Plastic Bags: Types, Uses and Production

Chapter One – What are Plastic Bags?

Plastic bags are versatile, one-piece containers crafted from polymer-based materials, designed for holding, transporting, and packaging numerous items. Though they may appear delicate due to their thin design, plastic bags are actually durable, waterproof, and maintain their form under normal usage. They are predominantly fabricated using diverse plastic polymers, with polyethylene and polypropylene leading as the most prevalent choices.

The diversity among plastic bags stems from factors such as the materials employed, their dimensions, hues, closure mechanisms, and styling. Polyethylene bags vary in density—ranging from high-density, low-density, medium, to linear. Conversely, polypropylene bags are distinguished by their transparency, excellent tensile strength, as well as resistance to heat and chemicals.

The polymers used to create plastic bags originate from processed natural gas or refined crude oil. These materials are subjected to high pressure and heat to isolate pure polyethylene chains, which are subsequently polymerized to produce resin pellets. Adjustments in heat and pressure during this stage yield resin pellets of varying densities, enabling the fabrication of different styles and types of plastic bags.

The creation of various plastic bag types is heavily reliant on the choice of specific resin pellets for the extrusion process. Each type of plastic exhibits distinct features and qualities, meaning that while plastic bags might seem similar at first glance, each kind possesses unique strengths, thickness, and durability.

Chapter Two – How are Plastic Bags Made?

Plastic bags start as resin pellets made from pure plastic, which is extracted from crude oil or cracked natural gas. The manufacturing process begins at the cracking plant, where naphtha (a product of crude oil) or ethane (a natural gas liquid) is converted into ethylene, a hydrocarbon used to produce various chemical products. Among these hydrocarbons, ethane is the less expensive option for producing ethylene.

Ethylene, which is produced as a gas during the cracking process, must be transformed into polyethylene resin through a process called polymerization. This involves applying pressure and using catalysts to induce a chemical reaction between ethylene molecules, forming polymer chains that are fundamental to all plastics. Plastics engineers adjust and configure these polymer chains to achieve specific physical properties in the plastic resins.

Polyethylene, a polymer consisting of long chains of ethylene monomers, is the most commonly used material for producing plastic bags. It results from the reaction of ethylene molecules in the presence of a catalyst that breaks the double bonds between carbon atoms.

Polypropylene is another plastic derived from crude oil, first introduced in 1951 and refined for commercial and industrial applications by 1957. Like polyethylene, polypropylene is produced from crude oil, from which the monomer propylene is extracted. Through a process called chain growth polymerization, propylene monomers are transformed into the polymer polypropylene.

During polypropylene production, propylene monomers are linked together to create a solid plastic material. To produce functional plastic bags, this polymer is blended with plasticizers, stabilizers, and fillers. The mixture is then melted, formed into pellets or bricks, and cooled for use in various applications.

How Plastic Bags are Made

Adding Color

Plastic coloring can be achieved through various methods, depending on customer requirements and the intended use of the plastic. The most common coloring techniques include masterbatch, cube blends, and precolored resin.

Masterbatches

Masterbatches, or color concentrates, are the most widely used method for coloring plastics. This process involves blending concentrated pigments with polymer resin and heating the mixture to ensure a thorough integration of color.

Cube Blends

Cube blending, also known as salt and pepper mixing, combines a masterbatch with polymers. This method allows for precise control over the final color distribution while maintaining the original properties of the polymer, as it remains solid during the process.

Precolored

Precoloring incorporates pigments directly into the plastic resin during polymerization. Precolored resins are convenient and quickly achieve the desired color and performance characteristics.

Extrusion

The extrusion process is employed to create the plastic film used in plastic bags. It starts with heating plastic resin pellets to over 500°F, turning them into a uniform molten liquid. The resin must be sufficiently pliable to pass through the circular die at the end of the extruder, forming the plastic film.

Hopper

Heating the resin to the correct melting point is crucial for shaping and forming plastic bags. Resin pellets are initially introduced into the extruder through the hopper, located at the top of the barrel above the auger. Early hoppers were simple cone-shaped structures that required manual loading of resin pellets.

Today’s hoppers have evolved with technological advancements, featuring various shapes and sophisticated feeding mechanisms that automate the loading process, eliminating the need for manual intervention.

Extruder

The extruder serves as the heating mechanism in the plastic bag manufacturing process. It features a long tube called the barrel, inside which is a helical screw known as the auger. Resin pellets are fed from the hopper into the barrel, where the auger gradually moves them toward the die. Heating elements positioned along the barrel melt the resin pellets into a molten liquid, which is then pushed through the die.

At the end of the barrel, the molten plastic is forced through a round, circular die before entering the cooling tower. A screening or breaker plate is placed before the die to ensure the purity of the molten resin by removing any potential contaminants.

