JP4424458B2 - Acrylic syrup and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an acrylic syrup containing a polymer obtained by polymerizing a monomer mainly composed of methyl methacrylate and a method for producing the same.
[0002]
[Prior art]
Acrylic syrup is an intermediate between methacrylic resin casting plates, optical materials such as optical transmission fibers and optical waveguides, acrylic artificial marble, artificial sealants, flooring materials, adhesives, adhesives, restoration materials such as cultural assets and stuffed animals, or medical materials. Used as a raw material. Of these, many methods for producing methyl methacrylate syrup mainly composed of methyl methacrylate have been introduced in patents, literatures, etc., and the production methods are roughly classified into the following two methods. One is a method of dissolving a separately prepared polymer in a monomer as disclosed in JP-A-49-104937 and JP-A-9-194673. This is a production method fundamentally different from the present invention. Further, since the polymer is once taken out and then dissolved again in the monomer, it is disadvantageous in terms of energy and economy. The other is a method in which the monomer is partially bulk polymerized and is also called a partial polymerization method. The partial polymerization method is further divided into a batch method and a continuous method.
[0003]
As a production method by a batch method among the partial polymerization methods, for example, in Japanese Patent Publication No. 36-3392, a raw material composed of a monomer mainly composed of methyl methacrylate and a chain transfer agent is heated to 80 ° C. Of 2,2′-azobisisobutyronitrile or benzoyl peroxide as a polymerization initiator, and simultaneously heated to 100 ° C. and polymerized for 27 to 50 minutes. A method for producing acrylic syrup by adding cold methyl methacrylate containing hydroquinone and quenching is disclosed. However, in this method, since the polymerization is stopped in a state where the polymerization initiator is not completely decomposed, the polymerization initiator remains in the obtained syrup, and even when a polymerization inhibitor is added, the storage stability is poor. It will be a thing. For example, benzoyl peroxide used as a polymerization initiator has a half-life at 100 ° C. of about 22 minutes. Therefore, when a predetermined viscosity is reached, 42 to 20% of the polymerization initiator is added to the product. Remains. In order to add a polymerization initiator in an amount necessary for the reaction at a time, the polymerization temperature is increased after the polymerization initiator is added once, so that the polymerization rate of the product, the molecular weight, and the product viscosity are greatly increased by a slight temperature change. Stable production cannot be performed due to influence.
[0004]
In Japanese Patent Publication No. 1-11652 and JP-A-9-67495, when producing syrup as an intermediate raw material for SMC or BMC, methyl methacrylate 89% by weight, methacrylic acid 5% by weight, trimethylolpropane trimethacrylate 6% A raw material containing 0.4 part of n-dodecyl mercaptan and 0.05 part of 2,2′-azobisisobutyronitrile is charged to 100 parts of monomer consisting of 100%, and polymerization is carried out at 80 ° C. A method for producing acrylic syrup containing a carboxylic acid is disclosed by adding hydroquinone and p-methoxyphenol as a polymerization inhibitor when a predetermined viscosity is reached and quickly cooling to room temperature to stop the polymerization. However, the polymerization initiator remains in the syrup obtained by this method, and even if a polymerization inhibitor is added, the storage stability is poor. Moreover, since a polymerization initiator in an amount necessary for the reaction is added at a time, it is difficult to control the reaction, and the polymerization rate, molecular weight, and product viscosity are greatly changed, so that it cannot be said to be an excellent production method. In addition, there is a high possibility that the polymerization initiator remains in the obtained syrup, and even if a polymerization inhibitor is added, the storage stability is inferior, and it causes coloring during storage. In order to prevent the polymerization initiator from remaining, a polymerization initiator having a short half-life at the polymerization temperature may be used. In this case, a large amount of polymerization initiator is required, resulting in a rapid progress of the polymerization reaction. It becomes very difficult to control. For this reason, the polymerization initiator that can be used in the batch method falls into a dilemma that the half-life at the polymerization temperature is limited.
