Effect of v-PP/r-PET composites filament on specific density, structural and mechanical properties

Polypropylene (PP) and its derivatives are widely used to produce molded plastics due to their lightweight and impact resistance. Polyethylene terephthalate (PET) is considered one of the essential engineering polymers for its applications, widely used to manufacture containers for liquids such as water, oil, etc. The compatibility of PP/PET increases the impermeability, thermic and mechanical properties. The present study shows the blend binary of recycling PET as refill into a v-PP matrix extruded in filament analyzing the specific density and XRD. The result XRD pattern exhibited the presence of miscibility of monoclinic α-form phase v-PP/r-PET peaks predominantly attributed to the crystallinity of baseline material. Likewise, crystallinity, and amorphous peaks are attributed to the semi-crystalline phase of r-PET with a triclinic structure. In the impact strength test, all specimens tested showed partial failure on the surface, majorly presenting cracks, and fractures.


Introduction
Mexico is a worldwide success story in PET recycling, with a rate of 56% of all PET plastic consumed in the country, only behind the European Union, which has a 57% rate in recovering and recycling this material [1].According to authors [2], the average Mexican citizen drinks and discards two bottles per day, which equals nearly six kilograms per capita annually, contributing to a yearly national total of about 750,000 tons of PET.The main destination of recycled PET is for application in the textile industry.PP is a low-cost and quickly processed material that is most used in the plastics processing industry, especially in the packing industry.PET is an excellent barrier property polymer, transparent, and has good mechanical strength, making it the primary polymer in manufacturing bottles and fibers [3].PET waste is currently a severe pollution problem in Mexico.PET may enhance the stiffing of PP at high temperatures.The compatibility of PP/PET by functionalization with stearic acid leads to a finely dispersed morphology, good adhesion between phases, better processability during extrusion, and superior mechanical properties due to a reduction in interfacial tension because of enhanced interactions between the polar components of the blend [4].Instead, since the 3-D printing process can generate parts with different percentages of porosity, this represents a weakness in the elongation property.Also, highfrequency increments in the 3-D filaments are unaffordable.Therefore, this research work aims to study the proportional effect of PET recycled as a filler in the PP matrix through the extruded fused filament, evaluating its structural and mechanical properties to achieve a material for possible print application 3-D.

Materials
To make the filament composites recycled PET was used as filler in the PP matrix.The Indelpro Company (plant Altamira) manufactured the Profax 6331 pallets.The manufacturer was providing a datasheet for the virgin material used as base line (v-PP).The filling material is from post-consumer recycled PET bottles flakes cutting.These flakes size from 0.4mm< D <8mm.The matrix base line material matrix from the manufacturer datasheet of v-PP used (see Table 1).

Blending and filament processing
The filament composites were obtained using various v-PP/r-PET ratios by percent weight, with r-PET contents ranging from 10 to 50 wt%, using a mass total of 400 g.The make of v-PP/ r-PET recycled mixtures which were carried out using a manual mixer through a cylindrical container.A precision balance Ohaus model Pioneer PA 3102 (American Manufacturing) was used for carrier mass proportionality.Both polymers were dry-mixed using an oven air force Shellab Sheldon (American Manufacturing) at 100°C for 60 minutes.
The filament composites were fabricated using an equipment Beutelspacher extrude with single screw heating in four zones (18036 series, 1.5 HP,220V, Mexico Manufacturing).The work heating zone temperatures extrude equipment from the hopper to die, were set to 175°C, 183°C, 193°C, and 203°C respectively, and screw speed was fixed at 15 rpm.The diameter of filament composites was 3 mm on average.Before the impact strength test by the Gardner tester, the filament was cut in small pallets (3 mm) through cutting machine model 180306, using a speed controller Baldor-DC-Driver (Beutelspacher Mexico).Subsequently, the pellets were placed on a molded mold using a thermal press (Carver, USA), applying a compression load of 3000 Lb-in.The dimensions of the specimen were 27 mm in diameter and 3 mm in thickness.

Testing and Characterization
To determinate the specific density of each filament composite, the Balance method was used, using an analytical balance Ohaus AR 2140.Analysis structural by XRD was obtained using the Diffractometer System Empyrean, measurement program Panalytical, scanning conditions minimum step size of 0.0001 with 2-θ from 10 to 100°.The evaluation of impact resistance by fast deformation plastic is used by Gardner Impact (Columbia, Maryland, USA) using a maximum load 8 poundsforce (35.58 J/m), carrier-out five replicas.To review the imagen, a microscope optical model 5306 (Konus Campus, Verona Italy) is used, with low magnification 40X is used.The Figure 1 shows the schematic experimental from obtaining filament composites using the extruder until determination of specific density.

Specific density of filament composite
Table 2 shows the value specific density of filament composite extruded in the laboratory.It observes that the mixture of 50/50 the proportion of r-PET is major compared with all fused other filaments composite specific density.This behavior can be attributed to the miscible dominant phase of r-PET matrix composite with high-density [6].

