JPH0414468B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a coiled electrode assembly for a battery. The electrode assembly has an electrode disposed within a separator pocket, the end surface of which has a uniform appearance. Therefore, the amount of debris generated by inserting the electrode assembly into the cell is minimized. In another aspect of the invention, the invention includes an improved method of manufacturing such electrode assemblies that is amenable to high speed manufacturing techniques.

[Technical background of the invention and its problems]

The use of coiled electrode or "jelly roll" battery configurations is well known in the field of battery construction. Such a construction involves coiling two flat electrodes with a separator layer between them about a common central axis. The coiled electrode assembly is then placed into the cylindrical electrode assembly, after which the electrolyte is added. This increases the contact surface area between the negative and positive electrodes over a comparable sized battery with a molded positive electrode. This increased contact surface area allows batteries with such coiled electrode assemblies to operate at higher discharge rates than batteries with molded positive electrodes.

Generally, two approaches have been taken to produce jellyroll batteries. One involves placing one electrode (typically the positive electrode) into a separator before winding the electrode assembly. This inserted electrode is combined with the other electrode of the cell and both electrodes are coiled together.

However, this combination process involves time-consuming manual work. The reason for this is that in order to manufacture a coiled electrode assembly whose end face has a uniform appearance as a whole, and to simplify the insertion and sealing of the electrode assembly into the battery case, the electrode pieces should be separated as much as possible. This is because it is desirable to wind it as uniformly as possible.
If such uniform electrode winding is not performed, as tends to occur when automated assembly processes are employed, there is an increased risk of separator tearing during the battery assembly process, and measures must be taken to address this. This will result in a large amount of waste. These difficulties are even more acute in batteries where minimal height variation from the height of a fully coiled electrode assembly is allowed within the battery case. Therefore, this problem is even more acute for small batteries where it is desirable to minimize wasted volume (ie, the volume not occupied by active battery material). Therefore, this first way of obtaining jelly roll batteries does not lend itself readily to manufacturing batteries, especially small batteries, on a large commercial scale. Additionally, if the positive electrode is an encapsulated electrode and the negative electrode is composed of a fragile material such as lithium, the negative electrode material may fracture and fall off the support (e.g., a metal screen) during battery manufacture; This contaminates the battery assembly equipment and reduces the electrochemical output of the manufactured battery.

The second common practice adopted in the past has been the union
Union Carbide Corporation
This method is representative of a commercial method for manufacturing nickel-cadmium batteries, which has been adopted by the company, and includes the step of wrapping both electrodes with a separator. More particularly, the method includes placing the electrodes end-to-end over a first separator piece. A second separator piece is then placed in register with the first separator piece and the longitudinal edges of the separator pieces are sealed together. Alternatively, the width of the first separator piece may be doubled, and half of the width of the first separator piece may be placed on top of the electrode, which is placed end-to-end on this first separator piece, along the length of the first separator piece. fold it. Sealing the open longitudinal edges of the folded separator pieces encases the electrodes, keeping them in an end-to-end arrangement. In either approach, the wrapped electrodes are overlapped by laterally folding the separator assembly between the ends of the electrodes. This folded assembly is then rolled up and inserted into the battery case. However, with this method, if the central lateral fold is not performed correctly, i.e., if the fold varies more than a few degrees from perpendicular to the length of the electrode assembly, the two electrodes will not fit properly. The disadvantage is that it will no longer be aligned. Such poor alignment may make it difficult to wind the electrodes, insert the wound electrode assembly into the battery case, and close the case. These difficulties become even more pronounced during battery assembly where, as in the first case described above, the tolerance of the battery case for positive errors in the height of the electrode assembly is minimal.

[Purpose of the invention]

It is therefore an object of the present invention to provide a method for manufacturing coiled electrode assemblies that is easily adaptable to high speed automated manufacturing techniques.

Another object of the present invention is to provide a method of manufacturing a coiled electrode assembly that has ends with a uniform appearance, thereby reducing the amount of debris produced when inserted into a battery. .

Yet another object of the invention is to provide a coiled electrode assembly with uniform appearance ends for a battery so that batteries are less likely to be manufactured with electrode assemblies containing torn separators. be.

