Microporous membranes obtained from PP/HDPE multilayer films by stretching
Introduction
Microporous membranes are commonly used in the separation processes such as battery separators and medical applications to control the permeation rate of chemical components. Due to the wide range of chemical structures, optimum physical properties, and low cost of polymers, they are the best candidates for the fabrication of microporous membranes.
Commercially available lithium battery separators are made from polyolefins such as polypropylene (PP) and polyethylene (PE). These materials are compatible with the cell chemistry and can be used for many cycles without significant degradation in properties [1]. Lithium (Li) batteries will generate heat if accidentally overcharged. Separator shutdown is a useful safety feature for restricting thermal reactions in Li-batteries [1], [2]. Shutdown occurs close to the melting temperature of the polymer, leading to pores collapse and restricting passage of current through the cell. PP separators melt around 160 °C whereas PE separators have shutdown temperature between 120 and 130 °C. If in a battery, the heat dissipation is slow, even after shutdown, the cell temperature may continue to increase before starting to cool [1]. Recently, manufacturers have started producing trilayer separators where a porous PE layer is sandwiched between two porous PP layers. In such a case, the PE layer has lower shutdown temperature while PP provides the mechanical stability at and above the shutdown temperature [1].
Three commercially available processes are used for making microporous membranes: solution casting (also known as extraction process), particle stretching, and dry-stretching [3]. In the extraction process, the polymeric raw material is mixed with a processing oil or plasticizer, this mixture is extruded and the plasticizer is removed through an extraction process [4]. In the particle stretch process, the polymeric material is mixed with particles, this mixture is extruded, and pores are formed during stretching at the interface of the polymer and solid particles [5]. Costly processes and difficulties in dealing with solvent and particle contaminations are main drawbacks of such methods. However, the dry-stretch process is based on the stretching of a polymer film containing a row-nucleated lamellar structure [6]. Three consecutive stages are carried out to obtain porous membranes by this technique: (1) creating a precursor film having a row-nucleated lamellar structure through shear and elongation-induced crystallization of the polymer having proper molecular weight and molecular weight distribution, (2) annealing the precursor film at temperatures near the melting point of the resin to remove imperfections in the crystalline phase and to increase lamellae thickness, and (3) stretching at low and high temperatures to create and enlarge pores, respectively [6], [7].
In fact, in this process, the material variables as well as the applied processing conditions are the key parameters that control the structure and the final properties of the fabricated microporous membranes [6]. The material variables include molecular weight, molecular weight distribution, and chain structure of the polymer. These factors mainly influence the row-nucleated structure in the precursor films in the first step of the formation of microporous membranes. According to Sadeghi et al. [8], [9], molecular weight was the main material parameter that controlled the orientation of the row-nucleated lamellar structure. The resins with high molecular weight developed larger orientation and thicker lamellae than those with low molecular weight. In our recent study [10], the addition of up to 10 wt% of a high molecular weight component to a low molecular weight resin enhanced the formation of the row-nucleated structure due to an increase in the nucleating sites. In Sadeghi et al. [11], a superior permeability was obtained by adding a small amount of a long-chain branched polypropylene (LCB-PP) to a linear polypropylene (L-PP). Recently [12], we investigated the effects of process conditions such as draw ratio (DR), air flow rate (AFR), and cast roll temperature on the structure of PP cast films and microporous membranes. A significant enhancement in orientation was observed by applying air cooling and increasing DR. An ordered stacked lamellar structure was seen for the films subjected to low air cooling whereas the films produced without air cooling showed a spherulitic structure.
There are two main industrial processes for the production of films: film blowing and cast film extrusion. It is well known that the thickness variation in blown films are considerably greater compared to cast films. For the preparation of porous membranes, obtaining a precursor film with good thickness uniformity is strongly recommended since any non-uniformity causes irregularities in the stress distribution in the following stretching process. In addition, compared to film blowing, cast film process has more flexibility in the supply of air cooling from both sides, leading to a more uniform lamellar structure in both surfaces.
