Study On Rapid Preparation And Repair Of Radar Absorbing Coatings Based On Frontal Polymerization

Stealth coatings which belong to a category of thick coatings(>400 ?m)play an important role in the modern advanced fighters and weapons systems.Once damaged,it can't be repaired rapidly by conventional repairing methods,which will cause serious consequence in wartime.Thus,developing a rapid repairing technology for stealth coatings is necessary and prospective.Stealth coatings,consisting of 80% solid fillers and hard and brittle alicyclic epoxy resin,are usually poor in ductile and easy to peer off,if not modified.Therefore,in this project,two different kinds of epoxy resin were synthesized for toughening the alicyclic epoxy resin.UV initiated frontal polymerization of such a composite system was performed and the effects of various factors on the frontal polymerization behavior were investigated,together with the structure and properties of the cured products characterized.The optimized epoxy resin system which could be cured by photo-induced frontal polymerization approach was further used as resin matrix for stealth coating with fast curing ability.The UV initiated frontal polymerization behavior of the stealth coating and the properties of the stealth coating were studied in order to obtain a kind of rapid,energy saving,environmental protection and high performance stealth coatings repair technology to meet the needs of the new aircrafts and other weapons with stealth coatings.The specific contents are as follows:(1)The modified polysiloxane was successfully synthesized by using methyl silicone oil containing low hydrogen and 4-vinyl epoxy cyclohexane in toluene solution under Karstedt catalyst.Then the curing of modified polysiloxane/epoxy system by UV initiated frontal polymerization in the horizontal direction was investigated.The results show that: as the photoinitiator concentration increases,the frontal velocity increases slightly,while the trigger time decreases rapidly.It decreases to 18 seconds when the concentration of photoinitiator increases to 6wt%.The frontal velocity and the maximum temperature rise rapidly with the increase of the concentration of thermal initiator.The frontal velocity is 3.93cm/cm the maximum temperature reaches up to 279.1 ? respectively when the concentration of thermal initiator is 5wt%.with the increase of modified polysiloxane,the frontal velocity and the maximum temperature experience a gradual drop but the propagating angle contibues to increase,which reaches up to 48° when the content of the epoxy polysiloxane is 20wt%;the results of FTIR and DSC measurements show that condensates cured by frontal polymerization have a very high curing degree while TG tests indicate that the condensates have excellent thermal stability;the impact testing and SEM analysis show that better toughening effect on epoxy resins can be achieved by using epoxy polysiloxane;compared with the traditional curing method,UV initiated frontal polymerization has an advantage in curing composite systems which have poor compatibility.This method is suitable for rapid curing of stealth coating with large amount of filler.(2)The flexible copolymers were prepared by using butyl acrylate and 4-vinyl cyclohexane as monomers.The curing of poly(4-vinyl epoxy cyclohexane-co-butyl acrylate)/ epoxy resin composites via UV initiated frontal polymerization was studied.The results show that: the trigger time decreases and the frontal velocity slightly increases with the increase of the concentration of photoinitiator,especially when the concentration reaches to 6wt%,the trigger time is only 10 seconds;the frontal velocity and the maximum temperature increase with the increase of the concentration of thermal initiator,the maximum temperature is 259.7? and the velocity is 2.72 cm/min;with the increase of the content of the copolymer,the frontal velocity and the maximum temperature of the fixed point experience a gradual drop;FTIR and DSC tests show that condensates cured by frontal polymerization have a very high curing degree;the impact test and SEM analysis show that the addition of soft copolymers is effective in toughening 221 epoxy resin;the hybrid system can fully meet the requirements of flexibility when used as the basic component of the radar absorbing coatings.(3)The poly(4-vinyl epoxy cyclohexane-co-butyl acrylate)/epoxy resin composite system was used as the basic component of the radar absorbing coatings.The results show that: the curing time of radar absorbing coatings decreases with the increase of the concentration of photoinitiator and thermal initiator;deep curing of the coating can be achieved in 120 s when photoinitiator and thermal initiator are carefully selected;when the content of flexible copolymers increases,the pencil hardness of the coatings deteriorates,the flexibility and impact resistance of the coating enhance,and the adhesion of the coating is kept unchanged;when the content of flexible copolymer is 35wt%,the pencil hardness of the coating is 4H,the flexibility is 7.5 mm,and impact strength is 45 Kg·cm;with the rise of the content of carbonyl iron powder,the pencil hardness and the radar absorbing performance of the coating become better,but the adhesion,flexibility and impact properties deteriorates;the performance of coatings resistance to acid,alkali and salt is excellent and no obvious changes happen within 20 days;flatting agent and defoaming agent can improve the condition of coating surface in the formula;after the repair of the damaged area of the coating,the property of the damaged area can be as well as the original one's.