Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 134-140
134
Effect of Bubble Dimensions on Shock Sensitivity in Gelled Slurry Explosives K. F. Keirstead Royal Military College of Canada, Kingston, Ontario K7L 2W3, Canada
D. De Keg“ fluid Dynamics Research Institute and Department of Chemical Engineering, University of Windsor, Windsor, Ontario N9B 3P4, Canada
Three commercial gelled slurry explosives (GSE), Powermex 300, Powermex 500, and Iremite H, were stored at 288, 300, 310, and 322 K. Cartridges were detonated at appropriate intervals for measurements of velocity of detonation (VOD), density, rigidity, and bubble parameters such as the void distribution f , the mean bubble diameter a, and the spacing factor E . Roughly 10000 photographs have been examined. The density at failure approached 1400 kg/m3, while the VOD dropped to about 3000 m/s. The bubble parameters did not seem to correlate with failure. Prepared GSE compositions have been used as a comparison between the homogeneous structure of small samples and the more heterogeneous internal structure of commercial samples. Based on the information generated, a “realistic” model relating storage temperature to time elapsed before failing is presented.
Introduction Gelled slurry explosives have largely displaced the use of dynamite in general excavation and rock mining applications because of low cost and safety considerations. Gelled slurry explosives consist essentially of concentrated solutions of ammonium nitrate and alkali metal nitrates and an appropriate selection of sensitized energetic fuels and other modifying additives. Bubble-sensitized slurries have the advantage of greater safety and lower cost, but they are subject to stability problems on extended storage. The stability problems are related to changes in bubble size as well as to the stability of the gel system itself. The properties of bubble change and stability of the gel system (gelling agent and crosslinking agent) are interrelated and interdependent. Some changes in bubble size take place in a stable gel by means of interbubble diffusion, but these will be accelerated if a deterioration takes place in the gel matrix. The sensitizing effect of a bubble system in ammonium nitrate gels may be extended by the use of an appropriate form of aluminum. Paint grade aluminum (micro size) acts as a sensitizer, while larger aluminum particles act mainly as a fuel. For sensitivity trials, three commercial explosives differing in aluminum content have been used: high aluminum (Iremite), low aluminum (Powermex 500), and no aluminum (Powermex 300). Detonation velocity obtained by firing trials, density, gel rigidity, mean bubble diameter, mean interbubble distance, and bubble size distribution measurements were obtained. Literature Survey The first commercial application of gelled slurries was reported in 1959 on the Mesabi Range in Minnesota. During the 1960’s the development of slurry explosives was particularly dynamic because of the wide range of formulations, densities, sensitivities, and consistencies they provided (Dick, 1972). Slurry explosives are based on concentrated solutions of mixed inorganic nitrates, usually those of calcium, am-
monium, and sodium, which may be thickened or gelled with natural gums, such as guar. Furthermore, aluminum powder, organic fuels, and solid oxidants may be introduced in this matrix. The sensitivity of slurry explosives may be increased by the incorporation of small air or gas bubbles. Air bubbles may be incorporated during the mixing of the explosive, or gas bubbles may be added to a more permanent form as small gas-filled silicate spheres. It has been shown that bubbles provide “hot-spots”, as the gas they contain is adiabatically compressed by the advancing detonation front and that these hot spots assist propagation of the detonation (Burgess and Hooper, 1977; Hay and Watson, 1968). The internal surface area is important with respect to sensitization (Chick, 1965). In addition to bubbles initially present in an explosive, cavitation ahead of the reaction front may produce additional bubbles. By means of high-speed photography it has been shown that cavities and bubbles are responsible for fast reactions in nitroglycerineand then help to sustain them by various processes such as adiabatic collapse of cavities by pressure waves from the deflagration front. Presentation of a large burning surface as the deflagration front enters the cavitated liquid and jetting during cavity collapse disperses liquid droplets in the heated cavity or produces high-impact pressures (Coley and Field, 1973). The particular effect of small bubbles (about 5 x m) may depend on the shock sensitivity of the matrix. In a study of explosive rates in thin films of liquid explosives by high-speed photography, Coley and Field (1973) emphasized the importance of bubble size; small closely packed cavities caused a buildup to a flash reaction moving at 1500 m s-’. Larger cavities may cause a marked increase in deflagration, but they do not necessarily lead to flash reactions. Rather and co-workers (in Taylor, 1952) estimated that in high-speed detonations the temperature in gas pockets may reach more than 10000 K and even in the condensed phase may be heated to several thousand degrees. Chau-
0196-4321/85/1224-0134$01.50/00 1985 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table I. Patents Cited on Iremite Cartridges 1-1.Clay, R. B.; Udy, L. L.; Orsenbach, W. 0. U S . Patent 3 249 474, May 3, 1966. 1-2. Clay, R. B. US. Patent 3453158, July, 1969. 1-3. Brockbank, S. M. U.S. Patent 3582411, June 1, 1971. 1-4. Christensen, C. E.; Clay, R. B.; Thornley, G . M. U.S. Patent 3610088, Oct 5, 1971. 1-5. Clay, R. B.; Udy, L. L. U.S. Patent 3660181, May 2, 1972. 1-6. Cook, M. A.; Clay, R. B.; Udy, L. L. US. Patent 3713917, Jan 30, 1973. 1-7. Murphy, C. H.; Udy, L. L. US. Patent 3783735, Jan 8, 1974. 1-8. Thornley, G. M.; Udy, L. L. U.S. Patent 3886010, May 27, 1975. 1-9. Jessop, H. A. U S . Patent 3 890 171, June 17, 1975.
