The effect of laboratory synthesized and modified layered double hydroxides on recycled and neat high density polyethylene matrix properties using stearic acid as an interface modifier

doi:10.21203/rs.3.rs-3140448/v1

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Research Article

The effect of laboratory synthesized and modified layered double hydroxides on recycled and neat high density polyethylene matrix properties using stearic acid as an interface modifier

https://doi.org/10.21203/rs.3.rs-3140448/v1

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published 02 Dec, 2023

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In this research, the effect of layered double hydroxides (LDHs) as nanofillers on recycled and neat high density polyethylene (HDPE) properties was investigated. The synthesis of unmodified and modified LDHs was achieved via one-pot hydrothermal co-precipitation method at a constant pH. The nanofillers were organically modified by using sodium dodecyl sulphate (SDS) as surfactant and stearic acid (SA) as an interface modifier in order to overcome the incompatibility problems with HDPE Matrix. The nanocomposites were prepared by melt blending and compression molding with 3 and 5 wt% filler loading rate. Thermal, structural, and morphological tests were conducted to analyze the performance of the LDHs and the compatibilizing agent on the dispersion, and on the properties of the studied composites. Results revealed that 3 wt% of unmodified LDHs filler reinforced all physical and mechanical properties of the obtained nanocomposites based on recycled and neat HDPE matrix. When the weight of LDHs was increased to 5 wt%, morphological observations showed that SA has prevented the aggregation of LDHs particles and improved their dispersion into the recycled HDPE matrix. The physical and mechanical properties of the composite materials were enhanced dramatically with the addition of stearic acid as interface modifier into the recycled polymeric matrix compared to SDS intercalating which improved only the izod impact strength. The formulations of r-HDPE/3%LDH and r-HDPE/5%LDH-1%SA were found to be the ideal combinations which could provide novel mechanical applications to meet industrial requirements at higher scale.

recycled HDPE matrix

LDHs

Stearic acid

SDS

polymer nanocomposites

During the recent years, the demand for polymers across the globe such as plastics has increased significantly due to rapid growth of world population and the lock-down caused by the COVID-19 pandemic. Plastics have become an indispensable ingredient to households and manufactories and their applications in many fields have surged dramatically, generating an enormous amount of waste which is released into environment. To prevent environment pollution, recycling and recovering plastics wastes has remained to be a major concern worldwide in the 21st century to sustain its circularity. Among the various plastic pollutants that threaten our planet, polyethylene (PE) based polymers especially high-density polyethylene (HDPE) which is considered as non-biodegradable polymer and accounted in the top three most used plastics in industry and has a wide variety of applications [1, 2].

HDPE is a thermoplastic polymer made from petroleum and it consumes a total of 1.75 kg of fossil fuels to manufacture just 1 kg of HDPE. Therefore manufactories using this polymer as raw material might face a shortage supply in order to maintain their activities so recycling is an alternative to generate new material resources from one hand and reduce the amount of wastes sent to landfill and curb raw material extraction and pollution from the other hand. Furthermore, the cost of elaborating recycled HDPE materials is almost the third less expensive than of manufacturing neat HDPE [3]. However, the recycling process is not always friendly to environment due to processing and separation difficulties. There are four dominant types of plastic recycling process namely, primary, secondary, tertiary, and quaternary recycling [1, 4].

The recycled and neat HDPE can be both enhanced by adding fillers in order to improve their physical and mechanical properties, tackle the quality degradation when necessary, and meet industrial requirements for various applications. Among the vast number of fillers under investigation in both academia and industry, layered double hydroxides (LDHs) are particularly interesting materials due to their low cost and can be easily intercalated with surfactants or compatibilizers to facilitate their dispersion in polymer matrix. LDHs are known as anionic clays or hydrotalcite-like compounds, a class of two dimensional inorganic nanomaterials with positively charged host layers and counter-anions found in the interlayer space. The general chemical formula of LDHs is [M^II _(1−x) M^III _x (OH)₂]^x+ [(Aⁿ⁻)_x/n mH2O]^x−, where M^II represents a divalent metal ion (i.e. Mg²⁺, Zn²⁺, etc.), and M^III represents a trivalent metal ion (i.e. Al³⁺, Cr³⁺, etc.). Aⁿ⁻ stands for an anion with a valency n (i.e. CO₃²⁻, Cl^–, etc.) or an organic anion (i.e. C₁₂H₂₅SO₄ ⁻), m is the number of moles of water per formula weight of compound and x is a stoichiometric coefficient, generally ranging between 0.2 and 0.4, which determines the layer charge density and the anion exchange capacity [5, 6]. There are several methods used to elaborate LDHs, the most common are namely the co-precipitation, urea hydrolysis, ion exchange and hydrothermal synthesis. The co-precipitation method is the most used and simplest among all available techniques to fabricate the hydrotalcite-like compounds from bimettalic and trimetallic or mixed salts [7–9]. The combination of two methods such as hydrothermal and co-precipitation in one-pot show better crystallization behaviour for the obtained products and it’s the one applied in our research.

Layered double hydroxides as inorganic fillers cannot be well exfoliated within the polymer matrix through simply melt blending due to the increasing number of aggregations related to stacking lamellar sheets of dried LDH powder and also to the nature of polymers which are hydrophobic while the LDHs have hydrophilic surfaces. To overcome the problem of incompatibility and create uniform distribution, sodium dodecyl sulfate (SDS) and stearic acid (SA) were selected among a variety of available organic modifiers. Another advantage by intercalating anionic surfactants is to expand the interlayer spacing of LDHs materials so as to make it easily accessible for the incorporation of large hydrophobic polymer chains [10].

