Pumps – Motor driven – Electric or magnetic motor
Patent
1989-10-12
1991-06-11
Smith, Leonard E.
Pumps
Motor driven
Electric or magnetic motor
4174234, 417DIG1, F04B 3714
Patent
active
050228293
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to vacuum apparatus, particularly vacuum pumping apparatus adapted to created a "high vacuum" and components of such vacuum apparatus which in use are subjected to vacuum, particularly a "high vacuum".
By the term "high vacuum" as used herein is meant a subatmospheric pressure of 1 .times. 10.sup.-4 torr or less.
BACKGROUND ART
Vacuum pumps capable of creating such pressures include diffusion and turbomolecular pumps. One of the problems in making such a pump and in making a vacuum pumping apparatus incorporating such a pump, is the selection of materials out of which the components of the pump and apparatus are to be made. Many materials which it would be otherwise desirable to use in the components outgas in a high vacuum at such a rate is to prevent the achievement of a desired degree of vacuum in the chamber to be evacuated. Accordingly, components that are to be exposed to high vacuum in use are typically made of metals such as stainless steel and aluminium, and in particular, components made of plastic materials are rarely if ever used in a high vacuum environment and thus advantages such as a reduction in the cost and weight of the vacuum apparatus typically associated with the use of plastics materials are not achieved.
DISCLOSURE OF INVENTION
We have now surprisingly discovered that plastic materials comprising liquid crystal polymer outgasses substantially less than other plastics we have tested and to an extent that makes possible their use at high vacuum in vacuum apparatus.
According to the present invention there is provided apparatus adapted to create or operate at a high vacuum, including at least one component of plastics material comprising liquid crystal polymer, which component in use is exposed to the vacuum.
The invention also provides a component of plastics material comprising liquid crystal polymer, for use in said apparatus. The apparatus including the component or components of said plastics material may have a combined surface area of at least 20 cm.sup.2 exposed in use to vacuum.
Typically, the plastics material may have fibrous or other reinforcement embedded therein. The fibres may for example be of mineral, for example glass, or carbon. The use of such strengthening may, however, increase the rate at which the plastics material outgasses under high vacuum, but generally not to an extent that renders the plastics material unsuitable for use in apparatus for creating high vacuum.
Typically, the said component is made by injection moulding or extrusion. The liquid crystal polymer is preferably an aromatic polyester or aromatic co-polyester and any commercially available liquid crystal polymer capable of being formed, for example by injection moulding into compounds having high flexural strength (e.g. greater than 75 MPa) may be employed in the invention. Such commercially available liquid crystal polymers are typically aromatic co-polyesters. One range of suitable liquid crystal polymer is available from Celanese Limited under the trade mark VECTRA. Each liquid crystal polymer within this range is believed to be an aromatic co-polyester consisting of phenyl and naphthyl rings attached together at the 1, 4 and the 1, 6 -positions respectively, by ester linkages. They can be processed on all conventional machinery without modification. Typical melt temperatures for injection moulding and extrusion are in the order of 285.degree. C. with injection mould temperatures usually held at about 100.degree. C. Components made from such liquid crystal polymers are able to withstand moderately hostile environments and may for example be employed in cryopumping apparatus even when exposed to cryogenic temperatures in the range of - 100.degree. C. to -196.degree. C. or in diffusion pumping apparatus even when exposed to temperatures in the order of 100.degree. C.
Other liquid crystal polymers that can be used in components of the invention are available under the trade mark XYDAR RC - 210 from the Dartco Technology Centre, Augusta, Ga.
Other suita
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Modern Plastics Encyclopedia, Oct. 1986, p. 42.
Caoutchoucs et Plastiques, Les polymeres a cristaux liquides (liquid crystal polymers), Apr. 1986, p. 70.
"Neus Liquid Crystal Polymer", Gummi Fasern Kunststoffe, vol. 39, No. 12, Dec. 1986, pp. 665, 668.
Pearlman Robert I.
Rosenblum David M.
Smith Leonard E.
Szczecina Jr. Eugene L.
The BOC Group plc
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