Synergistic Effects of Mixed Metal Stearate, Calcium Carbonate Particles and Recycled Low Density Polyethylene on The Mechanical, Thermal and Structural Performance of Polyvinylchloride Blend.

doi:10.21203/rs.3.rs-1352930/v1

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Synergistic Effects of Mixed Metal Stearate, Calcium Carbonate Particles and Recycled Low Density Polyethylene on The Mechanical, Thermal and Structural Performance of Polyvinylchloride Blend.

https://doi.org/10.21203/rs.3.rs-1352930/v1

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Thermo-mechanical recycling process is the cheapest way to recover plastic wastes such as LDPE with lower ecological impact, thus, the target in this work is to achieve high performance microcomposites prepared from polyvinyl chloride (PVC), recycled low density polyethylene (r-LDPE), calcium carbonate (CaCO₃) and Calcium/Zinc stearate (CaSt₂/ZnSt₂). The effects of two ratios of thermal stabilizers with different concentrations, on the mechanical properties and thermal stability of PVC and PVC/r-LDPE (1:1) blend were studied. The samples were characterized using infrared spectroscopy (FTIR), mechanical tests, thermal analysis and scanning electron microscopy (SEM). The addition of 5 phr of CaSt₂:ZnSt₂ = 9:1 into PVC (MC4) seems to produce an optimum tensile strength and elongation at break values. In addition, it is highlighted that MC4 showed a high thermal stability. Moreover, the incorporation of r-LDPE into PVC makes the PVC matrix stronger and more stable than pure PVC which yields to high mechanical and thermal performances. Furthermore, an outstanding synergistic effect can be showed when heat stabilizer rich in calcium combined with CaCO₃ and r-LDPE. This PVC/r-LDPE blend as waste composite can be used in several industry fields. Finally, we used DFT calculation to elucidate the dehydrochlorination mechanism of PVC in presence of Ca and Zn stearate.

PVC

r-LDPE

Thermal stabilizer

CaCO3

Density functional theory

The Novel industry technology influences positively on the performance of composite materials in the economic sector, especially thermoplastic polymers, blends and their composites [1]. For fast economic development and significant protection of the environment, recycled plastic waste technology is known as a clean energy source and plays a very important role in solid waste disposal [2, 3]. Agricultural plastic wastes (APW) is intrinsically difficult to recover and recycle in Algeria because the absence of APW systems and infrastructures [4, 5]. Polyvinylchloride (PVC) is one of the most important thermoplastic polymers used in industrial products, such as pipes, cables, food product container, construction applications, medical and electronic devices, due to high mechanical properties [6–8]. Lower thermal stabilization was observed for the PVC compared with other thermoplastic polymers such as : low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET) due to dehydrochlorination reaction of PVC around 100°C [9]. The pyrolysis of PVC characterizes a two interesting steps : (1) initial degradation of PVC due to the first dehydrochlorination of PVC at low temperature, (2) final degradation due to strongly cracking and decomposition of the PVC at higher temperatures [10, 11]. To decrease the pure PVC degradation, it should be mixed with thermal stabilizers to produce plastic with high thermal performance used in several applications [12, 13]. Incorporation of thermal stabilizer into PVC can make the zipper decomposition more difficult at both low and high temperatures, in which the chlorine atom can be absorbed by heat stabilizer [14].The combination of commercial calcium stearate (CaSt₂) and zinc stearate (ZnSt₂) has attracted the attention of several researchers, in which ZnSt₂ can substitute labile chlorine atom of PVC chain forming a strong Lewis acid, ZnCl₂, which in turn can react with HCl to perform dehydrochlorination reaction. On the other side, HCl is absorbed by CaSt₂ to generate CaCl₂ and fatty acid [15]. Moreover, calcium/zinc compounds can be considered as environmentally friendly thermal stabilizers compared with organo tin and lead compounds [16]. It is significant to develop new heat stabilizer of PVC that exhibited synergistic effect with commercial thermal stabilizers CaSt₂/ZnSt₂ [13, 17]. Li et al. found that new mixed Ca/Zn synthesized from tung oil fatty acid improved the thermal proprieties of PVC [18]. Moreover, Wang et al. studied the synergistic effect of tung-oil-based Ca/Zn and polyol in stabilizing polyvinylchloride [19]. The results show that the positive synergistic effect can be attributed to the hydroxyl and nitrogen groups of the heat stabilizers. In addition, Asawakosinchai et al. reported the 1,3-dimethyl-6-ami-nouracil (DAU) and eugenol are good thermal stabilizers of PVC compared with other heat stabilizers [20]. Recently, the synergistic effects of traditional heat stabilizer (CaSt₂/ZnSt₂) and Tung-oil-derived imide epoxidized ester (GEABTMI) on the thermal stabilization of PVC were successfully investigated by Wang et al. they found that the imide and epoxy functions of GEABTMI compound can scavenge free radicals and absorb HCl generated from PVC pyrolysis [21]. Li et al. studied the interesting performance of mechanical properties of the PVC composite reinforced with CaCO₃ nano-particles, which these particles considered as good dispersion agents in the PVC polymer system, and it can be an effective approach to resist the migration of plasticizer from the PVC [22]. It is well known that the addition of CaCO₃ fillers increase thermal stability of PVC polymer [23]. The results appeared that the CaCO₃ can absorb HCl to generate CaCl₂, CO₂ and H₂O [24]. Ahmad et al. investigated the synergistic effect of calcium carbonate (CaCO₃)/ layered double hydroxides (LDHs) on the thermal degradation of PVC [25]. Many researchers reported the incorporation of different polymers in PVC induced high thermal and mechanical performance [26–28]. Our previous experimental results revealed that thermal stability of the PVC/LDPE blend was improved significantly by increasing the LDPE loading to values above 50 wt% [29]. To the best of our knowledge there are no reports of degradation behavior of PVC, considering the synergistic effects of mixed metal stabilizers (CaSt₂/ZnSt₂) with different concentrations and different ratios of calcium/zinc stearates, Calcium carbonate (CaCO₃) as micro-filler and recycled low-density polyethylene (r-LDPE). In this paper, two ratios of calcium/zinc stearates metal Stabilizers, CaCO₃ particles and r-LDPE were added into PVC. This study investigates a new microcomposite based on PVC with high mechanical, thermal and morphological performance which can provide sufficient information to reveal the degradation process of PVC. Furthermore, we conducted density functional theory (DFT) calculations to elucidate the role of Ca-Zn in thermal stability of PVC.

