WO2015189793A1

WO2015189793A1 - Method for the synthesis at low temperature of acrylic resin for the production of polymethyl methacrylate cast in slabs

Info

Publication number: WO2015189793A1
Application number: PCT/IB2015/054411
Authority: WO
Inventors: Paolo Gozzi; Giorgio Tlustos
Original assignee: Plastidite S.R.L.
Priority date: 2014-06-11
Filing date: 2015-06-11
Publication date: 2015-12-17

Abstract

A method for the synthesis of acrylic resin by means of a radical reaction comprises: a loading and activation step that provides mixing and gradual heating of methyl methacrylate, one or more thermal initiators such as azobisisobutyronitrile and one or more mercaptan chain transfer agents in a discontinuous production reactor, in order to obtain a homogeneous mixture, a reaction step that provides incubation of the reaction, including stopping the mixing and maintaining the heat of the mixture and completion of the reaction with low-rpm regimen stirring of the mixture, and a cooling step that provides cooling and stirring of the homogeneous mixture to obtain a casting acrylic resin.

Description

"METHOD FOR THE SYNTHESIS AT LOW TEMPERATURE OF ACRYLIC RESIN FOR THE PRODUCTION OF POLYMETHYL METHACRYLATE CAST IN SLABS"

FIELD OF THE INVENTION

The present invention concerns a method for the synthesis of acrylic resin for the production of polymethyl methacrylate (PMMA), in particular for the synthesis of an acrylic resin, typically called acrylic syrup, suitable for the production of sheets of cast PMMA (acrylic glass).

BACKGROUND OF THE INVENTION

It is known that cast polymethyl methacrylate (PMMA) is traditionally produced using a radical reaction, extremely exothermal and difficult to control. The unsaturated monomer used is methyl methacrylate (MMA), the reaction is activated chemically by radicals deriving from thermally labile reagents called initiators, to obtain a suitably controlled synthesis to form oligomers with optimum molecular weight for subsequent use in casting to obtain the sheets of PMMA. The synthesis is activated thermally, for example at typical temperatures of 90-95°C, which allow an industrially acceptable productivity, so as to obtain a rapid radical propagation with formation of a considerable fraction of chains with a high molecular weight, with a continuous growth of the remaining oligomer fractions, and again with a continuous conversion of the monomer. During synthesis there is a progressive monomer reduction with consequent increase in oligomers; the reaction is typically very rapid and extremely exothermal, which determines a further self-triggering due to the development of heat which must be suitably disposed of with cooling methods. The synthesis is terminated at the desired step with energetic cooling of the whole mass produced. If the process is not interrupted sufficiently promptly, or if it is not suitably heat controlled, due to possible technical reasons on the plants, this would entail the loss of the whole batch and consequent irreversible damage to the reactor.

Syntheses are also known that are carried out in the absence of radical sources, exploiting the radicalizing property of the MMA monomer and overcoming its typical inhibition, due to the stabilization needed for storage and transport, in this case working at much higher temperatures, for example 130-160°C; however, these systems, which would obtain presumably performing acrylic resins, cause very extreme process situations with corresponding risks and dangers, as well as with considerable stress to the plant. On the contrary, operating at lower temperatures, for example 80°C, relatively controllable syntheses would be obtained, since there is in any case a very rapid radical kinetics in post-triggering. There is also another known process, which provides the physical mixing of the liquid monomer (MMA) with a suitable PMMA granule so as to dissolve it, preferably working hot, to obtain a thickening and obtain a castable acrylic syrup. Although this process is quick to execute and easily reproducible, it is not competitive in terms of economic costs.

Furthermore, document EP-A- 1.201.686 describes a methyl methacrylate syrup obtained using a method on a reduced scale, which provides to divide the step of loading the reactor, with an initial loading and a second loading with initiator additive in a subsequent step, with a ratio between the loads of 30:70 or 65:35. However, dividing the load entails disadvantages in that the conduction of the synthesis is not simple, and also takes a longer time. The method described in EP-A- 1.201.686 also provides to heat the initial load in the reactor and subsequently the whole load after the second load has been added, to a temperature between 95° and 110°C: however, this can lead to instability of the final product when stored. In particular, the initial load provides to use MMA and methacrylic acid, after which, when 100°C is reached, a mercaptan chain transfer agent is added and then a mixture of MMA and a polymerization initiator usable in polymerization at about 100°C, the whole is further heated and then a polymerization inhibitor is added, then it is all cooled to ambient temperature. The method described in EP-A- 1.201.686 therefore provides to use a co- monomer, that is, the methacrylic acid, which is slower reactively. The method described in EP-A- 1.201.686 also provides polymerization inhibitors, to stabilize and make the production of the syrup safe; otherwise, given the high temperatures, it would entail a progression of the polymerization also during the cooling step.

Document US-A-2,565, 141 describes a method, using suspension in water, for the production of polymethyl methacrylate granule or powder, used in the molding field. The method described in US-A-2,565, 141 is not intended for the production of a liquid resin or syrup using mass polymerization, to be used in the casting field. Moreover, this production method also provides to heat the reaction mixture to a high temperature of 1 10°C, with peaks of up to 125°C, with all the disadvantages described above.

Document US-A-4,877,853 also describes a method to make a polymethyl methacrylate granule or powder, used in the molding field, by means of an emulsion polymerization.

There is therefore a need to perfect a method for the synthesis of an acrylic resin for the production of PMMA, in particular for the production of casting acrylic syrup, which can overcome at least one of the shortcomings of the state of the art.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

Unless otherwise defined, all the technical and scientific terms used here and hereafter have the same meaning as commonly understood by a person with ordinary experience in the field of the art to which the present invention belongs. Even if methods and materials similar or equivalent to those described here can be used in practice and in the trials of the present invention, the methods and materials are described hereafter as an example. In the event of conflict, the present application shall prevail, including its definitions. The materials, methods and examples have a purely illustrative purpose and shall not be understood restrictively.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claim, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

According to some forms of embodiment, a method is provided for the synthesis of acrylic liquid resin by means of a radical reaction, easily controllable and reproducible. According to one form of embodiment, the method includes: -) a loading and activation step that provides:

i) supplying in a single load, in a discontinuous production reactor, methyl methacrylate, one or more thermal initiators of the azobisisobutyronitrile type and one or more mercaptan chain transfer agents, and mixing and gradual heating of the methyl methacrylate, of the one or more thermal initiators such as azobisisobutyronitrile and the one or more mercaptan chain transfer agents in the discontinuous production reactor, in order to obtain a homogeneous mixture, with a very accurate solubilization;

-) a reaction step that provides:

ii) incubation of the reaction, including stopping the mixing and maintaining the heat of the mixture;

iii) completion of the reaction with slow stirring, that is, low-rpm regimen stirring of the mixture;

-) a cooling step that provides:

iv) cooling and stirring of the reaction mixture to obtain a casting acrylic liquid resin.

According to one form of embodiment, the gradual heating i) provides to heat and maintain the mixture at a temperature between about 58°C and about 65°C.

According to another form of embodiment, stopping the mixing and maintaining the heat ii) occur at a temperature between about 55°C and about 60°C.

According to another form of embodiment, during the step when the mixing is stopped and the heat maintained ii) the temperature is kept at a value constantly comprised between 55°C and 60°C.

According to another form of embodiment, the slow stirring iii) of the mixture occurs at an rpm regimen comprised between 55 and 90 rpm.

These and other aspects, characteristics and advantages of the present disclosure will be better understood with reference to the following description and the attached claims. The present description is intended to describe the principles of the disclosure.

The various aspects and characteristics described in the present description can be applied individually where possible. These individual aspects, for example aspects and characteristics described in the attached dependent claims, can be the object of divisional applications. It is understood that any aspect or characteristic that is discovered, during the patenting process, to be already known, shall not be claimed and shall be the object of a disclaimer.

DETAILED DESCRIPTION OF SOME FORMS OF EMBODIMENT

We shall now refer in detail to the various forms of embodiment of the invention, each example of which is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, the characteristics shown or described insomuch as they are part of one form of embodiment can be adopted on, or in association with, other forms of embodiment to produce another form of embodiment. It is understood that the present invention shall include all such modifications and variants.

Forms of embodiment described here concern a method for the production of acrylic liquid resin, for the production of polymethyl methacrylate (PMMA) cast in sheets, by a radical reaction conducted exothermally at low temperature, in industrially acceptable times and easily controllable and reproducible.

In particular, forms of embodiment described here concern the production of acrylic liquid resin for subsequent casting of sheets of PMMA, usable directly or after storage.

