Ultrahigh molecular weight polyethylene film and preparation method thereof

Abstract

The invention discloses an ultra-high molecular weight polyethylene film and a preparation method thereof. The preparation method comprises the following steps: adding 20-40 parts by weight of a polyethylene mixture consisting of two or three ultra-high molecular weight polyethylene with different molecular weights and 0.1-0.5 part by weight of an antioxidant into an extruder, stirring and mixing, and heating to a preset temperature; wherein the molecular weight of the ultra-high molecular weight polyethylene is 600,000-3,000,000; the preparation method comprises the following steps: injecting 55-65 parts by weight of white oil into an extruder in batches, mixing and infiltrating the white oil with a polyethylene mixture, and shearing and melting to form a melt; detecting the temperature of the melt, and regulating the temperature by using white oil according to the temperature of the melt, so that the temperature of the melt is not higher than the set melting temperature; and shearing and dispersing the melt, and extruding to form the film. The ultra-high molecular weight polyethylene film prepared by the preparation method disclosed by the invention has excellent mechanical property and heat resistance, good film surface plasticization, no crystal point gel and no yellowing.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene film preparation technology, and specifically relates to an ultra-high molecular weight polyethylene film and its preparation method. Background Technology

[0002] Ultra-high molecular weight polyethylene (UHMWPE), due to its unique molecular structure, possesses properties such as high strength, wear resistance, chemical corrosion resistance, and excellent insulation. Its application in the battery field is expanding from a traditional auxiliary material to a core functional material. In batteries, UHMWPE is typically used in the form of films. However, existing UHMWPE films mainly use polyethylene with a molecular weight below 1.5 million. Polyethylene with a molecular weight above 1.5 million is difficult to process into UHMWPE films, especially polyethylene in the 1.7 million to 3 million molecular weight range, which is an extremely difficult polymer to process. This is mainly because its molecular chains often begin to degrade and break down before plasticizing and flowing. This results in poor mechanical properties and heat resistance in the finished film, failing to meet the application requirements of high-end solid-state lithium battery separators. Furthermore, traditional UHMWPE film preparation processes are inefficient, produce poor product quality, and suffer from problems such as yellowing of the film surface crystal points. Summary of the Invention

[0003] To address the aforementioned problems, this invention discloses an ultra-high molecular weight polyethylene film and its preparation method, thereby overcoming or at least partially solving the problems.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: This invention discloses a method for preparing an ultra-high molecular weight polyethylene film, the method comprising the following steps: Step 01: By weight, add 20 to 40 parts of a polyethylene mixture consisting of two or three different molecular weight ultra-high molecular weight polyethylenes and 0.1 to 0.5 parts of antioxidant into an extruder, stir and mix, and heat to a preset temperature; wherein, the molecular weight of the ultra-high molecular weight polyethylene is 600,000 to 3,000,000. Step 02: By weight, 55 to 65 parts of white oil are injected into the extruder in batches, mixed and impregnated with the polyethylene mixture, and sheared and melted to form a melt; Step 03: Detect the temperature of the melt and adjust the temperature using white oil according to the temperature of the melt so that the temperature of the melt does not exceed the set melting temperature; Step 04: Shear and disperse the melt, and extrude it into a film.

[0005] Further, step 02 includes: Step 021: Inject 65% to 75% of the white oil injection volume into the extruder through the injection port on the B3 barrel of the extruder; Step 022: The blend formed by the white oil and the polyethylene mixture is dispersed and sheared by the meshing disc and toothed disc in the extruder; Step 023: Inject 25% to 35% of the white oil into the extruder through the injection port on the B6 barrel of the extruder; Step 024: The blend of white oil and polyethylene is mixed and sheared by the shearing block and toothed disc in the extruder to form the melt.

[0006] Furthermore, the execution of step 03 includes: The temperature of the melt is checked once at preset intervals. If the temperature of the melt is not greater than the set melting temperature, no white oil is injected into the melt. If the temperature of the melt is greater than the set melting temperature, 1% of the amount of white oil injected in step 02 is injected into the melt. If the temperature of the melt is still greater than the set melting temperature in subsequent checks, the amount of oil injected into the melt is increased by 1% of the amount of white oil injected in step 02.

