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Abstract

In this study, optimized conditions were established for diatomite grinding, which is a natural inorganic mineral with inherently high oil absorption capacity. Diatomite surface was modified with a fluorocarbon chemical and stearic acid via facile methods for enhancing compatibility between polypropylene and diatomite. Polypropylene/diatomite composites were generated in a twin screw extruder with/without using compatibilizer, and nonwoven structures were produced via meltblown technique. Pore size and void content analyses showed that addition of diatomite led to thicker fibers (1–17 µm (the neat polypropylene) vs. 1–32 µm (2 wt.% diatomite containing polypropylene)). Diatomite incorporation into polypropylene resulted in a rigid and brittle structure and a worsened oil absorption property (rust inhibitor oil absorption capacity: 1184% ± 105% (the neat polypropylene) vs. 718% ± 78% and 1089% ± 136% (2 wt.% diatomite containing polypropylene)). Increasing oil viscosity resulted in increased discrepancy among the oil absorption capacities of the neat polypropylene and diatomite containing polypropylene. Analysis of variance tests showed no changes or statistically insignificant differences in oil absorbency.

Keywords

Diatomite polypropylene meltblown nonwoven oil absorbency

Introduction

Industrial oil sorbent textiles possessing a total turnover of $106.8 billion have an important position in technical textiles market. The most important reasons of this situation are the ever-increasing usage of oil and oil derivatives in industry and everyday life and the increased environmental conscious. Consequently, necessity of textile materials with improved oil absorbency property has been increasing day by day.

Sorbent materials should be both oleophilic and hydrophobic, i.e. they should have a high capacity to sorb oil and repel water. They are classified into three categories: natural organic vegetable-based, natural inorganic, and synthetic organic materials [1 –3]. Natural organic sorbent materials contain a lot of agricultural products such as sugarcane bagasse, saw dust, straw, milkweed, kenaf, cotton, wool, and kapok [4,5]. All of these products will become water wet and sink, carrying the oil with them [6]. It is known that these materials are subjected to a surface modification for improving their oil absorptive properties [7 –12]. Inorganic mineral sorbents include stearate-treated calcium carbonate, expanded perlite, expanded vermiculite, zeolite, exfoliated graphite, activated carbon, organoclay, basalt fibers, and diatomite (DE), which are also known as sinking sorbents [4,13 –15]. Mineral sorbents are generally disliked as they have numerous shortcomings, such as contamination of sea beds and harmful effects to aquatic habitats. They also tend to release some of the sorbed oil while sinking because of the low retention capacity of some of the solids [6,16 –18]. Synthetic organic materials are the most widely used sorbents, which are made from high-molecular-weight polymers, such as polyurethane and polypropylene (PP). They have good hydrophobic and oleophilic properties and high oil sorption capacity.

Absorbent minerals have been used since the 1950s for cleaning purposes of industrial spillages. DE, which is one of the absorbent minerals, represents a very rare occurrence—a silica mineral that has an elaborate structure worked by nature into a labyrinth of tiny holes. No other silica source that is mined or chemically prepared has such a structure [19]. The combination of the natural silica composition, the overall structure of the diatom particles, and the network of holes in the structure are responsible for its porous structure and thus high water and oil absorption capacity (150%–300%) [20,21].

DE has its origin from a siliceous, sedimentary rock consisting principally of the fossilized skeletal remains of diatom, a unicellular aquatic plant related to the algae, during the tertiary and quaternary periods [22,23]. DE consists of a wide variety of shape and sized diatoms, typically 10–200 mm, in a structure containing up to 80%–90% voids [24]. DE’s highly porous structure, low density, and high surface area resulted in a number of industrial applications as filtration media for various beverages and inorganic and organic chemicals as well as an absorbent for pet litter and oil spills. Although DE has a unique combination of physical and chemical properties, its use as an absorbent aid material in synthetic nonwoven structures has not been investigated so far.

Considering cost, oil absorption, and environmental issues, we tried to combine PP and DE in one single structure. To that end, DE was treated with a fluorosilane chemical, a fluorocarbon (FC) chemical, and a saturated fatty acid, and their surface properties related to water and oil wettability and absorbency were evaluated in our previous study [25]. In this study, the surface untreated/treated DEs were incorporated into PP matrix for generation of meltblown nonwoven structures and their oil absorption property was investigated for the first time.

Experimental

Materials

Materials and chemicals used in this study are given in Table 1.

Table 1.

Materials, chemicals, and their functions used in the study.

