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

Solid‐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.     

Transparent,Lightweight, and High Strength Polyethylene Films by a Scalable Continuous Extrusion and Solid‐State Drawing Process

The continuous production of transparent high strength ultra‐drawn high‐density polyethylene films or tapes is explored using a cast film extrusion and solid‐state drawing line. Two methodologies are explored to achieve such high strength transparent polyethylene films; i) the use of suitable additives like 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐ditertpentylphenol (BZT) and ii) solid‐state drawing at an optimal temperature of 105 °C (without additives). Both methodologies result in highly oriented films of high transparency (≈91%) in the far field. Maximum attainable modulus (≈33 GPa) and tensile strength (≈900 MPa) of both types of solid‐state drawn films are similar and are an order of magnitude higher than traditional transparent plastics such as polycarbonate (PC) and poly(methyl methacrylate). Special emphasis is devoted to the effect of draw down and pre‐orientation in the as‐extruded films prior to solid‐state drawing. It is shown that pre‐orientation is beneficial in improving mechanical properties of the films at equal draw ratios. However, pre‐orientation lowers the maximum attainable draw ratio and as such the ultimate modulus and tensile strength of the films. Potential applications of these high strength transparent flexible films lie in composite laminates, automotive or aircraft glazing, high impact windows, safety glass, and displays.     

Biobased plastics with insect‐repellent functionality

Natural insecticides/repellents, such as pyrethrum (derived from chrysanthemum plants), and insect repellent N,N‐Diethyl‐meta‐toluamide (DEET) were added to poly(lactic acid) (PLA) fibers through extrusion and spray coating on the PLA fabrics. Contact irritancy assay (CIA) showed that DEET‐treated PLA fabrics caused the lowest relative escape response of mosquitoes with an escape frequency of 33.3% ± 3.3%, indicating that DEET was less effect compared with natural insecticides/repellents. This was followed by the extruded natural pyrethrum‐treated PLA fabric with an escape frequency of 80% ± 6.3%. Finally, the PLA fabrics spray‐coated with natural pyrethrum caused the highest escape frequency of 98.3% ± 1.7%. Thus, it was found that pyrethrum/PLA fabrics functioned as a mosquito repellent better than DEET/PLA fabrics. In addition, TGA and tensile testing results demonstrated that pyrethrum was sufficiently thermally stable to be extrusion compounded with PLA. GPC results showed that DEET promoted de‐polymerization of PLA when co‐extruded. The results demonstrated that pyrethrum can be a viable additive for PLA to produce fibers that function as mosquito repellent to produce temporary garments that are compostable. The potential use of the developed biobased fibers with natural insect repellents is for single use of personal protection equipment (PPE) garments. POLYM. ENG. SCI., 59:E460–E467, 2019. © 2019 Society of Plastics Engineers     

Improvement of Flexural Strengths of Poly(L‐lactic acid) by Solid‐State Extrusion, 2. Extrusion Through Rectangular Die

Solid‐state extrusion of poly (L ‐lactic acid) (PLLA) through rectangular die was performed to produce high flexural strength plates that can be used as internal fixation devices. A single‐angle wedge‐shaped rectangular die was utilized having the die exit dimension of 4 mm × 1 mm. Billets were machined out from vacuum compression molded PLLA having different crystallinities to have various thicknesses and thus various imposed draw ratios. Solid‐state extrusion of billets was performed at various drawing rates at 130 °C, above glass transition and below melting temperature. Extruded plates had the width and thickness smaller than the die due to the further drawing outside the die. The decrease in width was larger than the decrease in thickness, and this became more prominent with increasing draw ratio and drawing rate, resulting in a significantly smaller aspect ratio. Contributions of die drawing and further drawing outside the die were estimated from the extruded plate dimensions, by which the drawing rate effect was attributed to the further drawing outside the die. As actual draw ratio increased, crystallinity, melting temperature, crystalline orientation factor, and birefringence increased. Throughout the whole process the decrease in molecular weight was largely suppressed to be about 10%. As billet crystallinity, draw ratio, and drawing rate increased, both flexural strength and flexural modulus increased up to the maxima of 202 MPa and 9.7 GPa, respectively. This enhancement in mechanical properties was correlated with structural developments.

