Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1996-03-20
1998-12-08
Green, Anthony
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106711, 106781, 106783, 106785, 156 34, 264333, C04B 1100
Patent
active
058463178
DESCRIPTION:
BRIEF SUMMARY
Described is a method of controlling the hydration behaviour of gypsum in the manufacture of composite materials, in particular board.
Gypsum-bound composite materials are increasingly produced according to the semi-dry method. The semi-dry method is a process in which the aggregates (e.g. chips or fibres) are mixed with the gypsum binder into a loose mixture. This mixture is spread to form a low-density fleece by means of automatic spreading equipment. Then, pressure is applied to compact the fleece so that the result after hydration is a more or less compact and solid board depending on the composition of the mixture and the amount of pressure applied. The different variations of the semi-dry method all share the same disadvantage: The short open time (OT) of commercial gypsum binders used in discontinuous systems is not sufficient to perform all operations (i.e. dosing, mixing, spreading, precompacting) properly before the beginning of hydration (BH) (see FIG. 1, curve 1). Therefore, retarders are added to the mixtures to be processed. However, this causes an undesirable side effect, i.e. the end of the hydration process (EH) is delayed (see FIG. 1, curve 2). This means that even longer compacting times are required, which has a marked negative impact on both continuous and discontinuous manufacturing. In those cases where a practicable solution has been found, the compacting presses required in the production plants are a considerable cost factor in the production process.
Several methods have already been proposed to avoid these disadvantages. DE-OS 37 10 907.3 describes such a method: It is proposed to add a retarder to the mixture that delays the hydration of the gypsum until a 1 m high stack of boards has accumulated for batch-processing in a compacting press of appropriate dimensions. A heavy clamping device is used to hold the stack of boards to ensure that the entire stack can be removed from the press to avoid that the press is blocked by the stack during the entire duration of the prolonged hydration process. This device is closed and locked as soon as the compaction process begins. After the compaction process, the stack is removed from the press together with the clamping device and left to harden outside the press. However, this compaction method requires an expensive special press and a sufficiently large number of clamping devices depending on the desired plant capacity. Additionally, boards pressed in stacks are marked by significant thickness tolerances, which means that a separate grinding process is usually required to calibrate the boards. This complicated and expensive method is described in detail by E. Frick (in Zement-Kalk-Gips, issue no. 7/1988).
Another method that can also be used for the continuous production of gypsum-bound composite materials is described in EP 171665 B2. The method of short-term overcompaction disclosed under this patent recommends a simplified continuous compaction system. However, this method has not been widely accepted in practice due to the long hydration times and the required long calibration unit. An additional disadvantage is that the method cannot be used for the production of highly resistant composite materials compacted to the point of incompressibility.
A method to reduce the hydration time without reducing the open time (OT) by adding activating energy during the hydration time (HT) is described in DE OS 36 42802. The activating energy can be thermal energy, electrical energy or electromechanical energy. Due to the very nature of the gypsum hydration process, it is not possible to accelerate the process by adding thermal energy. The result will rather be a delay, because the solubility of the semihydrate decreases as the temperature increases, and the dihydrate formation mechanism takes place via the dissolution phase. The addition of the entire quantity of mixing water and an accelerator contained in capsules is described in DE OS 40 31 935, while DE OS 39 29 703 describes a method to add the accelerator in the shape of ice granules. These
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Exhibit 1, Technical and Chemical Data (in the German language, with a copy f the English translation). No Date.
Exhibit 2, Wood Lexicon, "Reference Work for the Wood and Forestry Industry" (in the German language, with copy of the English translation). No Date.
Sattler Heinz
Schmelmer Bernhard
Thole Volker
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Green Anthony
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