talc nanocomposite

Accepted Manuscript Lightweight and strong microcellular injection molded PP/talc nanocomposite Guilong Wang, Guoqun Zhao, Guiwei Dong, Yue Mu, Chul B. Park PII:

S0266-3538(18)31277-6

DOI:

10.1016/j.compscitech.2018.09.009

Reference:

CSTE 7391

To appear in:

Composites Science and Technology

Received Date: 28 May 2018 Revised Date:

6 August 2018

Accepted Date: 11 September 2018

Please cite this article as: Wang G, Zhao G, Dong G, Mu Y, Park CB, Lightweight and strong microcellular injection molded PP/talc nanocomposite, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.09.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Lightweight and strong microcellular injection molded PP/talc

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nanocomposite

Guilong Wanga,*, Guoqun Zhaoa,*, Guiwei Donga, Yue Mua, Chul B. Parkb,*

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials

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a

(Ministry of Education), Shandong University, Jinan, Shandong 250061, China Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and

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b

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Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3G8, Canada

* Corresponding authors.

E-mail addresses: [email protected] (G. Wang), [email protected] (G. Zhao), [email protected] (C. Park).

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ABSTRACT: Lightweight is of great significance for reducing material and energy consumptions. Microcellular injection molding is an advanced technology for fabricating

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lightweight plastic structural components, but the deteriorated mechanical performance is a big challenge. In this study, we reported a facile and scalable way to fabricate the lightweight and strong microcellular polypropylene/talcum (PP/talc) component. Both PP/talc microcomposite and PP/talc nanocomposite were prepared by the twin-screw

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compounding, and the SEM images show a uniform dispersion of talc. The DSC analysis results demonstrate that either the micro or nano talc is very effective in promoting the

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crystallization of PP. The rheological tests show that both the micro talc and the nano talc lead to obviously enhanced viscoelastic properties of the PP melt, while the effect of the nano talc is much more pronounced than that of the micro talc. Thanks to the enhanced crystallization and improved viscoelastic behavior, both the microcomposite foam and the

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nanocomposite foam shows much refined cellular structure than the pure PP foam. The PP/talc microcomposite foam shows significantly improved strength but seriously deteriorated toughness, compared with the pure PP foam. In contrast, the PP/talc

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nanocomposite foam shows simultaneously improved strength, rigidity and toughness. Notably, the tensile toughness and the Gardner impact toughness of the PP/talc

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nanocomposite foam are dramatically enhanced by 226.1% and 166.2%, respectively. Taking into account the flexible and scalable features of the processing methodology, the lightweight and strong PP/talc nanocomposite foam shows a promising future to replace the solid structural components in many industrial applications such as automotive and consumer electronics. Key words: microcellular injection molding; polypropylene foam; nanocomposite; lightweight; mechanical properties 2

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1. Introduction In the past few decades, the use of plastics has been growing rapidly, and plastics

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have now become one of the most widely used materials. Nowadays, the plastic products have become so ubiquitous that it is unimaginable for a world without plastics. At present, most of the plastics are still synthesized from fossil resources. Excessive consumption of fossil resources brings a series of environmental and ecological problems. At the same

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time, the overuse of fossil-based plastics also poses a potential threat to the ecological environment [1,2]. As there is still a long way to go for replacing the fossil-based plastics

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with the bio-based plastics, saving the plastic consumption by reducing the plastic product weight is currently a much more realistic method to release the problems. Moreover, lightweight has become an essential pursuit in many industries such as automotive, 3C (computer, communication and consumer electronics) and aerospace, for saving materials

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and reducing energy consumption, or meeting some other functional requirements. Polypropylene (PP) is one of the most widely used polymers in plastic industry because of its good comprehensive mechanical properties, its outstanding temperature

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and chemical resistances, its low density and easy processing, as well as its low cost [3,4]. According to a new report from Research and Markets, the global production of PP was

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56.44 million tons in 2016 and is estimated to reach 75.72 million tons by 2022 [5]. In this context, lightweight of the PP products is of great significance for reducing the consumption of plastic materials and fossil resources. Microcellular foaming using the green blowing agent such as carbon dioxide and nitrogen is a promising methodology for achieving lightweight of plastic products, because it can not only save materials and reduce weight, but also offer beneficial properties such as better thermal and acoustic insulation as well as superior dielectric properties [6,7]. 3

