Dynamic gas transmission measuring apparatus

Gas transmission rates for and taken across plastic specimens are accurately measured in suitable apparatus for the purpose by a dynamic technique involving constant measurement of pertinent temperature and pressure data which is permitted to constantly change throughout the measurement testing.

BACKGROUND OF THE INVENTION 
There are several known gas transmission measuring devices available and in 
fairly extensive employment on and in the market. Prominent and widespread 
in application amongst these are The Dow Gas Transmission Cell (ASTM 
Designation D-1434-66T) available from Custom Scientific Instruments, Inc. 
of Whippany, N.J. 07981 and the Linde Permeability Cell (ASTM Designation 
D1434), also available from the same mentioned commercial source. 
The available gear for the indicated purpose, including those above 
specifically identified, generally require a constant pressure drop and 
temperature in order to obtain the desired permeation data. These 
requirements, as is well comprehended by those skilled in the art, impose 
some disadvantageous limitations and disabilities on the known apparatus 
and uses thereof. 
Thus, for indicated testing purposes, nothing in prior art appears to 
realistically concern itself with a truly dynamic way of measuring the 
transmission rate of various gases through curved or flat plastic sheets, 
liners, films, etc. in the same efficient and extremely reliable manner, 
well-adapted for commercial testings and investigations, in the style and 
means of implementation as is so crucially indigenous as is present and 
involved in the instant contribution to the art. 
FIELD AND PURVIEW OF THE INVENTION 
The present invention, and the principle aims and objectives attainable in 
its practice, pertain(s) and direct(s) to a novel and, in the overall, 
unprecedented and exceptionally efficient and technically accurate means 
and technique for measuring the gas transmission rate(s) of various 
plastics materials (including, for example but without limitation thereto, 
samples of tubular sections, sheets, film, laminates, plastic-coated paper 
and metal specimens, etc.) when subjected to permeation by various inert 
and/or corrosive gases and/or vapors. 
The achievement and provision of all indicated, with even more and 
additionally other benefits and advantages derivable in and from present 
practice appear and become more evident in the ensuing description and 
Specification. 
SUMMARY OF THE INVENTION 
The present invention, in its genesis and as derives from the discovery on 
which it is based, pertains to the indicated novel technique (as well as 
to the associated means combination in apparatus assembly for the 
implementation thereof) for dynamically determining the rate of 
transmission of a given gas (including vapors) by and through a sample of 
a given plastic material whose gas permeation characteristic is desired to 
be defined and measured which, in basic outline, comprises: taking a 
sample of given surface area subject to facially-transgressing permeation 
thereacross by the given gas of a given plastic material (of which said 
plastic constitutes at least a coated or laminate part if not the entirety 
of the sample) whose gas permeation rate characteristic is being (or 
wanted to be) determined; mounting said sample in a confined and retained 
manner in such a way that it can be exposed within its controlling 
confines to gas under a given (or pre-selected) maximum test pressure to 
be applied against the upstream side of the sample face to be thereby 
exposed with a reduced or (at least at test commencement, zero) gas 
pressure effective on and behind the opposing downstream side face of the 
sample; then, at constant given temperature, constantly applying for a 
given time period the gas involved in the test at said maximum pressure 
against the exposed upstream face of said sample; permitting the gas 
during said time period to permeate and transgress said sample; measuring 
continuously or from time-to-time within said test time period the 
dynamically changing pressure drop from upstream to downstream side faces 
of and across said sample; noting and collecting the critical involved 
time(s), temperature(s) and pressure(s) data; and, finally, calculating 
from the collected data according to appropriate mathematical and physical 
laws and principles relevant thereto the particularly-involved gas 
transmission rate of and for said plastic material. 
Apparatus embodimentation, as noted, of the contemplated technique is also 
here envisaged and intended as an integral part of the invention. 
Still other features and adaptations of beneficial import and salience are 
advantageously combinable in and made integral part(s) of the basic and 
above fundamentally-delineated efficient technique and means for dynamic 
gas transmission rate determination of plastics materials pursuant to the 
invention. 
Thus, various suitable procedures, modes of operation, precautions, 
instructions, parts, elements, sub-assemblies and overall assemblies plus 
other equipage and practices for utilization, as well as working details, 
embodimental parameters and other specifics of the invention are also set 
forth in the following Specification.

For expedience and enhanced clarity of: associated parts, elements and 
functions; subassemblies and assemblies; certain companion accessories, 
procedures and so forth; and results; reference is now thereto had to all 
such predominant componential features and manner and consequence(s) of 
their operations as they appear throughout the accompanying Figures 
included in the Drawing with explanation thereof in the following 
catalogued description of same as they are identified by their respective 
reference numeral(s) or letter designation(s) (i.e., "Ref. No(s).") 
therewith joined. 
______________________________________ 
Ref. 
No(s). Description With Relevant Corollary Explanation 
______________________________________ 
11 Length of plastic material tubing or pipe, such 
as a liner for plastic lined pipe, from which 
a specimen is to be taken for determination of 
the gas transmission rate of the involved plastic. 
12 Phantom line representation of hole saw cut to 
be taken to obtain specimen for testing. 
13 Cut out specimen or dish-like form (as is 
obtained from supply stock of tubular material). 
14 Center line of specimen 13. 
15 General designation of cell housing unit for 
mounting of specimen to be tested. 
16 Upper housing section of cell cut out along 
lower peripheral edge to conform to curved 
specimen to be mounted in the cell. 
17 Lower housing section of cell cut out along 
upper peripheral edge to conform to curved 
specimen to be mounted in the cell. 
18 Sealing sleeve (shown broken apart and in 
partial section in FIG. 4) to envelope 
upper and lower housing sections 16 and 17 
about the area of specimen mounting which 
usually is a more or less vertically central 
location in the cell. 
19 Flat specimen for testing (shown only in 
FIG. 5), as may be taken from sheet or 
film stock or coated paper or laminated 
metal specimens; it being advantageous for 
any sample to be tested to have a diameter 
between about 2-3 inches (ca. 5.08-7.62 
centimeters) while taken for the measure- 
ment purpose in a generally circular form, 
whether its contour be curved or flat. 
20 Sealing washers (shown only in FIG. 5) for 
assisting the firm and leak proof mounting 
of a flat or otherwise conformed sample to be 
tested. 
21 A bolt with a hexagonal head to secure the 
upper and lower halves together of the form 
of cell embodiment illustrated in FIGS. 6 
and 7 of the Drawing. 
22 The nut(s) for bolt(s) 21. 
23 The spring washer(s), or equivalent, to 
utilize with bolt(s) 21 and nut(s) 22 for 
clamping together of the cell unit shown in 
FIGS. 6 and 7. 
24 The top or upper clamping plate for assembly 
of the FIGS. 6 and 7 cell unit. 
25 The bottom or lower clamping plate for the 
FIGS. 6 and 7 cell unit. 
26 & 27 Upper and lower cover caps, respectively, 
for cell unit of FIGS. 6 and 7. 
28 Gas or vapor inlet port, identified by general 
designation. 
29 General designation of gas or vapor outlet 
port. 
30 General designation for thermal well to 
facilitate gas temperature measurement in 
the cell unit portrayed in FIGS. 6 and 7. 
31 Welded-on retainers or collars to engage with 
upper and lower clamping plates 24 and 25 to 
secure assembly of cell housing. 
32 Weld spots or lines. 
33 "O"-ring gaskets to facilitate sealing of 
circumenveloping sleeve unit 18. 
34 Gas (or vapor) supply (FIG. 8) 
35 Pipe or tubing lines or conduits for gas 
interconnections, etc., in assembly arrangement. 
36 Throttle valve. 
37 Inlet filter, advantageously of the molecular 
sieve variety. 
38 Outlet filter, also advantageously a molecular 
sieve. 
39 Pressure gauge or recorder. 
40 Regulating valve in gas conduits 35 on 
upstream side of cell. 
41 Stop (control) valve in main gas handling 
line on downstream side of cell 15. 
35b By-pass conduits. 
43 Stop (control) valve in by-pass evacuation 
line. 
40x Individual inlet control regulating valves 
in lines 35 upstream of cells 15. 
45 Receptacle or reservoir container for heat 
exchange liquid medium, such as a flat pan, 
drum or the like or equivalent for an oil 
(or other heat-exchanging liquid) medium. 
46 General designation of the heat exchanging 
fluid contained in the reservoir container 
45. 
47 Thermocouple unit or other equivalent and 
suitable temperature measuring means. 
48 Immersion heater for temperature elevation 
and regulation of heat-exchanging fluid 
medium 46. 
50 General designation of overall single cell 
test unit plan (FIG. 8 only). 
51 General designation of overall triple cell 
test unit layout (FIG. 9 only). 
______________________________________ 
With an overview of the several depictions, views and illustrations of the 
Drawing being maintained (especially in the light of the foregoing 
explanations of parts, components, etc., and other elucidations), the 
subsequent portion of this Specification now turns to a somewhat more 
cohesive and particularized disclosure and exposure of and coordinated 
amplification upon the invention; including therein most appropriate and 
expedient (or best) manners and means stemming from the foregoing in which 
the same may be advantageously and propitiously embodied and practiced. 
