Polymerization refers to a chemical reaction that results in bonding of two monomers to form a polymer (Tarle, 435). A polymer molecule is made up of hundreds, thousands, or even millions of atoms joined together to form a chain with an extended length at least an order of magnitude greater than its thickness (Peacock & Calhoun, 2006). Polymers have variations of molecular weight (length), branching, steric configuration, interconnections and chemical defects (Tarle, 435).
A polymer is made up of many elements. One can consider a polymer as a thread with every link of this string forming a fundamental unit which is in turn made of hydrogen, carbon, silicon and oxygen(Tarle, 435). They are mostly made up of hydrocarbons though other elements such as Sulphur, nitrogen and chlorine may also be found(Carraher & Charles, 2). The molecules are held together by covalent bonds that are branched, linear, or networked to form the polymeric material (Carraher & Charles, 2). While most polymers have carbon as their backbone, others have silicon as the main element holding the polymer (Carraher & Charles, 2). .
The science of polymerisation began way back in the 19th century where alumina-silica glass and phosphoric acid was widely applied to fill dentures (Carraher & Charles, 2). . However, the compounds had very poor mechanical properties and high solubility. This meant that they were not particularly suited in sensitive applications such as dentistry. Later in the mid 20th century, acrylic resins replaced the silica compounds as the only synthetic material (Carraher & Charles, 2). These comprised of polymethylmethacrylate compounds that had higher polymerization shrinkage than the former (Carraher & Charles, 2). Despite their superiority, bonding was difficult. Also, they were known to cause periodontitis and pulpitis (Carraher & Charles, 2). An improved orthophosphoric-acid compound was invented by Michael Buonocore. It had high adhesive capability but was not enough for specialised applications such as tooth replacement . Rafael Bowen’s developed a monomer in 1962 that greatly improved on the physical properties of the acrylic (Peacock & Calhoun, 26). He combined the acrylic resins with epoxy resulting in an aesthetic composite with greatly improved physical properties (Peacock & Calhoun, 26).. However, it required mixing with base paste, colors and catalyst thereby making it prone to mixing errors in processing. Calling it BIS-GMA resin (coined from glycidyl methacrylate diluted with glycol dimethacrylate and bisphenol A) he combined it with glass beads thereby making a hydrolytically unstable composite (Peacock & Calhoun, 26). This necessitated further work on the same. Eventually, a combination of fillers and resins, added to several cross linked polymer chains like ethylene glycol dimethacrylate and triethylene glycol dimethacrylate produced a successful dental composite in the 1970s (Peacock & Calhoun, 26).
Composites solved the problem of mixing errors as polymerisation could be done by electromagnetic radiation (Peacock & Calhoun, 26). Unfortunately, the particles were too large and restricted to only four colors. Besides, the resins were difficult to polish and the etching techniques, which required the use of acid, damaged the teeth (Peacock & Calhoun, 26). The resins were replaced with composites in the 1980s. These brought about better aesthetics but could fracture when exposed to high tension (Peacock & Calhoun, 26).
The process by which monomers form polymers is called polymerization. This occurs by one or two basic reactions namely addition and/or condensation. During addition polymerization, monomers are usually linked together to form a long, three dimensional polymer chain. On the other hand, polymer condensation involves small molecules being released to form small polymers chains. In the first steps of formation, the monomers double bond is broken (called curing) by means of heat, light or addition of other compounds (chemicals) (Peacock & Calhoun, 26).
The curing effect of a polymer is caused by presence of a photoinitiator (Vandewalle et al 300). This is a molecule that absorbs light and then spurs a chemical reaction that results in polymerisation. Once a photo-initiator absorbs light (mostly a carbonyl group with nonbonding electrons that can be promoted to an anti-bonding orbital) of an appropriate wavelength, an overlap of positive and negative atomic orbitals thereby forming a polymer chain (Vandewalle et al 300).
As noted, dental Composites can be cured by use of three means namely use of chemical, heat and light(Vandewalle et al 300).. These three elements result in production of free radicals that result in polymerisation. According to Micali and Basting (2004) such factors as light density, intensity of light source, and exposure and wavelength duration affect the amount of free radicals generated. The energy in these elements is paramount in determining the degree of cure and of flexural strength. Peutzfeldt & Asmussen (2005) asserts that the degree of cure and mechanical properties rises with increase in energy density. These parameters have to be adequate to facilitate compete polymerization. If not, then the materials will be of poor physical properties and hence sustain premature failure. Micali and Basting (2004) assert that the clinical performance of composite resins is greatly influenced by the quality of the light-curing unit used in photo-polymerisation. Mahn (2010) explains that composites which are not adequately irradiated, both in terms of quality and quality of light often result in increased bacterial colonization which reduces their bond strength, inferior physical properties, excessive wear and possible bulk fracture, postoperative sensitivity due to dissolution of uncured resin and recurrent caries at the interface (Mahn, 21).
