Acrylonitrile solutions of maleimides

Abstract: An acrylonitrile solution of maleimide manifesting transparency, precluding coloration and opacification during the course of handling, and excelling in stability, a method for the preparation of the acrylonitrile solution, and an acrylonitrile based copolymer obtained by using the acrylonitrile solution are provided. The acrylonitrile solution of maleimide is such that, when it is subjected to a forced coloration test, the differences Δ L, Δa, and Δb values (invariably as absolute values) each between L, a, and b values before and after the test are respectively not more than 5, not more than 5, and not more than 10. The acrylonitrile solution of maleimide is prepared by lowering the water content in the acrylonitrile solution to a level of not more than 0.1% by weight, or causing the acrylonitrile solution to permit coexistence therein of at least one member selected from the group consisting of alkyl-substituted hydroxybenzenes and hindered phenols and at least one member selected from the group consisting of phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides, or lowering the water content in the acrylonitrile solution to a level of not more than 0.3% by weight and causing the acrylonitrile solution to permit coexistence therein of at least one member selected from the group consisting of alkyl-substituted hydroxybenzenes, hindered phenols, phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to acrylonitrile solutions of maleimides The maleimides represented by N-phenyl maleimide and the like are used as a heat resistance enhancer for such acrylonitrile based copolymers as acrylonitrile-butadiene-styrene (ABS) resin, acrylonitrile-acrylic rubber-styrene (AAS) resin, acrylonitrile-styrene (AS) resin, and acrylonitrile-chlorinated polyethylene-styrene (ACS) resin. Description of the Prior Art: The maleimides which are in a solid state at room temperature have been heretofore handled generally in such forms as powder, flakes, and tablets. The maleimides in these solid forms, however, undergo gradual comminution and yield minute particles copiously during the course of transportation and consequently give rise to numerous problems such as deterioration of the work environment. For the solution of these problems, a method for enabling the maleimides to be handled securely in the form of acrylonitrile solutions obtained in advance by dissolving the maleimides together with a polymerization inhibition in acrylonitrile (JP-B-03-12,057; EP-A-0 323 244) has been proposed. Since this method which permits the maleimides to be handled in the form of acrylonitrile solutions is excellent in having solved the problems mentioned above, it has been finding growing acceptance for industrial applications. Since the acrylonitrile as the solvent for the acrylonitrile solutions of maleimides constitutes itself the source of the acrylonitrile component of the acrylonitrile based copolymers, the acrylonitrile solutions of maleimides, after being handled as for transportation, are used in their unmodified form for the production of the acrylonitrile based copolymers mentioned above. The maleimides are yellow and the acrylonitrile solutions obtained by dissolving the maleimides in acrylonitrile are yellow and transparent. So long as the acrylonitrile solutions of maleimides are as yellow and transparent as they were when they were fresh from the production line, the acrylonitrile based copolymers as finished products using the solutions pose no particular problem concerning the color characteristics. In other words, the acrylonitrile based copolymers as the finished products cannot induce such coloration as tends to impair their marketability. On no occasion, however, the acrylonitrile solutions of maleimides which are fresh from the production line are used immediately for the production of acrylonitrile based copolymers. Generally, the acrylonitrile solutions of maleimides after their production are transported by tank lorries and the like, stored in tanks and the like, or conveyed by the pipeline including pipe, valves, and nozzles (such as, for example, the polymer-feeding line from the storage tank for the acrylonitrile solutions of maleimides) before they are used for the production of acrylonitrile based copolymers. In the present invention, the transportation as by tank lorries, the storage as in tanks, and the conveyance as by the pipeline including pipe, valves, and nozzles (such as, for example, the polymer-feeding line from the storage tank for the acrylonitrile solutions of maleimides) will be referred to collectively as "handling." After the acrylonitrile solutions of maleimides have been handled over a specific period, they possibly change color from the inherent yellow color to a brown color, for example, gain in the degree of yellowness (namely, darken the yellow color), or occasionally opacify. In this invention, these phenomena will be referred to collectively as "discoloration." The acrylonitrile based copolymers obtained as finished products by using the acrylonitrile solutions of maleimides which have been discolored as described above are colored and have markedly impaired marketability. For the purpose of improving these finished products in marketability, the standards