The document summarizes key concepts in polymer rheology and viscosity relevant to polymer extrusion processes. It defines rheology and viscosity, describes how viscosity is measured using capillary and rotational viscometers, and models like the power law model that are used to characterize viscosity. Key factors that influence viscosity, like temperature, molecular weight, additives, and pressure are also summarized.
Acrylics are a family of transparent plastics that include polymethyl methacrylate (PMMA). PMMA was first synthesized in 1877 and commercialized in the 1930s for uses like aircraft canopies. It is produced through radical polymerization of methyl methacrylate. PMMA has good clarity, weatherability, and scratch resistance but limited chemical resistance. It finds wide use in glazing, lighting, medical devices, and coatings. Other acrylics include polyacrylamide, used as a flocculant and soil conditioner, and sodium polyacrylate, a super absorbent polymer used in diapers and water-retention products.
Natural Rubber is an elastic substance obtained from the latex sap of trees, especially those trees which belong to the genera Hevea and Ficus. It is having wide range of applications in the anthropogenic world. And it may undergo various kinds of degradation. Hence, Degradation is an important parameter when we study the application of NR. The slides consisting of various mechanism of degradation of Natural Rubber taken from various books and study materials.
Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer. It is produced through the hydrolysis of polyvinyl acetate, not by polymerization of vinyl alcohol. PVA has excellent film-forming, adhesive, and emulsifying properties. It is used in products like eye drops, contact lens solution, and as reinforcement in concrete. PVA dissolves in water due to hydrogen bonding between its hydroxyl groups and water molecules.
The document discusses various methods for improving the toughness of polymers, including rubber modification of thermoplastics. It describes how the inclusion of small, well-dispersed rubber particles can induce toughening mechanisms in a thermoplastic matrix like crazing, shear yielding, and cavitation. These particles prevent the growth of large cracks. Thermoplastic elastomers are also discussed as bimicrophasic materials that can be processed and recycled like thermoplastics but have rubber-like elasticity. Styrenic block copolymers and thermoplastic vulcanizates are provided as examples.
Poly Lactic Acid (PLA) is a biodegradable and compostable thermoplastic polymer made from renewable resources like corn, sugar beets and wheat. PLA is produced through fermentation of carbohydrates to lactic acid, then polymerization to form polylactic acid. It has physical properties comparable to polyethylene terephthalate but requires less fossil fuels to produce. While PLA has potential applications for single-use items and packaging due to its sustainability, its production also has criticisms related to energy usage and slowed degradation with certain additives.
Miscibility and Thermodynamics of Polymer BlendsAbhinand Krishna
Presentation includes classification of polymer blends based on miscibility, phase diagram of polymer blends and thermodynamics polymer blends which includes Gibbs energy theory and Flory-Huggins Theory
This document discusses biodegradable polymers. It begins by defining biodegradation as the process of converting polymers into harmless gaseous products via microorganisms and enzymes. It then notes that biodegradable polymers eliminate the need for disposal systems by degrading through natural biological processes. The document outlines the need for biodegradable polymers due to the large amount of non-biodegradable plastic waste produced annually. It proceeds to discuss various biodegradable polymers like biopol, polycaprolactone, polylactic acid, polyglycolic acid, and their characteristics, production processes, uses, and degradation mechanisms.
This document discusses various types of additives used in polymer processing and their functions. It describes additives like stabilizers, lubricants, plasticizers, fillers, fibers, coupling agents, antistatic agents, slip agents, anti-block agents, nucleating agents, optical brighteners, colorants, anti-aging additives, impact modifiers, flame retardants, blowing agents, and master batches. It provides examples and explains how each additive type alters polymer properties or facilitates processing to achieve the desired characteristics in final products.
Acrylics are a family of transparent plastics that include polymethyl methacrylate (PMMA). PMMA was first synthesized in 1877 and commercialized in the 1930s for uses like aircraft canopies. It is produced through radical polymerization of methyl methacrylate. PMMA has good clarity, weatherability, and scratch resistance but limited chemical resistance. It finds wide use in glazing, lighting, medical devices, and coatings. Other acrylics include polyacrylamide, used as a flocculant and soil conditioner, and sodium polyacrylate, a super absorbent polymer used in diapers and water-retention products.
Natural Rubber is an elastic substance obtained from the latex sap of trees, especially those trees which belong to the genera Hevea and Ficus. It is having wide range of applications in the anthropogenic world. And it may undergo various kinds of degradation. Hence, Degradation is an important parameter when we study the application of NR. The slides consisting of various mechanism of degradation of Natural Rubber taken from various books and study materials.
Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer. It is produced through the hydrolysis of polyvinyl acetate, not by polymerization of vinyl alcohol. PVA has excellent film-forming, adhesive, and emulsifying properties. It is used in products like eye drops, contact lens solution, and as reinforcement in concrete. PVA dissolves in water due to hydrogen bonding between its hydroxyl groups and water molecules.
The document discusses various methods for improving the toughness of polymers, including rubber modification of thermoplastics. It describes how the inclusion of small, well-dispersed rubber particles can induce toughening mechanisms in a thermoplastic matrix like crazing, shear yielding, and cavitation. These particles prevent the growth of large cracks. Thermoplastic elastomers are also discussed as bimicrophasic materials that can be processed and recycled like thermoplastics but have rubber-like elasticity. Styrenic block copolymers and thermoplastic vulcanizates are provided as examples.
Poly Lactic Acid (PLA) is a biodegradable and compostable thermoplastic polymer made from renewable resources like corn, sugar beets and wheat. PLA is produced through fermentation of carbohydrates to lactic acid, then polymerization to form polylactic acid. It has physical properties comparable to polyethylene terephthalate but requires less fossil fuels to produce. While PLA has potential applications for single-use items and packaging due to its sustainability, its production also has criticisms related to energy usage and slowed degradation with certain additives.
Miscibility and Thermodynamics of Polymer BlendsAbhinand Krishna
Presentation includes classification of polymer blends based on miscibility, phase diagram of polymer blends and thermodynamics polymer blends which includes Gibbs energy theory and Flory-Huggins Theory
This document discusses biodegradable polymers. It begins by defining biodegradation as the process of converting polymers into harmless gaseous products via microorganisms and enzymes. It then notes that biodegradable polymers eliminate the need for disposal systems by degrading through natural biological processes. The document outlines the need for biodegradable polymers due to the large amount of non-biodegradable plastic waste produced annually. It proceeds to discuss various biodegradable polymers like biopol, polycaprolactone, polylactic acid, polyglycolic acid, and their characteristics, production processes, uses, and degradation mechanisms.
This document discusses various types of additives used in polymer processing and their functions. It describes additives like stabilizers, lubricants, plasticizers, fillers, fibers, coupling agents, antistatic agents, slip agents, anti-block agents, nucleating agents, optical brighteners, colorants, anti-aging additives, impact modifiers, flame retardants, blowing agents, and master batches. It provides examples and explains how each additive type alters polymer properties or facilitates processing to achieve the desired characteristics in final products.
This document discusses the synthesis of poly(lactic acid) (PLA) biomaterials. There are two main synthetic methods - direct polycondensation and ring-opening polymerization of lactide monomers. Direct polycondensation includes solution and melt polycondensation, but yields PLA with low molecular weight. Ring-opening polymerization using metal catalysts is more common and can produce high molecular weight PLA, but the metal catalysts require removal. Recent research focuses on developing non-toxic catalysts and new polymerization conditions.
Elastomers are materials that can reversibly change length when a load is applied and return to their original dimensions once the load is removed. They include both natural and synthetic rubbers. Elastomers have properties that can be explained by a model sharing aspects of thermosets and thermoplastics, with reversible domains. Polyurethanes are high molecular weight polymers formed from reactions of polyisocyanates with polyols and chain extenders, producing urethane and urea bonds in the polymer structure. The interactions between hard and soft domains in the polyurethane structure influence the resulting mechanical properties.
Unsaturated polyester resin is prepared from diethylene glycol and maleic anhydride and can be cross-linked with styrene. It is used as a matrix material for fiber reinforced composites. The document discusses the preparation of unsaturated polyester resin and its composites with nano silica, glass fibers, and bagasse fibers. It is found that adding these fillers improves the mechanical, electrical and thermal properties of the composites compared to neat resin. Nano silica composites in particular exhibit better electrical properties than micro silica or neat resin composites.
- Polymers are giant molecules formed by linking together small repeating units called monomers via covalent bonds. There are three main types of polymerization: addition, condensation, and copolymerization.
- Properties of polymers depend on factors like the monomer type, the degree of polymerization, tacticity, and whether the polymer is crystalline or amorphous. Common polymers include polyethylene, polypropylene, nylon, polyethylene terephthalate.
- Natural rubbers are polymers of the monomer isoprene that provide flexibility and elasticity. However, natural rubber has limitations that are overcome through vulcanization, which introduces cross-links between polymer chains through the addition of sulfur.
