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Glossary

Brittle fracture
 
Failure or rupture of a material with little or no plastic flow or deformation. Usually this type of failure is associated with impact loads. However, many materials at low temperatures also show brittle fracture failures under static loads. Two common methods for determining resistance to brittle fracture are the Izod and Charpy impact tests.

Creep
 
Deformation that occurs over period of time when a material is subjected to constant stress.

Critical To Quality (CTQ Specifications)
 
Specific material performance requirements that are required to produce a quality product.

Dart impact test
 
Methods for determination of the energy that causes plastic film to fail under specified conditions of impact of a free-falling dart. This energy is expressed in terms of the weight (mass) of the missile falling from a specified height which would result in 50 % failure of specimens tested.

Density
 
The ratio of a material's mass to its volume at a given temperature and pressure. Water is 1.0 g/cm (8.35 lb./gal.) at 25C and 1 atmosphere of pressure. The American Society for Testing and Materials (ASTM) has identified three major classifications for polyethylene materials based on density values:
  • Type I materials (0.910 to 0.925 g/cc) - low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE).
  • Type II materials (0.926 to 0.940 g/cc) - medium-density copolymer polyethylene (MDPE).
  • Type III materials (0.941 to 0.965 g/cc) - high-density polyethylene (HDPE). Type III materials include both homopolymer polyethylene and copolymers of polyethylene with other alpha-olefin monomers (e.g. butene, hexene, or octene).
Ductility
 
Extent to which a material can sustain plastic deformation without rupture. Elongation and reduction of area are common indices of ductility.

Elastic limit
 
The greatest stress that can be applied to a material without causing permanent deformation.

Elasticity
 
Ability of a material to return to its original shape when the load causing deformation is removed.

Elongation
 
A measure of the ductility of a material, determined by a tension test.

Environmental stress crack resistance (ESCR)
 
The resistance of a grade or blend of polyethylene to cracking at a certain temperature and stress in the presence of a chemical.

Flexural modulus of elasticity
 
Alternate term for modulus in bending.

Flexural strength
 
Maximum fiber stress developed in a specimen just before it cracks or breaks in a flexure test. Flexural yield strength is reported instead of flexural strength for materials that do not crack in the flexure test. An alternate term is modulus of rupture.

HDPE
 
High Density Polyethylene resins range from 0.941 to 0.965 g/cm3 density.

Impact energy
 
Energy required to fracture a part subjected to shock loading as in an impact test. Alternate terms are impact value, impact strength, impact resistance and energy absorption.

Impact strength
 
Energy required to fracture a specimen subjected to shock loading, as in an impact test. Alternate terms are impact energy, impact value, impact resistance and energy absorption. It is an indication of the toughness of a material.

Impact test
 
A method for determining behavior of material subjected to shock loading in bending, tension or torsion. Impact tests can be conducted by Drop dart or with a pendulum as in the Izod or Charpy impact tests.

Izod impact test
 
A method for determining the behavior of materials when subjected to shock loading. The test specimen is supported as a cantilever beam is struck by a weight at the end of a pendulum. Impact strength is determined from the amount of energy required to fracture the specimen.

LDPE
 
Low Density Polyethylene resins range from 0.910 to 0.925 g/cm3 density.

LLDPE
 
Liner Low Density Polyethylene resins range from 0.900 to 0.939 g/cm3 density.

MDPE
 
Medium Density Polyethylene resins range from 0.926-0.940 g/cm3 density.

Melt index
 
Melt index (MI) is the weight of resin flowing out from a standard die for 10 minutes under a given temperature and pressure.

The most common device used for measuring rheological properties of melt polymers, the melt index is an empirical method that provides a single point measurement of viscosity. The melt index is actually the amount of material which flows through a capillary under a standard set of conditions (temperature, pressure and time).

In the melt index test, resin is heated at a specified temperature, and a standard weight is applied to a piston which extrudes the material through an orifice at very low shear rates. The test provides an indication of flowability, but it does not accurately indicate total relative flowability due to the shear rate at which the test is conducted.

Most often used as a simple, quick way to grade the relative differences between polymers in a quality control environment, melt index is not very sensitive to differences in molecular architecture. It therefore provides little useful or predictive information regarding how the material will behave in process.

Modulus of elasticity
 
The rate of change of strain as a function of stress. The slope of the straight line portion of a stress-strain diagram. Tangent modulus of elasticity is the slope of the stress-strain diagram at any point. Secant modulus of elasticity is stress divided by strain at any given value of stress or strain. It also is called stress strain ratio. Tangent and secant modulus of elasticity are equal up to the proportional limit of a material.

Depending on the type of loading represented by the stress-strain diagram, modulus of elasticity may be reported as compressive modulus of elasticity (or modulus of elasticity in compression), flexural modulus of elasticity (or modulus of elasticity in flexure), shear modulus of elasticity (or modulus of elasticity in shear), tensile modulus of elasticity (or modulus of elasticity in tension) or torsional modulus of elasticity (or modulus of elasticity in torsion). Modulus used alone generally refers to tensile modulus of elasticity. Shear modulus is almost always equal to torsional modulus and both are called modulus of rigidity. Moduli of elasticity in tension and compression are approximately equal and are known as Young's modulus. Modulus of rigidity is related to Young's modulus by the equation: E = 2G (1 + r) where E is Young's modulus (psi), G is modulus of rigidity (psi) and r is Poisson's ratio. Modulus of elasticity also is called elastic modulus and coefficient of elasticity.