[Summary of the invention]

The first feature of the present invention is that the first pocket is made up of two superimposed separator pieces and has a weldable edge; and the first pocket is made up of two superimposed separator pieces and has a weldable edge. and a second pocket overlaid on the first pocket such that the weldable edge overlaps at least a portion of the weldable edge of the first pocket. a first electrode disposed within the first pocket; and a second electrode disposed within the second pocket; There is provided a coiled electrode assembly for a battery, characterized in that the electrode assembly is wound in a coil shape around the axis so that the joined edge portion is included within the coiled electrode assembly.

A second feature of the present invention is the step of (a) preparing a first separator having a certain width and a certain length, and a second separator having a certain width and a certain length, and (b)
The first separator is superimposed on the second separator, and the first and second a seam by joining the separators of the seam together, and having a first pocket on one side of the seam for receiving a first electrode and a second pocket for receiving a second electrode on the other side of the seam. (c) bending the separator assembly along the seam so that the first pocket is superimposed on the second pocket; (d) the seam is winding a separator assembly in which an electrode is inserted into the pocket about an axis that crosses the seam so as to be contained in a plane perpendicular to the winding axis. A method of manufacturing a coiled electrode assembly is provided.

In practicing the invention, a first electrode is inserted into a first pocket and a second electrode is inserted into a second pocket before coiling the electrode assembly. Such insertion may occur after step (b) or after step (c).
It can be done independently after. Accordingly, in some embodiments of the invention, the step of (b') inserting a first electrode into said first pocket and inserting a second electrode into said second pocket is a step. Perform after (b) and before step (c).

In another embodiment of the method of the invention, the step of (c') inserting a first electrode into said first pocket and inserting a second electrode into said second pocket comprises step (c). and before step (c).

In yet another embodiment of the method of the invention, (b
″) inserting a first electrode into the first pocket is performed before step (c), and (c″) inserting a second electrode into the second pocket is performed before step (d). ) is executed before.

As used herein, the term "separator" refers to either different separator pieces, or portions of different separator pieces, or two or more discontinuous portions of a separator piece. Thus, in some embodiments, the battery separator assembly of the present invention has a single separator piece, and the separators of the separator piece are joined together. In another embodiment,
A separator assembly is made up of two or more different separator pieces joined together. The separator pieces constituting the separator of the present invention can have a length longer than a width, a width longer than a length, or a width and a length equal to each other.

Additionally, the term "pocket" as used herein refers to an enclosed portion formed by two separator pieces joined along at least a portion of one edge that is aligned and that receives an electrode. It is something. The two separator pieces can be different separator pieces or the same separator piece. Furthermore, in some embodiments of the invention, one separator piece can form both pockets or a portion thereof.

The use of separator assemblies formed by splicing provides an electrode assembly manufacturing method that is easily compatible with high speed automated manufacturing techniques. This is because the seams ensure that the pockets containing the electrodes are aligned with each other, thereby eliminating the need for manual fabrication of the electrodes. Additionally, the electrode assembly of the present invention is a coil with ends that have a uniform appearance, thus reducing the likelihood of torn separators being included after insertion into a battery case. This is accomplished by having a coiled electrode with uniform ends that does not have extended portions of separator that are prone to warping or tearing when full sealing forces are applied. Therefore, the rate of debris generated when the electrode assembly of the present invention is inserted into a battery case is the same as the rate of debris generated when a coiled electrode assembly having non-uniform ends is inserted into a battery case. will be lower than

The separators can be joined together by any means that performs fine wire joining to form a seam. This joint does not separate during the battery assembly process or in the environment within the battery. Examples of bonding means that can be used include adhesives, heat sealing, suturing, ultrasound, and the like. This seam can be either continuous or discontinuous along the separator assembly. If an adhesive is used, it must be able to bond rapidly, be chemically resistant to the battery components, and be stable over time. Examples of adhesives that can be employed in the present invention are polyamide high temperature melting, ultraviolet curing acrylic resin, epoxy, and the like. As is clear to those skilled in the art,
A suitable adhesive depends on the composition of the separator used,
Varies depending on other similar factors.

Most preferably, however, the separators are bonded by thermal or ultrasonic means. The reason is,
This is because the heat treatment or sonication creates a depression that extends along the seam of the separator assembly.
The recesses are convenient for holding the separator assembly.

