Surface Checks Research Articles

Using the Concept of Thin Dry Layer to Explain the Evolution of Thickness, Temperature, and Moisture Content during Convective Drying of Norway Spruce Boards

: This article presents an experimental device able to follow the wood temperature, moisture content, board thickness, and surface checking during convective drying. Tests have been carried out on Norway spruce (Picea abies) boards, with different sawing patterns and subjected to a more or less severe drying schedules. For some boards, we observed a first drying phase followed by a transient phase lasting several hours, during which the thickness reduces very slightly and the board temperatures depart from the wet bulb temperature. This period, which is not exactly a true constant drying rate period, can be analyzed using the concept of “thin dry shell.” In order to confirm our hypothesis, an analytical model was used and its results are compared with the experiment. This model takes into account external heat, mass transfer, and diffusion of heat and vapor between the exchange surface and the drying front. Very good agreement is obtained between calculated and experimental temperatures, which validates our equipment and brings physical explanations to different observed behavior; in particular, the existence of a transient first drying period with a thin dry layer is confirmed.

マイクロ波加熱によるスギ丸太の乾燥

Sugi logs were dried with the microwave (MW) heating. The temperature and moisture content distributions in the logs were measured during MW heating. And the surface stresses were measured at the same time as released strains on the surfaces. As a result, the temperature in the center layer was higher than that in the outside layer during drying because of rapid temperature decrease for irradiation stop in intermittent heating system of MW. The moisture content in the center layer was relatively low at the end of drying. The surface stress of dried log was compression for the tangential direction related to surface checks. During seasoning after MW drying, the moisture was removed mainly in the surface layer, and the surface stress changed to tensile stress. As a result, the surface checks occurred for all logs after seasoning in about two or three months.Some dried logs had been sawn into the boxed-heart square timbers. Many surface checks appeared on these timbers because of the existence of internal checks of sugi logs, which occurred by the MW drying. These check widths did not change largely for the efficiently dried timbers as the core part of sugi logs after MW drying.

スギ心持ち無背割り柱材における高温セット処理後の乾燥スケジュールが内部応力に及ぼす影響

The high-temperature setting method (HTS method) was developed in order to prevent surface checking and to reduce drying time for boxed-heart timber without back splitting. It's principle is based on the sudden formation of tension set at the surface of timber under high temperature and low relative humidity condition at the initial stage of kiln-drying.Using the HTS method, while reduction of surface checks was particularly obvious, honeycombs increased when moisture content of timber decreased. To examine the optimum condition of the HTS treatment and to clarify the mechanism of occurrence of honeycomb checks during subsequent kiln-drying, a more thorough knowledge of the internal stresses in timber kiln-dried is required.The purpose of this paper is to gain the knowledge of the internal stresses in timber just after the HTS treatment as well as after subsequent kiln-drying with various schedules after the HTS treatment.Testing materials were Japanese cedar (Cryptomeria japonica D. Don) green boxed-heart timber (132 × 132 × 3000mm) without back-splittings. The procedure used for strain analysis was the modified slicing technique.Main results were shown below.1. After the HTS treatment, the surface of timber already reversed to compressive stresses in half the number of testing materials.2. For the timber kiln-dried after the HTS treatment, while compressive strains at the surface significantly developed as drying proceeded, strain distributions tended to show characteristic concave curves which reveal that the shell of timber is stressed in compression and the core in tension.3. Under flat moisture gradients, strain gradients for the timber kiln-dried with medium-temperature schedule after the HTS treatment were steeper than that for the timber kiln-dried with high-temperature schedule after the HTS treatment.4. Strain distributions for timber kiln-dried with the conventional medium-temperature schedule without the HTS treatment were almost flat compared to that for timber kiln-dried after the HTS treatment.

Comparison of Vacuum Drying Characteristics of Radiata Pine Timber Using Different Heating Methods

Timber logs 14 × 14 × 240 cm long were dried in a vacuum kiln to compare vacuum drying characteristics using contact heating, radiofrequency heating, and the combination of both. Relationships between moisture content (MC) and time for vacuum conductive drying (VCD), RF/vacuum drying (RFVD), and hybrid vacuum drying (HVD) were close to linear. The hybrid vacuum drying offered the higher moisture removal rate than VCD and RFVD. Specific energy consumption curves as a function of MC comprised three sections; rapidly increasing, slightly decreasing, and moderately increasing. Average specific energy (kWh/kg of water removal) for VCD, RFVD, and HVD was 0.352, 0.520, and 0.461 kWh/kg, respectively. Transverse MC distribution represented the convex profile for VCD, the fluctuating profile for RFVD, and the flat profile for HVD. Transverse and longitudinal moisture gradient of HVD was much lower than those of VCD and RFVD. HVD has a very beneficial effect on preventing of surface checks. There were no end- and internal checks of dried timber in three vacuum drying methods.

