to 17 minutes the temperature increases gradually until it reaches 120° to 135°F. Then a sudden, sharp increase in temperature occurs until it reaches its peak at or about 350°F. This sudden, sharp rise in tempera- ture is known as kick-over. At this temperature, the resin-hardener mixture begins to solidify and change color from gray to light brown. The peak temperature (350°F) can be observed through the external change of the patch. The resin-hardener mixture begins to cool slowly because the materials conduct heat poorly. After kick- over, the mixture continues to harden and increase in strength. This process is called curing. Approximately 30 minutes after kick-over (the sharp rise in tempera- ture), the patch is strong and hard and cool enough to use. Pressure should not be restored to the system until the patch has cured. The patch is considered sufficiently cured when the bare hand can be placed on it without discomfort from heat. Several factors contribute to the control of kick- over. The most important factor is the temperature. Both the initial temperature of the activated resin mixture and the temperature of the atmosphere affect the kick-over time. Of these two temperatures, the initial temperature of the activated resin has the greater effect. When the temperature of the resin and the hardener before mixing is increased, the kickover time decreases. Conversely, when the temperature of the resin and hardener before mixing decreases, the kick-over time increases. A knowledge of the control of kick-over is neces- sary since it corresponds to the application of working time. This means, for example, that when the initial temperature of the mixture is 73°F, the patching ma- terial must be placed over the rupture within 12 min- utes. Once the resin and the hardener are mixed, the chemical reaction cannot be stopped. Therefore, the patch should be completely applied before kick-over occurs. Figure 2-24 shows the relationship of the kick- over time to the resin temperature. If you know the resin temperature at the time of mixing, you are able to determine the amount of time available to apply the patch before kick-over occurs. You can see in figure 2-24 that if the resin temperature is 80°F (point A), the kick-over will occur in less time than if the resin temperature were 60°F (point B). The difference in resin temperatures represents an application working time of 9 minutes instead of 18 minutes. NOTE If the initial resin temperature exceeds 80°F, the temperature should be reduced by artificial means to 73°F before mixing. This lowering of the temperature allows for addi- tional application working time. Advantages of the Plastic Patch From the damage control viewpoint, the main advantages of the plastic patch are (1) versatility, (2) simplicity, (3) effectiveness, (4) speed of application, and (5) durability. The plastic patch can be successfully applied to a variety of damaged surfaces, whether with smooth edges or jagged protruding edges. Since the plastic has excellent adhesive qualities, it can be readily ap- plied to steel, cast iron, copper, copper-nickel, brass, bronze, and galvanized metals. It is easy to prepare the plastic materials and to apply the plastic patch. By following the instructions outlined in the instruction manual that is included in the kit, anyone with little or no experience can readily prepare the materials and apply a plastic patch. A plastic patch is applied in much the same way as a battle dressing is used in first aid. If the materials are properly prepared and the application procedures are followed, the plastic patch will be 100 percent effective. If leakage occurs through a plastic patch, it is likely that propr prepa- ration and application procedures have not been fol- lowed. The speed of application varies somewhat with the size and type of rupture, and with local working conditions. When proper preparation procedures are followed, a simple patch can be applied to a 4-inch pipe by an inexperienced crew who have had the minimum amount of training and indoctrination in 10 minutes or less. The type and the size of the rupture or the shape and the size of the structure to which the patch is applied do not materially affect the time involved in patching, but some types of damage may require more initial preparation. The maximum period of effectiveness of a plastic patch is not known, but all indications are that a properly applied patch can last indefinitely or cer- tainly until permanent repairs can be made. The patch is relatively inert, being seriously affected only by excessive heat and concentrated acids. 2-16