Changes in flexural strength of solid‐state extruded PLLA plates as a function of draw ratio (the effect of drawing rate is co‐plotted by hollow symbols at corresponding draw ratio.).     

Rheology and extrusion foaming of chain‐branched poly(lactic acid)

In this study, the effect of macromolecular chain‐branching on poly(lactic acid) (PLA) rheology, crystallization, and extrusion foaming was investigated. Two PLA grades, an amorphous and a semi‐crystalline one, were branched using a multifunctional styrene‐acrylic‐epoxy copolymer. The branching of PLA and its foaming were achieved in one‐step extrusion process. Carbon dioxide (CO₂), in concentration up to 9%, was used as expansion agent to obtain foams from the two PLA branched using chain‐extender contents up to 2%. The foams were investigated with respect to their shear and elongational behavior, crystallinity, morphology, and density. The addition of the chain‐extender led to an increase in complex viscosity, elasticity, elongational viscosity, and in the manifestation of the strain‐hardening phenomena. Low‐density foams were obtained at 5–9% CO₂ for semi‐crystalline PLA and only at 9% CO₂ in the case of the amorphous PLA. Differences in foaming behavior were attributed to crystallites formation during the foaming process. The rheological and structural changes associated with PLA chain‐extension lowered the achieved crystallinity but slightly improved the foamability at low CO₂ content. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Processing‐structure‐property relationships in solid‐state shear pulverization: Parametric study of specific energy

Solid‐state shear pulverization (SSSP) is a unique processing technique for mechanochemical modification of polymers, compatibilization of polymer blends, and exfoliation and dispersion of fillers in polymer nanocomposites. A systematic parametric study of the SSSP technique is conducted to elucidate the detailed mechanism of the process and establish the basis for a range of current and future operation scenarios. Using neat, single component polypropylene (PP) as the model material, we varied machine type, screw design, and feed rate to achieve a range of shear and compression applied to the material, which can be quantified through specific energy input (E_p). As a universal processing variable, E_p reflects the level of chain scission occurring in the material, which correlates well to the extent of the physical property changes of the processed PP. Additionally, we compared the operating cost estimates of SSSP and conventional twin screw extrusion to determine the practical viability of SSSP. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Biocomposite films based on poly(lactic acid) and chitosan nanoparticles: Elaboration,microstructural and thermal characterization

Structured and fully bio‐based polymer assemblies based on chitosan micro‐ and nano‐particles and poly(lactic acid) (PLA) were developed using a continuous cast‐film extrusion process. The microstructure and thermal properties of the resulting biocomposite films are studied. Dispersion and size distribution of chitosan particles within the films were analyzed by optical microscopy and laser diffraction. Results show a homogeneous dispersion with no particles agglomeration, due to favorable physico–chemical interactions between chitosan particles and PLA and weak particle cohesion within the agglomerates. Differential scanning calorimetry experiments evidence a pronounced nucleating effect as well as an enhanced crystal growth rate, and a great increase in crystallinity of PLA in the presence of chitosan particles. Furthermore, in the case of chitosan nanoparticles, no reduction of PLA molecular weight occurred as revealed by gel permeation chromatography. The dispersion of nanosized chitosan particles in PLA thus appears to be an efficient way to control its crystallization behavior without degrading its molecular characteristics. POLYM. ENG. SCI., 59:E350–E360, 2019. © 2018 Society of Plastics Engineers     

Novel,synergistic composites of polypropylene and rice husk ash: Sustainable resource hybrids prepared by solid‐state shear pulverization