Material(s)	Explanation	Purpose
Polypropylene (PP)	A meltblown grade isotactic homopolymer (SEETEC H7912, MFI_{230℃/2.16 kg}: 1200 g/10 min, LG Chemicals, South Korea)	Polymer matrix
Diatomite (RDE)	Flux calcined type (Imerys, Celite 281, Istanbul, Turkey)	Filler
Orgaguard® FC 6000 (FC)	6 C FC compound, conc. 33% (Organik Kimya, Istanbul Turkey) [26]	Surface treatment
Stearic acid (SA)	>95% (Sigma–Aldrich)	Surface treatment
Maleic anhydride-grafted polypropylene (PP-g-MA)	Exxelor™ PO 1020 (MFI_{230℃/2.16 kg}: 430 g/10 min, MA graft level: <0.30%, Exxon chemicals)	Compatibilizer
Irganox® 1010	Ciba speciality chemicals	Antioxidant
Dynamar™ FX 9613	3M	Processing aid
Linseed oil	Density: 0.927–0.933 g/cm³, acidity index: 0.25–0.50 (as oleic acid), iodine index: 160–180, soaping index: 180–195, kinematic viscosity at 40℃: 35 cSt (Garanti Beziryağı San. ve Tic. A.S.) [27]	Contact angle measurement
Ethylene glycol	Anhydrous, 99.8% (Sigma–Aldrich)	Surface energy analysis
Formamide	ReagentPlus®, ≥99.0% (Sigma– Aldrich)
Diiodomethane	ReagentPlus®, 99% (Sigma–Aldrich)
Opet Fulmaster SHPD 15 W-40	Diesel oil, kinematic viscosity at 40℃: 115 cSt (Opet A.S.)	Oil absorption analysis
Castrol Hyspin AWH M68	Hydraulic oil, kinematic viscosity at 40℃: 68 cSt (Castrol)
Hangs. Rust Coat 10	Rust inhibitor oil, kinematic viscosity at 40℃: 3 cSt (Hangsterfer’s)

Methods

Before the methods are explained, a flowchart is given in Figure 1 for a better understanding of the processing steps used in this study.

Figure 1.

Flowchart of the processing steps.

Grinding

Considering the literature that there is an inverse proportionality between particle size and surface area [28,29], DE was ground for the purpose of enhancing specific surface area and thus to increase the absorption capacity of DE. Besides, reduction in particle size would enable easier processing during meltblown nonwoven production. Deniz [20] showed that DE could be ground easily although it contains high amount of silicon dioxide (SiO₂) meaning that there would be no problem in terms of grinding process.

DE samples were dry and wet ground in a Fritsch Pulverisette 5 planetary ball mill at 400 r/min using balls with 0.5 and 5 mm diameter. The material of the mortars and balls was zirconium oxide. The milling process was conducted in time intervals of 2 min milling, followed by a 4-min break to avoid excessive heating of the mortar and sample.

Fluorocarbon (FC) surface treatment

Here, 3.3 ml (10 wt.%) FC compound was added to 90 ml of distilled water containing 10 g of DE and stirred for 15 min using a magnetic stirrer. The slurry was repeatedly washed with distilled water and then centrifuged at 4000 r/min for 9 min. Then the slurry was dried and cured at 110℃ for 5 min [26].

Stearic acid (SA) surface treatment

DE was surface treated with stearic acid (SA) (10 wt.%) in dry state in a Cyclomix 5 (Hosokawa Micron, The Netherlands) high shear mixer. It can be used in the drying, coating, mixing, agglomeration, and densification of particulate materials and has a nominal volume capacity of 5 l. The mixer has four pairs of flat-bladed impellers placed from the bottom to the top (Figure 2). Related details can be found elsewhere [30]. The DE surface coating was carried out at 80℃ for 15 min with a rotation speed of 1500 r/min. After completing the coating process, a quick cooling to room temperature and discharging were carried out.

Figure 2.

Schematic diagram of a Cyclomix high shear mixer [30].

Generation of compounds and nonwoven structures

PP/DE compounds were generated using a co-rotating intermeshing twin screw extruder (Lab Type, Ø: 16 mm, L/D ratio: 40, Gülnar Plastik Makineleri San. ve Tic. A.Ş., İstanbul, Turkey). PP/DE physical mixtures together with processing aids were dried at 80℃ for 24 h in an air-circulating oven and fed to the extruder in hot state immediately after drying. The temperature of the extruder barrel zones was set to 20℃, 120℃, 130℃, 140℃, 140℃, and 140℃, and PP/DE compounds were generated at a screw speed of 300 r/min. Throughput was adjusted to 1.5 kg/h so that a torque value of 2 N·m was achieved. The notation and material amounts used are summarized in Table 2.

Table 2.

Notation and amounts for PP/diatomite (90/10 w/w) compounds.

Notation	Polypropylene amount (g)	Diatomite amount (g)	Exxelor™ PO 1015 amount (ppm)	Irganox® 1010 amount (ppm)	Dynamar™ FX9613 amount (ppm)
PP	1000	0	0	150	100
PP/RDE	900	100	0	150	100
PP/RDEM	900	100	2000	150	100
PP/RDEFC	900	100	0	150	100
PP/RDEFCM	900	100	2000	150	100
PP/RDESA	900	100	0	150	100
PP/RDESAM	900	100	2000	150	100
PP/GDE	900	100	0	150	100
PP/GDEM	900	100	2000	150	100
PP/GDEFC	900	100	0	150	100
PP/GDEFCM	900	100	2000	150	100
PP/GDESA	900	100	0	150	100
PP/GDESAM	900	100	2000	150	100

PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Nonwoven structures were generated by using meltblown production technique since it has been used for commercial manufacturing of oil absorbent pads. PP nonwovens containing untreated/treated DE were produced on a Biax Fiberfilm Meltblown System with an areal weight of 100 g/m². The production parameters employed are given in Table 3.

