Effect of Wheat Flour on the Viscosity of Urea-formaldehyde Adhesive Rui Ding n , Changhong Su, Yangong Yang, Chengfeng Li, Juncheng Liu School of Materials Science and Engineering, Shandong University of Technology; Zibo 255049; China article info Article history: Accepted 11 August 2012 Available online 25 August 2012 Keywords: Wheat flour Urea-formaldehyde adhesive Viscosity Additives abstract It is hoped to develop new additives substituting for wheat flour used in UF-glue, which needs to investigate the interaction of flour with UF-glue as reference. This paper tested and contrasted the effects of wheat flour, maize powder, wheat starch and its mixture with gluten on the properties of UF-glue through measuring glue viscosity and slab pre-press strength. The viscosity-increasing in UF-glue and slab pre-press strengthening efficiency ordered from great to small as: wheat gluten, wheat flour, simulated flour, wheat starch, maize powder. SEM images, IR and XRD spectra do not show clear evidence of chemical reaction between flour and UF-glue around room temperature. The starch separated from mixture of flour with UF-glue keeps its crystallitic properties such as polarizing and gelatinizes when heated enough. For same compositions the viscosity of the mixture of suspension with UF-glue is bigger than that of the mixture of dry powder with UF-glue. Wheat flour and other powders containing wheat proteins have stronger influence on UF-glue than on poly(vinyl acetate) latex; the reason for this fact needs further in-depth study. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Plywood used for furniture manufacturing and interior decora- tion must have very low formaldehyde emission to meet the requirements of environmental protection. Most plywood factories in China habitually use urea-formaldehyde adhesive (hereinafter referred to as UF-glue). The main method to reduce formaldehyde emission of plywood is using UF-glue that has a lower mole-ratio of formaldehyde (often referred to as environmental-friendly UF-glue), in its synthesis process the mole ratio of formaldehyde to urea is not greater than 1.4 typically [1–5]. Most of the current plywood factories apply plate-free hot-press technology. If the slabs are not flat, the loading trigger would be impeded to operate normally. So the slabs must be pre-pressed into stable shapes while the UF-glue in the slab does not cure at room temperature, producing sufficient sticking strength between the layers to tolerate external force during the slabs are moved, flipped, and trimmed, and not separate or warp by itself. UF-glue of low- cost, low mole ratio of formaldehyde to urea does not meet such initial- sticking requirements, so the plywood factories commonly add wheat flour to UF-glue in order to improve its stickiness and pre-press strength [6,7]. It is of great interesting to develop new additives that substitute for wheat flour used in UF-glue, in order that plywood production cost is reduced and wheat is saved to be applied in food or feed. Mineral powder such as bentonite is mostly concerned for this purpose [8–11]. Unfortunately, any additive has not succeeded to be not only cheaper but also not less efficient than flour till now. It is needed to thoroughly reveal the interaction mechanism of UF-glue with flour, as inspiring or referencing for the developing of such additives. This paper tested and contrasted the effects of wheat flour, maize powder, wheat starch and its mixture with gluten on the properties of UF-glue, and characterized using FTIR, SEM and XRD. 2. Materials and method 2.1. Materials The veneer and UF-glue are from Shuangxing plywood plant under Zibo Yufeng Industry Co., Ltd., in China. Size of the veneer is 290 mm 290 mm; thickness 1.5–1.6 mm, moisture content 12 wt%. Properties of the UF-glue are: solid content 47 wt%, viscosity 71 mPas, free formaldehyde content 4.98 wt%, pH value 7.5. Flour, from Zibo Good Hope Flour Co., Ltd. (manufactured at September 26, 2011), used as received. Wheat starch and gluten protein, are from Chengwu Shanhe Powder Co., Ltd. In Shandong province; gluten protein was smashed until its particles all pass 100 mesh screen (0.15 mm) before using. As a simulation and comparison to flour, a powder was prepared using 86 g wheat starch and 14 g gluten referred to as simulated flour with similar composition to flour. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijadhadh.2012.08.003 n Corresponding author. Tel.: þ86 533 2782232 3; fax: þ86 533 2781660. E-mail address: [email protected] (R. Ding). International Journal of Adhesion & Adhesives 41 (2013) 1–5
Transcript
International Journal of Adhesion & Adhesives 41 (2013) 1–5
Contents lists available at SciVerse ScienceDirect
effects of wheat flour, maize powder, wheat starch and its mixture with gluten on the properties of
UF-glue through measuring glue viscosity and slab pre-press strength. The viscosity-increasing in
UF-glue and slab pre-press strengthening efficiency ordered from great to small as: wheat gluten,
wheat flour, simulated flour, wheat starch, maize powder. SEM images, IR and XRD spectra do not show
clear evidence of chemical reaction between flour and UF-glue around room temperature. The starch
separated from mixture of flour with UF-glue keeps its crystallitic properties such as polarizing and
gelatinizes when heated enough. For same compositions the viscosity of the mixture of suspension with
UF-glue is bigger than that of the mixture of dry powder with UF-glue. Wheat flour and other powders
containing wheat proteins have stronger influence on UF-glue than on poly(vinyl acetate) latex; the
reason for this fact needs further in-depth study.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Plywood used for furniture manufacturing and interior decora-tion must have very low formaldehyde emission to meet therequirements of environmental protection. Most plywood factoriesin China habitually use urea-formaldehyde adhesive (hereinafterreferred to as UF-glue). The main method to reduce formaldehydeemission of plywood is using UF-glue that has a lower mole-ratio offormaldehyde (often referred to as environmental-friendly UF-glue),in its synthesis process the mole ratio of formaldehyde to urea is notgreater than 1.4 typically [1–5].
Most of the current plywood factories apply plate-free hot-presstechnology. If the slabs are not flat, the loading trigger would beimpeded to operate normally. So the slabs must be pre-pressed intostable shapes while the UF-glue in the slab does not cure at roomtemperature, producing sufficient sticking strength between thelayers to tolerate external force during the slabs are moved, flipped,and trimmed, and not separate or warp by itself. UF-glue of low-cost, low mole ratio of formaldehyde to urea does not meet suchinitial- sticking requirements, so the plywood factories commonlyadd wheat flour to UF-glue in order to improve its stickiness andpre-press strength [6,7]. It is of great interesting to develop newadditives that substitute for wheat flour used in UF-glue, in orderthat plywood production cost is reduced and wheat is saved to beapplied in food or feed. Mineral powder such as bentonite is mostly
ll rights reserved.
fax: þ86 533 2781660.
concerned for this purpose [8–11]. Unfortunately, any additive hasnot succeeded to be not only cheaper but also not less efficient thanflour till now. It is needed to thoroughly reveal the interactionmechanism of UF-glue with flour, as inspiring or referencing for thedeveloping of such additives. This paper tested and contrasted theeffects of wheat flour, maize powder, wheat starch and its mixturewith gluten on the properties of UF-glue, and characterized usingFTIR, SEM and XRD.
2. Materials and method
2.1. Materials
The veneer and UF-glue are from Shuangxing plywood plantunder Zibo Yufeng Industry Co., Ltd., in China. Size of the veneer is290 mm�290 mm; thickness 1.5–1.6 mm, moisture content12 wt%. Properties of the UF-glue are: solid content 47 wt%,viscosity 71 mPa�s, free formaldehyde content 4.98 wt%, pHvalue 7.5.
Flour, from Zibo Good Hope Flour Co., Ltd. (manufactured atSeptember 26, 2011), used as received.
Wheat starch and gluten protein, are from Chengwu ShanhePowder Co., Ltd. In Shandong province; gluten protein wassmashed until its particles all pass 100 mesh screen (0.15 mm)before using.
As a simulation and comparison to flour, a powder wasprepared using 86 g wheat starch and 14 g gluten referred to assimulated flour with similar composition to flour.
R. Ding et al. / International Journal of Adhesion & Adhesives 41 (2013) 1–52
Maize powder, prepared by smashing seeds of Pioneer MaizeNo.335 until the powder particles all pass 100 mesh screen(0.15 mm).
