Production of Filter Material from Polypropylene/Copolyamide Blend by Material Extrusion-Based Additive Manufacturing: Role of Production Conditions and ZrO₂ Nanoparticles

Abstract

The effect of technological conditions of the process and zirconia (ZrO₂) nanoparticles on the properties of fine-fibrous filter materials (FMs) obtained by matrix polymer extraction from a microfibrillar composite formed using the material extrusion-based additive manufacturing method from a polypropylene (PP)/copolyamide blend is studied. Different processing schemes were used for obtaining filaments for material extrusion: scheme I—the use of a single-screw extruder at the stage of compounding; scheme II—the use of a single-screw extruder at the stage of compounding and orientational stretching in the course of strand formation; scheme III—the use of a twin-screw extruder at the stage of compounding, scheme IV—the addition of ZrO₂ nanoparticles and use of a twin-screw extruder. It has been shown the possibility of reducing the diameters of the formed in situ PP microfibrils by using the twin-screw extruder, as well as additional orientation drawing. The introduction into the melt of ZrO₂ nanoparticles provides further improvement of the microstructure—the average diameter of the microfibrils is reduced by 1.4 times compared with the initial blend. Developed FMs are characterized by high efficiency of air purification from solid particles with a size of 0.3 μm. At the same time, the use of nanoadditives is the most effective—a two-layer FM with nanoparticles provides cleaning efficiency at the level of four- to six-layer materials without filler.

Introduction

Microfiltration by polymeric fine-fibrous filters is one of the simplest and most reliable and efficient methods of clearing of water, air, and industrial gaseous and liquid media from micron and submicron solid particles.¹ There exist a number of methods of production of nonwoven fabrics (NWF) made of micro- and nanosized fibers: melt blowing,^2,3 electrospinning of polymer melt or solution under electrostatic forces,^4,5 and reprocessing of immiscible polymer blends into composites characterized by micro- and nanofibrillar structure.⁶

Melt-blown NWF are mostly characterized by the fiber diameter of 1.0–20.0 μm. The maximum increase in the air velocity in the course of polymer melt blowing allowed the formation of NWF with the average fiber diameter of ca. 500 nm.⁷ Electrospinning provides nanofibrillar sheets with the individual filament diameter of above 10 nm. Application of electrospinning is limited by low production output due to low concentrations of polymer solutions and high toxic potential of solvents. To enhance the efficiency of filtration and to reduce hydraulic resistance of filter materials (FMs) based on micro- and nanosized fibers, the methods of melt blowing and electrospinning are combined.^8,9 However, this type of production is complicated because of incompatibility of the formation rates of the methods. NWF produced by aerodynamic or electrical formation are of uniform diameter distribution. However, chaotic (random) arrangement in the layer makes the possible formation of a certain number of pores larger of the nominal diameter.

The method of fine-fibrous material production by reprocessing of the immiscible polymer blends (in situ formation of micro- and nanofibrils of one component within the matrix of another one) has been realized now for a number of polymer pairs by means of extrusion, blow, and uniaxial tension. The structure of the composite monofilament or film leaving the die is a continuous phase filled with thin fibrils. After the matrix has been extracted from the composite by a solvent that is unreactive with respect to the dispersed polymer phase, bundles of micro- and nanofibers are left on the NWF made of them.^10–15

Dimensional characteristics of microfibers are determined by the rheological properties of the components of the blend and the degree of their compatibility at the interface, as well as the type of process equipment and processing parameters (mixing time and speed, geometry and dimensions of the screw, and the molding hole of the extruder, etc.). In the study by Jin et al.,¹⁰ to obtain an NWF characterized by a bimodal diameter distribution of the fibers, the method of melt blowing of three-component immiscible polymer blends was suggested to be applied (the ratio of two components of the dispersed phase was 50/5/45 wt.%). The matrix polymers were polystyrene (PS) or polyethylene oxide, and the dispersed phases were polyethylene (PE) and polyamide-6 (PA6).

