing, 2014,13: Pages 245-306. [3] H. Sterner, M. Groinig, M. Haselberger, et al. Improvement of an antenna sensor for occupant detection in passenger transportation. Procedia Engineering, 2014, 87:560-563. [4] D. Cerotti, M. Gribaudo, A. Bobbio. Markovian agents models for wireless sensor networks deployed in environmental protection. Reliability Engineering & System Safety, 2014, 130:149-158. [5] C. I. Wu , H. Y. Kung, C.H. Chen, et al. An intelligent slope disaster prediction and monitoring system based on WSN and ANP. Expert Systems with Applications, 2014,41: 4554-4562. [6] N. Amini, M. Sarrafzadeh, A. Vahdatpour, et al. Accelerometer-based on-body sensor localization for
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
38
health and medical monitoring applications. Pervasive and Mobile Computing, 2011,7: 746-760. [7] J. P. Wang, P. Xue, X. M. Tao. Strain sensing behavior of electrically conductive fibers under large deformation. Materials Science and Engineering, 2011,528:2863-2869. [8] J. Li, L. Tian, N. Pan, et al. Mechanical and electrical properties of the PA6/SWNTs nanofiber yarn by electrospinning. Polymer Engineering and Science, 2014,54:1618-1624. [9] X. J. Wang, X. L. Fu. Strain sensing using carbon fiber, Journal of Materials Research, 1999,14:790-802. [10] X. J. Wang, D. D. Chung. Short carbon fiber reinforced epoxy coating as a piezoresistive strain sensor for cement mortar[J], Sensors and Actuators A: Physical, 1998,71:208-212. [11] W. Qureshi, L. Guo, J. Peterson, et al. Knitted wearable stretch sensor for breathing monitoring application. Ambience’11, Bors, Sweden, 2011. [12] L. W. Li, B. Yang. Development of Knitted Fabric Sensors for Monitoring Human Respiration.The Ninth International Conference on Electronic Measurement ＆Instruments. New York: IEEE Press, 2009. [13] J. Wu, D. Zhou, C. O. Too, et al. Conducting polymer coated lycra. Synthetic Metals, 2005,155 : 698-701. [14] J. Tsang. Design and development of electrically conducting textile sensors for smart textiles and apparel. The Hongkong Polytechnic University, 2006. [15] J. Tsang, S. Leung, X. M. Tao, et al. Effect of fabrication temperature on strain-sensing capacity of polypyrrolecoated conductive fabrics. Polymer International, 2007,56 : 827-833. [16] Y. Li, X. Y. Cheng, M. Y. Leung, et al. A flexible strain sensor from polypyrrole-coated fabrics. Synthetic Metals,2005, 155:89-94. [17] J. W. Ryu, J. Y. Park, B. K. Kim, et al．Design and fabrication of a largely deformable sensorized polymer actuator. Biosensors and Bioelectronics, 2005, 21:822-826. [18] K. Kaneto. Conducting polymer soft actuators based on polypyrrole films. Smart structures and Materials, 2006,20:197-203. [19] E. C. Toni, J. M. Bridget. Can fabric sensors monitor breast motion?. Journal of Biomechanics, 2007,40:30563059. [20] J. H. Hong, Z. J. Pan, M. Yao. Preparation and properties of continuously produced conductive UHMWPE/PANI composite yarns based on in-situ polymerization. Synthetic Metals, 2014,193:117-124. [21]J. H. Hong, Z. J. Pan, L. Tian, et al. Continuous fabrication of conductive UHMWPE yarns based on in-situ polymerization with different doping acids. Synthetic Metals, 2015,209:512-520. [22] H. X. Wang, S. Y. Zhang. Principles and applications of transducer .Tianjin: Tianjin University press.2007:8. [23] J. H. Hong, Z. J. Pan, M. Li, et al. Effect of oxygen plasma pre-treatment on electroconductive property of UHMWPE /PANI composite fibers. Journal of Textile Research, 2013, 34:1-7. [24] H. Zhang, X. M. Tao, T. X. Yu, et al. Conductive knitted fabric as large-strain gauge under high temperature. Sensors and Actuators A: Physical, 2006,126 :129-140. [25] R. Holm, E. Holm, J. B. Williamson. Electric contacts theory and application. New York: Springer-Verlag Berlin and Heidelberg GmbH & Co. K, 2010:8-9. Jianhan Hong
1 2
Jianhan Hong, male, born in 1982, doctor, Lecturer of College of Textile and Garment, Shaoxing University.
3
Zhijuan Pan
4 5
Zhijuan Pan, female, born in 1967, assistant dean of College of Textile & Clothing Engineering, Soochow University,
6 7 8
professor, doctor supervisor.
9 10 11 12 13
14 15 16 17 18
Zhe Wang
Zhe Wang, male, born in 1986, PhD of textile materials, majored in advanced textile materials.
Mu Yao
Mu Yao, male, born in 1930, scientist and educationist of textile materials, academician of Chinese Academy of Engineering, professor, doctor supervisor of Xi’an Polytechnic University and Soochow University. Jianguang Chen
1 2
Jianguang Chen, male ,born in 1968, bachelor of engineering, general manager of Yancheng Silide Silk Co., Ltd..
