Development of Formaldehyde-Based Wood Adhesives with Co-Reacted Phenol/Soybean Flour

Ien resins, each with two replicates, were formulated from soy flour hydrolyzates and prepared with two hydrolyzation variables: phenol-to-soy flour ratios (w/w) of 4/4,3/4,2/4, l/4, and O/4 and sodium hydroxide-to-soy flour ratios (w/w) of 0.39 and 0.78. In addition, two commercially available phenol formaldehyde (PF) resins were used in the study as controls. The most ‘interesting result in the study of resin properties was gel time. When soy flour was hydrolyzed in the presence of phenol, the gel times were consistently longer with low caustic hydrolysis than that of high caustic hydrolysis. When soy flour was hydrolyzed without phenol, however, the gel times were longer with high caustic hydrolysis than that of low caustic hydrolysis. The internal bond (IB) strengths of oriented strandboard panels made with co-reacted soy hydrolyzate resins were highly comparable with boards made with the two phenolic control resins. When soy flour is hydrolyzed under high caustic conditions in the presence of phenol, acceptable co-reacted resins resulted; while under low caustic hydrolysis conditions with low phenol or no phenol at all, the best IB strengths were achieved. The overall high performance of the co-reacted resin prepared with phenol-to-soy flour ratio of l/4 under low caustic hydrolysis was the most interesting. It not only yielded the highest IB, attained one of the best dimensional stability results among all the co-reacted resins, and ranked high in modulus of elasticity. It also is significant that it was able to hydrolyze soy flour at low caustic content while maintaining a workableviscosity. With a 30-percent phenol substitution with soy flour hydrolyzate, a co-reacted soy hydrolyzate resin system as developed in the study could result in more than 20percent saving in material cost as compared to a conventional 2/l ratio (F/P) phenolic resin system used for bonding structural flakeboards. Introduction Adhesives based on soybean protein were importaut in the early development of the Douglas-fir plywood industry. Soybean adhesives produced a strong bond which, although not waterproof, was water resistant. At the peak in 1942, the amount of soybean glue used for gluing plywood, together with a small amount of casein glue, represented 85 percent of the total softwood plywood glue production (1). However, the development of highly durable, exterior-grade synthetic adhesives from petrochemicals resulted in a long-term decline in the production of soybean adhesives. Cold-press soybean adhesives fell from about 28 percent of the market in 1954 to almost zero in 1970; while hot-press soybean adhesives (containing dried blood) rose to a peak of 45 percent in 1956, then fell to near zero in 1973 (3). Nevertheless, intermittent declines in petroleum supplies and increases in petroleum prices, together with the worldwide interest in the development of environmentally friendly adhesives from renewable resources, have stimulated a renewed interest in soybean adhesives. Efforts have been made to find new ways to overcome limitations inherent in the use of soy flour when gluing wood. A recent study Green Chemistry for Adhesives l 13 (5) has shown that a concentrated soy protein-based resin was capable of finger jointing green lumber at room temperature when used with phenol-resorcinol-formaldehyde (PRP) resin in a modified finger-jointing operation. Altering the structure of the soy protein to reduce costs, Vajayendran (7,8) and Clay (2) have shown that a mixed hydrolyzed soy flour/PRF resin system can be applied to the two sides of lumber at room temperature in finger-joint operations. More recently, soybean-based adhesive resins were formulated by crosslinking hydrolyzed soy flour with 30 percent (w/w) of phenol formaldehyde (PF) prepolymer for bonding medium density fiberboard (MDF) and flakeboard that met the product performance requirements (6). It is generally recognized that for soy protein to become a useful base for adhesives, native soy protein needs to be hydrolyzed to break the internal bonds and uncoil the polar protein molecules. This exposes the functional groups of the protein complex and increases its water solubility and surface area. The soy protein hydrolyzate can then be readily mixed as a co-resin with conventional PRF or PF resins to facilitate the chemical reaction between functional groups as well as molecular entanglement of soy protein and PI@ or PF resins to form a highly crosslinked thermoset matrix. Almost all previously mentioned studies have followed this general approach. While combining polymers of different properties has been increasingly practiced by the plastic industry to obtain new and useful materials (4), it also hindered the development of highly reactive polymer blends. In this study, a new approach was used to hydrolyze lowcost soy flour in the presence of phenol. This hydrolyzate was then substituted for phenol in conventional phenolic resin synthesis. The objective of this new approach is to obtain a co-reacted resin system containing a soy flour hydrolyzate that is suitable for bonding exterior structural panels. Experimental Methods Alkaline Hydrolysis of Soy Flour Soy flour was hydrolyzed in a Parr reactor equipped with a stirrer drive system. The variables used in the hydrolysis reaction were: l five phenol-to-soy flour ratios (w/w): 4/4, 3/4, 2/4, l/4, and O/4, and l two sodium hydroxide-to-soy flour ratios (w/w): 0.39 and 0.78. Thus, 10 soy flour hydrolyzates were prepared. To initiate the hydrolysis, the ingredients (i.e., soy flour, phenol, and sodium hydroxide) were weighed and placed in the reactor. The concentration of the ingredient mix was adjusted to 46 percent by the addition of . water. After pre-mixing for 5 minutes at room temperature, the heater was turned on and temperature controlled at 120°C. After continuous agitation for 60 minutes, the hydrolysis reaction was terminated by removing the reactor