poly (butylene terephthalate) blends

poly (butylene terephthalate) blends

Polymer Testing 36 (2014) 69–74 Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest Materi...

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Polymer Testing 36 (2014) 69–74

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material properties

Study on the rheological, thermal and mechanical properties of thermoplastic polyurethane/poly (butylene terephthalate) blends Jintao Huang, Xiang Lu, Guizhen Zhang, Jinping Qu* National Engineering Research Center of Novel Equipment for Polymer Processing, The Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangdong, Guangzhou 510640, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 January 2014 Accepted 10 March 2014

Thermoplastic polyurethane (TPU)/poly (butylene terephthalate) (PBT) blends with different ratios were prepared by extrusion and injection molding. The morphology, dynamic viscoelastic, capillary rheological, thermal and mechanical properties of the blends were studied. Results showed that there was good compatibility between TPU and PBT. The capillary rheological properties showed that the apparent viscosity decreased with the TPU content. DSC analysis indicated that with increasing TPU content the crystallization temperature (Tc), the melting point (Tm) and the percent crystallinity (Xc) decreased. Mechanical properties showed that the addition of TPU could lead to a remarkable increase, about 368.18%, in impact strength with a small reduction in tensile and flexural strength of TPU/PBT blends. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Poly(butylene terephthalate) Thermoplastic polyurethane Blends Properties

1. Introduction In recent decades, polymer blending has received growing attention from both the industrial and scientific communities as it is widely accepted as an efficient method to achieve cost and performance balance [1,2]. Polymer blends are physical mixtures of structurally different polymers with no covalent bonds occurring between them. The constituent polymers adhere together through the action of secondary bond forces only [3]. They are important engineering materials with outstanding comprehensive performance by taking the advantages of each component. The degree of compatibility is one of the major factors in determining the final properties, and the other controlling factors include morphology, chemical composition, polymer crystal

* Corresponding author. Tel.: þ86 02087114578; fax: þ86 02087112503. E-mail address: [email protected] (J. Qu). http://dx.doi.org/10.1016/j.polymertesting.2014.03.006 0142-9418/Ó 2014 Elsevier Ltd. All rights reserved.

structure, molecular weight and processing [4-6]. A compatible polymer blend shows mechanical properties proportional to the ratio of the constituent of the blend, where incompatibility often results in a material with weak mechanical properties [7-9]. Poly (butylene terephthlate) (PBT) is an important engineering thermoplastic with many valuable attributes, including excellent electrical properties, high rigidity, low moisture absorption, broad chemical resistance and high rates of crystallization from the melt [10-13]. These characteristics make it very useful for manufacturing injectionmolded articles for electrical, domestic and automotive applications where the above mentioned characteristics are required [14]. However, PBT does not show sufficient toughness, in particular, high notched impact strength [15,16]. Therefore, to improve the fracture toughness, especially impact toughness, the incorporation of rubber or other elastomer is often required [17-19]. Up to now, PBT has been blended with polyethylene (PE), polycarbonate (PC), ethylene-propylene-diene (EPDM),

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acrylonitrile-butadiene-styrene (ABS), high-impact polystyrene (HIPS) and poly (ethylene–octene) (POE), paying attention to the effects of a compatibilizer on the morphology, microstructure, mechanical and thermal properties of compatibilized and uncompatibilized blends [3,8-10,20-22]. Thermoplastic polyurethane (TPU) has been extensively used because of its excellent mechanical properties such as high tensile strength, tear and abrasion resistance, solvent and oil resistance, and low temperature flexibility. TPU is a linear copolymer composed of microphase-separated soft and hard segments. The soft segments usually make up an elastomer matrix responsible for the elastic and low-temperature properties of TPU, whereas the hard segments act as multifunctional tie points that function as physical crosslinks and reinforcing fillers [23-26]. In this work, pellets of TPU and PBT were blended by extrusion and subsequent injection molding. The processability of the blends was studied by the rheological measurements and melt flow index (MFI). The structure and morphology were characterized by differential scanning calorimetry (DSC), dynamic mechanical properties analysis (DMA) and scanning electronic microscopy (SEM). The mechanical properties are discussed based on the observed structure and morphology.

