Handbook of Transition Metal Polymerization Catalysts

587 Pages·2010·93.033 MB·English

by  Ray Hoff, Robert T. Mathers

Appendix A
Pyrophoricity of Metal Alkyls
DENNIS B. MALPASS
Akzo Nobel Polymer Chemicals (retired), Magnolia Texas 77354

A.1 INTRODUCTION
Metal alkyls used with transition metal polyolefi n catalysts are typically highly
reactive with air. In fact, many of the commercially available metal alkyls used
in the polyolefi ns industry are pyrophoric; that is, they ignite spontaneously
upon exposure to air. 1 Most are also violently reactive with water.
Metal alkyls are often supplied as solutions, usually in aliphatic hydrocar-
bons, such as isopentane, hexane, and heptane. These solutions, with typical
metal alkyl concentrations of 10 – 25%, exhibit lower reactivity with air relative
to the neat (undiluted) metal alkyls and are perceived to be less hazardous.
Since such solutions contain large amounts of highly fl ammable, volatile
hydrocarbons, hazards of the solvent (toxicity, fl ash point, lower explosive
limit, etc.) should also be recognized and appropriate precautions taken.
Tests have been developed to gauge air reactivity of metal alkyl solutions
and results have been used to assign hazard classifi cations for transport.
However, results are frequently misinterpreted or misunderstood. The objec-
tives of this appendix chapter are
• To discuss the intent and meaning of pyrophoricity tests
• To describe procedures for testing pyrophoricity
• To provide results of pyrophoricity testing of metal alkyls and their
solutions
• To identify trends in pyrophoricity of metal alkyls.
Excerpted from a technical bulletin published by Akzo Nobel Polymer Chemicals
(publication number MA03.322.01/January 2003), with permission.

Handbook of Transition Metal Polymerization Catalysts Edited by Ray Hoff and
Robert T. Mathers
Copyright © 2010 John Wiley & Sons, Inc.
551
552 PYROPHORICITY OF METAL ALKYLS
Discussions of pryophoricity of metal alkyls in this chapter deal only with
compounds of aluminum, boron, magnesium, and zinc. Some metal alkyls
(e.g., organolead and many organotin compounds) are not reactive with air,
and others (e.g., potassium alkyls) are not soluble in hydrocarbons. Methods
discussed here are not applicable to such metal alkyls. Methods are, however,
applicable to lithium alkyls, which are used in industrial catalyst systems for
the manufacture of synthetic elastomers. Pyrophoricity of the commercially
available butyllithium solutions approximates that of triethylalumium and
increases in the series:
n-butyl>sec-butyl>tert-butyl

