Unsaturated esters containing additives to reduce and stabilize the yellowness index

Abstract

The present invention relates to a novel method for reducing the yellowness index of alkyl (meth)acrylates, especially MMA, and polymers made therefrom. The novel method achieves this effect even after long-term storage of the monomers. In the method, a specific aldehyde is added to the monomer composition. This addition can be carried out independently of the respective preparation of the alkyl (meth)acrylates and can therefore be realized simply and cheaply. Moreover, the corresponding monomer composition forms part of the present invention.

Description

[Technical field]

[0001] The present invention relates to a novel method for reducing the yellowness index of alkyl (meth)acrylates, especially MMA, and polymers made from said alkyl (meth)acrylates. The novel method achieves this effect even after long-term storage of the monomer. In the method, a specific aldehyde is added to the monomer composition. This addition can be carried out independently of the respective alkyl (meth)acrylate production process, and therefore can be realized simply and inexpensively.

[0002] Furthermore, the corresponding monomer compositions form part of the present invention.

[0003] prior art Currently, methyl methacrylate (MMA) is produced by various processes starting from C2, C3 or C4 units, still mostly starting from hydrogen cyanide and acetone via acetone cyanohydrin (ACH) formed as central intermediate. This process has the disadvantage of producing very large amounts of ammonium sulfate, the further processing of which is very costly. Further processes using raw material bases other than ACH are described in the relevant patent literature and have already been realized on an industrial scale. A further disadvantage is that the yellowness index of the produced C3-based MMA is not optimal. This yellowness index is indeed relatively low, but still leads to a slight yellowness that is disruptive, especially in the production of PMMA sheets, PMMA films or PMMA mouldings used in optically relevant applications.

[0004] The production of MMA based on C4 feedstocks starts with reactants such as isobutylene and tert-butanol, which are converted to the desired methacrylic acid derivatives through several process steps, where in the first step they are oxidized to methacrolein and in the second to methacrylic acid, and finally esterification gives the desired alkyl esters, in particular with methanol to give MMA. Details of this method are described, inter alia, 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, and in Krill and Ruehling et. al. "Viele Wege fuehren zum Methacrylsaeuremethylester", WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, doi.org / 10.1002 / ciuz.201900869.

[0005] Here, generally, isobutylene or tert-butanol is oxidized in a first step to methacrolein, which is then reacted with oxygen to give methacrylic acid. The resulting methacrylic acid is then converted to MMA with methanol. Details of this process are described, inter alia, 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.

[0006] In detail, three MMA production processes based on this can be distinguished. As raw materials, for example, tert-butanol is used, which is converted to isobutene by elimination of water, or methyl tert-butyl ether is converted to isobutene by elimination of methanol, or isobutene itself, which is available as a raw material, for example from a cracker. Overall, this gives rise to the following three routes: Method A, "tandem C4 direct oxidation" method, without intermediate isolation of methacrolein: in this case methacrolein is produced from isobutene in a first step, which is oxidized to methacrylic acid in a second step and finally esterified with methanol in a third step to give MMA.

[0007] Method B, "separate C4 direct oxidation" method: In this case, it is the same in that methacrolein is produced from isobutene in the first step, which is first isolated and intermediately purified in the second step, then oxidized to methacrylic acid in the third step, which is finally esterified with methanol in the fourth step to obtain MMA.

[0008] Method C, the "direct metha" or direct esterification method: Again, methacrolein is produced from isobutene in a first step, which is again first isolated and intermediately purified in a second step and then directly esterified with methanol and air in a third step to give MMA.

[0009] All the methods mentioned are well documented in the prior art, in particular in (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.

[0010] Although the reactant and by-product profile of C4-based MMA is very different from that obtained starting from C3 units, a slight but detrimental yellow color for optical end uses is also detectable in C4-based MMA without very laborious multi-stage purification with associated product losses. In principle, however, the yellow color in C4-based products, although detrimental, tends to be somewhat less pronounced than in C3-based products. This yellow color could be somewhat further reduced, albeit incompletely, by the provision of C4-based alternative processes.

[0011] In this alternative C4-based process, isobutylene or tert-butanol is oxidized in the gas phase with atmospheric oxygen over a heterogeneous catalyst to form methacrolein, which is then oxidatively esterified with methanol to obtain MMA. This process, developed by ASAHI, is described in particular in US Pat. Nos. 5,969,178 and 7,012,039. A particular disadvantage of this process is the very high amount of energy required. In one development of this process, methacrolein is obtained in a first step from propanal and formaldehyde. Such a process is described in WO 2014 / 170223. However, it is often found that, even with this optimization, C4-based MMA also often has a significant residual yellowness index.

