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 | Z

A

ABRASION RESISTANCE
Ability of a plastic to withstand mechanical action that tends to wear material from its surface.

ADAPTOR
A long heated cylindrical pipe used to convey molten resin from an extruder into an extrusion die.

ADDITIVE
Any substance compounded into resin to modify its properties. Additives can be antioxidants, colorants, pigments, light stabilizers, etc.

AIR RING
In blown film extrusion, a circular manifold mounted above the extrusion die used to distribute an even flow of air against a blown film bubble.

AMORPHOUS
Having no crystallinity. At processing temperatures most plastics are amorphous.

ANTIBLOCK AGENT
Additive incorporated in film to prevent the adhesion (sticking) between touching layers of film during fabrication, storage, or use. For example, these additives can be diatomaceous earth, silica, and talc.

ANTIMONY OXIDE
A white, odorless, fine powder widely used as a flame retardant in plastics.

ANTIOXIDANT
An additive which inhibits the degradation/oxidation of a material when exposed to ambient air, during processing and subsequently in the end-product form.

ANTISTATIC AGENT
Additive which imparts a slight degree of electrical conductivity to plastics, permitting the dissipation of static electricity.

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B

BACK PRESSURE
In extrusion, the resistance of molten polymer to flow forward, caused by a pressure difference between two points along the path of flow.

BAND HEATER
Electrical heating units fitted to extruder barrels, adaptors, dies, nozzles, etc. Utilized for heating the polymer to a desired temperature.

BAREFOOT
A resin or film containing no additives.

BARREL
The tubular portion of the extruder in which the extruder screw is placed and rotates.

BARRIER RESINS
Polymers which have very low permeability to gases.

BIODEGRADATION
The degradation of plastics by micro-organisms when buried in the soil. Some plastics can be modified to become biodegradable by the incorporation of a biodegradable additive such as corn starch.

BLEND
The mixing of polymers with other polymers or copolymers, us ually where the mixture results in the desired physical properties.

BLOCKING
An undesirable adhesion between layers of film or sheeting which may have developed during processing, or storage. Blocking can be prevented by adding antiblock agents to the resin.

BLOOM
A thin, greasy film on the surface of a plastic film or part usually caused by the exudation of an additive. Slip additives are designed to migrate or bloom to the surface of films.

BLOW HOLE
Blow-outs or loss of internal air from a blown film bubble usually due to a rupture caused by fisheyes, gels, or contamination.

BLOWN FILM TOWER
Apparatus for handling film in blown film extrusion between the extruder die and take-up equipment. The blown film -tube passes through the tower where it is cooled, sized, and gauged. Nip rolls are located at the top where the inflated tube is collapsed prior to winding.

BLOW-UP RATIO
The ratio of the final tube diameter to the die diameter in blown film extrusion.

BREAKER PLATE
A perforated plate located at the rear end of an extruder head or die adaptor serving to support the screen pack. The breaker plate also helps to generate back pressure in the extrusion process.

BULK DENSITY
The density (mass per unit of volume) of a resin in solid form (granular, nodular, pellet, powder, etc.) expressed in g/cc or lbs/ft3.

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C

CADMIUM PIGMENTS
Inorganic pigments based on cadmium sulphide and cadmium sulphoselenides used widely in polyethylene. Include cadmium maroon, -orange, -red, and -yellow.

CALCIUM CARBONATE
A filler and extender used in thermoplastics. It occurs naturally in the form of minerals such as calcite, chalk, limestone, marble, and whiting.

CARBON BLACK
A multi-functional pigment used in plastics as a conductor of electricity, a pigment, a filler-extender, and as a UV stabilizer.

CENTER FOLD
A roll of film that has a fold on one side and slit on the other in the machine direction allowing the film to be unfolded to twice its roll width.

CHANNEL DEPTH RATIO
In an extruder screw, the ratio of the depth of the first channel at the hopper end to the depth of the last channel in the metering section.

