Лако-красочные материалы — производство

Regardless of the fact that numerous investigations exist about the possibility of incorpo­rating the furan heterocycle into wood adhesive formulations, their industrial exploitation is still modest. The first suggestion concerning the use of 1 in partial substitution of formaldehyde in phenolic resins was put forward in 1958 by Baxter and Redfern [18] who proposed that the furfural units were incorporated into the polymer skeleton follow­ing condensation reactions such as:

The intermediate oligomers such as 13 were then subjected to methylolation with formal­dehyde to form phenolic-furanic-formaldehyde resins, according to:

The interest in this type of process was, of course, the decrease of formaldehyde content and, therefore, its lower release during the life cycle of the resin.

This approach was then extended by Pizzi’s group to other phenolic type adhesives such as phenol-resorcinol-formaldehyde networks [19]. In this work, it was shown that the addition of 1 gave cold setting resins with performances and costs comparable to those made using formaldehyde alone. Thus, the phenol-resorcinol-furfural-formaldehyde cold sets obtained appeared to have a lower bulk shrinkage compared to those prepared with­out 1. Moreover, it was established that the presence of furfural did not slow down the curing rate of the resins.

Stamm [20] studied the dimensional stabilization of different woods with 2. Thus, Douglas fir, Engelman spruce, loblolly pine, and yellow poplar woods were treated with 2 in the presence of zinc chloride, citric acid, or formic acid in order to induce their acid- catalyzed polymerization. It was established that the maximum antishrinking efficiency (around 72%) could be reached with a resin level of a minimum of 40% with respect to oven dried (OD) wood. The optimal amount of each acidic catalyst was also determined. The curing time was studied for each system and it was shown that the use of 1% zinc chloride and 6h of curing time at 120°C gave very satisfactory fracture moduli, toughness, abrasion resistance, and antishrinking behavior. The only limitation associated with the possible uses of these systems is the dark color of the final materials.

Dhamaney [21] showed that the addition of furfural into cashew nut shell liquid adhesives based on phenol-formaldehyde resins, using CuCl2 or CaCO3 as a ‘‘hardener,’’ gave good adhesive bonding for ordinary plywood. Johns et al. [22] prepared white fir flakeboards using an aqueous solution containing a mixture of ammonium lignosulfonate, 2, and maleic acid as a binder. Before bonding, the wood surface was activated by a nitric acid treatment. It was shown that the panels thus obtained possessed a higher elasticity modulus and lower thickness swell and water absorption compared with those prepared using classical phenol-formaldehyde binders. Nevertheless, the internal bonding and the rupture modulus were higher for panels obtained using conventional resins. It was also established that best surface activation was achieved using a 1.5% aqueous solution of nitric acid (25-40%) with respect to OD wood, since it gave the optimal mechanical properties for both high and low density panels.

Gupta et al. [23] prepared plywoods from Cedrus deodora and phenol-formaldehyde resins. They showed that the addition of 5% of 1 to this adhesive did not result in any appreciable improvement, but the concomitant addition of 10% of coconut shell powder gave very high failing loads and very low glue failures. Subsequently, in another context, Pizzi et al. [24] tested different aliphatic aldehydes and 1, in tannin-based adhesives, and showed that furfural could replace formaldehyde in the manufacture of adhesive resins for beam lamination. Roczniak [25] studied the thermal properties of phenol-formaldehyde-1 resins, as catalyzed by dichlorohydrin of glycerol, boric acid, hexamethylenetetramine (HMTA), or p-toluene sulfonic acid. Two main conclusions were drawn from this work: (i) p-toluene sulfonic acid gave a faster resinification rate and (ii) HMTA led to the highest thermal resistant resins. Krach and Gos [26] investigated the gluing of large dimension sawn wood structures using urea-melamine-furfural as a binder. They stated that the initial wood moisture (8 to 12%) and the time of adhesive spreading (10 to 90 min) did not influence significantly the strength properties of the glued junction.

