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     

Fabrication of poly(lactic acid) films with resveratrol and the diffusion of resveratrol into ethanol

Poly(94% L ‐lactic acid) (PLA) films containing 0, 1, and 3 wt % resveratrol (PR0, PR1, and PR3) were extruded in a pilot‐plant scale blown‐extrusion machine. Yellow‐colored PLA films with reduced crystallinity were obtained, and the films absorbed UV‐visible light at 350–200 nm. The diffusion kinetics of resveratrol into ethanol at 9, 23, 33, and 43°C displayed Fick's behavior, and diffusion coefficients between 10⁻¹³ and 10⁻¹⁰ cm²/s were obtained. According to the Arrhenius equation, the energy of activation for the diffusion of resveratrol from PR1 and PR3 films was 175 and 177 kJ/mol, respectively. The temperature of diffusion had a stronger effect on the reduction of the weight average molecular weight of PLA than the processing conditions and the contact time with ethanol. However, diffusion of resveratrol was not affected by the degradation of PLA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Epoxy functionalized poly(lactide) reactive modifier for blown film applications

Epoxy functionalized poly(lactide) (EF‐PLA) was synthesized by reacting PLA with a multifunctional epoxy polymer (MEP) using reactive extrusion processing. These polymers can function as a rheology modifier for PLA and a compatibilizer for other biopolyesters in blown film and foam applications. Model compound studies show that the epoxy functional group on the MEP reacts selectively with the carboxylic acid chain‐ends of PLA at processing temperatures below 200°C. An EF‐PLA containing up to 10% MEP was prepared without gel formation and reactively extruded with neat PLA to obtain three different product formulations containing MEP (0.25, 0.5, and 1.0%). These products showed significantly enhanced rheological properties compared to what has been reported by other groups and is currently used in the PLA blown film industry, the blending of MEP with PLA in a single step. These benefits are a result of how the MEP gets distributed in the material, and can lead to improved properties even at lower MEP concentrations. Our new materials showed significant strain hardening rheological behavior demonstrating that they can be readily blown into films and foams. A statistical simulation was developed to provide a fundamental understanding of the reaction as well as provide information on the molecular weight characteristics and reactivity of the EF‐PLA. The EF‐PLA molecule shows good potential for use as a rheology modifier and compatibilizer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42243.     

Optical properties of blown and cast polyethylene films: Surface versus bulk structural considerations

In this article we report on some surprising, and we believe new, findings regarding the factors affecting the optical properties (haze) of polyethylene blown and cast films. A comprehensive investigation of blown and cast films made from conventional Ziegler‐Natta catalyzed linear low density polyethylene (LLDPE) as well as metallocene‐catalyzed LLDPE (mLLDPE) resins was conducted. The large majority of the contribution to the total haze in the blown and cast films was observed to come from the surface roughness of the films, with the bulk (internal) contribution being relatively minor. Using a variety of analysis and characterization methods, including atomic force microscopy, small angle light scattering, and wide angle X‐ray scattering, we determined that the surface roughness in these films was a result of the development of distinct spherulitic‐like superstructures formed during the blown or cast film processing. Furthermore, these superstructures were observed only in the mLLDPE blown films, and not in the LLDPE blown films processed at similar conditions. Analysis of the rheological and molecular characteristics of these various mLLDPE and LLDPE resins revealed that the mLLDPE resins exhibited considerably lower molecular weight, narrower molecular weight distribution, lower zero shear viscosity, and lower melt elasticity compared with the LLDPE resins of similar melt index. These observations support our general finding and primary conclusion from this work that in polyethylene blown and cast films made using typical processing conditions, the optical haze properties are adversely affected because of enhanced surface roughness caused by the formation of spherulitic‐like superstructures in polymer melts that possess fast relaxing and low melt elasticity rheological characteristics. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2845–2864, 2000     

Reactive modification and compatibilization of poly(lactide) and poly(butylene adipate‐co‐terephthalate) blends with epoxy functionalized‐poly(lactide) for blown film applications

Biodegradable blown films comprising of poly(lactide) (PLA) and poly(butylene adipate‐co‐terephthalate) (PBAT) were produced using epoxy functionalized‐poly(lactide) (EF‐PLA) reactive modifiers for rheological enhancement and compatibilization. The epoxy groups on the EF‐PLA modifiers react with PBAT forming an in situ copolymer that localizes at the blend interphase resulting in compatibilization of the polymer blend components. The EF‐PLA modified polymer blends have improved melt strength and the resultant films showed better processability as seen by increased bubbled stability. This allowed for blown films with higher PLA content (70%) compared to the unmodified control films (40%). The static charge build‐up typically experienced with PLA film blowing was decreased with the inclusion of EF‐PLA yielding films with better slip and softness. The compatibilization effect of the EF‐PLA modifiers resulted in significant improvement in mechanical properties. For example, dart test performance was up to four times higher than the control, especially at higher PLA concentrations. Therefore, the rheological enhancement and compatibilization effects of the EF‐PLA reactive modifiers make them ideally suited to create high PLA content films. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43310.     

