Heterophasic ethylene-propylene copolymers: New insights on complex microstructure by combined molar mass fractionation and high temperature liquid chromatography

https://doi.org/10.1016/j.polymdegradstab.2019.109022Get rights and content

Highlights

  • High temperature interaction chromatography is used to separate components of heterophasic ethylene-propylene copolymers.

  • Two-dimensional liquid chromatography is used to track the changes in the alloy components before and after visbreaking.

  • There is evidence that the rubber phase may promote mobility of peroxide in the polypropylene matrix during degradation.

  • Fractionation and analysis of fractions confirms peroxide impact on polypropylene rather than ethylene-rich components.

Abstract

The present study investigates the changes in the microstructure of heterophasic ethylene propylene copolymers (HEPCs) with increasing ethylene content, before and after visbreaking. Three samples obtained from commercial gas process reactors at Sasol Polymers at varying times after the addition of the ethylene comonomer are further visbroken to produce three more samples. Bulk sample analyses indicate that the ethylene-rich rubber phase which is essential in aiding peroxide mobility during visbreaking as observed in narrower dispersities and lower peak molar masses with increasing ethylene content. Further investigations of the microstructure via offline coupling of preparative molar mass fractionation (pMMF) to advanced analytical techniques such as solvent gradient interaction chromatography (SGIC) reveal that ethylene-rich copolymer chains are not significantly affected by the peroxide as the polypropylene homopolymer. Consequently, the polypropylene (PP) homopolymer and polypropylene-rich fractions with high molar mass diminish after visbreaking. Furthermore, the increase in ethylene content was observed to reduce the impact of the peroxide on the fraction quantities before and after visbreaking implying that visbreaking affects more the polyolefin chains with more PP segments and less those with more ethylene comonomer.

Introduction

Impact resistant heterophasic ethylene-propylene copolymers (HEPCs) are commercially important materials due to their wide industrial application [[1], [2], [3], [4]] from automobile to refrigerator components. The material is an alloy of polypropylene (PP) and ethylene-propylene copolymers (EPCs) of varying ethylene contents with an array of block lengths. Commercial production of HEPCs requires a two-stage reactor system. In the first reactor the major component isotactic PP (iPP) homopolymer is produced. iPP is responsible for the toughness and rigid nature of the HEPC products. Ethylene and propylene monomers are then added to the second reactor [2,[5], [6], [7], [8], [9]] were complex EPCs with varying ethylene contents are produced. These copolymers give the alloy its impact resistance. Furthermore, the use of a Ziegler-Natta catalyst results in a complex mixture of copolymers as a result of the heterogenous active sites in the catalyst [[10], [11], [12]]. Owing to the close reactivities of the ethylene and propylene monomers, the copolymer produced in the second reactor consists of varying lengths of polyethylene and polypropylene.

Understanding the microstructure of these materials is challenging since they have complex distributions in molar mass and chemical composition [[13], [14], [15]]. The major parameters of polyolefin resins which are of fundamental importance are molar mass distribution (MMD) and chemical composition distribution (CCD). These parameters can be obtained independently by high-temperature size exclusion chromatography (HT-SEC) and a variety of crystallization and adsorption based-techniques for MMD and CCD respectively. The determination of these parameters independently often leads to inadequate information regarding the exact component information. More so, in HT-SEC, the dominant iPP matrix overshadows the smaller but crucial EPC components as a result of co-elution.

Despite the many studies that have been conducted on the morphology, microstructure, chemical composition, etc. of HEPCs [6,14,[16], [17], [18], [19], [20], [21], [22]], the manner in which the ethylene comonomer units insert themselves in between the PP segments is not yet fully understood. A fundamental understanding of this phenomenon can assist in the prediction of the material's properties as well as the production of resins with optimum properties. In the past high-temperature size exclusion chromatography (SEC) has been coupled to Fourier-transform infrared spectroscopy (FTIR) to monitor the ethylene and propylene contents as a function of molar mass [11,14].

To further improve on the HEPCs’ properties, the materials can be degraded using a peroxide. The technique is commonly referred to as visbreaking or controlled rheology (CR) and a notable decrease in the molar mass (MM) and narrowing of the molar mass distribution (MMD) of the polyolefin resin is the end result [23]. The degraded copolymers have higher melt flow rates, and this has obvious advantages in processing of the final product. Microstructural changes have been predominantly linked to molar mass changes of the iPP matrix and very little is known about the changes in the copolymer part of the alloys.

