A typical PVC compound is made up of multiple major and minor components. The major components include PVC resin, a primary and a secondary plasticizer. Minor components are typically used in small quantities to effect processability and quality of the end product. These include stabilizer, internal and external lubricant, and process aid to help manage PVC’s inherent low thermal stability and ensure that the polymer does not rapidly degrade at the shear rates and temperatures required for melt processing.
Primary plasticizers are used to impart flexibility and control the durometer of the base PVC polymer. While there is no covalent bond between PVC and the plasticizer, they are very compatible and become an integral part of the polymer matrix. Depending on the Shore A hardness required of the compound, the plasticizer content can range from 15 to 50% of the overall formulation.
There are numerous different plasticizer chemistries used commercially. Phthalate esters, particularly dialkyl phthalates have dominated the plasticizer market for almost 100 years. The dialkyl phthalate plasticizers are esters of ortho-phthalic acid with two adjacent ring substitutions. They are manufactured from phthalic anhydride via an esterification process with selected alcohols. The three prominent PVC plasticizers are DEHP, DINP and DIDP and they are all dialkyl ortho-phthalates.
Over the last 60 years, the most common plasticizer used to soften PVC compounds used for medical devices has been DEHP Di-2 ethyl hexyl phthalate. DEHP is economical and offers excellent performance (e.g., compatibility with PVC, plasticizer efficiency, low-temperature properties and low volatility). Also, the leaching of DEHP into blood has a stabilising effect on red cell membranes, reduces haemolysis and prolonging shelf life. This property that has led to the wide application of PVC in single use, pre-sterilised and disposable blood-contacting devices. Lastly, until 2018, DEHP was the only plasticizer mentioned in any European Pharmacopeia Monograph.
However, on the negative side DEHP has been battling detractors for decades and is probably one of the most studied substance in the medical device industry. As a result, there has been significant interest in non-DEHP alternatives for medical-grade PVC compounds.
Primary alternatives include monomeric plasticizer (all esters) manufactured by the reaction of selected acids with selected alcohols. Due to the polarity of the ester groups these plasticizers have a high level of compatibility with the PVC base polymer. These include:
- ATBC (Acetyl tri-butyl citrate) is an ester of citric acid, a natural occurring product in citrus fruits & the alcohol n-butanol
- DEHT(Di (2-ethylhexyl) terephthalate also commonly called dioctyl terephthalate and abbreviated as DOTP is a diester of terephthalic acid and 2-ethylhexanol.
- DINCH – 1,2-cyclohexane dicarboxylic acid diisononyl ester is manufactured by the catalytic hydrogenation of diisononyl phthalate (DINP) a phthalate plasticizer. DINP is manufactured by the esterification of phthalic acid & Isononyl alcohol
- DOA, dioctyl adipate also called DEHA di(2-ethylhexyl) adipate is a diester of adipic acid and 2-ethylhexanol.
- TOTM trioctyl trimellitate is a triester of trimellitic acid with 2-ethyl hexanol
Due to differences in chemistry and molecular weight subtle differences in compatibility with PVC exist. For example, molecular weight of the plasticizer alternatives vary form from a low of 371 g/mol for DOA to a high of 547 g/mol for TOTM.
In addition to molecular weight, it is known that extraction rates for the plasticizer are also dependent on nature of extractant, surface area in contact with extractant, temperature, flow rate, and contact period. Although the aqueous solubility of all the plasticizers are low, because they are not covalently bound in the PVC matrix, they can migrate into the contacting medium.
Our studies of extraction rate in saline solutions of the various plasticizers at 50°C for Shore A 65 PVC compounds over a 24hr period identified slight differences in extraction rates exist. TOTM, the highest molecular weight plasticizer, resisted extraction the most. ATBC, a citrate plasticizer, showed higher levels of extraction in comparison to the other plasticizers tested.
One of the key attributes of PVC compounds is its excellent clarity. Our studies on the effect of plasticizer type on the % haze of Shore A 75 compounds and highlight subtle differences in the compatibility of the various plasticizers with the PVC polymer matrix. The results show all plasticizers show very low % haze and have excellent clarity. However, the results also show that there are differences in the % haze of the various plasticizers. DINCH and, to lesser extent TOTM and DOA, generally show higher % haze values.
The quantity of plasticizer required to make a specified Shore A can also highlight subtle differences in the compatibility of the various plasticizers with PVC polymer matrix. For a Shore A 75 PVC compound the concentration of the plasticizer can vary from a low of 27% for DOA to a high of 32% for TOTM. In general, DEHT, DINCH & TOTM require higher levels of concentration than DEHP to match the same Shore A.
Selecting an alternative plasticizer to DEHP can also impact the cost of the PVC compound. The following are some generally guidelines on cost of alternative plasticizers relative to the cost of DEHP:
- DOA, TOTM & citrate plasticizer are more expensive than DEHP regardless of geography
- All non-phthalate plasticizers in Asia are generally more expensive that DEHP
- DEHT in Europe and the United States, and DINCH in Europe are generally the most competitive in cost relative to DEHP cost