Recently, Patti et al. critically reviewed the wide application of stearic acid as surface modifier in processing polymers and composites [11]. According to them, many research groups had investigated the performance of fatty acids including stearic acid as dispersing agents, surfactants, activators, lubricants, and softening agents but the main application of SA concerned the surface treatment. Treatment procedures of filler surfaces by stearic acid can be categorised in two main pathways namely, the dry methods where fillers and the fatty acid are mechanically mixed together above stearic acid melting temperature or they can also be directly mixed with the polymer matrix in an extruder or internal mixer above polymer melting point, and the wet methods where the treatment is applied to fillers surface through a good fatty acid solvent such as ethanol. The latter process is followed by final step of filtration and/or solvent evaporation before compounding.

Using stearic acid as coupling agent and surface modifier for polyethylene/clay composite has prevented agglomerations, inducing better dispersion of the filler into the polymeric matrix and yet enhancing the mechanical properties of the materials [2]. Similar results were obtained by Salmi et al. when added 1%wt of stearic acid to PP/SiO₂ nanocomposites whereas tensile strength and modulus of elasticity decrease as the amount of the fatty acid increases to 5%wt, concluding that stearic acid acted as lubricant and not as compatibilizing agent for that added weight [12]. Another study was conducted by Ma’ali and co-workers about the effect of stearic acid on the interfacial adhesion between recycled low density polyethylene and the filler [13]. It was registered that tensile properties were enhanced slightly.

LDH organic modification can be achieved successfully by following one of the main processes such as calcination/ memory effect [14, 15], anion exchange [5, 16], direct synthesis and delamination/restacking [8, 17]. Then, the OLDH is loaded into the polymer matrix with different molar and weight rations following various routes which were reported in Taviot-Guého et al. review [18] but the three dominant paths used in elaborating LDH-based polymer nanocomposites consist of in situ polymerization by incorporating polymers into the interlayer spacing of LDH, direct melt blending, and solvent casting by exfoliating LDHs sheets into convenient solvents. Direct melt processing is the most common at industrial scale.

Layered double hydroxides polymer based nanocomposites have been widely investigated with numerous polymers, including epoxy resins [19, 20], polyamide [21], PET [22], thermoplastic polyurethane (TPU) [23], PLA [24–26], PVC [27], PP [28], and PE [29–31]. Besides the considerable number of LDHs applications in many fields, it is very important to mention that organic layered double hydroxide nanofillers have a predominant role in the material chemistry field because of their wide range of industrial applications [32]. The modified layered double hydroxide has been used especially as flame retardant additive in HDPE nanocomposites, and dramatic improvements in thermal stability and flame retardancy were observed [33, 34].

Recently, Guo et al. critically reviewed LDHs applications as thermal stabilizers for PVC matrix by many research groups who had investigated the effect of LDHs nanofillers on a wide variety of polymers [27]. Their results contributed a lot in the development of new economic, environment-friendly, and non toxic PVC stabilizers replacing the conventional ones.

Another application of polymer/LDHs is in agriculture where Gomez and co-workers investigated the intercalation of organic ultraviolet absorbers anions into bimetallic and trimetallic LDHs for the protection of LDPE from degradation caused by difficult weather conditions and therefore extending its life cycle [31]. They have confirmed that these nanofillers are potential eco-friendly stabilizers for a wide range of polymers used as plastic materials and not only for polyethylene matrix. Modified LDHs proved to be a promising material for enhancing water purifications due to their high anion exchange capacity and high surface area. When the nanomaterials are combined with polymers, the capacity of sorption and separation is enhanced dramatically unlike toxicity level is decreased by using small amount of them and also when gathered with biopolymers such as PVA, chitosan, alginate, etc. [35, 36]. In food packaging applications, modified LDHs have been encapsulated into various polymers such as PET by ball milling technique, showing oxygen diffusion and permeability coefficient lower than pure PET [37], LDPE with 2%wt loading of OLDH enhanced the oxygen and moisture boundaries characteristics while preserving the quality of the film [38].

The purpose of this study was to investigate the effect of adding unmodified and modified LDHs using stearic acid (SA) as an interface modifier on the properties of neat and recycled high density polyethylene polymer nanocomposites by the melt processing method. In order to investigate the effect of stearic acid in the system, HDPE/MgAl-(SDS) LDH nanocomposites were elaborated as comparative samples. The preparation of organomodified MgAl-LDH via one-step hydrothermal co-precipitation route is reported. Stearic acid was selected due to its specific compatibilizing properties and is considered as good surfactant, cost effective, and also little quantities are recommended. Whereas, SDS was employed as a surfactant to enhance the basal space of LDHs for facilitating the intercalation of HDPE chain and its melting point is much higher than that of polyethylene. Then, in internal mixer using brabender plastograph, nanocomposites with recycled and neat high density polyethylene were formulated by adding directly the interface modifier (SA) to the polymer matrix and LDH filler. All elaborated fillers and polymer nanocomposites were characterized by FTIR, XRD, AFM, SEM, and TGA techniques. The morphology, thermal, and mechanical properties of the obtained composites were investigated to evaluate the effect of the dispersion of surface coated LDHs particles in the polymer matrix on the final properties. Despite the massive number of researches in previous literatures about LDHs based polymer, there is no study has discussed the effect of SA with LDHs as nanofillers on recycled high density polyethylene matrix properties.