[0012] As an alternative to this process, US Pat. No. 5,969,178 discloses a work-up in only one column, in which it is essential that the feed is located above the bottom. Low boiling components from the reactor discharge are removed from this column via the top. What remains at the bottom is a mixture of crude MMA and water, which is sent to further work-up. A mixture of methacrolein and methanol is finally removed from the column via a side stream for recycling to the reactor, the exact position of which must first be determined and can be adjusted by adding various sieve trays. However, US Pat. No. 5,969,178 itself points out that such a process is difficult to carry out due to the variety of azeotropes. Furthermore, in this case, in particular methacrylic acid, which is always present as a by-product, is of importance. According to this process, which is not mentioned in US Patent No. 5,969,178, methacrylic acid is separated, but since it remains in the waste phase, isolation may be of limited value, but it reduces the overall yield of methacrylic products of the process.

[0013] US Pat. No. 7,012,039 discloses a slightly different work-up of the oxidative esterification reactor effluent. Here, in a first distillation stage, methacrolein is distilled off at the top via sieve trays, and the MMA-containing aqueous mixture is sent from the bottom to a phase separator. In this phase separator, the mixture is adjusted to a pH value of about 2-3 by adding sulfuric acid. Separation of the aqueous, acidified with sulfuric acid, and the organic or oil phase is then carried out by centrifugation. In a further distillation, this oil phase is separated into high-boiling components and an MMA-containing phase, which is taken off at the top. From the MMA-containing phase, the low-boiling components are then separated in a third distillation. This is followed by a fourth distillation for final purification.

[0014] The problem with this method is that it requires the addition of large amounts of sulfuric acid, which can corrode parts of the plant. These components, especially the phase separator and the second distillation column, must therefore be made of suitable materials. Furthermore, US Pat. No. 7,012,039 is silent about the handling of the co-produced methacrylic acid and the remaining methanol remaining in the product. However, it can be assumed that the former is removed together in the distillation step, whereas methanol can only be partially obtained and returned together with methacrolein, the rest being lost probably in the third distillation step.

[0015] WO 2014 / 170223 describes a process similar to US Pat. No. 7,012,039. The only difference is that in the actual reaction, the pH value is adjusted in the circuit by adding a methanolic solution of sodium hydroxide. This serves in particular to protect the catalyst. Furthermore, the separation of the aqueous phase in the phase separation is easier due to the salt content. However, another consequence is that part of the methacrylic acid formed is present as a sodium salt, which is later separated and disposed of together with the aqueous phase. In the variant of adding sulfuric acid during phase separation, the free acid is indeed recovered. However, instead, sodium (hydrogen)sulfate is obtained, which can lead to other problems during disposal.

[0016] Finally, WO 2017 / 046110 teaches an optimized work-up in which the crude MMA obtained from the oxidative esterification is first separated into a heavy phase, from which an alcohol-containing light phase can then be distilled off and recycled again.A further feature of this process is that methacrolein is obtained here on the basis of propanal and formaldehyde, the former on the basis of C2 units, such as ethylene and synthesis gas.

[0017] Overall, regardless of the methacrolein feedstock base used, all of these methods result in MMA or alkyl methacrylates in general that exhibit a measurable yellow color as the monomers themselves.

[0018] As shown in the prior art, in various MMA processes, regardless of the raw material base, multiple separation steps are carried out, firstly to achieve isolation of the monomers according to specifications, and secondly to ensure that the color value of the final monomer product is sufficiently low so that a transparent polymer product can be finally produced.

[0019] Furthermore, the slight yellowness of the monomers, which in principle easily increases, for example during long-term storage in storage tanks or as a result of transport times for further processing, also leads to a yellowish color of downstream products, such as molding compounds and other polymers, such as plexiglass-based pellets and semi-finished products produced starting from MMA.

[0020] There is therefore a need for improvements to identify the cause of this yellow tint and to remove it as efficiently as possible from the corresponding alkyl methacrylates, in particular MMA, before polymerization.

