Chapter 7
Polymer and Other Degradation Studies Using Thermal Analysis Techniques 1
2,3
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John P. Redfern and Jay Powell 1
Rheometric Scientific (formerly Polymer Laboratories Ltd.) Thermal Sciences Division, Surrey Business Park, Kiln Lane, KT17 1JF Epsom, England Bio-Rad, Digilab Division, 237 Putnam Avenue, Cambridge, MA 02139 2
Polymer characterisation, stabilisation and degradation are very widely studied by Thermal Analysis (TA). Single techniques, such as thermogravimetric analysis (TG), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and dielectric thermal analysis (DETA) provide important information on the thermal behaviour of materials. However, to obtain a more complete profile of, say, polymer degradation gas analysis is required, particularly since all of the techniques listed give mainly physical information on the behaviour of materials. The use of a simultaneous thermal analyser (STA) which is usually a combined simultaneous measurement of weight and energy usually referred to as a TG-DSC instrument) coupled to a mass spectrometer or to an FTIR provides a very powerful system for such studies. Key elements of the design will be stressed - underlining the importance of the interface system and the advantages of a comprehensive software package. A number of specific studies are discussed in some detail and reference is made to other studies. To say that plastics impacts all our lives is to state the obvious. The plastics industry is big business - a $150 - 180bn dollar business (equivalent to the Gross Domestic Product of a country like Switzerland) and growing worldwide both through increased usage in more and more countries and through new applications. The major end uses are shown in Table 1. There is also growing concern about disposal of used plastics and of the impact on the environment. There is, therefore, a clearly identified need for reliable methods to study the stabilisation and characterisation of these materials, to obtain knowledge on their properties and behaviour, the effects of modifying structure, additives and processes to produce the most appropriate, cost effective material for a specific requirement. The study of the degradation behaviour 3
Corresponding author 0097-6156/94/0581-0081$08.36Α) © 1994 American Chemical Society In Hyphenated Techniques in Polymer Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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H Y P H E N A T E D TECHNIQUES IN P O L Y M E R CHARACTERIZATION
TABLE 1 Major End-Uses for Plastics Packaging Construction Electrical and Electronic Industries Automotive Industry Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 26, 2015 | http://pubs.acs.org Publication Date: December 9, 1994 | doi: 10.1021/bk-1994-0581.ch007
Annual World Sales of $150-180 bn TABLE 2a Thermoplastics
Thermosets
Melting Crystallisation Glass Transition (TJ
Curing Reactions
Expansion and Shrinkage Softening Strength Heat Capacity Thermal Conductivity Solvent and Water Retention
|
TABLE 2b Thermoplastics and Thermosets Ageing Thermal Stability Thermal Degradation Chemical Degradation is also important to contribute to our understanding of three possible decomposition scenarios, namely, in normal usage at operating temperatures, upon disposal or in accidental occurrences, for example in a fire. Therefore, there is a need to study the effects of ageing, the thermal stability, the degradation processes and the products of decomposition under a wide range of conditions. The techniques of thermal analysis are very significant to the wholefieldof polymers in that they provide essential information relating both to their characterisation and their degradation. Table 2a lists areas where Thermal Analysis (TA) provides information for characterisation while Table 2b lists areas where TA provides information on the degradation processes. TA techniques are a group of techniques in which the property of a sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. This programme may involve heating or cooling at a fixed rate of temperature change, or holding the temperature constant, or any sequence of these. The principal techniques are listed in Table 3. Of these the In Hyphenated Techniques in Polymer Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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7.
REDFERN & POWELL
Degradation Studies Using TGA
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most useful in the study of the degradation process are TG, DSC, Simultaneous Thermal Analysis or STA (a combined simultaneous thermobalance with a DSC measuring head incorporated, abbreviated as TG-DSC), STA-MS (a TG-DSC instrument coupled with a mass spectrometer) and a TG-FTIR or STA-FUR ( a thermobalance or a STA linked with an FTIR spectrometer). This review is limited primarily with the application of STA, STA-MS and STA-FTIR to the degradation of polymers and other materials. STA provides a precise simultaneous understanding of the physical phenomena of weight and energy change. The STA coupled to a mass spectrometer or to an FTIR instrument gives a more complete profile of the degradation process.
TABLE COMMON
TECHNIQUE
THERMAL
3
ANALYSIS
TECHNIQUES
ABBREVIATION
MONITORS
SINGLE TECHNIQUES
Thermogravimetry (Thermogravimetric Analysis)
TG
Mass
Differential Scanning Calorimetry
DSC
Energy
Differential Thermal Analysis
DTA
Temperature Difference
Thermomechanical Analysis
TMA
Dimensions
Dynamic Mechanical Thermal Analysis (Dynamic Thermal Analysis)
DMTA
Response to Oscillatory Load
Dielectric Thermal Analysis
DETA
Response to Alternating Current
Evolved Gas Analysis
EGA
Nature and/or Amount of Volatiles
Temperature Programmed Reduction
TPR
Solid-Reducing Gas Interaction
Thermogravimetry-Differential Scanning Calorimetry
TG-DSC
Mass and Energy
+ Mass Spectrometry
TG-DSC-MS
Mass, Energy & Gas Analysis
+ FTIR Spectroscopy
TG-DSC-FTIR
Mass. Energy & Gas Identification
DMA
SIMULTANEOUS TECHNIQUES
In Hyphenated Techniques in Polymer Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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HYPHENATED TECHNIQUES IN POLYMER CHARACTERIZATION
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Instrumentation A. Simultaneous Thermal Analyser (TG-DSQ. The instrumentation for STA is shown schematically in Fig. 1. The advantage of single sample simultaneous experiments have been discussed previously (i, 2). It is appropriate to summarise these advantages: 1 Removes uncertainty - a) precise correlation of events occurring on both TG and DSC assured, b) removes any problems relating to sample inhomogeneity or batch variations. 2 Gives fuller characterisation - ensures greater certainty in identifying thermal events. 3 Validates quantitative measurements from DSC for phase changes, melting and purity measurements. 4 Detects moisture content enabling in-situ dry starting weight to be known accurately. 5 Accurate TG temperature calibration using DSC/DTA standard materials.
M
G
GAS
IN
GAS OUT
Schematic diagram of simultaneous thermal analyser furnace and head. A, Ceramic tube to protect hangdown; B, movable baffle plates with gas ports; C, sample and reference crucibles; D, rigid heatfluxTG-DSC plate; G, gas entry tube; M, mineral insulated graded heating element; N, liquid nitrogen cooling jacket; R, four bore ceramic hangdown suspended from electronic microbalance; S,fixedcompartment divider with gas ports; T, side branch gas exit pipe; W, water-cooled cold finger.
Figure 1. Schematic diagram of a simultaneous thermal analyser. (Reproduced with Permission from Rheometric Scientific)
In Hyphenated Techniques in Polymer Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
In Hyphenated Techniques in Polymer Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
TG
ΔΤ ! capillary heater control*
inlet heater control
separator
molecular leak
rotary pump
rotary pump
turbo pump
QUADRUPOLE ANALYSER
τ
printer
recorder or data system
,Τίτττττί
! 8 analogue J outputs
microprocessor control unit + VDU + disc drive
ANALYSER SECTION
analyser hearing control
MASS SPECTROMETER PACKAGE INLET SECTION
Figure 2. Schematic diagram of a STA-MS system (Reproduced with Permission from Rheometric Scientific)
recorder or data system
balance control
DC amp.
MAIN UNIT with balance and furnace
temperature programmer
STA
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