Solid-state Shear Pulverization Research Articles

Effects of Polymer Properties on Solid-State Shear Pulverization: Thermoplastic Processability and Nanofiller Dispersibility.

Solid-state shear pulverization (SSSP) is an alternative polymer processing technique based on twin-screw extrusion with a continuous cooling system. In SSSP, low-temperature mechanochemistry modifies the macromolecular architecture and morphology, which in turn leads to physical property changes in the material. While a wide range of homopolymers, polymer blends, and polymer (nano)composites have been previously developed with SSSP, a fundamental understanding of how mechanochemistry affects polymer chain architecture and structure, and in turn, material properties, has not been elucidated. This paper conducts a systematic processing-structure-property relationship investigation of 10 thermoplastic polymers with varying properties, as they are subjected to consistent SSSP mechanochemical pulverization and nanocomposite compounding. Structural, mechanical, and thermal characteristics of the neat polymers are correlated to their response to SSSP by way of process covariants. Further, we investigate how SSSP processing parameters cause structural changes such as molecular weight reduction and filler dispersion level, which in turn dictate system properties like melt viscosity and thermal stability. Mechanochemical engagement with a high degree of physical contact during pulverization and compounding, characterized by the SSSP covariants exhibiting specific mechanical energy values above 4 kJ/g and an average screw temperature above 20 °C, is ensured when polymers have a glass transition temperature below the processing temperature (<50 °C) and high toughness (>40 MPa). Crystallinity and low thermal diffusivity (<0.2 mm2/s) are additional factors for engaged SSSP processing. Chain scission is an unavoidable outcome of SSSP, though the associated molecular weight reduction was <10% for 7 out of 10 polymers. The elucidated processing-structure-property relationships would allow the SSSP process for a given polymer system to be tailored to the specific needs for molecular structure alterations and performance improvements.

Solid‐state shear pulverization of post‐industrial ultra‐high molecular weight polyethylene: Particle morphology and molecular structure modifications toward conventional mechanical recycling

AbstractUltra‐high molecular weight polyethylene (UHMWPE) is one of the most prominent high‐performance thermoplastics for biomedical, leisure, and coating applications. Large‐scale recycling of UHMWPE is extremely difficult due to the high melt viscosity of the material as well as its exceptional chemical resistance and impact strength. There is a need for a commercially scalable methodology that can process the waste feedstock for mechanical recycling while sustaining the outstanding physical properties of the material. Solid‐state shear pulverization (SSSP) is a continuous, twin‐screw extruder‐based processing technique in which the low‐temperature application of shear and compressive forces impart changes in structure at different length scales to overcome the challenges of difficult‐to‐recycle polymers. This paper investigates the use of SSSP in mechanically recycling post‐industrial scrap UHMWPE (rUHMWPE) material from a local ski and snowboard manufacturer. The SSSP‐processed particles are flat, micron‐scale flakes with enhanced surface area, which can sinter very quickly when compression molded. The molded rUHMWPE samples in turn exhibit enhanced ductility and toughness compared to the as‐received scrap material, based on the tunable mechanochemical modification of the ethylene chains.

Polylactic Acid Chemical Foaming Assisted by Solid-State Processing: Solid-State Shear Pulverization and Cryogenic Milling.

A chemical foaming process of polylactic acid (PLA) was developed via the solid-state processing methods of solid-state shear pulverization (SSSP) and cryogenic milling. Based on the ability of solid-state processing to enhance the crystallization kinetics of PLA, chemical foaming agents (CFA) are first compounded before foaming via compression molding. Specifically, the effects of the pre-foaming solid-state processing method and CFA concentration were investigated. Density reduction, mechanical properties, thermal behavior, and cell density of PLA foams are characterized. Solid-state processing of PLA before foaming greatly increases the extent of PLA foaming by achieving void fractions approximately twice that of the control foams. PLA's improved ability to crystallize is displayed through both dynamic mechanical analysis and differential scanning calorimetry. The solid-state-processed foams display superior mechanical robustness and undergo low stress relaxation. The cell density of the PLA foams also increases with solid-state processing, especially through SSSP. Additionally, crosslinking of PLA during the pre-foaming processing step is found to result in the greatest enhancement of crystallization but decreased void fraction and foam effectiveness. Overall, SSSP and cryogenic milling show significant promise in improving chemical foaming in alternative biopolymers.