Materials

The high density polyethylene (SABIC® HDPE F00950) plastic, produced by Saudi Basic Industries Corporation (SABIC) was used as the neat matrix material. Its density was 0.923 g/cm3, and its melt flow index (MFI) was 0.75 g/10 min at 2.16 kg/190°C. It was provided in a granular form with an injection grade. Recycled HDPE was obtained from SOPHIPLAST National Company in Algeria. Its recycling cycle was 2 with a density of 0.952 g/cm3, and its melt flow index is 0.50 g/10 min at 2.16 Kg/190°C. Stearic acid (SA) [CH₃-(CH₂)₁₆-COOH] is a commercial product used without further purification, purchased from Henkel (Germany). SA has a molecular weight of 284.48 g/mol, melting point of 69.6°C, and evaporation temperature of 361°C. Sodium dodecyl sulphate (SDS) (C₁₂H₂₅NaO₄S) is a commercial product supplied by Panreac Quimica SA, Spain. SDS has a molecular weight (MW) of 288.38 g/mol. Sodium hydroxide (NaOH) was purchased from Honeywell, Sweden. NaOH has a molecular weight of 40 g/mol, melting point of 319°C, density 2.13g/mol. Sodium carbonate (Na₂CO₃₎ has molecular weight of 105.99 g/mol, melting point of 851°C. It was supplied by Sigma-Aldrich GmbH, Germany.

Magnesium chloride hexahydrate (MgCl₂.6H₂O) has a molecular weight of 203.3 g/mol. Magnesium nitrate hexahydrate Mg(NO₃)₂.6H₂O has a molecular weight of 256.41g/mol. Aluminium chloride hexahydrate (AlCl₃.9H₂O) has a molecular weight of 241.43g/mol and Aluminium nitrate hexahydrate Al(NO₃)₃.9H₂O has its molecular weight of 375.13g/mol. All chemical products (salts) were manufactured by VWR Chemicals, Belgium.

Synthesis of Mg₂Al-CO₃-LDH / Mg₂Al-SDS-LDH

Mg₂Al-SDS-LDH was synthesized in our Laboratory in a one step hydrothermal co-precipitation method according to the process described by Zhang et al. with minor modification inspired from the work of Leão et al. and the investigation of Li et al. [19, 23, 40]. Basically, the two salts were mixed together in 250 ml distilled water with 2/1 (Mg/Al) molar ratios. SDS was dissolved in 250 ml distilled water and heated at 65°C then the salts were dropwised into SDS solution under stirring. The pH was kept at 10 (± 0.5) by addition of NaOH. The reaction was kept at 65°C until the mixture was finished. Then transferred into Teflon-lined autoclave and heated at 80°C for 20 hours. The obtained white precipitate was filtered by centrifugation, washed several times with absolute ethanol and de-ionized water from chloride and sodium excess then dried at 50°C for 12 hrs. Finally, the obtained product was grounded to fine powder (63 micron). Mg₂Al-CO₃-LDH was elaborated as mentioned above without SDS. Instead, Na₂CO₃ was used. When washing the final product, AgNO₃ was used to verify the existence of chloride ions (Cl^− 1).

Nanocomposites preparations

Table 1

Composition of the prepared composites
Samples	r-HDPE (Wt %)	n-HDPE (Wt %)	LDHs (Wt %)	LDH-SDS (Wt %)	SA (Wt %)
1	-	100	-	-	-
2	100	-	-	-	-
3	95	-	5	-	-
4	96	-	3	-	1
5	94	-	5	-	1
6	97	-	3	-	-
7	-	97	3	-	-
8	-	97	-	3	-
9	97	-	-	3	-

The nanoparticles were incorporated into high density polyethylene by melt blending technique using an internal mixer (Brabender Plasticorder model MIXER 50 EHT) at 200°C and 60 rpm rotor speed for 10 min with varying in filler amount as 3 and 5 wt%, Table 1. The polymer was inserted and melted completely in the mixing chamber, and then layered double hydroxides and stearic acid (SA) were loaded into the molten polymer. After completion of mixing, the mixed composite was ejected from the mixing chamber, then cooled and crushed at room temperature. Specimens for testing were prepared in a Model ZWICK/ROELL Press under a pressure of 150 kg/cm², with a temperature of 190°C for both upper and lower platens.

Characterizations

The structure of HDPE and HDPE/LDH nanocomposites was identified by Fourier transform infrared (FTIR) spectroscopy and X-Ray diffraction (XRD) techniques. FTIR measurements were recorded by a Shimadzu IR Affinity-1S spectrophotometer on compression molded films in transmission mode by collecting scans at a resolution of 4 cm^− 1 over a spectral range from 4000 to 400 cm^− 1. However, LDHs synthesis and modification were performed on KBr films at the same above conditions. X-Ray diffraction analyses were performed by a Bruker D8 Advance diffractometer with a CuKα radiation (λ = 1.5406 Å) at room temperature, between incidence angles from 5° to 80°. The LDHs dispersion within the polymer and the effect of stearic acid on the nanocomposites morphology were studied using scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM). SEM micrographs were taken from thin films with a JEOL JCM-5000 NeoScope scanning electron microscope. AFM topographic images were recorded using an Axioscop 40 microscope with a frequency of 1 Hz at a scale of 60 µm. The RMS roughness data of the prepared composites was obtained directly from the software of the AFM.