Materials

The recycled LDPE Films (melting point: 128°C, Density: 0.9555 g/cm^− 3, and MFI: 0.92 (g/10min,190 ^oC/2.16kg)) used in the current study were collected from agricultural plastic waste (greenhouse) in Biskra, Algeria. An amorphous PVC white powder (4000M, K-value = 67–72) was purchased from the “Enterprise National de Pétrochimie (ENIP)”, Skikda, Algeria. Calcium stearate (CaSt₂, Ca content: 6.6–7.4%), zinc stearate (ZnSt₂, Zn content: 10–12%), and calcium carbonate (CaCO₃, 2500 mesh) were obtained from Nanjing OMYA Fine Chemical Ind. Co. Ltd. (Nanjing China). Bis (2-ethylhexyl) terephthalate (DOP, 98%) was obtained from Shanxi Sanwei Group Co., Ltd. Mixed Metal Stabilizers Calcium / Zinc stearate were prepared in the form of (CaSt₂: ZnSt₂ = 9:1 and CaSt₂: ZnSt₂ = 1:9).

Fabrication of microcomposites

Recycled LDPE was washed with detergent and water. All materials were dried in an oven at 80℃ for 12 hours before blending to remove humidity. PVC and PVC/r-LDPE (1:1) microcomposites using different concentrations of mixed metal stabilizers and calcium carbonate were extruded using the twin screw extruder, type MSH, at a processing temperature of 175°C, screw speed of 50 rpm for 10 min from the feed zone to die zone. All different samples are summarized in the Table 1, the samples obtained after extruding were cooled at room temperature and then it was pressed into a square mold with dimension of 200 x 200 x1mm, using a heated hydraulic press for 7 min at 170°C with 200-bar pressure. After cooling with water system to room temperature, the sample was cut off in altered form by the (Computer numerical control) CNC milling machine, before carrying out different characterizations.

