Antiblock. Substances added into plastic materials that retard or prevent ?lm layers from sticking together.
Blowup ratio. The ratio of ‘‘bubble’’ or ‘‘tube’’ diameter to extrusion die diameter. A primary consideration in determination of transverse directional (TD) shrinkage and a partial determinant of machine directional (MD) shrinkage.
Blown ?lm. Film that is fabricated by continuously pumping the polymer through a circular ori?ce (die). Once through the die, this polymer is then drawn upward and ?lled with air, creating a tube. The volume of air contained within the tube stretches the tube out to the desired width and, in conjunction with rate at which it is being pulled away from the die, the desired thickness is created.
Caliper. The thickness of a ?lm or bag, normally ex pressed in mils or as ‘‘gauge’’ (0.9 mil = 90 gauge = 0.0009 in.).
Cast ?lm. Film that is fabricated by continuously pumping the polymer through a straight slot ori?ce (die), then chilling this hot sheet of plastic immediately afterward by contact with a coiled roll. Film width is determined by the length of the slot. Thickness is determined by how fast the casting roll pulls the plastic away from the slot (or die).
Cling. The characteristic of stretch ?lm that makes it stick to itself or other clean objects.
Coef?cient of friction. A dimensionless numerical representation of the ease with which two objects or surfaces will slide against each other. Low numbers slide easily. Total range due to the test de?nition is 0–1.
Coextrusion. Fabrication of a multilayer ?lm by pumping the various materials through separate extruders and then merging the extrudates into a common die assembly. These die assemblies are constructed to maintain distinct material layers with fusion occurring at the boundaries, due to the pressures and temperature of the extrudates.
Copolymer. A plastic or polymeric whose chemical composition consists of more than one basic hydrocarbon type. The materials are chemically bonded and remain in combination, thus in?uencing the performance by their own distinct characteristics.
Crosslinked PE. A polyethylene that has been speci?cally treated by chemical or physical means to cause molecules to bond together in to strong chains.
Crystallization. The formation of distinct, ordered, and repeated molecular groups. Polyethylene forms varying amounts of crystalline structure depending on process conditions and original molecular makeup. The remaining portion is amorphous (which is the antonym of crystal line). Ice is crystalline; water is amorphous.
Dart drop. A method of measuring a ?lm’s impact strength or dynamic toughness. A hemispherical shaped, weighted ‘‘dart’’ is dropped onto a ?lm sample. The weight at which 50% of the samples are punctured is considered the WF50 reading.
Die. A device used in extrusion processes to shape the molten plastic. Circular dies are used for blown products, and slot dies are used for cast products.
Drawdown Ratio. Commonly expressed as the ratio of the width of the extrusion die ori?ce to the thickness of a particular ?lm. Frequently, however, and more correctly, it is a ratio involving die ori?ce, blowup ratio, and ?nal ?lm thickness as follows: die gap/(?lm gauge x blowup ratio).
Elastic recovery. The ability of a material to return to its original shape or size after having been deformed or subjected to strain.
Elmendorf tear. A testing method used to quantify a material’s resistance to tearing forces. Generally a sample is slit (initiated tear), then the force required to tear apart the sample is measured and given as the tear value.
Elongation at break. The strain or deformation required to break a sample. Generally expressed as a percentage by dividing the strain (distance) at which a sample breaks by the original sample length.
EVA. Abbreviation (acronym) for the copolymer ethylene vinyl acetate. Small percentages (1–18%) of vinyl acetate monomers are frequently polymerized with ethylene to provide greater extensibility or greater low temperature strength or to improve sealability.
Extrudate: Molten polymer.
Extrusion. The process of feeding, melting, and pump ing a material such that a desired shape or con?guration can be created. It is a continuous process and utilizes a device similar to a meat grinder.
Film yield. Generally expressed as the square inches of a ?lm (area) that weighs one pound if the ?lm is 1 mil (0.001 in.) thick. Coverage is on a per pound (lb 1) basis. This should not be confused with yield point, yield strength, or tensile yield, which are strength measurements. Polyethylene yields 30,000 in.2/lb at 1 mil.
Gauge. Used as a synonym for ?lm thickness or ?lm caliper. Sometimes expressed as 80 or 100 gauge, which equates to 0.8 or 1 mil, respectively, or 0.008 or 0.001 in., respectively.
Gauge band. A conformation irregularity found in rolls of material. A thick area in a ?lm will produce a raised or elevated ring in a ?nished roll of product. Conversely, a thin area will result in a soft ring in a ?nished roll.
Gloss. The shine or sparkle of a surface. In LDPE ?lm, gloss is described as the amount of light re?ected from the surface. Standard technique places a light source and a receiver at 451 angles from the surface. The number value produced is roughly the percentage of light re?ected from the source into the receiver.