Composites are specially suited for dental applications for a variety of reasons. Due to their tendency to withstand tension and high temperature, they are able to match the performance of natural teeth save for sensitivity. Composites can also be made to match the colour of the teeth. The polymer binders used to fill teeth improve their strength and polarization. Practically more than one monomer is mixed at different ratios to get an appropriate and easy to use thickness. A curing light from either halogen lights, light emitting diodes or plasma arc curing lights is beamed onto the composite to cure it.
The filling materials used in dental applications should be strong and compressive. They should be able to withstand high temperatures too. The material should be made to match the colour of the teeth. The polymer binders used to fill the tooth should be good paste so as to improve strength and polarization. Practically more than one monomer is mixed at different ratios to get an appropriate and easy to use thickness. A curing light is a beam that was made to quickly cure a resin. Composite resin was brought into the market to minimise negative aspect of the acrylic resins. Ravve, Abe (2000).The earlier composites required to be mixed with a base with a catalyst leading to problems in magnitude, Light ways of therapy have come a long way since 1970s (J.Dent 1999).Ultra violet rays were used to set off a special type of filling. We have plasma light that’s currently being applied on therapy is clinically applicable. It is a better method since the results revealed it had a similar therapy to the one of halogen yet the temperature rise was negligible. Some are using the Argon while others are using the laser light. The combination process and also the colour visibility. At first they used an ultraviolet light source. It was a required source of energy. But the polymerization had side effects which led to replacement with visible light which is currently in use and still being developed. With the current technique we are able to reduce the polymerization stress.
We have two methods that use small and low concentration light followed by a longer period of elevated intensity. (Chang-Jun Baek, 2008)Both physical and mechanical property of the compound depends on their structural arrangement. The dental composite is composed of three chemically different materials. The organic, non organic and the filler.
There’s a disperse phase of composite resins made with non-living filler material. So as to improve both the mechanical and physical properties of the compound, the filler particles are added to the organic stage. We have composites that are made up of polymer groups called hybrid composite resins. About 60% of it is glasses of different compositions and sizes. And these are the ones used by our dentists. We have flowable composites which are normally of low thickness. They contain lower inorganic filler. They have an advantage because of the tooth surface is highly wet thus ensuring ideal penetration due to their low thickness. There’s no air entrapment .That’s also an added advantage. We have condensable composites also which have a very high percentage of the filler. Their clinical behaviour is almost similar to the hybrid. Their condensability gives them an added advantage. This is because the contact point is easily achieved. The major areas where polymers are applied will depend on the chemical structure and processing of the polymers. Some could be very rigid; others flexible while some could also be very elastic. We have fibres polymers and textile polymers which are simply oriented forming elevated powerful fibres. We have natural polymers like proteins found in silk and wool. We also have carbohydrates like the ones found in cellulose. The other polymers are synthetic like the ones found in bullet proof vests. We use polymers for making packaging materials and coatings. Polymers are also very useful in the medical fields (Charles, 2003).
As mentioned earlier teeth fillers are also made of polymers. Polymers are also used in making implants. Most aesthetic dental coatings are made of polymers. The coatings are usually used to decorate the enamel surface of the teeth.
The composites have acquired a very high and important place in the filling equipments. These materials are able to protect the teeth arrangement in a better way because they are retained by cementing methods other than depending on cavity design. We should never forget that they are highly technique sensitive hence we need to control some aspects. We have to get the correct composite for each condition. The process followed in bonding should be cautiously done. Proper therapy is highly vital if reasonable results are to be achieved. Composite are not made of metal. So the dentists can merge and combine perfect colours to match your original teeth. This means no one else will know about your teeth apart from you and the dentist. Another advantage is that the resin usually holds up the remaining tooth structure stopping breakages and padding against high temperature changes. A composite can last up to 10 years. It is important to remember the more natural teeth structure you keep, the better teeth you will likely have in future. We have a growing demand of energy.