M Ł Mamiński 
  
M E Król 
  
M Grabowska 
  
P Głuszyński 
  
Simple urea-glutaraldehyde mix used as a formaldehyde-free adhesive: effect of blending with nano-Al 2 O 3

A formaldehyde-free adhesive based on urea (U) and glutaraldehyde (GA) was developed. High reactivity of the urea-glutaraldehyde mix at ambient temperature allows for its cold setting. The glues of GA/U molar ratio between 0.8 and 1.2 were examined. It was found that for the satisfactory performance of the system, blending with nano-Al 2 O 3 was necessary.

Introduction

Since formaldehyde has been classified as carcinogenic to human (IARC 2004), the requirements urea-formaldehyde resins must meet have become stricter and an urgent need for a reduction of its content and emission from wood based materials and adhesives occurred. E0 (known as F***) class means formaldehyde emission below 3 mg/100 g while that for Super E0 (also known as F****) is below 2 mg/100 g (JIS A 5905, JIS A 5908:2003).

There are several approaches allowing to overcome that problem: (1) development of adhesives based on renewable resources -e.g. soy proteins [START_REF] Kumar | Adhesives and plastics based on soy protein products[END_REF], tannins [START_REF] Ballerini | Non-toxic, zero emission tannin-glyoxal adhesives for wood panels[END_REF] or chitosane [START_REF] Umemura | Development of new natural polymer-based wood adhesives I: dry bond strenth and water resistance of konjac glucomannan, chitosan and their composites[END_REF]; (2) replacement of formaldehydeemitting adhesives -namely urea-formaldehyde -with other types of adhesives (e.g. polyurethanes); (3) addition of formaldehyde scavengers to the glue formulation (e.g. liquefied wood [START_REF] Medved | Liquefied wood as formaldehyde scavenger for wood-based panels[END_REF], acetoacetyl moieties [START_REF] Dziurka | The effect of introducing acrylic emulsions grafted with acetoacetyl groups on the properties of particleboards[END_REF] or urea which is used in industry up to date); (4) adhesive reformulating and/or partial substitution of formaldehyde with other aldehydes. In literature, there are some reports on the application of succinaldehyde or propionic aldehyde in adhesives (Manosuri and Pizzi 2006;[START_REF] Wang | Succinaldehyde induced water resistance improvements of UF wood adhesives[END_REF].

In the proposed solution, formaldehyde was replaced by glutaraldehyde -an industrial aldehyde used as a disinfecting agent or proteins-crosslinking agent [START_REF] Kupec | Effect of cross-linking waste protein with dialdehydes on its biodegradation under anaerobic conditions[END_REF]) -so that urea-glutaraldehyde (UGA) adhesive was obtained. However, it must be stressed that glutaraldehyde is less toxic than formaldehyde. Glutaraldehyde LD 50 value (oral, rat) is 1470 mg/kg (Sigma-Aldrich 2008), while that for formaldehyde is 100 mg/kg (NTIS 2005).

Also, since it is known that nanoparticles may greatly improve adhesive interactions with a substrate [START_REF] Zhai | Effect of nano-Al2O3 on adhesion strength of epoxy adhesive and steel[END_REF]) the prepared urea-glutraldehyde mix was blended with Al 2 O 3 nanopowder. The approach allowed for the development of a cold setting formaldehyde-free urea-based adhesive for wood bonding.

Materials and methods

The raw UGA mix was prepared by simply dissolving a calculated amount of urea in a respective amount of 50% aqueous glutaraldehyde solution (Aldrich) to finally achieve the required molar ratio of the reagents (e.g. 30 g of urea in 100 g of 50% GA solution for 1.0 GA/U molar ratio). Mixing (100 revolutions per minute) was kept at room temperature until complete dissolving of urea, so that the procedure required no typical and laborious resin synthesis.

Formulations with GA/U molar ratio of 0.8, 1.0 and 1.2 were prepared. 1%wt or 3%wt of nano-Al 2 O 3 (Aldrich) dispersions -based on solids -were obtained by addition of nanomaterial to the urea / glutaraldehyde solution followed by 30-min-ultrasonication. 1% aqueous oxalic acid (10 g of solution in 100 g of glue composition) used as hardener was added to the UGA mixture prior to bonding. Final solids contents were as follows: 58%, 56% and 60%, respectively, for 0.8, 1.0 and 1.2 GA/U molar ratio. After the time necessary for the system to achieve application viscosity of 1000 cP (Table 1), the glue was applied onto the specimens. Glue load was 160 g/m 2 . Solid beech specimens (density 620 kg/m 3 , 5.6% moisture content) cut according to EN 205 were bonded at ambient temperature (30 min, 1.5 N/mm 2 ). The manufactured samples -after 7 days in controlled climate (20°C, humidity 65%) -were subjected to dry shear strength tests (no. of samples was 20).

Results and discussion

From the data shown in Table 1 it is clear that reactivity of the adhesive is high -even at ambient temperature gellation occurs in 70 -100 min without any hardener. However, it can be easily shortened to minutes with small addition of an acid. The gellation experiments and physical analysis of the adhesive after curing showed that GA/U = 1.0 molar ratio was most suitable. The 0.8 and 1.2 options provided poor mechanical performance and brittleness of the cured polymer. Thus, wood bonding and blending with nano-Al 2 O 3 was performed for GA/U = 1.0 only. The strengths of the gluelines are shown in Table 2. The presented values obtained for the acid-cured series with Al 2 O 3 exceeded shear strength of the substrate and 100% wood failure was observed, while the controls exhibited adhesive failure and strengths lower by 52 and 34%. By comparing strengths of non-Al 2 O 3 blended series with and without hardener, it can be noticed that values of the hardened series are much higher, which suggests that acid hardener is necessary for the system. On the other hand, low strength values of the Al 2 O 3 blended series without hardener suggest that sole presence of nanoparticles did not provide significant changes in shear strengths. 

Conclusion

Thus, the observations allow for withdrawing the conclusions that by simple dissolving of urea in glutaraldehyde solution a formaldehyde-free cold-setting adhesive system can be made. Its acid-curing significantly improves the adhesive performance, however, a synergetic effect of small addition of Al 2 O 3 nanoparticles and acid curing is necessary for satisfactory shear strengths exceeding that of the substrate. 

Table 1

 1 

Table 2

 2 

Table 1 .

 1 Gelling times and application viscosity development of the studied UGA glues at 20ºC Tabelle 1 Gelierzeiten und Dauer bis zum Erreichen der georderten Viskosität der

	untersuchten UGA-Klebstoffe bei 20°C		
	GA/U molar ratio	Gelling time	1000 cP achieved after [min]
		1% oxalic acid	no hardener	
		[min]	[min]	
	0.8	7.5	70	5
	1.0	10	70	8
	1.2	16	100	13

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