Thermoplastic Laminated Composites Applied to Impact Resistant Protective Gear: Structural Design and Development

Laminated composites have been commonly applied to all fields. When made into laminated composites, Kevlar woven fabrics are able to provide the required functions. In this study, two types of TPU are incorporated to improve the intralayer features of Kevlar/TPU laminated composites. Hence, the Kevlar/TPU laminated composites consist of firmly bonded laminates while retaining flexibility of the fabrics. Being the interlayer of the laminated composites, the TPU layer provides adhesion while strengthening the tensile property, dynamic puncture resistance, and buffer strength of Kevlar/TPU laminated composites. The test results indicate that with a blending ratio of two types of TRU being 85/15 wt%, the Kevlar/TPU laminated composites exhibit a tensile strength of 18.08 MPa. When the stacking thickness is 1 mm, the tensile strength is improved to 357.73 N with the buffering strength reaching 4224.40 N. Notably, with a thickness being 1.2 mm, the laminated composites demonstrate a dynamic resistance being 672.15 N. In the meanwhile, functional Kevlar fabrics are allowed to keep the fiber morphology owing to the protection of TPU composite films. Considering the composition of protective gear, Kevlar/TPU laminated composites possess a powerful potential and are worthwhile exploring.


Introduction
There are increasing demands on composites in terms of good mechanical properties and a light weight because the development in composites has been flourishing. In the aviation, protection, and construction fields, composites can be effectively reinforced via the employment of lamination techniques and hot-pressing process, as well as a combination of multiple layers of materials, producing composites with the desired mechanical features and functions. Composed of fibers and polymers, the hot-pressing laminated composites take advantage of the fact that fibers can be combined with polymers, thereby achieving lamination bonding and external force dispersion. Moreover, polymer matrices do not alter their intrinsic attributes after they are embedded with fibers [1][2][3][4][5]. MTPU can be melted at lower temperatures to attain the adhesion function. Kevlar woven fabrics (DuPont, Miaoli, Taiwan) have a plain weave, composed of a yarn fineness being 1500 filament/strand, a warp/weft density being 15 ends (picks)/inch, and a thickness being 0.35 mm.

Preparation Process for TPU Composite Films and Kevlar/TPU Laminated Composites
TPU and MTPU are thermally baked at 40 • C for overnight, thereby removing the residual moisture in advance. A single-screw sheet extrusion process is employed to produce TPU films as Figure 1a-c shows the assembly. The TPU/MTPU ratios are 100/0, 95/5, 90/10, 85/15, and 80/20 wt%, and the resulting TPU films are tested and evaluated in order to acquire the optimal parameter. Next, the hot-pressing process is conducted to combine Kevlar woven fabrics, thereby producing Kevlar/TPU laminated composites. The hot pressing time and pressure are ten minutes and 50 kg/cm 2 , respectively. The upper and lower mold is 160 • C. Figure 1d shows the hot pressing equipment that is heated to 160 • C in advance while Figure 2 shows the diagram of the hot pressing process. To begin with, a sample is mounted in the mold set with a specified thickness (i.e., 1.0, 1.2, and 1.5 mm), and the mold is then placed in the hot pressing assembly. In order to evenly heat the materials for a better melting state, the upper and lower molds are sealed for the first two minutes. Next, the lower mold is moved back and forth three to four times, ensuring the sample is firmly adhered, and the lower mold is moved upward to heat the sample along with the upper mold the last time for six minutes. At last, the iron plates are removed for cooling, after which the Kevlar/TPU laminated composites are evaluated for morphology and applications.

