This glossary will be updated from time to
time. Terminology in several other languages will be added. Copyright © 1998-2001,
POLYDYNAMICS INC. (www.polydynamics.com). Copying for non-commercial use is permitted
provided that all copies carry the copyright.
This page best viewed in Internet Explorer.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | ZADAPTER: In an extruder, the portion of the die assembly that attaches the die to the extruder and provides a flow channel for the molten plastic between the extruder and the die.
ADDITIVE:Any substance that is added to another substance. Usually a material added in minor amounts to alter the properties of a resin or compound.
ADIABATIC:This adjective denotes a process in which no heat is added or removed. The term is used incorrectly to describe an extruder where the mechanical energy from the screw is sufficient to plastify the polymer and the barrel controller set-points are set so that little or no heating or cooling is required.
AIR GAP:In extrusion coating, the distance from the die opening to the nip formed by the pressure roll and the chill roll.
ALLOY: Polymer blend having a modified interface and/or morphology.
AMORPHOUS POLYMER: A polymer having no crystallinity. Polystyrene is an amorphous polymer while HDPE is semi-crystalline.
ANISOTROPY: The situation where properties vary according to the direction in which they are measured.
APPARENT SHEAR RATE: The shear rate determined in capillary viscometers without making a correction (Rabinowitsch) for shear thinning. It turns out that the apparent shear rate is equal to 4Q/p R3 where Q is the volumetric flow rate (m3/s) and R is the radius (m) of the capillary.
APPARENT VISCOSITY: The viscosity determined in capillary viscometry without making a correction (Rabinowitsch) for shear thinning. The apparent viscosity is equal to the shear stress divided by the apparent shear rate, which is , where DP is the pressure drop (Pa), Q is the volumetric flow rate (m3/s), L is the length (m) and R is the radius (m) of the capillary die.
ASTM: Abbreviation of American Society for Testing and Materials, an association for establishing standard testing and reporting procedures.
RETURN TO TOP
BACK-PRESSURE: The resistance that reduces forward flow in an extruder.
BAGLEY CORRECTION: A term used in capillary viscometry to describe the excess pressure drop in the entrance to the capillary due to extensional (elongational) viscosity. It might be negligible when very long capillaries are used (i.e. L/D > 35). If shorter capillaries are used the errors in viscosity measurement might be 10-30% or even higher.
BANBURY MIXER: A high-intensity batch mixer composed of a "figure-eight" shaped chamber and a pair of counter-rotating rotors that masticate the materials.
BARRIER SCREWS: Screws where a second flight (barrier), with a larger radial clearance than a normal flight, separates the solid bed from melt in the compression section. Barrier screws have significantly higher melting capacity than conventional screws.
BETA GAUGE: A device for measuring the thickness of plastic films, sheets or extruded shapes. It consists of a Beta-ray emitting source and a detecting element.
BLEND: An intimate combination of two or more polymer chains having different features, that are not bonded to each other.
BLOCK COPOLYMER: A block copolymer is made when one of the two monomers polymerized together to form a polymer exists as a long section or block in the polymer chain.
BLOW-UP RATIO: In blown film extrusion, the ratio of the final tube diameter to the die diameter.
BRANCHED POLYMERS: Polymers can be classified as linear or branched. Linear polymers have the monomeric units linked together, linearly, with little or no long chain branching. In branched polymers, side chains are attached to the molecular chain backbone. High-density polyethylene (HDPE) is linear, while low-density polyethylene (LDPE) contains both short and long chain branches. Linear LDPE (LLDPE) is a copolymer with controlled short chain branches. This results in polymer that is "stiffer" than LDPE in shear but "softer" in extension. In extension the LLDPE chains slide by without getting entangled since the chain branches are very short.
BREAKER PLATE: A perforated plate located at the end of the extruder, which usually supports screens for preventing contaminants from entering the die.
BULK DENSITY: Weight of a unit of a material, in powdered or granular form, including voids (air) inherent in the material.
RETURN TO TOP
CAPILLARY VISCOMETER: An instrument used to measure polymer melt viscosity. It consists of a heated reservoir used to melt the polymer, which is subsequently pushed by a piston and flows through a 1-mm- to 2-mm-diameter round die. From the force required to move the piston and the corresponding volumetric flow rate, the viscosity can be determined. The Rabinowitsch correction is necessary to account for the shear thinning effects and the Bagley correction to account for the excess pressure drop at the die entrance (see RABINOWITSCH CORRECTION and BAGLEY CORRECTION).
