Preparation and Properties of Flexible Cushioning Composites Based on Silicone Rubber and Warp—Knitted Spacer Fabric

This paper aims to develop a new type of flexible cushioning composite material that can be applied to cushioning protective appliances. Room temperature vulcanized silicone rubber (RTV silicone rubber) was filled into five groups of warp knitted spacer fabrics with different structural parameters, and flexible cushioning composites with filling rates of 30%, 50% and 70% were prepared. At the same time, there was a group of warp knitted spacer fabrics without silicone rubber as control. Then the infrared spectrum of silicone rubber was analyzed to test the compression and impact properties of flexible cushioning composite. It is found that the increase of filling rate enhances the compressive and impact properties of flexible cushioning composites. The flexible cushioning composite material can show better compression performance at a larger compression ratio. The structural parameters affecting the impact properties of flexible cushioning composites are arranged in descending order as fabric thickness, spacer tilt angle, spacer density, mesh number. The established mathematical model can provide a theoretical basis for the performance study of such composite materials. The compressive properties and impact properties of flexible buffer composites are not correlated.


Introduction
With the progress of science and technology and the development of economy, there are more and more kinds of buffer protective materials. At present, the buffer protective materials include expanded polystyrene (EPS), expanded polyethylene (EPE), sponge and polyurethane, but most of them are single and limited in performance [1]. Warp-knitted spacer fabric has excellent compressive elasticity, compression resistance and air permeability, moisture absorption performance and other comfort properties [2][3][4], so the warp-knitted spacer fabric is widely used in industrial, medical, clothing and other fields [5]. Xiaohui Zhang, Pibo Ma et al. [6] filled the silicone rubber containing two components into the fabric to make composite materials with different filling rates. The material was compressed for 3000 times at different rates, and the load-displacement curve was drawn. The results show that after the addition of silicone rubber, the bearing capacity of the warp knitted spacer fabric is improved, and the mechanical properties of the material are greatly improved. Ma Mingying et al. [7] carried out static compression test and dynamic impact performance test on flexible composites prepared by room temperature vulcanized silicone rubber (RTV silicone rubber) modified with different mass fractions of structure control agent and warp knitted spacer fabric. The results showed that the cushioning performance of spacer fabric filled with silicone rubber was improved, and the structure control agent had an effect on the compressive properties of the material.
The silk is compressed and bent, transferring the force around the point of force, forming the anisotropy of the warp knitted spacer fabric. However, the load capacity of warp knitted spacer fabric is not strong, easy to fall after stress. RTV silicone rubber compression or tensile excellent resilience, excellent weather resistance, less affected by temperature, and has good biocompatibility [8]. Therefore, silicone rubber and warp knitted spacer fabric composite can solve the fabric load capacity is not strong while giving the material flexibility and comfort [9].
In this paper, flexible cushioning composites filled with silicone rubber with filling rates of 30%, 50% and 70% were prepared and compared with unfilled warp knitted spacer fabrics. Then, the factors affecting the compressive properties of fabrics and flexible cushioning composites were studied from three aspects: filling rate, compression rate and spacer fiber arrangement density. Then, the factors affecting the impact performance of flexible cushioning composites were explored from the filling rate and the structural parameters of warp-knitted spacer fabrics. A mathematical model for predicting the maximum impact load of composites prepared by warp-knitted spacer fabrics with different structural parameters was established. The traditional EPS material in 50% compression ratio, compression stress can reach about 200 kPa [10]; polyurethane (PU) foam-in-place materials in 50% compression rate, the compression stress can reach about 20 kPa [11]; in this paper, the compressive stress of unfilled warp knitted spacer fabric is about 10 kPa at 50% compression rate. When the warp knitted spacer fabric is filled with silicone rubber, the minimum compressive stress can reach about 1500 kPa, proving the advantages of the composite in the field of cushioning. By this method, a composite material for protective cushioning apparatus such as insoles and protective cushions can be developed [12]. The research results can provide reference for the improvement and application of such materials.

