Effects of resin open time and melamine addition on cold pre-pressing performance of a urea–formaldehyde resin

The pre-pressing performance of urea formaldehyde resins has a significant impact on plywood production efficiency. This paper reports on the effects of the time elapsed after the addition of the hardener (“resin open time”) and of a melamine addition on the cold pre-pressing performance of UF resins. The changes in pH and viscosity, the thermal behaviour, and the structural composition as well as the performance during cold pre-pressing of veneers were investigated. Cold pre-pressing performance was measured by testing pre-pressed three-ply plywood stacks. The results showed that the cold pre-pressing performance is enhanced with longer time after preparation of the resin mix before application onto the veneers and also by longer pre-pressing times. All plywood stacks with at least 2 h pre-pressing time reached the so-called G1 grade (where the re-opened area of the cold pre-pressed plywood stack was 0), which meets the industrially accepted requirements for plywood fabrication. Further, the addition of melamine improved the cold pre-pressing behaviour compared to the investigated UF resin without melamine, showing an earlier start of the development of the pre-pressing shear strength and higher G grades at the various cold pressing times. There are two main reasons for the enhanced cold pre-pressing performance of the resin under the investigated conditions: (1) influence of the “resin open time”: a certain further condensation of the resin after preparing the glue mix (addition of hardener) leading to increased molar mass and viscosity ultimately transforming the resin from liquid-state to gel-state; (2) a higher methylol content in the resin after incorporation of melamine into the resin, which enhanced the formation of hydrogen bonds between the resin and the wood surface. These conclusions represent a feasible approach for the improvement of the cold pre-pressing properties and thus the practical applicability of UF resins with low molar ratios.     

On the performance of a melamine–urea–formaldehyde resin for decorative paper coatings

The decorative laminates industry is a highly competitive industrial sector. To be profitable, manufacturers of impregnated papers for surface laminated MDF and particleboards need to significantly reduce their production costs. Melamine formaldehyde resin (MF) formulations are commonly used for impregnation and coating of such papers, melamine being an important, but costly raw material used in high quantities. While MF is substituted by cheaper urea formaldehyde resins (UF) in the core impregnation, for paper surface films pure MF is used. Therefore, a further reduction in cost could be achieved if a portion of the melamine in the surface film was replaced by urea. In the present contribution, recent results of technological tests on paper laminates using a novel melamine–urea–formaldehyde resin (MUF) formulation are reported and their performance is compared to traditional surfaces made from MF.     

Improving the physical and mechanical properties of particleboards made from urea–glyoxal resin by addition of pMDI

The effect of pMDI on physical and mechanical properties of the particleboards made from urea–glyoxal resin was investigated. The nontoxic and ecofriendly urea–glyoxal (UG) resin was synthesized under weak acid conditions, and its different properties were measured. Then, pMDI at various contents (4, 6 and 8% on resin solids) was added to the UG resin prepared. The thermal and physicochemical properties of the resins prepared as well as their water absorption, flexural properties (flexural modulus and strength) and internal bond (IB) strength of the particleboard panels bonded with them were measured according to standard methods. According to the physicochemical results obtained, the addition of pMDI significantly accelerated the gel time and increased the viscosity and solids content of UG resins. Differential scanning calorimetry indicated that the addition of pMDI decreases the onset and curing temperatures of the UG resin. Physical analysis results of the panels indicated that the particleboards made from UG resins with isocyanate yielded lower water absorption when compared to those bonded with the control UG resins. Based on the findings of this research work, the mechanical properties of particleboard panels bonded with UG resins could be significantly enhanced by the addition of increasing percentages of pMDI. The panels having 8 wt% pMDI exhibited the highest flexural modulus, flexural strength and IB strength value and the lowest water absorption among all the panels prepared.     

Hydroxymethyl furfural-modified urea–formaldehyde resin: synthesis and properties

Considering the importance of urea–formaldehyde (UF) resins in the wood industry, this work reports on a new bio-based modification of UF resins. The use of 5-hydroxymethyl furfural (HMF) is motivated by the current concerns about the effects of formaldehyde on human health. UF and urea–HMF–formaldehyde (UHF) resins were synthesized by an alkaline-acid method and characterized by FTIR, thermogravimetric analysis, and differential scanning calorimetry. The UHF, as a newly modified polymeric resin, was thermally characterized, and it was found that its thermo-stability and char yield was improved. In order to investigate the performance of the UHF, the preparation of particleboards with the UHF as adhesive, as well as its film formation ability have been studied. The UHF films formed on wood panels were uniform without any crack. Film formation ability of the UHF resin was improved due to the presence of more hydroxyl groups as well as furan rings of the HMF moieties resulting in more activated groups to be bonded by wood. Furthermore, formaldehyde release of the particleboards bonded by UHF was significantly lower than that of which bonded by the UF resin. Lab particleboards using the UHF resins showed higher modulus of rupture, modulus of elasticity, and internal bond compared to boards with UF resins, as well as lower water absorption and thickness swelling. Based on these results UHF resin can be considered as a possible candidate as adhesive for wood and wood based panels.     

