Abstract
In this study, 3D spacer fabrics were prepared by chemical oxidation polymerization and then filled with polyurethane foam. Five different types of polyurethane conductive composite reinforced 3D spacer fabrics were prepared. Compression tests were performed on the prepared conductive composites under different test conditions to explore the effect of compression and impact factors on the electrical properties of the composites reinforced with 3D spacer fabrics. It is shown that the structure, thickness, and diameter of the spacer wire of the 3D spacer conductive fabric affect the conductivity of the conductive composites. At different compressive strains, the resistance change rate of the conductive compound decreases with increasing compressive period. As the compression period increases, the resistance change rate of the conducting compound gradually decreases. The change in mechanical hysteresis is larger than the change in electrical hysteresis in the compression recovery test for different strains. As the compression rate increases, the sensitivity of the conductive composite decreases, and there is a multivariate linear fit regression between the compression rate and the change in resistance. Fitting equations for the compression rate and resistance variations have been established, providing a theoretical basis for the theoretical study and application of conductive composites.
Keywords
Introduction
3D spacer fabric reinforced composite is a new type of textile structural composite material, which has the characteristics of high strength, heat insulation, good impact resistance, strong design ability, excellent overall performance, etc., and has been widely used in aerospace, construction, shipbuilding, and other fields.1,2 With the development of new materials and the continuous development of technology, the application demand of conductive composite materials is increasingly extensive and the range of applications is constantly being expanded, which leads to higher requirements for the conductivity and mechanical strength of conductive composite materials, and it has been difficult for traditional rigid conductive materials to meet the performance requirements of some high-tech product parts.3,4 Therefore, it is necessary and urgent to study and develop flexible conductive composites with mechanical and electrical properties.
Polyurethane conductive composites are promising for a wide range of applications such as anti-static packaging, electromagnetic shielding, microwave absorption, and flexible sensors due to their light weight, good resilience and flexibility.5–9 At present, the preparation of polyurethane conductive composites mainly focuses on uniformly mixing conductive materials such as graphene, conductive metal particles, and conductive carbon black powder into polyurethane slurry to prepare polyurethane conductive composites.10,11 This method is simple and easy, but microorganisms created by the foam material during the foaming process of polyurethane foam hinder the construction of a conductive network of conductive fillings. Therefore, a large number of conductive fillers need to be added in the preparation of conductive composites of the polyurethane foam to achieve better conductivity, but the addition of a large number of conductive fillers will greatly affect its mechanical properties. 12
Ding et al. 13 used conductive staple fibers filled with polyurethane foam to prepare conductive composite materials, in which the addition of cut conductive fibers significantly increased the conductivity of conductive foam composite materials and improved the wave absorption performance. Ugarte et al. 10 penetrated graphene into polyurethane using an ultrasonic assisted impregnation method to produce conductive polyurethane foam composites. The study found that conductive composites had different sensitivities under different compression loads, and conductive composites with different piezoresistive properties could be obtained by controlling the concentration of graphene to achieve controllable adjustment of sensing properties. Zhou et al. 14 used an in situ polymerization method to prepare conductive polyaniline/polyurethane foam composite materials with highly elastic porous polyurethane as the matrix. The electrical performance test of conductive composite materials showed that polyaniline could form a continuous conductive path in polyurethane foam and realize rapid electrical signal response under different compressive strains. From the perspective of cost reduction and environmental protection, Wang et al. 15 extracted lignin-based polyols from lignin and mixed them with diisocyanate doped with graphene to prepare a lignin-based polyurethane foam conductive composite material, and then studied the mechanical properties and electrical response of the foam material. The results show that the foam conductive composite has excellent compression performance and rapid response under different stimuli, and has good development prospects in the field of green flexible wearable.
