1. Are a Plastic Bottle's Cap and Body Made of the Same Plastic?
A plastic bottle consists of three parts: the body, the label, and the cap. According to educational materials from the Plastics Circulation Utilization Association, the body is made of PET (polyethylene terephthalate). The cap is usually made of polyethylene (PE) or polypropylene (PP), while labels are often polystyrene, though other label materials exist too. [1]
The US National Oceanic and Atmospheric Administration (NOAA) notes that many plastic bottle caps in the United States are made of polypropylene, which is hard and durable. However, because Japanese caps can be made from both PE and PP, it is not accurate to say all caps are one specific type. [1]
Since the materials are different, how does this change what happens when they are placed in water? [1]
2. How Does Density Make Bottle Caps Float in Water?
Whether an object floats or sinks depends on its "density." Density is the weight of an object compared to the same volume of water. If we set water's density to 1, objects with a density lower than 1 float, while those with a density higher than 1 sink. [1]
The Plastics Circulation Utilization Association provides approximate density values for these materials: PET (bottle body) is 1.37 to 1.40; PP (cap) is 0.90 to 0.92; PE (cap) is 0.91 to 0.97; and polystyrene (label) is 1.05 to 1.07. [1]
Comparing these numbers to water's density of 1, PET is about 40% heavier than the same volume of water, while PP caps are about 10% lighter (author's calculation). This means the cap floats and the bottle body sinks. Polystyrene labels also have a density greater than 1, so they sink too. Therefore, only the cap floats. [1]
Does this difference in floating and sinking help in real-world situations? [1]
3. How Do Recycling Plants Use Water to Separate Bottle Materials?
Even after removing the label and cap, a ring often remains on the neck of the bottle. The Plastics Circulation Utilization Association explains that in recycling plants, when bottles are crushed and washed, this difference in density separates the rings (from caps) from the bottle bodies. [1]
In water, fragments of PE or PP float to the surface, while PET fragments sink to the bottom. This means workers do not need to pick each piece out by hand; the difference in density does the sorting automatically. [1]
This method relies on the materials being different. If both the cap and the body were made of PET, they would not separate when put in water. Interestingly, this variation in cap material is not limited to plastic bottles. Glass bottles use completely different materials for their caps. [1]
4. What Metals Are Crown Caps on Glass Bottles Made Of?
Crown caps on glass bottles are made of tinplate steel or stainless steel, according to Japanese cap manufacturer documents. These materials allow for high-speed production and application, and they withstand the pressure from carbonated drinks as well as reduced pressure. [1]
The outer metal part is called the shell, and the inner lining is called the liner. Liners can be made of polyethylene resin, cork sheets, or PVC resin. Notice that PE, which is used in plastic bottle caps, also appears inside crown caps. [1]
The crown cap is said to have been invented by William Painter in Baltimore, USA, in 1892. He obtained three patents on February 2nd for a system using thin cork liners to maintain an airtight seal. [1] The number of crimps (folds) on the side is said to be 21 worldwide. However, a US university program notes that Painter’s first machine used 24 crimps. There is no clear record showing why it became 21, though some suggest it was a balance between grip strength and metal durability. [1]
While steel is used for glass bottles, PE and PP are used for plastic bottles. In the world of wine, yet another material is used. [1]
5. Why Did Wine Bottles Start Using Aluminum Screw Caps?
Aluminum screw caps for wine, such as "Stelvin," are said to have been developed in 1964 by Peter Wall of Yalumba Winery in Australia and LBM in France. They consist of an aluminum cap, a plastic liner, and a long metal skirt that mimics the foil once placed over old corks. [1]
A key moment for their adoption was in 2000, when 13 producers in Clare Valley, Australia, used these caps for premium Riesling wines. A study by the Australian Wine Research Institute in 2001 reported that these caps performed better than cork in most aspects. Note that this information comes from an aluminum manufacturer’s article, so it may favor aluminum. [1]
Counting from the invention of the crown cap in 1892, the development of screw caps happened 72 years later (author's calculation). So, how are plastic bottle caps changing today? [1]
6. How Are Bottle Caps Being Made Lighter and Recycled?
Suntory Beverage & Food International announced in September 2016 that they reduced the weight of PE caps from 2.05g to 1.85g, a reduction of about 10%. They also used 30% plant-derived polyethylene. The difference of 0.2g is about 9.8% of the original weight (author's calculation). For 550ml bottles, the company estimates a reduction of about 35% in petroleum-based raw materials and about 27% in CO2 emissions. These are the company’s own estimates, and the comparison conditions need to be checked in the announcement text. [1]
Technical verification for "cap-to-cap" recycling, where used caps are turned into new caps for the same purpose, has also been reported. Asahi Group Holdings, Sojitz Planet, and Nippon Closure conducted this verification, which was announced as the first initiative in the beverage industry. [1]
Making caps lighter, using plant-based materials, and recycling them back into caps are all efforts to use cap materials efficiently. [1]
Try It: Check If Your Cap Floats
You can test this at home. Put water in a bowl and float a washed plastic bottle cap on it. Try flipping it over or adding water to it and shaking it gently to see how the floating changes. If you want to test pieces of the bottle body, ask an adult to cut them for you, and be careful not to cut your hands on the sharp edges. [1]
If you find a crown cap from a glass bottle, try counting the crimps (folds) on its side to see if it matches the 21 folds mentioned in documents. [1]
You can also read "Plastic Density Separation Experiment" by the Plastics Circulation Utilization Association for density tables and experiment steps. According to these materials, based on density estimates, labels are expected to sink while caps float. [1]