Study On The Adhesion Mechanisms Of Nano-modified Polymers And Steel

Polyethylene is one of the most widely used corrosion-protection thermoplastic coating materials that protects steel from corrosion and performs very well.In spite of its many advantages,it exhibits poor adhesion property due to the nonpolar and inert surfaces,thus limiting the applicability.Various methods for surface modification have been made to improve the adhesion properties of polyethylene,such as corona discharge, plasma treatment,chemical etching,grafting functional groups,oxidizing chemical modifications to improve the surface characteristics.Such surface modification techniques are difficult to control,often cause problems with respect to uniformity and reproducibility,and present extra disadvantages.On the other hand,much attention has recently been paid to polymer nanocomposites,which can exhibit novel and enhanced properties.It was found that nanoparticles enhanced the adhesion strength between LDPE powder coating and steel substrate in the primary study.And it also was reported that nanoparticles improved the adhesion of epoxy resin and adhesives in some literatures. So,it is very important and meaningful to investigate the mechanisms of the adhesion strength between polymer and metal.However,as far as we know,very little effort has been made to study the mechanisms of nanoparticles in modifying the adhesion properties of polymer coating.Our work was focused on two different kinds of polymers to investigate the mechanisms of the adhesion strength between polymers and steel,which was divided into two parts.One selected polymer is low density polyethylene(LDPE)powder coating, which is nonpolar polymer.The other is epoxy adhesive,which is polar polymer. Nanoparticles used were nano-Al₂O₃,nano-CaCO₃,nano-SiO₂,which are very common.In Part One,the nano-modified LDPE powder coating was prepared by dispersing nanoparticles into LDPE in acetone.The influence of nanoparticles on the properties of nano-modified LDPE was studied by measuring or calculating matrix tensile strength, surface energy,residual strength,crystallinity with tensile testing machine,contact angle instrument,volumetric flask,XRD.The adhesion strength measurement samples were prepared by hot pressing the LDPE powder into thin coating between two steel cylinders. Then the adhesion strength was measured by a tensile testing machine.The influence of the nanoparticles dispersion,type and concentration of nanoparticles,the surface roughness of steel substrate,and the coating thickness on the adhesion strength of the nano-modified LDPE was studied by measuring the adhesion strength.The influence of nanoparticles on the interface morphology was observed by SEM and EDX to study the intact of LDPE and steel at the interface.The distribution of the some elements and chemical state was studied by EDX,XPS to investigate the chemical bonding at the interface.At last,the adhesion mechanisms of steel and LDPE modified by nanoparticles was proposed.The results were shown in the following:1.The influence of nanoparticles on the properties of LDPE was investigated.After the addition of nanoparticles into LDPE,the tensile strength decreased and the polarity of nano-modified LDPE increased while the volume variation decreased and then increased a little.2.The influence of nanoparticles on the adhesion properties of LDPE was investigated.The adhesion strength between LDPE and steel was enhanced obviously by nano-modification.The variety of the adhesion strength of the three kinds of nano-modified LDPE changed similar with increasing the nanoparticles concentration and get the highest strength 9.6 MPa,8.9 MPa,9.5 MPa with the addition of nano-Al₂O₃, nano-CaCO₃,nano-SiO₂ respectively.3.Surface roughness effected little on the adhesion strength of LDPE without nanoparticles modification.While the adhesion strength of LDPE increased much on the steel surface abraded by silicon carbide paper of 60 and 150 grits.And the adhesion strength increased much more on the polished steel surface.Those results meant that nanoparticles amplified the effect of roughness on the adhesion increase in some manners, nanoparticles increased the true adhesion strength which was higher than the practical adhesion strength because of the properties of LDPE,and the variation of the two adhesion strength was similar with the change of the nanoparticles concentration.4.The SEM observation of the interface showed that the surfaces of LDPE and steel contact closely whatever the