Comparison of microwave and thermal cure of an epoxy/amine matrix

Comparison of Microwave and Thermal Cure of an Epoxy/Amine Matrix

CATHERINE JORDAN, JOCELYNE G a y , * and JEAN-PIERRE PASCAULT

Laboratoire des Mathiawc Macromol6culaires (URA 507) INSA de Lyon

6962 1 Villeurbanne Cedex, France

and

CLAUDE MOFU?, MICHEL DELMOnE, and HENFU JULLIEN

CNRS, Luboratoire Organisation MoEculaire et Macromol6culaire 94320 Thiais, France

An investigation was camed out into the effect of a microwave cure on an epoxy prepolymer with a cycloaliphatic diamine mixture, as compared to a standard thermal cure. The microwave waveguide and process (propagation mode TE,,) were adjusted to obtain large homogeneous samples. The extent of reaction, x, was measured during the microwave processing by size exclusion chromatography and differential scanning calorimetry. A good estimate of x was found using a modified DiBenedetto equation correlating x and the glass transition temperature Tg. The homogeneity of the samples was checked during the last steps of cure, showing the efficiency of the microwave processing and waveguide. The influence of the nature of the mold (metallic or dielectric) on the reaction kinetic was also investigated. Samples cured by both thermal and microwave processing were characterized by dynamic and static mechanical properties and then compared with those of fully crosslinked networks, i.e., postcured at a high temperature.

INTRODUCTION on the classical Maxwell's equations, which can be

icrowave processing of materials is important M for various applications, among them crosslinking of polymer networks, curing of laminates, and joining and repairing of composites (1-3). The use of microwave processing is expected to reduce curing time because the microwave energy is supplied directly to the sample (no thermal lag and no heating of the container), and therefore the operating cost should be lower. In addition, an instant on-off control of the microwave power may be used to control the exothermic reaction. Generally, a reduction in resid- ual stress in the processed materials and an improve- ment in the final properties of the material are to be expected. The research in this area deals with three major topics:

i) The comprehension of the electromagnetic propct gation in the energy applicator and in the material The theoretical analysis of microwave behavior is based

*To whom correspondence should be addressed

solved using the appropriate boundary conditions. ii) The comprehension of thermal transfers in the

material and its environment The use of metal in an applicator is prohibited, and thus the mold must be made of a dielectric material that does not absorb microwaves; poly(tetrafluoroethylene), silicone, or glass are generally used. Consequently, the thermal behavior in a microwave differs greatly from that in a classical thermal environment, because of different heat capacities and heat conductivities of the materials. In a literature review, Mijovic and Wijaya (4) list the advantages and the drawbacks of the use of microwaves. They emphasize that crosslinking of large samples or irregular shapes, compared with the microwave wavelength, leads to temperature gradients inside the material. Temperature uniformity is essen- tial to provide homogeneous crosslinking in large samples. Therefore, it is necessary to check the homogeneousness of the final product (2).

iii) The physico-chemical analysis and the mechant cal characterization of the resulting material, so that structure-property relationship can be established. I t

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY 1995, Vol. 35, No. 3 233

Catherine Jordan, Jocelyne Galy, Jean-Pierre Pascault, Claude Mort5 Michel Delrnotte, and Henri JulIien

is well known that the final structure of a network depends on the reaction path and on the cure kinetics. The main question is to know whether or not the microwave process can alter the mechanism and kinetics of reaction. Actually, the answer is not defini- tively assessed. Some authors (5) find no difference in the kinetic parameters of epoxy-amine reactions, whereas others (6, 7) conclude that the rate of crosslinking and Ty were higher and the vitrification and gelation times were shorter in the microwave samples. An overall lower degree of cure was observed (6) in microwave samples and explained by the en- trapment of reactive functions within the network.

The aim of our work is to manufacture large home geneous epoxy networks using microwave processing and to compare the kinetic behavior of a classical thermal treatment and a microwave treatment. The epoxy-diamine system chosen for this study is well known. Various properties of this reactive system have been investigated as a function of the extent of reaction (8). This study is part of a larger project whose aim is the processing and characterization of epoxy/glass fiber unidirectional composites, cured in a microwave environment for comparison with a classical thermal cure in a hydraulic press.

