Investigation of Residue 

and Coating Stoichiometry on 

120~mm Combustible Cartridge Cases 

by Kestusis G. Chesonis, Pauline M. Smith, and William S.. Lu& 

ARL-TR-2337 September 2000 

200103128 i 

Approved for public release, distribution is unlid?d. 
. 

The findings in this report are not to be construed as an official 

Department of the Amy position unless so designated by other 

authorized documents. 

Citation of manufacturer’s or trade names does not constitute an 

official endorsement or approval of the use thereof. 

Destroy this report when it is no longer needed. Do not return 

it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, h4D 21005-5066 

ARJL-TR-2337 September 2000 

. 

Investigation of Residue and Coating 

Stoichiometry on 120~mm Combustible 

Cartridge Cases 

Kestutis G. Chesonis, Pauline M. Smith, and William S. Lum 

Weapons and Materials Research Directorate, AlU 

Approved for public release; distribution is unlimited. 
Abstract 

An investigation was conducted to determine the cause of coating residue found in the test 

gun chambers during qualification firing of 120~mm combustible cartridge case (CCC) 

ammunition for the MlAUA2 main battle tank. The CCC is coated with a clear epoxypolyamide topcoat and a highly pigmented aluminum epoxy basecoat. A laboratory analysis was 

performed on actual residues; parts of the CCC retain samples of actual coatings and laboratory 

control samples. Since there are six different resins used in this two-component coating system, 

production errors and deviations from correct mixing prior to application could have caused the 

residue. Any variation from the specified stoichiometry could also result in films with lower 

cross-link densities and lower glass transition temperatures (Tg). Rounds fired at 120 ‘C have 

more residues than identical rounds fired at ambientemperature; additional weeks of cure lower 

the amounts of residue. A differential screening calorimeter (DSC) analysis of the various 

samples revealed that a suspect lot of ammunition with higher levels of residue had lower Tg 

values. Fourier-transform infrared (FTIR) was effective in monitoring the degree of cure of the 

epoxy coating system. Analyzing the residue from actual firings showed similar features. 

Koenig hardness values showed variations caused by dry film thickness, but also showed 

insignificant changes caused by transformations in the stoichiometry of the clear topcoat from 

52.5 to 45 phr (pounds per hundred of resin). 

ii 
Acknowledgments 

The authors would like to thank Jae Shim and the U.S. Army Armament Research, 

Development, and Engineering Center (ARDEC), Picatinny, NJ, for sponsoring the study. We 

also thank Philip Patterson, Gume Rodriquez, Jeffrey Duncan, and Alan Teets of the U.S. Army 

Research Laboratory (ARL) for their assistance. A special thanks to Dr. Alex Hsieh of ARL for 

his hard work, technical comments, and guidance on this project. 

r 

I 

. 

. . . 

111 
hTENTIONALLY LEFT BLANK. 

iv 
Table of Contents 

3 

Acknowledgments 

. . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 

List of Figures ._......................................................................................................... vii 

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix 

1. Introduction . . . . . .._.....................-.......................................................-........................ 1 

2. Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-............................................................ . . . . . . . . . . 3 

3. Experimental . . . . . . . . . . . . . . . .._.......................................................................................... 3 

3.1 

3.2 

3.3 

3.4 

Materials .............................................................................................................. 3 

Thermal Analysis ................................................................................................. 5 

FTIR ..................................................................................................................... 6 

FTIR Application ................................................................................................. 6 

4. DSC Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-..................................... 7 

4.1 

4.2 

4.3 

4.4 

4.4.1 

4.4.2 

4.4.3 

Combustible Cartridge Cap Study ....................................................................... 

Topcoat Study.. .................................................................................................... 

Plastic Strips ........................................................................................................ 

Retain Samples ..................................................................................................... 

Room Temperature vs. Tg .............................................................................. 

Oven Cure Time vs. Tg (“C) and Pencil Hardness ........................................ 

Afected Cure Stoichiometly on Tg.. .............................................................. 

7 

7 

7 

10 

10 

11 

12 

5. FI’IR Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-................................... 13 

6. Koenig Pendulum Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

8. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-............... 21 

Distribution List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-......... 23 

Report Documentation Page .._........................................._............-......................... 25 

V 
INTENTIONALLY LEFT BLANK. 

vi 
Finure 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

Glass Transition Temperature of Control Cap and Caps 1 and 2 .............................. 