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Compared with the regular foaming process with the chemical blowing agent, microcellular foaming with the physical blow agent can produce plastic foams with much finer cells,

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which in turn offer better mechanical performance. Among the existing microcellular foaming technologies, microcellular injection molding shows many superiorities including its high efficiency and flexibility, its ability of directly molding complex component, and its product’s high shape and dimensional accuracy. However, to fabricate high-performance

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microcellular PP foams using microcellular injection molding is currently still very challenging due to the following reasons. First, PP is a linear hydrocarbon polymer that

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has poor melt strength and low viscoelastic properties. Thus, serious bubble collapse and coalescence phenomena generally occur at the rather high foaming temperature in microcellular injection molding, and they lead to an undesirable cellular structure and thus poor mechanical properties. Second, compared with the solid product, the foamed product

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usually shows inferior mechanical properties, particularly in ductility and toughness [8,9]. While the mechanical properties related to strength and modulus reduce linearly with the weight reduction, the mechanical properties related to ductility and toughness show an

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exponential decline with the weight reduction [10]. In addition, the microcellular injection molded product also suffers from serious surface defects such as silver streaks and swirl

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marks, which significantly limits its application. Modifying PP by blending, compositing or branching, combined with optimizing the cellular structure, is a critical methodology in improving the PP’s foaming ability and the PP foam's mechanical performance. The underlying mechanism for the improved foaming ability is that modification leads to increased melt viscoelasticity, enhanced melt strength and accelerated crystallization behavior, which in turn promotes cell nucleation and stabilizes cell growth. It has been demonstrated that the inorganic fillers such as talc 4

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[11,12], clay [12,13] and carbon fibers [14], and the organic fillers such as polytetrafluoroethylene (PTFE) [15], cellulose nanofibers [16] and sorbitol gelling agent

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[17], are all very effective in improving the PP’s foaming ability. In the earlier works [18–20], we have demonstrated that in-situ fibrillated polymer fibrils, such as polyethylene terephthalate (PET) fibrils and PTFE fibrils, are very effective in improving the PP matrix’s foaming ability and the PP foam’s mechanical properties. Significant reduction in

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toughness is a key challenge for the foamed components. It was reported that the ductility and toughness of the injection molded PP foam can be dramatically improved by

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achieving a special microcellular structure with a sub-micron scale immiscible secondary phase uniformly dispersed in the primary polymer matrix [9,21]. The mechanism underlying the phenomenon is that the special structure changes the fracture behavior form crack propagation across the matrix into shear yielding [21]. In a previous study, it

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was reported that the benzene trisamide-based nucleating agents were very effective in refining the PP foam’s cellular structure and enhance the PP foam’s mechanical properties in the foam injection molding process using chemical blowing agents [22]. The

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mechanism underlying this is that the needle-like benzene trisamide crystals can dramatically promote PP crystallization and improve melt strength, and thus lead to the

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refined cellular structure and enhanced mechanical properties of PP foams [22,23]. The talc-filled PP is versatile composite due to the relatively good mechanical performance combined with low cost, and it has been widely used in plastic industry. Talc is a good nucleating agent to promote PP crystallization and thus offers relatively high strength and stiffness [24–26]. In the perspective of foaming, talc has a very positive effect to improve the foaming ability of PP because of the enhanced crystallization behavior and melt strength [27–29]. While the strength and stiffness are enhanced by adding talc, the 5

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ductility and toughness are significantly decreased, especially for the foaming case [30]. Despite there are some researches about how to improve the mechanical properties of the

mechanical performance of the foamed PP/talc composite.

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unfoamed PP/talc composite, there is still seldom study about how to improve the

Towards fabricating lightweight and strong PP foam components, we conducted an experimental study to investigate the foaming behavior and mechanical properties of the

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PP/Talc micro/nano composites in foam injection molding process. To achieve the objective, the PP/talc microcomposite and nanocomposite were first prepared by

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twin-screw compounding, and the coupling agents were used to improve the talc dispersion. Second, the scanning electron microscope (SEM) was employed to check the PP/talc composite morphology. Afterwards, the differential scanning calorimeter (DSC) and the rheometer were used to investigate the effect of micro/nano talc on the

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crystallization and rheology behaviors of PP melt. Finally, foam injection molding experiments were conducted to prepare the pure PP foam and the PP/talc composite foam, and their mechanical properties were characterized by conducting tensile and

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impact tests. The prepared PP/talc nanocomposite foam shows outstanding mechanical properties characterized by the simultaneously enhanced strength, rigidity and toughness.