In this study, we fabricated a 3D spacer conducting fabric with uniform coating and good conductivity by chemically oxidizing a polymer with a pyrrole solution. The conductive network inside the polyurethane conductive composites can be uniformly distributed by filling the polyurethane foam with the fabric as reinforcement. There has been some progress in studying the mechanical properties of polyurethane 3D spacer fabric reinforced composites, but related studies of conductive composites reinforced with 3D spacer fabric are still scarce, which greatly limits the further development and applications of conductive composites. Therefore, the study of the electro-mechanical properties of polyurethane conductive composites reinforced with 3D spacer fabrics is an important reference for their application and development, and can provide directions for improvement in problems where it is difficult to match the electrical and mechanical properties of conductive composites. In this study, we have fabricated conductive composites reinforced with 3D spacer fabrics based on the mechanical properties of 3D spacer fabric reinforced composites. By testing the change of resistance of conductive composite reinforced with a 3D spacer fabric under mechanical action, the change law between mechanical and electrical properties of conductive composite materials can be better explored, so as to better understand the change mechanism of electrical properties of materials and the change rule of conductive properties under stress, which provides a theoretical basis for the development and application of conductive composites. This can lay the theoretical foundations for further research and applications of conductive composites in intelligent interaction, flexible sensors, antistaticity, electromagnetic shielding, and microwave absorption.
Experiment
Experimental Materials and Equipment
Experimental materials: pyrrole (Analytically pure, Shanghai Maclin Biochemical Technology Co., Ltd), p-toluene sulfonic acid (analytically pure, Tianjin Damao Chemical Reagent Factory), ferric chloride (Guangzhou Hewei Medical Technology Co., Ltd), sodium hydroxide (analytically pure, Tianjin Damao Chemical Reagent Factory), isocyanate and polyether-polyol (all purchased from Shandong Bluestar Dongda Chemical Co., Ltd), and 3D spacer fabric (Changshu Meiya Home Textile Co., Ltd). The selected spacer fabric is made of polyester fiber. Spacer fabrics with different thicknesses, different surface structures and different spacer filament diameters are selected; the specific parameters are shown in Table 1.
Structure and parameters of the chosen 3D spacer fabric.
a: hexagonal mesh; b: dense tissue; c: 300 D polyester high elastic yarn; d: polyester monofilament.
Experimental instruments: Shengli VC86D multimeter (Xi’an Shengli Instrument Co., Ltd), Shimadzu Autograph AGS-X100KN electronic universal testing machine (Japan Shimadzu Company), Scanning electron microscopy (Carl Zeiss, MERLIN Compact, Germany), 0–150 mm vernier caliper (Suzhou Huarui Technology Instrument Co., Ltd).
Preparation of Conductive Composites Reinforced with 3D Spacer Fabric
Preparation of 3D Spacer Conductive Fabric
The 3D spacer fabric was treated with 0.75 mol/L NaOH solution in an 80°C water bath for 30 min. Grooves were formed on the spacer fabric surface under a high temperature and alkaline environment, and the surface activity of the fabric was increased.16,17 As shown in Figure 1(a) and (b), SEM pictures of spaced fabric fibers before and after treatment with NaOH solution obviously show grooves on the surface of NaOH solution treated fibers. In our previous study on the electrical and mechanical properties of polypyrrole-coated three-dimensional spacer fabrics, the results showed that the conductive properties of polypyrrole/three-dimensional spacer fabrics were the best when 0.10 mol/L pyrrole, 0.40 mol/L oxidant FeCl3 solution, and 0.40 mol/L dopant p-toluene sulfonic acid were prepared within a 2-h reaction time. 18 Therefore, in this study, the NaOH solution treated fabric was then placed in 0.1 mol/L solution of pyrrole for 30 min for static impregnation, and a mixture of 0.4 mol/L solution of oxidized iron chloride and 0.4 mol/L solution of the dopant p-toluene sulfonic acid was added to the pyrrole solution, where the pyrrole was oxidized to form polypyrrole. When deposited on the surface of the 3D spacer fabric after 120 min, a 3D spacer fabric with good conductivity was prepared.
SEM pictures of fabric (a) untreated fabric, (b) NaOH solution treated fabric, (c) conductive fabric prepared without treatment with NaOH solution, (d) and (e) conductive fabric prepared by NaOH solution treatment, (f) fiber cross section of conductive fabric.
Figure 1(c)–(f) shows the surface morphology of the conductive fabric before and after NaOH solution treatment, and Figure 1(f) shows the cross section pictures of the conductive fabric. It can be clearly observed that the deposition of polypyrrole on the surface of the fabric after NaOH solution treatment is more uniform, which confirms the necessity of NaOH solution treatment of the 3D spacer fabric and the uniform distribution of polypyrrole on the surface of conductive fabric prepared by chemical oxidation method, which lays a foundation for the uniform distribution of conductive network in the conductive composites reinforced with 3D spacer fabric.
Preparation of Conductive Composites
The 3D spacer conductive fabric prepared above with good conductivity is used as the reinforcement, and the fabrication process of the conductive composites reinforced with 3D spacer fabric as shown in Figure 2. Firstly, the polyurethane foam solution was prepared by stirring the isocyanate and polyether polyol according to a mass ratio of 46.9:100. The water was used as a blowing agent and the ratio of the polyurethane foam solution to the water was 100:12.7. The mixed matrix solution was stirred for 15 min, after which the polyurethane foam solution to be foamed was slowly injected into the prepared conductive 3D spacer fabric from above. At room temperature, the polyurethane foam was allowed to stand for 1 h until it was sufficiently foamed and cured, then trimmed for use.
Preparation process of conductive composites reinforced with 3D spacer fabric.
The density is calculated from the formula:
where M is the mass (g) of the composite material, V is the volume (cm3) of the composite material, the density of the composite material is calculated and other specific parameters are shown in Table 2.
The density and fiber volume fraction of the composites.
Performance Test of Conductive Composites
The compression properties of conductive composites reinforced with 3D spacer fabric under different conditions were tested according to the experimental standard GB/T 8168-2008 “Compression Properties test of soft Polyurethane Foam,” and the selected sample size was 100 × 100 mm.
Compression Performance Test of Conductive Composites
The compression performance of the composite was tested at a speed of 2 mm/min. The resistance value measured by a multimeter when the compression displacement was moved by 1 mm during the compression process was recorded until the compression strain reached 70%; each sample was tested five times.
Cyclic Compression Performance Test of Conductive Composites
The 2# sample was selected to test the change of conductivity of the conductive composites under different compression strains for 10 cycles, and the compression speed of 2 mm/min was selected to test the change in resistance of the composites under 10%, 20%, 30%, and 40% compression strains. Each group was tested three times.
Compression Recovery Test for Conductive Composites
The 2# sample was chosen to investigate the change in resistance of conducting composites under different compressive strains and their recovery after compression at a compression rate of 2 mm/min. Each group was tested three times.
Compression Performance Test of Conductive Composites at Different Compression Speeds
The 2# sample was selected to investigate the strain resistance properties of the conductive composites at different compression speeds of 3, 5, and 8 mm/min, with three trials per group.
Results and Discussion
The conductive composites will deform under external load, causing a change in the internal conductance path, which in turn causes a corresponding change in the resistance under load. The variation of the electrical properties of conductive composites when they are deformed under compressive load is investigated as follows. First, the resistance R0 of the conductive composite material was recorded before the test, and then the resistance R of the conductive composite material was recorded during the compression process; the resistance change of the conductive composite material was characterized by (R − R0)/R0.
Compressive Strain Resistance Test for Conductive Composites
The strain resistance curve of the conductive composites reinforced with 3D spacer fabric upon compression is shown in Figure 3(a). The specific values of compression stroke and resistance change rate of different conductive composites are shown in Table 3. Under the action of pressure, the resistance change of the internal conductive fabric will cause the resistance change of the conductive composites.