Natural rubber is a type of polyisoprene made of cis-linked isoprene units that gives it an elastic structure. It is obtained commercially from the latex of the Para rubber tree. Rubber is vulcanized to improve its mechanical properties like tensile strength and hardness. Natural rubber composites are made by reinforcing rubber with fibers, fillers like carbon black, or nanoparticles to enhance properties. Natural rubber finds applications in tires, hoses, footwear and other products where elasticity and vibration absorption are important.
The basic knowledge of polymerization and further advancement in polymerization.
Different types of polymerization , uses and applications and reactions.
Rheology is the study of deformation and flow of matter. It involves measuring the viscosity and viscoelastic properties of materials under different conditions like temperature, pressure and shear rates. Various types of instruments called rheometers are used to measure rheological properties including rotational viscometers, capillary rheometers and other moving body viscometers. The document discusses different types of viscometers and rheometers used for measuring rheological properties of polymers and other materials.
This document discusses various types of additives used in plastics, including their purposes and applications. It describes additives like fillers, antioxidants, heat stabilizers, UV stabilizers, colorants, antistatics, flame retardants, cross-linking agents, blowing agents, lubricants and impact modifiers. Additives are used to improve processing, increase stability, obtain better properties like impact resistance and hardness, control factors like surface tension, reduce costs, and increase flame resistance of plastics. The document provides classifications and examples of different additive types.
Short Description related to the rubber filler properties and Rubber filler types ( Reinforcing fillers, Semi- reinforcing fillers and Non-reinforcing fillers). e.g.:- Carbon Black, Silica, Calcium Carbonate, Clay and Miscellaneous Fillers
Polyurethane is a polymer composed of organic units joined by carbamate links. It exists as both thermosetting and thermoplastic polymers. Polyurethane is used in applications such as flexible and rigid foams, fibers, elastomers, adhesives, coatings, and plastics. It is traditionally made by reacting a di- or polyisocyanate with a polyol. Polyurethane has properties including hardness, strength, resistance and is used in applications like furniture, appliances, composites, electronics, boats, and packaging due to its insulating and protective abilities. Some fungi are able to biodegrade polyurethane.
Polymer behaviour in solution & effect of molecular weight in polymerSyed Minhazur Rahman
Polymer chains of varying molecular weights exhibit different behaviors in solution. Higher molecular weight polymers swell more before dissolving and produce highly viscous solutions even at low concentrations. Their long, entangled chains confer properties like high strength, impact resistance, and chemical resistance. Lower molecular weight polymers dissolve immediately and yield low viscosity solutions. Their short chains act as plasticizers and impart softness, flexibility, and increased molecular mobility. A polymer's molecular weight determines the length of its chains and significantly impacts its solution behavior and material properties.
Determining molecular weights of polymers is important because it controls properties like solubility, elasticity, and mechanics. Polymers do not have uniform molecular weights but a distribution of different sizes. Molecular weight can be determined through various physical and chemical methods like end group analysis, light scattering, viscosity measurements, and gel permeation chromatography. These methods provide information about the number average molecular weight and distribution across molecules in a sample.
This document discusses fundamentals of polymer engineering, specifically polymer additives and blends. It defines additives as any substance added in small amounts to polymers to improve properties, facilitate processing, or reduce costs. Common additives include stabilizers, lubricants, fillers, plasticizers, and flame retardants. Fillers extend materials at low cost and can improve mechanical properties when well-dispersed. Polymer blends combine two or more polymers and offer benefits like extended temperature ranges, lighter weight, and improved toughness or barrier properties compared to the individual polymers. The classifications, functions, and examples of additives and blends in various polymer applications are covered in detail.
What is and what is the function of a rubber seal
The Increasing of the speed of mechanical systems, driven by the desire for greater productivity, leads to higher operating temperatures and reduced fluid viscosities. This, coupled with higher pressures, causes an increasing tendency for fluid to leak. This leak in fuel systems that handle highly flammable solvents cannot be overlooked as there is a high probability of a fire hazard.
For this reason it has become common practice to include a safe leak path in the system design, to an escape or collection point, in order to minimize risk.
Seals prevent fluid from escaping from a hollow cylinder when a shaft penetrates the cylinder wall. Most commonly, the axis will have a rotary or linear motion. If a seal is not made for functional requirements, or installed and maintained properly, it can fail, causing fluid loss. The two main functions of a seal are to keep the fluid in while keeping dirt and debris out.
It consists classification of polymerization techniques. What is bulk polymerization, how will the reaction proceed, and what are the advantages, disadvantages, and applications. Similarly, what is solution polymerization and how it will be carried out, what are the advantages, disadvantages, and applications behind it everything is explained in detail. Some of the related questions are also included for practice. All the contents taken from different websites and books are also mentioned.
Biopolymers are polymers that can be found in or manufactured by, living organisms. These also involve polymers that are obtained from renewable resources that can be used to manufacture Bioplastics by polymerization. Bioplastics are the plastics that are created by using biodegradable polymers
Polyurethane is a polymer made from organic compounds called isocyanates and polyols. It has many applications due to its versatile properties including flexibility, durability, impact resistance and insulation. Common uses include rigid and flexible foams for insulation and furniture, coatings, adhesives, elastomers and binders. Additives are used to modify properties and include flame retardants, colorants, and bacteriostats. Major applications sectors include construction, automotive, appliances, footwear and renewable energy like wind turbine blades.
To improve the properties of rubber, Charles Good in 1839 compounded the raw rubber with some chemicals and heated to 100 - 140°C. Finally the compounded and vulcanized rubber is draw in the form of sheet by calendaring process.
Practical Industrial Flow Measurement for Engineers and TechniciansLiving Online
This document provides an overview of basic fluid properties important for flow measurement. It discusses viscosity and different fluid types, including Newtonian and non-Newtonian fluids. It also describes ideal, laminar and turbulent flow profiles, and how the Reynolds number characterizes these behaviors. Key flow measurement parameters are introduced, such as volumetric and mass flow rates for single and multi-phase flows. The objectives are to describe fluid properties, flow profiles, and flow measurement concepts.
Rheology is the study of deformation and flow of matter. There are several types of rheological properties including stress, viscosity, viscoelastic modulus, creep, and relaxation times. Rheology is important in manufacturing pharmaceutical dosage forms and applications like ointments, syrups, suspensions, and emulsions where rheological properties influence acceptability, bioavailability, and handling. Materials can exhibit Newtonian, plastic, pseudo-plastic, or dilatant flow depending on the relationship between shear stress and shear rate. Viscometers are used to determine viscosity and classify fluids as Newtonian or non-Newtonian.
This document discusses the synthesis of poly(lactic acid) (PLA) biomaterials. There are two main synthetic methods - direct polycondensation and ring-opening polymerization of lactide monomers. Direct polycondensation includes solution and melt polycondensation, but yields PLA with low molecular weight. Ring-opening polymerization using metal catalysts is more common and can produce high molecular weight PLA, but the metal catalysts require removal. Recent research focuses on developing non-toxic catalysts and new polymerization conditions.
Elastomers are materials that can reversibly change length when a load is applied and return to their original dimensions once the load is removed. They include both natural and synthetic rubbers. Elastomers have properties that can be explained by a model sharing aspects of thermosets and thermoplastics, with reversible domains. Polyurethanes are high molecular weight polymers formed from reactions of polyisocyanates with polyols and chain extenders, producing urethane and urea bonds in the polymer structure. The interactions between hard and soft domains in the polyurethane structure influence the resulting mechanical properties.
Unsaturated polyester resin is prepared from diethylene glycol and maleic anhydride and can be cross-linked with styrene. It is used as a matrix material for fiber reinforced composites. The document discusses the preparation of unsaturated polyester resin and its composites with nano silica, glass fibers, and bagasse fibers. It is found that adding these fillers improves the mechanical, electrical and thermal properties of the composites compared to neat resin. Nano silica composites in particular exhibit better electrical properties than micro silica or neat resin composites.
- Polymers are giant molecules formed by linking together small repeating units called monomers via covalent bonds. There are three main types of polymerization: addition, condensation, and copolymerization.
- Properties of polymers depend on factors like the monomer type, the degree of polymerization, tacticity, and whether the polymer is crystalline or amorphous. Common polymers include polyethylene, polypropylene, nylon, polyethylene terephthalate.
- Natural rubbers are polymers of the monomer isoprene that provide flexibility and elasticity. However, natural rubber has limitations that are overcome through vulcanization, which introduces cross-links between polymer chains through the addition of sulfur.
Natural rubber is a type of polyisoprene made of cis-linked isoprene units that gives it an elastic structure. It is obtained commercially from the latex of the Para rubber tree. Rubber is vulcanized to improve its mechanical properties like tensile strength and hardness. Natural rubber composites are made by reinforcing rubber with fibers, fillers like carbon black, or nanoparticles to enhance properties. Natural rubber finds applications in tires, hoses, footwear and other products where elasticity and vibration absorption are important.
The basic knowledge of polymerization and further advancement in polymerization.
Different types of polymerization , uses and applications and reactions.
