Effect of reprocessing on the service life properties of glass fiber-reinforced in-house polypropylene waste composites

Abstract

Reprocessing of synthetic fiber-reinforced thermoplastics for reuse in the design of structural components applications is important owing to their non-biodegradability. This study evaluates the effect of reprocessing on the service life properties of in-house polypropylene wastes reinforced with E-glass fiber. Composite materials containing 10 mm length glass fiber (GF) and recycled polypropylene were manufactured with 10 wt% GF and subjected to five reprocessing cycles via extrusion and compression molding under similar conditions. Properties such as mechanical, melt flow index (MFI), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis were used to analyze the effect of reprocessing on the composites. The results obtained revealed that the composites’ properties diminished with successive reprocessing. The thermal study showed reductions in the thermal stabilities with a drop in onset degradation temperature from 338.07 to 245.15 °C after five runs, indicating an increase in the polymer chain molecular movement and less rigidity due to shorter chains provided by X-ray diffraction analysis with reduced peak intensities, average crystallite sizes, and degree of crystallinities but no destruction of the main functional groups was observed with the spectral studies. An increase in the MFI (6 g/10–17.88 g/10 min) indicated the possible loss of complex viscosity and damage in molecular weight for the reprocessed matrix. Reinforced in-house PP plastics can, therefore, be reprocessed twice only for the same product except with the addition of either virgin PP or other materials to compensate for the lost properties and help manufacturers save costs.

Graphical Abstract

Keywords:

• In-house waste plastics
• glass fiber-reinforced plastics
• plastic reprocessing

1. Introduction

The global use of thermoplastics is estimated at 85% of all consumed plastics [1]. This has necessitated their reprocessing in addition to increasing consumer demand, environmental awareness (non-biodegradable), cost of raw materials, and oil prices [2]. To improve stiffness and strength properties, polypropylene thermoplastics are usually reinforced with short glass fibers (GF) [3,4]. Thermoplastics have the advantage over thermosets being that they can be reprocessed by the application of heat after their service life or during manufacture, these have helped to reduce the problem of disposing of them in landfills [5–7].

With the risks and properties associated with fiberglass, its environmental hazards have become a serious concern; they remain intact for several years after deposition in landfills harming communities and workers [8]. The continuous production of glass fiber-reinforced PP without adequate recycling practices can release volatile organic compounds into the environment. Guidelines by Environmental Protection Agency (EPA) have achieved a reduction in air pollution through the regulation of Fiberglass production [9]. In an attempt to tackle this environmental challenge, different reprocessing techniques for recycling these used plastics have been developed [10,11]. Among them, mechanical recycling is more favored in terms of environmental and economic reasons because it involves the mechanical crushing and reprocessing of such used plastics to obtain new ones [12,13]. Recycling reduces environmental pollution and overdependence on new PP products. This approach is adopted because it is a valuable and economic practice, saving 20–50% in the market cost when compared with its virgin counterpart [14].

However, recycling or reprocessing of end-of-life plastics into new structural parts comes with its challenges, it leads to the loss of materials’ properties: discolorations and reductions in thermal and mechanical properties are part of the common degradation problems associated with the composites’ constituents due to extensive grinding and high-temperature mixing steps during reprocessing [15]. The extent of these degradation changes is still not fully known because minor changes or reduction in composites properties after reprocessing have been reported [16] others reported no changes after reprocessing [17,18], some registered improvements in mechanical properties which is dependent on the reinforcing fiber and the number of times those used plastics are reprocessed [19–22]. Thus, the property integrity of the reprocessed composites needs to be well determined if they are to be used for the same or a new product.

