Interesting Lu.
I'm going to run and check the chemical makeup real quick.
There are two types of glycerin: natural glycerin derived from oils and fats from such sources as coconuts and oil palms, and synthetic glycerin derived from petroleum.
Glycerin is a hygroscopic trihydroxy alcohol (HOCH2CHOHCH2OH)
Searching the chem database for Glycerin got me:C3H8O3
Polypropylene Glycol: C3H8O2
Other names for glycerin:
Glycerin; Glycerol; Glycerine; Glyceritol; Glycyl alcohol; Glyrol; Glysanin; Osmoglyn; Propanetriol; Trihydroxypropane; Synthetic glycerin; 90 Technical glycerin; Dagralax; Glycerin, anhydrous; Glycerin, synthetic; Ophthalgan; Synthetic glycerine; Vitrosupos; 1,2,3-Trihydroxypropane; 90 Technical glycerine; Clyzerin, wasserfrei; Grocolene; Moon; Star; Superol; Optim; Bulbold; Cristal; Croderol G7000; Dynamite glycerin; Emery 912; Glyceol; Glycerin USP; Glycon G 100, G 300; Incorporation factor, IFP; Lye glycerin; Polyhydric alcohol; Pricerine 9071; Shur-coal FCA; propane-1,2,3-triol
Other names for propylene glycol:
1,2-Propanediol; α-Propylene glycol; Methyl glycol; Methylethyl glycol; Methylethylene glycol; Monopropylene glycol; PG 12; Sirlene; 1,2-Dihydroxypropane; 1,2-Propylene Glycol; 2-Hydroxypropanol; 2,3-Propanediol; Propane-1,2-diol; Dowfrost; Propylene glycol usp; 1,2-Propylenglykol; Solar winter ban; Sentry Propylene Glycol; Isopropylene glycol; Ucar 35; Solargard P; Aliphatic alcohol; Chilisa FE; Ilexan P; Inhibited 1,2-propylene glycol; Prolugen; Propanediol; Trimethyl glycol; 1,2-Propandiol; 1,2-propanediol (propylene glycol); propylene glycol (1,2-propanediol); propylenglycol
tmi - but interesting