MXPA99003878A - A procedure for obtaining modified acrylic sheet of high resistance to impact - Google Patents

Abstract

Se describe un procedimiento para la botención de una lámina acrílica modificada de alta resistencia al impacto del tipo que comprende prepolimerizar una mezcla de monómeros con un polímero de monómero diénico disuelto, lo cual se realiza por lotes a temperaturas bajas sin que exista una separación del polímero de monómero diénico del resto de la mezcla. El procedimiento de la presente invención permite obtener una lámina acrílica modificada de alta resistencia al impacto cuya transmitancia se mantiene después de someterse a degradación acelerada, y cuyas propiedades de impacto no se pierden en más del 50%después de dicha degradación acelerada.

Description

"PROCEDURE FOR OBTAINING A MODIFIED ACRYLIC SHEET WITH HIGH IMPACT RESISTANCE" FIELD OF THE INVENTION The present invention relates to the techniques for obtaining plastic sheets, and more particularly, it is related to a modified acrylic sheet with high impact resistance and to a process for obtaining it.

BACKGROUND OF THE INVENTION Acrylic sheets have been used for many years for an infinity of applications that require translucent or transparent materials, whether industrial or consumer products, such as the manufacture of domes, patterns for illuminated signs, shelves for product display , office supplies, telephone booths, etc. However, it is known that the generic acrylic sheets commonly used for such applications, have very little resistance-to impact, so the articles made from them are very fragile. In accordance with the foregoing, polymeric compositions have been developed for a long time to obtain impact resistant sheets that can be used for the aforementioned applications. Most compositions of materials with impact resistance comprise at least one alkyl acrylate monomer mixed with some elastomeric material. It should be noted that such compositions may also include ethylenically unsaturated comonomers, such as other alkyl acrylates, styrene and its derivatives, or vinyl chloride; likewise, the elastomeric material can also be a homo-polymer or a co-polymer, such as polybutadiene, polyisoprene, polyisobutylene, butadiene-styrene copolymers, and butadiene-acrylonitrile copolymers. There are several methods for obtaining sheets that include acrylic monomers within their composition. However, most require a stage of polymerization of the monomers led to low conversions to obtain a prepolymer, known as syrup or honey due to its consistency, which is then subjected to a stage of curing at controlled temperature in which it is completed the polymerization of the monomers, finally obtaining the desired sheet with the characteristics corresponding to the composition from which it is split. It should be mentioned that there are also several methods to perform the curing of the prepolymers and obtain the sheets, such as curing in molds (known as "CELL CAST"), cured by calendering and continuous curing, among others. In the case of the acrylic compositions with improved impact resistance, as mentioned above, an elastomeric material is added to the monomer mixture normally used before performing the prepolymerization. However, these elastomeric materials, during the polymerization carried out at low conversion to form the prepolymer, become incompatible with the medium and tend to separate. The reason for such incompatibility is due to the fact that the elastomeric materials have differences in densities, viscosities and polarity with the prepolymer, which results in a tendency of the elastomeric materials to separate from the mixture, causing either a separation or an inversion of the elastomeric materials. phases. There is a wide variety of high impact strength compositions, such as those described in US Patent Nos. 4085166, 4530973, 5169903 and 5196483. However, most of these compositions require preparation from mixtures of materials, which include in addition, a large amount of monomers, difficult to control operating conditions and sophisticated equipment (high temperature and pressure reactors), so that said compositions make very expensive to obtain sheets with high impact resistance.