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2. Experimental section

2.1. Materials and Composite Preparation A commercial grade of linear PP (5703P, homopolymer) in pellet form with a melt flow index (MFI) of 10 g/10 min (230°C, 2.16 kg) was sup plied by the SABIC Innovative Plastics Canada Inc. The micro talc and nano talc were both provided by the Nippon Talc Company Limited, Japan. The micro talc had an average size of ~6.5 µm and the nano talc had an average size of ~350 nm. The graft polymer, maleic anhydride-grafted 6

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polypropylene (PP-g-MAH, CMG 5001), was supplied by Nantong Sunny Polymer New Materials Technology Co., Ltd., China, and the maleic anhydride content of PP-g-MAH

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was 0.5 wt%. A co-rotational twin-screw extruder (Trade name: TEM-26SS), manufactured by Toshiba Machine Co. Ltd., Japan, was used to prepare the PP/talc composites. The screw diameter and length-to-diameter ratio of the extruder were 26 mm and 40-fold,

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respectively. The additions of the micro talc and nano talc were 8.0 wt% and 3.0 wt%, respectively. The additions of PP-g-MAH in the two PP/talc composites are all 2.0 wt%. In

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prior to compounding, all the materials were dried at 140°C for 4 hours in order to remove the moisture. After drying, the materials were prematurely mixed by dry blending according to the formula. Thereafter, the mixture was added into the twin-screw extruder through the hopper for compounding. The compounding temperatures for the hoper zone

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to the die were 50, 150, 170, 180, 190, 190, 190, 190, 190, 180, and 170°C, respectively. The compounding discharge rate and screw speed were 20 kg/h and 200 rpm, respectively. The molten blends extruded from the die were shaped into a cylindrical stand,

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then drawn into a water bath for cooling, and finally pelletized in the cutting chamber. 2.2. Composite Morphology Analysis

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A scanning electron microscope (SEM, JSM-6060, JEOL, Japan) was used to analyze the morphology of the prepared PP/talc composites. To prepare the SEM specimen, the cylindrical PP/talc sample with a diameter of about 3 mm was first immersed into the liquid nitrogen for at least 10 min, and then the sample was broken using pliers. Finally, the cryofractured surface was coated with a thin layer of platinum using a sputter coater to prevent charging under the electron beam in SEM.

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2.3. DSC Analysis A TA-Q2000 differential scanning calorimeter (DSC) was used to investigate the

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non-isothermal crystallization behavior of the pure PP and the PP/talc composites under one atmospheric pressure. For measurement, the material of about 10 mg was firstly packaged using an aluminum pan and cap. Then, the measurement was carried out by putting it into a chamber under nitrogen atmosphere. The sample in the chamber was

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heated from 25 to 210°C at a rate of 10°C/min, and the temperature was remained at 210°C for 5  min to remove the sample’s thermal history completely. Afterwards, the

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chamber were cooled to 25°C at a rate of 10°C/min t o acquire the exothermic peak resulting from crystallization. Thereafter, the chamber was again heated to 210°C at a rate of 10°C/min to attain the endothermic peak caused b y crystal dissolution. To complete the measurement, the chamber was finally cooled to 25°C at a rate of 25°C/min. Based on the

the following equation:

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thermograms acquired in the cooling stage, the relative crystallinity, Xc, was calculated by


 = 100 ×  ⁄∆ × 

(1)

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where Hc is the measured enthalpy of crystallization, ∆Hf is the enthalpy of 100% crystalline (209 J/g), and Wp is the weight fraction of PP in the composite.

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2.4. Rheology Analysis

The ARES-G2 rotational rheometer, made by TA instruments, USA, was employed to characterize the rheological behavior of the pure PP and PP/talc composites. The dynamic frequency sweep (DFS) test and the dynamic temperature step (DTS) test were carried out to characterize the rheological behaviors as a function of frequency and as a function of temperature, respectively. For the DFS test, the dynamic strain sweep test was

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first carried out to identify the material’s strain limit of the linear viscoelastic region, and then the suitable preset strain used in the DFS test was determined to be 2%. Afterwards,

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the DFS test was conducted to measure the complex shear viscosity (η*), the storage modulus (G′) and the loss modulus (G″) over a frequency range from 0.1 to 100 rad/s at 190ºC. Finally, the DTS test was conducted with the temperature decreasing from 220 to 110ºC under a frequency of 0.63 rad/s.