CHROME PIGMENTS
Pigments based on basic lead chromate. Included are chrome yellow and orange. They have intense colors, good acid resistance, and heat stability.

COEFFICIENT OF FRICTION
Resistance to movement of-sliding or rolling surfaces of solid bodies in contact with each other.

CO-EXTRUSION
The technique of extruding two or more materials through a single die being fed by separate extruders.

COLORANT
Dyes or pigments which impart color to plastics. The dyes are synthetic or natural compounds of submicroscopic size, soluble in common solutions, yielding transparent colors. Pigments are organic and inorganic substances with larger particle sizes and are usually insoluble in common solvents.

COLOR CONCENTRATE
A plastics compound which contains a high percentage of pigment to be blended into base resins. The term masterbatch is sometimes used for color concentrate as well as for concentration of other additives.

COLORIMETER
An instrument for measuring and matching colors.

COMPATIBILITY
The ability of two or more substances to mix together without separation.

COMPRESSION RATIO
In an extruder screw, the ratio of the volume of the channel at the first flight of the screw to the volume at the last flight in the metering section.

CONVERTOR
A term used in the packaging industry. Converters buy plastic film or sheeting in the form of roll stock and convert it to useful forms by slitting, die cutting, heat sealing, etc. for resale to packaging firms.

CORE
Rigid cardboard tube onto which film is wound. Typical cores have either 3 inch or 6 inch inside diameters.

CORONA DISCHARGE
A method of rendering the surface of inert plastics such as polyethylene more receptive to inks, adhesives, or coatings by subjecting their surfaces to an electrical discharge. Typical method is to pass film over a grounded metal cylinder above which a high voltage electrode is spaced to leave a small air gap.—The corona discharge oxidizes the film leading to the formation of polar groups. The surface now becomes receptive to the coatings.

CREEP
Creep is the permanent deformation resulting from prolonged application of a stress below the material’s yield point.

CRYSTALLINITY
The state of molecular structure in some resins denoting uniformity and compactness of the molecular chain.

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D

DART DROP
Measurement of the puncture resistance/impact strength of film and its ability to withstand the shock of a falling “dart” without breaking. Expressed as gram weight of the heaviest dart which doesn’t break the film when dropped from a specified height.

DEGRADATION
A deleterious change in the chemical structure, physical properties, or appearance of a plastic caused by exposure to heat, light, oxygen, or weathering.

DELAMINATION
The separation of one or more layers in a laminate caused by the failing of the adhesive bond.

DENSITY
Weight per unit volume of a substance, usually reported in g/cc or lbs/ft3.

DIE
A steel block containing an orifice through which plastic is extruded, shaping the extrudate to the desired form.

DIE ADAPTOR
The part of an extrusion die which holds the die block.

DIE GAP
Distance between the metal faces forming the die opening.

DIE LAND
(See “Land”).

DIE LINES
Vertical or horizontal marks on the extrudate and in the finished product caused by damaged die elements or by contamination held-up in the die land.

DISCOLORATION
Any change from an initial color possessed by a plastic; a lack of uniformity in color where color should be uniform over the whole area of a plastic object.

DISPERSION
Fine division of particles of a resin or solid in suspension in another material.

DRAWDOWN
In extrusion, the process of pulling the extrudate away from the die at a linear speed higher than that at which the melt is emerging from the die, thus reducing the cross-sectional dimensions of the extrudate.

DYNE
Unit of measure for surface tension (treat level).

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E

ELASTICITY
The property of plastic materials by virtue of which they tend to recover their original size and shape after deformation.

ELASTOMER
A material which at room temperature can be stretched under low stress to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length.

ELONGATION
Deformation caused by stretching; the fractional increase in length of a material stressed in tension.

ERUCAMIDE
A fatty acid-based slip additive used in polyolefin resins.

ETHYLENE
A colorless, flammable gas derived by cracking of petroleum and natural gas fractions. Also serves as a monomer for polyethylene.

ETHYLENE VINYL ACETATE (EVA)
Copolymer member of the polyolefin family, derived from random copolymerization of vinyl acetate and ethylene.

EXTRUDER
A machine for producing more or less continuous lengths of plastic sections such as rods, sheets, tubes, profiles, and cable coatings by melting and pumping resin through a forming die.

EXTRUDER, SINGLE SCREW
Basic machine consists of a screw, barrel, drive mechanism, resin feed arrangement, and controls. The constantly turning screw augers the resin through the heated barrel where it is heated to proper temperature and blended into a homogeneous melt. Before the melt can leave the barrel, it must pass through a breaker plate and screen pack. The melt is then extruded through the die into the desired shape.

EXTRUDER, TWIN SCREW
Two screws, side by side, are placed within the extruder barrel, and they are either co-rotating or counter-rotating. Counter-rotating twin screw extruders are used primarily for processing PVC products such as pipe, siding, sheet, pellets, and film. The co-rotating units are used for compounding materials where thorough mixing and high output rates are important. The twin screw unit resembles a positive displacement screw pump. It conveys the material at low speeds with controlled shear. The positive action assures that all portions of the material experience a uniform residence time.

EXTRUSION
Compacting and melting a plastic material and forcing it through an orifice in a continuous fashion. In the extrusion process, the material is conveyed through the heated machine barrel by a helical screw, where it is heated and mixed to a homogeneous state and then forced through a die of the shape required for the finished product.
Blown Film: Process involves extruding a continuous thin-walled tube of plastic and inflating it immediately after it leaves the die. The pressure is such that the tube stretches, increasing its diameter and reducing its wall thickness to desired gauge. Air is trapped within the blow tube (bubble) between the die and collapsing rolls which convert it to lay-flat film to facilitate winding onto a roll.

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F

FEED SECTION
First section or zone of an extruder screw which is fed from the hopper and conveys solids to the melting zone.

FILLER
A material which is added to plastics to make it less costly. Filler can be inert or can alter various properties of the plastic.

FILM
Sheet material having a nominal thickness not greater than 10 mil.

FISHEYE
Small globular mass which has not blended completely into the surrounding material, resulting as a fault in film or sheet.

FLAME RETARDANT
Reactive compounds and additive compounds to render a polymer fire-resistant. Reactive compounds become an integral part of the polymer.

FLEXURAL MODULUS
Ratio of applied stress to strain in outer fibers of a plastic specimen during flexure.

FLEXURAL STRENGTH
Resistance of a plastic material to cracking or breaking during bending.

FLOW LINES
Distinctive surface marks caused when two flow fronts meet and weld together during molding or film extrusion.

FLOW RATE
The ASTM condition of 190°C and a load of 21.6 Kg used for determining the rate of flow of molten HDPE resins through a standard orifice. Also known as High Load Melt Index.

FROSTLINE
In the extrusion of blown film, a ring-shaped zone of frosty appearance located at the point where the resin solidifies, caused by a reduction in film temperature below the melting point of the resin.

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G

GEL
Small globular mass which has not blended completely into the surrounding material, resulting in a fault in the film or sheet.

GLASS TRANSITION TEMPERATURE
The temperature at which a reversible change occurs in an amorphous polymer when it is heated to a certain temperature and undergoes a rather sudden transition from a hard, glassy, or brittle condition to a flexible or elastomeric condition.

GLOSS
Brightness or luster of a plastic resulting from a smooth surface.

GUSSETS
The inward fold in the sides of bags which reduce the width of the bag and allow the bags to assume a rectangular form when opened.

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H

HAZE
Cloudiness in plastic film. Measured as percent haze, anything below 5% is generally considered high clarity.

HEATER BANDS
Electrical heating units shaped to fit extruder barrels, injection molding cylinders, and the like, for heating the plastic material to the desired temperature.