Structural analysis
Figure 2 shows the diffraction patterns using the Bragg method.The samples measured correspond to mixtures of 10%, 30% and 50% r-PET flakes in a commercial v-PP matrix.In general, it can be observed that is notable a most intense peak, while it is promoted by the contribution of the matrix that exhibits material crystalline as predominant.The comfortable crystallinity in commercial grade polypropylene (v-PP) presents the alpha phase with a monoclinic structure (α-form) and hexagonal beta lattice cell (β-form).The authors [8], they mention that the difference between the phases diffracts a small peak at the angle 16.18°; therefore, considering the theory, the material used as base line corresponds to the α-phase.In this case, the diffraction angles attributed to the presence of the α-form exhibiting 2-θ=14.20°(110), 16.91° (040), 18.50° (130), 21:22° (111) and 21.92 (131) similar at the chart JCPDS Card no.66-1214 corresponds to monoclinic crystalline phase [9], these positions of diffraction peaks, most of them coincide with the works of [10 -14].The α-form role plays predominant under normal processing conditions.The β-form is more evident when the PP is exposed to a high temperature gradient or when processed with a higher shear.It has been reported that when materials of mineral or organic nature are added the PP structure becomes purely β-form [14].Other authors such as [15], they mentioned in their publication the modified β-form phase compared to α-form, it presents high impact resistance at low and high environmental temperatures.However, the elastic limit and modulus is higher for the α-form phase.In its structure there are both crystalline and amorphous regions, which is why it is known as a two-phase system.In turn, in the amorphous region, both isotactic and atactic structures can occur [14].Isotactic PP presents three types of polymorphic crystalline forms, called α, β and γ forms, which have monoclinic, hexagonal, and triclinic geometries, respectively [14].On the other hand, poly (ethylene terephthalate) known as PET, is considered a thermoplastic resin with a semi-crystalline structure.In this case, it is observed that at an angle of ~23° there is a superposition of the diffraction peaks of v-PP and r-PET.The crystallinity of the peak (higher intensity and narrower peak) is directly related to the amount of r-PET filler added to the α-form matrix.In this context, the peaks attributed to the presence of r-PET 2-θ=17.64°,21.32°,25.95° in its crystalline phase [15]; however, 2-θ =~23.11°,28.66° and 42.71° presents the amorphous phase whose XRD signals are like those found by [16], while is considered as semi-crystalline structure.The r-PET presents a triclinic structure with lattice parameters of a=4.56Å, b=5.94Å and c=10.75Å,whose unit cell angles are α=98.5°,β=118° and γ=112° (17 -19].Although several authors have published that the mixture of v-PP and r-PET are not thermodynamically compatible, favoring phase separation [20].In this context, in filament processing by extrusion of r-PET filler within the α-form matrix remains stable.

Plastic deformation by impact
Authors [21] mentioned that the mechanical properties can be improved by filling material into a matrix primary.This improvement should be a linear exponential positive in the relationship at two or more characteristics of intrinsic materials.Such mechanical properties as elastic module and strength can be evaluated using mixture laws with the mechanical properties of the component materials [21].So, considering the r-PET impact stretch value of 40 KJ/m 2 [22] and the value of 21.3 J/m obtained from the manufacturer datasheet [5].As known, the uniformly distributed material filler (r-PET) in the matrix (v-PP) allows the load to pass through without cracking or delamination [23].However, in this case, the poor filler dispersion and agglomeration of r-PET into v-PP matrix filament shape promote the stress concentration.Alike-know according to ASTM D520-04 [214], the types of failure are: 1. Crack or cracks on one surface only (the plaque could still hold water), 2. Cracks that penetrate the entire thickness (water would probably penetrate through the plaque), 3. Brittle shatter (the plaque is in several pieces after impact), or, 4. Ductile failure (the plaques penetrated by a blunt tear).
Figure 3(a-d) shows the image of evaluating the impact test on a specimen using a 35.58 J/m by Gardner tester.Figure 3(a) shows the baseline of the PP specimen, which does not present a crack or fracture when there was some loading.On the contrary, the figures 3(b-d), in all specimens of mixtures presented failure, say, it is partially fractured through a single line that starts from the edge of the specimen towards the center all specimen of mixtures presented failure, is say, it is partially fractured through a single line that starts from the edge of the specimen towards the center.The fracture in all specimens can be attributed to poor filler dispersion and agglomeration, creating stress concentration points within the composite.The specimen-loaded cracks may initiate and propagate, resulting in a reduced load-bearing capacity in the fracture composites caused mainly by filler-matrix adhesion.The intensely bound particles can effectively convey the stress load across the interface.In inverse, fillermatrix debonding causes physical discontinuity that cannot withstand mechanical forces.The homogeneity of the filler in a composite system also depends on the mixing technique employed.The most popular technique for preparing a reinforced thermoplastic composite is extrusion using a twinscrew extruder, i.e., the design of the screw and the co-rotating mechanism determine the homogeneity level of a thermoplastic mixture.Hence, the filler agglomeration issue can be resolved if a good mixing technique is used in which the stress concentration point formation can be avoided, and the distribution of the load can be dispersed well [23].

Conclusions
Specific densities showed that the more significant the proportion of r-PET, the density increases on average by 1.025.On the other hand, the XRD results exhibited the presence of polycrystalamorphous, found two crystalline phases and one amorphous.The crystalline phases are attributed to the contribution of the v-PP matrix and r-PET refill (monoclinic and triclinic structure, respectively).Likewise, the amorphous phase found is formed by the chain structure of r-PET refill.The impact strength by the Gardner tester showed that the refill r-PET into the PP matrix did not improve mechanical properties, i.e., all coupons were fractured at 8-inch-pound loaded.

Figure 1 .
Figure 1.Schematic experimental general of filament composites obtained by extruder.

Figure 2 .
Figure 2. Diffraction pattern of mixture r-PET inserted in matrix v-PP

Table 2 .
Determination of specific density of extruded composites filaments v-PP a / r-PET b .