Preparation Of Porous Pendimethalin-loaded Microcapsules And Its Controllable Release Property

The microencapsulation in the field of agrochemicals has many advantages,such as abated toxicity and harmfulness,controlled release,improved storage and handle stability,odor-masking,and so on.The polymeric walls of the conventional microcapsules(MCs)are usually firm and compact,and the transfer of the core material primarily occurs through permeation into the outer environment,which leads to sustained slow release of the core materials and thus may be unsatisfactory in practical applications.Currently,controllable release behavior of the core material between the external environment and the MCs would be highly desired,and the on-demand release of the MCs have drawn widespread interest.In particular,porous materials have been applied in microencapsulation due to their large surface area and tunable porosity.In this study,porous pendimethalin-loaded MCs were fabricated with epoxy phenolic novolac resins(EPNs)as wall-forming materials by interfacial polymerization.The effects of the amount of cross-linking agent,the ratio of core material to shell material and the epoxy values of EPNs on the basic properties were investigated,and the mechanism of pore formation was also confirmed.In addition,the physicochemical properties were systematically characterized via various analytical approaches.The herbicidal activities of such MC samples were conducted using greenhouse pot culture experiment with the purpose of verifying the potential application of such porous MCs in the field of agriculture.The main contents are as the following aspects:1.The pendimethalin-loaded EPN-MCs were successfully prepared with DMP as cross-linking agent using interfacial polymerization.The obtained MCs had nearly spherical shapes with fine monodispersities and sizes in the range of 5-40 ?m,and encapsulation efficiencies of MC samples were all higher than 90%.The FTIR spectra suggested that the MC shells were composed with cured EPN-polymers and the pendimethalin did not react with EPN.SEM images showed that some small pores(ranging from 20 nm to 1300 nm)appeared on the MC surfaces and that both pore size and pore density could be controlled by regulating the amount of cross-linking agent,the ratio of core material to shell material or the epoxy values of EPNs.Furthermore,the mechanism of pore formation was because that the DMP aggregates deposit randomly on the loose MC wall layer when the temperature was higher than the Tscp of DMP.2.Both the pore sizes and pore density on MC surfaces markedly increased with increasing the percentage of DMP or the ratio of core material to shell material,and the influence of the DMP amount was most obvious.The diameter of the pores on the MC surfaces increased from 20 nm to 1000 nm as the percentage of DMP increased from 0.3 wt.% and 2.4 wt.%,and the pore density increased as well.Particularly,for the MC samples with a high core/shell ratio(15:1),a small proportion of the MCs exhibited irregular shapes and seemed to coalescence with each other when dried,we speculate that the amount of wall-forming material was relatively insufficient in this condition and that the mechanical strength of the capsule shell was too poor to maintain an undeformed morphology after the evaporation of water.The MCs had nearly spherical shapes with fine monodispersity but the pore sizes and pore density decreased with increasing amounts of EPN.3.The cured EPN shells had larger degrees of crosslinking with an increase in epoxy values from 0.42 mol to 0.53 mol/100 g,thus,forming a more compact cured EPN walls and leading to better thermal stabilities.Additionally,the porosity on the MC surfaces decreased with an increasing of epoxy value,which further leaded to the release rate of the core material decreased and thus resulted in a lower herbicidal control efficacy.4.Our results indicated that the differences of both pore size and pore density of the MCs determined their release properties and further regulated the bioactivity of the MCs.The cumulative release rate decreased from 80% to 39% as well as the poorer control efficacy with a decrease in pore size from 1000 nm to 200 nm.Generally,The large surface area of the MCs was larger with a higher porosity,leading to a larger contact area between the outer environment and the MCs,and further regulating the transfer rate of the core material and herbicidal control efficacy.Therefore,the porous MCs with different porosity could be facile obtained to meet various applications by adjusting the technology parameters in the process of preparation,and thus regulating the release of MCs to meet various applications.