Fusion characteristics

Ash fusion characteristics are important in ash deposition in boilers. Ash deposition occurring on the furnace walls is termed slagging, whereas accumulation on the superheater and other tubes is termed fouling. A variety of empirical indexes have been developed (60,61) to relate fouling and slagging to the ash chemical composition through parameters such as acidic and basic oxides content, sodium, calcium and magnesium, and sulfur. 

In the calorimetric approach, it is necessary to know the heat of fusion of the totally crystalline polymer. This can be obtained from melting-point depression measurements, as described in the following section. The basic idea depends on the fact that the melting temperature is independent of the size of the system, since it is an intensive property. The extent to which it is depressed by the presence of solvent can be used to calculate a heat of fusion characteristic of the crystallites, irrespective of how many are present. This is therefore the heat of fusion of the 100% crystalline polymer. The fractional crystallinity in an actual sample is then the ratio of its calorimetrically measured heat of fusion per gram to that of the 100% crystalline polymer. For example, if the actual polymer has a heat of fusion of 7 cal per gram, and the 100% crystalline polymer a heat of fusion of 10 cal per gram, then the fractional crystallinity is 0.7, and the percentage crystallinity is 70%. 

The results of monitoring the temperature profiles and ash fusion temperatures of the ash resulting from the test coals burned in the different boilers indicated that the effect of boiler temperature and ash fusion characteristics of the coal on the leaching and sorbent characteristics should be examined. This could be carried out through a comparison of the leaching and sorbent characteristics of the fly ash produced in the 11 boiler with that produced in 12 boiler and the fly ash produced in the A boiler with that produced in the C and D Boilers, respectively. 

Process aids are added primarily to PVC-U (including foam) to improve fusion characteristics and melt flow during the processing stage. Addition levels are typically from 1-6 phr with resultant improvements in output and surface finish. Transparency can also be improved in clear formulations. Chemically, these products are high molecular weight methyl methacrylate and alkyl acrylate copolymers. 

Finally, the ensemble-averaged quantity < 1F(Z. .. z r, /)>yy gives the probability of donors surviving to time t. Monte Carlo calculations have shown that in many cases the pair survival probability is approximated well by a single exponential except at very short times [79a, 81]. The rate constant /cd is related to the geometric and dif-fusional characteristics of the system by the equation [79a]... 

A variety of properties of the flux must be controlled, including fusion characteristics (fusion temperature range and sintering characteristics), insulation characteristics, flow properties of the powder, viscosity of the molten flux, and non-metallic absorption ability. The viscosity influences the consumption rate of flux, heat transfer in the mold, and non-metallic dissolution rate, and has been the subject of published and unpublished work over the last ten years. 

Matuana L.M. Kim J.W. Fusion Characteristics of Wood-Flour Filled Rigid PVC by Torque Rheometiy, Joirrnal of Vinyl Additive Technology, vol 13, No.l, (2007), pp. 7-13. 

A torque rheometer is used to determine fusion characteristics of PVC compounds. Components of formulation are first mixed in a beaker in an intensive mixer or ribbon blender. The rheometer is equipped with roller head and the premixed sample is added to a chamber having preselected temperature (typical temperatures used are 140 for flexible compounds, 180 for semirigid compounds, and 197°C for rigid compounds). The amount of the sample added is selected such that it occupies 65% of mixer bowl. Mixing is continued until torque peak is reached. Mixing time and maximum torque are two important results of testing. 

Processing Properties. The dry blend and fusion characteristics of an FPVC are related to the molecular weight and degree of branching of the... 

Despite the lack of quantitative agreement between theory and experiment, much of which can be attributed to experimental shortcomings and inaccessibility of the very long sequences, the data in Figs. 5.2 and 5.3 qualitatively show all of the major characteristics of the theoretical fusion curves. They can be expected to be typical of the fusion of random type copolymers, irrespective of the chemical nature and structure of the noncrystallizing chain units. Random ethylene copolymers, prepared by a completely different method, display similar fusion characteristics.(21) It is important, however, to assess the generality of the conclusions with other copolymers, rather than just depending on the results of ethylene copolymers. 

Particle size, loading level and coating level all have an influence on fusion characteristics, the final level of gelation and mechanical properties, particularly impact strength. 

Fusion characteristics are especially important for fabrication of chemically blown foams. Blowing agent decomposition temperature and kinetics must be carefully matched to paste fusion so as to produce ideally dispersed foam pores. 

Another factor affecting resin fusion characteristics is the presence (or lack) of an antistatic surface treatment on the resin particle surface. Some resin manufacturers use calcium stearate as an antistatic agent at levels ranging from 0.1 to 0.2 percent. Calcium stearate is one of the main additives used to speed fusion in rigid PVC compounds. Therefore, the presence of calcium stearate on the resin particle surface will... 

If the fusion characteristics of the resin stay consistent, output stays consistent. This is extremely important to a pipe producer. The objective of a pipe producer is to mn as close to the product dimensional tolerances as possible. If the resin particle size or bulk density changes during extrusion, there will be inconsistency in product output. In order to compensate for this, the pipe producer may have to mn slightly oversize on pipe dimensions in order to compensate for fusion-related output differences and to consistently meet minimum dimensional requirements. If a pipe producer must manufacture products an average of 2 percent overweight due to fusion-related output inconsistencies and the pipe producer consumes 50 million pounds of PVC compound in one year, the pipe manufacturer is arguably shipping one million pounds of free compound to customers each year. 

Wear Layer Clear PVC plastisols are typically used to coat the gel/ foam layer to provide a wear layer for resilient flooring products. Durability and clarity are the two characteristics considered most important for this application. Unfortunately, many of the resin characteristics that are a benefit to one of the above characteristics are often detriments to the other. Durability characteristics such as abrasion resistance are maximized through the use of high-molecular-weight resins. Unfortunately, lower-molecular-weight products often provide superior clarity due to improved fusion characteristics. 

The most common PVC copolymer resin type used in plastisol formulation is the PVC/vinyl acetate copolymer, and we will concentrate on the use of this type of copolymer resin. PVC/vinyl acetate copolymers are normally used to speed gelation and fusion in plastisol compounds. This provides benefits in many different plastisol applications. The downside to the improved gelation and fusion characteristics is that plastisol compositions containing PVC/vinyl acetate copolymas have worse shelf stability and poorer physical properties than the same compositions based solely on homopolymer resins of the same molecular weight. Another negative eharacter-istie of these resins is that they tend to be less heat-stable than their homopolymer cormterparts. 

Acrylic release agents in small amounts are generally described as not affecting fusion characteristics, thus ehminating the need for formulation lubricant adjustments. The polymeric nature of these materials means that plateout is not an issue for these materials. Additional benefits of acrylic release agents may include... 

The following properties of the plastisols were determined degassing properties, low and higher shear viscosity, viscosity stability and gelation/fusion characteristics. The stain resistance and tensile properties of fused vinyl sheets were also tested. 

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