Introduction
Advancements in organic chemistry have necessitated the manufacture of complex synthetic compounds, which are used in various aspects of life. Synthetic materials have become exceptionally useful in the field of medicine. In the medical practice, “synthetic products are used to manufacture bags for infusion, disposables, components of medical devices, and implants such as artificial heart valves” (Ripple, & Simons, 2007, p. 45). Thermoplastics are perfect examples of synthetic compounds that are being used in the field of medicine. A thermoplastic “is a polymer that becomes moldable above a specific temperature and returns to a solid phase upon cooling” (Drobny, 2007, p. 343). Synthetic materials are widely used in medical practice because of two main factors. First, synthetic materials have been found to be extremely hygienic (Drobny, 2007).
In the field of medicine, hygiene is a must. Medical instruments that are not made from synthetic materials were found to compromise the hygiene in many clinical settings. However, with the invention of synthetic compounds, hygiene is guaranteed. Secondly, synthetic materials are biocompatible (Drobny, 2007). The benefits of synthetic materials used in medicine are two-fold. On one side, synthetic materials guarantee patient safety; on the other hand, these materials are resistant to the effects of the biological environment in which they are used (Drobny, 2007). The use of synthetic materials in the medical field is not as rampant as in other fields such as construction, automobiles, and packaging. However, regardless of their minimal use in the medical field, their contribution is exceptionally huge. They have necessitated the manufacture of intelligent materials, which are essential in the generation of artificial organs. This is attributed to their capability to be adjusted to various biomedical requirements. This paper is going to discuss the medical applications of thermoplastics.
This paper is divided into four main parts. It will begin by detailing the history and properties of thermoplastics. The second part will focus on the latest technological advancements in the manufacture and use of thermoplastics. This part will discuss in detail current ideas that are under development and suggestions that have been put forward to enhance the medical use of thermoplastics. The third part will critique the current technology being used in the manufacture of thermoplastics. This section will evaluate some technological innovations that are being worked on and how they show promise. In addition, this section will detail why some of the current technological advancements might not succeed. Furthermore, section three will highlight obstacles that have to be overcome in order to realize greater success. The final part of the paper will summarize the findings and provide conclusions regarding the medical applications of thermoplastics.
A Brief History of Thermoplastic Elastomers
The development of thermoplastic materials having more or less elastic properties started in the late 1930s with the invention of plasticization of PVC at B. F. Goodrich company (Biron, 2007). This invention led to further interest in flexible plastics and eventually to the development of blends of PVC and NBR (butadiene-acrylonitrile rubber). The PVC/NBR blends, when properly formulated, have a rubber-like look and fill and bridge the gap between liquid and plasticized PVC and conventional cured elastomers. Thus, they can be considered as the precursors of thermoplastic elastomers, as we know them today. A major breakthrough occurred with the discovery of the basic diisocyanate reaction in 1937, which was first used to produce polyurethane fibers and then the development of some elastomeric polyurethanes.
Further development of thermoplastic polyurethanes continued through the 1950s and 1960s. In 1962, a group from the B.F. Goodrich Company presented an article on a virtually crossed linked polymer, which was soluble, had high tensile strength, good elasticity, and high abrasion resistance, and could be processed as a thermoplastic (Biron, 2007). Commercial polyurethane thermoplastic elastomers were first produced in the 1960s by the B.F. Goodrich Company, Mobay, and Upjohn in the US, and by Bayer A.G and Elastogram in Europe (Biron, 2007). During the last two decades of the 20th century, many new developments have taken place, such as functionalized styrenic thermoplastic elastomers. Others include new and improved thermoplastic vulcanizates (TPVs), softer single-phase melt-processable rubber (MPRs), and MPRs with improved physical properties, thermoplastic elastomers based on blends of natural rubber and polypropylene, and thermoplastic flouroelstomers.
Materials made from rubber consist of relatively long polymeric chains having a high degree of flexibility and mobility, which are joined into a network structure (Biron, 2007). “Their flexibility and mobility allow for very high deformability. When subjected to external stresses, the long chains may alter their configuration rather rapidly because of the high chain mobility. When the chains are linked to an outside network, the system has solid features, where the chains are prevented from flowing relative to each other under external stresses. As a result, typical rubber may be stretched up to 10 times its original length. On removal of the external stresses, it is rapidly restored to its individual dimensions, with essentially no residual or non recoverable strain” (Biron, 2007, p.35).
