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Part 3

Part 3. The work safety related to processing of composites. Styrene emissions Control methods. Background. Liquid thermosets containing organic solvents Open production methods (hand lay-up, spray lay up, filament winding) Labour intense processing methods, with several manual steps

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Part 3

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  1. Part 3

  2. The work safety related to processing of composites Styrene emissions Control methods

  3. Background • Liquid thermosets containing organic solvents • Open production methods (hand lay-up, spray lay up, filament winding) • Labour intense processing methods, with several manual steps • Small and medium sized companies, with low technical know-how, and limited resources • Unsaturated polyesters containing styrene are particularly commonly used thermosets

  4. Styrene emissions from unsaturated polyesters • Investigations started in 1980-1990 due to increased environmental awareness • 2500 persons were exposed to styrene in Finland • For 77 % the exposure was higher than the occupational exposure limit (20 ppm) • Neste Polyester had 1989 to close a production plant in Sweden due to styrene emissions

  5. Exposure to styrene in Finland 1992 Ref. Pfäffli et al. 1992

  6. Styrene health effects • Exposure by inhalation • Metabolisation through oxidation to styrene oxide • Neurotoxic: headache, dizziness, fatigue • Irritating effect on eyes and mucous membranes • Long exposure (10 - 20 years) harmful • Not carcinogenic • Very low odour threshold, 0.05 ppm • Exposure limit 20 ppm in most countries

  7. Health effects for other thermosets • Isocyanates: • Astma, allergenic sensibilisation • No warning (does not smell!) • Very low threshold value 0.05 mg/m3 • Epoxy: • Allergenic sensibilisation • Acrylates: • Irritating for eyes and skin • Astma, allergenic sensibilisation • Phenol-formaldehyde resins: • Irritating for eyes and respiratory tract • Allergenic sensibilisation • Threshold value 0.6 mg/m3

  8. Styrene exposure control techniques • Unsaturated polyester resin development • Processing technology developments • Closed mold techniques, spray gun development • Ventilation • Dilution ventilation, local ventilation • Respiratory protection • Good work practices and housekeeping • Personnel training and education

  9. Unsaturated polyester resin development Low styrene emission (LSE) resins: • Emission suppressed resins containing additives which prevents the styrene emission • High solid resins with reduced styrene content • Low vapour pressure monomer resins where styrene is substituted with an other compound • Chemically tailored resins with better compatibility between the resin and the styrene monomer

  10. Emission suppressed resins • Paraffin or wax compounds added to the resin • The additive separates after lay up, and forms a thin layer on the resin surface • No effect during spraying and rolling • Laminate interfacial bonding is reduced Paraffin covered laminate surface

  11. Delamination in paraffin LSE resins Good interlaminar adhesion Delaminated laminate

  12. High solid resins • Styrene content in conventional unsaturated polyesters is 38 - 45 weight % • Reduction of styrene content (< 35 weight %) gives a high solid resin • Reduced styrene content gives very high viscosities, which will make processing very difficult

  13. Low vapour pressure monomer resins • Styrene can be replaced with any compound which can participate in the free radical crosslinking reaction • A lower vapour pressure will decrease solvent emissions • p-Methyl styrene investigated • Higher prices, less commercial availability, unknown health properties, and effects on composite properties are main reasons for low use

  14. The styrene emission mechanism has been described by emission measurements • Gravimetric measurements • Styrene concentration measurements during lamination • On-line measurements by IR Ref.: Säämänen et al. 1991

  15. Styrene emission:Conventional resin - paraffin resin

  16. Styrene emission during the different phases of a lamination

  17. Styrene emission:Low vapour pressure resin p-methylstyrene p-methylstyrene with paraffin Solvent emission decreases during lamination with 2/3

  18. Styrene emissionvacuum injection - spray up lamination

  19. Work practice • Instruction of workers regarding the correct handling of the resins • Keep nose and mouth away from styrene sources! • Keep the lid on the bucket! • Do not leave dirty gloves, rollers and brushes in the work area! • Work upwind of the emission sources • Use personal respiratory protection

  20. Ventilation • Most important control technique for styrene emissions • Dilution ventilation and local ventilation: • In dilution ventilation large volumes of fresh air is blown into the work area • In local ventilation vapours are locally removed near the source • Ventilation requires heating of the air, and styrene removal from the outgoing air • Efficient ventilation is always a costly investment

  21. Local ventilationsmall objects

  22. Local ventilationlarge objects

  23. Dilution ventilationHorizontal displacement

  24. Spray booths Exhaust Air 2500 mm 2400 mm Exhaust 6000 mm

  25. Spray booth with automatically moving curtains and local fresh air supply Exhaust air Moving supply air unit Moving ceiling 1.6 m x 1.55 m Location of worker Moving curtains Decreases styrene exposure even more

