The following iiteraure may be useful for your research

his thesis contains a literature review chronicling the last half-century of

research to better understand the mechanisms that govern anomalous burning and to shed light on current research into plateau and related propellants. In addition to the review, a series of experiments investigating the use of nanoscale TiO2-based additives in AP-HTPB composite propellants was performed.The baseline propellant consisted of either 70% or 80% monomodal AP (223 μm) and 30% or 20% binder composed of IPDI-cured HTPB with Tepanol. Propellants’ burning rates were tested using a strand  bomb between 500 and 2500 psi (34.0-170.1 atm). 

Analysis of the burning rate data shows that the crystal phase and synthesis

method of the TiO2 additive are influential to plateau tailoring and to the apparent

effectiveness of the additive in altering the burning rate of the composite propellant.

Some of the discrepancy in the literature regarding the effectiveness of TiO2 as a

tailoring additive may be due to differences in how the additive was produced. Doping  the TiO2 with small amounts of metallic elements (Al, Fe, or Gd) showed additional effects on the burning rate that depend on the doping material and the amount of the dopant. 

Ref;TAILORING THE PLATEAU BURNING RATES OF COMPOSITE PROPELLANTS BY THE USE OF NANOSCALE ADDITIVES 

MATTHEW A. STEPHENS; THESIS FOR MS; Texas A&M University

https://repository.tamu.edu/bitstream/...1/.../STEPHENS-THESIS.pdf?...

The chemical structure of the diisocyanate determines its reactivity and suitability for preparation of a polyurethane from a polyol. Assuming HTPB is hydroxy terminated polybutadiene, then the polyol is a diol that will form a predominantly linear polyurethane (aside from side reactions) if a diisocyanate is used.

1. Aromatic isocyanates are more reactive than aliphatic: toluene diisocyanate (TDI), methanediphenyl diisocyanate (MDI) are more reactive than hexane-1,6-diisocyanate (HDI) and isophorone isocyanate.

2. Location of isocyanate in the structure: with TDI 2,4- isomer the 2-NCO is less reactive than the 4-NCO, with the 2,6- isomer both NCO are in the ortho position, less reactive. Differences in reaction rates are likely to result in differences in polymer structure.

3. MDI is available in several variants: the expected methane-4,4'-diphenyl diisocyanate is difunctional, however commercial MDI products differ from this precise structure and have fractional and ti-functionality. These structures can be chosen to give differing degrees of cross linking.

I have seen your fruitful comments. I was wondering about DDI, is it actually utilized nowadays? WHich advantages does it brings? Many thanks!