NH4 SSL-PF thermosetting resin and method of binding lignocellulosic material employing same
BACKGROUND 
     This invention relates to a novel method of bonding lignocellulosic materials and to a new and useful thermosetting binder employed in bonding same. 
     The most commonly used adhesive binders for bonding lignocellulosic materials into, for example, particleboard and waferboard are thermosetting urea-formaldehyde (UF) and phenol-formaldehyde (PF) resins; with the more expensive phenol-formaldehyde thermosetting resin being used for exterior grade products. As the UF and PF resins are petroleum based synthetic resins, from an economic standpoint, the reconstituted wood products industry has attempted to develop suitable interior and exterior adhesive substitutes from wood waste in order to solve environmental disposal problems and to reduce the industry&#39;s dependence upon the uncertain supplies and cost of petroleum based resins. 
     There have been numerous proposals for utilizing spent sulfite liquor (SSL), the by-product of the sulfite pulping process, as a adhesive binder for particleboard and waferboard but none of these proposals has to date, proven entirely satisfactory. 
     In context of employing ammonium lignosulfonate which can also be described as ammonium based spent sulfite liquor (NH 4  SSL), various attempts have been made to cross-link the PF with the NH 4  SSL. For example, Herschler in U.S. Pat. No. 2,786,008 issued Mar. 19, 1953 discloses the use of a binder consisting of an aqueous solution of phenol-formaldehyde with ammonium based sulfite liquor. Herschler recognized that ammonium based spent sulfite liquor which is acidic in character was not miscible with phenol-formaldehyde, since upon mixing, the resin would precipitate out. This was overcome by making the PF &#34;acid tolerant&#34; by first condensing the resin to the late A or early B stage and thereafter, adding an agent, such as oxalic acid, to remove the sodium oxalate so formed by means of filtration. In addition to being laborious and expensive, the adhesive obtained has been found to be slow curing. 
     Wiegand et al in Canadian Pat. No. 735,389 issued May 31, 1963 discloses an alkaline process in which water soluble alkaline kraft or lignosulfonate is mixed with water soluble high molecular weight PF. The water soluble mixture at high solids content may be used as a tackifying agent and adhesive for particleboard and fibreboard. The unique reactivity of NH 4  SSL towards PF was, however, not recognized as the best results were obtained with NaSSL and kraft lignin at high pH. The mixture as disclosed is slow curing at a temperature of 230° C. which was utilized to cure particleboard (normally cured 170° C.) with a product of low strength being obtained even with a large resin content. In addition, the SSL-PF could not be used as an additive where the SSL and PF were found incompatible. The compatibility, water solubility and mixing ratio considerations as taught by Wiegand et al are not considered controlling in accordance with our invention. 
     Shen et al in U.S. Pat. No. 4,265,846 issued May 5, 1981 recognized that chemical modification to the spent sulfite liquor is not desirable as it necessarily contributes to increased resin cost. Crude or low molecular weight ammonium based spent sulfite liquor was thus used alone as the thermosetting binder and hot pressed at 170° C. or higher for a time sufficient to decompose the NH 4  SSL into lignosulfonic acid and ammonia gas, whereafter the acid condensed and polymerized in the presence of liqnocellulosic materials to produce an exterior grade wood composite product. However, this decomposition process is slow, as approximately 14 minutes passing at 210° C. is required to produce an exterior grade 11.1 mm waferboard and approximately 8 minutes required when low molecular weight NH 4  SSL is utilized as the binder. Due to the long press cycles when compared with PF binders that have a press cycle of 3 to 4 minutes, it is unlikely this technique will receive signifigant acceptance by the industry. Moreover, NH 4  SSL as a resin suffers the disadvantage or resin sticking to the metal caul plates used during heat application and pressing. It has also been observed that if the carbohydrates are removed, NH 4  SSL looses its adhesive properties. 
     SUMMARY OF THE INVENTION 
     The NH 4  SSL-PF thermosetting binder of this invention is inexpensive, fast curing, and unlike prior techniques, can be produced in a one step operation and does not require modification or addition of alkali or buffer. Moreover, it has been found to have superior adhesion characteristics over NaSSL-PF and CaSSL-PF resins and also NH 4  SSL when used alone as a binder. 
     We have also found that the binder produced in accordance with our invention is less sensitive to fluctuations in pH when compared to composite wood products which employ NH 4  SSL alone as the binder, and that the current Canadian Standards Association requirements as set for particleboard and waferboard can be met using crude or fractionated NH 4  SSL. 
     In accordance with our invention, lignocellulosic materials are bonded into a formed lignocellulosic product such as particleboard or waferboard by mixing a thermosetting binder comprising an acid dispersion of ammonium based spent sulfite liquor, phenol and formaldehyde with said materials, and subjecting said mixtures to sufficient heat and temperature to cure said binder. 
     Although phenol and formaldehyde can be added to the NH 4  SSL separately, it is preferred that they be added together as phenol-formaldehyde. As indicated previously, phenol-formaldehyde precipitates when in acidic solution with NH 4  SSL and thus it is important to maintain the two in dispersion through rigourous stiring or agitation as otherwise the NH 4  SSL-PF separates with the loss of its adhesive properties. The phenol resin thus can be added direct to the acid NH 4  SSL. 
     The acidic NH 4  SSL-PF dispersion can also be in powder form and obtained by spray-drying the two together, or by dry mixing these previously separately dried components. It will also be apparant that NH 4  SSL and PF can be dispersed as contemplated in the process of our invention by adding them separately to a blender which has been previously charged with the lignocellulosic material. 
     While the ratio of NH 4  SSL measured on a dry weight basis can vary widely when practicing this process, say from 10:90 to 90:10, from a commercial standpoint a ratio of from 80:20 to 50:50 is preferred. Similarly, while acceptable modulus of rupture (MOR) tests on waferboard and particleboard have been obtained over a pH range from 3 to 7 for the NH 4  SSL-PF binder, a commercial operating range from pH 4 to 6 using crude NH 4  SSL and commercially available phenol-formaldahyde is also preferred. 
     Although crude NH 4  SSL can be used in the binder of this invention, we have additionally found that improved adhesive characteristics are obtained when fractionated low molecular weight NH 4  SSL is employed. 
    