Rheology is the study of deformation and flow of matter. It involves measuring the viscosity and viscoelastic properties of materials under different conditions like temperature, pressure and shear rates. Various types of instruments called rheometers are used to measure rheological properties including rotational viscometers, capillary rheometers and other moving body viscometers. The document discusses different types of viscometers and rheometers used for measuring rheological properties of polymers and other materials.
This document discusses various types of additives used in plastics, including their purposes and applications. It describes additives like fillers, antioxidants, heat stabilizers, UV stabilizers, colorants, antistatics, flame retardants, cross-linking agents, blowing agents, lubricants and impact modifiers. Additives are used to improve processing, increase stability, obtain better properties like impact resistance and hardness, control factors like surface tension, reduce costs, and increase flame resistance of plastics. The document provides classifications and examples of different additive types.
Short Description related to the rubber filler properties and Rubber filler types ( Reinforcing fillers, Semi- reinforcing fillers and Non-reinforcing fillers). e.g.:- Carbon Black, Silica, Calcium Carbonate, Clay and Miscellaneous Fillers
Polyurethane is a polymer composed of organic units joined by carbamate links. It exists as both thermosetting and thermoplastic polymers. Polyurethane is used in applications such as flexible and rigid foams, fibers, elastomers, adhesives, coatings, and plastics. It is traditionally made by reacting a di- or polyisocyanate with a polyol. Polyurethane has properties including hardness, strength, resistance and is used in applications like furniture, appliances, composites, electronics, boats, and packaging due to its insulating and protective abilities. Some fungi are able to biodegrade polyurethane.
Polymer behaviour in solution & effect of molecular weight in polymerSyed Minhazur Rahman
Polymer chains of varying molecular weights exhibit different behaviors in solution. Higher molecular weight polymers swell more before dissolving and produce highly viscous solutions even at low concentrations. Their long, entangled chains confer properties like high strength, impact resistance, and chemical resistance. Lower molecular weight polymers dissolve immediately and yield low viscosity solutions. Their short chains act as plasticizers and impart softness, flexibility, and increased molecular mobility. A polymer's molecular weight determines the length of its chains and significantly impacts its solution behavior and material properties.
Determining molecular weights of polymers is important because it controls properties like solubility, elasticity, and mechanics. Polymers do not have uniform molecular weights but a distribution of different sizes. Molecular weight can be determined through various physical and chemical methods like end group analysis, light scattering, viscosity measurements, and gel permeation chromatography. These methods provide information about the number average molecular weight and distribution across molecules in a sample.
This document discusses fundamentals of polymer engineering, specifically polymer additives and blends. It defines additives as any substance added in small amounts to polymers to improve properties, facilitate processing, or reduce costs. Common additives include stabilizers, lubricants, fillers, plasticizers, and flame retardants. Fillers extend materials at low cost and can improve mechanical properties when well-dispersed. Polymer blends combine two or more polymers and offer benefits like extended temperature ranges, lighter weight, and improved toughness or barrier properties compared to the individual polymers. The classifications, functions, and examples of additives and blends in various polymer applications are covered in detail.
What is and what is the function of a rubber seal
The Increasing of the speed of mechanical systems, driven by the desire for greater productivity, leads to higher operating temperatures and reduced fluid viscosities. This, coupled with higher pressures, causes an increasing tendency for fluid to leak. This leak in fuel systems that handle highly flammable solvents cannot be overlooked as there is a high probability of a fire hazard.
For this reason it has become common practice to include a safe leak path in the system design, to an escape or collection point, in order to minimize risk.
Seals prevent fluid from escaping from a hollow cylinder when a shaft penetrates the cylinder wall. Most commonly, the axis will have a rotary or linear motion. If a seal is not made for functional requirements, or installed and maintained properly, it can fail, causing fluid loss. The two main functions of a seal are to keep the fluid in while keeping dirt and debris out.
It consists classification of polymerization techniques. What is bulk polymerization, how will the reaction proceed, and what are the advantages, disadvantages, and applications. Similarly, what is solution polymerization and how it will be carried out, what are the advantages, disadvantages, and applications behind it everything is explained in detail. Some of the related questions are also included for practice. All the contents taken from different websites and books are also mentioned.
Biopolymers are polymers that can be found in or manufactured by, living organisms. These also involve polymers that are obtained from renewable resources that can be used to manufacture Bioplastics by polymerization. Bioplastics are the plastics that are created by using biodegradable polymers
Polyurethane is a polymer made from organic compounds called isocyanates and polyols. It has many applications due to its versatile properties including flexibility, durability, impact resistance and insulation. Common uses include rigid and flexible foams for insulation and furniture, coatings, adhesives, elastomers and binders. Additives are used to modify properties and include flame retardants, colorants, and bacteriostats. Major applications sectors include construction, automotive, appliances, footwear and renewable energy like wind turbine blades.
To improve the properties of rubber, Charles Good in 1839 compounded the raw rubber with some chemicals and heated to 100 - 140°C. Finally the compounded and vulcanized rubber is draw in the form of sheet by calendaring process.
Practical Industrial Flow Measurement for Engineers and TechniciansLiving Online
This document provides an overview of basic fluid properties important for flow measurement. It discusses viscosity and different fluid types, including Newtonian and non-Newtonian fluids. It also describes ideal, laminar and turbulent flow profiles, and how the Reynolds number characterizes these behaviors. Key flow measurement parameters are introduced, such as volumetric and mass flow rates for single and multi-phase flows. The objectives are to describe fluid properties, flow profiles, and flow measurement concepts.
Rheology is the study of deformation and flow of matter. There are several types of rheological properties including stress, viscosity, viscoelastic modulus, creep, and relaxation times. Rheology is important in manufacturing pharmaceutical dosage forms and applications like ointments, syrups, suspensions, and emulsions where rheological properties influence acceptability, bioavailability, and handling. Materials can exhibit Newtonian, plastic, pseudo-plastic, or dilatant flow depending on the relationship between shear stress and shear rate. Viscometers are used to determine viscosity and classify fluids as Newtonian or non-Newtonian.
This document discusses rheology, which is the science describing the flow and deformation of matter under stress. It defines key terms like viscosity, shear stress, shear rate, and classifies fluids as Newtonian or non-Newtonian based on their relationship between shear stress and shear rate. Newtonian fluids have a constant viscosity regardless of shear rate, while non-Newtonian fluids have variable viscosity. Plastic, pseudoplastic, and dilatant behaviors are described for non-Newtonian fluids. Thixotropy, which is a time-dependent decrease and recovery of viscosity under shear, is also discussed. The document concludes by explaining the operation and calibration of common viscometers.
Viscoelastic response of polymeric solids to sliding contactsPadmanabhan Krishnan
A polymeric solid is seen to produce its own signatures in sliding contacts. This has immense applications. The viscoelastic phenomena and signatures are discussed with the relevant models.
1. The chapter discusses key fluid properties including density, specific gravity, surface tension, vapor pressure, elasticity, and viscosity.
2. Density is defined as mass per unit volume and specific gravity is the ratio of the density of a liquid to the density of water.
3. Surface tension is caused by unbalanced cohesive forces at fluid surfaces which produce a downward force, while vapor pressure is the pressure produced by a fluid's vapor in an equilibrium state.
This document provides an overview of adhesive properties and testing methods. It discusses various rheological parameters including viscosity, flow behavior, and thixotropy. Viscosity describes a fluid's internal friction and resistance to flow. Materials can exhibit Newtonian or non-Newtonian flow, with pseudoplastic, dilatant, and plastic behaviors defined. Thixotropy refers to how viscosity changes over time with applied shear. Understanding these properties is important for choosing the appropriate adhesive for an application. The document aims to serve as an educational guide on testing and material characterization.
Fluid mechanics is the study of fluids either at rest or in motion. There are two main types of fluids: liquids and gases. Liquids have strong cohesive forces that allow them to retain their shape, while gases have negligible cohesive forces and are free to expand. Fluid properties include density, viscosity, and other thermodynamic properties. Viscosity describes a fluid's resistance to flow and is dependent on factors like temperature. Reynolds number is used to characterize different flow regimes from laminar to turbulent. Fluid mechanics has many applications in fields like engineering, biology, and meteorology.
Rheology is the investigation of the progression of issue, fundamentally in a fluid state, yet in addition as "delicate solids" or solids under conditions in which they react with plastic stream as opposed to distorting flexibly because of an applied power. Rheology is the study of misshapening and stream inside a material.
CHAPTER 6 Strength, creep and fracture of polymers.pptWeldebrhan Tesfaye
This document discusses creep, recovery, and stress relaxation in polymers. It begins with an introduction to deformation mechanisms in polymers and factors that influence polymer strength. It then covers stress-strain curves in polymers and different deformation modes. Specific topics discussed in more detail include glassy polymers, semicrystalline polymers, viscous flow, creep in polymers, stress relaxation, creep failure of polymers, creep modulus, and factors that influence creep resistance in plastics. Examples are provided to illustrate concepts.
