Xue-Chun Yang，Bing Wang，Jin-Zhuo Wu，Xiao-Qi Xing，Shi-Jun Yang

(College of Engineering and Technology，Northeast Forestry University，Harbin 150001，China)

1 Introduction

With the development of the society，the logistics industry has made a spurt of progress and the number of pallets used in the logistics industry has increased a lot.According to some statistics data，the number of trays has greatly increased in China，more than 20 million annually［1］.More than 70% of the total amount of trays are wooden pallets，which are widely used in the logistics industry because of the lower production cost and simple processing technology［2］.The size stability and strength of the wooden pallet is not so good because of the buckling deformation caused by moisture variation in daily use environment［3］.Therefore，wooden pallets are easily damaged in the using process and the useful life of pallet will be reduced，which lead to more consumption of wood.The increased demand for wood will intensify the contradiction between supply and demand of timber.

With an increased awareness of the fragility of our environment and the need for durability in wood products，new technologies have been developed to increase the service life of wood materials without using toxic chemicals［4］.The use of thermal treatments to enhance the moisture resistance and aboveground durability of solid wood materials has been studied for many years［5］.With the implementation of the Natural Forest Protection Project in our country，it is important for us to make efficient use of the limited timber resources and increase forest plantation.However，plantation wood has many defects because of its characteristics，so we need to improve its characters to make it suitable for pallet production.

In the early part of the 20th century，it was found that drying wood at high temperature increased dimensional stability and had a reduction in hygroscopicity［6－7］.Later，it was found that heating wood in molten metal between 140° C and 320° C reduced swelling in Sitka spruce by 60% and also increased resistance to microbiological attack［8－9］.The increase in stability and durability also increases brittleness and loss in some strength properties including toughness，modulus of rupture and work to failure.The treatments usually cause a darkening of the wood and the wood has a tendency to crack and split［10］.

Many works had been done to study the impacts of heat treatment on dimensional stability，strength，and durability.Tensile strength of particleboard decreased with an increase in the time and temperature conditions of post－h(huán)eat treatment［11］.Dimensionally stable woodbased composites also have a better inherent ability to withstand severe exposure conditions than do regular boards［12］.Prolonged heating at 175°C and 218°C for about 0.5 to 2 hours improved the performance of the board.The improvement increased with the increasing severity of treatment，but with a slight reduction in strength［13］.Heat treatment of hardboard increases its stiffness，bending strength，modulus of elasticity，and elastic bending strength［14－15］.Static bending properties and moisture absorption of particleboards were significantly improved by hot oil treatments than by dry heat treatments［16］.Nishikawa found that the strength properties of 12－mm－thick fiberboards of 0.70 g/cm3density were related to manufacturing conditions［17］.

Chen and Li［18］found that wood dimensional stability was better after the southern pine and birch had been done the heat treatment.The dimension stability of wood had been greatly enhanced when the heat treatment temperature was 210℃to 230℃.Tjeerdsma［19］found that wood at different stages of heat treatment suction performance had been reduced and its dimension stability had been improved through the infrared spectrum method.The domestic researches mainly focus on the influence of heat treatment of compressed wood fix and creep［20］and how to make the size stabilization of the real wood floor better［21］.Few studies have been done to analyze the influence of heat treatment on the wood dimensional stability，so it is important and necessary for us to investigate.

2 Materials and Methods

2.1 Experimental Materials

The larch pallet deck samples were selected from 1 meter above the breast high position in larch trees.These materials were made into small samples according to the GB1929.43－9l standard and the flawless ones were chosen for the following test［22］.The specification of the samples was 300 mm×70mm×10mm.

