Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1[36SS'~S
The present invention relates to a process for ren-
dering a powder dustless and to an aluminum powder which is both
dustless and of sensitizing grade suitable for use in explosives
and blasting agents.
U.S. Patent Nos. 3,838,064 and 3,838,092, issued
September 24, 1974, to John W. Vogt et al for, respectively,
"Process for Dust Control" and l'Dustless Compositions Containing
Fibrous Polytetrafluoroethylene" discuss techniques for rendering
powders dustless using polytetrafluoroethylene.
An object of the present invention is to provide a
process by which a powder can be made dustless.
Another object of the present invention is an aluminum
powder product which is both dustless and of sensitizing grade
suitable for use in explosives and blasting agents.
These as well as other objects which will become
: apparent in the discussion that follows are achieved, according
to the present invention, by providing a process including
mixing a wet, minus 200 mesh powder with liquid-dispersed, minus
60 mesh polytetrafluoroethylene particles, the mixing being
carried on for a time sufficient to make the powder dustless, as
determined by comparative pour test Y, when dry, the mixing
action being insufficient to provide a matrix of polytetraflu-
oroethylene fibers; and an aluminum powder which 1) is of sen-
; sitizing grade as determined by test X, and 2) is dustless as
determined by comparative pour test Y.
: A. Test X
In test X, the sensitizing action of candidatealuminum powders is assessed in the following formulation:
Ammonium nitrate 59.5 parts by weight
Water 28.7
Aluminum powder 10.0
Guar gum 1.5
pH buffer ~phosphate) 0.3
~1-
A suitable guar gum is Guartec 185, availab~e from General Mills
(Minneapolis, Minn.). After mixing, this formulation has a
density o 1.05-1.10 g/cc, and a p~ of 4.5. It is packed into
polyethylene tubes 1-1/4 inches in diameter and 16 inches long,
using a cardboard tube to assure a uniorm diameter along the
entire length. The charge thus prepared is initiated with a
standard $8 electric blasting cap. If detonation occurs, the
aluminum powder is considered herein to be of sensitizing grade.
B. Test Y
The comparative pour test Y is explained as follows.
Whereas the dusty nature of moderate to heavy dusting
powders is readily apparent from simple handling operations,
i.e., by scooping, tossing small quantities short distances into
the air, or transferring a small amount from hand to hand, the
dusting tendency of marginally dusty powders is not so apparent.
A simple test has been devised to aid in the visual iaentifica
tion of a "dusty".powder.
A standard dispensing burette of 500cc capacity is
employed. The cylindrical section of the burette used in our
tests measured approximately 51cm of the total 66cm burette
length. The cylinder had an internal diameter of about 3.7cm.
Candidate powder of questionable austiness is trans-
ferred into the burette, with stopcock closed, by spooning or
careful pouriny to within about 0.5cm of the top. The burette is
then attached to a burette support stand by means of a clamp
holder and an extension clamp. ~he clamp holder is affixed to
the support stand at a predetermined height so that when the
extension clamp holding the burette is rotated to turn the
burette upside down, the top of the burette is 25cm ~rom the base
of the support stand. The extension clamp may be left affixed to
the burette during cleaning and filling operations, and the clamp
holder to the support stand to maintain the same height for all
-- 2 --
tests.
After attaching the burette to the support stand, a
receptacle large enough to hold all the powder in the burette is
placed beneath the burette. A piece of aluminum sheet or other
material, large enough to cover the top of the burette, is held
by hand over the cylindrical opening of the burette as the
burette is rotated 180 to an upside-down position over the
receptacle. The stopcock is then opened to provide access of the
top of the powder column to the atmosphere. The sheet is then
removed from tha burette opening, allowing powder to fall into
the receptacle.
Using this technique, the individual (the operator)
running test Y loads the burette with Alcoa grade 1220 aluminum
powder having 90 weight-~ of its particles greater than 325 mesh
(U.S. Standard) and 10 weight-% less than 325 mesh. He then
observes how it dusts when the sheet is removed and it falls into
the receptacle. Next, the aluminum powder of unknown dusting
characteristic is loaded into the burette and it is allowed to
fall too into the receptacle from the standard height. By visual
comparision, the operator determines whether the dusting result-
ing upon the falling of the unknown into the receptacle is less
than that of the standard wherein the grade 1220 aluminum powder
is allowed to fall. If the dusting is less, then the un~nown is
classed "dustless as determined by comparative pour test Y".
Figure 1 is a flow diagram of an example of the process
of the invention.
Figure 2 is a graph of number-% of particles versus
channeI number or particle size.
Figures 3 to 7 are electron scanning photomicrographsl
with dimensions X, Y, and Z e~ualing~ respectively, 4, 10, and 5
microns.
Referring to Figure 1, there is illustrated a typical
process for producing al~inum Elake powder. The chapter enti-
tled "Aluminum Flake Pigment" by RolE Rolles, appearing in the
PIGMENT HANDBOOK, Volume I, John Wiley & Sons, Inc., 1973,
contains background material on such processes known by those
skilled in the art. The arrows in Figure ]L indicate the flow of
material through the process.
In this typical process, atomized aluminum, or alter-
natively aluminum foil scrap, is charged together with mineral
spirits and stearic acid into the ball mills 10 from a rec-tan- -
gular hopper 12. The ball milling flattens and comminutes the
aluminum charge to bring it into flake form, and, during this
milling, the flakes obtain a stearic acid coating to be given
what are referred to in the art as "leafing" properties. Essen-
tially, a leafing pigment tends to rise to the surface o~ coatings
into which it is incorporated. The ball mill discharge moves
through pipeline 14 into the upper sludge tank 15, which is
essentially a holding tank, and ~rom there down through screen
18. The o~ersized particles retained by screen 18 can be placed
in hopper car 12 for recycling.
The material which passes through screen 18 moves into
the lower sludge tank 20 and from there through pipeline 22,
pump 24, and then through pipeline 26 into the filter 28. The
filter cake obtained from this filtration operation can be mixed
with added solvent in mixer 32A and packed as a paste product at
packing sta~ion 30. Al~ernativeIy, the cake may be blended
without added solvent in mixer 32A and can be sent to a drier
operation 32B in order to make a dry product. A dry product can
be packed from pipeline 34 or else hopper car 36 can be used to
transfer charge material into a brush polishing operation at 38.
Again the discharge from the brush polishing operation can be
moved to a hopper 40 for packing in drums 42.
