Understanding Rheology of Thermosets

revised by A.J. Franck, TA Instruments 2 Rheology of Thermosets Rheological Testing of Thermosetting Polymers

General Considerations the controlled influence of heat and pressure over time. Thus, Thermoset Polymer Uses thermosets build their final struc- Thermoset polymers form the ture during processing, forming a matrix in filled plastics and fiber- three-dimensional internal struc- reinforced composites used in a tural network of highly crosslinked diversity of products. These range polymer chains. And the final from consumer items and auto product is insoluble and not ther- body panels to advanced compos- mally reformable. ites for printed circuit boards Elastomers share characteris- (PCBs), aerspace structural tics of thermoplastics and compo-nents such as the Space thermosets. Elastomers begin Shuttle payload bay door and jet as thermoplastic polymers with engine cowls and ducts, and ex- discrete chains that later develop pensive, high-performance sports a network of covalent crosslinks. equipment. Also, thermosets are However, elastomers are distin- used extensively as adhe-sives, guished from thermosets by the molding compounds, and surface fact that the crosslink network is coatings, including protective sol- formed in a separate post-polyme- der masks for PCBs. rization step called vulcanization. Another factor distinguish-ing the Thermosets versus Ther- three classes of polymers is the

moplastics and Elastomers glass transition temperature Tg – the temperature at which poly- Thermoset polymers are distin- mers reversibly transmute be- guished from elastomers and tween rubbery and glassy states. thermoplastic polymers in several For elastomers, the glass transi- ways. tion tempe-ature is below ambi- Thermoplastics are processed in ent temperatures; for thermoplas- the molten state, their final shape tics and thermosets, it is substan- and internal structure established tially above ambiant. by cooling, and they can be sof- tened and reshaped by reapplica- Studying the Crosslinking tion of heat and pressure. Also, Reaction their polymer chains, whether lin- ear or branched, remain di-screte The formation of a thermoset after molding. Thermoset poly- crosslinked network is shown mers usually go through three schematically in figure 1. stages. In the A-stage, some- times called a resole, the resin is Understanding of this pro-cess still soluble and fusible. In the B- has been advanced substantially stage, thermosets are nearly in- by use of rheological analysis – soluble, but are still thermoplas- measurements of resin viscosity, tic. They can, however, spend only shear modulus, and damping. a rela-tively short time in the mol- More sensitive than even Fourier ten state because the tempera- transform infrared spectroscopy tures that promote flow also cause for measuring extent of cure, the material to crosslink. The rheological testing has become crosslinking reaction is accom- a vital supplement to DSC, chro- plished in the final stages of po- matography, and wet chemical lymerization – the C-stage – dur- analysis in thermoset polymer ing molding of the product under re-search and develop-ment. The reasons for this reside in the na- AAN015 3 Rheology of Thermosets

ture of the processes involved larly epoxies, must be protected and the rheological changes in- from moisture and refrigerated to herent in these. forestall ”advancement”, that is, conti-nuation of the reaction. Thermoset Processing To fabricate a product, a multilayer Multistage Processing sandwich of prepreg is prepared, placed in a press or autoclave, Quite commonly, thermosetting and heated to ”cure”. This resins are processed in two or processing step must accom- more stages, as, for example, are plish several things: wet the those used in the production of fibers thoroughly with resin, con- fiber reinforced laminates. solidate the laminate to the de- First, a laminating varnish is pro- sired thickness, remove excess duced from resin monomer, cur- resin, exhaust trapped air, mois- ing agent, other functional addi- ture, and solvent to avoid poro- tives, and a suitable solvent. sity, and advance the cross-link- Then this varnish is applied to a ing reaction to the desir-ed level. reinforcing fabric made of glass As the temperature is raised, the fiber or other high modulus prepreg melts and its viscosity fibers, the solvent is removed, drops precipitously, then levels off and the resin is partially reacted and again climbs. This viscosity to form a ”prepreg”. At this point, behavior is the consequence of the B-stage, the prepreg is a par- the intervention of a competing tially soluble, low-melting ther- circumstance: The rising tem- moplastic solid composed of perature triggers further reaction unreacted monomer, adduct (the among monomer, curing agent, re-action product of a one-to-one and oligomers increasing the pre- ratio of resin and curing agent), preg’s viscosity. Because the and higher molecular weight reactants are multi-functional, oligomers. Some resins, particu- crosslinks form among the poly- mer chains quickly in all direc- tions. Suddenly the system gels and flow ceases. At this point, the matrix is solid and the thickness and shape of the composite are fixed. Additional heating is need- ed, however, for the matrix to form the balance of ist potential crosslinks and to develop, for the most part, ist ultimate level of stiff- ness. To accomplish this, an ad- ditional out-of-mold post-cure step is generally used. Single-stage Processing