Measurements of the mechanical properties were performed by Zwick/Roell Z100 tensile machine, by ASTM D638 using a crosshead speed of 5 mm/min for Young’s modulus and elongation measurement. The impact strength of the samples was evaluated by Resil Impactor apparatus according to ASTM ISO180 method. Thermogravimetric analysis (TGA) was conducted on a Perkin Elmer TGA 4000 device. Powder samples were placed and heated at a rate of 10°C/min under air or nitrogen atmosphere (100 ml/min) from 25 to 700°C. The rate of weight loss versus temperature was determined using derivative thermogravimetric (DTG) curve and the software associated to the analyzer.

FTIR and DRX Characterizations of LDH and LDH-SDS

The FTIR spectrum of LDHs and modified LDHs is illustrated in Fig. 1. The structure of LDHs shows a sharp peak at 1357 cm^− 1 and a small shoulder at 870 cm^− 1 which are assigned to the antisymmetric stretching mode (ν3) and the bending non-polar mode (ν2) of the CO₃²⁻, respectively [40]. The broad band at 3445 cm^− 1 is attributed to the stretching vibrations of the –OH group of the metal hydroxide, whereas the peak at 1633 cm^− 1 is attributed to the bending vibrations of the –OH group of the interlayer water. The peaks at 555, 680, and 790 cm^− 1 are associated to the stretching vibrations of metal hydroxides M-O and O-M-O (M: Mg or Al) however, the band at 453 cm^− 1 is due to the Al-O associated to the [AlO₆]³⁻. The shoulder at around 3000 cm^− 1 is due to the interaction between carbonate and the interlayer water. The presence of these peaks confirms the successful synthesis of LDHs.

The intercalation of SDS is confirmed by the appearance of new bands at 2852 and 2921 cm^− 1 in the structure of LDH-SDS which are related to the C-H stretching vibrations of –CH₂ and -CH₃ groups resulting from SDS anion and most importantly, the absorption peaks at 1064 and 1218 cm^− 1 are attributed to the symmetrical and asymmetrical stretching vibration of sulphate group, respectively [23, 41, 42].

The diffraction patterns for LDH and LDH-SDS are shown in Fig. 2, revealing basal reflections at 2θ = 11.63°, 23.43°, and 34.84° which are attributed to planes (003), (006), and (009). The planes (015), (018), (110), and (113) are the fingerprints of a crystalline structure type LDH [43]. The XRD patterns of the LDHs modified with anionic surfactant SDS, there was a shift toward lower angles of the (003) basal reflection at 2θ = 6.84° with an increase of the interlayer distance from 7.61 A° to 12.95 A°, calculated by using Bragg’s law: 2d.sinθ = n.λ where θ is the angle of incidence, d is the basal spacing, n is the order of diffraction, and λ value is 0.154 nm.

The inter lamellar spacing for MgAl-LDHs depends on the size of the intercalated anion (CO₃ ^− 2) and its reported value in literature is in good agreement to 0.761 nm which varies between 6.5 A° and 8.0 A° for pure LDH [26, 43–45]. For LDH-SDS basal spacing, Abd El-Fattah and Abd ElKader had registered 11.32 A° which is almost similar to our finding, above (12.95A°) [46]. They have concluded that the rise in d-spacing of the modified clay reveals that SDS had dispersed between the layered double hydroxide sheets, lowering the physical interactions between the clay layers and therefore it is easier for HDPE polymer chain to incorporate through the clay sheets. However, the XRD pattern of LDH-SDS illustrates a lower crystallinity and smaller intensity of basal reflections when compared to pure LDHs.

FTIR and DRX analyses as prepared samples for HDPE and its composites

The FTIR spectrum of r-HDPE (2) as demonstrated in Fig. 3 reveals two strong peaks located at a wave number range of 2937 − 2842 cm^− 1 which is attributed to C-H symmetric and asymmetric stretching vibration. Also, it shows a bending and a rocking vibration of –CH₂ that appeared at 1465 and 723 cm^− 1, respectively. All the four previous bands are the fingerprint and the main peaks present in HDPE chain structure [47]. The small broad band at 3373 cm^− 1 indicates the presence of OH group. Other peaks at 875, 557, and 418 cm^− 1 could be a possible explanation to justify on the existence of additives in the recycled polymer matrix. However, the recycled HDPE matrix has broadened the peak at 1465 cm^− 1 compared to the virgin one.

The impact of filler loading into the recycled polymer matrix is remarkable in almost all composites with the appearance of new peaks at 453, 553, 671, and 775 cm^− 1 which are attributed to the stretching vibrations of metal hydroxides (Mg and Al), r-HDPE/3%LDH (6) in Fig. 5. In addition to the previous peaks, a strong broad band at 3444 cm^− 1 is assigned to the stretching vibration of OH functional groups of LDH. The presence of LDHs peaks confirms its successful intercalation within the recycled polymer matrix. When increasing the weight of LDHs from 3 wt% to 5 wt%, there was a total disappearance of all peaks associated to LDHs in the composite r-HDPE/5%LDH (3) based on recycled high density polyethylene, Fig. 4. This could be a sign to bad compatibility caused by the agglomeration of the mineral clay particles.

The addition of the interface modifier SA to the recycled matrix at 5 wt% loading rate is depicted in composite r-HDPE/5%LDH-1%SA (5), Fig. 4. The characteristic peak of SA shifted from 1703 cm^− 1 to 1714 cm^− 1 and is associated to the carbonyl group C = O which was coupled with the peak at 1633 cm^− 1 due to vibration of OH in LDHs, generating a hydrogen bond between SA and OH groups of LDHs [12]. Meanwhile, SA presents multiple absorption peaks namely a peak at 1465 cm^− 1 which is attributed to the scissoring of -CH₂, a peak at 1301 cm^− 1 which is assigned to the stretching of C-O, and the peaks at 2918 cm^− 1 and 937 cm^− 1 are related to the asymmetric and symmetric stretching vibration of CH₂, respectively [2].

