3.1 Simulation and mechanism analysis of the formation process of composite
Figure 4 demonstrates the function mechanism of the natural lotion which has unique advantages. Traditional addition method can easily lead to the appearance of agglomerates of gum acacia. Friction electrification and electrostatic attraction can cause the powder to easily aggregate and form blocks. This effect can greatly weaken the effect of gum acacia as a compatibilizer. Insufficient contact between gum acacia and plastic particles makes it difficult to effectively enhance the bonding between the filler and the substrate.
Figure 5 demonstrates the promoting dispersion effect of natural lotion. Compounding process is a necessary technology to combine biomass filler and recycled plastic matrix into a composite. The compounding process and matched thermoforming process are used to prepare bio-composite products with low cost and high efficiency. The actual production process of biocomposite products is more sophisticated than academic theoretical research. Each link of the preparation process has a huge impact on the actual performance of the subsequent products. The actual mixing effect of biomass fillers and recycled plastic matrix is one of the most important links. In the mixing process of ingredients, biomass filler is prone to slagging and caking due to electrostatic attraction and centrifugal effect. These slags and cakes can cause additional agglomeration and voids in the internal structure of biocomposite to weaken the performance of products. In this paper, adding an appropriate amount of compatibilizer to improve the mixing effect of biomass filler and recycled plastic matrix based on the experimental experience is a novel research with extensive practical foreground.
The liquid transfer gun is used to drop NL evenly into the materials under high-speed stirring. NL is mainly used to adhere the biomass filler to the plastic matrix. Natural lotion can microencapsulate biomass filler by liquid film. Natural lotion can suppress the negative effect of filler agglomeration by strengthening the adhesion between the filler and the matrix. The biomass filler is in the form of fine powder. Small particles in the powder tend to form clusters due to electrostatic attraction. Under centrifugal action, clusters are prone to slagging and caking due to extrusion and other external forces. Slagging and caking result in excessive distribution of local biomass fillers in the mixture of ingredients, which will further lead to inhomogeneous structure in the composite. Heterogeneous structures such as agglomerates and voids can damage the matrix, which lead to the decline of material properties. NL makes the biomass filler adhere to the plastic particles, the biomass filler clusters on the surface of the plastic particles to form a new surface layer. This method can limit the free movement of biomass filler particles to avoid local aggregation and ultimately improve the mixing effect of ingredients.
3.2Analysis of fundamental physical properties of samples
As shown in Figs. 6, the effect of the amount of straw fillers added on the composites’ mechanical performance parameters such as tensile strength(TS), elongation at break(EAB) and Young’s modulus(YM). With the increase of the ratio of A to B1, the TS value of the composite increased first and then decreased. The EAB value of the composite gradually decreased and the YM value fluctuated. In the Fig. 4, the peak value of TS is 4.24 MPa and the peak value of YM is 2120.68 MPa. When the amount of straw fillers is appropriate, the granular fillers can play a role in promoting the internal stress transfer of the composite. Ibrahim’s research(Ibrahim et al. 2019) found that corn biomass fibers in thermoplastic composites can play a role as reinforcing fillers, improving multiple mechanical properties of composites such as tensile strength and Young’s modulus.
When A:B1 is very high, the three performance parameters of the composite are very low. Agglomerates cause destructive damage to the matrix, which leads to the failure of the filler/matrix interface. The gradual decrease of EAB value is due to the effect of fillers and aggregates on restricting the movement of long molecular chains in the matrix. Agglomeration also causes many stress concentration areas in the composite to weaken the ductility of the composite. When the amount of straw fillers is excessive, the agglomeration effect of fillers causes the aggregation of some fillers to damage the matrix and reduce the mechanical properties of the composites. In Prakash’s experiment (Prakash and Rajadurai 2016), it was found that as the amount of filler added increased, the aggregation of fillers would lead to a decrease in the tensile and flexural strength of bio-composite.
As shown in Figs. 7,with the increase of the ratio of A to B2, the TS value of the complex increased first and then decreased. The EAB value of the complex gradually decreased and the YM value fluctuated. In the Fig. 5, the peak value of TS is 4.31 MPa and the peak value of YM is 2151.09 MPa. Green waste fillers are mostly fibrous and straw fillers are mostly granular. The effect of fiber filler on stress transmission is poor, but it has certain deformation ability. When the amount of fibrous filler is not too much, the EAB value of the composite decreases relatively slowly. When the fibrous filler is excessive, the TS and EAB values of the sample drop sharply, and the YM value is at a lower level. This indicates that excessive fibrous filler can also cause filler/matrix interface failure. The morphology and particle size of each particle in the fibrous filler are more different. Some filler particles have very large particle sizes. These characteristics make the damage of fibrous filler to the matrix relatively large. Both scholars Mahalingam(Mahalingam and Babu 2022) and Alshahrani(Alshahrani and Prakash 2022) have found that severe aggregation of fillers can lead to failure of the filler/matrix interface, this abstract problem specifically manifests as a sudden decline in the mechanical properties of the material.
