Introduction and Purpose:
A study was performed to determine the effects of prolonged heating between 120º-300º C, on the expandability of Asbury 3393 intuminescent flake graphite. Since this product is routinely used in fire retardant applications the effect of above-ambient temperatures for extended time periods on the expandability, under test conditions, may be indicative of in-situ behavior. The results presented below may give users and formulators of graphite-containing fire retardants or graphite-containing fire stops some indication about:
1. The in situ behavior of graphite-containing articles which have been exposed to above normal temperatures from:
- proximity to furnaces or other central heating systems
- proximity to areas previously involved in a fire
- location in areas exposed to extreme heat from local weather conditions
- proximity to, or contact with pipe systems used for carrying process steam or other high temperature fluids.
2. The effect of high process temperatures on the expandability of treated flake graphite exposed to these processes.
Methods and Materials:
3393 is acid intercalated flake graphite which finds use as an intuminescent material in fire retardant applications. 3393 typically contains 2-3% sulfur, which is present as intercalated sulfuric acid. The nominal sizing of 3393 is -20 +50 mesh (primarily particles between 850 and 300 micrometers in size).
Ten samples of 3393 were placed into 5 cm. diameter Petri dishes and heat treated at the prescribed test temperature in a Thermolyne programmable muffle furnace. The furnace was pre-heated for all heat treatment temperatures. Samples were left in the furnace for between one and ten days each. For example, sample one was maintained at the test temperature for 24 hours, sample ten was maintained at the test temperature for 240 hours. After the prescribed test time, each sample was removed from the furnace and checked for percent heat expansion. Heat expansion was measured at 950 C using Asbury Graphite Mills Test Method E4-4.
The Raw Data:
Table 1 and Table 2, below, present the data collected.
The data in Table 1, “Initial Expansion at the Test Temperature” is the expansion realized by each sample due to the elevated temperature of the test conditions. This value will be somewhat indicative of the degree of “aging” that may occur as a result of a product’s exposure to above ambient temperature in service.
Table 1: Initial Expansion at the Aging-Test Temperature
The data in Table 1, above, shows the temperature-exfoliation effect resulting from treatment at the test temperature indicated.
Table 2 contains the raw data for the “overall expansion” of the test samples. The “overall expansion” is the sum of the volume increase realized in initial-expansion (see Table 1) and the final expansion. The final expansion is the measured exfoliation which occurs during the final heat treatment of the thermally aged test samples at 950ºC. This data was generated by testing expansion using Asbury method E4-4.
Table 2: Overall Expansion Ratio as a Function of Oven Residence Time and Temperature.
|3772||> 300||≥ 98||0.9||3.1||300:1||5 - 10|
|1721||> 300||≥ 98||0.9||3.5||300:1||1 - 6|
|3721||> 300||≥ 95||0.9||3.5||290:1||5 - 10|
|1722||> 300||≥ 95||0.9||3.5||290:1||1 - 6|
|3335||> 300||≥ 85||0.9||3.2||270:1||5 - 10|
|3577||> 300||≥ 85||0.9||3.4||270:1||1 - 6|
|3570||> 180||≥ 80||0.8||3.1||230:1||5 - 10|
|1395||> 180||≥ 80||0.8||3.5||230:1||1 - 6|
|3558||> 180||≥ 99||0.8||3.1||210:1||5 - 10|
|3626||> 75||≥ 80||0.6||3.0||160:1||5 - 10|
|3494||> 75||≥ 80||0.9||2.9||90:1||1 - 6|
|3538||< 75||≥ 80||1.4||2.6||60:1||5 - 10|
Explanation of data in Table 2: Treatment temperatures are listed along the top margin of the Table. The treatment period, in days, is listed along the left margin of the Table. The values reported are the volume expansion as performed by Method E4-4. As an example: the value at day 3, under the temperature = 200º column (220) is the overall expansion of the sample aged for three days(72 hours) at 200ºC.
Results and Discussion:
Heat/time effect on overall expansion
The 3393, treated flake graphite, had a 240:1 expansion ratio in the “as received” condition. Table 2 presents the overall expansion data for 3393 at the various test conditions.
Note that at 120 ºC, 140ºC, and 180ºC there is no change in the expansion ratio for any test time.
At 200º C the expansion ratio remains at 240:1 for 48 hours. After 3 days, the expansion ratio drops twenty points to 220:1 where it remains stable for the remainder of the testing period.
At 220º C the expansion ratio is reduced by 40 points after 24 hours at the test temperature. After 2 days, the expansion drops an additional 20 points where it remains stable at 180:1 for the remainder of the experiment.
At 240ºC the expansion is reduced by 60 points after 24 hours. No further reduction occurs, however, with the expansion leveling off at 180:1 for the remainder of the test period.
At 260ºC the expansion is reduced to 160:1 after 24 hours and remains stable at this level for the entire test duration.
At 300ºC the expansion ratio drops to 100:1 after 24 hours and remains stable for 48 hours. At 3 days, the expansion drops an additional 20 points to 80:1 after which it remains stable for the remainder of the test period.
Based on the experimental results, there may be two effects occurring at temperatures above 180ºC. One is the “premature expansion” of only a fraction of the material; the other is the reduction of the expansion ratio of the fraction of unexpanded graphite remaining.
Pre-mature or low temperature expansion
It is assumed that the “premature expanded” material is exfoliated as a result of temperature by the same mechanism which results in expansion at normal testing conditions, i.e. 950ºC. Based on qualitative observation, the expanded graphite residue appears to be expanded to a lesser extent than treated flake which is heated to the nominal 950ºC test temperature. In other words, the magnitude of the “c” axis expansion in individual flakes appears less in these particles. This observation does make sense since the heat flow which effects expansion will occur at a lower rate as the temperature drops. Less heat flow equates to a slower pressure rise of trapped intercallant. A slower rise in pressure will result in less gas-pressure induced impulse within the crystal corresponding to less “c” axis expansion.
The fact that some flake expands at low temperatures and some at higher temperatures does suggest that different expansion mechanisms may be at work. Intercalation of expanding reagent at different temperatures may occur at sites which are not crystallographically equivalent, i.e., intercalation between parting layers vs. intercalation between graphene layers. However, any in depth mechanistic discussion is beyond the scope of this report.
High temperature expansion of heat aged 3393
High temperature expansion of 3393 samples which have been heat aged above 200º C show reduced overall expansion ratio. As discussed above, the overall expansion reported on Table 2 is the total of the initial-expansion (at test temperature) and the final expansion at 950ºC.
It is interesting that at the different temperature regimes time-at- temperature has very little effect on the measured expansion. Essentially, the effect is the same for 24 hours as it is for 240 hours at a given temperature. This type of behavior is again indicative of a mechanistic effect which may be related to the crystallographic environment where intercallant materials reside.
The state of the heat aged material has not been determined. Aged residue may consist of:
- a mixture of pre expanded, and un-expanded flake which is partially depleted in intercallant due to heating effects.
- a mixture of pre expanded, and un-expanded flake.
- a mixture of pre expanded, partially depleted, and un-expanded flake
The quantitative characterization of heat aged flake is beyond the scope of this report but may be studied in the future. The above statements have been presented based only on the interpretation of the data generated from the testing described.
Expandable graphite is an effective intuminescent material for use in fire retardant applications. The data clearly suggests that expandable graphite is stable for extended periods of time at temperatures significantly above ambient. Even at temperatures approaching the limit of thermal stability of cellulostic materials acid intercalated flake graphite retains significant intumescence.
Albert V. Tamashausky
Director of Technical Services, Asbury