or demands imposed on the color of the acrylonitrile solutions of maleimides have been decisively gaining in exactitude in recent years. In the circumstances, such acrylonitrile solutions of maleimides as are appropriately used for the production of acrylonitrile based copolymers which sparingly incur discoloration due to aging during the course of handling, consequently preclude coloration, and excel in the color characteristics are strongly demanded. Hereinafter, the acrylonitrile solutions of maleimides will be occasionally referred to simply as "acrylonitrile solutions." An object of this invention, therefore, is to provide such acrylonitrile solutions of maleimides as are appropriately used for the production of acrylonitrile based copolymers which preclude coloration and excel in the color characteristics. SUMMARY OF THE INVENTION The objects mentioned above are accomplished by the following aspects (1) and (2) of the present invention. (1) An acrylonitrile solution of maleimide, as defined in claim 1, in which , in a forced coloration test, the values, Δ L, Δ a, and Δb, to be calculated in accordance with the following formulas = Δ L = |L1 - L2| (Absolute value) Δ a = |a1 - a1| (Absolute value) Δ b = |b1 - b2| (Absolute value) wherein L1, a1, and b1 are respectively the values of L, a, and b of the acrylonitrile solution before undergoing the forced coloration test and L2, a2, and b2 are respectively for the values of L, a, and b of the acrylonitrile solution after undergoing the forced coloration test, are respectively not more than 1, not more than 1, and not more than 2. Preferably an acrylonitrile solution of N-phenylmaleimide of the present invention has azobenzene and N,N-diphenyl hydrazine at a total content of not more than 100 ppm. (2) A method for the storage of an acrylonitrile solution of maleimide set forth in (1) mentioned above, characterized in that the molecular oxygen concentration of the gaseous phase portion of the acrylonitrile solution is adjusted to a level in the range of 0.01 to 10% by volume preparatorily to storing the solution as held in contact with a metal. It is well known that as one of the methods for quantitatively rating the difference between two colors, the so-called Hunter's L, a, b method has been proposed by Richard S. Hunter [Richard S. Hunter, The Measurement of Appearance, 2nd Ed., John Wiley & Sons, (1987)]. An instrument generally called a colorimeter which permits direct reading of the L, a, b values according to the Hunter's L, a, b method mentioned above is marketed and is readily available. We have decided to indicate the degree of discoloration of an acrylonitrile solution of maleimide by using the differences in L, a, and b values determined with a colorimeter designed on the basis of the Hunter's L, a, b method mentioned above. To be specific, we have established a method of forced coloration test for reproducing the discoloration by aging which occurs when the acrylonitrile solution is actually handled on a commercial scale and rated the degree of discoloration of the acrylonitrile solution in terms of the differences in the L, a, and b values before and after the test. We have consequently found that an acrylonitrile solution giving not more than 1, not more than 1, and not more than 2 respectively as the values of Δ L, Δ a, and Δ b calculated in accordance with the following formulas: Δ L = |L1 - L2| (Absolute value) Δ a = |a1 - all (Absolute value) Δ b = |b1 - b2| (Absolute value) wherein L1, a1, and b1 are respectively the values of L, a, and b before the test mentioned above and L2, a2, and b2 are respectively for the values of L, a, and b after the test, when copolymerized, produces an acrylonitrile based copolymer, more specifically an acrylonitrile-maleimide-containing copolymer, precluding discoloration, excelling in color characteristics, and enjoying high marketability. The acrylonitrile solution of maleimide of this invention incurs no or markedly diminished discoloration by aging during the course of handling thereof. We made a study in search of the cause for discoloration of the acrylonitrile solutions of maleimides, particularly N-phenyl maleimide. We have consequently demonstrated that the substances responsible for the discoloration are azobenzene (occasionally referred to briefly as "AZB" hereinafter) and N,N'-diphenyl hydrazine (occasionally referred to briefly as "DPH" hereinafter) . We have further found that acrylonitrile based copolymers, specifically acrylonitrile-N-phenyl maleimide-containing copolymers, obtained with an acrylonitrile solution of N-phenyl maleimide whose total content of AZB and DPH is not more than 500 ppm (based on the weight of the acrylonitrile solution) preclude discoloration and enjoy extremely high marketability. We seek to explain the formation of AZB and DPH in the acrylonitrile solution of N-phenyl maleimide as follows, though not intent on restricting the present invention by a theoretical discussion. When the acrylonitrile solution of N-phenyl maleimide is stored in a tank made of carbon steel or stainless steel, since the standard industry grade acrylonitrile contains water in a small amount such as, for example, in the approximate range of 0.5 to 1% by weight, this water acts and causes the N-phenyl maleimide to