EXPERIMENTAL

Materials

The epoxy prepolymer used was diglycidyl ether of bisphenol A (DGEBA): DER 332 from Dow Chemical. The curing agent was 4,4'-diamino-3,3'-dimethyldi- cyclohexylmethane (3DCM. Laromin C 260) from BASF. Figure 1 shows the structural formulae of both monomers. The reactants were used as received, with a stoichiometric ratio (aminohydrogen over epoxy functions) a/e of 1. The epoxy prepolymer and the comonomer, both liquid, were stirred under vacuum at room temperature for half an hour. The epoxy mixture ( 13 g) was then poured into PTFE parallelepipedic molds (inside dimensions: L = 96 mm, w = 16 mm, h = 8 mm). The choice of PTFE is justified by several advantages such as a low dielectric loss factor (3 x temperature-independent dielectric prop

erties, high temperature resistance, and also chemical inertness to epoxy and amine. The molds were then stored in a freezer at -30°C for less than 48 h. This temperature is low enough to prevent the reactive mixture from undergoing further reaction before the microwave processing. The kinetics of this system has been extensively studied in one of our laborate ries (9): For example, the extent of reaction reaches 17% after 5 h at 29°C. Another interest of this system is that the chemical path does not depend on the temperature: Only an epoxy-amine reaction occurs. Preliminary studies, in a microwave environment, showed that even a small evolution of reaction or a variation in temperature in the applicator, at the beginning of the experiment, led to nonreproducible samples (when the power, P, is monitored as a function of the temperature, T). This is the reason why particular attention was paid to sample storage.

Processing Method

Microwave

For any microwave use in chemical processing and mainly in polymer processing, the design of the Faraday cage, which allows the propagation of the wave and the conversion of the electromagnetic energy inside the material, is very important. Several microwave device techniques have been used to heat polymers and polymer composites (4, 10). They in- clude microwave ovens, single-mode resonant cavi- ties (4-71, single-mode traveling wave applicators (ll), with or without microwave pulse repetition of low frequency. Our objective is the achievement of the most homogeneous electric field inside the sample and around it. Consequently, the propagation mode chosen is the fundamental mode transverse electric, TE,, (1 2): The electric intensity has only a transverse component to the propagation direction. The applicator used is an oversized waveguide in which the prop agation mode remains TE,, (13), the wave frequency is 2.45 GHz. The FTFE container is set on a poly- propylene stand to define precisely the position of the polymer mixture in the plane of the electric field vector and of the wave path. The largest dimension of

DER 332 H = 0,03 DOW CHEMICAL 174 g/eq

3DCM BASF 238 g/mol

Fg. 1 . Molecular structure of the epoxy prepolyrner and the diarnine usedfor the synthesis of the epoxy networks.

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Comparison of Microwave and Thermal Cure of an Epoxy/Amine Matrix

the container is parallel to the electric field vector; its width is parallel to the wave path. The sample temperature was measured continuously using an in- frared pyrometer (RAY TEK mod. Thermalert SL230B A = 3.43 em), which gives a surface temperature, or by an optic fiber fluorometric thermometer, which gives the bulk temperature. The measurement was done in the middle of the sample.

The electromagnetic power of the incident wave was 200 W, whereas only a few watts were absorbed by the reactive volume. The power level was chosen so that the repeatability of the experiment was im- proved. The largest part of the incident power is a b sorbed by a water load regulated at 10°C at the end of the microwave line: this absorption only depends on the energy absorbed by the sample in the applicator.

Sampling was done at different reaction times to measure the extent of reaction. Before gelation, the sample was withdrawn directly inside the mixture in the applicator with a glass pipette and immediately dissolved in tetrahydrofuran (THF) solvent for chro- matographic analyses. For differential scanning calorimetry (DSC) measurements, the mold was re- moved from the applicator and quenched to avoid further cure. After gelation, only DSC measurements were performed.

Thermal

The thermal processing and the full characterization of samples cured in a stainless steel mold have been reported (8). In order to complete this work, a thermal cure of the reactive mixture was done in the PTFE mold in a regulated oven at 140°C until the maximum of temperature due to the exothermicity was obsewed. Then the samples were cooled at room temperature. The temperature was recorded by means of a thermocouple (K Type Chromel-Alumel) placed in the center of the mixture.