Warp-knitted spacer fabric structure
5 kinds of fabric on both sides of the organization physical map, which were numbered S1, S2, S3, S4, S5, as shown in Fig. 1. It can be seen from Fig. 1 that the front of S1 and S5 fabrics is regular mesh, and the mesh size is different. The front of S2 and S3 fabrics is irregular mesh, and the front and back of S4 fabrics are mesh. The reverse side of S1, S2 and S3 fabrics is velvet weave, and S5 fabric is plain weave.

Basic Parameters of Warp Knitted Spacer Fabric
The basic parameters of the five groups of warp knitted spacer fabrics are shown in Table 1.

Selection of RTV Silicone Rubber
RTV silicone rubber according to the curing mechanism and composition of different, can be divided into single component and two-component types, two-component is also divided into condensation and addition molding [13]. Different types of RTV silicone rubber have different advantages and disadvantages. The two-component needs to be added to the catalyst when it is used. Its advantages are small shrinkage, no heat released during the curing process, and no expansion. Curing together on the surface and inside and the vulcanization process is more uniform; the curing time of (a)S1, S2, S3, S4, S5 fabric front weave physical drawing (b)S1, S2, S3, S4, S5 fabric reverse weave physical drawing addition molding is mainly related to temperature, the higher the temperature, the faster the curing speed [14].
Based on the experimental requirements, two-component condensation RTV silicone rubber was selected as the filler of flexible buffer material. It generally packages silicone rubber and crosslinking agent into one component, and the catalyst is separately packaged into one component. In this experiment, RTV silicone rubber XC-107, purchased from Jinan Xingchi Chemical Co., Ltd., crosslinking agent and catalyst were tetraethyl orthosilicate, dibutyltin dilaurate, were purchased from Jinan Xingchi Chemical Co., Ltd. The viscosity of silica gel is 2000 Pa · s.
The relationship between the curing time of silicone rubber and the amount of catalyst is shown in Table 2. In order to facilitate batch operation, 4% catalyst dosage was selected in this experiment.

Flexible Buffer Material Preparation Process
The preparation process of the flexible cushioning composite material of silicone rubber filled warp knitted spacer fabric is shown in Fig. 2.
Firstly, the amount of RTV silicone rubber required under different filling rates is calculated. The calculation formula [6] is as follows.
where M is the mass of RTV silicone rubber, g; A% is the filling rate, namely the volume ratio of natural latex to fabric; V is the volume of warp knitted spacer fabric, the volume of warp knitted spacer fabric is the product of length, width and thickness. In this paper, the warp knitted spacer fabric is a square with a side length of 5 cm, and the thickness is Table 1, cm 3 ; is the density of RTV silicone rubber, 1.08 g / cm 3 .
A square warp-knitted spacer fabric with a side length of 5 cm was placed in the mold, and then the silicone rubber stock solution was weighed according to the calculated data, and then 4% catalyst was added. After stirring evenly with a glass rod, it was poured into the mold with warpknitted spacer fabric. The mixture in the mold was further treated with a scraper to ensure that the mixture could fully and evenly infiltrate the fabric, then it is placed in a vacuum drying oven to wait for bubbles to discharge. This method ensures the uniformity of silicone rubber penetration in the direction of thickness. Finally, it was allowed to stand at room temperature for 12 h, and the above steps were repeated many times to obtain a flexible buffer composite with a filling rate of 30%, 50%, and 70%. The number is numbered in 2.1, representing 5 groups of flexible buffer composites. Among them, the composite material with a silicone rubber filling rate of 30% in S1 is marked as S1 -30%, and so on.

Infrared Spectra
The infrared spectrum of RTV silicone rubber filler was tested by Nicolet 5700 Fourier transform infrared spectrometer of American Thermoelectric Company, and its characterization and solidification principle were analyzed [15].

Scanning Electron Microscope Observation
Scanning electron microscope (JSM-5610LV) was used to photograph the cross-section of flexible composites with different filling rates, and the distribution of RTV silicone rubber was observed with a magnification of 50 times.