Thermosets are also pro-cessed in a single-stage method called reaction injection molding (RIM). In use on a major scale since late in the seventies, RIM has become popular for a variety of automotive Figure 1: Schematic representation of structural development and recreational applications. during thermoset curing: (A) unreacted monomers; (B) formation of small branched molecules; (C) the gel point: a path of covalent In some respects, RIM resem- bonds exists across the sample; (D) the cured, crosslinked polymer bles thermoplastic injection with some unreacted groups and reactants. molding (TIM). The key difference, however, is that RIM uses polym- A.Franck 10/04 V1 4 Rheology of Thermosets

erization in the mold rather than ing. In structural laminates, resin cooling to form a solid polymer. flow influences porosity, cured part dimensional uniformity, and Other reaction molding processes process economics. In multilayer also use polymerization to solidify printed wiring boards, resin flow the molded piece; however, in properties also influence unifor- thermoset injection molding, for mity of etched circuit encap- example, reactants are heated to sulation, copper-to-epoxy adhe- around 200 °C to activate the reac- sion, and final press thickness. tion. In RIM, the reactants are Therefore, at each stage of the combined at a relatively lower tem- pro-cess, the rheological state perature (about 40°C) because the should be known to employ these reaction is activated by impinge- materials effectively and economi- ment mixing, not external heat. cally. In the RIM process, complex The rheology of thermosetting res- plastic parts are quickly produced ins can be studied using both directly from low viscosity react- steady shear and dynamic oscil- ants. Two or more liquids are latory tests (sometimes referred impin-gement mixed at low tem- to as dynamic mechanical analy- peratures and pres-sures as they sis (DMA) or dynamic mechani- enter the mold. Solid polymer cal rheological testing ( DMRT). forms by phase separation of seg- How steady shear, dynamic, and mented block copolymers (but transient tests are run is de- structural buildup by crosslinking scribed in a later section. occurs with some urethane sys- tems) and parts can be removed Steady shear measurements can from the mold in under a minute. characterize only the initial por- tion of a thermoset’s viscosity Process Rheology range. Near the gel point the Thermoset structural development steady shear viscosity in- is accompanied by enormous creases rapidly and becomes rheological changes: The resin unmeasurable. Eventually the changes from a low-melting ther- stiffening sample fractures or moplastic solid to a low viscosity tears. liquid, to a gel, and then to a stiff In contrast, dynamic oscillatory solid. measurements of the resin’s vis- Thus, the properties are vitally cosity can be made as the reac- important to successful process- tion proceeds through the gel point – and beyond – until the resin becomes a stiff solid. This is pos- sible because the measurements can be made at a strain ampli- liquid solid

oo tude low enough to prevent dis-

G ruption of the gel structure as it is

*, h*

h being formed. Steady shear vis- , o cosity data are compared to dy- h G namic viscosity data for a curing