The process o-f Figure 1 may be used, by those skilled
6~S~5
in the art, to manufacture a sensitizing grade aluminum powder.
Both surface area and surface chemistry of aluminum powders are
important factors to their sensitizing activity, as demonstrated,
for instance, in Examples 18 through 20. A minimum surface area
in the range of 2.0 to 2.5 m2/g is required for detonation under
the conditions of text ~. Other formulations, howe~er, would be
expected to demonstrate different threshold surface area levels,
either higher or lower than that of test X, depending on the
nature and ratios of ingredients used and the diameter and length
of the prepared charge. However, surface areas lower than 1.5 to
; 2.0 m2/g are generally ineffective to sensitize the majority of
blasting agent formulations presently known to the industry.
Aluminum powder having surface areas exceeding 2~5 m2/g
will not detonate in test X unless the surface is also hydro-
phobic. A coating of stearic acid has been found to produce this
necessary water-repelling property. The effectiveness of a
nonwettable surface is thought to be attributable to the ease
with which an air layer or bubbles are carried into the slurry
via this surface. These tiny volumes of air thus associated with
the aluminum surface are adiabatically compressed as the detona-
tion front passes through, and become hot spots that accelerate
chemical changes taking place in the explosive charge, especially
those at the aluminum surface whose location is a fuel-oxidizer
interface.
Further concerning the properties of sensitizing grade
aluminum powder, it is to be noted that when standard leafing
grade pigment powders such as Alcoa grades 302, 402 or 415 ~for
properties, see Tables 3 and 4 in the above-referenced chapter
"Aluminum Flake Pigment") are used in text X, the charge deto-
nates. Thus, these grades are termed sensitizing grades. Incontrast, when fuel-grade aluminum powder, such as Alcoa grade
101 atomized aluminum which is recited as Example 13 of Table I
3L~i55~ )
in U.S. 3,838,06~ or Alcoa grade 120 atomized aluminum (surface
area equals 0.1 to 0.3 m2/g), is used in test X, detonation does
not occur. The qualification "fuel-grade" is used to indicate an
aluminum powder which is provided in an explosive formulation
solely for the heat which it evolves when reacting and not for
purposes of sensitizing.
Test X may also be employed to assess degrees of sen-
sitizing activity among a group of aluminum sensitizer powders.
When Alcoa grade 120 is used in the formulation of text X,
portions of this 10 parts of aluminum powder may be replaced by
the candidate sensitizer until detonation occurs. The amount of
sensitizer grade needed to replace #120 in the formulation serves
as an index to the degree of sensitizing activity. Good sensi-
tizers such as Alcoa grade 1660 (surface area 5 m2/g, stearic
acid coated), and standard leafing grade pigment powders cause
detonation to occur when only 2-3% of the #120 fueI has been
replaced.
Furthe`r characterizing the product of the present
invention is the following test, herein referred to as the
'iHartmann Test", to determine the minimum explosive concentration
o~ dust in air. It has been developed by the U.S. Bureau of
Mines, and is described in their Report of Investigations No.
56~4, "Laboratory Equipment and Test Procedures for Evaluating
Explvsibility of Dusts". This test utilizes a Hartmann Explo-
sibility Tester, an apparatus capable of dispersing controlled
quantities of dust into a chaMber and igniting it with an induc-
tion ~park. As employed here, conditions of the test include a
1/4 inch electrode separation, lOOv charging voltage, 27.5
milliamperes current and a rupture disc of llcm filter paper
(Fisher Catalog No. 9-795). The following guide is used to
interpret the results as determined on the Hartmann apparatus for
aluminum powders:
S25
Minimum Explosive ~elative ~xplosive
Concentration ~Iaæard
. . . ~ . . .
0.02-0.06 oz/cu. ft. Severe
0.06-0.12 Strong
0.12-0.30 Moderate
0.30-0.60 Weak
~ 0.60 None
On the basis of this yuide and ~artmann Test results,
Alcoa grade 120 atomized powder rates "strong" as an explosive
hazard, and Alcoa grade 1220 ~a medium coarse grade) as a mod-
erate explosive hazard. Sensitizing grade aluminum powders as
produced dustless according to the teachings of this invention
most preferably do not ignite sufficiently to burst the paper
diaphragm when concentrations in excess of 0.60 ounce per cubic
foot are tested and are considered not to be an explosive hazard.
When lower polytetrafluoroethylene concentrations are used,
dustless aluminum powders according to the present invention will
represent weak relative explosive hazards and, at the most,
; moderate relative explosive hazards. Normal, dusty grades of
sensitizing aluminum powder, such as Alcoa grade 1660, rate as
severe explosive hazards.
As has already been emphasized above, the aluminum
powder product of the present invention combines two charac-
teristics. It is of sensitizing grade, and it is dustless.
This combination of properties has never before been obtained.
Aluminum powders which have been used previously as sensitizers
were always dusty, and consequently required special care to
avoid accidental explosions. The aluminum powder of sensitizing
grade and which is also dustless according to the present inven-
tion consequently represents an extremely important advance
in the art.
Also, as is indicated above, such a product is
obtained by mixing wet, minus 200 mesh aluminum powder with
liquid-dispersed, minus 60 mesh polytetrafluoroethylene particles,
the mixing being carried on for a time sufficient to make the
1~i5~
powder dustless, as determined by comparatlve pour test Y,
when dry, the mixing action bein~ insufficient to provide a
matrix of polytetrafluoroethylene fibers. The qualification
"minus" with reference to mesh sizes means that the particles
have sizes such that they will pass through the named mesh
size. In the case of minus 200 mesh powder, this means that
all the particles are less than 74 microns. Reference herein
is to U.S. Standard sieves. With reference to Figure l, a
wet, minus 2Q0 mesh aluminum powder can be found, for example,
in lower sludge tank 20, in pipelines 22 and 26, at the pump
24, and at the filter cake discharge of filter 28. When using
water-dispersed, minus 60 mesh polytetrafluoroethylene particles,
it is preferred to perform the mixing with the wet, minus 200
mesh aluminum powder after the filtration, because then one
does not have to be concerned with the problem of water getting
into the mineral spirit filtrate. However, mixing may also be
effectively accomplished at stages prior to filtration, for
example in lower sludge tank 20.
Preferred percentages of polytetrafluoroethylene lie
between 0.1% and 0.6~, as based on the weight of the aluminum
powder.
