During the electrospinning process the web of nanofibres is manufactured by means of electrostatic forces between two electrodes. The diameters of nanofibres usually differ and they depend on various parameters. The different fineness of fibres influences the structure of the web and herewith the end-use properties of such kind of nanomaterial. Analysis of nanofibres diameters distribution also shows big differences; even more, the distributions are not spread along the normal distribution. Understanding the influence of electrospinning parameters and the reason why the shapes of distributions are so sophisticated is very important. The goal of this paper is to analyse the distribution of diameter and to propose the new criterion for nanofibres diameter comparison and web of nanofibres estimation. In this paper the influence of covering time of support material on structure of PA6.6 nanofibre web has been investigated. It was estimated that this parameter does not have a significant influence on the average diameter of nanofibres, and only the structure of web has been influenced by the changes in covering time. According to the results provided the phenomena of nanofibres sticking on the support material at the time of electrospinning can be proved and explained.

Electrospinning is one of the most common processes used for nonwovens production. The diameters of electrospun fibres diameter may range from 10 nm to 1500 nm [

There are a lot of studies analyzing the influence of various parameters on nonwoven structure. The analysis of such studies shows that in some cases the different researches, analyzing the influence of the same factor on the structure of nanofibres, obtain different results. The reasons of different conclusions can be explained in several ways. According to one of them, in some cases the authors do not analyze the dispersions of nanofibres diameter. The other reason—quite often the authors skip the description of the main parameters which may have a significant influence on nonwoven structure. It is possible that the mismatches of the results occur because of the lack of common methodology for nanofiber diameter characterization.

While analyzing the literature sources we noticed that the measurements of nanofibres diameter are distributed in unclear distribution and the shapes of all distributions always differ. Ellison et al., Tsimpliaraki et al., and Gu et al. [

It is impossible to compare the average values when dispersions of diameter are different, so it is absolutely wrong way to estimate the web of nanofibres, in order to compare parameters having different kinds of distribution. According to mathematical statistic point the average value of nanofibres diameter (if they are distributed in not normal distribution) cannot characterize the nanofibres precisely, because changes in average values do not suppose changes in modal value and other characteristics. It means that in order to compare the diameter of nanofibres with an average value it is necessary to use the other characteristics too.

In our previous study [

In our previous study [

The main disadvantage of this method is sophistication of compound distribution calculation. For this reason the short-cut method of the quality of nonwoven structure estimation has been presented [

The obtaining of compound distribution confirms that the web of nanofibres at the electrospinning process consists of different kinds of nanofibres.

An 8% solution of PA6.6 in 85% formic acid was gently stirred at ambient temperature for 12 hours until the polymer granules dissolved completely. The nonwoven web from PA6.6 nanofibres had been formed using the electrospinning equipment “Nanospider” (Figure

Principal scheme and photo of electrospinning technique “Nanospider”: 1: upper electrode, 2: bath with polymer solution, 3: rotating roller with tines, 4: support material,

During all experiments, the ambient temperature was

The structure of received nanofibres web has been analyzed by Scanning Electron Microscopy (SEM)—SEM-FEI Quanta 200 (Netherlands). The value of diameter of PA6.6 nanofibres was measured from SEM images by LUCIA Image 5.0 programme, with an accuracy ± 0.01 nm. Nanofibres have been measured using the 5 different SEM images.

Nanofibres have been manufactured from PA6.6 solution of 8% concentration. The first series of experiments were carried out at the speed of support material 0.010 m/s, the second 0.006 m/s, and the third 0.002 m/s. With decreasing in speed the covering time of support material increased from 25 s to 125 s. The influence of the speed (and herewith the covering time) of support material on PA6.6 nanofibres web morphology is presented in Figure

SEM images of electrospun PA6.6 mats when the speed of support material was (a)

As is seen from Figure ^{2}. The results showed that the speed (also the covering time) of support material does not have significant influence on the average value of PA6.6 nanofibres diameter, as the difference is less than ± 3.5% (I

Distributions of PA6.6 nanofibres during the experiment: (a)

First of all Figure

Detailed distributions of the results of the I variant.

According to the results it is possible to assume that at the lowest speed of support material, the thicker nanofibres were formed and at the higher speed the thinner nanofibres were formed.

In previous work [

The following results have been received using the histograms presented in Figure

The next stage of our investigation has been aimed to check the hypothesis that the web from different diameter of nanofibres forms from single nanofibres which stick together on the support material. It is known that the surface area of cross-section of single nanofibre may be calculated using formula:

Fibre diameter distributions (a) from PVA [

In this figure few peaks can be noticed as well. According to this data one can guess that these peaks appeared for the sticking of PP nanofibres on the web. It means that this problem is typical for the whole electrospinning process regardless of the type of polymer—the similar results can be observed with PA6.6, PVA, and PP nanofibres.

At the last step of this study, the dependence of the density of nanofibres (in 100 ^{2}) on the covering time is presented in Figure

Dependence of the density of nanofibres (in 100 ^{2}) on the covering time.

During the analysis of the data presented in Figure

The distributions of electrospun nanofibres diameter are always different and the shapes of such distributions usually do not differ from the compound distribution, which consists of several normal distributions.

The speed of support material from 0.002 m/s till 0.010 m/s does not have a significant influence on the average value of PA6.6 nanofibres diameter. It only influences the structure of nanofibre web; that is, as the speed of the support material decreases, the more sticked nanofibres are formed.

For the web structure estimation the modal value of the first distribution, the percentage quantity of the first distribution, and the average diameter of two peaks of distribution—modal value and the value of the second highest peak—may be used.

The results of the tests confirm that the web of nanofibres consists of various nanofibres due to the sticking of several nanofibres during the process of their manufacturing.

The authors declare no possible conflict of interests.