What Is Cross-Linking?

In polymer chemistry, cross-linking is the formation of covalent or ionic bonds between polymers. This results in an interconnected network of polymers that is more rigid and resistant to mechanical stress, chemical attack, and heat. In industrial applications, cross-linking is done deliberately to boost the physical properties of a polymer designed for use in coating, elastomer, and composite industries, to name a few.

What Are Cross-Linkers?

Polymer cross-linking typically does not occur on its own, save for a few instances where heat, light, or other factors may facilitate a reaction. A cross-linker is used to cross-link polymers in industrial applications. This molecule contains functional groups on each end that react with chemical groups present on polymers to form cross-linked bonds.

Many types of cross-linkers are used commercially, with most designed to cross-link specific types of polymers. For example, diisocyanate cross-linkers are often used to react with hydroxyl or amine groups present on polyols and polyamines, respectively, to create polyurethane or polyurea polymers. For the remainder of this article, titanate and zirconate cross-linkers will be explored further. These provide a high degree of versatility to be used with several types of polymers, and they also impart vast performance improvements.

Titanate and Zirconate Cross-Linkers

Titanate and zirconate cross-linkers, such as Dorf Ketal’s Tyzor®, are a diverse class of organometallic molecules that vary based on the types of functional groups attached to a central titanium or zirconium atom via an ester linkage. The functional group attached to a titanate or zirconate cross-linker dictates several chemical properties of the molecule, such as its level of reactivity and its stability in the presence of water. Pictured below is Tyzor® AA-75.

How Do Titanate and Zirconate Cross-Linkers Work?

Titanate and zirconate cross-linkers function by reacting with nucleophilic chemical groups such as hydroxyl (-OH), carboxyl (-COOH), or amine (-NH2) groups. In some cases, titanate cross-linkers are designed to react with water and undergo hydrolysis to form a reactive hydroxy titanium chelate that is stable in water and forms complexes with hydroxyl, carboxyl, or amine groups. The general chemical reaction scheme of a titanate cross-linker with an organic polymer is shown below.

Why Use Titanate and Zirconate Cross-Linkers?

As mentioned previously, titanate and zirconate cross-linkers are extremely versatile. Since they react with hydroxyl, carboxyl, and amine groups, they are excellent cross-linkers for many different polymers used in industrial applications, such as the ones listed below.

 

• Polyamide
• Nitrocellulose
• Polyurethane
• Polyvinyl butyral
• Polyvinyl alcohol
• Cellulose acetate butyrate
• Cellulose acetate propionate
• Ethylene-vinyl acetate
• Cellulose
• Starch
• Guar

 

When cross-linking occurs, many of the polymer’s physical properties are improved. These properties typically increase the overall durability and resiliency of the chemistry and application they are used in. Examples of this are presented below.

Greater Mechanical Strength

Prior to being cross-linked, individual polymers may move freely past each other, allowing flexibility and deformation to occur in the chemistry or material they are used in. As mentioned, titanate and zirconate cross-linkers form bonds that create a fixed network of polymers. This network restricts the individual movement of polymers and serves as a better distribution for mechanical stress. Therefore, the cross-linked material is more rigid and resistant to deformation. It should be noted that while this property helps improve aspects such as impact resistance and tensile or compressive strength, it may cause embrittlement. Thus, carefully controlling the degree of cross-linking is important.

Improved Water Resistance

Water resistance is important for materials such as coatings, sealants, and adhesives engineered to perform in outdoor or marine environments that are constantly exposed to humidity or water. Once incorporated with polymers, titanate and zirconate cross-linkers form strong covalent bonds that make the chemistry resistant to hydrolysis. This improves the overall water resistance of the cross-linked system.

Enhanced Thermal Stability

Enhanced Thermal Stability Several materials that leverage the use of titanate and zirconate cross-linkers, such as heat-resistant coatings, also benefit from the strong covalent bonds that are formed. These bonds increase the overall thermal stability of the polymer network, increasing the heat resistance of the chemistry or material compared to before the polymer was cross-linked.

Example Applications of Titanate and Zirconate Cross-Linkers

Tyzor® Titanate and Zirconate Cross-Linkers

Tyzor® titanates and zirconates are ideal cross-linkers for improving the durability and efficiency of the polymer chemistry that constitutes many industrial products. Several Tyzor® options exist based on the relative chemical reactivity and water stability needed for each application. Recommendations for the specific applications above are listed within the linked articles. A full list of Tyzor® titanates and zirconates can also be viewed here. Click below to discuss your application and requirements for cross-linkers and one of our product experts will assist you further.