As indicated above, tests discussed here were developed primarily for
the purpose of hazard classifi cation for transport. Results are also useful
in comparing relative reactivity of metal alkyl solutions. Results, however,
should not be interpreted as predictive of whether solutions will self - ignite if
spilled. The tests are empirical and results should be regarded as semiquantita-
tive. Finding a solution to be pyrophoric under test conditions should n ot
be interpreted as an indication that the particular solution is certain to ignite
if spilled. Conversely, if a solution is found to be nonpyrophoric under test
conditions, it should n ot be concluded that the solution will not ignite if
spilled. So - called self - ignition limits are not easily determined, since variables
(wind speed, ambient temperature, humidity, surface onto which the spill
occurs, etc.) that may contribute to ignition in a spill are uncontrolled. Under
certain conditions, any commercially available solution of a metal alkyl could
ignite when spilled.
Though a solution may have been determined to be nonpyrophoric by test,
it will remain air reactive and must be handled under an inert atmosphere to
maintain product quality. While handling such solutions, workers must continue
to wear personal protective equipment to protect against exposure. Even solu-
tions that have been determined to be nonpyrophoric by test may cause burns.
In general, pyrophoricity of a metal alkyl increases as metal content
increases. Thus, trimethylaluminum (which contains about 37% Al) ignites
spectacularly when exposed to air, while tri -n - octylaluminum (which contains
about 7% Al) oxidizes slowly but usually does not ignite spontaneously. Nev-
ertheless, tri -n - octylaluminum is considered pyrophoric, since it tests positive
by the methods described below. Other trends relating degree of pyrophoricity
to solution and product characteristics will be developed below.
A.1.1 Sawdust Test
In the 1960s, the Bureau of Explosives (then part of the Association of Ameri-
can Railroads) developed the “ sawdust test ”1 to gauge pyrophoricity of air -
reactive liquids. The test involved introducing 800 mL of a metal alkyl solution
to the “ crater ” of a volcano - shaped cone of sawdust (∼ 4 L of dry, soft - wood
sawdust). If the sawdust ignited or was charred, the solution was deemed to
be pyrophoric. As concentration of the metal alkyl was reduced, a solution was
PYROPHORICITY OF METAL ALKYLS 553
achieved which did not cause ignition or charring. This concentration was
designated as the nonpyrophoric limit (NPL). Thus, the NPL from this test
may be defi ned as the m aximum concentration of metal alkyl that does not
ignite or char sawdust. The sawdust test is now obsolete and has been replaced
by tests based upon the effect of metal alkyls on a specifi c grade of fi lter paper
and/or silica.
A.1.2 Paper Char Test
A simpler, alternative method for gauging pyrophoricity of metal alkyl solu-
tions was developed in the mid - 1970s.2 The method was developed to provide
results i ntentionally comparable to the sawdust test. For this reason, the choice
of substrate was a grade of fi lter paper specifi cally chosen to simulate the cel-
lulosic composition of sawdust. This method has become known as the “ paper
char test ” and has been in use since the late 1970s in transport classifi cation
of metal alkyls.
The paper char test involves syringing a small quantity ( ∼ 0.5 mL) of
the metal alkyl solution onto the surface of dry Whatman No. 3 fi lter paper
(at ambient laboratory temperature, typically 22 – 25 ° C). The test must be
performed in a fume hood since profuse smoking or a small fi re may occur.
Personal protective equipment should be worn during testing, including
safety glasses, fi re - retardant coveralls, safety shoes, an aluminized suit with
face shield and hood, and protective gloves (lined polyvinyl chloride or
leather) that are impervious to metal alkyls. The fi lter paper is suspended
on the platform of a tripod to permit airfl ow above and below the paper.
A small indentation ( ∼ 2.5 cm diameter and ∼ 3 mm deep) should be made
in the center of the paper to form a small “ cup ” for the solution. Care must
be taken not to tear the paper when making the indentation. A small metal
pan containing a layer (∼ 2.5 cm) of vermiculite should be placed beneath the
tripod to catch drippings. The solution should be delivered in a steady stream
(not dropwise) within a few seconds to the indentation in the fi lter paper. The
rate of delivery, however, should be slow enough to prevent splattering or
overfl ow of the “ cup. ” The paper is then observed for about 1 min for ignition
or charring. Slight charring near the NPL may be more easily detected if the
fi lter paper is held up to sunlight. Tests are usually done in triplicate. If char-
ring is observed in any of the test fi lter papers, the solution is deemed pyro-
phoric at that concentration. Other grades of fi lter paper, common paper
towels, and so on, do not provide results comparable to the sawdust test and
should not be used. Also, wet paper dulls the sensitivity of the test.
As with the sawdust test, solution concentration may be decreased incre-
mentally until a concentration is achieved at which there is no char or ignition.
This concentration is deemed the NPL; that is, it is the m aximum concentration
that does not ignite or char the paper. Though reading char is somewhat sub-
jective near the NPL, test results are usually within ± 1% absolute. NPLs for
aluminum alkyls are summarized in Table A.1 . Data for metal alkyl derivatives
of boron, magnesium, and zinc are provided in Table A.2 .
st
Te e
PL by Sawdust 10 in hexane 15 in hexane 17 in isopentan 26 in benzene 15 in hexane
N

