The Technology of Curatives Used with Epoxy Resins for Composite Applications

Cured epoxy resins are strong, tough and inherently adhesive making them the material of choice for many composite applications. Their adhesive qualities allow them to stick to glass and carbon fiber surfaces as well as bond composites to each other and metals. Structure property relationships for epoxy resins are straight forward. In general, a lower number of epoxy groups per molecule and lower molecular weight lead to more liquid systems with lower temperature capability. A minimum of two epoxy groups per molecule are required to yield a crosslinked load-bearing material. Higher than two epoxy groups per molecule leads towards higher crosslinking and subsequent higher temperature capability. Almost limitless blending options of liquid and solid epoxy resins and lower and higher epoxy groups per molecule can balance process requirements with cured performance options such as operating temperature, toughness, and composite mechanical performance dominated by resin characteristics such as flexural, compressive and shear strengths as well as impact resistance. Epoxy resins must be reacted or cured to be converted from single molecules into a structural load-bearing material in a process called curing or crosslinking (and sometimes referred to as polymerization). In addition to structure property considerations in the cured resin, the curative chosen will determine the crosslinking reaction mechanism and reaction rate, thus controlling shelf life, pot life, cure cycle and general applicability to a fabrication method and raw material form. A wide adaptability to composite fabrication techniques has been made possible by a wide variety of curatives. This article will focus on curatives used for matrix resins used in composites. There are many ways to examine and discuss this subject such as by the nature of the curing reaction or by the chemical family of the curative. In this paper, the curatives will be discussed based upon the composite manufacturing method used for specific applications such as prepreg, filament winding, resin transfer molding, wet lay up and compression molding applications. The emphasis will be on why the chemical and physical attributes of certain types of curatives have found utility in a given application and less emphasis on specific chemical reaction mechanisms which have ;been covered in many excellent reviews(1,2). The examples shown in this paper are not exhaustive by any means: there are many additional materials used in the composites industry. Structures are shown to demonstrate similarities and differences in terms of physical form (liquid or solid), equivalent weight, viscosities where this is an important attribute and reactivity with a standard difunctional liquid bisphenol-A (bis-A) epoxy resin. Shorter gel times indicate faster reactions. Physical properties such as melting point, viscosity, amine content, epoxy content and particle size are shown as single values where in reality they are ranges.