ALD NANOSOLUTIONS, INC
Functionalized Nanocoatings for Filler Adhesion to Rubber Using Atomic Layer Deposition
Rubber can be a multifunctional material that can be tuned by the addition of various fillers. For example, automobile tires must provide traction, durability, ride comfort, fuel economy, road handling and heat resistance. Rubber alone can not meet these many needs. Consequently, a variety of additives are combined with rubber to define the tire properties. To control the stiffness, toughness and durability of the rubber-containing layers, fillers such as carbon-black and silica are added to the rubber. These fillers are heavy and contribute to tire weight. Ideally, lighter weight fillers such as polymeric aramid (Kevlar) fibers would be preferred for enhanced performance and greater fuel efficiency. One difficulty with replacing heavy filler materials with lighter weight aramid fibers is the lack of adhesion between the Kevlar fiber and the rubber. This proposed research will develop methods to coat Kevlar fibers with functionalized nanometer films that will couple the Kevlar fibers to the rubber. These nanometer coatings will provide better adhesion between the Kevlar filler and the rubber matrix, and will lead to performance advantages for tires, environmental benefits for tire fabrication and better fuel efficiency for cars. The methods are general and will be applicable to other applications.
The adhesion between polymeric fillers and rubber is critical in defining the durability and performance of rubber-based products. Despite its importance, there are no easy methods for adding adhesion promoters to polymeric fibers for coupling to rubber. For example, chopped Kevlar fibers are currently added to rubber with no surface pretreatment. The absence of adhesion promoters results from the lack of available methods to coat polymer fiber. Due to its structure, it cannot be coated using conventional deposition techniques such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). PVD requires line-of-sight and can not uniformly coat these entangled filaments. CVD also has difficulty with uniformity because thicker films will be deposited where the gas fluxes are highest on the outer surfaces of the chopped fiber. Only plasma polymerization methods have been moderately successful for modifying aramid tire cords to improve adhesion. However, they also require line-of-sight for uniformity and are very dependent on the plasma parameters. Without an adhesion layer, there is minimal adhesion between the polyaromatic Kevlar fiber and the polyisoprene rubber matrix.
To avoid line-of-sight and uniformity problems, this work will use atomic layer deposition (ALD) methods. ALD does not require line-of-sight and can coat very high aspect ratio structures with conformal and ultrathin films. In addition to providing conformal and uniform nanocoatings, ALD can also provide a platform for surface functionalization. The ALD film can be prepared with surface chemical species, such as hydroxyl groups, that facilitate coupling reactions that can add particular surface functional groups. A wide range of silane molecules can then couple to hydroxyl groups and introduce various chemical functional groups. Functional groups containing thiol (-SH) species on the surface of fillers will form strong disulfide linkages with sulfur in the vulcanized rubber. Surface functionalization of the filler with thiols, such as 3-mercapto-propyl-triethyoxysilane, can be achieved immediately after depositing the ALD nanocoating on the filler. This ALD nanocoating with thiol surface functionalization provides the best possible combination to obtain excellent adhesion between the Kevlar filler and the rubber matrix.