In the forest products industry，nondestructive evaluation technology has been developed and is used in structural product grading programs，which has resulted in engineered material with well－defined performance characteristics［23］.The nondestructive evaluation technique using stress－wave propagation characteristics has received considerable attention.Stress wave has been investigated extensively during the past few decades and has shown promise for predicting the mechanical properties of wood in dry conditions［24］.There is a good relationship between stress wave time measured in logs，cants，and the lumber produces from the logs.It is found that log stress wave grades have positive relationships with the lumber grades［25］.Logs with high stress wave grades will produce high grade lumber［26］.These findings indicate that the longitudinal stress wave technique has potential in sorting logs and cants for the production of high MOE products［27］.Stress wave transmission time has been recognized as a good indicator of wood strength and stiffness［28］.Ross and Brashaw investigated the cross correlation relationships between the properties of dry wood and measured NDT parameters in green wood for southern pine，a commonly used softwood species［29］.The results showed that the relationship between dry static bending MOE versus green stress wave velocity or the corresponding green MOE can directly be used to predict the dry static bending MOE.Stress wave devices can determine the information available from stress waves in veneer sheets.The distortion of the stress wave while passing a defect indicates that an estimate of the location and size of the defect can be obtained［30］.

The nature of the measured wood samples can be determined according to the result of the through time tested by the stress wave technique［31］.There is a liner relationship between the propagation time in wood tested by the stress wave and the strength of wood.When the wood strength is low，the propagation time is long［32］.After the stress wave test，the propagation time of the wood samples is divided into three groups according to the time length.Then，select one sample from each group to form a new group.Finally，a total of thirty－one groups are obtained.The wood samples sorting progress by stress wave is shown in Fig.1.

Fig.1 Samples grouping by stress wave technique

The 300 mm×70 mm×10 mm samples were heated and each sample was sawn into 10 small samples(70 mm×30 mm×10 mm).The small samples were used to do the immersion treatment to study the change of size stability.

2.2 Test Method

The larch deck samples were put in different temperatures and time conditions in order to analyze the size stability change of larch pallet decks.The differences between the heated and unheated samples were analyzed in water absorption(W)，the resistance of water absorption efficiency(RWE)，the volume expansion rate(S)and the anti－shrink efficiency(ASE).After the experiments，the best temperature and time condition to keep the decks’size stability can be obtained.The service life of the treated wooden pallets will be prolonged.

Three specimens were processed each time in this test under different temperatures and time conditions.There were 30 groups and 90 specimens in the heat treatment experiments in total.The tested temperature included 150℃，175℃，200℃，225℃，and 250℃.The time conditions of this heat treatment included 30 min，60 min，90 min，120 min，150 min，and 180 min.

When the heating equipment was naturally cooled to the room temperature(about 12 h)，the processed specimens were taken out.The small samples were sawn form the heated and unheated samples with specification of 70 mm×30 mm×10 mm.Each specimen was numbered and weighed.And then they were measured in length，width，and thickness.All the samples were soaked into water.The samples were measured and the water was changed every 12 h until the weight change of the samples were less than 0.02 g.In this test，the electronic balance accuracy is 0.0001 g and size measurement accuracy is 0.02 mm.

In this experiment，the measure accuracy of wood thickness，length and width was 0.02 mm.The measurement of the length and width were taken in the surface center place of the small samples.The thickness measurement was the mean thickness of the small samples in the four corners.The bibulous rate of the samples can be computed according to the samples’weight measured in different treatment conditions.

In this paper，absolute moisture content stands for the water absorption after the samples flooded in water.Bibulous rate is the moisture content when the samples are at the saturation state.

The bibulous rate calculation formula is:

where M is the quality of wood sample after water soakage，g;M0is the quality of the oven dry wood sample，g.

The resistance of water absorption efficiency stands for the ability of the heated wood samples’resistance to absorb the moisture from the surrounding environment.The greater the samples’resistance of water absorption efficiency is，the stronger the wood samples’ability of resistance to water absorption is.

The resistance of water absorption efficiency formula is:

where Wcis the bibulous rate of untreated wood sample;Wtis the bibulous rate of treated wood sample.

The volume expansion rate and the anti－shrink efficiency of the samples can be calculated according to the size difference of the samples.Before and after the soakage experiment，the length，width and thickness of the samples including heated and unheated were calculated.So we can get the size change to calculate the result.

