Horst

Horst, Factor Mnalysis of Data Matrices, Holt, Rinehart, and Winston, New York, 1965. 

Some plants have been using computer control for 20 years. Control systems in industrial use typically consist of individual feedback and feedforward loops. Horst and Enochs [Engineering h- Mining]., 181(6), 69-171 (1980)] reported that installation of single-variable automatic controls improved performance of 20 mineral processing plants by 2 to 10 percent. But interactions among the processes make it difficult for independent controllers to control the circuit optimally. 

Control of Crushers Lower-grade raw materials, higher energy costs, larger-scale operations, and more complex, capital-intensive plants make automatic control of size-reduction equipment more important (Suominen, 21st International Symposium—Applications of Computers and Operations Research in the Mineral Industry, 1011-1018). Benefits are increased productivity, process stability and safety, improved recoveiy of mineral values, and reduced costs [Horst and Enochs, Engineering Mining J., 181(6), 69-171 (1980)]. 

Steindorff, A., Balle, G., Horst, K. and Michel, R. U.S. Patent 2,413,477 September 3, 1940 assigned to General Aniline Film Corp. 

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

Thus, the role of the defect labile structures in the thermal degradation of PVC, which is the key to the Frye and Horst mechanism of stabilization, is itself debatable. The Frye and Horst mechanism—the substitution of labile chlorines by more stable groups—has also been criticized. 

Organic metal salts have frequently failed to produce an appreciable chemical stabilization effect, either during dehydrochlorination induction periods or in later decomposition stages. While this does not rule out the occurrence of Frye and Horst substitution reactions, it does suggest that these reactions may not be responsible for the observed retardation of color developments [126-128]. 

Horst mechanism of PVC stabilization. In fact, based on his observations Naqvi [134] has proposed an alternative model for the degradation and stabilization of PVC based on polar interactions within the polymer matrix, which will be discussed in the following section (Section VI). 

In conclusion, it may be said that a lot of literature has been published that favors the Frye and Horst mechanism of stabilization. Most of this is based on studies done on low-molecular weight model compound for al-lylicchlorines in PVC, i.e., 4-chloro-2-hexene. Although the large contribution of these studies toward understanding the mechanism of stabilization of PVC cannot be denied, the extrapolation of these results to the processes involved in the actual stabilization of the polymer should be done with extreme care. The polymer represents a complex mixture of macromolecules, which in the melt is not only physically a very different system compared to the low-molecular weight model compound, but invariably contains, apart from stabilizers, other additives, such as plasticizers, lubricants, processing aids, etc., that further complicate the situation. The criticism of the Frye and Horst mechanism is also based on solid experimental evidence, and hence, the controversy is still very much alive. 

Naqvi [134] has proposed an alternative model to the Frye and Horst mechanism for the degradation and stabilization of PVC. At room temperature, PVC is well below its glass transition temperature (about 81°C). The low thermal stability of the polymer may be due to the presence of undesirable concentrations of like-poles in the more or less frozen matrix with strong dipoles. Such concentrations, randomly distributed in the polymer matrix, may be considered to constitute weak or high energy spots in the polymer, the possible sites of initiation of thermal dehydrochlorination. 

Gegelova, N, B., Mudzhiri, Y. N., Knyazheva, V. M. and Topchiashvili, L. I., Determination of Intercrystalline Corrosion Tendency of Stainless Steel by Potentiodynamic Method , Zashch. Metal., 8, 420 (1972) C.A., 77, 121422n van der Horst, J. M. A., Grain Boundary Attack of 316L Stainless Steel in Ammonia-rich Environments , J. Electrochem. Soc., 119, 216C (1972)... 

Van der Horst, J. M. A., Grain Boundary Attack of 3II6L Stainless Steel in Ammonia-rich Environments , J. Electrochem. Soc., 120, 512 (1973)... 

We are grateful to Professor Andrew. Lyddiatt, Dr Horst Bierau and the University of Birmingham, UK, for various aspects of this collaborative work. 

Jameson C. J., Jameson A. K., ter Horst M. A. 14N spin relaxation studies of N2 in buffer gases. Cross sections for molecular reorientation and rotational energy transfer, J. Chem. Phys. 95 5799-808 (1991). 

Horst MN, Walker AN (1993) In Muzzarelli RAA (ed) Chitin Enzymology. Atec, Italy 109... 

HCBC = PerfonDance Index Nmisers range frcm 0 (Best) to 5 (Horst) n.a. = Not available. 

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