preface
Due to the risks and properties associated with fiberglass, its harm to the environment has become a serious problem. Fiberglass can remain intact in landfills for years, causing harm to communities and workers. Without effective recycling practices, continuous production of glass fiber reinforced polypropylene may release volatile organic compounds into the environment.
To address this environmental challenge, various reprocessing technologies have been developed to recycle used plastics. Mechanical recycling is preferred in environmental and economic terms because it involves mechanical shredding and reprocessing of waste plastics to obtain new plastics.
Recycling can reduce environmental pollution and reduce over-reliance on new polypropylene products. The mechanical recycling method is feasible and economical, saving 20-50% of the market cost compared to the original method.
Current post-treatment studies on composite properties are diverse, but most studies have looked at biofibers, plastics from open waste streams, or virgin polypropylene. Research on internal PP waste (industrial waste) reinforced with E glass fibers with a constant optimized average length and thermomechanical treatment is still quite limited or has not been fully explored, which is one of the main purposes of this study.
In this study, we collected recycled polypropylene (internal PP waste) as a matrix and prepared a composite using 10 wt% E glass fiber with an average length of 10 mm as a reinforcement. The composite is then crushed into granules, which are reprocessed five times by extrusion and compression molding after each reprocessing cycle, and their properties are studied.
We monitor changes in mechanical properties, rheological properties, structural characteristics, morphological characteristics, and thermal behavior to assess the effects of different extrusion and compression processes on material properties.
Through such research, plastic manufacturers can save costs, reduce internal waste generation, and greatly reduce their dependence on new materials by increasing the use of reprocessed materials in new products.
First, tensile properties
The results show that the tensile strength of the glass fiber reinforced polypropylene composite after multiple reprocessing gradually decreases. It decreased from 35 MPa for the first reworked composite (GFRPP1) to 14.95 MPa for the fifth reworked composite (GFRPP5), a decrease of 57.29%.
This reduction may be due to the fiber fracture that occurs during the crushing process of the composite, resulting in a reduction in the ratio of fiber length to diameter, which weakens the strength of the reprocessed composite.
The results of this study were also observed by Dickson et al. However, this reduction can also be due to the thermomechanical degradation of the substrate under the action of high processing temperatures and severe high shear forces. This can lead to the rupture of the polypropylene chain, which in turn leads to a decrease in the tensile strength of the composite.
The relative stiffness or elastic deformation resistance of the reprocessed glass fiber reinforced polypropylene composite is also shown. The observation results showed that the tensile modulus decreased significantly from 602.34 MPa of GFRPP1 to 470.7 MPa of GFRPP5, a decrease of 21.85%. Consistent with Beg and Pickering's report, the reduction in tensile modulus may be due to fiber breakage during reprocessing.
2. Bending strength and modulus
The results of glass fiber reinforced polypropylene composites in terms of flexural properties are shown in the figure. After several reworkings, the flexural strength and modulus were reduced from 54 MPa and 1482 MPa in the first reprocessing cycle to 26.44 MPa and 1424 MPa in the fifth reprocessing cycle. After the second reprocessing cycle, the plastic composite showed a relatively good flexural strength of 48.19 MPa.
Further reprocessing results in very low flexural strength and modulus, which is still unacceptable for high-quality products unless recycled polypropylene is mixed with virgin polypropylene or other materials are added that may improve flexural properties.
Due to the reduction in bending stiffness and strength during rework, the first and second reworked samples exhibit better and stronger composites than all other additional reworked composites.
3. Chemical and structural test results
The figure shows the X-ray diffraction results of glass fiber-reinforced internal PP waste after multiple extrusion runs. Peak intensity reflects the position and arrangement of atoms in the lattice structure, so continuous reprocessing results in a decrease in peak strength of reinforced composites. The peak intensity of the first extrusion run (GFRPP1) was 728 counts, while it dropped to 351 counts after the fifth run (GFRPP5).
The crystallinity of the first reworked composite material was 50.88%, which then gradually decreased to 46.27%. Multiple extrusion runs result in smaller and fewer polypropylene crystal structures, making them more amorphous in each successive extrusion run. This means that the degree of order is further reduced, resulting in a decrease in peak strength and crystallinity.
Using the Scherrer equation, the average crystallite size can be obtained, which also decreases with the increase of the post-processing cycle, while the peak position shifts slightly upward, and the decrease in the number and size of crystals also indicates the decrease in mechanical properties, as shown in the figure.
The study of PP/LDPE blends by Mofolken et al. found that the extrusion process leads to a decrease in the crystallinity index and a widening melting point, resulting in a decrease in the peak strength of polypropylene. In this work, a lower degree of crystallinity means a shorter chain length, which makes the flow of the chain very easy and therefore less heat energy is applied before it begins to break.
As can be seen from the spectrum, the loss or increase of peaks is not significant, which means that the nature of degradation is not oxidizing, but may be due to thermomechanical properties.
Since the decrease in mechanical properties and the increase in the melt flow index may be caused by the fracture of the chain, it is expected that the blocking of the end group will lead to a change in the FTIR spectral peak, which is also the innovation of this study.
In addition, the available functional groups were largely unaffected, contrary to the expected oxidative degradation due to the attraction of ambient oxygen during extrusion. It is suspected that shorter extrusion times may be responsible for the failure to generate smaller degradation byproducts (e.g., peroxy radicals), or that the processing temperature does not reach the degradation temperature to observe the destruction of the fundamental bonds.
4. Thermogravimetric analysis (TGA)
Thermogravimetric analysis is a common method to study the thermal stability and degradation behavior of materials by assessing the mass/weight changes of a sample at different temperatures and over time.
Thermogravimetric analysis can be performed in a controlled atmosphere to initiate chemical and physical changes by introducing heat into the sample, helping to characterize and identify the sample.
Due to the hydrophobicity of reinforced plastics, moisture in the environment does not cause a significant initial mass loss, mainly due to the drying (desorption) process, which can be observed in the thermal analysis diagram.
The initial degradation temperature (T Onset) of single-use reprocessed polypropylene was recorded at 338.07°C, and the maximum mass loss rate (T max) occurred at 426.02°C with a mass loss of 99.2%, showing the single-step decomposition pattern in all thermal analysis plots. The starting temperature of degradation provides a good insight into the thermal stability of reprocessed plastic composites.
The temperature at which the fifth reprocessed reinforced polypropylene begins to degrade is 245.15°C. As the processing cycle increases, the thermal stability of plastic composites decreases, which may be caused by the chain breaking mechanism, which means that the product performance decreases. This is possible because polypropylene backbones containing tertiary carbon are known to be vulnerable to attack, resulting in easy breakage.
A combined differential thermogravimetric (DTG) thermal analysis plot of a reworked glass fiber reinforced internal waste polypropylene composite is shown. DTG is used to determine the temperature at which the loss rate is maximal. From the curve, it can be seen that after each reprocessing cycle, the temperature of both TGA and DTG transitions to a lower temperature.
Studies of polypropylene biocomposites in the literature have reported similar one-step decomposition patterns and thermal properties to regions with lower TGA curves. Similarly, studies of polypropylene-montmorillonite nanocomposites have shown that the development of free radicals during chain breaking leads to accelerated degradation. The mass derivative curve of glass fiber reinforced polypropylene is also shown.
In the fifth reprocessing cycle, the maximum rate of mass loss occurred at 354.1°C, indicating that important components began to degrade and lose rapidly, eventually leading to a decrease in basic properties. In addition, the performance of polymer materials in the target application depends critically on thermal stability and degradability.
5. Melt Flow Index (MFI)
Rheological studies can provide important information about the behavior of melt processing during extrusion. It can determine the melt viscosity, molecular weight change and flow characteristics of polymers. In the current study, it was used to study the effects of multiple extrusions on composites, which has been successfully used in previous studies to estimate thermal and shear degradation of polymeric materials, as viscosity and molecular weight correlate with degradation.
The melting index (MFI) gradually increases with each reprocessing cycle. The MFI values of the first, second, third, fourth and fifth reprocessed PP composites were 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, due to the possible gradual reduction of the molecular weight of polypropylene throughout the recycling process.
Evaluate the fracture behavior after the test by observing the tensile fractures of GFRPP1 and GFRPP5. It was observed that alkali-free glass fibers were sufficiently immersed and covered by the polypropylene matrix (showing good wettability), resulting in enhanced adhesion at the fiber/matrix interface.
The structural integrity of the PP matrix is not compromised until the rigorous heat treatment, which also helps to adequately protect the alkali-free glass fibers. This is also a possible cause of better mechanical properties, as evidenced by the tensile strength results.
Scanning electron microscopy (SEM) images of GFRPP5 at 750x and 1000x magnifications are shown, showing signs of plastic deformation. Voids pulled out of the substrate indicate poor interface adhesion. This is thought to be the cause of the deterioration of mechanical properties, which is confirmed by the tensile strength (14.9 MPa).
Often, reprocessed or recycled fiber-reinforced plastics result in a significant reduction in fiber length (fiber wear), and as fiber length decreases, the tendency for fibers to be pulled out of the matrix is more pronounced.
Due to the multiple extrusion of glass fiber reinforced PP, the reinforced glass fiber is further exposed, which makes the matrix coverage performance worse, so its wettability becomes worse. It is understood that the wear of the fibers is also more significant, resulting in a decrease in the performance of the plastic, as highlighted in the previous section. This is because reinforcing fibers, having lost their good length, may now act as fillers rather than reinforcing materials.
6. Conclusion
In this study, glass fiber reinforced homemade polypropylene waste composites (GFRP) were prepared by melt mixing and compression molding using 10wt% glass fiber (GF) and 90wt% polypropylene with an average length of 10 mm. The composite was then reprocessed several times to assess the impact of reprocessing on the lifetime performance of the composite.
It can be concluded that internal waste PP composites that have been reprocessed many times are gradually lost in physical, mechanical, structural, rheological and thermal properties. This degradation may be due to fiber fracture and thermomechanical degradation of the matrix during multiple reprocessing, but the constituent functional groups of the composite are largely unaffected.
In the industrial sector, if reinforced homemade polypropylene plastic is to be used again to make the same product and maintain the same properties, it usually only needs to be reprocessed twice.
By increasing the number of reprocessing, virgin polypropylene or other materials can be added to compensate for the loss of performance. As a result, this helps manufacturers save costs and maintain the structural integrity of reinforced plastics.
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