dry et al. (1972) found the minimum size and temperature of these hot spots to be typically lo-' to lo4 m in diameter and 700-800 K, respectively. According to Koldunov et al. (1973), the heating of the gas is not the main cause of initiation and reaction development. Bubble size in gelled slurry explosives is mentioned in several basic patents. Clay (1969) mentioned that an examination of an aluminum-sensitized slurry revealed a range of bubble sizes from lo4 to m with a majority between 5 X lo4 m and 5 x m. Another patent (Brockbank and Clay, 1971) suggests that the number of bubbles is more important than the size. Graphs of detonation velocity vs. density (Brockbank and Clay, 1971) show an increase in detonation velocity from a density of lo3 kg m-3 to 1.1 X lo3 kg m-3 and a decline of detonation velocity from a density of 1.1X lo3 to 1.2 X lo3 kg m-3 in 1.25 X 10-'-m diameter charges. Prior to 1969 no substantial use had been made of calcium nitrate in GSE compositions because of the relative insensitivity of calcium nitrate in such compositions. Clay et al. (1972) found several advantages in the use of calcium nitrate. Calcium nitrate slurries can be made having relatively high density as compared with slurries made up primarily with ammonium and sodium nitrate. Calcium nitrate slurries require considerably less water to render them fluid due to the release of the water of crystallization normally present in calcium nitrate. The congelation temperature of "fudge" point of slurries containing calcium nitrate can be lowered; that is to say: larger total salt concentrations can be used without raising the fudge point. Dominating patents in the field of GSE have been issued to Cook and co-workers. Since a listing of Cook patents appear on the package of Iremite H used in the prresent work, a close study of Iremite patents has been made. A list of these patents is shown in Table I and referred to in the text as I-i, where i indicates the reference number in the table. A preferred guar is XG 492 formerly made by General Mills. This gum contains added hydroxyl groups. XG 492 promotes aeration of the liquid and stabilizes the foam (1-3 in Table I). Since calcium nitrate tends to inhibit cross-linking in unmodified guar compositions, the use of an oxidized guar is recommended in the presence of calcium nitrate, e.g., Guar 5-808 and 42-24 (1-3) from Stein, Hall & Co. Addition of cross-linking agent, e.g., metal salts such as borax, alkali, metal dichromates, or permanganates, tends to facilitate and expedite the thickening process. Sodium dichromate appears to be somewhat slower in action, giving less initial cross-linking of guar gum but forms a stiffer gel after 5 min (1-2). A cross-linking composition containing both an oxidizing agent and a reducing agent is said to be effective. A 50150 ( w t %) solution of potassium dichromate and potassium antimony tartrate or potassium dichromate in water and gallic acid at a concentration in
135
Table 11. Preparation of Ammonium Nitrate Gel. Composition of Stock Solution (Lignin Surfactant) ammonium nitrate, uncoated prills 0.250 kg sodium nitrate, coated prills 0.050 kg calcium nitrate, granular 0.050 kg water 0.150 kg 0.500 kg
the gel of 0.2 to 0.5% and at pH 4.3 to 5.0 represents a typical example. Several references are made in Iremite patents regarding the addition of ammonium nitrate prills to GSE. Mixing solids into the solution after its aeration breaks up and removes any undesirably large bubbles (1-3). Ammonium nitrate prills may be added in cracked prill form (1-6). Mixing temperatures mentioned in Iremite patents range from 330 to 360 K. Aluminum is used in GSE as an energetic fuel or as a sensitizer for detonation. The slurry blasting agent containing aluminum may be sensitized to full strength performance at relatively high densities (up to 1.5 X lo3 kg m-3) without resorting to aeration methods (Cook, 1968). Sensitivity in GSE is strongly dependent on the particle size of the solid explosive sensitizer. Thus the sensitivity of the slurry blasting agent plus aluminum increases as the surface area of the aluminum increases, or as its particle size decreases. Thin, flaky, and porous granular aluminum are better sensitizers than nonporous spherically shaped aluminum. Fine-grained ammonium nitrate and sodium nitrate prills give better sensitivity than coarse solid oxidizers (Cook, 1968). Hot spot sensitization is effective in GSE agents and explains why the sensitivity is markedly increased by coating the aluminum with thin solid films of hydrocarbon (hydrophobic coatings), (1-1). Three grades of aluminum are used in GSE (O'Dette, 1979). Paint grade aluminum used as a sensitizer is prepared by ball milling. The particle size is 5 X to 1 X m. Atomized aluminum is used as a fuel and is typically 50% -200 mesh. Because atomizing is a hazardous operation, the tendency is to use coarser forms of aluminum. Grain or flake aluminum is also used as a fuel and is produced by chopping aluminum foil. The particle size is -14 mesh to +45 mesh.