Rice husk ash (RHA) is an agrowaste byproduct resulting from the incineration of rice husks for power production; white RHA is ∼90 wt% or more silica, which makes it a potentially sustainable and inexpensive substitute for commercial (less “green”) silica filler. Past research on polypropylene (PP)‐RHA hybrids made by melt processing has yielded modest increments in Young's modulus, reduced yield strength, and drastic reductions in elongation at break relative to neat PP. Using the industrially scalable solid‐state shear pulverization (SSSP) process, PP‐RHA hybrids are made with 4–38 wt% RHA. As determined by microscopy and other methods, composites made by SSSP have much better RHA dispersion than composites reported in the literature made by twin‐screw extrusion. The superior dispersion leads to major enhancements in tensile modulus (up to 100% increases relative to neat PP) while maintaining the yield strength of neat PP and remarkably high values of elongation at break (e.g., 520% at 19 wt% RHA), far higher than composites made by melt processing. The properties of hybrids made by SSSP are competitive with and in some cases superior to those of PP hybrids made with commercial silica. The PP‐RHA hybrids also exhibit major increases in hardness, approaching that of polycarbonate in the case of a 38 wt% RHA hybrid. The 38 wt% RHA hybrid exhibits solid‐like rheology at low frequency. Nevertheless, all PP‐RHA hybrids made by SSSP exhibit viscosities at moderate to high shear rates that are little changed from that of neat PP. POLYM. COMPOS., 34:1211–1221, 2013. © 2013 Society of Plastics Engineers     

Penicillin Acylase‐Catalyzed Solid‐State Ampicillin Synthesis

The ability of immobilized penicillin acylase from E. coli to retain a remarkable catalytic activity in solid‐state systems has been demonstrated. Stabilization of immobilized penicillin acylase by inorganic salt hydrates allowed us to exploit nearly the whole catalytic activity of the enzyme at a very low water content. Using this technique, enzymatic synthesis of ampicillin in solid‐state systems was performed with high yields (up to 70% starting from equimolar mixture of reagents) and rates comparable to the corresponding values in homogeneous solutions and heterogeneous systems, “aqueous solution‐precipitate”. Peculiarities of the enzymatic solid‐state acyl transfer process, such as absence of the clear‐cut maximum on the ampicillin accumulation curves and dependence of the synthetic efficiency on the enzyme loading, have been observed. The space‐time yield of solid‐state enzymatic ampicillin synthesis was shown to be up to ten times higher compared to the homogeneous solutions and heterogeneous “aqueous solution‐precipitate” systems.     

Strain‐induced deformation mechanisms of polylactide plasticized with acrylated poly(ethylene glycol) obtained by reactive extrusion

This work aimed at identifying the tensile deformation mechanisms of an original grade of plasticized polylactide (pPLA) obtained by reactive extrusion. This material had a glass transition temperature of 32.6 °C and consisted of a polylactide (PLA) matrix grafted with poly(acryl‐poly(ethylene glycol)) (poly(Acryl‐PEG)) inclusions. pPLA behaved like a rubber‐toughened amorphous polymer at 20 °C, and its tensile behavior evolved toward a rubbery semicrystalline polymer with increasing temperature. The drawing of pPLA involved orientation of amorphous and crystalline chains, crystallization, and destruction of crystals. It was found that crystal formation and crystal destruction were in competition below 50 °C, resulting in a constant or slightly decreasing crystallinity with strain. Increasing temperature enhanced crystal formation and limited crystal destruction, resulting in an increased crystallinity with the strain level. Drawing yielded a transformation of the initial spherical poly(Acryl‐PEG) inclusions into ellipsoids oriented in the tensile direction. This mechanism may engender the formation of nanovoids within the inclusions due to a decreased density, assumed to be responsible for the whitening of the specimen. © 2015 Society of Chemical Industry     

Microcellular extrusion of PLA utilizing solid‐state nucleation in the gas‐saturated pellet extrusion process

