Residual monomers
Polymerization results in a distribution of molecular weights. Although monomers are usually toxic, the toxicity of the polymeric unit generally decreases as the molecular weight increases. Residual monomers result from incomplete polymerization; their concentration can be controlled by carefully regulating polymerization conditions.
Residual solvents
Solvents are often an integral part of manufacturing and may remain behind in fluid materials such as adhesives, adhesive removers, barrier pastes, gels, or lubricants. Some acrylate adhesive systems are now water-based, which eliminates the concern for residual solvents. When the solvent is an integral component of the final product, as with adhesive removers and certain barrier pastes, its presence and potential transdermal absorption must be addressed in labeling.
Degradation products
Materials may undergo degradation during manufacture, sterilization, or storage, or after application to or implantation in the body. During manufacture, heat may thermally degrade a material; polyvinyl chloride is especially susceptible to heat and may release hydrochloric acid, resulting in an autocatalytic unzipping process. During sterilization, polytetrafluorethylene is susceptible to irradiation breakdown, resulting in the release of hydrofluoric acid. Stored materials exposed to light and oxygen may suffer ultraviolet degradation or oxidation. Implanted materials, particularly metals, may corrode or be biologically degraded. Stabilizers added to the polymer can protect against degradation. Materials should be tested for degradation and biocompatibility only after both manufacture and a suitable ageing period.
By-products from irradiation
Gamma irradiation is becoming an increasingly common method of sterilization; 2-3 Mrad is the usual sterilization dose. It is also used to facilitate cross-linking in certain formulations. As a result, many materials undergo degradation. Polyglycolic acid, used in suture production, is virtually destroyed by irradiation. Most medical polymers decrease in molecular weight as a result of chain scission. Polypropylene and other polymers may undergo chain scission, cross-linking, and oxidation. Any material that is irradiation sterilized should be tested for biocompatibility afterward, although most medical polymers remain useful.
Sterilization residuals
Chemical sterilization with ethylene oxide has a long history of use; the main advantages are that the procedure is carried out at low temperatures and the sterilization facility need not deal with radioactive sources. Ethylene oxide, which is itself toxic, also degrades into toxic ethylene chlorohydrin and ethylene glycol. Even after extensive degassing, some materials do not release these toxic molecules. All materials that are to be ethylene oxide sterilized must be tested for toxic residuals. The principle concerns with the biocompatibility of polymers are additives, residual monomers, and contaminants that are leachable in the body. The result of the additives and contaminants being in plastic is that a range of toxic materials may be leached from many plastics. Identified toxic materials in polymers i.e.Aluminium, Acrylonitrile (monomer), Arsenic, Benzene, Benzoic peroxide, Bisphenol A, Cadmium, Carbon tetrachloride, Dibutyl tin, Epoxy curing agents, Ethylene dichloride, Ethylene oxide, Formaldehyde, Ketones and hydrocarbons, Lead, Mercaptobenzothiazole, Methyl chloride, Methylene chloride, Methylene dianiline, Nickel, PAHs on carbon black, Pyrene, Tricresyl phosphate, Triphenyl phosphate.
Plastics and their Additives
The finished plastic material is made by the chemical chain called as monomer and several other chemicals added to give its desired shape, color and several other properties which are known as additives. There are about 2000 additives utilized in various types of plastic and can be divided infollowing common major classes:Antiblocking agents, Antimicrobial agents, Antioxidants, Antistatic agents, Coloring agents, Fillers, Impact modifiers, Mold release agents, Plasticizers, Preservatives, Slip agents, Stabilizers (light and heat) etc.
Antioxidants
Antioxidants are additives that retard or inhibit the oxidative degradation of the plastic material within the intended processing and usage limits of the materials. ―Degradation is initiated by the action of highly reactive free radicals caused by heat, radiation, mechanical shear, or metallic impurities. The initiation of free radicals may occur during polymerization, processing, or fabrication. Once the first step of initiation occurs, propagation follows. The function of an antioxidant is to prevent the propagation steps of oxidation. It must be effective at low concentration, non toxic, conveniently and safely handled, and low in cost. Beside this, it must not impart undesirable characteristics to the system in which it is used. Antioxidants are classified as primary or secondary antioxidants depending on the method by which they prevent oxidation.
The most widely used antioxidants in plastics are phenolics. Phenolics are mainly used in polyolefins, styrenics, and engineering resins. Phenolics are generally stain resistant and include simple phenolics (BHT), various polyphenolics, and bisphenolics. A phenolic antioxidant may then be used for long term protection. Primary antioxidants are generally radical scavengers or H- donors i.e. hindered phenols such as BHT, Irganox1010, or Irganox 1076, cyanox 2246 and 425 and bisphenol A. Long term protection for the polymer, secondary antioxidants is typically hydroperoxide decomposers i.e. trivalent phosphorus compounds such as tris-nonylphenyl phosphate (TNPP) is the most commonly used organophosphite followed by tris (2, 4-di-tert-butylphenyl) phosphate (Irgafos 168). Organophosphite are used in polyolefins, styrenics, and engineering resins. Phosphite can improve colour and engineering resins stability, but can be corrosive if hydrolysed. Thioesters act as secondary antioxidants and also provide high heat stability to a variety of polymers (polyolefins and styrenics). Secondary antioxidants are typically used in synergistic combination with primary antioxidants. Lactone stabilizers are a new class of materials that are reputed to stop the autoxidation process before it starts.