The particular time over which mixing is conducted does
not appear to be of critical importance to the broader concept of
the invention. It appears that it is only necessary to dis-
tribute the polytetrafluoroethylene particles in the aluminum
powder, with greater amounts of mixing only contributing to
greater uniformity of the distribution. 21ixing times between
l/2 minute and 8 hours have been used. Post-treatment operations
to modify the nature of the dustless product, such as polishing
in polisher 38, also are not critical to the intentions of this
invention, so long as such operations are not so severe as to
again render the product dusty. Unlike the situation described
in the above-mentioned U.S. Patent Nos. 3,838,064 and 3,838,092,
microscopic study of the product of the present invention, as
resulting from the process, does not reveal that t~e aluminum
particles have been enmeshed in any matrix of polytetrafluoro-
ethylene fibers. No such matrix is in fac:t observed, with only
random fibers being sometimes found, in separated areas of the
product powders, and this is startling ancl unexpected in view of
the teachings of those patents.
Further illustrative of the present invention are the
following example~s:
General Nature of Examples:
1 through 4, - Successful processes for making
21 and 22,
32, 33, 36 dustless sensitizer according to
the invention.
- Control, Teflon-free run (dusty
sensitizer).
6 through 8 - Unsuccessful trials using K20 slurry.
9 through 11, - Unsuccessful trials using K10 powaer.
12B and 13
14 - Unsuccessful trial using another
2n
fibrillatible material, Swift 2185
fluid protein.
- Example of a traditional method of
making dustless powder, illustra~ing
loss of sensitizing activity by such
methods.
12A, 16, 17,- Properties of Teflon K.
23, 24, 37
18 - Illustration of importance of surface
area to sensitizing properties.
.30 19, 20 - Illustrations of importance of
chemical nature of aluminum surface
to sensitizing properties.
_ g _
s~
25, 26, 29, - Unsuccessful laboratory attempts at
and 30
making dustless powder.
27, 28, and - Successful laboratory attempts at
making dustless powder.
33 - Effect of Teflon concentration on
sensitivity, Hartmann Test.
34 - Covering test.
- Unsuccessful trial using Teflon 6.
38 - Al-Teflon size relationship.
Example 1
Filter cake e.g. from filter 28, consisting of ap-
proximateIy 80~ by weight aluminum flake pigment particles r 1%
stearic acid and the remainder mineral spirits, was charged to a
Stokes ~Pennwalt Corp., Philadelphia, Pa~) vacuum drier unit 32B
of 20 cubic feet capacity equipped with a rotary mixing blade.
Teflon K type 20 slurry, a DuPont (Wilmington, Delaware) product
consisting of Teflon (polytetrafluoroethylene) particles having
an a~erage diameter of about 0.2 micron dispersed in water, was
added to the drier in the amount of 0.5% Teflon solids on the
total aluminum weight. The drier was closed and mixing was
started and allowed to proceed for approximately one minute.
Vacuum (about 28 or 2-9 inches of mercury measured downwards from
atmospheric pressure as zero) was then applied and heating was
initiated by introducing steam (230F) into a jacket around the
drier.
After volatile materials had been distilled from the
drier charge, the drier was cooled while under vacuum by dis-
continuing the applicati-on of steam and instead introducing cold
water to the jacket. The drier was pressurized to atmospheric
pressure with inert gas, and the product was discharged. The
aluminum flake powder was dustless and was of sensitizing grade.
-- 10 --
~sæ~
Example 2
The procedure of Example 1 was repeated, except
that only 0.25~ Teflon based on aluminum weight was added to
the drier as Teflon K type 20 slurry. The product powder
was again dustless.
When subjected to test X, the dustless product
caused detonation, indicating that it is of sensitizer
quality.
When substituted for only part of the nonsensitizer
grade 120 powder in test X, the dustless product caused
detonation at a replacement le~el of only 3% ~that is, the
formulation contained 7% grade 120 and 3% dustless product,
maintaining a total of 10% aluminum), indicating that its sen-
sitizing activity is exceIlent.
When sub~ected to the Hartmann Test, the dustless
product did not ignite at a concentration of 0.813 oz./cubic
foot, indicating that it is not an explosive hazard.
When subjected to a Stability Test, the dustless
product generated no gas, indicating that the hydrophobic stearic
acid coating had not been disturbed by the Teflon treatment.
The Stability Test, devised to assess the reIative
degree of hydrophobicity of aluminum powders, is described as
follows:
The apparatus consists of a 250 ml Erlenmeyer flask
fitted with a one-hole rubber stopper, into which a micro-con-
denser is placed. A length of flexible tubing leads from the
condenser to a 25 ml burette, which is inverted in a beaker of
water to collect evolved gases.
l.Og candidate aluminum powder and 150 ml of a 20
(wei~ht) aqueous ammonium nitrate solution are added to the
Erlenmeyer flask. The rubber stopper with condenser is inserted,
and the flask is placed in an oil bath heated to 200F. All
flasks of a multi-t~s-t series are immersed to the same level in
the bath, and agitat:ion of the oil is adjusted for maximum
stirring without splashing around the flasks, 50 as not to disturb
temperature equilLbrium of protruding sections with the room
atmosphere.
Gas generated duriny the first hour of testing is
largely composed of expanded head-~pace gases forced from the
flask while th~ contents reach temperature equilibrium, and these
are allowed to escape by opening the stopcock of the gas-collec-
lQ tion burette. After one hour, water is drawn into the burette byapplying ~uction, the stopcock is closecl, and the generated gas
is collected and measured for a six-hour period. The burette is
refilled as needed.
A blank consisting oP a flask containing ammonium
nitrate solution but no aluminum, is included in every test
series to correct ox changes in atmospheric conditions. The
volume of gas generated by the blank is subtracted from that
generated by the other samples in test at that time.
Gassing occurs as a result of attack o~ the aluminum
surface according to the follow:ing reaction:
2~1 ~ 6H20 ~--~ 2Al(OH~3 + 3H2
Unprotected sur~aces generate gas in proportion to their surface
areas. AlcQa grade 120 medium-coarse atomized powderl with a
nominal surface area of 0.2 m2/g generates about 8-12 ml gas in
this te~t, while Alcoa grade 1401, a very fine atomized powder
with a nominal surace area of 1.0 m2/y, generates about 50-60 ml
~as Sensiti~ing grades, such as Alcoa grade 1660, whose surace
areas range from 3-6 m2/g but whose suraces are protected by a
hydrop~obic coating oE stearic acid, generate virtually no gas in
3Q this test, the experimental results typically alling in the
range 0.0 to 1.0 ml gas.