CARREAU MODEL: A mathematical expression describing the shear thinning behavior of polymers. It is more realistic than the power-law model because it fits the data very well at both high and low shear rates.
where: h0, l, n are curve fitting parameters and is the shear rate. Due to the mathematical complexities it is not possible to obtain analytical solutions with this model, but it is excellent for numerical simulations of flow processes.
CATALYST: A substance that changes the rate of a chemical reaction without itself undergoing a permanent change or becoming part of the molecular composition of the product.
COEFFICIENT OF FRICTION: A measure of the resistance to sliding of one surface in contact with another. Low values mean easy sliding. The coefficient of friction of a packed bed of plastic pellets on a polished screw surface is around 0.25, and about 0.4 on the barrel (rougher) surface. Pressure, temperature and surface characteristics affect the value of the coefficient of friction.
CO-EXTRUSION: The process used to form a multilayer structure from two or more polymers.
COGSWELLS METHOD: An approximate method for extensional viscosity measurement. It uses the excess pressure drop value in the die entrance (i.e. from the Bagley Correction). Reproducible measurements are usually possible in the elongation (extension, or stretch) rate range of 5 s-1 to 50 s-1.
COMMODITY RESIN: High-volume, low-priced resins like polyethylene (PE), polypropylene (PP), styrene (PS, etc), acrylic (PMMA), vinyl (PVC etc.).
COMPOUNDING: The combination of polymers with other materials either by means of mechanical (dry) blending or melt state blending.
COMPRESSION RATIO: In single-screw extruders the channel depth in the solids-conveying zone under the hopper is much larger than in the metering (pumping) zone. The depth ratio, usually in the range of 2.04.0, is referred to as the compression ratio. The low bulk density solid polymer bed is compressed as it is forced to go through a gradually decreasing depth and melts as it is sheared against the barrel wall.
COMPRESSION ZONE: The second zone in an extruder screw. It receives material from the feed zone and delivers it to the metering zone. It is sometimes referred to as the melting zone.
CONE AND PLATE INSTRUMENT (also called the Weissenberg Rheogoniometer): A device to measure viscosity by determining the torque necessary to rotate a cone over a flat plate with molten polymer in between. The angle is very small so that the rotational flow is nearly parallel. The device can also be used to determine the (first) normal stress difference by measuring the normal force tending to separate the cone from the plate surface (see NORMAL STRESSES).
CONSISTENCY INDEX: In the power-law viscosity model , which describes the reduction of viscosity as the shear rate increases (shear thinning), m is the consistency index (which is a function of temperature). It corresponds to the value of the viscosity for shear rate .
COPOLYMER: Two monomers polymerized together to form a polymer.
CORONA TREATMENT: Surface treatment of plastic parts by exposing them to an electrical corona discharge to increase their receptivity to inks, paints and adhesives.
COUETTE FLOW (also called DRAG FLOW): The flow between two surfaces caused by the movement of one relative to the other. The fluid is literally dragged by the moving wall. For parallel flat surfaces the resulting velocity profile is linear, varying from zero at the stationary wall to the velocity of the moving surface.
COX-MERZ RULE: Frequently, instead of (steady) viscosity measurements by a capillary and/or a cone-and-plate instrument, dynamic measurements are performed (easier) by applying a sinusoidal deformation in the cone-and-plate. The COX-MERZ rule states that the (steady) viscosity versus shear rate curve is virtually identical to the dynamic viscosity versus frequency curve. It is valid for most common polymers. Since it is easier to get the dynamic data over a very wide range of frequencies, it is used extensively in industry.
CREEPING FLOW: Flow at very low Reynolds Number i.e. Re << 1, where the dimensionless Reynolds number is defined as:
From Fluid Mechanics it is known that when Re is more than 2100 the flow is turbulent and below 2100 the flow is laminar. Molten polymer flows through channels and process equipment usually occur at Re = 10-4 10-2, that is under creeping flow conditions. The creeping flow assumption implies that the fluid inertia is negligible.
CROSSHEAD: A device for changing the extrudate flow direction, usually by 90°, so that it can be blow molded or coat wire or coat shapes.
CROSS-LINKING: Formation of a 3-dimensional network of polymer chains, which completely prevents flow (e.g. vulcanized rubber).
