Mechanisms of Plasticization

For a plasticizer to be effective, it must be thoroughly mixed and incorporated into the PVC polymer matrix. This is typically obtained by heating and mixing until either the resin dissolves in the plasticizer or the plasticizer dissolves in the resin. The plasticized material is then molded or shaped into the useful product and cooled. Different plasticizers will exhibit different characteristics in both the ease with which they form the plasticized material and in the resulting mechanical and physical properties of the flexible product.

Several theories have been developed to account for the observed characteristics of the plasticization process. A significant review of the theoretical treatment of plasticization is described by Sears and Darby [6]. In this treatment, plasticization is described by three primary theories, with some modifications.

According to the Lubricating Theory of plasticization, as the system is heated, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions (van der Waals’ forces). Here, the plasticizer molecules act as shields to reduce polymer-polymer interactive forces and prevent the formation of a rigid network. This lowers the PVC Tg and allows the polymer chains to move rapidly, resulting in increased flexibility, softness, and elongation.

The Gel Theory considers the plasticized polymer to be neither solid nor liquid but an intermediate state, loosely held together by a three-dimensional network of weak secondary bonding forces. These bonding forces acting between plasticizer and polymer are easily overcome by applied external stresses allowing the plasticized polymer to flex, elongate, or compress.

Free Volume is a measure of the internal space available within a polymer. As free volume is increased, more space or free volume is provided for molecular or polymer chain movement. A polymer in the glassy state has its molecules packed closely but is not perfectly packed.

 

 

The free volume is low and the molecules cannot move past each other very easily. This makes the polymer rigid and hard. When the polymer is heated to above the glass transition temperature, Tg, the thermal energy and molecular vibrations create additional free volume which allows the polymer molecules to move past each other rapidly. This has the effect of making the polymer system more flexible and rubbery. Free volume can be increased through modifying the polymer backbone, such as by adding more side chains or end groups. When small molecules such as plasticizers are added, this also lowers the Tg by separating the PVC molecules, adding free volume and making the PVC soft and rubbery. Molecules of PVC can then rapidly move past each other. If the plasticizer uniformly went into the PVC, it would behave similarly to an uncured rubber, with lots of creep and high compression set. For example, uncured tires do not hold their shape; they require a crosslinking cure to give them dimensional stability. Likewise, a thermoplastic elastomer such as PVC requires physical crosslinks which are meltable to make them thermoplastic. These meltable crosslinks are the PVC crystallites which give PVC a physical cure. Therefore, the plasticizer must not be a powerful solvent for all the PVC parts, but must be selective in enterring the amorphous PVC part and must not enter and destroy the crystalline part of PVC.

The mechanistic explanation of plasticization considers the interactions of the plasticizer with the PVC resin macromolecules. It assumes that the plasticizer molecules are not permanently bound to the PVC resin molecules but are free to self-associate and to associate with the polymer molecules at certain sites such as amorphous sites. As these interactions are weak, there is a dynamic exchange process whereby, as one plasticizer molecule becomes attached at a site or center, it is readily dislodged and replaced by another. Different plasticizers yield different plasticization effects because of the differences in the strengths of the plasticizer- polymer and plasticizer-plasticizer interactions. At low plasticizer levels, the plasticizer-PVC interactions are the dominant interactions, while at high plasticizer concentrations plasticizer- plasticizer interactions can become more significant. This can explain the observation of “anti plasticization”, wherein low plasticizer levels (< 15 phr) increase rigidity in PVC, as measured by modulus, tensile strength, elongation and low temperature properties.

For a plasticizer to be effective and useful in PVC, it must contain two types of structural components, polar and apolar. The polar portion of the molecule must be able to bind reversibly with the PVC polymer, thus softening the PVC, while the non-polar portion of the molecule allows the PVC interaction to be controlled so it is not so powerful a solvator as to destroy the PVC crystallinity. It also adds free volume, contributes shielding effects, and provides lubricity. Examples of polar components would be the carbonyl group of carboxylic ester functionality or, to a lesser extent, an aromatic ring; the non-polar portion could be the aliphatic side chain of an ester. The balance between the polar and non-polar portions of the molecule is critical to control its solubilizing effect; if a plasticizer is too polar, it can destroy PVC crystallites; if it is too non-polar, compatibility problems can arise. Useful tools in estimating plasticizer compatibility are the Apolar/Polar Ratio method developed by Van Veersen and Meulenberg [7] and the solubility parameter methods [8–11].

 

 

There is free plasticiser available not interacting with PVC, high chances are the paste PVC are clustered together ,normal particle size for paste resin is 5 micron to 25 micron, if most of the 5 micron cluster together then the surface area of PVC matrix is drastically reduced.

