7th International Electronic Conference on Synthetic Organic Chemistry (ECSOC-7), http://www.mdpi.net/ecsoc-7, 1-30 November 2003


[E005]

 

Curing of Solid Epoxy Resins with Dicyandiamide by DSC calorimetry.


Jaroslaw Gorczyk, Dariusz Bogdal

Department of Polymer Chemistry and Technology,
Krakow University of Technology,
ul. Warszawska 24, 31-155 Krakow, Poland

 

* e-mail: [email protected]

Abstract: Solid epoxy resins were synthesized both under conventional heating and microwave irradiation. Reactions were performed in a multi-mode microwave reactor “Plazmatronika”, microwave frequency-2.45 GHz, maximum of microwave power-300 W. The DSC curves of solid epoxy resins were recorded on a differential scanning calorimeter "Netzsch-DSC 200".


Keywords: Dicyandiamide, DSC, TG, Microwave irradiation, Polyaddition, Solid epoxy resin.



I. Introduction

Since epoxy resins were introduced to the market in the begining of 50-s, they have found a large variety of applications. Because of their unique properities, e.g. excellent chemical and electrical resistance, very good cohesion and adhesion to different kinds of materials and good heat resistance, they are willingly used as adhesives, coatings, casting compounds, composite materials and electrical laminates[1-2].

In this group, solid epoxy resins (Epoxy Value EV=0.25-0.02; Molecular Weight MW=1000-10000) are mostly used in industry of powder coatings. They are synthesized according to "advancement" process [3-5]. This method is based on polyaddition of bisphenol A (BPA) to low-molecular-weight epoxy resin (EV=0.58-0.35; MW=370-500) or middle-molecular-weight epoxy resin (EV=0.30-0.15; MW=500-1000) in the presence of a catalyst [6-7]. It is possible to synthesize such resins in a batch or continous process, under conventional heating or microwave irradiation [8-9].
Recently, we have shown that solid epoxy resins can be prepared under microwave irradiation which reduces the reaction time [10-11].

The convertion of meltable solid epoxy resin into crosslinked thermoset solid takes place in the reaction with a hardener, e.g. dicyandiamide. At moderate temperatures, dicyandiamide does not react with epoxy resins, but starting at about 180°C the reaction is very fast. The high temperature of the curing process promoting cyclization and structural rearrangement.
Zahir proposed the mechanism of reaction of epoxy groups incorporated in solid epoxy resin with amine groups in dicyandiamide [12]. The scheme of these reaction is shown in Figure 1.

Curing reaction of solid epoxy resin with dicyandiamide

Figure 1. The schematic reaction of epoxy functionalities with amine groups in dicyandiamide.

The initial DCDA/epoxide reaction results in the formation of N-alkyl dicyandiamides via amine/epoxide addition. These adducts cyclize by the intramolecular nucleophilic substitution of hydroksyls at the imide functionality. The 2-cyanimidooxazolidine formed by this reaction is difunctional epoxy chain extender. Tertiary amines, also formed in the reaction, act as a trifunctional cross-linkers.


II. Experimental part

Materials:

Syntheses of solid epoxy resins:

The general procedure for syntheses of solid epoxy resins can be described as follows:

Appropriate amount of BPA was added to low-molecular-weight epoxy resin (EV=0.57 mol/100 g) to obtain required increasing the molecular weight during reaction to the desired level. The calculated molar rate of BPA to low-molecular-weight epoxy resin was 3 : 4. The mixture was stirred at 160°C, in a multi-mode microwave reactor “Plazmatronika” (microwave frequency-2.45 GHz, maximum of microwave power-300 W), for time necessary to obtain EV about 0.11. In each case, 100 W of microwave power was used. Every 5 minutes a small sample of epoxy resin was taken from the mixture to determine the EV. After the reaction epoxy resin was cooled down and powdered.

Curing of solid epoxy resins:

The general procedure for curing of solid epoxy resins can be described as follows:

The powdered solid epoxy resin was put in a mortar and mixed with appropriate amount of a hardner (about 4 phr of dicyandiamide). After homogenization, a little amount of the composition (about 3.5 mg) was put into the DSC sample pan. The measurement was carried out under air atmosphere at a heating rate of 10°C/mn. To check the possibility of occurring some postcuring reactions, the second scan was conducted after cooling sample to the room temperature (see Figure 5).


III. Analytical part

Epoxy Value of synthesised resins was determined according to the PN-87/C-89085/13.

The GPC analyses were performed by means of GPC chromatograph "Knauer". A system of three columns was used: 2×PL-gel (300×7.5 mm; dimension of grains 3 mm and type of pore Mixed-E) with one precolumn. The refractrometer was used as a detector. Polystyrene standards were used to calibrate the chromatograph. Conditions of measurements: flow rate-0.8 mL/mn, temperature-30°C, solvent-tetrahydrofuran (THF).

The DSC measurements were carried out using a differential scanning calorimeter "Netzsch-DSC 200" with aluminum sample pans. Conditions of measurements: air atmosphere, heating rate-10°C/mn, the sample mass-3.5 mg.

Thermogravimetric analysis was performed with "Netzsch-TG 209". Conditions of measurements: temperature range-20-600°C, the sample mass-3.5 mg.


