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Expandable Flake Graphite

3393 Heat Aging Study

3393 Chemical Aging Study

Expandable Flake Graphite

Intercalation:

Expandable flake graphite, also known as intumescent flake graphite, or simply "expandable flake", is a form of intercalated graphite. Intercalation is a process whereby an intercallant material is inserted between the graphene layers of a graphite crystal or particle. After intercalation the resulting graphite material takes on new properties that are a function of the intercallant and the way it associates with the host (graphite) species. Both physical and chemical properties, including crystallographic structure, surface area, density, electronic properties, intumescent behavior, chemical reactivity, etc., may be affected by the intercallant.

A wide variety of chemical species have been used to intercalate graphite materials. These include halogens, alkali metals, sulfate, nitrate, various organic acids, aluminum chloride, ferric chloride, other metal halides, arsenic sulfide, thallium sulfide, etc.

The primary type of graphite intercalation compound described in this article is the "sulfate" intercalation compound sometimes referred to as "graphite bisulfate". This material is manufactured by treating highly crystalline natural flake graphite with a mixture of sulfuric acid and certain other oxidizing agents which aid in "catalysis" of the sulfate intercalation. The resultant product is a highly intumescent form of graphite that has proven useful in applications that include fire retardants, high performance gaskets, conductive fillers, electromagnetic pulse and radiation shielding, foundry products, and many others.

The primary reason for "bisulfate" intercalation is to impart the ability of the treated flake graphite to intumesce, exfoliate, or expand when heated. Exfoliation is a true volumetric expansion resulting from the crystallographic de-lamination that occurs parallel with the "c" crystallographic axis of the graphite flake. The figure, below-left, is a schematic of what occurs during expansion. The top illustration shows a single piece of flake graphite, which has been intercalated but not exfoliated. The bottom illustration is the same flake of graphite after being exposed to a temperature high enough to affect expansion/exfoliation. High temperature causes the expansion agent to gasify, producing enough pressure to push adjacent graphite layers apart. The scanning electron micrographs, below, show an actual graphite flake before and after expansion/exfoliation.

Flake Graphite, Unexpanded

expanded

Intercalated Flake Graphite, Expanded - Note the accordion-like morphology that results from the flake's expansion parallel to the "C" crystallographic axis.

expandable

Exfoliation of the flake graphite results in an overall decrease in bulk density, and an approximately 10 fold increase in surface area. Typically the increased surface area results in a material that has increased chemical reactivity. This reactivity is especially apparent in oxidation rate of uncompressed material.

Exfoliation: The Mechanism: The actual cause of expansion/exfoliation is the increase in volume, and resultant pressure, caused by the rapid heating of the intercallant. A simplified way to view the process is to model the intercallant as a liquid or solid phase that is fixed between the graphene layers. Heating of the treated graphite results in conversion of the intercallant, from a liquid or solid phase, to a gas phase. Gas formation results in an increase in volume of the intercallant of approximately 1000 fold. The pressure generated by this volume increase forces the adjacent graphene layers to separate, resulting in the accordion expansion observed.

expandable

The two Figures, left and right, illustrate schematically the process of expansion/exfoliation. The Figure on the left shows the expansion agent (blue spheres) in their metastable residence between adjacent graphene layers. The Figure on the right shows the expanded intercallant and the results of the expansion on the graphene layer spacing. Note that the interlayer dimensions, shown on the left are equal to approximately 3.35 Å (6.7 Å/2). The interlayer spacing, post exfoliation, is some value greater than 3.35 Å. An increase in graphene layer spacing is a result of the process.

Definition of the Expansion/Exfoliation Ratio: Asbury Graphite's definition of expansion ratio is the quotient of expandable graphite's post heating expansion volume and original sample mass. The "units" are reported simply as "expansion ratio", "exfoliation ratio", or "heat expansion". If one performs a dimensional analysis on the expansion quotient, the units are volume/mass, which reads like a specific volume. Due to certain complications such as volatile content the "expansion ratio" is not a specific volume.

