Plastic Bags

Chapter One – What are Plastic Bags?

A plastic bag is an unwoven single piece poly fabricated container for storing, carrying, and packaging of a wide variety of items and materials. Though they are very thin and easy to carry, plastic bags are strong, waterproof, and hold their shape under normal use. Plastic bags are made from a variety of plastic polymers with polyethylene and polypropylene being the most common.

The varieties and types of plastic bags are influenced by the materials used to manufacture them as well as their shape, size, color, types of closures, and style. Polyethylene plastic bags vary by their density between high, low, medium, and linear, while polypropylene bags are crystal clear, have exceptional tensile strength, and are heat and chemical resistant.

Polymers used to manufacture plastic bags come from cracked natural gas or refined crude oil that is pressurized and superheated to isolate the pure polyethylene chains that are polymerized to produce resin pellets. Variations in the amount of pressure and heat produces resin pellets of different densities to create the various styles and types of plastic bags.

The key to the production of the wide range of plastic bags is the types of resin pellets that are chosen for the extrusion process since there are differences in the characteristics and properties of the various types of plastics. Although all plastic bag material may seem the same, each type has its own strength, thickness, and resilience.

Chapter Two – How are Plastic Bags Made?

Plastic bags begin as resin pellets of pure plastic that have been extracted from crude oil or cracked natural gas. In the initial steps of the manufacture of plastics, the first stop is the cracking plant that turns naphtha, a crude oil product, or ethane, a natural gas liquid, into ethylene, a hydrocarbon used to produce several chemical products. Of the two hydrocarbons that are put through the cracking process, ethane is the least expensive that is used to produce ethylene.

Ethylene from the cracking process is a gas that has to be converted by pressure and catalysts into polyethylene resin, a process known as polymerization. The creation of plastics from ethylene is caused by a chemical reaction between molecules that react together to form polymer chains, which are the central part of all plastics. Plastics engineers manipulate and configure the polymer chains to produce specific physical properties in plastic resins.

Polyethylene, which has a long chain of ethylene monomers, is the polymer that is most commonly used to produce plastic bags. It is the result of the reaction of ethylene molecules in the presence of a catalyst that breaks the double bond of the carbon molecules.

Another form of plastic that is extracted from crude oil is polypropylene, which was first introduced in 1951 and perfected for commercial and industrial use in 1957. Like polyethylene, polypropylene is made from crude oil from which the monomer propylene is extracted. Using chain growth polymerization, the propylene monomer is transformed into the polymer polypropylene.

In the production of polypropylene, propylene monomers are strung together to form a solid plastic material. To be able to use polypropylene to make usable plastic bags, it is mixed with plasticizers, stabilizers, and fillers, which are mixed with the molten polypro and cooled to form pellets or bricks.

How Plastic Bags are Made

Adding Color

The coloring of plastics is completed in several different ways and is determined by the needs of the customer and how the plastic will be used. Although there are several methods for coloring plastic, the three that are used the most are masterbatch, cube blends, and precolored resin.

Masterbatches

Masterbatches, also known as concentrates, is the most common method for the colorization of plastics. The process involves the mixing of concentrated pigments with the polymer resin and heating them to ensure that they mix properly.

Cube Blends

Cube blend,, referred to as salt and pepper mixes, involves mixing a masterbatch blend with polymers and provides users with the ability to precision control the final output. Since the polymer remains solid in the process, it retains its original properties.

Precolored

Precoloring involves polymerizing the pigments into the plastic resin. Precolored resins are easy to use and quickly match the desired performance rate.

Extrusion

The extrusion process is used to create the plastic film that will form plastic bags. It begins by superheating plastic resin pellets by heating them to over 500o F to form a consistent molten liquid. The heated resin has to be pliable enough to fit through the circular die at the end of the extruder.

Hopper

The heating of the resin to the appropriate melting point is critical to the shaping and configuring of plastic bags. The initial insertion of the resin pellets into the extruder is through its hopper that is located at the beginning of the barrel above the auger. The first hoppers were simple cone shaped openings that were manually filled with resin pellets.

Modern hoppers have advanced with the times and come in a wide variety of shapes with different technologically advanced feeding mechanisms that remove the need for manually loading the hopper.

