Characterization of Talc using PXRD and TG analyzer

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Brief introduction:

Talc is a naturally occurring magnesium rich clay mineral which is stable, chemically inert and odorless. According to Mohs’ scale of hardness, talc is the softest mineral. It is composed of hydrated magnesium silicate having chemical formula Mg3Si4O10(OH)2. It has a soft, silky texture which acts as a solid lubricant and filler. Talc is widely used in paper manufacturing, plastic, paint/coatings, roofing, cementitious materials production, rubber, ceramic industries, food, pharmaceuticals and most importantly in various cosmetics and baby care products. Depending on the sources, pure talc is often associated with various minerals like calcite, magnesite, dolomite, kaolinite, chlorite, serpentine etc. As a component of various personal care products, Talc comes into direct human contact and therefore essentially requires composition analysis.

Experimental:

In the work presented herein, we have carried out a thorough phase analysis of talc obtained from various commercial talcum powders (CTP) and other talc samples received from different commercial sources, namely, CTP-1, CTP-2, CTP-3, sample A and sample B. In this regard, Powder X-ray diffraction (PXRD) and thermal analysis such as thermogravimetry (TG) and differential scanning calorimetry (DSC) were opted for simple, non-hazardous and rapid analysis. Two certified reference samples, magnesia (BCS-319/1) and SARM-32 were also analyzed to validate our investigation. For TG-DSC analysis, the samples were characterized using Perkin Elmer STA 6000 instrument. The experiments were carried out within the temperature range 50-995oC with a heating rate of 10oC/min under N2 atmosphere. For PXRD analysis, the samples were characterized using Bruker D2 Phaser analyzer having a Cu Kα X ray source (λ = 1.54184 Å). Loss on ignition (LOI) was also measured at 950oC to further corroborate the results obtained from TG analysis.

Results and Discussion:

Thermogravimetric study and phase analysis of different samples:

Figure-1

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Figure-2

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Figure-3

Figure-4

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Figure-5

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Figure-6

Table-1

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Key findings:

  • From TG-DSC curves of different samples (Figure 1-5) it was observed that dehydroxylation and decomposition of minor phases like kaolinite/chlorite/calcites etc. initiate from ~550oC whereas thermal decomposition of pure talc starts from ~820oC onwards to form enstatite or forsterite as the end products. Weight loss as observed in TG-DSC was concomitant with the result obtained by loss on ignition of each sample carried out at 950oC for 1h (shown in Table 1).  
  • X-ray diffraction pattern (Figure 6) clearly exhibits the signature peaks of talc and other components like chlorite, kaolinite, calcite/magnesium calcite etc. The weight loss due to thermal degradation and dehydroxylation of these phases are reflected in TG-DSC curves. Tentative phases with quantification based on PXRD data are presented in Table 1

Conclusion:

  • Stepwise thermal behaviour as observed in TG-DSC curve was successfully corroborated with X-ray diffraction pattern for the identification and quantification of different phases of talc samples obtained from different commercial sources. 
  • The cleanest matrix of CTP-3 can be clearly observed in PXRD pattern which is concomitant of corresponding TG-DSC curve and confirms high purity level for being used in personal care products.
  • Except the pure Talc phase, PXRD pattern identifies presence of various other phases. Depending upon the end use, presence of these all components can be justified. As for example, kaolinite enhances the coverage while reducing the sheen provided by talc, calcite absorbs moisture to minimize oiliness, magnesite chlorite and enstatite/forsterite acts as filler, enhance Mg availability and improve mechanical strength for cementitious products formulation. In recent times calcined talc powders are mostly used for the formulation of cementitious products.
  • Therefore, it can be concluded that analytical techniques such as PXRD and TGA can be successfully implemented for rapid preliminary screening and overall quality check of different talc samples prior to their application in various consumer products.

Contributed by Dr. Soumita Mukhopadhyay and Dr. Arijit Goswami under the guidance of Prof. Barun Kumar Gupta