Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2002-07-08
2004-05-11
Sanders, Kriellion A. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S718000, C524S720000, C525S289000, C525S290000, C525S309000, C525S310000, C525S440030, C525S445000
Reexamination Certificate
active
06734230
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related with techniques to obtain plastic sheets and more precisely, a modified acrylic sheet highly resistant to impact as well as the procedure to obtain this sheet.
2. Description of the Related Art
Acrylic sheets have been used for several years now for numerous applications requiring translucent or transparent materials, industrial or finished goods, as for the manufacturing of domes, molds for luminous advertisements, shelves for exhibiting products, office appliances, telephone booths, etc. Nevertheless, it is widely known that the generic acrylic sheets commonly used for such applications, have very little resistance to impact, therefore the goods manufactured with the latter are very fragile. Considering the above, polymeric compositions have been developed for some time now, which permit to obtain impact resistant sheets, which may be used for the aforementioned applications.
The majority of the compositions with impact resistant materials include at least one monomer of alkyl-acrylate mixed with some elastomeric material. It is worth mentioning that these compositions may also include ethylenically unsaturated co-monomers, or other alkyl-acrylates, styrene and its derivatives or vinyl chloride. Likewise, the elastomeric material may also be a homopolymer or a copolymer, like polybutadiene, polyisoprene, polyisobutylene, copolymers of butadiene-styrene, and butadiene-acrylonitrile polymers. There are various procedures to obtain sheets containing acrylic monomers in their composition. However, the majority requires a stage of polymerization of the monomers carried out at low conversions until obtaining a prepolymer, known as syrup or honey because of its consistency. Later it undergoes a stage of controlled temperature curing, in which the polymerization of the monomers is completed, finally obtaining the desired sheet with the characteristics corresponding to the composition of which it comes from.
It is worth mentioning that there also exist several methods to make the curing of the prepolymers and obtain the sheets, like the curing in molds (also known as “CELL CAST”), curing by rolling and continuous curing, amongst others.
In the case of acrylic compositions with improved resistance to impact, as mentioned above, an elastomeric material is added to the used monomer mix, usually before doing the prepolymerization. However, these elastomeric materials, during the polymerization executed at a low conversion in order to form the prepolymer, become incompatible with the medium and tend to separate themselves. The reason of this incompatibility is that the elastomeric materials have different degrees of density and viscosity than the prepolymer, the consequence is a tendency of the elastomeric materials to separate themselves from the mix, provoking a separation or inversion of phases.
There is an important variety of compositions with high impact resistance, like the ones described in the U.S. Pat. Nos. 4,085,166, 4,530,973, 5,169,903 and 5,196,483. However, the majority of these compositions need to be prepared from a mix of materials, including an important quantity of monomers, under difficult to control operation conditions and using sophisticated equipment (high temperature and pressure reactors). This means said compositions make the production of sheets with a high impact resistance very expensive.
One of the most complete works on acrylic compositions with high impact resistance is the one reported by Kitagawa, et. al., in the U.S. Pat. No. 4,287,317. It describes an acrylic polymer composition resistant to impact. In said patent, a continuous process is used to polymerize the components and form the honey. In the chapter on the state of the art, it is also stated that there are a wide variety of problems when doing mass polymerization of these types of materials, due to the aforementioned problem of separation and inversion of phases.
According to the description of Kitagawa, et. al., when they presented their patent request there was no procedure enabling to produce acrylic prepolymers modified with elastomeric materials with viscosity, which would enable the production of sheets, and at the same time obtain the characteristics of transparency and stability these sheets need. Therefore that patent describes a process to produce a stable polymer from the aforementioned mix of 100 parts in weight of a monomer or a mix of monomers containing from 80% to 100% in weight of methyl methacrylate and 0% to 20% in weight of an ethylenically unsaturated monomer, which can be copolymerized with methyl methacrylate, which are mixed with 1 to 20 parts in weight of an elastomeric polymer, until forming an homogeneous mix which is prepolymerized until it reaches 5 to 40% of conversion, through the addition of an appropriate initiator. A final composition will be obtained of 66% to 99 % in weight of methyl methacrylate, 0% to 20% in weight of the ethylenically unsaturated monomer, which can be copolymerized with the methyl methacrylate and 0.99% to 16.7% in weight of the elastomeric polymer. This allows in turn to produce a sheet with characteristics of high transparency and resistance to impact.
The procedure to obtain the prepolymer is very important since according to Kitagawa, et. al., the production of the prepolymer requires a strict control of flows and temperatures in two stages of reaction, which greatly increases the cost of operation.
Furthermore, the same patent mentions that the temperature of the prepolimerization reaction must be kept between 90 to 200° C., and states that lowering the temperature of the reaction results in an instability of the phases of the mix. This in turns means that the progress of the reaction won't be satisfactory, which causes a separation of the particles of the elastomeric polymer, and processing problems.
However, the fact of maintaining these strict conditions of flow and temperature control, results in an increase of the operation costs. It also forbids producing the prepolymer in a batch process. This is very inconvenient for applications in which the same equipment is used to obtain different types of sheets.
It is important to mention that the elastomeric polymer most used for reinforcing monomers is polybutadiene, which, like all polymers obtained from dienes, is chemically and thermally very unstable. This means that the transparency of the sheets obtained through polybutadiene mixes may be considerably affected by time. Concerning this point, in one of the examples of the U.S. Pat. No. 4,287,317, it says that the sheets obtained do not loose their transparency after 1000 hours of accelerated degradation. However, the parameters under which the accelerated degradation is made are not stated and, given the ease of degradation of the polybutadiene, it is very difficult to achieve a high stability of the latter, since no mention is made of the addition of a stabilizing agent in the composition. This means it is likely that the conditions of accelerated degradation were not drastic enough to insure proper transparency during the useful life of the sheet.
Considering the above, it has been tried to eliminate the inconveniences of the modified acrylic sheets currently used, by developing a modified acrylic sheet with a high impact resistance. This sheet, apart from resisting impact, would keep its transparency and resistance to impact properties throughout its useful life and would have characteristics suitable for submission to ulterior processes.
PURPOSES OF THE INVENTION
It is another purpose of the invention, to provide a procedure for the production of a prepolymer including a dissolved polymer of very high impact resistance, enabling the operation at a low temperature.
Another purpose of the invention is to provide a procedure to produce a prepolymer including a dissolved polymer of very high impact resistance, enabling the operation per batch. This will facilitate the production of prepolymers with different characteristics with the same equipmen
Escobar Sergio Colin
Loera Luis Enrique Mille
Ramirez Leopoldo Vilchis
Rodriguez Maria Sonia Limon
Romero Manuel Ahumada
Knobbe Martens Olson & Bear LLP
Plastiglas de Mexico, S.A. de C.V.
Sanders Kriellion A.
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