Unsaturated esters containing an additive for reducing and stabilizing the yellowness index

Abstract

This invention relates to a novel method for reducing the yellowing index of alkyl (meth)acrylates, particularly methyl methacrylate (MMA), and polymers derived from these alkyl (meth)acrylates. This novel method exhibits this effect even after relatively long monomer storage times. The method involves adding a specific aldehyde to the monomer composition. This can be done independently of the individual methods used to prepare the alkyl (meth)acrylate, and is therefore simple and inexpensive to implement. Furthermore, the corresponding monomer composition forms part of this invention.

Description

[Technical Field]

[0001] This invention relates to a novel method for reducing the yellowing index of alkyl (meth)acrylates, particularly methyl methacrylate (MMA), and polymers derived from these alkyl (meth)acrylates. This novel method exhibits this effect even after relatively long monomer storage times. The method involves adding a specific aldehyde to the monomer composition. This can be done independently of the individual methods used to prepare the alkyl (meth)acrylate, and is therefore simple and inexpensive to implement. Furthermore, the corresponding monomer composition forms part of this invention. [Previous Technology]

[0002] Currently, methyl methacrylate (MMA) is prepared by various methods from C2, C3, or C4 units, primarily from hydrogen cyanide and acetone via acetone cyanohydrin (ACH) to form a central intermediate. This method has the disadvantage of yielding very large quantities of ammonium sulfate, and its processing involves very high costs. Alternative methods using raw materials other than ACH are described in relevant patent literature and have been implemented on a large scale. Another disadvantage is that the preparation of MMA based on C3 does not achieve an optimal yellowing index. Although the yellowing index is relatively low, it still results in a detrimental slight yellowing, especially when producing PMMA sheets, films, or molded products for optical applications. Methods for preparing MMA based on C-4 raw materials begin with reactants such as isobutylene or tributanol, which are converted into the desired methacrylic acid derivative during several process stages. In this case, in the first stage, the reactants are oxidized to methacrolein, and in the second stage, they are oxidized to methacrylic acid. Finally, esterification is performed to provide the desired alkyl ester, particularly with methanol, to provide MMA. Further details of this method are given in particular in the following: Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Methacrylic Acid and Derivatives, DOI: 10.1002 / 14356007.a16_441.pub2 and Krill and Rühling et al. "Viele Wege führen zum Methacrylsäuremethylester" [Many Routes lead to Methyl Methacrylate], WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim, doi.org / 10.1002 / ciuz.201900869. Here, isobutylene or terbutanol is typically oxidized to methacrolein in the first stage, and this methacrolein is then reacted with oxygen to provide methacrylic acid. The resulting methacrylic acid and methanol were then converted into MMA.Further details of this method are given in Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Methacrylic Acid and Derivatives, DOI: 10.1002 / 14356007.a16_441.pub2. Specifically, three methods for preparing MMA based on this are distinguished. For example, the starting material used is tributanol, which is dehydrated to convert to isobutylene, or methyl tributyl ether, which is demethanolized to convert to isobutylene, or isobutylene itself, which can be used as a feedstock for a pyrolysis unit, for example. In summary, this leads to the following three routes: Method A, "tandem C4 direct oxidation" method, without the intermediate isopropenal: Here, in the first step, isopropenal is prepared from isobutylene and oxidized to methacrylic acid in the second step, followed by esterification with methanol in the third step to provide MMA. Method B, "Independent C4 Direct Oxidation" Method: This is the same in the first step, preparing methacrolein from isobutylene, and in the second step, isolating and purifying the intermediate, followed by oxidation to methacrylic acid in the third step, and finally esterifying with methanol in the fourth step to provide MMA. Method C, "Direct Meta Method" or Direct Oxidative Esterification Method: Here, methacrolein is also prepared from isobutylene in the first step, and here, isolating and purifying the intermediate is also done in the second step, followed by direct oxidative esterification with methanol and air in the third step to provide MMA. All of the methods described are well described in the prior art, for example (i) IHS Chemical Process Economics Program, Review 2015-05, RJ Chang, Syed Naqvi (ii) Vapor Phase Catalytic Oxidation of Isobutene to Methacrylic Acid, Stud. Surf. Sci. Catal. 