Several scholars have reported the effect of reprocessing on composites’ properties. For instance, Beg and Pickering [23] after carrying out a reprocessing study on wood fiber-reinforced polypropylene eight (8) times reported a linear reduction in the tensile modulus and strength from 4553 MPa and 41 MPa to 3800 MPa and 31 Mpa, respectively. The strain at the break however increased with additional reprocessing which made them conclude that the changes in mechanical properties were caused by the fiber fracture which took place during the reprocessing. A study on glass fiber-reinforced virgin polypropylene by Rafiq et al. [24] which was reprocessed four (4) times showed that the tensile properties were severely affected but the glass transition temperature (T_g) remained unaffected. The severe drop in mechanical properties was more pronounced in the third and fourth recycled composites compared to the first and second generations. They ascribed the decrease to changes in the size mix and reduced adhesion between the fibers and the polypropylene matrix but their study did not report the reason for the unaffected thermal properties. The effect of multiple extrusions was also studied by Nadali et al. [25] using wood flour and high-density polyethylene biocomposites. The obtained result revealed a lower modulus of elasticity and bending strength to the control but with increased impact strength. They concluded that fiber fracture and thermo-mechanical degradation may be responsible for the changes in the properties of the biocomposites.

Despite various reported findings on the effect of reprocessing on the performance properties of composites, the majority of the studies are on bio-fibers, plastics from open waste streams, or virgin polypropylene. In-house PP wastes (industrial scraps) reinforced with constant optimized average-length E-glass fibers and subjected to thermo-mechanical treatments are rather limited or not yet fully explored which is the main aim of this study.

In this present work, recycled polypropylene (in-house PP wastes) was collected as a matrix to produce a composite material using 10 wt% of 10 mm average length E-glass fibers as reinforcement. The composite was then shredded into granules and then reprocessed five (5) times by extrusion and compression molding and their properties were studied after each reprocessing cycle. Changes in the mechanical, rheological, structural, morphological, and thermal behaviors were monitored to assess the influence of the different extrusion and compression processes on the material properties. Thus, this will help plastic manufacturers in saving costs, reducing in-house wastes (production scraps) and overdependence on new materials will be greatly reduced by increasing the use of reprocessed materials in new products.

2. Materials and methods

2.1. Materials

Isotactic polypropylene in-house waste containers (industrial scrap) with 0.91 g/cm³ density were supplied by Proton Plastics, Kaduna-Abuja expressway, Niger State, Nigeria. E-Glass fibers (in chopped strand mat form) having a density of 1.9 g/cm³ were supplied by Fiber Glast Developments corporation, Ohio USA.

2.2. Preparation of composites

Before the composite fabrication, the polypropylene in-house waste samples (Figure 1) were crushed into flakes using a shredding machine (Taivhou Changda Machine Factory, China), washed with fresh distilled water for 15 min (to remove dirt and oils) and then dried at 80 °C for 2 h in an air circulating oven to remove residual moisture. Also, the E-glass fibers were unraveled and cut into individual fibers of an average length of 10 mm. Adequate caution was taken during the process of unraveling as fiberglass can result in painful and itchy irritation to the skin.

Figure 1. Polypropylene in-house waste plastics.

2.3. Reprocessing

The extrusion and successive reprocessing cycles of rPP were conducted in a 300 kg Deyang Dedong (Type Y 225 M − 6, China) single screw extruder with an extrusion speed of 50 rpm and 165 °C temperature. The granules with 10 wt.% glass fiber were repeatedly extruded and crushed five (5) times to have a very good understanding of how recycling affects the material’s responses thermo-mechanically. Preheating of the reinforced plastics was done for 5 min before the compression molding (Wenzhou Zhiguang Machine, China) at 165 °C (for all the platens) using a pressure of 2.5 MPa (25 bars) for 5 min to prepare the composite sheets with the dimensions of 240 mm × 240 mm × 4 mm for length, width, and thickness. Afterward, the sheets were compression molded in a cold press at 25 °C for 3 min using a pressure of 2.5 MPa. After each reprocessing cycle, samples were separated for shaping to different American Society for Testing Materials (ASTM) standards for analysis while the remaining materials were shredded and reprocessed all over again as presented in Figure 2. The ensuing samples were identified based on the number of times they are reprocessed as glass fiber-reinforced polypropylene 1 (GFRPP1)_, GFRPP2_, GFRPP3_, GFRPP4_, and GFRPP5. Where the numbers 1–5 denote the number of reprocessing cycles. The composite compositions, identifications, and the number of extrusion cycles are presented in Table 1.