One of the most complete works related to acrylic compositions with high impact resistance, is the one that Kitagawa reports, e. al., in US Patent No. 4287317, which discloses an impact-resistant acrylic polymer composition. In said patent, a continuous process is used to perform the prepolymerization of the components and thus form the honey. In the chapter on the state of the art, it is also established that there are a great variety of disadvantages that arise when performing the mass polymerization of this type of material, due to the aforementioned problem of phase separation and inversion. As described by Kitagawa, et. al., at the time of filing his patent application, there was no procedure that allowed obtaining acrylic prepolymers modified with elastomeric materials that had viscosities that allowed the manufacture of sheets, and that at the same time allowed to obtain the characteristics of transparency and stability that said sheets require, wherefore said patent discloses a process for obtaining a stable prepolymer from the aforementioned mixture of 100 parts by weight of a monomer or a mixture of monomers containing 80% to 100% by weight of methyl methacrylate and from 0% to 20% by weight of an ethylenically unsaturated monomer copolymerizable with methyl methacrylate, which are mixed with 1 to 20 parts by weight of an elastomeric polymer, to form a homogeneous mixture that is prepolymerized until reaching a 5 to a 40% conversion by the addition of a suitable initiator, so that a composition is obtained final end comprising from 66% to 99% by weight of methyl methacrylate, from 0% to 20% by weight of the ethylenically unsaturated monomer copolymerizable with methyl methacrylate and from 0.99% to 16.7% by weight of the elastomeric polymer, which allows In turn, obtain a sheet with characteristics of high transparency and impact resistance. The procedure for obtaining the prepolymer is of great importance, since according to Kitagawa, et. al., obtaining the prepolymer requires strict flow and temperature controls in two reaction stages, which increases the operating cost to a great extent. Inclusively, the same patent mentions that the temperature of the prepolymerization reaction must be maintained in a range of 90 to 200 ° C, and mentions that lowering the temperature of the reaction results in instability in the phases of the mixture, so that the progress of the reaction is not satisfactory, which results in a separation of the particles of the elastomeric polymer, causing problems of processability. However, the fact of maintaining such strict control conditions of flows and temperatures, results in an increase in operating costs, in addition to not allowing batch processing in the manufacture of the prepolymer, which is very inconvenient for applications in the same equipment is used to obtain different types of sheets. It is worth mentioning that the most used elastomeric polymer for the reinforcement of monomers is polybutadiene, which, like all polymers obtained from dienes, is chemically and thermally very unstable, so the transparency of the sheets obtained by mixing with polybutadiene It can be affected to a large extent over time. In this respect, in one of the examples of the North American Patent? O. No. 4287317, it is mentioned that the sheets obtained do not lose their transparency after 1000 hours of accelerated degradation. However, the parameters under which accelerated degradation is performed are not established, and given the ease of degradation of the polybutadiene, it is very difficult to achieve a high stability thereof, since the addition of a stabilizing agent in the composition, so it is likely that the conditions of accelerated degradation were not drastic enough to ensure adequate transparency during the useful life of the sheet. As a result of the above, it has been sought to eliminate the disadvantages presented by the modified acrylic sheets currently used, by developing a modified acrylic sheet with high impact resistance that, in addition to resisting impact, maintains its properties of transparency and impact resistance during its useful life and has suitable characteristics to be subjected to further processing.

OBJECTS OF THE INVENTION It is still another object of the present invention to provide a process for obtaining a prepolymer that includes a polymer dissolved in a very high impact resistance, which allows operation at low temperature. It is yet another object of the present invention to provide a process for obtaining a prepolymer which includes a dissolved polymer with very high impact resistance, which permits batch operation, thus facilitating the manufacture of prepolymers with various characteristics in the same equipment. Taking into account the shortcomings of the prior art, it is an object of the present invention to provide a modified acrylic sheet of high impact resistance that maintains its transparency throughout its useful life. It is another object of the present invention to provide a modified acrylic sheet with high impact resistance that maintains its transparency in finished products even after being subjected to further processing, such as thermal processing such as thermoforming or surface treatments. , as paste and / or application of inks.

It is a further object of the present invention to provide a modified acrylic sheet of high impact resistance, low molecular weight that provides optimal mechanical and flow properties for further processing.

