Amino-formaldehyde resins play a huge role in the contemporary world. They are actively used in civil engineering, aviation, textile, and other industries; this stimulates the development of methods for the research and analysis of various polymer materials.

Almost all synthetic polymers are polydisperse; therefore, their molecular weight is an average statistical value and is determined by the molecular weight distribution (MWD) type and the averaging technique. The specific feature of transport methods is that primary information is obtained as the time distribution curves of macromolecule concentration. Since these curves are direct transformations of MWD functions, the transport methods are very suitable for determining molecular weights and polydispersity coefficients of polymers.

Unlike other transport methods, chromatography combines a continuous sample fractionation with the fraction analysis. This is a heterophase process, in which fractionation is based on a difference in the interfacial distributions of substances moving together with a solvent (mobile phase) through a highly dispersed medium of the stationary phase. Chromatographic methods are usually classified according to the chosen type of mobile and stationary phases: gas chromatography covers methods, in which gas serves as the mobile phase and liquid serves as the liquid phase. In accordance with the separation mechanism, there are ion-exchange, adsorption, precipitation, partition, and exclusion chromatography techniques [5].

In adsorption (chromatographic) fractionation, a polymer is deposited on an inert material with a high specific surface (packing) and placed in a column. An elution mixture consisting of two liquids, solvent and nonsolvent, is passed through the column with packing and the adsorbed polymer. The first fraction having the minimum adsorption capacity, usually this is the fraction with the least molecular weight, eluates from the column. Sequential separation of elution portions allows isolation of up to 20–30 of polymer fractions [4].

The analysis of MWD was performed on a Waters liquid chromatograph (Fig. 1) equipped with a Waters 2414 differential refractometric detector and a PDA 996 diode array spectrophotometric detector and a PLgel 5 μm MIXED-C column. Analysis was carried out under the following conditions: eluent N-methylpyrrolidone + LiCl (1.0 g of LiCl/0.5 L of NMP), the elution rate 1 mL/min, Tcol = 70°C, and Tref = 50°C. The calibration dependence was obtained using standard polystyrene samples with molecular weights ranging from 580 to 3.7 × 106 Da. The resulting chromatograms were processed using the Empower program.

Fig. 1.
figure 1

Waters 2414 chromatograph.

Full size image

Polymer samples were dissolved in NMP + LiCl, and the polymer solution was filtered through an Anatop25 0.2 μm PTFE filter (Whatman).

Amino-formaldehyde resins modified at the synthesis stage were used as polymer samples.

In the synthesis of amino-formaldehyde resins, alkalis, in particular NaOH, is used to neutralize formalin. The Cannizzaro redox reaction is known to occur under these conditions; it involves reduction of a formaldehyde molecule with the simultaneous oxidation of another one:

$${\text{2C}}{{{\text{H}}}_{{\text{2}}}}{\text{O + }}{{{\text{H}}}_{{\text{2}}}}{\text{O}} \to {\text{C}}{{{\text{H}}}_{{\text{3}}}}{\text{OH + HCOOH}}{\text{.}}$$

As a result of the Cannizzaro reaction, pH of the reaction mixture (determined by adding sodium hydroxide) decreases gradually; this process is especially rapid in the presence of compounds acting as catalysts [1, 2]. Amino-formaldehyde resins were synthesized in the presence of a catalyst modifier preventing the Cannizzaro reaction. A salt of polyfunctional organic acids was used as a catalyst modifier.

Four samples of amino-formaldehyde resins (Table 1) were synthesized using various amounts of the LN catalyst modifier, melamine (M)- and carbamide (C)-to-formaldehyde (F) molar ratios, and amounts of diethylene glycol (DEG). The base case was the unmodified melamine resin.

Table 1.   Synthesized amino-formaldehyde resins
Full size table

The properties of the resulting amino-formaldehyde resins were studied and compared with those of the base case (the base case was the unmodified melamine resin containing 35 wt % melamine). The results are listed in Table 2.

Table 2.   Properties of amino-formaldehyde resins
Full size table

An analysis of the data presented in Table 2 makes it possible to infer that the resulting modified resins are characterized by a smaller content of free formaldehyde and a longer shelf life with the content of expensive melamine being lower.

Figure 2 shows the chromatograms of the amino-formaldehyde resins; black dashed lines denote the real chromatogram of N-methylpyrroldone + LiCl (the lowest curve), and the upper black dotted line denotes the same chromatogram magnified by 20 times.

Fig. 2.
figure 2

Chromatograms of amino-formaldehyde resins: (0) base case; (1) sample 1; (2) sample 2; (3) sample 3; and (4) sample 4.

Full size image

An analysis of the graphs shown ted in Fig. 2 indicates that the weight-average molecular weight (Mw) increases in the following sequence: base sample → sample 1 → sample 4, while polydispersity for samples 1–4 remains almost unchanged.

Table 3 presents the molecular weight characteristics of the amino-formaldehyde resins; here, Mn is the number-average molecular weight, Mw is the weight-average molecular weight, Mp is the molecular weight in the higher molecular peak, and PD is the Mw/Mn ratio, which is referred as the polydispersity index.

Table 3.   Molecular weight characteristics of amino-formaldehyde resins
Full size table

Based on the data presented in Table 3 it can be stated that the modified amino-formaldehyde resins have higher molecular weights compared with the resin currently used in the industry. Thus, the modification of amino-formaldehyde resins with the salts of polyfunctional organic acids makes it possible to eliminate one of the disadvantages of modern impregnating resins.