[0007] Furthermore, in step 03, the temperature of the white oil used to adjust the temperature of the melt is 40°C to 60°C.

[0008] Furthermore, the execution of step 03 includes: The temperature of the melt is detected once at preset intervals. The amount of white oil used for temperature control is determined by the following formula, and the amount of white oil is injected into the extruder to ensure that the temperature of the melt does not exceed the set melting temperature.

[0009] in, This indicates the mass flow rate ratio of white oil used for temperature control to the melt, and is dimensionless. This represents the reference mass flow rate ratio of white oil used for temperature control to the melt, and is dimensionless. The temperature difference is represented by e(t) = actual melt temperature - set melting temperature, in °C; t represents time, in seconds. Indicates proportional gain, dimensionless / ℃; This represents the integral gain, dimensionless / ℃·s; This represents the differential gain, dimensionless ·s / ℃.

[0010] Furthermore, the polyethylene mixture comprises 10 to 20 parts of ultra-high molecular weight polyethylene with a molecular weight of 1.7 million and 10 to 20 parts of ultra-high molecular weight polyethylene with a molecular weight of 3 million.

[0011] Furthermore, the preset temperature is 180℃ ~ 200℃.

[0012] Furthermore, the antioxidant is one or more of hindered phenolic antioxidants, phosphite antioxidants, and thioester antioxidants.

[0013] Furthermore, the screw speed in the extruder is 80 to 200 rpm.

[0014] Another aspect of the present invention discloses an ultra-high molecular weight polyethylene (UHMWPE) membrane, which is prepared by the above-described method for preparing UHMWPE membrane.

[0015] The advantages and beneficial effects of this invention are: In the preparation method of this invention, two or three different molecular weight ultra-high molecular weight polyethylenes and antioxidants are added to an extruder for mixing. White oil is added to the extruder in batches and mixed with ultra-high molecular weight polyethylene, so that the ultra-high molecular weight polyethylene is fully wetted and melted and plasticized. Finally, white oil is used to adjust the temperature so that the temperature of the melt does not exceed the set melting temperature, which timely neutralizes the excessive shear heat that causes the melt temperature to rise, and prevents excessive thermal degradation of ultra-high molecular weight polyethylene. As a result, the prepared ultra-high molecular weight polyethylene film has excellent mechanical properties and heat resistance, and the film surface is well plasticized, without crystal points or gels, and does not yellow. Attached Figure Description

[0016] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 This is a flowchart illustrating the steps of a method for preparing an ultra-high molecular weight polyethylene film in one embodiment of the present invention. Figure 2 This is a schematic diagram of the extruder in one embodiment of the present invention; Figure 3 This is a schematic diagram of the extruder in the comparative example.

[0017] In the diagram: 1. Main feed port; 2. Injection port A; 3. Injection port B; 4. Injection port C; 5. Multi-process element; 6. Meshing block element; 7. ZME single-head reverse pressure building toothed element; 8. Toothed disc element. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0019] The technical solutions provided by the various embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0020] One embodiment of the present invention provides a method for preparing an ultra-high molecular weight polyethylene film, such as... Figure 1 As shown, the preparation method includes the following steps: Step 01: By weight, add 20 to 40 parts of a polyethylene mixture consisting of two or three different molecular weights of ultra-high molecular weight polyethylene and 0.1 to 0.5 parts of antioxidant into an extruder, stir and mix, and heat to a preset temperature; wherein, the molecular weight of ultra-high molecular weight polyethylene is 600,000 to 3,000,000.

[0021] Specifically, such as Figure 2 As shown, 20 to 40 parts of a polyethylene mixture consisting of two or three different molecular weight ultra-high molecular weight polyethylenes and 0.1 to 0.5 parts of antioxidant are added to the extruder through the main feed port 1 on the extruder. The mixture is stirred and mixed in the extruder and heated to a preset temperature by thermal radiation so that the white oil added in the subsequent steps can better impregnate the polyethylene mixture.

[0022] Step 02: By weight, inject 55 to 65 parts of white oil into the extruder in batches, mix and impregnate it with the polyethylene mixture, and shear and melt it to form a melt.