This document discusses properties of fluids. It defines key fluid properties like density, specific gravity, vapor pressure, and viscosity. Density is defined as mass per unit volume, while specific gravity compares the density of a substance to that of water. An ideal gas is one that follows the ideal gas law relating pressure, temperature, and volume. Viscosity describes a fluid's resistance to flow and plays a dominant role in fluid mechanics. The document provides examples and discussions of these important fluid properties.
Creep is the slow, progressive deformation of a material under constant stress over time. It is dependent on both time and temperature. During a creep test, a constant stress is applied to a specimen and its deformation is measured over time. Typically, a creep curve will show an initial instantaneous elastic deformation followed by primary, secondary, and tertiary creep stages. The addition of nanoparticles like carbon nanotubes or clay to polymers can improve their creep resistance by acting as barriers to hinder molecular chain movement and reorientation under stress. Creep tests are used to characterize a material's creep performance by measuring its creep compliance over time under an applied load.
Polymer Rheology(Properties study of polymer)Haseeb Ahmad
This document discusses fundamentals of polymer rheology. It defines rheology as the study of flow of matter, primarily liquids but also soft solids. Rheology is important for characterizing polymers and understanding how polymer structure affects processing behavior. The document describes different types of fluids and their viscosity properties. It also discusses various rheological measurement techniques like rotational rheometers, capillary rheometers and melt flow indexers.
1. The document describes an experiment conducted to determine the rheological properties of viscosity and yield point of a drilling fluid sample using a Fann viscometer.
2. Key aspects of the experiment included preparing the mud sample, measuring its viscosity at 300 and 600 RPM, and determining its plastic viscosity and apparent viscosity. Calibration of the Marsh funnel and factors affecting rheological properties are also discussed.
3. Sources of potential error in measuring viscosity are described, such as improper mud weight, excess or insufficient fluid, and improper reading of the measuring scale.
This document describes an experiment conducted to determine the friction factor of water flowing through a pipe. The experiment measured the volumetric flow rate, velocity, temperature, and pressure drop of water flowing through a pipe. These measurements were used to calculate the Reynolds number, theoretical friction factor based on equations, and experimental friction factor. The results showed that at higher Reynolds numbers, the friction factor was lower, following trends in friction factor charts. Sources of error included inaccurate measurements of pressure drop and flow time. The experiment demonstrated how friction factor depends inversely on Reynolds number for turbulent flow in a pipe.
Adhesion is linked with surface forces like capillary pressure and is thus detrimental at the
nanoscale where body forces are negligible. It can lead to instant failure during fabrication and
operation but it can also lead to overtime failure because of induced friction and wear. However,
when it is possible, coating a device with hydrophobic materials reduces drastically that mechanism.
Understanding how adhesion works is crucial to design new systems and to enable new
technologies. Two models (JKR and DMT) are studied in this paper and model adhesion in different
cases. Photolithography and particularly the release step must be carefully designed to prevent
contamination and stiction. Materials must be chosen and designed wisely to prevent adhesion
failure during operation but lubricants can be used to reduce its impact as well as the impact of
friction and wear.
Stability Analysis of Journal Bearing Using Electro Rheological Fluid by Fini...ijsrd.com
in rotating machinery, the damping of structure which supports the rotating shaft has significant effect in machine vibration. Therefore by controlling the lubricant properties, the dynamic behavior of the system can be controlled. The objective of this paper is to study the dynamic behavior of a rotor supported by a journal bearing and fed with Electro-rheological (ER) fluid. ER fluids can be used to create ‘smart’ journal bearings & vibration controllers can be constructed to control the Stability of the ER fluid lubricated bearings. The ER fluid behaves like a Bingham fluid with a higher viscosity when electric field is applied, and restores its property when the field is removed. A reversible change in viscosity occurs in milliseconds with the electric field applied.
Fluid mechanics is the study of fluids and forces on them. It can be divided into fluid statics, kinematics, and dynamics. Fluid mechanics involves the properties of fluids, including that fluids continually deform under stress, take the shape of their container, and have indefinite shape and volume. Key terms include density, specific weight, specific volume, viscosity, compressibility, and dynamic viscosity. Viscosity measures a fluid's resistance to flow and internal friction. Dynamic viscosity describes the direct proportionality between shear stress and velocity gradient in a moving fluid.
This document discusses energy losses that occur in hydraulic systems. It begins by defining laminar and turbulent flows, and introduces the Reynolds number which determines the type of flow. It then explains that greater energy losses occur in turbulent flow compared to laminar flow. The document goes on to describe the Darcy-Weisbach equation for calculating head losses due to friction in pipes. Specific equations are provided to calculate losses for laminar and turbulent flow, taking into account factors like pipe roughness and Reynolds number. The purpose is to analyze energy losses that occur in components like valves and fittings so they can be properly accounted for in system design.
Lecture notes on Structure and Properties of Engineering Polymers
Course Objectives:
The main objective is to introduce polymers as an engineering material and emphasize the basic concepts of their nature, production and properties. Polymers are introduced at three levels; namely, the molecular level, the micro level, and macro-level. Through knowledge of all three levels, student can understand and predict the properties of various polymers and their performance in different products. The course also aims at introducing the students to the principles of polymer processing techniques and considerations of design using engineering polymers.
Finite element modeling and simulation with ANSYS Workbench ( PDFDrive ).pdfarpado
The document describes copyright and trademark information related to ANSYS, Inc. and its software products. It provides legal notices for ANSYS brands, logos, and software names that are trademarks or registered trademarks. It also contains standard copyright information for the book and limitations on copying or redistributing its content.
ANSYS Polyflow Tutorial Guide.pdf - Portal de Documentacion de ( PDFDrive ).pdfarpado
This document provides a tutorial guide for using ANSYS Polyflow to simulate fluid flow and heat transfer problems. It introduces the workflow in ANSYS Workbench for setting up a fluid flow analysis system, preparing the geometry, meshing, defining the simulation in Polydata, running the simulation in Polyflow, and post-processing the results in CFD-Post. Example applications covered include a 3D extrusion problem, a 2D axisymmetric extrusion problem, and a non-isothermal flow through a cooled die with conjugate heat transfer between the fluid and solid domains.
This document provides technical information about Viton fluoroelastomer, including its properties, types, curing systems, and performance comparisons. Viton was introduced in 1957 for aerospace sealing and has been used widely in automotive, chemical, and other industries due to its resistance to heat, chemicals, and degradation. It exists in various types that differ in fluorine content and fluid resistance properties. Curing systems like diamine, bisphenol, and peroxide provide different processing characteristics and vulcanizate properties for the various Viton types.
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1. 1
The Role of Rheology in Polymer Extrusion
John Vlachopoulos
Department of Chemical Engineering
McMaster University
Hamilton, Ontario, Canada
E-mail: vlachopj@mcmaster.ca
David Strutt
Polydynamics, Inc.
Hamilton, Ontario, Canada
1.0 Rheology
Rheology is the science of deformation and flow of materials [1]. The Society of
Rheology's Greek motto "Panta Rei" translates as "All things flow." Actually, all materials do
flow, given sufficient time. What makes polymeric materials interesting in this context is the
fact that their time constants for flow are of the same order of magnitude as their processing
times for extrusion, injection molding and blow molding. In very short processing times, the
polymer may behave as a solid, while in long processing times the material may behave as a
fluid. This dual nature (fluid-solid) is referred to as viscoelastic behavior.
1.1 Viscosity and Melt Flow Index
Viscosity is the most important flow property. It represents the resistance to flow.
Strictly speaking, it is the resistance to shearing, i.e., flow of imaginary slices of a fluid like the
motion of a deck of cards. Referring to Figure 1.1, we can define viscosity as the ratio of the
imposed shear stress (force F, applied tangentially, divided by the area A), and the shear rate
(velocity V, divided by the gap h)
=
h/V
A/F
=
RATESHEAR
STRESSSHEAR
= (1.1)
The Greek letters (tau) and (gamma dot) are conventionally used to designate the shear
stress and shear rate, respectively.
For flow through a round tube or between two flat plates, the shear stress varies linearly
from zero along the central axis to a maximum value along the wall. The shear rate varies
nonlinearly from zero along the central axis to a maximum along the wall. The velocity profile
is quasi-parabolic with a maximum at the plane of symmetry and zero at the wall as shown in
Figure 1.2, for flow between two flat plates.
2. 2
Figure 1.1. Simple shear flow.
Figure 1.2. Velocity, shear rate and shear stress profiles for flow between two flat plates.
3. 3
The viscosity in SI is reported in units of Pas (Pascalsecond). Before the introduction of
SI, poise was the most frequently used unit (1 Pas = 10 poise). Here are some other useful
conversion factors.
1 Pas = 1.45 10-4
lbf s/in2
= 0.67197 lbm/s ft = 2.0886 10-2
lbf s/ft2
The viscosity of water is 10–3
Pas while the viscosity of most polymer melts under
extrusion conditions may vary from 102
Pas to 105
Pas. The shear stress is measured in units of
Pa = (N/m2
) or psi (pounds (lbf) per square inch) and the shear rate in reciprocal seconds (s–1
).