CROSS MODEL: A mathematical expression describing the shear thinning behavior of polymers. It is more realistic than the power-law model because it fits the data very well at both high and low shear rates.
where: h0, l, n are curve fitting parameters and is the shear rate. Due to the mathematical complexities it is not possible to obtain analytical solutions with this model, but it is excellent for numerical simulations of flow processes. This model is very popular in injection molding simulations (cavity filling) and in the characterization of the flow behavior of polymers produced with metallocene catalysts.
CRYSTALLINITY: Ordered repeated structures (crystals) encountered in polymers below a certain temperature. Most polymers are semi-crystalline containing both crystalline and amorphous regions (see also GLASS TRANSITION and MELTING POINT).
RETURN TO TOP
DEBORAH NUMBER (De): The ratio of a characteristic material time to a characteristic process time. As the characteristic material time we can use the relaxation time and as a characteristic process time the inverse of the shear rate. Under usual extrusion conditions the relaxation might be 0.1 and the shear rate 100 s-1 and thus De = 10 (0.1 divided by 1/100). When De << 1, the polymer behaves as a purely viscous fluid while for De >> 1 as an elastic solid. The phenomena of extrudate swell and melt flow instability are evidence of polymer elasticity.
DESIGN OF EXPERIMENTS (DOE): A methodology for planning experiments where purposeful changes to the inputs (factors) to a process (or activity) are made in order to observe corresponding changes in the outputs (responses). A means of gaining knowledge about a process or activity with minimal effort.
DIE LIP BUILDUP (also known as DIE DROOL): The gradual formation of an initially liquid deposit at the edge of the die exit which solidifies 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 few days and the solid deposit must be removed from the die lips.
DIE SWELL: see EXTRUDATE SWELL
DIFFERENTIAL SCANNING CALORIMETRY (DSC): A thermal analysis technique, which measures the difference between a reference and a sample during a controlled temperature change. Changes in the heating rate can be converted into heat capacity and enthalpy changes. It is used to measure the Specific Heat (Heat Capacity Cp), Glass Transition Temperature (Tg), and Melting Temperature (Tm), and to probe the structure of polymer blends.
DISPERSIVE MIXING (also called INTENSIVE MIXING): An operation that reduces the size of agglomerates or liquid drops of a minor component within a major fluid matrix.
DISTRIBUTIVE MIXING (also called LAMINAR or EXTENSIVE mixing): An operation that increases the randomness of the spatial distribution of the minor component within the major fluid matrix.
DRAG FLOW: see COUETTE FLOW
DRAW RESONANCE: A phenomenon by which a continuous drawing process becomes unsteady, alternating between thick and thin sections. It is often encountered in fiber spinning and cast film production.
DYNAMIC MECHANICAL ANALYSIS (DMA): In this technique the response of a material to an oscillatory load is measured during a temperature cycle. It provides information on material modulus of elasticity, which in turn can be related to impact strength, tensile strength, toughness and creep rate.
RETURN TO TOP
ELECTRIC DISCHARGE MACHINING (EDM): A metalworking process in which controlled sparking is used to erode the workpiece.
ELONGATIONAL VISCOSITY (also called EXTENSIONAL VISCOSITY): The resistance to extension (while the (common) viscosity term describes the resistance to shearing). For Newtonian fluids the elongational viscosity is equal to 3 times the (common) viscosity (3 is known as the Trouton Ratio). Polymer melts are non-Newtonian and the elongational viscosity is usually between 3 and 100 times the viscosity. Melt strength is just a rough engineering measure of the elongational viscosity, determined from the force required to break an extruded strand of polymer by a pair of rollers (see MELT STRENGTH).
ENGINEERING RESINS: Resins for high-performance applications. This category includes polycarbonates (PC) and polyamides (PA) like nylon.
EXTRUDATE SWELL (also called DIE SWELL): Whenever a polymer melt emerges from a die the diameter or thickness is always larger than the diameter (or gap) of the die. At usual production throughputs, diameter or thickness ratios range from 1.20-1.40 for PVC to 1.50-2.00 for commercial grade Polyethylenes and much more for some polymers containing a high molecular weight tail. It is an indication of the elasticity of the polymer. The more elastic polymers give larger swell. Of course, by pulling the extrudate the swell is reduced and of course extrudates can be drawn down to diameters (or thicknesses) much smaller than the die diameter or gap.
RETURN TO TOP
FEED ZONE: The first zone of an extruder screw. It receives material from the hopper and delivers it to the compression zone.
