Study On The Mechanism Of Forming Micro-nano Structure On Polymer Surface By Stress Oscillation Method

Sharkskin bionic microstructure plays an irreplaceable role in the fields of hydrophobic,oil-repellent,anti-fouling,self-cleaning,anti-drag,anti-corrosion,anti-bacteria and other special application environments.The development of efficient and cost effective processing methods is of great social and commercial value.Based on the phenomenon and formation mechanism of sharkskin microstructures arising from the periodic interfacial stick-slip transition at the exit of the mouth die wall surface under high shear stress of polymers,this paper proposes for the first time a new processing method for extrusion of polymer microstructure functional surfaces by stress oscillation method using reverse thinking.By varying the polymer shear rate at the wall of the mouth die,the polymer can be made to oscillate with a certain frequency and amplitude in the flow channel of the mouth die.By purposely controlling the parameters of the stress oscillation so that the wall stressσfluctuates above and below the critical wall stressσ_c,a viscous-slip transition associated with the stress oscillation will occur at the polymer/mouth die wall interface and the desired sharkskin microstructure functional surface can be obtained.This paper systematically studied the process parameters affecting the microstructure of sharkskin by stress oscillation shaping,including head temperature,screw speed,head pressure fluctuation and cooling method.By selecting different additives,sharkskin structures with different surface microstructural characteristics are prepared,and the durability of the products is also investigated.Specific elements include the following:（1）A mathematical model of the instable flow was established.The stress tensor oscillation decreased with increasing length of the mouth die runner,based on which the length of the extrusion mouth die runner for the preparation of sharkskin structures based on the stress oscillation method was determined to be in the range of 20-40 mm.The effects of head temperature,screw speed and material relaxation time on stress oscillation and microstructure preparation were determined.（2）The extrusion head was designed on the basis of simulations.Linear low density polyethylene（LLDPE）was chosen as the raw material to prepare sharkskin microstructures by varying the processing parameters.The effects of screw speed,extruder head temperature,pressure fluctuation and cooling method on the hydrophobic properties of the microstructures were investigated.It was obtained that as the screw speed increased,the unstable flow of the melt was enhanced,the surface microstructure features of the product were more complete and the hydrophobic properties were improved.As the head temperature decreased,the hydrophobic properties of the extruded product improved,reaching a contact angle of 120°at a head temperature of 150℃and a screw speed of 80r/min.The contact angle of the samples prepared by sandblasting the inner wall surface of the head increased by 14°,while the fluctuation of the head pressure and the cooling method had less effect on the hydrophobic properties of the products.The experiments further confirmed the conclusion that the contact angle of microstructure surfaces with higher roughness is larger.（3）AC series Si O₂openers were added to LLDPE to study the effect of different additives on the hydrophobic properties of the products.When the additives were AC5 and AC15,the products showed good hydrophobic properties.The trend of the hydrophobic properties of the different additives varied with the change of temperature.AC5 showed good hydrophobic properties at a head temperature of 160℃,with a contact angle of 133°.While AC15 reached a contact angle of 128°at a head temperature of 155℃.In addition,the durability of the surface microstructure of the products was enhanced by the addition of Si O₂additives.The hydrophobic microstructure surfaces prepared by stress oscillation are simple and cost effective,and the prepared samples can be further enhanced with other microstructural processing methods.This offers a promising new method for the efficient and cost effective production of microstructure products such as tube interiors which are difficult to shape by conventional methods.