One remarkable property of polymeric liquids is their shear-thinning behavior (also
known as pseudo-plastic behavior). If we increase the rate of shearing (i.e., extrude faster
through a die), the viscosity becomes smaller, as shown in Figure 1.3. This reduction of
viscosity is due to molecular alignments and disentanglements of the long polymer chains. As
one author said in a recent article: "polymers love shear." The higher the shear rate, the easier it
is to force polymers to flow through dies and process equipment. During single-screw extrusion,
shear rates may reach 200 s–1
in the screw channel near the barrel wall, and much higher between
the flight tips and the barrel. At the lip of the die the shear rate can be as high as 1000 s–1
. Low
shear rate on a die wall implies slow movement of the polymer melt over the metal surface.
Some die designers try to design dies for cast film or blown film operations not having wall
shear rates less than, say 10 s–1
, to prevent potential hang-ups of the molten material. When the
wall shear stress exceeds 0.14 MPa, sharkskin (i.e. surface mattness) occurs in capillary
viscometer measurements using various HDPE grades. At very high shear rates, a flow
instability known as melt fracture occurs [2, 3].
Melt Index (MI), Melt Flow Index (MFI), or Melt Flow Rate (MFR) (for polypropylene)
refers to the grams per 10 minutes pushed out of a die of prescribed dimensions according to an
ASTM Standard [4] under the action of a specified load as shown in Figure 1.4. For PE (ASTM
D-1238) the load is 2.16 kg and the die dimensions are D = 2.095 mm and L = 8 mm. The
experiment is carried out at 190°C. For the PP, the same load and die dimensions are used, but
the experiment is carried out at 230°C.
Under the conditions of melt index measurement with a 2.16 kg load, the wall shear
stress can be calculated to be w = 1.94 104
Pa (= 2.814 psi) and the wall shear rate
approximately = (1838/) MI where is the melt density in kg/m3
. Assuming = 766
kg/m3
for a typical PE melt, we get = 2.4 MI. Low melt index means a high-molecular-
weight, highly viscous polymer. A high melt index means low-molecular-weight, low viscosity
polymer. When the melt index is less than 1, the material is said to have a fractional melt index.
Such materials are used for film extrusion. Most extrusion PE grades seldom exceed MI = 12;
however, for injection molding, MI is usually in the range of 5–100.
Viscosity can be measured by either capillary or rotational viscometers. In capillary
viscometers, the shear stress is determined from the pressure applied by a piston. The shear rate
is determined from the flow rate.
4. 4
Figure 1.3. Newtonian and shear-thinning viscosity behavior.
Figure 1.4. Schematic of a melt indexer.
5. 5
stressshear
L/R
P
=
cap
w
(1.2)
rateshearapparent
R
Q4
= 3a
(1.3)
where Pcap is pressure drop, L is capillary length, R is radius, and Q the volume flow rate.
The apparent shear rate corresponds to Newtonian behavior (constant viscosity fluids). A
correction is necessary (Rabinowitsch correction) for shear thinning fluids. For the power-law
model, the true (Rabinowitsch corrected) shear rate becomes
R
Q4
n4
1+n3
= 3
(1.4)
This means that for a material with power-law index n = 0.4 (very common), the relation
between apparent and true shear rate is
apparenttrue 1.375= (1.5)
When capillaries are relatively short (L/R < 50), the Bagley correction is necessary to
account for the excess pressure drop Pe at the capillary entry. The Bagley correction is usually
expressed as
w
e
B
2
P
n
(1.6)
where nB may vary from 0 to perhaps 20 when polymeric materials are extruded near the critical
stress for sharkskin. For a Newtonian fluid the value for nB is 0.587.
The Bagley corrected shear stress becomes
n+
R
L
2
P+P
=
B
ecap
w (1.7)
To apply the Bagley correction, measurements with at least two capillaries are needed.
The shear thinning behavior is frequently expressed by the power-law model
1-n
m= (1.8)
6. 6
where m is the consistency and n the power-law exponent. For n = 1, the Newtonian model
(constant viscosity) is obtained. The smaller the value of n, the more shear-thinning the polymer.
The usual range of power-law exponent values is between 0.8 (for PC) and 0.2 (for rubber
compounds). For various grades of PE, the range is 0.3 < n < 0.6. The consistency has values in
the usual range of 1000 Pasn
(some PET resins) to 100,000 Pasn
for highly viscous rigid PVC.
This power-law model gives a good fit of viscosity data at high shear rates but not at low shear
rates (because as goes to zero, the viscosity goes to infinity).
An approximate calculation of both m and n can be carried out by using two values of the
melt index (MI and HLMI). MI refers to standard weight of 2.16 kg and HLMI to “High Load”
melt index (frequently 10 kg or 21.6 kg). By manipulating the appropriate equations for pressure
drop, shear stress and flow rate, we have [1]:
n
MI
1838
)LL(8982
)MIlog()HLMIlog(
)LLlog()HLlog(
myConsistenc
nexponentlawPower
(1.9)
where LL is the standard load (usually 2.16 kg) and HL the high load (usually 10 kg or 21.6 kg).
Two other models are frequently used for better fitting of data over the entire shear rate
range:
Carreau-Yasuda
)(+1=
1n-
o
a a (1.10)
where o is the viscosity at zero shear and , a, and n are fitted parameters.
Cross model
)(+1
= n-1
o
(1.11)
where o is the zero shear viscosity and and n are fitted parameters.
With rotational viscometers (cone-and-plate or parallel plate), the shear stress is
determined from the applied torque and the shear rate from the rotational speed and the gap
where the fluid is sheared.
Capillary viscometers are usually used for the shear rate range from about 2 s–1
to
perhaps 3000 s–1
. Rotational viscometers are usually used for the range 10–2
to about 5 s–1
. At
higher rotational speeds, secondary flows and instabilities may occur which invalidate the simple
7. 7
shear assumption. For more information about viscosity measurements, the reader is referred to
Macosko [2].
The viscosity of polymer melts varies with temperature in an exponential manner
T)b(-exp= ref (1.12)
The value of the temperature sensitivity coefficient b ranges from about 0.01 to 0.1 °C-1
. For
common grades of polyolefins, we may assume that b = 0.015. This means that for a
temperature increase T = 10°C (18°F), the viscosity decreases by 14%.
The effects of various factors on viscosity are summarized in Figure 1.5 following
Cogswell [3]. Linear narrow molecular weight distribution polymers (e.g. metallocenes) are
more viscous than their broad distribution counterparts. Fillers may increase viscosity (greatly).
Pressure results in an increase in viscosity (negligible under usual extrusion conditions). Various
additives are available and are designed to decrease viscosity. The zero shear viscosity increases
dramatically with the weight average molecular weight:
Mconst= 3.4
wo (1.13)
For some metallocene PE with long chain branching, the exponent might be much higher
(perhaps 6.0).
In the above discussion of viscosity measurements, the assumption is made that the no-
slip condition on the die wall is valid. This is, however, not always the case. In fact, at shear
stress levels of about 0.1 MPa for PE, slip occurs. Wall slip is related to the sharkskin
phenomenon [5]. Wall slip is measured by the Mooney method [6] in which the apparent shear
rate (4Q / R3
) is plotted against 1/R for several capillaries having different radii. In the absence
of slip, the plot is horizontal. The slope of the line is equal to 4 (slip velocity).
1.2 Extensional Viscosity and Melt Strength
Extensional (or elongational) viscosity is the resistance of a fluid to extension. While it is
difficult to imagine stretching of a low viscosity fluid like water, polymer melts exhibit
measurable amounts of resistance. In fact, about 100 years ago, Trouton measured the resistance
to stretching and shearing of stiff liquids, including pitch, and found that the ratio of extensional
to shear viscosity is equal to 3.
3=e
(1.14)
This relation, known as the Trouton ratio, is valid for all Newtonian fluids and has a
rigorous theoretical basis besides Trouton's experiments.
8. 8
Figure 1.5. The influence of various parameters on polymer viscosity.
Figure 1.6. Extensional and shear viscosity as a function of stretch and shear rate, respectively.
9. 9
Measurement of elongational viscosity is considerably more difficult than measurement
of shear viscosity. One of the devices used involves extrusion from a capillary and subsequent
stretching with the help of a pair of rollers. The maximum force required to break the extruded
strand is referred to as melt strength. In practice, the terms extensional viscosity and melt
strength are sometimes confused. Extensional viscosity is a function of the stretch rate ( ), as
shown in Figure 1.6, and compared to the shear viscosity. Melt strength is more of an
engineering measure of resistance to extension. Several extrusion processes involve extension,
such as film blowing, melt spinning and sheet or film drawing. The stretch rates in film blowing
can exceed 10 s–1
, while in entry flows from a large reservoir into a smaller diameter capillary,
the maximum stretch rate is likely to be one order of magnitude lower than the maximum wall
shear rate (e.g. in capillary viscometry, approximately max 100 s-1
for max 1000 s-1
).