e
n
e
u
n Tol 12 12 19 26 20
L i
P
N

e
n
a
pt
n He 12 12 13 27 25 42 56 91 13 21 14 18 18 13 19 27
L i
P
N

e
n
a
x
e
n H 11 12 30 22 43 42 61 79 38 11 18 13 13 16 24
L i
P
N

e
n
a
nt
Isope 7 13 20 36 49 86 8 13 19
n
yls L i
k P
Al N
m
u
n
TABLE A.1 NPL s of Alumi Aluminum Alkyl Trimethylaluminum Triethylaluminum Tri - n - propylaluminum Tri - n - butylaluminum Triisobutylaluminum a Triisohexylaluminum Tri - n - hexylaluminum Tri - n - octylaluminum Tri - n - decylaluminum b Tricitronellylaluminum Isoprenylaluminum Diethylaluminum hydride Diisobutylaluminum hydride Dimethylaluminum chloride Dimethylaluminum bromide Diethylaluminum fl uoride Diethylaluminum chloride Diethylaluminum bromide Diethylaluminum iodide
554
e
n
a
x
e
h
n
20 i
19 25
18 25 20 37 37 49 50 20 29 35 28 28
17 20 16 21 15 22 25 25 26 51
4 8 0 0 6 ed.
1 1 2 2 2 ot
n
e
s
wi
p. er
uh
Di - n - propylaluminum chloride Di - n - butylaluminum chloride Diisobutylaluminum chloride Di - n - hexylaluminum chloride Diisohexylaluminum chloride Di - n - octylaluminum chloride Di - n - decylaluminum chloride Methylaluminum sesquichloride Methylaluminum sesquibromide Ethylaluminum sesquichloride Ethylaluminum sesquibromide Isobutylaluminum sesquichloride Ethylaluminum dichloride Ethylaluminum dibromide Isobutylaluminum dichloride Diethylaluminum ethoxide Diisobutylaluminum ethoxide a Isohexyl = 2 - methylpentyl group. b Citronelly = 3,7 - dimethyl - 6 - octenyl gro Note : wt% by paper char test, unless ot
555
F at
H h
t
T s
ata ne 1 in ndate
Other D 16 in THF a 100 a 100 12 in cyclohexa9 in n - pentane 100 13 in toluene; 2 22 in toluene cordingly, UN ma
c
A
Test a test.
dust xane xane e silic
L by Saw 20 in he 25 in he oric by th
P ph
N o
r
y
p
e
NPL in Heptane 18 30 70 14 20 31 31 29 shown to b
n
e
e
b
n e s
NPL iHexan 18 34 14 19 20 23 ar test, it haclass 4.2.
TABLE A.2 NPL s of Other Metal Alkyls NPL in Metal Alkyl Isopentane Triethylborane 17 Triisobutylborane Tri - n - butylborane Tri - n - octylborane Diethylboron methoxide Diethylboron isopropoxide sec -Butyllithium tert -Butyllithium n - Butylethylmagnesium n - Butylethylmagnesium n - butoxide b Dibutylmagnesium c Di - n - butylmagnesium Dimethylzinc Diethylzinc 17 Di - n - propylzinc Di - n - butylzinc a Though the neat product is nonpyrophoric by the paper chthe product be classifi ed as pyrophoric and assigned hazard b Product contains both n - butyl and sec - butyl groups. c Contains about 12 molar % triethylaluminum. Note : wt% by paper char test, unless otherwise noted.
556
PYROPHORICITY OF METAL ALKYLS 557
In rare cases, an oxide fi lm forms on the surface of the test solution and
prevents suffi cient contact with air. This sometimes occurs with metal alkyls
with high molecular weights or solvents with relatively high viscosity, such as
mineral oil. For these solutions, the standard paper char test procedure is
modifi ed slightly. When a crust forms, it may be necessary to agitate the solu-
tion in the indentation gently (using the tip of the syringe needle). It may also
be necessary to extend the observation time beyond the 1 - min standard. NPL
values thus obtained are usually lower than values obtained if the crust is not
disturbed.
Advantages of the paper char test relative to the sawdust test are as
follows:
• It can be run in a standard laboratory fume hood.
• It is simple to run with commonly available laboratory equipment.
• Small quantities are used (safer, reduced risk of exposure).
• Results are comparable (by design) to those from the sawdust test.
• Replicate tests can easily be run, improving accuracy and
reproducibility.
Both the paper char and sawdust tests rely upon the reactivity of a metal
