Expanded perlite/cork fillers applied to aerospace insulation materials

Our plain-English summary of Expanded perlite/cork fillers applied to aerospace insulation materials, by J.R.A. Pinto, N.B. Sanches, M.F. Diniz, et al (Anais Da Academia Brasileira de Ciências, 2018).

The study

• Authors: J.R.A. Pinto, N.B. Sanches, M.F. Diniz, R.S. Santos, J.I.S. Oliveira, R.C.L. Dutra
• Journal: Anais Da Academia Brasileira de Ciências
• Published: 2018;90(3):3197-3206
• Read it: PubMed · Publisher

Abstract

Insulations for rocket motors such as Flexible Thermal Protections (FTPs) and Rigid Thermal Protection (RTPs) act as thermal barriers against the hyperthermal environment from the solid propellant combustion. FTPs present dual function: to extend RTPs performance, and to attenuate the propellant contraction. FTPs used in the Brazilian Space Program have asbestos in their composition since the 70´s; however, they are hazardous for human health. In this context, a mixture of Expanded Perlite (PExp) and Cork Powder (CP) was investigated as a replacement for asbestos. Results showed reduction about 21 % in density and an increase of 13 % in the ablative properties. The low mechanical properties not interfere in this type of FTP due to your function of attenuate the propellant contraction.

In plain English

This laboratory materials study from Brazil tested asbestos-free fillers for the flexible thermal protection used inside solid-fuel rocket motors of the Brazilian Space Program. It is not a health study and reports no data on people. The reference material in current use contains asbestos fiber of a type the article does not name, plus other fillers. The authors call asbestos carcinogenic. They made four test materials, adding to 100 parts HTPB resin either 5 parts expanded perlite, 5 parts cork powder, or 5 parts perlite plus 3 or 5 parts cork. They measured strength, stretch, hardness, density and resistance to an oxy-acetylene torch. Because the reference data are confidential, torch results were reported only relative to the reference. Both perlite-cork mixtures resisted the torch better than the reference. The 5/5 mix had a relative burn time of 1.13 (plus or minus 0.07) and erosion rate of 0.87 (plus or minus 0.04), with the reference at 1.00. All four new materials were lighter than the reference but had lower tensile strength. The authors concluded that the 5/5 mixture was the most suitable replacement, since the insulation's design allows its lower mechanical properties.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory materials-testing study comparing experimental rubber formulations with an asbestos-containing reference insulation; not a study of human health
• Who was studied: Test specimens of four HTPB-resin formulations (PExp5, CP5, PExp5/CP3, PExp5/CP5) and the reference flexible thermal protection (FTP) used in rocket motors of the Brazilian Institute of Aeronautics and Space; ten samples were used for the tensile and elongation tests
• Exposure: The reference FTP contains asbestos fiber (type not specified) and other fillers. The test formulations replaced these with expanded perlite and/or cork powder at 3 to 5 parts per 100 parts HTPB resin
• Main results:
  • The perlite-cork mixtures gave better torch (ablation) resistance than the reference FTP. Normalized to the FTP (1.00), PExp5/CP3 had a burn time of 1.06 plus or minus 0.03 and an erosion rate of 0.94 plus or minus 0.04. PExp5/CP5 had a burn time of 1.13 plus or minus 0.07 and an erosion rate of 0.87 plus or minus 0.04
  • Perlite alone (PExp5) and cork alone (CP5) did not match the reference. Their normalized burn times were 0.93 and 0.96 (each plus or minus 0.08), and their erosion rates were 1.07 plus or minus 0.10 and 1.05 plus or minus 0.09
  • All test formulations were less dense than the reference FTP: 0.926 to 0.953 g cm-3 versus 1.151 g cm-3 (0.949 for PExp5/CP5)
  • Tensile strength was lower in all test formulations (1.49 to 1.84 MPa) than in the FTP (1.98 MPa). Elongation was higher than the FTP's 157 % for CP5 (179 %) and PExp5/CP5 (190 %)
  • Infrared analysis of surfaces after the torch test showed perlite exposed at the burned surface. The authors attribute the improved ablation resistance to this
• The authors' conclusion: The authors concluded that the PExp5/CP5 formulation is the most suitable to replace the existing FTP formulation, combining good ablative strength with low density, taking into account that the FTP design allows the lower mechanical properties obtained
• Limitations the authors state:
  • Reference FTP data could not be disclosed for confidentiality reasons, so ablation results were reported only as values normalized to the reference
  • The test formulations had low mechanical properties. The authors note that the fillers do not act as reinforcement

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