In general, a polymeric growth mechanism with the radical system provides an initiation, in which a radical active center is produced, generally thermally. The addition reaction of the monomeric species occurs on the radical active center, which also entails the formation of an analogous active center on the unit added. This allows the subsequent addition of various monomeric units and hence the propagation and formation of the polymer chain. Finally, there are the reactions that lead to the interruption of the propagation of the polymer chain, no longer able to add monomer units, thus creating chains that can no longer be increased. These reactions are called "termination reactions" if they entail the disappearance of the active center ("dead" chains), or "chain transfers" if it is simply transferred to another chain, thus allowing to propagate a new chain and blocking the growth thereof ("sleeping" chains).

According to some forms of embodiment, the method provides

-) a loading and activation step that provides: i) supplying in a single load, in a discontinuous production reactor, a monomer of methyl methacrylate (MMA), one or more thermal initiators of the nitrile family, in particular azobisisobutyronitrile (AIBN), and one or more chain transfer agents (CTA), typically of the mercaptan family, and mixing, advantageously at a low number of rpm, with gradual heating of the methyl methacrylate, of the one or more thermal initiators such as azobisisobutyronitrile and the one or more mercaptan chain transfer agents in a reactor, for between about 15 and 25 minutes in order to obtain a homogeneous mixture, well solubilized, at a temperature between about 58°C and about 65°C;

-) a reaction step, advantageously isothermal or similar to isothermal, that provides:

ii) incubation of the reaction, with stopping of the mixing and heat maintenance of the homogeneous mixture for between 40 minutes and 80 minutes, in particular between 40 minutes and 75 minutes, more in particular between 40 minutes and 70 minutes, even more in particular between 40 minutes and 60 minutes, at a temperature between 55°C and 60°C;

iii) completion of the reaction with slow stirring for between 10 minutes and 30 minutes, in particular between 12 minutes and 25 minutes, more in particular between 15 minutes and 20 minutes;

- a cooling step that provides:

iv) cooling and stirring, advantageously at high rpm regimen, of the homogeneous mixture at a temperature between 10°C and 20°C to obtain an acrylic liquid resin for casting, with momentarily blocked reactivity (typically defined "sleeping" pre-polymer).

The method according to the present invention is therefore advantageously an essentially isothermal production method and at low temperature, advantageously between 55°C and 60°C, to produce the acrylic resin for casting.

The method according to the present description allows to obtain a system of synthesis that achieves the final stability of the acrylic liquid resin produced, thanks to the fact that it is carried out at low temperatures.

The method according to the present description therefore has advantages in terms of controlling the synthesis at low temperatures, guaranteeing the stability of the final product in storage. Furthermore, the fact that the reagents are supplied to the reactor in a single load renders the preparation of the synthesis simpler and quicker, as a whole promoting better simplicity and shorter times of the synthesis. Therefore, the method according to the present description has greater operating simplicity. Moreover, since supplying the reagents in a single load entails the presence of a big mass in the reactor right from the start of synthesis, operating at low temperatures according to the present invention advantageously allows to control the exothermicity of the reaction, keeping it at levels such as to ensure stability of the final product.

With the method of the present invention it is possible to obtain sheets of PMMA with optimum optical quality, optimum resistance to aging during use, with chemical resistance within the typical standard of the material, optimum workability with laser cutting, very well thermoformable in subsequent workings. By inserting suitable colored pigments into the pre-polymer acrylic resin, it is also possible to obtain colored sheets of PMMA, with a transparent or solid effect. Advantageously, moreover, the sheets of PMMA obtained using the acrylic liquid resin according to the present invention are characterized by no shrinkage.

In possible implementations, the mixing i) in the loading and activation step can occur at an rpm regimen comprised between 60 and 125 rpm, in particular between 70 and 90 rpm. Examples of stirring rpm regimens in mixing i) are 70 rpm, 75 rpm, 85 rpm or intermediate values.

We believe that stopping the mixing and the heat maintenance of the mixture in the incubation ii) cause the reactions of radical polymerization to occur, with chain transfer, but remaining within a moderate range of temperature between

55°C and 60°C and therefore resulting in a more stable and controlled reaction, moderately exothermal, in practice essentially isothermal. The fact that the mixing is stopped means that the interaction between the polymer chains and between the chains and the one or more mercaptan chain transfer agents is not impeded by the mechanical action.

In possible implementations, the slow stirring iii) of the mixture can be carried out at an rpm regimen comprised between 55 and 90 rpm, in particular 65 and 85 rpm, more in particular between 70 and 80 rpm. Examples of rpm regimens in slow stirring iii) are 70 rpm, 75 rpm, 80 rpm or intermediate values.

