Type I plasticizers

Fig. 8. Single-shot laser-induced breakdown spectroscopy (LIBS) spectrum of type I plastic collected from a...

Type I plasticizers are used to soften the binder polymer chains, allowing them to stretch or deflect under an applied force. These additives can be accurately described as modifiers or binder solvents. Tg is a symbol that stands for glass transition temperature. While crystalline solids have a distinct change from solid to liquid, the binder polymer in the dry tape is a noncrystalline solid, having a gentle transformation from solid to liquid. While crystalline solids freeze and melt, the polymer matrix softens more and more until it is considered a liquid. In order to describe the softening characteristics of a polymer (or glass), a temperature is calculated... 

Two ways in which the Type I plasticizer can modify the of a polymer chain are by shortening the polymer chain length and by partially dissolving the polymer chain. Both of these mechanisms accomplish the same result, making the tape more flexible at a given temperature. As the becomes lower, often well below room temperature, the polymer chain is better able to stretch or reorient itself without fracturing. An example of polymer chain shortening is seen in the case of polypropylene carbonate modified by propylene car-... 

Large amounts of a Type I plasticizer will make the tape very elastic and can make the surface quite tacky. Excessive amounts of a Type I plasticizer will cause blocking, the immediate adhesion of one layer to another, due to the liquid-like character imparted to the tape. Excessive use of a Type I plasticizer has major drawbacks in that the adhesion of the polymer to the carrier surface increases while the yield stress of the polymer decreases. A tape that has been over-plasticized by a Type I plasticizer will stretch rather than release from the carrier surface. ... 

The interaction of the binder additive and the plasticizer(s) is usually, if not always, very desirable. Much of the interaction between these two components has already been addressed in this chapter in either the binder section or the plasticizer section. For the most part, the only purpose for adding the plasticizer, especially the Type I plasticizer, is to interact with the binder. Referred to previously as a binder solvent, the Type I plasticizer s role in the system is to lower the T of the binder to increase the flexibility of the dry tape, extending the leather-hard stage to longer working times. This in teraction with the binder is the only reason that a Type I plasticizer is used. 

The Type II plasticizer is also discussed in Section 2.5. The Type II plasticizer does not chemically react to any great extent with the binder or with the Type I plasticizer. The only real interaction between the Type II plasticizer (lubricant) and any other component (s) in the system is mechanical. As mentioned earlier, the lubricant physically inhibits some of the polymer ties from forming and facilitates permanent, plastic motion in both the fluid slip and the dried tape. 

After the dispersion milling process is complete—that is, after 24 or more hours of milling—the mill is opened and the plasticizer or plasticizers are poured onto the top of the slip. Most plasticizers are liquids and quite fluid, and therefore this is not a complicated procedure. The plasticizer or plasticizers form a thin film on the surface of the slip in the mill. The weighed quantity of binder, either in powder form or dissolved in one or both of the solvents, is then added to the layer of plasticizer(s). Usually the binder is more soluble in the plasticizer(s) than it is in the solvent(s), so it is good practice to introduce the binder to the plasticizer first. Remember from Chapter 2 that the whole point of the Type I plasticizer is to react with (dissolve) the binder. The plasticizer wets the binder if it is in powder form and aids in the dissolution process. It is also good practice to make sure that the binder, if in powder form, is stirred into the slip mixture before the mill is sealed and returned to the rollers. We have found large clumps of undissolved binder after 12 hours or more of mixing if we didn t follow this simple procedure. 

Usually about 12 hours are required for the complete dissolution of the binder and reaction of the binder with the Type I plasticizer (see the definition of a Type I plasticizer in Chapter 2). If the binder is pre-dissolved in a solvent mixture and the Type I plasticizer is added... 

The big picture in this case focuses mainly on the ability to plasticize these water-soluble polymers in the desired manner. Many plasticizers have been used and reported for both PVA and cellulose binders, but the vast majority fall into the category of Type II plasticizers. Glycerin and various glycols have been reported, and seen by the authors, to be very effective lubricants in the water-soluble tape casting systems. The earlier section on plasticizers, however, pointed out that while a Type II plasticizer is desired in some cases, the mandatory plasticizing agent found in all tapes is the Type I plasticizer or binder solvent. The requirements of a Type I plasticizer are solution of the binder polymer and low volatility. Water-soluble binders are plasticized (Type I) by water, which satisfies both of these criteria. 

Type I plasticizer An additive used to soften the binder polymer. The Type I plasticizer is a solvent for the binder and as such works to lower T of the polymer. The only difference between the solvent (s) ancf the Type I plasticizer is seen in the speed of evaporation. 

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