A Study on the Interfacial Characteristics of Nitramine Explosive-Polymer Binder

Transcription

1 169 A tuy on the Interfacial Characteristics of Nitramine Exlosive-Polymer Biner Jung. him, Hyoun. Kim, Keun D. Lee an Jeong K. Kim Agency for Defense Develoment (ADD) Yuseung P.O.Box 35-5, Daejon, Korea Keywor : Interfacial Characteristics, urface Free Energy, Exlosive Preare for Presentation at the 004 Annual Meeting, Austin, TX, Nov., 7-1 Coyright c J.. him, Agency for Defense Develoment AIChE shall not be resonsible for statements or oinions containe in aers or rinte in its ublications.

2 Abstract A Plastic Bone Exlosive (PBX) is mainly comose of nitramine exlosive an olymer biner. PBX is characterize by high velocity an ressure of etonation, low vulnerability an goo thermal stability. Many imortant alications of PBX require the goo ahesion between nitramine crystals an the biner. Ahesion eens on the surface characteristics of filler an biner. In orer for the better esign for ahesion, rofoun knowlege of the surface an interfacial characteristics of exlosive an biner is require. The influence of interfacial roerties of PBXs - such as interfacial tension, work of ahesion, sreaing coefficient an ensity - on imact sensitivity were investigate..

3 1. Introuction Interfacial roerties of Plastic Bone Exlosive (PBX) have been wiely investigate because these roerties have an imortant effect on sensitivity an erformance of PBX. Therefore, several investigators have stuie interfacial roerties such as interfacial tension, work of ahesion, sreaing coefficient, ilation etc. in PBX interface [1-3]. Nitramine-olymer comosites suffer from a roblem known as ewetting. When the ahesion between a nitramine crystals an a biner is not articularly strong an can be fail uner stress, ewetting occurs rather suenly an this leas to a significant ro in tensile strength of exlosives [4]. Dewetting aversely affects the erformance an sensitivity characteristics of an exlosive comosition. Vois, which are generate between nitramine an biner on ewetting, act as initiation sites if they are aiabatically comresse by an imact or a shockwave. In this stuy, exlosive (RDX) an 5 kins of olymers are selecte, since they are wiely use in many lastic bone exlosives. urface free energy etermine from wettability ata is a roer metho to escribe the roerties of a soli surface. ince the surface free energy of solis cannot be measure irectly, the contact angles of filler an biners are measure by Wilhelmy late metho an liqui enetration metho. An then the surface free energies are calculate from contact angle values by the metho of Kaelble. An interfacial tension an work of ahesion of PBXs are calculate by geometric mean metho. This aer iscusses an influence of interfacial roerties of PBXs on imact sensitivity.. Theoretical Backgrouns.1 Contact angle Contact angle of a series of test liquis such as water, glycerol, formamie, ethylene glycol, trycresyl hoshate an 1-ethoxy ethanol etc. on the surface of exlosive an

4 olymer secimen were measure using Cahn ynamic contact angle analyzer (DCA-31 system) [5]. Test samles for the Wihelmy late metho [6] were reare by iing microscoic sli cover glasses in the liqui uncure biner an then cure usie own to revent the buil u of biner on the sie use for measurement. Contact angles of ower were etermine by the liqui enetration metho [7] base on the Washburn s equation. F = r cosθ (1) 0 h krl cosθ t η = (). urface Free Energy urface free energy is an imortant hysicochemical roerty of a material that can be assesse inirectly from wettability measurement. In this stuy, the metho roose by Kaelble [8] is use for the surface energy analysis of solis. Accoring to the semiemirical theory, the intermolecular forces contributing to surface an interfacial tensions, an subsequent henomena such as wetting, coul be broken own into ineenent an aitive terms. For examle, a olar molecule such as an ester woul have two terms making u its surface tension, isersion force () an iolar interaction (), so that r + = r r (3) where r an r are the isersion an iolar contributions to the total surface tension. This rincile rouces a reasonable aroximation for the work of ahesion for interactions involving only isersion an iole forces.