Online measurements of crystallinity using Raman spectroscopy during blown film extrusion of a linear low‐density polyethylene

Real‐time, Raman spectroscopic measurements of crystallinity during the blown film extrusion of a linear low‐density polyethylene (LLDPE) are reported. The take‐up ratio (TUR), inflation air pressure, and cooling conditions were varied, and their effect on the development of crystallinity was studied. The results indicate that the crystallization process starts at the freeze line, increases along the film line, and finally plateaus. It was observed that with changing processing conditions, the differences in the profiles for crystallinity were evident when plotted as a function of time rather than distance along the machine direction. The influence of take‐up ratio and bubble inflation air pressure on the development of crystallinity clearly established the role of flow‐induced crystallization during film extrusion. Polym. Eng. Sci. 44:1484–1490, 2004. © 2004 Society of Plastics Engineers.     

Effect of cooling conditions on the mechanical properties of crystalline poly(lactic acid)

Appropriate cooling conditions in melt processing were found to provide crystalline poly(lactic acid) (PLA) with greater mechanical toughness in tensile tests. PLA films cooled near its glass transition temperature T_g showed ductile behavior, whereas those obtained by a quenching process exhibited brittle fracture. The content of gauche‐gauche (gg) conformer, which leads to low critical onset stress for shear yielding, increased in the films cooled near T_g. The crystallinity of the films hardly affected their mechanical toughness and proportion of gg conformer except for that with a high degree of crystallinity (>50%). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44960.     

In situ fibre‐reinforced composite films of poly(lactic acid)/low‐density polyethylene blends: effects of composition on morphology,transport and mechanical properties

This study aimed to investigate the effects of blend composition on packaging‐related properties of poly(lactic acid) (PLA) and low density polyethylene (LDPE) blown films. Blend films with PLA contents of 5–20 wt% were produced and compared. Scanning electron micrographs of cross‐sectional cryofractured surfaces of the blend films revealed that in situ fibre‐reinforced composites were obtained. Viscosity ratio of the polymer components of ca 1 confirmed that fibre formation was favourable for this blend system. PLA microdomains were dispersed throughout the film in forms of long fibres (length‐to‐diameter ratio > 100) and ribbons. The number of fibres and ribbons increased with an increase of PLA content. Critical content of PLA was found to be 20 wt% for effective improvement of both moduli and gas barrier properties. Incorporation of poly[ethylene‐co‐(methyl acrylate)] compatibilizer showed minimal effect on PLA structure. However, it did improve moduli and O₂ barrier properties when sufficient amount (1.5 pph) was used in 10 wt% PLA/LDPE. In short, flow behaviour, ratio of polymer components and degree of compatibility together played intricate roles in the morphology and hence mechanical and transport properties of PLA/LDPE immiscible blends. © 2017 Society of Chemical Industry     

The low temperature response of slit-seals in low density polyethylene thin films

The low temperature mechanical strength and the modes of failure of low density polyethylene (LDPE) films and hot air slit-seals have been assessed. Seven different LDPEs were blown into thin films (～30μm) and slit-sealed on-line, both the extrusion and sealing being undertaken using commercial equipment. When pulled in tension transverse to the extrusion direction, both the films and the slit-seals exhibited a ductile response at the higher temperatures, while at lower temperatures brittle failures were seen. The ductile-to-brittle transition temperatures were reasonably well defined with the slit-seal embrittling at significantly higher temperatures. The low temperature mechanical performance of LDPE films containing slit-seals is thus controlled by the seal. However, by using seven different LDPE resins with a range of melt flow rates, it was seen, that the low temperature response was enhanced with the low melt flow rate resins. Informed materials selection can therefore enable manufacturers to produce LDPE products containing slit-seals with good low temperature strength.     

