Toughening polyoxymethylene by polyolefin elastomer and compatibilizer

28 April 2017
Wenqing Yang, Xuan-Lun Wang, Xingru Yan, and Zhanhu Guo
Glycidyl methacrylate-grafted high-density polyethylene improved the compatibility of polyoxymethylene and polyolefin elastomer, in turn enhancing the properties of polyoxymethylene.

Polyoxymethylene (POM) is an excellent engineering thermoplastic with high stiffness, excellent flexural modulus and tensile strength, and good chemical resistance. For this reason, POM enjoys a wide range of application, including in the automotive, electronics, mechanical, and construction industries.1–4At the same time, POM possesses a high degree of crystallinity (about 70%), which leads to poor impact strength and thus susceptibility to fracture.4, 5

Compatibilizers are very important in modifying plastics because they serve to bridge different ingredients together. In particular, glycidyl methacrylate (GMA) improves the compatibility and thermal stability of thermoplastic blends. Moreover, research has shown that GMA copolymer acts as a reactive compatibilizer through reaction of epoxy groups in the GMA with hydroxyl (–OH) end groups.6 Because POM is a copolymer with hydroxymethyl (–CH2–OH) terminal groups, we hypothesized that GMA would react with the hydroxyl end groups of POM for good compatibility.

We applied glycidyl methacrylate-grafted high-density polyethylene (GMA-g-HDPE) as a reactive compatibilizer to POM/POE (polyolefin elastomer) blends to improve the toughness of POM.7 We prepared ternary blends of POM/POE/GMA-g-HDPE in various proportions using a co-rotating twin-screw extruder. We used a granulator to prepare granules of the blends, which were then dried. Injection molding was carried out in an injection-molding facility using a two-cavity mold. Accordingly, each injecting-molding cycle produced a dumbbell-shaped sample for tensile testing and a cuboid sample (80×10×4mm) with a B-type notch (depth of 2mm). The formulas of the blends were as follows: P-2.5C (POM/POE/GMA-g-HDPE, 97.5/2.5/1.25), P-5C (POM/POE/GMA-g-HDPE, 95/5/2.5), P-7.5C (POM/POE/GMA-g-HDPE, 92.5/7.5/3.75), P-10C (POM/POE/GMA-g-HDPE, 90/10/5), P-12.5C (POM/POE/GMA-g-HDPE, 87.5/12.5/6.25), P-15C (POM/ POE/GMA-g-HDPE, 85/15/7.5), and P-7.5 (POM/POE, 92.5/7.5).

We tested the morphology of the blends by scanning electron microscopy (SEM) and Fourier transform IR spectroscopy (FTIR). To prepare the samples for SEM testing, we fractured them at liquid nitrogen temperature and then sputtered a thin layer of gold onto them for better imaging. For the FTIR test, we compressed GMA-g-HDPE, P-7.5C, and P-7.5 into thin films using a tablet press before measuring them. Finally, we characterized the crystallization properties of the blends using differential scanning calorimetry (DSC), which we conducted under nitrogen atmosphere. We heated the samples from 40 to 200°C at 10°C/min, maintained them isothermally for 5min to eliminate the heat history effect, and then cooled them to 40°C at a rate of 10°C/min.

Figure 1 shows SEM images of the cryofractured surface of the samples. Pure POM is a brittle material and thus, when fractured at nitrogen temperature, it created debris. When POM was blended with POE, however, the distributed POE particles in the POM matrix absorbed energy by deformation, thereby increasing the resistance of the blends to fracture. The size of the dispersed phase increased as the content of elastomer increased. The particle number also increased with increasing elastomer content, from 0 to 7.5wt%, reaching a maximum at 7.5wt% content. Meanwhile, decreased interparticle distance led to increased impact strength. At a critical interparticle distance, when the content of the elastomer was relatively high, a sharp brittle-tough transition took place. The interface between the particles and matrix appears blurry in the SEM images due to enhanced compatibility between the POM matrix and the POE dispersed phase, enabled by GMA-g-HDPE. Additionally, we observed substantial agglomeration and broad particle-size distribution in the fracture surface of P-7.5 (compared with P-7.5C), attesting to the importance of compatibilizer in obtaining high-quality POM/POE blends.