Extruder

The extruder is the heating mechanism for the plastic bag producing process. It consists of a long tube referred to as the barrel in which is located a helical screw known as the auger. When the resin pellets fall from the hopper into the barrel, the auger slowly moves them along toward the die. Located along the barrel, on either side of it, are heating elements that slowly melt the resin pellets to form the molten liquid to be passed through the die.

At the end of the barrel is a round circular die through which the molten plastic is forced before entering the cooling tower. To ensure the purity of the molten resin, a screening or breaker plate is located before the die to remove any possible contaminants.

Die

As the molten plastic resin moves along the extruder barrel, pressure is exerted on it to force it through the screen or breaker plate and die. The breaker plate serves the functions of removing contaminants and protecting the die from the exceptionally high pressure created by the extrusion process.

The die can feed into the long cooling cylinder vertically or horizontally. The cooling cylinder is where the thickness of the bags is determined and where the bags are shaped. It is the aspect of the process that ensures that the bags are seamless and flawless without holes. Speed at which the molten material enters the cooling tower determines the thicknesses of the bags.

Cooling Tower

As the molten plastic resin leaves the extruder, it is forced through the die and brake plate into the bottom of the cooling tower cylinder. To move the molten material into the cooling tower, it is filled with compressed air that forms the shape and thickness of the bag. The process of inflating the molten plastic resin is very similar to inflating a gigantic balloon.

Slowly the tube of plastic is filled with air and moves into the cooling tower. During the process, the plastic material cools and takes on the dimensions of the completed plastic bags. The rate at which the movement takes place determines the thickness of the plastic film. When it expands rapidly, the plastic bags will have thin walls and be very light. If the molten plastic expands slowly and gently, the final bags will have thick walls and be more durable.

The tube of newly formed plastic film cools as it rises toward the nip rollers that tightly press the sides of the plastic bag together. The film of plastic stretches axially and radially creating a tube many times larger than the die ring. The nip rollers squeeze out any air bubbles, blisters, or other imperfections.

The diameter of the tube depends on the type of plastic being processed. The height of the cooling tower can be 25 to 35 feet and is another factor in determining the speed of the cooling process.

Gussets

Gussets are folds, creases, indentations, or expansions that are placed in the side of a plastic bag that increases its volume but makes it possible for the bag to lay flat when being stored. In the cooling tower before the bag material flattens against the walls of the tower, boards in the walls indent the fully inflated plastic as it moves toward the nip rollers.

The crease that is formed by the boards is in the form of two overlapping rectangles that are pressed together by the nip rollers as the bag material leaves the cooling tower. This particular type of gusset is referred to as a side gusset and is very common for plastic bags especially sturdier and stronger ones.

Nip Rollers

Nip rollers are located at the top of the cooling tower and provide traction, tension, and winding for placing the completed bag material onto rollers. As the inflated plastic film cools, it crystallizes at the frost line where it is cool enough to be flattened and transformed into lay flat or collapsed tubing. The nip rollers supply the force to pull the cooled plastic film upward such that it can be formed into rolls for further processing.

The purpose of nip rollers is to supply pressure to produce a solid and secure plastic bag winding. They control the hardness of the roll by removing air from the web of plastic from the cooling tower. In the design of nip rollers, the challenge is to make the winding sufficient for the nip to remove excess air and wind a hard firm roll but not damage the plastic film web.

Preventative Measures

During the production process, polyethylene material may be treated with additives such as:

• Anti-block – Blocking is when polyethylene sticks together. The anti-block additive roughens the surface of the plastic at a microscopic level making it easier to separate the bags.
• Anti-static – Much like various static sensitive applications, anti-static additives lower surface resistivity and static decay.
• Ultraviolet Inhibitor (UVI) – UV inhibitors prevent plastic bags from fading or degrading when exposed to the sun. They protect written and printed images on the bag from being damaged.

Complete Process for Plastic Bag Production

Thickness of Plastic Bags

The thicknesses, or gauges, of plastic bags are expressed in mils with one mil being one one thousandth of an inch, 0.001”. Mils are a metric unit for measuring thicknesses and are expressed as microns (µ), where 0.05 mm is equal to 50 µ. Mils are an English unit for thickness. Thicknesses are described as 2.0 mils or 0.002 inch or 50 µ.

The determination of the gauge of a plastic bag is based on three factors:

• Contents
• Protection
• Length of Use

Contents refers to what will be placed in the bag, while protection is in regard to how important it is to keep the contents safe. Length of use is the amount of time that the bag will be utilized.

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Chapter Three – Plastic Bag Material

Plastic bags are made from polyethylene, which begins as ethylene extracted from natural gas or oil. It is specially treated to become a polymer. Polyethylene is changed to fit how it will be used and the different types of bags to be produced.

Plastic material comes from polymers, which are easy to create and made from monomers. Polyethylene is a chain of carbon atoms that are mixed and bonded to hydrogen atoms. The structure of plastic makes it easy to change into various densities and shapes.

Plastic Bag Materials

High Density Polyethylene (HDPE)

HDPE is strong and has high tensile strength. It can withstand temperatures of 120° C or 248° F and is resistant to various solvents. The linear structure of the molecular chain creates a strong material capable of withstanding continual use.

Low Density Polyethylene (LDPE)

LDPE plastics are thick, flexible, and resistant to punctures and tears. Their thickness is measured in mils, or one millionth of an inch. The higher mil count means a thicker bag. Mil counts range from 0.9 mil for kitchen bags to an extra-thick 6.0 mil for contractor bags.

Linear Low Density Polyethylene (LLDPE)

LLDPE bags are similar to LDPE bags but have greater strength and puncture resistance. They are used for lighter gauged gusseted bags. LLDPE bags cost less than LDPE bags and are more environmentally friendly. They are made by copolymerizing ethylene with butene, hexene, and octene.

Medium Density Polyethylene (MDPE)

MDPE is stronger and harder but is still flexible enough for easy processing, even though it is much thicker and can be used for handle and loop bags. The density of MDPE makes it shock and drop resistant as well as stress-crack resistant.

Metallocene (mPE)

Metallocene is not a monomer like ethylene, propylene, or vinyl acetate. It is made of particles of charged metal ions combined with carbon atoms. It has the same density standards as polyethylene with lower densities being softer, while higher densities are more resilient. In the manufacturing process, it is blended with LDPE and LLDPE to make an enhanced poly film that seals rapidly and stronger. The downside of metallocene is its expense, which is caused by its being mixed with polyethylene.

Vinyl

Vinyl is made by combining chlorine and ethylene to produce ethylene dichloride, or EDC. The process for forming vinyl EDC involves creating vinyl chloride monomer (VCM), which is polymerized and turned into a PVC resin powder. The addition of modifiers and additives alter its physical properties. Vinyl is flexible, waterproof, durable, flame retardant, resistant to wear and tear, UV radiation, and stress. Its positive properties make it ideal for storing and preserving precious items.

Polypropylene

Polypropylene is made of long chains of propylene molecules, which provide flexibility to the material but allow it to keep its strength. It is resistant to fatigue and does not sustain damage from repeated use. Polypropylene has a low density, which makes it lightweight and yet have a high strength to weight ratio. Bags made of polypropylene have the same use as any of the other plastic bags. Its most common use is for tote and shopping bags.

Chapter Four – Types of Plastic Bags

There is a limitless number of plastic bag types with the number growing every year as new uses for them are developed. They can be flat, gusseted, anti-static, postal approved, protective hazard bags, and shrinkable.

Types of Plastic Bags

Flat Plastic Bags

Flat plastic bags are wide and long without depth. They can be heat sealed, tied, stapled, or taped shut and are a convenient way for storing various items, such as food and electronics.

Markable Plastic Bags

Markable bags have a white block for labeling the contents of the bag. They are convenient for storing small items for easy access.

Resealable Plastic Bags

Resealable bags have a means for closing the bag that can include zip track or some form of slider zipper. Large versions can be used for storage, while smaller ones can be used for packing lunches.

Gusseted Plastic Bags

Gusseted plastic bags have tucked in pleats on the sides so that they can expand when filled. Their gussets increase their volume and give them greater capacity without stretching or changing the shape of the bag. Gusseted plastic bags are similar to flat plastic bags in that they can be heat sealed, tied, stapled, or taped. They are sometimes referred to as flat bottom plastic bags due to the bottom that forms when the gussets are expanded. The special design of gusseted plastic bags makes it possible to use them for any type of application.

