Wet-Laid Formation and Strength Enhancement of Alkaline Battery

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Wet-Laid Formation and Strength Enhancement of Alkaline Battery Separators Using Polypropylene Fibers and Polyethylene/ Polypropylene Bicomponent Fibers as Raw Materials Bingxu Zhang, Qingxi Hou, Wei Liu, Zhihui Liang, Bing Wang, and Honglei Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01664 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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Industrial & Engineering Chemistry Research

1

Wet-Laid Formation and Strength Enhancement of

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Alkaline Battery Separators Using Polypropylene Fibers

3

and Polyethylene/Polypropylene Bicomponent Fibers as

4

Raw Materials

5

Bingxu Zhang, Qingxi Hou*, Wei Liu, Zhihui Liang, Wang Bing, Honglei Zhang

6

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science & Technology, Tianjin

7

300457, P. R. China

8

ABSTRACT: The handsheets of alkaline battery separator were made of

9

polypropylene

(PP)

fibers

and

polyethylene/polypropylene

bicomponent

10

sheath-core structure (EP) fibers in the wet-laid process followed by a hot-pressing

11

treatment. The effects of the process parameters in making the separator

12

handsheets on the formation and the resultant tensile strength, maximum pore

13

size, porosity and alkali resistance were investigated. The results showed that a

14

favorable formation could be obtained at the EP fibers addition level of not less

15

than 20%. In addition, the tensile strength, maximum pore size, porosity and alkali

16

resistance of the separator handsheets made of 60% PP fibers and 40% EP fibers

17

could meet the needs required in QB/T 4173 (2011), under the conditions of 1.5%

18

PEO and 4.0% PVA additions (on oven-dry fibers), the hot-pressing pressure of

19

0.5 MPa, and hot-pressing temperature of 135 °C for 90 s.

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Key words：：Polypropylene fiber; Polyethylene/polypropylene bicomponent fiber;

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Wet-laid process; Alkaline battery separator; Hot-pressing

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■ INTRODUCTION

*

Corresponding author. E-mail address: [email protected] (Q.X. Hou) 1

ACS Paragon Plus Environment


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The main cause for developing various types of clean cars such as fuel-cell

24

and electric vehicles is an environmental issue. Among clean cars, a hybrid electric

25

vehicle (HEV) with a rechargeable battery is considered to be the most promising

26

and, therefore, has been receiving much attention.1 For these attractive HEVs,

27

however, high performance rechargeable batteries are necessary. So far almost all

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commercial HEVs have been equipped with alkaline batteries because of their

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better combination of output power, capacity, long life, reliability and cost.2-3 The

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function of an alkaline battery separator, which plays a vital role in the application

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of alkaline batteries, is to separate the positive and negative electrodes in the

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batteries from direct contact so as to avoid a short circuit, load the electrolyte

33

solution, and allow the conductive ions to pass through the cell. Therefore, the

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quality of the battery separator has a great effect on discharge behavior,

35

self-discharge rate and cycle life of an alkaline battery.4-5 It is necessary for an

36

alkaline battery separator not only to possess a fairly good chemical stability and

37

affinity with the electrolyte but also to have certain properties such as a high

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mechanical strength, suitable pore size and porosity.6

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One of the core technologies of an alkaline battery is the use of a synthetic

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fiber-based separator, which possesses the properties of high porosity, high

41

strength, and solvent and corrosion resistance, and so on. Furthermore, the

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synthetic fiber-based separator especially adapts alkaline batteries to large

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capacity, low resistance, large output power and long period of storage, therefore,

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synthetic fibers have become the important raw materials in production of alkaline

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battery separators.7-9

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Currently widely used alkaline battery separators are typically manufactured

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from polyolefin, predominantly polyethylene (PE) or polypropylene (PP).10-12 U.S. 2


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Pat. No. 3,615,865 to Wetherill discloses a battery separator comprising a

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nonwoven mat of polypropylene fibers bonded with polyacrylic acid.12 J.P. Pat. No.

