Correlation between physical bonding and mechanical properties of wood–plastic composites: part 2: effect of thermodynamic factors on interfacial bonding at wood–polymer interface

Abstract

The main objective of this study was to find out if there is any significant correlation between physical properties and interfacial bonding of interphases in wood–plastic composites. To this end, high-density polyethylene (HDPE), mixture of 3% maleic anhydride grafted polyethylene (MAPE) and HDPE (coded as MHDPE) and polylactic acid (PLA) were separately interacted with veneers to identify factors underlying interfaces. Plastics were first melted at 180?°C and dispensed on wood surfaces so that the contact angle (CA) could be directly measured. Wood sanding moderately decreased the CAs of plastics in order of PLA, MHDPE, and HDPE. The treatment of veneers with MAPE comprehensively improved wetting, as the CA of HDPE was significantly reduced on the wood surface after the treatment. Thereafter, the interfacial shear strengths (IFSS) of the wood–polymer interface were determined using the automated bonding evaluation system. PLA had the highest IFSS both for unsanded and sanded veneers. Comparing both parts of this research finally revealed that applying sanding or/and MAPE treatments resulted in lower surface free energy and higher IFSS at the wood–polymer interface. However, our observations support the idea that, at higher temperatures, wetting of composites is mainly influenced by polymer properties rather than interfacial tension at the wood–polymer interface.     

On the correlation of some theoretical and experimental parameters in polycondensation cross-linked networks

The use of the equation f = km/αE, correlating number of degrees of freedom m of polymer segments between cross-linking nodes in polycondensation networks to the energy of interaction polymer segment/polymer segment, both within the same polymer and at different polymers interfaces, through measures of deflection in bending by dynamic thermomechanical analysis, yields a number of consequences of interest in the field of polycondensation-hardened networks and of their process of hardening. From this equation, regression equations correlating only two parameters are obtained, which render easier the determination of the parameters that are more difficult or lengthy to obtain by experimental means. The process of networking, hence of the reaction of polycondensation between the gel point and complete hardening of the network, can be followed by the determination of the average number of degrees of freedom m of the polymer segments between cross-linking nodes obtained through these equations. Even the equation of Carrothers can be adapted through the use of the average number of degrees of freedom of polymer segments between cross-linking nodes to describe the course of the polycondensation after the gel point and up to complete stable networking. The dependence from the temperature of m can be connected to both the rate constant of advancement of the network and to the correlation of the value of m of the system to its glass transition temperature. Peculiarities in gel point forecasting by Flory's and Carrothers' theories, which depend on the well-known existence of reactions of cyclization during polycondensation and by a thermodynamic temperature dependence not previously recognized, indicate that the gel point predicted by each theory fails to consider the existence of one and one only of these effects for each theory. On this theoretical basis, the combination of the two theories into a single, simple equation, which can still be used with ease at the applied level, leads to much better precision of forecasting of the gel point than any of the two theories from which the equation is derived and than any of the more complex theories in this field. © 1996 John Wiley & Sons, Inc. J Appl Polym Sci 63: 603–617, 1997     

On the correlation equations of liquid and solid 13C‐NMR,thermomechanical analysis,Tg, and network strength in polycondensation resins

Wide‐scope mathematical relationships have been established between the ¹³C‐NMR of liquid polycondensation resins, such as urea–formaldehyde and phenol–formaldehyde resins, and the strength of the network formed by the same resin when hardened under well‐defined conditions, the thermomechanical analysis deflection, the number average molecular mass and the number of degrees of freedom of the average polymer segment between crosslinking nodes in the hardened resin network, the resin network glass transition temperature, its solid‐phase ¹³C‐NMR proton‐rotating frame spin‐lattice relaxation time, and the homogeneous and heterogeneous polymer segment/polymer segment interfacial interaction energy calculated by molecular mechanics. These mathematical relationships allow the calculation of any of these parameters from any of the techniques listed, provided that all of the systems are used under well‐defined conditions. Under different conditions, the values of the numerical coefficients involved change; and, whereas the equations are still valid, a different set of coefficients needs to be recalculated. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1703–1709, 1999