Combined Microwave/ Thermal Processing

A first stage of cure using microwave radiation was camed out, and then the samples were cured in a thermoregulated oven at 190°C for 14 h to reach full cure.

Characterization of the Epoxy Networks: Epoxy Conversion

The extent of reaction, x, before gelation, was determined using size exclusion chromatography (Waters apparatus): the disappearance of the epoxy monomer was followed and the conversion in epoxy functions was calculated. A previous paper (8) details the calculation procedure.

A differential scanning calorimeter (Mettler TA 3000) was used to measure the glass transition temperature, Tgr at a heating rate of 10"C/min under an argon atmosphere. The degree of conversion can be obtained for highly crosslinked networks by deter- mining the Tg and using the DiBenedetto equation, in

the way used by Hale, et aL (14) and Pascault and Williams ( 15):

TgM is the glass transition temperature of a network, having a known extent of reaction x,,, and close to 1. The change in specific heat through the Tg of this network is ACpM. x' is the ratio x/xM and A' is the ratio ACpM/ACpo.

This equation is interesting since no adjustable parameter or statistical calculations are needed, as- suming a particular model for the epoxy network buildup. Thus, the epoxy conversion can be calculated when the SEC and FTIR methods provide the limits.

Multiple measurements of Tq were done on the same sample to test the homogeneousness of the microwave treatment. In addition, measurements of Tg were done on different sample cured under identi- cal conditions to test repeatability.

Characterization of the Epoxy Networks: Mechanical Measurements

Dynamic mechanical measurements were performed with a Rheometrics RDA 700 apparatus in torsion mode at 11 Hz. Storage shear modulus, G', loss shear modulus, G , and loss factor, tan 6 , were recorded during a temperature ramp from - 150°C to + 250°C. Samples (60 X 6 x 2 mm) were tailored in the center of the molded matrix.

Compression tests were performed with an Adamel Lhomargy (DY25) machine at room temperature. The parallelepipedic samples (15 X 6 X 45 mm) were de- formed in a compression rig at a strain rate of 3.7 X

s-'. The longitudinal and transverse deforma- tions were measured using precision strain gages (CEA-06-032WT- 120) from Vishay Micro Measure- ments. The Compression modulus, E,,, in the glassy state and the Poisson's ratio, v, were calculated.

RESULTS AND DISCUSSION

Homogeneous Aspect

Figure 2 shows the reduction in the changes in Tg. along the sample length, with increasing treatment time. A(Tg) represents the difference in Tg between the center and the two boundaries of a sample. The sample became more homogeneous when the curing time is increased from 9 to 15 min (at Po = 200 W). The temperatures at the sample center were higher than at the sample boundaries because of the slow heat conduction inside the sample. Optic fiber fluore metric thermometers placed in the boundaries and in the center of the sample showed this difference in temperature profiles. A n increase in treatment time allows heat transfer from the center to the boundaries, leading to similar values for Tg along the sample length.

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY 1995, Vol. 35, NO. 3 235

Catherine Jordan, Jocelyne Galy, Jean-Pierre Pasc

Kinetic Aspect A typical temperature-time profile is recorded in

Fkg. 3 for an incident power level Po of 200 W. This plot is similar to' those observed by several others (2, 3, 11, 16, 171, except for different times and temperatures. This is explained by the difference in reactivity (aromatic diamines are less reactive than cycloaliphatic diamines) and to the difference in the

-- " I

2.0-

l o -

0 1 t I 1 I 9 12 15

Time (min) Rg. 2. Differences in glass transition temperature between the center and the boundaries as a function of the time of treatment, Po = 200 W.

:ault, Claude Mork, Michel Delmotte, and Henri Jullien

microwave processing: The modes of propagation, the incident power, and the volumes of sample to be cured are different.