- 12 -
Example 3
The procedure of Example 2 was repeaked. At the end
of this cycle, the drier was opened to examine the product
powder r which was seen to be dustless. The drier was re-closed,
and the charge was subjected to a second drying cycle. At the
end of this "double-drying" operation, the product powder re-
mained as dustless as after the first c~cle.
Example 4
Filter cake of the kind used in Example 1 was premixed
in the mixer chamber 32A of Figure 1 with 0.16~ Teflon K type 20
(as total Teflon solids based on aluminum weight), and the
resultant mixture was charged to the drier unit 32B. After
drying in the standard manner as described in Example 1, the
product powder, when discharged ~rom the drier, was essentially
dustless.
When subjected to the Stability Test, this dustless
product generated no gas; when subjected to test X, it caused
detonation. It also caused detonation in test X when it was used
to replace 3% of grade 120 contained in that ormulation.
Example~ 5
Filter cake of the kind used in Example 1 was dried
without any Te~lon present. The pxoduct powder was extremely
dusty. It generatPd no gas when subjected to the Stability Test,
and caused detonation when subjected to test X. It also caused
detonation in test X when it was used to replace 3% of grade 120.
When subjected to the Hartmann Test, the product
powder ignited at a minimum explosive concentration of 0.045
oz./cu~ic oot, indicating that it is a severe explosive hazard,
necessitating special handling procedures in production opera-
~ions.
Example 6
The procedure of ~xample 2 was repeated, except tha~
- 13 -
s~
the sequence o~ operat.ions was altered. ~Ieat and vacuum were
applied to the drier for approximately two minutes before mixing
was started. This chan~e in sequence is the only point of
difference ~rom the procedure of Example 2.
The product powder was extremely dusty, havin~ the
visual appearance o~ the product resulting from Example 5. The
temperature inside the drier at the time mixing was started did
not exceed 110F, and the atmosphere inside the drier had not yet
reached full vacuum, according to indicatin~ probes and gages
attached to the drier, so it is most unlikely that any signifi-
cant amount o~ liquid had been lost from the drier charge during
this short period of time. The failure of this cycle to produce
dustless powder is thought to be due to an increase in size of
the Teflon particles as a result of the temperature increase.
Ex'ample 7
10~ by weight Cab-O-Sil M5 fumed silica, a product of
Cabot Corporation (Boston, Mass.), was mixed into Teflon K
type 20 dispersion, thereby absorbin~ water and producing a pasty
mass. This silica-Teflon paste was employed in a drier cycle
duplicating that of Example 2.
The product powder was extremely dusty, visually
resembling the product o~`Example 5.
Example 8
Filter cake was dried according to Example 5, and the
dusty product powder was left in the drier. Teflon K type 20
dispersion was added in the amount of 0.25~ Teflon solids on
aluminum weight. The drier was re-closed and a second drying
cycle was made in the same manner. At the end of this cycle, the
prod~ct powder remained extremeIy dusty.
Example 9
The procedure of Example 1 was followed~ except Tef-
lon K type 10 powder was substituted for Teflon K type 20 slurry.
- 14 -
~s~
The solid Teflon comprising types 10 and 20 are reportedly
identical chemically; however, the average particle size of
type 10 powder is 500 microns.
After drying, the product powder was extremely dusty,
and could not be distinguished visually from Teflon-free powder
as produced in Example 5.
Example 10
Aluminum powder as produced in Example 5 was trans-
ferred to brush polishing unit 38, and 0.5% by weight Teflon K
type 10 powder was added. The polishing unit contains rotating
brushes which maintain contact with the inner wall surface of the
polisher drum. See, for example/ U.S. Patent No. 1,930,683 of
Erwin Kramer issued October 17, 1933, for a "Polishing Machine
for Colored Pulverulent Bronze". During a polishing cycle, the
powder contents of the polisher are continually rubbed and
smeared. Frictional heat created by this process raises the
polisher temperature to 100-120F.
The aluminum and Teflon powders were polished together
for a period of 12 hours. At the end- of this time, the product
was extremeIy dusty, not being visually distinguishable from
powder of Example 5.
EXample ll
Teflon K type 10 powder was added to a slurry of
leafing quality (i.e. stearic acid coated) aluminum flake slurried
in mineral spirits, in the amount of 0.5% on total aluminum
weight. After agitating to disperse the Teflon particles, the
slurry was filtered. The Teflon-containing filter cake was
thereafter dried in the mann~r of Example 5. Powder produced as
a result of this sequence of treatments was extremely dusty.
Example 12A
Fifteen grams Teflon K type 10 powder and 60 ml mineral
spirits were charged to a one liter capacity vibratory mill
r~
containing 1/4 inch steel balls as grinding medium. This com-
position was milled for 15 minutes, after which the slurry was
examined. The Teflon was found to have absorbed most of the
mineral spirits--only about 10 ml drained free. The Teflon and
mineral spirits formed a soggy mass among lhe steel balls. The
entire mixture was allowed to air dry at room temperature.
After drying, much of the Teflon adhered to the steel
balls as a discontinuous, filmy coating, and had to be mechani-
cally dislodged. Under low power microscopic examination, the
Teflon had a film-like appearance with a small percentage of
tape-like fibers. The film had very poor tensile properties,
tearing easily under mild probing. No fine Teflon particles
could be found.
Example lZB
Teflon K type 10 powder, at a concentration of 0.3%
based on total aluminum weight, was charged into ball mill
unit 10 along with atomized al~inum powder, mineral spirits, and
stearic acid. After milling and filtering, filter cake from this
batch was dried as in Example 5. The` product powder was extremely
2Q dusty.
Qbservation of the screening operation indicates that
a large proportion of the Teflon was removed with oversize
aluminum particles. A discontinuous filmy coating on the screen,
which would not break apart or pass through the screen, was seen
to significantly slow the rate of screening.
Example 13
The procedure of Example 12B was repeated, except that
the proportion of Teflon K type 10 powder was increased to 0.75%
of the aluminum weight. After drying, the product powder was
again extremely dusty. Screening of the milled slurry was
especially difficult, resembling that of Example 12B.