HEAT SEALING
The process of joining two or more thermoplastic films or sheets by heating areas in contact with each other to the temperature at which fusion occurs, usually aided by pressure.

HIGH DENSITY POLYETHYLENE
This term is generally considered to include polyethylenes ranging in density from about 0.940 to 0.960 and over. Whereas the molecules in low density polyethylene are branched in random fashion, those in the higher density polyethylenes have fewer side branches, resulting in more rigid material with greater strength, hardness, chemical resistance, and higher softening temperature.

HMW-HIGH DENSITY POLYETHYLENE
High molecular weight high density polyethylene is usually defined as a polyethylene with a density of 0.940 or greater and a flow rate of 1 to 20 (190°C/21.6 Kg). The average molecular weight ranges from 200,000 to 500,000.

HOPPER
In polymer processing, the container holding a supply of molding material to be fed to the screw or ram. The hopper may be intermittently filled or continuously fed.

HOPPER BLENDER
Mixes material such as virgin resin, regrind, blowing agents, fillers, and colorants in desired proportions. Materials to be blended are metered in ratio to a mixing chamber and then discharged into the hopper of the processing machine.

HOPPER LOADER
A device for automatically feeding resins to hoppers of extruders, injection molding machines, and the like.

HOT TACK
Hot tack is a measure of the strength of a heat seal before the seal has cooled completely.

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I

IMMISCIBLE
Incapable of intermittently mixing, i.e., oil and water are immiscible.

IMPACT STRENGTH
Ability to withstand shock loading.

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J

J SHEET
Similar to center fold, j-sheet is folded on one edge of the roll and slit in the machine direction on the other side allowing the film to be unfolded. The difference is that the fold is not directly in the center of the film, so the width of the film on either side of the fold is asymetrical when opened.

K

L

LAND
1. The bearing surface along the top of the flights of a screw in a screw extruder.
2. The surface of an extrusion die parallel to the direction of melt flow.

LEAD CHROME PIGMENTS
A series of inorganic pigments including yellows, oranges, and greens, used in polyolefins and other plastics.

LENSING
A term which refers to a film defect that causes undesirable elongated thin voids in an extruded film. Sometimes referred to as windows or air pockets.

LINEAR POLYMER
A polymer in which the monomeric units are linked together in linear fashion with little or no long chain branching. Examples are linear low density polyethylene and high density polyethylene.

LINEAR LOW DENSITY POLYETHYLENE
See Linear Polymer. Includes polyethylenes ranging in density from 0.915 to 0.935.

LOW DENSITY POLYETHYLENE
This term is generally considered to include polyethylenes ranging in density from about 0.915 to 0.925. In low density polyethylenes, the ethylene monomeric units are linked in random fashion, with the main chains having long and short side branches. This branching prevents the formation of a closely knit pattern, resulting in material that is relatively soft, flexible, and tough, and which will withstand moderate heat.

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M

MANDREL
1. Core around which fiberglass impregnated with plastic resin is wound, as in filament winding.
2. The portion of an extrusion die that forms the hollow center in an extruded tube.

MASTERBATCH
A concentrated blend of pigment, additives, filler, etc. in a base polymer. Masterbatch is added in similar amounts to large volumes of material (the same as or compatible with the base polymer) to produce the desired formulation.

MELT
A thermoplastic stock which is in a molten state due to temperature.

MELT FRACTURE
A phenomenon of melt extrudate in which the surface appears rough or wavy upon exit from the die. Melt fracture may appear uniformly or in certain sections only.

MELT INDEX
The number of grams of a thermoplastic resin which can be forced through a 0.0825 inch orifice when subjected to 2160 grams force in ten minutes at 190°C.

MELT INSTABILITY
An instability in the melt flow through a die that causes irregularities in the finished part.

MELT STRENGTH
The elastic strength of a polymer in the molten state.

MELTING POINT
The temperature at which a resin changes from a solid to a liquid.