3 4
Yexing Zhang
5 6 7
Yexing Zhang, male, born in 1971, bachelor, general maneger of Jiangsu Shenghong Polytron Technologies Inc.
S0924-4247(15)30250-8 http://dx.doi.org/doi:10.1016/j.sna.2015.12.028 SNA 9454
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
3-10-2015 28-11-2015 2-12-2015
Please cite this article as: Jianhan Hong, Zhijuan Pan, ZheWang, Mu Yao, Jianguang Chen, Yexing Zhang, A large-strain weft-knitted sensor fabricated by conductive UHMWPE/PANI composite yarns, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.12.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
A large-strain weft-knitted sensor fabricated by conductive
2
UHMWPE/PANI composite yarns
3
Jianhan Honga,b,c, Zhijuan Panc,d,*,Zhe Wangc,d, Mu Yaoe, Jianguang Chen f, Yexing
4
Zhangg
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
a
College of Textile and Garment, Shaoxing University, Shaoxing 312000, China School of Textile and Clothing, Suzhou Institute of Trade & Commerce, Suzhou 215009, China c College of Textile & Clothing Engineering, Soochow University, Suzhou 215123, China d National Engineering Laboratory for Modern Silk (Suzhou), Suzhou 215123, China e Faculty of Textile & Material, Xi’an Polytechnic University, Xi’an 710048, China f Yancheng Silide Silk Co., Ltd., Yancheng 224311, China g Jiangsu Shenghong Polytron Technologies Inc., Suzhou 215228, China b
Highlights
Weft-knitted strain-resistance sensors from conductive UHMWPE/PANI composite yarns were obtained for the first time.
The resistance of the fabric sensors increased with increasing strain and then decreased when they were further stretched. The fabric sensors exhibited good linearity, sensitivity, and repeatability in the small-strain condition.
Abstract：In this paper, UHMWPE filament yarns were modified to improve their surface energy using atmospheric pressure plasma pretreatment. Then, a novel method based on in-situ polymerization was used for the continuous fabrication of conductive UHMWPE/PANI composite yarn with a conductivity of 0.87±0.1 S/cm. Cylindrical plain weft-knitted fabrics with different densities were fabricated using conductive UHMWPE/PANI composite yarns, and their strain-resistance performances were studied in detail. The results showed that the resistance of the fabrics increased with increasing strain at a relatively low strain range, and it also exhibited good linearity, sensitivity, and repeatability, which were enhanced by increasing density. When the strain was relatively high, the resistance of the fabrics increased with increasing strain and then decreased when the fabrics were further stretched. The repeatability of the fabrics in the large-strain condition was worse than that in the small-strain condition. The repeatability of the fabrics was very different when comparing the first stretching and the second stretching, and the difference then declined and showed good repeatability after repeated stretching three times. Key words：Conductive UHMWPE/PANI composite yarn; Weft-knitted fabric; Strain-resistance; Sensing character In recent decades, sensing technology has drawn increasing attention with the development of measuring techniques, control techniques, and automatic techniques. Sensors have played a more important role in many fields because of environmental requirements and the development of economy, science and technology. Various sensors have been widely used in production automation, energy, transportation, disaster prediction, environmental protection, medical health, and other fields for every aspect of human life [1-6]. With the optimal Corresponding author at: Soochow University, No. 199 Renai Road, Suzhou, PR China. Tel.: +86 13625273222. E-mail address: [email protected] (Z. Pan).
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
combination of sensing principles and material science, sensor technology is used for information monitoring based on a wide variety of functional materials. The sensing principle is based on the physical effect, chemical reaction, and biological reaction, and the functional material is the substantive basis of the sensor technology. In some sense, a sensor can be considered the functional material that can perceive many types of measured signals. Conductive fiber, as a type of functional material, plays a very important role in the sensor field. The resistance of the conductive fiber varies due to its structure change when the fiber was deformed. J. P. Wang[7] deposited conductive PPY on the surface of polyurethane fiber and tested its strain-resistance performance. J. Li [8] manufactured polyamide conductive nanofibers containing carbon nanotubes by using electrospinning and studied the variation of its resistance at different tensile conditions. X. J. Wang [9,10] used carbon fibers as strain sensors in a fiber reinforced cement composite to monitor its internal fracture. The conductive fiber has its own disadvantages of slight deformation, poor resilience, and low sensitivity when used as a strain sensor, owing to the limitation of its structure and properties. The deformation of the structures for the conductive fiber assemblies (woven fabric and knitted fabric) would first occur when they were stretched, for example, when extending the buckling of the yarn and the movement of the loops. Finally, the deformation of fiber occurred. So, the conductive fabric possessed of greater deformability. Furthermore, the resistance of fabric depends on not only on the resistance of fiber but also on its structure. Therefore, a greater variation space of resistance exists in the fabric sensor versus the fiber sensor. There are two main ways to prepare fabric sensors at present. First, they are manufactured using intrinsic conducting materials, such as metal fibers. W. Qureshi [11] prepared a knitted fabric strain sensor using conductive yarns containing metal fibers to monitor respiratory signals. L.W. Li [12] prepared a rib fabric sensor by using