undergo partial hydrolysis and first produce aniline and then this aniline, when activated, leads to the formation of AZB and DPH via the aniline radical. Incidentally, the maleimide to be used for the preparation of the acrylonitrile solution of maleimide is generally in the form of flakes of maleimide which are obtained by purifying crude maleimide by distillation in the presence of a polymerization inhibitor thereby obtaining molten maleimide, temporarily storing the molten maleimide in an intermediate tank, and then cooling and solidifying the molten maleimide by the treatment with a flaker, for example. Generally, the flakes of maleimide are stored as placed in a packing container such as, for example, a paper bag, a container bag, or an alumina container. At times, this storage extends over a period of several months. The flakes of maleimide are inevitably exposed to the ambience of air before they are dissolved in acrylonitrile. Specifically, the maleimide is exposed to the ambience of air during the step of storing the molten maleimide in the intermediate tank, the step of cooling and solidifying the molten maleimide into flakes, and the step of storing the flakes of maleimide. We have been ascertained by their study that the maleimide, when exposed to the ambience of air before it is dissolved in acrylonitrile, is disposed to form readily a peroxide as a source of radical and produce actually the peroxide in a very minute amount and that this peroxide directly passes into the acrylonitrile solution of maleimide, participates in the formation of substances responsible for coloration, and induces unfavorable results such as the discoloration of the acrylonitrile solution of maleimide. The idea of carrying out the steps mentioned above in the ambience of nitrogen may be conceived for the solution of this problem. This method, however, is incapable of effectively preventing the molten maleimide from being polymerized and is unrealistic from the viewpoint of economy, for example. We have further continued their study to find that the formation of the peroxide in the maleimide and the discoloration by aging of the acrylonitrile solution of maleimide can be very effectively prevented by quickly adding the combination of such antioxidants as generally defined as a primary antioxidant and a secondary antioxidant to the maleimide which is yet to be solidified, specifically to the maleimide which has been purified by distillation and is still in the molten state. PREFERRED EMBODIMENT OF THE INVENTION The present invention provides an acrylonitrile solution of maleimide, characterized by the fact that, in a forced coloration test performed by immersing a sample of the acrylonitrile solution of maleimide in a Pyrex test tube, with asize of 18 mm in inside diameter and 180 mm height, then introducing into the solution a preparatorily polished test piece made of SUS 304 and measuring 50 mm in length, 10 mm in width, and 2 mm in thickness, adjusting the molecular oxygen concentration of the gaseous phase of the test tube to a level in the range of 3 to 10% by volume (providing that the molecular oxygen concentration is obtained of the gas mixture excluding maleimide and acrylonitrile on the basis of volume), placing the test tube in an oil bath set at 70°C and allowing it to stand therein for three days (72 hours) , subsequently extracting the test piece from the test tube, and subjecting the acrylonitrile solution to the determination of the values, L, a, and b, the values, Δ L, Δ a, and Δ b, to be calculated in accordance with the following formulas = Δ L = |L1 - L2| (Absolute value), Δ a = |a1 - a1| (Absolute value), Δ b = |b1 - b2| (Absolute value) wherein L1, a1, and b1 are respectively the values of L, a, and b of the acrylonitrile solution before undergoing the forced coloration test mentioned above and L2, a2, and b2 are respectively for the values of L, a, and b of the acrylonitrile solution after undergoing the forced coloration test, are respectively not more than 1, not more than 1, and not more than 2. Incidentally, the L, a, and b values of the acrylonitrile solution of maleimide are determined by the following method. Method for determination of L, a, and b values By the use of a colorimeter produced by Nippon Denshoku Kogyo K.K. and marketed under product code of "Σ-80," a sample of the acrylonitrile solution of maleimide is adjusted to a concentration of 30% by weight and subsequently measured for the L, a, and b values by the transmission mode. A commercially available acrylonitrile (guaranteed reagent) is used as a control and the cell used for the test has a thickness of 10 mm. The maleimides which are used in this invention include N-methyl maleimide, N-ethyl maleimide, N-hexyl maleimide, N-octyl maleimide, N-dodecyl maleimide, N-benzyl maleimide, N-cyclohexyl maleimide, N-phenyl maleimide, N-nitrophenyl maleimide, N-methoxyphenyl maleimide, N-methylphenyl maleimide, N-carboxyphenyl maleimide, N-hydroxyphenyl maleimide, N-chlorophenyl maleimide, N-dimethylphenyl maleimide, N-dichlorophenyl maleimide, N-bromophenyl maleimide, and N-tribromophenyl maleimide, for example. Among other maleimides cited above, N-phenyl maleimide proves particularly advantageous for the use contemplated herein. This invention does not discriminate