Column-based Preparative Fractionation for Polyolefins

Preparative fractionation in a column packed with an inert support is a popular technique for the characterization of polyolefins because it facilitates the analysis of the fractions to obtain the bivariate distribution of a resin.

For decades, fractionation has been an important but challenging and tedious technique because it requires manually handling large volumes of toxic solvents at high temperature. The significant amount of time and effort demanded for its operation called for an automatic instrument that avoided solvent handling by the analyst. In 1996, a collaborative effort between a German Chemical Company and Polymer Char resulted in the development of the instrument PREP mc², an automated technique that was able to fractionate polyolefins according to their chemical composition (by temperature rising elution fractionation [TREF] or by crystallization analysis fractionation [CRYSTAF]) or according to their molar mass. Samples were placed into vessels and fractionation was performed automatically according to the selected method in less than 24 h. With this method fractionation takes place in the absence of a support and it has started to become evident that for some polymers, when fractionating by TREF, a support for the solution to adhere to is critical for a good separation of the fractions — as is the case with newly developed multiple reactor-catalyst resins.

In response to this, Polymer Char has introduced the new PREP C20, a column-based equipment for preparative fractionation of up to 20 g of polymer. It has been developed for polyolefins but it can be used for other polymers depending on the solvents needed for dissolution.

By adding a column as support for the solution to crystallize, the separation for each of the fractions is more appropriate for these new resins. Figure 1 illustrates the fractionation of a complex dual reactor LLDPE polyolefin that it had not been possible to fractionate properly before without using a support.

Figure 1: Fractionation of a complex sample using a column based preparative equipment. Separation of the fractions is achieved.

The instrument works as a pilot plant in complete automatic mode, eliminating the need for manual handling of high volumes of hot solvent. The software allows the analysts to create their own methods adjusted to their samples. Up to nine fractions can be obtained automatically, and it is possible to check the fractionation process thanks to interactive software. A schematic diagram of the process is shown in Figure 2.

Figure 2: Schematic diagram of the process.

Experimental

The main steps of fractionation are performed automatically following the selected method parameters:

1. Dissolution of the sample at high temperature.

2. Column loading with the sample at high temperature.

3. Crystallization step following a selected temperature ramp.

4. Elution and fraction collection pumping solvent at selected isothermal steps.

To perform a fractionation the user needs to place a dry sample in a vessel, select the method, and start the process. The vessel will be filled with solvent for dissolution and fractionation will take place according to the TREF technique. The fractions are collected in 1 or 2 litre bottles. Once the fractionation is finished, the equipment is cleaned and ready to start a new fractionation.

Figure 3: Column-based PREP C20 can fractionate up to 20 g of polymer.

The implementation of an infrared detector facilitates fractionation because it gives a reading of the concentration that is coming out of the column for each fraction, so the analysts can modify the fractionation methods according to their needs. Fractionation using sub ambient temperatures can be used without the need for liquid coolants.

Conclusions

• Fractionation of polyolefins has always been a difficult and physically demanding task because of the manual manipulation of large volumes of hot solvent.

• New complex polymers require a support for the fractionation to be performed properly.

• A new column-based fractionation equipment has been developed to enable the fractionation of large amounts of sample according to its chemical composition in a completely automatic way.

• Currently, fractionation takes place in TREF mode, but other techniques of fractionation such as thermal gradient interaction chromatography (TGIC), molar mass, and solvent gradient interaction chromatography (SGIC) will be implemented soon.

Polymer Characterization S.A.

Gustave Eiffel 8, Paterna, Valencia E-46980 Spain

Tel: +34 96 131 81 20 fax: +34 96 131 81 22

E-mail: info@polymerchar.com

Website: www.polymerchar.com