Thermal degradation of biopolymer binders

ARCHIVES of

ISSN (1897-3310) Volume 10 Issue Special1/2010 221-224

FOUNDRY ENGINEERING

42/1

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

Thermal degradation of biopolymer binders: the example of starch-poly(acrylic acid) B. Grabowska a,* Chair of Casting Process Engineering, Faculty of Foundry Engineering, AGH University of Science and Technology ul Reymonta 23, 30-059 Kraków, Poland *Contact in writing: e-mail: [email protected]

Received: 26.02.2010; accepted in revised form: 30.03.2010

Summary To characterise a polymer, it is of fundamental importance to determine its parameters, like the temperatures of destruction, vitrification, melting point, specific mass losses or polymorphic transformations, which frequently determine the quality of the product and its applications. Thermal analyses were conducted of samples of a biopolymer binder: a starch-poly(acrylic acid) composition and a moulding sand with a biopolymer binder previously hardened with microwaves. In order to determine the thermal stability of the examined samples by determining the destruction temperature and the thermal effects of transformations taking place during heating, FTIR spectroscopy and thermal analysis (DSC, DTG, TG) methods were used. In addition, volatile products of degradation were analysed using the thermogravimetry (TG) method coupled online with mass spectrometry (MS). These examinations were also aimed at identifying the changes that can take place in the moulding sand when it comes into contact with liquid metal. Key words: biopolymer binders, thermal degradation, destruction, moulding sand

1. Introduction During use, polymers are exposed to the effects of many physical factors, including physical stress, temperature, UV radiation, ultrasounds, electrical discharges and ionising radiation [1-4]. As a result of the action of these factors, the chemical and physical structure of polymers is destructively changed, which includes degradation. During degradation, the molar mass is reduced as a result of breaking polymer chains and forming new molecules with shorter chains. Polymer degradation usually occurs as a result of physical and biological factors (Fig. 1). The general mechanism of polymer degradation is radical in its course and the initiation which causes the degradation process may be due to heat, electromagnetic radiation, ionising radiation or mechanical stress (Fig. 2). Macro-radicals formed in the above fashion may undergo many reactions, including the depolymerisation process (the release of a monomer brought about by high temperature), the transfer of a free radical both in intra- and extra-molecular reactions, the dismutation reaction during which a new macroradical and a macromolecule with a double bond are formed, and also mutual recombination, forming branched or cross-linked structures.

Individual bonds within the macromolecule may dissociate into free radicals if energy exceeding the bond energy is supplied to them (the bond dissociation energy) (table 1) [1]. Organisms Radiation

Biodegradation

Photodegradation

NO2, SO2 NH3, CO H2O2, O2 metals

Environmental degradation Mechanical degradation

Thermal degradation Hydrolytic degradation Temperature

Stress Water

Fig. 1. Diversity of processes causing polymer degradation

ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 221-224

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¾

Fig. 2. A scheme of the reaction initiating the polymer thermal degradation process Table 1. Example magnitudes of bonding power in polymers Bond type

Dissociation energy (Ed) [kJ•mol-1]

C–C C–H C–O

348 412 360

C=O

743

O–H O–O

463 146

O–Si

466

2. Research methods Materials The following materials were used in the research conducted: ¾ binder: a biopolymer composition (KOMP. A) made up of a synthetic polymer: poly(acrylic acid) produced by BASF (Fig. 3a) and a natural polymer: starch by Xenon, in the 1:1 weight ratio (Fig. 3b and 3c); CH2OH O

CH2 CH C OH

OH

O

OH

OH

O

O OH O CH 2

O

OH O OH

a) b) c) Fig. 3 The overall structure: a) poly(acrylic acid), b) starch (amylose), c) starch (amylopectin)

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Preparing samples for examinations ¾ samples of moulding sand with a biopolymer binder, 1:1 weight ratio [8, 9]; ¾ cross-linking and hardening samples with microwaves: RM 2001 Pc microwave reactor by Plazmatronika (800 W, 90s) [8, 9]. Infrared spectroscopic examinations, FT-IR Infrared spectroscopic examinations were carried out using a Digilab Excalibur FTS 3000 Mx spectrometer with a DTGS detector, electrically cooled. The spectrometer is equipped with two attachments: ATR with a ZnSe crystal for multiple reflections and a transmission attachment.