Figure 2. Schematic representation of the recycling process.

Table 1. Composites identification number and the number of extrusion cycles.

Sample code	Fiber type	PP content (wt%)	Fiber content (wt%)	Number of cycle
GFRPP1	Glass fiber	90	10	1
GFRPP2	Glass fiber	90	10	2
GFRPP3	Glass fiber	90	10	3
GFRPP4	Glass fiber	90	10	4
GFRPP5	Glass fiber	90	10	5

2.4. Physical and mechanical characterizations

Tensile analysis was conducted according to ASTM D638-14 [26] using an electronic universal testing machine (Model: WDW-100KN No. 190536, made in China) furnished with a data acquisition system, and 100 KN load-cell to obtain the ultimate tensile strength, elongation at break, and tensile modulus of the material. Five dumb-bell-shaped specimens having 100 mm × 15 mm × 4 mm dimensions and 40 mm gauge length were tested at a cross-head speed of 50 mm/min in a standard laboratory of 23 °C and 50% relative humidity and the average value was recorded.

The hardness test was carried out using micro vickers hardness tester (Model: MV1-PC S/N. 07/2012-1329) according to ASTM E384-2003 [27]. The sample having 30 mm × 30 mm × 3 mm dimensions were placed on the flat surface and the load of 60 kg was maintained at a minimum of 10 to 15 s using a 1/16″ steel ball indenter. The analysis was done three times for each sample and the mean values were obtained.

The three-point flexural test was done to determine the bending properties of the samples with a universal/electronic flexural test machine (model number HD B6I5A-S, China) to determine the flexural modulus and strength according to ASTM D790-2008 [28] (ISO 4049.13) standard procedure with 2 mm/min cross-head speed. Five specimens were prepared for each of the samples. Rectangular-sized specimens cut into dimensions of 4 mm thickness, 30 mm width and 40 mm gauge length were used. The deformations in mm and loads in KN were recorded.

A Charpy toughness test was performed following ASTM D256-10e1 [29] on a pendulum impact tester (Resil Impactor Junior Series, manufactured by Ceast Torino, Italy), with a speed range of 2.9 − 3.7 m/s. A sample with dimensions of 100 mm × 13 mm and 4 mm thickness having 0.5 mm v-notch was used with a 2.75 J impact hammer (expandable to 25 J). The reported impact strength and absorbed energy values are the average results of five test specimens.

2.5 X-ray diffraction (XRD) analysis

This analysis was conducted on the MiniFlex 300+ diffractometer (USA) using the monochromatic Cu-Kα radiation (λ = 1.54059 nm) working under 30 kV and 10 mA. The X-ray patterns were observed in a step scan mode ranging from 5° to 80° in the 2θ angle with the step of 0.02° using a counting time of 12 s per step.

2.6. Fourier transform infrared (FTIR) spectroscopy

Changes in chemical properties due to reprocessing were observed with FT-IR spectroscopy on PP and glass fiber-reinforced PP pellets using ASTM E1252-98 (2013) [30]. The result spectra were recorded using an FTIR spectrometer (Perkin Elmer Spectrum 400, model Perkin Elmer Inc., USA) using a resolution of 4 cm⁻¹ over 20 scans. The scans were done in transmittance mode within a 4000–500 cm⁻¹ range.

2.7. Melt flow index (MFI)

Measurement of the melt flow index was done following ASTM D1238-13 (Procedure A) [31] with Dynisco LMI 4004 melt flow indexer (Malaysia), using an applied weight of 2.16 kg and 230 °C temperature.

2.8. Thermogravimetric analysis

The samples were subjected to thermogravimetric analysis to study how repeated reprocessing affects the composite’s thermal stability and degradation profile. A thermogravimetric analyzer (TGA) (Perkin Elmer TGA4000, Perkin Elmer Inc., USA) was used in this investigation with ASTM E1131-08(2014) [32]. An approximated weight of 13 mg was used for the tests applying a temperature rise of 10 °C/min from 50–850 °C. The analysis was carried out in a liquid nitrogen atmosphere using a 20 mL/min flow rate.

