Recycling

Question 1

Recyclable meaning

Answer 1

After a material has completed its life cycle in one component, it could be reprocessed, could reenter the materials cycle and reused in another compomemf

Question 2

Biodegradable meaning

Answer 2

By interactions with the environment, the material deteriorates and returns to virtually the same state in which it existed prior to processing

Question 3

Which metals are toxic and pose health risks if landfilled?

Answer 3

Lead and mercury

Question 4

Can all alloys be recycled

Answer 4

No, some are too comple or contaminated. The more a metal is recycled the more the quality decreases

Question 5

How can you design to increase recyclability of alloy items

Answer 5

Design so that they are easily dismantled. Joining dissimilar alloys can lead to contamination. Best to join similar alloys with welding over bolting or riveting

Avoid coatings as these can act as contaminantes

Question 6

How to separate allot types after dismantling and shredding

Answer 6

Magnetic or gravity techniques

Question 7

Why is alloy such a good recyclable material

Answer 7

Corrosion resistant so non biodegradable

Since not easily corrosive it can be totally reclaimed

Low energy required to recycle it

Question 8

Is glass biodegradable

Answer 8

No as is relatively inert so does not decompose

Question 9

Why is there not an economic drive to recycle glass

Answer 9

Its raw materials are readily available and inexpensive. To salvage glass it must be sorted by colour, type and composition which is time consuming and expensive.

Question 10

What is an advantage of using recycled glass

Answer 10

More rapid and increased production rates and a reduction in pollutant emissions

Question 11

Are polymers biodegradable

Answer 11

No as chemically and biologically inert. Biodegradable polymers exist but is expensive to produce.. some can be disposed of by combustion as do not release toxic polluting emissions

Question 12

What type of polymer is most recycled

Answer 12

Thermoplastics since they may be reformed upon heating. Sorting by type and colour is necessary. Process is complicated when fillers are added to modify original properties.

Question 13

Benefit and neg of recycled plastics

Answer 13

Recycled plastic is less costly but quality and appearance degrade with each recycle.

Question 14

Recycling of thermosets

Answer 14

More difficult as these materials are not easily remolded or reshaped due to crosslinks. Can be ground up and added to the virgin moulding material prior to processing. Recycled as filler material

Question 15

Recycling of rubber materials

Answer 15

When vulcanized (hardening of rubber by treating with chemicals), they are thermoset materials which makes chemical recycling difficult. They may also contain a variety of fillers. Some can be reshaped. The most recyclable alternative is thermoplastic elastomers as easily reshaped.

Question 16

Why are composites hard to recycle

Answer 16

Contain two or more phases/materials that are usually intermixed on a very fine scale and so hard to separate.

Question 17

Steps in recycling thermoset and thermoplastic matrix composites

Answer 17

Shedding/grinding to small particles
Sometimes this is used as a filler that is blended with a polymer before fabrication
Other techniques allow to separate fibres and/or matrix materials,
Sometimes matrix is volatized, others it is recovered as a monomer
Recovered fibres will have a short length from grinding process and experience a reduction in mechanical strength

Question 18

Galvanic series

Answer 18

Represents the relative reactivities of a number of metals and commercial alloys in seawater. The alloys near the top are cathodic and un reactive while those at the bottom are most anodic

Question 19

Why are most metals and alloys subject, to oxidation and corrosion (exist as compounds in nature)

Answer 19

They are more stable in an ionic state, there is a net decrease in free energy in going from metallic to oxidised states

Question 20

Passivity

Answer 20

When some metals or alloys under particular environmental conditions loose their chemical reactivity and become extremely inert. Caused by for action of highly adherent and very thin oxide film on metal surface which creates a protective barrier

Question 21

What impacts corrosion rate

Answer 21

Fluid velocity- increased velocity increases corrosion due to erosive effect

temp- increases rate of most chemical reactions

composition- increased concentration of corrosive species (e.g h+ ions in acid) increase corrosion.

However, for materials capable of passivation, raising corrosive content may result in active to passive transition, with a considerable reduction in corrosion

Question 22

How does increased fluid velocity impact corrosion rate

Answer 22

Increase it due to erosive effects.

Question 24

How does cold working impact corrosion

Answer 24

Increases it

Question 25

What is uniform attack corrosion

Answer 25

A form of electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface and often leaves behind scale or deposits. Oxidation and reduction reactions occur randomly over the surface

Question 26

Galvanic corrosion

Answer 26

Occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. Typically the most reactive metal will corrode before inert one and thus protect it.

Question 27

What’s an electrolyte

Answer 27

A liquid which contains both anions and cations which when electric potential is applied will move to their respective electrodes and an electric current can flow

Question 28

What does the rate of galvanic attack depend on

Answer 28

The relative anode to cathode surface areas that are exposed to the electrolyte. For a given cathode area, a smaller anode will corrode more rapidly than a larger one as corrosion rate depends on current density, not simply the current. There is a higher current density in the anode when it is smaller

Question 29

What measures can reduce the impact of galvanic corrosion

Answer 29

If coupling dissimilar metals is necessary, chose two that are close in galvanic series Avoid an unfavourable anode to cathode surface areas Electrically insulate dissimilar metals from each other Electrically connect a third, anoxic metal to the other two

Question 30

What is the primary corrosive agent

Answer 30

Moisture containing dissolved oxygen (gets reduced while metal is oxidised, water acts as electrolyte ) sulfur compounds and sodium chloride may also contribute.

Question 31

Why is freshwater usually less corrosive than sea water

Answer 31

Sea water has nacl (acts as electrolyte, increases electrical conductivity)

Question 32

How to reduce corrosion

Answer 32

Select materials once environment has been characterised Change the environment (e.g. increase or decrease the concentration of some species) Use inhibitors substances that when added at low concentrations , decrease the corrosiveness. (E.g. some redact and eliminate chemically active species, some attach themselves to corroding surface by oxidation or reduction to form a barrier) The design (e.g easily washable or drainable, exclusion of air) Physical barriers e.g films and coatings. Coating must have good surface adhesion. Should be non reactive and resistant to mechanical damage

Question 33

Cathodic protection

Answer 33

Involves supplying, from an external source, electrons to the metal to be protected, making it a cathode, so the reaction is forced in the reverse direction.

Question 34

What’s an example of a cathodic protection technique

Answer 34

Employ a galvanic couple. The metal to be protected is electrically connected to another metal that is more reactive in the particular environment. The latter experiences oxidation and, upon giving up electrons, protects the first metal. Often called sacrificial anode.

Question 35

Galvanizing (another cathodic protection)

Answer 35

A layer of zinc is applied to the surface of steel by hot dipping. Zinc is anodic so protect steel. Corrosion of steel is extremely slow as ration of anode to cathode surface area is quite large

Question 36

Impressed current (another cathodic protection)

Answer 36

Connect dc power source. The negative terminal is connected to the structure to be protected, the other is joined to an inert avoid (often graphite because of high conductivity p).

Question 37

Are ceramics corrosive

Answer 37

Very immune as already corroded as is compound between metallic and non metallic elements. If they do corrosion generally involves simple chemical dissolution.

Question 39

Degradation of polymeric materials

Answer 39

Degradation is physiochemical, involves physical as well as chemical phenomena. They may deteriorate by swelling or dissolution. Covalent bonds rupture due to heat, chemical reaction and radiation

Question 40

How does swelling cause polymer degradation

Answer 40

Liquid or solute diffuses into and is absorbed within the polymer. The small solute molecules fur and occupy positions a in polymer molecules. Thus structure is forced apart and specimen expands and swells. Furthermore, increase in chain separation results in a reduction of the secondary intermolecular bonding forces. So material becomes softer and more ductile. Liquid solute also lowers glass transition temp, which if enough will cause material to become rubbery and weak.

Question 41

How does dissolution cause polymer degradation

Answer 41

Polymer is completely soluble, may be throught if a continuation of swelling. The greater the similarity of chemical structure between solvent and polymer, the greater the likelihood of swelling and/or dissolution.e.g hydrocarbon rubbers absorb hydrocarbon liquids. Polymers can also experience a degradation process called scission

Question 42

How to decrease deterioration of polymers

Answer 42

Decrease temp, increase molecular weight, increase degree of crosslinks and crystallinity.

Question 43

Scission

Answer 43

The severance or rupture of molecular chain bonds. This causes a separation of chain segments at the point of scission and a reduction in the molecular weight (which has an impact in mechanical strength and resistance to chemical attack). Bond rupture may result from the exposure to radiation or heat and from chemical reactions .

Question 44

Weathering

Answer 44

Degradation of polymers from being exposed to outdoor conditions.

Question 45

Type of conditions that cause weathering

Answer 45

Oxidation initiated by ultraviolet radiation from the sun Some are susceptible to water absorption