Automated measurement of checks at wood surfaces

Surface checking often disfigures the appearance of wood, but methods of accurately and rapidly measuring checking in wood are not well developed. This paper describes the principal features of a software package that measures the dimensions and numbers of checks in scanned images of weathered wood surfaces. Procedures within the program analyze images searching for brightness minima that satisfy criteria that are characteristic of checks. A black-and-white image is then produced in which checks are shown black on a white background. Further procedures reduce noise in the black-and-white image eliminating artifacts and small checks invisible to the naked eye. The final stage of the analysis identifies and labels checks and quantifies their positions, sizes and shape. Using this software it has been possible to successfully quantify checking in a range of preservative treated wood specimens that have been subjected to natural weathering. The advantages of the system for quantifying checking in wood as well as its limitations are discussed.

Ind

One of the problems faced in mangium (Acacia mangium Willd.) wood utilization is its low drying rate. Some pre-treatment methods: steaming, boiling and microwave heating have been examined to improve its drying time. After treated with steaming, boiling and microwave heating, six various dimension of mangium samples were dried in oven. During drying process, the samples were weighed two hoursly in the first 24 hours to determine the pattern of moisture reduction. Microwave heating and boiling enhance its drying time in all sample sizes, while steaming was only effective for samples with thickness below 5 cm. Boiling leads to generate drying defects such as wrapping, twist, bow, and surface checks, while microwave heating and steaming methods caused minor drying defects.

Exploratory evaluation of oscillation drying for thick hemlock timbers

Abstract The study explores the effect of dry-bulb (Tdb ) and wet-bulb (Twb ) temperature oscillation in a conventional kiln on the drying characteristics of 105-mm-square Pacific coast hemlock. Five kiln-charges were dried using four oscillating drying schedules and a control. The base drying temperatures for the control were Twb =53°C, Tdb =60°C for the first drying phase and increased to Twb =65°C, Tdb =80°C in the second phase. These temperatures were oscillated at two amplitude and frequency combinations, namely, 3°C for 4 h (low) and 6°C for 8 h (high). The results indicated that the total impact of oscillating schedules on the drying rate was more pronounced at the early stages of the drying process. Oscillating Tdb at higher amplitudes increased drying rate by 12%, whereas the low amplitude counterpart reduced kiln residence time by 10–14%. The final moisture content variability decreased in both Twb oscillated schedules. Core and shell moisture content variation also slightly decreased in the low amplitude Twb oscillated schedules. High amplitude oscillated Tdb also showed 5% less casehardening than the control, whilst its low amplitude counterpart increased. The low amplitude Twb oscillated schedule had the least surface checks and splits.

A Vacuum Drying System for Green Hardwood Parts

A vacuum drying system was designed and fabricated and that system was used to dry green rough hardwood dimension. The red oak samples, 76.2 (long) × 7.62 (wide) × 2.54 (thick) cm, were dried from green moisture content (MC) to 7% MC in this system. They were dried at a pressure of 12 mm Hg and temperatures ranging from 30 to 50°C within 25 to 70 h. Drying quality tested included warp, internal checking, and surface checking. Moisture gradients along the length and thickness were measured. The standard prong test was used to assess the drying stresses. Vacuum drying was fast and the drying rate increased as the temperature increased. It was found that the general drying quality was good with no color change. Drying stresses including longitudinal and transverse stresses were small. There were no internal checks.