Anti-Static Plastic Bags

Anti-static plastic bags are manufactured with a special anti-static additive design to block static electrical charges and are designed for storage of electronic components. They come in various colors and plastic material densities.

Postal Approved Plastic Bags

Bags that have been postal approved must be able to work with the United Postal Services Scanners so that the bag will not delay the mail processing line. Manufacturers produce postal bags to protect the contents as well as meet USPS standards.

Biohazard Plastic Bags

The main purpose of biohazard bags is to protect workers who are required to handle hazardous materials. Biohazard bags are clearly marked with the biohazard symbol and can be clear, white, red, yellow, black, or blue depending on the design. They must be in compliance with CDC, DOT, EPA, and OSHA standards for handling medical waste. Biohazard bags are made of strong plastic material that cannot be punctured or torn.

Cleanroom Plastic Bags

Cleanroom requirements are very stringent and have to be met in order for the cleanroom to maintain its ISO classification. Bags for cleanrooms are designed to hold sensitive instruments and medical devices. The production of cleanroom bags requires the use of pure unused resin and not recycled plastics and must meet ISO 9 standards.

Wicketed Plastic Bags

Wicketed bags are arranged on top of each other on a wire wicket and are held in place by rubber washers or a clipboard header. They have a 1.5 inch lip and starter notches. Wicketed bags can be bottom gusseted, have butterfly vents, and perforations with resealable tape. The stacked wicketed design can hold over 200 bags and is customizable.

Shrink Plastic Bags

Shrink bags are designed to be used with shrink wrap machines. Items to be stored, shipped, or protected are placed in the bag and transferred to the shrink wrap machine that heats the fabric until it forms a tight seal around the contents. It is an excellent method for preserving sensitive items.

Vapor Corrosion Inhibitor Bags (VCI)

VCI bags are designed to protect ferrous metals against corrosion. They are normally clear to allow for inspection of the metal contents. VCI plastic bags create a barrier for the metal surface by dehumidification, which prevents exposure to moisture. Yellow VCI bags are used for ferrous metals such as iron, steel, and cast iron, while blue and clear VCI bags are designed for non-ferrous metals such as aluminum, copper, and brass. Metal parts placed in VCI bags are dry and corrosion free.

Tamper Evident Plastic Bags

Content for a tamper evident bag is loaded at the bottom and heat sealed. Once the contents are inserted, the bag can only be opened by cutting or tearing a perforated tear strip. The purpose of tamper evident bags is to ensure the security of the contents and show someone receiving the bag whether it has been opened.

Chapter Five – Plastic Bag Regulations

Over the last several years, plastic bags have received a great deal of negative press with some countries and states having enacted legislation regarding their use. The main emphasis, as of the moment, is recycling and avoiding throwing plastic bags into landfills.

State legislatures have enacted measures to reduce the presence of plastic bags at retail locations. The purpose is to control their effect on the environment which includes, oceans, rivers, lakes, forests, and wildlife habitats. A secondary aspect of the concern is relieving the pressure on waste management and landfills.

The present measures being considered are broad recycling programs, complete bans, or bans with fees attached to discourage plastic bag use.

Of the 50 states, 18 have enacted legislation that discourages or prohibits the use of plastic bags with California, Maine, and Delaware having the most regulations. The graphic below indicates states with highly restrictive legislation and bans as well as states that have not taken any action.

Over 95 pieces of legislation were enacted by various states in 2019 to regulate, control, and manage the use of plastic bags.

The main focus of the majority of states is recycling as an environmental initiative.

Aside from the states that have regulations regarding the use of plastic bags, several cities have bans on plastic bags or fees and bans. They include Boston, Chicago, Los Angeles, San Francisco, Seattle, Boulder, CO, Montgomery County, MD, New York, Portland, MA, and Washington, D. C.

Conclusion

• A plastic bag is an unwoven single piece poly fabricated receptacle for storing, carrying, packaging of a wide variety of items.
• A commonly used polymer for the fabrication of plastic bags is polyethylene, which is a long chain of ethylene monomers, the primary ingredient for a polymerization reaction.
• Plastic bags have become an essential part of marketing, product protection, and packaging.
• There is a limitless number of plastic bags with the number growing every year as new uses for them are developed.

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