KR20010068785A - Microporous membrane of high crystalline polypropylene and preparation method thereof - Google Patents

Abstract

PURPOSE: Provided is a method for manufacturing high crystalline polypropylene microporous film excellent in permeability and mechanical properties by using high crystalline polypropylene with crystallization degree of more than 50% and high isotacticity. CONSTITUTION: The method comprises: i) manufacturing precursor film by using high crystalline polypropylene having one or more of properties comprising a crystallization degree of more than 50%, an isotacticity of more than 95%, an atactic fraction of less than 5%, a density of more than 0.905g/cm¬3, a melting temperature of more than 164deg.C and a crystallization temperature of more than 125deg.C; ii) annealing the precursor film; iii) cold stretching the annealed film; iv) hot stretching the cold stretched film; and v) heat setting the hot stretched film.

Description

Microporous membrane of high crystalline polypropylene and preparation method

[Industrial use]

The present invention relates to a microporous membrane of high crystalline polypropylene and a method for preparing the same, and more particularly, to a high crystalline polypropylene (High Crystalline Polypropylene (HCPP)) having a crystallinity of 50% or more and a very high isotacticity. It relates to a method for producing a microporous membrane of high crystalline polypropylene by the step of preparing the original film using the annealing step, low temperature drawing step, high temperature drawing step and heat setting step.

[Prior art]

Microporous membranes are widely used in various fields such as filtration membranes such as air purification and water treatment, use as separators in electrolysis or batteries, gas exchange membranes, artificial organs, purification of various beverages, and purification of enzymes. In particular, microporous membranes are increasing in importance as separators in batteries (especially lithium ion batteries).

The separator basically serves to isolate the anode and the cathode and to prevent the short circuit caused by the fusion bonding between the two poles, while passing the electrolyte or the ions. Various kinds of separators are used according to the type of battery, the chemical system of the electrolyte, etc. However, various research and developments have recently been performed in lithium ion batteries because they require different characteristics from those used in other batteries. Conventional separator materials used in conventional batteries include cellulose and nonwoven fabrics, but these are difficult to satisfy the above-described characteristics required by batteries. Thus, new microporous separators using synthetic resin materials have been developed.

In the lithium ion battery, a highly active organic solvent is used as an electrolyte, and thus, polyolefin resins having low reactivity with organic solvents and low manufacturing costs are used. As for the materials other than polyolefin resins, as a separator for lithium ion batteries, There is no practical use.

There may be a number of processes that can be used on a theoretical or laboratory scale to prepare a precursor film using a polyolefin resin, but currently commercially available separator manufacturing methods include fillers or waxes and There are only two types of wet methods using solvents and dry methods without solvents. In addition, the stretching processes involved in forming the micropores from these methods include the uniaxial method and the biaxial method.

Compared with the wet method, the dry method is capable of producing a wide-format disc film, relatively easy to produce, and does not use a solvent, and thus has an excellent manufacturing environment and has an advantage of easy mass production. The method of manufacturing the microporous membrane by using the dry method includes a continuous cold stretching and hot stretching process as shown in US Patent Nos. 3,679,538 and 3,801,692. In general, these processes are a series of processes in which a film of high crystallinity and elasticity is continuously drawn at high temperature through a cold drawing process to form micropores, and then film formation is completed by heat setting. Include them. Special techniques for continuous cold drawing and hot drawing processes are disclosed in US Pat. Nos. 3,843,761 and 4,238,459. The disclosed techniques include methods of initially cold drawing an annealed disc film followed by multi-step high temperature drawing. In addition, U.S. Patent No. 5,013,439 discloses the use of a multi-step cold draw process in continuous cold draw and hot draw processes to produce membranes with reduced pore size and increased pore density. have. According to U.S. Patent Nos. 5,385,777 and 5,480,745, micropores are subjected to the above-described continuous multi-step cold drawing and high temperature drawing process using a polyethylene (PE) / polypropylene (PP) blend system to improve the safety of lithium ion batteries. Disclosing a film is disclosed.

