Thermogravimetric analysis for the characterization and comparative analysis of polyvinyl chloride (PVC) wall panel

. In this paper, thermogravimetric analysis of different types/grades of polyvinyl chloride (PVC) wall panel is carried out using thermogravimetric analyzer. The data output from the analyzer is subjected to data analysis software i.e., OriginPro 2018, from which thermal attributes are find out for different grades of PVC wall panel i.e., soft PVC, hard PVC, WPVC and SPVC. The thermogravimetric attributes i.e., onset degradation temperature T i , final degradation temperature T f ,, reaction interval and the peak temperature T P from the derivative thermogravimetric curve for the four basic types of PVC were comparatively analyzed for the characterization purpose. Beside the thermal characterization, the thermogravimetric analysis also disclosed the compositional attributes of the four types of PVC wall panel. The analysis in this paper characterized the SPVC as the most thermally stable PVC wall panel followed by hard, soft and WPVC.


Introduction
Polyvinyl chloride (PVC) is a widely used and has been accepted as universal polymer subject to its wide applicability in different engineering fields and applications. It is a thermoplastic type of polymer and due to its flexibility and ease of processability and printability, it is widely used for the aesthetics of walls in offices, homes and buildings in form of sheets with different attractive textures. Due to its recyclability, it is also used in various short life applications i.e., packaging and long life applications as well i.e., medical devices, plumbing and much more. [1,6] is rare research work on the qualitative and comparative analysis of different grades of PVC to comment on the thermal attributes of the said grades. [1] Thermal behavior of the polymers is an essential test since PVC is regarded as polymer thus susceptible to changing its properties subject to changing temperatures especially higher temperatures. The test carried out to predict the thermal behavior is the Thermogravimetric Analysis in which a sample weight is constantly monitored with changing temperatures in a controlled atmospheric condition particularly in the presence of inert gas. Monitoring the weight of the sample against constant heating rate with increasing temperature predicts the thermal behavior of the sample from the weight-temperature curve. The weight of the sample may not always reduce, it may increase subject to changing temperatures. The inert/purge gas commonly used is nitrogen which exhausts out any evaporated material from the sample. The most common curve obtain is a descending curve thus indicating a decrease in mass with increasing temperature. The curve also gives an idea of the amount of filler or different constituent materials used in a sample and is also used for comparative analysis of same material samples as in our case by plotting thermogravimetric curves of the samples thus to predict the high thermally stable material sample. [2,3,9] 2 Methodology As shown in Fig. 1, a thorough literature study is carried out followed by sampling i.e., reducing the raw panels into a sample size of 1cmx1cm for which mass is measured then for each grade of PVC through cutter. The requirement for sample to be tested is its mass rather than its length and width. A post sampling technique of finishing is also carried out to smooth out the edges of each sample using a grinder. The TG analyzer and samples ready to be subjected to TG analyzer are shown in the Fig. 2 and Fig. 3 respectively.  Thermogravimetric analyzer Model: TA Intruments:Q500 is used to carry out the analysis. The analyzer contains a source of constant heat rate capable to hit a temperature upto 1000 ° having a maximum ramp of 50 °/ . The analyzer has also a builtin accurate balance capable to handle masses upto 200mg but as per standard ASTM E1131, ISO 11358 the recommended mass of sample should range between 2 and 50 mg [4,7]. The balance constantly measures the mass with increasing temperature. The analyzer has builtin thermocouples which contantly measures the temperature as well. The standard operaitng 1 2

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3 procedure of Rev. 01/2019 KTM by advanced material science & engineering Center [8] is followed in this analysis, the data input to the analyzer is: The data output from the analyzer is a data set of temperature (°) and the corresponding % weight of the sample up to 600 °. The data analysis software used is Origin 2018, which plots the weight vs temperature curve. The former is plotted in % and on y-axis while the latter one on x-axis as per standard TG analysis [7,8]. This descending curve gives residual % weight at a specific temperature.
To characterize the said samples, the extrapolated onset temperature is calculated for each sample that denotes the temperature at which the weight loss begins in the TG curve. The extrapolated onset temperature (Ti) is calculated because it is a reproducible temperature calculation and it is specified to be used by ASTM® and ISO.
However, to have a better look for concluding the TGA curve (TGA-C), a derivative TG curve (DTG-C) is also plotted on the secondary y-axis to find out the peak temperature i.e., the inflection point at which rate of weight loss is the greatest [7], by taking the first derivative of %weight per unit degree of temperature i.e., The reason that nitrogen gas is used as purge gas is to avoid oxidation as in case of oxygen to avoid any distortion in the results. After the reaction interval is over i.e., the curve takes horizontal trend, the purge gas is shifted to oxygen to completely burn out the carbon present in the sample to know the carbon content approximately from the char residue that is burnt off and also determine the presence of other contents i.e., filler. The purpose to use both gases is that nitrogen provides more non-reactive atmosphere for the sample under consideration in thermogravimetric analysis thus to predict the degradation of the sample due to increasing temperature only, while oxygen is used to take away the weight of carbon present completely to have filler left only as the residue weight.
Other data variables calculated are discussed in the results and discussion section in detail.
The data output from the TG analyzer is input into Origin Pro 2018 as shown, the weight vs temperature data is plotted, then a first derivative of the %wt column is taken resulting in a curve that is smoothed out using adjacent averaging method. Then the weight and 1 st derivative of weight are plotted on double y-axis against temperature on x-axis. The thermogravimetric analysis curve TGA-C and derivative thermogravimetric curve DTG-C are obtained for soft PVC, hard PVC, WPVC and SPVC and discussed below:   The comparative analysis based on Fig. 4, Fig. 5, Fig. 6 and Fig. 7 for the thermal characterization of each grade of PVC wall panel and the comparative composition analysis is carried out using the screen recorder function in Origin Pro, the exact markings are made from which all the temperature and composition readings are extracted and tabulated below: 6 MATEC Web of Conferences 381, 02008 (2023) https://doi.org/10.1051/matecconf/202338102008 MTME-2023 The onset temperature Ti is the temperature at which thermal degradation of the sample starts that gradually increases and reaches the inflection point i.e., the peak temperature TP at which the rate of weight loss is maximum after which the rate decreases and reaches a nearly horizontal portion of the graph at final degradation temperature Tf. The curve area between Ti and Tf is called as the reaction interval during which the pyrolysis of the PVC occurs i.e., the thermal degradation of PVC between 260 and 300 ° with nitrogen as purge gas. Afterwards 300 °, low molecular hydrocarbons are left only which are thermally break down further till Tf [5]. As shown in Table 2, the lowest onset degradation temperature is that of soft and hard PVC i.e., 275.35 ° which indicates that the said are the least thermally stable, however the reaction interval of WPVC is the highest i.e., 196.17 ° with the highest weight loss % (91.41%) as well. Similarly, the peak temperature for WPVC is also the highest, thus making it also less thermally stable as compared to soft and hard PVC. Soon after the Tf, the gas is changed to oxygen to find out the carbon content of the sample at nearly 380 °. Soon after the combustion of char residue, the curve again goes nearly horizontal at 600 °, at which filler resides only. It is also worth it to mention that % residue and % filler is distinguished by the fact that % residue indicates the combine percentage of carbon as well as filler. The same is the reason that purge gas is shifted to oxygen to completely burn out the carbon content to know the % char and then to know the percent weight of filler when the curve takes horizontal trend again after losing the carbon. Now in case of soft and hard PVC, the % filler remaining is 70.36 and 66.93% respectively which only contains plasticizer and stabilizers etc while WPVC and SPVC have 8.59 and 79.46% filler remaining respectively which contains plasticizer, stabilizer and also wood flour in case of WPVC and calcium carbonate in case of SPVC. The reason for the least % filler remaining for WPVC is that the thermal stability of wood flour is weaker than other fillers or reinforcement thus the degradation of wood along with PVC content begins simultaneously thus also resulting in an unknown value of char i.e., the weight loss at reaction interval in case of WPVC is 91.41% which indicates the loss of PVC and wood flour both and makes it the highest of all panels. Referring to the SPVC, the onset temperature is (281.10 °) is less, the reaction interval (80.70°) is the least, the peak temperature (301.51 °) is the least, the char content is least (6.07%) with the highest amount of filler remaining at 600 ° i.e., 79.46% thus indicating that SPVC is highly thermally stable PVC panel.

Conclusions
− The highest thermally stable PVC wall panel is SPVC the reason for which is the presence of calcium carbonate thus making it very expensive as well. − The soft and hard PVC are nearly equally thermally stable, the later has a little edge over the former because of the reaction interval but a final decision can be made based on the price and mechanical properties of both as well. − The WPVC is the thermally least stable as compared to soft, hard and SPVC due to the presence of wood flour. The reaction interval and wood flour are the indications that as soon as the pyrolysis end, the wood content will catch fire thus making it a material that cannot be used for aesthetics of wall or flooring subject to safety precautions because it can possibly cause a fire hazard.