[0005]
On the other hand, with respect to the progress of the polymerization by mercaptans, for example, in Japanese Patent Publication No. 46-40693, a sulfur compound having active hydrogen is used as a chain transfer agent, such as mercaptans, and 65 to 105 ° C. without adding a polymerization initiator. Discloses a method for producing acrylic syrup by partial polymerization. In this method, a large amount of chain transfer agent is required to polymerize to a desired polymerization rate, and acrylic syrup containing a polymer having a high molecular weight cannot be obtained. Further, in order to obtain acrylic syrup containing a polymer having a high molecular weight, it is necessary to react for a long time using a small amount of chain transfer agent, which is not practical in any case.
[0006]
By the way, in the batch polymerization method described above, the polymerization is performed at a temperature lower than the boiling point of the in-system composition. On the other hand, it can be expected that the temperature change is reduced by carrying out the polymerization at the boiling point of the polymerization composition, but in reality, as the polymerization proceeds and the viscosity in the system rises, the liquid level rises due to foaming or the tank resulting therefrom. Problems such as the generation of internal deposits occur. In a large apparatus, since the area of the liquid surface is smaller than the volume of the reaction liquid, this tendency becomes remarkable. Therefore, it could be said that boiling point polymerization is impossible in practice. As means for solving these problems, JP-A-53-24380 is characterized in that 1 to 10,000 ppm of organosiloxanes are added when (co) polymerizing a monomer containing 70% by weight or more of methyl methacrylate. A method for producing a polymer is disclosed. However, as specified in the above patent specification, the organosiloxanes are basically incompatible with the acrylic monomer and the polymer, and the resulting molded product is cloudy. In addition, if organosiloxanes appear on the surface of the molded product, they may cause repelling during coating or cause dispersal of dust adhesion, so use with caution.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-mentioned problems of the conventional method, and to provide an acrylic syrup suitable for various uses and having a stable quality, and an efficient and simple method for producing the acrylic syrup.
[0008]
[Means for Solving the Problems]
As a result of intensive studies, the present inventors have found that an acrylic syrup suitable for various applications and having a stable quality can be efficiently and easily produced by a specific production method, and has reached the present invention. That is, the present invention includes (1) 20 to 70% by weight of a monomer raw material mainly composed of methyl methacrylate with a partial ester compound of aliphatic carboxylic acid having 1 to 30 carbon atoms and glycerin and / or butadiene. After adding an antifoaming agent composed of a polymer obtained from a monomer as a main component, the temperature is raised, and (2) when the temperature of the in-system composition reaches the boiling point and refluxing is started, (3) While maintaining the reflux, the remaining monomer is continuously or divided over 0.1 to 10 hours with a polymerization initiator having a half-life at the boiling point of 10 to 300 seconds. And (4) continuing the heating after the end of the addition, and (5) adding a hindered phenol polymerization inhibitor after the end of the heating. Further, the present invention provides (A) 39 to 90% by weight of a monomer having methyl methacrylate as a main component, and (B) a weight average measured by GPC, obtained from a monomer having a methyl methacrylate unit as a main component. 60 to 9% by weight of a polymer having a molecular weight of 20,000 to 500,000, (C) 0.0005 to 3.0% by weight of a mercaptan having 4 to 20 carbon atoms, and (D) an aliphatic carboxylic acid having 1 to 30 carbon atoms 0.0001 to 0.3% by weight of a polymer obtained from a partial ester compound of styrene and glycerin and / or a monomer containing butadiene as a main component, and (E) a hindered phenol polymerization inhibitor 0.001 to 1. The present invention relates to an acrylic syrup comprising 0% by weight (100% by weight in total) and having a viscosity at 25 ° C. of 10 to 500,000 mPa · s.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The acrylic syrup of the present invention and the production method thereof will be specifically described below. In the present invention, methyl methacrylate is an essential component as a monomer component, and other vinyl monomer components copolymerizable with methyl methacrylate can be optionally added and used. The monomer component is not particularly limited as long as it is a monomer copolymerizable with methyl methacrylate, unsaturated carboxylic acid such as acrylic acid, methacrylic acid, maleic acid and fumaric acid; ester of the unsaturated carboxylic acid , Nitrile, amide, imide and acid anhydrides; aromatic vinyl such as styrene; compounds having an ethylenic double bond such as vinyl carboxylate such as vinyl acetate.
