Aromatic polyols for simpler rigid PU formulations

A new collection of highly aromatic multi-functional polyols from Hexion could provide a range of useful properties in rigid polyurethane and PIR applications

The ability to mix and match polyols and diisocyanates allows polyurethane materials to be created with a versatile range of properties. Hexion has recently introduced a range of highly aromatic polyether polyols that can give useful performance advantages in rigid polyurethane and PIR applications.

This paper concentrates on the evaluation of two different polyol replacements, one in rigid PU applications and the second in spray foams. In both cases, the substitutions can produce formulations with a significant improvement in and flammability performance. There was a compressive strength improvement in the rigid formulation too. The foams produced in this work burnt significantly more slowly than the formulations they were compared with. It is possible that using Resonance polyols could simplify formulations by reducing the amount of fire retardant needed to meet standards.

Additionally, despite their significantly lower functionality than the sucrose-based polyether reference polyol used in this work, foams made with the modified formulations were dimensionally stable, and well within the physical specifications of PUR foams used in the industry.

The new polyols have greater reactivity than standard materials. Such formulations have shorter gel, rise and tack free times than the original formulations.

Although compression strength of foams produced with PL 203 and PL 205 were lower than references, their density is also lower than the references. Increasing the density of the test foams is likely to bring compression strengths closer to the reference.

We believe this paper shows that Resonance aromatic polyether polyols yield polyurethane test formulations with better flame retardance than polyols traditionally used in PU formulations. This could help to minimise the amount of flame retardants used in today’s formulations and meet standards in spray foams.

Inherent flame retardancy

Aromatic polyester polyols are usually included in polyurethane formulations because they improve the foam’s response to fire. They are widely used in polyurethane insulation foams.1 However, their functionality is limited, and so they are used in conjunction with highly functional polyether polyols. This gives polyurethanes that have a balance of physical properties, such as dimensional stability.

However, adding these polyether polyols increases the flammability compared with aromatic polyols, and this has to be compensated for in formulations. This is typically done by increasing levels of phosphorous or halogen-based flame retardants. Halogenated fire retardants are under regulatory scrutiny, and there is a desire in the industry to reduce or eliminate them from formulations².

Hexion’s Resonance polyether and aminated aromatic polyols are highly aromatic and have high functionality. Th e aromatic content ranges from 33 to 75% aromatic content across the PL91, TM91, PM91 and PM92 series.

PL series materials are liquid at room temperature and have between 32 and 33% aromatic content by weight. This is higher than some of the commonly use d aromatic polyester polyols.

The functionality of the Resonance polyols is significantly higher than aromatic polyester polyols, and ranges from 2.6 to 6.0. The Resonance TM (and TL) series are unusual because they contain aromatic nitrogen. This, coupled with their very high functionality, contributes to their fast reaction times, improved mechanical properties and good reaction to fire.^3,4,5

We carried out two evaluations. In the first, Resonance polyether polyols replaced sugar-based polyether polyol with Resonance PM and TM polyols in the rigid PU foam formulation, and the materials used can be seen in Table 1. In the second, Resonance PL91 203 and PL91 205 polyether polyols replaced sugar-based polyether and aromatic polyester polyols in a spray foam formulation. These are detailed in Table 2.

Foams were prepared in a similar manner for both studies. In Evaluation 1, the mixture was subject to mixing in a high torque mixer at 3,100 rpm for 10 seconds. Evaluation 2 was mixed for 5 seconds.

In both cases, the mixtures were transferred into an open cake box and allowed to rise freely. The foaming profile, including cream time, gel time, rise time and tack free time, were measured. Table 4 and 5 show the replacement levels of reference polyols with Resonance polyols and give important properties of the finished foams. The test methods used are shown in Table 6. weight of residual material after burning increased significantly. Table 4 shows that in formulation Mod 1, when 50% of the sucrose base d polyol was replaced with 50% of the blend that contains 30% of TM polyol the burning rate fell by about 53% and the weight retention increased 74% (Mod 1).

