The present invention generally relates to a delayed curing resin composition. More specifically, the present invention relates to a resin composition that allows the resin and catalyst to be mixed for an extended period creating a long pot life composition which cures only when subjected to elevated temperature.
Generally, epoxy coatings are well known in the art and due to their exceptional durability and structural properties epoxy based protective coatings have gained commercial acceptance as protective and decorative coatings for use on a wide variety of materials. For example, epoxy based protective coatings represent one of the most widely used methods of corrosion control. They are used to provide long term protection of steel, concrete, aluminum and other structures under a broad range of corrosive conditions, extending from atmospheric exposure to full immersion in highly corrosive environments. Further, epoxy coatings are readily available and are easily applied by a variety of methods including spraying, rolling and brushing. They adhere well to steel, concrete and other substrates, have low moisture vapor transmission rates and act as barriers to water, chloride and sulfate ion ingress, provide excellent corrosion protection under a variety of atmospheric exposure conditions and have good resistance to many chemicals and solvents. As a result, numerous industries including maintenance, marine, construction, architectural, aircraft and product finishing have adopted broad usage of epoxy coating materials.
The most common material utilized in the epoxy coating industry today is a multi-part epoxy material. In general the epoxy includes a first base resin matrix formed from a bisphenol material such as bisphenol A (BPA) and at least a second catalyst or hardener, although other components such as a pigment agent or an aggregate component may also be added. While the two parts remain separate, they remain in liquid form. After the two parts are mixed together, they begin a curing process that is typically triggered by exposure to heat, humidity or a ultra-violet light source, whereby the mixed material quickly begins to solidify. As a result, it is necessary to mix only a sufficient amount of compound such that it can be worked effectively before set up occurs. Accordingly, the use and application of these compounds is a tedious, slow and expensive proposition.
The hardeners are typically nitrogen-containing bases that are well known to the person skilled in the art as curing agents or curing accelerators for epoxy resins. Such systems have, however, only limited storage stability because those bases react with epoxides even at relatively low temperature, in some cases even at room temperature, which is manifested in an increase in the viscosity of the epoxy resin formulation and, on prolonged storage, results in hardening of the mixture. The greater the reactivity of the nitrogen-containing base, the lower the storage stability of the epoxy resin mixture and the shorter the pot life. For that reason, such systems are formulated as two-component systems, that is to say the epoxy resin and the nitrogen-containing base are stored separately and mixed only shortly before processing.
There has been no shortage of attempts at improving the storage stability of such systems by developing appropriate curing systems. The problem posed is particularly complex because, at the same time as the requirement for a high storage stability and a long pot life, there should not be any deterioration either in the reactivity at the desired curing temperature or in the properties of the fully cured materials. For example, adsorption techniques have been used to control and modify various types of chemical reactions. These techniques usually involve adsorbing a chemical reagent in an adsorbent material. Commonly used adsorbent materials for this purpose are materials having internal pore structure and active pore sites, and can consist of silica gel, certain types of carbon black, activated charcoal, and the like. In practice, when using an adsorbed chemical reagent in a process involving a controlled chemical reaction, the adsorbed chemical reagent and adsorbent is admixed with the reacting component at relatively low temperatures and subsequently heated to desorb the adsorbed component. Heating the adsorbent and adsorbate product desorbs the adsorbate reagent reactant making it available for a reaction with a reacting component. The mixture prior to being activated is relatively inert and fairly safe to handle.
While the aforementioned reagent adsorption solves many of the problems in regard to process control, handling etc., there is in most instances a slow escape of the chemical reagent from the adsorbent. In many instances, this slow escape of chemical reagent from the adsorbent creates problems. The desorbed chemical reagent if in a reactive environment or reactive medium will allow a slow reaction between reagents to proceed. If the rate of escape of reactant is large and the resulting reaction is exothermic there is a possibility that the exothermic heat effect will generate sufficient heat in the mixture to desorb and activate the entire mixture, or at least accelerate desorption. Further, the slow escape of chemical reagent will cause product deterioration, and shortened shelf life of an adsorbed component mixture. In general, depending on the type of adsorbent and adsorbate chemical reagent, the rate of escape of adsorbate from adsorbent will vary. Even a small escape of adsorbate is objectionable and in certain instances can cause very serious effects.
In the reagent adsorption techniques known to the prior art, adsorbed chemical reagent when present in a surrounding medium containing a reactive medium, is not rendered completely inert. In general, an adsorbent has an open pore structure. A portion of the adsorbed chemical reagent is in immediate contact with the reactive medium and is therefore in a potentially reactive position. The adsorbed chemical reagent molecules, even though attracted and held in the active pore sites by Van der Waals forces will often be dislodged from the adsorbent by the normal molecular vibration of the chemical components, and will be free to react with the reactive medium. The tendency to dislodge the adsorbed adsorbate and the seriousness of this effect will vary with the type of adsorbate, the adsorbent and the reactive medium and the other possible components having a tendency to displace the adsorbate. Normally, the function of the adsorbent is to prevent or delay a reaction between the adsorbed chemical reagent and a reactive surrounding medium.
The more efficiently this function is performed, in general, the more desirable is the adsorption system. Therefore, adsorption of chemical reagents known to the prior art will not produce complete inertness, of a chemical reagent. Further, if the adsorbent in the chemical reagent combination is selected so that the adsorbate is very securely attached to the adsorbent, thus producing a very inert adsorbent adsorbate combination, it may require an extremely powerful displacing agent or heat effect to activate the material. For example, when the adsorbent adsorbate must be heated to extremely high temperatures in order to desorb the adsorbed reagent, other reagents in the mixture may be decomposed. This effect may completely prohibit the use of an adsorbed chemical reagent. An example is a decomposable blowing agent in a foamable mixture that will cure at a relatively low temperature. If the mixture is heated high enough to desorb the blowing agent, the reaction may proceed too rapidly and scorch, burn or cure the resin poorly.
In view of the foregoing, there is a need for a delayed curing epoxy resin composition. Further, there is a need for a delayed curing epoxy resin wherein the hardener and resin components can be fully blended yet the curing reaction still be delayed to provide the composition with a long pot life. There is still a further need for method of lining a pipe system whereby a liner is fully wet out with a blended two part epoxy composition yet the curing reaction is delayed for an extended period allowing the wet out liner to be stored and installed before the reaction is activated.