Abstract:
A novel and improved EPDM formulation for a solid propellant rocket motor is described wherein hexadiene EPDM monomer components are replaced by alkylidene norbornene components and with appropriate adjustment of curing and other additives functionally-required rheological and physical characteristics are achieved with the desired compatibility with any one of a plurality of solid filler materials, e.g. powder silica, carbon fibers or aramid fibers, and with appropriate adhesion and extended storage or shelf life characteristics.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a novel composition and method for providing insulation for solid propellant rocket motors, and more particularly to EPDM compositions having fibrous components such as carbon fibers or powder fillers such as silica, or also containing Kevlar reinforcing fibers and suitable for internal and external insulation applications on such rocket motors.  
           [0003]    2. Background and Description of the Related Art  
           [0004]    It is generally accepted current industry practice to prepare insulations for solid propellant rocket motors from a polymeric base importantly composed of an EPDM (ethylene-propylene-diene monomer) terpolymer blend and containing as one of the diene monomer components of the EPDM blend a 1,4-hexadiene (HD).  
           [0005]    This EPDM terpolymer, which is commonly designated as the primary EPDM terpolymer since it is present in a higher concentration than the secondary EPDM terpolymer, has been established as a standard for solid propellant rocket motor insulations due to its superior ablation characteristics, excellent physical properties and processability.  
           [0006]    For instance, an exemplary carbon fiber-filled rocket motor insulation composed of NORDEL 1040 as the primary terpolymer is commonly known in the industry as the STW4-2868 thermal insulation and has the following composition as shown in Table 1:  
                                                                 TABLE 1                           STW4-2868 THERMAL INSULATION FORMULATION       (carbon fiber; parts by weight)                Ingredient   Function   Parts by Weight                            NORDEL 1040   Primary EPDM   80               terpolymer base           Neoprene FB   Secondary   20               polymer base           Zinc oxide   Activator   5           Sulfur   Curative   1           HAF carbon black   Pigment   1           MBT   Accelerator   1           AGERITE Resin D   Antioxidant   2           AGERITE HPS   Antioxidant   1           Tellurac   Accelerator   0.50           Sulfads   Accelerator   0.75           VCM carbon fibers   Filler   41                Total Parts by Weight   153.25                      
 
           [0007]    Alternatively, solid rocket motor insulations are also composed of compositions employing finely divided powder silica as a filler, with or without the added presence of a fibrous reinforcing agent.  
           [0008]    Exemplary silica-filled rocket motor insulations have also included NORDEL 1040 and NORDEL 2522 as the primary terpolymer in their formulations and the resulting compositions are respectively commonly known in the industry as the 053A and DL1375 thermal insulations. They have the following compositions shown in Table 2:  
                                                                 TABLE 2                           THERMAL INSULATION FORMULATION       (silica filled; parts by weight)                    DL1375   053A               (parts by   (parts by       Ingredient   Function   weight)   weight)                    NORDEL 1040   Primary EPDM       80           terpolymer base       NORDEL 2522   Primary EPDM   80           terpolymer base       Neoprene FB   Secondary   20   20           polymer base       Zinc oxide   Activator   5   5       Sulfur   Curative   1   1       AGERITE Resin   Antioxidant   2   2       D       AGERITE HPS   Antioxidant   1   1       Captax   Accelerator   1   1       Tellurac   Accelerator   0.5   0.5       Sulfads   Accelerator   0.75   0.75       HiSil 233   Filler   35.5   35.5            Total Parts by Weight   146.75   146.75                  
 
           [0009]    In addition, an EPDM terpolymer comprising the HD monomer is sold under the tradename NORDEL 2722E. An exemplary silica-filled rocket motor insulation comprising NORDEL 2722E as the secondary terpolymer is commonly known in the industry as the DL1552A thermal insulation and has the following composition as shown in Table 3:  
                                                     TABLE 3                           DL1552A THERMAL INSULATION       FORMULATION WITH SILICA                    Parts by       Ingredient   Function   Weight                    Buna EP T 3950 (Bayer Corp., Fiber,   Primary EPDM   75       Additives and Rubber Division of   terpolymer base       Orange, Texas)       NORDEL 2722E (DuPont Dow   Secondary EPDM   20       Elastomers)   terpolymer base           with high           ethylene content       WINGTACK 95 (hydrocarbon resin)   Tackifier   7       (Goodyear Tire and Rubber Co.,       Chemical Division of Beaumont,       Texas)       IRGANOX 1010 (tetrakis[methylene-3-   Antioxidant   1       (3′5′-di-tert-butyl-4′-hydroxyphenyl)       proprionate]methane) (Ciba Specialty       Chemicals, Additives Division,       Tarrytown, N.Y.)       TRYCOL DA-6 (decyl   Wetting agent   0.5       polyoxyethylene alcohol) (Chemical       Associates, Inc. of Copley, Ohio)       Stearic acid (including palmitic acid)   Cure activator   1       (Harwick Standard Distribution Corp.       of Akron, Ohio)       HiSil 233 (silica hydrate) (PPG   Reinforcing filler   45       Industries, Inc. of Lake Charles,       Louisiana)       Aluminum oxide C (Al 2 O 3 ) (Degussa   Reinforcing filler   0.3       Corporation of Ridgefield Park, N.J.)       N330 carbon black (Columbian   Pigment and   1       Chemicals Co. of Marietta, Ga.)   reinforcing filler       KALENE 1300 (butyl gum elastomer)   Co-vulcanizing   20       (Hardman Division of Harcros   plasticizer       Chemicals, Inc. of Belleville, N.J.)       HYPALON 20 (chlorosulfonated   Cure activator   5       polyethylene) (DuPont Dow       Elastomers)       AGERITE Resin D (polymerized   Antioxidant   0.25       trimethyl dihydroquinone) (R. T.       Vanderbilt Co., Inc. of Buena Park,       Ca.)       TZFD-88p (zinc oxide dispersed in an   Cure activator   2       EPDM binder) (Rhein Chemie Corp. of       Trenton, N.J.)       SP 1056 (bromomethyl alkylated   Curing agent   15       phenolic resin) (Schenectady Int&#39;l, Inc.       of Schenectady, N.Y.)            Total Parts by Weight   193.05                  
 
           [0010]    An exemplary aramid fiber filled rocket motor insulation comprising NORDEL 1040 is commonly known in the industry as R196 thermal insulation and has the following composition as shown in Table 4:  
                                                     TABLE 4                           R196 THERMAL INSULATION       FORMULATION WITH KEVLAR                    Parts by       Ingredient   Function   Weight                    NORDEL 1040 (EPDM terpolymer)   Polymer base   80       NATSYN 2200 (polyisoprene)   Polymer base   20       (Goodyear Tire and Rubber Co.,       Chemical Division of Akron, Ohio)       WINSTAY S (styrenated phenols)   Antioxidant   1.0       (Goodyear Tire and Rubber Co.,       Chemical Division of Akron, Ohio)       Dechlorane Plus 515   Flame retardant   40       (1,2,3,4,7,8,9,10,13,13,14,14-       dodecachloro-       1,4,4a,5,6,6q,7,10,10a,11,12,12a-       dodecahydro-1,4,7,10-       dimethanodibenzo (a,e) cyclooctene)       (Occidental Chemical Corporation of       Dallas, Texas)       Antimony oxide (Sb 2 O 3 ) (Harcros   Flame retardant/-   20       Chemicals, Inc. of Kansas City,   filler       Kansas)       ¼″ KEVLAR fiber (aramid staple fiber)   Fiber   20       (E. I. duPont de Nemours and Co., of       Wilmington, Delaware)       VAROX DBPH-50 (2,5-dimethyl-2,5-   Curing agent   2.5       di(t-butylperoxy)hexane on a carrier)       (R. T. Vanderbilt Co., Inc. of Buena       Park, Ca.)            Total Parts by Weight   183.5                  
 