Compression Performance
Referring to the constant deformation method in FZ / T 01051.1-1998 " Compression properties of textile materials and textile products Part 1: Determination of durable compression characteristics" [16], a double-column bench-top electronic testing machine of INSTRON company model 3367 was used. The cross-sectional diameter of the flat indenter was 5 cm, the compression rate was set to 25% and 50%, and the compression speed was 5 mm / min.

Impact Performance
According to Zhang Fule 's standardized test study on lowvelocity impact properties of textiles and composites [17], the prepared flexible buffer material samples were cut into 50 mm × 50 mm square by Instron CEAST 9340 drop weight impact tester, and the impact speed was set to 1 m / s. The impact head was used to impact the sample in free fall motion.

RTV Silicone Rubber Infrared Spectrum Analysis
The main chemical composition of room RTV silicone rubber is polydimethylsiloxane molecule with hydroxyl polydimethylsiloxane as terminal group, and the main component of catalyst is dibutyltin dilaurate, its structure is shown in Fig. 3.
From the structural formula of RTV silicone rubber, it can be seen that it contains Si-O-Si bond, Si-C bond, C-H bond and-OH bond; it can be seen from the structural formula of organotin salt that it contains Sn-O bond, C = O bond, C-O bond and C-C bond. The infrared spectra of RTV silicone rubber before and after curing are shown in Fig. 4.
From Fig. 4a, it can be seen that 1077 cm −1 and 1009 cm −1 are Si-O-Si asymmetric stretching vibration peaks, 1259 cm −1 and 786 cm −1 are Si-CH 3 and C-H stretching vibration peaks. Figure 4b shows that the characteristic absorption peak of the cured silicone rubber has a slight change compared with that before curing. The curing of silicone rubber is a vulcanization reaction under the action of a catalyst, and the hydroxyl group of the silicone rubber end group reacts with the vulcanizing agent to form a crosslinking structure. In this process, the alcohol produced by silicone rubber will diffuse out.

Flexible Cushioning Composite Material Internal Filler Adhesion
The cross sections of the untreated warp knitted spacer fabric and the cross sections of the silicone rubber filled warp knitted spacer fabric with different filling ratios are shown in Fig. 5. It can be seen from Fig. 5 that with the increase of silicone rubber filling rate, the interior of warp knitted spacer fabric is gradually filled. It can be seen from Fig. 5b that when the filling rate is 30%, most of the silicone rubber is attached to the spacer wire, while the space between the spacer wires is only filled with a small part of the silicone rubber, and the spacer wire is clearly visible. Figure 5c indicates that when the filling rate is 50%, the space between the partial spacers is filled with silicone rubber, and the silicone rubber wraps the spacers to form a column, supporting the warp knitted spacer fabric. Figure 5d shows that when the filling rate is 70%, the fabric spacer layer is almost filled with only a small amount of voids.

