In postindustrial polyolefin recovery streams, the blend of polypropylene-rich thermoforming scrap, polyethylene film edge trim, and mixed injection-molding purgings produces a recyclate whose notched impact toughness is frequently lower than that of virgin impact copolymers because the feedstock contains variable fractions of low-molecular-weight polypropylene, unpigmented and pigmented polyethylene, oxidized fines, and residual adhesive or label contamination. When ethylene-vinyl acetate copolymer grade EVA 1703 with a nominal vinyl acetate content of 17 wt% and a melt flow index of 3.0 g/10 min at 190 °C/2.16 kg per ISO 1133-1:2022 is added at levels from 5 wt% to 20 wt%, the impact response is controlled less by modifier concentration alone than by the distribution of EVA domains within the PP-continuous and PE-rich phases, the thermal history introduced during compounding, and the notch acuity of the finished test specimen. The postindustrial PP/PE feedstock is characterized by wide melt flow variability, often from 8 g/10 min to 35 g/10 min at 230 °C/2.16 kg, and by contamination that can include paper fiber, ink particles, silicone oils, and calcium carbonate fillers. These constituents alter both the nucleation and coalescence of the dispersed EVA phase and the plane-strain impact response measured by notched tests; therefore the addition of EVA 1703 must be interpreted as a rheology-sensitive modification strategy rather than a simple impact-modifier addition.
Impact energy dissipation in EVA 1703-modified PP/PE recyclate is governed by the competition between internal cavitation of the rubbery EVA domains, debonding at the EVA–matrix interface, and shear yielding of the PP-rich matrix. EVA 1703, with 17 wt% vinyl acetate content, has a crystalline melting range of approximately 80 °C to 90 °C when measured by differential scanning calorimetry at 10 °C/min under nitrogen in accordance with ISO 11357-3:2018, and its glass transition is expected in the range of −25 °C to −15 °C based on the vinyl acetate content. These transitions are significant because the dispersed EVA domains remain rubbery at low temperatures but may undergo phase separation from polyethylene-rich fractions during slow cooling. At low addition levels of 5 wt%, EVA 1703 typically forms discrete spherical inclusions with diameters below 1.5 µm if compounded at sufficient specific energy input. At loadings above 15 wt%, domain coarsening and partial continuity can occur, particularly when the PP/PE recyclate contains more than 25 wt% polyethylene film scrap. The resulting morphology determines whether the test specimen exhibits stress whitening around the notch root, whether crack propagation follows the EVA–matrix interface, and whether the absorbed energy is dissipated as shear yield of the bulk matrix or as brittle fracture through the PP-rich regions. Because the postindustrial recyclate is not a well-defined single polymer, the critical morphological parameter is not simply particle size; it is the interparticle ligament thickness in the PP-continuous regions, which is influenced by both EVA domain size and the local volume fraction of PE-rich inclusions.
On a 40 mm co-rotating twin-screw extruder with an L/D ratio of 48:1, the addition of EVA 1703 to washed postindustrial PP/PE flake requires controlled shear energy to generate the fine dispersed morphology that improves impact. The feed throat is flood-fed, the first barrel zone is set at 70 °C, and subsequent zones are ramped to 210 °C, with the die adapter set at 200 °C. Vacuum devolatilization at −0.08 MPa gauge is applied after the kneading section to remove moisture and low-molecular-weight volatiles; without effective venting, strand porosity and surface defects are observed on production lines processing reground film fractions. Screw speed must remain above 350 rpm because the viscosity ratio between EVA 1703 and a PP/PE recyclate with a melt flow index of 15 g/10 min at 230 °C/2.16 kg can be approximately 1.5:1 to 2.5:1 at 1000 s⁻¹, and the high-viscosity EVA phase remains as large agglomerates if specific energy input falls below approximately 0.22 kWh/kg. When screw speed exceeds 600 rpm, shear heating can raise melt temperature above 230 °C, at which point deacetylation of EVA 1703 begins to liberate acetic acid, causing surface splay, odor, and accelerated corrosion of carbon steel tooling if the acid is not neutralized by an appropriate acid scavenger such as calcium stearate at 0.1 wt% to 0.3 wt%. This processing window is significantly narrower than the typical 170 °C to 240 °C barrel range used for unmodified PP/PE recyclate, and it represents one of the primary operational boundaries for the compound.
