Preparation Of Waterborne Polyurethane Emulsion By High Gravity Emulsification

Waterborne polyurethane(WPU)is a new polyurethane emulsion system,which uses water instead of organic solvent as a dispersant in its preparation,thus greatly reducing the emission of volatile substances.Now it has attracted more and more attention.However,due to the presence of water,there are two main problems that limit the wider application of waterborne polyurethane.First of all,the introduction of hydrophilic groups reduced the water resistance of WPU film.Secondly,the solid content of WPU was low,making its use and transportation cost higher.Therefore,we should study the relationship between the synthesis process and the properties of WPU.At the same time,we should use efficient emulsification technology to reduce the water absorption rate and improve the solid content of emulsion.And that will help us to expand its application scope and save costs.High gravity is a kind of technology,which provides a centrifugal force field by equipment and utilizes the unique flow phenomena of multiphase fluid under high gravity to enhance mass transfer and micro-mixing between fluids.Different from the traditional internal emulsification and dispersion process,we can achieve rapid emulsification by adjusting the feed rate of polyurethane prepolymer and water,but there are few studies on its emulsification performance.This paper first explores the influence of the synthetic process on the properties of the emulsion,selects the formula,and then introduces the high gravity technology into the waterborne polyurethane emulsification process,hoping to improve the stability of the WPU emulsion,reduce the water absorption rate of the film,and improve the solid content of the WPU.The main contents of this paper are as follows:(1)WPU was synthesized by the self-emulsifying method.The effect of adding water on the stability of emulsion was first investigated.Then the effect of the synthesis process of polyurethane prepolymer on the properties and a solid content of WPU emulsion was emphatically explored.The relationship between R(NCO-:OH-),hydrophilic group(DMPA),the internal crosslinking agent(TMP)with emulsion stability,viscosity,solid content and water absorption of the film were studied.The results show that when R=1.3,DMPA and TMP are 3.4%and 3%respectively.The IPDI type waterborne polyurethane has good comprehensive properties.The particle size of the emulsion is about 150 nm,and the water absorption is about 16.2%.(2)The prepolymer synthesized by the optimized formula was emulsified by high gravity emulsification method,and the emulsification process of prepolymer in the high gravity machine was analyzed.The emulsification process was divided into feeding collision emulsification,filling area emulsification,wall collision emulsification,and cyclic emulsification.According to the analysis,the influence of the rotating speed,circulation time and water feed rate with the particle size and stability of the emulsion was investigated.The optimum process conditions are obtained:The emulsification time is 4-6min,the feed flow ratio of the water is 150mL/min,and the speed of the gravity machine is 2000r/min.The average size of polyurethane emulsion prepared by rotating packed bed is about 90nm,which is obviously smaller than that of waterborne polyurethane emulsion prepared by stirring.Under the condition of high gravity emulsification,the use of hydrophilic group can be reduced.It is found that when the hydrophilic group is at 3.2%,the emulsion can still be stable,the emulsion size is 161 nm,and the water absorption rate is 12.4%.(3)In order to verify the universal applicability of high gravity emulsification for waterborne polyurethane emulsion,we used the one-step method to synthesize polyurethane prepolymer and emulsified prepolymer in a RPB.The effects of cycle time,equipment rotation speed and deionized water feed flow on the particle size and stability of emulsion were investigated.The obtained optimal preparation conditions in the RPB are as follows:the emulsification time is 15 minutes,the feed flow rate of deionized water is 200 mL/min,and the rotating speed of the high gravity rotating bed is 2000 r/min.The waterborne polyurethane emulsion with solid content of 58%is prepared by means of high gravity emulsification.The average particle size of WPU is 231.8 nm.The emulsion showed obvious broad distribution trend by the electron microscope.The viscosity of WPU is about 140 mPa s,the water absorption rate is 16.4%,and the bonding strength to the metal is about 3.6 MPa.Tensile strength is 10.68 Mpa and elongation at break is 470%.(4)The relationship between the solid content and viscosity of the prepolymer prepared by one step process after emulsification by the rotating bed was studied.It was found that the viscosity of the emulsion was low.When the solid content was below 55%,and then increased rapidly.When the viscosity of the emulsion reached 60%,the emulsion had poor fluidity.The formation time of WPU emulsion with different solid content was studied.The results showed that if we do not use the auxiliaries the initial water loss of the emulsion with different solid content was close at the room temperature.With the high solid content emulsion drying,the film formed on the surface prevented the rapid evaporation of water.The water loss rate began to decrease,the final film forming time is close to that of a low solid content emulsion.Considering the WPU film with high solid content was thicker,which could reduce the number of construction times.Reduce the cost of labor power.(5)Finally,the efficiency of simultaneous removal of acetone in the process of emulsification in the high gravity machine was investigated.After 30 minutes of emulsification,few acetone contents in polyurethane emulsion was detected.(6)Small particle size WPU was used instead of deionized water to emulsify the prepolymer in high-gravity equipment.The solid content of the emulsion can reach 64.1%.The viscosity of the emulsion is 123.5 mPa s.The emulsion shows an obvious bimodal distribution,but the water resistance of the emulsion decreases,and the water absorption rate is about 23.5%.