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5,311,059 to Kubo discloses alkaline manganese batteries comprising a fibrous

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web having mixing rayon 1.5-denier and synthetic fiber 1.0-denier.13 When being

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used as raw material to make the separators by means of the traditional

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papermaking method used for plant fibers, PP fibers can only be interlaced with

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one another due to the absence of hydrogen bonding between the fibers. That is to

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say, there is no bonding force between PP fibers in the separator, resulting in a

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very low separator’s strength and a poor formation due to the lower density of PP

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fibers than that of water. However, PP fibers can be mixed with the low-melt fibers

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to make alkaline battery separators. When a separator made of PE and PP fibers

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is heated, the totally or partially melted PE fibers could act as binding ones to

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improve the separator’s strength.14 However, the totally melted PE fibers can lead

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to reducing in the separator’s porosity and subsequent battery quality. Among the

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low-melt fibers, polyethylene/polypropylene bicomponent sheath-core structure

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(EP) fibers with a PP core and a PE sheath are thought to be most promising.15

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Heating these fibers to the temperature above the PE melting point can make the

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fibers surface sticky, so that the bonding points between the fibers can be formed,

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resulting in an enhancement in the separator’s strength.14,16 The battery separator

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made of polyamide and EP fibers has a poor resistance to oxidation and may

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cause the self-discharge of the cells.15

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So far, a number of forming technologies to make alkaline battery separators

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using synthetic fibers as raw materials have involved the wet-laid process,14,17-19

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dry-laid process,20-22 and melt-blown process.23-24 The melt-blown process among

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them has been suggested as the best way of using the fibers with a small diameter 3



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and low strength, and the dry-laid one can be applied in making the separators

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with a relatively low quality. The wet-laid process to be applied in making alkaline

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battery separators has many advantages, such as a relative high tightness, better

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formation, and low production cost. However, due to the severe flocculation and

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poor dispersibility, only using low-density PP fibers to make the battery separators

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in the wet-laid process could lead to a very poor formation of the resultant

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separators. In addition, using PP and EP fibers simultaneously to make alkaline

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battery separators in the wet-laid process has been rarely reported.

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In this work, PP and EP fibers were selected to prepare alkaline battery

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separators by using the wet-laid process. The addition of EP fibers was mainly

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used to well disperse PP fibers in water and consequently to improve the

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separator’s formation as well as to increase the mechanical strength of the

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separator handsheets by hot-pressing treatment. In addition, PEO and PVA were

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also selected to add in the fiber suspension for further enhancing the tensile

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strength of the separator handsheets. The aim of this work is to improve the

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separator formation in the wet-laid process and the properties such as tensile

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strength, maximum pore size and porosity of the separator handsheet.

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■ MATERIALS AND METHODS

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Materials and chemicals. PP fibers (density of 0.91 g/cm3, alkali resistance

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of 0.88%, mean fiber length of 6.56 mm and mean fiber diameter of 2.06 µm) was

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obtained from an electronic ceramics company in Guangdong Province, China. EP

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fibers (PE/PP biocomponent sheath-core structure, Superior grade, density of 0.95

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g/cm3, alkali resistance of 0.32%, mean fiber length of 2.55 mm and mean fiber

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diameter of 14.1 µm) was obtained from the Complex New Material Technology

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(Shanghai, China) Co., Ltd. 4


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Polyethylene oxide (PEO, Mw = 5000000) was purchased from Sigma Aldrich

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Co., and polyvinyl alcohol (PVA) with a polymerization degree of 1700 and

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alcoholysis degree of 88% was supplied by Aladdin Reagent (Shanghai) Co., Ltd.

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N-butanol and potassium hydroxide were purchased from Sinopharm Chemical

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Reagent Co., Ltd. All chemicals used in the experiments were analytical-grade

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products.