Depending on the size of the test crucible, the test method requires that either one gram or ½ gram of intercalated graphite be expanded and the final expanded volume measured. Although the true density of expandable graphite is approximately 2.2 g/cm3 the bulk density is lower. The result is that a cubic centimeter of expandable flake graphite is close enough to 1 gram in mass to assume that the expansion ratio is the measure of the volume change realized when approximately one cubic center of intumescent flake graphite is exfoliated. The values reported for "heat expansion" are much easier to visualize when making this assumption.

Optimization of Expansion/Exfoliation Ratio: Not all bisulfate intercalated graphite expands equally. Differences in treatment type, particle size, etc., all have a bearing on the overall expansion of the product. Some applications that utilize expandable flake graphite require high expansion ratio, some applications require a low expansion ratio. In the following sections some of the factors that affect the magnitude of the expansion ratio will be discussed.

Expansion Vs Temperature: Most flake graphite exfoliation is affected by subjecting the intercalated product to heat energy. As explained above, heating of the intercallant results in a phase change that provides the inter-laminar pressure required to force layer planes to separate.

The maximum expansion potential of a specific grade of intercalated graphite can only be realized if the heating rate used to affect exfoliation is rapid. Slow heated or heating in "ramping" stages will typically produce low expansion and can even result in no expansion. Since the expansion process depends on harnessing the force developed by an expanding gas, it is important to develop maximum force by causing a sudden, rapid gas expansion. To do otherwise, i.e., slow heating and therefore slow expansion, will result in less than the threshold gas pressure required to affect expansion.

A good way to view the above concept is to model the flake graphite exfoliation process as the expansion of a gas in a cylinder with a loose fitting piston. The Figure below-left illustrates this model. Assume that the bottom of the piston and the bottom of the cylinder represent two adjacent graphene layers. Also assume that the clearance between the cylinder walls and piston represent the prismatic edges that define each graphene layer. Ultimately gas will escape through these edges. The blue spheres trapped between the piston and cylinder bottom represent the intercallant substance assumed to be bisulfate in this case. Let's slowly heat the system that consists of the cylinder/piston and its contents. The Figure to right illustrates this slow heating process.

expandable

Since the heating rate is low, the pressure rise is corresponding low. This is because gases are produced slowly and have time to "percolate" out of the system before pressure builds to a high enough level to cause delamination. The resultant material has not expanded to its full potential.

expandable Let's take the same system and apply heat at a much faster rate. The Figure to the left illustrates this scenario. When heat flow is high the gases produced by the vaporization of the intercallant do not have time to percolate out of the cylinder-piston clearance. The result is high pressure and the resultant force required to lift the piston, or exfoliate the graphite crystal, is realized.

The graph below is a plot of expansion ratio vs. heat treatment temperature for a +50 mesh (<300 micrometers) expandable flake graphite. The plotted data clearly shows the strong relationship of high temperature to high expansion. Note that the maximum expansion is at about 700 °C and higher.

graph

Expansion vs. Particle Size: The second, and equally important, material parameter that affects the expansion ratio is the particle size of the intercalated flake graphite. In general, particle size is directly proportional to expansion ratio. Large flakes typically have higher expansion ratios than smaller flakes. This phenomenon can be understood by looking at another simple model.

Let's assume that the a graphite flake can be modeled as a disc. This is a reasonable assumption since flake graphite is somewhat hexagonal in outline. The figure below illustrates flake graphite's discotic morphology. Although not to scale, the value of "h" shown below represents the interlayer or graphene layer spacing of the crystal. This space is where any vaporized intercalated species would exit the crystal. Discounting any defects or variation in interlayer spacing due to the insertion of the intercallant, the value of "h" is constant at 3.35 Å.