DETAILED DESCRIPTION The process for obtaining the prepolymer used in obtaining the modified high impact acrylic sheet of the present invention comprises a first mixing stage followed by a prepolymerization step. The mixing step consists in preparing a monomer mixture containing approximately 1 to 6 parts by weight of a polymer of a diene monomer with a particle size allowing its dissolution, preferably particles of approximately 1.5 cm 3; up to about 20 parts of an ethylenically unsaturated monomer copolymerizable with methyl methacrylate; and, about 70 to 99 parts of an alkyl acrylate or alkyl methacrylate, preferably methyl methacrylate. The monomer mixture formed must be agitated to form a single homogeneous phase in which the polymer of a diene monomer is dissolved in the other monomers. Additionally, at the beginning of the mixing, a mixture of stabilizing agents for ultraviolet light is incorporated, which includes stabilizers of the type containing a hindered amine, better known as HALS type (Hindered Amine Light Stabilizer) and stabilizers derived from the benzotriazole, which have a synergistic effect that allows to obtain a modified acrylic sheet of high impact resistance that has no variation in its transmittance after 1000 hours of subjection to accelerated degradation in exposure of Xenon ultraviolet light (ASTM G 26-92 ). In the preferred embodiment of the present invention, about 0.1 to 1.5% by weight is added with respect to the monomer mixture of the stabilizing agent mixture for ultraviolet light. In a preferred embodiment of the present invention, stabilizing agents for ultraviolet light of the HALS type are selected from bis- (1-octyloxy-2, 2, 6, 6, tetramethyl-4-piperidinyl) sebacate; dimethyl succinate polymer with 4-hydroxy-2, 2, 6, 6, tetramethyl-1-piperidine ethanol; bis (2, 2, 6, 6, -tetramethyl-4-piperidinyl) sebacate; 1, 3, 5-triazin-2,4,6-triamino, N, N '' '- [1, 2-ethanediylbis [[[4,6-bis [butyl] (1, 2, 2, 6, 6 pentamethyl-4-piperidinyl) amino] -1, 3, 5-triazin-2-yl] imino] -3,1-propanediyl]] - bis [N ', N "-dibutyl-N', N" -bis (1, 2, 2, 6, 6-pentamethyl-4-piperidinyl) -; poly- [[6- [(1,1,3,3, -tetramethyl butyl) amino] -s- triazin-2,4-diyl] [[(2, 2, 6, 6-tetramethyl-4-piperidyl)] imino] hexamethylene [(2, 2, 6, 6, -tetramethyl-4-piperidyl) imino]]; or, mixtures thereof; while the stabilizing agents for ultraviolet light derived from the benzotriazole are preferably selected from 2- (2 ', hydroxy-5-methyl-phenyl) benzotriazole; 2- (2H-benzotriazol-2-yl) -4,6-bis (1-methyl-1-phenylethyl) phenol; 2- (5-chloro-2H-benzotriazol-2-yl) -6- (1,1-dimethylethyl) -4-methylphenol; 2- (3 ', 5' -di-tert-butyl-2 '-hydroxyphenyl) -5-chlorobenzotriazole; 2- (2H-benzotriazol-2-yl) -4,6-bis (1,1-dimethylpropyl) phenol; or, mixtures thereof. In a specific embodiment of the present invention, the mixture of stabilizing agents for ultraviolet light is formed with 2- (2 ', idroxy-5-methyl-phenyl) benzotriazole and bis- (2, 2, 6, 6-tetramethyl-4) -piperidinyl) sebacate, said mixture preferably containing about 50% by weight of each. In a preferred embodiment of the present invention, the polymer of a diene monomer is polybutadiene, preferably of the high cis-type and the ethylenically unsaturated monomer is styrene. Once the polymer from a diene monomer has dissolved, an initiator agent is added in amounts of approximately 0.01 to 0.05% by weight with respect to the monomer mixture, and the temperature is raised to maintain it in a range of 70 to 95 ° C. , approximately, at atmospheric pressure. In a preferred embodiment of the present invention, the initiator agent is selected from among peroxide and azo type initiators, preferably being selected from terbutyl peroxypivalate and azo-bis-isobutyronitrile. The prepolymerization is carried out until reaching a viscosity of the prepolymer of 18 to 21 seconds in Ford Cup No. 6 at the reaction temperature, after which the prepolymer is cooled. The cold prepolymer of the present invention has a final viscosity of about 45 to 50 seconds in Ford Cup No. 6 at room temperature, with a conversion of about 8 to 30%, a particle size of a polymer of a diene monomer of 0.1. at about 1 micron, and an approximate molecular weight of 100,000 to 1,000,000 daltons with polydispersities approximately between 2.0 to 3.5. To obtain the desired sheet, a demolding agent, preferably sodium dioctylsulfosuccianate, is added to the prepolymer in amounts of 0.003% to 0.021% by weight with respect to the prepolymer, as well as pigments in various quantities according to the needs of the application. end of the sheet, for finally, adding more initiator in amounts of 0.15% to 2% by weight with respect to the prepolymer, approximately. Once the initiating agent is added to the prepolymer, the latter is subjected to a curing step, which can be carried out by any of the curing processes found in the state of the art, preferably by the mold curing process. Molding in molds consists of introducing the prepolymer into a mold that includes two tempered crystals perfectly polished and free of any imperfection, sealed by means of a joint made of a polymeric material suitable for the thickness of the sheet to be obtained; securing the mold by means of a plurality of pressure clamps which prevent the prepolymer from escaping from the mold, while allowing to achieve the exact thickness required for the sheet; and, completing the polymerization of the prepolymer by applying heat, using a heating means preferably selected from steam, air, water or infrared radiation. The polymeric material from which the mold gasket is made is preferably selected from polyvinyl chloride, ethylene vinyl acetate and polyolefins. ***** The sheet obtained after curing is a modified acrylic sheet with high impact resistance, which has an IZOD impact (ASTM D256) approximately between 1.0 and 1.6 (lb * ft) / inch, a Gardner impact ( ASTM 3029) of approximately 40 to 100 psi, and a transmittance greater than 88%. The transmittance is maintained after being subjected to 1000 hours of Xenon arc ultraviolet light exposure, while the impact resistance is reduced by up to 50% after accelerated degradation.