Table 1

Different composition of the PVC and PVC/LDPE microcomposites.
Sample	PVC (phr)	LDPE (phr)	CaCO₃ (phr)	Heat stabilizer ^(a)&(b) (phr)	Plasticizer DOP (phr)
MC0	100	-	20	-	25
MC1	100	-	20	2^a	25
MC2	100	-	20	2^b	25
MC3	100	-	20	5^a	25
MC4	100	-	20	5^b	25
MC5	100	-	20	10^a	25
MC6	100	-	20	10^b	25
MC7	100	100	20	2^a	25
MC8	100	100	20	2^b	25
MC9	100	100	20	5^a	25
MC10	100	100	20	5^b	25
MC11	100	100	20	10^a	25
MC12	100	100	20	10^b	25
^(a)Attribute to the CaSt₂: ZnSt₂ = 1:9 and ^(b) attribute to the CaSt₂: ZnSt₂₌ 9:1 heat stabilizer.

Table 1 Different composition of the PVC and PVC/r-LDPE microcomposites

Characterizatio n

Fourier transform infrared spectroscopy (FTIR)

The structural analysis of the PVC and PVC / r-LDPE microcomposite samples were carried out by Fourier Transform Infrared Spectroscopy Vertex 70v FTIR (Bruker Company, Billerica, MA, USA) coupled with ATR Golden Gate Diamond. The Samples were measured from 4000 to 400 cm^− 1 with a 4 cm ^− 1 spectrum resolution to obtain FTIR spectra.

Mechanical tests

The mechanical tests were conducted on samples with dimension of 40 × 10× 1 mm at ambient conditions, using a mechanical testing machine (Zwick / Roell, ISO 527, Germany) at a crosshead speed of 10 mm/min using ASTM D638. The standard dimension was 40 mm in length, 10 mm in width, and 1mm in thickness. Each tensile sample was performed five times to report the average value.

Heat ageing test. The samples were heated in air oven at 100 ± 1°C for 360 h (15 days), the heat ageing of tensile strength, young’s modulus and elongation-at-break values of the samples was carried out in accordance with ASTM D3045 to evaluate changes in the mechanical properties after ageing.

Investigation of initial thermal stabilization for microcomposites.

Congo Red Test

10.0 g of each PVC sample was put into an airtight tube and immersed in oil bath at 180°C refer to the GB/T 2917.1–2002 standard, furthermore, the PVC samples were controlled by Congo red paper and the thermal stability time was recorded when the color of the paper turned blue. The Congo red test was executed three times and an average value was reported.

Discoloration test

PVC and PVC/r-LDPE samples were cut into sheets having dimensions of 15.0 mm×15.0 mm ×2.0 mm according to the GB/T 9349 − 2002 standard. The samples were moved onto the ceramic plate in temperature-controlled oven (V 50 e, Prolabo) at 180°C, after that the samples were scanned every 10 min using (Epson Perfection V19) to evaluate the PVC discoloration during heating.

Thermogravimetric analysis (TGA and DTG). The TGA and DTG results of the microcomposites were obtained by using SDT Q600 (TGA/DSC simultaneous thermo gravimetric analyzer and differential scanning calorimeter) from TA Instruments under N₂ atmosphere with a heating rate of 10°C/min using 3 to 5 mg of the sample was analyzed. The temperature range was from 25 to 600°C.

Scanning electron microscopy (SEM). The morphology of the microcomposites was examined using scanning electron microscope (SEM) (JEOL JSM 6460LV) with an accelerating voltage of 20 kv. The samples were soaked in liquid nitrogen before fracturing and then were covered with a thin gold layer by sputtering using EDWARDS scan-coat on the surface and the cross-sections.

Calculation details. All the calculations were conducted by using the Gaussian 09 program package [30], we used hybrid functional M06-2X [31], because several theoretical studies of polymer pyrolysis reported that this functional is more viable in comparison with B3LYP [32–34], to reduce the computational cost, a 4-carbon PVC molecule and the polar part of thermal stabilizers CaSt₂ and ZnSt₂, were employed as a model reactants. Geometries of the reactants (R1, R2), transition states (TS1, TS2), and products (P1, P2) were optimized at M06-2X/6–31 + + G(d,p) level of theory. Frequencies were computed for all stationary points and used to compute the free energies at 298 K and 1 bar. Finally, we performed IRC calculation to confirm the connection between products and reactants.

Structure characterization of the PVC and PVC/r-LDPE microcomposites

FTIR tests were conducted to investigate and clarify the modifications in the functional groups of the microcomposites with different heat stabilizer ratios after the heat ageing at 100°C for 168 hours. From the FTIR spectra depicted in Fig. 1 and Fig. 2, the characteristic bands of PVC and PVC/r-LDPE microcomposites are well shown and the assignments of these bands are in line with those provided by several researchers corresponding with composites based on PVC [35–37].

