DE2132716A1 - Process for the production of monomeric esters of acrylic acid and substituted acrylic acid - Google Patents

Description

Priority: v. June 15, 1970 in USA Serial No. 1 55 224

The invention relates to a depolymerization process and more specifically a process for the preparation of monomeric esters of substituted or unsubstituted acrylic acids from the corresponding polymeric esters.

The acrylic acid ester resins are in a range between soft elastomers and hard plastics that are sawn and can be machined on a lathe. The materials generally have excellent initial coloring and are Resistant to light as they do not discolor with aging and do not decompose outdoors. You have relative good heat resistance as it can withstand temperatures up to

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about 66 C (150 F) show little or no color change and only tend to decompose at a temperature of about 260 C (500 F). Although they are thermoplastic materials and are therefore sensitive to some solvents, they have good resistance to acids, alkali, water and alcohol. In general, they have low acid numbers and therefore do not react with pigments or fillers and have good resistance to vegetable and mineral oils and fats.

The βlastomeric acrylic ester resins can be used as coatings be used in textile, leather and paper finishing. Some are also compatible with cellulosic ones Polymers and vinyl resins, to which they give improved stability to heat and light. Because of her excellent properties will be some of the harder ones Har »used as the entire film-forming material in heat-resistant white stoving enamels.

In general, because of their excellent clarity and transparency, acrylate polymers are used in windows, lenses, Instrument panels, optical parts such as contact lenses, reinforced plastics, protective coatings including lights Varnishes, paints and other finishes, adhesives, plasticizers and modifiers for venechiedene others Resins, lubricating oil additives, textile and leather finishing agents and coatings, to name just a few uses to name.

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Economically, it would be tremendously advantageous to so allow the use of any useless or contaminated acrylate monomers from polymerization processes along with any polymer chips or molded parts enable such a material to be depolymerized in order to re-polymerize it and then re-used for other moldings.

The present invention provides a new method for "cracking" or depolymerizing polymeric material in the polymer Form of chips, pieces, granules, powders, splits, sheets and / or of monomer material contaminated or unreacted by distillation, alone or in combination with one another, to a corresponding monomer material higher Getting quality in an effective, safe and economical way. The monomer obtained can, if desired, converted to useful polymers, either homopolymers or copolymers, after suitable purification, such as by distillation.

Acrylate type polymers have been in use for several years and are widely used in molding various articles by various molding processes. In fcaufe Such manufacturing processes generate a significant amount of waste polymer which is an important factor in determining the cost of the molded article. It was prepared various methods have been used in attempting to recover the monomer material and among these methods

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there is treating the acrylate polymer with heat to degrade the resin to form the starting monomer and to depolymerize. In the past, this was done by converting the polymeric acrylate into a suitable reaction kettle and heated therein to a temperature above 200 C to decompose it. This method is known as dry distillation. The condensation of the vapors in this process results in a relatively inferior, severely discolored monomer. Reference is made to US Pat. No. 2,030,901 in this regard. This The process cannot be advantageously adapted to industrial use, since the monomer obtained is rather impure and the colored substances contained therein are generally extremely difficult to remove. Also prevent some of the impurities contained therein the subsequent re-polymerization of the recovered Monomers.

In an attempt to solve this problem, several have been made Measures applied to this from the initial depolymerization process to purify resulting monomer. The methods used were for example one additional fractional distillation, steam distillation, or a combination of one or more fractional or steam distillation to remove the inferior Purify monomer.

One method (U.S. Patent 2,377,952) that uses was to obtain the monomer from the polymer and to

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cleaning was a seven-step process that consisted of that the polymer material was heated above the decomposition temperature, the vapors formed thereby condensing, the crude cracked monomer was steam distilled, then the resulting monomer was distilled with chlorine gas treated, the monomer then a water vapor distillation underwent and the aqueous layer from the monomer layer separated and finally the monomer was distilled under vacuum. The multistage and the manifold apparatus make such a process economically unsustainable, therefore, on an industrial scale, it is impossible to use polymer material or contaminated monomers with a to deal with such complex procedures.

