The Use of Nonmetallic Fraction Particles with the Double Purpose of Increasing the Mechanical Properties of Low-Density Polyethylene Composite and Reducing the Pollution Associated with the Recycling of Metals from E-Waste

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Introduction
The nonmetallic fraction (NMF) of printed circuit boards (PCBs) from end-of-life electrical and electronic equipment (EOL-EEE) contributes to more than 70 wt.% of the PCB material; the remaining amount of material corresponds to the different metals present in such devices' PCBs; these PCBs are the main component found in almost all electronic waste (e-waste) which grows at the highest level of waste streams in the world [1].
The NMF is constituted by fiberglass and thermosetting resins, such as epoxy resins and phenolic resins, mixed with brominated flame retardants, which are hazardous materials that must be recycled to reduce potential environmental contamination and human poisoning [2].By physical processes, valuable materials from PCBs can be obtained, which show very little contamination and, in some cases, are free of them; the remaining nonvaluable material is classified as a nonmetallic fraction.A sequence of operations which involves a crusher machine comminution followed by reverse froth flotation can generate particles in a size range that ensures the complete liberation and separation of the metallic fraction from the nonmetallic fraction; also in addition, these techniques are environmentally friendly when compared with chemical processes [2,3].When NMF particles obtained from the comminution process, without any further treatment, are added into a polymer matrix as a filler, a new composite which presents enhanced mechanical and physical properties as well as improved flame resistance due to the brominated flame retardants (BFRs) present in the NMF particles is obtained [4].Synthesized polymer matrix composite (PMC) with NMF particles as a reinforcing material results in better mechanical properties and lower production costs when compared to PMC generated using other common fillers such as calcium carbonate and talcum [2,[5][6][7][8].In the synthesis of PMC, low density polyethylene (LDPE) is used because of its combination of superior clarity, high stiffness, low density, low toxicity, and ease of processability, among other characteristics [9,10].Polymer matrix composites using NMF filler particles in the range of 300 µm have been synthesized that improve mechanical properties and thermal conductivity [11].
Polymers are materials with competitive economic and aesthetic properties since they are both of low weight and easy to mold in order to be adapted for their use in a variety of products in a wide number of applications improving the level of comfort; unfortunately, the majority of polymers are extremely flammable, which reduces their potential applications [12,13].Recycling of plastics is an activity that can reduce the environmental impact by reducing the use of petroleum and carbon dioxide emissions as well as reducing the number of polymers that end up as urban waste; 65-70% of the polymers waste ends up in landfills, while the remaining 30-35% is incinerated, which generates environmental contamination.Low-density polyethylene represents 22 wt.% of polymers in municipal solid waste [12,14].Mechanical recycling is a method that employs, in general, low-cost processes and noncomplex techniques to obtain new products; LDPE has several applications for its use in the fabrication of containers, tubing and plastic bags, among others [3,12].
Polymer matrix composites reinforced with the nonmetallic fraction of printed circuit boards (PCBs) can be synthesized in order to generate valuable new composites as a crucial part of the circular economy [15].The reinforced composite can be obtained when discrete particles are processed with a polymer such as low-density polyethylene (LDPE), which has a relatively low melting temperature and viscosity, a high strength-to-density ratio, and low toxicity.These properties improve the possibility for this material to be used in the production of PMC with increased properties [10,16,17].The properties of PMC such as those described in the present text are influenced by the interactions on the interfaces between the NMF particles and the polymer matrix.In order to improve the interfacial adhesion between the matrix and the NMF, a compatibilizing agent can be added, building bridges between the matrix and the NMF by means of primary or secondary bonding [18,19].
The composite produced using NMF particles can be fed into an additive manufacturing process, such as fused filament fabrication (FFF), an extrusion-based process in which melted extrudate material, with dispersed particles to reinforce the polymer, is deposited through a small nozzle of approximately 0.5 mm diameter, where a computer-assisted model can be built by gradual layerwise addition of the material in order to construct the final product [16,20].
The aim of this work is to generate a new polymer matrix composite using NMF particles as a filler, in order to: (1) Reutilize the non-valuable material obtained from the recycling of electronic devices.
(2) Produce a composite with improved properties like thermal stability, flammability resistance, yield strength, and ultimate tensile strength, among others.(3) Generate a simple process to produce the new composite that can be employed in the polymer industry as well as in additive manufacturing techniques (AMTs) allowing prototype creation and near-net-shape composite products.
In this way, a restorative industrial process which eliminates waste and pollution, circulates products at competitive value while reusing non-valuable material, generating zero residues, which in turn helps to nature regeneration, is achieved as in the circular economy tendencies [1].

