VACUUM APPARATUS

Abstract: Apparatus adapted to operate at a high vacuum (i.e. a pressure of or less than 1 x 10<-4> torr) includes at least one component of plastics material comprising liquid crystal polymer, the component being exposed in use to the vacuum. Plastics material comprising liquid crystal polymer has good outgassing properties. The component may for example comprise the base plate (2) of a vacuum deposition chamber.

Description: VACUUM APPARATUS 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 vacuuπ-''. By the term "high vacuum" as used herein is meant a subatmospheric pressure c -4 1 x 10 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 makin 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 Ke have now surprisingly discovered that plastic materials comprisir.c: liσ id crystal polymer outgaεseε substantially less than other plastics we have tested and to an extent that makes possible their use at high vacu in vac ac aratus. - z According:to the present invention there is provided apparatus adapted tc create or operate at a high vacuum, including at least one component of plastics material comprising liquid crystal polymer, which component in use exposed to the vacuum. The invention also provides a component of plastics material, comprising liσ crystal polymer, for use in said apparatus. The apparatus including the component or components of said plastics material may have a combined surfa 2 area of at least 20 cm exposed in use to vacuum. Typically, the plastics material may have fibrous or other reinforcement embedded therein. The fit_>res 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 genera 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. liquid crystal polymer is preferably an aromatic polyester or aromatic co-polyester and any commercially available liquid crystal polymer capable being formed, for example by injection moulding into compounds having high flexural strength (eg greater than 75 MPa) may be employe in the invention Such commercially available liquid crystal polymers are typically aromatic co-polyesters. One range of suitable liquid crystal polymer is available f Celanese Limited under the trade mark VECTOA. 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, ϊhey can be processed on all conventional machinery without modification. Typical melt temperatures fcr injection moulding and extrusion are in the order of 28Ξ C C with injecticr. mould temperatures usually held at about 100°C. Components made from such liquid crystal polymers are able to withstand moderately hostile environmen and may for example be employed in cryopumpinσ apparatus even wher. exposed cryogenic temperatures in the range of -10C C C tc -19 c cr in diffusion pumping apparatus even wher. exposed tc temperatures in the order cf 1CC C . Other liquid crystal polymers that can be used in components of the inventio are available under the trade mark XYDAR RC - 210 from the Dartco Technolocy Centre, Augusta, Georgia. Other suitable liquid crystal polymers are for example described in United States patent specification 4 614 629 which in particular discloses aromatic liquid crystalline co-polyesters such as co-polymers of polyethyleneterphthalate with poly-p-hydrobenzoic acid. The vacuum apparatus including component(s) in accordance with the invention may be employed in for example evacuating chambers in which a vacuum evaporation or sputtering operation is conducted. For example, the base plat of a vacuum deposition apparatus may be formed of plastics material comprisin liquid crystal polymer. Vacuum apparatus according to the invention may include a rotary vane vacuum pump, a diffusion vacuum pump, a dry vacuum pump or a turbomolecular vacuum, pump. The said plastics material, may be used in making the end plates of some such pumps, any other components of such pumps having a maximum, wall thickness of 9 mm or less or any tubing, piping, manifold, connector, adaptor or valve component associated with such pump. The casing of a turbomolecular pump may also be formed of a plastics material comprising liquid crystal polymer in accordance with the invention. Brief Description of Drawings The sole drawing illustrates part of a vacuum deposition apparatus having a base plate form of a plastics material comprising liquid crystal polymer. The drawing is not to scale. - A - Best Mode for Carrying Out the Invention Referring to the drawings, there is shown a vacuum deposition chamber of entirely conventional type save for the material out of which its base plate is formed. The apparatus comprises an annular base plate formed of a plasti material comprising a liquid crystal polymer. The top surface of the base plate 2 is engaged vacuum-tight to a bell jar 4. The apparatus additionally includes an assembly of electrodes and source of coating material generally indicates by the reference numeral 6 and a workholder ring 8 for a member tc be coated. The base plate 2 has a plurality of drillings 12 or other apertures formed therethrough through which electrical leads and the like ma pass. The base plate 2 defines a circular aperture 10 through which the apparatus is able to be evacuated using a conventional vacuum pumping εysterr to a pressure of 1 x 10 ~ torr or less. Typically, the outer edge of the base plate defines a circle whose diameter is in the order of 30 cm. ~~ while _2 the aperture 10 has a diameter in the order of 10 cm . Accordingly, in use, a relatively large surface area of the base plate is exposed to vacuum.. It is therefore important that the base plate should have good outgassing properties. The superior outgassing properties of plastics comprising liquid crystal polymers (in comparison with other plastics material] are illustrated in the examples given in the Table below:- 5 - Table: Outgassing Rate/10" 8 x bar 1 s ~ l cm Example No: Test Material 1st pump down after; 2nd pump down 4h lh VECTRA A230 0.8 ("C-filled brown") VECTRA A420 2.3 2.4 ("glass-filled brown") VECTRA A130 4.0 ("glass-filled white") 4 PEBAX MX 1057 5 PEBAX MX 1059 6 PEBAX MX 1060 7 PEBAX MX 1058 8 PEBAX 5533 SNOO 9 PEBAX 3535 SNOO 10 PEBAX 2533 SNOO 11 PORAFLON (ETFE) 12 Aluminium coated stainless steel 13 Stainless steel 1.65 C The results show the superiority of the three examples (1 to 3) according the invention over other plastics material tested (Examples 4 to 11) . The material of Example 1 is also superior to the stainless steel and. aluminium coated stainless steel of Examples 16 and 17.

Claims: CIAIMS 1. Apparatus adapted to create or operate at a high vacuum including at least one component of plastics material comprising liquid crystal polymer, said component being exposed in use to the vacuum. Apparatus as claimed in claim 1, in which said at least one component in 2 uussee ooff tthhee apparatus has a surface area of at least 20 c exposed to the vacuum. 3. Apparatus as claimed in claim 1 or claim 2, in which said at least one component employed in a vacuum pump forming part of said apparatus. 4. Apparatus as claimed in claim 3, in which said vacuum pump is a rotary vane vacuum pump, a diffusion vacuum pump, a dry vacuum pump, or a turbomolecular vacuum, pump. 5. Apparatus as claimed in claim 4, wherein the component or components of said plastics material comprise one or more of the end plates of the pump, or tubing, piping, connectors, adaptors, manifolds or valve components associated with the pump. 6. Apparatus as claimed in claim 1 or claim 2, in which said at least one component is the base plate of a vacuum deposition chamber. 7. A component of plastics material comprising liquid crystal polymer for use in the apparatus as claimed in any one of the preceding claims. 8. A component as claimed in claim 7, having reinforcing material embedded therein. 9. A component as claimed in claim 8, in which the reinforcing material iε of mineral or carbon fibre. 10. A component as claimed in claim 9, in which the mineral is glass. 11. A component as claimed in any one of claims 7 to 10, in which the liq crystal polymer is an aromatic polyester or aromatic co-polyester. 12. A component as claimed in claim 11, in which the aromatic co-polyeste consists of phenyl and naphthyl rings attached together at the 1,4 an 1,6 - positions respectively by ester linkages.

Inventors: DE HAAN GER, DE HAAN, GER
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1988
Main classification: B01J3
IPC codes: B01J3, F04B37