The evolution of the extent of reaction in the middle of the sample is superposed on this plot. Three re- gions are observed:

i) The first is a slow increase in temperature corre- sponding to the heating of the mixture and to the beginning of reaction. The heating rate is 1 l"C/min, between 0 and 7 min. The extent of reaction reaches 25%.

ii) The second region shows a very rapid temperature rise (4O"C/min), which is due to the exothermic epoxy-mine reaction ( A Htotal = 480 J/&. The heat is self-produced in the material by the chemical exothermicity. At this time (7 min) the temperature is near 100°C. which is close to the dielectric relaxation of the DGEBA prepolymer. Both phenomena are responsible for the sharp increase in temperature. The gel time can be characterized by the appearance of nonsoluble fraction in THF solvent: it occurs when the extent of reaction reaches 60% between 8 and 8.5 min and just before the maximum of temperature. The evolution of the extent of reaction with time is very fast; it reaches 91% at the maximum exothermicity.

iii) The third region shows the cooling of the crosslinked polymer by free convection. The Tg= 140°C and remains constant when the curing time is increased. The extent of reaction is thus 9 1 %.

The temperature inside the sample decreases rapidly after 9 min of treatment, preventing the reac-

1

0.8

0.6

0.4

0.2

0

Time (min) Flg. 3. The-ternperatureproJle and time-conversion profile of a DGEBA-3DCM mixture under Po = 200 W. (0) DSC measurements, ( ) SEC measurements, ( 0 ) gel point.

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Comparison ofMicrowave and Thermal Cure of an Epoxy/Amine Matrix

tion from proceeding. The quantity that most influences microwave heatability is the dielectric loss factor e". In fact, the changes in dielectric properties are representative of the alterations in molecular dipole moments as a result of epoxy-amine reaction: The dielectric loss factor E" decreases with an increase in the extent of reaction (18, 19). In the first reaction step, the viscosity of the mixture is low, allowing the dipoles to rotate freely and orient in the microwave field (4). With further reaction, the increasing viscosity leads to a decrease in the ease of dipole rotation. The final Tg is 140°C for the microwave network. However, it is the DSC measurement, and an increase in frequency increases Tg. Unfortunately, there are few data in the literature on high frequency (2.45 GHz) and temperature relaxation variations in epoxy-amine networks. Experiments reported on fully cured networks show that these polymers exhibit two relaxation processes: The main one, a, is associated to the glass transition, and the secondary one, p , is normally observed at low temperature, below Tg. and is due to limited motions of the main chain (hydroxy- ether groups in this kind of network) or to motions of side groups. The p process is supposed to follow an Arrhenius law (the temperature at the maximum of the peak is To = 223 K at a frequency of vo = 11 Hz, the activation energy is equal to 56 kJ/mol.) The a process is supposed to follow the Williams-Landel- Ferry equation: the coefficients C, and C, of this equation were found equal to 10.7 and 40.6"C, re- spectively (8). As proposed by Williams and Watts (20), the extrapolation to very high frequency shows that the two relaxations merge to give one process, up. At 2.45 GHz, the temperature of the ap relaxation is shifted to 370°C (Arrhenius value), that is to say, higher than the temperature of the experiment. Therefore, it appears that the network at 91% of conversion is in a glassy state when it is submitted to the microwave frequency. In the glassy state, the dipole cannot orient, and in consequence, cannot relax and heat the sample. e" values, at this point, are attributed to the glassy state of the network (2 1).

The DGEBA/3DCM mixture was also cured in a thermal oven at 140°C. in the same PTFE molds. The profile T = f ( t ) is quite similar to the microwave p r e file except that the heating rate in the first region of the curve is lower: it is S"C/min, and the temperature inside the sample reaches 220°C at the maximum of the peak.

In a metallic mold of large dimensions, the mixture is slowly heated (4"C/min) to the oven temperature (140°C). and a very small exothermicity of 25°C is observed.