Experimental Section Materials. The following gel slurry explosives were used for storage stability and sensitivity measurements: Powermex 300 and Powermex 500 (containing 4% Al) supplied by Canadian Industries Ltd. and Iremite H, containing 8% aluminum, which was supplied by IRECO, U.S.A. The following materials were used for the GSE preparations: ethylene glycol, BDH reagent; ammonium nitrate (uncoated technical prills) was provided by Canadian Industries Ltd. The sodium nitrate was Anachemia coated prills, technical grade. The calcium nitrate was Anachemia granular, reagent grade. The zinc nitrate used for pH adjustment was BDH reagent. The guar used for thickening was Jaguar A-40-F, special and Jaguar HP (hydroxyalkyl) supplied by Stein, Hall and Co. The surfactants were Lignosol XD-65, purified sodium lignosulfonate supplied by Reed Ltd., Chemical Division and Hyonic JN 400, sodium ether sulfate of a primary straight-chain alcohol, supplied by Nopco Chemical Division, Diamond Shamrock Chemical Co. The cross-linking agent was potassium pyroantimonate, supplied by Stein, Hall & co.; the aluminum was SA-23, fuel aluminum, blown powder, -200 mesh, and 5100G, fine sensitizer, hydrophobic coated with stearic acid provided by the Aluminum Co. of Canada.
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Table 111. Preparation of Ammonium Nitrate Gel. ComDosition of Stock Solution (Nonlignin Surfactant) ammonium nitrate, uncoated prills 0.1505 kg sodium nitrate, coated prills 0.0250 kg calcium nitrate, granular 0.1410 kg ethylene glycol 0.1410 kg 0.0425 kg water 0.5000 kg
Table IV. Composition of Explosive Slurry (Lignin Surfactant) s o h containing w t 90 ammonium nitrate 29.1 sodium nitrate 5.9 calcium nitrate 5.9 water 17.5 guar 0.9 1.1 ethylene glycol XD-65 0.3 aluminum 0.5 ammonium nitrate, prills 38.8 100.0
Table V. Composition of Explosive Slurry (Nonlignin Surfactant) s o h containing wt 90 17.3 ammonium nitrate 3.0 sodium nitrate 16.2 calcium nitrate 18.6 ethylene glycol 5.0 water 1.1 Guar HP-8 0.4 aluminum 0.2 Hyonic JN-400 38.2 ammonium nitrate, prills 100.0
Tables I1 and I11 list the preparation of the stock solutions. Tables IV and V list the preparation of the complete compositions. Ammonium nitrate refers to dissolved ammonium nitrate. Ammonium nitrate prills refers to solid ammonium nitrate added either before or after aeration of the gel. The pH of the stock solution was adjusted to the desired value by the addition of solid zinc nitrate as indicated by the pH meter. With rapid mechanical stirring the selected quantity of guar wetted with ethylene glycol was added over a period of 1-2 min. Stirring was continued for several minutes until the Weissenberg effect became apparent. The gel was aged for 24 h before use. Ammonium nitrate prills, either whole or crushed, were added either before or after aeration of the gel. In the latter situation the cross-linking agent was added before the addition of the prills. Compositions were brought from their initial density of 1.39 X lo3 kg m-3 to a density of 1.0 X lo3 kg m-3, before prill addition. The temperature of the gel was brought to the desired temperature in a thermostabxi water bath. The surfactant, XD-65 or Hyonic JN-400, was incorporated into the gel with a spatula, followed by 0.004 kg of aluminum powder. The stirring was done with the Doyon impeller (Keirstead et al., 1980). Stirring was done at a constant speed of 250 rpm, the density of the composition being checked at intervals with an aluminum “grease” pycnometer until the desired density was reached. The cross-linking agent was then added with stirring for about 30 s followed by addition of solid ammonium nitrate in either prill or crushed prill form. It is essential to incorporate the solid ammonium nitrate with a minimum amount of stirring. The
final density was about 1.2 X lo3 kg/m3. Small Diameter Slurry Elevated Temperature Aging and Densensitization Tests. Cartridges were stored at 288,300,310, and 322 K. A 0.05-m diameter was used for all samples and charges 0.80 m in length were used for firing trials. Two cartridges 0.40 m in length were placed end to end after trimming off one sealed end of each. The 0.80-m charge was fitted with a single no. 8 cap and a 0.76-m resistance probe through the center, end to end. As the explosion from the cap travels along the length of the charge, the resistance of the probe decreases and the voltage drop increases. An oscilloscope trace recorded by a Polaroid Land camera related voltage increase to time. When detonation failure takes place with a no. 8 cap, the trial is repeated with a larger booster in an attempt to initiate the detonation. After each firing, a duplicate sample was retrieved from constant temperature storage for physical testing and microscopic examination. Sampling Technique for Microscopic Examination (Powermex Samples). Microtome sections of frozen segments or from embedded samples, as well as razor cuts from frozen samples, tend to crumble. The following technique has been used with consistent and reproducible results. Three disks about 0.01 m in thickness are cut from commercial samples. One is removed from the center and the other two at a distance of about 0.025 m from each end. A segment, 0.01 m wide, is then cut from each disk and supported on a glass slide, resting on dry ice. After thorough chilling, a 0.003 m thick slice is removed from the segment with a scalpel and covered with a cover glass. Twelve photographs are then taken at regular intervals through the microscope (magnification 6 X 10) for bubble size determination. Bubble Size Distribution and Storage Position. The cylindrical cartridges used in the present study were stored in a horizontal position. When the cartridge finally fails to fire, the cartridge becomes limp and somewhat flattened. The change in shape suggests that air voids have migrated to the top side slowly during storage and over a short period of time when gel breakdown takes place. In an effort to measure parameters associated with migration toward the top (storage) side of the cartridge, successive photomicrographs in a series are always taken from the bottom (storage) side to the top (storage) side. Photograph no. 1 of the microscopic examination is near the bottom (storage) side and no. 12 is near the top one. Bubble Size Distribution by Photomicrographs. For the estimation of bubble size distribution the sample was photographed with a Bausch and Lomb microscope furnished with a Canon 35-mm camera using Plus-X Kodak film and a concentrated source of light. The camera was attached to an ocular furnished with a Weibel graticule to provide a system of points and lines for microstereology. The Weibel line was calibrated with a stage micrometer. With a magnification of 6 X 10, the Weibel line corresponds to 238 X lo4 m when 0.001 m on the print corresponds to 10.6 X lo4 m. The magnification may vary from 10.6 to 12.5 depending on the print enlargement. Thirty-six prints were made from each cartridge, twelve prints per location (slice). The diameter of each bubble was measured on the print with a metric rule and entered on a tally sheet according to 1 2 class sizes ranging from 0.0002 to 0.053 m. Each bubble was checked off on the print by a dot. For microstereological analysis, a count is made of the number of points (at either end of a Weibel line) falling on a bubble and of the number of intersections (“cuts”)
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
Table VI. GSE Preparation (Lignin Surfactant) index age f d x 106, m 0 2826.0 36.5 19-33 1 16.0 74.2 33 0.7 118.0 90 0.02 233.5
& X lo3, m 0.026 0.156 0.311 0.342
mixing temp, K 298
comments prills added before aeration
137
103.2 182.5 232.8
0.105 0.228
323
prills added before aeration
16.7 3.1
38.1 41.9
0.138 0.279
328
prills added after aeration; slow cooling
1
3.2
110.8
0.105
318
as above
14
0.5
124.5
0.191
318
as above
19-36
1 33 60
23-7
2 30
23- 11 23-12
3.9 0.1 0.05
of Weibel lines with bubbles. From the “points” and “cuts” the following parameters are obtained: volume of air, number of voids per unit length, the average chord intercept, the specific void surface, and the spacing factor (the average distance between bubbles, circumference to circumference). The volume fraction of a given component Vi can be estimated by imposing a point lattice on the section and counting the relative number of points lying on transsections of the component. The volume contribution of a component “a” is found from D