Figure 1 FTIR spectra of PVC microcomposites exposed to thermal ageing for t = 360 h under temperature T = 100°C:(a) OH water group superposed region, (b) CO carbonyl group superposed region

Figure 2 FTIR spectra of PVC/r-LDPE microcomposites exposed to thermal ageing for 360 h under temperature T = 100°C : (a) OH water group region, (b) CO carbonyl group region

The peaks in the region 2940–2960 cm^− 1 correspond to symmetric stretching C-H in the adjacent CH-Cl, but the peak at 2849 cm^− 1 in PVC/r-LDPE microcomposites is attributed to the asymmetric stretching, of C-H in -CH3 groups and in -CH2- groups. The peaks in the range 800–870 cm^− 1 can be attributed to the calcium carbonate group CaCO₃. Moreover, the asymmetric stretching of C-Cl can be related to the peaks in the range of 610–730 cm^− 1. Furthermore, the weak bands around 3620 and 3660 cm^− 1 in Fig. 1a and Fig. 2a, attributed to the hydroxyl group OH stretch of the water phase may result from adsorption of HCl by CaCO₃ particles. A strong peak at 1732 cm^− 1 in Fig. 1b and Fig. 2b is attributed to C = O stretch presented in DOP structure and it is probably due to the oxidation process during heat ageing test. Finally, we can conclude that PVC and PVC/r-LDPE microcomposites with different heat stabilizers concentrations are not significantly affected by thermal ageing at 100°C, as shown in Fig. 1 and Fig. 2. The difference of FTIR peaks between PVC and PVC/r-LDPE micro composites are visible in the superposed intensity and shape of characteristic bands represented above.

Mechanical properties of the PVC and PVC/r-LDPE microcomposites

The Tensile strength, Elongation at break and Young’s modulus results of PVC and PVC/r-LDPE microcomposites before and after ageing are displayed in Fig. 3a and b and Table 2. It showed that pure PVC gets more brittle, as compared with the PVC matrix contained heat stabilizers, CaCO₃, and r-LDPE polymer, which these additives make the material more mechanically stable.

Table 2

The mechanical results before and after heat ageing of PVC and PVC/LDPE microcomposites.
sample	Before ageing			After ageing
	Tensile Strength	Young’s Modulus	Elongation at break	Tensile Strength	Young’s Modulus	Elongation at break
	σ (MPa)	E (MPa)	Ɛ (%)	σ (MPa)	E (MPa)	Ɛ (%)
MC0	15,55 ± 0,8	30,33 ± 1,1	301,02 ± 15,2	12,2 ± 0,5	20,4 ± 1,3	250,5 ± 16,2
MC1	17,38 ± 0,5	40,24 ± 0,9	340,3 ± 20	15,6 ± 1	37,23 ± 0,9	300,09 ± 17,9
MC2	18 ± 0,7	44,23 ± 0,8	344,14 ± 21	17,6 ± 0,6	40,2 ± 1,4	305,76 ± 18,9
MC3	18,5 ± 0,8	56,41 ± 1	351,5 ± 19,6	17,9 ± 0,9	52,7 ± 1,6	320 ± 14
MC4	19,6 ± 1	60,32 ± 1,3	381,62 ± 19	19 ± 0,7	56,4 ± 0,6	360,34 ± 16,5
MC5	16,84 ± 1	36,03 ± 0,9	313,05 ± 22,5	15 ± 1,5	31,1 ± 0,7	287,91 ± 16,5
MC6	16,5 ± 0,4	42,27 ± 1,2	314,11 ± 17	15,9 ± 0,3	39,2 ± 1,2	290,7 ± 16,5
MC7	6,06 ± 0,5	95.14 ± 5,1	11,52 ± 0,6	5,50 ± 0,4	90,26 ± 4,9	10,4 ± 0,5
MC8	5,98 ± 0,3	99.20 ± 4	13,72 ± 0,9	5,60 ± 0,4	96,22 ± 5,7	12,5 ± 0,8
MC9	5,38 ± 0,3	115.33 ± 8,3	17,06 ± 12	4,98 ± 0,2	110 ± 8,6	16,12 ± 1,1
NC10	7.00 ± 0,5	130.19 ± 9	18,11 ± 1,4	6,80 ± 0,3	127,21 ± 10	17,5 ± 1
MC11	3,73 ± 0,2	80.20 ± 7,2	11 ± 0,5	3,63 ± 0,5	75,03 ± 5	10 ± 0,7
MC12	4,61 ± 0,4	65.44 ± 5,5	13,3 ± 11	4,58 ± 0,3	62 ± 3,9	12,6 ± 0,9

