JPH11286554A - Production of rubber-modified vinyl-based polymer composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a rubber-modified vinyl-based polymer composition in a slight consumption of dry energy, excellent in operability, productivity, uniform physical properties and in color tone. SOLUTION: A vinyl-based monomer is subjected to emulsion polymerization in the presence of a rubber-like polymer to give a rubber-modified polymer latex, which is coagulated to give a slurry. The slurry is dehydrated or treated as it is to be dehydrated by a specific pressing dehydrator to 10-30 wt.% water content. The dehydrated material and the melt of a vinyl-based polymer are supplied to a twin screw extruder with a vacuum vent and pelletized to produce the objective rubber-modified vinyl-based polymer composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a rubber-modified vinyl polymer composition such as an ABS resin. More specifically, an ABS resin is continuously prepared from a slurry obtained by coagulating a rubber-modified vinyl polymer latex obtained by emulsion polymerization and a vinyl polymer obtained by suspension, solution or bulk polymerization. And other methods for producing a rubber-modified vinyl polymer composition.

[0002]

2. Description of the Related Art Heretofore, generally, a method for producing a rubber-modified vinyl polymer composition such as an ABS resin is generally carried out by (1) slurry obtained by coagulating a rubber-modified vinyl polymer latex. (2) a method of dehydrating with a centrifugal dehydrator or a vacuum belt filter, drying with hot air, temporarily forming a dry powder, blending with a vinyl polymer, and pelletizing with an extruder; or (2)
After the slurry obtained by coagulating the rubber-modified vinyl polymer latex is dehydrated by a centrifugal dehydrator or a vacuum belt filter, the wet powder is supplied to a twin-screw extruder with a vacuum vent together with the vinyl polymer, There are two methods of drying and pelletizing.

However, it is difficult to reduce the moisture content of the wet powder obtained by a centrifugal dehydrator or a vacuum belt filter to 30% by weight or less because of the function of the apparatus. It is as high as about 50% by weight.
For this reason, the former method has a disadvantage that a large amount of drying energy is required. In addition, since dry powder is obtained by hot-air drying and pelletization is performed, the manufacturing process is complicated, the equipment cost is high, and the color tone tends to deteriorate in quality. In addition, there is a problem that there is a risk of dust explosion due to handling of dry powder.

On the other hand, the latter method is different from the former method in that
In terms of blending directly with the vinyl polymer pellets from the wet powder and pelletizing, the energy consumption is small, the process is simplified, and the above-mentioned disadvantages are improved. However, immediately after the wet powder, which is a rubber-modified vinyl polymer, and the pellets of the vinyl polymer are supplied to the extruder, the powder is not sufficiently melted. In the extruder equipped with a leak and a vent stuffer, there is a problem that small resin particles are entrained together with steam. In addition, entrained resin particles adhere to the vent stuffer.
As a result, there is a problem that resin contamination such as a discolored resin or a burnt resin is caused or color tone is deteriorated. Also,
Since the energy of the extruder is consumed for melting the vinyl polymer, there is a problem that the extrusion amount does not increase.

[0005]

DISCLOSURE OF THE INVENTION The present invention is excellent in operability and productivity, such as low consumption of drying energy and little resin leakage from a dehydrating slit or a vent stuffer, and uniform physical properties. An object of the present invention is to provide a method for producing a rubber-modified vinyl polymer composition having excellent color tone.

[0006]