alkyl solution with a cellulosic material in air as a measure of pyrophoricity.
Ignition need not occur for a solution to be adjudged pyrophoric. Hence, the
strict defi nition of the term “ pyrophoric ” may not be applicable to solutions
of metal alkyls using these tests, especially near the NPL.
A.1.3 Silica Test
In recent years, a supplemental test has been used in conjunction with the
paper char test to determine shipping classifi cation (see Section A.2 ). We shall
refer to this test as the “ silica test ” because it involves introducing a small
quantity of the metal alkyl solution to silica or diatomaceous earth in a por-
celain cup in air and observing whether the mixture ignites within 5 min. This
test is prescribed by United Nations (UN) regulations. 3,4 The silica test is
repeated 6 times. If a solution in any of the silica test ignites, it is classifi ed as
pyrophoric and assigned hazard classifi cation 4.2. In this situation, additional
testing by the paper char test would not be required. However, if none of the
tests results in ignition, the solution is deemed nonpyrophoric by the silica test.
In this case, UN regulations require additional testing by the paper char test
to confi rm a nonpyrophoric rating. If the solution is also nonpyrophoric by the
paper char test, the solution is then classifi ed as nonpyrophoric and assigned
hazard classifi cation 4.3. Only if all tests (silica and paper char) are negative
is the solution classifi ed as nonpyrophoric.
558 PYROPHORICITY OF METAL ALKYLS
A.2 HAZARD CLASSIFICATIONS
The UN Economic and Social Council ’ s Committee of Experts on Transport
of Dangerous Goods has adopted the paper char test in conjunction with the
silica test (as described above) for classifying solutions of metal alkyls for
international transport.3 ,4 Transport hazard class assignments are Division 4.2
(spontaneously combustible) and Division 4.3 (dangerous when wet). All
metal alkyls and their solutions are assigned to Packing Group I. Commonly
used UN identifi cation numbers are UN 2845 (pyrophoric liquids, organic),
UN 3051 (aluminum alkyls), UN 3052 ( “ aluminum alkyl halides ” ), and UN
3207 (organometallic compound, solution, water reactive).
Other transport regulatory agencies have accepted the UN recommenda-
tions. These include the U.S. Department of Transportation (DOT), Interna-
tional Maritime Organization (IMO), International Civil Aviation Organization
(ICAO), the European agency governing the international carriage of danger-
ous goods by road and rail (abbreviated from the French as ADR/RID), and
International Air Transport Association (IATA).
Note that UN regulations do not require that the NPL be determined by
any test. For this reason, the silica test has not been widely used to determine
the maxium concentration of a specifi c metal alkyl solution that does not ignite
under test conditions. Rather, it has been used simply to determine whether
a specifi c solution is pyrophoric or nonpyrophoric for transport classifi cation.
If adjudged to be nonpyrophoric, it does not necessarily mean that the specifi c
solution is at the NPL. Additional tests would be required to determine the
NPL.
A.3 TRENDS FROM PYROPHORICITY DATA
Using data from pyrophoricity testing, certain tendencies have been recog-
nized. Discussions below identify several trends. These interpretations are
based upon data from the paper char test. In some cases, trends may be used
to predict or estimate pyrophoricity of metal alkyls that have not been tested.
A.3.1 Effect of Metal Content
In general, NPLs tend to decrease with increasing metal content of the metal
alkyl. This trend is illustrated in Figure A.1 for a variety of R Al and R AlCl
3 2