FILMS: In the plastics and packaging industries, films are usually considered to be a web under 10 mils (0.010 inch or 250 microns) thick. Webs greater than 10 mils are considered sheet.
FISHEYE: Small globular mass which has not blended completely in the surrounding material.
FLIGHT: In an extruder, it is the helical metal left after machining the screw channels. The screw flight diameter is the barrel inside diameter minus a specified flight clearance to allow the screw to fit into the barrel. A rule of thumb is 0.001 inch (25 microns) of radial clearance for every inch (25 mm) of barrel diameter.
FLOW LINES OR STREAKS: Flow lines or streaks in the machine direction are visual defects in the form of continuous lines or streaks, which occur in the same axial location. They may appear and be very persistent after a change in material, screw or die.
FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR): In this technique infrared radiation is passed through a sample and the amount absorbed is measured as a function of wavelength. FTIR allows a wide range of wavelengths to be scanned quickly. By comparison to known spectra the polymer can be identified and its structure can be deduced. This technique is used extensively for polymer identification purposes.
FRACTIONAL MELT INDEX: A melt flow index of less than 1.0.
FRICTION COEFFICIENT: See COEFFICIENT OF FRICTION.
FRICTIONAL HEATING: Whenever a highly viscous polymer melt is forced to flow through a channel, considerable heat is generated by internal friction. The rotating screw supplies actually more than 70% of the heat required to melt a polymer in an extruder through this frictional heating mechanism. During flow through dies, the frictional heating can cause a significant local temperature increase (hot spots) and thermal degradation, with an eventual discoloration of the product, black specks, pinking, etc.
RETURN TO TOP
G' and G" : See STORAGE MODULUS and LOSS MODULUS
GEL PERMEATION CHROMATOGRAPHY (GPC): In this measurement technique separation of polymer fractions is effected by flowing a pulse of a polymer solution through a packed bed of porous particles. By measuring the polymer concentration in the effluent stream, and comparing to calibration standards, the molecular weight distribution can be determined.
GELS: In polymer science, a gel is defined as a 3-dimensional network of sufficient cross-link density that prevents flow. However, in extrusion practice the term "GEL" encompasses various visual defects in the final product, which sometimes appear as elongated ellipses and even includes various contaminants that are called "fisheyes" (see FISHEYE).
GLASS TRANSITION TEMPERATURE (Tg): The lowest temperature at which a polymer can be considered softened and possibly flowable. For HDPE and LDPE, it is 100°C and for PS +100°C.
GROOVED (BARREL) EXTRUDERS: The forward conveying action of a single-screw extruder can be increased by intentionally roughening the barrel surface (grooves) in the solids-conveying zone. Grooved extruders can produce rapid pressure rise, which can sometimes be high enough to damage the screw or barrel. Grooved extruders produce higher throughputs, but they might be susceptible to output instabilities and surging problems.
RETURN TO TOP
HEAT OF FUSION: The heat required to mobilize the molecules of a solid polymer to produce a fluid melt i.e. the heat required to destroy the solid crystal structure without increasing the temperature. For amorphous polymers like polystyrene (PS) the heat of fusion is zero. For LDPE it is about 130,000 J/kg, which is roughly equivalent to the heat required to raise the temperature of 1kg of LDPE by about 50°C.
HIGH LOAD MELT FLOW INDEX (HLMFI): This is a melt flow index test that uses a higher than the usual (2.16 kg) weight. For PE, the HLMFI weight is usually 10 kg, but sometimes 21.6 kg.
HOPPER: The container that holds the resin before it enters the extruder feed zone.
RETURN TO TOP
IMMISCIBLE: Incapable of mixing.
INTERFACIAL INSTABILITY IN COEXTRUSION: Highly irregular or sometimes regular waviness, which appears in coextruded structures at the polymer/polymer interface. Two types of interfacial instability are common: zigzag and wave.
INTRINSIC VISCOSITY (IV): The limiting value of viscosity (at infinite dilution) of a polymer in a solution, which is used in the determination of an average molecular weight. The viscosity average molecular weight lies between the weight average and number average molecular weight (see also MOLECULAR WEIGHT).
ISO: The International Organization for Standardization, a worldwide federation of national standards bodies from some 140 countries. Equivalent to ASTM.
ISO 9000: Family of standards concerned with "quality management". This means what the organization does to enhance customer satisfaction by meeting customer and applicable regulatory requirements.
ISOTROPY: The situation where properties are independent of the direction in which they are measured.
