Thermal properties of PLA and PLA/PBSA blends were investigated by calorimetric analysis (Q200 TA- DSC). Nitrogen, set at 50 mL/min, was used as purge gas for all measurements. Indium was used as a standard for temperature and enthalpy calibration of DSC. The materials used for DSC analysis have been cut from the ISO 5271-A dog-bone injection mold specimens. In order to evaluate if an eventual crystallization occurred during the specimen injection molding (affecting the mechanical behavior of the materials), the thermal properties were evaluated considering only the first DSC heating run. The sampling was carried out taking the material in the same region of the specimens to avoid differences ascribable to different cooling rates in the specimen thickness. The samples, with mass between 11.5 and 15 mg, were sealed inside aluminum pans before measurement. PBS granules were also analyzed in order to better understand how its thermal properties could affect the thermograms of the binary blends.
The samples were quickly cooled from room temperature to −50 °C and kept at this temperature for 1 min. Then the samples were heated at 10 °C/min to 200 °C to delete the thermal history then a second cooling scan from −70 °C to 190 °C, at 10 °C/min, was carried out. Melting temperature (Tm) and cold crystallization temperature (Tcc) of the blends were recorded at the maximum of the melting peak and at the minimum of the cold crystallization peak, respectively. The enthalpies of melting and cold crystallization were determined from the corresponding peak areas in the thermograms. Where possible the PLA and PBSA crystallinity were calculated according the following Equation:
where Xcc, is the crystallinity fraction of PLA or PBSA, ΔHm and ΔHcc are the melting and cold crystallization enthalpies respectively, while ΔH°m is the theoretical melting heat of 100% crystalline polymer. For PLA a ΔH°m value of 93 J/g [71 (link)] and for PBSA a ΔH°m value of 142 J/g were considered [8 (link)].
The heat deflection temperature or heat distortion temperature (HDT) corresponds to the temperature at which the polymeric material deforms under a specified load. This property is fundamental during the design and production of thermoplastic components. The HDT is also strictly correlated to the polymer crystallinity, in fact it is noteworthy that a highly crystalline polymer has an HDT value higher than its amorphous counterpart [72 (link)]. For this purpose, the determination of the deflection temperature under load (HDT) was carried out on a CEAST HV 3 (INSTRON, Canton, MA, USA) in accordance with ISO 75 (method A). A flexural stress of 1.81 MPa and a bath heating rate of 120 °C/h were used. The sample size was 80 mm × 10 mm × 4 mm. When the sample bar deflects by 0.34 mm, the corresponding bath temperature represents the HDT (Type A) value. At least five measurements were carried out and the average value was reported.
The samples were quickly cooled from room temperature to −50 °C and kept at this temperature for 1 min. Then the samples were heated at 10 °C/min to 200 °C to delete the thermal history then a second cooling scan from −70 °C to 190 °C, at 10 °C/min, was carried out. Melting temperature (Tm) and cold crystallization temperature (Tcc) of the blends were recorded at the maximum of the melting peak and at the minimum of the cold crystallization peak, respectively. The enthalpies of melting and cold crystallization were determined from the corresponding peak areas in the thermograms. Where possible the PLA and PBSA crystallinity were calculated according the following Equation:
where Xcc, is the crystallinity fraction of PLA or PBSA, ΔHm and ΔHcc are the melting and cold crystallization enthalpies respectively, while ΔH°m is the theoretical melting heat of 100% crystalline polymer. For PLA a ΔH°m value of 93 J/g [71 (link)] and for PBSA a ΔH°m value of 142 J/g were considered [8 (link)].
The heat deflection temperature or heat distortion temperature (HDT) corresponds to the temperature at which the polymeric material deforms under a specified load. This property is fundamental during the design and production of thermoplastic components. The HDT is also strictly correlated to the polymer crystallinity, in fact it is noteworthy that a highly crystalline polymer has an HDT value higher than its amorphous counterpart [72 (link)]. For this purpose, the determination of the deflection temperature under load (HDT) was carried out on a CEAST HV 3 (INSTRON, Canton, MA, USA) in accordance with ISO 75 (method A). A flexural stress of 1.81 MPa and a bath heating rate of 120 °C/h were used. The sample size was 80 mm × 10 mm × 4 mm. When the sample bar deflects by 0.34 mm, the corresponding bath temperature represents the HDT (Type A) value. At least five measurements were carried out and the average value was reported.
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