Effect of Filling Rate on Compressive Properties of Flexible Cushioning Composites
According to the experimental data, the load-compression strain curves of flexible buffer composites with different filling rates at a compression rate of 25% are shown in Fig. 6. There are three stages in the stress-strain curve of warp knitted spacer fabric during compression, which are linear stage, yield stage and collapse stage. Because the compression rate in the experiment is small, there is no third stage, namely collapse stage [18][19][20][21]. As shown in Fig. 6a, the unfilled warp knitted spacer fabric undergoes two stages during compression. In the first stage, due to the compression of surface hairiness, the load and compressive strain show a linear state and the slope is small. The load and compressive strain of the second stage show a nonlinear state, which is because the yarn inside the fabric begins to compress. Figure 6b shows that when the silicone rubber filling rate is 30%, the curve shows a nonlinear correlation. The main force in the composite is the spacer wire and the silicone rubber attached to the spacer wire, and the load capacity of the composite is about 2 times higher than that of the warp knitted spacer fabric. Figure 6c reveals that when the filling rate of silicone rubber increases to 50%, the composite material begins to show a linear state when the compressive strain is about 10% and the load capacity of the composite material is further improved. This is due to the filling rate of the composite material reaches 50%, the warp knitted spacer fabric part of the gap is filled, silicone rubber wrapped spacer filaments become columnar, supporting the fabric. From Fig. 6d, it can be seen that the compression stage of the composite material at a filling rate of 70% is similar to that at a filling rate of 50%, and the slope of the linear interval increases slightly. This phenomenon indicates that the pressure in the composite is mainly silicone rubber, but there is still a gap between the spacer wires of the composite.
When the compression ratio is 25%, the maximum load of the flexible cushioning composite material with the filling rate of 30%, 50% and 70% is 2.3 times, 3.9 times and 5.96 times that of the fabric respectively. Based on the above analysis, the composite material with the increase of filling rate, the ability to withstand load increases.
According to the experimental data, the load-compression strain curves of flexible cushioning composites with different filling rates at 50% compression ratio and the load-bearing histograms of composites with different filling rates at 50% compression ratio are drawn, as shown in Figs. 7 and 8. It can be seen from Fig. 7a that the load growth of the unfilled warp-knitted spacer fabric becomes slower when the compression rate becomes larger, because the fabric is gradually compressed and is about to enter the collapse stage, and the load that can be borne gradually reaches the maximum. When the warp knitted spacer fabric is filled with 30% silicone rubber, the slow elasticity of the composite increases. Therefore, when the compression rate becomes larger, the slope of the load curve becomes larger, and the load that can be borne becomes larger, which can be seen from (b). It can be found from (c) and (d) that when the filling rate is further increased, the load capacity of the material increases to varying degrees, but there is a collapse stage. Due to the increase of the filling rate of silicone rubber, the slow elasticity of the material decreases, and the load will be lost when subjected to large pressure.
As shown in Fig. 8, when the five groups of composite materials are subjected to a 50% compression ratio, that is, at a large compression ratio, the maximum load they can withstand gradually increases with the increase of the filling ratio, which is the same as when the compression ratio is 25%. The maximum load of the composites with 30%, 50% and 70% filling rate is about 3.5 times, 10.3 times and 37 times that of the fabric, respectively. When the filling rate becomes larger, the voids in the composite material are gradually filled, and more and more silicone rubbers are subjected to compression load. The higher the filling rate, the more load the silicone rubber bears, and the better the compression resistance.

Effect of Compression Ratio on Compression Properties of Flexible Cushioning Composites
In order to explore the influence of compressive strain on the compressive properties of composites, the load-compressive strain curves of composites with filling rates of 0%, 30%, 50% and 70% at compression rates of 25% and 50% are drawn as shown in Figs. 9 and 10. Figure 9 suggests that when the compression ratio is small, the load-compression strain curve of the composite material is basically linearly correlated, indicating that the material is not compacted during the compression process at a compression ratio of 25%, and the slope gradually increases as the filling ratio increases. This shows that there are still voids in the composite material when the compression ratio is small, and the flexible composite material has better slow elasticity. When the compression ratio is large, the compression curve enters the yield stage at about 30% of the compression strain and then quickly enters the linear stage with a higher slope. As the filling rate increases, the yield stage and the linear stage with a higher slope gradually move forward. It shows that with the increase of compressive strain, the flexible buffer material is gradually compacted, and the greater the filling rate, the faster the compaction. At this time, the flexible buffer material shows good compressive performance and limited elasticity. Summing up the above analysis, the flexible cushioning material has different stress conditions when subjected to different compression ratios. With the increase of compression strain, the maximum load of the composite material gradually increases. The composite material has good slow elasticity when the  Fig. 9 Load-compression strain curves of S1 under different compression ratios, the left is 25% compression ratio, the right is 50% compression ratio% compression strain is 8% -35%, and the flexible cushioning material has good compression resistance when the compression strain is 35% -50%. Figure 10 reveals that when the fabric filling rate is consistent, the load at 50% compression ratio is greater than the load at 25% compression ratio, which is consistent with the analysis of S1. When the filling rate and compression rate are small, the composite material is mainly spacer wire and a small amount of silicone rubber attached to the surface of spacer wire. Under pressure, when the filling rate and compression rate are large, the composite material is gradually compacted, and the silicone rubber in the gap of spacer wire is subjected to compression load. The hardness of the composite material increases, the slow elasticity gradually decreases, and the compression resistance is improved.

