Urea-formaldehyde Replacement Wood Adhesives Enter New Territory
SpecialChem |
Andrew Extance
- Apr 18, 2012
Gases are always getting up our nose, whether we know it or not. Many, largely developed, countries today seek to ensure that those gases are not harmful wherever possible. Formaldehyde is one gas that is largely perceived as toxic, and in homes it is widely produced by wood products containing urea-formaldehyde (UF) adhesive resins. Consequently, laws1 and voluntary standards2 have been developed to regulate emissions from those sources.
Composite and laminate wood panel manufacturers seeking to provide products that meet those rules must therefore reduce emissions from their UF resins, or replace them with other adhesives. When we're forced away from our favored solution, it often means compromising for an inferior product. But now, researchers say that they have developed alternatives which bond wood that even UF resins struggles to.
Figure 1: One of the earliest soy adhesive patents
One UF replacement option comes in the form of adhesives derived from soy flour, which first came into general use during the 1920s.3 Soy flour suitable for use in adhesives was, and still is, obtained by removing some or most of the oil from the soybean. This yields a residual soy meal that is subsequently ground into extremely fine soy flour. Typically, manufacturers use hexane to extract the majority of the non-polar oils from the crushed soybeans.
Figure 2: Unit cell of polyamidoamine-epichlorohydrin (PAE) polymers, which are used as wet strengthening agents for paper products and can also add wet strength to soy flour based wood adhesives
Two methods are popular for subsequently removing the residual hexane in the extracted soy flakes. The desolventiser toaster (DT) process treats the soy at a maximum temperature of about 120°C for 45-70 minutes, while the flash desolventiser system (FDS) reaches 70°C for less than 60 seconds. Manufacturers can then denature and hydrolyze the resulting soy flour, breaking down the protein structure to expose additional polar functional groups capable of bonding to yield wood adhesives. Unfortunately, such adhesives have poor water resistance. That means they can only be used indoors, which is why formaldehyde-based resins became popular.
This well-established technology offers an obvious starting point for tackling the formaldehyde problem. Over the last decade researchers have combined amine-epichlorohydrin (AE) adducts with traditional soy formulations to improve their water resistance.4 In fact, the resulting adhesives are effective and robust, giving commercial plywood systems better performance than traditional soy-based adhesives under both dry and wet conditions. However this was initially only true with some wood types, with maple, hickory and fumed yellow birch causing problems - though this last is difficult even for UF adhesives. Furthermore, bending and stiffness strengths of composite boards made using AE/soy adhesives has needed improving, while the AE additive is also expensive.
But now, researchers have largely resolved these issues by adding isocyanates to the formulation to act as an another cross-linking agent.5 The researchers prefer water-dispersible aliphatic isocyanates over aromatic isocyanates as they provide much lower viscosity adhesives, as well as FDS-processed soy flour, which has higher solubility/dispersibility in water. They combine these with polyamidoamine-epichlorohydrin (PAE) resins, which are well-known as being wet-strengthening agents for paper products.6
Adhesive composition
Panel Testing
Soy Flour
PAE
HDI
SMBS
Defoamer
Deionized Water
3-Cycle soak test pass rate
Dry shear strength (MPa)
Wet shear strength (MPa)
73.68
35
0.7
0.32
106.95
100.00%
2.2
0.9
73.68
52.5
0.7
0.32
99.17
25.00%
2.0
1.0
73.68
70
0.7
0.32
91.39
100.00%
2.7
1.5
73.68
17.5
3.5
0.7
0.28
120.95
25.00%
3.2
0.7
73.68
35
3.5
0.7
0.28
113.17
100.00%
3.1
1.5
73.68
17.5
7
0.7
0.28
127.17
50.00%
2.2
0.6
73.68
35
7
0.7
0.28
119.39
100.00%
2.5
1.3
Table 1: Adding HDI to AE/soy adhesives delivers water resistance using much lower levels of expensive PAE additives
For an adhesive containing 100 parts dry weight of the soy mixture and 20 parts PAE, particleboard modulus of rupture increased steadily as hexamethylenediisocyanate (HDI) was added up to a level of 7.5 parts. When the scientists used similar adhesives to make plywood, the boards passed all three-cycle soak, dry and wet shear strength and dry bond tests with PAE content of 10 parts and HDI content at the same level or half as much. However, properties worsened when PAE content was only 5 parts and isocyanate was 10 parts, providing a warning about replacing too much PAE with isocyanate. Plywood made from fumed yellow birch with soy adhesives containing both HDI and PAE passed all of the their three-cycle soak tests, while those made using exclusively PAE/soy adhesives did not pass any.
Figure 3: Increasing hexamethylenediisocyanate (HDI) content of an adhesive containing 100 parts soy flour/glycerol/water mixture, 20 parts PAE resin improves the modulus of rupture of a particleboard it is used to make.
It's somewhat ironic that isocyanates could now reduce wood panels' emissions of toxic molecules, when they can cause health concerns themselves when used in polyurethane adhesives. But by combining them with AE additives, the scientists involved in this work have provided a great example of how early 20th century technology can be brought up to date to solve 21st century problems.
How further might wood adhesives offering low formaldehyde emissions be improved?
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Davidson, G. "Process for Preparing Substances Composed in Part of Protein-containing Cells for the Manufacture of Adhesive", US Patent No. 1,724,695, August 13, 1929
Li, K. "Formaldehyde-free lignocellulosic adhesives and composites made from the adhesives", US Patent No. 7,252,735, August 7, 2007
Allen, A. J.; Wescott, J. M.; Varnell, D. F.; Evans, M. A. "Protein Adhesive Formulations with Amine-Epichlorohydrin and Isocyanate Additives", US Patent Application No. 20110293934, December 1, 2011
Spraul, B. K.; Brady R. L.; Allen, A. J. "Adhesive composition of low molecular weight polyaminopolyamide-epichlorohydrin (PAE) resin and protein", US Patent Application No. 20080050602, February 28, 2008
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