Vulcanisation of polymers & Biodegradable plastics
+ Rubber or related polymers sulfur / equivalent "curatives". more durable materials chemical process • These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains. • The vulcanized material is less sticky and has superior mechanical properties. • Hard vulcanized rubber is known as ebonite or vulcanite .
- Rubber is a long chain of polyene - Its main functional group is C=C group - Other functional groups are COOH, COOR,NH2 ,C=O,O-H,C-C group
- pyrolysis: reaction of natural rubber and Gutta-percha, forming a synthetic rubber 2-methylbuta-1,3-diene (isoprene) - the functional group of isoprene is C=C group - other functional groups is C-C group
Natural rubber is a completely amorphous polymer. • The potentially useful properties of raw latex rubber are limited by temperature dependence. • If the chains of rubber molecules are slightly cross-linked by sulfur atoms ,the desirable elastomeric properties of rubber are substantially improved • At 2 to 3% crosslinking a useful soft rubber, that no longer suffers stickiness and brittleness problems on heating and cooling, is obtained. • At 25 to 35% crosslinking a rigid hardrubber product is formed.
Vulcanization is generally irreversible, characterize the behavior of most modern polymers. • During vulcanization, some of these C-H bonds are replaced by chains of sulfur atoms that link with a reactive site of another polymer chain. • These bridges contain between one and eight atoms.
Number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. • Short crosslinks Better heat resistance. • Crosslinks with higher number of sulfur atoms • good dynamic properties but with lesser heat resistance. • Dynamic properties are important for flexing movements of the rubber article. • Without good flexing properties these movements will rapidly lead to formation of cracks and, ultimately, to failure of the rubber article.
Compression Molding • Economically most important method • Uses high pressure and temperature • The rubber article is intended to adopt the shape of the mold. • Hot Air Vulcanization / Microwave Heated Vulcanization • Continuous processes
Sulfur atoms form cross links • Prevent polymer chains to move independently • Increases strength and resiliency • Coiled polymer chains (from natural rubber) • Cross links lengthen the stretching distance • Hold the shape
Hold the basic structure and shape • Prevent over-contracting under cold • Prevent over-elongatedunder heat Natural rubber melt at 25 ℃!! • Compose of disulphide and C-S bonds • Dissociation E. slightly bellow C-C Bond • Breaking of cross link is required to break the polymer chains
Exist in solid state mainly e.g. Crack in low temperature, char or decompose under heat. • Thermosetting • Unable to melt Heat broken down bonds • Forming CO2, H2O and SO2 • No freely movingelectrons • No electricity transfer occur
Size • Huge polymer • Isoprene links with each other to form a large molecule • No weak van der Waal’s forces • Non-polar solvent molecule cannot attractand break the structure.
What vulcanized rubber can made?? ∵Vulcanization can turns the rubber into a strong elastic material and increases the strength and abrasion resistance The coverage of stiffness of vulcanized rubber depends on the quantity of sulphur added.
Uses of vulcanized rubber Making vehicle tires 5% sulphur is used in making tires
Uses of vulcanized rubber Making shoe soles
Uses of vulcanized rubber Making conveyor belts At the airport
Uses of vulcanized rubber Making conveyor belts At the supermarket
Vulcanisation - uses of polymer Making hockey pucks
Vulcanisation - uses of polymer Making bowling balls
Vulcanisation - uses of polymer Making clarinet mouthpieces
Vulcanisation - uses of polymer Making rubber condom (in the 19th Century)
Problem arising from Disposing of Non-Degradable Plastics: • Small pieces of plastic can get eaten by animals and which causes choking • Gets accumulated in the animal's body causing death • Fish can get trapped in plastic bags in marine habitat • Gives out poisonous smoke when burning which causes air pollution • Plastic will remain in the landfill for a long time which will cause shortage of land. • Many plastics are flammable, so there will be a fire risk of keeping plastics • Some of the additives in the plastics are reusable. However, it is very difficult to sort out plastics from rubbish. So the reuse of these useful substance is not feasible
Three important categories of degradable plastics: 1) Biopolymers 2) Synthetic biodegradable plastics 3) Photodegradable plastics
Biodegradable plastics • a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi and algae • decompose in natural aerobic and anaerobic environments (e.g. soil, compost, marine, etc.) • They may be composed of : • 1. Bioplastics, which are plastics whose components are derived from renewable raw materials • 2. Petroleum-based plastics which utilize an additive