1981, 7, 755-767. Even though the reactant and byproduct profiles of C4-based MMA differ significantly from those obtained from C3 units, yellowing was still detected in C4-based MMA with its highly complex and multi-stage purification process, which involved slight but still destructive product loss for optical end-use applications. Essentially, however, despite the destructive nature, C4-based products tend to yellow slightly less than C3-based products. While not entirely sufficient, this yellowing can be further reduced slightly by providing alternative C4-based methods.In this alternative C4-based method, MMA is obtained by gas-phase oxidation of isobutylene or terbutanol with atmospheric oxygen over a heterogeneous catalyst to obtain methacrolein, followed by oxidative esterification of methacrolein with methanol. This method (developed by ASAHI) is specifically described in publications US 5,969,178 and US 7,012,039. A particular drawback of this method is its very high energy requirement. In this development, methacrolein is obtained from propionaldehyde and formaldehyde in the first stage. Such a method is described in WO 2014 / 170223. However, even with this optimization, C4-based MMA often exhibits a noticeable residual yellowing index. As an alternative to this method, US 5,969,178 discloses a post-treatment in a single column, in which the feed must be located above the bottom of the column. Low-boiling components from the reactor output are removed from the top of this column. The residue at the bottom of the column is a mixture of crude MMA and water, which is sent to further post-treatment. Its precise position must first be determined via a side stream, which can be adjusted by adding various sieves. Finally, a mixture of methacrolein and methanol intended for recycling back to the reactor is drawn from the column. US 5,969,178 indicates that this method is difficult to carry out due to various azeotropes. Furthermore, methacrylic acid (which has always existed as a byproduct) plays a particularly important role. According to this method, although US 5,969,178 does not discuss this issue, methacrylic acid is removed in such a way that it remains in the phase sent for removal, and the value of individual separation is limited. However, this refers to a reduced overall yield of methacrylic acid products from this method. US 7,012,039 discloses the post-treatment of reactor outputs from oxidative esterification (slightly deviated). Here, in the first distillation stage, methacrolein is distilled off at the top of the column via sieves, and an aqueous MMA-containing mixture from the bottom of the column is passed into a phase separator. In the aforementioned phase separator, sulfuric acid is added to adjust the pH of the mixture to approximately 2 to 3. The sulfuric acid-acidified water is then separated from the organic / oil phase by centrifugation. In another distillation, this oil phase is separated into a high-boiling component and an MMA-containing phase exiting from the top of the column. The MMA-containing phase is then separated from the low-boiling component in a third distillation. A fourth distillation is then performed for final purification. A technical challenge of this method is the use of sulfuric acid, which requires large quantities and can corrode plant components. Therefore, these components, particularly the phase separator or the second distillation column, must be manufactured from suitable materials. Furthermore, US 7,012,039 does not address the treatment of concurrently produced methacrylic acid or residual methanol remaining in the product. However, it can be assumed that the former is removed during the distillation stage, with only a portion of the methanol and methacrolein returned, and the remainder likely lost in the third distillation stage. WO 2014 / 170223 describes a method similar to US 7,012,039.The only difference in the actual reaction is that the pH is adjusted in the circuit by adding a methanol solution of sodium hydroxide. This is particularly beneficial for protecting the catalyst. Furthermore, due to the salt content, removing the aqueous phase during phase separation is simpler. However, another consequence is that a portion of the methacrylic acid formed is in sodium salt form and is subsequently removed along with the aqueous phase. In the variant where sulfuric acid is added during phase separation, the free acid is indeed recovered, but sodium sulfate (hydrogen sulfate) is obtained, which presents other technical challenges during removal. Finally, WO 2017 / 046110 teaches an optimized post-treatment of crude MMA obtained from oxidative esterification: first, separation from the heavy phase, followed by distillation from this heavy phase to obtain a light phase containing alcohol, which can then be recycled. Furthermore, this method is characterized by the presence of methacrolein based on propionaldehyde and formaldehyde, with the former obtained from, for example, ethylene and syngas based on the C2 unit. In summary, regardless of the raw material basis of methacrolein used, all these methods result in MMA, or generally alkyl methacrylates, which, as monomers, exhibit measurable yellowing. As illustrated in the prior art, regardless of the raw material basis, various MMA methods involve numerous separation steps, firstly to isolate the monomers according to specifications, and secondly to achieve a sufficiently low chromaticity index in the final monomer product. Finally, a transparent polymer product can be produced in this manner. Furthermore, slight yellowing of the monomers typically increases during relatively long storage periods, such as in storage tanks, or due to transport time for further processing. Slight yellowing of the monomers also leads to yellowing in downstream products, such as molding compounds or other