Table 3.

Meltblown production parameters.

Production setting	Value
Extruder zone 1 temperature (℃)	300
Extruder zone 2 temperature (℃)	350
Extruder zone 3 temperature (℃)	400
Die temperature (℃)	353
Air temperature (℃)	370
Die hole diameter (cm)	0.038
Die-to collector distance (cm)	50
Air pressure (kPa)	65.5
Extruder speed (%)	30
Collector drum speed (m/min)	54

Characterization

Brunauer, Emmett and Teller analysis

The specific surface area of the DE samples was determined using multi-point measurements on a Micromeritics Gemini VII 2390 instrument by the Brunauer, Emmett and Teller (BET) gas adsorption method. The samples were dried at 150℃ for 24 h and degassed at 200℃ for 5 h under vacuum prior to measurement.

Particle size analysis

Particle size and particle size distribution analyses were performed on Mastersizer 2000 laser diffraction particle size analyzer (Malvern Instruments Ltd, UK) from wet dispersions. A micro-volume (200 mg) sample feeder collected triplicate measurements in 2-s intervals. The obscuration limits were set to 5%–15%.

Contact angle and surface energy analyses

In order to determine hydrophilic/hydrophobic and oleophilic/oleophobic character of the untreated/treated DE and DE incorporated meltblown structures, contact angle measurements were carried out on the Theta Lite Optical Goniometer (KSV Instruments Ltd, Finland). DE samples were prepared in a hydraulic pellet press and dried at 80℃ overnight in an air circulating oven before measurement. Contact angles were measured using the sessile drop method with 3 µl distilled water and linseed oil droplets for the judgment of hydrophobicity and oleophobicity, respectively. Measurements were taken after the water/linseed oil droplet came into contact with the DE for 30 s. In total, 10 values were recorded and their average was calculated.

Contact angles of ethylene glycol, formamide, and diiodomethane of the raw and surface-treated DEs were also measured for surface free energy calculations by using Van Oss-Chaudhury methods.

Melt flow index analysis

The melt flow index (MFI) values of the neat PP and PP/DE compounds were detected according to ASTM 1238 using an MFI analysis instrument (Devotrans EA3, Istanbul, Turkey) [31]. The flow measurement was conducted by pushing the samples through a 2-mm diameter die at 230℃ with 2.16 kg mass. Results are expressed in grams per 10 minutes (g/10 min).

Differential scanning calorimetry analysis

Melting peak temperature (T_m), melting enthalpy (ΔH_m), crystallization temperature (T_c), and % degree of crystallinity (χc) of the PP/DE compounds and nonwovens generated were determined with the help of a Perkin Elmer PYRIS Diamond™ differential scanning calorimeter. Heating and cooling scans were performed under nitrogen atmosphere at 10℃/min over the temperature range from 20℃ to 220℃. The first heating scans were considered for evaluation. Crystallinity of the samples was determined by adapting equation (1) as follows

χ c = (Δ H_{m} / Δ H_{m}^{\circ}) \times 100

(1) where ΔH_m is the enthalpy of fusion in the thermogram, and ΔH_m° the enthalpy of fusion of the completely crystalline material at the equilibrium melting temperature, T_m°. The value of 207 J/g has been taken as the enthalpy of melting of 100% crystalline PP [32].

Scanning electron microscope analysis

For morphological analysis, the samples were coated with gold (Au) using Emitech K950X sputter coater to avoid charge build-up and analyzed by LEO 440 scanning electron microscope equipped with a Quartz Xone energy dispersive X-ray (EDX) analysis system.

Pore size analysis

The pore sizes of the nonwoven structures were determined using a CFP-1005AQC capillary flow porometer (PMI Porous Materials, Inc., Ithaca, NY, USA) according to the ASTM F316-03 [33]. A Silwick by PMI with a surface tension of 20.1 dynes/cm was used as the immersion liquid. The sample immersed in the immersion liquid was pretreated by lowering it to 80 kPa below atmospheric pressure and deairing so that no bubbles remained in the sample. The measuring diameter was 20 mm. Dry air was passed through the sample and the gas pressure was increased in stages, measuring the gas flow rate at each point. The pressure P₅₀ (PSI) at which the flow rate with the sample immersed in the liquid (wet flow rate) was 50% of the flow rate when not immersed (dry flow rate) was determined, and the mean flow pore size was calculated by equation (2).

d_{50} = C \times r / P_{50}

(2)

In the formula, d₅₀ is the mean flow pore size (µm), r is the surface tension of the immersion liquid, which was 20.1 (dynes/cm), and the constant C is 0.451 (µm·cm·PSI/dynes). Measurement was performed three times and the average value was calculated. Minimum (d₁₀) and maximum (d₉₇) flow pore sizes were also calculated according to the equation (2) [34].

Void content analysis

Void content measurement of the meltblown nonwoven structures was carried out from the images acquired from a Leica DFC450 optical microscope (Leica, Germany, Magnification: ×40) using Image Proplus 6.0 program. This involved shining a strong light through a sample and observing any shadows formed by voids. Three replications were applied to obtain average values.

