This article provides an overview of resins and describes
how they are used in the adhesives and sealants industry.
Resin is a secretion of many plants, including coniferous trees.
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
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
reduce viscosity or hardness in contrast to a plasticizer, which generally will
not modify Tg
Types of Resins
Many 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, 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
Selecting 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 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
Secondary filters for choosing a resin include FDA clearances, odor, color and
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.