When ordinary solids, such as crystalline or glassy materials, are subjected to external forces, the distance between two atoms may be altered by only a few angstroms for the deformation to be recoverable. At higher deformations, such materials may flow out of the structure. The response of rubber is entirely intermolecular, that is, the externally applied forces are transmitted to the long chains through the linkages of their extremities, change their conformations, and each chain acts as an individual spring in response to the external forces.
On the other hand, “high molecular weight polymers form entanglements by molecular intertwining, with a spacing characteristic of the particular molecular structure” (Biron, 2007, p. 35). The spacing is expressed by molecular weight between entanglement s (Me). The table below shows the molecular weight between entanglements (Me) for polymeric melts (Biron, 2007).
Thus, it is evident that a high molecular weight polymeric melt will show transient rubber-like behavior even in the absence of permanent intermolecular bonds. In cross-linked elastomers, many of these entanglements are permanently locked in, and at a high degree of cross-linking, they may be regarded as fully equivalent to cross-links, and as such, they may contribute to the elastic response of the material. “A network is formed by the linking of polymer chains together, and this linkage may be either chemical or physical. Physical linking can be obtained by absorption of chains on the surface of finely divided particulate fillers or formation of small crystallites” (Biron, 2007, p. 56). In addition, “physical linking can be achieved through the coalescence of ironic centers, and coalescence of glassy blocks” (Biron, 2007, p. 56) These physical cross-links are, in general, not permanent and may disappear on swelling or increase in temperature. Physical, thermoreversible are present in most thermoplastic elastomers. Materials of this type are technologically very attractive because they can be processed as thermoplastics, yet they exhibit the behavior of rubber vulcanizates when cooled down to a sufficiently low temperature (Biron, 2007).
Thermoplastic Elastomers
Most thermoplastic elastomers are essentially phase-separated systems (Wood, 2004). Usually, one phase is hard and solid whereas the other is an elastomer. Often, the forces are bonded chemically by block or graft polymerization. In other cases, a fine dispersion of the phases is apparently sufficient. The hard phase gives thermoplastic elastomers their strength and represents the physical cross links. Without it, the elastomer phase would be free to flow under stress and the polymer would be practically unstable. On the other hand, the elastomer phase provides flexibility and elasticity to the system. Upon cooling or evaporation of the solvent, the hard phase solidifies and the material regains its strength and elasticity.
The individual polymers constituting the respective phases retain most of their characteristics so that each phase exhibits its specific glass transition temperature (Tg) or crystalline melting temperature (Tm) (Wood, 2004). These two temperatures determine the points at which the particular elastomer goes through transitions in its physical properties. The transition has three distinct regions. At very low temperatures, that is, below the glass transition of the elastomeric phase, both the phases are hard, so that the material is stiff and brittle (Wood, 2004). Above the Tg temperature, the elastomeric phase softens and the material is elastic, resembling a conventional vulcanized rubber. As the temperature increases, the modulus stays relatively constant until the point where the hard phase softens or melts. At this point, the material becomes a viscous fluid.
It is obvious that the service temperature range lies between Tg of the elastomeric phase and the Tg or Tm of the had phase. The exact values depend on the service conditions of the final product, for example, the amount of hardening that will be tolerated in the final product or the amount of stress applied. Thus, often the actual lower service temperature will be higher than Tg of the elastomer and the actual upper service temperature will be lower than Tg or Tm of the hard phase (Wood, 2004).
Methods of Synthesis of Thermoplastic Elastomers
Block Copolymers
A significant portion of industrially produced thermoplastic elastomers is represented by block copolymers, consisting of two or more polymer chains attached at their ends. A linear block comprises two or more polymer chains in sequence, whereas a star block copolymer comprises more than two linear blocks attached at a common branch point. “Most block copolymers are prepared by various controlled polymerization methods” (Wood, 2004, p. 24).
Anionic polymerization
This is a well established method for the synthesis of tailored block copolymers. To prepare well defined polymers, the technique requires high purity starting reagents and the use of high vacuum procedures to prevent accidental termination due to presence of impurities (Wood, 2004). In industrial practice, this method is used to prepare several important classes of block copolymers.