Compressive strain resistance curves of conductive composites: (a) strain resistance curves of five samples; (b) comparison strain resistance curves of samples 1# and 2#; (c) strain resistance curves of 3# and 4#; and (d) comparison strain resistance curves of 4# and 5#.
Compression stroke and resistance change rate of different conductive composites.
The spacer fabric composite undergoes certain deformations under pressure, resulting in a change in the structure of the conductive fabric inside the composite, and a change in the path of the conductive material inside the composite and the resistance of the composite under pressure. The effect of the compressive load on the composite consists of two main aspects: one is that the conductance path changes due to the deformation of the internal conductive fabric; the other is that the interface between the conductive fabric and the polyurethane is viscous under load, which causes a change in the resistance of the composite. Variations of strain and resistance in compression tests for different types of conductive composites are similar. The change in resistance during compression can be divided into three main phases:
Stage I: at the beginning of compression, when the composite strain is <15%, the resistance of the composite material is slightly reduced and the change is not significant, which is because at this stage, the conductive composites has just begun to be stressed, and the small deformation does not cause the change of the conductive fabric structure inside the composites, so the change of the resistance of the composites is not obvious.
Stage II: when the compressive strain of the composites is ≥15%, the resistance of the composites changes significantly. At this point, the composite is subjected to pressure and the structure of the reinforced conductive fabric inside the composite is significantly deformed, resulting in a large change in the internal conductance path of the composite and a large change in the resistance of the composite.
Stage III: when the compression strain is ≥55%, the resistance of the composites does not change much, at this time, the interior of the composites is compacted after compression. With the increase of the load, the structure of the internal enhanced conductive fabric can no longer change significantly. At this time the resistance of the composites tends to be stable, and there will be no greater change.
In Figure 3(b), the fabric structure of 3D spacer fabric 1# and 2# reinforced conductive composites is different. The fabric structure of the spacer fabric has an effect on the compressive strain and the change rate of surface resistance. According to Table 1, showing the structures and parameters of the selected 3D spacer fabrics, it can be seen that the surface structures of 1 and 2 are the same, but the fabric structures are different. The fabric structure of 2 is more compact. Therefore, according to Table 2, showing the density and fiber volume fraction of composites, it can be seen that the density and fiber volume fraction of 2# composites are greater than those of 1#. Therefore, according to the relationship given in this paper, it can be seen that the density of spacer filaments in the composite material prepared by the closer fabric structure is larger, and the number of spacer filaments per unit volume is higher, so it has a greater supporting effect. This composite material deforms less under the same pressure, thus exhibiting a smaller resistance change rate.
As shown in Figure 3(c), the thicknesses of 3# and 4# are different, which is used to explore the influence of spacer fabric thickness on compressive strain and the surface resistance change rate. In particular, the strain resistance of the thicker 3# conductive composite changes more significantly than that of the thicker 4# conductive composite, which depends mainly on the longer length of the internal spacer filament when the thicker fabric is subjected to pressure. When the composite is subjected to a compressive load, the deformation and bending of the inner spacer filament of the thicker spacer fabric undergoes a large change, 19 resulting in a change in the internal conductance path of the conductive composites. The greater the thickness of the reinforced conducting fabric, the more significant the change in the strain resistance of the conductive composites.
As shown in Figure 3(d), the diameters of spacer wires in the reinforced spacer fabric of the 4# and 5# composites are different, and the effects of spacer wire diameters on the compressive strain and surface resistance change rate can be observed. The 3D spacer fabrics formed from spacer filaments with larger diameter and greater rigidity have better compression resistance. 20 During compression of the spacer filaments, the spacer filaments are mainly under pressure, and the compression deformation is mainly along the axial compression of the spacer filament, which causes contact between the spacer filaments in the conducting compound and changes the resistance of the conductive fabric. Therefore, the diameters of spacer wires under pressure can affect the electrical properties of the conductive composite. When the diameter of the spacer wire of the 3D spacer fabric is small, the compressive strain of the conductive composite material increases and the resistance change rate increases.
