plastic realtive permittivity compared to vaccume permittivity

https://passive-components.eu/what-is-dielectric-constant-of-plastic-materials/

The dielectric constant (Dk) of a plastic or dielectric or insulating material can be defined as the ratio of the charge stored in an insulating material placed between two metallic plates to the charge that can be stored when the insulating material is replaced by vacuum or air. It is also called as electric permittivity or simply permittivity.

And, at times referred as relative permittivity, because it is measured relatively from the permittivity of free space (ε0).

Dielectric constant characterizes the ability of plastics to store electrical energy. Typical values of ε for dielectrics are:

Material Dielectric Constant (ε)
Vacuum 1.000
Dry Air 1.0059
Foam Polyethylene 1.6
Fluoropolymers 2.0
Polypropylene 2.1
Butyl Rubber 2.3
SBR 2.9
Silicone Rubber 3.2
Plexiglass 3.4
PVC 4.0
Glass 3.8-14.5
Distilled Water ~80
A dielectric constant of 2 means an insulator will absorb twice more electrical charge than vacuum.

Applications include:

Use of materials in the production of capacitors used in radios and other electrical equipment. Commonly used by circuit designers to compare different printed-circuit-board (PCB) materials.
Development of materials for energy storage applications.

For example, polymer-based dielectric composites are highly desirable for applications ranging from electronic packaging, embedded capacitors, to energy storage. These composites are highly flexible with a low process temperature and they exhibit a relatively high dielectric constant, low dielectric loss, high dielectric strength.

And why there's this wide range of different relative permittivity's value, I mean why plastic has a relative permittivity equal to 3 while the air has only 1 ?