In the Fig. 8, the peaks of TS, EAB and YM are 4.79 MPa, 9.44% and 2396.80 MPa respectively. With the increase of the ratio of B1 to D, the TS, EAB and YM value of the composite increased first and then decreased. Adding a proper amount of Natural lotion can promote the dispersion of fillers and weaken the agglomeration effect of fillers, but when the addition amount is too high, the appearance of special aggregates makes the performance of the composite decline. The filler is soaked with too much Natural lotion, soaked fillers bind to each other to form special aggregates. Special agglomerates make some fillers unable to adhere to the matrix, the effect of stress transfer by particles is poor. The same phenomenon also occurred in Vinod’s research (Vinod et al. 2020), he found that the performance of the material is poor due to the ineffective interfacial linkage between the matrix and fiber.
In the Fig. 9, the peaks of TS, EAB and YM are 4.06 MPa, 4.92% and 2027.86 MPa respectively. With the increase of the ratio of B2 to D, the TS, EAB and YM value of the composite increased first and then decreased. The small number of samples is due to the failure of sample preparation due to the blockage of the equipment feeding channel when the addition amount of NL exceeds 3.8%. Most of the green waste powder consists of relatively large sized fibers which are easily twisted into bundles during the mixing process. It is difficult for NL to penetrate into the interior of the bundles to achieve dispersion effect and redundant NL on bundles’ surface can also easily cause filler adhere to the inner wall of the equipment, leading to sedimentation of ingredients and failure of preparation experiment. This theory also explains the overall poor mechanical properties of composites prepared with Natural lotion and green waste. A similar phenomenon also occurred in Coltelli’s(Coltelli et al. 2008) research in which the mechanical properties of the composite decrease when excessive ester crosslinkers are added.
3.3 Analysis of thermal conductivity of samples
Figure 10 shows the change of thermal conductivity of the four composites. As the value of A:B1 decreases, the thermal conductivity of the composite decreases gradually. The agglomeration effect of straw fillers leads to cavities in the composites. With the gradual increase of straw fillers, the volume and quantity of cavities gradually increase. As thermal insulation phases, the cavities obviously inhibit the heat transfer in the material. As the value of A:B2 decreases, the thermal conductivity of the composite decreases sharply. Most of the green waste fillers are fibrous. When there are too many fibrous fillers, the aggregation leads to many gaps in the composite. Voids can act as insulation phases to reduce the thermal conductivity of materials. Alahnoori’s experiment(Alahnoori et al. 2023) found that the air in the voids between the filler particles can increase the thermal resistance of the material. Through comprehensive analysis combined with SEM characterization, it is found that the gaps are wide and deep, part of the gaps are widely distributed in the structure of the composite. As the value of B1:D increases, the thermal conductivity of the composite increases gradually. After adding Natural lotion, the thermal conductivity of the composite was improved. Straw fillers can fully contact with Natural lotion due to small particle size.
The filling of the granular fillers fully soaked by the oily additive improves the overall thermal conductivity of the composite. As the value of B2:D increases, the thermal conductivity of the composite fluctuates violently. The larger volume of fibrous fillers makes the wetting effect of Natural lotion poor. Aggregates of fiber fillers have poor thermal conductivity. When there are too many Natural lotion, some fibrous fillers adhere to each other to form larger clusters, resulting in a sudden decline in the thermal performance of the composite. Because natural fiber has good thermal insulation, the clusters which contain too many natural fibrous fillers directly reduce the thermal conductivity of the material. Vidal’s experiment (Vidal et al. 2023) found that natural fiber has a good heat insulation effect, which is similar to sheep wool and glass wool.
3.4 SEM characterization of samples
As shown in Figs. 11, the SEM morphology of images of 2 samples.There are many obvious and large defects in the structure of the following two composites, which reveals the disintegration of the filler/matrix interface. Figure a illustrates the internal structure of the composite containing excessive green waste. There are many deep gaps in the structure, the presence of deep gaps suggests more defects deep inside the composite. Figures b and c further illustrate the specific morphology of internal aggregates. Many fibrous fillers are agglomerated into bundles and part of the bundles strip out the matrix. Satapathy(Satapathy and Kothapalli 2018) also observed in SEM characterization that fibers detach from the matrix and many voids appear in the composite. Figures d, e and f illustrate the internal structure of the composite containing excessive straw fillers. There are shallow pits and hump-like aggregates in the internal structure of the composite. These phenomena indicate that the agglomeration effect of fillers causes certain damage to the structure of the composite. Figures g, h and i illustrate that the aggregates are hard and regular clusters composed of granular fillers.
As shown in Figs. 12, the SEM morphology of images of 3 samples.Figures a and b illustrate that proper amount of Natural lotion can improve the structure of the composite containing straw fillers. The shallow pits in the structure of the previous sample are filled and some protruding edges are exposed. Figures c and d illustrate that Natural lotion promote the adhesion of granular fillers to the matrix so that only small holes appear in the composite. Nagarajan(Nagarajan et al. 2013) found a similar phenomenon where the size of the voids in the composite was greatly reduced when additives were added to improve the compatibility between the filler and the matrix. Figures e, f and g illustrate that the internal structure of the composite containing excessive green waste. Figures h and i reveal that an appropriate amount of Natural lotion has a conscience effect on improving the structure of the composite containing fibrous fillers. Figures j, k and l illustrate that excessive Natural lotion have a negative effect on the structure of the composite containing straw fillers. Figures k and l demonstrate the special phenomenon of special agglomeration. Excessive Natural lotion penetrate into straw fillers and cause the self-adhesion of the filler. Previous literature has illustraten that only fillers in dispersed state can effectively strengthen the matrix, the clusters formed by self-adhesion damage the structure of the composite.
3.5 Analysis of microscopic morphology of samples
The micrographs of the samples’ surface are illustrated in the Fig. 13. Subfigure a illustrates the surface morphology of the composite containing excessive straw fillers, there are many bumps and pits on its surface. Subfigure b illustrates that deep gaps and bundle-like structure appear on the surface of samples containing excessive green waste fillers. The phenomena in these two subfigures can prove from another perspective that excessive filler cause serious filler agglomeration and further damage to the matrix. Zykova(Zykova et al. 2017) observed many filler agglomerates distributed in the microstructure of the composite in the microscopic image. Subfigure c illustrates the surface morphology of the composite containing excessive straw fillers added with excessive Natural lotion. A typical feature of its morphology is the appearance of clusters with wide distribution and large volume. The appearance of small holes on the surface indicates that excessive Natural lotion damage the integrity of the composite structure. The Natural lotion in the sample illustrated in subfigure d is appropriate. Some fillers enter the pit and make the pit shallow, only the contour of the pit edge bulge obviously. This phenomenon indicates that the surface morphology of the sample is relatively good.
3.6 TGA analysis of samples
The TGA analysis results of three samples are shown in Fig. 14. Subfigure a illustrates the thermal analysis diagram of the composite with appropriate amount of straw fillers. The initial pyrolysis temperature (Ti) on the TG curve is the medium (231.3 ℃). A sharp single peak appears on the DTA curve. The agglomeration of straw fillers leads to uneven heating inside the composite, the local part of the composite is prone to overheating and pyrolysis. The temperature corresponding to the peak on the DTG curve is higher(453.5 ℃). Due to the poor thermal conductivity inside the composite, it needs to be heated for a longer time to reach a higher temperature to make the composite undergo overall pyrolysis. Satapathy’s experiment(Satapathy and Kothapalli 2015) found that the temperature range for thermal decomposition of cellulose and lignin is between 351–381℃, this discovery explains why comprehensive pyrolysis of the composite requires a longer heating time.
Subfigure b illustrates the thermal analysis diagram of the biocomposite with excessive amount of oily additive. The initial pyrolysis temperature (Ti) on the TG curve is the lowest (228.9℃). A sharp single peak appears on the DTA curve. The agglomeration of straw fillers lead to uneven heating inside the composite, the local part of the composite is prone to overheating and pyrolysis. Special agglomeration caused by the self-adhesion of straw fillers soaked by natural lotion leads to the deterioration of thermal conductivity inside the composite, part of the Natural lotion seeping from the aggregates further induce the occurrence of local pyrolysis. The temperature corresponding to the peak on the DTG curve is highest(457.7℃). Specical agglomeration also has the effect of delaying the overall pyrolysis.
Subfigure c illustrates the thermal analysis diagram of the composite with appropriate amount of oily additive. The initial pyrolysis temperature (Ti) on the TG curve is the highest(241.4℃). A moderate single peak appears on the DTA curve. Natural lotion can promote the adhesion between filler and matrix to improve the thermal conductivity of the whole composite. Good thermal conductivity can distribute the heat gathered in the part to the whole to avoid local pyrolysis and improve the thermal stability of the whole composite. Biswal(Biswal et al. 2010) also has a similar inference, Biswal found that the thermal stability of the composite is attributed to the organic/inorganic interaction between the polymer and the filler. The use of compatibilizers to promote compatibility between the filler and the matrix can delay the volatilization of the product in the polymer matrix at the carbon bond fracture temperature. The temperature corresponding to the peak on the DTG curve is lowest(431.8℃). This phenomenon indicates that the improvement of the thermal conductivity of the composite can promote the heat transfer into all areas of the composite, which makes the overall pyrolysis occur at an early stage.