Fiber Reinforced Polymer (FRP) Composites Dr Antonis Michael Department of Civil Engineering Frederick University Cyprus Martix Materials: Introduction Hold fibers together Protect fibers against environment and mechanical abrasion Transfer transverse stresses and interlaminar shear stresses Transverse stiffness and strength are matrix dominated properties and they are temperature depended 1
Matrix Materials Types of matrix materials: polymers, metals and ceramics Polymer matrices most common Ease of fabrication Low tooling cost Unreinforced plastics: cost savings in tooling but have creep problems Reinforced plastics (with fibers): mechanical properties can be controlled, no significant creep problems 2
Resin and Matrix During processing the polymer is called resin (liquid form) After curing the polymer is called matrix (solid form) Resin selected based on mechanical, corrosion flammability and processing requirements (minimize cost) Thermoset Matrices Irreversible chemical transformation of a resin in an amorphous cross-linked polymer matrix Low viscosity Excellent impregnation of fibers High processing speed Most common resin systems due to lower cost 3
Thermoset Matrices Terms: Shelf life: time an un-mixed resin can be stored (refrigerated storage recommended) Pot life or gel time: time the mixed resin can be handled before viscosity grows Cure cycle: time it takes for the resin to fully solidify (chemical reactions completed). Depends on catalyst and reactivity of resin (minutes for high temperature up to hours at room temperatures). Heat accelerates reactions. Reactions are exothermic Thermoset Matrices Volumetric shrinkage from curing 4% for epoxies 8% for polyesters Problems due to shrinkage Internal stresses Cracking Dimensional variations Common types of thermosets: Polyesters, vinyl esters, epoxies and phenolics 4
Polyester Resins Low viscosity Clear liquids based on unsaturated polyesters Dissolved in styrene Not very resistant to UV light Can be formulated to have good resistance Resistant to chemical attacks. Applications: Chemical plants (tanks, pipes, ducts etc) Paper mills Polyester Resins (Isophthalic) Higher tensile and flexural properties (structural applications) High resistance to water Good resistance to gasoline, weak acids and alkali Low resistance to peroxides and hypochlorites 5
Vinyl Ester Resins Higher mechanical properties than polyesters Highly resistant to acids, alkalis, solvents, hypochlorites and peroxides Brominated versions have high flame resistance Epoxy Resins Highest mechanical properties Highest corrosion resistance Less shrinkable Excellent bond characteristics Resistant to water and heat Main applications in the aircraft industry 6
Epoxy Resins Construction applications Sealant and encapsulating compound (epoxy coated steel bars for better corrosion resistance) Polymer concrete overlays Comparison of Resin Matrices Properties and cost Polyester < Vinyl ester < Epoxy Low cost + High cost + properties best properties 7
Phenolic Resins Low flammability Low smoke production Good dimensional stability Good adhesive properties Applications: Fire critical applications Low cost (comparable to polyesters) Thermoplastic Matrices No chemical transformation during processing Softens during processing (usually heat) and returns to solid state 8
Thermoplastic Matrices Advantages High impact strength No refrigerated storage required Unlimited shelf and pot life Recyclable Ease of handling (no tackiness) Easy to repair: Repeated softening with heat (reshape) Thermoplastic Matrices Disadvantages High viscosity (difficulty in handling and fiber impregnation) Limited structural applications (low creep resistance) 9
Thermoplastic Matrix Types Polyamides Poly ether ether ketone (PEEK) (common matrix with high fracture toughness) LARC-TPI prepolymers used for fiber coating (when cured at 300º C it becomes a regular thermoplastic) Nylon Used with discontinuous fibers in injection-molded articles Creep Creep is the continuous growth of deformation with time under constant load Polymers creep at room temperature Creep properties depend on moisture and temperature 10
Manufacturing Methods for FRP Composites Hand Lay-Up Simplest most widely used method 11