The Effect of Structure Parameters of Warp Knitted Spacer Fabric on the Compressive Properties of Flexible Buffer Composites
Warp-knitted spacer fabrics are mainly composed of spacer filaments, which bear external pressure. Therefore, the arrangement density of spacer filaments affects the compression performance of fabrics to a certain extent. Comparison of compressive stress and spacer density of composites and fabrics with different filling ratios at 50% compressive strain is shown in Fig. 11. Comparing (a) and (b) of Fig. 11, it can be found that except for S4 fabric, the arrangement density of spacer filaments in the other four groups is proportional to the compressive stress, that is, the larger the arrangement density of spacer fabric, the greater the support force under the same compressive strain. The spacing wire arrangement density of S4 is the smallest, but its spacing wire arrangement is different from the other four, indicating that the spacing wire arrangement also has an effect on the compression performance of warp knitted spacer fabrics. Figure 11b, c and d show that the compressive stress of the composites after adding silicone rubber is no longer proportional to the arrangement density of spacer filaments. At the same time, when the filling rate of fillers in the composites increases, the range of compressive stress of the five groups of composites gradually decreases. It shows that the arrangement density of spacer filaments is not positively correlated with the compressive properties of composites, and with the increase of filling rate, the arrangement density of spacer filaments has less influence on the compressive properties of composites.
The thickness of the warp knitted spacer fabric also has a certain influence on the compressive properties of the composites. The compressive stress and thickness of the composites and fabrics with different filling rates at 50% compressive strain are shown in Fig. 12.
From Fig. 12, it can be seen that the compression performance of the warp knitted spacer fabric with a filling rate of 0% is inversely proportional to the thickness of the fabric, that is, when the thickness of the fabric is larger, the compression performance of the warp knitted spacer fabric Fig. 11 Effect of spacer density on the compressive properties of flexible composites, a spacer density of five microstructures, b, c, d, e compressive stress of five microstructures when the filling ratio is 0%, 30%, 50%, 70%, respectively is worse. The thickness of the warp knitted spacer fabric depends on the length of the spacer yarn. When the warpknitted spacer fabric is filled with silicone rubber, the spacer yarn is wrapped by silicone rubber, and the silicone rubber bears a certain pressure. Therefore, the compressive strain of the warp-knitted spacer fabric filled with silicone rubber is not inversely proportional to the thickness of the fabric.
According to the experimental data, the comparison of compressive stress and spacer tilt angle of composites and fabrics with different filling ratios at 50% compressive strain is shown in Fig. 13.
It can be seen from Fig. 13 that the inclined angles of spacer yarns of S1, S2 and S3 are the same, and the reason for the difference in compressive stress between these three unfilled warp-knitted spacer fabrics is the common influence of other fabric structural parameters. When the inclined angle of spacer filament is 45°, the compressive stress of warp knitted spacer fabric is larger. At the same time, when filled with silicone rubber, the compressive stress of the composite material is not regular.
According to the experimental data, the compressive stress and spacer diameter of composite materials and fabrics with different filling rates at 50% compressive strain are drawn as shown in Fig. 14.
The fineness of the spacer yarn also has a certain degree of influence on the compression performance of the warp knitted spacer fabric. When the spacer yarn is thicker, the fabric is stiffer and also has better compression performance.
However, the diameter difference of the spacer yarn of the five groups of warp knitted spacer fabrics is small, only between 0.02-6 μm, so it has little effect on the compression performance of the fabric.
According to the experimental data, the comparison of compressive stress and mesh number of composites and fabrics with different filling ratios at 50% compressive strain is shown in Fig. 15.
It can be seen from Fig. 15 that the compressive stress of the warp-knitted spacer fabric with a filling rate of 0% is the same as the trend of the number of meshes. At the same time, the size of the mesh also has a certain influence on the compressive stress of the fabric. When the mesh is small and dense, the warp-knitted spacer fabric will show better compression resistance. However, when it is filled with silicone rubber, the spacer wire can only be wrapped by less silicone rubber because of the small internal space of the mesh. Therefore, the compressive stress of the warpknitted spacer fabric filled with silicone rubber does not follow the law of unfilled change.