the maleimide to be used therein on account of the method to be adopted for the production thereof. The maleimides obtained by varying methods can be used herein. For example, the maleimides obtained by the method which comprises subjecting a maleamic acid, the precursor of a maleimide, to dehydrocyclization with an acid anhydride such as acetic anhydride, the method which comprises heating a maleamic acid together with an acid catalyst under a reduced pressure thereby effecting dehydrocyclization of the maleamic acid, the method which comprises heating a maleamic acid in an organic solvent while expelling from the reaction system the water formed by the resultant dehydrocyclization in the form of an azeotropic mixture with the organic solvent, and the method which comprises preparing as an organic solvent a mixed solvent consisting of an inert organic solvent such as benzene and a non-protonic polar organic solvent such as N',N-dimethyl formamide and heating a maleamic acid in the mixed solvent thereby effecting dehydrocyclization of the maleamic acid can be used. The maleimides produced by some, if not all, of these methods contain a residual acid component. This invention tolerates the presence in the maleimide of up to about 5% by weight of such an acid component. It nevertheless prefers the maleimide to be used after being refined until the content of the acid component thereof falls below 0.3% by weight. Advantageously, the acrylonitrile solution of maleimide of the present invention is such that the values of Δ L, Δ a, and Δ b are respectively not more than 1, not more than 1, and not more than 2, and particularly preferably not more than 0.5, not more than 0.5, and not more than 1. The dissolution of maleimide in acrylonitrile can be implemented by any arbitrary method such as, for example, throwing the maleimide into acrylonitrile. Properly, the solution of maleimide is carried out at a temperature not exceeding the boiling point of acrylonitrile (78.5°C). Since acrylonitrile has a high vapor pressure, it is generally advantageous to carry out the dissolution process at a temperature in the range of 30 to 70°C. Generally, the temperature at which the acrylonitrile solution is handled is in the range of 20 to 70°C. The materials of which the facilities (including pipelines) for handling the acrylonitrile solution are made are not limited particularly. The surfaces of the facilities exposed to the acrylonitrile solution may be passivated by glass lining, ceramic coating, etc. For handling the acrylonitrile solution on a commercial scale, it is economically favorable to use facilities which are made of such a general-purpose industrial material as carbon steel or stainless steel. The concentration of maleimide in the acrylonitrile solution is not particularly limited. It can be suitably selected in consideration of such factors as the temperature of handling, the form of handling (such as transportation or storage), the kind of the acrylonitrile based copolymer to be produced with the acrylonitrile solution, the method of this production, and the production conditions. In some cases, the concentration of maleimide is preferably high as when the acrylonitrile solution is used for the production of an acrylonitrile based copolymer having a high maleimide content, for example. Generally, the concentration of maleimide is in the range of 40 to 90% by weight. Preferably, the acrylonitrile solution of maleimide of this invention is such that the values of L, a, and b to be determined after the preparation of the solution are in the ranges, L = 90 to 100, a = -10 to -30, and b = 30 to 50. In the acrylonitrile solutions of maleimide according to this invention, those whose values of L, a, and b determined after the preparation thereof fall within the ranges, L = 90 to 100, a = -10 to -30, and b = 30 to 50 and whose values of Δ L, Δ a, and Δ b are respectively not more than 1, not more than 1, and not more than 2. The acrylonitrile solution of maleimide of this invention can be efficiently produced by the methods which will be described specifically hereinbelow. These methods are adopted advantageously when the acrylonitrile solution of maleimide is handled with facilities which are made of such a general-purpose industrial material as carbon steel or stainless steel. The water content in the acrylonitrile can be adjusted by any of the known methods such as, for example, precision distillation, physical adsorption by the use of silica gel or molecular sieve, or dehydration by the use of anhydrous sodium sulfate or anhydrous magnesium sulfate. Appropriately, the water content in the acrylonitrile solution is as low as possible. The lower limit of the water content is suitably decided in consideration of the cost for minimizing the water content besides the extent of discoloration by aging which the acrylonitrile solution is expected to tolerate. The water content in the acrylonitrile solution is lowered to a level of not more than 0.05% by weight. When the water content is thus lowered, the acrylonitrile solution to be prepared is such that the values of Δ L, Δ a, and Δ b thereof are respectively not more than 1, not more than 1, and not more than 2. The invention resides in enabling the acrylonitrile solution to coexist with an