Lectures part 2

1. Lectures part 2 Methods for cure monitoring Manufacturing methods for composites

2. Characterisation methods for the crosslinking in thermosets Physical methods Thermal methods Spectroscopic methods 2

3. Cure monitoring in composites • The crosslinking of thermoset resins is a fundamental process in the manufacturing of composites • This curing process must be done in a controlled manner in order to achieve good quality • It is necessary to follow and check the the cross-linking reaction during and after the cure of the composite • Important both in production and in R&D work 3

4. Methods to measure the degree of cure in composites • Techniques based on changes in physical properties: hardness, dielectric constant, viscosity • Techniques based on changes in chemical properties: reaction heat, chemical structure 4

5. Physical methods Barcol hardness Gel time Cure meters Rheology 5

6. Barcol hardness • Measurement of the surface hardness by pressing a needle into the laminate surface • Easy and fast method for quality control in production • 10-20 measurements needed per laminate for statistically reproductive results • Glas fibers near surface can disturb the measurement 6

7. Geltime measurement • Mixing of curing resin in a beaker, time for gelling is detected by a stopwatch • Most common method to characterise gel time by producers and end-users • Simple and fast method, which can be used by any-one • Subjective method • Must be done under standard conditions 7

8. Thermal Scanning Rheometer Gel-time detection by a oscillating probe 8

9. Rheological characterisation of the curing • Most accurate characterisation of the curing process is obtained by studying the rheological properties • Expensive instruments and skilled operators needed Bohlin CVO rheometer 9

10. Rheological methods • 1. Steady shearing flow properties • only in liquid phase • data before the gel point can be detected • can give too long gel times due to shear thinning near the gel point • 2. Dynamic shearing flow properties • oscillatory shearing flow • both in liquid,rubbery and glassy states • data after the gel point can be detected • Storage modulus G’ and loss modulus G’’ are measured • At the gel point is G’ = G’’ (tan  = 1) 10

11. Steady shearing flowShear rate 1.17 s-1, 80 ºC, 2 wt-% BPO Gel time 370 s, 6.2 min Harts 2 4 1 5 3 6 2 • Gel time 0/t -> 0 • 0 = viscosity at the start • t = viscosity at the time t Bohlin VOR, Cone & plate D = 30 mm, 5 º, 150 µm distance 11

12. Oscillatory shearing flow1 Hz, 80 ºC, 2 wt-% BPO Harts 2 Bohlin VOR, Cone & plate D = 30 mm, 5 º, 150 µm distance Gel time 350 s (5.8 min) 12

13. Thermal methods Differential scanning calorimetry 13

14. DSC • The exothermal heat H which is liberated in the crosslinking reaction is detected by DSC • Results depends on: • Heating rate (10 ºC/min) • Sample preparation • Sampling (10 - 15 mg) • Atmosphere (nitrogen most common) • Thermal history • Commonly used for quality control of laminates 14

15. Differential scanning calorimetry (DSC) Perkin-Elmer DSC 7 15

16. DSC Principle 16

17. Glass transition temperature The glass transition temperature can be detected from the DSC scan after a dynamic or isothermal scan Unsaturated polyesters have Tg’s at 50 - 70 ºC 17

18. Isothermal DSC scan • A liquid thermoset is cured in the DSC, at the curing temperature • Gives the total exothermal heat when assuming that all functional groups will react 18

19. Residual reactivity • A cured thermoset sample is analysed in the DSC • Any unreacted components will react during the scan, which can be detected as an exothermal heat 19

20. Degree of cure The degree of cure tcan be calculated from the isothermal scan and the residual reactivity scan: Htot = Hiso t = Ht / Htot 20

21. Spectroscopic methods Raman spectroscopy 21

22. Benefits with Raman spectroscopy • Well resolved spectra: simple calibration methods can be used • No sample preparation: can be easily adapted to on-line measurements • Weak water spectrum: works well with aqueous samples • NIR and visible wavelengths: low-cost fibre optics can be used 22

23. Process Raman spectrometer 23

24. Spectral changes in a unsaturated polyester after crosslinking ENDUR M 105 TB • Curing time 0.1 and 2 h • Sample thickness 0.5 cm • Measurement time 1 s • Laser effect 150 mW 24

25. The crosslinking reaction can be monitored ENDUR M 105TB • 2 wt-% MEKP • Gel time 20 min, 23oC • Sample thickness 0.5 cm • Postcuring 50oC, 24h MSK 2007-11-30 25

26. Postcuring effects can be detected ENDUR M 105TB • Curing time 30 h and 350 h • Postcuring 24h, 50oC • Measurement time10 s • Laser effect 150 mW MSK 2007-11-30 26

27. Laminates can also be analysed M 105 TB and glass fiber • The glass gives its own background • Longer measurement time 27

28. Gel coats MAXGUARD, clear gel coat • 0.5 mm thickness • Measurement time 1 s, 30 s 28

29. The fibre-resin interaction Surface treatment Fibre impregnation

30. Fibre – resin impregnation • A efficient and fast impregnation of the reinforcement by the resin is essential in all composite processing • Can be achieved mechanically by rolling or by the use of external pressure • Heating helps the impregnation • The impregnation is highly depending on the resin and the reinforcement characteristics

31. Effect of constituents on impregnation Resin Reinforcement Fibre surface treatment enhances impregnation A yarn with high twist is more difficult to impregnate Fabrics structure, a more dense structure will make impregnation more difficult Flow layers will enhance impregnation • Low viscosity enhances resin flow • Fillers and additives can enhance impregnation or make it more difficult

32. The reinforcement impregnation occurs during the processing of the composite

33. Manufacturing methods for composites

34. Open methods Hand lay-up Spraying Filament winding Compression moulding Pultrusion Closed methods Resin transfer moulding Vacuum bag infusion Vacuum infusion with flexible tooling Vacuum infusion with rigid tooling Autoclave processing Different manufacturing methods

35. A mould is needed for the making of the composite product

36. Prepreg • Preimpregnated fabrics: • A fabric is preimpregnated with the resin, and cuit into desired size • The prepreg is then processed by compression moulding, pultrusion or filmanet winding • Both thermoset prepregs and thermoplastic prepregs (GMT) are available

37. Processing methods • Fiber impregnation by mechanical action • Hand lay up • Spray lay up • Filament winding • Fiber impregnation by external pressure • RTM • Vacuum infusion • Autoclave, rubber bladder processing • Compression moulding (SMC and BMC)

38. Hand lay up MSK 2007-11-30 38

39. Mechanical impregnation of fibers Hand lay up

40. Reinforcement placement in mold • Pre-cutting to desired shape • Preforming by heat and pressure • Core material and inserts can be attached

41. Preformed glass fiber is oftem prepared in advantage with the desired shape

42. Spray up lamination CHOPPED ROVING + RESIN RESIN SPRAYED LAYER

43. Equipment for spray up lamination • Resin - peroxide mixing in spray • Roving chopper • 108 bar output pressure • Resin heating possible • 200 l resin drums

44. Filament winding Winding of impregnated reinforcement yarns onto a rotating mold (mandrel) A precise, highly efficient and automated process Only for closed geometries which can rotate such as pipes, pressure vessels Fiber volume fractions can be varied in a laminate 44

45. Filament winding - process principle Mandrel 4 degrees of freedom 45

46. Filament winding Polar winding pattern 46

47. Filament winding 47

48. Resin impregnation - dip through

49. Resin impregnation – drum type

50. Resin impregnation – closed type