Polymers for Advanced Technologies Volume 7 , p p . 743-748

A Novel Polypropylene Microporous

Wei Zhu, Xian Zhang, Chuntian Zhao, Wei Wu, Jianan Hou and Mao Xu* Polymer Physics Laborato y , Institute of Chemist y, Academia Sinica, Beijing 100080, China

ABSTRACT The structure and properties of a polypropylene micro- porous film prepared by biaxial stretching of non-porous polypropylene film of high p-cystal content were investigated. The porosity of these films can be as high as 3040%, and the average pore size was around 0.05 pm. The films were found to have the structure of a two-phase interpenetrating network; both the polypropylene and the pore regions were three-dimensionally continuous. The advantages of the biaxially stretched microporous films are the combination of high permeability to fluids with good mechanical properties and almost circular pore shape with narrow pore size distribution. The application of this microporous film for battery separators, filtration mem- branes and substrates of functional polymer composites is discussed.

KEYWORDS: microporous film; polypropylene; morphol- OSY

INTRODUCTION Membrane technology is one of the most attractive areas in modern science and technology. The basis of membrane technology is to prepare a variety of membranes with different special properties which may meet the needs of various applications. Micro- porous films are one of the basic and important membrane products.

Polypropylene is a low-cost polymer of large- scale production. Its high resistance to most of the

* To whom correspondence should be addressed. This paper has been presented at the Symposium on Construction of Measures for Environmental Safety Using Performance of Advanced Polymer in East Asia, Sappore, Japan, 2-6 October 1995.

CCC 1042-7147/96/08074346 0 1996 by John Wiley & Sons, Ltd.

conventional chemicals, good mechanical properties and moderate temperature range for application make it one of the ideal membrane polymers. It is supposed that polypropylene microporous films might be quite useful in many important applications.

Most of the polymeric membranes are now prepared by using the solution cast, or so-called phase inversion, method [ l l . The structure and properties of the membranes can be changed by controlling the composition of the solutions and the evaporation or subtraction of solvents in the gelation and solidification processes. However, the disadvan- tage of the solution method is that a large amount of waste solvents have to be dealt with, the reuse of which is usually very difficult. The solution method is also not convenient for polymers such as polypropy- lene which do not dissolve in conventional solvents.

A stretching method [2] has been developed for preparing microporous membranes. Polypropylene microporous films obtained by this method are now commercially available under the trade name Celgard. The preparation procedure involves the consecutive steps of uniaxial cold stretching and hot stretching a non-porous semicrystalline film.

The stretching method is economic and technolo- gically convenient because no solvents are required. However, only a uniaxial stretching method has been successful for polypropylene so far and, as a result, the pores are slit-like in shape, and the anisotropy of films in mechanical properties is obvious. The tensile strength in the lateral direction is very low, only about 10 MPa, so the films are very easy to split. These disadvantages limit the application in many impor- tant areas.

Efforts have been made in our laboratory to find a new route using a biaxial stretching technique for preparing polypropylene microporous film which may have sub-micrometer pore size and narrow size distribution, high permeability to gases and liquids

Received 28 December 1995 Revised 1 Februa y 1996

744 / Zhu ef al.

combined with good mechanical properties. We found it is possible to prepare these biaxially stretched polypropylene microporous films by using non-porous polypropylene films of high P-crystal content 131. In this work the structure and properties of polypropylene microporous films prepared in our laboratory by using a biaxial stretching technique and their possible applications are investigated and discussed.

EXPERIMENTAL Biaxially stretched polypropylene microporous films, Micpor, of thickness 25 pm were prepared in our laboratory. The technique used has been described elsewhere [31. A two-axis stretching tester made by Toyoseiki Co. Ltd was used to prepare specimens of different draw ratios. Commercially available poly- propylene microporous films, Celgard 2400, were also used for comparison.