Philippou et al. [27] studied the bonding of wood by graft polymerization. They produced white fir, Douglas fir, and bishop pine particleboards using 2 as well as mixtures of ammonium lignosulfonate with 2 or with formaldehyde as cross-linking agents. Before bonding, the wood surface was activated with different amounts of hydrogen peroxide (from 0.5 to 4% with respect to OD wood). The amount of the binder was kept constant in all experiments (7% with respect to OD wood). The internal bond strength of the materials obtained was found to increase with increasing amounts of hydrogen peroxide, whereas the thickness swelling followed the inverse trend. The use of both 2 alone and its mixture with ammonium lignosulfonate showed very good bonding capability. Bishop pine gave the highest internal bonding and white fir yielded the lowest thickness swelling and water absorption when the mixture of ammonium lignosulfonate with 2 was used as a binder. The least efficient adhesive was found to be the formaldehyde-lignosulfonate system. The differences between wood species were attributed to their different contents of extractives. In another study, Philippou et al. [28] studied the effect of the composition of the bonding materials on the properties of Douglas fir particleboards. Thus, the proportion between 2 and ammonium lignosulfonate was varied as follows: 10/0, 9/1, 8/2, 7/3, 6/4, 5/5, 2.5/7.5, and 0/10. In this work, the wood was also activated by hydrogen peroxide (2% w/w with respect to OD wood) and the catalysts used were ferric chloride and maleic acid. Ammonium lignosulfonate without 2 failed to develop resistance to boiling water whereas 2 without ammonium lignosulfonate gave good mechanical and water resistance proper­ties. However, the use of a mixture containing six parts of lignosulfonate and four parts of 2 yielded boards with the highest internal bond strength and water resistance values. Increasing the amount of resin with respect to wood was found to produce an increase in the elasticity and rupture moduli and a decrease in water absorption and thickness swelling. The boards prepared exhibited strength and resistance to cold and boiling water comparable to those made using classical phenol-formaldehyde resins. In a third investi­gation, Philippou et al. [29] studied the effect of the processing parameters on the mechan­ical properties of particleboards made from Douglas fir wood treated with ammonium lignosulfonate and 2 as a binder in the presence of maleic acid as a catalyst. They showed that increasing the pressing temperature from 121 to 177°C or the pressing time from 4 to 8 min, progressively enhanced the internal bond strength and the water resistance of the treated boards. The water resistance was found to be further improved by the addition of a small amount of wax (0.5% w/w with respect to OD wood) in the binder mixture.

Leitheiser et al. [30] prepared water dilutable furan resins as binders for particle­board and showed that the resulting composites could be used for exterior applications. These resins were readily water dilutable and had low viscosities, which made their appli­cation with conventional equipment an easy process. Kelley et al. [31] prepared wood panels from Acer saccharum var. Marsh. with various binders. They first activated the surface of the wood by nitric acid and bonded the particles with tannin, 2, and a mixture of the two, with and without maleic acid. In all cases, the particleboards obtained exhibited shear strengths as high as that obtained from a control system made with a conventional phenol-formaldehyde binder. However, the acidic treatment of wood appeared to have only a slight effect on the mechanical properties of the panels bonded with the tannin-2- maleic acid system. Subramanian et al. [32] subjected Douglas fir wood flakes to a nitric acid treatment followed by a grafting reaction with 2(1-aziridinyl)ethyl methacrylate and 2. They showed that the amount of carboxylic acid groups at the wood surface had increased substantially, thus enhancing its reactivity towards both reagents.

Philippou and Zavarin [33] studied the interactions between lignocellulosic materials, 2, and maleic acid in the presence or absence of hydrogen peroxide. They used white fir wood flour, microcrystalline cellulose, milled-wood lignin, and ammonium lignosulfonate and followed their interactions with the binder by differential scanning calorimetry (DSC) and concluded that a graft copolymerization between hydrogen peroxide activated wood, 2, and ammonium lignosulfonate had occurred. Balaba and Subramanian [34] studied the polymerization of 2 catalyzed by the surface acidity resulting from treating wood with nitric acid. They followed the polymerization by intrinsic viscosity measurements and showed that there were two reaction regimes. The first was found to obey zero order kinetics, with an activation energy of 53.4kJ/mol, whereas the second could not be exploited because of polymer precipitation following the formation of network structures.

In 1985, experiments on an industrial scale were carried out jointly at Quaker Oats Chemicals and Collins Pine Company particleboard plants [35]. In these trials 1 was used as an extender in a polymeric methylene diphenyl isocyanate (MDI) binder (1:MDI = 1:3 w/w). The main conclusions which could be reached from these trials were that savings in binder levels, pressing time, and temperature and drying requirements could be obtained compared with the corresponding performances of standard phenol — formaldehyde and urea-formaldehyde systems.

