Investigation of Phosphorus-containing Flame-retardant Systems for Rigid Polyurethane Foam
Student thesis: Doctoral Thesis
Related Research Unit(s)
In recent years, the increasing number of high-rise building fires that lead to significant property losses and deaths has attracted considerable attention. Many of these fires were mainly caused by the ignition of the insulation materials of organic exterior walls. A widely used building insulation material is the rigid polyurethane foam (RPUF), which features unique properties, such as low density, low thermal conductivity, high strength, excellent adhesion, and superior mechanical property. However, the inflammability of RPUFs presents a certain degree of danger in terms of fire safety. Hence, the flame retardancy and fire safety of this material must be improved. With regard to flame retardants, current development trends lean toward non-halogen and environmentally friendly products. Among the non-halogen flame retardants, the phosphorus-containing ones have received increasing attention because of their high flame retardancy. Previous reports have indicated that many kinds of phosphorus-containing compounds are used to improve the flame retardancy of RPUFs. However, the wide variety of phosphorus-containing compounds as a result of the variability of their phosphorus oxidation states feature different flame retardation mechanisms and efficiencies. Such variation makes the selection of effective flame-retardant systems for RPUFs confusing and complicated. Therefore, a systematic study into the influence of the phosphorus oxidation states and structure–property relationships of phosphorus-based compounds on the flame retardancy of RPUFs is extremely necessary in the development of RPUF insulation materials with high flame retardancy and fire safety.
In this dissertation, different kinds of phosphorus-containing polyols with similar structures and different phosphorus oxidation states were synthesized through molecular design, characterized, and then introduced into the RPUF matrix by employing "reactive" methods. The influence of phosphorus oxidation states on the thermal stability and flame retardancy of the RPUFs were evaluated and discussed. For comparison, different kinds of phosphorus-containing melamine salts with similar structures and different phosphorus oxidation states were synthesized and then added into the RPUF matrix by employing "additive" methods. Then, the influence of phosphorus oxidation states on the thermal stability and flame retardancy of the RPUFs were again evaluated and discussed. Expandable graphite (EG), which possesses high flame retardancy in the condensed phase, was used to further improve the flame retardancy of the RPUFs. The use of EG was aimed at understanding the effect of its synergy with the phosphorus-containing polyols and melamine salts on the flame retardancy of the RPUFs. The optimal molecular structure design of the polyurethane foam itself was also considered to improve the flame retardancy of the RPUFs. To achieve a high flame retardancy rating, flame retardation techniques for the structural, condensed, and gas phases were combined. Finally, the smoke production behaviors of the RPUFs and the smoke suppression properties of several kinds of smoke suppressants were preliminarily investigated. The research process employed for this dissertation is as follows.
1. Three reactive phosphorus-based polyols, namely, bis(4-hydroxybutyl) phenylphosphate (BHPP-1), bis(4-hydroxybutyl) phenylphosphonate (BHPP-2), and 2-carboxyethyl phenylphosphinic acid glycol ester (CEPPG), were successfully synthesized and characterized. RPUF samples with various phosphorus contents and oxidation states were prepared by the in situ polymerization between polyols and isocyanate. The flame retardancy and thermal properties of the composites were evaluated on the basis of the limiting oxygen index (LOI), cone calorimetry, and thermogravimetric analysis (TGA) results. The thermal degradation processes were investigated via real-time Fourier transform infrared spectroscopy (RT-FTIR). The LOI results indicated that the flame retardancy of the three compounds in the RPUF matrix followed the sequence of BHPP-2 > BHPP-1 > CEPPG at the same phosphorus contents. The peak heat release rate (PHRR) and the total heat release (THR) rate of the modified RPUF samples were significantly reduced compared with the rates of the pure RPUF. However, the smoke production of the modified foam increased obviously, which is an undesirable condition.
2. Three additive phosphorus-based melamine salts, namely, melamine phosphate (MP), melamine phosphite (MPi), and melamine hypophosphite (MHP), were successfully synthesized and characterized. RPUF samples with various phosphorus contents and phosphorus valences were prepared by mixing these compounds with the raw materials before the RPUF foaming process. The flame retardancy and thermal properties were evaluated on the basis of the LOI, UL-94, cone calorimetry, and TGA results. The thermal oxidative degradation of the selected RPUF samples was tested by RT-FTIR. The LOI results indicated that the flame retardancy of these three compounds in the RPUF matrix followed the sequence of MHP > MPi > MP at the same loadings and phosphorus contents. The PHRR and THR of the modified RPUF samples were significantly reduced compared with the rates of the pure RPUF.
3. EG and phosphorus-based polyols and phosphorus-based melamine salts as synergistic flame-retardant systems for RPUFs were prepared in varying proportions and subsequently studied. The flame retardancy and thermal properties of the RPUF/EG/phosphorus-containing polyol and RPUF/EG/phosphorus-containing melamine salt systems were evaluated on the basis of the LOI, cone calorimetry, and TGA results. In the former system, significantly improved LOI and reduced PHRR values were observed when EG and phosphorus-containing polyols were simultaneously added into the RPUF. The best synergistic flame-retardant effect was observed between the EG and the CEPPG. In the latter system, a synergistic flame-retardant effect was observed between the EG and the MP or MPi at a mass ratio of 1:1. However, the synergistic effect between the EG and the MHP was not obvious. The experimental analysis revealed that foam density was an important factor that affected the combustion properties of the RPUF. The PHRR, THR values, and combustion time increased with the increase of the foam density.
4. Polyisocyanurate-modified polyurethane (PIR-PUR) foams filled with flame retardants in the condensed phase (ammonium polyphosphate, APP) and flame retardants in the gas phase (dimethyl methyl phosphonate, DMPP) were prepared with different APP/DMMP ratios. The flame-retardant effects of APP and DMMP on the PIR-PUR foams were investigated through the LOI test, UL-94 vertical burning test, microscale combustion calorimeter, cone calorimetry, and TGA. The LOI results showed that the addition of 15 wt% APP into PIR-PUR could increase the LOI value from 23% to 29.5%. However, increasing the loading of APP into PIR-PUR further to 20 wt% could not increase the LOI value. After adding 3 wt% of DMMP and 12 wt% of APP into PIR-PUR, a high LOI value of 31.5% was achieved. The cone calorimetry results indicated that the presence of DMMP delayed the ignition of the specimen and the time to reach the maximal PHRR.
5. Melamine fumarate (MF), as a kind of fumaric acid (reported as smoke suppressant) derivative, was synthesized and subsequently characterized. A series of flame-retardant PIR-PUR foams filled with MF or several kinds of metal compounds, which have been used as smoke suppressants in previous reports, were prepared. The flame retardancy and smoke suppression properties were evaluated by employing the LOI test and cone calorimetry, respectively. For the smoke production behavior, all the additives showed a certain degree of smoke suppression effect on both the PIR-PUR and flame-retardant PIR-PUR systems. For MF, the total smoke release was reduced by 20% in the flame-retardant system. Cu2O possessed the highest smoke suppression efficiency in both the PIR-PUR and flame-retardant PIR-PUR systems. The cone calorimetry results revealed that all these additives did not have a heat retardation effect on the pure PIR-PUR system but showed a reduced heat release in the flame-retardant PIR-PUR system, except for ferrocene.
- Rigid polyurethane foam, Phosphorus-containing flame retardant, Phosphorus oxidation state, Flame-retardant mechanism, Smoke suppression