Microwave-assisted The Recycling Of Waste PET For Preparation Of Polyester Used In Powder Coatings

Polyester is widely used for its extraordinary performance,and the production is still increasing year by year.However,there are piles of abandoned polyester products, what have hazarded human's daily life,due to its chemical stability under the natural circumstance.Fortunately,the human beings' consciousness about environment protection is strengthening now,and many countries have proposed the road of sustainable development.Therefore,more and more attention has been paid on the recycling and reusing of polyester wastes.The development of polyester recycle technology and comprehensive utilization can cut down the cost of manufacturing polyester,reduce the excessive use of oil resources,and even have strategic significance to the environment protection and man's sustainable development.Poly(ethylene terephthalate),PET,is a versatile and the largest consumption product of polyester.At present,the two main methods on PET recycle are mechanical and chemical ways,and chemical recycling is the most important since the excellent properties and wide applications of its products.Attribute to the valuable products,the glycolysis of PET has been the most applicable and developable recycling ways.A novel PET chemical recycling technique focused on the fast and partially glycolysis employing a small amount of NPG was investigated.Microwave was introduced to assist the glycolysis for its rapid thermal effect and microwave catalytic effect.The glycolysis product was then esterified rapidly with microwave assisted to form the polyester resins which could be applied in epoxy-polyester powder coatings. The powder coatings were prepared using the esterified products,and the performances were tested according to the industry standards.The results of orthogonal experiments revealed that the main factors on PET glycolysis were the glycolysis reaction time under microwave,t.the applied microwave power.P.the mass ratio of NPG-PET.and the amount of ZnAc₂ as the catalyst,which were showed in a decreasing order.These factors were studied specifically,and the optimal glycolysis conditions were drawn as the glycolysis reaction time was 4min,the microwave power was 528W,the mass ratio of NPG-PET was 20%,and the amount of ZnAc₂ was 0.5%.The influences of the glycolysis reaction time,the microwave power,the mass ratio of NPG-PET during PET glycolysis could be concluded to the influence of the absorbed microwave energy of the system on glycolysis.Accordingly,a model on the effects of microwave energy during PET glycolysis was set up.The product of glycolized PET gained under the optimal conditions was characterized.The results of Gel Penetration Chromatography(GPC)showed that the PET glycolysis reaction was repeatable and the number average molecular weight ((?)_n)was 1200～300g/mol.The Thermal Gravity Analysis(TGA)and the Fourier Transform Infrared Spectroscopy(FTIR)results revealed that the small molecules such as NPG weren't contained in the glycolized product,what meant the ending of the glycolysis reaction under the optimal conditions,and the mainly glycolized product was polyester oligomer with hydroxyl ending groups.The glycolized product had to be esterified by polyacid or polyanhydride to produce the polyester with carboxyl ending groups which could be applied in powder coatings.The influences of adipic acid(AA)and pyromellitic dianhydride(PMDA) on the esterification were investigated.The esterification reaction,when the microwave power was 528W and the mass of antimony trioxide(Sb₂O₃)was 1.0%, finished in 3min whatever the amount of polyacid or polyanhydride was added, indicating the rapid esterification could be processed,which was also confirmed by TGA.However,the mass of AA and PMDA had played an important role on the number average molecular weight,the glass transition temperature(Tg),and the acid value(X)of the glycolized product.When AA was added with a constant amount, 11.7%,the change of the amount of PMDA could form polyester with various(?)_n,Tg, and acid value,and as the amount of PMDA increased,the(?)_n would decrease in a small range,while the Tg and X increased.If 7%PMDA was added,the esterified products.which could satisfy the requirements of the industrial epoxy-polyester powder coatings,with(?)_n=2100g/mol,Tg=58.5℃,X=70mgKOH/g would be collected.The main functions of PMDA introduced during the esterification process were to increase the(?)_n and Tg simultaneously,and to provide the crosslinking points to form well-crosslinked networks for improving the mechanical properties and the glossiness.This work focused on researching a rapid,efficient,and economic process to recycle PET waste,to decrease the piles of PET waste,and to support the environment protection and man's sustainable development.