Another known method (XJSA Patent 2 klZ 296) is to place a layer of polymer particles on a conveyor belt, spray it with water, and direct a high intensity flame such as an oxyacetylene flame at it to act directly on it. The flame cuts through the water coating on the polymer and has an instant depolymerizing effect on the polymer. The mixture is rapidly moved out of the flame zone with very little evaporation, placed in a suitable kettle and allowed to settle, forming two layers which are then separated to recover the monomer. The entire process takes place without volatilization of substantial quantities of the final product, even at temperatures in the range between 400 and 1000 ° C, which are far higher than the boiling points

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of the recovered monomers.

Another method involves batching the polymeric material placed in a lead-containing reaction kettle, the kettle is externally heated to the depolymerization temperature heated, namely to about 400 C, and the monomer vapor is fed to a condenser, wherein the Steam is condensed into a crude, liquid monomer. Carbon-containing finely divided particles form in the OiOsem system Deposits on the inside walls of the boiler and float on the surface of the lead. The monomer fumes deteriorated increasingly in their quality, and the boiler must be cleaned frequently, such as every 6 to 8 hours to prevent complete contamination and / or interruption of the process.

The present invention substantially completely overcomes the difficulties and disadvantages of the known ones Methods or at least minimizes them and provides a method for depolymerizing acrylate resins in a simple, effective, and economically feasible manner, using the process either on a batch basis or operated continuously using a minimum of labor and equipment and high quality monomer results that used in renewed polymerization reactions can be.

In short, there is obtained a process for the preparation of monomeric esters of acrylic acid and monomeric esters of substituted ones

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Acrylic acids from the appropriate! Polymer material by treating the polymer material in an inert atmosphere with a liquid metal which is maintained at or above the depolymerization temperature of the polymer material. The use of a molten metal for heating the polymeric material in contact therewith
has the advantage over using a rigid one
Heating surface, like the rigid inner surface of a heated metal kettle, that the surface of the molten metal can be attached by mechanical destruction and easily changed or constantly renewed. The disruption of the surface can occur in some cases
may be effected by allowing the polymeric material to fall onto that surface while continuously or continuously feeding it into the reaction vessel, or such disturbance may be by brushing or raking the surface, or most preferably by simply moving or
Stirring the molten metal.

The vapors generated by the contact of the polymer material with the surface of the molten metal are generated transferred into a condenser of the liquid-vapor contact type by suitable lines or tubes carrying steam, practice in being condensed and recovered. The steam are in the steam-carrying lines on one kept elevated temperature to avoid dust accumulation and undesirable To reduce deposits in different sections of these lines.

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It is a primary object of this invention to provide a process for the depolymerization of acrylate polymers which is essentially free from the disadvantages of the known methods discussed above.

Another aim is to have a continuous process for the depolymerization of substituted or unsubstituted To obtain acrylic ester that not only gives a high quality monomer but is also economical to perform and can be used with polymer material that is in a reactor in the form of chips, pieces, granules, powders, chips or sheets alone or in combination or with unreacted monomer, either contaminated or not contaminated.

Another aim is to find a method of manufacture of acrylic ester monomers from the corresponding polymer material using a continuous process Use of fewer stages than previously required, and get a monomer of comparable quality.

Yet another object is to produce an acrylic acid ester ionomer which, by depolymerizing the corresponding Polymer material was obtained, this monomer being of such a quality that there is a minimum of subsequent Purification treatment in a polymerization process to form polymer material of excellent quality can be used.

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These and other aspects and advantages of the present invention will be discussed in greater detail below Description and preferred embodiments explained

The present invention relates to a method of manufacture monomeric esters of acrylic acid and monomeric esters of substituted acrylic acids from the corresponding Polymer material and is characterized in that one the polymer material in an inert atmosphere with a liquid metal such as lead, tin, cadmium or various Alloys that are at or above the depolymerization temperature of the polymer material are held, brought into contact or treated, the surface of the liquid metal is constantly renewed. This is followed by the condensation of the vapors and formed in the process the recovery of the monomeric esters.