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Fiber morphology. The mean diameter and length of PP fibers were

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determined by using the Image-Pro Plus (IPP) software and the images of a SEM

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(JSM-IT300LV, JEOL, Japan) according to the method described in literature.25

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The EP fiber’s morphology was analyzed by using a Fiber Tester (Model 912,

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Lorentzen & Wettre Co. Ltd., Sweden).

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Determination of sedimentation volume. 400 mg fibers were added into

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800 mL deionized water to make a 0.05% fiber suspension. After being stirred for

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15 min, the fiber suspension was poured into a 1000 mL graduated cylinder. The

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sedimentation volume was determined according to the upper suspended volume

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of the fiber suspension.

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Preparation of alkaline battery separator handsheets. A series of 0.5%

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fiber suspensions with different proportions of the PP and EP fibers, i.e., the

116

addition levels of the EP fibers were 0%, 10%, 20%, 25%, 30% and 40%,

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respectively, were firstly made, and then the fiber suspensions were disintegrated

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for 20000 revolutions using a standard desintegrator (970154, Lorentzen & Wettre

119

Co. Ltd., Sweden). After pouring PEO (0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%

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on oven-dry fibers, respectively) and/or PVA (1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%,

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and 7.0% on oven-dry fibers, respectively) into the fiber suspension, stirring for 10

122

min was required. The handsheets of alkaline battery separator with a basis weight 5



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of 50 g/m2 (over-dry) were made by using a sheet former (RK-ZA-KWT, PTI,

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Austria) according to the standard method of ISO 5269–1(1998), then further

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dehydrated in a wet paper press machine (KRK SE0808098, MAVIS, UK) and

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subsequently dried in a drum dryer (2110, AMC, USA). Finally, the hot-pressing

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treatment of the separator handsheets was conducted by using a hot press

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machine (LX101-63, Lizhengxin Mechanical Equipment Co., China). The

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hot-pressing conditions were as follows: the pressure of 0.1 MPa, 0.2 MPa, 0.3

130

MPa, 0.4 MPa, 0.5 MPa, 0.55 MPa, and 0.6 MPa, respectively; the temperature of

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130 °C, 135 °C, 140 °C, 145 °C, 150 °C, 155 °C, and 160 °C, respectively; and the

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hot-pressing time of 90 s.

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Sulfonation treatment of alkaline battery separator handsheets. The

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alkaline battery separator handsheets were soaked with concentrated sulfuric acid

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(mass fraction of 98%) at room temperature for 5 min, 10 min, 15 min, 20 min, and

136

25 min, respectively. Then the sulfonated handsheets were taken out and

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thoroughly washed with deionized water until the pH of the filtrate was neutral.

138

Finally, the sulfonated separator handsheets were dried in an electric heating

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air-blowing drier (DGG-101-1, Tianjin Tianyu Experimental Instrument Co., China).

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DSC determination. The DSC determination of the separator handsheets

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was conducted by using a DSC (200F3, NETZSCH, Germany) under a nitrogen

142

atmosphere from 20 °C to 200 °C at a heating rate of 10 °C/min.

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Determination of the separators performance. Physical properties. The

144

basis weight, thickness and tensile index of the separator handsheets were

145

determined according to ISO 536 (1995), ISO 534 (1998) and ISO1924–2 (1994),

146

respectively. The pore size of the separator handsheet was detected by using a

147

Capillary Flow Porometer (Porolux 100, porometer NV, Germany) according to 6


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SJ/T 10171.10 (1991). The porosity of the separator handsheet was determined by

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using the n-butanol uptake test according to the literature method,26-27 as

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calculated by the following equation:

Porosity(%) = ( W 2 − W 1 ) / ρbV ×100

151

(1)

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where W2 and W1 are the weights of the wet and dry separator handsheet,

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respectively, ρb is the density of n-butanol, and V is the geometric volume of the

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separator handsheet.