- 16 -
5~5
Example 1~
The procedure of Example 1 was repeated, except that
the Teflon K type 20 slurry was replaced by Swift 21g5 fluid
colloid. The latter, a product of Swift Chemical Company, is a
refined collagen protein extract which has binding properties,
and which also ~ibrillates on being rubbed or smeared. A con-
centration of 10% protein solids on the aluminum weigh-t was
employed.
The product powder was somewhat reduced in dustiness,
but was still highIy dusty. Some hard clumps of powder occurred
within the batch, and these were very difficult to break apart.
Example 15
Filter cake dried according to Example 5 was later
mixed with diethanolamine in the amount of 10% based on aluminum
weight. Treatment with this organic liquid resulted in a dust-
less product. When subjected to the Stability Test, the powder
dispersed easily in the aqueous test solution and ~isually did
not appear to be hydrophobic. It generated 40 ml gas in this
test, indicating that the stearic acid coating had been dis-
turbed. When subjected to test X, it did not cause detonation,indicating that it had lost its sensitizing capabilities.
Example 16
Particles of Teflon K type 10 powder were placed on
the heating stage of a Fisher-Johns Melting Point Apparatus. The
instrument was turned on, and the particles were observed through
a magnifying glass. It was noted that, in the 40-50C range,
the dull, white particles passed through a transition wherein the
surface appeared smooth and highIy glossy. Further, touching
particles were seen to stick and fuse together. However, no
30 melting was observed; the particles essentially retained their
original shapes. When probed with a needle, particles s-tuck to
the point and could not be dislodged by shaking.
- 17 -
s
The temperature of the heating stage was allowed to
increase to 275C, but no further change in the visual appearance
of the particles was seen to occur.
Example 17
The experiment of Example 16 was repeated with a new
sample of Te1On K type 10, but flakes o aluminum powder were
sprinkled on the Teflon particles before turning on the appa-
ratus. When the heating stage reached the 40-50C range, the
Teflon particles were again seen to enter the transition noted in
Example 16. Aluminum flake particles lying on the larger par-
ticles of Teflon appeared to stick to th~ Teflon surfaces on this
oGcasion and could not be dislodged by probing with a needle.
Example 18
Samples of aluminum powder having various surface area
to mass ratios were coated with stearic acid and subjected to
test X. Results are ~abulated.
Surface Area Test X:Result
0.2 m2/g Did not detonate
1.0 Did not detonate
2.2 Incomplete detonation
2.5 Detonatea
4.2 Detonated
8.3 Detonated
This series of tests indicates khe existence of a threshold
surface area to mass ratio below which sensitizing activity is
unsatisfactory. For the formulation and charge diameter emplo~ed
in text X, this threshold value is close to 2.5 m2/g.
E ample 19
~ slurry of atomized aluminum powder in mineral spirits
was milled in a rotating ball mill in the absence of any lubri-
cant. Milling was continued until the aluminum particles had a
surface area -to mass ratio o~ approximately 4 m2/g. The slurry
was screened through a No. 170 standard U.S. Sieve to remove
coarse particles, and was subsequently filtered and dried. The
product powder had a measured surface area of 4.2 m2/g. One
- 18 -
s~
portion of this powder was then separated from the batch and the
particles were coated with stearic acid. Another poxtion was
coated with Vinsol Ester Gum, a product of ~lercules (Wilmington,
Delaware). A third portion was given an inorganic conversion
coating by treatment with a methanolic solution of phosphoric
acid r and was re-dried. A fourth portion was left untreated.
The four samples thus generated were each subjected to
test X. Only the stearic acid coated alum:inum powder caused a
detonation; the other three samples did not detonate.
Example:20
The procedure of Example 19 was repeated, with the
exception that a longer milling time was employed. The proauct
powder had a measured surface area of 8.3 m2/g.
Four samples were again generated: three coated and
one uncoated. On subjecting these to test X, only the stearic
acid coated aluminum powder caused a detonation, again, the other
three samples did not detonate.
~: Of the two groups of four samples generated in Ex-
amples l9 and 20, only the stearic acid coated powders were
completeIy hydrophobic. That is, when stirred by hand in a
beaker of water, the powder floated on the surface. The uncoated
: and conversion coated powders became wet and sank immediately,
whereas the organic coated powder was partially wettable, with
some sinking and some floating.
These two Examples l9 and 20 illustrate the importance
of the chemical nature of the aluminum surface towards the
powder's sensitizing activity.
Example 21
In this new modification of the process of the inven-
tion, the Teflon K type 20 slurry was added to a pr~slurriedmixture of aluminum flake in mineral spirits obtained e.g. from
lower sludge tank 20, at a concentration of 0.25% Teflon solids
-- 19 --
.
5~
based on aluminum weight. After this initial premix with the
aluminum slurry, the resultant slurry was filtered and the filter
cake transferred to mixing unit 32A for a very short mixing
period (approximately 10 minutes). This filter cake was then
charged to drier unit 32B at which point i~ was dried in a normal
manner. The product powder was dustless and of sensitizer grade.
The ~Iartmann Test indicates that the dustless product is not an
explosive hazard.
Example 22
The procedure of Example 21 was repeated, with the
exception that the concentration of Teflon K type 20 slurry was
~reduced to 0.10% Teflon solids based on aluminum weight. The
product powder was dustless and of sensitizer grade. When
subjected to the Hartmann Test, the product powder rated as a
moderate explosive hazard.
Example-23
Aliquots, approximately 10 ml in volume, were taken
from a single batch of Teflon K type 20 slurry and were placed
into two test tubes. One tube was held at room temperature and
the other was immersed for a period of two minutes, without
stirring or other agitation, into an oil bath maintained at
200F. Both tubes were thereafter held at room temperature for
approximately 2 hours.
A portion of each sample was analyzed on a Model B
Coulter Counter modified with a Tracor Econ II Series Pulse
Height Analyzer and log amplifier to categorize the particle
sizes into 256 channels, using a sodium chIoride electrolyte
(Abbott 6205-36) and a 70 micrometer aperture. A standard
latex, the majority of whose particles are about 3.5 micrometers
in diameter, was also analyzed to serve as an index maxker.
The results are illustrated graphically in Figure 2.
Number-~ is the number of particles of any given size divided by
- 20 -
~i5~
the total number of particles, times 100. rrhe stanclard late~ is
seen to peak in channel 14, indicating that particle~ 3.5 micro-
meters in diameter are registered by this channeI. The aliquot
of Teflon K type 20 slurry which had been maintained at room
temperature peaks off scale at the fine particle end; thus, the
average particle size is lower than 2 micrometers, which ap-
proximately defines the lower sensitivity limit of the Coultex
Coun~er with this particular aperture in place.