    
     In the following examples, unless otherwise indicated, all data obtained was based on aspen-poplar short wafers, a resin content expressed as a percentage of the wood content measured on a dry weight basis, and a press or platen temperature of 210° C. at 3.45 MPa when applied to a 11.1 mm waferboard having a 640 Kg/m density. 
     EXAMPLE 1 
     Comparison tests were run using crude and low fractionated NH 4  SSL binder as the resin type (U.S. Pat. No. 4,265,846) and two dispersed NH 4  SSL-PF binders; one using a crude NH 4  SSL component and the other a low fractionated NH 4  SSL component, with the results being set forth in table 1. Compared to crude and fractionated NH 4  SSL resin, the NH 4  SSL-PF resin had preferred MOR(dry) and Aged MOR(wet) test results. The CSA standard for waferboard is also included in this table. 
     
                       TABLE 1                                                     
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                                       WET                                
            RESIN      PRESS     DRY   AGED                               
            CONTENT    TIME      MOR   MOR                                
RESIN TYPE  (%)        (MIN.)    (MPa) (MPa)                              
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NH.sub.4 SSL (pH 2.5)                                                     
            4          15        19.4  9.3                                
NH.sub.4 SSL                                                              
            2.5        8         21.3  9.9                                
(low fractionated)                                                        
NH.sub.4 SSL (50%)                                                        
            2.5        3         23.2  11.6                               
PF(50) pH 6                                                               
NH.sub.4 SSL (50%)                                                        
            2.5        3         24.5  13.3                               
(low fractionated)                                                        
PF (50%)                                                                  
CSA STANDARD                                                              
            --         --        14.0  7.0                                
Can 3.0188.2-M78                                                          
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     EXAMPLE 2 
     Using a press cycle of 5 minutes, waferboards bonded with a SSL cooking base containing 70% PF component and 30% SSL Component based on dry weight were compared. As seen in table 2, all three SSL cooking bases passed the modulus elasticity (MOE) and internal bonding (IB) tests required by CSA, but both the NaSSL-PF and CaSSL-PF failed the Aged MOR, supporting the conclusion NH 4  SSL of our dispersion process has some unique reactive properties heretofore not recognized. 
     
                       TABLE 2                                                     
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            DRY     WET                                                   
            MOR     AGED MOR    MOE    IB                                 
RESIN TYPE  (MPa)   (MPa)       (GPa)  (kPa)                              
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NH.sub.4 SSL-PF                                                           
            22.8    11.2        3.94   455                                
NaSSL-PF    17.9    0           3.10   350                                
CaSSl-PF    20.8    2.8         4.10   450                                
CSA STANDARD                                                              
            14      7.0         2.70   280                                
Can 3.0188.2-M78                                                          
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     EXAMPLE 3 
     Table 3 sets forth the effect pH has on the adhesive properties of crude NH 4  SSL resin and the NH 4  SSL-PF dispersed resin of this invention where the dispersion ratio on a dry weight basis is 70% NH SSL and 30% PF. It has been further determined that the relationship of pH to NH 4  SSL-PF is substantially linear for MOR and Aged MOR whilst there is a significant drop in Aged MOR for NH 4  SSL alone when the pH is above 4. 
     