METERING ZONE
The final zone of an extruder barrel, in which the melt is conveyed at a uniform rate to the breaker plate or die.

MOISTURE VAPOR TRANSMISSION
The rate of permeation of water through a material at a specific temperature and relative humidity rate.

MOLECULAR WEIGHT (MW)
The sum of the atomic weights of all atoms in a molecule.

MOLECULAR WEIGHT DISTRIBUTION (MWD)
The relative amounts of polymers of different molecular weights (MW) that make up a specific polymer.

MIGRATION
The exudation of an ingredient from one material by another material; such as the migration of a plasticizer from one material into an adjacent material with a lower plasticizer content.

MODULUS OF ELASTICITY
The ratio of stress to strain below the yield point of the material.

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N

NECK-IN
In extrusion coating, the difference between the width of the extrusion die opening and the width of the coating on the substrate.

NIP ROLLS
In film blowing, a pair of rolls situated at the top of the tower which close the blown film envelope, seal air inside of it, and regulate the rate at which the film is pulled away from the extrusion die. One roll is usually covered with a resilient material, the other being bare metal.

NYLON
Generic name for all long-chain polymers which have recurring amide groups (-CONH-) as an integral part of the main polymer chain. Various types of nylons are described in subsequent listings, the numbers of which relate to the number of carbon atoms in the various reactants.

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O

OCTENE
A comonomer used in the production of linear low density polyethylenes.

OFFSET PRINTING
A printing process in which the image to be printed is first applied to an intermediate carrier such as a roll or plate, then is transferred to a plastic film or molded article.

ORANGE PEEL
An uneven surface texture of a plastic article or its finished coating somewhat resembling the surface of an orange, see Melt Fracture.

ORIENTATION
The process of stretching a hot plastic article to align the molecules, thus altering mechanical properties. When the stretching force is applied in one direction, the process is called uniaxial orientation. When stretching is in two directions, the term biaxial orientation is used. Upon reheating, an oriented film will shrink in the direction(s) of orientation. This property is useful in applications such as shrink packaging, and for improving the strength of molded or extruded articles such as pipe and fibers.

OXIDATION
In respect to polyethylene, the reaction of air or oxygen in polyethylene causing the formation of hydroxy groups which affects the physical properties adversely.

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P

PELLETS
Tablets of uniform size, consisting of resins or mixtures of resins with compounding additives which have been prepared for molding operations by shaping in a pelletizing machine or by extrusion and chopping into short segments.

PERFORATING
Processes by which plastic film or sheeting is provided with holes ranging from relatively large diameters for decorative effects (by means of punching or clicking) to very small, even invisible, sizes. The latter are attained by passing the material between rollers or plates, one of which is equipped with closely-spaced fine needles, or by spark erosion.

PERMEATION
Permeability is the property of a material, i.e., the degree to which it allows permeation to occur.

PERMEABILITY
The passage or diffusion of a gas, vapor, liquid, or solid through a barrier without physically or chemically affecting it.

PLASTIC (adj.)
An adjective indicating that the noun modified is made of, consists of, or pertains to plastics. (Noun) A material that in its finished state contains as an essential ingredient a synthetic polymer of high molecular weight, is a flexible or rigid solid but not an elastomer in its finished state, and at some stage in its manufacture or in its processing into finished articles can be shaped by flow or by in-situ polymerization or curing.

PLATE-OUT
The undesirable deposition of additives or pigments on machinery during processing of plastics

POLYETHYLENE
A family of resins obtained by polymerizing the gas ethylene. By varying the catalysts and methods of polymerization, properties such as density, melt index, crystallinity, degree of branching and crosslinking, molecular weight, and molecular weight distribution can be regulated over wide ranges. Further modifications are obtained by copolymerization, chlorination, and compounding additives.