We believe that the subsequent slow stirring iii) of the mass for between 10 minutes and 30 minutes allows to counter those phenomena typically defined as "gel effect" or Trommsdorff effect, which would entail a lack of control in the conduction of the synthesis, with non-homogeneous heat developments and consequently much more heterogeneous distributions of the oligomer molecular weights. This phenomenon causes sudden increases in viscosity, localized in the mass reaction medium (typically in "cells") with high conversion of the monomer to uncontrollable molecular weights due to auto-acceleration depending on the exothermicity which has been produced and which cannot be disposed of due to difficulties of diffusion. The process would therefore enter into "runaway" conditions, with the consequent loss of control of the system. The phenomenon can be attributed to a reduction in the kinetics of the termination reactions deriving from the difficult diffusion of the polymer chains which, for this reason, have a greater steric impediment; furthermore, the growth of the polymer chain is continually fed by the monomer, the diffusion of which is not impeded by the viscosity of the medium. On the contrary, slow stirring according to the present description allows to mechanically break the Trommsdorff cells that are formed, so that the process is more controllable in terms of temperature and viscosity, while still allowing a residual growth of the polymer chains (end reaction, with further and definitive increase in viscosity).

The presence of chain transfer agents in itself counters said phenomenon, chemically instead of mechanically, allowing a RAFT type polymerization ("Reversible Addition Fragmentation Transfer") which in the method according to the present invention does not need to be purified of the sulfur because the acrylic resin obtained from synthesis is colorless enough and allows to produce cast sheets of PMMA that are very stable to the aging effect over time and in the usual workings.

In possible implementations, the stirring and cooling iv) of the homogeneous mixture can occur at a higher rpm regimen than that provided in the slow stirring iii), for example comprised between 250 and 750 rpm, in particular 400 and 600 rpm, more in particular 450 and 550 rpm. Examples of rpm regimens in the stirring and cooling iv) are 480 rpm, 490 rpm, 500 rpm, 510 rpm, 520 rpm or intermediate values.

At the start of cooling of the reaction with stirring iv), it is possible for example to introduce other additives, such as UV adsorbers, possible release agents, possibly dyes to correct the optical effect of the acrylic resin.

During the stirring and cooling iv), it is possible for example to introduce possible cross-linkers, if necessary and provided the process is already cold.

We believe that the stirring and cooling iv) of the homogeneous mixture has the effect of inhibiting mechanically (stirring or mechanical mixing) and thermally (sudden cooling) the polymerization reaction, so that the reaction mixture becomes stable and controlled, and can thus be safely stored or can be used directly in the casting step for obtaining sheets of PMMA.

Applicant has found that the choice of initiators such as azobisisobutyronitrile, made according to suitable temperature and duration of the process according to the purposes of the present description, guarantees the production of radicals and the consumption of the monomer species present.

According to possible forms of embodiment, the one or more initiators such as azobisisobutyronitrile can have a dimerization trigger temperature comprised between 50°C and 90°C. The choice of the dimerization trigger temperature can influence the final properties of the PMMA, both optical and mechanical. In general, the lower the temperature, the more energetic the initiator.

In some forms of embodiment, the one or more initiators such as azobisisobutyronitrile in the mixing i) can have a quantity in the mixture comprised between 0.005 and 0.03 % w/w, in particular between 0.0075 and 0.025 % w/w, more in particular between 0.01 and 0.025 % w/w, even more in particular between 0.01 and 0.02 % w/w. Possible examples of quantities in the mixture of the one or more initiators such as azobisisobutyronitrile are 0.01 1 % w/w, 0.012 % w/w, 0.0125 % w/w, 0.013 % w/w, 0.015 % w/w, 0.0175 % w/w, 0.018 % w/w, 0.019 % w/w, 0.02 % w/w, 0.021 % w/w, 0.022 % w/w, 0.023 % w/w, 0.024 % w/w, 0.025 % w/w.

According to possible forms of embodiment, the one or more azobisisobutyronitrile initiators can have a physical form as a soluble granule, which advantageously can be very soluble and powderable. This physical form has the advantage that it prevents the formation of localized points of radical triggering, thus allowing more moderate and hence more controllable heat conditions of the reaction.

In some forms of embodiment, the one or more azobisisobutyronitrile initiators can be dissolved in an organic solvent or in MMA monomer.

According to some forms of embodiment, the one or more azobisisobutyronitrile initiators are preferably pre-mixed with the MMA monomer to give a liquid form to the radical initiation agent, preventing possible uncontrollable localized triggers.

