Graded polymer composites using twin-screw extrusion: A combinatorial approach to developing new energetic materials
Introduction
Composite materials are being fabricated for energetic applications using a very high solids loading of energetic particles (≫70 wt%), bound by a polymer. These materials have primarily been processed using batch techniques. By varying the solids content, particle sizes, and particle distributions, combined with smaller volume fractions of additives, it is possible to create a variety of formulations that exhibit a wide range of energy release rates, impact sensitivity, and thermal sensitivity.
Recently, alternative processing techniques that have been conventionally used for inert composite material systems have been pursued for the fabrication of composite energetic materials. In particular, a continuous processing technique known as Twin-Screw Extrusion (TSE) has been particularly attractive for increasing the safety and affordability of manufacturing composite energetic materials. This technology is fairly well understood for the processing of homogeneous energetic composite materials in a steady state and was originally developed in Germany during the mid-1960s and much later adopted by the United States [1], [2].
More recently, it has been shown that the TSE technique can be run in a transient state to produce controlled variations in material distribution resulting in non-energetic graded polymer composites for applications such as control and sensing [3]. The evolution of the gradient architecture was predicted using convolution process models based on residence distributions. A residence distribution is the result of an input disturbance to a continuous process whether it is a screw extruder or series of stirred tank reactors. It is a popular experimental and theoretical tool because it quantitatively reflects the combined effects of the process conditions on the material transport through the process (screw speed and design, throughput rate, temperature, ingredients, etc.). Using the residence distribution data in a novel approach, solutions to the convolution integral were employed in the time and volume domains to predict the one-dimensional structure of graded composite propellant produced using a twin-screw extruder.
Graded materials have been previously employed for combinatorial approaches to advanced materials research & development [4]. The combinatorial approach to developing advanced materials or for high-throughput screening of ingredient combinations and other parameters has been successfully demonstrated for small structures such as thin films and over short distances [4], [5], [6], [7]. Combinatorial and high-throughput experimental methods have also been applied to polymers and nanocomposites in recent years [8], [9], [10]. The experimental processes are quite specific as to material and objective, but the results are similar in that a graded structure, often with a continuous gradient architecture, is made and subjected to a variety of non-destructive testing. The gradients can be effected as compositional and/or processing conditions. From tests at different locations in the graded structures, it is possible to ascertain a number of properties through a range of the parameter space.
In this paper, the TSE technique is pursued as a combinatorial approach to the processing of new composite energetic materials. Using the previously developed convolution process model, it is shown that controlled gradient architectures can be predicted and then produced. The composition of these gradient architectures are then characterized, and then correlated to the combustion properties determined using burning rate testing. These results are compared with a conventional approach to determining the compositional dependence of burning rates using batch processing of formulations determined using a Kowalski–Cornell–Vining (KCV) design-of-experiments algorithm.
Section snippets
Prediction of gradient architectures fabricated using TSE processing
Burning rate is highly dependent upon oxidizer particle size and content among test conditions, etc. The functionally graded rocket motors conceived and developed during this project were linearly graded cylinders measuring 1.25 by 30 inch. Homogeneous compositions with 79 and 87 percent by weight comprised the two ends of the motor. Between the two was a continuous gradient from one fill content to the other measuring approximately 4.5 inch. It is beyond the scope of this paper to describe the
Experimental apparatus
Unique facilities have been assembled at the Naval Surface Warfare Center in Indian Head, MD (NAVSEAIHMD) to address the specific needs for processing energetic polymer composites in a TSE process [11]. One of the extruders employed is a Werner & Pfleiderer ZSK-40 (mm) featuring segmented and cantilevered screws. It has a process length to diameter (L/D) ratio of 28, which is similar to the machine employed at the University of Maryland in a previous investigation on graded polymer composites [3]
Acoustic strand burner test description
The best measure of a propellant's combustion properties is to test it in a motor; however, this is costly and inefficient during development. Instead, a good alternative is to conduct strand burning tests under a range of conditions (e.g. chamber pressure and temperature). Testing at different pressures allows determination of the burning rate exponent. The common method of strand burning is to test six-inch long strands 1/4 inch in thickness and report the average burning rate for
Comparison of compositional dependence between the combinatorial approach with the experimental response surface analysis (RSA)
It was desirable to compare the compositional dependence of properties obtained from the combinatorial approach with the most optimal mixture experiment approach for ascertaining the effects of the individual ingredients on the burning rates as produced over the range of feeding and extruding capability for the process. Response surface methods can quantify the contribution of individual ingredients and more importantly the combined effects of two or more ingredients. Most mixture designs are
Conclusions
A combinatorial approach based on Twin-Screw Extrusion (TSE) has been developed for evaluating new composite energetic materials. Materials with gradient architectures are produced by the TSE process. The combinatorial approach uses a convolution process model to predict the variation of composition in the gradient architecture. Experiments were developed for characterizing the variation in burning rate through the gradient architecture using strand burning tests. Comparisons of the burning
References (18)
Application of screw processors to the manufacture of energetic materials
(1987)- et al.
Twin-screw processing of plastic bonded explosives at naval surface warfare center
(1988) - et al.
Fabrication of particle-reinforced polymers with continuous gradient architectures using twin-screw extrusion processing
J Compos Mater
(2004) - et al.
Combinatorial methods for advanced materials research & development
Z Metallkunde
(2001) - et al.
The combinatorial approach: a useful tool for studying epitaxial processes in doped magnetic semiconductors
Macromol Rapid Commun
(2004) - et al.
Use of combinatorial materials development for polymer solar cells
Adv Mater Opt Electron
(2000) - et al.
Combinatorial approach to characterizing epoxy curing
Macromol Rapid Commun
(2004) A current perspective on high-throughput polymer science
J Mater Sci
(2003)- et al.
High throughput methods for nanocomposites materials research. Extrusion and visible optical probes
Polymeric Mater: Sci Eng
(2004)
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