Low-rainfall eucalypts as a potential plantation resource in south-eastern Australia for sawn appearance products

Summary Four eucalypt species were evaluated for their potential to produce sawn appearance products when grown in the low rainfall (400–600 mm y1) region of south-eastern Australia. The species were selected on the basis of their suitability for the region and the availability of sawlogs in existing plantations. The sampled trees were about 40 y-old with a minimum diameter at breast height over bark of 30 cm. A butt log 3.1 m long was harvested from ten or eleven trees of each of the following species: Eucalyptus astringens(Maiden) Maiden (brown mallet), E. cladocalyx F.Muell. (sugar gum), E. leucoxylon F.Muell. (yellow gum) and E. occidentalis Endl. (flat-topped yate). The logs were back-sawn with a conventional sawing system to produce boards mostly of 100 mm × 40 mm (nominal) cross-section. The recovery of green boards that met high-value appearance grades was poor. The recovery of select grade or better boards ranged from only 0.9% of log volume for E. leucoxylon to 8.1 % for E. astringens. The poor recoveries were mostly attributable to the small diameter of logs and the lack of silvicultural treatment in the stands from which they were cut. The results highlight the difficulties of growing high-value sawlogs in this region, without suitable silviculture. At best, given the expected long rotations, the economics of growing high-value sawlogs are likely to be marginal, and profitability will depend on other land management benefits provided by the trees. Improved genetic stock and appropriate silviculture will be critical if green recoveries (select grade and better) of at least 30–35% are to be achieved. Green recoveries of this magnitude are likely to be required for a viable sawn timber industry to be established in this region. Use of a conservative air-drying schedule resulted in little drying degrade, surface checking being the main drying defect observed. Given the level of surface checking found, in the absence of other defects 85% of boards would still have made select grade or better. Nevertheless, careful drying practices will be required to minimise surface checking in back-sawn boards of each of these species.

スギ心持ち無背割り柱材の高温乾燥における高温セット法の割れ防止効果について

スギ心持ち無背割り柱材の高温乾燥における高温セット法の割れ防止効果について

Density and shrinkage of four low-rainfall plantation-grown eucalypts

Summary Density and shrinkage values are presented for four eucalypt species (27-y-old Eucalyptus cladocalyx F.Muell. (sugar gum), 42-y-old E. occidentalis Endl. (swamp yate), 41-y-old E. astringens (Maiden) Maiden (brown mallet) and 40-y-old E. leucoxylon F. Muell. (yellow gum)) grown in low-rainfall plantations near Horsham in western Victoria. The mean basic densities for the four species were 752–827 kg m−3. The mean unit shrinkages were 0.37–0.41% for tangential shrinkage and 0.22–0.29% for radial shrinkage. These values are more or less comparable with those of wood sampled from mature trees of the same species. Density and shrinkage values were also compared with published data for E. delegatensis R.Baker (alpine ash). The four low-rainfall species have much higher densities than E. delegatensis (511 kg m−3), but their unit shrinkages are only slightly higher than those for E. delegatensis (0.35% tangential and 0.22% radial) and should be manageable with appropriate design allowances. Recoverable collapse, measured as the difference between shrinkage values (from green to 12% MC) before reconditioning (BR) and after reconditioning (AR), was evident in all four plantation-grown species (tangential, 2.4–4.1 %; radial, 0.4–1.3%). Despite this, steam reconditioning to recover collapse is not generally recommended because existing surface checks may reopen or become worse after steam reconditioning. In addition, the volume lost by not steaming is not large and ‘washboarding’ (localised earlywood collapse that results in a corrugated radial surface) does not occur with these species.

Industrial Kiln Drying and its Effect on Microstructure, Impregnation and Properties of Scots Pine Timber Impregnated for Above Ground Use. Part 1. Effects of Initial, Final Dryings and Preservative on Impregnation and Timber Quality

Summary Scots pine (Pinus sylvestris L.) planks were dried in industrial progressive, conventional batch and high temperature kilns. The quality of drying was assessed by measuring the final moisture content and its gradient, deformations, checks and internal stress of planks. The timber was rearranged in three charges and impregnated in an industrial autoclave with copper-based preservatives for above ground use (class AB):Kemwood ACQ 1900, Tanalith E and Wolmanit CX-8. The moisture content was determined after impregnation and the planks were divided into two charges; the first was dried in an industrial conventional batch kiln while the second was air dried. Assessment of the final product quality after kiln and air drying was carried out as after the initial drying. The penetration of preservatives was measured as well. The data were analysed by a statistical mixed model to ascertain the effects of drying and preservatives on the impregnation and timber quality. A strong relationship was found between the initial drying of Scots pine timber and its quality after impregnation and drying. The conventional batch and high temperature kiln drying ensured similar drying quality regarding the final moisture content and gradient, bow, twist and checks, but the high temperature drying provided significantly better (94.4%of the total sapwood area) penetration of the preservatives tested. It is suggested that changes in wood structure could be provoked during the high temperature drying, thus improving the subsequent impregnation and secondary drying. The progressive and conventional batch kiln drying led to similar penetration of the preservatives; the conventional batch kiln drying ensured significantly lower final moisture content, less twist and surface checks of the timber than the progressive kiln drying. Wolmanit CX-8 penetrated significantly better into the pine sapwood (96.3%) than Kemwood ACQ 1900 and Tanalith E (83.5 and 88.5%, respectively). The planks impregnated with Wolmanit CX-8 achieved the best quality regarding the final moisture content, its gradient and cup. The planks impregnated with Kemwood ACQ 1900 and Tanalith E showed similar quality regarding the penetration and some deformations after drying, but Kemwood ACQ 1900 impregnated planks had highest moisture content and gradient.