After the matrix polymers have been dissolved, the average diameter of PE fibers was 600 nm, and the average diameter of PA6 fibers was 9.0 μm thick. Formation of fibers from the melt blend of polybutylene terephthalate/polypropylene (PBTP/PP) at the ratio of 20/80 wt.% with the use of a capillary viscometer followed by thermal drawing allowed production of PBTP fibers of 600 nm in diameter and 100 μm in length.¹¹ Extrusion of polyethylene terephthalate (PET)/PP¹² and polytetrafluoroethylene/polylactide (PTFE/PLA)¹³ blends formed PET fibers of 2.0–9.2 μm in diameter and PTFE fibers of 100–500 nm.

Extraction of copolyamide (CPA) from PP/CPA composite film formed by a screw extruder through a flat swallow-tail-shaped die provided an NWF with ordered localization of PP fibers within the layer.^14,15 The microfibers ranged from a 10th to several micrometers, being of almost infinite length and strictly oriented along the extrusion direction. The use of NWF of this type as filters provided better cleaning of processing media compared with NWF characterized by a chaotic distribution of fibers.

An effective way to improve the functional properties of FM is the introduction into a blend of the third component (special substances or nanosized additives), which has a compatibilizing effect and allows to adjust the morphology of polymer dispersions.^12,14–17 Nanofillers in the melts of thermodynamically incompatible blends play a dual role. First, they reduce the amount of surface tension in the interfacial zone, improving the compatibility of the ingredients of the blend, which contributes to the formation of a finer and more homogeneous structure. Second, they give fibrous materials the unique properties inherent in these substances.

For instance, the introduction into the PP/CPA melt of nanoparticles of different chemical nature helped to reduce the diameters of microfibers in the layer, increase their hydrophilicity and specific surface area, which led to increased precision and productivity of FM based on them.¹⁵

The introduction of TiO₂ and Al₂O₃ nanoparticles into the PET/PP and PP/CPA blends, respectively, provided a decrease in the average diameter of polyester fibrils by about five times, and PP ones by two.^12,16,17 At the same time, the morphology of the system was determined by the content and the size of the particles, the sequence of mixing with the ingredients (pre-dispersion in the component of the dispersed phase or matrix or simultaneous mixing), as well as process parameters. The maximum effect was achieved under the condition of selective localization of nanoparticles at the interphase boundary.

Nanoparticles of pyrogenic silica and bifunctional substances based on it (Ag/SiO₂ and TiO₂/SiO₂) also have a modifying effect in the melt of the PP/CPA blend—microfibers acquire a regular cylindrical shape, their diameters decrease, and the uniformity of distribution increases.¹⁵ The use of silver-containing additives (Ag/SiO₂, composite material ZnO tetrapods/Ag nanoparticles) additionally gives the materials bactericidal properties.^15,18

A significant disadvantage of these FMs is the lack of strength in the transverse direction, due to which the removal of the matrix polymer and their operation is carried out in the presence of substrates of NWF. To eliminate this shortcoming in the formation of composite films with microfibrillar morphology, in particular from a PP/CPA blend, we previously¹⁹ proposed the use of material extrusion-based additive manufacturing (ME). After dissolving the matrix polymer, a multilayer fine-fiber FM was obtained from them, which is characterized by increased mechanical properties and an ordered arrangement of structural elements in the filter layer.

The basic structure unit of every layer was PP fibrils oriented in one direction, and the layers were arranged perpendicular to each other.¹⁹ Improving the filtration efficiency of FM was achieved by changing the size of the cells of the filtration grids and the pressure in front of the die during processing of the blend on a single-screw extruder.¹⁹

The aim of this work was to improve our previously proposed¹⁹ method of manufacturing multilayer FM from a PP/CPA blend by changing the conditions of compounding, the conditions of obtaining a strand, and the introduction of zirconia (ZrO₂) nanoparticles into the blend.