According to the present invention, a water content of 10 to 10 is provided.
In producing a rubber-modified vinyl polymer composition from 30% by weight of a rubber-modified vinyl polymer and a vinyl polymer, the vinyl polymer was first added in a molten state to a twin-screw extruder equipped with a vacuum vent. And then supplying the rubber-modified vinyl polymer, or simultaneously supplying the rubber-modified vinyl polymer and the molten vinyl polymer, a method for producing a rubber-modified vinyl polymer composition, To provide. Here, the rubber-modified vinyl polymer is obtained by dewatering a slurry obtained by coagulating a rubber-modified vinyl polymer latex obtained by emulsion polymerization of a vinyl monomer in the presence of a rubber-like polymer. Alternatively, it is preferable that the water content is dewatered to 10 to 30% by weight by a screw type or pressure filter type compression dehydrator as it is.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. In the present invention, a rubber-based polymer latex is obtained by emulsion-polymerizing a vinyl-based monomer or a monomer copolymerizable therewith in the presence of a rubber-like polymer. Next, a coagulant is added to the obtained rubber-modified vinyl polymer latex, and the polymer component is coagulated to obtain a slurry. The obtained slurry is subjected to preliminary dehydration, or washing and preliminary dehydration, and the cake-like polymer component is continuously recovered from the slurry. Alternatively, the process may proceed to the next step without going through the preliminary dehydration step. Next, the cake-like polymer component is dehydrated by a compression dehydrator until the water content becomes 10 to 30% by weight, and a powdery (wet powder) or cake-like rubber-modified vinyl-based polymer component is recovered. At this time, the cleaning may be performed simultaneously. When the polymer component obtained by the above-mentioned compression dehydrator is in a cake form, it can be returned to a powder (wet powder) by a crusher to improve the handling in the post-process. And to a twin screw extruder with a vacuum vent,
Supplying a melt of the vinyl-based polymer and additives such as a plasticizer as necessary, followed by and / or simultaneously supplying the rubber-modified vinyl-based polymer component recovered in the above step, and drying and blending; The rubber-modified vinyl polymer composition in the form of pellets is extruded from the tip.

The rubber-modified vinyl polymer latex of the present invention is obtained by emulsion polymerization of a vinyl monomer or a monomer copolymerizable therewith in the presence of a rubbery polymer. The polymerization conditions and the types and amounts of the polymerization initiator, the chain transfer agent, the additives, and the like used are not essentially different from those commonly used in the production of ABS resins. Examples of the rubbery polymer include polybutadiene, styrene-butadiene copolymer (preferably 5 to 60% by weight of styrene), styrene-isoprene copolymer, acrylonitrile-butadiene copolymer, and ethylene-α-polymer. Olefin-based copolymer, ethylene-α-olefin-polyene-based copolymer, acrylic rubber, butadiene-acryl-based copolymer, polyisoprene, styrene-butadiene block copolymer, hydrogenated butadiene-based copolymer, ethylene System ionomer, silicone rubber and the like. Preferred rubbery polymers include polybutadiene, styrene-butadiene copolymer, acrylic rubber, and silicone rubber (especially unsaturated group-containing silicone rubber is preferred). These rubbery polymers can be used alone or in combination of two or more. The rubbery polymer used in the production of the rubber-modified vinyl polymer is in a latex state to be subjected to emulsion graft polymerization.

When the rubber content (based on 100% resin) of the rubber-modified vinyl polymer of the present invention is 40 to 75% by weight, the effect of the present invention is further exhibited. Preferably it is 50 to 75% by weight. When the rubber content is less than 40% by weight, the water content can be stabilized by 3 in a screw type or pressure filter type compression dehydrator.
It is difficult to suppress the content to 0% by weight or less. In addition, since the physical properties of the final product are determined by the rubber content, the amount of the vinyl polymer that can be supplied at the time of drying with the twin-screw extruder cannot be increased, and the water content at the inlet of the extruder ( The ratio of the amount of water brought in to the total feed amount) increases, and stable operation cannot be performed. On the other hand, when the rubber content exceeds 75% by weight, the graft ratio becomes difficult to increase during polymerization, and quality may not be maintained.

On the other hand, the vinyl monomer used for the rubber-modified vinyl polymer is not particularly limited, but, for example, (meth) acrylic acid such as acrylic acid and methacrylic acid; methyl acrylate, ethyl acrylate Alkyl acrylate monomers such as acrylate and butyl acrylate; alkyl methacrylate monomers such as methyl methacrylate and ethyl methacrylate; vinyl cyanide monomers such as acrylonitrile and methacrylonitrile; styrene, α- Various monomers such as aromatic vinyl monomers such as methylstyrene; vinyl halide monomers such as vinyl chloride and vinyl bromide, and these may be used alone. Alternatively, two or more kinds can be used in combination. Preferred vinyl monomers are (meth) acrylic acid alkyl esters, vinyl cyanide monomers, and aromatic vinyl monomers.

