Article pubs.acs.org/IECR
New Way To Tailor Thermal Stability and Mechanical Properties of Thermoplastic Polyester Elastomer: Relations between Interfacial Structure and Surface Treatment of Spodumene Slag Jing Huang,† Yaxin Qiu,† Defeng Wu,*,†,§ and Jun Wang‡ †
School of Chemistry & Chemical Engineering and ‡Provincial Key Laboratories of Environmental Engineering & Materials, Yangzhou University, Yangzhou, Jiangsu 225002, China § Jinsen Photoelectric Material Co. Ltd., Yangzhou, Jiangsu 225009, China ABSTRACT: A new approach to improve the thermal stability and mechanical strength of thermoplastic polyester elastomer (TPEE) using spodumene slag as the filler is reported here. The thermal decomposition temperature of TPEE increases from 277 °C to about 350 °C by about 26% in the presence of pristine spodumene slag. The surface graft reaction further improves its barrier effect. Besides, spodumene slag can be used as a good reinforcement to TPEE, and also the surface treatment favors further improvement of its reinforcing effect. However, the presence of spodumene slag reduces the elastoplasticity and viscoplasticity of the system, leading to decreased tensile cycles and elongation levels. The morphological and crystallographic methods were then employed to further reveal the alteration of the phase separation structure of TPEE in the presence of spodumene slag, aiming at establishing structure−property relations of composites. This work can provide useful information on the preparation of new TPEE composites with tailorable properties.
1. INTRODUCTION Thermoplastic polyester elastomer (TPEE) is an interesting block copolymer material. The crystalline domains that are composed of hard polyester segments commonly determine its strength and modulus, and the amorphous ones that are composed of polyether segments determine its plasticity and low-temperature flexibility.1,2 It has therefore been used in the automotive and electronic fields3−5 because of its balanced performance. The composite technology is commonly used to improve or to control its mechanical strength and thermal stability, and finally to further extend its application fields. In recent years, many kinds of particles with fully different scales and dimensions, including silicates6−8 and carbonaceous particles,9−12 have been used as fillers to fabricate TPEE composites. The results reveal that the composite technology is a good option to tailor the overall performance of TPEE materials. Recently, some kinds of solid wastes such as slag and fly ash have attracted more and more attention in the polymer composite fields. This is because slag and fly ash are mainly composed of silicate, aluminosilicates, carbonates, and sulfates,13 which are very similar to the compositions of those commonly used fillers in the polymeric material industry. For instance, Cornacchia and co-workers14 reported that incorporation with electric-arc furnace slag could raise the strength and modulus of polypropylene effectively. Similar reinforcement was also reported on fly ash filled rubber materials.15 As for TPEE, the presence of mixed mica and fly ash could also © XXXX American Chemical Society
improve the mechanical and thermal properties to a certain degree.7 The reuse of those solid wastes in the polymer composite fabrication can provide an additional way to deal with solid wastes (besides those traditional ways such as landfills/incineration or use in the ceramic industry) and, on the other hand, also indicates a possible approach to fabricate polymer composite with environmentally friendly or economical characteristics. However, very few works could be found in the literature on this topic. With increasing demand for lithium in batteries for electronic products and electric vehicles, the slag yielded from extraction of lithium-rich ore, spodumene, or pegmatite has become a new member of industrial solid wastes. Spodumene slag is mainly composed of the oxides or salts of silicon, aluminum, and calcium, without any heavy metallic salts or oxides, and other hazardous substance.13 This makes it a candidate for the filler of plastics or rubbers. In the current work, the spodumene slag with and without surface treatment was used as the filler to prepare TPEE composites. The morphology, mechanical properties, and thermal properties of composites were then studied to understand the interfacial structure−property relations, aiming at finding a new approach to tailor the mechanical and thermal properties of TPEE. Received: Revised: Accepted: Published: A
March 2, 2017 May 6, 2017 May 9, 2017 May 9, 2017 DOI: 10.1021/acs.iecr.7b00904 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
2.3. Mechanical Property Characterizations. An Instron Mechanical Tester was used to examine the tensile properties. The crosshead speed of 50 mm min−1 were performed at a temperature of 25 °C using the dog-bone-shaped specimens (ASTM D638). Six tests with the same condition were measured to obtain the average strength and modulus values. The step cycle tensile tests were performed with the stretching rate of 6 mm s−1, at room temperature. Details of the method can be found in previous work.11 2.4. Thermal Property Characterizations. Thermal behaviors of TPEE and spodumene slag filled composites were recorded by a differential scanning calorimeter (DSC, Netzsch DSC-204F1, Germany). The sample (∼5 mg) was heated from room temperature to 200 °C, held for 5 min, and cooled to room temperature, during which the crystallization process was recorded. Then, the sample was heated to 200 °C again to record the melting process. All the experiments were performed under nitrogen with a heating/cooling rate of 10 °C min−1. Thermogravimetric analysis (TGA) was performed on a Netzsch Instrument (STA409PC, Germany). The samples (10−12 mg) were heated to 600 °C at a rate of 10 °C min−1 under nitrogen atmosphere, during which the mass loss was recorded. All TGA data are reproducible to ±1 °C. 2.5. Rheological Characterizations. Rheological tests were performed on a rotational rheometer (DHR-2, TA Instruments Co. Ltd.). The parallel plates were applied during small amplitude oscillatory shear (SAOS) flow. The dynamic strain sweep was first carried out (200 °C, 1 Hz) to determine the linear region, and then the dynamic frequency sweep was performed at a common strain level, 1%. The modulus and viscosity were recorded.
2. EXPERIMENTAL SECTION 2.1. Material Preparation. TPEE with the commercial brand of KP3340 was purchased from KOLON Co. Ltd., Korea. It is composed of hard poly(butylene terephthalate) (PBT, 50.69 wt %) and soft poly(tetramethylene glycol) (PTMEG, 49.31 wt %), with a number-average molecular weight (Mn) of about 36 000. The molar ratio of PBT/PTMEG is about 25/75 (measured by nuclear magnetic resonance (NMR)).11 The average size of discrete PBT domains dispersed in the continuous PTMEG phase is about 100 nm.16 The melting temperature (Tm) of TPEE is about 176 °C. Spodumene slag (white powder) was provided by Shandong Mingrui Chemical Engineering Co. Ltd., PRC. It is the residual after extraction of lithium mineral, with irregular particle shape (30 ± 5 μm)13 and a density of 1.95 g cm−3. It mainly consists of silicon and aluminum oxides, as well as calcium sulfate, accompanied by small quantities of residual carbonates of the metals lithium and zinc. Detailed chemical composition has been reported in a previous work.13 Polyethylene glycol (PEG) with an average Mn value of 800 was purchased from Aladdin Industrial Co. Ltd., PRC. A commonly used method for the surface functionalization of polyhydric silica or silicate17,18 was employed here to treat spodumene slag. In brief, a small amount of spodumene slag was mixed with dimethylformamide (DMF) (vol/vol 1/10) with vigorous stirring to obtain a stable suspension. Then PEG was dissolved in the suspension (wPEG/wslag 0.5/100), followed by the addition of dicyclohexylcarbodiimide (DCC)/dimethylaminopyridine (DMAP) as the catalyst (nDCC/nDMAP 5/1), and ultrasonicated for 30 min in ice−water. The grafted modification via esterification of the hydroxyl groups of PEG and spodumene slag was then maintained at 60 °C with stirring for about 6 h. Finally, the suspension system was filtered and washed with DMF, and then the filter cake was dried under vacuum. The composite samples containing pristine or treated spodumene slag were prepared by an internal mixer (Haake Polylab, Thermo Electron Co., USA) at 180 °C and 50 rpm for 6 min. The dog-bone-shaped samples (32 mm × 4 mm × 2 mm) were prepared by injection molding using a Haake minijet (Thermo Scientific Co., USA). The injection pressure and holding one are 600 and 500 bar, respectively. The barrel temperature and mold one are 220 and 35 °C, respectively. The composite samples with pristine spodumene slag and treated one are hereafter referred as to TPEECs and TPEECsP, respectively, where s is the weight ratio of spodumene slag. 2.2. Morphology and Structure Characterizations. The surface treatment of spodumene slag was evaluated by a Fourier transform infrared spectrometer (FT-IR, BRUKER Co., Germany) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific Co., USA). The FT-IR spectra were collected with the reflection mode. The XPS spectra were recorded with a pass energy of 20 at 0.05 eV steps at a pressure of