           [0011]    Numerous past efforts to develop effective replacements for these standard solid rocket motor insulation formulations have not been successful.  
           [0012]    The only manufacturer currently producing the foregoing primary EPDM terpolymer in adequate quantities to meet the demands of the rocket motor insulation industry is DuPont Dow Elastomers of Beaumont, Texas, which markets and sells an EPDM terpolymer comprising the HD monomer under the tradename NORDEL 1040 and Nordel 2522.  
           [0013]    However, the ability of the industry to produce STW4-2968, DL1375, 053A, DL1552A, R196 and other thermal insulations containing NORDEL 1040 and NORDEL 2522, and NORDEL 2722E as a primary or secondary EPDM terpolymer has recently been placed in jeopardy due to the announcement by DuPont of its intention to cease production of NORDEL 1040, 2522, 2722E and, generally, other EPDM polymers formed from 1,4-hexadiene. There is therefore a need in this industry, previously not satisfied, to find an effective alternate or a replacement for the above-described standard STW4-2868, DL1375,053A DL11552A and R196 thermal insulations. Development and formulation of a suitable primary EPDM terpolymer replacement is especially critical for these discontinued NORDEL insulation formulations.  
           [0014]    The requirements for an acceptable, functionally effective, insulation for solid propellant rocket motors are well known to be quite severe due to the extreme conditions to which the insulation is exposed. These conditions not only include exceedingly high temperatures but also severe ablative effects from the hot particles (as well as gases) that traverse and exit the rocket motor interior. Unless the insulation will withstand such conditions, catastrophic failure may (and has) occur.  
           [0015]    U.S. Pat. No. 3,347,047, an early patent describing asbestos fiber filled insulations, states that flame temperatures encountered in the combustion of propellants, particularly when used as source of propulsion, necessitating the confinement of the gases of combustion and ultimate release thereof through orifices, are usually accompanied by extremely turbulent flow conditions. All of these features place considerable stress and strain upon the member defining the escape passageway. While the combustion of the propellant in the case of rockets and the like will usually be of short duration, the temperatures and turbulence encountered have been found to very easily destroy even the strongest and most exotic alloys formed of iron, steel, titanium, magnesium, silicon, chromium, beryllium and the like. As a consequence, the projectile structure fails leading to total destruction thereof through explosion or in the event that only the exit passageway is destroyed, the projectile proceeds in an erratic uncontrollable path since its trajectory or path is at least in part dependent upon the contour of the passageway through which pass the gaseous products of combustion. That statement still remains fully applicable today.  
           [0016]    Therefore any replacement insulation should exhibit at least comparable temperature resistant and ablation characteristics and Theological and physical properties (e.g., Mooney viscosity) at least equivalent to that of STW4-2868, DL1375, 053A, DL1552A and R196, yet should not otherwise significantly alter the formulation techniques employed for the production of the such rocket motor thermal insulation. Additionally, due to the large and growing quantities of solid propellant rocket motor insulation required by the industry, any such replacement EPDM terpolymer candidate should be abundantly available now and into the foreseeable future.  
           [0017]    In addition, any replacement EPDM or like terpolymer should satisfy a number of other requirements including wettability of and bond strength with such diverse filler additives as a carbon fiber, aramid fiber, and a silica powder. It is also necessary that such additives be substantially homogeneously dispersed throughout the insulation composition as it is being produced. While standard mixing devices can be employed in the practice of this invention, such as a Banbury mixer, it is a common experience that substantially homogeneous distribution of fibrous additives is not achieved, or achieved only with difficulty, with many elastomeric compositions. Difficulties have been described as in, for instance, during mixing of the components, it can be observed that premature vulcanization may occur as well as other problems that may impede, or entirely frustrate, effective distribution of the various additives which are essential to the ultimate production of the insulation.  
           [0018]    Further, once formulated, the elastomeric composition must also possess acceptable shelf life characteristics such that it remains sufficiently pliable, without becoming fully cured, until used in application to the rocket motor casing. This requirement is essential because the production of a given lot of insulation may have to wait in storage for a number of months prior to use. Typically, the insulation may be stored in large rolls in an uncured, or at most a partially cured, state until ready for use. A number of curing agents are well known and are conventionally employed but still must be compatible with the overall EPDM formulation to permit satisfactory shelf life. This in turn requires a balancing of curing agent activity.  
           [0019]    In addition, the formulated insulation should be substantially odorless for obvious reasons and this can require special adjustment of the curing agent components.  
           [0020]    After application to the interior (or if desired the exterior) of the rocket motor casing, and subsequent curing thereof, an acceptable insulation must also exhibit satisfactory bonding characteristics to a variety of adjacent surfaces. Such surfaces include the internal surface of the rocket motor casing itself and the insulation must also exhibit adequate bonding characteristics between itself and the propellant grain, typically with an intermediate liner surface. In turn, the propellant grain in a solid propellant rocket motor is composed of a variety of materials notably including still another elastomer, various combustible materials, and such additional components as aluminum particles.  
           [0021]    A functionally acceptable solid propellant rocket motor insulation must meet those requirements and must also survive aging tests. Such rocket motors may be fully fabricated even many months before actual firing, and for tactical weapons especially sometimes even more than a year or even a plurality of years. For instance, strategic missiles may be stored in silos or submarine launch tubes for decades. Over that period of time, the insulation must continue to remain fully functional without unacceptable migration of its components to or from adjacent interfacial surfaces and adequately retain its elastomeric characteristics to prevent brittleness. This requirement also needs to be satisfied under wide temperature variations. The vibration and physical stress placed on a rocket motor at the time of launch, whether a ground launch or an air firing, is exceedingly high, and brittleness and cracking in the insulation is effectively intolerable, whether from premature or gradual overcure or whatever cause. Even at the end of the burn of the propellant grain within the rocket motor casing the insulation must remain substantially and functionally intact to avoid potentially catastrophic failures of the entire launch vehicle.  
           [0022]    In turn, this means that the insulation composition must meet the ablation limits for protection of the casing throughout the propellant burn without adding undue weight to the motor.  
           [0023]    A number of past patents have been granted proposing various solutions to the insulation formulation problem. These include U.S. Pat. No. 3,421,970 (generically describing elastomeric formulations with asbestos); U.S. Pat. No. 3,562,304 (generically describing an elastomeric formulation with asbestos fibers); U.S. Pat. No. 3,637,576 (describing an EPDM formulation with a norbornene component with asbestos fibers); U.S. Pat. No. 4,492,779 (generically describing elastomeric formulations with Kevlar fibers); U.S. Pat. No. 4,514,541 (generically a du Pont “master batch” formulation with Kevlar fibers, but not an insulation); U.S. Pat. No. 4,550,130 (generically describing a moldable carboxylic acid modified EPDM to enhance affinity to various fillers); U.S. Pat. No. 4,878,431 (generically describing an elastomeric formulation using the EPDM Nordel 1040, with Kevlar fibers); U.S. Pat. No. 5,364,905 (describing a technique for the in situ polycondensation formation of aramid fibers, but not referring to rocket motor insulations); U.S. Pat. No. 5,498,649 (describing a polyamide/maleic anhydride modified EPDM with Kevlar fibers for a rocket motor insulation); U.S. Pat. No. 5,821,284 (a Kevlar fiber filled insulation containing an EPDM illustrated by Nordel 2522 in combination with ammonium salts); and U.S. Pat. No. 5,830,384 (generically referring to EPDM&#39;s with a “dry water” silica additive for cooling purposes). None of these patents address nor effectively solve the problem faced by the present invention. In fact the frequent reference to Nordel 1040 or Nordel 2522 serves to confirm the observation that these particular elastomers are well-nigh the standard in the rocket motor insulation industry.  
           [0024]    Accordingly, the search for a functionally satisfactory elastomeric insulation composition requires discovery and implementation of an extraordinarily complex combination of characteristics. The criticality of the material selection is further demonstrated by the severity and magnitude of the risk of failure. Most insulations are of necessity “man-rated” in the sense that a catastrophic failure can result in the loss of human life—whether the rocket motor is used as a booster for launch of the space shuttle or is carried tactically underneath the wing of an attack aircraft. The monetary cost of failure in satellite launches is well-publicized and can run into the hundreds of millions of dollars.  
           [0025]    One well known potential point of failure is the appearance of voids or cracks in the insulation which could lead to the penetration of the rocket motor casing itself. The resultant dispersion of hot gases may not only lead to destruction of the motor generally or can at least lead to its being thrown of its intended course or trajectory with several unhappy results. In such events, either the vehicle itself will self-destruct, or will be intentionally destroyed, or the satellite will be launched into a useless orbit.  
           [0026]    Therefore, one of the most difficult tasks in the solid propellant rocket motor industry is the development of a suitable, acceptable insulation composition that will meet and pass a large number of test criteria to lead to its acceptability.  
           [0027]    Furthermore, any replacement EPDM terpolymers should not be susceptible to obsolescence issues nor discontinuance in future supply thereof.  
         SUMMARY OF THE INVENTION  
         [0028]    It is, therefore, an object of this invention to address a crucial long-standing need in the industry for an acceptable substitute for the STW4-2868, DL1375, 053A, DL1552A and R1961 insulations by providing a reformulated rocket motor thermal insulation notably comprising a suitable primary or secondary terpolymer replacement for the 1,4-hexadiene-based EPDM and one that minimizes the degree of modification to the existing formulation methods and also as to the ultimate functional properties of the STW4-2868, DL1375, DL 1552A, 053A and R196 thermal insulations.  
           [0029]    In accordance with the principles of this invention, these and other objects of the invention are attained by the discovery and provision of a rocket motor insulation formulation comprising, as a primary or secondary terpolymeric base, an EPDM terpolymer formed from at least one alkylidene norbornene, especially ethylidene norbornene (ENB) as the diene component.  
           [0030]    Exemplary EPDM terpolymers that may be used according to this invention comprise those having an alkylidene diene, particularly an ENB diene, component include KELTAN 4506, KELTAN 1446A, KELTAN 2308, NORDEL IP NDR-4520, and NORDEL IP NDR-4640, each of which may be substituted into the STW4-2868, DL1375, R196 and 053A insulation for the NORDEL 1040 without requiring significant modifications to the standard STW4-2868, DL1375, 053A, DL1552A and R196 thermal insulation formulation methods nor as to the resulting multitude of functionally acceptable properties. Other exemplary terpolymers include high-ethylene-content EPDM terpolymers formed from an ENB diene component are NORDEL IP NDR-3722p and BUNA EP T 2370, which may be substituted into the DL1552A for the NORDEL IP NDR-2722E without requiring significant modifications to the DL1552A formulation. Nordell IP NDR-3725 has also been used but the supplier (du Pont) has indicated that due to low demand it now prefers a different formulation, Nordel IP NDR-3722, with a lower diene content of about 0.5% versus about 2.5% for Nordel IP NDR-3725p.  
           [0031]    It has now been found that only a small proportion of ENB diene component is sufficient for incorporation in such elastomers, say from about 2 to about 10 wt. %, preferably from about 2 to about 7 wt. %, and with the balance of the olefin content of the composition composed of ethylene and propylene, with the ethylene forming from about 40 to about 80 wt. %, preferably from about 50 to about 75 wt. %, and with the remainder being propylene. Trace amounts of other dienes may also be present to induce branching in the elastomer. Generally, the only significant modification that is required involves the selection of a less reactive curing agent to offset the higher reactivity (relative to HD) of ethylidene norbornene (ENB). Furthermore, NORDEL IP NDR-3722 and BUNA EP T 2370 are not presently foreseen as being susceptible to obsolescence issues.  
           [0032]    Other objects, aspects and advantages of the invention will be apparent to those skilled in the art upon reading the specification and appended claims which, when read in conjunction with the accompanying drawings, explain the principles of this invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    The accompanying drawings serve to elucidate the principles of this invention. In such drawings:  
         [0034]    [0034]FIG. 1 is a schematic cross-sectional view of a rocket motor depicting various regions in which the insulation of this invention may be applied;  
         [0035]    [0035]FIG. 2 is a schematic of a char motor suitable for conducting the ablation tests reported herein; and  
         [0036]    [0036]FIG. 3 is a cut-away view of a solid propellant rocket motor illustrating the outer case, the internal insulation layer, the propellant grain, and the insulation of the ignitor closure structure. 
     
