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THERMAL PROPERTIES

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• Plays a vital role in evaluating the product performance & processibilty characteristics in polymers.

• Thermal analytical methods monitor differences in some sample property as the temperature increases, or differences in temperature between a sample and a standard as a function of added heat. These methods are usually applied to solids to characterize the materials.

THERMAL PROPERTIES

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• Heat Deflection Temperature (HDT)

• Vicat Softening Temperature (VSP)

• Thermal Endurance

• Thermal Conductivity

• Thermal Expansion

• Low Temperature Brittleness

• Flammability

• Melting Point, Tm, and Glass Transition, Tg (DSC)

• Thermomechanical Analysis

THERMAL PROPERTIES

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Heat Deflection Temperature

Defined as the temperature at which a standard test bar (5 x ½ x ¼ in ) deflects 0.010 inch under a stated load of either 66 or 264 psi.

Significance:

• HDT values are used to compare the elevated temperature performance of the materials under load at the stated conditions.

• Used for screening and ranking materials for short-term heat resistance.

• HDT values do not represent the upper temperature limit for a specific material or application.

• The data are not intended for use in design or predicting endurance at elevated temperatures.

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Test Methods, Specimen & Conditioning

Test Method:

• ASTMD 648, ISO 75 -1 and 75-2

Test Specimen:

• 127mm (5 in.) in length, 13mm (½ in.) in depth by any width from 3mm (⅛ in.) to 13mm ((½ in.)

Conditioning:

• 23 ± 2oC and 50 ± 5% RH for not less than 40 hrs prior to test.

Two replicate specimens are used for each test

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Apparatus for Determination of HDT

• Specimen Supports: Metal supports for the specimen of 100 ± 2mm

• Immersion Bath

• Deflection Measurement Device

• Weights: 0.455 MPa (66 psi) ± 2.5% or 1.82 MPa (264 psi) ± 2.5%.

• Temperature Measurement System

Apparatus

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Procedure

• Measure the width and depth of each specimen• Position the test specimens edgewise in the apparatus• Position the thermometer bulb sensitive part of the

temperature• Stir the liquid-heat transfer medium thoroughly• Apply the loaded rod to the specimen and lower the

assembly into the bath.• Adjust the load to obtain desired stress of 0.455 MPa (66 psi)

or 1.82 MPa (264 psi)• Five minutes after applying the load, adjust the deflection

measurement device to zero or record its starting position• Heat the liquid heat-transfer medium at a rate of 2.0 ±

0.2oC/min.• Record the temperature of the liquid heat-transfer medium at

which the specimen has deflected the specified amount at the specified fibre stress.

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Calculation

The weight of the rod used to transfer the force on the test

specimen is included as part of the total load. The load (P) is

calculated as:

P = 2Sbd2 / 3L

Where,

S = Max. Fibre stress in the specimen of 66 Psi / 264 Psi

b = Width of specimen

d = Depth of specimen

L = Width of span between support (4 in)

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• A bar of rectangular cross section is tested in the edgewise

position as a simple beam.• Load applied at the center to give maximum fibre stresses of

66 /264 psi. • The specimen is immersed under load in a heat-transfer

medium provided with a means of raising the temperature at 2

± 0.2oC/min. • The temperature of the medium is measured when the test bar

has deflected 0.25mm (0.010 in). • This temperature is recorded as the deflection temperature

under flexural load of the test specimen.

Results & Conclusion

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Factors influencing

• HDT of unannealed (heat treatment) specimen is

usually lower than that of annealed specimen.• Specimen thickness is directly proportional to HDT

because of the inherently low thermal conductivity

of plastic materials.• Higher the fibre stress or loading lower the HDT.• Injection moulded specimen tend to have a lower

HDT than compression – moulded specimen. • Compression moulded specimen are relatively

stress free.

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Vicat Softening Point (VSP)

Defined as the temperature at which a flat ended probe with 1 mm2 cross section penetrates a plastic specimen to 0.04 inch (1 mm) depth.

Significance

• Data obtained by this test method may be used to compare the heat-softening qualities of thermoplastic materials.

• This test method is useful in the areas of quality control, development and characterization of plastic materials.

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Test Methods, Specimen & Conditioning

Test Method: • ASTMD 1525 or ISO 306

Test Specimens : • The specimen shall be flat, between 3 and 6.5mm

thick and at least 10 by 10mm in area or 10mm in diameter.

Conditioning: • 23 ± 2oC and at 50 ± 5% relative humidity of not

less than 40 hrs

A minimum of two specimens shall be used to test each sample.