Results and Discussion
Figure 1 presents photographs of the Sample M1 particles obtained from the comminution of PCBs from EOL-EEEs.Figure 1a presents the material obtained from the sieve classification; as can be seen, this material has an acicular morphology, although the longitudinal section is larger than 250 µm, the transversal section can pass through the different sieves and only a few metallic particles are visible; this is due to the excessive amount of nonmetallic fraction particles, more than 70 wt.%[21,22], which cover the metallic particles making only a few of them observable.Figure 1b,c present the Sample M1 particles after the separation of fractions by a reverse froth flotation process; Figure 1b shows the tails products, the metallic faction, the largest particles of sample M1, with a plate-like morphology with an average size of 250 µm width and 1500 µm length; Figure 1c shows the concentration products, the nonmetallic fraction, consisting of white acicular fiberglass material with an average chemical composition of 37.40% C, 45.60% O, 0.24% Mg, 3.53% Al, 8.56% Si, 4.68% Ca, and 0.23% Fe, as well as resin particles in different colors; these particles have an average chemical composition of 52.61% C, 31.43%O; 2.06% Al, 3.74 Si, and 10.09% Br, among others.Despite the fact that the separation of the metallic fraction is not completely achieved, the nonmetallic fraction is adequate as a raw material for the conformation of the new composite.
In this way, a restorative industrial process which eliminates waste and pollu circulates products at competitive value while reusing non-valuable material, gener zero residues, which in turn helps to nature regeneration, is achieved as in the cir economy tendencies [1].