investigations. Prior to the SEM test, all surfaces were sputtered with gold to provide enhanced conductivity. 2.4. Dynamic mechanical properties analysis DMA was performed at temperatures ranging from 180  C to 150  C under nitrogen atmosphere by using a Netzsch DMA (model 242c, Germany), however, the pure PBT was tested at a temperature range from 100  C to 100  C, at the same heating rate and frequency. The samples had dimensions of 10 mm  4 mm  1 mm. Tests were performed in a single cantilever bending mode at a fixed frequency of 1 Hz and a heating rate of 3  C/min. 2.5. Melt flow index of blends Melt flow index (MFI) was performed with a Tinius Olsen MP993a Extrusion Plastometer (Melt Indexer) in accordance with ISO 1133-1. The test temperature and loading points were 230  C and 2.16 Kg, respectively. MFI value was determined by the following formula:

MFI ¼

m  600 ðg=10minÞ t

(1)

2. Experimental

where m is the average mass; and t is the time required for each cut segment.

2.1. Materials

2.6. Rheological properties analysis

PBT was Valox VX3101N provided by SABIC Innovative Plastics (China) Co., Ltd.. TPU (grade WHT1195; density ¼ 1.2 g/cm3) was supplied by Yantai Wanhua Polyurethanes.

The capillary measurements were performed using a Rheologic 5000 capillary rheometer (CEAST Co., Italy). The diameter of the circular die was 1 mm and L/D was 40:1, with a 90 entrance angle. The measurements were performed at 250  C (523 K). After being loaded into the barrel, the sample pellets were compacted and preheated for 3 min before the capillary experiments were started. Three replicate measurements were made to confirm good reproducibility of the results.

2.2. Preparation of the blends The PBT (4 h at 120  C) and TPU particles (4 h at 70  C) were dried in an air oven before processing to avoid possible moisture-degradation reactions. PBT was melt blended with TPU (0, 5, 10, 15, 20, 30 wt%, respectively) using a Brabender counter-rotating twin-screw extruder (Germany) with a screw diameter of 25 mm and a length/ diameter ratio of 20:1. The temperature profile was 165, 245, 250, 250, 250, 250, 255, 265  C, and the screw speed was 200 rpm. All the extrudated rods were immediately cooled in a water bath, pelletized and then dried in an air oven for 4 h at 120  C. The injection molding was carried out in a plastic injection molding machine (HTB110X/1, China) to obtain tensile, flexural and impact specimens for further tests. The barrel temperature profile of the injection molding was 250–245–245–240  C, and the mold temperature was 50  C. 2.3. Morphology observation The fracture surface was studied via SEM (HITACHI S3700N, Japan). The specimens (4 mm thick) were submerged in liquid nitrogen for approximately 15 min and fractured to expose the internal structure for SEM

2.7. Thermal properties analysis DSC was performed by using a Netzsch DSC (model 204c, Germany) equipped with a liquid nitrogen cooling accessory. Specimens were first heated from room temperature to 260  C, kept for 3 min to eliminate any thermal history, and then cooled to 30  C at 10  C/min under nitrogen atmosphere. The second scan was performed by reheating the specimens from 30  C to 260  C at 10  C/min. All thermal parameters provided were determined as the average of three repeats. 2.8. Mechanical properties testing Tensile and flexural test of pure PBT and the blends were conducted by using an INSTRON universal machine (model 5566, United States) in accordance with GBT 1447-2005. An INSTRON POE2000 pendulum impact tester was used in the impact test. All values were determined in an average of five repeats.

