One of the biggest myths concerning engineered wood flooring is moisture problem we normally have with wood is eliminated. 


NOT TRIUE

What some in the industry call Fiber Tear is many times the surface of the veneers were over dried or exposed to too much high heat for too long. 


The adhesive cannot absorb into the surface of the wood. 


The adhesive covers, settles into the small microscopic mountains and valleys on the surface of the veneers. 


The slightest change in Moisture Content that causes any swelling or shrinking, will product a shearing effect.

The fibers have the only bonding, and they break off when there is a Moisture Content change.


Wood, by its natural characteristic of hygroscopic absorption of moisture from the air, is always either gaining or losing moisture. Relative humidity  is always in flux and therefore, so is wood. It is either gaining or losing moisture content depending upon the relative humidity level in the environment the wood is installed. 


Many state this is a "Site Condition" but, that is not absolutely true. You will see small fibers on the adhesive. That is a strong indication of over drying or the veneer got too hot in the drying process. 


​Below is the  study, the Abstract and Conclusion of a study by a student for his PhD Degree. The entire study can be found using This Link, nearly 200 pages.


Actual shearing is a bond failure. Fiber tear is a bond failure due to over drying or over heating, too hot, of the veneer. 



Comparative Analysis of Inactivated Wood Surfaces 

Milan Sernek 

Wolfgang G. Glasser and Frederick A. Kamke, Co-Chairs 

ABSTRACT 

A wood surface, which is exposed to a high temperature condition, can experience inactivation. Surface inactivation reflects physical and chemical modifications of the wood surface. Consequently, these changes result in reduced ability of an adhesive to properly wet, flow, penetrate, and cure. Thus, an inactivated wood surface does not bond well with adhesives. 

The changes in surface chemistry, wettability, and adhesion of inactivated wood surfaces, including heartwood of yellow-poplar (Liriodendron tulipifera) and southern pine (Pinus taeda), were studied. Wood samples were dried from the green moisture content condition in a convection oven at five different temperature levels ranging from 50 to 200 °C. The comparative characterization of the surface was done by X-ray photoelectron spectroscopy (XPS), sessile drop wettability, and fracture testing of adhesive bonds. Additionally, several chemical treatments were utilized to improve wettability and adhesion of inactivated wood surfaces. 

The comparative analysis helped elucidate clear relationships between surface chemistry, wettability, and bond performance in regard to surface inactivation. XPS results showed that wood drying caused modification in wood surface chemistry. The oxygen to carbon ratio (O/C) decreased and the C1/C2 ratio increased with drying temperature. The C1 component is related to carbon-carbon or carbon-hydrogen bonds, and the C2 component represents single carbon- oxygen bond. A low O/C ratio and a high C1/C2 ratio reflected a high concentration of non-polar wood components (extractives/VOCs) on the wood surface, which modified the wood surface from hydrophilic to more hydrophobic. A hydrophobic wood surface repelled water and wettability of this surface was low (i.e., a high contact angle). Wettability was directly related to the O/C ratio and inversely related to the C1/C2 ratio.

Conclusion


This study dealt with heat-induced wood surface inactivation of yellow-polar and southern pine. The main objective of the study was identification of temperature and time exposure levels that cause wood surface inactivation for these two wood species. Additionally, chemical and physical characterization of wood surfaces in regard to inactivation was accomplished. Surface chemistry and wettability were evaluated by X-ray photoelectron spectroscopy (XPS) and liquid contact angle by means of the sessile drop technique. Bond performance was determined by fracture testing using two adhesive systems. Later, chemical treatment methods of reactivation were used to improve adhesion of inactivated wood surfaces. Finally, a simple comparative method was developed for the rapid identification of inactivated wood surfaces.

The results showed that experimental observation on surface chemistry of wood constituents corresponded to the theoretical interpretation. Cellulose had the highest value of the oxygen to carbon (O/C) ratio, followed by lignin, yellow-poplar extractives, and southern pine extractives. The C1/C2 ratio increased in the opposite order. The C1 component presents carbon, which is bonded to another carbon or hydrogen atom. The C2 component is carbon in C-O bond. A high O/C ratio or a low C1/C2 ratio presented a wood surface containing mostly polysaccharides, while a low O/C ratio and a high C1/C2 ratio reflected a high concentration of non-polar organic compounds with significant mobility; i.e., extractives, degraded VOCs, and possibly lignin on the wood surface. The removal of the extractives increased the O/C ratio and decreased the C1/C2 ratio of the wood surface. The assignment of the carbon C1s peak to extractives, VOCs, and lignin cannot be distinguished by XPS analysis. However, since lignin is relatively immobile, and solvent treatment reduced the C1 atomic percent, the increased C1/C2 ratio was likely the result of extractive/VOCs migration to the surface and residual products of the VOCs pyrolysis, which remained connected to the surface. 

The water contact angle observed on the wood surface decreased with time; an equilibrium was never reached. Southern pine exhibited a higher contact angle than yellow- poplar regardless of the temperature exposure. The extraction with acetone-water, which followed wood drying, improved wettability for both wood species. The extraction of the samples prior to drying did not improve wettability. This suggests that changes in surface energetics are related not only to extractives content but also to other factors, such as partial VOCs deposition on the wood surface. Wettability of the wood surface increased with the O/C ratio and it decreased with the C1/C2 ratio. 