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0037]    As shown in schematic FIG. 1, and its components in FIGS. 1 a - 8   b , illustrate the manner in which the inventive insulation may be applied to various parts of a rocket assembly. These include, but are not limited to, the system tunnel floor plate shear ply  10 , the LSC blast shield  12 , the stiffener stub hole plugs  14 , stiffener stub insulation  16 , the external joint weatherseal  18 , T-ring insulation  20 , aft dome internal insulation  22 , and center segment aft end internal insulation (underneath the flap)  24 .  
         [0038]    In FIG. 2, the schematic cross-section of a char motor test assembly is illustrated wherein the propellant is contained in the beaker  30 , and low velocity insulation test specimens located at  32  upstream of the throat  34 , with medium velocity test specimens located in the section at  36  and with high velocity insulation test specimens located in the region  38 . Generally, such a char test motor assembly permits the location of a plurality of different insulation formulation test specimens about the circumference at any of locations  32 ,  36  or  38 , in the conventional manner.  
         [0039]    [0039]FIG. 3 is a cut-away schematic view of a typical rocket motor illustrating the case  40 , the nozzle  42 , the propellant grain  44  with its center bore  46  and the internal insulation layer shown at  48 . The insulation of the ignitor closure structure is indicated at  50 .  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0040]    In accordance with one embodiment of this invention, the inventive rocket motor insulation formulation comprises, as a primary polymeric base, an EPDM terpolymer in which the diene component of the EPDM is composed of at least one alkylidene norbornene, and in particular ethylidene norbornene (ENB).  
         [0041]    The selected EPDM terpolymer should be substitutable into the STW4-2868, DL1375, 053A, DL1552A and R196, thermal insulation formulation (Tables 1 and 2) without requiring significant modification of the present techniques employed for the formulation thereof. It is a further highly desirable feature of the present invention that the insulation formulation may be composed of readily available commercial materials, provided that such compositions are properly assembled and blended together for the final insulation material.  
         [0042]    Suitable EPDM terpolymers having an ENB diene component for use in this invention include, without limitation, and as stated above, KELTAN 4506, KELTAN 1446A, KELTAN 2308, each of which is available from DSM of the Netherlands, and NORDEL 1P 4520 and NORDEL IP 4640, both of which are and continue to be available from DuPont Dow Elastomers.  
         [0043]    These materials have the following respective contents as derived from the manufacturer&#39;s data literature:  
                                                                             ENB   Ethylene               Content   Content           Wt %   Wt %   Mooney Viscosity                                        Keltan 4506   4.5   54   40           Keltan 1446A   7   59   10           Keltan 2308   2   74   24           Nordel IP 4520   5   51   20           Nordel IP 4640   5   55   40                      
 
         [0044]    The remaining content is propylene with traces of certain dienes used to produce branching in the molecular structure.  
         [0045]    In accordance with another preferred embodiment, the secondary EPDM terpolymer is/are NORDEL IP NDR-3722 and/or Buna EP T 2370, which include ENB as their diene monomers and are respectively available from DuPont Dow and Bayer Corporation and manufactured in relatively large capacities.  
         [0046]    An exemplary formulation is set forth in TABLE 5:  
                           TABLE 5                                   Ingredient   Parts by Weight                           Primary EPDM terpolymer   70-80           Secondary EPDM terpolymer   15-25           with ENB diene monomer           Tackifier    5-10           Antioxidant   1-3           Wetting agent   0-1           Curing activator    5-10           Silica filler   40-50           Pigment   0-3           Plasticizer   15-25           Curing agent   10-20                      
 