va = l-a
pt where Pt is the total number of points placed on the section and Pais the number of points lying on “an. If a linear density of particles NL is required, it is necessary to run test lines of total length LT over the microstructure. N represents the total number of intersections of particles by test lines, and in general, there will be two P intersections of particle perimeters to every one intersection of the particle, N. Thus PL= 2NL (Underwood, 1969; Shaefer, 1970). The estimation of void characteristics in a GSE is analogous to the ASTM specification (C457 71, 1979) for the estimation of void parameters in concrete, such as the spacing factor The derivation of the equations used in the calculation of void parameters is given by Powers (1949). For the Iremite H samples, the high aluminum content made bubble size estimates impossible. Figure 1 shows representative photomicrographs of the commercial samples. Density of GSE Samples, Powermex 300, Powermex 500, and Iremite H. The density of the GSE samples was determined with a “grease” aluminum pycnometer. A portion (0.005 kg) of a disk about 0.01 m in thickness was placed in the pycnometer having typically a volume of 11.75 X 10“ m3 and it was weighed. The remaining volume of the pycnometer was filled with toluene. From the weight of the sample, the volume of the pycnometer, the weight of the toluene, and the calculated volume of the toluene, one calculates the density of the GSE sample. The value of the density of the GSE sample is the average of 6 readings-two at each of the three sampling locations for each cartridge. Gel Rigidity. Gel rigidity typically changes during storage. A portion of a cartridge about 0.05 m in length is used for the determination of gel rigidity. The Gelometer (Goring and Young, 1955) and its use are described elsewhere (Keirstead and De Kee, 1980). Results and Discussion GSE Preparation. As shown in Table VI, the bubble size index f decreases quite rapidly over a period of 30 days and the average bubble diameter increases, as expected.
e.
Table VII. GSE Preparations (Nonlignin Surfactant) d x IO6, mixing f m temp, K comments index age 19-14 0 62.1 43.9 298 30 0.2 99.3 91.1 298 24 h to cross-linking 19-16 0 6.7 31 0.23 69.7 51.2 298 4 days to cross-linking 19-18 0 48.3
The same change took place more slowly in the gel matrix (Keirstead and De Kee, 1980), where f ranged from 0.95 to 19.75 as compared with the values of 0.7,0.1, and 3.1 shown in Table VI. Similar values are shown in Table VI1 for nonlignin dispersants. The f values for the commercial samples changed less rapidly with time. The bubble size index f is defined by f=d
(
I--
vl;tva)
where d is the bubble density given by 12
d =
EXi i=l
(3)
X i refers to the s u m of all bubbles in each class; X i = Xi/12 and Vl, V,, and V, represent the volume of the large bubbles, the volume of the small bubbles, and the total volume of the bubbles, respectively. These volumes are obtained by arbitrarily dividing the bubble count summaries for the different tests into two regions, one incorporating the classes 1 to 7 (7 being the class with an average bubble diameter of 100 pm) from which V, is calculated as I
v,= ;a1 cxjv;
(4)
where V jrepresents the volume associated with each class; that is, for class one where the average bubble diameter is 8.75 pm, V, = 350.77 pm3, and a second region (classes 8 to 12) from which VIis calculated in a similar way. The total volume V, is then obtained as
v,= v,4- v,
(5)
Qualitatively, one concludes that the prepared GSE’s are less stable with respect to bubble stability than the commercial preparations. The addition of the solid prills tends to accelerate the disappearance of small bubbles. In addition, the slow cooling of the commercial samples from about 348 to 293 K tends to retard the formation of ammonium nitrate crystals. Extensive crystal formation is observed in the GSE samples. According to present experience, the preparation of more stable GSE would require the use of equipment programmed for very slow temperature reduction.
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Ind. Eng. Chem. Prod. Res. Dev.. VoI. 24. No. 1. 1985
7 r
A
--
I
F i
1
B Figure 1. Photomicrographs of commercial samples: (a) initial sample of Powermex 5M); (h) Powermex 300 stored at 288 K after 872 days; (c) Powermex 500 stored at 288 K after 897 days; (d) voids in Iremite H by reflected light.
