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Increased production of nanofibrous materials by electroblowing from blends of hyaluronic acid and polyethylene oxide.

INTRODUCTION

Nanomaterials and nanofibers are developed for various potential applications [1], some of which have already been commercialized (e.g., air and liquid filters, medical stents, scaffolds for tissue engineering etc.). One of the methods used for the preparation of these materials is electrostatic spinning, a highly flexible method which can be scaled-up for industrial production [2, 3]. The development of new materials necessitates more efficient technological equipment and greater maximum production volumes [4, 5], The electrostatic spinning method applied in the production of nanofibrous and microfibrous materials uses a pair of electrodes connected to opposite electrostatic potentials [6, 7]. One of these electrodes comprises a spinning nozzle which distributes doses of the polymer solution used for fiber production. The forces of the strong electric field produce the so called Taylor cone [8] which propels the injected solution (in the form of a jet) toward the opposite, collector electrode. As the jet follows a very complicated trajectory, the solvent evaporates and the solution solidifies into fibers [9]. The process forms a continuous layer of randomly oriented fibers of small diameters on the collector (generally ranging from tens of nanometres to micrometres). Solvent evaporation rate is one of the key criteria for attaining the desired fiber quality and yield. In order to successfully produce fibers in a strong electric field and to attain sufficient efficiency of the spinning process, a set of environmental conditions, physical-chemical properties of the polymer solution as well as electrode geometries must be optimally set [10]. For this purpose, the core electrostatic spinning method is complemented with other technological elements. One such example is the implementation of airflow in the vicinity of the spinning nozzle. This enhanced form of electrospinning is called "air/gas assisted electro-spinning" or more simply "electroblowing" [11].

The combination of electrostatic spinning and airflow was probably first used in the making of Petrynov-Filters[R] [12]; the authors named their method "electro-gasodynamic injection." Its main advantages, as cited by the authors, are that the flowing air prevents the solution from overflowing the spinning nozzles and breaks up the solution into a large number of jets at the Taylor cone. Both of these advantages improve the stability of production and material quality. Another argument for incorporating airflow is that it supports the formation of the Taylor cone and helps initiate, stabilise and complete the spinning process [13]. Airflow positively influences fiber morphology in a way that leads to a narrow distribution of fiber diameters and small mean flow pore size, resulting in enhanced filtering efficiency, as described for a polycarbonate nanofibrous material by the authors of the article [14]. Electroblowing is used to spin blends which cannot be processed by standard methods due to for example the high viscosities of some polymer solutions even when used in low concentrations, as in the case of hyaluronic acid [15-17]. The combination of airflow and electrospinning is also used for the spinning of very thin nanofibers less than 100 nm thick. For example in Ref. 18, the researchers used a new method called "Supersonic Electronically Assisted Blowing" to break up the flying jet into fibers 20 to 50 nm thick. The authors of Ref. 19 studied the influence of relative humidity on evaporation and fiber solidification for an aqueous solution of PEO. Their results show that fiber diameters decreased from 253 to 144 nm when humidity was increased approximately from 5 to 50%. A further increase in humidity lead to the formation of bead defects. The decrease in fiber diameters with increased airflow was confirmed through experiments carried out by the authors of Ref. 20. The awarded patents also prove the feasibility of upscaling electroblowing and its industrial use [21-23], The advantages of electroblowing over classic electrospinning are already known, however, detailed studies of microscopic processes and their experimentally confirmed macroscopic effects are still lacking.

THEORY

During electrostatic spinning, the electrospun material changes its phase--the initial polymer solution in equilibrium of total weight [m.sub.L] transforms into the final polymer nanofibers in equilibrium of total weight [m.sub.s], the solvent of weight [m.sub.G] evaporating completely in the process. It follows from the law of conservation of mass that [m.sub.s] = [m.sub.L] - [m.sub.G] and if we differentiate this relation with respect to time, we obtain: [dm.sub.S]/dt = [dm.sub.I]/dt - [dm.sub.G]/dt. The term on the left side of the equation represents the increase in the weight of a nanofibrous material over time (i.e., production process); the first term on the right side represents the adjustable quantity of feed rate and the second term on the right represents evaporation rate. The higher the solvent evaporation rate the more efficient the process, allowing to increase the polymer solution dosing rate. This results in increased productivity of the spinning process. It is necessary to realize that: i) the [m.sub.s]/[m.sub.G] ratio, which represents solution concentration, is limited and ranges from approx. 0.01-0.3; ii) feed rate must be regulated as excessive feed rate would cause the solution to overflow the spinning electrodes or fibers to form with numerous defects due to insufficient solvent evaporation.