1) Biopolymers • Polymers that occurs in nature • made by living micro-organisms (such as paracoccus, bacillus and spirullum) • Examples: cellulose, starch, proteins, DNA, etc. • Some biopolymers, such as polylactic acid (PLA) poly-3-hydrobutyrate (PHB) can be used as plastics
Basic unit structure of PHAs General structure of Biopolymers: Polyhydroxyalkanoates (PHAs) • linear polyesters produced in nature by bacterial fermentation of sugar or lipids • biodegradeable and are used in the production of bioplastics - R=H or C chain up to 13C long - X=1 to 3 carbons - n=100 to 30,000 More than 150 different monomers can be combined within this family to give materials with extremely different properties Example: Poly(3-hydrobutanoic acid) (PHB): R=CH3 and X=1 - stiff, brittle, highly crystalline polymer Polyhydroxyoctanoate (PHO):R=CH3 and X=0 - soft, low melting thermoplastic polymer
Poly(3-hydrobutanoic acid) (PHB) • made by certain bacteria from glucose • When disposed, micro-organisms found in the soil and water are able to break it down into water, carbon dioxide, etc. • used mostly as shampoo bottles, plastic cups and packaging materials because it is waxy and non-toxic • as a material that slowly releases drugs into the body
Decomposition of Biodegradable Polymer Figure 1. This figure shows the first step of biodegradation. Depending on the type of biodegradation taking place, this process is initiated by heat, moisture, microbial enzymes, or other environmental factors.
Decomposition of Biodegradable Polymer Figure 2. This figure shows the second step which takes place when the short carbon chains pass through the cell walls of the bacteria or microbes and are used as an energy source. This is biodegradation, when the carbon chains are used as a food source and are converted into water, carbon dioxide or methane.
Advantages of using biodegradable polymer: 1. Renewable • Biopolymers are made from biomass, which is organic matter that breaks down and grow continuously. That makes bioplastics a product that is renewable 2. Break Down Faster • Biodegradable plastics will break down faster than non-biodegradable plastics, so there will be less plastic laying in the landfills 3. More Environmentally Friendly • Non-renewable raw materials, such as oil, coal and petroleum can be saved
Advantages of using biodegradable polymer: 4.Can Be Recycled Easier • When biodegradable plastics can be break down faster, they can be recycled much faster and easier, using a lot less energy 5. Requires Less Energy 6. Not Toxic • Biopolymers are very safe, and they contain no toxins at all. They breakdown into harmless substances, i.e. CO2 and H2O. There is no chemical leaching into rain water or the ground to threaten the health and safety of people or animals nearby
Limitation in the production and uses of PHB: • made by bacteria - the supply is limited to the concentration of bacteria • the cost - 15 times more expensive than polyethene • brittleness - easy to break a PHB shopping bag
2) Synthetic biodegradable plastics • Made by incorporating starch or cellulose into the polymers during production • Broken down into small pieces by micro-organisms’ consumption of starch or cellulose => larger surface area => speed up their iodegradation
Uses of synthetic biodegradable plastics: Pots Gardeners and farmers can place potted plants directly into the ground, and forget them. The pots will break down to carbon dioxide and water, eliminating double handling and recycling of conventional plastic containers.
Limitation in the use of synthetic biodegradable plastics: • Starch can be processed directly into a bioplastic but, because it is soluble in water, articles made from starch will swell and deform when exposed to moisture • Rate still too low for the large quantity of plastic waste generated
Photodegradable plastics • Degradation results from the action of natural daylight • Have light-sensitive functional groups (such as carbonyl groups) incorporated into their polymer chains • These groups will absorb sunlight and use the energy to break the chemical bonds in the polymer to form small fragments
The following structure is important to biodegrade it: Carbonyl group: For example, carbonyl groups have been incorporated in the process of manufacture into polyethene chains.
Limitation in the use of photodegradable plastics: (1) the photodegradation of plastics requires sunlight to decompose, so the if he buried in the soil, or sinking to the bottom, it will not decompose. (2) degradation of plastics in the light, there is a component of photosensitive promoter and its is a toxic, washed by rain, may be dissolved, thereby affecting the natural environment. (3) the photodegradation of the plastic and can not be 100% complete decomposition, only the larger pieces of plastic into smaller, while the debris can no longer decompose.