Preparation and characterization of urea–formaldehyde adhesives modified with glyoxalated tannin

European Journal of Wood and Wood Products - Urea–formaldehyde (UF) resins are the most important type of adhesive for the preparation of wood-based panels, due to its good performance and...     

Comparison of the migration of melamine from melamine–formaldehyde plastics (‘melaware’) into various food simulants and foods themselves

A variety of melaware articles were tested for the migration of melamine into the food simulant 3% w/v acetic acid as a benchmark, and into other food simulants, beverages and foods for comparison. The results indicate that the acidity of the food simulant plays a role in promoting migration, but not by as much as might have been anticipated, since 3% acetic acid gave migration values about double those obtained using water under the same time and temperature test conditions. In contrast, migration into the fatty food simulant olive oil was not detectable and at least 20-fold lower than with the aqueous food simulants. This was expected given the solubility properties of melamine and the characteristics of the melaware plastic. Migration levels into hot acidic beverages (apple juice, tomato juice, red-fruit tea and black coffee) were rather similar to the acetic acid simulant when the same time and temperature test conditions are used, e.g. 2?h at 70°C. However, migration levels into foods that were placed hot into melaware articles and then allowed to cool on standing were much lower (6–14 times lower) than if pre-heated food was placed into the articles and then maintained (artificially) at that high temperature in the same way that a controlled time–temperature test using simulants would be conducted. This very strong influence of time and especially temperature was manifest in the effects seen of microwave heating of food or beverage in the melaware articles. Here, despite the short duration of hot contact, migration levels were similar to simulants used for longer periods, e.g. 70°C for 2?h. This is rationalized in terms of the peak temperature achieved on microwave heating, which may exceed 70°C, counterbalancing the shorter time period held hot. There was also evidence that when using melaware utensils in boiling liquids, as for stovetop use of spatulas, the boiling action of circulating food/simulant can have an additional effect in promoting surface erosion, increasing the plastic decomposition and so elevating the melamine release.     

Ionic liquids as enhancers of urea-glyoxal panel adhesives as substitutes for urea–formaldehyde resins

Acid ionic liquids are used as hardeners of urea-glyoxal adhesives to enhance them to a level sufficient enough for substituting urea–formaldehyde adhesives used in wood panels.     

Improving the rheological and thermal properties of wheat dough by the addition of γ-polyglutamic acid

The rheological and thermal properties of wheat dough with the addition of γ-polyglutamic acid (PGA) (0.5, 1.0, 5.0 g kg⁻¹, w/w) was evaluated by the measurements of farinography, rapid visco analysis and differential scanning calorimetry. Adding 5.0 g kg⁻¹ PGA in wheat dough increased the mixing stability and raised the pasting temperature from 75.8 to 84.4 °C, but decreased the peak viscosity and breakdown. The water holding capacity of wheat dough increased with the addition of 5.0 g kg⁻¹ of PGA. At 5.0 g kg⁻¹ level, PGA caused significant declines in the enthalpy, onset and peak temperatures of ice-melting transition of wheat dough. Scanning electron microscopy showed that wheat bread with the addition of 1.0 and 5.0 g kg⁻¹ PGA exhibited microstructures with smoother surfaces. During storage, PGA retarded the staling process of wheat bread.     

Evaluation of cheese authenticity and proteolysis by HPLC and urea–polyacrylamide gel electrophoresis

Chromatographic and electrophoretic methods have been established as useful tools in characterising cheese ripening and in the detection of milk adulteration. The purpose of this work was to evaluate casein proteolysis of cheeses made from bovine, ovine or mixtures of bovine and ovine milks, as well as ovine cheese authenticity, for 30 days of ripening by HPLC and urea–polyacrylamide gel electrophoresis.Complementary information was obtained by both techniques when applied to the study of casein proteolysis during 30 days of ripening of ovine milk cheeses, ovine milk cheeses with 10% and 20% of bovine milk and bovine milk cheeses, manufactured according to the traditional Terrincho technology. For ovine cheeses, α-casein was the fraction that showed the higher degradation during cheese ripening. A similar behaviour was observed for ovine milk cheese with 10% of bovine milk. The profile for ovine milk cheese with 20% of bovine milk was more similar to that obtained for bovine cheese. Concerning bovine milk cheeses, electrophoresis was the most sensitive technique for the evaluation of proteolysis in these cheeses.Ten and 20% of bovine milk could be detected in ovine milk cheeses by urea–polyacrylamide gel electrophoresis and HPLC, respectively, even after 30 days of ripening.     