POLYETHYLENE TEREPHTHALATE
A saturated thermoplastic polyester resin made by condensing ethylene glycol and terephthalic acid, used for fibers and films (for example, DuPont’s “Mylar” film), and, more recently, for injection molded parts. It is extremely hard, wear-resistant, dimensionally stable, resistant to chemicals, and has good dielectric properties.

POLYMER
The product of polymerization of one monomer is called a homopolymer, monopolymer, or simply a polymer. When two monomers are copolymerized, the product is called a copolymer. The term terpolymer is used for polymerization products of three monomers.

POLYMER PROCESS AID (PPA)
Additives which are added or incorporated into plastics as a modifier to aid in the extrusion of film, pipe, sheet, etc.

POLYPROPYLENE
A tough, lightweight, rigid plastic made by the polymerization of high-purity propylene gas in the presence of an organometallic catalyst at relatively low pressures and temperatures.

POLYSTYRENE
Polymers of styrene (vinyl benzene). The homopolymer is water-white in color, has outstanding electrical properties, good thermal and dimensional stability, and is resistant to staining. However, it is somewhat brittle, and is often copolymerized or blended with other materials to obtain desired properties.

POLYVINYL ACETATE
A thermoplastic material composed of polymers of vinyl acetate in the form of a colorless solid. Used extensively in adhesives for paper and fabric coatings.

POLYVINYL ALCOHOL
A thermoplastic material composed of polymers of the hypothetical vinyl alcohol.

POLYVINYL CHLORIDE
Thermoplastic compounds formed by polymerization or copolymerization of vinyl or vinylidene chlorides and vinyl esters.

PURGING
In extrusion or injection molding, the cleaning of one color or type of material from the machine by forcing it out with the new color or material to be used in subsequent production, or with another compatible purging material.

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Q

R

RESIN
An organic substance of natural or synthetic origin characterized by being polymeric in nature. Most resins, though not all, are of high molecular weight and consist of long chain or network molecular structure. Usually resins are more soluble in their lower molecular weight forms.

RHEOLOGY
The study of the behavior of materials as they are deformed.

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S

SCRAP
All products of a processing operation which are not present in the primary finished articles. Scrap from operations can usually be reclaimed for reuse in the processor’s plant or for sale to a commercial reclaimer.

SCREEN
Woven metal screens are installed across the flow of plastic in an extruder. They are located between the tip of the screw and the die. Supported by a breaker plate, the screens strain out contaminants and increase back pressure.

SCREW
In extrusion, the shaft provided with helical grooves which conveys the material from the hopper outlet through the barrel and forces it out through the die.

SCREW FLIGHT
The helical metal thread of a screw in an extruder or injection molding machine.

SCREW SPEED
The rate of revolution (in RPM) of an extruder or injection molding machine screw.

SHARKSKIN
A surface irregularity of a blow molded container or film during extrusion. See Melt Fracture.

SHEAR
Shear is the product of shear rate and resistance time. It is often used to describe the degree of mixing experienced by a material.

SHEAR HEATING
Heat generated within the plastic melt as the polymer is sheared. It is caused by viscous dissipation of work.

SHEAR RATE
Rate of change of velocity across the flow channel.

SHEAR STRESS
Stress developed in a poly mer melt where a material is sheared.

SHELF LIFE
The length of time over which a product will remain fit for use during storage under specific conditions.

SHRINK FILM
A term sometimes used for pre-stretched or oriented film used in SHRINK PACKAGING.

SHRINK PACKAGING
An item or group of items packaged by wrapping in a film or bag, which when heated fits tightly around the contained article.

SINGLE WOUND SHEETING (SWS)
A single layer of plastic film that is wound on a roll.

STIFFNESS
The capacity of a material to resist strain where stressed.

STRAIN
In tensile testing, the ratio of the elongation to the gage length of the test specimen, that is, the change in length per unit of original length. The term is also used in a broader sense to denote a dimensionless number that characterizes the change in dimensions of an object during a deformation or flow process.