Experimental

Materials

PP TATREN HG 1007 (Slovnaft, Slovak Republic) was used as a component of minority constituent. The melt flow rate was 10 g/10 min (230°C/2.16 kg, ISO 1133-1). The majority constituent was CPA 6/66–3 (copolymer of PA6 and PA66 at the ratio of 50:50) produced by the Sverdlovsk chemical plant, Russia (viscosity rating in sulfuric acid was 2.46, the content of low-molecular compound was 3.5 wt.%). PP/CPA blend was of 20/80 wt.%. Rheological characteristics of the resulting polymers are reported in the study of Beloshenko et al.¹⁹

Zirconia (ZrO₂) is selected as a nanofiller that is one of the most popular materials in modern materials science due to its promising properties for the application in chemical industry, medicine, design of devices for filtering, and disinfection.²⁰ The content of ZrO₂ was 2.5 wt.% of PP. Nanoparticles of ZrO₂ were synthesized by a set of transformations: hydrogel of zirconium hydroxide—amorphous zirconium hydroxide—crystal zirconia.²¹ Hydrogen of zirconium hydroxide was produced by precipitation of zirconium oxychloride (the concentration was 0.55 M) in ammonia solution (the concentration was 13.3 M) at pH of 11 for 1 h at room temperature. The precipitate was filtered and washed up for elimination of residual ions of chlorine and ammonia. Washing of the precipitate was stopped at pH of 7. The filtered hydrogel of zirconium hydroxide was dried in a microwave oven at 700 W and the frequency of 2.45 GHz under constant disposal of water vapor. The dried amorphous xerogel was calcinated isothermally at 500°C for 2 h. The obtained crystal particles of ZrO₂ were of 12 nm in size.

Strand and filament preparation

The process of production of multilayered FMs of PP microfibrils is composed of three stages: granulation of the raw and the nanofilled blends (stage 1); extrusion of initial blends with the formation of a strand for a three-dimensional (3D) printer (stage 2); additive formation of the composite film and CPA extraction (stage 3).

To enhance the quality of the component mixing at the first stage, raw ingredients of the blend were reduced at the laboratory impeller breaker mill (Polymer Mash LTD, Ukraine) down to a particle size of ∼1 mm (Fig. 1). The resulting powders were mixed at a high-speed Henschel-type mixer for 5–10 min at the rotor speed of 1200 rpm. The mixtures were dried in vacuum in a drying oven at 80°C up to the content of water and volatile of 0.05 wt.%.

FIG. 1.

Scheme of PP/CPA, PP/CPA/ZrO₂ granule production. PP/CPA, polypropylene/copolyamide.

The blends were prepared using a single-screw extruder (Polymer Mash LTD; D = 25 mm, L/D = 16 and 45 rpm) at the temperatures within zones of 160-210-190°C or a twin-screw extruder (Polymer Mash LTD; D = 22 mm, L/D = 40 and 250 rpm) at the temperatures of 120-220-220-210-200-190°C. A strand pelletizer (Polymer Mash LTD) was used to obtained granules from the extrudate. The obtained granules were dried under the same conditions as powders.

The strand of 1750 ± 20 μm in diameter was formed by a single-screw extruder (Polymer Mash LTD; D = 27 mm, L/D = 30 and 30 rpm) (stage 2). The melt was filtered through a set of stainless steel meshes, and the mesh size was 20 and 60 μm (Fig. 2). Temperatures in the extruder zones were 160-200-205-200-190°C. The pressure at the mesh and in front of the die was 7.0 and 5.0 MPa, respectively. The formation was horizontal, with water cooling at 5°C to prevent an anisotropic upsetting. The prepared strand was dried up to the water content of 0.05 wt.% and kept in vacuum bags. The strand produced from granules by the single-screw extruder was further subjected to orientational stretching at 130°C.

FIG. 2.

Scheme of PP/CPA, PP/CPA/ZrO₂ strand production.

Composite film and FM preparation

For the additive formation of a composite film of defined shape, size, and structure from a strand (stage 3), 3D printer FlashForge Creator Pro (Jinhua City, Zhejiang, China) was used. The diameter of the extruder nozzle was 400 μm (Fig. 3). The parameters of the film printing were as follows: the nozzle temperature was 240°C, the build platform temperature was 60°C, model filling was 100%, and the printing rate was 100 mm/s. The layers were laid perpendicular to each other, the number of the layers was ranging from 2 to 6, the thickness of the upper layer was 150 μm, and the rest of the layers were 200 μm thick (Fig. 4). The upper layer was a lower thickness because of the deformation of the melt flow passing the nozzle. The operation was performed to form a single layer of the blend at the film surface and joining of the melt strands.