Both equilibrium states (polymer solution-solid fibers) are separated by a non-equilibrium, highly dynamic state occurring in the surrounding atmosphere (air at atmospheric pressure). This dynamic state is characterized by the initial formation of the Taylor cone and by the stable and unstable stages of the flight of the jet. During electrospinning, the energy of the electrostatic field causes the solution to leave the equilibrium state (if we disregard the constantly active evaporation of the solvent), i.e., the dynamic state is caused by supercritical forces of the external electrostatic field (it applies that E > [E.sub.C]). Fiber formation may also be initiated by mechanical energy alone, specifically by tangential forces of the flowing air (where [Q.sub.F] > 0 1/min and E = 0 V/m); this process is known as "solution blow spinning" [24, 25] or "gas jet spinning" [26]. A state of equilibrium is generally achieved by balancing temperatures, pressures and chemical potentials and in this case especially by complete solvent evaporation during the solidification of the flying jet.

The solution consisting of solvent molecules and polymer chains is characterized primarily by the interaction parameter [chi] which is related to the enthalpy of a polymer mixing with a solvent, dynamic viscosities [eta], conductivities s, surface tensions [gamma], temperatures [T.sub.L] and chemical potentials of both components (solvent [[micro].sub.SOL] and polymer Ppol). sec Fig. 1. The partial pressure of the present components acts within the solution [p.sub.L]. The solvent-air phase interface (air being the surrounding atmosphere) comprises a thin layer characterised by surface tension [gamma]. The external gas phase is characterized by temperature [T.sub.A], atmospheric pressure [p.sub.A] and partial pressure [p.sub.v], electrostatic field intensity E and the chemical potentials of the individual components of the gas phase [[mu].sub.G]. Furthermore, heated airflow, characterized by airflow temperature [T.sub.F] and volumetric flow rate [Q.sub.F], was used for electroblowing in the experimental part. The final solid phase (in the form of a nanofibrous material) is characterized by total weight [m.sub.s], fiber diameter [d.sub.S] and the area on the surface of the collector [A.sub.s] where the spun nanofibers are deposited. Having specified the quantities involved in the individual stages, we can identify process and solution quantities which influence solvent evaporation rate and thus process productivity.

EXPERIMENTAL

A 6% w/w solution of two polymers mixed in a ratio of 8:2 - hyaluronic acid ([M.sub.w] = 70 kDa, Contipro Pharma a.s.) and polyethylene oxide ([M.sub.w] = 400 kDa, Sigma-Aldrich) dissolved in water - was used in the experiments. The conductivity of the solution, measured with a conductivity meter (inoLab Cond 720, WTW), was [s.sub.1] = 5.1 mS/cm and [s.sub.2] = 5.4 mS/cm at temperatures of 24[degrees]C and 60[degrees]C respectively. Surface tension of the solution, measured with a tensiometer (K9, Kriiss), was [gamma] = 58.9 mN/m (at a temperature of 24[degrees]C). The dynamic viscosity of the solution was measured by a rheometer (AR G2, TA Instruments) at two temperatures and the values at shear rate of 100 [s.sup.-1] on the obtained curves were subtracted from one another; the viscosities were [[eta].sub.1] = 1.24 Pa s and [[eta].sub.2] = 0.55 Pa-s at temperatures of 24[degrees]C and 60[degrees]C, respectively.