         [0047]    The primary EPDM preferably has a sufficiently high diene content to provide a more reactive polymer to decrease cure time. Preferably, the alkylidene diene content of the primary EPDM is in a range of about 2-12 wt %, more preferably about 10-12 wt %. Additionally, the primary EPDM terpolymer preferably has a medium ethylene content of from about 56 wt % to about 65 wt %.  
         [0048]    In a more preferred embodiment, the primary and secondary terpolymer components, tackifier, antioxidant, wetting agent, curing activator, filler, pigment, plasticizer, and silica are the same as those set forth in Table 3 above and are present in the concentrations specified in Table 3. However, when following that formulation composition on simply a “drop in” basis, it was observed that an unsatisfactory scorch characteristic developed (e.g. a scorch time of about 5.9 minutes as against and 8 minute minimum time allowed per specification. Similarly, substituting Buna EP T 2370 in place of Nordel 2722E led to similar results of an unsatisfactory scorch time of 6.6 minutes and a high Mooney viscosity of 119. Consequently, according to this invention, a less reactive brominated phenolic resin curing agent, for instance, having a lower reactivity than that used in the DL1552A formulation (i.e., SP 1056 containing about 6 wt % bromine) was required and was selected in order to compensate for the observed higher reactivity of ENB. This then led to acceptable scorch characteristics and the use of cure temperatures of about 320° F. provided similar cure rates as were used for DL 1552A. The resulting physical properties were also acceptable. Preferably, the resin curing agent of the reformulated insulation formulation is SP 1055 (manufactured by Schenectady International), which contains about 3% bromine by weight.  
         [0049]    It will be observed that these materials may have significant variations in specific norbornene content, and also in the ethylene/propylene content ratio as well as in the Mooney viscosity, yet each has been found to be effective if selected for use in production of a rocket motor insulation.  
         [0050]    Adhesion-promoting secondary polymers that may also be used in the formulation include elastomer modifiers, especially polar polymers. Among suitable such secondary polymers are chlorosulfonated polyethylene, such as Hypalon 20 from DuPont Dow, and polychloroprene. Polychloroprene polymers are available from DuPont Dow under the tradenames Neoprene FB, Neoprene TW, and Neoprene GRT.  
         [0051]    An exemplary plasticizer for the inventive formulation is the EPDM-based Trilene 67A (Uniroyal).  
         [0052]    Tackifiers may also optionally be used. An example of a suitable tackifier is Akrochem P-133.  
         [0053]    One or more antioxidants are also preferably included within the inventive insulation formulation. Preferred antioxidants include polymerized 1,2-dihydro-2,2,4-trimethylquinoline (Agerite Resin D) and mixed octylated diphenylamines (Agerite Stalite S), each of which is available from R. T. Vanderbilt Co.  
         [0054]    Various powder silica fillers are well known in other elastomeric combinations and may be used, including HiSil 233.  
         [0055]    An exemplary carbon fiber is VMC carbon fiber.  
         [0056]    Suitable cure activators may include metal oxides, such as zinc oxide and magnesium oxide (e.g., Elastomag 170, from Morton Chemical Co.).  
         [0057]    The curing package preferably includes at least one phosphate cure accelerator, including by way of example, Rhenocure AP-5, Rhenocure AP-7, Rhenocure AP-3, Rhenocure ZADT/G, and Rhenocure S/G, which are available from Rhein Chemie and Accelerator VS, available from Akro Chem. Additional cure accelerators that may be used in combination with the phosphate cure accelerator include butyl zimate, Altax, Akroform Delta P.M., Sulfads. While the use of Accelerator VS was initially unacceptable because of the foul odor problem it generated, it has also been now found that such formulations can be prepared with no significant odor when about 1.0 phr magnesium oxide is added thereto.  
         [0058]    Sulfur curing agents are preferred for the formulation. A suitable sulfur-curing agent is Akrosperse IS-70 from Akro Chem. Elemental sulfur can also be used.  
         [0059]    Batches of insulation containing silica powder generally may be formulated in an internal mixer with the following two-pass mix procedure. In the first pass, the mixer speed may be set to approximately 40 rpm and the all of the components other than the curing agents and accelerators are added to the internal mixer. (The silica was added last in the first pass.) The mixing may be performed at a temperature of about 300° F. Suitable mixing times depend on the temperature and mixing speed, and are ascertainable to the skilled artisan without an undue amount of experimentation.  
         [0060]    In the second pass, the mixer speed was set to about 40 rpm. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added, then the remaining half of the master batch was added to the mixer. The second mixing step was performed at a temperature of approximately 180° F. to 190° F. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature prior to evaluating its Theological and physical properties.  
         [0061]    The shaping and curing of the inventive insulation may be performed in accordance with techniques known in the art.  
         [0062]    Exemplary new formulations containing a silica powder filler are set forth in TABLE 6 below with concentrations shown by weight.  
                                                                                                                                             TABLE 6                       EXAMPLES OF SILICA FILLED EPDM INSULATION FORMULATIONS                                Ingredient   RDL5338   RDL5342   RDL5343   RDL5347   RDL5363   RDL5255   RDL5335   RDL5201   RDL5220                    Keltan 4506   70.0   73.0   73.0   55.0   55.0                       Keltan 1446A   20.0   17.0   17.0       Keltan 2308               35.0   35.0       Nordel IP 4520                               80.0   80.0       Nordel IP 4640                       80.0   80.0       Hypalon 20   10.0   10.0   10.0   10.0   10.0       Neoprene FB                       20.0   20.0       Neoprene TW                               20.0   20.0       Neoprene GRT       Trilene 67A       Kraton L-2203       Akrochem P-133   5.0   5.0   5.0   5.0   5.0           5.0   5.0       Agerite Resin D                       2.0   2.0   2.0   2.0       Agerite Stalite S   2.0   2.0   2.0   2.0   2.0       HiSil 233   35.5   35.5   35.5   35.5   35.5   35.5   35.5   37.0   37.0       Elastomag 170   3.0   3.0   3.0   3.0   3.0   4.0   4.0   0.5   0.5       Zinc Oxide                               4.0   4.0       Butyl Zimate   0.50   0.50                           0.50       Rhenocure AP-5   3.50   3.50                           3.50       Rhenocure AP-7       Rhenocure AP-3                       2.90       3.85       Rhenocure ZADT/G                       0.80       1.00       Rhenocure S/G                       0.60       0.80       Altax           1.10   1.10   1.20       1.10       Akroform Delta P.M.                   0.45       0.20       Sulfads                   0.50       Accelerator VS           2.70   2.70       Akrosperse IS-70   1.10   1.10   1.20   1.20   1.40               1.00       Sulfur                       0.35   0.45   0.40                    Ingredient   RDL5370   RDL5380   RDL5381   RDL5194   RDL5222   RDL5273A   RDL5298   RDL5277   RDL5279               Keltan 4506       Keltan 1446A       Keltan 2308       Nordel IP 4520   80.0   80.0   80.0   90.0   90.0   90.0   80.0       Nordel IP 4640                               90.0   90.0       Hypalon 20               10.0   10.0   10.0   10.0       Neoprene FB       Neoprene TW       Neoprene GRT   20.0   20.0   20.0       Trilene 67A                           10.0       Kraton L-2203                               10.0   10.0       Akrochem P-133   5.0   5.0   5.0   5.0   5.0   5.0   5.0   9.0   9.0       Agerite Resin D   2.0   2.0   2.0   2.0   2.0   2.0   2.0   2.0   2.0       Agerite Stalite S       HiSil 233   37.0   37.0   37.0   35.5   35.5   35.5   36.5   38.0   38.0       Elastomag 170   0.50   0.50   0.50       Zinc Oxide   4.00   4.00   4.00   3.00   3.00   4.00   3.00   3.00   3.00       Butyl Zimate       0.50           0.40       0.15       Rhenocure AP-5       3.40           3.15       4.45       Rhenocure AP-7                               3.00       Rhenocure AP-3   3.85           3.85       Rhenocure ZADT/G   1.00           1.00               0.90       Rhenocure S/G   0.80           0.80       Altax           1.00           1.00           1.20       Akroform Delta P.M.                                   0.25       Sulfads                                   0.82       Accelerator VS           2.70           2.70       Akrosperse IS-70                   0.95   1.10   1.34   1.25   1.40       Sulfur   0.40   0.50   0.50   0.40               Ingredient   RDL5319   RDL5320A   RDL5350   RDL5351A   RDL5367   RDL5185   RDL5186   RDL5205   RDL5276               Keltan 4506       Keltan 1446A       Keltan 2308       Nordel IP 4520   40.0   40.0   45.0   45.0   45.0       Nordel IP 4640   50.0   50.0   45.0   45.0   45.0   80.0   80.0   80.0   80.0       Hypalon 20   10.0   10.0   10.0   10.0   10.0   10.0   10.0   10.0   10.0       Neoprene FB       Neoprene TW       Neoprene GRT       Trilene 67A                       10.0   10.0   10.0   10.0       Kraton L-2203       Akrochem P-133   5.0   5.0   5.0   5.0   5.0   5.0   5.0   5.0   5.0       Agerite Resin D                       2.0   2.0   2.0   2.0       Agerite Stalite S   2.0   2.0   2.0   2.0   2.0       HiSil 233   35.5   35.5   35.5   35.5   35.5   35.5   35.5   35.5   35.5       Elastomag 170       Zinc Oxide   3.0   4.0   3.0   4.0   3.0   3.0   3.0   3.0   3.0       Butyl Zimate   0.40       0.40                       0.40       Rhenocure AP-5   3.15       3.15                       3.50       Rhenocure AP-7                           3.00       Rhenocure AP-3                       3.85       3.85       Rhenocure ZADT/G                       0.80   0.80   1.00       Rhenocure S/G                       0.80       0.80       Altax       1.00       1.00   1.10       Akroform Delta P.M.                   0.35       Sulfads                   0.50       Accelerator VS       2.70       2.70       Akrosperse IS-70   1.00   1.00   1.00   1.00   1.00               1.05       Sulfur                       0.40   0.80   0.40                  
 
         [0063]    In addition, individual batches of insulation formulations containing silica filler were prepared with NORDEL IP NDR-3725 (Example 1, Table 8 below), BUNA EP T 2370 (Example 2), and NORDEL 2722E (Comparative Example) as the secondary EPDM terpolymers having high ethylene contents. However, in these formulations it has been observed that there is an increased reactivity of the ethylidene norbornene monomers as used in Examples 1 and 2 (compared to the 1,4-hexadiene monomer of the Comparative Example A), and accordingly, SP 1055 (3 wt % bromine) was used in Examples 1 and 2 and SP 1056 (6 wt % bromine) was used in the Comparative Example A (Table 8) in equal quantities for comparison purposes. In addition, another formulation, RDL 5654 was also prepared in much the same way, but substituting NORDEL IP NDR-3722p for NORDEL IP NDR-2722E, and SP 1055 was used in place of SP 1056.  
                                                     TABLE 7                           Ethylene   Propylene   Diene           Polymer   content %   content %   content % type   Viscosity                                NORDEL IP   71   26.5   2.5/ENB   25       NDR-3722       BUNA EP T 2370   71   26     3/ENB   16       NORDEL 2722E   72   22   6.4/HD     26                  
 
         [0064]    The batches were each formulated in a laboratory scale Reliable Rubber &amp; Plastics Machinery Company Model R-260 internal mixer having a net chamber volume of 4260 cubic centimeters. A 3000 gram batch was prepared by a two-pass mix procedure.  
         [0065]    In the first pass, the mixer speed was set at 40 rpm, and the primary and secondary polymer components, tackifier, and antidegradant were added to the mixing chamber in the concentrations set forth in Table 3 and masticated for one minute. Subsequently, the remaining ingredients were added (same concentrations as in Table 3), and the mixer speed was increased to 60 rpm. The batches were removed from the internal mixer after mixing for a total of approximately 7 minutes. The temperature at the time of removing the batch was 300° F. to 320° F. The master batch was sheeted out on a 6″×13″ Farrel Corporation laboratory two-roll mill and allowed to cool to room temperature.  
         [0066]    In the second pass, the mixer speed was set to 40 rpm. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added (same concentrations as set forth in Table 3), then the remaining half of the master batch was added to the mixer. After a total mix time of approximately 40 seconds, the final mix was removed from the mixer. The actual temperature at the time of removal was approximately 200° F. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature prior to evaluating its theological and physical properties.  
         [0067]    The rheological properties of these several formulations are reported in Table 8.  
                                                           TABLE 8                           RHEOLOGICAL PROPERTIES (silica filler)                Example 1   Example 2       Comparative       Property   [RDL5294]   [RDL5331]   RDL5654   Example A                    (1) Mooney viscosity (ML 1 + 4 at   63.4   61.0   65.4   68.8       212° F.) (ASTM D 1646)       (2) Mooney scorch (MS + 1 at 250° F.,   21.5   18.1   27.1   11.8       min.) (ASTM D 1646)       (3) Oscillating disk rheometer (ODR       at 320° F., 5° arc) (ASTM D 2084)       properties:       (a) ML (minimum torque, in.-lb.)   17.8   16.4   17.1   21.4       (ASTM D 2084)       (b) MH (maximum torque, at 2 hrs,   91.0   95.0   86.8   97.7       in.-lb.) (ASTM D 2084)       (c) Ts2, (time to 2 point rise above   2.3   1.8   2.4   1.5       minimum viscosity, in min.)       (ASTM D 2084)       (d) MC(90) (in.-lb.) (ASTM D   83.7   87.1   79.8   90.1       2084)       (e) Tc (90) (min) (ASTM E 2084)   74.0   73.5   77.1   75.5                  
 