Further, other copolymerizable monomers other than those described above can be used in combination. Examples of the other monomers include maleimide monomers such as maleimide, N-methylmaleimide, N-phenylmaleimide, and N-cyclohexylmaleimide. When a maleimide-based monomer is used, heat resistance is improved, and a rubber-modified vinyl polymer having excellent heat resistance can be obtained. The rubber-modified vinyl polymer latex of the present invention is obtained by emulsion polymerization of a vinyl monomer in the presence of a rubbery polymer.
Types of polymerization initiators, chain transfer agents, additives, etc. used
The amounts do not differ essentially from those conventionally used in the production of ABS resins. Alternatively, a vinyl (co) polymer obtained by (co) polymerizing only the vinyl monomer may be blended. Specific examples of the rubber-modified vinyl polymer include an ABS resin, an MBS resin, an AES resin, an AAS resin, and the like, and preferably an ABS resin.

In the present invention, a coagulant is added to the rubber-modified vinyl polymer latex obtained above, and the polymer component is coagulated to obtain a slurry. The solidification method is not particularly limited, but can be performed by a generally used method. At this time, heating may be performed if necessary. As the coagulant, those usually used for coagulating latex can be used. For example, inorganic acids such as hydrochloric acid and sulfuric acid,
Organic acids such as acetic acid and formic acid, and metal salts of these acids. Examples of the metal salts of the above acids include calcium chloride, aluminum chloride, aluminum sulfate, magnesium sulfate and the like.

The slurry obtained above is centrifugally dewatered,
The slurry is supplied to a preliminary dehydrator such as a horizontal belt filter for preliminary dehydration, or is washed with water and preliminarily dehydrated, and a cake-like polymer component is continuously recovered from the slurry. The water content of the recovered polymer component is generally 10 to 50% by weight. Further, the process may proceed to the next step without using the preliminary dehydrator. Next, the polymer component, after preliminary dehydration or as it is, is supplied to a screw type or pressure filter type compression dehydrator having a drainage device, and dehydrated until the water content becomes 10 to 30% by weight.
A powdery (wet powder) or cake-like polymer component is recovered. At this time, the cleaning may be performed simultaneously. The screw-type compression dehydrator is a device that compresses the above-mentioned polymer component while conveying it by a screw arranged in a cylinder (= barrel), and has a drainage function or a drainage function and a washing function. Further, a heating function may be provided. In addition, it is desirable to provide a punching plate or slit for drainage in the whole or a part of the barrel. Further, it is desirable to provide a screw having a compression ratio of 1.0 to 5.0 and a length of 3 to 15 times the screw diameter. Further, an inlet for injecting the cleaning water into the barrel can be provided.

On the other hand, a pressure filter type press dewatering machine has a room between a filter cloth and a diaphragm in a pressure filter. After supplying the slurry here, the slurry is squeezed by water pressure through a diaphragm to be dehydrated. It is desirable that the above-mentioned press dewatering machine is provided with a washing water inlet for washing the press cake. In addition, it is desirable to provide a nozzle for introducing pressurized air for removing surface water of the pressed cake.

The water content of the polymer component obtained by the above-mentioned compression dehydrator is 10 to 30% by weight, preferably 10 to 2% by weight.
5% by weight. If the water content is to be dehydrated to less than 10% by weight, in a screw-type compression dehydrator, the polymer component is easily melted and stable operation becomes difficult. In addition, a pressure filter type press dewatering machine requires a very high press pressure and press time, and is not industrial. On the other hand, when the water content exceeds 30% by weight, there is a problem that the operation in the next step becomes unstable, a large amount of energy is required for drying, and the processing capacity of the extruder is reduced. As a method for controlling the degree of dehydration of the rubber-modified vinyl polymer, in a screw type dehydrator, it can be controlled by the cone pressure at the tip of the dehydrator, the washing water temperature, and the barrel temperature. In the case of the pressure filter type, the pressure can be controlled by the pressure of the diaphragm, the temperature of the washing water, and the like. In addition, the water content is defined as a wet base percentage (wet base p).
(entry).

The polymer component obtained by the compression dehydrator may be in the form of a powder (wet powder). However, since the inside of the dehydrator is squeezed out by a large compressive force, the (Wet powder) It may be a solid cake with sticking. In this case, the powder (wet powder) can be easily returned to the powder (wet powder) by a crusher such as a hammer mill or a free mill, and the handling in the subsequent process can be improved.