stainless steel fibers and developed a respiratory monitoring system. The second way is to provide conductivity to the fabric using conductive treatment methods such as conductive polymer coating and chemical vapor deposition. J. Wu [13] studied the relationship between the strain and resistance of a polyurethane fabric coating with polypyrrole (PPy), and the fabric exhibited high sensitivity (the strain gauge factor was approximately 25). J. Tsang [14,15] prepared a conductive fabric coating with PPy using chemical vapor deposition based on a polyamide/polyurethane plain-knitted fabric, and the fabric possessed high sensitivity (the gauge factor was over 400 when the strain was less than 50%). Y. Li [16], J. W. Ryu [17] and K. Kalleto [18] also performed similar research, and the results showed that the strain sensor made of PPy conductive fabric possessed high sensitivity. From the aspect of fabric sensor applications, E. C. Toni [19] prepared a sensor using a polyamide/polyurethane fabric coating with PPy to monitor the vertical movement of the chest of the participants. The fabric sensor has many obvious advantages, such as low cost, lightweight, breathable, soft, wearable, large deformability, and easy recovery of deformation, versus many other types of sensors. So, the fabric sensor has been widely used in human detection, smart clothing, and other fields. The sensor prepared using metal fibers, which was mentioned above, possessed many shortcomings, such as difficult spinning, poor weaving properties, hard hand feel, and low wearing comfort. The sensor prepared based on the conductive post-treatment of common fabric also had a defect of lacking flexibility on the structure design, for example, it was difficult to implement the intertexture of conductive yarn and common yarn and to design a sensor that met different requirements. The purpose of this study was to solve the main problems of the two types of fabric sensors mentioned above. A fabric sensor with excellent conductivity was prepared using soft organic conductive yarn, which improved the processability for the post-processing of the yarn, the comfortable hand feel and the flexible structure design of the fabric. In previous studies, we developed a method of preparing conductive yarn continually based on in-situ polymerization, and the conductive UHMWPE/PANI composite yarn was also successfully prepared [20,21]. The results showed that the conductive UHMWPE/PANI composite yarn possessed good conductivity and durability. In this paper, four types of flexible weft-knitted fabric sensors with large strain capacities were prepared using the conductive UHMWPE/PANI composite yarn, and their strain-resistance properties were studied. Furthermore, the
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
conductive UHMWPE/PANI composite yarn’s applications in sensor technology were also studied. 1. Experimental 1.1 Preparation and characters of conductive UHMWPE/PANI composite yarn The UHMWPE filament yarn (444dtex/406f) used in this paper was provided by Xiangsheng High Strength Fiber Materials Inc.(Hangzhou, China). The detailed preparation process of conductive UHMWPE/PANI composite yarn was as follows: UHMWPE filament yarn was modified to improve its surface energy using an atmospheric pressure plasma pretreatment. Then, the yarn was successively immersed in an aniline monomers solution and the mixed reaction solution of oxidant (150 g/L ammonium persulfate) and doping acid (1.75 mol/L sulfuric acid) to make the yarn possess aniline monomers, oxidant, and doping acid on its surface. The yarn ran continuously (12 m/min) under traction so that it experienced a conductive treatment. In the end, the yarn was loosely placed in a standard environment so that the aniline reacts with the mixed reaction solution for at least 24 h to form PANI. The surface morphologies of the UHMWPE yarns before and after the conductive treatment were obtained using an S-4800 cold field emission scanning microscope (Hitachi Limited, Japan). The resistances of UHMWPE/PANI composite yarns were measured with an Agilent 4339B high resistance meter (Agilent Technologies Co., Ltd., USA) at 20℃and 65% RH, and the electrical conductivities of the UHMWPE/PANI composite yarns were calculated according to equation 1:
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
L ……（1） RS
where σ is the electrical conductivity (S/cm), R is the electrical resistance (Ω), and L and S are the length (cm) and cross section area (cm2) of UHMWPE/PANI composite yarns, respectively. To specify the variation in the resistance of the UHMWPE/PANI composite yarn along the length direction, the resistance test of the yarn began with the head of the yarn and was tested every one meter. The total measurements were 200 m. 1.2 Preparation of fabric sensors Weft-knitted fabric generates large deformation while it is under small stress, whereas it exhibits excellent recovery capability. The deformation of the weft-knitted fabric is mainly due to the transfer of the yarns in the loops for a certain range of deformation, which avoids the deformation of yarns and the fatigue damage of the knitted fabric after they are repeatedly stretched. Thus, in this paper, cylindrical plain weft-knitted fabric was selected to study its strain-resistance property. Fig. 1 shows the structure of the cylindrical plain weft-knitted fabric prepared by an E12 hand-operated flat knitting machine, and there are 60 wales for the length of 10 cm in the weft-knitted fabric when it was off the loom. The fabric was knitted using polyurethane yarns (26.6dtex) and conductive UHMWPE/PANI yarns to improve its recovery ability, and the mass percentage of polyurethane in the fabric was 4.4%. Four types of weft-knitted fabrics with different densities were prepared by adjusting the yarn’s sinking depth for the loop forming triangle in the knitting machine and to study the effect of fabric density on its sensing property.
