Formation Mechanism And Morphology Control Of Urea-formaldehyde Resin Flower-Like Microspheres

The flower-like microsphere refers to a 3D microsphere structure with a flower shape and is assembled by lamellar structure.If the lamellar structure is regarded as a "petal",the whole microsphere can be seen as a "flower-like microsphere" composed of "petals".Because of its special petal-like structure with large specific surface area and many reactive sites,flower-like microspheres are widely used in catalyst carriers,battery materials,microwave absorption,pH response and other fields.To date,almost all flower-like microspheres have been prepared with inorganic compounds,while flower-like microspheres prepared with organic materials are extremely rare.This is because metal and inorganic non-metal molecules have strong polarities and are more susceptible to crystallization.Organic matter is generally difficult to crystallize due to its large molecular weight and complex winding structure,that is,it is difficult to form an ordered structure.In this paper,formaldehyde and urea were used as raw materials,and the organic-based urea-formaldehyde resin flower-like microspheres with regular shape and particle size of 8～15 μm were prepared by simple precipitation polymerization.The morphological and structural changes of urea-formaldehyde flower-like microspheres were studied by SEM,FTIR,XRD,XPS and other test characterization methods.The results showed that pH is the key factor controlling the formation of flower-like microspheres.pH and temperature directly affect the reaction rate of formaldehyde and urea.The increase of pH(pH=4)and the decrease of temperature are beneficial to reduce the reaction rate of formaldehyde and urea.Further,the sedimentation and precipitation rate of the reduced product allows the urea-formaldehyde resin prepolymer to have sufficient time to crystallize and self-assemble to form an ordered structure,thereby generating flower-like microspheres.Different raw material ratios and different synthetic methods mainly affect the morphology of urea-formaldehyde resin microspheres from changing the structure of urea-formaldehyde resin.Finally,we proposed the formation mechanism of urea-formaldehyde resin flower microspheres by time tracking of urea-formaldehyde flower-like microspheres.This is the first possible formation theory for the formation of urea-formaldehyde flower-like microspheres,namely hydrogen bond-induced self-assembly theory.Using the formation mechanism,the thiourea resin flower-like microspheres were successfully prepared by the reaction of thiourea and formaldehyde.The successful preparation of thiourea resin flower microspheres verified the rationality of the theory,and a series of characterization of thiourea phenolic spheres by changing the reaction conditions,and explored the ability of thiourea resin as Ag+adsorbent.Finally,the maximum adsorption capacity of Ag+ was 8.1747 mmol/g with F4T1 resin with formaldehyde thiourea ratio of 4.The adsorption kinetics and static adsorption of the resin on Ag+adsorption were described by pseudo-second order model and Langmuir model,respectively.It has great guiding significance for the formation and subsequent application of other organic-based flower-like microspheres.