Plastic Bags

Specific materials for plastic bags abound, but polyethylene bags, polypropylene bags and vinyl bags are among the most popular and find great use as anything from medical plastic bags to plastic shopping bags. Beyond the basic material construction, plastic bags as a grouping may have little else in common due to the extreme variety in not only material choice, but also size, color, shape, closure and style which exhibit extreme variation. The selection often depends on the intended use of the bag. Clear plastic bags, for example, are a popular choice for plastic merchandise bags as they allow consumers to preview items before purchase. Resealable plastic bags and zip lock bags are also purpose driven as they provide users with a reliable closing and the option to reuse and reseal bags for extended use. While many of these factors are determined in manufacturing, post-production options such as printed plastic bags ensure the perfect fit, both functionally and aesthetically, for any application for all industrial, home or commercial locations.

Rigid, flexible, durable or tear away, plastic bags are a major part of the daily operations of many industrial and commercial outfits. Bags are practical and efficient for storage of parts, components and materials as they provide a safe and consistent barrier against moisture and other environmental contaminants. While some plastics are more susceptible to exposure to the elements, such as heat or UV light, others will last for extended periods of time with little to no change allowing their use in sterile and hermetic packaging of food and medical supplies as well as chemicals and biological materials. Pharmaceutical, retail, storage, sanitation, automotive, construction, electronics and chemical industries likewise take advantage of the many benefits afforded by plastic bags at a relatively low cost as compared to metal containment options. Big industry is not exclusive in its use of plastic bags which are also common household and office space goods. While the context and size may vary, plastic bags serve the same general purpose of storage, convenience and protection.

Although the specifics are not always listed, plastic bags employ a large number of different materials to accommodate the growing needs of the aforementioned applications. The word ‘plastic' simply refers to the use of materials derived from polymer resins. These are long chains of molecules that consist of several smaller monomers held together by covalent bonds formed during calculated chemical reactions. This general description encompasses a diverse field of precise plastic compositions that may be synthetic, natural or organic. Polyethylene and polypropylene are two specific polymers that are commonly used in the production of plastic bags. Both thermoplastics, they provide the durable yet lightweight high quality bag used in a number of industries. Vinyl bags are another popular option made of the polymer known as polyvinyl chloride. More rugged than polyethylene and polypropylene, this particular type of bag is well suited to harsher applications such as parts storage. Such use also requires a reliable seal, the design of which is most often a secondary step in the manufacturing of plastic bags. Before seams are heat-sealed, stitched or glued in place, the film encasement of the bag is formed through casting, winding, sheet extrusion or blown film extrusion with the latter being the most common. In this process polymer resins are melted into a viscous fluid which is then extruded through a die, producing a tube of plastic. Cool air is forced through the center of the die at uniform pressure to create a bubble of even thickness. Additional treatments may be applied after the material is cooled.

While material and necessary treatments are among the most important concerns to consider before selecting an application specific plastic bag, there are several other factors that come into play. The size of the bag must be suited to its potential use as bags can range from a capacity of several cubic inches to several hundred. The area of the bag is determined by length multiplied by height multiplied by diameter or width. It is important to look at specific dimensions and not just the total volume especially when dealing with irregularly shaped items. The style and closure type should also be considered as well as the overall cost. With increasing concerns for the environment, the recylability of a plastic should also be considered. In the United States alone, four out of five grocery bags used are plastic, but less than 0.5% of such consumer plastic is recycled. Reusable plastic bags made of thicker and recyclable plastics are gaining popularity as is the development of biodegradable polymer films in many applications. The variability of plastic bags gives consumers much to consider prior to use and purchasing, but also ensures that the proper container or carrier will be available no matter the task at hand.

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Plastic Bag Types

• Anti-static bags contain an anti-static additive that disseminates static electrical charges. Anti-static bags are ideal for short-term storage.
  
• Baggies are sacks or pouches made of thin and most often flexible polymer materials commonly generalized as plastics.
  
• Clear plastic bags are sacks or pouches made with transparent polymers such as low-density polyethylene and polypropylene.
• 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.
  
• 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.
  
• Electrically conductive bags, which provide exterior protection from static charges, are made from carbon-impregnated polyethylene.
  
• Flap lock bags, commonly known as "sandwich" bags, have a lip that is folded back with side seals.
  
• Flat bags provide versatile plastic packaging for items of many shapes and sizes that are heat-sealed on either the side or the bottom of the bag. Bottom-sealed bags provide extra support for heavier items.
  
• Food storage bags are bags made from polyethylene resin to maintain the freshness of the food items and are often sealed with slider grips or zippers. Food storage bags, which include sandwich bags, freezer bags and produce bags, increase the life of perishable foods.
  
• 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 include bottom-sealed, side-gusseted bags and side-sealed, bottom-gusseted bags.
  
• Header bags are side-weld bags that are loaded and sealed from the bottom. Header bags have a continuous seal along the top, which is 2-3" below the fold, and a hang hole.
  