In possible implementations, the one or more azobisisobutyronitrile initiators can be selected from a group consisting of:

2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4-dimethylpentanonitrile (CAS number 4419-1 1-8, for example available commercially from Dupont™ as Vazo® 52, dimerization trigger temperature 52°C);

- 2,2' - azobis(2- methylpropionitrile) (CAS number 78-67- 1, for example available commercially from Dupont^{1 M} as Vazo® 64, dimerization trigger temperature 64°C);

- 2- (2-cyanbutane-2-yldiazenyl)-2-methylbutanonitrile (CAS number 13472-08- 7, for example available commercially from Dupont™ as Vazo® 67, dimerization trigger temperature 67°C);

-l-(l-cyancyclohexyl)diazenylcyclohexane-l-carbonitrile (CAS number 2094- 98-6, for example available commercially from Dupont™ as Vazo® 88, dimerization trigger temperature 88°C).

In one form of embodiment, it is possible to use exclusively 2-(2-cyan-4- methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile (for example Vazo® 52). For example, 2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile can be comprised between 0.0075 and 0.025% w/w, in particular between 0.01 and 0.02% w/w with respect to the weight of the mixture. In one form of embodiment, it is possible to use both 2,2' - azobis(2- methylpropionitrile) (for example Vazo® 64), and also 2-(2-cyan-4- methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile (for example Vazo® 52). For example, it is possible to use a mixture of 2,2' - azobis(2- methylpropionitrile) and 2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile. An example of the proportion % w/w is 2,2' - azobis(2- methylpropionitrile) 90% w/w and 2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile 10% w/w, that is, a ratio of 9: 1 in weight.

In another possible form of embodiment, it is possible to use both 2,2' - azobis(2- methylpropionitrile) (for example Vazo® 64), and also 2-(2-cyan-4- methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile (for example Vazo® 52) and also -l-(l-cyancyclohexyl)diazenylcyclohexane-l-carbonitrile (Vazo® 88).

In another possible form of embodiment, the method does not provide to use polymerization inhibitors.

In another possible form of embodiment, the method does not provide to use co-monomers, such as mefhacrylic acid.

The reactions connected to the chain transfer mechanism stop the growth of a radical chain and transfer the radical activity to another chemical species, from which another chain starts to propagate. The chain transfer reaction therefore affects the length of the polymer chains produced. The one or more mercaptan chain transfer agents used in the method according to the present description are therefore specifically provided in the reaction medium to limit the length of the macro-molecules produced and therefore to reduce, or at least limit, the molecular weight, but without limiting the overall conversion rate from monomer to polymer, that is, obtaining the most regular oligomer distribution allowed in relation to the times industrially acceptable in terms of productivity, allowing a controlled RAFT polymerization. One advantage of the invention is also that it obtains a pre-polymer with a polydispersity index (PI) nearest the unitary value, that is, close to the mono-dispersed polymer, allowing a casting acrylic resin with a better rheological behavior under in-line pumping and a better regularity in sheet solidification, to minimize the internal tensions and defects that would ensue.

Given that the kinetics of final hardening into sheets can be considered in the first approximation independent of the molecular weight, since the better kinetics of the oligomers with high molecular weight is compensated by the greater mobility of those with low molecular weight, the synthesis according to the invention obtains a good conversion of the monomer to oligomers, a molecular weight of the oligomers that is not excessive, which would make the final viscosity higher and the process more difficult to perform, a molecular weight suitably near to the critical molecular weight which, together with a good polydispersity, determines a rheological behavior of the resin more similar to a Newtonian behavior than a viscoelastic behavior, with a smaller network "density" typical in determining this physical state. This gives a better workability both in transport to the production lines and in the subsequent steps of adding additives and coloring, and also in injection into the casting molds. In a parallel way, the molecular and rheological control makes it possible to modify the casting acrylic resin to obtain sheets with particular performance.

The viscosity chosen as optimum for subsequent casting can be controlled by extending the times of steps ii) incubation or preferably iii) completion.

The synthesis according to the present description does not compromise the times of the definitive polymerization in sheets, making the polymerization very progressive and regular, both in the increase in viscosity and in the gelation time and the final hardening.

In the present description the term mercaptans is intended as thiols, that is, organic compounds with -SH group, that is, an analog of alcohols with sulfur replacing oxygen.