[0023] Specifically, a portion of white oil is first injected into the extruder to mix and impregnate with the polyethylene mixture, allowing the white oil to migrate into the intersegments of the ultra-high molecular weight polyethylene molecules. Then, another portion of white oil is injected into the extruder, ensuring its uniform distribution within the melt of the polyethylene mixture, forming a three-dimensional spiderweb structure. Due to the high pressure within the extruder, the white oil can be injected into the extruder via a high-pressure metering injection system.

[0024] It should be noted that the length-to-diameter ratio of the extruder is 60 to 70:1, preferably 64:1. This range of length-to-diameter ratios ensures that the extruder has sufficient free volume, thereby allowing the polyethylene mixture to be fully impregnated.

[0025] Of course, in other embodiments, the white oil may be added to the extruder in three or other batches, which is also within the scope of protection of this invention.

[0026] Step 03: Detect the temperature of the melt and adjust the temperature using white oil to ensure that the melt temperature does not exceed the set melting temperature. The set melting temperature is 190℃ ~ 200℃.

[0027] Specifically, because the melt generates a large amount of shear heat during stirring and shearing within the extruder, leading to excessively high melt temperatures, the molecular chains of ultra-high molecular weight polyethylene (UHMWPE) are easily damaged when the melt temperature exceeds 200°C. Therefore, it is necessary to control the melt temperature to no higher than 200°C. That is, when the melt temperature is detected to be below 200°C, white oil is not injected into the extruder; when the melt temperature is detected to be above 200°C, such as... Figure 2 As shown, white oil is injected into the extruder through injection port C4 to lower the melt temperature. This method of temperature control using white oil serves two purposes: firstly, the white oil further wets the melt; secondly, its high specific heat capacity allows it to absorb more heat from the melt, making it easier to control the overall melt temperature within the desired range. Preferably, the temperature of the white oil used for temperature control is between 40°C and 60°C for better temperature control results.

[0028] Step 04: Shear and disperse the melt, and then extrude it into a film.

[0029] Specifically, such as Figure 2 As shown, the temperature-controlled melt is sheared and dispersed by the B12 and B13 barrels on the extruder to keep the melt refreshed, and then extruded under pressure through the B14 barrel. The barrel outlet of the extruder is connected in sequence to a melt pump, a casting die, and a film forming auxiliary device. The melt extruded from the extruder is pressurized again by the melt pump, and then extruded through the casting die to form a film. Finally, it is cooled and rolled into a roll by the film forming auxiliary device.

[0030] In summary, the preparation method of this embodiment involves mixing two or three different molecular weights of ultra-high molecular weight polyethylene (UHMWPE) and antioxidants in an extruder, and then adding white oil in batches to the extruder to mix with the UHMWPE. This allows the UHMWPE to melt and plasticize under full wetting. Finally, white oil is used to adjust the temperature so that the melt temperature does not exceed the set melting temperature, thus neutralizing the excessive shear heat that causes the melt temperature to rise and preventing excessive thermal degradation of the UHMWPE. As a result, the prepared UHMWPE film has excellent mechanical properties and heat resistance, and the film surface is well plasticized, without crystal points or gels, and does not yellow. The preparation method of this embodiment can process polyethylene with a molecular weight range of 1.7 million to 3 million. Furthermore, the porosity of the prepared ultra-high molecular weight polyethylene film is 30% to 50%, the tensile strength of the film in both the transverse and longitudinal directions is greater than 200 MPa, the puncture strength of the film is greater than 550 gf, the heat shrinkage rate of the film at 90°C for 1 hour is less than 2%, the closure temperature of the film is 120°C to 140°C, and the rupture temperature of the film is greater than 160°C.

[0031] In this embodiment, step 02 includes: Step 021, as follows Figure 2 As shown, 65% to 75% of the white oil injection volume is injected into the extruder through the injection port A2 on the B3 barrel of the extruder.

[0032] Step 022: The blend formed by the mixture of white oil and polyethylene is dispersed and sheared by the meshing disc and toothed disc in the extruder.