addition of nanoparticles or not.The surfaces morphology of the adhesive crack showed that adhesive crack usually occurred for the joints without nanoparticles additive,and mixture of adhesive and cohesive crack for nano nanoparticles modified joints.The XPS analysis of steel surface after adhesion test showed that the new polar groups,C-O and C=O were produced in LDPE after the addition of nanoparticles in LDPE.5.Based on those results,it can be suspected that the polar groups,matrix strength and residual strength influenced the practical adhesion strength,and polar groups were the main cause for the increase of the true adhesion strength.In Part Two,the nano-modified epoxy adhesive was prepared by mechanical dispersion of nanoparticles into epoxy adhesive.The influence of nanoparticles on the properties of the nano-modified epoxy adhesive was studied by measuring or calculating matrix tensile strength,surface energy with tensile testing machine,contact angle instrument.The influence of the nanoparticles dispersion,type and concentration of nanoparticles,the surface roughness of steel substrate,and the coating thickness on the adhesion strength of the nano-modified epoxy adhesive was studied by measuring the adhesion strength with pull-off adhesion tester.The influence of nanoparticles on the interface morphology was observed by SEM and EDX to study the intact of epoxy adhesive and steel at the interface.The distribution of the some elements and chemical state was studied to investigate the chemical bonding at the interface.At last,the adhesion mechanisms of steel and epoxy adhesive modified by nanoparticles was proposed.The results were shown in the following:1.The influence of nanoparticles on the properties of epoxy adhesive was investigated.The tensile strength of unmodified epoxy adhesive matrix was 6.77MPa. After the addition of nanoparticles into epoxy adhesive,the tensile strength increased a little when the concentration of nano-Al₂O₃ was 10%,the adhesion strength increased to 7.85 MPa.The tensile strength of nano-CaCO₃ modified epoxy adhesive got to the lowest value with 3%nano-CaCO₃ in epoxy adhesive.Then the tensile strength of nano-CaCO₃ modified epoxy adhesive increased to 5.2MPa with 10%nano-CaCO₃.The variation of the tensile strength of nano-SiO₂ modified epoxy adhesive was similar with that of nano-CaCO₃ modified epoxy adhesive.The lowest value was 4.44MPa with 2% nano-SiO₂ addition and higher than 6.77MPa with 4%nano-SiO₂ addition.2.The influence of nanoparticles type and concentration,the surface roughness of the steel on the properties of epoxy adhesive was investigated by pull-off adhesion test.The adhesion strength between epoxy adhesive and steel was enhanced obviously by nano-modification.The adhesion strength between pure epoxy adhesive and steel was 3.8MPa.After the addition of Al₂O₃ into epoxy adhesive,the adhesion strength got to the highest value 18.4MPa when the concentration of nano-Al₂O₃ was 2%then decreased a little to 8MPa when the concentration of nano-Al₂O₃ was 5%.The adhesion strength of nano-CaCO₃ modified epoxy adhesive got to the highest value 5.83MPa with 2% nano-CaCO₃ in epoxy adhesive.The adhesion strength of nano-SiO₂ modified epoxy adhesive increased with the concentration,and got the highest value 8.4MPa with 8% nano-SiO₂.The increase of adhesion strength of nano- Al₂O₃ modified epoxy adhesive on the steel surface abraded by silicon carbide paper of 150 grits was 4 times higher than that on whose surface was polished.3.The SEM observation of the interface showed that the surfaces of epoxy adhesive and steel contact closely whatever the addition of nanoparticles or not.It implied that surface roughness has little effect on the adhesion strength.The surfaces morphology of the adhesive crack showed that adhesive crack usually occurred for the joints without nanoparticles additive,and mixture of adhesive and cohesive crack for nanoparticles modified joints,which meant that the practical adhesion strength was comprised by the true adhesion strength and the cohesive strength of the matrix.4.The XPS analysis of steel surface after adhesion test showed that the new polar groups,O=C-O- was produced in epoxy adhesive after the addition of nanoparticles in epoxy adhesive,which was the main cause for the adhesion increase of the true adhesion. Moreover,the FTIR results showed that the numbers of the new polar groups was very little.At the end of the paper,we analyzed the problems and put forward the future works based on the present works.