The strain energy release rate obtained by the fracture test showed that southern pine was more susceptible to surface inactivation than yellow-poplar. Adhesive bond performance of southern pine dropped by a factor of two for samples exposed to high temperature. From a mechanical standpoint, the southern pine surface was inactive for PF adhesive when dried at 156°C or higher, and for PVA adhesives when dried at 187°C. Yellow-poplar surfaces did not show a significant inactivation phenomenon when exposed to drying temperatures up to 187°C. These specimens exhibited higher adhesive bond performance than southern pine specimens regardless of the drying temperature or adhesive used. 

Wood surface chemistry changed in regard to drying temperature. The oxygen to carbon ratio (O/C) decreased, and the C1/C2 ratio increased with temperature. Both yellow-poplar and southern pine surfaces indicated higher extractives contents, lignin content, and perhaps adsorbed VOCs, for samples exposed to higher temperatures, which modified the wood surface from hydrophilic to hydrophobic. 

Since the hydrophobic wood surface repelled water, wettability of this surface was low (i.e., a high contact angle). The highest contact angle was obtained on the surfaces that were exposed to the highest drying temperature. The contact angle increased with drying temperature and decreased with contact angle measurement time. Wood species affected wettability, whereby southern pine exhibited higher contact angles than yellow-poplar at all studied temperature exposures. Inactivation, as indicated by a high contact angle, occurred at a lower surface temperature during drying for southern pine than yellow-poplar. Wettability was crucial for good adhesion. The highest values of the Gmax were obtained at high cosè, (i.e., low contact angle), which presents good wettability. Gmax increased with cosè, regardless of wood species. 

Several chemical treatments improved the wettability of inactivated wood surfaces. Wettability of the treated surfaces does not necessarily correlate with adhesion, especially when evaluated with a liquid, which was not used for bonding. This suggests that the wettability should be evaluated by a contact angle measurement using the adhesive. The critical surface tension of an inactivated wood surface was lower than that of a fresh wood surface reported in the literature. 

Attempts to reverse surface inactivation involved aqueous solutions of xylanase, sodium hydroxide (NaOH), xylanase-NaOH, and hydroxymethylated resorcinol (HMR). Adhesion improvement due to surface chemical treatment was not evident for specimens bonded with PVA. Enzymatic treatment with xylanases did not improve adhesion. The HMR coupling agent was not operative on inactivated surfaces bonded with PF adhesive. NaOH was the most effective in restoring bonding ability of PF adhesive with inactivated wood surfaces. The maximum strain energy release rate (Gmax) of specimens treated with NaOH increased by a factor of three when compared with inactivated specimens. Of the chemical treatments employed by this study, NaOH was the most effective for improving adhesion, while HMR had the greatest influence on improving water wettability. 

The choice of the adhesive drastically impacted the adhesion of inactivated wood assemblies. The inclusion of HMR coupling agent into the PF adhesive mixture was unsuccessful in restoring the adhesion of inactivated wood surfaces. PMDI adhesive provided a three times higher Gmax than PF adhesive. Since this effect was similar to the effect of surface treatment with NaOH, the remedy for wood surface inactivation should be based on the usage of the adhesive with a better performance. 

The surface inactivation method for the detection of an inactivated wood surface is simple and useful. It distinguishes between inactivated and fresh wood surfaces prior to bonding based on wettability and absorption measurements. It might be possible to install in-line testing hardware to diagnose surface inactivation in real time. 


 Final Conclusions 

The comparative analysis of inactivated surfaces revealed clear relationships between wood surface chemistry, wettability, and adhesive bond performance. Extractives migration and VOCs degradation obviously play a significant role in heat-induced surface inactivation of southern pines. 

Solvent extraction after drying improves wettability, whereas, extraction prior to drying is less effective. Wettability is directly related to the O/C ratio and inversely related to the C1/C2 ratio, suggesting that increased concentration of non-polar substances; i.e., extractives and VOCs on a wood surface reduces wettability. Southern pine clearly has a lower wettability than yellow- poplar, which the comparison of XPS and solvent extraction results indicate is due to a greater concentration of extractives and degraded VOCs on the surface. 

Inactivation, as indicated by a high contact angle, occurs at a lower surface temperature during the drying of southern pine (about 150°C) than yellow-poplar (about 170°C). Adhesive bond performance, as determined by fracture mechanics testing, improves when contact angle decreases (èi < 90°). Bond performance of PVA adhesive is less affected by drying temperature than PF adhesive, at least with the adhesive formulations used in this research. In terms of adhesion, southern pine is susceptible to inactivation at temperatures above 156°C. Yellow- poplar does not show a significant surface inactivation for the investigated temperature range. 

Of the chemical treatments employed in this study, NaOH is the most effective for improving adhesion, while HMR has the greatest influence on improving water wettability. PMDI adhesive significantly increased fracture energy of bonded inactivated wood surfaces. However, the maximum improvement in adhesion, caused by surface treatment or by exchange of the adhesive mixture, approaches only 75% of the adhesion that is established when bonding fresh wood surfaces.