BLENDS OF POLY(PROPYLENE CARBONATE)/HYDROGENATED NITRILE BUTADIENE RUBBER: MORPHOLOGY AND THERMAL PROPERTIES

Ahmad Zohrevand, David Lepage, Dominic Rochefort, and Mickaël Dollé,

Department of Chemistry, University of Montreal, Montreal, QC Arnaud Prebé, Hutchinson Canada, Montreal, QC

Abstract

The morphology and thermal properties of poly(propylene carbonate) (PPC) and hydrogenated nitrile butadiene rubber (HNBR) blends obtained via a melt-mixing process were studied. Morphology of the blends with different compositions was observed by scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were performed to study miscibility and thermal stability of the blends. SEM image showed that PPC/HNBR blends are phase-separated at the microscopic scale and each phase showed characteristic Tg in DSC. The addition of HNBR is demonstrated as a mean to significantly improved thermal stability of PPC phase under air atmosphere.

Introduction

The development of high performance bio-based or biodegradable plastic materials called bioplastics has been receiving great attention from academic and industrial stakeholders to address environmental and economic issues related with the consumption of petroleum-based plastics. Improving or altering specific properties and physical characteristics of bioplastics are one of the important challenges for the researchers to extend their applications and to make them more competitive.

Poly(propylene carbonate) (PPC) (Figure 1) is a fully biodegradable aliphatic polycarbonate which is produced by copolymerization of carbon dioxide (CO2) and propylene oxide. Using CO2 as an important component of greenhouse gases to synthesize this polymer is another environmental and economic advantage of PPC. In addition to biodegradability and biocompatibility, it has excellent oxygen barrier properties and transparency. PPC is an amorphous polymer with glass transition temperature (Tg) around 20-45 °C depending on its molecular weight and regioregularity. This leads to brittleness at temperatures < 18 °C and poor dimensional stability at temperatures > 40 °C. Moreover, PPC is suffering from low thermal stability due to easy decomposition of cyclic carbonate at temperatures less than 200 °C. These drawbacks are the main limitation toward making high performance materials out of PPC and narrow its service and processing windows.

Figure 1. Chemical structure of PPC.

Among different approaches to improve physical

properties and thermal stability of PPC, melt blending with other polymers, reinforcing particles, and low-molecular-weight compounds is a simple and economic method which has been used widely. Many researchers have worked on blends of PPC with other biopolymers but few studies are reported using non-biodegradable ones [1-10]. Li et al. [7] recently reported effect of adding methacrylate-butadiene-styrene (MBS) as a core-shell rubber modifier on improving toughness and thermal stability of PPC. In another work [11], an environmental-friendly macromolecular plasticizer was added to PPC to decrease Tg, enhance toughness, and improve processability. Although efforts have been made to use PPC as plasticizer for rubbers [2, 3], rarely the effect of adding elastomers as modifiers to PPC has been investigated.

HNBR is an elastomer with excellent resistance to oils and chemically aggressive media. It has high thermal stability, high gas permeability, reasonable cost, and can be added to other polymers to modify their properties. In this work we studied effect of adding HNBR to PPC to modify the mechanical and thermal properties of the blend. Binary blends of PPC/HNBR were prepared by melt mixing in an internal mixer. In order to investigate miscibility of the components, and effect of HNBR on thermal stability of PPC, DSC and TGA analysis were performed. Morphology of the blends was also observed by SEM.

Experimental Materials

PPC (QPAC® 40) with density of 1.26 g/cm3 and Melt Flow Index (MFI) of 0.9 g/10 min (150°C and 2.16 kg) was purchased from Empower Materials Inc. (New Castle, DE). Based on the producer, QPAC® 40 has a weight average molecular weight (Mw) range between 150 and 350 kg/mole. HNBR (Zetpol® 2010L) with density of 0.95 g/cm3 and acrylonitrile (CAN) content of 36% was supplied by Zeon Chemicals Co. (Tokyo, Japan). According to the supplier, the elastomer has 96% degree of

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hydrogenation and Mooney viscosity (ML (1+4) at 100°C) of 50-65. All the resins were dried at 50°C for 3 h before use. Sample Preparation

Binary PPC/HNBR blends were prepared by melt mixing in an internal mixer (Scamex, Crosne, France) at 80°C and 10 rpm. Mixture of PPC granules and HNBR pieces were gradually inserted to the mixer and, mixing was stopped around 10 min after complete feeding when mixer torque had reached to a steady value. The blend samples were took out of the mixer and cooled down to room temperature. Table 1. Blend composition of PPC/HNBR binary blends.

Sample Code PPC [%wt] HNBR [%wt] B0 (Neat PPC) 100 - B5 95 5 B10 90 10 B20 80 20 B25 75 25 B50 50 50 B100 (Neat HNBR) - 100

Characterization

Morphology of the blends was observed by a field emission gun scanning electron microscope (FEG-SEM, JEOL 7600F FEG, Tokyo, Japan) operated at 2 kV. Before observation, the samples were cryo-microtomed (Cryostat CM1850, Leica Biosystems, Wetzlar, Germany) and covered by a sputter-coated gold layer.