Cationic (carbocationic) polymerization
This method is used to polymerize monomers that cannot be polymerized anionically. However, it applies to a limited number of monomers (Wood, 2004).
Controlled/ Living Radical Polymerization
This is the latest method used in the synthesis of thermoplastic elastomers. The aim of this method is to establish a dynamic equilibrium between a small fraction of growing free radicals and a large majority of dominant species (Wood, 2004). Generated free radicals propagate and terminate as in conventional free radical polymerization, although the presence of only a small proportion of radicals prevents premature termination.
Poly addition
This method is used for the synthesis of multi block thermoplastic polyurethanes using diisocyanate, long chain diol, and a chain extender (Wood, 2004). In addition, polymerization with Ziegler Natta catalyst is used for the synthesis of several block polyolefin based thermoplastic elastomers. Others include dynamic vulcanization, esterification and poly condensation, trans esterification, catalytic copolymerization, and direct copolymerization.
In order to develop superior mechanical properties in a two component polymeric system, the components should not be incompatible or mutually soluble. This also prevents the formation of a homogenous phase. Most of the currently known systems are compatible to the extent that a small degree of mixing takes place or interfacial bonding is developed directly, such as blocks or grafts. Polymer incompatibility arises from the very small entropy gained by mixing different kinds of chains.
Practically, all thermoplastic resins contain essential stabilizers that are added during or immediately after polymerization to prevent their degradation during monomer recovery, drying and compounding, and storage (Wood, 2004). The types and amounts of stabilizers used depend on the polymer. Other additives may be added during the different stages of processing and provide specific characteristics of the material during processing and application.
Antioxidants
Many organic materials including polymers undergo reduction reactions with oxygen. When polymers oxidize they lose their mechanical properties, such as tensile strength, and may become rough or scratched on the surface, or discolor. Most antioxidants are used in polyolefins and styrenics and impact modified styrenics, and to a lesser extent in polycarbonates, polyesters, polyamides, and polyacetals. The type and amount of the antioxidant used depends on the type of resin and application.
Light stabilizers
Generally, polymers deteriorate in the presence of sunlight, which results in cracking, embrittlement, chalking, discoloration, or loss of mechanical properties, or loss of mechanical properties such as tensile strength, elongation and impact strength. Photo degradation occurs as a result to exposure to ultraviolet light at wavelengths 290-400 nm. Different wavelengths may produce different types of degradation, depending on the polymer. Specialty chemicals, called light sterilizers or UV light sterilizers, are used to interfere with the physical and chemical of light induced polymer degradation. Stabilization of the polymer can occur by the use of additions that absorb UV radiation, preventing its absorption by the molecules of the polymers, or by the additives that decompose peroxides, or by quenchers that accept energy from chromophore and convert it to heat.
Nucleating Agents
Polymers crystallize from a melt under various conditions. The molecular structure of the polymer must be regular enough to allow crystalline ordering. The crystallization temperature must be below the melting temperature, but not the glass transition temperature of the polymer. Nucleation must occur prior to crystallization. The crystallization rate must be sufficiently high. Because of chain connectivity, polymers crystallize in such a way that only a limited crystallinity is obtained (Wood, 2004). Polymers, are therefore, often referred to as semi crystalline materials. This chain connectivity is the main reason why polymers crystallize at significantly lower temperatures than they melt.
When polymers crystallize from the melt, the polymer crystals organize from primary nucleus and form complex spherical macrostructures called spherulites. “Nucleation increases the crystallization temperature and the rate of crystallization; as a result, parts can be removed from the mould at higher temperatures, and molding cycle times are reduced” (Wood, 2004, p. 34). Nucleated materials have higher tensile strength, stiffness, flexural module and heat deflection temperatures, but impact temperature is generally lower. Clarity is enhanced by increasing the cooling rate and reduced spherulite size, which reduces the scattering of light as it passes through the material. Nucleating agents can be classified as inorganic additives, organic compounds or polymers.
Flame retardants
Many thermoplastics, particularly those with high carbon content are inherently inflammable due to their petrochemical origin. Polyolefins ignite when in contact with a flame and burn with a faintly luminous flame even after the ignition source is removed. Melting occurs due to the high temperature of the flame, producing burning drips. Thermoplastics containing halogens tend not to burn easily; some of them do not burn at all, the main factor being the content of the halogen. Flame retardants generally impact their properties to polymers in the condensed or gaseous phase. In the condensed phase, the additive can remove thermal energy from the substrate by functioning as a heat sink, or participating in char formation which creates a barrier heat and mass transfer (Wood, 2004). The additive can also provide flame redundancy by conduction, evaporation, mass dilution, or by participating in endothermic reactions.
Colorants
Colorants used in thermoplastics are pigments or dyes. Dyes are organic compounds that are soluble in the plastic, forming a molecular solution. They produce bright, intense colors and are transparent and easy to disperse and process (Wood, 2004). On the other hand, pigments are generally insoluble in thermoplastics, and color results from the dispersion of fine particles through the resin (Wood, 2004). They produce opacity or translucence in the final product. Pigments can be organic or inorganic compounds, and are available in a variety of forms, such as dry powders, color concentrates, liquids, and pre colored resins (Wood, 2004).
Medical Applications of Thermoplastics
The demand for medical devices, which are safe, cost effective, and durable, continues to rise. The use of thermoplastics in the manufacture of medical devices is also gaining momentum owing to their numerous benefits. The medical market consumes roughly 2 per cent of all thermoplastics (Ripple, & Simons, 2007). The use of thermoplastics in the field of medicine is subjected to the same requirements as the whole of the health industry: evolution of health services, compliance with specific standards and regulations, more rigorous cost controls, aging of the population, and increased resistance of pathogenic agents (Ripple, & Simons, 2007). The globalization of the medical industry offers new opportunities and challenges to the plastic suppliers. To reduce costs and improve their competiveness, manufactures of medical materials work in partnership with producers to examine, among other things, the possibilities to improve the products , materials, equipment and design methods.
An example of the possibilities for plastics to respond to a specific problem is the replacement of almost 100 per cent of the stoppers, capsules, joints and other closure, systems used for pharmaceutical products (Ripple, & Simons, 2007). The development of thermoplastics is the most significant opportunity for the development of thermoplastics but new possibilities could appear in medial self care systems and blood sample tubes. PVCs are used in the manufacture of tubes and pouches for blood. On the other hand, polypropylene is used for pharmaceutical and medical packaging. In addition, polystyrene is used in the manufacture of tubes and other test and laboratory equipment. Polyethylene is used in the manufacture of bacteriology equipment.
Furthermore, polyethylene is used in the processing of waterproof layers of sanitary towels and nappies. Moreover, thermoplastics are used in the production of gloves and general purpose films. It has taken a long time for the surgery sector to adopt plastic products. Engineering thermoplastics with high impact resistance, and some fiber reinforced grades are starting to be used in applications where commotional materials are predominant. For example, implants, the instruments used to insert suture clips, and the casings of apparatus are of paramount importance. Thermoplastics are used for internal prostheses and a blood pump used during open heart surgery operations. Examples include polycarbonate used in pump barrels and polypropylene for the manufacture of permeable membranes.
Plastics can bring solutions in the fields of single use products, unbreakable equipment and weight reduction, for example, in the syringes, tubes and pumps for drug injections and apparatus for ambulatory dialysis. The launch of new resins being rare, progress on materials intended for medial applications is by way of formulation, alloys and modifications of existing resins. Medical equipment and its packaging are undergoing significant evolution with regard to their design, which is being reexamined to reinforce the advantages of plastics. Plastics also make it possible to reexamine the policy of medical wastes (Ripple, & Simons, 2007). The incineration of plastics, provided that these resins are incinerated with negligible impact on the environment, poses fewer problems than the incineration of glass, which leaves significant residues (Ripple, & Simons, 2007).
State of the Art
Joining composite materials is a challenging due to the fact that traditional technologies are not directly transferrable to composite structures. The use of “thermoplastic films as hot adhesives, and fusion bonding as hot melt adhesives have been proposed as perfect replacements for mechanical fastening and thermosetting adhesive bonding” (Ageorges & Hou, 2000, p.34). Fusion bonding technology, which has led to the establishment of thermoplastic matrix composites, borrows a lot from the traditional joining technologies used in the thermoplastic polymer industry. Fusion bonding technology has introduced a number of new constituents which have enhanced the production of thermoplastic matrix composites. These advancements have boosted the mechanical resistance, and solvent resistance and service temperatures of the thermoplastics. Ageorges and Hou (2000) conducted a study to evaluate the state of the art of fusion bonding technology. They learnt that this technology has employed various promising bonding techniques. They include resistance welding, induction welding, and ultrasonic welding. Ageorges and Hou (2000) argue that an ideal structure would be joistless because joints are sources of weaknesses and additional weight.
