Optimization of corn-stalk skin flake-wood shaving composite technology

Haixiang Lu¨?Xianquan Zhang?Bo Yu

Optimization of corn-stalk skin flake-wood shaving composite technology

Haixiang Lu¨1?Xianquan Zhang1?Bo Yu1

We focused on the optimization of corn-stalk skin flake-wood shaving composite technology.We studied the effects of material-mixture ratio,glue content,hotpressing temperature,and hot-pressing time on the appearance,physical,and mechanicalproperties of the composite by the orthogonalexperimentmethod.Ourfindings yielded highly significantresults in allthree cases:the effects ofthe material-mixture ratio on 2 h of thickness swelling(2hTS) and the modulus of rupture(MOR);the effects of glue content on 2hTS,internal bond strength,and modulus of elasticity(MOE);and the effects ofhot-pressing temperature on MOR and MOE.Productoptimization is achieved when the ratio ofcorn stalk skin flake to wood shaving is3:7,the glue contentis12%,the hot-pressing temperature is150°C,and the hot-pressing time is 4.5 min.

Straw-based panel·Corn-stalk skin flake· Composite·Orthogonal experiment·Technology optimization

Introduction

Forest coverage in China is 20.36%.The domestic manufacture of wood-based panels has been affected to a certain degree by the shortage of the worldwide timberresources and the increasingly requirements of the relevant raw materials.Finding new types of alternative materials willgo along way toward solving raw materialshortagesand high costs(Yu etal.2005).Although more than 20 kinds of agricultural residues including wheat and rice straws and hemp and cotton stalks can be used for the manufacture particleboard(Gu and Gao 2002),the content of silicon dioxide and waxes on their surfaces will hamper bonding when using urea-formaldehyde(UF)resin(Inglesby et al. 2005;Halvarsson etal.2008).However,the bonding problem of straw-based panels was solved in the 1990s when isocyanate resin was introduced greatly facilitating particleboard manufacture(Pease 1997;Donnell1998).

Domestic production of corn stalks,one of the major crop straws in China,was 265 million tonnes,which accounts for 32.3%of the gross crop straw in 2009.Annual corn stalk production in China ranks second in the world, behind thatof the United States.Corn stalks have extensive applications such as returning to field,animalfeed,and fuel production(Li and Chan-Halbrendt 2009).On the other hand,traditional use of corn stalks is changing with rapid developmentof the ruraleconomy.The waste and burning straws accountfor31.31%ofthe totalcollectable resource according to Cui et al.(2013),and apart from a type of waste,it also causes negative impact to the environment (Hao 2011;Li and Peng 2004).

Corn stalks have been proven to be useful materials to manufacture particleboard because of the relatively high cellulose content,however,the integrative performance of straw-based panels is poor compared to wood-based panels.This study focuses on the technology ofcorn stalk skin flake-wood shaving composite,which can be comparable to usual shaving board by mixing corn stalk skin flakes with wood shavings,and optimizing the technology of the composite.

Materials and methods

Materials and reagents

Corn stalk was reaped from the Binxi Economic and Technological Development Zone in China.Since the mechanical strength of corn-stalk pulp is lower compared to the skin,the existence of the pulp will bring a negative impact to the final composite(Yang et al.1996).The skin was initially separated using a specific corn-stalk skin and pulp-separating apparatus followed by drying under the natural conditions for 1 month(Fig.1).Long strips of corn-stalk skin were smashed into tiny flakes using a pulverizer.The moisture contentand the mesh number of the finalcorn-stalk skin flakes,after being twice screened and dried,were 3%and 10–30 mesh,respectively(Fig.2).

The poplar-tree wood chips were smashed into shavings using a double-drum flaking machine.The moisture content of the wood shavings,after screening and drying,was 3%.

On the one hand,it is difficult for aldehyde adhesive to adhere because of the presence of wax coating and silicon compounds on the surface of corn stalk;on the other,the costof isocyanate resin is expensive.Therefore,we chose a compounded adhesive made of modified isocyanate and urea-formaldehyde resins developed by the Material Science and Engineering Institute of Northeast Forest University,China.

Experimentaldesign

Fig.1 Corn-stalk skin separated by a separating apparatus

Fig.2 The final corn-stalk skin flakes

The main factors that affect the physical and mechanical properties of the composites include the ratio of corn-stalk skin flakes to wood shavings,density of the particle board, content of the resin,hot-pressing temperature,and hotpressing time(Lu 1994).This experiment was based on a L9(34)orthogonal test(Cheng and Zhang 2001),and focused on four factors:the material mixture ratio,glue content,hot-pressing temperature,and hot-pressing time. Three levels were selected from each factor(Table 1).We studied the effects on the 2-h thickness swelling(2hTS), internal bond strength(IB),modulus of rupture(MOR), and modulus of elasticity(MOE).The size of the particle board was 32 cm.Each condition was repeated twice,and a total of 18 pieces of particle boards were obtained.In this study,all experiments were conducted under similar conditions for greater accuracy and precision.

