13. Enhancement of materials Flashcards
(polymer, wood, metal)
What is material enhancement?
a way of improving a material’s properties to better suit the requirements of the final product
What are the polymer additives?
- lubricants
- thermal antioxidants
- pigments
- antistatics
- flame retardants
- plasticisers
- fillers
- biodegradable plasticisers
- bio-batch additives
- antioxidants
- UV light stabilisers
What is the reason for adding lubricants to polymers?
- adding wax or calcium stearate reduces viscosity of molten polymer
- less sticky, allowing more intricate shapes to be formed
- allow moulding temp to be lowered
- saves energy
What is the reason for adding thermal antioxidants to polymers?
- prevent polymer oxidising or discolouring due to excessive heat during processing
What is the reason for adding pigments to polymers?
- tiny particles mixed into molten polymer
- give colour to final processed product
What is the reason for adding antistatics to polymers?
- reduce likelihood of polymer building up static charge
What is the reason for adding flame retardants to polymers?
- reduce likelihood of combustion or spread of fire (car engine components) or potential electrical fires ( e.g plug sockets)
What is the reason for adding plasticisers to polymers?
- allow plastics to become less hard and brittle at normal temperature use
- allow polymers to be easily formed at higher temperatures
What is an example of plasticisers being used?
- in LDPE food wrap, so it can be stretched over the fod
What is the reason for adding fillers to polymers?
- sawdust and wood flour provide bulk to the product
- less polymer required
- mineral fillers (chalk, clay, calcium carbonate) help increase thermal conductivity of polymer
- heat up and cool down more quickly = shorter mould cycle times
What is the reason for adding biodegradable plasticisers to polymers?
- make polymer more flexible, softer and easier to break down
- faster degradation time
What is the reason for adding bio-batch additives to polymers?
- oxy-degradable, photodegradable and hydro-degradable additives help reduce degradation time from 100s of years to a few years or months
What is the reason for adding antioxidants to polymers?
- help reduce environmental deterioration of polymer from exposure to oxygen
- help prevent increased brittleness and surface cracks
- help prevent pigment discolouration
When are antioxidants often used?
polymer garden furniture
What is the reason for adding UV light stabilisers to polymers?
- prevent polymer chains being broken down by sunlight
- UV attack causes colour loss and brittleness
What are UV light stabilisers used in?
- outdoor toys and play equipment
- garden furniture
- sports stadium seats
- synthetic grass playing surfaces
What are the wood enhancement methods?
- resins and laminations
- resins with fire retardant
- laminations
- preservatives
- pigments
- fire retardant preservatives
- modified natural polysaccharide
- structural composite lumber (SCL) and laminated veneer lumber (LVL)
When are resins and lamination used?
How is chipboard made and its properties?
- in engineered wood
- to enhance the properties of the useable parts of the tree such as sawdust, wood chips and fibres
- chipboard made from compressing wood chips with resin
- chipboard is very stable, and not really affected by temperature and humidity
- chipboard has a uniform strength across the board, with no grain problems like solid wood
What are resins with fire retardants?
- resin is impregnated with fire retardant cladding (e.g. for indoor flooring)
When are laminations used?
- veneers such as natural wood are laminated on to the board surface to enhance aesthetics
What are preservatives?
- protect wood from fungal and insect attack
- wood can be pre-treated with copper-based preservative
- penetrates the wood to protect the whole plank
- copper has excellent fungicidal properties
Why are pigments added?
- added to preservative to give different coloured shades to enhance the aesthetics
- e.g. outdoor decking
- can also be used on cheaper softwoods to make them look like hardwoods
Why are fire retardant preservatives used?
- used to pressure treat wood for roof joists, cladding etc and exhibition stands
- wood can also be treated to make it harder and more resistant in high wear situations such as flooring
How is modified natural polysaccharide used?
- wood is impregnated to cure within the wood cell structure
- results in increased hardness, toughness and stability
- often added to hardwood floor panels
How is structural composite lumber (SCL) and laminated veneer lumber (LVL) made, and what are their properties?
- made by layering strands (SCL) or veneers (LVL) of wood with resisns such as urea formaldehyde, pressing and heat curing them to produce a stable wood billet
- SCL and LVL are less prone to defects such as warping, splitting or shrinking, and have greater load-bearing properties
When is SCL and LVL used?
- used in structural appliactions including beams, joints and rafters
What is metal enhancement?
-metal properties can be enhanced through the controlled application of heating and cooling, known as heat treatment
What are the different metal enhancement methods?
- work hardening
- annealing
- case hardening
- hardening
- tempering
What is work hardening?
- when the metal is ‘cold worked’ by bending, rolling or hammering
What happens to the metal during work hardening?
- crystals within the metal are distorted and changed
- leading to improved tensile strength and hardness in the worked area
- when the metal crystals are distorted, they cannot move freely within the metal structure
- can result in less ductility, cracking or damage in the worked area
How can the affects of work hardening be removed?
- by annealing the metal
What is annealing?
- used to make the work-hardened metal easier to work by making it less brittle and more ductile
- the metal is heated and then cooled very slowly, allowing the metal crystals to grow and slowly move into place
What is case hardening?
- used for hardening the surface of steels with less than 0.4% carbon content
- produces an outer casing of greater hardness, improved wear resistance to surface indentation
- the inner core of the metal retains the original ‘softer’ properties
What is the two stage process of case hardening?
- carburising
- quenching
What is the carburising process?
- changes chemical composition of the surface of low carbon steel so that it can absorb more carbon to increase surface hardness
- steel placed in ceramic box packed with carbon
- the box is heated to 930 - 950 degrees
- the carbon atoms diffuse into the material’s structure to build up the surface carbon content
- depth of carbon layer determined by length of time the material is exposed to carbon
- longer time = thicker carbon layer
- product then heated to 760 degrees and then quenched
What is the quenching process?
- the hot metal is quenches in water to fast cool it and seal the hard surface case while not affecting the properties of the inner core
What is hardening?
- medium and high carbon steel are heated to alter their crystalline structure
- holding them at this temperature for a given time
- then quenching them in water, oil or salt water baths
- increases hardness but also brittleness
What is tempering?
- a heat treatment that reduces some of the excess hardness and brittleness of a hardened metal
- increases toughness and ductility
What is the tempering process?
- metal heated to below the critical point for a given time then slowly air cooled
- the exact temperature will determine the amount of hardness removed
- tempering colour on the metal indicates the temperature at which the brittleness is removed