Log epoxy resin in figure 2. h Being able to measure the viscos- ity throughout the thermoset cur- ing reaction is an important ca- pability. Still, it provides only a partial picture of the phenomenon tc Reaction time taking place. Because the mate- rial is viscoelastic, elasticity data are needed to complete the char- Figure 2: Measurement of the viscosity of a curing epoxy resin in acterization. These are obtained in a dynamic test concurrent with AAN015 5 Rheology of Thermosets

measurement of the viscosity. neously, measuring tan d in the Figure 3 shows the storage (G’) time scale of the developing gel. and loss (G”) moduli and com- Figure 4 shows dynamic oscilla- plex viscosity h* measured dur- tory measurements of tan d made ing an epoxy molding compound simultane-ously at three frequen- cure. Besides providing essen- cies on a crosslinking polydi- tial mini-mum viscosity data, the methylsiloxane (PDMS) resin. cross-over point of the two modu- Because tan d is independent of lus curves gives an estimate of frequency at the gel point, the the time at which the resin be- three curves pass through a sin- gins to gel. gle point and unambiguously de- But this is only an ap-proximate fine the instant of gelation. Such gel point. The problem is this: As measurements are readily made the transient gel network struc- using a Rheometric rheometer ture is forming, stress relaxation equipped with the MultiWave is also taking place. So, unless mode, described in detail in a later measurements on the gelling sys- section. tem are made extremely rapidly, the exact time of gelation will be Product Performance obscured by changes due to To derive optimum perform-ance stress relaxation. from a thermoset resin, both Recent work by H.H. Winter1 undercure and overcure must be shows that the cross-over point avoided. An undercured matrix of the storage and loss modulus may exhibit creep and tend to curves is not the true gel point. build up heat upon vibrat-ing; an Rather, the instant of gelation is overcured matrix may experi- when the critical gel exhibits ence room temperature perform- power law stress relaxation and ance problems because internal tan d momentarily becomes inde- stresses build up in the glassy pendent of frequency. This point polymer as it cools, and stress can be identified by making sev- cracks develop later. eral frequency sweeps simulta- A parameter known as the glass

transition temperature Tg, often measured during DSC and TGA,

7 6 has been used as an indicator of 10 10 Temperature ramp 5 C/min the extent of thermoset resin cure. For example, compared to a fully

106 cured control sample of known

105 good performance, a lower glass G' * [Pas] approx. gel point G'' h transition temperature indicates h* 105 undercure. But this does not necessarily 104 Moduli G', G'' [Pa] mean that a higher T indicates 104 g Minimum viscosity Complex viscosity overcure: The difference between complete cure and overcure can- not be detected by measuring T . 103 103 g 80 100 120 140 160 Temperature T [C] A higher glass transition tempera- ture may result form exposure later to temperatures above those of cure or postcure. The glass Figure 3: Measurement of the minimum viscosity and transition temperature can be approximate gel point for a curing epoxy molding compound lower if the prepreg were cut and

1 Fourier Transfom Mechanical Spectroscopy of Viscoelastic Materials with Transient Structure E.E. Holly, S.K. Venkataraman, F. Chambon and H.H. Winter, Journal of Non-Newtonian Fluid Mechanics, 27(1988)17-26

A.Franck 10/04 V1 6 Rheology of Thermosets

moduli (G’, G”) and damping behavior (tan d) of a polymer at a chosen oscillatory frequency over a sufficiently wide range of tem- gel point frequencies perature, the effect of the glass 1 rad/s transition can be clearly ob- 4 rad/s served. The glass transition is 16 rad/s detected as a sudden and con- siderable (several decades)

d change in the elastic modulus and 100

tan an attendant peak in the tan d curve. Besides being a more accurate

method for measuring Tg, DMRT reveals much about the material

30 32 34 36 38 40 before and after the glass trans- time t [min] ition because DMRT also meas- ures the rubbery plateau modu- lus. This is important because, at Figure 4: The precise gel point of a curing resin is identified by high thermoset cure levels, the the intersection of tan d curves generated in several rubbery plateau modulus is more sensitive than is T itself for de- simultaneous frequency sweeps g tecting, for example, small differ- laid up in a high relative humid- ences in a thermoset polymer cure ity. (This can be verified by level. retesting after drying the sample, since the effect is reportedly re- Also called the a transition, the versible.) And formulation errors glass transition is associated with can shift the glass transition tem- crank-shaft motion of major chain perature up or down. segments. Other transitions, des- ignated as b, g, etc. in order of To understand the inadequacy of descending temperature of occur-