Velocity of Detonation, Storage Time, Bubble Size Index f, Void Results, Rigidity, and Density. An extensive statistical stepwise regression analysis was performed to determine which of the variables measured are the important variables with respect to predicting the shelf life of the commercial samples (Powermex 300, Powermex 500, and Iremite H). There are many statistical packages available. Examples are MINITAB, SAS, BMDP, .... The BMDP package was chosen because of the straightforwardness of its control language and the self-explanatory computer output that it provides. A stepwise regression ( A M , 1979) was carried out on the independent variables, VOD, rigidity, density, ..., with age as the dependent variable. For Powermex 300 at 300 K, 50.32% of the variability in the age factor could be explained by the velocity of detonation. This increased to 61.89% if both the VOD and L (mean interbubble distance) were considered, Finally, 69.67% of the variability could he explained by the VOD, E , and the density. After deleting the data of 446 and 575 days affter manufacture (last two firings), on physical grounds, in order to eliminate the effect of a sudden increase in density due to material collapse, 51.7% of the variability in the age factor could be explained by the VOD and 74.85% of that variation could be explained by the VOD and rigidity. In the case of Powermex 300 stored a t 288 K, the VOD explained 83.6% of the variability in the age factor. Deletion of the data of 872 days after manufacture (last firing) resulted in a decrease of the variability to 80.7%. None
of the other variables was important in this case. For Powermex 300 stored a t 310 K, 73.03% of the variability of the age factor could be explained by the VOD; 78.7% could be explained by the VOD and the density. After deleting the data a t 173 days after manufacture (at which time failure occurred during firing), 69.78% of the variability of the age factor could be explained by the VOD and 76.61% could be explained by the VOD and the rigidity. The percentage increased to 81.24% if the variables VOD, rigidity, and density were considered, Similarly, the analysis of the rest of the data led us to conclude that the VOD is the variable to be considered, Variables such as E , rigidity, and density, in some of the cases considered, help to explain a certain (minor) percentage of the variability, but there was no observable trend to indicate which, or which group, of those variables would appear and under which conditions. In any case, the VOD even when combined with some (or all) of these other three variables accounted for the explanation of the overgreat majority of the discussed variability. We then proceeded to estimate after how many days the VOD would reach a value of 3000 m/s. As noted earlier, VOD values below 3000 m/s usually enhance failure. A semilogarithmic plot of this critical amount of days (D,) vs. the storage temperature, T,resulted in a straight line, allowing us to predict after how many days (De), a sample stored at a temperature T would fail. The results for Powermex 300 and Powermex 500 are shown in Figure 2. The equations representing this behavior are as follows: for Powermex 300, the number of days (D,) after which
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985
I 600500 -
A
Table VIII. Water Extraction of GSE Samples Solute, Weight 70 of the Sample time, h sample 0.5 1.0 2.0 pH 13.2 5.6 Powermex 300 8.7 11.2 14.0 5.1 Powermex 500 8.4 10.7 14.1 5.4 Iremite H 6.7 8.9
POWERMEX500 POWERMEX 300
q t-
I L
,001 0
I
I
1
I
I
800
400
I
I200
I
Dc{Jays)
Figure 2. Temperature-D, plots for Powermex 300 and Powermex 500.
one would expect failure for a given storage temperature is given by
D, = -9.17
X
lo3 In (T/324)
(6)
Similarly, for Powerinex 500, the equation is given by
D, = -1.07
X
lo4 In (T/325)
139
(7)
Such an equation can also serve as a guide for the Iremite H performance although one has to realize that in this case it is based on two points only since Iremite H was stored at only two temperatures (288 and 322 K). For the Iremite H case, the equation yields D, = -1.468 X lo4 In (T/329) (8) One of course has to bear in mind that the data used to generate those equations were from commercial samples and not from laboratory preparations. In addition, some of the data points in Figure 2 are the result of extrapolations. Nevertheless, we can recommend the use of those equations (6-8) as a practical guide to predict Powermex and Ireco shelf life, provided one is very careful with extrapolations. The form of those equations suggests that the uncertainty will increase with increasing storage temperature. Visible signs of approaching failure include flaky and wet to the touch samples and limp and easily flexible cartridges. Ahad (1974) pointed out that in a cross-linking agent-water-polymer system, the association factor varied with the polymer concentrations. Failure seems to result from loss of void structure and a breakdown of the gel structure (the specimen falls apart when handled). The moist and sticky appearance appears to be due to syneresis. Variations in density can be caused by random variations in prill content and by loss of air. At or near failure we observed a density rise due to the loss of air. We also observed a massive crystallization associated with the failure of Powermex 300 stored at 322 K. As far as rigidity is concerned, we observed that prior to failure the samples appear to be elastic. Near failure, they become less rigid. The method of measuring rigidity generates a sample compression. This could very well result in too high a reading.
