Increase In Melt Viscosity Research Articles

Some comments on the main fire retardancy mechanisms in polymer nanocomposites

AbstractBarrier formation and increasing the melt viscosity are addressed as the two main general fire retardancy mechanisms of polymer nanocomposites. They result in specific impacts on fire properties that consequentially cause varying flame retardancy efficiency in different fire tests. The barrier formation retards mainly flame spread (peak of heat release rate) in developing fires, but does not reduce fire load (total heat evolved), ignitability or flammability (limiting oxygen index, UL 94). Furthermore, this flame retardancy effect increases with increasing irradiation and vanishes with decreasing irradiation. The increased melt viscosity prevents dripping, which is beneficial or disadvantageous depending on the fire test used. In some test, it become the dominant influence, transforming self‐extinguishing samples into flammable materials or causing wicking. Advantages and the limits are sketched comprehensively for exploiting the main general fire retardancy mechanisms of polymer nanocomposites. It is concluded that barrier formation and changing the melt viscosity in nanocomposites are not sufficient for most applications, but must be accompanied by additional mechanisms in special systems or in combination with other flame retardants. Copyright © 2006 John Wiley & Sons, Ltd.

Thermal degradation of polyethylene film materials due to successive recycling

Mechanical recycling of plastic film involves subjecting plastic materials to a series of heat cycles that can potentially degrade the material, causing brittleness and increased melt viscosity. Plastic film recycling in the UK is in its infancy, in need of an increased understanding of how the physical properties of polymeric materials change before and during the process. Reliable data are required to estimate the behaviour of such film products when recycled. Measurements were made as to the changes in physical properties of four different varieties of polyethylene (PE) film products when subjected to a series of successive simulated heat cycles and evaluated after each step. Results showed that although changes in tensile properties were fairly small, changes in processing properties such as melt-flow index for highly branched or low-density PE are substantial and could be a concern during recycling operations.

Structure and properties of polyimide-bonded magnets processed from prepolymers based on diacetyl derivatives of aromatic diamines and dianhydrides

We report a novel method of polyimide (PI) synthesis from prepolymers based on dianhydrides and diacetyl derivatives of aromatic diamines that facilitate the preparation of a melt processable mixture at 300 ± 10°C of the prepolymer and magnetic Nd-Fe-B alloy to provide PI-bonded magnets with enhanced properties. It is shown that chemical structure of the prepolymers strongly influences viscosity behavior via crystallization of the oligoimide in the melt, leading to formation of PI with rigid-rod like structure. This structural ordering of the prepolymers based on diacetyl derivative of diamine used in this study, if not controlled, leads to exponential increase of melt viscosity with time, making it practically impossible to prepare melt processable mixture of the magnetic particles and the PI prepolymers at elevated temperatures. The results obtained demonstrate that appropriate dianhydrides and diacetyl derivatives of diamines that do not lead to crystallization of oligoimides in prepolymer mixture can be used under controlled processing conditions to prepare melt-processable PI-bonded magnets containing rigid-rod like PI structure that significantly increases thermal stability of the magnets. The temperature dependencies of the magnetic properties of the PI-bonded magnets under conditions that they are likely to encounter during their service life were found to be remarkably similar to that of commercial thermoplastic magnets such as injection-molded nylon magnets. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 478–485, 2006

Reinforced Thermoplastic Acetylated Starch with Layered Silicates

Montmorillonite (MMT) and organically modified montmorillonite (OMMT) were utilized to reinforce the thermoplastic acetylated starch (TPAS) composite prepared with glycerol as a plasticizer. After the addition of layered silicates, the restriction of the motion of the intercalated acetylated starch molecular chains by the clay layer sheets led to an increase in melt viscosity and equilibrium torque. As expected, the tensile strength and storage modulus of the TPAS composite were remarkably enhanced due to the interaction of layered silicates with the TPAS matrix, but the thermal stability of the TPAS composite was not obviously improved. The greater reinforcing effect of OMMT than that of MMT could be attributed to the better dispersion of OMMT in the TPAS matrix, resulting from the larger distance between OMMT layers after the modification by organic ammonium cations with long alkyl chains.