The volume expansion rate calculation formula is:

where V is the volume of soaked wood sample;V0is the volume of unsoaked wood sample.

The anti－shrink efficiency can evaluate the size stability of the modified wood.The anti－shrink efficiency stands for the ability of the heated samples’resistance to the change of the samples’volume caused by absorbing moisture from the surrounding environment.The greater the samples’anti－shrink efficiency is，the stronger the wood samples’ability of resistance to volume change is.So the samples’size stability will be better.

The anti－shrink efficiency calculation formula is:

where S1is the volume expansion rate of heated wood sample;S2is the volume expansion rate of unheated wood sample.

3 Results and Analyses

The samples were weighed after being soaked.The water absorption of the wood in different processing conditions can be obtained according to the experiment measurement data and formula(1).The samples were classified into different groups with different time and temperature conditions.

The bibulous rate of the samples after heat treatment can be calculated according to formula(1)and the weight change of samples.These wood samples were measured every 12 hours until their weight change were less than 0.02 g.

The results are shown in Fig.2.

Fig.2 Bibulous rate of the samples

According to the results of bibulous rate test and formula(2)，the resistance of water absorption efficiency of heated samples can be obtained in different experiment conditions.The results are shown in Fig.3.

Fig.3 shows that the anti bibulous rate of samples under the conditions of 175℃for 90 min，250℃for 90 min and 250℃for 180 min，are obviously better than that of samples under other conditions.The highest anti bibulous rate is 162.34%.Its processing condition of temperature and time is 175℃for 90min.It can be seen that the bibulous rate of samples under the same condition is the lowest.When processing conditions of temperature and time are 250℃for 90 min and 250℃for 180 min，the anti bibulous rate of samples are better.

Fig.3 RWE of the samples

The length，width，and thickness of every sample in every group before and after the soakage experiments were measured by the vernier caliper.Then the volume of each sample was calculated.According to the results of size measurement and formula(3)the volume expansion rate of every sample in different processing conditions can be obtained.

The results are shown in Fig.4.S is the volume expansion rate of heated and unheated samples.

Fig.4 Volume expansion rate of the samples

Fig.4 shows that the tenth group had the highest volume expansion rate which is 19.03%.The processing condition is 175℃for 60 min.The twentyninth group is the lowest which is 4.53%.The processing condition of temperature and time is 250℃for 90 min.In addition，the volume expansion rate of group twenty－seven and thirtieth is lower than other groups.The processing condition of temperature and time is 250℃for 90min and 250℃for 180 min.The volume expansion rates of groups fifteenth and seventeenth are higher than other groups.The processing conditions are 200℃for 90 min and 200℃for 150 min.It can be seen that there are no clear relationship between the volume expansion ratio and the processing conditions of temperature and time.But when the temperature is higher，the volume expansion is lower.

The anti－shrink efficiency of every sample in different processing conditions can be got according to the results of the volume expansion rate of samples and experimental data.Then we can know which treatment condition was good for wooden decks’dimension stability.

The results are shown in Fig.5.

Fig.5 Anti-shrink efficiency of the samples

Fig.5 shows the results of the anti－shrink efficiency of samples heated in different processing conditions.The anti－shrink efficiency of sample under the treatment condition of 250℃for 120 min is the highest which is 72.08%.The ASE under 175℃for 60 min is the lowest anti－shrink efficiency of samples which is－59.01%.It also can be seen that when the processing conditions of temperature and time are 150℃for 30 min;5℃for 90 min;0℃for 30 min;0℃for 180 min;0℃for 90 min and 250℃for 120 min，the anti－shrink efficiency is positive.It shows that the anti－shrink efficiency of samples is increased under these conditions.And there are no clear curve relationship between the anti－shrink efficiency and the processing conditions of temperature and time.

The change patterns of the volume change rate after heat treatment，RWE and ASE are shown in Figs.6(a－f).In Fig.6，(a)represents the change when the time condition is 30 min，(b)－(f)represent the changes for 60 min，90 min，120 min，150 min，and 180 min，respectively.

In Figs.6(a)－(f)，the variances of RWE and ASE are shown with the volume changes under different temperatures and times.In Fig.6(a)，the volume change is largest when the temperature is 200℃.When the temperature condition is 250℃，the RWE and ASE of the samples increased a lot.Fig.6(b)shows that when the time condition is 60 min，the ASE values are below zero.The ability of the heated samples’resistance to the change of the samples’volume decreases.

Fig.6(c)shows that when the time condition is 90 min，the best temperature condition is 250℃.At this condition，the volume change rate is the smallest and both of RWE and ASE increased a lot.The dimension stability of wood samples has been improved significantly by this heat treatment.Fig.6(d)shows that the ASE is above zero when the temperature condition is 250℃.At this temperature condition，the ASE and RWE are increased much more than the value when the time condition is 90 min.At these two conditions above，the value of ASE and RWE both has been increased.These two heat treatment conditions are better for wood dimension stability.

Fig.6(e)shows that the value of the RWE has been increased a lot.But the value of ASE is above zero only when the temperature is 225℃.If the time condition is 150 min，it is better to choose the 225℃temperature condition.Fig.6(f)shows that RWE increases at these heat treatment temperature conditions.It can be seen that the value of ASE increases with treating temperature increase.Both of ASE and RWE have maximal values when the temperature is 250℃.When the temperature is 250℃，the wood dimension stability has the largest increase.

Fig.6 RWE and ASE vary with volume change under different temperatures and times

4 Conclusions

The anti－shrink efficiency and dimension stability of larch pallet decks samples are increased after being heated in some temperature and time conditions mentioned above.

The wood dimension stability is increased obviously at the heat treatment conditions of 225℃for 150 min，250℃for 30min，250℃for 90 min，250℃for 120 min and 250℃for 180 min.Especially，when the condition is 250℃for 180 min，the value of volume change is smaller，and ASE and RWE are bigger.The dimension stability of wood is increased obviously.There is the best heat treatment condition for wood dimension stability.

It has reduced the damage of decks caused by the moisture change absorbed in the surrounding environment in the daily use of the wooden pallets.If wooden pallet decks are heated in these good temperature and time conditions，their dimension stability and use life will be increased.This measure can reduce the capital investment for buying new trays which can improve the enterprise’s benefit.

［1］Fu Yungang，Guo Yanfeng，Zhou Binghai.Development trend of pallet distribution.Packaging Engineering，2006，27(12):229－230.

［2］Lu Kaiyun.Tray and standardization.China Storage＆Transport，2007，2(2):75－78.

［3］Wang Xiaoxu.Influence of Different High－Temperature Treat on the Thermal wood Capability of Planted.Beijing:Beijing Forest University Press，2007.

［4］Rowell R M，Ibach R E，McSweeny J，et al.Understanding decay resistance，dimensional stability and strength changes in heat treated and acetylated wood.Proceedings of the European Conference on Wood Modification.Stockholm，Sweden.2009.489－502.

［5］Winandy J E，Smith W R，Winandy J E.Enhancing composite durability:using thermal treatments.Wood Protection，2006，23(5):195－199.

［6］Alen R，Kotulaninen R.Thermochemical behavior of Norway spruce(Picea abies)at 180℃－225℃.Wood Science Technol.，2002，36(7):163－171.

［7］Ayrilmis N，Winandy J E.Effects of post heat－treatment on surface characteristics and adhesive bonding performance of medium density fiberboard.Materials and Manufacturing Processes，2009，24(10):594－599.

［8］Ayrilmis N，Laufenberg T L，Winandy J E.Dimensional stability and creep behavior of heat－treated exterior medium density fiberboard.European Journal of Wood and Wood Products，2009，67(3):287－295.

［9］Stamm A J，Baechler R H.Decay resistance and dimensional stability of five modified woods.Forest Products Journal，1960，10(4):22－26.

［10］Korkut S，Budak?i M.The effects of high－temperature heat－treatment on physical properties and surface roughness of rowan(Sorbus aucuparia L.)wood.Wood Research，2010，55(1):67－78.

