During the last two decades, significant advances have been made in the development of biocompatible and biodegradable materials for biomedical applications, and in the case of the latter category, industrial applications, as well. In the biomedical field, the goal is to develop and characterize artificial materials or, in other words, "spare parts" for use in the human body to measure, restore, and improve physiologic function, and enhance survival and quality of life. Typically, inorganic (metals, ceramics, and glasses) and polymeric (synthetic and natural) materials have been used for such items as artificial heart-valves, (polymeric or carbon-based), synthetic blood-vessels, artificial hips (metallic or ceramic), medical adhesives, sutures, dental composites, and polymers for controlled slow drug delivery. The development of new biocompatible materials includes considerations that go beyond nontoxicity to bioactivity as it relates to interacting with and, in time, being integrated into the biological environment as well as other tailored properties depending on the specific "in vivo" application.
Biomimetics

The parallel field of "biomimetics" may be described as the "abstraction of good design from nature" or, plainly put, the "stealing of ideas from nature". The goal is to make materials for non-biological uses under inspiration from the natural world by combining them with manmade, non-biological devices or processes. This is fast becoming a new research frontier.
Biocompatible Polymers

One area of intense research activity has been the use of biocompatible polymers for controlled drug delivery. It has evolved from the need for prolonged and better control of drug administration. The goal of the controlled release devices is to maintain the drug in the desired therapeutic range with just a single dose. Localized delivery of the drug to a particular body compartment lowers the systemic drug level, reduces the need for follow-up care, preserves medications that are rapidly destroyed by the body, and increases patient comfort and/or improves compliance. In general, release rates are determined by the design of the system and are nearly independent of environmental conditions.
Degradable Polymers

A variety of natural, synthetic, and biosynthetic polymers are bio- and environmentally degradable. A polymer based on the C-C backbone tends to be nonbiodegradable, whereas heteroatom-containing polymer backbones confer biodegradability. Biodegradability can therefore be engineered into polymers by the judicious addition of chemical linkages such as anhydride, ester, or amide bonds, among others. Figure 1 provides a schematic representation of the types of polymer degradation. The mechanism for degradation is by hydrolysis or enzymatic cleavage resulting in a scission of the polymer backbone. Macroorganisms can eat and, sometimes, digest polymers, and also initiate a mechanical, chemical, or enzymatic aging.

Figure 1. Schematic representation of the types of polymer degradation.

Properties of Degradable Polymers that Make Them Suitable as Biomaterials

Biodegradable polymers with hydrolyzable chemical bonds are being researched extensively for biomedical, pharmaceutical, agricultural, and packaging applications. In order to be used in medical devices and controlled-drug-release applications, the biodegradable polymer must be biocompatible and meet other criteria to be qualified as a biomaterial-processable, sterilizable, and capable of controlled stability or degradation in response to biological conditions. The degradation products often define the biocompatibility of a polymer, not necessarily the polymer itself. Poly(esters) based on polylactide (PLA), polyglycolide (PGA), polycaprolactone (PCL), and their copolymers have been extensively employed as biomaterials. Degradation of these materials yields the corresponding hydroxy acids, making them safe for in vivo use.
Other Bio/Environmentally Degradable Polymers Other bio/environmentally degradable polymers include poly(hydroxyalkanoate)s of the PHB-PHV class, additional poly(ester)s, and natural polymers, particularly, modified poly(saccharide)s, e.g., starch, cellulose, and chitosan.
Chitosan

Chitosan is a technologically important biomaterial. Chitin is the second most abundant natural polymer in the world after cellulose. Upon deacetylation, it yields the novel biomaterial Chitosan, which upon further hydrolysis yields an extremely low molecular weight oligosaccharide (see figure 2). Chitosan possesses a wide range of useful properties. Specifically, it is a biocompatible, antibacterial and environmentally friendly polyelectrolyte, thus lending itself to a variety of applications including water treatment, chromatography, additives for cosmetics, textile treatment for antimicrobial activity, novel fibres for textiles, photographic papers, biodegradable films, biomedical devices, and microcapsule implants for controlled release in drug delivery.

Figure 2. Deacetylation of chitin to form chitosan and hydrolysis to form oligosaccharide.

