- Research article
- Open Access
Enhancing the mechanical and thermal properties of polypropylene composite by encapsulating styrene acrylonitrile with ammonium polyphosphate
© The Author(s) 2019
- Received: 21 September 2018
- Accepted: 16 January 2019
- Published: 30 January 2019
In recent decades, incorporating polypropylene (PP) within flame retardants has proved to be an effective method of improving the thermal stabilities of PP, but too much adversely affects the mechanical properties of this polymer materials. Herein we report a novel multifunctional flame retardant, (styrene acrylonitrile)–(titanate-modified ammonium polyphosphate) (SAN–TAPP), to simultaneously improve the mechanical properties and thermal stability of PP composites.
SAN–TAPP was synthesized by encapsulating SAN resins with functional titanate-modified APP (TAPP) and subsequently was incorporated into PP by a melt-blending process. The phase characteristics and morphology of SAN–TAPP were investigated, and the mechanical properties and thermal stability of different content of PP/SAN–TAPP composites were studied.
The results showed that the TAPP was almost entirely wrapped in the SAN resins and PP/SAN–TAPP composites exhibited the sea-island morphology. For the mechanical properties, the impact strength of PP/SAN–TAPP composite was significantly improved, especially 15 wt% SAN–TAPP filled PP/SAN–TAPP composite exhibiting 2.17 times higher than that of pure PP. And the tensile strength and modulus also increased by addition of SAN–TAPP. For the thermal stabilities, melting temperatures (Tm) and residual char yield were improved. Furthermore, the LOI value of PP/SAN–TAPP composites increased from 19.8 to 27.5%; The 15 and 20 wt% SAN–TAPP filled in PP/SAN–TAPP composites passed the V-2 test of UL-94, and exerted the similar effect on the flame retardancy to TAPP with the same loading.
These results revealed that a novel PP/SAN–TAPP composites with synthetically enhancement on the mechanical properties, thermal stabilities and flame retardancy, suggesting a strong correlation between the phase structure, mechanical and thermal properties.
- Mechanical properties
- Flame retardancy
Polypropylene (PP) has been widely used in the past decades due to its good mechanical properties, resistance to chemical agents, and excellent electrical insulation. Nevertheless, several critics such as low impact resistance, flammability and low thermal stabilities restrict its applications [1–4]. Improving its impact strength and thermal properties, has increasingly attracted the attention of many researchers. In recent decades, incorporating functional nanoparticles into PP has proved to be an effective method for improving thermal property of PP. However, a high content of nanoparticles may lead to the reduction of the mechanical properties, especially the elastic modulus, tensile strength and high-temperature creep deformation [5–8].
Blending PP with rigid polymers to synthesize a binary or ternary system is a traditional method to improve mechanical and thermal properties simultaneously [9–14]. Recently, rigid polymers of nylon-6 [15, 16], polymethyl methacrylate [17, 18], acrylonitrile–butadiene–styrene (ABS) [19–22] and styrene–acrylonitrile (SAN)  have been frequently reported. SAN copolymer plays an important role in many industries owing to its high weathering ability . Kim et al.  demonstrated that PC/SAN had developed useful mechanical properties. Yu et al. claimed that SAN can improve impact strength of isotactic polypropylene (iPP) . Also, other researches have explored the use of SAN as a reinforcing agent in polymer material .