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Fig. 2 Apparatus for Softening Temperature Determination

• Immersion Bath

• Heat-Transfer Medium

• Specimen Support

• Penetration-Measuring Device Masses: 10 ± 0.2N or 50 ±1.0N

• Temperature-Measuring Device

• Needle

Apparatus

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Procedure

• Prepare the immersion bath so that the temperature of the heat-transfer medium is between 20 and 23oC at the start of the test

• Place the specimen, which is at room temperature, on the specimen support.

• The needle should not be nearer than 3mm to the edge of the specimen.

• Gently lower the needle rod, without the extra mass, so that the needle rests on the surface of the specimen and holds it in position.

• Position the temperature-measuring device so that the sensing end is located within 10mm from where the load is applied to the surface of the specimen.

• Lower the assembly into the bath and apply the extra mass required to increase the load on the specimen to 10 ± 0.2N (Loading 1) or 50 ± 1.0N (Loading 2).

• After a 5-min waiting period, set the penetration indicator to zero. • Start the temperature rise. • Record the temperature of the bath when the needle has

penetrated 1 ± 0.01mm into the test specimen.

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Results & Conclusion

• Vicat softening temperature is expressed as the arithmetic mean of the temperature of penetration of all specimens tested.

• If the range of penetration temperatures for the individual test specimens exceeds 2oC, record the individual results and repeat the test, using at least two new specimens.

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Thermal Conductivity• Rate at which heat is transferred by conduction through a unit

cross sectional area of a material when a temperature gradient exists perpendicular to the area.

• `• The coefficient of thermal conductivity (K factor), is defined as

the quantity of heat that passes through a unit cube of the substance in a given unit time when the difference in temperature of the two faces is 10C.

• Mathematically, thermal conductivity is expressed as K = Qt/A(T1-T2)

• Q = amount of heat passing through a cross section, A causing a temperature difference, ∆T (T1-T2), t = thickness of the specimen.

• K is the thermal conductivity, typically measured as BTU.in / (hr.ft2.0F) indicates the materials ability to conduct heat energy.

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Significance

• Thermal conductivity is particularly important in applications such as headlight housings, pot handles & hair curlers that require thermal insulation or heat dissipation properties.

• Computerized mold-filling analysis programs requires special thermal conductivity data derived at higher temperatures than specified by most tests.

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Test Methods & Specimen

• Test method: Guarded hot plate test ASTM D177, ISO 2582

• Test Specimen: two identical specimens having plane surface of such size as to completely cover the heating unit surface

• The thickness should be greater than that for which the apparent thermal resistivity does not change by more than 2% with further increase in thickness

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Apparatus

The apparatus is broadly of two different categories of the following:

• Type I (low temperature) Temperature of cold plate : 21 K, Temperature of heating unit:<500 K

• Type II (High temperature) Temperature of heating unit range:>550 K -<1350K

• Heating units• Gap & Metering Area• Unbalance Detectors• Cooling units• Sensors for measuring Temperature difference• Clamping force• Measuring system for Temperature detector outputs

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Guarded Hot plate ApparatusCourtesy: Bayer Material Data Sheet

Guarded Hot plate Apparatus

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• Two test specimens are sandwiched between the heat source (main heater) & heat sink; one on either side of the heat source.

• The clamping force is so adjusted that the specimens remain in perfect contact with the heater & sink

• Guard heaters are provided to prevent heat flow in all except in the axial direction towards the specimen

• The time of stabilization of input & out put temperature is noted.

• Temperature difference between the hot & cold surfaces of the specimen should not be less that 5 K or suitable differences as required.

Procedure

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Calculation

The relationship between the quantity of heat flow and thermal conductivity is defined as

Q ~ K/ x

Q = Quantity of heat flowK = Thermal ConductivityX = The distance the heat must flow

Thermal conductivity is calculated as :

K = Qt / A (T1 – T2)

Q = Rate of heat flow (w)T = Thickness of specimen (m)A = Area under test (m2)T1 = Temperature of hot surface of specimen (k)T2 = Temperature of cold surface of specimen (k)

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• Thermal conductivity is calculated by using the value of rate of flow at a fixed temperature gradient.

• Data are obtained in the steady state

Results & Conclusion

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Factors influencing

• Crystallites have higher conductivity.

• As the density of the cellular plastic decreases, the conductivity also decreases up to a minimum value and rises again due to increased convection effects caused by a higher proportion of open cells.