[0021] In EP 3676241, it is explicitly proposed to adjust the pH value and water content in a specific manner during the oxidative esterification, and to further process the crude product of this stage in a further reactor, so that the water content during the post-treatment is higher and the pH value is lower than in the original reaction, in order to reduce the yellowness index. This method is indeed effective, but it has also proven to be complicated in terms of process engineering.

[0022] As a third raw material option, there are furthermore C2-based processes for the production of alkyl methacrylates, in particular MMA. These processes also contain methacrolein as an intermediate, which is produced from formaldehyde and propanal, the latter being obtained from ethylene. In the production of methacrolein by the C2 process, the desired 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 by the Mannich reaction. The methacrolein (MAL) thus synthesized can then be converted in a subsequent step into methacrylic acid by gas-phase oxidation or into methyl methacrylate by oxidative esterification. Such processes for the production of methacrolein are described, inter alia, in US Pat. No. 7,141,702, US Pat. No. 4,408,079, JP 3069420, JP 4173757, EP 0317909 and US Pat. No. 2,848,499.

[0023] Suitable processes for the preparation of methacrolein based on the Mannich reaction are generally known to the person skilled in the art and are, for example, the subject of a corresponding review in Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Acrolein and Methacrolein, DOI:10.1002 / 14356007.a01_149.pub2.

[0024] For this process to be economically viable, high yields and low specific energy requirements must be achieved. The yellow color of this C2-based MMA is also less than that of MMA based on alternative raw materials. Nevertheless, even in this case, it is currently very difficult to completely avoid yellowing, especially in the polymerized end product, and without the addition of bluing agents, it leads to a significant yellowing in colorless, transparent plastics made from the monomer, such as, for example, plexiglass sheets.

[0025] To sum up, it can be said that there is a wide variety of processes based on the raw materials ethylene, acetone and isobutene. Depending on the technology and post-processing methods, and depending on the various production processes, the resulting monomer quality, for example MMA, can actually differ in minor components.

[0026] In the C2-based LIMA process, the product is essentially free of (meth)acrylonitrile, but has a relatively high proportion of isobutyric acid methyl ester, also called methyl isobutyrate, in the ppm range. The content usually varies from 100 to 700 ppm. Other characteristic trace substances are dimethoxyisobutene and dimethoxyisobutane.

[0027] In the ALPHA process, also based on ethylene as a raw material, like the LIMA process, also based on C2, it can be said that, in principle, no (meth)acrylonitrile is present in this case. Instead, however, methyl propionate esters, also called methyl propionate, may be present at relatively high values ​​in the ppm range. The content varies from 10 to 100 ppm. In comparison with the LIMA process, the presence of methyl isobutyrate is low (tens of ppm). Other characteristic trace substances in the MMA obtained in the ALPHA process are pentanones, for example diethyl ketone and isopropyl methyl ketone, and ethanol.

[0028] In particular, the C3-based ACH sulfo process based on acetone as starting material essentially contains (meth)acrylonitrile, usually at concentrations between 30 and 250 ppm. Methyl propionate and isobutyric acid methyl ester are detected as well, but at lower concentrations than in the case of C2-based processes. Pentanones and ethanol, e.g. diethyl ketone and isopropyl methyl ketone, are either not detected or are only detected in the single-digit ppm range.

[0029] C4-based processes, especially those carried out as gas-phase processes, also contain certain other trace substances: here too, methyl isobutyrate and methyl propionate are detected, but in contrast, they are characterised by dimethylfuran and pyruvic acid, especially as trace components, which also affect the yellowness index of the isolated monomers.

[0030] A particular C4-based process, the Asahi process, must be highlighted here, which includes a direct liquid-phase oxidation as a second reaction step. In this MMA quality, methyl isobutyrate is again detected as a characteristic trace component.

[0031] In most processes, especially both C3 and C4 based processes, diacetyl is a coloring component that must be removed during the isolation process, but some of it does end up in the isolated MMA, where it can be found in commercial MMA at levels between 0 and just under 10 ppm.

[0032] Considering this inherent composition of the MMA monomer quality produced by various processes, reducing the yellow color of the product and preventing the yellow color during subsequent shipping and storage has been a particularly difficult task.

[0033] Thus, overall, there is a strong need for an effective and convenient method for preventing the yellowing of MMA, particularly regardless of the method of production of MMA.

[0034] assignment The present invention was therefore based on the problem of reducing the yellowness index of alkyl (meth)acrylates, in particular MMA, as easily as possible.

[0035] In particular, the objective here was to make it possible to achieve this reduction independent of the method for producing the alkyl (meth)acrylate.