The notched Izod measurement is performed in accordance with ASTM D256-23 using a pendulum impact tester of 5.5 J capacity on specimens of 63.5 mm × 12.7 mm × 3.2 mm with a notch radius of 0.25 mm and a notch angle of 45°. Because postindustrial PP/PE recyclate can contain fractions that show partial breaks rather than complete breaks, the operator records both break type and absorbed energy, and data from incomplete breaks are not directly compared with complete-break energies. In the Charpy configuration, ISO 179-1:2023 is applied to type 1 specimens of 80 mm × 10 mm × 4 mm cut from plaques injection-molded to ISO 294-1:2017 and conditioned for 40 h at 23 °C/50 % RH in accordance with ISO 291:2008. The edgewise notched Charpy value is generally more reproducible for recycled mixed polyolefins than flatwise Izod because it integrates the skin-core morphology generated by injection molding and is less sensitive to surface imperfections from residual contamination. At temperatures below −20 °C, the specimen is conditioned for 24 h in a temperature-controlled chamber with a stability of ±1 °C, and the pendulum energy is selected to produce a complete break without excessive plastic deformation; if the specimen slips or bends through the fixtures, the data are rejected. For postindustrial PP/PE recyclate, a coefficient of variation above 25 % in notched impact results is common because of local compositional drift, and ten specimens are tested per condition rather than five to improve confidence in the mean.
| Test | Standard | Specimen geometry | Conditioning | Reported value |
|---|---|---|---|---|
| Notched Izod impact | ASTM D256-23 | 63.5 mm × 12.7 mm × 3.2 mm | 40 h at 23 °C/50 % RH | kJ/m², break type |
| Charpy edgewise notched impact | ISO 179-1:2023 | 80 mm × 10 mm × 4 mm, type 1 | 40 h at 23 °C/50 % RH | kJ/m², complete or partial break |
| Instrumented puncture impact | ISO 6603-2 | 60 mm × 60 mm × 2 mm plaque | 40 h at 23 °C/50 % RH | maximum force, total energy |
| Tensile yield stress | ISO 527-2:2012 | Type 1A, thickness 4 mm | 40 h at 23 °C/50 % RH | MPa |
| Flexural modulus | ISO 178:2019 | 80 mm × 10 mm × 4 mm | 40 h at 23 °C/50 % RH | MPa |
| Melt flow index of PP/PE fraction | ISO 1133-1:2022 | Condition 230 °C/2.16 kg | N/A | g/10 min |
| Moisture content | ISO 15512:2019 | Granules or flake | N/A | wt% |
During injection molding of EVA 1703-modified recyclate on a 1200 kN clamp force machine with a 40 mm screw and a 3:1 compression ratio, melt temperature is controlled at 210 °C, mold temperature at 30 °C, holding pressure at 45 MPa, and holding time at 12 s. EVA 1703 lowers melt viscosity moderately at high shear, but the main processing change is a reduction in gate freeze time; therefore the holding pressure profile is shortened by 2 s to 3 s relative to unmodified PP/PE recyclate to avoid overpacking and gate blush. If gate blush appears, the first adjustment is to reduce holding pressure to 35 MPa and increase mold temperature to 40 °C rather than reduce injection speed, because higher shear promotes dispersion and gloss uniformity. A production-scale issue observed in such compounds is chalking on the mold surface after 8 h to 12 h runs when the vacuum vent is not maintained; the deposit is predominantly low-molecular-weight polyethylene wax and a small acetate fraction, not gross EVA degradation, but it alters micro-texture and can appear as haze on dark colored parts. The compound also exhibits moderate mold shrinkage anisotropy, with longitudinal shrinkage of 1.2 % to 1.6 % and transverse shrinkage of 0.8 % to 1.2 % at 3 mm thickness, measured after 48 h at 23 °C per ISO 294-4:2018; this anisotropy is greater than that of virgin impact copolymer and must be considered when reusing existing tooling originally designed for unmodified PP.
Residual polyethylene film in the postindustrial stream causes a distinct thermal and rheological interaction with EVA 1703. Because both components contain ethylene sequences, the EVA domains preferentially migrate toward the PE-rich regions during melt compounding. At barrel temperatures above 190 °C and with sufficient residence time, the PE phase swells with EVA, and the dispersed EVA domains lose their discrete spherical shape, forming elongated or interconnected structures that can exceed 5 µm in length. This morphology change does not necessarily reduce impact toughness; in thin-wall parts it can improve puncture energy by creating a more diffuse interface that absorbs energy during high-speed dart penetration under ISO 6603-2. However, in thick specimens tested under plane-strain notched Izod conditions, the same morphology can reduce resistance to crack initiation because the elongated domains act as stress concentrators perpendicular to the notch plane. The critical processing limit is therefore not solely the peak temperature but the combination of temperature, residence time, and PE content: if PE fraction exceeds 30 wt% of the recyclate, EVA 1703 loading above 10 wt% produces unstable morphology in a 48:1 L/D twin-screw extruder unless the screw configuration contains additional distributive mixing elements after the first kneading section. Published data for this specific configuration is limited, but the behavior is consistent with the thermodynamic compatibility of ethylene-rich segments in polyethylene and the tendency of vinyl acetate groups to interact with oxidized PE surfaces.