Figure 3a shows the variation of tensile behaviors of PVC microcomposites at different heat stabilizers ratios. The mechanical performance is remarkably decreased in the sample without thermal stabilizer MC0 after ageing. In addition, the sample MC4 exhibit better mechanical performance than the pure PVC and other samples, before and after ageing. It revealed that the optimum concentration of mixed stearate CaSt₂: ZnSt₂ = 9 :1 for the highest tensile strength, elongation at break and young’s modulus is 5 phr, this concentration probably leads to the good dispersion of CaSt₂ particles in PVC microcomposite. From a mechanical point of view, we can also see that 2 phr of thermal stabilizers MC1 and MC2 are slightly better than 10 phr, MC5 and MC6 with little favor of heat stabilizer with a high concentration of calcium, due to the interaction between the polar ends of Calcium stearate and the somewhat polar PVC chains [38].

Figure 3 Evolution of the mechanical properties of PVC and PVC/r-LDPE microcomposites before and after heat ageing

Figure 3b represents the influence of r-LDPE on the mechanical properties of the PVC microcomposites with 2, 5 and 10 phr of mixed metal stabilizers. It can be observed that the incorporation of r-LDPE into PVC decreases the tensile strength and elongation at break when it is compared with PVC micro composites. This deterioration is due to the crystalline structure part of the macromolecular chain of r-LDPE polymer and which makes the PVC polymer loses partially its flexibility [39]. On the other hand, Young’s Modulus values are doubly increased after addition of r-LDPE into PVC microcomposites. In addition, MC10 exhibits better mechanical properties than other samples, before and after ageing. In addition, it can be seen that the incorporation of r-LDPE enhances the mechanical stability of the PVC polymer after heat ageing. Hence, during the thermal treatment the PVC and short-chain r-LDPE radicals react to produce r-LDPE-g-PVC copolymers [26]. The mechanical properties of microcomposites based on PVC become higher with increasing the content of CaSt₂.

Table 2 The mechanical results before and after heat ageing of PVC and PVC/r-LDPE microcomposites

Effects of thermal stabilizers and r-LDPE on stabilizing PVC microcomposites

Figure 4a and b shows the results of Thermal stability time (Congo red test) of PVC microcomposites at 180°C and discoloration photos of PVC and PVC/r-LDPE microcomposites at 180°C for 110 min of three different concentrations of thermal stabilizers with (CaSt₂: ZnSt₂ = 9:1 and CaSt₂: ZnSt₂ = 1:9), respectively. As shown in Fig. 4a, the PVC microcomposites MC4 and MC6, when the thermal stabilizer is rich in calcium, it can relatively improve the thermal stability time (t = 160 min) [40, 41]. Whereas, in Fig. 4b thermal stability increased with increasing heat stabilizer concentrations and delayed the discoloration of PVC samples with a significantly resistance to discoloration of PVC/r-LDPE samples MC10 and MC12 which the incorporation of r-LDPE into PVC showed much better antidiscoloration compared with PVC microcomposites. In addition, the samples rich in Zinc stearate MC1, MC3, MC5, MC7, MC9, and MC11 quickly turned to dark color, the reason is that ZnSt₂ can remove initial coloration by substituting labile chlorine atoms from the PVC chain [42]. However, the heat stabilizer rich in calcium stearate increases the PVC stabilization time due to the inhibition of ZnCl₂ which is responsible of dehydrochlorination process [43]. As is known, CaSt₂ could react with ZnCl₂ to regenerate ZnSt₂ and CaCl₂ via ester exchange reaction. On the other side, a detailed computational study is required to understand the dehydrochlorination process in presence of CaSt₂ and ZnSt₂.

Figure 4 (a) Thermal stability time (Congo red test) of PVC microcomposites at 180°C. (b) Discoloration photos of PVC and PVC/r-LDPE microcomposites at 180°C for 110 min

Thermal properties of the PVC and PVC/r-LDPE microcomposites

The thermo-gravimetric curves of PVC and PVC/r-LDPE microcomposite with different metal mixed heat stabilizers ratios are plotted in Fig. 5a-d. The important temperatures and different thermal degradation levels of PVC and PVC/r-LDPE microcomposites are summarized in Table 3. As shown in the TGA graphs, thermal degradation of PVC and PVC/r-LDPE occurred in two major steps.