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Alkali resistance, alkali absorption rate and alkali absorption height. The

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alkali resistance, alkali absorption rate and alkali absorption height of the separator

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handsheet were detected according to SJ/T 10171.6 (1991), SJ/T 10171.7 (1991)

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and ISO 8787 (1991), respectively.

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SEM observation. The morphologies of the separator handsheets were

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observed by using a SEM (JSM-IT300LV, JEOL, Japan). Before observation, the

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test samples were oven-dried in a vacuum oven at 50 °C for 4 h, and then sputter

162

coated with gold.

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■ RESULTS AND DISCUSSION

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Effect of the addition level of EP fibers on the handsheet formation of

165

alkaline battery separator. The effect of the addition level of EP fibers on the

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handsheet formation of alkaline battery separator was investigated, and the result

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is shown in Figure 1. In Figure 1, a better formation can be found as the addition

168

level of the EP fibers was 20% or more, which is accordant with the result of about

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65% PP fibers observed by other researchers.16,28 This is because the EP fibers

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were shorter in length and had a higher density and larger diameter than the PP

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fibers, which effectively inhibited flocculation of the PP fibers in water. Therefore,

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from a better formation point of view, 20% or more addition level of the EP fibers 7



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was necessary to make alkaline battery separator handsheets.

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Selection of the temperature in the hot-pressing treatment of the

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separator handsheets. The DSC curves of the separator handsheets were shown

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in Figure 2. It can be found in Figure 2 that the melting temperature ranged from

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130 °C to 170 °C. According to previous studies,14,16,29 the PE sheath of the EP

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fibers would melt slightly beyond 130 °C, which is the main reason why the tensile

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index of the separator handsheet would increase after hot-pressing. That is

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because the bonding points between the fibers in the separator handsheet

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increased with an increase in the addition level of the EP fibers, resulting in an

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enhancement in the bonding strength of the separator handsheet. Furthermore,

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the EP fibers had a larger diameter and a shorter length than the PP fibers,

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resulting in an increment in the porosity and pore size of the separator handsheet,

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which can be confirmed according to the effect of the addition level of the EP fibers

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on the porosity and maximum pore size of the separator handsheets (Figure 3).

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However, as the hot-pressing temperature rose, the melting amount of the fibers

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increased, leading to a decrease in the porosity of the separator handsheet.

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Considering the porosity requirement for alkaline battery separators, the

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hot-pressing temperature ranging from 130 °C to 150 °C would be appropriate.

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Effect of hot-pressing temperature on the tensile strength, maximum

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pore size and porosity of the separator handsheets. The effect of hot-pressing

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temperature on the tensile strength, maximum pore size and porosity of the

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separator handsheets were explored, as shown in Figure 4. It can be found in

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Figure 4a that with the increase in hot-pressing temperature the tensile strength of

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the separator handsheets had an overall raising trend. When the hot-pressing

197

temperature exceeded 130 °C, the PE sheath of the EP fibers might begin melting 8


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and the PP fibers were then adhered to the EP fibers, resulting in an increased

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tensile index of the separator handsheets, which is consistent with the previous

200

study.14 Actually, there is no bonding strength between the PP fibers, thus the

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moderate melting of the PE sheath of the EP fibers becomes the main source of

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producing the bonding strength between the PP fibers. In other words, at that time

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the EP fibers acted as a binder between the PP fibers. However, as the

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hot-pressing temperature exceeded 135 °C, the PE sheath of the EP fibers could

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have completely melted, which led to changing the bonding mode between the

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fibers in the separator handsheets from ‘contact bonding’ to ‘layered-structure

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bonding’. Because the stress withstood by a layered-structure bonding is much

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smaller than that by a contact bonding,30 the tensile strength of the separator

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handsheets declined until the hot-pressing temperature reached 140 °C. As the

210

hot-pressing temperature was over 140 °C, the ‘layered-structure bonding’ could

211

have been improved greatly, leading to an ascendant trend of the tensile strength

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once again.

