Isocyanurates thermal stability

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

The epoxy resin component is made by a 2-stage process involving reaction of l-chloro-2,3-epoxypropane (epichlorhydrin) with isocyanuric acid to give the l,3,5-tris(2-hydroxy-3-chloropropyl) derivative, which is then treated with sodium hydroxide to eliminate hydrogen chloride to form the title compound. One batch contained more than the normal amount of hydroly sable chlorine, and when excess epichlorhydrin was distilled off, the residual material decomposed with explosive violence. It was later established that the abnormal chlorine content was associated with reduced thermal stability, and criteria for hydrolysable chlorine, epoxy content and pH have been set to prevent distillation of off-spec, material. 

With regard to reactive flame-retardants, two routes can be followed to improve thermal stability and fire behavior of PU foams use of brominated or phosphorus-containing polyol or, for rigid foams, the introduction inside polymer backbone of more thermally stable structure than urethane, mainly isocyanurate, but also uretidione rings or carbodiimide.19... 

The availability of PMDI also led to the development of polyurethane-modified isocyanurate (PUIR) foams by 1967. The PUIR foams have superior thermal stability and combustibility characteristics, which extend the use temperature of insulation foams well above 150°C. The PUIR foams are used in pipe, vessel, and solar panel insulation glass-fiber-reinforced PUIR roofing panels having superior dimensional stability have also been developed. More recently, inexpensive polyester polyols based on residues obtained in the production of dimethyl terephthalate (DMT) have been used in the formulation of rigid polyurethane and PUIR foams. 

Modification of poly(carbodiimide) foams with polyols afford hybride foams containing urethane sections. However, the thermal stabilities of the poly (urethane carbodiimide) foams are lower. Using isocyanate trimerization catalysts, such as l,3,5-tris(3-dimethylaminopropyl)hexahydro-s-triazine, in combination with the phospholene oxide catalyst gives poly(isocyanurate carbodiimide) foams with improved high temperature properties. The cellular poly(carbodiimide) foams derived from PMDI incorporate six-membered ring structures in their network polymer structure. ... 

Oxazolidone-Modified Isor anurate Foams. The 2-oxazolidone, or 2-oxazolidinone, linkage is considered to be a cyclic urethane linkage, but its thermal stability is much higher than that of a urethane linkage. Kordomenos et al (207) compared the thermal stabilities of urethane, oxazolidone and isocyanurate linkages in terms of activation energy by using model compounds. The results obtained were as follows. 

Imide-Modified Isocyanurate Foams. The imide linkage is a thermally stable linkage, and therefore, imide-modified isocyanurate foams have higher thermal stability and flame retardance than urethane-modified isocyanurate foams. R. Grieve (114) prepared such foams in a one-shot process by reacting a polycarboxylic acid anhydride with an organic polyisocyanate in the presence of a catalytic amount of a monomeric homocyclic polyepoxide and a tertiary amine. 

EO polymers at high temperatures is an important area of NLO polymer research. One way to increase the thermal stability of NLO polymers is a crosslinked polymer system where NLO dye is covalently attached to the polymer network at more than one site. A bifunctional molecule, such as a molecule with an amino group and a (N-ethyl, N-hydroxyethyl)amino group, is one example, which reacts with a trifunctional isocyanurate comonomer. This polymer is reported to be stable at 75 °C for more than three months. 

Triazine and Other Heterocyclic Ring Formation. Several types of reactions can be used to form heterocyclic rings in which multiple C-N bonds contribute high thermal stability. When these are used to cross-link heat-stable oligomers, the resulting thermoset polymers may have high thermal stability and other useful properties. These include cyanate/cyanurate, isocyanate/isocyanurate, hexaazatriphenylene trianhydride, and phtha-lonitrile/phthalocyanine. 

Increase in crosslink density, type of crosslinking and introduction of isocyanurate ring structures in the polymer-chain backbone has a strong beneficial effect on the thermal stability of polyurethanes and is discussed later in this chapter. It is known that thermal stability increases with increasing isocyanate content of urethane elastomers, and in addition to the formation of the thermally stable isocyanurate rings, stability is influenced by the different types of urethane-based groups formed, as shown in Table 3.9a. 

Isocyanurates and crosslinks based on such groups are known to possess the most thermally stable structures as they do not degrade below 270°C. PUs with high isocyanurate group contents, whilst possessing good thermal stability, can also be rigid, hard and sometimes brittle and hence were not suitable for use as elastomers in which flexibility and resistance to... 

A considerable increase in thermal stability occurs as a result of this technique of arranging for there to be a controlled amount of excess diisocyanate in the PU, when first cast, followed by conditioning and then further post-curing. These enhanced properties can be explained by proposing that the post-curing operation forms a series of isocyanurate structures in the PU which result in the following type of crosslink ... 

Isocyanurate structures are well known in the polyurethane field as possessing outstanding thermal stability and also have the particular property of not melting instead they decompose and eventually char when heated to high tempieratures. 

Of the two reactions, isocyanurate formation is the most widely utilized. Diisocyanates can be converted via this reaction into trifunctional isocyanurate derivatives and subsequently used to introduce branching and crosslinking into a polyurethane.These crosslinks have greater thermal stability than either allophonate or biuret linkages, and hence they are more useful in elevated temperature applications. 

As infrared characterization of solid residue and high boiling products has shown [146], carbodiimide functionalities are formed at the thermal decomposition of nylon-6 with melamine and its salts. An unusual mechanism of chain scission of nylon 6 through CH2-C(0) bonds [148] is likely to become operative in the presence of melamines (Scheme 3.4.1). The resultant isocyanurate chain ends undergo dimerisation to carbodiimide or trimerisation to N- alkylisocyanurate. Carbodiimide can also trimerise to N-aikylisotiiazine. These secondary reactions increase the reactions increase the thermal stability of the solid residue and increase the yield of the char. 

Polyurethanes show poor thermal stability due to the labile urethane Unkages and ester bonds. It is reported that the initial decomposition point of urethane formed by MDI and poly(ethylene adipate) is 227°C (Krol, 2008). The decomposition of the SMPU yields diisocyanate and polyols. The MDI hard segment undergoes secondary reactions and produces more stable urea and isocyanurate stmctures (Krol, 2008). SMPU is also prone to hydrolysis in the presence of small amoimts of moisture, yielding an amine and carbonic acid. The... 

---