The thermal approach of the curing of thermoset resins in specific conditions such as ultra-high frequency (UHF) electroheat leads to the examination of three main causes for the differences observed between classical curing in a thermoregulated oven and microwave curing in an applicator. The overall heat evolved by the chemical reaction is A Htotal = 480 J/g. This energy is used to increase the temperature of

the mixture itself and of the mold and is also trans- ferred to the environment.

i) The quantity of energy supplied to the reactive mixture and to the mold depends on the specific heats of both materials. The specific heat is about 1.2 to 1.8 J/g/"C for the mixture, 1.2 J/g/"C for the dielectric PTFE mold, and between 0.4 and 0.95 J/g/"C for metals. The specific mass of metals is generally higher than the specific mass of dielectric materials. Thus, taking into account the mold weights, higher heat capacities are obtained in the case of metallic molds.

ii) The thermal diffusivity of a metal is 150 times higher than the thermal diffusivity of a dielectric material. Consequently the metallic mold acts as a thermal buffer.

iii) Third, as mentioned before, one of the aims of microwave curing is the reduction in treatment time. The classical thermal cure is slow and only involves heat transfers from the source, the thermoregulated oven, to the mold and from the mold to the reactive mixture. The treatment time is governed by the time constant of the heat transfers, which leads to a sta- tionary phase. In the case of a microwave heating the effect of the heat transfers is severely reduced, because the dielectric electroheat induces internal heat generation in the transitory phase of the heat transfers.

Mechanical Characterization The loss factor (tan 6 ) and the shear storage modu-

lus ( G ' ) are recorded in Fig. 4 as a function of the temperature for a sample cured 15 min under microwave (Po = 200 W), and having an extent of reaction of 91% and for a fully cured sample (combined process: 15 min at Po = 200 W + 14 h at 190°C).

At low temperatures close to -50°C the /3 relaxation is observed, In this range of temperature, the decrease in G' is of low magnitude, but is propor- tional to the area below the p peak. A previous study (8) showed the dependence between the shape of the p relaxation and the extent of reaction. A direct consequence is that at room temperature the networks with the highest extent of reaction have the lowest modulus. The compression tests show that the compression modulus increases for lower extent of cure (see Table 1) . The fully cured networks have a lower modulus, as has been shown (8). The Poisson's ratio, v , is not influenced by the extent of reaction.

Near 170°C. in the rubbery state, as expected, an increase in shear modulus due to further cure is observed for the microwave sample. The rubbery modulus increases as the extent of cure increases. The fully cured network shows a mechanical transition temperature, T, = 203°C at 1 1 Hz, associated with the glass transition temperature of the network (equal to 183°C using DSC). It is higher than the mechanical transition temperature of the microwave sample owing to the higher extent of reaction. The rubbery modulus is a constant, showing that the extent of reaction is maximum.

POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY 1995, VOl. 35, NO. 3 237

Catherine Jordan, Jocelyne Galy, Jean-Pierre Pascault, Claude Mork Michel Delmotte, and Henri Jullien

-150 -110 -70 -30 10 50 90 130 170 210 250

Temperature ("c) Flg. 4. Dynamic mechanical spectrum G' , tan 6 as a-function of temperature, for microwave curing (Po = 200 W) (0) and combined microwave and thermal curing ( ).

Table 1. Compression Moduli in the Glassy State, Poisson's Ratio, Glass Transition Temperature, Extent of Reaction.

Curing Cycle Temperature ("C) Time GPa X

Y ("h) Power (W) E " C

Microwaves Microwaves +

thermal

200 200 t 190

15 min 3.15 0.36 131 89 15 min+ 1 4 h 2.9 0.36 186 100

It is important to note that the fully cured samples processed either by thermal processing (140"C, 1 h + 190°C. 14 h) (13) or combined processing (200 W, 15 min + 19O"C, 14 h) show the same viscoelastic behavior over a large range of temperature.

CONCLUSION

This work has shown that, if care is taken with the experimental parameters such as initial conversion (i.e. -OH content), H 2O content, initial temperature, uniformity of the electric field inside the applicator, minimum incident power, and curing time, large homogeneous epoxy networks can be obtained. Mi- crowave energy imposed on a dielectric mold induces a different thermal behavior for the reactive mixture: Heat transfer in the "mixture-mold environment" is strongly modified. Moreover, there is a cold surrounding compared with the hot surrounding in a thermal cure. The gelation time, an important parameter for the processing of epoxy networks, was determined

with a precision of 30 s on the temperature-time profile plotted during treatment. Under experimental conditions of microwave processing, it was not possi- ble to obtain fully cured DGEBA/3DCM networks: The vitrification time is shorter in this process because of the high frequency used. The Tg is increased to higher than the sample temperature. This is a drawback of microwave curing of high temperature networks. No direct influence of the microwave curing on the mechanical properties of the network was established. The only parameter that influences the mechanical properties is the extent of reaction. The extent of reaction is responsible for changes in the viscoelastic properties such as the shape of the p relaxation, which induces the decrease in compression modulus at room temperature for the highly crosslinked networks.