5 W W + W a = (4) s a The rincial relations for escribing the isersion an olar interactions between liquis an solis are state as follows: r = r + r =α + β (5) L L r = r + r =α + β (6) Wa = ( α α + β β ) (7) L L W a α L = α + β ( β / α ) (8) L L where α an β are square root of the resective isersion r an olar r P arts of r. From equation (8) we recognize a simle metho of grahical analysis wherein a lot of W a /α L versus (β L /α L ) efines α an an intercet at (β L /α L )=0 an β as a sloe..3 Interfacial Tension The geometric mean metho (Eqs. (9)) is use to calculate the interfacial tension between exlosive (RDX) an biner. When two materials are ahere by an interface, the more similar the two materials are, the lower the interfacial tension will be an the better the ahesion will be between them. r L 1 P P D D = r + r ( r r ) ( r r ) (9) 1

6 .4 reaing coefficient When soli-surface free-energy arameters are known, the sreaing coefficient () may be calculate to reict the interactions of biner with a exlosive []. During wet granulation, sreaing of biner over a ower mass is referre. The sreaing coefficient is the measure of the sreaing egree of one exlosive over another an is calculate as a ifference between work of ahesion (W a ) an work of cohesion (W c ). The sreaing coefficient of a biner over the exlosive ( 1 ) or exlosive over the biner ( 1 ) can be calculate. These calculations can be carrie out accoring to the methos of Wu (Eqs. (10) an (11)), 1 = r r 1 r1 + r r r 1 + r + r r (10) 1 = r r 1 r + r 1 r r 1 + r + r r 4 1 (11) where the subscrits 1 an refer to hases 1 an, resectively. r is the iserse hase of surface free energy, r is the olar hase of surface free energy. 3. Materials an Methos 3.1 Materials RDX was selecte as a moel exlosive because it is wiely use in many high energy PBX. Biners were 3 EVAs (UI Chem, UA) with VA contents ranging from 15 to 60%, Hytem (Zeon Chemical, UA) an Viton (DuPont, UA). Liquis use for wetting assessment were: istille water, glycerol (Alrich, UA), formamie, iioomethane, ethylene glycol, trycresyl hoshate, -ethoxy ethanol etc. an the surface tension roerties of test liqui is liste in table 1.

7 Table 1. urface Tension Proerties of Test Liquis at 0 Test liqui Density (g/cc) Viscosity urface Tension (yne/cm) (cp) r r Water Glycerol Formamie Ethylene glycol Dimethyl sulfoxie Trycresyl hoshate Dimethyl formamie ethoxy ethanol r 3. Wetting measurements For contact angle measurement, small ellet of EVA 15, EVA 31, an Viton were resse an EVA 60, HyTem were coate to glass microscoe slies ( cm). The exeriments were erforme by immersing the coate glass slies an ress samles (3 4 cm) in the vertical osition to a eth of about 10 mm in one of the liquis liste in table 1. Wetting of the ower was also etermine by means of a liqui enetration metho with the use of ynamic contact angle analyzer. The enetrating liquis were water, formamie, iioomethane, ethylene glycol an Trycresyl hoshate. Glass caillaries (100 mm long, inner iameter 9 mm) were fille with rie ower. 3.3 PBX (moling ower) PBX s are reare by moifie water-solvent slurry metho. The RDX is iserse in water with a suitable surfactant to form slurry. Then a lacquer solution comose of biner, in a suitable solvent, is ae to the slurry. The biner is coate onto the surface of the exlosive crystals by the system to istill off the solvent. The coate exlosive is filtere, washe, an rie.

8 3.4 Imact ensitivity Test A Julius-Peter s imact machine was use to etermine the imact sensitivity of the PBX formulations. Test was conucte in accorance with MIL-TD-1751A [9]. The H 50 value is the height in centimeters at which the robability of exlosion is 50%. 4. Results an iscussion 4.1 Contact angles of biners an exlosive The contact angles of nine teste liquis on the various biners an exlosive are shown in table. As we can see from table, the contact angles of biner an exlosive were in the range of yne/cm. Unfortunately, ue to the fact that as contact angle of the liquis was cosθ > 1, enetration with low-surface tension liquis, such as hexane, ecane an hexaecane, was not achievable. Table. Contact angles of biners an exlosive Test liqui r L EVA 15 EVA 31 EVA 60 HyTem4454 Viton RDX Water Glycerol Formamie Diioo methane Ethylene glycol Dimethyl sulfoxie Trycresyl hoshate Dimethyl formamie ethoxy ethanol urface free energy Accoring to equation (8), lotting of W a / αl against (β L /α L ) woul result in α s (square root of isersive soli surface free energy) as the intercet an β s (square root of olar soli surface free energy) from the sloe. The results are shown in figure 1 an table 3.