[0036] Furthermore, there has been a problem in that the decrease in yellowness index is persistent, i.e., exists even after long-term storage of a composition containing an alkyl (meth)acrylate.

[0037] Furthermore, it was a problem to provide an improved monomer product quality of alkyl(meth)acrylates in terms of yellowness index. In this connection, it was a problem to provide that this improvement in the optical product quality of the monomers, even after the poly(meth)acrylates thus produced have been obtained by polymerization, results in improved optical properties accompanied by a reduction in the yellowness index.

[0038] Additionally, it would be desirable for a method to sustainably reduce the Yellowness Index to be free of toxicological concerns, easy to use, and inexpensive.

[0039] A further object was to make it possible to carry out this method without making relatively large modifications to the production plant and without requiring relatively large investments.

[0040] Further problems not explicitly stated may become apparent from the detailed description of the invention, the claims, the examples or the overall context of the specification of the invention.

[0041] solution These problems have been solved by a novel method for reducing the yellowness index of alkyl (meth)acrylates. The method is characterized in that 0.5 to 500 ppm by weight of an aldehyde having the general formula R-HC=O is added to the alkyl (meth)acrylate. Surprisingly, the aldehyde can be selected relatively freely here. For example, according to the invention, aldehydes can be used having a group R with 1 to 20 carbon atoms and optionally up to 3 oxygen atoms as ether and / or hydroxy groups. Here, R can be a linear, branched or cyclic alkyl group, an aromatic group, an ether group or a combination of several of these groups.

[0042] Examples of typical linear alkyl groups include 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 these include isopropyl, isobutyl, tert-butyl or ethylhexyl. Cyclic alkyl groups can be, for example, cyclohexyl, cyclopentyl or methylcyclohexyl.

[0043] In addition to saturated alkyl groups, aromatic groups or combinations of aromatic and saturated alkyl groups can also be used. Examples of aromatic groups include phenyl or benzyl groups.

[0044] According to the invention it is also possible to use groups which contain a total of up to 20 carbon atoms and one or more additional oxygen atoms in the form of ether or hydroxy groups.

[0045] Aldehydes containing olefinic groups cannot be used according to the invention since they are potentially polymerization active, and in addition, they do not appear to show any effect, as can be judged from the various concentrations of methacrolein remaining in C2- or C4-MMA.

[0046] Furthermore, other heteroatoms in the aldehydes, such as in particular nitrogen or sulfur heteroatoms, are excluded, since for example they may be prone to oxidation and may themselves cause discolouration, and halogen atoms are unsuitable for reasons of reactivity and toxicological aspects.

[0047] The aldehydes are particularly preferably acetaldehyde, propanal, 3-methylpentanal, isobutanal or n-butanal, and n-pentanal.

[0048] The method of the present invention is particularly preferably used for the additive formulation of commercially conventional alkyl (meth)acrylates such as methyl methacrylate (MMA). However, additive formulation of other monomers is also possible, for example, in particular n- or tert-butyl methacrylate, ethylhexyl methacrylate, ethyl methacrylate or propyl methacrylate. Furthermore, the method can also be used for acrylates such as methyl acrylate or butyl acrylate. The yellowness index of important functional (meth)acrylates such as methacrylic acid, hydroxyethyl (meth)acrylate or hydroxypropyl (meth)acrylate can also be reduced. The alkyl (meth)acrylate is preferably methyl methacrylate.

[0049] According to the invention, 0.5 to 500 ppm by weight of aldehyde is added to each monomer composition. Here, the optimum amount depends on the (meth)acrylate to which the additive is to be incorporated and on the aldehyde used. This amount can be determined for each combination by a person skilled in the art with some simple manual experiments. For many of these combinations, a preferred amount of aldehyde added of 1 to 250 ppm by weight, particularly preferably 10 to 150 ppm by weight, has proven to be advantageous.

[0050] It is preferable to further add 1 to 300 ppm by weight of one or more polymerization stabilizers to the alkyl (meth)acrylates. It is preferable to use only one polymerization stabilizer. Polymerization stabilizers for (meth)acrylates are generally known to the person skilled in the art. Preferably, in combination with the process according to the invention, 2,4-dimethyl-6-tert-butylphenol (DMBP) or hydroquinone, very particularly preferably hydroquinone methyl ether (HQME) is used.

