RESINS AND BASE POLYMERS

INTRODUCTION TO RESINS

The term resin comes from resinous, the description of materials that are amorphous, brittle, and 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 as follows:

• Low 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
• Colors from water white to brown

Resins are used to modify the bulk rheology and surface adhesion properties of a variety of polymers for many applications, including adhesives, paints, inks, wax compounds, and chewing gum. In some cases, resins provide tackiness and an increase in specific adhesion, and in other cases they provide film forming and reduction in viscosity. 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.

There are many types of resins 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, either aliphatic (C5) or aromatic (C9), or dicyclopentadiene (dcpd), or mixtures of these. The monomers are polymerized using catalysts such as AlCl₃ or BF₃, or 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 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, either from wood sources or from citrus fruit. AlCl₃ also polymerizes these resins.

Choosing a Resin

With 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 very important criteria and, 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 around what stability is really required for the adhesive. With basic criteria of compatibility and stability in place, secondary filters come into play, such as: 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.