antioxidant effective in precluding the generation of peroxide. As the antioxidant, at least one member selected from the group consisting of alkyl-substituted hydroxy benzenes and hindered phenols and at least one member selected from the group consisting of phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides can be used by itself. Further, the acrylonitrile solution of this invention can be prepared with high efficiency by lowering the water content in the acrylonitrile to be used. In this case, it suffices to lower the water content in the acrylonitrile solution to a level of not more than 0.05% by weight. As typical examples of the alkyl-substituted hydroxy benzene mentioned above, 2,4-dimethyl-6-tert-butylphenol, 4-tert-butyl catechol, 2,5-di-tert-butyl hydroquinone, 2-tert-butyl hydroquinone, and 4,4'-thio-bis(6-tert-butyl-m-cresol) may be cited. As typical examples of the hindered phenol mentioned above, 2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butyl anilino)-1,3,5-triazine/ 2,2'-thiobis-(4-methyl-6-tert-butyl phenol), triethylene glycol-bis-[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl) propionate], pentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 2,2-thio-diethylenebis [3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], 1,6-hexane diol-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate], tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-isocyanurate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl) benzene, N' ,N'-hexamethylenebis (3,5-di-tert-butyl-4-hydroxyhydrocinnamide), and 3,5-di-tert-butyl-4-hydroxy-benzylphosphonate-diethyl ester may be cited. Among other hindered phenols enumerated above, 2/4-dimethyl-6-tert-butyl phenol, 4-tert-butyl catechol, 2/5-di-tert-butyl hydroquinone, 2-tert-dibutyl hydroquinone, 4,4'-thio-bis(6-tert-butyl-m-cresol), 2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butyl anilino)-1,3,5-triazine, 2,2'-thiobis-(4-methyl-6-tert-butyl phenol)-, and triethylene glycol-bis-[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl) propionate] are advantageously used. As typical examples of the phosphorous ester, triphenyl phosphite, tris(nonylphenyl)phosphite, triethyl phosphite, tris(2-ethylhexyl)phosphite, tridecyl phosphite, tris (tridecyl) phosphite, tristearyl phosphite, diphenyl mono(2-ethylhexyl) phosphite, diphenyl monodecyl phosphite, diphenyl monotridecyl phosphite, dilauryl hydrogen phosphite, dilauryl hydrogen phosphite, diphenyl hydrogen phosphite, tetraphenyl dipropylene glycol phosphite, tetraphenyl tetra(tridecyl)-pentaerythritol tetraphosphite, tetra(tridecyl)-4,4'-isopropylidene diphenyl phosphite, trilauryl trithiophosphite, bis (tridecyl)pentaerythritol diphosphite, bis(nonylphenyl) pentaerythritol phosphite, distearyl pentaerythritol diphosphite, tris(2,4-di-tert-butyl-phenyl) phosphite, hydrogenated bisphenol A-pentaerythritol phosphite polymer, and hydrogenated bisphenol A-phosphite polymer may be cited. As typical examples of the phosphoric ester, phosphine and phosphoric acid amide, ethyldiethyl phosphonoacetate, ethyl acid phosphate, β-chloroethyl acid phosphate, butyl acid phosphate, butyl pyrophosphate, butoxyethyl acid phosphate, 2-ethylhexyl acid phosphate, di(2-ethylhexyl) phosphate, ethylene glycol acid phosphate, (2-hydroxyethyl)methacrylate acid phosphate, tris (2-chloroethyl) phosphate, tris(dichloropropyl)phosphate, octyl dichloropropyl phosphate, phenyldichloropropyl phosphate, trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tricresyl phosphate, triphenyl phosphate, triethyl phosiphine, tri-n-butyl phosphine, tricyclohexyl phosphine, triphenyl phosphine, and hexamethyl phosphoric triamide may be cited. Among other phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides enumerated above, tris(nonylphenyl) phosphate, distearyl pentaerythritol diphosphite, tristearyl phosphite, and di (2-ethylhexyl) phosphate are advantageously used. The amount of at least one member selected from the group consisting of the alkyl-substituted hydroxy benzenes and hindered phenols mentioned above to be used is in the range of 0.0001 to 1% by weight, preferably 0.001 to 0.1% by weight, based on the amount of the maleimide. The amount of at least one member selected from the group consisting of phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides mentioned above to be used is in the range of 0.0001 to 1% by weight, preferably 0.001 to 0.1% by weight, based on the amount of the maleimide. When at least one member selected from the group consisting of alkyl-substituted hydroxy benzenes and hindered phenols is used in combination with at least one member selected from the group consisting of phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides, the ratio of combination of the two members is not particularly limited. It may be properly decided so that the amounts of the two members involved fall in the relevant ranges of use mentioned above. Among the combinations between at least one member selected from the group consisting of alkyl-substituted hydroxy benzenes and hindered phenols and at least one member selected from the group consisting of phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides, the following combinations are particularly advantageous. 