The aliquot of Teflon K type 20 slurry which had been
briefly heatedl however, is seen to peak in channels 15 and 16,
so is slightly coarser than 3.5 micrometers as registered by
channeI 14. This shift in particle size distribution after
heating indicates that some nonreversible coagulation has occur-
red and is consistent with the behavior of Teflon K type 10
;~ powder observed in Examples 16 and 17. The three samples regis-
tered very few particles beyond channel 30.
Example 24
Within days following receipt of a batch of Teflon K
type 20 from the manufacturerl equal volumes of the Teflon
slurry and mineral spirits were poured into a separatory funnel
and shaken together. The Teflon slurry dispersed easily into the
mineraI spirits, forming a singlel milky phase which did not
visually appear to separate for 30 minutes. The aqueous slurry
:separated slowlyl the'settled portion having the same appearance
as the original Teflon K type 20.
Five months later, the same experiment was repeated
using the same batch of Teflon K type 20, which had been stored
at 70 ~ 5F during the interim. On retesting, the Teflon slurry
; was seen to settle immediately and was almost completely settled
one minute following agitation. This indicates that a~ing
affects the dispersion characteristics of Teflon K type 20.
.
- 21 -
~S5~
Example 25
Based on the manufacturer's reported average diameter
of 0.2 micrometers for particles comprising Teflon K type 20, and
500 micrometers for particles comprising Teflon K type lO,
average surface areas of 13.6 and 0.0054 m2/g were calculated,
respectively, for the two products, based on an intrinsic density
of 2.2 g/ml for Teflon K. Thus, Teflon K type 20 has 2500 times
more surface area than does type lO, assuming that the particles
are spherical.
10Example 21 demonstrates that 0.25~ Teflon K solids on
aluminum weight, the Teflon being in the form of type 20 slurry,
adequately renders the aluminum powder dustless. An equivalent
~uantity of Teflon K type 10 to match this amount of type 20 in
surface area would be 625~ type lO on aluminum weight.
A sigma blade style mixing chamber, Type S-650/M, was
connected onto a Recording Plasti-Corder Torque Rheometer Typ~
PL Vl51 (C. W. Brabender Instruments, South Hackensack, N. J.).
Ninety grams aluminum sensitizer ~lake were placed into the
mixing chamber, which was heated to 90C by means of a circula-
ting oil bath. While mixing at 50 rpm, Teflon K type lO wassprinkled slowly into the aluminum powder until a total of 50g,
or 55.6~ on the aluminum weight, had been added. The powder was
mixed for a total of 75 minutes, but remained dusty. This high
concentration of Teflon K represents an economically prohibitive
level for this particular product; the fact that the product
powder remained dusty indicates that this relatively simple
mixing action, typical of that present in drier unit 32B, failed
to generate sufficient new surface area from the Teflon K type lO
particles to render the aluminum powder dustless.
30Example 26
The sigma blade style mixing chamber of Example 25 was
replaced by a roller blade style chamber REO-6/SB with ram
- 22 -
`` ~L0~5~
closure. Using this unit, pressure may be exerted on the chamber
contents as they are being mixed.
The chamber was filled to its normal capacity with a
mixture of 40g aluminum sensitizer flake containing 0.6% by
weight Teflon K type 10 powder. The ram closure was positioned,
the chamber heated to 90C by means of a circulating oil bath,
and the roller blade was rotated at 60 rpm. The torque resisting
rotation as measured by the Plasti-Corder was 35 meter-grams.
The run was terminated after 25 minutes mixing time; the product
powder was fluffy and slightly dusty.
Example 27
The experiment of Example 26 was repeated, except that
50g aluminum sensitizer flake-Teflon K type 10 mixture was
forced into the chamber by packing with the ram closure. Torque
increased to 180 meter-grams as the powder was initially mixed
while positioning the ram closure, then quickly fell off to 130
meter-grams after 90 seconds. After 5 minutes, the torque
gradually began to increase, and reached a value of 615 meter-
grams after 15 minutes mixing time. The torque gradually de-
creased as mixing continued, and had fallen to 255 meter-grams at
35 minutes, at which point mixing was terminated. The product
was a shiny metallic, putty-like composition completely devoid of
dust. It could ~ot be dispersed in water, so is useless as a
sensitizing agent for slurry blasting agents.
The product was examined with the aid of a scanning
electron microscope, but, surprisingly, no fiber network of the
type described in U.S. Patent Nos. 3,838,064 and 3,838,092 could
be found. The material resembled the product of Example 28.
Using a needle-pointed probe, the product of this
Example was pulled apart. This operation produced a fiber
matrix in the stretched zone, which`is illustrated in Figure 3.
Broken strands can be seen throughout the photomicrograph, many
- 23 -
of which appear to be lying loosely on aluminum fla]ce particles.
This is the type of behavior that would be expected if two
surfaces were weakly stuck together by particles of an elastic
substance and the surfaces were then separated, as, for example,
chewing gum stuck between two hands clapped together.
Example 28
Nine grams aluminum sensitizer flake and one gram
TefIon K type'10 were mildly ground togethe:r in an agate mortar
with a pestle. No heating was employed other than frictional
heat generated by the'rubbing action. The mixture quickly became
gummy and paste-like, visually resembling the product of Ex-
ample 27. It could not be dispersed in water, so also is useless
as a sensitizing agent for slurry bIasting agents. Scanning
eIectron photomicrographs of representative sections from this
product, in Figure 4 and also at 5000 magnifications (not shown),
did not disclose any fiber network.
Ten grams aluminum sensitizer flake were slurried in
100 ml methanol by stirring slowly with a Teflon-coated stir bar
over a magnetic stirring plate. Fifty milligrams Teflon K type 10
powder were added, and the slurry was transferred to a 500 ml
; round-bottom flask which was then affixed to a Rinco Model VE1000
Rotating Vacuum Evaporator (Rinco Instrument Company, Greenville,
Ill.). The flask was positioned in an oil bath heated to 85C,
rotation was initiated, and vacuum was drawn. After 30 minutes,
.
all of the methanol had been evaporated and the flask was removed
from the apparatus. The product powder was examined visually,
and was found to be extremeIy dusty.
