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The “Alien Family” of the Chemistry World

Release time:

2025-01-10 17:02

The “Alien Family” of the Chemistry World

Isocyanates—this name might sound like it belongs to the “Alien” family of the chemical world, but don’t be alarmed! In fact, they’re a group of versatile creators who excel at crafting all sorts of polyurethane masterpieces in the molecular realm.

Imagine the isocyanate family as a band: there’s the soloist, the monoisocyanate, playing its chemical melody all by itself; then there’s the duet, the diisocyanate, which collaborates seamlessly to create beautiful harmonies; and of course, there’s the polyisocyanate—a full symphony orchestra, crafting even more intricate musical compositions.

According to their molecular structure, isocyanates can be further classified into aliphatic, alicyclic, and aromatic types.

The primary diisocyanates used in the synthesis of polyurethane acrylates include toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), diphenylmethane diisocyanate (MDI), dicyclohexylmethane diisocyanate (HMDI), hexamethylene diisocyanate (HDI), and phenylene diisocyanate (XDI).

1. Toluene diisocyanate (TDI)

TDI, the chameleon of the chemical world, appears as a colorless or pale yellow, transparent liquid with a pungent, irritating odor. As a key member of the isocyanate family, TDI’s primary drawback in the manufacture of polyurethane products is its susceptibility to yellowing—a factor that requires careful attention during formulation design.

TDI has high reactivity, primarily due to the electron-withdrawing effect of the benzene ring in its structure, which makes the reaction process highly dynamic. When synthesizing products, we need to control the reaction conditions just as carefully as we would control the heat in a kitchen—otherwise, the reaction could proceed too rapidly, much like cooking rice over too high a flame, which can easily result in burning if you’re not careful.

TDI has two isomers—2,4 and 2,6—which are like identical twins. Industrially, TDI is a mixture of these two isomers. Common TDI grades on the market include TDI-80/20, TDI-100, and TDI-65/35. The numbers preceding the hyphen indicate the content of 2,4-TDI; these grades are often referred to simply as TDI-80, TDI-100, and TDI-65.

Although TDI boasts an affordable price, high reactivity, and produces polyurethanes with high hardness, excellent chemical resistance, and good abrasion resistance, it suffers from very poor yellowing resistance—this is because it readily forms colored quinone or azo structures during photoaging.

2. Diphenylmethane diisocyanate (MDI)

The main structural isomers of MDI include 4,4-MDI, 2,4-MDI, and 2,2-MDI. The commonly available products are pure MDI and liquid MDI.

Pure MDI is a white to slightly yellowish solid crystal at room temperature and requires low-temperature storage—it’s practically a “cold beauty.” However, it can be somewhat inconvenient to handle. As a result, commercially available liquid MDI has emerged on the market.

Because of MDI’s symmetrical structure, the cured films made from it exhibit good strength, wear resistance, and elasticity. Although MDI outperforms TDI in these aspects, its resistance to yellowing is somewhat unsatisfactory—indeed, it’s even inferior to that of TDI.

3. Benzene diisocyanate (XDI)

XDI consists of 71% meta-XDI and 29% para-XDI. XDI is an aromatic diisocyanate; however, due to the presence of a methylene spacer between the phenyl group and the isocyanate group, it does not yellow as readily as TDI or MDI. Its reactivity is faster than that of MDI, but its tendency to yellow and its lightfastness are slightly inferior to TDI yet superior to MDI.

4. Hexamethylene diisocyanate (HDI)

HDI is an aliphatic isocyanate. Since it does not contain a benzene ring, it exhibits excellent resistance to yellowing and exceptional softness. HDI’s reactivity is akin to that of a slow-burning athlete—under normal conditions, HDI is typically reacted with water to produce biuret diisocyanates, or it can be converted into trimers via catalysis for use.

5. Isophorone diisocyanate (IPDI)

IPDI is a cycloaliphatic isocyanate. Polyurethane resins made from IPDI exhibit excellent resistance to yellowing, flexibility, and good hardness. They are primarily used in the production of products that require resistance to yellowing and are relatively expensive, typically found in high-end applications.

6. Diisocyanate of dihexylmethane (HMDI)

HMDI is also a cycloaliphatic diisocyanate with lower reactivity than TDI. The polyurethane acrylates synthesized using HMDI exhibit excellent yellowing resistance, as well as good flexibility and hardness.

The reactivity of the -NCO group in diisocyanates toward alcohol hydroxyl groups depends on the structure of the diisocyanate. Aromatic diisocyanates are more reactive than aliphatic diisocyanates. If there is another group such as a -CH3 group adjacent to the -NCO group, the spatial steric hindrance will reduce the reactivity; thus, in TDI, the -NCO group at the 4-position is significantly more reactive than the one at the 2-position. In diisocyanates, the first -NCO group is more reactive than the second -NCO group.

The most widely used and highest-volume-consuming isocyanates are TDI and MDI, followed by HDI, IPDI, and HMDI; the usage of other isocyanates is relatively low.

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