In some forms of embodiment, the one or more mercaptan chain transfer agents, in mixing i), can be a quantity in the mixture comprised between 0.005 and 0.150 % w/w, in particular between 0.010 and 0.125 % w/w, more in particular between 0.012 and 0.11 % w/w, still more in particular between 0.015 and 0.1 % w/w. Depending on the type of mercaptan chain transfer agent chosen, it is possible to have a quantity in the mixture comprised between 0.01 % w/w and 0.025 % w/w, in particular between 0.015 % w/w and 0.02 % w/w, or between 0.075 % w/w and 0.125 % w/w, in particular between 0.08 % w/w and 0.1 % w/w.

Possible examples of the quantity in the mixture of the one or more mercaptan chain transfer agents are 0.015 % w/w, 0.016 % w/w, 0.017 % w/w, 0.018 % w/w, 0.019 % w/w, 0.02 % w/w. Other possible examples of the quantity in the mixture of the one or more mercaptan chain transfer agents are 0.08 % w/w, 0.085 % w/w, 0.09 % w/w, 0.095 % w/w, 0.1% w/w. According to possible forms of embodiment, the one or more mercaptan chain transfer agents can be chosen from the aliphatic mercaptans, in particular alkyl- mercaptans, for example methyl mercaptan (MM), ethyl mercaptan (EM), tert- butyl mercaptan (TBM), n-octyl mercaptan (NOM), or tert nonyl mercaptan (TNM), dodecyl mercaptan (DDM), for example t-dodecyl (TDM) or n-dodecyl mercaptan (NDM), or among the aromatic mercaptans, for example benzyl mercaptan (BZM). For example, NOM can be used exclusively, or it can be used as an alternative to NDM. NOM has, for example, a greater surface tension than NDM, and therefore does not create problems in thermoforming the sheets produced, and also allows less coloration without needing subsequent de- sulfuring, and a more advantageous cost of the process. NDM can be used, for example, instead of NOM to confer on the resin produced greater stability in storage. For example, NOM can be provided in a quantity between 0.015 and 0.02 % w/w and NDM can be provided in a quantity between 0.08 and 0.1 % w/w. The use of mixtures of two or more of said mercaptans, in some forms of embodiment, is not excluded.

Advantageously, the use of one or more mercaptan chain transfer agents makes the polymer chain a "sleeping" chain; in a subsequent casting step to obtain sheets of PMMA, it can be awakened thermally (the temperature of the water in the casting tanks for solidifying the sheets can be 50°C and 60°C, and performs this purpose). Therefore, thanks to the mercaptan chain transfer agents, the polymer chain is not dead, but still alive, although sleeping.

Using one or more mercaptans also allows to control the reaction in terms of viscosity, to control and block the exothermicity of the reaction, and advantageously to operate under conditions as near as possible to an isothermal condition, or in any case at a constant temperature within a limited range, and to obtain polymers with limited molecular weights (oligomers) and advantageously all identical or similar.

In possible implementations, the liquid mixture of pre-polymer or sleeping casting resin ("syrup", in jargon) obtained after stirring and cooling iv) can be used immediately or stored and used later, for the production of cast sheets of PMMA. To this purpose it may be provided to cast the pre-polymer in a mold immersed in water in order to control the exothermicity during solidification. This last operation can provide to cast the pre-polymer resin obtained in a mold in water at a temperature from 50°C to 60°C, in particular from 55°C to 57°C for thicknesses of 3 mm. This temperature awakens the pre-polymer mixture thermally. In particular, the increase in temperature supplies the polymer chain with the energy needed to release the one or more mercaptan chain transfer agents. The forming mold consists of sheets of glass separated by a polymeric gasket, for example PVC, which defines the thickness of the final sheet. The mixture is cast in the mold immersed in the hot water, as we said, from 50°C to 60°C, in particular from 55°C to 57°C for thicknesses of 3 mm, or gradually lower as the thicknesses increase, and polymerization is completed. The mixture can be cast in a vertical or horizontal mold.

According to possible forms of embodiment, which can be combined with all the forms of embodiment described here, the stirring and cooling iv) can last from 20 minutes to 40 minutes, in particular from 25 minutes to 35 minutes. In these forms of embodiment, after stirring and cooling iv), the casting pre-polymer resin obtained can be pumped directly to the casting line to produce cast sheets of PMMA.

According to other possible forms of embodiment, which can be combined with all the forms of embodiment described here, the stirring and cooling iv) can last from 45 minutes to 80 minutes, in particular from 50 minutes to 70 minutes, more in particular from 55 minutes to 65 minutes. In these forms of embodiment, after stirring and cooling the mixture obtained can be stored and kept safely in the warehouse, for example in tanks at 15°C, and subsequently used to produce cast sheets of PMMA.