Measurements
A stereo microscope is employed to observe the surface of samples while a scanning electron microscope (SEM) is employed to observe the micro-structure of samples. The thermal behavior of samples is observed as follows. Sample weighing 9~10 mg is sealed in an aluminum plate and measured using a DSC (Q20, TA Instruments, New Castle, USA). The DSC measurement is conducted in a temperature range that is increased from 40 • C to 200 • C by increments of 10 • C/min, during which, samples are observed for the melting status and crystallization morphology. The tensile strength of samples is measured at a test rate of 5 mm/min using the Instron 5566 Universal Tester (Norwood, USA) as specified in ASTM D638-10. Samples are dumbbell-shaped (Type IV). A drop weight impact tester (Changfata Industrial Company, Taichung, Taiwan) is used in both the dynamic puncture resistance and buffer strength tests. As for the dynamic puncture resistance test, the impact mass of the impactor is 20 kg, and the impactor is released from a specified height of 450 mm. Samples have a size of 100 mm × 100 mm. The transient maximal force that penetrates the sample is recorded as the anti-puncture value. In addition, as specified in ASTM D1596, the drop weight impact resistance measurement is conducted to evaluate the buffer strength of samples (100 mm × 100 mm). The impactor weighing 10 kg is released from a specified height of 200 mm in order to strike the sample, during which the sensor linked with a computer detects the maximal absorption capacity for buffer strength evaluation. For all measurements, ten samples for each specification are used. mm), and the mold is then placed in the hot pressing assembly. In order to even the materials for a better melting state, the upper and lower molds are sealed for th two minutes. Next, the lower mold is moved back and forth three to four times, en the sample is firmly adhered, and the lower mold is moved upward to heat the s along with the upper mold the last time for six minutes. At last, the iron plat removed for cooling, after which the Kevlar/TPU laminated composites are evalua morphology and applications.

Measurements
A stereo microscope is employed to observe the surface of samples while a scanning electron microscope (SEM) is employed to observe the micro-structure of samples. The thermal behavior of samples is observed as follows. Sample weighing 9~10 mg is sealed in an aluminum plate and measured using a DSC (Q20, TA Instruments, New Castle, USA). The DSC measurement is conducted in a temperature range that is increased from 40 °C to 200 °C by increments of 10 °C/min, during which, samples are observed for the melting status and crystallization morphology. The tensile strength of samples is measured at a test rate of 5 mm/min using the Instron 5566 Universal Tester (Norwood, USA) as specified in ASTM D638-10. Samples are dumbbell-shaped (Type IV). A drop weight impact tester (Changfata Industrial Company, Taichung, Taiwan) is used in both the dynamic puncture resistance and buffer strength tests. As for the dynamic puncture resistance test, the impact mass of the impactor is 20 kg, and the impactor is released from a specified height of 450 mm. Samples have a size of 100 mm × 100 mm. The transient maximal force that penetrates the sample is recorded as the anti-puncture value. In addition, as specified in ASTM D1596, the drop weight impact resistance measurement is conducted to evaluate the buffer strength of samples (100 mm × 100 mm). The impactor weighing 10 kg is released from a specified height of 200 mm in order to strike the sample,

Tensile Performance of TPU Composite Films
Composites consist of two or more than two materials with different characteristics, and the materials are bonded via physical, chemical, physical-chemical, or mechanical mechanisms. In this study, two types of TPU are combined to form TPU composite films, thereby bettering the tensile performance of the composite films as well as strengthening the adhesion level among laminates [22][23][24]. The films can be adjusted according to the thickness and the resulting films exhibit a more even morphology than other films made by molds of various thicknesses via hot pressing. Figure 3 shows the tensile stress-strain curves of TPU composite films, and all of the TPU composite films demonstrate good tensile features regardless of the TPU/MTPU blending ratios. In addition, the greater the MTPU content, the higher the tensile stress and strain of TPU composite films. However, with a TPU/MTPU ratio being 80/20, the tensile stress and strain start to decline. The TPU composite films show a raise in the tensile strain because