Haze. The lack of ?lm transparency. It can be induced by process considerations, inherently due to molecular con?gurations or created by pigmentation. It is measured by determination of the percentages of light not trans mitted through a ?lm sample.
Homopolymer. a plastic resin or polymer whose entire chemical molecular structure is of a single hydrocarbon group. It is made by feeding a single monomer to the reactor.
Impact strength. The ability of an object or material to resist rapidly applied destructive forces. Refer to ‘‘Dart drop.’’
LDPE. Low density polyethylene—any polyethylene homopolymer whose density is between 0.913 and 0.925 g/cm3 (g/mL).
Modulus. Short for modulus of elasticity, which is a numerical value re?ecting a material’s resistance to de formation. A ?lm with a high modulus is hard to stretch or elongate.
Monomer. The incremental or elemental chemical elements before polymerization. Ethylene is a monomer.
Neckdown. The ‘‘narrowing’’ tendency of a ?lm when it is being stretched or pulled. Occurs when ?lm is stretched in the machine direction, resulting in decreased transverse direction width.
Opaque. Film that is impervious to light: 100% haze level.
Polyisobutylene (PIB). Tacki?er additive in blown ?lm that gives it cling.
PSI. Abbreviation for pounds per square inch (lb/in.2). A unifying statistical measurement by which various thick ness of material can be tested, equated, and compared without regard to actual specimen thickness.
Puncture Performance. The relative comparison of a material’s resistance to failures caused by penetration; For example, how easily your ?nger pokes through a ?lm sample.
PVC. Abbreviation for polyvinyl chloride. In ?lm form, it is used as a meat or produce wrap, as stretch ?lm, and as a high clarity shrink wrap for consumer or retail packaging.
Polymer. Means ‘‘many members.’’ A structure generated by the repetitious joining of many of the same elementary units. Natural polymers are cotton, wood, or protein. Synthetic polymers are polyethylene, polystyrene, nylon, and so on.
Polyole?n. Polymers of basic unsaturated hydrocarbon chains containing at least one double bond (e.g., polyethylene, polypropylene, and polybutene). Not PVC or PVA. Reactor. The equipment used to transform ethylene gas to polyethylene.
Stress Relaxation. Primarily it is the phenomenon of force decay as a function of time. It means that the ‘‘rubber band’’ tying a stretch load together exerts less force as time passes. This rate of decrease varies greatly with different polymers. PVC has an early decay that is much greater and more rapid than that of LDPE.
Stretchability. A combination of factors related to the ability of a material to be stretched or elongated—for example, how easily and to what extent a ?lm stretches and whether it will return to its original length.
Tack. An adjective used to describe a ?lm’s resistance to slide against itself or another surface, or its resistance to separation from itself. Similar terms are surface adhesion or cohesion, surface seal, wetting of ?lm layers, blocking, or cling.
Tacki?ers. Chemical substances added to increase the tack of the parent or base material.
Tear resistance. The resistance of a ?lm to be torn. This is quanti?ed by Elmendorf tear testing, and is measured as the force required to propagate an initated tear in the MD or TD direction.
Tensile Ultimate. This is a stress level, measured in psi, beyond which the specimen will break. It is the laboratory measured, maximum stress (applied force) that the material will withstand. Tensile indicates that test is conducted with a pulling or tensioning type of loading.
Tensile yield. This is a stress level, measured in psi, beyond which permanent deformation occurs. ‘‘Tensile’’ indicates that the specimen is pulled or ‘‘tensioned’’ rather than compressed. Up to this stress or applied force level, a release of the force will result in the specimen returning to its original size. Pulling with a higher force will result in an elongated sample, even after the force is removed.
Tint. A slight coloring that allows light transmission and relatively good clarity.
Toughness. An overall strength measurement that takes into account both the amount of pull and the amount of elongation a sample can withstand. A tough material will resist breakage by both resisting force and by elongating. A brittle material (e.g., glass) will resist force, but will break instead of deforming; hence it has low toughness. LDPE, which is plastic or tough, both resists and elongates—hence is tougher than glass. The rate of force application has signi?cant effects on toughness, so comparisons must include testing data.
Ultimate Strength. Same as ‘‘Tensile ultimate.”
Wide web. Wrapping with a roll of ?lm that is approximately the same size as the load.
Wrap. One revolution of a machine turntable.
Yield stretch. Same as ‘‘Tensile yield.’’
Zippering. Lack of resistance to tear propagation of an initiated TD cut or tear. Once the ?lm has been torn or cut, the resulting slit rapidly opens and completes a full web break.
*Note: This information has been compiled from a variety of electronic sources including The Wiley Encyclopedia of Packaging Terminology, Third Edition. Discount Packaging Depot is not responsible for any errors, omissions or inaccuracies.