In addition, a method of manufacturing a lithium ion battery separator by laminating polyethylene and polypropylene using a dry method is disclosed in EP-A-715,364, 718,901 and 723,304, and US Pat. Nos. 5,240,655, 5,342,695 and 5,472,792, Japanese Patent Application Laid-Open No. 4-181651, and the like. As described above, in the dry method of manufacturing a separator using a crystalline polymer as a material, relatively weak amorphous regions are ruptured through cold drawing, and pores are formed. It has the advantage of no clean process, but the membrane manufactured by this process has a slight decrease in porosity and pore size of the membrane, difficulty in uniform control of pore size and shape, and an increase in elongation to maintain the shape of the membrane. There is also a limit to the difficulty in improving the porosity.

In addition, in the prior art, battery separators have not been manufactured using high crystalline polypropylene, but the researchers of the present invention have found that the air permeability of membranes made of high crystalline polypropylene is much better than that of membranes made of general purpose polypropylene. It is intended to use high crystalline polypropylene in the field of battery separators requiring more properties.

The present invention is to solve the above problems, the object of the present invention is

It is to provide a microporous membrane of highly crystalline polypropylene having a uniform pore size distribution, high pore density and porosity.

It is another object of the present invention to prepare a negative film using high crystalline polypropylene, annealing step, low temperature drawing step, high temperature drawing step and heat setting step, and fine pore having uniform pore size distribution, high pore density and porosity. It is to provide a method for producing a membrane.

Still another object of the present invention is to provide a battery separator, particularly a lithium ion or lithium ion polymer battery separator, having excellent permeability and mechanical properties for batteries using the prepared microporous membrane.

1 is a scanning electron microscope (Scanning Electron Microscopy, SEM) photograph of the microporous membrane prepared using the highly crystalline polypropylene of Example 1 of the present invention,

2 is a scanning electron micrograph of a microporous membrane prepared using the general polypropylene of Comparative Example 1 of the present invention.

In order to achieve the above object, the present invention

a) preparing a disc film of high crystalline polypropylene;

b) annealing the original film;

c) cold drawing the annealed disc film;

d) hot drawing the low temperature stretched film; And

e) heat setting the hot drawn film

It provides a method for producing a microporous membrane of high crystalline polypropylene by a dry method comprising a.

Hereinafter, the present invention will be described in more detail.

First, the high crystalline polypropylene used to prepare the microporous membrane of the present invention will be described in detail, and the method for producing the microporous membrane of high crystalline polypropylle will be described in detail, and then the battery separator using the microporous membrane of the present invention. This will be described in detail.

In the present invention, the highly crystalline polypropylene used to prepare the microporous membrane has a degree of crystallinity of about 50% or more, in contrast to the general degree of crystallinity of general-purpose polypropylene, which is generally used, at a density of about 0.905 g / cm. ³ or more) is used. Highly crystalline polypropylene of the present invention exhibits high density, melting temperature, heat of fusion and crystallization temperature due to its high isotactic compared to general general-purpose polypropylene, and has high rigidity, high heat resistance, high impact resistance, and high resistance. It has excellent mechanical properties such as scratch resistance and high dimensional stability.

There are several ways to distinguish general general purpose polypropylene and high crystalline polypropylene. Among them, classification is based on melting temperature, crystallization temperature, crystallinity, isotacticity, atactic fraction, etc. It can be said. Melting temperature and crystallization temperature are measured using a dynamic scanning calorimeter (DSC), which is a general general purpose polypropylene melting temperature of 160 to 163 ℃ and the crystallization temperature of 117 to 120 ℃, The melting temperature is above 164 ° C. and the crystallization temperature shows high value above 130 ° C. The degree of crystallinity is expressed as a percentage of the weight of the crystallized portion relative to the total weight of the polymer, and is measured using a dynamic scanning calorimeter or an X-ray diffractometer (XRD). Called propylene. The isotacticity of the polypropylene is measured by C ¹³ NMR and the like. The isotacticity of the highly crystalline polypropylene is about 95% or more, while the isotacticity of the general purpose polypropylene is 93 to 96%.

In addition, when the atactic fraction of polypropylene is measured using xylene, the general purpose polypropylene shows a value of 5% or more, but the high crystalline polypropylene of the present invention has an atactic fraction of less than 5%. Indicates.