Understanding the effect of peroxide-induced vis-breaking on the microstructure of HEPCs could be invaluable in predicting the ultimate properties of visbroken products although this is challenging [24,25]. Swart et al. [23] came to the conclusion that visbreaking also affects the ethylene-propylene copolymers with long ethylene sequences. In their study, they used preparative temperature rising elution fractionation (pTREF) to obtain several fractions for further analysis. pTREF is known to fractionate semi crystalline polyolefins according to crystallisability and this information can be related to chemical composition. However, pTREF has several drawbacks, one of which includes co-crystallization, as reported by Monrabal [26]. Coupling pTREF offline to interaction chromatography can significantly help separate the components with similar crystallisabilities or ethylene contents but with dissimilar chemical composition or dissimilar ethylene segment lengths. Other processes such as functionalization of the polyolefin chains by the peroxide can occur. Separation of these components can be achieved on silica using a non-polar/polar solvent gradient. However, this was not the focus of the present work.

Chemical composition separation is achieved on porous graphitic carbon (PGC) as reported in several works [[13], [14], [15],27]. Here, the iPP homopolymer can be separated from the EPCs. PP is largely not retained and elutes mainly in SEC mode with a small fraction being retained. Furthermore, the EPCs are retained according to the length of their constituent ethylene blocks. The longer the ethylene blocks the greater the retention.

Preparative fractionation is used to simplify the microstructural complexity as already reported in several works [1,14,19,23,[27], [28], [29], [30]]. In the process, smaller but important fractions are recognised when the fractionation is coupled offline to other analytical techniques. The chemical composition dimension is typically simplified by fractionating using crystallization-based techniques. pTREF has been applied on a variety of polyolefins including HEPCs [3,6,[10], [11], [12],19]. Preparative solution crystallization fractionation (pSCF) has also been applied using small laboratory setups to achieve chemical composition separation of impact polypropylene (IPC) samples [13]. The EPC copolymer is usually collected at low temperatures as the soluble fraction which is amorphous. Other copolymer fractions with longer ethylene sequences elute at slightly higher temperatures. Where the ethylene content is high, a lot of information can be lost since most ethylene containing chains elute in the room temperature fraction.

For fractionation according to molar mass, preparative molar mass fraction (pMMF) is applied [29,31,32]. The fractionation in this technique is based on solubility of polyolefin chains in a solvent/non-solvent mixture. Fractions are collected by simultaneously increasing the solvent and decreasing the non-solvent in the mixture or vice versa. One major drawback is that depending on the ratio or type of the solvent/non-solvent mixture, chemical composition tends to have an influence on the fractionation process. During pMMF, fractions with narrow molar mass distributions are obtained for further analysis. Although this technique is old, it is not used as frequently as pTREF. This partly due to the difficult nature of the fractionation technique. Recently, low density polyethylene (LDPE) has been fractionated according to molar mass and the fractions analysed using various techniques [29,32]. To the best of our knowledge, this technique of preparative fractionation has not been applied for the fractionation of HPECs. Therefore, the offline coupling of pMMF to advanced analytical techniques can provide a complementary perspective to that of pTREF.

Section snippets

Sample information and visbreaking

Sampling was done from commercial gas process reactors at Sasol Polymers, Secunda, South Africa. The ethylene contents and molar mass properties of the bulk HEPCs are indicated in Table 1. The ethylene content of the bulk samples was determined by 13C NMR. For simplified referencing, the following nomenclature will be used on the samples: in sample T180, T stands for time and 180 stands for the time of sampling in minutes after introduction of ethylene to the second reactor. The degraded

Results and discussion

The present study investigates the usefulness of combining preparative molar mass fractionation (pMMF) and high temperature solvent gradient interaction chromatography (HT-SGIC) as a tool to probe microstructural changes of HEPCs when the ethylene content is increased and changes after visbreaking. Understanding the chemical composition complexity of HEPCs has always been challenge when it comes to structure-property relations. Most studies on ethylene incorporation into the HEPCs resins has

Conclusion

Heterophasic ethylene-propylene copolymers (HEPCs) with increasing ethylene comonomer content have been analysed using molar mass and chemical composition separation and detection techniques. Three copolymers having ethylene contents of 0, 5.2 and 10.8 mol.% have been treated with 0.5 wt% peroxide before comparing the changes in the molecular structure. Obvious shift to low molar masses in the molar mass distributions of the three bulk samples after visbreaking was indicative of successful

Acknowledgement

The authors would like to thank Sasol Polymers and the NRF for funding and support.

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