Any metal can be used in the method of the invention between about 400 and about 450 ° C melts, taking economic considerations such as Ease of use and availability into account. Metals necessary for the here described Processes that are particularly suitable are, for example, lead, which melts at 3271 ^ C, cadmium, the melts at 320 ° C, and tin, which melts at 251.9 ° C. Metal alloys that melt up to about 400 C. also within the scope of the present invention. Examples such alloys are:

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White solder (Sn and Pb each in the range from ^ O to 60 ^ a and especially in a weight ratio of 50:50)

Cerobend (50 ^ Bi, 26.7 % Pb, 13.3 # Sn, 10 # Cd) Cerrotru (58 fo Bi, kZ fo Sn) ₄

In view of the above criteria, lead is the preferred metal for the process of the invention

In general, that which is fed into the reactor can be depolymerized Polymeric material be in any conveniently processable form, such as Schnitzel, pieces, granules, powder, shavings or sheets, individually or in combination with one another or together with liquid Monier, either in contaminated or non-contaminated form. The to fulfill this Apparatus used in a specific aspect of the invention is described in more detail herein, and in general a Hopper feeder can be used in conjunction with a conveyor belt to the reaction vessel. Size Pieces, such as arches or original bricks, can be broken into smaller, more useful pieces, to facilitate feeding into the depolymerization kettle. This of course depends on the size and shape the apparatus for charging the ReaktLonskesseis is used with polymer material. Many different types of polymer pieces can be placed in one at the same time Krackkessel are fed, provided that a

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suitable loading mechanism is available for these different types.

The chemical composition of the feed comprises polymeric compounds having the structure of the formulas below I and II, i.e. polymers of low molecular weight alkyl esters of acrylic acid and substituted. Acrylic acids:

GH,

CH

OR

Formula I.

CH,

OR

Formula II

12 3

In the above formulas I and II, R, R and R are low molecular weight alkyl groups having 1 to 6 carbon atoms, preferably 1 to h carbon atoms, and η is an integer greater than 1.

Representative compounds that fall within the structural formulas given above are, for example, polymethyl acrylate, Polyethyl acrylate, polymethyl methacrylate, polypropyl acrylate, Polybutyl acrylate, polypropyl methacrylate

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-Λ2 -

and polybutyl methacrylate. The first four given above Compounds are preferred in the practice of the present invention.

The most common impurities in polymer material of this type are, for example, Weihmacher, fillers, Dyes, pigments, polymerization inhibitors and other polymers used in molding processes, or those which can be used to stretch or modify the polymeric acrylates. In general, these contaminants affect the process does not work very well, as the polymer malarial evaporates on contact with the heated metal surface and the vapors are subsequently condensed while the solid or certain liquid impurities are on the liquid metal surface remain as a residue. This can lead to you getting these contaminants must periodically remove from the liquid metal surface.

ψ The rate of feed is of course a function of the size of the particles being fed, the size of the equipment used to depolymerize the particles and the temperature maintained on the heated liquid metal surface.

The reaction kettle can be of virtually any size and made of any suitable material that is specified in the Is able to withstand the temperatures required to keep the liquid metal in a molten state to keep. Examples include various steels (one

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finally stainless steels) which resist corrosion under the conditions in the reaction vessel, and especially those that withstand breaking at the temperatures in them. The types of steel used to manufacture are used by furnace combustion rooms quite useful, and an example of this is combustion chamber steel # 285, type C.

The reaction takes place in a non-oxidizing atmosphere, i.e. carried out in an inert atmosphere, generally in nitrogen gas, but other injection gases can also be used and argon and neon gas, for example, would serve the same purpose.