The solution was spun by the electrospinning and electroblowing methods using the 4SPIN[R] LABI device (Contipro Biotech s.r.o.) [27]. The solution was dosed by a spinning nozzle (21 gauge, Hamilton). The nozzle was connected to a highvoltage source with voltage set to V = 25 kV (E = 139 kV/m). The solution was dosed from a 20 mL syringe at rates ranging from 14 to 125 [micro]L/min. Dosing rates were optimised during all sample preparation processes so as to ensure that the processes would not be interrupted, that the solution would not flow over the edges of the spinning nozzle and that the forming nanofibers would be defectless. When airflow (dried atmospheric gas) of volumetric rates of [Q.sub.F] = 10-80 litres/minute and temperatures [T.sub.F1] = (29[+ or -] 1) [degrees]C and [T.sub.F2] = (60 [+ or -]1) [degrees]C were applied, an air nozzle was attached to the spinning nozzle - either an open pipe nozzle (inner diameter of 3 mm) or a Laval nozzle (inner diameter of 3 mm, MJ5, Silvent), see Fig. 2. For more detailed description, please see Supporting Information (part 1). An open pipe nozzle creates a turbulent flow at the mouth of the air nozzle. On the contrary, a Laval nozzle creates an accelerated flow with a smaller divergence angle [28]. A collector with a conductive surface of (225 X 225) [mm.sup.2] and a pre-weighted aluminium foil of identical size attached to it was installed 18 cm above the spinning nozzle. The experiments were conducted at two relative ambient air humidity levels of [R.sub.H1] = (24 [+ or -] 3) % and [R.sub.H2] = (35 [+ or -] 5) %. The ambient temperature was [T.sub.A] = (25 [+ or -] 3) [degrees]C. Each sample was deposited over 30 minutes. After the spinning process had been completed, the area on the collector covered with deposited nanofibers [A.sub.s] was measured and calculated. The weight of the nanofibrous layers [m.sub.s] was measured with analytical balances (XS204, Mettler Toledo). Nanofiber diameters [d.sub.s] were assessed by analysing images (a minimum of 50 measurements per sample in the ImageJ software) obtained with an electron microscope (Ultra Plus, Zeiss).

RESULTS AND DISCUSSION

Design of experiment is described in Supporting Information (part 2). More than 70 samples were prepared in the experimental part; for each one, the surface of the deposited area As was measured as well as the weight of the nanofibrous layer ms and fiber diameters [d.sub.s]. Figure 3 shows a curve representing the dependence of sample weight [m.sub.s] on airflow rate [Q.sub.F]; the data were obtained for both air nozzles (open pipe and Laval) and for two airflow temperatures [T.sub.F1] and [T.sub.F2]. The displayed dependence demonstrates that increasing airflow rate exponentially increases production - sample weight ms. Increasing airflow rate from 0 to 80 1/min resulted in an increase in production from 15 mg to 141 mg over 30 minutes, i.e., a more than 9-fold increase (we are comparing samples #1 and #5, see Table 1). In this case we compared the results of classic electro-spinning and electroblowing obtained at the same temperature. Air blowing in the vicinity of a forming fiber moves the molecules of the evaporating solvent away, decreasing their partial pressure [p.sub.v] and chemical potential [[micro].sub.G].

Furthermore, we see that heating the air flowing at 80 1/min from [T.sub.F1] to [T.sub.F2] (i.e., by 31[degrees]C) results in a further increase in production from 141 mg to 264 mg over 30 minutes (samples #5 and #59), which is an almost 1.9-fold increase. It is also a more than 17-fold increase compared to the spinning process without airflow--comparing results of samples #1 and #59, their scanning electron microscope images are shown in Fig. 4. Electro-blowing with airflow heated to a higher temperature has proven to be 17 times more productive for our solution than classic electrospinning. Local temperatures are a significant parameter of the spinning process. While passing through the spinning nozzle, the solution is heated to temperature [T.sub.L] by airflow at the temperature [T.sub.F]. An increase in the temperature of the solution results in increased thermal motion of individual polymer chain segments, lower polymer and solvent interaction parameter [chi], increased solubility of the polymer in the solvent and a change in polymer conformation toward less coiled and more spread out shapes. A lower interaction parameter causes the viscosity of a polymer solution to decrease as well and it applies that d[eta]/dT <0 [29]. If the spinning process is limited by the viscosity of the polymer solution, then increasing its temperature allows us to increase its concentration, resulting in a greater yield. Furthermore, surface tension decreases with increasing temperature (it applies that d[gamma]/dT <0), resulting in a lower critical value of the electrostatic field [E.sub.C] at which the spinning process initiates. A higher temperature also increases the partial pressures of the individual components of the solution (the total being [p.sub.L])--especially of the solvent due to the size of its molecules being many times smaller than those of the other components--as the kinetic energy of the molecules of the whole system increases; it applies that [dp.sub.l]/dT > 0 [30]. Evaporation rate is dependent on saturated vapour pressure, specifically the rate of evaporation increases with increasing saturated vapour tension. Therefore, evaporation rate increases with temperature. Surface tension y and the pressure within the solution [p.sub.L] do not directly affect solvent evaporation rate, only the initiation of the process. The temperature of airflow [T.sub.F] also locally raises the temperature of the atmosphere [T.sub.A], generally speeding up the evaporation of the solvent from a flying jet. The effect of increased temperature [T.sub.F], which results in a 56% reduction of the dynamic viscosity of the solution rj, was determined by rheological measurement (see Experimental section); this is a property of the solution that contributes to an increase in productivity.