Reactive mixing of PET and PET/PP blends with glycidyl methacrylate–modified styrene‐b‐(ethylene‐co‐olefin) block copolymers

AbstractStyrene‐b‐(ethylene‐co‐butylene)‐b‐styrene (SEBS) and styrene‐b‐(ethylene‐co‐propylene) (SEP, SEPSEP) block copolymers with different styrene contents and different numbers of blocks in the copolymer chain were functionalized by melt radical grafting with glycidyl methacrylate (GMA) and employed as compatibilizers for PET‐based blends. Binary blends of PET with both functionalized (SEBS‐g‐GMA, SEP‐g‐GMA, SEPSEP‐g‐GMA) and neat (SEBS, SEP, SEPSEP) copolymers (75 : 25 w/w) and ternary blends of PET and PP (75 : 25 w/w) with various amounts (2.5–10 phr) of both modified and unmodified copolymers were prepared in an internal mixer, and their properties were evaluated by SEM, DSC, melt viscosimetry, and tensile and impact tests. The roles of the chemical structure, grafting degree, and concentration of the various copolymers on blend compatibilization was investigated. The blends with the grafted copolymers showed a neat improvement of phase dispersion and interfacial adhesion compared to the blends with nonfunctionalized copolymers. The addition of grafted copolymers resulted in a marked increase in melt viscosity, which was accounted for by the occurrence of chemical reactions between the epoxide groups of GMA and the carboxyl/hydroxyl end groups of PET during melt mixing. Blends with SEPSEP‐g‐GMA and SEBS‐g‐GMA, at concentrations of 5–10 phr, showed a higher compatibilizing effect with enhanced elongation at break and impact resistance. The effectiveness of GMA‐functionalized SEBS was then compared to that of maleic anhydride–grafted SEBS. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2201–2211, 2005

The relation between rheological and mechanical properties of PA6 nano- and micro-composites

A series of nanocomposites was prepared with two different methods leading to different levels of exfoliation, and the dynamic properties were measured in the solid state and the melt. The results were compared with micro-composites containing various shapes of glass filler. The higher modulus of nanocomposites at higher concentrations is accompanied by an increase in melt viscosity and the occurrence of a yield stress in the melt. The modulus at room temperature and the melt viscosity are influenced by the aspect ratio and the concentration of the filler particles. These results were used to calculate the aspect ratios of the reinforcement using the Halpin–Tsai composite model and several modified Einstein viscosity models. The experiments showed that the Simha theory for oblate and prolate spheroids predicts platelet aspect ratios from melt viscosity data that are close to the results from the Halpin–Tsai composite model and from TEM observations. The results of the analysis show the advantages and disadvantages of the various shapes and sizes of fillers and the level of exfoliation, both from a processing and a mechanical properties point of view.

Growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting

To understand how frictional melting affects fault instability, we performed a series of high‐velocity friction experiments on gabbro at slip rates of 0.85–1.49 m s−1, at normal stresses of 1.2–2.4 MPa and with displacements up to 124 m. Experiments have revealed two stages of slip weakening; one following the initial slip and the other immediately after the second peak friction. The first weakening is associated with flash heating, and the second weakening is due to the formation and growth of a molten layer along a simulated fault. The two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin, continuous molten layer at the second peak friction. The frictional coefficient decays exponentially from 0.8–1.1 to 0.6 during the second weakening. The host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer. Melt viscosity increases notably soon after a molten layer forms. However, a fault weakens despite the increase in melt viscosity, and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer. Very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip‐weakening distance. Effect of frictional melting on fault motion can be predicted by solving a Stefan problem dealing with moving host rock/molten zone boundaries.

Fire behaviour of polyamide 6/multiwall carbon nanotube nanocomposites

Nanocomposites of polyamide 6 with 5 wt.% multiwall carbon nanotubes are investigated to clarify their potential as regards the fire retardancy of polymers. The nanocomposites are investigated using SEM, electrical resistivity, and oscillatory shear rheology. The pyrolysis is characterized using thermal analysis. The fire behaviour is investigated with a cone calorimeter using different external heat fluxes, by means of the limiting oxygen index and the UL 94 classification. The fire residue is characterized using SEM. The comprehensive fire behaviour characterization not only allows the materials’ potential for implementation in different fire scenarios and fire tests to be assessed, but also provides detailed insight into the active mechanisms. The increased melt viscosity of the nanocomposites and the fibre-network character of the nanofiller are the dominant mechanisms influencing fire performance. The changes are found to be adjuvant with respect to forced flaming conditions in the cone calorimeter, but also deleterious in terms of flammability.