polymers, such as granules based on plastic glass, and semi-finished products derived from MMA. Therefore, it is necessary to improve this by identifying the source of this yellowing and removing it from the corresponding alkyl methacrylate (especially MMA) as efficiently as possible before polymerization. EP 36 762 41, for reducing the yellowing index, explicitly proposes adjusting the pH and moisture content during oxidative esterification in a specific manner, and further processing the crude product from this stage in a separate reactor, wherein the moisture content during post-processing is higher than that of the original reaction, and the pH is lower than that of the original reaction. Although this procedure has proven effective, it is technically complex. As a third-feedback alternative, there are C2-based methods for preparing alkyl methacrylates (especially MMA). These methods also include methacrolein as an intermediate, prepared from formaldehyde and propionaldehyde, the latter obtained from ethylene. In this C2 method for preparing methacrolein, the target product is obtained from formalin and propionaldehyde in the presence of a secondary amine and an acid (usually an organic acid). In this case, the reaction is carried out via the Mannich reaction. The methacrolein (MAL) synthesized in this manner can then be converted to methacrylic acid in subsequent steps by gas-phase oxidation or to methyl methacrylate by oxidative esterification.Such methods for preparing methacrolein are particularly described in publications US 7,141,702, US 4,408,079, JP 3069420, JP 4173757, EP 0 317 909, and US 2,848,499. Methods based on the Mannich reaction suitable for preparing methacrolein are generally known to those skilled in the art and are the subject of corresponding commentaries, such as Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Acrolein and Methacrolein, DOI: 10.1002 / 14356007.a01_149.pub2. For the economical use of this method, high yield and low specific energy consumption should be achieved. Yellowing of C2-based MMA is also less than that of MMA based on alternative feedstock sources. Nevertheless, it is currently difficult to completely avoid yellowing (especially yellowing of the final polymerization product) and noticeable yellowing in colorless, transparent plastics (e.g., plastic glass sheets) produced from monomers without the addition of a blueing agent. In general, there are many methods based on raw materials such as ethylene, acetone, or isobutylene. Depending on the technology and post-processing methods, and the specific preparation methods used, the monomer properties produced (e.g., the monomer properties of MMA) can vary considerably depending on the trace components. For the C2-based LIMA method, it is generally said that (meth)acrylonitrile is absent in the product, but there is a relatively high content of methyl isobutyrate, also known as isobutyl ester, in the ppm range. The content typically varies between 100 and 700 ppm. Other trace characteristics include dimethoxyisobutylene and dimethoxyisobutane. For the C2-based ALPHA method (which, like the LIMA method, uses ethylene as a substrate), it is generally said that (meth)acrylonitrile is also absent in the product. However, relatively high levels of methyl propionate, also known as propionate methyl ester, may be present in the ppm range at this location. The concentration typically varies between 10 and 100 ppm. Compared to the LIMA method, there is a reduced level of isobutyrate methyl ester (tens of ppm). Other characteristic trace elements in MMA from the ALPHA method are pentanone (such as diethyl ketone or methyl isopropyl ketone) and ethanol. For the C3-based ACH-sulfo method (which is primarily based on acetone as a starting material), it can be said that (meth)acrylonitrile is typically present at concentrations ranging from 30 to 250 ppm. Methyl propionate and isobutyrate methyl ester are also detected, but at lower concentrations than in the C2-based method.Pentanone (e.g., diethyl ketone or methyl isopropyl ketone) and ethanol were not detected, or were only detected in single-digit ppm ranges. C4-based methods (especially gas-phase methods) further include other specific trace amounts. Trace amounts of methyl isobutyrate and methyl propionate were also detected here; however, unlike C4-based methods, dimethyl furan and pyruvic acid were specifically present as trace components, which also affected the yellowing index of the isolated monomer. Regarding specific C4-based methods, the Asahi method must be emphasized here, which includes direct oxidative liquid-phase oxidation as a second reaction step. With this MMA performance, methyl isobutyrate was again found as a characteristic trace component. In most methods, especially in the two C3-based and C4-based methods, diacetyl is a color-giving component that must be removed in the isolated method, but some of it enters the isolated MMA. Therefore, levels between 0 and 10 ppm can be detected in commercially available MMA. Given the specific composition of MMA monomers produced by various methods, reducing yellowing of the product and preventing subsequent yellowing during transportation and storage presents a particularly complex task. In short, there is a strong need to effectively and simply inhibit the yellowing of MMA. This need exists regardless of the method of MMA preparation. [Summary of the Invention]