When polymers are heated, reversible and irreversible changes take place. Reversible changes occur due to phase transitions, the disaggregation of supramolecular structures and the polymer transiting into the plastic state. Irreversible changes occur above the flow temperature, when polymers are thermally degraded, which is often accompanied by the emission of volatile lowmolecular substances. The polymer degradation temperature depends on the chemical structure of the macromolecule, the phase state of the polymer and the admixture content. The majority of polymers are thermally degraded at a temperature greater than their flow temperature (amorphous polymers) or melting point (crystalline polymers). During this process, quite a large quantity of low-molecule products of polymer degradation is emitted. Depending on the structure, carbon oxide (IV), carbon oxide (II), water, methane, ethane, propylene, acetylene and benzene may be produced. The final product is usually a carbonised polymer sample [1, 4-6].

CH2OH O

matrix: standard quartz sand from the Jaworzna-Szczakowa sand mine.

Thermal analysis examinations The NETZSCH model STA 449 F3 Jupiter® simultaneous thermal analyzer can be used to measure the mass change and transformation energetics of a wide range of materials. The toploading STA can be equipped with various easy exchangeable TG, TG-DTA or true TG-DSC sensors and with different furnaces to accommodate different application areas. The system employed for this work was equipped with a steel furnace capable of operation from -150 to 1,000°C. For control of the measurements as well as for data acquisition, modern digital electronics and the well established NETZSCH PROTEUS® 32bit Software are employed. Several Advanced Software packages like c-DTA® (calculated DTA-signal), Super-Res® (ratecontrolled mass change) or Thermokinetics® are available. Furthermore, combining both thermogravimetric and spectroscopic methods such as MS enables identification of the evolved gases.

3. FT-IR analysis

examinations

and

their

In order to explain processes taking place in the analysed biopolymer composition as a result of its heating, spectroscopic (FT-IR) examinations were conducted. Temperature spectra were recorded for a sample of the biopolymer composition at 25°C-180°C (the operating range of the temperature attachment of the IR spectroscope). Heating progressed continuously with the spectrum being recorded at the set temperature. Within the 25°C-180°C temperature range, the spectra discussed (Fig. 4) show a wide band in the 3,700-3,000 cm-1 wave number region which corresponds to the tensile vibrations of the hydroxyl group (the band of the free OH group from water and O-H...O=C hydrogen bonds). At the temperature of 120°C, a number of changes in spectrum shapes can be observed. The reduced intensity of the absorption band corresponding to the tensile vibration of -OH groups is mainly connected with water evaporating due to the temperature impact. At 120°C, two bands disappear in the 1,800-1,500 cm-1 area (vibrations of COO- and C-OH), and a new band appears: 1,700 cm-1 (C=O vibrations). These changes are connected with degradation processes


occurring due to the impact of the temperature: polymer chain fragment, and a result of this some bonds disappear and new ones are formed, which is obvious in the shape of the spectra.

to the progressive fragmentation of the polymer chain and the formation of new, shorter hydrocarbon chains in various structural configurations. In addition, the band in the 2300 cm-1 region, associated with the formation of CO2, can be seen to grow in intensity.

4. Thermal analysis examinations Figure 6 depicts the TG-DSC results for the sample KOMP.A. At sub-ambient temperatures, no effects were observed. Three mass loss steps of 8.1%, 25.5% and 41.7% occurred which were accompanied by endothermic effects visible in the DSC signal. Maxima in the rate of mass change occurred at 133°C, 276°C and 422°C.

