Fiber Web Meso-structure And Filtration Performance Of Spunbonded/Meltblown Nonwoven

The development of high efficiency air filtration material is one of the effective methods to deal with air pollution.It is an effective way of preparing high quality and high efficiency air filtration materials to optimize the meso-structure of fiber web of nonwoven and to improve its mechanical properties by compounding nonwovens produced using the same and/or different methods of web formation.The meso-structure of nonwoven has the characteristics of randomness and complexity.At present,the poor controllability of nonwoven web structure parameter in actual production makes it difficult to prepare the nonwovens which can meet the predetermined specifications.There is a complex nonlinear relationship between the parameters of the fiber web structure and the filtration performance of nonwoven,which is difficult to analyze using traditional methods such as mathematical statistics.The characteristics of fiber web of two thin nonwoven,namely spunbonded nonwoven and meltblown nonwoven,were studied using digital image processing technology,rough set and support vector machine.A few key structural parameters which most impact the filtration performance of the nonwovens were extracted and their regulating mechanism were revealed.Based on that,the formation mechanism of fiber web meso-structure in composite nonwovens including the variation regularity of pore size according to the layer number of composite and the composite mode were analyzed.The key structural parameters which most influence the filtration performance of composite nonwoven and the effect of compounding mode on their filtration performance were investigated,so as to provided a theoretical basis for preparing the high efficiency air filter media.The main results were as follows:1.The result of reduction on pore shape parameters for both spunbonded nonwoven and meltblown nonwoven shows that pore eccentricity and pore compactness is the two parameters which can best represent the characteristic of pore shape.The result of reduction on fiber web meso-structure parameters for spunbonded nonwoven shows that the thickness,fiber diameter,pore size,pore size variation coefficient and pore eccentricity are the five most informative fiber web parameters,while that for meltblown nonwoven shows that the thickness,fiber diameter,fiber diameter variation coefficient,pore size and pore eccentricity are the five most informative fiber web parameters.The difference on fiber web structure between spunbonded and meltblown nonwoven is as follows.First,the fiber diameter of spunbonded nonwoven is large and present asmall fluctuation,while that of melt blown nonwoven is small and fluctuates greatly.Secondly,The feature length of elemental plane of meltblown nonwoven is far greater than that of spunbonded nonwoven.Third,the pore size of meltblown nonwoven is more uniform than that of spunbonded nonwoven,although there was no remarkable difference in the pore size between these two nonwovens.Fourth,the porosity of meltblown nonwoven is larger than that of spunbonded nonwoven.2.In order to find out the effects of one single structural parameter of fiber assembly on pore structure of nonwoven,an image-based model of elemental plane was established,where the fiber number of elemental plane,fiber diameter and its variation coefficient,the degree of fiber orientation were taken as the model parameters.The results show that as the fiber number is increased,the pore size declines significantly and the variation coefficient of pore size showed a slightly increasing trend.When the fiber diameter increases,pore size reduces slightly and the variation coefficient of pore size increases slightly.The variation coefficient of pore size increases a little when the variation coefficient of fiber diameter increases.As the orientation degree of fiber web get larger,the pore eccentricity tends to increase significantly,and the variation coefficient of pore size declines slightly.Furthermore,the structural parameter of elemental plane that most affect the pore size is the fiber number.The conclusion of analysis based on elemental plane modeling is in agreement with the actually measured result of the fiber web structural parameters.3.The models based on support vector machine were established to predict the filtration performance using different combinations of the parameters within the reduction set of fiber web structural parameters for spunbonded/meltblown nonwoven as their inputs,and their predictive power were analyzed and compared.The results show that the predictive power of the model that take all fiber web structural parameters as its input parameters is quite high,regardless of the nonwoven type.In addition,the predictive power of the model that takes the five parameters of the reduction set as its inputs is found to be exceed that of the model that takes all fiber web structural parameters as its inputs on all indicators,which suggests that there exist fiber web structure parameters that have low impact on the filtration performance within the parameter set consisting of all fiber web structural parameters.Furthermore,the results show that the predictive power of the model that takes thickness,fiber diameter,and pore size as its input parameters is higher than that of any other model,regardless of the nonwoven type.The predictive accuracies(MPAs)of the predicted values for the two models are both higher than98%,and the variation coefficients(CVs)of the predicted values are both lower than 3%.Thissuggests that thickness,fiber diameter,and pore size are the three key parameters which most affect the filtration performances of spunbonded/meltblown nonwoven.4.Compare to that of the single layer nonwoven,the variation in fiber web structure of laminated composite nonwoven is given as follows according to the test results.First,the thickness and porosity reduce greatly.Secondly,the variation in the equivalent diameter of fiber depends on the compounding modes.The equivalent diameter of fiber in the composite nonwoven composed of the same type of nonwoven remains unchanged by the order magnitude,however that in those composite nonwoven composed of different type of nonwoven decreases significantly.Third,the feature length of elemental plane increases greatly.Fourth,the pore size become smaller,and the more the number of layer,the smaller the pore size.Moreover,the larger the variation coefficient of pore size of the compounding unit,the greater the pore size decreases.Fifth,the variation coefficient of pore size tends to increase,and the more the number of layer,the larger the variation coefficient of pore size.5.In the prediction of filtration performance of laminated composite nonwoven,the predictive power of the model based on support vector machine that take thickness,fiber diameter,and pore size as its input parameters is very high.The MPAs of the predicted values for all these models are higher than 97%,and the CVs of the predicted values are all lower than 3%.This indicates that composite nonwoven can be considered as a multi-stage filter composed of several single layer nonwoven,and thickness,fiber diameter,pore size are the three key parameters which most affect the filtration performances of composite nonwoven.6.The filtration efficiency and pressure drop of the composite nonwoven composed of only meltblown nonwoven are higher than those composed of only spunbonded nonwoven.The larger the layer number,the closer the filtration efficiency and pressure drop of the composite nonwoven composed of different type of nonwoven is to those of the composite nonwoven composed of only meltblown nonwoven.Especially,the mean filtration efficiency of every 15 composite nonwovens of type 3S1 M,type 2S2 M and type 1S3 M is 90.3985%、92.7092%、91.6731%,respectively,and they are valuable in application.The increasing of the layer number of composite can cause a decline in the filtering quality.The value of quality factor decreases significantly as the layer number of composite increases from one to two,and then it declines slightly as the layer number of composite increases from two to four.However there exists very small difference on quality factor among different kinds of laminated composite nonwoven with the same layer number.According to the comprehensive analysis,the optimal layer number of composite is four,and 2S2 M is considered as the optimal composite mode.