Plasticization lubricating theory

The effect of plasticizers has been explained by the lubricity, gel, and free volume theories. The lubricity theory states that the plasticizer acts as an internal lubricant and permits the... 

The viscous dissipation term is normally not important. Its significance has been considered in connection with lubrication theory (VI), flow through tubes (B20), extrusion of plastics melts (BIO), and viscometry in rotating-cylinder systems (W6). There is also an additional contribution to the energy flux vector describing energy transport by radiation. See discussion in connection with Eq. (29). 

By 1950 two theories had evolved to account for the major "flexibilizing" effect of plasticization the "lubricity" theory and the "gel" theory ( ). In addition, the "free volume" theory devised to explain fluid flow was also being adapted to plasticization (10). 

The lubricity theory explains the resistance of a polymer to deformation. Stiffness and rigidity are explained as the resistance of intermolecular friction. The plasticizer acts as a lubricant to facilitate movement of macromolecules over each other, thus giving the resin an internal lubricity. The gel theory is applied to predominantly amorphous polymers. It proposes that their rigidity and resistance to flex are due to an internal three-dimensional honeycomb structure or gel. The spatial dimensions of the cell in a brittle resin are small because their centers of attraction are closely spaced and deformation cannot be accommodated by internal movement in the cell-locked mass. Thus, the elasticity limit is low. Conversely, a thermoplastic or thermosetting polymer with widely separated points of attachment between its raacroraolecules is flexible without plasticization. 

Figures 2.29-2.31 show the mechanical performance of plasticized PVC containing 50 phr plasticizers of different lengths of their alcohol parts. The data come from technical literature available from the manufacturers literature. Figure 2.29 shows that Shore A linearly increases with the length of carbon atom chain in alcohol part of plasticizer. This data are in agreement with findings presented in Figure 10.2 for sebacates which shows that tensile strength decreases when number of carbon atoms in alcohol part of plasticizer increases. These data also follow simple models of plasticization mechanisms such as gel and lubricity theories but do not seem to account for compatibility between the plasticizer...

The first two theories which try to explain the plasticization of polymers were the gel theory and the lubricity theory. Both were developed simultaneorrsly and sometimes the authors to whom the theories are attributed were not aware of the differences between both... 

According to the lubricity theory, the function of a plasticizer is to reduce intermo-lecular friction between the polymer molecules. This theory has been presented in different ways, but the idea persisting in all of them is that when a plastic part is flexed, the polymer molecirles have to slip over each other. The plasticizer acts by lubricating the movement of the molecules and reducing their internal resistance to sliding. 

Some results, which were considered supporting the lubricity theory, were also found Verhoek, who measured vapor pressures of plasticizers in polymers, found that plasticizers reduce solvent retention, which was considered to support these theories (plasticizers fill voids and so they exclude solvents). Rententivity and permeability of cellulose acetate plasticized with different sulfonamides, phthalates and phosphates, were measured by Kirkpatrick who concluded that these properties depended on the attractive forces resulting between the plasticizer and the polymer. Houwink measured swelling of some butadiene-acrylonitrile copolymers with different acrylonitrile content when these were mixed with solvents of different polarities, and proved the importance of the polarity of the plasticizer and the polymer. Moreover, Houwink measured the brittle point of ethyl... 

The Lubrication Theory. The lubrication theory is based on the assiunp-tion that the rigidity of the resin arises from intermolecular friction binding the chains together in a rigid network. On heating, these firictional forces are weakened to allow the plasticizer molecules to lubricate the chains. Once incorporated into the polymer, the plasticizer molecules shield the chains from each other, thus preventing the reformation of the rigid network (1). 

Th0 GgI Theory. This theory extends the lubrication theory by having the plasticizer break the resin-resin attachments of a three-dimensional honeycomb or gel structure and by masking these centers of attachment from each other, preventing their reformation. This gel is formed by loose attachments occinring at intervals along the polymer chain. This facilitates the movement of plasticizer molecules, thus imparting flexibihty. 

Several theories have been developed to account for the observed characteristics of the plasticization process Daniels has recently published a review of plasticization mechanisms and theories [8]. Although most mechanistic studies of plasticization have focused on PVC, much of this information can be adapted to other polymer systems. The lubricating theory of plasticization holds that plasticizers act as molecular lubricants to facilitate polymer chain movement when a force is applied to the plastic. It starts with the assumption that the unplasticized polymer chains do not move freely because of surface irregularities and van der Waals attractive forces. As the system is heated and mixed, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions. Portions of the plasticizer molecule are strongly attracted to the polymer while other parts of the plasticizer molecule can shield the polymer chain and act as a lubricant. This reduction in intermolecular or van der Waals forces among the polymer chains increases the flexibility, softness, and elongation of the polymer. 