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Thermal Expansion (Coefficient of Linear Thermal

Expansion, CLTE)• Measures the change in length per unit length

of a material, per unit change in temperature.

• Expressed as in/in/0F or cm/cm/0C

• Mathematically, CLTE (α), between temperatures T1 and T2 for a specimen of length L0 at the reference temperature, is given by :

• α = (L2 – L1)/[L0(T2 – T1 )] = L/L0ΔT

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Significance

• Determines the rate at which a material expands as a function of temperature.

• The higher the value for this coefficient the more a material expands and contracts with temperature changes.

• Plastics tend to expand and contract anywhere from six to nine times more than materials that are metallic.

• The thermal expansion difference develops internal stresses and stress concentrations in the polymer, which allows premature failure to occur.

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Test Method: ASTMD 696

Test Specimen:

• 12.5 by 6.3mm (½ in. by ¼ in.) 12.5 by 3mm (½ by ⅛ in.), 12.5mm (½ in.) in diameter or 6.3mm (¼ in.) in diameter.

Conditioning: 23 ± 2oC and 50 ± 5% RH for not less than 40h prior to test.

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• A vitreous silica dilometer

• Dial gage

• The weight of the inner silica tube + the measuring device reaction shall not exert a stress > 70 kPa on the specimen so that the specimen is not distorted or appreciably indented.

• Scale or Caliper

• Controlled Temperature Environment

• Means shall be provided for stirring the bath

• Thermometer or thermocouple

Apparatus

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Procedure

• Measure the length of two conditioned specimen at room temperature

• Mount each specimen in a dilatometer, install the dilatometer in the –30oC control environment.

• Maintain the temperature of the bath in the range –32oC to –28oC ± 0.2oC until temperature of the specimen along the length is constant

• Record the actual temperature and the measuring device reading. • Change to the + 30oC bath, so that the top of the specimen is at least

50mm below the liquid level of the bath. • Maintain the temperature of the bath in the range from + 28 to 32oC ±

0.2oC • Record the actual temperature and the measuring device reading. • Change to –30oC and repeat the above procedure & measure the final

length of the specimen at room temperature.• If the change in length per degree of temperature difference due to

heating does not agree with the change length per degree due to cooling within 10% of their average investigate the cause of the discrepancy and if possible eliminate.

• Repeat the test until agreement is reached.

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• Calculate the CLTE over the temperature range as:

α = ΔL/LoΔT

α = Average coefficient of linear thermal expansion degree Celsius. ΔL = Change in length of test specimen due to heating or to cooling, Lo = Length of test specimen at room temperature (ΔL and

Lo being measured in the same units), and

ΔT = Temperature differences, oC, over which the change in the length of the specimen is measured.

• The values of α for heating and for cooling shall be averaged to give the value to be reported.

Calculation

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Result & Conclusion

• Provide a means of determining the CLTE of plastics, which are not distorted or indented by the thrust of the dilatometer on the specimen.

• The specimen is placed at the bottom of the outer dilatometer tube with the inner one resting on it.

• The measuring device, which is firmly, attached to the outer tube is in contact with top of the inner tube and indicates variations in the length of the specimen with changes in temperature.

• Temperature changes are brought about by immersing the outer tube in a liquid bath or other controlled temperature environment maintained at the desired temperature.

• The nature of most plastics and the construction of the dilatometer make –30 to +30oC a convenient temperature ranges for linear thermal expansion measurements of plastics.

• This range covers the temperatures in which plastics are most commonly used.

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• Thermal expansion is substantially affected • by the use of additives• especially fillers • Wt% Of loading

Lowers the coefficient of thermal expansion.

Factors influencing

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Differential Scanning

Calorimetry (DSC)• DSC measures the heat flow into or from a

sample as it is heated, cooled or held under isothermal conditions

• Applications of DSC includes characterization of

• Polymers• fibres• Elastomers• Composites• films• pharamaceuticals• foods• cosmetics

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• DSC provides the following important properties of materials

• Glass Transition Temp. (Tg)• Melting point (Tm)• Crystallization times & Temp.• Heats of melting & crystallization• Percent Crystallinities• Heat set temp.• OIT• Compositional Analysis• Heat capacities• Heats of cure• Thermal Stabilities

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DSC apparatus consists of

Furnace Temperature Sensor Differential Sensor Test Chamber Environment Temperature Controller Recording Device Sealed pans Balance

Apparatus

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Terminologies:

Glass Transition Temperature (Tg): it is defined as the temperature below

which the polymer is in the glassy state & above which it attains rubbery

state.