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3. Results and discussion 3.1. Morphology observation The SEM images of the fracture surface for the pure PBT, 5 wt% TPU/PBT, 15 wt% TPU/PBT, 30 wt% TPU/PBT blends are shown in Fig. 1 (a–d). As shown in Fig. 1, although the phase interface of TPU and PBT was more and more obvious as the TPU loading increased from 0 wt% to 30 wt%, the minor phase was, on the whole, hardly distinguishable. This indicates that there is good compatibility between TPU and PBT. 3.2. Dynamic mechanical properties analysis DMA data for blends may provide information about glass transition temperatures of components to give better understanding of the phase structure and interphase mixing of the blends [10,13,27]. Plots of loss factor (tan d) as a function of temperature for pure PBT and 30 wt% TPU/PBT are given in Fig. 2. Over the experimental temperature range, PBT exhibited one glass transition temperature at 72  C. Two transition temperatures (Tg1 and Tg2) were observed for 30 wt% TPU/PBT; the sharp dynamic mechanical damping peak at approximately 65  C was a result of the glass transition of PBT component, and the broad peak at approximately 38  C was associated with the glass transition temperature relaxation of the TPU component in the blend. With the addition of TPU, the glass transition temperature of PBT was slightly lower. Fig. 3 illustrates the temperature dependence of the storage modulus (E0 ) of the pure PBT and 30 wt% TPU/PBT.

Fig. 2. DMA loss factor (tan d) versus temperature of pure PBT and 30 wt% TPU/PBT.

As shown in Fig. 3, the dynamic mechanical properties of the blends are also affected by the composition. In the temperature range of 25–50  C, compared to the E0 of pure PBT, the decrease in E0 was 37.58% for 30 wt% TPU content. Furthermore, between 80 to 160  C, 30 wt% TPU/PBT still showed lower E0 than that of the pure PBT. 3.3. Melt flow index of blends The melt flow index (MFI) values of each sample are shown in Fig. 4. The MFI of pure PBT is 46.7 g/10min. As can be seen, for the TPU/PBT blends, the MFI increased with the

Fig. 1. SEM micrographs of fracture surfaces for (a) pure PBT, (b) 5 wt% TPU/PBT, (c) 15 wt% TPU/PBT, (d) 30 wt% TPU/PBT.

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Fig. 3. DMA storage modulus (E0 ) versus temperature of pure PBT and 30 wt % TPU/PBT.

addition of TPU, which implied a decrease of the viscosity of the blends. The change of MFI indicates a strengthening of the mobility of the molecule chains. 3.4. Rheological properties analysis The rheological properties of polymer materials are important for their processing and these are most often studied by using a capillary rheometry [28]. Thus, the rheological behavior of pure PBT and TPU/PBT blends were examined by a capillary rheometer, and the results at 250  C are given in Fig. 5. It shows that all these materials exhibit non-Newtonian and shear thinning characteristics in the range of applied shear rates [29]. In addition, with increasing the TPU content, the TPU/PBT becomes more sensitive to shear rate. Fig. 6 shows that, at all shear rates, apparent viscosity (ha) of TPU/PBT is lower than the pure PBT and, with the increase of the TPU content, the apparent viscosity decreased gradually. This shows that the processability of PBT is improved with the addition of TPU.

Fig. 4. The melt flow index (MFI) as a function of TPU content in TPU/PBT blends.

Fig. 5. Rheological curves for pure PBT and TPU/PBT blends at 250  C.

3.5. Thermal properties analysis Differential scanning calorimetry (DSC) thermograms of pure PBT and TPU/PBT blends are shown in Fig. 7 and Fig. 8, respectively. Table 1 shows the crystallization temperature (Tc), melting enthalpy (DHm) and percent crystallinity (Xc) of pure PBT and TPU/PBT blends. The Xc of pure PBT and TPU/PBT blends were determined by Eqs. (2) and (3), respectively [30]. The melting enthalpy of 100% crystalline o PBT (DHm ) is 145.5 J/g and Wf is the weight fraction of TPU in the composite [31].