RETURN TO TOP
K-VALUE OF PVC: A measure of the molecular weight of PVC based on measurements of viscosity of a PVC solution. It ranges usually between 35 and 80. Low K-values imply low molecular weight (which is easy to process but has inferior properties) and high K-values imply high molecular weight, (which is difficult to process, but has outstanding properties).
RETURN TO TOP
L/D RATIO: The ratio of the screw length to the screw diameter.
LEAKAGE FLOW: The flow through the clearance between the flight lands and the barrel wall.
LINEAR POLYMERS: Polymers can be classified as linear or branched. In linear polymers the monomeric units are linked together, linearly, with little or no long chain branching. In branched polymers, side chains are attached to the backbone of the molecular chain. High density polyethylene (HDPE) is linear while low density polyethylene (LDPE) is branched. Linear LDPE (LLDPE) is "stiffer" than LDPE in shear but "softer" in extension. In extension the LLDPE chains slide by without getting entangled (since the chain branches are very short).
LOSS MODULUS (denoted as G"): An indirect measure of polymer viscosity using a cone-and-plate instrument subjected to dynamic (sinusoidal) deformation (see also STORAGE MODULUS).
RETURN TO TOP
MASTERBATCH: A concentrated blend of pigments, additives, fillers, etc. in a base polymer. Masterbatch is added in small amounts to large volume material (the same as or compatible with the base polymer) to achieve desired properties.
MELT BLOCKAGE: The sudden drop of output rate of an extruder due to insufficient forward transport of the solid packed bed in the feeding zone of the machine.
MELT FLOW INDEX (also called MELT INDEX or MELT FLOW RATE): The number of grams of polymer that can be pushed out of a capillary die of standard dimensions (Diameter: 2.095 mm, Length: 8.0 mm) under the action of standard weight (2.16 kg for PE, at 190°C). in 10 minutes (ASTM Standard 1238). The usual melt index range is from less than 1.0 (called fractional) to more than 25 (up to 100 for injection molding). For PP it is usually called MELT FLOW RATE and the standard temperature is 230°C.
MELT FRACTURE: At higher throughput rates, extrudates usually become highly distorted and the head pressure shows significant fluctuations. This phenomenon is known as gross MELT FRACTURE. It is possible with some polymers to obtain grossly melt fractured extrudates without sharkskin, i.e. the surface remains smooth and glossy but overall the extrudate is distorted (see also SHARKSKIN).
MELT STRENGTH: A measure of the extensional viscosity of polymer melts. It represents the maximum tension that can be applied to the melt without rupture or tearing. Usually a capillary viscometer is used to extrude a polymer strand and the strand is pulled till rupture by a pair of rollers.
MELTING POINT: The temperature at which the structure of a crystalline polymer is destroyed to yield a liquid. For HDPE it is about 135°C, for LDPE it is about 110°C. It is not scientifically correct to talk about the melting point of an amorphous polymer like PS, because it has no crystalline structure. However, in extrusion practice it is often practical to use the glass transition temperature plus 50°C to define an equivalent melting point of such amorphous polymers. For PS this would be 100°C + 50°C = 150°C (see GLASS TRANSITION).
METALLOCENE CATALYZED POLYMERS: Commonly and erroneously called metallocene polymers. Most polyolefins are produced nowadays with the help of so-called Ziegler-Natta catalysts. Recent developments in metallocene catalysts give the possibility to tailor the structure in such a way as to produce polymers having significantly improved mechanical and physical properties. The better properties of blown film are apparently accompanied by the trade off of poorer processability than conventional materials.
METERING ZONE: The single screw extrusion process consists of three functional zones: the SOLIDS CONVEYING ZONE where the polymer pellets or powder are compacted and transported forward, the MELTING ZONE where the polymer melts mainly under the action of shear on the barrel wall, and the METERING ZONE (PUMPING ZONE) where the polymer is transported forward by DRAG FLOW caused by the rotating action of the screw.
MOLECULAR WEIGHT: Polymers are composed of long chain molecules. The monomer unit is repeated many times to give average molecular weights ranging from 50,000 to 500,000 for most common polymers. Of course, not all polymer chains are of the same length, so we have a MOLECULAR WEIGHT DISTRIBUTION (MWD). Different molecular weight averages are defined to express the breadth of the distribution. The number average molecular weight , Mn, is the sum of the individual molecular weights divided by their number. The weight average molecular weight, Mw, is the sum of the squares of the weights divided by the sum of the molecular weights. The POLYDISPERSITY INDEX (PDI) Mw/Mn (weight average / number average) would be 1.0 if all chains had exactly the same length (only theoretically possible). Usual grades of polymers have PI values from 1.5 to 30. Broad distribution polymers usually have lower viscosity, but higher elasticity.