Molecular weight
 
The sum of the atomic weights of all atoms in a molecule. In most non-polymeric materials the molecular weight is a fixed constant value. In polymers, the molecular weights of individual molecules vary widely so that they must be expressed as averages.

Molecular weight distribution
 
The relative distribution of large, medium and small molecular chains in a polyolefin resin is important to its properties. When the distribution consists of chains close to the average length, the resin is said to have a "Narrow molecular weight distribution". "Broad molecular weight distribution" polyolefin's are those resins with a wider variety of chain lengths. In general, resins with narrow molecular weight distributions have greater stress cracking resistance and better optical properties. Resins with broad molecular weight distributions generally have greater impact strength and greater

0ptiMISER®
 
A unique blend formulization and optimization program that uses key polyethylene resin properties to design blends that meet or exceed required physical and processing properties. The blending and optimization algorithms employed produce quality blends at the lowest cost. (patent pending)

Plasticity
 
Tendency of a material to remain deformed after reduction of the deforming stress to a value equal to or less than its yield strength.

Polyethylene
 
A family of resins prepared from the polymerization of ethylene gas using a variety of catalysts. Density, melt index, crystallinity, degree of branching and cross-linking, molecular weight and molecular weight distribution can be regulated over wide ranges. Further modifications are obtained by copolymerization, chlorination, and compounding additives.

Post-Consumer Plastic (PCR)
 
Any plastic that has entered the stream of commerce, served its intended purpose, and has now been diverted for recycling or export. This includes residential, commercial and institutional plastic.

Post-Industrial Plastic
 
Industrial or plant scrap created by a manufacturing process, like sprues, runners, excess parison material and reject parts from molding and extrusion used within the manufacturing facility or sold for reuse.

Processability
 
An improved method for predicting how a material will process vs. Melt Index. The viscosity of the material is measured at shear rates similar to what is experienced in the manufacturing process (I.e. extrusion, injection).

Reprocessed Plastic
 
A thermoplastic waste material from plant scrap or post consumer plastic that has been

Recycled Plastic
 
Any thermoplastic waste material, such as plant scrap, or post consumer plastic, which has been reclaimed by shredding, granulating or repelletizing.

Recycled Resin
 
Any previous molded material. Regrind, reprocessed, postconsumer or post industrial scrap.

Regrinds
 
Any thermoplastic waste material, such as plant scrap, or post consumer plastic, which has been reclaimed by shredding, granulating or repelletizing

Reprocessed Plastic
 
A thermoplastic waste material from plant scrap or post consumer plastic that has been pelletized.

Rheological properties
 
Material properties relating to the flow and deformation of "non-classical" materials such as rubber, molten plastics, polymer solutions, slurries and pastes, composites, and paints. These materials can exhibit varied and striking rheological properties that classical fluid mechanics and elasticity cannot describe. Important properties can be derived from the rheology of a material. I.e. steady state viscosity, temperature dependence, shear rate dependence, ESCR.

Sag resistance
 
Resistance to sagging during the thermoforming, heating and forming process. (High melt strength at low shear rates)

Secant modulus of elasticity
 
Ratio of stress to strain at any point on curve in stress-strain diagram. It is the slope of a line from the origin to any point on stress-strain curve.

Shear rate
 
A measure of the rate of deformation of a polymer melt. Shear rate is calculated from the flow rate and the geometry through which the polymer flows, reported in reciprocal seconds

Slow crack growth
 
Failure from cracks that initiate on the surface, and then grow slowly through the wall, as a result of long-term stress or strain.

Specific Gravity
 
The ratio of the weight of the molded piece as compared to the weight of an equal volume of water.

Stiffness
 
Measure of resistance of plastics to bending. It includes both plastic and elastic behavior, so it is an apparent value of elastic modulus rather than a true value.

Strain
 
Change per unit length in a linear dimension of a part or specimen, usually expressed in %. Strain as used with most mechanical tests is based on original length of the specimen.

Stress
 
Load on a specimen divided by the area through which it acts. As used with most mechanical tests, stress is based on original cross section area without taking into account changes in area due to applied load. This sometimes is called conventional or engineering stress. True stress is equal to the load divided by the instantaneous cross section area through which it acts.

Tensile impact energy
 
Energy required to break a plastic specimen in tension by a single swing of a calibrated pendulum. (ASTM D-1822). It is a measure of impact strength obtained in a tension impact test.

Tensile modulus of elasticity
 
Tangent or secant modulus of elasticity of a material subjected to tensile loading. Alternate terms are Young's modulus and modulus of elasticity in tension. Tensile modulus of elasticity is approximately equal to compressive modulus of elasticity within the proportional limit. An alternate term is Young's modulus.

Tensile strength
 
The ultimate strength of a material subjected to tensile loading. It is the maximum stress developed in a material in a tension test.

Tension test
 
Method for determining behavior of materials under axial stretch loading. Data from test are used to determine elastic limit, elongation, modulus of elasticity, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties. Tension tests at elevated temperatures provide creep data.

Toughness
 
Extent to which a material absorbs energy without fracture. It is usually expressed as energy absorbed in an impact test. The area under a stress-strain diagram also is a measure of toughness of a material.

Wide-specification PE
 
Wide-spec feedstock may include off-color, off-spec, transition and/or end-of-lot content. However, because of its widely varying properties, it requires accurate material characterization and an in-depth understanding of its potential behaviors to be used effectively.

Yield strength
 
Indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit.

Young's modulus
 
Alternate term for modulus of elasticity in tension or compression.

Virgin resin
 
Never processed other than that required for initial pellet manufacture.


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