X
|
STUDENT DIGITAL NEWSLETTER ALAGAPPA INSTITUTIONS |
|
Sherif F. Nagueh, MD, FACC, FAHA
This does not only include the net price of the adhesive but also the overall cost of the gluing system including glue spread virus encrypted files ciplox 500 mg low cost, capacity of the line (necessary press time) bacterial cell diagram buy cheap ciplox 500 mg line, and other parameters (Table 15) antibiotics for acne and birth control pills buy ciplox 500 mg on line. Environmental aspects can also have a significant influence on the choice of the adhesive system treatment for dogs back legs order ciplox 500mg fast delivery. Viscosity the viscosity of a glue mix is determined by the viscosity of the resin (mainly depending on the degree of condensation and the resin solids content) and the composition of the glue mix. If the viscosity or the degree of condensation of the resin is too low, a large portion of the resin might penetrate into the wood, causing a starved glue line. In such a case no true glue line can be formed and hence no bonding strength can be obtained. Conversely, at a too high viscosity there might be a lack of proper wetting by the adhesive of the wood surface opposite to that surface where the adhesive was applied, consequently with no or very low penetration into the word surface and hence no mechanical interlocking of the adhesive into the substrate. Besides the viscosity of the adhesive resin itself, the viscosity of the glue mix also plays an important role in the final result. A higher dilution of the resin gives a higher volume to be spread and with this a better distribution of the resin on the particles or fibers, and thus better bonding strength [337]. Flow Behavior the flowability of a resin depends on its viscosity and the solids content as well as the changes in the viscosity at elevated temperatures in the hardening glue line. A low flowability causes poor penetration of the resin into the wood surface and low bonding strengths. A too high flowability, on the other hand, leads to overpenetration of the resin into the wood and hence to starved glue lines. Flowability and hardening act against one another during the hot press curing process. Surface Tension and Wetting Behavior Aqueous adhesive resins behave similarly to water regarding surface tension and wetting behavior. The proper wetting of the wood surface is a precondition for achieving high adhesion strength between the resin and the wood surface. Reactivity the objective of the development of adhesive resins is to achieve as high reactivities as possible, while maintaining within acceptable limits other properties such as the storage stability of the resin or the pot life of the glue mix. The reactivities of the resin and of the glue mix are determined by various parameters: type of resin composition and preparation procedure type and amount of hardeners additives which might accelerate or retard the hardening process hardening temperature (press temperature, temperature in the glue line, temperature in the core layer) properties of the wood surfaces. The advantages and disadvantages of liquid and powder resins are summarized in Table 16. It is now possible to analyze the polydisperse nature of the resins as well as the individual structural elements in the resins, even semi quantitatively. The characterization of formaldehyde condensation resins was for several decades only possible with basic chemical methods, including elemental analysis [339]. The application of modern spectroscopic and chromatographic methods started as late as the 1970s. One of the reasons for this delay certainly is the fact that condensation resins themselves are still systems that might change during their preparation for analysis or during the analysis itself. Notwithstanding this, the chemical and structural composition of condensation resins is today well known. Laboratory Test Results the properties of a resin which can be determined by simple test methods are shown in Table 18. The solids content of a resin usually is determined by the so-called dish method at 120 C for 2 h [different times and sometimes lower temperatures (105 C) are often used as several variations of this method exist]. Even if it is a rather simple test, some deviations in the results might occur because not only does all water present as solvent in the liquid adhesive resin evaporate, but also a further condensation reaction with further water elimination takes place. The more severe the conditions during drying, the lower the solids content measured. Also some details of the test, such as the type of oven, the number of dishes in the oven at the same time, or recirculation of air or not, can influence the results of the test. The refractive index can be used as a quick method for the determination of solids content, however, the correlation between these two characteristic resin values is sometimes rather poor and not the same for all resins. The density is only important when using volumetric adhesive dosing systems, but not as a quality parameter of the adhesive. One of the most important characteristics is the reactivity of the adhesive resin. With some methods also the start and the end point of the gelling process, the duration of its time span, the