Thermoset Films Research Articles

Plastic deformation mechanisms in polyimide resins and their semi‐interpenetrating networks

AbstractThe plastic deformation mechanisms in both thermoplastic and thermoset polyimide resins and their semi‐interpenetrating networks (semi‐IPNs) were investigated. The fundamental tendency to undergo strain localization (crazing and shear banding) as opposed to a more diffuse (or homogeneous) deformation in these polymers was evaluated. In situ polarized light microscopic observation of crack‐tip deformation mechanisms in solvent‐cast films was conducted with a miniature testing device attached to the microscope stage. NASA LaRC TPI, a thermoplastic polyimide, was found to exhibit combined shear yielding and crazing under plane‐stress loading conditions. Contrarily, NR‐150B2 thermoplastic polyimide exhibited diffuse shear yeilding; no shear banding or crazing was found near the crack tip. Adding a small amount of thermoplastic component, either NR‐150B2 or LaRC TPI, was found to raise considerably the fracture toughness of PMR‐15 thermoset PI. PMR‐15 thermoset films also showed diffuse shear yielding, albeit with a much smaller deformation zone. This was consistent with its low toughness and could be understood on the basis of a limited extensibility of a highly crosslinked network. Numerical calculations were performed to confirm this low value of network chain draw ratio. The dimensions of the deformation zone ahead of the crack tip in a semi‐IPN with a thermoset matrix were increased as a higher weight fraction of the thermoplastic component was added. The deformation zone size of a thermoplastic matrix was found to decrease with an increasing amount of thermoset PMR‐15. Deformation also became more diffuse with a higher PMR‐15 content in LaRC TPI. Fracture toughness variations can be correlated with deformation size changes in these semi‐IPNs.

Mode I Interlaminar Fracture of Interleaved Graphite/Epoxy

Mode I interlaminar fracture of interleaved graphite/epoxy has been stud ied experimentally using double cantilever beam (DCB) specimens and optical micros copy. The interleaves were either thermoset or thermoplastic films placed at the midplane along the anticipated fracture plane. To examine the possible influence of constraints on the crack tip plastic zone due to the rigid composite plies, a large range of interleaf thick nesses was investigated. At each crack increment, the crack extension initiated close to the maximum load and progressed in a stable manner during the displacement controlled test ing. Fiber bridging was not observed on the fracture specimens, or indicated by the shape of the crack growth resistance curves. The thermoplastic interleaves enhanced the fracture toughness, G IC , to a much larger extent than the thermoset interleaves, due to a larger en ergy absorption capability in the fracture process. For the thermoplastic interleaves, the maximum fracture toughness was achieved at very small film thicknesses. At larger film thicknesses GIC remained large. For the thermoset film material, the enhancement of GIC was only nominal and the thicker thermoset interleaves failed in a film delamination mode, resulting in low GIC values.

Crack tip yield zone estimates in mode II interlaminar fracture of interleaved composites

An elastic stress analysis of the interleaved end notched flexure specimen was conducted by two-dimensional finite element modeling. The purpose of the study was to identify mechanisms that influence the enhancement of mode II interlaminar fracture toughness of interleaved composites. The study focused on a graphite/epoxy composite interleaved with thermoplastic and thermoset polymeric film materials. Crack tip yield zones were estimated from the elastic stress distribution beyond the crack tip and von Mises yield criterion. The dependence of yield zone height on interleaf film thickness agrees qualitatively with the trends observed experimentally for mode II interlaminar fracture toughness, G IIC , over a range of interleaf thickness. The plastic zone height and G IIC increase rapidly with adhesive thickness and attain a region of less variations at larger film thicknesses. The high yield strength thermoplastic interleaf material attained a constant plastic zone height and a relatively constant fracture toughness at much smaller film thicknesses than the lower yield strength thermoset interleaf material. Irwin's plastic zone model was examined and was found to be a promising candidate for initial design of interleaved composites.

The Processing and Properties of Multilayer Interconnection Structures Using Thermoset Films Derived from Bisbenzocyclobutene.

AbstractMultilayer interconnection structures incorporating a novel polymeric dielectric derived from a bis-benzocyclobutene(bis-BCB) monomer have been fabricated. This paper discusses the processing conditions for the construction of these circuits and describes electrical characteristics of the dielectric layers. The relative dielectric constant of the BCB film was 2.7. Thermal cycling produced no significant change in the conductance of three level metal via chains through two layers of the polymer.

15 Years experience in powder coating manufacturing and practical application

More than two decades after their frist start the thermosetting powder coatings became in 1985 a technically viable product and a commercial success. Powder coatings are 100% dry paints that contain no solvents. They are generally applied to metal substrates by means of electrostatic spray equipment that provides each powder particle with a small electric charge, which in turn makes it stick to the substrate. The coated objects then go into a high temperature oven (usualy 150 to 200°C), where the powder coating melts and reacts chemically while sintering together to a continuous smooth finish of a thermoset film.

Particles covered with a cured infusible thermoset film and process for their production: Johnson, C.K. and Armbruster, D.R. (Acme Resin Corporation, Forest Park, IL, USA) US Pat 4 439 489 (27 March 1984)

Particles covered with a cured infusible thermoset film and process for their production: Johnson, C.K. and Armbruster, D.R. (Acme Resin Corporation, Forest Park, IL, USA) US Pat 4 439 489 (27 March 1984)