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1
April 26, 2004
byRicky Magee
Columbian Chemicals Company
STSA – Life without CTAB
2
Outline
• Introduction
• Theory
• Results
• Comparison with CTAB surface area
• Surface Chemistry Effects
• New Developments
• Conclusions
3
Introduction
• Importance of Surface Area
• Traditional Surface Area Techniques
• Timeline of STSA at ASTM
4
Importance of Surface Area
• Surface area is one of the most important characteristics of the carbon black.
• Surface area of carbon black is a function of particle size, degree of aggregation and porosity. Therefore, surface area alone is not a reliable measure of particle size.
• In the absence of porosity, surface area values are an indication of a carbon black’s particle size (fundamental property).
• According to IUPAC convention, micropores are characterized by diameters less than 20 Å or 2 nm.
5
80 m2/g 100 m2/g 400 m2/g
Effect of Aggregation and Porosityon Surface Area
6
Traditional Surface Area Tests
Attribute CTAB Iodine NSASurface Type
Measured External Total Total
Affect of Oxidation Unknown Severe Minimal
Precision Poor Good Good
Difficulty High Low Low
Set-up Costs Medium Low High
7
Timeline of STSA at ASTM
• D5816 – STSA approved as ASTM standard in 1995.
• D1765 (CB Classification System)– In 1997, STSA was
added as a typical value in Table 1, with corresponding
CTAB values deleted.
• D6556 – Combined NSA (D4820) and STSA (D5816)
into a single standard in 2000. The NSA section was
modernized and data interpretation simplified.
• D3765 – In 2003, estimated CTAB values of SRB-6
carbon blacks was added to CTAB method.
8
Theory
• Nitrogen Adsorption
• Saturated Vapor Pressure
• de Boer t-values and Va-t plots
• Pore filling model
• Application of CB t Equation
9
Nitrogen Adsorption
• The concentration of nitrogen is expressed as
relative pressure (P/Po).
• A relative pressure of “0.0” is measured at
absolute vacuum, while a value of “1.0” is
measured at nitrogen’s saturated vapor
pressure (Po).
• The typical range for measuring NSA (BET) is
P/Po = 0.05 to 0.30.
10
Saturated Vapor Pressure
• Saturated vapor pressure is the pressure at
which nitrogen gas condenses.
• It is based on atmospheric pressure and the
temperature of the liquid nitrogen in the dewar.
• It is usually 10 - 20 mm Hg above ambient
pressure due to impurities.
• Critical for measuring accurate STSA values.
11
Saturated Vapor Pressure
Elevation
Sea Level 900 m
• Atm. Pressure 760 685
• Sat. Vapor Press. 775 700
• P/Po Value = 0.1 78 70
• P/Po Value = 0.2 155 140
• P/Po Value = 0.3 233 210
All values in mm Hg
12
Thickness Model
Small Particles Large Particle
13
Thickness Equations
13.99
de Boer t = 0.034 - log P/Po
CB t = 0.88 (P/Po)2 + 6.45 (P/Po) + 2.98
Carbon Black t curve based on N762
14
Va–t Plot
0 1 2 3 4 5 6 7 8 9
0
10
20
30
40
50
60
30 m /g 2
60 m /g 2
90 m /g 2
Thickness (Å)
Vol
. Ads
. (cc
/g)
15
Pore Filling Model
P/Po = 0.0 P/Po = 0.05
P/Po = 0.2 P/Po > 0.2
16
Adsorption Isotherms
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90
20
40
60
80
100
120
140
N110
N326
N660
N472
Vol
. Ads
orbe
d (c
c/g)
Relative Pressure
17
Va–t Plot for Standard Carbon Blacksbased on CB t equation
0 1 2 3 4 5 6 7 8 9 10 11 12
0
20
40
60
80
100
120P/P o = 0.2
N472
N110
N326
N660
P/P o = 0.5
Thickness (Å)
Vol
. Ads
orbe
d (c
c/g)
18
Results
• STSA versus CTAB
• Surface Chemistry
• Precision Statements
19
Tread Carbon Blacks
NSA STSA CTAB
N110 135.3 119.7 123.6
N121 122.8 116.5 120.4
N220 116.0 107.7 109.5
N234 117.2 111.2 115.6
N330 76.6 74.7 80.0
N339 91.0 88.5 94.8
All values in m2/g
20
Carcass Carbon Blacks
NSA STSA CTAB
N539 38.4 37.7 41.0
N550 38.1 38.2 38.6
N650 36.4 35.3 38.9
N660 33.0 33.1 34.9
N762 25.7 25.7 26.9
N787 29.7 29.6 31.0
All values in m2/g
21
CTAB versus STSA
0 20 40 60 80 100 120 1400
20
40
60
80
100
120
140
STSA (m2/g)
CT
AB
(m
2 /g)
R2 = 0.9985
22
Effect of Surface Oxidation on CTAB Measurements
Sample # 1 # 1 # 2 # 2 # 3 # 3
Oxygen (%) 2.0 1.9 1.5
STSA (m2/g) 86.4 85.7 90.8
CTAB (m2/g) 100.1 97.5 97.7
Difference -13.7 -11.8 -6.9Difference -13.7 -11.8 -6.9
23
Sample Sample # 1 # 1 # 2 # 2 # 3 # 3
STSA (mSTSA (m22/g) 86.3 86.3 90.0/g) 86.3 86.3 90.0 Initial Value 86.4 85.7 90.8Initial Value 86.4 85.7 90.8
CTAB (mCTAB (m22/g) 87.3 85.9 89.2/g) 87.3 85.9 89.2 Initial Value 100.1 97.5 97.7Initial Value 100.1 97.5 97.7
Difference -1.0 0.4 0.8Difference -1.0 0.4 0.8
Effect of Surface Oxidation on CTAB Measurements
24
Effect of Heat Treatment on ASTM SRB-5
00
55
1010
1515
2020
2525NSANSA
IodineIodine
N683N683N660N660N762N762N220N220N135N135 N330N330
%C
han
ge%
Ch
ange
25
Effect of Heat Treatment on ASTM SRB-5
--10.010.0
-7.5-7.5
-5.0-5.0
-2.5-2.5
0.00.0
2.52.5
5.05.0
N683N683N660N660N762N762N220N220N135N135 N330N330
%C
han
ge%
Ch
ange
STSASTSA
CTABCTAB
26
PrecisionN121 Control Chart
Run #
1 5 9 12 16 20
-2.5
-1.5
-0.5
0.5
1.5
2.5
CTAB
STSA
Dif
f. F
rom
Mea
n (m
2 /g)
27
Effect of Solution Agingon CTAB Solutions
0 10 20 30 40 50 60
Run #Run #
115
116
117
118
119
120
121
122
CT
AB
(m
2 /g)
28
Surface Area Precision Studyfrom Original STSA Paper
NSANSA STSASTSA CTABCTAB00
11
22
33
44
55
66 Between LabBetween Lab
WithinWithin LabLab
Per
cen
tP
erce
nt
29
Potential Errors in NSA/STSA Measurements
• Improper degassing time/temperature.