Typical DSC Scans of Epoxy Topcoat...................................................................... 

A Comparison of Caps With Laboratory Material.. ................................................... 

A Comparison of Caps, Residue, and Laboratory Control ........................................ 

Tg of Various Plastic Strips ....................................................................................... 

Tg of Epoxy Topcoat as a Function of A/B Mix Ratio.. ............................................ 

ATR Spectrum of Sample Cap .................................................................................. 

ATR Spectrum of Control Cap .................................................................................. 

FTIR Spectrum of Yuma Residue.. ............................................................................ 

FTIR Spectrum of Bis-phenol A-Epoxy.. .................................................................. 

Component A/B ......................................................................................................... 

Component A/B2 ....................................................................................................... 

Formula Comparison of 52 phr to 45 phr .................................................................. 

& 

8 

8 

9 

9 

10 

12 

13 

14 

14 

15 

17 

17 

18 

vii 
bJTENTIONALLY LEFT BLANK. 

. . . 

Vlll 
List of Tables 

3 

Table m 

1. Resin Composition ..................................................................................................... 3 

2. Topcoat Formulation .................................................................................................. 4 

3. Room Temperature vs. Glass Transition Temperature .............................................. 11 

4. Tg (“C) and Pencil Hardness vs. Oven Cure Time .................................................... 12 

ix 
. 

bTENTIONALLY LEFT BLANK. 

X 
1. Introduction 

The 120~mm family of tank ammunition used by the U.S. Army’s main battle tank, the 

Abrams MlA1/2, is the most advanced and lethal in the world. The two main types of 

ammunition are the kinetic energy (KE) and the high explosive anti-tank (HEAT); both have the 

propellants in an aluminum-coated combustible cartridge case, and only a small metal base case 

is discharged after firing. 

A major deficiency of the coating that existed in 1990 was its tendency to delaminate in 

sections when subjected to stress during uploading and downloading (Koehler 1990). He pointed 

out that premature ignition from these unprotected areas was possibly caused by their 

susceptibility to any burning residue left in the gun chamber. Another area of concern was that 

moisture penetrated the combustible cartridge in areas that lacked protective coatings. For these 

reasons, Picatinny Arsenal, NJ, requested that The Coatings Team evaluate this problem and 

recommend solutions. 

The final product looks similar to the type of metallic finish on many automobiles. The 

performance specification requires the coating system to be tough yet flexible, and able to endure 

repeated uploading and downloading during training and actual operations. The basecoat 

provides resistance to moisture and ignition from any possible residue left from previous firings, 

while the topcoat provides toughness and film integrity. To avoid any stress-induced 

delamination, coatings were formulated to balance the coefficient of thermal expansion and the 

elastic modulus between the basecoat and topcoat. Since these coatings are thermosets in nature, 

the stoichiometric ratio of the components and the degree of cure play an important role in 

determining the physical and mechanical properties of the coatings on the finished rounds. 

These coatings are applied by high-volume, low-pressure spray guns to the combustible 

cartridge c&es (CCC), consisting of nitiocellulose, wood pulp, and polyurethane resin. The 

average film thickness is controlled to 30-35 pm for the aluminum basecoat and 25-30 pm for 

1 
the clear topcoat. Both the topcoat and basecoat are force dried at 160-l 80 “F for 20-30 min to 

ensure production and handling ruggedness. 

The routine field testing conducted at the U.S. Army Test Center (ATC), Aberdeen Proving 

Ground (APG), MD, and at Yuma Proving Ground (YPG), AZ, involves firing coated 120-mm 

combustible cartridge cases conditioned at 120 “F, ambient, and subambient temperatures. 

Field observations revealed that the coating residues left behind in the chamber after ballistic 

firings were significantly higher for the rounds that were conditioned at 120 “F. Excess residues 

.were also evident when rounds were fired only seven days after a coatings application (not the 

usual time of 3-4 weeks). However, retesting a few weeks later gave acceptable results with 

minimal residue in all cases. A preliminary examination showed that the majority of these 

residues appeared to be the clear topcoat film with minor amounts of aluminum from the 

basecoat. Since the cross linkage of this epoxy system is continuous at ambient temperature, the 

industry-wide practice of evaluating film after a seven-day cure time is insufficient when the 

coating is subjected to high temperature and pressure in the gun chamber. 