34 35
Fig. 1 Structure of cylindrical plain weft-knitted fabric
36
Tab. 1 Density of weft-knitted fabrics
Sample
1#
2#
3#
4#
Courses per inch 1 2 3 4 5 6 7 8 9 10 11 12
18.2
19.0
21.1
22.3
1.3 Resistance testing of fabric sensors The resistance changes of the fabric sensors were recorded using a homemade apparatus to investigate their strain sensitivity, which is shown in Fig. 2. The homemade apparatus consisted of high resistance instrument 1 and two clamps (4 and 5). Clamp 4 was fixed on baseplate 2, and clamp 5 could reciprocate along the slideway that was fixed on the baseplate. Conductive fabric 6 could generate different tensile deformations when it was fixed between the two clamps. The fabric was held by the two clamps with a clamping length of 5 cm, and the resistance of the fabric was measured and recorded using the high resistance instrument. Then, clamp 5 moved 1-mm outward, and the resistance value was recorded again. The resistance value was recorded every 1 mm that clamp 5 moved by. To investigate the repeatability of the fabric sensor, the fabric returned to its original length after it was stretched to a certain length, and six repeated tests were performed. An elastic latex film was put in the middle of the fabric to prevent the connection of the fabric insides, which may distort the results.
13 14 15 16
1.4 Modeling of the conductive knitted fabric Figs. 3 a and b show the structure of the weft-knitted fabric and its equivalent electric circuit of a single loop,
17
respectively. RL1 is the length resistance of the needle loop, RL 2 is the length resistance of the limb, RL3 is the
18
length resistance of the sinker loop, and RC1 is the contact resistance of the adjacent loops (up sinker loop and down
19
needle loop). RC 2 is the contact resistance of the adjacent loops on the left and right sides. RC 3 is the contact
20 21 22 23 24 25 26
resistance of limbs. The length resistance of the loop depends on its electrical conductivity and length. The contact resistance of the loops depends on the contact length and the contact force, that is, the longer the contact length is, the greater the contact force is and the smaller the contact resistance is. When the fabric was stretched along the longitudinal direction, each section of the loop would transfer, the limb became longer and the circle loop became shorter; thus, the length resistance would change. Furthermore, the contact resistance would also change with the variation of contact length and contact force. Thus, the fabric could be used as a strain sensor due to its total resistance change with the variation of tensile elongation.
Fig. 2 Homemade apparatus for resistance testing of sensing fabrics
27 28
Fig. 3 Structure of (a) weft-knitted fabric and (b) the equivalent electric circuit of single loop
1 2 3
Weft plain construction is made up of continuous loops that are connected to each other in the transverse direction and are intermeshed together in the longitudinal direction. Thus, the weft plain construction has many sensor units that connect to each other repeatedly. The resistance of every sensor unit could be calculated as per Fig.
4
4. Suppose RC1 s on the left and right sides of the diagram were equal and so were RL 2 s; thus, RL1 and RC 3 would
5 6
act as the electrical bridges when the voltage was applied according to the figure. Because there was no current flowing through them, they had no influence on the total resistance of the sensor unit. Therefore, the resistance of
7
the sensor unit could be regarded as the two RC1 s that were connected in series and then were connected in parallel
8
with two RL 2 s that were connected in series or as two RC1 s that were connected in parallel and then were connected
9
in series with two RL 2 s that were connected in parallel. Equation 2 was used to measure the resistance of a sensing
10
element.
11 12
Fig. 4 Calculation of resistance
R11
13
RC1 R L 2 ……（2） 2
14
The resistance of the unit that made up one row and two wales could be measured in Fig. 4b. It was assumed
15
that the resistance of the same part (same color in Fig. 4b) of every loop was the same and that the values of RC 2
16
and RL3 had no influence on the total resistance of the unit. So, this unit ( R12 ) could be regarded as two R11 s that
17
were connected in parallel. R12 can be described by equation 3:
R12
18 19 20
The resistance of the unit that made up two rows and one wale could be measured in Fig. 4c. Thus, this unit ( R21 ) could be regarded as two R11 s that were connected in series, and R21 can be described by equation 4:
R 21 2R 11 R C 1 R L 2 ……（4）
21 22 23 24
R11 RC1 R L 2 ……（3） 2 4
All of the equivalent resistances of plain weft-knitted fabric with different sizes could be measured using the same method, as shown in Table 2. Tab. 2 Equivalent electric circuit of fabrics with different loops
course
1
wale 1
R11
RC1 R L 2 2
2
3
……
n
R21 2R11
R31 3R11
……
Rn1 nR11
…
R11 2 R11 R13 3 ……
m
R1m
2 3
R12
R11 m
3R11 2
……
R23
R33 R11
…… ……
R2 m
…… 3R R3 m 11 m
nR11 2 nR11 Rn 3 3 ……
……
Rnm
R22 R11
2 R11 3 …… 2 R11 m
R32
Rn 2
nR11 m
1 2
number of wales.