behavior of the resin during the test as well as the shape and strength of the gelled plug obtained are essential features of the gel time test. A long gel time can indicate a slow generation of cohesive bonding strength in the actual application of the resin. The temperature used for the gel time test should always be adjusted to the temperature of application of the resin. If the maximum temperature in a glue line during pressing is not higher than 70 C, then the gel test should be performed at such a temperature and not at water boiling point. This is recommended in order to better interpret the behavior of the resin or the resin glue mix under its conditions of industrial application. Chemical Composition of Adhesive Resins the various components and raw materials of the resins can be determined using different chemical methods (Table 19). Residual monomers such as free formaldehyde, unreacted urea, and residual phenol or methanol as a residual product of formaldehyde production can be determined by various methods. These results are the basis for correlations of resin structural composition with their molar composition, their preparation procedure, and the properties of the panels produced and hence to development and production of tailormade resins. Extensive information is available on the basic nature of resins and on the content of the various structural elements, including. This method divides the molecules according to their hydrodynamic volume, which is proportional to their molar mass. This choice influences the solubility of the resin, the behavior of the chromatographic columns, and the effectiveness of detection. Because in the oligomeric and polymeric regions of the resins no compounds with a special and singular molar mass and a clear molecular structure are available, similar or chemically related substances have to be used as calibration standards. However, differences in the hydrodynamic volumes even at the same molar mass cannot be excluded totally. This uncertain calibration of the columns also induces a great uncertainty in the calculation of molar mass averages on the basis of the chromatograms obtained. The eluent with the dissolved molecules passes a light scattering cell and the weight average molar mass is measured directly during each chromatographic run. During each run two curves are obtained: one is the concentration peak, and the other the light scattering peak, which is directly related to the actual molar mass average in the detection cell at each moment. Using these two curves, an individual calibration curve can be derived for each run. However, it must be taken into consideration that the light scattering signal can only be evaluated in the higher molar mass region and, therefore, the calibration curve is valid with sufficient accuracy only in this part of the chromatogram. Monitoring of Gelling and Hardening During gelling and hardening of the condensation resins in the hot press one can distinguish between the chemical advancement of the condensation reaction during curing of the thermosetting resin (build up of the three-dimensional network) and the progressive development of the mechanical strength of the joint (increase in cohesive bond strength). The test methods that are used to follow the progression of the hardening of the resin are shown in Table 20. The exothermic behavior of the curing process is then measured as a temperature difference or directly as heat flow. The data obtained that are of interest are: shape of the curve, temperatures of the onset and the top of an exothermic or an endothermic peak, slope of the upcurve, width of the peak. This sample then undergoes periodic oscillations, at the same time the sample is heated following a special temperature program. The curing of the resin leads to an increase in the strength of the sample which then can be correlated with the increase of the cohesive bonding strength. The curing of the resin leads to an increase in the strength of the sample which can then be correlated with the increase of the cohesive bonding strength as well as with the internal bond strength of wood particleboard using the same adhesive. The damping behavior of the torsion of a glass fiber probe impregnated with the resin is characteristic for the increase of stiffness. It enables bonds to be formed under highly controlled conditions; the joints that contain the bonds which are to be measured are pressed against heated blocks for a certain time, cooled within a few seconds, and pulled immediately thereafter in shear mode. Repetition of this procedure at different curing times and temperatures yields the points (a point for each specimen) of a near-isothermal strength development curve. Monitoring the effective strength increase (defined as the degree of mechanical curing) enables conclusions to be drawn about the suitability or not of a resin for a certain application. This indicates formation of entanglement networks of the resin in wood which is not possible on noninteractive substrates such as glass as in. In board manufacturing, when the press opens, a certain level of mechanical hardening and with this a certain bond strength is necessary to withstand the internal steam pressure in the pressed board.
