In: Moslemi, Al, ed. Inorganic bonded wood and fiber composite materials: Proceedings of the 2d international inorganic bonded wood and fiber composite materials conference; 1990 October 14-17; Moscow, ID. Madison, WI: Forest Products Research Society; 1991: 18-27. thermosetting resins. Hydration tem­ Technologies for Abstract perature curves illustrating the time- Existing technologies that reduce the related process of hydration are shown rapid production of production time of gypsum, magnesia, for b-gypsum hemihydrate, magnesi­ mineral-bondedwood and portland cement-bonded wood com­ um chloride cement, magnesium sul­ posite boards are reviewed. Experi­ fate cement, and portland cement in composite boards mental data that verify these technolo­ Figure 1. The course of hydration is gies are given, and new concepts are distinguished by two stages. The first presented. Adding an ammonium lig­ stage or "open time" begins with the nosulfonate retarder, an ammonium addition of water and ends when the Marull H. Simatupang sulfate accelerator, and gypsum dihy­ binder starts to set. During this period, Norbert Seddig drate to provide a crystallization nuclei the cement paste is still plastic and can be formed. Mixing and forming of the Christoph Habighorst resulted in an open time of 4 minutes and a setting time of 2 minutes for furnish must be performed within the Robert L. Geimer boards made with b-gypsum hemihy­ open time. Prior to the start of the sec­ drate. Steam injection during pressing ond stage or ''setting'' period, pressing of magnesia-bonded particleboards re­ of the green board should commence. duced press time to 15 seconds per mil­ Pressing should continue until the max­ limeter of board thickness. Adding car- imum hydration temperature is at­ bon dioxide as a gas injection or a tained. The time from mixing the water carbonate allowed the production of with the binder until the maximum hy­ rapid-setting cement-bonded particle- dration temperature is designated as boards. A fast-hardening cement mixture total hydration time. that is not retarded by soluble carbohy­ In the manufacture of rapid-setting drates and tannins can be obtained by boards, the course of hydration is al­ nixing portland cement, high alumina tered. Generally, the setting time is Short­ cement, and b-gypsum hemihydrate. ened. However, the open time must be Wood composites manufactured with sufficiently long enough to enable pro- iorganic binders may contain 30 to 90 per mixing and forming of the furnish. percent by weight of binder compo­ Retarders and accelerators am often nents. The three most important inor­ used to prolong the open time or short- ganic binders used in such composites en the setting time; retarders and accel­ (gypsum, magnesia cement, and port- erators may be added individually or in land cement) are generally crystalline combination. In general, additives in­ in nature. fluence the hydration reaction by mod­ The setting, hardening, or hydration ifying the crystallization process. A re­ of inorganic binders is a slow process tarder hampers the solution of the when compared to boards bonded with binder, thereby delaying hydration. An The authors are from the Federal Research Center for Forestry and Forest Products, Institute of Wood Chemistry and Chemical Technology of Wood Hamburg, Germany (Simatupang); Faculty of Natural Sciences, University of Hamburg, Hamburg, Germany (Sedding Habighorst); and USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin (Geimer). Figure 1. - Hydration temperature curve of gypsum, magnesium oxysulfate cement, magnesium oxychloride cement, and portland cement. 18 accelerator may enhance hydration by: fonate is added as plasticizer to reduce proximately 15 minutes, similar to pure 1) forming a supersaturated solution the water requirement. Potassium sul­ gypsum wallboard, but production line leading to early crystallization; 2) pro- fate, water-glass solution, or gypsum speed is limited to approximately 10 ducing crystallization nuclei, such as dihydrate are commonly used acceler­ meters per minute. Any attempt to in- occurs when gypsum dihydrate is added ators. Total cure time is approximately crease the setting rate of gypsum must to b-gypsum hemihydrate; 3) increas- 15 minutes at a production line speed of consider the time needed to mix and ing temperature, thus promoting the about 30 meters per minute. form the composite ingredients. Sev­ solubility of the binder and 4) direct The strength-enhancing advantages eral methods including the encapsula­ reaction with a binder component, such of mixing wood with an inorganic tion of gypsum dihydrate with starch as occurs when carbon dioxide (CO2) is binder were foreseen as early as 1880 (3) have been used to mask the effect of added