Plasticizers molecules

Solvation—Desolvation Equilibrium. From the observation of migration of plasticizer from plasticized polymers it is clear that plasticizer molecules, or at least some of them, are not bound permanently to the polymer as iu an internally plasticized resia, but rather an exchange—equiHbrium mechanism is present. This implies that there is no stoichiometric relationship between polymer and plasticizer levels, although some quasi-stoichiometric relationships appear to exist (3,4). This idea is extended later ia the discussion of specific iateractions. 

Specific Interactions. Ideas oa the subject of specific iateractions between PVC and a plasticizer molecule, as a basis of plasticization, can be considered a more detailed form of some of the ideas already discussed. Clearly some mechanism of attraction and interaction between PVC and plasticizer must exist for the plasticizer to be retained in the polymer after processing. 

A range of plasticizer molecule models and a model for PVC have been generated and energy minimized to observe their most stable conformations. Such models highlight the free volume iacrease caused by the mobiHty of the plasticizer alkyl chains. More detailed models have also been produced to concentrate on the polar region of the plasticizer and its possible mode of interaction with the polymer. These show the expected repulsion between areas on the polymer and plasticizer of like charge as weU as attraction between the negative portions of the plasticizer and positive portions of the PVC. 

Plasticizer molecules can undergo thermal degradation at high temperatures. Esters based on the more branched alcohol isomers are more susceptible to such degradation. This can, however, be offset by the incorporation of an antioxidant, and plasticizer esters for cable appHcations frequently contain a small amount of an antioxidant such as bisphenol A. 

Overall Assessment of Properties. Table 3 shows the effect for each criterion of increasing plasticizer concentration, increasing the size of the plasticizer molecule, increasing the linearity of the plasticizer molecule, and changing the acid constituent of the ester. An I indicates improved performance for a particular property, a P indicates poorer performance. I and P in parentheses indicate that any changes tend to be marginal. 

AplP ratio. The Ap/ P ratio is calculated by dividing the numbers of carbon atoms in aliphatic chains in a plasticizer molecule by the number of ester groups present. The Ap/P ratio correlates well with several properties of the plasticizers such as melting point, density, modulus and water absorption. 

As reviewed thermoplastics (TPs) being viscoelastic materials respond to induced stress by two mechanisms viscous flow and elastic deformation. Viscous flow ultimately dissipates the applied mechanical energy as frictional heat and results in permanent material deformation. Elastic deformation stores the applied mechanical energy as completely recoverable material deformation. The extent to which one or the other of these mechanisms dominates the overall response of the material is determined by the temperature and by the duration and magnitude of the stress or strain. The higher the temperature, the most freedom of movement of the individual plastic molecules that comprise the... 

After fabrication an initial collapsing of such a bottle should occur as soon as possible (no later than one to two hours after manufacture the sooner the better). Additional pressure is needed for this first-time collapse in order to create permanent fold rings and completely orient the plastic molecules (Chapter 8, PROCESSING AND PROP-... 

The high shear rate produces two other effects that significantly affect product performance. The plastics molecules become... 

Plastic molecules that can be packed closer together can more easily form crystalline structures in which the molecules align themselves in some orderly pattern. During processing they tend to develop higher strength in the direction of the molecules. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs (normally up to 85% crystalline and the rest amorphous). In this book and as usually identified by the plastic industry, they are called crystalline. 

Crystallization The formation of crystallites or groups of plastic molecules in an ordered structure within the plastic as the plastic is cooled from its amorphous state to a temperature below its crystallization temperature. 

Orientation and mobility Orientation requires considerable mobility of large segments of the plastic molecules. It cannot occur below the glass transition temperature (Tg). The plastic temperature is taken just above Tg. 

Plasticization has been explained by a variety of theories in an attempt to explain how the plasticizer reduces the rigidity of the final part. All theories rely on the premise that the plasticizer reduces the strength of the intermolecular forces between the polymer chains. The theories fall into two broad categories interference mechanisms and expansion mechanisms. The interference mechanisms state that plasticizer molecules interact only weakly with the polymer chains after separating the chains from one another, thereby reducing the overall cohesion of the material. The expansion mechanisms state that the reduced rigidity arises from an increase in the free volume of the system as the system expands to incorporate bulky,... 

Figure 22.6 Schematic diagram showing how plasticizer molecules interbed between polyvinyl chloride chains...

