Plastic bags are created from plastic resins through polymerization , the process in which a chemical reaction links together monomers to form a polymer. A monomer consists of molecules from the same organic substance. When linked together, monomers create polymers , solid substances composed of repeating units. In the plastics industry, the term polymer is synonymous with plastic. Polyethylene and polypropylene are two plastic resins frequently used in the production of plastic bags.
Polyethylene (PE), a light, chemically resistant thermoplastic used in packaging and insulation, represents the most common plastic resin. The polymerization of ethylene, a flammable, gaseous hydrocarbon found in petroleum, results in the production of polyethylene resin. Polyethylene resins used in the production of plastic bags include low density, linear low density, and high density resins. Low Density polyethylene (LDPE) remains the most common and least expensive plastic bag material. The resin is created through the polymerization of ethylene at very high temperatures and pressures. LDPE maintains its durability, flexibility, water resistance, and clarity under low temperatures, and its low melting point make it ideal for heat sealing.
Linear Low Density polyethylene (LLDPE) is produced at lower temperatures and pressures than LDPE through copolymerization, the polymerization of two distinct monomers. Copolymerization results in LLDPE’s crystalline structure, which provides LLDPE greater stiffness and a higher melting point than LDPE. Although it is more difficult to process, LLDPE maintains greater tensile strength and a greater resistance to stress cracking than LDPE.
High Density polyethylene (HDPE) maintains greater strength, resistance, and stiffness than either LDPE or LLDPE.
Polypropylene (PP) is a light, durable thermoplastic often used in packaging. Polypropylene contains polymers of propylene, a colorless, combustible gas found in petroleum. Although more expensive to process than polyethylene, processing of polypropylene remains quite easy. In addition, polypropylene is denser, stiffer, and stronger than polyethylene, and maintains a high melting point.
In addition to various resins, we have numerous sizes, shapes, styles, and features from which to choose. Common plastic bag shapes include flat and gusseted.
Flat bags provide versatile plastic packaging for items of many shapes and sizes. Flat bags are heat sealed on either the side or the bottom of the bag. Bottom-sealed bags provide extra support for heavier items.
Gusseted bags contain folds or pleats called gussets, which allow the bag and the bag opening to expand in order to accommodate large or bulky items. Gusseted bags consist of either bottom sealed, side gusseted bags or side sealed, bottom gusseted bags.
STYLES
Countless flat bag and gusseted bag styles exist. Some of the most familiar bags include those used in retail, food storage, and waste removal storage.
Retail bags include all bags used to store and carry merchandise. Supermarkets, department stores, and specialty shops offer different bag styles for employee and customer convenience. A few of the most common retail bag styles include the following: T-shirt bags , often found in grocery stores, are gusseted bags containing long handles to accommodate shoppers; the material of choice is high density polyethylene.
Die-cut bags are flat bags containing a hole at the top of the bag for carrying; die-cut bags are common in retail settings and trade shows. Patch handle bags are flat bags with a die-cut handle reinforced by a heat-sealed patch for added strength; patch handle bags are useful in carrying heavy items like books.
Drawstring bags contain either plastic or cotton drawstrings inserted inside the rim of the bag. The drawstrings allow for easy closure and provide handles with which to carry the bags.
TERMS
Additive – substance added to a polymer to increase the effectiveness, but not the strength, of the polymer. Strength increase is achieved by adding a reinforcement. Examples of additives include flame-retardants, anti-static compounds, pigments, and lubricants.
Blow extrusion – common process of creating plastic bags in which compressed air fills an extruded plastic tube in order to enlarge and thin out the resin.
Copolymer – a polymer made up of two monomers in which each repeating unit in the chain consists of units of both monomers.
Crazing - very thin cracks in a polymeric material caused by chemicals or other agents, such as ultraviolet radiation.
Degree of polymerization – the length of the molecular or monomeric units in a polymer chain; this length determines the properties of the polymer.
EVA (ethylene vinyl acetate ) – copolymer produced through the chemical reaction of ethylene and vinyl acetate; often added to plastic resins to increase the strength of the resin in temperatures below freezing.
Glass transition temperature (Tg )–reflects the temperature when a substance changes from a hard, glass to a rubber consistency; polymers become weak at temperatures below their transition temperature.
Grade – polymers originating from the same chemical family and produced from the same company; however, they vary in weight, additives, reinforcements, and the manner in which they are processed.
Heat sealing – fusing together two or more thermoplastic films, such as low density polyethylene, through the application of heat and pressure.
Light-weighting – the process of decreasing the weight of plastic by using less resin, while retaining the plastic’s strength and effectiveness.
Melting point – the temperature at which a substance converts from a solid into a liquid.
Monomer – the most basic polymeric unit, usually a liquid or a gas, consisting of molecules from the same organic substance; chained together, momomers form solid polymers.
Plasticizer – a chemical additive added to plastic resins to increase the plastic’s flexibility.
Polymer – two or more monomers bonded together through a chemical reaction; each polymer consists of a chain of repeating monomers.
Polyethylene – light, chemically resistant thermoplastic used in packaging and insulation. Polyethylene contains polymers consisting of ethylene, a colorless, odorless organic gas found in petroleum; ripening fruit also produces ethylene in order to regulate growth.
Polypropylene – light, durable thermoplastic with a high melting point often used in packaging. Polypropylene contains polymers consisting of propylene, a colorless, combustible gas found in petroleum.
“Poly bags” – bags derives from polyethylene or polypropylene, although it usually refers to polyethylene bags.
Reinforcement – substance added to a polymer to increase the strength of the plastic. Examples include clay, mica, and glass fibers.
Resin – a class of polymers, or plastics, chemically different to naturally occurring resin, a sticky substance obtained from certain trees and plants. Examples of resins include polyethylene, polyurethane, and acrylics.
Stress cracking – cracking occurring as a result of mechanical stress. In most cases, tiny cracks caused from exposure of the plastic to chemicals or ultraviolet radiation are already present. When stress is applied to the plastic, the cracks enlarge and spread, creating a greater fracture.
Terpolymer - a polymer made up of three monomers in which each repeating unit in the chain consists of units of all three monomers.
Thermoforming – the process of applying heat, pressure, or suction to create plastic sheets according to certain sizes and shapes.
Thermoplastic - category of plastics that have the potential to soften and reform when heated, and harden again during cooling; during the process, the plastic’s physical makeup does not change.
Thermoset – category of plastics that can not be reformed upon reheating; thermosets remain permanently hard.
UVI (ultraviolet inhibitor ) – plastic additive that increases the plastic’s resistance to the harmful effects of ultraviolet radiation, which include fading of color and strength decrease.
Vapor corrosive inhibitor – thermoplastic coating or film that safeguards sensitive items from harsh environmental conditions through the release of a vapor that forms a protective layer on the surface of the thermoplastic.
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Does LDPE provide barrier to Ink such as that from Pentel N50 marker? Is it to use the marker on the bag that comes in contact on the other side?
Posted by: Ma. Luisa S. Cagalingan | October 24, 2008 at 07:25 PM