| HTPB propellant has been widely used in all kinds of weapons,ammunitions and aerospace engines since its inception because of its excellent mechanical,safety and energy characteristics.With the upgrading of weapon system and aerospace technology,propellant,together with projectiles and engines,will be eliminated.Meanwhile,the natural aging problem of propellant is inevitable,which is bound to produce a large amount of propellant waste.Because the scrapped HTPB propellant still contains high-quality energetic solid components,such as ammonium perchlorate,aluminum powder,RDX,etc.,the necessary recovery can be realized.However,the binder system of HTPB propellant has lost its reuse value.In essence,it is a kind of ultra-high molecular weight crosslinked polyene polyurethane with stable physical and chemical properties,which is directly discharged into the environment without change for a long time and destroys the natural ecology.Therefore,effective measures must be taken for environmentally sound treatment.Based on the characteristics of green,safe,energy-saving and low consumption of biodegradation technology,and its research and application in polymers,this study tried to treat the binder system waste via microorganisms with the following series of systematic researches.The biodegradability of hydroxyl-terminated polybutadiene(HTPB)/toluene-2,4-diisocyanate(TDI)binder system was investigated with the sheet prepared according to the formula of binder system of HTPB propellant as the degradation substrate and the bacteriasource enrichment solution prepared with activated sludge and contaminated soil as the degradation medium.Besides,the degradation effect was analyzed through infrared spectroscopy(FT-IR),thermogravimetry(TG)and scanning electron microscopy(SEM).According to the results,the binder system was degraded to some extent,and the weight loss rate of the sheet was nearly 43% after 60 d.The main factor of degradation is that dioctyl adipate(DOA),a plasticizer component in the binder system,is decomposed,while the HTPB-based polyurethane,an essential component in the binder system,is difficult to biodegrade.Three pretreatment methods,i.e.polymerization,epoxidation,and UV irradiation,were discussed against the reason why HTPB-based polyurethane is difficult to degrade,and the pretreatment effect was characterized by FT-IR and gel permeation chromatography(GPC),so as to effectively treat the waste from the binder system with the biodegradation technology.The results showed that the HTPB-based polyurethane with a body structure was transformed into a linear product by depolymerization,the molecular weight of which was greatly reduced to be close to that of HTPB,and the urethane bond refractory to degrade was eliminated.On this basis,carbon-carbon double bonds in the depolymerized product were partially conversed by epoxidation,which introduced the strongly electronegative element oxygen,improved the hydrophilicity,and produced biologically sensitive oxygen-containing groups.The ability of the product to be degraded as a microbial carbon source was improved with the two-step pretreatment.However,the effect of UV irradiation on the binder system and the conversion of the double bonds were only found on the surface of the material,and the original body structure was not changed.Moreover,UV irradiation resulted in low oxidation efficiency,and curing and cross-linking of the depolymerized product,giving rise to an increase of molecular weight.Therefore,in this study,UV irradiation was not suitable for the pretreatment for biodegradation.The product from depolymerization and epoxidation pretreatment was utilized as the biodegradation substrate,and the biodegradation-effective bacteria were screened from the bacteria-source enrichment solution.Then,the degradability of the screened strains was evaluated by monitoring the weight loss rate after 100 d.The best two strains were selected.Next,degradation products and enzyme proteins in the degradation process were analyzed with GC/MS and SDS-PAGE,and the degradation process and mechanism were discussed.The results indicated that three strains of fungi and five strains of bacteria that were obtained after initial screening,acclimation,re-screening and isolation could grow with the product from epoxidation as the only carbon source and had certain degradability,which resulted in a weight loss of the epoxidation product by 11-29% after 100 d.Among them,Burkholderia contaminans and Candida palmioleophila resulted in a weight loss of the product from epoxidation by 29%and 25%,respectively.The degradation products of the two strains after 60 d showed similar characteristics,mainly composed of alkanes with different numbers of carbon atoms,accompanied by a few esters,alcohols and ketones formed.At least six and four induced enzymes were respectively produced due to the stimulation of the product from epoxidation.The degradation mechanism can be summarized as the combined effect of hydrolysis reaction and free radical reaction under the catalysis of biological enzyme.Specifically,the molecular chain of the product from epoxidation was firstly attacked by the epoxide hydrolase,and a series of transformations of the epoxide groups occurred,leading to in-situ formation of alcohols and ketones as well as chain breakage and formation of esters and carbon-carbon bond segments.In addition,the carbon-carbon bond segment was decomposed into alkyl radicals under the action of the enzyme catalyzing the free radical reaction,and underwent regular recombination and combination,generating normal alkanes with a large number of carbons and isomeric alkanes with a small number of carbons and realizing the main process of biodegradation.Moreover,the degradation also involved the reduction of carbon-carbon double bonds and the decomposition of the residual hard segment of the original HTPB-based polyurethane in the molecular chain of the epoxidation product.In the subsequent study,the degradation of the two strains on the depolymerized product and HTPB-based polyurethane was tested to further verify the effect of two-step pretreatment of depolymerization and epoxidation on improving the biodegradability of the binder system.The degradation conditions of the two strains were optimized with peptone or starch as primary carbon source and span-80 or sodium dodecylbenzene sulfonate as surfactant.The results show that Burkholderia contaminans had certain co-metabolism only when the peptone concentration was 5g/L,and the degradation weight loss of epoxidation products was higher than that without peptone as primary carbon source,which may be due to the decomposition of ammonia ester bonds by the protease produced during peptone digestion.Span-80 can improve the interface between Candida palmioleophila and epoxidation products,which promotes biodegradation with a slightly higher weight loss after degradation for 40 days than that without span-80.In this study,when HTPB-based polyurethane,the essential component of binder system,is difficult to biodegrade,the biodegradation of the binder system waste is realized through the pretreatment of depolymerization and epoxidation,obtaining an acceptable degradation effect.The method provides a technical idea for the treatment of the residual waste from HTPB propellant after the recovery of energetic components,and lays a certain theoretical and practical foundation.At the same time,by comparing the biodegradation characteristics of three types of polyurethane,discussing the degradation process and mechanism from binder system to small molecular alkane products,and comparing with the differences from previous researches on polyurethane biodegradation,the polyurethane biodegradation theory has been improved to some extent. |
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