What is an LPG cylinder made of?

As already aswered by others, the most common material is steel.  However, there are other alternatives.

Aluminium is frequently used for forklift sized cylinders.  Its lighter weight allows it to meet certain safety standards that relate to weight and safe lifting.

Composites are also becoming more popular.  Fiberglass, moulded plastic and even Kevlar are being used.

Here is an example description of one cylinder that actually bounces!

Liner. The inner part is called “liner” and made of recyclable HDPE by blow moulding.  Its main function is to keep the gas inside by providing a gas tight  barrier. Standard HDPE grades do not give sufficient barrier properties,  so the R&D team developed a special grade of HDPE which results in  permeation values that are so low that they are hard to measure.

The liner is chemically inert and can withstand temperatures between -40°C and +80°C

Fibers. The LoW8 technology uses ultra strong Aramide  fibers (also know under trade names Kevlar or Twaron). The selection of  Aramide over glass fiber is driven by performance and durability.  Looking at specific strength it even beats carbon fibers and you need 10  times more glass fiber to get the same strength. But it’s not only  strong, also processing Aramide fiber at high speeds is relatively easy.  Last but not least, the fatigue life of Aramide compared to glass fibers is much better.

It’s not only the type of fiber but also how to wind them on the liner.  And here lies one of the most spectacular innovation; the fibres are  processed dry and stay in position by winding them in pure geodetic  patterns. As a result of this the complete cylinder survives enormous  deformations when exposed to high impact loading.

Protective Elastomeric Coating. In order to protect the  Cylinder against extreme UV, wear and tear and all sorts of chemicals a  special grade of though elastomer has been composed. It’s actually so  resilient that the cylinder does not need any maintenance over it’s life  time. Adding colour pigments in the coating means you can have any  corporate colour.

Soft Grip Handle. New for 2015 is that both handle and  foot ring are replaceable! This means any owner can decide to have it’s  own design and, in the very unlikely situation it get’s damaged, they  are easy to replace by means of special tooling. Of course both handle  and foot ring are very robust and allow easy and stable stacking of the  LoW8 cylinders.

Torque Free End Boss. This patented design overcomes  any problem related to so called overtorquing. It’s simply impossible to  damage any part of the cylinder by applying too much torque on the  valve. Only with a simple tool you can place or remove the valve.

Valve. The valves used on the LoW8 are always fitted  with a Pressure Relief Device (PRD). This to warrant safety in case of  overfilling with LPG. Other features of the valves: all standard  connections available and even with a 80% auto fill stop and fluid  withdrawal (for forklifts) are available.

RFID. All LoW8 cylinders are fitted with a RFID Chip  which is placed between the liner and the fibers. Since the composite  material hardly absorbs radio waves, remote reading is done very easy  compared to metal cylinders