6. Plastics.ppt

7/27/2019 6. Plastics.ppt

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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses

6.4 General properties

6.5 Forming processes 6.6 Modes of failure (durability)

6.7 Typical applications


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PLASTICS

Polymer: A material formed of large moleculeswhich are built up (polymerised) from a largenumber of small molecules (monomers). Theusual (but not only) example is the organic

polymers.

Organic materials:These are materialsoriginating from living organisms. Strictly allmaterials containing carbon are defined asorganic.

Plastics: This term is used for a range of organicmaterials. Note that the term "plastic state" ofconcrete relates to the generally low modulus ofelasticity and high creep of plastics.


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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses





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Plastics are divided into two types:

Thermoplastics always soften when heated.

Thermosetting plastics which set, i.e.polymerise when mixed with a "hardener"

and will not soften when heated. Setting is

accelerated by catalysts, heat, pressure and

even radiation.


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Typical Thermoplastics and

Thermosets


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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses





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Plastics are used in different

categories of application: As formed products (e.g. parts of electrical

fittings). This is the only application for

thermoplastics. As textiles, e.g. Geotextiles.

As adhesives (e.g. epoxies)

As surface coatings (e.g. polyurethane paint) As a matrix for reinforced products (e.g. Glass

Reinforced Polyester - GRP or fibreglass)


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Plastic

windows and

fascias


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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses





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Properties of Plastics (1)

Strength: high for short term

Modulus: low

Creep: high

Behaviour in Fire


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Behaviour in fire: poor because of:

Creep and loss of strength at high temperatures.

High combustibility (fire retardant additives may

help - these give off small quantities of gases suchas chlorine which locally displace the oxygen)

Toxic fumes from combustion.

Hazards from burning liquid (e.g. droplets fromburning light fittings)


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Properties of Plastics (2)

Density: Similar to wood

Thermal Conductivity: Similar to wood

Electrical Conductivity: Insulator

Thermal Movement: High

Moisture Movement: LowPermeability: Surprisingly High

Cost: Material cost high but manufacture low


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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses





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FormingProcesses


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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses





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Modes of failure (durability) (1)

Biological Being organic many plastics arenutritious to some forms of animal/insect/fungus

etc. Biocides may be added during manufacture.

Oxidation Oxidation causes embrittlement andloss of strength. It is generally slow in the absence

of heat or sunlight. Anti-oxidants may be used.

Sunlight Most plastics are damaged by long term

exposure to ultraviolet light. The process is knownas degradation or photo-embrittlement. It may be

reduced by adding a uv absorber, e.g. carbon black

or silica fume.


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Modes of failure (durability) (2)

Water In permeable plastics there may be loss ofsome components through leaching. Osmotic

pressure from moisture ingress may also causesurface spalling.

Leaching There has been a specific problem withpvc leaching plasticiser when in contact withinsulating materials such as expanded polystyrene.Electrical wiring in loft spaces has been found tohave premature embrittlement of the pvcinsulation.


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PART 6 POLYMERS

6.1 Terminology

6.2 Types

6.3 Uses





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Typical Applications

Polymers in Concrete

Geotextiles

Plastic Pipes

Thermosetting Resins

Transparent Plastics


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Polymers in concrete (1)

Polymer Concrete is a mixture of resin (polyester

or epoxy, see below) and aggregate (normally

sand). It is used in thin sections for repairs. PIC Polymer Impregnated Concrete is rare and is

made by vacuum impregnating a monomer into

hardened concrete which is polymerised inside

concrete with heat or radiation. Only used infactory produced precast units.


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Polymers in concrete (2)

PMC Polymer Modified Concrete (PolymerPortland Cement Concrete). Catalysed polymer isadded into ordinary concrete at the mixer and

polymerises insitu. Used for concrete repairs withthicknesses of 50-100mm, overlays for bridgedecks etc.

Silanes and silicones. Used as coatings forhardened concrete.

Polymer reinforcement (e.g. polypropylene fibres)

Epoxy coated rebar


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Geotextiles

Plastic Pipes




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Geotextiles

These have 3 main functions:

As filters (e.g. to keep fine materials out of graveldrains)

As reinforcing nets (e.g. under roads)

As ties (e.g. in reinforced earth)

Non-woven materials are used as filters and forlight structural applications. Woven materials are

more expensive and are used for main structuralapplications.

The performance of the materials is adverselyaffected by the low elastic modulus and low

durability if exposed to sunlight.


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Drain Filter Layer

Topsoil

SubsoilGranular

Permeable pipe

for storm

drainage

Geotextile

filter


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Geotextiles










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Reinforced Earth

Precast slabs

Geotextile Ties


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Geotextiles










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Geotextiles

Plastic Pipes




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Plastic pipes.These are rapidly replacing conventional materials.They are almost universal for above ground wasteand are increasingly used for below ground andsupply applications. They are often resistant tochemicals and good at accommodating movementduring service.

High density polythene is often used for this.

Note that the water services have very strict controlson the types of polymer which may be used forpotable water supply because the polymerisation isnot normally complete and residual monomers can

be leached out and these may be toxic.


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Geotextiles

Plastic Pipes




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Thermosetting resins.

These are mixed on sites, e.g. for setting in

holding down bolts into existing slabs.

Epoxies. Two components must be mixed in exactproportions ("hooks and eyes"). Will cure down to

5oC.

Polyesters. Sets with a catalyst. More catalyst

gives faster setting. Can be recognised by the

characteristic "fibreglass" smell.


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Application of Polyester ResinBase Plate

Resin in

drilled hole

Threaded

bar


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For all Thermosetting resins.

The components are toxic - always wear gloves. Theorganic catalysts used with polyesters are particularlycarcinogenic. The dust arising from cutting/abrading the

hardened resin is carcinogenic. The odourless vapour whichevolves during curing of epoxy is toxic.

For low permeability the resins must be cured correctly inVERY DRY conditions. A white "bloom" on the surface

indicates moisture during curing.

The setting reactions are exothermic (especially polyesters).If use is delayed after mixing the set may be delayed by

placing the material in a shallow metal container.


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Geotextiles

Plastic Pipes




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Transparent plastics

These are used in windows etc. in place ofglass:

Acrylics (e.g. Perspex) are cheap but havepoor impact resistance.

Polycarbonates are more expensive but havegood impact resistance (may be used forbullet proof laminates).

Both have high thermal expansion.


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Polythene (polyethylene)

Polythene is used for many applications and

is available in HD (high density) form

which is less permeable. Black polythene lasts longer because it is

resistant to UV light.

1000 gauge polythene is 0.01", i.e.0.254mm thick.

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