[0051] Preferably, the process according to the invention is carried out such that the alkyl(meth)acrylate exhibits a drop in the yellowness index [D65 / 10] of at least 10% one hour after the addition of the aldehyde, particularly preferably at least 15%. Particularly preferably, the alkyl(meth)acrylate exhibits a drop in the yellowness index [D65 / 10] of at least 40% one hour after the addition of the aldehyde, such as isobutanal.

[0052] Surprisingly, it has been found that the yellowness index of alkyl (meth)acrylates can not only be significantly reduced within a short time by the simple addition of the described aldehydes. At least as surprisingly, it has been found that this reduction in yellowness index is still the same or at least detectably persistent even after storage for several days. This is observed even after storage at high temperatures, such as 40°C.

[0053] Preferably, the method according to the invention is carried out here such that the alkyl(meth)acrylate still exhibits a drop in the yellowness index [D65 / 10] of at least 10%, particularly preferably at least 15%, 8 days, preferably 1 month after the addition of the aldehyde, whereby during this period there is usually no or only a very slight increase in the yellowness index of the composition compared to the yellowness index 1 hour after the addition of the aldehyde.

[0054] Moreover, it was found, quite surprisingly, that the yellowness index of the polymers prepared from the alkyl (meth)acrylates additived according to the invention is also significantly reduced compared to the polymers prepared similarly but without the additive according to the invention. This effect remains stable even after the polymers are stored for a long period of time, for example for one month. Even after weathering the polymers, the color stabilization effect is easily measurable and surprisingly strong.

[0055] Basically, the method according to the invention can be used not only to reduce the yellowness index of pure alkyl (meth)acrylates such as MMA, but also to reduce the yellowness index of monomer mixtures based on various alkyl (meth)acrylates, where the aldehyde can be added to the monomer mixture or the monomer mixture can already contain one or more aldehydes according to the invention, so that the entire mixture has an aldehyde concentration according to the invention.

[0056] Furthermore, the effects of the present invention also extend to polymers produced from these monomer mixtures.

[0057] Surprisingly, it has also been found that many, more precisely all, of the aldehydes investigated, which correspond to the above explanation, exhibit the effect according to the invention.According to the tests carried out, for example, methanal, acetaldehyde, propanal, isobutanal or n-butanal, pentanal, 2-methylpentanal, decanal, dodecanal are particularly suitable.

[0058] Aromatic aldehydes such as benzaldehyde, 3-hydroxybenzaldehyde also show efficacy, although initially at a reduced level compared to aldehydes with pure alkyl groups, and therefore may be used according to the invention, but are less preferred.

[0059] In addition to the method according to the invention, a composition comprising at least 97.5% by weight of alkyl (meth)acrylate also forms part of the invention. The composition is characterized in that it comprises 0.5 to 500 ppm by weight of an aldehyde having the general formula R-HC=O, where the same applies to this aldehyde as stated above in connection with the method. Particularly preferred aldehydes according to the invention are isobutanal, n-pentanal or 3-methylpentanal.

[0060] Particularly preferably, but not exclusively, the alkyl (meth)acrylate is methyl methacrylate (MMA). In this case, the composition preferably comprises at least 99.5% by weight, ideally at least 99.9% by weight of MMA. Further monomers that may be included in the composition according to the invention have already been indicated in the description of the method.

[0061] According to the invention, the composition preferably comprises at least 97.5% by weight of alkyl (meth)acrylate and at least 0.5 to 500 ppm by weight of aldehyde. Preferably, the composition comprises 99.5% by weight, particularly preferably 99.8% by weight of alkyl (meth)acrylate and 1 to 300 ppm by weight, in particular 20 to 250 ppm by weight, very particularly preferably 10 to 130 ppm by weight, in particular 30 to 90 ppm by weight of aldehyde. The aldehyde in the composition is particularly preferably methanal, acetaldehyde, propanal, isobutanal or n-butanal, pentanal, 2-methylpentanal, decanal, dodecanal or a mixture of at least two of these aldehydes.

[0062] Preferably, the composition according to the invention further comprises 1 to 300 ppm by weight of a polymerization stabilizer, which is preferably 2,4-dimethyl-6-tert-butylphenol or hydroquinone, very particularly preferably hydroquinone methyl ether (HQME).

[0063] Surprisingly, it has further been found that by using the method according to the invention or by using the composition according to the invention, the color stabilization of functional or non-functional alkyl (meth)acrylates, in particular of the commercially very important MMA, can be achieved independently of the respective basic preparation method. However, it was particularly surprising here that the effect according to the invention occurs for MMA essentially independently of the preparation method, but the extent of the effect, in particular the extent of the color reduction, depends very largely on the basic preparation method. From experience, this can only be attributed to interactions with other components in the respective composition. However, it was by no means expected that this effect would be observed despite the very wide variety of secondary components in the alkyl (meth)acrylates.

[0064] For example, the effect of the present invention is particularly remarkable in MMA or other alkyl (meth)acrylates produced by the C3-based ACH process. This surprising effect is due to the composition containing acrylonitrile and / or methacrylonitrile, according to analysis. Particularly preferred here is the case where acrylonitrile and methacrylonitrile are present in the composition in a total amount of less than 300 ppm by weight, particularly less than 200 ppm by weight.

[0065] The effect according to the invention is particularly very pronounced in MMA or other alkyl (meth)acrylates produced by a C2-based process as well. This surprising effect is due, according to analysis, especially when the composition comprises at least two components selected from n-butanol, tert-butanol, methyl acrylate, isobutyric acid methyl ester, methyl propionate, 1,1-dimethoxyisobutene and ethyl methacrylate, in particular when the composition comprises n-butanol, tert-butanol, methyl acrylate, methyl propionate and ethyl methacrylate. Particularly preferred here is the case when all of these components are present, but n-butanol, tert-butanol, methyl acrylate, methyl propionate and ethyl methacrylate are present in the composition in a total amount of less than 5 ppm by weight. It is also preferred when n-butanol, tert-butanol, isobutyric acid methyl ester, methyl acrylate, methyl propionate and ethyl methacrylate are present in a total amount of less than 700 ppm by weight.

[0066] Similarly, the effect of the present invention is noticeable, although not as noticeable in other cases, especially with MMA or other alkyl (meth)acrylates produced by C4-based processes starting from isobutene, isobutanol or MTBE.

[0067] This surprising effect is due in particular to the composition comprising, by analysis, dimethylfuran, methyl pyruvate and / or diacetyl, preferably all three components, particularly preferably when the total of these three components is present in the composition at less than 30 ppm by weight, in particular less than 10 ppm by weight. [Brief description of the drawings]

[0068] [Figure 1] A diagram comparing the results of Examples 1 to 12 with Comparative Examples VB1, 2 and 3 for the stabilization of C2-, C3- or C4-MMA with various concentrations of isobutanal is shown (see also the results in Table 1). [Diagram 2]FIG. 1 shows the results of stabilization of C3-MMA with various concentrations of isobutanal over 8 weeks of storage (50° C.) for Examples 13-16 compared to Comparative Example VB4 (see also results in Table 2). [Diagram 3] FIG. 1 shows the results of stabilization of C4-MMA with various concentrations of isobutanal over 8 weeks of storage (50° C.) for Examples 17-20 compared to Comparative Example VB5 (see also results in Table 2). EXAMPLES

[0069] Decreasing yellowness index To investigate the reduction in the yellowness index of methyl methacrylate obtained by the C2, C3 or C4 process by the addition of the aldehyde isobutanal according to the invention, the methyl methacrylate was doped with an aldehyde, for example isobutanal. The procedure is initially for Examples 1 to 12. The yellowness index YI D65 / 10° was then measured according to DIN 6167 or at certain time points. The following raw materials were used to produce the doped methyl methacrylate samples: - Roehm methyl methacrylate derived from the C2 process produced by the LiMA method (hereinafter referred to as C2-MMA) - Methyl methacrylate manufactured by Roehm from the C3 process produced by the ACH method (hereinafter referred to as C3-MMA) - Roehm methyl methacrylate from the C4 process produced from isobutene, already stabilized with 50 ppm of hydroquinone monomethyl ether (hereafter referred to as C4-MMA) - Isobutanal manufactured by Merck KGaA

[0070] To prepare the isobutanal-doped methyl methacrylate samples, methyl methacrylate was charged into a glass beaker, in which hydroquinone monomethyl ether stabilizer was dissolved as required, and isobutanal was added. The mixture was homogenized with a magnetic stirrer for 1 hour. The yellowness index was then measured to evaluate the optical quality.

[0071] Comparative Example VB1 (Reference Example for Yellowness Index): C3-MMA without added aldehyde and stabilized with 50 ppm of hydroquinone monomethyl ether Example 1: C3-MMA stabilized with 50 ppm hydroquinone monomethyl ether and mixed with 12 ppm isobutanal Example 2: C3-MMA stabilized with 50 ppm hydroquinone monomethyl ether and mixed with 25 ppm isobutanal Example 3: C3-MMA stabilized with 50 ppm hydroquinone monomethyl ether and mixed with 60 ppm isobutanal Example 4: C3-MMA stabilized with 50 ppm hydroquinone monomethyl ether and mixed with 100 ppm isobutanal Comparative Example VB2 (Reference Example): C4-MMA containing no added aldehyde and already containing 50 ppm of hydroquinone monomethyl ether Example 5 C4-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 12 ppm of isobutanal Example 6: C4-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 25 ppm of isobutanal Example 7: C4-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 60 ppm of isobutanal Example 8: C4-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 100 ppm of isobutanal Comparative Example VB3 (Reference Example): C2-MMA, which does not contain any added aldehyde and already contains 50 ppm of hydroquinone monomethyl ether Example 9 C2-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 12 ppm of isobutanal Example 10: C2-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 25 ppm of isobutanal Example 11: C2-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 60 ppm of isobutanal Example 12: C2-MMA already contains 50 ppm of hydroquinone monomethyl ether and is mixed with 100 ppm of isobutanal

[0072] The Yellowness Index of the isobutanal doped methyl methacrylate samples C3-MMA, C4-MMA and C2-MMA was correlated to that of the pure methyl methacrylate (VB1, VB or VB3) from the C3, C4 and C2 processes, giving the percentage reduction in Yellowness Index relative to the starting value. The values ​​are shown in Table 1 and visually contrasted in Figure 1.

[0073] [Table 1]

[0074] Evaluation of yellowness index decrease over time To demonstrate the steady decrease of the yellowness index over time, samples of C3- or C4-MMA were mixed according to the invention with isobutanal as aldehyde and stored at 50° C. The corresponding tests are in Examples 9 to 16 or in the associated comparative examples VB4 and VB5. The yellowness index YI D65 / 10° was determined according to DIN 6167 after the time points specified in Table 2. The same raw materials were used as in Examples 1 to 12.

[0075] To prepare the isobutanal-doped samples for the study, methyl methacrylate was charged into a glass beaker, in which hydroquinone monomethyl ether stabilizer was dissolved as required, and isobutanal was added. The mixture was homogenized with a magnetic stirrer for 1 h. Afterwards, 25 ± 1 g of this solution was filled into a brown 30 mL narrow-mouth bottle and stored in an air-circulating drying cabinet at 50 °C.

[0076] To assess the optical quality, the yellowness index was measured at the start of storage at 50° C. and after storage times of 4 and 8 weeks, respectively, at the corresponding storage temperature of 50° C.

[0077] Comparative Example VB4 (reference example for yellowness index after storage): C3-MMA without added aldehyde, stabilized with 50 ppm hydroquinone monomethyl ether and stored for 8 weeks in an air-circulating drying cabinet at 50°C Example 13: C3-MMA stabilized with 50 ppm of hydroquinone monomethyl ether, mixed with 12 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Example 14: C3-MMA stabilized with 50 ppm of hydroquinone monomethyl ether, mixed with 25 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Example 15: C3-MMA stabilized with 50 ppm of hydroquinone monomethyl ether, mixed with 60 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Example 16: C3-MMA stabilized with 50 ppm of hydroquinone monomethyl ether, mixed with 100 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Comparative Example VB5 (Reference Example): C4-MMA without added aldehyde, already containing 50 ppm of hydroquinone monomethyl ether, stored for 8 weeks at 50°C in an air-circulating drying cabinet Example 17: C4-MMA already containing 50 ppm of hydroquinone monomethyl ether, mixed with 12 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Example 18: C4-MMA already containing 50 ppm of hydroquinone monomethyl ether, mixed with 25 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Example 19: C4-MMA already containing 50 ppm of hydroquinone monomethyl ether, mixed with 60 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks Example 20: C4-MMA already containing 50 ppm of hydroquinone monomethyl ether, mixed with 100 ppm of isobutanal, and stored in an air-circulating drying cabinet at 50°C for 8 weeks

[0078] The Yellowness Index of the isobutanal doped methyl methacrylate samples of C3-MMA and C4-MMA were related to the Yellowness Index of the pure methyl methacrylate from the C3 and C4 processes without storage and after 4 and 8 weeks of storage, respectively, to give the percentage loss of the Yellowness Index relative to the starting value. The values ​​are shown in Table 2 and are plotted in Figure 2 for C3-MMA and Figure 4 for C4-MMA, along with the percentage loss data.