Although a few authors have investigated the formation of porous membranes from polypropylene and high density polyethylene single layer films, no study has been performed on development of multilayer membranes using cast film process. In this study, following our previous studies [10], [12] and the ones conducted by Sadeghi et al. [8], [9], [11] on monolayer PP, we investigate the fabrication of microporous PP/HDPE/PP trilayer membranes using the cast film process. The role of process parameters and annealing on the shear and/or elongation-induced crystallization and orientation developed in the monolayer as well as the components in the multilayer cast films are examined and discussed. A detailed investigation of the structure, particularly in the cross-section, and performances of the trilayer microporous membranes has been carried out. In addition, the influence of the applied extension during the stretching steps is considered.
Section snippets
Materials
A commercial linear polypropylene (PP) and a commercial high density polyethylene (HDPE) were selected. PP5341E1 was supplied by ExxonMobil and had a melt flow rate (MFR) value of 0.8 g/10 min (under ASTM D1238 conditions of 230 °C and 2.16 kg). HDPE 19A was provided by NOVA Chemicals and had an MFR value of 0.72 g/10 min (under ASTM D1238 conditions of 190 °C and 2.16 kg). The main characteristics of the resins are shown in Table 1. The molecular weight and polydispersity index (PDI) of the HDPE
Rheological and film characterization
The complex shear viscosities as a function of frequency for the resins are shown in Fig. 1. The PP reveals larger viscosity compared to the HDPE in the Newtonian region (low frequencies) while the data cross over in the power-law region (high frequencies). It is well known that for the production of multilayer films the viscosity of the neat polymers should be close to each other in order to prevent instabilities and non-uniformities at the interface. In our case, the viscosities of the PP and
Conclusions
In this work, we have investigated the structure and performances of microporous membranes made from monolayer and trilayer films of the PP and HDPE. Our findings can be summarized as follows:
- •
Significant effects of cooling air flow rate (AFR), draw ratio (DR), and annealing on the crystal orientation of the PP and HDPE monolayer films as well as the components in the multilayer one were observed.
- •
At low AFR, the HDPE showed a twisted lamellar morphology whereas at high AFR an intermediate
Acknowledgements
Financial support from NSERC (Natural Science and Engineering Research Council of Canada) and from FQRNT (Fonds Québécois de Recherche en Nature et Technologies) is gratefully acknowledged. We also acknowledge the large infrastructure grant received from the Canadian Foundation for Innovation (Government of Canada and Province of Quebec), which allowed us to build the unique POLYNOV facility. We are also thankful to Messrs. P. Cigana, L. Parent and P.M. Simard for their technical help. Finally,
References (30)
- et al.
Solid State Ionics
(1994) - et al.
Analysis of microporous membranes obtained from polypropylene films by stretching
J. Membr. Sci.
(2007) - et al.
Microporous membranes obtained from polypropylene blend films by stretching
J. Membr. Sci.
(2008) - et al.
Effect of processing on the crystalline orientation, morphology, and mechanical properties of polypropylene cast films and microporous membrane formation
Polymer
(2009) - et al.
Crystallization kinetics of polypropylene: II. Effect of the addition of short glass fibres
Polymer
(1997) - et al.
An improved permanganic etchant for polyolefines
Polymer
(1982) - et al.
Characterizing co-continuous high density polyethylene/polystyrene blends
Polymer
(2001) - et al.
Probing nucleation and growth behavior of twisted kebabs from shish scaffold in sheared polyethylene melts by in situ X-ray studies
Polymer
(2007) - et al.
Biaxial orientation in HDPE films: comparison of infrared spectroscopy, X-ray pole figures and birefringence techniques
Polymer
(2005) - et al.
Oriented structure of PP/LLDPE multilayer and blends films
Polymer
(2005)
Flow-induced shish-kebab precursor structures in entangled polymer melts
Polymer
Oriented structure and anisotropy properties of polymer blown films: HDPE, LLDPE and LDPE
Polymer
The role of interlamellar chain entanglement in deformation-induced structure changes during uniaxial stretching of isotactic polypropylene
Polymer
Characterization of microporous separators for lithium-ion batteries
J. Power Source
Cited by (148)
Two layer film flow on an unsteady stretching cylinder
2024, Chinese Journal of PhysicsOne-step separation of fat globules based on size from bovine milk using a cross-flow microfiltration
2024, Separation and Purification TechnologyMicro/nano-structure skeleton assembled with graphene for highly sensitive and flexible wearable sensor
2023, Composites Part A: Applied Science and ManufacturingNanoadsorbents in nanofilter membrane
2023, Adsorption through Advanced Nanoscale Materials: Applications in Environmental Remediation