ESEM-EDXA (Pole Size Stubs After 12 Years Exposure)

ESEM images of CN treated southern pine, non-post-treatment steamed, fixed, and post-treatment steamed show solid deposits on the cell walls. Examples are shown in Figure 2. Thus, post-treatment steaming, fixation bath, and even outdoor exposure resulted in formation of solid deposits in CN treated wood. Closer examination by EDXA confirmed that the solids observed on CN treated poles exposed outdoors, were rich in copper, carbon and oxygen. Additionally, solid deposits were also rich in Ca, Si, Al, and Fe. Unlike the solids found in the laboratory steamed small samples, which were relatively rich in Cu and oxygen with a high Cu to oxygen ratio, the outdoor exposed samples had many more deposits in addition to the ones rich in both Cu and O.

XRD (Pole Size Stubs After 12 Years Exposure)

The XRD patterns contain two peaks at 2Q =36.50°and 42.20° as illustrated on the XRD patterns (Figure 3). These two peaks were earlier assigned to cuprous oxide. Their intensity in terms of count per second was lower than that observed from laboratory treated and conditioned wood. This may be due to the interference with other crystals in wood as revealed by the presence of significant quantities of Fe, Al, Si and Ca by the EDXA.

Qualitative Analysis

The pressure treatment with copper-naphthenate wood preservatives did not change or affect cellulose crystal lattice structure. Figure 3 shows the XRD patterns of 12-year old stub samples. Included in these samples are all the treatment variable extremes from Table 1. No significant modification in XRD patterns was noticeable after post-treatment steaming of untreated wood. XRD patterns for all CN-treated pole stubs contain the characteristic peaks at 2Q values of 36.5º and 42.1º, suggesting the presence of Cu₂O. These crystals were absent in the non-post-treatment steamed laboratory samples (Fig. 1C and 1D). These crystals could arise from four possible sources: Cu₂O in the original treating solution, treatment of hot wood, post-treatment steaming, or field exposure.

Analytical data are shown in Table 3. These data suggest post-steaming has no significant influence on copper reduction. In fact, post-steaming slightly reduced conversion. For example, air-dried, post-steamed samples (T18B) averaged about 10% conversion compared to T19B (no post-steaming) which averaged about 16% conversion.

The fixation process increases conversion compared as illustrated by the results for sample T17A vs. T19B for fixed and non-fixed air-dried material. A similar trend is evident for the steam-conditioned stock. The highest degree of conversion is obtained when steam-conditioned poles are treated at 140º F and given a final fixation bath. In general, the degree of conversion is greatest near the pole surface.

Summary and Conclusions

Conversion of Cu⁺⁺ to Cu⁺ in CN-treated southern pine was demonstrated. Conversion of the cupric copper to cuprous form ranged from 9 to 32 % (w/w) in large scale tests of stubs exposed for after 12 years in ground contact depending on assay zone depth and post-pre-treatment conditioning methods. Neither pre-treatment steam-conditioned or post-treatment steaming increased the occurrence of cuprous copper. Post-treatment fixation at elevated temperature increased the relative occurrence of Cu⁺. This study, coupled with the work by Barnes and Freeman (2000), indicates that regardless of the conversion of some small fractional amount of cupric copper found in copper naphthenate to cuprous oxide after 12 years of exposure, the pole stubs continued to perform satisfactorily with no colonization by decay fungi and subterranean termites.

Literature Cited

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