Multi-stage process for producing heat resistant...

Details Multi-stage process for producing heat resistant... Multi-stage process for producing heat resistant...

1998-09-14

2001-04-10

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymerizing in two or more physically distinct zones

C526S073000, C526S088000, C526S328000, C526S329700, C528S501000

Reexamination Certificate

active

06214942

ABSTRACT:

DESCRIPTION
1. Field of the Invention
The invention relates to a process for synthesis of polymethacrylate molding compounds with high deflection temperature under load and high thermal stability, wherein polymerization takes place below 120° C. in a first reactor stage and a second or further reactor stage or stages is or are operated in a temperature range of between 130 and 200° C. Residual monomers separated in a subsequent devolatilization stage can be recycled to the process. At the same time, the molding compound can be discharged.
2. Prior Art
Polymethacrylate molding compounds are usually synthesized by batchwise or continuous substance polymerization or by bead polymerization. The batch process, which is performed at low temperatures, for example between 20 and 50° C., is very time-consuming and labor-intensive, and so in recent years it has been progressively and almost completely superseded by continuous, albeit technically complex polymerization processes.
Continuous polymerization is performed, for example, in a zone in which the temperature increases from 130 to 250° C. in a polymerization system designed as an extruder. Before isolation of the polymerized product as extrudate, the unreacted monomer, which can represent as much as 40 wt % of the feed monomers, is removed in a devolatilization zone and recycled to polymerization.
To achieve high yields per unit volume and time, the polymerization is usually performed at temperatures of around 100° C. and in the presence of relatively high initiator concentrations (see, for example, Vieweg, Esser, Kunststoff-Handbuch [Plastics Handbook], Volume IX, pages 22 to 35, Carl Hanser Verlag, Munich, 1975 or Ullmanns Enzyklopädie der technischen Chemie [Encyclopedia of Industrial Chemistry], 4th Edition, Volume 19, pages 22 to 27, Verlag Chemie, Weinheim, 1980). The synthesis of polymethacrylate molding compounds by solution polymerization and subsequent evaporation of the solvent is also known.
In general, the methacrylate is copolymerized with a few per cent of an acrylic acid ester or with styrene to impart thermal stability to the polymethacrylate molding compounds.
Polymerization processes for the synthesis of thermoplastically processable polymethacrylate molding compounds are generally performed in the presence of transfer regulators, especially in the presence of mercaptans, which act as chain-termination regulators in the polymerization mechanism and thus influence the degree of polymerization and the chemical structure of the polymer end groups. With the mercaptans, which are the best known regulators, polymerized products are generally obtained that have a higher number of thermally more stable end groups than do polymethacrylates synthesized without regulators (see, for example, German Patent 1645232). The nature of the end group is of particular importance for the thermal stability of polymethacrylate molding compounds.
For example, it was shown in model experiments that polymethyl methacrylate polymer chains with saturated end groups as formed by transfer reactions are stable up to temperatures of higher than 300° C., whereas polymer chains terminated by disproportionation reactions and consequently having an olefinic double bond at the chains end already undergo thermal decomposition at about 250° C. [P. Cacioli et al., Polymer Bulletin 11, 325 (1984)]. Polymethyl methacrylate (PMMA) polymer chains terminated by recombination reactions are thermally stable only up to about 190° C.
To achieve satisfactory thermal stabilization, the incorporation of acrylic acid esters is advantageously combined with polymerization in the presence of mercaptans for industrial processes for synthesis of polymethacrylate molding compounds, while low molecular weight stabilizers are also added for better processing.
The deflection temperature under load of polymethacrylate molding compounds, which can be defined, for example, by the glass transition temperature Tg or the Vicat softening temperature VST per ISO 306, is decisively influenced by the tacticity of the polymethacrylate polymer chains, a parameter that can be varied within wide limits as a function of the polymerization process. For industrial purposes, high proportions of syndiotactic and heterotactic triads are desired, since they lead to molding compounds with high deflection temperature under load. In contrast, high proportions of isotactic triads in the polymer chains are not desired, because they act against high deflection temperature under load.
At low polymerization temperatures, mainly the desired syndiotactic triads are obtained in the polymer chains, but relatively high initiator concentrations must be used in such polymerization processes in order to achieve high yields per unit volume and time. Thereby there are increasingly formed polymer chains which, as already explained, are terminated by recombination reactions or disproportionation reactions and thus have poorer thermal stability. On the other hand, by polymerization at elevated temperatures, for example in a continuous polymerization process at about 180° C., good yields per unit volume and time of polymers with good thermal stability are already achieved with relatively low initiator concentrations. At these high polymerization temperatures, however, the polymerized product obtained has poor deflection temperature under load, due to high proportions of isotactic triads.
European Patent 245647 (=U.S. Pat. No. 4,877,853) teaches a technical method for overcoming this problem. Therein there is described a thermoplastically processable polyacrylate molding compound with simultaneously high deflection temperature under load and high thermal stability, wherein the content of syndiotactic triads is higher than 60% and the initiator/regulator ratio is preferably at most 1:2 or most preferably much lower, and wherein the polymethacrylate molding compound is synthesized at temperatures of between 0 and 100° C. by emulsion polymerization.
Continuous polymerization processes operating with relatively low conversions and high polymerization temperatures but good yields per unit volume and time are illustrated, for example, in Japanese Kokai Tokkyo Koho JP 04146903, wherein there are described the polymerization of methyl methacrylate up to conversions of 20 to 50%, the subsequent monomer devolatilization as well as the final polymerization of the polymer syrup to polymethyl methacrylate plates with good optical characteristics.
U.S. Pat. No. 2,974,125 relates to the synthesis of copolymerized products of methyl methacrylate, styrene and acrylonitrile, wherein the polymerization is performed at temperatures of between 130 and 155° C. up to conversions of between 10 and 50%. The unreacted monomers are preferably removed by devolatilization and the resulting polymerized products are characterized by high values of percentage elongation under load.
German Accepted Application 2504417 describes a process for synthesis of methyl methacrylate polymerized products containing at least 80 wt % of methyl methacrylate units, the monomers being reacted at temperatures of between 150 and 180° C. to a degree of conversion of about 50 to 80% in the agitating reactor and the unreacted monomers being removed by application of vacuum. The resulting polymerized products are largely free of residual monomers and oligomers up to a molecular weight of 1000 Dalton.
U.S. Pat. No. 4,711,938 relates to a continuous process for synthesis of polymethyl methacrylate or methyl-methacrylate-containing copolymerized products at polymerization temperatures of 140 to 170° C. and conversions of up to 50%, the residual monomers also being removed by application of a vacuum. The resulting polymerized products are characterized by high optical purity and narrow molecular weight distribution.
U.S. Pat. No. 3,637,545 describes the synthesis of polymethyl methacrylate by means of continuous bulk polymerization at temperatures of between 145 and 165
20
C. and conversions of between 40 and 60%. Once again the resid

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