Another parameter of interest was the influence of ambient pressures [p.sub.A] and [p.sub.w]. With the Laval nozzle, fibers are deposited over a smaller area As on the collector than with the open pipe nozzle, because using the open nozzle at airflow rates of [Q.sub.F] >60 1/min results in fibers landing outside the collector surface (see Fig. 5). Airflow from the Laval nozzle significantly narrows the cone during the chaotic stage of the spinning process and concentrates fibers in a smaller area As. That is why we presume that the partial pressures [p.sub.A] and [p.sub.v] are greater when using the Laval nozzle. This presumption is confirmed by lower productivity observed when using this air nozzle type (see Fig. 3). The evaporation of the solvent (water) from the solution occurs when the molecules of the solvent are sufficiently close to the surface, i.e., interface, and gain enough energy to overcome all the cohesive forces in the system. It applies that [p.sub.L] > [p.sub.A] + [p.sub.v] - Evaporation continues until equilibrium between liquid and vapour is achieved and vapour is saturated. The lower the pressure around the liquid, the greater the intensity of evaporation, which again results in a greater efficiency of the spinning process. The abovementioned processes are significantly influenced by local fluctuations in temperature, which may be considerable during electroblowing.

We also monitored the influence of relative ambient humidity which was maintained during the process at two levels--[R.sub.H1] and [R.sub.H2]- The dependence of sample weight on airflow rate at both humidity levels and with both air nozzles is shown in the graph in Fig. 6. In both cases, nanofiber production slightly increased with lower humidity (see Table 1 for a comparison of samples #5 and #44, #29 and #38). Generally, every system tends to achieve thermodynamic equilibrium and equalize chemical potentials of the individual components, in this case [[mu].sub.POL], [[mu].sub.SOL] and [[mu].sub.G]. The greater the differences between the chemical potentials of the individual material phases, the more intensive the processes that lead to their equalization. The significance of chemical potentials for the spinning process is described in greater detail in [31].

Average fiber diameters [d.sub.s] were calculated for all sample sets and their standard deviations were included in Table 2 and shown in the graph in Figure 7. The diameters doubled when temperature was raised to [T.sub.F2] (comparing the results of set #3 and #5) and the open pipe nozzle was used. When the Laval nozzle was used, the differences in average diameters were statistically insignificant. The influence of thermodynamic quantities on nanofiber morphology was not proven. It can be said that nanomaterial with very similar parameters was produced in all cases. The inconsistency with the cited literature [19] was probably caused by the experiment being conducted differently, specifically by adjusting the dosing rate as needed.

Furthermore, the critical values of high voltage at which the spinning process was initiated were recorded, see Figure 8. Higher airflow temperature (i.e., [T.sub.F2]) decreases the critical potential as the heated solution has lower viscosity (see Experimental section). The critical values decreased with airflow rate and at [Q.sub.F] > 10 1/min we observed fiber formation even without the electrostatic field acting (i.e., E = 0 V/m). Tangential forces of the flowing air mechanically act on a drop of the solution and help form the Taylor cone; these forces can initiate the spinning process by themselves without the electrostatic field acting. The process observed in these cases is called solution spinning (see Introduction). However, this process is highly unstable even at [Q.sub.F] = 80 1/min and does not produce defectless nanofibrous materials. A liquid layer of the solution forms on the collector and only several solid fibers form near the edges of the collector. The electrostatic field influences the spinning process in several ways, besides initiating it upon reaching the critical value. It can be presumed that an extremely high surface charge accumulates on the surface of the solution and the flying jet (the spinning nozzle is connected to a high-voltage source). This charge orientates the molecules of the polar solvent (water) in the direction of the solution-air interface. Increasing the concentration of solvent molecules at the phase interface aids their evaporation. It is demonstrated that the electrostatic field significantly influences solvent evaporation rate.