Nguyen and Zavarin [36] studied graft polymerization of 2 on cellulosic materials. They showed that 2 in an aqueous medium at pH 2.0 and 90° C did not copolymerize with the cellulose surface in the presence of H2O2/Fe2+. However, under the same conditions, poly2 was efficiently grafted onto cellulosic fibers and the amount of homopolymer of 2 was negligible. In these conditions, the amount of grafted poly2 reached 68% w/w with respect to OD fibers. They also showed that working at higher temperature and with more concentrated media yielded higher grafting efficiency. Sellers [37] prepared plywoods from southern pine (major structural species) and yellow poplar (most representative decorative species) using polymeric methyl diphenyl diisocyanate adhesive in the presence of 1 as a reactive diluent in order to reduce the adhesive costs. These formaldehyde-free plywood composites did not suffer delamination after accelerated-aging tests and, although the interfacial failure did not satisfy the requirements for structural plywood, they approached or exceeded requirements for decorative applications. Schultz [38] prepared an exterior plywood resin based on 2 and paraformaldehyde. Three-ply assemblies from yellow pine were bonded at different processing conditions and showed that the curing time necessary for these systems was longer than that which was generally required for conventional gluing systems. The use of veneers with a high moisture content (9.5 instead of 5.1%) had very negative effects on the strength properties of the plywood prepared. Pizzi [39] also prepared particleboard urea-furfural-formaldehyde binders. He concluded that a partial substitution of formaldehyde with 1 led to an enhanced CH2O emission and explained this unexpected feature in terms of two competitive reactions. In fact, he showed that in the resins which contained both formaldehyde and 1, the higher stability to hydrolysis of the 1-urea bonds induced the release of formaldehyde from the final product.

New adhesives from furfural-based diamines and diisocyanates were prepared by Holfinger and coworkers [40,41]. They produced flakeboards alternatively bonded with phenol-formaldehyde, MDI, and 5,50-ethylidene difurfuryl diisocyanate (14) adhesives and showed that the strength properties of flakeboards prepared with 14 were slightly lower than those based on MDI and higher than those prepared with phenol-formalde­hyde resins. Thus, the internal bond strength values of flakeboards bonded with MDI and 14 at 3% resin content, were 1.33 and 0.97 MPa, respectively [41], which are much higher than the value required by American standard ANSI/A208.1 (0.41 MPa).

Joshi and Singh [42] showed that about 30% of formaldehyde could be replaced by 1 (obtained from wheat straw) in the formulation of phenol-formaldehyde adhesives. They used these phenol-1-formaldehyde resins in the preparation of plywoods from Vateria indica and Toona ciliata and obtained materials with good resistance to boiling water. These authors mentioned, however, that 1 slowed down the curing rate of the resin and recommended longer condensation times compared with conventional phenol-formalde­hyde thermosets. Motawie et al. [43] prepared 1 by hydrolysis of Egyptian cotton straw and prepared different resins by the copolymerization of the in situ formed furfural with phenol, epichloridrin-phenol, or a bisphenol A-based epoxy prepolymer. The curing of these resins was investigated using phthalic or maleic anhydride at 170-185°C or using diamines at room temperature, both in the presence or absence of kaolin as an inorganic filler. Their properties appeared to be comparable to those of commercially available wood adhesives.

Ellis and Paszner [44] investigated the self-bonding of various lignocellulosic mate­rials possessing high hemicellulose content through the in situ generation of furanic derivatives by acid-catalyzed thermal conversion of some saccharidic units. They used seven different raw materials with increasing pentosan content, i. e., elm, aspen, oak, and birch woods as well as bagasse, sweetcorn cob, and feed corn cob, with pentosan contents of 18.8, 19.4, 20.2, 25.5, 27.2, 39.7, and 42.3%, respectively. The pressing temp­eratures, pressures, and times tested were in the ranges of 160 to 220°C, 14-20 kg/cm2 and 2-10 min, respectively. Ammonium sulfate and ammonium chloride were used as catalysts and their amounts were varied from 0 to 6% w/w with respect to the vegetable material. The bending strength of the materials obtained was directly proportional to the xylan content of the initial lignocellulosic source. The optimal amount of catalyst was found to be around 1.5% w/w based on the natural raw material and the optimal pressing time was established to be around 6 min. Increasing the wood particle size induced a drastic decrease in the bending load, whereas an increase in press plate temperature led to a substantial increase in the mechanical properties of these self-bonding composites.

Gos et al. [45] glued spruce wood (Picea excelsa L.) using three different adhesives, namely: (i) a phenol-resorcinol binder, (ii) carbamide-melamine-1 resins, and (iii) a poly(vinyl acetate) glue. They tested the bending elasticity of these glued woods in the temperature range of 20 to 150° C and a minimum loss of bending strength, when the temperature increased from 20 to 150°C, was observed when phenol-resorcinol or carba­mide-melamine-1 resins were used. Kim et al. [46] synthesized 1-modified phenol-formal­dehyde resol resins by partial substitution of formaldehyde by 1. They tested the performance of these resins using them as adhesives for oriented strandboards. They used 13C-NMR to establish the reaction mechanism between 1 and the other resin components and isolated and identified convincingly structures 15, 16, and 17. The use of 1 with 0.25 mole per mole of phenol in phenol-formaldehyde resol resins gave boards with properties very similar to those obtained by conventional gluing.