Effect of Filling Rate on Impact Properties of Flexible Cushioning Composites
According to the experimental data, the impact load-time curves of composite materials and fabrics with different filling rates at the impact speed of 1 m / s are drawn, as shown in Fig. 16. Figure 16 shows that the impact load-time curves of fabrics and composites have only one elastic stage, but the elastic forms are different. This is because when the warp knitted spacer fabric is impacted, the spacer yarn is quickly bent and the supporting force is small; the composite material is spacer yarn and elastic silicone rubber to withstand impact, and the filling of silicone rubber improves the impact resistance of warp knitted spacer fabric. When the impact speed is Fig. 15 Effect of mesh number on compressive properties of flexible composites a is the mesh number of five-component, b, c, d, e is the compressive stress of five-component when the filling ratio is 0%, 30%, 50%, 70%, respectively constant, the impact energy of the sample is the same. When the absorbed energy is more, the maximum impact load peak is smaller, indicating that the cushioning performance of the material is better. Therefore, it can be seen that the greater the filling rate of silicone rubber, the better the cushioning properties of composite materials.
In the compression resistance of the five groups of samples, the performance of S1 is better, and S1 has a small and dense mesh, larger spacing wire arrangement density, S1 than the rest of the sample has better mechanical properties, so choose S1 as the impact test sample.
Taking S1 as an example, the impact energy-time curves of composites with 0%,30%, 50% and 70% filling ratios are analyzed, as shown in Fig. 17.
The fabric impact energy-time curve can be divided into two parts: one is the energy absorbed by the material, which also includes the thermal energy generated by the material during the impact process and the vibration of the impact head. The other part is the energy that the material does not absorb, that is, the rebound energy. This part of the energy is first absorbed by the material in an elastic deformation manner during the impact process, and then this part of the energy is returned to the impact head. This process can be expressed by Formula [22] (2): E total is the total impact energy, J; E a is the energy absorbed by the composite material, J; E r is the unabsorbed energy (rebound energy) of the composite material, J. It can be seen (2) E total = E a + E r from the above formula that ignoring the vibration of the impact system during the impact process, the sum of the energy absorbed by the material and the unabsorbed energy is the total impact energy. The buffer material not only hopes to produce a smaller maximum impact load during the impact process, but also hopes to extend the length of the rebound zone and the gentle zone, that is, to delay the time of the composite material entering the dense zone. Figure 17 indicates that the rebound energy of the composite increases with the increase of the filling rate, and the elasticity is better. The length of the flat area of warp knitted spacer fabric after rebound energy is short, which is due to the limited support force of the spacer yarn of the warp knitted spacer fabric, which is rapidly compacted when the fabric is impacted. The length of the flat area of the composite with a filling rate of 50% is longer after experiencing rebound energy. This is because the composite has both silicone rubber support and sufficient internal voids, and the buffer space is large.

