Peter Siman | Polyketone | Stabilization

Polyketone Properties and Limitations

Aliphatic polyketone (PK) was first introduced to the market by Shell in the mid-1990’s under the brand name Carilon, and was discontinued in 2000. In 2013, it was re-launched by Hysung Corporation at small scale and in 2015 the company launched a 50,000 ton/year commercial plant. PK is a semi-crystalline polymer family resulting from alternating polyolefin and carbon monoxide monomers. PK can be made as a dipolymer of ethylene and carbon monoxide group or a terpolymer, where a small percentage of propylene monomer is introduced to the backbone.

Figure 1: Synthesis of PK, copolymer & terpolymer

Prior to Shell’s researchers, Drent and coworker, had already reported the use of the palladium catalyst for the synthesis of PK polymers.[1] Likewise, there was an intensive research and investigation of other catalysts as reported by Reppe [2,3] and Brubaker (DuPont)[4] where they have reported the use of nickel(II) catalyst in water at 200˚C and 200 atm. All these advances were much milder and came to replace those initially reported condition by Farbenfabriken Bayer (1941), that used much harsher conditions of 230˚C and 2000 atm.[5]

Apparently, the properties of the PK polymer are not only determined by the molecular weight, but also affected drastically from the chemical composition, as was reported by Sommazzi and Grbassi,[6] in which they reported that the more carbon monoxide presented in the backbone, the higher the melting point of the PK polymer (Figure 2).[6] While, increasing the propylene units in the terpolymer lowers the melting point and lowers the flexural module of the polymer (Figure 3).[6] Consequently, PK polymer producers can tailor its properties by varying the percentages of carbon monoxide and propylene in the backbone.

Figure 2: Dependence of melting temperature on CO content.

Figure 3: Dependence of melting temperature on propylene content.

In general, PK has mechanical properties close to those of engineering plastics such as polyamides and polyacetal (POM) with better stability towards solvents and chemicals (Table 1).[7] Specifically, towards acids and bases contrary to POM, this is accounted for the close packing of the ketone groups and the backbone in the crystal structure. As a result and in order to dissolve or solvate PK, polar and protic solvents are used so they can protonate or interact with the carbonyl group oxygen which then forces the solvation of the rest of the chain.

Similar to POM, aliphatic PK has an inherent property of high degree of crystallinity and smooth surface which in return grant PK polymers good abrasion resistance properties, which showed to be better than neat POM. PK abrasion resistance can be similar if not better that POM with PTFE compound. In this case, the use of PK can be an excellent solution to the limitation of PTFE migration and degradation over time. Another advantage over POM or PA is that PK has good barrier properties. Thus, PK manufacturers aimed at replacing POM with PK in the applications that require high abrasion resistance.

Figure 4: Abrasion comparison with POM (internal method).

Despite the above mentioned advantages of PK, costumers and compounders are not charging to use it due to the higher price tag and lower mechanical properties compared to POM. For example, unfilled PK has lower flexural module compared to POM, 1600 vs. 2500 respectively. Consequently, forcing suppliers and compounder to reinforce it with glass fibers or other fillers to increase the stiffness without hampering the abrasion resistance property of PK.

It seems that compounding PK is very delicate and challenging due to its chemical reactivity, thus, suppliers of PK recommend a good flush of barrel with neat polypropylene (PP), specifically before and after the use of polyamide (PA) or POM. PA and POM cause the PK to cross-link and thermoset which in many cased led to jammed barrels, nozzles, dies, etc.

For instance, a customer complained that he was unable to compound PK with GF without high shearing screw configuration and when he managed and tried to mold it, it cross linked immediately in the barrel. The reaction was so fast and severe that it looked like the polymer is cross-linking and setting instantly in the barrel upon heating. This required us to inspect and update the formula used.

Throughout the failure analysis, the first and immediate suspect was the heat stabilizer which could have cause and catalyze an Abramov reaction. Due to that instance and going back to the basic organic chemistry, it is important to remember that carbonyl group could undergo multiple reactions forcing us, the formulators, to take into account all these reactions into consideration while choosing additives in the formula along with PK.

Figure 5: Reactions of Carbonyls.[8]

After careful investigation and few trials, we have developed a method that can predict PK formula cross-linking (<3 minute) or stability for medium (3-9 minutes) or long time (>9 minutes). Since then, multiple heat and UV stabilizers, glass fiber with different sizing and coloring master batches were tested in order to pin point the compatible additives for PK. For instance, only one type of sizing was compatible with PK, while the others caused severe cross linking.

Figure 6: Effect of glass fiber sizing over PK

For instance we found that the colorant MB carrier will affect the stability of PK compound just like in the following case where two different carbon black master batches were used. While, the first caused instant cross linking of the PK (fig. 7), the other gave a stable compound (fig. 8).

Figure 7: Effect of incompatible carrier polymer over PK

Figure 8: Stability of PK with compatible carrier

In summary, PK is a promising material for replacement of toxic POM in abrasion resistant applications, however, this polymer is very challenging in terms of formulation and compounding and we have investigated its properties and compatibility with different materials, additives and fillers which led to stable compounds for extended injection molding periods which makes PK compounds safe and preferred choice for solvent, corrosion and wear resistance.

References:

Alperowicz, N., Chemical Week, 1995, Jan 25, 22.
Reppe, W. and Mangini, A., US Patent No. 2,577,208, 1951.
Reppe, W. and Mangini, A., G. Patent No. 880,297, 1948.
Brubaker, M.M., US Patent No. 2,495,286, 1950.
Ballauf, F., Bayer, 0. and Leichmann, L., G. Patent No. 863,711, 1941.
Sommazzi A. and Garbassi F., Prog. Polym. Sci., Vol. 22, 1547-1605, 1997.
Wakker, A., Kormelink, H.G. and Verbeke, P., Kunststoffe, 1995, 85, 1056.

8. https://www.masterorganicchemistry.com/2010/04/23/summary-sheet-3-reactions-of-carbonyls-the-big-picture/