Poly(ethylene Oxide)

Poly(ethylene oxide), PEO, a polymer with the repeat structural unit -CH2CH2O-, has applications in drug delivery. The material known as poly(ethylene glycol), PEG, is in fact PEO but has in addition hydroxyl groups at each end of the molecule. In contrast to high molecular weight PEO, in which the degree of polymerization, n, might range from 103 to 105, the range used most frequently for biomaterials is generally from 12 to 200, that is PEG 600 to PEG 9000, though grades up to 20,000 are commercially available. Key properties that make poly(ethylene oxide) attractive as a biomaterial are biocompatibility, hydrophilicity, and versatility. The simple, water-soluble, linear polymer can be modified by chemical interaction to form water-insoluble but water-swellable hydrogels retaining the desirable properties associated with the ethylene oxide part of the structure.

Poly(ethylene glycol)s first appeared in the U.S. Pharmacopoeia in 1950. Since then they have been used increasingly for a variety of pharmaceutical applications.
Multiblock Copolymers of Poly(ethylene Oxide) and Poly(butylene Terephthalate)

Multiblock copolymers of poly(ethylene oxide) (PEO) and poly(butylene terephthalate) (PBT) are also under development as prosthetic devices and artificial skin and as scaffolds for tissue engineering. These materials are subject to both hydrolysis (via ester bonds) and oxidation (via ether bonds). Degradation rate is influenced by PEO molecular weight and content. Additionally, the copolymer with the highest water uptake degrades most rapidly.

A widely used nondegradable polymer is ethylene-vinyl acetate copolymer. This copolymer displays excellent biocompatibility, physical stability, biological inertness, and processability. In drug delivery application these copolymers usually contain 30-50 weight percent vinyl acetate. Ethylene-vinyl acetate copolymer membrane acts as the rate-limiting barrier for the diffusion of the drug. In the Type II class of degradable polymers, the conversion of the hydrophobic substituents to hydrophilic side groups is a first step in the degradation process. A team of researchers has addressed the problem of fabricating open-pore, biodegradable polymer scaffolds for cell seeding or other tissue engineering applications. The material selected was the tyrosine-derived polycarbonate poly(DTE-co-DT carbonate), in which the pendant group via the tyrosine, an amino acid, is either an ethyl ester (DTE) or free carboxylate (DT). Through alteration of the ratio of DTE to DT, the material’s hydrophobic/hydrophilic balance and rate of in vivo degradation can be manipulated. It was shown that, as DT content increases, pore size decreases, the polymers become more hydrophilic and anionic, and cells attach more readily.
Water Swellable Polymer Networks

Water-swellable polymer networks may function as hydrogels at one end or as superabsorbers at the other extreme. Hydrogels are characterized by the pronounced affinity of their chemical structures for aqueous solutions in which they swell rather than dissolve. Such polymeric networks may range from being mildly absorbing, typically retaining 30 wt. % of water within their structure, to superabsorbing, where they retain many times their weight of aqueous fluids. Several synthetic strategies have been proposed to prepare absorbent polymers including:

· Polyelectrolyte(s) subjected to covalent cross-linking

· Associative polymers consisting of hydrophilic and hydrophobic components ("effective" cross-links through hydrogen bonding)

· Physically interpenetrating polymer networks yielding absorbent polymers of high mechanical strength.

Clearly, these strategies are not mutually exclusive, and efforts have focused on tailoring composite gels which are critically reliant on the balance between polymer-polymer and polymer-solvent interactions under various stimuli including changes in temperature, pH, ionic strength, solvent, concentration, pressure, stress, light intensity, and electric or magnetic fields. Such stimuli-responsive polymers, the so-called smart gels, continue to be the subject of extensive investigation for applications in diverse fields. These applications range from biomedical (controlled drug release, ocular devices, and biomimetics), agricultural (soil additive to conserve water, plant root coating to increase water availability, and seed coating to increase germination rates), and personal care (diapers and adult hygiene products), to industrial (thickener, gelling agent, cable wrap, specialty packaging, tack reduction for natural rubber, and fine coal dewatering).

Absorbent polymers may be of synthetic (petrochemical) origin where the effects of morphology and porosity affects the absorbent properties. Aldrich also offers an extensive selection of polymers of natural (starches, etc.) and semisynthetic (cellulose ethers, etc.) origins for use in the synthesis of multicomponent hydrogels. To aid in designing your application-specific hydrogel, Aldrich offers over 1,500 monomers and a wide selection of cross-linking agents.

Chemically this material is the same as grey iron and is iron-carbon-silicon alloy. It is one of the more recent developments in cast iron technology and has been around since 1948. As the name suggests, it was developed to overcome the brittle nature of grey and white irons. It is also quite ductile in the as-cast form and negates the need for long heat treatments such as those required to produce malleable iron. Chemistry Like grey iron, ductile iron is an iron-carbon-silicon alloy. Typical carbon content is in the range 3 to 4%, while silicon content is slightly higher than grey iron at 2 to 3%. It is also common for ductile irons to contain significant additions of nickel. Ductile iron is often made from scrap, pig iron and recycled ductile iron. Formation of flake graphite (and a grey iron structure) is avoided through the addition of small amount of magnesium to the melt. The interaction is extremely vigorous as the melt temperature of iron is higher then the boiling temperature for magnesium. Ferrosilicon and other additives may also be used to promote graphitisation, control nodule size and promote formation of spherical graphite particles. Such additions may be added to the ladle rather than the furnace, a process called ladle inoculation. Structure The main difference between ductile iron and grey iron is the morphology of the graphite particles which take on a nodular or almost spherical form after suitable treatments are made to the melt. They resemble those found in malleable iron, but are more spherical. Similar to grey iron the matrix may be ferritic, pearlitic or martensitic depending factors such as chemistry and other process variables. Designations The most commonly used system for these materials is outlined in ASTM A536. Designations consist of three numbers and cover 5 grades. The fist number relates to the minimum tensile strength (in kips per square inch), the second to the minimum yield strength (in kips per square inch), while the last refer to elongation during the tensile test (in per cent). The grades of ductile iron are as follows: · Grade 60-40-18 · Grade 65-45-12 · Grade 80-55-06 · Grade 100-70-03 · Grade 120-90-02 Separate ASTM standards pertain to austenitic ductile irons (ASTM A439 and A571) and special purpose ductile irons (ASTM A476, A716, A395 and A667).

Key Properties

As with most grades of cast iron, ductile irons display: · Good hardness and good wear resistance · Good corrosion resistance · Have tensile and yields strengths that vary widely across the various grades. · Have compressive strengths that can be utilised more widely (than tensile strengths), with values tending to be about twice the tensile strength. · Impact strengths are better than grey irons, with lower grades approaching values common for mild steel. · Fatigue strengths are approximately 40 to 50% of tensile strengths. · Electrical resistivities are significantly lower compared to grey irons · Corrosion resistance is similar to grey iron · Machinability is dependent on hardness, with ferritic grades machining better. Heat Treatments Ductile irons can be heat treated similarly to grey irons, i.e. they can be: · Normalised – resulting in increased strength · Annealed – to relieve internal stresses and aid machining · Stress relieved – to remove internal stresses from uneven cooling and other effects · Quench hardened – to produce a stronger, harder, more wear resistant material. Pearlitic matrix ductile irons produce the best results for quench hardening. Prime candidates for this operation include the grades 80-55-06 and 100-70-03. Heat treatments are often integral in the production process Typically: · Grade 60-40-18 – is annealed · Grade 65-45-12 – is used as-cast · Grade 80-55-06 - is used as-cast · Grade 100-70-03 – is normalised · Grade 120-90-02 – is quenched and tempered

Refractory materials can support high temperatures, thermal strength and the contact with aggressive environments, for this reason they are widely used in the cement, glass and steel industry. Commercial AZS (alumina-zirconia-silica) refractories are a good alternative in refractory materials for the glass industry because they can support the aggressive conditions during liquid processing of glass. However, another problem encountered in glass industry is contamination by refractory material that fall into the molten glass, which can produce a series of defects in the final product. This research was conducted to develop new formulations of AZS refractories with different amounts of ZrO2 with the purpose of improving the characteristics, properties and the work conditions in the glass melting furnaces and, at the same time, lower the costs this type of refractories. The results obtained indicate that the composition with low content of ZrO2 can provide better properties than the commercial product, with some modifications in the particle size distribution.

