Introduction

Sand casting is currently among the most developing fields of producing parts for machinery and equipment. It is a broad section of industry that uses the knowledge of many often separate fields of science, such as material engineering, metallurgy, chemistry, environmental protection, computer modelling, or mechanics. The key challenge in the foundry technology is the attainment of good quality large-sized castings, masses of which often reach several dozen tonnes. Cast parts of iron and steel are used in the energy, shipyard, and machine industries [1]. Due to large sizes, often complicated shapes, and single-time nature of the production, forging is a highly advanced field, which requires application of specialist tools and solutions, including the appropriate technological process supported with the results of computer simulations [2, 3]. Therefore, major challenges in the foundry processes include the development of new technologies using environment-friendly materials, as well as skilful planning of the foundry sands, the key component of which is the binder. The binder has the capacity of binding matrix particles and provides the bulk sand with a specific mechanical strength [4]. The binder in the compound is cross-linked with physical or chemical agents, so that the binding strength in the binder matrix system is sufficient to keep the predefined shape to the foundry sands (mould) when it is cast with liquid metal, and as a result to obtain a defect-free casting [5, 6].

The development of a new binder and its implementation in the fully operating industrial cycle requires, however, broad primary research. The research involves the preliminary development of the binder composition, its cross-linking mechanism, including the capacity of binding matrix particles [6]. The application of a new binder based on synthetic and natural polymers in the foundry technology undoubtedly brings along many benefits, yet raises a question as to its behaviour under the impact of a sudden temperature shock, while casting. Issues related to the determination of the degradation process and the accompanying thermal effect are particularly important here [7]. Moreover, significant issues include determination of the impact of gas products generated during thermal decomposition on the final cast quality and the environmental impact [8, 9]. Thermal analysis carried out within this study will, therefore, allow for determining the degradation process of the polymer binder (Part 1 of the publication) in order to better understand the phenomena occurring in the foundry sands bound with a polymer binder during the process of sand casting with liquid metal; this research area is the subject of Part 2 of this publication.

This paper presents TG–DSC analysis in oxygen and oxygen-free conditions for two polymer compositions, as the actual degradation and destruction process of the cross-linked polymer binder inside the mould occurs with limited oxygen access, under the impact of high temperature of casting with a liquid metal alloy. When casting the mould, due to strong stresses at the liquid metal/mould interface phases and the spreading heat wave, one can also expect oxidative degradation. Furthermore, hardened mould is a non-uniform system, which can include blocked air bubbles, by which the oxygen presence may cause thermal decomposition according to oxidative degradation. New polymer binders BioCo subjected to thermal analysis are three-component aqueous polymer compositions containing water and poly(acrylic acid) (PAA) and modified polysaccharide (sodium salt of carboxymethyl starch—CMS, dextrin—D). The analysed PAA/CMS and PAA/D systems are thus complex, and the determination of the course of their thermal degradation requires the literature research in the area of input polymer components [10–14]. The literature indicates that poly(acrylic acid) heated to the temperature of approx 140 °C begins to lose (solvent and constitution) water, while dehydration to anhydride—as a result of intramolecular reaction of forming a six-membered ring—occurs at the temperature of approx 250 °C. An intermolecular reaction is also possible, which can result in cross-linked structure formation. Further quick PAA decomposition takes place at the temperature of approx 400 °C. Moreover, thermal degradation of acrylic polymers often occurs with characteristic cross-linking reactions, hence endothermic effects visible on DSC curves (in the range 123–183 °C) related to the aforementioned dehydration, formation of short polymer chains, carbon dioxide release, whereas monomer is released in small quantities—approximately 0.3 % of input compound [12, 13]. In the case of polysaccharides, within the temperature range of 300–450 °C, weak C–O–C and C–C bonds in the glycoside ring decompose. Next, with temperature increase, side chains fall apart, which together with C–O bond cracking leads to depolymerisation of the sugar chain. Finally, at the temperature of approx 500 °C, dehydrated polymer undergoes complete destruction, whereas gas products and carbonised material are formed [14, 15].

Considering the literature data presented above, and accounting for the presence of aqueous polymer compositions: PAA, CMS, and D, an attempt was made to determine the course of their thermal degradation. The knowledge obtained from TG–DSC thermal analysis on thermal stability of the polymer binders analysed will allow for passing onto the next phase of research on determining their thermal properties in the binder-mineral matrix (foundry sands) system. Thermal analyses (TG–DSC, TG–MS, and TG–IR) to determine the course of thermal degradation process in respect of new binders in the foundry sands, including thermal effects accompanying the process and the impact of gases generated during thermal destruction of moulds on the quality of the final casts, as well as environmental impact, will be the subject of Part 2 of the publication. Results of the study will also prove helpful to verify physical and chemical properties in order to create a database of materials used in foundry technology of products. Information contained in the database will make it easier to search for materials by their physical and chemical properties (search criteria). The material database can be one of the modules of a technological knowledge management system [16].

Experimental

Materials

Thermal analysis involved two polymer compositions cross-linked with microwave radiation. Their characterisation and cross-linking conditions are provided in Table 1.

Table 1 Characteristics of compositions and their cross-linking parameters
Full size table

Thermal examinations

The thermal examinations were carried out using a NETZSCH STA 449 F3 Jupiter® thermal analyser which supports simultaneous TG and DSC measurements, thus providing two independent signals recorded in the same measurement conditions, namely at/in the same heating rate (10 °C min−1), atmosphere, and gas flow rate (40 mL min−1). The measurements for the sample were taken in an oxidising atmosphere (air) and an oxygen-free one (argon). The sample submitted to the TG–DSC thermal analysis weighed approximately 15 mg. Platinum crucibles were used, as they allowed measurements up to 1000 °C.

Results and discussion

TG–DSC thermal analysis

In an air atmosphere, decomposition of cross-linked PAA/CMS polymer composition occurs with five mass losses (Fig. 1a). Within the temperature range of 20–100 °C, the remains of solvent water evaporate, followed by constitution water. Next, within the temperature range of 100–220 °C, intermolecular dehydration reaction takes place. In this temperature range, mainly reversible processes occur: physical transformations and reversible reactions. On the basis of TG curves, it can be stated that thermal degradation process starts at the temperature of approx 220 °C. Next, polymer chains decompose, including decomposition of side chains and glycosidic bonds (temperature range of ~220–300 °C). Further on, progressive decomposition takes place with intensive generation of gas products of destruction, including in the combustion reactions in an oxygen atmosphere (temperature range of ~300–500 °C). At the DSC curve, at the temperature of 424 and 497 °C, there are two exothermic effects visible, which may be a result of composition combustion process (Fig. 1a). Moreover, at the DSC curve, one can notice three endothermic effects (due to the adopted scale, they are invisible in the figure). A sample of the PAA/CMS composition actually completely decomposes at the temperature of 1000 °C (remaining sample mass 2.9 %).

Fig. 1
figure 1

TG–DSC curves of PAA/CMS composition: a in an oxidising atmosphere, b in oxygen-free atmosphere

Full size image

In the oxygen-free atmosphere, PAA/CMS decomposition is different (Fig. 1b). At the TG curve of the composition, five mass losses can be observed, while at the DSC curve, six endothermic effects and one exothermic effect are visible. The last small mass loss (5.0 %) at the temperature of maximum rate loss of 748 °C, not accompanied by any thermal effects, is most probably related to further transformations of compounds containing sodium atoms, formed during CMS decomposition. The remaining part of the composition sample mass (approx 18.9 %) which did not decompose at the temperature of 1000 °C probably contains carbonised coal and low molecular weight compounds containing sodium atoms.

Key information obtained by analysing the TG–DSC curves for the PAA/CMS composition is presented in Table 2.

Table 2 Analysis of TG–DSC curves of PAA/CMS composition in an oxidising and argon atmosphere
Full size table

The course of TG–DSC curves indicates that the process of thermal decomposition of cross-linked PAA/D composition must also be analysed while considering degradation of both polymer components (Fig. 2), as in the case of PAA/CMS. Together with the temperature increase, degradation in the air occurs gradually (Fig. 2a). Four phases of decomposition can be identified. Within the temperature range of 22–215 °C, one can notice two endothermic effects, probably related to the process of solvent water evaporation (up to 100 °C), followed by dehydration. The temperature of 215 °C can be considered as the beginning of polymer chain decomposition. Within the temperature range of 422–515 °C, three exothermic effects are visible, which may be a result of combustion process, whereas the last one is the strongest. At the temperature of approx 500 °C, PAA/D undergoes complete destruction (strong exothermic effect is observed).

Fig. 2
figure 2

TG–DSC curves of PAA/D composition: a in an oxygen atmosphere, b in oxygen-free atmosphere

Full size image

In the oxygen-free atmosphere, PAA/D decomposition occurs with four mass losses (TG curve, Fig. 2b). DSC curve shows six endothermic effects and one exothermic effect accompanying pyrolysis. The remaining part of the sample mass (approx 25.6 %) which did not decompose at the temperature of 1000 °C probably contains carbonised coal.

Key information obtained by analysing the TG–DSC curves for the PAA/D composition is presented in Table 3.

Table 3 Analysis of TG–DSC curves of PAA/D composition in an oxidising and argon atmosphere
Full size table

Conclusions

On the basis of the obtained results of TG–DSC, it can be stated that the course of decomposition is complex (mixed degradation mechanism), which is a result of the structure, as well as physical and chemical properties of polymer compositions analysed. At the same time, it depends on the conditions in which decomposition occurs. With temperature increase, physical and chemical changes occur in the analysed samples of PAA/CMS and PAA/D polymer compositions.

Within the temperature range of 20–100 °C, evaporation of physically bound water occurs, followed by constitution water, and above the temperature of 100 °C, intermolecular dehydration reaction takes place with formation of intermolecular anhydride bonds among carboxyl groups of neighbouring polymer chains (cross-linking), which is a characteristic of thermal decomposition of poly(acrylic acid). On the basis of TG curves, it can be stated that thermal degradation process starts at the temperature of approx 220 °C. Degradation of modified starch begins with decomposition of weak C–O–C bonds in the glycoside bond (polysaccharides)—temperature range: ~220–300 °C. In this temperature range, polyacrylic chains also get fragmented. Furthermore, side chains can break off, and bonds may crack in the poly(acrylic acid) chain in the –COOH group or in carboxymethyl starch within the –CH2O–CH2O–CH2–COONa group.

Above the temperature of 300 °C, intensive cracking of chemical bonds occurs, which results in breaking of most bonds (temperature range III ~300–500 °C). The process becomes massive, which has an effect of releasing volatile low molecular weight inorganic and organic compounds and finally leads to complete destruction of a sample. At the temperature of approx 550 °C, only carbonised form remains, which can contain carbonised coal, whereas if sodium atoms form part of the composition, low molecular weight compounds are formed containing sodium atoms.