[0003] Therefore, the technical problem solved by the present invention is to reduce the yellowing index of (meth)acrylates, particularly MMA, in the simplest and most feasible way. Here, a particular technical problem is that this reduction should be achievable regardless of the method of preparation of (meth)acrylates. Another technical problem is that the reduction in yellowing index should be persistent, even after long storage periods of compositions containing (meth)acrylates. Another technical problem is to provide monomeric properties of (meth)acrylates with improved yellowing index. In this regard, this technical problem arises after polymerization to provide the resulting poly(meth)acrylate, where the improved optical properties of the monomer should also result in improved optical properties with a reduced yellowing index. Furthermore, the method for maintaining the reduction in yellowing index should be toxicologically harmless and simple to use and inexpensive. Another technical problem is that the above-mentioned methods should be feasible without relatively large modifications to the production plant and without relatively large investments. Further technical problems not explicitly stated may be apparent from the description of the invention, the claims, the examples, or the context of the description of the invention.

Implementation Method

[0005] The aforementioned technical problem has been solved by a novel method for reducing the yellowing index of (meth)acrylates. This method is characterized by adding 0.5 to 500 ppm by weight of an aldehyde having the general formula R-HC=O to the (meth)acrylate. Surprisingly, the aldehyde is relatively freely selectable. For example, in this invention, an aldehyde having a group R is available, which has between 1 and 20 carbon atoms as an ether group and / or a hydroxyl group and optionally up to 3 oxygen atoms. Here, R can be a straight-chain, branched, or cyclic alkyl group, an aromatic group, an ether group, or a combination of two or more of these groups. Common examples of straight-chain alkyl groups are ethyl, propyl, n-butyl, n-hexyl, or n-dodecyl. Branched alkyl groups include alkyl groups having one or more, for example, tertiary or quaternary carbon atoms. Examples of branched alkyl groups are isopropyl, isobutyl, or tertiary butyl, or ethylhexyl. Cyclic alkyl groups can be, for example, cyclohexyl, cyclopentyl, or methylcyclohexyl. Besides saturated alkyl groups, aromatic groups or combinations of aromatic groups and saturated alkyl groups are also usable. Examples of aromatic groups are phenyl or benzyl. In this invention, groups containing a total of up to 20 carbon atoms and one or more additional oxygen atoms in the form of ether groups or hydroxyl groups can also be used. In this invention, aldehydes containing olefinic groups are not usable because they are potentially polymerizable. Furthermore, they do not appear to exhibit any effect, as can be determined from different concentrations of residual methacrolein in C2- or C4-MMA. In addition, other heteroatoms, such as nitrogen or sulfur heteroatoms in particular, are excluded from the above aldehydes because they may be dominated by oxidative sensitivity (e.g.) and thus cause discoloration. Halogen atoms are therefore unsuitable for reactivity reasons and from a toxicological point of view. The above aldehydes are preferably acetaldehyde, propionaldehyde, 3-methylpentanal, isobutyraldehyde, and n-pentanal. This method is particularly suitable for adding commercially available (meth)acrylates, such as methyl methacrylate (MMA). However, other monomers may also be added, such as, in particular, butyl or tributyl methacrylate, ethylhexyl methacrylate, ethyl methacrylate, or propyl methacrylate. This method can also be used with acrylates, such as methyl acrylate or butyl acrylate. It can also reduce the yellowing index of important functional (meth)acrylates, such as methacrylic acid, hydroxyethyl (meth)acrylate, or hydroxypropyl (meth)acrylate. Alkyl (meth)acrylates are preferably methyl methacrylate. According to the invention, the aldehyde is added to the individual monomer composition at amounts between 0.5 and 500 ppm by weight. Here, the optimal amount depends on the (meth)acrylate to be added and the aldehyde used. Those skilled in the art to which this invention pertains determine this amount for each combination using simple manual experiments. For many of these combinations, a preferred amount of aldehyde added between 1 and 250 ppm by weight, particularly preferably between 10 and 150 ppm by weight, has proven advantageous.Preferably, one or more polymerization stabilizers between 1 and 300 ppm by weight are added to the (meth)acrylate. Preferably, only one polymerization stabilizer is used. The polymerization stabilizers used for the (meth)acrylate are generally known to those skilled in the art to which this invention pertains. Preferably, 2,4-dimethyl-6-tert-butanol (DMBP) or hydroquinone, very preferably hydroquinone methyl ether (HQME), are used in conjunction with the method according to the invention. Preferably, the method according to the invention is carried out in such a manner