To establish feasible characterization techniques for the quality control of the coated 

cartridge cases, differential scanning calorimetry @SC) and Fourier-transform infrared (FTIR) 

spectrometer characterization techniques were employed to determine the state of cure and cure 

chemistry of the clear topcoat. The focus was on identifying any differences in the coated 

specimens obtained from coating residues, production runs, production retains, and laboratoryprepared materials. The goal of this project was to identify and verify the causes of the firing 

residue and to provide a quick and reliable procedure for better quality control of the production 

rounds. 

2 
7 

i 

2. Formulation 

The topcoat epoxy is formulated at 52.5 parts by weight of the polyamide per 100 parts by 

weight of epoxy resin. This polyamide content is within the manufacturer’s recommended ratio, 

45:55, for high performance coating application. The resin composition is shown in Table 1. 

The structural formula for the epoxy resin is 

A. Bisphenol A-epichlorohydrin (DGEBA) resins 

?I *CL-CH , t oe ~~~o~cH*x-cH2 It o~f~~o-cH2-c~~cH? 

3 3 n 

0” (DGEBA) 

Trade names 

Ciba-Geigy Araldite AY 105. 6004. Dow DER 332, Shell Epon 828 

2-3 Ciba-Geigy Araldite 6071. Dow DER 661. Shell Epon 1001 

5-6 Ciba-Geigy Araldite 6084, Dow DER 664. Shell Epon 1004 

while for the polyamide resin, it is 

R-(-C-NH-RI-)-NH-R2-N-I&. 

Table 1. Resin Composition 

Resin 

Epoxy resin 

Polyamide resin 

WPE 

Total (Epoxide equivalent weight) Amine 

Solids Viscosity 100% Value 

75% 22-27 450-550 - 

70% 0.3-0.8 poise - 

at 40 OC 161-173 

3. Experimental 

3.1 Materials. The U.S. Army Research Laboratory (ART-,) Coatings Team received the 

following samples for evaluation: 

3 
(1) One control cap and two from the lots with excessive residue, labeled cap No. 1 and 

cap No. 2. These caps attach the projectiles to the combustible cartridge cases. 

(2) Wet retains from the topcoat vendor, identified as component A and component B. 

Basecoat samples were not available. 

(3) Film thickness strips from the Iowa Army Ammunition Plant of various production lots, 

topcoat, basecoat, and a combination of both coats. 

(4) Two residues from an actual firing at Yuma Proving Grounds labeled as Yuma No. 7 

and Yuma No. 8. 

A sample of the polyamide-epoxy topcoat was prepared in the laboratory as a control. The 

formulation is given in Table 2. Films were allowed to cure at ambient temperature and were 

primarily used for establishing a baseline and for comparative purposes. 

Table 2. Topcoat Formulation 

Comnonent A 

Ingredients 

Epoxy resin 

Flow/leveling resin 

Solvent A 

Solvent B 

Ingredients 

(wt %) 

57.9 

0.8 

19.7 

5.6 

Solvent C 

Component B 

16.0 

Ingredients Ingredients 

(wt %) 

Polyamide 38.8 

Solvent A 14.5 

Solvent B ! 43.1 

late: Mix ratio is 1: 1 of part A to part B by volume. 

4 
3.2 Thermal Analysis. The glass transition temperature (Tg) is one of the main 

characteristic temperatures of an amorphous solid. When an amorphous polymer is cooled or 

heated through the glass transition region, it undergoes structural relaxation. Tg can be defined 

as the temperature in amorphous polymers where a second-order phase transition is detected as a 

distinct increase in the rate of change of the polymer with temperature (TA Instruments 1997). 

All experiments were performed on a TA Instrument DSC 2920. DSC is an analytical method 

that measures the change in heat flow between the sample and an inert reference as a function of 

time and temperature (TA Instruments 1995). The selected sample and an inert reference are 

subjected to controlled conditions of time, temperature, atmosphere, and pressure. Both are 

placed in hermetic pans that sit on raised platforms on a constantan disk. Heat is transferred 

through this disk, which is used as the primary transfer element. The cell measures and monitors 

differential heat flow. In developing a film for any type of coating, the effect of temperature is 

crucial, and the distinctive features of the performance of the coating are controlled by glass 

transition temperature. The transition is detected as a distinct increase in the rate of change of 

the polymers’ thermoplasticity with temperature (Turi 1997). 