Oil absorption measurements

The oil absorption capacity was measured by the gravimetric method. Meltblown structures were conditioned at 80℃ for 24 h in an air circulating oven prior to the measurements. ASTM D1483 standard was used as the test method which involves the titration of linseed oil onto a known amount of DE (1 g) until free oil appears [35]. Besides, three different oils (rust inhibitor oil, hydraulic oil, and diesel oil) were used for oil absorption measurements. The average of three measurements was used for analysis.

Statistical analysis

Oil absorption test results were statistically evaluated in Design Expert® 6.06 software by considering the grinding, compatibilizer, surface treatment, and oil type as independent variables and oil absorption property as the dependent variable. In order to judge the statistical importance of the variations, analysis of variance (ANOVA) tests were applied. The “p” values were examined as to whether the parameters were significant or not. If the “p” value of a parameter is greater than 0.05 (p > 0.05), the parameter will not be important and should be ignored.

Results and discussion

Particle size and morphology of the DE

In our previous study [25], we have shown that silica (SiO₂) was the main constituent of the DE (89.70%). The particle size analyses results of the DE samples are presented in Table 4. Particle size is defined as equivalent diameter of a spherical particle and given as d₁₀, d₅₀, and d₉₀ values. (d₁₀: diameter where 10% of the population lies below this value, d₅₀: diameter where 50% of the population lies below this value (mean particle size), and d₉₀: diameter where 90% of the population lies below this value).

Table 4.

Particle size distribution of the diatomite samples.

Notation	ΣTime (min)	Ø (mm)	d₁₀ (µm)	d₅₀ (µm)	d₉₀ (µm)
RDE	–	–	8.53	20.26	37.54
GDED48-Ø5	48	5	1.55	4.69	11.57
GDED78-Ø5	78	5	1.70	20.30	94.70
GDED120-Ø5	120	5	0.80	4.70	21.00
GDED240-Ø5	240	5	0.72	4.12	10.26
GDED48-Ø0.5	48	0.5	2.31	10.20	58.07
GDED78-Ø0.5	78	0.5	2.40	10.50	63.19
GDED120-Ø0.5	120	0.5	2.12	10.20	77.56
GDED48Ø5 + Ø0.5	48	5 + 0.5	1.94	10.62	49.39
GDED48Ø5 + D48Ø0.5	48 + 48	5 + 0.5	1.29	6.77	37.73
GDED48Ø5 + D78 Ø0.5	48 + 78	5 + 0.5	0.98	4.96	33.93
GDEW48-Ø5	48	5	0.97	5.30	18.39

RDE: raw diatomite; GDE: ground diatomite; D: dry grinding; W: wet grinding; Ø: bead diameter.

The mean particle size (d₅₀) of the raw DE was determined to be 20.26 µm. Its d₁₀ and d₉₀ values were 8.53 and 37.54 µm, respectively. Particle size of the DEs decreased to a large extent after grinding. Processing conditions were optimized according to the dry grinding. Two different beads (Ø: 0.5 and 5 mm) and four different grinding times (48 min (active grinding: 16 min), 78 min (active grinding: 26 min), 120 min (active grinding: 40 min), and 240 min (active grinding: 80 min) were employed. In addition, combined grindings were also performed by using different beads. Especially, the mean particle size (d₅₀) was taken into account for judging the grinding performance. According to the results, the smallest particle sizes were obtained with a grinding time of 240 min (d₁₀: 0.72 µm, d₅₀: 4.12 µm, d₉₀: 10.26 µm). A grinding time of 48 min delivered similar results (d₁₀: 1.55 µm, d_50: 4.69 µm, and d₉₀: 11.57 µm). Wet grinding, which was also performed with a grinding time of 48 min, exhibited nearly the same results (d₁₀: 0.97 µm, d₅₀: 5.30 µm, d₉₀: 18.39 µm for GDEW48-Ø5). Considering the particle sizes obtained and the easiness of the process, dry grinding with a total grinding time of 48 min was decided to use for compound and nonwoven generation. It was observed that usage of smaller beads together or separately with 5 mm beads did not work in terms of particle size reduction.

The morphological analysis showed that the diatom frustules in the raw DE were mostly well preserved, having cylindrical or disk shape (Figure 3, RDE). In spite of a short processing time of 48 min, dry grinding obviously deteriorated the porous structure of the DE (Figure 3, GDED), while wet grinding damaged it to less extent (Figure 3, GDEW).

Figure 3.

SEM images of the neat (RDE), dry ground (GDED), and wet ground (GDEW) diatomite.

Surface area of the DE

We have shown in our previous study that surface area of the DE increased after grinding only to a small extent (RDE: 0.23 ± 0.06 m²/g, GDE: 2.25 ± 0.03 m²/g) [25]. Pore size of the DE increased after grinding (RDE: 163.7 Å, GDE: 201.8 Å). Grinding did not change the pore volumes considerably. As indicated in the literature [36 –38], calcination carried out at high temperatures up to 1600℃ causes the generation of siloxane bridges on the DE backbone surface which leads to closing of the pores. Closed pores on the DE surface were not able to be detected by the BET N₂-adsorption method and thus very low surface area was detected.