that, one hour after the addition of the aldehyde, the (meth)acrylate has a yellowing index [D65 / 10] reduced by at least 10%, preferably at least 15%. Preferably, one hour after the addition of the aldehyde (e.g., isobutyraldehyde), the (meth)acrylate has a yellowing index [D65 / 10] reduced by at least 40%. Surprisingly, it has been found that simply adding the aforementioned aldehyde can significantly reduce the yellowing index of the (meth)acrylate in a short time. At least equally surprisingly, this reduction in yellowing index has been found to be persistent, so that it can still be detected to the same or at least similar degree even after several days of storage. This is still observable even after storage at high temperatures, such as 40°C. Preferably, the method of the present invention is carried out in such a manner that, 8 days after the addition of the aldehyde, preferably 1 month later, the (meth)acrylate still has a yellowing index [D65 / 10] reduced by at least 10%, particularly at least 15%. During this period, the yellowing index of the composition is usually not detected to increase or only increases very slightly, compared to the yellowing index one hour after the addition of the aldehyde. Very surprisingly, it has also been found that the yellowing index of polymers produced by the addition of the (meth)acrylate of the present invention is significantly reduced compared to polymers produced similarly without the (meth)acrylate of the present invention. This effect remains stable even after long polymer storage times (e.g., one month). Even after weathering tests of the polymer, color stability is easily measurable and surprisingly strong. In principle, the method according to the invention can be used not only to reduce the yellowing index of pure (meth)acrylates (such as MMA), but also to reduce the yellowing index of monomer mixtures mainly composed of various (meth)acrylates. In this case, aldehydes can be added to the monomer mixture, or one or more aldehydes have been added to the blended monomers according to the invention to obtain a total mixture of aldehydes with the concentrations of the invention. Polymers produced from these monomer mixtures also exhibit the effects of the invention. Surprisingly, many (more precisely, all) aldehydes corresponding to those described above as being studied have also been found to exhibit the effects of the invention. According to the tests conducted, for example, formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, pentanal, 2-methylpentanal, decanal, and dodecaldehyde are particularly suitable. Aromatic aldehydes such as benzaldehyde and 3-hydroxybenzaldehyde also show effects, although the initial reduction effect is similar to that of aldehydes with pure alkyl groups.Therefore, these are available, but less than preferred, in this invention. In addition to the method of this invention, compositions containing at least 97.5% by weight of (meth)acrylate also form part of this invention. These compositions of this invention are characterized by containing an aldehyde having the general formula R-HC=O at a concentration between 0.5 and 500 ppm by weight. This also applies to the aforementioned aldehyde in the context of this method. In this invention, the preferred aldehyde is isobutyraldehyde, n-pentanaldehyde, or 3-methylpentanaldehyde. Preferably – but not limitingly – the (meth)acrylate is methyl methacrylate (MMA). In this case, the composition preferably contains at least 99.5% by weight, ideally at least 99.9% by weight of MMA. Other monomers of this invention that may be present in the composition have been identified in the description relating to the method. According to the invention, the composition preferably comprises at least 97.5% by weight of (meth)acrylate and an aldehyde at a concentration between 0.5 and 500 ppm by weight. Preferably, the composition contains 99.5% by weight, particularly 99.8% by weight, of alkyl (meth)acrylate and an aldehyde comprising between 1 and 300 ppm by weight, particularly between 20 and 250 ppm by weight, and very preferably between 10 and 130 ppm by weight, particularly between 30 and 90 ppm by weight. The aldehyde in the composition is preferably formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, pentanal, 2-methylpentanal, decanal, dodecaldehyde, or a mixture of at least two of these aldehydes. The composition of the present invention preferably additionally contains a polymerization stabilizer between 1 and 300 ppm by weight. This is preferably 2,4-dimethyl-6-tert-butanol or hydroquinone, and very preferably hydroquinone methyl ether (HQME). Surprisingly, it has also been found that color stability of functional or non-functional alkyl (meth)acrylates, especially (commercially most importantly) MMA, can be achieved independently of the specific preparation methods described below, using the method of the present invention or the composition according to the present invention. However, what is particularly surprising is that the effects of this invention on MMA are essentially independent of the preparation method, but related to the degree of the effect, especially the degree of decolorization, and