Table 3

Interested decomposition temperatures and weight loss levels of PVC and PVC/LDPE microcomposites
Sample	Decomposition temperature
	T ^Onset (°C)	T^10% (°C)	First stage			Second stage
	T ^Onset (°C)	T^10% (°C)	Tmax (°C)	Mass loss (%)	T_range /(°C)	Tmax (°C)	Mass loss (%)	T_range /(°C)
MC1	277.28	285.36	399.54	54.07	277–343	468.30	66.50	442–490
MC2	278.72	286.43	301.22	5 3.10	278–350	464.63	67.50	440–493
MC3	281.13	284.10	288.00	54.00	281–335	474.49	66.30	441–494
MC4	285.55	291.29	302.91	51.09	286–355	473.75	64.21	444–498
MC5	283.30	292.24	296.23	52.51	283–330	470.27	63.00	445–502
MC6	284.61	295.56	300.32	52.23	284–339	470.02	62.10	446–501
MC7	283.52	289.83	290.20	31.00	283–315	490.6	67.40	468–505
MC8	284.48	293.53	305.80	34.00	285–317	491.01	70.50	463–508
MC9	281.00	286.72	290.03	31.45	281–323	490.55	64.60	470–509
MC10	290.20	297.72	304.82	31.35	290–325	492.51	63.50	474–511
MC11	277.22	285.44	292.65	31.90	276–322	490.06	68.55	471–508
MC12	286.42	296.06	300.19	31.50	286–324	492.23	64.85	472–510

Figure 5a and b shows the first degradation of PVC begins around 277°C with remarkably lose weight which is attributed to the dehydrochlorination of PVC and the second degradation is related to the scission of polyene sequences [10, 44].The onset degradation temperatures of PVC microcomposites are in the range of 276–290˚C. It is observed that the samples MC1 and MC2 have lower decomposition temperatures as compared to any other PVC microcomposite, due to a small amount of heat stabilizer incorporated in the PVC matrix, moreover, as long as the heat stabilizer is increased, the PVC degradation is delayed significantly in the MC4, MC5 and MC6 samples, in which MC4 sample shifts to higher values. As expected, the thermal stability of samples containing high calcium concentration are much better than samples with high zinc concentration into mixed metal stabilizer, due to the ability of CaSt₂ to absorb more HCl which indeed leads to much less the dehydrochlorination, and more stability of PVC microcomposites [45].

Figure 5 (a, b) TGA and DTG curves of PVC and (c, d) TGA and DTG curves of PVC/r-LDPE microcomposites

Figure 5c and d represented the influencing of r-LDPE on the thermal degradation of PVC, which plays the same role as heat stabilizer. Similarly to PVC degradation, it can be seen that the optimum concentration of mixed stearate CaSt₂ : ZnSt₂ = 9 :1 for the degradation of PVC/r-LDPE microcomposite is 5 phr, in addition, it can be concluded that the incorporation of r-LDPE into PVC enhances the values of onset degradation temperatures up to 290°C and retards the degradation process [36], accordingly to the mechanism which was proposed by Thongpin et al. [46], and Sombatsompop et al. [26], in particular through the starting of the initiation co-cross-linking process which results in macro-radical recombination reactions which lead to producing more short chains, PVC grafted with r-LDPE at high temperatures.

Table 3 Interested decomposition temperatures and weight loss levels of PVC and PVC/r-LDPE microcomposites

Fracture surface morphology of the PVC and PVC/r-LDPE microcomposites

Figure 6 presents SEM micrographs for fracture surfaces of PVC and PVC/r-LDPE with CaCO₃ particles and CaSt₂: ZnSt₂ = 9:1. Figure 6a and c shows that the hydrophilic CaCO₃ micro-particles were highly aggregated in the PVC matrix, with several voids present, leading to a decrease in the interfacial adhesion between CaCO₃ particles and PVC matrix [47]. Figure 6b and d reveals a good compatibility between CaCO₃ microparticles and PVC/r-LDPE mixtures compared with PVC matrix, due to the well distribution of CaCO₃ in the blend. As the PVC was blended with r-LDPE, the CaCO₃ were well dispersed in the PVC/r-LDPE blend, which led to a strong interfacial interaction between PVC/r-LDPE and CaCO₃ microparticles [48]. These findings are in accordance with the mechanical behaviors of microcomposites.