Tg alone as a performance gauge, rence, arise from vibrations and

what Tg is must be understood. rotations. Material toughness and The glass transition is a tempera- impact resistance correlate with ture-induced morphological the magnitude of tan d at the sec- change in a polymer between a ondary (b, g) peaks. brittle, glassy state and a viscous, rubbery state. The glass transi- For many materials, as the dy- namic oscillatory frequency is tion temperature Tg is the tem- perature at which this transition increased, transitions occur at occurs. higher temperatures. Also, some transitions shift different amounts, Because the material ex-periences depending on their degree of fre- a substantial change in rigidity in quency dependence. This helps a short span of temperatures, locate some transitions in the glass transition is a key fac- multiphase resin systems (such tor in deciding the use-fulness as those modified by addition of of a polymer. So, merely know- elastomers or thermoplastics) if ing the temperature at which it one component is more fre- occurs (all that DSC and TGA quency-dependent than another. provide) is not enough. The Shifts in the glass transition tem- modulus must be measured as perature and the b peak can indi- a function of temperature as well. cate the effect of compounding or This is accomplished most ef- process changes on product per- fectively using dynamic me- formance. In general, the tem- chanical rheological testing . perature of the secondary transi- In the course of measuring the tion shifts more than does the

AAN015 7 Rheology of Thermosets

glass transition temperature as from measurements of the cur- frequency is changed. ing resin’s dynamic viscosity pro- file as a function of temperature How dynamic mechanical testing and time. can be employed to study the criti- cal aspects of thermoset rheol- Rheometers are designed to fa- ogy during resin curing and solid cilitate making these measure- state finished product performan- ments: A time/cure mode allows ce is described in the following test temperatures to be advanced sections. (ramped) at rates of 1° to 5°C per minute in eight separately control- Sample Selection and led heat zones. Each zone can Preparation be programmed for a ramp rate, final temperature, total time, If the immediate application in- measure time, and percent strain. volves use of the neat resin, then Thus, almost any cure cycle can tests should be run on material in be conveniently simulated. And that form. If, however, prepregs are rheometers are equipped with a used, then tests should be made MultiWave mode that provides the on the staged resin mechanically most precise measurement of the extracted form the prepreg, or di- gel point available by measur-ng rectly on die-cut samples of lami- tan d in several simultaneous fre- nates (for example, a three-ply quency sweeps. laminate of prepreg stacked with the middle and top ply oriented at From a series of measure-ments 45° and 90° with respect to the at different temperature ramping fiber orientation of the bottom ply). rates (generally 1° to 5°C per minute), the storage (G’) and loss The die size is dictated by the (G”) moduli and the complex vis- diameter of the parallel plates to cosity h* are derived, and the fol- be used for the dynamic mechani- lowing key information is ob- cal tests. Disposable plates, tained: should be used for tests that cause the resin to cure since the • Initial viscosity resin can adhere strongly to the • Minimum viscosity plates. The sample should be pressed (under vacuum, if possi- • Gel point ble) to reduce ist bulk and hold • Optimum heating rate the plies together. To avoid slip- page, the test should be begun at a temperature at which the Initial Viscosity resin is tacky. The initial viscosity, the first point Powdered resin samples should of the viscosity profile, indicates be pulverized, dried, and pressed the onset of matrix melting, and in a mold, forming a wafer of a is not influenced by heating rate. size matching the test plate’s di- It is a useful indicator of degree of ameter. resin advancement for quality con- Generating Viscosity trol. Profiles Minimum Viscosity

Developing the proper cure level The minimum viscosity represents is perhaps the most critical step the time period and temperature for a thermoset polymer. And both range over which resistance to under-curing and overcuring must flow is lowest. Knowing this and be avoided. the time needed to reach the mini- mum viscosity is critical in vari- The most effective and economi- ous processes: In laminate pro- cal way to do this is to design a duction, the resin viscosity must thermoset cure cycle derived

A.Franck 10/04 V1 8 Rheology of Thermosets

be low enough to wet the embed- ded fibers uniformly, but not so low as to cause excessive bleed- ing at the laminate edges. And in the encapsulation of integrated cir- cuits by transfer molding, the vis- cosity must be low enough to 103 avoid damaging the fragile wires Heating rate that connect the circuits to the o

* [Pa s] 3 C/min frame. The time interval during h 2 oC/min which the viscosity is at or near o 102 1 C/min ist minimum value constitutes the processing window.

