摘要:

The excellent high-temperature mechanical properties and other desirable characteristics of polybenzimidazole (PBI) polymer systems make these systems attractive candidates for development as ablative heat-shield materials. This paper describes the formulation of several new low-density polybenzimidazole composites. The proposed structure of the basic linear PBI prepolymer and of several highly cross-linked PBI polymers are presented. The cross-linked PBI's were obtained either thermally (by postcuring to a high temperature) or chemically (by the use of either preoxidized polyfunctional amines or triphenyl trimeasate as a comonomer in the polymerization).

ISSN:0022-233X    

DOI:10.1080/10601326908053830

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The ablative degradation performance of elastomeric shield materials is interpreted in terms of a hypothesis based upon the study of the ablation residue which is correlated to seven parameters.

ISSN:0022-233X    

DOI:10.1080/10601326908053831

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The differential equations describing the thermochemical ablation-indepth process and the boundary layer transport processes which couple at the ablative surface are presented. Physical interpretations of the relationships are given along with the results of sensitivity analyses which pinpoint the properties of importance in ablation performance predictions. Char thermophysical properties are shown to have a considerable influence. Based on this observation, comparisons and discussion of recent ablative char thermal conductivity results are given for high-performance carbonphenolic and graphite-phenolic composites. Physical and chemical changes in the chars which influence the thermal transport properties are summarized and recommendations offered concerning the use of the information in ablation predictions. Finally, the important thermal and aerodynamic forces which may lead to thermomechanical and thermostructural erosion of the ablative are summarized.

ISSN:0022-233X    

DOI:10.1080/10601326908053832

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The development of a high-heating-rate thermogravimetric analyzer for use with ablative plastics is described. Data are presented for the decomposition of a nylon-phenolic ablator.

ISSN:0022-233X    

DOI:10.1080/10601326908053833

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

A multistep kinetic process is postulated to describe the pyrolysis of nylon 6–6, phenolic CTL-91LD (a phenolic novalac resin), and their composites. Kinetic coefficients are derived from thermogravimetric analysis data (TGA) at heating rates varying from 3–60°C/min for each component. Three correlation techniques are used to obtain consistent coefficients. A two-step kinetic equation is derived to describe the phenolic pyrolysis and a single-step kinetic equation is found to adequately represent nylon 6–6 pyrolysis. These equations are then combined to obtain an overall pyrolysis rate equation for the composite.

ISSN:0022-233X    

DOI:10.1080/10601326908053834

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Pyrolysis-gas chromatography has been used to locate the degradation front in pieces of typical phenolic ablative materials. This technique gives quantitative results for per cent phenolic resin versus distance normal to the surface. The charred material is carefully machined so as to remove successive 0.020-in. cuts starting with the charred face of the material. Each cut is then analyzed by the combined techniques of pyrolysis and gas chromatography. A plot of per cent phenolic resin versus distance from the undecomposed back face gives a profile of the degree of degradation of the ablative plastic. Samples are weighed on the pyrolysis ribbon before and after pyrolysis giving total per cent volatiles. Divergence of the per cent volatiles and per cent phenolic resin curves near the charred surface of the material is due to the presence of the pyrolysis products near the surface.

ISSN:0022-233X    

DOI:10.1080/10601326908053835

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

In the temperature range of 500–2500°F, nonequilibrium flow analysis predicted only a small change in pyrolysis gas composition as the gases flow through the char zone. Essentially all of the reactions took place in the temperature range of 2000–2500°F. Comparing nonequilibrium flow with the two limiting cases, the energy absorbed in the char zone for frozen flow was two-thirds that of nonequilibrium flow, and equilibrium flow was about four times that of nonequilibrium flow at the 2500°F level.

ISSN:0022-233X    

DOI:10.1080/10601326908053836

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

An arc-image furnace has been used to evaluate polymeric ablators in a closely controlled experimental environment over wide temperature, pressure, and thermal flux ranges under conditions where gas phase reactions outside the char do not interfere with reactions occurring within the char.

ISSN:0022-233X    

DOI:10.1080/10601326908053837

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Earth and planetary entry vehicles require heat shield materials whose transparency to radio frequency (rf) signals is unaffected by the ablation process. Teflon or fused silica are currently used for this purpose, but these materials are heavy and difficult to attach. For applications such as Mars entry vehicles or lifting bodies where low heating rates prevail, low-density, rf-transparent ablators are desired. To fill this need, Martin Marietta Corporation has developed a rf-transparent composition designated SLA-220. This ablator, a silica-filled elastomeric silicone, has a density of 0.25 g/cm3and forms a siliceous rather than a carbonaceous char. Pyrolysis of SLA-220 at slow heating yields a pure silica residue. As heating rate increases, the volatile pyrolysis products tend to undergo secondary decomposition and to deposit carbon while passing through the hot char. Therefore, rf signal attenuation increases with heating rate. Electrical properties of SLA-220 (dielectric constant, signal attenuation) have been measured before and after ablation. Ablation testing has been conducted at heating rates ranging from 6 to 60 Btu/ft2-sec. Thermal properties (conductivity, specific heat, expansion) and mechanical properties (strength, elastic modulus, elongation) have been determined. The properties of SLA-220 are compared with those of 0.50 and 0.70 g/cm3porous Teflon ablators to demonstrate the advantages of SLA-220 over other low-density rf-transparent ablative systems.

ISSN:0022-233X    

DOI:10.1080/10601326908053838

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Exploring the other planets of our solar system with atmospheric probes and landers places new requirements on polymeric systems. Because of a preoccupation with weight, designers of capsule components are switching over to plastics with increasing frequency. Ablative heat shields, aeroshell structures, antennas, insulators, electronics packaging, parachutes, and many specialty items could be made from plastics which are tailored to the specific environmental needs. In spite of generally poor practices which limit the reproducibility of commercial products, much is being done to delineate the problems set forth by these environments. This paper outlines the specific environments expected for Mars and Venus capsules and discusses some of the efforts in process, or prospective, within or for the National Aeronautics and Space Administration (NASA), to provide the understanding necessary to tailor new composites for these specific environments. Although much of the work is oriented toward ablative heat shield development, the application is generally similar for all of the other capsule components. Based on this work, an idealized polymer would be a reproducible high-temperature plastic with low volatile content, low-temperature flexibility, and a low number of reactive sites after cure. Though we are some distance from this ideal, progress is being made toward this objective.

ISSN:0022-233X    

DOI:10.1080/10601326908053839

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor