Lectures part 2 Methods for cure monitoring Manufacturing methods for composites MSK

2 Characterisation methods for the crosslinking in thermosets 1.Physical methods 2.Thermal methods 3.Spectroscopic methods MSK

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 MSK

4 Methods to measure the degree of cure in composites 1.Techniques based on changes in physical properties: hardness, dielectric constant, viscosity 2.Techniques based on changes in chemical properties: reaction heat, chemical structure MSK

5 Physical methods 1.Barcol hardness 2.Gel time 3.Cure meters 4.Rheology MSK

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 measurements needed per laminate for statistically reproductive results Glas fibers near surface can disturb the measurement MSK

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 MSK

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

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 MSK

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) MSK

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

12 Oscillatory shearing flow 1 Hz, 80 ºC, 2 wt-% BPO Harts 2 Gel time 350 s (5.8 min) Bohlin VOR, Cone & plate D = 30 mm, 5 º, 150 µm distance MSK

13 Thermal methods Differential scanning calorimetry MSK

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 ( mg) Atmosphere (nitrogen most common) Thermal history Commonly used for quality control of laminates MSK

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

MSK DSC Principle

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

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 MSK

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 MSK

20 Degree of cure The degree of cure  t can be calculated from the isothermal scan and the residual reactivity scan: (1)  H tot =  H iso (2)  t =  H t /  H tot MSK

21 Spectroscopic methods Raman spectroscopy MSK

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 MSK

23 Process Raman spectrometer MSK

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 MSK

MSK The crosslinking reaction can be monitored ENDUR M 105TB 2 wt-% MEKP Gel time 20 min, 23 o C Sample thickness 0.5 cm Postcuring 50 o C, 24h MSK

MSK Postcuring effects can be detected ENDUR M 105TB Curing time 30 h and 350 h Postcuring 24h, 50 o C Measurement time10 s Laser effect 150 mW MSK

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

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

The fibre-resin interaction Surface treatment Fibre impregnation MSK

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 MSK

Effect of constituents on impregnation Resin Low viscosity enhances resin flow Fillers and additives can enhance impregnation or make it more difficult 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 MSK

The reinforcement impregnation occurs during the processing of the composite MSK

Manufacturing methods for composites MSK

Different manufacturing methods 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 MSK

A mould is needed for the making of the composite product MSK

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 MSK

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) MSK

Hand lay up MSK MSK

Mechanical impregnation of fibers Hand lay up MSK

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

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

Spray up lamination CHOPPED ROVING + RESIN SPRAYED LAYER RESIN MSK

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

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 MSK

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

46 Filament winding Polar winding pattern MSK

47 Filament winding MSK

Resin impregnation - dip through MSK

Resin impregnation – drum type MSK

Resin impregnation – closed type MSK

51 Mandrel removal after winding Removable mandrel 1.Treatment with release agent 2.Release tape winding 3.Water-soluble sand and salt mandrels 4.Melting metal alloy mandrels 5.Casted plaster mandrels Integrated mandrel 1.Termoplastic liner mandrel 2.Metal liner mandrel MSK

52 Filament winding - product examples Liner for smoke chimney 6 m diameter 25 min production time Wind mill blade 8 m x 38 m Speeds up to 1 m/s 100 kg/h MSK

Pressure vessels Filament winding Carbon fibers and epoxy Much lower weight than metal Not sensitive for corrosive environments Better work safety and comfort

Fiber impregnation by external pressure Autoclave processing – fibre compaction under very high pressure in a chamber, commonly used for pre pregs Resin transfer moulding – fibre impregnation and compaction by a pressurised mould Vacuum infusion – fibre impregnation and compaction by external pressure (surrounding air) MSK

Autoclave processing Used for consolidisation of prepregs by high pressure and curing by heating MSK MSK

Aircraft Epoxy resins Carbon, aramid and glass fibers Honey comb core sandwich (Al) Prepreg lay up with autoclave cure Wings, control surfaces, hatches, covers, floors JAS Gripen SAAB 2000

Rubber bladder consolidation MSK MSK

RTM - Resin transfer molding Resin injection into a closed mold, containing the reinforcement, with the aid of pressure Rigid, metal molds are used Mold can be heated if necessary Large, complex and highly integrated components can be produced Low investment and mold costs Good work environment Medium length to long length series MSK

RTM principle Insertion of preformed reinforcement Resin injection by external pressure Product demoulding Curing under pressure Heating possible for resin curing MSK

RTM automated process MSK

Bicycle fram manufacture by bladder moulding process MSK MSK

Vacuum infusion molding Resin injection into a closed mold, containing the reinforcement, with the aid of vacuum Processing at room temperature Higher external pressure seals the mold As upper mold can either a vacuum bag or a rigid shell be used Lower investment cost Only one high quality surface is obtained MSK