Next, a twin screw extruder equipped with a vacuum vent,
A melt of vinyl polymer beads and / or pellets obtained by a suspension, solution or bulk polymerization method and, if necessary, additives such as a plasticizer are supplied, and the rubber-modified vinyl polymer recovered in the above step is supplied. The coalesced components are fed sequentially or simultaneously, preferably simultaneously, blended and dried, and extruded from the tip to obtain a pellet-shaped rubber-modified vinyl polymer composition.

The twin-screw extruder equipped with a vacuum vent used in the present invention can use any type of screw engagement or non-engagement. Further, the screw can be rotated in the same direction or in different directions. Preferably, it is a meshing co-directional twin screw extruder. The twin screw extruder with a vacuum vent has a function of discharging water as a liquid, that is, a method having a dewatering slit or a mechanical filter, or a method without a slit or the like, and water is discharged from an atmospheric vent and / or a vacuum vent. It is also possible to use a method of removing only.

The barrel configuration of the twin-screw extruder with a vacuum vent includes a feed barrel for receiving a raw material,
A slit barrel having a slit for discharging liquid and / or vapor, a standard barrel having heating means,
It is configured by combining each split barrel of water, a vent barrel for vaporizing residual monomers (a vent stuffer can also be attached), and a press-fitting barrel to which a side feeder can be attached, if necessary.

The screw element of the twin screw extruder equipped with a vacuum vent includes a forward screw, a reverse screw, a forward kneading disk, an orthogonal kneading disk, a reverse kneading disk, or a divided twin screw of a seal ring. Element. It is not necessary to combine all of these, and the screws selected from the resistance screws (reverse feed screw, reverse feed kneading disk, orthogonal kneading disk, progressive kneading disk, seal ring) and the progressive screw are combined. Configuration.

On the other hand, the vinyl polymer of the present invention is obtained by suspending, solution or bulk polymerization of a vinyl monomer. The conditions of this polymerization, the polymerization initiator used, the chain transfer agent and the additive The kind and amount of such as are not essentially different from those commonly used in the production of general vinyl resins. Examples of the vinyl monomer constituting the vinyl polymer include, but are not particularly limited to, (meth) acrylic acid such as acrylic acid and methacrylic acid; and alkyl acrylate such as methyl acrylate, ethyl acrylate, and butyl acrylate. Ester monomers; alkyl methacrylate monomers such as methyl methacrylate and ethyl methacrylate; vinyl cyanide monomers such as acrylonitrile and methacrylonitrile; aromatic vinyl monomers such as styrene and α-methylstyrene. Various monomers such as vinyl halide monomers such as vinyl chloride and vinyl bromide, and two or more of these can be used in combination. Preferred vinyl monomers include (meth) acrylic acid alkyl ester monomers, vinyl cyanide monomers, and aromatic vinyl monomers. Further, other copolymerizable monomers other than those described above can be used in combination. Examples of other copolymerizable monomers include maleimide, N
Maleimide monomers such as -methylmaleimide, N-phenylmaleimide, and N-cyclohexylmaleimide. When a maleimide-based monomer is used, heat resistance is improved, and a polymer having excellent heat resistance can be obtained. Specific examples of the vinyl polymer include an AS resin and an MS resin (methyl methacrylate-styrene copolymer), and preferably an AS resin. The shape of the vinyl polymer used in the present invention may be beads, pellets, powders, slurries, etc., which have high operability and are industrially available.

In the present invention, the amount of the vinyl-based polymer to be added is 10% (in terms of solid content) of the rubber-modified vinyl-based polymer.
The amount is preferably 30 to 600 parts by weight, more preferably 50 to 500 parts by weight, based on 0 parts by weight. If the amount is less than 30 parts by weight, the fluidity is low, and stable operation of the extruder becomes difficult. On the other hand, if it exceeds 600 parts by weight, the impact resistance of the obtained composition is lowered, which is not preferable.