Gel Point

101 Complex viscosity The gel point is the temperature or time at which the first set of 0 1000 2000 3000 4000 5000 6000 covalent bonds connects across time t [min] the sample (the onset of a three- dimensional network) and the molecular weight becomes infi- nitely large. The gel point is im- Figure 5: Measurement of the viscosity profile of a curing portant in determining the time thermoset resin as a function of heat-up rate and temperature at which pres- sure should be applied. Pressure is needed during lami- nation to squeeze the plies to- gether, and timing is critical: Sub- stantial pressure must not be ap- plied until after the minimum vis- cosity has been reached, else the resin will bleed excessively at the edges; but it must be applied be- Isothermal cure at different temperatures fore the gel point to assure ad-

106 equate compaction.

105 TGDDM + DDS 100/30 Pressure is used also to control outgassing of volatile by-products 104 173 C generated in some thermoset polymer reactions such as those 103 165 C 152 C involved in the manufacture of 102 143 C polyimides. * [Pas] 129 C h 101 Excessive pressure must be avoided during the lamination of 100

viscosity circuit boards that include an ox-

10-1 ide-prepreg interfacial treatment since sme oxides are fragile and -2 10 can be crushed, causing dela-

10-3 mination. 103 104 time t [s] Optimum Heating Rate

The heating rate must be optimized Figure 6: Isothermal scans at various temperatures are used to help because it affects the time decide on the time interval and temperature at which to impose needed to reach the minimum vis- isothermal holds during a thermoset resin production cure cycle cosity, and the time and tempera- ture intervals over which the mini- AAN015 9 Rheology of Thermosets

mum viscosity extends. Too fast

4 a heating rate can create un- 10 wanted temperature differentials 180 Temperature Pressure within the part, irregular curing 160 Viscosity throughout the piece, and possi- 103 140 ble porosity problems. Figure 5

120 shows the effect of heat-up rate on the viscosity profile.

100 2 * [Pas] 10 h 80 Isothermal Scans

60 Viscosity

Temperature T [°C] 101 40 Once the minimum visco-sity and gel point have been determined, 20 a series of isothermal scans 0 100 should be run at temperature in- 0 1 2 3 4 tervals beginning with the mini- Time t [hours] mum viscosity temperature and continuing up to the gel point tem- perature in intervals of about 10°C. These scans can be made quickly Figure 7: A thermoset resin production cure cycle in which on the ARES since it can can hydraulic pressure is induced in the resin to reduce gas record one data point a second. bubble nucleation in the cured matrix This information will help in de- ciding the time and temperature at which to impose isothermal holds during the process. An iso- thermal hold at minimum viscos- ity extends the time the resin will remain near the minimum viscos- ity and determines the dimen- sions of the processing window. The time at or near the minimum viscosity is a key factor in con- trolling overall flow and avoiding problems such as bulk porosity. A second hold near the gel point Temperature ramp 2 C/min is sometimes used to eli-minate Test frequency 10 rad/s surface porosity. Typical isother- mal scans are shown in figure 6. Using the Viscosity Profile

A dynamic mechanical viscosity profile offers unparalleled utility in G', G'' [Pa] every phase of thermoset polymer 100000 technology, including research, product development; process w/o Zn-stearate development and control; and w Zn-stearate quality control. Some examples

1 2 3 4 5 6 7 8 9 follow. time t [min] Research

Dynamic mechanical testing is employed extensively in thermo- Figure 8: The addition of zinc stearate lowers resin set composites research to pro- viscosity at the injection mold barrel temperature without vide needed material properties for affecting the cure time