[0033] Specifically, such as Figure 2 As shown, the B4 and B5 barrels of the extruder are equipped with a multi-process element 5 and a meshing block element 6. The white oil is used to impregnate the ultra-high molecular weight polyethylene through the B4 and B5 barrels of the extruder, thereby improving the homogenization of the mixed raw materials and completing the initial plasticization of the ultra-high molecular weight polyethylene.

[0034] Step 023, as follows Figure 2 As shown, 25% to 35% of the white oil is injected into the extruder through the injection port B3 on the B6 barrel of the extruder.

[0035] Step 024: The blend of white oil and polyethylene is mixed and sheared by the shearing block and toothed disc in the extruder to form a melt.

[0036] Specifically, such as Figure 2As shown, ZME single-head reverse pressure-building toothed elements 7 and toothed disk elements 8 are arranged in the B7 to B10 barrels of the extruder. Through the distribution and mixing effect of the B7 and B8 barrels on the extruder, more small molecule white oil can enter between the molecular chains of ultra-high molecular weight polyethylene, thereby further swelling the melt and increasing the free volume between the ultra-high molecular weight polyethylene molecular chains. Through the strong shearing effect of the B9 and B10 barrels on the extruder, the large entanglements of the fully swollen ultra-high molecular weight polyethylene molecular chains are untied, thereby improving the plasticity of the melt.

[0037] Furthermore, the execution of step 03 includes: The temperature of the melt is checked at preset intervals. Specifically, the temperature of the melt inside the extruder is detected by a temperature sensor. If the melt temperature is not higher than the set melting temperature, no white oil is injected into the melt. If the melt temperature is higher than the set melting temperature, 1% of the white oil injection amount in step 02 (i.e., 0.55 to 0.65 parts by mass) of white oil is injected into the melt. If the melt temperature is still higher than the set melting temperature in subsequent checks, the amount of oil injected into the melt is increased by 1% of the white oil injection amount in step 02. The preset time is 1 to 3 minutes, preferably 2 minutes; the set melting temperature is 190°C to 200°C.

[0038] For example, when the melt temperature is detected to be higher than 200°C for the first time, 1% of the white oil injected in step 02 is sprayed into the melt. When the melt temperature is detected to be higher than 200°C for the second time, 2% of the white oil injected in step 02 is sprayed into the melt. When the melt temperature is detected to be higher than 200°C for the third time, 3% of the white oil injected in step 02 is sprayed into the melt, and so on, until the melt temperature is lower than 200°C.

[0039] In another embodiment of the present invention, the execution of step 03 includes: The temperature of the melt is checked at preset time intervals, which are set as needed. The amount of white oil used for temperature control is determined using the following formula, and the amount of white oil is injected into the extruder to ensure that the melt temperature does not exceed the set melting temperature. During this process, the melt temperature is checked once, and a certain amount of white oil is injected into the melt. The amount of white oil injected each time is determined according to the following formula.

[0040]

[0041] in, This indicates the mass flow rate ratio of white oil used for temperature control to the melt, and is dimensionless. This represents the reference mass flow rate ratio of white oil used for temperature control to the melt, and is dimensionless. Representing the temperature difference, e(t) = the actual temperature of the melt (T). out- Set melting temperature, unit: °C; t represents time, unit: seconds; Indicates proportional gain, dimensionless / ℃; This represents the integral gain, dimensionless / ℃·s; This represents the differential gain, dimensionless, in s / ℃. It should be noted that... The values ​​of the set melting temperature and the preset time t are: value The value and The value is determined based on the actual situation.

[0042] Specifically, the following selections The value is 0.133, the melting temperature is set to 200℃, and the preset time t is 5 s. 0.005 For 0.001 and The value is 0.0005 to explain the above formula.

[0043] For the proportional term When the detected melt temperature is too high, that is... ,at this time ,but This indicates that the amount of white oil needs to be increased; when the detected melt temperature is too low, that is... ,at this time ,but This indicates that the amount of white oil needs to be reduced. For example, hour, ,but At this point, the amount of white oil needs to be increased by 1%; hour, ,but At this point, the amount of white oil needs to be reduced by 1%.

[0044] For the integral term When the detected melt temperature remains consistently high, i.e. ,at this time ,but This indicates that the amount of white oil needs to be continuously increased; when the detected melt temperature remains consistently low, that is... ,at this time ,but This indicates a need to continuously reduce the amount of white oil. For example, if the melt temperature is detected to be 1°C higher than normal for 30 consecutive seconds, ,but At this point, the amount of white oil needs to be increased by 3%.

[0045] For differential terms When the detected melt temperature rises rapidly, that is... ,at this time ,but This indicates that white oil needs to be added in advance to prevent the melt from heating up too quickly; when the detected melt temperature drops rapidly, that is... ,at this time ,but This indicates that the amount of white oil needs to be reduced in advance to prevent the melt from cooling down too quickly. For example, when the detected melt temperature rises at a rate of 0.5 ℃ / s, ,but At this point, the amount of white oil needs to be slightly increased; when the detected melt temperature decreases at a rate of 1 ℃ / s, ,but At this point, the amount of white oil needs to be slightly reduced.

[0046] Understandably, when determining the amount of white oil used for temperature control using the above formula, the amount of white oil is first determined based on the initially detected melt temperature. During subsequent melt temperature detection, the amount of white oil is further adjusted based on the continuous state and rate of change of the melt temperature. In this way, it can not only take into account the situation where the melt temperature is continuously too high or too low, but also the situation where the melt temperature rises or falls rapidly, ensuring rapid and precise control of the melt temperature to keep it at a reasonable temperature.

[0047] Preferably, as shown in Table 1, the temperature (T) of the melt is detected every 5 seconds. out ), and substitute this temperature into the above formula to calculate the corresponding Then according to The value is used to execute the corresponding control action.

[0048] Table 1

[0049] It should be noted that when the initial temperature of the detected melt is >210℃, a step response is used to determine... =0.18, meaning the ratio of the amount of white oil injected to the mass flow rate of the melt is 0.18:1; when the initial temperature of the melt is detected to be <190℃, the injection of white oil is completely cut off.

[0050] In one comparative example, the extruder used was a twin-screw extruder with a length-to-diameter ratio of 48:1, such as... Figure 3As shown, white oil is injected into the extruder only through the injection ports on barrels B3 and B7 to impregnate the polyethylene mixture, without using white oil for temperature control. In this comparative example, due to the small length-to-diameter ratio of the injection molding machine, the free volume of the extruder is low, resulting in insufficient impregnation of the polyethylene mixture and poor plasticization. In addition, the heat generated by localized excessive shearing causes the molecular chains in the ultra-high molecular weight polyethylene film to break, resulting in ultra-high molecular weight polyethylene with a molecular weight of less than 1.5 million. Furthermore, the prepared film is yellowed and has an orange peel texture on the surface, and the prepared film has defects such as more gel points and low melt strength.

[0051] In a preferred embodiment of the present invention, the polyethylene mixture comprises 10 to 20 parts of ultra-high molecular weight polyethylene with a molecular weight of 1.7 million and 10 to 20 parts of ultra-high molecular weight polyethylene with a molecular weight of 3 million. The ultra-high molecular weight polyethylene film prepared from this composition has better quality and higher strength.

[0052] In this embodiment, the preset temperature is 180℃ ~ 200℃, at which the polyethylene mixture can achieve a better wetting effect.

[0053] Furthermore, the antioxidant is one or more of hindered phenolic antioxidants, phosphite antioxidants, and thioester antioxidants. Of course, other types of antioxidants may also be used, and they are also within the scope of protection of this invention.

[0054] In addition, the screw speed in the extruder is 80 to 200 rpm, which allows the polyethylene mixture, antioxidants and white oil to be better dispersed, and the melt to be better sheared.

[0055] In another embodiment of the present invention, an ultra-high molecular weight polyethylene (UHMWPE) film is provided. The UHMWPE film is prepared by the UHMWPE film preparation method described above. The UHMWPE film has excellent mechanical properties and heat resistance, and the film surface is well plasticized, without crystal points or gel, and does not yellow.

