Glue line

In gluing, the adhesive must not saturate veneers or wood chips, but must remain in the glue line on the surface of the chips or between the pHes. The adhesives are generally of high viscosity so that they remain in the glue line. Thickeners and extenders, such as powdered pecan shells and wheat flour, are often used. 

Terpene phenolic resins can also be added to solvent-borne CR adhesives to increase open tack time and to provide a softer glue line than /-butyl phenolic resins. To provide adequate hot bond strength, these resins are used in combination with a polyisocyanate curing agent. 

Fully hydrated potassium ion coordinates about 10-11 molecules of water, whereas sodium coordinates about 16-17 molecules [115]. The ionic mobility of potassium is about 50% greater than that of sodium. In simple terms, this means that more of the water in a potassium-catalyzed resin will be available as free water for viscosity reduction and that movement of water from a glue line into the wood will have less effect in moving the adhesive off of the glue line with it. 

This makes a mix with a viscosity of 5000-7000 centipoise. The total mix solids are 40% and the resin solids in the mix are 26%. A mix like this would be used on Douglas fir veneer at the rate of about 55 pounds per 1000 square feet of veneer surface (double glue line basis). The 43% solids resin would be used at about 500 cps viscosity. 

Oriented strandboard (OSB) resins are quite similar to plywood resins, though they tend to be lower in alkalinity and higher in solids. Sinee OSB resins are sprayed onto dry wood in small discrete droplets rather than applied in glue lines, the problems associated with holding the glue on the wood surfaee are not usually issues of coneem. Adhesive dry-out and loss of flow displace over-penetration as major difficulties. 

Disadvantages of the phenolic resins are the necessary longer press times compared to UF-resins, and the dark color of the glue line and the board surface. There is also the possibility of a higher moisture pickup of the boards when stored at higher relative humidity due to the hygroscopicity of the high alkali content of the resin. 

Special resins consist of a two-phase system composed of a mix of a highly condensed, and no longer soluble, PF-resin with a standard type PF-resin [63]. Another two-phase resin is composed of a highly condensed PF-resin, still in an aqueous solution, and of a PF dispersion [64]. The purpose of such special resins is the gluing of wet wood, where the danger of overpenetration of the resin into the wood surface exists and would cause a starved glue line. 

The penetration behavior of resins into the wood surface also is influenced by various parameters, like wood species, amount of glue spread, press temperature and pressure and hardening time. The temperature of the wood surface and of the glue line and hence the viscosity of the resin (which itself also depends on the already reached degree of hardening) influence the penetration behavior of the resin [79]. 

Acid-induced gelling reactions of PF resins can cause severe deterioration of the wood substrate and therefore have lost any importance in the wood adhesives field. Pizzi et al. [85] describe a procedure for the neutralization of acid-hardened PF glue lines by partly using as hardener a mix of p-toluene sulfonic acid with a... 

Polyurethane adhesives are formed by the reaction of various types of isoeyanates with polyols. The polar urethane group enables adhesion to various surfaees. Depending on the raw materials, glue lines with rubber-like elastic to brittle-hard behavior ean be aehieved. The presence of reactive terminal groups provides a ehemieally hardened adhesive. When polymerized to a high enough molecular weight, the adhesive ean be physically rather than chemically hardened, i.e. a hot melt. 

The influence of wood includes a variety of topics. Wood bonding is often described as a chain of several links wood (substance), wood surface, interface between wood and adhesive, surface of the glue line (boundary layer), glue line itself. As with all chains, the weakest link determines the strength of the chain. In wood gluing, in most cases, the interphase is the weakest link. 

During the production of wood-based panels, part of the adhesive penetrates into the wood surface. An overpenetration causes starved glue lines, whereas a low penetration limits the contact surface between wood and the adhesive low penetration often is the consequence of bad wetting behavior. 

Fig. 2.3.7 Lower GARField profiles showing a UF (urea formaldehyde) glue line acting as a barrier to water transport for up to 24 h. The glue line is at 800 pm on the scale. Wood is above and below this. The water reservoir is beyond 1300 pm. The profiles shown were recorded after 20 (thin line), 100 and 1400 (thick line) min of exposure to water. Upper plots of the magnetization signal intensity in the lower and upper wood layers as a function of time for three glues urea formaldehyde (squares), phenolic resorcinol formaldehyde (triangles), and poly (vinyl acetate) (diamonds).

The gluability of the lignin-epoxy resin adhesives was found to be improved by the addition of calcium carbonate (50% by weight) to the liquid resin. This must be attributed to the nature of the weak alkali in calcium carbonate as a cure accelerator, and to the reinforcement effect of fillers. Since wood surfaces are acidic, the addition of alkaline fillers effectively alters the pH of the glue line. 

After veneers have been unitized from small strips to full sheets of the desired width, usually 4 feet by 8 feet in softwood plywood and various widths and lengths for hardwood custom manufactured plywood, they are ready to be fed through a glue application which coats one or both sides of veneer sheets with liquid adhesive. These applicators control the amount of adhesive transferred to the veneer. The amount of glue mix applied per 100 square feet of surface area (single glue line basis) will approximate 4-5 pounds and the amount of resin solids 1-1.5 pounds. 

The combined panels of the desired number of plies or layers are now transferred either directly to the hot press or to a cold press and then to the hot press. The cold prepress tacks the veneer adhesive glue line together so the panels require smaller openings in the hot press, thus permitting larger numbers of hot press openings per cycle of production. 

In hardwood plywood, cold presses without heat application are occasionally used where curved plywood in particular is desired. Dielectric heat curing of the plywood glue lines as a means to increase productivity is sometimes used in conjunction with this process. The acid salt becomes critically important in these cases. 

The exterior durability of softwood veneer species in this country has been demonstrated. There have been some difficulties encountered in the long term exterior durability of some Asian veneer species when bonded with phenolic resins. Extractives interfering with the cure of the resin directly or the resin bonded to the extractives rather than the wood cause failures along the glue line. These species shrink and swell more than native softwoods. Stresses are greater and breaks in the wood hydrogen... 

Another area of real concern deals with HEW Occupational and Health Standards on hazardous substances listing formaldehyde and phenol. With phenolic adhesives, this is not a problem since all of the resin solids and non-volatile solids are usually tied up in the set glue line. 

Well over 95 percent of the hardwood plywood production in the United States uses urea-formaldehyde as the adhesive bonding agent between veneers for reasons of quality, intended use and economics. In this industry, formaldehyde evolution is a subject of concern because normally all of the adhesive formaldehyde released under elevated temperatures is not tied up in the set glue line. This is an area that is of real concern to the particleboard industry which uses urea-formaldehyde as its binder. 

Figure 3. The absorption of alcohol-water solvent by wood near glue line (0 min open assembly time and 15 min closed time)...

In contrast, compare with the picture of a joint made with the same glue and wood but after 60 minutes open time and 0 minutes closed, Figure 4. The very thick glue line and almost total lack of solvent absorption are quite apparent. The wood failure in testing this joint was very low, while that of the previous figure was very high. 

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