Tackifiers (natural, synthetic)

1. Natural Tackifiers Stabilization
2. 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).

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.

Stabilization

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 Aging

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 at 40°C

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 end-use performance.

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 storage.