A.Franck 10/04 V1 10 Rheology of Thermosets

finite element and finite difference 200 analyses. And dynamic viscosity profiles provide vital data for stud- 180 ies of cure reaction kinetics, proc- 160 ess modeling using ”expert” sys-

140 tems, and advanced curing con- cepts such as ”gas diffusion con- 120 trol.” 100 Diffusion control reduces gas bub- 80 ble nucleation in the curing ma- Temperature T [°C] trix by inducing hydraulic pres- 60 sure in the resin during the gas 40 volatilization stage of the cure cycle. Figure 7 shows a typical 0 50 100 150 200 matrix resin cure cycle incorpo- Time t [min] rating diffusion control, superim- posed on the viscosity profile. Product Development Figure 9: A graphite-epoxy production cure cycle Viscosity profiles are used to great advantage in ther-moset product development to evaluate, for example, • new or different resin formulations

• fiber types, finishes, and weaves • materials from alternati ve suppliers; and • functional additives Figure 8 shows how the viscosity profile during cure has been used

320 to evaluate, at a typical screw

300 barrel temperature, the beneficial

280 effect of adding 1% zinc stearate

260 to a phenolic injection molding

240 compound. The zinc stearate low-

220 ered the viscosity considerably at Press Molding Cycle the barrel temperature without af- 200 Temperature T [°C] fecting cure time. 180 -50 0 50 100 150 200 250 300 350 400 450 Process Development Time t [min] and Control

300 Developing Production Cure Cycles

250 A dynamic mechanical viscosity profile provides the rheological Post cure cycle 200 data needed to construct a pro- Temperature T [°C] duction cure cycle. Figure 9

0 50 100 150 200 250 300 350 400 450 500 550 shows a typical cure cycle deve- Time t [min] loped for a graphite/epoxy lami- nate. Figure 10: The cure and post-cure cycles for a polyimide resin Developing Post-Cure Cycles

AAN015 11 Rheology of Thermosets

In many cases, especially with bismaleimides and polyimides, a post-cure step is used. This in-

5 volves heating the laminate for an 10 Temperature extended time outside the mold No advancement to increase the crosslink density advanced 4 800 10 and raise the glass transition tem- further advanced perature of the matrix, thereby improving functional properties. A 600 3 10 typical post-cure cycle in a circu- * [Pas] h lating air oven for a polyimide resin

2 400 B-stage advanced 10 composite used for a jet engine component is shown in figure 10. Viscosity Tempertaure T [C]

200 101 Quality Control

The cure step is the most critical 0 100 element in ther-moset process- 0 20 40 60 80 100 120 140 160 180 ing. Accordingly, maintenance of Time t [min] strict quality control standards on the incoming materials, the prepreg especially, is important. Figure 11: B-stage resin advancement alters the viscosity Two areas of major concern are profile of a curing thermoset resin degree of resin advancement and the effects of moisture. Prepreg Advancement

Prepregs are partially re-acted by the supplier to differentdegrees to develop particular properties (stiff- ness, tack, drape) needed for a given application. The degree of reaction advancement in the B- staged prepreg is controlled mainly by stoichiometry and stor- age conditions, including duration, humidity, and teperature. Ideally, prepregs should be stored only a short time, and then in a cold, dry

1000 environment. Any lapses in sup- 1000 Test frequency 10 rad/s plier quality control or improper storage may cause the reaction 0% rel. humdity 800 to advance, changing the materi- 100 75% rel. humidity al’s response in a standard cure cycle. 600 10

* [Pas] Tests on prepreg samples imme- h diately prior to use can quickly 400 1 identify advancement level differ- viscosity

Temperature T [C] ences so that adjustments can be 0.1 200 made to the cure cycle. Figure 11 shows viscosity profiles of an epoxy resin at three different lev- 0.01 0 20 40 60 80 100 120 140 160 180 200 els of minimum viscosity and the time t [min] reaction rate (reflected in the slope of the viscosity curve during the isothermal hold), and the sooner Figure 12: Viscosity profiles of a dry epoxy resin and the gelation occurs. same resin after extended storage in a high humidity