There is considerable evidence that the disparagement of the epistemological values and perspectives of traditional (particularly indigenous) communities that view nature/land and culture/values as indivisible (Claus et al antimicrobial 7287 msds ciplox 500 mg sale. In successfully opposing mining and logging operations on their traditional lands how long for antibiotics for acne to work order ciplox 500 mg without prescription, many indigenous groups have also reproduced and transformed their identities and worlds (Poirier antimicrobial bed sheets buy ciplox 500mg without a prescription, 2010) through innovative practices around their land-based resources (Haglund et al antibiotics for dogs dental infection proven 500mg ciplox. The adoption of soil conservation measures often faces cultural barriers when their implementation is perceived as a cost to local farmers, while benefits accrue at regional to global levels (Knowler & Bradshaw, 2007). Farmer decisions are strongly influenced by socio-economic factors (role of subsidies, quotas, cost savings) (Boardman et al. In such contexts, participatory approaches have been found to be effective in promoting the adoption of soil conservation measures (Bewket, 2007), with economically- and environmentallybeneficial outcomes (Shiferaw & Holden, 2000). Certification Eco-certification (or eco-labelling) is a voluntary instrument that has been applied to certain crops and forest products. In principle, eco-certification enables consumers who prefer "green goods" to identify the good and purchase them in a price-differentiated market, which can address the environmental problems associated with production of goods by creating incentives for producers, otherwise difficult to handle with regulatory instruments alone (Lambin et al. Studies examining the impacts of eco-certification schemes have found limited economic benefits of certification, but significant social and environmental benefits. In comparing certified and non-certified coffee growers and their land-use practices, certified coffee growers were found to be adopting environmental-friendly practices in Colombia (Rueda & Lambin, 2013) and they had a higher biodiverse coffee farms in Mexico (Mas & Dietsch, 2004). While there is little evidence of positive environmental or socio-economic impacts of forest product certification, at the global level (Dauvergne & Lister, 2010), positive local impacts have been documented in Brazil, Malaysia and Indonesia (Durst et al. Natural capital accounting broadly follows the accounting conventions of balance sheets and profit and loss accounts to reflect natural assets and service flows respectively, as well as exposure to natural capital risk (Trucost, 2013). For example, Denkstatt (2016) used the Natural Capital Protocol to review water replenishment options for the Coca Cola Company showing, for example, that wetland restoration provided particularly high benefits beyond those linked to water conservation alone. In a similar vein, Hugo Boss used the natural capital accounting framework to assess the effects on ecosystems services of the supply chains for their cotton, wool and leather fashion goods (Zeller et al. In their case, cotton cultivation and sheep farming accounted for large shares of monetized natural capital impacts for the clothing sector, while tanning processes dominated environmental costs for footwear. The assessment is being used to promote environmental provenance in the supply chain for their products, including the use of natural, less environmentally-burdensome substitute materials and processes. Despite these notable efforts, systematic reviews of the empirical evidence on direct correlation between corporate social responsibility and prevention of land degradation are scarce. Since the mid-1990s, various methodologies have been developed for assessing protected area management effectiveness. Assessment data from all over the world have now been collated in the Global Database for Protected Area Management Effectiveness which contains records of almost 18000 assessments of protected area management effectiveness. The database includes information about the methodologies and indicators used, and records details of individual assessments. As of January 2015, nearly 18,000 of the assessments had been collated in the database, representing over 9000 protected areas, with 3,666 sites having multiple assessments. The differences in proportion of protected area assessed for effectiveness, by country and region, are given in Panel A and B in Figure 6. A systematic literature review of impact evaluation papers that used a composite-single indicator for measuring effectiveness, (Coad et al. It remains unclear whether this lack of correlation with the impact of protected areas in some studies is real, meaning either that protected area management has no impact on biodiversity outcomes or more plausibly that good management (as measured by protected area management effectiveness scores) is necessary but not sufficient to ensure effective conservation (Carranza et al. Further