Laundress is correct. STPP left out in humid conditions will slowly revert to TSP or similarly less effective laundry additive.
In terms of hashness, TSP is more alkaline than STPP, but less alkaline than sodium carbonate (washing soda). It's an excellent degreaser for hard surfaces, and is used to help prepare painted surfaces for repainting (it cleans it well and slightly etches/roughens the surface to accept a new coat more readily).
The main reason why TSP is not as effective as STPP lies in its chemistry. TSP is basically 1/3 of the STPP molecule. It has only one phosphate group, vs. the three phosphate groups of the STPP molecule. Those three phosphate groups surround and bind hard water minerals and associated soil components, and the various other charges on the molecule help keep the entire combination suspended in water so it can be rinsed away. TSP has only one phosphate group, which also binds tightly to hard water minerals and associated soil components, but lacks the other charges to keep it suspended in water. So when combined with the minerals it falls out of solution and forms a cement-like precipitate on fabrics and washer parts. Washing soda also does this. Most people say it looks like lint on fabrics. That's because it's still moist and hasn't had a chance to accumulate on a hard surface. It does look like cement on washer parts, after it accumulates and dries in many layers.
It would not be incorrect to describe STPP as the high energy version of TSP. That energy, in a sense, is stored as a function of its more complex and ordered structure, is used to help bind minerals and keep them in solution. But it also means that the STPP molecule is somewhat unstable, and its energy content favors its eventual dissociation into TSP, given moisture, air, heat, and time, just as a piece of iron will rust or a piece of wood will burn, both releasing energy in the process. When the complex STPP molecule is broken down to the simpler TSP molecules, energy is released and the Universe is happy ;-)
Way back when, when I discovered that the missing ingredient in modern laundry detergents was phosphate, I bought about 20 boxes of TSP at the hardware store. Shortly thereafter I learned that STPP, not TSP, was the missing ingredient, and got a lot of that instead. I still have the TSP, which I guess will come in handy when I have to repaint the house.