Decomposition of some polymeric material forms solid carbonaceous materials including carbon dust, and these materials are deposited on the surface of the molten metal. These deposits can also originate from impurities in the starting polymer. Although the mechanical destruction of the molten metal surface by the introduction of the polymer material into the Reaction kettle can be used to renew the surface in some cases (for example, when the polymer material is relatively dense as a result of heavy pigmentation or when the polymer material exerts great force on the molten one Metal is brought), the surface is renewed but preferably by moving or stirring the molten metal at or near the surface. the Renewal holds a strong holdover from the

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molten metal upright to the polymeric material deposited thereon, and this ensures that a large rate of depolymerization during considerable periods of time despite the accumulation of carbonaceous materials or other contaminants the molten material is maintained.

An important aspect of the present invention resides in the fact that the molten metal, and especially the upper layer of which is constantly agitated, either by a mechanical stirrer or by a mechanical stirrer installed in the kettle by any other suitable form of movement. Moving the molten metal serves many purposes, including a constant renewal of the metal surface and the maintenance of a high heat transfer from the heated liquid metal to the polymer material. The thermal conductivity of the moving liquid metal surface area to the polymer material is increased and reduces the polymer residence time, resulting in a substantially reduced deposit on the surface of the liquid metal formed for a certain volume of raw monomer in comparison with such a system without Moving leads. In addition, with the agitation, some of the dust build-up in the reactor goes along with the vapors and can easily be removed at a later stage of the process. If some of the liquid condensate too recycled to the condenser, as is preferred it can be passed through a suitable filter. The proportion of the condensate that is not returned

can also be filtered before distillation.

The molten metal surface can be maintained at a temperature from about 350 to about 700 ° C, more specifically the temperature can be from about 1200 to about 600 ° C, preferably from about 50 to about 550 ° C, and most preferably from about 500 to 530 ° C being held. There is no noticeable depolymerization below 400 ° C. Above 700 C the decomposition produces excessive amounts of undesirable products which have low condensation temperatures and some of these products do not condense under normal condenser conditions. The production of these undesirable products reduces the yield and quality of the recovered monomer.

The carbon deposits and impurities on the Molten metal surfaces must generally be removed periodically. Jßin plant reactor middle Size, such as a steel kettle 3 »6 m in diameter with a diameter stirrer of 3 B) the. works with 10 to 12 rpm, can with one molten lead bath with a depth of 45 cm with a Feed rate of 1360 kg poly (methyl methacrylate) operated per hour for a period of 5 days before the apparatus becomes necessary turn off the lead surface for removal of the To wipe off contamination residues.

A carbonaceous residue is almost always formed,

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regardless of the method of heat application. The residue generally forms on the heat transfer surface and / or is deposited thereon, and without agitating the molten metal, the carbonaceous metal decreases Residue that accumulates on the metal surface, quickly the effectiveness of heat transfer. In addition, if there is no movement, the formation of a carbonaceous Residue it is necessary to switch off the equipment fairly frequently and to clean the system, such as for example every 6 to 8 hours.

The molten metal can be made according to any conventional one known heating method can be kept at the desired temperature, for example by direct gas or Fuel oil flame directed at the bottom of the reactor or by electrical heating elements using nichrome wires (trade name of Driver-Harris Company, Harrison, New Jersey) and voltage regulators. It has been found that electric heaters are particularly suitable because they Maintain a more even heat distribution and increase the life of the reactor, since there is no direct The flame is directed at the reaction kettle itself.

The polymer residence time in the reactor depends on the rate of depolymerization starting, which is a puncture of the temperature of the molten metal, with the depolymerization The higher the temperature, the faster it runs above the lowest depolymerization temperature

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lies. If the input speed, i.e. the feed speed of the polymer material equivalent to the rate of cracking, as determined by the temperature of the molten one Metal can be adjusted, the residence time is at a minimum and also the production is more carbonaceous Residues. The rate of depolymerization is essentially dependent on the intensity of stirring, provided that the stirring is sufficient to continuously or continuously produce a fresh, exposed to provide molten metal surface of the polymer material by removing the layer of solid, finely divided impurities, which gradually builds up on the molten metal surface is destroyed.

