Processing and application of PPO

PPO forms one of a group of rigid, heat-resistant, more-or-less selfextinguishing polymers with a good electrical and chemical resistance, low water absorption and very good dimensional stability. This has led to a number of applications in television such as tuner strips, microwave insulation components and transformer housings. The excellent hydrolytic stability has also led to applications in water distribution and water treatment applications such as in pumps, water meters, sprinkler systems and hot water tanks. It is also used in valves of drink vending machines. 

Unfortunately for PPO its price is too great to justify more them very restricted application and this led to the introduction of the related tmd cheaper Noryl materials in 1966 by the General Electric Corporation. These will be discussed in the next section. In recent years the only sources of unmodified PPO have been the USSR (Aryloxa) and Poland (Biapen). 

---

One of the most important applications of PPO, although rarely reported, is its role in synthetic processes, such as the biosynthesis of betalains. Several researchers reported the hydroxylation of tyrosine to dopa, which can then be oxidized to dopaquinone, through a PPO from Portulaca grandiflora and from Beta vulgaris. Thus, a dioxygenase activity complements the constitutive PPO activity and the initiation of this dioxygenase... 

Flame-retardant polystyrene is used primarily in expanded foam for building insulation. Rubber-modified styrenic polymers are flame retarded for use in a number of applications, such as enclosures for electronics and business equipment. By far the largest volume flame-retardant HIPS application is television enclosures (Figure 29.1) these are made primarily from flame-retardant HIPS [3]. Flame-retardant HIPS has an attractive balance of mechanical properties, processability and cost. Flame-retardant styrenic blends such as HIPS-PPO and PC-ABS also find utility in a number of electrical applications such as printers, computers and monitors. These blends have received increasing attention recently because of their ability to be flame retarded with nonhalogen flame retardants (see Section 7). 

As already mentioned earher (see Section 7.1.1), because of its high glass-transition temperature, that is, T = 208°C, PPO has to be melt-processed at elevated temperatures. As a result, degradation of the polymer may occur at such temperatures (particularly through oxidation reactions at the methyl substituents), furthermore, upon cooling from the liquid to the rubber state, two unwanted events can take place (i) the polymer crystallizes and (ii) the molecular motions are frozen and the rubbery polymer turns to a glass. As a consequence, the material becomes brittle and cannot be used for practical applications. Fortunately, PPO exhibits unusual and remarkable blending properties [36]. 

PPO homopolymer shows considerable chain stiffness, high and Consequently a relatively difficult processing which limited its use in a wider range of applications. Various polymers including HIPS, PP, Nylon and PPS are compounded with PPOs to achieve a desired processability and mechanical properties. Polystyrenes are known to show miscibility with PPO over the complete compositional... 

PPO Blends. As with cyanate ester, both the tan 5 and of the PPO resin are lower than those of epoxy. For processibility and cost reasons, a blend of approximately 50 percent PPO and 50 percent epoxy is generally used.This gives electrical performance similar to that of BT, but with a cost penalty of only 20 to 50 percent above standard epoxy. It is likely that PPO will find wide use in those applications snch as high-performance workstations that need a small improvement in electrical performance. Unfortnnately, the characteristics of a PPO-epoxy blend are often inadequate for very high-speed applications such as supercomputers and wireless (RF) applications. 

The present chapter is a brief summary of the work, on the application of Poly (2, 6-dimethyl-1, 4-phenylene oxide) (PPO) and modified PPO carried out at the Industrial Membrane Research Institute, University of Otawa, Canada [1]. The approach has been to seek a better understanding of the processes that govern gas and water vapour transport across the membrane. 

Gadelle et al. (1995) investigated the solubilization of various aromatic solutes irbfftRSS-b-PEO (ABA)/PPO-bPEO-bPPO (BAB) triblock copolymers. According to the experimental results, they indicated two different solubilization processes. To understand better the mechanism for solubilization in the polymeric surfactant solutions, it was postulated that (1) the addition of apolar solutes promotes micellization of the polymeric surfactant molecules, (2) the central core of the polymeric micelles contains some water molecules, and (3) solubilization is initially a replacement process in which water molecules are displaced from the micellar core bythesolubilizate. Adetailed discussion of the solubilization process can be found in the next section and the pharmaceutical application section of this chapter. 

Polypropylene oxide (PPO) is a recently developed resin with an application that is rapidly expanding. It requires a comparatively long drying time since it contains superfine particles and has high affinity for water. Of various kinds of polymers, this is the one that requires the most difficult processing techniques. The paddle dryer is found to process this material economically. 

Although the above triblock polymeric surfactants have been widely used in various applications in emulsions and suspensions, some doubt has arisen on how effective these can be. It is generally accepted that the PPO chain is not sufficiently hydrophobic to provide a strong anchor to a hydrophobic surface or to an oil droplet. Indeed, the reason for the surface activity of the PEO-PPO-PEO triblock copolymers at the o/w interface may stem from a process of rejection anchoring of the PPO chain, since it is not soluble both in oil and water. 

---