Ammonium chloride (AR), China Pharmaceutical Group che-mical reagent Co., Ltd.. It was used as received.
As a contrast to UF-glue, poly(vinyl acetate) latex is preparedin laboratory through such method as mentioned by Nagasawa[12], which is shorted as PV-ltx hereinafter.
2.2. Experiment method
100.0 g UF-glue is added into a plastic cup followed by addingwheat flour or other powder for comparing; then adding 2.0 gammonium chloride (solid reagent). Stir this mixture with anelectric mixer of 300 w power at 260–290 rpm speed for 10minutes, so the finished glue is formed.
Veneer is coated with finished glue on one side, two coated veneerlayers are composited as a slab and loaded 1 MPa pressure at roomtemperature for 1 h, forming a pre-pressed slab. Several pre-pressedslabs were used to measure pre-press strength by electronic universaltesting machine(WDW1020, Changchun Kexin Test Instrument Co.,Ltd., in China) based on China national standards GB9846-88 (Ply-wood-Determination of Glue Bond Strength,1988); other pre-pressedslabs were hot-pressed at 120 1C for 3 minutes then they were usedto measure hot-press strength by the same equipment and method asthe measuring of pre-press strength.
About 30 g finished glue is put in stainless steel plate andvacuum dried at 37 1C and 0.098MPa below atmospheric pressure,water content of the dried samples being 5.3–6.5 wt%. The reason ofchoosing so low drying temperature as 37 1C is not resulting morereactions between UF-glue and the added powder in drying than inpre-press. Grind a little dried sample with agate mortar and analyzethem using following instruments: Dutch FEI Sirion 200 Hot FieldEmission Scanning Electron Microscope, Thermo Electron’s Nicolet5700 Fourier Transform Infrared Spectrometer, and Bruker D8ADVANCE Polycrystalline X-ray Diffractometer.
A part of the finished glue is used for viscosity measuring withNDJ-79 Rotational Viscometer.
0 5 10 15 20 25 30 35amount of powder added, g
Fig. 2. Effects of powder addition on slab pre-press strength.
3. Results and discussion
3.1. Effects of powder addition on viscosity of finished glue
Viscosity test results of the finished glues with different kindand amount of powder are shown in Fig. 1. It is seen clearly thatthe viscosity-increasing effect by same amount ordered fromgreat to small are: wheat gluten, wheat flour, simulated flour,wheat starch, maize powder. Wheat gluten has the greatestviscosity-increasing effect just like that when it is suspended inwater its protein molecules link each other to form webs whichincrease suspension viscosity[13]. The simulated flour has lessviscosity-increasing effect than wheat flour although both aresimilar for their protein and starch contents, this may be mainlyattributed to the water soluble and/or swell-able pentosanscontained in flour rather than in simulated flour[14]. Maizepowder has the least viscosity-increasing effect mainly becauseof its protein different from wheat protein.
3.2. Effects of powder addition on strength of slabs
Strength test results of the pre-pressed slabs added differentkind and amount of powder are shown in Fig. 2. It is clearly thatthe order for bond-enhancing ability is same as for viscosity-increasing effect seen in Fig. 1. Wheat gluten has prominentlygreater bond-enhancing ability than wheat flour but it is too
expensive for plywood factories to use. Simulated flour has lessbond-enhancing ability than wheat flour likely due to one or bothof following facts: (1) the wheat starch and gluten that composethe simulated flour contain much less water-soluble substancesthan wheat flour [15]; (2) the structure and characteristicsof gluten protein is altered when separated from flour [16].Maize powder is the last in this order and wheat starch is thepenultimate, meeting the facts that maize has no gluten proteinand commercial wheat starch contains a little gluten proteininevitably.
Parts of pre-pressed slabs were hot-pressed at 120 1C for3 minutes then they were measured for hot-press strength. Testresults of hot-press strength are shown in Tab.1.
The data in Table 1 show that the hot-press strength of slabsadhered with glue of different powder at same dosage is nearlyequal to each other; the date difference is basically come fromexperimental error. In other words the hot-press strength of slabsis not obviously affected by the kind variation of fine plantpowders though the pre-press strength of slabs is markedlyaffected by this variation. This may be due to that the hot-pressstrength of slabs is rather contributed by the hot curing of UF-gluethan by the plant powder.