evidence indicates that the rate of conversion of landscape is lower in protected areas. They argued that approximately 10% of the protected forests would have been deforested had they not been protected. Based on an assessment of the impacts of anthropogenic threats to 93 protected areas in 22 tropical countries, the parks were found to be an effective mean to protect tropical biodiversity by stopping land clearing, and to a lesser degree by mitigating logging, hunting, fire and grazing (Bruner et al. In Dana Reserve, Jordan, degradation has been partially reversed by agreeing with local farmers and herders to reduce stocking density of goats by 50% and providing alternative livelihood options through ecotourism and craft development (Schneider & Burnett, 2000). On the other hand, protected areas are not always effective in halting land degradation. Focusing on understanding causes of land degradation and deforestation in the Wildlife Reserve of Bontioli (Burkina Faso), Dimobe et al. They concluded that this was due to the lack of long-term adaptive management and conservation strategies in the communal areas and recommended strengthening the scientific foundation for greater involvement of local populations and staff in conservation and management activities. Indigenous protected areas as a response Globally, 18% of land is formally recognized as either owned by, or designated for, indigenous peoples and local communities. Within the 18%, 10% is owned by indigenous peoples and local communities and 8% is designated for (or "controlled by") indigenous peoples and local communities (Rights and Resources Initiative, 2015). For example, Australia has included Indigenous Protected Areas as a key part of the National Reserve System, in recognition that indigenous Australians have managed their country for tens of thousands of years. Specific responses to climate change mitigation and adaption based on land-use types have been discussed in earlier sections (such as cropland in Section 6. Given the pervasive influence of climate change on socioecological systems, climate change adaptation planning has important implications for land resource management and conservation (Lawler, 2009). Climate change adaption depends on a variety of factors including: land-use domains; adaptation purpose, timing and planned horizon; form and measures of adaptation. The four principles underpinning the framework are: (i) socially-focused but ecologically-constrained; (ii) actor-centric but context-aware; (iii) process-focused but outcome and/or action-oriented; and (iv) iterative and messy, but linear for convenience (Moser & Ekstrom, 2010). The uncertain and varying nature of climate change impacts in different places and land-use systems necessitates adaptive management, which has often been referred to as a critical adaptation strategy for resource management (Lawler, 2009). For example, scenario planning allows managers and planners to evaluate multiple potential scenarios of change, for a given system, in order to develop alternative management goals and strategies (Peterson et al. In the context of climate change and managing forests in the future, Millar et al. For example, restoring ecosystem functions of a degraded land through restoration would increase resilience of the system (Julius et al. Many industrialized countries have developed comprehensive national adaptation assessments. In addition, adaptation to climate change has been increasingly considered in regional- and local-level planning. However, in a systematic review of climate change adaptation literature comprised of 39 studies from developed countries between 2006 and 2009, Ford et al. Those adaptation interventions that are found in practice are localized (municipality level) and funded through higher-level government interventions mostly concentrated on transportation, infrastructure and utility sectors and based on non-structural adaptation responses. In addition, their review highlighted that stakeholder engagement in adaptation planning and implementation, and adaptation actions did not focus on vulnerable populations (Ford et al. Addressing land degradation through climate change adaptation planning requires a broad-base integrated and adaptive approach involving all affected stakeholders. As countries are affected differently by climate change-induced land degradation, adaptation plans and their effectiveness will vary depending on the socioeconomic context of the place or system in question. For example, in a survey of 127 agro-pastoralist households in Kenya, Speranza et al. They conclude that building adaptive capacity through extension services, maintaining infrastructure and embedding indigenous knowledge in adaptation plans would be effective adaptation measures for agropastoral communities (Speranza et al. Indigenous communities have adapted to change for centuries and their practices and knowledge provide effective responses in land management responses (Fisher, 2013).
Cheap ciplox 500 mg on line. Antimicrobial resistance: a global health issue.