FIG. 3.

Scheme of PP/CPA, PP/CPA/ZrO₂ FM production. FM, filter material.

FIG. 4.

A schematic of the printed specimens showing the infill orientation and the specimen and layer dimensions.

Thus, to produce composition films (a semi-finished FM), three technological schemes were used: scheme I—the use of a single-screw extruder at the stage of compounding; scheme II—the use of a single-screw extruder at the stage of compounding and orientational stretching in the course of strand formation; scheme III—the use of a twin-screw extruder at the stage of compounding. The composition films of PP/CPA/ZrO₂ were produced by scheme III (hereinafter referred to as scheme IV).

The FM was produced by extraction of CPA from the composite films by 70% ethyl alcohol solution at the temperature of 70 ± 5°C in a Soxhlet extractor.

Characterization

The microstructure of the strands and the FMs was tested by scanning electron microscope JEOL JSM-5500LV (Japan) at the accelerating voltage of 10 kV. The average fiber diameter of each sample was determined using the ImageJ software at 3000 × .

The distribution of ZrO₂ particles in the studied samples was examined with scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (JEOL JSM-6010LA, Japan) operating in the high vacuum mode at an accelerating voltage of 8 kV. The surfaces were sputtered with carbon using a coater Q150R ES (Quorum Technologies, UK).

The differential scanning calorimetry (DSC) was performed with a device DSC 2920 (TA Instruments, New Castle, DE) under heating from 40°C to 210°C at the rate of 20°C/min. The samples of 7–8 mg in mass were cut of polymeric materials and placed into standard aluminum pans. During measurement, the DSC cell was flushed by dry nitrogen (20 mL/min).

The filter retention of FM was estimated by a counter of aerosol particles within the diameter range of (0.3–1.0) μm. Effectiveness of the filtration was established as the ratio of the retained particles of the fixed size to their amount in the air. Five samples of two-, four-, and six-layer FM, made according to schemes 1–4 (total 50 samples), were used. The measurements were performed on the nominal filter area of 25 cm² at 20°C in triplicate for each diameter of particles. The relative error in determining the amount of retained particles did not exceed 4% (at an initial concentration of up to 1500 dm⁻³).

Results and Discussion

Effect of strands formation conditions on the PP/CPA microstructure

When the melts of thermodynamically incompatible polymer blends are processed by extrusion devices, dispersed-phase particles are elongated into fibrils due to the presence of a shear force field within the area between the extruder screw and the extruder walls. Detailed mechanisms for mixing of immiscible polymers are provided elsewhere.²²

It is already well known that when dispersed minor polymer droplets, immersed in the immiscible matrix, undergo shear deformation, they tend to deform and extend into long thin threads. As a result, particle radius decreases while the capillary instabilities at the interface increase. When the capillary number approaches to the critical value, the elongated nanofibrils tend to disintegrate and break up.

The study of the microstructure of PP left after CPA extraction from the composition strands gives evidences that microfibrillar morphology is formed in the melt PP/CPA blend for all types of equipment used in the work, taking into account the selected extrusion conditions (Fig. 5). At the same time, as is observed in SEM images, a significant amount of PP is in the form of finely dispersed particles resulting from decomposition of the finest fibrils that are the most instable thermodynamically.¹⁴

FIG. 5.

SEM images of PP microfibrils and histograms of their diameter distribution after CPA extraction from PP/CPA strands produced by schemes I (a), II (b), and III (c). SEM, scanning electron microscopy.

The degree of dispersion of a component depends on the extrusion conditions of the composition strands. The average diameter of PP microfibrils is 1.68 μm for strands obtained from the blend obtained according to scheme 1 (Fig. 5a). During orientation drawing (scheme 2), the average diameter of microfibrils decreases to 1.45 μm (Fig. 5b). A significant proportion of microfibrils (≈20%) have a diameter of <1000 nm. The use of a twin-screw extruder for compounding (scheme 3) is the most effective due to finer dispersion: the average diameter of microfibrils is 1.40 μm, and the homogeneity of the diameter distribution is the highest (Fig. 5c).

Effect of ZrO₂ nanoparticles on the strand microstructure

The analysis of PP left after CPA extraction from strands of nanofilled blend gives evidences that the incorporation of ZrO₂ nanoparticles does not modify the type of composite morphology (Fig. 6). From the histograms of the distribution of PP microfibrils by diameters, it can be observed that the introduction of ZrO₂ nanoparticles increases the degree of dispersion and deformation of the polymer droplets of the dispersed phase. The average diameter of PP microfibrils decreases more than two times compared with the initial blend and is 640 nm. It is important to state that the presence of the nanoparticles in the structure results in the emergence of microfibrils characterized by the diameter of 200–400 nm, the fraction is 25%, so the thermodynamic stability of liquid PP fibers of small diameters is substantially increased (Fig. 6).

FIG. 6.

SEM image of PP microfibrils and histogram of their diameter distribution after extraction of CPA from PP/CPA/ZrO₂ strands.

Studies of the morphology of PP/CPA/ZrO₂ composite strands by energy dispersion analysis showed that the compounding on a twin-screw extruder provides a uniform distribution of nanoparticles of zirconium dioxide in both phases (Fig. 7).

FIG. 7.

ZrO₂ mapping of PP/CPA/ZrO₂ strands before removal of CPA (a) and FM (b).

Advancing of the morphology of the studied system is determined by the formation of specific bonds between the amines of CPA molecules and the oxide nanoparticles of ZrO₂. A significant effect has been achieved due to prevailing localization of nanoparticles at the component interface because of high polarity and low wetting by the melt of nonpolar PP.

It is known that injection of third substances enables specific interactions (hydrogen, dipole, etc.) with one or both ingredients of the blend, which enhances compatibility of the polymers at the interphase boundary.²² Reduced surface tension and formation of functional bonds between the polymer macromolecules and nanoparticles determine an increase in the degree of dispersion and deformation of the droplets of the dispersed phase component into fibers. Thus, the process of microfibril formation in immiscible polymer blends is substantially facilitated.^11,12,14,15 Another factor that contributes to the reduction of the microfibril diameter is an enhancement of the stability of liquid streams of PP before the decomposition to droplets. The reason is that the amplitude of the excitation wave is quenched at the nanoparticles localized at microfibrils.¹⁴

In melting endotherms of the strands of the unmodified and ZrO₂—modified blends, one asymmetrical melting peak is recorded because of close melting temperatures of the components (Fig. 8). The addition of zirconia obviously determines an increase in the melting temperature maximum from 165.2°C to 168.4°C.

FIG. 8.

Melting endotherms of the PP, CPA, as well as PP/CPA and PP/CPA/ZrO₂ strands. Color images are available online.

Structure and features of the FM

As reported previously,¹⁹ the PP/CPA blend is stably reprocessed into the composite film by ME due to high adhesion to the substrate and cohesion between the layers. An analogous behavior is demonstrated by nanofilled blend of PP/CPA/ZrO₂. After the polymer matrix extraction, the samples of nonwoven fine-fibrous materials have been obtained that can be used as filters.

The microscopic tests have shown that the basic elements of the structure are PP microfibrils. The average diameter of the microfibrils of the upper layer is higher of that of strands (Fig. 9). For instance, the use of a single-screw extruder with subsequent orientational stretching (Fig. 9a), a twin-screw extruder (Fig. 9b), and the addition of ZrO₂ nanoparticles (Fig. 9c) at the compounding stage leads to the formation of an average size of PP fibrils of 3.68, 2.28, and 1.39 μm, respectively. Compared with the strands, the content of zirconium nanoparticles in the FM decreases slightly (by 10%), there are areas with a more intense signal (Fig. 7b), which indicates the formation of ZrO₂ aggregates.

FIG. 9.

SEM image of the upper layer of FM and histograms of diameter distribution of PP microfibrils after CPA extraction from the double-layered film of PP/CPA produced by schemes II (a), III (b), and IV (c).

