Thanks to Mr. YASHODHAN KANADE for his studies on this topic.
We want to share with you this analysis:
Plasticizer is a substance, liquid or solid, incorporated in a polymer to increase its flexibility, workability or distensibility.
A plasticizer reduces melt viscosity during processing.
In case of PVC products glass transition temperature (Tg) & elastic modulus also reduces.
Liquids are usually termed as “plasticizers” where as solids like nitrile rubbers, compatible polyesters and some polyacrylates are termed as “plastifiers or Flexibilizers”.
Soft or flexible PVC products like extruded soft tubing, profiles & sheeting, calendared thin sheets etc make use of suspension grade PVC.
Since plasticizers are dissolved in S PVC (PVC is not dissolved in plasticizers) during mixing process, higher the porosity in PVC grain, higher is the extent of plasticizers absorption. Hence more the plasticizer requirement higher is the K Value of resin used, as porosity increases with K value. Thus for soft PVC K-67 and for very soft PVC products K-70 or K-73 suspension PVC resins are used.
On the other hand for plastisols and organosols, emulsion and micro suspension grade PVC is used. Slurry of PVC in plasticizers along with additives is used in processes like rotational casting and coating to produce inflatable products and vinyl rexine or 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 that it is not as powerful as a solvator to destroy the crystallinity in PVC. It also adds free volume, contribute shielding effect and provide lubricity. The balance between polar and non-polar portions of the plasticizer molecule is critical to control the solubilizing effect.
If plasticizer is too polar, it can destroy crystallites, on the other hand, too non-polar plasticizer will cause compatibility problem in PVC.
An ideal plasticizer should be:
- Cost effective, Stable, Low in colour
- Compatible with PVC
- Readily dispersible in PVC
- Low in volatility
- Low in odour
- Low in toxicity
- Have good permanence
- Must not interact unfavorably with other additives
- Provide other specific properties required in the product like weatherability.
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, not to be readily squeezed out of plasticized PVC by moderate pressure. They are mainly responsible for providing flexibility. Examples are – Monomeric, polymeric, epoxy, specialty flame retardants 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 along with the primary plasticizer to certain extent. This reduces the cost. Examples are – Chlorinated paraffin oils having various extent of chlorination.
Sub Classification of plasticizers
1. General purpose plasticizer [GP] - They provide desired flexibility to PVC, balancing overall quality with low cost. Examples are DIHP, DOP (DEHP), DINP and DIDP. They can be used along with secondary plasticizers to reduce cost. DIDP is 76% less evaporating than DOP.
DIDP is usually used in washing machine pipes, as it offers resistance to soap water.
2. Performance plasticizer [PP] – They contribute to secondary desired properties beyond GP type, but are somewhat costly. Performance criteria includes :
- PP – SS –strong solvators have higher polarity / aromacity such as DBP, DHP, BHP and BBP. They are low molecular weight & volatile liquids. Due to high polarity, they destroy PVC crystallinity.
- BBP is recommended for non staining applications like vinyl flooring.
- Benzoates are non phthalate plasticizers. Examples are Polypropylene Glycol Di Benzoate (PGDB), DPGDB. They are petrol resistant and find application in petrol tubing.
- PP – LT - low temperature plasticizers such as Di octyl adipates (DOA), DOS & DOZ are less solvating and have higher diffusivity. They are aliphatic dibasic esters. They are used in fridge/deep fridge gaskets.
- PP – LV - low volatility plasticizers are high mole weight plasticizers such as Tri Octyl Tri Mellitate (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 high molecular weight and have highly branched isomeric structure. Like Di iso decyl phthalate (DIDP), DTDP etc.
- Polyester plasticizers are noted for its outstanding performance in this category.
- SP – ST – plasticizers with stabilizing function, Epoxidized Soybean oil (ESBO) and linseed oil (ELSO) improves thermal and UV stability. They are usually added with primary stabilizer.
- SP – FR - fire resistant stabilizers are tricresyl phosphate [TCP], tri alkyl 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 application
- 1,2 Cyclohexane dicarboxylic acid diisononyl ester (DINCH) used for food application.
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.