Fifteen Douglas-fir plywood panels were treated with a copper-zinc-arsenic additive preservative (CZAA) in an ammoniacal solution in a full-cell process. Individual plies from the centers of the treated panels were assayed for CuO. ZnO, and As2 O5 on the basis of Cu, Zn, and As content by atomic adsorption spectrophotometry. Retentions of preservative oxides and their distribution in the plies as judged from copper-containing compounds indicated that treated panels could be well protected against biodegradation.Microscopic identification of species revealed that inner plies were made from fir, spruce, and hemlock. The preservative in them, monitored by copper-containing compounds, was evenly spread in the core plies

Rak, J.

English

Wood and Fiber Science (E-Journal)

DISTRIBUTION O F  TOXIC ELEMENTS IN DOUGLAS-FIR PLYWOOD TREATED WITH CZAA PRESERVATIVE J .  Rnk Research Scientist, Eastern Forest Products Laboratory. XOO Montreal Road, Ottawa, CANADA, KIA OW5 (Received 22 July 1977) ABSTRACT Fifteen Douglas-fir plywood panels were treated with a copper-zinc-arsenic additive preservative (CZAA) in an ammoniacal solution in a full-cell process. Individual plies from the centers of the treated panels were assayed for CuO. ZnO, and As,O, on the basis of Cu,  Zn, and As content by atomic adsorption spectrophotometry. Retentions of preservative oxides and their distribution in the plies as judged from copper-containing compounds indicated that treated panels could be well pro- tected against biodegradation. Microscopic identification of species revealed that inner plies were made from fir, spruce, and hemlock. The preservative in them. monitored by copper-containing compounds, was evenly spread in the core plies. Kcy~t .or i ls :  Pscuilot.sr~,yci tnrnzic~sii, Ahirc, sp. ,  Pic,c,u sp. ,  T.tr~gu sp.. L0ri.u sp., preservative treat- ments, copper-zinc-arsenic-additive preservative (CZAA), plywood, atomic adsorption spectropho- tometry, microscopic examination. INTRODUCTION Plywood treated with preservatives is becoming an important commodity in many applications, particularly in houses with wood foundations. In these con- structions Douglas-fir [Ps~udotsuga mrnziesii (Mirb.) Franco] plywood is used because of its satisfactory strength properties. The plywood must be treated with preservatives prior to its use. Inner plies of plywood panels are often made from softwood species such as spruce, fir, or hemlock: these belong mostly to the category of difficult-to-penetrate species. The plywood must retain high levels of preservatives [at least 9.6 kg/& 0.6 lblft") of inorganic preservatives in oxide form], which penetrate with difficulty into refractory species in the inner plies. Treatment of such plywood is therefore a difficult task. In our tests we were interested in the retention and distribution of ammoniacal CZAA preservative (copper-zinc-arsenic-additives) under industrial conditions. From an industrial charge of 1.6-cm (%-inch) Douglas-fir plywood, fifteen panels were randomly chosen and investigated. The aim of this work was twofold: first, to define retention levels of toxic elements and oxides and their penetration in individual plies of each panel, and second, to determine whether the preservative was evenly spread throughout the wood in the plies at the microscopic level. MATERIALS A N D  METHODS Fifteen panels of Douglas-fir plywood, five-ply, 1.6 cm (% inch) thick, treated with CZAA (containing 0.6% wlw CuO, 0.9% wlw ZnO, and 0.7% wlw As,OS) to a liquid preservative retention from 592.7 to 825.0 kglm" with the average 682.5 kg1m:l (37.0 to 5 1.5 Iblft:'. with the average 42.6 Iblft:'), under conditions described earlier (Krzyzewski et al. 1977) were fully air-dried and then analyzed for reten- tion and distribution of toxic elements in individual plies. Microscopic exami- Nood und Fihrr.  11(1), 1979. pp. 2-9 J. Huh-DISTRIBUTION OF TOXIC ELEMENTS 3 nations were also carried out on individual plies to identify the wood species and to detect preservative penetration at the nlicroscopic level. All analyses were carried out on specimens 19 mm in diameter taken approx- imately 10 cm (4 inches) from the geometrical center of each full-sized panel. Individual plies from these specimens were separated by a scalpel and examined microscopically (from 15 to 20 microsections from each ply of the specimen). The larger, remaining part of each ply from the specimen was then milled in a Wiley mill and analysed by AAS for copper, zinc, and arsenic. Each of five plies was marked as follows: two surface plies as I and V (face and back), two plies below the surface plies as 11 and IV (cross-band) and the ply in the center as 111 (core). Copper, zinc. and arsenic were detected by atomic adsorption spectrophotom- etry (by Perkin-Elmer Spectrophotometer Model 303) from solutions prepared from 16 N nitric acid extractions of powdered samples (0.5 gIl0 ml of acid) from individual plies. The method, reported separately (Rak 1977), allowed complete extraction of each element (particularly arsenic, which is difficult to extract) from the treated wood substance of the isolated plies. Development and use of special, modified standards for arsenic determination was a necessary condition for its correct, quantitative determination (Rak 1977). Copper and zinc were determined with ordinary AAS standard solutions. An electrodeless discharge lamp for ar- senic was used at a wavelength of 193.7 nm. with argon-hydrogen flame. A hollow cathode lamp was used for zinc and copper determination, at wavelengths of 2 13.9 nm and 324.7 nrn respectively, with air-acetylene flame. Retentions of toxic elements were expressed in kilograms per cubic meter of treated wood. Oxide retentions (CuO, ZnO, and As,O,) were calculated from these figures. Retentions of individual toxic elements (Cu, Zn, and As) were tabulated along with total oxide retentions. Oxide retentions in individual plies were statistically evaluated and average values (with minimum and maximum) were plotted for face and back plies, both cross-band plies and cores, which were combined from fifteen firll-sized panels and plotted as histograms (Fig. 1 ) .  The species from which every individual ply had been derived was identified micro- scopicall y. Fifteen to twenty cross sections, approximately 20 wm thick, cut from water- swollen pieces of individual plies, were stained with chrome azurol S (0.5% wlw in 5% aqueous solution of sodium acetate) to measure penetration of copper- containing compounds of the preservative into the walls of treated wood cells. To find out whether the microscopic stain can carry the copper compound from its original location inside the cell walls (after treatment) further into the wall during the staining process and therefore make an artefact we carried out the following test: A small block of white spruce was treated under intermittent vacuum and atmospheric pressure with the CZAA preservative. After air-drying, the block was microtomed and cross sections were stained with chrome azurol S. One section was placed in the microscope and a boundary between treated and un- treated cells was found. Continuous observation began immediately as a drop of stain was put on the wood section. A first micrograph was taken within 3 min after staining. The preservative did not spread any further during the next 30 min of continuous microscopic observation. Then a second matched micrograph was 4 WOOD AND FIBER, SPRING 1979, V.  1 I ( 1 )  T CROSS-SECTION OF PLYWOOD PANEL: -2:- 111- 7 ivy- V' PLIES I + V  II  + I V  I l l  (plies) FACE AND BACK CROSS BAND CORE FIG. I .  Oxide distribution in combined plies of CZAA-treated Douglas-fir plywood. (The range is given by the minimum and maximum values.) taken. Numerous groups of partially treated cell walls similar to those photo- graphed were found on the other microsections. Figure 2 shows an example of a group of cell walls after staining, a few of which (cell walls C ,  D, E,  and F) were only partially penetrated by the preser- vative and stained dark blue. About half of the thickness of the walls of these cells was not penetrated at all, so the neighboring cells (A and B) were not penetrated. Figure 2 is a typical micrograph of the boundary between penetrated and nonpenetrated cell walls of spruce latewood. The observation confirmed that the staining technique distinguishes between fully and partially penetrated cell walls. It also confirms that the original location of the copper-containing com- pounds can be determined exactly, by the stain used for microscopic examination, without any further movement of the copper within the cell wall. RESULTS Mic,ro.sc,opic exuminution 1d thc plies The face and back plies (I + V) were Douglas-fir, except that four veneers were larch. The inner (I1 + IV and 111) plies were made from spruce, fir, or hemlock-all difficult-to-penetrate species according to MacLean (1935). Spruce was found in 17 plies (37.8%), fir in 24 plies (53.3%), and hemlock in 4 plies (8.9%). Results are given in Table 1 .  