Results and Discussion
Figure 1 presents photographs of the Sample M1 particles obtained from the com nution of PCBs from EOL-EEEs.Figure 1a presents the material obtained from the classification; as can be seen, this material has an acicular morphology, although the gitudinal section is larger than 250 µm, the transversal section can pass through the ferent sieves and only a few metallic particles are visible; this is due to the exce amount of nonmetallic fraction particles, more than 70 wt.%[21,22], which cover the tallic particles making only a few of them observable.Figures 1b,c present the Sampl particles after the separation of fractions by a reverse froth flotation process; Figu shows the tails products, the metallic faction, the largest particles of sample M1, w plate-like morphology with an average size of 250 µm width and 1500 µm length; F 1c shows the concentration products, the nonmetallic fraction, consisting of white aci fiberglass material with an average chemical composition of 37.40% C, 45.60% O, 0 Mg, 3.53% Al, 8.56% Si, 4.68% Ca, and 0.23% Fe, as well as resin particles in differen ors; these particles have an average chemical composition of 52.61% C, 31.43%O; 2 Al, 3.74 Si, and 10.09% Br, among others.Despite the fact that the separation of the me fraction is not completely achieved, the nonmetallic fraction is adequate as a raw ma for the conformation of the new composite.Figure 2 presents photographs of the raw material before and after the blending cess.Figures 2a,b show the LDPE pellets and the nonmetallic fraction before the blen process, respectively; the particle size and morphology difference between LDPE NMF are evident, with polymer pellets having a 5 mm average diameter, while the N has an average particle size of 250 µm width and 1500 µm length.Figure 2c present material after the blending process; as can be seen, the NMF particles are attached t LDPE pellets in a homogeneous way, indicating a well-mixed product.Figure 2d pre an LDPE pellet with several NMF particles attached to it; the size difference betwee raw materials is evident.Figure 2 presents photographs of the raw material before and after the blending process.Figure 2a,b show the LDPE pellets and the nonmetallic fraction before the blending process, respectively; the particle size and morphology difference between LDPE and NMF are evident, with polymer pellets having a 5 mm average diameter, while the NMF has an average particle size of 250 µm width and 1500 µm length.Figure 2c presents the material after the blending process; as can be seen, the NMF particles are attached to the LDPE pellets in a homogeneous way, indicating a well-mixed product.Figure 2d presents an LDPE pellet with several NMF particles attached to it; the size difference between the raw materials is evident.
The system LDPE pellets/NMF particles has a size difference ratio of more than one order of magnitude; in general, the blending of different size particles generates nonhomogeneous mixing [23]; nevertheless, in this case a good blending is attainable.This homogeneous distribution of particles is possible due, among other characteristics, to the temporal static electricity that is generated when polyethylene particles are rubbed with each other inside the blending cone; this static electricity attaches the nonmetallic fraction particles to the surface of the polyethylene pellets and this particle attachment remains for some period of time in what looks like a large sphere of polyethylene covered by smaller particles of NMF, as can be seen in Figure 2d.
The system LDPE pellets/NMF particles has a size difference ratio of more than one order of magnitude; in general, the blending of different size particles generates nonhomogeneous mixing [23]; nevertheless, in this case a good blending is attainable.This homogeneous distribution of particles is possible due, among other characteristics, to the temporal static electricity that is generated when polyethylene particles are rubbed with each other inside the blending cone; this static electricity attaches the nonmetallic fraction particles to the surface of the polyethylene pellets and this particle attachment remains for some period of time in what looks like a large sphere of polyethylene covered by smaller particles of NMF, as can be seen in Figure 2d.The attachment effect in the blending of these particles remains during a period of time, after which the NMF starts falling apart from the LDPE and a segregation process occurs.In order to take advantage of the homogeneous blending obtained, the mixture must be fed into the screw extruder, to produce the new composite, at most a few minutes after the blending process has been performed.The segregated NMF is recovered to be reused in posterior blending processes.
Figure 3 presents a set of filaments with different NMF particles weight content after the extrusion process and coiling.As can be seen, as the amount of NMF increases, the tone of the filament color becomes darker; the original white LDPE tone becomes greener at the higher particles content, showing a good distribution of the NMF particles along all The attachment effect in the blending of these particles remains during a period of time, after which the NMF starts falling apart from the LDPE and a segregation process occurs.In order to take advantage of the homogeneous blending obtained, the mixture must be fed into the screw extruder, to produce the new composite, at most a few minutes after the blending process has been performed.The segregated NMF is recovered to be reused in posterior blending processes.
Figure 3 presents a set of filaments with different NMF particles weight content after the extrusion process and coiling.As can be seen, as the amount of NMF increases, the tone of the filament color becomes darker; the original white LDPE tone becomes greener at the higher particles content, showing a good distribution of the NMF particles along all of the composite filament.Polyethylene material can be considered as molecular networks, amorphous in nature, arranged in entangled chains; its mechanical properties depend on its molecular conformation and crystallinity.When the polyethylene matrix is reinforced with nonmetallic fraction particles, these are randomly oriented and act as physical crosslinks increasing the mechanical properties of the composite material [24].
of the composite filament.Polyethylene material can be considered as molecular networks, amorphous in nature, arranged in entangled chains; its mechanical properties depend on its molecular conformation and crystallinity.When the polyethylene matrix is reinforced with nonmetallic fraction particles, these are randomly oriented and act as physical crosslinks increasing the mechanical properties of the composite material [24].4d,e show the longitudinal section of the composite.As can be seen in Figure 4a, several NMF particles with different particle sizes are distributed in the polymer matrix; this distribution is obtained by means of a good mechanical mixing and the extrusion process, which redistributes the particles in the polymer matrix as the mixed raw material flows through the screw extruder.At higher magnifications, Figure 4b highlights a detailed view of the morphology of the NMF particles.The filler particles composed mainly of fiberglass filaments with a round morphology are presented as a single or joined, by the epoxy resin, strand; the insert at the upper-right end of Figure 4b shows the transversal view of individual strands of fiberglass joined together.This micrograph also highlights that the acicular particles were oriented in the flow direction, since the length of the fiberglass strands is perpendicular to the observed section, indicating that the extrusion process not only redistributes the particles but also orients them along the flow of the material.
Figure 4d presents a long particle at the upper-left section of the micrograph; this particle, a fiberglass strand, lies almost parallel to the surface of the composite round bar.A more detailed observation, in Figure 4e, shows that the particle was fractured in several parts along its length; this effect occurs due to the fact that, as the material flows through the extrusion barrel, the forces that some of the NMF experiment are strong enough to break them apart; as the NMF continue their travels in the extrusion barrel, they become oriented in the flow direction of the extrusion process.Figures 4c,f present the EDS spectrums taken to the fiberglass and the epoxy resin, respectively; the highlighted numbers 1 and 2 in Figure 4 correspond to the fiberglass and brominated resin, respectively, as can be appreciated in the EDS spectrums.As can be seen, strand particles present silicon and aluminum as their main elements, which correspond to the fiberglass particles; on the other hand, the resin that joins the fiberglass strands is rich in bromine, which comes from the brominated flame retardant present in the PCBs [25].Figure 4 presents SEM sputter coating micrographs of the transversal and longitudinal sections of the composite; several particles with different particle sizes distributed over the matrix are observable.Figure 4a,b display the transversal section; Figure 4d,e show the longitudinal section of the composite.As can be seen in Figure 4a, several NMF particles with different particle sizes are distributed in the polymer matrix; this distribution is obtained by means of a good mechanical mixing and the extrusion process, which redistributes the particles in the polymer matrix as the mixed raw material flows through the screw extruder.At higher magnifications, Figure 4b highlights a detailed view of the morphology of the NMF particles.The filler particles composed mainly of fiberglass filaments with a round morphology are presented as a single or joined, by the epoxy resin, strand; the insert at the upper-right end of Figure 4b shows the transversal view of individual strands of fiberglass joined together.This micrograph also highlights that the acicular particles were oriented in the flow direction, since the length of the fiberglass strands is perpendicular to the observed section, indicating that the extrusion process not only redistributes the particles but also orients them along the flow of the material.
Figure 4d presents a long particle at the upper-left section of the micrograph; this particle, a fiberglass strand, lies almost parallel to the surface of the composite round bar.A more detailed observation, in Figure 4e, shows that the particle was fractured in several parts along its length; this effect occurs due to the fact that, as the material flows through the extrusion barrel, the forces that some of the NMF experiment are strong enough to break them apart; as the NMF continue their travels in the extrusion barrel, they become oriented in the flow direction of the extrusion process.Figure 4c,f present the EDS spectrums taken to the fiberglass and the epoxy resin, respectively; the highlighted numbers 1 and 2 in Figure 4 correspond to the fiberglass and brominated resin, respectively, as can be appreciated in the EDS spectrums.As can be seen, strand particles present silicon and aluminum as their main elements, which correspond to the fiberglass particles; on the other hand, the resin that joins the fiberglass strands is rich in bromine, which comes from the brominated flame retardant present in the PCBs [25].
Figure 5 presents SEM micrographs of a cut section of the polymer matrix composite.Figure 5a shows an almost full view of the composite sample; a slender white contour is observable in the transversal section of the sample.This contour delimits different phases in the composite microstructure, i.e., the LDPE polymer matrix and the NMF; this contour appears to have the same thickness around the different phases, indicating a continuous interface boundary, which ensures a good adhesion between the matrix and the NMF particles [6]. Figure 5 presents SEM micrographs of a cut section of the polymer matrix composite.Figure 5a shows an almost full view of the composite sample; a slender white contour is observable in the transversal section of the sample.This contour delimits different phases in the composite microstructure, i.e., the LDPE polymer matrix and the NMF; this contour appears to have the same thickness around the different phases, indicating a continuous interface boundary, which ensures a good adhesion between the matrix and the NMF particles [6].