MTPU (the adhesive type TPU) is well mixed with TPU and contributes greater tensile properties. Nevertheless, when the content of MTPU exceeds the limit, there is subsequently a smaller space for TPU chain linking, which in turn has a negative influence over the tensile stress and strain. MTPU content, the higher the tensile stress and strain of TPU composite films. However, with a TPU/MTPU ratio being 80/20, the tensile stress and strain start to decline. The TPU composite films show a raise in the tensile strain because MTPU (the adhesive type TPU) is well mixed with TPU and contributes greater tensile properties. Nevertheless, when the content of MTPU exceeds the limit, there is subsequently a smaller space for TPU chain linking, which in turn has a negative influence over the tensile stress and strain.  Figure 4 shows the DSC curves of TPU composite films. To testify to the thermal behaviors of TPU composite films during the melting process, the melting and crystallization characteristic peaks are observed. Figure 4a shows that with the incorporation of MTPU, the melting characteristic peak of TPU composite films has a lower and wider peak. Figure 4b shows the comparable melting and crystallization characteristic peaks, and in the meanwhile, the crystallization degree shows a declining trend after MTPU and TPU are blended. TPU has a structure consisting of hard and soft segments that are alternatively aligned. Hard segments provide crystallization sites while soft segments help with the expansion of the TPU composite films [25,26]. Specifically, the soft segments could be entangled with each other, which strengthens the blending level of TPU and MTPU subsequently. In this study, the aim is to explore the application feasibility of combining TPU composite films and fabrics. Hence, MTPU is used to bond laminates as an adhesive and its resilient attribute improves the protection of the fabric layers.   Figure 4 shows the DSC curves of TPU composite films. To testify to the thermal behaviors of TPU composite films during the melting process, the melting and crystallization characteristic peaks are observed. Figure 4a shows that with the incorporation of MTPU, the melting characteristic peak of TPU composite films has a lower and wider peak. Figure 4b shows the comparable melting and crystallization characteristic peaks, and in the meanwhile, the crystallization degree shows a declining trend after MTPU and TPU are blended. TPU has a structure consisting of hard and soft segments that are alternatively aligned. Hard segments provide crystallization sites while soft segments help with the expansion of the TPU composite films [25,26]. Specifically, the soft segments could be entangled with each other, which strengthens the blending level of TPU and MTPU subsequently. In this study, the aim is to explore the application feasibility of combining TPU composite films and fabrics. Hence, MTPU is used to bond laminates as an adhesive and its resilient attribute improves the protection of the fabric layers.

DSC Observation of TPU Composite Films
MTPU content, the higher the tensile stress and strain of TPU composite films. Ho with a TPU/MTPU ratio being 80/20, the tensile stress and strain start to decline. Th composite films show a raise in the tensile strain because MTPU (the adhesive typ is well mixed with TPU and contributes greater tensile properties. Nevertheless, wh content of MTPU exceeds the limit, there is subsequently a smaller space for TPU linking, which in turn has a negative influence over the tensile stress and strain.  Figure 4 shows the DSC curves of TPU composite films. To testify to the th behaviors of TPU composite films during the melting process, the meltin crystallization characteristic peaks are observed. Figure 4a shows that wi incorporation of MTPU, the melting characteristic peak of TPU composite films lower and wider peak. Figure 4b shows the comparable melting and crystall characteristic peaks, and in the meanwhile, the crystallization degree shows a de trend after MTPU and TPU are blended. TPU has a structure consisting of hard an segments that are alternatively aligned. Hard segments provide crystallization sites soft segments help with the expansion of the TPU composite films [25,26]. Specifica soft segments could be entangled with each other, which strengthens the blendin of TPU and MTPU subsequently. In this study, the aim is to explore the appl feasibility of combining TPU composite films and fabrics. Hence, MTPU is used to laminates as an adhesive and its resilient attribute improves the protection of the layers.   Kevlar/TPU laminated composites have a multi-layered structure and the alternatively aligned TPU films serve as adhesive layers. As a result, the laminated composites are wellbonded while preserving the softness of fabrics and films concurrently. Similarly, the threelayered Kevlar/TPU laminated composites exhibit the softness attribute. The combination of TPU composite films and Kevlar woven fabrics successfully retains the mechanical properties of Kevlar woven fabrics. Despite the conduction of the sheet extrusion process and hot-pressing lamination, TPU composite films still keep the flexible attribute. Figure 5 demonstrates the surface observation of Kevlar/TPU laminated composites where Kevlar fabrics have a woven structure. The good mechanical properties of hotpressing laminated composites are attributed to the interlacing warp and weft yarns. The risers mean the point where the warp and weft yarns are crossed, whether it is with the warp or the weft yarns being on top of the opposite yarn. Subsequently, TPU infiltrates along the path offered by the risers. Comprising the composites, the constituent fibers are ubiquitous throughout the whole structure, and as such attain isotropic reinforcement. Namely, Kevlar/TPU laminated composites have a multi-layered structure and the alternatively aligned TPU films serve as adhesive layers. As a result, the laminated composites are well-bonded while preserving the softness of fabrics and films concurrently. Similarly, the three-layered Kevlar/TPU laminated composites exhibit the softness attribute. The combination of TPU composite films and Kevlar woven fabrics successfully retains the mechanical properties of Kevlar woven fabrics. Despite the conduction of the sheet extrusion process and hot-pressing lamination, TPU composite films still keep the flexible attribute.

Tensile Properties of Kevlar/TPU Laminated Composites
Figures 6 and 7 demonstrate the tensile test results of Kevlar/TPU laminated composites. As shown in Figure 6, Kevlar/TPU laminated composites are affixed by a pair of clamps, and then are stretched by the clamps in opposite directions until samples are broken. During the tensile test, Kevlar woven fabrics are the subject that is first broken. In the meanwhile, a small proportion of fibers remain embedded in the TPU composite films without being exposed during and after the tensile test. Figure 7 shows the results of tensile stress and strain tests, the broken samples demonstrate a trivial amount of Kevlar fibers embedded in TPU composite films, thereby limiting the expansion of TPU composite films. This result is in conformity with the results of the tensile strain test. To sum up, the tensile strain of Kevlar/TPU laminated composites is only 295.94%, which is decreased by 60.32%. Kevlar/TPU laminated composites exhibit two-stage breakage, which suggests that Kevlar woven fabrics and TPU composite films are separately damaged in order. By contrast, the Kevlar/TPU laminated composites exhibit greater tensile stress than that of TPU composite films (85/15 wt%), which is 49.96 Mpa and 18.08   Figure 6, Kevlar/TPU laminated composites are affixed by a pair of clamps, and then are stretched by the clamps in opposite directions until samples are broken. During the tensile test, Kevlar woven fabrics are the subject that is first broken. In the meanwhile, a small proportion of fibers remain embedded in the TPU composite films without being exposed during and after the tensile test. Figure 7 shows the results of tensile stress and strain tests, the broken samples demonstrate a trivial amount of Kevlar fibers embedded in TPU composite films, thereby limiting the expansion of TPU composite films. This result is in conformity with the results of the tensile strain test. To sum up, the tensile strain of Kevlar/TPU laminated composites is only 295.94%, which is decreased by 60.32%. Kevlar/TPU laminated composites exhibit two-stage breakage, which suggests that Kevlar woven fabrics and TPU composite films are separately damaged in order. By contrast, the Kevlar/TPU laminated composites exhibit greater tensile stress than that of TPU composite films (85/15 wt%), which is 49.96 Mpa and 18.08 Mpa, respectively. As Kevlar/TPU laminated composites are composed of two different structures with corresponding materials, delamination occurs inevitably. Two constituent materials confine each other while strengthening the whole structure of Kevlar/TPU laminated composites. Subsequently, the embedded Kevlar fibers exert reinforcement over the residual TPU composite films. Although Kevlar/TPU laminated composites have multiple layers, they demonstrate two maximal tensile strengths exclusively, indicating that fabrics and films are bonded in a one-piece state.

