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Evaluation of Plasticisers for PVC for Medical Devices

7 Discussion

7.1 Uniformity of the compounds

All the compounds are apparently uniform, as there is no sign of phase separation.

7.2 Visual assessment of the prepared mouldings and films

The injection mouldings prepared from the compounds plasticised with DEHP, DINP, DEHA, DEHS, TEHTM and ATBC look all alike, i.e. transparent, very slightly yellow and with a smooth and shiny surface. The benzoate and polyadipate plasticised mouldings are slightly more yellow. However, with all the compounds the yellowness is only visible by the naked eye through the approx. 4 mm thick mouldings, not through any of the film samples.

The injection mouldings from the compound plasticised with EAC terpolymer have an uneven dull surface with indications of material flow directions. Because of the dull surface character it is difficult to compare the yellowness to that of the other compounds. Immediately the yellowness looks more like that of the benzoate and the polyadipate plasticised compounds. However, this yellowness is not visible in the film thickness.

The EVAC mouldings are colourless. The surface is almost as shiny as that of the first mentioned six compounds. However, indications of material flow directions are slightly visible near the injection inlet point.

Apart from the fact that all of the prepared film samples look fairly homogeneous, it makes no sense to visually assess the roll milled film samples, as they have been prepared by a method that is not representative for the industrial manufacture of PVC films in large quantities.

7.3 Evaluation of the test results

7.3.1 Hardness

For comparison reasons the PVC test compounds were prepared with the attempt to obtain the same hardness as near as possible to Shore A 80.

The hardness of the reference DEHP plasticised compound has been determined to be 82. That of the alternative compounds differs between 78 and 84, except the EAC terpolymer plasticised compound, which has a Shore A hardness of 94 (a value that is beyond the interval valid for Shore A). The hardness of the chosen alternative plastic material, EVAC, is 79.

The hardness values found for the test compounds plasticised with DEHP, DEHA, DEHS, TEHTM and polyadipate are within two units from the values that could be expected according to the literature (ref. 19).

Transformation from Shore A values to British Standard Softness Numbers can be found in the literature e.g. ref. 19 and 20.

It seems likely that any wanted specified interesting hardness of PVC compounds plasticised with the selected alternative Plasticisers - with the exception of the EAC terpolymer - is possible. Obviously it is possible to purchase an EVAC with a Shore A hardness of approximately the reference value of 80.

7.3.2 Density

The density of the tested compounds varies from that of the DEHP plasticised compound with from -39 kg/m³ to +60 kg/m³ (from -3 % to +5 %) for the PVC compounds. The density of the EVAC is 951 kg/m³, which is to be expected as this material is based on olefins.

7.3.3 Mechanical properties

7.3.3.1 Tensile strength

The tensile strength of injection moulded samples of the PVC compounds vary from 12.6 MPa to 19.7 MPa. The strength of the DEHP plasticised compound is 14.7 MPa. The highest value (19.7 MPa) was found at the compound plasticised with the EAC terpolymer. Among the monomer plasticised compounds the highest value is 16.8 MPa.

With a tensile strength value of 6.3 MPa the strength of EVAC is less than half that of the reference PVC compound. For some Medical applications, the EVAC probably results in uninterestingly weak compounds.

Due to different crosshead speeds during testing it is expected that the tensile strength values determined on the film samples are significantly higher than the values determined on the injection moulded specimens. The test results show 14 – 40 % higher tensile strength values determined on the film samples (at crosshead speed 200 mm/min) than on the injection moulded specimens (at crosshead speed 100 mm/min).

The results determined on film specimens for DEHP, DEHA and TEHTM do not contradict with the expected values found in the literature (ref. 19).

The tensile strength of the EAC terpolymer plasticised PVC compound is 34 - 38 % higher than that of the reference compound; however, the samples of this compound is significantly thicker.

Determined on the 4 mm thick specimens at 100 mm/min crosshead speed the highest tensile strength value among the PVC compounds is approx. 34 % higher than that of the reference compound; the lowest one is approx. 14 % lower than that of the reference compound. Determined on the film samples at 200 mm/min crosshead speed the highest value is approx. 38 % higher and the lowest value approx. 28 % lower than that of the reference compound.

7.3.3.2 Tensile strain at break

The tensile strain at break of injection moulded samples of the PVC compounds vary from 102 % to 279 %. The strain at break of the DEHP plasticised compound is 234 %. The highest value (279 %) was found at the compound plasticised with the DEHA and the lowest value (102 %) at the EAC terpolymer plasticised compound.

With a tensile strain at break of more than 934 % the strain of EVAC is more than four times that of the reference PVC compound.

Due to different crosshead speeds during testing it is expected that the tensile strain values determined on the roll milled films (at crosshead speed 200 mm/min) are significantly higher than the values determined on the injection moulded specimens (at crosshead speed 100 mm/min). This is confirmed by the test results.

The tensile strain at break values determined on film specimens for DEHP, DEHA and TEHTM do not contradict significantly with the expected values found in the literature (ref. 19), actually they are some 19 % higher.

The tensile strain at break of the EAC terpolymer plasticised PVC compound determined at 200 mm/min crosshead speed is 20 % lower than that of the reference compound; determined at 100 mm/min it is 56 % lower.

The actual values found are 333 % and 102 % respectively; the former is more than three times the latter. For the other compounds the high-speed results are 37 - 93 % higher than the low-speed values. Apparently the EAC terpolymer plasticised compound is significantly more sensitive to strain speed than the other ones.

7.3.3.3 Cold flexibility temperature

The determined cold flexibility temperature values are very close to the expected values. However, for the EVAC and the PVC compounds plasticised with benzoate and EAC terpolymer no information on expected values was available.

The cold flexibility temperature are approx. -20° C for PVC compounds plasticised with DEHP, DINP, TEHTM and ATBC. For DEHA and DEHS plasticised compounds the value are significantly below -40° C. For the benzoate plasticised compound it is approx. -3° C, and for the compound plasticised with polyadipate it is -10° C. In comparison with DEHP, the benzoate and the polyadipate seem critical as substitutes for DEHP.

The cold flexibility temperature of the compound plasticised with EAC terpolymer is as high as 11° C! This high value probably disqualifies the EAC terpolymer as a substitute for DEHP for PVC for Medical Devices.

The value for EVAC is approx. -36° C.

7.3.4 Light transmission

Light transmission values of 90 to 92 % in the visible wavelength range 450 nm to 800 nm does not disqualify any of the tested PVC compounds nor the EVAC.

7.4 Cost

The current cost relations of the used plasticiser products are shown in table 11.

Table 11
Plasticiser cost

7.5 Comments to each of the tested plasticising substances

DINP

DINP is not available in Medical grades. Concern on the available purity has been expressed. The current price is as expected. Two CAS numbers exist.

DEHA

The current price of the used DEHA is more than twice the expected. According to a Danish EPA project, DEHA is considered more aqua-toxic than DEHP (ref. 31 and 32).

DEHS

The current price of the used DEHS is in the low end of the expected price interval.

TEHTM: Trimellitates are reported to migrate to the blood faster than DEHP (ref. 33), which, however, contradicts with the experience of the project partners. The price of the used TEHTM is significantly lower than expected.

Benzoate: According to correspondence with Velsicol Chemicals, which produces benzoates, they are reluctant to consider Benzoflex 9-88 as a suitable plasticiser for PVC for Medical products. One obvious drawback is the poor resistance to extraction by water, which is an important consideration for PVC Medical products. The price of the used benzoate is slightly lower than expected

Polyadipate

Polyadipates have successfully been used as Plasticisers for Medical grade PVC since around 1980. The price of the used polyadipate product is as expected.

ATBC

The price of the used ATBC is twice the expected.

EAC Terpolymer

The price of the used Elvaloy product is as expected.

7.6 Films versus tubings

In this project all the specimens were prepared only by injection moulding and the film samples only by roll milling, so this gives no basis for judging the ability of the prepared compounds to be processed by extrusion and calendering. So far no arguments were found to distinguish between compounds for tubings and compounds for films.

7.7 Preliminary ranking of the investigated substances and compounds

Based on the performed tests in this project DINP, DEHA and ATBC appear to give compounds with similar properties as DEHP.

A proper raking of the other plasticising substances is not possible to make as the drawbacks found concern different properties. The application in question of the final product will determine the required materials properties.

The investigated polyadipate shows a drawback in that the cold flexibility temperature of the compound is some 10ºC higher than that of the DEHP plasticised compound.

TEHTM shows a decrease in the tensile strain at break and can only be used if this is acceptable for the application in question.

DEHS gives compounds with a decreased tensile strength, and the tensile strain at break is also lower at least in film thickness. Depending on the specific application DEHS might be an interesting substitute for DEHP.

It would be relevant to verify this apparent similarity in a succeeding investigation as a new round of compounding and characterisation, before one can select appropriate alternatives for the next tasks, e.g. the biological assessment etc.

7.8 General remarks

In the project period a new plasticiser has been reported. BASF has introduced di-isononyl-cyclohexane-1,2 dicarboxylate under the trade name Hexamoll® DINCH. They claim it to be suitable in PVC applications that are particularly sensitive from the toxicological point of view. Information on the product can be found on their homepage on Internet (ref. 35).

The possibilities for replacing plasticised PVC with other plastic materials are multiple and many attempts have been made during recent years. However, each single application has to be investigated on its own. This might very well include a total materials selection process. An example on a newly reported replacement material for certain Medical applications is Dow Chemical’s work with a metallocene catalysed polyolefin film called Corvelle (ref. 36).

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