Absorbance

180oC 160oC

DTG /(%/min) DSC /(mW/mg)

TG /%

↓ exo

120oC

1.6

0

120 132.7 °C

1.4 -2

100oC

100

1.2

-8.11 % 276.2 °C

80oC

80

Sample: KOMP.A

-25.48 % 217.7 °C

50oC

1.0 422.4 °C

-4

0.8

144.6 °C

0.6

60

25oC

-41.72 % 443.0 °C

40

0.2 880 J/g

-6

0.4 -8

36.1 J/g

0.0

20

-10

4000

3500

3000

2500

2000

1500

1000

Wavenumber, cm-1

Fig. 4. FT-IR temperature spectra for the biopolymer composition

Absorbance

Figure 5 shows the FT-IR temperature spectra at the 300°C500°C range for a moulding sand composed of a biopolymer binder on a quartz matrix within the 4,000-800 cm-1 range.

500oC

400oC

300oC

4000

3500

3000

2500

2000

1500

1000

Wavenumber, cm-1

Fig. 5. FT-IR temperature spectra for the moulding sand with a biopolymer binder hardened with microwaves Between the 3,700 cm-1 and 2900 cm-1 wave numbers, the absorption corresponding to tensile vibrations of hydroxyl groups within hydrogen bonds gradual can be seen to gradually fade. Conversely, the band in the 2900 cm-1 region, connected with C-H symmetrical and asymmetric tensile vibrations, changes its shape and becomes more intensive. This may be due

-100

0

100

200 Temperature /°C

300

400

500

Fig. 6. Temperature-dependent mass change (TG), rate of mass change (DTG, dashed line) and heat flow rate (DSC) of the sample biopolymer composition in the hardened state. At the -100°C-0°C temperature range no polymorphic transformations were detected. The shape of thermal curves is complex because the degradation process progresses in stages, which is due to the structure and the physical chemistry properties of the two-component biopolymer composition in question. Shapes of thermal curves support the claim that the degradation process starts at the temperature of 132°C. To analyse the volatile products of decomposition, the thermogravimetry (TG) method coupled online with mass spectrometry (MS) was used in this research. Within the 140°C400°C temperature range, signals of low molecular weights were detected, proving the occurrence of processes of degradation, polymer chain fragmentation and the formation of low molecular compounds (molar masses: 15, 18, 22, 37, 38, 39, 41, 43, 44, 45, 46, 51 and 55 g/mol), during which mainly the following low molecular compounds are formed: H2O, CO2 and alkyl radicles among others CH3. (Fig. 7) [10]. A microscopic photograph taken of a moulding sand sample with a biopolymer binder baked at the temperature of 500°C shows carbonisations of the binder resulting from the action of high temperature, which means that the polymer has suffered partial destruction (Fig. 8).


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TG /%

QMID /A

100.0 2

148 °C

10 -9

90.0

274 °C s:1, m:18

5

382 °C 2

80.0

10 -10

s:1, m:15

5

70.0

439 °C 2

s:1, m:44 60.0

50.0

structural configurations. It can also be said that during baking, low molecule organic compounds (alkyl radicles) and inorganic ones (mainly CO2 and H2O) are formed. Microscopic examinations have confirmed that at 500°C, the polymer under consideration suffered partial destruction.

10 -11

s:1, m:22

Bibliography

5

s:1, m:43 s:1, m:39 s:1, m:41

2 10 -12 5

40.0

s:1, m:37 s:1, m:45 s:1, m:38 s:1, m:46

282-U-09

s:1, m:51 s:1, m:55

2 10 -13

30.0

Komp A 100

200

300 Temperature /°C

400

5 500

Fig. 7. Mass changes (TG) and MS signals of a sample biopolymer composition

Fig. 8. The morphology of a moulding sand with a biopolymer binder made using quartz sand (baking temperature of 500°C)

5. Summary Infrared (IR) spectroscopic examinations at higher temperatures have confirmed the results of the thermal analysis (DSC) aimed at determining the thermal stability of the polymer. Based on the research conducted, it can be said that, starting at the temperature

[1]

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of 130°C, a gradual process of degradation occurs, associated with the progressive fragmentation of polymer chains and the formation of new, shorter hydrocarbon chains in various

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