In this study, we explore the use of solid‐state nucleation in polymer pellets as a means to create microcellular PLA foams in extrusion. This is achieved by using gas‐saturated PLA pellets as input to the extruder. Foam density, bubble size, and bubble density is reported and compared with microcellular foams created in the gas‐injection extrusion process. PLA pellet gas concentrations between 17 and 29 mg CO₂/g PLA was found to produce quality microcellular foams in this process. Gas concentrations within this range were achieved by varying methods that included partial saturation, desorption from full saturation, and blending saturated with unsaturated pellets. This gas concentration window that produced microcellular foams was found to be independent of the saturation and desorption process used to achieve the desired concentration. We further compare the pressure drop and pressure drop rate of the gas‐saturated pellet extrusion process showing that similar foams can be produced at pressures orders of magnitude lower than the alternative gas‐injection extrusion processes. Investigations into extrusion pressures support the hypothesis that the gas‐saturated pellet extrusion process utilizes solid‐state nucleation in the feed section of the extruder to achieve high bubble density foams. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Extrusion blown films of poly(lactic acid) chain‐extended with food grade multifunctional epoxies

The effectiveness and efficiency of two food grade multifunctional epoxies with low and high epoxy equivalent weights in chain extending/branching poly(lactic acid) (PLA) were studied in a torque rheometer. Processing PLA and chain extender (CE) at 200°C for 300 s not only chain‐extended PLA effectively as indicated by a significant increase in the mixing torque as well as PLA's melt viscosity and molecular weight, but also branched it leading to its reduced crystallinity. Chain extension occurred through the ring opening reaction of epoxy groups in the CE with PLA's hydroxyl and/or carboxyl groups. CE with lower epoxy equivalent weight was more efficient due to its higher reactivity. Secondly, the processabilities of PLA films chain‐extended and branched with various amounts of the most efficient CE were assessed. Like in torque rheometer, chain extension and branching also occurred during film production as indicated by PLA's increased molecular weight and decreased crystallinity when blended with CE. However, film manufacture was feasible only for blends with up to 0.5% CE, becoming unprocessable above this content due to chain entanglement leading to increased viscosity. Chain extension/branching of PLA was beneficial in overcoming film's brittleness since its impact strength increased almost linearly with the CE content. POLYM. ENG. SCI., 59:2211–2219, 2019. © 2019 Society of Plastics Engineers     

Solid‐phase extrusion of polyamide‐6 by using combined deformation schemes

Potentialities of new combined deformation schemes, including the solid state extrusion through conical die (ED) and equal‐channel multiple angle extrusion (ECMAE) implemented in different sequence to modify structure and properties of semicrystalline polymers, have been studied for polyamide‐6 as an example. It is shown that deformation by the ED‐ECMAE scheme gives the best complex of physical and mechanical properties. A significant improvement in elastic and strength properties of polyamide‐6 with the conserved high level of plastic characteristics has been observed. There was only a slight anisotropy and dispersion of microhardness across the extrudates. A more uniform oriented structure with lamellae orientation along extrudate's axis has been formed in semicrystalline polymer because the ED‐ECMAE scheme implementation. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Improvement of blown film extrusion of poly(Lactic Acid): Structure–Processing–Properties relationships

The blown extrusion of poly(lactic acid) (PLA) presents several challenges mainly due to the poor shear and elongation properties of this biopolymer. This article highlights some promising routes to enhance the processability of PLA for blown extrusion. To achieve this objective, various formulations of PLA with multifunctionalized epoxy, nucleating agents, and plasticizer were elaborated and studied on the basis of their linear viscoelasticity and elongational properties. We further characterized both the structure and thermomechanical properties of blown films produced with these PLA formulations. Stability charts for the film blowing of neat and modified PLA were thus established at different processing conditions. On the basis of these results, we managed to achieve a large enhancement of the blown processing windows of PLA with high blow‐up ratio (BUR) and take‐up ratio attained. We were able to demonstrate that a higher kinetic of crystallization can also be reached for chain‐extended and branched PLA formulated with adequate amounts of nucleating agents and plasticizers. Induced crystallization during process was also demonstrated. Through this work, blown films with interesting thermomechanical and mechanical properties have been elaborated using an optimal formulation for PLA. POLYM. ENG. SCI., 54:840–857, 2014. © 2013 Society of Plastics Engineers     