The addition of SDS to the recycled HDPE is clearly illustrated in Fig. 5, composite r-HDPE/3%LDH-SDS (9), showing two peaks at 1087 cm^− 1 and 1153 cm^− 1 which were shifted from 1064 cm^− 1 and 1218 cm^− 1, respectively and are attributed to the stretching vibrations of the sulphate group. In addition, SDS shifted the peaks of CH group from 2960 cm^− 1 to 2966 cm^− 1. Thus, confirms the presence of physical interactions between SDS and recycled HDPE matrix. However, there was a total disappearance of LDHs native peaks as mentioned above in the obtained nanocomposite with the addition of SDS. Therefore, there was a remarkable change occurred with the addition of LDHs/SA compared to LDHs/SDS as nanofillers to the recycled HDPE matrix by showing all the peaks of LDHs with different intensities. As a result, SA exhibited better compatibility than SDS.

These results are confirmed by XRD patterns (Figs. 6 and 7) where LDHs basal reflection (003) peak is observed in composites r-HDPE/3%LDH-1%SA (4), r-HDPE/5%LDH-1%SA (5), r-HDPE/3%LDH (6), and n-HDPE/3%LDH (7) and is absent in composites r-HDPE/5%LDH (3), n-HDPE/3%LDH-SDS (8), and r-HDPE/3%LDH-SDS (9) at around 11.5 degree, indicating that the ideal filler loading weight is 3 wt% for the r-HDPE. The XRD patterns of both neat and recycled HDPE exhibit two main peaks in the region of 21̊ and 24˚ degree (two theta), which are attributed to (110) and (200) crystal planes, respectively [33]. After introducing modified and unmodified LDHs at a content of 3 wt% into pure HDPE matrix, their reflections were shifted to lower values (Table 2) and therefore a bigger increase in d-spacing (4.173 Å) was recorded with SDS at 21.27° (two theta). Similarly, the addition of unmodified LDHs to r-HDPE with the same loading rate has shifted also the reflection to smaller value (21.23°). These findings justify the effectiveness of both SA and SDS on enhancing the dispersion of LDHs (3 wt%) into HDPE matrix. However, the effect of stearic acid on shifting HDPE basal reflections is considerable. The addition of the fatty acid moved the plane (110) from 21.23° to 21.14° (two theta) corresponding to an increase of d-spacing from 4.181 to 4.199 A°. When increasing the filler weight to 5 wt%, SA shifted the previous peak from 21.72° to 21.23° (two theta) corresponding to a soar in d-spacing from 4.088 to 4.188 A°. This shifting phenomenon is probably related to the interaction at the interface between LDHs and HDPE which could be generated by chemical reaction of the SA acting as both interface modifier and surfactant.

The addition of 3wt% of unmodified LDHs into the virgin HDPE increased the d-spacing from 4.06 to 4.125 Å in composite. Further increase of d-spacing (4.173) was registered with the addition of SDS modified LDHs into the neat polymer matrix in the composite n-HDPE/3%LDH-SDS (8). This is to confirm the exfoliation and dispersion behaviour of the modified LDHs into HDPE polymeric matrix. Contrary to the addition of LDH-SDS with 3 wt% loading rate where d-spacing remains constant (21.47˚) in composites r-HDPE (2) and r-HDPE/3%LDH-SDS (9), the addition of SA into the recycled matrix exhibited lower reflections and the best was found with 3%LDH-1%SA loading rate. As a result, SA has prevented the agglomeration of LDH lamellar and improved their dispersion into HDPE matrix at higher filler loading, r-HDPE/5%LDH-1%SA (5) [48]. Moreover, the characteristic (003) pattern of the mineral clay is not detected in n-HDPE/3%LDH-SDS (8) composite although an increase of d-spacing was registered. Most likely is because of the content of SDS was too low to be coated on LDHs surface or the spacing between layers is too large [41]. Botan et al. justified the absence of the mineral clay peaks in the nanocomposite DRX pattern to the complete breakdown of LDHs structure when the polymer chains are intercalated into the interlayer space of the hydrotalcite-like compound [49]. In the research of Khanal et al. the absence of LDHs basal reflection was ascribed to the agglomeration of its sheets when increasing the filling weight which is in agreement with our findings in r-HDPE/5%LDH (3) composite, Fig. 6 [30]. This behaviour decreased the intensity of the composite peaks and lowered its crystallinity.

Table 2

XRD of all recycled and neat HDPE based nanocomposites
Composition	2 Theta (degree) of (110)	d spacing (A°)
n-HDPE (1)	21.87	4.06
n-HDPE/3%LDH (7)	21.52	4.125
n-HDPE/3% LDH-SDS (8)	21.27	4.173
r-HDPE (2)	21.47	4.135
r-HDPE/3% LDH (6)	21.23	4.181
r-HDPE/3%LDH-1%SA (4)	21.14	4.199
r-HDPE/3% LDH-SDS (9)	21.47	4.135
r-HDPE/5%LDH (3)	21.72	4.088
r-HDPE/5%LDH-1%AS (5)	21.23	4.188

