Adhesives and Sealants 101: Resins
The term resin comes from resinous, the description of materials that are amorphous and brittle yet soft and tacky. Rosin from trees is the easiest example to relate to - gymnasts and baseball players use it for grip. Rosin derivatives form the oldest group of tackifier resins.
The term resin is sometimes used to describe materials like polyvinyl chloride (PVC) and acrylonitrile butadiene styrene (ABS), which do not fit the description used here for tackifier resins, including molecular weight less than 20,000 Mz (typically less than 5,000 Mz); amorphous; melting points from liquid to 180°C; glass-transition temperatures (Tg) from liquid to 130°C; and colors from water white to brown.
Resins are used to modify the bulk rheology and surface-adhesion properties of polymers used in applications including adhesives, paints, inks, wax compounds, and chewing gum.
In some cases, resins provide tackiness and an increase in specific adhesion; in other cases, they provide film forming and viscosity reduction. When mixed with a polymer, a compatible resin will increase the Tg and reduce viscosity or hardness in contrast to a plasticizer, which generally will not modify Tg.
Types of ResinsMany types of resins are available from a range of suppliers. There are also a number of ways that they can be grouped. The most common way is by the feedstock used to produce them: hydrocarbon resins, rosin resins and terpene resins.
Hydrocarbon resins are generally made from petroleum-based feedstocks that are either aliphatic (C5), aromatic (C9), dicyclopentadiene (dcpd) or mixtures of these. The monomers are polymerized using catalysts such as AlCl3 or BF3, or, as in the case of dcpd, thermal polymerization can be employed. These resins can be sold as-is or after they have been hydrogenated to reduce either color or levels of unsaturation.
Rosin acids are derived from the tree root (wood rosin) or sap (gum rosin), or from the kraft process (tall oil rosin). Rosin acids can be tackifiers themselves, but it is more common to use their ester derivatives.
Terpene resins are derived from terpene feedstocks, which means either wood sources or citrus fruit. AlCl3 also polymerizes these resins.
Selecting a ResinWith so many resins available, how does one choose the right one for a particular application? Basic criteria narrow down the choice, and then a number of secondary filters are applied. Resin suppliers really sell two basic properties: compatibility and stability.
In most formulations, especially adhesives, the polymer is the starting point of the mixture, which means that the resin must have a known and understood compatibility with the polymer and with other ingredients. Viscosity, color and oxidative stability are important criteria which, if not understood, can affect application and end-use properties, resulting in blocked nozzles, poor coatings, and failed or discolored adhesives. Stability is proportional to cost, so choices are made based on what stability is really required for the adhesive.
Secondary filters for choosing a resin include FDA clearances, odor, color and cost.
New resins are often developed as a response to market and polymer changes. Trends such as biodegradability and repulpability will result in new polymers with a different structure that will require different resins and other additives. Advances in radiation-curing techniques will also result in new polymers and resins with the required ability to add adhesion and modify rheology without impacting curing mechanisms.