EXAMPLE 1

A discontinuous reactor or batch production reactor is made available, which can work at atmospheric pressure. The reactor can be provided with a mechanical stirrer, for example a system with a disc-type impeller with an oil-dynamic motor, coil-type heating system and external cooling jacket.

The reactor can be sized for example to work a batch load of 1000 kg. An example of the sizes of the vessel of the reactor are: diameter 1160 mm, height 1270 mm, diameter of the disc-type impeller 730 mm, positioned at 20% of the height from the bottom. A loading step is provided, in which a load is prepared, introducing into the reactor about 1000 kg of MMA monomer at a temperature of about 15°C-20°C and about 200 g of initiator, for example AIBN, such as Vazo® 52, carefully pre- dissolved in MMA (w/w = 10%) for about 5 minutes, and simultaneously introducing about 190 g of mercaptan chain transfer agent, for example liquid NOM.

Subsequently there is an activation step, in which the heating system is activated gradually until it reaches about 60-62°C in about 20-25 minutes, with stirring at about 100 rpm to homogenize the load and components loaded, after which heating is set at a temperature of about 55°C.

An incubation step is subsequently provided, stopping the stirring motor and maintaining the heat at 55°C, in which mixing is suspended and the load is kept stationary at about 55°C for about 70 minutes. Afterward, a step is provided to complete the reaction, in which slow stirring is started, with a low number of rpm, about 75 rpm, for about 15-20 minutes, again at 55°C.

The slow stirring homogenizes the resin formed, also thermally, at low rpm, so as to "dissolve" the Trommsdorff cells, yet still allowing a reactive conclusion. Finally, a cooling step is provided in which the number of rpm of the stirrer is increased, to about 500 rpm to definitively break the Trommsdorff cells, heating is stopped and cooling is started with cooling liquid, for example water, at about 10°C (produced by a chiller), to take the load to 10°C-15°C and obtain the sleeping pre-polymer resin. In this step other products can also be added, such as UV adsorbers, possible release agents, possible blue dyes to reduce the weak yellowish shade possibly visible on the edges of the sheets produced. Only when it is cold can cross-linker agents be introduced, if necessary.

The finished resin can have a viscosity between 30 and 50 seconds, cup DIN 5321 1 0 (diameter) 4 mm at 25°C and specific weight of about 1 kg/liter. Immediately before the cooling step, an operation to control the viscosity can be carried out, for example with Brookfield process viscosimeters inserted in the reactor, and data transmission and control on a PLC (Programmable Logic Controller) interface. The cooling and stirring operation can last about 30 minutes, in the case of a subsequent use directly in casting, and provides to pump the resin directly to the casting line. The total duration can therefore be about 2.5 hours, to produce 1 ton of resin to be cast directly. These times are compatible with normal productivity required in an industrial field and allow to obtain a competitive process, also in economic terms. In particular, the sleeping pre- polymer resin can be used to make the PMMA sheets. This last operation can be performed by a mold immersed in water at 57°C. It is possible for example to obtain sheets of PMMA sized 1500x3000 mm and 3 mm thick. For example, it may be possible to carry out as many as 5 production cycles a day, that is, a production of 5 tons a day, with a batch reactor of the type in question.

EXAMPLE 2

As in example 1, with the difference that the cooling and stirring operation can last about 60 minutes, in the case of storage in tanks kept at 15°C - in this way, the radical reaction is definitively "extinguished", thus preventing possible and dangerous subsequent reaction triggering - and later use. The total duration can therefore be about 3 hours, to produce 1 ton of resin to be stored.

EXAMPLE 3

As in example 1 or 2, with the difference that the 200 g of AIBN also contain Vazo® 64.

EXAMPLE 4

As in example 1, 2 or 3, with the difference that the 200 g of AIBN also contain Vazo® 88.

EXAMPLE 5

As in example 1, 2, 3 or 4, with the difference that the 190 g of NOM are replaced by 900 g of NDM.