213

The

hot-pressing

decreased

the

diameter

of

the

EP

fibers

and

214

correspondingly increased the tightness of the separator handsheets, as a result,

215

the maximum pore size and porosity of the handsheets were both decreased

216

(Figures 4b and c). Moreover, it can be clearly seen from Figure 4 that the

217

maximum pore size had a rapid decrease as hot-pressing temperature exceeded

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150 °C.

219

Effect of hot-pressing pressure on the tensile strength, maximum pore

220

size and porosity of the separator handsheets. Figure 5 shows the effect of

221

hot-pressing pressure on the tensile strength, maximum pore size and porosity of

222

the separator handsheets. It can be found in Figure 5a that the tensile index had a 9



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rapid increase at a hot-pressing pressure of lower than 0.5 MPa and a

224

subsequently flat trend beyond 0.5 MPa. Contrary to the trend of tensile index, the

225

maximum pore size and porosity had a rapid decrease at a hot-pressing pressure

226

of lower than 0.5 MPa and a subsequently flat trend beyond 0.5 MPa (Figures 5b

227

and c). In addition, the tensile index, maximum pore size and porosity all rose as

228

the addition level of the EP fibers in the separator handsheets increased at the

229

same hot-pressing pressures. When the proportion of the EP fibers was 40%, the

230

tensile index, maximum pore size and porosity of the resultant separator

231

handsheets reached 33.00 Nm/g, 40.91 µm and 45.71%, respectively. When the

232

pressure rose to 0.5 MPa or more, almost no changes happened to the tensile

233

index, maximum pore size, and porosity. The events above can be explained as

234

follows: under a certain addition level of the EP fibers, high hot-pressing pressure

235

could result in a large contact area and a consequently strong bonding between

236

the fibers in the separator handsheets; as a result, the pore size and porosity

237

declined. The explanation above can be also confirmed by the SEM observation

238

shown in Figure 6. It can be found from Figures 6a to c that more coating which

239

came from the melted PE sheath of the EP fibers appeared on the surface of the

240

separators handsheets, leading to the gradually decreased porosity and pore size

241

of the handsheets. Meanwhile, the bonding between the fibers in the separator

242

handsheets could be enhanced, resulting in the improved tensile strength of the

243

separator handsheets.

244

Optimization of the hot-pressing temperature and pressure for the

245

separator handsheets. Figure 7 further shows the changes of the surface and

246

cross section of the separator handsheets at a hot-pressing pressure of 0.5 MPa

247

and temperatures of 135 °C and 150 °C as follows: on the surface of the separator 10


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248

handsheets, both the melting amount of the EP fibers and consequently the

249

binding area at a hot-pressing temperature of 150 °C (Figure 7b) were obviously

250

larger than that at 135 °C (Figure 7a); on the cross section of the separator

251

handsheets, almost all the fibers were more tightly closed with each other at

252

150 °C (Figure 7d) than that at 135 °C (Figure 7c), both leading to the decreased

253

porosity and maximum pore size.

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In general, the requirements of the maximum pore size and porosity for

255

alkaline battery separators are less than 50 µm (QB/T 4173 (2011)) and 40% or

256

more,26,31 respectively. Based on the discussions above, the appropriate

257

hot-pressing pressure and temperature were 0.5 MPa and 135 °C, respectively.

258

Effect of PEO addition on fiber dispersion. It can be found in Figure 8 that

259

with the increase in sedimentation time the sedimentation volume of fibers in the

260

fiber suspension has a reducing trend. The sedimentation volume decreased

261

rapidly when the sedimentation time was less than 225 s, and then went flatly

262

beyond 225 s. The above can be explained as follows: the PP fibers flocculated

263

constantly before 225 s, and flocculated completely over 225 s, which is consistent

264

with the fact that the tensile index reduced with increasing the PEO dosage.