RETURN TO TOP
NEWTONIAN FLUIDS: Fluids which exhibit constant viscosities independent of the shear rate. Water, glycerin, oil and other small molecule fluids are Newtonian.
NON-NEWTONIAN FLUIDS: Fluids having viscosities that depend on the shear rate. Polymer solutions and melts are non-Newtonian fluids. They also exhibit other non-Newtonian properties such as stress relaxation and normal stresses.
NORMAL STRESSES: Polymer melts when sheared (i.e. when subjected to tangential forces) give rise to perpendicular (NORMAL) STRESSES. This means that when a fluid is flowing in a tube it is less compressed in the axial direction than in the radial direction. These NORMAL STRESSES are responsible for the phenomenon of EXTRUDATE SWELL at the exit of the die. Polymers containing a high molecular weight tail tend to give larger NORMAL STRESSES (i.e. they are more elastic).
RETURN TO TOP
PLATE-OUT: The undesirable deposition of additives or pigments on machinery parts during processing of plastics.
POISEUILLE FLOW (also called PRESSURE FLOW): The flow of a fluid caused by a pressure difference. The resulting velocity profile in a tube is parabolic for Newtonian fluids and somewhat "flatter" for polymer melts. The pressure drop is linear in the direction of flow for tubes or channels with parallel walls.
POLYDISPERSITY INDEX (PDI): The ratio of weight average to number average molecular weight (Mw/Mn) (see also MOLECULAR WEIGHT).
POWER-LAW MODEL: A simple mathematical expression describing the shear thinning behavior of polymers:
Where m is the consistency index, n is the power-law index, and the shear rate (for polymer melts 0.2 < n < 0.8). The power-law model does a good job in fitting high shear rate viscosity data but a poor job for fitting low shear rate viscosity data. In fact for the power-law model gives which is, of course, unrealistic. The usefulness of the power-law model is derived from the fact that several analytical solutions are possible and many practical flow problems occur at high shear rates where the power-law model gives a reasonable fit of viscosity data.
PRESSURE FLOW (also called POISEUILLE FLOW): The flow of a fluid caused by a pressure difference. The resulting velocity profile in a tube is parabolic for Newtonian fluids and somewhat "flatter" for polymer melts. The pressure drop is linear in the direction of flow for tubes or channels with parallel walls. In the metering section of an extruder screw, pressure flow is the relatively backward flow of material down the screw channel caused by pressure in the head.
PSEUDOPLASTIC FLOW: This term is synonymous to shear thinning flow, i.e. viscosity decreases as the shear rate increases.
PURGING: Cleaning of one type of material or color from an extruder by forcing it out with the new material or color or by using special purging substances. Purging is more efficient when the purging material is more viscous than the material being replaced.
PYROMETER: A device for measuring high temperatures, usually by radiation. Radiation devices have the advantage of not having to touch the material being measured.
RETURN TO TOP
RABINOWITSCH CORRECTION: When viscosity data are obtained from a capillary viscometer, they require a correction to account for the fact that the viscosity decreases as the shear rate increases. Without this (RABINOWITSCH) correction the viscosity is referred to as APPARENT VISCOSITY. Errors up to 10-20% in viscosity are common when this correction has not been made. For the power-law viscosity model the Rabinowitsch correction gives (i.e. for n=0.45 m(true) = 0.89 m(apparent)).
RANDOM COPOLYMER: A random copolymer occurs when one of the two monomers polymerized together to form a polymer is in a random or statistical distribution in the polymer chain.
REGRIND: Waste material that has been reclaimed by shredding or granulating.
RELAXATION: Whenever polymer melts are subjected to mechanical work they develop stresses which do not become immediately zero when the mechanical influence is removed. The time required for the stresses to relax is referred to as RELAXATION TIME. There is no universally accepted definition of the most characteristic relaxation time for a polymer melt. Some authors use the l of the Carreau or Cross viscosity models, but this is considered a poor choice by theoreticians. The best choice is perhaps the "longest relaxation time" determined in dynamic measurements as where G' is the storage modulus, h' is the dynamic viscosity and w is the frequency. Polymers with higher molecular weights have long relaxation times. When the polymer solidifies without the stresses having been relaxed, the product includes FROZEN-IN STRESSES which will be released when the plastic part is reheated. The accompanying changes in dimensions, shrinkage or warpage may be significant.