Example 30
150 mg Teflon K type 20 (50 mg Teflon solids) were
added dropwise to 100 ml methanol being stirred magnetically as
; in Example 29. The Teflon was seen to disperse easily into the
- 24 -
methanol, initially forming a milky slurry; however, after five
minutes, the milky characteristics had disappeared, and the
Teflon particles were seen to be coagulated in the form o:E
clumps, -Eilm particles and strings as the methanol became more
clarified. Ten minutes after adding the Teflon, the methanol was
essentially clear with the Teflon coagulum sluggishly moving in
the stirred medium. At this point, lOg aluminum sensitizer flake
were added, the slurry was transferred to a 500 ml round-bottom
flask, and the methanol was evaporated on the Model VE1000 as in
Example 29. The product powder was seen to be extremely dusty.
Example-31
Ten grams aluminum sensitizer flake were slurried in
100 ml methanol by stirring magnetically as in Example 29.
150 mg Teflon K type 20 was added dropwise to this slurry over a
period o~ 10-15 seconds, and the slurry was immediately trans-
ferred to a round-bottom flask which was then affi~ed to the
Model VE1000 Evaporator unit. The slurry was dried as in Ex-
ample 29. The product powder was seen to stick to the flask
walls as the slurry became thicker and ultimately dried, with no
tumbling of the powder within the rotating flask being noticed.
The product powder had to be dislodged from the flask walls by
scraping; it was found to be essentially dustless, in marked
contrast to the product powders of Examples 29 and 30.
Example.32
The:process of Example 22 was repeated using Teflon K
type 20 from the original batch, but which had been stored for a
period of three months si.nce the trial of Example 22 had been
made. This batch of Teflon slurry was seen to be less easily
dispersible in mineral spirits immediately prior ~o the second
run than pxior to the first run, as judged by shaking a small
quantity with mineral spirits in a test tube. Apparently, the
Teflon particles had coagulated slightly during the interim.
- 25 -
s~æ~
The product powder from this second run, using the
identical concentra-tion and procedure of Example 22, WA5 dusty.
The Teflon concentration had to be increased to 0.15% Teflon
solids on aluminum weight in order to produce a dustless product.
This need for a higher concentration of Teflon is thouyht to be
attributabIe to ongoin~, slow coa~ulation oE Teflon particles and
consequent reduction in the total number of particles available
to stick the aluminum flakes together.
Example~ 33
~ number of batches of dustless aluminum flake sen-
sitizer were produced by the addition of Teflon K type 20 slurry
to the lower sludge tank 20 of Figure 1. A different concentra-
tion of Teflon was added to each batch. The dustless products
were subjected to Sensitivity Test X, wherein portions of the
nonsensitizer grade 120 fuel were replaced by the product dust-
less sensitizer, as described in Example 2. The products were
also subjected to the Hartmann Test. The results are herein
tabulated.
- 26 -
s~
~ o
E~ ~1
~ ~ u ~
N
P~ ~I ~ CCI 00
.~
N Ul ~
~Q O
~-rl O
aJ ~ ~
U~ ~ ~I d~
~ ~ ~ ~ ~ ~ ~ U~
X 0'-1
~ Q)
~-'
~1 o a
~ I
t) ~ 3
,1-,1 o ~ ,1 ,~
~Q ~ ~ ~.q
~ ~ ~ u~ to lQ rn
~ o Q a a
u ~
~ ~3
O ~
~ ~ a) o ~ ~ In
O q~ O - - . .
--I ~ O ~; O O O O
.~, ~1 ~
a)-- 0~o
-- 27 --
~5~
As is readily apparent, the sensiti~ing quality of the
product decreases as the Teflon concentration increases. The
Hartmann Test results also reflect this increasing tenacity with
which the aluminum particles are held together by the Teflon. By
extrapolation, a Teflon concentration may eventually be reached
wherein the product powder no longer quali:Eies as a sensitizer as
defined by test ~.
Example 34
An attempt was made to determine covering area on water
for the products of Example 33. The covering test, which is
based on spreading a thin film one flake thick on the surface of
water and measuring the area covered, is described on pages 18-22
of the book, "Aluminum Paint and Powder", by J. D. Edwards and
R. I. Wray. According to this test, the dusty control product of
Example 33 had an apparent covering oE 7800 cm2/g. The remaining
dustless products, containing various amounts of Teflon, could
not be tested by this technique, the Teflon preventing the fla]~es
from spreading on the water surface.
Example :35
The procedure of Example 1 was followed, except 0~3~
Teflon 6 powder on aluminum weight was substituted for Teflon K
type 20. Teflon 6 is a product of DuPont, having a reported
particle size of 500 + 150 micrometer, and was previously used
successfully by the inventors of U.S. 3,838,064 and U.S. 3,383,092
in producing various dustless products.
A~ter drying, the product powder was extremely dusty,
and could not be distinguished visually from Teflon-free powder
as produced in Example 5.
Example 36
The procedure of Example 1 was followed, except Tef-
lon 42 slurry was substitu~ed for Teflon K type 20. An amount of
Teflon 42 was added to yield 0.3~ Teflon solids on aluminum
- 28 -
~65~
weight. Teflon 42 is a product of DuPont, consisting of a
dispersion of 0.05-0.5 micrometer polytetrafluoroethylene par-
ticles suspended in water.
Ater drying, the product powder was dustless.
Example 37
Portions of the dustless sensitizer product of Ex~
ample 33, to which 0.16~ Teflon had been aclded on aluminum
; weight, were added to half-fill each of three 25 ml capacity
glass screw-cap vials. The caps were then screwed tight. One
vial was left at room temperature (22 + 1C), one was placed in
;- a refrigerator (3 + 3C), and one was placed in ~he freezer
compartment of a refrigerator (-14 + 2~C). Thereafter, once a
day, each vial was visually examined by shaking it briefly with
two or three quick snaps of the wrist, then replacing it in its
environment. After one day of storage~ all three samples were
similar in their dustless behavior. After four days, the samples
stored at room temperature and refrigerated temperature remained
dustless, but the freezer-stored sample was somewhat dusty.
AEter seven days, the former two samples were still dustless, but
the freezer-stored sample had developed strong dusty tendencies.
While the glass walls of the vials holding the former two samples
remained clean and free of aluminum flakes, the wall of the
freezer-stored vial was internally coated with loose aluminum
powder. This indicates that freezing temperatures (below 0C~
affect the adhesive bond by which the Teflon particles hold
aluminum flakes together, whereas temperatures above freezing
have little or no effect on these bonds.