4-Tert-butyl catechol and distearyl pentaerythritol diphospite, Triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate] and distearyl pentaerythritol diphosphite, 4-Tert-butyl catechol and tristearyl phosphite. The discoloration of the acrylonitrile solution of maleimide can be effectively prevented by adjusting the molecular oxygen concentration of the gaseous phase part of the acrylonitrile solution (providing that the molecular oxygen concentration is obtained of the gas mixture excluding maleimide and acrylonitrile on the basis of volume; this remark similarly applies hereinafter) to a level in the range of 0.01 to 10% by volume. A deviation from the range mentioned above is unfavorable because the discoloration will not be effectively precluded if the molecular oxygen concentration exceeds 10% by volume and the acrylonitrile solution of maleimide will suffer inclusion of monomers other than maleimide and acrylonitrile and incur discoloration owing to the contamination of containers such as storage tanks if the molecular oxygen concentration is less than 0.01% by volume. It is, therefore, appropriate that the molecular oxygen concentration is in the range of 0.01 to 10% by volume, preferably 0.1 to 8% by volume, and more preferably 1 to 7% by volume. The gas mixture excluding maleimide and acrylonitrile mentioned above generally comprises molecular oxygen and such inert gases as nitrogen, carbon dioxide, helium, and argon. Since nitrogen is advantageously used as the inert gas, the gas mixture consisting of molecular oxygen and nitrogen may be cited as an appropriate example of the gas mixture excluding maleimide and acrylonitrile. Further , the discoloration of the acrylonitrile solution can be diminished more effectively by allowing a metal-inactivating agent to exist in the acrylonitrile solution. As typical examples of the metal-inactivating agent mentioned above, N,N'-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl] hydrazine, 2,2'-oxamide bis[ethyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], N,N'-disalicylidene-1,2-propane diamine, benzotriazole, tollyl triazole, tollyl triazole-potassium salt, and mercapto benzotriazole-sodium salt may be cited. Among other metal-inactivating agents enumerated above, N,N'-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionyl] hydrazine, and 2,2'-oxamide bis[ethyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] are advantageously used. These metal-inactivating agents may be used either singly or in the form of a mixture of two or more members. The amount of the metal-inactivating agent to be added is in the range of 0.0001 to 1% by weight, preferably 0.001 to 0.1% by weight. The acrylonitrile solution of N-phenyl maleimide of this invention whose total content of AZB and DPH is not more than 500 ppm (based on the weight of the acrylonitrile solution) is used advantageously for the production of an acrylonitrile based copolymer, specifically an acrylonitrile-N-phenyl maleimide-containing copolymer, which precludes coloration and excels in color characteristics. The acrylonitrile solution of N-phenyl maleimide whose total content of AZB and DPH is not more than 100 ppm is appropriate in particular. Preferably, the total content of AZB and DPH in the acrylonitrile solution is as low as possible. Depending on the application for which the acrylonitrile solution is intended, it suffices to adjust the total content to the neighborhood of 20 ppm. The acrylonitrile solution of N-phenyl maleimide whose total content of AZB and DPH is not more than 500 ppm, preferably not more than 100 ppm, can be prepared by adopting without any modification the method which is used for the preparation of the acrylonitrile solution of maleimide whose values of Δ L, Δ a, and Δ b are respectively, preferably not more than 1, not more than 1, and not more than 2. The maleimide to be used herein is not particularly limited. Any of the maleimides obtained by various known methods can be adopted as already pointed out. Since the maleimide which is obtained by such a known method (occasionally containing an organic solvent used in the process of reaction) (hereinafter referred to as "crude maleimide") suffers such extraneous substances as, for example, phosphoric acid that has been used as an acid catalyst to remain therein, it is generally refined by such treatments as washing with water and distillation and converted into a finished maleimide. To be specific, the refinement may be implemented by a series of operations such as, for example, (1) a procedure of washing the crude maleimide with water thereby depriving it of the organic solvent and subsequently distilling the washed maleimide, (2) a procedure of treating the crude maleimide with such an inorganic acid as sulfuric acid, washing the acid-treated maleimide with water, and thereafter separating the organic solvent therefrom, and (3) washing the crude maleimide with water, recrystallizing the washed maleimide as by concentration, and subsequently separating the produced crystals by filtration. The acrylonitrile solution of maleimide of this invention enjoys transparency and appreciably diminished discoloration. The use of this acrylonitrile solution of maleimide, therefore, permits the production of an acrylonitrile based copolymer, an acrylonitrile-maleimide-containing