Plastic high vacuum piping components

A method for making vacuum components from plastic material, and in particular, PVC and CPVC, where the plastic component is exposed to a high vacuum or ultra high vacuum for an extended period of time, by which the outgas rate drops considerably to levels that are well within required limits for high vacuum and ultra high vacuum applications. Residual gas analysis may be used to certify this low outgas level for the plastic components.

BACKGROUND OF THE INVENTION 
1. Field of the Invention (Technical Field), 
The invention relates specifically to the discovery that certain low cost 
thermoplastics such as Polyvinylchloride (PVC) and chlorinated 
polyvinylchloride (CPVC) can be excellent materials for the construction 
of high and ultra high vacuum components. Prior art vacuum use of these 
materials has been restricted to vacuum pump exhaust lines and rough 
vacuum. It has been conclusively determined through the use of residual 
gas analysis that the prior bias against the use of these materials in 
this service relates to surface impurities that are either a residue of 
manufacture, a non vacuum compatible lubricant or a joining adhesive, not 
continuous outgas or permeation of the PVC or CPVC plastic material 
itself. Subsequently, when these materials are exposed to high vacuum 
pressures, the surface impurities are gradually eliminated. This type of 
surface contamination would, by its presence, render any material 
unsuitable for high and ultra high vacuum service. The invention uses an 
outgas reduction process to demonstrate the capability of PVC and CPVC 
plastic compounds to be suitable for use in the construction of high and 
ultra high vacuum systems, where vacuum pressures from 9.times.10.sup.-4 
torr to 1.times.10.sup.-11 torr are required. The present invention has 
also determined the relative purity level of the vacuum processed plastic 
components through the use of residual gas analysis. It should be noted 
that not all thermoplastic materials exhibit adequate high and ultra high 
vacuum performance. It is expected that through the inventions evaluation 
of thermoplastics that other cost effective materials may be found in 
addition to PVC and CPVC. 
2. Background Art 
The vacuum industry has considered low cost thermoplastics such as PVC, or 
CPVC to be unsuitable as a material of construction for high and ultra 
high vacuum components. This relates primarily to the perceived outgassing 
rate of such plastic materials and, secondly, to the perceived rate at 
which gases are able to permeate through the plastic material into the 
contained vacuum pressure space. "Outgassing" is the tendency of materials 
to release gases over time. The rate at which materials give off gas, or 
outgas, is particularly a problem in high and ultra high vacuum 
applications where systems are almost exclusively fabricated from 
expensive polished stainless steel components which have low outgassing 
characteristics. Metal components for high and ultra high vacuum piping 
are often supplied with a helium leak rate certificate but little or no 
information is currently supplied with vacuum piping components which 
specifies the outgas/permeation rate that these components produce. Low 
cost PVC and CPVC components are typically used in the construction of 
liquid transfer piping systems. Because these components are manufactured 
in very high quantity they are low in cost to end users. 
SUMMARY OF THE INVENTION 
Certain low cost thermoplastic materials such as PVC and CPVC are excellent 
low cost high and ultra high vacuum materials. We have discovered that 
when a clean PVC or CPVC component is exposed to high and ultra high 
vacuum pressures for a period of time, the outgas rate for the PVC or CPVC 
component drops dramatically to levels which are well within required 
limits for high and ultra high vacuum applications where vacuum pressures 
from 9.times.10.sup.-4 torr to 1.times.10.sup.-11 torr are required. 
Subsequently, it is a purpose of this invention to show that low cost 
materials such as PVC and. CPVC are suitable, and in certain cases 
superior to stainless steel, for high and ultra high vacuum component 
service. This is demonstrated through outgas reduction of surface 
impurities by exposing said materials to high and ultra high vacuum and 
through residual gas analysis determination of the outgas levels for said 
materials while exposed to high and ultra high vacuum pressures. 
It is also the intent of this invention to provide the user of these 
processed plastic components with documentation that shows the type and 
relative amount of contamination that the components would contribute to 
their vacuum process at a given vacuum pressure prior to the 
fabrication/installation of their system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention is directed to a new use of thermoplastic material for the 
construction of components for high and ultra high vacuum environment 
generation and containment. According to the invention, it has been proven 
that certain low cost thermoplastics such as PVC, and CPVC, can replace 
expensive metal components in high and ultra high vacuum systems through 
the use of residual gas analysis evaluation. When these thermoplastic 
materials are exposed to high vacuum pressures, the surface impurities 
that initially are damaging to high and ultra high vacuum pressures are 
gradually eliminated. The present invention has used an outgas reduction 
process to demonstrate the capability of PVC and CPVC plastic compounds to 
be suitable for use in the construction of high and ultra high vacuum 
systems, where vacuum pressures from 9.times.10.sup.-4 torr to 
1.times.10.sup.-11 torr are required. Additional thermoplastic materials 
can be used according to the invention provided they meet the criteria of 
low permeability to prevent gas molecules from migrating through the 
material into the contained high or ultra high vacuum and low outgassing 
characteristics which is the tendency of a material to give off gas 
molecules into a vacuum. It should be noted that not all thermoplastic 
materials exhibit adequate high and ultra high vacuum performance. The 
method of the invention determines the relative purity level of the vacuum 
processed plastic components through the use of residual gas analysis. It 
is expected that through the inventions evaluation of thermoplastics that 
other cost effective materials may be found in addition to PVC and CPVC. 