In the past antibiotics lactose intolerance order ciplox 500 mg amex, Ximena has been a recipient of numerous awards antibiotic use in poultry order ciplox 500mg without prescription, including the Research Training Program Stipend Scholarship antibiotic youtube ciplox 500mg on line, supported by the Australian Government Research (2017); Plant Insect Ecosystems Legends of Entomology Award for Masters Student Achievement (2013); and Outstanding Graduate Research Assistant of the Year antibiotics for boils purchase 500 mg ciplox amex, sponsored by the Golden Key Honour Society, Kansas State University Chapter (2013). Her focus is on the insect causal agents of two emergent diseases, including thousand cankers disease of walnut and drippy blight disease of red oak. Her work has also looked at insecticide resistance in the European elm scale, the life history of the kermes scale involved in drippy blight disease, and insect involvement in the spread of a bacterial tree pathogen. Forest Service Rocky Mountain Research Station, where she integrates basic entomology, microbiology, and ecology to answer applied research questions that aid management of insect-associated diseases in urban and natural forests. Kunle was born and raised in Lagos, Nigeria, where he received his elementary and high school education. Kunle proceeded to earn a master of science degree in entomology at Auburn University, Alabama, graduating in summer 2015 under the direction of Dr. In the future, Kunle hopes to secure an academic faculty position, where he intends to use the triad of research, teaching, and extension to address food security, especially through crop protection and pest management. His dissertation research focused on several aspects of urban pest behavior, physiology, evolution, and management. Specifically, he evaluated the efficacy and exposure risks associated with total release foggers ("bug bombs") used for German cockroach control. In addition, he evaluated bed bug host-attraction and several behavioral and physiological factors responsible for preventing gene flow among different host-associated lineages (human- and bat-associated) of bed bugs. During his graduate tenure, Zach received numerous national and international level awards for his work relating to insect behavior, physiology, and integrated pest management. He has also taught several undergraduate entomology courses and has actively engaged the public in both extension and outreach activities throughout his career, including events such as BugFest and pest control association meetings. Zach is currently a postdoctoral research scholar at North Carolina State University, working with Dr. Coby Schal on the relationship among urban pest biology, management, and human health. He began his career studying molecular biology, focusing on developmental genes associated with gastrulation in Drosophila melanogaster, and epigenetic mechanisms. During his four years at Texas A&M University, Freddy contributed with nine manuscripts, six of them as first author. Currently, he is a postdoctoral researcher in Department of Entomology and Nematology at the Citrus Research and Education Center, University of Florida, working with Dr. The main goal of this research is to address the effectiveness of pesticide applications to control Diaphorina citri on Candidatus Liberibacter asiaticus-infected citrus groves and evaluating the effect of Candidatus Liberibacter asiaticus inoculation frequency on citrus greening progression and plant defense response. Daniela Pezzini graduated from the Federal University of Santa Maria in Brazil with a B. While an undergraduate, she acquired multiple experiences as a research assistant in an entomology laboratory. Her research project involves nine states of the North Central Region and aims to address questions related to the increasing threat posed by stink bugs in soybean. Specifically, it will fill the gap of literature on stink bug species composition and abundance in the region and develop sampling recommendations for growers and researchers. Daniela is passionate about agriculture and enjoys being able to help farmers better protect their crops. Hillary Fischer found her passion for understanding the mechanisms of host-plant resistance while working as an undergraduate research assistant in Dr. Her dissertation research aims to understand the role of primary metabolism in plants for defense against aphids. Specifically, she investigates how singlet oxygen, a reactive oxygen species produced during photosynthesis, can serve as a signaling molecule for plant defense. Singlet oxygen has been largely overlooked in plant defense to herbivores due to a short half-life, making it difficult to study. Her research will improve techniques to more accurately and efficiently study singlet oxygen and elucidate its role in signaling for aphid resistance. She was president of the Arkansas Entomological Society from 2016 to Spring 2018, during which time she organized the annual entomology meeting for the state. She also served as president of the Isely-Baerg Entomology Club for the 2017-2018 term. After graduating, she hopes to continue in research focusing on the molecular aspects of host-plant