R. Ding et al. / International Journal of Adhesion & Adhesives 41 (2013) 1–5 3
3.3. SEM observation
The SEM images of 6 samples are shown in Fig. 3. (a) and (c) showthat wheat flour and maize powder are mainly built up by ellipsoidstarch granules, with smooth surface and diameters ranging from 1 to
Table 1Hot-press strength of slabs adhered with glue of different powder addition.
Powder
added, g
Hot-press strength, MPa
wheat
flour
maize
powder
wheat
starch
wheat
gluten
simulated
flour
0 1.05 1.05 1.05 1.05 1.05
6 1.17 1.22 1.23 1.29 1.19
12 1.36 1.34 1.35 1.37 1.34
20 1.43 1.45 1.41 1.49 1.46
30 1.34 1.26 1.23 1.31 1.35
Fig. 3. SEM images of six samples. (a) flour, (b)mixture of UF-glue and flour, (c) Maiz
of simulated flour and UF-glue.
20 micrometers, attached some irregular protein particles. Fig. 3(d) shows that UF-glue is built up by irregular aggregates of nano-particles, with uneven surface. Fig. 3 (b) ,(e) and (f) show that themixtures of UF-glue and flour/maize powder/ simulated flour are alsobuilt up by irregular aggregates of nano-particles, among them thereare some ellipsoidal starch granules , meaning that no obviousreaction occurs between starch and UF-glue around room tempera-ture though the starch granule surfaces become rough due to beingcoated with nano-particles.
To observe whether wheat starch gelatinizes in UF-glue atroom temperature, 300 ml distilled water was added into amixture of 20 g wheat flour with 100 g UF-glue (47 wt% solidcontent) and 2.0 g ammonium chloride, after uniformed bystirring it was kept stationary to settle in a graduated cylinderfor 1 hr; the wheat gluten and UF-glue mainly suspended in theupper section and the wheat starch mainly settled to the bottomsection. The upper and bottom sections were separated bydumping. When a little drop of the bottom section was coated
e powder, (d) UF-glue, (e) mixture of Maize powder and UF-glue and (f) mixture
Fig. 6. XRD spectra of (a) flour, (b) UF-glue, (c) mixture of flour and UF-glue,
(d) maize powder
R. Ding et al. / International Journal of Adhesion & Adhesives 41 (2013) 1–54
on a slide and viewed under a polarizing microscope, the imageshows that there is a polarization cross on every starch particle(asshown in Fig. 4), which means the crystallite morphology ofwheat starch has not been damaged; the viscosity of bottomsection fluid was 0.038 Pa�s, but it rised to 1.56 Pa�s after heatingto 75 1C, and the sample changed from a non-adhesive fluid intoan adhesive paste, i.e. it was gelatinized. These phenomenamanifest that wheat starch dose neither gelatinize nor reactchemically with UF-glue at room temperature.
3.4. FTIR analysis
IR spectra of 3 samples are shown in Fig. 5. Since the mixture ofUF-glue and flour contains UF-glue as main part and flour assecondary part, the overall spectra shape of the mixture is closerto UF-glue than to flour. The first spectra change is one of the dual-valley representing stretch-vibration of amide carbonyl (CQO)moves from 1553.7 cm�1 for UF-glue to 1550.6 cm�1 for themixture , it is possibly resulted from instrumental errors. The deepervalley at 1157 cm�1 may be due to the wheat protein containingmore glutamine [17] that increasing the glutamine amount in themixture. No clear evidence of reaction between wheat flour and UF-glue around room temperature exists in the IR spectra.
3.5. XRD analysis
XRD spectra of four samples are shown in Fig. 6, steep peaksindicate that all these samples contain crystallites, distributedwider in flour (a) and in maize powder (b) than in UF-glue (c) and
Fig. 4. polarizing micro-image of wheat starch separated from mixture of wheat
flour with UF-glue.