It is clear that modifications and/or additions of parts may be made to the method for the synthesis of acrylic resin for the production of polymethyl methacrylate as described heretofore, without departing from the field of the invention as defined by the claims. It is also clear that, although the present invention has been described with reference to some examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of method for the synthesis of acrylic resin, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

Claims

1. Method for the synthesis of acrylic liquid resin by means of a radical reaction, said method comprising:

- a loading and activation step that provides:

i) supplying in a single load, in a discontinuous production reactor, methyl methacrylate, one or more thermal initiators of the azobisisobutyronitrile type and one or more mercaptan chain transfer agents, and mixing and gradual heating of the methyl methacrylate, of the one or more thermal initiators such as azobisisobutyronitrile and the one or more mercaptan chain transfer agents in the discontinuous production reactor, in order to obtain a homogeneous mixture, in which the gradual heating provides to heat and keep the mixture at a temperature between about 58°C and about 65°C;

- a reaction step that provides:

ii) incubation of the reaction, including stopping the mixing at a temperature between about 55°C and about 60°C and maintaining the heat of the mixture during which the temperature is maintained at a value constantly comprised between 55°C and 60°C;

iii) completion of the reaction with low-rpm regimen stirring of the mixture comprised between 55 and 90 rpm;

- a cooling step that provides:

iv) cooling and stirring of the homogeneous mixture to obtain a casting acrylic liquid resin.

2. Method as in claim 1, wherein the mixing and gradual heating i) last between 15 and 25 minutes.

3. Method as in claim 1 or 2, wherein the mixing i) occurs at a regimen comprised between 60 and 125 rpm.

4. Method as in any claim hereinbefore, wherein the mixing is stopped and heat maintained ii) for between 40 and 80 minutes.

5. Method as in any claim hereinbefore, wherein the slow stirring iii) of the mixture occurs for between 10 and 30 minutes.

6. Method as in any claim hereinbefore, wherein stirring and cooling iv) are at a temperature between 10°C and 20°C.

7. Method as in any claim hereinbefore, wherein stirring and cooling iv) occur at a regimen comprised between 250 and 750 rpm.

8. Method as in any claim hereinbefore, wherein stirring and cooling iv) last from 20 to 40 minutes.

9. Method as in claim 8, wherein the casting pre-polymer resin obtained after stirring and cooling iv) is used immediately after for the production of cast sheets of polymethyl methacrylate.

10. Method as in any claim from 1 to 7, wherein stirring and cooling iv) last from 45 to 80 minutes.

1 1. Method as in claim 10, wherein the casting pre-polymer resin obtained after stirring and cooling iv) is stored and used at a later time, for the production of cast sheets of PMMA.

12. Method as in any claim hereinbefore, said method comprising casting the pre-polymer mixture obtained after stirring and cooling iv) by water molding.

13. Method as in any claim hereinbefore, wherein the one or more initiators such as azobisisobutyronitrile have a dimerization triggering temperature comprised between 50°C and 90°C.

14. Method as in any claim hereinbefore, wherein, in the mixing i), the quantity of the one or more initiators such as azobisisobutyronitrile in the mixture is comprised between 0.005 and 0.03 % w/w.

15. Method as in any claim hereinbefore, wherein the one or more initiators such as azobisisobutyronitrile have a physical form of easily soluble granules.

16. Method as in any claim hereinbefore, wherein the one or more initiators such as azobisisobutyronitrile are pre-mixed with the methyl methacrylate monomer to give a pre-solution, before the mixing i).

17. Method as in any claim hereinbefore, wherein the one or more initiators such as azobisisobutyronitrile are selected from a group which consists of:

- 2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4-dimethylpentanonitrile;

- 2,2' - azobis(2- methylpropionitrile);

- 2- (2-cyanbutane-2-yldiazenyl)-2-methy lbutanonitrile;

- 1 -( 1 -cy ancy clohexy l)diazeny lcyclohexane- 1 -carbonitrile.

18. Method as in any claim hereinbefore, wherein the azobisisobutyronitrile is 2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile.

19. Method as in any claim hereinbefore, wherein the one or more initiators such as azobisisobutyronitrile are 2-(2-cyan-4-methylpentane-2-yl)diazenyl-2,4- dimethylpentanonitrile and 2,2'-azobis(2-methylpropionitrile) in a 9: 1 ratio in weight.

20. Method as in any claim hereinbefore, wherein, in the mixing i), the quantity of the one or more mercaptan chain transfer agents is comprised between 0.005 and 0.15 % w/w.

21. Method as in any claim hereinbefore, wherein the one or more mercaptan chain transfer agents are chosen from aliphatic mercaptans, in particular alkyl- mercaptans.

22. Method as in claim 21, wherein the one or more mercaptan chain transfer agents are chosen from methyl mercaptan, ethyl mercaptan, tert-butyl mercaptan, n-octyl mercaptan, tert nonyl mercaptan, dodecyl mercaptan.

23. Method as in any claim hereinbefore, wherein said method does not provide to use polymerization inhibitors.

24. Acrylic liquid resin obtainable by means of a method as in any claim hereinbefore.