Electromechanical Properties of Conductive Composites Under Cyclic Compression
Cyclic compression tests can be used to evaluate the linearity, repeatability, stability, and durability of conductive composites. Figure 4 shows the resistance change rate of the conductive composite during 10 cycles of compression at different compression strains. As can be seen from Figure 4, the resistance variation of the conductive composite under different compressive strains decreases at the beginning of compression and increases with the increase in the composite strain after unloading. The resistance change rate was found to be 0.98% for different compressive strains and was found to be small after 10 cycles of stretching at 10% strain. The rate of change in resistance during cyclic tensile recovery at 40% strain is 12.36%, which is much higher than at 10% strain. This is because when the compressive strain of 10% and 20% is small, the internal structure of the conductive composites changes little when it is subjected to the load, and the conductive path is easily recovered when the load is removed. Therefore, the resistance of the composite material does not change significantly under the repeated cyclic compression of small strain. The electrical resistance of the conductive composite varies greatly under 10 compressions at pressures of 30% and 40%.
Compressive strain resistance curves of conductive composites for 10 cycles under different compressive strains: (a) 10% strain; (b) 20% strain; (c) 30% strain; and (d) 40% strain.
According to the comparison between the resistance change and recovery curve of the first compression and that of the 10th compression, the resistivity change of the conductive composite during the first compression cycle is greater than the resistance change rate of the composite material during the recovery, and the resistance change rate of the conductive composites during the recovery gradually decreases with the increase in the compression cycle.
In order to explore the linear relationship between the number of compressions and the change in resistivity before and after cyclic compression of the composite, a regression analysis was performed on the change in resistivity and the number of cyclic compresses before and after each cycle. Various fitting attempts were made for the compression times and resistance variation rates for different strains. Finally, a binomial equation was chosen as the fitting model and its fitting curve is shown in Figure 5. The fitted equations are shown in Table 4. The curve of resistance change rate before and after cyclic compression conforms to the curve of binomial equation, and the R2 of all equations fitted by binomial equation is greater than 0.99, which indicates that the binomial equation as a fitting model has a good fitting relationship with the test data. Therefore, by fitted equations we can predict the resistance change of the conductive composite reinforced with 3D spacer fabric under different compressions, which can be used as a reference for future studies.
Fitted curves of the resistance change rate and the number of compressions before and after cyclic compression of the composite.
Equations for the resistance change rate and the number of compressions before and after cyclic compression for a binomial fit to conductive composites.
After that, the 3D surface diagram of the conductive composite before and after cyclic compression was drawn, as shown in Figure 6. As the compressive strain increases and the number of compressive cycles increases, the resistance of the conductive composite changes more and more significantly. The law of variation is the same before and after cyclic compression recovery, and the rate of change of resistance is larger after strain recovery. It can be clearly observed that the resistance change rate is large for multiple cycles of compression and 40% compression strain. The electrical properties of the composite are stable under 10 cycles of compression, indicating that the number of compression cycles and the deformation under the pressure load affect the conductivity of the composite.
3D curved surface of cyclic compression times, compression strain and resistance change rate of conductive composite: (a) before each cyclic compression; and (b) after each cyclic compression.