Therefore, in order to prepare the microporous membrane satisfying the properties required by the present invention, the highly crystalline polypropylene used is 50% or more in crystallinity, 95% or more in isotacticity and less than 5% in atactic fraction, 0.905 g in density. It is preferable to use a material which satisfies any one or more of physical properties of / cm ³ or more, a melting temperature of 164 ° C or more, and a crystallization temperature of 125 ° C or more. The above properties of the highly crystalline polypropylene of the present invention are not related to each other but are correlated with each other. In particular, the crystallinity and isotacticity in the above properties are very important to satisfy the properties required for the microporous membrane of the present invention. Must satisfy In addition, when the microporous membrane is prepared using the high crystalline polypropylene of the present invention, the air permeability of the microporous membrane prepared using the high crystalline polypropylene of the present invention to improve the conductivity of ions such as lithium ions through the prepared membrane is It is preferable that it is 500 second / 100 cc or less. In order to improve the stability of the membrane, the separator for a lithium ion battery containing the highly crystalline polypropylene of the present invention preferably has a high membrane breaking temperature, and most preferably a membrane breaking temperature of 163 ° C or higher.

That is, when the microporous membrane is manufactured using the polypropylene having no physical properties described above, the uniformity and porosity of the pores of the prepared pore membrane are not excellent and the permeability of the pores is lowered. Preference is given to using highly crystalline polypropylene which satisfies the values of the described properties.

As described above, a method of manufacturing a separator using the dry method is a method of forming pores by orienting a polymer crystal part in a predetermined direction and then rupturing a relatively weak amorphous part through cold drawing. Therefore, the characteristics of the microporous membrane to be produced are determined not only by the uniform orientation and degree of the polymer crystal portion, but also by the crystallinity of the polymer used, and thus the permeation of the membrane produced using the high crystalline polypropylene. To improve the properties and mechanical properties.

That is, in the present invention, by using high crystalline polypropylene instead of general general-purpose polypropylene having a crystallinity of less than 50%, the degree of orientation is improved and the microporous membrane having a very excellent orientation uniformity is used. Therefore, the pore membrane prepared using the high crystalline polypropylene has a very uniform pore size distribution, high pore density and porosity compared to the conventional pore membrane, thereby not only having improved permeability properties, but also high polypropylene. Due to the crystallinity, the prepared pore membrane has improved mechanical properties.

When the pore membrane prepared using the high crystalline polypropylene of the present invention is used as a separator for a lithium ion battery, the air permeability is excellent and the permeability is remarkably improved, and thus the charge and discharge characteristics are excellent, and in particular, the low temperature characteristics are excellent. Due to the high melting temperature of the highly crystalline polypropylene, the breakage temperature of the membrane is high and the stability of the battery is also significantly improved.

In addition, the microporous membrane of the present invention may be prepared using the above-mentioned high crystalline polypropylene alone to satisfy the required physical properties according to the use, or lamination together with a compound such as high crystalline polypropylene and polyolefin having excellent physical properties. Or may be prepared by blending. In the present invention, the compound used to prepare the microporous membrane together with the highly crystalline polypropylene includes general purpose polypropylene, polyethylene, ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM), and the like. One or more ethylene propylene copolymers may be used. In the case of the polyethylene, high density polyethylene, low density polyethylene, linear low density polyethylene, or the like may be used alone or together. In addition, the microporous membrane of the highly crystalline polypropylene of the present invention or a microporous membrane prepared by laminating or blending one or more compounds that can be used in the preparation of the microporous membrane to the high crystalline polypropylene has properties such as conductivity to ions. It is excellent and can be used for the separator for lithium ion batteries or the separator for lithium ion polymer batteries. The air permeability of the separator for a lithium ion battery manufactured using the fixed polypropylene of the present invention is preferably 500 seconds / 100 cc or less in order to easily conduct lithium ions through the membrane.

Next, a method for producing a microporous membrane by the dry method using the high crystalline polypropylene of the present invention will be described.

In the present invention, a method of preparing a microporous membrane using high crystalline polypropylene is as follows.

(1) Preparation of Disc Film:

The high crystalline polypropylene of the present invention alone, or the polyolefin containing the high crystalline polypropylene by a conventional method using an extruder attached to a T-die or a tubular die Prepare the original film. At this time, the extrusion conditions may be different depending on the compound used to prepare the microporous membrane, but the extrusion temperature is 150 to 300 ℃, the cooling roll (roll) temperature is 0 to 150 ℃, winding speed is 5 to 100m / min desirable.