Keywords

Sintering Process, Zirconia, Chemical Attack Resistance, Scanning Electron Microscopy, X-ray Diffraction

Introduction Refractory materials can be considered how resistant products to the generation of high temperatures of the process at the furnaces and reactors [1, 2]. The reliability of a refractory for specific applications is determined for his chemical attack resistance to the molten slags and molten products, as soon as his mechanic and thermal resistance in use [3]. By other hand, it knows that all refractory materials are destroyed when they are exposed to the characteristic conditions of each application and a common factor in the destruction is the temperature [4]. For this reason, new refractory materials with high temperature resistance and very high resistance in contact with aggressive environments are required [5, 6]. The glass industry has serious problems of contamination [7, 8] because during the initial melting glass, the refractory brick can be attacked in the most vulnerable points as in the matrix that surrounds the gross particles. The dissolution process of the matrix frees the gross particles and they remain floating in the glass, passing subsequently to the final product [9, 10]. The principal interest of this work is the development of refractory bricks with good mechanical properties and good corrosion resistance [11, 12], taking like departure point the commercial product composition (AZS = alumina-zirconia-silica). Experimental Procedure Raw Materials Selection and Characterization The raw materials selected for the production of AZS compositions were alumina, zirconia and silica, proceeding of two sources: silica sand and silica fume. The difference between raw materials is their particle size, the silica sand presents an approximate particle size of 150 mm while the silica fume is lower than 45 mm, the change was carried out with the objective to obtain homogeneity in the particle sizes of the raw materials with the formulations be prepared. According to it found in the literature [13-15], the AZS refractory materials with alumina-zirconia-silica are used for the glass melting furnaces, and the other side, the presence of carbides provides refractories with better properties such as greater corrosion resistance by molten materials. With the purpose of knowing the characteristics of each raw material, the following analysis techniques were used: x-ray diffraction, scanning electron microscopy and differential thermal analysis. These analysis were carried out taking a representative quantity of each composite of them. Manufacture of AZS Formulations AZS refractories were produced at the laboratory with Al2O3 (43%), ZrO2 (20 and 37%) and SiO2 (20 and 37%), at the same time, AZS refractories with small contents of silicon carbide and magnesium oxide were manufactured with the purpose of obtaining a product with better properties. See Table 1. Table 1. AZS compositions (weight percent of each raw materials) Composition Al2O3 ZrO2 SiO2 (silica sand) SiC MgO 1* 43 37 20 -- -- 2** 43 20 37 -- -- 3*** 39 33 18 10 -- 4*** 39 33 18 -- 10 * commercial composition ** low amount of ZrO2 *** AZS with SiC and MgO respectively All raw materials were dried at a temperature of about 110°C during 18 h, three grams of each one of the compositions presented in the Table 1, were mixed, homogenized and pressed to form specimens of 2.5 x 0.5 x 0.5 cm with 13350 N of load, sufficient to compact the dust and to obtain the samples. The burning was carried out at a temperature of 1450°C during 12, 18, 24 and 48 h. The results obtained from RXD and SEM characterization of the AZS prepared indicate the need to change some experimental conditions because the adequate refractory phases formation was not obtained, due to that direct bond among the grains is absent and the material remains without reacting, which can produce problems when the product is in use [16]. New AZS compositions were prepared with the addition of an organic compound (methocel) and silicoaluminate (clay) like bonding materials in the composition 1 (AZS= 43-37-20) with the objective of improving the bond among the grains at the moment of the compaction and press [17]; the similar alumina, zirconia and silica particle sizes (<45>mm) were obtained with the change of the silica sand (150 mm) by silica fume (<45>mm). These formulations were sintered to 1450°C during 12, 18, 24 and 48 h (Table 2). By another side, the pressed load was increased from 13350 to 44500 N to improve the specimen compaction. With the results obtained from characterization of these products with bonding materials, we can see that the addition of these compounds was not necessary for an adequate phases formation, however, the use of silica fume provides homogeneity at the particle sizes and better mullite formation. By other hand, the comparative analysis between commercial AZS and the AZS compositions produced at the laboratory (composition 1: AZS= 43-37-20 and composition 2: AZS= 43-20-37) was obtained with a commercial brick characterized by x-ray diffraction, scanning electron microscopy, differential thermal analysis and the static test of penetration and attack with molten glass. The static test was carried out for 4 h at a temperature of 1450ºC in a gas-air furnace [18]. Table 2. AZS modify compositions. Composition Al2O3 ZrO2 SiO2 (Silica sand) SiO2 (Silica fume) Methocel Clay 1 43 37 20 -- -- -- 2 42.1 36.3 19.6 -- 2 -- 3 42.1 36.3 19.6 -- -- 2 4 43 37 -- 20 -- -- 5 42.1 36.3 -- 19.6 2 -- 6 42.1 36.3 -- 19.6 -- 2 These compositions were compacted with two loads 13350 y 44500 N to compare them. Results and Discussion Figures 1 and 2 present the X-Ray diffraction patterns of two compositions with high and low percentage of zirconium oxide, respectively. Although in all cases badeleyite, quartz and corundum (from raw materials) without refractory properties were obtained, also the presence of zircon and mullite is detected. These are highly refractory phases and they can provide excellent properties to the final product, as it knows that the refractory phase formation with