Preparation Of Amine-Based Thermo-Sensitive Curing Agents And Their Application In Epoxy Powder Coatings

Powder coating is a pure solid coating that does not contain volatile organic compounds.It has unique advantages in economy,environmental protection,high efficiency and application performance.Curing agent is an important component of epoxy powder coatings.It can promote crosslinking of epoxy resin under certain conditions to form an insoluble and infusible three-dimensional network structure of high polymer.Curing agent plays a decisive role in its aggregation state,therefore,the development of new curing agent will effectively broaden the application range of powder coating.The high curing temperature of traditional epoxy powder coating curing agent severely limits the development and application of powder coating.Lowering the curing temperature of epoxy powder coating is an important problem that needs to be solved urgently in the powder coating industry.Hydroxyl-terminated resins（HR-A,HR-B,HR-C）have good compatibility,gloss and fluidity,and both ends contain highly reactive hydroxyl groups,which are easy to be modified.Anhydride is used to react with hydroxyl-terminated resin to obtain carboxyl terminated resin.The carboxyl-terminated resin is then neutralized by ethylenediamine（EDA）to obtain an amine-based thermo-sensitive curing agent with modified resin as the main chain.Under higher temperature conditions,it releases highly active ethylenediamine,which reacts with epoxy resin,the epoxy resin is cross-linked and cured.The curing behavior of the amine-based thermo-sensitive curing agent and powder epoxy resin NPES-604 is studied.Meanwhile,the application and performance of the prepared powder coatings on the surface of metal materials and plywoods are investigated.The specific research content and results are as follows:（1）In order to study the influence of different anhydrides（aromatic anhydrides,unsaturated anhydrides and alicyclic anhydrides）on the acid values and softening points of the modified resins,as well as the influence of different anhydrides on the release temperatures of ethylenediamine.Phthalic anhydride（PA）,tetrahydrophthalic anhydride（THPA）and 4-methylhexahydrophthalic anhydride（Me HHPA）are selected to perform esterification reaction with the hydroxyl group of the hydroxyl-terminated resins to obtain the esterification products HR-A/PA,HR-A/THPA,HR-A/Me HHPA,HR-B/PA and HR-C/PA,their acid values are 2.10mmol/g,2.07 mmol/g,2.06 mmol/g,1.24 mmol/g and 0.27 mmol/g.Preferably,the carboxyl-terminated resins HR-A/PA,HR-A/THPA and HR-A/Me HHPA produced by the esterification reaction are neutralized with EDA to form salt.The thermo-sensitive curing agents HR-A/PA/EDA,HR-A/THPA/EDA and HR-A/Me HHPA/EDA containing ammonium carboxylate structure were obtained.That softening points are 75.0℃,65.5℃ and 62.5℃,and the decomposition temperatures are 97.8℃,122.6℃ and 110.9℃,the release amounts are 8.2%,8.3%and 8.6%;（2）HR-A/PA/EDA has a suitable softening point and a low decomposition temperature of ammonium carboxylate.Therefore,HR-A/PA/EDA is preferred as a curing agent to study its curing behavior with epoxy resin NPES-604.By studying the effects of different curing agent dosages and different curing temperatures on the coating gel rate,it is determined that the curing agent dosage is 30 phr.When curing conditions are 100℃/8h,120℃/6h and 150℃/4h,the coating gel ratios are 58%,76%and 97%.The effect of solidification temperature on solidification behavior was studied by monitoring the change of gel rate.The results show that when the amount of solidifying agent added was 30 phr,the experimental value and the melt diffusion model were as follows:c（%）=100[C_equi-aexp（-bt）]fits well;（3）The powder coatings are used for metal-based materials,the coatings obtained under the conditions of 120℃/6h and 150℃/4h curing show excellent mechanical properties and acid,alkali and salt resistance.To study the influence of different fillers on its mechanical properties and acid,alkali and salt resistance,the better filler system is 60 phr BaSO₄ and 0.5 phr fumed silica;（4）When it is applied to plywood surface,good decoration,stain resistance and mechanical properties can be obtained.At the same time,the powder state of the powder coating system is level 1 after 28 days storage experiment,showing good storage stability.