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Polyesters

Polyesters. Brent Strong. Polyesters (Unsaturated). The most common type of resin for composites The least expensive composite resin The easiest-to-cure composite resin Polyesters are made from two types of monomers: Di-acids Glycols. Polyester polymerization. Monomers. A. Acids A

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Polyesters

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  1. Polyesters Brent Strong

  2. Polyesters (Unsaturated) • The most common type of resin for composites • The least expensive composite resin • The easiest-to-cure composite resin • Polyesters are made from two types of monomers: • Di-acids • Glycols

  3. Polyester polymerization Monomers A Acids A (di-acids) G Glycols G (di-alcohols) A G A G Polyester polymer A G A

  4. Polyester polymerizationn • Di-acids have active OH groups on both ends • Glycols have active H groups on both ends • One end of the di-acid (the OH group) reacts with one end of the glycol (the H group) to form water (H−OH) • The water separates from the polymer and condenses out as a liquid • These type of polymerization reactions are called condensation reactions

  5. Polyesters polymerization HO G OH Glycol HO-G HO G OH Glycol G G Glycol Glycol

  6. Customizing the Polyester − Acids

  7. Customizing the Polyester− Glycols

  8. Customizing the Polyester− Solvents

  9. Customizing the Polyester− Adding Other Monomers or Resins

  10. Polyesters − specific molecules RepeatingUnit n The number of repeating units is usually shown by an n

  11. Polyesters − crosslinking (curing) C C C C Unsaturated portion Polyesters must have unsaturated portions to crosslink

  12. Initiators (catalysts) • Initiators are sometimes called catalysts. • The crosslinking reaction is begun when an initiator reacts with the double bond. • The most common initiators are peroxides. • The peroxides are effective initiators because they split into free radicals (that is, they have unshared electrons) which react easily with the double bonds. • Free radicals have unshared electrons.

  13. Polyesters − crosslinking (curing) I Initiator ● C C C C

  14. Polyesters − crosslinking (curing) I C ● C ● C ● C

  15. Polyesters − crosslinking (curing) Bond (2 electrons) I C C C C ● Unshared electron

  16. Polyesters − crosslinking (curing) I C C C C * Free radical (unshared (unbonded) electron) Free radicals react readily with any Carbon-carbon double bond they encounter

  17. Polyesters − Reaction Problem • To react and form a crosslink, the free radical on the polymer needs to encounter (collide with) a double bond on another polymer • The polymers are long and entangled (highly viscous), thus they don’t move very quickly • The polymers are bulky and it is hard to get the free radical into the area of the double bond • The chances of lining up just right are not good

  18. Polyesters − Reaction Solution • Dissolve (dilute) the polymer with a solvent so that the polymers can move around freely • Ideally, the solvent will react during the crosslinking reaction so that it does not need to be removed from the solid • These types of solvents are called “reactive solvents” or “reactive diluents” or “co-reactants” • Added benefit: • The solvent will also reduce the viscosity so that the polymer will wet the fibers more easily

  19. Styrene • Styrene is the most common solvent for polyesters • The styrene reacts (is consumed) during the crosslinking reaction because the styrene contains a double bond and reacts with the free radical • The styrene serves as a bridge molecule between the polymer chains (as part of the crosslink) • There may be as many as 8 styrene molecules in a bridge

  20. Polyester − forming the crosslink I C C C C Styrene New bonds (crosslink) C C C C * New free radical The styrene is a bridge molecule between the polyester polymers The new free radical is available to react with another styrene

  21. Crosslinking Reaction • Called addition or free radical crosslinking reaction • Proceeds as a chain reaction • Once started, it will keep going unless specifically terminated • Doesn’t need more initiator • Makes its own reactive sites

  22. Continuing and Terminating • Once started, each free radical-based reaction will continue until the new free radical site stops colliding with double bonds • Runs out of reactive diluent • Stops encountering other polymers with double bonds • Perhaps because the polymers get so long they don’t move much (the polymer becomes a rigid solid) • Post-curing can induce some movement in solids and increase the amount of crosslinking • The free radical site might cease to exist • React with another growing chain’s free radical site • React with another free radical in the solution • From an initiator (danger of adding too much initiator) • From ozone (using wax to help exclude air)

  23. Inhibitors • Inhibitors are added, usually by the resin manufacturer, to slow down the crosslinking reaction • Inhibitors typically absorb free radicals • Inhibitors protect the polymer during storage because sunlight, heat, contaminants, etc. can start the curing reaction • Molders must add sufficient initiator to overcome the inhibitors and to cause the crosslinking to occur

  24. Promotors (accelerators) • Added to the polymer to make the initiator work more efficiently or at a lower temperature • Each type of peroxide has a temperature at which it will break apart into free radicals • These temperatures are usually above room temperature • For room temperature curing, a chemical method for breaking apart peroxides is needed • The most common promoters (accelerators) are cobalt compounds and analines (DMA) • Never add a promoter directly into an initiator

  25. Additives • Additives are components (usually minor) that have various functions that are not related to the curing reaction • The most common types of additives are: • Fillers (to lower cost and/or give stiffness) • Thixotropes (to control viscosity) • Pigments • Fire retardants • Surfactants (to promote surface wetting) • UV inhibitors/Anti-oxidants

  26. Factors influencing cure • Mix ratios • Resin, initiator, inhibitor, accelerator, solvent • Fillers, pigments, other additives • Storage time after activation • Thickness of the part • Cure time • Humidity • Temperature

  27. Thermal effects • The rate (speed) of chemical reactions increases as the temperature is increased • Arrhenius equation: exponential relationship between rate and temperature • Rate roughly doubles for each 10°C rise • Molecular collisions are required • Heat increases molecular movement • Highly reactive entities (like free radicals) have successful reactions with almost every collision • Some reactions create heat (exothermic)

  28. Time-Temperature Curve in the sample at constant applied temperature 149 300 93 200 oC oF Temperature 100 38 15 0 5 10 Time, min

  29. Temperature-Viscosity Curve Gel Point Solids Liquid-Solid Line Viscosity Region B Region A Liquids Thinning due to temperature Crosslinking Time/Temperature Combination (What is seen)

  30. Thickness-Exotherm Curve Heat Buildup Thickness Heat builds up because of the poor thermal conductivity of polymers

  31. Peroxide content – Temperature and Time Curves 150 310 140 290 130 270 Peak Exotherm, oC °F 120 0.5% cobalt naphthenate 250 110 230 100 210 2.0 0.5 1.0 0 1.5 % methyl ethyl ketone peroxide (MEKP)

  32. Accelerator Content − Time Curve 300 250 Gel Time, Min At 0.25% MEKP 200 150 100 At 0.50% MEKP 50 .004 .008 .012 .016 .020 % Cobalt Naphthenate

  33. Polyesters

  34. Thank you A. Brent Strong

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