Plasticizers- Phthalic Acid Esters and Phthalic anhydride

Plasticizers, not only to Phthalic Acid Esters and Phthalic anhydride but a wide variety and kinds, are often added to a lot of engineering and modern polymers and general plastics to improve their process ability, improve their flow ability and injection molding characteristics and also to improve their elasticity or reduce brittleness in them. The form in which plastics are used before injection molding is short pellets, which seem to have been cut out from a long wire extruded shape. To actually mold these plastics, the injection molding machine and its operator need to perform a lot of tasks, such as mounting the mold on injection machine with the help of a crane, screwing the mold onto the machine and then clamping it securely, feeding the properly dried plastic material to the hopper unit. From there onwards setting the injection parameters into the screen, which range from the opening closing distance of the mold with differential high and low pressures, setting the screw velocities and screw displacements, setting of the heating units of the screw to proper temperatures and then molding parts.

Below is a picture of a cute kid playing with flexible plastic toy.


Anyhow, in today’s post, our main motive is to study about plasticizers, and not injection molding. So lets begin by discussing what are plasticizers and why are they added to plastics. The basic understanding about uses of plasticizing will provide us with a good base to understand Phthalic Acid Esters and Phthalic anhydride more clearly.
Plasticizers are organic substances of low volatility that are added to plastics compounds to improve their flexibility, extensibility, and  processability. They increase flow and thermoplasticity of plastic materials by decreasing the viscosity of polymer melts, the glass transition temperature (Tg) the melting temperature, and the elasticity modulus of finished products.

Plasticizers are particularly used for polymers that are in a glassy state at room temperature. These rigid polymers become flexible by strong interactions between plasticizer molecules and chain units, which lower their brittle-tough transition or brittleness temperature (Tb) (the temperature at which a sample breaks when struck) and their Tg value, and extend the temperature range for their rubbery or viscoelastic state behavior.


Side Note: In this regards, Preparation of aniline and itaconic anhydride may be something I will ike to discuss in my coming posts. Also, I will try to provide maximum information on synthesis of alcohols, synthesis of acetanilide and synthesis of alkenes. Plastic making is similar to production of acids and some chemicals in many regards. So to understand it completely, we also need to look more into methods such as oxidation of benzaldehyde, 4-bromotoluene, preparation of amides, preparation of aldehydes, synthesis of amines, phenylacetyl chloride and such similar processes which are in really high demand today.

Mutual miscibility between plasticizers and polymers is an important criterion from a practical point of view. If a polymer is soluble in a plasticizer at a high concentration of the polymer, the plasticizer is said to be a primary plasticizer. Primary plasticizers should gel the polymer rapidly in the normal processing temperature range and should not exude from the plasticized material. Secondary plasticizers, on the other hand, have lower gelation capacity and limited compatibility with the polymer. In this case, two phases are present after plasticization process—one phase where the polymer is only slightly plasticized, and one phase where it is completely plasticized. Polymers plasticized with secondary plasticizers do not, therefore, deform homogeneously when stressed as compared to primary plasticizers.
The deformation appears only in the plasticizer-rich phase and the mechanical properties of the system are poor. Unlike primary plasticizers, secondary plasticizers cannot be used alone and are usually employed in combination with a primary plasticizer.

Plasticizer properties are determined by their chemical structure because they are affected by the polarity and flexibility of molecules. The polarity and flexibility of plasticizer molecules determine their interaction with polymer segments. Plasticizers used in practice contain polar and nonpolar groups, and their ratio determines the miscibility of a plasticizer with a given polymer.
Plasticizers for PVC can be divided into two main groups according to their nonpolar part. The first group consists of plasticizers having polar groups attached to aromatic rings and is termed the polar aromatic group. Plasticizers such as phthalic acid esters and tricresyl phosphate belong to this group. An important characteristic of these substances is the presence of the polarizable aromatic ring. It has been suggested that they behave like dipolar molecules and form a link between chlorine atoms belonging to two polymer chains or to two segments of the same chain.
Plasticizers belonging to this group are introduced easily into the polymer matrix. They are characterized by ability to produce gelation rapidly and have a temperature of polymerplasticizer miscibility that is low enough for practical use. These plasticizers are therefore called solvent-type plasticizers, and their kerosene extraction (bleeding) index is very low. They are, however, not recommended for cold-resistant materials.

