Formulation Overview Part 2: Oligomers
Now that we have an idea of what we want, it’s time to build our formula. The first element to consider is the oligomer, as it provides the foundation of the formula. Oligomers are molecular complexes that make up polymers, and there are many different types of oligomers that possess many different properties. To start off with, urethane, epoxy, polyester and miscellaneous oligomers all comprise their own families. We’ll learn more about the oligomers within each group below.
Urethane oligomers are based on urethane chemistry, which means that an isocyanate has reacted with a hydroxyl group. The urethane oligomers family contains several subcategories, depending on which isocyanate and which polyol (a compound containing hydroxyl groups), is used. This categorical division is illustrated below.
There are two types of urethane oligomers based on the isocyanate; aliphatic and aromatic. Each group conveys differing characteristics. For instance:
- Aliphatic oligomers possess superior UV resistance but are more expensive.
- Aromatic oligomers possess superior chemical resistance and adhesion to glass, and are less expensive. However, they do not possess good UV resistance and can yellow when exposed to UV light.
There are also several different types of urethane oligomers based on the polyol. Listed below are three of the most common:
- Polyethers give good all-around properties for the price.
- Polyesters are slightly better at toughness and are better with outdoor stability.
- Polycarbonates provide excellent outdoor stability and UV resistance.
Generally, urethane acrylates are higher-end oligomers. Comprised of both hard and soft segments, urethane acrylates yield good chemical resistance, strength, and flexibility, and can display superior toughness in comparison to other oligomers. Additionally, the strength and flexibility of urethane acrylates can be adjusted based on the isocyanate and polyol, providing flexibility in formulating. Some examples of polyester urethanes include polyester urethane acrylates and methacrylates. Examples of polyether urethanes include polyether urethane acrylates and methacrylates.
Polyester oligomers are based on polyester monomers and are very versatile, like the urethanes. These oligomers are typically produced in a solvent process. During this process, the solvent is stripped from the oligomer, which can cause the product to have a higher color than other processes. Some products in this class, especially those with low 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. Most epoxy acrylates do not have any free epoxy groups left; rather, they are named for the base epoxy resin that makes up the backbone. There are a few types of epoxy oligomers:
- Aromatic epoxies are generally low in molecular weight. These oligomers have some limitations, including high viscosity, lack of flexibility and the propensity to yellow. The most common type of aromatic epoxy is bisphenol A epoxy acrylates.
- Aliphatic epoxies tend to be more flexible than aromatics and are sometimes dilutable with water.
- Epoxidized oil acrylates tend to be flexible and relatively low in viscosity. These oligomers also offer low cost and good pigment wetting. However, they tend to have a lower cure speed.
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.
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Classifying oligomers into families is a helpful tool for understanding the physical and chemical properties each confers 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 on 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.
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.