The manufacturing process significantly influences the properties of thermoplastics. “The manufacture of relatively large and complex thermoplastics has been limited by high melt resin viscosity” (Ageorges & Hou, 2000, p. 840). According to Ageorges and Hou (2000), the manufacture of thermoplastics from historically acceptable materials is likely to disappear. For this reason, it is important to manufacture reliable and cost effective thermoplastics. “Fusion, bonding, which is also known as welding, significantly enhances the efficiency of the welded bond. Welding has eliminated the stress concentrations generated by holes required for mechanical fasteners” (Ageorges & Hou, 2000, p. 841). Furthermore, welding eliminates surface requirements and minimizes processing times. However, it should be noted that welding has a number of demerits. They include distortion of the laminates, delamination, and uneven heating. These problems limit bonding of large components. Fusion bonding is classified on the basis of the method used to introduce heat.
Ultrasonic Welding
In this technique, “joints are held under pressure and subjected to ultrasonic vibrations perpendicular to the contact area” (Ageorges & Hou, 2000, p. 844). This technology shoulder however not be used in flexible polymers. This is due to the fact that flexible polymers absorb energy. During ultrasonic welding, “vibrational energy often concentrates around surface asperities which dissipate heat” (Ageorges & Hou, 2000, p. 844).
Induction Welding
Induction welding is achieved by subjecting materials on an induction field. The induction field is often produced by a coil. Eddy currents are introduced in the conductive materials by the induction field. On the other hand, heating is achieved through I2R heating. Generally, “eddy currents are often generated in the outer layer of the conductor, as the frequency of the induction field rises” (Ageorges & Hou, 2000, p. 841). During the use of induction fields in polymers, RF is applied to tapes of thermoplastics filled with iron particles.
Resistance Welding
Resistance welding, “also known as electro resistance fusion or resistive implant welding or electro fusion, involves trapping a conductive implant between the two parts to be joined “(Holden, Rytger, & Quirk, 2004, p. 843). It is important to appreciate the fact that joining technologies vary. “Fusion boding provides a huge potential for volume intensive applications in which short processing cycles are necessary” (Holden, Rytger, & Quirk, 2004, p. 852). However, few studies have been conducted on fatigue characteristics of fusion bonded thermoplastic bonds. Process integration is a significantly important phenomenon of joining technology. For this reason, the design code should have the necessary requirements of various joining technologies. This will ensure that the requirements are integrated at every stage of the design process.
According to Kalisvaart and Wright (2010), plasticized PVCs used in medical devices have excellent price performance balance. However, several factors are limiting the use of these PVCs. First, when PVCs are incinerated, they release harmful chemicals in the environment. Secondly, some derivatives of PVCs have negative health effects when they enter the human body. Polymers scientists argue that “perfect replacements for PVC tubes can be made by combining elastomeric block copolymer and polypropylene” (Kalisvaart and Wright, 2010, p. 5). In addition, “this combination does not harm the body, and does not release harmful dioxins when incinerated” (Kalisvaart and Wright, 2010, p. 5).
Scientists have been contemplating on the possibility of manufacturing kink resistant, flexible medical tubing by using styrenic block copolymers. Styrenic block copolymers contain polypropylene end blocks, which are chemically bonded to a rubber mid block. Usually, the rubber mid block is manufactured by mixing polybutadiene and polyisoprene. In addition, this rubber mid blocks can also be made from the hydrogenated polyolefin versions. “Rubber and polystyrene blocks are highly incompatible, and during phase separation, there is the formation of polystyrene domains. These domains form physical cross links, which can be broken and reestablished by using a combination of shear and temperature” (Kalisvaart and Wright, 2010, p. 8). Furthermore, “the shear that is used depends on the type of rubber midblock and the molecular weight of the styrene” (Kalisvaart and Wright, 2010, p. 5). Styrenic block copolymers containing hydrogen as a functional group have been invented recently. This group has enhanced rubber segment mid blocks due to the presence of a high content of butylenes when compared to traditional mid blocks. As a result, the new family of hydrogenated styrenic block copolymers has high processability and compatibility with polypropylene. In addition, a binary blend resulting from the combination of molecules with low molecular weight forms a dense continuous network of these materials. This improves the transparency of the materials. In addition, increased compatibility also broadens Tm below room temperature.