Die

As the molten plastic resin progresses through the extruder barrel, pressure is applied to push it through the screen or breaker plate and die. The breaker plate serves to remove contaminants and protect the die from the high pressure generated during the extrusion process.

The die directs the molten plastic into a cooling cylinder, which can be oriented either vertically or horizontally. This cooling cylinder determines the thickness of the bags and shapes them accordingly. It is crucial for ensuring that the bags are seamless and free of holes. The speed at which the molten plastic enters the cooling cylinder affects the final thickness of the bags.

Cooling Tower

As the molten plastic resin exits the extruder, it is forced through the die and breaker plate into the bottom of the cooling tower cylinder. Compressed air is introduced to shape and form the plastic, similar to inflating a large balloon.

As the plastic tube is inflated with air, it moves upward through the cooling tower. During this time, the plastic cools and takes on the dimensions of the final plastic bags. The speed of this movement affects the thickness of the plastic film: rapid expansion results in thinner, lighter bags, while slower expansion creates thicker, more durable bags.

The newly formed plastic film continues to cool as it rises toward the nip rollers, which press the sides of the plastic bag together. This process stretches the film both axially and radially, creating a tube much larger than the die ring. The nip rollers also eliminate any air bubbles, blisters, or other imperfections in the film.

The diameter of the plastic tube depends on the type of plastic being processed, and the height of the cooling tower, which can range from 25 to 35 feet, plays a role in the cooling speed.

Gussets

Gussets are folds, creases, or indentations added to the sides of a plastic bag to increase its volume while allowing it to lay flat for storage. In the cooling tower, boards embedded in the walls create these gussets by indenting the fully inflated plastic as it moves toward the nip rollers.

The gusset created by these boards forms two overlapping rectangles, which are pressed together by the nip rollers as the bag material exits the cooling tower. This type of gusset, known as a side gusset, is commonly used in plastic bags, particularly those designed to be sturdier and more durable.

Nip Rollers

Nip rollers, positioned at the top of the cooling tower, provide the necessary traction, tension, and winding to transfer the completed bag material onto rollers. As the inflated plastic film cools, it crystallizes at the frost line, where it becomes cool enough to be flattened into lay-flat or collapsed tubing. The nip rollers apply the force needed to pull the cooled plastic film upward, forming it into rolls for further processing.

The nip rollers play a crucial role in creating a solid and secure winding of the plastic bag material. They regulate the firmness of the roll by removing air from the plastic film as it exits the cooling tower. The challenge in designing nip rollers is to ensure they provide enough pressure to remove excess air and produce a firm roll without damaging the plastic film.

Preventative Measures

During the production process, polyethylene material may be treated with various additives, including:

• 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 measured in mils, with one mil equal to one-thousandth of an inch (0.001"). Mils are a unit used to quantify thickness in the English system, whereas microns (µ) are used in the metric system, with 0.05 mm equivalent to 50 µ. For example, a thickness of 2.0 mils is also 0.002 inch or 50 µ.

The gauge of a plastic bag is determined by three key factors:

• Contents
• Protection
• Length of Use

Contents refer to the items that will be placed in the bag, protection pertains to the importance of keeping these items secure, and length of use indicates how long the bag will be used.

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Chapter Three – What materials are commonly used in plastic bags?

Plastic bags are manufactured from polyethylene, which starts as ethylene derived from natural gas or oil. This ethylene undergoes a transformation into polymer form. Polyethylene is then modified to suit different applications and types of plastic bags.

Plastic materials are created from polymers, which are constructed from monomers. Polyethylene consists of long chains of carbon atoms bonded with hydrogen atoms. This polymer structure allows plastic to be easily adjusted into various densities and shapes.

Plastic Bag Materials

High Density Polyethylene (HDPE)

HDPE is known for its strength and high tensile strength. It can endure temperatures up to 120°C or 248°F and exhibits resistance to various solvents. Its linear molecular structure results in a robust material capable of withstanding prolonged use.

Low Density Polyethylene (LDPE)

LDPE plastics are characterized by their thickness, flexibility, and resistance to punctures and tears. Thickness is measured in mils, with one mil equating to one thousandth of an inch. A higher mil count indicates a thicker bag, with typical ranges from 0.9 mil for kitchen bags to up to 6.0 mil for heavy-duty contractor bags.

Linear Low Density Polyethylene (LLDPE)

LLDPE bags, while similar to LDPE bags, offer enhanced strength and puncture resistance. They are commonly used for lighter gauge gusseted bags. LLDPE bags are more cost-effective and environmentally friendly compared to LDPE bags. They are produced through the copolymerization of ethylene with butene, hexene, and octene.