(2) Annealing of Film:

In order to increase the crystallinity and elasticity recovery rate of the prepared original film, it is annealed at a temperature below the melting point of the polyolefin in a drying oven. The annealing is preferably performed for 30 seconds to 1 hour below the melting point of the polyolefin used because the melting point of the general polyolefin than the high crystalline polypropylene is lower.

(3) Low temperature stretching:

The annealed film is stretched uniaxially or biaxially at temperatures below room temperature using a roll or other stretching machine to produce microcracks. The elongation at low temperature elongation may vary depending on the degree of formation of micropores, but is preferably 10 to 150%.

(4) high temperature stretching:

The low temperature stretched film is uniaxially or biaxially stretched using a roll or other stretching machine at a temperature below the melting point of the polymer to form micropores having a desired size and impart mechanical properties to the membrane. The high temperature stretching conditions may vary depending on the raw material of the film, the high temperature stretching temperature is preferably 100 to 200 ℃ and the elongation is preferably 30 to 200%.

(5) heat fixing:

The high temperature stretched film is heat-fixed for a predetermined time as it is under tension under the melting point of the polymer for film production to stabilize the micropores formed. The heat setting is preferably performed for about 1 minute to 1 hour at a temperature similar to the high temperature stretching temperature.

The steps used in the method for producing the microporous membrane of the present invention describe the entire process for the preparation of the membrane having optimum physical properties, and according to the membrane having the required final physical properties, some steps are omitted or generally used in the preparation of the membrane. Not only can additional steps be added, but the order of the above steps can also be altered to produce membranes.

Air permeability, porosity, pore size, shut-down (SD) temperature, and melt-integrity temperature for the microporous membrane prepared by the above method Measure physical properties such as).

The microporous membrane prepared by the present invention exhibits a very uniform pore size distribution, high pore density and porosity compared to a membrane made of conventional general-purpose polypropylene. ) Can be used as a battery separator in the case of extrusion to a certain thickness because of the improved mechanical properties. In particular, in the case of manufacturing a separator for a lithium ion polymer battery using the microporous membrane of the present invention, both sides of the microporous membrane of the present invention having a predetermined thickness are coated with a polymer electrolyte on a gel by a conventional method to prepare a separator. do. As the polymer electrolyte used to prepare the separator for the lithium ion polymer battery, polyvinylidene fluoride (PVDF), polyurethane, polyethylene oxide, polyacrylonitrile, polymethyl Acrylate (polymethylacrylate), polyacrylamide, polyvinylacetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate, etc. may be used in combination of one or more Alternatively, a copolymer prepared by copolymerizing two or more compounds may be used.

EXAMPLE

The following presents preferred examples and comparative examples to aid in understanding the invention. However, the following examples are merely provided to more easily understand the present invention, and the present invention is not limited to the following examples.

(Example 1)

Preparation of Microporous Membrane Using Highly Crystalline Polypropylene

Melt index 2.0g / 10min, density 0.91g / cc, melt temperature measured using DSC, 167 ° C crystallization temperature, 135 ° C, crystallinity 58%, isometry measured by C ¹³ NMR Highly crystalline polypropylene having a taxitic fraction of about 2% after tacticity was dissolved in 98.5% and xylene was used as a raw material of the microporous membrane. Take the high crystalline polypropylene from a single screw extruder with a T-die with an extrusion temperature of 220 ° C. and a cold roll temperature of a winding device with a temperature of 70 ° C. and a winding speed of 50 m / min. A precursor film was prepared using an up device, and the draw down rate (DDR) of the prepared original film was 90. The prepared original film was annealed at 160 ° C. for 1 hour in a drying oven. The annealed film was uniaxially stretched at a draw ratio of 50% relative to the initial length at room temperature using a roll stretching method. After performing the stretching at room temperature, using a roll stretching method, a high-temperature uniaxial stretching at a stretching ratio of 100% at 140 ° C. was performed, followed by thermal fixation for about 10 minutes under tension at 140 ° C., followed by cooling to prepare a microporous membrane. It was. The physical properties of the obtained microporous membrane were measured and the results are shown in Table 1, and the scanning electron micrograph (Scanning Electron Microscopy, SEM) photograph of the pore membrane is shown in FIG. 1.

