The first element to consider in a formula is the oligomer or oligomer package suitable to the fundamental properties an application requires. Oligomers are molecular complexes that make up polymers, each with varying characteristics and properties. Types of oligomers include urethane, epoxy, polyester, and specialty oligomers which are usually designed to obtain a specific property or adhere to the requirements of a specific application. We will learn more about the oligomers within each group below.
Urethane oligomers, based on urethane chemistry, form when an isocyanate has reacted with a hydroxyl group. The subcategories of urethane oligomers vary depending on which isocyanate and which polyol, compound containing hydroxyl groups, is used. This categorical division is explained below.
There are two types of urethane oligomers based on the isocyanate; aliphatic and aromatic. Each group conveys differing characteristics:
Urethane oligomers can also vary based on the type of polyol used. Listed below are three of the most common groups of oligomers by polyol:
Generally, urethane acrylate oligomers are higher in cost, but provide superior toughness in comparison to other oligomers. Comprised of both hard and soft segments, urethane acrylates yield good chemical resistance, high strength, and more flexibility. Moreover, the strength and flexibility of oligomers can be adjusted based on the isocyanate and polyol, providing flexibility when formulating. Some examples of polyester urethanes include polyester urethane acrylates and methacrylates. Examples of polyether urethanes include polyether urethane acrylates and methacrylates.
Like urethanes, polyester oligomers are prepared by polyester monomers. These oligomers are typically produced in a solvent process, in which the solvent is stripped from the oligomer and causes the product to have higher color than other processes. Some products in this class, especially those with lower molecular weight, can have high irritancy. In short, polyester oligomers offer good performance – though usually not as good as urethane oligomers – as well as considerable cost savings.
The term “epoxy oligomer” may be somewhat misleading because they are named for the base epoxy resin that makes up the backbone and do not actually have any free epoxy groups left. Types of epoxy oligomers include:
These oligomers include melamine, silicone, and others that do not classify easily into other oligomer families. Typically, these are specialty items chosen for specific properties, like high thermal stability. Some oligomers are hydrophobic, while others are water-dilutable.
Classifying oligomers into families is a helpful tool for understanding the physical and chemical properties each gives to a formulation. Additionally, there are a few properties to consider for all oligomers, including molecular weight and functional groups.
Oligomers are produced with varying molecular weight; in general, the higher the molecular weight, the higher the viscosity. They are also produced with different amounts of functionality—mono-functional, di-functional, tri-functional, and higher multifunctional products all exist. The higher the functionality, generally, the higher the viscosity as well. High-functional products will yield faster cure speeds, less flexibility, higher strength, more shrinkage, and higher chemical resistance.
Another important variation amongst oligomers centers around the functional group; acrylate vs. methacrylate. Methacrylates tend to cure at a slower rate than acylates. However, methacrylates are generally higher in Tg, help to increase tensile strength, have less chance of causing skin irritation, and exhibit less shrinkage than acrylates. Oligomers are a foundational element in any formulation and can make up as little as 5% to as much as 70% of a formula. This concentration is generally dictated by the viscosity of the oligomer and the desired viscosity of the final product. In selecting oligomers for a formulation, it’s important to choose a product that provides most of the properties you will need. UV and chemical resistance, tensile strength, toughness, propensity to yellow, and shrinkage are all significant points to consider.
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If you are exploring oligomer options, our team of Applications Engineers will be happy to help you evaluate your requirements and choose the appropriate product to meet your needs.