Water Extraction of Powermex 300, Powermex 500, and Iremite H. Leaching of 0.075-m lengths of 0.05-m diameter cartridges were performed using 0.4 L of water at room temperature. The weights of solute recovered after 0.5, 1.0, and 2 h are given in Table VIII. A fresh sample was used for each determination. As shown in Table VIII, Iremite H is significantly less sensitive to water than Powermex 300 and Powermex 500 for treatments of 1h or less. The greatest difference between Iremite and Powermex in gel structures appears when the samples are digested in boiling water. Powermex 500 disintegrates in boiling water in about three minutes. Prolonged boiling appears to have little effect on the gel structure of Iremite. The gelling agent in Iremite appears to have unusual stability properties. According to Iremite patents, the gelling agent may be an oxidized guar (Stein, Hall 5-808 or 44-24) recommended for use in compositions containing calcium nitrate. Aluminum Content of Iremite H and Powermex 500. Aluminum was extracted from Iremite by digesting in a hot solution of nitric acid. Powermex 500 was digested in boiling water. The aluminum content of Iremite and Powermex 500 was found to be respectively 8.2 and 3.9%. Iremite aluminum is probably fine shredded foil shredded in a hammermill. The aluminum is essentially a fuel, but it may contain about 25% sensitizing grade. Powermex 500 contains fairly coarse aluminum and is described as coarse atomized. Calcium Content of Iremite H, Powermex 500, and Powermex 300. Iremite patents cite the advantages of GSE compositions containing calcium nitrate. Calcium was determined by atomic absorption spectrophotometry in a digested sample and was found to be 12.9% as calcium nitrate hydrate, Ca(NOJ2.4H20. Calcium determinations were also obtained for the Powermex samples. The results were 15.1% for Powermex 500 and 10.5% for Powermex 300. Conclusions About 900 days of testing commercial GSE samples and the examination of about 10000 photographs leads to the following conclusions. The velocity of detonation (VOD) of all samples decreases with time to a value of about 3000 m/s, after which failure occurs. The rigidity of the samples decreases with time as well, indicating a gradual weakening of the gel structure. The method used to measure the rigidity may yield values that are too high, as suggested in the Discussion. The density fluctuates and usually shows an increase near the time of failure due to an ‘‘abrupt’’ m-assive loss of air bubbles. The void results ( f , d , and L ) are statistically not important. That is, as long as the distribution of air bubbles is “within bounds”, it does not seem to affect the explosive sample performance. However, the distribution (f) must affect the “structural state” of the material, which in turn is responsible for the change in the VOD. A model predicting the shelf life of the commercial samples as a function of their storage temperature has been presented. The Iremite samples, by far, outlast the Powermex compositions.
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 140-144
140
The prepared GSE samples show an increase with time, both in the mean bubble size and in the mean interbubble distance. The commercial samples exhibit considerable variations in internal structure from point to point and in bubble distribution trends. Acknowledgment The authors wish to express their gratitude to Dr. Bauer and Mr. Heater at Queen's University, Kingston for their contribution with respect to the firing of the commercial samples. They also acknowledge the help of (i) J. O'Dette of the Aluminum Co. of Canada, for the examination of the aluminum content of the samples, (ii) S. De Kee, for help provided with the statistical analysis, and (iii) Reed Ltd., Quebec, for the calcium determinations. Finally, the authors wish to acknowledge financial support through Contract DREV 14/2: Defence Research Establishment, Valcartier, Quebec. Registry No. Powermex 300, 94294-02-7; Powermex 500, 94294-03-8; Iremite H, 94293-95-5. Literature Cited