CONCLUSIONS

This article introduces theoretical principles that influence the production of nanofibrous materials from aqueous solutions using the electrospinning and electroblowing methods. As these are both drying processes, evaporation rate was determined as the key production parameter. The influence of process and solution quantities on evaporation rate was demonstrated. Solution dosing rates in the experiments were optimized so as to ensure a stable process resulting in a fast and stable production of defectless nanofibrous materials. The results of the experiments confirmed the theoretical presumptions and the described principles--the production of nanofibers increased with increasing volumetric rate of air flowing around the spinning electrode, with increasing airflow temperature, with decreasing relative air humidity and with the supercritical value of the acting electrostatic field. Our findings significantly emphasize the advantages of the electroblowing method over the classic electrospinning method and support its superior suitability for both laboratory work and potential industrial production.

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Marek Pokorny, (1) Vladimir Rassushin, (2) Lucie Wolfova, (1) Vladimir Velebny (1)

(1) R&D Department, Contipro Biotech S.R.O, 561 02 Doini Dobrouc, Czech Republic

(2) National Research Centre, Kurchatov Institute, Moscow 123182, Russia

Additional Supporting Information may be found in the online version of this article.

Correspondence to: M. Pokorny; e-mail marek.pokomy@contipro.com Contract grant sponsor: Technology Agency of the Czech Republic; contract grant number: TA02011238: Novel wound dressings based on nanofibres and staple microfibres of hyaluronan and chitin/chitosan-glucan complex.

DOI 10.1002/pen.24322

TABLE 1. Process parameters and properties of deposited
nanofibrous layers for the compared samples.

                                         Airflow

               Air       Feed rate      [Q.sub.F]
Sample [#]    nozzle   [[micro]L/min]    [1/min]

1              open         9-15            0
5              open        70-90           80
59             open       130-160          80
44             open        90-125          80
29            Laval        50-70           80
54            Laval        90-115          80
38            Laval        50-75           80

               Air temp.     Humidity     Weight

                   TV        [R.sub.H]   [m.sub.s]
Sample [#]    [[degrees]C]      [%]        [mg]

1                  0            37         15.6
5                  29           36         140.9
59                 60           30         264.1
44                 29           24         183.9
29                 29           36         110.0
54                 60           30         184.4
38                 29           26         108.0

               Dep. area     Fiber diam.

               [S.sub.A]      [d.sub.s]
Sample [#]    [[cm.sup.2]]      [nm]

1                  50            65
5                 530            101
59                530            179
44                530            143
29                 90            125
54                 64            295
38                 54            225

TABLE 2. Average nanofiber diameters calculated for sample
sets prepared with the specified process parameters.

                         Air             Air             Fiber
                      humidity          temp.          diameters

Sample      Air                       [T.sub.F]
set [#]    nozzle   [R.sub.H] [%]   [[degrees]C]    [d.sub.s] [nm]

1           Open    24 [+ or -] 3   29 [+ or -] 1   140 [+ or -] 18
2          Laval    24 [+ or -] 3   29 [+ or -] 1   162 [+ or -] 32
3           Open    35 [+ or -] 5   29 [+ or -] 1    85 [+ or -] 14
4          Laval    35 [+ or -] 5   29 [+ or -] 1   116 [+ or -] 19
5           Open    35 [+ or -] 5   60 [+ or -] 1   175 [+ or -] 16
6          Laval    35 [+ or -] 5   60 [+ or -] 1   151 [+ or -] 4


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Author:Pokorny, Marek; Rassushin, Vladimir; Wolfova, Lucie; Velebny, Vladimir
Publication:Polymer Engineering and Science
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Date:Aug 1, 2016
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