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Higher magnifications with a slight tilt of the sample.Figure 5b-d show the NMF particles surrounded by the LDPE.As the material is cut in order to take a sample, the polymer slips over the cutting blade and, since NMF particles have higher mechanical properties than the matrix polymer, these are not cut, generating protrusions in the new generated surface.These protrusions are semi-exposed NMF particles showing that NMF can resist the shear stress applied when cutting the polymer matrix.As can be seen, in the polymer composite's surface, the NMF particles are distributed over the LDPE matrix.These particles are aligned in the extrusion direction (longitudinal direction of the material) as presented in Figure 4.In this manner, the NMF particles improve the mechanical properties of the new composite.
Another characteristic that can be observed is the interface between the matrix and the NMF particles; there are no discontinuities along the particles' surface, indicating a good adhesion of these with the LDPE.An interesting observation from Figures 4 and 5 is the interface between particles and the polymer matrix: a continuum in the material around the particles is observable, indicating a good adhesion between the NMF and the LDPE material due to the employed combination of LLDPE, slip resin and LDPE, thus indicating a good synthesized process.LLDPE and slip resin are added to the LDPE pellets to improve interfacial adhesion and prevent debonding when the final product is subject to a load [19].moisture present on the sample, as well as to some low volatile material, as the TGA temperature increases from room temperature to 500 K; the extrusion process takes place at 450 K. Figure 6b presents the TGA of the LDPE composite.As can be seen, the weight loss of the samples is very low, with the slope of the curve displaying only a slight decrease, even smaller than that present in the NMF particles (Figure 6a); this behavior is maintained all the way from room temperature up to 741 K and this effect is due to the release of the superficial or bounded moisture of the NMF particles that stay on the surface of the Higher magnifications with a slight tilt of the sample.Figure 5b-d show the NMF particles surrounded by the LDPE.As the material is cut in order to take a sample, the polymer slips over the cutting blade and, since NMF particles have higher mechanical properties than the matrix polymer, these are not cut, generating protrusions in the new generated surface.These protrusions are semi-exposed NMF particles showing that NMF can resist the shear stress applied when cutting the polymer matrix.As can be seen, in the polymer composite's surface, the NMF particles are distributed over the LDPE matrix.These particles are aligned in the extrusion direction (longitudinal direction of the material) as presented in Figure 4.In this manner, the NMF particles improve the mechanical properties of the new composite.
Another characteristic that can be observed is the interface between the matrix and the NMF particles; there are no discontinuities along the particles' surface, indicating a good adhesion of these with the LDPE.An interesting observation from Figures 4 and 5 is the interface between particles and the polymer matrix: a continuum in the material around the particles is observable, indicating a good adhesion between the NMF and the LDPE material due to the employed combination of LLDPE, slip resin and LDPE, thus indicating a good synthesized process.LLDPE and slip resin are added to the LDPE pellets to improve interfacial adhesion and prevent debonding when the final product is subject to a load [19].
Thermal stability of the new composite was obtained by a thermogravimetric analysis (TGA).Because of the presence of brominated products in the raw material, a sample of the NMF particles was analyzed prior to the extrusion process for health and safety considerations in regard to the process and the people involved [26,27].Figure 6 presents the TGA plots of the NMF particles and the LDPE/NMF composite.Figure 6a shows the TGA of the nonmetallic fraction in order to determine the safety of the process.As can be seen, the NMF weight loss is very low, and is due to the release of superficial and bounded moisture present on the sample, as well as to some low volatile material, as the TGA temperature increases from room temperature to 500 K; the extrusion process takes place at 450 K. Table 1 presents the linear burning rate (V) and the melt dripping velocity of the ymer matrix composite reinforced with 3, 6, and 10 wt.% NMF particles as a filler; the filler condition is also included for reference.As can be seen, the linear burning rate o samples undergoes a slight decrease as the NMF particles increase, going from 17 mm/min.Polyethylene polymer combustion usually starts from thermal pyrolysis mechanism of random free radical chain scission [13].As the composite is on fire, the particles, fiberglass, and brominated resins reach the large interfacial boundaries o flaming polymer and agglomerate among them, forming an insulation barrier th duces the high flammability of the LDPE.Additionally, a melt dripping effect is pre Figure 6b presents the TGA of the LDPE composite.As can be seen, the weight loss of the samples is very low, with the slope of the curve displaying only a slight decrease, even smaller than that present in the NMF particles (Figure 6a); this behavior is maintained all the way from room temperature up to 741 K and this effect is due to the release of the superficial or bounded moisture of the NMF particles that stay on the surface of the LDPE composite, as well as to the degradation of the carbon-carbon bond chains of the LDPE [19].Only a 0.5 wt.% loss occurs in this temperature interval.The graph corresponding to the composite with 10 wt.% NMF filler displays a second stage of weight loss at 573 K; since the composite has a higher amount of NMF filler compared with the other analyzed composites' conditions, a higher amount of NMF is exposed on the material's surface due to the softening of the material as the temperature increases; having more exposed particles on the material's surface means more low volatile materials can be released [2,25].As the temperature increases above 741 K, the weight loss increases significantly, indicating the decomposition of the polymer matrix by the cleave of the carbon-carbon bonds in the chains of the polymer matrix.At 772 K, all of the polymer material has been decomposed, and only the NMF remains as a solid material.A total of 0, 1.6 and 6.3 wt.% is the sample material weight that remains above 772 K, for the composites with 0, 3, and 10 wt.% of the filler, respectively.
Table 1 presents the linear burning rate (V) and the melt dripping velocity of the polymer matrix composite reinforced with 3, 6, and 10 wt.% NMF particles as a filler; the non-filler condition is also included for reference.As can be seen, the linear burning rate of the samples undergoes a slight decrease as the NMF particles increase, going from 17 to 16 mm/min.Polyethylene polymer combustion usually starts from thermal pyrolysis by a mechanism of random free radical chain scission [13].As the composite is on fire, the NMF particles, fiberglass, and brominated resins reach the large interfacial boundaries of the flaming polymer and agglomerate among them, forming an insulation barrier that reduces the high flammability of the LDPE.Additionally, a melt dripping effect is present, and dripping might either cause secondary blazes, fire pools that maintain the combustion process or, as the material burns, dripping might occur in which the fuel material and heat will be removed from the pyrolysis zone, extinguishing the fire.The melt dripping of the polymer is modified when a filler is introduced into the polymer matrix composite, increasing its viscosity and limiting the flow of the molten material as well as reducing the melt drip velocity [28,29], as can be seen in Table 1; a greater amount of NMF particles reduces the melt drip velocity from 3.64 down to 1.60, increasing the detachment time between drops and indicating an improvement in the flame-retardant performance of the LDPE composite [17,30].Dripping of molten fuels occurs under the flame heat, which is maintained by the pyrolysis gasses from the melting material.Molten material drops can appear either as a continuous downward flow of fuel or as discrete drops that become detached from the fuel.Drops of molten polyethylene can often carry a flame that continuously peels off from the drip and then becomes a bright yellow flame, posing a significant fire hazard [17].Figure 7 shows photographs of the composite after the burning test; image inserts present a molten drop being detached from the polymer material.As can be seen, the raw LDPE material tends to drip in a continuum form; as the NMF content increases, the continuum dripping effect is limited and the drain reduced, indicating a positive effect of the NMF on the LDPE composite.The addition of NMF particles into the LDPE polymer improves its flame-retardant performance, reducing the large calorific value and fast flame propagation of the raw low-density polyethylene.Table 2 presents the measured and calculated density of the LDPE/NMF-par composites; calculated density was obtained with the rule of mixture; calculated de presents a slightly increase as the number of NMF particles increases; since the NMF ticles are heavier than the LDPE pellets, the density increases as these particles are i duced in the polymer matrix.