Frank (2012) argues that the medical field is a perfect market for thermoplastics owing to a need to shift from glass equipment. Frank (2012, p. 1) notes that “flouro polymer resins are used to make catheters, sutures, inhaler components, bio containment vessels, and heat shrinks tubing”. This is due to the fact that flouro polymers have low water absorption, and are biocompatible. In addition, they have excellent chemical resistance and barrier properties. Thermoplastic vulcanizate elastomers have been found to have high resilience and purity, and some experts think that they are perfect replacements for rubber in the manufacture of medical devices. They are currently being used to make durometers.
Discussion
For many years, one prime objective in thermoplastics research was the economical polymerization of polyisoprene with a high cis-1, 4, structure (Rubber News, 2012). That is, the production of a synthetic version of natural rubber. In the mid 1950s, this work was stimulated by papers describing the synthesis on a semi commercial scale using both Ziegler type catalysts and lithium metal initiators (Rubber News, 2012). With these lithium based initiators and with properly purified monomers, there are no chain termination or chain transfer steps. Thus, when all the original monomer was consumed, the polymer chain still remained active. It could initiate further polymerization if more monomer was added. In 1957, a process was described for the manufacture of polystyrene polydiene block copolymers using alkyl lithium initiators (Rubber News, 2012). About this time, triblock copolymers were also reported in which the polymerization initiator was dysfunctional.
It has been found that thermoplastic vulcanizate elastomers are more resilient when compared to other thermoplastic elastomers. They perform well in high temperature ends and are more chemical resistant. Thermoplastic vulcanizate elastomers exhibit lower oxygen absorption than other thermoplastic elastomers. As a result, they are very useful in the manufacture of pharmaceutical contents. Thermoplastic vulcanizate elastomers can be sterilized by using different methods. Sterilization can either be conducted using steam or gamma radiation processes. Furthermore, autoclaving can be employed. Hygiene is very important when dealing with medical devices. However, maintaining good hygienic standards in the clinical setting is partly dependent on the materials used in the manufacture of medical devices.
The use of thermoplastic vulcanizate elastomers in the manufacture of medical devices enhances the possibility of maintaining high hygienic standards. First, thermoplastic vulcanizate elastomers can be cleaned using steam or gamma radiation unlike most thermoplastic elastomers. The use of thermoplastic vulcanizate elastomers does not only enhance the hygiene of medical devices, but also produces long lasting medical equipment. On the other hand, Fusion bonding technology, which has led to the establishment of thermoplastic matrix composites, borrows a lot from the traditional joining technologies used in the thermoplastic polymer industry. Fusion bonding technology has introduced a number of new constituents which have enhanced the production of thermoplastic matrix composites.
Moreover, the new family of hydrogenated styrenic block copolymers has high processability and compatibility with polypropylene. In addition, a binary blend resulting from the combination of molecules with low molecular weight forms a dense continuous network of these materials. This improves the transparency of the materials. Furthermore, increased compatibility also broadens Tm below room temperature.
Since the anionic synthesis of first polystyrene triblock copolymers demonstrating properties of thermoplastic elastomers began, considerable research effort has sought to elucidate the relationships between their structure, morphology and properties. Block copolymers are macromolecules composed of linear arrangements of blocks; a block is composed of units which are chemically different, with respect to either chemical constitution or stereochemistry, from adjacent portions of the macromolecules. The unique properties of many block copolymers are a consequence of the basic thermodynamic incompatibility of the blocks, which results in micro phase separation into domains. The properties of thermoplastic elastomers, which are generally block copolymers, are a direct consequence of the composition dependent morphology of these copolymers. Thermoplastic elastomers exhibit the elastomeric property of long range, reversible extensibility. This property is characteristic of cross chemically linked elastomers. However, because the chains are interconnected by physical cross links from the thermoplastic hard domains, these domains can be made to flow at elevated temperatures.