Xc ¼

DHm  100% DHmo

(2)

Xc ¼

DHm   100% DHmo 1  Wf

(3)

The intermolecular interaction between two components can lead to depression of the crystallization rate, reduction of Tc, and increase of degree of undercooling. As

Fig. 6. Effect of the TPU content on apparent viscosity of pure PBT and TPU/ PBT blends.

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Table 1 Crystallization temperature (Tc), melting enthalpy(DHm), percent crystallinity (Xc) and melting temperature(Tm) for pure PBT and TPU/PBT blends. Samples

Tc ( C)

DHm (J/g)

Xc (%)

Tm1 ( C)

Tm2 ( C)

Pure PBT 5 wt% TPU/PBT 10 wt% TPU/PBT 15 wt% TPU/PBT 20 wt% TPU/PBT 30 wt% TPU/PBT

198.2 192.9 187.3 177.2 174.1 164.5

50.2 38.4 37.5 32.4 29.3 11.5

34.5 26.4 25.8 22.3 20.1 7.9

218.3 213.8 211.1 204.5 201.8 199.7

226.4 222.7 218.9 210.9 208.0

regularity of PBT resin, so that the crystallinity of the blends was reduced, resulting in a decrease of the peak intensity.

Fig. 7. DSC non-isothermal crystallization curves of pure PBT and TPU/PBT blends.

shown in Fig. 7, the peak temperature (Tc) corresponding to the crystallization of the blend from the melt decreases compared with the pure PBT when TPU is the continuous phase in the blend. This may be because that TPU hinders the transfer and movement of the crystallizable molecular chains of PBT. As shown in Fig. 8, pure PBT and TPU/PBT blends (TPU content no more than 15 wt%) exhibit two melting peaks, and the peak of pure PBT at higher temperatures is dominant compared with that at lower temperatures [10]. The origin of these peaks has been ascribed to the presence of different morphologies and simultaneous melting and reorganization of the crystallites [32]. With the increase of TPU content, the melting point of all blends is reduced. This is because a growing number of branched or crosslinking structure appeared in the blends with the increase of TPU content. Moreover, the peak intensity of the curve showed a trend of decline with increasing TPU content, especially when the content was from 20 to 30 wt %. On the one hand, this is because an increase in the amount of TPU enhances the blend interface reaction, giving better compatibility between TPU and PBT [33]. On the other hand, due to its flexibility, the TPU disrupted the

Fig. 8. DSC melting curves of pure PBT and TPU/PBT blends.

3.6. Mechanical properties testing The phase morphology and the interfacial adhesion between component polymers influence the mechanical properties of polymer blends. Multiphase morphology with lack of adhesion between the component polymers leads to premature failure and, thus, to poor mechanical strength. On the other hand, the enhanced interfacial adhesion leads to good mechanical strength. Fig. 9 shows the dependence of tensile strength and flexural strength of blends as a function of TPU content. It can be seen that, with increasing TPU content, the tensile strength and flexural strength decrease monotonously. The tensile strength and flexural strength of blends are decreased by 43.86% and 49.13%, respectively, with the 30 wt% TPU addition to PBT. The impact strength for pure PBT and TPU/PBT blends with different TPU loadings are summarized in Fig. 10. When the TPU content was 30 wt%, there is a marked increase in impact strength by approximately 368.18% over that of pure PBT from 2.2 kJ/m2 to 10.3 kJ/m2. It can be seen that, by introducing TPU to the blends, a significant improvement of impact strength is obtained. 4. Conclusions This paper investigates the effect of TPU content on the properties of TPU/PBT blends. Results showed that there was good compatibility between TPU and PBT. With the

Fig. 9. Tensile strength and flexural strength of pure PBT and TPU/PBT blends.

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Fig. 10. Impact strength of pure PBT and TPU/PBT blends.

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