ACKNOWLEDGMENTS

This research is supported by the CNRS/PIRSEM (Centre National de la Recherche Scientifique), the

238 POLYMER ENGINEERING AND SCIENCE, MID-FEBRUARY 1995, VOl. 35, NO. 3

Comparison of Microwave and Thermal Cure of a n Epoxy/ Arnine Matrix

AFME (Agence Franqaise pour la Maitrise de 1'En- ergie), EDF (Electricit6 de France), PSA (Peugeot SC- ciet6 Anonyme), and by the RNUR (R6gie National des Usines Renault). The authors express their apprecia- tion for this financial support. They also thank M. L. Outifa (CNRS/OMM) for helpful discussions.

NOMENCLATURE

a/e = Stoichiometric ratio. E,, G' = Storage shear modulus. G = Loss shear modulus. Po = Incident power. t = Time. tan S = Loss factor. T =Temperature. Tg = Glass transition temperature. v = Poisson's ratio. E ' = Dielectric storage factor. E" = Dielectric loss factor.

= Compression modulus in the glassy state.

REFERENCES

1. V. K. Varadan and V. V. Varadan, Polym Eng. Sci., 31, 470 (1991).

2. S. M. Singer, J. Jow, J . D. DeLong, and M. C . Hawley, S M P E Quarterly. 20, 14 (1989).

3. D. A. Lewis, J . C. Hedrick, J . E. McGrath, andT. C. Ward, Arner. Chem SOC., Polym Chem Prep., p. 222, New Orleans (1989).

4. J . Mijovic and J. Wijaya, Polym Cornpos., 11, 47 (1990). 5. J. Mijovic, A. Fishbain and J. Wijaya, Macromolecules,

25, 986 (1992).

6. E. Marand, K. R. Baker, and J. D. Graybeal, Macro molecules, 25, 2243 (1992).

7. J . Wei, M. C. Hawley. J. Jow, and J. D. DeLong. SAMPE J., 27, 1, 33 (1991).

8. C. Jordan, J. Galy, and J . P. Pascault, J. AppL Polyrn Sci., 46. 859 (1992).

9. D. Verchere, H. Sautereau, J. P. Pascault, C. C. Riccardi, S. M. Moschiar, and R. J. J. Williams, Macro molecules, 23, 725 (1990).

10. J. Asmussen. Amer. Chem SOC., Polym Mater. Sci. Eng. Prep., 341 (1991).

11. F. M. Thuillier and H. Jullien, Makromol Chem, Macro rnoL Syrnp., 25, 63 (1989).

12. L. Outifa, H. Jullien, and M. Delmotte, Arner. Chern Soc.. Polym Mater. Sci. Eng. Prep., 424 (1991).

13. A. J . Berteaud, R. Clement, C. Merlet, and C. Leclerco. French pat. no. 2,523.797 (Sept. 23, 1983).

14. A. Hale, C. W. Macosko, and H. E. Bair, Macromolecules, 24, 2610 (1991).

15. J . P. Pascault and R. J. J . Williams, J. PoIyrn Sci., 28, 85

16. 9. Le Van and A. Gourdenne, Eur. Polym J., 23. 10. 777 ( 1987).

17. J . Jow, J. D. D e h n g , and M. C. Hawley, SAMPE Quar- terly. 20, 46 (1989).

18. M. C. Finzel and M. C. Hawley, and J. Joy, Polym Eng. Sci., 31, 1240 (1991).

19. P. Urro, H. Jullien, and V. Duhot, Ann Compos., 1-2, 137 (1990).

20. G . Williams and D. C. Watts, NMR? Basic Principles and Progress, 4, 27 1, Springer Verlag, Heidelberg (197 1).

21. M. B. M. Mangion and G. P. Johari. Macromolecules. 23, 3687 ( 1990).

Received October 2 I , 1992

Revision received October 1993

(1990).

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Comparison of microwave and thermal cure of an epoxy/amine matrix

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