Production of thermoplastic starch and poly (butylene adipate-co-terephthalate) films assisted by solid-state shear pulverization

A novel processing technique involving Solid-State Shear Pulverization (SSSP) was used to produce thermoplastic starch (TPS) and Poly (Butylene Adipate-co-Terephthalate) (PBAT) films to improve processability and produce well-dispersed blends. Four different compositions (50−80 wt% TPS content) were processed using two different production routes. In one instance, the compositions were pre-treated by SSSP before melt extrusion (SSSPE). Secondly, starch was initially plasticized and thereafter blended with PBAT by melt extrusion (EXT) method. Flat films were produced using both routes and processability, visual and tactical aspects, mechanical and optical properties, crystallinity, and water absorption behavior were evaluated. High starch content films (70 and 80 wt%) prepared based on SSSP incorporation showed easier processability, and better visual aspect and mechanical integrity than EXT ones. However, EXT films with 50 and 60 wt% of starch presented higher elongation at break and lower water absorption due to finer dispersion of TPS and better starch plasticization.

The role of nanofiller size and polymer chain configuration on the properties of polypropylene/graphite nanoplates composites

Abstract This study aimed to evaluate the impacts of different polypropylene (PP) chain configurations, i.e., linear and branched form, on the rheological, mechanical, and electrical properties of PP-based, graphite nanoplates (GnP)-reinforced nanocomposites. The incorporation of GnP with different particle size gave us constructive information about the influences of filler's dimension on the abovementioned properties. A solid-state shear pulverization (SSSP) technique followed by conventional melt mixing was employed to get GnP-filled samples prepared for better filler dispersion. Due to higher interfacial surface area, the obtained mechanical properties from finer GnP were more satisfying. However, the electrical and rheological percolation thresholds (the formation of conductive pathways) took place at a lower content of larger GnP, which is mainly due to its higher aspect ratio compared to GnP. Replacement of linear PP with branched one did not lead to conspicuous alterations in tensile modulus and strength. Based on rheological evaluations, however, the presence of side branches hindered the development of GnP sound network throughout PP matrix, so that in comparison to linear PP-based sample the augmentation of electrical conductivity of branched PP-based composites against filler loading demonstrated slower rates.

Residence Time Distribution and Specific Mechanical Energy in Solid‐State Shear Pulverization: Processing‐Structure‐Property Relationships in a Chilled Extruder

Solid‐state shear pulverization (SSSP) is a continuous polymer processing methodology based on a modified twin‐screw extruder. The unique application of low barrel temperatures and mechanochemistry has contributed to the development of an extensive range of polymer‐based materials, from environmentally responsible polymer blends to nanocomposites, for more than 30 years. The complex processing‐structure‐processing relationships in SSSP can be elucidated by way of integrated, measured covariants that capture the interplay between numerous processing parameters. Residence time distribution and specific mechanical energy are evaluated in a base case polypropylene (PP) study under a full factorial experiment involving screw design, screw speed, and throughput parameters. These factors are in turn correlated to dispersion morphology and thermal property results from a parallel study based on a model PP/carbon black composite. This investigation highlights the tunability of SSSP processing parameters for tailored output with desired purposes and applications. In particular, enhanced residence distribution can be achieved with low screw speed and high throughput settings, leading to high levels of material mixing and shear. POLYM. ENG. SCI., 60:503–511, 2020. © 2019 Society of Plastics Engineers

Recycled Cellulose Polypropylene Composite Feedstocks for Material Extrusion Additive Manufacturing.

Many types of consumer-grade packaging can be used in material extrusion additive manufacturing processes, providing a high-value output for waste plastics. However, many of these plastics have reduced mechanical properties and increased warpage/shrinkage compared to those commonly used in three-dimensional (3D) printing. The addition of reinforcing materials can lead to stiffer parts with reduced distortion. This paper presents work in the reinforcement of recycled polypropylene using cellulose waste materials to generate a green composite feedstock for extrusion-based polymer additive manufacturing. Recycled polypropylene/waste paper, cardboard, and wood flour composites were made using a solid-state shear pulverization process. Fourier transform infrared and thermogravimetric analysis were utilized to qualitatively analyze the amount of filler incorporated into the 3D-printed materials. Recycled polymer composites had increased levels of filler incorporated in the printed parts compared to the virgin polymer composites based on the thermal gravimetric analysis. The dynamic mechanical analysis showed a ca. 20–30% increase in storage modulus with the addition of cellulose materials. Tensile strength was not significantly increased with the addition of 10 wt % cellulose, but the elastic modulus increased 38% in virgin polypropylene. The analysis of fracture surfaces revealed that failure initiates at the interface, suggesting that the interfacial strength is weaker than the filler strength.

Isolating the effect of polymer-grafted nanoparticle interactions with matrix polymer from dispersion on composite property enhancement: The example of polypropylene/halloysite nanocomposites

Interfacial effects can significantly perturb the properties of nanocomposites. Modification of filler surfaces can improve filler dispersion and compatibilize the polymer-filler interface, leading to property enhancements relative to composites made with unmodified filler. Here, we have isolated the effect of interactions between matrix polymer and polymer-grafted nanoparticles (PGNs) from dispersion quality and studied how the interactions affect the properties of polypropylene (PP)/halloysite nanotube hybrids in the absence of entanglements between matrix PP and grafted PP. We prepared PP hybrids with pristine halloysite nanotubes (p-HNT) or PP-grafted halloysite nanotubes (PP-g-HNT) up to 10 wt% filler using melt mixing or solid-state shear pulverization (SSSP). For melt-mixed samples, PGN-type filler yields better property enhancement than pristine filler, consistent with the combination of improved dispersion and polymer-filler interactions. As shown by microscopy and rheology, hybrids prepared by SSSP with p-HNT and PP-g-HNT fillers have very similar, high levels of dispersion, eliminating dispersion quality as a factor in property improvements. In comparison with well-dispersed PP/p-HNT nanocomposites at the same filler level, well-dispersed PP/PP-g-HNT nanocomposites prepared by SSSP exhibit enhanced tensile strength but the same values within error of Young's modulus and elongation at break when there is no entanglement between matrix polymer and grafted chains. Notably, at the same filler level, well-dispersed PP/PP-g-HNT nanocomposites exhibit higher crystallization rate and nucleation efficiency (NE) that is 2.5–4.5 times higher than well-dispersed PP/p-HNT hybrids. The impact of grafted PP chains on NE is so strong that at the same filler level but with worse dispersion, PP/PP-g-HNT hybrids made by melt mixing exhibit NE values 2.1 to 3.4 times higher than well-dispersed PP/p-HNT nanocomposites made by SSSP.

Crystallinity and Property Enhancements in Neat Polylactic Acid by Chilled Extrusion: Solid‐State Shear Pulverization and Solid‐State/Melt Extrusion

Polylactic acid (PLA) is a biopolymer of significant interest to both industry and the scientific community, but an incomplete understanding of the practical processing‐structure‐property relations is limiting its application range. The study applies alternative, chilled extrusion technologies called solid‐state shear pulverization (SSSP) and solid‐state/melt extrusion (SSME) to neat, commercial PLA, and investigates the effect of the resulting morphological features upon the thermomechanical behavior. Although conventional, heated twin‐screw extrusion leads to significant thermal degradation of PLA chains, which in turn facilitates crystal growth due to enhanced chain mobility, chilled SSSP imparts chain defects and branching, which serve as heterogeneous nucleation sites in the polymer and promotes the formation of a rigid amorphous phase. A hybrid SSME process brings unique interplay of both chain architecture effects, resulting in a synergistic thermomechanical behavior involving the α' crystal polymorph formation. POLYM. ENG. SCI., 59:E286–E295, 2019. © 2019 Society of Plastics Engineers