Frequently the extensional viscosity is plotted as a function of stretching time (increasing)
without reaching a steady value (strain hardening).
The excess pressure encountered in flow from a large reservoir to a smaller diameter
capillary is due to elongational viscosity. In fact, Cogswell [3] has developed a method for
measurement of elongational viscosity e from excess pressure drop Pe (i.e., the Bagley
correction):
e
2
2
2
e
2
e
P)1n(3
4
at
32
P)1n(9
(1.15)
Shear and extensional viscosity measurements reveal that LLDPE (which is linear) is
"stiffer" than LDPE (branched) in shear, but "softer" in extension. In extension, the linear
LLDPE chains slide by without getting entangled. However, the long branches of the LDPE
chains result in significantly larger resistance in extension. In the film blowing process, LDPE
bubbles exhibit more stability because of their high extensional viscosity. Typical LDPE and
LLDPE behavior in shear and extension is shown in Figure 1.7. LDPE is often blended with
LLDPE to improve the melt strength and consequently bubble stability in film blowing. Most PP
grades are known to exhibit very low melt strength. However, recent advances in polymer
chemistry have led to the production of some high-melt-strength PP grades.
1.3 Normal Stresses and Extrudate Swell
Stress is defined as force divided by the area on which it acts. It has units of lbf/in2
(psi)
in the British system or N/m2
(Pascal, Pa) in SI. When a force is acting tangentially on a surface,
the corresponding stress is referred to as shear stress. When a force is perpendicular (normal) to
a surface, it is termed normal stress. Pressure is a normal stress. When a fluid is forced to flow
through a conduit, it is acted upon by the normal (pressure) forces and it exerts both normal and
shear (stress) forces on the conduit walls. For flow through a planar die as shown in Figure 1.2,
the shear stress is zero at the midplane and maximum at the wall, while the corresponding
velocity profile is quasi-parabolic. Weissenberg discovered in the 1940s that polymer solutions
10. 10
Figure 1.7. Schematic representation of LDPE and LLDPE behavior in shear and extension.
Figure 1.8. (a) Rod climbing (Weissenberg) effect in polymeric fluids, (b) extrudate swell.
11. 11
and melts, when subjected to shearing, tend to develop normal stresses that are unequal in the x
(direction of flow), y and z (normal directions). But, why are these elusive forces generated?
Because the flow process results in anisotropies in the microstructure of the long molecular
chains of polymers. Any further explanation of the physical origin of normal stresses is likely to
be controversial. Here is perhaps an oversimplification: shearing means motion of a fluid in a
slice-by-slice manner. If the imaginary slices were made of an extensible elastic material (like
slices of rubber), shearing would also result in extension in the flow direction and uneven
compression in the other two directions. So, when an (elastic) polymer solution or melt is forced
to flow, it is less compressed in the direction of flow than in the other two normal directions.
The so-called First Normal Stress Difference N1 is defined as the normal stress in the
direction of the flow ( xx ) minus the perpendicular ( yy )
yyxx1 -=N (1.16)
The Second Normal Stress Difference is
zzyy2 -=N (1.17)
Experiments show that N1 is positive for usual polymers (i.e. extensive, while the compressive
pressure forces are negative). N2 is negative and of the order of 20% of N1 for most common
polymers.
The normal stress differences can be very large in high-shear-rate extrusion. Some
authors suggest a variation for the normal stress difference at the wall in the form
b
ww1 A=N (1.18)
The stress ratio
w
w1
R
2
N
=S (1.19)
can reach a value of 10 or more at the onset of melt fracture.
The rod-climbing (Weissenberg) effect observed (Figure 1.8 (a)) when a cylinder rotates
in a polymeric liquid is due to some sort of "strangulation" force exerted by the extended
polymer chains, which results in an upward movement normal to the direction of rotation
(normal stress difference). The extrudate swell phenomenon [7] (see Figure 1.8 (b)) is due
mainly to the contraction of exiting polymer that is under extension in the die. The uneven
compression in the various directions results in a number of unusual flow patterns and
instabilities. The secondary flow patterns observed by Dooley and co-workers [8] are due to the
second normal stress difference. Bird et al. [9] in their book state: "A fluid that's
macromolecular is really quite weird, in particular the big normal stresses the fluid possesses
give rise to effects quite spectacular."
12. 12
The phenomenon of extrudate swell (also known as die swell) has been studied by several
researchers. While the primary mechanism is release of normal stresses at the exit, other effects
are also important. The amount of swell is largest for zero length dies (i.e. orifices). It decreases
for the same throughput with the length of the die due to fading memory as the residence time in
the die increases. Even Newtonian fluids exhibit some swell upon exiting dies (13% for round
extrudates, 19% for planar extrudates). This is due to streamline rearrangement at the exit. The
amount of swell can be influenced by thermal effects due to viscosity differences between the
walls and center of a die. Maximum thermal swell can be obtained when a hot polymer flows
through a die having colder walls. Swell ratio of about 5% on top of other mechanisms can be
obtained from temperature differences.
Several attempts have been made to predict extrudate swell numerically through
equations relating the swell ratio d/D (extrudate diameter / die diameter) to the first normal stress
difference at the wall N1w. Based on the theory of rubber elasticity, the following is obtained [7]
2
1
24
ww1 3
D
d
2
D
d
32N
(1.20)
Based on stress release for a Maxwell fluid exiting from a die, Tanner’s equation applies [7]
2
1
6
ww1 113.0
D
d
22N
(1.21)
Although Equation 1.21 has a more rigorous derivation and theoretical basis, the rubber
elasticity theory (Equation 1.20) gives better predictions.
1.4 Stress Relaxation and Dynamic Measurements
After cessation of flow, the stresses become immediately zero for small molecule
(Newtonian) fluids like water or glycerin. For polymer melts, the stresses decay exponentially
after cessation of flow. Stress relaxation can be measured in a parallel plate or a cone-and-plate
rheometer by applying a given shear rate level (rotation speed/gap) and measuring the stress
decay after the rotation is brought to an abrupt stop. Such tests, however, are not performed
routinely because of experimental limitations.
Dynamic measurements involve the response of a material to an imposed sinusoidal
stress or strain on a parallel plate or cone-and-plate instrument. A perfectly elastic material that
behaves like a steel spring, by imposition of extension (strain), would develop stresses that
would be in-phase with the strain, because
γstrain(G)modulus=τstress (1.22)
13. 13
However, for a Newtonian fluid subjected to a sinusoidal strain, the stress and strain will not be
in-phase because of the time derivative (strain rate) involved
= (1.23)
)90+t(sin=
tcos=t)sin(
d
d
=
td
d
=
o
oo
(1.24)
where is frequency of oscillation. That is, a Newtonian fluid would exhibit 90 phase
difference between stress and strain. Polymeric liquids that are partly viscous and partly elastic
(viscoelastic) will be 0 90 out of phase.
We can define
part)(elastic
modulus
storage
strainmaximum
stressphase-in
=ωG' (1.25)
part)(viscous
modulus
loss
strainmaximum
stressphase-of-out
=ωG" (1.26)
where ranges usually from 0.01 to 103
rad/s. Larger G implies more elasticity. Further, we
can define
viscositydynamicthe
G"
='
(1.27)
G
="
(1.28)
and the magnitude of the complex viscosity
)"+(=|
2/122*
| (1.29)
An empirical relationship called the "Cox-Merz rule" states that the shear rate
dependence of the steady state viscosity is equal to the frequency dependence of the complex
viscosity *
, that is
|)(|=)(
*
(1.30)
14. 14
The usefulness of this rule, which holds for most polymers, is that while steady measurements of
shear viscosity are virtually impossible above 5 s–1
with rotational instruments, the dynamic
measurements can easily be carried out up to 500 rad/s (corresponds to = 500 s–1
) or even
higher. Thus, the full range of viscosity needed in extrusion can be covered.
Some typical results involving narrow and broad molecular-weight-distribution samples
are shown in Figure 1.9. The relative behavior of G versus can be used to identify whether a
sample is of narrow or broad molecular weight distribution [6]. In fact, from the "crossover
point" where G = G, it is possible to get a surprisingly good estimate of the polydispersity
Mw/Mn for PP [10].
1.5 Constitutive Equations
These are relations between stresses and strains (deformations). In its simplest form, the
Newtonian equation is
fluid= (1.31)
where is viscosity and = du/dy, the shear rate.
For a shear thinning material of the power-law type, we have
nn-1
m=m== (1.32)
where m is consistency and n the power-law exponent.
However, the above expressions, when inserted into the equation of conservation of
momentum, cannot predict viscoelastic effects such as normal stresses, stress relaxation or
extrudate swell. The simplest way to develop viscoelastic constitutive equations is to combine a
model for an elastic solid
solidG= (1.33)
with that for a Newtonian fluid
dflui= (1.34)
By differentiating Equation 1.33 and adding the two strain rates, we get
=+
G
(1.35)
or
15. 15
Figure 1.9. Storage modulus G and dynamic viscosity * behavior of broad and narrow
molecular weight distribution polymers.