Reduction in percolation threshold of injection molded high‐density polyethylene/exfoliated graphene nanoplatelets composites by solid state ball milling and solid state shear pulverization

Previous work showed that high density polyethylene (HDPE)/exfoliated graphene nanoplatelets (GNP) nanocomposites fabricated with melt extrusion followed by injection molding had a relatively high percolation threshold of between 10 and 15 vol % GNP loading. To lower the percolation threshold of injection molded HDPE/GNP nanocomposites, two special processing methods were investigated: solid state ball milling (SSBM) and solid state shear pulverization (SSSP). Results have confirmed that the percolation threshold of HDPE/GNP nanocomposites could be reduced to between 3 and 5 vol % GNP loading by these two approaches. The mechanism by which SSBM and SSSP are capable of producing lower percolation is to coat the polymer surface with GNP platelets which facilitates the formation of conductive networks during injection molding. However, it was found that HDPE/GNP nanocomposites obtained from these two techniques exhibited lower mechanical properties at high GNP loadings. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of lactic acid chain length on thermomechanical properties of star‐LA‐xylitol resins and jute reinforced biocomposites

Star‐shaped bio‐based resins were synthesized by direct condensation of lactic acid (LA) with xylitol followed by end‐functionalizing of branches by methacrylic anhydride with three different LA chain lengths (3, 5 and 7). The thermomechanical and structural properties of the resins were characterized by ¹³C NMR, Fourier transform IR spectroscopy, rheometry, DSC, dynamic mechanical analysis (DMA), TGA and flexural and tensile tests. An evaluation of the effect of chain length on the synthesized resins showed that the resin with five LAs exhibited the most favorable thermomechanical properties. Also, the resin's glass transition temperature (103 °C) was substantially higher than that of the thermoplast PLA (ca 55 °C). The resin had low viscosity at its processing temperature (80 °C). The compatibility of the resin with natural fibers was investigated for biocomposite manufacturing. Finally, composites were produced from the n5‐resin (80 wt% fiber content) using jute fiber. The thermomechanical and morphological properties of the biocomposites were compared with jute‐PLA composites and a hybrid composite made of the impregnated jute fibers with n5 resin and PLA. SEM and DMA showed that the n5‐jute composites had better mechanical properties than the other composites produced. Inexpensive monomers, good thermomechanical properties and good processability of the n5 resin make the resin comparable with commercial unsaturated polyester resins. © 2017 Society of Chemical Industry     

Iminium Salts in Solid‐State Syntheses Giving 100% Yield

Numerous reaction types in the field of iminium salts are performed in the gas‐solid and solid‐solid techniques in order to reach 100% yield. The stoichiometric runs are waste‐free and do not require costly workup. Frequently, iminium salts were avoided, as acid catalysis was dispensable. Thioureas and α‐halogenated ketones give a variety of 2‐aminothiazoles via thiuronium salts in quantitative yield. A new intramolecular solid‐state thermal condensation is reported. Enaminoketones are synthesized quantitatively from anilines and 1,3‐diketones without catalysis and those can be used for quantitative solid‐state 4‐cascade reactions. Solid paraformaldehyde is used to produce methylene imines and internally trapped methylene iminium salts. Benzoylhydrazones are produced again without catalysis in the solid state. Vacuum and ball‐mill techniques are particularly useful in the production of highly sensitive iminium salts. Hexahydro‐1,3,5‐triazines cyclorevert upon exposure to HCl gas to give solid arylmethylene iminium chlorides as new versatile reagents. These are used in arylaminomethylations of β‐naphthol and of themselves to give Troeger's bases in 3‐cascades. More direct are 4‐cascade Troeger's base syntheses by dissolving hexahydro‐1,3,5‐triaryltriazines in trifluoroacetic acid. Alkylations of imines with trimethyloxonium tetrafluoroborate and triphenylmethyl cation give highly sensitive quaternary iminium salts in the ball‐mill. The products are characterized by spectroscopic techniques and density functional theory (DFT) calculations at the B3LYP 6‐31G* level. Molecular movements in the crystal and surface passivation are investigated with atomic force microscopy (AFM) techniques.     