Scanning electron micrographs of the cryofractured surface of (A) polyoxymethylene (POM), (B) P-2.5C (POM/POE/GMAg-HDPE, 97.5/2.5/1.25), (C) P-5C (POM/POE/GMA-g-HDPE, 95/5/2.5), (D) P-7.5C (POM/POE/GMA-g-HDPE, 92.5/7.5/3.75), (E) P-10C (POM/POE/GMA-g-HDPE, 90/10/5), (F) P-12.5C (POM/POE/GMA-g-HDPE, 87.5/12.5/6.25), (G) P-15C (POM/POE/GMA-g-HDPE, 85/15/7.5), and (H) P-7.5 (POM/POE, 92.5/7.5). POE: Polyolefin elastomer. GMA-g-HDPE: Glycidyl methacrylate-grafted high-density polyethylene.

Figure 2 shows FTIR spectra of GMA-g-HDPE, P-7.5C, and P-7.5. GMA-g-HDPE has epoxy functional groups that are evident in the peaks at around 841 and 900cm−1.6 However, we observed no epoxy functional groups at 841 and 900cm−1 in the P-7.5C sample. Because the epoxy functional groups could only react with the terminal hydroxyl groups of POM in POM/POE/GMA-g-HDPE, we conclude that GMA-g-HDPE reacted with POM and formed a new graft copolymer.


Fourier transform IR spectra of (a) GMA-g-HDPE, (b) P-7.5C, and (c) P-7.5.

Figure 3 shows differential scanning calorimeter (DSC) curves of the samples. We calculated the POM crystalline fraction (Fc) in the blend system based on Equation 1.8

Our results indicate that the crystal structure remained unchanged with the incorporation of elastomer and compatibilizer. The crystallinity of the POM/POE blends initially decreased from 75.68% for pure POM to 73.73% for P-7.5C, but increased to 74.87% for P-15C. All the POM/POE blends exhibited lower crystallinity than pure POM. We attribute this result to the fact that fewer POM chain segments were available for crystallization (because some chain segments adhered to the interface in the amorphous state and the gross chain segments were constant). Indeed, the SEM microstructures show that at a lower dosage (0–7.5wt%) of elastomer, the interfacial area of POM and POE increased with increasing elastomer content, in turn decreasing the crystallinity of the POM/POE blends. When the dosage of elastomer was over 7.5wt%, the interfacial area decreased owing to agglomerations, thus increasing the crystallinity of the POM/POE blends. However, the crystallinity of P-7.5 (75.24%) was only a little less than that of pure POM (75.68%). We attribute this result to weak interfacial adhesion in the absence of compatibilizer, and thus reduced interfacial adhesion of POM molecule segments in the amorphous state.


Differential scanning calorimeter curves of (a) POM, (b) P-2.5C, (c) P-5C, (d) P-7.5C, (e) P-10C, (f) P-12.5C, (g) P-15C, and (h) P-7.5.

In summary, we investigated the effect of the reactive compatibilizer GMA-g-HDPE on the morphological and thermal properties of POM/POE blends. Due to the reaction between the epoxy groups of GMA and the hydroxyl end groups of POM, GMA-g-HDPE showed good compatibility with the POM matrix, thus enhancing the properties of the composite. As a next step, we plan to experiment with other plastics as flexibilizers together with GMA-grafted copolymers as compatibilizers to toughen POM.


Authors

Wenqing Yang
The University of Tennessee

Wenqing Yang is a graduate student of materials science and engineering.

Xuan-Lun Wang
The University of Tennessee

Xuan-Lun Wang is a professor of materials science and engineering.

Xingru Yan
The University of Tennessee

Xingru Yan is currently a graduate researcher in Zhanhu Guo's Integrated Composites Laboratory. She is working in the area of polymer processing and multifunctional polymer nanocomposites.

Zhanhu Guo
The University of Tennessee

Zhanhu Guo is currently an associate professor of chemical and biomolecular engineering and director of the Integrated Composites Laboratory. His research focuses on multifunctional nanocomposites.


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DOI:  10.2417/spepro.006889