Not all plastics contain plasticizers, so I would say that fnordpiglet is correct. For example, low density polyethylene (commonly used for jugs holding milk and water) is flexible without addition of any plasticizers. For this reason it is also used in laboratory equipment where plasticizer contamination would interfere with experiments:
Low density polyethylene (LDPE) is a high molecular weight polyolefin material. Like all polyolefins, LDPE is nontoxic, non-contaminating and exhibits a high degree of break resistance.
The polymerization of polyethylene results in an essentially straight chain, high molecular weight hydrocarbon. The polyethylenes are classified according to the relative degree of branching (side chain formation) in their molecular structures, which can be controlled with selective catalysts. LDPE has more side branching than HDPE resulting in a less-dense 3-D structure. As a result, LDPE is naturally very flexible without the addition of plasticizers and melts at a relatively low temperature (85°C).
Low density polyethylene (LDPE) is a high molecular weight polyolefin material. Like all polyolefins, LDPE is nontoxic, non-contaminating and exhibits a high degree of break resistance.
The polymerization of polyethylene results in an essentially straight chain, high molecular weight hydrocarbon. The polyethylenes are classified according to the relative degree of branching (side chain formation) in their molecular structures, which can be controlled with selective catalysts. LDPE has more side branching than HDPE resulting in a less-dense 3-D structure. As a result, LDPE is naturally very flexible without the addition of plasticizers and melts at a relatively low temperature (85°C).
https://www.thermofisher.com/us/en/home/life-science/lab-pla...