The high flammability of PP limits its applications, thus improving the fire retardancy of PP is the focus of many researches. In recent decades, adding flame retardants (FRs) into polymer materials is well known the main approaches. As a member of polymeric flame retardant additives, ammonium polyphosphate (APP), has received great attention due to its synergistic effect between phosphorus (P) and nitrogen (N), and highly effective catalyzing carbonization effect to promote the char formation. Besides, APP is as an intumescent flame retardant (IFR) with unmatched halogen-free, low-smoke and low-toxicity. However, like many other flame retardants additives, high APP content in a polymer (such as 20 wt%) results in the deterioration of its mechanical properties due to the thermodynamic incompatibility between APP and the polymer matrix [28–30]. Very recently, to overcome this problem, many researchers have covalently grafted polymeric flame retardant groups onto the polymer matrix or have modified the flame retardant with functional groups. For instance, Wang et al. wrapped ammonium polyphosphate with melamine-containing polyphosphazene (PZMA@APP) to improve flame retardancy and mechanical performance of EP composites . Shao et al. modified APP via an ion exchange reaction with ethylene diamine, and obtained a novel flame retardant of polypropylene . While modified APP has high flame retardancy, its low cross-linking would result in the deterioration of physical properties and thermal stabilities; this remains a problem for phosphorus-containing flame retardants. Therefore, it is necessary to further modify the phosphorus-containing flame retardant system to enhance both the thermal stabilities and mechanical properties of PP.
In this study, APP modified with titanate coupling agent (TAPP) was wrapped with SAN to produce a multifunctional flame retardant, SAN–TAPP; subsequently SAN–TAPP was incorporated into PP to obtain PP/SAN–TAPP composites. In this system, SAN is utilized to enhance the mechanical strength as a rigid body, and most importantly, it is expected to exert synergistic effect with TAPP to improve thermal properties and flame resistance of PP. Treating APP with titanate coupling agent aims to modify the interface between SAN and APP. For comparison, PP/(SAN + TAPP) was prepared by a one-step melt-blending process and PP/TAPP was also synthesized. The mechanical and thermal properties and flame retardancy of all three (PP, PP/TAPP, and PP/SAN–TAPP composites) were characterized by impact and tensile testing, thermos gravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X-ray diffraction (XRD) and flammability properties.
The composition of pure PP, PP/TAPP20 and PP/SAN–TAPP composites
PP/(SAN10 + TAPP10)
Synthesis of TAPP
Initially, APP was added into ethanol in a weight ratio of 1:3 with stirring. 10 wt% titanate coupling agent relative to APP/ethanol mixture was added dropwise. The mixture was magnetically stirred for 40 min at 50°C and then heated at 80 °C to remove all water. The dried samples were grounded for characterization and were grafted onto SAN.
Synthesis of SAN–TAPP
SAN–TAPP was prepared by melt-blending SAN with TAPP. Initially, pure SAN and TAPP at the weight ratio of 1:1 were premixed in a high speed mixer (SHR-10A, Coperion Heng AO Machinery, Nanjing, China). Then the mixtures were fed into a twin screw co-rotating extruder (SHJ-36, Coperion Heng AO Machinery, Nanjing, China) with L/D 40 operating at a speed of 30 rpm/min. Compounding was carried out at 165, 175, 180, 185, 190, 195 and 190 °C in sequential heating zones. It was then cooled, cut, and finally dried at 90 °C for 8 h to remove all water.
Preparation of PP/SAN–TAPP composites
PP/SAN–TAPP composites with different loading of SAN–TAPP were also synthesized by a melt-mixing process. Pure PP, SAN–TAPP, SMA and CP were mixed, melt-blended, cooled, cut, and then dried. The processing temperatures were set as 160, 180, 190, 200, 200, 200, 200, 210, and 210 °C and the screw rotating speed was 30 rpm/min.
Some extrudates were immediately molded by an injection molding machine (TC-150-P, Tiancheng Machinery Co. Ltd., China) at 180, 195, and 205 °C in sequential zones from hopper to mold to obtain specific sheets (dog bone-shaped specimens (150 mm × 10 mm × 4 mm) and rectangular samples (80 mm × 10 mm × 4 mm)) for mechanical and thermal testing and morphological examination.
The phase constituents of TAPP, SAN–TAPP, PP/TAPP20 and PP/SAN–TAPP composites were evaluated using an X-ray diffractometer (XRD, Philips PC-APD) with a CuKα (40 mA and 40 kV) radiation source of 0.154 nm wavelength at room temperature of 25 °C. The functional groups were examined using a Fourier transform infrared spectroscope (FTIR, Nicolet, 170SX, Wisconsin, USA) in the wave number range of 400 to 4000 cm−1 by pressing the samples and KBr into a membrane.
Mechanical properties testing
Measurements of tensile properties of PP/SAN–TAPP composites were carried out on a universal testing machine (WDW-100, Tianjin Meites Testing machine factory, China) using dog bone-shaped specimens (150 mm × 10 mm × 4 mm) according to the standard of GB/T 1040.2-2006 at room temperature. The assay was performed under a liner deformation loading rate of 50 mm/min until mechanical failure occurred. Three replicates were performed for each measurement. The impact strength was assessed on a beam impact testing machine (XJJ-5, Chengde Shipeng Testing Machine Co. LTD, China) at ambient temperature using rectangular samples (80 mm × 10 mm × 4 mm) in terms of GB/T 1043.1-2008 standard. For each measurement, three specimens were used.
The morphologies of SAN–TAPP, pure PP and PP/SAN–TAPP composites containing 10 and 20 wt% SAN–TAPP were characterized by scanning electron microscopy (SEM, S-900, Hitachi, Japan) at magnifications of 2000×, operating at an accelerating voltage of 5 kV. The specimens were cryogenically fractured in liquid nitrogen, and the fracture surfaces were coated with platinum to a depth of 10 Å.
Thermal deformation behavior and viscosity analysis
The thermal properties of the composites were determined using a differential scanning calorimeter (DSC, Q2000, TA instruments Inc., USA). Samples were subjected to a stream of pure nitrogen flowing at a rate of 50 ml/min and heated at 10 °C/min from 25 to 220 °C. Thermogravimetric analysis (TGA) measurements were carried out with a thermal analyzer (Q5000, TA instruments Inc., USA) from 30 to 700 °C at a heating rate of 10 °C/min under N2 atmosphere.
The heat deflection temperature (HDT) and vicat softening temperature (VST) of pure PP, PP/TAPP20 and PP/SAN–TAPP composites were assessed using a thermal deformation and vicat softening temperature tester (XWB-300B, Chengde Shipeng Testing Machine co. LTD, China) with silicone oil as warming medium. To test HDT values, rectangular samples (80 mm × 10 mm × 4 mm) were scanned from 25 °C to deformation temperature at a heating rate of 120 °C/h under a perpendicular loading weight of 75 g (bending normal stress: 0.45 MPa) in line with GB/T1634.2-2004. The VST values of all specimens were measured under a loading weight of 1000 g, heating from 25 °C to vicat softening temperature at a rate of 50 °C/h in terms of GB/T 1633-2000. The flame-retardant performance was characterized by vertical burning test (UL-94) and limiting oxygen index (LOI). Vertical burning ratings of these samples were determined using a CZF-5 instrument (Nanjing Qionglei Instrument Co., China) with a sample size of 125 mm × 12.5 mm × 3 mm according to ISO 1210-1992. Limiting oxygen indexes (LOI) of all samples (130 mm × 6.5 mm × 3 mm) were determined on a JF-3 oxygen index meter (Nanjing Jiangning Analysis Instrument Co., China) according to ASTM D2863-2012 standard.
Characterization of SAN–TAPP
XRD analysis of PP/SAN–TAPP composites
FTIR analysis of PP/SAN–TAPP composites
Scanning electron microscopy
Impact property plays a critical role in engineering applications. The result demonstrates that the toughness was significantly enhanced by adding SAN–TAPP into the PP matrix, as a function of the content of SAN–TAPP, clearly indicating that SAN–TAPP can serve as a rigid reinforcer in PP matrix. The enhancement in impact strength of PP/SAN–TAPP composites was mostly attributed to the formation of a sea-island morphology. When PP/SAN–TAPP composites are subjected to impact loading, the “islands” were pulled out as the load transferring to, accompanied by void growth at the interface or cavitation of SAN–TAPP, and finally resulting in more energy absorption and effective resistance to crack propagation [20, 41]. The impact strength of 10 wt% SAN- and 10 wt% TAPP-filled PP/SAN–TAPP composite was lower than that of 20 wt% SAN–TAPP filled PP/SAN–TAPP composite, which could be attributed to the dispersion of more and larger TAPP particles in PP matrix. Moreover, 20 wt% SAN–TAPP filled PP/SAN–TAPP composite exhibited a more refined and homogeneous morphology in comparison to 10 wt% SAN- and 10 wt% TAPP-filled PP/SAN–TAPP composite. García et al.  confirmed that the large particles would create large voids that may destroy the structural integrity of a polymer matrix, ultimately resulting in specimen failure. Thus, smaller particles are more desirable. In addition, in 20 wt% SAN–TAPP filled PP/SAN–TAPP composite, the TAPP was wrapped in SAN such that SAN served as a shell and connected more tightly connect with the PP matrix, leading to higher resistance to separation when the composites were subjected to impact loading .
The reason for maximum reinforcement in tensile strength and tensile modulus at 10 wt% or 15 wt% SAN–TAPP filled PP/SAN–TAPP composite could be the dispersion of SAN–TAPP in the PP matrix. With respect to the distribution and the size of SAN–TAPP in PP matrix, 10 wt% and 15 wt% SAN–TAPP filled PP/SAN–TAPP composite had a more homogeneous morphology than that of other PP/SAN–TAPP composites. This decreased the stress-concentration points in the interfacial regions, and resulted in the reinforcement of tensile properties.
Thermal properties of PP/SAN–TAPP composites
Degradation temperature of pure PP, PP/TAPP20 and PP/SAN–TAPP composites
T (°C) at 10% weight loss
T (°C) at maximum weight loss
Flammability properties of pure PP, PP/APP20 and PP/SAN–TAPP composites
The LOI and UL-94 values of pure PP, PP/TAPP20 and PP/SAN–TAPP composites
PP/(SAN10 + TAPP10)
In our study, SAN resins encapsulated functional titanate-modified APP (TAPP) to produce a flame retardant for PP, SAN–TAPP, which is then added to PP composites to simultaneously improve their mechanical properties and thermal stability. The XRD result demonstrated that SAN–TAPP had no obvious effect on the crystal form of PP. According to SEM images, PP/SAN–TAPP composites had a sea-island morphology with irregular spheres and dark cavities. Impact strength of PP/SAN–TAPP composites was significantly improved, especially for15 wt% SAN–TAPP filled PP/SAN–TAPP composites. Their tensile strength and modulus were also higher than pure PP. The improvement in mechanical strength is most likely due to sea-island morphology and even dispersion of SAN–TAPP in the PP matrix. The DSC, TG and HDT results demonstrated that Tm, HDT and residual char yield were increased by the addition of SAN–TAPP. Furthermore, the LOI value of the PP composites increased with addition of SAN–TAPP. The 15 and 20 wt% SAN–TAPP filled PP/SAN–TAPP composites passed the V-2 test of UL-94, and exerted the similar effect on the flame retardancy as TAPP with the same loading. The enhancement of thermal stabilities is probably due to the cooperative reinforcement effect of TAPP and SAN and the interfacial reaction (interaction) of SAN–TAPP and PP.
TY, LZ and FW initiated and designed the study. YJL, XLW, ZZ and JZW collected the literatures and drafted the manuscript. TJ and JXC analyzed the data. All authors contributed to literatures analysis and manuscript finalization. All authors read and approved the final manuscript.
This work was partially supported by the National Natural Science Foundation of China (81673691, 81603381), the Guangdong Natural Science Foundation (2016A030313008), the Shenzhen Science and Technology Innovation Committee (JCYJ20160518094706544), and the Applied Basic Research Program of Sichuan Province (2018JY0445).
The authors declare that they have no competing interests.
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