摘要:

Ablative polymers and polymeric composites serve an important function in aerospace technology. They protect aerodynamic surfaces, propulsion structures, and ground equipment from degradative effects of very high temperatures or incident heating rates. This protective function is accomplished by a self-regulating heat and mass transfer process known as ablation. By this relatively new process, thermal energy is expended via sacrificial loss of material. The amount of energy absorbed, dissipated, and blocked depends critically upon both materials and environmental variables. including the properties of the polymeric material, the thermal-chemical-mechanical aspects of the environment, and the complex interaction of the ablator with its hyperenvironment. Important thermochemical and thermo-physical parameters of the ablation process are discussed. In addition, a state-of-the-art of polymeric ablators is presented, with emphasis on the advantages, limitations, and current problems associated with ablative plastics. The need for charring, noncharring, inorganic, low-modulus, cryogenic, processible, and low-cost polymers is discussed in considerable detail. Lastly, the service uses developed for ablative polymers during the past decade are reported to illustrate the importance of this new class of engineering materials.

ISSN:0022-233X    

DOI:10.1080/10601326908053818

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Recent laboratory research concerned with boundary layer, surface, and internal ablation mechanisms is examined, particularly in relation to their coupling with the imposed heating environment. Emphasis is devoted to defining ablator response at heating conditions representative of those encountered by the atmospheric entry of manned NASA vehicles. Experimental results are presented for four ablative composites (Teflon, phenolic nylon, epoxy-novolac, and silicone elastomer), which differ significantly in chemical formulation and mode of degradation. The influence of both test stream variables and chemical composition on material behavior is demonstrated and discussed. Future areas of ablation research are delineated by calculations that illustrate the relative importance of the various heat accommodation mechanisms available to a charring ablator at entry speeds above 15 km/sec.

ISSN:0022-233X    

DOI:10.1080/10601326908053819

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Ablative materials have been used widely by the Navy to protect both the internal and external surfaces of solid-propellant missile propulsion systems. Plastics and rubber-based ablators have been employed for this purpose because of their relatively low density and high thermal insulation performance. To aid in the development and selection of these materials, lab oratory-scale thermal tests have evolved. An oxyacetylene burner test found to be useful for this purpose has gained widespread acceptance and is now a standard procedure known as ASTM 285–65T. Standardization of other test procedures and techniques is in process. These include a method for determining the total enthalpy of electric plasma arc heaters and recommended practices for installing thermocouples in ablative test specimens. An advanced technique in use at the Naval Ordnance Laboratory (NOL) known as the “alpha rod test” provides a continuous record of ablation rate and internal temperature. It also enables one to compute the effective thermal diffusivity as a function of temperature, which is suitable for predicting internal temperature histories in ablators. Under ideal conditions, agreement to within 5–10% of experimental data has been obtained. Using the concept of “constructive thermal degradation,” NOL has also developed new, novel epoxide resin-curing agent systems equal to or better than phenolics in ablation performance.

ISSN:0022-233X    

DOI:10.1080/10601326908053820

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Aerospace technology has established the demand for thermally stable polymers which retain useful properties at high temperatures. The reliability of a re-entry shield is directly dependent on the performance of the polymers used as ablators. Polymeric ablation materials are sacrificial and are classified as subliming and charring ablators. Subliming polymeric ablators function by (a) depolymerization-vaporization, (b) pyrolysis-evaporation, and (c) melting-vaporization. Polymers which depolymerize quantitatively and reversibly are ideal subliming ablators. Under hyperthermal conditions, charring polymeric ablators must generate a large amount of char residue which will act as a binder at high temperatures for the reinforcements embedded in the char layer. Generally, the higher the thermal stability of the polymer, the higher the yield of char. The maximum thermal stability in polymers is usually obtained when: (a) thermally unreactive ring structures constitute a major portion of the polymer, (b) maximum use is made of resonance stabilization, (c) high bond energies exist between the atoms in the polymer, and (d) the cohesive energy density between polymer chains is high. Carbocyclic polymers, including the graphites, have high thermal stabilities but offer difficulties of fabrication. Thermally stable heteroaromatics which pass through tractable prepolymer stages are more readily prepared and fabricated; their char-forming properties in relation to structure are discussed. The values of “true” and “practical” thermal stabilities as applied to ablators are also considered. The need for a multidiscipline “oecumenical” approach to solving difficult problems is stressed. Some recent developments in the methods of syntheses of thermally stable polymers, including isotropic graphitic structures, are surveyed.

