Plastics are complex materials composed of polymers. Their diverse chemical structures mean they cannot be treated uniformly, especially after use. Proper identification of the specific polymer is necessary for effective recycling and disposal because different plastic types require distinct processing methods. Knowing the type of plastic ensures collected items are sorted correctly and incompatible materials do not contaminate the recycling stream.
Decoding the Resin Identification Codes
The most straightforward method for identifying the specific polymer in an item is by locating the Resin Identification Code (RIC). This numerical system, established by the plastics industry, is typically found molded into the bottom of a product. The code consists of a number surrounded by a chasing arrows symbol. This identification is necessary because each polymer must be melted and processed separately during recycling.
The codes 1 through 6 each correspond to a specific plastic type. Code 1 represents Polyethylene Terephthalate (PET), which is widely used for clear items like soda and water bottles due to its transparency and strength. High-Density Polyethylene (HDPE) is designated by the number 2 and is typically found in opaque, more rigid containers such as milk jugs, detergent bottles, and shampoo bottles.
Number 3 identifies Polyvinyl Chloride (PVC), a versatile material used in both rigid forms, like piping and window frames, and flexible forms, such as medical tubing. Low-Density Polyethylene (LDPE) is categorized under the number 4. It is commonly utilized for flexible packaging films, including plastic grocery bags, shrink wrap, and bread bags.
Polypropylene (PP) is found under code 5, distinguished by its high melting point and resistance to chemicals. This makes it suitable for yogurt containers, bottle caps, and some food packaging. Code 6 signifies Polystyrene (PS), which can appear as brittle, rigid plastic in disposable cutlery and CD cases, or in its foamed state, known as Expanded Polystyrene (EPS), often used for packaging peanuts and disposable cups.
The final category, number 7, is designated as “Other.” This is a catch-all for any plastic that does not fit into the first six groups, including mixtures, composites, and newer bioplastics. While the RIC system standardizes the classification of plastic resins, the presence of a code does not guarantee that the item is accepted by local recycling programs.
Identifying Plastics Without a Code
When an item lacks a visible Resin Identification Code, simple visual and tactile inspection can provide clues to the polymer type based on its inherent physical properties. The clarity and appearance of the plastic often offer the first indication of its composition. Polyethylene Terephthalate (PET) is characterized by its glass-like transparency and is frequently used for clear food and beverage containers. In contrast, High-Density Polyethylene (HDPE) is typically opaque, exhibiting a milky white or semi-translucent appearance, and has a smooth, somewhat waxy feel.
Flexibility and texture are key properties that help distinguish between plastic families. Low-Density Polyethylene (LDPE) is notably tough and highly flexible, commonly encountered as thin films and plastic wraps. Polypropylene (PP) also displays good flexibility and is known for its resistance to material fatigue. It can be bent many times without breaking, often showing a white crease line when sharply bent.
Polystyrene (PS) can be identified by its rigidity, as the non-foamed version is brittle and tends to snap rather than bend, such as in a disposable plastic fork. Polyvinyl Chloride (PVC) is unique because it is engineered to be either very rigid, like a piece of plumbing pipe, or very soft and pliable. By linking these observable characteristics to common household applications, one can make an educated assessment of the polymer type even without the presence of a stamped code.
The Float Test for Density
A simple density test, often called the float test, provides a method to differentiate between major groups of plastic polymers at home. This test is based on the principle that an object less dense than the liquid it is in will float, while a denser object will sink. Tap water, which has a density of approximately 1.0 gram per cubic centimeter, serves as the initial testing medium.
When a small piece of plastic is submerged in tap water, the low-density polyolefins will float, while the higher-density plastics will sink. The polymers that typically float include High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP).
Conversely, plastics like Polyethylene Terephthalate (PET), Polyvinyl Chloride (PVC), and Polystyrene (PS) all have a density greater than water and will sink. To further distinguish between the plastics that float, a salt water solution can be used, as it has a higher density than plain water. By observing which items float or sink in the different liquids, this procedure effectively narrows the possibilities.