Surface modification of the DE

In order to increase physical interactions between hydrophilic DE and hydrophobic PP and thus contributing to enhanced compatibility in PP/DE compounds, hydrophobic surface treatments were performed on the DE. According to the results obtained from our previous study [25], it can be clearly said that the both surface treatment chemicals were successfully attached to the DE samples’ surfaces either physically (SA) or chemically (FC).

Wettability and oil absorptivity of the DE

Our previous study gives a detailed report on wettability and oil absorption properties of the raw, ground, and surface-treated DEs [25]. It was shown that the inherently hydrophilic and oleophilic structure of the DE could be changed to either a hydrophobic/oleophobic or hydrophobic/oleophilic one by FC or SA treatment, respectively. It was determined that the grinding process reduced the oil absorption property of the DE.

Surface energy

Only the raw, i.e. unground DEs were examined since the measurements are done in pressed form and hence particle size would not affect the surface energy. PP and raw DE delivered low and high surface energies due to their hydrophobic and hydrophilic character, respectively (Table 5). Surface treatment with FC chemical or SA reduced the DE’s surface energy substantially. It points out that PP and surface-treated DE would be compatible with each other, when they are mixed in melt stage together.

Table 5.

Surface energy values of the materials used.

Sample	Mean surface energy at 20℃ (dynes/cm)
PP	29 ± 6
RDE	67 ± 3
RDEFC	10 ± 5
RDESA	14 ± 3

PP: polypropylene; RDE: raw diatomite; FC: fluorocarbon; SA: stearic acid.

MFI analysis of the PP/DE compounds

It is known in the literature that maleic anhydride was used for PP and silica composites as a compatibilizer [39 –41]. Therefore, we used a maleic anhydride-grafted PP in addition to the DE surface treatment for achieving an enhanced compatibility between PP and DE.

MFI value is of importance in terms of materials processing. As shown in Table 6, incorporation of DE reduced the MFI values. Usage of compatibilizer reduced them further meaning that the processing in melt stage would be difficult. But, considering the low amount of DE used in the final product, i.e. nonwoven, it can be said that the addition of DE would not affect the processability.

Table 6.

MFI values of the PP/DE (90/10 w/w) compounds.

Notation	MFI (g/10 min)
PP	1198 ± 21
PP/RDE	1021 ± 10
PP/RDEM	996 ± 24
PP/RDEFC	1000 ± 19
PP/RDEFCM	917 ± 42
PP/RDESA	1000 ± 34
PP/RDESAM	960 ± 32
PP/GDE	946 ± 47
PP/GDEM	1011 ± 11
PP/GDEFC	1089 ± 17
PP/GDEFCM	1004 ± 33
PP/GDESA	982 ± 38
PP/GDESAM	961 ± 36

PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Thermal behavior of the PP/DE compounds and nonwovens

Figures 4 and 5 present the differential scanning calorimetry (DSC) thermograms of the raw DE containing PP compounds and nonwovens, representatively. Thermal behavior results of all compounds and nonwovens are given in Table 7. The neat PP has a melting temperature of 164.4℃, a crystallization temperature of 122.0℃, and a crystallinity value of 30.6%. Independent of the grinding process, surface treatment, and compatibilizer usage, addition of DE led to only a slight change in thermal behavior of the PP/DE (90/10 w/w) compounds. 2 wt.% DE containing nonwoven samples delivered similar results. But, the values were different from those of the compounds. Nonwoven samples exhibited slightly lower T_m and T_c values than those of the compounds. But, their degrees of crystallinity values were determined to be too low when compounds were considered, although DE incorporation changed them only slightly. Generally, it can be said that incorporation of DE into PP did not affect the thermal behavior of the compounds and nonwovens.

Figure 4.

DSC thermograms of the PP/raw diatomite (90/10 w/w) compounds. PP: polypropylene; RDE: raw diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Figure 5.

DSC thermograms of the PP/raw diatomite (98/2 w/w) nonwovens. PP: polypropylene; RDE: raw diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Table 7.

DSC data of the PP/DE (90/10 w/w) compounds.

Notation	T_m/T_c (℃)		Crystallinity (%)
Notation	Compound (90/10)	Nonwoven (98/2)	Compound (90/10)	Nonwoven (98/2)
PP	164.4/122.0	162.8/120.0	30.6	5.2
PP/RDE	166.0/123.6	161.5/118.4	33.4	3.6
PP/RDEM	164.4/122.6	162.2/117.9	28.0	4.4
PP/RDEFC	165.1/120.9	161.3/117.8	30.0	4.4
PP/RDEFCM	167.8/119.8	162.0/119.1	32.7	4.9
PP/RDESA	165.1/121.4	161.6/119.6	32.1	5.1
PP/RDESAM	164.7/122.5	162.7/120.3	28.1	3.9
PP/GDE	165.3/119.3	163.0/118.4	28.6	4.3
PP/GDEM	165.9/120.7	161.9/117.6	27.6	4.8
PP/GDEFC	165.5/121.5	162.4/119.6	33.0	5.9
PP/GDEFCM	166.3/121.9	161.1/120.6	34.2	6.0
PP/GDESA	165.0/123.8	162.3/121.4	26.3	3.7
PP/GDESAM	162.2/122.1	162.0/119.7	28.6	4.2

PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Morphology of the PP/DE compounds and nonwovens

Before going into details related to the analysis results of the DE containing PP meltblown composite structures, it should be pointed out that high torque values were recorded and rigid and brittle structures were obtained during meltblowing process, when the DE amount was adjusted to equal or higher than 2 wt.%. In addition, increasing the DE amount led to a distinctive separation of the PP and DE as if the DE was sprayed separately as a powder mass through the spinnerets of the meltblown equipment. It can be said that observation of high torque values is related with the DE loading amount rather than its particle size, since even the raw DE has the biggest particle size of 37 µm, while the die diameter of the meltblown equipment is 380 µm (Table 3). In order to verify that rigid and brittle structures were obtained, we performed an EDX analysis on the cross section of a 2 wt.% DE incorporated PP nonwoven, which is presented in Figure 6. It can be said that the elements Si and O belonging to DE accumulated more on the fiber periphery. Therefore, DE-embedded PP meltblown nonwovens presented a rigid and brittle structure since DE has a typical Mohs hardness of 5–6. To our knowledge, this phenomenon is the first observation in the literature and could be attributed to the incompatibility between PP and DE and to hot air flow with very high attenuation speeds, which is used for melt blowing of primary filaments into very fine fibers. Because of these problems mentioned above, we only generated the PP/DE nonwovens with the highest possible amount of 2 wt.% DE.

Figure 6.

EDX analysis of the 2 wt.% raw diatomite containing PP nonwoven.

Scanning electron microscope (SEM) images of the raw DE containing PP compounds and nonwovens in Figures 7 and 8, respectively. The compounds and nonwovens with the ground DEs were not shown here. PP/DE compounds exhibited DE domains varying between 2 and 8 µm independent of DE surface treatment and compatibilizer (Figure 7). A distinctive observation of the DE particles was only achieved on the PP/RDEFC and PP/RDESAM nonwovens due to the low amount of DE. Generally, it can be said that the PP/DE nonwovens showed smaller DE particles with several micrometers (Figure 8). Considering the initial mean particle sizes of the raw and ground DEs (d₅₀: 20.26 µm (RDE), 4.69 µm (GDE), and d₉₀: 37.54 µm (RDE), 11.57 µm (GDE)), it is obvious that the DE particle size reduced to a large extent when processed with PP in melt stage. Besides, the neat PP nonwoven showed a fiber diameter ranging from 1 to 17 µm, while fiber diameter of the DE containing PP composite nonwovens changed from 1 to 32 µm indicating a fiber thickening, which will be discussed in pore size and void content analyses.

Figure 7.

SEM images of the PP/raw diatomite (90/10 w/w) compounds. PP: polypropylene; RDE: raw diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Figure 8.

SEM images of the PP/raw diatomite (98/2 w/w) nonwovens. PP: polypropylene; RDE: raw diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Pore size and void content of the PP/DE nonwovens

Table 8 shows the minimum (d₁₀), maximum (d₉₇), and mean (d₅₀) flow pore diameter of the meltblown composite structures. The neat PP has the following pore sizes: 13 ± 4 µm (d₁₀), 44 ± 4 µm (d₉₇), and 23 ± 4 µm (d₅₀). Raw DE containing PP composites exhibited considerably increased minimum, maximum, and mean flow pore sizes independent of the DE surface treatment and usage of compatibilizer. The PP/ground DE composite nonwoven structures delivered similar results. It is known that pore diameter of a fibrous nonwoven structure is closely related to the fiber diameter [42,43]. Rising pore size is caused by higher fiber diameter by widening interfiber space. Void content analysis delivered supporting data.

Table 8.

Minimum (d₁₀), maximum (d₉₇), mean (d₅₀) flow pore diameter of the neat and 2 wt.% diatomite containing polypropylene meltblown composite structures.

Notation	Pore size analysis
Notation	Minimum pore size (µm)	Maximum pore size (µm)	Mean pore size (µm)	Void content (%)
PP	13 ± 4	44 ± 4	23 ± 4	1.2 ± 0.3
PP/RDE	31 ± 6	155 ± 6	73 ± 6	46.7 ± 2.5
PP/RDEM	30 ± 8	142 ± 8	72 ± 8	20.9 ± 1.0
PP/RDEFC	30 ± 3	157 ± 3	79 ± 3	52.0 ± 1.6
PP/RDEFCM	32 ± 11	180 ± 11	82 ± 11	9.6 ± 1.2
PP/RDESA	30 ± 8	187 ± 8	78 ± 8	25.6 ± 1.4
PP/RDESAM	30 ± 5	149 ± 5	69 ± 5	21.8 ± 0.7
PP/GDE	14 ± 7	193 ± 7	45 ± 7	24.3 ± 1.1
PP/GDEM	15 ± 4	223 ± 4	173 ± 4	17.9 ± 1.0
PP/GDEFC	33 ± 2	155 ± 2	85 ± 2	23.5 ± 1.2
PP/GDEFCM	33 ± 6	165 ± 6	77 ± 6	43.1 ± 1.8
PP/GDESA	31 ± 7	168 ± 7	84 ± 7	16.8 ± 0.7
PP/GDESAM	33 ± 4	191 ± 4	89 ± 4	37.4 ± 2.1

PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Void content results of the nonwoven structures are also given in Table 8. Selected optical microscopy images (on the left) and corresponding software images (on the right) of the raw DE containing PP used for the void content calculations are displayed in Figures 9 and 10. The neat PP showed a void content of 1.2% ± 0.3%, while DE containing PP nonwoven structures exhibited higher void contents varying between 9.6% ± 1.2% and 52.0% ± 1.6%. Accordingly, addition of the DE increased interfiber voids. Void content calculations are based on background gray level of the images. As might be expected, thicker fibers lead to an increased background gray level due to having a lower surface coverage, which result in higher void content.

Figure 9.

Optical microscopy (left) and software (right) images of the PP/raw diatomite (98/2 w/w) nonwovens without compatibilizer. PP: polypropylene; RDE: raw diatomite; FC: fluorocarbon; SA: stearic acid.

Figure 10.

Optical microscopy (left) and software (right) images of the PP/raw diatomite (98/2 w/w) nonwovens with compatibilizer. PP: polypropylene; RDE: raw diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Considering both the pore size and the void content analyses, it can be said that incorporating DE into PP resulted in thicker fibers.

Wettability and oil absorptivity of the PP/DE nonwovens

Contact angle measurements of the PP/DE nonwovens with oil were not able to be performed since oil droplet spread out quickly once it contacted the nonwoven surface due to oleophilic character of the PP.

Oil absorption property of the PP/DE composite nonwovens was tested with four oils. As shown in Figure 11, linseed oil absorption of the neat PP is very high, being 3068% ± 90%. But, addition of DE worsened the linseed oil absorption capacity to a large extent independent of the grinding, surface treatment, and compatibilizer usage. Individual samples such as PP/RDEFC, PP/GDE, PP/GDEFC, and PP/GDESAM presented relatively higher linseed oil absorption values between 1750% ± 108% and 1809% ± 8% within the DE containing samples. But, considered as a whole, they changed between 1299% ± 110% and 1570% ± 54%.

Figure 11.

Linseed oil absorbency of the PP/DE (98/2 w/w) nonwovens. PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

The purpose of using linseed oil was to meet the standard’s requirement [35], although it is not an oil type, which is encountered in industrial oil spills. In order to observe the oil absorption behavior of the nonwovens under real conditions, we used three synthetic-based oils additionally, which have different kinematic viscosities. Rust inhibitor oil delivered the following results (Figure 12): The neat PP had an oil absorption value of 1184% ± 105%. Independent of the grinding, surface treatment, and compatibilizer usage, DE containing nonwovens showed reduced oil absorption capacities changing between 718% ± 78% and 1089% ± 136%. PP/RDEM and PP/RDEFC nonwovens presented the highest oil absorption values within the DE containing samples. Usage of hydraulic oil delivered similar results as follows (Figure 13): Oil absorption capacity of the neat PP nonwoven was determined to be 2566% ± 167%, and incorporation of DE reduced the oil absorption values to a large extent. The general trend did not change when diesel oil was used (Figure 14); The neat PP exhibited an oil absorption value of 2921% ± 86% and DE added nonwovens showed reduced oil absorption capacities between 1276% ± 24% and 1523% ± 42%. Paying closer attention to all oil absorption results, it is noticeable that the discrepancies among the values of the neat PP and PP/DE nonwovens increase. This observation could be explained with the viscosity values of the oils used. Increasing kinematic viscosity resulted in increased discrepancy among the oil absorption values of the neat PP and DE containing PP. Accordingly, addition of DE reduced the oil absorbency more adversely when more viscous oil was used.

Figure 12.

Rust inhibitor oil absorbency of the PP/DE (98/2 w/w) nonwovens. PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Figure 13.

Hydraulic oil absorbency of the PP/DE (98/2 w/w) nonwovens. PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Figure 14.

Diesel oil absorbency of the PP/DE (98/2 w/w) nonwovens. PP: polypropylene; RDE: raw diatomite; GDE: ground diatomite; M: maleic anhydride; FC: fluorocarbon; SA: stearic acid.

Reduction of oil absorbency in DE containing PP composite nonwovens could be attributed to the fiber structure, pore size, and void content. Incorporation of DE led to thicker fibers having a so-called rigid sheath layer on the fiber surface resulting in a rigid and brittle structure. This structure probably acted as a barrier and led to a substantial reduction in oil absorbency.

The test data were analyzed based on the ANOVA statistical tool, and the results were presented in Tables 9 and 10. In order to judge the effect of surface treatment, SA and FC treatments were evaluated separately. For the SA surface treatment, the factor of oil type significantly affects the oil absorption property at the 99% confidence level. For the FC surface treatment, the factor of oil type influences the nonwoven oil absorption property significantly at the 95% confidence level.

Table 9.