The picture below shows the percentage use of various plasticizers world-wide


The second group consists of plasticizers having polar groups attached to aliphatic chains and is called the polar aliphatic group. Examples are aliphatic alcohols and acid or alkyl esters of phosphoric acid (such as trioctyl phosphate). Their polar groups interact with polar sites on polymer molecules, but since their aliphatic part is rather bulky and flexible other polar sites on the polymer chain may be screened by plasticizer molecules. This reduces the extent of intermolecular interactions between neighboring polymer chains.
Polar aliphatic plasticizers mix less well with polymers than do polar aromatics and, consequently, may exude (bloom) from the plasticized polymer more easily. Their polymer miscibility temperature is higher than that for the first group. These plasticizers are called oil-type plasticizers, and their kerosene extraction index is high. Their plasticization action is, however, more pronounced than that of polar aromatic plasticizers at the same molar concentration. Moreover, since the aliphatic portions of the molecules retain their flexibility over a large temperature range, these plasticizers give a better elasticity to finished products at low temperature, as compared to polar aromatic plasticizers, and allow the production of better cold-resistant materials. In PVC they also cause less coloration under heat exposure.
In practice plasticizers usually belong to an intermediate group. Mixtures of solvents belonging to the two groups discussed above are used as plasticizers to meet the requirements for applications of the plasticized material.
Plasticizers can also be divided into groups according to their chemical structure to highlight their special characteristics. Several important plasticizers in each group (with their standard abbreviations) are cited below.

Phthalic Acid Esters and Phthalic anhydride

Di(2-ethyl hexyl) phthalate (DOP) and diisooctyl phthalate (DIOP) are largely used for PVC and copolymers of vinyl chloride and vinyl acetate as they have an affinity to these polymers, produce good solvation, and maintain good flexibility of finished products at low temperature. The use of n-octyl-ndecyl phthalate in the production of plastics materials also allows good flexibility and ductility at low temperature. Diisodecyl phthalate (DDP), octyl decyl phthalate (ODP), and dicapryl phthalate (DCP) have a lower solvency and are therefore used in stable PVC pastes. Butyl octyl phthalate (BOP), butyl decyl phthalate (BDP, and butyl benzyl phthalate (BBP) have a good solvency and are used to adjust melt viscosity and fusion time in the production of high-quality foams. They are highly valued for use in expandable plasticized PVC.
Dibutyl phthalate (DBP) is not convenient for PVC plasticization because of its relatively high volatility. It is a good gelling agent for PVC and vinyl chloride-vinyl acetate copolymer (PVCA) and so is sometimes used as a secondary plasticizer in plasticizer mixers to improve solvation. DBP is mainly used for cellulose-based varnishes and for adhesives. It has a high dissolving capacity for cellulose nitrate (CN).
Dimethyl phthalate (DMP) also has high dissolving capacity for CN. It has good compatibility with cellulose esters and are used in celluloid made from CN and plastic compounds or films made from other cellulosic polymers, cellulose acetate (CA), cellulose acetate-butyrate (CAB), cellulose acetate-propionate (CAP), and cellulose propionate (CP). It is light stable but highly volatile. Diethyl phthalate (DEP) possesses properties similar to DMP and is slightly less volatile.

So that it all that is to Phthalic Acid Esters and Phthalic anhydride, which are widely used as Plasticizers for various high grade plastics. Of course, I had to prepare a lot of information about these in general so you can understand these two better.

I know I am not a master in this field as my primary field of expertise is Injection Molding Troubleshooting. So if you find any mistakes, then point them out and leave me comments below, I will correct them so other readers do not face same problems as you did. Remember to bookmark Molding Plastic Components  and keep checking periodically for more updates on Plastic Injection Mold Design.


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