Sixth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-6), http://www.mdpi.org/ecsoc-6, 1-30 September 2002

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[E001]

	Determination of Chain Branching in Solid Epoxy Resins Using 1H NMR Spectrometry Jarosław Górczyk, Dariusz Bogdał Department of Polymer Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Cracow, 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-2,45GHz, maximum of microwave power-300W). The H¹ NMR spectra were made before and after the reaction of solid epoxy resin with Trichloroacetyl isocyanate. It allowed us to determine the degree of branching.

Keywords: H¹ NMR spectrometry, solid epoxy resin, microwave irradiation, polyaddition

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

I. Introduction

Epoxy resins are produced from the begining of 50-ths. Because of their unique properities, e.g. excellent chemical and electrical resistance, very good adhesion to different kinds of materials and good heat resistance, they are willingly used in various kinds of everyday life¹⁾. Solid epoxy resins (Epoxy Value EV=0,25-0,02; M_n=1000-10000), (which are used in industry of varnishes and dyes), are mostly synthesised in according to indirect method^2,3,4). This method, called "advancement" process is based on polyaddition of Bisphenol A to Low-molecular-weight epoxy resin (Epoxy Value EV=0,58-0,35; M_n=370-500) or Middle-molecular-weight epoxy resin (Epoxy Value EV=0,30-0,15; M_n=500-1000) in the presence of a catalyst^5,6). It is possible to synthesize such resins in a batch or continous process, under conventional heating or microwave irradiation.

Schematic reaction of synthesis solid epoxy resins is shown in Figure 1.

Figure 1. "Advancement" process (polyaddition of Bisphenol A to Low-molecular-weight epoxy resin).

As we can see in a Figure 1 each molecule contains as many aliphatic hydroxyl groups as there are repeat units in the linear structure. These hydroxyls are a potential brenching points. So that for every branch point in the nonlinear structure there is one hydroxyl-carbinol methine pair less in comparison with the linear structure of equal molecular weight. The H¹ NMR method of determining branching points relies on involving reaction between hydroxyl groups presented in the solid epoxide resin and Trichloroacetyl isocyanate^7,8,9,10). The urethane group is formed as a result of that reaction. For each sample the H¹ NMR spectra are made two times before and after the reaction. Hypothetical H¹ NMR spectra before (Figure 2) and after (Figure 3) the reaction of epoxy resin with Trichloroacetyl isocyanate are shown below

Figure 2. H¹ NMR spectrum before the reaction of solid epoxy resin with Trichloroacetyl isocyanate.


Figure 3. H¹ NMR spectrum after the reaction of solid epoxy resin with Trichloroacetyl isocyanate.


The total number of methine protons Z_methine can be describe as a fifth part of sum the reduced integration of methine and methylene signals before reaction:

Z_methine = 1/5 · (Z_4.1 - Z_4.3)

The number of methine protons in a branched molecules Z_branched however can be describe as a difference between Z_methine and the reduced integration of methine signal after reaction with Trichloroacetyl isocyanate:

Z_branched = Z_methine - Z_5.5

where:
            Z_4.1-reduced integration of methylene signal before reaction
            Z_4.3-reduced integration of methine signal before reaction
            Z_5.5-reduced integration of methine signal after reaction

So that the degree of branching f can be calculated according to the formula:

f = (Z_4.1 - Z_4.3 - 5Z_5.5) / (Z_4.1 - Z_4.3)

It is also easy to estimate the number of branche points per moleculer r according to the formula:

r = f · (M_n - 340.4) / 284.4

Persentage of chain branching can be expressed as a ratio of the number of branche points per moleculer r to the number of repeat units n. Where:

n = 2 · (EEW - 170.2) / 284.4

All appropriate data are stored in Table 2.

II. Experimental part

Materials

• Bisphenol A ("Shell"),
• Low-molecular-weight epoxy resin Ruetapox 0162 ("Bakelite AG"),
• 2-methylimidazole ("Fluka"),
• Trichloroacetyl isocyanate ("Fluka").

Syntheses of epoxy resins

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

Appropriate amount of Bisphenol A was added to Low-molecular-weight epoxy resin (Epoxy Value EV=0.57 mol/100 g) to obtain required increasing the molecular weight during reaction to the desired level. The calculated molar rate of Bisphenol A to Low-molecular-weight epoxy resin was 3 : 4. The mixture was stirred at 160°C, in a Multi-mode microwave reactor “Plazmatronika” - Figure 2 (microwave frequency - 2,45GHz, maximum of microwave power - 300W), for time necessary to obtain Epoxy Value about 0,11. 100W of microwave power was used. Every 5 minutes a small sample of epoxy resin was taken from the mixture to determine the Epoxy Value. After the reaction epoxy resin was cooled down and powdered.

	Plazmatronika microwave reactor implements novell Concentrated Microwave Field (CMF) which provides the microwave field focused into the reaction vessel. The temperature can be measured at the bottom of the reaction chamber using Infra-red thermometry with fine beam focusing (http://www.plazmatronika.pl).
Figure 4. A Multi-mode microwave reactor "Plazmatronika".

    III. Analytical part

Epoxy Values of synthesised resins were determined according to the PN-87/C-89085/13. All GPC analyses were obtained on a GPC chromatograph (“Knauer”). 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. Results of all analyses are presented in Table 1.

The general procedure for determination the degree of branching of solid epoxy resins can be described as follows:

Aproximately 10 mg of solid epoxy resin was dissolved in 0.5 ml of Deuterochloroform. The solution was scanned on a Merkury-300 "Varian" spectrometer and the peaks were integrated. Then 10 mg of Trichloroacetyl isocyanate was added and after 5 minutes a few drops of Deuterium oxide. The solution was scanned second time and the peaks were integrated. Results of all analyses are presented in Table 2.

IV. Results and discussion

Table 1. Results of determination of molecular weight distribution of solid epoxy resins by GPC chromatography.

GPC analyses shows that all synthesized solid epoxy resins have comparable molecular weight and polydispersity.

Table2. Results of determination of chain branching in solid epoxy resins by H¹ NMR spectrometry.

The degree of branching is in the range from 0.36 to 1.19. It is consistent with the literature data. As one can see it is better not to use too much catalyst. The catalyst content 5·10^-3[mol] has already caused on a large scale ariseing the chain branching. The optimal reaction conditions for processes provided under microwave irradiation were found to be: 40 minutes, temperature 160°C, content of catalyst 1·10^-3[mol] and 100W of microwave power. Shortening of the reaction time for all reactions provided in microwave reactor in comparison to conventional heating were observed. Important is that all solid epoxy resins synthesized both under microwave irradiation and conventional heating have comparable physico-chemical properties.

V. References