The density value for the LDPE pellets is 0.918 g/cm 3 ; since the extruded comp is conformed with three different kinds of pellets, i.e., LDPE, LLDPE, and slip resin measured density presents a higher value than the calculated density of the low-de polyethylene.The density value for the NMF is taken as 2.24 g/cm 3 ; this density value obtained after the separation process of the metallic fraction from the nonmetallic frac LDPE/NMF-particles composites show densities with small changes compared t LDPE; these changes in density are presented by the amount of filler material add the polymer and, also, since NMF particles are a mixture of several materials, main berglass, but also resins and other non-valuable materials, this mixture of NMF par has different density values and has an effect on the composite density obtained.The posite density will change as the NMF particles have more or less fiberglass and mo less other kind of materials.Table 2 presents the measured and calculated density of the LDPE/NMF-particles composites; calculated density was obtained with the rule of mixture; calculated density presents a slightly increase as the number of NMF particles increases; since the NMF particles are heavier than the LDPE pellets, the density increases as these particles are introduced in the polymer matrix.The density value for the LDPE pellets is 0.918 g/cm 3 ; since the extruded composite is conformed with three different kinds of pellets, i.e., LDPE, LLDPE, and slip resin, the measured density presents a higher value than the calculated density of the low-density polyethylene.The density value for the NMF is taken as 2.24 g/cm 3 ; this density value was obtained after the separation process of the metallic fraction from the nonmetallic fraction.LDPE/NMF-particles composites show densities with small changes compared to the LDPE; these changes in density are presented by the amount of filler material added to the polymer and, also, since NMF particles are a mixture of several materials, mainly fiberglass, but also resins and other non-valuable materials, this mixture of NMF particles has different density values and has an effect on the composite density obtained.The composite density will change as the NMF particles have more or less fiberglass and more or less other kind of materials.Figure 8 presents the mechanical properties, i.e., ultimate tensile strength (UTS) and yield strength (S 0 ) at 100 mm elongation of the new composite as a function of the amount of NMF particles added.The addition of NMF particles to the polymer matrix increases both the UTS and the S 0 values of the composite, indicating that the NMF has a significant effect on the mechanical properties.For example, for the material with 6 wt.% of NMF, the one with the best properties obtained, the UTS went from 20 MPa for the LDPE without any filler material to more than 68 MPa, while the S 0 went from 10 MPa to 46 MPa; similar values for LDPE composites were reported by [31].This increment is indicative of the contribution of the NMF particles to the improvement in the mechanical properties; as the number of NMF particles increases in the polymer matrix, the strength increases as well until a certain point, after which, further increasing the content of NMF particles results in decreasing mechanical properties, indicating a saturation point on the new composite.Although the mechanical properties decrease, their value stays above the UTS and S 0 values of the raw LDPE.
the agglomeration of the particles [32], acting out as large particles, whi gaps between them, reducing in this way the stress-bearing area [31], and mechanical properties of the composite.The amount and morphology of portant characteristics that contribute to the enhancement of the compo another effect is the interfacial adhesion between the NMF particles and t trix [2,33].Despite HDPE/NMF-particles composites having low mecha due to weak interfacial adhesion [6], the LDPE/NMF-particles composit this article present both good mechanical properties and a good union be particles and the polymer matrix since no evidence of void spaces in th detected during its microstructural characterization.The LDPE matrix pletely the NMF particles and no discontinuities are presented, as can be 4 and 5.The lack of void spaces or discontinuities indicates a good uni tween the nonmetallic fraction particles and the low density polyethylene surface between the polyethylene matrix and the NMF particles is presen versal as in the longitudinal section of the LDPE/NMF particles composit Previous works have shown that low reinforcement is achieved when NMF particles and high-density polyethylene is conformed, and this is compatibility of HDPE and NMF as well as to the agglomeration of the fi Beyond the saturation point, as the number of NMF increases, the mechanical properties continue to decrease; one effect that produces this loss of mechanical properties is the agglomeration of the particles [32], acting out as large particles, which causes larger gaps between them, reducing in this way the stress-bearing area [31], and decreasing the mechanical properties of the composite.The amount and morphology of the filler are important characteristics that contribute to the enhancement of the composite properties; another effect is the interfacial adhesion between the NMF particles and the polymer matrix [2,33].Despite HDPE/NMF-particles composites having low mechanical properties due to weak interfacial adhesion [6], the LDPE/NMF-particles composites analyzed for this article present both good mechanical properties and a good union between the NMF particles and the polymer matrix since no evidence of void spaces in the material was detected during its microstructural characterization.The LDPE matrix surrounds completely the NMF particles and no discontinuities are presented, as can be seen in Figures 4 and 5.The lack of void spaces or discontinuities indicates a good union interface between the nonmetallic fraction particles and the low density polyethylene; this continuum surface between the polyethylene matrix and the NMF particles is presented in the transversal as in the longitudinal section of the LDPE/NMF particles composite.
Previous works have shown that low reinforcement is achieved when a composite of NMF particles and high-density polyethylene is conformed, and this is due to the low compatibility of HDPE and NMF as well as to the agglomeration of the filler that creates different zones of stress concentrations [2,6].Nevertheless, when using low-density polyethylene pellets blended with NMF particles as described in the present work, a considerable improvement in mechanical properties is achieved due to a good interfacial union between the polymer matrix and the NMF particles.The slip resin is distributed over the polymer matrix and as it cools down, the head group of the slip resin becomes oriented towards the polymer while the tails group is oriented over the polymer's surface and also over the NMF particles, generating a good adhesion of these with the polymer matrix [34].Additionally, the NMF, mainly fiberglass particles, were oriented along the extrusion barrel, and fragmented reducing their length-diameter (L/D) ratio, during the extrusion process, thus increasing the amount of particles dispersed in the polymer matrix and reducing the gap between particles.As a result, the mechanical properties of the new composite are greater than the material without NMF particles.Polymer matrix composites reinforced with up to 6 wt.% of nonmetallic fraction particles from printed circuit boards present an improvement in their mechanical properties of more than 200%.At the same time, NMF particles form an insulation barrier when the polymer is combusted, reducing its high flammability.
The nonmetallic fraction from PCBs can be reused in the creation of reinforced polymer matrix composites in order to reduce the amount of discarded material from the PCB recycling process, reducing the environmental contamination and health risk associated with its disposal, and giving a value to the new product conformed, generating a zero residues process.

