Crystallization, thermal properties, rheological properties, mechanical properties, and morphology of thermoplastic polyurethanes/calcium sulfate whiskers composites

The research shows that thermoplastic polyurethane (TPU) which is easy to process has great application potential in practical application. In this study, a calcium sulfate whisker (CSW) with high strength and stability was used as a reinforcing filler to improve the processability of flexible TPU. In this paper, TPU/CSW composites were prepared by the melt blending method, and the effects of modified CSW on the thermal, crystalline, and melt properties, as well as the rheological and mechanical properties of TPU composites were investigated. The results showed that when the mass content of modified CSW was 10%, the maximum values of ΔH c and ΔH m were 222.81°C and 9.24 J/g, respectively, and the thermal properties of the composites were the most stable. When the content of modified CSW is 5%, the maximum impact strength, bending strength, and tensile strength are 50.5 KJ/m2, 38.8 MPa, and 927.7 MPa, respectively, and the mechanical properties of the composites are the best. The excellent stability and high strength of TPU/CSW composites show great promise for various applications in transportation and building materials.


Introduction
Thermoplastic polyurethane (TPU) is a synthetic organic polymer material composed of oligomeric polyols and diisocyanate hard segment, which possesses excellent properties such as high strength, strong tensile resistance, and easy processability, making it widely utilized in aerospace, aviation, and mechanical parts industries [1][2][3] .However, the mechanical strength and thermal stability of TPU are poor, which limits its application and development in various fields [4][5] .In recent years, domestic and foreign research on TPU blending modification technology has been more active through the addition of modifiers to improve the thermal stability and processability of TPU, because of maintaining the original excellent properties of TPU [6] .
Calcium sulfate whisker (CSW) is a new inorganic nano-filler with a nearly perfect crystal structure, which is not only cost-effective but also has special properties such as excellent thermal stability, insulation, high tensile strength, and elastic modulus, witnessing wide applications as an excellent filler to enhance the machinability and thermal stability of polymers [7][8] .However, as the modification of TPU with inorganic materials becomes more common, it is not possible to improve the mechanical, crystalline, and processability properties of TPU at the same time.The reason is that CSW whiskers have a high aspect ratio, and CSW is often agglomerated within TPU when preparing TPU/CSW composites [9] .Improving the overall performance of CSW in TPUs, including dispersion, mechanical properties, and processability, poses a significant challenge in current research [10] .
Previous studies have demonstrated that silane coupling agents are commonly employed to enhance the dispersion of inorganic fillers and further promote the nucleation and crystallization of composites, acting as a "bridge" between inorganic and organic materials [11] .He et al. [12] prepared isotactic polypropylene (IPP)/CSW composites by surface modification of CSW using γ-aminopropyl triethoxysilane (KH550).The results showed it can improve the interfacial compatibility between the CSW filler and the IPP matrix, and then improve the mechanical properties, thermal stability, and crystalline properties of the IPP/CSW composite system.Jayachitra et al. [13] performed three types of silane modifications on lignocellulosic raw fibers extracted from coconut inflorescences, namely, KH550, KH560, and KH570 before hybridization.The results showed that KH570 silane-modified inflorescence fibers blended with glass fibers and reinforced epoxy composites were found to exhibit maximum tensile and flexural strengths of 102.6 MPa and 166.89MPa, respectively.
To address these issues, the TPU/CSW composite was subjected to surface modification by using KH570 and subsequently prepared through a melt blending and extrusion pelletizing process.The melt was blended with TPU to enhance the interfacial compatibility between CSW and TPU while reducing the agglomeration of CSW in the TPU matrix and improving the mechanical processability of the composites.The resulting TPU/CSW composites were then analyzed for their thermomechanical properties, crystallization, melting behaviors, rheological properties, and mechanical properties to investigate the effects of different levels of KH550-modified CSW on the thermal stability, crystallization, and rheological and mechanical properties of the TPU/CSW composite system.

Experiment
2.1 Materials TPU (903TU) was purchased from Shenzhen Plastic Sea Rubber & Plastic Technology Co., Ltd.Calcium sulfate (CSW with a length of 30~90 μm, diameter of 2~3 μm, and effective mass fraction of 99%) was obtained from Jinan Qingyuyuan New Material Co., Ltd.

Preparation of CSW modifier and CSW surface modification
We measured 17 mL of anhydrous ethanol, 2 mL of distilled water, and 1 mL of KH570.Then we transfered the mixture to a beaker and stir magnetically for 4 h at room temperature to obtain the formulated modifier.
The CSW was placed in a blast oven at 100°C for 12 h.250 g of CSW was weighed and 20% of the solid volume of the modifier was slowly added drop by drop, and the reaction was carried out with magnetic stirring at 60°C for 3 h.Afterward, the precipitate was washed and dried at 100°C for 8 h to obtain the modified CSW.