Tensile Properties of Kevlar/TPU Laminated Composites
Mpa, respectively. As Kevlar/TPU laminated composites are composed of two different structures with corresponding materials, delamination occurs inevitably. Two constituent materials confine each other while strengthening the whole structure of Kevlar/TPU laminated composites. Subsequently, the embedded Kevlar fibers exert reinforcement over the residual TPU composite films. Although Kevlar/TPU laminated composites have multiple layers, they demonstrate two maximal tensile strengths exclusively, indicating that fabrics and films are bonded in a one-piece state.   Figure 8 shows the tensile properties of Kevlar/TPU laminated composites as related to the thickness of samples, including 1 mm, 1.2 mm, and 1.5 mm. The tensile strength of samples is inversely proportional to the thickness of samples, decreasing from 357.73 N to 311.18 N. There is a significant association between the thickness of samples and the density of the lamination [27]. It is apparent that TPU layers show a greater compression capacity while strengthening the intralayer bonding force remarkably. The Kevlar/TPU laminated composite has a thickness of 0.5 mm, and three layers account for an original thickness of 1.5 mm. By contrast, the three-layered Kevlar/TPU laminated composites are hot pressed into a thickness of 1 mm, which suggests that TPU is melted to infiltrate Kevlar woven fabrics, achieving a more compact intra-layer structure. Therefore, this specified group exhibits maximal tensile strength. However, when hot pressed in a mold with a thickness of 1.2 or 1.5 mm, there is less restriction in the space for the laminated composites, especially the 1.5-mm group. The brief melting-cooling intervals of the hot Mpa, respectively. As Kevlar/TPU laminated composites are composed of two different structures with corresponding materials, delamination occurs inevitably. Two constituent materials confine each other while strengthening the whole structure of Kevlar/TPU laminated composites. Subsequently, the embedded Kevlar fibers exert reinforcement over the residual TPU composite films. Although Kevlar/TPU laminated composites have multiple layers, they demonstrate two maximal tensile strengths exclusively, indicating that fabrics and films are bonded in a one-piece state.   Figure 8 shows the tensile properties of Kevlar/TPU laminated composites as related to the thickness of samples, including 1 mm, 1.2 mm, and 1.5 mm. The tensile strength of samples is inversely proportional to the thickness of samples, decreasing from 357.73 N to 311.18 N. There is a significant association between the thickness of samples and the density of the lamination [27]. It is apparent that TPU layers show a greater compression capacity while strengthening the intralayer bonding force remarkably. The Kevlar/TPU laminated composite has a thickness of 0.5 mm, and three layers account for an original thickness of 1.5 mm. By contrast, the three-layered Kevlar/TPU laminated composites are hot pressed into a thickness of 1 mm, which suggests that TPU is melted to infiltrate Kevlar woven fabrics, achieving a more compact intra-layer structure. Therefore, this specified group exhibits maximal tensile strength. However, when hot pressed in a mold with a thickness of 1.2 or 1.5 mm, there is less restriction in the space for the laminated composites, especially the 1.5-mm group. The brief melting-cooling intervals of the hot  Figure 8 shows the tensile properties of Kevlar/TPU laminated composites as related to the thickness of samples, including 1 mm, 1.2 mm, and 1.5 mm. The tensile strength of samples is inversely proportional to the thickness of samples, decreasing from 357.73 N to 311.18 N. There is a significant association between the thickness of samples and the density of the lamination [27]. It is apparent that TPU layers show a greater compression capacity while strengthening the intralayer bonding force remarkably. The Kevlar/TPU laminated composite has a thickness of 0.5 mm, and three layers account for an original thickness of 1.5 mm. By contrast, the three-layered Kevlar/TPU laminated composites are hot pressed into a thickness of 1 mm, which suggests that TPU is melted to infiltrate Kevlar woven fabrics, achieving a more compact intra-layer structure. Therefore, this specified group exhibits maximal tensile strength. However, when hot pressed in a mold with a thickness of 1.2 or 1.5 mm, there is less restriction in the space for the laminated composites, especially the 1.5-mm group. The brief melting-cooling intervals of the hot pressing process hamper TPU from infiltrating the composites, which in turn compromises the tensile performance of Kevlar/TPU laminated composites. pressing process hamper TPU from infiltrating the composites, which in turn compromises the tensile performance of Kevlar/TPU laminated composites.  