In the present invention, the vinyl polymer is melted together with the rubber-modified vinyl polymer component recovered in the above step, for example, by a single screw extruder or the like, and then supplied to the twin screw extruder equipped with a vacuum vent. I do. As a specific example of the supply method, first, a molten vinyl polymer is supplied to a feed port of an extruder, and a rubber-modified vinyl polymer component is supplied from a barrel in the middle of the extruder by a side feeder, or Feeding the rubber-modified vinyl polymer component and the molten vinyl polymer together to the feed port of a co-rotating twin-screw extruder equipped with a vacuum vent, 50% by weight of the molten vinyl polymer
A method in which the above and a rubber-modified vinyl polymer are supplied to a feed port of an extruder, and the remaining molten vinyl polymer is supplied from a side feeder. Preferably, the above and the above methods. In these cases, processing aids can be added as appropriate. As this processing aid, metal soaps such as ethylenebisstearylamide and magnesium stearate can be used.

In the present invention, in the system in which beads and / or pellets of the vinyl polymer are melt-added, the wet powder of the rubber-modified vinyl polymer and the beads and / or pellets of the vinyl polymer are fed before being fed. The calculated water content in the mixed state is preferably 9% by weight or less, more preferably 8% by weight or less. This water content is 9% by weight
If the pressure exceeds the limit, a small amount of resin will be entrained from venting up by a vacuum vent or an atmospheric vent with a vent stuffer, and stable operation cannot be performed. In the present invention, the water content is 10
By blending 30% by weight of the rubber-modified vinyl polymer and the melt of the vinyl polymer, an effect that a final product can be obtained with a simple process and with low energy consumption can be obtained. The product of the rubber-modified vinyl polymer composition produced in the present invention and having excellent physical properties and uniform color tone can be processed by various molding methods and used in a wide range of fields.

[0025]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof. In the examples, parts and% are by weight unless otherwise specified.

Example 1 As a rubber-modified vinyl polymer latex, styrene,
A polymer latex consisting of 60% polybutadiene, 30% styrene, and 10% acrylonitrile obtained by graft copolymerizing acrylonitrile with polybutadiene was used as a raw material. The solid content concentration of the rubber-modified vinyl polymer latex was 30%. In a coagulation tank with a stirrer having an inner volume of 100 liters (tank diameter: 500 mm, two-stage 45-degree inclined paddle blades, blade diameter: 250 mm), the mixture was stirred at 180 revolutions / minute, and the latex was continuously supplied at 14 liters / minute.
0.08 kg / min of concentrated sulfuric acid and 3 l / min of water were added. The coagulation bath temperature is increased by blowing steam.
The temperature was adjusted to 85 ° C.

The coagulated slurry continuously overflowed and discharged from the coagulation tank is heated at 90 ° C. for 40 minutes,
Dehydration was performed using a basket-type centrifugal dehydrator. The water content of the dehydrated slurry was 38%. The dewatered slurry was supplied to a screw-type pressing dehydrator and pressed to obtain a cake. The water content of the cake is 1
8%. After crushing this cake and making it wet powder,
Two-stage vacuum vent, one-stage slit, co-rotating twin-screw extruder [TEM48BS, manufactured by Toshiba Machine Co., Ltd.]
g / min. FIG. 1 shows a schematic view of the screw type dehydrator and the twin screw extruder used. In addition, dried AS (acrylonitrile-styrene copolymer) pellets as a vinyl polymer were melted by a single screw extruder, and 2.
The wet powder was simultaneously supplied at a rate of 45 kg / min, dried and pelletized. The temperature of the molten AS was kept at 180 ° C. The wet powder obtained by crushing the cake was treated with ethylene bisstearylamide as a processing aid in an amount of 3.5% based on the dry resin.
% Added and mixed. The rotation speed of the twin screw extruder was 300 rotations / minute. The barrel structure of the twin screw extruder was arranged as shown in FIG. The barrel temperature is from the next barrel of the feed barrel, slit barrel (no heating), 180 ° C, 200 ° C, 200 ° C, 220 ° C, 22
0 ° C, 200 ° C, 200 ° C, the die plate part is 2
20 ° C. Table 1 shows the results.