copolymer to be exact, which precludes coloration and excels in color characteristics. By the same token, the use of the acrylonitrile solution of N-phenyl maleimide of this invention whose total content of AZB and DPH is not more than 500 ppm permits the production of an acrylonitrile based copolymer, an acrylonitrile-N-phenyl maleimide-containing copolymer to be exact, which precludes coloration and excels in color characteristics. The term "acrylonitrile based copolymer" as used in this invention refers to copolymers which are obtained by the use of the acrylonitrile solution of maleimide mentioned above (including the acrylonitrile solution of N-phenyl maleimide mentioned above) . The term embraces copolymers of maleimides with acrylonitrile, and copolymers of maleimides with acrylonitrile and monomers which are copolymerizable therewith. As representative compounds of the copolymerizable monomer, methacrylic esters such as, for example, methyl methacrylate, ethyl methacrylate, propyl methacrylate, cyclohexyl methacrylate, and isobonyl methacrylate; acrylic esters such as, for example, methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate; aromatic vinyl compounds such as, for example, styrene and α-methyl styrene; and conjugate dienes such as, for example, butadiene may be cited. Among other copolymerizable monomers enumerated above, methyl methacrylate, methyl acrylate, styrene, and α-methyl styrene prove advantageous in exhibiting excellent reactivity and permitting the production of resins which excel in heat-resistance. As other concrete examples of the copolymer mentioned above, acrylonitrile-butadiene-styrene (ABS) resin, acrylonitrile-acrylic rubber-styrene (AAS) resin, and acrylonitrile-chlorinated polyethylene-styrene (ACS) resin which contain maleimide as one component of copolymer may be cited. The acrylonitrile based copolymers of this invention are invariably known to the art. They can be easily produced by the known method of polymerization except that the acrylonitrile solution of maleimide of this invention is used in place of the known acrylonitrile solution of maleimide. These acrylonitrile based copolymers, when necessary, can incorporate therein various known additives. Now, this invention will be described more specifically below with reference to working examples and controls. Synthesis of N-phenyl maleimide An N-phenyl maleamic acid obtained from aniline and maleic anhydride was left reacting in an o-xylene solvent at a temperature of not less than 130°C in the presence of an acid catalyst. The reaction was allowed to proceed while the water formed thereby was expelled in the form of a mixture with o-xylene from the system by distillation. The reaction mixture obtained by the resultant ring-closure conversion into imide was separated into an organic solvent layer containing therein N-phenyl maleimide and a catalyst layer. The organic solvent layer was washed with water. The organic solvent layer thus obtained was distilled under a reduced pressure to expel o-xylene. The residue by the distillation was further distilled to obtain a yellow N-phenyl maleimide. This product, on analysis by liquid chromatography, was found to have a purity of not less than 99.5%. Comparative Example 1 Two Pyrex test tubes with a size of 18 mm in inside diameter and 180 in height, were prepared. In each of the test tubes, an acrylonitrile solution having an N-phenyl maleimide concentration of 60% by weight, a water content of 0.1% by weight, an antioxidant (a-1) content of 500 ppm (based on N-phenyl maleimide; which similarly applies hereinafter), and an antioxidant (b-1) content of 500 ppm was prepared by dissolving 9 g of the N-phenyl maleimide obtained as above in 6 g of acrylonitrile having a water content of 0.005% by weight, then adding 4.5 mg of pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate] [antioxidant (a-1)] and 4.5 mg of tristearyl phosphite [antioxidant (b-1)] to the resultant solution, and adjusting the water content of the produced mixture. When the acrylonitrile solution in one of the test tubes was tested for the L, a, and b values after the preparation, L = 99.3, a = -21.0, and b = 39.0 were obtained. The other test tube, with the gaseous phase part of the acrylonitrile solution displaced with nitrogen gas containing 7% by volume of molecular oxygen, was used for the forced coloration test contemplated by this invention. After 3 days' (72 hours') standing, the test piece was extracted from the test tube and the acrylonitrile solution left in the test tube was measured for the L, a, and b values. The results were L = 98.6, a = -19.8, and b = 41.2 and the differences in the L, a, and b values before and after the test were Δ L = 0.7, Δ a = 1.2, and Δ b = 2.2. The solution after the test was transparent. Example 1 An acrylonitrile solution shown in Table 1 was prepared by following the procedure of Comparative Example 1 while using triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate] [antioxidant (a-2)] and distearyl pentaerythritol diphosphite [antioxidant (b-3)] instead as antioxidants and 2,2'-oxamide bis[ethyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] [inactivating agent (c-2)] as a metal-inactivating agent. The acrylonitrile solution was tested to determine the