are very related to the preparation method described below. Empirically, this can be attributed solely to the interaction with other components in a particular composition. Nevertheless, one should not expect to detect this effect despite the very different second component in the (meth)acrylate. For example, the effects of this invention are particularly pronounced in MMA or other (meth)acrylates, especially those prepared using a C3-based ACH method. Analysis suggests that this surprising effect can be particularly attributed to the fact that the composition contains acrylonitrile and / or methacrylonitrile. This is especially preferred when the total amount of acrylonitrile and methacrylonitrile in the composition is less than 300 ppm by weight, particularly less than 200 ppm by weight.The effects of this invention are particularly pronounced in MMA or other (meth)acrylates, especially those prepared by a C2-based method. Analysis suggests that this unexpected effect can be attributed in particular to the fact that the composition contains at least two components selected from the following: n-butanol, tributanol, methyl acrylate, methyl isobutyrate, methyl propionate, 1,1-dimethoxyisobutylene, and ethyl methacrylate, especially when the composition includes n-butanol, tributanol, methyl acrylate, methyl propionate, and ethyl methacrylate. It is particularly preferred that all of these components are present, but that less than 5 ppm by weight of n-butanol, tributanol, methyl acrylate, methyl propionate, and ethyl methacrylate are present in the composition. It is also preferred that less than 700 ppm by weight of n-butanol, tributanol, methyl isobutyrate, methyl acrylate, methyl propionate, and ethyl methacrylate are present. Although not as pronounced as in other cases, the effects of the present invention are equally evident in MMA or other (meth)acrylates, particularly those prepared from isobutylene, isobutanol, or MTBE using a C4-based method. Analysis suggests that this unexpected effect can be particularly attributed to the fact that the composition contains dimethylfuran, methyl pyruvate, and / or diacetyl, preferably all three components. It is especially preferred when the total amount of these three components is less than 30 ppm by weight, particularly less than 10 ppm by weight, in the composition. [Example] Reduction in Yellowing Index To determine the reduction in the yellowing index of methyl methacrylate from C2, C3, or C4 methods due to the addition of isobutyraldehyde according to the present invention, methyl methacrylate is doped with an aldehyde (e.g., isobutyraldehyde). This procedure initially pertains to Examples 1 to 12. Then, and / or at specified time points, the yellowing index YI D65 / 10° is determined according to DIN 6167. The following raw materials were used to prepare doped methyl methacrylate samples: - Methyl methacrylate from Röhm's C2 method, prepared by the LiMA method (hereinafter referred to as C2-MMA), - Methyl methacrylate from Röhm's C3 method, prepared by the ACH method (hereinafter referred to as C3-MMA), - Methyl methacrylate from Röhm's C4 method, prepared from isobutylene and stabilized with 50 ppm hydroquinone monomethyl ether (hereinafter referred to as C4-MMA), - Isobutyraldehyde from Merck KGaA. To prepare the isobutyraldehyde-doped methyl methacrylate samples for research, methyl methacrylate was initially added to a glass beaker, and if necessary, the stabilizer hydroquinone monomethyl ether was dissolved therein, along with isobutyraldehyde. The mixture was homogenized with a magnetic stirrer for one hour. The yellowing index was then measured to evaluate optical properties. Comparative Example CE1 (Reference Example for Yellowing Index): C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether, without the addition of aldehydes.Example 1: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 12 ppm isobutyraldehyde. Example 2: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 25 ppm isobutyraldehyde. Example 3: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 60 ppm isobutyraldehyde. Example 4: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 100 ppm isobutyraldehyde. Comparative Example CE2 (Reference Example): C4-MMA, already containing 50 ppm hydroquinone monomethyl ether, without added aldehyde. Example 5: C4-MMA, already containing 50 ppm hydroquinone monomethyl ether, blended with 12 ppm isobutyraldehyde. Example 6: C4-MMA, already containing 50 ppm hydroquinone monomethyl ether, blended with 25 ppm isobutyraldehyde. Example 7: C4-MMA, containing 50 ppm of hydroquinone monomethyl ether and blended with 60 ppm of isobutyraldehyde. Example 8: C4-MMA, containing 50 ppm of hydroquinone monomethyl ether and blended with 100 ppm of isobutyraldehyde. Comparative Example CE3 (Reference Example): C2-MMA, containing 50 ppm of hydroquinone monomethyl ether, without added aldehyde. Example 9: C2-MMA, containing 50 ppm of hydroquinone monomethyl ether and blended with 12 ppm of isobutyraldehyde. Example 10: C2-MMA, containing 50 ppm of hydroquinone monomethyl ether and blended with 25 ppm of isobutyraldehyde. Example 11: C2-MMA, containing 50 ppm of