J. RuL-DISTRIBUTION OF TOXIC ELEMENTS FIG. 2. Microscopic cross section of CZAA-treated white spruce from the small block (not from the plywood) with ~~npenet ra ted  (A and B) and partially penetrated (C. D. E. F) cell w;~lls of latewood tracheids. Chrome Azurol-S \taining. Microscopic examination of the inner plies for distribution of copper-containing compounds from the center of the full-sized treated panels indicated that woocl tissues in the plies were thoroughly penetrated. Distrihritiotz of' toxic. r1rtncrit.s in tl1c1 plic1.s Retentions of toxic elements (in kgim", Table 2) in the face and back plies were from 2.52 to 6.30 for copper, from 1.14 to 2.01 for zinc, and from 0.63 to 1.39 for arsenic. In the cross-band plies, retentions varied from 1.47 to 3.15 for copper, from 1.33 to 3.21 for zinc, and from 0.79 to 2.03 for arsenic. Core retentions of copper varied from 1.94 to 3.36, zinc from 1.15 to 5.37, and arsenic from 0.63 to 2.67. These retentions vary over a wide range. When we calculate oxide reten- tions from these figures (Table 3), evaluate them statistically using a t-test, and plot them as histograms for the individual plies (Fig. I), we obtain a definite trend: the copper oxide retention, high in the face and back plies, drops significantly (at the 99% probability level) in the cross-band plies and remains at the lower level in the cores. On the other hand, arsenic oxide, the retention of which is less than half that of copper in the face and back plies, significantly increases (at the 95% probability level) in the cross-band plies and stays at this higher level in the cores. The zinc oxide retention, low in the face and back plies, increases in the cross-band plies, but the increase is not significant at the 95% probability level. 6 WOOD AND FIBER, SPKING 1979, V. I ] ( ] )  ' ~ A B I . E  I .  S p e ( . i ~ s  idrrltifi(.(ition in individucrl plies of  trruted Douglas-jir plywood pntrels Ply no. Panel no. I I1 I11 IV V I D F S F D 2 D F F F D 3 D S S S D 4 D S S H L 5 D H F S D 6 D F F F D 7 D F F S * 8 D S F F L 9 D F F S D 10 D F F S D I I D S S S L 12 D F F F D 13 D F S H L 14 D S S F D 15 D F H F D C01)E: -- I) Uouglas-fir IP\rudor.$ugo ~rrrrtz~uaii ( M ~ r b . )  Fr;incoI k Fir (Ahie.5 Sp. )  S Spruce (Picro  Sp.) H Hemlock ( T s u ~ o  Sp.) 1. Larch (Larix Sp. )  M i \ \ ~ n p  \peclmen. The largest difference between copper oxide on one hand and zinc oxide and arsenic pentoxide on the other is in the face and back plies, and narrows toward the center of the panels (See Fig. 1). Oxides of all three toxic elements were combined for individual plies in each panel. The data (Table 2) show that total oxide retentions (kglm" in the face and back plies varied from 7.03 to 11.78, in the cross-band plies from 5.37 to 10.26, and in the cores from 5.40 to 12.41. These data, particularly averages of total oxide retentions in combined plies, (9.28 for the face and back plies, 7.48 for the cross-band plies, and 8.01 for the cores), closely approach the total oxide reten- tion (9.6 kg/mZ%, 0.6 Ibift:') required by CSA Standard 080-15 for treated plywood in ground contact exposures. DISCUSSION Microscopic examination of individual plies of the CZAA-treated Douglas-fir plywood was carried out on samples from the center of full-sized panels. This location is considered as the most difficult-to-reach by the penetrating preser- vative and therefore to be a significant position for evaluation of preservative penetration of the whole panel. The copper-containing compounds of the preser- vative, detected by microscopic examination, were present in all plies of all panels and were well spread throughout each ply. Although the spread of other elements (zinc and arsenic) was not microscopically tested, we presume that they also were well spread in each ply, even in variable amounts as detected by the AAS analysis. If the retentions of preservative oxides were higher as would be the case in treatments with higher contents of preservative salts, the product would J .  Huh-DISTRIBUTION O F  TOXIC EL.EMENTS 7 TABI E 2. Eletnc,nttrl trnd total oxide rett~ntions in individuul p1ic.s of CZAA-treoted Douglas-jir ply- M , O O ~  P ( I I I C / S .  Panel no Element Ply no. I + v 11 + IV 111 I + v I1 + IV 111 1 + v I1 + IV I11 I + v I1 + IV 111 I + v 11 + IV 111 I + V  I1 + IV 111 I + V  I1 + 1 v  111 I + v I1 + IV 111 I + V  I1 + IV 111 I + V  I1 + IV 111 1 + v  11 + IV 111 1 + v  I1 + IV 111 I + V  I1 + IV I11 I + v 11 + IV I11 1 + v I1 + 1V 111 Total oxtdes 8 WOOD A N D  FIBER, SPRING 1979, V.  