The Durability of Composites for Outdoor Applications Dr Cris Arnold.

Transcription

1 The Durability of Composites for Outdoor Applications Dr Cris Arnold

2 Use of Composites Outside 2

3 Use of Composites Outside 3

4 Use of Composites Outside 4

5 Use of Composites Outside

6 Use of Composites Outside 6

7 Benefits of Composites Composites don t corrode Composites are water-resistant Composites don t rot We ve been using them in outside applications for decades No problems end of presentation Well, not quite! 7

8 Important Effects Elevated Temperature Oxygen Water UV radiation Other liquids / gases Mechanical loading, including creep and fatigue

9 Effects of Elevated Temperatures Polymers lose stiffness and strength Thermal / oxidative degradation Thermal expansion effects cause local stresses

10 Important Terms: Polymers HDT (Heat Deflection Temperature) Temperature at which material loses a certain proportion of its short term stiffness Related to Tg or melting point of polymer (or fibres) Polymer composites retain stiffness to higher temperatures, so HDT of a composite is often much higher than that of the polymer matrix

11 Important Terms: Polymers Maximum Use Temperature (UL) Temperature at which material would lose half its strength after 10 years. Properties measured at higher temperatures and extrapolated. Measure of thermal degradation. UL of composite is normally quite close to UL of matrix (unidirectional composites an exception)

12 MATERIAL HDT UL Polypropylene ~ 60 C ~ 100 C Glass-filled Polypropylene ~ 130 C ~ 140 C Nylon 66 ~ 100 C ~ 100 C PEEK ~ 190 C ~ 230 C Polyester ~ 130 C ~ 150 C Glass Reinforced Polyester ~ 250 C ~ 180 C Epoxy ~ 170 C ~ 140 C Carbon Fibre / Epoxy > 250 C ~ 170 C Phenolic ~ 180 C ~ 160 C

13 Thermal Fatigue Temperature fluctuations can cause fatigue cracks Thermal expansion along fibres is much less than across fibres, or in matrix Mismatch of thermal expansion causes stresses which can lead to cracking

14 X-Ray micrographs showing crack development due to temperature cycles

15 Effects of Weathering Predominant effect is UV damage of polymer matrix and polymer fibres. Oxygen, water, temperature fluctuations are generally secondary effects.

16 Spectrum of Sunlight

17 UV Degradation UV radiation causes chain scission of polymers The broken ends are often highly reactive free radicals These can then undergo reactions with the polymer, oxygen, water to cause more chain breakage Leads to embrittlement, loss of strength, material removal

18 Assessing Weathering Resistance Outdoor exposure tests Slow! Can speed up using hot, sunny locations Can also use mirrors

19 Assessing Weathering Resistance Accelerated test chambers Faster Need to ensure appropriate spectrum Slow mechanisms not seen

20 UV B

21 UV A

22 UV Stabilisers Can protect to some extent with stabilisers that: Screen material from UV (pigments that reflect or absorb UV) Absorb UV (degrade it to heat) Harmlessly mop up free radicals

23 UV Resistance PEEK - excellent Conventional Polyesters good (some yellowing with time) Nylons - moderate Vinyl Esters moderate (more degradation due to OH groups) Epoxy resins only moderate resistance (need surface coatings for good protection) PP poor (needs stabilising)

24 UV Resistance Glass fibres not affected Carbon fibres not affected and give UV screening due to dark colour Aramid (Kevlar) moderate / poor resistance

25 Methods to Measure Effects of Weathering Mechanical testing Microscopy, especially of the surface 25 Glass / polyester composite after 12 years outdoor exposure

26 Methods to Measure Effects of Weathering FTIR Spectroscopy Shows chemical changes due to degradation Can also be used to measure CO 2 evolution during weathering 26

27 Methods to Measure Effects of Weathering DSC Can show oxidation resistance, especially for thermoplastics 27

28 Methods to Measure Effects of Weathering Colour changes Surface hardness Weight gain / loss 28

29 Effects of Liquids on Composites Softening of the polymer matrix. Chemical attack of the polymer. Chemical attack of the reinforcement. Chemical attack at the interface between polymer and reinforcement.

30 Effects of Liquids on Composites Physical swelling of the reinforcement. Physical degradation of the interface between polymer and reinforcement. Internal pressures generated from osmotic effects. Stresses generated during sorption and desorption.

31 Solubility Determined by chemical similarity between material and liquid For water, more -OH groups in material lead to higher solubility Liquids will also collect at cracks, voids, debonded interfaces

32 32 Material Water uptake (%, by weight) Polyethylene 0.1 Polypropylene 0.1 PEEK 0.2 Polycarbonate 0.35 Polysulphone 0.4 Bismaleimide 0.8 Polyester Vinyl ester Nylon 6, Polyimide 1.3 Phenolic Epoxy Cellulose (fibres)

33 Swelling Expansion of polymer due to liquid absorption Determined by: Solubility Molecular volume of liquid Stiffness of polymer Different swelling of polymer and fibres can lead to significant stresses at interface

34 Interface failure of glass / polyester composite due to swelling

35 Plasticisation Absorbed liquids can cause plasticisation of matrix (and fibres) Leads to reduction in stiffness and increase in creep rate Noticeable with aramid fibre composites Increase in toughness and ductility (with more brittle matrix materials)

36 1640 MPa 1470 MPa 36

37 Diffusion Diffusion important as well as solubility Determines time required for effects to become important Fast diffusion can lead to problems with absorption / desorption Harder to model and predict with composites than other materials

38 Diffusion Diffusion through matrix Diffusion through fibres Capilliary wicking along fibre / matrix interface Permeation to voids / microcracks

39 Chemical Degradation of Matrix Hydrolysis of resin can occur Ester groups especially susceptible Higher temperatures accelerate degradation Alkaline conditions worse Stresses increase degradation (?)

40 Chemical Degradation of Fibres Carbon Fibres very resistant apart from very strong oxidising agents Aramid fibres and natural fibres can be subject to hydrolysis at high temperatures and in acid / alkaline conditions Glass fibres attacked in acid conditions

41 Environmental Stress Corrosion Cracking of Glass Fibre Composites 41

42 ESCC of GFRP In acidic conditions, Chemical degradation of fibres Al H H H + 3 H + + Al 3+ = SiO 4

43 ESCC of GFRP Degradation accelerated at regions of high stress Leads to rapid cracking in a planar manner (unusual for composites)

44 Osmotic Blistering (boat-pox) 44

45 Osmotic Blistering Widely investigated problem, mainly an issue with glass fibre / polyester composite boats, also swimming pools Lots of incorrect and partly-correct information out there 45

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49 Osmotic Blistering Water will permeate through the material and collect at any void / debond Fibre Water diffusion Debond Matrix More water diffusion Debond propagates

50 Osmotic Blistering Hydrolysis causes degradation products to dissolve in this water pocket O O R C O R' + OH - R C O - + HO R' + H 2 O O R C + OH - OH

51 Osmotic Blistering Hydrolysis causes degradation products to dissolve in this water pocket Other additives can also dissolve in these pockets Fibre Debond Water diffusion Degradation products Matrix

52 Fibre Water diffusion Osmotic Debond Blistering Degr prod Increased diffusion of water due to Matrix osmosis More water diffusion Debond propagates Pressure builds up

53 Fibre Debond Matrix Water diffusion Degradation Osmotic Blistering products Pressure builds up, causing cracks and blisters More water diffusion Debond propagates Pressure builds up

54 Osmotic Blistering Repair is very costly Worse in low salinity / high temperatures Can be reduced by eliminating voids and un-wetted areas and with good gel-coat Vinyl esters and isophthalic polyesters are better than orthophthalic polyesters Barrier coatings will slow the problem but not prevent it

55 Water Ingress into Carbon / Epoxy Composites Aerospace composites will operate in humid environments for decades Water is known to be absorbed into epoxies, causing Plasticisation Stiffness and strength reductions Swelling 55

56 Dry 1% water content

57 Improved Design Methods Current design methodologies assume worst case hot/wet saturation To safely refine designs, a model of water ingress into complex components has been developed

58 Improved Design Methods This predicts moisture content and resulting properties during service life

59 Improved Design Methods Need to fully understand the water sorption / desorption and the variation with: Temperature Humidity / water contact Fibre volume fraction Fibre orientation Fibre architecture Working towards improved design methods

60 Modelling at scales of fibre, weave, component. 60

61 Galvanic Corrosion Carbon fibre composites can cause rapid corrosion of steel if there is electrical contact H 2 O + O 2 > OH - M > M + e- <= 61

62 Galvanic Corrosion Carbon fibre composites can cause rapid corrosion of steel if there is electrical contact H 2 O + O 2 > OH - M > M + e- <= 62

63 Galvanic Corrosion Carbon fibre composites can cause rapid corrosion of steel if there is electrical contact 63

64 Galvanic Corrosion Carbon fibre composites can cause rapid corrosion of steel if there is electrical contact Quite well-known about and requires electrical insulation of bolts 64

65 Conclusions Composites are generally resistant to the effects of outdoor exposure Far fewer problems than steel! A wide variety of effects do occur Complex physical / chemical interactions Improved understanding, modelling, design and information will help minimise problems. 65

66 Thank you for your attention