[0079] [Table 2]

[0080] Comparative Example VB1 (Reference Example): C3-MMA without added aldehyde and stabilized with 50 ppm of hydroquinone monomethyl ether Examples 21 to 34: C3-MMA was stabilized with 50 ppm hydroquinone monomethyl ether, mixed with the aldehydes as shown in Table 3, stored for 8 weeks at 50°C in an air circulating drying cabinet, and the yellowness index was measured after 4 or 8 weeks. The results are shown in Table 3.

[0081] Notes on Example 27: The measurement of the Yellowness Index after the direct addition of 10 ppm by weight of dodecanal is likely due to measurement error. The decrease in the Yellowness Index after 4 or 8 weeks is consistent with other examples and is an expected result of the present invention.

[0082] Examples 35 to 40 C3-MMA was stabilized with 50 ppm of hydroquinone monomethyl ether, mixed with n-pentanal as shown in Table 4, stored for 8 weeks at 50°C in an air circulating drying cabinet, and the yellowness index was measured after 4 or 8 weeks. The results are shown in Table 4.

[0083] [Table 3]

[0084] [Table 4]

Claims

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

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

3. 3. The method according to claim 1, wherein the aldehyde is added to the alkyl (meth)acrylate in an amount of 1 to 150 ppm by weight.

4. 3. The method according to claim 1 or 2, further comprising adding to said alkyl (meth)acrylate one or more polymerization stabilizers, preferably DMBP (2,4-dimethyl-6-tert-butylphenol) or HQME (hydroquinone monomethyl ether) in an amount of 1 to 300 ppm by weight.

5. 3. The method of claim 1 or 2, wherein the alkyl (meth)acrylate exhibits at least a 5% decrease in Yellowness Index [D65 / 10] one hour after addition of the aldehyde.

6. 6. The method of claim 5, wherein the alkyl (meth)acrylate exhibits at least a 15% decrease in Yellowness Index [D65 / 10] one hour after addition of the aldehyde.

7. 3. The method according to claim 1, wherein the aldehyde is methanal, acetaldehyde, propanal, isobutanal, n-butanal, pentanal, 2-methylpentanal, decanal, dodecanal, benzaldehyde, or 3-hydroxybenzaldehyde.

8. 1. A composition comprising at least 97.5% by weight of an alkyl (meth)acrylate, the composition comprising 0.5 to 500 ppm by weight of an aldehyde of the general formula R-HC=O, wherein R has 1 to 20 carbon atoms and optionally has up to 3 oxygen atoms as ether and / or hydroxy groups, and R is a linear, branched or cyclic alkyl group, an ether group, an aromatic group, or a combination of several of these groups.

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

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

11. The composition of claim 10, wherein the composition comprises 10 to 130 ppm by weight of the aldehyde.

12. 10. The composition according to claim 8 or 9, wherein the aldehyde is methanal, acetaldehyde, propanal, isobutanal, n-butanal, pentanal, 2-methylpentanal, decanal, dodecanal, or a mixture of at least two of these aldehydes.

13. 10. The composition of claim 8 or 9, wherein the composition comprises acrylonitrile and / or methacrylonitrile, and the acrylonitrile and methacrylonitrile are present in the composition in a total amount of less than 200 ppm by weight.

14. 10. The composition of claim 8 or 9, wherein the composition comprises dimethylfuran, methyl pyruvate and diacetyl, and the dimethylfuran, diacetyl and methyl pyruvate are present in the composition in a total amount of less than 30 ppm by weight.

15. 10. The composition of claim 8 or 9, wherein the composition comprises at least two components selected from n-butanol, tert-butanol, isobutyric acid methyl ester, methyl acrylate, 1,1-dimethoxyisobutene, methyl propionate, and ethyl methacrylate, and wherein n-butanol, tert-butanol, isobutyric acid methyl ester, methyl acrylate, 1,1-dimethoxyisobutene, methyl propionate, and ethyl methacrylate are present in a total amount of less than 700 ppm by weight.

16. The composition according to claim 8 or 9, wherein the composition further comprises 1 to 300 ppm by weight of a polymerization stabilizer, preferably HQME (hydroquinone monomethyl ether).

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