The present invention will be more clearly illustrated by means of the following examples, which are presented for illustrative purposes, and do not limit it.

EXAMPLES Example 1. In an atmospheric dissolver tank provided with agitator with two marine-type propellers rotating at 1000 rpm, a mixture of 81 parts of methyl methacrylate, 15 parts of styrene monomer and 4 parts of high cis polybutadiene was prepared, as well as 0.6% by weight of a mixture of 2- (2 ', idroxy-5-methyl-phenyl) benzotriazole and bis- (2, 2, 6, 6-tetramethyl-4-piperidinyl) sebacate in a 1: 1 ratio. The mixture was stirred for 5 hours at room temperature, after which a homogeneous solution was obtained. The monomer mixture was then transferred to an agitated tank reactor of batch processing, at atmospheric pressure and with temperature control by means of water, where 0.013% of tertbutyl peroxypivalate was added as initiator. The temperature was maintained at 82 ° C while the mixture was stirred at 300 rpm, until a prepolymer with viscosity at 82 ° C of 19 seconds was obtained in Ford Cup No. 6. The prepolymer was cooled to room temperature (20 ° C. ), after which I report a viscosity of 47 seconds in Ford Cup No. 6, a conversion of 10% and a particle size of the dissolved polybutadiene of 0.1 microns.

Subsequently, 0.3% by weight of the same initiator was added, as well as 0.0045% of sodium dioctyl sulfosuccyanate as release agent. The prepolymer was subjected to vacuum and by the method of curing in molds a sheet 3 mm thick and 244 cm by 122 cm was obtained. The sheet obtained had no surface defects, showed high gloss and impact resistance. The IZOD impact presented by the blade was 1.2 ft / inch and the Gardner impact of 90 I-pulg with a transmittance of 88%. The impact property was maintained at 50% of the original impact after subjecting it to 1000 hours of Xenon arc ultraviolet light exposure (ASTM G 26-92). It should be mentioned that a sheet formed from methyl methacrylate as a single monomer generally has a Gardner impact strength of 1.6 Ib-in. The material was tested for functionality by heating the sheet at 180 ° C for 10 minutes and then thermoforming it to a 30 cm cant keeping the impact and light transmission.

Example 2. A mixture of 96 parts of methyl methacrylate and 4 parts of polybutadiene was prepared, in the same manner as in Example 1, with the dissolution time of 4 hours, this mixture was again transferred to the reactor described in example 1 , 0.02% azobisisobutyronitrile initiator and 0.2% of a mixture of 2- (2 ', hydroxy-5-methyl-phenyl) benzotriazole and bis- (2, 2, 6,6-tetramethyl-4-piperidinyl) sebacate, were added. this mixture was reacted at 82 ° C for 45 minutes until a conversion of 10% was achieved. As in Example 1, an initiator and a release agent were added to the cold prepolymer, 0.05% by weight of azobisisobutyronitrile instead of a peroxide initiator and 0.0045% of sodium dioctylsulfosuccianate as release agent. From the prepolymer, again a sheet was made with the same dimensions as for Example 1, which was also free of surface defects, exhibited high brightness and impact resistance IZOD of 0.52 lb-ft / in., Gardner Impact of 60 lb. -pulg and 80% light transmission. The impact property was maintained at 50% of the original impact after subjecting it to 1000 hours of Xenon arc ultraviolet light exposure (ASTM G 26-92). The material was subjected to thermoforming under the same conditions as in Example 1 and showed no change in its impact and light transmission properties after this test.

