Effect of boron nitride on melt fracture phenomena and processability of lldpe during extrusion

Melt fractures related to processing instabilities limit processing rates in many commercially important polymer processing operations, such as fiber spinning, film blowing, extrusion, and various coating flows. Therefore, melt fracture is responsible for deteriorating the quality and the mechanical properties of final products for rates greater than a critical processing one at which melt fracture occurs. In this study, a commercial linear low-density polyethylene (LLDPE) was modified by adding a small amount of boron nitride (BN) during extrusion in order to improve processability. Capillary rheometry was used to assess processability at various temperatures, levels of applied shear rate, and the length-to-diameter (L/D) ratio for both the pure resin and resins containing boron nitride. Also, parallel-plate rheometry was used to evaluate the dynamic rheological properties of these resins. The relationship between the characteristic relaxation time and the critical shear rate for the onset of melt fracture and slip is discussed.     

Boron nitride as a processing aid for the extrusion of polyolefins and fluoropolymers

The influence of a new processing additive (fine particles of boron nitride) on the processability of polyolefins and fluoropolymers in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and an extruder. Two metallocene polyethylenes and several Teflon® fluoropolymers were tested using these two pieces of equipment. The additive had a significant effect on the extrudate appearance of polyethylene and fluoropolymer particularly in the crosshead dies. It was found to eliminate surface melt fracture and to postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.005% to 0.5%). To explain the possible mechanism for the effect of the additive on the processability of the resins, rheological measurements using both parallel‐plate and sliding‐plate rheometers were carried out. The rheology of the resins did not seem to change significantly with the addition of boron nitride except for the low‐shear‐rate (low‐frequency) range, where the behavior of the filled resin was found to be similar to that of a crosslinked polymer or a phase‐separated entangled blend. Practical wire coating and tubing extrusion studies for these resins were also carried out.     

Combining boron nitride with a fluoroelastomer: An enhanced polymer processing additive

The effect of a new processing additive (boron nitride powder in combination with a fluoroelastomer) on the rheology and processability of molten polymers is studied. The equipment used includes an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross‐head) and a parallel‐plate rheometer. Metallocene polyethylenes with and without boron nitride (BN) and fluoroelastomer are tested in extrusion. First, it is demonstrated that BN is a superior processing aid compared to conventional fluoropolymer ones. Second, it is found that the combination of BN powders with a small amount of a fluorelastomer improves even further the processability of molten polymers (melt fracture performance).     

Gross melt fracture elimination: The role of surface energy of boron nitride powders

Boron nitride (BN) is an effective processing aid for the extrusion of polyethylenes. It postpones the onset of gross melt fracture to significantly high shear rates not previously attained with conventional fluoropolymers. However, BN particles containing relatively high amounts of boron oxide (B₂O₃) do not perform well as processing aids. A reliable procedure has been developed for measurement of surface energy of powders using the capillary rise technique through the use of Washburn's equation. It is based on finding the contact angle from liquid penetration experiments with polar and non‐polar liquids. Both the dispersive and non‐dispersive components of surface energy are determined. With this technique, the surface energy of a number of different powders has been assessed. These results of the surface energy of BN powders have been found to correlate well with the critical shear rate for the onset of melt fracture, indicating the important role that surface energy plays in gross melt fracture elimination.     

The effect of surface energy of boron nitride on polymer processability

Flow instabilities manifest themselves as distortions on the extrudate surface (melt fracture). They are usually observed at high production rates in many polymer processing operations. Certain fluoropolymers/fluoroelastomers have long been used as processing aids for surface melt fracture elimination. Recent developments have shown that a small amount of boron nitride (BN) powder may successfully eliminate surface melt fracture and also delay the onset of gross melt fracture to higher rates. It has also been reported that a combination of BN and fluoropolymer/fluoroelastomer enhances the effectiveness of the polymer processing even further. The main objective of the present work was to measure the surface properties of a number BN powders, mainly surface energy, in order to gain a better understanding of its performance as a processing aid. Based on this study, it can be concluded that surface energy plays an important role in deciding the possible interactions between the processing aid, polymer melt and the extruding surface. It is observed that the lubricious nature of BN along with an optimum balance of its polar (non‐dispersive) and non‐polar (dispersive) components of surface energy renders BN a successful processing aid in eliminating both sharkskin and gross melt fracture phenomena. Polym. Eng. Sci. 44:1543–1550, 2004. © 2004 Society of Plastics Engineers.     