Example 2 Except that molten AS was first supplied to a feed port, and then a rubber-modified vinyl polymer cake was supplied to the next barrel.
It carried out similarly to Example 1. The temperature of the molten AS is 180
C. The barrel structure of the twin-screw extruder was arranged as shown in FIG. The temperature of the barrel is 180 ° C, 180 ° C, 200 ° C, 2 ° C from the next barrel of the feed barrel.
The temperature was 20 ° C., 220 ° C., 200 ° C., 200 ° C., 200 ° C., and the die plate portion was 220 ° C. Table 1 shows the results. Example 3 It carried out similarly to Example 1 except having used the pressure filter (made by LAROX) as the compression dehydrator. The moisture content of the wet powder obtained by crushing the cake was 25%. Table 1 shows the results.

Comparative Example 1 The same operation as in Example 1 was carried out, except that the dried AS pellets were directly supplied to a twin screw extruder without melting. Table 1 shows the results. Comparative example 2 It carried out similarly to Example 1 except having supplied the slurry which came out of the basket type centrifugal dehydrator to the twin-screw extruder without passing through a pressing machine. The water content of the slurry after dehydration is
38%. However, in the extruder, the air was slightly vented up by the vacuum vent, and the extruded amount was 14
It could only flow up to 0 kg / h. Table 1 shows the results.

[0030]

[Table 1]

Examples 1 to 3 are examples carried out within the scope of the present invention. The extruder has a good color tone, can operate the extruder stably at a high flow rate, and achieves the object of the present invention. ing. On the other hand, in Comparative Example 1, only a low flow was able to flow as compared with the Example. There is also a problem that there is a lot of resin leakage from the dewatering slit and the vent stuffer. In Comparative Example 2, venting occurred in the extruder, and stable operation could not be performed, and the extrusion amount could not be increased. Further, the color tone (particularly, whiteness and yellowness) deteriorated. In Examples and Comparative Examples, the color tone was measured according to ASTM D-1003. Also,
Resin leakage from the dehydration slit and vent stuffer
Weight ratio of leaked resin dry weight to extrusion amount (%)
It was expressed by.

[0032]

According to the method for producing a rubber-modified vinyl polymer composition from a rubber-modified vinyl polymer and a vinyl polymer, a melt of the vinyl polymer is supplied to a twin-screw extruder equipped with a vacuum vent. In addition, the rubber-modified vinyl polymer is pressed and dewatered by a specific pressing and dewatering machine and supplied to the extruder, so that the consumption of drying energy is small and the resin from the dewatering slit or vent stuffer is supplied. It is possible to provide a method for producing a product of a rubber-modified vinyl polymer composition which is excellent in operability and productivity such as little leakage, and has excellent physical properties and uniform color tone.

[Brief description of the drawings]

FIG. 1 is a schematic view of a screw type dehydrator and a twin screw extruder used in the present invention.

FIG. 2 is a barrel configuration diagram of a twin-screw extruder with a vacuum vent used in Example 1.

FIG. 3 is a barrel configuration diagram of a twin-screw extruder with a vacuum vent used in Example 2.

[Explanation of symbols]

1: Screw type dehydrator 2: Feed barrel 3: Dehydration barrel 4: Discharge box 5: Screw 6: Feed port 7: Screw groove bottom 8: Cone (resistor) 9: Motor 10: Transmission 11: Heat medium 12: Outlet 13: Weight feeder [rubber-modified vinyl polymer (AB
S :): 14: twin screw extruder 15: single screw extruder [vinyl polymer (AS resin)
16: Extruder supply port 17: Dewatering slit 18: Crusher

Claims (2)

[Claims] In producing a rubber-modified vinyl polymer composition from a rubber-modified vinyl polymer having a water content of 10 to 30% by weight and a vinyl polymer, a vinyl screw extruder equipped with a vacuum vent is first used. Adding a rubber-based polymer in a molten state and then supplying a rubber-modified vinyl polymer, or simultaneously supplying a rubber-modified vinyl polymer and a molten vinyl polymer, A method for producing a polymer-based composition. 2. A slurry obtained by coagulating a rubber-modified vinyl polymer latex obtained by emulsion-polymerizing a vinyl-based monomer in the presence of a rubber-like polymer with a rubber-modified vinyl polymer is subjected to dehydration. 2. The method for producing a rubber-modified vinyl polymer composition according to claim 1, wherein the water content is dehydrated to 10 to 30% by weight by a screw type or pressure filter type compression dehydrator as it is.