The porosity of the films is defined as their volume content of pores and was determined according to the following equation:

where Vf is the gross volume of the film specimen which consists of two parts, the volume occupied by the polymer resin V, and the volume occupied by the pores Vp.

The pore size was determined by a combination of the bubble-pressure and the solvent permeability methods [41. An integral effluent flux-pore size curve can be obtained (Fig. 1). The average pore size Qav is defined as the size corresponding to 50% integral flux. The pore size distribution can be expressed by the differential form of the curve, or characterized quantitatively by a parameter R90, which is defined as:

where Q90 is the pore size corresponding to 90% integral flux as indicated in Fig. 1.

The permeation properties of the films were characterized by the permeation coefficient P for gases or liquids:

P = Qd/pAt (3)

where Q is the volume of the effluent permeated through a film of area A and thickness d in a time period t under pressure difference p across the film.

The tensile measurements were performed with a tester Instron 1122 at an extension speed of 10 cm/ min. Dumbbell specimens with a width of 5 mm were used, and their effective length was 3 cm. Film specimens of diameter 13 mm were fixed in a filter holder without a support screen for burst strength measurements.

A scanning electron microscope Hitachi S530 and a transmission electron microscope Hitachi H700 were used for morphological studies. For observa-

0 400 800 1200 160C /a11 SlZC

FIGURE 1. Integral flux versus pore size curve from bubble- pressure and solvent permeation measurements.

tions of the internal structure the specimens were first filled with crosslinked polyacrylic acid and then treated with lead nitrate.

RESULTS AND DISCUSSION The polymorphism of isotactic polypropylene is well known. There are three main crystal modifications: the monoclinic a form, hexagonal p form and the triclinic y form. It is accepted that the a modification is the thermodynamically stable one, and the commercial polypropylene grades have crystallized mostly in the a modification under the usual industrial thermal conditions. A small amount of p modification may form in company with the a modification under conventional thermal conditions, especially at higher supercooling [5,61. A high content of ,L? modification can be obtained under special crystallization conditions 17, 81, or using selective p- nucleating agents [9-121.

Usually an X-ray diffraction method is used to characterize the relative amount of the ,&crystals, and a K parameter was defined by the formula 1131:

K = H(@300)/[H(@300) + H(all0) (4) + H(a040) + H(a130)]

where the Hs are the heights of four main peaks in the 28 range of 10-20" (Fig. 2(a)). P(300) is the strongest peak for /?-crystals, and the other three are for a- modifications. Figure 2(b) shows the typical X-ray diffraction pattern of our polypropylene specimens with high @-crystal content. The K value in this case is larger than 0.9.

After stretching, the &crystalline polypropylene film becomes opaque due to the scattering of micropores. Figure 3 gives the typical dependence of porosity on the stretching ratio for these films. The porosity of the films increases rapidly at first, reaches a maximum and then keeps almost unchanged for a while and decreases with further stretching depend- ing on the technical conditions. Porosity as high as 30- 40% is usually obtained for these polypropylene microporous films. The permeability of the films to gases and liquids changes in a similar way. Figure 4 shows the dependence of the permeation coefficient of alcohol on the stretching degree. It was interesting to

A Novel Polypropylene Microporous Film / 745

E.00K n

VI

?; 4 .80

-. . . .. . . .. . 5 . n 0 10.nL4 2 0 . 0 0 39.u0 4 1 . a

FIGURE 2. Wide angle x-ra diffraction (WAXD) patterns of polypropylene samples with dikrent 0-crystal contents: (a) K = 0.44 and (b) K = 0.94.

find that the content of /3-crystal decreases with stretching (Fig. 5). It may imply that the existence of @crystal is really important and essential for pore formation, and it seems that the pore formation is based on the spending of P-crystals.