3 4 5 6 7
Thus, planar weft plain fabric could be considered as a sensor that is made up of many of the same sensor units, which are connected with each other in a transverse direction and a longitudinal direction. However, the current transferred from one loop row to another in the longitudinal direction in the cylindrical plain weft-knitted fabric and also moved along the fiber length because the fabric was knitted using a single yarn. As shown in Fig. 5, the current transferred from one loop row to the next row in a corkscrew pattern.
8 9
Fig. 5 Resistance network of cylindrical plain weft-knitted fabric
Note: in the subscript of the resistance, the previous number represents the number of rows, and the next number represents the
10
Thus, the total resistance of the cylindrical plain weft-knitted fabric ( Rtotal ) could be regarded as the total
11
resistance of the plain weft-knitted fabric ( Rnm ) with the same rows and wales connected in parallel with the total
12
length resistance of the yarn ( RL ), and Rtotal could be calculated using equation 5:
13 14 15 16 17 18 19 20 21 22 23
Rtotal Rnm // RL
Rnm R L ……（5） R nm RL
Based on the above analysis, it is assumed that the resistance of the loops with the same color in Fig. 5 was all of the same, the resistance of the cylindrical plain weft-knitted fabric depended on the total length resistance of the yarn, the length resistance of the limb, and the contact resistance of the adjacent row loops and had nothing to do with the length resistance of the needle loop, the length resistance of the sinker loop, the contact resistance of the longitudinal adjacent loops, and the contact resistance of the right-and-left limbs. Actually, every length resistance and contact resistance in the whole fabric were not identical with those mentioned above because the conductivity of the yarn was not the same in the length direction, and the contact force and contact area of the loops were also not completely consistent with each other due to the difference in size and the morphology of the loops. Thus, the total resistance of the cylindrical plain weft-knitted fabric was affected by the length resistance of the needle loop, the length resistance of the sinker loop, the contact resistance of the longitudinal adjacent loops, and the contact
1 2 3 4 5 6 7 8
resistance of the right-and-left limbs. The greater these resistances were, the higher the total resistance of the fabric was. 1.5 Sensing performance of conductive weft-knitted fabric Many sensing performance specifications could be used to evaluate the property of a sensor. The linearity, sensitivity, and repeatability were proposed in this paper. Linearity was defined by the percentage of the maximum deviation between the calibration curve with the fitting straight-line and the full scale of the sensor under definite conditions, which can be described by the following equation [22]:
L
9
10
Ymax ……（6） YF S
where L is the linearity, Ymax is the maximum deviation between the calibration curve and the fitting
11
straight-line, and YF S is the full scale of the sensor.
12 13 14
Sensitivity could be defined as the ratio of output and input variation in a stable state, which is usually represented by a gauge factor. The sensitivity is the ratio of the change in electrical resistance and the strain in this paper, which can be described by the following equation:
G
15 16 17 18 19 20 21
where G is the gauge factor, R is the difference in the resistance before and after stretching, L is the variation of the sample’s length, and L0 is the original length of the sample. Repeatability could be defined as the inconsistency of the characteristic curves of the input quantity that continuously changes within the full scale range in the same direction under the same condition, which could be represented by the percentage of the maximum value of the standard deviation of two or three times and full scale [22]. The following equation was used to measure the repeatability of a sensor.
k
22
23
R / R 0 ……（7） L / L 0
2 ~ 3 100 ……（8） YF S
where k is the repeatability and is the standard deviation.
24 25
Repeatability is the percentage of the maximum value of the standard deviation of two times during the tensile process and full scale in this paper.
26
2. Results and discussion
27 28 29 30 31 32 33 34 35 36
2.1 UHMWPE/PANI composite yarn Fig. 6 shows the morphology of UHMWPE yarns before and after the atmospheric pressure plasma pretreatment and the conductive treatment. It can be seen from Fig. 6b that the surface roughness of the UHMWPE fiber was improved effectively after the atmospheric pressure plasma pretreatment, which is helpful for more aniline and reaction solution absorption and is uniformly scattered on the surface of fibers during conductive treatment. Furthermore, the improvement of surface roughness for UHMWPE fiber contributes to the uniformity and fastness of PANI on the fiber surface, which is in favor of the enhancement of conductivity and the durability of the composite conductive yarns, and this phenomenon is consistent with the previous study [23]. A complete PANI layer was coated on the surface of the UHMWPE fiber, and the gaps between fibers were also filled with PANI, which contributed to the fibers’ bond in the UHMWPE filament yarn, as shown in Fig. 6c.
1 2 3
Fig. 6 Surface morphology of (a) UHMWPE, (b) plasma pretreated UHMWPE, and (c) conductive UHMWPE/PANI
4 5 6 7 8
Fig. 7 shows the distribution of electrical conductivity for the UHMWPE/PANI conductive yarn within 200 meters in the length direction. It can be seen that the conductive yarn exhibited excellent electrical conductivity, that the average conductivity was 0.87 S/cm and that the deviation was approximately 0.1 S/cm, which further indicated that conductive UHMWPE/PANI composite yarn with high conductivity and uniformity could be prepared using the continuous conductive treatment method based on in-situ polymerization.