Strain Resistance Test for Compressive Recovery of Conductive Composites
Both the polyurethane foam material and the 3D spacer fabric have good compression recovery, as well as some compression recovery in the fabricated conductive composite. Conductive composites are inevitably subjected to external pressure loads when in use, and deformation occurs after being subjected to the load, with the deformation of the composite slowly recovering with the passage of time after the release of the load. A compression rate of 2 mm/min was used to investigate the resistance of the conductive composite under different compressive strains. The strain curves of the compressive recovery of the conductive composite at different strains are shown in Figure 7. As the compressive strain increases, the resistance change rate of the conductive compound during compressive recovery shows a slow increase and gradually stabilizes, which is consistent with the trend of the resistance change rate under compressive stress. Under the compression strain of 10%, the resistance change rate of the conductive composite is 0.32%, and the strain is 0.43%. At 40%, the resistance change rate is 1.42% and the strain is 3.96%. During compression, the change in mechanical hysteresis is greater than the change in electrical hysteresis, mainly because the polyurethane foam is the first to be stressed when the conductive composite is subjected to a pressure load. When compressive strain is increased, the polyurethane foam in the composite is irreversibly broken and damaged, 21 while the internal conductive fabric structure of the reinforcement is not. It is able to recover its own structure with increasing time, which makes the mechanical hysteresis of conductive composites larger than the electrical hysteresis.
Strain resistance curves of conductive composite under compression recovery under different strains: (a) 10% strain; (b) 20% strain; (c) 30% strain; and (d) 40% strain.
Strain Resistance Test of Conductive Composites at Different Compression Rates
The variation of the electrical properties of the composite at different compression rates determines the application domain and application scenarios of conductive composites. Therefore, it is important to study the electrical properties of conductive composites at different compression rates. In Figure 8(a) stress–strain curves at different compression rates show that with the increase in the compression rate, the compressive stress of the conductive composite gradually increases; as reflected in the tensile strain resistance curve in Figure 8(b), when the stress change is small at the beginning of compression, the strain of the conductive structure inside the conductive composite is small, and the resistance change rate is small. As the compression rate increases, the resistance change rate of the conductive compound gradually increases. This indicates that the resistance of the conductive composite rapidly reacts with the increase in the strain rate, and the sensitivity of the conductive composite increases with the increase in the strain rate.
(a) Stress–strain curve at different compression speeds; (b) strain resistance curve at different compression speeds; and (c) fitted curve of strain resistance change rate at different compression speeds.
In order to more accurately explore the relationship between different compression speeds and the resistance changes of conductive composite, the fitting curves are shown in Figure 8(c), and the fitting equations are shown in Table 5, with R2 greater than 0.99, indicating that the prediction equations of strain resistance under different compression rates were reliable. The fitted equations can be used to predict the variation of strain and resistance for different compression rates.
Fitted equation of strain resistance change rate at different compression speeds.
Conclusions
Five different types of conductive composites reinforced with 3D spacer fabric were prepared by oxidizing the spacer fabric to give it electrical conductivity and then filling it with polyurethane foam, to explore the laws of change and the factors that influence the mechanical and electrical properties of conductive composites under pressure load. The results show that the microstructure, thickness, and diameter of the spacer fibers of the 3D spacer fabric have a significant effect on the conductivity of the conductive composite, but the structure of the reinforced 3D spacer fabric does not have a significant effect on the conductivity of the composite. The larger the thickness of the reinforced conductive fabric, the more pronounced the change in the strain resistance of the composite. The smaller the diameter of the spacer filament of the 3D spacer fabric, the larger the resistance change rate of the conductive composite. The cyclic compression properties of conductive composites under different strains indicate that the change in the resistance of the composite under repeated cyclic compression is not significant when the compressive strains of 10% and 20% are small. The resistivity of the conductive composite varies considerably under 10 compressions at the compression strains of 30% and 40%. As the compression period increases, the resistance change rate of the conductive compound gradually decreases.
The rates of resistance change before and after cyclic compression are well fitted by a binomial equation model, which provides a theoretical reference for the study of cyclic compression and resistance change in conductive composites reinforced with 3D spacer fabrics. Compression recovery tests on conductive composites under various strains have shown that mechanical hysteresis varies more than electrical hysteresis. As the compression rate increases, the resistance of the conductive composite changes rapidly, that is, the sensitivity of the conductive composite increases as the strain rate increases. In addition, a multilinear fit regression relation between the compressive rate and the resistance variation has been found, which provides a theoretical foundation for the theoretical study and application of conductive composites.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