resistance. Her research interests include plant-insect and plant-pathogen interactions as well as host plant resistance. Her project involves investigations of the mechanistic basis of plant resistance against above-ground and below-ground organisms in rice. Primarily, she studies the effects of mycorrhizal fungi, a symbiotic soil-borne organism, on rice resistance to insect herbivores and pathogens. Lina has been actively involved in the Entomological Society of America since 2013. Currently, she serves as student representative to the Governing Board and participates in both the Student Affairs Committee and the Early Career Professionals Committee. She is very grateful to the society for the many opportunities it has provided her. She has had a lifelong interest on science and looks forward to sharing this passion with younger generations. Emily has focused her career on leveraging entomological knowledge to best serve people. Emily utilizes simulation models of ecosystems to optimize integrated pest management strategies, a technique she learned while on an American Scandinavian Foundation Fellowship working with Dr. Dosdall Scholarship, Danks Scholarships, Graduate Research Travel Scholarships, Biological Survey of Canada Scholarship, Ed Becker Conference Travel Award Pauline Deschodt Dr. Dosdall Memorial Scholarship Pauline is a PhD candidate at Simon Fraser University in the Cory lab. Her work focuses on pathogen ecology and evolution, examining within and between host competition. Her work involves investigating the effects of mixed pathogen infections within a single host, as well as pathogen interactions at the host population level. She is primarily interested in the effects of pathogen competition on replication and secondary transmission. She uses laboratory as well as field experiments to explore factors affecting pathogen competition and disease outcome. Pauline hopes to understand the ecology of pathogen competition at the host and community level. He is interested in the field of integrated pest management, specifically for the control of forest pests. She is a behavioural ecologist with experience and expertise in 3D high resolution live insect imaging, insect molecular biology, insecticide toxicology, integrated pest management, chemical ecology, and biological control. Joanna is transitioning into public health and medical entomology research, with interest in zoonoses and vector-borne diseases, where she can bring her research skills and training in behavioural ecology to study arthropod disease-vectors including mosquitoes and ticks. Dosdall Scholarship, Danks Scholarships, Graduate Research Travel Scholarships, Biological Survey of Canada Scholarship, Ed Becker Conference Travel Award Catherine Little Graduate Research Travel Scholarship (Ph. She is investigating interactions between olfactory reception, visual cues, and evolution of host preference in Drosophila suzukii. It is a serious pest of cruciferous crops, most notably canola, and if populations are not controlled it may spread from Ontario to the major canola-producing regions of Canada.
In the mid-1970s infection earring hole ciplox 500mg online, polysulfide polymer chemistry was advanced when Products Research and Chemical Corporation introduced a polyoxypropylene urethane backbone with mercaptan terminal groups [7] infection quality control staff in a sterilization unit of a hospital buy ciplox 500 mg fast delivery. Molecular weight regulation and minimization of side reactions are important features 600 mg antibiotic generic ciplox 500mg otc. The backbone is significantly different from that of conventional polysulfide polymers infection 8 weeks after birth 500 mg ciplox with visa, yet the curing chemistry is the same and the cured product is a polysulfide rubber. The polythioether polymer has excellent resistance to fuel and organic solvents and has better thermal stability than that of conventional polysulfides. Polythioethers can be terminated with mercaptan, hydroxyl, silyl, and nonreactive end groups. In general, polymers in the range 400 to 500 P are used in sealants and adhesives, while lower-viscosity polymers are used for coatings and casting compounds. The tensile properties of unfilled polysulfide polymers are poor but are improved by suitable reinforcement with pigments and fillers. Both the molecular weight of the liquid polysulfide polymer and oxidative curing influence the physical properties. Cured liquid polysulfide compositions have excellent resistance to many oils and solvents. Systems must be properly formulated and cured to obtain maximum solvent resistance. The glass transition temperature (Tg) of polysulfides depends on the hydrocarbon moiety and the length of the polysulfide chain. The amount of cross-linking monomer is small, and therefore it does not influence Tg. The thermal stability of polysulfide polymers depends on the polymer backbone and the curative used to vulcanize the polymer. Commercially available polysulfides have an ethyl formal disulfide backbone, and this regulates the upper temperature limits. In an acid-catalyzed hydrolytic attack, formaldehyde is released, which in turn reduces the disulfide bond to mercaptan. The