The observed morphologies of triblock copolymers generally correspond to domains of the minor component dispersed in a matrix of the major component; the critical parameters determining domain morphology are the volumes fractions of the components, molecular weights, and the degree of incompatibility of the different blocks, as well as molecular architecture. Because the blocks are chemically aligned to each other, the dimensions of the phase separation are restricted; thus, domain sizes of the minor component are in the order of hundreds of angstroms. For a typical copolymer composition of 10,000-15, 000g/mol polystyrene end block molecular weights and 50,000-70,000g/mol molecular weights, the equilibrium domain morphology corresponds to a fine dispersion of spheres of thermoplastic polystyrene, chemically bonded to the surrounding matrix of elastic polybutadiene chains (Rubber News, 2012). Thus, it can be concluded that in this type of network, the elastic chains are held together only by the thermoplastic domains.
Several universities and educational institutions are conducting research on thermoplastics in order to understand the mixing process, much of which is totally ignored by the industry at large, except where they are directly involved. Akron University continues to publish many papers on mixing and blending, more particularly using batch mixers and with a developing interest in the intercalation and exfoliation of nano fillers in rubbery materials (Satri, 2010).
The Delft University of technology has revolutionized the thermoplastics industry by inventing a vacuum infusion process for anionic plymaide-6 composites (Satri, 2010). This technology can be used to produce large and thick composites, a feature that cannot be achieved by the present state of the art melt processing. Vacuum infusion process has necessitated the production of large structural composites, which can be recycled, and can be welded. This is attributed to several unique features associated with vacuum infusion process. First, anionic plymaide-6 matrix can be polymerized in situ around the fibers. As a result, a strong chemical fiber to matrix bond is produced (Satri, 2010). This property gives thermoplastic composites a solid fatigue resistance. Owing to the promising features of the vacuum infusion process, the current research focuses on the production of thermoplastic composites using vacuum infusion processes (Satri, 2010).
Certainly whilst the use as of curable rubbers may grow as more of the world becomes developed, it will now grow at the same rate as the usage of elastomeric materials generally. Thermoplastic elastomers, as their properties are improved, will replace curable elastomers in many areas currently the preserve of cured materials.
Conclusion
This paper noted that synthetic materials have become exceptionally useful in the field of medicine. In the medical practice, “synthetic products are used to manufacture bags for infusion, disposables, components of medical devices and implants such as artificial heart valves” (Ripple, & Simons, 2007, p. 45). Thermoplastics are perfect examples of synthetic compounds that are being used in the field of medicine. The benefits of synthetic materials used in medicine are two fold. On one side, synthetic materials guarantee patient safety; on the other hand, these materials are resistant to the effects of the biological environment, in which they are used. Most thermoplastic elastomers are essentially phase separated systems. Usually, one phase is hard and solid whereas the other is an elastomer. Often, the forces are bonded chemically by block or graft polymerization.
The medical market consumes roughly 2 per cent of all thermoplastics. The use of thermoplastics in the field of medicine is subjected to the same requirements as the whole of the health industry: evolution of health services, compliance with specific standards and regulations, more rigorous cost controls, aging of the population, and increased resistance of pathogenic agents. The globalization of the medical industry offers new opportunities and challenges to the plastic suppliers. To reduce costs and improve their competiveness, manufactures of medical materials work in partnership with producers to examine, among other things, the possibilities to improve the products, materials, equipment and design methods. Medical equipment and its packaging are undergoing significant evolution with regard to their design, which is being reexamined to reinforce the advantages of plastics. Plastics also make it possible to reexamine the policy of medical wastes. The incineration of plastics, provided that these resins are incinerated with negligible impact on the environment, poses fewer problems than the incineration of glass, which leaves significant residues.
Joining composite materials is a challenging due to the fact that traditional technologies are not directly transferrable to composite structures. A number of state of the art technologies are being tested at the moment. One such technology is the use of thermoplastic films as hot adhesives. In addition, “fusion bonding as hot melt adhesives has been proposed as a perfect replacement for mechanical fastening and thermosetting adhesive bonding” (Ageorges & Hou, 2000, p.34). Another technology involves the manufacture of kink resistant, flexible medical tubing by using styrenic block copolymers. In addition, the use of thermoplastic vulcanizate elastomers is increasingly gaining momentum. They perform well in high temperature ends, and are more chemical resistant. Thermoplastic vulcanizate elastomers exhibit lower oxygen absorption than other thermoplastic elastomers. As a result, they are very useful in the manufacture of pharmaceutical contents.
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