The condensation of the vapor after contacting the polymer material with the heated metal can be done in any suitable manner. However, care should be taken the formation of undesirable deposits, such as carbonaceous deposits, which lead to it tend to accumulate in the still, minimized or avoided.

The increase in rack capacity by moving the Improvement in the rate of heat transfer be attributable. One of the by-products of the cracking reaction is the carbonaceous residue, which is a « Layer, foam or film on top of the melted Metal loberf laughs “forms between the polymer and the metal. The build-up of this layer gradually reduces the

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BATH ORIGINAL

speed of heat transfer from the melted Metal to the polymer. When moving, the movement of the molten metal surface destroys this layer and largely restores the rate of heat transfer to what it was before the formation of the Layer was present.

Preferably, the intensity of the agitation or stirring is sufficient to achieve the maximum capacity of the specific depolymerization system under the operating conditions, such as feed rate, temperature of the melted Metal and evaporation rate.

Without agitation, essentially all of the carbonaceous residue remains in the reaction kettle. If you exercise moderately, the carbonaceous residue will be carried away in the form of dust with the fumes. If one moves at such a low intensity that the layer of impurities on the molten metal is not destroyed to provide fresh surface for contact with the polymer material, no dust is carried out, and in this respect the work is similar to that without moving "If." As the intensity of moving is increased beyond that of destroying the contaminant layer and exposing fresh, molten metal surface, the entrainment of carbonaceous dust increases increasingly, until ο is reached at the point where more than 6ü '?> deep of carbonaceous residue, which occurs during depolymerization was formed with the

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BATH ORIGINAL

Vapors are discharged together into the condensation section of the apparatus. The residue removal from the Reacting in this manner is quite desirable as there is less frequent cleaning of the molten metal surface is required.

The best results were achieved by working with vigorous agitation of the molten metal and maintaining a minimal polymer residence time in the reactor by continuously feeding the polymer chips at a rate equal to the rate of depolymerization at the specific temperature of the molten metal used in the reactor equals.

In general, it is a means of condensing what is formed Steam suitable for the method according to the invention. A liquid-vapor contact condenser turned out to be particularly suitable as this construction eliminates dry spots where high-boiling fractions lead to condensation tend. In addition, the steam lines are heated in a conventional manner to help cause condensation the high-boiling fraction of the distillate in the steam lines by particles of the carbonaceous residue around as such condensation would cause condensation to build up on the inside walls of the steam pipes Form rubber-like finely divided precipitates.

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BATH ORIGINAL

Because some of the carbonaceous residue formed in the reactor through the steam lines to the condenser when the molten lead is agitated, the resulting monomeric ester contains a small one Percentage of carbonaceous solids with less Particle size. It is desirable to have these solids before transferring the monomer to a distillation column for further purification, if such purification is desired to remove. Commonly a common type of filtration is used in known distillation processes becomes effective to remove this particulate matter.

The following table gives a comparison of the compositions of two representative samples of crude methyl methacrylate monomer obtained by the cracking process according to the present invention on the one hand and in a typical industrially used batch process for depolymerization on the other hand obtained, wherein in the latter process was fed discontinuously and none Moving the molten metal occurred. The analyzes were obtained by gas-liquid chromatography, the two raw monomer samples according to the present invention being the result of filtration of the condensed Monomer vapors were.

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Table I. Sample A, Sample B, Components conventional
raw methyl
methacrylate, 90.62 92.54 Methyl methacrylate .87.34 1.18 0.03 Ethyl acrylate 0.97 0.12 0.06 Ethyl methacrylate 0.26 0.44 0.24 Methacrylic acid 0.38 0.09 ■ o, 54 Methyl acrylate 0.18 0.51 0.54 water 1.72 O, o4 0.01 acetone 2.11 0.23 0.14 Methyl isobutyrate 0.28 0.02 0.09 Methanol 6.17 6.03 higher boiling
Fabrics 6.20 0.47 0.27 other components 0.61

all in all

100.05

99.89

99.98

From this table it can be seen that when using the method according to the present invention, a raw,
cracked monomer of greater than $ 90 purity is easily obtained.