9 10 11 12 13 14
15 16 17 18 19 20
composite yarn
Fig. 7 Variation of electrical conductivity of UHMWPE/PANI conductive yarn in length direction
2.2 Effect of tensile strain on the resistance of fabric sensor Fig. 8 shows the variation in the resistance of weft-knitted fabrics with different densities under the condition of small strain. It can be seen that these weft-knitted fabrics have a similar strain-resistance change rule, and the resistance of all of the fabrics increased with the increase in strain.
Fig. 8 Strain-resistance curves of weft-knitted fabric with different densities in small strain
The variation in the resistance of weft-knitted fabrics with different densities under the condition of large strain was shown in Fig. 9. The sensing property of the weft-knitted fabrics under the condition of large strain was significantly different than that under the condition of small strain. The resistance of weft-knitted fabrics increased with the increase in strain at the initial stage of tension, whereas when the strain arrived at a certain point, the
1
resistance would decrease when the strain was further increased.
2 3
Fig. 9 Strain-resistance curves of weft-knitted fabric with different densities in large strain
4 5 6 7 8 9
The resistance of weft-knitted fabric depends mostly on the total length resistance of yarn, the length resistance of limbs, and the contact resistance of the adjacent row loops. The total length resistance of yarn was constant during the deformation. When the cross-sectional area and conductivity of the yarn were fixed, the length resistance was only affected by the length of limbs, which was determined by the applied force in the course of tension. The greater force applied, the greater length resistance due to the longer yarn transferred from the sinker loop and needle loop, as shown in Fig. 10, which were recorded by a 3D optical microscope (Keyence, Japan) in different stretching.
10
11 12 13 14 15 16 17
Fig. 10 Changes in structure of sensing fabric after (a) 0%, (b) 6%, (c) 10%, (d) 20%, (e) 30% and (f) 40% stretching
The influencing factors of contact resistance were very complex. The contact resistance increased under low tensile stress due to the decrease in contact area, which was caused by the decrease in the sinker loop, the needle loop, and the length of contact yarn. When the tensile stress reached a certain level, the contact area increased due to more fibers transferring to the contact surface of the yarn as the extrusion deformation of yarn occurred, which led to the decrease in contact resistance [24], as shown in Fig. 11.
1 2
Fig. 11 Movement of fibers in two yarns
3
The contact resistance was also significantly affected by the contact force beside the contact area. The contact
4
resistance can be described by the equation: Rc KFc m , where Rc is the contact resistance, Fc is the contact
5 6 7 8 9 10 11
force, K is the constant related to material, and m is the constant related to contact condition [25]. The contact resistance decreased exponentially with increasing contact force. In conclusion, the resistance of the weft-knitted fabric sensor was highly dependent on the length resistance and contact resistance, which was affected by the length of limbs and the contact area between loops, and the resistance increased with increasing strain under a small strain range. As the strain reached a certain point, the contact resistance decreased due to the increase in contact force and contact area when further increasing the strain, which led to a decrease in the total resistance of the weft-knitted fabric sensor.
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
2.3 The linearity of weft-knitted fabric sensor The resistance of weft-knitted fabrics with different densities increased linearly with the increase in strain under the condition of small strain, as shown in Fig. 8. However, the resistance and the strain were not linearly related under the condition of large strain. Thus, the linearity of the weft-knitted fabric sensor was studied within only 10% strain. By the regression analysis of the experiment data, the fitting equations of resistance and strain and their correlation coefficients were obtained, as shown in Table 3. The correlation coefficient of the regression equations increased with the increase in fabric density, which indicated that the linearity of the weft-knitted fabric sensor increased with the increase in its density. This phenomenon may be explained in that the strain of every loop of fabric with high density was relatively small versus the fabric with low density because the applied strain was the same, and the variation in the resistance for the weft-knitted fabric sensor was highly dependent on the variation in loop length and the contact area between up-and-down loops, which possess relatively high linearity. When the strain of a single loop became larger, the contact force between up-and-down loops would play a more important role in changing the total resistance of the weft-knitted fabric, whereas the variation in resistance caused by contact force was non-linear, which was the reason why the linearity of weft-knitted fabric with low density was relatively low.
28 29 30 31 32 33
Tab. 3 The fitting results of the strain sensing characters of sensing fabrics with different densities
Sample Fitted equation Correlation 1# y=0.0117x+0.1794 0.9716 2# y=0.0189x+0.1278 0.9939 3# y=0.023x+0.1593 0.9971 4# y=0.0252x+0.1513 0.9973 2.4 The sensitivity of weft-knitted fabric sensor The sensitivity was an important indicator of a sensor. The greater the sensitivity is, the more sensitive the sensor is. The sensitivity was measured within only 10% of the strain because high linearity was required. The gauge factor of the sensing fabrics with different densities as measured according to equation 7, and the results were shown in Table 4. The gauge factor increased with the increase in the density of the sensing fabric because the fabric with high density possessed more sensor elements and more obvious changes than that of the fabric with relatively
1 2
low density. Tab. 4 The gauge factor of sensing fabrics with different densities
Sample
1#
2#
3#
4#
Gauge factor
7.38
13.26
13.69
15.47
3 4
Thus, to prepare the weft-knitted fabric with high sensitivity, the sinking depth should be decreased to improve the density of weft-knitted fabric.