                       TABLE 3                                                     
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RESIN TYPE  PRESS TIME  pH     MOR   Aged MOR                             
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NH.sub.4 SSL                                                              
            8           3      21    9                                    
NH.sub.4 SSL                                                              
            8           5      18    3                                    
NH.sub.4 SSL-PF                                                           
            5           3      23    13                                   
NH.sub.4 SSL-PF                                                           
            5           7      21    9                                    
CSA STANDARD                     14.0                                     
                                     7                                    
Can 3.0188.2-M78                                                          
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     EXAMPLE 4 
     A test was run to determine the effect of molecular weight on the bonding properties of NH SSL-PF, using a press time of 5 minutes each. 
     The reactivity of NH 4  SSL-PF increased when low molecular weight NH 4  SSL was used. The reactivity was excellent whether or not the low molecular weight NH 4  SSL contained some reducing carbohydrate. When crude NH 4  SSL alone was utilized as adhesive (U.S. Pat. No. 4,265,846) the presence of carbohydrate was found indispensable to the bonding. The instability of the NH 4  SSL-PF mixtures is however increased with utilization of low molecular weight NH 4  SSL with a high level of carbohydrates. Higher pH and a small quantity of organic solvent, such as dimethyl formaldehyde was found to help stabilize the resin mixture. 
     
                       TABLE 4                                                     
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                     MOR     Aged MOR                                     
RESIN TYPE           (MPa)   (MPa)                                        
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Crude NH.sub.4 SSL with 30% PF                                            
                     23      11                                           
High molecular weight NH.sub.4 SSL                                        
                     22      10                                           
with 30% PF                                                               
Low molecular weight NH.sub.4 SSL                                         
                     26      13                                           
with 30% PF                                                               
Desugarized low molecular                                                 
                     24      12                                           
weight NH.sub.4 SSL with 30% PF                                           
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     EXAMPLE 5 
     Well cured phenol-formaldehyde adhesive is a stable cross-linked polymer which is resistant to water hydrolysis and will not emit formaldehyde gas. However, during manufacture of wood composites, if very short press cycles are used or if for some reason, poor quality control is exercised, the phenol-formaldehyde resin may be undercured. As a result the undercured phenol-formaldehyde bonded product will emit small amounts of formaldehyde gas. During experimentation with NH 4  SSL-PF resin adhesive, it was observed that the lignin-based adhesive emits much less formaldehyde as compared to phenol-formaldehyde resin when &#34;undercured&#34; by the same conditions. Less phenolic resin alone would not appear to explain this large reduction in formaldehyde emission. It is beleived that the ammonium ion from NH 4  SSL reacts with the free formaldehyde to form a stable hexamethlenetertramine complex. This formaldehyde scavenging effect of NH 4  SSL is illustrated on table 5 where interior grade particleboard was pressed employing a 8.5% resin content and a 10% mat moisture content. Formaldehyde emission was measured using the 2-hour Desiccator Test. 
     
                       TABLE 5                                                     
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                                FORMALDAHYDE                              
RESIN    TEMP.    TIME    MOR   EMISSION                                  
TYPE     (C.)     (MIN.)  (GPa) (mg/l)                                    
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PF       210      4       25.1  0.31                                      
NH.sub.4 SSL-PF                                                           
         210      4       20.1  0.06                                      
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     EXAMPLE 6 
     While PF resins are utilized exclusively for the manufacture of exterior grade wood composite products, it was found that NH 4  SSL-PF can be utilized under certain conditions for the manufacture of interior grade wood composites. Table 6 summarizes the test results for particleboards bonded with a short press cycle of 3-4 minutes at 180° and 210° C. The resin coated particles were pressed at 10 percent moisture content (MC). The board had no wet strength as it disintegrated in boiling water. It was however suprising to find that even with this short press cycle and high M.C. content (waferboard is made at 3 to 5 percent M.C.) a board which would meet CSA requirements for interior grade particleboard was obtained. Interestingly, the NH 4  SSL-PF resin cost at current market cost was less expensive than the urea-formaldehyde cost. 
     
                       TABLE 6                                                     
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                                   Aged                                   
            TEMP.    TIME    MOR   MOR   IB                               
RESIN TYPE  (°C.)                                                  
                     (MIN.)  (GPa) (GPa) (kPa)                            
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NH.sub.4 SSL-PF                                                           
            210      3       21    0     530                              
(70:30)                                                                   
NH.sub.4 SSL-PF                                                           
            180      4       19    0     521                              
(60:40)                                                                   
CSA STANDARD                 14                                           
Can 3-0188.1-M78                                                          
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