Recently, Schneider et al. [47] fabricated particleboards using poly2-urea-formalde — hyde adhesives (P2-U-F). They observed that the curing time needed for P2-U-F was double that necessary for classical urea-formaldehyde resins. They also established that P2-U-F produced boards with lower strength properties, but with higher water resistance, if classical processing conditions were used. However, at higher resin contents, P2-U-F gave boards with better mechanical properties. The following optimal conditions were derived to produce particleboards: a blending time of 10 min, a press platen temperature of 150°C, 15% of P2-U-F resin with respect to OD softwood, 1.4 min of pressing time per millimeter thickness, and a board density of 0.67kg/dm3.

Dao and Zavarin [48,49] prepared boards using wood powder and 2 or poly2 as binders. The wood species was white fir (Abies concolor) which was used as powder screened to 80 mesh. Compound 2, poly2, and wood were subjected to chemical activation with hydrogen peroxide/ferrous ions or nitric acid. It was established that an increase in the degree of polymerization of poly2 yielded boards with increased strength properties and that poly2 gave materials with higher strength and water resistance properties than those obtained using 2. They also showed that the addition of the activator to poly2, rather than to wood, was more efficient. Finally, they also isolated the acetone-soluble fraction of poly2 (about 73%) and used it as a binder for the same wood samples. They found that the tensile properties of the corresponding boards exceeded, by over 50%, those of composites prepared with conventional phenol-resorcinol-formaldehyde resins.

Abd El Mohsen et al. [50] modified classical urea-formaldehyde resins by adding different amounts of 2 and used them as binders for beech-based plywoods. These mod­ified resins gave materials with higher shear strength properties (100% increase) in comparison to unmodified adhesives. They also established the following optimal formulations: addition of 30, 45, and 60% of 2 to classical urea-formaldehyde resins and 3, 4.5, and 6% of p-toluene sulfonic acid as a hardener, respectively. Coppock [51] prepared durable wood adhesives from furfural-based diols, diamines, and diisocyanates. She then made plywoods or particleboards using modified urea-formaldehyde resins, with 3 and 4 as binders and found that the materials thus obtained showed acceptable mechan­ical properties. These properties were not improved by the addition of further modifiers, such as 5,5′-ethylidene furfuryl amine (18). Measurements using DSC showed that 3 did not react under alkaline conditions, but readily resinified at pH values below 3.0. These materials were found to have lower formaldehyde emission compared with those made with unmodified resins. The mechanical performances of flakeboards made with 14 exceeded the industrial standard requirements and were equivalent to those prepared using MDI. Finally, materials based on 14 in the presence of 3 or 18 as modifiers were obtained and found to have better performances in comparison to those prepared without these additives.

Suzuki et al. [52] prepared wood-meal/plastic composites with an average thickness of 4 mm using urea-2 and phenol-1 resins as binders. The molar ratio between urea and 2 was varied from 9:1 to 1:9. The amount of formaldehyde emission decreased with increasing quantities of added 2 and the optimal ratios were found to lie between 2:1 and 1:2. Hexamethylenetetramine was added to phenol-formaldehyde resins which were formulated with a molar ratio of 1:3. The bending strengths of composites prepared using urea-2 adhesives were substantially higher than those made using phenol-formaldehyde binder. More recently, Raknes [53] studied the natural aging of 14 different commercial adhesives used in plywood manufacturing. He glued spruce (Picea abies) pieces and subjected them to 30 years of natural aging! He concluded that the shear strength and the water resistance of samples bonded with ‘‘furfurylated’’ urea-formaldehyde resins (Cascorit 1250 and Dynorit L166, manufactured by Casco Wood Adhesives, Sweden) were still satisfactory.

Kim et al. [54] explored the possibility of using 2 as a cobinder in conventional urea — formaldehyde adhesives. They successfully prepared water-insoluble poly2 as oil-in-water emulsions and added them to urea-formaldehyde in different proportions. The ensuing mixtures were used to produce particleboards from a mixture of southern pine and hard­woods (75/25). The resin content of these panels was 8% w/w based on OD wood particles and the catalyst used was ammonium sulfate at a level of 0.3% w/w with respect to the dry resins. The optimal quantity of added 2 was found to be in the range of 20-30% with respect to conventional urea-formaldehyde resins. These formulations gave panels with increased strength and low formaldehyde emission. Russian investigators [55-58] used 5 as a binder for fir (Abies) plywoods and showed that the properties of these materials met the Russian standard requirements if pressing time of about 10 min, pressing temperature of 160°C, and a platen pressure of 1.8 MPa were used. Thus, the shear strength of the plywoods reached almost 1.5 MPa, and their water uptake did not exceed 39%. The use of clay as a filler (up to 40% w/w with respect to the binder) decreased substantially the final properties of the materials [57]. Mezhov et al. [59] also studied the furfural emission from plywoods prepared using 5 as a binder (produced in situ by reaction of 1 with acetone) and showed that it was much lower than that allowed, i. e., 3-5mg/100g of plywood instead of 10mg/100g.