Referring to FIG. 1, the PVC/CPVC ball valve 1 is shown to illustrate the 
discovery that expensive stainless steel high and ultra high vacuum valves 
can be replaced, in many high and ultra high vacuum applications, by a low 
cost PVC ball valve that has been modified for high and ultra high vacuum 
service by replacing the standard valve lubricant with a vacuum compatible 
lubricant such as "KRYTOX" grease from Dupont. 
Referring to FIG. 2 a 8" diameter PVC cross 2 is shown to illustrate the 
discovery that expensive large diameter stainless steel piping components 
can be replaced, in many high and ultra high vacuum applications, by a low 
cost PVC component that is actually superior to stainless steel in certain 
applications. Many other sizes and shapes of piping components can be 
used. 
Referring to FIG. 3, a 12" section of 4" diameter flexible PVC tubing 3, is 
shown to illustrate our discovery that an expensive section of metal 
bellows tubing can be replaced by a low cost flexible PVC section in 
certain high and ultra high vacuum piping applications where a flexible 
connection is required. 
Referring to FIG. 4 an injection molded CPVC molecular drag pump rotor 4, 
is shown to illustrate a complex shape, high and ultra high vacuum pump 
component that can be molded from plastic much more economically than it 
can be machined from aluminum or stainless steel. 
Referring to FIG. 5 a molded PVG molecular drag pump stator/housing 5, is 
shown to illustrate further the ability to produce the two major 
components of a high vacuum molecular drag pump from molded plastic 
instead of machined metal at a dramatic reduction in cost. 
Referring to FIG. 6, a typical system for high and ultra high vacuum outgas 
reduction/evaluation of thermoplastic piping component material or 
material interior surface contamination is shown. In this example, 
flexible PVC tubing 3 is capped 9 and connected to a high and ultra high 
vacuum outgas reduction manifold 8 and then subsequently evacuated by a 
high and ultra high vacuum pump 7 until a specific high and ultra high 
vacuum pressure, and or an outgas rate for each atomic mass unit between 0 
and 100 is attained on a residual gas analysis device 6 which is connected 
to a residual gas sensor 10. The said analysis device and sensor used in 
the invention was the model "PPT" from MKS instruments connection to an 
ALR "flyer" model PC. The results of this process are stored for customer 
distribution. 
Referring to FIG. 7, there is shown a typical system for high and ultra 
high vacuum outgas reduction/evaluation of entire thermoplastic piping 
components interior and exterior surfaces. In this example, the PVC/ CPVC 
ball valve 1 is placed into a high and ultra high vacuum outgas reduction 
chamber 13 and evacuated by a high and ultra high vacuum pump 7 until a 
specific high and ultra high vacuum pressure and/or a specific outgas rate 
for each atomic mass unit between 0 and 100 is attained on a residual gas 
analysis device 6 which is connected to a residual gas sensor 10. 
Referring to FIG. 8, a residual gas analysis bar chart is shown with bar 
levels which represent the partial pressure reading scale levels 11 of 
individual atomic mass units such as the Atomic Mass Unit Four--Helium 12 
and Atomic Mass Unit Twenty-Eight--Nitrogen 14. The partial pressure for 
each atomic mass unit added together is equal to the total vacuum 
pressure. Through careful monitoring of individual atomic mass unit 
partial pressures, it is possible to determine both the purity of the 
components which are undergoing high and ultra high vacuum outgas 
reduction and the integrity/effectiveness of the high and ultra high 
vacuum outgas reduction/evaluation system. 
Referring to FIG. 9, an actual residual gas analysis initial outgas 
reduction spectra 16 is shown. This actual data was taken 6 hours after 2 
8" diameter PVC tees were connected to a high and ultra high vacuum outgas 
reduction manifold. The spectra shows numerous atomic mass units present 
in the composition of thermoplastic material or surface impurities 
outgassing into the manifold system. 
Referring to FIG. 10, an outgas reduction spectra 17 is shown 72 hours 
after connecting the 8" PVC tees to the high and ultra high vacuum outgas 
reduction manifold. The only significant atomic mass unit outgas 
contributors are Seventeen and Eighteen which are the secondary and 
primary atomic mass units for water. These partial pressure levels for 
water vapor are very low for an unheated vacuum chamber showing that it 
may be easier to remove water vapor molecules from a thermoplastic system 
than it is in a stainless steel or aluminum chamber.