• Medical plastic bags are used for a number of practical applications in hospitals, doctors' offices, pharmacies, medical labs and other health-care facilities for the storage or containment or objects and materials.
• Patch handle bags are flat bags with a die-cut handle reinforced by a heat-sealed patch for added strength.
  
• Plastic merchandise bags are sacks or pouches used in commercial and retail settings to store and protect goods and other items.
• Plastic shopping bags are a cultural convenience product designed for the transportation of purchased goods from the store to home or other location.
• Poly bags encompass a wide range of sacks, pouches and flexible containment systems made of organic, synthetic or processed plastic materials known as polymers.
• Polyethylene bags are waterproof, translucent plastic receptacles employed in a number of industrial, commercial and domestic settings for the protection, storage or transport of goods.
• Polypropylene bags are sacks or pouches made of completely transparent, strong, rigid material that doesn't stretch but provides a good vapor and moisture barrier.
• Printed plastic bags are mass manufactured or custom made storage containers to which permanent labels and designs are added.
• Resealable plastic bags include all baggies designed with a closure mechanism that can open and be re-fastened for continued product containment. 
• Retail bags include all bags used to store and carry merchandise.
  
• Side weld bags are sealed on the side and have no bottom seal.
  
• Vinyl bags are sacks or pouches made of a specific thermoplastic polymer known as polyvinyl chloride, or PVC.
• Waste removal bags include all trash can liners and bags, as well as bags used to remove hazardous waste. Waste removal bags are often manufactured from high density polyethylene resin because of the strength and cost-effectiveness of the resin, but may also be made out of linear low density resin because of its stretching ability.
  
• Zip lock bags are plastic sacks or pouches that have an interlocking or slide lock closure mechanism.

Plastic Bag Terms

Additive - Substance added to a polymer to increase the effectiveness, but not the strength, of the polymer. Examples of additives include flame-retardants, anti-static compounds, pigments and lubricants.
 
Alloy - Also referred to as a "blend" or "hybrid," it is two chemically dissimilar polymers bonded together to form a new substance. However, each polymeric unit is representative of only one monomer.
 
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.

Ethylene Vinyl Acetate (EVA) - A copolymer produced through the chemical reaction of ethylene and vinyl acetate. EVA is often added to plastic resins to increase the strength of the resin in temperatures below freezing.

Gauge - A term referring to the thickness of the material. The smaller the gauge number, the thinner the material.

Glass Transition Temperature (Tg) - Term that 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.
 
High Density Polyethylene (HDPE) - A plastic material whose thickness ranges from .941 -.965 g/cm³. HDPE is more expensive to process, but maintains greater strength, resistance and stiffness than either LDPE or LLDPE.
 
Light-Weighting - The process of decreasing the weight of plastic by using less resin, while retaining the strength and effectiveness of the plastic.
 
Linear Low Density Polyethylene (LLDPE) - A plastic material that is produced at lower temperatures and pressures than LDPE through copolymerization, resulting in a crystalline structure responsible for 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.
 
Lip - The longer side of the bag that allows the bag to be opened more easily.
 
Low Density Polyethylene (LDPE) - The most common and least expensive plastic bag material that maintains a density of .910-.925 g/cm³. LDPE maintains its durability, flexibility, water resistance and clarity under low temperatures, and its low melting point makes it ideal for heat sealing.
 
Melting Point - The temperature at which a substance converts from a solid into a liquid.
 
Metallocene - A puncture resistant material that is thinner and stronger than LDPE.
 
Monomer - The most basic polymeric unit, usually a liquid or a gas, consisting of molecules from the same organic substance. When chained together, monomers form solid polymers.
 
Plasticizer - A chemical added to plastic resins to increase the flexibility of the plastic.
 
Polyethylene (PE) - The most common plastic resin, it is a light, chemically resistant thermoplastic used in packaging and insulation. PE resins used in the production of plastic bags include low density, linear low density and high density resins.

Polymer - Two or more monomers bonded together through a chemical reaction. Each polymer consists of a chain of repeating monomers.
 
Polypropylene (PP) - Light, durable thermoplastic with a high melting point that is often used in packaging. PP contains polymers consisting of propylene, a colorless, combustible gas found in petroleum.
 
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.
 
Slip - An additive that aids in decreasing the slippage of stacked packaged goods and prevents bags from sticking together.
 
Star Seal - A bottom seal for liners that combines four sections into a star design. Star seals are the strongest seals, and they maximize carrying capacity.
 
Stress Cracking - Cracking that occurs 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.
 
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 specified sizes and shapes.
 
Thermoplastic - Category of plastics that has the potential to soften and reform when heated and to harden again during cooling. During the process, the physical makeup of the plastic does not change.
 
Thermoset - A category of plastics that cannot be reformed upon reheating. Thermosets remain permanently hard.
 
Ultraviolet Inhibitor (UVI) - A plastic additive that increases the resistance of the plastic to the harmful effects of ultraviolet radiation, such as 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.