The Heat and Pressure Method

FIGURE 4.15 A diagram of how heat and pressure are applied to a polymeric sample to turn it into a thin film. 

Some labs use heat and pressure to turn awkwardly shaped parts into thin films, and then take their sample spectra via ATR (see later in this chapter). 

The thickness of heat and pressure films depends upon a number of variables, and the only way to know if a film is of the proper thickness is to measure its spectrum. If 

FIGURE 4.16 An example of a hydraulic press used in the heat and pressure method of analyzing polymers in transmission. (Photo courtesy of PIKE Technologies.) 

FIG U RE 4.17 The infrared spectrum of a polystyrene-butadiene co-polymer obtained using the heat and pressure method. 

---

A second way of taking the spectra of polymeric samples in transmission is the heat and pressure method. This method works by compressing a polymer and heating it to above its glass transition temperature, where it will soften and flow to form a thin film. A diagram of how the technique works is shown in Figure 4.15. 

The sample may need to be heated and pressed for 5 or more minntes to form a film. Once the pressnre is released and the polymer film cools, it may stick to the platens. To avoid this problem, the sample can be placed between sheets of Teflon or Teflon-coated alnminnm foil before being placed in the press. Sample clamps such as those shown in Fignre 4.7 can be used to hold polymer films in the infrared beam. The backgronnd spectrnm would be run on the empty film holder. An example of a polymer whose spectrum was obtained using the heat and pressure method is shown in Figure 4.17. 

FIGURE 4.18 A constant thickness film maker, used with the heat and pressure method to make polymer films of reproducible thickness. (Photo courtesy Specac Inc.)... 

Another issue with the heat and pressure method is that it alters polymer morphology. This information is lost once the polymer softens and flows. A final problem with the heat and pressure method is that it does not work on all polymers. Polymers with very high glass transition temperatures or melting points, such as PTFE, are difficult to analyze. Additionally, cross-linked or thermoset polymers cannot be analyzed by this method because they will not soften and flow. 

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the... 

There have been developments in synthetic methods. First, related to the heat and grind method, gel/colloid methods of producing reaction precursors are giving much more control and more reproducibility of the final product. Secondly, hydrothermal methods are finding utility. In these, reactions are carried out at high temperature and pressure conditions in the presence of a solvent (not necessarily water, although this has been most commonly used). For instance, Fe"Fe2"F8 H20 was prepared in this way using liquid HF as a solvent. 

Suspension Polymers. Methacrylate suspension polymers are characterized by thek composition and particle-size distribution. Screen analysis is the most common method for determining particle size. Melt-flow characteristics under various conditions of heat and pressure are important for polymers intended for extmsion or injection molding appHcations. Suspension polymers prepared as ion-exchange resins are characterized by thek ion-exchange capacity, density (apparent and wet), solvent sweUing, moisture holding capacity, porosity, and salt-spHtting characteristics (105). 

Composite resins can be cured using a variety of methods. Intraoral curing can be done by chemical means, where amine—peroxide initiators are blended in the material to start the free-radical reaction. Visible light in the blue (470—490 nm) spectmm is used to intraoraHy cure systems containing amine—quin one initiators (247). Ultraviolet systems were used in some early materials but are no longer available (248). Laboratory curing of indirect restorations can be done by the above methods as well as the additional appHcation of heat and pressure (249,250). 

It is the ultimate objective of a refinery to transform the fractions from the distillation towers into streams (intermediate components) that eventually become finished products. This also is where a refinery makes money, because only through conversion can most low-value fractions become gasoline. The most widely used conversion method is called cracking because it uses heat and pressure to "crack"... 

CM is the most common method of molding TSs. In this process, material is compressed into the desired shape using a press containing usually a two-part closed mold and is cured with heat and pressure. This process is not generally used with TPs. TM, also called compression-transfer molding is a... 

In Chapter 3, the reaction system is discussed using the heat and mass balances, and interaction with the equipment. Scale-up affects both temperature and pressure profiles, which vary with types of reactor systems and sizes. Relevant test methods for scale-up and for process design are covered, including discussions on the methods as well as the relative advantages and disadvantages. Typical approaches for safe design and for defensive measures are presented. The theoretical and experimental subjects in Chapters 2 and 3 are illustrated by the use of examples. 

In practice, every chemical reaction carried out on a commercial scale involves the transfer of reactants and products of reaction, and the absorption or evolution of heat. Physical design of the reactor depends on the required structure and dimensions of the reactor, which must take into account the temperature and pressure distribution and the rate of chemical reaction. In this chapter, after describing the methods of formulating optimization problems for reactors and the tools for their solution, we will illustrate the techniques involved for several different processes. 

In order to develop a method for the design of distillation units to give the desired fractionation, it is necessary, in the first instance, to develop an analytical approach which enables the necessary number of trays to be calculated. First the heat and material flows over the trays, the condenser, and the reboiler must be established. Thermodynamic data are required to establish how much mass transfer is needed to establish equilibrium between the streams leaving each tray. The required diameter of the column will be dictated by the necessity to accommodate the desired flowrates, to operate within the available drop in pressure, while at the same time effecting the desired degree of mixing of the streams on each tray. 

Once the boundary conditions have been implemented, the calculation of solution molecular dynamics proceeds in essentially the same manner as do vacuum calculations. While the total energy and volume in a microcanonical ensemble calculation remain constant, the temperature and pressure need not remain fixed. A variant of the periodic boundary condition calculation method keeps the system pressure constant by adjusting the box length of the primary box at each step by the amount necessary to keep the pressure calculated from the system second virial at a fixed value (46). Such a procedure may be necessary in simulations of processes which involve large volume changes or fluctuations. Techniques are also available, by coupling the system to a Brownian heat bath, for performing simulations directly in the canonical, or constant T,N, and V, ensemble (2,46). 

---