The separator pieces used in the separator assembly of the present invention can be any material that is capable of transporting ions, maintains its physical and chemical properties within the battery environment, absorbs the battery electrolyte, and is easily wetted by the electrolyte. Can be composed of any substance. Such materials will be well known to those skilled in the art. When a separator assembly is composed of one or more types of separator pieces and those separator pieces are thermally bonded, at least one separator piece is capable of being thermally bonded, and the other separator pieces are made of a material that can undergo such thermal bonding. It must be composed of Preferably, all separator pieces used can be thermally bonded. Examples of thermally bondable materials that can be employed include polyethylene with minute pores and polypropylene with minute pores. The latter is the preferred thermally bondable material.

The electrode assembly of the present invention is, for example, a lithium-
MnO ₂ battery, lithium-iron sulfide battery, lithium-
It can be employed in a wide range of batteries, such as non-aqueous batteries such as polycarbonate batteries, and aqueous batteries such as alkaline-MnO ₂ batteries, nickel-cadmium batteries, lead-acid batteries, etc.

The electrode assembly of the present invention is typically preferably manufactured as follows. overlapping a first rectangular separator and a second rectangular separator and joining both separators together along a thin line extending in the middle along the width of the separators and parallel to the longitudinal direction of the separators; to form a separator assembly. This separator assembly has two pockets. Each pocket is adapted to receive an electrode. The pockets extend parallel to the length of the separator assembly, and the joining seams of the separator assembly serve as separations. The pockets are arranged so that when the separator assembly is folded along the seam, the electrodes placed in the pockets overlap each other.

In some embodiments of the method of the invention, one separator piece can be folded so that it constitutes both separators. Therefore, in one embodiment shown in Figures 2A, 2B, 2C and 2D, the cross section is V.
Fold one separator piece so that each wall of the V-shape forms a separator. In a second embodiment somewhat similar to this, but detailed in Figures 3A, 3B and 3C, one separator piece consists of a first separator and two discontinuities of the same separator piece. Fold the one separator piece into a shape to form a second separator.

Once the separator assembly is formed, the separator assembly is folded along the seam so that the first pocket overlaps the second pocket. The folded separator assembly with electrodes inserted therein is coiled about an axis transverse to the seam so that the seam is contained in a plane perpendicular to the axis. The coiled electrode assembly is placed in a case to form a battery. However, if desired, the open edges of the separator assembly opposite the folded seam can and should be joined prior to coiling the electrode assembly. is preferable.

In some embodiments of the invention, one or both of the electrodes can be placed into a pocket before folding the electrode assembly. However, the insertion of the electrode can also take place after its bending. Additionally, as will be apparent to those skilled in the art, the connecting tabs connecting the electrodes to the respective terminals of the battery case may be removed prior to placing the electrodes into the separator assembly.
Or it can be attached to those electrodes after being inserted. In those embodiments, if the tabs are attached after inserting the electrodes, attachment can be done before or after folding the separator assembly along its seams.

The method of the present invention creates a seam extending along the length of the separator assembly, which serves as a pivot line during folding of the separator assembly, resulting in a time-consuming process of manual electrode assembly. is resolved. The method of the invention can therefore be easily adapted to high speed automated manufacturing techniques.

Also, because the finished separator assembly has at least one joined edge, it is easy to assemble the assembly so that the ends of the coiled assembly have an overall uniform appearance. Can be wound into a coil. Thus, tearing of the separator during battery assembly operations and the difficulty of sealing the battery are reduced. Therefore, compared to currently commercially employed manufacturing methods,
The amount of waste generated in the manufacture of such electrochemical cells is reduced.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to the drawings.

First, reference is made to FIG. 1A. This figure shows a separator assembly 2 made by overlapping a first separator 4 and a second separator 6 and joining the separators together along their longitudinal directions to form a central seam 8. It is shown. The seam 8 extends in the middle of the width of the separator parallel to the longitudinal direction of the separator. In this embodiment, each separator 4, 6 consists of a different separator piece. Therefore, in this embodiment, separators made of different materials can be employed. As is clear from the description of this first embodiment, the first separator 4 is folded between two electrodes, and the second separator 6 is located outside these electrodes. Seam 8 is formed by applying heat as much as possible. This joining forms a first pocket 10 for receiving the first electrode and a second pocket 12 for receiving the second electrode.