Figure 9 shows the dynamic anti-puncture strength and buffer strength of Kevlar/TPU laminated composites. The thickness of samples is specified in both tests. Figure 9a shows two impactors for the corresponding tests. The needle-type impactor is for the dynamic anti-puncture test while the rounded impactor is used in the buffer strength test. Figure 9b shows the anti-puncture strength of Kevlar/TPU laminated composites as related to the thickness, and the anti-puncture strength first increases and then decreases. With the thickness increasing from 1 mm, 1.2 mm to 1.5 mm in order, the tensile strength declines to 632.30 N, 672.14 N, and then 648.52 N correspondingly. By contrast, the buffer strength of Kevlar/TPU laminated composites exhibits different trends. Figure 9c demonstrates the buffer strength test assembly. Right after the rounded impactor strikes the sample, the sensor transmits the residual buffer strength, which is then recorded to evaluate how the sample buffers the impact force. Finally, Figure 9d shows the buffer strength of Kevlar/TPU laminated composites as related to the thickness of samples. The test results indicate that a rise in the thickness adversely affects the buffer level of Kevlar/TPU laminated composites, which decreases from 4224.39 N to 4168.89 N. Two different trends in the two different tests (puncture resistance and impact resistance tests) are attributed to the damage mechanisms. With a needle-like impactor, the puncture resistance of composites is dependent on the overall structure. As for Kevlar/TPU laminated composites, the 1.2-mm (thick) group has a lower intralayer density than the 1mm group, and therefore the multiple Kevlar fabrics may block the needle-like impactor. Nonetheless, the 1.5-mm group with a lower density of adhesion layer may show the displacement of woven fabric structure, which is responsible for a decreasing trend in the puncture resistance of the Kevlar/TPU laminated composites of a 1.5-mm thickness [28][29][30].

Dynamic Puncture Resistance and Impact Resistance of Kevlar/TPU Laminated Composites
As for the impact resistance performance, it is the TPU layer that provides the dominate cushion effect. The 1-mm group is composed of a more tightly formed TPU layer that comes through the three-layered Kevlar/TPU laminated composites, and the TPU layer thus contributes higher impact resistance (buffer effect). By contrast, the 1.2-and 1.5mm groups consist of a comparatively lower structural density, so the corresponding  Figure 9 shows the dynamic anti-puncture strength and buffer strength of Kevlar/TPU laminated composites. The thickness of samples is specified in both tests. Figure 9a shows two impactors for the corresponding tests. The needle-type impactor is for the dynamic anti-puncture test while the rounded impactor is used in the buffer strength test. Figure 9b shows the anti-puncture strength of Kevlar/TPU laminated composites as related to the thickness, and the anti-puncture strength first increases and then decreases. With the thickness increasing from 1 mm, 1.2 mm to 1.5 mm in order, the tensile strength declines to 632.30 N, 672.14 N, and then 648.52 N correspondingly. By contrast, the buffer strength of Kevlar/TPU laminated composites exhibits different trends. Figure 9c demonstrates the buffer strength test assembly. Right after the rounded impactor strikes the sample, the sensor transmits the residual buffer strength, which is then recorded to evaluate how the sample buffers the impact force. Finally, Figure 9d shows the buffer strength of Kevlar/TPU laminated composites as related to the thickness of samples. The test results indicate that a rise in the thickness adversely affects the buffer level of Kevlar/TPU laminated composites, which decreases from 4224.39 N to 4168.89 N. Two different trends in the two different tests (puncture resistance and impact resistance tests) are attributed to the damage mechanisms. With a needle-like impactor, the puncture resistance of composites is dependent on the overall structure. As for Kevlar/TPU laminated composites, the 1.2-mm (thick) group has a lower intralayer density than the 1-mm group, and therefore the multiple Kevlar fabrics may block the needle-like impactor. Nonetheless, the 1.5-mm group with a lower density of adhesion layer may show the displacement of woven fabric structure, which is responsible for a decreasing trend in the puncture resistance of the Kevlar/TPU laminated composites of a 1.5-mm thickness [28][29][30].