low amount of ZrO2 is feasible at similar time and temperature. This observation indicates that the refractory phases formation is independent of the amount of ZrO2 [19]. Figure 1. Composition 1 sintered to 1450°C, load of 13350 N. (B-badeleyite,Q-quartz, Cr-cristobalite, C-corundum, ZS-zircon, M-mullite). Figure 2. Composition 2 sintered to 1450C. (B-badeleyite, ZS-zircon, C-corundum, Q-quartz, Cr-cristoballite and M-mullite). On the other hand, the formulations with carbides and magnesium oxide additions did not present an adequate behavior, because they are not consolidate a refractory matrix and the low melting point phases formation is present, respectively. The XRD patterns of the composition with magnesium oxide addition is detected the cordierite formation, this product has a low melting point (about 1390ºC), which represents a critical problem when the refractory material is in use because the mechanical properties, such as refractariety and mechanical resistant will be decrease. Microscopic characterization of the sintered compositions indicates the presence of large grains of SiO2 (Figure 3). The presence of zircon around the grains is detected by EDS (Figure 4), however, a good bond in the microstructure is not observed, due to the separation between grain and grain, and the presence of the characteristic “needles” of mullite is not detected [20]. Although, with these sinter conditions, temperature and times, we can find refractory phases, it does not mean that product obtained had very good physical properties. The presence of a weak bond between the grains due to a bad compaction of the product because the particle size distribution is not adequate; it can observed to low sinter temperatures. Figure 3. Composition 1 (1450°C) 18 hours, the microstructure presents separation between grains of SiO2. Due to the original raw materials (alumina, zirconia and silica sand) can not produce a good product due to the different particle sizes. The particle size of the silica sand is larger than the particle sizes of alumina and zirconia and then some modifications were considered [17]: · Bonding materials: clay or methocel (2%) · Replace of silica sand by silica fume (from 150 to 45 µm respectively). · Change of pressed pressure from 13350 to 44500 N The phases found in this case are the same that in the previous compositions but in this case the quartz combines easily with the badeleyite to produce zircon, and the free quartz tends to be transformed completely in cristobalite. The addition of bonding materials, such as methocel or clay, does not alter the formation of phases, because these materials only contribute to the compaction of the product (Figures 5 and 6). They can help to form a better bond between particles and improve the sintering. Figure 4. Composition 4 (1450°C) 18 h, the separation of the grains of SiO2 (dark) and zircon formation (clear). Figure 5. Composition 1 sintered to 1450°C during 18 h with addition of methocel and clay, load 13350 N, (B-badeleyite, C-corundum, Q-quartz, ZS-zircon, M-mullite). Figure 6. Composition 1 (43-37-20): 1450°C during 18 hours without addition of methocel and clay, load 13350 and 44500 N. (B-badeleyite, C-corundum, ZS-zircon, M-mullite). Figure 7 represents a model of sintering process, the starting point is consisting of contacting particles, the initial bonds range from point contacts to highly deformed interfaces. With sintering the contacts grow in size, and in the initial stage there is extensive loss of surface area. As the pore structure becomes rounded the discrete particles are less evident and the intermediate stage of sintering occurs. This is characterized by a tubular, rounded pore structure that is open to the compact surface; gas can permeate through the open pore space. Consequently, many sintered structures are sintered to this stage only to preserve desirably pore structures [21]. Figure 7. Model of the interparticle bond as the ceramic microstructure is transformed during the sintering process. (a) Loose powder (start of bond growth). (b) Initial stage (the pore volume shrinks). (c) Intermediate stage (grain boundaries form at the contacts). (d) Final stage (pores become smoother). After evaluating the microstructure of the modified compositions, the best results were found for the addition of silica fume and pressed pressure of 44500 N, independently of the addition of bonding product (Figures 8 and 9). Figure 8. Composition 1: 18 h, the use of silica fume can provide a good bond between grains of Al2O3 (dark grains). Figure 9. Composition 1, 18 h, the microstructure presents some pores. The particle size of the raw materials is very significant since it contributes to the phases formation owing to provide a good bond between grains of the same size, and then a final product with better physical properties is obtained [22, 23]. This fact can be seen with a static test of the refractory in contact with molten glass for the compositions 1, 2 and the commercial product; in the Figure 10 we can see the three compositions after the static test and they indicate the penetration of the molten glass. The microscopic analysis of three compositions indicates a good behavior of the composition 2 with low content of ZrO2 because the molten glass penetration is about 1.5 cm of the hot face (region in contact with the glass), for this reason the penetration is absent at the cold face (the other region of the sample), which indicates that this composition can present good corrosion resistance. (a) (b) (c) Figure 10. Static test to 1450°C during 4 h, (a) composition 1:43-37-20, (b) composition 4:43-37-20, (c) commercial product We can see the microstructure without pores and homogeneous phases distribution conducting to good properties and characteristics of the refractory phases. The development of new Al2O3-ZrO2-SiO2 (AZS) product with low ZrO2 allows to have refractory materials with good mechanical and thermal properties; therefore the AZS with 20% of ZrO2 has economic advantages since the ZrO2 is very expensive and the contain in the commercial product is about 37%.