5 6 7 8 9 10 11 12 13 14
2.5 The repeatability of weft-knitted fabric sensor The strain-resistance performance of the weft-knitted fabric was measured under the condition of small strain, and the fabric reverted to the original length after the first test. Six cycles were performed to measure the repeatability of a weft-knitted fabric sensor. Fig. 12 shows the variation of the resistance in several extension processes and the increase in strain. Within 10% of strain, the weft-knitted fabric sensors with different densities all possessed excellent repeatability, and a slight difference was found between the resistances of the same fabric sensor that were tested under the same strain at different extension and recovery times. This phenomenon could be attributed to the polyurethane filament with excellent resilience that was used to prepare the weft-knitted fabrics, and the 10% strain was far less than the ultimate elongation of the weft-knitted fabrics, which makes a contribution to the full recovery of the loops of the fabrics.
15
16
1
2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23
Fig. 12 The repeatability of the sensing character of sensing fabrics in small strain
The repeatability error of the sensing character of sensing fabrics with different densities in small strain was shown in Table 5. It can be seen that the repeatability errors were smaller than 8% and decreased with the increase in the density of the weft-knitted fabrics. The elongation of the single loop of the weft-knitted fabric with relatively high density was less than that of the weft-knitted fabric with relatively low density when the strain was the same, which contributed to the recovery of the fabric and went against the error of the resistance. So, the fabric with high density possessed higher repeatability. The elongation of the single loop of the weft-knitted fabric with low density was larger, and the fabric had difficulty returning to its original shape and size, which lead to a larger repeatability error. Tab. 5 Repeatability error of the sensing character of sensing fabrics with different densities in small strain
Sample
1#
2#
3#
4#
k /%
7.56
7.23
7.02
6.36
Fig. 13 shows the repeatability of the sensing character of sensing fabrics in large strain. It can be seen that the repeatability of the sensing fabric in large strain was significantly different from that in small strain. In the first testing cycle, the resistance of the fabric is remarkably different between the extension and recovery time. In addition, the resistance of the weft-knitted fabric tested the first time was also very different from the second time. The resistance tested the first time was smaller than that of the second time when the strain was smaller than a certain value, and the opposite results occurred when further increasing the strain. Furthermore, the difference of the resistance changing curve obtained in the second time is very slight with that tested in the third time. The resistance changing curve that was tested the third time agreed with the following three times very well, which indicated that the resistance of the fabric showed good repeatability after the fabric experienced repeated stretching three times. This phenomenon can be attributed to that relatively large irreversible deformation that occurred in the fabric after the first stretch, and the deformation decreased with the increase in the extension processes.
1
2
3
4
1 2 3
Fig. 13 The repeatability of sensing character of sensing fabrics in large strain
3. Conclusion
4 5 6 7 8 9 10 11 12 13 14 15 16
Conductive UHMWPE/PANI composite yarn with a conductivity of 0.87±0.1 S/cm was continuously prepared using in-situ polymerization based on the UHMWPE filament yarns with an atmospheric pressure plasma pretreatment. The conductive UHMWPE/PANI composite yarns were used to prepare the cylindrical plain weftknitted fabric with different densities. The strain-resistance property of the cylindrical plain weft-knitted fabric was also studied. The results show that the resistance of weft-knitted fabric increased with increasing strain and exhibited excellent linearity, sensitivity, and repeatability under the condition of small strain. Furthermore, the linearity, sensitivity, and repeatability increased with the increase in the density of the weft-knitted fabric. Under large strain, the resistance of the weft-knitted fabric increased with the increase in strain in the early stage and then decreased when further increasing the strain after the strain reached a certain value. The weft-knitted fabric sensor possessed relatively poor repeatability due to the irreversible deformation of the UHMWPE/PANI yarn and the damage of the structure of the PANI on the yarn surface. Additionally, the resistance of the weft-knitted fabric tested the first time was significantly different from that of the second time for large strain, whereas the resistance of the fabric showed good repeatability after the fabric experienced repeated stretching three times.
17
Funding
18 19 20 21
This work was supported by the Natural Science Foundation of Jiangsu Province（BK20150360, BK20141267）, the Cooperative Innovation Fund of Jiangsu Province（BY2014059-12）, the Technology Innovation Fund of Science and Technology Enterprises of Jiangsu Province（BC2014166）, and the Science and Technology Support Program Funds of Suzhou（SG201444）.