FIG. 1B shows the separator assembly of FIG. 1A. That is, the first electrode 14 is inserted into the first pocket 10 and the second electrode 18 is inserted into the second pocket 12. A first electrode tab 16 is shown extending from the end of the separator assembly 2 and a second electrode tab 20 is shown extending from the other side of the separator assembly, although the connecting tabs can be adjusted as desired. It can be located along the sides of the separator assembly or at either end of the separator assembly, depending on the location. Although the tabs 16 are shown extending from the separator assembly in FIG. 1B, it will be appreciated by those skilled in the art that the tabs 16 are
Because the tabs 16 extend as such, the tabs 16 need not initially extend beyond the separator assembly prior to winding.

FIG. 1C shows the separator assembly of FIG. 1B after it has been folded along the center seam 8 so that the pockets overlap. The folded separator assembly is wound about an axis transverse to the seam such that the seam is contained in a plane perpendicular to the winding axis. The electrode assembly thus wound can then be inserted into the battery case. If desired, the open edge 2 of the folded separator assembly
By joining 2, the electrodes 14 and 18 can be completely surrounded. The sealing is preferably done by heating.

Figures 2A, 2B, 2C and 2D show another embodiment of the method of the invention. In this example, the first
and the second separator consists of a single separator piece. As shown in FIG. 2A, by bending the separator piece 24 into a V-shaped cross section, first and second side walls 26 and 28 corresponding to the first and second separators, respectively, are formed. The separators are stacked as shown in FIGS. 2A and 2B, and the first electrode 30 having the first connecting tab 32 is inserted into the folded portion of the separator piece 24, and the first and second side walls 26, 28
by joining them together along seam 34,
The electrode 30 is enclosed within the first pocket 25. The seam 34 extends parallel to the length of the separators 26, 28 and along the middle of their width. The seam can be formed by heat, ultrasound, adhesives, sutures, or other means.

a second electrode 36 having a second tab 38;
Put it in your pocket 40. Next, this separator assembly is connected so that the first electrode 36 is connected to the second electrode 30.
Seam 34 (second C
Fold it along the lines shown in the figure. The folded separator assembly is rolled around the axis that crosses the seam, and then placed into the battery case. Depending on your wishes,
The open edges 42 of the separator assembly can be joined before the separator assembly is rolled.

Figures 3A, 3B and 3C show another embodiment of the method of the invention, in which the first and second separators consist of one separator piece. As shown in Figure 3A, the upper wall 4 corresponding to the second separator
The upper walls 48, 50 are connected to the lower wall in a position such that the longitudinal edges of the upper walls 8, 50 are located along a centerline extending longitudinally along the lower wall 46 corresponding to the first separator. The separator piece 44 is bent so as to be superimposed on the separator piece 46. A first pocket 52 is defined by lower wall 46 and upper wall 48, and a second pocket 54 is defined by lower wall 46 and upper wall 50.

As shown in FIG. 3B, first conductive tab 58
A first electrode 56 having a conductive tab 62 is placed in the second pocket 54 and a second electrode 60 having a second conductive tab 62 is placed in the first pocket 52. after that,
A seam 64 is formed by joining the longitudinal edges of the two upper walls 48, 50, ie, the middle of the width of the second separator, to the middle of the width of the first separator, ie, the longitudinal edge of the lower wall 46. Form. The seam extends parallel to the longitudinal direction of the separator (3rd B
figure). The separator assembly thus produced is bent in either direction along the seam 64 so that the first electrode 56 is superimposed on the second electrode 60. If desired, before coiling and placing the electrode assembly in the battery case.
Open longitudinal edge 6 of the separator assembly
6 can be joined together.

FIG. 4 shows a partial longitudinal cross-sectional view of a battery 58 made in accordance with one embodiment of the present invention.

The cover portion has an inner cover 70 and an outer cover 76. The inner cover 70 has a hole 72 in the center. Air is removed from the interior of the battery case 74 through this hole, and electrolyte is introduced into the case 74.
The outer cover 76 can withstand the battery's sealing forces without being detrimentally deformed. Conductive tab 78 attached to first electrode 80
is fixed in electrical contact with the inner cover 70. As will be apparent to those skilled in the art, other means of electrically connecting the electrodes and the battery case may also be employed. Case side wall 82 and covers 70,7
A sealing gasket 77 is placed in compression between the edges of 6.