• Improper sample weight.
• Inaccurate or changing Po value.
30
NSA/STSA Control Chartusing ASTM B-6 (N220)
101.5
102.0
102.5
103.0
103.5
104.0
104.5
105.0
105.5
106.0
106.5
107.0
107.5
108.0
108.5
109.0
109.5
110.0
110.5
111.0
111.5
112.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Surf
ace
Are
a (m
2/g)
Mean = 109.6 ± 1.1
(ASTM = 110.0 ± 1.6)
Mean = 105.4 ± 2.1
(ASTM = 105.4 ± 2.9)
Data collected over a 4 month period
31
NSA/STSA Control Chart with Po Outliers Removed (P >20mm Hg)
101.5
102.0
102.5
103.0
103.5
104.0
104.5
105.0
105.5
106.0
106.5
107.0
107.5
108.0
108.5
109.0
109.5
110.0
110.5
111.0
111.5
112.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Surf
ace
Are
a (m
2/g)
Mean = 109.5 ± 1.1
(Previous = 109.6 ± 1.1)
Mean = 105.4 ± 1.5 (Previous = 105.4 ± 2.1)
32
Effect of Dewar Stability
• A single sample of ASTM B-6 (N220) degassed at
300°C then run multiple times, measuring the Po after
each run using the standard Gemini (600 ml) and a
large volume (2 L) dewars.
33
Modified Gemini
34
Effect of Dewar Stability – 1 Hr. Equilibration Time
NSA (m2/g) STSA (m2/g)
1 Hour Equil. Mean 3 Mean 3
Dewar #1 - Std (600 ml) 110.0 1.83 104.7 4.86
Dewar #2 - Std (600 ml) 110.0 1.23 104.3 2.16
Dewar #3 - Large (2 L)* 109.9 0.18 105.4 0.57
Dewar #3 - Large (+15 mm)* 109.9 0.15 105.3 0.33
* = Filled and covered overnight before analysis
35
Effect of Dewar Stability – 2 Hr. Equilibration Time
NSA (m2/g) STSA (m2/g)
1 Hour Equil. Mean 3 Mean 3
Dewar #1 - Std (600 ml) 110.2 0.60 105.2 0.87
Dewar #2 - Std (600 ml) 109.9 0.99 104.2 1.95
Dewar #3 - Large (2 L)* 109.9 0.15 105.3 0.33
Dewar #3 - Large (+15 mm)* 109.9 0.15 105.3 0.33
* = Filled and covered overnight before analysis
36
Po Summary
• A minimum 2 hour dewar equilibration is required
(longer is better).
• Large volume dewars allow improved precision.
• Other Po options exist for newer, higher-end
instruments.
• Changes to D6556 are required based on this study.
37
Analysis Time
Standard ValueStandard Method
(D6556)Modified Method
(3 pt.)
Sample ID NSA STSA NSA STSA NSA STSA
(m2/g) (m2/g) (m2/g) (m2/g) (m2/g) (m2/g)
A-6 (N134) 143.9 135.7 142.1 133.7 142.7 133.7
B-6 (N220) 110.0 105.4 109.4 104.6 108.8 105.3
C-6 (N326) 78.3 79.2 78.3 79.1 77.6 79.8
D-6 (N762) 30.6 29.6 30.4 29.0 30.7 29.2
E-6 (N660) 36.0 35.1 35.5 34.7 35.1 33.8
F-6 (N683) 35.3 34.1 34.7 33.2 34.6 32.8
Mean Values 72.4 69.9 71.7 69.1 71.6 69.1
38
Analysis Time
Standard Method (D6556) Modified Method (3 pt.)
Sample ID Analysis Degassing Total Analysis Degassing Total
Time (min.)
Time (min.)
Time (min.)
Time (min.)
Time (min.)
Time (min.)
A-6 (N134) 33 30 63 20 10 30
B-6 (N220) 30 30 60 17 10 27
C-6 (N326) 23 30 53 17 10 27
D-6 (N762) 20 30 50 15 5 20
E-6 (N660) 19 30 49 15 5 20
F-6 (N683) 20 30 50 14 5 19
Mean Values 24.2 30.0 54.2 16.3 7.5 23.8
39
Conclusions
STSA provides the following advantages STSA provides the following advantages
over CTAB:over CTAB: Improved precision and accuracy, provided
proper attention to Po
Less affected by surface oxidation
Less operator time
Measured simultaneously with NSA
No reagent preparation
Recommended