[0010]
In the present invention, an antifoaming agent is added to the initial charge of the monomer raw material containing methyl methacrylate as an essential component, and then the temperature is raised to the boiling point of the monomer raw material. Before raising the temperature after adding the antifoaming agent, the temperature may be raised after contacting the charged raw material with 200 to 1000 vol% of an inert gas with respect to the initial charged raw material to replace the dissolved oxygen concentration. Nitrogen can be illustrated as an inert gas. Antifoaming agents that do not adversely affect the polymerization reaction and the resulting product, suppress the activity of substances that stabilize bubbles, improve the bubble removal of liquids, break bubbles on the surface, and A substance having the property of reducing the liquid viscosity is selected. As such an antifoaming agent, an antifoaming agent comprising a polymer obtained from a monomer mainly composed of a partial ester compound of an aliphatic carboxylic acid having 1 to 30 carbon atoms and glycerin and / or butadiene is used in the present invention. Used. Specifically, Caprylic acid monoglyceride, lauric acid monoglyceride, myristic acid monoglyceride, palmitic acid monoglyceride described in Big Chemie Japan Co., Ltd. “Plastic Additive”, Kao Corporation “Kao Plastic Lubricant”, etc. Partial ester compounds of fatty acids having 1 to 30 carbon atoms such as linolenic acid monoglyceride, oleic acid monoglyceride, stearic acid monoglyceride and behenic acid monoglyceride; butadiene rubber, nitrile rubber, styrene-butadiene rubber, styrene-butadiene-styrene elastomer, etc. It is a monomer polymer mainly composed of butadiene.
[0011]
The defoamer addition amount is 0.0001 to 0.3% by weight with respect to the total amount of the monomer raw material (total amount of the monomer initially charged and the monomer added later) in order to exhibit the required performance. It is desirable to add. Even if added over 0.3%, there is no significant difference in the defoaming effect. The added antifoaming agent suppresses the inclusion of bubbles when producing the final product and improves the defoaming at the time of molding, such as casting plates, optical materials such as optical transmission fibers and optical waveguides, acrylic artificial marble, The effect of reducing the appearance defect rate or mechanical defects can also be expected in final products such as artificial sealants, flooring materials, adhesives, adhesives, restoration materials such as cultural properties and stuffing, and medical materials.
[0012]
In the present invention, the antifoaming agent is added to the initial charge of the monomer raw material, and then the temperature is raised to the boiling point of the monomer raw material. When the boiling point is reached, reflux begins. The purpose of the reflux is to remove oxygen brought into the system by adding additional raw materials to the outside of the system, whereby the polymerization reaction can be performed stably.
[0013]
As the chain transfer agent to be added in the total amount at the start of reflux, mercaptans that do not inhibit the polymerization reaction and obtain a product with a desired molecular weight are used. For example, 1-butanethiol, 2,2-dimethylethanethiol, 4-20 carbon atoms such as 1-octanethiol, 2,2-dimethylhexanethiol, 1-dodecanethiol, 2,2-dimethyldecanethiol, 1-hexadecanethiol, 1-octadecanethiol, benzenethiol, thiocresol, thionaphthol At least one of the mercaptans is used. The addition amount of the chain transfer agent is 0.0005 to 3.0% by weight, preferably 0.03 to 1.5% by weight, based on the total amount of the monomer raw materials.
[0014]
After adding the chain transfer agent, the remaining monomer is added continuously or in portions over 0.1 to 10 hours together with a polymerization initiator having a half-life at the boiling point of 10 to 300 seconds. The polymerization initiator used in the present invention is a polymerization initiator having a half-life of 10 to 300 seconds at the reaction temperature. By using such a polymerization initiator, the polymerization initiator is completely consumed, The storage stability of the obtained syrup can be improved. Such polymerization initiators include various constants described in Nippon Oil & Fats Co., Ltd. “Organic Peroxide” Material 13th Edition, Atchem Yoshitomi Co., Ltd. Technical Materials, Wako Pure Chemical Industries, Ltd. “Azo Polymerization Initiators”, etc. For example, 2,2′-azobisisobutyronitrile, 2,2′-azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (2,4dimethyl-4) -Methoxyvaleronitrile), 1,1'-azobiscyclohexanecarbonitrile, lauroyl peroxide, benzoyl peroxide, t-butyl pivalate, t-butyl peroxy-2-ethylhexanoate, diisopropyl peroxydicarbonate and bis ( 4-t-butylcyclohexyl) peroxydicarbonate and the like are exemplified.
