A Brief Study on Toxic Combustion Products of the Polymers Used in High-Pressure Oxygen Systems
One likely cause of polymer ignition in a high-pressure oxygen system is the adiabatic-compression heating of polymers caused by pneumatic impact. Oxidative pyrolysis or combustion of polymers in a high-pressure oxygen system could generate toxic gases. This paper investigates the feasibility of using the NASA pneumatic-impact system to conduct adiabatic-compression combustion tests and determines the toxic combustion products produced from the burning of five selected polymers. Five polymers commonly used in high-pressure oxygen systems, Zytel(Registered TradeMark) 42 (Nylon 6/6), Buna N (nitrile rubber), Witon(Registered TradeMark) A (copolymer of vinylidene fluoride and hexafluoropropylene), Neoflon(Registered TradeMark) (polychlorotrifluoroethylene), and Teflon(Registered TradeMark) (polytetrafluoroethylene), were tested in the NASA pneumatic-impact test system at 17.2-MPa oxygen pressure. The polymers were ignited and burned; combustion products were collected in a stainless-steel sample bottle and analyzed using various methods. The results show that the NASA pneumatic-impact system is an appropriate test system to conduct adiabatic-compression combustion tests and to collect combustion products for further chemical analysis. The composition of the combustion product gas generated from burning the five selected polymers are presented and discussed.
Combustion Characteristics of Fluoropolymer Coated Boron Powders
ABSTRACT The problem of sluggish combustion reactivity of Boron (B) can be overcome by inclusion of fluoropolymers. In this paper, three commercial fluoropolymers; PVDF (59 wt% F), Viton (66 wt% F) and THV (72 wt% F) coated (Ca. 4 wt%) B powders (Ca. 1-µm in size) were prepared and their combustion characteristics have been investigated. Among the fluoropolymer-coated powders, THV coated B provided the highest average improvement in heat of oxidation, reactivity in terms of pressure generation and combustion temperature followed by Viton and PVDF coated B. The sequence of reactivity enhancement has been explained by the variances in gasification efficiencies of the boric oxide shell, induced by the thermal decomposition of the respective fluoropolymer coatings. THV with higher fluorine and lower hydrogen content, supposedly promotes better gasification of the boric oxide shell by releasing more fluorine rich alkanes/alkenes during thermal decomposition. However, PVDF predominantly produces HF, which apparently less effective in gasification of the boric oxide shell, resulting in limited improvement of the measured properties.
Predictability of Gaseous Impact Ignition Sensitivity from Autoignition Temperature Data
An analysis of cumulative binomial probabilities for a quantal-type test like ASTM G74 indicates that for a probability of reaction in a single trial of 0.05, the probability of obtaining zero reactions in 20 trials is only approximately 36 percent, Because the lack of precision with the ASTM G74 test logic could be potentially misleading, an investigation was conducted to determine if pneumatic impact sensitivity could be predicted more reliably from other, more precise test methods. Pneumatic impact sensitivity tests were conducted following ASTM G74, but employing more rigorous test logics; autoignition temperature tests were conducted per ASTM G72. Polytetrafluoroethylene (PTFE), Viton A, polyethylene, polyoxymethylene polyamide, and silicone were the polymers used. After a fixed number of pneumatic impact tests was performed at set pressures, a plot of reaction frequency over a 3.4 to 24.1 MPa impact pressure range indicated that PTFE, Viton A, and silicone would outperform other polymers. The 50 percent pneumatic impact reactivity level was determined with the Bruceton test method. The data obtained resulted in a strong correlation between the 50 percent reactivity level and the autoignition temperature of polymers, indicating similar ignition mechanisms.