265

Figure 9 shows the effect of adding PEO on the sedimentation volume of

266

fibers in the fiber suspension. It can be seen from this figure that the sedimentation

267

volume had an increasing trend, especially when the PEO dosage was lower than

268

1.5%. This can be attributed to that the addition of PEO might increase the

269

viscosity of the fiber suspension and therefore prevented the PP fibers from

270

flocculating, thus making PP fibers and EP fibers uniformly mixed in the

271

suspension.16

272

Effects of PEO and PVA additions on tensile strength of the separator 11



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273

handsheets. It can be found in Figure 10a that the tensile index of the separator

274

handsheets made of 60% PP fibers and 40% EP fibers just reached 33.00 Nm/g

275

at a hot-pressing pressure of 0.5 MPa and temperature of 135 °C for 90 s,

276

suggesting that it did not meet the needs of more than 40.00 Nm/g required in

277

QB/T 4173 (2011). Consequently, PEO and PVA were selected as dispersant and

278

reinforcing agent, respectively, to add to the fiber suspension for further enhancing

279

the tensile strength of the resultant separator handsheets in this work. Figure 1h

280

shows the formation of the resultant separator handsheets, seemingly no obvious

281

change compared to that shown in Figure 1g.

282

Figure 10a shows the effect of the PEO dosage on the tensile index of the

283

separator handsheets. It can be seen that the tensile index had a sharp increase

284

until the PEO dosage reached 1.5% (on oven-dry fibers) and that the maximal

285

tensile index of 40.29 Nm/g could be got at the PEO dosage of 1.5%. This is

286

because with an increase in the PEO dosage the viscosity of the fiber suspension

287

rose and the movement of the PP fibers in the fiber suspension was blocked,

288

resulting in the improved formation and resultant tensile index, which is consistent

289

with the previous report.32 However, when the PEO dosage exceeded 1.5% in the

290

handsheet-making process, the drainage time of the fiber suspension on the

291

formation web was so prolonged that the PP fibers could flocculate again,28 which

292

led to a poor formation and consequently decreased tensile strength of the

293

resultant separator handsheets.

294

Figure 10b shows the effect of the PVA dosage on the tensile index and

295

maximum pore size of the separator handsheets at the PEO dosage of 1.5% (on

296

oven-dry fibers). It can be clearly seen in Figure 10b that the tensile index

297

increased obviously until the addition level of PVA was up to 4.0% (on oven-dry 12


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298

fibers) and then went slowly, in which the tensile index increased from 40.29 Nm/g

299

to 45.16 Nm/g, an increment of 12.1%. That can be attributed to the good bonding

300

property of the PVA between the fibers in the separator handsheets in the

301

hot-pressing treatment. However, the maximum pore size went completely

302

opposite to the tensile index, because the existence of PVA improved the bonding

303

area between the fibers, resulting in a decrease in the pore size of the separator

304

handsheet. When the PEO and PVA dosages were 1.5% and 4.0%, respectively,

305

the maximum pore size decreased from 40.31 µm to 34.19 µm, a decrement of

306

15.2%. As the addition level of PVA was further raised, the maximum pore size did

307

not meet the needs (30-50 µm) required in QB/T 4173 (2011). Therefore, at the

308

addition level of 1.5% PEO, the appropriate dosage of PVA was 4.0% on oven-dry

309

fibers.

310

Performance of the separator handsheets under the optimal conditions.

311

Based on all discussions above, the separator handsheets were also made of 60%

312

PP fibers and 40% EP fibers under the optimum conditions as follows: 1.5% PEO;

313

4.0% PVA; hot-pressing pressure of 0.5 MPa; and hot-pressing temperature of

314

135 °C for 90 s. The performance of the separator handsheets was then tested,

315

and the results are listed in Table 1. It can be seen in Table 1 that the tensile

316

strength, maximum pore size, porosity and alkali resistance of the separator

317

handsheets were qualified compared to the standard values set in QB/T 4173

318

(2011). In order to further compare the performance of the separator handsheets

319

to that of some commercial separators available in the market and the standard

320

values in QB/T 4173 (2011), the above separator handsheets were then

321

experienced a sulphonation treatment to increase their absorbing capability. The

322

result of the corresponding comparison is listed in Table 2, in which the data for 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

323

two commercial separators are cited in literatures.18,33 It can be seen in Table 2

324

that the tensile index and alkali absorption rate of the separator handsheets were

325

lower than those of the American product but met the needs of QB/T 4173 (2011),

326

and the alkali resistance of the separator handsheets was the highest among the

327

three samples.

328

■ CONCLUSIONS

329

The handsheets of alkaline battery separators were successfully made of PP

330

fibers and EP fibers in the wet-laid process followed by a hot-pressing treatment.

331

Both the formation issue that exists in the wet-laid process and the poor

332

corresponding tensile strength of the resultant alkaline battery separators can be

333

solved. A better formation of the separator handsheet could be obtained when the

334

addition of the EP fibers was not less than 20%. The properties such as tensile

335

strength, maximum pore size, porosity and alkali resistance of the separator

336

handsheets made of 60% PP fibers and 40% EP fibers could meet the needs

337

required in QB/T 4173 (2011) under the conditions of 1.5% PEO and 4.0% PVA

338

additions (on oven-dry fibers), the hot-pressing pressure of 0.5 MPa, and

339

hot-pressing temperature of 135 °C for 90 s.

340

■ AUTHOR INFORMATION

341

Corresponding Author

342

343 344 345 346

*Email: [email protected] (Q. X. Hou) ■ REFERENCES

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Properties and Failure Mechanism of PP/PE Core/Sheath Bicomponent and PP

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Spunbond Fabrics in Uniaxial Conditions. Text. Res. J. 2010, 80, 1759-1767.

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via centrifugal spinning as a separator for li-ion batteries. J. Power Sources. 2015,

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18


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Captions of Figures Figure 1. Images of the handsheet formation of alkaline battery separator with different addition levels of the EP fibers: (a) 100% PP; (b) 10% EP + 90% PP; (c) 15% EP + 85% PP; (d) 20% EP + 80% PP; (e) 30% EP + 70% PP; (f) 35% EP + 65% PP; (g) 40% EP + 60% PP；(h) (40% EP + 60% PP)/(1.5% PEO + 4.0% PVA). Figure 2. DSC curves of the separator handsheets: T1 was the initial melting temperature of PE sheath of the EP fibers; T2 was the initial melting temperature of the PP fibers and PP core of the EP fibers. Figure 3. Effect of the addition level of the EP fibers on the porosity and maximum pore size of the separator handsheets at hot-pressing pressure of 0.5 MPa and temperature of 135 °C for 90 s. Figure 4. Effect of hot-pressing temperature on the tensile index (a), maximum pore size (b) and porosity (c) of the separator handsheets at hot-pressing pressure of 0.5 MPa for 90 s. Figure 5. Effect of hot-pressing pressure on the tensile index (a), maximum pore size (b) and porosity (c) of the separator handsheets at hot-pressing temperature of 135 °C for 90 s. Figure 6. SEM images of the separator handsheets made of 60% PP fibers and 40% EP fibers under the conditions of hot-pressing pressure of 0.1 MPa (a), 0.3 MPa (b) and, 0.5 MPa (c) and hot-pressing temperature of 135 °C for 90 s. Figure 7. SEM images of the surfaces (a, 135 °C; b, 150 °C) and cross sections (c, 135 °C; d, 150 °C) of the separator handsheets made of 60% PP fibers and 40% EP fibers under the conditions of a pressure of 0.5 MPa and 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperatures of 135 °C and 150 °C for 90 s. Figure 8. Relationship of the sedimentation volume of the fibers in the fiber suspension with time. Figure 9. Effect of adding PEO on the sedimentation volume of fibers in the fiber suspension. Figure 10. Effects of PEO (a) and PVA (b) dosages on the tensile index and/or maximum pore size of the separator handsheets at hot-pressing pressure of 0.5 MPa and temperature of 135 °C for 90 s.

Captions of Tables Table 1. Performance of the separator handsheets. Table 2. The performance comparison of commercial separators and the separator handsheets expericed a sulphonation treatment.

20


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Figure 1.

a

b

c

d

e

f

g

h

21



Figure 2.

Heat Flow Endo Up

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

20% EP + 80% PP 30% EP + 70% PP 40% EP + 60% PP (40% EP + 60% PP)/1.5% PEO (40% EP + 60% PP)/1.5% PEO/4% PVA

T1=130 oC

20

40

60

80

100

120

140

T2=170 oC

160

Temperature, oC

22


180

200

Page 23 of 33

Figure 3.

60 55 40% EP + 60 %PP 1.5% PEO

50

50

45 45

40

40

35 Maximum pore size Porosity

30 25

35 30

20

30

40

50

60

ES fiber dosage, %(on oven-dry fibers)

23


Maximum pore size, µm

55

Porosity, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



a

48

b

50

Maximum pore size, µm

Figure 4.

45

45 30% EP + 70% PP 35% EP + 65% PP 40% EP + 60% PP

42 39 36 33 30 130

135

140

145

150

155

30% EP + 70% PP 35% EP + 65% PP 40% EP + 60% PP

40 35 30

27

25

160

130

135

o

c

140

52 30% EP + 70% PP 35% EP + 65% PP 40% EP + 60% PP

48 44 40 36 32 130

145

150

155

Hot-pressing tempeture, oC

Hot-pressing temperature, C

Porosity, %

Tensile index, N·m/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

135

140

145

150

155

160

Hot-pressing tempeture, oC

24


160

Page 25 of 33

34

b

32 30 28 30% EP + 70% PP 35% EP + 65% PP 40% EP + 60% PP

26 24 22

80 30% EP + 70% PP 35% EP + 65% PP 40% EP + 60% PP

70 60 50 40 30

0.1

0.2

0.3

0.4

0.5

0.6

0.1

c

0.2

65 30% EP + 70% PP 35% EP + 65% PP 40% EP + 40% PP

60 55 50 45 40 35 0.1

0.3

0.4

0.5

Hot-pressing pressure, MPa


Porosity, %

a

Maximum pore size, µ m

Figure 5.

Tensile index, N·m/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.3

0.4

0.5

0.6


25


0.6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6.

26


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Figure 7.

27



Figure 8.

700

Sedimentation volume, mL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

650 1.5% PEO 3.0% PEO

600 550

40% EP + 60% PP

500 450 0

100

200

300

400

Sedimentation time, s

28


500

600

Page 29 of 33

Figure 9.

560

Sedimentation volume, mL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


540 40% EP + 60% PP sedimentation time of 2 min

520 500 480 460 440 420 0.0

0.5

1.0

1.5

2.0

2.5

PEO dosage, % (on oven-dry fibers)

29


3.0


Figure 10.

42 40% EP + 60% PP

b Tensile index, N·m/g

40 38 36 34 32

42 46 39

45 40% EP + 60% PP 1.5% PEO

44

36 33

43 42

30 Tensile index Maximum pore size

41

27

40 24 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

1

2

3

4

5

6

7

PVA dosage, % (on oven-dry fibers)

PEO dosage, % (on oven-dry fibers)

30


8

Maximum pore size, µ m

a Tensile index, N·m/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1

2

Basis weight (g/cm ) Maximum pore size (µm) Porosity (%) Tensile index (Nm/g) Alkali resistance (%) *

Standard valuea

Test sample*

30 ~ 50 30 ~ 50 ≥ 40 ≥ 40

Wet-Laid Formation and Strength Enhancement of Alkaline Battery

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