2.9. Morphological characterization

To study how reprocessing affects the damage mechanism of the composites, tensile fractured specimens were viewed using SEM, model JOEL-JSM 7600 F operated at 20 kV and magnifications of 750× and 1000× using ASTM E986-04(2017) [33]. The samples with appropriate sizes were fitted in the specimen compartment and solidly mounted on a holder. Before observation, the samples were made to be electrically conductive by coating them with platinum on their surfaces. This was achieved with a gold-palladium alloy in an argon-inert environment.

3. Results and discussion

3.1. Tensile properties

The tensile strength and modulus of recycled polypropylene at different reprocessing cycles are presented in Figure 3.

Figure 3. Tensile strength and modulus of in-house waste PP composites at different reprocessing cycles.

The result showed that the tensile strength progressively decreased from 35 MPa for the first reprocessed composite (GFRPP1) down to 14.95 MPa for the five times reprocessed composite (GFRPP5) which represents a percentage reduction of 57.29%. The progressive decrease in the tensile strength might be linked to the fiber breakage that happened during the shredding of the composite, as a result of this, the fiber aspect ratio was reduced thereby reducing the strength of the reprocessed composite. Related outcomes were also seen by Dickson et al. [34], Morán et al. [35] and Migneault et al. [36] Also, the decrease may, however, be due to the high processing temperature added with the extreme and aggressive high shear exposure leading to the thermo-mechanical degradation of the matrix, which may have induced chain scission phenomenon and a resulting reduction in the chain length of the polypropylene thereby worsening the tensile strength of the composite as also reported by Navarro et al. [37]

The comparative stiffness or resistance of the reprocessed glass fiber-reinforced polypropylene composite to elastic deformation is also shown in Figure 3. It is observed that the tensile modulus significantly reduced from 602.34 MPa for GFRPP1 to 470.7 MPa for GFRPP5 which is a 21.85% reduction. The reduction in tensile modulus might be due to the fiber fracture that occurred during reprocessing as Beg and Pickering reported [23] or due to thermo-mechanical degradation of the polypropylene matrix as also reported by Achukwu et al. [38] who studied the impact of recycling in-house polypropylene wastes without the addition of reinforcements.

3.2. Elongation at break

This is the strain on test specimens at the break. The higher the value, the tougher and more ductile the material will be. It is seen that the elongation at break (Figure 4) decreased as a result of reprocessing indicating that the composite became more brittle after reprocessing, thus reducing the toughness properties. The first reprocessed had an elongation at a break of 27.44% which reduced to 18.22% after the fifth reprocessing cycle, representing a drop of 34% in value. An important service life property is thus lost after repeated reprocessing encouraging the need to refresh the plastic being processed with either virgin materials or other materials of importance that can help restore the lost property.

Figure 4. Elongation at break of in-house waste PP composites at different reprocessing cycles.

The decreasing elongation at break means that the brittleness of the composite samples might lead to catastrophic failure during use. Jansson et al. [39] who simulated the recycling of polypropylene materials from post-consumer goods reported a drop in the breaking elongation after a successive aging process which reverted more or less to the original value after extrusion.

3.3. Impact strength

Toughness properties are very important for plastic manufacturers; good toughness ensures the preservation of the plastics when exposed to sudden shock and impact. The impact strength of glass fiber-reinforced in-house PP is shown in Figure 5. An impact strength of 12.43 kJ/m² was obtained for the first processed cycle which gradually decreased to 6.42 kJ/m² after five reprocessing cycles. This decrease could be credited to both the fiber’s average length reduction during the extrusion molding and the thermo-mechanical degradation effect.

Figure 5. Impact strength of in-house waste PP at different reprocessing cycles.

Bahlouli et al. [40] reported a drop in impact energy after nine cycles for their virgin and their toughened polypropylene filled with talc. In their study, the impact strength was intensely degraded after the first run at an approximated temperature of −20 °C, after the first run. The obtained value for the first cycle is, however, lower than the impact strength of virgin PP reinforced with glass fiber because of previous thermal stresses. Since impact resistance is a critical factor for consideration when the design of structural parts is made, the reduction of this property may lead to reduced resistance to deformation, crack propagation or even rupture, knowing that the impact strength is a derivation of the impact energy.

