Nail Holding Performance of Self-Tapping Screws on Masson Pine and Chinese Fir Dimension Lumbers

Screw connection is a type most commonly applied to timber structures.As important commercial tree species in China,Masson pine and Chinese fir have the potential to prepare wood structures.In this study,the effects of the diameter of the self-tapping screw and the guiding bores on the nail holding performance on different sections of Masson pine and Chinese fir dimension lumbers were mainly explored.The results showed that:(1)The nail holding strength of the tangential section was the maximum,followed by that of the radial section,and that of the cross section was the minimum.(2)The nail holding strength of Masson pine was higher than that of Chinese fir.(3)The nail holding strength grew with the increase in the diameter of self-tapping screws,but a large diameter would lead to plastic cracking of the timber,thus further affecting the nail holding strength.Masson pine and Chinese fir reached the maximum nail holding strength when the diameter of self-tapping screws was 3.5 mm.(4)Under a large diameter of screws,prefabricated guiding bores could mitigate timber cracking and improve its nail holding strength.(5)Prefabricated guiding bores were more necessary for the screw connection of Masson pine.The results obtained could provide a scientific basis for the screw connection design of Masson pine and Chinese fir timber structures.

Study on the Soy Protein-Based Adhesive Cross-Linked by Glyoxal

Based on the ESI-MS and ^(13)C-NMR analysis of the forms of glyoxal in acidic and alkaline solutions,the soy-based adhesive cross-linked by glyoxal was prepared in this work.The results showed that glyoxal existed in water in different forms at different pH levels.Under alkaline conditions,glyoxal transformed to glycolate through the intramolecular disproportionation reaction.Under acidic conditions,although some of glyoxal transformed to glycolate as what happened under alkaline conditions,most of glyoxal molecules existed in the form of fiveor six-membered cyclic ether structure.No ethylene tetraol or free aldehyde group was actually detected under these conditions.Although glyoxal reacted with soy protein under both acidic and alkaline conditions,alkaline conditions were more favorable for the improvement of mechanical performance and water resistance of soybased adhesives than acid conditions.

Plasma Treatment Induced Chemical Changes of Alkali Lignin to Enhance the Performances of Lignin-Phenol-Formaldehyde Resin Adhesive

Alkali lignin was processed by plasma and then used in modification of phenol formaldehyde resin in this study.Chemical structural changes of lignin which was processed by plasma as well as bonding strength,tensile property,curing performance and thermal property of the prepared phenol formaldehyde resin which was modified by the plasma processed lignin were analyzed.Results demonstrated that:(1)Alkali lignin was degraded after the plasma processing.The original groups were destroyed,and the aromatic rings collected abundant free radicals and oxygen-containing functional groups like hydroxyls,carbonyls,carboxyls and acyls were introduced into increase the reaction activity of lignin significantly.(2)The introduction of alkali lignin decreased the free formaldehyde content and increased bonding strength and toughness of the prepared phenol formaldehyde resin,especially after the introduction of lignin treated with plasma.(3)The introduction of alkali lignin led to high curing temperature for the prepared phenol formaldehyde resin,but that was reduced by the plasma processed alkali lignin.(4)The introduction of alkali lignin could also increase thermal stability of phenol formaldehyde resin,but that was modified by plasma processed alkali lignin was better than the unprocessed lignin.Based on the results,the plasma processed lignin was used to modify phenol formaldehyde resin,which could increase the strength and toughness of phenol formaldehyde resin significantly.

Competitive polycondensation of model compound melamine-urea-formaldehyed(MUF)resin system by^(13)C NMR

Melamine-urea-formaldehyde(MUF)resin is an excellent adhesive in the field of wood adhe-sives,however the competition mechanism is questionable which affects the structure control and performance optimization of the resin.In this study,the competitive resin synthesis poly-condensation reaction of MUF system under alkaline condition was studied based on the model compound 1,3-dihydroxymethyl urea(UF_(2))and melamine(M)system,and the competitive reac-tion mechanism in the system was deduced by^(13)C NMR quantitative analysis.The results show that the energy barrier of hydroxymethylation of melamine is lower than that of urea,and the priority of hydroxymethylation is lower;the addition of melamine results in a large amount of hy-drolysis of UF_(2),and the formed free formaldehyde,resulting in hydroxymethylation of melamine;there is obvious polycondensation reaction in UF_(2)+M system,mainly from the relationship be-tween hydroxymethylurea and melamine or hydroxymethylmelamine.The type I bridge bond structure of polycondensation mainly comes from the reaction of UF_(2) and M,which is difficult to form the type II bridge bond.At low molar ratio,the formation of bridge bond is superior to that of ether bond.With the increase of molar ratio,the formation of ether bond shows advantages,but there is obvious competition between them.There may be competitive presence of the UF self-condensation products,melamine-formaldehyde(MF)self-condensation products and MUF co-condensed products after the polycondensation reaction.