Synthesis of high molecular weight epoxy resins under microwave irradiation

Jarosław Górczyk^a), Dariusz Bogdał^*a) , Jan Pielichowski^a), Piotr Penczek^b)

^a) Department of Polymer Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Cracow, Poland

^b) Industrial Chemistry Research Institute, ul. Rydygiera 8, 01-793 Warsaw, Poland

* e-mail: pcbogdal@cyf-kr.edu.pl
 

Received: 15 August 2001 / Uploaded 22 August 2001
 
 

Abstract: A new method of synthesis of high molecular weight epoxy resins is presented. All reactions were performed in a multi-mode microwave reactor “Plazmatronika” (microwave frequency - 2,45GHz, maximum of microwave power - 300W). Shortening of the reaction time for all processes performed in the microwave reactor, in comparison to conventional heating, was observed.  Keywords: epoxy resin, microwave

• Introduction 
• Experimental part 
• Materials 
• Synthesis of epoxy resins 
• Analytical part 
• Results and discussion 
• References

I. Introduction

High molecular weight epoxy resins (epoxy number LE=0,25-0,02; M_n=1000-10000) are mostly used in industry of varnishes and dyes. There are two different methods of synthesis of high molecular weight epoxy resins¹⁾:

1.Direct method based on polycondensation of bisphenol A and epichlorohydrin under normal²⁾ or higher³⁾ pressure in the presence of NaOH as a catalyst.

2. Indirect method based on polyaddition of bisphenol A to low molecular weight epoxy resin^4,5) (epoxy number LE=0,58-0,35; M_n=370-500) or middle-molecular-weight epoxy resin (epoxy number LE=0,30-0,15; M_n=500-1000).

The direct method is performed in the solution so it is necessary to remove solvent, water and NaCl after reaction. In the indirect method, a pure product is obtained so all the costly procedures of purification are eliminated. Usually in industry this two reactions are performed under conventional heating.

In conventional thermal processing, energy is transferred to the material through convection, conduction and radiation. Transfers of energy rely on diffusion of heat from the surfaces of the material. Microwave energy is delivered directly to material through molecular interaction with the electromagnetic field. A transfer of energy does not rely on diffusion of heat. It is possible to achieve rapid and uniform heating even in materials with low thermal conductivity (e.g. epoxy resin). The significant result is the reducing of processing time.

Schematic reaction of synthesis of high molecular weight epoxy resins is shown in Figure 1.

Figure 1. Polyaddition of bisphenol A to low-molecular-weight epoxy resin.

II. Experimental part

Materials

• bisphenol A (cz.d.a.)
• low molecular weight epoxy resin

Synthesis of epoxy resins

The general procedure for the synthesis of high molecular weight epoxy resins can be described as follows: a stoichiometric amount of bisphenol A was added to low molecular weight epoxy resin, which already contained catalyst:
(molar rate: m _{(bisphenol A)} : m _{(epoxy resin)} = 3 : 7).
 

The mixture was stirred under nitrogen at different temperatures: 140, 160 and 170°C, in a Multi-mode Microwave Reactor “Plazmatronika” - Figure 2 (microwave frequency - 2,45GHz, maximum of microwave power - 300W), for time required to obtain epoxy numbers about 0,11.  Normally 40% of full microwave’s power was used. Every 5 minutes a small sample of epoxy resin was taken from the mixture to determine the epoxy number. After the reaction the epoxy resin was cooled down and powdered.  Plazmatronika Microwave Reactor implements novell Concentrated Microwave Field (CMF) which provides the microwave field focused onto the reaction vessel.  The temperature can be measured at the bottom of the reaction chamber using Infra-red thermometry with fine beam focusing.  (http://plazmatronika.pl/)

Figure 2. A Multi-mode Microwave Reactor « Plazmatronika ».

III. Analytical part

Epoxy numbers of synthesised resins were determined according to the PN-87/C-89085/13. All GPC analyses were obtained on a GPC chromatograph (“Knauer”). A system of three columns was used: 2×PL-gel (300×7,5 mm; dimension of grains 3mm and type of pore Mixed-E) with one precolumn. The temperature of measurement was 30°C and THF was a solvent.

IV. Results and discussion

In the Figure 3 GPC chromatograms of the reaction extent, performed in microwave reactor (160°C, 20 minutes, 40% power), are presented. One can observe an increase of molecular weight through the reaction time.

Figure 3. GPC chromatograms of high molecular weight epoxy resins synthesised in a microwave reactor (160°C, 40% power).

Results of all analyses are presented in Table 1.

GPC analysis shows that all synthesised high molecular weight epoxy resins have comparable molecular weight and polydispersion.

The optimal reaction conditions for the microwave processes were found to be: 15-20 minutes, temperature between 160-170°C, using 40% of full power. Shortening of the reaction time for all processes provided in the microwave reactor, in comparison to conventional heating, was observed.

It is necessary to use an atmosphere of nitrogen to improve the colour of the synthesised epoxy resins.
 

Reaction heating	Temperature of reaction	Time of reaction	LE	GPC analysis
	[°C]	[min]	[mol/100g]	Mn	Mw	Pd
microwave	170	15	0,107	2180	5660	2,59
conventional	170	30	0,111	2210	5730	2,59
microwave	160	20	0,112	2200	5680	2,58
conventional	160	40	0,110	2210	5700	2,58
microwave	140	50	0,113	2190	5610	2,56
conventional	140	70	0,108	2220	5680	2,56

Table 1. Results of GPC analysis of high molecular weight epoxy resins.

V. References

¹⁾ Brojer Z., Hertz Z., Penczek P.: “Żywice epoksydowe”, Warszawa, WNT, 1982 

²⁾ Pat. USA 2 992 239 (1961)

³⁾ Pat. USA 2 694 694 (1954)

⁴⁾ Brojer Z.: Polimery 1980, 25, 205

⁵⁾ Csillag L., Antal I., Dolp H. R.: Polimery 1974, 19, 578