Mechanical and Morphological Properties of Polypropylene/Polyoxymethylene Blends

Sirirat Wacharawichanant, Tanakorn Siripattanasak

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Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Bangkok, Thailand.

DOI: 10.4236/aces.2013.33026                

Abstract

This work studies the mechanical and morphological properties of polypropylene (PP)/polyoxymethylene (POM) blends. The PP/POM blends were prepared by melting-blend with an internal mixer. The contents of PP were used at 10%, 20%, 30%, 70%, 80% and 90% by weight. The phase morphology of the blends was observed by scanning electron microscope (SEM) due to the mechanical properties depended on the phase morphology. The results show the decrease of the impact strength, Young’s modulus and tensile strength of POM/PP blends with increasing of PP content up to 30 wt% and then increase again with a high PP content. The percent strain at break of the blends increases after adding PP in a range of 70 - 90 wt%. SEM study reveals that the POM/PP blends clearly demonstrate a two-phase matrix-particle microstructure. The results also show that the domain size of dispersed PP or POM phase increases with increasing PP or POM content. The POM domain size is smaller than the PP domain size which leads to a little change of mechanical results of PP.

Keywords

Polypropylene; Polyoxymethylene; Morphology; Mechanical Properties

1. Introduction

Blending two polymers to produce new and high-performance polymeric materials with targeted properties is a popular and attractive topic. The final aim is to promote synergism among the immiscible polymer pairs in order to form blends with enhanced or even new tailored properties with respect to those of the different polymers [1-4].

Polyoxymethylene (POM) is an excellent engineering thermoplastic, which is considered as an alternative for metals in many applications because of its remarkable mechanical and self-lubricating characteristics [5]. It is a lightweight, low-friction, and wear-resistant thermoplastic with good physical and processing properties and capable of operating at temperatures in excess of 90˚C [6]. The POM blends have been less studied, for example, Wang [7] studied the thermal stability of POM and Its blends with poly(ethylene-methylacrylate) (EMA) or poly (styrene-butadiene-styrene) (SBS). The results were reported that POM blends with SBS or EMA had similar degradation behavior as POM, but addition of SBS accelerated the POM degradation significantly. Degradation in POM and SBS/POM occurred in amorphous phase while EMA/POM degraded in both amorphous and crystal phase.

Polypropylene (PP) is one of the most widely used commodity polymers with the high heat distortion temperature and economic merits. However, the use of the PP is restricted by the low impact strength at the subambient temperature [8]. A blend of POM/PP is significant interesting, thus this work studies the mechanical and morphological properties of PP/POM blends. The phase morphology of one polymer and the blends was observed by scanning electron microscope (SEM).

2. Experimental

2.1. Materials

POM (DURACON M90-44) with the melt flow rate of 8.9 g/10min and specific gravity of 1.41 was produced by Polyplastics Company. PP (Mophen HP400K) was produced by HMC Polymer Company with the melt flow rate of 4 dg/min.

2.2. Sample Preparation

All types of polymers were dried before blending, POM and PP was dried in an oven at 110˚C for 4 h. PP/POM blends were prepared by melt blending in an internal mixer at 200˚C and a rotor speed of 50 rpm for 10 min. The copolymer contents were 10, 20, 30, 70, 80 and 90 wt% POM content. The samples for tensile and Izod impact tests were prepared by a compression molding at 200˚C for 20 min. Dumbbell samples for tensile test and rectangular samples for Izod impact test.

2.3. Sample Characterization

The impact test was performed with a Zwick/material testing August-Nagelstr.11.D-89079 Ulm at room temperature. Tensile tests were conducted according to ASTM D 638 with a universal tensile testing machine (LR 50 k from Lloyd instruments) at a crosshead speed of 50 mm/min. Each value obtained represented the average of five samples.

The thermal stability and decomposition temperature of polymer blends were measured by thermogravimetric analysis (TGA) (Model SDT Q600, TA Instruments, England). The temperature was in the range of 30˚C - 600˚C at a heating rate of 10 ˚C/min under nitrogen atmosphere.

SEM (Model Maxim 2000S, CamScan Analytical, England) was taken to study the morphology of the POM/PP blends. The accelerating voltage of SEM observation is 15 kV. The impact-fractured surfaces of the POM/PP blends obtained from impact test were examned. All specimens were coated with gold before SEM observations.

3. Results and Discussion

The notched Izod impact strength of POM/PP blends is presented in Figure 1. It can be seen that the impact strength of POM/PP blends decreases with increasing PP contents until 30 wt% and then slightly increases but the impact strength is still lower than neat POM. Moreover, the result shows that the neat POM is higher impact strength than the neat PP so the addition of PP may affect the decrease of impact strength of the blends.

Figure 2 illustrates the trend of the tensile strength of POM/PP blends. The tensile strength of the blends decreases continuously with increasing PP content and small changes in a range of 70 - 100 wt% PP. The neat POM is higher tensile strength than the neat PP so the addition of PP may affect the decrease of tensile strength of the blends same as the impact strength. Figure 3 presents Young’s modulus of POM/PP blends, it can be seen that Young’s modulus decreases continuously with increasing PP content up to 30 wt% and then slightly in-

Conflicts of Interest

The authors declare no conflicts of interest.

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