Low‐temperature compounding of flax fibers with polyamide 6 via solid‐state shear pulverization: Towards viable natural fiber composites with engineering thermoplastics

Low‐temperature compounding of natural fiber/thermoplastic composites via solid‐state shear pulverization (SSSP) is explored for the first time, with a goal of processing temperature‐sensitive natural fibers with high temperature‐melting engineering thermoplastics without fiber degradation. The model study was based on polyamide 6 (PA6) as the matrix material and short flax fibers as the filler materials; flax fiber type was varied to provide a range of comparison. Composite structural characterization was conducted using computer tomography, optical microscopy, and scanning electron microscopy, while mechanical property measurements were performed on injection molded specimens in both tension and bending. SSSP demonstrated robust and effective processing results in model PA6/flax composites, especially when compared with conventional extrusion. SSSP was able to isolate unmodified scutched fibers into individual elementary fibers with minimal scission, and effectively distribute them in the polymer matrix in situ. The dispersed and distributed filler morphology led to mechanical property enhancements, including 230% and 40% increases in Young's modulus and tensile strength, respectively, compared with neat PA6, at a 20 vol% fiber content. POLYM. COMPOS., 40:3285–3295, 2019. © 2018 Society of Plastics Engineers

Poliolefiny napelniane odpadami sztywnej pianki poliuretanowej i kompatybilizowane metoda pulweryzacji scinaniem w stanie stalym podczas wytlaczania

Low density polyethylene (PE-LD), high density polyethylene (PE-HD), PE-LD/PE-HD blend at aratio of 1:1, and polypropylene (PP) were filled with finely-ground waste of rigid polyurethane-polyisocyanurate (PUR-PIR) foam in an amount up to 40 wt %. The components were homogenized with two-roll-mill and then, after granulation, subjected to additional homogenization using the method of solid-state shear pulverization (so-called S3P process) during extrusion. The specimens for comparative studies were obtained by injection molding. The changes in chemical structure (using FT-IR spectroscopy), mechanical properties (tensile and impact tests), rheological properties (MFR, rotational rheometry), and thermal properties (Vicat temperature) of the investigated compositions were determined. It was found that the pulverization has afavorable effect on the characteristics of the examined systems, although the improvement is not significant and clearly depends on the type of matrix polymer and foam content.

Direct Use of Natural Antioxidant-rich Agro-wastes as Thermal Stabilizer for Polymer: Processing and Recycling

Antioxidant-rich agro-wastes such as grape pomace waste (GW), turmeric shavings and waste, coffee grounds, and orange peel waste are used as-received for the first time as thermo-oxidative stabilizers for polymer. Relative to neat low density polyethylene (LDPE), a well-dispersed hybrid made by solid-state shear pulverization with 4 wt % GW results in 62 and 44 °C increases in temperatures corresponding to 10 and 20% mass loss in air (T10% and T20%), respectively. Such enhancements are superior to those obtained by adding 1 wt % synthetic antioxidant Irganox I1010 to LDPE by melt mixing. Relative to neat LDPE, hybrids with well-dispersed agro-waste exhibit enhanced Young’s modulus, equal or enhanced tensile strength, and relatively small reduction in elongation at break. Reprocessing or recycling sometimes leads to enhanced antioxidant activity: relative to a hybrid before melt extrusion, 92/8 wt% LDPE/TW exhibits major increases in T10% and T20% after two and six melt extrusion passes, which is consisten...