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2.5. Foaming Process and Conditions

The low-pressure foam injection molding process was conducted to fabricate the

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foams using a 50-ton Arburg Allrounder 270 injection molding machine (ARBURG, Germany) equipped with the Mucell SCF delivery system (Trexel Inc., USA). The injection mold has a rectangular mold cavity (132 × 108 mm) of 3.0 mm thickness, a cold sprue and a fan gate, as Fig. 1 shows. For foaming, nitrogen (N2), was used as the blowing agent. In

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prior to formal experiments, a series of trial experiments had been conducted to explore the optimum foaming conditions. For gas injection, the injection pressure and dosage were set to be 20 MPa and 0.3 wt%, respectively. The melt temperature and mold

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temperature were 180 and 170ºC, respectively. A shot size of 45 cm3 was used to achieve a weight reduction of 20%, and a high injection speed of 50 cm3/s was used to increase

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pressure drop rate and thus to enhance cell nucleation.

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2.6. Foam Structure Characterization

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Fig. 1. The cavity structure of the injection mold used in foam injection molding.

The same SEM equipment described in section 2.2 was used to observe the foam’s cellular morphology. To ensure an intact cellular structure, a small piece cut from the foamed sample was first immersed into the liquid nitrogen for 10 min, and then it was

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broken with pliers. Thereafter, the cryofractured surface was coated with a thin layer of platinum using a sputter coater to eliminate sample charging. In order to fully display the internal cellular structure of the foamed sample, the specimens used for the SEM

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observation were taken from three different positions of the foamed sample, as Fig. 1

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shows.

With the obtained SEM images, a commercial software, Image-Pro Plus, was used to count the cell number and measure the cell size. At least, two hundred cells are used for calculated the average cell size. Based on the counted cell number, n, in a specific area, A, the cell population density, N, was calculated with the following equation:  ⁄

= 

(2)



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2.7. Mechanical Property Measurement A computer controlled electronic universal testing machine (5940, Instron, USA) was

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used to conduct the tensile test under the standard environment, according to the ASTM D638. In measurement, the speed of stretching was set to be 10 mm/min. The standard specimens used in measurements were punched from the foamed sample using a cutting die. For each case, at least ten samples were tested, and the average value was reported.

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A Gardner type impact tester (G0001, IDM Instruments, Australia) was used to characterize the impact strength of the foamed sample according to the ASTM D4226. In a

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normal test procedure, a 3.6 kg striker was released from a predetermined height to drop onto the top of the impactor, forcing a nose of specified radius through the test sample. In prior to the formal test, a number of trial tests were first carried out to bracket the pass/fail energy level and the corresponding reasonable drop height. Based on the predetermined

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drop height, a series of 20 formal tests were conducted. In the formal tests, if a test sample passes, the drop height will be increased by one unit, or else, the drop height will be decreased by one unit. The results from the formal tests were used to calculate the mean

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failure height, the point at which 50% of the test samples would fail under the impact. In prior to any testing, all of the samples were vacuumed for over one week, and were

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then left for at least one month in standard environmental conditions to ensure that air was the only gas in the foam, and to avoid the effects of environmental fluctuations on the testing results.

3. Results and discussions 3.1. Composite Morphology Fig. 2 shows the morphology of the pure PP and PP/talc composites. Thanks to the coupling agent, PP-g-MAH, which helps to improve the compatibility of the talc with the PP 11

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matrix, the PP/talc micro and nano composites all show a very uniform dispersion of the talc particles. For the nanocomposite, the average size and thickness of the nano talc are 320 nm and 90 nm, respectively, and the talc’s population density is about 2.4 × 1012

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particles/cm3. Regarding the microcomposite, the average size and thickness of the micro are 4.8 µm and 0.52 µm, respectively, and its population density is about 8.3 × 1010 particles/cm3.

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It was expected that the uniformly dispersed talc would play a positive role in two aspects for fabricating the lightweight and strong PP foams. First, as a reinforcing phase,

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the talc could improve the rigidity and strength of the PP matrix. Compared with the micro talc, it was believed that the nano talc should be much more effective in improving the PP matrix’s mechanical properties because of its much larger specific surface area. At the same time, the talc was also used as heterogeneous nucleating agents which would

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improve the foaming ability of the PP matrix. The improved cellular morphology would in

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turn benefit the weight reduction and mechanical properties of the PP foams.

