Plastic is the most popularly suggested element in tpt.  And I agree that we should have it.  Here are some ideas on what tpt plastic would be like as well as suggestions for 2 new elements that plastic would create:

Plastic (aka, PLCT):

PLCT is a solid.  It is has the same optical properties as GLAS and melts at the same temperature as GLAS.  Unlike GLAS; however, it can't be broken into it's powdered form (see below) by pressure.  Like WOOD, it can only be broken into it's powdered form by high-velocity particles.  PLCT can be made by mixing GEL and OIL.  Both the GEL and the OIL become PLCT upon contact with each other, just like CLST and NITR become TNT upon contact with each other.  PLCT also has the same electricity-blocking properties as INSL, but, unlike INSL, is not flammable.  One pixel of it, just like one pixel of INSL, doesn't block SPRK with a ctype of GOLD.  PLCT will be useful in making prisms that can withstand high pressures and in making non-flammable electricity-blockers that are not walls.

Microplastics (aka, MCPS):

MCPS is the powdered form of PLCT (see above.)  Created when PLCT is bombarded with high-velocity particles.  MCPS melts at the same temperature as PLCT and when molten MCPS is cooled it turns back into PLCT.  MCPS has the same optical properties and the same electricity-blocking properties as PLCT.  When MCPS mixes with WATR, DSTW, or SLTW, PWTR (see below) is formed.

Polluted water (aka, PWTR):

PWTR is a new type of tpt water.  It can be made by mixing MCPS (see above) with WATR, DSTW, or SLTW.  These water varients absorb the MCPS upon contact (like WATR absorbs SALT upon contact) and become PWTR.  BUBW and FRZW; however, do not absorb MCPS or become PWTR in any way other than by means of CONV or console.  PWTR, unlike other water varients, is immune to GEL.  In fact, when under pressure, it splits into MCPS and GEL.  It doesn't have the same optical properties as PLCT (see above) or MCPS but does have the same electricity-blocking properties as PLCT and MCPS.  Unlike DSTW, it can't be contaminated by other water varients (not even FRZW can contaminate it.)  As well as being useful to spread environmental messages, it can also be used as a non-conductive water varient that can't be contaminated.

NOTE:  The PWTR splitting into MCPS and GEL was inspired by the book "Sneaky uses for everyday things."  There was this bit where you could use milk to make DIY plastic and DIY glue.  I was going to suggest PWTR split into PLCT and GEL, but that would be too hard to code.  So I suggested it split into MCPS and GEL instead.  But GEL is actually glue, so at least we have that.  I do not own this book and therefore am not advertising it.

Molten plastic exists in real life.  Also tpt is only supposed to be 75%-80% realistic.

Point taken.  But plastic melting into LAVA is cooler.  And PLCT combines the properties of GLAS and INSL without being breakable or meltable.  This opens up new opportunities.