Résumé

Le quatrième chapitre est sous la forme d’un article scientifique rédigé en anglais et publié dans la revue Journal of Cellular Plastics en 2012, volume 48, numéro 4, pages 341-354. L’article présente les résultats de caractérisations obtenus à la suite de la fabrication de films cellulaires en utilisant l’approche qui y est décrite. En premier lieu, la méthodologie de fabrication est détaillée; extrusion, pressage, étirage bi-axial et traitement de pression. Ensuite, les résultats de caractérisations sont caractérisés discutés et les points majeurs à en tirer sont les suivants :

 La configuration de la vis est adéquate à la distribution et dispersion de carbonate de calcium de diamètre moyen de 3 µm dans une matrice de polypropylène;

 Diminuer la vitesse de refroidissement des films pressés de 1200°C/min à 0,78°C/min permet de faire augmenter la cristallinité des films de 43,5% à 49,4%;

 La méthodologie de fabrication a permis d’obtenir des films cellulaires avec une distribution de tailles avantageuses au faible ratio de densité de 0,8 et un haut taux de cristallinité, deux caractéristiques favorables à un éventuel effet piézoélectrique.

Morphology Development of Polypropylene Cellular Films for Piezoelectric Applications

Hugues Gilbert-Tremblay1,2_{, Frej Mighri1,2*, Denis Rodrigue}1,2,3

1 _{CREPEC, Center for Applied Research on Polymers and Composites}

2Department of Chemical Engineering, Laval University, QC, Canada, G1V 0A6

3 _{CQMF, Quebec Centre on Functional Materials}

* Correspondence author: Frej.mighri@gch.ulaval.ca

4.2 Abstract

In the present work, the dielectric nature of polypropylene and the softness of the cellular structure in the thickness direction of polypropylene cellular films were combined together to create a low cost and easily processable piezoelectric material. The effects of processing parameters, polymer crystallinity, filler type and concentration on the final structure of the cellular films were investigated. Three grades of calcium carbonate (CaCO3) filler having an average particle size of 0.7, 3 and 12 mm

were used. An optimized cellular film was developed by biaxial stretching and inflating a film made from three hot-pressed polypropylene sheets filled with 20 wt% of calcium carbonate particles with an average size of 12 µm. The cells have an average length of 35 µm and height of 4 µm with a density ratio of 0.8. Its particle size distribution compares favorably with those available in open literature.

4.3 Introduction

In recent years, cellular polypropylene (PP) thin films have attracted a lot of attention for their use as piezoelectric ferroélectrets. These 50–100 µm thick films are filled with gas cells of lengths reaching 100 µm and heights up to 10 µm. After proper electric charging, they present a good piezoelectric d33

coefficient up to 1200 pC/N at low frequency for single films [11], and up to 2010 pC/N when they are developed in multi-layer systems [26] [43]. Their piezoelectricity can be compared favorably to lead zirconate titanate (PZT) d33 coefficient, which is around 170 pC/N [46]. They are also far more

affordable and easier to produce [47]. They are promising multifunctional materials able to convert electric or magnetic field into mechanical movement and vice versa, and could be used to manufacture basic hardware like speakers, microphones, keyboards, as well as movement detection and security systems [9]. They could also be used in more technologically advanced systems such as

Piezoelectricity is not intrinsic to polypropylene cellular films like in conventional piezoelectric ceramics. Polymeric cellular films become piezoelectric after proper charging in an electrical field sufficient to create dielectric barrier discharge in the cells [16] [28] [34]. Because of the dielectric nature of polypropylene, charges are retained at the walls of cells after the discharge. Opposite electrical charges on each side of the cells create macroscopic electrical dipoles, which render the cellular film piezoelectric.

A lot of work has been accomplished on post-fabrication treatments like pressure inflation and charging parameters [31] [48] [30] [49]. However, all this work has been done on industrial cellular PP films, such as those produced by Du Pont USA [31], Nan Ya Plastics [49], Treofan [30] and VTT Processes [48]. To the best of our knowledge, very little information is available in literature on the processing conditions and parameters of the films themselves and how these parameters could affect the final cellular morphology and properties, which are primordial for the piezoelectric efficiency. Generally, cellular films are produced by stretching polymer extruded sheets filled with solid particles [21] [12]. During bi-axial stretching, filler particles, which are solid, act as mechanical stress concentrators and high stress regions propagate cracks surrounding these particles. Since mechanical stresses are applied in two directions, these cracks transform into surfaces and create cells around filler particles [20]. To ease the process, fillers with bad compatibility with the polymer matrix must be chosen. The most used material for cellular ferroélectret research is PP filled with CaCO3. PP is a good charge retaining dielectric material and has a good resistance to fatigue, which

is important for piezoelectric materials that will sustain several cyclic stresses. It is also inexpensive, easy to process and has well-known processing conditions. CaCO3 is used as filler because of its

availability, its low cost, and especially its prism shape, which is adequate for stress concentration. The quality of the film’s piezoelectric effect can be enhanced by various treatments. Nonetheless, the very own characteristics of the films, such as filler content and properties, crystallinity as well as stretching parameters cannot be neglected. The scope of this work is to study film processing parameters affecting the cellular morphology and film crystallinity in order to produce cellular PP films suitable for piezoelectric use. It provides a blueprint for researchers to produce cellular PP films with a wider range of characteristics than industrial films.

Aiming to develop a cellular film used in a wide range of piezoelectric applications that would be produced in large quantities, conditions and parameters were chosen to mimic industrial ones. The

final film must be filled with large cells without any bulk polymer zone. Larger cells (height up to 10 µm) are known to lower Young’s modulus the most. They are softer than smaller cells because of their easily compressible central area. The desired final cell must have a large surface area with an eye-shaped cross-section [23].