Example 38
A drop of Teflon K type 20 was placed on a metal stage
of the scanning electron microscope r and a small amount of
sensitizer yrade aluminum flake powder was sprinkled onto a
section of this drop. The stage was left to air dry several
- 29 -
s~z~
days, then was examined under the electron beam. At 200 magni-
ications, the Teflon was seen to have dried into ~he form of a
thin, cracked film. At the greater magniEication in Figure 5,
this ilm appears at area lO and is seen to be composed of
rounded particles, each of which is much s]maller than one micro-
meter in diameter. Their size is significantly smaller than e~en
the smallest aluminum flakes.
* * * * *
The preceding examples illustrate a number of points
concerning this process and the products made therefromD Perhaps
the most important single finding, surprising in view of the
teachings of U.S. 3;838,064 and 3,838,092 (hereafter referred to
as the Vogt patents, or individually as Vogt 64 or Vogt 92), is
the critical nature of what appears to be particle-to-particle
adhesion in the present invention. Vogt 64 (column 8, lines 1~3)
and Vogt 92 (column 2, lines 67-71) teach that particle-to-
particle adhesion is unimportant to the operation or products of
their inventions, and that the critical aspect is the formation
of a fiber matrix which holds or loosely traps normally dusty
material (Vogt 92r column 4, lines 20-23 and lines 69-75).
~xamples 16 and 17 demonstrate the adhesive qualities
of Teflon K, which especially become apparent in the 40-50C
temperature range. Example 23 demonstrates a shift in particle
size distribution as a result of causing Teflon X particles to
irreversibly stick together while dispersed in an aqueous medium.
This adhesive characteristic is thought to be responsible for the
dustless product obtained in Example 31, where mixing was delib-
erately held to the minimum necessary for dispersion in order to
avoid mechanical working of Teflon K particles. By inference,
then, it seems likeIy that the dustless products of Examples 1,
2, 3, 4, 21, 22, 27, 28, 3I, 32, 33, and 36 acquire their dust-
less properties through flakes of aluminum being held together by
s~
one or more Teflon K particles (which may be highly deEormedl as
in the case of Examples 27 and 28). These weak adhesive bonds
between flakes are seen to be further weakened by freezing
temperatures, as demonstrated in Example 37. The bonding is also
seen to become stronger when more Teflon K particles are present
in mixture with aluminum 1akes, as Example 33 illustrates.
The latter condition is also surprising, because the
Vogt patents teach that the physical properties of powders
rendered dustless by entrapment in a fiber matrix are essentially
unchanged from those of the original dusty powders (Vogt 64,
column 3, line 12; column 7/ lines 65-71; and Vogt 92, column 4,
lines 29-32). The inability to perorm a covering test on the
products of the instant invention, as in Example 34, is contrary
to Vogt 64 (column 7, lines 65-71) expectations for powder simply
trapped in a fiber matrix.
Some fibers are found in dustless sensitizer prepared
according to the procedures of the instant invention, but these
seem to be an incidental consequence of the process, resulting
from the sort of mechanical action most dramatically illustrated
:~ 20 in Example 27. A typical scanning electron photomicrograph is
illustrated in Figure 6. A comparison photomicrograph of un-
treated (dusty) sensitizer is illustrated in Figure 7. Aside
from a few random fibers seen in Figure 6, the dustless and dusty
sensitizars ha~e a similar microscopic appearance.
A second criterion necessary to the successful function
of the instant process is adequate dispersion of Teflon particles
throughout the mass of aluminum flakes under conditions which
minimize a premature coagulation of Teflon particles. The
critical consequences of premature coagulation are readily
apparent by comparing the performance of dispersed Teflon par-
ticles (Teflon ~-20 or Teflon 42) with that of pre-coagulated
Teflon powder (Teflon K-10 or Teflon 6) under similar mixing
- 31 -
5~
conditions. In comparing Examples 1 with 9, 21 with 11, 31 with29, and 36 with 35, a dustless product is obtained using a Teflon
dispersion whereas pre-coagulated Teflon powder produced dusty
product in every instance under identical mixing procedures.
Examples 6, 7, 8 and 30 further illustrate the importance of
dispersion before coagulation occurs, while Example 32 illus-
trates the effect of a lesser degree of coagulation.
Premature coagulation of the Teflon particles is
destructive to the process, most probably through a reduction in
Teflon surface available to bond aluminum flakes together. This
drawback may be remedied by mixing the coagulated Teflon par-
ticles under conditions which physically deform the Teflon,
thereby creating new surfaces to which aluminum flakes may
adhere. The need for such forceful mixing is demonstrated by
comparing Example 26 with 27, and Examples 27 and 28 with those
employing milder mixing techniques, such as Examples 25, 9, 10,
11, and 35.
The conventional mixing techniques employed in Examples
` 9, 10, and 11 are apparently inefficient in creating significant
new Teflon surfaces, thus leading to the critical nature to the
instant process of adequate dispersion of high surface area
Teflon particles before they coagulate. Assuming a Teflon
density of 2.2 g/cc; an average Teflon particle to be of spher-
ical dimensions having a 0.2 micrometer diameter; an aluminum
flake density of 2.7 g/cc; and an average aluminum Elake particle
to be of cylindrical dimensions having an 8 micrometer diameter
and a one micrometer altitude: then, by calculation, a mixture
of 0.25% by weight Teflon in aluminum contains 30-40 Teflon
particles for ea~h aluminum flake present. Thus, it is seen that
an adequate number of Teflon particles are present to tack
toyether aluminum flakes, with no need to further increase Teflon
surface area by working, smearing, kneading or mastication.
- 32 ~
S25
Various modifications may be made in the invention
without departing from the spirit thereof, or the scope of the
claims and therefore, the exact form shown is to be taken as
illustrative only and not in a limiting sense, and it is
desired that only such limitations shall be placed thereon as
are imposed by the prior art, or are specifically set forth
in the appended claims.
The following are trade marks used in this dis-
closure:
Abbott
Alcoa
Cab-O-Sil
Cabot
Coulter Counter
DuPont
Fisher
Fisher-Johns
Guartec
; Hercules
Pennwalt
Plasti-Corder
Rinco
Stokes
Swift
Teflon
Tracor Econ
Vinsol
- 33 -