The instrument was calibrated according to the manufacturer’s procedure, using Indium as 

the standard reference material. Samples were weighed (approximately 8-10 mg) in ahtminum 

pans, hermetically sealed, and loaded into the DSC cell. The glass transition temperature was 

determined using the midpoint temperature, which is the point on the thermal curve 

corresponding to one half of the heat flow difference between the extrapolated onset point of 

transition and the end inflection change (ASTM 1997a). Experiments were performed on the 

cured films at a heating rate of 10 “Urnin from -25 “C to 200 “C in a nitrogen atmosphere. 

Samples from the 120~mm cartridge were lightly scraped with clean razor blades. 

The samples from the coated plastic strips basically used for film thickness control in 

production were selected to establish a baseline. These samples were previously punched, and 

the clear plastic strip was used as reference. To duplicate results, DMA was performed on 

several strips, but the results were inconsistent because of material variation from the clear 

plastic strips. Although there are various ways to deter-r-nine Tg, and other techniques were 

5 
considered, DSC seems to be the more practical one in this case. The coatings on the plastic 

strips were scraped with a razor, and experiments were performed on the cured films at a heating 

rate of 10 “Urnin from -25 “C to 200 “C. 

3.3 FTIR. An FTIR spectrometer (model 5DXE3, Nicolet Instruments, Madison, WI) was 

used to collect and process infrared wavelength absorbance/transmission spectra of the 

specimens. The spectrometer radiates a broad band of infrared light through the specimen. 

Depending on their chemical bonding, individual materials will absorb, transmit, or reflect the 

infrared (IR) light of various wavelengths. From the spectrum produced, information about 

chemical bonding is obtained from the location of group frequency peaks (Haslam 1965). Most 

spectra contain additional “fingerprint” peaks that are unique to a particular molecular structure. 

This technique is appropriate for all qualitative and some quantitative analyses of most materials. 

An II2 spectrum is capable of indicating the chemical composition and/or bonding of organic, 

polymeric, and many inorganic substances (Haslam 1965). In this case, the FTIR was used to 

detect the specific functional epoxy groups remaining in a cured coating film. The epoxy resin 

of interest is commonly encountered in the coatings industry and is found in many formulations. 

Epoxy resins contain one or more terminal epoxy groups in their molecules, which appear in the 

IR trace as a low-intensity band at 10.9 u (915 cm-‘) (Haslam 1965). Monitoring this 

absorbance band is useful in estimating the degree of cure. As the coating cures, the terminal 

epoxy ring opens, resulting in a reduction and/or loss of intensity at this frequency (Tatsumiya 

et al. 1997). During cure, the terminal epoxy group disappears, and their 

appearance/disappearance can be followed by their intensity. Furthermore, to aid in the 

identification process, the specimen’s spectrum is compared with a computer database of 

standard polymeric materials from an IR reference library. 

3.4 FTIR Application. An attenuated total reflectance (ATR) accessory was attached to the 

FTIR for the initial analysis. This surface measuring technique involves placing the sample in 

direct contact with an internally reflecting crystalline plate. For this analysis, the crystalline 

plate was composed of zinc selenide (ZnSe). The samples were prepared for analysis by 

stripping the cured coating film from the casing and clamping it onto the crystalline plate. 

6 
Sufficient pressure was applied to hold the sample in intimate contact with the crystalline plates 

for direct analysis. Additionally, an JR analysis was run on residual coated materials. The 

residues were thoroughly mixed with potassium bromide (KBr) and Tetrahydrofuran (THF), then 

oven dried at 105 “C to drive off residual solvents. The dried material was compressed into thin 

KBr pellets for spectral analysis. A blank KBr pellet was used as a background spectrum. 

4. DSC Results 

4.1 Combustible Cartridge Cap Study. A control cap was applied from an acceptable 

production lot and two caps from production lots that showed excess residue. These caps hold 

the projectile and are attached with adhesives to the cartridge casing. Figure 1 shows samples 

labeled as cap No. 1 and cap No. 2 with low Tg values, an average of 60.1 “C and 60.2 “C. The 

control cap as had an average of 72.0 “C. The significant differences in Tg ‘between cap No. 1, 

cap No. 2, and the control indicates deviations in stoichiometry of component A to component B. 