Furthermore, depending on the product requirements, additional

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Borrowed stages can be applied using different purification methods, although the purity of the after the product obtained by the present process comes close to that of monomer obtained by conventional Methods obtained from the original reactants.

The invention is further illustrated by the following examples. In these all mean parts and percentages Parts by weight and percentages by weight if nothing otherwise expressly stated.

All of the following examples were made in an inert atmosphere carried out in a laboratory-sized reaction kettle with heated steam lines carrying the vapors a condenser of the liquid-vapor contact type with the possibility of a countercurrent or balanced flow and with a product return flow, about the carbon residue that normally settles on the walls of this facility, to limit.

The reactor was a closed kettle 30 cm in diameter and with a flat bottom made of stainless steel and had 15 cm high steel side walls and a bowl-shaped top part flanged thereon, also made of stainless steel. The facility was with a Equipped with a stirring blade, which ran parallel to the bottom of the reactor, was slightly smaller in diameter and right

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had angularly arranged teeth which protruded upwardly at regular intervals along the two arms of the agitator which extended from the central shaft, the shaft extending along the central axis of the steel reactor. The molten metal used in the reactor was molten lead that was held at a depth of about 1 inch.

The bowl-shaped top of the reaction vessel stainless steel was with a feed connection for the polymer feed and a connection for the steam outlet, i.e. for the outlet of the monomer. A nitrogen tank was connected to the feed connection, and nitrogen was metered into this feed compound at a rate sufficient to to prevent the monomer vapor from entering the compound. The side walls and the top of the reactor were appropriately isolated.

All of the heat supplied to the reaction kettle was supplied by external electrical heating means, the controlled by voltage regulators. Heating tapes were placed on the top of the reaction kettle used to reduce heat loss from there, and also in the steam outlet line to the condenser, premature vapor condensation and the formation of To reduce deposits.

- 2k -

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The composite granulated feed comprised polymeric Methylmethacrylsfc different shape, such as molding powder _t (colored or uncolored), small pieces of waste sheets, and various other types of polymer waste, which had been reduced to a granules-shaped charge, which was suitable for use in 'the apparatus described .

example 1

In this example, a granule-like charge of polymerizing methyl methacrylate was continuously fed into the reaction kettle at a rate of 53 g / min over a period of hk 1/2 hours. Nitrogen was also introduced into the same line as the polymer at a rate of about 1.5 liters per minute to prevent monomer from escaping through this line and to provide an inert component in the steam content in the kettle. The nitrogen content of the steam in the reaction vessel was about k volume. The molten lead was kept at a temperature of 500 to 530 C and stirring was carried out at a speed of 80 rpm. The monomer was condensed, filtered and distilled.

Example 2

Pieces of poly (methyl methacrylate) in sheet form with a size of k χ 7 »5 cm and with a thickness of 3 mm to 12 mm were continuously for 47 1/2 hours in

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fed into the reactor at a rate of k5 1/2 g / min, the agitation and other reactor conditions being essentially the same as those in Example 1. The monomer vapor was condensed and filtered to remove carbonaceous dust and distilled.

The results of Examples 1 and 2 are summarized in Table II below.

Table II

Example type of distillate, percentage total

Charging percentage rate of percentage

dsr feed methyl set

meth-carbon acrylate in the leg of the sending distillate

1 granules 97.25 85.7 0.67

2 schnitzel
of 4x7.5 cm
and with one
Thickness of

3 - 12 mm 99.1 90.6 0.06

Example 3

a) As in Example 1, a granular charge of poly (methyl methacrylate) was continuously fed into the reactor described above, but at a stirrer speed of 117 rpm. The average lead temperature during this experiment was 556 C. Die

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Speed of the reactor under these conditions was 160 g / min. The monomer vapor was condensed, filtered and distilled. The yield and quality of the distillate were comparable to those of Example 1. The plant operated effectively for about 5 days before an interruption.

b) Under the same conditions as in part a), but without agitation, the maximum cracking speed was 75 g / min and it was necessary to shut down after eight hours of operation in order to purify the molten lead.