Salt baths are often used instead of conventional atmospheric furnaces for many heat treatment applications. The heating medium is a hot liquid compound, normally referred to as ‘salt’, and is contained in a metal pot, heated by external electrical currents or submerged heaters. The items to be treated are immersed and heated rapidly and uniformly to temperatures of up to 700°C. One of the advantages of this process is that air is excluded from the piece being treated. Unfortunately, the baths need to be regularly inspected and this means the removal of the salt, which solidifies as it cools down.
Dealing with Used Salt by Conventional Methods

Until now it has been common practice to pump the hot liquid salt into drums and allow it to cool and harden before cutting open the drum and breaking up the salt for disposal. When the bath is refilled, new granular salt is needed, which needs to be gradually melted. The only way to save the old salt is to grind it down to its original consistency and reuse it. However, this is a time-consuming task, and can lead to hot spots around the heating elements in the bath if not done properly. If not reused, the salt must be properly disposed of, so increasing the costs of the inspection process.
New technology for Dealing with Used Salt

However, Southport-based Mannings has come up with an answer to this problem. It has developed technology to pump out the hot liquid, keep it hot and pump it back into the bath after it has been inspected or repaired. The whole process takes less than two days.
Case Study

One customer for this service was Flight Refuelling, a company founded to develop air-to-air refuelling equipment that has supplied the Royal Air Force for more than 30 years. It specialises in the design and manufacture of structural aircraft parts and has a reputation for innovation and high-quality engineering. It also uses one of the largest salt baths in the UK, and consequently spends a lot of time and money on inspection. The company turned to Mannings to see if the cost and time could be reduced.
Furnace Details

For this project, Mannings used a large 12-tonne capacity holding tank fitted with electric heating elements. The power was provided by 50kVA transformers and the required temperature regulated in zones within the tank. The hot liquid salt was pumped into the holding tank using a special pump designed and built by Mannings. The client was impressed - Andy Wills, Facilities and Safety Manager at Flight Refuelling, said, ‘We were very impressed by the time and cost savings that resulted from this new method and our next inspection will be carried out in the same way.’
Scalability of Technology

Mannings has developed a range of specialised equipment to cater for all sizes of salt baths. Alan Bannister, Site Superintendent at Mannings, said, ‘Our thermal and environmental expertise is combined by this technology and we believe that there is considerable potential for this application.’