         [0068]    The physical characteristics of these formulations are reported as follows in Table 9.  
                                                                   TABLE 9                           PHYSICAL PROPERTIES                    Example   Example                   Specification   1   2       Comparative       Properties   Requirement   [RDL5294]   [RDL5331]   RDL 5654   Example A                    Shore A hardness   69-79   73.8   74.2   72.4   71.8       (ASTM D 2240)       Ash content (%)   20-26   22.6   22.4   22.2   22.7       (ASTM D 297)       Specific gravity   1.04-1.07   1.051   1.058   1.056   1.054       (ASTM D 792)       Tensile strength,      1450 min.   2420   2480   2210   2280       perpendicular (psi;       ASTM D 412)       Elongation parallel (%;        450 min    703   693   643   636       ASTM D 412)       100% modulus (psi)   —   347   370   377   371       Tear resistance (pli)        170 min.   233   238   230   221       (ASTM D 624)       Specific heat (BTU   —   0.445   0.442   —   0.443       (lb.) −1  (° F.) −1  (ASTM E       1269)       Thermal conductivity   —   0.134   0.133   —   0.129       (BTU (ft) −1  (in.) −1  (° F.) −1         (ASTM E 1225)                  
 
         [0069]    Measurements were also made of the resulting ablation rates for these formulations.  
                                                   TABLE 10                           MATERIAL ABLATION RATE (MILS/SECOND)                        Comparative           Example 1   Example 2   Example A                        Low Mach Region (0.0028)   3.00   3.13   3.40       Medium Mach Region (0.0318-   7.34   7.94   7.60       0.0386)       High Mach Region (0.0397-   19.60   17.37   20.11       0.0634)                  
 
         [0070]    The ablation tests for Table 10 were performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the three velocity regions of the motor (low, medium, and high Mach) were net molded using conventional compression molding techniques. The low Mach region had space for 15 specimens, while the medium and high Mach regions had space for 4 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 11.7 seconds and 936 psi, respectively.  
         [0071]    The cure characteristics and processability of the inventive Examples closely matched those of the Comparative Example A. The cure rates, as indicated by the 90 percent cure time tc(90), of Examples 1 and 2 were equivalent to that of the Comparative Example A. Similarly, the physical properties and ablative performance of the materials prepared in accordance with Examples 1 and 2 were substantially equivalent to those of the Comparative Example and were well within the specification requirements. Thus these results indicate that the new formulations have the capability of being substituted for the old standard to-be-discontinued insulations.  
         [0072]    In addition, a comparison was made between DL1552A and RDL 5654 in a low mach test char motor using TP-H1148 propellant and a 30.8 second firing tme at an average pressure of 881 psi. The following results indicate that RDL 5654 is a viable replacement for DL1552A.  
                                                   TABLE 10A                           COMPARISON OF THE MATERIAL AFFECTED       RATE (MAR) OF DL1552A AND RDL5654       IN THE LOW MACH CHAR MOTOR                        RDL5654               D-5 DL1552A   Average           Mach No.   Average MAR   MAR           Range   (mils/sec.)   (mils/sec.)                        Test motor       HPCAT-07   MIR-05       Low velocity test section   0.0027   3.83   3.86       Medium velocity test   0.0027-0.0081   3.79   3.80       section       High velocity test section   0.0094-0.044   5.36   5.49                  
 
         [0073]    In the following TABLE 11 there are set forth examples of the new EPDM insulation formulations but now containing carbon fibers.  
         [0074]    Batches of carbon fiber containing insulations generally may be formulated in a mixer via a two-pass mix procedure. In the first pass, the mixer speed may be set to approximately 40 rpm and the all of the components other than the curing agents, accelerators, and carbon fiber are added to the internal mixer. As the mixing proceeds, the temperature will increase up to about 300° F., or even higher. Suitable mixing times depend on the temperature and mixing speed, and are ascertainable to the skilled artisan without an undue amount of experimentation.  
         [0075]    In the second pass, the mixer speed may be set to about 40 rpm., but this speed is dependent on the size of the mixer. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added, then the remaining half of the master batch was added to the mixer. The second mixing step may be performed at a temperature rising to approximately 180° F. to 190° F. but below the temperature where the accelerator would become unduly activated. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature. Then the material is dissolved in a suitable solvent, such as, by way of example, hydrocarbons such as hexane, heptane, and/or cyclohexane. The carbon fiber was then mixed with, for example, a sigma-blade mixer. The material is then sheeted out and the solvent allowed to evaporate at ambient atmosphere or in an oven. Throughout this process care must be taken that the frangible carbon fibers are not themselves fractured and broken up so as to become shortened and less effective as a result.  
         [0076]    The shaping and curing of the inventive insulation may be performed in accordance with techniques known in the art.  
         [0077]    The following Table 11 presents a number of examples of the novel EPDM formulations with carbon fibers.  
                                                                                                                                                                                                                                             TABLE 11                       EXAMPLES OF NEW EPDM FORMULATIONS WITH CARBON FIBER                                INGREDIENT   RDL5421   RDL5420A   RDL5377A   RDL5444A   RDL5443   RDL5436   RDL5408   RDL5409               Keltan 4506       Keltan 1446A       Keltan 2308       Nordel IP 4520   40.0   40.0   45.0   45.0   45.0       80.0   80.0       Nordel IP 4640   50.0   50.0   45.0   45.0   45.0   80.0       Hypalon 20   10.0   10.0   10.0   10.0   10.0   10.0       Neoprene FB       Neoprene TW       Neoprene GRT                           20.0   20.0       Trilene 67A                       10.0       Akrochem P-133   5.0   5.0   5.0   5.0   5.0   5.0   5.0   5.0       Agerite Resin D                       2.0   2.0   2.0       Agerite Stalite S   2.0   2.0   2.0   2.0   2.0       HiSil 233   3   3   3   3   3   3   3   3            VMC Carbon Fiber   All formulations adjusted to have 26.75% VMC Carbon Fiber in finished product                C.B. N330   1   1   1   1   1   1   1   1           Elastomag 170                           0.50   0.50       Zinc Oxide   4.0   5.0   5.0   5.0   4.0   4.0   4.00   4.00       Butyl Zimate   0.40       0.40                   0.50       Rhenocure AP-5   3.15       3.15                   3.40       Rhenocure AP-7                       3.00       Rhenocure AP-3                           3.85       Rhenocure ZADT/G                       0.80   1.00       Rhenocure S/G                           0.80       Altax       1.00       1.00   1.10       Akroform Delta P.M.                   0.35       Sulfads                   0.50       Accelerator VS       2.70       2.70       Akrosperse IS-70   1.00   1.00   1.00   1.00   1.00       Sulfur                       0.80   0.40   0.50                    Ingredient   RDL5445   RDL5410   RDL5375   RDL5395   RDL5394   RDL5435   RDL5434   RDL5376A                    Keltan 4506       70.0   73.0   55.0   55.0                   Keltan 1446A       20.0   17.0       Keltan 2308               35.0   35.0       Nordel IP 4520   80.0                           80.0       Nordel IP 4640                       80.0   80.0       Hypalon 20   10.0   10.0   10.0   10.0   10.0       Neoprene FB                       20.0   20.0       Neoprene TW                               20.0       Neoprene GRT       Trilene 67A   10.0       Akrochem P-133   5.0   5.0   5.0   5.0   5.0           5.0       Agerite Resin D   2.0                   2.0   2.0   2.0       Agerite Stalite S       2.0   2.0   2.0   2.0       HiSil 233   3   3   3   3   3   3   3   3            VMC Carbon Fiber   All formulations adjusted to have 26.75% VMC Carbon Fiber in finished product            C.B. N330   1   1   1   1   1   1   1   1       Elastomag 170                               0.5       Zinc Oxide   4.0   4.0   4.0   4.0   4.0   4.0   4.0   5.0       Butyl Zimate   0.15   0.50                       0.50       Rhenocure AP-5   4.45   3.50                       3.50       Rhenocure AP-7       Rhenocure AP-3                       2.90       Rhenocure ZADT/G                       0.80       Rhenocure S/G                       0.60       Altax           1.10   1.10   1.20       1.10       Akroform Delta P.M.                   0.45       0.20       Sulfads                   0.50       Accelerator VS           2.70   2.70       Akrosperse IS-70   1.34   1.10   1.20   1.20   1.40           1.00       Sulfur                       0.35   0.45                  
 