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Fig. 2. The pure PP morphology under different magnifications: (a) ×1000, (b) ×2000, and (c) ×5000, the nanocomposite morphology under different magnifications: (d) ×1000, (e) ×2000, and (f) ×5000, and the microcomposite morphology under different magnifications:

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(g) ×1000, (h) ×2000, and (i) ×5000.

3.2. Crystallization Behavior

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Crystallization plays a pivotal role in foaming for the semi-crystalline polymers [31–34]. Generally, improved crystallization leads to enhanced cell nucleation and increased

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expansion ratio of foams [35–37]. Thus, it is essential to investigate the crystallization behavior of the pure PP and its composites for clarifying the foaming behavior. Fig. 3a plots the thermograms of the pure PP and PP/talc composites in the cooling process at a cooling rate of 10ºC/min. It is observed that the presence of either the micro talc or the

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nano talc increases the onset and peak temperatures of crystallization by about 5ºC. It clearly demonstrates that both the talc is a good nucleating agent in promoting crystallization and increasing the nucleation rate. Compared with the micro talc, it seems

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that the nano talc is more effective in promoting crystallization due to the corresponding higher peak temperature of crystallization. Basically, crystallization at a higher

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temperature, as the molecular chain mobility is much stronger, means that the crystal structure is more perfect [38,39]. This in turn will definitely benefit the material’s mechanical properties, particularly considering the refined crystal structure. According to the cooling thermograms in Fig. 3a, the degrees of crystallization of the pure PP, the PP/talc microcomposite and the PP/talc nanocomposite are calculated to be 37.1%, 36.5% and 37.0%, respectively. The slightly decreased degree of crystallization with the presence of talc is attributed to the fact that a large quantity of talc and hence the 13

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increased crystals restrict the full growth of crystals.

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Fig. 3. The DSC curves of the pure PP and the PP/talc composites: (a) exotherms at a cooling rate of 10 ºC/min, and (b) endotherms at a heating rate of 10ºC/min.

Fig. 3b shows the thermograms of the pure PP and PP/talc composites in the heating process at a heating rate of 10ºC/min. It is noticed that the PP/talc nanocomposite has the

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highest melting peak, followed sequentially by the PP/talc microcomposite and the pure PP. The result confirms that the talc induces more perfect crystals that shows higher melting temperature. Moreover, it can be seen from Fig. 3b that the presence of talc leads

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to a much wider melting peak, and this phenomenon is more pronounced for the nano talc.

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It is believed that the wider melting behavior is beneficial for broadening the processing window for foaming [40–42]. 3.3. Rheological Behavior

The rheological behavior also shows a significant effect on the foaming of polymers. Generally, an enhanced viscoelastic performance, including the increased storage modulus, the reduced loss tangent, and the improved melt strength, benefits the foaming of polymers. To clarify the effect of talc on the foaming behavior of the PP matrix, it is thus

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necessary to investigate the rheological behavior of the pure PP and the PP/talc composites. Fig. 4a–d show the responses of the storage modulus (G′), the loss modulus

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(G″), the loss tangent (tan δ) and the complex viscosity (η*) as a function of the shearing frequency from 0.1 to 100 rad/s. From Fig. 4a&b, it is observed that both G′ and G″ increases significantly with the presence of the talc, and the nano talc is much effective in raising G′ and G″ than the micro talc, particularly considering the much less addition of the

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nano talc than that of the micro talc. It is worth noting that the increase of the storage modulus is much more pronounced than that of the loss modulus, and hence tan δ, equal

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to G″/ G′, is decreased in the presence of either the micro or the nano talc, as Fig. 4c shows. Actually, tan δ is an important analysis criteria in deciding the polymer melt’s viscoelastic response behavior. For pure PP, tan δ, much larger than 1.0 at low shearing frequencies, reduces obviously with the increase of the shearing frequency, and this

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phenomenon indicates the pure PP melt behaves like a liquid with relatively weak elasticity. With the presence of talc, tan δ is dramatically decreased particularly at low frequency range, and it becomes almost independent of the shearing frequency over the

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whole range, in particular for the case of nano talc (Fig. 4c). With the nano talc, tan δ can even reduce to less than 1.0, which means the sol-gel transition point has been achieved,

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and thus the PP/talc melt behaves more like a solid rather like a liquid any more. In Fig. 4d, it can be found that η* reduces gradually with increasing in the shearing frequency for all the materials, because of the shear-thinning effect. The addition of talc leads to an obvious increase of η* because the rigid talc can restrict the soft molecule mobility.