TGA experiments were carried out using a Thermogravimetric Analyzer (Q500, TA Instruments, New Castle, DE) under air atmosphere. With the exception of the isothermal test, 5-10 mg of the samples were heated from 30 to 600 °C with rate of 10 °C/min under air. In isothermal TGA experiments, samples were heated up to 240°C using the same rate and kept at this temperature for 60 min.

DSC analysis was done using a calorimeter (DSC 1®, Mettler-Toledo, Greifensee, Switzerland) under nitrogen atmosphere. Samples were heated from -60 to 150 °C at a rate of 10 °C/min.

Results and Discussion

Figure 2 shows representative SEM images of PPC/HNBR blends at different compositions. As can be seen, the blends of two polymers have a two-phase morphology in which each component forms distinct domains with clear interface indicating immiscible blends. However, it should be noticed that there is no separation or cavities at the interface between PPC and HNBR phases showing a good adhesion between the phases. Figure 2 (a)

and (b) show that HNBR droplets are dispersed in the PPC matrix and, size of the droplets increases with HNBR content. At the composition of PPC/HNBR 50/50 %wt (Figure 2 (c)), the blend has a co-continuous morphology.

Figure 3 depicts DSC thermograms of the samples

recorded during heating at 10 °C/min. Both HNBR and PPC are amorphous polymers with glass transition temperatures (Tg) around -25.2 and 22.1 °C respectively and no melting peaks. DSC results shows that blend of PPC and HNBR in all the studied compositions have two separate Tg corresponding to the polymer components.

Figure 2. SEM micrographs of PPC/HNBR binary blends:

(a) 90/10, (b) 75/25, and (c) 50/50 %wt.

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Figure 3. DSC curves of the neat polymers and binary

blends. Table 2 reports Tg values corresponding to PPC and HNBR components of the blends. As can be seen, difference between Tg of PPC and HNBR phases of the blends increases by HNBR content. These are indicating that PPC and HNBR are immiscible which is in agreement with the morphology observed by SEM.

Figure 4 shows TGA thermograms and their derivatives for both neat polymer components as well as the binary blends. The 5% weight loss temperature (T5%) and decomposition peak temperature (Tmax) of neat PPC are 242.3 and 280.3 °C respectively. TGA analysis shows that adding HNBR significantly improves thermal stability of PPC under air atmosphere. For instance, adding 5 %wt of HNBR increased T5% and Tmax to 255.2 and 296.4 °C respectively. Further, thermal stability of the blends slightly increases by adding more HNBR. Thermal decomposition of PPC takes places via two main paths: i) unzipping or backbiting, and ii) random chain scission [12]. It has been reported that decomposition under air mainly follows chain scission path and, yields to combustion products [13]. In order to increase thermal stability of PPC, it has been blended with other polymers such as poly(vinyl alcohol) (PVA) which are able to form hydrogen bond with carbonyl groups or/and terminal hydroxyl groups of PPC [10]. This bonding between the Table 2. Tg corresponding to each component of the

samples analyzed by DSC. Sample Code Tg(HNBR)

[°C] Tg(PPC)

[°C] B0 - 22.1 B5 -31.2 20.5 B10 -31.0 22.8 B20 -31.0 24.4 B25 -30.7 24.6 B50 -28.0 29.3 B100 -25.2 -

Figure 4. (a) TGA thermograms and, (b) derivative TGA

analysis. polymers stabilizes the chains and increase PPC degradation temperature. Although similar interactions are possible with HNBR’s acrylonitrile moieties, it’s unlikely to be the sole mechanism for stabilization due to the importance of the phase separation (Figure 2). In addition to such chemical interactions, different mechanisms can be proposed to explain the improved thermal stability of the PPC/HNBR blends. These include the barrier effect of the more stable component (HNBR) which its domains form tortuous paths that can slow the transfer of decomposition products, its interaction with volatile products (e.g. reaction and adsorption), and any physical/chemical interaction which could hinder mobility of the other polymer component (PPC) molecular chain [14].

In order to gain a better insight on this thermal stabilization of PPC by HNBR, an isothermal TGA was carried out to avoid any kinetic complications during measurement (two phases may have different heat transfer rates). Figure 5 shows the results of this isothermal TGA run of neat PPC, neat HNBR, and B10 blend sample. As it can be seen, PPC shows a rapid mass reduction at 240°C, indicative of its fast decomposition rate. HNBR, on the

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Figure 6. Isothermal TGA thermograms of neat polymers

and B10 blend obtained at 240°C under air. other hand, does not show any significant change in mass with time under the same conditions. Interestingly, B10 sample which contains 10 %wt. of HNBR shows a much slower decomposition rate compared to neat PPC (B0). It shows that adding HNBR kinetically slows down decomposition of PPC. Better understanding about contribution of the aforementioned different mechanism is subject of our future studies.

Conclusions

In the present work, morphological and thermal properties of melt-processed PPC/HNBR binary blends were studied. SEM observations showed that, the blends have two-phase morphologies in which HNBR domain are dispersed into PPC. At PPC/HNBR 50/50 %wt morphology turns to a co-continuous one. Two distinct Tg corresponding to each polymer components and their shifts with HNBR content, realized by DSC, was attributed to the immiscibility of the blend. Decomposition temperature of the PPC phase increased with HNBR content.

Acknowledgment

Authors would acknowledge financial support provided by Natural Science and Engineering Research Council of Canada (NSERC), Hutchinson SA, and Total SA.

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