Investigation On Dynamic Cushioning Property Of EPP

Expanded polypropylene is one of the most important polymer foams.It has good capability of energy absorption,high deformation recovery rate,light weight and degradability.At present,it is commonly used in automobile,packaging,architecture and other fields.When the expanded polypropylene is compressed,the cellular micropores are squeezed so as to cause its volume change,which consumes the impact energy to protect the products from damage,and shows that the expanded polypropylene plays an important role in cushioning applications.The cushioning ability is different for the expanded polypropylene foams with different densities,and the thickness is also a factor affecting its cushioning property.Therefore,it is of important research significance and economic value to study the effect of density and thickness on the cushioning property of expanded polypropylene,in view of rational use,cost reduction and resource saving.The related mechanical theory of expanded polypropylene is summarized.The acceleration-time curves of polypropylene foams with different thicknesses and densities are obtained by means of dynamic impact tests and related standards.Then by changing the impact height to obtain different impact energy,the evaluating indicators of contact force,maximum displacement,maximum strain,and energy absorption are obtained for the expanded polypropylene foams with the different ratios of impact energy to thickness.Meanwhile,by using the stress-energy method,the fitting formulas of maximum acceleration-static stress curves are established for the expanded polypropylene foams with different thicknesses and densities under different drop heights.The influence of thicknesses on dynamic cushioning performance of expanded polypropylene is studied by analyzing the experimental results.For the expanded polypropylene foams with same density,the maximum contact force decreases and the contact time increases with the increases of specimens' thickness,but the energy absorption values of all specimens vary little,when theimpact energy was constant.When the ratio of impact energy to thickness is fixed,the increase of impact energy makes the specimen engender larger displacement to absorb energy with the increase of the thickness.Therefore with the increase of thickness,the maximum displacement and energy absorption increase,while there is no obvious change for maximum contact force and maximum strain.When the thickness is constant,the maximum contact force,maximum displacement,maximum strain and the energy absorption of the specimens increase with increasing ratios of impact energy to thickness.The influence of density on dynamic cushioning performance of expanded polypropylene is also studied.For the expanded polypropylene foams with the same thickness and the given density,the maximum contact force,maximum displacement,maximum strain and absorption energy increase with the increase of impact energy.When the impact energy is fixed,the maximum contact force and contact time becomes larger,and the maximum displacement and maximum strain decrease with the increase of density,while there is no significant change for energy absorption of specimens.In the range of selected impact energy and density,the greater the impact force transferred to the protected product.From the curves of maximum acceleration versus static stress obtained by experiments for the expanded polypropylene,it is shown that the maximum accelerations corresponding to the minimum points on the maximum acceleration-static stress curves decrease and the corresponding static stresses increase with the increase of thickness for the specimens with the same density under the same drop heights.Under the same drop height,the maximum acceleration and static stress corresponding to the minimum points of maximum acceleration-static stress curves increase with the increase of density for the specimens with the same thickness.For the specimens with the same thickness and density,the maximum accelerations corresponding to the minimum points of maximum acceleration-static stress curves increase and the corresponding static stresses decrease with the increase of drop heights.The maximum acceleration-static stress curves for the expanded polypropylene with different thicknesses and densities are established under different drop heights by using the stress-energy method.It is seen that the quadratic polynomial and exponential functions can well fit the experimental results.Based on the experimental results,theparameters of these functional relationships are calculated by using the least square method to obtain the final empirical formulas.It is shown that the quadratic polynomial formulas are more consistent with the experimental results than the exponential ones.