PVC Plasticizers Introduction
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In this article we have taken the time to talk about plasticizers for PVC, specifically DOP and DOTP, to understand why choose one over the other.
To begin with, it is necessary to understand that the plasticizer serves as mechanism to solvate PVC, which means that it provides flexibility by altering its physical properties. The theory explains that the plasticizer is located between the PVC particles to form a bridge that allows mobility between its molecules. The greater the amount of plasticizer, the greater the flexibility we will obtain.
This hardness is measured in Shore A scale (for flexible products) and Shore D scale (for rigid products) by the use of a durometer. Shore A scale reaches a maximum of 95 points before preferring to refer to Shore D scale.
Due to its excellent polarity with Polyvinyl Chloride (PVC) molecule, both DOTP and DOP can alter the softening temperature of PVC, which is roughly 86°C in a rigid product. As we increase the amount of plasticizer for PVC in a formulation, we will go from a rigid material, to a semi-rigid, until ending up with a completely flexible material, where we will have considerably reduced the softening temperature of the PVC in question.
It is important to note that PVC has a limit of additive acceptance just as it happens with plasticizer before presenting bleeding or processing problems.
Similarly, to obtain an excellent process result with plasticized PVC, the key lies in its addition during mixing process with high-speed blades that generate high shear and thus increase the temperature of the resin used. This in turn will allow the PVC resin particle to open, resembling a sponge, in order to adsorb the plasticizer and additives that the formulator choose to add.
During mixing process, not following a gradual addition, controlling the mixing temperatures and order of addition of additives for PVC, may generate agglomerations that, in turn, will result in surface defects (lumps) in the final product. The molecular weight of the PVC resin used must also be taken into consideration, since high values are usually prefered for a greater absorption capacity.
The concept of solvation is very important when referring to the polarity of the polymer with the plasticizer. Depending on the type of plasticizer used for PVC, we will have certain degree of compatibility, which translates into the amount that we must add to a formula to achieve the desired hardness. For example, we will need different amounts of plasticizer to achieve 65 Shore A hardness depending on its type: DOP, DOTP, DINP, etc.
It is commonly believed that PVC plasticizers only serves to provide flexibility to the material, which is incorrect. PVC Plasticizers can also have effects on resistance to chemical attack extraction, good sub-zero temperature properties, electrical properties, lubrication properties during processing, etc. It is important to use the grade that best suits your final product and your production process.
PVC plasticizer: DOTP versus DOP
There is great controversy about the hazards that DOP plasticizer presumes and that is the reason producers are migrating towards the use of DOTP. Studies done to date have not really concluded DOP causing any disease in humans. Tests carried out on rodents at a higher dose and a long period of intravenous exposure, have shown some effect, but this hardly applies when a finished product is used by humans
However, and due to the movement of the industry for products with a lower degree of toxicity, DOTP is preferred when talking medical and/or food applications.
Although DOP and DOTP have the same molecular weight, there is a difference in the arrangement of their molecular chains with DOP having an "Ortho" position compared to DOTP having a "meta" position. This difference is why the industry has called DOTP a terephthalate instead of a conventional phthalate.
From the commercial point of view, DOP has been preferred, of course when it is possible to use it, due to its higher solvation with PVC and faster absorption during the mixing process (fastest drying). Normally, DOP tends to be the cheapest PVC plasticizer, mainly due to the economies of scale achieved by this material, which means that its offer is greater than DOTP. In Plastisol, DOP is preferred because it does not generate so many “pin holes” defects because it doesn’t affect viscosity, allowing volatile substances to leave the paste.
Among the advantages of DOTP we find its compliance with regulations such as RoHS or California's Prop 65 List because of its low toxicity. In addition, DOTP is a PVC plasticizer that provides excellent mechanical properties at low temperatures, for which it is an added value that many companies are willing to pay.
In color and appearance, both PVC plasticizers are transparent liquids that do not contain volatiles levels to be concerned of. They also confer a very mild odor.
We hope this article has been helpful. If you have questions or comments please do not hesitate to expose them to your technical agent, or through our social network. You can visit our PVC plasticizers section.
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