Morphology and toughness enhancements in recycled high‐density polyethylene (rHDPE) via solid‐state shear pulverization (SSSP) and solid‐state/melt extrusion (SSME)

ABSTRACTSolid‐state, mechanochemical polymer processing techniques are explored as an effective and sustainable solution to appearance and performance issues commonly associated with recycled plastic products. Post‐consumer high‐density polyethylene (HDPE) from milk jugs is processed via conventional twin screw extrusion (TSE), solid‐state shear pulverization (SSSP), and solid‐state/melt extrusion (SSME), and compared to the as‐received and virgin forms regarding output attributes and mechanical properties, as well as morphology. Solid‐state processing methods, particularly SSME with a harsh screw configuration, produce samples with consistent appearance and melt flow characteristics. Tensile ductility/toughness and impact toughness are enhanced by up to 11‐fold as compared to the as‐received sample, to a level near and above those of an equivalent virgin HDPE. Calorimetry, optical microscopy, X‐ray scattering, and rheology characterization reveal that the mechanical improvements result from a favorable combination of physical and molecular changes in rHDPE, such as impurity size reduction, spherulite size enlargement, and chain branching. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43070.

Green polypropylene/waste paper composites with superior modulus and crystallization behavior: Optimizing specific energy in solid-state shear pulverization for filler size reduction and dispersion

Solid-state shear pulverization (SSSP) is a continuous process that overcomes challenges in producing well-dispersed polymer composites that cannot be made by twin-screw melt extrusion. We use SSSP to produce 85/15wt% polypropylene/waste paper biocomposites with polypropylene pellets and 2-cm-square waste paper pieces as starting material. Single-pass SSSP achieves effective filler size reduction and dispersion within the polypropylene matrix. We determine how waste paper size reduction and composite properties are functions of specific energy input and tune specific energy input by SSSP screw design and throughput. Composites made at moderate to high specific energy input (14–35kJ/g) have 25 to nearly 50% of filler particles at sub-micron size; relative to neat polypropylene, composites exhibit a 70% increase in Young’s modulus, retention of neat polypropylene yield strength, and a ∼50% reduction in crystallization half-time. Estimates indicate that the cost of such biocomposite materials made by SSSP is less than that of virgin polypropylene.

Comparison of polyolefin biocomposites prepared with waste cardboard, microcrystalline cellulose, and cellulose nanocrystals via solid-state shear pulverization

As a significant part of municipal solid waste (MSW), waste cardboard (CB) is a sustainable, inexpensive, and rich source of cellulose. Previous studies of polyolefin/CB composites have reported modest enhancement to major reduction in modulus and major reduction in elongation at break values relative to neat polymer. Here, green hybrids of low density polyethylene (LDPE) and polypropylene (PP) with 5–25 wt% CB are made by solid-state shear pulverization (SSSP), which achieves both size reduction of 2–3 cm sized CB pieces to the micron level and dispersion in polymer. The properties obtained with CB incorporation in LDPE and PP are compared and contrasted with those obtained with incorporation of microcrystalline cellulose (MCC) and cellulose nanocrystal (CNC). Polyolefin composites with CB made by SSSP exhibit major enhancement in Young's modulus (63% and 71% increases for 10 wt% CB in LDPE and 15 wt% CB in PP, respectively). The PP/CB composites exhibit a broad range of property enhancements relative to neat PP, including a nearly 50% nucleating efficiency, as much as an 8% increase in PP crystallinity, and a factor of ∼3 decrease in crystallization half-time. Well-dispersed CB particles improve LDPE and PP thermo-oxidative stability as shown by thermogravimetric analysis (∼5–20 °C increase in 20% mass loss temperature in air with 15–20 wt% CB addition) and isothermal shear flow rheology. Similarly, post-SSSP high-temperature, long-time melt mixing results in no apparent degradation of LDPE/CB and MCC composites whereas LDPE/CNC composites show major degradation. When incorporated into polyolefin composites, low cost, cellulose-rich MSW can often produce reinforcement similar to glass fibers and thus has potential as filler for structural composite applications.