3.4. Flexural strength and modulus

The bending properties of glass fiber-reinforced PP plastics composites are presented in Figure 6. Multiple reprocessing reduced the bending strength and modulus from 54 MPa and 1482 MPa for the first cycle to 26.44 MPa and 1424 MPa for the fifth cycle. After the second processing cycle, the plastic composites exhibited reasonable flexural strength at a value of 48.19 MPa. Further reprocessing beyond that resulted in very low flexural strength and modulus which remains unacceptable for a quality product, except with the blending of recycled polypropylene with the virgin PP counterparts, or the addition of other materials that may be useful for the improvement of flexural properties. Colucci et al. [41] reported similar behavior in their mechanical recycling of glass fiber-reinforced polypropylene composites. They established that after a one-time mechanical processing cycle, the flexural strength decreased by 27.5% when compared to the materials that were not recycled. This behavior is similar to what is obtained in this study.

Figure 6. Flexural strength and modulus of in-house waste PP composites at different reprocessing cycles.

Because the flexural stiffness and strength decreased upon reprocessing, the first and second reprocessed samples presented better and stronger composites than all other additionally reprocessed composites. The summary of all the physical and mechanical properties is presented in Table 2 for ease of comparison alongside the deviations from their mean values.

Table 2. Summary of physical and mechanical properties of glass fiber reinforced in-house waste PP reprocessed five times.

Material	Density (g/cm^3)	Hardness (Vickers)	Impact strength	Flexural properties		Tensile properties
			IS (kJ/m^2)	FS (MPa)	FM (MPa)	TS (MPa)	TM (MPa)	EB (%)
GFRPP1	2.1 ± 0.2	34.2 ± 1.8	12.4 ± 1.6	54.1 ± 2.5	1472 ± 28	35.0 ± 3.0	602.3 ± 23	27.4 ± 1.2
GFRPP2	1.9 ± 0.3	28.0 ± 1.2	10.3 ± 0.8	48.2 ± 3.5	1465 ± 20	27.7 ± 2.5	550.4 ± 18	25.4 ± 1.0
GFRPP3	1.9 ± 0.2	22.5 ± 1.4	9.55 ± 0.9	37.4 ± 2.0	1450 ± 16	17.6 ± 1.2	511.7 ± 14	19.6 ± 1.0
GFRPP4	1.8 ± 0.2	20.0 ± 1.1	8.67 ± 0.7	29.7 ± 3.1	1440 ± 22	17.6 ± 0.1	489.7 ± 08	18.8 ± 0.2
GFRPP5	1.8 ± 0.1	13.9 ± 1.2	6.42 ± 1.0	26.4 ± 2.6	1414 ± 18	14.9 ± 2.0	470.7 ± 09	18.2 ± 0.3

IS = Impact Strength, TS = Tensile Strength, TM = Tensile Modulus, EB = Elongation at Break, FS = Flexural strength, and FM = Flexural Modulus.

3.5. Chemical and structural test results

3.5.1 X-ray diffraction (XRD) analysis

The use of X-ray as a crystallographic technique is well presented in Figure 7 and Table 3. It shows the X-ray diffractions of glass fiber-reinforced in-house PP wastes exposed to several extrusion runs. As peak intensity tells about the position of atoms within a lattice structure and has to do with the arrangement of atoms, successive reprocessing was found to lead to reductions in the peak intensities of the reinforced composites. The first extrusion run (GFRPP1) has a peak intensity of 728 counts which decreased to 351 counts after the fifth run (GFRPP5).

Figure 7. X-ray diffraction analysis for first and fifth reprocessed in-house waste PP composites.

Table 3. Crystallinity, crystallite sizes, peak positions, and intensities of reprocessed GF/PP for different cycles.