The scanning electron micrograph (Fig. 6) shows the surface structure of our microporous film prepared by biaxial stretching. It is clear that the pores on the surface are almost circular in shape and their size is in the range of hundreds of angstroms. They are not slit-like as in the case of uniaxially stretched samples. The internal structure of the films is quite complicated. A tortuous pore network is formed in these films. Figure 7 gives the transmission electron micrographs for sections along different

, 4. 0 2 . 0 3 . 0 4 . 0 5. c I’

STRETCH I NG RATIO

FIGURE 3. Dependence of porosity on the stretching degree.

0 2 . 0 3 . 0 STRETCH1 N G RATIO

4

FIGURE 4. Dependence of permeation coefficient of the film to alcohol on the stretching degree.

directions. In sections parallel to the film surface, a polypropylene fibril network was observed with pores separated by these fibrils (Fig. 7(a)). The size of the pore sections was mostly in the range of hundreds of thousands of angstroms. Figure 7(b) shows a section in the thickness direction of the film samples, and a layer structure was observed for both the polypropylene phase and the pore phase. The thickness of these layers was usually several thou- sands of angstroms. There were deviations in layer thickness in different local places. Some fibrils connecting the neighboring polypropylene layers and also some pore paths through the polypropylene layers can be observed. It means that both the polypropylene regions and the pore regions form three dimensional continuous networks in the film, so these films have the structure of a two-phase interpenetrating network (IPN). In studies of poly- mer blends and composites it is accepted that the IPN structure of the two components gives the optimized way to combine the useful properties of the components in their blends or composites. Our polypropylene microporous films are now really in this case.

Table 1 gives the general characteristics of these biaxially stretched polypropylene microporous films. The average pore size of these new polypropylene microporous films is usually about 0.05 pm. Data for a Celgard film of similar average pore size, the

2 0 6 .

0 .4

0.2 .

3

8 -

1 0 2 0 3. 0, 4 0 5 0 0.0 ‘ STRETCHIN(> RATIO

FIGURE 6. Dependence of the @-crystal content of the film on the stretching degree.

746 / Zhu et al.

FIGURE 6. Scanning electron micrograph of the surface of the biaxially stretching polypropylene microporous film.

Celgard 2400, are also given for comparison. The porosity of Micpor is equal to or larger than that of Celgard 2400, and it has an obviously narrower size distribution than the Celgard sample.

Other characteristic features of these biaxially stretched microporous films are the high permeability to gases and liquids and good mechanical properties. The permeation coefficient to nitrogen for Micpor is equal to or larger than that of the corresponding Celgard sample, and the mechanical properties of Micpor are rather uniform in all directions within the film plane. The tensile strength of the films is always larger than 60 MPa, which is about six times higher than the lateral tensile strength of the Celgard specimens. Micpor also shows a higher burst strength than Celgard.

It is clear that the advantages of this novel polypropylene microporous film are the combination of high permeability and good mechanical properties, high porosity and almost circular pore shape, the sub- micrometer pore size and narrow size distribution, its excellent resistance to acids, basis and most chemicals, moderate temperature range of application and easy sealing and bonding.

According to these specifications, Micpor may have many important end uses. For example, it can be used as battery separators, for filtration of gases and liquid specimens, and also as films for functional medical use and substrate of film composites.

The function and properties of a polypropylene microporous film that make it important for use as battery separators are the effective separation of

FIGURE 7. Transmission electron micrographs for sections parallel (a) and perpendicular (b) to the plane of the microporous film. The arrow indicates the thickness direction.

positive and negative electrodes, high permeability of ions and low internal electric resistance, high chemical stability and wide temperature range and uniform sub-micrometer pore size. The advantages over previous polypropylene films are obvious. Its good mechanical properties and narrow size distribu- tion make it reliable in battery production and application, that is to prevent mechanical break and particle migration and formation of fibrous crystals between the electrodes.

