Controlling Gel Time Drift in Unsaturated Polyester Resins Promoted with Cobalt Accelerators

By Richard A. Reichert, Jr.

Most unsaturated polyester resins (UPR) used for room temperature cure applications are designed to work as a two-component system. The first component is the pre-promoted resin and the second component is the organic peroxide initiator. The pre-promoted resin is comprised of the unsaturated polyester resin, reactive diluent (typically styrene) and the accelerator. It may also contain additional additives, such as rheology modifiers, secondary accelerators, pigments, fillers, inhibitors, flame retardants, anti-skinning agents, as well as other ingredients, depending on the specific application and type of fabrication process used.

Pre-promoted resins are very practical because only one additional component, the initiator, must be added to cure the resin. However, pre-promoted resins can be susceptible to a shelf life stability phenomenon known as gel time drift. Gel time drift is defined as a change in the resin’s measured gel time compared to the original gel time measured at the time of its manufacture.

Gel time drift is typically associated with a progressive increase in the resin’s gel time and is attributed to the loss of the accelerator’s activity over time. The loss of accelerator activity can also affect the cure time for the part. Control agents are used to prevent gel time drift in both pre-promoted and customer compounded UPR.
Let’s take a close look at the gel time drift phenomenon as it relates to cobalt accelerated pre-promoted unsaturated polyester resins and how “feeder type” cobalt control agents are used to eliminate gel time drift.

Gel Time Drift
Gel time drift is a common occurrence in pre-promoted unsaturated polyester resins. The UPR’s curing activity diminishes as the product ages. The loss of curing activity is prevalent in cobalt promoted resins containing thixotropic agents, pigments, fillers and flame retardants. It is not uncommon in standard systems as well. A classic example of gel time drift is a thixotroped pre-promoted resin for a gel coat or laminate application that has a gel time of 15 minutes at the time of its manufacture, but after several weeks has a gel time of 20 to 30 minutes or even longer. In severe cases, this affect can be observed after several days.

The most common cause of gel time drift is through the loss of active cobalt from the organic resin phase, where the cobalt is needed to activate the initiator in order to form free radicals at room temperature. These radicals in turn begin the crosslinking (curing) reaction between the unsaturated polyester double bonds and the styrene diluent. Cobalt accelerators are metal carboxylate liquids that reside in the organic resin phase. Gel time drift takes place when the cobalt is lost from the organic phase and adsorbed onto the surface of particles in the system (i.e., fumed silica, pigment, filler, etc.).

In general, the cobalt is electronically bound there to passivate the ionic charge of the particle surface. Bound to the particle in this way, its ability to catalyze initiator dissociation is severely reduced. As a result, the gel and cure times of such resins can be profoundly affected. For all practical purposes the cobalt is lost, no longer available to activate the initiator. Using a “feeder type” cobalt control agent is the most common approach to eliminating gel time drift commonly evoked through the adsorption/extraction mechanism.

Control Agent
Historically, a “feeder type” cobalt control agent has proven the most effective solution for eliminating the gel time drift phenomenon in cobalt promoted unsaturated polyester resins.

This type of control agent provides a controlled rate of cobalt solubility and is designed to supply both initial and continuous activity. It is referred to as a “feeder type” control agent because it slowly “feeds” cobalt back into the organic phase of the resin at the same time soluble cobalt is being lost through adsorption. Thus, an essentially level cobalt concentration is maintained in the organic phase. This type of control agent consists of a soluble cobalt in the form of a cobalt carboxylate (typically cobalt neodecanoate) and a dispersed inorganic cobalt hydroxide. The cobalt carboxylate provides the initial activity of the control agent. The cobalt hydroxide supplies continuous activity by reacting with acid groups in the system to generate soluble cobalt, slowly feeding it back into the system.

The “feeder type” control agent is not intended as a replacement for the normal cobalt accelerator utilized, but should be used as a companion product to the standard cobalt accelerators (cobalt naphthenate, cobalt octoate, and cobalt neodecanoate). The actual ratio of accelerator to control agent in the pre-promoted resin is determined empirically by measuring gel and cure time drift with the fully formulated resin. Typical starting ratios of accelerator to control agent are 2:1 and 3:1 depending on formulation and shelf life requirements. “Feeder type” control agents are referenced by the total percent cobalt metal in the product, typically about twenty-one percent. They may also require special handling instructions to insure product consistency because of the potential for settling.

Experimental Determination
“Feeder type” control agents are a proven and effective solution for eliminating the gel time drift phenomenon in cobalt promoted unsaturated polyester resins. Gel time drift is caused by the loss of active cobalt accelerator to the resin system. The “feeder type” control agent provides a mechanism to maintain a consistent concentration of active cobalt accelerator in the resin throughout its shelf life. “Feeder type” control agents are used in combination with standard cobalt accelerators. The ratio of standard cobalt accelerator to control agent is formulation dependent and determined experimentally.

Richard A. Reichert Jr. is a senior research chemist at OMG Americas: 440.808.7228; Richard.reichert@na.omgi.com.