Role of different plasticizers in PVC products

Understanding Plasticizers in PVC

We would like to share this analysis with you:
A plasticizer is a substance, liquid or solid, incorporated in a polymer to increase its quality, such as flexibility, workability or distensibility. A plasticizer reduces melt viscosity during processing.
In PVC products, the glass transition temperature (Tg) and elastic modulus of PVC are also reduced.
Liquids are usually called “plasticizers”, whereas solids like nitrile rubbers, compatible polyesters and some polyacrylates are called plastifiers or flexibilizers.
Soft or flexible PVC products like extruded soft tubing, profiles and sheeting, thin calendared sheets, etc. make use of suspension-grade PVC. Since plasticizers are dissolved in SPVC (PVC is not dissolved in plasticizers) during the mixing process, the higher the porosity of PVC grain, the higher the rate of plasticizer absorption. Thus, the more the plasticizer requirement, the higher the K value of resin used, as porosity increases with the K value. Therefore, K-67 Suspension PVC resins are used for soft PVC products and K-70 or K-73 suspension PVC resins are used for very soft PVC products.
 
On the other hand, for plastisols and organosols, emulsion and micro-suspension grade PVC is used. PVC slurry in plasticizers and additives are used in processes like rotational casting and coating to produce inflatable products and rexine or in artificial leather manufacturing.
 

What is the chemical nature of plasticizers?

An effective plasticizer molecule for PVC has two types of structural components:

• Polar component
• Non-polar component.

The polar component of the molecule must be able to bind reversibly [not like vulcanized rubber] with the PVC polymer, thus softening the PVC. The non-polar component of the molecule allows the PVC interaction to be controlled, so it is not as powerful as a solvator to destroy the crystallinity in PVC. It also adds free volume, aids the shielding effect and provides lubricity. The balance between polar and non-polar portions of the plasticizer molecule is critical to control the solubilizing effect.
 
If a plasticizer is too polar, it can destroy crystallites; however, a too non-polar plasticizer will cause compatibility problems in PVC.
An ideal plasticizer should be:

• Cost-effective, stable, light in color
• Compatible with PVC
• Readily dispersible in PVC
• Low in volatility
• Low in odor
• Low in toxicity
• Have good permanence
• Must not interact unfavorably with other additives
• Have other specific properties required by the product like weather resistance

Plasticizers are broadly classified as primary and secondary plasticizers:

Primary plasticizers are low-volatility liquids, whose polarity is such that they are sufficiently compatible with PVC and will not be readily squeezed out of plasticized PVC by moderate pressure. They are mainly responsible for providing flexibility. Examples include: monomeric, polymeric, epoxy, specialty flame-retardant plasticizers, etc.
Secondary plasticizers are also low-volatility liquids, whose compatibility is such that it will exude if used alone. However, it can be used with the primary plasticizer to certain extent. This reduces the cost. Examples include: chlorinated paraffin oils with various degrees of chlorination.
 

Sub-classification of plasticizers

1. General purpose plasticizer [GP] - They provide the desired flexibility to PVC, overall balancing quality with a low cost. Examples are DIHP, DOP (DEHP), DINP and DIDP. They can be used along with secondary plasticizers to reduce the cost. DIDP evaporates 76% less than DOP.
DIDP is usually used in washing machine pipes, as it offers resistance to soap water.

2. Performance plasticizer [PP] – They offer secondary desired properties beyond GP type but are somewhat costly.
Performance criteria include:
PP-SS – strong solvators have higher polarity/aromaticity such as DBP, DHP, BHP and BBP. They have a low molecular weight and are volatile liquids. Due to their high polarity, they destroy PVC crystallinity.
BBP is recommended for non-staining applications like vinyl flooring.
Benzoates are non-phthalate plasticizers for PVC. Examples are Polypropylene glycol dibenzoate (PGDB), DPGDB. They are petrol resistant and are used in petrol tubing.
PP-LT – low-temperature plasticizers such as Dioctyl adipates (DOA), DOS and DOZ are less solvating and have higher diffusion. They are aliphatic dibasic esters. They are used in fridge/deep freezer gaskets.
PP-LV – low-volatility plasticizers are high mole-weight plasticizers such as Trioctyl trimellitate (TOTM), TIOTM and polyesters [polymeric]. They are used in cables and wires.

3. Specialty plasticizers [SP] – They have specialty characteristics like:
SP-LD – low-diffusion plasticizers. They have a high molecular weight and have a highly branched isomeric structure. Like Diisodecyl phthalate (DIDP), DTDP, etc.
Polyester plasticizers are noted for their outstanding performance in this category.
SP- ST – plasticizers with a stabilizing function, Epoxidized soybean oil (ESBO) and linseed oil (ELSO) improve thermal and UV stability. They are usually added with the primary stabilizer.
SP-FR – fire-resistant stabilizers are tricresyl phosphate (TCP), trialkyl phosphate (TAP), etc. They are generally used in FR cables. 

4. Non-phthalate food-grade plasticizers Acetyl tributyl citrate plasticizers (ATBC) used for food and medical applications.
1,2 Cyclohexane dicarboxylic acid diisononyl ester (DINCH) used for food applications.

5. Plasticizers for wires and cables as per temperature rating
60 deg C building application - DIOP, DOP, DINP, DIDP
75 deg C cord and fixture - DINP, DIDP
80 deg C appliance - DINP / DIDP blends
90 deg C appliance - DUDP, DTDP, TOTM
105 deg C appliance - TIOTM, TOTM, polyesters
Computer wires - TIOTM, TOTM, polyesters

All plasticizers for wires and cables contain appropriate antioxidants.


Thanks to Mr. YASHODHAN KANADE for his studies on this topic.