Dynamic Puncture Resistance and Impact Resistance of Kevlar/TPU Laminated Composites
As for the impact resistance performance, it is the TPU layer that provides the dominate cushion effect. The 1-mm group is composed of a more tightly formed TPU layer that comes through the three-layered Kevlar/TPU laminated composites, and the TPU layer thus contributes higher impact resistance (buffer effect). By contrast, the 1.2-and 1.5-mm groups consist of a comparatively lower structural density, so the corresponding threelayered Kevlar/TPU laminated composites demonstrate lower impact energy dissipation in the impact resistance test [31][32][33].
Polymers 2023, 15, x FOR PEER REVIEW 9 of 13 three-layered Kevlar/TPU laminated composites demonstrate lower impact energy dissipation in the impact resistance test [31][32][33].  Figure 10a shows that the 1-mm group has the least number of fibers appearing in the surface, which is owing to a compact material structure. However, Figure 10b,c shows that 1.2-and 1.5-mm groups demonstrate higher puncture resistance with a small amount of fiber embedded in TPU. The fibers are thus evenly formed into a circular state over the surface due to the needle-like impactor, and the fiber counts are lower than that in Figure 10a. Figure 11a shows that both the thin TPU composite film and the Kevlar woven fabric are damaged at the punctured position, while Figure 11b-d shows that when the fabric is punctured, the needle penetrates against the fabric, which generates strong friction and causes the fibers to become finer until they are broken. The puncture resistance can be confirmed by the fiber surface showing much hairiness in the image [34]. Figure 11e,f shows that the thin film is squeezed after being punctured and ruptured layer by layer around the damaged position. The extruded thin film is also damaged due to the slippage and fracture of the fibers at the bottom layer. The photographs of the fibers and sheets are shown in Figure 11g, the fibers are embedded in the films and bonded to each other. Limiting the thickness of the Kevlar/TPU laminated composites by the hot-pressing method can improve the bonding effect between layers and achieve a good reinforcing effect in the mechanical property tests [35,36]. The proposed Kevlar/TPU laminated composites only allow a lower content of the TPU blend infiltrating Kevlar fibers. The interweaving of woven fabrics strengthens the intralyer adhesive mechansim, and the interweaving of warp yarns and weft yarns generate rugged surface. The convex parts of woven fabrics are combined with TPU, which simultaneously drawing the concave parts  Figure 10a shows that the 1-mm group has the least number of fibers appearing in the surface, which is owing to a compact material structure. However, Figure 10b,c shows that 1.2-and 1.5-mm groups demonstrate higher puncture resistance with a small amount of fiber embedded in TPU. The fibers are thus evenly formed into a circular state over the surface due to the needle-like impactor, and the fiber counts are lower than that in Figure 10a. Figure 11a shows that both the thin TPU composite film and the Kevlar woven fabric are damaged at the punctured position, while Figure 11b-d shows that when the fabric is punctured, the needle penetrates against the fabric, which generates strong friction and causes the fibers to become finer until they are broken. The puncture resistance can be confirmed by the fiber surface showing much hairiness in the image [34]. Figure 11e,f shows that the thin film is squeezed after being punctured and ruptured layer by layer around the damaged position. The extruded thin film is also damaged due to the slippage and fracture of the fibers at the bottom layer. The photographs of the fibers and sheets are shown in Figure 11g, the fibers are embedded in the films and bonded to each other. Limiting the thickness of the Kevlar/TPU laminated composites by the hot-pressing method can improve the bonding effect between layers and achieve a good reinforcing effect in the mechanical property tests [35,36]. The proposed Kevlar/TPU laminated composites only allow a lower content of the TPU blend infiltrating Kevlar fibers. The interweaving of woven fabrics strengthens the intralyer adhesive mechansim, and the interweaving of warp yarns and weft yarns generate rugged surface. The convex parts of woven fabrics are combined with TPU, which simultaneously drawing the concave parts of the woven fabrics. According to the peeling characteristic that is previously discussed, this manner of combination retains the flexible attribute of the laminated composites while protecting and strengthening the fiber structure. of the woven fabrics. According to the peeling characteristic that is previously discussed, this manner of combination retains the flexible attribute of the laminated composites while protecting and strengthening the fiber structure.