Morphology by AFM and SEM

The morphology and surface properties of r-HDPE/LDHs nanocomposites are investigated by atomic force microscopy. The taken images are illustrated in Fig. 8 where the dispersion of the filler in the recycled matrix is in agreement with the above discussed FTIR and XRD results. From the Table 3, there was a decrease in all RMS values related to the addition of LDHs into recycled HDPE matrix and the smallest one was registered with 3 wt% filler loading rate at 106 nm in r-HDPE/3%LDH (6) composite. The addition of 3wt% of unmodified LDHs to the neat polymer matrix also decreased the Roughness data but the RMS value increased when the nanocomposite was filled with SDS modified LDHs. The effect of 1 wt% of stearic acid on surface treatment is clearly demonstrated by enhancing the dispersion of the nano clays into r-HDPE and preventing their aggregations when the weight is increased from 3 to 5 wt% in the composite r-HDPE/5%LDH-1%SA (5) compared to the pristine composite r-HDPE/5%LDH (3) without the addition of the fatty acid, decreasing the roughness value for almost 38 nm within these formulations. On the other hand, the addition of SDS intercalated LDHs presented the highest RMS value (475 nm) among all nanocomposites based on recycled high density polyethylene matrix, r-HDPE/3%LDH-SDS (9) although it did not enhance the roughness data in the composite n-HDPE/3%LDH-SDS (8). Thus, stearic acid helped LDHs particles to be dispersed much better than SDS did. Benabid et al. explained that the homogenious dispersion is observed when the filler particles are coated by stearic acid in its molten state [50]. According to Salmi et al. and Mohammedi et al., this behavior is controlled by the nature of the interface modifier SA which presented interactions between the composite components confirming its intercalation between LDHs layers [12, 51]. Moreover, these findings justify the presence of a monolayer at the surface in presence of SDS.

Table 3

Surface roughness values of HDPE/LDHs composites
Material	Average surface roughness (nm)	RMS (nm)
n-HDPE (1)	163.439	228.864
r-HDPE (2)	609.304	790.282
r-HDPE/5%LDH (3)	124.735	161.421
r-HDPE/3%LDH-1%SA (4)	139.811	181.608
r-HDPE/5%LDH-1%SA (5)	96.814	123.559
r-HDPE/3%LDH (6)	78.537	106.371
n-HDPE/3%LDH (7)	184.771	215.827
n-HDPE/3%LDH-SDS (8)	217.906	298.626
r-HDPE/3%LDH-SDS (9)	345.878	475.772

These results are also confirmed with scanning electron microscopy (SEM) images in Fig. 9 where a homogeneous dispersion of the layered double hydroxides in the recycled high density polyethylene polymer matrix was observed with the addition of stearic acid and the best one was obtained within r-HDPE/5%LDH-1%SA (5) composite. Nguyen at al. found that the surface treatment of bentonite and silica with stearic acid improved the adhesion between the filler particles and the polyethylene matrix [2]. The effect of the fatty acid on surface tratment was related to its binding behaviour which reduced the agglomeration of filler particles.

Mechanical properties

The mechanical properties results of neat and recycled HDPE filled by organically modified and unmodified LDH nanocomposites are presented in Table 4. The obtained results have helped us to better evaluate the type of interactions between the polymer matrix and the filler, generating changes in tensile strength, elongation at break, impact strength, and elastic modulus values. From the Table 4, the elongation at break and tensile strength increased when virgin and recycled HDPE are filled with 3 wt% of LDH. This confirms the nanoscale dispersion of unmodified LDHs into the polymer matrix. By increasing the filling rate to 5 wt%, the latter decreased due to the agglomeration of the mineral clay particles. However, at the same loading rate a remarkable improvement was obtained when the stearic acid (SA) is used as modifier agent and the best was found within r-HDPE/5%LDH-1%SA (5). SA has demonstrated to be an essential ingredient to tackle the higher filler content dispersion within recycled polymer matrix. This finding was reported in the work of Salmi et al. with a different system where the fatty acid is used as interface modifier between the SiO₂ filler and polymer matrix [12]. According to their results there were enhancements in elongation at break and impact strength due to the good surface interaction of the filler with stearic acid. These findings are also in agreement with the work of Nguyen et al. when treating the fillers with stearic acid, the mechanical properties of the polyethylene films were enhanced considerably due to the lubricant behaviour of the fatty acid [2]. On the other hand, in our study, the elongation at break and tensile strength plummeted when LDH was organically modified by SDS and then mixed with HDPE polymer matrix (r-HDPE-3%LDH-SDS). This was attributed to the lower physical interactions between the mineral clay particles and the polymer matrix due to the higher distance between LDH sheets caused by the intercalation of the organic surfactant SDS [46]. Whereas, the addition of 3 wt% of LDH-SDS to the recycled high density polyethylene matrix in the composite namely r-HDPE/3%LDH-SDS has improved the impact strength by 20%. As a result, SDS plays the role of a monolayer which absorbs external forces and therefore affects directly the enhancement of the izod impact strength.

Table 4

Mechanical properties of HDPE/Clay nanocomposites
Sample name	Tensile strength at break, (MPa)	Young’s modulus, (MPa)	Elongation at break, (%)	Impact strength a_K _(KJ/m²₎
n-HDPE (1)	22.43 ± 1.71	1220.35 ± 50.38	411.97 ± 18.5	65.23 ± 3.16
r-HDPE (2)	14.33 ± 3.49	308.72 ± 28.91	427.46 ± 38.37	58.55 ± 8.87
n-HDPE/ (3% LDH) (7)	25.16 ± 2.4	1392.48 ± 20.51	418.4 ± 52	19.1 ± 4.3
r-HDPE/ (3% LDH) (6)	17.8 ± 2.54	355.85 ± 55	455.93 ± 99.65	57.07 ± 13.46
r-HDPE/ (3% LDH-1% SA) (4)	15.3 ± 1.37	341.97 ± 29.23	439.08 ± 48.01	47.35 ± 6.83
r-HDPE/ (5% LDH) (3)	12.56 ± 1.1	364.43 ± 8.06	346.64 ± 38.37	44.36 ± 3.5
r-HDPE/ (5% LDH-1% SA) (5)	18.65 ± 0.49	356.18 ± 8.78	538.71 ± 12.57	44.42 ± 5.09
n-HDPE/ (3% LDH-SDS) (8)	22.3 ± 0.72	1084.81 ± 9.47	139.12 ± 14.36	53.7 ± 2.18
r-HDPE/ (3% LDH-SDS) (9)	13.76 ± 1.15	330.19 ± 35.37	392.53 ± 34.92	70.14 ± 4.03

