There is often more to a roll of filament than just a pure strand of the polymer mentioned on the box. Sometimes you get important clues on the box, sometimes you have to speculate.
Additives can be divided into two main categories, heterogeneous and homogeneous. Heterogeneous additives feature most of the non-plastic additives one can find in filaments such as fibers, metals, and other materials. Homogeneous additives are more ubiquitous and feature colorants, multi-polymer mixes, and other compounds that may or may not be advertised by a vendor.
Carbon Fibers and Glass Fibers behave very similarly when added to a filament. They strengthen the print like a fiber. Annealing time is usually reduced significantly due to the additional spherulite nucleation sites provided by the fibers.
These are much easier to print.,
ESD-Safe filaments are designed to produce slightly conductive printed parts. This is often achieved with the usage of Carbon Nanotubes.
Many filaments can be found in their “natural” color, usually a semi-transparent beige or tan color. Every other color involves the addition of some kind of colorant.
White filament is often made with the addition of Titanium Dioxide, and has been blamed for a lack of strength. TODO: investigate if this is true/find examples.
Sometimes you mix polymers together. Sometimes this is advertised with a +, sometimes it is not.
Filament blends are used to improve or change the base plastic's behavior in a certain way. For example, several kinds of PLA+ make only small sacrifices in printing difficulty in exchange for the increased temperature resistance/durability of mixed plastics such as ABS, PC, or PETG.
This also applies in the reverse direction. Some ABS variants are sold with a significant amount of PETG or PLA added to lower printing difficulty while maintaining the overall characteristics of ABS.
Overall, these adulterated filaments make no promises about their ratios, characteristics, or behavior. They vary from vendor to vendor and possibly even from batch to batch. Do not expect blended filaments to reach anywhere close to the strength of a filament made from the pure base polymer.
TODO: Boron Carbide, Stone, Glass, Metal (copper, iron), etc.