Medium Density Polyethylene (MDPE)

MDPE offers greater strength and rigidity while remaining flexible enough for easy processing. Its increased thickness makes it suitable for handle and loop bags. The higher density of MDPE enhances its resistance to shock, drops, and stress-cracking.

Metallocene (mPE)

Metallocene is distinct from traditional monomers like ethylene, propylene, or vinyl acetate. Instead, it consists of particles of charged metal ions combined with carbon atoms. Metallocene maintains similar density standards to polyethylene, with lower densities being softer and higher densities offering greater resilience. During manufacturing, it is blended with LDPE and LLDPE to create a superior poly film that seals quickly and securely. However, the cost of metallocene is higher due to its blending 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 composed of long chains of propylene molecules, giving the material both flexibility and strength. It is highly resistant to fatigue and can endure repeated use without significant damage. With its low density, polypropylene is lightweight yet offers a high strength-to-weight ratio. Although polypropylene bags are versatile and can serve various purposes like other plastic bags, they are most commonly used for tote and shopping bags.

Chapter Four – What are the different types of plastic bags?

The variety of plastic bags is virtually limitless, expanding annually as new applications are discovered. They can come in many forms, including flat, gusseted, anti-static, postal-approved, protective hazard bags, and shrinkable types.

Types of Plastic Bags

Flat Plastic Bags

Flat plastic bags are characterized by their wide and long dimensions without depth. These bags can be sealed using heat, ties, staples, or tape, making them a versatile option for storing a range of items, from food to electronics.

Markable Plastic Bags

Markable bags feature a white block designed for labeling the contents. This makes them ideal for organizing and storing small items, ensuring easy identification and 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 equipped with a special additive that prevents static electrical charges, making them ideal for storing electronic components. These bags come in various colors and densities of plastic material to suit different needs.

Postal Approved Plastic Bags

Postal-approved bags are designed to be compatible with United States Postal Service (USPS) scanners to ensure smooth processing and avoid delays. These bags are manufactured to both protect their contents and meet USPS standards for efficient mail handling.

Biohazard Plastic Bags

Biohazard bags are designed to protect workers from hazardous materials. These bags are clearly marked with the biohazard symbol and come in various colors such as clear, white, red, yellow, black, or blue, depending on their intended use. They must comply with standards set by the CDC, DOT, EPA, and OSHA for handling medical waste. Made from robust plastic, biohazard bags are resistant to punctures and tears, ensuring safe containment of dangerous substances.

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 stacked on a wire wicket and secured with rubber washers or a clipboard header. They feature a 1.5-inch lip and starter notches for ease of use. These bags can include bottom gussets, butterfly vents, perforations, and resealable tape. The wicketed design allows for holding over 200 bags and can be customized to fit specific needs.

Shrink Plastic Bags

Shrink bags are used with shrink wrap machines to securely enclose items. The items are placed in the bag, which is then heated in the machine. The heat causes the bag to shrink tightly around the contents, creating a secure seal. This method is ideal for protecting and preserving sensitive items during storage or shipping.

Vapor Corrosion Inhibitor Bags (VCI)

VCI (Vapor Corrosion Inhibitor) bags are designed to protect ferrous metals from corrosion. These bags are typically clear, allowing for easy inspection of the metal inside. They work by creating a moisture-resistant barrier that prevents exposure to humidity, which can lead to corrosion. Yellow VCI bags are used for ferrous metals like iron, steel, and cast iron, while blue and clear VCI bags are intended for non-ferrous metals such as aluminum, copper, and brass. Metal parts stored in VCI bags remain dry and free from corrosion.

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 – What are the regulations regarding plastic bags?

In recent years, plastic bags have faced significant criticism, prompting several countries and states to introduce legislation aimed at their reduction. Currently, the focus is on enhancing recycling efforts and minimizing plastic bag disposal in landfills.

Legislative measures have been implemented to decrease the prevalence of plastic bags in retail environments. These regulations aim to mitigate their environmental impact on oceans, rivers, lakes, forests, and wildlife habitats, while also easing the burden on waste management systems and landfills.

Proposed measures include broad recycling initiatives, outright bans, or bans with associated fees to deter the use of plastic bags.

Out of all 50 states, 18 have implemented legislation aimed at reducing or banning plastic bags, with California, Maine, and Delaware having the most stringent regulations. The accompanying graphic highlights states with strict legislation and bans, as well as those that have yet to take action.

In 2019 alone, over 95 pieces of legislation were introduced across various states to regulate and manage plastic bag use.

Most of these legislative efforts focus primarily on promoting recycling as a key environmental strategy.

In addition to state-level regulations, several cities have implemented their own bans or fees related to plastic bags. Notable examples include Boston, Chicago, Los Angeles, San Francisco, Seattle, Boulder (CO), Montgomery County (MD), New York City, 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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