Polyethylene Matrix
Low-density polyethylene (LDPE) (Commercial name: PX 20020 X/P) grade film extrusion, Linear low-density polyethylene (LLDPE) (Commercial name BDL 92020 S), and slip resin (Erucamide pellets) from PEMEX-Petroquímica (México), were used as a raw material; all of these materials were donated by Orion Plastic in Estado de México, México.A mix of these three material pellets, in an industrial secret proportion, is the raw material used to produce the new composite.These mixed pellets generate the mechanical and physical properties required for LDPE products, and also allow for the correct integration of the NMF filler into the polymer matrix.

Nonmetallic Fraction
The nonmetallic fraction was obtained from arcade video games' printed circuit boards (PCBs) sourced from arcade machines in Estado de México, México.PCBs were crushed in a blade crusher machine and then the comminuted PCBs were sieve classified into three different groups of particles: Group 1: particles which passed the screen sieve # 50 but not # 60, this group, which is to be called from here on Sample M1, contains a particle size distribution of 250 µm; Group 2: particles which do not pass the screen sieve # 50; and, finally, Group 3: particles which pass the screen sieve # 60.Sample M1 retains the particles with a size that ensures a good liberation of the metallic from the nonmetallic fraction of the PCBs as described by Estrada-Ruiz R. et.al. [3].NMF particles with the 250 µm particle size distribution were employed to produce different composites [6,9,12], and therefore the NMF particles from Sample M1 were selected for the synthesis of the new composite.
Sample M1 particles were processed in a reverse froth flotation column to separate them into two different classes: a metallic fraction and a nonmetallic fraction, by taking advantage of the superficial properties of the particles.A detailed analysis of the separation of fractions is presented by Estrada-Ruiz R.H. et.al. [3] and by Flores-Campos R. et.al. [34].The particles recovered in the concentration zone correspond to the NMF particles employed as a filler for the generation of the new composite.A concentration process to obtain the NMF, including the sample M1 particles, was performed using a water pulp with the particles of the group 1; thus, it is considered an environmentally friendly process.

