Resumen de Characterization of mineral polyolefin concentrates for plastic formulations
Julio Alejandro Carrero Primiani
Mineral concentrates can be described, in general, as a compound of 70 to 80 w/wt of mineral filler, loaded in a polyolefin carrier. These products are commonly used by manufacturers to introduce mineral fillers in a wide variety of plastic articles, in some cases reaching loadings of 75 w/wt, depending on the type of material and final application.
The addition of mineral concentrates modifies the overall performance of plastics. The extent of such modification depends on the amount added, but more importantly, on the characteristics of the polymer, filler and additives that compose it. Therefore, the characterization of such products is of high interest. An important limitation concerning the characterization of filled polymers is that, when mixed, the filler often impedes the analysis of the polymer, and vice versa. As a result, the characterization is unreliable, or simply not possible.
Methods for the suitable characterization of mineral concentrates, specifically for the determination of type and amount of antioxidant additives as well as polymer and filler characteristics were developed in this work. Results indicate that these methods represent a viable way to accurately characterize mineral concentrates, avoiding current characterization limitations.
Such methods were applied for the characterization of commercially available PE based and PP based mineral concentrates of calcium carbonate and talc. Based on these results, a study on how the characteristics of the mineral concentrate affect the thermal, rheological and mechanical properties of LLDPE films was performed.
-Additive Characterization:
Having the means for the determination of type and amount of additives is important for the plastics industry because of quality control, research and development or for regulatory reasons. One of the major concerns regarding the characterization of additives in highly filled polymers was the possibility that filler particles could limit the extraction of additives from the polymer matrix.
A method for additive characterization that combined extraction of the additives by means of PLE and quantification by HPLC was developed in this work for the determination of type and amount of antioxidant additives in mineral concentrates. To evaluate the method, extraction recoveries of filled PE and PP were compared with their unfilled counterparts.
The extraction recoveries of Irganox 1010, Irganox 1076 and Irgafos 168 from non filled PE were the same as those obtained for PE filled with 80% w/wt of CaCO3 sample. Similar behavior was observed for non filled PP relative to the PP sample filled with 80% w/wt of CaCO3.
Additive recovery was considerably lower when extracting from talc filled PE and PP in relation to their non filled counterparts. The relation between recovery yields and mixing time was investigated for these samples and it was found that as processing time increased new peaks were detected in the HPLC chromatogram at the same time that the amount of antioxidant additives Irganox 1010, Irganox 1076 and Irgafos 168 decreased. Based on these results it was proposed that the presence of talc could have somehow accelerated the rate of antioxidant consumption during sample processing, thus, less antioxidant was left to be extracted from the polyolefin.
This method was employed to determine the amount of antioxidant additives Irganox 1010, Irganox 1076 and Irgafos 168 in commercial PE based and PP based mineral concentrates of calcium carbonate and talc.
-Polymer and Filler Characterization:
One important limitation regarding the characterization of the polymer and filler components of mineral concentrates is that when mixed the polymer often impedes or limits the analysis of the filler by conventional means, and vice versa. As of this, characterization of polymer and filler is not possible or in the best case scenario not reliable and this represents a concern for the plastics industry.
A separation procedure based in solvent extraction prior centrifuge was developed in this work with the objective to separate the polymer and filler components of the mineral concentrate. By doing this it was possible to perform an individual characterization on the LLDPE and CaCO3 of PE/CaCO3 mineral concentrates, avoiding the interference issue mentioned above. The separation procedure was evaluated by comparing the initial properties of the polymer and filler with those obtained after performing the separation.
For the polymer, the following characteristics were considered: melting point, endotherm of melt, molecular weight, molecular weight distribution and chain architecture, measured by DSC, GPC-IR and 13C NMR, respectively; and for the filler: surface area, particle size distribution and surface treatment, measured by laser diffraction and by evaluating their hydrophobicity to water/ethanol solutions.
Results indicate that little or none variation was measured over the mentioned properties when applying the method on PE/CaCO3 mineral concentrates (20/80 w/w). Also, the method developed in this work allowed for an accurate determination of the polymer and filler characteristics of commercial LLDPE/CaCO3 mineral concentrates designed for film applications.
During the development of the separation procedure it was found that GPC and DSC are techniques that allow branch content determination in polyethylene but, without external calibration, obtain quantitative information about type and distribution of branches is not possible. However, 13C NMR allowed the quantitative determination of type, content and distribution of branching in polyethylene.-Effect of Mineral Concentrate Characteristics on LLDPE Films:
The effect that the characteristics of mineral concentrate have on the overall properties of a plastic article was studied for LLDPE films loaded with 35% of three different commercially available mineral concentrates of PE/CaCO3. The study was based on comparing the mechanical (tensile strength, tear and impact resistance) thermal (melting point, endotherm of fusion and oxidation resistance) and rheological properties (melt volume rate) of filled and non filled samples.
Previous sample preparation, the different mineral concentrates (M1, M2 and M3) were characterized by means of the methods developed in this work.
In general, the performance of the LLDPE film was considerably modified by the addition of 35% w/w of mineral concentrates, especially in regards of the rheological and mechanical properties and, in lesser extent, of the thermal properties:
• The resulting melting temperature and the melting endotherm of the filled films were similar to the non filled counterpart, independently of the characteristics of the mineral concentrate, as measured by DSC. The oxidation resistance of the films prepared with M2, which was the mineral concentrate with the higher amount of antioxidant additives Irganox 1010 and Irgafos 168, was considerably higher than those films prepared with mineral concentrates M1 and M3, and even higher than the non filled film. These results indicate that the additive content of the mineral concentrate helps improve the oxidation resistance of the material.
• For all samples, the addition of CaCO3 increased the viscosity of the LLDPE film as evidenced by a reduction in the melt flow rate. Additionally, It was observed that the ease of flow was further reduced as the surface area of the filler was higher, which was the case for the films prepared with the mineral concentrate M2.
• A significant reduction in the tensile strength was observed for the filled films, both in the machine and cross direction. Tear resistance in cross direction was slightly decreased, and no clear pattern was observed for the tear resistance in machine direction. As for impact resistance, all filled samples showed an important increment in this property.
It is worth noting that the film prepared with the mineral concentrate M2, which has the narrowest particle size distribution, the higher hydrophobiicity to water/ethanol solutions and the higher surface area among the mineral concentrates used for this study, showed best performance in all mechanical tests. These results seem to indicate that the individual characteristics of the mineral concentrate can further influence the performance of filled films.