terminal mercaptan group can react with a hydroxyl group to give a monosulfide bond. The degradation results in weight loss and loss of flexibility due to the monosulfide structure formation. Calcium oxide can neutralize formic acid and absorb water and is therefore an effective stabilizer. Practical cure rates cannot be achieved in anhydrous formulations by metal dioxide curing agents. Thermal instability can also arise when the mercaptan group reacts with the metal oxide. Polysulfide sealants cured using manganese dioxide and chromate salts provide continuous service at 250 F. Tobolsky has studied extensively the viscoelastic properties of polysulfide polymers [9]. Polysulfide polymers have the unique ability to relieve internal stress or stress between mercaptan and disulfide linkages. The relaxation times (in hours) for polysulfide polymers at 80 C for some curing agents are 0. The ability of polysulfide polymers to relieve stress is extremely valuable in maintaining adhesion in joints subjected to joint movement. Suitable fillers should be dispersed and suitable additives should be incorporated into the formulation. Curing agent, curing modifier, filler, plasticizer, and adhesion additive are discussed briefly below. Mercaptan-terminated liquid polysulfide polymers are polymerized to rubbery solids by oxidizing agents. The curing process involves the oxidation of the terminal mercaptan groups in the polysulfide polymers to form the corresponding disulfide. Typical accelerators are amines, inorganic bases, water, dinitrobenzene, and sulfur. Fillers increase the strength, impart needed rheological properties, and reduce the cost of sealants. Tensile properties are increased significantly, depending on the type of filler, its particle size, and the type of cure. Calcium carbonates (wet or dry ground limestone, precipitated), carbon blacks (furnace, thermal), calcined clays, silica and silicate fillers, and rutile titanium dioxide are typical fillers used in polysulfide sealants. One-part 20 50 25 2 3 Building 35 40 20 2 3 Insulating glass 30 50 15 2 3 Aircraft 65 25 5 2 3 Casting compound 35 35 27 - 3 Table 2 Ingredient Chemical Composition of a Typical Aircraft Sealant Function(s) Filler, reinforcer Filler, opacifier Vehicle Viscosity adjuster Curing agent Modulus adjuster Viscosity adjuster Percent 26. The plasticizer must be compatible with the cured sealant, should have low volatility, and must be safe. Table 1 lists five types of sealant formulations suggested by Panek that are useful in several end applications [10]. Generally, the integral fuel tank polysulfide sealant consists of two parts: 9 parts by weight of sealant base components mixed with 1 part by weight of accelerator. Most sealants, especially those used in building, applications, contain adhesion additives. Masonary primers generally contain a chlorinated rubber or a modified phenolic resin either alone or in combination with an additional plasticizer. Thin layers of silanes give good polysulfide adhesion to metals, glass, and ceramic substrates. Therfore, the equipment used in coating and ink manufacture is applicable to sealant manufacture. Viscosities of up to 60,000 P are common in polysulfide sealant, and therefore heavy equipment is generally used in its manufacture. Mechanical and physical quality control tests on polysulfide sealants can produce erroneous and misleading results, especially in predicting long-term performance. Problems such as poor adhesion, inadequate cure, and short working life can frequently occur, resulting in tedious and costly repair. The composition of polysulfide sealants can be determined by centrifuging thinned polysulfide sealants and resolving them into components. Nuclear magnetic resonance spectroscopy and gel permeation chromatography can be used for vehicle analysis. Paul has studied the effects of environment on the performance of polysuifide sealants [15]. Numerous data can be obtained by monitoring the effects of various environmental conditions. This polymer has a molecular weight of about 1000 with viscosity in the range 7 to 12 P. The epoxy resins most widely used in adhesive formulations have viscosities the range 80 to 200 P with an epoxy equivalent of 175 to 210. The general reaction of a polysulfide polymer, epoxy resin, and amine hardner is these adhesives are used in the construction, electrical, and transportation industries because of flexibility, adhesion to many substrates, and chemical resistance. The epoxy-modified polysulfide has improved adhesion due to chemical reactions that increase electronic attraction forces. Formation of thiourethane is responsible for excellent adhesion of polysulfide onto polyurethane coatings. Macknight, Polymeric Sulfur and Related Polymers, WileyInterscience, New York, 1965. Phenolic resins were the first true synthetic polymers to be developed commercially. Notwithstanding this, even now their structure is far from completely clear, because the polymers derived from the reaction of phenol with formaldehyde differ in one important aspect from other polycondensation products. Polyfunctional phenols may react with formaldehyde in both the ortho and para positions to the hydroxyl group.
References