［11］Zhang Xiangquan，Lu Renshu，Wang Weihong，et al.Heat post－treatment to reduce thickness swelling of particleboard from fast－sowing poplars.Forestry Research，1997，8(3):188－190.

［12］Boonstra M J，Van A J.Optimization of a two－stage heat treatment process:durability aspects.Wood Science and Technology，2007，41(3):31－57.

［13］Li Xianjun，Cai Zhiyong，Mou Qunying，et al.Effects of heat treatment on some physical properties of Douglas fir(Pseudotsuga Menziesii)wood.Advanced Materials Research，2011，197(198):90－95.

［14］Yildiz S，Gumuskaya E.The effects of thermal modification on crystalline structure of cellulose in soft and hardwood.Build Environ，2007，42(1):62－67.

［15］Winandy J E，Smith W R.Enhancing composite durability:using thermal treatments.Forest Prod.Society，2006，21(3):195－199.

［16］Rowell R M，Ibach R E，McSweeny J，et al.Understanding decay resistance，dimensional stability and strength changes in heat－treated and acetylated wood.Wood Material Science＆Engineering，2009，4(1－2):14－22.

［17］Nishikawa K，Matsumoto A，Niiro O.Manufacturing of medium－density fiberboards by the wet process.Journal of the Hokkaido Forest Products Research Institute，1979(4):1－5.

［18］Chen Renwang，Li Huiming.Dimensional stability of heat treated lumber.China Wood Industry，2010，24(9):48－49.

［19］Tjeerdsma B F，Militz H.Chemical changes in hydrothermal treated wood:FTIR analyses of combined hydrothermal and dry heat－treated wood.Holz alRohunWerkstoff，2005，63(2):102－111.

［20］Li Jian，Liu Yixing.Study on the fixation of compressive deformation of wood in the transverse direction by high temperature steam treatment.Journal of Northeast Forestry University，2007，28(4):4－8.

［21］Gu Lianbai，Li Tao.Technology and application on heattreated solid wood flooring.China Wood Industry，2007，21(3):4－7.

［22］Wang Songyung，Hung Chungpin.Electromagnetic shielding efficiency of the electric field of charcoal from six wood species.Journal of Wood Science，2003，49(5):450－454.

［23］Celzard A，Treusch O.Electrical and elastic properties of new monolithic wood based carbon materials.Journal of Materials Science，2005，40(1):63－70.

［24］Blomberg J.Effects of semi－isostatic densification of wood on the variation in strength properties with density.Wood Science and Technology，2005，39(5):339－350.

［25］Han Guangping，Wu Qinglin.Stress－wave velocity of wood－based panels:effect of moisture，product type，and material direction.Forest Products Journal，2006，56(1):28－33.

［26］Ross R J，Geske E A，Larson G L，et al.Transverse Vibration Nondestructive Testing Using a Personal Computer.Madison:Forest Products Laboratory，WI，1991.

［27］Mcdonald K A，Green D W，Schad K C.Relationship between log and lumber modulus of elasticity.Eur.J.Wood Prod.，1997，47(2):89－92.

［28］Wang Xiping，Ross R J，Green D W，et al.Wave sorting of red maple logs for structural quality.Wood Science Technology，2004，37(6):531－537.

［29］Rippy R C，Wagner F G，Gorman T M，et al.Stress－wave analysis of Douglas－fir logs for veneer properties.Eur.J.Wood Prod.，2000，50(4):49－52.

［30］Ross R J，Brashaw B K，Pellerin R F.Nondestructive evaluation of wood.Eur.J.Wood Prod.，1998，48(1):15－19.

［31］Ross R J，Susan W W，William V S，et al.A stress wave based approach to NDE of logs for assessing potential veneer quality.Eur.J.Wood Prod.，1999，49(8):60－62.

［32］Yang Xuechun，Wang Lihai.Wood Non－Destructive Testing by Using Stress Wave.Beijing:Science Press，2011.2－7.