IV. Results and discussion

All synthesized high molecular weight epoxy resins, were characterized by standard analitycal mathod, e.g. epoxy value content. The GPC analyzes show that all resins have comparable molecular weight distribution. Reaction conditions and results of chromatographic analyzes are shown in Table 1.

Table 1. Reaction conditions and results of chromatographic analyses of synthesized solid epoxy resins.

Solid epoxy
resin sample
Reaction
heating
Reaction
time

[min]
Catalyst
content
[1·10-3mol]
Epoxy
Value

[mol/100g]
GPC analysis
Mn
Mw
Pd
A160A Microwave 65 0.5 0.110 2140 3780 1.77
A160B 40 1 0.113 2150 3930 1.83
A160C 20 5 0.104 2470 3390 1.83
K160A Conventional 120 0.5 0.106 1790 3130 1.75
K160B 80 1 0.111 2180 4000 1.84
K160C 35 5 0.100 2380 5010 2.10

Uncured high molecular weight epoxy resin have good thermal stability till 330°C. The degradation of the sample in an air atmosphere occurred in one step process to ca. 50%, what is shown in Figure 2. Results of TG analyzis are shown in Table 2.

TG analysis of unreacted solid epoxy resin

Figure 2. TG analyzis of uncured "Bisphenol A-type" solid epoxy resin.


Table 2. Thermal stability of solid epoxy resin synthesized under microwave irradiation analyzed by means of TG.

Solid epoxy
resin sample
T3% T5% T10% T20% T50% Residual mass
[%]
A160B 328 370 407 424 442 12.9

Based on the DSC analyzes (see results in Table 3), it is evident that the reaction of curing solid epoxy resins, for the lowest catalyst content occures in temperature interval of 177-200°C, whereas for the highest catalyst content occures in temperature range of 165-189°C (data for solid epoxy resins synthesized under microweve irradiation).
Appropriate data for solid epoxy resins synthesized under conventional heating are 178-200°C (for the lowest catalyst content), and 157-187°C (for the highest catalyst content).
It was impossible to estimate the activation energy and enthalpy of curing because of the small endothermic peak which occurs at about 209°C, and was connected with melting of DCDA.

Curing of solid epoxy resins with dicyandiamide - DSC analyses

Figure 3. DSC curves of compositions of solid epoxy resins (synthesized under microwave irradiation) with dicyandiamide.


Curing of solid epoxy resins with dicyandiamide - DSC analyses

Figure 4. DSC curves of compositions of solid epoxy resins (synthesized under conventional heating) with dicyandiamide.

The glass transition temperature Tg is a parameter which can be used to show the differences in crosslinking density or degree of cure. With increasing crosslinking, the freedom of movement in the polymer network decreases. As a results, the shift of Tg to higher temperature is observed.
In our experiments we have observed, that with the rising of the catalyst concentration in the solid epoxy resin structure, the T
g temperature is also increasing. These shifts are connected with the different contents of catalyst in solid epoxy resins. During synthezes of high molecular weight epoxy resin, we were used three different catalyst contents, e.g. 0.5, 1 and 5·10-3mol of 2-methylimidazole. This compound is often used as an accelerator of curing reaction.

Table 3. Results of DSC analyzes of solid epoxy resins cured with dicyanodiamide.

Solid epoxy
resin sample
Tg Tons Tmax
A160A 61.2 176.8 199.5
A160B 61.7 175.7 199.0
A160C 64.4 164.9 188.8
K160A 53.5 178.2 200.0
K160B 58.7 173.2 199.8
K160C 69.0 156.8 187.1

Tg-glass transition temperature, Tons-temperature onset of cure, Tmax-temperature of maximum exothermic reaction.

Curing of solid epoxy resins with dicyandiamide - DSC analyses

Figure 5. DSC analyzes of a postcuring reaction of solid epoxy resin.

After first cycle of heating, the second scan was conducted after cooling sample to the room temperature. In this way, we wanted to check, if some post curing reactions occur. As one can see in Figure 5, no significant effect was observed. The sample of solid epoxy resin was completely cured.


Acknowledgement

The Polish State Committee supported the research for Scientific Research Grant No. 7T09B06521.


V. References

[1] E. Bryan, "Chemistry and Technology of Epoxy Resins", London, Chapman and Hall, 1994.

[2] P. Czub, Z. Bończa-Tomaszewski, P. Penczek, J. Pielichowski, "Chemia i Technologia Żywic Epoksydowych", Warszawa, WNT, 2002.

[3] L. Csillag, L. Antal, H.R. Dolp, Polimery, 19, 578 (1974).

[4] Z. Brojer, Polimery, 25, 205 (1980).

[5] B. Szczepaniak, J. Rejdych, Polimery, 27, 236 (1982).

[6] US Pat 6,022,931 (2000).

[7] P. Penczek, B. Szczepaniak, Acta Polymerica, 42, 112 (1991).

[8] US Pat 6,262,189 B1 (2001).

[9] US Pat 3,006,891 (1961).

[10] D. Bogdał, J. Pielichowski, P. Penczek, J. Górczyk, G. Kowalski, Polimery, 47, 842 (2002).

[11] D. Bogdał, J. Pielichowski, P. Penczek, A. Prociak, Advances in Polymer Science, 163, 193 (2003).

[12] M. Gilbert, N. Schneider, W. MacKnight, Macromolecules, 24, 360 (1991).