When 70% of the reference was replaced by the TM/RL blend, burn rate fell by about 59%, and weight retention increased by 111%. These results are exceptional by any standard.

The reference formulation is a mixture of sucrose-based polyol PEP GS2, an APP and a Mannich polyol, which is quite common for a spray foam. The catalyst package consists of a tertiary amine, potassium salt for trimerisation and bismuth-based gel catalyst.

Solstice LBA and water were used as blowing agents. Polymeric MDI Lupranate M20S with average functionality of 2.7 was the isocyanate. Foams were prepared at an isocyanate index of 120. The formulation was free of flame retardant additives. Dicarboxylic dimethylesters (DBE) esters were used as a viscosity modifier.

Replacing 75% of the PEP GS2 in Mod 3, which contains 50% of PM91 the burn rate fell by about 65% and the amount of mass retained increased by 94% compared to the reference. These results are shown graphically in Figure 1.

Model spray foam

In the Evaluation 2 study, a model spray rigid PUR foam formulation was used to evaluate the effect of Resonance PL91 203 and PL91 205 on PU system reactivity, foam density, compressive strength properties and resistance to flammability.

Table 5 provides formulation details and summarises the results of physical and fire testing. The Hexion materials produced foams with similar cream times but faster reactivity. This is shown by shorter gel, rise and tack free times in Table 6. These could be altered by modifying the catalyst packages if needed. Foam density at around 1.78 lb/cu ft (28.5 kg/m3) with the Resonance materials was lower than the 1.9 lb/cu ft (30.4 kg/m3) of the standard. Future work will be done to optimise this.

All foams had uniform cell shape. Formulations with Resonance had finer cells as was the case in Evaluation 1. The dimensional stability at 70°F/ 95% relative humidity (RH) is well within the industry standard for PUR foams.

The compression stress for the reference foams were significantly higher than the four foams produced from PL91 203 and PL91 205. But normalising the compression stress data to the density of the reference foam brings the values of the modified foams quite close to that of the reference (Ref. 2). The normalised compressive stress at 10% strain is very close to the reference.

Burn properties

The value of the Resonance PL91 203 and PL 91 205 polyols is evident in their ability to influence the burn properties of the foam. For instance, 100% replacement of sugar-based PEP with PL91 203 and PL91 205 reduces the burn rate from 140 mm/min to 93 mm/min and 85 mm/min respectively. This is accompanied by gain in mass retention from 64% to >70%.

Total replacement of sugar-based PEP and 33% of aromatic polyester polyol with higher amounts of PL91 203 and PL91 205 in the formulation also resulted in significant reduction in burn rate. This is shown in Table 5. The lower mass retention for the Mod 7 warrants a repeat of the test. Another outstanding result is that while flame propagated along the length of 6/6 of the reference foam samples (Ref. 2), the flame did not propagate the whole of the length of a number of the modifications. This is shown in Fig. 2 & 3.

In conclusion, aromatic polyester polyols PM91, TM91 and PL91 can replace a number of traditional sugar-based polyols and polyether polyols in rigid and spray foam applications. Such formulations produce foams with acceptable physical properties and good flame retardance without using halogenated or phosphorous based materials in the foam formulation.

The full details of these formulations are available in the paper presented at the CPI meeting in Florida in 2019.

REFERENCES

1. E. Dominguez Rosado et al., Polymer Degradation and Stability, 2002, 78, 1

2. Environ Health Perspect 2004, 112, 9

3. G. Viswanathan et al. PU Magazine, Jun/Jul 2019, vol 16, no. 3

4. G. Viswanathan et al. Novel Nitrogen Containing Aromatic Polyols for Rigid Polyurethane Foams, 2018 Polyurethanes Technical Conference

5. M.F. Sonnenschein, Polyurethanes: Science, Technology, Markets, and Trends; Wiley

Authors

Ganapathy Viswanathan, Pravin Kukkala, Steve Crain, Malik Rizmanova, Basilissi Luca, Hexion; and Ibrahim Sendijarevic, Troy Polymers