Preparation of TPU/CSW composites
The TPU was placed in a blast drying oven at 100°C for 12 h.The additional amounts of KH570 modified CSW were set to 0%, 5%, 10%, 15%, and 20% of the mass of TPU.Using the melt co-mixing method, the dried TPU and modified CSW were put into a high-speed mixer and mixed well and then put into a twin-screw extruder (medium control temperature set at 185 ~ 230°C) for extrusion, and finally, the samples were pelletized with a cutting machine.The pellets were dried in an oven at 100°C for 12 h and then put into a plastic injection moulding machine for injection moulding to obtain the required sample strips.

Characterization
The TG-DTG curve was obtained by using a thermogravimetric analyzer (TGA-55, TA Instruments, USA).Enthalpic properties of samples were conducted by using NETZSCH DSC214.The rheological properties were tested by using a Thermo Fisher Scientific MARSIII rotational rheometer.The morphologies of samples were characterized by scanning electron microscopy (FEI Quanta 250).The samples were evaluated by tensile test according to GB/T1040-79, using a universal material testing machine (SNAS, MTS Industrial Systems Co., Ltd., China)   1 and Table 1, which overall show that the thermal decomposition process of TPU/CSW composites with different CSW fractions mainly occurs from 300°C to 450°C.When the temperature reaches 352.3°C and 400.6°C, two weight losses occur, mainly due to the decomposition of TPU.The TDTG peak1 appearing at 320°C ~ 360℃ is the maximum weight loss peak of the composite.As the CSW content gradually increased, the DTG peak temperature shifted to the left, and the peak height gradually decreased, i.e., the temperature at which the TPU/CSW composites started to decompose shifted towards lower temperatures.As shown in Table 1, the T2%, T5%, and T10% of TPU/CSW composites gradually decreased with increasing CSW content, because CSW as a TPU filler enhanced the intermolecular forces between matrix and filler, produced a rapid heat transfer effect, and acted as a thermal catalyst for degradation, resulting in lower T2%, T5%, and T10%.However, the residual carbon rate of TPU/CSW composites gradually increased with the increase of CSW content, with the maximum residual carbon rate of 21.17% for TPU/CSW-20%, indicating that the filling of CSW reduced the mass loss of TPU composites and facilitated the formation of residual carbon, and the low thermal transfer efficiency could not degrade HDPE/CSW composites effectively.The DSC curves and the melting and crystallization data of TPU/CSW composites with different CSW contents are shown in Figure 2 and Table 2, respectively.The TPU/CSW composites exhibit a single melting or crystallization peak, but with an increase in CSW content, the peak gradually increases and shifts towards higher temperatures.From the data presented in Table 2, it can be observed that as the content of CSW increases, there is a gradual increase in the crystallization and melting temperatures of TPU/CSW composites.Specifically, there is an increase of 9.2°C and 8.74°C in the melting and crystallization temperatures, respectively.In addition, the values of both melt enthalpy (∆Hc) and crystallization enthalpy (∆Hm) exhibited an initial increase followed by a decrease upon increasing CSW content, but both were greater than those of pure TPU, indicating that the thermal stability of the TPU/CSW composites increased with the addition of CSW.Notably, the largest values of ∆Hc and ∆Hm for TPU/CSW-10% were 222.81°C and 9.24 J/g, respectively, indicating that the TPU/CSW composites had the greatest thermal stability at 10% CSW content.factor τ for TPU/CSW composites with different additions.Polymer rheology measurements are a valuable technique for evaluating the flow and deformation characteristics of polymeric materials, specifically, which serve as an effective method for gaining insights into microstructural information of filled particles within a polymer melt.The energy storage modulus of the melt G′ represents the amount of recoverable energy stored in the melt and the loss modulus G″ represents the amount of non-recoverable energy released from the melt.Figure 3 displays the energy storage modulus G′, loss modulus G″, complex viscosity η*, and loss factor τ of TPU/CSW composites with different CSW contents.The results reveal a non-Newtonian characteristic of the TPU/CSW blended system, where shear viscosity decreases with increasing shear rate.The G′, G″, η*, and τ of the pure TPU are all lower than those of the TPU/CSW composites.The value of G′ and G″ for TPU/CSW composites exhibits a decline as the frequency increases, while the value of η* primarily exhibits a decrease with frequency.This is due to the reduced duration of shear force action with increasing frequency, leading to a rise in energy loss and an increase in resistance to the movement of molecular chains within the material.As a result of the latter, the deformation of macromolecular chains is unable to keep pace with the changes in shear external force.The system then changes to an elastic behavior, with G' and G" decreasing initially and subsequently increasing as shear frequency rises.In the low-frequency region, the TPU macromolecular chains have enough time to deform.The viscoelasticity decreases, and G′, G″, and η* also decrease.As shown in Figure 4, with the addition of CSW content, the impact and bending strengths of TPU composites increased compared to pure TPU, with the highest impact, bending, and tensile strengths of 50.5 KJ/m 2 , 38.8 MPa, and 927.7 MPa, respectively, when the CSW content was added at 5%.This phenomenon can be explained by the special structure of TPU and the increasing content of CSW whiskers, which increases the concentration in the plastic phase of TPU, resulting in complete microphase separation and improving the mechanical properties of TPU/CSW composites to some extent.When the mass fraction of CSW content is greater than 5%, CSW will appear as agglomerates and have an uneven dispersion in the composite process.When the composite is subjected to external forces, a large internal stress will be generated within it, resulting in the concentration of internal forces.Dry cracking will occur when the force has been sustained throughout the process, which will lead to a reduction in the tensile strength, bending strength, and impact strength of TPU/CSW composites.The mechanical properties are then reduced.