In this connection, the basic principles and limitations of: gas 
transmission rates and the mathematics and physics, such as ideal gas 
laws, therewith associated; the taking of plastic materials samples of 
testing; the handling and usage of both inert and possibly reactive gases 
and vapors; valving, pumps; temperature controls; vacuum systems; 
gas-drying assemblies; data taking, recording and application; suitable 
materials of construction for handling various materials of the type here 
involvable; and so forth are so widely comprehended by those skilled in 
the art that greatly elaborated detailing and/or fundamentals-explanation 
of all the basics thereof is not herein made or attempted; the same being 
unnecessary for thorough understanding and recognition of the advance 
possibilitated for achievement and realization by and with the development 
in and of the outstanding dynamic gas transmission rate procedure and 
realization improvement that is according to and in keeping with the 
present invention. 
TICULARIZED OPERATION AND USE DESCRIPTION OF THE INVENTION 
As is clearly evident in and readily-enough deducible from the foregoing 
description and disclosure, the present invention in basic essence and 
substance contemplates the provision for use and application of a dynamic 
system for determination of gas transmission rate(s) through plastic 
materials in which the pressure drop across the sample of plastic being 
tested and the rate of transmission through the plastic sample is 
continuously changing throughout the test run for the desired measurement. 
The present technique, most advantageously, offers what in effect is a 
self-monitoring system with the maximum temperature(s) and pressure(s) 
capable of utilization limited only by the withstanding ability against 
such factors of the plastic material, per se, being tested. 
Some of the characteristics and particulars of the instant contribution to 
the art that, perhaps, are not completely-abundantly-plain in and from the 
foregoing Specification are now more precisely expostulated, including 
some optimum features prescribable for practice of the invention. 
The basic and outstanding advantage and hitherto unknown benefit of the 
present invention is its capability, not available in other known systems, 
to dynamically measure the gas transmission rate without dependence on 
gathering data only at given pressure differentials across the sample 
undergoing test. By virtue of the instant development, only the 
temperature need be held constant while permitting the pressure 
differential to constantly change as the gas or vapor involved in the 
testing is permeating the sample. 
The accessories and means utilized in embodimentation of the invention, 
such as valves, conduits, pumps, heaters, filters, etc., are of the 
generally standard and widely-utilized and -available types and styles 
commonly employed and as individual preference may dictate or select for 
laboratory and test equipment usage. It is usually desirable for the 
materials of construction employed to be of an inherently 
corrosion-resisting nature, such as stainless steel, Monel metal and so 
forth. The actual cell housing is often beneficially fabricated from such 
a material as "HASTALLOY" (Reg. TM). In this connection, an ordinarily 
convenient size for a cell unit built along the lines demonstrated in 
FIGS. 6 and 7 of the Drawing is something on the order of about 4 inches 
(ca. 10.16 centimeters) in width and about 12 inches (ca. 30.48 
centimeters) in height, both overall. While the sample mounting section of 
the cell and general shape of the sample to be tested is usually 
preferably cylindrical and circular, respectively, other sample-holding 
cell chamber cross-sectional configurations and corresponding sample 
shapes may be utilized, including square, rectangular, other polygonal 
such as triangular and otagonal, etc. As is readily apparent, when 
flammable gases or vapors are involved, it is desirable to utilize 
explosion-proof motors, connectors and other items of equipment utilized. 
In performance of the testing in keeping with preferable practice of the 
present invention, it is generally most beneficial and conductive of best 
results to allow any given run to proceed for a time period sufficient to 
allow the overall system, insofar as concerns upstream applied pressure 
relative to downstream accumulated pressure about the testing cell, to 
reach an equilibrium point or (and oftentimes even better) until the 
downstream accumulated gas or vapor pressure achieved by permeation across 
the sample under test equals the upstream pressure applied on the face of 
the sample undergoing measurement. Depending on particular temperature and 
pressure conditions utilized, the appropriate time period for this may be 
as long as 4-6 days and sometimes even longer. 
