Composites- Thermoset Matrices Flashcards
Steps in curing
Resin and curing agents as monomers. Curing starts and dimers/trimers/oligomers start to form. Gelation occurs when sufficient crosslinking to cause loss of fluidity. Further reaction results in more crosslinking, improving most properties but reducing toughness.
Shelf life
The time the unmixed, unreacted resin system can be stored without degradation.
Pot life
Aka gel time. The time the mixed resin can be handled before the viscosity increases to a point where processing is no longer possible
Cure cycles
Application of heat and/or pressure over time (ramps, dwells, etc) for curing of thermosets. Can vary according to the reactivity of the resin and the presence of a catalyst/initiator. Vary from few minutes to several hours, room temp or high temp. Reactions often exothermic. Viscosity decreases initially but then increases as curing starts. Optional post-cure can be done to increase crosslinking.
Why would you do a post-cure?
Unreacted monomers would eventually react and the properties of the part would change in service otherwise.
Common thermosets used as matrices in order of increasing modulus, T performance and cost
Unsaturated polyesters
Vinyl esters
Epoxies
Thermosetting polyimides
Bismaleimides
Cyanate esters
Phenolics (generally cheaper and low-med performances but very good T resistance)
Unsaturated polyesters stats
Tensile modulus: 1.5-2GPa
Tensile strength: >20MPa
Strain at break: 1%
Tg: <0°C
Processing temperature: RT
Unsaturated polyesters
Low cost and reasonable modulus. Widely used in production of lower performance composite components. More polyester resins used as matrices than all other thermosetting polymers added together. Unsaturated means it has a C=C between the two end Ester groups. Resin typically consists of a linear unsaturated polyester, a solvent/crosslinking agent (styrene common), a radical initiator (MEKP common (methyl ethyl ketone peroxide). Suffer from poor: thermal resistance, shrinkage on cure, environmental resistance
Vinyl esters stats
Tensile modulus: 2-3GPa
Tensile strength: >60MPa
Strain at break: 1%
Tg: up to 120°C
Processing temperature: RT-80°C
Vinyl esters
Slightly higher cost and performance. Specific polyesters with reactive groups at the end of chain (unsaturated vinyl bonds (C=C)). Curing is same as for unsaturated polyesters. Viscosity of vinyl esters generally lower than for unsaturated polyesters. Start with monomer with epoxy groups at chain ends (triangle of 2C and O). React with 2(HOC(=O)C(=CH2)CH3) to get C=C bonds at ends. Reactive groups at ends means reduced crosslinking than UPs so tougher, better environmental resistance, reduced shrinkage. Often have backbone phenylene rings
Epoxies stats
Tensile modulus: 3.5-4GPa
Tensile strength: >60MPa
Strain at break: 2%
Tg: up to 250°C
Processing temperature: 50-200°C
Epoxies
Higher cost, modulus, T performance. Most common matrix for high performance composites. Contain reactive triangular epoxy group at chain ends which readily react with a curing agent to form the polymer. Typically use epoxy resin with two or more epoxy groups. Properties can vary widely. Common approach is to react a di-epoxy (e.g DGEBA) with a di-amine (e.g DDS). Generally have high modulus and strength, low shrinkage, high T resistance, good chemical resistance
Epoxy functionality >2
Like tri-functional or tetra-functional epoxies. More than 2 epoxy groups implies extensive crosslinking. Diamine curing agent can react twice per amine group. Curing agents with amine functionality >2 also implies extensive crosslinking. Highly crosslinked multifunctional epoxy resins like these used for improved performance and routinely used in pre-pregs for aerospace. The high performance epoxies and curing agents all have backbone phenylene rings
Thermosetting polyimides stats
Tensile modulus: 2-3GPa
Tensile strength: >60MPa
Strain at break: 2%
Tg: up to >300°C
Processing temperature: >300°C
Thermosetting polyimides
Much higher cost, higher modulus, high T performance. Some can have continuous service temperatures up to 370°C. Other properties similar to epoxies so only find use where both good mechanical properties and high T capability are necessary (near exhaust of aircraft or in rocketry). Exact structures not easy to find. Generic structure has at one end wing from N bonded to two C=O groups as part of a ring (see slide 12). Other end has wing out from phenyl ring
Bismaleimides stats
Tensile modulus: 4-4.5GPa
Tensile strength: >50MPa
Strain at break: 1%
Tg: >350°C
Processing temperature: >200°C
Bismaleimides
(BMI). Very high cost and performance. React through opening the unsaturated C=C bond through heating, with or without a curing agent. Ends are two rings with N bonded to two C=Os each bonded to a C which form C=C. Result is a highly crosslinked, very high performance thermoset. BMIs expensive but used where very high T performance is required
Cyanate esters stats
Tensile modulus: 3-3.5GPa
Tensile strength: >70MPa
Strain at break: 1%
Tg: >400°C
Processing temperature: 200°C
Cyanate esters
Very high cost and performance. Cyanate esters resins react through the isocyanate (-NCO) groups along and at the end of chains. This bond trimerises to create very stable cyanurate ring (alternating C and N in C3N3). The highly crosslonked structure leads to excellent properties. Very high cost and tend to be used in high T and electrical applications. Low dielectric loss applications such as radomes and antennas
Phenolics stats
Tensile modulus: 3-4GPa
Tensile strength: >50MPa
Strain at break: 1%
Tg: >250°C
Processing temperature: 100-150°C
Phenolics
Generally cheaper and low-med performance but very good T resistance. Phenolic resin was first commercially available synthetic polymer (Bakelite). Fire resistant because on heating they form a char layer which prevents further burning. Used in nose cones for spacecraft and rockets. Find use where fire-resistance and low smoke generation important (internal panels in spacecraft). Looks like use phenol and methandi-ol and water to eventually achieve highly crosslinked structure