Effects of melt‐extension and annealing on row‐nucleated lamellar crystalline structure of HDPE films

The row‐nucleated lamellar crystalline structure of high‐density polyethylene (HDPE) films was prepared by applying elongation stress to HDPE melt during T‐die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long‐period lamellar spacing, crystallite size, and degree of crystallinity. The contribution of melt‐extension represented by draw‐down‐ratio (DDR) to the overall orientation was found to be most noticeable than other processing variables. Meanwhile, the long‐period lamellar spacing, the crystallite size, and the degree of crystallinity were influenced predominantly by the annealing temperature. Finally, the processing (melt extension and annealing temperature) – structure (lamellar crystalline structure) – property (hard elasticity) relationship of HDPE films was investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3326–3333, 2007     

Die design for rigid PVC—the effect of die land length on extrudate swell

PVC profile extrusion compounds have a unique morphology. While other polymers gradually decrease in extrusion die swell with increasing length/thickness (L/D) ratio, PVC profile extrusion compounds have a low die swell, quite independent of the die's L/D ratio in the range of 5 to 20. The fact that the die land length can be changed without changing the extrudate swell is an important consideration, which makes die design and balancing dies simpler and easier for PVC profile extrusion compounds. While other polymers substantially increase extrudate swell with increased shear rate, the swell of the PVC profile compounds is not much affected by shear or extrusion rate. This unique behavior allows wider processing latitude in profile extrusion and faster extrusion rates than with other polymers. Another unique factor in the rheology of PVC profile extrusion compounds is that extrusion die swell increases with increasing melt temperature, while other polymers have decreasing die swell with increasing melt temperature. The unusual rheology of PVC profile extrusion compounds is attributed to its unique melt morphology, where the melt flow units are 1 um bundles and molecules that have low surface to surface interaction and entanglement at low processing temperatures but increased melting and increased entanglement at higher processing temperatures. Other polymers, unlike PVC, have melt flow at the molecular level.     

Poly(lactic acid)/montmorillonite blown films: Crystallization,mechanics, and permeation

The study focuses on the development and characterization of poly(lactic acid) (PLA)/montmorillonite (clay) nanocomposite films. Samples of 0%, 1%, 3%, and 6% (by weight) clay were shear‐mixed, melt‐blended, and blown‐film processed. Afterward, the effects of clay on the kinetics of cold‐crystallization, mechanical properties, and the oxygen barriers are investigated using differential scanning calorimetry, dynamic mechanical analyzer, and permeation tester, respectively. Through the traditional Avrami analysis, clay is found to accelerate the crystallization process with a higher crystallization rate constant. The Avrami exponent obtained for composites is higher than the neat PLA although all samples show a decreased Avrami exponent with increase of the crystallization temperature. At the same time, the clay exhibits reinforcement effects on the glassy modulus of the composites and influences the cold‐crystallization event, similar to the calorimetric results. In addition, the oxygen permeation slightly decreases on adding the clay. With 3% clay concentration, the permeation coefficient is reduced by 24%. The implication of the results is discussed in the article. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45260.     

Structure and improved properties of PPC/PBAT blends via controlling phase morphology based on melt viscosity

Poly(propylene carbonate) (PPC)/poly(butylenes adipate-co-terephthalate) (PBAT) blends with various composition ratios were prepared via melt mixing using a twin-screw extruder. The effect of melt viscosities of polymers on mechanical behavior, interfacial interaction, thermal properties, rheological responses, and phase morphology was investigated. Results showed that the phase morphology and properties of PPC/PBAT blends were affected by the composition of the blends and the melt viscosities of the two polymers. Results of tensile tests, FTIR, and dynamic rheological measurement of PBAT-rich blends exhibited a better mechanical properties, intermolecular interactions, and compatibility when compared with PPC-rich blends due to the differences of their melt viscosities. Incorporating of PBAT effectively improved the T_g of PPC and the thermal stability of the blends. The T_c of PPC/PBAT blends markedly increased from 37.5 to 66.8 °C with addition of only 10 wt% PPC, indicating an enhanced crystallization ability of PBAT. The improvement of T_c was helpful for blown film extrusion. SEM microphotographs showed that the size of the dispersed phase particles is much smaller and the distribution is more uniform for PBAT-rich blends, compared with that in PPC-rich blends. The processing stability of blown film extrusion was improved by blending PPC with PBAT. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48924.     