STRESS
The force producing or tending to produce deformation divided by the area over which the force is applied. As generally defined in tensile testing (engineering stress), stress is the ratio of applied load to the original cross-sectional area.

SURFACE TENSION
A fluid in contact with a surface exhibits phenomena, due to mole cular attractions, which appears to arise from a tension in the surface of the fluid. It may be expressed as dynes per cm or as ergs per sq.cm.

SURGING
In extrusion, an unstable pressure build­up leading to variable output and waviness of the surface of the extrudate. In extreme cases, the flow of extrudate may even cease momentarily at intervals.

SYNERGISM
A phenomenon wherein the effect of a combination of two additives is greater than the effect that could be expected from the known performance of each additive used singly.

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T

TEAR RESISTANCE
Resistance of a material to a force acting to initiate and then propagate a failure at the edge of a test specimen.

TEAR PROPAGATION RESISTANCE
Resistance of a material to a force acting to propagate an initiated tear in the material.

TENSILE STRENGTH
The maximum tensile stress sustained by the specimen before failure in a tension test. Usually expressed in pounds per square inch or megapascals. The cross-sectional area used is that of the original specimen, not at the point of rupture.

THERMAL STABILITY
Ability of a polymer to maintain its initial physical and chemical properties at elevated temperature.

TITANIUM DIOXIDE
A white powder available in two crystalline forms, the anatas e and rutile types. Both are widely used as opacifying pigments in thermosets and thermoplastics.

TRANSITION SECTION
In an extruder, the section of the screw that contains material in both the solid and molten state.

TREATER
Equipment used for preparing resin-impregnated reinforcements including means for the delivery of a continuous web or strand to a resin tank, controlling the amount of resin pickup, drying and/or partially curing the resin, and rewinding the impregnated reinforcement. Term also used to refer to equipment and process used to render a surface of inert plastics like polyethylene more receptive to inks, adhesives, or coatings.

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U

ULTIMATE STRENGTH
The maximum stress developed in a tensile/compression specimen.

ULTRAVIOLET (UV) STABILIZER
Chemical agents which absorb or screen out radiation beyond the violet end of the spectrum of electromagnetic radiation. Such radiation has sufficient energy to initiate reactions leading to the degradation of many plastics. These agents are often combined with other additives, e.g., heat stabilizers and antioxidants, with which they act in synergistic fashion. UV stabilizers can be UV absorbers or radical scavengers.

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V

VAPOR BARRIER
A layer of material through which water vapor will not pass readily or at all.

VINYL ACETATE
A colorless liquid obtained by the reaction of ethylene and acetic anhydride in the presence of a catalyst. It is the monomer for polyvinyl acetate, and a comonomer and intermediate for many members of the vinyl plastics family.

VIRGIN MATERIAL
Any plastic compound or resin that has not been subjected to use or processing other than that required for its original manufacture.

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W

WATER VAPOR TRANSMISSION
The amount of water vapor passing through a given area and thickness of a plastic sheet or film in a given time, when the sheet or film is maintained at a constant temperature and when its faces are exposed to certain different relative humilities. The result is usually expressed as grams per 24 hours per square meter.

WELD LINE
A flaw on a molded plastic article or blown film caused by the incomplete fusion of two flow fronts which meet during the molding or extrusion operation.

WRINKLE
A surface imperfection in plastic films that has the appearance of a crease or wrinkle.

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X

Y

YELLOWNESS INDEX
A measure of the tendency of plastics to turn yellow upon long-term exposure to light or heat.

YIELD POINT
In tensile testing, yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress. This is the point at which permanent deformation of the stressed specimen begins to take place.

YIELD STRENGTH
The stress at the yield point of a specimen, usually expressed in pounds per square inch or megapascals.

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Z

ZINC OXIDE
An amorphous white or yellowish powder, used as a pigment in plastics. It is said to have the greatest ultra-violet light absorbing power of all commercially available pigments.

ZINC STEARATE
A white powder used as a lubricant and antioxidant synergist.

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