Moreover, results showed that the soar of filler content directly leads to the increase of Young Modulus with all composites based on recycled HDPE contrary to those built with virgin HDPE, presenting higher elasticity for composite films due to their compact structure. However, both recycled and virgin HDPE based composites with unmodified LDH showed a negative impact resistance trend at loading rates 3 wt% and 5 wt%. The lowest value was registered at n-HDPE3%LDH with threefold decrease from the virgin one but at r-HDPE-3%LDH (6), the impact strength declined slightly. This concludes that the intercalation of clays decreased the ductility of the polymer films, leading to breakable composites under filling weight.

Contrary to the elongation at break, the izod impact strength values were kept almost steady with the addition of stearic acid into the recycled polymer formulations at both 3 and 5 wt%. In contrast, the incorporation of 3 wt% LDH-SDS into r-HDPE considerably improved the impact strength by 20% that was the highest among all registered values. The reinforcing efficiency of the layered double hydroxides as fillers in high density polyethylene polymer nanocomposites was dependant on its loading rate which should not exceed 3%wt in order to avoid clays aggregation, enhance their dispersion and to obtain the optimum mechanical properties such as tensile strength, Young’s modulus, and elongation at break.

Thermo gravimetric Analyses (TGA)

The TGA analyses were performed in an inert nitrogen atmosphere employing a thermogravimetric analyser. As discussed in the literature, TGA among the most commonly used methods to evaluate the thermal behaviour of the polymer nanocomposites. Curves of all samples are illustrated in Figs. 10–12 and thermal properties are summarized in Table 5. Previous investigations found that the introduction of clay fillers into the polymer matrix increased the thermal stability [30, 52].

All TGA measurements have a similar trend of thermal stability curves. From the Table 5, the incorporation of 3 wt% of unmodified LDHs did not improve remarkably the T_50% (the temperature at which 50% mass loss occurs) and T_max (the temperature at the rate of maximum decomposition) weight loss and remained almost steady (1 to 2°C) for both neat and virgin HDPE polymer matrix. Further increase in filler content to 5 wt% enhanced the thermal stability at T_max by 4°C for r-HDPE, leading to an increase of residue at high temperature (500°C-600°C) due to the complete collapse of clay structure and the formation of metal oxides in the endothermic decomposition of LDHs layers, thus delaying the final decomposition of the nanocomposites, Figs. 10 and 11 [39]. The addition of 1 wt% of SA to 3 wt% and 5 wt% of clay improved the T_max by 5°C and 2°C, respectively due to the nano-dispersion of filler in the matrix. The present findings are in agreement with the above mechanical, morphological, XRD, and FTIR results.

In comparison with the addition of the fatty acid, the intercalation of SDS within LDHs reduced the T_50% by 8°C to 477°C when using recycled HDPE polymer matrix but did not improve the maximum decomposition temperature (T_max) and remained unchanged of the composites containing both recycled and pure HDPE.

Table 5

Thermogravimetric analyses of (n-r) HDPE/LDH nanocomposites
Samples	T_1%	T_5%	T_20%	T_50%	T_max	Weight loss (%)	Residual weight
n-HDPE (1)	353	455	478	489	491	55.81	3.14
r-HDPE (2)	399	445	470	485	485	50	2.71
n-HDPE/ (3% LDH) (7)	251	450	477	490	492	55.92	2.95
r-HDPE/ (3% LDH) (6)	287	445	473	486	487	50	2.78
r-HDPE/ (3% LDH-1% SA) (4)	186	404	465	484	490	64.59	2.58
r-HDPE/ (5% LDH) (3)	288	431	469	486	489	56.47	2.48
r-HDPE/ (5% LDH-1% SA) (5)	236	436	472	487	487	50	2.81
n-HDPE/ (3% LDH-SDS) (8)	249	455	480	491	491	50	3.21
r-HDPE/ (3% LDH-SDS) (9)	161	387	451	477	485	66.95	2.22

Moreover, TGA showed also an early decomposition for all nanocomposites containing LDHs based filler with different loading rate compared to the pristine HDPE, leading to a decreasing in initial decomposition temperature of the nanocomposites. This drop in T_1% which is the loss of 1% from the original sample weight is attributed to the dehydration of LDHs (25°C-120°C) and the loss of lamellar hydroxyls (200°C-370°C) water molecules and the poor resistance of the organic surfactants at the lower temperature [23, 46, 53]. The effect of SDS intercalated LDH on recycled HDPE matrix considerably plummeted the thermal stability of the composite (Fig. 12).

