Introduction to Plasticizers for PVC
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In this article, we have taken the time to discuss PVC plasticizers, specifically DOP and DOTP, to understand why one might be used over the other.
To begin, it is important to understand that a PVC plasticizer is an additive that solvates this polymer, meaning it provides flexibility by altering its physical properties. The theory explains that the plasticizer positions itself between PVC particles to form a bridge that allows molecular mobility; the greater the amount of plasticizer, the higher the flexibility achieved.
This hardness is in turn controlled by a Shore A measurement scale (for flexible products) and a Shore D scale (for rigid products) using a durometer; the Shore A scale reaches a maximum of 95 points before we prefer to refer to a Shore D scale.
Due to their excellent polarity with the polyvinyl chloride (PVC) molecule, both DOTP and DOP can alter the softening temperature of PVC, which is normally around 86°C in a rigid product. As the amount of PVC plasticizer in a formulation is increased, the material transitions from rigid to semi-rigid, and eventually to fully flexible, significantly lowering the softening temperature of the PVC in question.
It is important to note that, as with any additive, PVC has a limit to the amount of plasticizer it can accept before issues such as bleeding or processing problems arise.
Similarly, to achieve excellent processing results with plasticized PVC, the key is its addition during a high-speed mixing process that generates significant shear, thereby increasing the temperature of the resin. This allows the resin particles to open up, like a sponge, enabling them to absorb the PVC plasticizers and other additives the formulator wishes to include.
Failure to follow a gradual addition sequence, controlling mixing temperatures and the order in which PVC additives are added, can cause agglomeration and encapsulation problems, which in turn will result in surface defects in the final product. The molecular weight of the material must also be taken into consideration. PVC resin used because high values are normally used that are directly related to the absorption capacity of the resin used.
The concept of solvation is very important when referring to polymer–plasticizer polarity. Depending on the type of PVC plasticizer used, a certain degree of compatibility is achieved, which determines the amount that must be added to a formulation to reach the desired hardness. For example, different quantities will be required to achieve a Shore A 65 hardness depending on the plasticizer used, whether DOP, DOTP, DINP, etc.
It is commonly believed that a PVC plasticizer only provides flexibility to the material, which is a misconception. PVC plasticizers can also affect chemical extraction resistance, low-temperature performance, electrical properties, lubrication during processing, and more. It is important to use the grade that best suits your final product and production process.
Plasticizer for PVC: DOTP vs DOP
DOP (dioctyl phthalate or DEHP) is classified in Europe as a substance of very high concern (SVHC under REACH) due to its reprotoxicity category 1B classification, and is listed under California Prop 65. Its use is restricted in toys under EN 71-3, CPSIA (United States), and NOM-015-SCFI (Mexico), as well as in food contact and medical devices. These regulatory frameworks have prompted the plastics industry to migrate toward technically equivalent alternatives without the aforementioned restrictions, with DOTP being the preferred option because it maintains very similar mechanical and process performance to DOP.
However, due to the industry's shift towards products with lower levels of toxicity, this is why DOTP is the preferred option for medical and food applications.
Although the plasticizers DOP and DOTP have the same molecular weight, there is a difference in the arrangement of the ester groups on the benzene ring: DOP has its two ester groups in the 'ortho' (1,2) position, characteristic of orthophthalic acid, while DOTP has them in the 'para' (1,4) position, characteristic of terephthalic acid. Therefore, DOTP is not classified as an orthophthalate under REACH or California Prop 65. This difference is why the industry has referred to DOTP as a terephthalate rather than a conventional phthalate.
From a commercial point of view, DOP has been preferred, of course, when it is possible to use it, due to its greater solvating power on the PVC and therefore faster absorption during the mixing process (better drying). Normally DOP is the cheapest PVC plasticizer , mainly because of the economies of scale achieved by this material, which result in a greater supply than the DOTP. In Plastisol the DOP is preferred because it does not generate as many pinhole defects due to its different viscosity and preference for releasing gases trapped in the mixture, although care must be taken regarding the market to which it is directed.
Among the advantages of DOTP, We found the previously discussed issue of toxicity, which allows it to comply with the substance restrictions of the EU RoHS Directive or California's Prop 65 list. Furthermore, the DOTP is a PVCplasticizer that provides excellent mechanical properties at low temperatures, which is an added value that many companies are willing to pay for.
In terms of color and appearance, both PVC plasticizers are transparent liquids that do not contain volatiles that could pose a concern to the user. They also have a very mild odor, which is generally pleasant for plant operators.
We hope you found this article helpful and enjoyed it. If you have any questions or comments, please raise it with your technical representative, on our social media or through our website. Visit our section on PVC plasticizers.
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Chemical Additives de México, S.A. de C.V.