         [0078]    Silica Powder Filled Insulation Properties  
         [0079]    TABLE 12 reports rheological properties measured for the silica powder thermal insulation formulations that were set forth in TABLE 6.  
                                                                             TABLE 12                           RHEOLOGICAL PROPERTIES (silica powder)                Oscillating disk rheometer (ODR at 320° F.,           5° arc) (ASTM D 2084)                            ts2,                           (time           Mooney           to 2           viscosity           point           (ML 1 + 4   ML   MH   rise       Silica-Filled   at 212° F.)   (minimum   (maximum   above       EPDM   (ASTM   torque,   torque, at 1   ML, in   Tc (90)       Formulation   D 1646)   in.-lb.)   hrs, in.-lb.)   min.)   (min)                    RDL 5338   72.0   17.7   93.9   6.0   43.5       RDL 5342   78.5   21.3   97.8   7.0   45.0       RDL 5343   78.8   19.9   98.0   4.3   44.5       RDL 5347   84.0   21.7   94.5   3.4   38.0       RDL 5363   83.0   22.9   87.2   3.3   44.2       RDL 5255   85.7   19.4   103.2   3.6   28.5       RDL 5335   87.7   23.4   93.4   2.7   38.5       RDL 5201   73.0   16.4   89.0   6.2   31.5       RDL 5220   71.2   16.0   110.0   5.9   40.0       RDL 5370   80.0   20.3   104.6   5.6   32.0       RDL 5380   80.3   21.0   107.4   7.3   56.0       RDL 5381   79.6   21.0   104.0   4.8   36.0       RDL 5194   62.0   13.4   90.9   8.7   44.0       RDL 5222   61.9   13.5   92.8   6.3   45.6       RDL 5273A   64.5   14.7   97.1   3.8   42.0       RDL 5298   51.0   11.0   91.0   7.1   49.0       RDL 5277   72.4   18.9   98.5   1.9   42.6       RDL 5279   75.0   17.3   93.0   2.7   18.8       RDL 5319   87.8   19.9   97.0   6.3   43.0       RDL 5320A   86.5   20.7   103.0   4.0   41.6       RDL 5350   83.0   22.0   103.0   6.8   46.1       RDL 5351A   84.6   19.8   105.6   3.5   37.0       RDL 5367   86.0   22.3   96.7   3.2   43.3       RDL 5185   79.5   18.7   90.0   8.4   40.9       RDL 5186   80.6   19.3   95.5   4.1   43.0       RDL 5205   79.9   19.4   94.0   8.0   41.2       RDL 5276   81.6   18.5   96.7   6.2   42.5                  
 
         [0080]    TABLE 13 reports various physical properties measured for the silica powder thermal insulation formulations that were set forth in TABLE 6.  
                                                 TABLE 13                           PHYSICAL PROPERTIES            Silica-Filled                                   EPDM       Formulation   A   B   C   D   E   F   G               RDL 5338   2280   646   2260   658   220   62.0   1.06       RDL 5342   1910   588   1790   582   206   63.4   1.06       RDL 5343   2330   655   2040   619   221   63.2   1.06       RDL 5347   2100   621   2060   630   271   68.6   1.06       RDL 5363   2920   610   2820   608   270   72.0   1.06       RDL 5255   2430   703   2330   695   223   69.4   1.10       RDL 5335   2560   725   2600   739   232   65.2   1.10       RDL 5201   2150   772   2220   779   190   70.2   1.10       RDL 5220   2170   634   2020   620   194   70.2   1.10       RDL 5370   2550   700   2340   688   201   70.4   1.10       RDL 5380   2610   688   2300   663   206   69.4   1.11       RDL 5381   2420   700   2470   701   207   69.0   1.11       RDL 5194   1920   734   1920   748   201   71.8   1.05       RDL 5222   2100   703   1960   653   218   71.2   1.05       RDL 5273A   1420   612   1430   613   241   70.4   1.06       RDL 5298   1910   592   1870   586   223   71.6   1.06       RDL 5277   2010   614   2190   632   181   66.4   1.05       RDL 5279   2390   672   2370   687   199   64.8   1.05       RDL 5319   2440   659   2270   647   205   71.2   1.05       RDL 5320A   2250   657   2220   653   224   71.6   1.06       RDL 5350   2350   688   2420   707   233   64.2   1.06       RDL 5351A   2270   681   2250   699   226   65.2   1.06       RDL 5367   2440   621   2430   625   238   70.4   1.06       RDL 5185   2310   776   2230   771   208   71.4   1.06       RDL 5186   2390   725   2290   718   221   71.6   1.06       RDL 5205   1990   729   2120   773   207   71.6   1.06       RDL 5276   2270   653   2210   656   223   71.2   1.06                                                                                  
 
         [0081]    Carbon-Fiber Containing Insulation Formulation Properties  
         [0082]    Next, Table 14 lists the rheological properties measured according to the indicated tests for the carbon fiber-containing insulation formulations set forth in TABLE 11.  
                                                                             TABLE 14                           RHEOLOGICAL PROPERTIES                Oscillating Disk Rheometer (ODR at 300° F.,           5° arc) (ASTM D 2084)                            ts2,               Mooney           (time to           viscosity       MH   a 2 point       Carbon-   (ML 1 + 4   ML   (maximum   rise       Fiber   at 212° F.)   (minimum   torque,   above   Tc (90%       EPDM   (ASTM D   torque,   at 1 hr,   ML; in   cure)       Formulation   1646)   in.-lb.)   in.-lb.)   min.)   (min)                    RDL 5421   72.4   23.3   111   3.5   30.2       RDL 5420A   70.0   21.2   119   2.9   37.0       RDL 5377A   68.0   18.8   102   2.8   36.5       RDL 5444A   67.4   21.0   110   3.2   28.0       RDL 5443   74.5   25.7   101   1.6   33.5       RDL 5436   80.0   25.8   98.2   2.6   32.0       RDL 5408   83.0   29.2   95.5   2.5   16.5       RDL 5409   83.8   25.8   99.0   3.5   25.5       RDL 5445   56.0   15.8   82.0   3.4   39.2       RDL 5410   81.0   18.0   103   4.8   34.5       RDL 5375   55.0   21.0   100   2.5   31.7       RDL 5395   64.7   29.2   87.4   3.2   35.0       RDL 5394   67.0   25.1   89.5   1.8   36.0       RDL 5410   81.0   18.0   103   4.8   34.5       RDL 5434   43.7   12.1   136   1.9   18.8       RDL 5435   39.0   9.1   118   2.3   19.0                  
 
         [0083]    Table 15 reports the physical properties measured for the carbon fiber-containing insulation formulations set forth in TABLE 11.  
                                                                           TABLE 15                           PHYSICAL PROPERTIES            Carbon-Fiber                               EPDM Form.   A   B   C   D   E   F                    RDL 5421   1750   3.34   820   4.59   84.4   1.097       RDL 5420A   1600   3.12   871   4.60   84.2   1.106       RDL 5377A   1540   3.11   814   4.03   82.6   1.072       RDL 5444A   2030   5.01   884   6.30   84.2   1.097       RDL 5443   2070   5.37   799   7.21   83.2   1.090       RDL 5436   1500   2.48   721   4.05   84.0   1.098       RDL 5408   1060   3.77   525   5.61   81.2   1.131       RDL 5409   1112   2.95   668   3.35   83.4   1.142       RDL 5445   1160   3.58   725   4.71   84.4   1.096       RDL 5410   1500   2.88   719   3.85   83.8   1.063       RDL 5375   2010   4.38   881   6.85   80.6   1.076       RDL 5395   2650   4.55   1090   4.98   86.1   1.078       RDL 5394   2218   3.80   982   4.84   87.2   1.088       RDL 5435   1690   4.17   905   6.03   83.4   1.127       RDL 5434   1470   4.41   786   5.82   83.6   1.126       RDL 5410   1500   2.88   719   3.85   83.8   1.063       RDL 5435   1690   4.17   905   6.03   83.4   1.127       RDL 5434   1470   4.41   786   5.82   83.6   1.126                                                                          
 
         [0084]    The following TABLE 16 shows the results of ablation tests with the silica powder-containing formulations of Tables 6, 12 and 13.  
                             TABLE 16                           MATERIAL ABLATION RATE (MILS/SECOND)                Silica powder   Low Velocity,           Filled   0.003 Mach                       RDL 5338   3.65           RDL 5343   3.14           RDL 5347   2.31           RDL 5363   2.82           RDL 5255   2.76           RDL 5335   2.42           RDL 5220   3.66           RDL 5370   3.44           RDL 5380   3.24           RDL 5298   3.36           RDL 5319   3.23           RDL 5320   2.81           RDL 5350   3.38           RDL 5351   3.35           RDL 5367   3.31           RDL 5186   2.34                      
 