Particle Blending
In order to produce a polymer matrix composite, the different particles must be mixed together.The mixture of these particles must ensure a controlled segregation to obtain homogeneous properties of the consolidated composite product [5].Nonmetallic fraction particles with an average size distribution of 250 µm and the previously mixed polyethylene pellets were mixed together in a cone mixer machine with a blender screw, parallel to the container wall, rotating on its own axis during its travel along the mixer contour.The mixing process was carried out immediately before the extrusion process and the operation time was set up to 2 min.Several extrusion samples were obtained, for which the number of particles added was one to six, and 10 wt.% in order to determine the condition that results in the better composite properties; a condition without a filler was also produced as reference material.

Composite Synthesis
Once mixed, the raw material was fed into a screw extruder; a barrel temperature profile of 428 to 448 K from the feeding zone to the die head was maintained during the whole process duration; this working temperature is lower than the decomposition temperature for the brominated materials, 500 K, which ensures a safe process for the personnel involved.The angular velocity of the screw was fixed at 10 rpm.As the material moves from the hopper to the die, its temperature increases and the NMF becomes homogeneously distributed as the solid paste is being generated.When the material leaves the extruder, the solid paste is pulled through a cooling section; afterwards, the generated filament is coiled.By varying the coiler velocity, the filament diameter is controlled.Another option for the final product is to mold the material right as it comes out of the extrusion die.

Characterization
The microstructure of the synthesized composites was characterized by a scanning electron microscope (SEM) in a JEOL JSM 10LV microscope operated at 20 kV; some samples were tilted since a tilted sample generates a balance between the incoming electron charge and the electron charge leaving the surface of the polymer composite; by this process it was possible to obtain a charge-free image.Chemical analyses were determined by energy dispersive spectroscopy (EDS), using an Oxford Inca X-ray energy dispersive spectrometer attached to the microscope system.Also, a Dino-Lite digital microscope was employed to obtain photographs of the synthesized composites.
The thermal stability of both the NMF and the new composite was obtained by thermogravimetric analysis (TGA) by a TA Instruments SDT Q600.The samples were scanned starting at room temperature and up to 500 K in a nitrogen atmosphere with a flow rate of 200 mL/min and a heating rate of 10 K/min.The sample size was kept nearly the same for all the tests performed.The sample weight was in the range of 25-30 mg.
The flammability of the LDPE reinforced with NMF particles was studied in accordance with the standard test ASTM D 635.Ten samples of each condition were obtained with dimensions of 125 mm × 13 mm × 3 mm; the samples were held in the horizontal position, and with their traverse axis inclined at 45 • ; the burner was adjusted to produce a yellow-tipped blue flame of 20 mm, and then the burner was inclined at 45 • and moved so the flame impinged the free end of the sample to a depth of approximately 6 mm, maintaining it during 30 s; after that time, the burner was removed.Once the flame reached the 25 mm mark, the burning time was started; when the flame reached the 100 mm mark, the burning time was stopped.All samples' tests were performed inside a fume hood.The linear burning rate (V) was determined in the range of 25 mm to 100 mm; furthermore, the dripping detachment generation was evaluated.