[0015]
The polymerization initiators can be used alone or in combination of two or more, and an amount necessary for obtaining a desired polymerization rate is added. The polymerization initiator can be used either by adding it alone or by mixing it with a monomer raw material. The viscosity of acrylic syrup prepared according to the present invention is affected by the polymerization rate, molecular weight, and composition of unsaturated monomer units copolymerizable with methyl methacrylate in the polymer, but to satisfy the required viscosity range The amount of polymerization initiator used for the total amount of monomer raw material is 5. 0.times.10.sup.- ^{5 to} 2.0% by weight is preferred. More preferably, 0 × 10 ^{−4 to} 1.0% by weight.
[0016]
Although the addition time of the additional monomer and the polymerization initiator can exceed 10 hours, it is not preferable from the viewpoint of productivity because the process time from preparation to product removal becomes long. In addition, it is known that polymerization proceeds little by little when mercaptans are used as chain transfer agents. If the temperature is increased in a state where mercaptans are added to the raw material initially charged, stable production cannot be performed because the polymerization rate varies depending on the rate of temperature increase. In addition, it is not preferable to prepare a chain transfer agent at the same time as the polymerization initiator because polymerization may occur in the raw material tank due to a redox reaction between the polymerization initiator and the chain transfer agent.
[0017]
In the present invention, after the addition of the additional monomer and the polymerization initiator is completed, heating is continued for a certain period of time to complete the polymerization reaction, and then a polymerization inhibitor is added, followed by cooling and taking out the product. By adding a polymerization inhibitor at the end of heating, polymerization by mercaptans can be completely prevented during the cooling operation, and it can be produced more safely and can produce acrylic syrup of stable product quality. Moreover, since the storage stability of acrylic syrup is improved by adding a polymerization inhibitor at the end of heating, it is not necessary to inactivate mercaptans remaining in the acrylic syrup.
[0018]
In order to avoid polymerization and coloring of the obtained syrup, a hindered phenol polymerization inhibitor is used in the present invention. Examples of the hindered phenol polymerization inhibitor include 2,6-di-t-butyl-4-methylphenol, 6-t-butyl-2,4-dimethylphenol, 4,4′-thiobis- (6-t -Butyl-3-methylphenol) and / or 2,2'-methylenebis- (4-methyl-6-t-butylphenol). These hindered phenol polymerization inhibitors can be used alone or in combination of two or more. The amount of the hindered phenol polymerization inhibitor added is preferably 0.001 to 1.0% by weight, more preferably 0.005 to 0.3% by weight, based on the total amount of the monomer raw materials.
[0019]
The acrylic syrup obtained as described above is obtained from (A) 39 to 90% by weight of a monomer having methyl methacrylate as a main component and (B) a monomer having a methyl methacrylate unit as a main component. , A polymer having a weight average molecular weight of 20,000 to 500,000 as measured by GPC, 60 to 9% by weight, (C) at least one compound selected from mercaptans having 4 to 20 carbon atoms, 0.0005 to 3.0% by weight %, (D) 0.0001 to 0.3% by weight of a polymer obtained from a partial ester compound of an aliphatic carboxylic acid having 1 to 30 carbon atoms and glycerin and / or a monomer based on butadiene, and ( E) It consists of 0.001 to 1.0% by weight (100% by weight in total) of a hindered phenol polymerization inhibitor, and has a viscosity at 25 ° C. of 10 to 500,000 mPa · s.
[0020]
Acrylic syrup obtained by the method of the present invention is a cast plate, optical materials such as optical transmission fibers and optical waveguides, acrylic artificial marble, artificial sealants, flooring materials, adhesives, adhesives, restoration materials such as cultural assets and stuffed or It can be used as an intermediate material for medical materials. Fillers, fiber reinforcements, low shrinkage agents, lubricants, plasticizers, thickeners, diluents such as organic solvents, crosslinking agents, leveling agents, anti-settling agents, mold release agents, antioxidants, UV absorption as necessary Known additives such as agents, pigments and / or dyes can also be mixed and used. The present invention will be illustrated more specifically, but the present invention is not limited thereto.