Wetting out a pigment for several days by simply storing the manually prepared pigmented PVC paste (DOP content 39%) makes for almost optimum dispersion, which requires very little shear (see Fig. 88, p. 162). The wetting of the surface of the pigment particles by the plasticizer molecules thus determines the outcome of the dispersion process. 

External plasticizers are not permanent. Plasticizer molecules associate with one another eventually creating preferred migration routes to the material s surface where the plasticizer is rubbed or washed away. The preferential association of plasticizers also leaves some sites less flexible and creates variations in the material s stress-strain and expansion-contraction behaviors. 

Plasticization is the process in which the plasticizer molecules neutralize the secondary valence bonds, known as van der Waal s force between the polymer molecules. It increases the mobility of the polymer chains and reduces the crystallinity. These phenomena become evident in reduced modulus or stiffness, increased elongation and flexibility, and lowering of the brittle or softening temperature of the plasticized product. The effect of plasticizers on polymers is the subject of the first chapter by E. H. Immergut and H. F. Mark. 

However, it is also important to consider the intermolecular forces between the plasticizer molecules themselves and between plasticizer and polymer. Unless all these interactions-i.e., plasticizer-plasticizer, plasticizer-polymer, and polymer-polymer-are the same order of magnitude, there can be no plasticizing action. 

It is readily apparent from Figures 2, 3, and 4 that a plasticizer molecule will have much more difficulty in penetrating the crystalline regions, where there is a... 

In addition to size and molecular weight, one of the most important factors which determines plasticizer efficiency is the rate of diffusion of the plasticizer in the polymer matrix. In view of the dynamic solvation-desolvation between the plasticizer molecules and the polymer chains, the higher the diffusion rate, the greater the efficiency of the compound as a plasticizer. However, high diffusion rates are usually encountered with small molecules the smaller the plasticizer molecule, the greater its volatility and, therefore, the rate at which it is lost from the plasticized product. 

Permanence. The permanence of a plasticizer-i.e., its tendency to remain in the plasticized material, depends on the size of the plasticizer molecule and on its rate of diffusion in the polymer. The larger the plasticizer molecule, the lower its vapor pressure, or volatility and, therefore, the greater its permanence. This accounts for the popularity of certain polymeric plasticizers, such as polyesters, in spite of their relatively high price. Other factors, such as polarity and hydrogen bonding, will also, of course, affect the vapor pressure of the plasticizer. 

The rate of diffusion of the plasticizer molecules within the polymer matrix will also determine plasticizer permanence. Unfortunately, while a high rate of diffusion provides for greater plasticizer efficiency, it results in low plasticizer p ermanence. 

Effect of Plasticizer Shape. Extended plasticizer molecules such as aliphatic chains with a high degree of flexibility, usually lower Tg or Tc much more than bulky plasticizer molecules, such as those containing ring structures. However, Table XIII shows that the comparison between aliphatic and aromatic ester plasticizers is also determined by the length of the alcohol residues and by separation between the ester groups. 

Effect of Branching. A comparison of branched vs, linear plasticizer molecules usually shows that the linear molecules are more efficient in lowering Tg than the branched molecules. This effect is shown in Figure 15, where the open circles represent the branched phthalates and the full circles, the corresponding linear compounds. The branched esters give lower AT values in every case. 

Effect of Internal Mobility (Flexibility). It seems that the internal mobility or flexibility of the plasticizer molecule plays an important, if not the most important, role in determining plasticizer efficiency. This appears to be true irrespective of the polymer which is being plasticized, unless there are overriding physical factors involved, such as polymer crystallinity. In general, the lowering of T0 will be proportional to the temperature difference between (Tg)polymer and (Tg)plasticizer- This is illustrated in Table XIV. This table also shows that if the polymer itself is quite flexible, such as polychloroprene (Neoprene), the plasticizer efficiency is quite small, and may even result in negative AT values. 

It is worth noting that if we substitute Equation 6 into Equation 9, equating chain ends to plasticizer molecules, we obtain... 

Now the definition of 0 is generalized to include the free volume per plasticizer molecule. Equation 14a is equivalent to the one proposed by Fujita and Kishimoto (6) for the effect of plasticizer on T0 as shown in Table II. Originally was defined... 

From these measurements it follows that the solvent molecules are in a liquidlike state even below glass temperature. This indicates that the solvent molecules are mobile within the numerous cavities of the polymer as in a liquid. With decreasing temperature more and more of them are fixed against each other and against the chain segments, until finally all plasticizer molecules are immobilized. 

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