| Pests such as diseases,insects and weeds are the critical factors limiting the production and quality of rice in China,and need to be controlled during rice growth annually.Moreover,pesticide application is the most effective and frequently used practice to suppress the damages by rice pests.At present the major method to vehicle pesticides to rice field is the foliar spray by spraying machines in rice growing areas of China.Due to the limitation of small plantation scale by family as a unit,also the limited conditions for technology and equipment of pesticide application in countryside,the pesticide application is very low in working efficacy,and also labor and time consuming.Moreover,foliar application by spraying apparatus often produce mist drift and pollute air environment.The development of new pesticide formulations and application technologies should be studied toward the direction of high efficacy,economical and simplified application.The development of water surface spread oil is one of the attempts to dress this issue,and has been registered in China recently.However,the application of this new formulation is frustrated due to the drawback of this new formulation,such as uneven distribution of oil membrane on water surface in rice field,uncomplete liberty of oil from pack-bottle and absorbing upon rice leave when throwing into rice field.Hereby,a new concept about oil spreading tablet is proposed to overcome above short comes.Oil spreading tablet can be broadcasted easily and form oil membrane on water surface in rice fields.The first problem for initiative of oil spread tablet is the screening for favorable solvent for pesticide.Twelve solvents from biological sources were selected as candidates,HPLC was used to compare their solubility for chlorpyrifos and abamectin.The solubility of methyl laurate were 1850mg/mL and 14.14 mg/mL for chlorpyrifos and abamection respectively.The solubility of turpentine based vegetable ois(ND-45)were 1850 mg/mL and 41.66 mg/mL for chlorpyrifos and abamection.ND-45 has higher solubility for these two insecticides than tested solvents.Epoxy soybean oil and liquid paraffin have the lowest solubility for two pesticides.However,turpentine based vegetable oil has pungent odor,and formulated tablet with turpentine based vegetable oil also emit unpleasant odor.Nevertheless,the solubility of chlorpyrifos and abamectin in methyl laurate is lower than that in ND-45,it meets the requirement for formulating oil spread tablet.Methyl laurate is adopted to formulate oil spread tablet in this study.The core technology for initiative of oil spread tablet is to solidify the liquid solvent oil and the pesticide solved in solvent.A lot of water soluble materials such as urea,ammonium sulfate,calcium chloride,sodium benzoate and sodium bicarbonate,etc.which have comparable high water solubility,are test for their absorbing capacity for solvent(methyl laurate)in this experiment.The absorbing capacity for methyl laurate by the these water soluble agents are too low to absorb large volume of solvent in formulating oil spread tablet.Hydrophilic polymers,such as polyethylene glycol,polyvinylpyrrolidone,hydroxyethyl cellulose,sodium carboxymethyl cellulose,and soluble starch,etc.are tested for their oil absorbing capacity,however the absorbing quantities are also very low and very unsatisfactory.We have to consider white carbon black which is frequently used as absorbing material to formulate wettable powder and water dispensable granular with liquid pesticides.White carbon black has very high absorbing quantity for methyl laurate 4.15 g/g,much higher than water soluble materials and hydrophilic polymers,white carbon black can absorb 418%solvent of itself quantity.However when white carbon black is incorporated into the oil spread tablet as solvent absorbing agent,the tablet has higher density and drop to water bottom rapidly due to its weight when applied to water surface.Mehtyl laurate and pesticide is brought to water bottom by white carbon black,it doesn’t spread onto the water surface,and no oil membrane is formed on water surface.Given this,carrier materials with low specific gravity should be adopted.Hollow micro-sphere glass beads are novel material which have very low specific gravity(only 1/8 of water density)and also excellent flowability and dispersibility,can meet this requirements.The result shows that hollow micro-sphere glass beads have high absorbing capacity for methyl laurate(4.18 g/g)comparable with white carbon black.It can absorb 418%solvent of itself weight.The tablets formulated with hollow micro-sphere glass beads don’t drop down to water bottom,and have very good dispensability on water surface.Therefore,hollow micro-sphere glass beads can be adopted in the formulation of oil spread tablets.It is required that water spreading tablet could automatically stretch to form membrane after being applied to the rice field,and good surfactant is needed to promote the spreading of the drugs on the water surface.In this research screening selection were conducted on 32 kinds of surfactants and it was found through analysis on the spreading speed and distance on water surface of methyl laurate affected by different surfactants that the SP-3708 and#33 surfactants had spread at the highest speeds and to the farthest distances in addition that the spreading speeds of methyl laurate after being applied to water surface were 8.49cm/s and 8.96cm/s respectively with the spreading distances of the oil membrane within 5 seconds as 42.5cm and 44.8cm respectively,significantly better than other surfactants.We also plan to adopt this kind of surfactant for creating and producing water spreading tablets because of the application in water surface spreading oils.Furthermore,other components like dispersant,disintegrant,and binder of the water surface spreading oil had also been selected during the research,and finally the formula for the 5%chlorpyrifos water surface spreading oil has been proposed as having 5%as chlorpyrifos,42%as methyl laurate,6%as octadecanol,2%as SP-3708 surfactant,20%as hollow glass microsphere,6%as D-518 dispersant,8%as citric acid,8%as dicarbonate,1.5%as sodium carboxymethyl starch,and 1.5%as filling carriers.This formula was taken to produce the product of 5%chlorpyrifos water surface spreading tablets.The self-made product is white and in the form of tablet with the average weight of each tablet as 0.1 g as well as good stability in hot storage and water surface diffusivity.However,this research has only conducted primary exploration on the creation and production of water surface spreading tablets,so further studies still need to be made on the membrane spreading performance on water surface of the rice field,effect of prevention in field and the controlling indexes of the quality of the products. |
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