With a feedstock containing less than 15 wt% PE film and an EVA 1703 addition of 8 wt%, scanning electron microscopy of cryo-fractured surfaces etched with xylene at 40 °C for 10 min shows EVA domain diameters between 0.5 µm and 1.5 µm. Under these conditions, the notched Izod at 23 °C typically shows a transition from partial break to complete break, and the absorbed energy increases relative to the unmodified recyclate by a factor of 1.5 to 2.2. The improvement is attributed to cavitation of EVA domains and associated shear yielding of the PP matrix; this is evidenced by stress whitening around the notch root and by increased fracture surface roughness when the EVA domains are fine. When the domain size exceeds 2.5 µm, the stress-whitening zone is larger but the crack propagation energy is lower because cavitation occurs along the weakest interfaces before the surrounding matrix can yield. The target is therefore not merely a minimum modifier loading but a narrow particle size distribution below 2 µm and an interparticle ligament thickness that permits stress overlap between adjacent cavitated domains.
Interfacial adhesion between EVA 1703 and PP/PE recyclate is weak without an interfacial agent; the polar vinyl acetate comonomer contributes a solubility parameter closer to polyethylene than to polypropylene but does not create covalent bonding. The addition of maleic anhydride grafted polypropylene at 1 wt% to 3 wt% increases Charpy notched impact energy mainly by reducing EVA domain size and preventing coalescence during injection molding, not by increasing interfacial adhesion alone. According to the brittle-to-tough transition concept, the matrix ligament thickness between dispersed rubbery particles must fall below a critical value; in PP-rich matrices the critical value is commonly cited in the range 0.2 µm to 0.4 µm for ligament-based shear yielding under quasi-static conditions. With EVA 1703, the lower melt flow index and higher viscosity require higher shear energy than typical ethylene-octene or ethylene-propylene rubber modifiers; thus a compounder that can disperse 10 wt% of an ethylene-octene modifier may not obtain fine dispersion with EVA 1703 without increasing screw speed by 15 % to 25 %. The same effect appears in capillary rheometry: at 200 °C and 1000 s⁻¹, EVA 1703 is typically 20 % to 40 % more viscous than a 20 g/10 min PP/PE recyclate, so the shearing stress transferred to the discrete phase is lower unless the matrix viscosity is also raised by using lower-melt-flow polypropylene or by decreasing barrel temperature within the safe processing window. This rheological mismatch is one reason why EVA 1703 cannot be treated as a direct drop-in replacement for metallocene-catalyzed ethylene-octene impact modifiers in recycled polyolefin compounds.
| Parameter | Lower limit | Upper limit | Basis |
|---|---|---|---|
| Compounding barrel temperature | 180 °C | 220 °C | Complete EVA melting; avoid deacetylation above 230 °C |
| Screw speed, 40 mm L/D 48:1 | 350 rpm | 600 rpm | Fine dispersion without excessive shear heating |
| Specific energy input | 0.22 kWh/kg | 0.28 kWh/kg | Required for domain size below 2 µm |
| Vacuum devolatilization | −0.08 MPa gauge | N/A | Remove moisture and low-molecular-weight volatiles |
| Injection mold temperature | 30 °C | 40 °C | Balance surface gloss and impact morphology |
| Moisture before compounding | N/A | 0.10 wt% | Prevent strand porosity and surface defects |
| EVA 1703 loading | 5 wt% | 20 wt% | Upper bound set by stiffness loss above 20 wt% |
When the modified recyclate is exposed to prolonged heat aging at 100 °C for 500 h in air, the impact toughness declines more rapidly than virgin impact copolymer because the vinyl acetate segments are susceptible to thermo-oxidative degradation; this is measured by a shift from complete break to partial break in notched Izod and by an increase in the infrared carbonyl index. The use of a hindered phenolic antioxidant at 0.1 wt% to 0.3 wt% and a phosphite secondary antioxidant at 0.1 wt% extends the time to 50 % loss of impact energy to roughly 800 h, but published data for this specific postindustrial PP/PE recyclate configuration is limited. Pre-drying of the recyclate is required when moisture content exceeds 0.10 wt% as measured by ISO 15512:2019; drying at 80 °C for 2 h in a desiccant dryer reduces moisture-related surface defects. EVA 1703 itself should not be pre-dried above 60 °C to avoid pellet agglomeration. The compound should not be blended with amine-based antistats or amine-cured epoxy residues because the acetate groups can react with basic nitrogen species; the result is discoloration and a measurable reduction in both melt flow index and notched Izod energy. For regulatory compliance, the recyclate must be tested against REACH Annex XVII restrictions for PAH and cadmium if it is intended for consumer durables, and against RoHS Directive 2011/65/EU Annex II for lead and brominated flame retardants if electrical or electronic equipment housings are produced. If food-contact use is contemplated, the postindustrial PP/PE recyclate is generally not suitable without additional decontamination and may be subject to FDA 21 CFR 177.1520 and EU Regulation (EC) No 1935/2004, which impose purity and migration limits that are difficult to satisfy with mixed postindustrial streams.