ISSN:0022-233X    

DOI:10.1080/10601326908053821

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

A method has been developed which permits a complete molecular structure description of a polymeric material. Very good molecular descriptions have been achieved for certain epoxy-novolac resin-alicyclic anhydride polymers, which were catalyzed by tertiary amine, by using refined analytical techniques and computer correlation of the analytical data. The method and results are described. The chemical compositions of cured polymers containing various ratios of epoxy-novolac to anhydride are illustrated. The information generated by this work (Contract NAS 7–344) was one element of the NASA-Ames Research Center program to improve char-forming heat shield polymers.

ISSN:0022-233X    

DOI:10.1080/10601326908053822

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

A series of polymers based on the reaction products of bis-(a-chloro-ptolyl)ether, formaldehyde, and phenols have been synthesized. The char yields of these polymers, determined by thermogravimetric analysis, vary between 50 and 66%. The data indicate that char yield is influenced by the degree of cross-linking and that the char structure still possesses the phenyl ether unit. Incorporation of thermally stable fillers or substituents on the phenol can produce desired properties without weakening the polymer system. The high char yield and char stability of these polymer systems suggest potential merits in ablative applications.

ISSN:0022-233X    

DOI:10.1080/10601326908053823

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The effect of chlorine and polynuclear structures on the thermal degradation of polyphenylene has been examined. The thermal dehalogenation reaction has been utilized to cross-link the polymer. Composites stabilized in this manner were evaluated by exposure to an oxyacetylene torch.

ISSN:0022-233X    

DOI:10.1080/10601326908053824

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The synthesis of tractable polyphenylenes and their conversion to thermosetting resins by curing with p-xylylene glycol is described as suitable for the fabrication of reinforced laminates. Ablative performance tests are correlated with composition and the method of preparation of the polymer.

ISSN:0022-233X    

DOI:10.1080/10601326908053825

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Laminated reinforced plastics using carbon or silica fabrics and phenolic resins are widely used for ablative liners of rocket nozzles and for other applications requiring protection of structural elements in a high thermal environment. Under cyclic heating conditions, the phenolic resin chars, thereby providing ablation cooling; on cool-down, cracks and/or delaminations result in the majority of applications. On a reheat cycle, as in a multistart engine or other pulsed environment, a burn-through or other catastrophic failure may occur in the cracked areas.

ISSN:0022-233X    

DOI:10.1080/10601326908053826

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

High-strength, readily processable, char-forming, insulative materials are being sought for application in ablative heat shields for re-entry vehicles. Toward this end, the family of epoxy resins has been evaluated. The structure of epoxy resins and curing agents in terms of their functionality, aromaticity, and chemical nature is discussed in relation to its effect on ablative properties. The concept of controlled, constructive thermal degradation is extremely important in ablative epoxide compositions. Bridged Diels-Alder adducts based upon cyclic dienes and maleic anhydride perform well as epoxy resin curing agents in this respect. The mechanism of thermal ablative degradation of these systems is discussed in terms of in situ thermal control and char-forming reactions. The position of attachment of glycidyl groups, as well as the nature and position of other sub-stituents around the aromatic nucleus, has little effect in general upon the ablative properties of epoxy resins. Thermal and ablative data of both benzene and naphthalene derivatives are given. A new epoxy resin based upon 2-nitro resorcinol has been synthesized. This resin gives significant char increases, both quantitatively and qualitatively, over conventional epoxides. The unusual mechanism of polymerization and thermal degradation of this resin is discussed.

ISSN:0022-233X    

DOI:10.1080/10601326908053827

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor