Tackifiers (natural, synthetic)
- Natural Tackifiers Stabilization
- Synthetic Tackifiers Stabilization
Natural Tackifiers Stabilization
Rosin-based tackifiers find continued use in hot melt adhesives because of their
unique property of providing compatibility to a wide variety of adhesive components.
Unmodified rosins, however, are easily oxidized on account of their main constituent,
abietic acid (Figure 2), containing a conjugated double bond (a), tertiary carbon
atoms (b), and allylic hydrogen atoms (c).
Figure 1 : Sites of potential oxidation (abietic acid)
Because of their tendency to degrade, natural rosins must be modified to meet the
stability requirements of the hot adhesives. Improved stability can be developed
by several processes such as hydrogenation, disproportionation, and dimerization,
followed by esterification of the rosin with glycerol or pentaerythritol.
Esterification Catalyst and Bleaching Agent
Various catalyst systems such as TNPP, calcium acetate, zinc acetate, and zinc oxide
are used to accelerate the esterification reaction times.
Catalyst offers the shortest reaction time and produces a resin with reduced color
and lower acid number. In addition, they imparts a "carry through" antioxidant
effect by offering improved heat stability of the final resin.
Figure 2 : Rosin Esterification : influence of catalyst
As shown in the autoxidation mechanisms of adhesives, hydroperoxides are formed
during degradation. These hydroperoxides are stable at ambient temperatures, and
may accumulate in the raw material during storage. Upon heating, they may decompose
and induce further degradation. Therefore, the determination of hydroperoxide content
is a good method for quality control for these materials.
Figure 3 : Hydroperoxide Formation of Rosin Ester During
Hydroperoxide content correlates well with melt viscosity and color changes after
heat treatment: ROOH formation (Figure 3) and color development (Figure 4) present
similar profiles during oven aging. The color increase of tackifiers after heat
treatment is caused by hydroperoxide decomposition and subsequent chromophore formation.
Figure 4 : Gardner Color of Rosin Ester During Aging
A suitable stabilizer system should therefore be used to provide long-term oxidation
resistance to the tackifier and to ensure consistency of its key properties.
Synthetic Tackifiers Stabilization
Synthetic hydrocarbon resins make up another major group of tackifier resins. Synthetic
tackifier resins consist of hydrocarbon resins (C5-, C9-, aliphatic or aromatic),
a collection of modified or special resins, primarily phenolics, and polyterpenes.
Stabilization at low temperatures
As with rosin esters, synthetic tackifiers tend to oxidize during shipment and storage
unless they contain an antioxidant. This oxidation is usually observed as an accumulation
of hydroperoxides, which greatly influences tackifier properties that relate to
Figure 5 exhibits hydroperoxide formation in a C5-aliphatic hydrocarbon resin after
low temperature aging at 40°C.
Figure 5 : Hydroperoxide Formation in a C5-aliphatic
Hydrocarbon Resin after 15 Days Oven Aging at 40°C
Stabilization at high temperatures
Because hydroperoxides lead to discoloration at high temperatures, the use of antioxidants
can also prevent discoloration during long-term storage.
Figure 6 : Yellowness of a Hydrogenated Hydrocarbon Resin
After Oven Aging at 180°C.
Other benefits that are obtained with the use of antioxidants are excellent viscosity
stability and skin prevention. These results are applicable for other types of synthetic
tackifier resins as well.
It is strongly recommended that all tackifier resins (natural or synthetic) should
be protected by the addition of stabilizers as early as possible in their life cycle,
i.e. in the resin manufacturing process, to prevent hydroperoxide formation during