Plastic Bags

Chapter One – What are Plastic Bags?

A plastic bag is an unwoven single piece poly fabricated receptacle for storing, carrying, 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. They 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 material, shape, size, color, closure, and style. Polyethylene plastic bags vary by their density between high, low, medium, and linear, while polypropylene bags have exceptional chemical strength and resistance.

Plastic bags are manufactured using extrusion, which pushes melted plastic into a die that determines the final thickness of the bag.

Chapter Two – How are Plastic Bags Made?

The production of plastic bags begins with a polymer. A commonly used polymer for the fabricating plastic bags is polyethylene, which is a long chain of ethylene monomers, the primary ingredient for the polymerization reaction. Polyethylene involves the reaction of ethylene molecules in the presence of a catalyst to break the double bond of the carbon molecules.

Another polymer used in the process is polypropylene, which is made by breaking the double bond of carbon in a propylene molecule. The third carbon atom of propylene enhances the complexity of polypropylene and gives it its unique properties.

The raw material for making plastic comes from oil or gas production. To get the pure polyethylene, oil or gas is superheated and pressurized, which separates the polyethylene chain.

How Plastic Bags are Made

The color for plastic bags is added during the production of the resin pellets, which arrives at the production facility pre-colored.

Extrusion

The polymer material extracted from oil or gas is formed into pellets, or nurdles, for the extrusion process.

Hopper

The resin or nurdles are poured into a hopper that feeds them into the extruder.

Extruder

The extruder uses a screw to move the resin toward the die. As it moves down the extruder barrel, it is heated to approximately 500o F. At the end of the barrel, before the die, is a screen plate to remove any possible contaminants.

Die

As the molten plastic exits the extruder and passes through the breaker plate, it enters the die that can be positioned horizontally or vertically.

Cooling Tower

Air is forced through the center of the die into the molten plastic to blow it up like a balloon. As the molten plastic is filled with air, it continues upward and expands in the cooling tower. The tube of film cools as it rises toward the nip rollers. The film of plastic stretches axially and radially creating a tube many times larger than the die ring. 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 to enable the plastic film sufficient time to cool.

Gussets

Gussets, triangular shapes in the side of the bag, which increases a plastic bag’s volume but allows it to lay flat when being stored, are added by gusset boards before the bag is flattened in the cooling tower. The boards indent the sides of the fully inflated film. Gussets allow the bag to expand out for bulky, big, or square items.

Nip Rollers

As the tubular shape reaches the top of the cooling tower, guides flatten it into a laid flat form. If it has gussets, gusset boards indent the sides of the tube before it is flattened. Nip rollers at the top of the cooling tower move the solidified and cooled poly film out of the cooling tower.

The complete process for plastic or poly bag manufacturing is explained in the video below from the production of the polymer from gas or oil to the final product.

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 thickness, or gauge, of plastic bags is measured in millimeters, which are referred to as mils. One mil is one one thousandth of an inch, 0.001”. The chart below, provided by Multi-Pak, Inc., is a list of some of the standard thicknesses, gauges.

Mil Thickness	Thousandth of an Inch Thickness	Best For...
1/2 mil	.0005"	Short-term use for lightweight products. A grocery store bag is ½ mil. A dry cleaning bag is ¾ mil.
3/4 mil	.00075"
1 mil	.001"	Storing, displaying, and shipping everyday items such as food or clothes. A bread bag is 1.5 mils thick. A trade show bag or retail boutique bag is usually 2 mils.
1.5 mils	.0015"
2 mils	.002"
3 mils	.003"	Providing scuff protection and tear resistance during shipping, especially for heavy or abrasive items. Industrial pallet covers are 3 mils.
4 mils	.004"
5 mils	.005"

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.

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 120o C or 248o 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 bags have tucked in pleats on the sides so that they can expand when filled. The gussets increase the volume of the bag for greater capacity without stretching or changing the shape of the bag.

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.