A coiled electrode assembly 84 including a first electrode 80 and a second electrode 86 that are spirally wound is arranged inside the case 74 . The first electrode 80 is a first separator piece formed by a first separator piece (a part of the first separator 88) overlaid on a second separator piece (a part of the second separator 90). It is placed in the pocket of the person. Second
The electrode 86 is connected to the first and second separators 88, 9
0 is placed in a second pocket formed by another overlapping portion of the 0. The separators can be constructed of the same or different materials as desired. As can be seen from Figure 1C, the first
and the second pocket are joined along their respective aligned edges (indicated by seam 8). The electrode assembly 84 is coiled about an axis transverse to the alignment edge such that the alignment edge is contained within a plane perpendicular to the central winding axis of the electrode assembly.

The examples described below represent specific embodiments of the invention, but are not intended to limit the scope of the invention in any way.

Example 1 200 batteries were made as follows. Two pieces of underwoven polypropylene separators with a length of about 12.7 cm, a width of about 3.0 cm, and a thickness of about 0.025 mm are stacked on top of each other, and along a seam extending parallel to the length direction along the middle of the width of those members, Heat bonding was performed using a soldering iron. A negative electrode composed of pressed lithium on a stretched stainless steel holder,
It was inserted into one of the pockets formed by the heat-bonded separators. Its negative pole is about length
It was 8.6cm long and about 0.7cm wide. A positive electrode member made by pressing a manganese dioxide positive electrode mixture onto a stainless steel screen and having a length of about 10.8 cm and a width of about 0.8 cm was inserted into the opposing pockets. The electrode assembly thus made was bent along the central seam.
The open longitudinal edges of the separator assembly were then heat sealed. The bent electrode assembly is then wound into a coil around the axis that crosses the seam, and is approximately 1.14 cm in diameter and 1.06 cm in height.
cm in a cylindrical battery container or battery case. A non-aqueous electrolyte was then added prior to sealing by conventional means. By using this embodiment of the method of the invention, the handling and winding of the completed electrode assembly can be done quickly, and the produced battery can be compared to a battery produced by conventional production methods that surround only the positive electrode. Found to have comparable quality.

Example 2 Fifty batteries were manufactured as follows. A polypropylene separator having a length of about 12.7 cm, a width of about 4.5 cm, and a thickness of about 0.025 mm was bent in half along its longitudinal direction to form a V-shaped cross section. Composed of pressed lithium on an expanded stainless steel cage, approximately 8.6cm long and approximately 8.6cm wide.
A 0.7 cm negative electrode member was inserted into the V-shaped separator, and the overlapping edges of the separator were heat-bonded along a seam extending along the mid-width, parallel to the length of the element. I created a collective. It is composed of manganese dioxide and has a length of approximately 10.8
A positive electrode member having a width of about 0.8 cm and a width of about 0.8 cm was placed in the assembly, and the positive electrode member was encapsulated by heat sealing the open longitudinal edges of the separator assembly.
The electrode assembly thus obtained was bent along the central seam and then wound into a coil around an axis that crossed the seam, with a diameter of approximately 1.14 cm and a height of approximately 1.06 cm.
cm in a cylindrical steel container. Electrolyte was then added to seal the cell. By using this method of the invention, the handling and coiling of the completed electrode assembly can be done quickly, and the produced battery can be compared to the battery produced by the conventional production method, which only surrounds the positive electrode. Found to have comparable quality.

[Brief explanation of drawings]

Figures 1A, 1B and 1C are perspective views showing particularly preferred embodiments of the method of the invention; Figures 2A, 2B and 2
Figures C and 2D are perspective views showing a preferred embodiment of the method of the present invention, Figures 3A, 3B, and 3C are perspective views showing another preferred embodiment of the method of the present invention, and Figure 4 is a perspective view showing a preferred embodiment of the method of the present invention. FIG. 2 is a partial longitudinal sectional view of a battery of the present invention using a separator assembly of the type manufactured by the method shown in FIG. 2... Separator assembly, 4, 6, 88, 90...
Separator, 10, 12, 25, 40, 52, 5
4...Pocket, 14, 18, 30, 36, 56,
60, 80... Electrode, 24, 44... Separator piece,
84...electrode assembly.

Claims (1)

[Scope of Claims] 1. A first pocket consisting of two overlapping separator pieces and having a joinable edge;
Consists of two overlapping separator pieces,
a second pocket having a weldable edge and superimposed over the first pocket such that the weldable edge overlaps at least a portion of the weldable edge of the first pocket; a separator assembly having: a first electrode disposed inside the first pocket; and a second electrode disposed inside the second pocket; 1. A coiled electrode assembly for a battery, characterized in that it is wound in a coil shape around an axis so that the joined edges are included in a plane perpendicular to the transverse axis. 2. The electrode assembly according to claim 1, wherein the separator piece constituting the first pocket and the separator piece constituting the second pocket are composed of one separator piece. . 3. The separator piece according to claim 2, wherein the separator piece is made of a thermally bondable material, and the bondable edges of the rolled separator assembly are thermally bonded. Electrode assembly. 4. The electrode assembly according to claim 3, wherein the separator piece is made of porous polypropylene. 5. The electrode assembly according to claim 1, wherein the second separator is configured as a different component from the first separator. 6. The electrode assembly according to claim 5, wherein the separator pieces constituting the second separator are made of a different material from the separator pieces constituting the first separator. 7. The electrode assembly according to claim 5, wherein at least one separator piece is made of a thermally bondable material. 8. The electrode assembly according to claim 7, wherein at least one separator piece is made of porous polypropylene. 9. Claim 2, wherein the first electrode is comprised of lithium, and the second electrode is comprised of a compound selected from the group consisting of manganese dioxide and carbon polyfluoride. The electrode assembly described. 10 (a) preparing a first separator having a certain width and a certain length and a second separator having a certain width and a certain length; (b) preparing the second separator on the second separator; A seam is formed by overlapping the first and second separators and joining the first and second separators to each other along the widthwise middle of the separators and along at least a portion of a thin line extending parallel to the longitudinal direction of the separators. on one side of the seam.
(c) constructing a separator assembly having a first pocket for receiving an electrode on the other side of the seam and a second pocket for receiving an electrode on the other side of the seam; (d) folding the separator assembly along the seam so that the first pockets overlap, and (d) folding an axis across the seam so that the seam is contained in a plane perpendicular to the winding axis. A method for manufacturing a coiled electrode assembly for a battery, comprising: a step of winding a separator assembly in which an electrode is inserted into a pocket, centering on the separator assembly; 11. After step (b) and before step (c): (b') inserting a first electrode into the first pocket;
Inserting a second electrode into the second pocket.
The method described in item 0. 12. After step (c) and before step (d): (c') inserting a first electrode into the first pocket;
Inserting a second electrode into the second pocket.
The method described in item 0. 13 After step (b) and before step (c), (b'') inserting a first electrode into the first pocket, and after step (c), performing step ( Claim 1, characterized in that, before step d), (c'') inserting a second electrode into the second pocket.
The method described in item 0. 14. Claim 10, characterized in that the first separator and the second separator prepared in step (a) are each composed of a first separator piece and a second separator piece that are different from each other. the method of. 15. The method of claim 14, wherein the width of the first separator piece and the width of the second separator piece are equal. 16. The method of claim 15, wherein the first separator piece and the second separator piece are constructed of different materials. 17. The method of claim 14, wherein the first separator piece and the second separator piece are bonded by heat. 18. The method of claim 17, wherein at least one separator piece is comprised of porous polypropylene. 19. The method according to claim 10, characterized in that the first separator and the second separator provided in step (a) consist of one separator piece. 20 Before joining in step (b), fold the separator piece in half along its length,
Claim 19 characterized in that the cross-section is V-shaped, with two walls corresponding respectively to the first separator and the second separator.
The method described in section. 21 Two upper walls corresponding to the second separator are positioned above the lower wall corresponding to the first separator, the longitudinal edges of the upper walls being along the center of the width and extending along the length of the lower wall. 20. A method as claimed in claim 19, characterized in that the separator pieces are folded in such a way that they are arranged parallel to each other. 22. A method according to claim 19, characterized in that the first separator and the second separator are joined in step (b). 23. The method of claim 22, wherein the separator pieces are comprised of porous polypropylene.