First order transitions: In a first-order transition there is a transfer of heat

between system and surroundings and the system undergoes an abrupt

volume change eg. Melting point (Tm), Crystallization Temperature (Tc)

Second order transitions: In a second-order transition, there is no

transfer of heat, but the heat capacity does change. The volume changes to

accommodate the increased motion of the wiggling chains, but it does not

change discontinuously..

• Samples: Powder, Liquids, crystal

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Procedure

DSC apparatus consists of two sealed pans sample and reference aluminum pans

The pans are heated, or cooled, uniformly while the heat flow difference between the two is monitored.

This can be done at a constant temperature (isothermally), but is more commonly done by changing the temperature at a constant rate, called temperature scanning.

The instrument detects differences in the heat flow between the sample and reference & plots the differential heat flow between the reference and sample cell as a function of temperature.

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Specimen mass appropriate of 5-mg is taken in the pan

Intimate thermal contact between the pan and specimen is

established for reproducible results.

Heat the sample at a rate of 10oC/min under inert gas

atmosphere from 50oC below to 30oC above the melting

point to erase the thermal history .

The selection of temperature and time are critical when

effect of annealing is studied.

Hold temperature for 10min.

Cool to 50oC below the peak crystallization temperature at

a rate of 10oC/min and record the cooling curve.

Repeat heating as soon as possible under inert purge gas

at a rate of 10oC/min, and record the heating curve.

First Order Transitions (Tc, Tm)

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Use a specimen mass of 5-mg.

Perform and record a preliminary thermal cycle as up

to a temperatures 30oC above the extrapolated end

temperature, Te, to erase previous thermal history,

heating at a rate of 20oC/min.

Hold temperature for 10min.

Quench cool to 50oC below the transition

temperature of interest.

Hold temperature for 10min.

Repeat heating at a rate of 20oC/min, and record the

heating curve until all desired transition have been

completed.

For Second order Transition (Tg)

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Typical DSC curves

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Measurement of various Properties/Explanations

• Heat Capacity

• Heating the sample & Reference pans, the the difference in heat output of the two heaters is plotted against temperature. i.e the heat absorbed by the polymer against temperature.

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Dividing,

Heat Capacity

• The heat flow at a given temperature is represented units of heat, q supplied per unit time, t.

• The heating rate is temperature increase T per unit time, t.

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Glass Transition

• Property of the amorphous region

• Below Tg: Disordered amorphous solid with immobile molecules

• Above Tg: Disordered amorphous solid in which portions of molecules can wiggle around

• A second order transition ( Increase in heat capacity but there is no transfer of heat

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We call crystallization an exothermic transition.

Crystallization

• Above Tg, the polymers are in mobile conditions.

• When they reach the right temperature, they gain enough energy to move into very ordered arrangements, which we call crystals,

• When polymers fall into these crystalline arrangements, they give off heat.

• When this heat is dumped out, there is drop in the heat flow as a big dip in the plot of heat flow versus temperature:

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Melting

Above Tc, we reach the polymer's melting temperature, or Tm, those polymer crystals begin to fall apart, that is they melt.

The chains come out of their ordered arrangements, and begin to move around freely.

Melting is a first order transition (Tm).

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Putting it all together

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multiply this by the mass of the sample

Polymer crystallinity

Measure the area of under the melting of the polymer.

Plot of heat flow per gram of material, versus temperature.

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X 100% = Xc

Where H’= Heat of Fusion determined from DSC thermogram

H*m= Heat of fusion of a 100% crystalline sample

Degree of crystallinity is given by

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Results & Conclusion

• DSC thermograms provides an elaborate picture of various transitions in a polymer.

• The degree of crystallinity in a polymer sample, specific heat etc. can be determined.

• Any side reaction (for example, crosslinking, thermal degradation or oxidation) shall also be reported and the reaction identified if possible.

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Factors affecting

• Addition of fillers affects the transitions in DSC

• Previous thermal history of the samples also affects the DSC transitions.

• There should be proper contact between the samples & pans

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Thermo Gravimetric

Analysis (TGA)

Changes in weight of the specimen is recorded as the specimen is heated in air or in a controlled atmosphere such as nitrogen

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Terminologies Highly volatile matter – moisture, plasticizer, residual

solvent or other low boiling (200oC or less)

components. Medium volatile matter – medium volatility materials

such as oil and polymer degradation products. In

general, these materials degrade or volatilize in the

temperature range 200 to 750oC. Combustible material – oxidizable material not volatile

(in the unoxidized from) at 750oC, or some stipulated

temperature dependent on material. Carbon is an

example of such a material. Ash – nonvolatile residues in an oxidizing atmosphere

which may include metal components, filler content or

inert reinforcing materials. Mass loss plateau – a region of a thermogravimetric

curve with a relatively constant mass.

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Thermogravimetric curves (thermograms) provide information regarding

polymerization reactions, the efficiencies of stabilizers and activators, the

thermal stability of final materials, and direct analysis.

Provides a general technique to determine the amount of highly volatile matter,medium volatile matter, combustible material and ash content of compounds.

This test method is useful in performing a compositional analysis in polymers

This test method is applicable to solids and liquids.

Significance

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Apparatus

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Procedure

Establish the inert (nitrogen) and reactive (air oxygen) gases at the desired flow rates in the range of 10 to 100mL/min.

Switch the purge gas to the inert (nitrogen) gas. Zero the recorder and tare the balance. Open the apparatus to expose the specimen holder. Prepare the specimen of 10 to 30mg and carefully place it in the

specimen holder. Position the specimen temperature sensor Enclose the specimen holder. Record the initial mass. Initiate the heating program within the desired temperature

range. Record the specimen mass change continuously over the

temperature interval. The mass loss profile may be expressed in either milligrams or

mass percent of original specimen mass. Once a mass loss plateau is established in the range 600 to

1200oC, depending on the material, switch from inert to reactive environment.

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Highly volatile matter content may be determined by the following equation

V = [(W-R)/W] x 100% (1)

Where:

V = highly volatile matter content, as received basis (%),

W = original specimen mass (mg), and

R = mass measured at Temperature X (mg).

Calculation

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Medium volatile matter content can be determined using the following

equation:

O = [(R-S)/W] x 100% (2)

Where:

O = medium volatile matter content, as-received basis, %

R = mass measured at Temperature X, (mg),

S = mass measured at Temperature Y, (mg), and

W = original specimen mass, (mg).

Calculation

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Combustible material content may be calculated by the following equation:

C = [(S-T)/W] x 100% (3)

Where:

C = combustible material content, as-received basis, (%),

S = mass measured at Temperature Y, (mg),

T = mass measured at Temperature Z, (mg) and

W = original specimen mass, (mg).

Calculation

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The ash content may be calculated using the following equation:

A = (T/W) x 100% (4)

Where:

A = ash content, as received basis, (%),

T = mass measured at Temperature Z, (mg) and

W = original specimen mass.

Calculation

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Factors influencing

• Oil-filled elastomers have such high molecular weight oils and such low molecular weight polymer content that the oil and polymer may not be separated based upon temperature stability.

• Ash content materials (metals) are slowly oxidized at high temperatures and in an air atmosphere, so that their mass increases (or decreases) with time. Under such conditions, a specific temperature or time region must be identified for the measurement of that component.

• Polymers, especially neoprene and acrylonitrile butadiene rubber (NBR), carbonize to a considerable extent, giving low values for the polymer and high rubber values.

Others, such as calcium carbonate, release CO2 upon decomposition at interference is dependent upon the type and quantity of pigment present.

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Dynamic Mechanical Analysis

(DMA)

DMA is a technique in which a substance while under an oscillating load is measured as a function of temperature or time as the substance is subjected to a controlled temperature program in a controlled atmosphere.

Dynamic Mechanical Analysis (DMA) examines materials between -170°C and +1000°C.

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DMA - Storage Modulus

Storage Modulus

DSC Tg

Hard andBrittle Elastic andDeformable Fluid

Tm

105

106

107

108

109

1010

Temperature

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DMA for Testing Recyclates Virgin ABS and ABS with 25%

Recyclate

-150 Temperature ( C) 100

E’

E”

DMA of ABS With Recyclates

The high inherent sensitivity of DMA provides a means of testing polymers with

recyclates

In this example of virgin ABS and ABS with 25% recyclates, the effects

of the recyclates can be easily observed

Testing was done with 3-point bending probe

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FOURIER TRANSFORM INFRARED TECHNIQUE

Identification of plastic through structural Identification of plastic through structural analysisanalysis

Identification of additives, fillers, etc.Identification of additives, fillers, etc. Polymer blend analysisPolymer blend analysis Monomer content analysis on plasticsMonomer content analysis on plastics Compatibility studies on blendsCompatibility studies on blends Curing of polymersCuring of polymers Degradation studiesDegradation studies

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