This discussion refers to external plasticization only. Several theories, varyiag ia detail and complexity, have been proposed ia order to explain plasticizer action. Some theories iavolve detailed analysis of polarity, solubiHty, and iateraction parameters and the thermodynamics of polymer behavior, whereas others treat plasticization as a simple lubrication of chains of polymer from each other, analogous to the lubrication of metal parts by oil. Although each theory is not exhaustive, an understanding of the plasticization process can be gained by combining ideas from each theory, and an overall theory of plasticization must include all these aspects. 

The Free Volume Theory. This extends the lubricity and gel theories and also allows a quantitative assessment of the plasticization... 

The free volume theory originated some years later than the lubricity and the gel theories, when the evolution of different properties of polymers as a function of temperature, specific volume, thermal expansion coefficients, or viscosity was attempted to be explained.The relationships between these properties and some variables corresponding to polymer stracture, such as molecular weight or terminal groups content, the presence of another monomer and, of course, the presence of plasticizers, was also explained. For plasticized polymers the theory attempted to explain the diminution of the glass transition temperature with the plasticizer content. This theory is a contribution of different authors, but it was postulated by Fox and Floiy. The theory is still being used to explain some properties of plasticized polymers, i.e., viscoelastic properties. ... 

A plasticiser is a material incorporated in a plastic to increase its workability and flexibility or distensibility. The melt viscosity, elastic modulus and Tg of a plastic are lowered by a plasticiser addition. There are several theories to explain plasticiser effects such as the lubricity, gel, and free volume. Plasticisers are essentially nonvolatile solvents and therefore, polymer and plasticiser compatibility is very important and the solubility parameter difference (A8) should be less than 1.8. When present in small amounts plasticisers generally act as antiplasticisers, (i.e., they increase the hardness and decrease the elongation of polymers). Figure 6.12 illustrates the effect of plasticiser on modulus. Increasing concentration of the plasticiser shifts the transition from the high modulus (glassy) plateau region to the low, i.e., to occur at lower temperature [9]. 

The Free Volume Theory. The Free Volume Theory is a further extension of the lubricity and gel theories and can be used to explain both external and internal plasticization. Free volume is a measure of the internal space available in a polymer for the movement of the polymer chain, which imparts flexibility to the resin. Plasticizers increase the free volume of the resin and ensure that free volume is maintained as the resin-plasticizer mixture is cooled from the melt, preventing interactions between neighboring polymer chains. For the plasticized resins, free volume can arise from motion of the chain ends, side chains, or the main chain. The fact that free volume increases with molecular motion is useful in explaining internal plasticization achieved by side-chain addition, where each side chain acts as a small molecule and free volume of the system is increased. 

The actual reduction in Tg per weight of plasticizer added is called the plasticizer efficiency (in some cases the amount of added plasticizer can be as high as 70%). Compatibility or miscibility of the plasticizer with the polymer is a requirement. Theories of the mode of action include lubrication of the chains of PVC, inhibiting interchain attraction or increasing the free volume of the polymer. 

The idea of a weak boundary layer is a theory of non-adhesion rather than one of adhesion. However, weak boundary layers certainly occur on some substrates. Examples include lubricating oil on metals and rust on iron, but a less obvious case lies in the tendency of foreign material in plastics to diffuse to the surface such materials might be the various additives discussed in Section 2.4 or simply low molar mass polymer. In either case the remedy is to remove the weak boundary layer with a solvent or by abrasion. 

Plasticization occurs as a result of the action of plasticizer on PVC. The physical properties of PVC gradually changes from a rigid solid to a soft gel or viscous liquid. The action of plasticizer on PVC is a complex phenomena and involves many interactions. Four principal theories have been proposed to account for the main effects plasticizers produce. These are lubricity, gel, mechanistic and free volume theories (22). For more detailed theories the reader is referred to several textbooks which cover the subject in more detail (23-25). Plasticization occurs in a number of steps. The steps of plasticization may not occur for every polymer-plasticizer combination but various steps of plasticization for two main PVC applications, namely plastisol and dry blending are shown in Figure 1. 

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