Interaction of Verapamil Hydrochloride with Carbopol 934P and Its Effect on the Release Rate of the Drug and the Water Uptake of the Polymer Matrix

In the present study, investigation of the possibility of interaction of verapamil hydrochloride with Carbopol 934P using differential scanning calorimetric analysis and Fourier transform infrared analysis was performed. The effect of the drug-to-polymer ratio, the electrolyte concentration, and the pH of the medium on the extent of interaction of the drug with the polymer using 23 factorial design was investigated. The study also investigated the effect of this interaction on the rate of water uptake of the matrix or the rate of release of verapamil hydrochloride from the swelling polymer matrix. Results revealed that the drug-to-polymer ratio had the most influential effect on both the extent of interaction between the drug and the polymer and the rate of water uptake of the polymer matrix. On the other hand, the pH of the medium had the most significant effect on the rate of drug release. Interaction of the tertiary amine nitrogen of the drug with the anionic carboxyl group on the polymer, forming an insoluble complex, reduced the rate of drug release. This interaction also led to neutralization of the carboxyl group and suppression of the electrostatic repulsion between the anionic groups, which reduced the uncoiling and chain relaxation of the polymer and consequently decreased the swelling of the matrix. The application of the designed experiment allowed the quantification of the effect of each of the studied variables on the investigated responses through the calculation of their coefficient in the response surface equation and checking of their significance.

くん煙熱処理によるスギ（Ｃｒｙｐｔｏｍｅｒｉａ ｊａｐｏｎｉｃａ Ｄ．Ｄｏｎ）およびカラマツ（Ｌａｒｉｘ ｋａｅｍｐｆｅｒｉ Ｃａｒｒｉｅｒｅ）材の材質変化 処理温度と時間の影響

In the present study, sugi (Cryptomeria japonica D. Don) and karamatsu (Larix kaempferi Carriere) logs were smoke-heated with different temperatures and times using a modified food smoker. After smoke heating, several wood qualities were examined, and then the effects of treatment temperature and time of smoke heating on wood quality were discussed. Moisture contents (MC) were decreased with increase of temperature and time in each treatment. The difference in MC between heartwood and sapwood became small due to great decrease of MC in sapwood with increase in time, resulted in uniform distribution of MC around fiber saturation point. However, prolonged treatment caused tension stress at surface layer due to drying, which led to the increase in frequent occurrence of surface checks. In the treatment at 60°C, relative degree of crystallinity (RDC) increased by smoke heating over 40 hours in karamatsu wood, but not changed in sugi wood. However, RDC of both species increased with increase in time at 80 and 100°C treatments. Equilibrium moisture content (EMC) did not change in the treatment at 60°C. In the treatments at 80 and 100°C, however, EMC decreased with increase in treating time. This trend corresponds to increase of RDC. In both species, almost no change of sapwood color occurred in the treatment at 60°C. However, total color difference increased with increase in treating time at 80 and 100°C; the values showed more than 5, which means appreciable color change. Bending properties showed almost no change in the treatments at 60 and 80°C in both species. In karamatsu wood, however, specific Young's modulus increased with increase of time in the treatment at 100°C. These results suggest that no thermal degradation of wood occurred by smoke heating within 100 hours at a temperature inside the log below 100°C.

VACUUM-PRESS DRYING OF THICK SOFTWOOD LUMBERS

ABSTRACT In vacuum-press drying of softwood species, wood lemperaiure exhibited a low temperature gradient, and plateau temperature of core lasied during all stages of drying. The drying curves were close to “linear”. The drying rates of the short lumbers were higher for red pine and western hemlock, lower for white pine and similar for larch lumber compared to the long ones. Transverse and longitudinal moisture gradients were small for western hemlock and red pine, and great for larch and white pine lumber. Energy consumption curves were split into three sections: increasing moderately, fairly constant and increasing rapidly. Casehardening stress of dried lumber occurred very slightly. Dried lumbers exhibited strong tendency for fine end checking, slight surface checking and no internal checking. Shrinkage appeared to be low.