(Example 2)

Preparation of Microporous Membranes Composed of Highly Crystalline Polypropylene / Polyethylene Laminates

The same high crystalline polypropylene and high density polyethylene as used in Example 1 were used as raw materials for the original film. The melt index of the high density polyethylene was 0.3 g / 10 min and the density was 0.964 g / cc. The original film was prepared using the same extruder, T-die and winding apparatus as in Example 1, in the case of polyethylene, the extrusion temperature was 210 ° C., the cold roll temperature of the winding device was 80 ° C., and the winding speed was 50 m / min. At this time, the winding ratio of the original film produced was 90. The prepared high crystalline polypropylene and high density polyethylene original film had a thickness of 10 μm, respectively. The prepared two original films were laminated in the order of polypropylene / high density polyethylene / polypropylene at high temperature using a press having a temperature of 130 ° C. and a pressure of 50 kg / cm ² . The laminated original film was annealed at 125 ° C. for about 1 hour in the same drying oven as in Example 1. The annealed film was uniaxially stretched at a draw ratio of 50% at room temperature and 100% at 125 ° C in the same manner as in Example 1, and then heat-fixed and cooled for about 10 minutes under tension at a temperature of 125 ° C. Microporous membranes were prepared. The physical properties of the obtained microporous membrane were measured and the results are shown in Table 1 below.

(Example 3)

Preparation of Microporous Membranes Composed of Highly Crystalline Polypropylene / Polyethylene Blends

The same high crystalline polypropylene and high density polyethylene as used in Examples 1 and 2 were blended to 70/30% by weight of polypropylene / high density polyethylene and used as a raw material of the original film. The original film was prepared using the same extruder, T-die and winding apparatus as in Example 1, the extrusion temperature is 210 ℃, the cold roll temperature of the winding device is 80 ℃ and the winding speed was 50m / min, the production Ratio of the original film thus obtained was 90. The obtained raw film was annealed at 125 ° C. for about 1 hour in the same drying oven as in Example 2. The annealed film was uniaxially stretched at a draw ratio of 50% at room temperature and 100% at 125 ° C in the same manner as in Example 1, and then heat-fixed and cooled for about 10 minutes under tension at a temperature of 125 ° C. Microporous membranes were prepared. The physical properties of the obtained microporous membrane were measured and the results are shown in Table 1 below.

(Comparative Example 1)

Preparation of Microporous Membrane Using General-Purpose Polypropylene

Melt index 2.0g / 10min, density 0.90g / cc, melt temperature measured using DSC is 162 ° C, crystallization temperature is 119 ° C, crystallinity is 48%, isometry measured by C ¹³ NMR The microporous membrane was prepared in the same manner and conditions as in Example 1 using general purpose polypropylene having a taxity of 95% and an atactic fraction of about 6% measured after dissolving in xylene. The physical properties of the obtained microporous membrane were measured and the results are shown in Table 1, and the scanning electron micrograph (Scanning Electron Microscopy, SEM) photograph results of the pore membrane are shown in FIG. 2.

Example 1 Example 2 Example 3 Comparative Example 1 Thickness (㎛) 25 25 25 25 Pore size (μm) 0.3 x 0.1 0.3 x 0.1 0.3 x 0.1 0.2 x 0.1 Porosity (%) 48 42 44 37 Breathability (sec / 100cc) 160 380 310 530 Non-Resolution Temperature (℃) 165 135 134 161 Membrane breaking temperature (℃) 168 168 168 163

Measurement of physical properties of Table 1 was performed by the following criteria.

Breathability: JIS P8117 Porosity: ASTM D2873 Pore Size: SEM, TEM

As shown in Table 1, it can be seen that the air permeability and porosity of the membrane prepared using the highly crystalline polypropylene of Example 1 were significantly improved compared to the membrane of Comparative Example 1 prepared using the general-purpose polypropylene. It can be seen that the membrane breaking temperature, which is considered important when used as a separator, is high. In addition, as shown in the SEM photographs of FIGS. 1 and 2, the fine pore membrane prepared using the high crystalline polypropylene of the present invention has a uniform pore size and pore size compared to the pore membrane prepared using general polypropylene. It can be seen that the density is remarkably high.

