Epoxy resins are typically cured with hardeners or curing agents, and when cured, the resins are known for their thermal and chemical resistance. The cured epoxy resins also display good mechanical properties but they lack toughness and tend to be very brittle upon cure. The lack of toughness of the resins is especially true as the crosslink density or Tg of the resins increases.
Epoxy resins are typically used for preparing high solids ambient cure coating compositions. High solids ambient cure epoxy coatings that have good toughness are also known, but an improvement in resin toughness most often comes at the expense of other properties of the resin. Heretofore, it has been difficult to develop ambient cure coatings that combine all of the good application properties as well as toughness in one system.
Several methods have been proposed to improve the flexibility of high solids ambient cure epoxy coatings but each method has at least one disadvantage associated with it that has limited its wide application.
For example, aliphatic backbone modification can be used to impart flexibility in these systems. The aliphatic modification can be introduced to the thermoset network through the epoxy or curing agent. The disadvantage of this approach is that the glass transition temperature, chemical resistance and corrosion resistance are all negatively impacted by the aliphatic chain segment. Cure speed can also be negatively impacted if an aliphatic epoxy is utilized to modify the system.
Another known process involves the use of plasticizers, which can be added to a high solids ambient cure epoxy coating to increase its flexibility. Much like the aliphatic backbone modification this can negatively impact coating performance. Plasticizers have the added disadvantage of either volatilizing or leaching from the film with time causing embrittlement.
Still another known method is the use of carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber. CTBN is a second phase toughener used in high solids ambient cure epoxy coatings. Because the second phase morphology is a function of cure conditions the performance of CTBN modified systems is a function of the cure schedule. CTBN also increases the viscosity of the modified resin thus limiting its use in high solids systems, as large amounts are needed to attain a significant improvement in toughness and ductility. CTBN is also known to depress the glass transition temperature of the system.
Recently, there have been several studies related to increasing the fracture resistance or toughness of epoxy resins by adding to the epoxy resin various block copolymers. Much of the previous work is focused on the use of amphiphilic diblock copolymers having an epoxy miscible block and an epoxy immiscible block in which the epoxy miscible block is poly(ethylene oxide) (PEO) and the immiscible block is a saturated polymeric hydrocarbon. Although effective at providing templated epoxies with appealing property sets, the known block copolymer materials are too expensive to be used in some applications.
For example, Journal of Polymer Science, Part B: Polymer Physics, 2001, 39(23), 2996-3010 discloses that the use of a poly(ethylene oxide)-b-poly(ethylene-alt-propylene) (PEO-PEP) diblock copolymer provides micellar structures in cured epoxy systems; and that block copolymers self-assembled into vesicles and spherical micelles can significantly increase the fracture resistance of model bisphenol A epoxies cured with a tetrafunctional aromatic amine curing agent. And, Journal of the American Chemical Society, 1997, 119(11), 2749-2750 describes epoxy systems with self-assembled microstructures brought using amphiphilic PEO-PEP and poly(ethylene oxide)-b-poly(ethyl ethylene) (PEO-PEE) diblock copolymers. These block copolymer containing-systems illustrate characteristics of self-assembly.
Other block copolymers incorporating an epoxy-reactive functionality in one block have been used as modifiers for epoxy resins to achieve nanostructured epoxy thermosets. For example, Macromolecules, 2000, 33(26) 9522-9534 describes the use of poly(epoxyisoprene)-b-polybutadiene (BIxn) and poly(methylacrylate-co-glycidyl methacrylate)-b-polyisoprene (MG-I) diblock copolymers that are amphiphilic in nature and are designed in such a way that one of the blocks can react into the epoxy matrix when the resin is cured. Also, Journal of Applied Polymer Science, 1994, 54, 815 describes epoxy systems having submicron scale dispersions of poly(caprolactone)-b-poly(dimethylsiloxane)-b-poly(caprolactone) triblock copolymers.
While some of the previously known diblock and triblock copolymers mentioned above are useful for improving the toughness of epoxy resins, the preparation of such previously known block copolymers is complicated. The previously known block copolymers require multiple steps to synthesize and therefore are less economically attractive from a commercial standpoint.
Still other self-assembled amphiphilic block copolymers for modifying thermosetting epoxy resins to form nanostructured epoxy thermosets are known. For example, Macromolecules, 2000, 33, 5235-5244 and Macromolecules, 2002, 35, 3133-3144, describe the addition of a poly(ethylene oxide)-b-poly(propylene oxide) (PEO-PPO) diblock and a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock to an epoxy cured with methylene dianiline, where the average size of the dispersed phase in the diblock-containing blends is of the order of 10-30 nanometers (nm). And, a polyether block copolymer such as a PEO-PPO-PEO triblock is also known to be used with an epoxy resin as disclosed in Japanese Patent Application Publication No. H9-324110.
While some of the previously known diblock and triblock copolymers mentioned above are useful for improving the toughness of epoxy resins, there is still a need in the ambient cure high solids coatings industry to further enhance the toughness of the epoxy resin used in ambient cure high solids coatings applications while maintaining all other crucial properties of the resin.
It is therefore desired to provide an alternative block copolymer that is useful for improving the toughness of thermosetting epoxy resins by a self assembly process without any of the disadvantages of the previously known block copolymers.