Effective nucleating chemical agents for the crystallization of poly(trimethylene terephthalate)

AbstractThis study focused on the crystallization promotion of poly(trimethylene terephthalate) (PTT), with an aim at engineering thermoplastics applications. The effects of organic sodium (Na) salts, including Na stearate, Na benzoate, disodium‐p‐phenolsulfonate (2Na‐p‐PS), disodium‐p‐hydroxybenzoate (2Na‐p‐HB), and the sodium ionomer of poly(ethylene‐co‐methacrylic acid) (Na‐EMAA), were investigated as nucleating chemical agents with differential scanning calorimetry and capillary viscometry. For comparison, the effect of fine talc powder was also examined. The chemical agents were generally more effective than fine talc powder. Na stearate and Na benzoate caused large‐scale decomposition of PTT. 2Na‐p‐PS was quite thermally stable and caused little decomposition. 2Na‐p‐HB was the most efficacious of the nucleating chemical agents and caused mild decomposition. Na‐EMAA was the most thermally stable and induced an increase in melt viscosity. A remarkable improvement in the crystallization rate of PTT was successfully attained at a minimum polymer decomposition cost by the introduction of a suitable amount of 2Na‐p‐PS, 2Na‐p‐HB, or Na‐EMAA or by the concurrent proper incorporation of both of the latter two agents. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 590–601, 2004

Thermoplastic olefin/clay nanocomposites: Morphology and mechanical properties

AbstractThermoplastic olefin (TPO)/clay nanocomposites were made with clay loadings of 0.6–6.7 wt %. The morphology of these TPO/clay nanocomposites was investigated with atomic force microscopy, transmission electron microscopy (TEM), and X‐ray diffraction. The ethylene–propylene rubber (EPR) particle morphology in the TPO underwent progressive particle breakup and decreased in particle size as the clay loading increased from 0.6 to 5.6 wt %. TEM micrographs showed that the clay platelets preferentially segregated to the rubber–particle interface. The breakup of the EPR particles was suspected to be due to the increasing melt viscosity observed as the clay loading increased or to the accompanying chemical modifiers of the clay, acting as interfacial agents and reducing the interfacial tension with a concomitant reduction in the particle size. The flexural modulus of the injection moldings increased monotonically as the clay loading increased. The unnotched (Izod) impact strength was substantially increased or maintained, whereas the notched (Izod) impact strength decreased modestly as the clay loading increased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 928–936, 2004

Preparation and properties ofin situ polymerized poly(ethylene terephthalate)/fumed silica nanocomposites

We have prepared poly(ethylene terephthalate) (PET) nanocomposites filled with two different types of fumed silicas, hydrophilic (FS) and hydrophobic (MFS) silicas of 7-nm diameter, byin situ polymerization. We then investigated the morphological changes, rheological properties, crystallization behavior, and mechanical properties of the PET nanocomposites. Transmission electron microscopy (TEM) images indicate that the dispersibility of the fumed silica was improved effectively byin situ polymerization; in particular, MFS had better dispersibility than FS on the non-polar PET polymer. The crystallization behavior of the nanocomposites revealed a peculiar tendency: all the fillers acted as retarding agents for the crystallization of the PET nanocomposites. The incorporation of fumed silicas increased the intrinsic viscosities (IV) of the PET matrix, and the strong particleparticle interactions of the filler led to an increased melt viscosity. Additionally, the mechanical properties, toughness, and modulus of the nanocomposites all increased, even at low filler content.

Influence of molecular weight on the domain coarsening rates in linear polyethylene–poly(ethylene‐co‐1‐octene) blends

AbstractThe coarsening rates of the two‐phase morphologies of linear polyethylene/poly(ethylene‐co‐1‐octene) blends were determined as functions of molecular weight. Samples with cocontinuous morphologies that were prepared through solution blending were annealed in the melt state for various times, and, subsequently the length scales of the morphologies were determined with a line‐intersection method. Length‐scale data were multiplied by a function that normalized for the effects of differences in zero‐shear‐rate viscosity and thermal energy; after normalization, the data largely fell on one trend line within the bounds of experimental error. This indicated that the principal effect of increasing molecular weight was to slow the coarsening rate through an increase in melt viscosity, with little effect from the thermodynamic compatibility of the two polymers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1655–1661, 2003

Functionalization of LDPE by Melt Grafting with Glycidyl Methacrylate and Reactive Blending with Polyamide‐6