Volatile characteristics of 50 peaches and nectarines evaluated by HP–SPME with GC–MS

Using HS–SPME–GC–MS, characteristics of the volatiles of 50 peaches and nectarines representing different germplasm origins were investigated. Ten of these peaches and nectarines were studied in two successive years. Eighty-four compounds were identified. Volatile composition was relatively consistent, but the amount of total volatiles and certain individual compounds varied between years. Moreover, the composition of volatiles and their contents depended on genotypic background and germplasm origin. Total volatiles in wild peaches and a Chinese local cultivar ‘Wutao’ were much higher than in the other groups. All the peaches and nectarines could be classified into four groups by principal component analysis of the volatiles (excluding C₆ compounds): ‘Ruipan 14’ and ‘Babygold 7’ with high contents of lactones, Chinese wild peaches and ‘Wutao’ with high contents of terpenoids and esters, seven cultivars with American or European origins with high content of linalool, and others without characteristic composition of volatiles.     

Melamine–formaldehyde curing acceleration by TiO2-based silver-white pigments catalysis

European Journal of Wood and Wood Products - The preparation of melamine–formaldehyde (MF) resins for paper impregnation for wood panels laminating was found to be catalyzed by TiO2-mica...     

Elucidation of key aroma enhancement in cloudy lemon juices by the addition of peel oil using GC–MS-Olfactometry

Volatile substances play a crucial role in the consumer preference of lemon juices. The effects of heat treatment and addition of lemon peel oil to lemon juice (Citrus limon Burm. cv. Kutdiken) on the aroma compounds were elucidated for the first time. Total phenolic, antioxidant activity, acidity, pH and colour properties were also studied. Results revealed that the aroma of the juice samples comprised mostly the terpene compounds. Limonene, γ-terpinene, β-pinene and α-pinene were the influential terpenes responsible for the distinctive odour of the lemon juice. The addition of the lemon peel oil increased the amounts of the aroma and aroma-active compounds. Besides, the heat treatment had a positive effect on the total phenolics and the antioxidant activity but it caused significant reduction of the aroma compounds. Sensory analysis revealed that the most preferred sample was the juice with lemon peel oil added after pasteurisation.     

Study of the effect of adding the powder of waste PET bottles and borax pentahydrate to the urea formaldehyde adhesive applied on?plywood

In the present study, the effect of adding the powder of waste PET (polyethylene terephthalate) bottles and borax pentahydrate in equal amounts to the urea-formaldehyde (UF) adhesive on the bonding characteristics and free formaldehyde content was studied. Experiments were conducted on three layered plywood of beech (Fagus orientalis) and poplar (Populus x eureamericana), which were produced using urea-formaldehyde adhesive to which powdered PET bottle and borax pentahydrate were added in certain amounts. Furthermore, the bending strength of the produced plywood was analyzed according to BS EN 320, the withdrawal shear strength according to BS EN 314-2, and the free formaldehyde content according to EN 717-2. The obtained results were then compared. It can be seen that the addition of powdered PET bottle and borax pentahydrate to the adhesive mixture used on the plywood statistically increased its bending strength and withdrawal shear strength but significantly reduced the free formaldehyde content.     

Cold-set whey protein–flaxseed gum gels induced by mono or divalent salt addition

Mixed cold-set whey protein isolate (WPI)–flaxseed gum (FG) gels, induced by the addition of CaCl₂ or NaCl at fixed ionic strength (150 mM), were evaluated with respect to their mechanical properties, water-holding capacity (WHC) and SEM microscopy. They were prepared by mixing FG and thermally denatured (90 °C/30 min) WPI solutions at room temperature, but the gels were formed at 10 °C using two methods of salt incorporation: diffusion through dialysis membranes and direct addition. The mixed systems formed using dialysis membranes showed phase separation with the development of two (axial) layers, and the CaCl₂-induced gels presented radial phase separation. In general the CaCl₂-induced gels were less discontinuous, stronger, and showing lower WHC and deformability than the NaCl-induced gels. An increase in the FG concentration reduced the gel strength and WHC for both systems, which was associated with a prevailing phase separation between the biopolymers over the gelation process. Using direct salt addition, apparently none of the mixed gels showed macroscopic phase separation, but the NaCl-induced gels showed much higher hardness and elasticity than the CaCl₂-induced gels. Since the gelation process occurred more quickly by direct salt addition, and more effectively for the divalent salts, the more fragile structure of the CaCl₂-induced gels was a consequence of disruption of the cross-link interactions of the aggregates during the agitation used to homogenize the salt added.