4.4 Experimental

4.4.1 Materials

PP homopolymer in pellets form (Pro-fax 6323), provided by Lyondell Basell Industries, was used in this study. The use of PP in pellets form was preferred over PP powder in order to be closer to industrial conditions where PP pellets are largely used over powders. Three industrial grades of CaCO3, provided by OMYA Canada, were used. Commercial names for these three grades are

Omyacarb UF, Snowhite 3 and Snowhite 12 with average particle size diameters of 0.7, 3.2 and 12 microns, respectively. Figure 4.4-1 shows the particle-size distribution of each grade provided by the manufacturer.

Figure 4.4-1 : Particle-size distribution (manufacturer data). 4.4.2 Film processing

A twin-screw corotating 18 mm extruder (Leistriz ZSE18HP-40D), a slit die and a calendaring system were used to produce sheets having a thickness between 0.30 and 0.35 mm. Screw configuration (Figure 4.4-2) was optimized to ensure a maximum mixing of the polymer matrix and the solid CaCO3

filler. Good dispersion and even distribution of CaCO3 filler are both very important to produce a

uniform cellular morphology. To achieve a good mixing, five sections of kneading blocks were added to the screws for a total of 12 kneading elements, nearly half of the total elements. A reverse kneading block was inserted at the end of the second kneading section in order to increase the pressure and to ensure complete melting and better initial mixing. Another reverse kneading block was inserted at the end of the fifth section in order to increase the residence time in the last intense mixing section. The first three kneading sections accomplish a more distributive mixing role while the last two, with disks having a 90° stagger angle, are more dispersive. The extruder has nine adjustable temperature zones, including the die. The temperature profile used for all film

compositions was: 165–180–185–190–190–190–190–190–185°C (from the feeding zone section to the die).

Figure 4.4-2 : Screws configuration.

The screw speed was set constant at 250 rpm and PP was added at the main hopper of the extruder at a rate of 3.33 kg/h. CaCO3 filler was also fed to the main hopper of the extruder using a

mixing of PP and CaCO3 prior to the extrusion since PP pellets and CaCO3 powder cannot produce a

stable homogenous mixture.

Compression molding was used to develop multilayer films from the extruded sheets. Square samples (85 x 85 mm2_{) were cut from the center of the extruded sheets and stacks of three such}

squares were hot-pressed in a 85 x 85 x 0.9 mm3 _{steel frame using a Carver 15 tons, 2 platens Auto}

Series press. Compression load was around 3175 kg (7000 lbs) for 2 min at 195°C, well over the melting point of PP (163°C). Also, Teflon sheets were inserted between each side of the frame and the platens to avoid sample sticking. The purpose of stacking multiple films is to have multiple layers of cells in the final cellular film. Having multiple layers of cells maximizes the use of the available film volume and also the area of cell walls needed for the storing of electrical charges. To maximize the piezoelectric signal, the whole film volume must be cellular so there is no bulk polymer non- contributing zone. Multiple cell layers also provide a cellular film with stable and flexible structure that could not be achieved by a single layer of bigger cells that could collapse under stress. Three different cooling ways were used in order to verify the effect of cooling rate on multilayer film crystallinity: (i) using fresh water at around 5°C circulating near the surface of the platens (36°C/min), (ii) by quenching the pressed samples right out of the press during 5 min between 2 mm thick steel plates previously cooled down in liquid nitrogen and (1200°C/min), and (iii) by letting the film cooled down in the press left under ambient conditions (0.78°C/min). The pressed samples were then stretched bi-axially using a Brückner Karo IV Labstretcher. Depending on the initial thickness of the sample, the stretching ratios in the machine and transverse directions varied from 1:3.8 to 1:4.2. Despite all samples being pressed in the same mold under the same pressure, initial sample thickness varied from 880 to 950 mm. The ratios were adjusted aiming to obtain a final thickness between 50 and 60 mm. Samples were preheated for 50 s at 155°C [19], then stretched at a speed of 3 m/min. After stretching, the samples were subjected to a pressure treatment to inflate the cells. Adequate pressure treatment can enhance the piezoelectricity [35]. The chosen treatment was the same as that proposed by Zhang [26]. The samples were put into a gas tight chamber where the pressure and temperature are controlled. Vacuum was applied for the first 5 min to evacuate air and mostly humidity, a nuisance to piezoelectricity [41]. Nitrogen pressure was then elevated to 15 bars at ambient temperature. After 3 h, the temperature was increased to 100°C for 1 h. Finally, the pressure was quickly released to atmospheric pressure and the sample was cooled down slowly in the press chamber left under ambient conditions.

4.4.3 Film characterization

Scanning electron microscopy (SEM) was done using a JEOL JSM-840A in order to study the quality of CaCO3 filler dispersion and also to observe cells morphology of the produced films. SEM scans of

the cellular film were analyzed with the ImageJ software. To avoid cell structure modification, the samples were first soaked in liquid nitrogen for 5 min before preparation for SEM observations. Polarized optical microscopy (POM), with a 40 X zoom and 25°C/min cooling rate, was also used to observe the crystalline zones in the polymer matrix. The crystallinity of PP/CaCO3

samples was evaluated by differential scanning calorimetry (DSC) using a Perkin Elmer DSC 7. Samples of 5–10 mg were heated at a rate of 10°C/min from 70°C to 195°C. The theoretical value of the melting enthalpy for 100% crystalline PP is 209 J/g [45]. It should be noted that the weight of CaCO3 filler was excluded during the calculation of PP/CaCO3 crystallinity.

Density characterization was done using a Quantachrome Ultrapyc 1200e gas pycnometer (with nitrogen). The reported values correspond to the average of a minimum of five measurements for each sample.