Errors can occur either during the production of A and B or during the mixing process. Figure 2 

shows typical DSC scans for these coatings. Experiments were performed on the cured film at a 

heating rate of 10 “Urnin from -25 “C to 200 “C in a nitrogen atmosphere. Each samtile had 

two runs. The first run erased thermal history and established the baseline; however, the 

recorded Tg was fkom the second run. 

4.2 Topcoat Study. The difference in Tg values for the caps and control samples prepared 

in the laboratory are shown in Figure 3. A comparison of DSC scans for the residue, caps, and 

laboratory prepared samples are shown in Figure 4. The Tg values of 71-73 “C for the gun 

residue compares favorably with results for the control cap, indicating that this lot of paint was 

mixed to correct ratios. A lower Tg would indicate more polyamide and less epoxy, so the 

coating would be more flexible and less hard. 

c 

4.3 Plastic Strips. During the spray application, narrow strips of flexible plastic were 

coated with basecoat only, topcoat only, and a combination of both coats on dummy rounds to 

adjust the film thickness. The initial effort concentrated primarily on the topcoat because the 

7 
--l_l---“-.- 

GLASS TRANSfi’lON TEMPERATURE 

I”.“.---- ---I 

ii 

f0CAP 1 

WAP2 

1 GxxwTAOL 

iP 

i 

Figure 1. Glass Transition Temperature of Control Cap and Caps 1 and 2. 

0.00 

-0.02 i 

I 

-0.08 ‘1 

-0.10 1 

50 

Em up 

1 

0 50 100 

Temperature (“C) 

Figure 2. Typical DSC Scans of Epoxy Topcoat. 

8 
Glass Transition Temperature 

i 

ISI.( 

O.! 

0.l 

-0.1 

\-- 

I 

--A-- 62.27”C(l) 

I 

--“‘-------k~‘~ t ___.__.____ cae?_-- ---- 

i. 

‘.-.-.-- ---.____ 

--- -..___ h- 7f22”W 

i -=JJ.~.-F _._ yurreresidue7-------..-- --.... -__ 

A is * --.-- ------- ..___._______ 

3- 

5- 

-1.n- 

-50 0 50 

--A -.- -2_ I 72.04X(l) 

1 -%@&llgK , labstandard 

----___--.- 

- I I I , 

100 150 IO 

Exe up Temperature (“C) Universal V2.4F TA Instruments 

Figure 4. A Comparison of Caps, Residue, and Laboratory Control. 

Control cap 

60 

55 Cap 

IO Lab raw material@ Control cap 0 Cap 11 

Figure 3. A Comparison of Caps With Laboratory Material. 

__ Stdca.026 

---- cap253 

Yum residue 8 ---- ____-.- +- 

9 
residue that remained in the chamber after firing was a relatively clear film with minor amounts 

of aluminum from the basecoat. These plastic production strips have been analyzed on a daily 

basis for several months to determine precision and methodology. 

The average Tg value is shown in Figure 5. Each column in the graph represents a 

production batch. From January 1999-May 1999, approximately 70 plastic strips with Tg values 

ranging from 66 “C to 72 “C have been analyzed. 

DSC X-RAY STRIPS (TOPCOAT) 

Figure 5. Tg of Various Plastic Strips. 

4.4 Retain Samples. 

4.41 Room Temperature vs. Tg. Retain samples prepared and air dried in the laboratory 

had Tg values in the upper sixties. The difference between sample A and sample B is the 

production lot of the polyamide. Since samples were tested over time from 4 days to 17 days, Tg 

increased over time and appeared to level off after approximately 17 days. Values for these 

10 
samples are shown in Table 3. This data indicate values that are within the defined parameter 

and also show that the composition of the retain sample is acceptable. 