Figure 2 .
Figure 2. Photographs of the raw material.(a) LDPE pellets, (b) NMF particles, (c) LDPE plus NMF after the blending process and (d) LDPE plus NMF at higher magnifications.

Figure 2 .
Figure 2. Photographs of the raw material.(a) LDPE pellets, (b) NMF particles, (c) LDPE plus NMF after the blending process and (d) LDPE plus NMF at higher magnifications.

Figure 3 .
Figure 3. Filaments of LDPE/NMF-particle composites showing an increased color tone as the NMF content increases.

Figure 4
Figure 4 presents SEM sputter coating micrographs of the transversal and longitudinal sections of the composite; several particles with different particle sizes distributed over the matrix are observable.Figures 4a,b display the transversal section; Figures4d,eshow the longitudinal section of the composite.As can be seen in Figure4a, several NMF particles with different particle sizes are distributed in the polymer matrix; this distribution is obtained by means of a good mechanical mixing and the extrusion process, which redistributes the particles in the polymer matrix as the mixed raw material flows through the screw extruder.At higher magnifications, Figure4bhighlights a detailed view of the morphology of the NMF particles.The filler particles composed mainly of fiberglass filaments with a round morphology are presented as a single or joined, by the epoxy resin, strand; the insert at the upper-right end of Figure4bshows the transversal view of individual strands of fiberglass joined together.This micrograph also highlights that the acicular particles were oriented in the flow direction, since the length of the fiberglass strands is perpendicular to the observed section, indicating that the extrusion process not only redistributes the particles but also orients them along the flow of the material.Figure4dpresents a long particle at the upper-left section of the micrograph; this particle, a fiberglass strand, lies almost parallel to the surface of the composite round bar.A more detailed observation, in Figure4e, shows that the particle was fractured in several parts along its length; this effect occurs due to the fact that, as the material flows through the extrusion barrel, the forces that some of the NMF experiment are strong enough to break them apart; as the NMF continue their travels in the extrusion barrel, they become oriented in the flow direction of the extrusion process.Figures4c,fpresent the EDS spectrums taken to the fiberglass and the epoxy resin, respectively; the highlighted numbers 1 and 2 in Figure4correspond to the fiberglass and brominated resin, respectively, as can be appreciated in the EDS spectrums.As can be seen, strand particles present silicon and aluminum as their main elements, which correspond to the fiberglass particles; on the other hand, the resin that joins the fiberglass strands is rich in bromine, which comes from the brominated flame retardant present in the PCBs[25].

Figure 3 .
Figure 3. Filaments of LDPE/NMF-particle composites showing an increased color tone as the NMF content increases.

Figure 4 .
Figure 4. SEM micrographs of the transversal section of the composite, (a,b); the longitudinal section of the composite, (d,e) and the EDS spectrums of the NMF particles for fiberglass, (c) and brominated resin, (f).

Figure 4 .
Figure 4. SEM micrographs of the transversal section of the composite, (a,b); the longitudinal section of the composite, (d,e) and the EDS spectrums of the NMF particles for fiberglass, (c) and brominated resin, (f).

Figure 5 .
Figure 5. SEM micrographs of the polymer composite, (a) full view (longitudinal and transversal sections), (b-d) transversal views of the material at higher magnifications.

Figure 5 .
Figure 5. SEM micrographs of the polymer composite, (a) full view (longitudinal and transversal sections), (b-d) transversal views of the material at higher magnifications.
its flame-retardant performance, reducing the large calorific value and fast flame p gation of the raw low-density polyethylene.

Figure 7 .
Figure 7.Samples after rate of burning test with different NMF particles content.Inserts pre drop formation for each condition.

Figure 7 .
Figure 7.Samples after rate of burning test with different NMF particles content.Inserts present a drop formation for each condition.

Figure 8 .
Figure 8. Mechanical properties as a function of the NMF particles content added trix.

Figure 8 .
Figure 8. Mechanical properties as a function of the NMF particles content added to the LDPE matrix.

Table 1 .
Linear burning rate and dripping formation as a function of the NMF particles content in the polymer matrix composite.

Table 2 .
Density of the polymer matrix composite as a function of the number of NMF particl

Table 2 .
Density of the polymer matrix composite as a function of the number of NMF particles.