ANOVA test results for the polypropylene/diatomite (98/2 w/w) nonwoven structures.

Source	Sum of squares	DF	Mean square	F value	Prob > F
Model	1617951.00	19	85155.33	3.910438	0.0974
A	56357.04	1	56357.041	2.587985	0.1830
B	45153.38	1	45153.37	2.073499	0.2233
C	3675.37	1	3675.37	0.168778	0.7023
D	1394988.00	2	697494.04	32.02979	0.0035
AB	8778.37	1	8778.37	0.403114	0.5600
AC	25807.04	1	25807.04	1.185091	0.3375
AD	30865.58	2	15432.79	0.708693	0.5452
BC	715.04	1	715.04	0.032836	0.8650
BD	18327.25	2	9163.62	0.420805	0.6826
CD	7972.75	2	3986.37	0.183059	0.8393
ABC	3432.04	1	3432.04	0.157604	0.7116
ABD	3595.75	2	1797.87	0.082561	0.9223
ACD	18283.58	2	9141.79	0.419802	0.6831
Residual	87105.67	4	21776.41
Cor Total	1705057.00	23

A: grinding; B: compatibilizer; C: stearic acid surface treatment; D: oil type.

Table 10.

ANOVA test results for the polypropylene/diatomite (98/2 w/w) nonwoven structures.

Source	Sum of squares	DF	Mean square	F value	Prob > F
Model	1,531,368.00	21	72,922.29	9.330123	0.1011
A	80,041.50	1	80,041.50	10.24100	0.0853
B	580.16	1	580.16	0.074230	0.8108
C	4930.66	1	4930.66	0.630860	0.5103
D	1,215,317.00	2	607,658.29	77.74750	0.0127
AB	48.16	1	48.16	0.006163	0.9446
AC	13,254.00	1	13,254.00	1.695798	0.3226
AD	41,487.75	2	20,743.87	2.654098	0.2737
BC	26,136.00	1	26,136.00	3.343999	0.2090
BD	27,723.58	2	13,861.79	1.773562	0.3605
CD	49,838.08	2	24,919.04	3.188294	0.2388
ABC	21,122.67	1	21,122.66	2.702563	0.2419
ABD	13,943.08	2	6971.54	0.891982	0.5285
ACD	2693.25	2	1346.62	0.172295	0.8530
BCD	34,252.75	2	17,126.37	2.191253	0.3134
Residual	15,631.58	2	7815.79
Cor Total	1,547,000.00	23

A: grinding; B: compatibilizer; C: fluorocarbon surface treatment; D: oil type.

Conclusions

It was determined that the porous structure of the DE disappeared completely in dry grinding while wet grinding destroyed the porous structure to a small extent. A total grinding time of more than 48 min did not change particle size distribution of the DE noticeably.

Application of a grinding process, a surface treatment, and usage of a compatibilizer was revealed to be dispensable for DE containing PP composite nonwoven structures since DE particle size already reduced during melt compounding and melt blowing processes to a large extent and more than 2 wt.% DE could not be embedded to the PP nonwoven.

It was shown that the addition of DE led to increased fiber diameters due to adversely affecting the melt blowing process and a rigid and brittle structure by DE accumulation on the PP surface. As a consequence, DE on the surface acted as a barrier and worsened the oil absorbency of meltblown nonwoven structures. Reduction in oil absorbency was more distinctive when more viscous oil was used. ANOVA tests delivered statistically insignificant differences in terms of oil absorbency property independent of grinding, surface treatment, compatibilizer usage, and oil type used.

Footnotes

Acknowledgments

The authors would like to thank Mr. Tuncay Saka from Organik Kimya A.Ş. for providing fluorocarbon chemical, Mr. Tayfun Tümer from Mikron’s A.Ş. for providing the coating facility Cyclomix, Ms. Menekşe Sarıhan from ERNAM (Erciyes University Nanotechnology Research Center) for SEM analysis, and Mr. Serkan Göğüş from Mogul Tekstil Sanayi ve Ticaret Ltd Sti. for pore size measurements.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) (Project Number: 113M512).

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Oil absorbency of diatomite-embedded polypropylene meltblown composite structures

Abstract

Keywords

Introduction

Experimental

Materials

Methods

Grinding

Fluorocarbon (FC) surface treatment

Stearic acid (SA) surface treatment

Generation of compounds and nonwoven structures

Characterization

Brunauer, Emmett and Teller analysis

Particle size analysis

Contact angle and surface energy analyses

Melt flow index analysis

Differential scanning calorimetry analysis

Scanning electron microscope analysis

Pore size analysis

Void content analysis

Oil absorption measurements

Statistical analysis

Results and discussion

Particle size and morphology of the DE

Surface area of the DE

Surface modification of the DE

Wettability and oil absorptivity of the DE

Surface energy

MFI analysis of the PP/DE compounds

Thermal behavior of the PP/DE compounds and nonwovens

Morphology of the PP/DE compounds and nonwovens

Pore size and void content of the PP/DE nonwovens

Wettability and oil absorptivity of the PP/DE nonwovens

Conclusions

Footnotes

Acknowledgments

Declaration of Conflicting Interests

Funding

References