Figure 1.10. Reptation model of polymer chain motion.
16. 16
=
G
+ (1.36)
=
G
has dimensions of time (relaxation constant).
=+ (1.37)
This is known as the Maxwell model. Viscoelastic models must be expressed in three
dimensions and in a proper mathematical frame of reference that moves and deforms with the
fluid. The result is a very complicated expression involving dozens of derivatives [11,12].
The most powerful constitutive equation is the so-called K-BKZ integral model that
involves more than two dozen experimentally fitted parameters (see, for example: Mitsoulis
[13]). Current trends involve the development of models based on macromolecular motions. De
Gennes proposed the snake-like motion of polymer chains called reptation, illustrated in Figure
1.10. Based on the reptation concept, Doi and Edwards [2] developed a constitutive equation
which leaves much to be desired before it can be used for prediction of viscoelastic flow
phenomena. Several attempts have been made to fix the Doi-Edwards theory. The most
prominent researcher in the area is G. Marrucci (see, for example: Marrucci and Ianniruberto
[14]).
The most talked about viscoelastic model recently is the Pom-Pom polymer model,
developed by T.C.B. McLeish and R.G. Larson [15]. The motivation for its development was
that the K-BKZ equation fails to predict the observed degree of strain hardening in planar
extension when the kernel functions are adjusted to fit the observed degree of strain softening in
shear. The failure to describe the rheology of long-chain branched polymers suggests that some
new molecular insight is needed into the nonlinear relaxation processes that occur in such melts
under flow. The Pom-Pom model uses an H-polymer structure, in which molecules contain just
two branch points of chosen functionality – a “backbone” which links two pom-poms of q arms
each, as shown in Figure 1.11.
The Pom-Pom model exhibits rheological behavior remarkably similar to that of
branched commercial melts like LDPE. It shows strain hardening in extension and strain
softening in shear. It can describe both planar and uniaxial extension. The constitutive equation
is integro-differential. For successful application at least 32 parameters must be obtained by
fitting experimental rheological data. Of course, best fitting 32 or more parameters of a
complicated constitutive equation is a mathematical challenge of its own.
Modeling of the viscoelastic behavior of polymers has always been a very controversial
subject. The viscoelastic constitutive equations have contributed towards the understanding of
the various mechanisms of deformation and flow, but unfortunately have not provided us with
quantitative predictive power. Very often the predictions depend on the model used for the
computations and are not corroborated with experimental observations. Some viscoelastic flow
problems can be solved with the appropriate constitutive equations, but this is still an area of
academic research with limited practical applications at the moment.
17. 17
Figure 1.11. Pom-Pom polymer model idealized molecules.
Figure 1.12. LLDPE extrudates obtained from a capillary at apparent shear rates of 37, 112, 750
and 2250 s-1
.
18. 18
1.6 Sharkskin, Melt Fracture and Die Lip Build-Up
The term sharkskin refers to the phenomenon of loss of surface gloss of an extrudate, also
sometimes termed surface mattness. The surface usually exhibits a repetitious wavy or ridged
pattern perpendicular to the flow direction. It occurs at a critical stress level of at least 0.14 MPa
(21 psi) for most common polymers extruded through capillary dies. With some additives,
lubricants, processing aids or die coatings, the onset of sharkskin can be shifted to a higher shear
stress level, with values up to 0.5 MPa being reported.
The prevailing point of view is that sharkskin originates near the die exit and is due to
stick-slip phenomena. A critical shear stress near the exit in conjunction with a critical
acceleration results in skin rupture of the extrudate [16,17]. There was some disagreement over
whether slip between the polymer and the die wall causes or helps avoid sharkskin [18].
However, it is now believed that it is slip which helps to postpone sharkskin to higher flow rates.
Good adherence is also thought to be potentially beneficial, but stick-slip is always detrimental.
Minute amounts of (expensive) fluorocarbon polymers are used as processing aids with
LLDPE. The proposed mechanism is that they deposit on the die surface and allow continuous
slip. More recently boron nitride has been introduced for the same purpose [19]. Other remedies
for postponing the onset of sharkskin to higher throughput rates involve reducing the wall shear
stress by heating the die lips to reduce the polymer viscosity and by modifying the die exit to
include a small exit angle (flaring).
At higher throughput rates, extrudates usually become highly distorted and the pressure
in a capillary viscometer shows significant fluctuations. This phenomenon is known as gross
melt fracture.
Figure 1.12 shows LLDPE extrudates for increasing shear rates, illustrating the
progression from smooth surface to sharkskin and then melt fracture [20]. It is possible with
some polymers to obtain melt fractured extrudates without sharkskin, i.e. the surface remains
smooth and glossy but overall the extrudate is distorted.
Proposed mechanisms for melt fracture include entry flow vortex instability, elastic
instability during flow in the die land for stress ratios greater than about 10 (see Equation 1.19),
stick-slip phenomena and other interactions between the polymer and the metal die wall.
Probably more than one mechanism is responsible.
Die lip build-up (also known as die drool) refers to the gradual formation of an initially
liquid deposit at the edge of the die exit which solidifies and grows and may partially obstruct
the flow of the extruded product and/or cause defective extrudate surface. Depending on the
severity of the problem, continuous extrusion must be interrupted every few hours or days and
the solid deposit removed from the die lips. The causative mechanisms are not really known.
Observations suggest that the formation of die lip build-up is not continuous but intermittent.
Tiny droplets of material come out of the die or perhaps from a rupturing extrudate surface.
Some studies suggest that the build-up is rich in lower molecular weight polymer fractions,
waxes and other additives [21].
19. 19
Remedies for reducing die lip build-up include repairing missing plating and surface
imperfections from die lips, removing moisture from the feed material, lowering the extrudate
temperature and adding stabilizer to the resin. Fluorocarbon processing aids will sometimes also
be helpful, as they are with sharkskin. The melt fracture remedy of small die exit angles (flaring)
is also known to reduce build-up, for polyethylenes and polycarbonate.
1.7 Rheological Problems in Coextrusion
1.7.1 Layer-To-Layer Non-Uniformity
Layer non-uniformity in coextrusion flows is caused mainly by the tendency of the less
viscous polymer to go to the region of high shear (i.e. the wall) thereby producing encapsulation.
Figure 1.13 illustrates this phenomenon for rod and slit dies [22]. Complete encapsulation is
possible for extremely long dies (this is not encountered in coextrusion practice). Differences in
wall adhesion and viscoelastic characteristics of polymers are also contributing factors. Weak
secondary flows caused by viscoelastic effects (from the second normal stress difference) have
been demonstrated to produce layer non-uniformities even in coextrusion of different colored
streams of the same polymer [23]. Reduction of this defect can be achieved by choosing
materials with the smallest possible differences in viscosity and viscoelasticity (G, G, extrudate
swell), or by changing the stream temperatures to bring the polymer viscosities closer to one
another.
Layer non-uniformity can also arise in feedblock cast film coextrusion, in which melt
streams are merged into a single stream in a feedblock prior to entering the flat die for forming.
Uneven flow leakage from the flat die manifold to the downstream sections of the die can lead to
encapsulation of the more viscous polymer by the less viscous, or even the reverse! The
technique of feedblock profiling is used to counteract the natural tendency for encapsulation
from viscosity differences. This involves contouring the feedblock flow passages for regions of
high or low volumetric throughput, as shown in Figure 1.14. Feedblock profiling combined with
elimination of uneven flow leakage from the feeding section of a flat die (or the use of this
leakage to counteract the natural tendency for encapsulation) can be used to produce layer-to-
layer uniformity in the extrudate. The problem is much more complex in coextrusion of many
layers, as profiling for one layer will disrupt the other layers. The influence of a feedblock design
change is virtually impossible to predict at present, even with the use of the most powerful 3-D
finite element flow simulation packages on powerful supercomputers.
1.7.2 Interfacial Instability
Interfacial instability in coextrusion refers to two common types of defects consisting of
highly irregular or sometimes regular waviness which appears in coextruded structures at the
polymer/polymer interface. The effect is to significantly reduce the optical quality of coextruded
film. It is an internal defect, which distinguishes it from sharkskin, which is a surface defect.
20. 20
Figure 1.13. Layer-to-layer flow rearrangement as a function of time.
Figure 1.14. Feedblock profiling and the resultant effects.
21. 21
The most frequently encountered type of interfacial instability is zig-zag (also known as
die-land) instability, which appears as chevrons pointing in the flow direction. It is initiated in
the die land and is characterized by a critical interfacial shear stress, in the range of 30 kPa to 80
kPa (roughly ¼ to ½ of the critical wall shear stress level for sharkskin). Figure 1.15 shows the
effect of this instability on film clarity [24]. This problem can arise even if adjacent layers are of
the same material. The mechanism responsible has not been conclusively identified. Apparently
there is amplification of certain disturbance wavelengths under high stress conditions [25].
Viscoelasticity is probably a contributing factor, i.e. the value of interfacial normal stress
difference is important. Unfortunately this is impossible to measure and difficult to calculate
accurately. The most reliable means of diagnosing zig-zag instability at present is to calculate
interfacial shear stress using simulation software.