Solid‐State Conversion of Single Crystals: The Principle and the State‐of‐the‐Art

Solid‐state conversion of single crystals from polycrystalline materials has the advantages of cost‐effectiveness, chemical homogeneity, and versatility over the conventional melt growth and solution growth methods, particularly for systems with high melting points, incongruent melting, high reactivity (volatility), and phase transformations at high temperature. Nevertheless, for commercial production, this technique has only been successful in a few limited systems, in particular ferroelectric systems. This is mostly because of the difficulty in controlling the microstructure, particularly suppressing grain growth in the polycrystal during its conversion. This article describes the principle and the current status of the solid‐state conversion of single crystals. We first introduce the recently developed principle of microstructural evolution to explain the basis of the microstructure control in polycrystals for solid‐state conversion. We then report recent technical developments in fabricating single crystals by the solid‐state single crystal growth (SSCG) method and their physical properties. The SSCG method is expected to be studied and utilized more widely in fabricating single crystals with complex compositions as a strong alternative to the melt growth and solution growth methods.     

Effect of the annealing procedure and the molecular weight on the crystalline phase morphology and thermal properties of polylactide

The thermomechanical properties of poly(lactide) (PLA) are strongly related to its semicrystalline microstructure and morphology. Thermal annealing is a strategy to improve the crystallinity of PLA. However, the different techniques and specimen types needed for each kind of characterization could lead to misleading conclusions. In this work, annealed samples of three PLA grades with different molecular weights were studied by DSC, wide angle X‐ray scattering and polarized optical microscopy (POM) and the results are related to their thermomechanical and impact properties. Special focus is put on the POM results obtained by different approaches and the suitability of each of them to be related to the thermomechanical properties. By annealing medium molecular weight PLA specimens at 140 °C an important increase of the heat distortion temperature was obtained, which was not related to the spherulite size but to the combination of high crystallinity degree together with high α/α′ crystal type ratio. However, the impact properties of annealed PLA decreased with increase in the annealing temperature according to an increment in crystallinity and in the α/α′ crystal ratio. © 2019 Society of Chemical Industry     

Solvent‐ and thermal‐induced crystallization of poly‐L‐lactic acid in supercritical CO2 medium

The effect of different annealing treatments with supercritical carbon dioxide (SCCO₂) on the structural and mechanical properties of semicrystalline poly‐L ‐lactic acid (L ‐PLA) was investigated. 2000, 27,000, 100,000, and 350,000 g mol^?1 molecular weight L ‐PLA polymers were used in the study. The solid‐state processing of L ‐PLA at temperatures lower than the effective melting point led to solvent‐ and thermal‐induced crystallization. Solvent‐induced and isothermal crystallization mechanisms could be considered similar regarding the increase of polymer chain mobility and mass‐transfer in the amorphous region; however, quite different microstructures were obtained. SCCO₂ solvent‐induced crystallization led to polymers with high crystallinity and melting point. On the contrary, SCCO₂ thermal‐induced crystallization led to polymers with high crystallinity and low melting point. For these polymers, the hardness increased and the elasticity decreased. Finally, the effect of dissolving SCCO₂ in the molten polymer (cooling from the melt) was analyzed. Cooling from the melt led to polymers with high crystallinity, low melting point, low hardness, and low elasticity. Distinctive crystal growth and nucleation episodes were identified. This work also addressed the interaction of SCCO₂‐drug (triflusal) solution with semicrystalline L ‐PLA. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009