  26. Routes to safe work environment • Good work practices and housekeeping • Personnel training and education • Processing technology developments (Closed mold techniques, spray gun development) • Ventilation • Unsaturated polyester resins with lower emissions • Respiratory protection

  27. Adhesion in polymers and composites

  28. Bolts versus adhesives Bolted car fender Laminated one piece front bumper

  29. Adhesives in composite products Benefits with adhesives: • Uniform stress and strain distribution over the joint • Maintained integrity and strength of substrate • No holes, rivets or fastening elements which weakens the substrates • Better aesthetics • Better design possibilities • Component symmetry is not disturbed

  30. Definitions • Adhesive = A polymeric material which forms a structural bond between two materials • Adhesion = occurs when two surfaces are held together • Adherend = the surface of the substrate which are held together

  31. Adhesive joints Idealized joint Realistic joint Four bonding surfaces, three materials, two interfaces Four bonding surfaces, three materials, two interphases

  32. The adhesive joint – the realistic case Characteristics: • Surface roughness and topography • Different properties for adherend near surface than in bulk • Interphase will not have zero thickness

  33. Structural adhesives • Epoxy based: • one component or two component adhesives with excellent adhesion to rigid materials such as metals, thermoplastics or composites • Low creep, high stiffness, low shringae, high tensile shear strength, poor peel strength, modest impact strength • Operating temperature up to 150 – 250 °C • Polyurethane based: • softer adhesives for flexible joints, suitable for thermoplastic • Isocyanate bonds are formed • Metylmethacrylate based: • excellent adhesion to metals, composites and thermoplastics, with very fast setting • Cyanoacrylates: • Cure by moisture in air, very fast • Silicones: • Low temperature adhesives • Very flexible

  34. Curing/setting of an adhesive Depends on the type of adhesive: • Thermoplastic adhesive (hot melt) by melting and cooling • Solvent evaporation from thermoplastic adhesive • Thermoset adhesive is cured by thermal or catalytical initiation • In-situ polymerisation of adhesive molecules, to a large adhesive polymers

  35. Adhesion mechanisms • Surface wetting: • Hydrophobic surfaces dislike water • Hydrophilic surfaces like water • Surface compatibility: • Similar surfaces like each other • Consider the chemical characteristics • Mechanical treatment of surface • Increases surface roughness

  36. Similar mechanisms as for fiber-matrix interface • Micromechanical interlocking • Electrostatic (dipole) interaction • Chemical bonding • Chain entangling • Transcrystallisation

  37. Adhesion by interdiffusion • Aherend surface is softened by melting or by the use of solvent so that adhesive polymer molecules can interdiffuse into the adherend • Possible for thermoplastic adherends always • For thermosetadherends the intediffusion must be done before the thermoset is fully cured -> on wet thermoset surface

  38. Surface treatment before adhesive application • Cleaning • Primer application • Chemical treatment: • Sizeing treatment on glass fibres • Plasma treatment • -can induce new functional groups • Corona discharge treatment

  39. Measurement of surface attraction • Surface energy/surface tension gives a measure of the attraction of an adhesive to the adherend • Contact angle measurements is the most common way to determine surface attraction: • The angle formed by a drop of a liquid on a smooth surface is measured • The angle indicates the attractive or repulsive forces between liquid and surface • Measurement with different liquids gives and indication of the surface attraction

  40. Contact angle measurements High attraction Low attraction A high surface angle is obtained as the liquid is repelled from the surface • A low surface angle is obtained as the liquid is pulled into contact with the surface

  41. The Lotus effectWater-repellent surfaces Good wetting to the surface the droplet stays Poor wetting to the surface the droplet rolls off 42

  42. Mechanical failure of adhesives

  43. Loading modes for adhesives Shear Tensile Shear Flexural with interlaminar shear

  44. Adhesion failure modes Cohesive failure Adhesive failure The failure occurs at the interface, leaving the adhesive and adherend separated when the surface is observed macroscopically • The adhesive or the adherend fails in the bulk of the material • The interphase is intact Adhesive strength is often limited by the bulk strength of adhesive or adherend

  45. Cohesive failure

  46. Measurement of adhesion • Failure mode must be defined • Tests must be done under conditions similar as during the use conditions • Time scale for failure must be considered • Temperature effects must be considered • Ageing effects (long term adhesive strength) must be considered

  47. Measurement methods • Lap-shear specimen • Double lap-shear specimen Both tensile and shear failure!

  48. Peel test • An adhesive joint is peeled in different angles 90 degree peel test, often used for tapes T-test or 180 degree test, often used for flexible materials

  49. Wedge test • Used for rigid substrates • Crack propagation can be detected • Environmental effects can be evaluated • Qualitative test

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