STSA – Life without CTAB
April 26, 2004
by
Ricky Magee
Columbian Chemicals Company
1
Outline
•
Introduction
•
Theory
•
Results
• Comparison with CTAB surface area
• Surface Chemistry Effects
• New Developments
•
2
Conclusions
Introduction
3
•
Importance of Surface Area
•
Traditional Surface Area Techniques
•
Timeline of STSA at ASTM
Importance of Surface Area
4
•
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.
Effect of Aggregation and Porosity
on Surface Area
100 m2/g
5
80 m2/g
400 m2/g
Traditional Surface Area Tests
Attribute
CTAB
Iodine
NSA
Surface 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
6
Timeline of STSA at ASTM
7
•
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.
Theory
8
•
Nitrogen Adsorption
•
Saturated Vapor Pressure
•
de Boer t-values and Va-t plots
•
Pore filling model
•
Application of CB t Equation
Nitrogen Adsorption
9
•
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.
Saturated Vapor Pressure
10
•
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.
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
11
All values in mm Hg
Thickness Model
Small Particles
12
Large Particle
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
13
Va–t Plot
60
Vol. Ads. (cc/g)
50
40
90 m2/g
30
60 m2/g
20
10
2
30 m /g
0
0
1
2
3
4
5
6
Thickness (Å)
14
7
8
9
Pore Filling Model
P/Po = 0.0
P/Po = 0.2
15
P/Po = 0.05
P/Po > 0.2
Adsorption Isotherms
140
Vol. Adsorbed (cc/g)
N472
120
100
N110
N326
80
N660
60
40
20
0
0.0
16
0.1
0.2
0.3
0.4
0.5
0.6
Relative Pressure
0.7
0.8
0.9
Va–t Plot for Standard Carbon Blacks
based on CB t equation
120
P/P o = 0.5
P/P o = 0.2
N472
Vol. Adsorbed (cc/g)
100
80
N110
60
N326
40
20
N660
0
0
17
1
2
3
4
5
6
7
Thickness (Å)
8
9
10
11
12
Results
18
•
STSA versus CTAB
•
Surface Chemistry
•
Precision Statements
Tread Carbon Blacks
19
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
Carcass Carbon Blacks
20
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
CTAB versus STSA
140
CTAB (m 2/g)
120
100
80
60
40
R2 = 0.9985
20
0
0
21
20
40
60
80
STSA (m2/g)
100
120
140
Effect of Surface Oxidation on
CTAB Measurements
Sample
#1
22
#2
#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
-6.9
Effect of Surface Oxidation on
CTAB Measurements
#1
#3
STSA (m2/g)
86.3
86.3
90.0
Initial Value
86.4
85.7
90.8
CTAB (m2/g)
87.3
85.9
89.2
Initial Value
100.1
97.5
97.7
-1.0
0.4
0.8
Difference
23
Sample
#2
Effect of Heat Treatment on
ASTM SRB-5
25
NSA
Iodine
%Change
20
15
10
5
0
N135
24
N330 N220 N762 N660 N683
Effect of Heat Treatment on
ASTM SRB-5
5.0
%Change
2.5
0.0
-2.5
-5.0
STSA
-7.5
CTAB
-10.0
N135 N330 N220 N762 N660 N683
25
Precision
N121 Control Chart
Diff. From Mean (m2/g)
2.5
CTAB
STSA
1.5
0.5
-0.5
-1.5
-2.5
1
26
5
9
12
Run #
16
20
Effect of Solution Aging
on CTAB Solutions
122
CTAB (m 2/g)
121
120
119
118
117
116
115
0
27
10
20
30
Run #
40
50
60
Surface Area Precision Study
from Original STSA Paper
Within Lab
Between Lab
Percent
6
5
4
3
2
1
0
NSA
28
STSA
CTAB
Potential Errors in
NSA/STSA Measurements
29
•
Improper degassing time/temperature.
•
Improper sample weight.
•
Inaccurate or changing Po value.
NSA/STSA Control Chart
using ASTM B-6 (N220)
112.0
Mean = 109.6 ± 1.1
111.5
111.0
(ASTM = 110.0 ± 1.6)
110.5
110.0
109.5
109.0
Surface Area (m2/g)
108.5
108.0
107.5
107.0
106.5
106.0
105.5
105.0
104.5
104.0
Mean = 105.4 ± 2.1
103.5
103.0
(ASTM = 105.4 ± 2.9)
102.5
102.0
101.5
1
30
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Data collected over a 4 month period
22
23
24
25
26
27
28
29
30
31
NSA/STSA Control Chart with
Po Outliers Removed (DP >20mm Hg)
112.0
Mean = 109.5 ± 1.1
111.5
111.0
(Previous = 109.6 ± 1.1)
110.5
110.0
109.5
109.0
Surface Area (m2/g)
108.5
108.0
107.5
107.0
106.5
106.0
105.5
105.0
104.5
104.0
Mean = 105.4 ± 1.5
(Previous = 105.4 ± 2.1)
103.5
103.0
102.5
102.0
101.5
1
31
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
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.
32
Modified Gemini
33
Effect of Dewar Stability – 1 Hr.
Equilibration Time
NSA (m2/g)
1 Hour Equil.
STSA (m2/g)
Mean
3s
Mean
3s
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
34
* = Filled and covered overnight before analysis
Effect of Dewar Stability – 2 Hr.
Equilibration Time
NSA (m2/g)
1 Hour Equil.
STSA (m2/g)
Mean
3s
Mean
3s
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
35
* = Filled and covered overnight before analysis
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.
•
36
Changes to D6556 are required based on this study.
Analysis Time
Standard Value
Standard Method
(D6556)
Modified Method
(3 pt.)
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
Sample ID
37
Analysis Time
Standard Method (D6556)
Sample ID
38
Modified Method (3 pt.)
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
Conclusions
STSA provides the following advantages
over CTAB:

39
Improved precision and accuracy, provided
proper attention to Po

Less affected by surface oxidation

Less operator time

Measured simultaneously with NSA

No reagent preparation