Common Compositional Problems

If it is suspected that the composition of the product is not correct, for example that the antioxidant or correct cure system has not been incorporated into the compound, then the appropriate species specific tests that have been discussed earlier can be carried out. If possible, it is advisable to conduct the analysis alongside that of a sample known to be of the correct composition. This ensures that no product specific matrix effects/interference reactions affect the data obtained and hence the conclusions reached. 

A surprisingly large number of failure problems can be solved by carrying out a simple series of tests including a polymer identification check by IR, a bulk compositional analysis by TGA and a molecular weight determinatioa 

Most of the failures that are associated with the use of either some or too much regrind are related to a reduction in the strength properties of the plastic, but another reasonably common type of failure is odour, especially in packaging applications. This odour is invariably due to oxygenated degradation products of the plastic and can be detected and analysed by headspace GC-MS. 

---

The contributions of Richard Block to the serum protein problem originated from the hypothesis of Kossel. From recent data on the amino acid composition of the proteins found in animal sera, a formulation is derived which reflects the properties of a continuous system of molecular species originating from a common biosynthetic pathway, as if from mixed polymers of monomeric peptides of lower molecular weight. Indirect evidence of this is found in the amino acid interrelationship, and direct evidence is limited to the isolation of peptides of common composition, whose primary structures are still under investigation. These findings suggest that undifferentiated proteins may be continuous systems rather than discrete molecular species. 

The sample may be solid, liquid, or gas, although the first two forms are most commonly used. Problems associated with sampling are the same as in any other method of analysis. Once the sample is secured, it is packaged in an appropriate container. An important point to keep in mind is that the sample and standards should be as similar as possible in matrix composition. For example, when analyzing rocks the sample is generally pulverized to a fine powder, and the standards are also preferably made from finely powdered standard rocks (such as those... 

Whenever buffers or other mobile-phase additives are used, you should carefully check the solubility in the mobile phase. This is especially true for gradient applications or when the mobile-phase composition is generated automatically by the HPLC instrument. Buffer predpitation is one the most common column problems. A predpitated buffer can be redissolved by slowly purging the column with a mobile phase, in which the buffer is soluble. 

The most common VLE problem is to calculate the temperature and vapor composition yj that is in equilibrium with a liquid at a known total pressure of the system P and with a... 

The more common environmental problem with metal joints is attack by water. In addition to alloy composition, pre-treatment and primer, durability is affected by the adhesive used (see Pre-treatment of metals prior to bonding. Pre-treatment of polymers. Primers for adhesive bonding). Sell has ranked adhesives in the order of durability they provide with aluminium substrates ... 

Protease-containing liquid aqueous detergents are well known, especially in the context of laundry washing, where they attack protein-based stains. A commonly encountered problem in such protease-containing liquid aqueous detergents is the degradation second enzymes (e.g., lipase, amylase, and cellu-lase, or protease itself) by the proteolytic enzyme in the composition. As a... 

How to detect that recursion is useless in some non-minimal sub-cases Step 4 (Syntactic introduction of the recursive atoms) creates a non-recursive case if at least one example Ej leads to a non-admissible procComp(,,yp atom. Howto invent or re-use appropriate predicates How to implement invented predicates The predicate invention problem is tackled at four points during the synthesis. At Steps 2 and 3, the Database Method re-uses predefined predicates this amounts to using domain knowledge. At Step 6, the Synthesis Method (see Section 14.2.4 below) invents new composition operators. As of now, there is no method yet of re-using common composition operators. At Step 7, the PaP Method synthesizes discriminants that are in terms of predicates other than rfn. How to discover which parameters are auxiliary parameters Due to our restriction to version 3 of the divide-and-conquer schema, auxiliary parameters are not taken into account. See Section 14.2.2 below on how to achieve this. 

Extmsion is a common way for soHd products such as plastics to emerge from closed manufacturing systems. Normally a polymer is hot when extmded and may contain additives and oligomers that are volatile at elevated tempera-tures. The result is Aiming at the extmder head. These fumes can result in employee annoyance, housekeeping problems, and, at worst, depending on composition, health ha2ards. 

Thermosetting unsaturated polyester resins constitute the most common fiber-reinforced composite matrix today. According to the Committee on Resin Statistics of the Society of Plastics Industry (SPl), 454,000 t of unsaturated polyester were used in fiber-reinforced plastics in 1990. These materials are popular because of thek low price, ease of use, and excellent mechanical and chemical resistance properties. Over 227 t of phenoHc resins were used in fiber-reinforced plastics in 1990 (1 3). PhenoHc resins (qv) are used when thek inherent flame retardance, high temperature resistance, or low cost overcome the problems of processing difficulties and lower mechanical properties. 

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

Unfilled Tooth Restorative Resins. UnfiUed reskis were some of the first polymer materials iatroduced to repak defects ki anterior teeth where aesthetics were of concern. They have been completely replaced by the fiUed composite reskis that have overcome the problems of poor color StabUity, low physical strength, high volume shrinkage, high thermal expansion, and low abrasion resistance commonly associated with unfiUed reskis. 

The relative volatiHties Ot) are defined by Eq. (13-33), is the mini-mum-reflux ratio (L v + i/D)min,. nd q describes the thermal condition of the feed (e.g., 1.0 for a bubble-point feed and 0.0 for a saturated-vapor feed). The Xi p values are available from the given feed composition. The 0 is the common root for the top-section equations and the bottom-section equations developed by Underwood for a column at minimum reflux with separate zones of constant composition in each section. The common root value must fall between 06/, and Ot/, where hk and Ik stand for heavy key and light key respectively. The key components are the ones that the designer wants to separate. In the butane-pentane splitter problem used in Example 1, the light key is /1-C4 and the heavy key is i-C. ... 

The previous analysis has shown that the properties of unidirectional fibre composites are highly anisotropic. To alleviate this problem, it is common to build up laminates consisting of stacks of unidirectional lamina arranged at different orientations. Clearly many permutations are possible in terms of the numbers of layers (or plies) and the relative orientation of the fibres in each... 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

The steady-state design equations (i.e., Equations (14.1)-(14.3) with the accumulation terms zero) can be solved to find one or more steady states. However, the solution provides no direct information about stability. On the other hand, if a transient solution reaches a steady state, then that steady state is stable and physically achievable from the initial composition used in the calculations. If the same steady state is found for all possible initial compositions, then that steady state is unique and globally stable. This is the usual case for isothermal reactions in a CSTR. Example 14.2 and Problem 14.6 show that isothermal systems can have multiple steady states or may never achieve a steady state, but the chemistry of these examples is contrived. Multiple steady states are more common in nonisothermal reactors, although at least one steady state is usually stable. Systems with stable steady states may oscillate or be chaotic for some initial conditions. Example 14.9 gives an experimentally verified example. 

---