Table 3. Room Temperature vs. Glass Transition Temperature 

Sample ID Cure Time at Sample A 

Room Temperature T?3 

Sample B 

Tg 

(days) (“Cl (“Cl 

Wet retain samples 4 66.6 65.3 

8 67.2 68.7 

Sample A and B 12 68.0 68.7 

14 68.8 68.7 

I 17 I 70.8 I 68.9 

4.4.2 Oven Cure Time vs. Tg (“C) and Pencil Hardness. To determine the effect of time 

and temperature of force drying during production, samples were prepared in the laboratory and 

force dried at 160 “F for a length of time. Values for these samples are shown in Table 4. This 

data suggest that cure proceeds at 160 OF for a long time, and the final cure is proportional to 

high temperature and longer time. Although DSC results yield reliable Tg values at 160 “F for 

30 min, these coatings were not fully cured. We also evaluated the coating using the American 

Society for Testing and Materials (ASTM) pencil test method for film hardness (ASTM 1999), 

which also verified inadequate cure. Since the ultimate properties of the epoxy network are 

achieved during the final stages of cure, it is critical that cure attain full completion. The pencil 

hardness scale is as follows: 

soft + Hard 

11 
Table 4. Tg (“C) and Pencil Hardness vs. Oven Cure Time 

Cure Temp. 

(min) 

160 “F (20) 

160 “F (30) 

160 “F (40) 

Pencil Hardness 

HB 

F 

H 

Tg 

(“Cl 

63.2 

75.4 

78.0 

4.4.3 Affected Cure Stbichiometry on Tg. Since a shorter cure time leads to more residue, it 

was assumed that harder films and higher Tg would have less residue. To prove this, the ratio of 

epoxy to polyamide was varied. The expected glass transition temperature of the topcoat with 

component A/component B, at mix ratio of 1: 1, should have a Tg of approximately 70 k 2 “C. A 

mix sensitivity study proved that even a 15% variation had adverse effects on Tg. Figure 6 

displays the study. 

Sensitivity Study 

1.5:l.O 1 .I!31 .o 1 .O:l .o 

A:B mix ratio 

1.0:1.15 

Figure 6. Tg of Epoxy Topcoat as a Function of A/B Mix Ratio. 

The primary vendors, Primex and Alliant Techsysystems (ATK), also participated in 

analyzing these films. A correlation study was done on samples that were sprayed on aluminum 

panels at Alliant and air dried for 16 days. Good correlation was established between both 

laboratories with Tg results within 4 “C. 

12 
5. FTIR Results 

Using the control cap as standard, the suspect cap No. 1 and residue from the firings were 

analyzed and compared. The resulting IR spectrum of each sample is shown in Figures 7 and 8. 

The ATR analysis of the control cap and the sample cap No. 1 indicate similar functional group 

composition with minor spectral differences. Also, as shown, the epoxy absorbance band at 

915cm-’ has a very low intensity, suggesting that the cure is relatively complete (Federation of 

Societies for Paint Technology 1969). 

The residue Corn the firings was analyzed to identify its components. Figure 9 represents a 

typical lR spectrum of the residue. Analyzing the composition of the residue from the Yuma 

firing indicates that it has compositional characteristics similar to those of the control sample 

cap. When comparing spectra, the relative intensities of individual absorption and the overall 

intensities of the spectra could vary due to specimen thickness variation. 

0.0834 

0.070 : 

II. 

o.om i 

o.oxl j 

0.040 -j 

0.030 4 

on?0 4 

i 

O.OlOi A 

P 

O.OOUj 

4 

-0.010 f 

4&l 3oin 2&l mio 

Warrenumbers (cm-l) -- 

“-----. 

Figure 7. ATR Spectrum of Sample ,Cap. 

13 
Figure 8. ATR Spectrum of Control Cap. 

Wavenumbers fcm-11 

Figure 9. FTIR Spectrum of Yuma Residue. 

Figure 10 is a reference spectrum of an uncured Bis-phenol A-Epoxy resin. A library search 

was done after each analysis, and this spectrum was a computer match to the samples in the 

library search. 

14 
Figure 10. FTIR Spectrum of Bis-phenol A-Epoxy. 

6. Koenig Pendulum Hardness 

The wet retains and the laboratory controls were analyzed fbrther to determine if a physical 

or mechanical preliminary screening test could indicate stoichiometric variations. Koenig testing 

was done to distinguish the difference between dry film thickness (DFT) and the various ranges 

of coating stoichiometry. The objective was to use the Koenig testing as a simple and quick 

screening prior to DSC testing; then, if these distinctions were possible, Koenig testing would 

complementhe DSC technique. 

