Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Patent
1995-05-26
1997-11-04
Marquis, Melvyn I.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
528500, 528502F, 159 401, 159 404, 159 431, 159 132, 159 271, 159 274, 159 90, 202236, 202237, C08G 7734
Patent
active
056841256
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for purifying liquid polysiloxane containing impurities as a low-molecular-weight fraction, by evaporating the low-molecular-weight fraction.
Such a method for purifying polysiloxane is disclosed in U.S. Pat. No. 2,834,754. In that case, impure polysiloxane is kneaded at a temperature of 75.degree.-175.degree. C. in a mixer while gas ("stripping gas") is continuously fed to the kneaded material. This removes volatile constituents which are present in the polysiloxane as a result of the synthesis. However, this known method yields a product whose purity still leaves something to be desired, i.e. the purified product still contains too large an amount of low-molecular-weight constituents for certain applications.
U.S. Pat. No. 4,933,432 describes a method for reducing the amount of oligomeric siloxanes which are present as impurity in a polysiloxane, in which method the polysiloxane product is immersed in an organic liquid, in which liquid the oligomeric siloxanes are in fact soluble but not the polysiloxane, after which the immersed product is exposed to ultrasonic vibrations and the immersed product is then removed from the liquid. The polysiloxane product is a moulded product of so-called silicone rubber. In the introduction to the last-mentioned U.S. Patent Specification, applications of silicone-rubber products are mentioned, in which the problem of the undesirable effect of impurities originating from the synthesis of the polysiloxane may occur. In applications in electrical equipment, for example as sealing material in switchboxes, the problem occurs that low-molecular-weight components may evaporate during use or in the long term from the polysiloxane and deposit on components which should be electrically conducting. Although polysiloxane is an ideal sealing material per se, the phenomenon of the evaporation and redeposition of low-molecular-weight material on electrical components has a disadvantageous effect on the service life of the electrical device, for example a switch.
U.S. Pat. No. 4,356,116 discloses a method for devolatilizing polysiloxanes by heating this material at a temperature of at least 200.degree. C. under vacuum for at least 5 minutes. This process yields a devolatilized room temperature silicone composition wherein the volatiles content does not exceed 0.003 g/cm.sup.3.
U.S. Pat. No. 3,926,927 relates to a method for stripping unreacted monomers from a slurry of copolymerized acrylonitrile. According to the main claim this slurry should fulfil special conditions with respect to the solids content. The slurry is feeded downwardly through a treatment zone having a sequence of surfaces each having therein perforations through which the slurry can flow downward onto the next surface in the sequence. Steam is forced upward through the perforations in countercurrent flow to the slurry to strip unreacted monomers from that slurry. The flow rate of the steam is controlled in a special way. At a certain flow rate, the slurry will not flow downward through the perforations. This period takes 5-15 seconds. After this period, the steam flow is controlled in such a way that the slurry flows downward through the perforations. This period takes 1-10 seconds.
Polysiloxanes or silicones are synthetic organic silicon oxide compounds which are composed of the elements silicon, oxygen, carbon and hydrogen. They are used as lubricants, hydraulic liquids, agents for rendering water-repellant, varnishes and synthetic rubbers. Silicones are chemically inert and extremely stable at high temperatures. The synthesis of many linear and cyclic polysiloxanes by hydrolysis, polycondensation and polymerization is known from the prior art. Polysiloxanes are suitable for electrical applications and vacuum applications, and also for applications in the textile, paper, rubber and clothing industry. Because of the inert properties and the lack of toxicity, silicones are also used for medical purposes, for example, in dental laboratories (prostheses) and for surgical applications. A
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Bayer Aktiengesellschaft
Marquis Melvyn I.
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