L, a, and b values and the differences of the L, a, and b values before and after the test in the same manner as in Comparative Example 1. The results are shown in Table 1. Example 2 An acrylonitrile solution shown in Table 1 was prepared by following the procedure of Comparative Example 1 while using triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate] [antioxidant (a-2)] and distearyl pentaerythritol diphosphite [antioxidant (b-3)] instead as antioxidants and 2,2'-oxamide bis[ethyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] [inactivating agent (c-2)] as a metal-inactivating agent. The acrylonitrile solution was tested to determine the L, a, and b values and the differences of the L, a, and b values before and after the test in the same manner as in Comparative Example 1. The results are shown in Table 1. Example 1 2 Antioxidant a - 2 a - 2 Kind Amount added 1000 1000 Kind Kind b - 3 b - 3 b - 3 b - 3 Amount added 1000 1000 Metal-inactivating agent Kind c - 2 c - 2 Amount added 1000 500 Water content in solution 0.05 0.002 Molecular oxygen concentration 5 5 Clarity of solution after test transparent transparent L, a, and b values 99.3 99.3 L 99.3 99.3 a -21.0 -21.0 b 39.0 39.0 Difference before and after test Δ L 0.3 0.1 Δ a 0.3 0.1 Δ b 0.5 0.3 Controls 1 to 3 Acrylonitrile solutions shown in Table 2 were prepared. The acrylonitrile solutions were tested to determine the L, a, and b values and the differences of the L, a, and b values before and after the test in the same manner as in Comparative Example 1. The results are shown in Table 2. Control 1 2 3 Antioxidant Kind - - - Amount added Kind - - - Amount added Metal-inactivating agent Kind - - - Amount added Water content in solution 0.15 0.2 0.8 Molecular oxygen concentration 21 5 5 Clarity of solution after test transparent transparent white cloudy L.a.b. values L 99.3 99.3 99.3 a -21.0 -21.0 -21.0 b 39.0 39.0 39.0 Difference before and after test ΔL 6.2 3.6 10.7 Δa 6.5 5.2 6.7 Δb 10.4 8.5 10.5 Control 5 An acrylonitrile solution was produced by following the procedure of Control 1 while changing the molecular oxygen concentration in the gaseous phase part of the acrylonitrile solution to 7% by volume. The acrylonitrile solution was tested to determine the L, a, and b values and the differences of the L, a, and b values before and after the test in the same manner as in Control 1. The results are shown in Table 3. Example Control 5 Antioxidant Kind - Amount added - Kind Amount added Metal-inactivating agent - Kind Amount added Water content in solution 0.15 Molecular oxygen concentration 5 7 Clarity of solution after test transparent L, a, and b values L 99.3 99.3 a -21.0 b 39.0 Difference before and after test Δ L 3.4 Δ a 5.1 Δ b 9.2

Claims: An acrylonitrile solution of maleimide, characterised in that the acrylonitrile solution additionally contains not more than 0.05% by weight of water, 0.001 to 1% of at least one member selected from alkyl-substituted hydroxybenzenes and hindered phenols to the maleimide and 0.001 to 1% of at least one member selected from phosphorous esters, phosphoric esters, phosphine, and phosphoric acid amides to the maleimide; and in which, in a forced coloration test , the values, ΔL, Δa, and Δb are respectively 1 or less, 1 or less and 2 or less, the values to be calculated in accordance with the following formulae: ΔL = |L1 - L2| (Absolute value) Δa = a1 - a2 | (Absolute value) Δb = |b1 - b2 |(Absolute value) wherein L1, a1, and b1 are respectively the values of L, a, and b of the acrylonitrile solution before undergoing the forced coloration test, and L2, a2, and b2, are respectively the values of L, a, and b of the acrylonitrile solution after undergoing the forced coloration test, and the above-described forced coloration test is carried out as follows : (a) the L, a and b values of the maleimide solution are determined using a colorimeter, and then a test piece of SUS 304 is polished and placed in a test tube containing maleimide solution, the molecular oxygen concentration of the gaseons phase above the solution being adjusted to a level in the range of 3 to 10% by volume, in this instance the molecular oxygen being determined based on the gas mixture excluding any maleimide and acrylonitrile present ; (b) the test tube containing the test solution and sample is placed in an oil bath set at 70°C and is allowed to stand therefor for three days; and (c) subsequently, the test piece is extracted from the solution and the L, a and b values of the solution are determined using the colorimeter. An acrylonitrile solution of maleimide as claimed in Claim 1 having azobenzene and N, N,N-diphenyl hydrazine at a total content of not more than 100ppm. A method for the storage of an acrylonitrile solution of maleimide as claimed in Claim 1 or Claim 2, characterised in that the molecular oxygen concentration of the gaseous phase portion of the solution is adjusted to a level in the range of 0.01 to 10% by volume preparatorily to storing the solution as held in contact with a metal.

Inventors: KISHINO KAZUO, KITA YUICHI, NAKAYAMA KINICHI, SHIMOYAMA FUMIOKI, KISHINO, KAZUO, KITA, YUICHI, NAKAYAMA, KINICHI, SHIMOYAMA, FUMIOKI
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 1996
Main classification: C07D207
IPC codes: C07D207, C07C255