The main role of a plasticizer in impression compound materials is to enhance the flow of the material at mouth temperature.

Impression compounds are thermoplastic materials used primarily for preliminary impressions in dentistry. They consist of a base (resin or wax), fillers, and plasticizers. The plasticizer serves to reduce the brittleness and increase the flexibility of the compound when heated. This results in improved malleability and flow, allowing the material to adapt closely to the oral tissues without cracking or breaking.

Key points to note:
- Plasticizers lower the softening temperature, which makes the compound soft and pliable at mouth temperature.
- This increased flow ability allows for better accuracy in capturing fine anatomical details.
- Unlike other components that may affect setting time or strength, plasticizers specifically influence the plasticity and viscosity of the material when warmed.
- This characteristic is essential for patient comfort and the clinical success of impression taking with impression compounds.

Incorrect options explained:
- Increasing setting time is not the role of plasticizers; impression compounds are thermoplastic and do not undergo a chemical set.
- Improving strength is generally related to fillers, not plasticizers.
- Antimicrobial properties are provided by additives or specific agents but are not related to plasticizers.

Reference:
Stewart's Clinical Removable Partial Prosthodontics, Volume 1, Chapter 7: Impression Materials / Page 145