Purpose of the study:
The purpose of this study is to determine the effects of boiling alkali solution (sodium hydroxide) on the expansion characteristics of 3393 expandable flake graphite. Any change in the expansion ratio of material treated by the method outlined below may indicate changes which could occur in similar chemical environments encountered in situ.
3393 is acid treated (intercalated) flake graphite. Any reduction in the intercalated acid content is expected to reduce the ratio of expansion. One possible way to reduce intercalated acid content may be through chemical neutralization. Such neutralization may occur as a result of interaction between the un-expanded, intercalated flake graphite and components utilized in the matrix into which the graphite is added.
The reaction between sodium hydroxide and sulfuric acid is: 2NaOH + H2SO4 = Na2SO4 + 2H2O, which is a standard neutralization reaction. At 2-molar NaOH concentration, the amount of sulfuric acid (96%) which can be neutralized by 100 milliliters of alkali solution is approximately 10 grams. The total quantity of “sulfate” intercalate present in 100 grams of 3393 is expected to be no more than 4 grams.
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 (bisulfate intercalation). The nominal sizing of 3393 is -20 +50 mesh (primarily particles between 850 and 300 micrometers in size).
Fifty grams of 3393 flake graphite was added to 100 ml of 2-molar sodium hydroxide (80grams NaOH/liter). The mixture was heated to boiling and allowed to boil for one hour. After boiling the mixture was allowed to stand for approximately 24 hours.
After 24 hours the solution was decanted and the graphite residue washed repeatedly to remove any residual sodium hydroxide. The washed, alkali-treated graphite was then transferred to a glass dish and placed into a drying oven at 105-110 C. After allowing sufficient time for drying the alkali-washed-expandable-graphite was tested for expansion ratio using the standard Asbury method (Asbury Test Method E4-4). The expansion ratio of alkali washed 3393 was compared to an aliquot of the original 3393, which was not treated with sodium hydroxide.
The Table below presents the results of the experiment.
|3393 Untreated||3393 Treated with NaOH|
|Carbon/Loss on ignition||98.68 %||99.04%|
|Sulfur||2.66 %||2.36 %|
|pH(10grams in 100 ml water)||2.35||9.94|
|Expansion Ratio Test 1||240:1||220:1|
Results and Discussion:
The most obvious result of the alkali treatment of 3393 is the apparent reduction of expansion from 240:1 to 220:1. This equates to a total reduction in expansion of 8.3 percent. A similar procedure, using cold 1-molar NaOH was done previously and no effect on expansion was observed. However, this current procedure utilized a higher concentration of base along with higher treatment temperature (102 C boiling temperature). Therefore it is not surprising that this more chemically aggressive environment resulted in a slightly diminished exfoliation of the flake graphite.
It is the opinion of the author that the reduction in expansion ratio observed in this study resulted from the neutralization of the intercalated materials which affect expansion. Although a discussion of the precise mechanism of expansion and the exact crystallographic/molecular position of intercalents within graphene layers is beyond the scope of this article, suffice it to say that rapid solid-to-gas or liquid-to-gas phase transformation of these intercalents cause the separation of the graphite crystal parallel with the “C” crystallographic axis. Any reduction of these intercallant species, from consumption by an alkali material for example, will reduce the amount of intercallant available for gasification within the crystal.
Sulfuric acid decomposes into water and sulfur dioxide (or trioxide) gases at high temperature. One mole of sulfuric acid will yield approximately two moles of gaseous decomposition products. Upon neutralization with an alkali, however, the products of neutralization are one liquid and one solid (water and sodium sulfate in this case). Essentially the intercalated water is the only one of the two neutralization products that can “flash” vaporize under the test conditions and act as an effective expansion agent. Sodium sulfate has to pass through two separate phase transformations before the gas phase is reached so its effectiveness as an expansion agent may be limited.
Since the total reduction in expansion ratio in this experiment is only 8.3%, it is suspected that only “edge neutralization” of intercalated materials has occurred as a result of the reaction between the “treated” graphite and the sodium hydroxide solution. The term “edge neutralization” is somewhat vague, but is used here to describe the reaction between the alkali solution and the intercalated acid which resides at close proximity to the prismatic edges of the flake graphite used in this study, in other words acid close to the surface but not at the surface (surface neutralization certainly does occur but is not expected to effect the overall expansion).
Note that the pH of the alkali-treated 3393 is 9.94. This high pH was realized even after repeated washing of the flake with distilled water (washing and soaking was done over a 2 day period). The persistence of a high pH is perhaps indicative of basic or alkali functionality induced on the graphite surface due to contact of the graphite with the highly concentrated hydroxyl solution (NaOH).
Based on the results of this study the intumescent stability of Asbury 3393 expandable flake graphite is only moderately affected by hot concentrated sodium hydroxide. In the method described above more than enough alkali reagent was available to neutralize 100% of intercalated acid. However, the total acid consumed by the reaction resulted in a decrease in exfoliation of only 8.3%. It is expected that a similar stability will be exhibited toward other high pH materials.
A laboratory study has shown that Asbury 3393 expandable flake graphite exhibits a high degree of chemical stability toward concentrated sodium hydroxide solution. 3393 and similar expandable graphite materials are expected to show good in situ chemical stability toward other basic systems. Most products into which these types of material are added are much less chemically aggressive then the solution utilized in this study. Based on the test results reported above, and based on past studies using less concentrated alkali, expandable graphite is expected to remain stable regarding its expansion functionality when exposed to high pH environments.
Albert V. Tamashausky
Director of Technical Services, Asbury