A good reference for test operation with a typical system implementation in 
accordance with the present invention involves the following operational 
steps: 
(a) Set high temperature cut-out controls to desired maximum temperature 
limits; 
(b) Set the oil (or other heat-exchanging media) bath temperature heater 
(48 in FIG. 9) such as to effect the desired control temperature for the 
cell unit(s) 15 within the oil bath 46 in reservoir container 45; 
(c) Turn on the agitator when one is employed (as 49 in FIG. 9); 
(d) Switch on the bath heater (as 48 in FIG. 9); 
(e) Set the desired pressure of gas or vapor to be applied over the 
upstream face of the sample 13 by regulation of throttle (or equivalent) 
valve 36; 
(f) Open the main gas valve(s) feeding to the system for pressure 
regulation thereinto (as 40 in FIG. 8 and 40x in FIG. 9); 
(g) Open the inlet pressure valve 36; 
(h) Check the inlet or upstream gas or vapor being applied and regulate 
same to get desired reading (frequently, depending on precise installation 
involved and plastic material sample being tested on the order of about 
150 pounds per square inch absolute (i.e., "psia"); 
(i) Start up the vacuum pump 44; 
(j) Open the gas inlet valves 40 and/or 40x to the cell unit(s) 15; 
(k) Open the cell by-pass valve 43 in by-pass line 35b for .ltoreq.30 or 
seconds--then close same; 
(l) Hold the oil or other bath temperature at the desired set point for 24 
or so hours; 
(m) Close the downstream gas outlet valve(s) 41; 
(n) Turn off pump 44, meanwhile 
(o) Noting and/or recording all pertinent time, temperature and pressure 
data; 
(p) Wait until the system reaches equilibrium or until collection volume 
cell pressures reach a predetermined suitable maximum (such as, say, about 
50 psia), continuing appropriate data gathering, then 
(q) With the obtained data, calculate the desired gas transmission rate 
value of the involved sample. 
To obtain the gas transmission rate figure or value characteristic of the 
involved plastic material sample tested, it is advantageous to use 
formulae derivable from the clasic Ideal Gas Law using for the purpose the 
information contained in the Appendix to ASTM Method D-1434-66 (1972) 
entitled "Gas Transmission Rate of Plastic Film and Sheeting" as a model 
to follow. Thus, the following equations that apply are developed: 
##EQU1## 
wherein: 
n=the number of moles of gas (or vapor) involved at given time "t"; 
P=the applied pressure in mm of Hg; 
V=the involved volume in cm.sup.3 ; 
R=a universal gas constant; and 
T=the involved temperature in .degree.K. 
t=value at time t. 
From Equation (1), there is associated the expression 
EQU RTn=P.sub.t V.sub.t, (2) 
in which the actual moles of gas transmitted is the change of "n" with "t" 
(time) per the expressive denotation: 
EQU dn/dt, (2') 
so that: 
##EQU2## 
Since the downstream (or "collecting") volume is constant in systems 
embodied in accordance with the present invention, it logically follows 
that: 
EQU -dv/dt=0 (4) 
providing for the mathematical statement that 
##EQU3## 
Accordingly, since 
##EQU4## 
then and thereby letting 
##EQU5## 
wherein: 
P.sub.2 =the pressure at time t.sub.2 ; and 
P.sub.1 =the pressure at time t.sub.1. 
It logically follows from the foregoing, utilizing the appropriate calculus 
thereto and therefor, that: 
##EQU6## 
Since the GTR is a function of the "driving pressure" (i.e., "P.sub.d ", 
which in actuality is the difference in pressure on each side of the 
transmitting membrane or like or equivalent) and the involved transmitting 
area (i.e., "A"), it can be concluded that: 
##EQU7## 
wherein: 
P.sub.d =P.sub.i -P.sub.t, in which 
P.sub.i =the inlet (or upstream) pressure; and 
P.sub.t =the pressure at any given time "t". 
With respect to the above given Equation (7), it can be taken into account 
that when the thickness of a given sample undergoing test (taken in mils, 
i.e., 0.001 inch or 0.00254 centimeter per mil) is utilized in place of 
unity over the "P.sub.d A" term, the resulting value obtained is the 
"permeation coefficient" for the involved material. 
In any event and in further regard of Equation (7), it can be set forth 
that for time periods in which the pressure of the transmitted gas (or 
vapor) is increasing linearly with time, the value according to following 
Equation (8) can be obtained when P.sub.t is averaged over the time period 
t.sub.1 to t.sub.2, namely: 
##EQU8## 
Substituting Equation (8') above into Equation (7), it is thereby 
mathematically provided that: 
##EQU9## 
One common system of units for reporting GTR's, at least in the United 
States of America, is in cm.sup.3 /24 hr. atm. per 100 in.sup.2. 
From Equations (5) and (7), it can be stated that: 
##EQU10## 
Pursuing that to arrive at figures or values in the above indicated units 
for GTR: 
##EQU11## 
Substituting for R and T in Equation (12): 
##EQU12## 
Accordingly and in conclusion for calculation purposes: 
##EQU13## 
wherein .degree.F=the test temperature taken in .degree.F. 