Importance of superior dispersion versus filler surface modification in producing robust polymer nanocomposites: The example of polypropylene/nanosilica hybrids

With polymer nanocomposites, achieving highly effective dispersion of agglomerated nanofiller and major or optimal property enhancements remain challenges. A commonly posited solution is to improve the polymer-filler surface thermodynamic compatibility; this approach has led to significant improvements in some cases, but it has not provided a general solution. We address the question of whether achieving a metastable, well-dispersed state is better than compatibilization in attaining the goal of major property enhancements. We use solid-state shear pulverization to produce well-dispersed polypropylene (PP) nanocomposites with up to 8 or 9 wt% pristine nanosilica (p-NS) or organically modified nanosilica (m-NS). Microscopy shows that as-received, tens-of-micron-sized p-NS and m-NS agglomerates undergo very good dispersion, with ∼10–100 nm size-range nanofiller in hybrids. Rheology is consistent with very good dispersion, with only 92/8 wt% PP/p-NS indicating incipient nanofiller network formation. The PP/p-NS hybrids have superior Young's modulus and tensile strength. Relative to PP, modulus increases by 22% and 12% and tensile strength by 19% and 14% for 99/1 wt% PP/p-NS and 99/1 wt% PP/m-NS, respectively. The PP/p-NS hybrids have the largest increases in modulus (46% at 8 wt% p-NS) and tensile strength (22% at 6 wt% p-NS). Upon melting and crystallization, both PP/p-NS and PP/m-NS result in PP β-crystal formation at 1 wt% nanosilica, with p-NS having a greater effect. The PP/p-NS hybrid shows larger increases in thermal stability and nucleating efficiency for PP crystallization. Thus, with very good dispersion, unmodified nanofiller in a metastable dispersed state can result in more robust nanocomposites than when modified nanofiller is used to compatibilize the polymer–nanofiller interface.

Sustainable Green Hybrids of Polyolefins and Lignin Yield Major Improvements in Mechanical Properties When Prepared via Solid-State Shear Pulverization

Lignin, a byproduct of paper and pulp industries, is a sustainable, inexpensive biomaterial with potential as composite filler. Past research on polyolefin/lignin composites made by melt processing has led to modest increases in Young’s modulus and drastic reductions in tensile strength and elongation at break relative to neat polymer. Here, green hybrids of low density polyethylene (LDPE) and polypropylene (PP) with 5–30 wt % lignin are made by solid-state shear pulverization (SSSP). Microscopy shows that SSSP leads to superior lignin dispersion and suppressed degradation when compared to melt-mixed composites reported in the literature. Composites made by SSSP exhibit major improvements in Young’s modulus (81% and 62% increases for 30 wt % lignin in LDPE and PP, respectively, relative to neat polymer), tensile strength equal to or better than that of neat LDPE and near that of neat PP, and much better strain at break than reported in the literature for polyolefin/lignin composites. The SSSP-produced hybrids exhibit major increases in hardness, with 70/30 wt % PP/lignin hybrids reaching values near that of polycarbonate. Well-dispersed lignin improves LDPE and PP thermo-oxidative stability as shown by thermogravimetric analysis (∼35 °C increase in 20% mass loss temperature in air with 20 wt % lignin addition) and isothermal shear flow rheology. Lastly, SSSP-processed composites exhibit slightly improved crystallizability and melt viscosities at moderate to high shear rates that differ relatively little from those of neat LDPE and PP.

Dispersion and Property Enhancements in Polyolefin/Soy Flour Biocomposites Prepared via Melt Extrusion Followed by Solid‐State Shear Pulverization

Biocomposites of low‐density polyethylene (LDPE) and polypropylene (PP) with 5–40 wt% soy flour (SF) are produced by two‐step single‐screw extrusion (SSE) followed by solid‐state shear pulverization (SSSP). The SSE‐SSSP approach overcomes limitations with melt mixing, e.g poor SF dispersion and degradation, and limitations with single‐step SSSP. Microscopy shows that SF is well dispersed in SSE‐SSSP composites but agglomerated and degraded in melt‐mixed composites. The SSE‐SSSP composites exhibit major improvements in Young's modulus relative to neat polymer, including 74 and 43% increases in 80/20 wt% LDPE/SF and 95/5 wt% PP/SF composites, respectively. Relative to neat polymer, SSE‐SSSP composites exhibit the largest improvements in Young's modulus and best tensile strengths reported for polyolefin/SF composites. Crystallization and viscosity are only slightly affected by SF in the composites. At 20% and higher mass loss, char can result in greater thermo‐oxidative stability of 80/20 wt% polyolefin/SF composites relative to neat polymer.