from the heater and cooling it to room temperature with running tap waterz The hydro&ate was then transferred into containers and stored at room temperature before use. Viscosity, pH, solid content, and alkaline content of the hydrolyzates were determined. Resin Preparation All phenol-based, co-reacted soy flour hydrolyzate resins were synthesized in the laboratory. A total of 10 resins, based on the 10 hydrolyzates, were formulated. The general resin formulation conditions were: 1. the molar ratio of formaldehyde-to-(phenol + soy flour hydrolyzate)-to-sodium hydroxide was controlled at 1.67/l/0.45, 2. the solid content of the reaction mixture was maintained at 46 percent by water addition, 3. the soy flour replaced 30 percent of phenol by weight, and 4. the molar weight of soy flour was assumed to be equal to that of phenol. To prepare each resin all phenol, soy flour hydrolyzates, and water were placed in a resin reaction kettle. Formaldehyde was added in four equal parts at 5min. intervals. ‘I% initiate the reaction, the mixture was heated and maintained at 80°C to promote the addition reaction for the formation of the methyl01 phenols. At 20 minutes reaction time, the reaction temperature was raised gradually to 95°C in 40-min. intervals, and this temperature was maintained for 30 minutes to promote the condensation reaction. Additional sodium hydroxide was introduced with four consecutive additions of 0.3 moles sodium hydroxide each at 40-min. intervals beginning at the reaction time of 90 minutes. The sodium hydroxide additions were also accompanied with a 10°C temperature decrease after each sodium hydroxide addition. All reactions were terminated by rapidly cooling the mixture to 25°C at 240 minutes. Gel time, pH, solid content, and viscositywere determined. The gel time was measured with a Sunshine gel timer at 1 OO”C, and theviscosity was measured at room temperature (25°C). Flakeboard Manufacture All panels were made in the laboratory with mixed hardwood flakes obtained from a local flakeboard plant. Two flakeboards were fabricated for each resin adhesive. Thus, a total of 20 panels were made. To prepare each panel, flakes were weighed and placed in a rotating drum-type blender. The resin blend, 14 l Wood Adhesives 2000 ‘Iable I.-Physical p.roperties of resin adhesives. Sample ID P/S ratioa Solid content PH Viscosity Gel time (WrwJ (W (qrs) bw High caustic hydrolysis 1OOH (44) 5 1 . 5 12.89 320 43.8 75 H (3/4) 5 1 . 5 12.62 3 8 0 40.5 50H ia41 5 1 . 6 12.82 370 36.9 25 H (114) 5 1 . 7 12.77 ,510 36.7 OH (W4) 5 2 . 2 12.72 4 1 0 43.2 Low caustic hydrolysis IOOH (4/4) 51.1 12.64 7 1 0 5 6 . 3 75H (3/4) 5 1 . 2 12.61 420 55.6 50H (W41 5 1 . 5 12.68 4 7 0 48.2 25 H I 114 1 5 1 . 7 12.81 4 1 0 49.2 OH (W41’ 5 1 . 6 12.80 4 6 0 35.3 Phenol formaldehyde resin n-A 5 5 . 0 1 0 . 2 6 0 0 1 7 . 2 P P B 5 1 . 0 11.10 3 4 0 2 2 . 1 p Phenol-to-soy flour ratio in hydrolysis reaction. in amounts equal to 4.5 percent of the ovendry weight of flakes, were then weighed and applied by air-atomizing nozzles. Average moisture content of the flakes after spraying was 11 percent. After blending, the randomly oriented flakes were carefully felted into a 17.5 by 20-in. box to form the mat. The mat was transferred immediately to a 20by 20-in. single-opening hot press with the platen temperature regulated at 188°C. Sufficient pressure (about 550 psi) was applied so that the platens closed to 0.5-m stops in approximately 45 seconds. Press time was 4 minutes, 15 seconds after closure. All panels were hot-stacked in a wood box overnight immediately after removing from the hot press. Sampling and Rsting The flakeboards, removed from the hot-stack box, were trimmed to 14by 16.5-in. panels. After trimming, each panel was cut to yield three static-bending specimens (2-by 14-in.), two-dimensional stability test specimens (2by 14-in.), and twelve specimens (2by IL-in.) for tensile strength perpendicular to the face (IB). Bending and IB specimens were evaluated according to ASTM D1037-72. For durability evaluations, an ovendry to vacuum pressure soak test (ODVPS) was employed with the following constraints: 1. dried at 100°C oven for 24 hours, 2 . placed in a pressure cylinder and flooded with tap watel; 3 . vacuumed in 27 +2 in. of mercury for 1 hour, and 4. put under 90 rt 10 psi for 2 hours. The procedure was developed by the American Plywood Association and designated as APA ‘l&t Method P-1 for linear expansion (LE) evaluation. Linear expansion and thickness swell (TS) values are based on the change from the ovendry condition to the end of the ODVPS cycle. Results as&Discussion Resin Properties Average physical properties of co-reacted soy hydro1yzateJPF resin adhesives are summarized in Table 1. On average, the resin properties, with exception of gel time, showed little difference among the 10 resins prepared in the study. Solid content ranged from 41.4 percent to 44.6 percent, and pH ranged from 12.61 to 12.89. The results suggest that the soy flour hydrolyzates prepared in presence of various amounts of phenol at two caustic content levels had no significant effect on resin properties when the same proportion of formaldehyde, phenol, soy flour hydrolyzate, and sodium hydroxide content were maintained at the final co-reacted soy resins. In comparison with the property of the two PF resins included in the study as controls, the co-reacted soy resins had lower solid content, but higher resin PH. Average resin viscosity ranged from 320 to 610 cps that were felt in line with the targeted viscosity of 300 to 450 cps in resin synthesis. In general, the viscosity was comparable with that of the PF resins. The most interesting result in the study of resin properties was gel time. When soy flour was hydrolyzed in the presence of phenol, the gel times were consistently longer with low caustic hydrolysis than with high Green Chemistry for Adhesives l 15 Ykble Z.-Average physical and mechanical properties of flakeboard made with soy hydrolyzate/phenolic resin adhesives.