Example 3. A prepolymer was prepared with styrene monomer only, by reacting it in a stirred tank reactor for batch processing in the presence of 0.005% by weight of azobisisobutyronitrile as the initiating agent, at a temperature of 82 ° C for 60 minutes, which after being cooled presented a conversion of 8% and a viscosity of 35 seconds Ford Cup No. 6. On the other hand, a second prepolymer was prepared according to that described in example 1, and was prepared for curing using 0.003% of dioctyl terephthalate as release agent and 1.5% of terbutyl peroxypivalate. Additionally, 5% by weight of the prepolymer prepared only with styrene was added and a sheet was formed as described in example 1. The sheet obtained was free of surface defects, had high gloss and high impact resistance. The Gardner impact for this sheet was 65 lb-in, while the IZOD impact was 1.0 lb ft / in and the transmittance 50%. As can be seen, the transmission of light from this type of sheet was low because in some applications this is required, for example in the case of domes, cancelería for bathrooms and illuminated signs.

Example 4. A ready-to-cure prepolymer was prepared according to that described in Example 3. However, in this case 3% of a paste (70% Ti02 / 30% Dioctyl terephthalate) was added to obtain a white sheet. The sheet was thermoformed at a temperature of 180 ° C for 12 minutes to obtain a luminous advertisement where the transmission and diffusion of light was uniform throughout its surface. The IZOD impact of the thermoformed light advertisement was 0.98 lb-ft / in, while the Gardner impact was 60 lb-in. It was observed that the material maintained its brightness, and showed no defects when subjected to heating. As can be clearly seen from the examples, a consequence of the process of the present invention is that a sheet with optimum processability characteristics is obtained, that is, when a sheet obtained by the process of the present invention is subsequently subjected to processes of heat deformation, such as thermoforming, the sheet maintains its characteristics of transparency and impact resistance. It should be noted that in example 2, which does not include the ethylenically unsaturated monomer copolymerizable with the monomer of the alkyl acrylate or alkyl methacrylate type, the sheet obtained, although it did not fulfill the desired transmittance characteristics, maintained its impact properties after being thermoformed. In accordance with what has been described above, it can be seen that the process for forming a prepolymer of the present invention has been designed to allow the obtaining of modified acrylic sheets with high impact resistance, and it is obvious to any person skilled in the art that the embodiments of the modified impact-resistant acrylic sheet described above are only illustrative but not limiting of the present invention, since numerous changes of consideration in their details are possible without departing from the scope of the invention. Even though a specific embodiment of the invention has been illustrated and described, it should be emphasized, that numerous modifications to it are possible, such as various pigments and initiating or demolding agents. Therefore, the present invention should not be considered as restricted except by what is required by the prior art and by the spirit of the appended claims.

Claims (1)

