Rozprawa naukowa nr 423

Żywice aminowe zajmują dominującą pozycję w zakresie ogólnego zużycia klejów w drzewnictwie, zwłaszcza w odniesieniu do wytwarzania różnego rodzaju tworzyw drewnopochodnych. Powszechność ich stosowania wynika z wielu korzystnych właściwości, m.in. wodorozpuszczalności, dużej reaktywności zapewniającej możliwość przystosowywania klejów do zróżnicowanych warunków nakładania i utwardzania, bezbarwnych spoin, dużej odporności termicznej oraz na działanie czynników biotycznych. 
Niestety, najtańsze z klejów aminowych, bazujące na żywicach UF, cechują się niewystarczającą odpornością spoin na działanie wody oraz warunków atmosferycznych, dlatego są wykorzystywane głównie w produkcji sklejki suchotrwałej i płyt wiórowych do zastosowań wewnętrznych. Podstawową przyczyną zmniejszania się wytrzymałości spoin z klejów UF poddanych testom starzeniowym są procesy hydrolitycznego rozpadu powodowane oddziaływaniem wody i kwasowej pozostałości utwardzacza znajdującego się w spoinach klejowych.
Wad tych w znacznym stopniu pozbawione są relatywnie drogie żywice MF, które cechuje duża wytrzymałość i odporność spoin na hydrolityczny rozpad, jednakże wykazują one niezadowalającą stabilność podczas transportu oraz składowania. Wysoce stabilne żywice MF można uzyskać poprzez nadanie im postaci sproszkowanej, co jednak istotnie podwyższa ich koszt, sprawiając, że nie znajdują one szerszego zastosowania w technologiach produkcji tworzyw drzewnych.
Zwiększenie stabilności żywic MF, przy zachowaniu innych korzystnych właściwości użytkowych, uzyskuje się poprzez ich modyfikację mocznikiem, otrzymując w ten sposób o wiele tańsze polikondensaty MUF.

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1. WSTĘP

2. PRZEGLĄD LITERATURY

3. CEL I ZAKRES PRACY

4. METODYKA I OPIS BADAŃ 
4.1. Założenia 
4.2. Synteza żywic klejowych 
4.3. Podstawowe właściwości fizykochemiczne
4.4. Właściwości fizykomechaniczne
4.5. Budowa i struktura ciekłych żywic MUF
4.6. Charakterystyka procesu żelowania i utwardzania
4.7. Badania właściwości utwardzonych żywic
4.8. Badania właściwości fizykomechanicznych i higienicznych sklejki

5. WYNIKI
5.1. Modelowe żywice i analiza ich właściwości technologicznych
5.2. Właściwości fizykomechaniczne
5.3. Struktura fizyczna żywic MUF
5.4. Struktura molekularna żywic MUF
5.5. Charakterystyka procesów żelowania i utwardzania
5.6. Właściwości utwardzonych żywic
5.7. Właściwości sklejki