Novel synthesis of branched polypropylene via solid-state shear pulverization

As synthesized by Ziegler–Natta or metallocene catalysis, commercial polypropylene (PP) is linear and has low melt strength. A common approach for improving melt strength is to incorporate long-chain branches (LCBs). We describe the discovery of a novel approach to prepare LCB PP by subjecting linear PP to solid-state shear pulverization (SSSP) with benzoyl peroxide (BPO) as the lone additive. Depending on BPO content, LCB PP can be formed by radical reactions during SSSP or post-SSSP melt extrusion. Using shear rheology, we demonstrated that LCB PP was formed during SSSP of samples with 4 and 6 wt% BPO. Relative to linear PP, the post-SSSP sample (purified of residual BPO) made with 6 wt% BPO exhibited enhanced shear thinning behavior, a decreased dependence of storage modulus (G′) on frequency (ω) at low ω, and a deviation from linear polymer behavior in its van Gurp–Palmen curve. For samples that were prepared with low levels of BPO (0.5–1.5 wt%), LCBs were not formed (within error) during SSSP; instead, LCBs were formed during post-SSSP melt extrusion (with residual BPO). While a sample with 1.5 wt% BPO showed no deviation from linear polymer behavior when tested immediately after SSSP, the same sample after post-SSSP melt extrusion demonstrated enhanced shear thinning behavior, decreased dependence of G′ on ω at low ω, deviation from linear polymer behavior in its van Gurp–Palmen plot, and improved crystallization and tensile properties (as is expected for branched PP). We also showed that the LCB formation is achieved by taking advantage of the near-ambient temperature conditions associated with SSSP and is unattainable via conventional melt processing of PP.

Effects of process method and quiescent coarsening on dispersed-phase size distribution in polymer blends: comparison of solid-state shear pulverization with intensive batch melt mixing

We compare how solid-state shear pulverization (SSSP), batch melt mixing (BMM), and static melt-state annealing affect the morphology of an immiscible blend of polypropylene (PP) and ethylene-α-olefin copolymer (EOC). For 85/15 wt% PP/EOC blends, SSSP and BMM led to log-normal distributions of dispersed-phase particle size. The SSSP blend had smaller average particle diameters (e.g., D n = 0.24 μm) and a narrower particle size distribution (e.g., D w/D n = 1.17; D v/D n = 1.56) than the BMM blend (D n = 0.28 μm; D w/D n = 1.25; D v/D n = 1.79). The fact that BMM is subject to thermodynamically and flow-induced coalescence while SSSP is not, can lead to smaller particle sizes and a narrower distribution by SSSP. Although annealing at 200 °C for 30 and 90 min led to continuous growth of average particle size in the BMM blend, the particle-size dispersity remained virtually unchanged. In contrast, after 30-min annealing at 200 °C, the SSSP blend showed less growth in $$D_{{_{n} }}^{{^{3} }}$$ than the BMM blend but a dramatic increase in particle-size dispersity and a loss of the log-normal size distribution. Between 30 and 90 min, there was at most slight growth in $$D_{{_{n} }}^{{^{3} }}$$ , consistent with partial compatibilization caused by in situ block copolymer formation during SSSP, and major reductions in D w/D n and D v/D n close to those of the BMM blend and recovery of the log-normal size distribution. These results suggest that caution should be used in correlating immiscible blend properties to a particular average particle size as that value may not reflect possible complexity of the underlying size distribution.