In line with indications previously herein made, and as is intrinsically 
evident in the foregoing equational formula for GRT calculation, one great 
advantage in adaptation of the present invention to previously known 
techniques is that, even though the involved pressure differential is 
changing all of the time throughout the time period involved in the test 
(i.e., the ".DELTA.P-P.sub.d " factor, as is the rate of moles of gas or 
vapor permeated through the sample a result of the "dn/dt" factor so as to 
cause constantly differing conditions in performance of the test), the 
same accurate GRT value will always be found. 
To particularly illustrate tests run in practice of the present invention, 
three (3) individual samples having a generally dished and curvilinear 
contour with a thickness of about 1/4 inch (ca. 0.635 centimeter) were cut 
into about 21/2 inch (ca. 6.35 centimeters) diameter from a nominal 3-inch 
(ca. 7.62 centimeters) diameter section of tubular extrudate of "TEFLON" 
(Reg. TM) Brand polytetrafluoroethylene material obtained from E. I. 
duPont deNemours & Co., Inc. of Wilmington, Del. 19899. Usually an 
assembly analogous to that depicted in FIG. 9 of the Drawing, the samples 
(after precise average thickness determination of each) were loaded into 
the three (3) accommodating cells utilized and, after assembly therein, 
were placed in the constant temperature oil bath. The above-described 
start up procedure first using nitrogen gas; after which the gas 
transmission rate was determined per the foregoing Equation (14). 
Following this, another run was made on identical samples with more 
nitrogen gas; and then yet another run with helium gas. 
The results obtained are set forth in the graphical representation of FIG. 
10 of the accompanying Drawing. In connection with that, it is to be noted 
that if the log of the difference between the inlet (or upstream) and 
outlet (or downstream) pressures of the gas utilized in each test are 
plotted versus time, the slope of the resulting linear curve is 
proportional to the associated gas transmission rate. 
In correlation with the indicated results, the following tabulation sets 
forth the determined permeation coefficients (available in the indicated 
Equations) for each sample and gas tested with literature values given for 
same. 
TABLE 
______________________________________ 
Obtained Permeation Coefficients Gotten 
In Testing vs. Literature Values* 
______________________________________ 
Gas Line N-1 In Line N-2 In Literature Value 
Involved 
FIG. 10 Graph 
FIG. 10 Graph 
Given As L-1 
______________________________________ 
Nitrogen 
1,900 2,400 1,500 
______________________________________ 
Line H-1 In Literature Value 
Literature Value 
FIG. 10 Graph 
Given As L-2 Given As L-3 
______________________________________ 
Helium 11,100 9,500 16,500 
______________________________________ 
*Note: 
In the given determinations, allowances for various manufacturing 
processes of involved materials and miniscule density differences were no 
taken into account; even though and notwithstanding, such differences are 
obviously and clearly of a very small order of magnitude. 
By way of recapitulation, it is plain from the foregoing that, in 
comparison with the system and embodimentation thereof of the present 
invention (here, referred to, as a matter of convenience in 
identification, as a "DyGTC"), the predominant equivalent units of prior 
art apparatus and procedure for the same purpose are quite inferior. These 
are the above mentioned "Dow Gas Transmission Cell" (presently called, 
also for convenience, the "DGCT") and the "Linde Permeability Cell" 
(likewise presently called the "LPC"). 
Thus, as has been brought forth: 
(i) The DGTC and the LPC require a constant .DELTA.P and temperature in 
order to secure accurate data; whereas the present DyGTC requires only 
constant temperature (which may result in shorter actual test run 
consummation times to arrive at suitable GRT values); 
(ii) The DGTC is temperature limited--usually operable at only about normal 
Room Temperatures, this being due to the differences in expansion 
coefficients involved for the three (3) diverse materials utlized in 
operation thereof; 
(iii) The DGTC is pressure limited (this being usually at atmospheric 
pressure or less), whereas the DyGTC as well, candidly compared, as the 
LPC is (are) pressure and temperature limited according to the 
characteristics in such particulars of the particular plastic material in 
the sample being tested; and 
(iv) The LPC is not self-monitoring, whereas both the DyGTC and the DGTC 
are. 
In connection with the immediate foregoing and regardless of its 
operational superiority, DyGTC units pursuant to the present invention are 
commonly found to be only approximately half to one-third as costly to 
embody or procure as are DGTC and/or LPC apparatus installations. 
Many changes and modifications can be readily made in and adapted to 
embodiments and practices in accordance with the present invention without 
substantially departing from its apparent and intended spirit and scope, 
all in pursuance and accordance with same as it is set forth and 
delineated in the hereto-appended Claims.