22
References
23 24 25 26 27 28 29 30 31 32 33
[1] S. S. Varghese, S. Lonkar, K.K. Singh, et al. Recent advances in graphene based gas sensors. Sensors and Actuators B: Chemical, 2015, 218:160-183. [2] M. Aliofkhazraei, N. Ali. Recent Developments in miniaturization of sensor technologies and their applications. Comprehensive Materials Processing, 2014,13: Pages 245-306. [3] H. Sterner, M. Groinig, M. Haselberger, et al. Improvement of an antenna sensor for occupant detection in passenger transportation. Procedia Engineering, 2014, 87:560-563. [4] D. Cerotti, M. Gribaudo, A. Bobbio. Markovian agents models for wireless sensor networks deployed in environmental protection. Reliability Engineering & System Safety, 2014, 130:149-158. [5] C. I. Wu , H. Y. Kung, C.H. Chen, et al. An intelligent slope disaster prediction and monitoring system based on WSN and ANP. Expert Systems with Applications, 2014,41: 4554-4562. [6] N. Amini, M. Sarrafzadeh, A. Vahdatpour, et al. Accelerometer-based on-body sensor localization for
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
38
health and medical monitoring applications. Pervasive and Mobile Computing, 2011,7: 746-760. [7] J. P. Wang, P. Xue, X. M. Tao. Strain sensing behavior of electrically conductive fibers under large deformation. Materials Science and Engineering, 2011,528:2863-2869. [8] J. Li, L. Tian, N. Pan, et al. Mechanical and electrical properties of the PA6/SWNTs nanofiber yarn by electrospinning. Polymer Engineering and Science, 2014,54:1618-1624. [9] X. J. Wang, X. L. Fu. Strain sensing using carbon fiber, Journal of Materials Research, 1999,14:790-802. [10] X. J. Wang, D. D. Chung. Short carbon fiber reinforced epoxy coating as a piezoresistive strain sensor for cement mortar[J], Sensors and Actuators A: Physical, 1998,71:208-212. [11] W. Qureshi, L. Guo, J. Peterson, et al. Knitted wearable stretch sensor for breathing monitoring application. Ambience’11, Bors, Sweden, 2011. [12] L. W. Li, B. Yang. Development of Knitted Fabric Sensors for Monitoring Human Respiration.The Ninth International Conference on Electronic Measurement ＆Instruments. New York: IEEE Press, 2009. [13] J. Wu, D. Zhou, C. O. Too, et al. Conducting polymer coated lycra. Synthetic Metals, 2005,155 : 698-701. [14] J. Tsang. Design and development of electrically conducting textile sensors for smart textiles and apparel. The Hongkong Polytechnic University, 2006. [15] J. Tsang, S. Leung, X. M. Tao, et al. Effect of fabrication temperature on strain-sensing capacity of polypyrrolecoated conductive fabrics. Polymer International, 2007,56 : 827-833. [16] Y. Li, X. Y. Cheng, M. Y. Leung, et al. A flexible strain sensor from polypyrrole-coated fabrics. Synthetic Metals,2005, 155:89-94. [17] J. W. Ryu, J. Y. Park, B. K. Kim, et al．Design and fabrication of a largely deformable sensorized polymer actuator. Biosensors and Bioelectronics, 2005, 21:822-826. [18] K. Kaneto. Conducting polymer soft actuators based on polypyrrole films. Smart structures and Materials, 2006,20:197-203. [19] E. C. Toni, J. M. Bridget. Can fabric sensors monitor breast motion?. Journal of Biomechanics, 2007,40:30563059. [20] J. H. Hong, Z. J. Pan, M. Yao. Preparation and properties of continuously produced conductive UHMWPE/PANI composite yarns based on in-situ polymerization. Synthetic Metals, 2014,193:117-124. [21]J. H. Hong, Z. J. Pan, L. Tian, et al. Continuous fabrication of conductive UHMWPE yarns based on in-situ polymerization with different doping acids. Synthetic Metals, 2015,209:512-520. [22] H. X. Wang, S. Y. Zhang. Principles and applications of transducer .Tianjin: Tianjin University press.2007:8. [23] J. H. Hong, Z. J. Pan, M. Li, et al. Effect of oxygen plasma pre-treatment on electroconductive property of UHMWPE /PANI composite fibers. Journal of Textile Research, 2013, 34:1-7. [24] H. Zhang, X. M. Tao, T. X. Yu, et al. Conductive knitted fabric as large-strain gauge under high temperature. Sensors and Actuators A: Physical, 2006,126 :129-140. [25] R. Holm, E. Holm, J. B. Williamson. Electric contacts theory and application. New York: Springer-Verlag Berlin and Heidelberg GmbH & Co. K, 2010:8-9. Jianhan Hong
1 2
Jianhan Hong, male, born in 1982, doctor, Lecturer of College of Textile and Garment, Shaoxing University.
3
Zhijuan Pan
4 5
Zhijuan Pan, female, born in 1967, assistant dean of College of Textile & Clothing Engineering, Soochow University,
6 7 8
professor, doctor supervisor.
9 10 11 12 13
14 15 16 17 18
Zhe Wang
Zhe Wang, male, born in 1986, PhD of textile materials, majored in advanced textile materials.
Mu Yao
Mu Yao, male, born in 1930, scientist and educationist of textile materials, academician of Chinese Academy of Engineering, professor, doctor supervisor of Xi’an Polytechnic University and Soochow University. Jianguang Chen
1 2
Jianguang Chen, male ,born in 1968, bachelor of engineering, general manager of Yancheng Silide Silk Co., Ltd..
3 4
Yexing Zhang
5 6 7
Yexing Zhang, male, born in 1971, bachelor, general maneger of Jiangsu Shenghong Polytron Technologies Inc.