1. NOVELTY OF THE INVENTION CLAIMS 1. - A process for obtaining a modified high impact resistance acrylic sheet of the type comprising homogeneously mixing at least one monomer of the type of acrylates or alkyl methacrylates, an ethylenically unsaturated monomer copolymerizable with the monomer of the alkyl acrylate type or alkyl methacrylates and a diene monomer polymer, followed by a bulk prepolymerization of the components of the previously obtained monomer mixture, by the use of a first initiating agent, thus forming a prepolymer which is subsequently subjected to a curing step by means of traditional methods for forming sheets, in the presence of a second initiating agent; characterized in that the prepolymerization is carried out batchwise at a temperature of 70 to 95 ° C, approximately, at atmospheric pressure, without there being a separation of the diene monomer polymer from the rest of the mixture. 2. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 1, further characterized in that the monomer mixture comprises approximately 70 to 99 parts by weight of the alkyl acrylate or alkyl methacrylate; up to about 20 parts by weight of the copolymerizable ethylenically unsaturated monomer with the alkyl acrylates or alkyl methacrylates; and, about 1 to 6 parts by weight of the diene monomer polymer. 3. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 2, further characterized in that the polymer of diene monomer is polybutadiene. 4. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 3, further characterized in that the polybutadiene is of the high cis content type. 5. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 2, further characterized in that the ethylenically unsaturated monomer is styrene. 7. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 2, further characterized in that the monomer of the type of the alkyl acrylates or alkyl methacrylates is methyl methacrylate. 8. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 1, further characterized in that at the beginning of the mixing step, a mixture of stabilizing agents for ultraviolet light including stabilizers is incorporated. of the HALS type and benzotriazole derivatives, to improve the transparency of the sheets obtained from the prepolymer. 9. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 8, further characterized in that the stabilizing agents for ultraviolet light of the HALS type are selected from bis- (1-octyloxy-2, 2, 6, 6, etramethyl-4-piperidinyl) sebacate; dimethyl succinate polymer with 4-hydroxy-2, 2, 6, 6, tetramethyl-1-piperidine ethanol; bis (2, 2, 6, 6, -tetramethyl-4-piperidinyl) sebacate; 1, 3, 5-triazin-2,4,6-triamino, N, N '' '- [1,2-ethanediylbis [[[4, 6-bis] [butyl (1,2,2,6,6-pentamethyl-4-piperidinyl) amino] -1,3,5-triazin-2-yl] imino] -3,1-propanediyl]] - bis [N ', N] '' - dibutil - N ', N! Ia (1, 2, 2, 6, 6-pentamethyl-4-piperidinyl) -; poly- [[6- [(1, 1, 3, 3, -tetramethylbutyl) amino] -s- triazin-2,4-diyl] [[(2,2,6,6-tetramethyl-4-piperidyl)] imino] hexamethylene [(2, 2, 6, 6, -tetramethyl-4-piperidyl) imino]]; or, mixtures thereof. 10. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 8, further characterized in that the stabilizing agents for ultraviolet light derived from benzotriazole are selected from 2- (2 ', hydroxy-5). -methyl-phenyl) benzotriazole; 2- (2H-benzotriazol-2-yl) -4,6-bis (1-methyl-1-phenylethyl) phenol; 2- (5-chloro-2H-benzotriazol-2-yl) -6- (1, 1-dimethylethyl) -4-methylphenol; 2- (3 ', 5' -di-tert-butyl-2 '-hydroxyphenyl) -5-chlorobenzotriazole; 2- (2H-benzotriazol-2-yl) -4,6-bis (1,1-dimethylpropyl) phenol; or, mixtures thereof. 11. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 8, further characterized in that the mixture of stabilizing agents for ultraviolet light is added in amounts of approximately 0.2 to 1.5% by weight with with respect to the mixture of monomers 12. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 11, further characterized in that the mixture of stabilizing agents for ultraviolet light is formed with 2- (2) ', hydroxy-5-methylphenyl) benzotriazole and bis- (2,2,6,6-tetramethyl-4-piperidinyl) sebacate. 13. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 12, further characterized in that the mixture contains 50% by weight of each of the stabilizing agents for ultraviolet light, approximately. 14. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claims 2 and 12, further characterized in that the monomer mixture comprises approximately 70 to 99 parts by weight of the alkyl acrylate or alkyl methacrylate and about 1 to 6 parts by weight of the diene monomer polymer. 15. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 1, further characterized in that the first initiating agent is added in amounts of approximately 0.01 to 0.05% by weight with respect to the mixture of monomers. 16. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 15, further characterized in that the first initiator agent is selected between initiators of peroxide type and azo type. 17. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 12, further characterized in that the first initiator agent is selected from terbutyl peroxypivalate and azo-bis-isobutyronitrile. 18. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 1, further characterized in that the prepolymerization is carried out until reaching a viscosity of the prepolymer of 18 to 21 seconds in Ford Cup No .6 at the reaction temperature. 19. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 18, further characterized in that the cold prepolymer has a final viscosity of approximately 45 to 50 seconds in Ford Cup No. 6. 20 .