REYNOLDS NUMBER: A dimensionless quantity defined as:
which is equivalent to the ratio of INERTIA forces to VISCOUS forces. The flow is turbulent when the Reynolds number is more than 2100 for tubes. Below 2100 the flow is laminar (i.e. streamlines without disturbances). For molten polymer flow, the Reynolds Number is usually in the range 10-4 to 10-2 (see also CREEPING FLOW).
RHEOLOGY: The science of deformation and flow of materials including polymers. Viscosity, elongational viscosity, normal stresses, relaxation time, G', G", etc. are rheological properties.
RETURN TO TOP
SHARKSKIN (also known as SURFACE MATTNESS): The failure of an extrudate to exhibit smooth and glossy appearance. The surface usually exhibits a repetitious wavy or ridged surface pattern perpendicular to the flow direction.
SHEAR FLOW: The sliding of imaginary fluid slices parallel to each other, like a deck of cards. Shearing occurs whenever fluids flow through tubes and channels. The velocity is zero right at the wall surface and maximum at the center. So the fluid is being sheared as it flows through a tube or channel.
SHEAR RATE: The velocity gradient, i.e., velocity divided by the gap measured in reciprocal seconds, s-1. In screw extruder channels, the shear rate can usually reach 100 s-1 or more. In flow through extrusion dies, it might reach 500 s-1 or more, and in injection molding more than 5000 s-1.
SHEAR STRESS: A tangential force divided by the area (FORCE/AREA) on which it is applied. The shear stress is equal to the viscosity multiplied by the shear rate (measured in units of pressure, i.e., MPa or psi). At the die lips under usual production conditions, the shear stress may reach values of 0.2 MPa (29.0 psi) or more. The usually accepted value for the onset of sharkskin in capillaries is 0.14 MPa (20.3 psi), although higher values are reported in industrial production. With additives the critical shear stress value might be pushed up to 0.5 MPa (72.5 psi).
SHEAR THINNING: The reduction of the viscosity as the shear rate increases, which is exhibited by polymeric liquids. Shear thinning is due to molecular chain alignments in the direction of flow and disentanglements.
SHEAR VISCOSITY: The ordinary viscosity that is the ratio of shear stress to the shear rate (see also VISCOSITY).
SHEET: In the plastics and packaging industries, sheets are usually considered to be a web greater than 10 mils (0.010 inch or 250 microns) thick. Webs smaller than 10 mils are considered films.
SI UNITS: "Systeme International" units, established in 1960, based partly on the metric system, which was used in Europe for a long time. In SI the six base units are: metre (length), kilogram (mass), second (time), ampere (electric current), degree Kelvin (temperature) and candela (luminous intensity).
SLIP, SLIPPAGE: When fluids flow it is assumed that the velocity at a surface is zero (or equal to the surface velocity if the surface moves). Virtually all polymer melts exhibit some slippage on the surface, especially when the shear stress levels are high, e.g., over 0.1 MPa (14.5 psi). Stick-slip phenomena are responsible for the onset of sharkskin whenever polymers are extruded at shear stresses higher than 0.14 MPa (20.3 psi). Some additives and processing aids promote slippage. Slippage is beneficial for delaying the appearance of sharkskin at higher throughput rates.
SOLIDS CONVEYING ZONE: The single screw extrusion process consists of three functional zones: the SOLIDS CONVEYING ZONE where the polymer pellets or powder are compacted and transported forward, the MELTING ZONE, where the polymer melts mainly under the action of shear on the barrel wall and the METERING ZONE (PUMPING ZONE) where the polymer is transported forward by DRAG FLOW caused by the rotating action of the screw.
SPE: Abbreviation for Society of Plastics Engineers.
SPI: Abbreviation for Society of Plastics Industry.
STARVE FEEDING: Feeding of an extruder at a rate below the full capacity of the machine. This results in output determined by the feeder and not by the extruder or the process parameters.