As indicated in the above figure, "r" is the radius of the disc element illustrated. The total area available for gas escape from a single layer is equal to: 2prh. The volume from which intercallant gas is produced is equal to: pr2h. If one looks at the ratio of edge area to disc volume, 2prh /pr2h, it can be seen that the ratio varies as 2/r. You can see that as the radius decreases the ratio 2/r gets very large. The Figure below presents a graphical representation of the (edge area/disc volume) vs. particle radius.

graph

As illustrated by the plotted values, the smaller the particle radius the higher the value of the edge area/disc volume ratio, and the larger the particle radius the smaller the edge area/disc volume ratio.

As the particles get smaller intercallant gases have a more efficient escape pathway. Rapid gas escape prevents development of the pressure required to push adjacent graphene layers apart. The result: low expansion ratio.

In addition to the "pressure" effect modeled above, the thickness of the graphite flakes also has a significant effect on expansion ratio. Since the interlayer spacing is the same regardless of the flake thickness, thicker flake will expand proportionally more than thinner flakes. When grinding, screening, or otherwise reducing the particle size of flake graphite, cleavage parallel to the basal plane (basal pinacoid) occurs. The result is the creation of thinner flakes with subsequent reduction of the expansion potential of those flakes.

The effect of particle size on graphite exfoliation has good empirical support. The Figure to the left presents a graph of Particle Size vs. Expansion Ratio. This data was generated by screen fractionation of a sample of commercially available intercalated graphite. The various fractions were exposed to a furnace temperature of 950 ºC to affect exfoliation and the volume of the expanded flake was compared. Note that the same sample mass was used for each test (Asbury test method E4-4).

The Graph shows a clear trend in the data; as particle size decreases the exfoliation volume decreases accordingly. Formulators who must chose the correct expansion potential for a specific application can use this property variable to their advantage. Specific expansion ratios can be adjusted by mixing different grades of differing expansion potential to control the ultimate expansion ratio of the finished product.

Asbury Graphite Mills is a major world wide supplier of expandable flake graphite. Numerous grades are available, which vary in particle size distribution, carbon content, pH, and expansion ratio. In addition to the 10 standard products listed in this Web site we have the ability to provide customers with tailor made materials designed to precisely fit the application at hand. For technical inquires about expandable graphite please feel free to contact the Asbury Technical Services Department at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it . Inquiries regarding pricing and availability should be directed to the Asbury Sales Department at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it .

Heat Aging of 3393 Expandable Flake Graphite

A Study Performed by the Technical Services Department of Asbury Graphite Mills, Inc. Asbury, NJ, USA

Albert V. Tamashausky

Director of Technical Services

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:

Product Description

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).

Test method:

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

Temperature C	120	140	180	200	220	240	260	300
Pre-expansion	0	0	10	10	20	20	40	50

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.

Temperature	120	140	180	200	220	240	260	300
Day	Expansion
1	240	240	240	240	200	180	160	100
2	240	240	240	240	180	180	160	100
3	240	240	*	220	180	180	160	80
4	240	240	240	220	180	180	160	80
5	240	240	240	220	180	180	160	80
6	240	240	240	220	180	180	160	80
7	240	240	240	220	180	180	160	80
8	240	*	240	220	180	180	160	80
9	*	*	240	220	180	180	160	80
10	240	240	240	220	180	180	160	80

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.

Expansion

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.

Conclusion:

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 Graphite Mills, Inc.

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High pH Chemical Aging of Asbury 3393 Expandable Flake Graphite

An original research report from the Technical services Laboratory of Asbury Graphite Mills, Inc. Asbury, NJ, USA

Albert V. Tamashausky

Director of Technical Services

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.

Product Description:

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).

Method:

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.

Results:

The Table below presents the results of the experiment.

	3393 Untreated	3393 Treated with NaOH
Moisture	2.51 %	-
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
Test 2	240:1	220:1
Test 3	240:1	220:1
Test 4	240:1

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.

Closing Comments:

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.

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