Cracking is the process of breaking down long hydrocarbon molecules into smaller, more useful hydrocarbon molecules. Cracking is a form of thermal decomposition. To crack a hydrocarbon, it is heated to a very high temperature, and either mixed with steam, or passed over a catalyst (called catalytic cracking).
When cracking, one of the products is a small, unsaturated hydrocarbon (usually ethene) as well as a larger, saturated hydrocarbon. If we crack decane (C10H22), for instance, at 500 Celcius, and using a catalyst, we would recieve ethene (C2H4), as well as octane (C8H18).
Making Polymers from Alkenes
A monomer is another word for a small unsaturated hydrocarbon (alkene) molecule. A polymer is made up of many momomers. To create a ploymer, from monomers, the reaction is called polymerization.
When ploymerization occures, the double bond of the unsaturated (alkene) monomer, opens up to bond with the other monomers.
To name a new polymer, the word poly is added to the original hydrocarbon. For example, ethene, polymerized makes poly(ethene).
The Properties of Polymers
LDPE stands for low desity poly(ethene), where as HDPE stands for high density poly(ethene). Basically, HDPE is stronger than LDPE.
To form LDPE, a trace of oxygen needs to be present, in addition with high pressure. The poly(ethene) chains are randomly branched, and cannot pack closely together.
To form HDPE, however, we'd use a catalyst at 50 Celcius, and slightly higher pressure than LDPE. The poly(ethene) chains are straighter and can pack closely together.
Thermosoftening polymers are plastics that, when heated, can be reshaped. This is because weak intermolecular forces are present, only. Thermosoftening plastics consist of tangled polymer chains.
Thermosetting polymers are plastics that, when heated, cannot be reshaped, but char instead. This is due to the strong covalent cross links (it also has weak intermolecular forces). Thermosetting plastics are bonded together, not tangled together.
New and Useful Polymers
Smart polymers (also known as designer polymers) are polymers designed with special useful properties.
Light-sensitive plasters have a special film, that when pealed off, stop the stickiness, and can therefore be peeled off, pain-free. Hydrogels are polymers that form a framework, or matrix, to trap water. Wound dressings (to allow wound to heal in sterile, moist conditions) and some contact lenses are made of hydrogels.
Poly(ethene therepthlate) more commonly known as PET, is a plastic that is strong, and transparent, and ideal for drink bottles. It can be recycled, too.
Most polymers are not biodegradable. This means microbes can't break them down, and they take up space in landfills. PLA, or poly(lactic acid), are biodegradable, and contain cornstarch. Although biodegradable, deforestation for cornstarch farms, and fields used for plastic and food, recycling bags instead of throwing them away, remains the best option, to some.