hydroquinone monomethyl ether and blended with 60 ppm of isobutyraldehyde. Example 12: C2-MMA, already containing 50 ppm of hydroquinone monomethyl ether, was doped with 100 ppm of isobutyraldehyde. The yellowing indices of methyl methacrylate samples of C3-MMA, C4-MMA, and C2-MMA doped with isobutyraldehyde were respectively correlated with the yellowing indices (CE1, CE2, and CE3) of pure methyl methacrylate from the C3, C4, and C2 methods, respectively. This produced a percentage reduction in the yellowing index compared to the initial value. These values ​​are presented in Table 1 and visually compared in Figure 1. Evaluation of the decrease in yellowing index over time: To demonstrate the stable decrease in the yellowing index over a relatively long period of time, samples of C3- and C4-MMA according to the present invention were doped with isobutyraldehyde as an aldehyde and stored at 50°C. The corresponding experiments can be found in Examples 9 to 16 and the related comparative Examples CE4 and CE5. The yellowing index YI D65 / 10° was determined according to DIN 6167 at the time points specified in Table 2. The same raw materials as in Examples 1 to 12 were used.To prepare samples doped with isobutyraldehyde for research purposes, methyl methacrylate was initially added to a glass beaker, and if necessary, hydroquinone monomethyl ether was dissolved therein, along with isobutyraldehyde. The mixture was homogenized with a magnetic stirrer for one hour. Then, 25 ± 1 g of the solution was filled into brown 30 ml narrow-necked flasks and stored at 50°C in an air-circulating desiccator. To evaluate optical properties, the yellowing index was measured at the beginning of storage at 50°C and after 4 and 8 weeks of storage at the corresponding storage temperature of 50°C. Comparative Example CE4 (reference example for yellowing index after storage): C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether, without added aldehyde, and stored at 50°C in an air-circulating desiccator for 8 weeks. Example 13: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and doped with 12 ppm isobutyraldehyde, and stored at 50°C in an air-circulating desiccator for 8 weeks. Example 14: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 25 ppm isobutyraldehyde, and stored for 8 weeks at 50°C in an air-circulating drying chamber. Example 15: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 60 ppm isobutyraldehyde, and stored for 8 weeks at 50°C in an air-circulating drying chamber. Example 16: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether and blended with 100 ppm isobutyraldehyde, and stored for 8 weeks at 50°C in an air-circulating drying chamber. Comparative Example CE5 (Reference Example): C4-MMA, already containing 50 ppm hydroquinone monomethyl ether, without added aldehydes, and stored for 8 weeks at 50°C in an air-circulating drying chamber. Example 17: C4-MMA containing 50 ppm hydroquinone monomethyl ether and blended with 12 ppm isobutyraldehyde, stored at 50°C in an air-circulating drying chamber for 8 weeks. Example 18: C4-MMA containing 50 ppm hydroquinone monomethyl ether and blended with 25 ppm isobutyraldehyde, stored at 50°C in an air-circulating drying chamber for 8 weeks. Example 19: C4-MMA containing 50 ppm hydroquinone monomethyl ether and blended with 60 ppm isobutyraldehyde, stored at 50°C in an air-circulating drying chamber for 8 weeks. Example 20: C4-MMA containing 50 ppm hydroquinone monomethyl ether and blended with 100 ppm isobutyraldehyde, stored at 50°C in an air-circulating drying chamber for 8 weeks. The yellowing indices of C3-MMA and C4-MMA-doped isobutyraldehyde methyl methacrylate samples were correlated with the yellowing indices of pure methyl methacrylate from the C3 and C4 methods, respectively, under various conditions (without storage and after 4 or 8 weeks of storage). This resulted in a percentage decrease in the yellowing index compared to the initial value.These values ​​can be found in Table 2, and the C3-MMA graph is shown in Figure 2 and the C4-MMA graph is shown in Figure 4 at the reduced percentage specifications. Comparative Example CE1 (Reference Example): C3-MMA stabilized with 50 ppm hydroquinone monomethyl ether, without added aldehyde. Examples 21 to 34: C3-MMA stabilized with 50 ppm hydroquinone monomethyl ether, mixed with aldehyde as shown in Table 3, and stored at 50°C in an air-circulating desiccator for 8 weeks, with the yellowing index measured after 4 and 8 weeks. The results can be found in Table 3. Comments on Example 27: The measurement of the yellowing index after the direct addition of 10 ppm by weight of dodecyl aldehyde appears to be attributable to measurement error. The reduction in the yellowing index after 4 / 8 weeks in this example is consistent with that in other examples, and the results are as expected by the present invention. Examples 35 to 40: C3-MMA, stabilized with 50 ppm hydroquinone monomethyl ether, was mixed with n-pentanal as shown in Table 4 and stored at 50°C in an air-circulating desiccator for 8 weeks, with the yellowing index measured after 4 and 8 weeks. The results are shown in Table 4. [Simplified Explanation of the Diagram]

