R45M has a higher molecular weight and functionality (OH sites) than R45HTLO. It also has an antioxidant included in the formulation, whereas R45HTLO does not unless it is specified at time of order (which I did when I purchased 4 drums).
R45M is no longer readily available. Most HTPB will be lower molecular weight, and slightly lower viscosity. However, some sources will not be high quality Total/Cray Valley R45HTLO; some will be imported; others may be surplus and not stored correctly to guarantee full activity. I've tested those scenarios and it's a waste of time and money. (For me, the RCS HTLO did not work as well as the direct purchase I made).
It's important to keep track of all of the hydroxyl values (or hydroxyl numbers) of the resin and all liquid additives which contain -OH sites (tepanol, crosslinkers, etc.). We typically use the unitless "equivalent weight" to calculate the correct amount of curative (-NCO sites) to bond the resin and additives (-OH sites). If the chemical source doesn't directly specify EW, you need to convert.
Here are my short notes on how to do that. R45M has an EW of ~1370 and R45HTLO has an EW of ~1190. You must adjust for more curative when switching from R45M to R45HTLO.
Other factors can effect the quality of the binder. Moisture in any ingredient or in the environment while mixing will consume curative quickly. It's best to dry the solids and to vacuum dehydrate the liquids before adding the curative. The "cure index" may be adjusted to allow for moisture with a value slightly >1. On the other hand, an index somewhat <1 will produce softer, stretchier binder at the expense of reduce strength.
You can send me a PM if you need additional info on formulation which might not be allowed here in the open forum. To get access to the restricted Research section, you need to be NAR or TRA L2 certified and a US Person.