Melt processing of poly(L‐lactic acid) in the presence of organomodified anionic or cationic clays

Poly(L ‐lactic acid) (PLA) films are in use for various types of food packaging; however, a wider range of applications would be possible if the barrier properties of these films could be improved. To make such improvements, combinations of PLA with two nanofillers, laurate‐intercalated Mg‐Al layered double hydroxide (LDH‐C₁₂) and a cationic organomodified montmorillonite (MMT) clay (Cloisite® 30B), were investigated. The dispersion of these fillers in PLA by melt processing was explored using two methods, either by mixing the nanofillers with PLA granulate immediately before extrusion or by preparation and subsequent dilution of PLA‐nanofiller masterbatches. After melt processing of these materials, PLA molecular weight, thermal stability, film transparency, morphology, and permeability characteristics were determined. Direct addition of LDH‐C₁₂ drastically reduced the PLA molecular weight. Although this reduction in molecular weight was still very significant, it was less when a PLA/LDH‐C₁₂ masterbatch was processed. In contrast, there was no significant reduction in PLA molecular weight when processing with Cloisite® 30B. However, film transparency was compromised when either LDH or MMT nanofillers were used. Evidence from DSC analyses showed a significant increase in heat of fusion when LDH‐C₁₂ was dispersed in PLA compared with Cloisite® 30B, likely indicating a difference in nucleating properties. Complementary optical purity analyses suggested that racemization as a result of processing could influence the PLA crystallinity as determined by DSC in certain cases. A reduction in thermal stability when incorporating LDH‐C₁₂ could be a direct result of PLA molecular weight reduction. XRD and TEM analyses showed that both Cloisite® 30B‐ and LDH‐C₁₂‐based PLA composites yielded exfoliated and intercalated morphologies, but nanofiller agglomeration was also seen when LDH‐C₁₂ was used. PLA/Cloisite® 30B nanocomposite films exhibited significant enhancement in oxygen and water vapor barrier properties, but no such improvement was found in PLA/LDH‐C₁₂ nanocomposite films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Rheological,mechanical, optical,and transport properties of blown films of polyamide 6/residual monomer/montmorillonite nanocomposites

Exfoliated nanocomposites of polyamide 6 (PA6) with residual monomer and an organically treated montmorillonite (3 and 5 wt %) were produced by twin‐screw extrusion. The composites had their steady state, dynamic, and transient rheological properties measured by parallel‐plates rheometry; their exfoliation level was characterized by wide angle X‐rays diffraction (WAXD) and transmission electron microscopy (TEM). The characterization showed as follows: (i) the nanoclay's lamellas were well dispersed and distributed thru the PA6, (ii) the postpolymerization of the residual monomer produced more branched chains than linear ones in the pure PA6, (iii) the nanoclay's lamellas acted as entanglement points in the nanocomposites, and (iv) the molecular weight of the PA6 in the nanocomposites decreased. Blown films of the nanocomposites were produced by single screw extrusion; the die pressure during the film blowing of the nanocomposites strongly decreased. The tensile mechanical properties of the blown films were also measured. Along the machine direction (MD), the best mechanical properties were obtained with the 5 wt % nanocomposite, whereas along the transverse direction (TD), the 3 wt % nanocomposite had the best behavior. The glass transition temperature (T_g) of the blown films was measured by dynamic mechanical thermal analyses (DMTA). The 5 wt % nanocomposite had the highest T_g of all the films. The optical properties were measured by spectrophotometry; the nanoclay decreased the films' haze, but the level of transmittance was not affected. The water vapor and oxygen permeability rates of the nanocomposites films were found to be lower than in the pure PA6 blown film as a result of a tortuosity effect. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Physical extruder foaming of poly(lactic acid)—processing and foam properties

The article describes extrusion foaming of poly(lactic acid) (PLA) using carbon dioxide in the supercritical state as foaming agent emphasizing the steps required to establish a stable extrusion process. Low melt strength of PLA plays a role in optimizing processing conditions. The tests included PLA grades of different viscosity in addition to a chain extender. Processing at low temperature is possible due to the plasticizing effect of the CO₂ on the PLA melt and a sufficiently low melt temperature is also a prerequisite in production of stable foams due to improved melt strength. Foams were characterized by density, cell structure, crystallinity, and mechanical properties in compression. Low density, microcellular foams with density down to 20–30 kg/m³ were obtained for three different PLA grades. Varying die temperature and pressure drop rate we can explain observed abrupt drops in density with increasing CO₂ content by the interplay between cell nucleation and gas diffusivity at given temperatures. An effect on melt strength similar to using a chain extender is achieved by lowering the melt temperature at the die. Observed variations in sample crystallinity do not correlate with foam density. The PLA foams have good energy absorption capability. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Structure,mechanical, and electrical properties of high‐density polyethylene/multi‐walled carbon nanotube composites processed by compression molding and blown film extrusion