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1.0E+04 104

1.0E+04 104

Pure PP PP/nano-talc PP/micro-talc

1.0E+02 102 1.0E+01 101 0.1

1

1.0E+03 103 1.0E+02 102 1.0E+01 101 100 0.1

10

Frequency (rad/s)

d 105 1.0E+05 Pure PP PP/nano-talc PP/micro-talc

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1.0E+04 104

0 0.1

1

10

Frequency (rad/s)

100

Pure PP PP/nano-talc PP/micro-talc

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1.0E+03 103

1

10

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5

tan δ

1

Frequency (rad/s)

η* (Pa·s)

c

Pure PP PP/nano-talc PP/micro-talc

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1.0E+03 103

G″ (Pa)

1.0E+05 b 105

G′ (Pa)

a 105 1.0E+05

1.0E+02 102 0.1 100

1

10

100

Frequency (rad/s)

Fig. 4. The rheological behavior of the pure PP and the PP/talc composites at 190ºC over

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a shearing frequency from 0.1 to 100 rad/s: (a) the storage modulus, G′, (b) the loss modulus, G″, (c) the loss tangent, tan δ, and (d) the complex viscosity, η*.

To investigate the polymer’s rheological behavior as a function of temperature, the

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temperature sweep test was conducted, with the temperature reducing from 220 to 100ºC

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at a rate of 2ºC/min. Fig. 5a–d plot the responses of the storage modulus (G′), the loss modulus (G″), the loss tangent (tan δ) and the complex viscosity (η*) as a function of the temperature. It is observed that G′, G″, and η* increases sharply while tan δ decreases sharply as the temperature reduces to a certain critical value for each material. This sudden change is owing to the crystallization of polymer melt. For the PP/talc composite, the critical transition temperature is about 10ºC higher than that of the pure PP. This phenomenon indicates that the PP/talc composite melt can crystallize at a much higher

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temperature than the pure PP melt due to the heterogeneous crystal nucleation, and it agrees with the DSC data, as Fig. 3a shows. Compared with the pure PP melt, the PP/talc

PP/talc

nanocomposite

shows

higher

viscoelastic

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composites shows a higher G′, a higher G″, and a higher η* before crystallization, and the properties

than

the

PP/talc

microcomposite. These findings are consistent with the oscillatory testing data, as Fig. 4 shows. From the Fig. 5c, it is noticed that the presence of talc can obviously reduce the

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loss tangent of the PP melt, and at the same time decrease the dependence of the loss tangent on the temperature. Notably, the effect of the nano talc is much more pronounced

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than the micro talc in reducing the tan δ, and this should be related with the larger specific surface area of the nano particles. In particular, the tan δ of the PP/talc nanocomposite melt before crystallization maintains at an almost constant value that is less than 1 over the temperature range from 220 to 140ºC, and hence it once again clearly demonstrates

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that the viscoelastic response behavior of the PP/talc nanocomposite has transferred from

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the liquid-like style to the solid-like style.

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a 107 1.0E+07

1.0E+06 b 106 Pure PP PP/nano talc PP/micro talc

G′

1.0E+05 105

Pure PP PP/nano talc PP/micro talc

1.0E+05 105

G″

1.0E+06 106

1.0E+04 104 1.0E+03 103

1.0E+03 103 1.0E+02 102 100

120

140

160

180

200

1.0E+02 102 220 100

120

Temperature (ºC)

d 107 1.0E+07

5 Pure PP PP/nano-talc PP/micro-talc

η* (Pa·s)

1.0E+06 106

3

1.0E+05 105

2

1.0E+04 104

1

180

200

220

Pure PP PP/nano-talc PP/micro talc

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tan δ

4

160

Temperature (ºC)

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c

140

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1.0E+04 104

1.0E+03 103

0 100

120

140

160

180

Temperature (ºC)

200

1.0E+02 102 220 100

120

140

160

180

200

220

Temperature (ºC)

Fig. 5. The rheological behavior of the pure PP and the PP/talc composites at 190ºC over

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a shearing frequency from 0.1 to 100 rad/s: (a) the storage modulus, G′, (b) the loss modulus, G″, (c) the loss tangent, tan δ, and (d) the complex viscosity, η*.

