Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-09-03
2004-07-20
Egwim, Kelechi C. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S416000, C524S417000, C524S560000, C525S340000
Reexamination Certificate
active
06765046
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of molding compositions, especially of products based on polymethyl methacrylate (PMMA) that are so stable to yellowing and highly transparent that even the edge view of panel materials prepared from them is color-neutral and shows no yellow cast.
STATE OF THE ART
Moldings from polymethyl methacrylate (PMMA) are distinguished by good weather resistance and outstanding transparency. PMMA panel material that is produced from molding compositions by extrusion or by injection molding appears practically color-neutral in face view. Of course this does not apply to the edge view. Since a panel viewed from the side has very great visual thickness, an otherwise imperceptible yellow cast becomes clearly visible here. Bluing agents that are added to the molding composition prior to processing are used in the trade to eliminate the yellowish edge impression. This produces a bluish edge impression that is indeed associated with high transparency by an observer in purely visual evaluation, but measurement of the degree of transmission, for example by the method of DIN 5033 (D65/10) shows that the degree of transmission declines with increasing concentration of bluing agent used. This solution is also not ideal since many observers also see a dirty gray or even a red violet tinge of color because of the subjectivity in purely visual evaluation through a very great thickness (edge view). The amount of bluing agent needed to compensate for the yellow cast is also so great that certain fields of application have to be excluded because of government regulations. Thus, for example, the standards enacted for use in food contact (Regulations of the Food and Drug Administration, FDA, for the USA or the corresponding EU directives for the region of the European Union) allow the use of only a few colorants in everyday commodities made of PMMA. Furthermore, the concentration of the bluing agents is limited to very low amounts, often less then 10 ppb (=“level of toxicological insignificance”).
However, such small quantities of bluing agents are insufficient to compensate completely for the yellowish edge cast. Accordingly, so far there are no panels or moldings made of PMMA molding compositions known in the state of the art whose edge cast could be called color-neutral without containing bluing agents in amounts above the thresholds set by the government.
A great number of general methods have been proposed for preventing or reducing a yellow cast in thermoplastics. For example, the use of antioxidants that are to prevent discoloration of plastic molding compositions under thermal stress, among other effects, are known (for example, see Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 3, page 133, Wiley, New York, 1978). Antioxidants in some cases are also added to the molding compositions prior to processing, in the compounding step. Thus, according to Japanese Application Kokai Tokkyo Koho JP 60 123 547, a reduction of discoloration of copolymers of methyl methacrylate, styrene, and maleic anhydride monomer units is observed at high temperatures under injection molding conditions when at least one phosphaphenanthrene derivative and additionally a sterically hindered phenol, an ester of thiopropionic acid, or an ester of phosphoric acid are added to such copolymers prior to processing as stabilizers against oxidative degradation.
Jpn. Kokai Tokkyo Koho JP 60 120 735 describes copolymers of methyl methacrylate, vinylaromatics, and copolymerized cyclic anhydrides, to which are added esters of phosphoric acid and other stabilizers based on sterically hindered phenols to increase thermal stability and to prevent the discoloration of such copolymers under thermal stress in the melt, for example in injection molding.
Jpn. Kokkai Tokkyo Koho JP 03 167 245 claims the stabilization of copolymers of methyl methacrylate, N-substituted maleimides, and other copolymerizable monomers with compounds chosen from the group consisting of alkyl-substituted triaryl phosphites, dialkyl pentaerythritoldiphosphites, and phosphaphenanthrene derivatives.
Jpn. Kokkai Tokkyo Koho JP 63 163 306 includes copolymers of methyl methacrylate and C
8
- to C
20
-alkyl methacrylate as core material for optical fibers that contain stabilizers consisting of phosphites, for example such as sterically hindered diaryl pentaerythritoldiphosphites, or thiophosphites, to prevent the discoloration of copolymers under thermal stress.
Sterically hindered phenols and organic phosphites are claimed in all of the Japanese patents mentioned here, or organic phosphites combined with sterically hindered phenols. Inorganic reducing phosphorus compounds are not mentioned.
German Utility Patent 2 95 04 693.7 also describes the use of sterically hindered organic phosphite compounds in molding compositions of copolymers consisting of the monomer units of alkyl methacrylate, vinylaromatic, and maleic anhyd ride, as well as alkyl acrylate optionally. The organic phosphorus compounds are sometimes also added to the finished, granulated, or ground polymer prior to further processing.
Inorganic phosphorus compounds with reducing action are also known from the state of the art as stabilizers against yellowing of molding compositions under thermal stress.
In EP-A 576 877 a polymer based on polymethacrylimide and polyacrylimide with low yellowing is described, with salts of phosphinic acid or phosphonic acid already having been added during the imidization reaction. The amounts used of the phosphorus compounds are high, presumably to compensate for loss of activity in the subsequent processing steps. It is preferred to use amounts of 0.1-1 wt. % based on the amount of polymer to be imidized.
EP-A 0 516 131 A1 describes weather-resistant plastic compositions that contain a UV absorber and a hypophosphite. UV absorbers can be included in amounts of 0.01 to 10 wt. %, and hypophosphite in an amount of 0.001 to 100 wt. % based on the UV absorber contained, i.e. in an extremely wide range of 0.1 ppm to 10 wt. %. The weather-resistance additives are suitable for a number of plastics, including also polymethyl methacrylate.
Agents are also known for reducing discoloration, especially yellowing, for impact strength-modified PMMA molding compositions. These molding compositions contain particularly large numbers of polymerization auxiliaries from their production such as emulsifiers and buffer salts especially, for example when a latex dispersion with a core-shell structure is embedded as toughening phase in a hard PMMA matrix.
EP-A 465 049 (U.S. Pat. No.5,063,259) describes a method for producing clear, impact strength-modified acrylic plastics with improved color quality, particularly lower yellowing, by adding reducing compounds, for example such as sodium hypophosphite, to emulsion polymers, preferably with a core/shell structure, based on methyl methacrylates. The amounts used can be 50-250 ppm according to claims, based on the emulsion. The emulsion polymers are then incorporated in a polymethyl methacrylate matrix. It is also described how to add the Na hypophosphite first in the polymer matrix (100 ppm) and then to mix it 1:1 with the modifier, so that a final concentration of 50 ppm is obtained.
On the other hand, German Patent Application 1 95 44563.5 describes adding a much smaller amount of inorganic phosphorus compound with reducing action in a later phase of processing, namely in the compounding step, whereby yellowing under thermal stress, for example during the production of moldings, is largely suppressed.
German Patent Application 1 95 44562.7 teaches similarly with regard to the mode of addition of the inorganic reducing phosphorus compounds. They are added to a molding composition based on poly(meth)acrylimide in the compounding step in an amount of 0.01 to 1 wt. %, and they improve especially the color stability under thermal stress.
Apparently by their reducing action, they prevent the formation of chromophores from the amide and imide structural units caused by oxidation.
Albrecht Klaus
Jaksch Hermann
Mohr Ernst
Numrich Uwe
Vetter Heinz
Egwim Kelechi C.
Roehm GmbH & Co. KG
LandOfFree
Process for producing polymethylmethacrylate molding... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing polymethylmethacrylate molding..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing polymethylmethacrylate molding... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3187225