Adhesive bonding and miscellaneous chemical manufacture – Methods – Plaster board making
Patent
1998-08-07
2000-03-28
Aftergut, Jeff H.
Adhesive bonding and miscellaneous chemical manufacture
Methods
Plaster board making
106772, 264333, B28B 152, C04B 2814
Patent
active
060426659
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the production of composite boards and shaped bodies, totally consisting of gypsum or gypsum mixed with reinforcement materials and fillers. Instead of the commonly known calcium sulfate hemihydrate (CaSO.sub.4 .times.1/2 H.sub.2 O; hereinafter hemihydrate), calcium sulfate dihydrate (CaSO.sub.4 .times.2 H.sub.2 O; hereinafter dihydrate) is used as base material in the process according to the invention.
For the production of gypsum-based composite boards essentially methods of two kinds are known [P. Takacs: Faipar (in English: "Woodworking Industry") 8, 140-149 (1993)]. According to one of these, hemihydrate being the base material of the process is produced from dihydrate in a separate gypsum-calcinating furnace. This binding material is mixed with water required for the hardening of hemihydrate, fillers and reinforcement materials as well as other conventional additives. A mat is formed from the mixture, which is then pressed. During compression the gypsum hardens and the board is formed out. The moisture content of the product is adjusted by an artificial final drying. The process is called as wet, half-dry or dry process, depending on the amount of water used for mixing-in as well as the technological site and time of water addition.
According to the other method, a dihydrate flour is utilized as base material. A suspension is prepared from dihydrate, fillers and reinforcement materials, other conventional as well as water. Therefrom, a mat is first prepared, which is then dehydrated. Dihydrate forming the main constituent of the mat is first calcinated (dehydrated) in a heat-press or autoclave, then the hemihydrate obtained is rehydrated with water at the same place, simultaneously with the decrease in temperature and pressure conditions. The moisture content of the hardened board is adjusted similarly by a final drying.
There exists no difference in the basic principle of these traditional techniques. Each of them is based on the dehydration and rehydration of gypsum known for several hundreds of years. The basic principle is not altered by carrying out the procedures of crystal water loss and re-incorporation (de- and rehydration) in a single step or several steps, respectively.
Thus, as it is known e.g. from the German published patent application No. 34 19558 A1, the processes of dehydration and rehydration are carried out in the same autoclave. In this process, an aqueous suspension is prepared from a mixture containing finely ground dihydrate, a fibrous material (e.g. waste paper fibres) in an amount of not more than 10% by weight as well as substances controlling the hardening. After being applied onto an endless screen belt, the dilute slurry-like suspension is dehydrated to a moisture content of less than 40% by weight in a belt press by the simultaneous use of vacuum and pressure. After cutting the endless mat to the desired size, the raw boards are first calcinated in an autoclave, then rehydrated, in a single run. The temperature of the autoclave is adjusted first to 135-145.degree. C., then below 100.degree. C. to transform the hemihydrate formed again to dihydrate. The temperature of 142.degree. C. and pressure of 4 bar defined in an example of the description is one point of the saturated vapour curve.
Other known processes may be considered to be the further developed variants of the former basic conception. Thus, e.g. according to the European published patent application No. 0 173 351 A2, a difference consists only in that the hemihydrate is prepared in a heat-press but not in an autoclave. It appears from the European published patent application No. 0 309 709 A1 that in the first step the boards are calcinated at 100-170.degree. C. under atmospheric pressure in an autoclave, then they are hardened, i.e. rehydrated, with water at the same place. The process according to European published patent application No. 0 294 566 A1 is a further example of diminishing the drawbacks of former processes; according to this water containing also hardening
REFERENCES:
patent: 4213931 (1980-07-01), Trutnev et al.
patent: 4840688 (1989-06-01), Vogt
patent: 5015449 (1991-05-01), Kosolowski
patent: 5041333 (1991-08-01), Conroy
Hadnagy Jozsef
Juhasz Zoltan A.
Kiraly Bela
Szabo Peter
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