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Overall, adding the plate-shaped talc particles leads to significantly enhanced viscoelasticity of PP melt, particularly at relatively low frequency range. The mechanism

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underlying this phenomenon is that the added large amount of talc particles, acting as physical bonding node, significantly increase the entanglements and movement resistances of polymer chains. At a certain critical concentration, the added particles in PP melt can even form a kind of spatially network structure, which can largely prevent the relaxation of polymer chains at low frequencies, and hence enhance the elasticity of polymer melt. As the nano talc particles have much larger specific areas than the micro talc particles, they are more pronounced in improving the viscoelasticity of PP melt. 18

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3.4. Foam Morphology Microcellular injection molding experiments were conducted using the pure PP and

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the PP/talc composites to check their foaming ability. Fig. 6 shows the comparison of the foam’s cellular morphology at different locations of the foamed sample. Compared with the pure PP foam, the PP/talc composite foams show obviously refined cellular structure with reduced cell size and increased cell number. Moreover, the pure PP foams suffers from a

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serious non-uniform cellular structure at different regions of the sample. With the presence of talc, the uniformity of the foam’s cellular structure is greatly improved, in particular for

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the nano-scale talc case.

Fig. 6. The cellular morphology of the pure PP foam and the PP/talc composite foam

To quantitatively compare the cellular structure, the cell density and cell size of the prepared foams were measured, and the results are given in Fig. 7. It can be clearly seen

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that the cell density was increased by several folds while the average cell size was obviously decreased from around 100 µm to about 50 µm by adding talc into the PP matrix.

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For the pure PP foam, the cell density reduces gradually while the cell size increased continuously as the sampling location becomes farther from the injection gate. It indicates that the cellular morphology of the pure PP foam is very non-uniform. With the addition of talc, the changes of both cell density and cell size with the sampling location reduce

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obviously, which confirms that the cellular structure becomes more uniform, particular for the PP/talc nanocomposite. It is expected that the dramatically improved cellular

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morphology will obviously benefit the foam’s mechanical properties [43,44].

Fig. 7. The quantitative cellular structure information of the pure PP and PP/talc foams: (a)

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cell density, and (b) cell size.

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The refined cellular structure of PP foams by adding talc particles can be owing to the following reasons. First, the plate-shaped talc particles, acting as heterogeneous nucleating agents, can reduce the energy barrier of cell nucleation, and hence increase cell nuclei [45,46]. Second, adding talc particles leads to enhanced viscoelasticity of PP melt, and hence increases local energy variation, which in turn also promotes cell nucleation [47,48]. Third, talc particles can improve PP melt strength, and hence lead to reduced cell coalescences and collapses [49, 50]. 20

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3.5. Mechanical Properties The tensile tests were conducted to evaluate the mechanical properties of the

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prepared foams. Fig. 8a plots the stress-strain curves obtained in the tensile tests. Totally, the presence of either micro talc or nano talc leads to the improved strength and rigidity of the foam. This can be owing to the fact that the rigid and strong phase of talc inhibits the extension, conformational transition and movement of the polymer chain through the

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pinning effect. It is noticed that the PP/talc microcomposite foam shows the best tensile strength (Fig. 8a) and rigidity (Fig. 8c). Compared with the pure PP foam, the tensile

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strength and modulus are increased by 12.4% and 20.6%, respectively. In contrast, for the PP/talc nanocomposite foam, the tensile strength and modulus are improved by 9.1% and 15.6%, respectively, compared with the pure PP foam. The more pronounced effect of the micro talc in improving the strength and modulus are owing to the following reasons. First,

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the addition of the micro talc (8 wt%) is much higher than that of the nano talc (3 wt%). Moreover, as the talc size reduces from the micro scale to the nano scale, it becomes less rigid and more flexible. In addition, the addition of talc refines the cellular structure, which

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in turn helps to improve the mechanical properties. Notably, compared with the pure PP foam, the elongation at break of the PP/talc

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microcomposite foam is obviously decreased, while that of the PP/talc nanocomposite is dramatically increased. It indicates that the foam’s ductility is decreased by adding the micro-scale talc, but increased by adding the nano-scale talc. In terms of the tensile toughness, which is an index to describe the ductility, the PP/talc microcomposite foam’s ductility is decreased by 18.7% while the PP/talc nanocomposite foam’s ductility is increased by 226.1%, as Fig. 8d shows. The reason for the deteriorated ductility of the microcomposite foam is that the talc size is so big that it seriously destroys the continuity 21