to portland cement. when a German patent was issued that an accelerator to allow ample open time. This paper reviews existing technol- described a light-weight, gypsum­ Bucking (2) reported a method spe­ ogies that reduce the production time of bonded wood wool board (17). Wood cific to the production of three-layer gypsum, magnesia, and portland ce- composite boards must be pressed or gypsum-bonded particleboards. The re­ ment-bonded wood composite boards. clamped until the gypsum has hard- quired make water for the entire board It also reports results of our research ened sufficiently to withstand the was deposited in the middle layer. In carried out in Hamburg, Germany, and springback forces exerted by the wood addition to air-dried fine wood parti­ Madison, Wisconsin, that verify these particles or fiber. Gypsum particleboa­ cles and gypsum hemihydrate, the fur­ technologies and introduce new concepts. rds are made commercially in a discon­ nish for the outer layers contained gyp- tinuous process requiring 2 hours of sum dihydrate as a nuclei builder. During Gypsum-bonded composite boards clamping time (26). The long press time pressing, moisture diffused from the Gypsum wallboard is produced in is needed because of the retarders used middle layer to both outer layers, acti­ large quantities throughout the world to obtain the required open time. Con- vating the setting of the gypsum hemi­ for use as sheathing material in interior tinuous production of gypsum-bonded hydrate. A retarder can be added to applications. These boards are made particleboard on a pilot-plant scale has regulate the setting time of the gypsum from foamed b-gypsum hemihydrate been reported by Bucking (2), but the hemihydrate in the middle layer. Ac­ (plaster of paris) and derive their bend- 6-minute press time is still too long for cording to Bucking, the setting time of ing strength from a thin layer of paper profitable commercial operation. gypsum is generally 2.5 times the open overlay Addition of additives regulate Gypsum fiberboard has less Spring- time. Since the open time was regulated the setting behavior and permit pro- back than gypsum-bonded particlebo­ to 24 minutes, the setting or pressing duction on a continuous basis using a ard and is produced commercially on a time was 6 min. In another variation of belt conveyor (36). Generally, lignosul- continuous basis. Cure times are ap- this method, water was added as gran­ ulated ice (1). In our laboratory, we examined com­ binations of ammonium lignosulfonate as a retarder and ammonium sulfate as a latent accelerator for the production of gypsum particleboards. Three meth­ ods to mask the acceleration effect were applied: 1) encapsulation of ammoni­ um sulfate with polyvinyl-propylene (PVP);2) absorption of ammonium sul­ fate in fine wood particles, and 3) ab­ sorption of ammonium sulfate in a super absorber. In all cases, an ammo­ nium lignosulfonate retarder solution was incorporated into the furnish by mixing with the wood particles. In the first method, encapsulation of ammo­ nium sulfate with the water soluble PVP proved to be very effective in mask­ ing the acceleration effect. The masking effect lasted approximately 5 to 6 min­ utes, which is too long to be used in a continuous process but may have ap plication in production with a single- opening discontinuous press. Figure 2. -Schematicof total hydration time of regulated gypsum using various In the second method, we treated a additives. Plain area represents open time; shaded area represents setting small portion of fine wood particles time. with an ammonium sulfate solution and 19 subsequently dried at 4°C. The ammo­ by reacting magnesium hydroxide with tion of inogonic-bonded wood compos­ nium sulfate was deposited as crystals carbon dioxide and heating the formed ites. The setting time is about the same as in the wood particles. The treated wood reaction product. magnesium oxychloride cement (7). particles were then mixed with the re­ Magnesium oxide is often referred to Cement prepared from caustic-cal­ mainder of the furnish and used to as magnesia. Magnesium oxide may be ined magnesia and carbonic acid is make gypsum-bonded particleboards. either caustic-calcined magnesia or dead- designated as magnesium oxycarbon­ The results of bending strength tests burned magnesia. Dead-burned mag­ ate cement. It is prepared by obtaining showed the feasibility of using a combi­ nesia is also called heavy magnesia. The magnesium carbonate trihydrate from nation of a retarder and a latent acceler­ choice of the magnesia form to make the treatment of a magnesium hydroxide ator. The ammonium lignosulfonate magnesia cement is predicated by the slurry with exhaust gas from burners or prolonged the open time of the binder chemical