AbstractLow‐density polyethylene (LDPE) was functionalized by melt radical grafting with glycidyl methacrylate (GMA) and employed for reactive blending with polyamide‐6 (PA6). The effect of the reaction procedure on the grafting degree of LDPE‐g‐GMA samples (0.5–12.5 wt.‐% GMA) was analyzed as a function of the concentration of GMA monomer, radical initiator (BTP), and addition of styrene as co‐monomer. Optimized grafting conditions were obtained when the amount of the monomer is below 10 wt.‐% and that of peroxide in the range 0.2–0.4 wt.‐%. Binary blends of PA6 with LDPE‐g‐GMA (3.5 wt.‐% GMA) and with LDPE at various compositions (80/20, 67/33, 50/50 wt.‐%) were prepared in an internal mixer and their properties were evaluated by torque, SEM and DSC analyses. Morphological examination by SEM showed a large improvement of phase dispersion and interfacial adhesion in PA6/LDPE‐g‐GMA blends as compared with PA6/LDPE blends. The average diameter of dispersed polyolefin particles was about 0.4 μm for LDPE‐g‐GMA contents < 50 wt.‐%. A marked increase of melt viscosity was observed for the compatibilized blends depending on the concentration of grafted polyolefin, and it was accounted for by the reaction between the epoxy groups of GMA and the carboxyl/amine end‐groups of PA6. The variation of torque was thus related to the molar ratio of reactive group concentration. The analysis of crystallization and melting behavior pointed out marked differences in the phase structure of the blends due to the presence of the functionalized polyolefin. Finally, the in situ formation of a graft copolymer between LDPE‐g‐GMA and PA6 was investigated by means of a selective dissolution method (Molau test) and by FT‐IR and DSC analyses.SEM micrograph of fracture surface of PA6/LDPE‐g‐GMA 50/50 blend.magnified imageSEM micrograph of fracture surface of PA6/LDPE‐g‐GMA 50/50 blend.

Rheological features and molecular architecture of polyethylenes

Compared to other polymers, highly increased melt viscosity, elasticity, and flow activation energy were observed in several chromium-based polyethylenes. Findings reported in the literature suggest that these effects could, at least theoretically, be accounted for by the polydispersity of these polyethylenes. However, the data presented here would appear to indicate a much stronger, evident connection with long chain branching than with polydispersity.

Melt Rheological Properties Of Polypropylene/lldpe Copolymer Blends

Melt rheological properties of the blends of polypropylene (PP) with LLDPE copolymer, at blending ratios of 10 to 40% LLDPE copolymer were studied. Data obtained from a capillary rheometer is presented to describe the effect of blending ratio on shear stress, shear rate, melt viscosity, and melt elasticity parameters. In general, blending of PP with LLDPE copolymer results in an increase in its viscosity. At blend composition corresponding to 40% LLDPE copolymer content, a maximum increase in melt viscosity, accompanied by minimum melt elasticity, is observed. The blends studied showed a pseudoplastic, nonnewtonian behavior.

Parameters affecting the chain extension and branching of PET in the melt state by polyepoxides

AbstractThis article describes the chemical modification of polyethylene terephthalate (PET) with a variety of compounds containing reactive glycidyl group(s). Four different modifiers, namely, diglycidyl ether of bisphenol‐A (DGEBA), N,N′‐bis[3(carbo‐2′,3′‐epoxypropoxy) phenyl] pyromellitimide (BGPM), triglycidyl glycerol (TGG), and triglycidyl isocyanurate (TGIC) were compared for their reactivity toward PET in the melt phase. It was found that the presence of tertiary nitrogen in the structure of the epoxide modifiers plays the role of in‐built catalyst for their reaction with the end groups of PET. TGIC as a modifier was selected for the detailed investigation of the simultaneously occurring degradation and chain extension/branching reactions in a batch‐melt mixer. The reactions were followed by torque changes, analyzing the products for residual carboxyl content, and by determining insoluble content. It is shown that the rate of the reactive modification of PET melt by TGIC depends upon stoichiometry, temperature, rate of shear, and the chemical composition and the molecular weight (MW) of the PET resin. In general, the results indicate an increase in melt viscosity and insoluble content, whereas an overall decrease in carboxyl content occurs, as defined by the choice of mixing conditions and stoichiometry. Analysis of the batch kinetic data can be useful to define the process requirements for carrying out the reactive modification in continuous extrusion equipment. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 643–652, 2003

Melt Rheological Behaviour of Short Sisal Fibre Reinforced Polypropylene Composites