S/No	Samples	Highest peak intensities (Counts)	Peak position (2θ)	Average crystallite size (nm)	Degree of crystallinity (%)
1	1st cycle	728.06	14.48	44.94	50.88
2	2nd cycle	653.25	14.72	44.21	49.54
3	3rd cycle	608.40	15.48	43.64	48.96
4	4th cycle	405.72	18.56	42.11	48.05
5	5th cycle	351.71	22.52	40.58	46.27

To obtain the crystallinity index, the various peaks were integrated using the origin software for peak analysis, and the calculation was obtained using Eq. (1)(1) $$\mathrm{Crystallinity}=\frac{\mathit{Area}\mathrm{ }of\mathrm{ }\mathit{Crystalline}\mathrm{ }\mathit{peaks}}{\mathit{Area}\mathrm{ }of\mathrm{ }\mathit{all}\mathrm{ }\mathit{peaks}\mathrm{ }(Cr\mathit{ystalline}+\mathit{Amorphous})}\mathrm{ }\times \mathrm{ }100$$(1) . (1) $$\mathrm{Crystallinity}=\frac{\mathit{Area}\mathrm{ }of\mathrm{ }\mathit{Crystalline}\mathrm{ }\mathit{peaks}}{\mathit{Area}\mathrm{ }of\mathrm{ }\mathit{all}\mathrm{ }\mathit{peaks}\mathrm{ }(Cr\mathit{ystalline}+\mathit{Amorphous})}\mathrm{ }\times \mathrm{ }100$$(1)

The first reprocessed composites recorded a crystallinity of 50.88% which gradually reduced to 46.27%. Multiple extrusion runs make the crystal structure of PP smaller and fewer in number, making them more amorphous with each successive extrusion run. This implies that the degree of orderliness is further reduced which resulted in lower peak intensities and the degree of crystallinity. Scherrer’s Equation (Eq. (2)(2) $$\text{D =}\frac{\text{Kλ}}{\text{βCosθ}}$$(2) ) was used to obtain the average crystallite sizes which also reduced with increasing reprocessing cycle accompanied by a slightly higher shift in the peak positions as shown in Table 3. The reduction in crystal quantities and size also describes the reduced mechanical properties as presented in Figures 3–6. (2) $$\text{D =}\frac{\text{Kλ}}{\text{βCosθ}}$$(2) where:

D = Crystallites size (nm)

K = 0.9 (Scherrer constant)

λ = 1.54059 nm (wavelength of the x-ray sources)

β = Full-Width Half Maximum (FWHM in radians)

θ = Peak position (radians)

The research of Mofokeng et al. [42] on the blend ratio of PP/LDPE found that the extrusion process caused a reduction in the crystallinity index and a widening of the melting point resulting in decreased peak intensities for the polypropylene. In this work, lower crystallinity produces shorter chains which makes chain mobility very easy, thus, less amount of energy is applied in the form of heat before disintegration sets in.

3.5.2. Fourier-transform infrared (FTIR) spectroscopy

The absorption bands for GFRPP1 and GFRPP5 are both shown in Figure 8. The peak at 2950 cm⁻¹ position represents the C-H stretch bond while the medium CH₂ bending vibration from the polypropylene backbone occurred at 1457 cm⁻¹. This was closely followed by the CH₃ bending vibration which occurred at approximately 1375 cm⁻¹. From the two spectra, there is no loss or addition of peaks as can be seen meaning that the nature of the degradation is not oxidative but may be thermo-mechanical. Since the drop in the mechanical properties and increases in the melt flow index are likely due to chain scission, one would have expected the capping of the end groups to lead to changes in the FTIR peaks, but this is where the novelty of this work exists.

Figure 8. Combined FTIR spectra of glass fiber reinforced in-house waste PP reprocessed five times.

Also, the available functional groups are majorly unaffected as against expectations that environmental oxygen would have been attracted during the extrusion process leading to oxidative degradation; it is suspected that short extrusion time might be responsible for the non-generation of smaller degradation by-products such as peroxy-radicals or the processing temperature is not up to the degradation temperatures to observe the destruction of the essential bonds. Esmizadeh et al. [43] also reported no formation of absorption peaks related to PP oxidation after several extrusion runs. Touati et al. [44] related research also recorded that the neat PP and the nanocomposites recorded no change in the chemical structure with the FT-IR spectra after four (4) reprocessing cycles, thus, validating the finding in this study.