(R = C to C alkyl) compounds in hexane or heptane.
1 10
A.3.2 Effect of Solvent
NPLs increase as the boiling point (bp) of the solvent increases. Stated differ-
ently, pyrophoricity of hydrocarbon solutions of metal alkyls tends to increase
as the vapor pressure (or volatility) of the solvent increases. A plot of NPLs
PYROPHORICITY OF METAL ALKYLS 559
(R Al and R AlCl, where R = C to C )
3 2 1 10
100
90
80
70
e
n
a 60
pt
e
H 50
n
L i 40
P
N 30
20
10
0
0 5 10 15 20 25 30 35 40
Al Content in Neat Al Alkyl
Figure A.1 NPL as function of Al content.
(R = C to C alkyl)
1 2
45
40
35
30
%) TNHAL
wt 25 TIBAL
L ( 20 TEAL
P TMAL
N 15
10
5
0
0 50 100 150 200 250
bp of Hydrocarbon (∞C)
Figure A.2 NPLs of R Al in various hydrocarbons (R = C to C ).
3 1 6
for a range of R Al (R = C to C alkyl) compounds versus bp of the solvent
3 1 6
is shown in Figure A.2 . Though data are limited for some of the R Als, the
3
trend of increasing NPLs as the atmospheric bp of the solvent increases is
clear. Results suggest that trimethylaluminum (TMAL) solutions are slightly
less pyrophoric than triethylaluminum (TEAL) solutions by the paper char
test. This result is anomalous since TMAL contains about 58% more alumi-
num than TEAL.
560 PYROPHORICITY OF METAL ALKYLS
A.3.3 Effect of Ligand
Incorporation of oxygen - containing ligands into an aluminum alkyl tends to
increase the NPL (lower pyrophoricity). For example, if an ethoxide ligand
replaces one of the ethyl groups in TEAL, the NPL increases from 12% for
TEAL to 28% for diethylaluminum ethoxide (both in heptane). Similarly,
triisobutylaluminum has an NPL of 22%, while diisobutylaluminum ethoxide
has an NPL of 51% (both in hexane). The same trend is also observed in
other series of metal alkyls. Note from Table A.2 that triethylborane
(TEB) and n - butyl(ethyl)magnesium (BEM) have NPLs of 18 and 14%,
respectively, in heptane. However, alkoxide derivatives of TEB and BEM
[diethylboron methoxide (DEB - M), diethylboron isopropoxide (DEB - IP),
and n - butyl(ethyl)magnesium n - butoxide (BEM - B)] are nonpyrophoric by
the paper char test as neat products.
Methylaluminoxanes (methylaluminum compounds containing Al – O – Al
linkages) have grown in importance in recent years because of their use as
cocatalysts for single - site catalysts.5 Methylaluminoxanes are most commonly
produced by controlled reaction of trimethylaluminum with water. Since meth-
ylaluminoxanes contain oxygen, they also show diminished pyrophoricity. (See
Chapter 1 for additional information on properties and compositions of alkyl-
aluminoxanes.) Limited data on pyrophoricity testing of solutions of methyl-
aluminoxanes have shown them to be nonpyrophoric by the paper char test
at the concentrations (28 – 35%) commonly supplied in the merchant market.
A.3.4 Effect of Temperature
A few NPLs were conducted at elevated temperatures (35 – 55 ° C). 2 Results
indicate that NPLs decrease slightly as temperature increases. This is consis-
tent with the fi nding that pyrophoricity increases (or NPLs decrease) as the
vapor pressure of the solvent increases (see Figure A.2 ).
A.4 SUMMARY OF TRENDS
In general, pyrophoricity of metal alkyls is inversely proportional to NPLs;
that is, pyrophoricity increases as NPLs decrease. The following may be helpful
in understanding trends for pyrophoricity of metal alkyls:
Metal content ↑ NPL ↓ Pyrophoricity ↑
Oxygen content ↑ NPL ↑ Pyrophoricity ↓
Solvent vapor pressure ↑ (or bp↓ ) NPL ↓ Pyrophoricity ↑
Temperature ↑ NPL ↓ Pyrophoricity ↑
As suggested above, using trends allows one to estimate an NPL for an
unknown system. For example, knowing that the NPL for TIBAL in heptane
(bp = 98 ° C) is 26%, one may conclude that the NPL for TIBAL in dodecane