These facts may be due to the process of extraction of CPA. One of the reasons for an increase in the microfibrils size in the filter layer is a coalescence of single liquid PP fibrils in the course of composite film formation because the melt fibers leaving the nozzle are flattened.

One of the FM characteristics is retention potential. The results of the estimate of air cleaning from solid particles of 0.3/1.0 μm in size through the designed FM give evidence that the retention potential differs with respect to the scheme of FM production and the number of layers (Table 1).

Table 1.

The Efficiency of Air Cleaning from Solid Particles by the Developed Filter Materials

Scheme of FM production	No. of layers	Filtration efficiency, % (by the particle size, μm)
		0.3	0.4	0.5	0.6	0.7	0.8	0.9	1.0
I	2	82.1	84.2	86.3	89.3	92.4	97.7	98.0	98.9
	4	95.0	96.5	97.1	98.9	97.8	98.5	99.8	99.9
	6	96.0	96.9	97.7	99.4	99.7	99.8	100	100
II	2	84.0	88.6	91.3	94.0	99.5	99.8	99.9	100
	4	95.9	97.6	97.9	99.3	99.9	99.9	100	100
	6	97.3	97.8	98.6	99.8	100	100	100	100
III	2	83.8	88.4	90.1	93.3	99.4	96.6	99.7	99.9
	4	95.2	97.2	97.2	99.1	99.8	99.8	99.9	100
	6	96.2	97.5	97.9	99.7	100	100	100	100
IV	2	94.1	97.3	98.9	99.9	100	100	100	100
a	1	78.6	83.5	85.9	87.8	89.3	91.9	97.4	99.4

^a Filter material made of a film produced by extrusion of polypropylene/copolyamide (thickness −350 ± 20 μm).

FM, filter material.

Application of a twin-screw extruder at the stage of blend compounding (scheme III) or single-screw extruder followed by orientational stretching (scheme II) in the course of strand production provides better results compared with a FM prepared from blend obtained using a single-screw extruder (scheme I). Irrespective of the scheme of production, the cleaning efficiency is enhanced as the number of layers is increased. This fact is associated with the formation of a more perfect structure.¹⁹ It should be noted that the precision and the efficiency of filtration with a FM produced by ME technique is better of the parameters of a material formed by extrusion.

FM composed of structural units in the form of PP microfibrils filled with the ZrO₂ nanoparticles (scheme IV) is characterized by a substantially higher precision and efficiency (Table 1). This fact is a result of the reduced average size of PP microfibrils after the use of a nanoadditive. The FM pore size is lower when the component size is smaller and the form is more homogeneous when the component structures of a filter layer are more uniform and regular.¹ Besides, it is known that precision filtration can be due to a number of physical and chemical processes, namely effect of touchdown, adsorption, and Brownian diffusion.¹

The substantial effect of adsorption is confirmed by an increase in the specific surface of FM formed by PP nanofibrils up to 240 m²/g, as reported previously.¹⁵ That is why fine-fibrous filters can entrap the particles that are five times smaller of the pore size. It should be noted that double-layered FMs containing the ZrO₂ nanoparticles are of the same air cleaning efficiency as four to six layers' thick filters without a filler (even higher, if the particle size is >0.5 μm). In prospect, this fact determines the reduction of material and printing time consumption without a loss of the functional parameters of the product.

Conclusions

The developed new method of production of multilayered fine-fibrous FMs made of PP/CPA blend by 3D printing allows enhancement of the filter abilities and expansion of the application spectrum compared with that produced by extrusion. The dependence of the PP fiber formation process on the conditions of PP/CPA monofilament formation for 3D printing (single-screw or twin-screw extruders, presence/absence of orientational drawing) is established, and the possibility of adjusting the diameters of the dispersed phase fibrils is shown.

The effect of nanoadditive of ZrO₂ on the morphology of fibrous materials (strands after CPA extraction, FMs) is registered that is an increase in the mass fraction of PP fibers and the reduction of their average size.

It is shown that FMs made of the PP/CPA blend modified by the ZrO₂ nanoparticles are of better retention potential when cleaning a gas medium. At the same time, the two-layer FM with nanoparticles provides cleaning efficiency at the level of four- to six-layer materials without filler.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.