Ahad, E. J. Appl. Potym. Sci. 1974, 78. 1587. American Society of Testing Materials, ASTM 1979, C457-71. Brockbank, S. M.; Clay, R. 6. US. Patent 3582411, 1971. Burgess, J. A.; Hwper, G. Phys. Techno/. 1977, 8(6), 257. Chaudry, M. M.; Field, J. E.; Heavens, S. N.; Coley, M. 10th International Congress on High-speed Photography, Nice, 1972. Chick. Fourth Symposium on Detonation, 1965, p 349. Clay, R. 6. US. Patent 3453 158, 1969. Clay, R. 6.; Cook, M. A,; M y , L. L. U S . Patent 3660181, 1972. Coley. G. D.; Field, J. E. Proc. R . SOC. London, Ser. A 1973, 335, 67. Cook, M. A. Ind. Eng. Chem. 1988, 60(7), 44. Dick, R. A. Information Circular 8560, U S . Department of the Interior, Bureau of Mines, 1972. Goring, D. A. 1.; Young, E. G. Can. J . Chem. 1855, 3 , 480. Hay, J. E.; Watson, R. W. Ann. N.Y. Acad. Sci. 1988, 752,621. Keirstead. K. F.: De Kee, D. Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 91. Keirstead, K. F.; De Kee, D.; Carreau, P. Can. J. Chem. Eng. 1980, 58, 549. Koldunov; Shevdov; Dremin. Combust. Explos. Shock Waves (USSR)1973, 9 , 255. O'Dette, J. Personal communication, Aluminum Co. of Canada, Kingston, 1979. Powers, T. C. Proc. Highway Res. Board 1949, 29, 184. Shaefer. A. Microskopion 1870, 7, 18. Taylor, J. "Detonation in Condensed Explosives"; Oxford Press, 1952. Underwood, E. E. J. Microsc. 1980, 89(2), 161.
Received for review January 17, 1984 Revised manuscript received October 10, 1984 Accepted October 20, 1984
Afifi, A. A,; Azen, S. P. "Statistical Analysis"; Academic Press: New York, 1979.
Absorption and Desorption of Water by Some Common Fibers John F. Fuzek Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, Tennessee 37662
All fibers, whether hydrophilic or hydrophobic, absorb some water from an atmosphere having a relative humidity above 0%. The equilibrium amounts of water absorbed at different relative and absolute humidiiies and at different temperatures are presented for some commonly used fibers. The amounts of water remaining in the fibers after equilibrium desorption are also shown. Consideration is given to the kinetics of absorption and desorption. The effect of water on some physical properties of the fibers as well as on subjective properties, such as comfort, is presented. Fibers that are considered include polyesters, nylons, acrylics, modacrylics, cellulosics, and polyolefins as well as natural fibers.
Introduction The interactions of moisture and fibers result in many technical and commercial consequences. The resulting weight changes can affect the blend level in fiber blends as well as the commercial weight of the fibers. Changes in mechanical properties as a result of moisture can influence the behavior of textile products under different atmospheric conditions. Fiber swelling caused by moisture can influence the rate of heat transfer and moisture-vapor transfer through a textile fabric. Consequently, changes will occur in the comfort perception as well as in the dimensional stability of the fabric. Moisture in a fiber reduces the fiber's glass-transition temperature; this reduction results in impaired wash-wear behavior and in changes in the aesthetics of the fabric. Furthermore, high levels of moisture in a fiber usually result in low static propensity. Because of these effects of moisture on fibers, the amount of water held at equilibrium, the rate at which water is absorbed and desorbed, and the effect that the amount of water has on fiber properties are of technical and commercial importance. These factors have been well established for the natural fibers cotton and wool (Speakman, 1936;Urquhart, 1924). Since the amount of moisture taken up by most synthetic fibers is substantially less than that taken up by cellulosic fibers and by wool, studies involving the effect of moisture on synthetic fibers have been generally less thorough. The objective of this 0196-4321/85/1224-0140$01.50/0
Table I % R.H.
10 20 30 40 50 60 70 80 90 95
salt system H3POJ/ZH20 CH3COOK CaCl2-6H20 Zn(N03)2.6H20 NaHS04.H20
NH4N03 NH4Cl+ KNO, (equimolar amounts) (NH4M04 ZnSO4-7HZO Na2SO3.7H20
paper is to present data showing the amount of water absorbed and desorbed in a few synthetic fibers and the rates at which these sorptions occur; the effect of water on some of the physical properties of these fibers will also be discussed. Experimental Section Fiber samples were scoured with a 1% neutral soap solution at 40 "C for 30 min. The samples were then washed with water until they were free of soap. They were then allowed to air-dry for 24 h. For the absorption studies, 1-g samples of scoured fibers were placed in weighing bottles in a desiccator with P205 and dried under a vacuum for 2-3 days to obtain the dry weight of each sample. The samples were then placed in desiccators containing a salt-water system to maintain 1985 American Chemical Society