STORAGE MODULUS (denoted as G' ): The ratio of shear stress to strain (deformation) when dynamic (sinusoidal) deformation is applied in a cone-and-plate rheometer. It relates to the elasticity of the polymer melt. G", the loss modulus, also determined in dynamic (sinusoidal) measurements relates to the viscous behaviour. So, G' and G" together give an idea of the dual nature of the polymer melt (partly elastic solid and partly viscous fluid). Measurements of G' and G" provide information on polymer structure and might be related to molecular weight distribution, cross-linking, etc.. Computer software like RHEOMWD can infer the breadth of the molecular weight distribution from G' and G" data.
STRAIN: A technical term synonymous with deformation.
SURGING: An instability of melt pressure and flow rate in an extruder, which can be detected by a pressure gage at the tip of the screw (or at the die adapter), or by dimensional product variations.
RETURN TO TOP
THERMAL MECHANICAL ANALYSIS (TMA): In this technique, a sample is deformed under a static load as its temperature is being changed. Glass transition and softening points can be measured. The amount of orientation can also be measured by TMA.
THERMOGRAVIMETRIC ANALYSIS (TGA): This technique is used to measure a variety of polymeric phenomena involving weight changes, such as sorption of gases, desorption of contaminants (monomers, solvents, and additives) and degradation. In TGA, a sample is placed on a balance beam in an oven. From the weight versus temperature curves, kinetic and other studies are carried out.
THERMOPLASTICS: Polymers that can be melted by heating and solidified by cooling and may be remelted repeatedly. PE, PP, PVC and all extrudable polymers are thermoplastics.
THERMOSETS: Materials that undergo chemical reaction and can be hardened by application of heat and pressure. They cannot be softened again to make them flowable. Typical plastics in this family are melamine, urea, epoxies and phenolics.
TROUTON RATIO: The ratio of elongational (extensional) viscosity to (shear) viscosity for Newtonian fluids is 3. Polymers do not obey this relation. Ratios can range up to 100 for melts and up to perhaps 10,000 for solutions.
RETURN TO TOP
VISCOELASTICITY: The dual nature of polymers, partly viscous fluid and partly elastic solid, is referred to as viscoelasticity. In flowing polymers viscoelasticity is responsible for time-dependent properties, such as stress relaxation, normal stresses, very large elongational viscosities, and numerous unusual phenomena such as extrudate swell, entry flow vortices and some flow instabilities.
VISCOSITY: The resistance to flow of a fluid (strictly speaking the resistance to shearing). It is defined as the ratio of shear stress (Tangential Force/Area) to shear rate (velocity/gap). The viscosity of a polymer decreases as the shear rate increases. This property is referred to as pseudoplastic behavior or shear thinning. The viscosity of a polymer at (near) zero shear for a polymer like PE might be 5,000 to 10,000 Pa.s while during flow in an extrusion channel it could be much lower (i.e. 500 Pa.s or less). Melt flow index corresponds to just one point on a viscosity curve (actually inverse). High viscosity implies low melt index and high molecular weight. Viscosity is measured in units of Pa.s or poise. 1 Pa.s = 10 poise. The viscosity of water is 10-3 Pa.s (1 centipoise) and for a typical polymer melt at least one million times larger (i.e. over 1000 Pa.s or 10,000 poise)
VISCOUS DISSIPATION (or VISCOUS HEATING): see FRICTIONAL HEATING
RETURN TO TOP
WEISSENBERG NUMBER: The product of a characteristic material time and shear rate. It has the same meaning as the Deborah Number under certain conditions (see DEBORAH NUMBER).
WEISSENBERG RHEOGONIOMETER: see CONE AND PLATE INSTRUMENT
WELDLINES (also known as PARTING LINES): Weldlines are formed because of flow interruptions by obstructions in a melt flow field. Because of the high viscosity, the diffusion of polymer molecules, after they have been separated, is very slow and the weldline remains a line of mechanical weakness and may be visible to the naked eye. Such defects are common in injection molded products and are often encountered in extruded pipes, bottles, and film.
RETURN TO TOP
ZERO SHEAR VISCOSITY: The asymptotic viscosity value at zero shear rate (i.e., the maximum value). As the shear rate increases, the viscosity decreases due to alignments of molecular chains in the direction of flow and molecular chain disentanglements. The zero shear viscosity is proportional to the 3.4 power of the weight average molecular weight (i.e. h0 = constant * Mw3.4 for most common polymers). This means that if we increase the molecular weight of a polymer from 100,000 to 200,000 the viscosity will increase by 23.4 = 10.55 times. Obviously it is extremely difficult to extrude polymers having very high molecular weight.
RETURN TO TOP