A.Franck 10/04 V1 12 Rheology of Thermosets

Moisture Effects

Moisture adsorbed during im- proper storage of a prepreg can 0.07 create quality problems in thermo- set processing by changing reac- 0.06 tion kinetics and there-by chang-

0.05 ing the viscosity profile of the prepreg. Also, trapped moisture 0.04

d can cause matrix porosity. By Modulus G' testing before use, moisture-re- tan d tan 0.03 lated cure problems can be Modulus G' [Pa] 0.02 avoided, or at least minimized. The effects of moisture are often 0.01 reversible, so the material may be 1E10 salvaged by drying before curing.

-200 -100 0 100 200 300 400 Figure 12 shows the viscosity pro- Temperature T [°C] files of a dry epoxy resin and the same resin after extended stor- age in a high humidity. Figure 13: Temperature sweep on a cured epoxy-graphite Evaluating Product composite. The glass transition is manifested in a sudden and Performance considerable decrease in the storage modulus with an attendant peak in the tan d curve Key factors in thermoset polymer product performance are the cured resin’s modulus and glass transition temperature. The dy- namic moduli (G’, G” or E’, E”) are measured directly on solid samples with or without fiber re- inforcements at a selected fre- quency as a function of tempera- ture. And from these measu- rements tan d is automatically calculated. The glass transition is detected as a sudden and con- siderable decrease in the storage 1E10 1 modulus and an attendant peak

in the tan d curve. The symbol Tg denotes the temperature at which this transition occurs. 1E9 0.1 Figure 13 shows the tan d and storage modulus curves, the d glass transition and secondary Tan G' [Pa] transition of an epoxy-graphite 1E8 0.01 sample measured in bending on the Solids Analyzer. The glass second run high to low transition temperature is not a first run low to high constant, so care must be taken 1E7 during its measure-ment to avoid -150 -100 -50 0 50 100 150 200 generating misleading data. For Temperature T [°C] example, two tests made at dif- ferent heating rates can produce different values of the glass tran- sition temperature. The slower the Figure 14: Different values of the glass transition measurement, the lower is the temperature are obtained if the transition is approached glass transition temperature. while the sample is being cooled compared to when the sample is being heated AAN015 13 Rheology of Thermosets

Also, for valid comparisons to be made, the glass transition tem- perature must be approached from the same direction to avoid hys- teresis during the test. So, data from test samples should be com- 1E11 100 pared to data from tests made Frequencz 1Hz under the same conditions (in-

1E10 10 cluding oscillation frequency) on good-performing control samples.

1 1E9 Figure 15 shows the effect on the

d glass transition temperature of

0.1 tan going from a low to high tempera- 1E8 ture and then repeating the test as the sample cools, using a

Storage Modulus G' [Pa] Azelaic acid & BOMA 0.01 1E7 BOMA cured and a cured plus postcured DDM sample of PMR-15 polymimide. BTDA 1E-3 -150 -100 -50 0 50 100 150 200 250 300 350 Figure 16 shows the effects of Temperature T [°C] various curing agents on the glass transition temperature of an epoxy resin. Figure 15: Temperature sweeps are an effective means for The data in figures 14 through 16 detecting the effects of different curing agents on the were obtained in bending and tor- viscoelastic properties of thermoset resins sion. Figure 15 shows the glass transition temperature as a func- tion of cure level measured in ten- sion following the procedure out- 1 lined in ASTM D5026. 109

Curing Temperature 93 °C

135 °C d 350 °C 0.1 8 10 tan Modulus G' [Pa]

0.01 107 -200 -150 -100 -50 0 50 100 150 200 250 300 Temperature T [oC]

Figure 16: The cure level of a thermoset polymer as a function of cure temperature measured in torsion. Tests can also be run in compression or bending.

A.Franck 10/04 V1 Keywords: thermopsets, processing, elasticity, cure, viscosity, performance, gel point, vitrification, glass transition

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