The structure and properties of melt mixed high‐density polyethylene/multi‐walled carbon nanotube (HDPE/MWCNT) composites processed by compression molding and blown film extrusion were investigated to assess the influence of processing route on properties. The addition of MWCNTs leads to a more elastic response during deformations that result in a more uniform thickness distribution in the blown films. Blown film composites exhibit better mechanical properties due to the enhanced orientation and disentanglement of MWCNTs. At a blow up ratio (BUR) of 3 the breaking strength and elongation in the machine direction of the film with 4 wt % MWCNTs are 239% and 1054% higher than those of compression molded (CM) samples. Resistivity of the composite films increases significantly with increasing BURs due to the destruction of conductive pathways. These pathways can be recovered partially using an appropriate annealing process. At 8 wt % MWCNTs, there is a sufficient density of nanotubes to maintain a robust network even at high BURs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42665.     

Effect of mixing poly(lactic acid) with glycidyl methacrylate grafted poly(ethylene octene) on optical and mechanical properties of the blown films

Poly(lactic acid) (PLA) was plasticized with acetyl tributyl citrate (ATBC). The plasticized PLA was further blended with poly(ethylene octene) grafted with glycidyl methacrylate (POE‐g‐GMA denoted as GPOE) using a twin‐screw extruder and the extruded samples were blown using the blown thin film technique. Both ATBC and GPOE significantly influenced the physical properties of the films. Compared to neat PLA, the elongation at break and tear strength of the films were significantly improved. The cavitation and large plastic deformation observed in films subjected to the tear test were the important energy‐dissipation process, which led to a torn PLA film. Moreover, the PLA/ATBC/GPOE blown films had better transparency and water tolerance than that of neat PLA. GPOE could act as a tear resistance modifier for PLA blown films. These findings contributed new knowledge to the additives area and gave important implications for designing and manufacturing polymer packaging materials. POLYM. ENG. SCI., 55:2801–2813, 2015. © 2015 Society of Plastics Engineers     

Compatibilization of poly(lactic acid)/ethylene‐propylene‐diene rubber blends by using organic montmorillonite as a compatibilizer

This work focuses on phase morphology and properties of immiscible poly(lactic acid)/ethylene‐propylene‐diene rubber (PLA/EPDM) blends compatibilized with organic montmorillonite (OMMT). Effect of OMMT loading on phase morphology, mechanical properties, and blown film bubble stability was investigated. Transmission electron micrographs show that a large number of OMMT nanolayers locate at interfacial region between PLA and EPDM phase, as well as in EPDM phase due to higher affinity of OMMT with EPDM. Scanning electron micrographs show that EPDM domain size decreases largely with increasing OMMT loading, which is associated with reduction of interfacial energy and inhibition of coalescence by the OMMT locating at the interface, acting as an emulsifier to enwrap the discrete domains. As OMMT loading increases from 0 to 1 phr, elongation at break increases from 20.4 to 151.7% and notched impact strength is enhanced from 8.2 to 31.7 kJ?m^?2. The reduced EPDM domain is the main reason for enhanced toughness of PLA/EPDM/OMMT samples according to crazing with shear yielding mechanism. However, with more than 2 phr of OMMT, the toughness decreases largely due to excessive stress concentration and OMMT aggregation. Attempts were made to produce ductile films from the PLA/EPDM/OMMT nanocomposites by using blown film extrusion. Improvement in blown film bubble stability and tensile ductility of PLA/EPDM/OMMT films also shows that OMMT is an efficient compatibilizer, as well as a processing aid for PLA/EPDM blends. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44192.     

Modification and extrusion coating of polylactic acid films

Polyethylene (PE) extrusion coating on paper substrates are the traditional packing material for coffee cups and take‐out food containers. It is difficult to recycle the PE/paper laminates and the thin polymer films remain in landfills after the decomposition of their paper substrates. Disposal of plastic materials is causing serious effects on our environment and wildlife. Demand for compostable or biodegradable plastic packaging products is increasing because of consumer pressure and legislation. Biodegradable polylactic acid (PLA) is regarded as one of the most promising biopolymers with a large market potential, but its applications are limited by poor thermal stability, mechanical properties and processibility. We utilize modified gelatin as additives to improve PLA's performance without compromizing the biomass origin and compostable properties of the material. Extrusion coating, or extrusion casting, of polylactic acid (PLA) films onto paper substrates to form PLA/paper laminates was achieved by modification of the polymer with a plant or animal gelatin. Various paper substrates with thin PLA coatings were prepared using a conventional extrusion coating equipment for the fabrication of take‐out food containers and coffee cups. Melt rheology of PLA and adhesion of the resulting thin film were greatly improved in the presence of a small amount of gelatin in the polymer matrices. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42472.