As shown in Table 1, the laminate membrane prepared in Example 2 was found to have better air permeability and porosity than the membrane prepared from the general-purpose polypropylene of Comparative Example 1, in particular, the membrane breaking temperature determined by polypropylene. It can be seen that the high is very advantageous in terms of battery stability.

It can be seen that the membrane prepared in Example 3 also has better air permeability and porosity than the membrane prepared in Comparative Example 1, and the membrane breaking temperature also shows a high value similar to that of the membrane composed of the laminate of Example 2. It can be seen that it is very advantageous in terms of stability.

Since the microporous membrane prepared by the method for producing a microporous membrane of the highly crystalline polypropylene of the present invention has a very uniform pore size distribution, high pore density and porosity, a battery separator, particularly lithium ion or polymer having excellent permeability and mechanical properties, It can be used as a separator for lithium ion polymer batteries.

Claims (15)

At least 50% of crystallinity, at least 95% of isotacticity and less than 5% of atactic fraction, at least one of the physical properties consisting of a density of at least 0.905g / cm ³ , a melting temperature of at least 164 ℃ and a crystallization temperature of at least 125 ℃. Satisfactory microporous membrane of high crystalline polypropylene. The method of claim 1, A microporous membrane further comprising at least one member selected from the group consisting of general purpose polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, and ethylene propylene copolymer. The method according to claim 1 or 2, Microporous membrane with air permeability of 500 sec / 100 cc or less. a) at least 50% of crystallinity, at least 95% of isotacticity and at least 5% of atactic fraction, at least 0.905 g / cm ³ , at least 164 ° C of melting temperature and at least 125 ° C of crystallization temperature Preparing an original film of high crystalline polypropylene satisfying the above; b) annealing the original film; c) cold drawing the annealed disc film; d) hot drawing the low temperature stretched film; And e) heat setting the hot drawn film Method for producing a microporous membrane comprising a. The method of claim 4, wherein A method for producing a microporous membrane further comprising at least one member selected from the group consisting of general purpose polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, and ethylene propylene copolymer. At least 50% of crystallinity, at least 95% of isotacticity and less than 5% of atactic fraction, at least one of the physical properties consisting of a density of at least 0.905g / cm ³ , a melting temperature of at least 164 ℃ and a crystallization temperature of at least 125 ℃. Separation membrane for lithium ion battery of satisfactory high crystalline polypropylene. The method of claim 6, A separator for a lithium ion battery further comprising at least one member selected from the group consisting of general-purpose polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, and ethylene propylene copolymer. The method according to claim 6 or 7, Separation membrane for lithium ion batteries with air permeability of 500 sec / 100 cc or less. The method according to claim 6 or 7, The separator for lithium ion batteries whose membrane breaking temperature is 163 degreeC or more. At least 50% of crystallinity, at least 95% of isotacticity and less than 5% of atactic fraction, at least one of the physical properties consisting of a density of at least 0.905g / cm ³ , a melting temperature of at least 164 ℃ and a crystallization temperature of at least 125 ℃. A separator for lithium ion polymer batteries of satisfactory high crystalline polypropylene. The method of claim 10, Separation membrane for a lithium ion polymer battery further comprising at least one member selected from the group consisting of general-purpose polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, and ethylene propylene copolymer. The method according to claim 10 or 11, wherein Separation membrane for a lithium ion polymer battery prepared by coating a polymer electrolyte on a gel on both sides of the separator. The method according to claim 10 or 11, wherein Separation membrane for lithium ion polymer batteries with air permeability of 500 sec / 100 cc or less. The method according to claim 10, wherein Separation membrane for lithium ion polymer batteries whose membrane breaking temperature is 163 ° C or higher. The method of claim 12, The polymer electrolyte on the gel is polyvinylidene fluoride, polyurethane, polyethylene oxide, polyacrylonitrile, polymethyl acrylate, polyacrylamide, polyvinylacetate, polyvinylpyrrolidone, polytetraethylene glycol diacrylate and The separator for lithium ion polymer batteries which is at least 1 sort (s) chosen from the group which consists of a copolymer containing these 2 or more types.