Fig. 5. FTIR spectra of (a) flour, (b) UF-glue, (c) mixture of flour and UF-glue.
Table 2Viscosity of related samples.
Sample Viscosity, Pa�s
Wheat
gluten
Wheat
flour
Simulated
flour
Wheat
starch
Maize
powder
40 wt% suspension 8.6 0.49 0.34 0.11 0.049
20 g powder with 100 g
47 wt% UF-glue
4.8 2.2 2.1 1.2 1.1
50 g suspension with 70 g
67 wt% UF-glue
6.3 2.9 2.6 1.4 1.3
20 g powder with100 g
39 wt% PV-ltx
4.3 1.7 1.5 1.3 1.2
50 g suspensions with
70 g 56 wt% PV-ltx
5.9 2.5 2.3 1.6 1.4
in the mixture of flour with UF-glue (c). The overall spectra shapeof (c) is much more similar to (b) than to flour (a); diffraction ofthe mixture of flour with UF-glue (c) is weaker a bit than that ofUF-glue (b) , which may be due to that UF resin concentration in(c) is lower than in (b). From the XRD spectra we can neither seeevidence of reaction between UF-glue and flour around roomtemperature.
3.6. Further comparison about tackifying effects
When the solid content of the UF-glue is 47 wt%, its viscosity is71 mPa�s; after adding 20 g grain powder into 100 g UF-glue thesolid content of finished glue roses to (47þ20)/(100þ20)¼56 wt%.
Its composition is nearly same as the mixture of 50 g 40 wt%grain powder suspensions in water with 70 g 67 wt% UF-glue.
In the same principle, the composition of a mixture of 20 ggrain powder with100 g 39 wt% PV-ltx basically equals to themixture of 50 g 40 wt% grain powder suspensions in water with70 g 56 wt% PV-ltx. The viscosity of 39 wt% PV-ltx is 86 mPa�s; of56 wt% PV-ltx is 2.1 Pa�s; of 67 wt% UF-glue is 3.9 Pa�s. Viscosityvalues of 40 wt% suspensions of the 5 grain powders in water and
R. Ding et al. / International Journal of Adhesion & Adhesives 41 (2013) 1–5 5
mixtures of 50 g suspension with 70 g 67 wt% UF-glue and with70 g 56 wt% PV-ltx are shown in Table 2.
The data in Table 2 show that the viscosity-increasing rate ofsuch a grain powder as wheat gluten, wheat flour and simulatedflour on UF-glue is bigger than on PV-ltx; in other wards wheatflour and other powders containing wheat proteins have strongerinfluence on UF-glue than on PV-ltx. The viscosity of the mixtureof 50 g 40 wt% suspension with 70 g 67 wt% UF-glue is biggerthan that of the mixture of 20 g dry powder with 100 g 47 wt%UF-glue; which is similar to that of mixtures containing PV-ltx;this may be due to that hydration extent of grain molecules play akey role to tackify UF glue and PV-ltx; the hydration extent ofgrain molecules is higher when grain powder is added into waterto form suspension than when grain powder is added in dry stateinto UF-glue and into PV-ltx.
4. Conclusions
The tackifying effect on UF-glue and slab pre-press strength-ening efficiency of grain powders ordered from big to small asfollows: wheat gluten, wheat flour, simulated flour, wheat starch,and maize powder.
SEM images, IR and XRD spectra do not show clear evidence ofchemical reaction between wheat flour and UF-glue around roomtemperature. The starch separated from mixture of flour with UF-glue keeps its crystallite properties such as polarizing andgelatinizes when head enough. Test dada show that for samecompositions the viscosity of the mixture of suspension with UF-glue is bigger than that of the mixture of dry powder with UF-glue. Hydration extent of grain molecules plays a key role totackify UF glue and poly(vinyl acetate) latex ; the hydrationextent of grain molecules is higher when grain powder is addedinto water forming suspension than when grain powder is addeddirectly into UF-glue and into poly(vinyl acetate) latex. Wheatflour and other powders containing wheat proteins have strongerinfluence on UF-glue than on poly(vinyl acetate) latex; the reasonfor this fact needs further in-depth researching.
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