3.6. Thermogravimetric analysis (TGA)

The thermogravimetric analysis evaluates weight/mass change (gain or loss) as a function of time, and temperature, especially in a controlled atmosphere. The introduction of heat into a substance can trigger chemical and physical changes that can be of great assistance in sample characterization and identification [45]. It is used here for thermal stability studies and estimation of product life span. Figures 9 and 10 show the thermal properties of glass fiber-reinforced in-house waste PP subjected to five extrusion runs. Because of the hydrophobic nature of the reinforced plastics, there was no prominent initial weight loss of ambient moisture which is mostly seen on thermograms due to the drying (desorption) process. The onset degradation temperature (T_Onset) for the one-time reprocessed PP was recorded at 338.07 °C (Figure 9a) and the maximum rate of mass loss (T_max) occurred at 426.02 °C (Figure 9b) with a mass loss of 99.2% (Figure 9a) showing a one-step decomposition pattern for all the thermograms. The onset of degradation offers a good knowledge of the thermal stability of the reprocessed plastic composites.

Figure 9. (a) TGA thermogram of glass fiber reinforced in-house waste PP for first and fifth reprocessed cycles (b) combined TGA and DTG thermograms of glass fiber reinforced in-house waste PP reprocessed once.

Figure 10. (a) DTG thermograms of glass fiber reinforced in-house waste PP for the first and fifth cycle, (b) combined TGA and DTG thermograms of glass fiber reinforced in-house waste PP reprocessed five times.

The onset of degradation for the fifth reprocessed reinforced PP (Figure 9a) occurred at 245.15 °C. There are reductions in the thermal stabilities of the plastic composites with increasing processing cycles possibly due to the chain scission mechanism, meaning a reduction in product performance. This is possible because the PP backbone having the tertiary carbons are known to be prone to attacks leading to their easy breakdown. The combined Derivative Thermogravimetry (DTG) thermograms for the reprocessed glass fiber-reinforced in-house waste PP composites are represented in Figure 10a. DTG is used to determine the temperature where the rate of weight loss is maximum. It can be seen from the curve that there are shifts to lower temperatures for both the TGA and DTG after each reprocessing cycle.

A one-step degradation pattern and the movement of thermal properties to lower regions in the TGA curves have also been reported in the literature on polypropylene biocomposites [43] and polypropylene–montmorillonite nanocomposites [46] indicating the occurrence of speeded degradation prompted by radicals’ development during chain scission. Figure 10b also shows the weight derivative curves of the glass-fiber-reinforced PP. The maximum rate of mass loss taking place at 354.1 °C was for the fifth reprocessed cycle displaying rapid commencement of degradation and loss of important components which finally leads to the drop of essential performance properties. It was also reported that the performance of polymer materials in target applications depends critically upon thermal stability and degradation [47,48].

3.7. Melt flow index (MFI)

Rheological studies have been found to provide important knowledge of the melt-processing behavior in the extrusion process [49]. It also assists in the determination of the melt viscosity, changes in molecular weight, and flow properties of polymers. In the present context, it is used to study the effect of multiple extrusions being previously successful in the estimation of thermal and shear degradation of polymeric materials since it is inversely related to viscosity and molecular weight [50]. Figure 11 shows the melt flow index of the differently processed glass fiber-reinforced in-house waste PP composites as a function of reprocessing cycles. The MFI increased progressively with every reprocessing cycle. The first, second, third, fourth, and fifth reprocessed PP composites had values of 6 g/10 min, 7.45 g/10 min, 10.8 g/10 min, 12.82 g/10 min, and 17.88 g/10 min respectively and this is because of the possible gradual loss in the polypropylene molecular weight throughout the recycling process.

Figure 11. Melt flow index of glass fiber reinforced in-house waste PP composites reprocessed five times.