For a plasticizer to be effective, it must be thoroughly mixed and incorporated into the PVC polymer matrix. This is typically obtained by heating and mixing until either the resin dissolves in the plasticizer or the plasticizer dissolves in the resin. The plasticized material is then molded or shaped into the useful product and cooled. Different plasticizers will exhibit different characteristics in both the ease with which they form the plasticized material and in the resulting mechanical and physical properties of the flexible product.

Several theories have been developed to account for the observed characteristics of the plasticization process. A significant review of the theoretical treatment of plasticization is described by Sears and Darby [6]. In this treatment, plasticization is described by three primary theories, with some modifications.

According to the Lubricating Theory of plasticization, as the system is heated, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions (van der Waals’ forces). Here, the plasticizer molecules act as shields to reduce polymer-polymer interactive forces and prevent the formation of a rigid network. This lowers the PVC Tg and allows the polymer chains to move rapidly, resulting in increased flexibility, softness, and elongation.

The Gel Theory considers the plasticized polymer to be neither solid nor liquid but an intermediate state, loosely held together by a three-dimensional network of weak secondary bonding forces. These bonding forces acting between plasticizer and polymer are easily overcome by applied external stresses allowing the plasticized polymer to flex, elongate, or compress.

Free Volume is a measure of the internal space available within a polymer. As free volume is increased, more space or free volume is provided for molecular or polymer chain movement. A polymer in the glassy state has its molecules packed closely but is not perfectly packed.

 

 

The free volume is low and the molecules cannot move past each other very easily. This makes the polymer rigid and hard. When the polymer is heated to above the glass transition temperature, Tg, the thermal energy and molecular vibrations create additional free volume which allows the polymer molecules to move past each other rapidly. This has the effect of making the polymer system more flexible and rubbery. Free volume can be increased through modifying the polymer backbone, such as by adding more side chains or end groups. When small molecules such as plasticizers are added, this also lowers the Tg by separating the PVC molecules, adding free volume and making the PVC soft and rubbery. Molecules of PVC can then rapidly move past each other. If the plasticizer uniformly went into the PVC, it would behave similarly to an uncured rubber, with lots of creep and high compression set. For example, uncured tires do not hold their shape; they require a crosslinking cure to give them dimensional stability. Likewise, a thermoplastic elastomer such as PVC requires physical crosslinks which are meltable to make them thermoplastic. These meltable crosslinks are the PVC crystallites which give PVC a physical cure. Therefore, the plasticizer must not be a powerful solvent for all the PVC parts, but must be selective in enterring the amorphous PVC part and must not enter and destroy the crystalline part of PVC.

The mechanistic explanation of plasticization considers the interactions of the plasticizer with the PVC resin macromolecules. It assumes that the plasticizer molecules are not permanently bound to the PVC resin molecules but are free to self-associate and to associate with the polymer molecules at certain sites such as amorphous sites. As these interactions are weak, there is a dynamic exchange process whereby, as one plasticizer molecule becomes attached at a site or center, it is readily dislodged and replaced by another. Different plasticizers yield different plasticization effects because of the differences in the strengths of the plasticizer- polymer and plasticizer-plasticizer interactions. At low plasticizer levels, the plasticizer-PVC interactions are the dominant interactions, while at high plasticizer concentrations plasticizer- plasticizer interactions can become more significant. This can explain the observation of “anti plasticization”, wherein low plasticizer levels (< 15 phr) increase rigidity in PVC, as measured by modulus, tensile strength, elongation and low temperature properties.

For a plasticizer to be effective and useful in PVC, it must contain two types of structural components, polar and apolar. The polar portion of the molecule must be able to bind reversibly with the PVC polymer, thus softening the PVC, while the non-polar portion of the molecule allows the PVC interaction to be controlled so it is not so powerful a solvator as to destroy the PVC crystallinity. It also adds free volume, contributes shielding effects, and provides lubricity. Examples of polar components would be the carbonyl group of carboxylic ester functionality or, to a lesser extent, an aromatic ring; the non-polar portion could be the aliphatic side chain of an ester. The balance between the polar and non-polar portions of the molecule is critical to control its solubilizing effect; if a plasticizer is too polar, it can destroy PVC crystallites; if it is too non-polar, compatibility problems can arise. Useful tools in estimating plasticizer compatibility are the Apolar/Polar Ratio method developed by Van Veersen and Meulenberg [7] and the solubility parameter methods [8–11].

The issue faced yesterday was low viscosity, what cause low viscosity ?

 

There is free plasticiser available not interacting with PVC, high chances are the paste PVC are clustered together ,normal particle size for paste resin is 5 micron to 25 micron, if most of the 5 micron cluster together then the surface area of PVC matrix is drastically reduced.