[0021]
【Example】
The polymerization rate was determined by the gravimetric method by putting the sample into a large amount of cold hexane and purifying and drying under reduced pressure. The molecular weight of the polymer was measured by 8010 type gel permeation chromatography (GPC) manufactured by Tosoh Corporation. The viscosity was measured at 25 ° C. using a B-type viscometer.
Example 1
Into a 1.5 liter separable four-necked flask equipped with a thermometer, reflux condenser, metering pump, and stirrer, a monomer polymer (mainly 458 g of methyl methacrylate, 10.9 g of methacrylic acid and butadiene) A mixture consisting of 0.090 g of a trade name BYK-A515 (BIC Chemie Co., Ltd.) was added, and the temperature was raised while stirring at 100 rpm. When the temperature reached 100 ° C., 2.6 g of 1-dodecanethiol as a chain transfer agent was quickly added, and then 0.058 g of 2,2′-azobis (2,4-dimethylvaleronitrile) as a polymerization initiator was dissolved. 472 g of the methyl methacrylate was added dropwise at 2.6 g / min using a metering pump over 3 hours. After completion of the dropwise addition, heating was continued for 0.15 hours, and 0.2 g of 2,6-di-t-butyl-4-methylphenol as a polymerization inhibitor was added to terminate the polymerization. Thereafter, the mixture was cooled to room temperature to obtain a colorless and transparent acrylic syrup. As the polymerization progressed, the viscosity of the reaction solution increased and foam was generated, but bubbles immediately broke off at the interface, and no foam phase was observed at the end and end of the polymerization. The polymerization rate of the obtained syrup was 31.3%, and the weight average molecular weight (Mw) measured by gel permeation chromatography (GPC) was 103,000. The viscosity at 25 ° C. was 4,200 mPa · s.
[0022]
Example 2
Acrylic syrup was obtained by performing the same reaction as in Example 1 except that 0.09 g of BYK-A515 in Example 1 was changed to 0.045 g of BYK-A515. Bubbles were generated in the reaction solution as the polymerization proceeded, but the bubbles were well removed into the gas phase, and no bubble phase was observed even at the end of the polymerization. The obtained acrylic syrup was colorless and transparent, the polymerization rate was 31.4%, and the weight average molecular weight (Mw) was 10.0 million. The viscosity at 25 ° C. was 4,700 mPa · s.
[0023]
Example 3
Acrylic syrup was obtained in the same manner as in Example 1 except that BYK-A515 in Example 1 was changed to 2.83 g of stearic acid monoglyceride. Bubbles were generated in the reaction solution as the polymerization proceeded, but the bubbles were very well removed from the gas phase, and no bubble phase was observed even at the end of the polymerization. The obtained acrylic syrup was colorless and transparent, the polymerization rate was 32.2%, the weight average molecular weight (Mw) was 10.0 million, and the viscosity at 25 ° C. was 6,100 mPa · s.
[0024]
Example 4
Acrylic syrup was obtained in the same manner as in Example 1 except that BYK-A515 in Example 1 was changed to 2.83 g of caprylic acid monoglyceride. Bubbles were generated in the reaction solution as the polymerization proceeded, but the bubbles were well removed into the gas phase, and no bubble phase was observed even at the end of the polymerization. The obtained acrylic syrup was colorless and transparent, the polymerization rate was 33.0%, and the weight average molecular weight (Mw) was 10.0 million. The viscosity at 25 ° C. was 6,000 mPa · s.
[0025]
Example 5
In the same apparatus as in Example 1, 458 g of methyl methacrylate, 10.9 g of methacrylic acid, and 0.090 g of BYK-A515 were charged, and the temperature was increased while stirring at 100 rpm. When the temperature reached 101 ° C., 1.1 g of 1-dodecanethiol as a chain transfer agent was quickly added, and then 0.030 g of 2,2′-azobis (2,4-dimethylvaleronitrile) as a polymerization initiator was dissolved. 472 g of methyl methacrylate was added from a metering pump over 3 hours. After completion of the dropwise addition, heating was continued for 0.25 hour, and 0.2 g of 6-tert-butyl-2,4-dimethylphenol as a polymerization inhibitor was added to terminate the polymerization. Thereafter, the mixture was cooled to room temperature to obtain a colorless and transparent acrylic syrup. As the polymerization progressed, the viscosity of the reaction solution increased and foam was generated, but bubbles immediately broke off at the interface, and no foam phase was observed at the end and end of the polymerization. The polymerization rate of the obtained syrup was 24.3%, and the weight average molecular weight (Mw) measured by gel permeation chromatography (GPC) was 203,000. The viscosity at 25 ° C. was 6,100 mPa · s.