- A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 19, further characterized in that the cold prepolymer has a conversion of about 8 to 30%. 21. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 20, further characterized in that the cold prepolymer has a polymer particle size of a diene monomer of 0.1 to 1 microns, approximately . 22. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 21, further characterized in that the cold prepolymer has a molecular weight in the approximate number of 100,000 to 1,000,000 daltons. 23.- A procedure for obtaining a modified acrylic sheet with high impact resistance, according to claim 22, further characterized in that the cold prepolymer has polydispersities of about 2.0 to 3.5. 24. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 1, further characterized in that the at least one demolding agent and at least one pigment prior to the stage is added to the prepolymer. of curing. 25. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 24, further characterized in that the release agent is added in amounts of 0.003% to 0.021% by weight with respect to the prepolymer, approximately . 26. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 25, further characterized in that the release agent is preferably sodium dioctylsulfosuccianate. 27. A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 1, further characterized in that the second initiator agent is added in amounts of 0.15% to 2% by weight with respect to the prepolymer, approximately. 28.- A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 27, further characterized in that the second initiating agent is selected among initiators of peroxide type and azo type. 29. - A process for obtaining a modified acrylic sheet with high impact resistance, according to claim 27, further characterized in that the second initiating agent is selected among initiators of peroxide type and azo type. 30. A process for obtaining a modified acrylic sheet with high impact resistance, in accordance with claim 27, further characterized in that the step of curing the prepolymer is carried out by the method of curing in molds. 31.- A modified acrylic sheet of high impact resistance that is obtained from mixing approximately 70 to 99 parts by weight of an alkyl acrylate or alkyl methacrylate; up to about 20 parts by weight of an ethylenically unsaturated monomer copolymerizable with the alkyl acrylates or alkyl methacrylates; and, about 1 to 6 parts by weight of a polymer of diene monomer, preferably butadiene, characterized in that it is obtained by curing a prepolymer obtained by a batch mass prepolymerization process, at a temperature of about 70 to 95 ° C. and atmospheric pressure, said sheet, having an IZOD impact (ASTM D256) of 1.0 to 1.6 (lb * ft) / in, approximately; a Gardner impact (ASTM 3029) of approximately 40 to 100 psi; and, a transmittance greater than 88%. 32. - A modified acrylic sheet with high impact resistance, in accordance with the claim 31, further characterized in that it includes from 0.2 to 1.5% by weight of a mixture of stabilizing agents for ultraviolet light of the Hals type and benzotriazole derivatives so that the transmittance properties of the sheet are maintained after being subjected to 1000 hours of light exposure Xenon arc ultraviolet. 33.- A modified acrylic sheet with high impact resistance, in accordance with the claim 32, further characterized in that the stabilizing agents for ultraviolet light of the Hals type are selected from bis- (1-octyloxy-2, 2, 6, 6, tetramethyl-4-piperidinyl) sebacate; dimethyl succinate polymer with 4-hydroxy-2, 2, 6, 6, tetramethyl-1-piperidine ethanol; bis (2, 2, 6, 6, -tetramethyl-4-piperidinyl) sebacate; 1,3,5-triazin-2,4,6-triamino, N, N '' '- [1,2-ethanediylbis [[[4,6-bis [butyl (1, 2,2,6,6- pentamethyl-4-piperidinyl) amino] -1, 3, 5-triazin-2-yl] imino] -3,1-propanediyl]] - bistN ^ N'1-dibutyl-N ', N "bis (1, 2, 2, 6, 6-pentamethyl-4-piperidinyl) -; poly- [[6- [(1, 1, 3, 3, -tetramethylbutyl) amino] -s- triazin-2,4-diyl] [[( 2, 2, 6,6-tetramethyl-4-piperidyl) imino] hexamethylene [(2, 2, 6, 6, -tetramethyl-4-piperidyl) imino]], or mixtures thereof. modified acrylic with high impact resistance, in accordance with the claim 32, further characterized in that the stabilizing agents for ultraviolet light derived from the benzotriazole are selected from 2- (2 ', hydroxy-5-methyl-phenyl) benzotriazole; 2- (2H-benzotriazol-2-yl) -4,6-bis (1-methyl-1-phenylethyl) phenol; 2- (5-chloro-2H-benzotriazol-2-yl) -6- (1,1-dimethylethyl) -4-methylphenol; 2 - (3 ', 5' -di-tert-butyl-2 '-hydroxyphenyl) -5-chlorobenzotriazole; 2- (2H-benzotriazol-2-yl) -4,6-bis (1,1-dimethylpropyl) phenol; or, mixtures thereof. 35.- A modified acrylic sheet with high impact resistance, according to claim 32, further characterized in that the mixture of stabilizing agents for ultraviolet light is formed with 2- (2 ', hydroxy-5-methyl-phenyl) benzotriazole and bis- (2, 2, 6, 6-tetramethyl-4-piperidinyl) sebacate. 36.- A modified acrylic sheet with high impact resistance, in accordance with the claim 35, further characterized in that the mixture of stabilizing agents contains 50% by weight of each of the stabilizing agents for ultraviolet light, approximately. 37.- A modified acrylic sheet with high impact resistance, in accordance with the claim 36, further characterized in that the impact properties of the sheet are lost by 50% at the most after being subjected to 1000 hours of Xenon ultraviolet light exposure. 38.- A modified acrylic sheet with high impact resistance, in accordance with the claim 31, further characterized in that the sheet, once subjected to a heat deformation process, maintains its properties of transparency and impact resistance.