Zig-zag instability problems are remedied by reducing interfacial shear stresses. The
following actions are beneficial:
decrease the total output rate (this reduces stresses everywhere)
increase the skin layer thickness (this will shift the interface away from the wall
where the shear stress is maximum)
decrease the viscosity of the skin layer, i.e. by raising its temperature or by using a
less viscous polymer (this reduces stresses everywhere)
increase the die gap (this reduces stresses everywhere)
Viscosity matching of layers is a popular remedy that does NOT always work. In fact, as
recommended above, it is often advisable to intentionally mismatch the viscosities by using a
low viscosity resin for the skin layer.
The less common type of interfacial instability is “wave” pattern instability, which
appears as a train of parabolas spanning the width of the sheet and oriented in the flow direction.
It occurs when a fast moving polymer stream merges with a much slower moving stream in a
coextrusion feedblock. When the skin layer is thin relative to the second layer (i.e. the skew of
the coextruded structure is small), the wave instability can be more pronounced. Large
differences in extensional viscosities between adjacent layers can also make the defect more
likely, as can large extensional viscosity of the skin layer. The instability is aggravated by
whatever flow or geometrical asymmetries might be present in the feedblock and die. As well,
dies with larger lateral expansion ratios (die lip width divided by manifold entry width) and
longer channel lengths (from feed slot vanes to die manifold) are more susceptible [26].
1.8 Troubleshooting With the Help of Rheology
Rheological measurements (viscosity, elongational viscosity, G and G) can be used for
(a) material characterization, (b) determination of processability, and (c) as input data for
computer simulations [1].
In material characterization, rheology has an advantage over other methods because of its
sensitivity to certain aspects of the structure such as the high molecular weight tail and
branching. Also, in many instances, rheological characterization can be a lot faster than other
methods such as GPC.
22. 22
Figure 1.15. The effect of interfacial instability on contact clarity of coextruded films (top)
versus see-through clarity (bottom).
Figure 1.16. Simulation prediction of pressure build-up in extruder.
23. 23
With careful rheological measurements, it is possible to determine whether, or under
what conditions, a material will be processable. Blend ratios, or additive quantities necessary to
facilitate processing can be determined. Many problems can be avoided by a thorough
rheological characterization, before the material is introduced into the extruder hopper. For the
relative benefits of on-line, in-line or off-line rheometry, the reader is referred to Kelly et al.
[27].
Rheological measurements are absolutely necessary as input for computer simulations.
The viscosity must be measured over the shear rate range that is anticipated in the real process,
and then fitted to a proper model (power-law, Carreau-Yasuda or Cross). Figure 1.16 shows a
prediction of pressure build-up in an extruder made using viscosity data [28]. Other
measurements are necessary, whenever viscoelastic simulations are undertaken.
Rheology is used for troubleshooting purposes in a great variety of situations. Here are
some frequently encountered ones:
Processability of material A versus material B. A frequently asked question from rheology
consultants is: "Materials A and B have the same Melt Index (MI), virtually identical viscosity
curves and virtually identical molecular weight distributions (measured by Gel Permeation
Chromatography (GPC)). Yet, they behave very differently in extrusion through the same
machine. Why?" The reason is that processability is often determined by small amounts of high
molecular weight fractions or branching which are not detectable by conventional GPC methods
and do not cause any measurable differences in MI or the viscosity curve. To detect the
differences it is recommended that G and G be determined and compared. Occasionally, first
normal stress difference measurements (N1) might be necessary, and since these are difficult and
expensive, extrudate swell measurements are recommended. Larger G, N1 or extrudate swell
implies the presence of a higher-molecular-weight tail. For processing involving extension
(blown film, melt spinning, sheet and film drawing), measurements of extensional viscosity (or
melt strength) are recommended.
Final product properties are poor. These may include impact resistance, optics, warpage,
brittleness, etc. Again, rheological measurements may have to be carried out on samples from
the raw material and from the final product for comparison purposes. This is aimed at detecting
any degradation or other modification that might have occurred during extrusion.
Material is prone to sharkskin. Determine the viscosity of material at the processing temperature
(in the lip region). Materials that are not very shear thinning are prone to sharkskin at relatively
low throughput rates. To reduce shear stress, increase die temperature or use additives that
promote slippage (e.g. fluorocarbon polymers).
Bubble instability in film blowing. One of the causes might be low melt strength of the material.
Measure extensional viscosity and/or melt strength. Compare with other materials that show
better bubble stability. Choose a higher melt strength material. Increase cooling to lower bubble
temperature and thereby increase melt strength.
24. 24
Draw resonance in melt spinning or drawing of cast film. Draw resonance refers to periodic
diameter or thickness variation. Low-elasticity materials are more prone to this type of
instability. Measure G and choose more elastic resin grades (higher G).
Poor blending of two polymers. When the viscosity difference between two polymers to be
blended is large (say, over five times), blending is difficult because the shear stress exerted by
the matrix on the higher viscosity dispersed phase is not large enough to cause breakup. Use a
matrix of higher viscosity or an extensional flow mixer [1].
1.9 Concluding Remarks
Polymer resins are frequently sold on the basis of density and Melt Index (MI).
However, MI is only just one point on an (apparent) viscosity curve. Plastics extrusion involves
shear rates usually up to 1000 s–1
, and viscosity measurements are called for to determine the
shear thinning behavior. For the analysis of some processes, knowledge of extensional viscosity
and/or melt strength may be needed. The level of elasticity is indicated by the normal stress
differences and dynamic modulus measurements (G and G.
Rheology is an excellent tool for materials characterization and miscellaneous
troubleshooting purposes. However, understanding of the problem is absolutely necessary for
the successful application of rheological methods for pinpointing the root causes of various
extrusion defects.
25. 25
1.10 References
1. J. Vlachopoulos and J.R. Wagner (eds.), The SPE Guide on Extrusion Technology and
Troubleshooting, Society of Plastics Engineers, Brookfield CT (2001).
2. C.W. Macosko, Rheology: Principles, Measurements and Applications, VCH Publishers,
New York (1994).
3. F.N. Cogswell, Polymer Melt Rheology, Woodhead Publishing, Cambridge, England (1996).
4. A.V. Chenoy and D.R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker,
New York (1996).
5. S.G. Hatzikiriakos, Polym. Eng. Sci., 34, 1441 (1994).
6. J.M. Dealy and K.F. Wissbrun, Melt Rheology and Its Role in Plastics Processing, Chapman
and Hall, London (1996).
7. J. Vlachopoulos, Rev. Def. Beh. Mat., 3, 219 (1981).
8. B. Debbaut, T. Avalosse, J. Dooley, and K. Hughes, J. Non-Newt. Fluid Mech., 69, 255
(1997).
9. R.B. Bird, R.C. Armstrong, and O. Hassager, Dynamics of Polymeric Liquids, Vol. I, Wiley,
New York (1987).
10. S.W. Shang, Adv. Polym. Tech., 12, 389 (1993).
11. R.I. Tanner, Engineering Rheology, Oxford Engineering Science, Oxford, England (2000).
12. D.G. Baird and D.J. Collias, Polymer Processing Principles and Design, Wiley, New York
(1998).
13. E. Mitsoulis, J. Non-Newt. Fluid Mech., 97, 13 (2001).
14. G. Marrucci and G. Ianniruberto, J. Rheol., 47, 247 (2003).
15. T.C.B. McLeish and R.G. Larson, J. Rheol., 42, 81 (1998).
16. R. Rutgers and M. Mackley, J. Rheol., 44, 1319 (2000).
17. M.M. Denn, Ann. Rev. Fluid Mech., 33, 265 (2001).
18. A.V. Ramamurthy, Proceedings of Xth Intl. Cong. Rheo., Sydney (1988).
19. E.C. Achilleos, G. Georgiou and S.G. Hatzikiriakos, J. Vinyl Addit. Techn., 8, 7 (2002).
20. R. H. Moynihan, PhD thesis, Dept. of Chem. Eng., Virginia Tech. (1990).
21. J.D. Gander and J. Giacomin, Polym. Eng. Sci., 37, 1113 (1997).
22. N. Minagawa and J.L. White, Polym. Eng. Sci., 15, 825 (1975).
23. J. Dooley, PhD thesis, U. Eindhoven, Netherlands (2002).
24. R. Shroff and H. Mavridis, Plas. Tech., 54 (1991).
25. J. Perdikoulias and C. Tzoganakis, Plas. Eng., 52, #4, 41 (1996).
26. R. Ramanathan, R. Shanker, T. Rehg, S. Jons, D.L. Headley and W.J. Schrenk, SPE ANTEC
Tech. Papers, 42, 224 (1996).
27. A.L. Kelly, M. Woodhead, R.M. Rose, and P.D. Coates, SPE ANTEC Tech. Papers, 45,
1979 (1999).
28. NEXTRUCAD, Polydynamics, Inc., http://www.polydynamics.com.