9 The α an β values are converte to their corresoning isersive (r ) an olar free energy (r ) term. The total soli surface free energy (r s ) is the sum of r s an r s. Also the critical surface tension (r c ) of soli are calculate using contact angle. A lot of cosθ versus r, the known surface tension of liqui, is usually mae an the line extraolate to cosθ=1. It can be foun from table 3 that surface free energy of EVA was range of 36. to yne/cm, an increasing soli surface free energy with ecreasing VA content an r s of HyTem4454 was yne/cm an r s of viton was yne/cm. Also, r s of RDX was 35.5 yne/cm. We known that the r s of exlosive similar to that of olymer biner. Table 3. urface free energy of biners an exlosive Material r C α s β s r s r s P r s EVA EVA EVA HyTem Viton RDX α s =5.55 β s =.11 γ s = α s =4.65 β s =4.06 r s =38.10 W A /α L 7 W A /α L β L /α L β L /α L Figure 1. urface free energy of RDX Figure. urface free energy of HyTem4454

10 4.3 Effects of interfacial roerties on imact sensitivity The geometric mean metho (Eqs. (9)) is use to calculate the interfacial tension between exlosive an biner. Materials with similar surface energy roerties will have low interfacial tension when joine an more energy will be require to searate or ewet the surfaces. It is therefore esirable to have a low interfacial tension between RDX an biner. The sensitivity characteristics of a PBX were influence by these roerties. The interfacial tension, work of ahesion, ensity an sreaing coefficient between RDX an biner are eicte in table 4. The results show the imact sensitivity factor (H 50 ) of the PBX increases with ecreasing interfacial tension, while work of ahesion, ensity an sreaing coefficient is not as straightforwar as one woul exect. From table 4, it is easy to know that the major contribution factor to H 50 is interfacial tension, comare to any other factors. We acknowlege that a esensitizing efficiency eens strongly on coating efficiency. Consequently, a lower interfacial tension of biner to exlosive coul guarantee a better coating an thus a better esensitizing efficiency. Table 4. Interfacial roerties of RDX-base PBXs. PBXs Materials Interfacial Tension (yne/cm) Work of Ahesion (yne/cm) Density (g/cm3) reaing Coefficient RDX-EVA RDX-EVA RDX-EVA RDX-HyTem RDX-Viton H 50 (cm) 5. Conclusions The surface free energies of exlosive an five biners have been etermine from the contact angle measurements. The interfacial characteristics between exlosive an biner

11 were calculate from the surface free energies. These values have been use to reict ahesion between exlosive an each biner as well as the imact sensitivity of PBX. The major contribution factor to imact sensitiveness of PBX was the interfacial tension, comare to other surface roerties. Reference 1. H. Hengjian, D. Haishan, Z. Ming, C. Kemei an Z. Yufen, A Quasi-Theoretic Metho for the election of Desensitizers in the Designs of Exlosive Formulations, 003 International Autumn eminar on Proellants, Exlosive an Pyrotechnics, Vol. V,.54(003).. O. Planinsek, R. Pisek, A Trojak,. rcic, The utilization of surface free-energy arameters for the selection of a suitable biner in fluiize be granulation, International Journal of harmaceutics, Vol. 07, Issues 1-,.77-87(000). 3. J.. him, H.. Kim an K.D. Lee, The Analysis of surface free energy of RDX/EVA from contact angle measurements, KIMT, Vol. 3, art.,.19-30(000). 4. E.. Barak an T. Kaully, ensitivity of resse PBX: Influence of biner an mechanical roerties, Proceeings of the 35 th International Conference of ICT,.(99) 1-7(004). 5. Cahn Instruments Inc., DCA-31 Instruction manual, L. Wilhelmy, Ann. Phy., Vol. 119,.177(1963). 7. Wu,. : Polymer Interface an Ahesion, Dekker, NY(198). 8. D.H. Kaelble, P.J. Dynes an E.H. Cirlin, Interfacial bining an environmental stability of olymer matrix comosites, J. Ahesion, Vol. 6,.3-48(1974). 9. Mil-TD-1751A, afety an Performance tests for the Qualification of Exlosives, 001.