Morphological characterization of TPU/CSW composites
Figure 5 (a) shows the SEM image of pure TPU, which exhibits strong continuity and flat cross-section.Figures 5 (b)-(e) show that the KH570-modified CSW particles are uniform in size, CSW is well dispersed in the TPU matrix, and there is no gap at the interface between CSW and the matrix, indicating good compatibility between the two.Figures 5 (b)-(c) show that the SCSW exhibits a uniform insertion of the sharp end into the TPU matrix in a radioactive manner, and more CSW whiskers can be observed with the increase of the filling amount.A more uneven fracture surface near the CSW is found in Figure 5 (c), because more CSW appear to be agglomerated in the TPU matrix, hindering the dispersion of the CSW, which explains the decrease in mechanical properties mentioned above.

Conclusions
In this work, TPU/CSW composites were prepared by using the melt mixing method.The incorporation of modified CSW of a silane coupling agent (KH570) at a content of 20% resulted in a maximum residual carbon rate of 21.17% in the TPU/CSW composites.The thermal stability of the TPU/CSW composites initially increased and then decreased with increasing modified CSW content.The maximum values of ΔHc and ΔHm at 222.81°C and 9.24 J/g were achieved when the modified CSW content was 10%.The fluidity of the composites exhibited a gradual decrease as the modified CSW content increased, primarily due to agglomeration.The mechanical properties of the TPU/CSW composites were best when the modified CSW content was 5%, with maximum impact, flexural strength, and tensile strength of 50.5 KJ/m 2 , 38.8 Mpa, and 927.7 MPa, respectively.These results contribute to the advancement of high-strength and easily processed polymers in various fields.

Figure 1 .
Figure 1.Curves of TPU/CSW composites with different additions.Table 1. TG and DTG data of TPU/CSW composites with different additions.

Figure 2 .
Figure 2. DSC curves of TPU/CSW composites with different additions.Table2.DSC melting and crystallization data of pure TPU and TPU/CSW composites.

Figure 3 .
Figure 3. (a) Energy storage modulus G′; (b) Loss modulus G″; (c) Complex viscosity η*; (d) Lossfactor τ for TPU/CSW composites with different additions.Polymer rheology measurements are a valuable technique for evaluating the flow and deformation characteristics of polymeric materials, specifically, which serve as an effective method for gaining insights into microstructural information of filled particles within a polymer melt.The energy storage modulus of the melt G′ represents the amount of recoverable energy stored in the melt and the loss modulus G″ represents the amount of non-recoverable energy released from the melt.Figure3displays the energy storage modulus G′, loss modulus G″, complex viscosity η*, and loss factor τ of TPU/CSW composites with different CSW contents.The results reveal a non-Newtonian characteristic of the TPU/CSW blended system, where shear viscosity decreases with increasing shear rate.The G′, G″, η*, and τ of the pure TPU are all lower than those of the TPU/CSW composites.The value of G′ and G″ for TPU/CSW composites exhibits a decline as the frequency increases, while the value of η* primarily exhibits a decrease with frequency.This is due to the reduced duration of shear force action with increasing frequency, leading to a rise in energy loss and an increase in resistance to the movement of molecular chains within the material.As a result of the latter, the deformation of macromolecular chains is unable to keep pace with the changes in shear external force.The system then changes to an elastic behavior, with G' and G" decreasing initially and subsequently increasing as shear frequency rises.In the low-frequency region, the TPU macromolecular chains have enough time to deform.The viscoelasticity decreases, and G′, G″, and η* also decrease.

Table 1 .
TG and DTG data of TPU/CSW composites with different additions.

Table 2 .
DSC melting and crystallization data of pure TPU and TPU/CSW composites.