         [0085]    The ablation tests were performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the three velocity regions of the motor (low, medium, and high Mach) were neat molded using conventional compression molding techniques. The low Mach region had space for 15 specimens, while the medium and high Mach regions had space for 4 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 12.1 seconds and 860 psi, respectively.  
         [0086]    The following TABLE 17 shows the results of ablation tests with the carbon fiber-containing formulations of Tables 11, 14 and 15.  
                                 TABLE 17                           MATERIAL ABLATION RATE (MILS/SECOND)                        High Velocity,       Carbon fiber   Low Velocity,   Medium Velocity,   Avg. 0.01-0.15       EPDM   0.003 Mach   Avg. 0.03-0.09 Mach   Mach               RDL 5421   3.38   11.4   18.7       RDL 5420   2.87   12.2   19.2       RDL 5377   3.86   11.5   17.6       RDL 5444   3.29   10.7   18.4       RDL 5443   3.22   12.5   19.9       RDL 5436   2.63   11.0   15.3       RDL 5408   3.91   11.2   16.5       RDL 5409   4.12   10.6   15.5       RDL 5410   3.55   10.9   19.9       RDL 5375   2.97   12.3   16.0       RDL 5395   3.00   11.7   20.8       RDL 5394   3.05   11.7   20.2       RDL 5435   3.90   11.4   15.1       RDL 5434   3.90   11.3   15.3       RDL 5376   4.27   12.9   14.7       RDL 5445   3.45   11.3   18.3                  
 
         [0087]    The ablation tests were also performed, as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the low velocity region of the motor was molded using conventional compression molding techniques. The low Mach region had space for 15 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 12.1 seconds and 860 psi, respectively.  
         [0088]    From the characteristics measured in these tables it can be seen that a special combination of properties is required for a suitable insulation. And it is important to the consideration of this invention that the formulations are effective as to both of the commonly used solid additives silica powder and carbon fibers.  
         [0089]    With reference to the results set forth in the foregoing tables it is presently considered in Table 13 (silica filled EPDM) that the minimum acceptable values for tensile strength (parallel and perpendicular, values A and C) should be at least about 1600, and are preferably close to equal (within about plus or minus 10%). The elongation (parallel and perpendicular, Table 13) should be in the range of about 550-850. For tear strength, the value of about 170 is presently seen as a minimum characteristic. Similarly, an ODR-measured maximum torque MH (Table 12) for the silica powder filled insulations should be at least about 85 in-lb. and at most about 120 in-lb., preferably at most about 115 in-lb. is currently thought to be appropriate. The ODR torque ML has a useful lower limit of about 5 in-lb., preferably about 10. The scorch test Ts result (Tables 12 and 14) should be at least 1.5. For silica filled insulations, the tear strength (Table 13) should be at least about 170. For carbon fiber containing insulations, the elongation should exhibit a minimum of at least about 2%, preferably at least about 2.5%, especially for measurement B in Table 15. Also, for the carbon fiber insulations (Table 14) the Mooney viscosity ML should be below about 90 Mooney units.  
         [0090]    Considering the combined properties of the individual insulations noted in the above Tables, the silica-filled insulation 5273A is not presently preferred nor are the carbon-fiber containing insulations 5408, 5409, 5434 and 5445.  
         [0091]    In addition, formulations were also made with using aramid fibers as the filler material. An exemplary EPDM terpolymer for use with Kevlar fibers and comprising ENB as its diene component is NORDEL IP NDR-4640 (available from DuPont Dow Elastomers), may be substituted into the R196 for the NORDEL 1040 without requiring significant modifications to the R196 formulation. Generally, the one significant modification that is required with the use of aramid fibers involves a reduction in the amount of the curing agent, e.g., peroxide, again to offset the increased reactivity (relative to HD) of the alkylidene norbornenes. Furthermore, NORDEL IP NDR-4640 is not presently foreseen as being susceptible to obsolescence issues.  
         [0092]    Thus, in accordance with a further embodiment of this invention utilizing aramid fibers, the inventive rocket motor insulation formulation comprises, as a polymeric base, an EPDM/polyisoprene blend in which the diene component of the EPDM is based on at least one alkylidene norbornene, such as ethylidene norbornene (ENB), in effect in place of and without the above-described primary EPDM. The alkylidene norbornene content of such an EPDM may be from about 2-10 wt %. The selected EPDM terpolymer should be substitutable into the R196 thermal insulation formulation (Table 4) without requiring significant modification of the formulation. In accordance with a preferred embodiment, the EPDM terpolymer is NORDEL IP NDR-4640, which includes ENB as its diene monomer, is available from DuPont Dow Elastomers, and is manufactured in a relatively large capacity.  
         [0093]    An exemplary aramid fiber formulation is set forth in TABLE 18:  
                           TABLE 18                                   Ingredient   Parts by Weight                           EPDM terpolymer with ENB   79.5-80.5           diene monomer component           Polyisoprene   19.5-20.5           Antioxidant   0.95-1.05           Halogen-containing flame   39.5-40.5           retardant           Metal oxide flame retardant   19.5-20.5           KEVLAR fiber (aramid staple)   19.5-20.5           fiber)           Curing agent   1.45-1.55                      
 
         [0094]    In a more preferred embodiment, the polyisoprene, antioxidant, flame retardants, and aramid, e.g. KEVLAR, fibers are the same as those set forth in Table 4 above and are present in the concentrations specified in Table 4. The metal oxide may be present in fine powdery form so as to also serve as a filler. Varox DBPH-50 is also a suitable peroxy curing agent, although the substitution of ethylidene norbornene for 1,4-hexadiene requires a reduction in curing agent concentration to offset the increased reactivity of ethylidene norbornene. By providing the reformulated thermal insulation formulation with a peroxide concentration in a range of from about 1 to about 2.5 phr, more preferably about 1.5 phr, curing effect is achieved similar to the properties of R196.  
         [0095]    In general, the said EPDM/polyisoprene combination may be used with about 60-90 wt % of the norbornene EPDM and about 40-10 wt % polyisoprene, again with suitable additives being present in functionally desired amounts.  
         [0096]    Suitable additives that may be added as functionally required or as desired include one or more of the following, in various combinations: fillers, antidegradants, curing agents, plasticizers, processing aids, and pigments, bonding agents, fibers, and flame retardants. Two classes of curing systems that may be used are sulfur based curing agents in combination with organic accelerators, and peroxide curing agents.  
         [0097]    The shaping and curing of the such aramid inventive insulations may be performed in accordance with techniques known in the art.  
       Examples of Aramid Fiber Insulations  
       [0098]    Individual batches of insulation formulations were prepared USING NORDEL IP NDR-4640 (Example, in Table 20) and NORDEL IP NDR-1040 (Comparative Example B).  
                               TABLE 19                           Ethylene   Propylene   Diene           Polymer   content   content   content/type   Viscosity                   NORDEL IP   55   40   5/ENB   40       NDR-4640       NORDEL   55   41   4/HD   40       1040                  
 
         [0099]    The batches were each formulated in a laboratory scale Reliable Rubber &amp; Plastics Machinery Company Model R-260 internal mixer having a net chamber volume of 4260 cubic centimeters. A 3000 gram batch was prepared by a two-pass mix procedure.  
         [0100]    In the first pass, the mixer speed was set at 40 rpm, and 80 parts by weight of the corresponding EPDM component set forth in Table 3, 20 parts by weight of NATSYN 2200 polyisoprene, and 1.0 parts by weight of Wingstay S were added to the mixing chamber and masticated for one minute. Subsequently, 40 parts by weight of Dechlorane Plus 515, 20 parts by weight of Sb 2 O 3 , and 20 parts by weight of ¼ inch KEVLAR aramid fibers were added, and the mixer speed was increased to 60 rpm. The batches were removed from the internal mixer after mixing for a total of approximately 7 minutes. The temperature at the time of removing the batch was 300° F. to 320° F. The master batch was sheeted out on a 6″×13″ Farrel Corporation laboratory two-roll mill and allowed to cool to room temperature.  
         [0101]    For the Example and Comparative Example B, the fibers were dispersed in a two-roll mill to reduce agglomerations of fibers. The dispersion was accomplished by tightening the nip (separation between the two rolls) to 0.030 inches on the laboratory two-roll mill and passing the rubber through the nip a minimum of six times. This dispersion step was performed after the first pass but before the second pass. The rubber was then allowed to cool before the second pass.  
         [0102]    In the second pass, the mixer speed was set to 40 rpm. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added at a peroxide level of 1.5 phr, then the remaining half of the master batch was added to the mixer. After a total mix time of approximately 40 seconds, the final mix was removed from the mixer. The actual temperature at the time of removal was approximately 200° F. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature prior to evaluating its rheological and physical properties.  
         [0103]    Table 20 gives the measured rheological properties of these examples.  
                                           TABLE 20                           RHEOLOGICAL PROPERTIES                    Comparative       Property   Example   Example 1B                    (1) Mooney viscosity (ML 1 + 4 at   58.2   51.3       212° F.) (ASTM D 1646)       (2) Mooney scorch (MS +1 at 270° F.,   52.0   27.8       min.) (ASTM D 1646)       (3) Oscillating disk rheometer (ODR at       320° F., 5° arc) (ASTM D 2084)       properties:       (a) ML (minimum torque, in.-lb.)   11.3   13.3       (ASTM D 2084)       (b) MH (maximum torque, at 2 hrs,   61.5   55.0       in.-lb.) (ASTM D 2084)       (c) Ts2, (time to 2 point rise above   4.3   3.3       minimum viscosity, in min.)       (ASTM 2084)       (d) MC(90) (in.-lb.) (ASTM 2084)   56.4   50.8       (e) Tc(90) (min) (ASTM E 2084)   65.5   63.0                  
 