Figure 6 SEM images of the fracture surfaces of (a,c) MC4 and (b,d) MC10 microcomposites

DFT calculation results

Geometries optimization of reactants, transitions state and products were depicted in Fig. 7a. PVC polymer, may be regarded as polar, interacted with the polar part of thermal stabilizers. The strong hydrogen bond between H of PVC and O of Cast₂ and Znst₂ is 2.29 and 2.35 Å, respectively, reveals the stability of reactants. In addition, Ca and Zn formed electrostatic bond with Cl by 2.87 and 2.49 Å, respectively. The simultaneous transfer of the chlorine and the hydrogen to the thermal stabilizers leads to the formation of unsaturated groups in PVC and takes place via TS1 and TS2 which lies 29.6 and 25.0 kcal/mol, respectively, above reactants. The free energy profiles of the dehydrochlorination process are displayed in Fig. 7b, we found that dehydrochlorination process can occur via a concerted mechanism with four- member ring transition state and the free activation energy with ZnSt₂ is smaller than that with CaSt₂. Our DFT results are consistent with previous theoretical and experimental results [45, 49].

Figure 7 (a) Optimized geometries of reactants, transition states and products for the thermal dehydrochlorination of PVC with different thermal stabilizers. Distance given in A°.Atom color code: H in white, C in grey, Cl in green, O in red, Ca in yellow and Zn in blue and (b) Relative free energies of the dehydrochlorination process with (blue) CaSt₂ and (red) ZnSt₂ at the M06-2X/6–31 + + G(p,d) level

This work highlights the importance of different additives, such as mixed metal calcium/zinc stearate, CaCO₃ particles, and r-LDPE with their positive effects to enhancing the polyvinylchloride (PVC) properties. The CaSt₂/ZnSt₂ stearate as heat stabilizer does not affect alone on PVC degradation and mechanical properties, but the addition of r-LDPE in the PVC helps to upgrade the performance of the latter to be used in several fields. The optimum ratio of (CaSt₂: ZnSt₂ = 9:1) was 5phr which can yield good thermal stability and high mechanical performance of PVC microcomposite before and after the heat ageing test. r-LDPE as a thermoplastic polymer, with 50 phr content, exhibited the best young modulus before and after heat ageing, and confirmed the ability to be a highly effective compound to protecting the thermal stability of the PVC. Thermal analysis revealed the excellent synergistic effects of CaCO₃, heat stabilizer rich in calcium and r-LDPE for more thermal stability of PVC polymer.

SEM micrographs show that the CaCO₃ microparticles is well dispersed in the PVC/r-LDPE blend, which leads to a strong interfacial interaction between PVC and r-LDPE. Finally, DFT calculation revealed that dehydrochlorination process can occur by the simultaneous transfer of the chlorine and the hydrogen to the thermal stabilizers via a concerted mechanism with four- member ring transition state with a low free energy barrier in favor to ZnSt₂. This can help polymerists to design new thermal stabilizers for PVC.

Acknowledgements

The authors are grateful to the staff of plastic laboratory of (Entreprise des Industries du Câble ENICAB), Biskra, Algeria, for suppling the PVC polymer and their additives of this work, the (Centre de Recherche Scientifique & Technique en Analyse Physico-Chimiques CRAPC), Tipaza, Algeria, for thermal analysis. The authors are also thankful acknowledgments to Dr. Antoine Kervoelen and Anthony Magueresse for supporting this experimental work by the characterization methods.

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Synergistic Effects of Mixed Metal Stearate, Calcium Carbonate Particles and Recycled Low Density Polyethylene on The Mechanical, Thermal and Structural Performance of Polyvinylchloride Blend.

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Abstract

Figures

Introduction

Experimental

Materials

Fabrication of microcomposites

Fourier transform infrared spectroscopy (FTIR)

Mechanical tests

Congo Red Test

Discoloration test

Results And Discussion

Structure characterization of the PVC and PVC/r-LDPE microcomposites

Mechanical properties of the PVC and PVC/r-LDPE microcomposites

Effects of thermal stabilizers and r-LDPE on stabilizing PVC microcomposites

Thermal properties of the PVC and PVC/r-LDPE microcomposites

Fracture surface morphology of the PVC and PVC/r-LDPE microcomposites

DFT calculation results

Conclusions

Declarations

Acknowledgements

References

Supplementary Files

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