The successful synthesis of LDHs and SDS-LDHs was first confirmed by structure analyses and then the nanofillers were mixed into recycled or virgin high density polyethylene matrix by melting at different weight ratios to form thermoplastic nancocomposites with the addition of SDS as surfactant and SA as an interface modifier. The properties of the obtained nanocomposites were further investigated by a variety of techniques such as FTIR, XRD, SEM, and AFM. Results proved that 3 wt% of unmodified LDHs enhanced all mechanical properties of the obtained nanocomposites based on r-HDPE and n-HDPE matrix. However, the impact of inorganic LDHs on the recycled HDPE showed greater improvement than on the neat one. The effectiveness of choosing SA as an interface modifier was confirmed by morphological and structure analyses, improving the dispersion of LDHs particles and reducing their aggregation into the polymeric matrix. AFM topographic images illustrated that the addition of 1 wt% of SA decreased the roughness data of the obtained composites surfaces. This binding behaviour has effectively affected the mechanical properties of the composites where an enhancement of Young’s modulus at 3% and 5 wt% of unmodified LDHs within the recycled HDPE was revealed. The combination of 1 wt% SA with 5 wt% of LDHs loading into composites based on recycled HDPE increased all mechanical properties (tensile strength by 30%, Young’s modulus by 15%, elongation at break by 26%) and kept the impact strength values steady. Whereas, composites with 3 wt% of LDHs loading showed that stearic acid has less improved all above mechanical properties. On the other hand, the presence of physical interactions in r-HDPE/3%LDH-SDS (9) revealed that SDS acted as a monolayer which has enhanced the impact strength by 23% as compared to unmodified LDHs in the composite r-HDPE/3%LDH (6). As a result, inorganic fillers such as LDHs with 3 wt% loading rate can be used as good reinforcing materials and the composite r-HDPE/3% LDH-1% SA (4) improved the thermal stability by 5°C. The interfacial modification by stearic acid with 1 wt% loading rate improved the mechanical properties of the final nanocomposites based on recycled high density polyethylene but the mechanism of adhesion still unclear. The existence of physical interactions between all composite components with the addition of SDS and SA need to be discussed in detail for future work especially with other recycled polymers and a variety of nanofillers at different weight ratios. These findings reinforce recent studies which demonstrated that surface modification techniques have improved the physical and mechanical properties of recycled polymers to fulfil the requirement for various applications by industrials and enhance the quality of recycled plastic polymers.

Acknowledgements

This research was supported by the General Directorate for Scientific Research and Technological Development (DGRSDT), Algerian Ministry of Scientific Research, Emerging Materials Research Unit (URME), Ferhat ABBAS University of Setif-1.

CRediT author contributions statement

DERRADJ Mourad: Investigation, Formal analysis, Methodology, Resources, Writing – Original Draft.

ZOUKRAMI Fouzia: Conceptualization, Methodology, Supervision, Validation.

GUERBAA Hadjira: Resources, Supervision. BOUCHAOUI Abdelmounaim: Investigation

The authors declare that they have no known conflicts of interest or personal relationships that could have appeared to influence the work reported in this paper.

Data and code availability

The data that support the findings of this research are available from the corresponding author, M. DERRADJ, upon reasonable request.

Supplementary information

Not Applicable

Ethical approval

Not Applicable

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The effect of laboratory synthesized and modified layered double hydroxides on recycled and neat high density polyethylene matrix properties using stearic acid as an interface modifier

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials and methods

Materials

Synthesis of Mg₂Al-CO₃-LDH / Mg₂Al-SDS-LDH

Nanocomposites preparations

Characterizations

Results and discussions

FTIR and DRX Characterizations of LDH and LDH-SDS

FTIR and DRX analyses as prepared samples for HDPE and its composites

Morphology by AFM and SEM

Mechanical properties

Thermo gravimetric Analyses (TGA)

Conclusion and perspectives

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

Samples	r-HDPE (Wt %)	n-HDPE (Wt %)	LDHs (Wt %)	LDH-SDS (Wt %)	SA (Wt %)
1	-	100	-	-	-
2	100	-	-	-	-
3	95	-	5	-	-
4	96	-	3	-	1
5	94	-	5	-	1
6	97	-	3	-	-
7	-	97	3	-	-
8	-	97	-	3	-
9	97	-	-	3	-

Samples	r-HDPE (Wt %)	n-HDPE (Wt %)	LDHs (Wt %)	LDH-SDS (Wt %)	SA (Wt %)
1	-	100	-	-	-
2	100	-	-	-	-
3	95	-	5	-	-
4	96	-	3	-	1
5	94	-	5	-	1
6	97	-	3	-	-
7	-	97	3	-	-
8	-	97	-	3	-
9	97	-	-	3	-

The effect of laboratory synthesized and modified layered double hydroxides on recycled and neat high density polyethylene matrix properties using stearic acid as an interface modifier

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials and methods

Materials

Synthesis of Mg2Al-CO3-LDH / Mg2Al-SDS-LDH

Nanocomposites preparations

Characterizations

Results and discussions

FTIR and DRX Characterizations of LDH and LDH-SDS

FTIR and DRX analyses as prepared samples for HDPE and its composites

Morphology by AFM and SEM

Mechanical properties

Thermo gravimetric Analyses (TGA)

Conclusion and perspectives

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

Synthesis of Mg₂Al-CO₃-LDH / Mg₂Al-SDS-LDH

Samples	r-HDPE (Wt %)	n-HDPE (Wt %)	LDHs (Wt %)	LDH-SDS (Wt %)	SA (Wt %)
1	-	100	-	-	-
2	100	-	-	-	-
3	95	-	5	-	-
4	96	-	3	-	1
5	94	-	5	-	1
6	97	-	3	-	-
7	-	97	3	-	-
8	-	97	-	3	-
9	97	-	-	3	-