Araújo et al. [51] showed that the increment of the melt flow index means a drop in the melt viscosity and consequently a negative change in the plastic’s molecular weight. They reported a polymer viscosity decrease caused by a reduction in intermolecular interactions and entanglements which leads to reductions in molecular weight and the length of the polymer chain. The analysis of complex viscosity and molecular weight determination could not be directly carried out in this study, but the progressive increase in melt flow index has been established to be inversely proportional to molecular weight and viscosity [52].

3.8. Morphological property

3.8.1. Scanning electron microscopy (SEM)

The observations of the tensile fractured surface of GFRPP1 and GFRPP5 was conducted to evaluate the fracture behavior after the test. Figure 12a and c show the SEM images of GFRPP1 at 750× and 1000× magnifications respectively. It is observed that the E-glass fibers were well soaked and covered by the polypropylene matrix (showing good wetting has taken place) leading to greater fiber/matrix interfacial adhesion. The unaffected structural integrity of the PP matrix prior to severe heat treatment must have also contributed to their ability to fully protect the E-glass fibers. This was adjudged to be the possible cause for the better mechanical properties as proven by the tensile strength result (35 MPa). However, Figure 12b and d showed the SEM micrographs of GFRPP5 at 750× and 1000× magnifications having signs of plastic deformation. Subsequent pull-out from the matrix led to the appearance of voids which indicated a poor interfacial adhesion. This was ascribed to be responsible for the depreciation in the mechanical properties as proven by the tensile strength (14.9 MPa). Usually, the reprocessing or recycling of fiber-reinforced plastic leads to a clear reduction in the fiber length (fiber attrition), however, fiber debonding starts at the fiber ends, and the shorter the fiber, the easier for the fiber to be pulled out of the matrix [53].

Figure 12. SEM fractography of tensile fractured specimens for the first (GFRPP1) and fifth time reprocessed composite (GFRPP5) at magnifications of 750× (a and b) and 1000× (c and d).

As repeated extrusion of the glass fiber-reinforced PP can be seen to have further exposed the reinforcing glass fibers, this made the binding matrix not to have good covering properties, thus, its poor wetting abilities. Fiber attrition is also understood to be more prominent and has contributed to the reductions in the plastics’ performance properties as highlighted in the sections before now. This is because the reinforcing fibers after the loss of their good length might now function as fillers rather than reinforcements [54].

4. Conclusion

In this research work, glass fiber-reinforced in-house polypropylene waste composites (GFRPP) were manufactured by melt mixing and compression molding using a 10 mm average length of 10 wt% glass fiber (GF) and 90 wt% polypropylene. This composite was then reprocessed to evaluate the impact of reprocessing on the service life properties of the composite. It can therefore be concluded that:

The physical, mechanical, structural, rheological, and thermal properties of reprocessed in-house waste PP composites are lost after each reprocessing. These decreases were possibly due to the fiber breakage and thermo-mechanical degradation of the matrix during the multiple reprocessing but the constituent functional groups are majorly unaffected.

In the industry, reinforced in-house PP plastics can be reprocessed only two times if they are to be used in the same state for the manufacture of the same product. Increased reprocessing requires the addition of either virgin PP or other materials to compensate for the lost properties. Thus, helping manufacturers to save costs and maintain the structural integrity of the reinforced plastics.

Acknowledgments

The authors are grateful for the research support from the Scientific and Equipment Development Institute (SEDI), Enugu, Nigeria, the Department of Polymer and Textile Engineering, Ahmadu Bello University, Zaria, Nigeria, and the collaborative research work with School of Mechanical Engineering, College of Engineering, Universiti Teknologi MARA (UiTM) Shah Alam Malaysia, Centre of Polymer Composites Research and Technology (PoCresT), Institute of Science (IOS), UiTM, Shah Alam Malaysia, and the School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia (USM), Engineering Campus, Nibong Tebal, Penang, Malaysia.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The data supporting the findings of this research are available within the article. Additional raw data that support the findings are also available from the corresponding author, upon reasonable request.