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of the PP matrix. Furthermore, the micro talc is so rigid that it may seriously inhibit the shear slipping of the polymer chain. In addition, the micro talc could result in severe stress

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concentration, which is also harmful to the toughness. In contrast, the above problems can be effectively mitigated by reducing the talc size. As the talc size reaches the nano scale, it tends to become more flexible and deformable, which helps to enhance the deformability of the composite. Moreover, the nano-scale talc can act as a role of lubricant, which

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benefits the shear slipping of the polymer chains and hence leads to the increased elongation at break. In addition, compared with the micro talc, the nano talc has much

and thus benefits the toughness.

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larger specific surface areas, which can consume more energy during plastic deformation

To evaluate the ductility of the injection molded foams at a relatively high deformation rate, the Gardner impact testing was conducted. Fig. 9 shows the measured Gardner

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impact strength of the pure PP foam and the PP/talc composite foam. It can be found that the PP/talc nanocomposite foam exhibits the best ductility, sequentially followed by the pure PP foam and the PP/talc microcomposite foam. The result is consistent with the

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tensile toughness data obtained in the tensile tests (Fig. 8d). Compared with the pure PP foam, the PP/talc microcomposite foam’s impact strength was decreased by 45.3%, which

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is much more pronounced than the reduction of the tensile toughness in the tensile testing. This is probably due to the fact that the stress concentration caused by the micro talc and its destructive effect on the matrix continuity become more serious under the higher deformation rate in the Gardner impact tests. In contrast, the PP/talc nanocomposite foam’s impact strength is 166.2% higher than the pure PP foam’s. It is worth mentioning that the PP/talc nanocomposite foam shows a very similar cellular structure with the PP/talc microcomposite foam, as Fig. 6 shows. Therefore, the sharply improved impact 22

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strength should definitely be correlated with the reduction of the talc size. As discussed above, the nano talc can refine the PP crystals, promote shear slipping of polymer chains

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and result in large interfaces, and all of these factors contribute to the obviously enhanced ductility. Based on both the tensile test and the Gardner impact test, it is clear that modifying the PP matrix with the nano-scale talc can lead to simultaneously enhanced

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strength and toughness of the PP foam.

Fig. 8. The tensile mechanical properties of the foams: (a) stress-strain curves, (b) tensile strength, (c) tensile modulus, and (d) tensile toughness.

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Fig. 9. The Gardner impact strength of the pure PP foam and the PP/talc composite foam.

4. Conclusions

In summary, we carried out a study to investigate the effect of talc on the foaming behavior and the foam’s mechanical properties in the microcellular injection molding. The PP/talc microcomposite and the PP/talc nanocomposite were prepared by the twin-screw

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compounding. The DSC characterization results demonstrate that both the micro talc and the nano talc are very effective in promoting the crystallization of the PP matrix, and lead to more perfect crystals. The rheological analysis data indicates that the PP melt’s

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viscoelasticity can be significantly increased with the presence of talc, particularly for the

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nano-scale talc. Thanks to the enhanced crystallization and viscoelastic properties, the introduction of either the micro talc or the nano talc leads to obviously refined foam structure with increased cell density and reduced cell size. Compared with the pure PP foam, the PP/talc microcomposite foam shows obviously improved strength and modulus, but seriously reduced ductility, probably because the micro talc results in serious stress concentration, poor shear slipping ability of polymer chains and a discontinuous polymer matrix. In contrast, the PP/talc nanocomposite foam

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shows simultaneously improved strength, rigidity and toughness. Notably, the tensile toughness and the Gardner impact toughness are enhanced by 226.1% and 166.2%,

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respectively. Thus, lightweight and strong microcellular injection molded PP/talc nanocomposite is achieved, which shows a promising future in many industrial applications such as automotive and consumer electronics. Acknowledgements

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The authors are grateful to the National Natural Science Foundation of China (NSFC, Grant No. 51405267), the Shandong Provincial Natural Science Foundation of China

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(Grant No. ZR2014EEQ017), the Young Scholars Program of Shandong University (Grant No. 2017WLJH23), the Fundamental Research Funds of Shandong University, and the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (Grant No. P2018-002) for the funding support.

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