The synergistic effect of boron nitride particle and die on the capillary extrusion processability of mLDPE

The synergistic effects of boron nitride (BN) powder and die on the rheology and processability of metallocene‐catalyzed low density polyethylene (mLDPE) were investigated. The processability in the extrusion process is closely related to the interfacial properties between the polymer melts and the die wall. BN powder was added to mLDPE to reduce the friction coefficient and surface energy. Adding 0.5 wt% BN powder to mLDPE was very effective in improving the processability and the extrudate appearance. To study the effect of die surface property, three different dies were applied in capillary extrusion. One was conventional tungsten carbide (TC) die, and the others were hot‐pressed BN (hpBN) die and hot‐pressed BN composite (hpBNC) die. The applications of these BN dies were quite effective in delaying surface melt fracture (sharkskin) and postponing gross melt fracture to higher shear rate compared to the TC die. These improvements result from the fact that BN dies reduce the wall shear stress significantly and promote slip. The synergistic effect of processability could be obtained when both BN powder and hpBN die were used together.     

The effect of nanoclays on the processibility of polyolefins

The influence of a new processing additive (particles of organically modified nanoclays) on the processibility of polyolefins in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer. Ziegler Natta and metallocene polyethylenes and one polypropylene were tested using these two pieces of equipment. It was found that the nanoclay additive had a significant effect on the extrudate appearance of polyethylene. It eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.05 to 0.5 wt%). To explain the possible mechanism for the effect of the additive on the processibility of the resins, shear and extensional rheological measurements were carried out for the pure resins as well those loaded with the nanoclay additives. It seems that the presence of nanoclays suppresses the development of extensional stresses to such high levels that can cause melt fracture phenomena. Finally, it was found that the combination of nanoclays with traditional processing aids such as fluoropolymers produce an enhanced processing aid that can increase the critical shear rates for the onset of melt fracture to levels much higher than the individual constituents when they are used independently. POLYM. ENG. SCI., 45:1098–1107, 2005. © 2005 Society of Plastics Engineers     

A new processing aid for the extrusion of polyolefins

The influence of a new processing additive (a composition of fine particles of boron nitride) on the rheology and processability of molten polymers is studied. The equipment used includes both an Instron capillary rheometer with special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer and a parallelplate rheometer. Two metallocene polyethylenes with several types of boron nitride powders varying in particle size distribution are tested at various concentration levels. The powder having the finest particle size was found to have the greatest influence in the processing of polyolefins using crosshead dies and tips. As a result, its use eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on the additive concentration. The performance of the boron nitride in eliminating melt fracture is compared with that of a typical fluoroelastomer. It is shown that boron nitride is a superior processing aid.     

Prediction of elongation viscosity of polymer melts

Melt extension flow is a common flow pattern during polymer processing, such as entrance converging flow in die extrusion or runner injection of polymer melts from an extruder barrel, blow molding, blowing film and melt spinning. Extensional viscosity is one of the important characterizations of the flow characteristics for polymer fluids. A new extension viscosity equation was established based on White‐Metzner model, Vinogradov‐Malkin viscosity equation and a new relaxation time equation in the present paper. The melt elongation viscosities of metallocene linear low‐density polyethylene (mLLDPE) and polyvinyl butyral (PVB) resins at 130°C were estimated applying this viscosity equation, and the predictions were compared with the measured data of mLLDPE and PVB resins at 130°C reported from reference. The results showed that calculations were close to the experimental data. The parameters in this equation were easy to be determined and the equation was convenient to use for estimating the extension viscosity of polymer melts. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

A study of “worms” melt fracture in polyethylene extrusion blow molding process using capillary rheometry

During the polyethylene (PE) blow molding process of large size drums, string‐like defects, which are referred to here as worm melt fracture, can sometimes be observed on the extrudate surface. Such string‐like defects, in various shapes and sizes, are also observed in the capillary extrusion at high shear rates after the slip‐stick transition. The PE resin with broader molecular weight distribution (MWD) exhibits a greater degree of worm melt fracture while narrow MWD PE resin, which has higher slip velocity and a uniform slip layer, shows a lesser degree of worm melt fracture. It is hypothesized that the worm melt fracture is related to the die build‐up. Based on the mechanism of the fast die build‐up, it is proposed that the cohesive slip layer, which is a failure within the polymer melts at an internal surface, could emerge out from the die as these string‐like materials attached on the extrudates. The broader MWD resin, which has more small polymer chains and a lower plateau modulus, is postulated to have a weaker polymer melt, which then makes it easier to have such an internal failure and consequently have more string‐like defects at high shear rates. POLYM. ENG. SCI., 56:650–656, 2016. © 2016 Society of Plastics Engineers     

双壁工业配件的吹塑成型

挤出吹塑一直被用于生产瓶、罐等包装容器,现正越来越多地用于生产工业配件。双壁吹塑制件已逐步应用于各行各业。双壁制件的吹塑主要以HDPE、PP、工程塑料及各种共混料为原料。本文着重介绍了双壁工业配件挤出吹塑的方法及设备设计要点。     

POLYFLOW在高分子成型加工中的应用研究进展

在聚合物制品外形控制和性能调控过程中,高分子熔体的流动分析对于研究高分子成型工艺至关重要.POLYFLOW软件在高分子成型加工中的应用日渐广泛.本文深入分析了POLYFLOW在挤出、吹塑和热成型等方面的应用,重点介绍了挤出过程中的正向流动仿真和逆向口模设计,最后对POLYFLOW在高分子成型加工中的应用研究方向进行展望...     

Melt fracture of two broad molecular weight distribution high‐density polyethylenes

The melt fracture instabilities of two broad molecular weight distribution (MWD) high‐density polyethylenes (one Ziegler–Natta and one metallocene HDPEs) are studied as functions of the temperature and geometrical details and type of die (cylindrical, slit, and annular). It is found that sharkskin and other melt fracture phenomena are distinctly different for these resins, despite their almost identical rheology. It is also found that the critical conditions for the onset of various melt fracture phenomena depend significantly on the type of die used for their study. For example, sharkskin melt fracture in slit and capillary extrusion was obtained at much small critical shear stress values compared with those found in annular extrusion. Moreover, the metallocene HDPE shows significant slip at the die wall in the sharkskin flow regime. On the other hand, the Ziegler–Natta HDPE has shown no sign of slip. These differences are discussed on the basis of differences in their MWDs that influence their melt elasticity. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Reinforced thermal conductivity and mechanical properties of in situ microfibrillar composites through multistage stretching extrusion

To surmount difficulty of the melt processing and deterioration of mechanical properties of polymer composites induced by high fraction of the reinforced fibers and thermal conductive fillers, polyethylene (PE)/boron nitride (BN)/polyamide 6 (PA6) and PE/BN/poly(‐hydroxybenzate‐co‐DOPO‐benzenediol dihydrodiphenyl ether terephthalate) (PHDDT) in situ microfibrillar composites were prepared through multistage stretching extrusion. The experimental results showed that both the tensile and impact strength of the PE/BN/PA6 and PE/BN/PHDDT composites were improved. Meanwhile, the thermal conductivities of the PE/BN, PE/BN/PA6, and PE/BN/PHDDT composites were also reinforced. Based on the equation proposed by Y. Agari, the new modified equations can well predict the thermal conductivity of the composites prepared through multistage stretching extrusion with different number of laminating‐multiplying elements. In addition, it was found that PHDDT can act as a “processing aid” to reduce the viscosity of the PE/BN composites. POLYM. COMPOS., 38:2663–2669, 2017. © 2015 Society of Plastics Engineers     

Impact of organoclay and maleated polyethylene on the rheology and instabilities in the extrusion of high density polyethylene

The impact of organoclay on the rheology and extrusion of high density polyethylene (HDPE) was studied. Organoclay effect was studied at very low clay loading (≤0.1 wt %) while serving as a processing aid. A special design slit die with three transducers was used in the study of the extrusion melt instabilities. The rheological results showed that normal stress difference of HDPE was reduced during steady shear rate and stress growth tests when organoclay (≤0.1 wt %) was added. The extensional strain and stress growth of HDPE reduced with the addition of organoclay. So, organoclay (≤0.1 wt %) has an effect on the shear and extensional rheology of HDPE. The intensity of the melt instability was characterized with both a moment analysis and distortion factor (DF) from an advanced Fourier transform analysis. Both showed the same trends in the characterization of the pressure fluctuations in the die. Generally, addition of organoclay (≤0.1 wt %) to HDPE led to the reduction in DF. The ratio of first and second moment analyses became reduced as well. The results quantified the extent of elimination of gross melt fracture in HDPE by organoclay. Also, the extrusion pressure was reduced with organoclay (≤0.1 wt %) inclusion hence more throughput. There was a good correlation between rheology and extrusion. Both showed that the platy‐like organoclay streamlined the melt flow. However, the maleated polyethylene added as a compatibilizer did not give substantial synergistic effect. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Poly (alpha-methylstyrene) as a processing aid for rigid poly (vinyl chloride)

Poly (alpha-methylstyrene) (PAMS) is a new approach to process improvement in rigid poly (vinyl chloride) (PVC)—the use of a low molecular weight polymer as a process aid. PAMS can reduce fusion time, melt viscosity and improve heat stability of rigid PVC compounds. The reduced melt viscosity allows the addition of fillers to PVC without adverse effect on extrusion rates. This polymer also improves the resistance to melt fracture and shear “burning” at high shear rates. When used within the optimum concentration range, physical properties such as tensile strength, Izod impact and heat distortion are maintained. In addition, PAMS gives, improved production rates in both pipe extrusion and injection molding processes without sacrifice in properties.     