6. PODSUMOWANIE

7. WNIOSKI

8. LITERATURA

SUMMARY

The objective of the presented studies was to investigate the physicochemical ageing processes occurring during storage time under normal conditions of adhesive MUF resins designed for the production of waterproof plywood.
The work included the following procedures: conception development, synthesis of highly stable MUF resins and the performance of a multi-dimensional study of resin properties as a function of the storage time and the molar ratio between substrates and the type of the applied modifier. It was also intended to determine the physicomechanical and hygienic properties (formaldehyde and VOC, TD/GC-MS method) of adhesive glue lines in the plywood.
The research work was concentrated on the recognition of the physical properties (reflected light microscopy: OM, POM, NIC, granulometric methods DLS, NIBS, LALLS) and chemical properties (gel chromatography GPC, X-ray diffraction WAXS, SAXS methods, spectroscopy MALDI-TOF-MS, 1H NMR and 13C NMR), and structure of liquid polycondensates, gelling and curing processes (DSC), and properties of hardened
glue mixtures (TGA, DSC, NAXS, the drop method, microscopy: SEM-SE, AFM-TM).
The research object consisted of MUF resins synthetized in laboratory according to the developed conception by a three-step method excluding the operation of concentration with the application of formalin with a high methanol content and variable molar ratio F:M:U of (4.6-2.8):1.0:1.0, or appropriately modified potato starch or phenol.
On the basis of the obtained results, 20 conclusions and generalizations have been formulated. The studied resins were characterized by a very high stability during a period of 18 months storage time in normal conditions. At the same time, the increase of substrates molar ratio caused an increase of resin stability. So far, the suitability of liquid adhesive MUF resins for use during 10 to 18 months has not been reported in the available literature.
During the ageing of resins, there followed an increase of their viscosity and the initial adhesion, with a slight increse of reactivity and a decrease of formaldehyde content and water tolerance. There were no significant changes in the pH value and in dry matter content.
Model resins were characterized by similar rheological properties specific of non-Newtonian rheostable shear-thinning liquids whose curves ran in a way similar to the Ostwald-de Waele model. During resins ageing, they showed increasing deviations from the given rheological model and an increase in the activation energy of viscous flow. In case of polycondensates modified by starch, some symptoms characteristic of phisicoelastic fluids were identified. It was found that macroscopic ageing processes occurring in resins during storage time took place in a similar way, independent of the molar ratio of substrates and the modifier type with a distinct differentiation in the time of their occurrence.
Separate stages of polycondensates ageing have been distinguished. They included the initial, the main and the final stages which took place alternatively towards physical gelation or separation of the phases.
It has been found that during storage, resins were out of order and kept changing the following dispersion systems: the molecular and semicolloidal systems (at most 5 nm); the nanosuspension-colloidal systems (thin dispersion in the range of 5-1000 nm), the microsuspension (thick dispersion above 1000 nm) and in the final phase of minisuspension phase (above 10000 nm) dispersed in water monomers and oligomers of melamine and urea, as well as other compounds such as accompanying methoxyl ether, hemiacetals, polymeric methylene glycol, methanol, and as modifiers.
Already in the initial stage, resins can be classified to polydispersion hydrocolloidal particles in which the dispersed phase consists of oligomers developed in such oligomers which are subject to aggregation by creating a nanosuspension with mean diameters of 2-12 nm (DLS, NIBS) with a minimal participation (below 0.01%) of 500-4000 nm aggregates in bi-modal and tri-modal distributions (DLS).
Physicochemical processes occurring during the ageing process of resins consisted in the successive combining of oligomer particles into spherical clusters showing flowerlike or berry-like forms and ribbon-like form superaggregates (OM, POM, NIC) clusters created successively already after the termination of their synthesis.
During ageing process, regardless of the molar ratio of substrates and the modifier type, resins were characterized by an amorphous structure (WAXS, SAXS). The chemical structure of resin polycondensates included methylolated derivatives of melamin and urea, methylene and dimethyl ether bonds of oligomers in the combinations of urea–urea, melamine–melamine, melamine–urea, as well as compounds with methoxy
groups (MALDI-TOF, NMR).
In reference to molecular structure, an increase was observed in the number and weight of average molar masses and mass dispersion (GPC) and a reduction of the share of mono-substituted methylolated groups in the increasing of the number of methylene and dimethyl ether bonds and a slight increase of the substitutions of the amide groups (NMR). A high stability of resins during their storage was caused by the participation of
methyl alcohol playing the role of a buffer, blocking and depolymerization agent of polymeric methylene glycols (NMR).
Resin destabilization followed by intensive coalescence processes of aggregates leading to phase separation or to the formation of networks in the physical gelation process of gel development and then, by further separation of the diffusing phase (syneresis). Phase separation was characteristic of resins with higher average molecular masses and a higher dispersion coefficient (GPC).
In the studies referring to the curing process, no effect was found to be exerted by the storage time, either on the gelation process (DSC), or on the properties of hardened polycondensates such as thermoresistance (TGA, DSC) and phase composition (WAXS). Resins hardened at the level of molecular structure were characterized by an amorphous structure (WAXS). Independent of the storage time of resin, the molar ratio and the modifier type, the hardened adhesive layer was characterized by a similar wettability and high quality of adhesion effectiveness which suggested the possibility of a strong interaction between resin and wood.
No differences were found in the microscopic picture (SEM-SE) of adhesive resin layers with different storage time made on beech wood both by pressing or by free-hardening. Free-hardened layers were characterized by a hemispherical structure. Glue film, independently of resin storage time, showed only negligible differences in the texture and topography (SEM-SE, AFM-TM) and the ratio of RMS (AFM).
Plywood fulfilled the requirements of PN-EN-314-02:2001 in terms of strength and water resistance of adhesive glue lines, regardless of resin storage time, the molar ratio and the applied modifiers. The glueing effect obtained with resin stored even longer than 12 months represented a fully satisfactory, waterproof plywood. The storage time did not affect the resin on the level of formaldehyde emission from the produced plywood.
The increase of substrate molar ratio caused a higher formaldehyde emission. The method of producing plywoood with the use of resins in the form of solidified layers of verneers significantly reduced the emission of formaldehyde, regardless of resins storage time.
There was no effect of plywood manufacturing methods on the molar ratio of resin on the level of VOC emission from the plywood (TD-GC/MS). The extent of VOC emission depended on the wood type and on the seasoning time of veneer and plywood. The main compounds emitted from pine plywood included monoterpenes and carbonyl compounds. The highest emissions among terpenes were shown by α-pinene and Δ3-carene, while among carbonyl compounds, it was done by hexanal.
In the range of instrumental methods used in the analysis of amino adhesive resins, unusually important in a research work are the following aspects: the structure of resins using reflectance optical microscopy in Nomarski contrast, photon correlation spectroscopy (DLS class technique, NIBS), composition of molecular masses of oligomers and monomers by mass spectroscopy MALDI-TOF applying two independent matrices or microscopic examination of adhesive films with the use of atomic force microscopy (AFM).