Polymers + processes

Define polymer

A long molecular chain made up of monomers who are mainly made up of carbon and hydrogen atoms

DURING POLYMERISATION THE DOUBLE BOND BETWEEN CARBON ATOMS BREAK SUCH THAT EACH CARBON CAN BE ATTACHED TO THE NEXT MONOMERS OREPONDING CARBON ATOM.

What does chain length of a polymer effect and how do we measure it?

Flow properties

Mechanical properties

Chain weight

Two main types of bonds in polymers?

Covalent bonds = strong = the ‘glue’ of all of the chains

Van der Waals forces = weak = lightly connect the chains to eachother and are caused by dislike charges in each.

PLASTIC DEFORMATION OCCURS WHEN THE VVW FORCES BREAK AND RCONNECT WITH ANOTHER POINT IN THE NEIGHBOURING CHAIN BECAUSE NOW THERE IS NO MEMORY OF WHERE IT WAS BEFORE.
UNLESS CROSAS LINKED IN WHIXCH CASE THESE ACT AS ANCHORS

What are the main types of chained networks + info

Linear chains = most dense

Branched chains = less dense due to spacing between chains caused by irregular branch system

Cross-linked chains = least dense due to permanent strong covalent bonds between chains preventing close compact (dense) structure

Linear chain polymer examples

Acrylic (PMMA)

Nylon

HDPE

And more…

Branched chain polymer examples

LDPE

Cross linked polymer examples

VULCANISED rubber

Rubber initially is linear but once vulcanised covalent bonds form between chains thus turning it cross-linked.

Epoxy resin

What are the main types of polymer chain network structures (i.e. how the networks are packed together - larger scale.)

Crystalline = lattice like and ordered.

Amorphous = disordered

Semi-crystalline = MOST COMMON and is a mix of the two

Nucleation site = point in SCP where crystalline structure originates.

3 types of polymers + link to network type + how this explains properties

Thermosets = hard/strong therefore full of cross links as covalent bonds are strong.

Thermoplastics = soft therefore linear and branched as vvw forces are weak. AND BREAK AT LOWER TEMPS!!

Elastomers = PARTIALLY cross-linked - more = stiffer. (But u need at least some or wont ‘spring back’ like thermoplastics. - the coiled up partially cross linked polymers allign (large strain) when stretched out by strain breaking vvw forces, stress removed, vvw forces rebroken and anchor covelnt bonds bring back to original shape.

Mechanical properties of polymers explained

Taking semi crystalline system (most common) - when a tensile load is applied to it, the amorphous ares are the first to stretch due to their lack of structure.

NOTE: the longer the chains the more the entanglement in these amorphous regions ad hence the increase in stiffness.

Also, as is the case with most materials, lower temperatures = stronger but more brittle (glassy) and vv for higher temps.

Visco-elastic behaviour of polymers

If u apply a load for time t then remove it, the polymer could return to its natural shape i.e. strain = 0 (elastic) or if in liquidy form, as the force is being applied during time t the strain increases linearly and once the force removed it stays at that strain (like honey). Or you can have both when it will strain then keep straining then try return back to its original shape but fail (lie melted cheese) = visco-elastic. which most polymers are visco elastic not either, both.

Main advantages of polymer forming over other material forming

1. Geometry can be much more complex

2. Less energy required to form than say metals

3. Easier to work with than say timbers

4. Lower energy tends to be because of lower temps therefore also = easier handling

5. Material finishing often not required - self finishing processes.

Extrusion

Hopper/barrel + Archimedes screw + die nozzle produces EXTRUDATE

Produces uniform cross-sectional area item which can then be cut to size if necessary.

For thermoplastics cos requires heating and compression of pellets.

Allows for pigment addition

Sheets vs Films + how theyre formed

Both extruded through SLIT die extrusion - MANIFOLD spreads the melt to fill the whole slit

Sheet = sounds thick therefore is thicker than film. (0.5-12.5mm)

Film = thin (<0.5mm)

Injection moulding + main disadv

RECIPROCATING SCREW = archimdedes screw but also rams shots into mould

2 part mould and hollow items cannot be produced - then stuck together by other means across the PARTING LINE

Just as u have ramming of screw - same motor system o other side to continuously open and close 2 part mould

Extrusion blow moulding

PARISON = hollow extruded polymer melt tube

Pinches at top then sealed and then air blown into parison from other side

Open 2 part mould.

Injection blow moulding

Parison injected rather than extruded - formed around a blowing rod IN ITS OWN MINI MOULD rather than initially being hollow like extrusion.

Put parison in bigger main mould./replace old mould and keep parison in same place

Blowing rod HAS A ONE WAY VALVE - blowing air in expanding parison into main mould shape.

Remove mould and blowing rod and done.

Thermo/Vacuum forming

Flat sheet is heated

Place over mould

CLAMP IN PLACE

Vacuum sucks air from between mould and sheet - pulling sheet into place

Let cool and remove from mould.

NOTE: issues include:

1. no overhangs allowed matter of fact DRAFT needed = opposite of overhang.

2. No sharp corners allowed

3. Air tubes in mould for vacuum to function

4. Generally limited geometry