[0056] The above description is merely a specific embodiment of the present invention. Under the teachings of the present invention, those skilled in the art can make other improvements or modifications based on the above embodiments. Those skilled in the art should understand that the above specific description is only to better explain the purpose of the present invention, and the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing an ultra-high molecular weight polyethylene film, characterized in that, The preparation method includes the following steps: Step 01: By weight, add 20 to 40 parts of a polyethylene mixture consisting of two or three different molecular weight ultra-high molecular weight polyethylenes and 0.1 to 0.5 parts of antioxidant into an extruder, stir and mix, and heat to a preset temperature; wherein, the molecular weight of the ultra-high molecular weight polyethylene is 600,000 to 3,000,000. Step 02: By weight, 55 to 65 parts of white oil are injected into the extruder in batches, mixed and impregnated with the polyethylene mixture, and sheared and melted to form a melt; Step 03: Detect the temperature of the melt and adjust the temperature using white oil according to the temperature of the melt so that the temperature of the melt does not exceed the set melting temperature; Step 04: Shear and disperse the melt, and extrude it into a film.

2. The method for preparing ultra-high molecular weight polyethylene film according to claim 1, characterized in that, Step 02 includes: Step 021: Inject 65% to 75% of the white oil injection volume into the extruder through the injection port on the B3 barrel of the extruder; Step 022: The blend formed by the white oil and the polyethylene mixture is dispersed and sheared by the meshing disc and toothed disc in the extruder; Step 023: Inject 25% to 35% of the white oil into the extruder through the injection port on the B6 barrel of the extruder; Step 024: The blend of white oil and polyethylene is mixed and sheared by the shearing block and toothed disc in the extruder to form the melt.

3. The method for preparing ultra-high molecular weight polyethylene film according to claim 1, characterized in that, The execution of step 03 includes: The temperature of the melt is checked once at preset intervals. If the temperature of the melt is not greater than the set melting temperature, no white oil is injected into the melt. If the temperature of the melt is greater than the set melting temperature, 1% of the amount of white oil injected in step 02 is injected into the melt. If the temperature of the melt is still greater than the set melting temperature in subsequent checks, the amount of oil injected into the melt is increased by 1% of the amount of white oil injected in step 02.

4. The method for preparing ultra-high molecular weight polyethylene film according to claim 3, characterized in that, In step 03, the temperature of the white oil used to adjust the temperature of the melt is 40℃ ~ 60℃.

5. The method for preparing ultra-high molecular weight polyethylene film according to claim 1, characterized in that, The execution of step 03 includes: The temperature of the melt is detected once at preset intervals. The amount of white oil used for temperature control is determined by the following formula, and the amount of white oil is injected into the extruder to ensure that the temperature of the melt does not exceed the set melting temperature. in, This indicates the mass flow rate ratio of white oil used for temperature control to the melt, and is dimensionless. This represents the reference mass flow rate ratio of white oil used for temperature control to the melt, and is dimensionless. The temperature difference is represented by e(t) = actual melt temperature - set melting temperature, in °C; t represents time, in seconds. Indicates proportional gain, dimensionless / ℃; This represents the integral gain, dimensionless / ℃·s; This represents the differential gain, dimensionless ·s / ℃.

6. The method for preparing ultra-high molecular weight polyethylene film according to claim 1, characterized in that, The polyethylene mixture comprises 10 to 20 parts of ultra-high molecular weight polyethylene with a molecular weight of 1.7 million and 10 to 20 parts of ultra-high molecular weight polyethylene with a molecular weight of 3 million.

7. The method for preparing ultra-high molecular weight polyethylene film according to claim 1, characterized in that, The preset temperature is 180℃ ~ 200℃.

8. The method for preparing ultra-high molecular weight polyethylene film according to claim 1, characterized in that, The antioxidant is one or more of hindered phenolic antioxidants, phosphite antioxidants, and thioester antioxidants.

9. The method for preparing ultra-high molecular weight polyethylene film according to any one of claims 1 to 8, characterized in that, The screw speed in the extruder is 80 to 200 rpm.

10. A type of ultra-high molecular weight polyethylene film, characterized in that, The ultra-high molecular weight polyethylene film is prepared by the method for preparing ultra-high molecular weight polyethylene film according to any one of claims 1 to 9.

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