[0026]
Example 6
In the same apparatus as in Example 1, 472 g of methyl methacrylate and 2.83 g of oleic acid monoglyceride were charged, and the temperature was raised while stirring at 80 rpm. When the temperature reached 101 ° C., 3.4 g of 1-dodecanethiol as a chain transfer agent was added, and then 0.041 g of 2,2′-azobis (2,4-dimethylvaleronitrile) as a polymerization initiator was dissolved. 472 g of methyl methacrylate was added from a metering pump over 3 hours. After completion of the dropwise addition, the heating was continued for 0.25 hours, and 0.5 g of 2,6-di-t-butyl-4-methylphenol as a polymerization inhibitor was added to terminate the polymerization. Thereafter, the mixture was cooled to room temperature to obtain a colorless and transparent acrylic syrup. As the polymerization progressed, the viscosity of the reaction solution increased and foam was generated, but bubbles immediately broke off at the interface, and no foam phase was observed at the end and end of the polymerization. The polymerization rate of the obtained syrup was 27.6%, and Mw measured by GPC was 79,000. The viscosity at 25 ° C. was 500 mPa · s.
[0027]
Example 7
In the same apparatus as in Example 1, 472 g of methyl methacrylate and 0.090 g of BYK-A515 were charged, and the temperature was increased while stirring at 100 rpm. When the temperature reached 101 ° C., 3.4 g of 1-dodecanethiol as a chain transfer agent was added, and then 0.102 g of 2,2′-azobis (2,4-dimethylvaleronitrile) as a polymerization initiator was dissolved. 472 g of methyl methacrylate was added from a metering pump over 3 hours. After completion of the dropwise addition, heating was continued for 0.25 hour, and 1.0 g of 2,6-di-t-butyl-4-methylphenol as a polymerization inhibitor was added to terminate the polymerization. Thereafter, the mixture was cooled to room temperature to obtain a colorless and transparent acrylic syrup. As the polymerization progressed, the viscosity of the reaction solution increased and foam was generated, but bubbles immediately broke off at the interface, and no foam phase was observed at the end and end of the polymerization. The polymerization rate of the obtained syrup was 40.4%, and Mw measured by GPC was 80000. The viscosity at 25 ° C. was 50,000 mPa · s.
[0028]
Comparative Example 1
The same reaction as in Example 1 was performed except that BYK-A515 in Example 1 was not added. As the polymerization progressed, bubbles were generated due to an increase in the viscosity of the reaction solution. As the polymerization reaction proceeded, bubbles started to form from the interface, and a foam phase with a height of about 5 mm was formed, but the polymerization reaction was not stopped. The polymerization rate of the obtained syrup was 32.1%, and Mw measured by GPC was 101,000. The viscosity at 25 ° C. was 5,000 mPa · s.
[0029]
【The invention's effect】
According to the present invention, acrylic syrup having desired characteristics can be produced stably, and the obtained syrup has excellent moldability and has great industrial significance.

Claims (3)

(1) The main component is a partial ester compound of aliphatic carboxylic acid having 1 to 30 carbon atoms and / or glycerin and / or butadiene in 20 to 70% by weight of the monomer raw material containing 97.7% by weight or more of methyl methacrylate. (2) When the temperature of the composition in the system reaches the boiling point and the reflux is started, the total amount of the chain transfer agent is reduced. (3) While maintaining reflux, the remaining monomer is added continuously or in portions over 0.1 to 10 hours with a polymerization initiator having a half-life at the boiling point of 10 to 300 seconds. And (4) continuing the heating after the end of the addition, and (5) adding a hindered phenol polymerization inhibitor after the end of the heating. The production method according to claim 1, wherein 0.0001 to 0.3% by weight of an antifoaming agent is added to the total amount of the monomer charged first and the monomer added later. The production method according to claim 1, wherein the chain transfer agent is at least one compound selected from mercaptans having 4 to 20 carbon atoms.