The rough edge of the particle boards was cut after being hotpressed,and they were placed indoors for48 h to eliminate internal stress.The physical and mechanical properties,such as the density,moisture content,2hTS,IB, MOR,and MOE,were measured according to the Particle Board National Standards of the People’s Republic of China(Chinese Standards GB/T4897 2003).

Results

The results indicate that many factors contribute to the physical and mechanical properties of the composite (Table 2).The order of high-to-low importance factors for differentcomposites is as follows:(1)for2hTS,the order is glue content,materialmixture ratio,hot-pressing time,and hot-pressing temperature;(2)for IB,the order is glue content,material mixture ratio,hot-pressing temperature, and hot-pressing time;(3)for MOR,the order is hotpressing temperature,materialmixture ratio,glue content, and hot-pressing time;(4)for MOE,the order is hotpressing temperature,glue content,hot-pressing time,and material mixture ratio.

Table 1 Orthogonal factor level of composite

Table 2 Variance analysis for the factors of composite

Discussion

Material mixture ratio

The effectofthe materialmixture ratio on 2hTS and MOR was found to be highly significant;the effect of material mixture ratio on IB was significant;and the effect of material mixture ratio on MOE was insignificant.The mechanical properties,such as the IB,MOR,and MOE of the composite,decreased in different degrees with increasing corn-stalk skin flakes;however,the performance of 2hTS increased(Fig.3).The decreasing trend of MOR with the increasing material mixture ratio is shown in Fig.4.This phenomenon could be explained mainly by the lower content of cellulose in cornstalk skin flakes,together with the carbonization of the corn-stalk skin flakes.

Fig.3 Relationship between material mixture ratio and 2hTS

Glue content

The effectof the glue contenton 2hTS,IB,and MOE was found to be highly significant,and the effect of the glue content on MOR was also found to be significant.It is evidentfrom Fig.5 thatthe IB increases rapidly with glue content increasing from 10 to 12%.With further increase of the glue contentfrom 12 to 14%,the increasing trend of IB becomes slightly less significant.The uneven fracture surface of the composites can be visualized in the scanning electron microscopy image(Fig.6).Itis worth noting thatthe resin should be aerosolized adequately,and the cornstalk skin flakes/wood shavings should be uniformly agitated to avoid a negative effect on the particle board, when the two types of materials are not adhered well to each other.Taking the costof the resin into consideration, an optimum glue content of 12%is recommended.

Fig.4 Relationship between material mixture ratio and MOR

Fig.5 Relationship between glue content and IB

Fig.6 SEM image of the composite

Fig.7 Relationship between hot-pressing temperature and IB

Fig.8 Relationship between hot-pressing temperature and MOR

Fig.9 Relationship between hot-pressing time and MOR

Fig.10 Relationship between hot-pressing time and MOE

Table 3 Properties between optimized technology and national standard of Class A

Hot-pressing temperature

The effectofthe hot-pressing temperature on MOR and MOE wasfound to behighly significant,theeffectofthe hot-pressing temperature on IB wasfound to be significant,and the effectof the hot-pressing temperature on 2hTS was found to be insignificant.Allthepropertieswereimproved to differentextents with increasing hot-pressing temperature from 130 to 150°C. With furtherincreasesin temperature,the increasing trend ofthe IB becomes slightly less significant(Fig.7),and the otherthree properties were also found to decrease to differentextents.

For example,the MOR of the composite peaked at 150°C,but it decreased sharply as the temperature increased to 170°C(Fig.8).The increase in hot-pressing temperature will increase the recoverable and plastic deformations of the shavings.The solidification of the resin will also be accelerated to make the shaving board less compact with improved mechanical properties(Gu et al. 1999).However,the corn-stalk skin flakes will degrade, and the chemicalbonds between the resin and flakes willbe destroyed with increasing hot-pressing temperature.The optimum hot-pressing temperature was 150°C.

Hot-pressing time

The effect of hot-pressing time on the composites was insignificantcompared with the other three factors.In Figs.9 and 10,both the MOR and MOE show optimum properties at 4.5 min.The increase in hot-pressing time is conducive to the solidification of the resin and the evaporation of the waterfrom the composite;however,the single factorcannot determine the quality of the composite.From all three levels,a hot-pressing time of 4.5 min is recommended.

Conclusion

Within the experimental range,the highly significant effects on the properties of the composite are as follows:the material mixture ratio on the 2hTS and MOR;the glue content on the 2hTS,IB,and MOE;and the hot-pressing temperature on the MOR and MOE.The optimum technology development strategy is as follows:the ratio of corn-stalk skin to wood particles,the glue content,the hotpressing temperature,and the hot-pressing time are 3:7, 12%,150°C,and 4.5 min,respectively.The physicaland mechanical properties of the composite meet the requirements of National Standard(Table 3).

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29 October 2013/Accepted:27 February 2014/Published online:9 May 2015

?Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

The online version is available at http://www.springerlink.com

Corresponding editor:Yu Lei

?Xianquan Zhang zhangxianquan2013@aliyun.com

1Northeast Forestry University,Harbin 150040, People’s Republic of China