[0004] [Figure 1] The results of Examples 1 to 12 and Comparative Examples CE1, 2 and 3 are compared from the viewpoint of the stability of C2-, C3- and C4-MMA and various concentrations of isobutyraldehyde (see also the results in Table 1). [Figure 2] The results of Examples 13 to 16, which show the stability of C3-MMA with various concentrations of isobutyraldehyde within 8 weeks of storage (at 50°C), are compared with Comparative Example CE4 (see also the results in Table 2). [Figure 3] The results of Examples 17 to 20, which show the stability of C4-MMA with various concentrations of isobutyraldehyde within 8 weeks of storage (at 50°C), are compared with Comparative Example CE5 (see also the results in Table 2).

Claims

1. A method for reducing the yellowing index of (meth)acrylate, wherein 0.5 to 500 ppm by weight of an aldehyde having the general formula R-HC=O is added to the (meth)acrylate, wherein R has between 1 and 20 carbon atoms and, optionally, up to 3 oxygen atoms as an ether group and / or a hydroxyl group, and R is a straight-chain, branched, or cyclic alkyl, aromatic, ether group, or a combination of two or more of these groups.

2. The method of claim 1, wherein the (meth)acrylate is methyl methacrylate.

3. The method of claim 1 or 2, wherein the aldehyde is added to the (meth)acrylate between 1 and 150 ppm by weight.

4. The method of claim 1 or 2, wherein one or more polymerization stabilizers are additionally added to the (meth)acrylate between 1 and 300 ppm by weight.

5. The method of claim 1 or 2, wherein the (meth)acrylate has a yellowing index [D65 / 10] reduced by at least 5% after 1 hour of addition of the aldehyde.

6. The method of claim 5, wherein after 1 hour of adding the aldehyde, the (meth)acrylate has a yellowing index reduced by at least 15% [D65 / 10].

7. The method of claim 1 or 2, wherein the aldehyde is acetaldehyde, propionaldehyde, isobutyraldehyde, pentanal, 2-methylpentanal, decanal, dodecaldehyde, benzaldehyde, or 3-hydroxybenzaldehyde.

8. A composition containing at least 97.5% by weight of (meth)acrylate alkyl ester, wherein the composition contains an aldehyde having the general formula R-HC=O in an amount between 0.5 and 500 ppm by weight, wherein R has between 1 and 20 carbon atoms and at most 3 oxygen atoms, optionally as an ether group and / or a hydroxyl group, and R is a straight-chain, branched, or cyclic alkyl, ether group, aromatic group, or a combination of two or more of these groups.

9. The composition of claim 8, wherein the (meth)acrylate is methyl methacrylate (MMA), and wherein the composition contains at least 99.5% by weight of MMA.

10. A composition as claimed in claim 8 or 9, wherein the composition comprises at least 99.8% by weight of alkyl (meth)acrylate and the aldehyde in the range of 1 to 250 ppm by weight.

11. The composition of claim 10, wherein the composition includes the aldehyde in amounts between 10 and 130 ppm by weight.

12. A composition of claim 8 or 9, wherein the aldehyde is acetaldehyde, propionaldehyde, isobutyraldehyde, pentanal, 2-methylpentanal, decanal, dodecaldehyde, or a mixture of at least two of these aldehydes.

13. A composition as claimed in claim 8 or 9, wherein the composition contains acrylonitrile and / or methacrylonitrile, wherein the composition contains a total of less than 200 ppm by weight of acrylonitrile and methacrylonitrile.

14. A composition as claimed in claim 8 or 9, wherein the composition contains dimethylfuran, methyl pyruvate and diacetyl, wherein the composition contains a total of less than 30 ppm by weight of dimethylfuran, diacetyl and methyl pyruvate.

15. A composition as claimed in claim 8 or 9, wherein the composition contains at least two components selected from the following: n-butanol, tributanol, methyl isobutyrate, methyl acrylate, 1,1-dimethoxyisobutene, methyl propionate and ethyl methacrylate, wherein the total amount of n-butanol, tributanol, methyl isobutyrate, methyl acrylate, 1,1-dimethoxyisobutene, methyl propionate and ethyl methacrylate is less than 700 ppm by weight.

16. A composition as claimed in item 8 or 9, wherein the composition additionally includes a polymerization stabilizer between 1 and 300 ppm by weight.

Images