I I ( I )  T A B L ~  3. Oxide retc~ntions in c.ornbinrd plies - Plies Statistic CuO ZnO A\,Os (kgim? (kgirn') ( k w m 7 )  I + V  minimum 3.15 1.42 0.97 average, m 5.58 2.10 1.60 maximum 7.89 2.50 2.12 standard deviation, s 1.32 0.39 0.33 coeff. var., % 23.67 18.48 20.42 11 + I V  minimum average, .i- maximum standard deviation, s coeff. var . ,  % 111 minimum average, i maximum standard deviation, s coeff. var., % probably be sufficiently protected for ground contact exposures against wood- decaying fungi, such as Basidiomycetes. However, under biodegrading factors we also include so-called soft rot, caused mainly by cellulose-decomposing microorganisms from the Ascomycetes and Frrngi i tnperfwti .  It was shown years ago (Findlay and Savory 1954) and con- firmed more recently (Hulme and Butcher 1977) that preservatives containing large amounts of copper can be used successfully to protect wood against soft rot fungi that decompose material from the surface. Our analyses suggest that CZAA-treated plywood retained high levels of copper oxide in the face and back plies (on the average 5.6 kgim", 0.35 Iblft") and satisfactory levels of it in the cross-band plies and cores (on the average 3.0 kglm" 0.2 Iblft:]). Combining these findings with the established fact that copper is well fixed in CZAA-treated wood (Rak 1976), we presume that the distribution of copper in the CZAA-treated plywood may also offer a good chance of withstanding exposure to conditions favoring soft rot. CONCLUSIONS Analyses of 15 Douglas-fir plywood panels (1.6 cm, % inch) treated with CZAA preservative in an industrial treating plant show: I. Oxide retentions in individual plies analyzed on specimens taken from the center of the panels were always highest for CuO, lower for ZnO, and lowest for As,O,. 2. From the face and back plies to the cross-band plies As,O, and ZnO retentions increased while CuO retentions decreased. This difference between the face and back plies ( I  + V) and the cross-band plies ( I 1  + IV) was statistically significant for CuO and As,O,. Variations in oxide retentions from the cross- band plies to the cores (111) were not statistically significant for all oxides. .I. Kuh-DISTRIBUTION OF TOXIC ELEMENTS 9 3. The face and back plies retained substantially more CuO than As,O,. How- ever, this difference was smaller in the cross-band plies. 4. The inner plies were made from difficult-to-treat species: spruce, fir, and hem- lock. Microscopic examinations of these plies showed that the copper-con- taining compounds of the preservative were evenly spread even in the cores. 5 .  Retention of toxic elements in individual plies and the spread of preservative at microscopic levels as monitored by copper-containing compounds would suggest good protection of CZAA-treated plywood against biodegradaiion. Treatments with higher salt concentrations in the treating solution would prob- ably result in oxide retentions sufficient for protection of the plywood even in ground contact exposures. 6. The chrome azurol-S staining method appeared to be reliable for detection of copper-containing compounds penetrating cell walls to various degrees. and was suitable for microscopic examination of CZAA-treated wood. R E F E R E N C E S  CSA S I A N D A R D  080 O N  Woo11 PRLSERVA I I O N .  1974. Canadian Standards Association. Rexdale. Ont. F I N D L A Y .  W. P. K., A N D  J .  Ci. SAVORY. 1954. M o d s f i ~ u l e .  Die Zersetrung von Holz durch niedere Pilze. Holz Roh-Werkct. 12(8):293-296. Hur M L .  M. A , ,  A N D  J .  A .  BUICHF.R.  1977. Soft-rot control in hardwoods treated with chromated copper arsenate preservatives I. Treatment problems. Mates Org. 12(2):81-95. K R ~ Y L E W S K I .  J . ,  J .  RAK.  T. SZABO. A N D  M. H U I  w t .  1977. Industrial treatments of Douglas-fir plywood with two ammoniacal preservatives. Ottawa. Environ. Can. Inf. Rep. O P X 0 0 0 .  M A c I , ~ ~ A N .  J .  D. 1935. Manual of preservative treatment of wood by pressure. U.S. Dep. Agric. Misc. Publ. No. 224. RAK.  J .  1976. Leaching of toxic elements from spruce treated with ammoniacal solutivns of copper- rinc-arsenic preservatives. Wood Sci. Technol. 10(1):47:56. 1977. Complex analytical determination of arsenic by atomic adsorption spectrophotometry for wood preservation purposes. (in preparation for press). The Wood Technician: A Comment (Continued from page I )  1. Improved communication with students and teachers in the public system regarding the profession of wood technology. 2. More direct influence upon the direction of wood technician programs. J .  HOWARD SMITH 5189 Gloria Avenue Encino, Culiforniu 9 1436 26 July 1979 

Distribution of Toxic Elements in Douglas-Fir Plywood Treated with Czaa Preservative

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Distribution of Toxic Elements in Douglas-Fir Plywood Treated with Czaa Preservative

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