Effect of Structural Parameters of Warp Knitted Spacer Fabric on Impact Properties of Flexible Cushioning Composites
This experiment adopts constant speed impact, so the maximum impact load is selected to represent the impact performance of flexible cushioning composite material. The smaller the impact load is, the better the impact resistance Fig. 17 Tmpact energy-time curve of S1 fabric and flexible cushioning composite material., the small figure is the impact energy-time curve of S1 unfilled warp knitted spacer fabric of flexible cushioning composite material is. The impact load after filling the silicone rubber is less affected by the diameter of the spacer wire, and the difference in the diameter of the spacer wire in this experiment is small, and the significance of the impact on the experiment is not strong. Therefore, the independent variables affecting the impact load are determined as fabric thickness, spacer wire arrangement density, spacer wire inclination angle, and mesh number. The dependent variable is the maximum impact load, and these factors are numbered, as shown in Table 3.
The impact performance prediction model of flexible cushioning composite was established according to the formulated variables. SPSS software was used to analyze the data by multiple linear regression analysis, and the nonstandardized coefficients were obtained. The relationship model between the maximum impact load of silicone rubber flexible buffer material with filling rate of 30%, 50% and 70% and the structural parameters of warp knitted spacer fabric was established, as shown in Table 4.
The coefficient of determination R 2 is greater than 0.95, so the independent variable and the dependent variable have a high degree of fitting, and the model is reliable. According to this model, the maximum impact load of composites prepared by warp knitted spacer fabrics with different structural parameters can be predicted.
In order to explore the influence of warp-knitted spacer fabric structure parameters on the impact properties of composites, SPSS software was used for data analysis. The standardized coefficients of each independent variable are shown in Table 6.
From Table 5, it can be seen that the influence of the structural parameters of warp knitted spacer fabrics on the impact properties of composites with filling rates of 30%, 50% and 70% is ranked as follows: x 1 > x 3 > x 2 > x 4 , that is, fabric thickness > spacer tilt angle > spacer arrangement density > mesh number. The thickness of the fabric has the greatest influence on the impact performance of the composite, because the greater the thickness of the same area, the more the amount of silicone rubber, and the greater the total thickness, the longer the time behind the contact material under the impact head, and the more the dispersion energy. The second is the tilt angle of the spacer wire. The smaller the tilt angle of the spacer wire, the easier the vertical impact force will deform the fabric. There are also many voids between the composites with low filling rate, so this law is also followed [23].

Correlation Between Low-velocity Impact and Compressive Properties of Flexible Cushioning Composites
The correlation between compression data and low velocity impact data of two kinds of flexible cushioning materials is tested by using MATLAB and Pearson coefficient. The parameters are shown in Table 6. Table 6 shows that the significant coefficient of the compressive load and impact load of the composite material is greater than 0.05, and the absolute value of the correlation coefficient is less than 0.9, so the compressive performance Spacer filament density y Shock load x 3 Tilt angle of spacer wire  of the composite material is not related to the cushioning performance. Therefore, the quality of one performance cannot determine the quality of the other.

Conclusion
(1) Flexible cushioning composites with different filling rates were prepared by using RTV silicone rubber. The composites possessed both the flexibility and comfort of warp knitted spacer fabric and the good mechanical properties of silicone rubber. (2) Silicone rubber can improve the load capacity of warp knitted spacer fabrics, and with the increase of silicone rubber filling rate, the load capacity of flexible cushioning composites is also improved to some extent. (3) When the compression ratio is low, the low filling rate flexible cushioning composite material with more voids is not compacted, showing good retarding performance. When the compression ratio is high, the slow compression performance of the flexible cushioning composite material is reduced. With the increase of the filling rate, the material is gradually compacted and the compressive performance is enhanced. (4) Warp-knitted spacer fabric spacing wire arrangement density, spacing wire tilt angle, spacing wire diameter and compression performance of flexible cushioning composite material correlation is relatively small, warp-knitted spacer fabric compression performance is inversely proportional to the fabric thickness, is proportional to the number of mesh, and with the filling rate, the greater the spacing wire arrangement density on the compression performance of flexible cushioning composite material is smaller. (5) The impact resistance of warp-knitted spacer fabric is improved by filling silicone rubber, and the greater the filling rate, the better the cushioning performance of the flexible cushioning composites. (6) The influence of structural parameters of warp knitted spacer fabric on the impact properties of flexible cushioning composites is ranked from large to small as fabric thickness, spacer inclination angle, spacer arrangement density and mesh number. (7) There is no correlation between the impact properties and compressive properties of flexible cushioning composites, and it is impossible to judge the quality of another performance by one of them. (8) Finally, a new type of composite material suitable for the field of buffer protection was prepared by filling silicone rubber with warp-knitted spacer fabric. The preparation process is simple and the cost is low. It can be widely used in insoles, cushions, etc. However, the process takes a long time, and there is a need for a vacuum environment during the preparation process.