The method for Koenig pendulum film hardness testing used by the ARL Coating 

Technology Team is the ASTM method (ASTM 1995). The procedure is based on a pendulum 

resting on a coated surface set into an oscillation motion. The Koenig hardness consists of both 

the damping time and the corresponding number of oscillations required for the amplitude to 

decrease from 6” to 3”. 

The principle behind the test is based on the fact that the amplitude of oscillations decreases 

more rapidly when supported on softer films rather than on harder surfaces. A photo sensor is 

15 
accurately positioned to register swings of the pendulum. An automatic electronic counter 

registers the signals from the photo sensor, and the test result is shown on the digital display. 

But this procedure is limited to certain conditions. The surface of the coating should be level 

and free of disturbances. Dust, poor leveling properties, and coarse pigment agglomerations can 

cause false measurements. The substrate should not be deformed or vibrate under the load of the 

pendulum. DFT is crucial and should be at least 1.2 mil (30.5 urn) to minimize substrate 

influences. Temperature and relative humidity must be carefully controlled. 

The epoxy topcoat was applied at two different film thicknesses on glass panels and air dried 

at room temperature and humidity. The instrument used was a Byk-Gardner from Paul N. 

Gardner Co., Inc., model no. 5854. The Koenig apparatus was calibrated daily according to 

specifications. Hardness measurements were considered on cured sample films from 3 to 

30 days. 

The graph in Figure 11 shows a typical pattern of the cure rate for both formulas; the 

laboratory formula is labeled component A&I. The thicker draw downs made using a 3.2-mil 

blade demonstrate a much slower cure rate than the draw downs using a 2.0-mil blade. 

Figure 12 represents the cure rate of the component A/B2 formula. The difference between 

the two formulas is the production lot of the polyamide. The illustration confirms the cure rate is 

the same as that in Figure 11. 

. 

The laboratory control designated KC-86-1 was made using 100 parts by weight of epoxy to 

45.0 parts by weight of polyamide; KC-87-l was made using 100 parts by weight of epoxy to 

52.5 parts by weight of polyamide. 

16 
KONIG PENDULUM FILM HARDNESS 

Component A/E 

? 

- 14t 

- 135 

- 130 

- 12: 

- 120 

- 115 

-110 

,105 

3 I 14 20 

Dry time (days) 

+ 1z2.0 MIL BLADEt2=3.2 ML MAD 

Figure 11. Component A/B. 

Figure 12. Component A/B2. 

During the period (3-30 days), we could not tell the difference between 45.0 lb per hundred 

of resin (phr) and 52.5 phr was indistinguishable. This is clearly shown in Figure 13, where the 

two lines are indistinguishable. Therefore, the Koenig test, though simple and quick, was not a 

good screening test for the DSC test. 

17 
Laboratory Formula (KC-66-1 & KC-67-l) 45.0 parts versus 52.5 parts 

0 2 4 6 8 10 12 14 

Dry time days) 

“KC66-l-r-KCBG-1.1 KC67-1 +-KC67-1.11 

Figure 13. Formula Comparison of 52 phr to 45 phr. 

7. Conclusions 

The current epoxy coating was developed to offer maximum film deformation resistance 

from normal usage, to ensure resistance to moisture, and to minimize any residues left in the gun 

chamber from previous firings. The ARL-developed coating has met all of these requirements. 

However, sufficient data does not exist to compare variables in coating formulations to the 

amount of residue left after firings. It has been noted that residue is more prevalent when “hot” 

(120 “F) rounds are fired and when the rounds are fired only seven days after applying the 

coating. A few weeks later, these same rounds show minimal residue. 

To the formulator, this indicates that the current system has sufficient toughness and 

flexibility; however, with some modification to a higher Tg value, residue would be minimized 

when the rounds are fired at hot temperatures. This could be accomplished by adjusting the 

topcoat resin ratios from 52.5 to 45 parts by weight of polyamide per 100 parts of the epoxy 

resin. 

18 
DSC is a valuable quality control tool for tracking cure vs. time, provided that the ratios of 

epoxy and polyamides are consistent. This thermal study showed that incomplete film curing 

and higher temperature are the primary causes of gun tube residue. Based on the DSC data and 

observations at field testing, rounds should be allowed to cure for a minimum of three weeks 

before firing. 

19 
. 

20 
8. References 

American Society for Testing and Materials (ASTM). “Standard Test Methods for Hardness of 

Organic Coatings by Pendulum Damping Tests.” Designation D4366-92a, vol. 6.01, 

pp. 594596,1995. 