Simulation of drying stresses in wood

A number of rheological models of drying wood have been presented during the last ten years. At the material point level, most models show high accuracy. The models in themselves, however, have no value unless used for improving wood drying, technique, quality, etc. To achieve this, they have to be implemented in applicable programs/ methods. In this article, the implementation of a material model calibrated on small clear wood specimen is presented. The strain (cup-deformations) and the internal stresses (indicator of surface checks and internal collapse) of the boards cross-section are calculated for given moisture histories. Comparisons with test results concerning cupping show that the model can describe the deformation of a board cross-section. It is also shown that in many cases a simple analytical solution can be used to determine the cupping. Comparisons between two-dimensional and one-dimensional calculations show that the one-dimensional formulation in many cases is suf®cient to describe the maximum internal stresses during drying as function of time. (Less)

木質材料 遠赤外線くん煙熱処理スギ心持ち正角の材質 含水率・干割れ・動的ヤング率について

Boxed heart square timbers of sugi (Cryptomeria japonica D. Don) were treated by smoke-heating system with increased far-infrared radiation for about 50 hours at a temperature inside the square timbers of 70°C-80°C. After smoke-heat treatment (SHT), wood qualities (moisture content, surface check, dynamic Young's modulus) were examined. Average moisture contents of boxed heart square timbers were largely decreased by SHT, but moisture contents near the pith still remained high. No surface checks occurred in 29 specimens (24.8%) of 117 square timbers smoke-heated. In addition, decrease of dynamic Young's modulus by SHT was not recognized. The results obtained indicate that SHT has effect on pre-drying of square timbers, without any damages, followed by kiln drying.

Vessels as surface stress raisers during drying ofEucalyptus diversicolor F. Muell

When surface checks form in karri (Eucalyptus diversicolor F. Muell), they almost invariably do so in the bottoms of vessels exposed on the surfaces of the boards. It is postulated that the vessels act as stress raisers, and thus surface checks form at lower values of average surface stress and strain than they would otherwise. Surface checks were seen to form at acoustic emission levels approximately one-third of those corresponding to surface check formation in other eucalypts. Analytical and finite element stress analysis techniques suggested a stress concentration factor at the bottoms of the vessels of approximately three. A trial drying of karri showed a close correlation between predicted surface instantaneous strain using the stress concentration factor found from the stress analysis in a model of drying timber, and measured acoustic emissions.

Collapse and internal checking in the latewood of Eucalyptus regnans F.Muell

Tangential shrinkage was measured on longitudinal-tangential slices of separate earlywood and latewood from one board of Eucalyptus regnans F.Muell. at various temperatures. Large amounts of collapse shrinkage were measured in the latewood slices, and lesser amounts in the earlywood slices. Collapse shrinkage in the earlywood was found only when the slices were dried at temperatures above a minimum temperature (the “collapse threshold temperature”). End-coated board sections approximately 200 mm long were rapidly dried at dry-bulb temperatures below the collapse threshold temperature for earlywood. Incipient internal checks were found in the latewood of these boards. Board sections dried at higher temperatures showed internal checks starting in both early and latewood. A non-linear drying simulation model was modified to take the heterogeneous nature of wood into account. This model predicted that internal checks would form in this wood even if it were dried sufficiently slowly to avoid surface checking.

Acoustic emission related to strain energy during drying of Eucalyptus regnans boards

During timber drying, surface differential shrinkage within a board causes a high surface tensile stress and associated instantaneous strain. Acoustic Emission (AE) is generated when localised irreversible stress release events act to reduce the strain energy imparted to the material. A nonlinear one-dimensional drying model was used to calculate through-thickness moisture, stress and strain profiles during drying. The surface stress and instantaneous strain were used to calculate the strain energy at the surface. AE was measured during drying and the relationship between strain energy and the cumulative count (total ringdown counts) was investigated. The cumulative count is related to the unrecoverable strain energy rather than the elastic or recoverable strain energy. The cumulative count is not a useful measure of the propensity for surface checking. However the peak AE rate values are closely related to the surface instantaneous strain.