Effect of the sol fraction and hydrostatic deformation on the viscoelastic behavior of prestrained highly filled elastomers

Hydrolysis of particleboard bonded with urea-formaldehyde resin for recycling

In this study, the removal of urea-formaldehyde (UF) resin adhesives from waste wood particleboards (PBs) via hydrolysis was discussed, particularly the use of this application to combat environmental issues often encountered in recycling projects. Herein, the conditions required for producing PBs with poor binding properties were examined. Additionally, we determined the appropriate formaldehyde: urea (F/U) mole ratios, namely, 0.95, 1.05, and 1.15, required for generating UF resins that can be characterized and used as PB binders. The resulting values were compared with those obtained for a high mole ratio of UF resin (F/U = 2.0) as well as a commercially available PB sample for binding. Aqueous hydrochloric acid (HCl) solutions of various concentrations and water were used to leach the adhesive from the wood residues, and the effectiveness of these leaching agents was determined using a combined scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) technique in addition to the Kjeldahl method. Swelling tests were performed on the UF resins to measure the sol fraction (ω_sol) and evaluate the network behavior of the resulting resins. Our results showed that factors, such as solid content, density, viscosity, and gel time, were necessary for generating an effective adhesive; herein, we determined that a solid content between 37.17 and 56.57%, density between 1.45 and 1.54 g/cm³, viscosity ranging from 115-444 MPa.s, and gel time between 8.50 and 13.13 min were feasible. Whereas the physical properties of the resulting PB (i.e., the density and moisture content) fulfilled the criteria established by the Japanese Industrial Standard, as laid out in the document entitled JIS A 5908: Particleboards (2003), the mechanical properties failed to pass the aforementioned standard as low bending strength and weak internal bonding were noted for the PBs produced. The use of hydrolyzing agents successfully decomposed the UF resin adhesives by altering their nitrogen (N) content; confirmation of this was obtained through SEM-EDS analysis along with the Kjeldahl method. Swelling tests showed that despite containing a reasonable amount of nitrogen owing to its dissolution in either HCl or water, the ω_sol parameter was heavily influenced by the concentration of the hardener and type of F/U mole ratio adhesive used for the PB under investigation. These results indicate that wood residues can be used as raw materials for recycling PBs.

Influence of Thermally-Accelerated Aging on the Dynamic Mechanical Properties of HTPB Coating and Crosslinking Density-Modified Model for the Payne Effect

Hydroxyl terminated polybutadiene (HTPB) coating is widely used in a solid rocket motor, but an aging phenomenon exists during long-term storage, which causes irreversible damage to the performance of this HTPB coating. In order to study the effect of aging on the dynamic mechanical properties of the HTPB coating, the thermally-accelerated aging test was carried out. The variation of maximum elongation and crosslinking density with aging time was obtained, and a good linear relationship between maximum elongation and crosslinking density was found by correlation analysis. The changing regularity of dynamic mechanical properties with aging time was analyzed. It was found that with the increase of aging time, Tg of HTPB coating increased, Tα, tan β and tan α decreased, and the functional relationships between the loss factor parameters and crosslinking density were constructed. The storage modulus and loss modulus of HTPB coating increased with the increase of aging time, and decreased with the increase of pre-strain. The aging enhanced the Payne effect of HTPB coating, while the pre-strain had a weakening effect. In view of the Payne effect of HTPB coating, the crosslinking density was introduced into Kraus model as aging evaluation parameter, and the crosslinking density modified models with and without pre-strain were established. The proposed models can effectively solve the problem that the Kraus model has a poor fitting effect under the condition of small strain (generally less than 1%) and on the loss modulus, which have improved the correlations between the fitting results and the test results.