The properties and advantages of the films used for filtration are their sub-micrometer pore size,

TABLE 1 . General Characteristics of Polypropylene Microporous Films

Properties

Average pore size, (pm) Pore size distribution, R90 Porosity (%) Tensile strength, MD/TD (MPa) Elongation (%) Burst strength (MPa) Permeation coefficient (N2) (1 0-3 ml/cm/sec/atm

Micpor Celgard 2400 0.05 0.05 <2 >3

32-40 32 60-1 20 140/10 50-1 00 50/ 1 00

0.60 0.27 2.5-5.0 2.5

A Novel Polypropylene Microporous Film / 747

TABLE 2. Properties of Polyaniline (PANI/Polypropylene (PP) Composites

Properties PP substrate PANI/PP composite Pure PAN/ film

Tensile stren th (MPa) 60-1 20 60-1 00 110

Electrical conductivity (S/cm) - Young's rnohus (GPa) 0.4-1.2 0.7-1 .O 2.2 Elongation at break (%) 50-1 00 60-1 80 8

5-6 10

TABLE 3. Properties of Ion Exchange Membrane Composites with Micpor Substrate ~~ ~~~

Strong acidic Ion exchange membrane composite Strong basic anionic cationic

Thickness (prn) Exchange capacity (mmol/ ) Swelling in water, MD/TD 7x1 Water uptake (%) Electric resistivity (0 crn2)a Transport nurnberb Tensile strength (MPa)

30 60

<0.5/1 .O <2.0/4.0 20-25 30-40

0.96-0.97 0.96-0.98 >60 >70

3.0-3.2 3.2-3.5

3.0-3.5 1 .O-3.0

0.5N NaCl aqueous solution at 25°C. 0.5N/1 .ON NaCl aqueous solutions at 25°C.

narrow size distribution and good mechanical proper- ties. They are also supposed to be much cheaper than the conventional ultrafiltration membranes. The main use areas might be in biotechnology, and the pharmaceutical, food and electronic industries.

Another important and interesting application of these films is to make polymer composites. The continuous pore network ensures the formation of fine dispersed IPN structure, when the second component is introduced. The IPN structure gives composites the best combination of the properties of the two constituent components.

For example, it is possible to prepare composite damping tapes by introducing a high damping material. Figure 8 gives the dynamic mechanical data for a polybutyl acrylate/polypropylene compo- site; it has large loss tangent value in a wide temperature range. Of course, you can choose a polymer to match your needs.

Conductive polymers can also be introduced to fill the pore network for making electric conductive composite films. The results for polyaniline/poly- propylene composites are given in Table 2 [141. These composite films exhibit the mechanical properties of the original polypropylene films and are not brittle like the pure polyaniline. At the same time, the electric conductivity of these composites is almost as high as that of the pure conductive polymer speci- mens. They are of the same order of magnitude. It means that flexible, tough and highly electric conductive composites can be made on the bases of these microporous films.

Another example of composites based on these polypropylene microporous films is the ion exchange membrane. The advantages here are the high exchange capacity and high conductivity and very small degree of swelling by water, which ensures the maintaining of high selectivity of the ion exchange polymers (Table 3). In comparison with available

commercial products one important characteristic of these polypropylene-based ion exchange membranes is their thinness due to the excellent mechanical properties of the substrate.

i \\

1 emperaNre(T)

FIGURE 8. Dynamic mechanical data for a polybutyl acrylate/polypropylene composite.

748 / Zhu et al.

CONCLUSIONS The porosity of the polypropylene microporous film, Micpor, prepared by biaxial stretching of non-porous polypropylene film of high @-crystal content, can be as high as 3040%, and the average pore size is around 0.05 pm. These films have the structure of a two-phase interpenetrating network; both the polypropylene region and the pore region are three-dimensionally continuous. The advantages of these biaxially stretched films are the combination of high perme- ability to fluids with good mechanical properties and almost circular pore shape with narrow pore size distribution. Polypropylene microporous films with these specifications are promising in many important applications.

ACKNOWLEDGEMENTS We are grateful to the National Natural Science Foundation of China and the Science Foundation of Polymer Physics Laboratory, Academia Sinica, for supporting this work.

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