We use plastic bags every day for various purposes, such as carrying our shopping or disposing of waste. Have you ever wondered how these ubiquitous items are made? The process of transforming raw materials into finished plastic bags is fascinating, involving the steps of melting, shaping, and sealing.
Raw Materials: The Building Blocks of Plastic Bags
The primary material used in manufacturing plastic bags is polyethylene (PE), a type of plastic belonging to the polyolefin family. There are several types of PE, including:
- Low-Density Polyethylene (LDPE): Characterized by low density, high softness, and excellent transparency. It is typically used for making plastic bags, cling film, and other flexible packaging.
- High-Density Polyethylene (HDPE): Offers high rigidity, high strength, and heat resistance. HDPE is commonly used to manufacture various containers, pipes, toys, and similar rigid products.
- Linear Low-Density Polyethylene (LLDPE): Known for its high toughness and puncture resistance. LLDPE is typically used in producing high-performance films such as stretch film and agricultural film.
In addition to polyethylene, other additives are used during the production of plastic bags. These can include colorants to make the bags, stabilizers to increase the life span of the bag, or processing aids to improve the consistency of the bag.
Where does PE come from?
Polyethylene is derived from crude oil or natural gas. The process begins with extracting ethylene, a gaseous hydrocarbon, from these sources through a process called cracking. Cracking involves heating the crude oil or natural gas to high temperatures, breaking down the large hydrocarbon molecules into smaller ones, including ethylene.
Ethylene is then transported to plastic manufacturing facilities, often in liquid form. Upon arrival, it undergoes polymerization, a process that links the ethylene molecules together to form long chains of polyethylene. These polyethylene chains are then formed into small pellets. These pellets are then transported to plastic bag manufacturing plants. These pellets are then ready to undergo the melting and shaping processes.
From Pellet to Plastic Bag
1. Extrusion (Melting and Shaping):
The first step is extrusion, a process that melts and shapes the polyethylene pellets into a continuous film.
- Feeding: PE pellets are fed into a machine called an extruder.
- Melting: Raw material pellets are fed into the film blowing machine. Inside the machine, under intense heat and pressure, the pellets melt into a viscous, molten liquid.
- Film Blowing (Extrusion & Inflation): The molten plastic is extruded through a circular die. Simultaneously, air is blown into the center of the extruded tube, inflating it like a balloon into a cylindrical bubble. The amount of air pressure controls the size and thickness of the film.
- Stretching: The film bubble undergoes controlled longitudinal and transverse stretching, which significantly enhances the film’s strength and clarity. The stretched film is then cooled to set its structure, resulting in a uniform-thickness plastic film.
2. Printing: The plastic film passes through a printing press to apply the desired graphics or text. Flexographic printing, the most commonly used printing method, is favored for high-volume orders. Gravure printing offers superior precision and more vibrant colors.
3. Sealing (Bottom Sealing): The continuous printed film moves through a machine that uses heat and pressure to form a bottom seal. The type of seal varies depending on the bag’s intended use.
4. Cutting: The sealed film is then cut into individual bags using cutting devices, such as rotary blades or lasers.
5. Winding & Packaging:
- Winding/Stacking: The finished bags are either wound onto rolls or stacked in piles, depending on the required packaging format for distribution.
- Packaging for Shipment: The rolls or stacks of bags are then packaged into boxes or bundled for shipment.
6. Quality Control (QC): Rigorous quality control checks are performed throughout the manufacturing process. These checks ensure the bags meet specified standards for strength, thickness, and print quality. (Key QC Standard: Tensile Strength ≥ 15 MPa, Elongation at Break ≥ 300%).
From Nondegradable to Biodegradable Plastic Bags
Conventional plastic bags pose a serious environmental threat. Their production relies on fossil fuels, depleting precious resources. The extraction, transportation, and processing of these fuels release significant greenhouse gases, accelerating climate change. While recycling programs exist, proper disposal is only part of the solution.
The answer? Biodegradable plastic bags.
What Makes a Bag Biodegradable?
Unlike traditional polyethylene, biodegradable bags are made from polymers that microorganisms (like bacteria and fungi) can consume and convert into water, carbon dioxide (CO₂), methane (CH₄ – in anaerobic conditions), and biomass. This decomposition process relies heavily on specific environmental conditions (temperature, moisture, microbial activity). The production of plastic bags usually goes through the above process, and whether plastic bags can be biodegradable depends on the raw materials used.
Materials of Biodegradable Plastic Bags:
Bio-based Plastics:
Polylactic Acid (PLA): Derived from fermented plant sugars (corn starch, sugarcane). PLA bags are typically rigid and transparent but can be brittle. They require industrial composting facilities (high temperatures of 55-70°C and specific humidity) for timely decomposition (typically 90-180 days), breaking down primarily into CO₂ and water.
Polypropylene carbonate (PPC): A copolymer of CO₂ and propylene oxide (PO). It has a series of features such as being non-toxic, biodegradable, having good biological adaptability, strong toughness and excellent gas barrier properties. PPC has a wide range of applications. Currently, PPC is widely used in packaging, mulch film, biomedicine and other fields. It has the potential to replace polyethylene(PE), polypropylene and other materials in the preparation of disposable materials.
Starch Blends: Thermoplastic starch (TPS) is often blended with other biodegradable polyesters (like PBAT or PLA) to improve processability and mechanical properties while enhancing biodegradability. (The starch component readily absorbs water and initiates breakdown.)
Fossil-based but Biodegradable Plastics:
Polybutylene Adipate Terephthalate (PBAT): A copolymer of butylene adipate (PBA) and butylene terephthalate (PBT), combining the characteristics of PBA and PBT. It has good ductility and elongation at break, as well as good heat resistance and impact performance. In addition, it also has excellent biodegradability. It is frequently blended with PLA or starch to improve flexibility and toughness in compostable bags. PBAT degrades effectively in industrial compost.
The Manufacturing Shift for Biodegradable Plastic Bags
In order to make biodegradable bags, crucial adjustments are necessary.
Material Handling & Drying: Many biodegradable resins (especially PLA and starch blends) are highly hygroscopic (absorb moisture). Pellets must be thoroughly dried before extrusion to prevent water-induced breakdown during processing, which ruins the melt and weakens the final film.
Extrusion & Blowing Parameters: Biodegradable polymers often have different melting points, viscosities, and thermal stability compared to PE. Processing temperatures are generally lower (e.g., PLA ~160-190°C vs LDPE ~160-230°C). Melt strength can be lower, requiring careful control of extrusion rates, blow-up ratios (BUR), and cooling to achieve stable bubbles and consistent film gauge.
Additives: Additives must also be biodegradable and compostable certified to maintain the integrity of the final bag’s environmental claim. This includes colorants and anti-block agents.
Printing: Bio-based inks compatible with the biodegradable films are essential.
Certification & Testing: Rigorous quality control includes verifying biodegradability and compostability according to international standards (e.g., EN 13432, ASTM D6400, ISO 17088). Tests confirm complete disintegration within a defined timeframe in industrial composting conditions and the absence of ecotoxicity in the resulting compost.
Adopting Biodegradable Plastic Bags
The journey from pellet to plastic bag involves a complex process of melting, shaping, and sealing. While plastic bags offer convenience and utility, their environmental impact cannot be ignored. From the consumption of fossil fuels to the persistent problem of plastic pollution, it’s clear that a shift towards more sustainable practices is essential.
As companies, we have a responsibility to move beyond merely acknowledging this impact and actively seek solutions.
The shift towards certified compostable plastic bags – encompassing garbage bags, garment bags, mailers, and more – presents a crucial step forward. By choosing bags made from materials like PLA, PBAT, or starch blends, designed to break down efficiently in industrial composting facilities, businesses can significantly reduce their reliance on finite fossil resources and contribute to a circular economy. While challenges exist, particularly in ensuring proper disposal through accessible composting infrastructure and consumer education, the potential environmental benefits are substantial.
