2.1.2 - Tertiary amines

Tertiary amines are the most widely used catalysts in the manufacture of cellular and solid PU's. Some of the more commonly used tertiary amines are shown in Table 2.3. In PU foams, tertiary amines control the blowing and polymerization reactions, and play a relevant role in PU end properties.

In agreement with its chemical behavior, flowability, and PU end properties, tertiary amines may be classified as: Gel catalysts, Blow catalysts, Delayed action catalysts, Skin cure catalysts, and Reactive catalysts.

Gel catalysts - Gellation catalysts promote the polyaddition reaction of isocyanate with polyol. These catalysts are non-sterically hindered tertiary amines having the free electronic pair available on the nitrogen atom; they activate the C=N moiety of the isocyanate group. This activated complex quickly reacts with the active hydrogen atom of the polyol, forming the PU. DABCO or TEDA is a strong general-purpose gel catalyst, due to the absence of steric hindrance, otherwise, dimethylcyclohexylamine (DMCHA) is sterically hindered but it is a strong base, widely used in PU rigid foams.

Blow catalysts - These catalysts promote the blowing reaction between the isocyanate and water, forming polyurea and carbonic gas, which acts as a blowing agent. They are non-sterically hindered tertiary amines with ethylene groups between the two (nitrogen or oxygen) active centers. These two active centers are able to chelate water (Figure 2.6a), thus increasing its reactivity.

Delayed action catalysts - Foam flowability is very important to properly fill the mold, especially with large and complicated profiles. On the other hand, good curing is essential to perform short demolding times and to avoid fingerprinting when the pad is taken out of the mold. To combine excellent in-mold flowability and fast curing, delayed-action catalysts were developed. When starting reactivity is too high, delayed-action blow catalysts should be employed; on the other hand, delayed-action gellation catalysts are used to delay the viscosity build-up of the reacting mixture allowing to flow easily in the mould cavity. The process to manufacture a delayed-action catalyst involves reacting a specified tertiary amine with a carboxylic acid. The resulting compound is composed of a salt and an excess of the starting amine. The salt has limited or no catalytic activity and when this blend is used in a foam formulation, the free amine launches the reaction, but when the reactions have progressed, with heat generation, the salt dissociates to yield back the amine and the acid. At this time the catalyst is de-blocked and has recovered the original activity. The released organic acid reacts with isocyanate-forming carbon dioxide and carbon monoxide, supplying an auxiliary source of blowing agent, resulting in less dense with more open cell foams. Normally, carboxylic acids such as formic and 2-ethylhexanoic acids are used as blocking agents. The unblocking temperature depends on the acid used. Strong acids require higher temperatures than the weak ones. In commercial applications the unblock temperature is located between 30oC and 60oC. There is some corrosion linked to the use of formic acid and the foams produced tend to be very closed and are therefore difficult to open.

Skin cure catalysts - The skin cure catalysts provide additional isocyanate reaction at the surface, improving the overall appearance of the part and eliminating defects such as fingerprint. There are two categories of surface cure catalysts. The first includes tertiary amines that have high vapor pressure and volatilize from the developing foam to the foam mold surface where they provide additional reactivity. Typical examples of these catalysts include triethylamine (TEA), N-methylmorpholine (NMM), and N-ethylmorpholine (NOR). The second group includes tertiary amines that are incompatible with the developing foam and migrate to the mold surface and provide the same effect as above. Typical amines of this category are modified morpholines.

Reactive catalysts - Reactive amine catalysts, as DMEA, DMAEE and TMAEE that contain a hydroxyl group, are able to react with isocyanates becoming chemically a portion of the polymer matrix. The disadvantage of polymer-bonded catalysts may be the deterioration of foam physical properties, especially when the foam is exposed to hot and humid climates. They are used in special applications, such as dashboard production, in order not to migrate into the PVC skin, this causing its discoloration.

2.1.2.1 - Catalysis mechanism

Since Otto Bayer's pioneering work, the catalysis mechanism of isocyanates reactions with alcohols (gel reaction) or with water (blow reaction) was studied by several authors and especially by Baker and Farkas. Baker postulated the formation of a complex consisting of an isocyanate and a tertiary amine catalyst, followed by the attack by nucleophilic reagents (water or alcohols) (Figure 2.2).

Figure 2.2 - Tertiary amines catalysis Baker's mechanism

Farkas based his theory on the initial formation of a complex between the nucleophilic reagent (alcohol or water) and the tertiary amine (Figure 2.3). This complex then reacts with the isocyanate, and the gel reaction (with polyol), or blow reaction (with water) occurs, with regeneration of the tertiary amine. In this mechanism the amine basicity is the predominant factor.

Figure 2.3 - Tertiary amine catalysis Farkas' mechanism

2.1.2.2 - Gel and blow catalysis

Alcohol interacts with tertiary amines like TEDA (triethylenediamine) through a single hydrogen bond (Figure 2.4).

Figure 2.4 - Interaction of water (R=H) and alcohol with TEDA

Water is able to interact with tertiary amines through different modes. Water interacts with TEDA (a strong gelling catalyst) through a single hydrogen bond like an alcohol. On the other hand, tertiary amines of higher blowing activity such as BDMAEE [bis(2-dimethylaminoethyl)ether] or PMDETA (N,N,N',N',N''-pentamethyl-diethylenetriamine) (Figure 2.5) are able to chelate water (Figure 2.6). This indicates that a single nitrogen-water hydrogen bond does not result in effective catalysis of the blowing reaction. The water-chelated complex, which is not feasible with TEDA, may then be the selective high-activity catalyst precursor.

BDMAEE

PMDETA

PMDPTA (N,N,N',N',N"-pentamethyldipropylenetriamine) is also unable to chelate water (Figure 2.6). Water chelation by the central nitrogen and one of the terminal nitrogens is disfavored by close contact between water hydrogens and hydrogens form the three-carbon bridge. These different modes of water binding provide an explanation for the experimental observation that acyclic tertiary amines with hetero atoms separated by a two-carbon bridge favor the blowing reaction more than tertiary amines with hetero atoms separated by a three-carbon bridge.

BDMAEE

PMDPTA

Table 2.4 includes the catalytic activity of many tertiary amines. The gelling and blowing catalytic activities were measured by a titration method in a standardized reaction system containing TDI, diethylene glycol and water. TEDA is a strong gelling catalyst. BDMAEE and PMDETA are strong blowing catalysts. TMHMDA, TMEDA and DMAEMP exhibit moderate activity between the gelling and blowing reactions. ROCONHC6H6 and R'NHCONHC6H6 are reactive tertiary amines.

2.1.3 - Organometallic catalysts