Examples h to 7

The same equipment as in the previous examples was used with different stirrer speeds. The feed material was the same as in Example 1, Table III shows the effect of varying the agitation intensity on the residence time of the polymer material in the reactor and the residue distribution between the reaction vessel and the condenser section of the apparatus shown.

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IV V VI VII

Stirring, rpm 60 80 96 II7

Feeding speed, g / min 7 ^ .5 53 66.8 75.0

Molten lead temperature

513- 500- 595-515-5 ^ 0 535 608 532

Carbon in
Reactor, percentage of feed 0.55 0.61 0.39 0.22

Carbon in
Distillate, percentage of feed 0.06 0.06 0.17 0.38

Total carbon,

Percentage of

Charge 0.61 0.67 0.56 0.60

Examples 8 to 1h

In these examples a granular feed was used of poly (methyl methacrylate) fed into the same reactor in which the stirrer with 117 U / mln was operated. Various feed rates and lead bath temperatures are shown below Table IV shown. It is worth noting that the steam in the reactor is a fairly constant temperature in the region of 100 ° C, i.e. the cracking temperature.

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Table IV

Example lead feed steam production temperature rate temperature speed / min door, ⁰ C speed des

Monomers in kg / h / cm of lead surface

8th 451 10.1 386 7.2 9 457 29.9 400 21.1 10 466 49.8 400 35.4 11 471 69 * 9 402 49.4 12th 495 90.0 396 63.6 13th 526 115.0 4 03 81.3 14th 552 160.0 394 113.4

The condensed monomer (the production rate the same is given in the last column of the table) is filtered and distilled and yields a high purity monomer yield comparable to that of the previous examples.

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Claims (1)

P a t θ η t a n Proverbs 1.) Process for the preparation of monomeric esters of acrylic acid C- ^ or substituted acrylic acids, especially for the preparation of monomeric methyl methacrylate, from a corresponding polymer, especially from polymethyl methacrylate, characterized in that the polymer material is in an inert atmosphere with a molten metal surface which is kept at or above the depolymerization temperature of the polymer material, brings into contact, wherein the surface of the molten metal is constantly renewed, and the resulting vapors condense. 2,) Method according to claim 1, characterized in that lead, tin or cadmium are used as molten metal and keeps this at a * temperature of at least about 400 ° C. 3.) Method according to claims 1 and 2, characterized in that that the surface of the molten metal is constantly renewed by mechanical agitation or stirring. k.) Method according to claim 1 to 3, characterized in that the vapors are condensed in a condenser of the contact type from the group of the counter-current or direct current condensers, a portion of the condensate being returned through the condenser, whereby the BiI- 109885/1972 dung undesired deposits in it decreased. 5.) Process according to claim 1 to k, characterized in that the vapors from the reactor are passed through heated steam lines to the condenser. 6.) The method according to claim 5, characterized in that the temperature of the steam lines to above 250 C, preferably at about 300 to about 400 ° C. · 7) The method of claim 1 to 6, characterized in that one maintains the molten metal at a temperature of about 400 to about 65O ⁰ C. 8.) The method according to claim 1 to 7 »characterized in that one al» inert atmosphere nitrogen, argon or helium used. 9 ·) Method according to claim 2 to 8, characterized in that W is used as the molten metal lead. 10.) The method according to claim 9 »characterized in that the liquid lead is kept at about 450 to about 550 ° C and nitrogen, helium or argon is used as the inert atmosphere. 11.) The method according to claim 9 and 10, characterized by lehnet, that the temperature of the steam lines to the condenser is maintained at at least about 300.degree. 1098 85/1972 -31- 12.) The method according to claim 9 to 11, characterized in that the temperature of the liquid lead to about 500 holds up to about 53O ° C and uses nitrogen as the inert atmosphere. 109885/1972