American Society for Testing and Materials (ASTM). “Standard Test Methods for Glass 

Transition Temperature by Differential Scanning Calorimetry or Differential Thermal 

Analysis.” General Tests Methods, Designation E 1356-91, vol. 14.02, pp. 851-854, 1997a. 

American Society for Testing and Materials (ASTM). “Standard Test Methods for Transition 

Temperature of Polymers by Thermal Analysis.” Plastics, Designation D 3418-97, 

vol. 8.02, pp. l-4, 1997b. 

American Society for Testing and Materials (ASTM). “Paint-Tests for Chemical, Physical, and 

Optical: Appearance.” Paints, Related Coatings, and Aromatics, Designation D3363-92a, 

vol. 6.01, pp. 378-379, 1999. 

Federation of Societies for Paint Technology. “lR Spectroscopy, Its Use in the Coatings 

Industry.” Inpared Spectroscopy Committee of Chicago Society for Paint Technology, 

vol. 35, pp. 126130. 

Haslam, J., and H. A. Willis. Identzfzcation and Analysis of Plastics. New Jersey: 

D. Van Nostrand, 1965. 

Koehler, Captain C. J. “120-mm Tank Main Gun Ammunition: An Accident Waiting to 

Happen.” ARMOR, 1990. 

TA Instruments. “DSC 2920 Differential Scanning Calorimeter,” 1995. 

Tatsumiya, S., K. Yokokawa, and K. Miki. “A Dynamic DSC Study of the Curing Process of 

Epoxy Resins.” Journal of Thermal Analysis, vol. 49, pp. 123-129,1997. 

Turi, E. Thermal Characterization of Polymeric Materials, Second Edition. California: 

Academic Press, 1997. 

21 
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT NPE AND DATES COVERED 

September 2000 Final, Nov 98 - Jun 99 

4. TlTLE AND SUBTITLE 5. FUNDING NUMBERS 

Investigation of Residue and Coating Stoichiometry on 120~mm Combustible 

Cartridge Cases 622624H1800 

6. AUTHOR(S) 

Kestutis G. Chesonis, Pauline M. Smith, and William S. Lum 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

ATTN: AMSRL-WM-MA 

Aberdeen Proving Ground, MD 2 1005-5066 

8. PERFORMING ORGANlZATlON 

REPORT NUMBER 

ARL-TR-2337 

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AGENCY REPORT NUMBER 

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-- 

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Approved for public release; distribution is unlimited. 

12b. DISTRIBUTION CODE 

13. ABSTRACT(Maximum 200 words) 

An investigation was conducted to determine the cause of coating residue found in the test gun chambers during 

qualification firing of 120~mm combustible cartridge case (CCC) ammunition for the MlAl/A2 main battle tank. The 

CCC is coated with a clear epoxy-polyamide topcoat and a highly pigmented aluminum epoxy basecoat. A laboratory 

analysis was performed on actual residues; parts of the CCC retain samples of actual coatings and laboratory control 

samples. Since there are six different resins used in this two-component coating system, production errors ant 

deviations from correct mixing prior to application could have caused the residue. Any variation from the specified 

stoichiometry could also result in films with lower cross-link densities and lower glass transition temperatures (Tg) 

Rounds fried at 120 “C have more residues than identical rounds fired at ambient temperature; additional weeks of curr 

lower the amounts of residue. A differential screening calorimeter (DSC) analysis of the various samples revealed that s 

suspect lot of ammunition with higher levels of residue had lower Tg values. Fourier-transform infrared (FTIR) war 

effective in monitoring the degree of cure of the epoxy coating system Analyzing the residue from actual firings 

showed similar features. Koenig hardness values showed variations caused by dry film thickness, but also showei 

insignificant changes caused by transformations in the stoichiometry of the clear topcoat from 52.5 to 45 phr (pound 

per hundred of resin). 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

120~mm combustible cartridge case, epoxy coating, DSC, Tg, Koening Hardness Tester, 27 

FTIR, ammunition 16. PRICE CODE 

17. SECURIN CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LlMlTATlON OF ABSTRACT 

OF REPORT OF THIS PAGE OF ABSTRACT 

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL 

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Prescribed by ANSI Std. 239-18 298-l 02 
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