Conclusions
In this study, Kevlar/TPU laminated composites were successfully prepared via the sheet extrusion process and hot-pressing process. For the starter, TPU composite films were formed with a greatly even structure because of the employment of the sheet extrusion process. In particular, with a TPU/MTPU blending ratio of 85/15 wt%, TPU composite films attained the optimal tensile properties. Next, the TPU composite films were melted to infiltrate the Kevlar woven fabrics during the hot-pressing process, which bonded the composites well. The mechanical properties of TPU composite films were highly improved with the incorporation of Kevlar woven fabrics. The 1-mm-thick group exhibited a maximal tensile strength as high as 357.73 N. According to the dynamic antipuncture test and buffer strength test, the presence of TPU was proven to firmly adhere to different laminates. Moreover, TPU composite films demonstrated different extents in the anti-puncture strength and buffer strength performances when the intralayer density and failure mode were changed. In conclusion, the proposed Kevlar/TPU laminated of the woven fabrics. According to the peeling characteristic that is previously discussed, this manner of combination retains the flexible attribute of the laminated composites while protecting and strengthening the fiber structure.

Conclusions
In this study, Kevlar/TPU laminated composites were successfully prepared via the sheet extrusion process and hot-pressing process. For the starter, TPU composite films were formed with a greatly even structure because of the employment of the sheet extrusion process. In particular, with a TPU/MTPU blending ratio of 85/15 wt%, TPU composite films attained the optimal tensile properties. Next, the TPU composite films were melted to infiltrate the Kevlar woven fabrics during the hot-pressing process, which bonded the composites well. The mechanical properties of TPU composite films were highly improved with the incorporation of Kevlar woven fabrics. The 1-mm-thick group exhibited a maximal tensile strength as high as 357.73 N. According to the dynamic antipuncture test and buffer strength test, the presence of TPU was proven to firmly adhere to different laminates. Moreover, TPU composite films demonstrated different extents in the anti-puncture strength and buffer strength performances when the intralayer density and failure mode were changed. In conclusion, the proposed Kevlar/TPU laminated

Conclusions
In this study, Kevlar/TPU laminated composites were successfully prepared via the sheet extrusion process and hot-pressing process. For the starter, TPU composite films were formed with a greatly even structure because of the employment of the sheet extrusion process. In particular, with a TPU/MTPU blending ratio of 85/15 wt%, TPU composite films attained the optimal tensile properties. Next, the TPU composite films were melted to infiltrate the Kevlar woven fabrics during the hot-pressing process, which bonded the composites well. The mechanical properties of TPU composite films were highly improved with the incorporation of Kevlar woven fabrics. The 1-mm-thick group exhibited a maximal tensile strength as high as 357.73 N. According to the dynamic anti-puncture test and buffer strength test, the presence of TPU was proven to firmly adhere to different laminates. Moreover, TPU composite films demonstrated different extents in the antipuncture strength and buffer strength performances when the intralayer density and failure mode were changed. In conclusion, the proposed Kevlar/TPU laminated composites can be used as an interlayer of the protective gear and embody powerful flexibility, qualifying Kevlar/TPU laminated composites as a reinforcement material.