Tekapet polymers

Classified in Biology

Written at on English with a size of 5.98 KB.

 

Q4) a) Discuss the difference between sapwood and heartwood in terms of natural durability and resistance against fungal and insect attack.

Sapwood:

• Width varies from species to species, rate of growth & age of tree

• Except for very young trees (sapwood = whole radius) sapwood typically represents 20 to 50% of the total radius

Heartwood:

• Heartwood advances to include former sapwood cells

• The acidity of the heartwood increases & extractives are formed & colouration changes take place

• Resistance to fungal & insect attack increases

Many timbers develop gums & resins in the heartwood

(10 marks)

b) List and briefly describe the primary, secondary and tertiary functions provided by the cells in a growing tree and state the characteristic differences between softwoods and hardwoods.

Functions provided by the cells:

• The structure of timber is connected to the functions performed within the growing tree

• Primary function – conduction of water& dissolved mineral salts from the roots to the leaves

• Secondary function – mechanical support of the tree as a whole

• Tertiary function – serves as a food store during the winter to supply spring growth

• Cellular structure specially designed to perform the functions listed above

• Each different kind of tree has evolved its own individual way of performing these functions

• Wood cells are of three main types according to adaptations for conduction, mechanical support & food storage

Characteristic differences between softwoods and hardwoods:

• Hardwoods – water conducting elements distributed throughout the annual ring

• Softwoods – water conducting & strengthening elements segregated in the springwood & summerwood respectively


Q5) a) Produce a table to indicate the environmental impact of using polymers in the construction industry and discuss the importance of carbon in the production process.

Item Comment

Availability of raw material Limited – by-product of oil

Extraction process Possible pollution problems

Energy consumed High – due to chemical processes

Health & safety Fire hazard

Waste disposal Many do not break down & require some form of recycling

The importance of carbon:

• Lightest element that has four electrons in the outer shell – good for bonding

• Wants to share electrons with other atoms to produce an octet (8 electrons in the outer shell)- covalent bonding

• Easiest with atoms of similar size

• Bonding may result in regular patterns of atoms – (crystals), irregular arrangements (amorphous) or long chains (fibres)


b) What is meant by the term ‘visco-elastic’ behaviour and how do thermoplastic and thermosetting polymers differ from each other in terms of their chains and overall properties?

Visco-elastic:

• Mechanical properties lie between:

• Hookean – stress ∝ strain

• Newtonian – stress ∝ rate of strain

• Termed visco-elastic – stress is a function of strain & time.

• Thermoplastic – all chains of the molecules are separate & can slide over each other Long chain molecules held together by Van der Waals forces, which are relatively weak

• Chemical bonding along the length of the chain is very strong

• Heating weakens the intermolecular forces & the material soften and becomes a viscous melt – cooling allows re-solidification

• Process may be repeated

• Thermosetting – chains become cross-linked producing a solid material which cannot be softened & resists flow

• Formed in a two-stage chemical process:

• 1.) Long chain polymerised molecules produced

• 2.) Cross-linking takes place

• Reaction can take place at room temperatures or at elevated temperatures & pressures

• Rigid structure – mechanical properties affected by application of heat

• Thermosets cannot be softened by heating & products must be moulded prior to polymerisation

• Achieved by mixing a resin & hardener & moulding quite quickly

• Material begins to degrade with the application of excessive heat

• Comprise large three-dimensional molecules – higher strength & stiffness than thermoplastics

• Increased rigidity leads to increased brittleness – require toughening using fibres to give impact resistance

Entradas relacionadas: