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DETERMINATION OF XYLENE SOLUBLES IN POLYPROPYLENE BY FTIR

IP.com Disclosure Number: IPCOM000129736D
Publication Date: 2005-Oct-07
Document File: 3 page(s) / 17K

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Abstract

The determination of low molecular weight components in polyolefins such as atactic polypropylene has been historically determined by extraction using xylene, hence the name “xylene solubles.” The laboratory method is based on gravimetry, using hot xylene to dissolve the polymer, careful cooling to precipitate the insoluble fraction, evaporation of the remaining xylene solution and then weighing to determine the soluble fraction. It is desirable to replace this method with an instrumental method, in this case, mid-infrared Fourier transform spectrometry (FTIR). The main drivers for development of an FTIR method are decreased labor and analysis time, although benefits from reduction in xylene use and exposure will also be an advantage.

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Determination of Xylene Solubles in Polypropylene by FTIR

The determination of low molecular weight components in polyolefins such as atactic polypropylene has been historically determined by extraction using xylene, hence the name “xylene solubles.”  The laboratory method is based on gravimetry, using hot xylene to dissolve the polymer, careful cooling to precipitate the insoluble fraction, evaporation of the remaining xylene solution and then weighing to determine the soluble fraction.  It is desirable to replace this method with an instrumental method, in this case, mid-infrared Fourier transform spectrometry (FTIR).  The main drivers for development of an FTIR method are decreased labor and analysis time, although benefits from reduction in xylene use and exposure will also be an advantage. 

The standard test for the determination of xylene solubles is based on an ASTM method[i], which was originally issued in 1994 and revised in 1998. 

Summary of Lab Method

1.       A weighed sample is dissolved in orthoxylene at reflux conditions.

2.       The solution is cooled under controlled conditions and maintained at 25ºC so that crystallization of the insoluble fraction takes place.

3.       The insoluble portion is removed by filtration.

4.       The orthoxylene is evaporated from the filtrate, leaving the soluble fraction as residue.

5.       The percent xylene solubles (XS) is determined gravimetrically.

The time required to run this method is approximately 3 hours.  If the ASTM method is followed strictly, then for samples with less than 12% XS a cooling period of 16-20 hours is recommended. 

The ASTM method lists several sources of possible error:

·         Materials with similar solubility to XS, such as additives, may interfere.

·         Insoluble gels present in the polymer may cause incomplete extraction.

·         The polymer sample must be dried or else moisture can interfere.

·         The cooling rate and time has an influence on the test.

·         The filter or aluminum pan may leak.

The ASTM document includes the results of an inter-laboratory study using 7 laboratories and 5 materials.  In the best case of a single laboratory, single analyst, the standard deviation varied from 0.202 for 3.58 % XS to 1.22 for 22.3 % XS.  For a more realistic case, the between-laboratory precision varied from 0.408 to 1.63 for the same materials.  The ASTM method also defines a critical interval of 2.8 times the standard deviation.  Any two tests must exceed this critical interval for the results to be considered different. 

Summary of FTIR Method

The FTIR procedure can be summarized as follows (steps 1-3 involve analyst time, or are typically manual operations, while steps 4-10 are typically automated):

1.       Press a plaque of material using typical methods used for polyolefins and quench at a lower temperature as compared to that used to prepare the plaque in order to stabilize the material.

2.       Cut a hole in the plaque at a predetermined location.

3.       Mount the plaque in a sample holder, such as...