Thermal conductivity of boron nitride‐filled thermoplastics: Effect of filler characteristics and composite processing conditions

The thermal conductivity of boron nitride (BN)‐filled poly(butylene terephthalate) (PBT) was investigated as a function of particle size, aspect ratio, surface area, surface chemistry, and concentration of BN as well as composite processing methods and conditions. In the low filler concentration region, a larger BN surface area resulted in lower thermal conductivity of the composites as a result of phonon scattering at interfaces. In the high filler concentration region the ease in forming filler networks, as reflected by the aspect ratio of BN, played a more dominant role. A percolation‐like behavior was observed when BN networks were formed while the thermal conductivity at close vicinity of the percolation threshold was not completely governed by the scaling law of classic percolation theory. High shear force employed in extrusion was effective in dispersing BN agglomerates into fine platelets while also inducing PBT degradation. When a low screw speed was used in extrusion followed by injection molding, the samples exhibited significantly lower thermal conductivity, which may be attributed to flow‐induced orientation of BN platelets in the direction perpendicular to the heat flow, relatively low concentration of filler at sample surfaces (skin‐core effect), and agglomeration of the BN platelets. POLYM. COMPOS. 26:778–790, 2005. © 2005 Society of Plastics Engineers     

Boron nitride and fluoropolymer combinations: Interactions and their performance as processing aids

In this work, we studied the adsorption capacity of boron nitride (BN) for fluoropolymer and polyethylene (PE) to gain a better understanding of the interactions and the performance of BN and fluoropolymer, and their combinations as processing aids in the extrusion of Ziegler‐Natta PEs. We found that BN has a relatively high adsorption capacity for both PE and fluoropolymer. As a result, simultaneous compounding of BN and fluoropolymer into the host polymer causes fluoropolymer to be trapped within the bulk of the polymer, and prevents the fluoropolymer particles from coating the die wall during flow. This limits the effectiveness of fluoropolymer and BN as processing aids. To avoid this interaction, we added fluoropolymer separately in a dry form just prior to extrusion. In this case, the synergistic effect of BN and fluoropolymer as a combination processing aid was evident. POLYM. ENG. SCI., 45:669–677, 2005. © 2005 Society of Plastics Engineers     

Corotating twin-screw extrusion of poly(lactic acid) PLA/poly(butylene succinate) PBS/ micro-lamellar talc blends for extrusion blow molding of biobased bottles for alcoholic beverages

Extrusion blow molding is a well-established technology for the manufacture of fossil-based plastic bottles. The process is, however, still little used for the manufacture of bottles with a low environmental footprint, especially those based on bioplastic from renewable sources. In this context, the objective of this work is precisely the study and experimental design of poly(lactic acid) PLA/poly(butylene succinate) PBS/micro-lamellar talc compounds for the manufacturing of bioplastic bottles, basically for wine packaging. In particular, the design was carried out to ensure, primarily, an adequate processability of the bioplastic material in the blowing process. Second, the compound was loaded with different micro-lamellar talc content so as to achieve protection from the environmental factors, which is of paramount importance to ensure a long shelf-life to wine. The bio-derived polyester resins are very complex to transform, as they are subject to thermo-hydrolytic degradation phenomena during the processing of the polymer melt. Processability is further limited in the presence of high micro-lamellar talc content that increases the melt viscosity, thus making the material even more difficult to shape by extrusion blow molding. The experimental analysis involved the use of a co-rotating twin-screw extruder for the manufacture of the bioplastic compounds. The compounds were first subjected to thermo-rheological and physical characterization tests. Second, it was tested in the extrusion blow molding process. The experimental results have shown that blends based on bio-derived polyester resins can be adequately processed by extrusion blow molding, showing extremely stable rheological behavior both during the extrusion phase of the parison and the subsequent blowing process of the parison itself. These blends have, therefore, an interesting potential to be used as an alternative with a low environmental footprint to oil-based plastics in the production of wine bottles.     

Real‐time diagnosing polymer processing in injection molding using ultrasound

Ultrasonic diagnosing technique with a new high‐temperature ultrasonic transducer is developed to real‐time diagnose polymer processing and its morphology changes in injection molding processing. Compared with the previous researches, the new technique can provide more and accurate information. In this study, ultrasound diagnosis shows that longitudinal wave can real‐time characterize the data of the injection process and polymer morphology changes, including melt flow arrival time, the part ejection time, filling and packing stages, polymer solidification process, and the morphology changes during polymer crystallization. Shear waves can real‐time diagnose Young's and shear storage modulus, anisotropy property of polymer in injection molding. During our research, real‐time ultrasonic diagnosis shows that the storage modulus along the vertical direction is larger than that of the parallel to the melt flow direction under our setup injection conditions. Scanning electron microscopy and dynamic mechanical analysis measurements present that it is because the crystalline lamellas of HDPE are parallel arrangement and grow in a vertical to melt flow direction owing to injection shear force under a certain injection conditions. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012