The melt rheological properties of short sisal fibre reinforced polypropylene composites have been studied using a capillary rheometer. The dependence of the relative composition of component reinforcements and their aspect ratio and also the extent of chemical modification on the overall rheological behaviour have been studied. Incorporation of sisal fibre into polypropylene results in an increase in melt viscosity and a decrease in melt elasticity. The melt viscosity was found to have increased with fibre loading. Incorporation of fibres decreased the extrudate deformation and die-swell and this improvement was more prominent at higher fibre loading. A comparison was made between the two techniques employed for the preparation of the composites, i.e. melt mixing in a Haake Rheocord and solution mixing using toluene-xylene mixture as the solvent. It is found that composites prepared by solution mixing showed a higher viscosity as compared to melt mixed composites. Various chemical modifications using chemicals such as isocyanate, maleic anhydride and permanganate treatment have been carried out to improve the interaction between the matrix and fibre. The influence of chemical treatment on the viscosity of the system has been analysed. The fibre breakage analysis during extrusion was carried out using optical microscopy. The morphology of extrudate has been studied by optical and scanning electron microscopes. Master curves have been generated using modified viscosity and shear rate functions that contain the melt flow index (MFI) as a parameter. A comparison was made between theoretical and experimental viscosity values and it was found that experimental viscosity is slightly higher than theoretical viscosity because of the misalignment of fibres.

Orientation of Aromatic Ion Exchange Diamines and the Effect on Melt Viscosity and Thermal Stability of PMR-15/Silicate Nanocomposites

Nanocomposites of PMR-15 polyimide and a diamine modified silicate were prepared by the addition of the silicate to PMR- 15 resin. The orientation of the ion exchange diamine within the silicate gallery was evaluated by x-ray diffraction and found to depend on the clay cation exchange capacity. The melt viscosity of the oligomer and the uncured nanocomposite was measured. The melt viscosity exhibited a dependence on the orientation of the diamine in the silicate interlayer and, in some cases, on the chain length of the diamine. A correlation was observed between the oligomer melt viscosity and the crosslinking enthalpy, where nanocomposites with an increased melt viscosity exhibited a decrease in enthalpy on crosslinking. After crosslinking, a poorer nanocomposite thermal oxidative stability was also observed, compared to the less viscous systems. The melt viscosity was tailored by coexchange of an aromatic diamine and an aliphatic amine, increasing the thermal oxidative stability of the nanocomposite. PMR-15/silicate nanocomposites were investigated as a matrix for carbon fabric reinforced composites. Dispersion of an organically modified silicate into the PMR-15 matrix greatly enhanced the thermal oxidative stability of the polymer matrix composite.

Spatial variation of structural hierarchy in injection molded PVDF and blends of PVDF with PMMA. Part II. Application of microbeam WAXS pole figure and SAXS techniques

The influence of blend composition on the processing–structure–property relationships in injection molded PVDF and blends of PVDF with PMMA was investigated. Local crystalline order and chain orientation were studied using newly developed microbeam wide angle X-ray pole figure camera. The depth profiling of lamellar texture was investigated using SAXS combined with microtomy. These techniques provided a general picture on the macro and microstructural spatial gradients developed in the injection molded parts as influenced by the process condition and the composition of the blends. The injection molded PVDF samples were found to exhibit a typical three layer morphology. The skin layer is formed as a result of the extensional flow with dominant c-axis orientation along the flow direction (FD). The crystal lamellae in this region are mainly extended in the direction perpendicular to the FD. The “shear zone” formed under the shear flow possesses “shish–kebab” structure. The “shish” structure is formed under high shear stress with “kebab” lateral overgrowth that occurs at subsequent stages. The chains in the shear zone are highly oriented in the FD with the local symmetry axis slightly tilted inwards towards the core. This tilt angle tends to be the largest at the interior portions of the shear crystallized zone essentially following an approximate parabolic profile indicating that this feature of the structure is established by the spatial variation of the flow kinematics during injection. The microbeam X-ray pole figure results indicate that the a- and b-axes of the α crystals are distributed in a plane close to the ND–TD plane in the skin and shear zone. The level of orientation gradually decreases towards the mid symmetry plane at the interior of the samples from the slow cooling that takes place in these regions. The addition of small amount of diluent PMMA was found to enhance the chain orientation levels in the PVDF. This was attributed to the increase of melt viscosity as well as reduction of “self” entanglements of the crystallizable PVDF chains.

Influence of modified rheology on the efficiency of intumescent flame retardant systems

Optimal concentration of boroxo siloxane elastomer synergist was determined by LOI and cone calorimetric measurements on ammonium polyphosphate and pentaerythritol containing intumescent flame retardant for polypropylene. The increased viscosity of the melt and plasticity of intumescent char due to boroxo siloxane elastomer could be proved by a thermal scanning rheometric investigation. It is presumed, the increased melt viscosity is created by the product of boroxo siloxane-pentaerythritol, formed during the compound preparation, while the improved char plasticity is the result of product formed at high temperature from boroxo siloxane and ammonium polyphosphate.