         [0104]    Table 21 reports the physical properties of these aramid fiber examples.  
                                                   TABLE 21                           PHYSICAL PROPERTIES                Specification       Comparative       Properties   Requirement   Example   Example B                    Density (lbs/in 3 ) (ASTM D    0.040 min   0.0422   0.0421       792)       Shore A hardness (ASTM D   70-85   75.8   74.4       2240)       Tensile strength, parallel (psi)   700 min   1085   1050       (ASTM D 412)       Elongation parallel (%)    10 min   27.3   24.6       (ASTM D 412)       Tensile strength, perpendicular   —   461   367       (psi) (ASTM D 412)       Elongation, perpendicular (%)   —   109   126                  
 
         [0105]    Table 22 reports the results from a test to measure the ablation rates for these aramid fiber examples.  
                                           TABLE 22                           MATERIAL ABLATION RATE (MILS/SECOND)                    Comparative           Example   Example B                        Low Mach Region (0.0028)   3.58   3.43       Medium Mach Region (0.039-0.0970)   10.55   10.83       High Mach Region (0.109-0.140)   16.59   21.99                  
 
         [0106]    The ablation tests were performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the three velocity regions of the motor (low, medium, and high Mach) were net molded using conventional compression molding techniques. The low Mach region had space for 15 specimens, while the medium and high Mach regions had space for 4 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 12.0 seconds and 842 psi, respectively.  
         [0107]    Although the Mooney scorch time of the Example was somewhat longer than that of the Comparative Example (R196), at the cure temperature of 320° F., the cure rates, as indicated by the 90 percent cure time tc(90), of the two materials were equivalent. Similarly, the physical properties of the materials prepared in accordance with the Example and Comparative Example B were substantially equivalent and well within the specification requirements. Additionally, the ablative performance of the material prepared in accordance with the inventive Example was equivalent or better than the ablative performance of the Comparative Example B (R196).  
         [0108]    In the course of experimentation leading to this invention, a number of other candidate elastomeric formulations were tried in the hopes that the path to a new acceptable insulation would not be overly difficult. In fact, such other formulations proved to be unsuccessful and this is demonstrated by the following illustrative unsuccessful examples.  
                                                                                                                                                                                                                                                                   Total Wt. = 3000 grams                    PARTS                       by       CATEGORY   Component Id.   weight   Wt %   Grams                    RUBBER COMPONENT FORMULA RDL5191            Polymers   1   Nordel IP NDR-   80.00   54.22   1626.57               4640       Plasticizers   2   Neoprene FB   20.00   13.55   406.64       Antioxidants   3   Agerite Resin D   2.00   1.36   40.66       Fillers   4   HiSil 233   35.50   24.06   721.79       Activator   5   Zinc Oxide   4.00   2.71   81.33           6   2nd Pass Addi-               tions       Accelerators   7   Rhenocure S/G   0.80   0.54   16.27           8   Rhenocure AP-3   3.85   2.61   78.28           9   Rhenocure   1.00   0.68   20.33               ZADT/G       Curative   10   Sulfur   0.40   0.27   8.13               Totals:   147.55   100.00   3000.00                    Mixing Instructions, first pass:       Add Nordel and zinc oxide       Add Hisil and antioxidant       Add Neoprene FB       Dump about 280F       2nd Pass Additions       Mix MB with curitives in Brabender       Dump about 190 or below                    RUBBER COMPONENT FORMULA RDL5570            Polymers   1   Nordel IP NDR-   50.00   33.89   1016.60               4640           2   Nordel IP NDR-   40.00   27.11   813.28               4520       Plasticizers   3   Hypalon 20   10.00   6.78   203.32           4   Akrochem P-133   5.00   3.39   101.66       Antioxidants   5   Agerite Stalite S   2.00   1.36   40.66       Fillers   6   HiSil 233   35.50   24.06   721.79       Activator   7   Zinc Oxide   4.00   2.71   81.33           8   2 nd  Pass Addi-               tions       Accelerators   9   Methyl Ethyl   2.00   1.36   40.66               Tuads           10   Monex or Unads   1.00   0.68   20.33           11   Sulfads   1.00   0.68   20.33           12   Butyl Zimate   1.50   1.02   30.50       Curative   13   Sulfur   0.50   0.34   10.17               Totals:   152.50   103.35   3100.64                    Mixing Instructions, first pass:       Mix EPDMs and Hypalon together       Add antioxidants 5 &amp; 6, Add silica, zinc oxide       Add Akrochem resin.       Dump about 250-300       2nd pass Additions       Mix MB with curatives in Brabender       Dump about 190 or below.                    RUBBER COMPONENT FORMULA RDL 5571            Polymers   1   Nordel IP NDR-   50.00   33.89   1016.00               4640           2   Nordel IP NDR-   40.00   27.11   813.28               4520       Plasticizers   3   Hypalon 20   10.00   6.78   203.32           4   Akrochem P-133   5.00   3.39   101.66       Antioxidants   5   Agerite Stalite S   2.00   1.36   40.66       Fillers   6   HiSil 233   35.50   24.06   721.79       2nd Pass Additions   8       Accelerators   9   Methyl Tuads   0.75   0.51   15.25           10   Altax   1.50   1.02   30.50           11   Sulfads   0.75   0.51   15.25           12   Butyl Zimate   1.50   1.02   30.50       Curative   13   Sulfur   0.50   0.34   10.17               Totals:   151.50   102.68   3080.31                    Mixing Instructions, first pass:       Mix EPDMs and Hypalon together       Add antioxidants 5 &amp; 6, Add silica, zinc oxide       Add Akrochem resin.       Dump about 250-300°       2nd Pass Additions       Mix MB with curatives in Brabender       Dump about 190° or below.                    RUBBER COMPONENT FORMULA RDL 5572            Polymers   1   Nordel IP NDR-   50.00   33.89   1016.60               4640           2   Nordel IP NDR-   40.00   27.11   813.28               4520       Plasticizers   3   Hypalon 20   10.00   6.78   203.32           4   Akrochem P-133   5.00   3.39   101.66       Antioxidants   5   Agerite Stalite S   2.00   1.36   40.66       Fillers   6   HiSil 233   35.50   24.06   721.79       Activator   7   Zinc Oxide   4.00   2.71   8.133           8   2nd Pass Addi-               tions       Accelerators   9   Methyl Tuads   3.00   2.03   61.00           10   Methyl Zimate   3.00   2.30   61.00           11   Sulfasan R   2.00   1.36   40.66           12   Butyl Zimate   3.00   2.03   61.00       Curative   13   Sulfur   0.50   0.34   10.17               Totals:   158.00   107.08   3212.47                    Mixing Instructions, first pass:       Mix EPDMs and Hypalon together       Add antioxidants 5 &amp; 6, Add silica, zinc oxide       Add Akrochem resin.       Dump about 250-300°       2nd Pass Additions       Mix MB with curatives in Brabender       Dump about 190° or below.                    RUBBER COMPONENT FORMULA RDL 5172            Polymers   1   Nordel IP NDR-   80.00   54.22   1626.57               4640           2   Trilene 67A   10.00   6.78   203.32       Plasticizers   3   Hypalon 20   10.00   6.78   203.32           4   Akrochem P-133   5.00   3.39   101.66       Antioxidants   5   Agerite Resin D   2.00   1.36   40.66       Fillers   6   HiSil 233   35.50   24.06   721.79       Activator   7   Zinc Oxide   3.00   2.03   61.00       2nd Pass Additions   8       Accelerators   9   Captax   1.00   0.68   20.33           10   Tellurac   0.50   0.34   10.17           11   Sulfads   0.75   0.51   15.25       Curative   12   Sulfur   1.00   0.68   20.33               Totals:   148.75   100.81   3024.40                    Mixing Instructions, first pass:       Mix Nordel and Hypalon together       Add antioxidants 4 &amp; 6, Add silica, zinc oxide       Add Trilene and Akrochem resin.       Dump about 250-300°       2nd Pass Additions       Mix MB with curatives in Brabender       Dump about 190° or below                  
 
         [0109]    As indicated below, for various reasons, the above-described experimental test insulations proved to be unsuitable formulations for production of rocket motor insulations.  
                                                   TABLE 23                           Unsuccessful Experiment Results            Rheological Properties                Formulation   ML   MH   ts2   Tc (90)   Rejected because:               RDL5571   25.3   138.3   2.3   21.3   MH too high to match                           existing standard                           insulation       RDL5572   22.6   173.9   2.5   25.7   MH too high to match                           existing standard                           insulation       RDL5570   25.6   128.6   2.7   29.5   MH too high to match                           existing standard                           insulation       RDL5191   26.8   133.6   3.5   25.4   MH too high to match                           existing standard                           insulation       RDL5172   39.0   128.0   1.0   25.0   Uses the old cure system                           as in TABLE 2, but                           product is much too                           scorchy to be useful;                           cures at too low                           temperature                  
 
         [0110]    As these unsuccessful formulations indicate, and faced with the discontinuance of the availability of the hitherto thought to be essential component for the present standard insulation formulations, the development of a suitable and acceptable solid propellant rocket motor insulation that will provide the required performance criteria has not been an easy task.  
         [0111]    The foregoing detailed description of the preferred embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Many modifications and variations within the scope of this invention will be apparent to practitioners skilled in this art. The illustrated embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is therefore intended that the scope of the invention cover various modifications and equivalents included within the spirit and scope of the appended claims.